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diff --git a/old/54612-8.txt b/old/54612-8.txt deleted file mode 100644 index 05fb98d..0000000 --- a/old/54612-8.txt +++ /dev/null @@ -1,24349 +0,0 @@ -The Project Gutenberg EBook of The Principles of Biology, Volume 1 (of 2), by -Herbert Spencer - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: The Principles of Biology, Volume 1 (of 2) - -Author: Herbert Spencer - -Release Date: April 26, 2017 [EBook #54612] - -Language: English - -Character set encoding: ISO-8859-1 - -*** START OF THIS PROJECT GUTENBERG EBOOK PRINCIPLES OF BIOLOGY, VOL 1 *** - - - - -Produced by Keith Edkins, MFR, Adrian Mastronardi and the -Online Distributed Proofreading Team at http://www.pgdp.net -(This file was produced from images generously made -available by The Internet Archive/American Libraries.) - - - - - -Transcriber's note: Text enclosed by underscores is in italics (_italics_). - A single underscore NH_3 introduces a subscript, - and a caret 30^a a superscript. - - * * * * * - - - - -THE PRINCIPLES OF -BIOLOGY - -BY - -HERBERT SPENCER - -[Illustration] - -IN TWO VOLUMES - -VOLUME I - - -NEW YORK AND LONDON -D. APPLETON AND COMPANY -1910 - - - - -COPYRIGHT, 1866, 1898, -BY D. APPLETON AND COMPANY. - -PREFACE - -TO THE REVISED AND ENLARGED EDITION. - - -Rapid in all directions, scientific progress has during the last generation -been more rapid in the direction of Biology than in any other; and had this -work been one dealing with Biology at large, the hope of bringing it up to -date could not have been rationally entertained. But it is a work on the -_Principles_ of Biology; and to bring an exposition of these up to date, -seemed not impossible with such small remnant of energy as is left me. -Slowly, and often interrupted by ill-health, I have in the course of the -last two years, completed this first volume of the final edition. - -Numerous additions have proved needful. What was originally said about -vital changes of matter has been supplemented by a chapter on "Metabolism." -Under the title "The Dynamic Element in Life," I have added a chapter which -renders less inadequate the conception of Life previously expressed. A gap -in preceding editions, which should have been occupied by some pages on -"Structure," is now filled up. Those astonishing actions in cell-nuclei -which the microscope has of late revealed, will be found briefly set forth -under the head of "Cell-Life and Cell-Multiplication." Further evidence and -further thought have resulted in a supplementary chapter on "Genesis, -Heredity, and Variation"; in which certain views enunciated in the first -edition are qualified and developed. Various modern ideas are considered -under the title "Recent Criticisms and Hypotheses." And the chapter on "The -Arguments from Embryology" has been mainly rewritten. Smaller increments -have taken the shape of new sections incorporated in pre-existing chapters. -They are distinguished by the following section-marks:--§ 8a, § 46a, § 87a, -§ 100a, § 113a, § 127a, §§ 130a-130d. There should also be mentioned a -number of foot-notes of some significance not present in preceding -editions. Of the three additional appendices the two longer ones have -already seen the light in other shapes. - -After these chief changes have now to be named the changes necessitated by -revision. In making them assistance has been needful. Though many of the -amendments have resulted from further thought and inquiry, a much larger -number have been consequent on criticisms received from gentlemen whose aid -I have been fortunate enough to obtain: each of them having taken a -division falling within the range of his special studies. The part -concerned with Organic Chemistry and its derived subjects, has been looked -through by Mr. W. H. Perkin, Ph.D., F.R.S., Professor of Organic Chemistry, -Owens College, Manchester. Plant Morphology and Physiology have been -overseen by Mr. A. G. Tansley, M.A., F.L.S., Assistant Professor of Botany, -University College, London. Criticisms upon parts dealing with Animal -Morphology, I owe to Mr. E. W. MacBride, M.A., Fellow of St. John's -College, Cambridge, Professor of Zoology in the McGill University, -Montreal, and Mr. J. T. Cunningham, M.A., late Fellow of University -College, Oxford. And the statements included under Animal Physiology have -been checked by Mr. W. B. Hardy, M.A., Fellow of Gonville and Caius -College, Cambridge, Demonstrator of Physiology in the University. Where the -discoveries made since 1864 have rendered it needful to change the text, -either by omissions or qualifications or in some cases by additions, these -gentlemen have furnished me with the requisite information. - -Save in the case of the preliminary portion, bristling with the -technicalities of Organic Chemistry (including the pages on "Metabolism"), -I have not submitted the proofs, either of the new chapters or of the -revised chapters, to the gentlemen above named. The abstention has resulted -partly from reluctance to trespass on their time to a greater extent than -was originally arranged, and partly from the desire to avoid complicating -my own work. During the interval occupied in the preparation of this volume -the printers have kept pace with me, and I have feared adding to the -entailed attention the further attention which correspondence and -discussion would have absorbed: feeling that it was better to risk minor -inaccuracies than to leave the volume unfinished: an event which at one -time appeared probable. I make this statement because, in its absence, one -or other of these gentlemen might be held responsible for some error which -is not his but mine. - -Yet another explanation is called for. Beyond the exposition of those -general truths constituting the Principles of Biology as commonly accepted, -the original edition of this work contained sundry views for which -biological opinion did not furnish any authority. Some of these have since -obtained a certain currency; either in their original forms or in modified -forms. Misinterpretations are likely to result. Readers who have met with -them in other works may, in the absence of warning, suppose, to my -disadvantage, that I have adopted them without acknowledgment. Hence it -must be understood that where no indication to the contrary is given the -substance is unchanged. Beyond the corrections which have been made in the -original text, there are, in some cases, additions to the evidence or -amplifications of the argument; but in all sections not marked as new, the -essential ideas set forth are the same as they were in the original edition -of 1864. - - BRIGHTON, - - _August, 1898_. - - - - -PREFACE. - - -The aim of this work is to set forth the general truths of Biology, as -illustrative of, and as interpreted by, the laws of Evolution: the special -truths being introduced only so far as is needful for elucidation of the -general truths. - -For aid in executing it, I owe many thanks to Prof. Huxley and Dr. Hooker. -They have supplied me with information where my own was deficient;[1] and, -in looking through the proof-sheets, have pointed out errors of detail into -which I had fallen. By having kindly rendered me this valuable assistance, -they must not, however, be held committed to any of the enunciated -doctrines that are not among the recognized truths of Biology. - -The successive instalments which compose this volume, were issued to the -subscribers at the following dates:--No. 7 (pp. 1-80) in January, 1863; No. -8 (pp. 81-160) in April, 1863; No. 9 (pp. 161-240) in July, 1863; No. 10 -(pp. 241-320) in January, 1864; No. 11 (pp. 321-400) in May, 1864; and No. -12 (pp. 401-476) in October, 1864. - - _London, September 29th, 1864._ - - - - -CONTENTS OF VOL. I. - - ----- - - - CHAPTER PAGE - - I. ORGANIC MATTER 3 - II. THE ACTIONS OF FORCES ON ORGANIC MATTER 27 - III. THE RE-ACTIONS OF ORGANIC MATTER ON FORCES 45 - III^A. METABOLISM 62 - IV. PROXIMATE CONCEPTION OF LIFE 78 - V. THE CORRESPONDENCE BETWEEN LIFE AND ITS CIRCUMSTANCES 91 - VI. THE DEGREE OF LIFE VARIES AS THE DEGREE OF 101 - CORRESPONDENCE - VI^A. THE DYNAMIC ELEMENT IN LIFE 111 - VII. THE SCOPE OF BIOLOGY 124 - - PART II.--THE INDUCTIONS OF BIOLOGY. - I. GROWTH 135 - II. DEVELOPMENT 162 - II^A. STRUCTURE 181 - III. FUNCTION 197 - IV. WASTE AND REPAIR 213 - V. ADAPTATION 227 - VI. INDIVIDUALITY 244 - VI^A. CELL-LIFE AND CELL-MULTIPLICATION 252 - VII. GENESIS 269 - VIII. HEREDITY 301 - IX. VARIATION 320 - X. GENESIS, HEREDITY, AND VARIATION 336 - X^A. GENESIS, HEREDITY, AND VARIATION--_Concluded_ 356 - XI. CLASSIFICATION 374 - XII. DISTRIBUTION 395 - - PART III.--THE EVOLUTION OF LIFE. - I. PRELIMINARY 415 - II. GENERAL ASPECTS OF THE SPECIAL-CREATION-HYPOTHESIS 417 - III. GENERAL ASPECTS OF THE EVOLUTION-HYPOTHESIS 431 - IV. THE ARGUMENTS FROM CLASSIFICATION 441 - V. THE ARGUMENTS FROM EMBRYOLOGY 450 - VI. THE ARGUMENTS FROM MORPHOLOGY 468 - VII. THE ARGUMENTS FROM DISTRIBUTION 476 - VIII. HOW IS ORGANIC EVOLUTION CAUSED? 490 - IX. EXTERNAL FACTORS 499 - X. INTERNAL FACTORS 508 - XI. DIRECT EQUILIBRATION 519 - XII. INDIRECT EQUILIBRATION 529 - XIII. THE CO-OPERATION OF THE FACTORS 549 - XIV. THE CONVERGENCE OF THE EVIDENCES 554 - XIV^A. RECENT CRITICISMS AND HYPOTHESES 559 - - APPENDICES. - A. THE GENERAL LAW OF ANIMAL FERTILITY 577 - B. THE INADEQUACY OF NATURAL SELECTION, ETC. 602 - C. THE INHERITANCE OF FUNCTIONALLY-WROUGHT MODIFICATIONS: 692 - A SUMMARY - D. ON ALLEGED "SPONTANEOUS GENERATION" AND ON THE 696 - HYPOTHESIS OF PHYSIOLOGICAL UNITS - - - - -PART I. - -THE DATA OF BIOLOGY. - -CHAPTER I. - -ORGANIC MATTER. - - -§ 1. Of the four chief elements which, in various combinations, make up -living bodies, three are gaseous under all ordinary conditions and the -fourth is a solid. Oxygen, hydrogen, and nitrogen are gases which for many -years defied all attempts to liquefy them, and carbon is a solid except -perhaps at the extremely high temperature of the electric arc. Only by -intense pressures joined with extreme refrigerations have the three gases -been reduced to the liquid form.[2] There is much significance in this. -When we remember how those redistributions of Matter and Motion which -constitute Evolution, structural and functional, imply motions in the units -that are redistributed; we shall see a probable meaning in the fact that -organic bodies, which exhibit the phenomena of Evolution in so high a -degree, are mainly composed of ultimate units having extreme mobility. The -properties of substances, though destroyed to sense by combination, are not -destroyed in reality. It follows from the persistence of force, that the -properties of a compound are _resultants_ of the properties of its -components--_resultants_ in which the properties of the components are -severally in full action, though mutually obscured. One of the leading -properties of each substance is its degree of molecular mobility; and its -degree of molecular mobility more or less sensibly affects the molecular -mobilities of the various compounds into which it enters. Hence we may -infer some relation between the gaseous form of three out of the four chief -organic elements, and that comparative readiness displayed by organic -matters to undergo those changes in the arrangement of parts which we call -development, and those transformations of motion which we call function. - -Considering them chemically instead of physically, it is to be remarked -that three out of these four main components of organic matter, have -affinities which are narrow in their range and low in their intensity. -Hydrogen, it is true, may be made to combine with a considerable number of -other elements; but the chemical energy which it shows is scarcely at all -shown within the limits of the organic temperatures. Of carbon it may -similarly be said that it is totally inert at ordinary heats; that the -number of substances with which it unites is not great; and that in most -cases its tendency to unite with them is but feeble. Lastly, this chemical -indifference is shown in the highest degree by nitrogen--an element which, -as we shall hereafter see, plays the leading part in organic changes. - -Among the organic elements (including under the title not only the four -chief ones, but also the less conspicuous remainder), that capability of -assuming different states called allotropism, is frequent. Carbon presents -itself in the three unlike conditions of diamond, graphite, and charcoal. -Under certain circumstances, oxygen takes on the form in which it is called -ozone. Sulphur and phosphorus (both, in small proportions, essential -constituents of organic matter) have allotropic modifications. Silicon, -too, is allotropic; while its oxide, silica, which is an indispensable -constituent of many lower organisms, exhibits the analogue of -allotropism--isomerism. No other interpretation being possible we are -obliged to regard allotropic change as some change of molecular -arrangement. Hence this frequency of its occurrence among the components of -organic matter is significant as implying a further kind of molecular -mobility. - -One more fact, that is here of great interest for us, must be set down. -These four elements of which organisms are almost wholly composed, exhibit -certain extreme unlikenesses. While between two of them we have an -unsurpassed contrast in chemical activity; between one of them and the -other three, we have an unsurpassed contrast in molecular mobility. While -carbon, until lately supposed to be infusible and now volatilized only in -the electric arc, shows us a degree of atomic cohesion greater than that of -any other known element, hydrogen, oxygen, and nitrogen show the least -atomic cohesion of all elements. And while oxygen displays, alike in the -range and intensity of its affinities, a chemical energy exceeding that of -any other substance (unless fluorine be considered an exception), nitrogen -displays the greatest chemical inactivity. Now on calling to mind one of -the general truths arrived at when analyzing the process of Evolution, the -probable significance of this double difference will be seen. It was shown -(_First Principles_, § 163) that, other things equal, unlike units are more -easily separated by incident forces than like units are--that an incident -force falling on units that are but little dissimilar does not readily -segregate them; but that it readily segregates them if they are widely -dissimilar. Thus, the substances presenting these two extreme contrasts, -the one between physical mobilities, and the other between chemical -activities, fulfil, in the highest degree, a certain further condition to -facility of differentiation and integration. - - -§ 2. Among the diatomic combinations of the three elements, hydrogen, -nitrogen and oxygen, we find a molecular mobility much less than that of -these elements themselves; at the same time that it is much greater than -that of diatomic compounds in general. Of the two products formed by the -union of oxygen with carbon, the first, called carbonic oxide, which -contains one atom[3] of carbon to one of oxygen (expressed by the symbol -CO) is a gas condensible only with great difficulty; and the second, -carbonic acid, containing an additional atom of oxygen (CO_{2}) assumes a -liquid form also only under a pressure of about forty atmospheres. The -several compounds of oxygen with nitrogen, present us with an instructive -gradation. Nitrous oxide (N_{2}O), is a gas condensible only under a -pressure of some fifty atmospheres; nitric oxide (NO) is a gas which -although it has been liquefied does not condense under a pressure of 270 -atmospheres at 46.4° F. (8° C.): the molecular mobility remaining -undiminished in consequence of the volume of the united gases remaining -unchanged. Nitrogen trioxide (N_{2}O_{3}) is gaseous at ordinary -temperatures, but condenses into a very volatile liquid at the zero of -Fahrenheit; nitrogen tetroxide (N_{2}O_{4}) is liquid at ordinary -temperatures and becomes solid at the zero of Fahrenheit; while nitrogen -pentoxide (N_{2}O_{5}) may be obtained in crystals which melt at 85° and -boil at 113°. In this series we see, though not with complete uniformity, a -decrease of molecular mobility as the weights of the compound molecules are -increased. The hydro-carbons illustrate the same general truth still -better. One series of them will suffice. Marsh gas (CH_{4}) is gaseous -except under great pressure and at very low temperatures. Olefiant gas -(C_{2}H_{4}) and ethane (C_{2}H_{6}) may be readily liquefied by pressure. -Propane (C_{3}H_{8}) becomes liquid without pressure at the zero of -Fahrenheit. Hexane (C_{5}H_{12}) is a liquid which boils at 160°. And the -successively higher multiples, heptane (C_{7}H_{16}), octane (C_{8}H_{18}), -and nonane (C_{9}H_{20}) are liquids which boil respectively at 210°, 257°, -and 302°. Pentadecan (C_{15}H_{32}) is a liquid which boils at 270°, while -paraffin-wax, which contains the still higher multiples, is solid. There -are three compounds of hydrogen and nitrogen that have been obtained in a -free state--ammonia (NH_{3}) is gaseous, but liquefiable by pressure, or by -reducing its temperature to -40° F., and it solidifies at -112° F.; -hydrazine (NH_{2}--NH_{2}) is liquid at ordinary temperatures, but -hydrozoic acid (N_{3}H) has so far only been obtained in the form of a -highly explosive gas. In cyanogen, which is composed of carbon and -nitrogen, (CN)_{2}, we have a gas that becomes liquid at a pressure of four -atmospheres and solid at -30° F. And in paracyanogen, formed of the same -proportions of these elements in higher multiples, we have a solid which -does not fuse or volatilize at ordinary temperatures. Lastly, in the most -important member of this group, water (H_{2}O), we have a compound of two -difficultly-condensible gases which assumes both the fluid state and the -solid state within ordinary ranges of temperature; while its molecular -mobility is still such that its fluid or solid masses are continually -passing into the form of vapour, though not with great rapidity until the -temperature is raised to 212°. - -Considering them chemically, it is to be remarked of these diatomic -compounds of the four chief organic elements, that they are, on the -average, less stable than diatomic compounds in general. Water, carbonic -oxide, and carbonic acid, are, it is true, difficult to decompose. But -omitting these, the usual strength of union among the elements of the -above-named substances is low considering the simplicity of the substances. -With the exception of acetylene and possibly marsh gas, the various -hydro-carbons are not producible by directly combining their elements; and -the elements of most of them are readily separable by heat without the aid -of any antagonistic affinity. Nitrogen and hydrogen do not unite with each -other immediately save under very exceptional circumstances; and the -ammonia which results from their union, though it resists heat, yields to -the electric spark. Cyanogen is stable: not being resolved into its -components below a bright red heat. Much less stable, however, are several -of the oxides of nitrogen. Nitrous oxide, it is true, does not yield up its -elements below a red heat; but nitrogen tetroxide cannot exist if water be -added to it; nitrous acid is decomposed by water; and nitric acid not only -readily parts with its oxygen to many metals, but when anhydrous, -spontaneously decomposes. Here it will be well to note, as having a bearing -on what is to follow, how characteristic of most nitrogenous compounds is -this special instability. In all the familiar cases of sudden and violent -decomposition, the change is due to the presence of nitrogen. The explosion -of gunpowder results from the readiness with which the nitrogen contained -in the nitrate of potash, yields up the oxygen combined with it. The -explosion of gun-cotton, which also contains nitrogen, is a substantially -parallel phenomenon. The various fulminating salts are all formed by the -union with metals of a certain nitrogenous acid called fulminic acid; which -is so unstable that it cannot be obtained in a separate state. -Explosiveness is a property of nitro-mannite, and also of nitro-glycerin. -Iodide of nitrogen detonates on the slightest touch, and often without any -assignable cause. And the bodies which explode with the most tremendous -violence of any known, are the chloride of nitrogen (NCl_{3}) and hydrazoic -acid (N_{3}H). Thus these easy and rapid decompositions, due to the -chemical indifference of nitrogen, are characteristic. When we come -hereafter to observe the part which nitrogen plays in organic actions, we -shall see the significance of this extreme readiness shown by its compounds -to undergo changes. Returning from these facts parenthetically introduced, -we have next to note that though among the diatomic compounds of the four -chief organic elements, there are a few active ones, yet the majority of -them display a smaller degree of chemical energy than the average of -diatomic compounds. Water is the most neutral of bodies: usually producing -little chemical alteration in the substances with which it combines; and -being expelled from most of its combinations by a moderate heat. Carbonic -acid is a relatively feeble acid: the carbonates being decomposed by the -majority of other acids and by ignition. The various hydro-carbons are but -narrow in the range of their comparatively weak affinities. The compounds -formed by ammonia have not much stability: they are readily destroyed by -heat, and by the other alkalies. The affinities of cyanogen are tolerably -strong, though they yield to those of the chief acids. Of the several -oxides of nitrogen, it is to be remarked that, while those containing the -smaller proportions of oxygen are chemically inert, the one containing the -greatest proportion of oxygen (nitric acid) though chemically active, in -consequence of the readiness with which one part of it gives up its oxygen -to oxidize a base with which the rest combines, is nevertheless driven from -all its combinations by a red heat. - -These diatomic compounds, like their elements, are to a considerable degree -characterized by the prevalence among them of allotropism; or, as it is -more usually called when displayed by compound bodies--isomerism. Professor -Graham finds reason for thinking that a change in atomic arrangements of -this nature, takes place in water, at or near the melting point of ice. In -the various series of hydro-carbons, differing from each other only in the -ratios in which the elements are united, we find not simply isomerism but -polymerism occurring to an almost infinite extent. In some series of -hydro-carbons, as, for example, the terpenes, we find isomerism and at the -same time a great tendency to undergo polymerisation. And the relation -between cyanogen and paracyanogen is, as we saw, a polymeric one. - -There is one further fact respecting these diatomic compounds of the chief -organic elements, which must not be overlooked. Those of them which form -parts of the living tissues of plants and animals (excluding water which -has a mechanical function, and carbonic acid which is a product of -decomposition) belong for the most part to one group--the -carbo-hydrates.[4] And of this group, which is on the average characterized -by comparative instability and inertness, these carbo-hydrates found in -living tissues are among the most unstable and inert. - - -§ 3. Passing now to the substances which contain three of these chief -organic elements, we have first to note that along with the greater atomic -weight which mostly accompanies their increased complexity, there is, on -the average, a further marked decrease of molecular mobility. Scarcely any -of them maintain a gaseous state at ordinary temperatures. One class of -them only, the alcohols and their derivatives, evaporate under the usual -atmospheric pressure; but not rapidly unless heated. The fixed oils, though -they show that molecular mobility implied by an habitually liquid state, -show this in a lower degree than the alcoholic compounds; and they cannot -be reduced to the gaseous state without decomposition. In their allies, the -fats, which are solid unless heated, the loss of molecular mobility is -still more marked. And throughout the whole series of the fatty acids, in -which to a fixed proportion of oxygen there are successively added higher -equimultiples of carbon and hydrogen, we see how the molecular mobility -decreases with the increasing sizes of the molecules. In the amylaceous and -sugar-group of compounds, solidity is the habitual state: such of them as -can assume the liquid form, doing so only when heated to 300° or 400° F.; -and decomposing when further heated, rather than become gaseous. Resins and -gums exhibit general physical properties of like character and meaning. - -In chemical stability these triatomic compounds, considered as a group, are -in a marked degree below the diatomic ones. The various sugars and kindred -bodies, decompose at no very high temperatures. The oils and fats also are -readily carbonized by heat. Resinous and gummy substances are easily made -to render up some of their constituents. And the alcohols, with their -allies, have no great power of resisting decomposition. These bodies, -formed by the union of oxygen, hydrogen, and carbon, are also, as a class, -chemically inactive. Formic and acetic are doubtless energetic acids; but -the higher members of the fatty-acid series are easily separated from the -bases with which they combine. Saccharic acid, too, is an acid of -considerable power; and sundry of the vegetable acids possess a certain -activity, though an activity far less than that of the mineral acids. But -throughout the rest of the group, there is shown but a small tendency to -combine with other bodies; and such combinations as are formed have usually -little permanence. - -The phenomena of isomerism and polymerism are of frequent occurrence in -these triatomic compounds. Starch and dextrine are probably polymeric. -Fruit-sugar and grape-sugar, mannite and sorbite, cane-sugar and -milk-sugar, are isomeric. Sundry of the vegetal acids exhibit similar -modifications. And among the resins and gums, with their derivatives, -molecular re-arrangements of this kind are not uncommon. - -One further fact respecting these compounds of carbon, oxygen and hydrogen, -should be mentioned; namely, that they are divisible into two classes--the -one consisting of substances that result from the destructive decomposition -of organic matter, and the other consisting of substances that exist as -such in organic matter. These two classes of substances exhibit, in -different degrees, the properties to which we have been directing our -attention. The lower alcohols, their allies and derivatives, which possess -greater molecular mobility and chemical stability than the rest of these -triatomic compounds, are rarely found in animal or vegetal bodies. While -the sugars and amylaceous substances, the fixed oils and fats, the gums and -resins, which have all of them much less molecular mobility, and are, -chemically considered, more unstable and inert, are components of the -living tissues of plants and animals. - - -§ 4. Among compounds containing all the four chief organic elements, a -division analogous to that just named may be made. There are some which -result from the decomposition of living tissues; there are others which -make parts of living tissues in their state of integrity; and these two -groups are contrasted in their properties in the same way as are the -parallel groups of triatomic compounds. - -Of the first division, certain products found in the animal excretions are -the most important, and the only ones that need be noted; such, namely, as -urea, kreatine, kreatinine. These animal-bases exhibit much less molecular -mobility than the average of the substances treated of in the last section: -being solid at ordinary temperatures, fusing, where fusible at all, at -temperatures above that of boiling water, and having no power to assume a -gaseous state. Chemically considered, their stability is low, and their -activity but small, in comparison with the stabilities and activities of -the simpler compounds. - -It is, however, the nitrogenous constituents of living tissues, that -display most markedly those characteristics of which we have been tracing -the growth. Albumen, fibrin, casein, and their allies, are bodies in which -that molecular mobility exhibited by three of their components in so high a -degree is reduced to a minimum. These substances are known only in the -solid state. That is to say, when deprived of the water usually mixed with -them, they do not admit of fusion, much less of volatilization. To which -add, that they have not even that molecular mobility which solution in -water implies; since, though they form viscid mixtures with water, they do -not dissolve in the same perfect way as do inorganic compounds. The -chemical characteristics of these substances are instability and inertness -carried to the extreme. How rapidly albumenoid matters decompose under -ordinary conditions, is daily seen: the difficulty of every housewife being -to prevent them from decomposing. It is true that when desiccated and kept -from contact with air, they may be preserved unchanged for long periods; -but the fact that they can be only thus preserved, proves their great -instability. It is true, also, that these most complex nitrogenous -principles are not absolutely inert, since they enter into combinations -with some bases; but their unions are very feeble. - -It should be noted, too, of these bodies, that though they exhibit in the -lowest degree that kind of molecular mobility which implies facile -vibration of the molecules as wholes, they exhibit in high degrees that -kind of molecular mobility resulting in isomerism, which implies permanent -changes in the positions of adjacent atoms with respect to each other. Each -of them has a soluble and an insoluble form. In some cases there are -indications of more than two such forms. And it appears that their -metamorphoses take place under very slight changes of conditions. - -In these most unstable and inert organic compounds, we find that the -molecular complexity reaches a maximum: not only since the four chief -organic elements are here united with small proportions of sulphur and -sometimes phosphorus; but also since they are united in high multiples. The -peculiarity which we found characterized even diatomic compounds of the -organic elements, that their molecules are formed not of single equivalents -of each component, but of two, three, four, and more equivalents, is -carried to the greatest extreme in these compounds, which take the leading -part in organic actions. According to Lieberkühn, the formula of albumen is -C_{72}H_{112}SN_{18}O_{22}. That is to say, with the sulphur there are -united seventy-two atoms of carbon, one hundred and twelve of hydrogen, -eighteen of nitrogen, and twenty-two of oxygen: the molecule being thus -made up of more than two hundred ultimate atoms. - - -§ 5. Did space permit, it would be useful here to consider in detail the -interpretations that may be given of the peculiarities we have been -tracing: bringing to their solution, the general mechanical principles -which are now found to hold true of molecules as of masses. But it must -suffice briefly to indicate the conclusions which such an inquiry promises -to bring out. - -Proceeding on these principles, it may be argued that the molecular -mobility of a substance must depend partly on the inertia of its molecules; -partly on the intensity of their mutual polarities; partly on their mutual -pressures, as determined by the density of their aggregation; and (where -the molecules are compound) partly on the molecular mobilities of their -component molecules. Whence it is to be inferred that any three of these -remaining constant, the molecular mobility will vary as the fourth. Other -things equal, therefore, the molecular mobility of molecules must decrease -as their masses increase; and so there must result that progression we have -traced, from the high molecular mobility of the uncombined organic -elements, to the low molecular mobility of those large-moleculed substances -into which they are ultimately compounded. - -Applying to molecules the mechanical law which holds of masses, that since -inertia and gravity increase as the cubes of the dimensions while cohesion -increases as their squares, the self-sustaining power of a body becomes -relatively smaller as its bulk becomes greater; it might be argued that -these large, aggregate molecules which constitute organic substances, are -mechanically weak--are less able than simpler molecules to bear, without -alteration, the forces falling on them. That very massiveness which renders -them less mobile, enables the physical forces acting on them more readily -to change the relative positions of their component atoms; and so to -produce what we know as re-arrangements and decompositions. - -Further, it seems a not improbable conclusion, that this formation of large -aggregates of elementary atoms and resulting diminution of self-sustaining -power, must be accompanied by a decrease of those dimensional contrasts to -which polarity is ascribable. A sphere is the figure of equilibrium which -any aggregate of units tends to assume, under the influence of simple -mutual attraction. Where the number of units is small and their mutual -polarities are decided, this proclivity towards spherical grouping will be -overcome by the tendency towards some more special form, determined by -their mutual polarities. But it is manifest that in proportion as an -aggregate molecule becomes larger, the effects of simple mutual attraction -must become relatively greater; and so must tend to mask the effects of -polar attraction. There will consequently be apt to result in highly -compound molecules like these organic ones, containing hundreds of -elementary atoms, such approximation to the spherical form as must involve -a less distinct polarity than in simpler molecules. If this inference be -correct, it supplies us with an explanation both of the chemical inertness -of these most complex organic substances, and of their inability to -crystallize. - - -§ 6. Here we are naturally introduced to another aspect of our subject--an -aspect of great interest. Professor Graham has published a series of -important researches, which promise to throw much light on the constitution -and changes of organic matter. He shows that solid substances exist under -two forms of aggregation--the _colloid_ or jelly-like, and the -_crystalloid_ or crystal-like. Examples of the last are too familiar to -need specifying. Of the first may be named such instances as "hydrated -silicic acid, hydrated alumina, and other metallic peroxides of the -aluminous class, when they exist in the soluble form; with starch, dextrine -and the gums, caramel, tannin, albumen, gelatine, vegetable and animal -extractive matters." Describing the properties of colloids, Professor -Graham says:--"Although often largely soluble in water, they are held in -solution by a most feeble force. They appear singularly inert in the -capacity of acids and bases, and in all the ordinary chemical relations." * -* * "Although chemically inert in the ordinary sense, colloids possess a -compensating activity of their own arising out of their physical -properties. While the rigidity of the crystalline structure shuts out -external impressions, the softness of the gelatinous colloid partakes of -fluidity, and enables the colloid to become a medium of liquid diffusion, -like water itself." * * * "Hence a wide sensibility on the part of colloids -to external agents. Another and eminently characteristic quality of -colloids is their mutability." * * * "The solution of hydrated silicic -acid, for instance, is easily obtained in a state of purity, but it cannot -be preserved. It may remain fluid for days or weeks in a sealed tube, but -is sure to gelatinize and become insoluble at last. Nor does the change of -this colloid appear to stop at that point; for the mineral forms of silicic -acid, deposited from water, such as flint, are often found to have passed, -during the geological ages of their existence, from the vitreous or -colloidal into the crystalline condition (H. Rose). The colloid is, in -fact, a dynamical state of matter, the crystalloidal being the statical -condition. The colloid possesses _energia_. It may be looked upon as the -primary source of the force appearing in the phenomena of vitality. To the -gradual manner in which colloidal changes take place (for they always -demand time as an element) may the characteristic protraction of -chemico-organic changes also be referred." - -The class of colloids includes not only all those most complex nitrogenous -compounds characteristic of organic tissues, and sundry of the -carbo-hydrates found along with them; but, significantly enough, it -includes several of those substances classed as inorganic, which enter into -organized structures. Thus silica, which is a component of many plants, and -constitutes the spicules of sponges as well as the shells of many -foraminifera and infusoria, has a colloid, as well as a crystalloid, -condition. A solution of hydrated silicic acid passes in the course of a -few days into a solid jelly that is no longer soluble in water; and it may -be suddenly thus coagulated by a minute portion of an alkaline carbonate, -as well as by gelatine, alumina, and peroxide of iron. This last-named -substance, too--peroxide of iron--which is an ingredient in the blood of -mammals and composes the shells of certain _Protozoa_, has a colloid -condition. "Water containing about one per cent. of hydrated peroxide of -iron in solution, has the dark red colour of venous blood." * * * "The red -solution is coagulated in the cold by traces of sulphuric acid, alkalies, -alkaline carbonates, sulphates, and neutral salts in general." * * * "The -coagulum is a deep red-coloured jelly, resembling the clot of blood, but -more transparent. Indeed, the coagulum of this colloid is highly suggestive -of that of blood, from the feeble agencies which suffice to effect the -change in question, as well as from the appearance of the product." The -jelly thus formed soon becomes, like the last, insoluble in water. Lime -also, which is so important a mineral element in living bodies, animal and -vegetal, enters into a compound belonging to this class. "The well-known -solution of lime in sugar forms a solid coagulum when heated. It is -probably, at a high temperature, entirely colloidal." - -Generalizing some of the facts which he gives, Professor Graham says:--"The -equivalent of a colloid appears to be always high, although the ratio -between the elements of the substance may be simple. Gummic acid, for -instance, may be represented by C^{12}H^{22}O^{11}; but, judging from the -small proportions of lime and potash which suffice to neutralize this acid, -the true numbers of its formula must be several times greater. It is -difficult to avoid associating the inertness of colloids with their high -equivalents, particularly where the high number appears to be attained by -the repetition of a small number. The inquiry suggests itself whether the -colloid molecule may not be constituted by the grouping together of a -number of smaller crystalloid molecules, and whether the basis of -colloidality may not really be this composite character of the molecule." - - -§ 7. A further contrast between colloids and crystalloids is equally -significant in its relations to vital phenomena. Professor Graham points -out that the marked differences in volatility displayed by different -bodies, are paralleled by differences in the rates of diffusion of -different bodies through liquids. As alcohol and ether at ordinary -temperatures, and various other substances at higher temperatures, diffuse -themselves in a gaseous form through the air; so, a substance in aqueous -solution, when placed in contact with a mass of water (in such way as to -avoid mixture by circulating currents) diffuses itself through this mass of -water. And just as there are various degrees of rapidity in evaporation, so -there are various degrees of rapidity in diffusion: "the range also in the -degree of diffusive mobility exhibited by different substances appears to -be as wide as the scale of vapour-tensions." This parallelism is what might -have been looked for; since the tendency to assume a gaseous state, and the -tendency to spread in solution through a liquid, are both consequences of -molecular mobility. It also turns out, as was to be expected, that -diffusibility, like volatility, has, other things equal, a relation to -molecular weight--other things equal, we must say, because molecular -mobility must, as pointed out in § 5, be affected by other properties of -atoms, besides their inertia. Thus the substance most rapidly diffused of -any on which Professor Graham experimented, was hydrochloric acid--a -compound which is of low molecular weight, is gaseous save under a pressure -of forty atmospheres, and ordinarily exists as a liquid, only in -combination with water. Again, "hydrate of potash may be said to possess -double the velocity of diffusion of sulphate of potash, and sulphate of -potash again double the velocity of sugar, alcohol, and sulphate of -magnesia,"--differences which have a general correspondence with -differences in the massiveness of their molecules. - -But the fact of chief interest to us here, is that the relatively -small-moleculed crystalloids have immensely greater diffusive power than -the relatively large-moleculed colloids. Among the crystalloids themselves -there are marked differences of diffusibility; and among the colloids -themselves there are parallel differences, though less marked ones. But -these differences are small compared with that between the diffusibility of -the crystalloids as a class, and the diffusibility of the colloids as a -class. Hydrochloric acid is seven times as diffusible as sulphate of -magnesia; but it is fifty times as diffusible as albumen, and a hundred -times as diffusible as caramel. - -These differences of diffusibility manifest themselves with nearly equal -distinctness, when a permeable septum is placed between the solution and -the water. The result is that when a solution contains substances of -different diffusibilities, the process of dialysis, as Professor Graham -calls it, becomes a means of separating the mixed substances: especially -when such mixed substances are partly crystalloids and partly colloids. The -bearing of this fact on the interpretation of organic processes will be -obvious. Still more obvious will its bearing be, on joining with it the -remarkable fact that while crystalloids can diffuse themselves through -colloids nearly as rapidly as through water, colloids can scarcely diffuse -themselves at all through other colloids. From a mass of jelly containing -salt, into an adjoining mass of jelly containing no salt, the salt spread -more in eight days than it spread through water in seven days; while the -spread of "caramel through the jelly appeared scarcely to have begun after -eight days had elapsed." So that we must regard the colloidal compounds of -which organisms are built, as having, by their physical nature, the ability -to separate colloids from crystalloids, and to let the crystalloids pass -through them with scarcely any resistance. - -One other result of these researches on the relative diffusibilities of -different substances has a meaning for us. Professor Graham finds that not -only does there take place, by dialysis, a separation of _mixed_ substances -which are unlike in their molecular mobilities; but also that _combined_ -substances between which the affinities are feeble, will separate on the -dialyzer, if their molecular mobilities are strongly contrasted. Speaking -of the hydrochloride of peroxide of iron, he says, "such a compound -possesses an element of instability in the extremely unequal diffusibility -of its constituents;" and he points out that when dialyzed, the -hydrochloric acid gradually diffuses away, leaving the colloidal peroxide -of iron behind. Similarly, he remarks of the peracetate of iron, that it -"may be made a source of soluble peroxide, as the salt referred to is -itself decomposed to a great extent by diffusion on the dialyzer." Now this -tendency to separate displayed by substances which differ widely in their -molecular mobilities, though usually so far antagonized by their affinities -as not to produce spontaneous decomposition, must, in all cases, induce a -certain readiness to change which would not else exist. The unequal -mobilities of the combined atoms must give disturbing forces a greater -power to work transformations than they would otherwise have. Hence the -probable significance of a fact named at the outset, that while three of -the chief organic elements have the greatest atomic mobilities of any -elements known, the fourth, carbon, has the least atomic mobility of known -elements. Though, in its simple compounds, the affinities of carbon for the -rest are strong enough to prevent the effects of this great difference from -clearly showing themselves; yet there seems reason to think that in those -complex compounds composing organic bodies--compounds in which there are -various cross affinities leading to a state of chemical tension--this -extreme difference in the molecular mobilities must be an important aid to -molecular re-arrangements. In short, we are here led by concrete evidence -to the conclusion which we before drew from first principles, that this -great unlikeness among the combined units must facilitate differentiations. - - -§ 8. A portion of organic matter in a state to exhibit those phenomena -which the biologist deals with, is, however, something far more complex -than the separate organic matters we have been studying; since a portion of -organic matter in its integrity, contains several of these. - -In the first place no one of those colloids which make up the mass of a -living body, appears capable of carrying on vital changes by itself: it is -always associated with other colloids. A portion of animal-tissue, however -minute, almost always contains more than one form of protein-substance: -different chemical modifications of albumen and gelatine are present -together, as well as, probably, a soluble and insoluble modification of -each; and there is usually more or less of fatty matter. In a single -vegetal cell, the minute quantity of nitrogenous colloid present, is -imbedded in colloids of the non-nitrogenous class. And the microscope makes -it at once manifest, that even the smallest and simplest organic forms are -not absolutely homogeneous. - -Further, we have to contemplate organic tissue, formed of mingled colloids -in both soluble and insoluble states, as permeated throughout by -crystalloids. Some of these crystalloids, as oxygen,[5] water, and perhaps -certain salts, are agents of decomposition; some, as the saccharine and -fatty matters, are probably materials for decomposition; and some, as -carbonic acid, water, urea, kreatine, and kreatinine, are products of -decomposition. Into the mass of mingled colloids, mostly insoluble and -where soluble of very low molecular mobility or diffusive power, we have -constantly passing, crystalloids of high molecular mobility or diffusive -power, that are capable of decomposing these complex colloids, or of -facilitating decompositions otherwise caused; and from these complex -colloids, when decomposed, there result other crystalloids (the two chief -ones extremely simple and mobile, and the rest comparatively so) which -diffuse away as rapidly as they are formed. - -And now we may clearly see the necessity for that peculiar composition -which we find in organic matter. On the one hand, were it not for the -extreme molecular mobility possessed by three out of the four of its chief -elements; and were it not for the consequently high molecular mobility of -their simpler compounds; there could not be this quick escape of the waste -products of organic action; and there could not be that continuously active -change of matter which vitality implies. On the other hand, were it not for -the union of these extremely mobile elements into immensely complex -compounds, having relatively vast molecules which are made comparatively -immobile by their inertia, there could not result that mechanical fixity -which prevents the components of living tissue from diffusing away along -with the effete matters produced by decomposition. - - -§ 8a. Let us not omit here to note the ways in which the genesis of these -traits distinguishing organic matter conforms to the laws of evolution as -expressed in its general formula. - -In pursuance of the belief now widely entertained by chemists that the -so-called elements are not elements, but are composed of simpler matters -and probably of one ultimate form of matter (for which the name "protyle" -has been suggested by Sir W. Crookes), it is to be concluded that the -formation of the elements, in common with the formation of all those -compounds of them which Nature presents, took place in the course of Cosmic -Evolution. Various reasons for this inference the reader will find set -forth in the Addenda to an essay on "The Nebular Hypothesis" (see _Essays_, -vol. I, p. 155). On tracing out the process of compounding and -re-compounding by which, hypothetically, the elements themselves and -afterwards their compounds and re-compounds have arisen, certain cardinal -facts become manifest. - -1. Considered as masses, the units of the elements are the smallest, though -larger than the units of the primordial matter. Later than these, since -they are composed of them, and since they cannot exist at temperatures so -high as those at which the elements can exist, come the diatomic -compounds--oxides, chlorides, and the rest--necessarily larger in their -molecules. Above these in massiveness come the molecules of the -multitudinous salts and kindred bodies. When associated, as these commonly -are, with molecules of water, there again results in each case increase of -mass; and unable as they are to bear such high temperatures, these -molecules are necessarily later in origin than those of the anhydrous -diatomic compounds. Within the general class of triatomic compounds, more -composite still, come the carbohydrates, which, being able to unite in -multiples, form still larger molecules than other triatomic compounds. -Decomposing as they do at relatively low temperatures, these are still more -recent in the course of chemical evolution; and with the genesis of them -the way is prepared for the genesis of organic matter strictly so called. -This includes the various forms of protein-substance, containing four chief -elements with two minor ones, and having relatively vast molecules. -Unstable as these are in presence of heat and surrounding affinities, they -became possible only at a late stage in the genesis of the Earth. Here, -then, in that chemical evolution which preceded the evolution of life, we -see displayed that process of integration which is the primary trait of -evolution at large. - -2. Along with increasing integration has gone progress in heterogeneity. -The elements, regarding them as compound, are severally more heterogeneous -than "protyle." Diatomic molecules are more heterogeneous than these -elements; triatomic more heterogeneous than diatomic; and the molecules -containing four elements more heterogeneous than those containing three: -the most heterogeneous of them being the proteids, which contain two other -elements. The hydrated forms of all these compounds are more heterogeneous -than are the anhydrous forms. And most heterogeneous of all are the -molecules which, besides containing three, four, or more elements, also -exhibit the isomerism and polymerism which imply unions in multiples. - -3. This formation of molecules more and more heterogeneous during -terrestrial evolution, has been accompanied by increasing heterogeneity in -the aggregate of compounds of each kind, as well as an increasing number of -kinds; and this increasing heterogeneity is exemplified in an extreme -degree in the compounds, non-nitrogenous and nitrogenous, out of which -organisms are built. So that the classes, orders, genera, and species of -chemical substances, gradually increasing as the Earth has assumed its -present form, increased in a transcendent degree during that stage which -preceded the origin of life. - - -§ 9. Returning now from these partially-parenthetic observations, and -summing up the contents of the preceding pages, we have to remark that in -the substances of which organisms are composed, the conditions necessary to -that re-distribution of Matter and Motion which constitutes Evolution, are -fulfilled in a far higher degree than at first appears. - -The mutual affinities of the chief organic elements are not active within -the limits of those temperatures at which organic actions take place; and -one of these elements is especially characterized by its chemical -indifference. The compounds formed by these elements in ascending grades of -complexity, become progressively less stable. And those most complex -compounds into which all these four elements enter, together with small -proportions of two other elements which very readily oxidize, have an -instability so great that decomposition ensues under ordinary atmospheric -conditions. - -Among these elements out of which living bodies are built, there is an -unusual tendency to unite in multiples; and so to form groups of products -which have the same chemical elements in the same proportions, but, -differing in their modes of aggregation, possess different properties. This -prevalence among them of isomerism and polymerism, shows, in another way, -the special fitness of organic substances for undergoing re-distributions -of their components. - -In those most complex compounds that are instrumental to vital actions, -there exists a kind and degree of molecular mobility which constitutes the -plastic quality fitting them for organization. Instead of the extreme -molecular mobility possessed by three out of the four organic elements in -their separate states--instead of the diminished, but still great, -molecular mobility possessed by their simpler combinations, the gaseous and -liquid characters of which unfit them for showing to any extent the process -of Evolution--instead of the physical properties of their less simple -combinations, which, when not made unduly mobile by heat, assume the unduly -rigid form of crystals; we have in these colloids, of which organisms are -mainly composed, just the required compromise between fluidity and -solidity. They cannot be reduced to the unduly mobile conditions of liquid -and gas; and yet they do not assume the unduly fixed condition usually -characterizing solids. The absence of power to unite together in polar -arrangement, leaves their molecules with a certain freedom of relative -movement, which makes them sensitive to small forces, and produces -plasticity in the aggregates composed of them. - -While the relatively great inertia of these large and complex organic -molecules renders them comparatively incapable of being set in motion by -the ethereal undulations, and so reduced to less coherent forms of -aggregation, this same inertia facilitates changes of arrangement among -their constituent molecules or atoms; since, in proportion as an incident -force impresses but little motion on a mass, it is the better able to -impress motion on the parts of the mass in relation to one another. And it -is further probable that the extreme contrasts in molecular mobilities -among the components of these highly complex molecules, aid in producing -modifiability of arrangement among them. - -Lastly, the great difference in diffusibility between colloids and -crystalloids, makes possible in the tissues of organisms a specially rapid -re-distribution of matter and motion; both because colloids, being easily -permeable by crystalloids, can be chemically acted on throughout their -whole masses, instead of only on their surfaces; and because the products -of decomposition, being also crystalloids, can escape as fast as they are -produced: leaving room for further transformations. So that while the -composite molecules of which organic tissues are built up, possess that low -molecular mobility fitting them for plastic purposes, it results from the -extreme molecular mobilities of their ultimate constituents, that the waste -products of vital activity escape as fast as they are formed. - -To all which add that the state of warmth, or increased molecular -vibration, in which all the higher organisms are kept, increases these -various facilities for re-distribution: not only as aiding chemical -changes, but as accelerating the diffusion of crystalloid substances. - - - - -CHAPTER II. - -THE ACTIONS OF FORCES ON ORGANIC MATTER. - - -§ 10. To some extent, the parts of every body are changed in their -arrangement by any incident mechanical force. But in organic bodies, and -especially in animal bodies, the changes of arrangement produced by -mechanical forces are usually conspicuous. It is a distinctive mark of -colloids that they readily yield to pressures and tensions, and that they -recover, more or less completely, their original shapes, when the pressures -or tensions cease. Evidently without this pliability and elasticity, most -organic actions would be impossible. Not only temporary but also permanent -alterations of form are facilitated by this colloid character of organic -matter. Continued pressure on living tissue, by modifying the processes -going on in it (perhaps retarding the absorption of new material to replace -the old that has decomposed and diffused away), gradually diminishes and -finally destroys its power of resuming the outline it had at first. Thus, -generally speaking, the substances composing organisms are modifiable by -arrested momentum or by continuous strain, in far greater degrees than are -inorganic substances. - - -§ 11. Sensitiveness to certain forces which are quasi-mechanical, if not -mechanical in the usual sense, is seen in two closely-related peculiarities -displayed by organic matter as well as other matter which assumes the same -state of molecular aggregation. - -Colloids take up by a power called "capillary affinity," a large quantity -of water: undergoing at the same time great increase of bulk with change of -form. Conversely, with like readiness, they give up this water by -evaporation; resuming, partially or completely, their original states. -Whether resulting from capillarity, or from the relatively great -diffusibility of water, or from both, these changes are to be here noted as -showing another mode in which the arrangements of parts in organic bodies -are affected by mechanical actions. - -In what is termed osmose, we have a further mode of an allied kind. When on -opposite sides of a permeable septum, and especially a septum of colloidal -substance, are placed miscible solutions of different densities, a double -transfer takes place: a large quantity of the less dense solution finds its -way through the septum into the more dense solution; and a small quantity -of the more dense finds its way into the less dense--one result being a -considerable increase in the bulk of the more dense at the expense of the -less dense. This process, which appears to depend on several conditions, is -not yet fully understood. But be the explanation what it may, the process -is one that tends continually to work alterations in organic bodies. -Through the surfaces of plants and animals, transfers of this kind are ever -taking place. Many of the conspicuous changes of form undergone by organic -germs, are due mainly to the permeation of their limiting membranes by the -surrounding liquids. - -It should be added that besides the direct alterations which the imbibition -and transmission of water and watery solutions by colloids produce in -organic matter, they produce indirect alterations. Being instrumental in -conveying into the tissues the agents of chemical change, and conveying out -of them the products of chemical change, they aid in carrying on other -re-distributions. - - -§ 12. As elsewhere shown (_First Principles_, § 100) heat, or a raised -state of molecular vibration, enables incident forces more easily to -produce changes of molecular arrangement in organic matter. But besides -this, it conduces to certain vital changes in so direct a way as to become -their chief cause. - -The power of the organic colloids to imbibe water, and to bring along with -it into their substance the materials which work transformations, would not -be continuously operative if the water imbibed were to remain. It is -because it escapes, and is replaced by more water containing more -materials, that the succession of changes is maintained. Among the higher -animals and higher plants its escape is facilitated by evaporation. And the -rate of evaporation is, other things equal, determined by heat. Though the -current of sap in a tree is partly dependent on some action, probably -osmotic, that goes on in the roots; yet the loss of water from the surfaces -of the leaves, and the consequent absorption of more sap into the leaves by -capillary attraction, must be a chief cause of the circulation. The -drooping of a plant when exposed to the sunshine while the earth round its -roots is dry, shows us how evaporation empties the sap-vessels; and the -quickness with which a withered slip revives on being placed in water, -shows us the part which capillary action plays. In so far, then, as the -evaporation from a plant's surface helps to produce currents of sap through -the plant, we must regard the heat which produces this evaporation as a -part-cause of those re-distributions of matter which these currents effect. -In terrestrial animals, heat, by its indirect action as well as by its -direct action, similarly aids the changes that are going on. The exhalation -of vapour from the lungs and the surface of the skin, forming the chief -escape of the water that is swallowed, conduces to the maintenance of those -currents through the tissues without which the functions would cease. For -though the vascular system distributes nutritive liquids in ramified -channels through the body; yet the absorption of these liquids into -tissues, partly depends on the escape of liquids which the tissues already -contain. Hence, to the extent that such escape is facilitated by -evaporation, and this evaporation facilitated by heat, heat becomes an -agent of re-distribution in the animal organism.[6] - - -§ 13. Light, which is now known to modify many inorganic compounds--light, -which works those chemical changes utilized in photography, causes the -combinations of certain gases, alters the molecular arrangements of many -crystals, and leaves traces of its action even on substances that are -extremely stable,--may be expected to produce marked effects on substances -so complex and unstable as those which make up organic bodies. It does -produce such effects; and some of them are among the most important that -organic matter undergoes. - -The molecular changes wrought by light in animals are of but secondary -moment. There is the darkening of the skin that follows exposure to the -Sun's rays. There are those alterations in the retina which cause in us -sensations of colours. And on certain eyeless creatures that are -semi-transparent, the light permeating their substance works some effects -evinced by movements. But speaking generally, the opacity of animals limits -the action of light to their surfaces; and so renders its direct -physiological influence but small.[7] On plants, however, the solar rays -that produce in us the impression of yellow, are the immediate agents of -those molecular changes through which are hourly accumulated the materials -for further growth. Experiments have shown that when the Sun shines on -living leaves, they begin to exhale oxygen and to accumulate carbon and -hydrogen--results which are traced to the decomposition, by the solar rays, -of the carbonic acid and water absorbed. It is now an accepted conclusion -that, by the help of certain classes of the ethereal undulations -penetrating their leaves, plants are enabled to separate from the -associated oxygen those two elements of which their tissues are chiefly -built up. - -This transformation of ethereal undulations into certain molecular -re-arrangements of an unstable kind, on the overthrow of which the -stored-up forces are liberated in new forms, is a process that underlies -all organic phenomena. It will therefore be well if we pause a moment to -consider whether any proximate interpretation of it is possible. Researches -in molecular physics give us some clue to its nature. - -The elements of the problem are these:--The atoms[8] of several ponderable -matters exist in combination: those which are combined having strong -affinities, but having also affinities less strong for some of the -surrounding atoms that are otherwise combined. The atoms thus united, and -thus mixed among others with which they are capable of uniting, are exposed -to the undulations of a medium that is so rare as to seem imponderable. -These undulations are of numerous kinds: they differ greatly in their -lengths, or in the frequency with which they recur at any given point. And -under the influence of undulations of a certain frequency, some of these -atoms are transferred from atoms for which they have a stronger affinity, -to atoms for which they have a weaker affinity. That is to say, particular -orders of waves of a relatively imponderable matter, remove particular -atoms of ponderable matter from their attachments, and carry them within -reach of other attachments. Now the discoveries of Bunsen and Kirchoff -respecting the absorption of particular luminiferous undulations by the -vapours of particular substances, joined with Prof. Tyndall's discoveries -respecting the absorption of heat by gases, show very clearly that the -atoms of each substance have a rate of vibration in harmony with ethereal -waves of a certain length, or rapidity of recurrence. Every special kind of -atom can be made to oscillate by a special order of ethereal waves, which -are absorbed in producing its oscillations; and can by its oscillations -generate this same order of ethereal waves. Whence it appears that immense -as is the difference in density between ether and ponderable matter, the -waves of the one can set the atoms of the other in motion, when the -successive impacts of the waves are so timed as to correspond with the -oscillations of the atoms. The effects of the waves are, in such case, -cumulative; and each atom gradually acquires a momentum made up of -countless infinitesimal momenta. Note, further, that unless the members of -a chemically-compound molecule are so bound up as to be incapable of any -relative movements (a supposition at variance with the conceptions of -modern science) we must conceive them as severally able to vibrate in -unison or harmony with those same classes of ethereal waves that affect -them in their uncombined states. While the compound molecule as a whole -will have some new rate of oscillation determined by its attributes as a -whole; its components will retain their original rates of oscillation, -subject only to modifications by mutual influence. Such being the -circumstances of the case we may partially understand how the Sun's rays -can effect chemical decompositions. If the members of a diatomic molecule -stand so related to the undulations falling on them, that one is thrown -into a state of increased oscillation and the other not; it is manifest -that there must arise a tendency towards the dislocation of the two--a -tendency which may or may not take effect, according to the weakness or -strength of their union, and according to the presence or absence of -collateral affinities. This inference is in harmony with several -significant facts. Dr. Draper remarks that "among metallic substances -(compounds) those first detected to be changed by light, such as silver, -gold, mercury, lead, have all high atomic weights; and such as sodium and -potassium, the atomic weights of which are low, appeared to be less -changeable." As here interpreted, the fact specified amounts to this; that -the compounds most readily decomposed by light, are those in which there is -a marked contrast between the atomic weights of the constituents, and -probably therefore a marked contrast between the rapidities of their -vibrations. The circumstance, too, that different chemical compounds are -decomposed or modified in different parts of the spectrum, implies that -there is a relation between special orders of undulations and special -orders of molecules--doubtless a correspondence between the rates of these -undulations and the rates of oscillation which some of the components of -such molecules will assume. Strong confirmation of this view may be drawn -from the decomposing actions of those longer ethereal waves which we -perceive as heat. On contemplating the whole series of diatomic compounds, -we see that the elements which are most remote in their atomic weights, as -hydrogen and the noble metals generally, will not combine at all, or do so -with great difficulty: their vibrations are so unlike that they cannot keep -together under any conditions of temperature. If, again, we look at a -smaller group, as the metallic oxides, we see that whereas those metals -which have atoms nearest in weight to the atoms of oxygen, cannot be -separated from oxygen by heat, even when it is joined by a powerful -collateral affinity; those metals which differ more widely from oxygen in -their atomic weights, can be de-oxidized by carbon at high temperatures; -and those which differ from it most widely combine with it very -reluctantly, and yield it up if exposed to thermal undulations of moderate -intensity. Here indeed, remembering the relations among the atomic weights -in the two cases, may we not suspect a close analogy between the -de-oxidation of a metallic oxide by carbon under the influence of the -longer ethereal waves, and the de-carbonization of carbonic acid by -hydrogen under the influence of the shorter ethereal waves? - -These conceptions help us to some dim notion of the mode in which changes -are wrought in light in the leaves of plants. Among the several elements -concerned, there are wide differences in molecular mobility, and probably -in the rates of molecular vibration. Each is combined with one of the -others, but is capable of forming various combinations with the rest. And -they are severally in presence of a complex compound into which they all -enter, and which is ready to assimilate with itself the new compound -molecules they form. Certain of the ethereal waves falling on them when -thus arranged, cause a detachment of some of the combined atoms and a union -of the rest. And the conclusion suggested is that the induced vibrations -among the various atoms as at first arranged, are so incongruous as to -produce instability, and to give collateral affinities the power to work a -rearrangement which, though less stable under other conditions, is more -stable in the presence of these particular undulations. There seems, -indeed, no choice but to conceive the matter thus. An atom united with one -for which it has a strong affinity, has to be transferred to another for -which it has a weaker affinity. This transfer implies motion. The motion is -given by the waves of a medium that is relatively imponderable. No one wave -of this imponderable medium can give the requisite motion to this atom of -ponderable matter: especially as the atom is held by a positive force -besides its inertia. The motion required can hence be given only by -successive waves; and that these may not destroy each other's effects, it -is needful that each shall strike the atom just when it has completed the -recoil produced by the impact of previous ones. That is, the ethereal -undulations must coincide in rate with the oscillations of the atom, -determined by its inertia and the forces acting on it. It is also requisite -that the rate of oscillation of the atom to be detached, shall differ from -that of the atom with which it is united; since if the two oscillated in -unison the ethereal waves would not tend to separate them. And, finally, -the successive impacts of the ethereal waves must be accumulated until the -resulting oscillations have become so wide in their sweep as greatly to -weaken the cohesion of the united atoms, at the same time that they bring -one of them within reach of other atoms with which it will combine. In this -way only does it seem possible for such a force to produce such a transfer. -Moreover, while we are thus enabled to conceive how light may work these -molecular changes, we also gain an insight into the method by which the -insensible motions propagated to us from the Sun, are treasured up in such -ways as afterwards to generate sensible motions. By the accumulation of -infinitesimal impacts, atoms of ponderable matter are made to oscillate. -The quantity of motion which each of them eventually acquires, effects its -transfer to a position of unstable equilibrium, from which it can -afterwards be readily dislodged. And when so dislodged, along with other -atoms similarly and simultaneously affected, there is suddenly given out -all the motion which had been before impressed on it. - -Speculation aside, however, that which it concerns us to notice is the -broad fact that light is an all-important agent of molecular changes in -organic substances. It is not here necessary for us to ascertain _how_ -light produces these compositions and decompositions. It is necessary only -for us to observe that it _does_ produce them. That the characteristic -matter called chlorophyll, which gives the green colour to leaves, makes -its appearance whenever the blanched shoots of plants are exposed to the -Sun; that the petals of flowers, uncoloured while in the bud, acquire their -bright tints as they unfold; and that on the outer surfaces of animals, -analogous changes are induced; are wide inductions which are enough for our -present purpose. - - -§ 14. We come next to the agency of chief importance among those that work -changes in organic matter; namely, chemical affinity. How readily vegetal -and animal substances are modified by other substances put in contact with -them, we see daily illustrated. Besides the many compounds which cause the -death of an organism into which they are put, we have the much greater -number of compounds which work those milder effects termed -medicinal--effects implying, like the others, molecular re-arrangements. -Indeed, most soluble chemical compounds, natural and artificial, produce, -when taken into the body, alterations that are more or less manifest in -their results. - -After what was shown in the last chapter, it will be manifest that this -extreme modifiability of organic matter by chemical agencies, is the chief -cause of that active molecular re-arrangement which organisms, and -especially animal organisms, display. In the two fundamental functions of -nutrition and respiration, we have the means by which the supply of -materials for this active molecular re-arrangement is maintained. - -The process of animal nutrition consists partly in the absorption of those -complex substances which are thus highly capable of being chemically -altered, and partly in the absorption of simpler substances capable of -chemically altering them. The tissues always contain small quantities of -alkaline and earthy salts, which enter the system in one form and are -excreted in another. Though we do not know specifically the parts which -these salts play, yet from their universal presence, and from the -transformations which they undergo in the body, it may be safely inferred -that their chemical affinities are instrumental in working some of the -metamorphoses ever going on. - -The inorganic substance, however, on which mainly depend these -metamorphoses in organic matter, is not swallowed along with the solid and -liquid food, but is absorbed from the surrounding medium--air or water, as -the case may be. Whether the oxygen taken in, either, as by the lowest -animals, through the general surface, or, as by the higher animals, through -respiratory organs, is the immediate cause of those molecular changes which -are ever going on throughout the living tissues; or whether the oxygen, -playing the part of scavenger, merely aids these changes by carrying away -the products of decompositions otherwise caused; it equally remains true -that these changes are maintained by its instrumentality. Whether the -oxygen absorbed and diffused through the system effects a direct oxidation -of the organic colloids which it permeates, or whether it first leads to -the formation of simpler and more oxidized compounds, which are afterwards -further oxidized and reduced to still simpler forms, matters not, in so far -as the general result is concerned. In any case it holds good that the -substances of which the animal body is built up, enter it in either an -unoxidized or in a but slightly oxidized and highly unstable state; while -the great mass of them leave it in a fully oxidized and stable state. It -follows, therefore, that, whatever the special changes gone through, the -general process is a falling from a state of unstable chemical equilibrium -to a state of stable chemical equilibrium. Whether this process be direct -or indirect, the total molecular re-arrangement and the total motion given -out in effecting it, must be the same. - - -§ 15. There is another species of re-distribution among the component -matters of organisms, which is not immediately effected by the affinities -of the matters concerned, but is mediately effected by other affinities; -and there is reason to think that the re-distribution thus caused is -important in amount, if not indeed the most important. In ordinary cases of -chemical action, the two or more substances concerned themselves undergo -changes of molecular arrangement; and the changes are confined to the -substances themselves. But there are other cases in which the chemical -action going on does not end with the substances at first concerned, but -sets up chemical actions, or changes of molecular arrangement, among -surrounding substances that would else have remained quiescent. And there -are yet further cases in which mere contact with a substance that is itself -quiescent, will cause other substances to undergo rapid metamorphoses. In -what we call fermentation, the first species of this communicated chemical -action is exemplified. One part of yeast, while itself undergoing molecular -change, will convert 100 parts of sugar into alcohol and carbonic acid; and -during its own decomposition, one part of diastase "is able to effect the -transformation of more than 1000 times its weight of starch into sugar." As -illustrations of the second species, may be mentioned those changes which -are suddenly produced in many colloids by minute portions of various -substances added to them--substances that are not undergoing manifest -transformations, and suffer no appreciable effects from the contact. The -nature of the first of these two kinds of communicated molecular change, -which here chiefly concerns us, may be rudely represented by certain -visible changes communicated from mass to mass, when a series of masses has -been arranged in a special way. The simplest example is that furnished by -the child's play of setting bricks on end in a row, in such positions that -when the first is overthrown it overthrows the second, the second the -third, the third the fourth, and so on to the end of the row. Here we have -a number of units severally placed in unstable equilibrium, and in such -relative positions that each, while falling into a state of stable -equilibrium, gives an impulse to the next sufficient to make the next, -also, fall from unstable to stable equilibrium. Now since, among mingled -compound molecules, no one can undergo change in the arrangement of its -parts without a molecular motion that must cause some disturbance all -round; and since an adjacent molecule disturbed by this communicated -motion, may have the arrangement of its constituent atoms altered, if it is -not a stable arrangement; and since we know, both that the molecules which -are changed by this so-called catalysis _are_ unstable, and that the -molecules resulting from their changes are _more_ stable; it seems probable -that the transformation is really analogous, in principle, to the familiar -one named. Whether thus interpretable or not, however, there is good reason -for thinking that to this kind of action is due a large amount of vital -metamorphosis. Let us contemplate the several groups of facts which point -to this conclusion.[9] - -In the last chapter (§ 2) we incidentally noted the extreme instability of -nitrogenous compounds in general. We saw that sundry of them are liable to -explode on the slightest incentive--sometimes without any apparent cause; -and that of the rest, the great majority are very easily decomposed by -heat, and by various substances. We shall perceive much significance in -this general characteristic when we join it with the fact that the -substances capable of setting up extensive molecular changes in the way -above described are all nitrogenous ones. Yeast consists of vegetal cells -containing nitrogen,--cells that grow by assimilating the nitrogenous -matter contained in wort. Similarly, the "vinegar-plant," which greatly -facilitates the formation of acetic acid from alcohol, is a fungoid growth -that is doubtless, like others of its class, rich in nitrogenous compounds. -Diastase, by which the transformation of starch into sugar is effected -during the process of malting, is also a nitrogenous body. So too is a -substance called synaptase--an albumenous principle contained in almonds, -which has the power of working several metamorphoses in the matters -associated with it. These nitrogenized compounds, like the rest of their -family, are remarkable for the rapidity with which they decompose; and the -extensive changes produced by them in the accompanying carbo-hydrates, are -found to vary in their kinds according as the decompositions of the -ferments vary in their stages. We have next to note, as having here a -meaning for us, the chemical contrasts between those organisms which carry -on their functions by the help of external forces, and those which carry on -their functions by forces evolved from within. If we compare animals and -plants, we see that whereas plants, characterized as a class by containing -but little nitrogen, are dependent on the solar rays for their vital -activities; animals, the vital activities of which are not thus dependent, -mainly consist of nitrogenous substances. There is one marked exception to -this broad distinction, however; and this exception is specially -instructive. Among plants there is a considerable group--the Fungi--many -members of which, if not all, can live and grow in the dark; and it is -their peculiarity that they are very much more nitrogenous than other -plants. Yet a third class of facts of like significance is disclosed when -we compare different portions of the same organism. The seed of a plant -contains nitrogenous substance in a far higher ratio than the rest of the -plant; and the seed differs from the rest of the plant in its ability to -initiate, in the absence of light, extensive vital changes--the changes -constituting germination. Similarly in the bodies of animals, those parts -which carry on active functions are nitrogenous; while parts that are -non-nitrogenous--as the deposits of fat--carry on no active functions. And -we even find that the appearance of non-nitrogenous matter throughout -tissues normally composed almost wholly of nitrogenous matter, is -accompanied by loss of activity: what is called fatty degeneration being -the concomitant of failing vitality. One more fact, which serves to make -still clearer the meaning of the foregoing ones, remains--the fact, namely, -that in no part of any organism where vital changes are going on, is -nitrogenous matter wholly absent. It is common to speak of plants--or at -least all parts of plants but the seeds--as non-nitrogenous. But they are -only relatively so; not absolutely. The quantity of albumenoid substance in -the tissues of plants, is extremely small compared with the quantity -contained in the tissues of animals; but all plant-tissues which are -discharging active functions have some albumenoid substance. In every -living vegetal cell there is a certain part that includes nitrogen as a -component. This part initiates those changes which constitute the -development of the cell. And if it cannot be said that it is the worker of -all subsequent changes undergone by the cell, it nevertheless continues to -be the part in which the independent activity is most marked. - -Looking at the evidence thus brought together, do we not get an insight -into the actions of nitrogenous matter as a worker of organic changes? We -see that nitrogenous compounds in general are extremely prone to decompose: -their decomposition often involving a sudden and great evolution of energy. -We see that the substances classed as ferments, which, during their own -molecular changes, set up molecular changes in the accompanying -carbo-hydrates, are all nitrogenous. We see that among classes of -organisms, and among the parts of each organism, there is a relation -between the amount of nitrogenous matter present and the amount of -independent activity. And we see that even in organisms and parts of -organisms where the activity is least, such changes as do take place are -initiated by a substance containing nitrogen. Does it not seem probable, -then, that these extremely unstable compounds have everywhere the effect of -communicating to the less unstable compounds associated with them, -molecular movements towards a stable state, like those they are themselves -undergoing? The changes which we thus suppose nitrogenous matter to produce -in the body, are clearly analogous to those which we see it produce out of -the body. Out of the body, certain carbo-hydrates in continued contact with -nitrogenous matter, are transformed into carbonic acid and alcohol, and -unless prevented the alcohol is transformed into acetic acid: the -substances formed being thus more highly oxidized and more stable than the -substances destroyed. In the body, these same carbo-hydrates, in continued -contact with nitrogenous matter, are transformed into carbonic acid and -water: substances which are also more highly oxidized and more stable than -those from which they result. And since acetic acid is itself resolved by -further oxidation into carbonic acid and water; we see that the chief -difference between the two cases is, that the process is more completely -effected in the body than it is out of the body. Thus, to carry further the -simile used above, the molecules of carbo-hydrates contained in the tissues -are, like bricks on end, not in the stablest equilibrium; but still in an -equilibrium so stable, that they cannot be overthrown by the chemical and -thermal forces which the body brings to bear on them. On the other hand, -being like similarly-placed bricks that have very narrow ends, the -nitrogenous molecules contained in the tissues are in so unstable an -equilibrium that they cannot withstand these forces. And when these -delicately-poised nitrogenous molecules fall into stable arrangements, they -give impulses to the more firmly-poised non-nitrogenous molecules, which -cause them also to fall into stable arrangements. It is a curious and -significant fact that in the arts, we not only utilize this same principle -of initiating extensive changes among comparatively stable compounds, by -the help of compounds much less stable, but we employ for the purpose -compounds of the same general class. Our modern method of firing a gun is -to place in close proximity with the gunpowder which we wish to decompose -or explode, a small portion of fulminating powder, which is decomposed or -exploded with extreme facility, and which, on decomposing, communicates the -consequent molecular disturbance to the less-easily decomposed gunpowder. -When we ask what this fulminating powder is composed of, we find that it is -a nitrogenous salt.[10] - -Thus, besides the molecular re-arrangements produced in organic matter by -direct chemical action, there are others of kindred importance produced by -indirect chemical action. Indeed, the inference that some of the leading -transformations occurring in the animal organism, are due to this so-called -catalysis, appears necessitated by the general aspect of the facts, apart -from any such detailed interpretations as the foregoing. We know that -various amylaceous and saccharine matters taken as food do not appear in -the excreta, and must therefore be decomposed in their course through the -body. We know that these matters do not become components of the tissues, -but only of the contained liquids and solids; and that thus their -metamorphosis is not a direct result of tissue-change. We know that their -stability is such that the thermal and chemical forces to which they are -exposed in the body, cannot alone decompose them. The only explanation open -to us, therefore, is that the transformation of these carbo-hydrates into -carbonic acid and water, is due to communicated chemical action. - - -§ 16. This chapter will have served its purpose if it has given a -conception of the extreme modifiability of organic matter by surrounding -agencies. Even were it possible, it would be needless to describe in detail -the immensely varied and complicated changes which the forces from moment -to moment acting on them, work in living bodies. Dealing with biology in -its general principles, it concerns us only to notice how specially -sensitive are the substances of which organisms are built up to the varied -influences that act upon organisms. Their special sensitiveness has been -made sufficiently manifest in the several foregoing sections. - - - - -CHAPTER III. - -THE RE-ACTIONS OF ORGANIC MATTER ON FORCES. - - -§ 17. Re-distributions of Matter imply concomitant re-distributions of -Motion. That which under one of its aspects we contemplate as an alteration -of arrangement among the parts of a body, is, under a correlative aspect, -an alteration of arrangement among certain momenta, whereby these parts are -impelled to their new positions. At the same time that a force, acting -differently on the different units of an aggregate, changes their relations -to one another; these units, reacting differently on the different parts of -the force, work equivalent changes in the relations of these to one -another. Inseparably connected as they are, these two orders of phenomena -are liable to be confounded together. It is very needful, however, to -distinguish between them. In the last chapter we took a rapid survey of the -re-distributions which forces produce in organic matter; and here we must -take a like survey of the simultaneous re-distributions undergone by the -forces. - -At the outset we are met by a difficulty. The parts of an inorganic mass -undergoing re-arrangement by an incident force, are in most cases -passive--do not complicate those necessary re-actions that result from -their inertia, by other forces which they themselves originate. But in -organic matter the re-arranged parts do not re-act in virtue of their -inertia only. They are so constituted that an incident force usually sets -up in them other actions which are much more important. Indeed, what we may -call the indirect reactions thus caused, are so great in their amounts -compared with the direct re-actions, that they quite obscure them. - -The impossibility of separating these two kinds of reaction compels us to -disregard the distinction between them. Under the above general title, we -must include both the immediate re-actions and those re-actions mediately -produced, which are among the most conspicuous of vital phenomena. - - -§ 18. From organic matter, as from all other matter, incident forces call -forth that re-action which we know as heat. More or less of molecular -vibration necessarily results when, to the forces at work among the -molecules of any aggregate, other forces are added. Experiment abundantly -demonstrates this in the case of inorganic masses; and it must equally hold -in the case of organic masses. In both cases the force which, more -markedly than any other, produces this thermal re-action, is that which -ends in the union of different substances. Though inanimate bodies admit of -being greatly heated by pressure and by the electric current, yet the -evolutions of heat, thus induced are neither so common, nor in most cases -so conspicuous, as those resulting from chemical combination. And though in -animate bodies there are certain amounts of heat generated by other -actions, yet these are secondary to the heat generated by the action of -oxygen on the substances composing the tissues and the substances contained -in them. Here, however, we see one of the characteristic distinctions -between inanimate and animate bodies. Among the first there are but few -which ordinarily exist in a condition to evolve the heat caused by chemical -combination; and such as are in this condition soon cease to be so when -chemical combination and genesis of heat once begin in them. Whereas, among -the second there universally exists the ability, more or less decided, thus -to evolve heat; and the evolution of heat, in some cases very slight and in -no cases very great, continues as long as they remain animate bodies. - -The relation between active change of matter and re-active genesis of -molecular vibration, is clearly shown by the contrasts between different -organisms, and between different states and parts of the same organism. In -plants the genesis of heat is extremely small, in correspondence with their -extremely small production of carbonic acid: those portions only, as -flowers and germinating seeds, in which considerable oxidation is going on, -having decidedly raised temperatures. Among animals we see that the -hot-blooded are those which expend much force and respire actively. Though -insects are scarcely at all warmer than the surrounding air when they are -still, they rise several degrees above it when they exert themselves; and -in mammals, which habitually maintain a temperature much higher than that -of their medium, exertion is accompanied by an additional production of -heat. - -This molecular agitation accompanies the falls from unstable to stable -molecular combinations; whether they be those from the most complex to the -less complex compounds, or whether they be those ultimate falls which end -in fully oxidized and relatively simple compounds; and whether they be -those of the nitrogenous matters composing the tissues or those of the -non-nitrogenous matters diffused through them. In the one case as in the -other, the heat must be regarded as a concomitant. Whether the -distinction, originally made by Liebig, between nitrogenous substances as -tissue-food and non-nitrogenous substances as heat-food, be true or not in -a narrower sense, it cannot be accepted in the sense that tissue-food is -not also heat-food. Indeed he does not himself assert it in this sense. The -ability of carnivorous animals to live and generate heat while consuming -matter that is almost exclusively nitrogenous, suffices to prove that the -nitrogenous compounds forming the tissues are heat-producers, as well as -the non-nitrogenous compounds circulating among and through the tissues: a -conclusion which is indeed justified by the fact that nitrogenous -substances out of the body yield heat, though not a large amount, during -combustion. But most likely this antithesis is not true even in the more -restricted sense. The probability is that the hydrocarbons and -carbo-hydrates which, in traversing the system, are transformed by -communicated chemical action, evolve, during their transformation, not heat -alone but also other kinds of force. It may be that as the nitrogenous -matter, while falling into more stable molecular arrangements, generates -both that molecular agitation called heat and such other molecular -movements as are resolved into forces expended by the organism; so, too, -does the non-nitrogenous matter. Or perhaps the concomitants of this -metamorphosis of non-nitrogenous matter vary with the conditions. Heat -alone may result when it is transformed while in the circulating fluids, -but partly heat and partly another force when it is transformed in some -active tissue that has absorbed it; just as coal, though producing little -else but heat as ordinarily burnt, has its heat partially transformed into -mechanical motion if burnt in a steam-engine furnace. In such case the -antithesis of Liebig would be reduced to this--that whereas nitrogenous -substance is tissue-food _both_ as material for building-up tissue and as -material for its function; non-nitrogenous substance is tissue-food _only_ -as material for function. - -There can be no doubt that this thermal re-action which chemical action -from moment to moment produces in the body, is from moment to moment an aid -to further chemical action. We before saw (_First Principles_, § 100) that -a state of raised molecular vibration is favourable to those -re-distributions of matter and motion which constitute Evolution. We saw -that in organisms distinguished by the amount and rapidity of such -re-distributions, this raised state of molecular vibration is conspicuous. -And we here see that this raised state of molecular vibration is itself a -continuous consequence of the continuous molecular re-distributions it -facilitates. The heat generated by each increment of chemical change makes -possible the succeeding increment of chemical change. In the body this -connexion of phenomena is the same as we see it to be out of the body. Just -as in a burning piece of wood, the heat given out by the portion actually -combining with oxygen, raises the adjacent portion to a temperature at -which it also can combine with oxygen; so, in a living animal, the heat -produced by oxidation of each portion of organized or unorganized -substance, maintains the temperature at which the unoxidized portions can -be readily oxidized. - - -§ 19. Among the forces called forth from organisms by re-action against the -actions to which they are subject, is Light. Phosphorescence is in some few -cases displayed by plants--especially by certain fungi. Among animals it is -comparatively common. All know that there are several kinds of luminous -insects; and many are familiar with the fact that luminosity is a -characteristic of various marine creatures. - -Much of the evidence is supposed to imply that this evolution of light, -like the evolution of heat, is consequent on oxidation of the tissues or of -matters contained in them. Light, like heat, is the expression of a raised -state of molecular vibration: the difference between them being a -difference in the rates of vibration. Hence it seems inferable that by -chemical action on substances contained in the organism, heat or light may -be produced, according to the character of the resulting molecular -vibrations. Some experimental evidence supports this view. In -phosphorescent insects, the continuance of the light is found to depend on -the continuance of respiration; and any exertion which renders respiration -more active, increases the brilliancy of the light. Moreover, by separating -the luminous matter, Prof. Matteucci has shown that its emission of light -is accompanied by absorption of oxygen and escape of carbonic acid. The -phosphorescence of marine animals has been referred to other causes than -oxidation; but it may perhaps be explicable without assuming any more -special agency. Considering that in creatures of the genus _Noctiluca_, for -example, to which the phosphorescence most commonly seen on our own coasts -is due, there is no means of keeping up a constant circulation, we may -infer that the movements of aerated fluids through their tissues, must be -greatly affected by impulses received from without. Hence it may be that -the sparkles visible at night when the waves break gently on the beach, or -when an oar is dipped into the water, are called forth from these creatures -by the concussion, not because of any unknown influence it excites, but -because, being propagated through their delicate tissues, it produces a -sudden movement of the fluids and a sudden increase of chemical action. - -Nevertheless, in other phosphorescent animals inhabiting the sea, as in the -_Pyrosoma_ and in certain _Annelida_, light seems to be produced otherwise -than by direct re-action on the action of oxygen. Indeed, it needs but to -recall the now familiar fact that certain substances become luminous in the -dark after exposure to sunlight, to see that there are other causes of -light-emission. - - -§ 20. The re-distributions of inanimate matter are habitually accompanied -by electrical disturbances; and there is abundant evidence that electricity -is generated during those re-distributions of matter that are ever taking -place in organisms. Experiments have shown "that the skin and most of the -internal membranes are in opposite electrical states;" and also that -between different internal organs, as the liver and the stomach, there are -electrical contrasts: such contrasts being greatest where the processes -going on in the compared parts are most unlike. It has been proved by du -Bois-Reymond that when any point in the longitudinal section of a muscle is -connected by a conductor with any point in its transverse section, an -electric current is established; and further, that like results occur when -nerves are substituted for muscles. The special causes of these phenomena -have not yet been determined. Considering that the electric contrasts are -most marked where active secretions are going on--considering, too, that -they are difficult to detect where there are no appreciable movements of -liquids--considering, also, that even when muscles are made to contract -after removal from the body, the contraction inevitably causes movements of -the liquids still contained in its tissues; it may be that they are due -simply to the friction of heterogeneous substances, which is universally a -cause of electric disturbance. But whatever be the interpretation, the fact -remains the same:--there is throughout the living organism, an unceasing -production of differences between the electric states of different parts; -and, consequently, an unceasing restoration of electric equilibrium by the -establishment of currents among these parts. - -Besides these general, and not conspicuous, electrical phenomena common to -all organisms, vegetal as well as animal, there are certain special and -strongly marked ones. I refer, of course, to those which have made the -_Torpedo_ and the _Gymnotus_ objects of so much interest. In these -creatures we have a genesis of electricity which is not incidental on the -performance of their different functions by the different organs; but one -which is itself a function, having an organ appropriate to it. The -character of this organ in both these fishes, and its largely-developed -connexions with the nervous centres, have raised in some minds the -suspicion that in it there takes place a transformation of what we call -nerve-force into the force known as electricity. Perhaps, however, the true -interpretation may rather be that by nervous stimulation there is set up in -these animal-batteries that particular transformation of molecular motion -which it is their function to produce. - -But whether general or special, and in whatever manner produced, these -evolutions of electricity are among the reactions of organic matter called -forth by the actions to which it is subject. Though these re-actions are -not direct, but seem to be remote consequences of changes wrought by -external agencies on the organism, they are yet incidents in that general -re-distribution of motion which these external agencies initiate; and as -such must here be noticed. - - -§ 21. To these known modes of motion, has next to be added an unknown one. -Heat, Light, and Electricity are emitted by inorganic matter when -undergoing changes, as well as by organic matter. But there is manifested -in some classes of living bodies a kind of force which we cannot identify -with any of the forces manifested by bodies that are not alive,--a force -which is thus unknown, in the sense that it cannot be assimilated to any -otherwise-recognized class. I allude to what is called nerve-force. - -This is habitually generated in all animals, save the lowest, by incident -forces of every kind. The gentle and violent mechanical contacts, which in -ourselves produce sensations of touch and pressure--the additions and -abstractions of molecular vibration, which in ourselves produce sensations -of heat and cold, produce in all creatures that have nervous systems, -certain nervous disturbances: disturbances which, as in ourselves, are -either communicated to the chief nervous centre, and there arouse -consciousness, or else result in mere physical processes set going -elsewhere in the organism. In special parts distinguished as organs of -sense, other external actions bring about other nervous re-actions, that -show themselves either as special sensations or as excitements which, -without the intermediation of distinct consciousness, beget actions in -muscles or other organs. Besides neural discharges following the direct -incidence of external forces, others are ever being caused by the incidence -of forces which, though originally external, have become internal by -absorption into the organism of the agents exerting them. For thus may be -classed those neural discharges which result from modifications of the -tissues wrought by substances carried to them in the blood. That the -unceasing change of matter which oxygen and other agents produce throughout -the system, is accompanied by production of nerve-force, is shown by -various facts;--by the fact that nerve-force is no longer generated if -oxygen be withheld or the blood prevented from circulating; by the fact -that when the chemical transformation is diminished, as during sleep with -its slow respiration and circulation, there is a diminution in the quantity -of nerve-force; by the fact that an excessive expenditure of nerve-force -involves excessive respiration and circulation, and excessive waste of -tissue. To these proofs that nerve-force is evolved in greater or less -quantity, according as the conditions to rapid molecular change throughout -the body are well or ill fulfilled, may be added proofs that certain -special molecular actions are the causes of these special re-actions. The -effects of the vegeto-alkalies put beyond doubt the inference that the -overthrow of molecular equilibrium by chemical affinity, when it occurs in -certain parts, causes excitement in the nerves proceeding from those parts. -Indeed, looked at from this point of view, the two classes of nervous -changes--the one initiated from without and the other from within--are seen -to merge into one class. Both of them may be traced to metamorphosis of -tissue. The sensations of touch and pressure are doubtless consequent on -accelerated changes of matter, produced by mechanical disturbance of the -mingled fluids and solids composing the parts affected. There is abundant -evidence that the gustatory sensation is due to the chemical actions set up -by particles which find their way through the membrane covering the nerves -of taste; for, as Prof. Graham points out, sapid substances belong to the -class of crystalloids, which are able rapidly to permeate animal tissue, -while the colloids which cannot pass through animal tissue are insipid. -Similarly with the sense of smell. Substances which excite this sense are -necessarily more or less volatile; and their volatility being the result of -their molecular mobility, implies that they have, in a high degree, the -power of getting at the olfactory nerves by penetrating their mucous -investment. Again, the facts which photography has familiarized us with, -show that those nervous impressions called colours, are primarily due to -certain changes wrought by light in the substance of the retina. And -though, in the case of hearing, we cannot so clearly trace the connexion of -cause and effect, yet as we see that the auditory apparatus is one fitted -to intensify those vibrations constituting sound, and to convey them to a -receptacle containing liquid in which nerves are immersed, it can scarcely -be doubted that the sensation of sound proximately results from molecular -re-arrangements caused in these nerves by the vibrations of the liquid: -knowing, as we do, that the re-arrangement of molecules is in all cases -aided by agitation. Perhaps, however, the best proof that nerve-force, -whether peripheral or central in origin, results from chemical change, lies -in the fact that most of the chemical agents which powerfully affect the -nervous system, affect it whether applied at the centre or at the -periphery. Various mineral acids are tonics--the stronger ones being -usually the stronger tonics; and this which we call their acidity implies a -power in them of acting on the nerves of taste, while the tingling or pain -following their absorption through the skin, implies that the nerves of the -skin are acted on by them. Similarly with certain vegeto-alkalies which are -peculiarly bitter. By their bitterness these show that they affect the -extremities of the nerves, while, by their tonic properties, they show that -they affect the nervous centres: the most intensely bitter among them, -strychnia, being the most powerful nervous stimulant.[11] However true it -may be that this relation is not a regular one, since opium, hashish, and -some other drugs, which work marked effects on the brain, are not -remarkably sapid--however true it may be that there are relations between -particular substances and particular parts of the nervous system; yet such -instances do but qualify, without negativing, the general proposition. The -truth of this proposition can scarcely be doubted when, to the facts above -given, is added the fact that various condiments and aromatic drugs act as -nervous stimulants; and the fact that anæsthetics, besides the general -effects they produce when inhaled or swallowed, produce local effects of -like kind--first stimulant and then sedative--when absorbed through the -skin; and the fact that ammonia, which in consequence of its extreme -molecular mobility so quickly and so violently excites the nerves beneath -the skin, as well as those of the tongue and the nose, is a rapidly-acting -stimulant when taken internally. - -Whether a nerve is merely a conductor, which delivers at one of its -extremities an impulse received at the other, or whether, as some now -think, it is itself a generator of force which is initiated at one -extremity and accumulates in its course to the other extremity, are -questions which cannot yet be answered. All we know is that agencies -capable of working molecular changes in nerves are capable of calling forth -from them manifestations of activity. And our evidence that nerve-force is -thus originated, consists not only of such facts as the above, but also of -more conclusive facts established by direct experiments on -nerves--experiments which show that nerve-force results when the cut end of -a nerve is either mechanically irritated, or acted on by some chemical -agent, or subject to the galvanic current--experiments which prove that -nerve-force is generated by whatever disturbs the molecular equilibrium of -nerve-substance. - - -§ 22. The most important of the re-actions called forth from organisms by -surrounding actions, remains to be noticed. To the various forms of -insensible motion thus caused, we have to add sensible motion. On the -production of this mode of force more especially depends the possibility of -all vital phenomena. It is, indeed, usual to regard the power of generating -sensible motion as confined to one out of the two organic sub-kingdoms; or, -at any rate, as possessed by but few members of the other. On looking -closer into the matter, however, we see that plant-life as well as -animal-life, is universally accompanied by certain manifestations of this -power; and that plant-life could not otherwise continue. - -Through the humblest, as well as through the highest, vegetal organisms, -there are ever going on certain re-distributions of matter. In Protophytes -the microscope shows us an internal transposition of parts, which, when not -immediately visible, is proved to exist by the changes of arrangement that -become manifest in the course of hours and days. In the individual cells of -many higher plants, an active movement among the contained granules may be -witnessed. And well-developed cryptogams, in common with all phanerogams, -exhibit this genesis of mechanical motion still more conspicuously in the -circulation of sap. It might, indeed, be concluded _a priori_, that through -plants displaying much differentiation of parts, an internal movement must -be going on; since, without it, the mutual dependence of organs having -unlike functions would be impossible. Besides keeping up these motions of -liquids internally, plants, especially of the lower orders, move their -external parts in relation to each other, and also move about from place to -place. There are countless such illustrations as the active locomotion of -the zoospores of many _Algæ_, the rhythmical bendings of the _Oscillatoræ_, -the rambling progression of the _Diatomaceæ_. In fact many of these -smallest vegetals, and many of the larger ones in their early stages, -display a mechanical activity not distinguishable from that of the simplest -animals. Among well-organized plants, which are never locomotive in their -adult states, we still not unfrequently meet with relative motions of -parts. To such familiar cases as those of the Sensitive plant and the -Venus' fly-trap, many others may be added. When its base is irritated the -stamen of the Berberry flower leans over and touches the pistil. If the -stamens of the wild _Cistus_ be gently brushed with the finger, they spread -themselves: bending away from the seed-vessel. And some of the -orchid-flowers, as Mr. Darwin has shown, shoot out masses of pollen on to -the entering bee, when its trunk is thrust down in search of honey. - -Though the power of moving is not, as we see, a characteristic of animals -alone, yet in them, considered as a class, it is manifested to an extent so -marked as practically to become their most distinctive trait. For it is by -their immensely greater ability to generate mechanical motion, that animals -are enabled to perform those actions which constitute their visible lives; -and it is by their immensely greater ability to generate mechanical motion, -that the higher orders of animals are most obviously distinguished from the -lower orders. Though, on remembering the seemingly active movements of -infusoria, some will perhaps question this last-named contrast, yet, on -comparing the quantities of matter propelled through given spaces in given -times, they will see that the momentum evolved is far less in the -_Protozoa_ than in the _Metazoa_. These sensible motions of animals are -effected in sundry ways. In the humblest forms, and even in some of the -more developed forms which inhabit the water, locomotion results from the -oscillations of whip-like appendages, single or double, or from the -oscillations of cilia: the contractility resides in these waving hairs that -grow from the surface. In many _Coelenterata_ certain elongations or tails -of ectodermal or endodermal cells shorten when stimulated, and by these -rudimentary contractile organs the movements are effected. In all the -higher animals, however, and to a smaller degree in many of the lower, -sensible motion is generated by a special tissue, under a special -excitement. Though it is not strictly true that such animals show no -sensible motions otherwise caused, since all of them have certain ciliated -membranes, and since the circulation of liquids in them is partially due to -osmotic and capillary actions; yet, generally speaking, we may say that -their movements are effected solely by muscles which contract solely -through the agency of nerves. - -What special transformations of force generate these various mechanical -changes, we do not, in most cases, know. Those re-distributions of liquid, -with the alterations of form sometimes caused by them, that result from -osmose, are not, indeed, incomprehensible. Certain motions of plants which, -like those of the "animated oat," follow contact with water, are easily -interpreted; as are also such other vegetal motions as those of the -Touch-me-not, the Squirting Cucumber, and the _Carpobolus_. But we are -ignorant of the mode in which molecular movement is transformed into the -movement of masses, in animals. We cannot refer to known causes the -rhythmical action of a Medusa's disc, or that slow decrease of bulk which -spreads throughout the mass of an _Alcyonium_ when one of its component -individuals has been irritated. Nor are we any better able to say how the -insensible motion transmitted through a nerve, gives rise to sensitive -motion in a muscle. It is true that Science has given to Art several -methods of changing insensible into sensible motion. By applying heat to -water we vaporize it, and the movement of its expanding vapour we transfer -to solid matter; but evidently the genesis of muscular movement is in no -way analogous to this. The force evolved in a galvanic battery or by a -dynamo, we communicate to a soft iron magnet through a wire coiled round -it; and it would be possible, by placing near to each other several magnets -thus excited, to obtain, through the attraction of each for its neighbours, -an accumulated movement made up of their separate movements, and thus -mechanically to imitate a muscular contraction. But from what we know of -organic matter there is no reason to suppose that anything analogous to -this takes place in it. We can, however, through one kind of molecular -change, produce sensible changes of aggregation such as possibly might, -when occurring in organic substance, cause sensible motion in it. I refer -to change that is allotropic or isomeric. Sulphur, for example, assumes -different crystalline and non-crystalline forms at different temperatures, -and may be made to pass backwards and forwards from one form to another, by -slight variations of temperature: undergoing each time an alteration of -bulk. We know that this allotropism, or rather its analogue isomerism, -prevails among colloids--inorganic and organic. We also know that some of -these metamorphoses among colloids are accompanied by visible -re-arrangements: instance hydrated silicic acid, which, after passing from -its soluble state to the state of an insoluble jelly, begins, in a few -days, to contract and to give out part of its contained water. Now -considering that such isomeric changes of organic as well as inorganic -colloids, are often rapidly produced by very slight causes--a trace of a -neutral salt or a degree or two rise of temperature--it seems not -impossible that some of the colloids constituting muscle may be thus -changed by a nervous discharge: resuming their previous condition when the -discharge ceases. And it is conceivable that by structural arrangements, -minute sensible motions so caused may be accumulated into large sensible -motions. - - -§ 23. But the truths which it is here our business especially to note, are -independent of hypotheses or interpretations. It is sufficient for the ends -in view, to observe that organic matter _does_ exhibit these several -conspicuous reactions when acted on by incident forces. It is not requisite -that we should know _how_ these re-actions originate. - -In the last chapter were set forth the several modes in which incident -forces cause re-distributions of organic matter; and in this chapter have -been set forth the several modes in which is manifested the motion -accompanying this re-distribution. There we contemplated, under its several -aspects, the general fact that, in consequence of its extreme instability, -organic matter undergoes extensive molecular re-arrangements on very slight -changes of conditions. And here we have contemplated, under its several -aspects, the correlative general fact that, during these extensive -molecular re-arrangements, there are evolved large amounts of energy. In -the one case the components of organic matter are regarded as falling from -positions of unstable equilibrium to positions of stable equilibrium; and -in the other case they are regarded as giving out in their falls certain -momenta--momenta that may be manifested as heat, light, electricity, -nerve-force, or mechanical motion, according as the conditions determine. - -I will add only that these evolutions of energy are rigorously dependent on -these changes of matter. It is a corollary from the primordial truth which, -as we have seen, underlies all other truths, (_First Principles_, §§ 62, -189,) that whatever amount of power an organism expends in any shape, is -the correlate and equivalent of a power which was taken into it from -without. On the one hand, it follows from the persistence of force that -each portion of mechanical or other energy which an organism exerts, -implies the transformation of as much organic matter as contained this -energy in a latent state. And on the other hand, it follows from the -persistence of force that no such transformation of organic matter -containing this latent energy can take place, without the energy being in -one shape or other manifested. - - - - -CHAPTER III^{A.} - -METABOLISM. - - -§ 23a. In the early forties the French chemist Dumas pointed out the -opposed actions of the vegetal and animal kingdoms: the one having for its -chief chemical effect the decomposition of carbon-dioxide, with -accompanying assimilation of its carbon and liberation of its oxygen, and -the other having for its chief chemical effect the oxidation of carbon and -production of carbon-dioxide. Omitting those plants which contain no -chlorophyll, all others de-oxidize carbon; while all animals, save the few -which contain chlorophyll, re-oxidize carbon. This is not, indeed, a -complete account of the general relation; since it represents animals as -wholly dependent on plants, either directly or indirectly through other -animals, while plants are represented as wholly independent of animals; and -this last representation though mainly true, since plants can obtain direct -from the inorganic world certain other constituents they need, is in some -measure not true, since many with greater facility obtain these materials -from the decaying bodies of animals or from their _excreta_. But after -noting this qualification the broad antithesis remains as alleged. - -How are these transformations brought about? The carbon contained in -carbon-dioxide does not at a bound become incorporated in the plant, nor -does the substance appropriated by the animal from the plant become at a -bound carbon-dioxide. It is through two complex sets of changes that these -two ultimate results are brought about. The materials forming the tissues -of plants as well as the materials contained in them, are progressively -elaborated from the inorganic substances; and the resulting compounds, -eaten and some of them assimilated by animals, pass through successive -changes which are, on the average, of an opposite character: the two sets -being constructive and destructive. To express changes of both these -natures the term "metabolism" is used; and such of the metabolic changes as -result in building up from simple to compound are distinguished as -"anabolic," while those which result in the falling down from compound to -simple are distinguished as "katabolic." These antithetical names do not -indeed cover all the molecular transformations going on. Many of them, -known as isomeric, imply neither building up nor falling down: they imply -re-arrangement only. But those which here chiefly concern us are the two -opposed kinds described. - -A qualification is needful. These antithetic changes must be understood as -characterizing plant-life and animal-life in general ways rather than in -special ways--as expressing the transformations in their totalities but not -in their details. For there are katabolic processes in plants, though they -bear but a small ratio to the anabolic ones; and there are anabolic -processes in animals, though they bear but a small ratio to the katabolic -ones. - -From the chemico-physical aspect of these changes we pass to those -distinguished as vital; for metabolic changes can be dealt with only as -changes effected by that living substance called protoplasm. - - -§ 23b. On the evolution-hypothesis we are obliged to assume that the -earliest living things--probably minute units of protoplasm smaller than -any the microscope reveals to us--had the ability to appropriate directly -from the inorganic world both the nitrogen and the materials for -carbo-hydrates without both of which protoplasm cannot be formed; since in -the absence of preceding organic matter there was no other source. The -general law of evolution as well as the observed actions of _Protozoa_ and -_Protophyta_, suggest that these primordial types simultaneously displayed -animal-life and plant-life. For whereas the developed animal-type cannot -form from its inorganic surroundings either nitrogenous compounds or -carbo-hydrates; and whereas the developed plant-type, able to form -carbo-hydrates from its inorganic surroundings, depends for the formation -of its protoplasm mainly, although indirectly, on the nitrogenous compounds -derived from preceding organisms, as do also most of the plants devoid of -chlorophyll--the fungi; we are obliged to assume that in the beginning, -along with the expending activities characterizing the animal-type, there -went the accumulating activities characterizing both of the vegetal -types--forms of activity by-and-by differentiated. - -Though the successive steps in the artificial formation of organic -compounds have now gone so far that substances simulating proteids, if not -identical with them, have been produced, yet we have no clue to the -conditions under which proteids arose; and still less have we a clue to the -conditions under which inert proteids became so combined as to form active -protoplasm. The essential fact to be recognized is that living matter, -originated as we must assume during a long stage of progressive cooling in -which the infinitely varied parts of the Earth's surface were slowly -passing through appropriate physical conditions, possessed from the outset -the power of assimilating to itself the materials from which more living -matter was formed; and that since then all living matter has arisen from -its self-increasing action. But now, leaving speculation concerning these -anabolic changes as they commenced in the remote past, let us contemplate -them as they are carried on now--first directing our attention to those -presented in the vegetal world. - - -§ 23c. The decomposition of carbon-dioxide (§ 13)--the separation of its -carbon from the combined oxygen so that it may enter into one or other form -of carbo-hydrate,--is not now ordinarily effected, as we must assume it -once was, by the undifferentiated protoplasm; but is effected by a -specialized substance, chlorophyll, imbedded in the protoplasm and -operating by its instrumentality. The chlorophyll-grain is not simply -immersed in protoplasm but is permeated throughout its substance by a -protoplasmic network or sponge-work apparently continuous with the -protoplasm around; or, according to Sachs, consists of protoplasm holding -chlorophyll-particles in suspension: the mechanical arrangement -facilitating the chemical function. The resulting abstraction of carbon -from carbon-dioxide, by the aid of certain ethereal undulations, appears to -be the first step in the building up of organic compounds--the first step -in the primary anabolic process. We are not here concerned with details. -Two subsequent sets of changes only need here to be noted--the genesis of -the passive materials out of which plant-structure is built up, and the -genesis of the active materials by which these are produced and the -building up effected. - -The hydrated carbon which protoplasm, having the chlorophyll-grain as its -implement, produces from carbonic acid and water, appears not to be of one -kind only. The possible carbo-hydrates are almost infinite in number. -Multitudes of them have been artificially made, and numerous kinds are made -naturally by plants. Though perhaps the first step in the reduction of the -carbon from its dioxide may be always the same, yet it is held probable -that in different types of plants different types of carbo-hydrates -forthwith arise, and give differential characters to the compounds -subsequently formed by such types: sundry of the changes being katabolic -rather than anabolic. Of leading members in the group may be named dextrin, -starch, and the various sugars characteristic of various plants, as well as -the cellulose elaborated by further anabolism. Considered as the kind of -carbo-hydrate in which the products of activity are first stored up, to be -subsequently modified for divers purposes, starch is the most important of -these; and the process of storage is suggested by the structure of the -starch-grain. This consists of superposed layers, implying intermittent -deposits: the probability being that the variations of light and heat -accompanying day and night are associated now with arrest of the deposit -and now with recommencement of it. Like in composition as this stored-up -starch is with sugar of one or other kind, and capable of being deposited -from sugar and again assuming the sugar form, this substance passes, by -further metabolism, here into the cellulose which envelopes each of the -multitudinous units of protoplasm, there into the spiral fibres, annuli, or -fenestrated tubes which, in early stages of tissue-growth, form channels -for the sap, and elsewhere into other components of the general structure. -The many changes implied are effected in various ways: now by that simple -re-arrangement of components known as isomeric change; now by that taking -from a compound one of its elements and inserting one of another kind, -which is known as substitution; and now by oxidation, as when the -oxy-cellulose which constitutes wood-fibre, is produced. - -Besides elaborating building materials, the protoplasm elaborates -itself--that is, elaborates more of itself. It is chemically distinguished -from the building materials by the presence of nitrogen. Derived from -atmospheric ammonia, or from decaying or excreted organic matter, or from -the products of certain fungi and microbes at its roots, the nitrogen in -one or other combination is brought into a plant by the upward current; and -by some unknown process (not dependent on light, since it goes on equally -well if not better in darkness) the protoplasm dissociates and appropriates -this combined nitrogen and unites it with a carbo-hydrate to form one or -other proteid--albumen, gluten, or some isomer; appropriating at the same -time from certain of the earth-salts the requisite amount of sulphur and in -some cases phosphorus. The ultimate step, as we must suppose, is the -formation of living protoplasm out of these non-living proteids. A cardinal -fact is that proteids admit of multitudinous transformations; and it seems -not improbable that in protoplasm various isomeric proteids are mingled. If -so, we must conclude that protoplasm admits of almost infinite variations -in nature. Of course _pari passu_ with this dual process--augmentation of -protoplasm and accompanying production of carbo-hydrates--there goes -extension of plant-structure and plant-life. - -To these essential metabolic processes have to be added certain ancillary -and non-essential ones, ending in the formation of colouring matters, -odours, essential oils, acrid secretions, bitter compounds and poisons: -some serving to attract animals and others to repel them. Sundry of these -appear to be excretions--useless matters cast out, and are doubtless -katabolic. - -The relation of these facts here sketched in rude outline to the doctrine -of Evolution at large should be observed. Already we have seen how (§ 8a), -in the course of terrestrial evolution, there has been an increasingly -heterogeneous assemblage of increasing heterogeneous compounds, preparing -the way for organic life. And here we may see that during the development -of plant-life from its lowest algoid and fungoid forms up to those forms -which constitute the chief vegetal world, there has been an increasing -number of complex organic compounds formed; displayed at once in the -diversity of them contained in the same plant and in the still greater -diversity displayed in the vast aggregate of species, genera, orders, and -classes of plants. - - -§ 23d. On passing to the metabolism characterizing animal life, which, as -already indicated, is in the main a process of decomposition undoing the -process of composition characterizing vegetal life, we may fitly note at -the outset that it must have wide limits of variation, alike in different -classes of animals and even in the same animal. - -If we take, on the one hand, a carnivore living on muscular tissue (for -wild carnivores preying upon herbivores which can rarely become fat obtain -scarcely any carbo-hydrates) and observe that its food is almost -exclusively nitrogenous; and if, on the other hand, we take a graminivorous -animal the food of which (save when it eats seeds) contains comparatively -little nitrogenous matter; we seem obliged to suppose that the parts played -in the organic processes by the proteids and the carbo-hydrates can in -considerable measures replace one another. It is true that the quantity of -food and the required alimentary system in the last case, are very much -greater than in the first case. But this difference is mainly due to the -circumstance that the food of the graminivorous animal consists chiefly of -waste-matter--ligneous fibre, cellulose, chlorophyll--and that could the -starch, sugar, and protoplasm be obtained without the waste-matter, the -required bulks of the two kinds of food would be by no means so strongly -contrasted. This becomes manifest on comparing flesh-eating and -grain-eating birds--say a hawk and a pigeon. In powers of flight these do -not greatly differ, nor is the size of the alimentary system conspicuously -greater in the last than in the first; though probably the amount of food -consumed is greater. Still it seems clear that the supply of energy -obtained by a pigeon from carbo-hydrates with a moderate proportion of -proteids is not widely unlike that obtained by a hawk from proteids alone. -Even from the traits of men differently fed a like inference may be drawn. -On the one hand we have the Masai who, during their warrior-days, eat flesh -exclusively; and on the other hand we have the Hindus, feeding almost -wholly on vegetable food. Doubtless the quantities required in these cases -differ much; but the difference between the rations of the flesh-eater and -the grain-eater is not so immense as it would be were there no substitution -in the physiological uses of the materials. - -Concerning the special aspects of animal-metabolism, we have first to note -those various minor transformations that are auxiliary to the general -transformation by which force is obtained from food. For many of the vital -activities merely subserve the elaboration of materials for activity at -large, and the getting rid of waste products. From blood passing through -the salivary glands is prepared in large quantity a secretion containing -among other matters a nitrogenous ferment, ptyaline, which, mixed with food -during mastication, furthers the change of its starch into sugar. Then in -the stomach come the more or less varying secretions known in combination -as gastric juice. Besides certain salts and hydrochloric acid, this -contains another nitrogenous ferment, pepsin, which is instrumental in -dissolving the proteids swallowed. To these two metabolic products aiding -solution of the various ingested solids, is presently added that product of -metabolism in the pancreas which, added to the chyme, effects certain other -molecular changes--notably that of such amylaceous matters as are yet -unaltered, into saccharine matters to be presently absorbed. And let us -note the significant fact that the preparation of food-materials in the -alimentary canal, again shows us that unstable nitrogenous compounds are -the agents which, while themselves changing, set up changes in the -carbo-hydrates and proteids around: the nitrogen plays the same part here -as elsewhere. It does the like in yet another viscus. Blood which passes -through the spleen on its way to the liver, is exposed to the action of "a -special proteid of the nature of alkali-albumin, holding iron in some way -peculiarly associated with it." Lastly we come to that all-important organ -the liver, at once a factory and a storehouse. Here several metabolisms are -simultaneously carried on. There is that which until recent years was -supposed to be the sole hepatic process--the formation of bile. In some -liver-cells are masses of oil-globules, which seem to imply a carbo-hydrate -metamorphosis. And then, of leading importance, comes the extensive -production of that animal-starch known as glycogen--a substance which, in -each of the cells generating it, is contained in a plexus of protoplasmic -threads: again a nitrogenous body diffused through a mass which is now -formed out of sugar and is now dissolved again into sugar. For it appears -that this soluble form of carbo-hydrate, taken into the liver from the -intestine, is there, when not immediately needed, stored up in the form of -glycogen, ready to be re-dissolved and carried into the system either for -immediate use or for re-deposit as glycogen at the places where it is -presently to be consumed: the great deposit in the liver and the minor -deposits in the muscles being, to use the simile of Prof. Michael Foster, -analogous in their functions to a central bank and branch banks. - -An instructive parallelism may be noted between these processes carried on -in the animal organism and those carried on in the vegetal organism. For -the carbo-hydrates named, easily made to assume the soluble or the -insoluble form by the addition or subtraction of a molecule of water, and -thus fitted sometimes for distribution and sometimes for accumulation, are -similarly dealt with in the two cases. As the animal-starch, glycogen, is -now stored up in the liver or elsewhere and now changed into glucose to be -transferred, perhaps for consumption and perhaps for re-deposit; so the -vegetal starch, made to alternate between soluble and insoluble states, is -now carried to growing parts where by metabolic change it becomes cellulose -or other component of tissue and now carried to some place where, changed -back into starch, it is laid aside for future use; as it is in the turgid -inside leaves of a cabbage, the root of a turnip, or the swollen -underground stem we know as a potato: the matter which in the animal is -used up in generating movement and heat, being in the plant used up in -generating structures. Nor is the parallelism even now exhausted; for, as -by a plant starch is stored up in each seed for the subsequent use of the -embryo, so in an embryo-animal glycogen is stored up in the developing -muscles for subsequent use in the completion of their structures. - - -§ 23e. We come now to the supreme and all-pervading metabolism which has -for its effects the conspicuous manifestations of life--the nervous and -muscular activities. Here comes up afresh a question discussed in the -edition of 1864--a question to be reconsidered in the light of recent -knowledge--the question what particular metabolic changes are they by which -in muscle the energy existing under the form of molecular motion is -transformed into the energy manifested as molar motion? - -There are two views respecting the nature of this transformation. One is -that the carbo-hydrate present in muscle must, by further metabolism, be -raised into the form of a nitrogenous compound or compounds before it can -be made to undergo that sudden decomposition which initiates muscular -contraction. The other is the view set forth in § 15, and there reinforced -by further illustrations which have occurred to me while preparing this -revised edition--the view that the carbo-hydrate in muscle, everywhere in -contact with unstable nitrogenous substance, is, by the shock of a small -molecular change in this, made to undergo an extensive molecular change, -resulting in the oxidation of its carbon and consequent liberation of much -molecular motion. Both of these are at present only hypotheses, in support -of which respectively the probabilities have to be weighed. Let us compare -them and observe on which side the evidence preponderates. - -We are obliged to conclude that in carnivorous animals the katabolic -process is congruous with the first of these views, in so far that the -evolution of energy must in some way result solely from the fall of complex -nitrogenous compounds into those simpler matters which make their -appearance as waste; for, practically, the carnivorous animal has no -carbo-hydrates out of which otherwise to evolve force. To this admission, -however, it should be added that possibly out of the exclusively -nitrogenous food, glycogen or sugar has to be obtained by partial -decomposition before muscular action can take place. But when we pass to -animals having food consisting mainly of carbo-hydrates, several -difficulties stand in the way of the hypothesis that, by further -compounding, proteids must be formed from the carbo-hydrates before -muscular energy can be evolved. In the first place the anabolic change -through which, by the addition of nitrogen, &c., a proteid is formed from a -carbo-hydrate, must absorb an energy equal to a moiety of that which is -given out in the subsequent katabolic change. There can be no dynamic -profit on such part of the transaction as effects the composition and -subsequent decomposition of the proteid, but only on such part of the -transaction as effects the decomposition of the carbo-hydrate. In the -second place there arises the question--whence comes the nitrogen required -for the compounding of the carbo-hydrates into proteids? There is none save -that contained in the serum-albumen or other proteid which the blood -brings; and there can be no gain in robbing this proteid of nitrogen for -the purpose of forming another proteid. Hence the nitrogenizing of the -surplus carbo-hydrates is not accounted for. One more difficulty remains. -If the energy given out by a muscle results from the katabolic consumption -of its proteids, then the quantity of nitrogenous waste matters formed -should be proportionate to the quantity of work done. But experiments have -proved that this is not the case. Long ago it was shown that the amount of -urea excreted does not increase in anything like proportion to the amount -of muscular energy expended; and recently this has been again shown. - -On this statement a criticism has been made to the following -effect:--Considering that muscle will contract when deprived of oxygen and -blood and must therefore contain matter from which the energy is derived; -and considering that since carbonic acid is given out the required carbon -and oxygen must be derived from some component of muscle; it results that -the energy must be obtained by decomposition of a nitrogenous body. To this -reasoning it may be objected, in the first place, that the conditions -specified are abnormal, and that it is dangerous to assume that what takes -place under abnormal conditions takes place also under normal ones. In -presence of blood and oxygen the process may possibly, or even probably, be -unlike that which arises in their absence: the muscular substance may begin -consuming itself when it has not the usual materials to consume. Then, in -the second place, and chiefly, it may be replied that the difficulty raised -in the foregoing argument is not escaped but merely obscured. If, as is -alleged, the carbon and oxygen from which carbonic acid is produced, form, -under the conditions stated, parts of a complex nitrogenous substance -contained in muscle, then the abstraction of the carbon and oxygen must -cause decomposition of this nitrogenous substance; and in that case the -excretion of nitrogenous waste must be proportionate to the amount of work -done, which it is not. This difficulty is evaded by supposing that the -"stored complex explosive substance must be, in living muscle, of such -nature" that after explosion it leaves a "nitrogenous residue available for -re-combination with fresh portions of carbon and oxygen derived from the -blood and thereby the re-constitution of the explosive substance." This -implies that a molecule of the explosive substance consists of a complex -nitrogenous molecule united with a molecule of carbo-hydrate, and that time -after time it suddenly decomposes this carbo-hydrate molecule and thereupon -takes up another such from the blood. That the carbon is abstracted from -the carbo-hydrate molecule can scarcely be said, since the feebler -affinities of the nitrogenous molecule can hardly be supposed to overcome -the stronger affinities of the carbo-hydrate molecule. The carbo-hydrate -molecule must therefore be incorporated bodily. What is the implication? -The carbo-hydrate part of the compound is relatively stable, while the -nitrogenous part is relatively unstable. Hence the hypothesis implies that, -time after time, the unstable nitrogenous part overthrows the stable -carbo-hydrate part, without being itself overthrown. This conclusion, to -say the least of it, does not appear very probable. - -The alternative hypothesis, indirectly supported as we saw by proofs that -outside the body small amounts of change in nitrogenous compounds initiate -large amounts of change in carbonaceous compounds, may in the first place -be here supported by some further indirect evidences of kindred natures. A -haystack prematurely put together supplies one. Enough water having been -left in the hay to permit chemical action, the decomposing proteids forming -the dead protoplasm in each cell, set up decomposition of the -carbo-hydrates with accompanying oxidation of the carbon and genesis of -heat; even to the extent of producing fire. Again, as shown above, this -relation between these two classes of compounds is exemplified in the -alimentary canal; where, alike in the saliva and in the pancreatic -secretion, minute quantities of unstable nitrogenous bodies transform great -quantities of stable carbo-hydrates. Thus we find indirect reinforcements -of the belief that the katabolic change generating muscular energy is one -in which a large decomposition of a carbo-hydrate is set up by a small -decomposition of a proteid.[12] - - -§ 23f. A certain general trait of animal organization may fitly be named -because its relevance, though still more indirect, is very significant. -Under one of its aspects an animal is an apparatus for the multiplication -of energies--a set of appliances by means of which a minute amount of -motion initiates a larger amount of motion, and this again a still larger -amount. There are structures which do this mechanically and others which do -it chemically. - -Associated with the peripheral ends of the nerves of touch are certain -small bodies--_corpuscula tactus_--each of which, when disturbed by -something in contact with the skin, presses on the adjacent fibre more -strongly than soft tissue would do, and thus multiplies the force producing -sensation. While serving the further purpose of touching at a distance, the -_vibrissæ_ or whiskers of a feline animal achieve a like end in a more -effectual way. The external portion of each bristle acts as the long arm of -a lever, and the internal portion as the short arm. The result is that a -slight touch at the outer end of the bristle produces a considerable -pressure of the inner end on the nerve-terminal: so intensifying the -impression. In the hearing organs of various inferior types of animals, the -otolites in contact with the auditory nerves, when they are struck by -sound-waves, give to the nerves much stronger impressions than these would -have were they simply immersed in loose tissue; and in the ears of -developed creatures there exist more elaborate appliances for augmenting -the effects of aerial vibrations. From this multiplication of molar actions -let us pass to the multiplication of molecular actions. The retina is made -up of minute rods and cones, so packed together side by side that they can -be separately affected by the separate parts of the images of objects. As -each of them is but 1/10,000th of an inch in diameter, the ethereal -undulations falling upon it can produce an amount of change almost -infinitesimal--an amount probably incapable of exciting a nerve-centre, or -indeed of overcoming the molecular inertia of the nerve leading to it. But -in close proximity are layers of granules into which the rods and cones -send fibres, and beyond these, about 1/100th of an inch from the retinal -layer, lie ganglion-cells, in each of which a minute disturbance may -readily evolve a larger disturbance; so that by multiplication, single or -perhaps double, there is produced a force sufficient to excite the fibre -connected with the centre of vision. Such, at least, judging from the -requirement and the structure, seems to me the probable interpretation of -the visual process; though whether it is the accepted one I do not know. - -But now, carrying with us the conception made clear by the first cases and -suggested by the last, we shall appreciate the extent to which this general -physiological method, as we may call it, is employed. The convulsive action -caused by tickling shows it conspicuously. An extremely small amount of -molecular change in the nerve-endings produces an immense amount of -molecular change, and resulting molar motion, in the muscles. Especially is -this seen in one whose spinal cord has been so injured that it no longer -conveys sensations from the lower limbs to the brain; and in whom, -nevertheless, tickling of the feet produces convulsive actions of the legs -more violent even than result when sensation exists: clearly proving that -since the minute molecular change produced by the tickling in the -nerve-terminals cannot be equivalent in quantity to the amount implied by -the muscular contraction, there must be a multiplication of it in those -parts of the spinal cord whence issue the reflex stimuli to the muscles. - -Returning now to the question of metabolism, we may see that the processes -of multiplication above supposed to take place in muscle, are analogous in -their general nature to various other physiological processes. Carrying -somewhat further the simile used in § 15 and going back to the days when -detonators, though used for small arms, were not used for artillery, we may -compare the metabolic process in muscle to that which would take place if a -pistol were fired against the touch-hole of a loaded cannon: the cap -exploding the pistol and the pistol the cannon. For in the case of the -muscle, the implication is that a nervous discharge works in certain -unstable proteids through which the nerve-endings are distributed, a small -amount of molecular change; that the shock of this causes a much larger -amount of molecular change in the inter-diffused carbo-hydrate, with -accompanying oxidation of its carbon; and that the heat liberated sets up a -transformation, probably isomeric, in the contractile substance of the -muscular fibre: an interpretation supported by cases in which small rises -and falls of temperature cause alternating isomeric changes; as instance -Mensel's salt. - -Ending here this exposition, somewhat too speculative and running into -details inappropriate to a work of this kind, it suffices to note the most -general facts concerning metabolism. Regarded as a whole it includes, in -the first place, those anabolic or building-up processes specially -characterizing plants, during which the impacts of ethereal undulations are -stored up in compound molecules of unstable kinds; and it includes, in the -second place, those katabolic or tumbling-down changes specially -characterizing animals, during which this accumulated molecular motion -(contained in the food directly or indirectly supplied by plants), is in -large measure changed into those molar motions constituting animal -activities. There are multitudinous metabolic changes of minor kinds which -are ancillary to these--many katabolic changes in plants and many anabolic -changes in animals--but these are the essential ones.[13] - - - - -CHAPTER IV.[14] - -PROXIMATE CONCEPTION OF LIFE. - - -§ 24. To those who accept the general doctrine of Evolution, it need -scarcely be pointed out that classifications are subjective conceptions, -which have no absolute demarcations in Nature corresponding to them. They -are appliances by which we limit and arrange the matters under -investigation; and so facilitate our thinking. Consequently, when we -attempt to define anything complex, or make a generalization of facts other -than the most simple, we can scarcely ever avoid including more than we -intended, or leaving out something which should be taken in. Thus it -happens that on seeking a definite idea of Life, we have great difficulty -in finding one that is neither more nor less than sufficient. Let us look -at a few of the most tenable definitions that have been given. While -recognizing the respects in which they are defective, we shall see what -requirements a more satisfactory one must fulfil. - -Schelling said that Life is the tendency to individuation. This formula, -until studied, conveys little meaning. But we need only consider it as -illustrated by the facts of development, or by the contrast between lower -and higher forms of life, to recognize its significance; especially in -respect of comprehensiveness. As before shown, however (_First Principles_, -§ 56), it is objectionable; partly on the ground that it refers not so much -to the functional changes constituting Life, as to the structural changes -of those aggregates of matter which manifest Life; and partly on the ground -that it includes under the idea Life, much that we usually exclude from it: -for instance--crystallization. - -The definition of Richerand,--"Life is a collection of phenomena which -succeed each other during a limited time in an organized body,"--is liable -to the fatal criticism, that it equally applies to the decay which goes on -after death. For this, too, is "a collection of phenomena which succeed -each other during a limited time in an organized body." - -"Life," according to De Blainville, "is the two-fold internal movement of -composition and decomposition, at once general and continuous." This -conception is in some respects too narrow, and in other respects too wide. -On the one hand, while it expresses what physiologists distinguish as -vegetative life, it does not indicate those nervous and muscular functions -which form the most conspicuous and distinctive classes of vital phenomena. -On the other hand, it describes not only the integrating and disintegrating -process going on in a living body, but it equally well describes those -going on in a galvanic battery; which also exhibits a "two-fold internal -movement of composition and decomposition, at once general and continuous." - -Elsewhere, I have myself proposed to define Life as "the co-ordination of -actions."[15] This definition has some advantages. It includes all organic -changes, alike of the viscera, the limbs, and the brain. It excludes the -great mass of inorganic changes; which display little or no co-ordination. -By making co-ordination the specific character of vitality, it involves the -truths, that an arrest of co-ordination is death, and that imperfect -co-ordination is disease. Moreover, it harmonizes with our ordinary ideas -of life in its different grades; seeing that the organisms which we rank as -low in their degrees of life, are those which display but little -co-ordination of actions; and seeing that from these up to man, the -recognized increase in degree of life corresponds with an increase in the -extent and complexity of co-ordinations. But, like the others, this -definition includes too much. It may be said of the Solar System, with its -regularly-recurring movements and its self-balancing perturbations, that -it, also, exhibits co-ordination of actions. And however plausibly it may -be argued that, in the abstract, the motions of the planets and satellites -are as properly comprehended in the idea of life as the changes going on in -a motionless, unsensitive seed: yet, it must be admitted that they are -foreign to that idea as commonly received, and as here to be formulated. - -It remains to add the definition since suggested by Mr. G. H. Lewes--"Life -is a series of definite and successive changes, both of structure and -composition, which take place within an individual without destroying its -identity." The last fact which this statement brings into view--the -persistence of a living organism as a whole, in spite of the continuous -removal and replacement of its parts--is important. But otherwise it may be -argued that, since changes of structure and composition, though -concomitants of muscular and nervous actions, are not the muscular and -nervous actions themselves, the definite excludes the more visible -movements with which our idea of life is most associated; and further that, -in describing vital changes as _a series_, it scarcely includes the fact -that many of them, as Nutrition, Circulation, Respiration, and Secretion, -in their many subdivisions, go on simultaneously. - -Thus, however well each of these definitions expresses the phenomena of -life under some of its aspects, no one of them is more than approximately -true. It may turn out that to find a formula which will bear every test is -impossible. Meanwhile, it is possible to frame a more adequate formula than -any of the foregoing. As we shall presently find, these all omit an -essential peculiarity of vital changes in general--a peculiarity which, -perhaps more than any other, distinguishes them from non-vital changes. -Before specifying this peculiarity, however, it will be well to trace our -way, step by step, to as complete an idea of Life as may be reached from -our present stand-point; by doing which we shall both see the necessity for -each limitation as it is made, and ultimately be led to feel the need for a -further limitation. - -And here, as the best mode of determining what are the traits which -distinguish vitality from non-vitality, we shall do well to compare the two -most unlike kinds of vitality, and see in what they agree. Manifestly, that -which is essential to Life must be that which is common to Life of all -orders. And manifestly, that which is common to all forms of Life, will -most readily be seen on contrasting those forms of Life which have the -least in common, or are the most unlike.[16] - - -§ 25. Choosing assimilation, then, for our example of bodily life, and -reasoning for our example of that life known as intelligence; it is first -to be observed, that they are both processes of change. Without change, -food cannot be taken into the blood nor transformed into tissue; without -change, there can be no getting from premisses to conclusion. And it is -this conspicuous display of changes which forms the substratum of our idea -of Life in general. Doubtless we see innumerable changes to which no notion -of vitality attaches. Inorganic bodies are ever undergoing changes of -temperature, changes of colour, changes of aggregation; and decaying -organic bodies also. But it will be admitted that the great majority of the -phenomena displayed by inanimate bodies, are statical and not dynamical; -that the modifications of inanimate bodies are mostly slow and unobtrusive; -that on the one hand, when we see sudden movements in inanimate bodies, we -are apt to assume living agency, and on the other hand, when we see no -movements in living bodies, we are apt to assume death. Manifestly then, be -the requisite qualifications what they may, a true idea of Life must be an -idea of some kind of change or changes. - -On further comparing assimilation and reasoning, with a view of seeing in -what respect the changes displayed in both differs from non-vital changes, -we find that they differ in being not simple changes; in each case there -are _successive_ changes. The transformation of food into tissue involves -mastication, deglutition, chymification, chylification, absorption, and -those various actions gone through after the lacteal ducts have poured -their contents into the blood. Carrying on an argument necessitates a long -chain of states of consciousness; each implying a change of the preceding -state. Inorganic changes, however, do not in any considerable degree -exhibit this peculiarity. It is true that from meteorologic causes, -inanimate objects are daily, sometimes hourly, undergoing modifications of -temperature, of bulk, of hygrometric and electric condition. Not only, -however, do these modifications lack that conspicuousness and that rapidity -of succession which vital ones possess, but vital ones form an _additional_ -series. Living as well as not-living bodies are affected by atmospheric -influences; and beyond the changes which these produce, living bodies -exhibit other changes, more numerous and more marked. So that though -organic change is not rigorously distinguished from inorganic change by -presenting successive phases; yet vital change so greatly exceeds other -change in this respect, that we may consider it as a distinctive character. -Life, then, as thus roughly differentiated, may be regarded as change -presenting successive phases; or otherwise, as a series of changes. And it -should be observed, as a fact in harmony with this conception, that the -higher the life the more conspicuous the variations. On comparing inferior -with superior organisms, these last will be seen to display more rapid -changes, or a more lengthened series of them, or both. - -On contemplating afresh our two typical phenomena, we may see that vital -change is further distinguished from non-vital change, by being made up of -many _simultaneous_ changes. Nutrition is not simply a series of actions, -but includes many actions going on together. During mastication the stomach -is busy with food already swallowed, on which it is pouring out solvent -fluids and expending muscular efforts. While the stomach is still active, -the intestines are performing their secretive, contractile, and absorbent -functions; and at the same time that one meal is being digested, the -nutriment obtained from a previous meal is undergoing transformation into -tissue. So too is it, in a certain sense, with mental changes. Though the -states of consciousness which make up an argument occur in series, yet, as -each of them is complex, a number of simultaneous changes have taken place -in establishing it. Here as before, however, it must be admitted that the -distinction between animate and inanimate is not precise. No mass of dead -matter can have its temperature altered, without at the same time -undergoing an alteration in bulk, and sometimes also in hygrometric state. -An inorganic body cannot be compressed, without being at the same time -changed in form, atomic arrangement, temperature, and electric condition. -And in a vast and mobile aggregate like the sea, the simultaneous as well -as the successive changes outnumber those going on in an animal. -Nevertheless, speaking generally, a living thing is distinguished from a -dead thing by the multiplicity of the changes at any moment taking place in -it. Moreover, by this peculiarity, as by the previous one, not only is the -vital more or less clearly marked off from the non-vital; but creatures -possessing high vitality are marked off from those possessing low vitality. -It needs but to contrast the many organs cooperating in a mammal, with the -few in a polype, to see that the actions which are progressing together in -the body of the first, as much exceed in number the actions progressing -together in the body of the last, as these do those in a stone. As at -present conceived, then, Life consists of simultaneous and successive -changes. - -Continuance of the comparison shows that vital changes, both visceral and -cerebral, differ from other changes in their _heterogeneity_. Neither the -simultaneous acts nor the serial acts, which together constitute the -process of digestion, are alike. The states of consciousness comprised in -any ratiocination are not repetitions one of another, either in composition -or in modes of dependence. Inorganic processes, on the other hand, even -when like organic ones in the number of the simultaneous and successive -changes they involve, are unlike them in the relative homogeneity of these -changes. In the case of the sea, just referred to, it is observable that -countless as are the actions at any moment going on, they are mostly -mechanical actions that are to a great degree similar; and in this respect -differ widely from the actions at any moment taking place in an organism. -Even where life is nearly simulated, as by the working of a steam-engine, -we see that considerable as is the number of simultaneous changes, and -rapid as are the successive ones, the regularity with which they soon recur -in the same order and degree, renders them unlike those varied changes -exhibited by a living creature. Still, this peculiarity, like the -foregoing ones, does not divide the two classes of changes with precision; -since there are inanimate things presenting considerable heterogeneity of -change: for instance, a cloud. The variations of state which this -undergoes, both simultaneous and successive, are many and quick; and they -differ widely from one another both in quality and quantity. At the same -instant there may occur change of position, change of form, change of size, -change of density, change of colour, change of temperature, change of -electric state; and these several kinds of change are continuously -displayed in different degrees and combinations. Yet when we observe that -very few inorganic objects manifest heterogeneity of change comparable to -that manifested by organic objects, and further, that in ascending from low -to high forms of life, we meet with an increasing variety in the kinds of -changes displayed; we see that there is here a further leading distinction -between vital and non-vital actions. According to this modified conception, -then, Life is made up of heterogeneous changes both simultaneous and -successive. - -If, now, we look for some trait common to the nutritive and logical -processes, by which they are distinguished from those inorganic processes -that are most like them in the heterogeneity of the simultaneous and -successive changes they comprise, we discover that they are distinguished -by the _combination_ among their constituent changes. The acts which make -up digestion are mutually dependent. Those composing a train of reasoning -are in close connection. And, generally, it is to be remarked of vital -changes, that each is made possible by all, and all are affected by each. -Respiration, circulation, absorption, secretion, in their many -sub-divisions, are bound up together. Muscular contraction involves -chemical change, change of temperature, and change in the excretions. -Active thought influences the operations of the stomach, of the heart, of -the kidneys. But we miss this union among non-vital activities. Life-like -as may seem the action of a volcano in respect of the heterogeneity of its -many simultaneous and successive changes, it is not life-like in respect of -their combination. Though the chemical, mechanical, thermal, and electric -phenomena exhibited have some inter-dependence, yet the emissions of -stones, mud, lava, flame, ashes, smoke, steam, take place irregularly in -quantity, order, intervals, and mode of conjunction. Even here, however, -it cannot be said that inanimate things present no parallels to animate -ones. A glacier may be instanced as showing nearly as much combination in -its change as a plant of the lowest organization. It is ever growing and -ever decaying; and the rates of its composition and decomposition preserve -a tolerably constant ratio. It moves; and its motion is in immediate -dependence on its thawing. It emits a torrent of water, which, in common -with its motion, undergoes annual variations as plants do. During part of -the year the surface melts and freezes alternately; and on these changes -depend the variations in movement, and in efflux of water. Thus we have -growth, decay, changes of temperature, changes of consistence, changes of -velocity, changes of excretion, all going on in connexion; and it may be as -truly said of a glacier as of an animal, that by ceaseless integration and -disintegration it gradually undergoes an entire change of substance without -losing its individuality. This exceptional instance, however, will scarcely -be held to obscure that broad distinction from inorganic processes which -organic processes derive from the combination among their constituent -changes. And the reality of this distinction becomes yet more manifest when -we find that, in common with previous ones, it not only marks off the -living from the not-living, but also things which live little from things -which live much. For while the changes going on in a plant or a zoophyte -are so imperfectly combined that they can continue after it has been -divided into two or more pieces, the combination among the changes going on -in a mammal is so close that no part cut off from the rest can live, and -any considerable disturbance of one chief function causes a cessation of -the others. Hence, as we now regard it, Life is a combination of -heterogeneous changes, both simultaneous and successive. - -When we once more look for a character common to these two kinds of vital -action, we perceive that the combinations of heterogeneous changes which -constitute them, differ from the few combinations which they otherwise -resemble, in respect of _definiteness_. The associated changes going on in -a glacier, admit of indefinite variation. Under a conceivable alteration of -climate, its thawing and its progression may be stopped for a million -years, without disabling it from again displaying these phenomena under -appropriate conditions. By a geological convulsion, its motion may be -arrested without an arrest of its thawing; or by an increase in the -inclination of the surface it slides over, its motion may be accelerated -without accelerating its rate of dissolution. Other things remaining the -same, a more rapid deposit of snow may cause great increase of bulk; or, -conversely, the accretion may entirely cease, and yet all the other actions -continue until the mass disappears. Here, then, the combination has none of -that definiteness which, in a plant, marks the mutual dependence of -respiration, assimilation, and circulation; much less has it that -definiteness seen in the mutual dependence of the chief animal functions; -no one of which can be varied without varying the rest; no one of which can -go on unless the rest go on. Moreover, this definiteness of combination -distinguishes the changes occurring in a living body from those occurring -in a dead one. Decomposition exhibits both simultaneous and successive -changes, which are to some extent heterogeneous, and in a sense combined; -but they are not combined in a definite manner. They vary according as the -surrounding medium is air, water, or earth. They alter in nature with the -temperature. If the local conditions are unlike, they progress differently -in different parts of the mass, without mutual influence. They may end in -producing gases, or adipocire, or the dry substance of which mummies -consist. They may occupy a few days or thousands of years. Thus, neither in -their simultaneous nor in their successive changes, do dead bodies display -that definiteness of combination which characterizes living ones. It is -true that in some inferior creatures the cycle of successive changes admits -of a certain indefiniteness--that it may be suspended for a long period by -desiccation or freezing, and may afterwards go on as though there had been -no breach in its continuity. But the circumstance that only a low order of -life can have its changes thus modified, serves but to suggest that, like -the previous characteristics, this characteristic of definiteness in its -combined changes, distinguishes high vitality from low vitality, as it -distinguishes low vitality from inorganic processes. Hence, our formula as -further amended reads thus:--Life is a definite combination of heterogenous -changes, both simultaneous and successive. - -Finally, we shall still better express the facts if, instead of saying _a_ -definite combination of heterogeneous changes, we say _the_ definite -combination of heterogeneous changes. As it at present stands, the -definition is defective both in allowing that there may be _other_ definite -combinations of heterogeneous changes, and in directing attention to the -heterogeneous changes rather than to the definiteness of their combination. -Just as it is not so much its chemical elements which constitute an -organism, as it is the arrangement of them into special tissues and organs; -so it is not so much its heterogeneous changes which constitute Life, as it -is the co-ordination of them. Observe what it is that ceases when life -ceases. In a dead body there are going on heterogeneous changes, both -simultaneous and successive. What then has disappeared? The definite -combination has disappeared. Mark, too, that however heterogeneous the -simultaneous and successive changes exhibited by such an inorganic object -as a volcano, we much less tend to think of it as living than we do a watch -or a steam-engine, which, though displaying changes that, serially -contemplated, are largely homogeneous, displays them definitely combined. -So dominant an element is this in our idea of Life, that even when an -object is motionless, yet, if its parts be definitely combined, we conclude -either that it has had life, or has been made by something having life. -Thus, then, we conclude that Life is--_the_ definite combination of -heterogeneous changes, both simultaneous and successive. - - -§ 26. Such is the conception at which we arrive without changing our -stand-point. It is, however, an incomplete conception. This ultimate -formula (which is to a considerable extent identical with one above -given--"the co-ordination of actions;" seeing that "definite combination" -is synonymous with "co-ordination," and "changes both simultaneous and -successive" are comprehended under the term "actions;" but which differs -from it in specifying the fact, that the actions or changes are -"heterogeneous")--this ultimate formula, I say, is after all but a rude -approximation. It is true that it does not fail by including the growth of -a crystal; for the successive changes this implies cannot be called -heterogeneous. It is true that the action of a galvanic battery is not -comprised in it; since here, too, heterogeneity is not exhibited by the -successive changes. It is true that by this same qualification the motions -of the Solar System are excluded, as are also those of a watch and a -steam-engine. It is true, moreover, that while, in virtue of their -heterogeneity, the actions going on in a cloud, in a volcano, in a glacier, -fulfil the definition; they fall short of it in lacking definiteness of -combination. It is further true that this definiteness of combination -distinguishes the changes taking place in an organism during life from -those which commence at death. And beyond all this it is true that, as well -as serving to mark off, more or less clearly, organic actions from -inorganic actions, each member of the definition serves to mark off the -actions constituting high vitality from those constituting low vitality; -seeing that life is high in proportion to the number of successive changes -occurring between birth and death; in proportion to the number of -simultaneous changes; in proportion to the heterogeneity of the changes; in -proportion to the combination subsisting among the changes; and in -proportion to the definiteness of their combination. Nevertheless, -answering though it does to so many requirements, this definition is -essentially defective. _The definite combination of heterogeneous changes, -both simultaneous and successive_, is a formula which fails to call up an -adequate conception. And it fails from omitting the most distinctive -peculiarity--the peculiarity of which we have the most familiar experience, -and with which our notion of Life is, more than with any other, associated. -It remains now to supplement the conception by the addition of this -peculiarity. - - - - -CHAPTER V. - -THE CORRESPONDENCE BETWEEN LIFE AND ITS CIRCUMSTANCES. - - -§ 27. We habitually distinguish between a live object and a dead one, by -observing whether a change which we make in the surrounding conditions, or -one which Nature makes in them, is or is not followed by some perceptible -change in the object. By discovering that certain things shrink when -touched, or fly away when approached, or start when a noise is made, the -child first roughly discriminates between the living and the not-living; -and the man when in doubt whether an animal he is looking at is dead or -not, stirs it with his stick; or if it be at a distance, shouts, or throws -a stone at it. Vegetal and animal life are alike primarily recognized by -this process. The tree that puts out leaves when the spring brings increase -of temperature, the flower which opens and closes with the rising and -setting of the sun, the plant that droops when the soil is dry and -re-erects itself when watered, are considered alive because of these -induced changes; in common with the acorn-shell which contracts when a -shadow suddenly falls on it, the worm that comes to the surface when the -ground is continuously shaken, and the hedgehog that rolls itself up when -attacked. - -Not only, however, do we look for some response when an external stimulus -is applied to a living organism, but we expect a fitness in the response. -Dead as well as living things display changes under certain changes of -condition: instance, a lump of carbonate of soda that effervesces when -dropped into sulphuric acid; a cord that contracts when wetted; a piece of -bread that turns brown when held near the fire. But in these cases, we do -not see a connexion between the changes undergone and the preservation of -the things that undergo them; or, to avoid any teleological -implication--the changes have no apparent relations to future events which -are sure or likely to take place. In vital changes, however, such relations -are manifest. Light being necessary to vegetal life, we see in the action -of a plant which, when much shaded, grows towards the unshaded side, an -appropriateness which we should not see did it grow otherwise. Evidently -the proceedings of a spider which rushes out when its web is gently shaken -and stays within when the shaking is violent, conduce better to the -obtainment of food and the avoidance of danger than were they reversed. The -fact that we feel surprise when, as in the case of a bird fascinated by a -snake, the conduct tends towards self-destruction, at once shows how -generally we have observed an adaptation of living changes to changes in -surrounding circumstances. - -A kindred truth, rendered so familiar by infinite repetition that we forget -its significance, must be named. There is invariably, and necessarily, a -conformity between the vital functions of any organism and the conditions -in which it is placed--between the processes going on inside of it and the -processes going on outside of it. We know that a fish cannot live long in -air, or a man under water. An oak growing in the ocean and a seaweed on the -top of a hill, are incredible combinations of ideas. We find that each kind -of animal is limited to a certain range of climate; each kind of plant to -certain zones of latitude and elevation. Of the marine flora and fauna, -each species is found only between such and such depths. Some blind -creatures flourish in dark caves; the limpet where it is alternately -covered and uncovered by the tide; the red-snow alga rarely elsewhere than -in the arctic regions or among alpine peaks. - -Grouping together the cases first named, in which a particular change in -the circumstances of an organism is followed by a particular change in it, -and the cases last named, in which the constant actions occurring within an -organism imply some constant actions occurring without it; we see that in -both, the changes or processes displayed by a living body are specially -related to the changes or processes in its environment. And here we have -the needful supplement to our conception of Life. Adding this all-important -characteristic, our conception of Life becomes--The definite combination of -heterogeneous changes, both simultaneous and successive, _in correspondence -with external co-existences and sequences_. That the full significance of -this addition may be seen, it will be necessary to glance at the -correspondence under some of its leading aspects.[17] - - -§ 28. Neglecting minor requirements, the actions going on in a plant -pre-suppose a surrounding medium containing at least carbonic acid and -water, together with a due supply of light and a certain temperature. -Within the leaves carbon is being appropriated and oxygen given off; -without them, is the gas from which the carbon is taken, and the -imponderable agents that aid the abstraction. Be the nature of the process -what it may, it is clear that there are external elements prone to undergo -special re-arrangements under special conditions. It is clear that the -plant in sunshine presents these conditions and so effects these -re-arrangements. And thus it is clear that the changes which primarily -constitute the plant's life, are in correspondence with co-existences in -its environment. - -If, again, we ask respecting the lowest protozoon how it lives; the answer -is, that while on the one hand its substance is undergoing disintegration, -it is on the other hand absorbing nutriment; and that it may continue to -exist, the one process must keep pace with, or exceed, the other. If -further we ask under what circumstances these combined changes are -possible, there is the reply that the medium in which the protozoon is -placed, must contain oxygen and food--oxygen in such quantity as to produce -some disintegration; food in such quantity as to permit that disintegration -to be made good. In other words--the two antagonistic processes taking -place internally, imply the presence externally of materials having -affinities that can give rise to them. - -Leaving those lowest animal forms which simply take in through their -surfaces the nutriment and oxygenated fluids coming in contact with them, -we pass to those somewhat higher forms which have their tissues slightly -specialized. In these we see a correspondence between certain actions in -the digestive sac, and the properties of certain surrounding bodies. That a -creature of this order may continue to live, it is necessary not only that -there be masses of substance in the environment capable of transformation -into its own tissue, but also that the introduction of these masses into -its stomach, shall be followed by the secretion of a solvent fluid which -will reduce them to a fit state for absorption. Special outer properties -must be met by special inner properties. - -When, from the process by which food is digested, we turn to the process by -which it is seized, the same general truth faces us. The stinging and -contractile power of a polype's tentacle, correspond to the sensitiveness -and strength of the creatures serving it for prey. Unless that external -change which brings one of these creatures in contact with the tentacle, -were quickly followed by those internal changes which result in the coiling -and drawing up of the tentacle, the polype would die of inanition. The -fundamental processes of integration and disintegration within it, would -get out of correspondence with the agencies and processes without it, and -the life would cease. - -Similarly, when the creature becomes so large that its tissue cannot be -efficiently supplied with nutriment by mere absorption through its lining -membrane, or duly oxygenated by contact with the fluid bathing its surface, -there arises a need for a distributing system by which nutriment and oxygen -may be carried throughout the mass; and the functions of this system, being -subsidiary to the two primary functions, form links in the correspondence -between internal and external actions. The like is obviously true of all -those subordinate functions, secretory and excretory, that facilitate -oxidation and assimilation. - -Ascending from visceral actions to muscular and nervous actions, we find -the correspondence displayed in a manner still more obvious. Every act of -locomotion implies the expenditure of certain internal forces, adapted in -amounts and directions to balance or out-balance certain external forces. -The recognition of an object is impossible without a harmony between the -changes constituting perception, and particular properties co-existing in -the environment. Escape from enemies implies motions within the organism, -related in kind and rapidity to motions without it. Destruction of prey -requires a special combination of subjective actions, fitted in degree and -succession to overcome a group of objective ones. And so with those -countless automatic processes constituting instincts. - -In the highest order of vital changes the same fact is equally manifest. -The empirical generalization that guides the farmer in his rotation of -crops, serves to bring his actions into concord with certain of the actions -going on in plants and soil. The rational deductions of the educated -navigator who calculates his position at sea, form a series of mental acts -by which his proceedings are conformed to surrounding circumstances. Alike -in the simplest inferences of the child and the most complex ones of the -man of science, we find a correspondence between simultaneous and -successive changes in the organism, and co-existences and sequences in its -environment. - - -§ 29. This general formula which thus includes the lowest vegetal processes -along with the highest manifestations of human intelligence, will perhaps -call forth some criticisms which it is desirable here to meet. - -It may be thought that there are still a few inorganic actions included in -the definition; as, for example, that displayed by the mis-named -storm-glass. The feathery crystallization which, on a certain change of -temperature, takes place in its contained solution, and which afterwards -dissolves to reappear in new forms under new conditions, may be held to -present simultaneous and successive changes that are to some extent -heterogeneous, that occur with some definiteness of combination, and, above -all, occur in apparent correspondence with external changes. In this case -vegetal life is simulated to a considerable extent; but it is _merely_ -simulated. The relation between the phenomena occurring in the storm-glass -and in the atmosphere respectively, is not a correspondence at all, in the -proper sense of the word. Outside there is a thermal change; inside there -is a change of atomic arrangement. Outside there is another thermal change; -inside there is another change of atomic arrangement. But subtle as is the -dependence of each internal upon each external change, the connexion -between them does not, in the abstract, differ from the connexion between -the motion of a straw and the motion of the wind that disturbs it. In -either case a change produces a change, and there it ends. The alteration -wrought by some environing agency on this or any other inanimate object, -does not tend to induce in it a secondary alteration which anticipates some -secondary alteration in the environment. But in every living body there is -a tendency towards secondary alterations of this nature; and it is in their -production that the correspondence consists. The difference may be best -expressed by symbols. Let A be a change in the environment, and B some -resulting change in an inorganic mass. Then A having produced B, the action -ceases. Though the change A in the environment is followed by some -consequent change _a_ in it; no parallel sequence in the inorganic mass -simultaneously generates in it some change _b_ that has reference to the -change _a_. But if we take a living body of the requisite organization, and -let the change A impress on it some change C; then, while in the -environment A is occasioning _a_, in the living body C will be occasioning -_c_; of which _a_ and _c_ will show a certain concord in time, place, or -intensity. And while it is _in_ the continuous production of such concords -or correspondences that Life consists, it is _by_ the continuous production -of them that Life is maintained. - -The further criticism to be expected concerns certain verbal imperfections -in the definition, which it seems impossible to avoid. It may fairly be -urged that the word _correspondence_ will not include, without straining, -the various relations to be expressed by it. It may be asked:--How can the -continuous _processes_ of assimilation and respiration correspond with the -_co-existence_ of food and oxygen in the environment? or again:--How can -the act of secreting some defensive fluid correspond with some external -danger which may never occur? or again:--How can the _dynamical_ phenomena -constituting perception correspond with the _statical_ phenomena of the -solid body perceived? The only reply is, that we have no word sufficiently -general to comprehend all forms of this relation between the organism and -its medium, and yet sufficiently specific to convey an adequate idea of the -relation; and that the word _correspondence_ seems the least objectionable. -The fact to be expressed in all cases is that certain changes, continuous -or discontinuous, in the organism, are connected after such a manner that -in their amounts, or variations, or periods of occurrence, or modes of -succession, they have a reference to external actions, constant or serial, -actual or potential--a reference such that a definite relation among any -members of the one group, implies a definite relation among certain members -of the other group. - - -§ 30. The presentation of the phenomena under this general form, suggests -that our conception of Life may be reduced to its most abstract shape by -regarding its elements as relations only. If a creature's rate of -assimilation is increased in consequence of a decrease of temperature in -the environment, it is that the relation between the food consumed and the -heat produced, is so re-adjusted by multiplying both its members, that the -altered relation in the environment between the quantity of heat absorbed -from, and radiated to, bodies of a given temperature, is counterbalanced. -If a sound or a scent wafted to it on the breeze prompts the stag to dart -away from the deer-stalker, it is that there exists in its neighbourhood a -relation between a certain sensible property and certain actions dangerous -to the stag, while in its body there exists an adapted relation between the -impression this sensible property produces, and the actions by which danger -may be escaped. If inquiry has led the chemist to a law, enabling him to -tell how much of any one element will combine with so much of another, it -is that there has been established in him specific mental relations, which -accord with specific chemical relations in the things around. Seeing, then, -that in all cases we may consider the external phenomena as simply in -relation, and the internal phenomena also as simply in relation; our -conception of Life under its most abstract aspect will be--_The continuous -adjustment of internal relations to external relations_.[18] - -While it is simpler, this formula has the further advantage of being -somewhat more comprehensive. To say that it includes not only those -definite combinations of simultaneous and successive changes in an -organism, which correspond to co-existences and sequences in the -environment, but also those structural arrangements which _enable_ the -organism to adapt its actions to actions in the environment, is going too -far; for though these structural arrangements present internal relations -adjusted to external relations, yet the _continuous adjustment_ of -relations cannot be held to include a _fixed adjustment_ already made. -Life, which is made up of _dynamical_ phenomena, cannot be described in -terms that shall at the same time describe the apparatus manifesting it, -which presents only _statical_ phenomena. But while this antithesis serves -to remind us that the distinction between the organism and its actions is -as wide as that between Matter and Motion, it at the same time draws -attention to the fact that, if the structural arrangements of the adult are -not properly included in the definition, yet the developmental processes by -which those arrangements were established, are included. For that process -of evolution during which the organs of the embryo are fitted to their -prospective functions, is the gradual or continuous adjustment of internal -relations to external relations. Moreover, those structural modifications -of the adult organism which, under change of climate, change of occupation, -change of food, bring about some re-arrangement in the organic balance, may -similarly be regarded as progressive or continuous adjustments of internal -relations to external relations. So that not only does the definition, as -thus expressed, comprehend all those activities, bodily and mental, which -constitute our ordinary idea of Life; but it also comprehends both those -processes of development by which the organism is brought into general -fitness for such activities, and those after-processes of adaptation by -which it is specially fitted to its special activities. - -Nevertheless, so abstract a formula as this is scarcely fitted for our -present purpose. Reserving it for use where specially appropriate, it will -be best commonly to employ its more concrete equivalent--to consider the -internal relations as "definite combinations of simultaneous and successive -changes;" the external relations as "co-existences and sequences;" and the -connexion between them as a "correspondence." - - - - -CHAPTER VI. - -THE DEGREE OF LIFE VARIES AS THE DEGREE OF CORRESPONDENCE. - - -§ 31. Already it has been shown respecting each other component of the -foregoing definition, that the life is high in proportion as that component -is conspicuous; and it is now to be remarked, that the same thing is -especially true respecting this last component--the correspondence between -internal and external relations. It is manifest, _a priori_, that since -changes in the physical state of the environment, as also of those -mechanical actions and those variations of available food which occur in -it, are liable to stop the processes going on in the organism; and since -the adaptive changes in the organism have the effects of directly or -indirectly counter-balancing these changes in the environment; it follows -that the life of the organism will be short or long, low or high, according -to the extent to which changes in the environment are met by corresponding -changes in the organism. Allowing a margin for perturbations, the life will -continue only while the correspondence continues; the completeness of the -life will be proportionate to the completeness of the correspondence; and -the life will be perfect only when the correspondence is perfect. Not to -dwell in general statements, however, let us contemplate this truth under -its concrete aspects. - - -§ 32. In life of the lowest order we find that only the most prevalent -co-existences and sequences in the environment, have any simultaneous and -successive changes answering to them in the organism. A plant's vital -processes display adjustment solely to the continuous co-existence of -certain elements and forces surrounding its roots and leaves; and vary only -with the variations produced in these elements and forces by the Sun--are -unaffected by the countless mechanical movements and contacts occurring -around; save when accidentally arrested by these. The life of a worm is -made up of actions referring to little else than the tangible properties of -adjacent things. All those visible and audible changes which happen near -it, and are connected with other changes that may presently destroy it, -pass unrecognized--produce in it no adapted changes: its only adjustment of -internal relations to external relations of this order, being seen when it -escapes to the surface on feeling the vibrations produced by an approaching -mole. Adjusted as are the proceedings of a bird to a far greater number of -co-existences and sequences in the environment, cognizable by sight, -hearing, scent, and their combinations: and numerous as are the dangers it -shuns and the needs it fulfils in virtue of this extensive correspondence; -it exhibits no such actions as those by which a human being counterbalances -variations in temperature and supply of food, consequent on the seasons. -And when we see the plant eaten, the worm trodden on, the bird dead from -starvation; we see alike that the death is an arrest of such correspondence -as existed, that it occurred when there was some change in the environment -to which the organism made no answering change, and that thus, both in -shortness and simplicity, the life was incomplete in proportion as the -correspondence was incomplete. Progress towards more prolonged and higher -life, evidently implies ability to respond to less general co-existences -and sequences. Each step upwards must consist in adding to the -previously-adjusted relations of actions or structures which the organism -exhibits, some further relation parallel to a further relation in the -environment. And the greater correspondence thus established, must, other -things equal, show itself both in greater complexity of life, and greater -length of life: a truth which will be fully perceived on remembering the -enormous mortality which prevails among lowly-organized creatures, and the -gradual increase of longevity and diminution of fertility which we meet -with on ascending to creatures of higher and higher developments. - -It must be remarked, however, that while length and complexity of life are, -to a great extent, associated--while a more extended correspondence in the -successive changes commonly implies increased correspondence in the -simultaneous changes; yet it is not uniformly so. Between the two great -divisions of life--animal and vegetal--this contrast by no means holds. A -tree may live a thousand years, though the simultaneous changes going on in -it answer only to the few chemical affinities in the air and the earth, and -though its serial changes answer only to those of day and night, of the -weather and the seasons. A tortoise, which exhibits in a given time nothing -like the number of internal actions adjusted to external ones that are -exhibited by a dog, yet lives far longer. The tree by its massive trunk and -the tortoise by its hard carapace, are saved the necessity of responding to -those many surrounding mechanical actions which organisms not thus -protected must respond to or die; or rather--the tree and the tortoise -display in their structures, certain simple statical relations adapted to -meet countless dynamical relations external to them. But notwithstanding -the qualifications suggested by such cases, it needs but to compare a -microscopic fungus with an oak, an animalcule with a shark, a mouse with a -man, to recognize the fact that this increasing correspondence of its -changes with those of the environment which characterizes progressing life, -habitually shows itself at the same time in continuity and in complication. - -Even were not the connexion between length of life and complexity of life -thus conspicuous, it would still be true that the life is great in -proportion as the correspondence is great. For if the lengthened existence -of a tree be looked upon as tantamount to a considerable amount of life; -then it must be admitted that its lengthened display of correspondence is -tantamount to a considerable amount of correspondence. If, otherwise, it be -held that notwithstanding its much shorter existence, a dog must rank above -a tortoise in degree of life because of its superior activity; then it is -implied that its life is higher because its simultaneous and successive -changes are more complex and more rapid--because the correspondence is -greater. And since we regard as the highest life that which, like our own, -shows great complexity in the correspondences, great rapidity in the -succession of them, and great length in the series of them; the equivalence -between degree of life and degree of correspondence is unquestionable. - - -§ 33. In further elucidation of this general truth, and especially in -explanation of the irregularities just referred to, it must be pointed out -that as the life becomes higher the environment itself becomes more -complex. Though, literally, the environment means all surrounding space -with the co-existences and sequences contained in it: yet, practically, it -often means but a small part of this. The environment of an entozoon can -scarcely be said to extend beyond the body of the animal in which the -entozoon lives. That of a freshwater alga is virtually limited to the ditch -inhabited by the alga. And, understanding the term in this restricted -sense, we shall see that the superior organisms inhabit the more -complicated environments. - -Thus, contrasted with the life found on land, the lower life is that found -in the sea; and it has the simpler environment. Marine creatures are -affected by fewer co-existences and sequences than terrestrial ones. Being -very nearly of the same specific gravity as the surrounding medium, they -have to contend with less various mechanical actions. The sea-anemone fixed -to a stone, and the acalephe borne along in the current, need to undergo no -internal changes such as those by which the caterpillar meets the varying -effects of gravitation, while creeping over and under the leaves. Again, -the sea is liable to none of those extreme and rapid alterations of -temperature which the air suffers. Night and day produce no appreciable -modifications in it; and it is comparatively little affected by the -seasons. Thus its contained fauna show no marked correspondences similar to -those by which air-breathing creatures counterbalance thermal changes. -Further, in respect to the supply of nutriment, the conditions are more -simple. The lower tribes of animals inhabiting the water, like the plants -inhabiting the air, have their food brought to them. The same current which -brings oxygen to the oyster, also brings it the microscopic organisms on -which it lives: the disintegrating matter and the matter to be integrated, -co-exist under the simplest relation. It is otherwise with land animals. -The oxygen is everywhere, but the sustenance is not everywhere: it has to -be sought; and the conditions under which it is to be obtained are more or -less complex. So too with that liquid by the agency of which the vital -processes are carried on. To marine creatures water is ever present, and by -the lowest is passively absorbed; but to most creatures living on the earth -and in the air, it is made available only through those nervous changes -constituting perception, and those muscular ones by which drinking is -effected. Similarly, after tracing upwards from the _Amphibia_ the widening -extent and complexity which the environment, as practically considered, -assumes--after observing further how increasing heterogeneity in the flora -and fauna of the globe, itself progressively complicates the environment of -each species of organism--it might finally be shown that the same general -truth is displayed in the history of mankind, who, in the course of their -progress, have been adding to their physical environment a social -environment that has been growing ever more involved. Thus, speaking -generally, it is clear that those relations in the environment to which -relations in the organism must correspond, themselves increase in number -and intricacy as the life assumes a higher form. - - -§ 34. To make yet more manifest the fact that the degree of life varies as -the degree of correspondence, let me here point out, that those other -distinctions successively noted when contrasting vital changes with -non-vital changes, are all implied in this last distinction--their -correspondence with external co-existences and sequences; and further, that -the increasing fulfilment of those other distinctions which we found to -accompany increasing life, is involved in the increasing fulfilment of this -last distinction. We saw that living organisms are characterized by -successive changes, and that as the life becomes higher, the successive -changes become more numerous. Well, the environment is full of successive -changes, and the greater the correspondence, the greater must be the number -of successive changes in the organism. We saw that life presents -simultaneous changes, and that the more elevated it is, the more marked the -multiplicity of them. Well, besides countless co-existences in the -environment, there are often many changes occurring in it at the same -moment; and hence increased correspondence with it implies in the organism -an increased display of simultaneous changes. Similarly with the -heterogeneity of the changes. In the environment the relations are very -varied in their kinds, and hence, as the organic actions come more and more -into correspondence with them, they too must become very varied in their -kinds. So again is it even with definiteness of combination. As the most -important surrounding changes with which each animal has to deal, are the -definitely-combined changes exhibited by other animals, whether prey or -enemies, it results that definiteness of combination must be a general -characteristic of the internal ones which have to correspond with them. So -that throughout, the correspondence of the internal relations with the -external ones is the essential thing; and all the special characteristics -of the internal relations, are but the collateral results of this -correspondence. - - -§§ 35, 36. Before closing the chapter, it will be useful to compare the -definition of Life here set forth, with the definition of Evolution set -forth in _First Principles_. Living bodies being bodies which display in -the highest degree the structural changes constituting Evolution; and Life -being made up of the functional changes accompanying these structural -changes; we ought to find a certain harmony between the definitions of -Evolution and of Life. Such a harmony is not wanting. - -The first distinction we noted between the kind of change shown in Life, -and other kinds of change, was its serial character. We saw that vital -change is substantially unlike non-vital change, in being made up of -_successive_ changes. Now since organic bodies display so much more than -inorganic bodies those continuous differentiations and integrations which -constitute Evolution; and since the re-distributions of matter thus carried -so far in a comparatively short period, imply concomitant re-distributions -of motion; it is clear that in a given time, organic bodies must undergo -changes so comparatively numerous as to render the successiveness of their -changes a marked characteristic. And it will follow _a priori_, as we found -it to do _a posteriori_, that the organisms exhibiting Evolution in the -highest degree, exhibit the longest or the most rapid successions of -changes, or both. Again, it was shown that vital change is distinguished -from non-vital change by being made up of many _simultaneous_ changes; and -also that creatures possessing high vitality are marked off from those -possessing low vitality, by the far greater number of their simultaneous -changes. Here, too, there is entire congruity. In _First Principles_, -§ 156, we reached the conclusion that a force falling on any aggregate is -divided into several forces; that when the aggregate consists of parts that -are unlike, each part becomes a centre of unlike differentiations of the -incident force; and that thus the multiplicity of such differentiations -must increase with the multiplicity of the unlike parts. Consequently -organic aggregates, which as a class are distinguished from inorganic -aggregates by the greater number of their unlike parts, must be also -distinguished from them by the greater number of simultaneous changes they -display; and, further, that the higher organic aggregates, having more -numerous unlike parts than the lower, must undergo more numerous -simultaneous changes. We next found that the changes occurring in living -bodies are contrasted with those occurring in other bodies, as being much -more _heterogeneous_; and that the changes occurring in the superior living -bodies are similarly contrasted with those occurring in inferior ones. -Well, heterogeneity of function is the correlate of heterogeneity of -structure; and heterogeneity of structure is the leading distinction -between organic and inorganic aggregates, as well as between the more -highly organized and the more lowly organized. By reaction, an incident -force must be rendered multiform in proportion to the multiformity of the -aggregate on which it falls; and hence those most multi-form aggregates -which display in the highest degree the phenomena of Evolution structurally -considered, must also display in the highest degree the multiform actions -which constitute Evolution functionally considered. These heterogeneous -changes, exhibited simultaneously and in succession by a living organism, -prove, on further inquiry, to be distinguished by their _combination_ from -certain non-vital changes which simulate them. Here, too, the parallelism -is maintained. It was shown in _First Principles_, Chap. XIV, that an -essential characteristic of Evolution is the integration of parts, which -accompanies their differentiation--an integration shown both in the -consolidation of each part, and in the union of all the parts into a whole. -Hence, animate bodies having greater co-ordination of parts than inanimate -ones must exhibit greater co-ordination of changes; and this greater -co-ordination of their changes must not only distinguish organic from -inorganic aggregates, but must, for the same reason, distinguish higher -organisms from lower ones, as we found that it did. Once more, it was -pointed out that the changes constituting Life differ from other changes in -the _definiteness_ of their combination, and that a distinction like in -kind though less in degree, holds between the vital changes of superior -creatures and those of inferior creatures. These, also, are contrasts in -harmony with the contrasts disclosed by the analysis of Evolution. We saw -(_First Principles_, §§ 129-137) that during Evolution there is an increase -of definiteness as well as an increase of heterogeneity. We saw that the -integration accompanying differentiation has necessarily the effect of -increasing the distinctness with which the parts are marked off from each -other, and that so, out of the incoherent and indefinite there arises the -coherent and definite. But a coherent whole made up of definite parts -definitely combined, must exhibit more definitely combined changes than a -whole made up of parts that are neither definite in themselves nor in their -combination. Hence, if living bodies display more than other bodies this -structural definiteness, then definiteness of combination must be a -characteristic of the changes constituting Life, and must also distinguish -the vital changes of higher organisms from those of lower organisms. -Finally, we discovered that all these peculiarities are subordinate to the -fundamental peculiarity, that vital changes take place in correspondence -with external co-existences and sequences, and that the highest Life is -reached, when there is some inner relation of actions fitted to meet every -outer relation of actions by which the organism can be affected. But this -conception of the highest Life, is in harmony with the conception, before -arrived at, of the limit of Evolution. When treating of equilibration as -exhibited in organisms (_First Principles_, §§ 173, 174), it was pointed -out that the tendency is towards the establishment of a balance between -inner and outer changes. It was shown that "the final structural -arrangements must be such as will meet all the forces acting on the -aggregate, by equivalent antagonistic forces," and that "the maintenance of -such a moving equilibrium" as an organism displays, "requires the habitual -genesis of internal forces corresponding in number, directions, and -amounts, to the external incident forces--as many inner functions, single -or combined, as there are single or combined outer actions to be met." It -was shown, too, that the relations among ideas are ever in progress towards -a better adjustment between mental actions and those actions in the -environment to which conduct must be adjusted. So that this continuous -correspondence between inner and outer relations which constitutes Life, -and the perfection of which is the perfection of Life, answers completely -to that state of organic moving equilibrium which we saw arises in the -course of Evolution and tends ever to become more complete. - - - - -CHAPTER VI^A. - -THE DYNAMIC ELEMENT IN LIFE. - - -§ 36a. A critical comparison of the foregoing formula with the facts proves -it to be deficient in more ways than one. Let us first look at vital -phenomena which are not covered by it. - -Some irritant left by an insect's ovipositor, sets up on a plant the morbid -growth named a gall. The processes in the gall do not correspond with any -external co-existences or sequences relevant to the plant's life--show no -internal relations adjusted to external relations. Yet we cannot deny that -the gall is alive. So, too, is it with a cancer in or upon an animal's -body. The actions going on in it have no reference, direct or indirect, to -actions in the environment. Nevertheless we are obliged to say that they -are vital; since it grows and after a time dies and decomposes. - -A kindred lesson meets us when from pathological evidence we turn to -physiological evidence. The functions of some important organs may still be -carried on for a time apart from those of the body as a whole. An excised -liver, kept at a fit temperature and duly supplied with blood, secretes -bile. Still more striking is the independent action of the heart. If -belonging to a cold-blooded animal, as a frog, the heart, when detached, -continues to beat, even until its integuments have become so dry that they -crackle. Now though under such conditions its pulsations, which ordinarily -form an essential part of the linked processes by which the correspondence -between inner and outer actions is maintained, no longer form part of such -processes, we must admit that the continuance of them implies a vital -activity. - -Embryological changes force the same truth upon us. What are we to say of -the repeated cell-fissions by which in some types a blastula, or -mulberry-mass, is formed, and in other types a blastoderm? Neither these -processes nor the structures immediately resulting from them, show any -correspondences with co-existences and sequences in the environment; though -they are first steps towards the organization which is to carry on such -correspondences. Even this extremely small fulfilment of the definition is -absent in the cases of rudimentary organs, and especially those rudimentary -organs which after being partly formed are absorbed. No adjustment can be -alleged between the inner relations which these present and any outer -relations. The outer relations they refer to ceased millions of years ago. -Yet unquestionably the changes which bring about the production and -absorption of these futile structures are vital changes. - -Take another class of exceptions. What are we to say of a laugh? No -correspondence, or part of a correspondence, by which inner actions are -made to balance outer actions, can be seen in it. Or again, if, while -working, an artisan whistles, the making of the sounds and the -co-ordination of ideas controlling them, cannot be said to exhibit -adjustment between certain relations of thoughts, and certain relations of -things. Such kinds of vital activities lie wholly outside of the definition -given. - -But perhaps the clearest and simplest proof is yielded by contrasting -voluntary and involuntary muscular actions. Here is a hawk adapting its -changing motions to the changing motions of a pigeon, so as eventually to -strike it: the adjustment of inner relations to outer relations is -manifest. Here is a boy in an epileptic fit. Between his struggles and the -co-existences and sequences around him there is no correspondence whatever. -Yet his movements betray vitality just as much as do the movements of the -hawk. Both exhibit that principle of _activity_ which constitutes the -essential element in our conception of life. - - -§ 36b. Evidently, then, the preceding chapters recognize only the _form_ of -our conception of life and ignore the _body_ of it. Partly sufficing as -does the definition reached to express the one, it fails entirely to -express the other. Life displays itself in ways which conform to the -definition; but it also displays itself in many other ways. We are obliged -to admit that the element which is common to the two groups of ways is the -essential element. The essential element, then, is that special kind of -energy seen alike in the usual classes of vital actions and in those -unusual classes instanced above. - -Otherwise presenting the contrast, we may say that due attention has been -paid to the connexions among the manifestations, while no attention has -been paid to that which is manifested. When it is said that life is "the -definite correspondence of heterogeneous changes, both simultaneous and -successive, in correspondence with external co-existences and sequences," -there arises the question--Changes of what? Within the body there go on -many changes, mechanical, chemical, thermal, no one of which is the kind of -change in question; and if we combine in thought so far as we can these -kinds of changes, in such wise that each maintains its character as -mechanical, chemical, or thermal, we cannot get out of them the idea of -Life. Still more clearly do we see this insufficiency when we take the more -abstract definition--"the continuous adjustment of internal relations to -external relations." Relations between what things? is the question then to -be asked. A relation of which the terms are unspecified does not connote a -thought but merely the blank form of a thought. Its value is comparable to -that of a cheque on which no amount is written. If it be said that the -terms cannot be specified because so many heterogeneous kinds of them have -to be included, then there comes the reply that under cover of this -inability to make a specification of terms that shall be adequately -comprehensive, there is concealed the inability to conceive the required -terms in any way. - -Thus a critical testing of the definition brings us, in another way, to the -conclusion reached above, that that which gives the substance to our idea -of Life is a certain unspecified principle of activity. The dynamic element -in life is its essential element. - - -§ 36c. Under what form are we to conceive this dynamic element? Is this -principle of activity inherent in organic matter, or is it something -superadded? Of these alternative suppositions let us begin with the last. - -As I have remarked, in another place, the worth of an hypothesis may be -judged from its genealogy; and so judged the hypothesis of an independent -vital principal does not commend itself. Its history carries us back to the -ghost-theory of the savage. Suggested by experiences of dreams, there -arises belief in a double--a second self which wanders away during sleep -and has adventures but comes back on waking; which deserts the body during -abnormal insensibility of one or other kind; and which is absent for a long -period at death, though even then is expected eventually to return. This -indwelling other-self, which can leave the body at will, is by-and-by -regarded as able to enter the bodies of fellow men or of animals; or again, -by implication, as liable to have its place usurped by the intruding -doubles of fellow men, living or dead, which cause fits or other ills. -Along with these developments its quality changes. At first thought of as -quite material it is gradually de-materialized, and in advanced times comes -to be regarded as spirit or breath; as we see in ancient religious books, -where "giving up the ghost" is shown by the emergence of a small floating -figure from the mouth of a dying man. This indwelling second self, more and -more conceived as the real self which uses the body for its purposes, is, -with the advance of intelligence, still further divested of its definite -characters; and, coming in mediæval days to be spoken of as "animal -spirits," ends in later days in being called a vital principle. - -Entirely without assignable attributes, this something occurs in thought -not as an idea but as a pseud-idea (_First Principles_, Chap. II). It is -assumed to be representable while really unrepresentable. We need only -insist on answers to certain questions to see that it is simply a name for -an alleged existence which has not been conceived and cannot be conceived. - -1. Is there one kind of vital principle for all kinds of organisms, or is -there a separate kind for each? To affirm the first alternative is to say -that there is the same vital principle for a microbe as for a whale, for a -tape-worm as for the person it inhabits, for a protococcus as for an oak; -nay more--is to assert community of vital principle in the thinking man and -the unthinking plant. Moreover, asserting unity of the vital principle for -all organisms, is reducing it to a force having the same unindividualized -character as one of the physical forces. If, on the other hand, different -kinds of organisms have different kinds of vital principles, these must be -in some way distinguished from one another. How distinguished? Manifestly -by attributes. Do they differ in extension? Evidently; since otherwise that -which animates the vast _Sequoia_ can be no larger than that which animates -a yeast-plant, and to carry on the life of an elephant requires a quantity -of vital principle no greater than that required for a microscopic monad. -Do they differ otherwise than in amount? Certainly; since otherwise we -revert to the preceding alternative, which implies that the same quality of -vital principle serves for all organisms, simple and complex: the vital -principle is a uniform force like heat or electricity. Hence, then, we have -to suppose that every species of animal and plant has a vital principle -peculiar to itself--a principle adapted to use the particular set of -structures in which it is contained. But dare anyone assert this -multiplication of vital principles, duplicating not only all existing -plants and animals but all past ones, and amounting in the aggregate to -some millions? - -2. How are we to conceive that genesis of a vital principle which must go -along with the genesis of an organism? Here is a pollen-grain which, -through the pistil, sends its nucleus to unite with the nucleus of the -ovule; or here are the nuclei of spermatozoon and ovum, which, becoming -fused, initiate a new animal: in either case failure of union being -followed by decomposition of the proteid materials, while union is followed -by development. Whence comes that vital principle which determines the -organizing process? Is it created afresh for every plant and animal? or, if -not, where and how did it pre-exist? Take a simpler form of this problem. A -protophyte or protozoon, having grown to a certain size, undergoes a series -of complex changes ending in fission. In its undivided state it had a vital -principle. What of its divided state? The parts severally swim away, each -fully alive, each ready to grow and presently to subdivide, and so on and -so on, until millions are soon formed. That is to say, there is a -multiplication of vital principles as of the protozoa animated by them. A -vital principle, then, both divides and grows. But growth implies -incorporation of something. What does the vital principle incorporate? Is -it some other vital principle external to it, or some materials out of -which more vital principle is formed? And how, in either case, can the -vital principle be conceived as other than a material something, which in -its growth and multiplication behaves just as visible matter behaves? - -3. Equally unanswerable is the question which arises in presence of life -that has become latent. Passing over the alleged case of the mummy wheat, -the validity of which is denied, there is experimental proof that seeds -may, under conditions unfavourable to germination, retain for ten, twenty, -and some even for thirty years, the power to germinate when due moisture -and warmth are supplied. (_Cf._ Kerner's _Nat. Hist. of Plants_, i, 51-2). -Under what form has the vital principle existed during these long -intervals? It is a principle of activity. In this case, then, the principle -of activity becomes inactive. But how can we conceive an inactive activity? -If it is a something which though inactive may be rendered active when -conditions favour, we are introduced to the idea of a vital principle of -which the vitality may become latent, which is absurd. What shall we say of -the desiccated rotifer which for years has seemed to be nothing more than a -particle of dust, but which now, when water is supplied, absorbs it, swells -up, and resumes those ciliary motions by which it draws in nutriment? Was -the vital principle elsewhere during these years of absolute quiescence? If -so, why did it come back at the right moment? Was it all along present in -the rotifer though asleep? How happened it then to awaken at the time when -the supply of water enabled the tissues to resume their functions? How -happened the physical agent to act not only on the material substance of -the rotifer, but also on this something which is not a material substance -but an immaterial source of activity? Evidently neither alternative is -thinkable. - -Thus, the alleged vital principle exists in the minds of those who allege -it only as a verbal form, not as an idea; since it is impossible to bring -together in consciousness the terms required to constitute an idea. It is -not even "a figment of imagination," for that implies something imaginable, -but the supposed vital principle cannot even be imagined. - - -§ 36d. When, passing to the alternative, we propose to regard life as -inherent in the substances of the organisms displaying it, we meet with -difficulties different in kind but scarcely less in degree. The processes -which go on in living things are incomprehensible as results of any -physical actions known to us. - -Consider one of the simplest--that presented by an ordinary vegetal cell -forming part of a leaf or other plant-structure. Its limiting membrane, -originally made polyhedral by pressure of adjacent cells, is gradually -moulded "into one of cylindrical, fibrous, or tabular shape, and -strengthening its walls with pilasters, borders, ridges, hooks, bands, and -panels of various kinds" (Kerner, i, 43): small openings into adjacent -cells being either left or subsequently made. Consisting of -non-nitrogenous, inactive matters, these structures are formed by the -inclosed protoplast. How formed? Is it by the agency of the nucleus? But -the nucleus, even had it characters conceivably adapting it to this -function, is irregularly placed; and that it should work the same effects -upon the cell-wall whether seated in the middle, at one end, or one side, -is incomprehensible. Is the protoplasm then the active agent? But this is -arranged into a network of strands and threads utterly irregular in -distribution and perpetually altering their shapes and connexions. Exercise -of fit directive action by the protoplasm is unimaginable. - -Another instance:--Consider the reproductive changes exhibited by the -_Spirogyra_. The delicate threads which, in this low type of Alga, are -constituted of single elongated cells joined end to end, are here and there -adjacent to one another; and from a cell of one thread and a cell of -another at fit distance, grow out prominences which, meeting in the -interspace and forming a channel by the dissolution of their adjoined -cell-walls, empty through it the endochrome of the one cell into the other: -forming by fusion of the two a zygote or reproductive body. Under what -influence is this action initiated and guided? There is no conceivable -directive agency in either cell by which, when conditions are fit, a -papilla is so formed as to meet an opposite papilla. - -Or again, contemplate the still more marvellous transformation occurring in -_Hydrodictyon utriculosum_. United with others to form a cylindrical -network, each sausage-shaped cell of this Alga contains, when fully -developed, a lining chromatophore made of nucleated protoplasm with -immersed chlorophyll-grains. This, when the cell is adult, divides into -multitudinous zoospores, which presently join their ends in such ways as to -form a network with meshes mostly hexagonal, minute in size, but like in -arrangement to the network of which the parent cell formed a part. -Eventually escaping from the mother-cell, this network grows and presently -becomes as large as the parent network. Under what play of forces do these -zoospores arrange themselves into this strange structure? - -Kindred insoluble problems are presented by animal organisms of all grades. -Of microscopic types instance the Coccospheres and Rhabdospheres found in -the upper strata of sea-water. Each is a fragment of protoplasm less than -one-thousandth of an inch in diameter, shielded by the elaborate protective -structures it has formed. The elliptic coccoliths of the first, severally -having a definite pattern, unite to form by overlapping an imbricated -covering; and of the other the covering consists of numerous -trumpet-mouthed processes radiating on all sides. To the question--How does -this particle of granular protoplasm, without organs or definite structure, -make for itself this complicated calcareous armour? there is no conceivable -answer. - -Like these _Protozoa_, the lowest _Metazoa_ do things which are quite -incomprehensible. Here is a sponge formed of classes of monads having among -them no internuncial appliances by which in higher types cooperation is -carried on--flagellate cells that produce the permeating currents of water, -flattened cells forming protective membranes, and amoeboid cells lying free -in the gelatinous mesoderm. These, without apparent concert, build up not -only the horny network constituting the chief mass of their habitation, but -also embodied spicules, having remarkable symmetrical forms. By what -combined influences the needful processes are effected, it is impossible to -imagine. - -If we turn to higher types of _Metazoa_ in which, by the agency of a -nervous system, many cooperations of parts are achieved in ways that are -superficially comprehensible, we still meet with various actions of which -the causation cannot be represented in thought. Lacking other calcareous -matter, a hen picks up and swallows bits of broken egg-shells; and, -occasionally, a cow in calf may be seen mumbling a bone she has -found--evidently scraping off with her teeth some of its mass. These -proceedings have reference to constitutional needs; but how are they -prompted? What generates in the cow a desire to bite a substance so unlike -in character to her ordinary food? If it be replied that the blood has -become poor in certain calcareous salts and that hence arises the appetite -for things containing them, there remains the question--How does this -deficiency so act on the nervous system as to generate this vague desire -and cause the movements which satisfy it? By no effort can we figure to -ourselves the implied causal processes. - -In brief, then, we are obliged to confess that Life in its essence cannot -be conceived in physico-chemical terms. The required principle of activity, -which we found cannot be represented as an independent vital principle, we -now find cannot be represented as a principle inherent in living matter. -If, by assuming its inherence, we think the facts are accounted for, we do -but cheat ourselves with pseud-ideas. - - -§ 36e. What then are we to say--what are we to think? Simply that in this -direction, as in all other directions, our explanations finally bring us -face to face with the inexplicable. The Ultimate Reality behind this -manifestation, as behind all other manifestations, transcends conception. -It needs but to observe how even simple forms of existence are in their -ultimate natures incomprehensible, to see that this most complex form of -existence is in a sense doubly incomprehensible. - -For the actions of that which the ignorant contemptuously call brute -matter, cannot in the last resort be understood in their genesis. Were it -not that familiarity blinds us, the fall of a stone would afford matter for -wonder. Neither Newton nor anyone since his day has been able to conceive -how the molecules of matter in the stone are affected not only by the -molecules of matter in the adjacent part of the Earth but by those forming -parts of its mass 8,000 miles off which severally exercise their influence -without impediment from intervening molecules; and still less has there -been any conceivable interpretation of the mode in which every molecule of -matter in the Sun, 92 millions of miles away, has a share in controlling -the movements of the Earth. What goes on in the space between a magnet and -the piece of iron drawn towards it, or how on repeatedly passing a magnet -along a steel needle this, by some change of molecular state as we must -suppose, becomes itself a magnet and when balanced places its poles in -fixed directions, we do not know. And still less can we fathom the physical -process by which an ordered series of electric pulses sent through a -telegraph wire may be made to excite a corresponding series of pulses in a -parallel wire many miles off. - -Turn to another class of cases. Consider the action of a surface of glass -struck by a cathode current and which thereupon generates an order of rays -able to pass through solid matters impermeable to light. Or contemplate the -power possessed by uranium and other metals of emitting rays imperceptible -by our eyes as light but which yet, in what appears to us absolute -darkness, will, if passed through a camera, produce photographs. Even the -actions of one kind of matter on another are sufficiently remarkable. Here -is a mass of gold which, after the addition of 1-500th part of bismuth, has -only 1-28th of the tensile strength it previously had; and here is a mass -of brass, ordinarily ductile and malleable, but which, on the addition of -1-10,000th part of antimony, loses its character. More remarkable still are -the influences of certain medicines. One-hundredth of a grain of -nitro-glycerine is a sufficient dose. Taking an average man's weight as 150 -pounds, it results that his body is appreciably affected in its state by -the 115-millionth part of its weight of this nitrogenous compound. - -In presence of such powers displayed by matter of simple kinds we shall see -how impossible it is even to imagine those processes going on in organic -matter out of which emerges the dynamic element in Life. As no separate -form of proteid possesses vitality, we seem obliged to assume that the -molecule of protoplasm contains many molecules of proteids, probably in -various isomeric states, all capable of ready change and therefore -producing great instability of the aggregate they form. As before pointed -out (§ 4), a proteid-molecule includes more than 220 equivalents of several -so-called elements. Each of these undecomposed substances is now recognized -by chemists as almost certainly consisting of several kinds of components. -Hence the implication is that a proteid-molecule contains thousands of -units, of which the different classes have their respective rates of -inconceivably rapid oscillation, while each unit, receiving and emitting -ethereal undulations, affects others of its kind in its own and adjacent -molecules: an immensely complex structure having immensely complex -activities. And this complexity, material and dynamic, in the -proteid-molecule we must regard as raised to a far higher degree in the -unit of protoplasm. Here as elsewhere alternative impossibilities of -thought present themselves. We find it impossible to think of Life as -imported into the unit of protoplasm from without; and yet we find it -impossible to conceive it as emerging from the cooperation of the -components. - - -§ 36f. But now, having confessed that Life as a principle of activity is -unknown and unknowable--that while its phenomena are accessible to thought -the implied noumenon is inaccessible--that only the manifestations come -within the range of our intelligence while that which is manifested lies -beyond it; we may resume the conclusions reached in the preceding chapters. -Our surface knowledge continues to be a knowledge valid of its kind, after -recognizing the truth that it is only a surface knowledge. - -For the conclusions we lately reached and the definition emerging from -them, concern the _order_ existing among the actions which living things -exhibit; and this order remains the same whether we know or do not know the -nature of that from which the actions originate. We found a distinguishing -trait of Life to be that its changes display a correspondence with -co-existences and sequences in the environment; and this remains a -distinguishing trait, though the thing which changes remains inscrutable. -The statement that the continuous adjustment of internal relations to -external relations constitutes Life as cognizable by us, is not invalidated -by the admission that the reality in which these relations inhere is -incognizable. - -Hence, then, after duly recognizing the fact that, as pointed out above, -Life, even phenomenally considered, is not entirely covered by the -definition, since there are various abnormal manifestations of life which -it does not include, we may safely accept it as covering the normal -manifestations--those manifestations which here concern us. Carrying with -us the definition, therefore we may hereafter use it for guidance through -all those regions of inquiry upon which we now enter. - - - - -CHAPTER VII. - -THE SCOPE OF BIOLOGY. - - -§ 37. As ordinarily conceived, the science of Biology falls into two great -divisions, the one dealing with animal life, called Zoology, and the other -dealing with vegetal life, called Botany, or more properly to be called -Phytology. But convenient as is this division, it is not that which arises -if we follow the scientific method of including in one group all the -phenomena of fundamentally the same order and putting separately in another -group all the phenomena of a fundamentally different order. For animals and -plants are alike in having structures; and animals and plants are alike in -having functions performed by these structures; and the distinction between -structures and functions transcends the difference between any one -structure and any other or between any one function and any other--is, -indeed, an absolute distinction, like that between Matter and Motion. -Recognizing, then, the logic of the division thus indicated, we must group -the parts of Biology thus:-- - -1. An account of the structural phenomena presented by organisms. This -subdivides into:-- - - _a._ The established structural phenomena presented by individual - organisms. - - _b._ The changing structural phenomena presented by successions of - organisms. - -2. An account of the functional phenomena which organisms present. This, -too, admits of subdivision into:-- - - _a._ The established functional phenomena of individual organisms. - - _b._ The changing functional phenomena of successions of organisms. - -3. An account of the actions of Structures on Functions and the re-actions -of Functions on Structures. Like the others, this is divisible into:-- - - _a._ The actions and re-actions as exhibited in individual organisms. - - _b._ The actions and re-actions as exhibited in successions of - organisms. - -4. An account of the phenomena attending the production of successions of -organisms: in other words--the phenomena of Genesis. - -Of course, for purposes of exploration and teaching, the division into -Zoology and Botany, founded on contrasts so marked and numerous, must -always be retained. But here recognizing this familiar distinction only as -much as convenience obliges us to do, let us now pass on to consider, more -in detail, the classification of biologic phenomena above set down in its -leading outlines. - - -§ 38. The facts of structure shown in an individual organism, are of two -chief kinds. In order of conspicuousness, though not in order of time, -there come first those arrangements of parts which characterize the mature -organism; an account of which, originally called Anatomy, is now called -Morphology. Then come those successive modifications through which the -organism passes in its progress from the germ to the developed form; an -account of which is called Embryology. - -The structural changes which any series of individual organisms exhibits, -admit of similar classification. On the one hand, we have those inner and -outer differences of shape, that arise between the adult members of -successive generations descended from a common stock--differences which, -though usually not marked between adjacent generations, become great in -course of multitudinous generations. On the other hand, we have those -developmental modifications, seen in the embryos, through which such -modifications of the descended forms are reached. - -Interpretation of the structures of individual organisms and successions of -organisms, is aided by two subsidiary divisions of biologic inquiry, named -Comparative Anatomy (properly Comparative Morphology) and Comparative -Embryology. These cannot be regarded as in themselves parts of Biology; -since the facts embraced under them are not substantive phenomena, but are -simply incidental to substantive phenomena. All the truths of structural -Biology are comprehended under the two foregoing subdivisions; and the -comparison of these truths as presented in different classes of organisms, -is simply a _method_ of interpreting them. - -Nevertheless, though Comparative Morphology and Comparative Embryology do -not disclose additional concrete facts, they lead to the establishment of -certain abstract facts. By them it is made manifest that underneath the -superficial differences of groups and classes and types of organisms, there -are hidden fundamental similarities; and that the courses of development in -such groups and classes and types, though in many respects divergent, are -in some essential respects, coincident. The wide truths thus disclosed, -come under the heads of General Morphology and General Embryology. - -By contrasting organisms there is also achieved that grouping of the like -and separation of the unlike, called Classification. First by observation -of external characters; second by observation of internal characters; and -third by observation of the phases of development; it is ascertained what -organisms are most similar in all respects; what organisms otherwise unlike -are like in important traits; what organisms though apparently unallied -have common primordial characters. Whence there results such an -arrangement of organisms, that if certain structural attributes of any one -be given, its other structural attributes may be _empirically_ predicted; -and which prepares the way for that interpretation of their relations and -genesis, which forms an important part of _rational_ Biology. - - -§ 39. The second main division of Biology, above described as embracing the -functional phenomena of organisms, is that which is in part signified by -Physiology: the remainder being distinguishable as Objective Psychology. -Both of these fall into subdivisions that may best be treated separately. - -That part of Physiology which is concerned with the molecular changes going -on in organisms, is known as Organic Chemistry. An account of the modes in -which the force generated in organisms by chemical change, is transformed -into other forces, and made to work the various organs that carry on the -functions of Life, comes under the head of Organic Physics. Psychology, -which is mainly concerned with the adjustment of vital actions to actions -in the environment (in contrast with Physiology, which is mainly concerned -with vital actions apart from actions in the environment) consists of two -quite distinct portions. Objective Psychology deals with those functions of -the nervo-muscular apparatus by which such organisms as possess it are -enabled to adjust inner to outer relations; and includes also the study of -the same functions as externally manifested in conduct. Subjective -Psychology deals with the sensations, perceptions, ideas, emotions, and -volitions that are the direct or indirect concomitants of this visible -adjustment of inner to outer relations. Consciousness under its different -modes and forms, being a subject-matter radically distinct in nature from -the subject-matter of Biology in general; and the method of self-analysis, -by which alone the laws of dependence among changes of consciousness can be -found, being a method unparalleled by anything in the rest of Biology; we -are obliged to regard Subjective Psychology as a separate study. And since -it would be very inconvenient wholly to dissociate Objective Psychology -from Subjective Psychology, we are practically compelled to deal with the -two as forming an independent science. - -Obviously, the functional phenomena presented in successions of organisms, -similarly divide into physiological and psychological. Under the -physiological come the modifications of bodily actions that arise in the -course of generations, as concomitants of structural modifications; and -these may be modifications, qualitative or quantitative, in the molecular -changes classed as chemical, or in the organic actions classed as physical, -or in both. Under the psychological come the qualitative and quantitative -modifications of instincts, feelings, conceptions, and mental processes in -general, which occur in creatures having more or less intelligence, when -certain of their conditions are changed. This, like the preceding -department of Psychology, has in the abstract two different aspects--the -objective and the subjective. Practically, however, the objective, which -deals with these mental modifications as exhibited in the changing habits -and abilities of successive generations of creatures, is the only one -admitting of investigation; since the corresponding alterations in -consciousness cannot be immediately known to any but the subjects of them. -Evidently, convenience requires us to join this part of Psychology along -with the other parts as components of a distinct sub-science. - -Light is thrown on functions, as well as on structures, by comparing -organisms of different kinds. Comparative Physiology and Comparative -Psychology, are the names given to those collections of facts respecting -the homologies and analogies, bodily and mental, disclosed by this kind of -inquiry. These classified observations concerning likenesses and -differences of functions, are helpers to interpret functions in their -essential natures and relations. Hence Comparative Physiology and -Comparative Psychology are names of methods rather than names of true -subdivisions of Biology. - -Here, however, as before, comparison of special truths, besides -facilitating their interpretation, brings to light certain general truths. -Contrasting functions bodily and mental as exhibited in various kinds of -organisms, shows that there exists, more or less extensively, a community -of processes and methods. Hence result two groups of propositions -constituting General Physiology and General Psychology. - - -§ 40. In these divisions and subdivisions of the first two great -departments of Biology, facts of Structure are considered separately from -facts of Function, so far as separate treatment of them is possible. The -third great department of Biology deals with them in their necessary -connexions. It comprehends the determination of functions by structures, -and the determination of structures by functions. - -As displayed in individual organisms, the effects of structures on -functions are to be studied, not only in the broad fact that the general -kind of life an organism leads is necessitated by the main characters of -its organization, but in the more special and less conspicuous fact, that -between members of the same species, minor differences of structure lead to -minor differences of power to perform certain actions, and of tendencies to -perform such actions. Conversely, under the reactions of functions on -structures in individual organisms, come the facts showing that functions, -when fulfilled to their normal extents, maintain integrity of structure in -their respective organs; and that within certain limits increases of -functions are followed by such structural changes in their respective -organs, as enable them to discharge better their extra functions. - -Inquiry into the influence of structure on function as seen in successions -of organisms, introduces us to such phenomena as Mr. Darwin's _Origin of -Species_ deals with. In this category come all proofs of the general truth, -that when an individual is enabled by a certain structural peculiarity to -perform better than others of its species some advantageous action; and -when it bequeaths more or less of its structural peculiarity to -descendants, among whom those which have it most markedly are best able to -thrive and propagate; there arises a visibly modified type of structure, -having a more or less distinct function. In the correlative class of facts -(by some asserted and by others denied), which come under the category of -reactions of function on structure as exhibited in successions of -organisms, are to be placed all those modifications of structure which -arise in races, when changes of conditions entail changes in the balance of -their functions--when altered function externally necessitated, produces -altered structure, and continues doing this through successive generations. - - -§ 41. The fourth great division of Biology, comprehending the phenomena of -Genesis, may be conveniently separated into three subdivisions. - -Under the first, comes a description of all the special modes whereby the -multiplication of organisms is carried on; which modes range themselves -under the two chief heads of sexual and asexual. An account of Sexual -Multiplication includes the various processes by which germs and ova are -fertilized, and by which, after fertilization, they are furnished with the -materials, and maintained in the conditions, needful for their development. -An account of Asexual Multiplication includes the various processes by -which, from the same fertilized germ or ovum, there are produced many -organisms partially or totally independent of one another. - -The second of these subdivisions deals with the phenomena of Genesis in the -abstract. It takes for its subject-matter such general questions as--What -is the end subserved by the union of sperm-cell and germ-cell? Why cannot -all multiplication be carried on after the asexual method? What are the -laws of hereditary transmission? What are the causes of variation? - -The third subdivision is devoted to still more abstract aspects of the -subject. Recognizing the general facts of multiplication, without reference -to their modes or immediate causes, it concerns itself simply with the -different rates of multiplication in different kinds of organisms and -different individuals of the same kind. Generalizing the numerous contrasts -and variations of fertility, it seeks a rationale of them in their -relations to other organic phenomena. - - -§ 42. Such appears to be the natural arrangement of divisions and -subdivisions which Biology presents. It is, however, a classification of -the parts of the science when fully developed; rather than a classification -of them as they now stand. Some of the subdivisions above named have no -recognized existence, and some of the others are in quite rudimentary -states. It is impossible now to fill in, even in the roughest way, more -than a part of the outlines here sketched. - -Our course of inquiry being thus in great measure determined by the present -state of knowledge, we are compelled to follow an order widely different -from this ideal one. It will be necessary first to give an account of those -empirical generalizations which naturalists and physiologists have -established: appending to those which admit of it, such deductive -interpretations as _First Principles_ furnishes us with. Having done this, -we shall be the better prepared for dealing with the leading truths of -Biology in connexion with the doctrine of Evolution. - - - - -PART II. - -THE INDUCTIONS OF BIOLOGY. - - - - -CHAPTER I. - -GROWTH. - - -§ 43. Perhaps the widest and most familiar induction of Biology, is that -organisms grow. While, however, this is a characteristic so uniformly and -markedly displayed by plants and animals, as to be carelessly thought -peculiar to them, it is really not so. Under appropriate conditions, -increase of size takes place in inorganic aggregates, as well as in organic -aggregates. Crystals grow; and often far more rapidly than living bodies. -Where the requisite materials are supplied in the requisite forms, growth -may be witnessed in non-crystalline masses: instance the fungous-like -accumulation of carbon that takes place on the wick of an unsnuffed candle. -On an immensely larger scale, we have growth in geologic formations: the -slow accumulation of deposited sediment into a stratum, is not -distinguishable from growth in its widest acceptation. And if we go back to -the genesis of celestial bodies, assuming them to have arisen by Evolution, -these, too, must have gradually passed into their concrete shapes through -processes of growth. Growth is, indeed, as being an integration of matter, -the primary trait of Evolution; and if Evolution of one kind or other is -universal, growth is universal--universal, that is, in the sense that all -aggregates display it in some way at some period. - -The essential community of nature between organic growth and inorganic -growth, is, however, most clearly seen on observing that they both result -in the same way. The segregation of different kinds of detritus from each -other, as well as from the water carrying them, and their aggregation into -distinct strata, is but an instance of a universal tendency towards the -union of like units and the parting of unlike units (_First Principles_, -§ 163). The deposit of a crystal from a solution is a differentiation of -the previously mixed molecules; and an integration of one class of -molecules into a solid body, and the other class into a liquid solvent. Is -not the growth of an organism an essentially similar process? Around a -plant there exist certain elements like the elements which form its -substance; and its increase of size is effected by continually integrating -these surrounding like elements with itself. Nor does the animal -fundamentally differ in this respect from the plant or the crystal. Its -food is a portion of the environing matter that contains some compound -atoms like some of the compound atoms constituting its tissues; and either -through simple imbibition or through digestion, the animal eventually -integrates with itself, units like those of which it is built up, and -leaves behind the unlike units. To prevent misconception, it may be well to -point out that growth, as here defined, must be distinguished from certain -apparent and real augmentations of bulk which simulate it. Thus, the long, -white potato-shoots thrown out in the dark, are produced at the expense of -the substances which the tuber contains: they illustrate not the -accumulation of organic matter, but simply its re-composition and -re-arrangement. Certain animal-embryos, again, during their early stages, -increase considerably in size without assimilating any solids from the -environment; and they do this by absorbing the surrounding water. Even in -the highest organisms, as in children, there appears sometimes to occur a -rapid gain in dimensions which does not truly measure the added quantity of -organic matter; but is in part due to changes analogous to those just -named. Alterations of this kind must not be confounded with that growth, -properly so called, of which we have here to treat. - -The next general fact to be noted respecting organic growth, is, that it -has limits. Here there appears to be a distinction between organic and -inorganic growth; but this distinction is by no means definite. Though that -aggregation of inanimate matter which simple attraction produces, may go on -without end; yet there appears to be an end to that more definite kind of -aggregation which results from polar attraction. Different elements and -compounds habitually form crystals more or less unlike in their sizes; and -each seems to have a size that is not usually exceeded without a tendency -arising to form new crystals rather than to increase the old. On looking -at the organic kingdom as a whole, we see that the limits between which -growth ranges are very wide apart. At the one extreme we have monads so -minute as to be rendered but imperfectly visible by microscopes of the -highest power; and at the other extreme we have trees of 400 to 500 feet -high and animals of 100 feet long. It is true that though in one sense this -contrast may be legitimately drawn, yet in another sense it may not; since -these largest organisms arise by the combination of units which are -individually like the smallest. A single plant of the genus _Protococcus_, -is of the same essential structure as one of the many cells united to form -the thallus of some higher Alga, or the leaf of a phænogam. Each separate -shoot of a phænogam is usually the bearer of many leaves. And a tree is an -assemblage of numerous united shoots. One of these great teleophytes is -thus an aggregate of aggregates of aggregates of units, which severally -resemble protophytes in their sizes and structures; and a like building up -is traceable throughout a considerable part of the animal kingdom. Even, -however, when we bear in mind this qualification, and make our comparisons -between organisms of the same degree of composition, we still find the -limit of growth to have a great range. The smallest branched flowering -plant is extremely insignificant by the side of a forest tree; and there is -an enormous difference in bulk between the least and the greatest mammal. -But on comparing members of the same species, we discover the limit of -growth to be much less variable. Among the _Protozoa_ and _Protophyta_, -each kind has a tolerably constant adult size; and among the most complex -organisms the differences between those of the same kind which have reached -maturity, are usually not very great. The compound plants do, indeed, -sometimes present marked contrasts between stunted and well-grown -individuals; but the higher animals diverge but inconsiderably from the -average standards of their species. - -On surveying the facts with a view of empirically generalizing the causes -of these differences, we are soon made aware that by variously combining -and conflicting with one another, these causes produce great irregularities -of result. It becomes manifest that no one of them can be traced to its -consequences, unqualified by the rest. Hence the several statements -contained in the following paragraphs must be taken as subject to mutual -modification. - -Let us consider first the connexion between degree of growth and complexity -of structure. This connexion, being involved with many others, becomes -apparent only on so averaging the comparisons as to eliminate differences -among the rest. Nor does it hold at all where the conditions are radically -dissimilar, as between plants and animals. But bearing in mind these -qualifications, we shall see that organization has a determining influence -on increase of mass. Of plants the lowest, classed as Thallophytes, usually -attain no considerable size. Algæ, Fungi, and the Lichens formed by -association of them count among their numbers but few bulky species: the -largest, such as certain Algæ found in antarctic seas, not serving greatly -to raise the average; and these gigantic seaweeds possess a considerable -complexity of histological organization very markedly exceeding that of -their smaller allies. Though among Bryophytes and Pteridophytes there are -some, as the Tree-ferns, which attain a considerable height, the majority -are but of humble growth. The Monocotyledons, including at one extreme -small grasses and at the other tall palms, show us an average and a maximum -greater than that reached by the Pteridophytes. And the Monocotyledons are -exceeded by the Dicotyledons; among which are found the monarchs of the -vegetal kingdom. Passing to animals, we meet the fact that the size -attained by _Vertebrata_ is usually much greater than the size attained by -_Invertebrata_. Of invertebrate animals the smallest, classed as -_Protozoa_, are also the simplest; and the largest, belonging to the -_Annulosa_ and _Mollusca_, are among the most complex of their respective -types. Of vertebrate animals we see that the greatest are Mammals, and that -though, in past epochs, there were Reptiles of vast bulks, their bulks did -not equal that of the whale: the great Dinosaurs, though as long, being -nothing like as massive. Between reptiles and birds, and between -land-vertebrates and water-vertebrates, the relation does not hold: the -conditions of existence being in these cases widely different. But among -fishes as a class, and among reptiles as a class, it is observable that, -speaking generally, the larger species are framed on the higher types. The -critical reader, who has mentally checked these statements in passing them, -has doubtless already seen that this relation is not a dependence of -organization on growth but a dependence of growth on organization. The -majority of Dicotyledons are smaller than some Monocotyledons; many -Monocotyledons are exceeded in size by certain Pteridophytes; and even -among Thallophytes, the least developed among compound plants, there are -kinds of a size which many plants of the highest order do not reach. -Similarly among animals. There are plenty of Crustaceans less than -_Actiniæ_; numerous reptiles are smaller than some fish; the majority of -mammals are inferior in bulk to the largest reptiles; and in the contrast -between a mouse and a well-grown _Medusa_, we see a creature that is -elevated in type of structure exceeded in mass by one that is extremely -low. Clearly then, it cannot be held that high organization is habitually -accompanied by great size. The proposition here illustrated is the converse -one, that great size is habitually accompanied by high organization. The -conspicuous facts that the largest species of both animals and vegetals -belong to the highest classes, and that throughout their various -sub-classes the higher usually contain the more bulky forms, show this -connexion as clearly as we can expect it to be shown, amid so many -modifying causes and conditions. - -The relation between growth and supply of available nutriment, is too -familiar a relation to need proving. There are, however, some aspects of it -that must be contemplated before its implications can be fully appreciated. -Among plants, which are all constantly in contact with the gaseous, liquid, -and solid matters to be incorporated with their tissues, and which, in the -same locality, receive not very unlike amounts of light and heat, -differences in the supplies of available nutriment have but a subordinate -connexion with differences of growth. Though in a cluster of herbs -springing up from the seeds let fall by a parent, the greater sizes of some -than of others is doubtless due to better nutrition, consequent on -accidental advantages; yet no such interpretation can be given of the -contrast in size between these herbs and an adjacent tree. Other conditions -here come into play: one of the most important being, an absence in the one -case, and presence in the other, of an ability to secrete such a quantity -of ligneous fibre as will produce a stem capable of supporting a large -growth. Among animals, however, which (excepting some _Entozoa_) differ -from plants in this, that instead of bathing their surfaces the matters -they subsist on are dispersed, and have to be obtained, the relation -between available food and growth is shown with more regularity. The -_Protozoa_, living on microscopic fragments of organic matter contained in -the surrounding water, are unable, during their brief lives, to accumulate -any considerable quantity of nutriment. _Polyzoa_, having for food these -scarcely visible members of the animal kingdom, are, though large compared -with their prey, small as measured by other standards; even when aggregated -into groups of many individuals, which severally catch food for the common -weal, they are often so inconspicuous as readily to be passed over by the -unobservant. And if from this point upwards we survey the successive grades -of animals, it becomes manifest that, in proportion as the size is great, -the masses of nutriment are either large, or, what is practically the same -thing, are so abundant and so grouped that large quantities may be readily -taken in. Though, for example, the greatest of mammals, the arctic whale, -feeds on such comparatively small creatures as the acalephes and molluscs -floating in the seas it inhabits, its method of gulping in whole shoals of -them and filtering away the accompanying water, enables it to secure great -quantities of food. We may then with safety say that, other things equal, -the growth of an animal depends on the abundance and sizes of the masses of -nutriment which its powers enable it to appropriate. Perhaps it may be -needful to add that, in interpreting this statement, the proportion of -competitors must be taken into account. Clearly, not the absolute, but the -relative, abundance of fit food is the point; and this relative abundance -very much depends on the number of individuals competing for the food. Thus -all who have had experience in fishing in Highland lochs, know that where -the trout are numerous they are small, and that where they are -comparatively large they are comparatively few. - -What is the relation between growth and expenditure of energy? is a -question which next presents itself. Though there is reason to believe such -a relation exists, it is not very readily traced: involved as it is with so -many other relations. Some contrasts, however, may be pointed out that -appear to give evidence of it. Passing over the vegetal kingdom, throughout -which the expenditure of force is too small to allow of such a relation -being visible, let us seek in the animal kingdom, some case where classes -otherwise allied, are contrasted in their locomotive activities. Let us -compare birds on the one hand, with reptiles and mammals on the other. It -is an accepted doctrine that birds are organized on a type closely allied -to the reptilian type, but superior to it; and though in some respects the -organization of birds is inferior to that of mammals, yet in other -respects, as in the greater heterogeneity and integration of the skeleton, -the more complex development of the respiratory system, and the higher -temperature of the blood, it may be held that birds stand above mammals. -Hence were growth dependent only on organization, we might infer that the -limit of growth among birds should not be much short of that among mammals; -and that the bird-type should admit of a larger growth than the -reptile-type. Again, we see no manifest disadvantages under which birds -labour in obtaining food, but from which reptiles and mammals are free. On -the contrary, birds are able to get at food that is fixed beyond the reach -of reptiles and mammals; and can catch food that is too swift of movement -to be ordinarily caught by reptiles and mammals. Nevertheless, the limit of -growth in birds falls far below that reached by reptiles and mammals. With -what other contrast between these classes, is this contrast connected? May -we not suspect that it is connected (partially though not wholly) with the -contrast between their amounts of locomotive exertion? Whereas mammals -(excepting bats, which are small), are during all their movements supported -by solid surfaces or dense liquids; and whereas reptiles (excepting the -ancient pterodactyles, which were not very large), are similarly restricted -in their spheres of movement; the majority of birds move more or less -habitually through a rare medium, in which they cannot support themselves -without relatively great efforts. And this general fact may be joined with -the special fact, that those members of the class _Aves_, as the _Dinornis_ -and _Epiornis_, which approached in size to the larger _Mammalia_ and -_Reptilia_, were creatures incapable of flight--creatures which did not -expend this excess of force in locomotion. But as implied above, and as -will presently be shown, another factor of importance comes into play; so -that perhaps the safest evidence that there is an antagonism between the -increase of bulk and the quantity of motion evolved is that supplied by the -general experience, that human beings and domestic animals, when overworked -while growing, are prevented from attaining the ordinary dimensions. - -One other general truth concerning degrees of growth, must be set down. It -is a rule, having exceptions of no great importance, that large organisms -commence their separate existences as masses of organic matter more or less -considerable in size, and commonly with organizations more or less -advanced; and that throughout each organic sub-kingdom, there is a certain -general, though irregular, relation between the initial and the final -bulks. Vegetals exhibit this relation less manifestly than animals. Yet -though, among the plants that begin life as minute spores, there are some -which, by the aid of an intermediate form, grow to large sizes, the immense -majority of them remain small. While, conversely, the great Monocotyledons -and Dicotyledons, when thrown off from their parents, have already the -formed organs of young plants, to which are attached stores of highly -nutritive matter. That is to say, where the young plant consists merely of -a centre of development, the ultimate growth is commonly insignificant; but -where the growth is to become great, there exists to start with, a -developed embryo and a stock of assimilable matter. Throughout the animal -kingdom this relation is tolerably manifest though by no means uniform. -Save among classes that escape the ordinary requirements of animal life, -small germs or eggs do not in most cases give rise to bulky creatures. -Where great bulk is to be reached, the young proceeds from an egg of -considerable bulk, or is born of considerable bulk ready-organized and -partially active. In the class Fishes, or in such of them as are subject to -similar conditions of life, some proportion usually obtains between the -sizes of the ova and the sizes of the adult individuals; though in the -cases of the sturgeon and the tunny there are exceptions, probably -determined by the circumstances of oviposition and those of juvenile life. -Reptiles have eggs that are smaller in number, and relatively greater in -mass, than those of fishes; and throughout this class, too, there is a -general congruity between the bulk of the egg and the bulk of the adult -creature. As a group, birds show us further limitations in the numbers of -their eggs as well as farther increase in their relative sizes; and from -the minute eggs of the humming-bird up to the immense ones of the -_Epiornis_, holding several quarts, we see that, speaking generally, the -greater the eggs the greater the birds., Finally, among mammals (omitting -the marsupials) the young are born, not only of comparatively large sizes, -but with advanced organizations; and throughout this sub-division of the -_Vertebrata_, as throughout the others, there is a manifest connexion -between the sizes at birth and the sizes at maturity. As having a kindred -meaning, there must finally be noted the fact that the young of these -highest animals, besides starting in life with bodies of considerable -sizes, almost fully organized, are, during subsequent periods of greater or -less length, supplied with nutriment--in birds by feeding and in mammals by -suckling and afterwards by feeding. So that beyond the mass and -organization directly bequeathed, a bird or mammal obtains a further large -mass at but little cost to itself. - -Were exhaustive treatment of the topic intended, it would be needful to -give a paragraph to each of the incidental circumstances by which growth -may be aided or restricted:--such facts as that an entozoon is limited by -the size of the creature, or even the organ, in which it thrives; that an -epizoon, though getting abundant nutriment without appreciable exertion, is -restricted to that small bulk at which it escapes ready detection by the -animal it infests; that sometimes, as in the weazel, smallness is a -condition to successful pursuit of the animals preyed upon; and that in -some cases, the advantage of resembling certain other creatures, and so -deceiving enemies or prey, becomes an indirect cause of restricted size. -But the present purpose is simply to set down those most general relations -between growth and other organic traits, which induction leads us to. -Having done this, let us go on to inquire whether these general relations -can be deductively established. - - -§ 44. That there must exist a certain dependence of growth on organization, -may be shown _a priori_. When we consider the phenomena of Life, either by -themselves or in their relations to surrounding phenomena, we see that, -other things equal, the larger the aggregate the greater is the needful -complexity of structure. - -In plants, even of the highest type, there is a comparatively small mutual -dependence of parts: a gathered flower-bud will unfold and flourish for -days if its stem be immersed in water; and a shoot cut off from its -parent-tree and stuck in the ground will grow. The respective parts having -vital activities that are not widely unlike, it is possible for great bulk -to be reached without that structural complexity required for combining the -actions of parts. Even here, however, we see that for the attainment of -great bulk there requires such a degree of organization as shall -co-ordinate the functions of roots and branches--we see that such a size as -is reached by trees, is not possible without a vascular system enabling the -remote organs to utilize each other's products. And we see that such a -co-existence of large growth with comparatively low organization as occurs -in some of the marine _Algæ_, occurs where the conditions of existence do -not necessitate any considerable mutual dependence of parts--where the near -approach of the plant to its medium in specific gravity precludes the need -of a well-developed stem, and where all the materials of growth being -derived from the water by each portion of the thallus, there requires no -apparatus for transferring the crude food materials from part to part. -Among animals which, with but few exceptions, are, by the conditions of -their existence, required to absorb nutriment through one specialized part -of the body, it is clear that there must be a means whereby other parts of -the body, to be supported by this nutriment, must have it conveyed to them. -It is clear that for an equally efficient maintenance of their nutrition, -the parts of a large mass must have a more elaborate propelling and -conducting apparatus; and that in proportion as these parts undergo greater -waste, a yet higher development of the vascular system is necessitated. -Similarly with the prerequisites to those mechanical motions which animals -are required to perform. The parts of a mass cannot be made to move, and -have their movements so co-ordinated as to produce locomotive and other -actions, without certain structural arrangements; and, other things equal, -a given amount of such activity requires more involved structural -arrangements in a large mass than in a small one. There must at least be a -co-ordinating apparatus presenting greater contrasts in its central and -peripheral parts. - -The qualified dependence of growth on organization, is equally implied when -we study it in connexion with that adjustment of inner to outer relations -which constitutes Life as phenomenally known to us. In plants this is less -striking than in animals, because the adjustment of inner to outer -relations does not involve conspicuous motions. Still, it is visible in the -fact that the condition on which alone a plant can grow to a great size, -is, that it shall, by the development of a massive trunk, present inner -relations of forces fitted to counterbalance those outer relations of -forces which tend continually, and others which tend occasionally, to -overthrow it; and this formation of a core of regularly-arranged woody -fibres is an advance in organization. Throughout the animal kingdom this -connexion of phenomena is manifest. To obtain materials for growth; to -avoid injuries which interfere with growth; and to escape those enemies -which bring growth to a sudden end; implies in the organism the means of -fitting its movements to meet numerous external co-existences and -sequences--implies such various structural arrangements as shall make -possible these variously-adapted actions. It cannot be questioned that, -everything else remaining constant, a more complex animal, capable of -adjusting its conduct to a greater number of surrounding contingencies, -will be the better able to secure food and evade damage, and so to increase -bulk. And evidently, without any qualification, we may say that a large -animal, living under such complex conditions of existence as everywhere -obtain, is not possible without comparatively high organization. - -While, then, this relation is traversed and obscured by sundry other -relations, it cannot but exist. Deductively we see that it must be -modified, as inductively we saw that it is modified, by the circumstances -amid which each kind of organism is placed, but that it is always a factor -in determining the result. - - -§ 45. That growth is, _cæteris paribus_, dependent on the supply of -assimilable matter, is a proposition so continually illustrated by special -experience, as well as so obvious from general experience, that it would -scarcely need stating, were it not requisite to notice the qualifications -with which it must be taken. - -The materials which each organism requires for building itself up, are not -of one kind but of several kinds. As a vehicle for transferring matter -through their structures, all organisms require water as well as solid -constituents; and however abundant the solid constituents there can be no -growth in the absence of water. Among the solids supplied, there must be a -proportion ranging within certain limits. A plant round which carbonic -acid, water, and ammonia exist in the right quantities, may yet be arrested -in its growth by a deficiency of potassium. The total absence of lime from -its food may stop the formation of a mammal's skeleton: thus dwarfing, if -not eventually destroying, the mammal; and this no matter what quantities -of other needful colloids and crystalloids are furnished. - -Again, the truth that, other things equal, growth varies according to the -supply of nutriment, has to be qualified by the condition that the supply -shall not exceed the ability to appropriate it. In the vegetal kingdom, the -assimilating surface being external and admitting of rapid expansion by the -formation of new roots, shoots, and leaves, the effect of this limitation -is not conspicuous. By artificially supplying plants with those materials -which they have usually the most difficulty in obtaining, we can greatly -facilitate their growth; and so can produce striking differences of size in -the same species. Even here, however, the effect is confined within the -limits of the ability to appropriate; since in the absence of that solar -light and heat by the help of which the chief appropriation is carried on, -the additional materials for growth are useless. In the animal kingdom this -restriction is rigorous. The absorbent surface being, in the great majority -of cases, internal; having a comparatively small area, which cannot be -greatly enlarged without reconstruction of the whole body; and being in -connexion with a vascular system which also must be re-constructed before -any considerable increase of nutriment can be made available; it is clear -that beyond a certain point, very soon reached, increase of nutriment will -not cause increase of growth. On the contrary, if the quantity of food -taken in is greatly beyond the digestive and absorbent power, the excess, -becoming an obstacle to the regular working of the organism, may retard -growth rather than advance it. - -While then it is certain, _a priori_, that there cannot be growth in the -absence of such substances as those of which an organism consists; and -while it is equally certain that the amount of growth must primarily be -governed by the supply of these substances; it is not less certain that -extra supply will not produce extra growth, beyond a point very soon -reached. Deduction shows to be necessary, as induction makes familiar, the -truths that the value of food for purposes of growth depends not on the -quantity of the various organizable materials it contains, but on the -quantity of the material most needed; that given a right proportion of -materials, the pre-existing structure of the organism limits their -availability; and that the higher the structure, the sooner is this limit -reached. - - -§ 46. But why should the growth of every organism be finally arrested? -Though the rate of increase may, in each case, be necessarily restricted -within a narrow range of variation--though the increment that is possible -in a given time, cannot exceed a certain amount; yet why should the -increments decrease and finally become insensible? Why should not all -organisms, when supplied with sufficient materials, continue to grow as -long as they live? To find an answer to this question we must revert to the -nature and functions of organic matter. - -In the first three chapters of Part I, it was shown that plants and animals -mainly consist of substances in states of unstable equilibrium--substances -which have been raised to this unstable equilibrium by the expenditure of -the forces we know as solar radiations, and which give out these forces in -other forms on falling into states of stable equilibrium. Leaving out the -water, which serves as a vehicle for these materials and a medium for their -changes; and excluding those mineral matters that play either passive or -subsidiary parts; organisms are built up of compounds which are stores of -force. Thus complex colloids and crystalloids which, as united together, -form organized bodies, are the same colloids and crystalloids which give -out, on their decomposition, the forces expended by organized bodies. Thus -these nitrogenous and carbonaceous substances, being at once the materials -for organic growth and the sources of organic energy, it results that as -much of them as is used up for the genesis of energy is taken away from the -means of growth, and as much as is economized by diminishing the genesis of -energy, is available for growth. Given that limited quantity of nutritive -matter which the pre-existing structure of an organism enables it to -absorb; and it is a necessary corollary from the persistence of force, that -the matter accumulated as growth cannot exceed that surplus which remains -undecomposed after the production of the required amounts of sensible and -insensible motion. This, which would be rigorously true under all -conditions if exactly the same substances were used in exactly the same -proportions for the production of force and for the formation of tissue, -requires, however, to be taken with the qualification that some of the -force-evolving substances are not constituents of tissue; and that thus -there may be a genesis of force which is not at the expense of potential -growth. But since organisms (or at least animal organisms, with which we -are here chiefly concerned) have a certain power of selective absorption, -which, partially in an individual and more completely in a race, adapts the -proportions of the substances absorbed to the needs of the system; then if -a certain habitual expenditure of force leads to a certain habitual -absorption of force-evolving matters that are not available for growth; and -if, were there less need for such matters, the ability to absorb matters -available for growth would be increased to an equivalent extent; it follows -that the antagonism described does, in the long run, hold even without this -qualification. Hence, growth is substantially equivalent to the absorbed -nutriment, minus the nutriment used up in action. - -This, however, is no answer to the question--why has individual growth a -limit?--why do the increments of growth bear decreasing ratios to the mass -and finally come to an end? The question is involved. There are more causes -than one why the excess of absorbed nutriment over expended nutriment must, -other things equal, become less as the size of the animal becomes greater. -In similarly-shaped bodies the masses, and therefore the weights, vary as -the cubes of the dimensions; whereas the powers of bearing the stresses -imposed by the weights vary as the squares of the dimensions. Suppose a -creature which a year ago was one foot high, has now become two feet high, -while it is unchanged in proportions and structure; what are the necessary -concomitant changes? It is eight times as heavy; that is to say, it has to -resist eight times the strain which gravitation puts upon certain of its -parts; and when there occurs sudden arrest of motion or sudden genesis of -motion, eight times the strain is put upon the muscles employed. Meanwhile -the muscles and bones have severally increased their abilities to bear -strains in proportion to the areas of their transverse sections, and hence -have severally only four times the tenacity they had. This relative -decrease in the power of bearing stress does not imply a relative decrease -in the power of generating energy and moving the body; for in the case -supposed the muscles have not only increased four times in their transverse -sections but have become twice as long, and will therefore generate an -amount of energy proportionate to their bulk. The implication is simply -that each muscle has only half the power to withstand those shocks and -strains which the creature's movements entail; and that consequently the -creature must be either less able to bear these, or must have muscles and -bones having relatively greater transverse dimensions: the result being -that greater cost of nutrition is inevitably caused and therefore a -correlative tendency to limit growth. This necessity will be seen still -more clearly if we leave out the motor apparatus, and consider only the -forces required and the means of supplying them. For since, in similar -bodies, the areas vary as the squares of the dimensions, and the masses -vary as the cubes; it follows that the absorbing surface has become four -times as great, while the weight to be moved by the matter absorbed has -become eight times as great. If then, a year ago, the absorbing surface -could take up twice as much nutriment as was needed for expenditure, thus -leaving one-half for growth, it is now able only just to meet expenditure, -and can provide nothing for growth. However great the excess of -assimilation over waste may be during the early life of an active organism, -we see that because a series of numbers increasing as the cubes, overtakes -a series increasing as the squares, even though starting from a much -smaller number, there must be reached, if the organism lives long enough, a -point at which the surplus assimilation is brought down to nothing--a point -at which expenditure balances nutrition--a state of moving equilibrium. The -only way in which the difficulty can be met is by gradual re-organization -of the alimentary system; and, in the first place, this entails direct cost -upon the organism, and, in the second place, indirect cost from the -carrying of greater weight: both tending towards limitation. There are two -other varying relations between degrees of growth and amounts of expended -force; one of which conspires with the last, while the other conflicts with -it. Consider, in the first place, the cost at which nutriment is -distributed through the body and effete matters removed from it. Each -increment of growth being added at the periphery of the organism, the force -expended in the transfer of matter must increase in a rapid progression--a -progression more rapid than that of the mass. But as the dynamic expense of -distribution is small compared with the dynamic value of the materials -distributed, this item in the calculation is unimportant. Now consider, in -the second place, the changing proportion between production and loss of -heat. In similar organisms the quantities of heat generated by similar -actions going on throughout their substance, must increase as the masses, -or as the cubes of the dimensions. Meanwhile, the surfaces from which loss -of heat takes place, increase only as the squares of the dimensions. Though -the loss of heat does not therefore increase only as the squares of the -dimensions, it certainly increases at a smaller rate than the cubes. And to -the extent that augmentation of mass results in a greater retention of -heat, it effects an economization of force. This advantage is not, however, -so important as at first appears. Organic heat is a concomitant of organic -action, and is so abundantly produced during action that the loss of it is -then usually of no consequence: indeed the loss is often not rapid enough -to keep the supply from rising to an inconvenient excess. It is chiefly in -respect of that maintenance of heat which is needful during quiescence, -that large organisms have an advantage over small ones in this relatively -diminished loss. Thus these two subsidiary relations between degrees of -growth and amounts of expended force, being in antagonism, we may conclude -that their differential result does not greatly modify the result of the -chief relation. - -Comparisons of these deductions with the facts appear in some cases to -verify them and in other cases not to do so. Throughout the vegetal -kingdom, there are no distinct limits to growth except those which death -entails. Passing over a large proportion of plants which never exceed a -comparatively small size, because they wholly or partially die down at the -end of the year, and looking only at trees that annually send forth new -shoots, even when their trunks are hollowed by decay; we may ask--How does -growth happen here to be unlimited? The answer is, that plants are only -accumulators: they are in no very appreciable degree expenders. As they do -not undergo waste there is no reason why their growth should be arrested by -the equilibration of assimilation and waste. Again, among animals there -are sufficient reasons why the correspondence cannot be more than -approximate. Besides the fact above noted, that there are other varying -relations which complicate the chief one. We must bear in mind that the -bodies compared are not truly similar: the proportions of trunk to limbs -and trunk to head, vary considerably. The comparison is still more -seriously vitiated by the inconstant ratio between the constituents of -which the body is composed. In the flesh of adult mammalia, water forms -from 68 to 71 per cent., organic substance from 24 to 28 per cent., and -inorganic substance from 3 to 5 per cent.; whereas in the foetal state, the -water amounts to 87 per cent., and the solid organic constituents to only -11 per cent. Clearly this change from a state in which the force-evolving -matter forms one-tenth of the whole, to a state in which it forms two and a -half tenths, must greatly interfere with the parallelism between the actual -and the theoretical progression. Yet another difficulty may come under -notice. The crocodile is said to grow as long as it lives; and there -appears reason to think that some predaceous fishes, such as the pike, do -the same. That these animals of comparatively high organization have no -definite limits of growth, is, however, an exceptional fact due to the -exceptional non-fulfilment of those conditions which entail limitation. -What kind of life does a crocodile lead? It is a cold-blooded, or almost -cold-blooded, creature; that is, it expends very little for the maintenance -of heat. It is habitually inert: not usually chasing prey but lying in wait -for it; and undergoes considerable exertion only during its occasional -brief contests with prey. Such other exertion as is, at intervals, needful -for moving from place to place, is rendered small by the small difference -between the animal's specific gravity and that of water. Thus the crocodile -expends in muscular action an amount of force that is insignificant -compared with the force commonly expended by land-animals. Hence its -habitual assimilation is diminished much less than usual by habitual waste; -and beginning with an excessive disproportion between the two, it is quite -possible for the one never quite to lose its advance over the other while -life continues. On looking closer into such cases as this and that of the -pike, which is similarly cold-blooded, similarly lies in wait, and is -similarly able to obtain larger and larger kinds of prey as it increases in -size; we discover a further reason for this absence of a definite limit. To -overcome gravitative force the creature has not to expend a muscular power -that is large at the outset, and increases as the cubes of its dimensions: -its dense medium supports it. The exceptional continuance of growth -observed in creatures so circumstanced, is therefore perfectly explicable. - - -§ 46a. If we go back upon the conclusions set forth in the preceding -section, we find that from some of them may be drawn instructive -corollaries respecting the limiting sizes of creatures inhabiting different -media. More especially I refer to those varying proportions between mass -and stress from which, as we have seen, there results, along with -increasing size, a diminishing power of mechanical self-support: a relation -illustrated in its simplest form by the contrast between a dew-drop, which -can retain its spheroidal form, and the spread-out mass of water which -results when many dew-drops run together. The largest bird that flies (the -argument excludes birds which do not fly) is the Condor, which reaches a -weight of from 30 to 40 lbs. Why does there not exist a bird of the size of -an elephant? Supposing its habits to be carnivorous, it would have many -advantages in obtaining prey: mammals would be at its mercy. Evidently the -reason is one which has been pointed out--the reason that while the weight -to be raised and kept in the air by a bird increases as the cubes of its -dimensions, the ability of its bones and muscles to resist the strains -which flight necessitates, increases only as the squares of the dimensions. -Though, could the muscles withstand any tensile strain they were subject -to, the power like the weight might increase with the cubes, yet since the -texture of muscle is such that beyond a certain strain it tears, it results -that there is soon reached a size at which flight becomes impossible: the -structures must give way. In a preceding paragraph the limit to the size of -flying creatures was ascribed to the greater physiological cost of the -energy required; but it seems probable that the mechanical obstacle here -pointed out has a larger share in determining the limit. - -In a kindred manner there results a limitation of growth in a land-animal, -which does not exist for an animal living in the water. If, after comparing -the agile movements of a dog with those of a cow, the great weight of which -obviously prevents agility; or if, after observing the swaying flesh of an -elephant as it walks along, we consider what would happen could there be -formed a land-animal equal in mass to the whale (the long Dinosaurs were -not proportionately massive) it needs no argument to show that such a -creature could not stand, much less move about. But in the water the strain -put upon its structures by the weights of its various parts is almost if -not quite taken away. Probably limitation in the quantity of food -obtainable becomes now the chief, if not the sole, restraint. - -And here we may note, before leaving the topic, something like a converse -influence which comes into play among creatures inhabiting the water. Up to -the point at which muscles tear from over-strain, larger and smaller -creatures otherwise alike, remain upon a par in respect of the relative -amounts of energy they can evolve. Had they to encounter no resistance from -their medium, the implication would be that neither would have an advantage -over the other in respect of speed. But resistance of the medium comes into -play; and this, other things equal, gives to the larger creature an -advantage. It has been found, experimentally, that the forces to be -overcome by vessels moving through the water, built as they are with -immersed hinder parts which taper as fish taper, are mainly due to what is -called "skin-friction." Now in two fish unlike in size but otherwise -similar skin-friction bears to the energy that can be generated, a smaller -proportion in the larger than in the smaller; and the larger can therefore -acquire a greater velocity. Hence the reason why large fish, such as the -shark, become possible. In a habitat where there is no ambush (save in -exceptional cases like that of the _Lophius_ or Angler) everything depends -on speed; and if, other things equal, a larger fish had no mechanical -advantage over a smaller, a larger fish could not exist--could not catch -the requisite amount of prey. - - -§ 47. Obviously this antagonism between accumulation and expenditure, must -be a leading cause of the contrasts in size between allied organisms that -are in many respects similarly conditioned. The life followed by each kind -of animal is one involving a certain average amount of exertion for the -obtainment of a given amount of nutriment--an exertion, part of which goes -to the gathering or catching of food, part to the tearing and mastication -of it, and part to the after-processes requisite for separating the -nutritive molecules--an exertion which therefore varies according as the -food is abundant or scarce, fixed or moving, according as it is -mechanically easy or difficult to deal with when secured, and according as -it is, or is not, readily soluble. Hence, while among animals of the same -species having the same mode of life, there will be a tolerably constant -ratio between accumulation and expenditure, and therefore a tolerably -constant limit of growth, there is every reason to expect that different -species, following different modes of life, will have unlike ratios between -accumulation and expenditure, and therefore unlike limits of growth. - -Though the facts as inductively established, show a general harmony with -this deduction, we cannot usually trace it in any specific way; since the -conflicting and conspiring factors which affect growth are so numerous. - - -§ 48. One of the chief causes, if not the chief cause, of the differences -between the sizes of organisms, has yet to be considered. We are introduced -to it by pushing the above inquiry a little further. Small animals have -been shown to possess an advantage over large ones in the greater ratio -which, other things equal, assimilation bears to expenditure; and we have -seen that hence small animals in becoming large ones, gradually lose that -surplus of assimilative power which they had, and eventually cannot -assimilate more than is required to balance waste. But how come these -animals while young and small to have surplus assimilative powers? Have all -animals equal surpluses of assimilative powers? And if not, how far do -differences between the surpluses determine differences between the limits -of growth? We shall find, in the answers to these questions, the -interpretation of many marked contrasts in growth that are not due to any -of the causes above assigned. For example, an ox immensely exceeds a sheep -in mass. Yet the two live from generation to generation in the same fields, -eat the same grass, obtain these aliments with the same small expenditure -of energy, and differ scarcely at all in their degrees of organization. -Whence arises, then, their striking unlikeness of bulk? - -We noted when studying the phenomena of growth inductively, that organisms -of the larger and higher types commence their separate existences as masses -of organic matter having tolerable magnitudes. Speaking generally, we saw -that throughout each organic sub-kingdom the acquirement of great bulk -occurs only where the incipient bulk and organization are considerable; and -that they are the more considerable in proportion to the complexity of the -life which the organism is to lead. - -The deductive interpretation of this induction may best be commenced by an -analogy. A street orange-vendor makes but a trifling profit on each -transaction; and unless more than ordinarily fortunate, he is unable to -realize during the day a larger amount than will meet his wants; leaving -him to start on the morrow in the same condition as before. The trade of -the huxter in ounces of tea and half-pounds of sugar, is one similarly -entailing much labour for small returns. Beginning with a capital of a few -pounds, he cannot have a shop large enough, or goods sufficiently abundant -and various, to permit an extensive business. He must be content with the -half-pence and pence which he makes by little sales to poor people; and if, -avoiding bad debts, he is able by strict economy to accumulate anything, it -can be but a trifle. A large retail trader is obliged to lay out much money -in fitting up an adequate establishment; he must invest a still greater sum -in stock; and he must have a further floating capital to meet the charges -that fall due before his returns come in. Setting out, however, with means -enough for these purposes, he is able to make many and large sales; and so -to get greater and more numerous increments of profit. Similarly, to get -returns in thousands merchants and manufacturers must make their -investments in tens of thousands. In brief, the rate at which a man's -wealth accumulates is measured by the surplus of income over expenditure; -and this, save in exceptionably favourable cases, is determined by the -capital with which he begins business. Now applying the analogy, we may -trace in the transactions of an organism, the same three ultimate elements. -There is the expenditure required for the obtainment and digestion of food; -there is the gross return in the shape of nutriment assimilated or fit for -assimilation; and there is the difference between this gross return of -nutriment and the nutriment that was used up in the labour of securing -it--a difference which may be a profit or a loss. Clearly, however, a -surplus implies that the force expended is less than the force latent in -the assimilated food. Clearly, too, the increment of growth is limited to -the amount of this surplus of income over expenditure; so that large growth -implies both that the excess of nutrition over waste shall be relatively -considerable, and that the waste and nutrition shall be on extensive -scales. And clearly, the ability of an organism to expend largely and -assimilate largely, so as to make a large surplus, presupposes a large -physiological capital in the shape of organic matter more or less developed -in its structural arrangements. - -Throughout the vegetal kingdom, the illustrations of this truth are not -conspicuous and regular: the obvious reason being that since plants are -accumulators and in so small a degree expenders, the premises of the above -argument are but very partially fulfilled. The food of plants (excepting -Fungi and certain parasites) being in great measure the same for all, and -bathing all so that it can be absorbed without effort, their vital -processes result almost entirely in profit. Once fairly rooted in a fit -place, a plant may thus from the outset add a very large proportion of its -entire returns to capital; and may soon be able to carry on its processes -on a large scale, though it does not at first do so. When, however, plants -are expenders, namely, during their germination and first stages of growth, -their degrees of growth _are_ determined by their amounts of vital capital. -It is because the young tree commences life with a ready-formed embryo and -store of food sufficient to last for some time, that it is enabled to -strike root and lift its head above the surrounding herbage. Throughout the -animal kingdom, however, the necessity of this relation is everywhere -obvious. The small carnivore preying on small herbivores, can increase in -size only by small increments: its organization unfitting it to digest -larger creatures, even if it can kill them, it cannot profit by amounts of -nutriment exceeding a narrow limit; and its possible increments of growth -being small to set out with, and rapidly decreasing, must come to an end -before any considerable size is attained. Manifestly the young lion, born -of tolerable bulk, suckled until much bigger, and fed until half-grown, is -enabled by the power and organization which he thus gets _gratis_, to catch -and kill animals big enough to give him the supply of nutriment needed to -meet his large expenditure and yet leave a large surplus for growth. Thus, -then, is explained the above-named contrast between the ox and the sheep. A -calf and a lamb commence their physiological transactions on widely -different scales; their first increments of growth are similarly contrasted -in their amounts; and the two diminishing series of such increments end at -similarly-contrasted limits. - - -§ 49. Such are the several conditions by which the phenomena of growth are -determined. Conspiring and conflicting in endless unlike ways and degrees, -they in every case qualify more or less differently each other's effects. -Hence it happens that we are obliged to state each generalization as true -on the average, or to make the proviso--other things equal. - -Understood in this qualified form, our conclusions are these. First, that -growth being an integration with the organism of such environing matters as -are of like natures with the matters composing the organism, its growth is -dependent on the available supply of them. Second, that the available -supply of assimilable matter being the same, and other conditions not -dissimilar, the degree of growth varies according to the surplus of -nutrition over expenditure--a generalization which is illustrated in some -of the broader contrasts between different divisions of organisms. Third, -that in the same organism the surplus of nutrition over expenditure differs -at different stages; and that growth is unlimited or has a definite limit, -according as the surplus does or does not rapidly decrease. This -proposition we found exemplified by the almost unceasing growth of -organisms that expend relatively little energy; and by the definitely -limited growth of organisms that expend much energy. Fourth, that among -organisms which are large expenders of force, the size ultimately attained -is, other things equal, determined by the initial size: in proof of which -conclusion we have abundant facts, as well as the _a priori_ necessity that -the sum-totals of analogous diminishing series, must depend upon the -amounts of their initial terms. Fifth, that where the likeness of other -circumstances permits a comparison, the possible extent of growth depends -on the degree of organization; an inference testified to by the larger -forms among the various divisions and sub-divisions of organisms. - - - - -CHAPTER II. - -DEVELOPMENT.[19] - - -§ 50. Certain general aspects of Development may be studied apart from any -examination of internal structures. These fundamental contrasts between the -modes of arrangement of parts, originating, as they do, the leading -external distinctions among the various forms of organization, will be best -dealt with at the outset. If all organisms have arisen by Evolution, it is -of course not to be expected that such several modes of development can be -absolutely demarcated: we are sure to find them united by transitional -modes. But premising that a classification of modes can but approximately -represent the facts, we shall find our general conceptions of Development -aided by one. - -Development is primarily _central_. All organic forms of which the entire -history is known, set out with a symmetrical arrangement of parts round a -centre. In organisms of the lowest grade no other mode of arrangement is -ever definitely established; and in the highest organisms central -development, though subordinate to another mode of development, continues -to be habitually shown in the changes of minute structure. Let us glance at -these propositions in the concrete. Practically every plant and every -animal in its earliest stage is a portion of protoplasm, in the great -majority of cases approximately spherical but sometimes elongated, -containing a rounded body consisting of specially modified protoplasm, -which is called a nucleus; and the first changes that occur in the germ -thus constituted, are changes that take place in this nucleus, followed by -changes round the centres produced by division of this original centre. -From this type of structure, the simplest organisms do not depart; or -depart in no definite or conspicuous ways. Among plants, many of the -simplest _Algæ_ and _Fungi_ permanently maintain such a central -distribution; while among animals it is permanently maintained by creatures -like the _Gregarina_, and in a different manner by the _Amoeba_, -_Actinophrys_, and their allies: the irregularities which are many and -great do not destroy this general relation of parts. In larger organisms, -made up chiefly of units that are analogous to these simplest organisms, -the formation of units ever continues to take place round nuclei; though -usually the nuclei soon cease to be centrally placed. - -Central development may be distinguished into _unicentral_ and -_multicentral_; according as the product of the original germ develops more -or less symmetrically round one centre, or develops without subordination -to one centre--develops, that is, in subordination to many centres. -Unicentral development, as displayed not in the formation of single cells -but in the formation of aggregates, is not common. The animal kingdom shows -it only in some of the small group of colonial _Radiolaria_. It is feebly -represented in the vegetal kingdom by a few members of the _Volvocineæ_. On -the other hand, multicentral development, or development round -insubordinate centres, is variously exemplified in both divisions of the -organic world. It is exemplified in two distinct ways, according as the -insubordination among the centres of development is partial or total. We -may most conveniently consider it under the heads hence arising. - -Total insubordination among the centres of development, is shown where the -units or cells, as fast as they are severally formed, part company and lead -independent lives. This, in the vegetal kingdom, habitually occurs among -the _Protophyta_, and in the animal kingdom, among the _Protozoa_. Partial -insubordination is seen in those somewhat advanced organisms, that consist -of units which, though they have not separated, have so little mutual -dependence that the aggregate they form is irregular. Among plants, the -Thallophytes very generally exemplify this mode of development. Lichens, -spreading with flat or corrugated edges in this or that direction as the -conditions determine, have no manifest co-ordination of parts. In the -_Algæ_ the Nostocs and various other forms similarly show us an -unsymmetrical structure. Of _Fungi_ we may say that creeping kinds display -no further dependence of one part on another than is implied by their -cohesion. And even in such better-organized plants as the _Marchantia_, the -general arrangement shows no reference to a directive centre. Among animals -many of the Sponges in their adult forms may be cited as devoid of that -co-ordination implied by symmetry: the units composing them, though they -have some subordination to local centres, have no subordination to a -general centre. To distinguish that kind of development in which the whole -product of a germ coheres in one mass, from that kind of development in -which it does not, Professor Huxley has introduced the words "_continuous_" -and "_discontinuous_;" and these seem the best fitted for the purpose. -Multicentral development, then, is divisible into continuous and -discontinuous. - -From central development we pass insensibly to that higher kind of -development for which _axial_ seems the most appropriate name. A tendency -towards this is vaguely manifested almost everywhere. The great majority -even of _Protophyta_ and _Protozoa_ have different longitudinal and -transverse dimensions--have an obscure if not a distinct axial structure. -The originally spheroidal and polyhedral units out of which higher -organisms are mainly built, usually pass into shapes that are subordinated -to lines rather than to points. And in the higher organisms, considered as -wholes, an arrangement of parts in relation to an axis is distinct and -nearly universal. We see it in the superior orders of Thallophytes; and in -all the cormophytic plants. With few exceptions the _Coelenterata_ clearly -exhibit it; it is traceable, though less conspicuously, throughout the -_Mollusca_; and the _Annelida_, _Arthropoda_, and _Vertebrata_ uniformly -show it with perfect definiteness. - -This kind of development, like the first kind, is of two orders. The whole -germ-product may arrange itself round a single axis, or it may arrange -itself round many axes: the structure may be _uniaxial_ or _multiaxial_. -Each division of the organic kingdom furnishes examples of both these -orders. In such _Fungi_ as exhibit axial development at all, we commonly -see development round a single axis. Some of the _Algæ_, as the common -tangle, show us this arrangement. And of the higher plants, many -Monocotyledons and small Dicotyledons are uniaxial. Of animals, the -advanced are without exception in this category. There is no known -vertebrate in which the whole of the germ-product is not subordinated to a -single axis. In the _Arthropoda_, the like is universal; as it is also in -the superior orders of _Mollusca_. Multiaxial development occurs in most of -the plants we are familiar with--every branch of a shrub or tree being an -independent axis. But while in the vegetal kingdom multiaxial development -prevails among the highest types, in the animal kingdom it prevails only -among the lowest types. It is extremely general, if not universal, among -the _Coelenterata_; it is characteristic of the _Polyzoa_; the compound -Ascidians exhibit it; and it is seen, though under another form, in certain -of the inferior Annelids. - -Development that is axial, like development that is central, may be either -continuous or discontinuous: the parts having different axes may continue -united, or they may separate. Instances of each alternative are supplied by -both plants and animals. Continuous multiaxial development is that which -plants usually display, and need not be illustrated further than by -reference to every garden. As cases of it in animals may be named all the -compound _Hydrozoa_ and _Actinozoa_; and such ascidian forms as the -_Botryllidæ_. Of multiaxial development that is discontinuous, a familiar -instance among plants exists in the common strawberry. This sends out over -the neighbouring surface, long slender shoots, bearing at their extremities -buds that presently strike roots and become new individuals; and these by -and by lose their connexions with the original axis. Other plants there are -that produce certain specialized buds called bulbils, which separating -themselves and falling to the ground, grow into independent plants. Among -animals the fresh-water polype very clearly shows this mode of development: -the young polypes, budding out from its surface, severally arrange their -parts around distinct axes, and eventually detaching themselves, lead -separate lives, and produce other polypes after the same fashion. By some -of the lower _Annelida_, this multiplication of axes from an original axis, -is carried on after a different manner: the string of segments -spontaneously divides; and after further growth, division recurs in one or -both of the halves. Moreover in the _Syllis ramosa_, there occurs lateral -branching also. - -Grouping together its several modes as above delineated, we see that - - { Unicentral - { Central { or { Continuous - { { Multicentral { or - { { Discontinuous - DEVELOPMENT is { or - { - { { Uniaxial - { Axial { or { Continuous - { Multiaxial { or - { Discontinuous - -Any one well acquainted with the facts, may readily raise objections to -this arrangement. He may name forms which do not obviously come under any -of these heads. He may point to plants that are for a time multicentral but -afterwards develop axially. And from lower types of animals he may choose -many in which the continuous and discontinuous modes are both displayed. -But, as already hinted, an arrangement free from such anomalies must be -impossible, if the various kinds of organization have arisen by Evolution. -The one above sketched out is to be regarded as a rough grouping of the -facts, which helps us to a conception of them in their totality; and, so -regarded, it will be of service when we come to treat of Individuality and -Reproduction. - - -§ 51. From these most general external aspects of organic development, let -us now turn to its internal and more special aspects. When treating of -Evolution as a universal process of things, a rude outline of the course of -structural changes in organisms was given (_First Principles_, §§ 110, 119, -132). Here it will be proper to describe these changes more fully. - -The bud of any common flowering plant in its earliest stage, consists of a -small hemispherical or sub-conical projection. While it increases most -rapidly at the apex, this presently develops on one side of its base, a -smaller projection of like general shape with itself. Here is the rudiment -of a leaf, which presently spreads more or less round the base of the -central hemisphere or main axis. At the same time that the central -hemisphere rises higher, this lateral prominence, also increasing, gives -rise to subordinate prominences or lobes. These are the rudiments of -stipules, where the leaves are stipulated. Meanwhile, towards the other -side of the main axis and somewhat higher up, another lateral prominence -arising marks the origin of a second leaf. By the time that the first leaf -has produced another pair of lobes, and the second leaf has produced its -primary pair, the central hemisphere, still increasing at its apex, -exhibits the rudiment of a third leaf. Similarly throughout. While the germ -of each succeeding leaf thus arises, the germs of the previous leaves, in -the order of their priority, are changing their rude nodulated shapes into -flattened-out expansions; which slowly put on those sharp outlines they -show when unfolded. Thus from that extremely indefinite figure, a rounded -lump, giving off from time to time lateral lumps, which severally becoming -symmetrically lobed gradually assume specific and involved forms, we pass -little by little to that comparatively complex thing--a leaf-bearing shoot. - Internally, a bud undergoes analogous changes; as witness this -account:--"The general mass of thin-walled parenchymatous cells which -occupies the apical region, and forms the _growing point_ of the shoot, is -covered by a single external layer of similar cells, which increase in -number by the formation of new walls in one direction only, perpendicular -to the surface of the shoot, and thus give rise only to the _epidermis_ or -single layer of cells covering the whole surface of the shoot. Meanwhile -the general mass below grows as a whole, its constituent cells dividing in -all directions. Of the new cells so formed, those removed by these -processes of growth and division from the actual apex, begin, at a greater -or less distance from it, to show signs of the differentiation which will -ultimately lead to the formation of the various tissues enclosed by the -epidermis of the shoot. First the pith, then the vascular bundles, and then -the cortex of the shoot, begin to take on their special characters." -Similarly with secondary structures, as the lateral buds whence leaves -arise. In the, at first, unorganized mass of cells constituting the -rudimentary leaf, there are formed vascular bundles which eventually become -the veins of the leaf; and _pari passu_ with these are formed the other -tissues of the leaf. Nor do we fail to find an essentially parallel set of -changes, when we trace the histories of the individual cells. While the -tissues they compose are separating, the cells are growing step by step -more unlike. Some become flat, some polyhedral, some cylindrical, some -prismatic, some spindle-shaped. These develop spiral thickenings in their -interiors; and those, reticulate thickenings. Here a number of cells unite -together to form a tube: and there they become almost solid by the internal -deposition of woody or other substance. Through such changes, too numerous -and involved to be here detailed, the originally uniform cells go on -diverging and rediverging until there are produced various forms that seem -to have very little in common. - -The arm of a man makes its first appearance in as simple a way as does the -shoot of a plant. According to Bischoff, it buds-out from the side of the -embryo as a little tongue-shaped projection, presenting no differences of -parts; and it might serve for the rudiment of some one of the various other -organs that also arise as buds. Continuing to lengthen, it presently -becomes somewhat enlarged at its end; and is then described as a pedicle -bearing a flattened, round-edged lump. This lump is the representative of -the future hand, and the pedicle of the future arm. By and by, at the edges -of this flattened lump, there appear four clefts, dividing from each other -the buds of the future fingers; and the hand as a whole grows a little more -distinguishable from the arm. Up to this time the pedicle has remained one -continuous piece, but it now begins to show a bend at its centre, which -indicates the division into arm and forearm. The distinctions thus rudely -indicated gradually increase: the fingers elongate and become jointed, and -the proportions of all the parts, originally very unlike those of the -complete limb, slowly approximate to them. During its bud-like stage, the -rudimentary arm consists only of partially-differentiated tissues. By the -diverse changes these gradually undergo they are transformed into bones, -muscles, blood-vessels, and nerves. The extreme softness and delicacy of -these primary tissues, renders it difficult to trace the initial stages of -the differentiations. In consequence of the colour of their contents, the -blood-vessels are the first parts to become distinct. Afterwards the -cartilaginous parts, which are the bases of the future bones, become marked -out by the denser aggregation of their constituent cells, and by the -production between these of a hyaline substance which unites them into a -translucent mass. When first perceptible, the muscles are gelatinous, pale, -yellowish, transparent, and indistinguishable from their tendons. The -various other tissues of which the arm consists, beginning with very -faintly-marked differences, become day by day more definite in their -qualitative appearances. In like manner the units composing these tissues -severally assume increasingly-specific characters. The fibres of muscle, at -first made visible in the midst of their gelatinous matrix only by -immersion in alcohol, grow more numerous and distinct; and by and by they -begin to exhibit transverse stripes. The bone-cells put on by degrees their -curious structure of branching canals. And so in their respective ways with -the units of skin and the rest. - -Thus in each of the organic sub-kingdoms, we see this change from an -incoherent, indefinite homogeneity to a coherent, definite heterogeneity, -illustrated in a quadruple way. The originally-like units called cells, -become unlike in various ways, and in ways more numerous and marked as the -development goes on. The several tissues which these several classes of -cells form by aggregation, grow little by little distinct from each other; -and little by little put on those structural complexities that arise from -differentiations among their component units. In the shoot, as in the limb, -the external form, originally very simple, and having much in common with -simple forms in general, gradually acquires an increasing complexity, and -an increasing unlikeness to other forms. Meanwhile, the remaining parts of -the organism to which the shoot or limb belongs, having been severally -assuming structures divergent from one another and from that of this -particular shoot or limb, there has arisen a greater heterogeneity in the -organism as a whole. - - -§ 52. One of the most remarkable inductions of embryology comes next in -order. And here we find illustrated the general truth that in mental -evolution as in bodily evolution the progress is from the indefinite and -inexact to the definite and exact. For the first statement of this -induction was but an adumbration of the correct statement. - -As a result of his examinations von Baer alleged that in its earliest stage -every organism has the greatest number of characters in common with all -other organisms in their earliest stages; that at a stage somewhat later -its structure is like the structures displayed at corresponding phases by a -less extensive assemblage of organisms; that at each subsequent stage -traits are acquired which successively distinguish the developing embryo -from groups of embryos that it previously resembled--thus step by step -diminishing the group of embryos which it still resembles; and that thus -the class of similar forms is finally narrowed to the species of which it -is a member. This abstract proposition will perhaps not be fully -comprehended by the general reader. It will be best to re-state it in a -concrete shape. Supposing the germs of all kinds of organisms to be -simultaneously developing, we may say that all members of the vast -multitude take their first steps in the same direction; that at the second -step one-half of this vast multitude diverges from the other half, and -thereafter follows a different course of development; that the immense -assemblage contained in either of these divisions very soon again shows a -tendency to take two or more routes of development; that each of the two or -more minor assemblages thus resulting, shows for a time but small -divergences among its members, but presently again divides into groups -which separate ever more widely as they progress; and so on until each -organism, when nearly complete, is accompanied in its further modifications -only by organisms of the same species; and last of all, assumes the -peculiarities which distinguish it as an individual--diverges to a slight -extent to the organisms it is most like. - -But, as above said, this statement is only an adumbration. The order of -Nature is habitually more complex than our generalizations represent it as -being--refuses to be fully expressed in simple formulæ; and we are obliged -to limit them by various qualifications. It is thus here. Since von Baer's -day the careful observations of numerous observers have shown his -allegation to be but approximately true. Hereafter, when discussing the -embryological evidence of Evolution, the causes of deviations will be -discussed. For the present it suffices to recognize as unquestionable the -fact that whereas the germs of organisms are extremely similar, they -gradually diverge widely, in modes now regular and now irregular, until in -place of a multitude of forms practically alike we finally have a multitude -of forms most of which are extremely unlike. Thus, in conformity with the -law of evolution, not only do the parts of each organism advance from -indefinite homogeneity to definite heterogeneity, but the assemblage of all -organisms does the same: a truth already indicated in _First Principles_. - - -§ 53. This comparison between the course of development, in any creature, -and the course of development in all other creatures--this arrival at the -conclusion that the course of development in each, at first the same as in -all others, becomes stage by stage differentiated from the courses in all -others, brings us within view of an allied conclusion. If we contemplate -the successive stages passed through by any higher organism, and observe -the relation between it and its environment at each of these stages; we -shall see that this relation is modified in a way analogous to that in -which the relation between the organism and its environment is modified, as -we advance from the lowest to the highest grades. Along with the -progressing differentiation of each organism from others, we find a -progressing differentiation of it from its environment; like that -progressing differentiation from the environment which we meet with in the -ascending forms of life. Let us first glance at the way in which the -ascending forms of life exhibit this progressing differentiation from the -environment. - -In the first place, it is illustrated in _structure_. Advance from the -homogeneous to the heterogeneous, itself involves an increasing distinction -from the inorganic world. Passing over the _Protozoa_, of which the -simplest probably disappeared during the earliest stages of organic -evolution, and limiting our comparison to the _Metazoa_, we see that low -types of these, as the _Coelenterata_, are relatively simple in their -organization; and the ascent to organisms of greater and greater complexity -of structure, is an ascent to organisms which are in that respect more -strongly contrasted with the structureless environment. In _form_, again, -we see the same truth. An ordinary characteristic of inorganic matter is -its indefiniteness of form; and this is also a characteristic of the lower -organisms, as compared with the higher. Speaking generally, plants are less -definite than animals, both in shape and size--admit of greater -modifications from variations of position and nutrition. Among animals, the -simplest Rhizopods may almost be called amorphous: the form is never -specific, and is constantly changing. Of the organisms resulting from the -aggregation of such creatures, we see that while some, as the -_Foraminifera_, assume a certain definiteness of form, in their shells at -least, others, as the Sponges, are very irregular. The Zoophytes and the -_Polyzoa_ are compound organisms, most of which have a mode of growth not -more determinate than that of plants. But among the higher animals, we find -not only that the mature shape of each species is very definite, but that -the individuals of each species differ little in size. A parallel increase -of contrast is seen in _chemical composition_. With but few exceptions, and -those only partial ones, the lowest animal and vegetal forms are -inhabitants of the water; and water is almost their sole constituent. -Desiccated _Protophyta_ and _Protozoa_ shrink into mere dust; and among the -Acalephes we find but a few grains of solid matter to a pound of water. The -higher aquatic plants, in common with the higher aquatic animals, -possessing as they do increased tenacity of substance, also contain a -greater proportion of the organic elements; further they show us a greater -variety of composition in their different parts; and thus in both ways are -chemically more unlike their medium. And when we pass to the superior -classes of organisms--land-plants and land-animals--we see that, chemically -considered, they have little in common either with the earth on which they -stand or the air which surrounds them. In _specific gravity_ too, we may -note a like truth. The simplest forms, in common with the spores and -gemmules of higher ones, are as nearly as may be of the same specific -gravity as the water in which they float; and though it cannot be said that -among aquatic creatures, superior specific gravity is a standard of general -superiority, yet we may fairly say that the higher orders of them, when -divested of the appliances by which their specific gravity is regulated, -differ more from water in their relative weights than do the lowest. In -terrestrial organisms, the contrast becomes marked. Trees and plants, in -common with insects, reptiles, mammals, birds, are all of a specific -gravity considerably less than that of the earth and immensely greater than -that of the air. Yet further, we see the law fulfilled in respect of -_temperature_. Plants generate but extremely small quantities of heat, -which are to be detected only by delicate experiments; and practically they -may be considered as having the same temperature as their environment. The -temperature of aquatic animals is very little above that of the surrounding -water: that of the invertebrata being mostly less than a degree above it, -and that of fishes not exceeding it by more than two or three degrees; save -in the case of some large red-blooded fishes, as the tunny, which exceed it -in temperature by nearly ten degrees. Among insects the range is from two -to ten degrees above that of the air: the excess varying according to their -activity. The heat of reptiles is from four to fifteen degrees more than -the heat of their medium. While mammals and birds maintain a heat which -continues almost unaffected by external variations, and is often greater -than that of the air by seventy, eighty, ninety, and even a hundred -degrees. Once more, in greater _self-mobility_ a progressive -differentiation is traceable. The chief characteristic by which we -distinguish dead matter is its inertness: some form of independent motion -is our most familiar proof of life. Passing over the indefinite border-land -between the animal and vegetal kingdoms, we may roughly class plants as -organisms which, while they exhibit that kind of motion implied in growth, -are not only devoid of locomotive power, but with some unimportant -exceptions are devoid of the power of moving their parts in relation to -each other; and thus are less differentiated from the inorganic world than -animals. Though in those microscopic _Protophyta_ and _Protozoa_ inhabiting -the water we see locomotion produced by ciliary action; yet this -locomotion, while rapid relatively to the sizes of their bodies, is -absolutely slow. Of the _Coelenterata_ a great part are either permanently -rooted or habitually stationary; and so have scarcely any self-mobility but -that implied in the relative movements of parts; while the rest, of which -the common jelly-fish serves as a sample, have mostly but little ability to -move themselves through the water. Among the higher aquatic -_Invertebrata_,--cuttlefishes and lobsters, for instance,--there is a very -considerable power of locomotion; and the aquatic _Vertebrata_ are, -considered as a class, much more active in their movements than the other -inhabitants of the water. But it is only when we come to air-breathing -creatures that we find the vital characteristics of self-mobility -manifested in the highest degree. Flying insects, mammals, birds, travel -with velocities far exceeding those attained by any of the lower classes of -animals. Thus, on contemplating the various grades of organisms in their -ascending order, we find them more and more distinguished from their -inanimate media, in _structure_, in _form_, in _chemical composition_, in -_specific gravity_, in _temperature_, in _self-mobility_. It is true that -this generalization does not hold with complete regularity. Organisms which -are in some respects the most strongly contrasted with the environing -inorganic world, are in other respects less contrasted than inferior -organisms. As a class, mammals are higher than birds; and yet they are of -lower temperature and have smaller powers of locomotion. The stationary -oyster is of higher organization than the free-swimming medusa; and the -cold-blooded and less heterogeneous fish is quicker in its movements than -the warm-blooded and more heterogeneous sloth. But the admission that the -several aspects under which this increasing contrast shows itself, bear -variable ratios to each other, does not conflict with the general truth -that as we ascend in the hierarchy of organisms, we meet with not only an -increasing differentiation of parts but also an increasing differentiation -from the surrounding medium in sundry other physical attributes. It would -seem that this trait has some necessary connexion with superior vital -manifestations. One of those lowly gelatinous forms, so transparent and -colourless as to be with difficulty distinguished from the water it floats -in, is not more like its medium in chemical, mechanical, optical, thermal, -and other properties, than it is in the passivity with which it submits to -all the influences and actions brought to bear upon it; while the mammal -does not more widely differ from inanimate things in these properties, than -it does in the activity with which it meets surrounding changes by -compensating changes in itself. And between these extremes, these two kinds -of contrast vary together. So that in proportion as an organism is -physically like its environment it remains a passive partaker of the -changes going on in its environment; while in proportion as it is endowed -with powers of counteracting such changes, it exhibits greater unlikeness -to its environment.[20] - -If now, from this same point of view, we consider the relation borne to its -environment by any superior organism in its successive stages, we find an -analogous series of contrasts. Of course in respect of degrees of -_structure_ the parallelism is complete. The difference, at first small, -between the little-structured germ and the little-structured inorganic -world, necessarily becomes greater, step by step, as the differentiations -of the germ become more numerous and definite. How of _form_ the like -holds is equally manifest. The sphere, which is the point of departure -common to all organisms, is the most generalized of figures; and one that -is, under various circumstances, assumed by inorganic matter. But as it -develops it loses all likeness to inorganic objects in the environment; and -eventually becomes distinct even from nearly all organic objects in its -environment. In _specific gravity_ the alteration, though not very marked, -is still in the same direction. Development being habitually accompanied by -a relative decrease in the quantity of water and an increase in the -quantity of constituents that are heavier than water, there results a small -augmentation of relative weight. In power of maintaining a _temperature_ -above that of surrounding things, the differentiation from the environment -that accompanies development is marked. All ova are absolutely dependent -for their heat on external sources. The mammalian young one is, during its -uterine life, dependent on the maternal heat; and at birth has but a -partial power of making good the loss by radiation. But as it advances in -development it gains an ability to maintain a constant temperature above -that of surrounding things: so becoming markedly unlike them. Lastly, in -_self-mobility_ this increasing contrast is no less decided. Save in a few -aberrant tribes, chiefly parasitic, we find the general fact to be that the -locomotive power, totally absent or very small at the outset, increases -with the advance towards maturity. The more highly developed the organism -becomes, the stronger grows the contrast between its activity and the -inertness of the objects amid which it moves. - -Thus we may say that the development of an individual organism, is at the -same time a differentiation of its parts from each other, and a -differentiation of the consolidated whole from the environment; and that in -the last as in the first respect, there is a general analogy between the -progression of an individual organism and the progression from the lowest -orders of organisms to the highest orders. It may be remarked that some -kinship seems to exist between these generalizations and the doctrine of -Schelling, that Life is the tendency to individuation. For evidently, in -becoming more distinct from one another and from their environment, -organisms acquire more marked individualities. As far as I can gather from -outlines of his philosophy, however, Schelling entertained this conception -in a general and transcendental sense, rather than in a special and -scientific one. - - -§ 54. Deductive interpretations of these general facts of development, in -so far as they are possible, must be postponed until we arrive at the -fourth and fifth divisions of this work. There are, however, one or two -general aspects of these inductions which may be here conveniently dealt -with deductively. - -Grant that each organism is at the outset relatively homogeneous and that -when complete it is relatively heterogeneous, and it necessarily follows -that development is a change from the homogeneous to the heterogeneous--a -change during which there must be gone through all the gradations of -heterogeneity that lie between these extremes. If, again, there is at first -indefiniteness and at last definiteness, the transition cannot but be from -the one to the other of these through all intermediate degrees of -definiteness. Further, if the parts, originally incoherent or uncombined, -eventually become relatively coherent or combined, there must be a -continuous increase of coherence or combination. Hence the general truth -that development is a change from incoherent, indefinite homogeneity, to -coherent, definite heterogeneity, becomes a self-evident one when -observation has shown us the state in which organisms begin and the state -in which they end. - -Just in the same way that the growth of an entire organism is carried on by -abstracting from the environment substances like those composing the -organism; so the production of each organ within the organism is carried on -by abstracting from the substances contained in the organism, those -required by this particular organ. Each organ at the expense of the -organism as a whole, integrates with itself certain kinds and proportions -of the matters circulating around it; in the same way that the organism as -a whole, integrates with itself certain kinds and proportions of matters at -the expense of the environment as a whole. So that the organs are -qualitatively differentiated from each other, in a way analogous to that by -which the entire organism is qualitatively differentiated from things -around it. Evidently this selective assimilation illustrates the general -truth, set forth and illustrated in _First Principles_, that like units -tend to segregate. It illustrates, moreover, the further aspect of this -general truth, that the pre-existence of a mass of certain units produces a -tendency for diffused units of the same kind to aggregate with this mass -rather than elsewhere. It has been shown of particular salts, A and B, -co-existing in a solution not sufficiently concentrated to crystallize, -that if a crystal of the salt A be put into the solution, it will increase -by uniting with itself the dissolved atoms of the salt A; and that -similarly, though there otherwise takes place no deposition of the salt B, -yet if a crystal of the salt B is placed in the solution, it will exercise -a coercive force on the diffused atoms of this salt, and grow at their -expense. Probably much organic assimilation occurs in the same way. -Particular parts of the organism are composed of special units or have the -function of secreting special units, which are ever present in them in -large quantities. The fluids circulating through the body contain special -units of this same order. And these diffused units are continually being -deposited along with the groups of like units that already exist. How -purely physical are the causes of this selective assimilation, is, indeed, -shown by the fact that abnormal constituents of the blood are segregated in -the same way. The chalky deposits of gout beginning at certain points, -collect more and more around those points. And similarly in numerous -pustular diseases. Where the component units of an organ, or some of them, -do not exist as such in the circulating fluids, but are formed out of -elements or compounds that exist separately in the circulating fluids, the -process of differential assimilation must be of a more complex kind. Still, -however, it seems not impossible that it is carried on in an analogous way. -If there be an aggregate of compound atoms, each of which contains the -constituents A, B, C; and if round this aggregate the constituents A and B -and C are diffused in uncombined states; it may be suspected that the -coercive force of these aggregated compound atoms A, B, C, may not only -bring into union with themselves adjacent compound atoms A, B, C, but may -cause the adjacent constituents A and B and C to unite into such compound -atoms, and then aggregate with the mass. - - - - -CHAPTER II^A. - -STRUCTURE.[21] - - -§ 54a. As, in the course of evolution, we rise from the smallest to the -largest aggregates by a process of integration, so do we rise by a process -of differentiation from the simplest to the most complex aggregates. The -initial types of life are at once extremely small and almost structureless. -Passing over those which swarm in the air, the water, and the soil, and are -now some of them found to be causes of diseases, we may set out with those -ordinarily called _Protozoa_ and _Protophyta_: the lowest of which, -however, are either at once plants and animals, or are now one and now the -other. - -That the first living things were minute portions of simple protoplasm is -implied by the general theory of Evolution; but we have no evidence that -such portions exist now. Even admitting that there are protoplasts (using -this word to include plant and animal types) which are without nuclei, -still they are not homogeneous--they are granular. Whether a nucleus is -always present is a question still undecided; but in any case the types -from which it is absent are extremely exceptional. Thus the most general -structural traits of protoplasts are--the possession of an internal part, -morphologically central though often not centrally situated, a general mass -of protoplasm surrounding it, and an inclosing differentiated portion in -contact with the environment. These essential elements are severally -subject to various complications. - -In some simple types the limiting layer or cortical substance can scarcely -be said to exist as a separate element. The exoplasm, distinguished from -the endoplasm by absence or paucity of granules, is continually changing -places with it by the sending out of pseudopodia which are presently drawn -back into the general mass: the inner and outer, being unsettled in -position, are not permanently differentiated. Then we have types, -exemplified by _Lithamoeba_, constituted of protoplasm covered by a -distinct pellicle, which in sundry groups becomes an outer shell of various -structure: now jelly-like, now of cellulose, now siliceous or calcareous. -While here this envelope has a single opening, there it is perforated all -over--a fenestrated shell. In some cases an external layer is formed of -agglutinated sand-particles; in others of imbricated plates, as in -Coccospheres; and in many others radiating spicules stand out on all sides. -Throughout sundry classes the exoplasm develops cilia, by the wavings of -which the creatures are propelled through the water--cilia which may be -either general or local. And then this cortical layer, instead of being -spherical or spheroidal, may become plano-spiral, cyclical, crosier-shaped, -and often many-chambered; whence there is a transition to colonies. - -Meanwhile the inclosed protoplasm, at first little more than a network or -foamwork containing granules and made irregular by objects drawn in as -nutriment, becomes variously complicated. In some low types its continuity -is broken by motionless, vacant spaces, but in higher types there are -contractile vacuoles slowly pulsing, and, as we may suppose, moving the -contained liquid hither and thither; while there are types having many -passive vacuoles along with a few active ones. In some varieties the -protruded parts, or pseudopodia, into which the protoplasm continually -shapes itself, are comparatively short and club-shaped; in others they are -long and fine filaments which anastomose, so forming a network running here -and there into little pools of protoplasm. Then there are kinds in which -the protoplasm streams up and down the protruding spicules: sometimes -inside of them, sometimes outside. Always, too, there is included in the -protoplasm a small body known as a centrosome. - -Lastly, we have the innermost element, considered the essential -element--the nucleus. According to Prof. Lankester, it is absent from -_Archerina_, and there are types in which it is made visible only by the -aid of special reagents. Ordinarily it is marked off from the surrounding -protoplasm by a delicate membrane, just as the protoplasm itself is marked -off by the exoplasm from the environment. Most commonly there is a single -nucleus, but occasionally there are many, and sometimes there is a chief -one with minor ones. Moreover, within the nucleus itself there have of late -years been discovered remarkable structural elements which undergo -complicated changes. - -These brief statements indicate only the most general traits of an immense -variety of structures--so immense a variety that Prof. Lankester, in -distinguishing the classes, sub-classes, orders, and genera in the briefest -way, occupies 37 quarto pages of small type. And to give a corresponding -account of _Protophyta_ would require probably something like equal space. -Thus these living things, so minute that unaided vision fails to disclose -them, constitute a world exhibiting varieties of structure which it -requires the devotion of a life to become fully acquainted with. - - -§ 54b. If higher forms of life have arisen from lower forms by evolution, -the implication is that there must once have existed, if there do not still -exist, transitional forms; and there follows the comment that there _do_ -still exist transitional forms. Both in the plant-world and in the -animal-world there are types in which we see little more than simple -assemblages of _Protophyta_ or of _Protozoa_--types in which the units, -though coherent, are not differentiated but constitute a uniform mass. In -treating of structure we are not here concerned with these unstructured -types, but may pass on to those aggregates of protoplasts which show us -differentiated parts--_Metaphyta_ and _Metazoa_: economizing space by -limiting our attention chiefly to the last. - -When, half a century ago, some currency was given to the statement that all -kinds of organisms, plant and animal, which our unaided eyes disclose, are -severally composed of myriads of living units, some of them partially, if -not completely, independent, and that thus a man is a vast nation of minute -individuals of which some are relatively passive and others relatively -active, the statement met, here with incredulity and there with a shudder. -But what was then thought a preposterous assertion has now come to be an -accepted truth. - -Along with gradual establishment of this truth has gone gradual -modification in the form under which it was originally asserted. If some -inhabitant of another sphere were to describe one of our towns as composed -exclusively of houses, saying nothing of the contained beings who had built -them and lived in them, we should say that he had made a profound error in -recognizing only the inanimate elements of the town and disregarding the -animate elements. Early histologists made an analogous error. Plants and -animals were found to consist of minute members, each of which appeared to -be simply a wall inclosing a cavity--a cell. But further investigation -proved that the content of the cell, presently distinguished as protoplasm, -is its essential living part, and that the cell-wall, when present, is -produced by it. Thus the unit of composition is a protoplast, usually -enclosed, with its contained nucleus and centrosome. - - -§ 54c. As above implied, the individualities of the units are not wholly -lost in the individuality of the aggregate, but continue, some of them, to -be displayed in various degrees: the great majority of them losing their -individualities more and more as the type of the aggregate becomes higher. - -In a slightly organized Metazoon like the sponge, the subordination is but -small. Only those members of the aggregate which, flattened and united -together, form the outer layer and those which become metamorphosed into -spicules, have entirely lost their original activities. Of the rest nearly -all, lining the channels which permeate the mass, and driving onwards the -contained sea-water by the motions of their whip-like appendages, -substantially retain their separate lives; and beyond these there exist in -the gelatinous substance lying between the inner and outer layers, which is -regarded as homologous with a mesoderm, amoeba-form protoplasts which move -about from place to place. - -Relations between the aggregate and the units which are in this case -permanent, are in other cases temporary: characterizing early stages of -embryonic development. For example, drawings of Echinoderm larvæ at an -early stage, show us the potential independence of all the cells forming -the blastosphere; for in the course of further development some of these -resume the primitive amoeboid state, migrate through the internal space, -and presently unite to form certain parts of the growing structures. But -with the progress of organization independence of this kind diminishes. - -Converse facts are presented after development has been completed; for with -the commencement of reproduction we everywhere see more or less resumption -of individual life among the units, or some of them. It is a trait of -transitional types between _Protozoa_ and _Metazoa_ to lead an aggregate -life as a plasmodium, and then for this to break up into its members, which -for a time lead individual lives as generative agents; and sundry low kinds -of plants possessing small amounts of structure, have generative -elements--zoospores and spermatozoids--which show us a return to unit life. -Nor, indeed, are we shown this only in the lowest plants; for it has -recently been found that in certain of the higher plants--even in -Phænogams--spermatozoids are produced. That is to say, the units resume -active lives at places where the controlling influence of the aggregate is -failing; for, as we shall hereafter see, places at which generation -commences answer to this description. - -These different kinds of evidence jointly imply that the individual lives -of the units are subordinate to the general life in proportion as this is -high. Where the organism is very inferior in type the unit-life remains -permanently conspicuous. In some superior types there is a display of -unit-life during embryonic stages in which the co-ordinating action of the -aggregate is but incipient. With the advance of development the unit-life -diminishes; but still, in plants, recommences where the disintegrating -process which initiates generation shows the coercive power of the -organization to have become small. - -Even in the highest types, however, and even when they are fully developed, -unit-life does not wholly disappear: it is clearly shown in ourselves. I do -not refer simply to the fact that, as throughout the animal kingdom at -large and a considerable part of the vegetal kingdom, the male generative -elements are units which have resumed the primitive independent life, but I -refer to a much more general fact. In that part of the organism which, -being fundamentally an aqueous medium, is in so far like the aqueous medium -in which ordinary protozoon life is carried on, we find an essentially -protozoon life. I refer of course to the blood. Whether the tendency of the -red corpuscles (which are originally developed from amoeba-like cells) to -aggregate into _rouleaux_ is to be taken as showing life in them, may be -left an open question. It suffices that the white corpuscles or leucocytes, -retaining the primitive amoeboid character, exhibit individual activities: -send out prolongations like pseudopodia, take in organic particles as food, -and are independently locomotive. Though far less numerous than the red -corpuscles, yet, as ten thousand are contained in a cubic millimetre of -blood--a mass less than a pin's head--it results that the human body is -pervaded throughout all its blood-vessels by billions of these separately -living units. In the lymph, too, which also fulfils the requirements of -liquidity, these amoeboid units are found. Then we have the curious -transitional stage in which units partially imbedded and partially free -display a partial unit-life. These are the ciliated epithelium-cells, -lining the air-passages and covering sundry of the mucous membranes which -have more remote connexions with the environment, and covering also the -lining membranes of certain main canals and chambers in the nervous system. -The inner parts of these unite with their fellows to form an epithelium, -and the outer parts of them, immersed either in liquid or semi-liquid -(mucus), bear cilia that are in constant motion and "produce a current of -fluid over the surface they cover:" thus simulating in their positions and -actions the cells lining the passages ramifying through a sponge. The -partially independent lives of these units is further seen in the fact that -after being detached they swim about in water for a time by the aid of -their cilia. - - -§ 54d. But in the _Metazoa_ and _Metaphyta_ at large, the associated units -are, with the exceptions just indicated, completely subordinated. The -unit-life is so far lost in the aggregate life that neither locomotion nor -the relative motion of parts remains; and neither in shape nor composition -is there resemblance to protozoa. Though in many cases the internal -protoplasm continues to carry on vital processes subserving the needs of -the aggregate, in others vital processes of an independent kind appear to -cease. - -It will naturally be supposed that after recognizing this fundamental trait -common to all types of organisms above the _Protozoa_ and _Protophyta_, the -next step in an account of structure must be a description of their organs, -variously formed and combined--if not in detail yet in their general -characters. This, however, is an error. There are certain truths of -structure higher in generality than any which can be alleged of organs. We -shall see this if we compare organs with one another. - -Here is a finger stiffened by its small bones and yet made flexible by the -uniting joints. There is a femur which helps its fellow to support the -weight of the body; and there again is a rib which, along with others, -forms a protective box for certain of the viscera. Dissection reveals a set -of muscles serving to straighten and bend the fingers, certain other -muscles that move the legs, and some inconspicuous muscles which, -contracting every two or three seconds, slightly raise the ribs and aid in -inflating the lungs. That is to say, fingers, legs, and chest possess -certain structures in common. There is in each case a dense substance -capable of resisting stress and a contractile substance capable of moving -the dense substance to which it is attached. Hence, then, we have first to -give an account of these and other chief elements which, variously joined -together, form the different organs: we have to observe the general -characters of _tissues_. - -On going back to the time when the organism begins with a single cell, then -becomes a spherical cluster of cells, and then exhibits differences in the -modes of aggregation of these cells, the first conspicuous rise of -structure (limiting ourselves to animals) is the formation of three layers. -Of these the first is, at the outset and always, the superficial layer in -direct contact with the environment. The second, being originally a part of -the first, is also in primitive types in contact with the environment, but, -being presently introverted, forms the rudiment of the food-cavity; or, -otherwise arising in higher types, is in contact with the yelk or food -provided by the parent. And the third, presently formed between these two, -consists at the outset of cells derived from them imbedded in an -intercellular substance of jelly-like consistence. Hence originate the -great groups classed as epithelium-tissue, connective tissue (including -osseous tissue), muscular tissue, nervous tissue. These severally contain -sub-kinds, each of which is a complex of differentiated cells. Being brief, -and therefore fitted for the present purposes, the sub-classification given -by Prof. R. Hertwig may here be quoted;-- - - "The physiological character of epithelia is given in the fact that they - cover the surfaces of the body, their morphological character in that - they consist of closely compressed cells united only by a cementing - substance. - - "According to their further functional character epithelia are divided - into glandular epithelia (unicellular and multicellular glands), sensory, - germinal, and pavement epithelia. - - "According to the structure are distinguished one-layered (cubical, - cylindrical, pavement epithelia) and many-layered epithelia, ciliated and - flagellated epithelia, epithelia with or without cuticle. - - "The physiological character of the connective tissues rests upon the - fact that they fill up spaces between other tissues in the interior of - the body. - - "The morphological character depends upon the presence of the - intercellular substance. - - "According to the quantity and the structure of the intercellular - substance the connective substances are divided into (1) cellular (with - little intercellular substance); (2) homogeneous; (3) fibrillar - connective tissue; (4) cartilage; (5) bone. - - "The physiological character of muscular tissue is contained in the - increased capacity for contraction. - - "The morphological character is found in the fact that the cells have - secreted muscle-substance. - - "According to the nature of the muscle-substance are distinguished smooth - and cross-striated muscle-fibres. - - "According to the character and derivation of the cells - (muscle-corpuscles) the musculature is divided into epithelial - (epithelial muscle-cells, primary bundles) and connective-tissue muscle - cells (contractile fibre-cells). - - "The physiological character of nervous tissue rests upon the - transmission of sensory stimuli and voluntary impulses, and upon the - co-ordination of these into unified psychic activity. - - "The conduction takes place by means of nerve-fibres (non-medullated and - medullated fibrils and bundles of fibrils); the co-ordination of stimuli - by means of ganglion-cells (bipolar, multipolar ganglion-cells)." - (_General Principles of Zoology_, pp. 117-8.) - -But now concerning cells out of which, variously modified, obscured, and -sometimes obliterated, tissues are formed, we have to note a fact of much -significance. Along with the cell-doctrine as at first held, when attention -was given to the cell itself rather than to its contents, there went the -belief that each of these morphological units is structurally separate from -its neighbours. But since establishment of the modern view that the -essential element is the contained protoplasm, histologists have discovered -that there are protoplasmic connexions between the contents of adjacent -cells. Though cursorily observed at earlier dates, it was not until some -twenty years ago that in plant-tissues these were clearly shown to pass -through openings in the cell-walls. It is said that in some cases the -openings are made, and the junctions established, by a secondary process; -but the implication is that usually these living links are left between -multiplying protoplasts; so that from the outset the protoplasm pervading -the whole plant maintains its continuity. More recently sundry zoologists -have alleged that a like continuity exists in animals. Especially has this -been maintained by Mr. Adam Sedgwick. Numerous observations made on -developing ova of fishes have led him to assert that in no case do the -multiplying cells so-called--blastomeres and their progeny--become entirely -separate. Their fission is in all cases incomplete. A like continuity has -been found in the embryos of many Arthropods, and more recently in the -segmenting eggs and blastulæ of Echinoderms. The _syncytium_ thus formed is -held by Mr. Sedgwick to be maintained in adult life, and in this belief he -is in agreement with sundry others. Bridges of protoplasm have been seen -between epithelium-cells, and it is maintained that cartilage-cells, -connective tissue cells, the cells forming muscle-fibres, as well as -nerve-cells, have protoplasmic unions. Nay, some even assert that an ovum -preserves a protoplasmic connexion with the matrix in which it develops. - -A corollary of great significance may here be drawn. It has been observed -that within a vegetal cell the strands of protoplasm stretched in this or -that direction contain moving granules, showing that the strands carry -currents. It has also been observed that when the fission of a protozoon is -so nearly complete that its two halves remain connected only by a thread, -currents of protoplasm move through this thread, now one way now the other. -The inference fairly to be drawn is that such currents pass also through -the strands which unite the protoplasts forming a tissue. What must happen? -So long as adjacent cells with their contents are subject to equal -pressures no tendency to redistribution of the protoplasm exists, and there -may then occur the action sometimes observed inside the strands within a -cell: currents with their contained granules moving in opposite directions. -But if the cells forming a portion of tissue are subject to greater -pressure than the cells around, their contained protoplasm must be forced -through the connecting threads into these surrounding cells. Every change -of pressure at every point must cause movements and counter-movements of -this kind. Now in the _Metazoa_ at large, or at least in all exhibiting -relative motions of parts, and especially in all which are capable of rapid -locomotion, such changes of pressure are everywhere and always taking -place. The contraction of a muscle, besides compressing its components, -compresses neighbouring tissues; and every instant contractions and -relaxations of muscles go on throughout the limbs and body during active -exertion. Moreover, each attitude--standing, sitting, lying down, turning -over--entails a different set of pressures, both of the parts on one -another and on the ground; and those partial arrests of motion which result -from sitting down the feet alternately when running, send jolts or waves of -varying pressure through the body. The vital actions, too, have kindred -effects. An inspiration alters the stress on the tissues throughout a -considerable part of the trunk, and a heart-beat propels, down to the -smallest arteries, waves which slightly strain the tissues at large. The -component cells, thus subject to mechanical disturbances, small and great, -perpetual and occasional, are ever having protoplasm forced into them and -forced out of them. There are gurgitations and regurgitations which, if -they do not constitute a circulation properly so called, at least imply an -unceasing redistribution. And the implication is that in the course of -days, weeks, months, years, each portion of protoplasm visits every part of -the body. - -Without here stating specifically the bearings of these inferences upon the -problems of heredity, it will be manifest that certain difficulties they -present are in a considerable degree diminished. - - -§ 54e. Returning from this parenthetical discussion to the subject of -structure, we have to observe that besides facts presented by tissues and -facts presented by organs, there are certain facts, less general than the -one and more general than the other, which must now be noted. In the order -of decreasing generality an account of organs should be preceded by an -account of systems of organs. Some of these, as the muscular system and the -osseous system, are co-extensive with tissues, but others of them are not. -The nervous system, for example, contains more than one kind of tissue and -is constituted of many different structures: besides afferent and efferent -nerves there are the ganglia immediately controlling the viscera, and there -are the spinal and cerebral masses, the last of which is divisible into -numerous unlike parts. Then we have the vascular system made up of the -heart, arteries, veins, and capillaries. The lymphatic system, too, with -its scattered glands and ramifying channels has to be named. And then, not -forgetting the respiratory system with its ancillary appliances, we have -the highly heterogeneous alimentary system; including a great number of -variously-constructed organs which work together. On contemplating these -systems we see their common character to be that while as wholes they -cooperate for the carrying on of the total life, each of them consists of -cooperative parts: there is cooperation within cooperation. - -There is another general aspect under which structures must be -contemplated. They are divisible into the universal and the -particular--those which are everywhere present and those which occupy -special places. The blood which a scratch brings out shows us that the -vascular system sends branches into each spot. The sensation accompanying a -scratch proves that the nervous system, too, has there some of its ultimate -fibrils. Unobtrusive, and yet to be found at every point, are the ducts of -the lymphatic system. And in all parts exists the connective tissue--an -inert tough substance which, running through interspaces, wraps up and -binds together the other tissues. As is implied by this description, these -structures stand in contrast with local structures. Here is a bone, there -is a muscle, in this place a gland, in that a sense-organ. Each has a -limited extent and a particular duty. But through every one of them ramify -branches of these universal structures. Every one of them has its arteries -and veins and capillaries, its nerves, its lymphatics, its connective -tissue. - -Recognition of this truth introduces what little has here to be said -concerning organs; for of course in a work limited to principles no -detailed account of these can be entered upon. This remainder truth is -that, different as they may be in the rest of their structures, all organs -are alike in certain of their structures. All are furnished with these -appliances for nutrition, depuration and excitation: they have all to be -sustained, all to be stimulated, all to be kept clean. It has finally to be -remarked that the general structures which pervade all the special -structures at the same time pervade one another. The universal nervous -system has everywhere ramifying through it the universal vascular system -which feeds it; and the universal vascular system is followed throughout -all its ramifications by special nerves which control it. The lymphatics -forming a drainage-system run throughout the other systems; and in each of -these universal systems is present the connective tissue holding their -parts in position. - - -§ 54f. So vast and varied a subject as organic structure, even though the -treatment of it is limited to the enunciation of principles, cannot, of -course, be dealt with in the space here assigned. Next to nothing has been -said about plant-structures, and in setting forth the leading traits of -animal-structures the illustrations given have been mostly taken from -highly-developed creatures. In large measure adumbration rather than -exposition is the descriptive word to be applied. - -Nevertheless the reader may carry away certain truths which, exemplified in -a few cases, are exemplified more or less fully in all cases. There is the -fundamental fact that the plants and animals with which we are -familiar--_Metaphyta_ and _Metazoa_--are formed by the aggregation of units -homologous with _Protozoa_. These units, often conspicuously showing their -homology in early embryonic stages, continue some of them to show it -throughout the lives of the highest type of _Metazoa_, which contain -billions of units carrying on a protozoon life. Of the protoplasts not thus -active the great mass, comparatively little transformed in low organisms, -become more and more transformed as the ascent to high organisms goes on; -so that, undergoing numerous kinds of metamorphoses, they lose all likeness -to their free homologues, both in shape and composition. The cell-contained -protoplasts thus variously changed are fused together into tissues in which -their individualities are practically lost; but they nevertheless remain -connected throughout by permeable strands of protoplasm. Arising by -complication of the outer and inner layers of the embryo and growing more -unlike as their units become more obscured, these tissues are formed into -systems, which develop into sets of organs. Some of the resulting -structures are localized and special but others are everywhere interfused. - -While the first named of these facts are displayed in every _Metazoon_, and -while the last named are visible only in _Metazoa_ of considerably -developed structures, a gradual transition is shown in intermediate kinds -of _Metazoa_. Of this transition it remains to say that it is effected by -the progressive development of auxiliary appliances. For example, the -primitive foot-cavity is a sac with one opening only; then comes a second -opening through which the waste-matter of the food is expelled. The -alimentary canal between these openings is at first practically uniform; -afterwards in a certain part of its wall arise numerous bile-cells; these -accumulating form a hollow prominence; and this, enlarging, becomes in -higher types a liver, while the hollow becomes its duct. In other gradual -ways are formed other appended glands. Meanwhile the canal itself has its -parts differentiated: one being limited to swallowing, another to -triturating, another to adding various solvents, another to absorbing the -prepared nutriment, another to ejecting the residue. Take again the visual -organ. The earliest form of it is a mere pigment-speck below the surface. -From this (saying nothing here of multiple eyes) we rise by successive -complications to a retina formed of multitudinous sensory elements, lenses -for throwing images upon it, a curtain for shutting out more or less light, -muscles for moving the apparatus about, others for adjusting its focus; -and, finally, added to these, either a nictitating membrane or eyelids for -perpetually wiping its surface, and a set of eyelashes giving notice when a -foreign body is dangerously near. This process of elaborating organs so as -to meet additional requirements by additional parts, is the process pursued -throughout the body at large. - -Of plant-structures, concerning which so little has been said, it may here -be remarked that their relative simplicity is due to the simplicity of -their relations to food. The food of plants is universally distributed, -while that of animals is dispersed. The immediate consequences are that in -the one case motion and locomotion are superfluous, while in the other case -they are necessary: the differences in the degrees of structure being -consequences. Recognizing the locomotive powers of minute _Algæ_ and the -motions of such other _Algæ_ as _Oscillatoria_, as well as those movements -of leaves and fructifying organs seen in some Phænogams, we may say, -generally, that plants are motionless; but that they can nevertheless carry -on their lives because they are bathed by the required nutriment in the air -and in the soil. Contrariwise, the nutriment animals require is distributed -through space in portions: in some cases near one another and in other -cases wide apart. Hence motion and locomotion are necessitated; and the -implication is that animals must have organs which render them possible. In -the first place there must be either limbs or such structures as those -which in fish, snakes, and worms move the body along. In the second place, -since action implies waste, there must be a set of channels to bring -repairing materials to the moving parts. In the third place there must be -an alimentary system for taking in and preparing these materials. In the -fourth place there must be organs for separating and excreting -waste-products. All these appliances must be more highly developed in -proportion as the required activity is greater. Then there must be an -apparatus for directing the motions and locomotions--a nervous system; and -as fast as these become rapid and complex the nervous system must be -largely developed, ending in great nervous centres--seats of intelligence -by which the activities at large are regulated. Lastly, underlying all the -structural contrasts between plants and animals thus originating, there is -the chemical contrast; since the necessity for that highly nitrogenous -matter of which animals are formed, is entailed by the necessity for -rapidly evolving the energy producing motion. So that, strange as it seems, -those chemical, physical, and mental characters of animals which so -profoundly distinguish them from plants, are all remote results of the -circumstance that their food is dispersed instead of being everywhere -present. - - - - -CHAPTER III. - -FUNCTION. - - -§ 55. Does Structure originate Function, or does Function originate -Structure? is a question about which there has been disagreement. Using the -word Function in its widest signification, as the totality of all vital -actions, the question amounts to this--does Life produce Organization, or -does Organization produce Life? - -To answer this question is not easy, since we habitually find the two so -associated that neither seems possible without the other; and they appear -uniformly to increase and decrease together. If it be said that the -arrangement of organic substances in particular forms, cannot be the -ultimate cause of vital changes, which must depend on the properties of -such substances; it may be replied that, in the absence of structural -arrangements, the forces evolved cannot be so directed and combined as to -secure that correspondence between inner and outer actions which -constitutes Life. Again, to the allegation that the vital activity of every -germ whence an organism arises, is obviously antecedent to the development -of its structures, there is the answer that such germ is not absolutely -structureless. - -But in truth this question is not determinable by any evidence now -accessible to us. The very simplest forms of life known (even the -non-nucleated, if there are any) consist of granulated protoplasm; and -granulation implies structure. Moreover since each kind of protozoon, even -the lowest, has its specific mode of development and specific -activity--even down to bacteria, some kinds of which, otherwise -indistinguishable, are distinguishable by their different reactions on -their media--we are obliged to conclude that there must be constitutional -differences between the protoplasms they consist of, and this implies -structural differences. It seems that structure and function must have -advanced _pari passu_: some difference of function, primarily determined by -some difference of relation to the environment, initiating a slight -difference of structure, and this again leading to a more pronounced -difference of function; and so on through continuous actions and reactions. - - -§ 56. Function falls into divisions of several kinds according to our point -of view. Let us take these divisions in the order of their simplicity. - -Under Function in its widest sense, are included both the statical and the -dynamical distributions of force which an organism opposes to the forces -brought to bear on it. In a tree the woody core of trunk and branches, and -in an animal the skeleton, internal or external, may be regarded as -passively resisting the gravity and momentum which tend habitually or -occasionally to derange the requisite relations between the organism and -its environment; and since they resist these forces simply by their -cohesion, their functions may be classed as _statical_. Conversely, the -leaves and sap-vessels in a tree, and those organs which in an animal -similarly carry on nutrition and circulation, as well as those which -generate and direct muscular motion, must be considered as _dynamical_ in -their actions. From another point of view Function is divisible into the -_accumulation of energy_ (latent in food); the _expenditure of energy_ -(latent in the tissues and certain matters absorbed by them); and the -_transfer of energy_ (latent in the prepared nutriment or blood) from the -parts which accumulate to the parts which expend. In plants we see little -beyond the first of these: expenditure being comparatively slight, and -transfer required mainly to facilitate accumulation. In animals the -function of _accumulation_ comprehends those processes by which the -materials containing latent energy are taken in, digested, and separated -from other materials; the function of _transfer_ comprehends those -processes by which these materials, and such others as are needful to -liberate the energies they contain, are conveyed throughout the organism; -and the function of _expenditure_ comprehends those processes by which the -energy is liberated from these materials and transformed into properly -co-ordinated motions. Each of these three most general divisions includes -several more special divisions. The accumulation of energy may be separated -into _alimentation_ and _aeration_; of which the first is again separable -into the various acts gone through between prehension of food and the -transformation of part of it into blood. By the transfer of energy is to be -understood what we call _circulation_; if the meaning of circulation be -extended to embrace the duties of both the vascular system and the -lymphatics. Under the head of expenditure of energy come _nervous actions_ -and _muscular actions_: though not absolutely co-extensive with expenditure -these are almost so. Lastly, there are the subsidiary functions which do -not properly fall within any of these general functions, but subserve them -by removing the obstacles to their performance: those, namely, of -_excretion_ and _exhalation_, whereby waste products are got rid of. Again, -disregarding their purposes and considering them analytically, the general -physiologist may consider functions in their widest sense as the -correlatives of tissues--the actions of epidermic tissue, cartilaginous -tissue, elastic tissue, connective tissue, osseous tissue, muscular tissue, -nervous tissue, glandular tissue. Once more, physiology in its concrete -interpretations recognizes special functions as the ends of special -organs--regards the teeth as having the office of mastication; the heart as -an apparatus to propel blood; this gland as fitted to produce one requisite -secretion and that to produce another; each muscle as the agent of a -particular motion; each nerve as the vehicle of a special sensation or a -special motor impulse. - -It is clear that dealing with Biology only in its larger aspects, -specialities of function do not concern us; except in so far as they serve -to illustrate, or to qualify, its generalities. - - -§ 57. The first induction to be here set down is a familiar and obvious -one; the induction, namely, that complexity of function is the correlative -of complexity of structure. The leading aspects of this truth must be -briefly noted. - -Where there are no distinctions of structure there are no distinctions of -function. A Rhizopod will serve as an illustration. From the outside of -this creature, which has not even a limiting membrane, there are protruded -numerous processes. Originating from any point of the surface, each of -these may contract again and disappear, or it may touch some fragment of -nutriment which it draws with it, when contracting, into the general -mass--thus serving as hand and mouth; or it may come in contact with its -fellow-processes at a distance from the body and become confluent with -them; or it may attach itself to an adjacent fixed object, and help by its -contraction to draw the body into a new position. In brief, this speck of -animated jelly is at once all stomach, all skin, all mouth, all limb, and -doubtless, too, all lung. In organisms having a fixed distribution of parts -there is a concomitant fixed distribution of actions. Among plants we see -that when, instead of a uniform tissue like that of many _Algæ_, everywhere -devoted to the same process of assimilation, there arise, as in the higher -plants, root and stem and leaves, there arise correspondingly unlike -processes. Still more conspicuously among animals do there result varieties -of function when the originally homogeneous mass is replaced by -heterogeneous organs; since, both singly and by their combinations, -modified parts generate modified changes. Up to the highest organic types -this dependence continues manifest; and it may be traced not only under -this most general form, but also under the more special form that in -animals having one set of functions developed to more than usual -heterogeneity there is a correspondingly heterogeneous apparatus devoted to -them. Thus among birds, which have more varied locomotive powers than -mammals, the limbs are more widely differentiated; while the higher -mammals, which rise to more numerous and more involved adjustments of inner -to outer relations than birds, have more complex nervous systems. - - -§ 58. It is a generalization almost equally obvious with the last, that -functions, like structures, arise by progressive differentiations. Just as -an organ is first an indefinite rudiment, having nothing but some most -general characteristic in common with the form it is ultimately to take; so -a function begins as a kind of action that is like the kind of action it -will eventually become, only in a very vague way. And in functional -development, as in structural development, the leading trait thus early -manifested is followed successively by traits of less and less importance. -This holds equally throughout the ascending grades of organisms and -throughout the stages of each organism. Let us look at cases: confining our -attention to animals, in which functional development is better displayed -than in plants. - -The first differentiation established separates the two -fundamentally-opposed functions above named--the accumulation of energy and -the expenditure of energy. Passing over the _Protozoa_ (among which, -however, such tribes as present fixed distributions of parts show us -substantially the same thing), and commencing with the lowest -_Coelenterata_, where definite tissues make their appearance, we observe -that the only large functional distinction is between the endoderm, which -absorbs nutriment, and the ectoderm which, by its own contractions and -those of the tentacles it bears, produces motion: the contractility being -however to some extent shared by the endoderm. That the functions of -accumulation and expenditure are here very incompletely distinguished, may -be admitted without affecting the position that this is the first -specialization which begins to appear. These two most general and most -radically-opposed functions become in the _Polyzoa_, much more clearly -marked-off from each other: at the same time that each of them becomes -partially divided into subordinate functions. The endoderm and ectoderm are -no longer merely the inner and outer walls of the same simple sac into -which the food is drawn: but the endoderm forms a true alimentary canal, -separated from the ectoderm by a peri-visceral cavity, containing the -nutritive matters absorbed from the food. That is to say, the function of -accumulating force is exercised by a part distinctly divided from the part -mainly occupied in expending force: the structure between them, full of -absorbed nutriment, effecting in a vague way that transfer of force which, -at a higher stage of evolution, becomes a third leading function. -Meanwhile, the endoderm no longer discharges the accumulative function in -the same way throughout its whole extent; but its different portions, -oesophagus, stomach and intestine, perform different portions of this -function. And instead of a contractility uniformly diffused through the -ectoderm, there have arisen in the intermediate mesoderm some parts which -have the office of contracting (muscles), and some parts which have the -office of making them contract (nerves and ganglia). As we pass upwards, -the transfer of force, hitherto effected quite incidentally, comes to have -a special organ. In the ascidian, circulation is produced by a muscular -tube, open at both ends, which, by a wave of contraction passing along it, -sends out at one end the nutrient fluid drawn in at the other; and which, -having thus propelled the fluid for a time in one direction, reverses its -movement and propels it in the opposite direction. By such means does this -rudimentary heart generate alternating currents in the nutriment occupying -the peri-visceral cavity. How the function of transferring energy, thus -vaguely indicated in these inferior forms, comes afterwards to be the -definitely-separated office of a complicated apparatus made up of many -parts, each of which has a particular portion of the general duty, need not -be described. It is sufficiently manifest that this general function -becomes more clearly marked-off from the others, at the same time that it -becomes itself parted into subordinate functions. - -In a developing embryo, the functions or more strictly the structures which -are to perform them, arise in the same general order. A like primary -distinction very early appears between the endoderm and the ectoderm--the -part which has the office of accumulating energy, and the part out of which -grow those organs that are the great expenders of energy. Between these two -there presently arises the mesoderm in which becomes visible the rudiment -of that vascular system, which has to fulfil the intermediate duty of -transferring energy. Of these three general functions, that of accumulating -energy is carried on from the outset: the endoderm, even while yet -incompletely differentiated from the ectoderm, absorbs nutritive matters -from the subjacent yelk. The transfer of energy is also to some extent -effected by the rudimentary vascular system, as soon as its central cavity -and attached vessels are sketched out. But the expenditure of energy (in -the higher animals at least) is not appreciably displayed by those -ectodermic and mesodermic structures that are afterwards to be mainly -devoted to it: there is no sphere for the actions of these parts. Similarly -with the chief subdivisions of these fundamental functions. The distinction -first established separates the office of transforming other energy into -mechanical motion, from the office of liberating the energy to be so -transformed. While in the layer between endoderm and ectoderm are arising -the rudiments of the muscular system, there is marked out in the ectoderm -the rudiment of the nervous system. This indication of structures which are -to share between them the general duty of expending energy, is soon -followed by changes that foreshadow further specializations of this general -duty. In the incipient nervous system there begins to arise that contrast -between the cerebral mass and the spinal cord, which, in the main, answers -to the division of nervous actions into directive and executive; and, at -the same time, the appearance of vertebral laminæ foreshadows the -separation of the osseous system, which has to resist the strains of -muscular action, from the muscular system, which, in generating motion, -entails these strains. Simultaneously there have been going on similar -actual and potential specializations in the functions of accumulating -energy and transferring energy. And throughout all subsequent phases the -method is substantially the same. - -This progress from general, indefinite, and simple kinds of action to -special, definite, and complex kinds of action, has been aptly termed by -Milne-Edwards, "the physiological division of labour." Perhaps no metaphor -can more truly express the nature of this advance from vital activity in -its lowest forms to vital activity in its highest forms. And probably the -general reader cannot in any other way obtain so clear a conception of -functional development in organisms, as he can by tracing out functional -development in societies: noting how there first comes a distinction -between the governing class and the governed class; how while in the -governing class there slowly grow up such differences of duty as the civil, -military, and ecclesiastical, there arise in the governed class fundamental -industrial differences like those between agriculturists and artizans; and -how there is a continual multiplication of such specialized occupations and -specialized shares of each occupation. - - -§ 59. Fully to understand this change from homogeneity of function to -heterogeneity of function, which accompanies the change from homogeneity of -structure to heterogeneity of structure, it is needful to contemplate it -under a converse aspect. Standing alone, the above exposition conveys an -idea that is both inadequate and erroneous. The divisions and subdivisions -of function, becoming definite as they become multiplied, do not lead to a -more and more complete independence of functions; as they would do were the -process nothing beyond that just described; but by a simultaneous process -they are rendered more mutually dependent. While in one respect they are -separating from each other, they are in another respect combining with each -other. At the same time that they are being differentiated they are also -being integrated. Some illustrations will make this plain. - -In animals which display little beyond the primary differentiation of -functions, the activity of that part which absorbs nutriment or accumulates -energy, is not immediately bound up with the activity of that part which, -in producing motion, expends energy. In the higher animals, however, the -performance of the alimentary functions depends on the performance of -various muscular and nervous functions. Mastication and swallowing are -nervo-muscular acts; the rhythmical contractions of the stomach and the -allied vermicular motions of the intestines, result from the reflex -stimulation of certain muscular coats caused by food; the secretion of the -several digestive fluids by their respective glands, is due to nervous -excitation of them; and digestion, besides requiring these special aids, is -not properly performed in the absence of a continuous discharge of energy -from the great nervous centres. Again, the function of transferring -nutriment or latent energy, from part to part, though at first not closely -connected with the other functions, eventually becomes so. The short -contractile tube which propels backwards and forwards the blood contained -in the peri-visceral cavity of an ascidian, is neither structurally nor -functionally much entangled with the creature's other organs. But on -passing upwards through higher types, in which this simple tube is replaced -by a system of branched tubes, that deliver their contents through their -open ends into the tissues at distant parts; and on coming to those -advanced types which have closed arterial and venous systems, ramifying -minutely in every corner of every organ; we find that the vascular -apparatus, while it has become structurally interwoven with the whole body, -has become unable properly to fulfil its office without the help of offices -that are quite separated from its own. The heart, though mainly automatic -in its actions, is controlled by the nervous system, which takes a share in -regulating the contractions both of the heart and the arteries. On the due -discharge of the respiratory function, too, the function of circulation is -directly dependent: if the aeration of the blood is impeded the vascular -activity is lowered; and arrest of the one very soon causes stoppage of the -other. Similarly with the duties of the nervo-muscular system. Animals of -low organization, in which the differentiation and integration of the vital -actions have not been carried far, will move about for a considerable time -after being eviscerated, or deprived of those appliances by which energy is -accumulated and transferred. But animals of high organization are instantly -killed by the removal of these appliances, and even by the injury of minor -parts of them: a dog's movements are suddenly brought to an end, by cutting -one of the main canals along which the materials that evolve movements are -conveyed. Thus while in well-developed creatures the distinction of -functions is very marked, the combination of functions is very close. From -instant to instant the aeration of blood implies that certain respiratory -muscles are being made to contract by nervous impulses passing along -certain nerves; and that the heart is duly propelling the blood to be -aerated. From instant to instant digestion proceeds only on condition that -there is a supply of aerated blood, and a due current of nervous energy -through the digestive organs. That the heart of a mammal may act, its -muscle substance must be continuously fed with an abundant supply of -arterial blood. - -It is not easy to find an adequate expression for this double -re-distribution of functions. It is not easy to realize a transformation -through which the functions thus become in one sense separated and in -another sense combined, or even interfused. Here, however, as before, an -analogy drawn from social organization helps us. If we observe how the -increasing division of labour in societies is accompanied by a closer -co-operation; and how the agencies of different social actions, while -becoming in one respect more distinct, become in another respect more -minutely ramified through one another; we shall understand better the -increasing physiological co-operation that accompanies increasing -physiological division of labour. Note, for example, that while local -divisions and classes of the community have been growing unlike in their -several occupations, the carrying on of their several occupations has been -growing dependent on the due activity of that vast organization by which -sustenance is collected and diffused. During the early stages of social -development, every small group of people, and often every family, obtained -separately its own necessaries; but now, for each necessary, and for each -superfluity, there exists a combined body of wholesale and retail -distributors, which brings its branched channels of supply within reach of -all. While each citizen is pursuing a business that does not immediately -aim at the satisfaction of his personal wants, his personal wants are -satisfied by a general agency which brings from all places commodities for -him and his fellow-citizens--an agency which could not cease its special -duties for a few days, without bringing to an end his own special duties -and those of most others. Consider, again, how each of these differentiated -functions is everywhere pervaded by certain other differentiated functions. -Merchants, manufacturers, wholesale distributors of their several species, -together with lawyers, bankers, &c., all employ clerks. In clerks we have a -specialized class dispersed through various other classes; and having its -function fused with the different functions of these various other classes. -Similarly commercial travellers, though having in one sense a separate -occupation, have in another sense an occupation forming part of each of the -many occupations which it aids. As it is here with the sociological -division of labour, so is it with the physiological division of labour -above described. Just as we see in an advanced community, that while the -magisterial, the clerical, the medical, the legal, the manufacturing, and -the commercial activities, have grown distinct, they have yet their -agencies mingled together in every locality; so in a developed organism, we -see that while the general functions of circulation, secretion, absorption, -excretion, contraction, excitation, &c., have become differentiated, yet -through the ramifications of the systems apportioned to them, they are -closely combined with one another in every organ. - - -§ 60. The physiological division of labour is usually not carried so far as -wholly to destroy the primary physiological community of labour. As in -societies the adaptation of special classes to special duties, does not -entirely disable these classes from performing one another's duties on an -emergency; so in organisms, tissues and structures that have become fitted -to the particular offices they have ordinarily to discharge, often remain -partially able to discharge other offices. It has been pointed out by Dr. -Carpenter, that "in cases where the different functions are highly -specialized, the general structure retains, more or less, the primitive -community of function which originally characterized it." A few instances -will bring home this generalization. - -The roots and leaves of plants are widely differentiated in their -functions: by the roots, water and mineral substances are absorbed; while -the leaves take in, and decompose, carbonic acid. Nevertheless, by many -botanists it is held that some leaves, or parts of them, can absorb water; -and in what are popularly called "air-plants," or at any rate in some kinds -of them, the absorption of water is mainly and in some cases wholly carried -on by them and by the stems. Conversely, the underground parts can -partially assume the functions of leaves. The exposed tuber of a potato -develops chlorophyll on its surface, and in other cases, as in that of the -turnip, roots, properly so called, do the like. In trees the trunks, which -have in great measure ceased to produce buds, recommence producing them if -the branches are cut off; sometimes aerial branches send down roots to the -earth; and under some circumstances the roots, though not in the habit of -developing leaf-bearing organs, send up numerous suckers. When the -excretion of bile is arrested, part goes to the skin and some to the -kidneys, which presently suffer under their new task. Various examples of -vicarious functions may be found among animals. The excretion of carbonic -acid and absorption of oxygen are mainly performed by the lungs, in -creatures which have lungs; but in such creatures there continues a certain -amount of cutaneous respiration, and in soft-skinned batrachians like the -frog, this cutaneous respiration is important. Again, when the kidneys are -not discharging their duties a notable quantity of urea is got rid of by -perspiration. Other instances are supplied by the higher functions. In man -the limbs, which among lower vertebrates are almost wholly organs of -locomotion, are specialized into organs of locomotion and organs of -manipulation. Nevertheless, the human arms and legs do, when needful, -fulfil, to some extent, each other's offices. Not only in childhood and old -age are the arms used for purposes of support, but on occasions of -emergency, as when mountaineering, they are used by men in full vigour. And -that legs are to a considerable degree capable of performing the duties of -arms, is proved by the great amount of manipulatory skill reached by them -when the arms are absent. Among the perceptions, too, there are examples of -partial substitution. The deaf Dr. Kitto described himself as having become -excessively sensitive to vibrations propagated through the body; and as so -having gained the power of perceiving, through his general sensations, -those neighbouring concussions of which the ears ordinarily give notice. -Blind people make hearing perform, in part, the office of vision. Instead -of identifying the positions and sizes of neighbouring objects by the -reflection of light from their surfaces, they do this in a rude way by the -reflection of sound from their surfaces. - -We see, as we might expect to see, that this power of performing more -general functions, is great in proportion as the organs have been but -little adapted to their special functions. Those parts of plants which show -so considerable an ability to discharge each others' offices, are not -widely unlike in their minute structures. And the tissues which in animals -are to some extent mutually vicarious, are tissues in which the original -cellular composition is still conspicuous. But we do not find evidence that -the muscular, nervous, or osseous tissues are able in any degree to perform -those processes which the less differentiated tissues perform. Nor have we -any proof that nerve can partially fulfil the duty of muscle, or muscle -that of nerve. We must say, therefore, that the ability to resume the -primordial community of function, varies inversely as the established -specialization of function; and that it disappears when the specialization -of function becomes great. - - -§ 61. Something approaching to _a priori_ reasons may be given for the -conclusions thus reached _a posteriori_. They must be accepted for as much -as they seem worth. - -It may be argued that on the hypothesis of Evolution, Life necessarily -comes before organization. On this hypothesis, organic matter in a state of -homogeneous aggregation must precede organic matter in a state of -heterogeneous aggregation. But since the passing from a structureless state -to a structured state, is itself a vital process, it follows that vital -activity must have existed while there was yet no structure: structure -could not else arise. That function takes precedence of structure, seems -also implied in the definition of Life. If Life is shown by inner actions -so adjusted as to balance outer actions--if the implied energy is the -_substance_ of Life while the adjustment of the actions constitutes its -_form_; then may we not say that the actions to be formed must come before -that which forms them--that the continuous change which is the basis of -function, must come before the structure which brings function into shape? -Or again, since in all phases of Life up to the highest, every advance is -the effecting of some better adjustment of inner to outer actions; and -since the accompanying new complexity of structure is simply a means of -making possible this better adjustment; it follows that the achievement of -function is, throughout, that for which structure arises. Not only is this -manifestly true where the modification of structure results by reaction -from modification of function; but it is also true where a modification of -structure otherwise produced, apparently initiates a modification of -function. For it is only when such so-called spontaneous modification of -structure subserves some advantageous action, that it is permanently -established. If it is a structural modification that happens to facilitate -the vital activities, "natural selection" retains and increases it; but if -not, it disappears. - -The connexion which we noted between heterogeneity of structure and -heterogeneity of function--a connexion made so familiar by experience as to -appear scarcely worth specifying--is clearly a necessary one. It follows -from the general truth that in proportion to the heterogeneity of any -aggregate, is the heterogeneity it will produce in any incident force -(_First Principles_, § 156). The energy continually liberated in the -organism by decomposition, is here the incident force; the functions are -the variously modified forms produced in its divisions by the organs they -pass through; and the more multiform the organs the more multiform must be -the differentiations of the force passing through them. - -It follows obviously from this, that if structure progresses from the -homogeneous, indefinite, and incoherent, to the heterogeneous, definite, -and coherent, so too must function. If the number of different parts in an -aggregate must determine the number of differentiations produced in the -energies passing through it--if the distinctness of these parts from one -another, must involve distinctness in their reactions, and therefore -distinctness between the divisions of the differentiated energy; there -cannot but be a complete parallelism between the development of structure -and the development of function. If structure advances from the simple and -general to the complex and special, function must do the same. - - - - -CHAPTER IV. - -WASTE AND REPAIR. - - -§ 62. Throughout the vegetal kingdom, the processes of Waste and Repair are -comparatively insignificant in their amounts. Though all parts of plants -save the leaves, or other parts which are green, give out carbonic acid; -yet this carbonic acid, assuming it to indicate consumption of tissue, or -rather of the protoplasm contained in the tissue, indicates but a small -consumption. Of course if there is little waste there can be but little -repair--that is, little of the interstitial repair which restores the -integrity of parts worn by functional activity. Nor, indeed, is there -displayed by plants in any considerable degree, if at all, that other -species of repair which consists in the restoration of lost or injured -organs. Torn leaves and the shoots that are shortened by the pruner, do not -reproduce their missing parts; and though when the branch of a tree is cut -off close to the trunk, the place is in course of years covered over, it is -not by any reparative action in the wounded surface but by the lateral -growth of the adjacent bark. Hence, without saying that Waste and Repair do -not go on at all in plants, we may fitly pass them over as of no -importance. - -There are but slight indications of waste in those lower orders of animals -which, by their comparative inactivity, show themselves least removed from -vegetal life. Actiniæ kept in an aquarium, do not appreciably diminish in -bulk from prolonged abstinence. Even fish, though much more active than -most other aquatic creatures, appear to undergo but little loss of -substance when kept unfed during considerable periods. Reptiles, too, -maintaining no great temperature, and passing their lives mostly in a state -of torpor, suffer but little diminution of mass by waste. When, however, we -turn to those higher orders of animals which are active and hot-blooded, we -see that waste is rapid: producing, when unchecked, a notable decrease in -bulk and weight, ending very shortly in death. Besides finding that waste -is inconsiderable in creatures which produce but little insensible and -sensible motion, and that it becomes conspicuous in creatures which produce -much insensible and sensible motion; we find that in the same creatures -there is most waste when most motion is generated. This is clearly proved -by hybernating animals. "Valentin found that the waking marmot excreted in -the average 75 times more carbonic acid, and inhaled 41 times more oxygen -than the same animal in the most complete state of hybernation. The stages -between waking and most profound hybernation yielded intermediate figures. -A waking hedgehog yielded about 20.5 times more carbonic acid, and consumed -18.4 times more oxygen than one in the state of hybernation."[22] If we -take these quantities of absorbed oxygen and excreted carbonic acid, as -indicating something like the relative amounts of consumed organic -substance, we see that there is a striking contrast between the waste -accompanying the ordinary state of activity, and the waste accompanying -complete quiescence and reduced temperature. This difference is still more -definitely shown by the fact, that the mean daily loss from starvation in -rabbits and guinea-pigs, bears to that from hybernation, the proportion of -18.3:1. Among men and domestic animals, the relation between degree of -waste and amount of expended energy, though one respecting which there is -little doubt, is less distinctly demonstrable; since waste is not allowed -to go on uninterfered with. We have, however, in the lingering lives of -invalids who are able to take scarcely any nutriment but are kept warm and -still, an illustration of the extent to which waste diminishes as the -expenditure of energy declines. - -Besides the connexion between the waste of the organism as a whole and the -production of sensible and insensible motion by the organism as a whole, -there is a traceable connexion between the waste of special parts and the -activities of such special parts. Experiments have shown that "the starving -pigeon daily consumes in the average 40 times more muscular substance that -the marmot in the state of torpor, and only 11 times more fat, 33 times -more of the tissue of the alimentary canal, 18.3 times more liver, 15 times -more lung, 5 times more skin." That is to say, in the hybernating animal -the parts least consumed are the almost totally quiescent motor-organs, and -the part most consumed is the hydro-carbonaceous deposit serving as a store -of energy; whereas in the pigeon, similarly unsupplied with food but awake -and active, the greatest loss takes place in the motor-organs. The -relation between special activity and special waste, is illustrated, too, -in the daily experiences of all: not indeed in the amount of decrease of -the active parts in bulk or weight, for this we have no means of -ascertaining; but in the diminished ability of such parts to perform their -functions. That legs exerted for many hours in walking and arms long -strained in rowing, lose their powers--that eyes become enfeebled by -reading or writing without intermission--that concentrated attention, -unbroken by rest, so prostrates the brain as to incapacitate it for -thinking; are familiar truths. And though we have no direct evidence to -this effect, there is little danger in concluding that muscles exercised -until they ache or become stiff, and nerves of sense rendered weary or -obtuse by work, are organs so much wasted by action as to be partially -incompetent. - -Repair is everywhere and always making up for waste. Though the two -processes vary in their relative rates both are constantly going on. Though -during the active, waking state of an animal waste is in excess of repair, -yet repair is in progress; and though during sleep repair is in excess of -waste, yet some waste is necessitated by the carrying on of certain -never-ceasing functions. The organs of these never-ceasing functions -furnish, indeed, the most conclusive proofs of the simultaneity of repair -and waste. Day and night the heart never stops beating, but only varies in -the rapidity and vigour of its beats; and hence the loss of substance which -its contractions from moment to moment entail, must from moment to moment -be made good. Day and night the lungs dilate and collapse; and the muscles -which make them do this must therefore be kept in a state of integrity by a -repair which keeps pace with waste, or which alternately falls behind and -gets in advance of it to a very slight extent. - -On a survey of the facts we see, as we might expect to see, that the -progress of repair is most rapid when activity is most reduced. Assuming -that the organs which absorb and circulate nutriment are in proper order, -the restoration of the body to a state of integrity, after the -disintegration consequent on expenditure of energy, is proportionate to the -diminution in expenditure of energy. Thus we all know that those who are in -health, feel the greatest return of vigour after profound sleep--after -complete cessation of motion. We know that a night during which the -quiescence, bodily and mental, has been less decided, is usually not -followed by that spontaneous overflow of energy which indicates a high -state of efficiency throughout the organism. We know, again, that -long-continued recumbency, even with wakefulness (providing the wakefulness -is not the result of disorder), is followed by a certain renewal of -strength; though a renewal less than that which would have followed the -greater inactivity of slumber. We know, too, that when exhausted by labour, -sitting brings a partial return of vigour. And we also know that after the -violent exertion of running, a lapse into the less violent exertion of -walking, results in a gradual disappearance of that prostration which the -running produced. This series of illustrations conclusively proves that the -rebuilding of the organism is ever making up for the pulling down of it -caused by action; and that the effect of this rebuilding becomes more -manifest, in proportion as the pulling down is less rapid. From each -digested meal there is every few hours absorbed into the mass of prepared -nutriment circulating through the body, a fresh supply of the needful -organic compounds; and from the blood, thus occasionally re-enriched, the -organs through which it passes are ever taking up materials to replace the -materials used up in the discharge of functions. During activity the -reintegration falls in arrear of the disintegration; until, as a -consequence, there presently comes a general state of functional languor; -ending, at length, in a quiescence which permits the reintegration to -exceed the disintegration, and restore the parts to their state of -integrity. Here, as wherever there are antagonistic actions, we see -rhythmical divergences on opposite sides of the medium state--changes which -equilibrate each other by their alternate excesses. (_First Principles_, -§§ 85, 173.) - -Illustrations are not wanting of special repair that is similarly ever in -progress, and similarly has intervals during which it falls below waste and -rises above it. Every one knows that a muscle, or a set of muscles, -continuously strained, as by holding out a weight at arm's length, soon -loses its power; and that it recovers its power more or less fully after a -short rest. The several organs of the special sensations yield us like -experiences. Strong tastes, powerful odours, loud sounds, temporarily unfit -the nerves impressed by them for appreciating faint tastes, odours, or -sounds; but these incapacities are remedied by brief intervals of repose. -Vision still better illustrates this simultaneity of waste and repair. -Looking at the Sun so affects the eyes that, for a short time, they cannot -perceive the things around with the usual clearness. After gazing at a -bright light of a particular colour, we see, on turning the eyes to -adjacent objects, an image of the complementary colour; showing that the -retina has, for the moment, lost the power to feel small amounts of those -rays which have strongly affected it. Such inabilities disappear in a few -seconds or a few minutes, according to circumstances. And here, indeed, we -are introduced to a conclusive proof that special repair is ever -neutralizing special waste. For the rapidity with which the eyes recover -their sensitiveness, varies with the reparative power of the individual. In -youth the visual apparatus is so quickly restored to its state of -integrity, that many of these _photogenes_, as they are called, cannot be -perceived. When sitting on the far side of a room, and gazing out of the -window against a light sky, a person who is debilitated by disease or -advancing years, perceives, on transferring the gaze to the adjacent wall, -a momentary negative image of the window--the sash-bars appearing light and -the squares dark; but a young and healthy person has no such experience. -With a rich blood and vigorous circulation, the repair of the visual nerves -after impressions of moderate intensity, is nearly instantaneous. - -Function carried to excess may produce waste so great that repair cannot -make up for it during the ordinary daily periods of rest; and there may -result incapacities of the over-taxed organs, lasting for considerable -periods. We know that eyes strained by long-continued minute work lose -their power for months or years: perhaps suffering an injury from which -they never wholly recover. Brains, too, are often so unduly worked that -permanent relaxation fails to restore them to vigour. Even of the motor -organs the like holds. The most frequent cause of what is called "wasting -palsy," or atrophy of the muscles, is habitual excess of exertion: the -proof being that the disease occurs most frequently among those engaged in -laborious handicrafts, and usually attacks first the muscles which have -been most worked. - -There has yet to be noticed another kind of repair--that, namely, by which -injured or lost parts are restored. Among the _Hydrozoa_ it is common for -any portion of the body to reproduce the rest; even though the rest to be -so reproduced is the greater part of the whole. In the more -highly-organized _Actinozoa_ the half of an individual will grow into a -complete individual. Some of the lower Annelids, as the _Nais_, may be cut -into thirty or forty pieces and each piece will eventually become a perfect -animal. As we ascend to higher forms we find this reparative power much -diminished, though still considerable. The reproduction of a lost claw by a -lobster or crab, is a familiar instance. Some of the inferior _Vertebrata_ -also, as lizards, can develop new limbs or new tails, in place of those -which have been cut off; and can even do this several times over, though -with decreasing completeness. The highest animals, however, thus repair -themselves to but a very small extent. Mammals and birds do it only in the -healing of wounds; and very often but imperfectly even in this. For in -muscular and glandular organs the tissues destroyed are not properly -reproduced, but are replaced by tissue of an irregular kind which serves to -hold the parts together. So that the power of reproducing lost parts is -greatest where the organization is lowest; and almost disappears where the -organization is highest. And though we cannot say that in the intermediate -stages there is a constant inverse relation between reparative power and -degree of organization; yet we may say that there is some approach to such -a relation. - - -§ 63. There is an obvious and complete harmony between the first of the -above inductions and the deduction which follows immediately from first -principles. We have already seen (§ 23) "that whatever amount of power an -organism expends in any shape, is the correlate and equivalent of a power -that was taken into it from without." Motion, sensible or insensible, -generated by an organism, is insensible motion which was absorbed in -producing certain chemical compounds appropriated by the organism under the -form of food. As much energy as was required to raise the elements of these -complex atoms to their state of unstable equilibrium, is given out in their -falls to a state of stable equilibrium; and having fallen to a state of -stable equilibrium they can give out no further energy, but have to be got -rid of as inert and useless. It is an inevitable corollary "from the -persistence of force, that each portion of mechanical or other energy which -an organism exerts, implies the transformation of as much organic matter as -contained this energy in a latent state;" and that this organic matter in -yielding up its latent energy, loses its value for the purposes of life, -and becomes waste matter needing to be excreted. The loss of these complex -unstable substances must hence be proportionate to the quantity of expended -force. Here, then, is the rationale of certain general facts lately -indicated. Plants do not waste to any considerable degree, for the obvious -reason that the sensible and insensible motions they generate are -inconsiderable. Between the small waste, small activity, and low -temperature of the inferior animals, the relation is similarly one -admitting of _a priori_ establishment. Conversely, the rapid waste of -energetic, hot-blooded animals might be foreseen with equal certainty. And -not less manifestly necessary is the variation in waste which, in the same -organism, attends the variation in the heat and mechanical motion produced. - -Between the activity of a special part and the waste of that part, a like -relation may be deductively inferred; though it cannot be inferred that -this relation is equally definite. Were the activity of every organ quite -independent of the activities of other organs, we might expect to trace out -this relation distinctly; but since increased activity in any organ or -group of organs, as the muscles, necessarily entails increased activity in -other organs, as in the heart, lungs, and nervous system, it is clear that -special waste and general waste are too much entangled to admit of a -definite relation being established between special waste and special -activity. We may fairly say, however, that this relation is quite as -manifest as we can reasonably anticipate. - - -§ 64. Deductive interpretation of the phenomena of Repair, is by no means -so easy. The tendency displayed by an animal organism, as well as by each -of its organs, to return to a state of integrity by the assimilation of new -matter, when it has undergone the waste consequent on activity, is a -tendency which is not manifestly deducible from first principles; though it -appears to be in harmony with them. If in the blood there existed -ready-formed units exactly like in kind to those of which each organ -consists, the sorting of these units, ending in the union of each kind with -already existing groups of the same kind, would be merely a good example of -Segregation (_First Principles_, § 163). It would be analogous to the -process by which, from a mixed solution of salts, there are, after an -interval, deposited separate masses of these salts in the shape of -different crystals. But as already said (§ 54), though the selective -assimilation by which the repair of organs is effected, may result in part -from an action of this kind, the facts cannot be thus wholly accounted for; -since organs are in part made up of units which do not exist as such in the -circulating fluids. We must suppose that, as suggested in § 54, groups of -compound units have a certain power of moulding adjacent fit materials into -units of their own form. Let us see whether there is not reason to think -such a power exists. - -"The poison of small-pox or of scarlatina," remarks Mr. (now Sir James) -Paget, "being once added to the blood, presently affects the composition of -the whole: the disease pursues its course, and, if recovery ensue, the -blood will seem to have returned to its previous condition: yet it is not -as it was before; for now the same poison may be added to it with -impunity." ... "The change once effected, may be maintained through life. -And herein seems to be a proof of the assimilative force in the blood: for -there seems no other mode of explaining these cases than by admitting that -the altered particles have the power of assimilating to themselves all -those by which they are being replaced: in other words, all the blood that -is formed after such a disease deviates from the natural composition, so -far as to acquire the peculiarity engendered by the disease: it is formed -according to the altered model." Now if the compound molecules of the -blood, or of an organism considered in the aggregate, have the power of -moulding into their own type the matters which they absorb as nutriment; -and if they have the power when their type has been changed by disease, of -moulding materials afterwards received into the modified type; may we not -reasonably suspect that the more or less specialized molecules of each -organ have, in like manner, the power of moulding the materials which the -blood brings to them into similarly specialized molecules? The one -conclusion seems to be a corollary from the other. Such a power cannot be -claimed for the component units of the blood without being conceded to the -component units of every tissue. Indeed the assertion of this power is -little more than an assertion of the fact that organs composed of -specialized units _are_ capable of resuming their structural integrity -after they have been wasted by function. For if they do this, they must do -it by forming from the materials brought to them, certain specialized units -like in kind to those of which they are composed; and to say that they do -this, is to say that their component units have the power of moulding fit -materials into other units of the same order. - - -§ 65. What must we say of the ability an organism has to re-complete itself -when one of its parts has been cut off? Is it of the same order as the -ability of an injured crystal to re-complete itself. In either case new -matter is so deposited as to restore the original outline. And if in the -case of the crystal we say that the whole aggregate exerts over its parts a -force which constrains the newly-integrated molecules to take a certain -definite form, we seem obliged, in the case of the organism, to assume an -analogous force. If when the leg of a lizard has been amputated there -presently buds out the germ of a new one, which, passing through phases of -development like those of the original leg, eventually assumes a like shape -and structure, we assert only what we see, when we assert that the entire -organism, or the adjacent part of it, exercises such power over the forming -limb as makes it a repetition of its predecessor. If a leg is reproduced, -where there was a leg, and a tail where there was a tail, there seems no -alternative but to conclude that the forces around it control the formative -processes going on in each part. And on contemplating these facts in -connexion with various kindred ones, there is suggested the hypothesis, -that the form of each species of organism is determined by a peculiarity in -the constitution of its units--that these have a special structure in which -they tend to arrange themselves; just as have the simpler units of -inorganic matter. Let us glance at the evidences which more especially -thrust this conclusion upon us. - -A fragment of a Begonia-leaf imbedded in fit soil and kept at an -appropriate temperature, will develop a young Begonia; and so small is the -fragment which is thus capable of originating a complete plant, that -something like a hundred plants may be produced from a single leaf. The -friend to whom I owe this observation, tells me that various succulent -plants have like powers of multiplication. Illustrating a similar power -among animals, we have the often-cited experiments of Trembley on the -common polype. Each of the four pieces into which one of these creatures -was cut, grew into a perfect individual. In each of these, again, bisection -and tri-section were followed by like results. And so with their segments, -similarly produced, until as many as fifty polypes had resulted from the -original one. Bodies when cut off regenerated heads; heads regenerated -bodies; and when a polype had been divided into as many pieces as was -practicable, nearly every piece survived and became a complete animal. -What, now, is the implication? We cannot say that in each portion of a -Begonia-leaf, and in every fragment of a Hydra's body, there exists a -ready-formed model of the entire organism. Even were there warrant for the -doctrine that the germ of every organism contains the perfect organism in -miniature, it still could not be contended that each considerable part of -the perfect organism resulting from such a germ, contains another such -miniature. Indeed the one hypothesis negatives the other. The implication -seems, therefore, to be that the living particles composing one of these -fragments, have an innate tendency to arrange themselves into the shape of -the organism to which they belong. We must infer that the active units -composing a plant or animal of any species have an intrinsic aptitude to -aggregate into the form of that species. It seems difficult to conceive -that this can be so; but we see that it _is_ so. Groups of units taken from -an organism (providing they are of a certain bulk and not much -differentiated into special structures) _have_ this power of re-arranging -themselves. Manifestly, too, if we are thus to interpret the reproduction -of an organism from one of its amorphous fragments, we must thus interpret -the reproduction of any minor portion of an organism by the remainder. When -in place of its lost claw a lobster puts forth a cellular mass which, while -increasing in bulk, assumes the form and structure of the original claw, we -cannot avoid ascribing this result to a play of forces like that which -moulds the materials contained in a piece of Begonia-leaf into the shape of -a young Begonia. - - -§ 66. As we shall have frequent occasion hereafter to refer to these units -which possess the property of arranging themselves into the special -structures of the organisms to which they belong; it will be well here to -ask by what name they may be most fitly called. - -On the one hand, it cannot be in those chemical compounds characterizing -organic bodies that this specific property dwells. It cannot be that the -molecules of albumin, or fibrin, or gelatine, or other proteid, possess -this power of aggregating into these specific shapes; for in such case -there would be nothing to account for the unlikenesses of different -organisms. If the proclivities of proteid molecules determined the forms of -the organisms built up of them or by them, the occurrence of such endlessly -varied forms would be inexplicable. Hence what we may call the _chemical -units_ are clearly not the possessors of this property. - -On the other hand, this property cannot reside in what may be roughly -distinguished as the _morphological units_. The germ of every organism is a -minute portion of encased protoplasm commonly called a cell. It is by -multiplication of cells that all the early developmental changes are -effected. The various tissues which successively arise in the unfolding -organism, are primarily cellular; and in many of them the formation of -cells continues to be, throughout life, the process by which repair is -carried on. But though cells are so generally the ultimate visible -components of organisms, that they may with some show of reason be called -the morphological units; yet we cannot say that this tendency to aggregate -into special forms dwells in them. In many cases a fibrous tissue arises -out of a nucleated blastema, without cell-formation; and in such cases -cells cannot be regarded as units possessing the structural proclivity. But -the conclusive proof that the morphological units are not the building -factors in an organism composed of them, is yielded by their independent -homologues the so-called unicellular organisms. For each of these displays -the power to assume its specific structure. Clearly, if the ability of a -multicellular organism to assume its specific structure resulted from the -cooperation of its component cells, then a single cell, or the independent -homologue of a single cell, having no other to cooperate with, could -exhibit no structural traits. Not only, however, do single-celled organisms -exhibit structural traits, but these, even among the simplest, are so -distinct as to originate classification into orders, genera, and species; -and they are so constant as to remain the same from generation to -generation. - -If, then, this organic polarity (as we might figuratively call this -proclivity towards a specific structural arrangement) can be possessed -neither by the chemical units nor the morphological units, we must conceive -it as possessed by certain intermediate units, which we may term -_physiological_. There seems no alternative but to suppose that the -chemical units combine into units immensely more complex than themselves, -complex as they are; and that in each organism the physiological units -produced by this further compounding of highly compound molecules, have a -more or less distinctive character. We must conclude that in each case some -difference of composition in the units, or of arrangement in their -components, leading to some difference in their mutual play of forces, -produces a difference in the form which the aggregate of them assumes. - -The facts contained in this chapter form but a small part of the evidence -which thrusts this assumption upon us. We shall hereafter find various -reasons for inferring that such physiological units exist, and that to -their specific properties, more or less unlike in each plant and animal, -various organic phenomena are due. - - - - -CHAPTER V. - -ADAPTATION. - - -§ 67. In plants waste and repair being scarcely appreciable, there are not -likely to arise appreciable changes in the proportions of already-formed -parts. The only divergences from the average structures of a species, which -we may expect particular conditions to produce, are those producible by the -action of these conditions on parts in course of formation; and such -divergences we do find. We know that a tree which, standing alone in an -exposed position, has a short and thick stem, has a tall and slender stem -when it grows in a wood; and that also its branches then take a different -inclination. We know that potato-sprouts which, on reaching the light, -develop into foliage, will, in the absence of light, grow to a length of -several feet without foliage. And every in-door plant furnishes proof that -shoots and leaves, by habitually turning themselves to the light, exhibit a -certain adaptation--an adaptation due, as we must suppose; to the special -effects of the special conditions on the still growing parts. In animals, -however, besides analogous structural changes wrought during the period of -growth, by subjection to circumstances unlike the ordinary circumstances, -there are structural changes similarly wrought after maturity has been -reached. Organs that have arrived at their full sizes possess a certain -modifiability; so that while the organism as a whole retains pretty nearly -the same bulk, the proportions of its parts may be considerably varied. -Their variations, here treated of under the title Adaptation, depend on -specialities of individual action. In the last chapter we saw that the -actions of organisms entail re-actions on them; and that specialities of -action entail specialities of re-action. Here it remains to be pointed out -that these special actions and re-actions do not end with temporary -changes, but work permanent changes. - -If, in an adult animal, the waste and repair in all parts were exactly -balanced--if each organ daily gained by nutrition exactly as much as it -lost daily by the discharge of its function--if excess of function were -followed only by such excess of nutrition as balanced the extra waste; it -is clear that there would occur no change in the relative sizes of organs. -But there is no such exact balance. If the excess of function, and -consequent excess of waste, is moderate, it is not simply compensated by -repair but more than compensated--there is a certain increase of bulk. This -is true to some degree of the organism as a whole, when the organism is -framed for activity. A considerable waste giving considerable power of -assimilation, is more favourable to accumulation of tissue than is -quiescence with its comparatively feeble assimilation: whence results a -certain adaptation of the whole organism to its requirements. But it is -more especially true of the parts of an organism in relation to one -another. The illustrations fall into several groups. The growth of muscles -exercised to an unusual degree is a matter of common observation. In the -often-cited blacksmith's arm, the dancer's legs and the jockey's crural -adductors, we have marked examples of a modifiability which almost every -one has to some extent experienced. It is needless to multiply proofs. The -occurrence of changes in the structure of the skin, where the skin is -exposed to unusual stress of function, is also familiar. That thickening of -the epidermis on a labourer's palm results from continual pressure and -friction, is certain. Those who have not before exerted their hands, find -that such an exercise as rowing soon begins to produce a like thickening. -This relation of cause and effect is still better shown by the marked -indurations at the ends of a violinist's fingers. Even in mucous membrane, -which ordinarily is not subject to mechanical forces of any intensity, -similar modifications are possible: witness the callosity of the gums which -arises in those who have lost their teeth, and have to masticate without -teeth. The vascular system furnishes good instances of the increased growth -that follows increased function. When, because of some permanent -obstruction to the circulation, the heart has to exert a greater -contractile force on the mass of blood which it propels at each pulsation, -and when there results the laboured action known as palpitation, there -usually occurs dilatation, or hypertrophy, or a mixture of the two: the -dilatation, which is a yielding of the heart's structure under the -increased strain, implying a failure to meet the emergency; but the -hypertrophy, which consists in a thickening of the heart's muscular walls, -being an adaptation of it to the additional effort required. Again, when an -aneurism in some considerable artery has been obliterated, either -artifically or by a natural inflammatory process; and when this artery has -consequently ceased to be a channel for the blood; some of the adjacent -arteries which anastomose with it become enlarged, so as to carry the -needful quantity of blood to the parts supplied. Though we have no direct -proof of analogous modifications in nervous structures, yet indirect proof -is given by the greater efficiency that follows greater activity. This is -manifested alike in the senses and the intellect. The palate may be -cultivated into extreme sensitiveness, as in professional tea-tasters. An -orchestral conductor gains, by continual practice, an unusually great -ability to discriminate differences of sound. In the finger-reading of the -blind we have evidence that the sense of touch may be brought by exercise -to a far higher capability than is ordinary.[23] The increase of power -which habitual exertion gives to mental faculties needs no illustration: -every person of education has personal experience of it. Even from the -osseous structures evidence may be drawn. The bones of men accustomed to -great muscular action are more massive, and have more strongly marked -processes for the attachment of muscles, than the bones of men who lead -sedentary lives; and a like contrast holds between the bones of wild and -tame animals of the same species. Adaptations of another order, in which -there is a qualitative rather than a quantitative modification, arise after -certain accidents to which the skeleton is liable. When the hip-joint has -been dislocated, and long delay has made it impossible to restore the parts -to their proper places, the head of the thigh-bone, imbedded in the -surrounding muscles, becomes fixed in its new position by attachments of -fibrous tissue, which afford support enough to permit a halting walk. But -the most remarkable modification of this order occurs in united ends of -fractured bones. "False joints" are often formed--joints which rudely -simulate the hinge structure or the ball-and-socket structure, according as -the muscles tend to produce a motion of flexion and extension or a motion -of rotation. In the one case, according to Rokitansky, the two ends of the -broken bone become smooth and covered with periosteum and fibrous tissue, -and are attached by ligaments that allow a certain backward and forward -motion; and in the other case the ends, similarly clothed with the -appropriate membranes, become the one convex and the other concave, are -inclosed in a capsule, and are even occasionally supplied with synovial -fluid! - -The general truth that extra function is followed by extra growth, must be -supplemented by the equally general truth, that beyond a limit, usually -soon reached, very little, if any, further modification can be produced. -The experiences which we colligate into the one induction thrust the other -upon us. After a time no training makes the pugilist or the athlete any -stronger. The adult gymnast at last acquires the power to perform certain -difficult feats; but certain more difficult feats no additional practice -enables him to perform. Years of discipline give the singer a particular -loudness and range of voice, beyond which further discipline does not give -greater loudness or wider range: on the contrary, increased vocal exercise, -causing a waste in excess of repair, is often followed by decrease of -power. In the exaltation of the perceptions we see similar limits. The -culture which raises the susceptibility of the ear to the intervals and -harmonies of notes, will not turn a bad ear into a good one. Lifelong -effort fails to make this artist a correct draftsman or that a fine -colourist: each does better than he did at first, but each falls short of -the power attained by some other artists. Nor is this truth less clearly -illustrated among the more complex mental powers. A man may have a -mathematical faculty, a poetical faculty, or an oratorical faculty, which -special education improves to a certain extent. But unless he is unusually -endowed in one of those directions, no amount of education will make him a -first-rate mathematician, a first-rate poet, or a first-rate orator. Thus -the general fact appears to be that while in each individual certain -changes in the proportions of parts may be caused by variations of -functions, the congenital structure of each individual puts a limit to the -modifiability of every part. Nor is this true of individuals only: it -holds, in a sense, of species. Leaving open the question whether, in -indefinite times, indefinite modifications may not be produced by -inheritance of functionally wrought adaptations; experience proves that -within assigned times, the changes wrought in races of organisms by changes -of conditions fall within narrow limits. Though by discipline, aided by -selective breeding, one variety of horse has had its locomotive power -increased considerably beyond the locomotive powers of other varieties; yet -further increase takes place, if at all, at an inappreciable rate. The -different kinds of dogs, too, in which different forms and capacities have -been established, do not now show aptitudes for diverging in the same -directions at considerable rates. In domestic animals generally, certain -accessions of intelligence have been produced by culture; but accessions -beyond these are inconspicuous. It seems that in each species of organism -there is a margin for functional oscillations on all sides of a mean state, -and a consequent margin for structural variations; that it is possible -rapidly to push functional and structural changes towards the extreme of -this margin in any direction, both in an individual and in a race; but that -to push these changes further in any direction, and so to alter the -organism as to bring its mean state up to the extreme of the margin in that -direction, is a comparatively slow process.[24] - -We also have to note that the limited increase of size produced in any -organ by a limited increase of its function, is not maintained unless the -increase of function is permanent. A mature man or other animal, led by -circumstances into exerting particular members in unusual degrees, and -acquiring extra sizes in these members, begins to lose such extra sizes on -ceasing to exert the members; and eventually lapses more or less nearly -into the original state. Legs strengthened by a pedestrian tour, become -relatively weak again after a prolonged return to sedentary life. The -acquired ability to perform feats of skill disappears in course of time, if -the performance of them be given up. For comparative failure in executing a -piece of music, in playing a game at chess, or in anything requiring -special culture, the being out of practice is a reason which every one -recognizes as valid. It is observable, too, that the rapidity and -completeness with which an artificial power is lost, is proportionate to -the shortness of the cultivation which evoked it. One who has for many -years persevered in habits which exercise special muscles or special -faculties of mind, retains the extra capacity produced, to a very -considerable degree, even after a long period of desistance; but one who -has persevered in such habits for but a short time has, at the end of a -like period, scarcely any of the facility he had gained. Here too, as -before, successions of organisms present an analogous fact. A species in -which domestication continued through many generations, has organized -certain peculiarities; and which afterwards, escaping domestic discipline, -returns to something like its original habits; soon loses, in great -measure, such peculiarities. Though it is not true, as alleged, that it -resumes completely the structure it had before domestication, yet it -approximates to that structure. The Dingo, or wild dog of Australia, is one -of the instances given of this; and the wild horse of South America is -another. Mankind, too, supplies us with instances. In the Australian bush -and in the backwoods of America, the Anglo-Saxon race, in which -civilization has developed the higher feelings to a considerable degree, -rapidly lapses into comparative barbarism: adopting the moral code, and -sometimes the habits, of savages. - - -§ 68. It is important to reach, if possible, some rationale of these -general truths--especially of the last two. A right understanding of these -laws of organic modification underlies a right understanding of the great -question of species. While, as before hinted (§ 40), the action of -structure on function is one of the factors in that process of -differentiation by which unlike forms of plants and animals are produced, -the reaction of function on structure is another factor. Hence, it is well -worth while inquiring how far these inductions are deductively -interpretable. - -The first of them is the most difficult to deal with. Why an organ exerted -somewhat beyond its wont should presently grow, and thus meet increase of -demand by increase of supply, is not obvious. We know, indeed, (_First -Principles_, §§ 85, 173,) that of necessity, the rhythmical changes -produced by antagonistic organic actions cannot any of them be carried to -an excess in one direction, without there being produced an equivalent -excess in the opposite direction. It is a corollary from the persistence of -force, that any deviation effected by a disturbing cause, acting on some -member of a moving equilibrium, must (unless it altogether destroys the -moving equilibrium) be eventually followed by a compensating deviation. -Hence, that excess of repair should succeed excess of waste, is to be -expected. But how happens the mean state of the organ to be changed? If -daily extra waste naturally brings about daily extra repair only to an -equivalent extent, the mean state of the organ should remain constant. How -then comes the organ to augment in size and power? - -Such answer to this question as we may hope to find, must be looked for in -the effects wrought on the organism as a whole by increased function in one -of its parts. For since the discharge of its function by any part is -possible only on condition that those various other functions on which its -own is immediately dependent are also discharged, it follows that excess in -its function presupposes some excess in their functions. Additional work -given to a muscle implies additional work given to the branch arteries -which bring it blood, and additional work, smaller in proportion, to the -arteries from which these branch arteries come. Similarly, the smaller and -larger veins which take away the blood, as well as those structures which -deal with effete products, must have more to do. And yet further, on the -nervous centres which excite the muscle a certain extra duty must fall. But -excess of waste will entail excess of repair, in these parts as well as in -the muscle. The several appliances by which the nutrition and excitation of -an organ are carried on, must also be influenced by this rhythm of action -and reaction; and therefore, after losing more than usual by the -destructive process they must gain more than usual by the constructive -process. But temporarily-increased efficiency in these appliances by which -blood and nervous force are brought to an organ, will cause extra -assimilation in the organ, beyond that required to balance its extra -expenditure. Regarding the functions as constituting a moving equilibrium, -we may say that divergence of any function in the direction of increase, -causes the functions with which it is bound up to diverge in the same -direction; that these, again, cause the functions which they are bound up -with, also to diverge in the same direction; and that these divergences of -the connected functions allow the specially-affected function to be carried -further in this direction than it could otherwise be--further than the -perturbing force could carry it if it had a fixed basis. - -It must be admitted that this is but a vague explanation. Among actions so -involved as these, we can scarcely expect to do more than dimly discern a -harmony with first principles. That the facts are to be interpreted in some -such way, may, however, be inferred from the circumstance that an extra -supply of blood continues for some time to be sent to an organ that has -been unusually exercised; and that when unusual exercise is long continued -a permanent increase of vascularity results. - - -§ 69. Answers to the questions--Why do these adaptive modifications in an -individual animal soon reach a limit? and why, in the descendants of such -animal, similarly conditioned, is this limit very slowly extended?--are to -be found in the same direction as was the answer to the last question. And -here the connexion of cause and consequence is more manifest. - -Since the function of any organ is dependent on the functions of the organs -which supply it with materials and stimuli; and since the functions of -these subsidiary organs are dependent on the functions of organs which -supply them with materials and stimuli; it follows that before any great -extra power of discharging its function can be gained by a -specially-exercised organ, a considerable extra power must be gained by a -series of immediately-subservient organs, and some extra power by a -secondary series of remotely-subservient organs. Thus there are required -numerous and wide-spread modifications. Before the artery which feeds a -hard-worked muscle can permanently furnish a large additional quantity of -blood, it must increase in diameter; and that its increase of diameter may -be of use, the main artery from which it diverges must also be so far -modified as to bring this additional quantity of blood to the branch -artery. Similarly with the veins; similarly with the structures which -remove waste-products; similarly with the nerves. And when we ask what -these subsidiary changes imply, we are forced to conclude that there must -be an analogous group of more numerous changes ramifying throughout the -system. The growth of the arteries primarily and secondarily implicated, -cannot go to any extent without growth in the minor blood-vessels on which -their nutrition depends; while their greater contractile power involves -enlargement of the nerves which excite them, and some modification of that -part of the spinal cord whence these nerves proceed. Thus, without tracing -the like remote alterations implied by extra growth of the veins, -lymphatics, glandular organs, and other agencies, it is manifest that a -large amount of rebuilding must be done throughout the organism, before any -organ of importance can be permanently increased in size and power to a -great extent. Hence, though such extra growth in any part as does not -necessitate considerable changes throughout the rest of the organism, may -rapidly take place; a further growth in this part, requiring a re-modelling -of numerous parts remotely and slightly affected, must take place but -slowly. - -We have before found our conceptions of vital processes made clearer by -studying analogous social processes. In societies there is a mutual -dependence of functions, essentially like that which exists in organisms; -and there is also an essentially like reaction of functions on structures. -From the laws of adaptive modification in societies, we may therefore hope -to get a clue to the laws of adaptive modification in organisms. Let us -suppose, then, that a society has arrived at a state of equilibrium -analogous to that of a mature animal--a state not like our own, in which -growth and structural development are rapidly going on, but a state of -settled balance among the functional powers of the various classes and -industrial bodies, and a consequent fixity in the relative sizes of such -classes and bodies. Further, let us suppose that in a society thus balanced -there occurs something which throws an unusual demand on one industry--say -an unusual demand for ships (which we will assume to be built of iron) in -consequence of a competing mercantile nation having been prostrated by -famine or pestilence. The immediate result of this additional demand for -iron ships is the employment of more workmen, and the purchase of more -iron, by the ship-builders; and when, presently, the demand continuing, the -ship-builders find their premises and machinery insufficient, they enlarge -them. If the extra requirement persists, the high interest and high wages -bring such extra capital and labour into the business as are needed for new -ship-building establishments. But such extra capital and labour do not come -quickly; since, in a balanced community, not increasing in population and -wealth, labour and capital have to be drawn from other industries, where -they are already yielding the ordinary returns. Let us now go a step -further. Suppose that this iron-ship-building industry, having enlarged as -much as the available capital and labour permit, is still unequal to the -demand; what limits its immediate further growth? The lack of iron. By the -hypothesis, the iron-producing industry, like all the other industries -throughout the community, yields only as much iron as is habitually -required for all the purposes to which iron is applied: ship-building being -only one. If, then, extra iron is required for ship-building, the first -effect is to withdraw part of the iron habitually consumed for other -purposes, and to raise the price of iron. Presently, the iron-makers feel -this change and their stocks dwindle. As, however, the quantity of iron -required for ship-building forms but a small part of the total quantity -required for all purposes, the extra demand on the iron-makers can be -nothing like so great in proportion as is the extra demand on the -ship-builders. Whence it follows that there will be much less tendency to -an immediate enlargement of the iron-producing industry; since the extra -quantity will for some time be obtained by working extra hours. -Nevertheless if, as fast as more iron can be thus supplied, the -ship-building industry goes on growing--if, consequently, the iron-makers -experience a permanently-increased demand, and out of their greater profits -get higher interest on capital, as well as pay higher wages; there will -eventually be an abstraction of capital and labour from other industries to -enlarge the iron-producing industry: new blast-furnaces, new rolling-mills, -new cottages for workmen, will be erected. But obviously, the inertia of -capital and labour to be overcome before the iron-producing industry can -grow by a decrease of certain other industries, will prevent its growth -from taking place until long after the increased ship-building industry has -demanded it; and meanwhile, the growth of the ship-building industry must -be limited by the deficiency of iron. A remoter restraint of the same -nature meets us if we go a step further--a restraint which can be overcome -only in a still longer time. For the manufacture of iron depends on the -supply of coal. The production of coal being previously in equilibrium with -the consumption; and the consumption of coal for the manufacture of iron -being but a small part of the total consumption; it follows that a -considerable extension of the iron manufacture, when it at length takes -place, will cause but a comparatively small additional demand on the -coal-owners and coal-miners--a demand which will not, for a long period, -suffice to cause enlargement of the coal-trade, by drawing capital and -labour from other investments and occupations. And until the permanent -extra demand for coal has become great enough to draw from other -investments and occupations sufficient capital and labour to sink new -mines, the increasing production of iron must be restricted by the scarcity -of coal, and the multiplication of ship-yards and ship-builders must be -checked by the want of iron. Thus, in a community which has reached a state -of moving equilibrium, though any one industry directly affected by an -additional demand may rapidly undergo a small extra growth, yet a growth -beyond this, requiring as it does the building-up of subservient -industries, less directly and strongly affected, as well as the partial -unbuilding of other industries, can take place only with comparative -slowness. And a still further growth, requiring structural modifications of -industries still more distantly affected, must take place still more -slowly. - -On returning from this analogy, we see more clearly the truth that any -considerable member of an animal organism, cannot be greatly enlarged -without some general reorganization. Besides a building up of the primary, -secondary, and tertiary groups of the subservient parts, there must be an -unbuilding of sundry non-subservient parts; or, at any rate, there must be -permanently established a lower nutrition of such non-subservient parts. -For it must be remembered that in a mature animal, or one which has reached -a balance between assimilation and expenditure, there cannot (supposing -general conditions to remain constant) be an increase in the nutrition of -some organs without a decrease in the nutrition of others; and an organic -establishment of the increase implies an organic establishment of the -decrease--implies more or less change in the processes and structures -throughout the entire system. And here, indeed, is disclosed one reason why -growing animals undergo adaptations so much more readily than adult ones. -For while there is surplus nutrition, it is possible for -specially-exercised parts to be specially enlarged without any positive -deduction from other parts. There is required only that negative deduction -implied in the diminished growth of other parts. - - -§ 70. Pursuing the argument further, we reach an explanation of the third -general truth; namely that organisms, and species of organisms, which, -under new conditions, have undergone adaptive modifications, soon return to -something like their original structures when restored to their original -conditions. Seeing, as we have done, how excess of action and excess of -nutrition in any part of an organism, must affect action and nutrition in -subservient parts, and these again in other parts, until the re-action has -divided and subdivided itself throughout the organism, affecting in -decreasing degrees the more and more numerous parts more and more remotely -implicated; we see that the consequent changes in the parts remotely -implicated, constituting the great mass of the organism, must be extremely -slow. Hence, if the need for the adaptive modification ceases before the -great mass of the organism has been much altered in its structure by these -ramified but minute reactions, we shall have a condition in which the -specially-modified part is not in equilibrium with the rest. All the -remotely-affected organs, as yet but little changed, will, in the absence -of the perturbing cause, resume very nearly their previous actions. The -parts that depend on them will consequently by and by do the same. Until at -length, by a reversal of the adaptive process, the organ at first affected -will be brought back almost to its original state. Reconsidering the -above-drawn analogy between an organism and a society, will enable us -better to recognize this necessity. If, in the case supposed, the extra -demand for iron ships, after causing the erection of some additional -ship-yards and the drawing of iron from other manufactures, were to cease; -the old dimensions of the ship-building trade would be quickly returned to: -discharged workmen would seek fresh occupations, and the new yards would be -devoted to other uses. But if the increased need for ships lasted long -enough, and became great enough, to cause a flow of capital and labour from -other industries into the iron-manufacture, a falling off in the demand for -ships, would much less rapidly entail a dwindling of the ship-building -industry. For iron being now produced in greater quantity, a diminished -consumption of it for ships would cause a fall in its price, and a -consequent fall in the cost of ships: thus enabling the ship-builders to -meet the competition which we may suppose led to a decrease in the orders -they received. And since, when new blast-furnaces and rolling-mills, &c., -had been built with capital drawn from other industries, its transference -back into other industries would involve great loss; the owners, rather -than transfer it, would accept unusually low interest, and an excess of -iron would continue to be produced; resulting in an undue cheapness of -ships, and a maintenance of the ship-building industry at a size beyond the -need. Eventually, however, if the number of ships required still -diminished, the production of iron in excess would become very -unremunerative: some of the blast-furnaces would be blown out; and as much -of the capital and labour as remained available would be re-distributed -among other occupations. Without repeating the steps of the argument, it -will be clear that were the enlargement of the ship-building industry great -enough, and did it last long enough to cause an increase in the number of -coal-mines, the ship-building industry would be still better able to -maintain itself under adverse circumstances; but that it would, though at a -more distant period, end by sinking down to the needful dimensions. Thus -our conclusions are:--First, that if the extra growth caused by extra -activity in a particular industry has lasted long enough only to remodel -the proximately-affected industries; it will dwindle away again after a -moderate period, if the need for it disappears. Second, that a long period -must be required before the re-actions produced by an enlarged industry can -cause a re-construction of the whole society, and before the countless -re-distributions of capital and labour can again reach a state of -equilibrium. And third, that only when such a new state of equilibrium is -eventually reached, can the adaptive modification become a permanent one. -How, in animal organisms the like argument holds, need not be pointed out. -The reader will readily follow the parallel. - -That organic types should be comparatively stable, might be anticipated on -the hypothesis of Evolution. The structure of any organism being a product -of the almost infinite series of actions and reactions to which ancestral -organisms have been exposed; any unusual actions and reactions brought to -bear on an individual, can have but an infinitesimal effect in permanently -changing the structure of the organism as a whole. The new set of forces, -compounded with all the antecedent sets of forces, can but inappreciably -modify that moving equilibrium of functions which all these antecedent sets -of forces have established. Though there may result a considerable -perturbation of certain functions--a considerable divergence from their -ordinary rhythms--yet the general centre of equilibrium cannot be sensibly -changed. On the removal of the perturbing cause the previous balance will -be quickly restored: the effect of the new forces being almost obliterated -by the enormous aggregate of forces which the previous balance expresses. - - -§ 71. As thus understood, the phenomena of adaptation fall into harmony -with first principles. The inference that organic types are fixed, because -the deviations from them which can be produced within assignable periods -are relatively small, and because, when a force producing deviation ceases, -there is a return to something like the original state; proves to be an -invalid inference. Without assuming fixity of species, we find good reasons -for anticipating that kind and degree of stability which is observed. We -find grounds for concluding, _a priori_, that an adaptive change of -structure will soon reach a point beyond which further adaptation will be -slow; for concluding that when the modifying cause has been but a short -time in action, the modification generated will be evanescent; for -concluding that a modifying cause acting even for many generations, will do -but little towards permanently altering the organic equilibrium of a race; -and for concluding that on the cessations of such cause, its effects will -become unapparent in the course of a few generations. - - - - -CHAPTER VI. - -INDIVIDUALITY. - - -§ 72. What is an individual? is a question which many readers will think it -easy to answer. Yet it is a question that has led to much controversy among -Zoologists and Botanists, and no quite satisfactory reply to it seems -possible. As applied to a man, or to any one of the higher animals, which -are all sharply-defined and independent, the word individual has a clear -meaning: though even here, when we turn from average cases to exceptional -cases--as a calf with two heads and two pairs of fore-limbs--we find -ourselves in doubt whether to predicate one individuality or two. But when -we extend our range of observation to the organic world at large, we find -that difficulties allied to this exceptional one meets us everywhere under -every variety of form. - -Each uniaxial plant may perhaps fairly be regarded as a distinct -individual; though there are botanists who do not make even this admission. -What, however, are we to say of a multiaxial plant? It is, indeed, usual to -speak of a tree with its many branches and shoots as singular; but strong -reasons may be urged for considering it as plural. Every one of its axes -has a more or less independent life, and when cut off and planted may grow -into the likeness of its parent; or, by grafting and budding, parts of this -tree may be developed upon another tree, and there manifest their specific -peculiarities. Shall we regard all the growing axes thus resulting from -slips and grafts and buds, as parts of one individual or as distinct -individuals? If a strawberry-plant sends out runners carrying buds at their -ends, which strike root and grow into independent plants that separate from -the original one by decay of the runners, must we not say that they possess -separate individualities; and yet if we do this, are we not at a loss to -say when their separate individualities were established, unless we admit -that each bud was from the beginning an individual? Commenting on such -perplexities Schleiden says--"Much has been written and disputed concerning -the conception of the individual, without, however, elucidating the -subject, principally owing to the misconception that still exists as to the -origin of the conception. Now the individual is no conception, but the mere -subjective comprehension of an actual object, presented to us under some -given specific conception, and on this latter it alone depends whether the -object is or is not an individual. Under the specific conception of the -solar system, ours is an individual: in relation to the specific conception -of a planetary body, it is an aggregate of many individuals." ... "I think, -however, that looking at the indubitable facts already mentioned, and the -relations treated of in the course of these considerations, it will appear -most advantageous and most useful, in a scientific point of view, to -consider the vegetable cell as the general type of the plant (simple plant -of the first order). Under this conception, _Protococcus_ and other plants -consisting of only one cell, and the spore and pollen-granule, will appear -as individuals. Such individuals may, however, again, with a partial -renunciation of their individual independence, combine under definite laws -into definite forms (somewhat as the individual animals do in the globe of -the _Volvox globator_[25]). These again appear empirically as individual -beings, under a conception of a species (simple plants of the second order) -derived from the form of the normal connexion of the elementary -individuals. But we cannot stop here, since Nature herself combines these -individuals, under a definite form, into larger associations, whence we -draw the third conception of the plant, from a connexion, as it were, of -the second power (compound plants--plants of the third order). The simple -plant proceeding from the combination of the elementary individuals is then -termed a bud (_gemma_), in the composition of plants of the third order." - -The animal kingdom presents still greater difficulties. When, from sundry -points on the body of a common polype, there bud out young polypes which, -after acquiring mouths and tentacles and closing up the communications -between their stomachs and the stomach of the parent, finally separate from -the parent; we may with propriety regard them as distinct individuals. But -when in the allied compound _Hydrozoa_, we find that these young polypes -continue permanently connected with the parent; and when by this continuous -budding-out there is presently produced a tree-like aggregation, having a -common alimentary canal into which the digestive cavity of each polype -opens; it is no longer so clear that these little sacs, furnished with -mouths and tentacles, are severally to be regarded as distinct individuals. -We cannot deny a certain individuality to the polypedom. And on discovering -that some of the buds, instead of unfolding in the same manner as the rest, -are transformed into capsules in which eggs are developed--on discovering -that certain of the incipient polypes thus become wholly dependent on the -aggregate for their nutrition, and discharge functions which have nothing -to do with their own maintenance, we have still clearer proof that the -individualities of the members are partially merged in the individuality of -the group. Other organisms belonging to the same order, display still more -decidedly this transition from simple individualities to a complex -individuality. In the _Diphyes_ there is a special modification of one or -more members of the polypedom into a swimming apparatus which, by its -rhythmical contractions, propels itself through the water, drawing the -polypedom after it. And in the more differentiated _Physalia_ various -organs result from the metamorphosis of parts which are the homologues of -individual polypes. In this last instance, the individuality of the -aggregate is so predominant that the individualities of its members are -practically lost. This combination of individualities in such way as to -produce a composite individual, meets us in other forms among the -ascidians. While in some of these, as in the _Clavelina_ and in the -_Botryllidæ_, the animals associated are but little subordinated to the -community they form, in others they are so combined as to form a compound -individual. The pelagic ascidian _Doliolum_ is an example. "Here we find a -large individual which swims by contractions of circular muscular bands, -carries a train of smaller individuals attached to a long dorsal process of -the test. These are arranged in three rows: those constituting the lateral -row have wide mouths and no sexual organs or organs of locomotion--they -subserve the nutrition of the colony, a truth which is illustrated by the -fact that as soon as they are properly developed the large individual (the -mother) loses her alimentary canal;" while from the median row are -eventually derived the sexual zoids. - -On the hypothesis of Evolution, perplexities of this nature are just such -as we might anticipate. If Life in general commenced with minute and simple -forms, like those out of which all organisms, however complex, now -originate; and if the transitions from these primordial units to organisms -made up of groups of such units, and to higher organisms made up of groups -of such groups took place by degrees; it is clear that individualities of -the first and simplest order would merge gradually in those of a larger and -more complex order, and these again in others of an order having still -greater bulk and organization. Hence it would be impossible to say where -the lower individualities ceased and the higher individualities commenced. - - -§ 73. To meet these difficulties, it has been proposed that the whole -product of a single fertilized germ shall be regarded as a single -individual; whether such whole product be organized into one mass, or -whether it be organized into many masses that are partially or completely -separate. It is urged that whether the development of the fertilized germ -be continuous or discontinuous (§ 50) is a matter of secondary importance; -that the totality of living tissue to which the fertilized germ gives rise -in any one case, is the equivalent of the totality to which it gives rise -in any other case; and that we must recognize this equivalence, whether -such totality of living tissue takes a concrete or a discrete arrangement. -In pursuance of this view, a zoological individual is constituted either by -any such single animal as a mammal or bird, which may properly claim the -title of a _zoon_, or by any such group of animals as the numerous _Medusæ_ -that have been developed from the same egg, which are to be severally -distinguished as _zooids_. - -Admitting it to be very desirable that there should be words for expressing -these relations and this equivalence, it may be objected that to apply the -word individual to a number of separate living bodies, is inconvenient: -conflicting so much, as it does, with the ordinary conception which this -word suggests. It seems a questionable use of language to say that the -countless masses of _Anacharis Alsinastrum_ (now _Eloidea canadensis_) -which, within these few years, have grown up in our rivers, canals, and -ponds, are all parts of one individual: and yet as this plant does not seed -in England, these countless masses, having arisen by discontinuous -development, must be so regarded if we accept the above definition. - -It may be contended, too, that while it does violence to our established -way of thinking, this mode of interpreting the facts is not without its -difficulties. Something seems to be gained by restricting the application -of the title individual, to organisms which, being in all respects fully -developed, possess the power of producing their kind after the ordinary -sexual method, and denying this title to those incomplete organisms which -have not this power. But the definition does not really establish this -distinction for us. On the one hand, we have cases in which, as in the -working bee, the whole of the germ-product is aggregated into a single -organism; and yet, though an individual according to the definition, this -organism has no power of reproducing its kind. On the other hand, we have -cases like that of the perfect _Aphis_, where the organism is but an -infinitesimal part of the germ product, and yet has that completeness -required for sexual reproduction. Further, it might be urged with some show -of reason, that if the conception of individuality involves the conception -of completeness, then, an organism which possesses an independent power of -reproducing itself, being more complete than an organism in which this -power is dependent on the aid of another organism, is more individual. - - -§ 74. There is, indeed, as already implied, no definition of individuality -that is unobjectionable. All we can do is to make the best practicable -compromise. - -As applied either to an animate or an inanimate object, the word individual -ordinarily connotes union among the parts of the object and separateness -from other objects. This fundamental element in the conception of -individuality, we cannot with propriety ignore in the biological -application of the word. That which we call an individual plant or animal -must, therefore, be some concrete whole and not a discrete whole. If, -however, we say that each concrete living whole is to be regarded as an -individual, we are still met by the question--What constitutes a concrete -living whole? A young organism arising by internal or external gemmation -from a parent organism, passes gradually from a state in which it is an -indistinguishable part of the parent organism to a state in which it is a -separate organism of like structure with the parent. At what stage does it -become an individual? And if its individuality be conceded only when it -completely separates from the parent, must we deny individuality to all -organisms thus produced which permanently retain their connexions with -their parents? Or again, what must we say of the _Hectocotylus_, which is -an arm of the Cuttle-fish that undergoes a special development and then, -detaching itself, lives independently for a considerable period? And what -must we say of the larval nemertine worm the pilidium of which with its -nervous system is left to move about awhile after the developing worm has -dropped out of it? - -To answer such questions we must revert to the definition of life. The -distinction between individual in its biological sense, and individual in -its more general sense, must consist in the manifestation of Life, properly -so called. Life we have seen to be, "the definite combination of -heterogeneous change, both simultaneous and successive, in correspondence -with external co-existences and sequences." Hence, a biological individual -is any concrete whole having a structure which enables it, when placed in -appropriate conditions, to continuously adjust its internal relations to -external relations, so as to maintain the equilibrium of its functions. In -pursuance of this conception, we must consider as individuals all those -wholly or partially independent organized masses which arise by -multicentral and multiaxial development that is either continuous or -discontinuous (§ 50). We must accord the title to each separate aphis, each -polype of a polypedom, each bud or shoot of a lowering plant, whether it -detaches itself as a bulbil or remains attached as a branch. - -By thus interpreting the facts we do not, indeed, avoid all anomalies. -While, among flowering plants, the power of independent growth and -development is usually possessed only by shoots or axes; yet, in some -cases, as in that of the Begonia-leaf awhile since mentioned, the appendage -of an axis, or even a small fragment of such appendage, is capable of -initiating and carrying on the functions of life; and in other cases, as -shown by M. Naudin in the _Drosera intermedia_, young plants are -occasionally developed from the surfaces of leaves. Nor among forms like -the compound _Hydrozoa_, does the definition enable us to decide where the -line is to be drawn between the individuality of the group and the -individualities of the members: merging into each other, as these do, in -different degrees. But, as before said, such difficulties must necessarily -present themselves if organic forms have arisen by insensible gradations. -We must be content with a course which commits us to the smallest number of -incongruities; and this course is, to consider as an individual any -organized mass which is capable of independently carrying on that -continuous adjustment of inner to outer relations which constitutes Life. - - - - -CHAPTER VI^A. - -CELL-LIFE AND CELL-MULTIPLICATION. - - -§ 74a. The progress of science is simultaneously towards simplification and -towards complication. Analysis simplifies its conceptions by resolving -phenomena into their factors, and by then showing how each simple mode of -action may be traced under multitudinous forms; while, at the same time, -synthesis shows how each factor, by cooperation with various other factors -in countless modes and degrees, produces different results innumerable in -their amounts and varieties. Of course this truth holds alike of processes -and of products. Observation and the grouping into classes make it clear -that through multitudinous things superficially unlike there run the same -cardinal traits of structure; while, along with these major unities, -examination discloses innumerable minor diversities. - -A concomitant truth, or the same truth under another aspect, is that Nature -everywhere presents us with complexities within complexities, which go on -revealing themselves as we investigate smaller and smaller objects. In a -preceding chapter (§§ 54a, 54b) it was pointed out that each primitive -organism, in common with each of the units out of which the higher and -larger organisms are built, was found a generation ago to consist of -nucleus, protoplasm, and cell-wall. This general conception of a cell -remained for a time the outcome of inquiry; but with the advance of -microscopy it became manifest that within these minute structures processes -and products of an astonishing nature are to be seen. These we have now to -contemplate. - -In the passages just referred to it was said that the external layer or -cell-wall is a non-essential, inanimate part produced by the animate -contents. Itself a product of protoplasmic action, it takes no part in -protoplasmic changes, and may therefore here be ignored. - - -§ 74b. One of the complexities within complexities was disclosed when it -was found that the protoplasm itself has a complicated structure. Different -observers have described it as constituted by a network or reticulum, a -sponge-work, a foam-work. Of these the first may be rejected; since it -implies a structure lying in one plane. If we accept the second we have to -conceive the threads of protoplasm, corresponding to the fibres of the -sponge, as leaving interstices filled either with liquid or solid. They -cannot be filled with a continuous solid, since all motion of the -protoplasm would be negatived; and that their content is not liquid seems -shown by the fact that its parts move about under the form of granules or -microsomes. But the conception of moving granules implies the conception of -immersion in a liquid or semi-liquid substance in which they move--not a -sponge-work of threads but a foam-work, consisting everywhere of septa -interposed among the granules. This is the hypothesis which sundry -microscopists espouse, and which seems mechanically the most feasible: the -only one which consists with the "streaming" of protoplasm. Ordinarily the -name protoplasm is applied to the aggregate mass--the semi-liquid, hyaline -substance and the granules or microsomes it contains. - -What these granules or microsomes are--whether, as some have contended, -they are the essential living elements of the protoplasm, or whether, as is -otherwise held, they are nutritive particles, is at present undecided. But -the fact, alleged by sundry observers, that the microsomes often form rows, -held together by intervening substance, seems to imply that these minute -bodies are not inert. Leaving aside unsettled questions, however, one fact -of significance is manifest--an immense multiplication of surfaces over -which inter-action may take place. Anyone who drops into dilute sulphuric -acid a small nail and then drops a pinch of iron filings, will be shown, by -the rapid disappearance of the last and the long continuance of the first, -how greatly the increasing of surfaces by multiplication of fragments -facilitates change. The effect of subdivision in producing a large area in -a small space, is shown in the lungs, where the air-cells on the sides of -which the blood-vessels ramify, are less than 1/100th of an inch in -diameter, while they number 700,000,000. In the composition of every tissue -we see the same principle. The living part, or protoplasm, is divided into -innumerable protoplasts, among which are distributed the materials and -agencies producing changes. And now we find this principle carried still -deeper in the structure of the protoplasm itself. Each microscopic portion -of it is minutely divided in such ways that its threads or septa have -multitudinous contacts with those included portions of matter which take -part in its activities. - -Concerning the protoplasm contained in each cell, named by some cytoplasm, -it remains to say that it always includes a small body called the -centrosome, which appears to have a directive function. Usually the -centrosome lies outside the nucleus, but is alleged to be sometimes within -it. During what is called the "resting stage," or what might more properly -be called the growing stage (for clearly the occasional divisions imply -that in the intervals between them there has been increase) the centrosome -remains quiescent, save in the respect that it exercises some coercive -influence on the protoplasm around. This results in the radially-arranged -lines constituting an "aster." What is the nature of the coercion exercised -by the centrosome--a body hardly distinguishable in size from the -microsomes or granules of protoplasm around--is not known. It can scarcely -be a repelling force; since, in a substance of liquid or semi-liquid kind, -this could not produce approximately straight lines. That it is an -attractive force seems more probable; and the nature of the attraction -would be comprehensible did the centrosome augment in bulk with rapidity. -For if integration were in progress, the drawing in of materials might well -produce converging lines. But this seems scarcely a tenable interpretation; -since, during the so-called "resting stage," this star-like structure -exists--exists, that is, while no active growth of the centrosome is going -on. - -Respecting this small body we have further to note that, like the cell as a -whole, it multiplies by fission, and that the bisection of it terminates -the resting or growing stage and initiates those complicated processes by -which two cells are produced out of one: the first step following the -fission being the movement of the halves, with their respective completed -asters, to the opposite sides of the nucleus. - - -§ 74c. With the hypothesis, now general, that the nucleus or kernel of a -cell is its essential part, there has not unnaturally grown up the dogma -that it is always present; but there is reason to think that the evidence -is somewhat strained to justify this dogma. - -In the first place, beyond the cases in which the nucleus, though -ordinarily invisible, is said to have been rendered visible by a re-agent, -there are cases, as in the already-named _Archerina_, where no re-agent -makes one visible. In the second place, there is the admitted fact that -some nuclei are diffused; as in _Trachelocerca_ and some other Infusoria. -In them the numerous scattered granules are supposed to constitute a -nucleus: an interpretation obviously biassed by the desire to save the -generalization. In the third place, the nucleus is frequently multiple in -cells of low types; as in some families of Algæ and predominantly among -Fungi. Once more, the so-called nucleus is occasionally a branching -structure scarcely to be called a "kernel." - -The facts as thus grouped suggest that the nucleus has arisen in conformity -with the law of evolution--that the primitive protoplast, though not -homogeneous in the full sense, was homogeneous in the sense of being a -uniformly granular protoplasm; and that the protoplasts with diffused -nuclei, together with those which are multi-nucleate, and those which have -nuclei of a branching form, represent stages in that process by which the -relatively homogeneous protoplast passed into the relatively heterogeneous -one now almost universal. - -Concerning the structure and composition of the developed nucleus, the -primary fact to be named is that, like the surrounding granular cytoplasm, -it is formed of two distinct elements. It has a groundwork or matrix not -differing much from that of the cytoplasm, and at some periods continuous -with it; and immersed in this it has a special matter named chromatin, -distinguished from its matrix by becoming dyed more or less deeply when -exposed to fit re-agents. During the "resting stage," or period of growth -and activity which comes between periods of division, the chromatin is -dispersed throughout the ground-substance, either in discrete portions or -in such way as to form an irregular network or sponge-work, various in -appearance. When the time for fission is approaching this dispersed -chromatin begins to gather itself together: reaching its eventual -concentration through several stages. By its concentration are produced the -chromosomes, constant in number in each species of plant or animal. It is -alleged that the substance of the chromosomes is not continuous, but -consists of separate elements or granules, which have been named -chromomeres; and it is also alleged that, whether in the dispersed or -integrated form, each chromosome retains its individuality--that the -chromomeres composing it, now spreading out into a network and now uniting -into a worm-like body, form a group which never loses its identity. Be this -as it may, however, the essential fact is that during the growth-period the -chromatin substance is widely distributed, and concentration of it is one -of the chief steps towards a division of the nucleus and presently of the -cell. - -During this process of mitosis or karyokinesis, the dispersed chromatin -having passed through the coil-stage, reaches presently the star-stage, in -which the chromosomes are arranged symmetrically about the equatorial plane -of the nucleus. Meanwhile in each of them there has been a preparation for -splitting longitudinally in such way that the halves when separated contain -(or are assumed to contain) equal numbers of the granules or chromomeres, -which some think are the ultimate morphological units of the chromosomes. A -simultaneous change has occurred: there has been in course of formation a -structure known as the _amphiaster_. The two centrosomes which, as before -said, place themselves on opposite sides of the nucleus, become the -terminal poles of a spindle-shaped arrangement of fibres, arising mainly -from the groundwork of the nucleus, now continuous with the groundwork of -the cytoplasm. A conception of this structure may be formed by supposing -that the radiating fibres of the respective asters, meeting one another and -uniting in the intermediate space, thereafter exercise a tractive force; -since it is clear that, while the central fibres of the bundle will form -straight lines, the outer ones, pulling against one another not in straight -lines, will form curved lines, becoming more pronounced in their curvatures -as the distance from the axis increases. That a tractive force is at work -seems inferable from the results. For the separated halves of the split -chromosomes, which now form clusters on the two sides of the equatorial -plane, gradually part company, and are apparently drawn as clusters towards -the opposing centrosomes. As this change progresses the original nucleus -loses its individuality. The new chromosomes, halves of the previous -chromosomes, concentrate to found two new nuclei; and, by something like a -reversal of the stages above described, the chromatin becomes dispersed -throughout the substance of each new nucleus. While this is going on the -cell itself, undergoing constriction round its equator, divides into two. - -Many parts of this complex process are still imperfectly understood, and -various opinions concerning them are current. But the essential facts are -that this peculiar substance, the chromatin, at other times existing -dispersed, is, when division is approaching, gathered together and dealt -with in such manner as apparently to insure equal quantities being -bequeathed by the mother-cell to the two daughter-cells. - - -§ 74d. What is the physiological interpretation of these structures and -changes? What function does the nucleus discharge; and, more especially, -what is the function discharged by the chromatin? There have been to these -questions sundry speculative answers. - -The theory espoused by some, that the nucleus is the regulative organ of -the cell, is met by difficulties. One of them is that, as pointed out in -the chapter on "Structure," the nucleus, though morphologically central, is -not central geometrically considered; and that its position, often near to -some parts of the periphery and remote from others, almost of itself -negatives the conclusion that its function is directive in the ordinary -sense of the word. It could not well control the cytoplasm in the same ways -in all directions and at different distances. A further difficulty is that -the cytoplasm when deprived of its nucleus can perform for some time -various of its actions, though it eventually dies without reproducing -itself. - -For the hypothesis that the nucleus is a vehicle for transmitting -hereditary characters, the evidence seems strong. When it was shown that -the head of a spermatozoon is simply a detached nucleus, and that its -fusion with the nucleus of an ovum is the essential process initiating the -development of a new organism, the legitimate inference appeared to be that -these two nuclei convey respectively the paternal and maternal traits which -are mingled in the offspring. And when there came to be discerned the -karyokinesis by which the chromatin is, during cell-fission, exactly halved -between the nuclei of the daughter-cells, the conclusion was drawn that the -chromatin is more especially the agent of inheritance. But though, taken by -themselves, the phenomena of fertilization seem to warrant this inference, -the inference does not seem congruous with the phenomena of ordinary -cell-multiplication--phenomena which have nothing to do with fertilization -and the transmission of hereditary characters. No explanation is yielded of -the fact that ordinary cell-multiplication exhibits an elaborate process -for exact halving of the chromatin. Why should this substance be so -carefully portioned out among the cells of tissues which are not even -remotely concerned with propagation of the species? If it be said that the -end achieved is the conveyance of paternal and maternal qualities in equal -degrees to every tissue; then the reply is that they do not seem to be -conveyed in equal degrees. In the offspring there is not a uniform -diffusion of the two sets of traits throughout all parts, but an irregular -mixture of traits of the one with traits of the other. - -In presence of these two suggested hypotheses and these respective -difficulties, may we not suspect that the action of the chromatin is one -which in a way fulfils both functions? Let us consider what action may do -this. - - -§ 74e. The chemical composition of chromatin is highly complex, and its -complexity, apart from other traits, implies relative instability. This is -further implied by the special natures of its components. Various analyses -have shown that it consists of an organic acid (which has been called -nucleic acid) rich in phosphorus, combined with an albuminous substance: -probably a combination of various proteids. And the evidence, as summarised -by Wilson, seems to show that where the proportion of phosphorized acid is -high the activity of the substance is great, as in the heads of -spermatozoa; while, conversely, where the quantity of phosphorus is -relatively small, the substance approximates in character to the cytoplasm. -Now (like sulphur, present in the albuminoid base), phosphorus is an -element which, besides having several allotropic forms, has a great -affinity for oxygen; and an organic compound into which it enters, beyond -the instability otherwise caused, has a special instability caused by its -presence. The tendency to undergo change will therefore be great when the -proportion of the phosphorized component is great. Hence the statement that -"the chemical differences between chromatin and cytoplasm, striking and -constant as they are, are differences of degree only;" and the conclusion -that the activity of the chromatin is specially associated with the -phosphorus.[26] - -What, now, are the implications? Molecular agitation results from -decomposition of each phosphorized molecule: shocks are continually -propagated around. From the chromatin, units of which are thus ever falling -into stabler states, there are ever being diffused waves of molecular -motion, setting up molecular changes in the cytoplasm. The chromatin stands -towards the other contents of the cell in the same relation that a -nerve-element stands to any element of an organism which it excites: an -interpretation congruous with the fact that the chromatin is as near to as, -and indeed nearer than, a nerve-ending to any minute structure stimulated -by it. - -Several confirmatory facts may be named. During the intervals between -cell-fissions, when growth and the usual cell-activities are being carried -on, the chromatin is dispersed throughout the nucleus into an irregular -network: thus greatly increasing the surface of contact between its -substance and the substances in which it is imbedded. As has been remarked, -this wide distribution furthers metabolism--a metabolism which in this case -has, as we infer, the function of generating, not special matters but -special motions. Moreover, just as the wave of disturbance a nerve carries -produces an effect which is determined, not by anything which is peculiar -in itself, but by the peculiar nature of the organ to which it is -carried--muscular, glandular or other; so here, the waves diffused from the -chromatin do not determine the kinds of changes in the cytoplasm, but -simply excite it: its particular activities, whether of movement, -absorption, or structural excretion, being determined by its constitution. -And then, further, we observe a parallelism between the metabolic changes -in the two cases; for, on the one hand, "diminished staining capacity of -the chromatin [implying a decreased amount of phosphorus, which gives the -staining capacity] occurs during a period of intense constructive activity -in the cytoplasm;" and, on the other hand, in high organisms having nervous -systems, the intensity of nervous action is measured by the excretion of -phosphates--by the using up of the phosphorus contained in nerve-cells. - -For thus interpreting the respective functions of chromatin and cytoplasm, -yet a further reason may be given. One of the earliest general steps in the -evolution of the _Metazoa_, is the differentiation of parts which act from -parts which make them act. The _Hydrozoa_ show us this. In the hydroid -stage there are no specialized contractile organs: these are but incipient: -individual ectoderm cells have muscular processes. Nor is there any -"special aggregation of nerve-cells." If any stimulating units exist they -are scattered. But in the _Medusa_-stage nerve-matter is collected into a -ring round the edge of the umbrella. That is to say, in the undeveloped -form such motor action as occurs is not effected by a specialized part -which excites another part; but in the developed form a differentiation of -the two has taken place. All higher types exhibit this differentiation. Be -it muscle or gland or other operating organ, the cause of its activity lies -not in itself but in a nervous agent, local or central, with which it is -connected. Hence, then, there is congruity between the above interpretation -and certain general truths displayed by animal organization at large. We -may infer that in a way parallel to that just indicated, cell-evolution -was, under one of its aspects, a change from a stage in which the exciting -substance and the substance excited were mingled with approximate -uniformity, to a stage in which the exciting substance was gathered -together into the nucleus and finally into the chromosomes: leaving behind -the substance excited, now distinguished as cytoplasm. - - -§ 74f. Some further general aspects of the phenomena appear to be in -harmony with this interpretation. Let us glance at them. - -In Chapters III and IIIA of the First Part, reasons were given for -concluding that in the animal organism nitrogenous substances play the part -of decomposing agents to the carbo-hydrates--that the molecular disturbance -set up by the collapse of a proteid molecule destroys the equilibrium of -sundry adjacent carbo-hydrate molecules, and causes that evolution of -energy which accompanies their fall into molecules of simpler compounds. -Here, if the foregoing argument is valid, we may conclude that this highly -complex phosphorized compound which chromatin contains, plays the same part -to the adjacent nitrogenous compounds as these play to the carbo-hydrates. -If so, we see arising a stage earlier that "general physiological method" -illustrated in § 23f. It was there pointed out that in animal organisms the -various structures are so arranged that evolution of a small amount of -energy in one, sets up evolution of a larger amount of energy in another; -and often this multiplied energy undergoes a second multiplication of like -kind. If this view is tenable, we may now suspect that this method -displayed in the structures of the _Metazoa_ was initiated in the -structures of the _Protozoa_, and consequently characterizes those -homologues of them which compose the _Metazoa_. - -When contemplated from the suggested point of view, karyokinesis appears to -be not wholly incomprehensible. For if the chromatin yields the energy -which initiates changes throughout the rest of the cell, we may see why -there eventually arises a process for exact halving of the chromatin in a -mother-cell between two daughter-cells. To make clear the reason, let us -suppose the portioning out of the chromatin leaves one of the two with a -sensibly smaller amount than the other. What must result? Its source of -activity being relatively less, its rate of growth and its energy of action -will be less. If a protozoon, the weaker progeny arising by division of it -will originate an inferior stirp, unable to compete successfully with that -arising from the sister-cell endowed with a larger portion of chromatin. By -continual elimination of the varieties which produce unequal halving, -necessarily at a disadvantage if a moiety of their members tend continually -to disappear, there will be established a variety in which the halving is -exact: the character of this variety being such that all its members aid -the permanent multiplication of the species. If, again, the case is that of -a metazoon, there will be the same eventual result. An animal or plant in -which the chromatin is unequally divided among the cells, must have tissues -of uncertain formation. Assume that an organ has, by survival of the -fittest, been adjusted in the proportions and qualities of its parts to a -given function. If the multiplying protoplasts, instead of taking equal -portions of chromatin, have some of them smaller portions, the parts of the -organ formed of these, developing less rapidly and having inferior -energies, will throw the organ out of adjustment, and the individual will -suffer in the struggle for life. That is to say, irregular division of the -chromatin will introduce a deranging factor and natural selection will weed -out individuals in which it occurs. Of course no interpretation is thus -yielded of the special process known as karyokinesis. Probably other modes -of equal division might have arisen. Here the argument implies merely that -the tendency of evolution is to establish _some_ mode. In verification of -the view that equal division arises from the cause named, it is pointed out -to me that amitosis, which is a negation of mitosis or karyokinesis, occurs -in transitory tissues or diseased tissues or where degeneracy is going on. - -But how does all this consist with the conclusion that the chromatin -conveys hereditary traits--that it is the vehicle in which the -constitutional structure, primarily of the species and secondarily of -recent ancestors and parents, is represented? To this question there seems -to be no definite answer. We may say only that this second function is not -necessarily in conflict with the first. While the unstable units of -chromatin, ever undergoing changes, diffuse energy around, they may also be -units which, under the conditions furnished by fertilization, gravitate -towards the organization of the species. Possibly it may be that the -complex combination of proteids, common to chromatin and cytoplasm, is that -part in which the constitutional characters inhere; while the phosphorized -component, falling from its unstable union and decomposing, evolves the -energy which, ordinarily the cause of changes, now excites the more active -changes following fertilization. This suggestion harmonizes with the fact -that the fertilizing substance which in animals constitutes the head of the -spermatozoon, and in plants that of the spermatozoid or antherozoid, is -distinguished from the other agents concerned by having the highest -proportion of the phosphorized element; and it also harmonizes with the -fact that the extremely active changes set up by fertilization are -accompanied by decrease of this phosphorized element. Speculation aside, -however, we may say that the two functions of the chromatin do not exclude -one another, but that the general activity which originates from it may be -but a lower phase of that special activity caused by fertilization.[27] - - -§ 74g. Here we come unawares to the remaining topic embraced under the -title Cell-Life and Cell-Multiplication. We pass naturally from asexual -multiplication of cells to sexual multiplication--from cell-reproduction to -cell-generation. The phenomena are so numerous and so varied that a large -part of them must be passed over. Conjugation among the _Protophyta_ and -_Protozoa_, beginning with cases in which there is a mingling of the -contents of two cells in no visible respect different from one another, and -developing into a great variety of processes in which they differ, must be -left aside, and attention limited to the terminal process of fertilization -as displayed in higher types of organisms. - -Before fertilization there occurs in the ovum an incidental process of a -strange kind--"strange" because it is a collateral change taking no part in -subsequent changes. I refer to the production and extrusion of the "polar -bodies." It is recognized that the formation of each is analogous to -cell-formation in general; though process and product are both dwarfed. -Apart from any ascribed meaning, the fact itself is clear. There is an -abortive cell-formation. Abortiveness is seen firstly in the diminutive -size of the separated body or cell, and secondly in the deficient number of -its chromosomes: a corresponding deficiency being displayed in the group of -chromosomes remaining in the egg--remaining, that is (on the hypothesis -here to be suggested), in the sister-cell, supposing the polar body to be -an aborted cell. It is currently assumed that the end to be achieved by -thus extruding part of the chromosomes, is to reduce the remainder to half -the number characterizing the species; so that when, to this group in the -germ-cell, the sperm-cell brings a similarly-reduced group, union of the -two shall bring the chromosomes to the normal number. I venture to suggest -another interpretation. In doing this, however, I must forestall a -conclusion contained in the next chapter; namely, the conclusion that -gamogenesis begins when agamogenesis is being arrested by unfavourable -conditions, and that the failing agamogenesis initiates the gamogenesis. Of -numerous illustrations to be presently given I will, to make clear the -conception, name only one--the formation of fructifying organs in plants at -times when, and in places where, shoots are falling off in vigour and -leaves in size. Here the successive foliar organs, decreasingly fitted -alike in quality and dimensions for carrying on their normal lives, show us -an approaching cessation of asexual multiplication, ending in the aborted -individuals we call stamens; and the fact that sudden increase of nutrition -while gamogenesis is being thus initiated, causes resumption of -agamogenesis, shows that the gamogenesis is consequent upon the failing -agamogenesis. See then the parallel. On going back from multicellular -organisms to unicellular organisms (or those homologues of them which form -the reproductive agents in multicellular organisms), we find the same law -hold. The polar bodies are aborted cells, indicating that asexual -multiplication can no longer go on, and that the conditions leading to -sexual multiplication have arisen. If this be so, decrease in the chromatin -becomes an initial cause of the change instead of an accompanying incident; -and we need no longer assume that a quantity of precious matter is lost, -not by passive incapacity, but by active expulsion. Another anomaly -disappears. If from the germ-cell there takes place this extrusion of -superfluous chromatin, the implication would seem to be that a parallel -extrusion takes place from the sperm-cell. But this is not true. In the -sperm-cell there occurs just that failure in the production of chromatin -which, according to the hypothesis above sketched out, is to be expected; -for, in the process of cell-multiplication, the cells which become -spermatozoa are _left_ with half the number of chromosomes possessed by -preceding cells: there is actually that impoverishment and declining vigour -here suggested as the antecedent of fertilization. It needs only to imagine -the ovum and the polar body to be alike in size, to see the parallelism; -and to see that obscuration of it arises from the accumulation of cytoplasm -in the ovum. - -A test fact remains. Sometimes the first polar body extruded undergoes -fission while the second is being formed. This can have nothing to do with -reducing the number of chromosomes in the ovum. Unquestionably, however, -this change is included with the preceding changes in one transaction, -effected by one influence. If, then, it is irrelevant to the decrease of -chromosomes, so must the preceding changes be irrelevant: the hypothesis -lapses. Contrariwise this fact supports the view suggested above. That -extrusion of a polar body is a process of cell-fission is congruous with -the fact that another fission occurs after extrusion. And that this occurs -irregularly shows that the vital activities, seen in cell-growth and -cell-multiplication, now succeed in producing further fission of the -dwarfed cell and now fail: the energies causing asexual multiplication are -exhausted and there arises the state which initiates sexual multiplication. - -Maturation of the ovum having been completed, entrance of the spermatozoon, -sometimes through the limiting membrane and sometimes through a micropyle -or opening in it, takes place. This instantly initiates a series of -complicated changes: not many seconds passing before there begins the -formation of an aster around one end of the spermatozoon-head. The growth -of this aster, apparently by linear rangings of the granules composing the -reticulum of the germ-cell, progresses rapidly; while the whole structure -hence arising moves inward. Soon there takes place the fusion of this -sperm-nucleus with the germ-nucleus to form the cleavage-nucleus, which, -after a pause, begins to divide and subdivide in the same manner as cells -at large: so presently forming a cluster of cells out of which arise the -layers originating the embyro. The details of this process do not concern -us. It suffices to indicate thus briefly its general nature. - -And now ending thus the account of genesis under its histological aspect, -we pass to the account of genesis under its wider and more significant -aspects. - - - - -CHAPTER VII. - -GENESIS. - - -§ 75. Having, in the last chapter but one, concluded what constitutes an -individual, and having, in the last chapter, contemplated the histological -process which initiates a new individual, we are in a position to deal with -the multiplication of individuals. For this, the title Genesis is here -chosen as being the most comprehensive title--the least specialized in its -meaning. By some biologists Generation has been used to signify one method -of multiplication, and Reproduction to signify another method; and each of -these words has been thus rendered in some degree unfit to signify -multiplication in general. - -Here the reader is indirectly introduced to the fact that the production of -new organisms is carried on in fundamentally unlike ways. Up to quite -recent times it was believed, even by naturalists, that all the various -processes of multiplication observable in different kinds of organisms, -have one essential character in common: it was supposed that in every -species the successive generations are alike. It has now been proved, -however, that in many plants and in numerous animals, the successive -generations are not alike; that from one generation there proceeds another -whose members differ more or less in structure from their parents; that -these produce others like themselves, or like their parents, or like -neither; but that eventually, the original form re-appears. Instead of -there being, as in the cases most familiar to us, a constant recurrence of -the same form, there is a cyclical recurrence of the same form. These two -distinct processes of multiplication, may be aptly termed _homogenesis_ and -_heterogenesis_.[28] Under these heads let us consider them. - -There are two kinds of homogenesis, the simplest of them, probably once -universal but now exceptional, being that in which there is no other form -of multiplication than one resulting from perpetual spontaneous fission. -The rise of distinct sexes was doubtless a step in evolution, and before it -took place the formation of new individuals could have arisen only by -division of the old, either into two or into many. At present this process -survives, so far as appears, among _Bacteria_, certain _Algæ_, and sundry -_Protozoa_; though it is possible that a rarely-occurring conjugation has -in these cases not yet been observed. It is a probable conclusion, however, -that in the _Bacteria_ at any rate, the once universal mode of -multiplication still survives as an exceptional mode. But now passing over -these cases, we have to note that the kind of genesis (once supposed to be -the sole kind), in which the successive generations are alike, is sexual -genesis, or, as it has been otherwise called--_gamogenesis_. In every -species which multiplies by this kind of homogenesis, each generation -consists of males and females; and from the fertilized germs they produce -the next generation of similar males and females arises: the only needful -qualification of this statement being that in many _Protophyta_ and -_Protozoa_ the conjugating cells or protoplasts are not distinguishable in -character. This mode of propagation has the further trait, that each -fertilized germ usually gives rise to but one individual--the product of -development is organized round one axis and not round several axes, -Homogenesis in contrast with heterogenesis as exhibited in species which -display distinct sexuality, has also the characteristic that each new -individual begins as an egg detached from the maternal tissues, instead of -being a portion of protoplasm continuous with them, and that its -development proceeds independently. This development may be carried on -either internally or externally; whence results the division into the -oviparous and the viviparous. The oviparous kind is that in which the -fertilized germ is extruded from the parent before it has undergone any -considerable development. The viviparous kind is that in which development -is considerably advanced, or almost completed, before extrusion takes -place. This distinction is, however, not a sharply-defined one: there are -transitions between the oviparous and the viviparous processes. In -ovo-viviparous genesis there is an internal incubation; and though the -young are in this case finally extruded from the parent in the shape of -eggs, they do not leave the parent's body until after they have assumed -something like the parental form. Looking around, we find that homogenesis -is universal among the _Vertebrata_. Every vertebrate animal arises from a -fertilized germ, and unites into its single individuality the whole product -of this fertilized germ. In the mammals or highest _Vertebrata_, this -homogenesis is in every case viviparous; in birds it is uniformly -oviparous; and in reptiles and fishes it is always essentially oviparous, -though there are cases of the kind above referred to, in which viviparity -is simulated. Passing to the _Invertebrata_, we find oviparous homogenesis -universal among the _Arachnida_ (except the Scorpions, which are -ovo-viviparous); universal among the higher _Crustacea_, but not among the -lower; extremely general, though not universal, among Insects; and -universal among the higher _Mollusca_ though not among the lower. Along -with extreme inferiority among animals, we find homogenesis to be the -exception rather than the rule; and in the vegetal kingdom there appear to -be no cases, except among the _Algæ_ and a few aberrant parasites like the -_Rafflesiaceæ_, in which the centre or axis which arises from a fertilized -germ becomes the immediate producer of fertilized germs. - -In propagation characterized by unlikeness of the successive generations, -there is asexual genesis with occasionally-recurring sexual genesis; in -other words--_agamogenesis_ interrupted more or less frequently by -_gamogenesis_. If we set out with a generation of perfect males and -females, then, from their ova arise individuals which are neither males nor -females, but which produce the next generation from buds. By this method of -multiplication many individuals originate from a single fertilized germ. -The product of development is organized round more than one centre or axis. -The simplest form of heterogenesis is that seen in most uniaxial plants. -If, as we find ourselves obliged to do, we regard each separate shoot or -axis of growth as a distinct individual, homogenesis is seen in those which -have absolutely terminal flowers; but in all other uniaxial plants, the -successive individuals are not represented by the series A, A, A, A, &c., -but they are represented by the series A, B, A, B, A, B, &c. For in the -majority of plants which were classed as uniaxial (§ 50), and which may be -conveniently so distinguished from other plants, the axis which shoots up -from the seed, and substantially constitutes the plant, does not itself -flower but gives lateral origin to flowering axes. Though in ordinary -uniaxial plants the fructifying apparatus _appears_ to be at the end of the -primary, vertical axis; yet dissection shows that, morphologically -considered, each fructifying axis is an offspring from the primary axis. -There arises from the seed a sexless individual, from which spring by -gemmation individuals having reproductive organs; and from these there -result fertilized germs or seeds that give rise to sexless individuals. -That is to say, gamogenesis and agamogenesis alternate: the peculiarity -being that the sexual individuals arise from the sexless ones by continuous -development. The _Salpæ_ show us an allied form of heterogenesis in the -animal kingdom. Individuals developed from fertilized ova, instead of -themselves producing fertilized ova, produce, by gemmation, strings of -individuals from which fertilized ova again originate. In multiaxial -plants, we have a succession of generations represented by the series A, B, -B, B, &c., A, B, B, B, &c. Supposing A to be a flowering axis or sexual -individual, then, from any fertilized germ it casts off, there grows up a -sexless individual, B; from this there bud-out other sexless individuals, -B, and so on for generations more or less numerous, until at length, from -some of these sexless individuals, there bud-out seed-bearing individuals -of the original form A. Branched herbs, shrubs, and trees, exhibit this -form of heterogenesis: the successive generations of sexless individuals -thus produced being, in most cases, continuously developed, or aggregated -into a compound individual, but being in some cases discontinuously -developed. Among animals a kind of heterogenesis represented by the same -succession of letters, occurs in such compound polypes as the _Sertularia_, -and in those of the _Hydrozoa_ which assume alternately the polypoid form -and the form of the _Medusa_. The chief differences presented by these -groups arise from the fact that the successive generations of sexless -individuals produced by budding, are in some cases continuously developed, -and in others discontinuously developed; and from the fact that, in some -cases, the sexual individuals give off their fertilized germs while still -growing on the parent-polypedom, but in other cases not until after leaving -the parent-polypedom and undergoing further development. Where, as in all -the foregoing kinds of agamogenesis, the new individuals bud out, not from -any specialized reproductive organs but from unspecialized parts of the -parent, the process has been named, by Prof. Owen, _metagenesis_. In most -instances the individuals thus produced grow from the outsides of the -parents--the metagenesis is external. But there is also a kind of -metagenesis which we may distinguish as internal. Certain _entozoa_ of the -genus _Distoma_ exhibit it. From the egg of a _Distoma_ there results a -rudely-formed creature known as a sporocyst and from this a redia. -Gradually, as this divides and buds, the greater part of the inner -substance is transformed into young animals called _Cercariæ_ (which are -the larvæ of _Distomata_); until at length it becomes little more than a -living sac full of living offspring. In the _Distoma pacifica_, the brood -of young animals thus arising by internal gemmation are not _Cercariæ_, but -are like their parent: themselves becoming the producers of _Cercariæ_, -after the same manner, at a subsequent period. So that now the succession -of forms is represented by the series A, B, A, B, &c., now by the series A, -B, B, A, B, B, &c., and now by A, B, B, C, A. Both cases, however, -exemplify internal metagenesis in contrast with the several kinds of -external metagenesis described above. That agamogenesis which is carried on -in a reproductive organ--either an ovarium or the homologue of one--has -been called, by Prof. Owen, _parthenogenesis_. It is the process familiarly -exemplified in the _Aphides_. Here, from the fertilized eggs laid by -perfect females there grow up imperfect females, in the ovaria of which are -developed ova that though unfertilized, rapidly assume the organization of -other imperfect females, and are born viviparously. From this second -generation of imperfect females, there by-and-by arises, in the same -manner, a third generation of the same kind; and so on for many -generations: the series being thus symbolized by the letters A, B, B, B, B, -B, &c., A. Respecting this kind of heterogenesis it should be added that, -in animals as in plants, the number of generations of sexless individuals -produced before the re-appearance of sexual ones, is indefinite; both in -the sense that in the same species it may go on to a greater or less extent -according to circumstances, and in the sense that among the generations of -individuals proceeding from the same fertilized germ, a recurrence of -sexual individuals takes place earlier in some of the diverging lines of -multiplication than in others. In trees we see that on some branches -flower-bearing axes arise while other branches are still producing only -leaf-bearing axes; and in the successive generations of _Aphides_ a -parallel fact has been observed. Lastly has to be set down that kind of -heterogenesis in which, along with gamogenesis, there occurs a form of -agamogenesis exactly like it, save in the absence of fecundation. This is -called true parthenogenesis--reproduction carried on by virgin mothers -which are in all respects like other mothers. Among silk-worm-moths this -parthenogenesis is exceptional rather than ordinary. Usually the eggs of -these insects are fertilized; but if they are not they are still laid, and -some of them produce larvæ. In certain _Lepidoptera_, however, of the -groups _Psychidæ_ and _Tineidæ_, parthenogenesis appears to be a normal -process--indeed, so far as is known, the only process; for of some species -the males have never been found. - -A general conception of the relations among the different modes of Genesis, -thus briefly described, will be best given by the following tabular -statement. - - GENESIS is - { Oviparous - { or - Homogenesis, which is usually Gamogenesis { Ovo-viviparous - { or - { Viviparous - or - { Gamogenesis - { alternating - Heterogenesis, which is { with { Parthenogenesis - { Agamogenesis { or { Internal - { Metagenesis { or - { External - -This, like all other classifications of such phenomena, presents anomalies. -It may be justly objected that the processes here grouped under the head -agamogenesis, are the same as those before grouped under the head of -discontinuous development (§ 50): thus making development and genesis -partially coincident. Doubtless it seems awkward that what are from one -point of view considered as structural changes are from another point of -view considered as modes of multiplication.[29] There is, however, nothing -for us but a choice of imperfections. We cannot by any logical dichotomies -accurately express relations which, in Nature, graduate into one another -insensibly. Neither the above, nor any other scheme, can do more than give -an approximate idea of the truth. - - -§ 76. Genesis under every form is a process of negative or positive -disintegration; and is thus essentially opposed to that process of -integration which is the primary process in individual evolution. Negative -disintegration occurs in those cases where, as among the compound -_Hydrozoa_, there is a continuous development of new individuals by budding -from the bodies of older individuals; and where the older individuals are -thus prevented from growing to a greater size, or reaching a higher degree -of integration. Positive disintegration occurs in those forms of -agamogenesis where the production of new individuals is discontinuous, as -well as in all cases of gamogenesis. The degrees of disintegration are -various. At the one extreme the parent organism is completely broken up, or -dissolved into new individuals; and at the other extreme each new -individual forms but a small deduction from the parent organism. _Protozoa_ -and _Protophyta_ show us that form of disintegration called spontaneous -fission: two or more individuals being produced by the splitting-up of the -original one. The _Volvox_ and the _Hydrodictyon_ are plants which, having -developed broods within themselves, give them exit by bursting; and among -animals the one lately referred to which arises from the _Distoma_ egg, -entirely loses its individuality in the individualities of the numerous -_Distoma_-larvæ with which it becomes filled. Speaking generally, the -degree of disintegration becomes less marked as we approach the higher -organic forms. Plants of superior types throw off from themselves, whether -by gamogenesis or agamogenesis, parts that are relatively small; and among -superior animals there is no case in which the parent individuality is -habitually lost in the production of new individuals. To the last, however, -there is of necessity a greater or less disintegration. The seeds and -pollen-grains of a flowering plant are disintegrated portions of tissue; as -are also the ova and spermatozoa of animals. And whether the fertilized -germs carry away from their parents small or large quantities of nutriment, -these quantities in all cases involve further negative or positive -disintegrations of the parents. - -Except in spore-producing plants, new individuals which result from -agamogenesis usually do not separate from the parent-individuals until they -have undergone considerable development, if not complete development. The -agamogenetic offspring of those lowest organisms which develop centrally, -do not, of course, pass beyond central structure; but the agamogenetic -offspring of organisms which develop axially, commonly assume an axial -structure before they become independent. The vegetal kingdom shows us this -in the advanced organization of detached bulbils, and of buds that root -themselves before separating. Of animals, the _Hydrozoa_, the _Trematoda_, -and the _Salpæ_, present us with different kinds of agamogenesis, in all of -which the new individuals are organized to a considerable extent before -being cast off. This rule is not without exceptions, however. The -statoblasts of the _Plumatella_ (which play the part of winter eggs), -developed in an unspecialized part of the body, furnish a case of -metagenesis in which centres of development, instead of axes, are detached; -and in the above-described parthenogenesis of moths and bees, such centres -are detached from an ovarium. - -When produced by gamogenesis, the new individuals become (in a -morphological sense) independent of the parents while still in the shape of -centres of development, rather than axes of development; and this even -where the reverse is apparently the case. The fertilized germs of those -inferior plants which are central, or multicentral, in their development, -are of course thrown off as centres; and the same is usually the case even -in those which are uniaxial or multiaxial. In the higher plants, of the two -elements that go to the formation of the fertilized germ, the pollen-cell -is absolutely separated from the parent-plant under the shape of a centre, -and the egg-cell, though not absolutely separated from the parent, is still -no longer subordinate to the organizing forces of the parent. So that when, -after the egg-cell has been fertilized by matter from the pollen-tube, the -development commences, it proceeds without parental control: the new -individual, though remaining physically united with the old individual, -becomes structurally and functionally separate: the old individual doing no -more than supply materials. Throughout the animal kingdom, the new -individuals produced by gamogenesis are obviously separated in the shape of -centres of development wherever the reproduction is oviparous: the only -conspicuous variation being in the quantity of nutritive matter bequeathed -by the parent at the time of separation. And though, where the reproduction -is viviparous, the process appears to be different, and in one sense is so, -yet, intrinsically, it is the same. For in these cases the new individual -really detaches itself from the parent while still only a centre of -development; but instead of being finally cast off in this state it is -re-attached, and supplied with nutriment until it assumes a more or less -complete axial structure. - - -§ 77. As we have lately seen, the essential act in gamogenesis is the union -of two cell-nuclei, produced in the great majority of cases by different -parent organisms. Nearly always the containing cells, often called -_gametes_, are unlike: the sperm-cell being the male product, and the -germ-cell the female. But among some _Protozoa_ and many of the lower -_Algæ_ and _Fungi_, the uniting cells show no differentiation. Sexuality is -only nascent. - -There are very many modes and modifications of modes in which these cells -are produced; very many modes and modifications of modes by which they are -brought into contact; and very many modes and modifications of modes by -which the resulting fertilized germs have secured to them the fit -conditions for their development. But passing over these divergent and -re-divergent kinds of sexual multiplication, which it would take too much -space here to specify, the one universal trait is this coalescence of a -detached portion of one organism with a more or less detached portion of -another. - -Such simple _Algæ_ as the _Desmidieæ_, which are sometimes called -unicellular plants, show us a coalescence, not of detached portions of two -organisms, but of two entire organisms: the entire contents of the -individuals uniting to form the germ-mass. Where, as among the -_Confervoideæ_, we have aggregated cells whose individualities are scarcely -at all subordinate to that of the aggregate, the gamogenetic act is often -effected by the union "of separate motile protoplasmic masses produced by -the division of the contents of any cell of the aggregate. These -free-swimming masses of protoplasm, which are quite similar to (but -generally smaller than) the agamogenetic 'zoospores' of the same plants, -and to the free-swimming individuals of many _Protophyta_, are apparently -the primitive type of gametes (conjugating cells); but it is noteworthy -that such a gamete nearly always unites with one derived from another cell -or from another individual. The same fact holds with regard to the gametes -of the Protophytes themselves, which are formed in the same way from the -single cell of the mother individual. In the higher types of -_Confervoideæ_, and in _Vaucheria_, we find these equivalent, -free-swimming, gametes replaced by sexually differentiated sperm- and -germ-cells, in some cases arising in different organs set apart for their -production, and essentially representing those found in the higher plants. -Transitional forms, intermediate between these and the cases where -equivalent gametes are formed from any cell of the plant are also known." - -Recent investigations concerning the conjugation of _Protozoa_ have shown -that there is not, as was at one time thought, a fusion of two -individualities, but a fusion of parts of their nuclei. The macro-nucleus -having disappeared, and the micro-nucleus having broken up into portions, -each individual receives from the other one of these portions, which -becomes fused with its own nuclear matter. So that even in these humble -forms, where there is no differentiation of sexes, the union is not between -elements that have arisen in the same individual but between those which -have arisen in different individuals: the parts being in this case alike. - -The marvellous phenomena initiated by the meeting of sperm-cell and -germ-cell, or rather of their nuclei, naturally suggest the conception of -some quite special and peculiar properties possessed by these cells. It -seems obvious that this mysterious power which they display of originating -a new and complex organism, distinguishes them in the broadest way from -portions of organic substance in general. Nevertheless, the more we study -the evidence the more are we led towards the conclusion that these cells -are not fundamentally different from other cells. The first fact which -points to this conclusion is the fact recently dwelt upon (§ 63), that in -many plants and inferior animals, a small fragment of tissue which is but -little differentiated, is capable of developing into an organism like that -from which it was taken. This implies that the component units of tissues -have inherent powers of arranging themselves into the forms of the -organisms which originated them. And if in these component units, which we -distinguished as physiological, such powers exist,--if, under fit -conditions, and when not much specialized, they manifest such powers in a -way as marked as that in which the contents of sperm-cells and germ-cells -manifest them; then, it becomes clear that the properties of sperm-cells -and germ-cells are not so peculiar as we are apt to assume. Again, the -organs emitting sperm-cells and germ-cells have none of the specialities of -structure which might be looked for, did sperm-cells and germ-cells need -endowing with properties unlike those of all other organic agents. On the -contrary, these reproductive centres proceed from tissues characterized by -their low organization. In plants, for example, it is not appendages that -have acquired considerable structure which produce the fructifying -particles: these arise at the extremities of the axes where the degree of -structure is the least. The cells out of which come the egg and the -pollen-grains, are formed from undifferentiated tissue in the interior of -the ovule and of the stamen. Among many inferior animals devoid of special -reproductive organs, such as the _Hydra_, the ova and spermatozoa originate -from the interstitial cells of the ectoderm, which lie among the bases of -the functional cells--have not been differentiated for function; and in the -_Medusæ_, according to Weismann, they arise in the homologous layer, save -where the medusoid form remains attached, and then they arise in the -endoderm and migrate to the ectoderm: lack of specialization being in all -cases implied. Then in the higher animals these same generative agents -appear to be merely modified epithelium-cells--cells not remarkable for -their complexity of structure but rather for their simplicity. If, by way -of demurrer to this view, it be asked why other epithelium-cells do not -exhibit like properties; there are two replies. The first is that other -epithelium-cells are usually so far changed to fit them to their special -functions that they are unfitted for assuming the reproductive function. -The second is that in some cases, where they are but little specialized, -they _do_ exhibit the like properties: not, indeed, by uniting with other -cells to produce new germs but by producing new germs without such union. I -learn from Dr. Hooker that the _Begonia phyllomaniaca_ habitually develops -young plants from the scales of its stem and leaves--nay, that many young -plants are developed by a single scale. The epidermal cells composing one -of these scales swell, here and there, into large globular cells; form -chlorophyll in their interiors; shoot out rudimentary axes; and then, by -spontaneous constrictions, cut themselves off; drop to the ground; and grow -into Begonias. Moreover, in a succulent English plant, the _Malaxis -paludosa_, a like process occurs: the self-detached cells being, in this -case, produced by the surfaces of the leaves.[30] Thus, there is no -warrant for the assumption that sperm-cells and germ-cells possess powers -fundamentally unlike those of other cells. The inference to which the facts -point, is, that they differ from the rest mainly in not having undergone -functional adaptations. They are cells which have departed but little from -the original and most general type: such specializations as some of them -exhibit in the shape of locomotive appliances, being interpretable as -extrinsic modifications which have reference to nothing beyond certain -mechanical requirements. Sundry facts tend likewise to show that there does -not exist the profound distinction we are apt to assume between the male -and female reproductive elements. In the common polype sperm-cells and -germ-cells are developed in the same layer of indifferent tissue; and in -_Tethya_, one of the sponges, Prof. Huxley has observed that they occur -mingled together in the general parenchyma. The pollen-grains and -embryo-cells of plants arise in adjacent parts of the meristematic tissue -of the flower-bud; and from the description of a monstrosity in the -Passion-flower, recently given by Mr. Salter to the Linnæan Society, it -appears both that ovules may, in their general structure, graduate into -anthers, and that they may produce pollen in their interiors. Moreover, -among the lower _Algæ_, which show the beginning of sexual differentiation, -the smaller gametes, which we must regard as incipient sperm-cells, are -sometimes able to fuse _inter se_, and give rise to a zygote which will -produce a new plant. All which evidence is in perfect harmony with the -foregoing conclusion; since, if sperm-cells and germ-cells have natures not -essentially unlike those of unspecialized cells in general, their natures -cannot be essentially unlike each other. - -The next general fact to be noted is that these cells whose union -constitutes the essential act of gamogenesis, are cells in which the -developmental changes have come to a close--cells which are incapable of -further evolution. Though they are not, as many cells are, unfitted for -growth and metamorphosis by being highly specialized, yet they have lost -the power of growth and metamorphosis. They have severally reached a state -of equilibrium. And while the internal balance of forces prevents a -continuance of constructive changes, it is readily overthrown by external -destructive forces. For it almost uniformly happens that sperm-cells and -germ-cells which are not brought in contact disappear. In a plant, the -egg-cell, if not fertilized, is absorbed or dissipated, while the ovule -aborts; and the unimpregnated ovum eventually decomposes: save, indeed, in -those types in which parthenogenesis is a part of the normal cycle. - -Such being the characters of these cells, and such being their fates if -kept apart, we have now to observe what happens when they are united. In -plants the extremity of the elongated pollen-cell applies itself to the -surface of the embryo-sac, and one of its nuclei having, with some -protoplasm, passed into the egg-cell, there becomes fused with the nucleus -of the egg-cell. Similarly in animals, the spermatozoon passes through the -limiting membrane of the ovum, and a mixture takes place between the -substance of its nucleus and the substance of the nucleus of the ovum. But -the important fact which it chiefly concerns us to notice, is that on the -union of these reproductive elements there begins, either at once or on the -return of favourable conditions, a new series of developmental changes. The -state of equilibrium at which each had arrived is destroyed by their mutual -influence, and the constructive changes, which had come to a close, -recommence. A process of cell-multiplication is set up; and the resulting -cells presently begin to aggregate into the rudiment of a new organism. - -Thus, passing over the variable concomitants of gamogenesis, and confining -our attention to what is constant in it, we see:--that there is habitually, -if not universally, a fusion of two portions of organic substance which are -either themselves distinct individuals, or are thrown off by distinct -individuals; that these portions of organic substance, which are severally -distinguished by their low degree of specialization, have arrived at states -of structural quiescence or equilibrium; that if they are not united this -equilibrium ends in dissolution; but that by the mixture of them this -equilibrium is destroyed and a new evolution initiated. - - -§ 78. What are the conditions under which Genesis takes place? How does it -happen that some organisms multiply by homogenesis and others by -heterogenesis? Why is it that where agamogenesis prevails it is usually -from time to time interrupted by gamogenesis? A survey of the facts -discloses certain correlations which, if not universal, are too general to -be without significance. - -Where multiplication is carried on by heterogenesis we find, in numerous -cases, that agamogenesis continues as long as the forces which result in -growth are greatly in excess of the antagonist forces. Conversely, we find -that the recurrence of gamogenesis takes place when the conditions are no -longer so favourable to growth. In like manner where there is homogenetic -multiplication, new individuals are usually not formed while the preceding -individuals are still rapidly growing--that is, while the forces producing -growth exceed the opposing forces to a great extent; but the formation of -new individuals begins when nutrition is nearly equalled by expenditure. A -few out of the many facts which seem to warrant these inductions must -suffice. - -The relation in plants between fructification and innutrition (or rather, -between fructification and such diminished nutrition as makes growth -relatively slow) was long ago asserted by a German biologist--Wolff, I am -told. Since meeting with this assertion I have examined into the facts for -myself. The result has been a conviction, strengthened by every inquiry, -that some such relation exists. Uniaxial plants begin to produce their -lateral, flowering axes, only after the main axis has developed the great -mass of its leaves, and is showing its diminished nutrition by smaller -leaves, or shorter internodes, or both. In multiaxial plants two, three, or -more generations of leaf-bearing axes, or sexless individuals, are produced -before any seed-bearing individuals show themselves. When, after this first -stage of rapid growth and agamogenetic multiplication, some gamogenetic -individuals arise, they do so where the nutrition is least;--not on the -main axis, or on secondary axes, or even on tertiary axes, but on axes that -are the most removed from the channels which supply nutriment. Again, a -flowering axis is commonly less bulky than the others: either much shorter -or, if long, much thinner. And further, it is an axis of which the terminal -internodes are undeveloped: the foliar organs, which instead of becoming -leaves become sepals, and petals, and stamens, follow each other in close -succession, instead of being separated by portions of the still-growing -axis. Another group of evidences meets us when we observe the variations of -fruit-bearing which accompany variations of nutrition in the plant regarded -as a whole. Besides finding, as above, that gamogenesis commences only when -growth has been checked by extension of the remoter parts to some distance -from the roots, we find that gamogenesis is induced at an earlier stage -than usual by checking the nutrition. Trees are made to fruit while still -quite small by cutting their roots or putting them into pots; and luxuriant -branches which have had the flow of sap into them diminished, by what -gardeners call "ringing," begin to produce flower-shoots instead of -leaf-shoots. Moreover, it is to be remarked that trees which, by flowering -early in the year, seem to show a direct relation between gamogenesis and -increasing nutrition, really do the reverse; for in such trees the -flower-buds are formed in the autumn. That structure which determines these -buds into sexual individuals is given when the nutrition is declining. -Conversely, very high nutrition in plants prevents, or arrests, -gamogenesis. It is notorious that unusual richness of soil, or too large a -quantity of manure, results in a continuous production of leaf-bearing or -sexless shoots; and a like result happens when the cutting down of a tree, -or of a large part of it, is followed by the sending out of new shoots: -these, supplied with excess of sap, are luxuriant and sexless. Besides -being prevented from producing sexual individuals by excessive nutrition, -plants are, by excessive nutrition, made to change the sexual individuals -they were about to produce, into sexless ones. This arrest of gamogenesis -may be seen in various stages. The familiar instance of flowers made barren -by the transformation of their stamens into petals, shows us the lowest -degree of this reversed metamorphosis. Where the petals and stamens are -partially changed into green leaves, the return towards the agamogenetic -structure is more marked; and it is still more marked when, as occasionally -happens in luxuriantly-growing plants, new flowering axes, and even -leaf-bearing axes, grow out of the centres of flowers.[31] The anatomical -structure of the sexual axis affords corroborative evidence: giving the -impression, as it does, of an aborted sexless axis. Besides lacking those -internodes which the leaf-bearing axis commonly possesses, the flowering -axis differs by the absence of rudimentary lateral axes. In a leaf-bearing -shoot the axil of every leaf usually contains a small bud, which may or may -not develop into a lateral shoot; but though the petals of a flower are -homologous with leaves, they do not bear homologous buds at their bases. -Ordinarily, too, the foliar appendages of sexual axes are much smaller than -those of sexless ones--the stamens and pistils especially, which are the -last formed, being extremely dwarfed; and it may be that the absence of -chlorophyll from the parts of fructification is a fact of like meaning. -Moreover, the formation of the seed-vessel appears to be a direct -consequence of arrested nutrition. If a gloved-finger be taken to represent -a growing shoot, (the finger standing for the pith of the shoot and the -glove for the peripheral layers of meristem and young tissue, in which the -process of growth takes place); and if it be supposed that there is a -diminished supply of material for growth; then, it seems a fair inference -that growth will first cease at the apex of the axis, represented by the -end of the glove-finger; and supposing growth to continue in those parts of -the peripheral layers of young tissue that are nearer to the supply of -nutriment, their further longitudinal extension will lead to the formation -of a cavity at the extremity of the shoot, like that which results in a -glove-finger when the finger is partially withdrawn and the glove sticks to -its end. Whence it seems, both that this introversion of the apical -meristem may be considered as due to failing nutrition, and that the ovules -growing from its introverted surface (which would have been its outer -surface but for the defective nutrition) are extremely aborted homologues -of external appendages: both they and the pollen-grains being either -morphologically or literally quite terminal, and the last showing by their -dehiscence the exhaustion of the organizing power.[32] - -Those kinds of animals which multiply by heterogenesis, present us with a -parallel relation between the recurrence of gamogenesis and the recurrence -of conditions checking rapid growth: at least, this is shown where -experiments have thrown light on the connexion of cause and effect; namely, -among the _Aphides_. These creatures, hatched from eggs in the spring, -multiply by agamogenesis, which in this case is parthenogenesis, throughout -the summer. When the weather becomes cold and plants no longer afford -abundant sap, perfect males and females are produced; and from gamogenesis -result fertilized ova. But beyond this evidence we have much more -conclusive evidence. For it has been shown, both that the rapidity of the -agamogenesis is proportionate to the warmth and nutrition, and that if the -temperature and supply of food be artificially maintained, the agamogenesis -continues through the winter. Nay more--it not only, under these -conditions, continues through one winter, but it has been known to continue -for four successive years: some forty or fifty sexless generations being -thus produced. And those who have investigated the matter see no reason to -doubt the indefinite continuance of this agamogenetic multiplication, so -long as the external requirements are duly met. Evidence of another kind, -complicated by special influences, is furnished by the heterogenesis of the -_Daphnia_--a small crustacean commonly known as the Water-flea, which -inhabits ponds and ditches. From the nature of its habitat this little -creature is exposed to very variable conditions. Besides being frozen in -winter, the small bodies of water in which it lives are often unduly heated -by the summer Sun, or dried up by continued drought. The circumstances -favourable to the _Daphnia's_ life and growth, being thus liable to -interruptions which, in our climate, have a regular irregularity of -recurrence; we may, in conformity with the hypothesis, expect to find both -that the gamogenesis recurs along with declining physical prosperity and -that its recurrence is very variable. I use the expression "declining -physical prosperity" advisedly; since "declining nutrition," as measured by -supply of food, does not cover all the conditions. This is shown by the -experiments of Weismann (abstracted for me by Mr. Cunningham) who found -that in various _Daphnideæ_ which bring forth resting eggs, sexual and -asexual reproduction go on simultaneously, as well as separately, in the -spring and summer: these variable results being adapted to variable -conditions. For not only are these creatures liable to die from lack of -food, from the winter's cold, and from the drying up of their ditches, &c., -as well as from the over-heating of them, but during this period of -over-heating they are liable to die from that deoxygenation of the water -which heat causes. Manifestly the favourable and unfavourable conditions -recurring in combinations that are rarely twice alike, cannot be met by any -regularly recurring form of heterogenesis; and it is interesting to see how -survival of the fittest has established a mixed form. In the spring, as -well as in the autumn, there is in some cases a formation of resting or -winter eggs; and evidently these provide against the killing off of the -whole population by summer drought. Meanwhile, by ordinary males and -females there is a production of summer eggs adapted to meet the incident -of drying up by drought and subsequent re-supply of water. And all along -successive generations of parthenogenetic females effect a rapid -multiplication as long as conditions permit. Since life and growth are -impeded or arrested not by lack of food only, but by other unfavourable -conditions, we may understand how change in one or more of these may set up -one or other form of genesis, and how the mixture of them may cause a mixed -mode of multiplication which, originally initiated by external causes, -becomes by inheritance and selection a trait of the species.[33] And then -in proof that external causes initiate these peculiarities, we have the -fact that in certain _Daphnideæ_ "which live in places where existence and -parthenogenesis are possible throughout the year, the sexual period has -disappeared:" there are no males. - -Passing now to animals which multiply by homogenesis--animals in which the -whole product of a fertilized germ aggregates round a single centre or axis -instead of round many centres or axes--we see, as before, that so long as -the conditions allow rapid increase in the mass of this germ-product, the -formation of new individuals by gamogenesis does not take place. Only when -growth is declining in relative rate, do perfect sperm-cells and germ-cells -begin to appear; and the fullest activity of the reproductive function -arises as growth ceases: speaking generally, at least; for though this -relation is tolerably definite in the highest orders of animals which -multiply by gamogenesis, it is less definite in the lower orders. This -admission does not militate against the hypothesis, as it seems to do; for -the indefiniteness of the relation occurs where the limit of growth is -comparatively indefinite. We saw (§ 46) that among active, hot-blooded -creatures, such as mammals and birds, the inevitable balancing of -assimilation by expenditure establishes, for each species, an almost -uniform adult size; and among creatures of these kinds (birds especially, -in which this restrictive effect of expenditure is most conspicuous), the -connexion between cessation of growth and commencement of reproduction is -distinct. But we also saw (§ 46) that where, as in the Crocodile and the -Pike, the conditions and habits of life are such that expenditure does not -overtake assimilation as size increases, there is no precise limit of -growth; and in creatures thus circumstanced we may naturally look for a -comparatively indeterminate relation between declining growth and -commencing reproduction.[34] There is, indeed, among fishes, at least one -case which appears very anomalous. The male parr, or young of the male -salmon, a fish of four or five inches in length, is said to produce milt. -Having, at this early stage of its growth, not one-hundredth of the weight -of a full-grown salmon, how does its production of milt consist with the -alleged general law? The answer must be in great measure hypothetical. If -the salmon is (as it appears to be in its young state) a species of -fresh-water trout that has contracted the habit of annually migrating to -the sea, where it finds a food on which it thrives--if the original size of -this species was not much greater than that of the parr (which is nearly as -large as some varieties of trout)--and if the limit of growth in the trout -tribe is very indefinite, as we know it to be; then we may reasonably infer -that the parr has nearly the adult form and size which this species of -trout had before it acquired its migratory habit; and that this production -of milt is, in such case, a concomitant of the incipient decline of growth -naturally arising in the species when living under the conditions of the -ancestral species. Should this be so, the immense subsequent growth of the -parr into the salmon, consequent on a suddenly-increased facility in -obtaining food, removes to a great distance the limit at which assimilation -is balanced by expenditure; and has the effect, analogous to that produced -in plants, of arresting the incipient reproductive process. A confirmation -of this view may be drawn from the fact that when the parr, after its first -migration to the sea, returns to fresh water, having increased in a few -months from a couple of ounces to five or six pounds, it no longer shows -any fitness for propagation: the grilse, or immature salmon, does not -produce milt or spawn. - -We conclude, then, that the products of a fertilized germ go on -accumulating by simple growth, so long as the forces whence growth results -are greatly in excess of the antagonist forces; but that when diminution of -the one set of forces or increase of the other, causes a considerable -decline in this excess and an approach towards equilibrium, fertilized -germs are again produced. Whether the germ-product be organized round one -axis or round the many axes that arise by agamogenesis, matters not. -Whether, as in the higher animals, this approach to equilibrium results -from that disproportionate increase of expenditure entailed by increase of -size; or whether, as in most plants and many inferior animals, it results -from absolute or relative decline of nutrition; matters not. In any case -the recurrence of gamogenesis is associated with a decrease in the excess -of tissue-producing power. We cannot say, indeed, that this decrease always -results in gamogenesis: some organisms multiply for an indefinite period by -agamogenesis only. The Weeping Willow, which has been propagated throughout -Europe, does not seed in Europe; and yet, as the Weeping Willow, by its -large size and the multiplication of generation upon generation of lateral -axes, presents the same causes of local innutrition as other trees, we -cannot ascribe the absence of sexual axes to the continued predominance of -nutrition. Among animals, too, the anomalous case of the _Tineidæ_, a group -of moths in which parthenogenetic multiplication goes on for generation -after generation, seems to imply that gamogenesis does not necessarily -result from an approximate balance of assimilation by expenditure. What we -must say is that an approach towards equilibrium between the forces which -cause growth and the forces which oppose growth, is the chief condition to -the recurrence of gamogenesis; but that there appear to be other -conditions, in the absence of which approach to equilibrium is not followed -by gamogenesis. - - -§ 79. The above induction is an approximate answer to the question--_When_ -does gamogenesis recur? but not to the question which was propounded--_Why_ -does gamogenesis recur?--_Why_ cannot multiplication be carried on in all -cases, as it is in many cases, by agamogenesis? As already said, biologic -science is not yet advanced enough to reply. Meanwhile, the evidence above -brought together suggests a certain hypothetical answer. - -Seeing, on the one hand, that gamogenesis recurs only in individuals which -are approaching a state of organic equilibrium; and seeing, on the other -hand, that the sperm-cells and germ-cells thrown off by such individuals -are cells in which developmental changes have ended in quiescence, but in -which, after their union, there arises a process of active cell-formation; -we may suspect that the approach towards a state of general equilibrium in -such gamogenetic individuals, is accompanied by an approach towards -molecular equilibrium in them; and that the need for this union of -sperm-cell and germ-cell is the need for overthrowing this equilibrium, and -re-establishing active molecular change in the detached germ--a result -probably effected by mixing the slightly different physiological units of -slightly different individuals. The several arguments which support this -view, cannot be satisfactorily set forth until after the topics of Heredity -and Variation have been dealt with. Leaving it for the present, I propose -hereafter to re-consider it in connexion with sundry others raised by the -phenomena of Genesis. - -But before ending the chapter, it may be well to note the relations between -these different modes of multiplication, and the conditions of existence -under which they are respectively habitual. While the explanation of the -teleologist is untrue, it is often an obverse to the truth; for though, on -the hypothesis of Evolution, it is clear that things are not arranged thus -or thus for the securing of special ends, it is also clear that -arrangements which _do_ secure these special ends tend to establish -themselves--are established by their fulfilment of these ends. Besides -insuring a structural fitness between each kind of organism and its -circumstances, the working of "natural selection" also insures a fitness -between the mode and rate of multiplication of each kind of organism and -its circumstances. We may, therefore, without any teleological implication, -consider the fitness of homogenesis and heterogenesis to the needs of the -different classes of organisms which exhibit them. - -Heterogenesis prevails among organisms of which the food, though abundant -compared with their expenditure, is dispersed in such a way that it cannot -be appropriated in a wholesale manner. _Protophyta_, subsisting on diffused -gases and decaying organic matter in a state of minute subdivision, and -_Protozoa_, to which food comes in the shape of extremely small floating -particles, are enabled, by their rapid agamogenetic multiplication, to -obtain materials for growth better than they would do did they not thus -continually divide and disperse in pursuit of it. The higher plants, having -for nutriment the carbonic acid of the air and certain mineral components -of the soil, show us modes of multiplication adapted to the fullest -utilization of these substances. A herb with but little power of forming -the woody fibre requisite to make a stem that can support wide-spreading -branches, after producing a few sexless axes produces sexual ones; and -maintains its race better, by the consequent early dispersion of seeds, -than by a further production of sexless axes. But a tree, able to lift its -successive generations of sexless axes high into the air, where each gets -carbonic acid and light almost as freely as if it grew by itself, may with -advantage go on budding-out sexless axes year after year; since it thereby -increases its subsequent power of budding-out sexual axes. Meanwhile it may -advantageously transform into seed-bearers those axes which, in consequence -of their less direct access to materials absorbed by the roots, are failing -in their nutrition; for it thus throws off from a point at which sustenance -is deficient, a migrating group of germs that may find sustenance -elsewhere. The heterogenesis displayed by animals of the Coelenterate type -has evidently a like utility. A polype, feeding on minute annelids and -crustaceans which, flitting through the water, come in contact with its -tentacles, and limited to that quantity of prey which chance brings within -its grasp, buds out young polypes which, either as a colony or as dispersed -individuals, spread their tentacles through a larger space of water than -the parent alone can; and by producing them, the parent better insures the -continuance of its species than it would do if it went on slowly growing -until its nutrition was nearly balanced by its waste, and then multiplied -by gamogenesis. Similarly with the _Aphis_. Living on sap sucked from -tender shoots and leaves, and able thus to take in but a very small -quantity in a given time, this creature's race is more likely to be -preserved by a rapid asexual propagation of small individuals, which -disperse themselves over a wide area of nutrition, than it would be did the -individual growth continue so as to produce large individuals multiplying -sexually. And then when autumnal cold and diminishing supply of sap put a -check to growth, the recurrence of gamogenesis, or production of fertilized -ova which remain dormant through the winter, is more favourable to the -preservation of the race than would be a further continuance of -agamogenesis. On the other hand, among the higher animals living on food -which, though dispersed, is more or less aggregated into large masses, this -alternation of gamic and agamic reproduction ceases to be useful. The -development of the germ-product into a single organism of considerable -bulk, is in many cases a condition without which these large masses of -nutriment could not be appropriated; and here the formation of many -individuals instead of one would be fatal. But we still see the beneficial -results of the general law--the postponement of gamogenesis until the rate -of growth begins to decline. For so long as the rate of growth continues -rapid, there is proof that the organism gets food with facility--that -expenditure does not seriously check accumulation; and that the size -reached is as yet not disadvantageous: or rather, indeed, that it is -advantageous. But when the rate of growth is much decreased by the increase -of expenditure--when the excess of assimilative power is diminishing so -fast as to indicate its approaching disappearance--it becomes needful, for -the maintenance of the species, that this excess shall be turned to the -production of new individuals; since, did growth continue until there was a -complete balancing of assimilation and expenditure, the production of new -individuals would be either impossible or fatal to the parent. And it is -clear that "natural selection" will continually tend to determine the -period at which gamogenesis commences, in such a way as most favours the -maintenance of the race. - -Here, too, may fitly be pointed out the fact that, by "natural selection," -there will in every case be produced the most advantageous proportion of -males and females. If the conditions of life render numerical inequality of -the sexes beneficial to the species, in respect either of the number of the -offspring or the character of the offspring; then, those varieties of the -species which approach more than other varieties towards this beneficial -degree of inequality, will be apt to supplant other varieties. And -conversely, where equality in the number of males and females is -beneficial, the equilibrium will be maintained by the dying out of such -varieties as produce offspring among which the sexes are not balanced. - - -NOTE.--Such alterations of statement in this chapter as have been made -necessary by the advance of biological knowledge since 1864 have not, I -think, tended to invalidate its main theses, but have tended to verify -them. Some explanations to be here added may remove remaining difficulties. - -Certain types, which are transitional between _Protozoa_ and _Metazoa_, -exhibit under its simplest form the relation between self-maintenance and -race-maintenance--the integration primarily effecting the one and the -disintegration primarily effecting the other. Among the _Mycetozoa_ a -number of amoeba-like individuals aggregate into what is called a -plasmodium; and while, in some orders, they become fused into a mass of -protoplasm through which their nuclei are dispersed, in other orders -(_Sorophora_) they retain their individualities and simply form a coherent -aggregate. These last, presumably the earliest in order of evolution, -remain united so long as the plasmodium, having a small power of -locomotion, furthers the general nutrition; but when this is impeded by -drought or cold, there arise spores. Each spore contains an amoeboid -individual; and this, escaping when favourable conditions return, -establishes by fission and by union with others like itself a new colony or -plasmodium. Reduced to its lowest terms, we here see the antagonism between -that growth of the coherent mass of units which accompanies its physical -prosperity, and that incoherence and dispersion of the units which follows -unfavourable conditions and arrest of growth, and which presently initiates -new plasmodia. - -This antagonism, seen in these incipient _Metazoa_ which show us none of -that organization characterizing the _Metazoa_ in general, is everywhere in -more or less disguised forms exhibited by them--must necessarily be so if -growth of the individual is a process of integration while formation of new -individuals is a process of disintegration. And, primarily, it is an -implication that whatever furthers the one impedes the other. - -But now while recognizing the truth that nutrition and innutrition (using -these words to cover not supply of nutriment only but the presence of other -influences favourable or unfavourable to the vital processes) primarily -determine the alternations of these; we have also to recognize the truth -that from the beginning survival of the fittest has been shaping the forms -and effects of their antagonism. By inheritance a physiological habit which -modifies the form of the antagonism in a way favourable to the species, -will become established. Especially will this be the case where the lives -of the individuals have become relatively definite and where special organs -have been evolved for casting off reproductive centres. The resulting -physiological rhythm may in such cases become so pronounced as greatly to -obscure the primitive relation. Among plants we see this in the fact that -those which have been transferred from one habitat to another having widely -different seasons, long continue their original time of flowering, though -it is inappropriate to the new circumstances--the reproductive periodicity -has become organic. Similarly in each species of higher animal, development -of the reproductive organs and maturation of reproductive cells take place -at a settled age, whether the conditions have been favourable or -unfavourable to physical prosperity. The established constitutional -tendency, adapted to the needs of the species, over-rides the -constitutional needs of the individual. - -Even here, however, the primitive antagonism, though greatly obscured, -occasionally shows itself. Instance the fact that in plants where -gamogenesis is commencing a sudden access of nutrition will cause -resumption of agamogenesis; and I suspect that an illustration may be found -among human beings in the earlier establishment of the reproductive -function among the ill-fed poor than among the well-fed rich. - -One other qualification has to be added. In plants and animals which have -become so definitely constituted that at an approximately fixed stage, the -proclivity towards the production of new individuals becomes pronounced, it -naturally happens that good nutrition aids it. Surplus nutriment being -turned into the reproductive channel, the reproduction is efficient in -proportion as the surplus is great. Hence the fact that in fruit trees -which have reached the flowering stage, manuring has the effect that though -it does not increase the quantity of blossoms it increases the quantity of -fruit; and hence the fact that well-fed and easy-living races of men are -prolific. - - - - -CHAPTER VIII. - -HEREDITY. - - -§ 80. Already, in the last two chapters, the law of hereditary transmission -has been tacitly assumed; as, indeed, it unavoidably is in all such -discussions. Understood in its entirety, the law is that each plant or -animal, if it reproduces, gives origin to others like itself: the likeness -consisting, not so much in the repetition of individual traits as in the -assumption of the same general structure. This truth has been rendered so -familiar by daily illustration as almost to have lost its significance. -That wheat produces wheat--that existing oxen have descended from ancestral -oxen--that every unfolding organism eventually takes the form of the class, -order, genus, and species from which it sprang; is a fact which, by force -of repetition, has acquired in our minds almost the aspect of a necessity. -It is in this, however, that Heredity is principally displayed: the -manifestations of it commonly referred to being quite subordinate. And, as -thus understood, Heredity is universal. The various instances of -heterogenesis lately contemplated seem, indeed, to be at variance with this -assertion. But they are not really so. Though the recurrence of like forms -is, in these instances, not direct but cyclical, still, the like forms do -recur; and, when taken together, the group of forms produced during one of -the cycles is as much like the groups produced in preceding cycles, as the -single individual arising by homogenesis is like ancestral individuals. - -While, however, the general truth that organisms of a given type uniformly -descend from organisms of the same type, is so well established by infinite -illustrations as to have assumed the character of an axiom; it is not -universally admitted that non-typical peculiarities are inherited. Many -entertain a vague belief that the law of Heredity applies only to main -characters of structure and not to details; or, at any rate, that though it -applies to such details as constitute differences of species, it does not -apply to smaller details. The circumstance that the tendency to repetition -is in a slight degree qualified by the tendency to variation (which, as we -shall hereafter see, is but an indirect result of the tendency to -repetition), leads some to doubt whether Heredity is unlimited. A careful -weighing of the evidence, however, and a due allowance for the influences -by which the minuter manifestations of Heredity are obscured, may remove -this scepticism. - -First in order of importance comes the fact that not only are there -uniformly transmitted from an organism to its offspring, those traits of -structure which distinguish the class, order, genus, and species; but also -those which distinguish the variety. We have numerous cases, among both -plants and animals, where, by natural or artificial conditions, there have -been produced divergent modifications of the same species; and abundant -proof exists that the members of any one sub-species habitually transmit -their distinctive peculiarities to their descendants. Agriculturists and -gardeners can furnish unquestionable illustrations. Several varieties of -wheat are known, of which each reproduces itself. Since the potato was -introduced into England there have been formed from it a number of -sub-species; some of them differing greatly in their forms, sizes, -qualities, and periods of ripening. Of peas, also, the like may be said. -And the case of the cabbage-tribe is often cited as showing the permanent -establishment of races which have diverged widely from a common stock. -Among fruits and flowers the multiplication of kinds, and the continuance -of each kind with certainty by agamogenesis, and to some extent by -gamogenesis, might be exemplified without end. From all sides evidence may -be gathered showing a like persistence of varieties among animals. We have -our distinct breeds of sheep, our distinct breeds of cattle, our distinct -breeds of horses: each breed maintaining its characteristics. The many -sorts of dogs which, if we accept the physiological test, we must consider -as all of one species, show us in a marked manner the hereditary -transmission of small differences--each sort, when kept pure, reproducing -itself not only in size, form, colour, and quality of hair, but also in -disposition and speciality of intelligence. Poultry, too, have their -permanently-established races. And the Isle of Man sends us a tail-less -kind of cat. Even in the absence of other evidence, that which ethnology -furnishes would suffice. Grant them to be derived from one stock, and the -varieties of man yield proof upon proof that non-specific traits of -structure are bequeathed from generation to generation. Or grant only their -derivation from several stocks, and we still have, between races descended -from a common stock, distinctions which prove the inheritance of minor -peculiarities. Besides seeing the Negroes continue to produce Negroes, -copper-coloured men to produce men of a copper colour, and the fair-skinned -races to perpetuate their fair skins--besides seeing that the broad-faced -and flat-nosed Calmuck begets children with broad faces and flat noses, -while the Jew bequeaths to his offspring the features which have so long -characterized Jews; we see that those small unlikenesses which distinguish -more nearly-allied varieties of men, are maintained from generation to -generation. In Germany, the ordinary shape of skull is appreciably -different from that common in Britain: near akin though the Germans are to -the British. The average Italian face continues to be unlike the faces of -northern nations. The French character is now, as it was centuries ago, -contrasted in sundry respects with the characters of neighbouring peoples. -Nay, even between races so closely allied as the Scotch Celts, the Welsh -Celts, and the Irish Celts, appreciable differences of form and nature have -become established. - -The fact that sub-species and sub-sub-species thus exemplify the general -law of inheritance which shows itself in the perpetuation of ordinal, -generic, and species peculiarities, is strong reason for the belief that -this general lay is unlimited in its application. This has the support of -still more special evidences. They are divisible into two classes. In the -one come cases where congenital peculiarities, not traceable to any obvious -causes, are bequeathed to descendants. In the other come cases where the -peculiarities thus bequeathed are not congenital, but have resulted from -changes of functions during the lives of the individuals bequeathing them. -We will consider first the cases that come in the first class. - - -§ 81. Note at the outset the character of the chief testimony. Excluding -those inductions that have been so fully verified as to rank with exact -science, there are no inductions so trustworthy as those which have -undergone the mercantile test. When we have thousands of men whose profit -or loss depends on the truth of their inferences from perpetually-repeated -observations; and when we find that their inferences, handed down from -generation to generation, have generated an unshakable conviction; we may -accept it without hesitation. In breeders of animals we have such a class, -led by such experiences, and entertaining such a conviction--the conviction -that minor peculiarities of organization are inherited as well as major -peculiarities. Hence the immense prices given for successful racers, bulls -of superior forms, sheep that have certain desired peculiarities. Hence the -careful record of pedigrees of high-bred horses and sporting dogs. Hence -the care taken to avoid intermixture with inferior stocks. As quoted by Mr. -Darwin, Youatt says the principle of selection "enables the agriculturist -not only to modify the character of his flock but to change it altogether." -Lord Somerville, speaking of what breeders have done for sheep, says:--"It -would seem that they have chalked upon a wall a form perfect in itself and -then given it existence." That most skilful breeder, Sir John Sebright, -used to say, with respect to pigeons, that "he would produce any given -feather in three years, but it would take him six years to obtain head and -beak." In all which statements the tacit assertion is, that individual -traits are bequeathed from generation to generation, and may be so -perpetuated and increased as to become permanent distinctions. - -Of special instances there are many besides that of the often-cited -Otto-breed of sheep, descended from a single short-legged lamb, and that of -the six-fingered Gratio Kelleia, who transmitted his peculiarity, in -different degrees, to several of his children and to some of his -grandchildren. In a paper contributed to the _Edinburgh New Philosophical -Journal_ for July, 1863, Dr. (now Sir John) Struthers gives cases of -hereditary digital variations. Esther P----, who had six fingers on one -hand, bequeathed this malformation along some lines of her descendants for -two, three, and four generations. A---- S---- inherited an extra digit on -each hand and each foot from his father; and C---- G----, who also had six -fingers and six toes, had an aunt and a grandmother similarly formed. A -collection of evidence published by Mr. Sedgwick in the _Medico-Chirurgical -Review_ for April and for July, 1863, in two articles on "The Influence of -Sex in limiting Hereditary Transmission," includes the following -cases:--Augustin Duforet, a pastry-cook of Douai, who had but two instead -of three phalanges to all his fingers and toes, inherited this malformation -from his grandfather and father, and had it in common with an uncle and -numerous cousins. An account has been given by Dr. Lepine, of a man with -only three fingers on each hand and four toes on each foot, and whose -grandfather and son exhibited the like anomaly. Béchet describes Victoire -Barré as a woman who, like her father and sister, had but one developed -finger on each hand and but two toes on each foot, and whose monstrosity -re-appeared in two daughters. And there is a case where the absence of two -distal phalanges on the hands was traced for two generations. The various -recorded instances in which there has been transmission from one generation -to another, of webbed-fingers, of webbed-toes, of hare-lip, of congenital -luxation of the thigh, of absent patellæ, of club-foot, &c., would occupy -more space than can here be spared. Defects in the organs of sense are also -not unfrequently inherited. Four sisters, their mother, and grandmother, -are described by Duval as similarly affected by cataract. Prosper Lucas -details an example of amaurosis affecting the females of a family for three -generations. Duval, Graffe, Dufon, and others testify to like cases coming -under their observation.[35] Deafness, too, is occasionally transmitted -from parent to child. There are deaf-mutes whose imperfections have been -derived from ancestors; and malformations of the external ears have also -been perpetuated in offspring. Of transmitted peculiarities of the skin and -its appendages, many cases have been noted. One is that of a family -remarkable for enormous black eyebrows; another that of a family in which -every member had a lock of hair of a lighter colour than the rest on the -top of the head; and there are also instances of congenital baldness being -hereditary. From one of our leading sculptors I learn that his wife has a -flat mole under the foot near the little toe, and one of her sons has the -same. Entire absence of teeth, absence of particular teeth, and anomalous -arrangements of teeth, are recorded as traits that have descended to -children. And we have evidence that soundness and unsoundness of teeth are -transmissible. - -The inheritance of tendencies to such diseases as gout, consumption, and -insanity is universally admitted. Among the less-common diseases of which -the descent has been observed, are ichthyosis, leprosy, pityriasis, -sebaceous tumours, plica polonica, dipsomania, somnambulism, catalepsy, -epilepsy, asthma, apoplexy, elephantiasis. General nervousness displayed by -parents almost always re-appears in their children. Even a bias towards -suicide appears to be sometimes hereditary. - - -§ 82. To prove the transmission of those structural peculiarities which -have resulted from functional peculiarities, is, for several reasons, -comparatively difficult. Changes produced in the sizes of parts by changes -in their amounts of action, are mostly unobtrusive. A muscle which has -increased in bulk is usually so obscured by natural or artificial clothing, -that unless the alteration is extreme it passes without remark. Such -nervous developments as are possible in the course of a single life, cannot -be seen externally. Visceral modifications of a normal kind are observable -but obscurely, or not at all. And if the changes of structure worked in -individuals by changes in their habits are thus difficult to trace, still -more difficult to trace must be the transmission of them: further hidden, -as this is, by the influences of other individuals who are often otherwise -modified by other habits. Moreover, such specialities of structure as are -due to specialities of function, are usually entangled with specialities of -structure which are, or may be, due to selection, natural or artificial. In -most cases it is impossible to say that a structural peculiarity which -seems to have arisen in offspring from a functional peculiarity in a -parent, is wholly independent of some congenital peculiarity of structure -in the parent, whence this functional peculiarity arose. We are restricted -to cases with which natural or artificial selection can have had nothing to -do, and such cases are difficult to find. Some, however, may be noted. - -A species of plant that has been transferred from one soil or climate to -another, frequently undergoes what botanists call "change of habit"--a -change which, without affecting its specific characters, is yet -conspicuous. In its new locality the species is distinguished by leaves -that are much larger or much smaller, or differently shaped, or more -fleshy; or instead of being as before comparatively smooth, it becomes -hairy; or its stem becomes woody instead of being herbaceous; or its -branches, no longer growing upwards, assume a drooping character. Now these -"changes of habit" are clearly determined by functional changes. Occurring, -as they do, in many individuals which have undergone the same -transportation, they cannot be classed as "spontaneous variations." They -are modifications of structure consequent on modifications of function that -have been produced by modifications in the actions of external forces. And -as these modifications re-appear in succeeding generations, we have, in -them, examples of functionally-established variations that are hereditarily -transmitted. - -Evidence of analogous changes in animals is difficult to disentangle. Only -among domesticated kinds have we any opportunity of tracing the results of -altered habits; and here, in nearly all cases, artificial selection has -obscured them. Still, there are some facts which seem to the point. Mr. -Darwin, while ascribing almost wholly to "natural selection" the production -of those modifications which eventuate in differences of species, -nevertheless admits the effects of use and disuse. He says--"I find in the -domestic duck that the bones of the wing weigh less and the bones of the -leg more, in proportion to the whole skeleton, than do the same bones in -the wild duck; and I presume that this change may be safely attributed to -the domestic duck flying much less, and walking more, than its wild parent. -The great and inherited development of the udders in cows and goats in -countries where they are habitually milked, in comparison with the state of -these organs in other countries, is another instance of the effect of use. -Not a single domestic animal can be named which has not in some country -drooping ears; and the view suggested by some authors, that the drooping is -due to the disuse of the muscles of the ear, from the animals not being -much alarmed by danger, seems probable." Again--"The eyes of moles and of -some burrowing rodents are rudimentary in size, and in some cases are quite -covered up by skin and fur. This state of the eyes is probably due to -gradual reduction from disuse, but aided perhaps by natural selection." ... -"It is well known that several animals belonging to the most different -classes, which inhabit the caves of Styria and of Kentucky, are blind. In -some of the crabs the footstalk of the eye remains, though the eye is gone; -the stand for the telescope is there, though the telescope with its glasses -has been lost. As it is difficult to imagine that eyes, though useless, -could be in any way injurious to animals living in darkness, I attribute -their loss wholly to disuse."[36] The direct inheritance of an acquired -peculiarity is sometimes observable. Mr. Lewes gives a case. He "had a -puppy taken from its mother at six weeks old, who, although never taught -'to beg' (an accomplishment his mother had been taught), spontaneously took -to begging for everything he wanted when about seven or eight months old: -he would beg for food, beg to be let out of the room, and one day was found -opposite a rabbit hutch begging for rabbits." Instances are on record, too, -of sporting dogs which spontaneously adopted in the field, certain modes of -behaviour which their parents had learnt. - -But the best examples of inherited modifications produced by modifications -of function, occur in mankind. To no other cause can be ascribed the rapid -metamorphoses undergone by the British races when placed in new conditions. -In the United States the descendants of the immigrant Irish lose their -Celtic aspect, and become Americanized. This cannot be ascribed to mixture, -since the feeling with which Irish are regarded by Americans prevents any -considerable amount of intermarriage. Equally marked is the case of the -immigrant Germans who, though they keep very much apart, rapidly assume the -prevailing type. To say that "spontaneous variation" increased by natural -selection, can have produced this effect, is going too far. Peoples so -numerous cannot have been supplanted in the course of two or three -generations by varieties springing from them. Hence the implication is that -physical and social conditions have wrought modifications of function and -structure, which offspring have inherited and increased. Similarly with -special cases. In the _Cyclopædia of Practical Medicine_, Vol. II., p. 419, -Dr. Brown states that he "has in many instances observed in the case of -individuals whose complexion and general appearance has been modified by -residence in hot climates, that children born to them subsequently to such -residence, have resembled them rather in their acquired than primary mien." - -Some visible modifications of organs caused by changes in their functions, -may be noted. That large hands are inherited by those whose ancestors led -laborious lives, and that those descended from ancestors unused to manual -labour commonly have small hands, are established opinions. It seems very -unlikely that in the absence of any such connexion, the size of the hand -should have come to be generally regarded as some index of extraction. That -there exists a like relation between habitual use of the feet and largeness -of the feet, we have strong evidence in the customs of the Chinese. The -torturing practice of artificially arresting the growth of the feet, could -never have become established among the ladies of China, had they not seen -that a small foot was significant of superior rank--that is of a luxurious -life--that is of a life without bodily labour. There is evidence, too, that -modifications of the eyes, caused by particular uses of the eyes, are -inherited. Short sight appears to be uncommon among peasants; but it is -frequent among classes who use their eyes much for reading and writing, and -is often congenital. Still more marked is this relation in Germany. There, -the educated are notoriously studious, and judging from the numbers of -young Germans who wear spectacles, there is reason to think that congenital -myopia is very frequent among them. - -Some of the best illustrations of functional heredity, are furnished by -mental characteristics. Certain powers which mankind have gained in the -course of civilization cannot, I think, be accounted for without admitting -the inheritance of acquired modifications. The musical faculty is one of -these. To say that "natural selection" has developed it by preserving the -most musically endowed, seems an inadequate explanation. Even now that the -development and prevalence of the faculty have made music an occupation by -which the most musical can get sustenance and bring up families; it is very -questionable whether, taking the musical career as a whole, it has any -advantage over other careers in the struggle for existence and -multiplication. Still more if we look back to those early stages through -which the faculty must have passed before definite perception of melody was -arrived at, we fail to see how those possessing the rudimentary faculty in -a somewhat greater degree than the rest, would thereby be enabled the -better to maintain themselves and their children. There is no explanation -but that the habitual association of certain cadences of speech with -certain emotions, has slowly established in the race an organized and -inherited connection between such cadences and such emotions; that the -combination of such cadences, more or less idealized, which constitutes -melody, has all along had a meaning in the average mind, only because of -the meaning which cadences had acquired in the average mind; and that by -the continual hearing and practice of melody there has been gained and -transmitted an increasing musical sensibility. Confirmation of this view -may be drawn from individual cases. Grant that among a people endowed with -musical faculty to a certain degree, spontaneous variation will -occasionally produce men possessing it in a higher degree; it cannot be -granted that spontaneous variation accounts for the frequent production, by -such highly-endowed men, of men still more highly endowed. On the average, -the children of marriages with others not similarly endowed, will be less -distinguished rather than more distinguished. The most that can be expected -is that this unusual amount of faculty shall re-appear in the next -generation undiminished. How then shall we explain cases like those of -Bach, Mozart, and Beethoven, all of them sons of men having unusual musical -powers who were constantly exercising those powers, and who greatly -excelled their fathers in their musical powers? What shall we say to the -facts that Haydn was the son of an organist, that Hummel was born to a -music master, and that Weber's father was a distinguished violinist? The -occurrence of so many cases in one nation within a short period of time, -cannot rationally be ascribed to the coincidence of "spontaneous -variations." It can be ascribed to nothing but inherited developments of -structure caused by augmentations of function. - -But the clearest proof that structural alterations caused by alterations of -function are inherited, occurs when the alterations are morbid. I had -originally named in this place the results of M. Brown-Sequard's -experiments on guinea-pigs, showing that those which had been artificially -made epileptic had offspring which were epileptic; and I name them again -though his inference is by many rejected. For, as exemplified a few pages -back, strong evidence is often disregarded for trivial reasons by those who -dislike the conclusion drawn. Just naming this evidence and its possible -invalidity, let me pass to some results of experiences recently set forth -by Dr. Savage, President of the Neurological Society. In an essay on -"Heredity and Neurosis" published in _Brain_, Parts LXXVII, LXXVIII, 1897, -he says:--"We recognise the transmission of a tendency to develop gout, and -we recognise that the disease produced by the individual himself differs -little from that which may have been inherited." [That is, acquired gout -may be transmitted as constitutional gout.] "I have seen several patients -whose history I have been able to examine carefully, in whom mental tricks -have been transmitted from one generation to another." In the "musical -prodigies" descending from musical parents, "there seemed to be a -transmission of a greatly increased aptitude or tendency which is all one -is contending for." "Though there is, in my opinion, power to transmit -acquired peculiarities, yet the tendency is to transmit a predisposition." -(pp. 19-21.) And an authority on nervous diseases who is second to -none--Dr. Hughlings Jackson--takes the same view. The liability to -consumption shown by children of consumptive parents, which no one doubts, -shows us the same thing. It is admitted that consumption may be produced by -conditions very unfavourable to life; and unless it is held that the -disease so produced differs from the disease when inherited, the conclusion -must be that here, too, there is a transmission of functionally-produced -organic changes. This holds true whether the production of tubercle is due -to innate defect or whether it is due to the invasion of a bacillus. For in -this last case the consumptive diathesis must be regarded as a state of -body more than usually liable to invasion by the bacillus, and this is the -same when acquired as when transmitted. - - -§ 83. Two modified manifestations of Heredity remain to be noticed. The one -is the re-appearance in offspring of traits not borne by the parents, but -borne by the grandparents or by remoter ancestors. The other is the -limitation of Heredity by sex--the restriction of transmitted peculiarities -to offspring of the same sex as the parent possessing them. - -Atavism, which is the name given to the recurrence of ancestral traits, is -proved by many and varied facts. In the picture-galleries of old families, -and on the monumental brasses in the adjacent churches, are often seen -types of feature which are still, from time to time, repeated in members of -these families. It is a matter of common remark that some constitutional -diseases, such as gout and insanity, after missing a generation, will show -themselves in the next. Dr. Struthers, in his above-quoted paper "On -Variation in the Number of Fingers and Toes, and in the Phalanges in Man," -gives cases of malformations common to grandparent and grandchild, but of -which the parent had no trace. M. Girou (as quoted by Mr. Sedgwick) -says--"One is often surprised to see lambs black, or spotted with black, -born of ewes and rams with white wool, but if one takes the trouble to go -back to the origin of this phenomena, it is found in the ancestors." -Instances still more remarkable, in which the remoteness of the ancestors -copied is very great, are given by Mr. Darwin. He points out that in -crosses between varieties of the pigeon, there will sometimes re-appear the -plumage of the original rock-pigeon, from which these varieties descended; -and he thinks the faint zebra-like markings occasionally traceable in -horses have probably a like meaning. - -The other modified manifestation of heredity above referred to is the -limitation of heredity by sex. In Mr. Sedgwick's essays, already named, -will be found evidence implying that there exists some such tendency to -limitation, which does or does not show itself distinctly according to the -nature of the organic modification to be conveyed. On joining to the -evidence he gives certain bodies of allied evidence we shall, I think, find -the inconsistences comprehensible. - -Beyond the familiar facts that in ourselves, along with the essential -organs of sex there go minor structures and traits distinctive of sex, such -as the beard and the voice in man, we have numerous cases in which, along -with different sex-organs there go general differences, sometimes immense -and often conspicuous. We have those in which (as in sundry parasites) the -male is extremely small compared with the female; we have those in which -the male is winged and the female wingless; we have those, as among birds, -in which the plumage of males contrasts strongly with that of females; and -among butterflies we have kindred instances in which the wings of the two -sexes are wholly unlike--some, indeed, in which there is not simply -dimorphism but polymorphism: two kinds of females both differing from the -male. How shall we range these facts with the ordinary facts of -inheritance? Without difficulty if heredity results from the proclivity -which the component units contained in a germ-cell or a sperm-cell have to -arrange themselves into a structure like that of the structure from which -they were derived. For the obvious corollary is that where there is -gamogenesis there will result partly concurring and partly conflicting -proclivities. In the fertilized germ we have two groups of physiological -units, slightly different in their structures. These slightly-different -units severally multiply at the expense of the nutriment supplied to the -unfolding germ--each kind moulding this nutriment into units of its own -type. Throughout the process of development the two kinds of units, mainly -agreeing in their proclivities and in the form which they tend to build -themselves into, but having minor differences, work in unison to produce an -organism of the species from which they were derived, but work in -antagonism to produce copies of their respective parent-organisms. And -hence ultimately results an organism in which traits of the one are mixed -with traits of the other; and in which, according to the predominance of -one or other group of units, one or other sex with all its concomitants is -produced. - -If so, it becomes comprehensible that with the predominance of either -group, and the production of the same sex as that of the parent whence it -was derived, there will go the repetition not only of the minor sex-traits -of that parent but also of any peculiarities he or she possessed, such as -monstrosities. Since the two groups are nearly balanced, and since -inheritance is never an average of the two parents but a mixture of traits -of the one with traits of the other, it is not difficult to see why there -should be some irregularity in the transmission of these monstrosities and -constitutional tendencies, though they are most frequently transmitted only -to those of the same sex.[37] - - -§ 84. Unawares in the last paragraph there has been taken for granted the -truth of that suggestion concerning Heredity ventured in § 66. Anything -like a positive explanation is not to be expected in the present stage of -Biology, if at all. We can look for nothing beyond a simplification of the -problem; and a reduction of it to the same category with certain other -problems which also admit of hypothetical solutions only. If an hypothesis -which sundry widespread phenomena have already thrust upon us, can be shown -to render the phenomena of Heredity more intelligible than they at present -seem, we shall have reason to entertain it. The applicability of any method -of interpretation to two different but allied classes of facts, is evidence -of its truth. - -The power which many animals display of reproducing lost parts, we saw to -be inexplicable except on the assumption that the units of which any -organism is built have a tendency to arrange themselves into the shape of -that organism (§ 65). This power is sufficiently remarkable in cases where -a lost limb or tail is replaced, but it is still more remarkable in cases -where, as among some annelids, the pieces into which an individual is cut -severally complete themselves by developing heads and tails, or in cases -like that of the _Holothuria_, which having, when alarmed, ejected its -viscera, reproduces them. Such facts compel us to admit that the components -of an organism have a proclivity towards a special structure--that the -adult organism when mutilated exhibits that same proclivity which is -exhibited by the young organism in the course of its normal development. As -before said, we may, for want of a better name, figuratively call this -power organic polarity: meaning by this phrase nothing more than the -observed tendency towards a special arrangement. And such facts as those -presented by the fragments of a _Hydra_, and by fragments of leaves from -which complete plants are produced, oblige us to recognize this proclivity -as existing throughout the tissues in general--nay, in the case of the -_Begonia phyllomaniaca_, obliges us to recognize this proclivity as -existing in the physiological units contained in each undifferentiated -cell. Quite in harmony with this conclusion, are certain implications since -noticed, respecting the characters of sperm-cells and germ-cells. We saw -sundry reasons for rejecting the supposition that these are -highly-specialized cells and for accepting the opposite supposition, that -they are cells differing from others rather in being unspecialized. And -here the assumption to which we seem driven by the _ensemble_ of the -evidence, is, that sperm-cells and germ-cells are essentially nothing more -than vehicles in which are contained small groups of the physiological -units in a fit state for obeying their proclivity towards the structural -arrangement of the species they belong to. - -If the likeness of offspring to parents is thus determined, it becomes -manifest, _à priori_, that besides the transmission of generic and specific -peculiarities, there will be a transmission of those individual -peculiarities which, arising without assignable causes, are classed as -"spontaneous." For if the assumption of a special arrangement of parts by -an organism, is due to the proclivity of its physiological units towards -that arrangement; then the assumption of an arrangement of parts slightly -different from that of the species, implies physiological units slightly -unlike those of the species; and these slightly-unlike physiological units, -communicated through the medium of sperm-cell or germ-cell, will tend, in -the offspring, to build themselves into a structure similarly diverging -from the average of the species. - -But it is not equally manifest that, on this hypothesis, alterations of -structure caused by alterations of function must be transmitted to -offspring. It is not obvious that change in the form of a part, caused by -changed action, involves such change in the physiological units throughout -the organism that these, when groups of them are thrown off in the shape of -reproductive centres, will unfold into organisms that have this part -similarly changed in form. Indeed, when treating of Adaptation (§ 69), we -saw that an organ modified by increase or decrease of function, can but -slowly re-act on the system at large, so as to bring about those -correlative changes required to produce a new equilibrium; and yet only -when such new equilibrium has been established, can we expect it to be -_fully_ expressed in the modified physiological units of which the organism -is built--only then can we count on a complete transfer of the modification -to descendants. Nevertheless, that changes of structure caused by changes -of action must also be transmitted, however obscurely, appears to be a -deduction from first principles--or if not a specific deduction, still, a -general implication. For if an organism A, has, by any peculiar habit or -condition of life, been modified into the form A', it follows that all the -functions of A', reproductive function included, must be in some degree -different from the functions of A. An organism being a combination of -rhythmically-acting parts in moving equilibrium, the action and structure -of any one part cannot be altered without causing alterations of action and -structure in all the rest; just as no member of the Solar System could be -modified in motion or mass, without producing rearrangements throughout the -whole Solar System. And if the organism A, when changed to A', must be -changed in all its functions; then the offspring of A' cannot be the same -as they would have been had it retained the form A. That the change in the -offspring must, other things equal, be in the same direction as the change -in the parent, appears implied by the fact that the change propagated -throughout the parental system is a change towards a new state of -equilibrium--a change tending to bring the actions of all organs, -reproductive included, into harmony with these new actions. Or, bringing -the question to its ultimate and simplest form, we may say that as, on the -one hand, physiological units will, because of their special polarities, -build themselves into an organism of a special structure; so, on the other -hand, if the structure of this organism is modified by modified function, -it will impress some corresponding modification on the structures and -polarities of its units. The units and the aggregate must act and re-act on -each other. If nothing prevents, the units will mould the aggregate into a -form in equilibrium with their pre-existing polarities. If, contrariwise, -the aggregate is made by incident actions to take a new form, its forces -must tend to re-mould the units into harmony with this new form. And to say -that the physiological units are in any degree so re-moulded as to bring -their polar forces towards equilibrium with the forces of the modified -aggregate, is to say that when separated in the shape of reproductive -centres, these units will tend to build themselves up into an aggregate -modified in the same direction. - - -NOTE.--A large amount of additional evidence supporting the belief that -functionally produced modifications are inherited, will be found in -Appendix B. - - - - -CHAPTER IX. - -VARIATION. - - -§ 85. Equally conspicuous with the truth that every organism bears a -general likeness to its parents, is the truth that no organism is exactly -like either parent. Though similar to both in generic and specific traits, -and usually, too, in those traits which distinguish the variety, it -diverges in numerous traits of minor importance. No two plants are -indistinguishable; and no two animals are without differences. Variation is -co-extensive with Heredity. - -The degrees of variation have a wide range. There are deviations so small -as to be not easily detected; and there are deviations great enough to be -called monstrosities. In plants we may pass from cases of slight alteration -in the shape of a leaf, to cases where, instead of a flower with its calyx -above the seed-vessel, there is produced a flower with its calyx below the -seed-vessel; and while in one animal there arises a scarcely noticeable -unlikeness in the length or colour of the hair, in another an organ is -absent or a supernumerary organ appears. Though small variations are by far -the most general, yet variations of considerable magnitude are not -uncommon; and even those variations constituted by additions or -suppressions of parts, are not so rare as to be excluded from the list of -causes by which organic forms are changed. Cattle without horns are -frequent. Of sheep there are horned breeds and breeds that have lost their -horns. At one time there existed in Scotland a race of pigs with solid feet -instead of cleft feet. In pigeons, according to Mr. Darwin, "the number of -the caudal and sacral vertebræ vary; as does the number of the ribs, -together with their relative breadth and the presence of processes." - -That variations, both small and large, which arise without any specific -assignable cause, tend to become hereditary, was shown in the last chapter. -Indeed the evidence which proves Heredity in its smaller manifestations is -the same evidence which proves Variation; since it is only when there occur -variations that the inheritance of anything beyond the structural -peculiarities of the species can be proved. It remains here, however, to be -observed that the transmission of variations is itself variable; and that -it varies both in the direction of decrease and in the direction of -increase. An individual trait of one parent may be so counteracted by the -influence of the other parent, that it may not appear in the offspring; or, -not being so counteracted, the offspring may possess it, perhaps in an -equal degree or perhaps in a less degree; or the offspring may exhibit the -trait in even a still higher degree. Among illustrations of this, one must -suffice. I quote it from the essay by Sir J. Struthers referred to in the -last chapter. - -"The great-great-grandmother, Esther P---- (who married A---- L----), had a -sixth little finger on one hand. Of their eighteen children (twelve -daughters and six sons), only one (Charles) is known to have had digital -variety. We have the history of the descendants of three of the sons, -Andrew, Charles, and James. - -"(1.) Andrew L---- had two sons, Thomas and Andrew; and Thomas had two sons -all without digital variety. Here we have three successive generations -without the variety possessed by the great-grandmother showing itself. - -"(2.) James L----, who was normal, had two sons and seven daughters, also -normal. One of the daughters became Mrs. J---- (one of the informants), and -had three daughters and five sons, all normal except one of the sons, James -J----, now æt. 17, who had six fingers on each hand.... - -"In this branch of the descendants of Esther, we see it passing over two -generations and reappearing in one member of the third generation, and now -on both hands. - -"(3.) Charles L----, the only child of Esther who had digital variety, had -six fingers on each hand. He had three sons, James, Thomas, and John, all -of whom were born with six fingers on each hand, while John has also a -sixth toe on one foot. He had also five other sons and four daughters, all -of whom were normal. - -"(a.) Of the normal children of this, the third generation, the five sons -had twelve sons and twelve daughters, and the four daughters have had four -sons and four daughters, being the fourth generation, all of whom were -normal. A fifth generation in this sub-group consists as yet of only two -boys and two girls who are also normal. - -"In this sub-branch, we see the variety of the first generation present in -the second, passing over the third and fourth, and also the fifth as far as -it has yet gone. - -"(b.) James had three sons and two daughters, who are normal. - -"(c.) Thomas had four sons and five daughters, who are normal; and has two -grandsons, also normal. - -"In this sub-branch of the descent, we see the variety of the first -generation, showing itself in the second and third, and passing over the -fourth, and (as far as it yet exists) the fifth generation. - -"(d.) John L---- (one of the informants) had six fingers, the additional -finger being attached on the outer side, as in the case of his brothers -James and Thomas. All of them had the additional digits removed. John has -also a sixth toe on one foot, situated on the outer side. The fifth and -sixth toes have a common proximal phalange, and a common integument invests -the middle and distal phalanges, each having a separate nail. - -"John L---- has a son who is normal, and a daughter, Jane, who was born -with six fingers on each hand and six toes on each foot. The sixth fingers -were removed. The sixth toes are not wrapped with the fifth as in her -father's case, but are distinct from them. The son has a son and daughter, -who, like himself, are normal. - -"In this, the most interesting sub-branch of the descent, we see digital -increase, which appeared in the first generation on one limb, appearing in -the second on two limbs, the hands; in the third on three limbs, the hands -and one foot; in the fourth on all the four limbs. There is as yet no fifth -generation in uninterrupted transmission of the variety. The variety does -not yet occur in any member of the fifth generation of Esther's -descendants, which consists, as yet, only of three boys and one girl, whose -parents were normal, and of two boys and two girls, whose grandparents were -normal. It is not known whether in the case of the great-great-grandmother, -Esther P----, the variety was original or inherited."[38] - - -§ 86. Where there is great uniformity among the members of a species, the -divergences of offspring from the average type are usually small; but -where, among the members of a species, considerable unlikenesses have once -been established, unlikenesses among the offspring are frequent and great. -Wild plants growing in their natural habitats are uniform over large areas, -and maintain from generation to generation like structures; but when -cultivation has caused appreciable differences among the members of any -species of plant, extensive and numerous deviations are apt to arise. -Similarly, between wild and domesticated animals of the same species, we -see the contrast that though the homogeneous wild race maintains its type -with great persistence, the comparatively heterogeneous domestic race -frequently produces individuals more unlike the average type than the -parents are. - -Though unlikeness among progenitors is one antecedent of variation, it is -by no means the sole antecedent. Were it so, the young ones successively -born to the same parents would be alike. If any peculiarity in a new -organism were a direct resultant of the structural differences between the -two organisms which produced it; then all subsequent new organisms produced -by these two would show the same peculiarity. But we know that the -successive offspring have different peculiarities: no two of them are ever -exactly alike. - -One cause of such structural variation in progeny, is functional variation -in parents. Proof of this is given by the fact that, among progeny of the -same parents, there is more difference between those begotten under -different constitutional states than between those begotten under the same -constitutional state. It is notorious that twins are more nearly alike than -children borne in succession. The functional conditions of the parents -being the same for twins, but not the same for their brothers and sisters -(all other antecedents being constant), we have no choice but to admit that -variations in the functional conditions of the parents, are the antecedents -of those greater unlikenesses which their brothers and sisters exhibit. - -Some other antecedent remains, however. The parents being the same, and -their constitutional states the same, variation, more or less marked, still -manifests itself. Plants grown from seeds out of one pod, or animals -produced at one birth, are not alike. Sometimes they differ considerably. -In a litter of pigs or of kittens, we rarely see uniformity of markings; -and occasionally there are important structural contrasts. I have myself -recently been shown a litter of Newfoundland puppies, some of which had -four digits to their feet, while in others there was present, on each -hind-foot, what is called the "dew-claw"--a rudimentary fifth digit. - -Thus, induction points to three causes of variation, all in action -together. We have heterogeneity among progenitors, which, did it act -uniformly and alone in generating, by composition of forces, new -deviations, would impress such new deviations to the same extent on all -offspring of the same parents; which it does not. We have functional -variation in the parents, which, acting either alone or in combination with -the preceding cause, would entail the same structural variations on all -young ones simultaneously produced; which it does not. Consequently there -is some third cause of variation, yet to be found, which acts along with -the structural and functional variations of ancestors and parents. - - -§ 87. Already, in the last section, there has been implied some relation -between variation and the action of external conditions. The above-cited -contrast between the uniformity of a wild species and the multiformity of -the same species when cultivated or domesticated, thrusts this truth upon -us. Respecting the variations of plants, Mr. Darwin remarks that "'sports' -are extremely rare under nature, but far from rare under cultivation." -Others who have studied the matter assert that if a species of plant which, -up to a certain time, has maintained great uniformity, once has its -constitution thoroughly disturbed, it will go on varying indefinitely. -Though, in consequence of the remoteness of the periods at which they were -domesticated, there is a lack of positive proof that our extremely variable -domestic animals have become variable under the changed conditions implied -by domestication, having been previously constant; yet competent judges do -not doubt that this has been the case. - -Now the constitutional disturbance which precedes variation, can be nothing -else than an overthrowing of the pre-established equilibrium of functions. -Transferring a plant from forest lands to a ploughed field or a manured -garden, is altering the balance of forces to which it has been hitherto -subject, by supplying it with different proportions of the assimilable -matters it requires, and taking away some of the positive impediments to -its growth which competing wild plants before offered. An animal taken from -woods or plains, where it lived on wild food of its own procuring, and -placed under restraint while artificially supplied with food not quite like -what it had before, is an animal subject to new outer actions to which its -inner actions must be adjusted. From the general law of equilibration we -found it to follow that "the maintenance of such a moving equilibrium" as -an organism displays, "requires the habitual genesis of internal forces -corresponding in number, directions, and amounts, to the external incident -forces--as many inner functions, single or combined, as there are single or -combined outer actions to be met" (_First Principles_, § 173); and more -recently (§ 27), we have seen that Life itself is "the definite combination -of heterogeneous changes, both simultaneous and successive, in -correspondence with external co-existences and sequences." Necessarily, -therefore, an organism exposed to a permanent change in the arrangement of -outer forces must undergo a permanent change in the arrangement of inner -forces. The old equilibrium has been destroyed; and a new equilibrium must -be established. There must be functional perturbations, ending in a -re-adjusted balance of functions. - -If, then, change of conditions is the only known cause by which the -original homogeneity of a species is destroyed; and if change of conditions -can affect an organism only by altering its functions; it follows that -alteration of functions is the only known internal cause to which the -commencement of variation can be ascribed. That such minor functional -changes as parents undergo from year to year are influential on the -offspring, we have seen is proved by the greater unlikeness that exists -between children born to the same parents at different times, than exists -between twins. And here we seem forced to conclude that the larger -functional variations produced by greater external changes, are the -initiators of those structural variations which, when once commenced in a -species, lead by their combinations and antagonisms to multiform results. -Whether they are or are not the direct initiators, they must still be the -indirect initiators. - - -§ 87a. In the foregoing sentence those pronounced structural variations -from which may presently arise new varieties and eventually species, are -ascribed to "the larger functional variations produced by greater external -changes"; and this limitation is a needful one, since there is a constant -cause of minor variations of a wholly different kind. - -There are the variations arising from differences in the conditions to -which the germ is subject, both before detachment from the parent and -after. At first sight it seems that plants grown from seeds out of the same -seed-vessel and animals belonging to the same litter, ought, in the absence -of any differences of ancestral antecedents, to be entirely alike. But this -is not so. Inevitably they are subject from the very outset to slightly -different sets of agencies. The seeds in a seed-vessel do not stand in -exactly the same relations to the sources of nutriment: some are nearer -than others. They are somewhat differently exposed to the heat and light -penetrating their envelope; and some are more impeded in their growth by -neighbours than others are. Similarly with young animals belonging to the -same litter. Their uterine lives are made to some extent unlike by unlike -connexions with the blood-supply, by mutual interferences not all the same, -and even by different relations to the disturbances caused by the mother's -movements. So, too, is it after separation from the parent plant or animal. -Even the biblical parable reminds us that seeds fall into places here -favourable and there unfavourable in various degrees. In respect of soil, -in respect of space for growth, in respect of shares of light, none of them -are circumstanced in quite the same ways. With animals the like holds. In a -litter of pigs some, weaker than others, do not succeed as often in getting -possession of teats. And then in both cases the differences thus initiated -become increasingly pronounced. Among young plants the smaller, outgrown by -their better-placed neighbours, are continually more shaded and more left -behind; and among the litter the weakly ones, continually thrust aside by -the stronger, become relatively more weakly from deficient nutrition. - -Differentiations thus arising, both before and after separation from -parents, though primarily differences of growth, entail structural -differences; for it is a general law of nutrition that when there is -deficiency of food the non-essential organs suffer more than the essential -ones, and the unlikenesses of proportion hence arising constitute -unlikenesses of structure. It may be concluded, however, that variations -generated in this manner usually have no permanent results. In the first -place, the individuals which, primarily in growth and secondarily in -smaller developments of less-important organs, are by implication inferior, -are likely to be eliminated from the species. In the second place, -differences of structure produced in the way shown do not express -differences of constitution--are not the effects of somewhat divergent -physiological units; and consequently are not likely to be repeated in -posterity. - - -§ 88. We have still, therefore, to explain those variations which have no -manifest causes of the kinds thus far considered. These are the variations -termed "spontaneous." Not that those who apply to them this word, or some -equivalent, mean to imply that they are uncaused. Mr. Darwin expressly -guards himself against such an interpretation. He says:--"I have hitherto -sometimes spoken as if the variations--so common and multiform in organic -beings under domestication, and in a lesser degree in those in a state of -nature--had been due to chance. This, of course, is a wholly incorrect -expression, but it serves to acknowledge plainly our ignorance of the cause -of each particular variation." Not only, however, do I hold, in common with -Mr. Darwin, that there must be some cause for these apparently-spontaneous -variations, but it seems to me that a definite cause is assignable. I think -it may be shown that unlikenesses must necessarily arise even between the -new individuals simultaneously produced by the same parents. Instead of the -occurrence of such variations being inexplicable, the absence of them would -be inexplicable. - -In any series of dependent changes a small initial difference often works a -marked difference in the results. The mode in which a particular breaker -bursts on the beach, may determine whether the seed of some foreign plant -which it bears is or is not stranded--may cause the presence or absence of -this plant from the Flora of the land; and may so affect, for millions of -years, in countless ways, the living creatures throughout the land. A -single touch, by introducing into the body some morbid matter, may set up -an immensely involved set of functional disturbances and structural -alterations. The whole tenor of a life may be changed by a word of advice; -or a glance may determine an action which alters thoughts, feelings, and -deeds throughout a long series of years. In those still more involved -combinations of changes which societies exhibit, this truth is still more -conspicuous. A hair's-breadth difference in the direction of some soldier's -musket at the battle of Arcola, by killing Napoleon, might have changed -events throughout Europe; and though the type of social organization in -each European country would have been now very much what it is, yet in -countless details it would have been different. - -Illustrations like these, with which pages might be filled, prepare us for -the conclusion that organisms produced by the same parents at the same -time, must be more or less differentiated, both by insensible initial -differences and by slight differences in the conditions to which they are -subject during their evolution. We need not, however, rest with assuming -such initial differences: the necessity of them is demonstrable. The -individual germ-cells which, in succession or simultaneously, are separated -from the same parent, can never be exactly alike; nor can the sperm-cells -which fertilize them. When treating of the instability of the homogeneous -(_First Principles_, § 149), we saw that no two parts of any aggregate can -be similarly conditioned with respect to incident forces; and that being -subject to forces that are more or less unlike, they must become more or -less unlike. Hence, no two ova in an ovarium or ovules in a seed-vessel--no -two spermatozoa or pollen-cells, can be identical. Whether or not there -arise other contrasts, there are certain to arise quantitative contrasts; -since the process of nutrition cannot be absolutely alike for all. The -reproductive centres must begin to differentiate from the very outset. Such -being the necessities of the case, what will happen on any successive or -simultaneous fertilizations? Inevitably unlikenesses between the respective -parental influences must result. Quantitative differences among the -sperm-cells and among the germ-cells, will insure this. Grant that the -number of physiological units contained in any one reproductive cell, can -rarely if ever be exactly equal to the number contained in any other, -ripened at the same time or at a different time; and it follows that among -the fertilized germs produced by the same parents, the physiological units -derived from them respectively will bear a different numerical ratio to -each other in every case. If the parents are constitutionally quite alike, -the variation in the ratio between the units they severally bequeath, -cannot cause unlikenesses among the offspring. But if otherwise, no two of -the offspring can be alike. In every case the small initial difference in -the proportions of the slightly-unlike units, will lead, during evolution, -to a continual multiplication of differences. The insensible divergence at -the outset will generate sensible divergences at the conclusion. Possibly -some may hence infer that though, in such case, the offspring must differ -somewhat from each other and from both parents, yet that in every one of -them there must result a homogeneous mixture of the traits of the two -parents. A little consideration shows that the reverse is inferable. If, -throughout the process of development, the physiological units derived from -each parent preserved the same ratio in all parts of the growing organism, -each organ would show as much as every other, the influence of either -parent. But no such uniform distribution is possible. It has been shown -(_First Principles_, § 163), that in any aggregate of mixed units -segregation must inevitably go on. Incident forces will tend ever to cause -separation of the two orders of units from each other--will tend to -integrate groups of the one order in one place and groups of the other -order in another place. Hence there must arise not a homogeneous mean -between the two parents, but a mixture of organs, some of which mainly -follow the one and some the other. And this is the kind of mixture which -observation shows us. - -Still it may be fairly objected that however the attributes of the two -parents are variously mingled in their offspring, they must in all of them -fall between the extremes displayed in the parents. In no characteristic -could one of the young exceed both parents, were there no cause of -"spontaneous variation" but the one alleged. Evidently, then, there is a -cause yet unfound. - - -§ 89. Thus far we have contemplated the process under its simplest aspect. -While we have assumed the two parents to be somewhat unlike, we have -assumed that each parent has a homogeneous constitution--is built up of -physiological units which are exactly alike. But in no case can such a -homogeneity exist. Each parent had parents who were more or less -contrasted--each parent inherited at least two orders of physiological -units not quite identical. Here then we have a further cause of variation. -The sperm-cells or germ-cells which any organism produces, will differ from -each other not quantitatively only but qualitatively. Of the -slightly-unlike physiological units bequeathed to it, the reproductive -cells it casts off cannot habitually contain the same proportions; and we -may expect the proportions to vary not slightly but greatly. Just as, -during the evolution of an organism, the physiological units derived from -the two parents tend to segregate, and produce likeness to the male parent -in this part and to the female parent in that; so, during the formation of -reproductive cells, there will arise in one a predominance of the -physiological units derived from the father, and in another a predominance -of the physiological units derived from the mother. Thus, then, every -fertilized germ, besides containing different _amounts_ of the two parental -influences, will contain different _kinds_ of influences--this having -received a marked impress from one grandparent, and that from another. -Without further exposition the reader will see how this cause of -complication, running back through each line of ancestry, must produce in -every germ numerous minute differences among the units. - -Here, then, we have a clue to the multiplied variations, and sometimes -extreme variations, that arise in races which have once begun to vary. Amid -countless different combinations of units derived from parents, and through -them from ancestors, immediate and remote--amid the various conflicts in -their slightly-different organic polarities, opposing and conspiring with -one another in all ways and degrees; there will from time to time arise -special proportions causing special deviations. From the general law of -probabilities it may be concluded that while these involved influences, -derived from many progenitors, must, on the average of cases, obscure and -partially neutralize one another; there must occasionally result such -combinations of them as will produce considerable divergences from average -structures; and, at rare intervals, such combinations as will produce very -marked divergences. There is thus a correspondence between the inferable -results and the results as habitually witnessed. - - -§ 90. Still there remains a difficulty. It may be said that admitting -functional change to be the initiator of variation--granting that the -physiological units of an organism long subject to new conditions, will -tend to become modified in such way as to cause change of structure in -offspring; yet there will still be no cause of the supposed heterogeneity -among the physiological units of different individuals. There seems -validity in the objection, that as all the members of a species whose -circumstances have been altered will be affected in the same manner, the -results, when they begin to show themselves in descendants, will show -themselves in the same manner: not multiform variations will arise, but -deviations all in one direction. - -The reply is simple. The members of a species thus circumstanced will _not_ -be similarly affected. In the absence of absolute uniformity among them, -the functional changes caused in them will be more or less dissimilar. Just -as men of slightly-unlike dispositions behave in quite opposite ways under -the same circumstances; or just as men of slightly-unlike constitutions get -diverse disorders from the same cause, and are diversely acted on by the -same medicine; so, the insensibly-differentiated members of a species whose -conditions have been changed, may at once begin to undergo various kinds of -functional changes. As we have already seen, small initial contrasts may -lead to large terminal contrasts. The intenser cold of the climate into -which a species has migrated, may cause in one individual increased -consumption of food to balance the greater loss of heat; while in another -individual the requirement may be met by a thicker growth of fur. Or, when -meeting with the new foods which a new region furnishes, accident may -determine one member of the species to begin with one kind and another -member with another kind; and hence may arise established habits in these -respective members and their descendants. Now when the functional -divergences thus set up in sundry families of a species have lasted long -enough to affect their constitutions, and to modify somewhat the -physiological units thrown off in their reproductive cells, the divergences -produced by these in offspring will be of divers kinds. And the original -homogeneity of constitution having been thus destroyed, variation may go on -with increasing facility. There will result a heterogeneous mixture of -modifications of structure caused by modifications of function; and of -still more numerous correlated modifications, indirectly so caused. By -natural selection of the most divergent forms, the unlikenesses of parents -will be rendered more marked, and the limits of variation wider. Until at -length the divergences of constitutions and modes of life, become great -enough to lead to segregation of the varieties. - - -§ 91. That variations must occur, and that they must ever tend, both -directly and indirectly, towards adaptive modifications, are conclusions -deducible from first principles; apart from any detailed interpretations -like the above. That the state of homogeneity is an unstable state we have -found to be a universal truth. Each species must pass from the uniform into -the more or less multiform, unless the incidence of external forces is -exactly the same for all its members, which it never can be. Through the -process of differentiation and integration, which of necessity brings -together, or keeps together, like individuals, and separates unlike ones -from them, there must nevertheless be maintained a tolerably uniform -species, so long as there continues a tolerably uniform set of conditions -in which it may exist. But if the conditions change, either absolutely by -some disturbance of the habitat or relatively by spread of the species into -other habitats, then the divergent individuals that result must be -segregated by the divergent sets of conditions into distinct varieties -(_First Principles_, § 166). When, instead of contemplating a species in -the aggregate, we confine our attention to a single member and its -descendants, we see it to be a corollary from the general law of -equilibration that the moving equilibrium constituted by the vital actions -in each member of this family, must remain constant so long as the external -actions to which they correspond remain constant; and that if the external -actions are changed, the disturbed balance of internal changes, if not -overthrown, cannot cease undergoing modification until the internal changes -are again in equilibrium with the external actions: corresponding -structural alterations having arisen. - -On passing from these derivative laws to the ultimate law, we see that -Variation is necessitated by the persistence of force. The members of a -species inhabiting any area cannot be subject to like sets of forces over -the whole of that area. And if, in different parts of the area, different -kinds or amounts or combinations of forces act on them, they cannot but -become different in themselves and in their progeny. To say otherwise, is -to say that differences in the forces will not produce differences in the -effects; which is to deny the persistence of force. - - - - -CHAPTER X. - -GENESIS, HEREDITY, AND VARIATION. - - -§ 92. A question raised, and hypothetically answered, in §§ 78 and 79, was -there postponed until we had dealt with the topics of Heredity and -Variation. Let us now resume the consideration of this question, in -connexion with sundry others which the facts suggest. - -After contemplating the several methods by which the multiplication of -organisms is carried on--after ranging them under the two heads of -Homogenesis, in which the successive generations are similarly produced, -and Heterogenesis, in which they are dissimilarly produced--after observing -that Homogenesis is nearly always sexual genesis, while Heterogenesis is -asexual genesis with occasionally-recurring sexual genesis; we came to the -questions--why is it that some organisms multiply in the one way and some -in the other? and why is it that where agamogenesis prevails it is usually, -from time to time, interrupted by gamogenesis? In seeking answers to these -questions, we inquired whether there are common to both Homogenesis and -Heterogenesis, any conditions under which alone sperm-cells and germ-cells -arise and are united for the production of new organisms; and we reached -the conclusion that, in all cases, they arise only when there is an -approach to equilibrium between the forces which produce growth and the -forces which oppose growth. This answer to the question--_when_ does -gamogenesis recur? still left unanswered the question--_why_ does -gamogenesis recur? And to this the reply suggested was, that the approach -towards general equilibrium in organisms, "is accompanied by an approach -towards molecular equilibrium in them; and that the need for this union of -sperm-cell with germ-cell is the need for overthrowing this equilibrium, -and re-establishing active molecular change in the detached germ--a result -probably effected by mixing the slightly-different physiological units of -slightly-different individuals." This is the hypothesis which we have now -to consider. Let us first look at the evidences which certain inorganic -phenomena furnish. - -The molecules of any aggregate which have not a balanced arrangement, -inevitably tend towards a balanced arrangement. As before mentioned (_First -Principles_, § 100), amorphous wrought iron, when subject to continuous -jar, begins to arrange itself into crystals--its atoms assume a condition -of polar equilibrium. The particles of unannealed glass, which are so -unstably arranged that slight disturbing forces make them separate into -small groups, take advantage of that greater freedom of movement given by a -raised temperature, to adjust themselves into a state of relative rest. -During any such re-arrangement the aggregate exercises a coercive force -over its units. Just as in a growing crystal the atoms successively -assimilated from the solution, are made by the already crystallized atoms -to take a certain form, and even to re-complete that form when it is -broken; so in any mass of unstably-arranged atoms which passes into a -stable arrangement, each atom conforms to the forces exercised on it by all -the other atoms. This is a corollary from the general law of equilibration. -We saw (_First Principles_, § 170) that every change is towards -equilibrium; and that change can never cease until equilibrium is reached. -Organisms, above all other aggregates, conspicuously display this -progressive equilibration; because their units are of such kinds, and so -conditioned, as to admit of easy re-arrangement. Those extremely active -changes which go on during the early stages of evolution, imply an immense -excess of the molecular forces over those antagonist forces which the -aggregate exercises on the molecules. While this excess continues, it is -expended in growth, development, and function: expenditure for any of these -purposes being proof that part of the force constituting molecular tensions -remains unbalanced. Eventually, however, this excess diminishes. Either, as -in organisms which do not expend much energy, decrease of assimilation -leads to its decline; or, as in organisms which expend much energy, it is -counterbalanced by the rapidly-increasing reactions of the aggregate -(§ 46). The cessation of growth when followed, as in some organisms, by -death, implies the arrival at an equilibrium between the molecular forces -and those forces which the aggregate opposes to them. When, as in other -organisms, growth ends in the establishment of a moving equilibrium, there -is implied such a decreased preponderance of the molecular forces, as -leaves no surplus beyond that which is used up in functions. The declining -functional activity characteristic of advancing life, expresses a further -decline in this surplus. And when all vital movements come to an end, the -implication is that the actions of the units on the aggregate and the -reactions of the aggregate on the units are completely balanced. Hence, -while a state of rapid growth indicates such a play of forces among the -units of an aggregate as will produce active re-distribution, the -diminution and arrest of growth shows that the units have fallen into such -relative positions that re-distribution is no longer so facile. When, -therefore, we see that gamogenesis recurs only when growth is decreasing, -or has come to an end, we must say that it recurs only when the organic -units are approximating to equilibrium--only when their mutual restraints -prevent them from readily changing their arrangements in obedience to -incident forces. - -That units of like forms can be built up into a more stable aggregate than -units of slightly unlike forms, is tolerably manifest _à priori_. And we -have facts which prove that mixing allied but somewhat different units, -_does_ lead to comparative instability. Most metallic alloys exemplify this -truth. Common solder, which is a mixture of lead and tin, melts at a much -lower temperature than either lead or tin. The compound of lead, tin, and -bismuth, called "fusible metal," becomes fluid at the temperature of -boiling water; while the temperatures at which lead, tin, and bismuth -become fluid are, respectively, 612°, 442°, and 497° F. Still more -remarkable is the illustration furnished by potassium and sodium. These -metals are very near akin in all respects--in their specific gravities, -their atomic weights, their chemical affinities, and the properties of -their compounds. That is to say, all the evidences unite to show that their -units, though not identical, have a close resemblance. What now happens -when they are mixed? Potassium alone melts at 136°, sodium alone melts at -190°, but the alloy of potassium and sodium is liquid at the ordinary -temperature of the air. Observe the meaning of these facts, expressed in -general terms. The maintenance of a solid form by any group of units -implies among them an arrangement so stable that it is not overthrown by -the incident forces. Whereas the assumption of a liquid form implies that -the incident forces suffice to destroy the arrangement of the units. In the -one case the thermal undulations fail to dislocate the parts; while in the -other case the parts are so dislocated by the thermal undulations that they -fall into total disorder--a disorder admitting of easy re-arrangement into -any other order. For the liquid state is a state in which the units become -so far free from mutual restraints, that incident forces can change their -relative positions very readily. Thus we have reason to conclude that an -aggregate of units which, though in the main similar to one another, have -minor differences, must be more unstable than an aggregate of homogeneous -units. The one will yield to disturbing forces which the other successfully -resists. - -Now though the colloidal molecules of which organisms are mainly built, are -themselves highly composite; and though the physiological units compounded -out of these colloidal molecules must have structures far more involved; -yet it must happen with such units, as with simple units, that those which -have exactly like forms will admit of arrangement into a more stable -aggregate than those which have slightly-unlike forms. Among units of this -order, as among units of a simpler order, imperfect similarity must entail -imperfect balance in anything formed of them, and consequent diminished -ability to withstand disturbing forces. Hence, given two organisms which, -by diminished nutrition or increased expenditure, are being arrested in -their growths--given in each an approaching equilibrium between the forces -of the units and the forces of the aggregate--given, that is, such a -comparatively balanced state among the units that re-arrangement of them by -incident forces is no longer so easy; and it will follow that by uniting a -group of units from the one organism with a group of slightly-different -units from the other, the tendency towards equilibrium will be diminished, -and the mixed units will be rendered more modifiable in their arrangements -by the forces acting on them: they will be so far freed as to become again -capable of that re-distribution which constitutes evolution. - -And now let us test this hypothesis by seeing what power it gives us of -interpreting established inductions. - - -§ 93. The majority of plants being hermaphrodites, it has, until quite -recently, been supposed that the ovules of each flower are fertilized by -pollen from the anthers of the same flower. Mr. Darwin, however, has shown -that the arrangements are generally such as to prevent this. Either the -ovules and the pollen are not ripe simultaneously, or obstacles prevent -access of the one to the other. At the same time he has shown that there -exist arrangements, often of a remarkable kind, which facilitate the -transfer of pollen by insects from the stamens of one flower to the pistil -of another. Similarly, it has been found that among the lower animals, -hermaphrodism does not usually involve the production of fertile ova by the -union of sperm-cells and germ-cells developed in the same individual; but -that the reproductive centres of one individual are united with those of -another to produce fertile ova. Either, as in _Pyrosoma_, _Perophora_, and -in many higher molluscs, the ova and spermatozoa are matured at different -times; or, as in annelids, they are prevented by their relative positions -from coming in contact. - -Remembering the fact that among the higher classes of organisms, -fertilization is always effected by combining the sperm-cell of one -individual with the germ-cell of another; and joining with it the above -fact that among hermaphrodite organisms, the germ-cells developed in any -individual are usually not fertilized by sperm-cells developed in the same -individual; we see reason for thinking that the essential thing in -fertilization, is the union of specially-fitted portions of _different_ -organisms. If fertilization depended on the peculiar properties of -sperm-cell and germ-cell, as such; then, in hermaphrodite organisms, it -would be a matter of indifference whether the united sperm-cells and -germ-cells were those of the same individual or those of different -individuals. But the circumstance that there exist in such organisms -elaborate appliances for mutual fertilization, shows that unlikeness of -derivation in the united reproductive centres, is the desideratum. Now this -is just what the foregoing hypothesis implies. If, as was concluded, -fertilization has for its object the disturbance of that approaching -equilibrium existing among the physiological units separated from an adult -organism; and if, as we saw reason to think, this object is effected by -mixture with the slightly-different physiological units of another -organism; then, we at the same time see that this object will not be -effected by mixture with physiological units belonging to the same -organism. Thus, the hypothesis leads us to expect such provisions as we -find. - - -§ 94. But here a difficulty presents itself. These propositions seem to -involve the conclusion that self-fertilization is impossible. It apparently -follows from them, that a group of physiological units from one part of an -organism ought to have no power of altering the state of approaching -balance in a group from another part of it. Yet self-fertilization does -occur. Though the ovules of one plant are generally fertilized by pollen -from another plant of the same kind, yet they may be, some of them, -fertilized by pollen of the same plant; and, indeed, there are plants in -which self-fertilization is the rule: even provision being in some cases -made to prevent fertilization by pollen from other individuals. And though, -among hermaphrodite animals, self-fertilization is usually negatived by -structural or functional arrangements, yet in certain _Entozoa_ there -appear to be special provisions by which the sperm-cells and the germ-cells -of the same individual may be united, when not previously united with those -of another individual. Nay, it has even been shown that in certain -Ascidians the contents of oviduct and spermiduct of the same individual -produce, when united, fertile ova whence evolve perfect individuals. -Certainly, at first sight, these facts do not consist with the above -supposition. Nevertheless there is something like a solution. - -In the last chapter, when considering the variations caused in offspring -from uniting elements representing unlike parental constitutions, it was -pointed out that in an unfolding organism, composed of slightly-different -physiological units derived from slightly-different parents, there cannot -be maintained an even distribution of the two orders of units. We saw that -the instability of the homogeneous negatives the uniform blending of them; -and that, by the process of differentiation and integration, they must be -more or less separated; so that in one part of the body the influence of -one parent will predominate, and in another part of the body the influence -of the other parent: an inference which harmonizes with daily observation. -We also saw that the sperm-cells or germ-cells produced by such an organism -must, in virtue of these same laws, be more or less unlike one another. It -was shown that through segregation, some of the sperm-cells or germ-cells -will get an excess of the physiological units derived from one side, and -some of them an excess of those derived from the other side: a cause which -accounts for the unlikenesses among offspring simultaneously produced. Now -from this segregation of the different orders of physiological units, -inherited from different parents and lines of ancestry, there arises the -possibility of self-fertilization in hermaphrodite organisms. If the -physiological units contained in the sperm-cells and germ-cells of the same -flower, are not quite homogeneous--if in some of the ovules the -physiological units derived from the one parent greatly predominate, and in -some of the ovules those derived from the other parent; and if the like is -true of the pollen-cells; then, some of the ovules may be nearly as much -contrasted with some of the pollen-cells in the characters of their -contained units, as were the ovules and pollen-cells of the parents from -which the plant proceeded. Between part of the sperm-cells and part of the -germ-cells, the community of nature will be such that fertilization will -not result from their union; but between some of them, the differences of -constitution will be such that their union will produce the requisite -molecular instability. The facts, so far as they are known, seem in harmony -with this deduction. Self-fertilization in flowers, when it takes place, is -not so efficient as mutual fertilization. Though some of the ovules produce -seeds, yet more of them than usual are abortive. From which, indeed, -results the establishment of varieties that have structures favourable to -mutual fertilization; since, being more prolific, these have, other things -equal, greater chances in the "struggle for existence." - -Further evidence is at hand supporting this interpretation. There is reason -to believe that self-fertilization, which at the best is comparatively -inefficient, loses all efficiency in course of time. After giving an -account of the provisions for an occasional, or a frequent, or a constant -crossing between flowers; and after quoting Prof. Huxley to the effect that -among hermaphrodite animals, there is no case in which "the occasional -influence of a distinct individual can be shown to be physically -impossible;" Mr. Darwin writes--"from these several considerations and from -the many special facts which I have collected, but which I am not here able -to give, I am strongly inclined to suspect that, both in the vegetable and -animal kingdoms, an occasional intercross with a distinct individual is a -law of nature ... in none, as I suspect, can self-fertilization go on for -perpetuity." This conclusion, based wholly on observed facts, is just the -conclusion to which the foregoing argument points. That necessary action -and the re-action between the parts of an organism and the organism as a -whole--that power of an aggregate to re-mould the units, which is the -correlative of the power of the units to build up into such an aggregate; -implies that any differences existing among the units inherited by an -organism, must gradually diminish. Being subject in common to the total -forces of the organism, they will in common be modified towards congruity -with these forces, and therefore towards likeness with one another. If, -then, in a self-fertilizing organism and its self-fertilizing descendants, -such contrasts as originally existed among the physiological units are -progressively obliterated--if, consequently, there can no longer be a -segregation of different physiological units in different sperm-cells and -germ-cells; self-fertilization will become impossible. Step by step the -fertility will diminish, and the series will finally die out. - -And now observe, in confirmation of this view, that self-fertilization is -limited to organisms in which an approximate equilibrium among the organic -forces is not long maintained. While growth is actively going on, and the -physiological units are subject to a continually-changing distribution of -forces, no decided assimilation of the units can be expected: like forces -acting on the unlike units will tend to segregate them, so long as -continuance of evolution permits further segregation; and only when further -segregation cannot go on, will the like forces tend to assimilate the -units. Hence, where there is no prolonged maintenance of an approximate -organic balance, self-fertilization may be possible for some generations; -but it will be impossible in organisms distinguished by a sustained moving -equilibrium. - - -§ 95. The interpretation which it affords of sundry phenomena familiar to -breeders of animals, adds probability to the hypothesis. Mr. Darwin has -collected a large "body of facts, showing, in accordance with the almost -universal belief of breeders, that with animals and plants a cross between -different varieties, or between individuals of the same variety but of -another strain, gives vigour and fertility to the offspring; and on the -other hand, that _close_ interbreeding diminishes vigour and fertility,"--a -conclusion harmonizing with the current belief respecting -family-intermarriages in the human race. Have we not here a solution of -these facts? Relations must, on the average of cases, be individuals whose -physiological units are more nearly alike than usual. Animals of different -varieties must be those whose physiological units are more unlike than -usual. In the one case, the unlikeness of the units may frequently be -insufficient to produce fertilization; or, if sufficient to produce -fertilization, not sufficient to produce that active molecular change -required for vigorous development. In the other case, both fertilization -and vigorous development will be made probable. - -Nor are we without a cause for the irregular manifestations of these -general tendencies. The mixed physiological units composing any organism -being, as we have seen, more or less segregated in the reproductive centres -it throws off; there may arise various results according to the degrees of -difference among the units, and the degrees in which the units are -segregated. Of two cousins who have married, the common grandparents may -have had either similar or dissimilar constitutions; and if their -constitutions were dissimilar, the probability that their married -grandchildren will have offspring will be greater than if their -constitutions were similar. Or the brothers and sisters from whom these -cousins descended, instead of severally inheriting the constitutions of -their parents in tolerably equal degrees, may have severally inherited them -in very different degrees: in which last case, intermarriages among the -cousins will be less likely to prove infertile. Or the brothers and sisters -from whom these cousins descended, may severally have married persons very -like, or very unlike, themselves; and from this cause there may have -resulted, either an undue likeness, or a due unlikeness, between the -married cousins.[39] These several causes, conspiring and conflicting in -endless ways and degrees, will work multiform effects. Moreover, -differences of segregation will make the reproductive centres produced by -the same nearly-related organisms, vary considerably in their amounts of -unlikeness; and therefore, supposing their amounts of unlikeness great -enough to cause fertilization, this fertilization will be effective in -various degrees. Hence it may happen that among offspring of nearly-related -parents, there may be some in which the want of vigour is not marked, and -others in which there is decided want of vigour. So that we are alike shown -why in-and-in breeding tends to diminish both fertility and vigour: and why -the effect cannot be a uniform effect, but only an average effect. - - -§ 96. While, if the foregoing arguments are valid, gamogenesis has for its -main result the initiation of a new development by the overthrow of that -approximate equilibrium arrived at among the molecules of the -parent-organisms, a further result appears to be subserved by it. Those -inferior organisms which habitually multiply by agamogenesis, have -conditions of life that are simple and uniform; while those organisms which -have highly-complex and variable conditions of life, habitually multiply by -gamogenesis. Now if a species has complex and variable conditions of life, -its members must be severally exposed to sets of conditions that are -slightly different: the aggregates of incident forces cannot be alike for -all the scattered individuals. Hence, as functional deviation must ever be -inducing structural deviation, each individual throughout the area occupied -tends to become fitted for the particular habits which its particular -conditions necessitate; and in so far, _un_fitted for the average habits -proper to the species. But these undue specializations are continually -checked by gamogenesis. As Mr. Darwin remarks, "intercrossing plays a very -important part in nature in keeping the individuals of the same species, or -of the variety, true and uniform in character:" the idiosyncratic -divergences obliterate one another. Gamogenesis, then, is a means of -turning to positive advantage the individual differentiations which, in its -absence, would result in positive disadvantage. Were it not that -individuals are ever being made unlike one another by their unlike -conditions, there would not arise in them those contrasts of molecular -constitution, which we have seen to be needful for producing the fertilized -germs of new individuals. And were not these individual differentiations -ever being mutually cancelled, they would end in a fatal narrowness of -adaptation. - -This truth will be most clearly seen if we reduce it to its purely abstract -form, thus:--Suppose a quite homogeneous species, placed in quite -homogeneous conditions; and suppose the constitutions of all its members in -complete concord with their absolutely-uniform and constant conditions; -what must happen? The species, individually and collectively, is in a state -of perfect moving equilibrium. All disturbing forces have been eliminated. -There remains no force which can, in any way, change the state of this -moving equilibrium; either in the species as a whole or in its members. But -we have seen (_First Principles_, § 173) that a moving equilibrium is but a -transition towards complete equilibration, or death. The absence of -differential or un-equilibrated forces among the members of a species, is -the absence of all forces which can cause changes in the conditions of its -members--is the absence of all forces which can initiate new organisms. To -say, as above, that complete molecular homogeneity existing among the -members of a species, must render impossible that mutual molecular -disturbance which constitutes fertilization, is but another way of saying -that the actions and re-actions of each organism, being in perfect balance -with the actions and re-actions of the environment upon it, there remains -in each organism no force by which it differs from any other--no force -which any other does not meet with an equal force--no force which can set -up a new evolution among the units of any other. - -And so we reach the remarkable conclusion that the life of a species, like -the life of an individual, is maintained by the unequal and ever-varying -actions of incident forces on its different parts.[40] An individual -homogeneous throughout, and having its substance everywhere continuously -subject to like actions, could undergo none of those changes which life -consists of; and similarly, an absolutely-uniform species, having all its -members exposed to identical influences, would be deprived of that -initiator of change which maintains its existence as a species. Just as, in -each organism, incident forces constantly produce divergences from the mean -state in various directions, which are constantly balanced by opposite -divergences indirectly produced by other incident forces; and just as the -combination of rhythmical functions thus maintained, constitutes the life -of the organism; so, in a species, there is, through gamogenesis, a -perpetual neutralization of those contrary deviations from the mean state -which are caused in its different parts by different sets of incident -forces; and it is similarly by the rhythmical production and compensation -of these contrary deviations, that the species continues to live. The -moving equilibrium in a species, like the moving equilibrium in an -individual, would rapidly end in complete equilibration, or death, were not -its continually-dissipated forces continually re-supplied from without. -Besides owing to the external world those energies which, from moment to -moment, keep up the lives of its individual members, every species owes to -certain more indirect actions of the external world, those energies which -enable it to perpetuate itself in successive generations. - - -§ 97. What evidence still remains may be conveniently woven up along with a -recapitulation of the argument pursued through the last three chapters. Let -us contemplate the facts in their synthetic order. - -That compounding and re-compounding through which we pass from the simplest -inorganic substances to the most complex organic substances, has several -concomitants. Each successive stage of composition presents us with -molecules that are severally larger or more integrated, that are severally -more heterogeneous, that are severally more unstable, and that are more -numerous in their kinds (_First Principles_, § 151). And when we come to -the substances of which living bodies are formed, we find ourselves among -innumerable divergent groups and sub-groups of compounds, the units of -which are large, heterogeneous, and unstable, in high degrees. There is no -reason to assume that this process ends with the formation of those complex -colloids which constitute organic matter. A more probable assumption is -that out of the complex colloidal molecules there are evolved, by a still -further integration, molecules which are still more heterogeneous, and of -kinds which are still more multitudinous. What must be their properties? -Already the colloidal molecules are extremely unstable--capable of being -variously modified in their characters by very slight incident forces; and -already the complexity of their polarities prevents them from readily -falling into such positions of equilibrium as results in crystallization. -Now the organic molecules composed of these colloidal molecules, must be -similarly characterized in far higher degrees. Far more numerous must be -the minute changes that can be wrought in them by minute external forces; -far more free must they remain for a long time to obey forces tending to -re-distribute them; and far greater must be the number of their kinds. - -Setting out with these physiological units, the existence of which various -organic phenomena compel us to recognize, and the production of which the -general law of Evolution thus leads us to anticipate; we get an insight -into the phenomena of Genesis, Heredity, and Variation. If each organism is -built of certain of these highly-plastic units peculiar to its -species--units which slowly work towards an equilibrium of their complex -proclivities, in producing an aggregate of the specific structure, and -which are at the same time slowly modifiable by the re-actions of this -aggregate--we see why the multiplication of organisms proceeds in the -several ways, and with the various results, which naturalists have -observed. - -Heredity, as shown not only in the repetition of the specific structure but -in the repetition of ancestral deviations from it, becomes a matter of -course; and it falls into unison with the fact that, in various inferior -organisms, lost parts can be replaced, and that, in still lower organisms, -a fragment can develop into a whole. - -While an aggregate of physiological units continues to grow by the -assimilation of matter which it moulds into other units of like type; and -while it continues to undergo changes of structure; no equilibrium can be -arrived at between the whole and its parts. Under these conditions, then, -an un-differentiated portion of the aggregate--a group of physiological -units not bound up into a specialized tissue--will be able to arrange -itself into the structure peculiar to the species; and will so arrange -itself, if freed from controlling forces and placed in fit conditions of -nutrition and temperature. Hence the continuance of agamogenesis in -little-differentiated organisms, so long as assimilation continues to be -greatly in excess of expenditure. - -But let growth be checked and development approach its completion--let the -units of the aggregate be severally exposed to an almost constant -distribution of forces; and they must begin to equilibrate themselves. -Arranged, as they will gradually be, into comparatively stable attitudes in -relation to one another, their mobility will diminish; and groups of them, -partially or wholly detached, will no longer readily re-arrange themselves -into the specific form. Agamogenesis will be no longer possible; or, if -possible, will be no longer easy. - -When we remember that the force which keeps the Earth in its orbit is the -gravitation of each particle in the Earth towards every one of the group of -particles existing 92,000,000 of miles off; we cannot reasonably doubt that -each unit in an organism acts on all the other units, and is reacted on by -them: not by gravitation only but chiefly by other energies. When, too, we -learn that glass has its molecular constitution changed by light, and that -substances so rigid and stable as metals have their atoms re-arranged by -forces radiated in the dark from adjacent objects;[41] we are obliged to -conclude that the excessively-unstable units of which organisms are built, -must be sensitive in a transcendant degree to all the forces pervading the -organisms composed of them--must be tending ever to re-adjust, not only -their relative attitudes but their molecular structures, into equilibrium -with these forces. Hence, if aggregates of the same species are differently -conditioned, and re-act differently on their component units, their -component units will be rendered somewhat different; and they will become -the more different the more widely the re-actions of the aggregates upon -them differ, and the greater the number of generations through which these -different re-actions of the aggregates upon them are continued. - -If, then, unlikenesses of function among individuals of the same species, -produce unlikenesses between the physiological units of one individual and -those of another, it becomes comprehensible that when groups of units -derived from two individuals are united, the group formed will be more -unstable than either of the groups was before their union. The mixed units -will be less able to resist those re-distributing forces which cause -evolution; and may thus have restored to them the capacity for development -which they had lost. - -This view harmonizes with the conclusion, which we saw reason to draw, that -fertilization does not depend on any intrinsic peculiarities of sperm-cells -and germ-cells, but depends on their derivation from different individuals. -It explains the facts that nearly-related individuals are less likely to -have offspring than others, and that their offspring, when they have them, -are frequently feeble. And it gives us a key to the converse fact that the -crossing of varieties results in unusual vigour. - -Bearing in mind that the slightly-different orders of physiological units -which an organism inherits from its parents, are subject to the same set of -forces, and that when the organism is fully developed this set of forces, -becoming constant, tends slowly to re-mould the two orders of units into -the same form; we see how it happens that self-fertilization becomes -impossible in the higher organisms, while it remains possible in the lower -organisms. In long-lived creatures which have tolerably-definite limits of -growth, this assimilation of the somewhat-unlike physiological units is -liable to go on to an appreciable extent; whereas in organisms which do not -continuously subject their component units to constant forces, there will -be much less of this assimilation. And where the assimilation is not -considerable, the segregation of mixed units may cause the sperm-cells and -germ-cells developed in the same individual, to be sufficiently different -to produce, by their union, fertile germs; and several generations of -self-fertilizing descendants may succeed one another, before the two orders -of units have had their unlikenesses so far diminished that they will no -longer do this. The same principles explain for us the variable results of -union between nearly-related organisms. According to the contrasts among -the physiological units they inherit from parents and ancestors; according -to the unlike proportions of the contrasted units which they severally -inherit; and according to the degrees of segregation of such units in -different sperm-cells and germ-cells; it may happen that two kindred -individuals will produce the ordinary number of offspring or will produce -none; or will at one time be fertile and at another not; or will at one -time have offspring of tolerable strength and at another time feeble -offspring. - -To the like causes are also ascribable the phenomena of Variation. These -are unobtrusive while the tolerably-uniform conditions of a species -maintain tolerable uniformity among the physiological units of its members; -but they become obtrusive when differences of conditions, entailing -considerable functional differences, have entailed decided differences -among the physiological units, and when the different physiological units, -differently mingled in every individual, come to be variously segregated -and variously combined. - -Did space permit, it might be shown that this hypothesis is a key to many -further facts--to the fact that mixed races are comparatively plastic under -new conditions; to the fact that pure races show predominant influences in -the offspring when crossed with mixed races; to the fact that while mixed -breeds are often of larger growth, pure breeds are the more hardy--have -functions less-easily thrown out of balance. But without further argument -it will, I think, be admitted that the power of this hypothesis to explain -so many phenomena, and to bring under a common bond phenomena which seem so -little allied, is strong evidence of its truth. And such evidence gains -greatly in strength on observing that this hypothesis brings the facts of -Genesis, Heredity, and Variation into harmony with first principles. We see -that these plastic physiological units, which we find ourselves obliged to -assume, are just such more integrated, more heterogeneous, more unstable, -and more multiform molecules, as would result from continuance of the steps -through which organic matter is reached. We see that the differentiations -of them assumed to occur in differently-conditioned aggregates, and the -equilibrations of them assumed to occur in aggregates which maintain -constant conditions, are but corollaries from those universal principles -implied by the persistence of force. We see that the maintenance of life in -the successive generations of a species, becomes a consequence of the -continual incidence of new forces on the species, to replace the forces -that are ever being rhythmically equilibrated in the propagation of the -species. And we thus see that these apparently-exceptional phenomena -displayed in the multiplication of organic beings, fall into their places -as results of the general laws of Evolution. We have, therefore, weighty -reasons for entertaining the hypothesis which affords us this -interpretation. - - - - -CHAPTER X^A. - -GENESIS, HEREDITY, AND VARIATION - -_CONCLUDED_. - - -§ 97a. Since the foregoing four chapters were written, thirty-four years -ago, the topics with which they deal have been widely discussed and many -views propounded. Ancient hypotheses have been abandoned, and other -hypotheses, referring tacitly or avowedly to the cell-doctrine, have been -set forth. Before proceeding it will be well to describe the chief among -these. - -Most if not all of them proceed on the assumption, shown in § 66 to be -needful, that the structural characters of organisms are determined by the -special natures of units which are intermediate between the chemical units -and the morphological units--between the invisible molecules of -proteid-substances and the visible tissue-components called cells. - -Four years after the first edition of this volume was published, appeared -Mr. Darwin's work, _The Variation of Animals and Plants under -Domestication_; and in this he set forth his doctrine of Pangenesis. -Referring to the doctrine of physiological units which the preceding -chapters work out, he at first expressed a doubt whether his own was or was -not the same, but finally concluded that it was different. He was right in -so concluding. Throughout my argument the implication everywhere is that -the physiological units are all of one kind; whereas Mr. Darwin regards his -component units, or "gemmules," as being of innumerable unlike kinds. He -supposes that every cell of every tissue gives off gemmules special to -itself, and capable of developing into similar cells. We may here, in -passing, note that this view implies a fundamental distinction between -unicellular organisms and the component cells of multicellular organisms, -which are otherwise homologous with them. For while in their essential -structures, their essential internal changes, and their essential processes -of division, the _Protozoa_ and the component units of the _Metazoa_ are -alike, the doctrine of Pangenesis implies that though the units when -separate do not give off invisible gemmules the grouped units do. - -Much more recently have been enunciated the hypotheses of Prof. Weismann, -differing from the foregoing hypotheses in two respects. In the first place -it is assumed that the fragment of matter out of which each organism arises -consists of two portions--one of them, the germ-plasm, reserved within the -generative organ of the incipient individual, representing in its -components the structure of the species, and gives origin to the germs of -future individuals; and the other of them, similarly representative of the -specific structure, giving origin to the rest of the body, or soma, but -contains in its components none of those latent powers possessed by those -of the germ-plasm. In the second place the germ-plasm, in common with the -soma-plasm, consists of multitudinous kinds of units portioned out to -originate the various organs. Of these there are groups, sub-groups, and -sub-sub-groups. The largest of them, called "idants," are supposed each to -contain a number of "ids"; within each id there are numerous -"determinants"; and each determinant is made up of many "biophors"--the -smallest elements possessing vitality. Passing over details, the essential -assumption is that there exists a separate determinant for each part of the -organism capable of independent variation; and Prof. Weismann infers that -while there may be but one for the blood and but one for a considerable -area of skin (as a stripe of the zebra) there must be a determinant for -each scale on a butterfly's wing: the number on the four wings being over -two hundred thousand. And then each cluster of biophors composing a -determinant has to find its way to the place where there is to be formed -the part it represents. - -Here it is needless to specify the modifications of these hypotheses -espoused by various biologists--all of them assuming that the structural -traits of each species are expressed in certain units intermediate between -morphological units and chemical units. - - -§ 97b. A true theory of heredity must be one which recognizes the relevant -phenomena displayed by all classes of organism. We cannot assume two kinds -of heredity, one for plants and another for animals. Hence a theory of -heredity may be first tested by observing whether it is equally applicable -to both kingdoms of living things. Genesis, heredity, and variation, as -seen in plants, are simpler and more accessible than as seen in animals. -Let us then note what these imply. - -Already in § 77 I have illustrated the power which some plants possess of -developing new individuals from mere fragments of leaves and even from -detached scales. Striking as are the facts there instanced, they are -scarcely more significant than some which are familiar. The formation of -cauline buds, presently growing into shoots, shows us a kind of inheritance -which a true theory must explain. As described by Kerner, such buds arise -in Pimpernel, Toad-flax, etc., below the seed-leaves, even while yet there -are no axils in which buds usually grow; and in many plants they arise from -intermediate places on the stem: that is, without definite relations to -pre-existing structures. How fortuitous is their origin is shown when a -branch is induced to bud by keeping it wrapped round with a wet cloth. Even -still better proved is the absence of any relation between cauline buds and -normal germs by the frequent growth of them out of "callus"--the tissue -which spreads over wounds and the cut ends of branches. It is not easy to -reconcile these facts with Mr. Darwin's hypothesis of gemmules. We have to -assume that where a cauline bud emerges there are present in due -proportions gemmules of all the parts which will presently arise from -it--leaves, stipules, bracts, petals, stamens, anthers, etc. We have to -assume this though, at the time the bud originates, sundry of these organs, -as the parts of flowers, do not exist on the plant or tree. And we have to -assume that the gemmules of such parts are duly provided in a portion of -adventitious callus, far away from the normal places of fructification. -Moreover, the resulting shoot may or may not produce all the parts which -the gemmules represent; and when, perhaps after years, flowers are produced -on its side shoots, there must exist at each point the needful proportion -of the required gemmules; though there have been no cells continually -giving them off. - -Still less does the hypothesis of Prof. Weismann harmonize with the -evidence as plants display it. Plant-embryogeny yields no sign of -separation between germ-plasm and soma-plasm; and, indeed, the absence of -such separation is admitted. After instancing cases among certain of the -lower animals, in which no differentiation of the two arises in the first -generation resulting from a fertilized ovum, Prof. Weismann continues:-- - - "The same is true as regards the higher plants, in which the first shoot - arising from the seed never contains germ-cells, or even cells which - subsequently become differentiated into germ cells. In all these - last-mentioned cases the germ-cells are not present in the first person - arising by embryogeny as special cells, but are only formed in much later - cell-generations from the offspring of certain cells of which this first - person was composed." (_Germ-Plasm_, p. 185.) - -How this admission consists with the general theory it is difficult to -understand. The units of the soma-plasm are here recognized as having the -same generative powers as the units of the germ-plasm. In so far as one -organic kingdom and a considerable part of the other are concerned the -doctrine is relinquished. Relinquishment is, indeed, necessitated even by -the ordinary facts, and still more by the facts just instanced. Defence of -it involves the assertion that where buds arise, normal or cauline, there -exist in due proportion the various ids with their contained -determinants--that these are diffused throughout the growing part of the -soma; and this implies that the somatic tissue does not differ in -generative power from the germ-plasm. - -The hypothesis of physiological units, then, remains outstanding. For -cauline buds imply that throughout the plant-tissue, where not unduly -differentiated, the local physiological units have a power of arranging -themselves into the structure of the species. - -But this hypothesis, too, as it now stands, is inadequate. Under the form -thus far given to it, it fails to explain some accompanying facts. For if -the branch just instanced as producing a cauline bud be cut off and its end -stuck in the ground, or if it be bent down and a portion of it covered with -earth, there will grow from it rootlets and presently roots. The same -portion of tissue which otherwise would have produced a shoot with all its -appendages, constituting an individual, now produces only a special part of -an individual. - - -§ 97c. Certain kindred facts of animal development may now be considered. -Similar insufficiencies are disclosed. - -The often-cited reproduction of a crab's lost claw or a lizard's tail, Mr. -Darwin thought explicable by his hypothesis of diffused gemmules, -representing all organs or their component cells. But though, after simple -amputation, regrowth of the proximate part of the tail is conceivable as -hence resulting, it is not easy to understand how the remoter part, the -components of which are now absent from the organism, can arise afresh from -gemmules no longer originated in due proportion. Prof. Weismann's -hypothesis, again, implies that there must exist at the place of -separation, a ready-provided supply of determinants, previously latent, -able to reproduce the missing tail in all its details--nay, even to do this -several times over: a strong supposition! The hypothesis of physiological -units, as set forth in preceding chapters, appears less incompetent: -reproduction of the lost part would seem to be a normal result of the -proclivity towards the form of the entire organism. But now what are we to -say when, instead of being cut off transversely, the tail is divided -longitudinally and each half becomes a complete tail? What are we to say -when, if these two tails are similarly dealt with, the halves again -complete themselves; and so until as many as sixteen tails have been -formed? Here the hypothesis of physiological units appears to fail utterly; -for the tendency it implies is to complete the specific form, by -reproducing a single tail only. - -Various annulose animals display anomalies of development difficult to -explain on any hypothesis. We have creatures like the fresh-water _Nais_ -which, though it has advanced structures, including a vascular system, -branchiæ, and a nervous cord ending with cephalic ganglia, nevertheless -shows us an ability like that of the _Hydra_ to reproduce the whole from a -small part: nearly forty pieces into which a _Nais_ was cut having -severally grown into complete animals. Again we have, in the order -_Polychætæ_, types like _Myrianida_, in which by longitudinal budding a -string of individuals, sometimes numbering even thirty, severally develop -certain segments into heads, while increasing their segments in number. In -yet other types there occurs not longitudinal gemmation only, but lateral -gemmation: a segment will send out sideways a bud which presently becomes a -complete worm. Once more, _Syllis ramosa_ is a species in which the -individual worms growing from lateral buds, while remaining attached to the -parent, themselves give origin to buds; and so produce a branched aggregate -of worms. How shall we explain the reparative and reproductive powers thus -exemplified? It seems undeniable that each portion has an ability to -produce, according to circumstances, the whole creature or a missing part -of the creature. When we read of Sir J. Dalyell that he "cut a _Dasychone_ -into three pieces; the hindermost produced a head, the anterior piece -developed an anus, and the middle portion formed both a head and a tail" we -are not furnished with an explanation by the hypothesis of gemmules or by -the hypothesis of determinants; for we cannot arbitrarily assume that -wherever a missing organ has to be produced there exists the needful supply -of gemmules or of determinants representing that organ. The hypothesis that -physiological units have everywhere a proclivity towards the organic form -of the species, appears more congruous with the facts; but even this does -not cover the cases in which a new worm grows from a lateral bud. The -tendency to complete the individual structure might be expected rather to -restrain this breaking of the lines of complete structure. - -Still less explicable in any way thus far proposed are certain remedial -actions seen in animals. An example of them was furnished in § 67, where -"false joints" were described--joints formed at places where the ends of a -broken bone, failing to unite, remain moveable one upon the other. -According to the character of the habitual motions there results a rudely -formed hinge-joint or a ball-and-socket joint, either having the various -constituent parts--periosteum, fibrous tissue, capsule, ligaments. Now Mr. -Darwin's hypothesis, contemplating only normal structures, fails to account -for this formation of an abnormal structure. Neither can we ascribe this -local development to determinants: there were no appropriate ones in the -germ-plasm, since no such structure was provided for. Nor does the -hypothesis of physiological units, as presented in preceding chapters, -yield an interpretation. These could have no other tendency than to restore -the normal form of the limb, and might be expected to oppose the genesis of -these new parts. - -Thus we have to seek, if not another hypothesis, then some such -qualification of an existing hypothesis as will harmonize it with various -exceptional phenomena. - - -§ 97d. In Part II of the _Principles of Sociology_, published in 1876, will -be found elaborated in detail that analogy between individual organization -and social organization which was briefly sketched out in an essay on "The -Social Organism" published in 1860. In §§ 241-3 a parallel is drawn between -the developments of the sustaining systems of the two; and it is pointed -out how, in the one case as in the other, the components--here organic -units and there citizens--have their activities and arrangements mainly -settled by local conditions. One leading example is that the parts -constituting the alimentary canal, while jointly fitted to the nature of -the food, are severally adapted to the successive stages at which the food -arrives in its progress; and that in an analogous way the industries -carried on by peoples forming different parts of a society, are primarily -determined by the natures of things around--agriculture, pastoral and -arable, special manufactures and minings, ship-building and fishing: the -respective groups falling into fit combinations and becoming partially -modified to suit their work. The implication is that while the organization -of a society as a whole depends on the characters of its units, in such way -that by some types of men despotisms are always evolved while by other -types there are evolved forms of government partially free--forms which -repeat themselves in colonies--there is, on the other hand, in every case a -local power of developing appropriate structures. And it might have been -pointed out that similarly in types of creatures not showing much -consolidation, as the _Annelida_, many of the component divisions, largely -independent in their vitalities, are but little affected in their -structures by the entire aggregate. - -My purpose at that time being the elucidation of sociological truths, it -did not concern me to carry further the biological half of this comparison. -Otherwise there might have been named the case in which a supernumerary -finger, beginning to bud out, completes itself as a local organ with bones, -muscles, skin, nail, etc., in defiance of central control: even repeating -itself when cut off. There might also have been instanced the above-named -formation of a false joint with its appurtenances. For the implication in -both cases is that a local group of units, determined by circumstances -towards a certain structure, coerces its individual units into that -structure. - -Now let us contemplate the essential fact in the analogy. The men in an -Australian mining-camp, as M. Pierre Leroy Beaulieu points out, fall into -Anglo-Saxon usages different from those which would characterize a French -mining-camp. Emigrants to a far West settlement in America quickly -establish post-office, bank, hotel, newspaper, and other urban -institutions. We are thus shown that along with certain traits leading to a -general type of social organization, there go traits which independently -produce fit local organizations. Individuals are led into occupations and -official posts, often quite new to them, by the wants of those around--are -now influenced and now coerced into social arrangements which, as shown -perhaps by gambling saloons, by shootings at sight, and by lynchings, are -scarcely at all affected by the central government. Now the physiological -units in each species appear to have a similar combination of capacities. -Besides their general proclivity towards the specific organization, they -show us abilities to organize themselves locally; and these abilities are -in some cases displayed in defiance of the general control, as in the -supernumerary finger or the false joint. Apparently each physiological -unit, while having in a manner the whole organism as the structure which, -along with the rest, it tends to form, has also an aptitude to take part in -forming any local structure, and to assume its place in that structure -under the influence of adjacent physiological units. - -A familiar fact supports this conclusion. Everyone has at hand, not -figuratively but literally, an illustration. Let him compare the veins on -the backs of his two hands, either with one another or with the veins on -another person's hands, and he will see that the branchings and -inosculations do not correspond: there is no fixed pattern. But on -progressing inwards from the extremities, the distribution of the veins -becomes settled--there is a pattern-arrangement common to all persons. -These facts imply a predominating control by adjacent parts where control -by the aggregate is less easy. A constant combination of forces which, -towards the centre, produces a typical structure, fails to do this at the -periphery where, during development, the play of forces is less settled. -This peripheral vascular structure, not having become fixed because one -arrangement is as good as another, is in each determined by the immediately -surrounding influences. - - -§ 97e. And now let us contemplate the verifications which recent -experiments have furnished--experiments made by Prof. G. Born of Breslau, -confirming results earlier reached by Vulpian and adding more striking -results of kindred nature. They leave no longer doubtful the large share -taken by local organizing power as distinguished from central organizing -power. - -The independent vitality shown by separated portions of ventral skin from -frog-larvæ may be named as the first illustration. With their attached -yolk-cells these lived for days, and underwent such transformations as -proved some structural proclivity, though of course the product was -amorphous. Detached portions of tails of larvæ went on developing their -component parts in much the same ways as they would have done if remaining -attached. More striking still was the evidence furnished by experiments in -grafting. These proved that the undifferentiated rudiment of an organ will, -when cut off and joined to a non-homologous place in another individual, -develop itself as it would have done if left in its original place. In -brief, then, we may say that each part is in chief measure autogenous. - -These strange facts presented by small aggregates of organic matter, which -are the seats of extremely complex forces, will seem less incomprehensible -if we observe what has taken place in a vast aggregate of inorganic matter -which is the seat of very simple forces--the Solar System. Transcendently -different as this is in all other respects, it is analogous in the respect -that, as factors of local structures, local influences predominate over the -influences of the aggregate. For while the members of the Solar System, -considered as a whole, are subordinate to the totality of its forces, the -arrangements in each part of it are produced almost wholly by the play of -forces in that part. Though the Sun affects the motions of the Moon, and -though during the evolution of the Earth-and-Moon system the Sun exercised -an influence, yet the relations of our world and its satellite in respect -of masses and motions were in the main locally determined. Still more -clearly was it thus with Jupiter and his satellites or Saturn with his -rings and satellites. Remembering that the ultimate units of matter of -which the Solar System is composed are of the same kinds, and that they act -on one another in conformity with the same laws, we see that, remote as the -case is from the one we are considering in all other respects, it is -similar in the respect that during organization the energies in each -locality work effects which are almost independent of the effects worked by -the general energies. In this vast aggregate, as in the minute aggregates -now in question, the parts are practically autogenous. - -Having thus seen that in a way we have not hitherto recognized the same -general principles pervade inorganic and organic evolution, let us revert -to the case of super-organic evolution from which a parallel was drawn -above. As analogous to the germinal mass of units out of which a new -organism is to evolve, let us take an assemblage of colonists not yet -socially organized but placed in a fertile region--men derived from a -society (or rather a succession of societies) of long-established type, who -have in their adapted natures the proclivity towards that type. In passing -from its wholly unorganized state to an organized state, what will be the -first step? Clearly this assemblage, though it may have within the -constitutions of its units the potentialities of a specific structure, will -not develop forthwith the details of that structure. The inherited natures -of its units will first show themselves by separating into large groups -devoted to strongly-distinguished occupations. The great mass, dispersing -over promising lands, will make preparations for farming. Another -considerable portion, prompted by the general needs, will begin to form a -cluster of habitations and a trading centre. Yet a third group, recognizing -the demand for wood, alike for agricultural and building purposes, will -betake themselves to the adjacent forests. But in no case will the primary -assemblage, before these separations, settle the arrangements and actions -of each group: it will leave each group to settle them for itself. So, too, -after these divisions have arisen. The agricultural division will not as a -whole prescribe the doings of its members. Spontaneous segregation will -occur: some going to a pastoral region and some to a tract which promises -good crops. Nor within each of these bodies will the organization be -dictated by the whole. The pastoral group will separate itself into -clusters who tend sheep on the hills and clusters who feed cattle on the -plains. Meanwhile those who have gravitated towards urban occupations will -some of them make bricks or quarry stone, while others fall into classes -who build walls, classes who prepare fittings, classes who supply -furniture. Then along with completion of the houses will go occupation of -them by men who bake bread, who make clothing, who sell liquors, and so on. -Thus each great group will go on organizing itself irrespective of the -rest; the sub-groups within each will do the same; and so will the -sub-sub-groups. Quite independently of the people on the hills and the -plains and in the town, those in the forest will divide spontaneously into -parties who cut down trees, parties who trim and saw them, parties who -carry away the timbers; while every party forms for itself an organization -of "butty" or "boss," and those who work under him. Similarly with the -ultimate divisions--the separate families: the arrangements and -apportionments of duties in each are internally determined. Mark the fact -which here chiefly concerns us. This formation of a heterogeneous aggregate -with its variously adapted parts, which while influenced by the whole are -mainly self-formed, goes on among units of essentially the same natures, -inherited from units who belonged to similar societies. And now, carrying -this conception with us, we may dimly perceive how, in a developing embryo, -there may take place the formation, first of the great divisions--the -primary layers--then of the outlines of systems, then of component organs, -and so on continually with the minor structures contained in major -structures; and how each of these progressively smaller divisions develops -its own organization, irrespective of the changes going on throughout the -rest of the embryo. So that though all parts are composed of physiological -units of the same nature, yet everywhere, in virtue of local conditions and -the influence of its neighbours, each unit joins in forming the particular -structure appropriate to the place. Thus conceiving the matter, we may in a -vague way understand the strange facts of autogenous development disclosed -by the above named experiments. - - -§ 97f. "But how immeasurably complex must be the physiological units which -can behave thus!" will be remarked by the reader. "To be able to play all -parts, alike as members of the whole and as members of this or that organ, -they must have an unimaginable variety of potentialities in their natures. -Each must, indeed, be almost a microcosm within a microcosm." - -Doubtless this is true. Still we have a _consensus_ of proofs that the -component units of organisms have constitutions of extremely involved -kinds. Contemplate the facts and their implications. (1) Here is some large -division of the animal kingdom--say the _Vertebrata_. The component units -of all its members have certain fundamental traits in common: all of them -have proclivities towards formation of a vertebral column. Leaving behind -the great assemblage of Fishes with its multitudinous types, each having -special units of composition, we pass to the _Amphibia_, in the units of -which there exist certain traits superposed upon the traits they have in -common with those of Fishes. Through unknown links we ascend to incipient -Mammalian types and then to developed Mammalian types, the units of which -must have further superposed traits. Additional traits distinguish the -units of each Mammalian order; and, again, those of every genus included in -it; while others severally characterize the units of each species. -Similarly with the varieties in each species, and the stirps in each -variety. Now the ability of any component unit to carry within itself the -traits of the sub-kingdom, class, order, genus, species, variety, and at -the same time to bear the traits of immediate ancestors, can exist only in -a something having multitudinous proximate elements arranged in innumerable -ways. (2) Again, these units must be at once in some respects fixed and in -other respects plastic. While their fundamental traits, expressing the -structure of the type, must be unchangeable, their superficial traits must -admit of modification without much difficulty; and the modified traits, -expressing variations in the parents and immediate ancestors, though -unstable, must be considered as capable of becoming stable in course of -time. (3) Once more we have to think of these physiological units (or -constitutional units as I would now re-name them) as having such natures -that while a minute modification, representing some small change of local -structure, is inoperative on the proclivities of the units throughout the -rest of the system, it becomes operative in the units which fall into the -locality where that change occurs. - -But unimaginable as all this is, the facts may nevertheless in some way -answer to it. As before remarked, progressing science reveals complexity -within complexity--tissues made up of cells, cells containing nuclei and -cytoplasm, cytoplasm formed of a protoplasmic matrix containing granules; -and if now we conclude that the unit of protoplasm is itself an -inconceivably elaborate structure, we do but recognize the complexity as -going still deeper. Further, if we must assume that these component units -are in every part of the body acting on one another by extremely -complicated sets of forces (ethereal undulations emanating from each of the -constituent molecules) determining their relative positions and actions, we -are warranted by the discoveries which every day disclose more of the -marvellous properties of matter. When to such examples as were given in -§ 36e we add the example yielded by recent experiments, showing that even a -piece of bread, after subjection to pressure, exhibits diamagnetic -properties unlike those it previously exhibited, we cannot doubt that these -complex units composing living bodies are all of them seats of energies -diffused around, enabling them to act and re-act so as to modify one -another's states and positions. We are shown, too, that whatever be the -natures of the complex forces emanating from each, it will, as a matter of -course, happen that the power of each will be relatively great in its own -neighbourhood and become gradually smaller in parts increasingly remote: -making more comprehensible the autogenous character of each local -structure. - -Whatever be their supposed natures we are compelled to ascribe extreme -complexity to these unknown somethings which have the power of organizing -themselves into a structure of this or that species. If gemmules be -alleged, then the ability of every organ and part of an organ to vary, -implies that the gemmules it gives off are severally capable of receiving -minute modifications of their ordinary structures: they must have many -parts admitting of innumerable relations. Supposing determinants be -assumed, then in addition to the complexity which each must have to express -in itself the structure of the part evolved from it, it must have the -further complexity implied by every superposed modification which causes a -variation of that part. And, as we have just seen, the hypothesis of -physiological units does not relieve us from the need for kindred -suppositions. - -One more assumption seems necessary if we are to imagine how changes of -structure caused by changes of function can be transmitted. Reverting to -§ 54d, where an unceasing circulation of protoplasm throughout an organism -was inferred, we must conceive that the complex forces of which each -constitutional unit is the centre, and by which it acts on other units -while it is acted on by them, tend continually to remould each unit into -congruity with the structures around: superposing on it modifications -answering to the modifications which have arisen in those structures. -Whence is to be drawn the corollary that in course of time all the -circulating units,--physiological, or constitutional if we prefer so to -call them--visiting all parts of the organism, are severally made bearers -of traits expressing local modifications; and that those units which are -eventually gathered into sperm-cells and germ-cells also bear these -superposed traits. - -If against all this it be urged that such a combination of structures and -forces and processes is inconceivably involved, then the reply is that so -astonishing a transformation as that which an unfolding organism displays -cannot possibly be effected by simple agencies. - - -§ 97g. But now let it be confessed that none of these hypotheses serves to -render the phenomena really intelligible; and that probably no hypothesis -which can be framed will do this. Many problems beyond those which -embryology presents have to be solved; and no solution is furnished. - -What are we to say of the familiar fact that certain small organs which, -with the approach to maturity, become active, entail changes of structure -in remote parts--that after the testes have undergone certain final -developments, the hairs on the chin grow and the voice deepens? It has been -contended that certain concomitant modifications in the fluids throughout -the body may produce correlated sexual traits; and there is proof that in -many of the lower animals the period of sexual activity is accompanied by a -special bodily state--sometimes such that the flesh becomes unwholesome and -even poisonous. But a change of this kind can hardly account for a -structural change in the vocal organs in Man. No hypothesis of gemmules or -determinants or physiological units enables us to understand how removal of -the testes prevents those developments of the larynx and vocal cords which -take place if they remain. - -The inadequacy of our explanations we at once see in presence of a -structure like a peacock's tail-feather. Mr. Darwin's hypothesis is that -all parts of every organ are continually giving off gemmules, which are -consequently everywhere present in their due proportions. But a completed -feather is an inanimate product and, once formed, can add to the -circulating fluids no gemmules representing all its parts. If we follow -Prof. Weismann we are led into an astounding supposition. He admits that -every variable part must have a special determinant, and that this results -in the assumption of over two hundred thousand for the four wings of a -butterfly. Let us ask what must happen in the case of a peacock's feather. -On looking at the eye near its end, we see that the minute processes on the -edge of each lateral thread must have been in some way exactly adjusted, in -colour and position, so as to fall into line with the processes on adjacent -threads: otherwise the symmetrical arrangement of coloured rings would be -impossible. Each of these processes, then, being an independent variable, -must have had its particular determinant. Now there are about 300 threads -on the shaft of a large feather, and each of them bears on the average -1,600 processes, making for the whole feather 480,000 of these processes. -For one feather alone there must have been 480,000 determinants, and for -the whole tail many millions. And these, along with the determinants for -the detailed parts of all the other feathers, and for the variable -components of all organs forming the body at large, must have been -contained in the microscopic head of a spermatozoon! Hardly a credible -supposition. Nor is it easy to see how we are helped by the hypothesis of -constitutional units. Take the feather in its budding state and ask how the -group of such units, alike in structure and perpetually multiplying while -the unfolding goes on, can be supposed by their mutual actions so to affect -one another as eventually to produce the symmetrically-adjusted processes -which constitute the terminal eye. Imagination, whatever licence may be -given, utterly fails us. - -At last then we are obliged to admit that the actual organizing process -transcends conception. It is not enough to say that we cannot know it; we -must say that we cannot even conceive it. And this is just the conclusion -which might have been drawn before contemplating the facts. For if, as we -saw in the chapter on "The Dynamic Element in Life," it is impossible for -us to understand the nature of this element--if even the ordinary -manifestations of it which a living body yields from moment to moment are -at bottom incomprehensible; then, still more incomprehensible must be that -astonishing manifestation of it which we have in the initiation and -unfolding of a new organism. - -Thus all we can do is to find some way of symbolizing the process so as to -enable us most conveniently to generalize its phenomena; and the only -reason for adopting the hypothesis of physiological units or constitutional -units is that it best serves this purpose. - - - - -CHAPTER XI. - -CLASSIFICATION. - - -§ 98. That orderly arrangement of objects called Classification has two -purposes, which, though not absolutely distinct, are distinct in great -part. It may be employed to facilitate identification, or it may be -employed to organize our knowledge. If a librarian places his books in the -alphabetical succession of the author's names, he places them in such way -that any particular book may easily be found, but not in such way that -books of a given nature stand together. When, otherwise, he makes a -distribution of books according to their subjects, he neglects various -superficial similarities and distinctions, and groups them according to -certain primary and secondary and tertiary attributes, which severally -imply many other attributes--groups them so that any one volume being -inspected, the general characters of all the neighbouring volumes may be -inferred. He puts together in one great division all works on History; in -another all Biographical works; in another all works that treat of Science; -in another Voyages and Travels; and so on. Each of his great groups he -separates into sub-groups; as when he puts different kinds of Literature -under the heads of Fiction, Poetry, and the Drama. In some cases he makes -sub-sub-groups; as when, having divided his Scientific treatises into -abstract and concrete, putting in the one Logic and Mathematics and in the -other Physics, Astronomy, Geology, Chemistry, Physiology, &c.; he goes on -to sub-divide his books on Physics, into those which treat of Mechanical -Motion, those which treat of Heat, those which treat of Light, of -Electricity, of Magnetism. - -Between these two modes of classification note the essential distinctions. -Arrangement according to any single conspicuous attribute is comparatively -easy, and is the first that suggests itself: a child may place books in the -order of their sizes, or according to the styles of their bindings. But -arrangement according to combinations of attributes which, though -fundamental, are not conspicuous, requires analysis; and does not suggest -itself till analysis has made some progress. Even when aided by the -information which the author gives on his title page, it requires -considerable knowledge to classify rightly an essay on Polarization; and in -the absence of a title page it requires much more knowledge. Again, -classification by a single attribute, which the objects possess in -different degrees, may be more or less serial, or linear. Books may be put -in the order of their dates, in single file; or if they are grouped as -works in one volume, works in two volumes, works in three volumes, &c., the -groups may be placed in an ascending succession. But groups severally -formed of things distinguished by some common attribute which implies many -other attributes, do not admit of serial arrangement. You cannot rationally -say either that Historical Works should come before Biographical Works, or -Biographical Works before Historical Works; nor of the sub-divisions of -creative Literature, into Fiction, Poetry, and the Drama, can you give a -good reason why any one should take precedence of the others. - -Hence this grouping of the like and separation of the unlike which -constitutes Classification, can reach its complete form only by slow steps. -I have shown (_Essays_, Vol. II., pp. 145-7) that, other things equal, the -relations among phenomena are recognized in the order of their -conspicuousness; and that, other things equal, they are recognized in the -order of their simplicity. The first classifications are sure, therefore, -to be groupings of objects which resemble one another in external or -easily-perceived attributes, and attributes that are not of complex -characters. Those likenesses among things which are due to their possession -in common of simple obvious properties, may or may not coexist with further -likenesses among them. When geometrical figures are classed as curvilinear -and rectilinear, or when the rectilinear are divided into trilateral, -quadrilateral, &c., the distinctions made connote various other -distinctions with which they are necessarily bound up; but if liquids be -classed according to their visible characters--if water, alcohol, sulphuret -of carbon, &c., be grouped as colourless and transparent, we have things -placed together which are unlike in their essential natures. Thus, where -the objects classed have numerous attributes, the probabilities are that -the early classifications, based on simple and manifest attributes, unite -under the same head many objects that have no resemblance in the majority -of their attributes. As the knowledge of objects increases, it becomes -possible to make groups of which the members have more numerous properties -in common; and to ascertain what property, or combination of properties, is -most characteristic of each group. And the classification eventually -arrived at is of such kind that the objects in each group have more -attributes in common with one another than they have in common with any -excluded objects; one in which the groups of such groups are integrated on -the same principle; and one in which the degrees of differentiation and -integration are proportioned to the degrees of intrinsic unlikeness and -likeness. And this ultimate classification, while it serves to identify the -things completely, serves also to express the greatest amount of knowledge -concerning the things--enables us to predicate the greatest number of facts -about each thing; and by so doing implies the most precise correspondence -between our conceptions and the realities. - - -§ 99. Biological classifications illustrate well these phases through which -classifications in general pass. In early attempts to arrange organisms in -some systematic manner, we see at first a guidance by conspicuous and -simple characters, and a tendency towards arrangement in linear order. In -successively later attempts, we see more regard paid to combinations of -characters which are essential but often inconspicuous, and an abandonment -of a linear arrangement for an arrangement in divergent groups and -re-divergent sub-groups. - -In the popular mind, plants are still classed under the heads of Trees, -Shrubs, and Herbs; and this serial classing according to the single -attribute of magnitude, swayed the earliest observers. They would have -thought it absurd to call a bamboo, thirty feet high, a kind of grass; and -would have been incredulous if told that the Hart's-tongue should be placed -in the same great division with the Tree-ferns. The zoological -classifications current before Natural History became a science, had -divisions similarly superficial and simple. Beasts, Birds, Fishes, and -Creeping-things are names of groups marked off from one another by -conspicuous differences of appearance and modes of life--creatures that -walk and run, creatures that fly, creatures that live in the water, -creatures that crawl. And these groups were thought of in the order of -their importance. - -The first arrangements made by naturalists were based either on single -characters or on very simple combinations of characters; as that of -Clusius, and afterwards the more scientific system of Cesalpino, -recognizing the importance of inconspicuous structures. Describing -plant-classifications, Lindley says:--"Rivinus invented, in 1690, a system -depending upon the formation of the corolla; Kamel, in 1693, upon the fruit -alone; Magnol, in 1720, on the calyx and corolla; and finally, Linnæus, in -1731, on variations in the stamens and pistil." In this last system, which -has been for so long current as a means of identification (regarded by its -author as transitional), simple external attributes are still depended on; -and an arrangement, in great measure serial, is based on the degrees in -which these attributes are possessed. In 1703, some thirty years before the -time of Linnæus, our countryman Ray had sketched the outlines of a more -advanced system. He said that-- - - Plants are either - Flowerless, or - Flowering; and these are - Dicotyledones, or - Monocotyledones. - -Among the minor groups which he placed under these general heads, "were -Fungi, Mosses, Ferns, Composites, Cichoraceæ, Umbellifers, Papilionaceous -plants, Conifers, Labiates, &c., under other names, but with limits not -very different from those now assigned to them." Being much in advance of -his age, Ray's ideas remained dormant until the time of Jussieu; by whom -they were developed into what has become known as the Natural System: a -system subsequently improved by De Candolle. Passing through various -modifications in the hands of successive botanists, the Natural System is -now represented by the following form, which is based upon the table of -contents prefixed to Vol. II. of Prof. Oliver's translation of the _Natural -History of Plants_, by Prof. Kerner. His first division, Myxothallophyta (= -Myxomycetes), I have ventured to omit. The territory it occupies is in -dispute between zoologists and botanists, and as I have included the group -in the zoological classification, agreeing that its traits are more animal -than vegetal, I cannot also include it in the botanical classification. - -Here, linear arrangement has disappeared: there is a breaking up into -groups and sub-groups and sub-sub-groups, which do not admit of being -placed in serial order, but only in divergent and re-divergent order. Were -there space to exhibit the way in which the Alliances are subdivided into -Orders, and these into Genera, and these into Species, the same principle -of co-ordination would be still further manifested. - - PHYLA. CLASSES. ALLIANCES. - SUB-PHYLA. SUB-CLASSES. - - THALLOPHYTA - I. Schizophyta - 2. Cyanophyceæ. Blue-green Algæ. - 3. Schizomycetes. - II. Dinoflagellata - Peridineæ - 4. - III. Bacillariales - 5. - IV. Gamophyceæ - I. Chlorophyceæ - 6. Protococcoideæ. - 7. Siphoneæ. - 8. Confervoideæ. - 9. Conjugatæ. - 10. Charales. - 11. Phæophyceæ. - 12. Dictyotales. - 13. Florideæ, Red Seaweeds. - V. Fungi - I. Phycomycetes - 14. Oomycetes. - 15. Zygomycetes. - II. Mesomycetes - 16. - 17. - III. Mycomycetes - 18. - 19. - Additional group of Fungi, Lichenes. - ARCHEGONIATÆ - I. Bryophyta - 20. Hepaticæ, Liverworts. - 21. Musci, Mosses. - II. Pteridophyta - Vas. Cryptogams - 22. Filices, Ferns. - 23. Hydropterides, Rhizocarps. - 24. Equisetales, Horse-tails. - 25. Lycopodiales, Club-mosses. - PHANEROGAMIA (Flowering Plants.) - GYMNOSPERMÆ - I. Cycadales, Cycads - 26. - II. Coniferæ - 27. - III. Gnetales - 28. - ANGIOSPERMÆ - I. Monocotyledons - 29. Liliifloræ. - 30. Scitamineæ. - 31. Gynandræ. - 32. Fluviales. - 33. Spadicifloræ. - 34. Glumifloræ. - II. Dicotyledons - I. Monochlamydæ - 35. Centrospermæ. - 36. Protiales. - 37. Daphnales. - 38. Santalales. - 39. Rafflesiales. - 40. Asarales. - 41. Euphorbiales. - 42. Podostemales. - 43. Viridifloræ. - 44. Amentales. - 45. Balanophorales. - II. Monopetalæ - 46. Caprifoliales. - 47. Asterales. - 48. Campanales. - 49. Ericales. - 50. Vaccinales. - 51. Primulales. - 52. Tubifloræ. - III. Polypetalæ - 53. Ranales. - 54. Parietales. - 55. Malvales. - 56. Discifloræ. - 57. Crateranthæ. - 58. Myrtales. - 59. Melastomales. - 60. Lythrales. - 61. Hygrobiæ. - 62. Passifloræ. - 63. Pepones. - 64. Cactales. - 65. Ficoidales. - 66. Umbellales. - -On studying the definitions of these primary, secondary, and tertiary -classes, it will be found that the largest are marked off from one another -by some attribute which connotes sundry other attributes; that each of the -smaller classes comprehended in one of these largest classes, is marked off -in a similar way from the other smaller classes bound up with it; and that -so, each successively smaller class has an increased number of co-existing -attributes. - - -§ 100. Zoological classification has had a parallel history. The first -attempt which we need notice, to arrange animals in such a way as to -display their affinities, is that of Linnæus. He grouped them thus:[42]-- - - CL. 1. MAMMALIA. _Ord._ Primates, Bruta, Feræ, Glires, Pecora, Belluæ, - Cete. - - CL. 2. AVES. _Ord._ Accipitres, Picæ, Anseres, Grallæ, Gallinæ, Passeres. - - CL. 3. AMPHIBIA. _Ord._ Reptiles, Serpentes, Nantes. - - CL. 4. PISCES. _Ord._ Apodes, Jugulares, Thoracici, Abdominales. - - CL. 5. INSECTA. _Ord._ Coleoptera, Hemiptera, Lepidoptera, Neuroptera, - Diptera, Aptera. - - CL. 6. VERMES. _Ord._ Intestina, Mollusca, Testacea, Lithophyta, - Zoophyta. - -This arrangement of classes is obviously based on apparent gradations of -rank; and the placing of the orders similarly betrays an endeavour to make -successions, beginning with the most superior forms and ending with the -most inferior forms. While the general and vague idea of perfection -determines the leading character of the classification, its detailed -groupings are determined by the most conspicuous external attributes. Not -only Linnæus but his opponents, who proposed other systems, were "under the -impression that animals were to be arranged together into classes, orders, -genera, and species, according to their more or less close external -resemblance." This conception survived until the time of Cuvier. -"Naturalists," says Agassiz, "were bent upon establishing one continual -uniform series to embrace all animals, between the links of which it was -supposed there were no unequal intervals. The watchword of their school -was: _Natura non facit saltum_. They called their system _la chaine des -êtres_." - -The classification of Cuvier, based on internal organization instead of -external appearance, was a great advance. He asserted that there are four -principal forms, or four general plans, on which animals are constructed; -and, in pursuance of this assertion, he drew out the following scheme. - - First Branch. ANIMALIA VERTEBRATA. - Cl. 1. Mammalia. - Cl. 2. Birds. - Cl. 3. Reptilia. - Cl. 4. Fishes. - - Second Branch. ANIMALIA MOLLUSCA. - Cl. 1. Cephalapoda. - Cl. 2. Pteropoda. - Cl. 3. Gasteropoda. - Cl. 4. Acephala. - Cl. 5. Brachiopoda. - Cl. 6. Cirrhopoda. - - Third Branch. ANIMALIA ARTICULATA. - Cl. 1. Annelides. - Cl. 2. Crustacea. - Cl. 3. Arachnides. - Cl. 4. Insects. - - Fourth Branch. ANIMALIA RADIATA. - Cl. 1. Echinoderms. - Cl. 2. Intestinal Worms. - Cl. 3. Acalephæ. - Cl. 4. Polypi. - Cl. 5. Infusoria. - -But though Cuvier emancipated himself from the conception of a serial -progression throughout the Animal Kingdom, sundry of his contemporaries and -successors remained fettered by the old error. Less regardful of the -differently-combined sets of attributes distinguishing the different -sub-kingdoms, and swayed by the belief in a progressive development which -was erroneously supposed to imply a linear arrangement of animals, they -persisted in thrusting organic forms into a quite unnatural order. The -following classification of Lamarck illustrates this. - - -INVERTEBRATA. - - I. APATHETIC ANIMALS. } - } - Cl. 1. Infusoria. } Do not feel, and move only by their - Cl. 2. Polypi. } excited irritability. No brain, no - Cl. 3. Radiaria. } elongated medullary mass; no senses; - Cl. 4. Tunicata. } forms varied; rarely articulations. - Cl. 5. Vermes. } - - II. SENSITIVE ANIMALS. } Feel, but obtain from their sensations - } only perceptions of objects, a - Cl. 6. Insects. } sort of simple ideas, which they are - Cl. 7. Arachnids. } unable to combine to obtain complex - Cl. 8. Crustacea. } ones. No vertebral column; a brain - Cl. 9. Annelids. } and mostly an elongated medullary - Cl. 10. Cirripeds. } mass; some distinct senses; muscles - Cl. 11. Conchifera. } attached under the skin; form symmetrical, - Cl. 12. Mollusks. } the parts being in pairs. - - -VERTEBRATA. - - { Feel; acquire preservable ideas; - III. INTELLIGENT ANIMALS. { perform with them operations by which - { they obtain others; are intelligent in - Cl. 13. Fishes. { different degrees. A vertebral column; - Cl. 14. Reptiles. { a brain and a spinal marrow; distinct - Cl. 15. Birds. { senses; the muscles attached to the - Cl. 16. Mammalia. { internal skeleton; form symmetrical, - { the parts being in pairs. - -Passing over sundry classifications in which the serial arrangement -dictated by the notion of ascending complexity, is variously modified by -the recognition of conspicuous anatomical facts, we come to classifications -which recognize another order of facts--those of development. The -embryological inquiries of Von Baer led him to arrange animals as -follows:-- - - I. Peripheric Type. (RADIATA.) _Evolutio radiata._ The development - proceeds from a centre, producing identical parts in a radiating order. - - II. Massive Type. (MOLLUSCA.) _Evolutio contorta._ The development - produces identical parts curved around a conical or other space. - - III. Longitudinal Type. (ARTICULATA.) _Evolutio gemina._ The development - produces identical parts arising on both sides of an axis, and closing up - along a line opposite the axis. - - IV. Doubly Symmetrical Type. (VERTEBRATA.) _Evolutio bigemina._ The - development produces identical parts arising on both sides of an axis, - growing upwards and downwards, and shutting up along two lines, so that - the inner layer of the germ is inclosed below, and the upper layer above. - The embryos of these animals have a dorsal cord, dorsal plates, and - ventral plates, a nervous tube and branchial fissures. - -Recognizing these fundamental differences in the modes of development, as -answering to fundamental divisions in the animal kingdom, Von Baer shows -(among the _Vertebrata_ at least) how the minor differences which arise at -successively later embryonic stages, correspond with the minor divisions. - -Like the modern classification of plants, the modern classification of -animals shows us the assumed linear order completely broken up. In his -lectures at the Royal Institution, in 1857, Prof. Huxley expressed the -relations existing among the several great groups of the animal kingdom, by -placing them at the ends of four or five radii, diverging from a centre. -The diagram I cannot obtain; but in the published reports of his lectures -at the School of Mines the groups were arranged as on the following page. -What remnant there may seem to be of linear succession in some of the -sub-groups contained in it, is merely an accident of typographical -convenience. Each of them is to be regarded simply as a cluster. And if -Prof. Huxley had further developed the arrangement, by dispersing the -sub-groups and sub-sub-groups on the same principle, there would result an -arrangement perhaps not much unlike that shown on the page succeeding this. - - VERTEBRATA - - (_Abranchiata_) - Mammalia - Aves - Reptilia - (_Branchiata_) - Amphibia - Pisces. - - MOLLUSCA ANNULOSA - - Cephalopoda Heteropoda } _Articulata._ - Gasteropoda-dioecia } Insecta Arachnida - } Myriapoda Crustacea - { Pulmonata Gasteropoda-monoecia - { Pteropoda _Annuloida._ - Lamellibranchiata Annellata Scoleidæ - Echinodermata Trematoda - Rotifera Tæniadæ - Turbellaria - Nematoidea - - COELENTERATA - - Hydrozoa Actinozoa. - - PROTOZOA - - Infusoria Spongiadæ Gregarinidæ - _Noctilucidæ_ Foraminifera _Thallassicollidæ_ - -In the woodcut, the dots represent orders, the names of which it is -impracticable to insert. If it be supposed that when magnified, each of -these dots resolves itself into a cluster of clusters, representing genera -and species, an approximate idea will be formed of the relations among the -successively-subordinate groups constituting the animal kingdom. Besides -the subordination of groups and their general distribution, some other -facts are indicated. By the distances of the great divisions from the -general centre, are rudely symbolized their respective degrees of -divergence from the form of simple, undifferentiated organic matter; which -we may regard as their common source. Within each group, the remoteness -from the local centre represents, in a rough way, the degree of departure -from the general plan of the group. And the distribution of the sub-groups -within each group, is in most cases such that those which come nearest to -neighbouring groups, are those which show the nearest resemblances to -them--in their analogies though not in their homologies. No such scheme, -however, can give a correct conception. Even supposing the above diagram -expressed the relations of animals to one another as truly as they can be -expressed on a plane surface (which of course it does not), it would still -be inadequate. Such relations cannot be represented in space of two -dimensions, but only in space of three dimensions. - - _Mammalia_ - _Aves_ - _Reptilia_ - - VERTEBRATA - \ - _Amphibia_\ _Pisces_ - \ - \ _Arachnida_ - \ _Insecta_ - \ _Crustacea_ - \ - \ Articulata - \ | - \ | _Myriapoda_ - \ | - \ ANNULOSA - \ | - \ |_Annelida_ - \ _Scolecida_ - \ | - \ Annuloida - \ | / - \ _Echinodermata_ - \ | / - _Pteropoda_ _Cephalopoda_\ | | - _Gasteropoda dioecia_ | | - _Gasteropoda monoecia_ _Pulmonata_ | | - \ | | - MOLLUSCA------------- \ | | - \ \ | | - _Lamellibranchiata_ \ \ | | - \ \ | | - _Brachiopoda_ \ \ | | - \ \ _Gregarinida_ - Molluscoida------------ - _Rhizopoda_ \ - _Ascidioida_ _Polyzoa_ / \ - / PROTOZOA - / _Spongida_ _Infusoria_ - _Hydrozoa_ / - / - COELENTERATA - - _Actinozoa_ - -§ 100a. Two motives have prompted me to include in its original form the -foregoing sketch: the one being that in conformity with the course -previously pursued, of giving the successive forms of classifications, it -seems desirable to give this form which was approved thirty-odd years ago; -and the other being that the explanatory comments remain now as applicable -as they were then. Replacement of the diagram by one expressing the -relations of classes as they are now conceived, is by no means an easy -task; for the conceptions formed of them are unsettled. Concerning the -present attitude of zoologists, Prof. MacBride writes:-- - - "They all recognize a certain number of phyla. Each phylum includes a - group of animals about whose relation to each other no one entertains a - doubt. Each zoologist, however, has his own idea as to the relationship - which the various phyla bear to each other. - - "The phyla recognized at present are:-- - - (1) Protozoa. - (2) Porifera (Sponges). - (3) Coelenterata. - (4) Echinodermata. - { Cestodes. - (5) Platyhelminthes { Trematodes. - { Turbellaria. - (6) Nemertea. - (7) Nematoda. - (8) Acanthocephala (Echinorhyncus). - (9) Chætognatha (Sagitta). - (10) Rotifera. - (11) Annelida (Includes Leeches and Gephyrea, Chætifera). - (12) Gephyrea, Achæta. - { Tracheata (Peripatus, Myriapods, Insects). - (13) Arthropods { Arachnids. - { Crustacea. - { Pycnogonida. - (14) Mollusca. - (15) Polyzoa (Including Phoronis). - (16) Brachiopoda. - (17) Chordata (Includes Balanoglossus and Tunicates. Some - continental zoologists do not admit Balanoglossus)." - - [This last phylum of course includes the _Vertebrata_.] - -Though under present conditions, as above implied, it would be absurd to -attempt a definite scheme of relationships, yet it has seemed to me that -the adumbration of a scheme, presenting in a vague way such relationships -as are generally agreed upon and leaving others indeterminate, may be -ventured; and that a general impression hence resulting may be useful. On -the adjacent page I have tried to make a tentative arrangement of this -kind. - -At the bottom of the table I have placed together, under the name "Compound -_Protozoa_," those kinds of aggregated _Protozoa_ which show no -differentiations among the members of groups, and are thus distinguished -from _Metazoa_; and I have further marked the distinction by their -position, which implies that from them no evolution of higher types has -taken place. Respecting the naming of the sub-kingdoms, phyla, classes, -orders, &c., I have not maintained entire consistency. The relative values -of groups cannot be typographically expressed in a small space with a -limited variety of letters. The sizes of the letters mark the -classificatory ranks, and by the thickness I have rudely indicated their -zoological importance. In fixing the order of subordination of groups I -have been aided by the table of contents prefixed to Mr. Adam Sedgwick's -_Student's Text Book of Zoology_ and have also made use of Prof. Ray -Lankester's classifications of several sub-kingdoms. - - _Placental_-----+ - | _Aves_ - _Mammalia_----+ | _Arachnida_ - | | | - _Implacental_---+ | _Insecta_ | _Crustacea_ - | | | | | - VERTEBRATA | | | | | - | | | | | - _Reptilia_ _Chilopoda_ - | | - _Amphibia_ ARTHROPODA | - | | - | _Diplopoda_ - _Pisces_ | - | _Chætopoda_ - _Cephalochorda_ | | - CHORDATA ANNELIDA - _Urochorda_ | | _Echiuroidea_ - _(Tunicata)_ | | _Hirudinea_ - +---------+ _Archiannelida_ - | | - BRACHIOPODA | | ROTIFERA - _Dibranchiata_ | | | | - _Cephalopoda_ | | +----+ - _Tetrabranchiata_ | | | _Crinoidea_ - MOLLUSCA ---------------+ | | | +------ ECHINODERMATA - _Scaphopoda_ | | | | | _Asteroidea_ - _Solenogastres_ | | | | | _Echinoidea_ - _Gasteropoda_ | | | | | _Holothuroidea_ - _Lammellibranchiata_----+ | | | | _Enteropneusta_ - | | | | | - | | | | | _Acanthocephala_ - POLYZOA --------------------+ | | | | | +--- NEMATHELMINTHES - | | | | | | | _Nematomorpha_ - | | | | | | | _Nematoda_ - | | | | | | | - - _Zoantharia_ Ctenophora - _Rugosa_ _Acalephos_ - _Alcyonaria_ COELENTERATA _Hydromedusae_ - Actinozoa Hydrozoa | - | | |-------- NEMERTEA - | | | - | +----------+ _Turbellaria_ - ----------------| - | - PROTOZOA PLATYHELMINTHES - _Corticata_ _Trematoda_ - _Gymnomyxa_ _Cestoda_ - | | - | | - Myxomycetes | _Triaronia_ - | _Vobrocina_ _Calcarea_ - _Foraminifera_ | PORIFERA - Compound Protozoa _Demospongiae_ - _Radiolaria_ - -Let me again emphasize the fact that the relationships of these diverging -and re-diverging groups cannot be expressed on a flat surface. If we -imagine a laurel-bush to be squashed flat by a horizontal plane descending -upon it, we shall see that sundry of the upper branches and twigs which -were previously close together will become remote, and that the relative -positions of parts can remain partially true only with the minor branches. -The reader must therefore expect to find some of the zoological divisions -which in the order of nature are near one another, shown in the table as -quite distant. - - -§ 101. While the classifications of botanists and zoologists have become -more and more natural in their arrangements, there has grown up a certain -artificiality in their abstract nomenclature. When aggregating the smallest -groups into larger groups and these into groups still larger, they have -adopted certain general terms expressive of the successively more -comprehensive divisions; and the habitual use of these terms, needful for -purposes of convenience, has led to the tacit assumption that they answer -to actualities in Nature. It has been taken for granted that species, -genera, orders, and classes, are assemblages of definite values--that every -genus is the equivalent of every other genus in respect of its degree of -distinctness; and that orders are separated by lines of demarcation which -are as broad in one place as another. Though this conviction is not a -formulated one, the disputes continually occurring among naturalists on the -questions, whether such and such organisms are specifically or generically -distinct, and whether this or that peculiarity is or is not of ordinal -importance, imply that the conviction is entertained even where not avowed. -Yet that differences of opinion like these arise and remain unsettled, -except when they end in the establishment of sub-species, sub-genera, -sub-orders, and sub-classes, sufficiently shows that the conviction is -ill-based. And this is equally shown by the impossibility of obtaining any -definition of the degree of difference which warrants each further -elevation in the hierarchy of classes. - -It is, indeed, a wholly gratuitous assumption that organisms admit of being -placed in groups of equivalent values; and that these may be united into -larger groups which are also of equivalent values; and so on. There is no -_à priori_ reason for expecting this; and there is no _à posteriori_ -evidence implying it, save that which begs the question--that which asserts -one distinction to be generic and another to be ordinal, because it is -assumed that such distinctions must be either generic or ordinal. The -endeavour to thrust plants and animals into these definite partitions is of -the same nature as the endeavour to thrust them into linear series. Not -that it does violence to the facts in anything like the same degree; but -still, it does violence to the facts. Doubtless the making of divisions and -sub-divisions, is extremely useful; or rather, it is necessary. Doubtless, -too, in reducing the facts to something like order they must be partially -distorted. So long as the distorted form is not mistaken for the actual -form, no harm results. But it is needful for us to remember that while our -successively subordinate groups have a certain general correspondence with -the realities, they tacitly ascribe to the realities a regularity which -does not exist. - - -§ 102. A general truth of much significance is exhibited in these -classifications. On observing the natures of the attributes which are -common to the members of any group of the first, second, third, or fourth -rank, we see that groups of the widest generality are based on characters -of the greatest importance, physiologically considered; and that the -characters of the successively-subordinate groups, are characters of -successively-subordinate importance. The structural peculiarity in which -all members of one sub-kingdom differ from all members of another -sub-kingdom, is a peculiarity that affects the vital actions more -profoundly than does the structural peculiarity which distinguishes all -members of one class from all members of another class. Let us look at a -few cases. - -We saw (§ 56), that the broadest division among the functions is the -division into "the _accumulation of energy_ (latent in food); the -_expenditure of energy_ (latent in the tissues and certain matters absorbed -by them); and the _transfer of energy_ (latent in the prepared nutriment or -blood) from the parts which accumulate to the parts which expend." Now in -the lowest animals, united under the general name _Protozoa_, there is -either no separation of the parts performing these functions or very -indistinct separation: in the _Rhizopoda_, all parts are alike accumulators -of energy, expenders of energy and transferers of energy; and though in the -higher members of the group, the _Infusoria_, there are some -specializations corresponding to these functions, yet there are no distinct -tissues appropriated to them. Similarly when we pass from simple types to -compound types--from _Protozoa_ to _Metazoa_. The animals known as -_Coelenterata_ are characterized in common by the possession of a part -which accumulates energy more or less marked off from the part which does -not accumulate energy, but only expends it; and the _Hydrozoa_ and -_Actinozoa_, which are sub-divisions of the _Coelenterata_, are contrasted -in this, that in the second these parts are much more differentiated from -one another, as well as more complicated. Besides a completer -differentiation of the organs respectively devoted to the accumulation of -energy and the expenditure of energy, animals next above the _Coelenterata_ -possess rude appliances for the transfer of energy: the peri-visceral sac, -or closed cavity between the intestine and the walls of the body, serves as -a reservoir of absorbed nutriment, from which the surrounding tissues take -up the materials they need. And then out of this sac originates a more -efficient appliance for the transfer of energy: the more highly-organized -animals, belonging to whichever sub-kingdom, all of them possess -definitely-constructed channels for distributing the matters containing -energy. In all of them, too, the function of expenditure is divided between -a directive apparatus and an executive apparatus--a nervous system and a -muscular system. But these higher sub-kingdoms are clearly separated from -one another by differences in the relative positions of their component -sets of organs. The habitual attitudes of annulose and molluscous -creatures, is such that the neural centres are below the alimentary canal -and the hæmal centres above. And while by these traits the annulose and -molluscous types are separated from the vertebrate, they are separated from -each other by this, that in the one the body is "composed of successive -segments, usually provided with limbs," but in the other, the body is not -segmented, "and no true articulated limbs are ever developed." - -The sub-kingdoms being thus distinguished from one another, by the presence -or absence of specialized parts devoted to fundamental functions, or else -by differences in the distributions of such parts, we find, on descending -to the classes, that these are distinguished from one another, either by -modifications in the structures of fundamental parts, or by the presence or -absence of subsidiary parts, or by both. Fishes and _Amphibia_ are unlike -higher vertebrates in possessing branchiæ, either throughout life or early -in life. And every higher vertebrate, besides having lungs, is -characterized by having, during development, an amnion and an allantois. -Mammals, again, are marked off from Birds and Reptiles by the presence of -mammæ, as well as by the form of the occipital condyles. Among Mammals, the -next division is based on the presence or absence of a placenta. And -divisions of the _Placentalia_ are mainly determined by the characters of -the organs of external action. - -Thus, without multiplying illustrations and without descending to genera -and species, we see that, speaking generally, the successively smaller -groups are distinguished from one another by traits of successively less -importance, physiologically considered. The attributes possessed in common -by the largest assemblages of organisms, are few in number but -all-essential in kind. Each secondary assemblage, included in one of the -primary assemblages, is characterized by further common attributes that -influence the functions less profoundly. And so on with each lower grade. - - -§ 103. What interpretation is to be put on these truths of classification? -We find that organic forms admit of an arrangement everywhere indicating -the fact, that along with certain attributes, certain other attributes, -which are not directly connected with them, always exist. How are we to -account for this fact? And how are we to account for the fact that the -attributes possessed in common by the largest assemblages of forms, are the -most vitally-important attributes? - -No one can believe that combinations of this kind have arisen fortuitously. -Even supposing fortuitous combinations of attributes might produce -organisms that would work, we should still be without a clue to this -special mode of combination. The chances would be infinity to one against -organisms which possessed in common certain fundamental attributes, having -also in common numerous non-essential attributes. - -Nor, again, can any one allege that such combinations are necessary, in the -sense that all other combinations are impracticable. There is not, in the -nature of things, a reason why creatures covered with feathers should -always have beaks: jaws carrying teeth would, in many cases, have served -them equally well or better. The most general characteristic of an entire -sub-kingdom, equal in extent to the _Vertebrata_, might have been the -possession of nictitating membranes; while the internal organizations -throughout this sub-kingdom might have been on many different plans. - -If, as an alternative, this peculiar subordination of traits which organic -forms display be ascribed to design, other difficulties suggest themselves. -To suppose that a certain plan of organization was fixed on by a Creator -for each vast and varied group, the members of which were to have many -different modes of life, and that he bound himself to adhere rigidly to -this plan, even in the most aberrant forms of the group where some other -plan would have been more appropriate, is to ascribe a very strange motive. -When we discover that the possession of seven cervical vertebræ is a -general characteristic of mammals, whether the neck be immensely long as in -the giraffe, or quite rudimentary as in the whale, shall we say that -though, for the whale's neck, one vertebra would have been equally good, -and though, for the giraffe's neck, a dozen would probably have been better -than seven, yet seven was the number adhered to in both cases, because -seven was fixed upon for the mammalian type? And then, when it turns out -that this possession of seven cervical vertebræ is not an -absolutely-universal characteristic of mammals (there is one which has -eight), shall we conclude that while, in a host of cases, there was a -needless adherence to a plan for the sake of consistency, there was yet, in -some cases, an inconsistent abandonment of the plan? I think we may -properly refuse to draw any such conclusion. - -What, then, is the meaning of these peculiar relations of organic forms? -The answer to this question must be postponed. Having here contemplated the -problem as presented in these wide inductions which naturalists have -reached; and having seen what proposed solutions of it are inadmissible; we -shall see, in the next division of this work, what is the only possible -solution. - - - - -CHAPTER XII. - -DISTRIBUTION. - - -§ 104. There is a distribution of organisms in Space, and there is a -distribution of organisms in Time. Looking first at their distribution in -Space, we observe in it two different classes of facts. On the one hand, -the plants and animals of each species have their habitats limited by -external conditions: they are necessarily restricted to spaces in which -their vital actions can be performed. On the other hand, the existence of -certain conditions does not determine the presence of organisms that are -fit for them. There are many spaces perfectly adapted for life of a high -order in which only life of a much lower order is found. - -While, in the inevitable restriction of organisms to environments with -which their natures correspond we find a _negative_ cause of distribution, -there remains to be found that _positive_ cause whence results the presence -of organisms in some places appropriate to them and their absence from -other places equally appropriate or more appropriate. Let us consider the -phenomena as thus classed. - - -§ 105. Facts which illustrate the limiting influence of surrounding -conditions are abundant, and familiar to all readers. It will be needful, -however, here to cite a few typical ones of each order. - -The confinement of different kinds of plants and different kinds of -animals, to the media for which they are severally adapted, is the broadest -fact of distribution. We have extensive groups of plants that are -respectively sub-aerial and sub-aqueous; and of the sub-aqueous some are -exclusively marine, while others exist only in rivers and lakes. Among -animals we similarly find some classes confined to the air and others to -the water; and of the water-breathers some are restricted to salt water and -others to fresh water. Less conspicuous is the fact that within each of -these contrasted media there are further widespread limitations. In the -sea, certain organisms exist only between certain depths, and others only -between other depths--the limpet and the mussel within the littoral zone, -and numerous kinds at the bottom of the ocean; and on the land, there are -Floras and Faunas peculiar to low regions and others peculiar to high -regions. Next we have the familiar geographical limitations made by -climate. There are temperatures which restrict each kind of organism -between certain isothermal lines, and hygrometric states which prevent the -spread of each kind of organism beyond areas having a certain humidity or a -certain dryness. Besides such general limitations we find much more special -limitations. Some minute vegetal forms occur only in snow. Hot springs have -their peculiar _Infusoria_. The habitats of certain Fungi are mines or -other dark places. And there are creatures unknown beyond the water -contained in particular caves. After these limits to distribution imposed -by physical conditions, come limits imposed by the presence or absence of -other organisms. Obviously, graminivorous animals are confined within -tracts which produce plants fit for them to feed on. The great carnivores -cannot exist out of regions where there are creatures large enough and -numerous enough to serve for prey. The needs of the sloth limit it to -certain forest-covered spaces; and there can be no insectivorous bats where -there are no night-flying insects. To these dependences of the -relatively-superior organisms on the relatively-inferior organisms which -they consume, must be added certain reciprocal dependences of the inferior -on the superior. Mr. Darwin's inquiries have shown how generally the -fertilization of plants is due to the agency of insects, and how certain -plants, being fertilizable only by insects of certain structures, are -limited to regions inhabited by insects of such structures. Conversely, the -spread of organisms is often bounded by the presence of particular -organisms beyond the bounds--either competing organisms or organisms -directly inimical. A plant fit for some territory adjacent to its own, -fails to overrun it because the territory is pre-occupied by some plant -which is its superior, either in fertility or power of resisting -destructive agencies; or else fails because there lives in the territory -some mammal which browses on its foliage or bird which devours nearly all -its seeds. Similarly, an area in which animals of a particular species -might thrive, is not colonized by them because they are not fleet enough to -escape some beast of prey inhabiting this area, or because the area is -infested by some insect which destroys them, as the tsetse destroys the -cattle in parts of Africa. Yet another more special series of limitations -accompanies parasitism. There are parasitic plants that flourish only on -trees of some few species, and others that have particular animals for -their habitats--as the fungus which is fatal to the silk-worm, or that -which so strangely grows out of a New Zealand caterpillar. Of -animal-parasites various kinds lead lives involving specialities of -distribution. We have kinds which use other creatures for purposes of -locomotion, as the _Chelonobia_ uses the turtle, and as a certain _Actinia_ -uses the shell inhabited by a hermit-crab. We have the parasitism in which -one creature habitually accompanies another to share its prey, like the -annelid which takes up its abode in a hermit-crab's shell, and snatches -from the hermit-crab the morsels of food it is eating. We have again the -commoner parasitism of the _Epizoa_--animals which attach themselves to the -surfaces of other animals, and feed on their juices or on their secretions. -And once more, we have the equally common parasitism of the -_Entozoa_--creatures which live within other creatures. Besides being -restricted to the bodies of the organisms it infests, each species has -usually still narrower limits of distribution; in some cases the infested -organisms furnish fit habitats for the parasites only in certain regions, -and in other cases only when in certain constitutional states. There are -more indirect modes in which the distributions of organisms affect one -another. Plants of some kinds are eaten by animals only in the absence of -kinds that are preferred to them; and hence the prosperity of such plants -partly depends on the presence of the preferred plants. Mr. Bates has shown -that some South American butterflies thrive in regions where insectivorous -birds would destroy them, did they not closely resemble butterflies of -another genus which are disliked by those birds. And Mr. Darwin gives cases -of dependence still more remote and involved. - -Such are the chief negative causes of distribution--the inorganic and -organic agencies that set bounds to the spaces which organisms of each -species inhabit. Fully to understand their actions we must contemplate them -as working not separately but in concert. We have to regard the physical -influences, varying from year to year, as now producing an extension or -restriction of the habitat in this direction and now in that, and as -producing secondary extensions and restrictions by their effects on other -kinds of organisms. We have to regard the distribution of each species as -affected not only by causes which favour multiplication of prey or of -enemies within its own area, but also by causes which produce such results -in neighbouring areas. We have to conceive the forces by which the limit is -maintained, as including all meteorologic influences, united with the -influences, direct or remote, of numerous co-existing species. - -One general truth, indicated by sundry of the above illustrations, calls -for special notice--the truth that all kinds of organisms intrude on one -another's spheres of existence. Of the ways in which they do this the -commonest is invasion of territory. That tendency which we see in the human -races, to overrun and occupy one another's lands, as well as the lands -inhabited by inferior creatures, is a tendency exhibited by all classes of -organisms in various ways. Among them, as among mankind, there are -permanent conquests, temporary occupations, and occasional raids. Every -spring an inroad is made into the area which our own birds occupy, by birds -from the South; and every winter the fieldfares of the North come to share -the hips and haws of our hedges, and thus entail on our native birds some -mortality. Besides these regularly-recurring incursions there are irregular -ones; as of locusts into countries not usually visited by them, or of -certain rodents which from time to time swarm into areas adjacent to their -own. Every now and then an incursion ends in permanent settlement--perhaps -in conquest over indigenous species. Within these few years an American -water-weed has taken possession of our ponds and rivers, and to some extent -supplanted native water-weeds. Of animals may be named a small kind of red -ant, having habits allied to those of tropical ants, which has of late -overrun many houses in London. The rat, which must have taken to infesting -ships within these few centuries, furnishes a good illustration of the -readiness of animals to occupy new places that are available. And the way -in which vessels visiting India are cleared of the European cockroach by -the kindred _Blatta orientalis_, shows us how these successful invasions -last only until there come more powerful invaders. Animals encroach on one -another's spheres of existence in further ways than by trespassing on one -another's areas: they adopt one another's modes of life. There are cases in -which this usurpation of habits is slight and temporary; and there are -cases where it is marked and permanent. Grey crows often join gulls in -picking up food between tide-marks; and gulls may occasionally be seen many -miles inland, feeding in ploughed fields and on moors. Mr. Darwin has -watched a fly-catcher catching fish. He says that the greater titmouse -sometimes adopts the practices of the shrike, and sometimes of the -nuthatch, and that some South American woodpeckers are frugivorous while -others chase insects on the wing. Of habitual intrusions on the occupations -of other creatures, one case is furnished by the sea-eagle, which, besides -hunting the surface of the land for prey, like the rest of the hawk-tribe, -often swoops down upon fish. And Mr. Darwin names a species of petrel that -has taken to diving, and has a considerably modified organization. The -last cases introduce a still more remarkable class of facts of kindred -meaning. This intrusion of organisms on one another's modes of life goes to -the extent of intruding on one another's media. The great mass of flowering -plants are terrestrial, and (aside from other needs) are required to be so -by their process of fructification. But there are some which live in the -water, and protrude their flowers above the surface. Nay, there is a still -more striking instance. At the sea-side may be found an alga a hundred -yards inland, and a phænogam rooted in salt water. Among animals these -interchanges of media are numerous. Nearly all coleopterous insects are -terrestrial; but the water-beetle, which like the rest of its order is an -air-breather, has aquatic habits. Water appears to be an extremely unfit -medium for a fly; and yet Mr. [now Sir John] Lubbock has discovered more -than one species of fly living beneath the surface of the water and coming -up occasionally for air. Birds, as a class, are specially fitted for an -aerial existence; but certain tribes of them have taken to an aquatic -existence--swimming on the surface of the water and making continual -incursions beneath it, and some kinds have wholly lost the power of flight. -Among mammals, too, which have limbs and lungs implying an organization for -terrestrial life, may be named kinds living more or less in the water and -are more or less adapted to it. We have water-rats and otters which unite -the two kinds of life, and show but little modification; hippopotami -passing the greater part of their time in the water, and somewhat more -fitted to it; seals living almost exclusively in the sea, and having the -mammalian form greatly obscured; whales wholly confined to the sea, and -having so little the aspect of mammals as to be mistaken for fish. -Conversely, sundry inhabitants of the water make excursions on the land. -Eels migrate at night from one pool to another. There are fish with -specially-modified gills and fin-rays serving as stilts, which, when the -rivers they inhabit are partially dried-up, travel in search of better -quarters. And while some kinds of crabs do not make land-excursions beyond -high-water mark, other kinds pursue lives almost wholly terrestrial. - -Guided by these two classes of facts, we must regard the bounds to each -species' sphere of existence as determined by the balancing of two -antagonist sets of forces. The tendency which every species has to intrude -on other areas, other modes of life, and other media, is restrained by the -direct and indirect resistance of conditions, organic and inorganic. And -these expansive and repressive energies, varying continually in their -respective intensities, rhythmically equilibrate each other--maintain a -limit that perpetually oscillates from side to side of a certain mean. - - -§ 106. As implied at the outset, the character of a region, when -unfavourable to any species, sufficiently accounts for the absence of this -species; and thus its absence is not inconsistent with the hypothesis that -each species was originally placed in the regions most favourable to it. -But the absence of a species from regions that _are_ favourable to it -cannot be thus accounted for. Were plants and animals localized wholly with -reference to the fitness of their constitutions to surrounding conditions, -we might expect Floras to be similar, and Faunas to be similar, where the -conditions are similar; and we might expect dissimilarities among Floras -and among Faunas, proportionate to the dissimilarities of their conditions. -But we do not find such anticipations verified. - -Mr. Darwin says that "in the Southern hemisphere, if we compare large -tracts of land in Australia, South Africa, and western South America, -between latitudes 25° and 35°, we shall find parts extremely similar in all -their conditions, yet it would not be possible to point out three faunas -and floras more utterly dissimilar. Or again we may compare the productions -of South America south of lat. 35° with those north of 25°, which -consequently inhabit a considerably different climate, and they will be -found incomparably more closely related to each other, than they are to the -productions of Australia or Africa under nearly the same climate." Still -more striking are the contrasts which Mr. Darwin points out between -adjacent areas that are totally cut off from each other. "No two marine -faunas are more distinct, with hardly a fish, shell, or crab in common, -than those of the eastern and western shores of South and Central America; -yet these great faunas are separated only by the narrow, but impassable, -isthmus of Panama." On opposite sides of high mountain-chains, also, there -are marked differences in the organic forms--differences not so marked as -where the barriers are absolutely impassable, but much more marked than are -necessitated by unlikenesses of physical conditions. - -Not less suggestive is the converse fact that wide geographical areas which -offer decided geologic and meteorologic contrasts, are peopled by -nearly-allied groups of organisms, if there are no barriers to migration. -"The naturalist in travelling, for instance, from north to south never -fails to be struck by the manner in which successive groups of beings, -specifically distinct, yet clearly related, replace each other. He hears -from closely allied, yet distinct kinds of birds, notes nearly similar, and -sees their nests similarly constructed, but not quite alike, with eggs -coloured in nearly the same manner. The plains near the Straits of Magellan -are inhabited by one species of Rhea (American Ostrich), and northward the -plains of La Plata by another species of the same genus; and not by a true -ostrich or emu, like those found in Africa and Australia under the same -latitude. On these same plains of La Plata, we see the agouti and bizcacha, -animals having nearly the same habits as our hares and rabbits and -belonging to the same order of Rodents, but they plainly display an -American type of structure. We ascend the lofty peaks of the Cordillera and -we find an alpine species of bizcacha; we look to the waters, and we do not -find the beaver or musk-rat, but the coypu and capybara, rodents of the -American type. Innumerable other instances could be given. If we look to -the islands off the American shore, however much they may differ in -geological structure, the inhabitants, though they may be all peculiar -species, are essentially American." - -What is the generalization implied by these two groups of facts? On the one -hand, we have similarly-conditioned, and sometimes nearly-adjacent, areas, -occupied by quite different Faunas. On the one hand, we have areas remote -from one another in latitude, and contrasted in soil as well as climate, -occupied by closely-allied Faunas. Clearly then, as like organisms are not -universally, or even generally, found in like habitats, nor very unlike -organisms in very unlike habitats, there is no manifest pre-determined -adaptation of the organisms to the habitats. The organisms do no occur in -such and such places solely because they are either specially fit for those -places, or more fit for them than all other organisms. - -The induction under which these facts come, and which unites them with -various other facts, is a totally-different one. When we see that the -similar areas peopled by dissimilar forms, are those between which there -are impassable barriers; while the dissimilar areas peopled by similar -forms, are those between which there are no such barriers; we are at once -reminded of the general truth exemplified in the last section--the truth -that each species of organism tends ever to expand its sphere of -existence--to intrude on other areas, other modes of life, other media. And -we are shown that through these perpetually-recurring attempts to thrust -itself into every accessible habitat, each species spreads until it reaches -limits which are for the time insurmountable. - - -§ 107. We pass now to the distribution of organic forms in Time. Geological -inquiry has established the truth that during a Past of immeasurable -duration, plants and animals have existed on the Earth. In all countries -their buried remains are found in greater or less abundance. From -comparatively small areas multitudinous different types have been exhumed. -Every exploration of new areas, and every closer inspection of areas -already explored, brings more types to light. And beyond question, an -exhaustive examination of all exposed strata, and of all strata now covered -by the sea, would disclose types immensely out-numbering those at present -known. Further, geologists agree that even had we before us every kind of -fossil which exists, we should still have nothing like a complete index to -the past inhabitants of our globe. Many sedimentary deposits have been so -altered by the heat of adjacent molten matter, as greatly to obscure the -organic remains contained in them. The extensive formations once called -"transition," and now re-named "metamorphic," are acknowledged to be -formations of sedimentary origin, from which all traces of such fossils as -they probably included have been obliterated by igneous action. And the -accepted conclusion is that igneous rock has everywhere resulted from the -melting-up of beds of detritus originally deposited by water. How long the -reactions of the Earth's molten nucleus on its cooling crust, have been -thus destroying the records of Life, it is impossible to say; but there are -strong reasons for believing that the records which remain bear but a small -ratio to the records which have been destroyed. Thus we have but extremely -imperfect data for conclusions respecting the distribution of organic forms -in Time. Some few generalizations, however, may be regarded as established. - -One is that the plants and animals now existing mostly differ from the -plants and animals which have existed. Though there are species common to -our present Fauna and to past Faunas, yet the _facies_ of our present Fauna -differs, more or less, from the _facies_ of each past Fauna. On carrying -out the comparison, we find that past Faunas differ from one another, and -that the differences between them are proportionate to their degrees of -remoteness from one another in Time, as measured by their relative -positions in the sedimentary series. So that if we take the assemblage of -organic forms living now, and compare it with the successive assemblages of -organic forms which have lived in successive geologic epochs, we find that -the farther we go back into the past, the greater does the unlikeness -become. The number of species and genera common to the compared -assemblages, becomes smaller and smaller; and the assemblages differ more -and more in their general characters. Though a species of brachiopod now -extant is almost identical with a species found in Silurian strata, though -between the Silurian Fauna and our own there are sundry common genera of -molluscs, yet it is undeniable that there is a proportion between lapse of -time and divergence of organic forms. - -This divergence is comparatively slow and continuous where there is -continuity in the geological formations, but is sudden, and comparatively -wide, wherever there occurs a great break in the succession of strata. The -contrasts which thus arise, gradually or all at once, in formations that -are continuous or discontinuous, are of two kinds. Faunas of different eras -are distinguished partly by the absence from the one of type's present in -the other, and partly by the unlikenesses between the types common to both. -Such contrasts between Faunas as are due to the appearance or disappearance -of types, are of secondary significance: they possibly, or probably, do not -imply anything more than migrations or extinctions. The most significant -contrasts are those between successive groups of organisms of the same -type. And among such, as above said, the differences are, speaking -generally, small and continuous where a series of conformable strata gives -proof of continued existence of the type in the locality; while they are -comparatively large and abrupt where the adjacent formations are shown to -have been separated by long intervals. - -Another general fact, referred to by Mr. Darwin as one which palæontology -has made tolerably certain, is that forms and groups of forms which have -once disappeared from the Earth, do not reappear. Passing over the few -species which have continued throughout the whole period geologically -recorded, it may be said that each species after arising, spreading for an -era, and continuing abundant for an era, eventually declines and becomes -extinct; and that similarly, each genus during a longer period increases in -the number of its species, and during a longer period dwindles and at last -dies out. After making its exit neither species nor genus ever re-enters. -The like is true even of those larger groups called orders. Four types of -reptiles which were once abundant have not been found in modern formations, -and do not at present exist. Though nothing less than an exhaustive -examination of all strata, can prove conclusively that a type of -organization when once lost is never reproduced, yet so many facts point to -this inference that its truth can scarcely be doubted. - -To frame a conception of the total amount and general direction of the -change in organic forms during the time measured by our sedimentary series, -is at present impossible--the data are insufficient. The immense contrast -between the few and low forms of the earliest-known Fauna, and the many and -high forms of our existing Fauna, has been commonly supposed to prove, not -only great change but great progress. Nevertheless, this appearance of -progress may be, and probably is, mainly illusive. Wider knowledge has -shown that remains of comparatively well-organized creatures really existed -in strata long supposed to be devoid of them, and that where they are -absent, the nature of the strata often explains their absence, without -assuming that they did not exist when these strata were formed. It is a -tenable hypothesis that the successively-higher types fossilized in our -successively-later deposits, indicate nothing more than successive -migrations from pre-existing continents to continents that were step by -step emerging from the ocean--migrations which necessarily began with the -inferior orders of organisms, and included the successively-superior orders -as the new lands became more accessible to them and better fitted for -them.[43] - -While the evidence usually supposed to prove progression is thus -untrustworthy, there is trustworthy evidence that there has been, in many -cases, little or no progression. Though the orders which have existed from -palæozoic and mesozoic times down to the present day, are almost -universally changed, yet a comparison of ancient and modern members of -these orders shows that the total amount of change is not relatively great, -and that it is not manifestly towards a higher organization. Though nearly -all the living forms which have prototypes in early formations differ from -these prototypes specially, and in most cases generically, yet ordinal -peculiarities are, in numerous cases, maintained from the earliest times -geologically recorded, down to our own time; and we have no visible -evidence of superiority in the existing genera of these orders. In his -lecture "On the Persistent Types of Animal Life," Prof. Huxley enumerated -many cases. On the authority of Dr. Hooker he stated "that there are -Carboniferous plants which appear to be generically identical with some now -living: that the cone of the Oolitic _Araucaria_ is hardly distinguishable -from that of an existing species; that a true _Pinus_ appears in the -Purbecks and a _Juglans_ in the chalk." Among animals he named palæozoic -and mesozoic corals which are very like certain extant corals; genera of -Silurian molluscs that answer to existing genera; insects and arachnids in -the coal-formations that are not more than generically distinct from some -of our own insects and arachnids. He instanced "the Devonian and -Carboniferous _Pleuracanthus_, which differs no more from existing sharks -than these do from one another;" early mesozoic reptiles "identical in the -essential characters of their organization with those now living;" and -Triassic mammals which did not differ "nearly so much from some of those -which now live, as these differ from one another." Continuing the argument -in his "Anniversary Address to the Geological Society" in 1862, Prof. -Huxley gave many cases in which the changes that have taken place, are not -changes towards a more specialized or higher organization--asking "in what -sense are the Liassic Chelonia inferior to those which now exist? How are -the Cretaceous Ichthyosauria, Plesiosauria, or Pterosauria less embryonic -or more differentiated species than those of the Lias?" While, however, -contending that in most instances "positive evidence fails to demonstrate -any sort of progressive modification towards a less embryonic or less -generalized type in a great many groups of animals of long-continued -geological existence," Prof. Huxley added that there are other groups, -"co-existing with them under the same conditions, in which more or less -distinct indications of such a process seem to be traceable." And in -illustration of this, he named that better development of the vertebræ -which characterizes some of the more modern fishes and reptiles, when -compared with ancient fishes and reptiles of the same orders; and the -"regularity and evenness of the dentition of the _Anoplotherium_ as -contrasting with that of existing Artiodactyles."[44] - -The facts thus summed up do not show that higher forms have not arisen in -the course of geologic time, any more than the facts commonly cited prove -that higher forms have arisen; nor are they regarded by Professor Huxley as -showing this. Were those which have survived from palæozoic and mesozoic -days down to our own day, the only types; and did the modifications, rarely -of more than generic value, which these types have undergone, give no -better evidences of increased complexity than are actually given by them; -then it would be inferable that there has been no appreciable advance. But -there now exist, and have existed during the more recent geologic epochs, -various types which are not known to have existed in earlier epochs--some -of them widely unlike these persistent types and some of them nearly allied -to these persistent types. As yet, we know nothing about the origins of -these new types. But it is possible that causes like those which have -produced generic differences in the persistent types, have, in some or many -cases, produced modifications great enough to constitute ordinal -differences. If structural contrasts not exceeding certain moderate limits -are held to mark only generic distinctions; and if organisms displaying -larger contrasts are regarded as ordinally or typically distinct; it is -obvious that the persistence of a given type through a long geologic period -without apparently undergoing deviations of more than generic value, by no -means disproves the occurrence of far greater deviations in other cases; -since the forms resulting from such far greater deviations, being regarded -as typically distinct forms, will not be taken as evidence of great change -in an original type. That which Prof. Huxley's argument proves, and that -only which he considers it to prove, is that organisms have no innate -tendencies to assume higher forms; and that "any admissible hypothesis of -progressive modification, must be compatible with persistence without -progression through indefinite periods." - -One very significant fact must be added concerning the relation between -distribution in Time and distribution in Space. I quote it from Mr. -Darwin:--"Mr. Clift many years ago showed that the fossil mammals from the -Australian caves were closely allied to the living marsupials of that -continent. In South America a similar relationship is manifest, even to an -uneducated eye, in the gigantic pieces of armour like those of the -armadillo, found in several parts of La Plata; and Professor Owen has shown -in the most striking manner that most of the fossil mammals, buried there -in such numbers, are related to the South American types. This relationship -is even more clearly seen in the wonderland collection of fossil bones made -by MM. Lund and Clausen in the caves of Brazil. I was so much impressed -with these facts that I strongly insisted, in 1839 and 1845, on this 'law -of the succession of types,'--on 'this wonderful relationship in the same -continent between the dead and the living.' Professor Owen has subsequently -extended the same generalization to the Mammals of the Old World. We see -the same law in this author's restorations of the extinct and gigantic -birds of New Zealand. We see it also in the birds of the caves of Brazil. -Mr. Woodward has shown that the same law holds good with sea-shells, but -from the wide distribution of most genera of molluscs, it is not well -displayed by them. Other cases could be added, as the relation between the -extinct and living landshells of Madeira, and between the extinct and -living brackish-water shells of the Aralo-Caspian Sea." - -The general results, then, are these. Our knowledge of distribution in -Time, being derived wholly from the evidence afforded by fossils, is -limited to that geologic time of which some records remain--cannot extend -to those remoter times the records of which have been obliterated. From -these remaining records, which probably form but a small fraction of the -whole, the general facts deducible are these:--That such organic types as -have lived through successive epochs, have almost universally undergone -modifications of specific and generic values--modifications which have -commonly been great in proportion as the period has been long. That besides -the types which have persisted from ancient eras down to our own era, other -types have from time to time made their appearance in the ascending series -of strata--types of which some are lower and some higher than the types -previously recorded; but whence these new types came, and whether any of -them arose by divergence from the previously-recorded types, the evidence -does not yet enable us to say. That in the course of long geologic epochs -nearly all species, most genera, and a few orders, have become extinct; and -that a species, genus, or order, which has once disappeared from the Earth -never reappears. And, lastly, that the Fauna now occupying each separate -area of the Earth's surface is very nearly allied to the Fauna which -existed on that area during recent geologic times. - - -§ 108. Omitting sundry minor generalizations, the exposition of which would -involve too much detail, what is to be said of these major generalizations? - -The distribution in Space cannot be said to imply that organisms have been -designed for their particular habitats and placed in them; since, besides -the habitat in which each kind of organism is found there are commonly -other habitats, as good or better for it, from which it is absent--habitats -to which it is so much better fitted than organisms now occupying them, -that it extrudes these organisms when allowed the opportunity. Neither can -we suppose that the purpose has been to establish varieties of Floras and -Faunas; since, if so, why are the Floras and Faunas but little divergent in -widely-sundered areas between which migration is possible, while they are -markedly divergent in adjacent areas between which migration is impossible? - -Passing to distributions in Time, there arise the questions--why during -nearly the whole of that vast period geologically recorded have there -existed none of those highest organic forms which have now overrun the -Earth?--how is it that we find no traces of a creature endowed with large -capacities for knowledge and happiness? The answer that the Earth was not, -in remote times, a fit habitation for such a creature, besides being -unwarranted by the evidence, suggests the equally awkward question--why -during untold millions of years did the Earth remain fit only for inferior -creatures? What, again, is the meaning of extinction of types? To conclude -that the saurian type was replaced by other types at the beginning of the -tertiary period, because it was not adapted to the conditions which then -arose, is to conclude that it could not be modified into fitness for the -conditions; and this conclusion is at variance with the hypothesis that -creative skill is shown in the multiform adaptations of one type to many -ends. - -What interpretations may rationally be put on these and other general facts -of distribution in Space and Time, will be seen in the next division of -this work. - - - - -PART III. - -THE EVOLUTION OF LIFE. - - - - -CHAPTER I. - -PRELIMINARY. - - -§ 109. In the foregoing Part, we have contemplated the most important of -the generalizations to which biologists have been led by observation of -organisms; as well as some others which contemplation of the facts has -suggested to me. These Inductions of Biology have also been severally -glanced at on their deductive sides; for the purpose of noting the harmony -existing between them and those primordial truths set forth in _First -Principles_. Having thus studied the leading phenomena of life separately, -we are prepared for studying them as an aggregate, with the view of -arriving at the most general interpretation of them. - -There is an _ensemble_ of vital phenomena presented by each organism in the -course of its growth, development, and decay; and there is an _ensemble_ of -vital phenomena presented by the organic world as a whole. Neither of these -can be properly dealt with apart from the other. But the last of them may -be separately treated more conveniently than the first. What interpretation -we put on the facts of structure and function in each living body, depends -entirely on our conception of the mode in which living bodies in general -have originated. To form some conclusion respecting this mode--a -provisional if not a permanent conclusion--must therefore be our first -step. - -We have to choose between two hypotheses--the hypothesis of Special -Creation and the hypothesis of Evolution. Either the multitudinous kinds of -organisms which now exist, and the far more multitudinous kinds which have -existed during past geologic eras, have been from time to time separately -made; or they have arisen by insensible steps, through actions such as we -see habitually going on. Both hypotheses imply a Cause. The last, certainly -as much as the first, recognizes this Cause as inscrutable. The point at -issue is, how this inscrutable Cause has worked in the production of living -forms. This point, if it is to be decided at all, is to be decided only by -examination of evidence. Let us inquire which of these antagonist -hypotheses is most congruous with established facts. - - - - -CHAPTER II. - -GENERAL ASPECTS OF THE SPECIAL-CREATION-HYPOTHESIS.[45] - - -§ 110. Early ideas are not usually true ideas. Undeveloped intellect, be it -that of an individual or that of the race, forms conclusions which require -to be revised and re-revised, before they reach a tolerable correspondence -with realities. Were it otherwise there would be no discovery, no increase -of intelligence. What we call the progress of knowledge, is the bringing of -Thoughts into harmony with Things; and it implies that the first Thoughts -are either wholly out of harmony with Things, or in very incomplete harmony -with them. - -If illustrations be needed the history of every science furnishes them. The -primitive notions of mankind as to the structure of the heavens were wrong; -and the notions which replaced them were successively less wrong. The -original belief respecting the form of the Earth was wrong; and this wrong -belief survived through the first civilizations. The earliest ideas that -have come down to us concerning the natures of the elements were wrong; and -only in quite recent times has the composition of matter in its various -forms been better understood. The interpretations of mechanical facts, of -meteorological facts, of physiological facts, were at first wrong. In all -these cases men set out with beliefs which, if not absolutely false, -contained but small amounts of truth disguised by immense amounts of error. - -Hence the hypothesis that living beings resulted from special creations, -being a primitive hypothesis, is probably an untrue hypothesis. It would be -strange if, while early men failed to reach the truth in so many cases -where it is comparatively conspicuous, they reached it in a case where it -is comparatively hidden. - - -§ 111. Besides the improbability given to the belief in special creations, -by its association with mistaken beliefs in general, a further -improbability is given to it by its association with a special class of -mistaken beliefs. It belongs to a family of beliefs which have one after -another been destroyed by advancing knowledge; and is, indeed, almost the -only member of the family surviving among educated people. - -We all know that the savage thinks of each striking phenomenon, or group of -phenomena, as caused by some separate personal agent; that out of this -conception there grows up a polytheistic conception, in which these minor -personalities are variously generalized into deities presiding over -different divisions of nature; and that these are eventually further -generalized. This progressive consolidation of causal agencies may be -traced in the creeds of all races, and is far from complete in the creed of -the most advanced races. The unlettered rustics who till our fields, do not -let the consciousness of a supreme power wholly absorb the aboriginal -conceptions of good and evil spirits, and of charms or secret potencies -dwelling in particular objects. The earliest mode of thinking changes only -as fast as the constant relations among phenomena are established. Scarcely -less familiar is the truth, that while accumulating knowledge makes these -conceptions of personal causal agents gradually more vague, as it merges -them into general causes, it also destroys the habit of thinking of them as -working after the methods of personal agents. We do not now, like Kepler, -assume guiding spirits to keep the planets in their orbits. It is no longer -the universal belief that the sea was once for all mechanically parted from -the dry land; or that the mountains were placed where we see them by a -sudden creative act. All but a narrow class have ceased to suppose sunshine -and storm to be sent in some arbitrary succession. The majority of educated -people have given up thinking of epidemics of punishments inflicted by an -angry deity. Nor do even the common people regard a madman as one possessed -by a demon. That is to say, we everywhere see fading away the -anthropomorphic conception of Cause. In one case after another, is -abandoned the ascription of phenomena to a will analogous to the human -will, working by methods analogous to human methods. - -If, then, of this once-numerous family of beliefs the immense majority have -become extinct, we may not unreasonably expect that the few remaining -members of the family will become extinct. One of these is the belief we -are here considering--the belief that each species of organism was -specially created. Many who in all else have abandoned the aboriginal -theory of things, still hold this remnant of the aboriginal theory. Ask any -well-informed man whether he accepts the cosmogony of the Indians, or the -Greeks, or the Hebrews, and he will regard the question as next to an -insult. Yet one element common to these cosmogonies he very likely retains: -not bearing in mind its origin. For whence did he get the doctrine of -special creations? Catechise him, and he is forced to confess that it was -put into his mind in childhood, as one portion of a story which, as a -whole, he has long since rejected. Why this fragment is likely to be right -while all the rest is wrong, he is unable to say. May we not then expect -that the relinquishment of all other parts of this story, will by and by be -followed by the relinquishment of this remaining part of it? - - -§ 112. The belief which we find thus questionable, both as being a -primitive belief and as being a belief belonging to an almost-extinct -family, is a belief not countenanced by a single fact. No one ever saw a -special creation; no one ever found proof of an indirect kind that a -special creation had taken place. It is significant, as Dr. Hooker remarks, -that naturalists who suppose new species to be miraculously originated, -habitually suppose the origination to occur in some region remote from -human observation. Wherever the order of organic nature is exposed to the -view of zoologists and botanists, it expels this conception; and the -conception survives only in connexion with imagined places, where the order -of organic nature is unknown. - -Besides being absolutely without evidence to give it external support, this -hypothesis of special creations cannot support itself internally--cannot be -framed into a coherent thought. It is one of those illegitimate symbolic -conceptions which are mistaken for legitimate symbolic conceptions (_First -Principles_, § 9), because they remain untested. Immediately an attempt is -made to elaborate the idea into anything like a definite shape, it proves -to be a pseud-idea, admitting of no definite shape. Is it supposed that a -new organism, when specially created, is created out of nothing? If so, -there is a supposed creation of matter; and the creation of matter is -inconceivable--implies the establishment of a relation in thought between -nothing and something--a relation of which one term is absent--an -impossible relation. Is it supposed that the matter of which the new -organism consists is not created for the occasion, but is taken out of its -pre-existing forms and arranged into a new form? If so, we are met by the -question--how is the re-arrangement effected? Of the myriad atoms going to -the composition of the new organism, all of them previously dispersed -through the neighbouring air and earth, does each, suddenly disengaging -itself from its combinations, rush to meet the rest, unite with them into -the appropriate chemical compounds, and then fall with certain others into -its appointed place in the aggregate of complex tissues and organs? Surely -thus to assume a myriad supernatural impulses, differing in their -directions and amounts, given to as many different atoms, is a -multiplication of mysteries rather than the solution of a mystery. For -every one of these impulses, not being the result of a force locally -existing in some other form, implies the creation of force; and the -creation of force is just as inconceivable as the creation of matter. It is -thus with all attempted ways of representing the process. The old Hebrew -idea that God takes clay and moulds a new creature, as a potter moulds a -vessel, is probably too grossly anthropomorphic to be accepted by any -modern defender of the special-creation doctrine. But having abandoned this -crude belief, what belief is he prepared to substitute? If a new organism -is not thus produced, then in what way is one produced? or rather--in what -way does he conceive a new organism to be produced? We will not ask for the -ascertained mode, but will be content with a mode which can be consistently -imagined. No such mode, however, is assignable. Those who entertain the -proposition that each kind of organism results from a divine interposition, -do so because they refrain from translating words into thoughts. They do -not really believe, but rather _believe they believe_. For belief, properly -so called, implies a mental representation of the thing believed, and no -such mental representation is here possible. - - -§ 113. If we imagine mankind to be contemplated by some being as -short-lived as an ephemeron, but possessing intelligence like our own--if -we imagine such a being studying men and women, during his few hours of -life, and speculating as to the mode in which they came into existence; it -is manifest that, reasoning in the usual way, he would suppose each man and -woman to have been separately created. No appreciable changes of structure -occurring in any of them during the time over which his observations -extended, this being would probably infer that no changes of structure were -taking place, or had taken place; and that from the outset each man and -woman had possessed all the characters then visible--had been originally -formed with them. The application is obvious. A human life is ephemeral -compared with the life of a species; and even the period over which the -records of all human lives extend, is ephemeral compared with the life of a -species. There is thus a parallel contrast between the immensely-long -series of changes which have occurred during the life of a species, and -that small portion of the series open to our view. And there is no reason -to suppose that the first conclusion drawn by mankind from this small part -of the series visible to them, is any nearer the truth than would be the -conclusion of the supposed ephemeral being respecting men and women. - -This analogy, suggesting as it does how the hypothesis of special creations -is merely a formula for our ignorance, raises the question--What reason -have we to assume special creations of species but not of individuals; -unless it be that in the case of individuals we directly know the process -to be otherwise, but in the case of species do not directly know it to be -otherwise? Have we any ground for concluding that species were specially -created, except the ground that we have no immediate knowledge of their -origin? And does our ignorance of the manner in which they arose warrant us -in asserting that they arose by special creation? - -Another question is suggested by this analogy. Those who, in the absence of -immediate evidence of the way in which species arose, assert that they -arose not in a natural way allied to that in which individuals arise, but -in a supernatural way, think that by this supposition they honour the -Unknown Cause of things; and they oppose any antagonist doctrine as -amounting to an exclusion of divine power from the world. But if divine -power is demonstrated by the separate creation of each species, would it -not have been still better demonstrated by the separate creation of each -individual? Why should there exist this process of natural genesis? Why -should not omnipotence have been proved by the supernatural production of -plants and animals everywhere throughout the world from hour to hour? Is it -replied that the Creator was able to make individuals arise from one -another in a natural succession, but not to make species thus arise? This -is to assign a limit to power instead of magnifying it. Either it was -possible or not possible to create species and individuals after the same -general method. To say that it was not possible is suicidal in those who -use this argument; and if it was possible, it is required to say what end -is served by the special creation of species which would not have been -better served by the special creation of individuals. Again, what is to be -thought of the fact that the immense majority of these supposed special -creations took place before mankind existed? Those who think that divine -power is demonstrated by special creations, have to answer the question--to -whom demonstrated? Tacitly or avowedly, they regard the demonstrations as -being for the benefit of mankind. But if so, to what purpose were the -millions of these demonstrations which took place on the Earth when there -were no intelligent beings to contemplate them? Did the Unknowable thus -demonstrate his power to himself? Few will have the hardihood to say that -any such demonstration was needful. There is no choice but to regard them, -either as superfluous exercises of power, which is a derogatory -supposition, or as exercises of power that were necessary because species -could not be otherwise produced, which is also a derogatory supposition. - - -§ 113a. Other implications concerning the divine character must be -recognized by those who contend that each species arose by divine fiat. It -is hardly supposable that Infinite Power is exercised in trivial actions -effecting trivial changes. Yet the organic world in its hundreds of -thousands of species shows in each sub-division multitudinous forms which, -though unlike enough to be classed as specifically distinct, diverge from -one another only in small details which have no significance in relation to -the life led. Sometimes the number of specific distinctions is so great -that did they result from human agency we should call them whimsical. - -For example, in Lake Baikal are found 115 species of an amphipod, -_Gammarus_; and the multiplicity becomes startling on learning that this -number exceeds the number of all other species of the genus: various as are -the conditions to which, throughout the rest of the world, the genus is -subject. Still stranger seems the superfluous exercise of power on -examining the carpet of living forms at the bottom of the ocean. Not -dwelling on the immense variety of creatures unlike in type which live -miles below the surface in absolute darkness, it will suffice to instance -the _Polyzoa_ alone: low types of animals so small that a thousand of them -would not cover a square inch, and on which, nevertheless, there has been, -according to the view we are considering, an exercise of creative skill -such that by small variations of structure more than 350 species have been -produced! - -Kindred illustrations are furnished by the fauna of caverns. Are we to -suppose that numerous blind creatures--crustaceans, myriapods, spiders, -insects, fishes--were specially made sightless to fit them for the Mammoth -Cave? Or what shall we say of the _Proteus_, a low amphibian with -rudimentary eyes, which inhabits certain caves in Carniola, Carinthia and -Dalmatia and is not found elsewhere. Must we conclude that God went out of -his way to devise an animal for these places? - -More puzzling still is a problem presented to the special-creationist by a -batrachian inhabiting Central Australia. In a region once peopled by -numerous animals but now made unfit by continuous droughts, there exists a -frog which, when the pools are drying up, fills itself with water and -burrowing in the mud hibernates until the next rains; which may come in a -year or may be delayed for two years. What is to be thought of this -creature? Were its structure and the accompanying instinct divinely planned -to fit it to this particular habitat? - -Many such questions might be asked which, if answered as the current theory -necessitates, imply a divine nature hardly like that otherwise assumed. - - -§ 114. Those who espouse the aboriginal hypothesis entangle themselves in -yet other theological difficulties. This assumption that each kind of -organism was specially designed, carries with it the implication that the -designer intended everything which results from the design. There is no -escape from the admission that if organisms were severally constructed with -a view to their respective ends, then the character of the constructor is -indicated both by the ends themselves, and the perfection or imperfection -with which the organisms are fitted to them. Observe the consequences. - -Without dwelling on the question recently raised, why during untold -millions of years there existed on the Earth no beings endowed with -capacities for wide thought and high feeling, we may content ourselves with -asking why, at present, the Earth is largely peopled by creatures which -inflict on one another so much suffering? Omitting the human race, whose -defects and miseries the current theology professes to account for, and -limiting ourselves to the lower creation, what must we think of the -countless different pain-inflicting appliances and instincts with which -animals are endowed? Not only now, and not only ever since men have lived, -has the Earth been a scene of warfare among all sentient creatures; but -palæontology shows us that from the earliest eras geologically recorded, -there has been going on this universal carnage. Fossil structures, in -common with the structures of existing animals, show us elaborate weapons -for destroying other animals. We have unmistakable proof that throughout -all past time, there has been a ceaseless devouring of the weak by the -strong. How is this to be explained? How happens it that animals were so -designed as to render this bloodshed necessary? How happens it that in -almost every species the number of individuals annually born is such that -the majority die by starvation or by violence before arriving at maturity? -Whoever contends that each kind of animal was specially designed, must -assert either that there was a deliberate intention on the part of the -Creator to produce these results, or that there was an inability to prevent -them. Which alternative does he prefer?--to cast an imputation on the -divine character or to assert a limitation of the divine power? It is -useless for him to plead that the destruction of the less powerful by the -more powerful, is a means of preventing the miseries of decrepitude and -incapacity, and therefore works beneficently. For even were the chief -mortality among the aged instead of among the young, there would still -arise the unanswerable question--why were not animals constructed in such -ways as to avoid these evils? why were not their rates of multiplication, -their degrees of intelligence, and their propensities, so adjusted that -these sufferings might be escaped? And if decline of vigour was a necessary -accompaniment of age, why was it not provided that the organic actions -should end in sudden death, whenever they fell below the level required for -pleasurable existence? Will any one who contends that organisms were -specially designed, assert that they could not have been so designed as to -prevent suffering? And if he admits that they could have been made so as to -prevent suffering, will he assert that the Creator preferred making them in -such ways as to inflict suffering? - -Even as thus presented the difficulty is sufficiently great; but it appears -immensely greater when we examine the facts more closely. So long as we -contemplate only the preying of the superior on the inferior, some good -appears to be extracted from the evil--a certain amount of life of a higher -order, is supported by sacrificing a great deal of life of a lower order. -So long, too, as we leave out all mortality but that which, by carrying off -the least perfect members of each species, leaves the most perfect members -to survive and multiply; we see some compensating benefit reached through -the suffering inflicted. But what shall we say on finding innumerable cases -in which the suffering inflicted brings no compensating benefit? What shall -we say when we see the inferior destroying the superior? What shall we say -on finding elaborate appliances for furthering the multiplication of -organisms incapable of feeling, at the expense of misery to organisms -capable of happiness? - -Of the animal kingdom as a whole, more than half the species are parasites. -"The number of these parasites," says Prof. Owen, "may be conceived when it -is stated that almost every known animal has its peculiar species, and -generally more than one, sometimes as many as, or even more kinds than, -infest the human body." This parasitism begins among the most minute -creatures and pervades the entire animal kingdom from the lowest to the -highest. Even _Protozoa_, made visible to us only by the microscope, are -infested, as is _Paramoecium_ by broods of _Sphærophrya_; while in large -and complex animals parasites are everywhere present in great variety. More -than this is true. There are parasites upon parasites--an arrangement such -that those which are torturing the creatures they inhabit are themselves -tortured by indwelling creatures still smaller: looking like an ingenious -accumulation of pains upon pains. - -But passing over the evils thus inflicted on animals of inferior dignity, -let us limit ourselves to the case of Man. The _Bothriocephalus latus_ and -the _Tænia solium_, are two kinds of tape-worm, which flourish in the human -intestines; producing great constitutional disturbances, sometimes ending -in insanity; and from the germs of the _Tænia_, when carried into other -parts of the body, arise certain partially-developed forms known as -_Cysticerci_, _Echinococci_, and _Coenuri_, which cause disorganization -more or less extensive in the brain, the lungs, the liver, the heart, the -eye, &c., often ending fatally after long-continued suffering. Five other -parasites, belonging to a different class, are found in the viscera of -man--the _Trichocephalus_, the _Oxyuris_, the _Strongylus_ (two species), -the _Ancylostomum_ and the _Ascaris_; which, beyond that defect of -nutrition which they necessarily cause, sometimes induce certain -irritations that lead to complete demoralization. Of another class of -_entozoa_, belonging to the subdivision _Trematoda_, there are five kinds -found in different organs of the human body--the liver and gall-duct, the -portal vein, the intestine, the bladder, the eye. Then we have the -_Trichina spiralis_, which passes through one phase of its existence -imbedded in the muscles and through another phase of its existence in the -intestine; and which, by the induced disease _Trichinosis_, has lately -committed such ravages in Germany as to cause a panic. To these we must add -the Guinea-worm, which in some part of Africa and India makes men miserable -by burrowing in their legs; and the more terrible African parasite the -_Bilharzia_, which affects 30 per cent. of the natives on the east coast -with bleeding of the bladder. From _entozoa_, let us pass to _epizoa_. -There are two kinds of _Acari_, one of them inhabiting the follicles of the -skin and the other producing the itch. There are creatures that bury -themselves beneath the skin and lay their eggs there; and there are three -species of lice which infest the surface of the body. Nor is this all. -Besides animal parasites there are sundry vegetal parasites, which grow and -multiply at our cost. The _Sarcina ventriculi_ inhabits the stomach, and -produces gastric disturbance. The _Leptothrix buccalis_ is extremely -general in the mouth, and may have something to do with the decay of teeth. -And besides these there are microscopic fungi which produce ringworm, -porrigo, pityriasis, thrush, &c. Thus the human body is the habitat of -parasites, internal and external, animal and vegetal, numbering, if all are -set down, between two and three dozen species; sundry of which are peculiar -to Man, and many of which produce great suffering and not unfrequently -death. What interpretation is to be put on these facts by those who espouse -the hypothesis of special creations? According to this hypothesis, all -these parasites were designed for their respective modes of life. They were -endowed with constitutions fitting them to live by absorbing nutriment from -the human body; they were furnished with appliances, often of a formidable -kind, enabling them to root themselves in and upon the human body; and they -were made prolific in an almost incredible degree, that their germs might -have a sufficient number of chances of finding their way into the human -body. In short, elaborate contrivances were combined to insure the -continuance of their respective races; and to make it impossible for the -successive generations of men to avoid being preyed on by them. What shall -we say to this arrangement? Shall we say that "the head and crown of -things," was provided as a habitat for these parasites? Shall we say that -these degraded creatures, incapable of thought or enjoyment, were created -that they might cause human misery? One or other of these alternatives must -be chosen by those who contend that every kind of organism was separately -devised by the Creator. Which do they prefer? With the conception of two -antagonist powers, which severally work good and evil in the world, the -facts are congruous enough. But with the conception of a supreme -beneficence, this gratuitous infliction of pain is absolutely incompatible. - - -§ 115. See then the results of our examination. The belief in special -creations of organisms arose among men during the era of profoundest -darkness; and it belongs to a family of beliefs which have nearly all died -out as enlightenment has increased. It is without a solitary established -fact on which to stand; and when the attempt is made to put it into -definite shape in the mind, it turns out to be only a pseud-idea. This -mere verbal hypothesis, which men idly accept as a real or thinkable -hypothesis, is of the same nature as would be one, based on a day's -observation of human life, that each man and woman was specially -created--an hypothesis not suggested by evidence but by lack of -evidence--an hypothesis which formulates ignorance into a semblance of -knowledge. Further, we see that this hypothesis, failing to satisfy men's -intellectual need of an interpretation, fails also to satisfy their moral -sentiment. It is quite inconsistent with those conceptions of the divine -nature which they profess to entertain. If infinite power was to be -demonstrated, then, either by the special creation of every individual, or -by the production of species by some method of natural genesis, it would be -better demonstrated than by the use of two methods, as assumed by the -hypothesis. And if infinite goodness was to be demonstrated, then, not only -do the provisions of organic structure, if they are specially devised, fail -to demonstrate it, but there is an enormous mass of them which imply -malevolence rather than benevolence. - -Thus the hypothesis of special creations turns out to be worthless by its -derivation; worthless in its intrinsic incoherence; worthless as absolutely -without evidence; worthless as not supplying an intellectual need; -worthless as not satisfying a moral want. We must therefore consider it as -counting for nothing, in opposition to any other hypothesis respecting the -origin of organic beings. - - - - -CHAPTER III. - -GENERAL ASPECTS OF THE EVOLUTION-HYPOTHESIS. - - -§ 116. Just as the supposition that races of organisms have been specially -created, is discredited by its origin; so, conversely, the supposition that -races of organisms have been evolved, is credited by its origin. Instead of -being a conception suggested and accepted when mankind were profoundly -ignorant, it is a conception born in times of comparative enlightenment. -Moreover, the belief that plants and animals have arisen in pursuance of -uniform laws, instead of through breaches of uniform laws, is a belief -which has come into existence in the most-instructed class, living in these -better-instructed times. Not among those who have disregarded the order of -Nature, has this idea made its appearance; but among those who have -familiarized themselves with the order of Nature. Thus the derivation of -this modern hypothesis is as favourable as that of the ancient hypothesis -is unfavourable. - - -§ 117. A kindred antithesis exists between the two families of beliefs, to -which the beliefs we are comparing severally belong. While the one family -has been dying out the other family has been multiplying. As fast as men -have ceased to regard different classes of phenomena as caused by special -personal agents, acting irregularly; so fast have they come to regard these -different classes of phenomena as caused by a general agency acting -uniformly--the two changes being correlatives. And as, on the one hand, the -hypothesis that each species resulted from a supernatural act, having lost -nearly all its kindred hypotheses, may be expected soon to die; so, on the -other hand, the hypothesis that each species resulted from the action of -natural causes, being one of an increasing family of hypotheses, may be -expected to survive. - -Still greater will the probability of its survival and establishment -appear, when we observe that it is one of a particular genus of hypotheses -which has been rapidly extending. The interpretation of phenomena as -results of Evolution, has been independently showing itself in various -fields of inquiry, quite remote from one another. The supposition that the -Solar System has been evolved out of diffused matter, is a supposition -wholly astronomical in its origin and application. Geologists, without -being led thereto by astronomical considerations, have been step by step -advancing towards the conviction that the Earth has reached its present -varied structure by modification upon modification. The inquiries of -biologists have proved the falsity of the once general belief, that the -germ of each organism is a minute repetition of the mature organism, -differing from it only in bulk; and they have shown, contrariwise, that -every organism advances from simplicity to complexity through insensible -changes. Among philosophical politicians, there has been spreading the -perception that the progress of society is an evolution: the truth that -"constitutions are not made but grow," is seen to be a part of the more -general truth that societies are not made but grow. It is now universally -admitted by philologists that languages, instead of being artificially or -supernaturally formed, have been developed. And the histories of religion, -of science, of the fine arts, of the industrial arts, show that these have -passed through stages as unobtrusive as those through which the mind of a -child passes on its way to maturity. If, then, the recognition of evolution -as the law of many diverse orders of phenomena, has been spreading; may we -not say that there thence arises the probability that evolution will -presently be recognized as the law of the phenomena we are considering? -Each further advance of knowledge confirms the belief in the unity of -Nature; and the discovery that evolution has gone on, or is going on, in so -many departments of Nature, becomes a reason for believing that there is no -department of Nature in which it does not go on. - - -§ 118. The hypotheses of Special Creation and Evolution, are no less -contrasted in respect of their legitimacy as hypotheses. While, as we have -seen, the one belongs to that order of symbolic conceptions which are -proved to be illusive by the impossibility of realizing them in thought; -the other is one of those symbolic conceptions which are more or less fully -realizable in thought. The production of all organic forms by the -accumulation of modifications and of divergences by the continual addition -of differences to differences, is mentally representable in outline, if not -in detail. Various orders of our experiences enable us to conceive the -process. Let us look at one of the simplest. - -There is no apparent similarity between a straight line and a circle. The -one is a curve; the other is defined as without curvature. The one encloses -a space; the other will not enclose a space though produced for ever. The -one is finite; the other may be infinite. Yet, opposite as the two are in -their characters, they may be connected together by a series of lines no -one of which differs from the adjacent ones in any appreciable degree. -Thus, if a cone be cut by a plane at right angles to its axis we get a -circle. If, instead of being perfectly at right angles, the plane subtends -with the axis an angle of 89° 59', we have an ellipse which no human eye, -even when aided by an accurate pair of compasses, can distinguish from a -circle. Decreasing the angle minute by minute, this closed curve becomes -perceptibly eccentric, then manifestly so, and by and by acquires so -immensely elongated a form so as to bear no recognizable resemblance to a -circle. By continuing this process the ellipse changes insensibly into a -parabola. On still further diminishing the angle, the parabola becomes an -hyperbola. And finally, if the cone be made gradually more obtuse, the -hyperbola passes into a straight line as the angle of the cone approaches -180°. Here then we have five different species of line--circle, ellipse, -parabola, hyperbola, and straight line--each having its peculiar properties -and its separate equation, and the first and last of which are quite -opposite in nature, connected together as members of one series, all -producible by a single process of insensible modification. - -But the experiences which most clearly illustrate the process of general -evolution, are our experiences of special evolution, repeated in every -plant and animal. Each organism exhibits, within a short time, a series of -changes which, when supposed to occupy a period indefinitely great, and to -go on in various ways instead of one way, give us a tolerably clear -conception of organic evolution at large. In an individual development, we -see brought into a comparatively infinitesimal time, a series of -metamorphoses equally great with each of those which the hypothesis of -evolution assumes to have taken place during immeasurable geologic epochs. -A tree differs from a seed in every respect--in bulk, in structure, in -colour, in form, in chemical composition. Yet is the one changed in the -course of a few years into the other: changed so gradually, that at no -moment can it be said--Now the seed ceases to be and the tree exists. What -can be more widely contrasted than a newly-born child and the small, -semi-transparent, gelatinous spherule constituting the human ovum? The -infant is so complex in structure that a cyclopædia is needed to describe -its constituent parts. The germinal vesicle is so simple that it may be -defined in a line. Nevertheless, nine months suffice to develop the one out -of the other; and that, too, by a series of modifications so small, that -were the embryo examined at successive minutes, even a microscope would not -disclose any sensible changes. Aided by such facts, the conception of -general evolution may be rendered as definite a conception as any of our -complex conceptions can be rendered. If, instead of the successive minutes -of a child's foetal life, we take the lives of successive generations of -creatures--if we regard the successive generations as differing from one -another no more than the foetus differs in successive minutes; our -imaginations must indeed be feeble if we fail to realize in thought, the -evolution of the most complex organism out of the simplest. If a single -cell, under appropriate conditions, becomes a man in the space of a few -years; there can surely be no difficulty in understanding how, under -appropriate conditions, a cell may, in the course of untold millions of -years, give origin to the human race. - -Doubtless many minds are so unfurnished with those experiences of Nature -out of which this conception is built, that they find difficulty in forming -it. Looking at things rather in their statical than in their dynamical -aspects, they never realize the fact that, by small increments of -modification, any amount of modification may in time be generated. The -surprise they feel on finding one whom they last saw as a boy, grown into a -man, becomes incredulity when the degree of change is greater. To such, the -hypothesis that by any series of changes a protozoon can give origin to a -mammal, seems grotesque--as grotesque as Galileo's assertion of the Earth's -movement seemed to his persecutors; or as grotesque as the assertion of the -Earth's sphericity seems now to the New Zealanders. But those who accept a -literally-unthinkable proposition as quite satisfactory, may not -unnaturally be expected to make a converse mistake. - - -§ 119. The hypothesis of evolution is contrasted with the hypothesis of -special creations, in a further respect. It is not simply legitimate -instead of illegitimate, because representable in thought instead of -unrepresentable; but it has the support of some evidence, instead of being -absolutely unsupported by evidence. Though the facts at present assignable -in _direct_ proof that by progressive modifications, races of organisms -which are apparently distinct from antecedent races have descended from -them, are not sufficient; yet there are numerous facts of the order -required. Beyond all question unlikenesses of structure gradually arise -among the members of successive generations. We find that there is going on -a modifying process of the kind alleged as the source of specific -differences: a process which, though slow, does, in time, produce -conspicuous changes--a process which, to all appearance, would produce in -millions of years, any amount of change. - -In the chapters on "Heredity" and "Variation," contained in the preceding -Part, many such facts were given, and more might be added. Although little -attention has been paid to the matter until recent times, the evidence -already collected shows that there take place in successive generations, -alterations of structure quite as marked as those which, in successive -short intervals, arise in a developing embryo--nay, often much more marked; -since, besides differences due to changes in the relative sizes or parts, -there sometimes arise differences due to additions and suppressions of -parts. The structural modification proved to have taken place since -organisms have been observed, is not less than the hypothesis -demands--bears as great a ratio to this brief period, as the total amount -of structural change seen in the evolution of a complex organism out of a -simple germ, bears to that vast period during which living forms have -existed on the Earth. - -We have, indeed, much the same kind and quantity of direct evidence that -all organic beings have arisen through the actions of natural causes, which -we have that all the structural complexities of the Earth's crust have -arisen through the actions of natural causes. Between the known -modifications undergone by organisms, and the totality of modifications -displayed in their structures, there is no greater disproportion than -between the observed geological changes, and the totality of geological -changes supposed to have been similarly caused. Here and there are -sedimentary deposits now slowly taking place. At this place a shore has -been greatly encroached on by the sea during recorded times; and at another -place an estuary has become shallower within some generations. In one -region an upheaval is going on at the rate of a few feet in a century; -while in another region occasional earthquakes cause slight variations of -level. Appreciable amounts of denudation by water are visible in some -localities; and in other localities glaciers are detected in the act of -grinding down the rocky surfaces over which they glide. But these changes -are infinitesimal compared with the aggregate of changes to which the -Earth's crust testifies, even in its still extant systems of strata. If, -then, the small changes now being wrought on the Earth's crust by natural -agencies, yield warrant for concluding that by such agencies acting through -vast epochs, all the structural complexities of the Earth's crust have been -produced; do not the small known modifications produced in races of -organisms by natural agencies, yield warrant for concluding that by natural -agencies have been produced all those structural complexities which we see -in them? - -The hypothesis of Evolution then, has direct support from facts which, -though small in amount, are of the kind required; and the ratio which these -facts bear to the generalization based on them, seems as great as is the -ratio between facts and generalization which, in another case, produces -conviction. - - -§ 120. Let us put ourselves for a moment in the position of those who, from -their experiences of human modes of action, draw differences respecting the -mode of action of that Ultimate Power manifested to us through phenomena. -We shall find the supposition that each kind of organism was separately -designed and put together, to be much less consistent with their professed -conception of this Ultimate Power, than is the supposition that all kinds -of organisms have resulted from one unbroken process. Irregularity of -method is a mark of weakness. Uniformity of method is a mark of strength. -Continual interposition to alter a pre-arranged set of actions, implies -defective arrangement in those actions. The maintenance of those actions, -and the working out by them of the highest results, implies completeness of -arrangement. If human workmen, whose machines as at first constructed -require perpetual adjustment, show their increasing skill by making their -machines self-adjusting; then, those who figure to themselves the -production of the world and its inhabitants by a "Great Artificer," must -admit that the achievement of this end by a persistent process, adapted to -all contingencies, implies greater skill than its achievement by the -process of meeting the contingencies as they severally arise. - -So, too, it is with the contrast under its moral aspect. We saw that to the -hypothesis of special creations, a difficulty is presented by the absence -of high forms of life during immeasurable epochs of the Earth's existence. -But to the hypothesis of evolution, absence of them is no such obstacle. -Suppose evolution, and this question is necessarily excluded. Suppose -special creations, and this question can have no satisfactory answer. Still -more marked is the contrast between the two hypotheses, in presence of that -vast amount of suffering entailed on all orders of sentient beings by their -imperfect adaptations to their conditions of life, and the further vast -amount of suffering entailed on them by enemies and by parasites. We saw -that if organisms were severally designed for their respective places in -Nature, the inevitable conclusion is that these innumerable kinds of -inferior organisms which prey on superior organisms, were intended to -inflict all the pain and mortality which results. But the hypothesis of -evolution involves us in no such dilemma. Slowly, but surely, evolution -brings about an increasing amount of happiness. In all forms of -organization there is a progressive adaptation, and a survival of the most -adapted. If, in the uniform working out of the process, there are evolved -organisms of low types which prey on those of higher types, the evils -inflicted form but a deduction from the average benefits. The universal -multiplication of the most adapted must cause the spread of those superior -organisms which, in one way or other, escape the invasions of the inferior; -and so tends to produce a type less liable to the invasions of the -inferior. Thus the evils accompanying evolution are ever being -self-eliminated. Though there may arise the question--Why could they not -have been avoided? there does not arise the question--Why were they -deliberately inflicted? Whatever may be thought of them, it is clear that -they do not imply gratuitous malevolence. - - -§ 121. In all respects, then, the hypothesis of evolution contrasts -favourably with the hypothesis of special creation. It has arisen in -comparatively-instructed times and in the most cultivated class. It is one -of those beliefs in the uniform concurrence of phenomena, which are -gradually supplanting beliefs in their irregular and arbitrary concurrence; -and it belongs to a genus of these beliefs which has of late been rapidly -spreading. It is a definitely-conceivable hypothesis; being simply an -extension to the organic world at large, of a conception framed from our -experiences of individual organisms; just as the hypothesis of universal -gravitation was an extension of the conception which our experiences of -terrestrial gravitation had produced. This definitely-conceivable -hypothesis, besides the support of numerous analogies, has the support of -direct evidence. We have proof that there is going on a process of the kind -alleged; and though the results of this process, as actually witnessed, are -minute in comparison with the totality of results ascribed to it, yet they -bear to such totality a ratio as great as that by which an analogous -hypothesis is justified. Lastly, that sentiment which the doctrine of -special creations is thought necessary to satisfy, is much better satisfied -by the doctrine of evolution; since this doctrine raises no contradictory -implications respecting the Unknown Cause, such as are raised by the -antagonist doctrine. - -And now, having observed how, under its most general aspects, the -hypothesis of organic evolution commends itself to us by its derivation, by -its coherence, by its analogies, by its direct evidence, by its -implications; let us go on to consider the several orders of facts which -yield indirect support to it. We will begin by noting the harmonies between -it and sundry of the inductions set forth in Part II. - - - - -CHAPTER IV. - -THE ARGUMENTS FROM CLASSIFICATION. - - -§ 122. In § 103, we saw that the relations which exist among the species, -genera, orders, and classes of organisms, are not interpretable as results -of any such causes as have usually been assigned. We will here consider -whether they are interpretable as the results of evolution. Let us first -contemplate some familiar facts. - -The Norwegians, Swedes, Danes, Germans, Dutch, and Anglo-Saxons, form -together a group of Scandinavian races, which are but slightly divergent in -their characters. Welsh, Irish, and Highlanders, though they have -differences, have not such differences as hide a decided community of -nature: they are classed together as Celts. Between the Scandinavian race -as a whole and the Celtic race as a whole, there is a distinction greater -than that between the sub-divisions which make up the one or the other. -Similarly, the several peoples inhabiting Southern Europe are more nearly -allied to one another, than the aggregate they form is allied to the -aggregates of Northern peoples. If, again, we compare these European -varieties of Man, taken as a group, with that group of Eastern varieties -which had a common origin with it, we see a stronger contrast than between -the groups of European varieties themselves. And once more, ethnologists -find differences of still higher importance between the Aryan stock as a -whole and the Mongolian stock as a whole, or the Negro stock as a whole. -Though these contrasts are partially obscured by intermixtures, they are -not so much obscured as to hide the truths that the most-nearly-allied -varieties of Man are those which diverged from one another at -comparatively-recent periods; that each group of nearly-allied varieties is -more strongly contrasted with other such groups that had a common origin -with it at a remoter period; and so on until we come to the largest groups, -which are the most strongly contrasted, and of whose divergence no trace is -extant. - -The relations existing among the classes and sub-classes of languages, have -been briefly referred to by Mr. Darwin in illustration of his argument. We -know that languages have arisen by evolution. Let us then see what grouping -of them evolution has produced. On comparing the dialects of adjacent -counties in England, we find that their differences are so small as -scarcely to distinguish them. Between the dialects of the Northern counties -taken together, and those of the Southern counties taken together, the -contrast is stronger. These clusters of dialects, together with those of -Scotland and Ireland, are nevertheless so similar that we regard them as -one language. The several languages of Scandinavian Europe, including -English, are much more unlike one another than are the several dialects -which each of them includes; in correspondence with the fact that they -diverged from one another at earlier periods than did their respective -dialects. The Scandinavian languages have nevertheless a certain community -of character, distinguishing them as a group from the languages of Southern -Europe; between which there are general and special affinities that -similarly unite them into a group formed of sub-groups containing -sub-sub-groups. And this wider divergence between the order of languages -spoken in Northern Europe and the order of languages spoken in Southern -Europe, answers to the longer time that has elapsed since their -differentiation commenced. Further, these two orders of modern European -languages, as well as Latin and Greek and certain extinct and spoken -languages of the East, are shown to have traits in common which unite them -into one great class known as Aryan languages; radically distinguished from -the classes of languages spoken by the other main divisions of the human -race. - - -§ 123. Now this kind of subordination of groups which we see arises in the -course of continuous descent, multiplication, and divergence, is just the -kind of subordination of groups which plants and animals exhibit: it is -just the kind of subordination which has thrust itself on the attention of -naturalists in spite of pre-conceptions. - -The original idea was that of arrangement in linear order. We saw that even -after a considerable acquaintance with the structures of organisms had been -acquired, naturalists continued their efforts to reconcile the facts with -the notion of a uni-serial succession. The accumulation of evidence -necessitated the breaking up of the imagined chain into groups and -sub-groups. Gradually there arose the conviction that these groups do not -admit of being placed in a line. And the conception finally arrived at, is -that of certain great sub-kingdoms, very widely divergent, each made up of -classes much less divergent, severally containing orders still less -divergent; and so on with genera and species. - -Hence this "grand fact in natural history of the subordination of group -under group, which from its familiarity does not always sufficiently strike -us," is perfectly in harmony with the hypothesis of evolution. The extreme -significance of this kind of relation among organic forms is dwelt on by -Mr. Darwin, who shows how an ordinary genealogical tree represents, on a -small scale, a system of grouping analogous to that which exists among -organisms in general, and which is explained on the supposition of a -genealogical tree by which all organisms are affiliated. If, wherever we -can trace direct descent, multiplication, and divergence, this formation of -groups within groups takes place; there results a strong presumption that -the groups within groups which constitute the animal and vegetal kingdoms, -have arisen by direct descent, multiplication, and divergence--that is, by -evolution. - - -§ 124. Strong confirmation of this inference is yielded by the fact, that -the more marked differences which divide groups are, in both cases, -distinguished from the less marked differences which divide sub-groups, by -this, that they are not simply greater in _degree_, but they are more -radical in _kind_. Objects, as the stars, may present themselves in small -clusters, which are again more or less aggravated into clusters of -clusters, in such manner that the individuals of each simple cluster are -much closer together than are the simple clusters gathered into a compound -cluster: in which case, the trait that unites groups of groups differs from -the trait that unites groups, not in _nature_ but only in _amount_. But -this is not so either with the groups and sub-groups which we know have -resulted from evolution, or with those which we here infer have resulted -from evolution. In both cases the highest or most general classes, are -marked off from one another by fundamental differences that have no common -measure with the differences that mark off small classes. Observe the -parallelism. - -We saw that each sub-kingdom of animals is distinguished from other -sub-kingdoms, by some unlikeness in its main plan of organization; such as -the presence or absence of a peri-visceral cavity. Contrariwise, the -members of the smallest groups are united together, and separated from the -members of other small groups, by modifications which do not affect the -relations of essential parts. That this is just the kind of arrangement -which results from evolution, the case of languages will show. - -On comparing the dialects spoken in different parts of England, we find -scarcely any difference but those of pronunciation: the structures of the -sentences are almost uniform. Between English and the allied modern -languages there are divergences of structure: there are some unlikenesses -of idiom; some unlikenesses in the ways of modifying the meanings of verbs; -and considerable unlikenesses in the uses of genders. But these -unlikenesses are not sufficient to hide a general community of -organization. A greater contrast of structure exists between these modern -languages of Western Europe, and the classic languages. Differentiation -into abstract and concrete elements, which is shown by the substitution of -auxiliary words for inflections, has produced a higher specialization, -distinguishing these languages as a group from the older languages. -Nevertheless, both the ancient and modern languages of Europe, together -with some Eastern languages derived from the same original, have, under all -their differences of organization, a fundamental likeness; since in all of -them words are formed by such a coalescence and integration of roots as -destroys the independent meanings of the roots. These Aryan languages, and -others which have the _amalgamate_ character, are united by it into a class -distinguished from the _aptotic_ and _agglutinate_ languages; in which the -roots are either not united at all, or so incompletely united that one of -them still retains its independent meaning. And philologists find that -these radical traits which severally determine the grammatical forms, or -modes of combining ideas, characterize the primary divisions among -languages. - -So that among languages, where we know that evolution has been going on, -the greatest groups are marked off from one another by the strongest -structural contrasts; and as the like holds among groups of organisms, -there results a further reason for inferring that these have been evolved. - - -§ 125. There is yet another parallelism of like meaning. We saw (§ 101) -that the successively-subordinate groups--classes, orders, genera, and -species--into which zoologists and botanists segregate animals and plants, -have not, in reality, those definite values conventionally given to them. -There are well-marked species, and species so imperfectly marked that some -systematists regard them as varieties. Between genera strong contrasts -exist in many cases, and in other cases contrasts so much less decided as -to leave it doubtful whether they imply generic distinctions. So, too, is -it with orders and classes: in some of which there have been introduced -sub-divisions, having no equivalents in others. Even of the sub-kingdoms -the same truth holds. The contrast between the _Coelenterata_ and the -_Mollusca_, is far less than that between the _Coelenterata_ and the -_Vertebrata_. - -Now just this same indefiniteness of value, or incompleteness of -equivalence, is observable in those simple and compound and re-compound -groups which we see arising by evolution. In every case the endeavour to -arrange the divergent products of evolution, is met by a difficulty like -that which would meet the endeavour to classify the branches of a tree, -into branches of the first, second, third, fourth, &c., orders--the -difficulty, namely, that branches of intermediate degrees of composition -exist. The illustration furnished by languages will serve us once more. -Some dialects of English are but little contrasted; others are strongly -contrasted. The alliances of the several Scandinavian tongues with one -another are different in degree. Dutch is much less distinct from German -than Swedish is; while between Danish and Swedish there is so close a -kinship that they might almost be regarded as widely-divergent dialects. -Similarly on comparing the larger divisions, we see that the various -languages of the Aryan stock have deviated from their original to very -unlike distances. The general conclusion is manifest. While the kinds of -human speech fall into groups, and sub-groups, and sub-sub-groups; yet the -groups are not equal to one another in value, nor have the sub-groups equal -values, nor the sub-sub-groups. - -If, then, when classified, organisms fall into assemblages such that those -of the same grade are but indefinitely equivalent; and if, where evolution -is known to have taken place, there have arisen assemblages between which -the equivalence is similarly indefinite; there is additional reason for -inferring that organisms are products of evolution. - - -§ 126. A fact of much significance remains. If groups of organic forms have -arisen by divergence and re-divergence; and if, while the groups have been -developing from simple groups into compound groups, each group and -sub-group has been giving origin to more complex forms of its own type; -then it is inferable that there once existed greater structural likenesses -between the members of allied groups than exists now. This, speaking -generally, proves to be so. - -Between the sub-kingdoms the gaps are extremely wide; but such distant -kinships as may be discerned, bear out anticipation. Thus in the formation -of the germinal layers there is a general agreement among them; and there -is a further agreement among sundry of them in the formation of a gastrula. -This simplest and earliest likeness, significant of primitive kinship, is -in most cases soon obscured by divergent modes of development; but sundry -sub-kingdoms continue to show relationships by the likenesses of their -larval forms; as we see in the trochophores of the _Polyzoa_, _Annelida_, -and _Mollusca_--sub-kingdoms the members of which by their later structural -changes are rendered widely unlike. - -More decided approximations exist between the lower members of classes. In -tracing down the _Crustacea_ and the _Arachnida_ from their more complex to -their simpler forms, zoologists meet with difficulties: respecting some of -these simpler forms, it becomes a question which class they belong to. The -_Lepidosiren_, about which there have been disputes whether it is a fish or -an amphibian, is inferior, in the organization of its skeleton, to the -great majority of both fishes and amphibia. Widely as they differ from -them, the lower mammals have some characters in common with birds, which -the higher mammals do not possess. - -Now since this kind of relationship of groups is not accounted for by any -other hypothesis, while the hypothesis of evolution gives us a clue to it; -we must include it among the supports of this hypothesis which the facts of -classification furnish. - - -§ 127. What shall we say of these leading truths when taken together? That -naturalists have been gradually compelled to arrange organisms in groups -within groups, and that this is the arrangement which we see arises by -descent, alike in individual families and among races of men, is a striking -circumstance. That while the smallest groups are the most nearly related, -there exist between the great sub-kingdoms, structural contrasts of the -profoundest kind, cannot but impress us as remarkable, when we see that -where it is known to take place evolution actually produces these -feebly-distinguished small groups, and these strongly-distinguished great -groups. The impression made by these two parallelisms, which add meaning to -each other, is deepened by the third parallelism, which enforces the -meaning of both--the parallelism, namely, that as, between the species, -genera, orders, classes, &c., which naturalists have formed, there are -transitional types; so between the groups, sub-groups, and sub-sub-groups, -which we know to have been evolved, types of intermediate values exist. And -these three correspondences between the known results of evolution and the -results here ascribed to evolution, have further weight given to them by -the fact, that the kinship of groups through their lowest members is just -the kinship which the hypothesis of evolution implies. - -Even in the absence of these specific agreements, the broad fact of unity -amid multiformity, which organisms so strikingly display, is strongly -suggestive of evolution. Freeing ourselves from pre-conceptions, we shall -see good reason to think with Mr. Darwin, "that propinquity of descent--the -only known cause of the similarity of organic beings--is the bond, hidden -as it is by various degrees of modification, which is partly revealed to us -by our classifications." When we consider that this only known cause of -similarity, joined with the only known cause of divergence (the influence -of conditions), gives us a key to these likenesses obscured by -unlikenesses; we shall see that were there none of those remarkable -harmonies above pointed out, the truths of classification would still yield -strong support to our conclusion. - - - - -CHAPTER V. - -THE ARGUMENTS FROM EMBRYOLOGY. - - -§ 127a. Already I have emphasized the truth that Nature is always more -complex than we suppose (§ 74a)--that there are complexities within -complexities. Here we find illustrated this truth under another aspect. -When seeking to formulate the arguments from Embryology, we are shown that -the facts as presented in Nature are not to be expressed in the simple -generalizations we at first make. - -While we recognize this truth we must also recognize the truth that only by -enunciation and acceptance of imperfect generalizations can we progress to -perfect ones. The order of Evolution is conformed to by ideas as by other -things. The advance is, and must be, from the indefinite to the definite. -It is impossible to express the totality of any natural phenomenon in a -single proposition. To the primary statement expressing that which is most -dominant have to be added secondary statements qualifying it. We see this -even in so simple a case as the flight of a projectile. The young artillery -officer is first taught that a cannon-shot describes a curve treated as a -parabola, though literally part of an extremely eccentric ellipse not -distinguishable from a parabola. Presently he learns that atmospheric -resistance, causing a continual decrease of velocity, entails a deviation -from that theoretical path which is calculated on the supposition that the -velocity is uniform; and this incorrectness he has to allow for. Then, -further, there comes the lateral deviation due to wind, which may be -appreciable if the wind is strong and the range great. To introduce him all -at once to the correct conception thus finally reached would be impossible: -it has to be reached through successive qualifications. And that which -holds even in this simple case necessarily holds more conspicuously in -complex cases. - -The title of the chapter suggests a metaphor, which is, indeed, something -more than a metaphor. There is an embryology of conceptions. That this -statement is not wholly a figure of speech, we shall see on considering -that cerebral organization is a part of organization at large; and that the -evolving nervous plexus which is the correlative of an evolving conception, -must conform to the general law of change conformed to in the evolution of -the whole nervous structure as well as in the evolution of the whole bodily -structure. As the body has at first a rude form, very remotely suggesting -that which is presently developed by the superposing of modifications on -modifications; so the brain as a whole and its contained ideas together -make up an inner world answering with extreme indefiniteness to that outer -world to which it is brought by successive approximations into tolerable -correspondence; and so any nervous plexus and its associated hypothesis, -which refer to some external group of phenomena under investigation, have -to reach their final developments by successive corrections. - -This being the course of discovery must also be the course of exposition. -In pursuance of this course we may therefore fitly contemplate that early -_formula_ of embryological development which we owe to von Baer. - - -§ 128. Already in § 52, where the generalization of von Baer respecting the -relations of embryos was set forth, there was given the warning, above -repeated with greater distinctness, that it is only an adumbration. - -In the words of his translator, he "found that in its earliest stage, every -organism has the greatest number of characters in common with all other -organisms in their earliest stages; that at a stage somewhat later, its -structure is like the structures displayed at corresponding phases by a -less extensive multitude of organisms; that at each subsequent stage, -traits are acquired which successively distinguished the developing embryo -from groups of embryos that it previously resembled--thus step by step -diminishing the class of embryos which it still resembles; and that thus -the class of similar forms is finally narrowed to the species of which it -is a member." - -Assuming for a moment that this generalization is true as it stands, or -rather, assuming that the qualifications needed are not such as destroy its -correspondence with the average facts, we shall see that it has profound -significance. For if we follow out in thought the implications--if we -conceive the germs of all kinds of organisms simultaneously developing, and -imagine that after taking their first step together, at the second step one -half of the vast multitude diverges from the other half; if, at the next -step, we mentally watch the parts of each great assemblage beginning to -take two or more routes of development; if we represent to ourselves such -bifurcations going on, stage after stage, in all the branches; we shall see -that there must result an aggregate analogous, in its arrangement of parts, -to a tree. If this vast genealogical tree be contemplated as a whole, made -up of trunk, main branches, secondary branches, and so on as far as the -terminal twigs; it will be perceived that all the various kinds of -organisms represented by these terminal twigs, forming the periphery of the -tree, will stand related to one another in small groups, which are united -into groups of groups, and so on. The embryological tree, expressing the -developmental relations of organisms, will be similar to the tree which -symbolizes their classificatory relations. That subordination of classes, -orders, genera, and species, to which naturalists have been gradually led, -is just that subordination which results from the divergence and -re-divergence of embryos, as they all unfold. On the hypothesis of -evolution this parallelism has a meaning--indicates that primordial kinship -of all organisms, and that progressive differentiation of them, which the -hypothesis alleges. But on any other hypothesis the parallelism is -meaningless; or rather, it raises a difficulty; since it implies either an -effect without a cause or a design without a purpose. - - -§ 129. This conception of a tree, symbolizing the relationships of types -and a species derived from the same root, has a concomitant conception. The -implication is that each organism, setting out from the simple nucleated -cell, must in the course of its development follow the line of the trunk, -some main branch, some sub-branch, some sub-sub-branch, &c., of this -embryological tree; and so on till it reaches that ultimate twig -representing the species of which it is a member. It must in a general way -go through the particular line of forms which preceded it in all past -times: there must be what has been aptly called a "recapitulation" of the -successive ancestral structures. This, at least, is the conclusion -necessitated by the generalization we are considering under its original -crude form. - -Von Baer lived in the days when the Development Hypothesis was mentioned -only to be ridiculed, and he joined in the ridicule. What he conceived to -be the meaning of these groupings of organisms and these relations among -their embryological histories, is not obvious. The only alternative to the -hypothesis of Evolution is the hypothesis of Special Creation; and as he -did not accept the one it is inferable that he accepted the other. But if -he did this he must in the first place have found no answer to the inquiry -why organisms specially created should have the embryological kinships he -described. And in the second place, after discovering that his alleged law -was traversed by many and various nonconformities, he would have been -without any explanation of these. Observe the positions which were open to -him and the reasons which show them to be untenable. - -If it be said that the conditions of the case necessitated the derivation -of all organisms from simple germs, and therefore necessitated a -morphological unity in their primitive states; there arises the obvious -answer, that the morphological unity thus implied, is not the only -morphological unity to be accounted for. Were this the only unity, the -various kinds of organisms, setting out from a common primordial form, -should all begin from the first to diverge individually, as so many radii -from a centre; which they do not. If, otherwise, it be said that organisms -were framed upon certain types, and that those of the same type continue -developing together in the same direction, until it is time for them to -begin putting on their specialities of structure; then the answer is, that -when they do finally diverge they ought severally to develop in direct -lines towards their final forms. No reason can be assigned why, having -parted company, some should progress towards their final forms by irregular -or circuitous routes. On the hypothesis of design such deviations are -inexplicable. - -The hypothesis of evolution, however, while it pre-supposes those kinships -among embryos in their early forms which are found to exist, also leads us -to expect nonconformities in their courses of development. If, as any -rational theory of evolution implies, the progressive differentiations of -types from one another during past times, have resulted from the direct and -indirect effects of external conditions--if races of organisms have become -different, either by immediate adaptations to unlike habits of life, or by -the mediate adaptations resulting from preservation of the individuals most -fitted for such habits of life, or by both; and if most embryonic changes -are significant of changes that were undergone by ancestral races; then -these irregularities must be anticipated. For the successive changes in -modes of life pursued by successive ancestral races, can have had no -regularity of sequence. In some cases they must have been more numerous -than in others; in some cases they must have been greater in degree than in -others; in some cases they must have been to simpler modes, in some cases -to more complex modes, and in some cases to modes neither higher nor lower. -Of two cognate races which diverged in the remote past, the one may have -had descendants that have remained tolerably constant in their habits, -while the other may have had descendants that have passed through -widely-aberrant modes of life; and yet some of these last may have -eventually taken to modes of life like those of the other races derived -from the same stock. And if the metamorphoses of embryos indicate, in a -general way, the changes of structure undergone by ancestors; then, the -later embryologic changes of such two allied races will be somewhat -different, though they may end in very similar forms. An illustration will -make this clear. Mr. Darwin says: "Petrels are the most aërial and oceanic -of birds, but in the quiet sounds of Tierra del Fuego, the _Puffinuria -berardi_, in its general habits, in its astonishing power of diving, its -manner of swimming, and of flying when unwillingly it takes flight, would -be mistaken by any one for an auk or grebe; nevertheless, it is essentially -a petrel, but with many parts of its organization profoundly modified." Now -if we suppose these grebe-like habits to be continued through a long epoch, -the petrel-form to be still more obscured, and the approximation to the -grebe-form still closer; it is manifest that while the chicks of the grebe -and the _Puffinuria_ will, during their early stages of development, -display that likeness involved by their common derivation from some early -type of bird, the chick of the _Puffinuria_ will eventually begin to show -deviations, representative of the ancestral petrel-structure, and will -afterwards begin to lose these distinctions and assume the grebe-structure. - -Hence, remembering the perpetual intrusions of organisms on one another's -modes of life, often widely different; and remembering that these -intrusions have been going on from the beginning; we shall be prepared to -find that the general law of embryonic parallelism is qualified by -irregularities which are mostly small, in many cases considerable, and -occasionally great. The hypothesis of evolution accounts for these: it does -more--it implies the necessity of them. - - -§ 130. The substitutions of organs and the suppressions of organs, are -among those secondary embryological phenomena which harmonize with the -belief in evolution but cannot be reconciled with any other belief. Some -embryos, during early stages of development, possess organs that afterwards -dwindle away, as there arise other organs to discharge the same functions. -And in other embryos organs make their appearance, grow to certain points, -have no functions to discharge, and disappear by absorption. - -We have a remarkable instance of substitution in the temporary appliances -for respiration, which some embryos exhibit. During the first phase of its -development, the mammalian embryo possesses a system of blood-vessels -distributed over what is called the _area vasculosa_--a system of vessels -homologous with one which, among fishes, serves for aërating the blood -until the permanent respiratory organs come into play. Now since this -system of blood-vessels, not being in proximity to an oxygenated medium, -cannot be serviceable to the mammalian embryo during development of the -lungs, as it is serviceable in the embryo-fish during development of the -gills, this needless formation of it is unaccountable as a result of -design. But it is quite congruous with the supposition that the mammalian -type arose out of lower vertebrate types. For in such case the mammalian -embryo, passing through states representing in a general way those which -its remote ancestors had in common with the lower _Vertebrata_, develops -this system of vessels in like manner with them. An instance more -significant still is furnished by certain _Amphibia_. One of the facts -early made familiar to the natural-history student is that the tadpole -breathes by external branchiæ, and that these, needful during its aquatic -life, dwindle away as fast as it develops the lungs fitting it for -terrestrial life. But in one of the higher _Amphibia_, the viviparous -Salamander, these transformations ordinarily undergone during the free life -of the larva, are undergone by the embryo in the egg. The branchiæ are -developed though there is no use for them: lungs being substituted as -breathing appliances before the creature is born. - -Even more striking than the substitutions of organs are the suppressions of -organs. Mr. Darwin names some cases as "extremely curious; for instance, -the presence of teeth in foetal whales, which when grown up have not a -tooth in their heads;... It has even been stated on good authority that -rudiments of teeth can be detected in the beaks of certain embryonic -birds." Irreconcilable with any teleological theory, these facts do not -even harmonize with the theory of fixed types which are maintained by the -development of all the typical parts, even where not wanted; seeing that -the disappearance of these incipient organs during foetal life spoils the -typical resemblance. But while to other hypotheses these facts are -stumbling-blocks, they yield strong support to the hypothesis of evolution. - -Allied to these cases, are the cases of what has been called retrograde -development. Many parasitic creatures and creatures which, after leading -active lives for a time, become fixed, lose, in their adult states, the -limbs and senses they had when young. It may be alleged, however, that -these creatures could not secure the habitats needful for them, without -possessing, during their larval stages, eyes and swimming appendages which -eventually become useless; that though, by losing these, their organization -retrogresses in one direction, it progresses in another direction; and -that, therefore, they do not exhibit the needless development of a higher -type on the way to a lower type. Nevertheless there are instances of a -descent in organization, following an apparently-superfluous ascent. Mr. -Darwin says that in some genera of cirripedes, "the larvæ become developed -either into hermaphrodites having the ordinary structure, or into what I -have called complemental males, and in the latter, the development has -assuredly been retrograde; for the male is a mere sack, which lives for a -short time, and is destitute of mouth, stomach, or other organ of -importance, excepting for reproduction." - - -§ 130a. But now let us contemplate more closely the energies at work in the -unfolding embryo, or rather the energies which the facts appear to imply. - -Whatever natures we ascribe to the hypothetical units proper to each kind -of organism, we must conclude that from the beginning of embryonic -development, they have a proclivity towards the structure of that organism. -Because of their phylogenetic origin, they must tend towards the form of -the primitive type; but the superposed modifications, conflicting with -their initial tendency, must cause a swerving towards each successively -higher type. To take an illustration:--If in the germ-plasm out of which -will come a vertebrate animal there is a proclivity towards the primitive -piscine form, there must, if the germ-plasm is derived from a mammal, be -also from the outset a proclivity towards the mammalian form. While the -initial type tends continually to establish itself the terminal type tends -also to establish itself. The intermediate structures must be influenced by -their conflict, as well as by the conflict of each with the proclivities -towards the amphibian and reptilian types. This complication of tendencies -is increased by the intervention of several other factors. - -There is the factor of economy. An embryo in which the transformations have -absorbed the smallest amount of energy and wasted the smallest amount of -matter, will have an advantage over embryos the transformations of which -have cost more in energy and matter: the young animal will set out with a -greater surplus of vitality, and will be more likely than others to live -and propagate. Again, in the embryos of its descendants, inheriting the -tendency to economical transformation, those which evolve at the least cost -will thrive more than the rest and be more likely to have posterity. Thus -will result a continual shortening of the processes. We can see alike that -this must take place and that it does take place. If the whole series of -phylogenetic changes had to be repeated--if the embryo mammal had to become -a complete fish, and then a complete amphibian, and then a complete -reptile, there would be an immense amount of superfluous building up and -pulling down, entailing great waste of time and of materials. Evidently -these abridgments which economy entails, necessitate that unfolding embryos -bear but rude resemblances to lower types ancestrally passed -through--vaguely represent their dominant traits only. - -From this principle of economy arise several derivative principles, which -may be best dealt with separately. - - -§ 130b. In some cases the substitution of an abridged for an unabridged -course of evolution causes the entire disappearance of certain intermediate -forms. Structural arrangements once passed through during the unfolding are -dropped out of the series. - -In the evolution of these embryos with which there is not laid up a large -amount of food-yolk there occurs at the outset a striking omission of this -kind. When, by successive fissions, the fertilized cell has given rise to a -cluster of cells constituting a hollow sphere, known as a _blastula_, the -next change under its original form is the introversion of one side, so as -to produce two layers in place of one. An idea of the change may be -obtained by taking an india-rubber ball (having a hole through which the -air may escape) and thrusting in one side until its anterior surface -touches the interior surface of the other side. If the cup-shaped structure -resulting be supposed to have its wide opening gradually narrowed, until it -becomes the mouth of an internal chamber, it will represent what is known -as a _gastrula_--a double layer of cells, of which the outer is called -epiblast and the inner hypoblast (answering to ectoderm and endoderm) -inclosing a cavity known as the _archenteron_, or primitive digestive sac. -But now in place of this original mode of forming the _gastrula_, there -occurs a mode known as delamination. Throughout its whole extent the single -layer splits so as to become a double layer--one sphere of cells inclosing -the other; and after this direct formation of the double layer there is a -direct formation of an opening through it into the internal cavity. There -is thus a shortening of the primitive process: a number of changes are left -out. - -Often a kindred passing over of stages at later periods of development may -be observed. In certain of the _Mollusca_, as the _Patella chiton_, the egg -gives origin to a free-swimming larva known as a trochosphere, from which -presently comes the ordinary molluscous organization. In the highest -division of the Molluscs, however, the Cephalopods, no trochosphere is -formed. The nutritive matter laid up in the egg is used in building up the -young animal without any indication of an ancestral larva. - - -§ 130c. Among principles derived from the principle of economy is the -principle of pre-adaptation--a name which we may appropriately coin to -indicate an adaptation made in advance of the time at which it could have -arisen in the course of phylogenetic history. - -How pre-adaptation may result from economy will be shown by an illustration -which human methods of construction furnish. Let us assume that building -houses of a certain type has become an established habit, and that, as a -part of each house, there is a staircase of given size. And suppose that in -consequence of changed conditions--say the walling in of the town, limiting -the internal space and increasing ground-rents--it becomes the policy to -build houses of many stories, let out in flats to different tenants. For -the increased passing up and down, a staircase wider at its lower part will -be required. If now the builder, when putting up the ground floor, follows -the old dimensions, then after all the stories are built, the lower part of -the staircase, if it is to yield equal facilities for passage, must be -reconstructed. Instead of a staircase adapted to those few stories which -the original type of house had, economy will dictate a pre-adaptation of -the staircase to the additional stories. - -On carrying this idea with us, we shall see that if from some type of -organism there is evolved a type in which enlargement of a certain part is -needed to meet increased functions, the greater size of this part will -begin to show itself during early stages of unfolding. That unbuilding and -rebuilding which would be needful were it laid down of its original size, -will be made needless if from the beginning it is laid down of a larger -size. Hence, in successive generations, the greater prosperity and -multiplication of individuals in which this part is at the outset somewhat -larger than usual, must eventually establish a marked excess in its -development at an early stage. The facts agree with this inference. - -Referring to the contrasts between embryos, Mr. Adam Sedgwick says that "a -species is distinct and distinguishable from its allies from the very -earliest stages." Whereas, according to the law of von Baer, "animals so -closely allied as the fowl and duck would be indistinguishable in the early -stages of development," "yet I can distinguish a fowl and a duck embryo on -the second day by the inspection of a single transverse section through the -trunk." This experience harmonizes with the statement of the late Prof. -Agassiz, that in some cases traits characterizing the species appear at an -earlier period than traits characterizing the genus. - -Similar in their implications are the facts recently published by Dr. E. -Mehnert, concerning the feet of pentadactyle vertebrates. A leading example -is furnished by the foot in the struthious birds. Out of the original five -digits the two which eventually become large while the others disappear, -soon give sign of their future predominance: their early sizes being in -excess of those required for the usual functional requirements in birds, -and preparing the way for their special requirements in the struthious -birds. Dr. Mehnert shows that a like lesson is given by the relative -developments of legs and wings in these birds. Ordinarily in vertebrates -the fore limbs grow more rapidly than the hind limbs; but in the ostrich, -in which the hind limbs or legs have to become so large while the wings are -but little wanted, the leg development goes in advance of the -wing-development in early embryonic stages: there is a pre-adaptation. - -Much more striking are examples furnished by creatures whose modes of -existence require that they shall have enormous fertility--require that the -generative system shall be very large. Ordinarily the organs devoted to -maintenance of the race develop later than the organs devoted to -maintenance of the individual. But this order is inverted in certain -_Entozoa_. To these creatures, imbedded in nutritive matters, -self-maintenance cost nothing, and the structures devoted to it are -relatively of less importance than the structures devoted to -race-maintenance, which, to make up for the small chance any one germ has -of getting into a fit habitat, have to produce immense numbers of germs. -Here the rudiments of the generative systems are the first to become -visible--here, in virtue of the principle of pre-adaptation, a structure -belonging to the terminal form asserts itself so early in the developmental -process as almost to obliterate the structure of the initial form. - -It may be that in some cases where the growth of certain organs goes in -advance of the normal order, the element of time comes into play--the -greater time required for construction. To elucidate this let us revert to -our simile. Suppose that the staircase above instanced, or at any rate its -lower part, is required to be of marble with balusters finely carved. If -this piece of work is not promptly commenced and pushed on fast, it will -not be completed when the rest of the house is ready: workmen and tools -will still block it up at a time when it should be available. Similarly -among the parts of an unfolding embryo, those in which there is a great -deal of constructive work must early take such shape as will allow of this. -Now of all the tissues the nervous tissue is that which takes longest to -repair when injured; and it seems a not improbable inference that it is a -tissue which is slower in its histological development than others. If this -be so, we may see why, in the embryos of the higher vertebrates, the -central nervous system quickly grows large in comparison to the other -systems--why by pre-adaptation the brain of a chick develops in advance of -other organs so much more than the brain of a fish. - - -§ 130d. Yet another complication has to be noted. From the principle of -economy, it seems inferable that decrease and disappearance of organs which -were useful in ancestral types but have ceased to be useful, should take -place uniformly; but they do not. In the words of Mr. Adam Sedgwick, "some -ancestral organs persist in the embryo in a functionless rudimentary -(vestigial) condition and at the same time without any reference to adult -structures, while other ancestral organs have disappeared without leaving a -trace."[46] This anomaly is rendered more striking when joined with the -fact that some of the structures which remain conspicuous are relatively -ancient, while some which have been obliterated are relatively modern--_e. -g._, "gill slits [which date back to the fish-ancestor], have been retained -in embryology, whereas other organs which have much more recently -disappeared, _e. g._ teeth of birds, fore-limbs of snakes [dating back to -the reptile ancestor], have been entirely lost."[47] Mr. Sedgwick ascribes -these anomalies to the difference between larval development and embryonic -development, and expresses his general belief thus:-- - - "The conclusion here reached is that, whereas larval development must - retain traces (it may be very faint) of ancestral stages of structure - because they are built out of ancestral stages, embryonic development - need not necessarily do so, and very often does not; that embryonic - development in so far as it is a record at all, is a record of structural - features of previous larval stages. Characters which disappear during - free life disappear also in the embryo, but characters which though lost - by the adult are retained in the larva may ultimately be absorbed into - the embryonic phase and leave their traces in embryonic development."[48] - -To set forth the evidence justifying this view would encumber too much the -general argument. Towards elucidation of such irregularities let me name -two factors which should I think be taken into account. - -Abridgment of embryonic stages cannot go on uniformly with all disused -organs. Where an organ is of such size that progressive diminution of it -will appreciably profit the young animal, by leaving it a larger surplus of -unused material, we may expect progressive diminution to occur. -Contrariwise, if the organ is relatively so small that each decrease will -not, by sensibly increasing the reserve of nutriment, give the young animal -an advantage over others, decrease must not be looked for: there may be a -survival of it even though of very ancient origin. - -Again, the reduction of a superfluous part can take place only on condition -that the economy resulting from each descending variation of it, is of -greater importance than are the effects of variations simultaneously -occurring in other parts. If by increase or decrease of any other parts of -the embryo, survival of the animal is furthered in a greater degree than by -decrease of this superfluous part, then such decrease is unlikely; since it -is illegitimate to count upon the repeated concurrence of favourable -variations in two or more parts which are independent. So that if changes -of an advantageous kind are going on elsewhere in the embryo a useless part -may remain long undiminished. - -Yet another cause operates, and perhaps cooperates. Embryonic survival of -an organ which has become functionless, may readily happen if, during -subsequent stages of development, parts of it are utilized as parts of -other organs. In the words of Mr. J. T. Cunningham:-- - - "It seems to be a general fact that a structure which in metamorphosis - disappears completely may easily be omitted altogether in embryonic - development, while one which is modified into something else continues to - pass more or less through its original larval condition." (_Science - Progress_, July, 1897, p. 488.) - -One more factor of considerable importance should be taken into account. A -disused organ which entails evil because construction of it involves -needless cost, may entail further evil by being in the way. This, it seems -to me, is the reason why the fore-limbs of snakes have disappeared from -their embryos. When the long-bodied lizard out of which the ophidian type -evolved, crept through stiff herbage, and moved its head from side to side -to find openings, there resulted alternate bends of its body, which were -the beginnings of lateral undulations; and we may easily see that in -proportion as it thus progressed by insinuating itself through interstices, -the fore-limbs, less and less used for walking, would be more and more in -the way; and the lengthening of the body, increasing the undulatory motion -and decreasing the use of the fore-limbs, would eventually make them -absolute impediments. Hence besides the benefit in economy of construction -gained by embryos in which the fore-limbs were in early stages a little -less developed than usual, they would gain an advantage by having, when -mature, smaller fore-limbs than usual, leading to greater facility of -locomotion. There would be a double set of influences causing, through -selection, a comparatively rapid decrease of these appendages. And we may I -think see also, on contemplating the kind of movement, that the fore-limbs -would be more in the way than the hind limbs, which would consequently -dwindle with such smaller rapidity as to make continuance of the rudiments -of them comprehensible. - - -§ 131-132. So that while the embryonic law enunciated by von Baer is in -harmony with the hypothesis of evolution, and is, indeed, a law which this -hypothesis implies, the nonconformities to the law are also interpretable -by this hypothesis. - -Parallelism between the courses of development in species allied by remote -ancestry, is liable to be variously modified in correspondence with the -later ancestral forms passed through after divergence of such species. The -substitution of a direct for an indirect process of formation, which we -have reason to believe will show itself, must obscure the embryonic -history. And the principle of economy which leads to this substitution -produces effects that are very irregular and uncertain in consequence of -the endlessly varied conditions. Thus several causes conspire to produce -deviations from the general law. - -Let it be remarked, finally, that the ability to trace out embryologic -kinships and the inability to do this, occur just where, according to the -hypothesis of Evolution, they should occur. We saw in § 100a that -zoologists are agreed in grouping animals into some 17 phyla--_Mollusca_, -_Arthropoda_, _Echinodermata_, &c.--each of which includes a number of -classes severally sub-divided into orders, genera, species. All the members -of each phylum are so related embryologically, that the existence of a -common ancestor of them in the remote past is considered certain. But when -it comes to the relations among the archaic ancestors, opinion is -unsettled. Whether, for instance, the primitive _Chordata_, out of which -the _Vertebrata_ emerged, have molluscan affinities or annelidan -affinities, is still a matter in dispute. With regard to the origins of -various other types no settled conclusions are held. Now it is clear that -on tracing down each branch of the great genealogical tree, kinships would -be much more manifest among the recently-differentiated forms than among -those forms which diverged from one another in the earliest stages of -organic life, and had separated widely before any of the types we now know -had come into existence. - - - - -CHAPTER VI. - -THE ARGUMENTS FROM MORPHOLOGY. - - -§ 133. Leaving out of consideration those parallelisms among their modes of -development which characterize organisms belonging to each group, that -community of plan which exists among them when mature is extremely -remarkable and extremely suggestive. As before shown (§ 103), neither the -supposition that these combinations of attributes which unite classes are -fortuitous, nor the supposition that no other combinations were -practicable, nor the supposition of adherence to pre-determined typical -plans, suffices to explain the facts. An instance will best prepare the -reader for seeing the true meaning of these fundamental likenesses. - -Under the immensely-varied forms of insects, greatly elongated like the -dragon-fly or contracted in shape like the lady-bird, winged like the -butterfly or wingless like the flea, we find this character in -common--there are primarily seventeen segments.[49] These segments may be -distinctly marked or they may be so fused as to make it difficult to find -the divisions between them, but they always exist. What now can be the -meaning of this community of structure throughout the hundred thousand -kinds of insects filling the air, burrowing in the earth, swimming in the -water? Why under the down-covered body of a moth and under the hard -wing-cases of a beetle, should there be discovered the same number of -divisions? Why should there be no more somites in the Stick-insect, or -other Phasmid a foot long, than there are in a small creature like the -louse? Why should the inert _Aphis_ and the swift-flying Emperor-butterfly -be constructed on the same fundamental plan? It cannot be by chance that -there exist equal numbers of segments in all these multitudes of species. -There is no reason to think it was _necessary_, in the sense that no other -number would have made a possible organism. And to say that it is the -result of _design_--to say that the Creator followed this pattern -throughout, merely for the purpose of maintaining the pattern--is to assign -an absurd motive. No rational interpretation of these and countless like -morphological facts, can be given except by the hypothesis of evolution; -and from the hypothesis of evolution they are corollaries. If organic forms -have arisen from common stocks by perpetual divergences and -re-divergences--if they have continued to inherit, more or less clearly, -the characters of ancestral races; then there will naturally result these -communities of fundamental structure among creatures which have severally -become modified in multitudinous ways and degrees, in adaptation to their -respective modes of life. To this let it be added that while the belief in -an intentional adhesion to a pre-determined pattern throughout a whole -group, is negatived by the occurrence of occasional deviations from the -pattern; such deviations are reconcilable with the belief in evolution. As -pointed out in the last chapter, ancestral traits will be obscured more or -less according as the superposed modifications of structure, have or have -not been furthered by the conditions of life and development to which the -type has been subjected. - - -§ 134. Besides these wide-embracing and often deeply-hidden homologies, -which hold together different animals, there are the scarcely-less -significant homologies between different organs of the same animal. These, -like the others, are obstacles to the supernatural interpretations and -supports of the natural interpretation. - -One of the most familiar and instructive examples is furnished by the -vertebral column. Snakes, which move sinuously through and over plants and -stones, obviously need a segmentation of the bony axis from end to end; and -inasmuch as flexibility is required throughout the whole length of the -body, there is advantage in the comparative uniformity of this -segmentation. The movements would be impeded if, instead of a chain of -vertebræ varying but little in their lengths, there existed in the middle -of the series some long bony mass that would not bend. But in the higher -_Vertebrata_, the mechanical actions and reactions demand that while some -parts of the vertebral column shall be flexible, other parts shall be -inflexible. Inflexibility is specially requisite in that part of it called -the sacrum; which, in mammals and birds, forms a fulcrum exposed to the -greatest strains the skeleton has to bear. Now in both mammals and birds, -this rigid portion of the vertebral column is not made of one long segment -or vertebra, but of several segments fused together. In man there are five -of these confluent sacral vertebræ; and in the ostrich tribe they number -from seventeen to twenty. Why is this? Why, if the skeleton of each species -was separately contrived, was this bony mass made by soldering together a -number of vertebræ like those forming the rest of the column, instead of -being made out of one single piece? And why, if typical uniformity was to -be maintained, does the number of sacral vertebræ vary within the same -order of birds? Why, too, should the development of the sacrum be by the -round-about process of first forming its separate constituent vertebræ, and -then destroying their separateness? In the embryo of a mammal or bird, the -central element of the vertebral column is, at the outset, continuous. The -segments that are to become vertebræ, arise gradually in the adjacent -mesoderm, and enwrap this originally-homogeneous axis or notochord. Equally -in those parts of the spine which are to remain flexible, and in those -parts which are to grow rigid, these segments are formed; and that part of -the spine which is to compose the sacrum, having acquired this segmental -structure, loses it again by coalescence of the segments. To what end is -this construction and re-construction? If, originally, the spine in -vertebrate animals consisted from head to tail of separate moveable -segments, as it does still in fishes and some reptiles--if, in the -evolution of the higher _Vertebrata_, certain of these moveable segments -were rendered less moveable with respect to one another, by the mechanical -conditions they were exposed to, and at length became relatively immovable; -it is comprehensible why the sacrum formed out of them, should continue -ever after to show its originally-segmented structure. But on any other -hypothesis this segmented structure is inexplicable. "We see the same law -in comparing the wonderfully complex jaws and legs in crustaceans," says -Mr. Darwin: referring to the fact that those numerous lateral appendages -which, in the lower crustaceans, most of them serve as legs, and have like -shapes, are, in the higher crustaceans, some of them represented by -enormously-developed claws, and others by variously-modified foot-jaws. "It -is familiar to almost every one," he continues, "that in a flower the -relative position of the sepals, petals, stamens, and pistils, as well as -their intimate structure, are intelligible on the view that they consist of -metamorphosed leaves arranged in a spire. In monstrous plants we often get -direct evidence of the possibility of one organ being transformed into -another; and we can actually see in embryonic crustaceans and in many other -animals, and in flowers, that organs, which when mature become extremely -different, are at an early stage of growth exactly alike." ... "Why should -one crustacean, which has an extremely complex mouth formed of many parts -consequently always have fewer legs; or conversely, those with many legs -have simpler mouths? Why should the sepals, petals, stamens, and pistils in -any individual flower, though fitted for such widely-different purposes, be -all constructed on the same pattern?" - -To these and countless similar questions, the theory of evolution furnishes -the only rational answer. In the course of that change from homogeneity to -heterogeneity of structure displayed in evolution under every form, it will -necessarily happen that from organisms made up of numerous like parts, -there will arise organisms made up of parts more and more unlike: which -unlike parts will nevertheless continue to bear traces of their primitive -likeness. - - -§ 135. One more striking morphological fact, near akin to some of the facts -dwelt on in the last chapter, must be here set down--the frequent -occurrence, in adult animals and plants, of rudimentary and useless organs, -which are homologous with organs that are developed and useful in allied -animals and plants. In the last chapter we saw that during the development -of embryos, there often arise organs which disappear on being replaced by -other organs discharging the same functions in better ways; and that in -some cases, organs develop to certain points and are then re-absorbed -without performing any functions. Very generally, however, the -partially-developed organs are retained throughout life. - -The osteology of the higher _Vertebrata_ supplies abundant examples. -Vertebral processes which, in one tribe, are fully formed and ossified from -independent centres, are, in other tribes, mere tubercles not having -independent centres of ossification. While in the tail of this animal the -vertebræ are severally composed of centrum and appendages, in the tail of -that animal they are simple osseous masses without any appendages; and in -another animal they have lost their individualities by coalescence with -neighbouring vertebræ into a rudimentary tail. From the structures of the -limbs analogous facts are cited by comparative anatomists. The undeveloped -state of certain metacarpal bones, characterizes whole groups of mammals. -In one case we find the normal number of digits; and, in another case, a -smaller number with an atrophied digit to make out the complement. Here is -a digit with its full number of phalanges; and there a digit of which one -phalange has been arrested in its growth. Still more remarkable are the -instances of entire limbs being rudimentary; as in certain snakes, which -have hind legs hidden beneath the integument. So, too, is it with dermal -appendages. Some of the smooth-skinned amphibia have scales buried in the -skin. The seal, which is a mammal considerably modified in adaptation to an -aquatic life, and which uses its feet mainly as paddles, has toes that -still bear external nails; but the manatee, which is a much more -transformed mammal, has nailless paddles which, when the skin is removed, -are said, by Humboldt, to display rudimentary nails at the ends of the -imbedded digits. Nearly all birds are covered with developed feathers, -severally composed of a shaft bearing fibres, each of which, again, bears a -fringe of down. But in some birds, as in the ostrich, various stages of -arrested development of the feathers may be traced: between the -unusually-elaborated feathers of the tail, and those about the beak which -are reduced to simple hairs, there are transitions. Nor is this the extreme -case. In the _Apteryx_ we see the whole of the feathers reduced to a -hair-like form. Again, the hair which commonly covers the body in mammals -is, over the greater part of the human body almost rudimentary, and is in -some parts reduced to mere down--down which nevertheless proves itself to -be homologous with the hair of mammals in general, by occasionally -developing into the original form. Numerous cases of aborted organs are -given by Mr. Darwin, of which a few may be here added. "Nothing can be -plainer," he remarks, "than that wings are formed for flight, yet in how -many insects do we see wings so reduced in size as to be utterly incapable -of flight, and not rarely lying under wing-cases, firmly soldered -together?" ... "In plants with separated sexes, the male flowers often have -a rudiment of a pistil; and Kölreuter found that by crossing such male -plants with an hermaphrodite species, the rudiment of the pistil in the -hybrid offspring was much increased in size; and this shows that the -rudiment and the perfect pistil are essentially alike in nature." And then, -to complete the proof that these undeveloped parts are marks of descent -from races in which they were developed, there are not a few direct -experiences of this relation. "We have plenty of cases of rudimentary -organs in our domestic productions--as the stump of a tail in tailless -breeds--the vestige of an ear in earless breeds--the re-appearance of -minute dangling horns in hornless breeds of cattle." (_Origin of Species_, -1859, pp. 451, 454.) - -Here, as before, the teleological doctrine fails utterly; for these -rudimentary organs are useless, and occasionally even detrimental; as is -the _appendix vermiformis_, in Man--a part of the cæcum which is of no -value for the purpose of absorption but which, by detaining small foreign -bodies, often causes severe inflammation and death. The doctrine of typical -plans is equally out of court; for while, in some members of a group, -rudimentary organs completing the general type are traceable, in other -members of the same group such organs are unrepresented. There remains only -the doctrine of evolution; and to this, these rudimentary organs offer no -difficulties. On the contrary, they are among its most striking evidences. - - -§ 136. The general truths of morphology thus coincide in their -implications. Unity of type, maintained under extreme dissimilarities of -form and mode of life, is explicable as resulting from descent with -modification; but is otherwise inexplicable. The likenesses disguised by -unlikenesses, which the comparative anatomist discovers between various -organs in the same organism, are worse than meaningless if it be supposed -that organisms were severally framed as we now see them; but they fit in -quite harmoniously with the belief that each kind of organism is a product -of accumulated modifications upon modifications. And the presence, in all -kinds of animals and plants, of functionally-useless parts corresponding to -parts that are functionally-useful in allied animals and plants, while it -is totally incongruous with the belief in a construction of each organism -by miraculous interposition, is just what we are led to expect by the -belief that organisms have arisen by progression. - - - - -CHAPTER VII. - -THE ARGUMENTS FROM DISTRIBUTION. - - -§ 137. In §§ 105 and 106, we contemplated the phenomena of distribution in -Space. The general conclusions reached, in great part based on the evidence -brought together by Mr. Darwin, were that, "on the one hand, we have -similarly-conditioned, and sometimes nearly-adjacent, areas, occupied by -quite different Faunas. On the other hand, we have areas remote from each -other in latitude, and contrasted in soil as well as climate, which are -occupied by closely-allied Faunas." Whence it was inferred that "as like -organisms are not universally, or even generally, found in like habitats; -nor very unlike organisms, in very unlike habitats; there is no manifest -pre-determined adaptation of the organisms to the habitats." In other -words, the facts of distribution in Space do not conform to the hypothesis -of design. At the same time we saw that "the similar areas peopled by -dissimilar forms, are those between which there are impassable barriers; -while the dissimilar areas peopled by similar forms, are those between -which there are no such barriers;" and these generalizations appeared to -harmonize with the abundantly-illustrated truth, "that each species of -organism tends ever to expand its sphere of existence--to intrude on other -areas, other modes of life, other media." - -By way of showing still more clearly the effects of competition among races -of organisms, let me here add some recently-published instances of the -usurpations of areas, and changes of distribution hence resulting. In the -_Natural History Review_ for January, 1864, Dr. Hooker quotes as follows -from some New Zealand naturalists:--"You would be surprised at the rapid -spread of European and other foreign plants in this country. All along the -sides of the main lines of road through the plains, a _Polygonum_ -(_aviculare_), called 'Cow Grass,' grows most luxuriantly, the roots -sometimes two feet in depth, and the plants spreading over an area from -four to five feet in diameter. The dock (_Rumex obtusifolius_ or _R. -crispus_) is to be found in every river bed, extending into the valleys of -the mountain rivers, until these become mere torrents. The sow-thistle is -spread all over the country, growing luxuriantly nearly up to 6000 feet. -The water-cress increases in our still rivers to such an extent, as to -threaten to choke them altogether ... I have measured stems twelve feet -long and three-quarters of an inch in diameter. In some of the mountain -districts, where the soil is loose, the white clover is completely -displacing the native grasses, forming a close sward.... In fact, the young -native vegetation appears to shrink from competition with these more -vigorous intruders." "The native (Maori) saying is 'as the white man's rat -has driven away the native rat, so the European fly drives away our own, -and the clover kills our fern, so will the Maoris disappear before the -white man himself.'" - -Given this universal tendency of the superior to overrun the habitats of -the inferior,[50] let us consider what, on the hypothesis of evolution, -will be the effects on the geographical relationships of species. - - -§ 138. A race of organisms cannot expand its sphere of existence without -subjecting itself to new external conditions. Those of its members which -spread over adjacent areas, inevitably come in contact with circumstances -partially different from their previous circumstances; and such of them as -adopt the habits of other organisms, necessarily experience re-actions more -or less contrasted with the re-actions before experienced. Now if changes -of organic structure are caused, directly or indirectly, by changes in the -incidence of forces; there must result unlikenesses of structure between -the divisions of a race which colonizes new habitats. Hence, in the absence -of obstacles to migration, we may anticipate manifest kinships between the -animals and plants of one area, and those of areas adjoining it. This -inference corresponds with an induction before set down (§ 106). In -addition to illustrations of it already quoted from Mr. Darwin, his pages -furnish others. One is that species which inhabit islands are allied to -species which inhabit neighbouring main lands; and another is that the -faunas of clustered islands show marked similarities. "Thus the several -islands of the Galapagos Archipelago are tenanted," says Mr. Darwin, "in a -quite marvellous manner, by very closely related species; so that the -inhabitants of each separate island, though mostly distinct, are related in -an incomparably closer degree to each other than to the inhabitants of any -other part of the world." Mr. Wallace has traced "variation as specially -influenced by locality" among the _Papilionidæ_ inhabiting the East Indian -Archipelago: showing how "the species and varieties of Celebes possess a -striking character in the form of the anterior wings, different from that -of the allied species and varieties of all the surrounding islands;" and -how "tailed species in India and the western islands lose their tails as -they spread eastward through the archipelago." During his travels on the -Upper Amazons, Mr. Bates found that "the greater part of the species of -_Ithomiæ_ changed from one locality to another, not further removed than -100 to 200 miles;" that "many of these local species have the appearance of -being geographical varieties;" and that in some species "most of the local -varieties are connected with their parent form by individuals exhibiting -all the shades of variation." - -Further general relationships are to be inferred. If races of organisms, -ever being thrust by pressure of population into new habitats, undergo -modifications of structure as they diverge more and more widely in Space, -it follows that, speaking generally, the widest divergences in Space will -indicate the longest periods during which the descendants from a common -stock have been subject to modifying conditions; and hence that, among -organisms of the same group, the smaller contrasts of structure will be -limited to the smaller areas. This we find: "varieties being," as Dr. -Hooker says in his _Flora of Tasmania_, "more restricted in locality than -species, and these again than genera." Again, if races of organisms spread, -and as they spread are altered by changing incident forces; it follows that -where the incident forces vary greatly within given areas, the alterations -will be more numerous than in equal areas which are less-variously -conditioned. This, too, proves to be the fact. Dr. Hooker points out that -the relatively uniform regions have the fewest species; while in the most -multiform regions the species are the most numerous. - - -§ 139. Let us consider next, how the hypothesis of evolution corresponds -with the facts of distribution, not over different areas but through -different media. If all forms of organisms have descended from some -primordial form, it follows that since this primordial form must have -inhabited some one medium out of the several media now inhabited, the -peopling of other media by its descendants implies migration from one -medium to others--implies adaptations to media quite unlike the original -medium. To speak specifically--water being the medium in which the lowest -living forms exist, the implication is that the earth and the air have been -colonized from the water. Great difficulties appear to stand in the way of -this assumption. Ridiculing those who alleged the uniserial development of -organic forms, who, indeed, laid themselves open to ridicule by their many -untenable propositions, Von Baer writes--"A fish, swimming towards the -shore desires to take a walk, but finds his fins useless. They diminish in -breadth for want of use, and at the same time elongate. This goes on with -children and grandchildren for a few millions of years, and at last who can -be astonished that the fins become feet? It is still more natural that the -fish in the meadow, finding no water, should gape after air, thereby, in a -like period of time developing lungs; the only difficulty being that in the -meanwhile, a few generations must manage without breathing at all." -Though, as thus presented, the belief in a transition looks laughable; and -though such derivation of terrestrial vertebrates by direct modification of -piscine vertebrates, is untenable; yet we must not conclude that no -migrations of the kind alleged can have taken place. The adage that "truth -is stranger than fiction," applies quite as much to Nature in general as to -human life. Besides the fact that certain fish actually do "take a walk" -without any obvious reason; and besides the fact that sundry kinds of fish -ramble about on land when prompted by the drying-up of the waters they -inhabit; there is the still more astounding fact that one kind of fish -climbs trees. Few things seem more manifestly impossible, than that a -water-breathing creature without efficient limbs, should ascend eight or -ten feet up the trunk of a palm; and yet the _Anabas scandens_ does as -much. To previous testimonies on this point Capt. Mitchell has recently -added others. Such remarkable cases of temporary changes of media, will -prepare us for conceiving how, under special conditions, permanent changes -of media may have taken place; and for considering how the doctrine of -evolution is elucidated by them. - -Inhabitants of the sea, of rivers, and of lakes, are many of them left from -time to time partially or completely without water; and those which show -the power to change their media temporarily or permanently, are in very -many cases of the kinds most liable to be thus deserted by their medium. -Let us consider what the sea-shore shows us. Twice a day the rise and the -fall of the tide covers and uncovers plants and animals, fixed and moving; -and through the alternation of spring and neap tides, it results that the -exposure of the organisms living low down on the beach, varies both in -frequency and duration: while some of them are left dry only once a -fortnight for a very short time, others, a little higher up, are left dry -during two or three hours at several ebb tides every fortnight. Then by -small gradations we come to such as, living at the top of the beach, are -bathed by salt-water only at long intervals; and still higher to some which -are but occasionally splashed in stormy weather. What, now, do we find -among the organisms thus subject to various regular and irregular -alterations of media? Besides many plants and many fixed animals, we find -moving animals of numerous kinds; some of which are confined to the lower -zones of this littoral region, but others of which wander over the whole of -it. Omitting the humbler types, it will suffice to observe that each of the -two great sub-kingdoms, _Mollusca_ and _Arthropoda_, supplies examples of -creatures having a wide excursiveness within this region. We have -gasteropods which, when the tide is down, habitually creep snail-like over -sand and sea-weed, even up as far as high-water mark. We have several kinds -of crustaceans, of which the crab is the most conspicuous, running about on -the wet beach, and sometimes rambling beyond the reach of the water. And -then note the striking fact that each of the forms thus habituated to -changes of media, is allied to forms which are mainly or wholly -terrestrial. On the West Coast of Ireland marine gasteropods are found on -the rocks three hundred feet above the sea, where they are only at long -intervals wetted by the spray; and though between gasteropods of this class -and land-gasteropods the differences are considerable, yet the -land-gasteropods are more closely allied to them than to any other -_Mollusca_. Similarly, the two highest orders of crustaceans have their -species which live occasionally, or almost entirely, out of the water: -there is a kind of lobster in the Mauritius which climbs trees; and there -is the land-crab of the West Indies, which deserts the sea when it reaches -maturity and re-visits it only to spawn. Seeing, thus, how there are many -kinds of marine creatures whose habitats expose them to frequent changes of -media; how some of the higher kinds so circumstanced, show a considerable -adaptation to both media; and how these amphibious kinds are allied to -kinds that are mainly or wholly terrestrial; we shall see that the -migrations from one medium to another, which evolution pre-supposes, are by -no means impracticable. With such evidence before us, the assumption that -the distribution of the _Vertebrata_ through media so different as air and -water, may have been gradually effected in some analogous manner, would not -be altogether unwarranted even had we no clue to the process. We shall -find, however, a tolerably distinct clue. Though rivers, and lakes, and -pools, have no sensible tidal variations, they have their rises and falls, -regular and irregular, moderate and extreme. Especially in tropical -climates, we see them annually full for a certain number of months, and -then dwindling away and drying up. The drying up may reach various degrees -and last for various periods. It may go to the extent only of producing a -liquid mud, or it may reduce the mud to a hardened, fissured solid. It may -last for a few days or for months. That is to say, aquatic forms which are -in one place annually subject to a slight want of water for a short time, -are elsewhere subject to greater wants for longer times: we have gradations -of transition, analogous to those which the tides furnish. Now it is well -known that creatures inhabiting such waters have, in various degrees, -powers of meeting these contingencies. The contained fish either bury -themselves in the mud when the dry season comes, or ramble in search of -other waters. This is proved by evidence from India, Guiana, Siam, Ceylon; -and some of these fish, as the _Anabas scandens_, are known to survive for -days out of the water. But the facts of greatest significance are furnished -by an allied class of _Vertebrata_, almost peculiar to habitats of this -kind. The _Amphibia_ are not, like fish, usually found in waters that are -never partially or wholly dried up; but they nearly all inhabit waters -which, at certain seasons, evaporate, in great measure or -completely--waters in which most kinds of fish cannot exist. And what are -the leading structural traits of these _Amphibia_? They have two -respiratory systems--pulmonic and branchial--variously developed in -different orders; and they have two or four limbs, also variously -developed. Further, the class _Amphibia_ consists of two groups, in one of -which this duality of the respiratory system is permanent, and the -development of the limbs always incomplete; and in the other of which the -branchiæ disappear as the lungs and limbs become fully developed. The -lowest group, the _Perennibranchiata_, have internal organs for aerating -the blood which approach in various degrees to lungs, until "in the -_Siren_, the pulmonic respiration is more extensive and important than the -branchial;" and to these creatures, having a habitat partially aërial and -partially aquatic, there are at the same time supplied, in the shallow -water covering soft mud, the mechanical conditions which render swimming -difficult and rudimentary limbs useful. In the higher group, the -_Caducibranchiata_, we find still more suggestive transformations. Having -at first a structure resembling that which is permanent in the -perennibranchiate amphibian, the larva of the caducibranchiate amphibian -pursues for a time a similar life; but, eventually, while the branchial -appendages dwindle the lungs grow: the respiration of air, originally -supplementary to the respiration of water, predominates over it more and -more, till it replaces it entirely; and an additional pair of legs is -produced. This having been done, the creature either becomes, like the -_Triton_, one which quits the water only occasionally; or, like the Frog, -one which pursues a life mainly terrestrial, and returns to the water now -and then. Finally, if we ask under what conditions this metamorphosis of a -water-breather into an air-breather completes itself, the answer is--it -completes itself at the time when the shallow pools inhabited by the larvæ -are being dried up, or in danger of being dried up, by the summer's -sun.[51] - -See, then, how significant are the facts when thus brought together. There -are particular habitats in which animals are subject to changes of media. -In such habitats exist animals having, in various degrees, the power to -live in both media, consequent on various phases of transitional -organization. Near akin to these animals there are some that, after passing -their early lives in the water, acquire more completely the structures -fitting them to live on land, to which they then migrate. Lastly, we have -closely-allied creatures, like the Surinam toad and the terrestrial -salamander, which, though they belong by their structures to the class -_Amphibia_, are not amphibious in their habits--creatures the larvæ of -which do not pass their early lives in the water, and yet go through these -same metamorphoses! Must we then think, like Von Baer, that the -distribution of kindred organisms through different media presents an -insurmountable difficulty? On the contrary, with facts like these before -us, the evolution-hypothesis supplies possible interpretations of many -phenomena that are else unaccountable. After seeing the ways in which such -changes of media are in some cases gradually imposed by physical -conditions, and in other cases voluntarily commenced and slowly increased -in the search after food; we shall begin to understand how, in the course -of evolution, there have arisen strange obscurations of one type by the -externals of another type. When we see land-birds occasionally feeding by -the water-side, and then learn that one of them, the water-ouzel, an -"anomalous member of the strictly terrestrial thrush family, wholly -subsists by diving--grasping the stones with its feet and using its wings -under water"--we are enabled to comprehend how, under pressure of -population, aquatic habits may be acquired by creatures organized for -aërial life; and how there may eventually arise an ornithic type in which -the traits of the bird are very much disguised. On finding among mammals -some that, in search of prey or shelter, have taken to the water in various -degrees, we shall cease to be perplexed on discovering the mammalian -structure hidden under a fish-like form, as it is in the _Cetacea_ and the -_Sirenia_: especially on finding that in the sea-lion and the seals there -are transitional forms. Grant that there has ever been going on that -re-distribution of organisms which we see still resulting from their -intrusions on one another's areas, media, and modes of life; and we have an -explanation of those multitudinous cases in which homologies of structure -are complicated with analogies. And while it accounts for the occurrence in -one medium of organic types fundamentally organized for another medium, the -doctrine of evolution accounts also for the accompanying unfitnesses. -Either the seal has descended from some mammal which little by little -became aquatic in its habits, in which case the structure of its hind limbs -has a meaning; or else it was specially framed for its present habitat, in -which case the structure of its hind limbs is incomprehensible. - - -§ 140. The facts respecting distribution in Time, which have more than any -others been cited both in proof and in disproof of evolution, are too -fragmentary to be conclusive either way. Were the geological record -complete, or did it, as both Uniformitarians and Progressionists have -commonly assumed, give us traces of the earliest organic forms; the -evidence hence derived, for or against, would have had more weight than any -other evidence. As it is, all we can do is to see whether such fragmentary -evidence as remains, is congruous with the hypothesis. - -Palæontology has shown that there is a "general relation between lapse of -time and divergence of organic forms" (§ 107); and that "this divergence is -comparatively slow and continuous where there is continuity in the -geological formations, but is sudden and comparatively wide wherever there -occurs a great break in the succession of strata." Now this is obviously -what we should expect. The hypothesis implies structural changes that are -not sudden but gradual. Hence, where conformable strata indicate a -continuous record, we may anticipate successions of forms only slightly -different from one another; while we may rationally look for marked -contrasts between the groups of forms fossilized in adjacent strata, where -there is evidence of a great blank in the record. - -The permanent disappearances of species, of genera, and of orders, which we -saw to be a fact tolerably-well established, is also a fact for which the -belief in evolution prepares us. If later organic forms have in all cases -descended from earlier organic forms, and have diverged during their -descent, both from their prototypes and from one another; then it follows -that such of them as become extinct at any epoch, will never re-appear at a -subsequent epoch; since there can never again arise a concurrence and -succession of conditions such as those under which each type was evolved. - -Though comparisons of ancient and modern organic forms, prove that many -types have persisted through enormous periods of time, without undergoing -great changes; it was shown that such comparisons do not disprove the -occurrence in other organic forms, of changes great enough to produce what -are called different types. The result of inductive inquiry we saw to be, -that while a few modern higher types yield signs of having been developed -from ancient lower types; and that while there are many modern types which -_may_ have been thus developed, though we are without evidence that they -have been so; yet that "any admissible hypothesis of progressive -modification must be compatible with persistence without progression -through indefinite periods." Now these results are quite congruous with the -hypothesis of evolution. As rationally interpreted, evolution must in all -cases be understood to result, directly or indirectly, from the incidence -of forces. If there are no changes of conditions entailing organic changes, -organic changes are not to be expected. Only in organisms which fall under -conditions leading to additional modifications answering to additional -needs, will there be that increased heterogeneity which characterizes -higher forms. Hence, though the facts of palæontology cannot be held -conclusive proof of evolution, yet they are congruous with it; and some of -them yield it strong support. - - -§ 141. One general truth respecting distribution in Time, is profoundly -significant. If, instead of contemplating the relations among past forms of -life taken by themselves, we contemplate the relations between them and the -forms now existing, we find a connexion which is in harmony with the belief -in evolution but irreconcilable with any other belief. - -Note, first, how full of meaning is the close kinship existing between the -aggregate of organisms now living, and the aggregate of organisms which -lived in the most recent geologic times. In the last-formed strata, nearly -all the imbedded remains are those of species which still flourish. Strata -a little older contain a few fossils of species now extinct, though, -usually, species greatly resembling extant ones. Of the remains found in -strata of still earlier date, the extinct species form a larger percentage; -and the differences between them and the allied species now living are more -marked. That is to say, the gradual change of organic types in Time, which -we before saw is indicated by the geological record, is equally indicated -by the relation between existing organic types and organic types of the -epochs preceding our own. The evidence completely accords with the belief -in a descent of present life from past life. Doubtless such a kinship is -not incongruous with the doctrine of special creations. It may be argued -that the introduction, from time to time, of new species better fitted to -the somewhat changed conditions of the Earth's surface, would result in an -apparent alliance between our living Flora and Fauna, and the Floras and -Faunas that lately lived. No one can deny it. But on passing from the most -general aspect of the alliance to its more special aspects, we shall find -this interpretation completely negatived. - -For besides a close kinship between the aggregate of surviving forms and -the aggregate of forms which have died out in recent geologic times; there -is a peculiar connexion of like nature between present and past forms in -each great geographical region. The instructive fact, before cited from Mr. -Darwin, is the "wonderful relationship in the same continent between the -dead and the living." This relationship is not explained by the supposition -that new species have been at intervals supernaturally placed in each -habitat, as the habitat became modified; since, as we saw, species are by -no means uniformly found in the habitats to which they are best adapted. It -cannot be said that the marsupials imbedded in recent Australian strata, -having become extinct because of unfitness to some new external condition, -the existing marsupials were then specially created to fit the modified -environment; since sundry animals found elsewhere are so much more in -harmony with these new Australian conditions that, when taken to Australia, -they rapidly extrude the marsupials. While, therefore, the similarity -between the existing Australian Fauna and the Fauna which immediately -preceded it over the same area, is just that which the belief in evolution -leads us to expect; it is a similarity which cannot be otherwise accounted -for. And so is it with parallel relations in New England, in South America, -and in Europe. - - -§ 142. Given, then, that pressure which species exercise on one another, in -consequence of the universal overfilling of their respective -habitats--given the resulting tendency to thrust themselves into one -another's areas, and media, and modes of life, along such lines of least -resistance as from time to time are found--given besides the changes in -modes of life, hence arising, those other changes which physical -alterations of habitats necessitate--given the structural modifications -directly or indirectly produced in organisms by modified conditions; and -the facts of distribution in Space and Time are accounted for. That -divergence and re-divergence of organic forms, which we saw to be shadowed -forth by the truths of classification and the truths of embryology, we see -to be also shadowed forth by the truths of distribution. If that aptitude -to multiply, to spread, to separate, and to differentiate, which the human -races have in all times shown, be a tendency common to races in general, as -we have ample reason to assume; then there will result those kinds of -spacial relations and chronological relations among the species, and -genera, and orders, peopling the Earth's surface, which we find exist. The -remarkable identities of type discovered between organisms inhabiting one -medium, and strangely modified organisms inhabiting another medium, are at -the same time rendered comprehensible. And the appearances and -disappearances of species which the geological record shows us, as well as -the connexions between successive groups of species from early eras down to -our own, cease to be inexplicable. - - - - -CHAPTER VIII. - -HOW IS ORGANIC EVOLUTION CAUSED? - - -§ 143. Already it has been necessary to speak of the causes of organic -evolution in general terms; and now we are prepared for considering them -specifically. The task before us is to affiliate the leading facts of -organic evolution, on those same first principles conformed to by evolution -at large. - -Before attempting this, however, it will be instructive to glance at the -causes of organic evolution which have been from time to time alleged. - - -§ 144. The theory that plants and animals of all kinds were gradually -evolved, seems to have been at first accompanied only by the vaguest -conception of cause--or rather, by no conception of cause properly so -called, but only by the blank form of a conception. One of the earliest who -in modern times (1735) contended that organisms are indefinitely -modifiable, and that through their modifications they have become adapted -to various modes of existence, was De Maillet. But though De Maillet -supposed all living beings to have arisen by a natural, continuous process, -he does not appear to have had any definite idea of that which determines -this process. In 1794, in his _Zoonomia_, Dr. Erasmus Darwin gave reasons -(sundry of them valid ones) for believing that organized beings of every -kind, have descended from one, or a few, primordial germs; and along with -some observable causes of modification, which he points out as aiding the -developmental process, he apparently ascribes it, in part, to a tendency -given to such germ or germs when created. He suggests the possibility "that -all warm-blooded animals have arisen from one living filament, which THE -GREAT FIRST CAUSE endued with animality, with the power of acquiring new -parts, attended with new propensities, directed by irritations, sensations, -volitions, and associations; and thus possessing the faculty of continuing -to improve by its own inherent activity." In this passage we see the idea -to be, that evolution is pre-determined by some intrinsic proclivity. "It -is curious," says Mr. Charles Darwin, "how largely my grandfather, Dr. -Erasmus Darwin, anticipated the erroneous grounds of opinion, and the views -of Lamarck." One of the anticipations was this ascription of development to -some inherent tendency. To the "plan général de la nature, et sa marche -uniforme dans ses opérations," Lamarck attributes "la progression évidente -qui existe dans la composition de l'organisation des animaux;" and "la -_gradation_ régulière qu'ils devroient offrir dans la composition de leur -organisation," he thinks is rendered irregular by secondary causes. -Essentially the same in kind, though somewhat different in form, is the -conception put forth in the _Vestiges of Creation_; the author of which -contends "that the several series of animated beings, from the simplest and -oldest up to the highest and most recent, are, under the providence of God, -the results, _first_, of an impulse which has been imparted to the forms of -life, advancing them, in definite times, by generation, through grades of -organization terminating in the highest dicotyledons and vertebrata;" and -that the progression resulting from these impulses, is modified by certain -other causes. The broad contrasts between lower and higher forms of life, -are regarded by him as implying an innate aptitude to give birth to forms -of more perfect structures. The last to re-enunciate this doctrine has been -Prof. Owen; who asserts "the axiom of the continuous operation of creative -power, or of the ordained becoming of living things." Though these words do -not suggest a very definite idea, yet they indicate the belief that organic -progress is a result of some in-dwelling tendency to develop, -supernaturally impressed on living matter at the outset--some ever-acting -constructive force which, independently of other forces, moulds organisms -into higher and higher forms. - -In whatever way it is formulated, or by whatever language it is obscured, -this ascription of organic evolution to some aptitude naturally possessed -by organisms, or miraculously imposed on them, is unphilosophical. It is -one of those explanations which explain nothing--a shaping of ignorance -into the semblance of knowledge. The cause assigned is not a true -cause--not a cause assimilable to known causes--not a cause that can be -anywhere shown to produce analogous effects. It is a cause unrepresentable -in thought: one of those illegitimate symbolic conceptions which cannot by -any mental process be elaborated into a real conception. In brief, this -assumption of a persistent formative power inherent in organisms, and -making them unfold into higher types, is an assumption no more tenable than -the assumption of special creations: of which, indeed, it is but a -modification; differing only by the fusion of separate unknown processes -into a continuous unknown process. - - -§ 145. Besides this intrinsic tendency to progress which Dr. Darwin -ascribes to animals, he says they have a capacity for being modified by -processes which their own desires initiate. He speaks of powers as "excited -into action by the necessities of the creatures which possess them, and on -which their existence depends;" and more specifically he says that "from -their first rudiment or primordium, to the termination of their lives, all -animals undergo perpetual transformations; which are in part produced by -their own exertions, in consequence of their desires and aversions, of -their pleasures and their pains, or of irritations, or of associations; and -many of these acquired forms or properties are transmitted to their -posterity." While it embodies a belief for which much may be said, this -passage involves the assumption that desires and aversions, existing before -experiences of the actions to which they are related, were the originators -of the actions, and therefore of the structural modifications caused by -them. In his _Philosophie Zoologique_, Lamarck much more specifically -asserts "le _sentiment intérieur_," to be in all creatures that have -developed nervous systems, an independent cause of those changes of form -which are due to the exercise of organs: distinguishing it from that simple -_irritability_ possessed by inferior animals, which cannot produce what we -call a desire or emotion; and holding that these last, along with all "qui -manquent de système nerveux, ne vivent qu'à l'aide des excitations qu'ils -reçoivent de l'extérieur." Afterwards he says--"je reconnus que la nature, -obligée d'abord d'emprunter des milieux environnants la _puissance -excitatrice_ des mouvements vitaux et des actions des animaux imparfaits, -sut, en composant de plus en plus l'organisation animale, transporter cette -puissance dans l'intérieur même de ces êtres, et qu'à la fin, elle parvint -à mettre cette même puissance à la disposition de l'individu." And still -more definitely he contends that if one considers "la _progression_ qui se -montre dans la composition de l'organisation," ... "alors on eût pu -apercevoir comment les _besoins_, d'abord réduits à nullité, et dont le -nombre ensuite s'est accru graduellement, ont amené le penchant aux actions -propres à y satisfaire: comment les actions devenues habituelles et -énergiques, ont occasionné le développement des organes qui les exécutent." - -Now though this conception of Lamarck is more precisely stated, and worked -out with much greater elaboration and wider knowledge of the facts, it is -essentially the same as that of Dr. Darwin; and along with the truth it -contains, contains also the same error more distinctly pronounced. Merely -noting that desires or wants, acting directly only on the nervo-muscular -system, can have no immediate influence on very many organs, as the -viscera, or such external appendages as hair and feathers; and observing, -further, that even some parts which belong to the apparatus of external -action, such as the bones of the skull, cannot be made to grow by increase -of function called forth by desire; it will suffice to point out that the -difficulty is not solved, but simply slurred over, when needs or wants are -introduced as independent causes of evolution. True though it is, as Dr. -Darwin and Lamarck contend, that desires, by leading to increased actions -of motor organs, may induce further developments of such organs; and true, -as it probably is, that the modifications hence arising are transmissible -to offspring; yet there remains the unanswered question--Whence do these -desires originate? The transference of the exciting power from the exterior -to the interior, as described by Lamarck, begs the question. How comes -there a wish to perform an action not before performed? Until some -beneficial result has been felt from going through certain movements, what -can suggest the execution of such movements? Every desire consists -primarily of a mental representation of that which is desired, and -secondarily excites a mental representation of the actions by which it is -attained; and any such mental representations of the end and the means, -imply antecedent experience of the end and antecedent use of the means. To -assume that in the course of evolution there from time to time arise new -kinds of actions dictated by new desires, is simply to remove the -difficulty a step back. - - -§ 146. Changes of external conditions are named, by Dr. Darwin, as causes -of modifications in organisms. Assigning as evidence of original kinship, -that marked similarity of type which exists among animals, he regards their -deviations from one another, as caused by differences in their modes of -life: such deviations being directly adaptive. After enumerating various -appliances for procuring food, he says they all "seem to have been -gradually produced during many generations by the perpetual endeavour of -the creatures to supply the want of food, and to have been delivered to -their posterity with constant improvement of them for the purposes -required." And the creatures possessing these various appliances are -considered as having been rendered unlike by seeking for food in unlike -ways. As illustrating the alterations wrought by changed circumstances, he -names the acquired characters of domestic animals. Lamarck has elaborated -the same view in detail: using for the purpose, with great ingenuity, his -extensive knowledge of the animal kingdom. From a passage in the -_Avertissement_ it would at first sight seem that he looks upon direct -adaptation to new conditions as the chief cause of evolution. He says--"Je -regardai comme certain que le _mouvement des fluides_ dans l'intérieur des -animaux, mouvement qui c'est progressivement accéléré avec la composition -plus grande de l'organisation; et que _l'influence des circonstances_ -nouvelles, à mesure que les animaux s'y exposèrent en se répandant dans -tous les lieux habitables, furent les deux causes générales qui ont amené -les différents animaux à l'état où nous les voyons actuellement." But -elsewhere the view he expresses appears decidedly different from this. He -asserts that "dans sa marche, la nature a commencé, et recommence encore -tous les jours, par former les corps organisés les plus simples;" and that -"les premières ébauches de l'animal et du végétal étant formées dans les -lieux et les circonstances convenables, les facultés d'une vie commençante -et d'un mouvement organique établi, ont nécessairement développé peu à peu -les organes, et qu'avec le temps elles les ont diversifies ainsi que les -parties." And then, further on, he puts in italics this proposition:--"_La -progression dans la composition de l'organisation subit, çà et là, dans la -série générale des animaux, des anomalies opérées par l'influence des -circonstances d'habitation, et par celle des habitudes contractées._" -These, and sundry other passages, joined with his general scheme of -classification, make it clear that Lamarck conceived adaptive modification -to be, not the cause of progression, but the cause of irregularities in -progression. The inherent tendency which organisms have to develop into -more perfect forms, would, according to him, result in a uniform series of -forms; but varieties in their conditions work divergences of structure, -which break up the series into groups: groups which he nevertheless places -in uni-serial order, and regards as still substantially composing an -ascending succession. - - -§ 147. These speculations, crude as they may be considered, show much -sagacity in their respective authors, and have done good service. Without -embodying the truth in definite shapes, they contain adumbrations of it. -Not directly, but by successive approximations, do mankind reach correct -conclusions; and those who first think in the right direction, loose as may -be their reasonings, and wide of the mark as their inferences may be, yield -indispensable aid by framing provisional conceptions and giving a bent to -inquiry. - -Contrasted with the dogmas of his age, the idea of De Maillet was a great -advance. Before it can be ascertained how organized beings have been -gradually evolved, there must be reached the conviction that they _have_ -been gradually evolved; and this conviction he reached. His wild notions -about the way in which natural causes acted in the production of plants and -animals, must not make us forget the merit of his intuition that animals -and plants _were_ produced by natural causes. In Dr. Darwin's brief -exposition, the belief in a progressive genesis of organisms is joined with -an interpretation having considerable definiteness and coherence. In the -space of ten pages he not only indicates several of the leading classes of -facts which support the hypothesis of development, but he does something -towards suggesting the process of development. His reasonings show an -unconscious mingling of the belief in a supernaturally-impressed tendency -to develop, with the belief in a development arising from the changing -incidence of conditions. Probably had he pursued the inquiry further, this -last belief would have grown at the expense of the first. Lamarck, in -elaborating this general conception, has given greater precision both to -its truth and to its error. Asserting the same imaginary factors and the -same real factors, he has traced out their supposed actions in detail; and -has, in consequence, committed himself to a greater number of untenable -positions. But while, in trying to reconcile the facts with a theory which -is only an adumbration of the truth, he laid himself open to the criticisms -of his contemporaries; he proved himself profounder than his contemporaries -by seeing that natural genesis, however caused, has been going on. If they -were wise in not indorsing a theory which fails to account for a great part -of the facts; they were unwise in ignoring that degree of congruity with -the facts, which shows the theory to contain some fundamental verity. - -Leaving out, however, the imaginary factors of evolution which these -speculations allege, and looking only at the one actual factor which Dr. -Darwin and Lamarck assign as accounting for some of the phenomena; it is -manifest, from our present stand-point, that this, so far as it is a cause -of evolution, is a proximate cause and not an ultimate cause. To say that -functionally-produced adaptation to conditions originates either evolution -in general, or the irregularities of evolution, is to raise the further -question--why is there a functionally-produced adaptation to -conditions?--why do use and disuse generate appropriate changes of -structure? Neither this nor any other interpretation of biologic evolution -which rests simply on the basis of biologic induction, is an ultimate -interpretation. The biologic induction must itself be interpreted. Only -when the process of evolution of organisms is affiliated on the process of -evolution in general, can it be truly said to be explained. The thing -required is to show that its various results are corollaries from first -principles. We have to reconcile the facts with the universal laws of the -re-distribution of matter and motion. - - - - -CHAPTER IX. - -EXTERNAL FACTORS. - - -§ 148. When illustrating the rhythm of motion (_First Principles_, § 83) it -was pointed out that besides the daily and annual alternations in the -quantities of light and heat which any portion of the Earth's surface -receives from the Sun, there are alternations which require -immensely-greater periods to complete. Reference was made to the fact that -"every planet, during a certain long period, presents more of its northern -than of its southern hemisphere to the Sun at the time of its nearest -approach to him; and then again, during a like period, presents more of its -southern hemisphere than of its northern--a recurring coincidence which, -though it causes in some planets no sensible alterations of climate, -involves, in the case of the Earth, an epoch of 21,000 years during which -each hemisphere goes through a cycle of temperate seasons, and seasons that -are extreme in their heat and cold." Further, we saw that there is a -variation of this variation. The slow rhythm of temperate and intemperate -climates, which takes 21,000 years to complete itself, undergoes -exaggeration and mitigation during epochs that are far longer. The Earth's -orbit slowly alters in form: now approximating to a circle, and now -becoming more eccentric. During the period in which the Earth's orbit has -least eccentricity, the temperate and intemperate climates which repeat -their cycle in 21,000 years, are severally less temperate and less -intemperate, than when, some one or two millions of years later, the -Earth's orbit has reached its extreme of eccentricity. - -Thus, besides those daily variations in the quantities of light and heat -received by organisms, and responded to by variations in their functions; -and besides the annual variations in the quantities of light and heat which -organisms receive, and similarly respond to by variations in their -functions; there are variations that severally complete themselves in -21,000 years and in some millions of years--variations to which there must -also be responses in the changed functions of organisms. The whole vegetal -and animal kingdoms, are subject to quadruply-compounded rhythms in the -incidence of the forces on which life primarily depends--rhythms so -involved in their slow working round that at no time during one of these -vast epochs, can the incidence of these various forces be exactly the same -as at any other time. To the direct effects so produced on organisms, have -to be added much more important indirect effects. Changes of distribution -must result. Certain redistributions are occasioned even by the annual -variations in the quantities of the solar rays received by each part of the -Earth's surface. The migrations of birds thus caused are familiar. So, too, -are the migrations of certain fishes: in some cases from one part of the -sea to another; in some cases from salt water to fresh water; and in some -cases from fresh water to salt water. Now just as the yearly changes in the -amounts of light and heat falling on each locality, yearly extend and -restrict the habitats of many organisms which are able to move about with -some rapidity; so must the alterations of temperate and intemperate -climates produce extensions and restrictions of habitats. These, though -slow, must be universal--must affect the habitats of stationary organisms -as well as those of locomotive ones. For if, during an astronomic era, -there is going on at any limit to a plant's habitat, a diminution of the -winter's cold or summer's heat, which had before stopped its spread at that -limit; then, though the individual plants are fixed, yet the species will -move: the seeds of plants living at the limit, will produce individuals -which survive beyond the limit. The gradual spread so effected, having gone -on for some ten thousand years, the opposite change of climate will begin -to cause retreat. The tide of each species will, during one half of a long -epoch, slowly flow into new regions, and then will slowly ebb away from -them. Further, this rise and fall in the tide of each species will, during -far longer intervals, undergo increasing rises and falls and then -decreasing rises and falls. There will be an alteration of spring tides and -neap tides, answering to the changing eccentricity of the Earth's orbit. - -These astronomical rhythms, therefore, entail on organisms unceasing -changes in the incidence of forces in two ways. They directly subject them -to variations of solar influences, in such a manner that each generation is -somewhat differently affected in its functions; and they indirectly bring -about complicated alterations in the environing agencies, by carrying each -species into the presence of new physical conditions, new soil and surface. - - -§ 149. The power of geological actions to modify everywhere the -circumstances in which plants and animals are placed, is conspicuous. In -each locality denudation slowly uncovers different deposits, and slowly -changes the exposed areas of deposits already uncovered. Simultaneously, -the alluvial beds in course of formation, are qualitatively affected by -these progressive changes in the natures and proportions of the strata -denuded. The inclinations of surfaces and their directions with respect to -the Sun, are at the same time modified; and the organisms existing on them -are thus having their thermal conditions continually altered, as well as -their drainage. Igneous action, too, complicates these gradual -modifications. A flat region cannot be step by step thrust up into a -protuberance without unlike climatic changes being produced in its several -parts, by their exposures to different aspects. Extrusions of trap, -wherever they take place, revolutionize the localities; both over the areas -covered and over the areas on to which their detritus is carried. And where -volcanoes are formed, the ashes they occasionally send out modify the -character of the soil throughout large surrounding tracts. - -In like manner alterations in the Earth's crust cause the ocean to be ever -subjecting the organisms it contains to new combinations of conditions. -Here the water is being deepened by subsidence, and there shallowed by -upheaval. While the falling upon it of sediment brought down by -neighbouring large rivers, is raising the sea-bottom in one place, in -another the habitual rush of the tide is carrying away the sediment -deposited in past times. The mineral character of the submerged surface on -which sea-weeds grow and molluscs crawl, is everywhere occasionally -changed; now by the bringing away from an adjacent shore some previously -untouched strata; and now by the accumulation of organic remains, such as -the shells of pteropods or of foraminifera. A further series of alterations -in the circumstances of marine organisms, is entailed by changes in the -movements of the water. Each modification in the outlines of neighbouring -shores makes the tidal streams vary their directions or velocities or both. -And the local temperature is from time to time raised or lowered, because -some far-distant change of form in the Earth's crust has wrought a -divergence in those circulating currents of warm and cold water which -pervade the ocean. - -These geologically-caused changes in the physical characters of each -environment, occur in ever-new combinations, and with ever-increasing -complexity. As already shown (_First Principles_, § 158), it follows from -the law of the multiplication of effects, that during long periods each -tract of the Earth's surface increases in heterogeneity of both form and -substance. So that plants and animals of all kinds are, in the course of -generations, subjected by alterations in the crust of the Earth, to sets of -incident forces differing from previous sets, both by changes in the -proportions of the factors and, occasionally, by the addition of new -factors. - - -§ 150. Variations in the astronomical conditions joined with variations in -the geological conditions, bring about variations in the meteorological -conditions. Those slow alternations of elevation and subsidence which take -place over immense areas, here producing a continent where once there was a -fathomless ocean, and there causing wide seas to spread where in a long -past epoch there stood snow-capped mountains, gradually work great -atmospheric changes. While the highest parts of an emerging surface of the -Earth's crust exist as a cluster of islands, the plants and animals which -in course of time migrate to them have climates that are peculiar to small -tracts of land surrounded by large tracts of water. As, by successive -upheavals, greater areas are exposed, there begin to arise sensible -contrasts between the states of their peripheral parts and their central -parts. The breezes which daily moderate the extremes of temperature near -the shores, cease to affect the interiors; and the interiors, less -qualified too in their heat and cold by such ocean-currents as approach the -coast, acquire more decidedly the characters due to their latitudes. Along -with the further elevations which unite the members of the archipelago into -a continent, there come new meteorologic changes, as well as exacerbations -of the old. The winds, which were comparatively uniform in their directions -and periods when only islands existed, grow involved in their distribution, -and widely-different in different parts of the continent. The quantities of -rain which they discharge and of moisture which they absorb, vary -everywhere according to the proximity to the sea and to surfaces of land -having special characters. - -Other complications result from variations of height above the sea: -elevation producing a decrease of heat and consequently an increase in the -precipitation of water--a precipitation which takes the shape of snow where -the elevation is very great, and of rain where it is not so great. The -gatherings of clouds and descents of showers around mountain tops, are -familiar to every tourist. Inquiries in the neighbouring valleys prove that -within distances of a mile or two the recurring storms differ in their -frequency and violence. Nay, even a few yards off, the meteorological -conditions vary in such regions: as witness the way in which the condensing -vapour keeps eddying round on one side of some high crag, while the other -side is clear; or the way in which the snowline runs irregularly to -different heights, in all the hollows and ravines of each mountain side. - -As climatic variations thus geologically produced, are compounded with -those which result from slow astronomical changes; and as no correspondence -exists between the geologic and the astronomic rhythms; it results that the -same plexus of actions never recurs. Hence the incident forces to which the -organisms of every locality are exposed by atmospheric agencies, are ever -passing into unparalleled combinations; and these are on the average ever -becoming more complex. - - -§ 151. Besides changes in the incidence of inorganic forces, there are -equally continuous, and still more involved, changes in the incidence of -forces which organisms exercise on one another. As before pointed out -(§ 105), the plants and animals inhabiting each locality are held together -in so entangled a web of relations, that any considerable modification -which one species undergoes, acts indirectly on many other species, and -eventually changes, in some degree, the circumstances of nearly all the -rest. If an increase of heat, or modification of soil, or decrease of -humidity, causes a particular kind of plant either to thrive or to dwindle, -an unfavourable or favourable effect is wrought on all such competing kinds -of plants as are not immediately influenced in the same way. The animals -which eat the seeds or browse on the leaves, either of the plant primarily -affected or those of its competitors, are severally altered in their states -of nutrition and in their numbers; and this change presently tells on -various predatory animals and parasites. And since each of these secondary -and tertiary changes becomes itself a centre of others, the increase or -decrease of each species produces waves of influence which spread and -reverberate and re-reverberate throughout the whole Flora and Fauna of the -locality. - -More marked and multiplied still, are the ultimate effects of those causes -which make possible the colonization of neighbouring areas. Each intruding -plant or animal, besides the new inorganic conditions to which it is -subject, is subject to organic conditions different from those to which it -has been accustomed. It has to compete with some organisms unlike those of -its preceding habitat. It must preserve itself from enemies not before -encountered. Or it may meet with a species over which it has some advantage -greater than any it had over the species it was previously in contact with. -Even where migration does not bring it face to face with new competitors or -new enemies or new prey, it inevitably experiences new proportions among -these. Further, an expanding species is almost certain to invade more than -one adjacent region. Spreading both north and south, or east and west, it -will come among the plants and animals, here of a level district and there -of a hilly one--here of an inland tract and there of a tract bordered by -the sea. And while different groups of its members will thus expose -themselves to the actions and reactions of different Floras and Faunas, -these different Floras and Faunas will simultaneously have their organic -conditions changed by the intruders. - -This process becomes gradually more active and more complicated. Though, in -particular cases, a plant or animal may fall into simpler relations with -the living things around than those it was before placed in, yet it is -manifest that, on the average, the organic environments of organisms have -been advancing in heterogeneity. As the number of species with which each -species is directly or indirectly implicated, multiplies, each species is -oftener subject to changes in the organic actions which influence it. These -more frequent changes severally grow more involved. And the corresponding -reactions affect larger Floras and Faunas, in ways increasingly complex and -varied. - - -§ 152. When the astronomic, geologic, meteorologic, and organic agencies -which are at work on each species of plant and animal are contemplated as -becoming severally more complicated in themselves, and as co-operating in -ways that are always partially new; it will be seen that throughout all -time there has been an exposure of organisms to endless successions of -modifying causes which gradually acquire an intricacy scarcely conceivable. -Every kind of plant and animal may be regarded as for ever passing into a -new environment--as perpetually having its relations to external -circumstances altered, either by their changes with respect to it when it -remains stationary, or by its changes with respect to them when it -migrates, or by both. - -Yet a further cause of progressive alteration and complication in the -incident forces, exists. All other things continuing the same, every -additional faculty by which an organism is brought into relation with -external objects, as well as every improvement in such faculty, becomes a -means of subjecting the organism to a greater number and variety of -external stimuli, and to new combinations of external stimuli. So that each -advance in complexity of organization, itself becomes an added source of -complexity in the incidence of external forces. - -Once more, every increase in the locomotive powers of animals, increases -both the multiplicity and the multiformity of the actions of things upon -them, and of their reactions upon things. Doubling a creature's activity -quadruples the area that comes within the range of its excursions; thus -augmenting in number and heterogeneity, the external agencies which act on -it during any given interval. - -By compounding the actions of these several orders of factors, there is -produced a geometric progression of changes, increasing with immense -rapidity. And there goes on an equally rapid increase in the frequency with -which the combinations of the actions are altered, and the intricacies of -their co-operations enhanced. - - - - -CHAPTER X. - -INTERNAL FACTORS. - - -§ 153. We saw at the outset (§§ 10-16), that organic matter is built up of -molecules so unstable, that the slightest variation in their conditions -destroys their equilibrium, and causes them either to assume altered -structures or to decompose. But a substance which is beyond all others -changeable by the actions and reactions of the forces liberated from -instant to instant within its own mass, must be a substance which is beyond -all others changeable by the forces acting on it from without. If their -composition fits organic aggregates for undergoing with special facility -and rapidity those re-distributions of matter and motion whence result -individual organization and life; then their composition must make them -similarly apt to undergo those permanent re-distributions of matter and -motion which are expressed by changes of structure, in correspondence with -permanent re-distributions of matter and motion in their environments. - -In _First Principles_, when considering the phenomena of Evolution at -large, the leading characters and causes of those changes which constitute -organic evolution were briefly traced. Under each of the derivative laws of -force to which the passage from an incoherent, indefinite homogeneity to a -coherent, definite heterogeneity, conforms, were given illustrations drawn -from the metamorphoses of living bodies. Here it will be needful to -contemplate the several resulting processes as going on at once, in both -individuals and species. - - -§ 154. Our postulate being that organic evolution in general commenced with -homogeneous organic matter, we have first to remember that the state of -homogeneity is an unstable state (_First Principles_, § 149). In any -aggregate "the relations of outside and inside, and of comparative nearness -to neighbouring sources of influence, imply the reception of influences -that are unlike in quantity, or quality, or both; and it follows that -unlike changes will be produced in the parts thus dissimilarly acted upon." -Further, "if any given whole, instead of being absolutely uniform -throughout, consists of parts distinguishable from one another--if each of -these parts, while somewhat unlike other parts, is uniform within itself; -then, each of them being in unstable equilibrium, it follows that while the -changes set up within it must render it multiform, they must at the same -time render the whole more multiform than before;" and hence, "whether that -state with which we commence be or be not one of perfect homogeneity, the -process must equally be towards a relative heterogeneity." This loss of -homogeneity which the special instability of organic aggregates fits them -to display more promptly and variously than any other aggregates, must be -shown in more numerous ways in proportion as the incident forces are more -numerous. Every differentiation of structure being a result of some -difference in the relations of the parts to the agencies acting on them, it -follows that the more multiplied and more unlike the agencies, the more -varied must be the differentiations wrought. Hence the change from a state -of homogeneity to a state of heterogeneity, will be marked in proportion as -the environing actions to which the organism is supposes it is only are -complex. This transition from a uniform to a multiform state, must continue -through successive individuals. Given a series of organisms, each of which -is developed from a portion of a preceding organism, and the question is -whether, after exposure of the series for a million years to changed -incident forces, one of its members will be the same as though the incident -forces had only just changed. To say that it will, is implicitly to deny -the persistence of force. In relation to any cause of divergence, the whole -series of such organisms may be considered as fused together into a -continuously-existing organism; and when so considered, it becomes manifest -that a continuously-acting cause will go on working a -continuously-increasing effect, until some counteracting cause prevents any -further effect. - -But now if any primordial organic aggregate must, in itself and through its -descendants, gravitate from uniformity to multiformity, in obedience to the -more or less multiform forces acting on it; what must happen if these -multiform forces are themselves undergoing slow variations and -complications? Clearly the process, ever-advancing towards a temporary -limit but ever having its limit removed, must go on unceasingly. On those -structural changes wrought in the once homogeneous aggregate by an original -set of incident forces, will be superposed further changes wrought by a -modified set of incident forces; and so on throughout all time. Omitting -for the present those circumstances which check and qualify its -consequences, the instability of the homogeneous must be recognized as an -ever-acting cause of organic evolution, as of all other evolution. - -While it follows that every organism, considered as an individual and as -one of a series, tends thus to pass into a more heterogeneous state; it -also follows that every species, considered as an aggregate of individuals, -tends to do the like. Throughout the area it inhabits, the conditions can -never be absolutely uniform: its members must, in different parts of the -area, be exposed to different sets of incident forces. Still more decided -must this difference of exposure be when its members spread into other -habitats. Those expansive and repressive energies which set to each species -a limit that perpetually oscillates from side to side of a certain mean, -are, as we lately saw, frequently changed by new combinations of the -external factors--astronomic, geologic, meteorologic, and organic. Hence -there from time to time arise lines of diminished resistance, along which -the species flows into new localities. Such portions of the species as thus -migrate, are subject to circumstances unlike its previous average -circumstances. And from multiformity of the circumstances, must come -multiformity of the species. - -Thus the law of the instability of the homogeneous has here a three-fold -corollary. As interpreted in connexion with the ever-progressing, -ever-complicating changes in external factors, it involves the conclusion -that there is a prevailing tendency towards greater heterogeneity in all -kinds of organisms, considered both individually and in successive -generations; as well as in each assemblage of organisms constituting a -species; and, by consequence, in each genus, order, and class. - - -§ 155. When considering the causes of evolution in general, we further saw -(_First Principles_, § 156), that the multiplication of effects aids -continually to increase that heterogeneity into which homogeneity -inevitably lapses. It was pointed out that since "the several parts of an -aggregate are differently modified by any incident force;" and since "by -the reactions of the differently modified parts the incident force itself -must be divided into differently modified parts;" it follows that "each -differentiated division of the aggregate thus becomes a centre from which a -differentiated division of the original force is again diffused. And since -unlike forces must produce unlike results, each of these differentiated -forces must produce, throughout the aggregate, a further series of -differentiations." To this it was added that, in proportion as the -heterogeneity increases, the complications arising from this multiplication -of effects grow more marked; because the more strongly contrasted the parts -of an aggregate become, the more different must be their reactions on -incident forces, and the more unlike must be the secondary effects which -these initiate; and because every increase in the number of unlike parts -adds to the number of such differentiated incident forces, and such -secondary effects. - -How this multiplication of effects conspires, with the instability of the -homogeneous, to work an increasing multiformity of structure in an -organism, was shown at the time; and the foregoing pages contain further -incidental illustrations. In § 69 it was pointed out that a change in one -function must produce ever-complicating perturbations in other functions; -and that, eventually, all parts of the organism must be modified in their -states. Suppose that the head of a bison becomes much heavier, what must be -the indirect results? The muscles of the neck are put to greater exertions; -and its vertebræ have to bear additional tensions and pressures, caused -both by the increased weight of the head, and by the stronger contractions -of the muscles that support and move it. These muscles also affect their -special attachments: several of the dorsal spines suffer augmented strains; -and the vertebræ to which they are fixed are more severely taxed. Further, -this heavier head and the more massive neck it necessitates, require a -stronger fulcrum: the whole thoracic arch, and the fore-limbs which support -it, are subject to greater continuous stress and more violent occasional -shocks. And the required strengthening of the fore-quarters cannot take -place without the centre of gravity being changed, and the hind limbs being -differently reacted upon during locomotion. Any one who compares the -outline of the bison with that of its congener, the ox, will see how -profoundly a heavier head affects the entire osseous and muscular systems. -Besides this multiplication of mechanical effects, there is a -multiplication of physiological effects. The vascular apparatus is modified -throughout its whole structure by each considerable modification in the -proportions of the body. Increase in the size of any organ implies a -quantitative, and often a qualitative, reaction on the blood; and thus -alters the nutrition of all other organs. Such physiological correlations -are exemplified in the many differences which accompany difference of sex. -That the minor sexual peculiarities are brought about by the physiological -actions and reactions, is shown both by the fact that they are commonly but -faintly marked until the fundamentally distinctive organs are developed, -and that when the development of these is prevented, the minor sexual -peculiarities do not arise. No further proof is, I think, needed, that in -any individual organism or its descendants, a new external action must, -besides the primary internal change which it works, work many secondary -changes, as well as tertiary changes still more multiplied. That tendency -towards greater heterogeneity which is given to an organism by disturbing -its environment, is helped by the tendency which every modification has to -produce other modifications--modifications which must become more numerous -in proportion as the organism becomes more complex. Lastly, among the -indirect and involved manifestations of this tendency, we must not omit the -innumerable small irregularities of structure which result from the -crossing of dissimilarly-modified individuals. It was shown (§§ 89, 90) -that what are called "spontaneous variations," are interpretable as results -of miscellaneously compounding the changes wrought in different lines of -ancestors by different conditions of life. These still more complex and -multitudinous effects so produced, are further illustrations of the -multiplication of effects. - -Equally in the aggregate of individuals constituting a species, does -multiplication of effects become the continual cause of increasing -multiformity. The lapse of a species into divergent varieties, initiates -fresh combinations of forces tending to work further divergences. The new -varieties compete with the parent species in new ways; and so add new -elements to its circumstances. They modify somewhat the conditions of other -species existing in their habitat, or in the habitat they have invaded; and -the modifications wrought in such other species become additional sources -of influence. The Flora and Fauna of every region are united by their -entangled relations into a whole, of which no part can be affected without -affecting the rest. Hence, each differentiation in a local assemblage of -species, becomes the cause of further differentiations. - - -§ 156. One of the universal principles to which we saw that the -re-distribution of matter and motion conforms, is that in any aggregate -made up of mixed units, incident forces produce segregation--separate -unlike units and bring together like units; and it was shown that the -increasing integration and definiteness which characterizes each part of an -evolving organic aggregate, as of every other aggregate, results from this -(_First Principles_, § 166). It remains here to say that while the actions -and reactions between organisms and their changing environments, add to the -heterogeneity of organic structures, they also give to the heterogeneity -this growing distinctness. At first sight the reverse might be inferred. It -might be argued that any new set of effects wrought in an organism by some -new set of external forces, must tend more or less to obliterate the -effects previously wrought--must produce confusion or indefiniteness. A -little consideration, however, will dissipate this impression. - -Doubtless the condition under which alone increasing definiteness of -structure can be acquired by any part of an organism, either in an -individual or in successive generations, is that such part shall be exposed -to some set of tolerably-constant forces; and doubtless, continual change -of circumstances interferes with this. But the interference can never be -considerable. For the pre-existing structure of an organism prevents it -from living under any new conditions except such as are congruous with the -fundamental characters of its organization--such as subject its essential -organs to actions substantially the same as before. Great changes must kill -it. Hence, it can continuously expose itself and its descendants, only to -those moderate changes which do not destroy the general harmony between the -aggregate of incident forces and the aggregate of its functions. That is, -it must remain under influences calculated to make greater the definiteness -of the chief differentiations already produced. If, for example, we set out -with an animal in which a rudimentary vertebral column with its attached -muscular system has been established; it is clear that the mechanical -arrangements have become thereby so far determined, that subsequent -modifications are extremely likely, if not certain, to be consistent with -the production of movement by the actions of muscles on a flexible central -axis. Hence, there will continue a general similarity in the play of forces -to which the flexible central axis is subject; and so, notwithstanding the -metamorphoses which the vertebrate type undergoes, there will be a -maintenance of conditions favourable to increasing definiteness and -integration of the vertebral column. Moreover, this maintenance of such -conditions becomes secure in proportion as organization advances. Each -further complexity of structure, implying some further complexity in the -relations between an organism and its environment, must tend to specialize -the actions and reactions between it and its environment--must tend to -increase the stringency with which it is restrained within such -environments as admit of those special actions and reactions for which its -structure fits it; that is, must further guarantee the continuance of those -actions and reactions to which its essential organs respond, and therefore -the continuance of the segregating process. - -How in each species, considered as an aggregate of individuals, there must -arise stronger and stronger contrasts among those divergent varieties which -result from the instability of the homogeneous and the multiplication of -effects, need only be briefly indicated. It has already been shown (_First -Principles_, § 166), that in conformity to the universal law that mixed -units are segregated by like incident forces, there are produced -increasingly-definite distinctions among varieties, wherever there occur -definitely-distinguished sets of conditions to which the varieties are -respectively subject. - - -§ 157. Probably in the minds of some, the reading of this chapter has been -accompanied by a running commentary, to the effect that the argument proves -too much. The apparent implication is, that the passage from an indefinite, -incoherent homogeneity to a definite, coherent heterogeneity in organic -aggregates, must have been going on universally; whereas we find that in -many cases there has been persistence without progression. This apparent -implication, however, is not a real one. - -For though every environment on the Earth's surface undergoes changes; and -though usually the organisms which each environment contains, cannot escape -certain resulting new influences; yet occasionally such new influences are -escaped, by the survival of species in the unchanged parts of their -habitats, or by their spread into neighbouring habitats which the change -has rendered like their original habitats, or by both. Any alteration in -the temperature of a climate or its degree of humidity, is unlikely to -affect simultaneously the whole area occupied by a species; and further, it -can scarcely fail to happen that the addition or subtraction of heat or -moisture, will give to a part of some adjacent area, a climate like that to -which the species has been habituated. If, again, the circumstances of a -species are modified by the intrusion of some foreign kind of plant or -animal, it follows that since the intruders will probably not spread -throughout its whole habitat, the species will, in one or more localities, -remain unaffected by them. Especially among marine creatures, must there -frequently occur cases in which modifying causes are continually eluded. -Comparatively uniform as are the physical conditions to which the sea -exposes its inhabitants, it becomes possible for such of them as live on -widely-diffused food, to be widely distributed; and wide distribution -generally prevents the members of a species from being all subject to the -same cause. Our commonest cirriped, for instance, subsisting on minute -creatures everywhere dispersed through the water; needing only to have some -firm surface on which to build up its shell; and in scarcely any danger -from surrounding animals; is able to exist on shores so widely remote from -one another, that nearly every change in the incident forces must fall -within narrower areas than that which the species occupies. Nearly always, -therefore, a portion of the species will survive unmodified. Its -easily-transported germs will take possession of such new habitats as have -been rendered fitter by the change that has unfitted some parts of its -original habitat. Hence, on successive occasions, while some parts of the -species are slightly transformed, another part may continually escape -transformation by migrating hither and thither, where the simple conditions -needed for its existence recur in nearly the same combinations as before. -And it will so become possible for it to survive, with insignificant -structural changes, throughout long geologic periods. - - -§ 158. The results to which we find ourselves led, are these. - -In subordination to the different amounts and kinds of forces to which its -different parts are exposed, every individual organic aggregate, like all -other aggregates, tends to pass from its original indistinct simplicity -towards a more distinct complexity. Unless we deny the persistence of -force, we must admit that the lapse of an organism's structure from an -indefinitely homogeneous to a definitely heterogeneous state, must be -cumulative in successive generations, if the forces causing it continue to -act. And for the like reasons, the increasing assemblage of individuals -arising from a common stock, is also liable to lose its original -uniformity; and, in successive generations, to grow more pronounced in its -multiformity. - -These changes, which would go to but a comparatively small extent were -organisms exposed to constant external conditions, are kept up by the -continual changes in external conditions, produced by astronomic, geologic, -meteorologic, and organic agencies: the average result being, that on -previous complications wrought by previous incident forces, new -complications are continually superposed by new incident forces. And hence -simultaneously arises increasing heterogeneity in the structures of -individuals, in the structures of species, and in the structures of the -Earth's Flora and Fauna. - -But while, in very many or in most cases, the ever-changing incidence of -forces is ever adding to the complexity of organisms, and to the complexity -of the organic world as a whole; it does this only where its action cannot -be eluded. And since, by migration, it is possible for a species to keep -itself under conditions that are tolerably constant, there must be a -proportion of cases in which greater heterogeneity of structure is not to -be expected. - -To show, however, that there must arise a certain average tendency to the -production of greater heterogeneity is not sufficient. Aggregates might be -rendered more heterogeneous by changing incident forces, without having -given to them that kind of heterogeneity required for carrying on life. -Hence it remains now to inquire how the production and maintenance of this -kind of heterogeneity is insured. - - - - -CHAPTER XI. - -DIRECT EQUILIBRATION. - - -§ 159. Every change is towards a balance of forces; and of necessity can -never cease until a balance of forces is reached. When treating of -equilibration under its general aspects (_First Principles_, Part II., -Chap. xxii.), we saw that every aggregate having compound movements tends -continually towards a moving equilibrium; since any unequilibrated force to -which such an aggregate is subject, if not of a kind to overthrow it -altogether, must continue modifying its state until an equilibrium is -brought about. And we saw that the structure simultaneously reached must be -"one presenting an arrangement of forces that counterbalance all the forces -to which the aggregate is subject;" since, "so long as there remains a -residual force in any direction--be it excess of a force exercised by an -aggregate on its environment, or of a force exercised by its environment on -the aggregate, equilibrium does not exist; and therefore the -re-distribution of matter must continue." - -It is essential that this truth should here be fully comprehended; and to -the end of insuring clear comprehension of it, some re-illustration is -desirable. The case of the Solar System will best serve our purpose. An -assemblage of bodies, each of which has its simple and compound motions -that severally alternate between two extremes, and the whole of which has -its involved perturbations, that now increase and now decrease, is here -presented to us. Suppose a new factor were brought to bear on this moving -equilibrium--say by the arrival of some wandering mass, or by an additional -momentum given to one of the existing masses--what would be the result? If -the strange body or the extra energy were very large, it might so derange -the entire system as to cause its collapse. But what if the incident -energy, falling on the system from without, proved insufficient to -overthrow it? There would then arise a set of perturbations which would, in -the course of an enormous period, slowly work round into a modified moving -equilibrium. The effects primarily impressed on the adjacent masses, and in -a smaller degree on the remoter masses, would presently become complicated -with the secondary effects impressed by the disturbed masses on one -another; and these again with tertiary effects. Waves of perturbation would -continue to be propagated throughout the entire system; until, around a new -centre of gravity, there had been established a set of planetary motions -different from the preceding ones. The new energy must gradually be used up -in overcoming the energies resisting the divergence it generates; which -antagonizing energies, when no longer opposed, set up a counter-action, -ending in a compensating divergence in the opposite direction, followed by -a re-compensating divergence, and so on. Now though instead of being, like -the Solar System, in a state of _independent_ moving equilibrium, an -organism is in a state of _dependent_ moving equilibrium (_First -Principles_, § 170); yet this does not prevent the manifestation of the -same law. Every animal daily obtains from without, a supply of energy to -replace the energy it expends; but this continual giving to its parts a new -momentum, to make up for the momentum continually lost, does not interfere -with the carrying on of actions and reactions like those just described. -Here, as before, we have a definitely-arranged aggregate of parts, called -organs, having their definitely-established actions and reactions, called -functions. These rhythmical actions or functions, and the various compound -rhythms resulting from their combinations, are so adjusted as to balance -the actions to which the organism is subject: there is a constant or -periodic genesis of energies which, in their kinds, amounts, and -directions, suffice to antagonize the energies the organism has constantly -or periodically to bear. If, then, there exists this moving equilibrium -among a set of internal actions, exposed to a set of external actions, what -must result if any of the external actions are changed? Of course there is -no longer an equilibrium. Some energy which the organism habitually -generates, is too great or too small to balance some incident energy; and -there arises a residual energy exerted by the environment on the organism, -or by the organism on the environment. This residual or unbalanced energy, -of necessity expends itself in producing some change of state in the -organism. Acting directly on some organ and modifying its function, it -indirectly modifies dependent functions and remotely influences all the -functions. As we have already seen (§§ 68, 69), if this new energy is -permanent, its effects must be gradually diffused throughout the entire -system; until it has come to be equilibrated in producing those structural -rearrangements whence result a counter-balancing energy. - -The bearing of this general truth on the question we are now dealing with -is obvious. Those modifications upon modifications, which the unceasing -mutations of their environments have been all along generating in -organisms, have been in each case modifications involved by the -establishment of a new balance with the new combination of actions. In -every species throughout all geologic time, there has been perpetually -going on a rectification of the equilibrium, which has been perpetually -disturbed by the alteration of its circumstances; and every further -heterogeneity has been the addition of a structural change entailed by a -new equilibration, to the structural changes entailed by previous -equilibrations. There can be no other ultimate interpretation of the -matter, since change can have no other goal. - -This equilibration between the functions of an organism and the actions in -its environment, may be either direct or indirect. The new incident force -may either immediately call forth some counteracting force, and its -concomitant structural change; or it may be eventually balanced by some -otherwise-produced change of function and structure. These two processes of -equilibration are quite distinct, and must be separately dealt with. We -will devote this chapter to the first of them. - - -§ 160. Direct equilibration is that process currently known as -_adaptation_. We have already seen (Part II., Chap, v.), that individual -organisms become modified when placed in new conditions of life--so -modified as to re-adjust the powers to the requirements; and though there -is great difficulty in disentangling the evidence, we found reason for -thinking (§ 82) that structural changes thus caused by functional changes -are inherited. In the last chapter, it was argued that if, instead of the -succession of individuals constituting a species, there were a -continuously-existing individual, any functional and structural divergence -produced by a new incident action, would increase until the new incident -action was counterpoised; and that the replacing of a continuously-existing -individual by a succession of individuals, each formed out of the modified -substance of its predecessor, will not prevent the like effect from being -produced. Here we further find that this limit towards which any such -organic change advances, in the species as in the individual, is a new -moving equilibrium adjusted to the new arrangement of external forces. - -But now what are the conditions under which alone, direct equilibration can -occur? Are all the modifications that serve to re-fit organisms to their -environments, directly adaptive modifications? And if otherwise, which are -the directly adaptive and which are not? How are we to distinguish between -them? - -There can be no direct equilibration with an external agency which, if it -acts at all, acts fatally; since the organism to be adapted disappears. -Conversely, some inaccessible benefit which a small modification in the -organism would make accessible, cannot by its action tend to produce this -modification: the modification and the benefit do not stand in dynamic -relation. The only new incident forces which can work the changes of -function and structure required to bring any animal or plant into -equilibrium with them, are such incident forces as operate on this animal -or plant, either continuously or frequently. They must be capable of -appreciably changing that set of complex rhythmical actions and reactions -constituting the life of the organism; and yet must not usually produce -perturbations that are fatal. Let us see what are the limits to direct -equilibration hence arising. - - -§ 161. In plants, organs engaged in nutrition, and exposed to variations in -the amounts and proportions of matters and forces utilized in nutrition, -may be expected to undergo corresponding variations. We find evidence that -they do this. The "changes of habit" which are common in plants, when taken -to places unlike in climate or soil to those before inhabited by them, are -changes of parts in which the modified external actions directly produce -modified internal actions. The characters of the stem and shoots as woody -or succulent, erect or procumbent; of the leaves in respect of their sizes, -thicknesses, and textures; of the roots in their degrees of development and -modes of growth; are obviously in immediate relation to the characters of -the environment. A permanent difference in the quantity of light or heat -affects, day after day, the processes going on in the leaves. Habitual rain -or drought alters all the assimilative actions, and appreciably influences -the organs that carry them on. Some particular substance, by its presence -in the soil, gives new qualities to some of the tissues; causing greater -rigidity or flexibility, and so affecting the general aspect. Here then we -have changes towards modified sets of functions and structures, in -equilibrium with modified sets of external forces. - -But now let us turn to other classes of organs possessed by plants--organs -which are not at once affected in their actions by variations of incident -forces. Take first the organs of defence. Many plants are shielded against -animals that would else devour them, by formidable thorns; and others, like -the nettle, by stinging hairs. These must be counted among the appliances -by which equilibrium is maintained between the actions in the organism and -the actions in its environment; seeing that were these defences absent, the -destruction by herbivorous animals would be so much increased, that the -number of young plants annually produced would not suffice, as it now does, -to balance the mortality, and the species would disappear. But these -defensive appliances, though they aid in maintaining the balance between -inner and outer actions, cannot have been directly called forth by the -outer actions which they serve to neutralize; for these outer actions do -not continuously affect the functions of the plant even in a general way, -still less in the special way required. Suppose a species of nettle bare of -poison-hairs, to be habitually eaten by some mammal intruding on its -habitat. The actions of this mammal would have no direct tendency to -develop poison-hairs in the plant; since the individuals devoured could not -bequeath changes of structure, even were the actions of a kind to produce -fit ones; and since the individuals which perpetuated themselves would be -those on which the new incident force had not fallen. Organs of another -class, similarly circumstanced, are those of reproduction. Like the organs -of defence these are not, during the life of the individual plant, variably -exercised by variable external actions; and therefore do not fulfil those -conditions under which structural changes may be directly caused by changes -in the environment. The generative apparatus contained in every flower acts -only once during its existence; and even then, the parts subserve their -ends in a passive rather than an active way. Functionally-produced -modifications are therefore out of the question. If a plant's anthers are -so placed that the insect which most commonly frequents its flowers, must -come in contact with the pollen, and fertilize with it other flowers of the -same species; and if this insect, dwindling away or disappearing from the -locality, leaves behind no insects having such shapes and habits as cause -them to do the same thing efficiently, but only some which do it -inefficiently; it is clear that this change of its conditions has no -immediate tendency to work in the plant any such structural change as shall -bring about a new balance with its conditions. For the anthers, which, even -when they discharge their functions, do it simply by standing in the way of -the insect, are, under the supposed circumstances, left untouched by the -insect; and this remaining untouched cannot have the effect of so modifying -the stamens as to bring the anthers into a position to be touched by some -other insect. Only those individuals whose parts of fructification so far -differed from the average form that some other insect could serve them as -pollen-carrier, would have good chances of perpetuating themselves. And on -their progeny, inheriting the deviation, there would act no external force -directly tending to make the deviation greater; since the new circumstances -to which re-adaptation is required, are such as do not in the least alter -the equilibrium of functions constituting the life of the individual plant. - - -§ 162. Among animals, adaptation by direct equilibration is similarly -traceable wherever, during the life of the individual, an external change -generates some constant or repeated change of function. This is -conspicuously the case with such parts of an animal as are immediately -exposed to diffused influences, like those of climate, and with such parts -of an animal as are occupied in its mechanical actions on the environment. -Of the one class of cases, the darkening of the skin which follows exposure -to one or other extreme of temperature, may be taken as an instance; and -with the other class of cases we are made familiar by the increase and -decrease which use and disuse cause in the organs of motion. It is needless -here to exemplify these: they were treated of in the Second Part of this -work. - -But in animals, as in plants, there are many indispensable offices -fulfilled by parts between which and the external conditions they respond -to, there is no such action and reaction as can directly produce an -equilibrium. This is especially manifest with dermal appendages. Some -ground exists for the conclusion that the greater or less development of -hairs, is in part immediately due to increase or decrease of demand on the -passive function, as forming a non-conducting coat; but be this as it may, -it is impossible that there can exist any such cause for those immense -developments of hairs which we see in the quills of the porcupine, or those -complex developments of them known as feathers. Such an enamelled armour as -is worn by _Lepidosteus_, is inexplicable as a direct result of any -functionally-worked change. For purposes of defence, such an armour is as -needful, or more needful, for hosts of other fishes; and did it result from -any direct reaction of the organism against any offensive actions it was -subject to, there seems no reason why other fishes should not have -developed similar protective coverings. Of sundry reproductive appliances -the like may be said. The secretion of an egg-shell round the substance of -an egg, in the oviduct of a bird, is quite inexplicable as a consequence of -some functionally-wrought modification of structure, immediately caused by -some modification of external conditions. The end fulfilled by the -egg-shell, is that of protecting the contained mass against certain slight -pressures and collisions, to which it is liable during incubation. How, by -any process of direct equilibration, could it come to have the required -thickness? or, indeed, how could it come to exist at all? Suppose this -protective envelope to be too weak, so that some of the eggs a bird lays -are broken or cracked. In the first place, the breakages or crackings are -actions which cannot react on the maternal organism in such ways as to -cause the secretion of thicker shells for the future: to suppose that they -can, is to suppose that the bird understands the cause of the evil, and -that the secretion of thicker shells can be effected by its will. In the -second place, such developing chicks as are contained in the shells which -crack or break, are almost certain to die; and cannot, therefore, acquire -appropriately-modified constitutions: even supposing any relation could -exist between the impression received and the change required. Meanwhile, -such eggs as escape breakage are not influenced at all by the requirement; -and hence, on the birds developed from them, there cannot have acted any -force tending to work the needful adjustment of functions. In no way, -therefore, can a direct equilibration between constitution and conditions -be here produced. Even in organs that can be modified by certain incident -actions into correspondence with such incident actions, there are some -re-adjustments which cannot be effected by direct balancing. It is thus -with the bones. The majority of the bones have to resist muscular strains; -and variations in the muscular strains call forth, by reaction, variations -in the strengths of the bones. Here there is direct equilibration. But -though the greater massiveness acquired by bones subject to greater -strains, may be ascribed to counter-acting forces evoked by forces brought -into action; it is impossible that the acquirement of greater lengths by -bones can be thus accounted for. It has been supposed that the elongation -of the metatarsals in wading birds, has resulted from direct adaptation to -conditions of life. To justify this supposition, however, it must be shown -that the mechanical actions and reactions in the legs of a wading bird, -differ from those in the legs of other birds; and that the differential -actions are equilibrated by the extra lengths. There is not the slightest -evidence of this. The metatarsals of a bird have to bear no appreciable -strains but those due to the superincumbent weight. Standing in the water -does not appreciably alter such strains; and even if it did, an increase in -the lengths of these bones would not fit them any better to meet the -altered strains. - - -§ 163. The conclusion at which we arrive is, then, that there go on in all -organisms, certain changes of function and structure that are directly -consequent on changes in the incident forces--inner changes by which the -outer changes are balanced, and the equilibrium restored. Such -re-equilibrations, which are often conspicuously exhibited in individuals, -we have reason to believe continue in successive generations; until they -are completed by the arrival at structures fitted to the modified -conditions. But, at the same time, we see that the modified conditions to -which organisms may be adapted by direct equilibration, are conditions of -certain classes only. That a new external action may be met by a new -internal action, it is needful that it shall either continuously or -frequently be borne by the individuals of the species, without killing or -seriously injuring them; and shall act in such way as to affect their -functions. And we find that many of the environing agencies--evil or -good--to which organisms have to be adjusted, are not of these kinds: being -agencies which either do not immediately affect the functions at all, or -else affect them in ways that prove fatal. - -Hence there must be at work some other process which equilibrates the -actions of organisms with the actions they are exposed to. Plants and -animals that continue to exist, are necessarily plants and animals whose -powers balance the powers acting on them; and as their environments change, -the changes which plants and animals undergo must necessarily be changes -towards re-establishment of the balance. Besides direct equilibration, -there must therefore be an indirect equilibration. How this goes on we have -now to inquire. - - - - -CHAPTER XII. - -INDIRECT EQUILIBRATION. - - -§ 164. Besides those perturbations produced in any organism by special -disturbing forces, there are ever going on many others--the reverberating -effects of disturbing forces previously experienced by the individual, or -by ancestors; and the multiplied deviations of function so caused imply -multiplied deviations of structure. In § 155 there was re-illustrated the -truth, set forth at length when treating of Adaptation (§ 69), that an -organism in a state of moving equilibrium, cannot have extra function -thrown on any organ, and extra growth produced in such organ, without -correlative changes being entailed throughout all other functions, and -eventually throughout all other organs. And when treating of Variation -(§ 90), we saw that individuals which have been made, by their different -circumstances, to deviate functionally and structurally from the average -type in different directions, will bequeath to their joint offspring, -compound perturbations of function and compound deviations of structure, -endlessly varied in their kinds and amounts. - -Now if the individuals of a species are thus necessarily made unlike in -countless ways and degrees--if in one individual the amount of energy in a -particular direction is greater than in any other individual, or if here a -peculiar combination gives a resulting action which is not found elsewhere; -then, among all the individuals, some will be less liable than others to -have their equilibria overthrown by a particular incident force previously -unexperienced. Unless the change in the environment is so violent as to be -universally fatal to the species, it must affect more or less differently -the slightly-different moving equilibria which the members of the species -present. Inevitably some will be more stable than others when exposed to -this new or altered factor. That is to say, those individuals whose -functions are most out of equilibrium with the modified aggregate of -external forces, will be those to die; and those will survive whose -functions happen to be most nearly in equilibrium with the modified -aggregate of external forces. - -But this survival of the fittest[52] implies multiplication of the fittest. -Out of the fittest thus multiplied there will, as before, be an -overthrowing of the moving equilibrium wherever it presents the least -opposing force to the new incident force. And by the continual destruction -of the individuals least capable of maintaining their equilibria in -presence of this new incident force, there must eventually be reached an -altered type completely in equilibrium with the altered conditions. - - -§ 165. This survival of the fittest, which I have here sought to express in -mechanical terms, is that which Mr. Darwin has called "natural selection, -or the preservation of favoured races in the struggle for life." That there -goes on a process of this kind throughout the organic world, Mr. Darwin's -great work on the _Origin of Species_ has shown to the satisfaction of -nearly all naturalists. Indeed, when once enunciated, the truth of his -hypothesis is so obvious as scarcely to need proof. Though evidence may be -required to show that natural selection accounts for everything ascribed to -it, yet no evidence is required to show that natural selection has always -been going on, is going on now, and must ever continue to go on. -Recognizing this as an _à priori_ certainty, let us contemplate it under -its two distinct aspects. - -That organisms which live, thereby prove themselves fit for living, in so -far as they have been tried, while organisms which die, thereby prove -themselves in some respects unfitted for living, are facts no less manifest -than is the fact that this self-purification of a species must tend ever to -insure adaptation between it and its environment. This adaptation may be -either so _maintained_ or so _produced_. Doubtless many who have looked at -Nature with philosophic eyes, have observed that death of the worst and -multiplication of the best, tends towards maintenance of a constitution in -harmony with surrounding circumstances. That the average vigour of any race -would be diminished did the diseased and feeble habitually survive and -propagate; and that the destruction of such, through failure to fulfil some -of the conditions to life, leaves behind those which are able to fulfil the -conditions to life, and thus keeps up the average fitness to the conditions -of life; are almost self-evident truths. But to recognize "Natural -Selection" as a means of preserving an already-established balance between -the powers of a species and the forces to which it is subject, is to -recognize only its simplest and most general mode of action. It is the more -special mode of action with which we are here concerned. This more special -mode of action, Mr. Darwin has been the first to recognize as an -all-important factor, though, besides his co-discoverer Mr. A. R. Wallace, -some others have perceived that such a factor is at work. To him we owe due -appreciation of the fact that natural selection is capable of _producing_ -fitness between organisms and their circumstances. He has worked up an -enormous mass of evidence showing that this "preservation of favoured races -in the struggle for life," is an ever-acting cause of divergence among -organic forms. He has traced out the involved results of the process with -marvellous subtlety. He has shown how hosts of otherwise inexplicable -facts, are accounted for by it. In brief, he has proved that the cause he -alleges is a true cause; that it is a cause which we see habitually in -action; and that the results to be inferred from it are in harmony with the -phenomena which the Organic Creation presents, both as a whole and in its -details. Let us glance at a few of the more important interpretations which -the hypothesis furnishes. - -A soil possessing some ingredient in unusual quantity, may supply to a -plant an excess of the matter required for certain of its tissues; and may -cause all the parts formed of such tissues to be abnormally developed. -Suppose that among these are the hairs clothing its surfaces, including -those which grow on its seeds. Thus furnished with somewhat longer fibres, -its seeds, when shed, are carried a little further by the wind before they -fall to the ground. The plants growing from them, being rather more widely -dispersed than those produced by other individuals of the same species, -will be less liable to smother one another; and a greater number may -therefore reach maturity and fructify. Supposing the next generation -subject to the same peculiarity of nutrition, some of the seeds borne by -its members will not simply inherit this increased development of hairs, -but will carry it further; and these, still more advantaged in the same way -as before, will, on the average, have still more numerous chances of -continuing the race. Thus, by the survival, generation after generation, of -those possessing these longer hairs, and the inheritance of successive -increments of growth in the hairs, there may result a seed deviating -greatly from the original. Other individuals of the same species, subject -to the different physical conditions of other localities, may develop -somewhat thicker or harder coatings to their seeds: so rendering their -seeds less digestible by the birds which devour them. Such thicker-coated -seeds, by escaping undigested more frequently than thinner-coated ones, -will have additional chances of growing and leaving offspring; and this -process, acting in a cumulative manner season after season, will produce a -seed diverging in another direction from the ancestral type. Again, -elsewhere, some modification in the physiologic actions of the plant may -lead to an unusual secretion of an essential oil in the seeds; rendering -them unpalatable to creatures which would otherwise feed on them: so giving -an advantage to the variety in its rate of multiplication. This incidental -peculiarity, proving a preservative, will, as before, be increased by -natural selection until it constitutes another divergence. Now in such -cases, we see that plants may become better adapted, or re-adapted, to the -aggregate of surrounding agencies, not through any _direct_ action of such -agencies on them, but through their _indirect_ action--through the -destruction by them of the individuals least congruous with them, and the -survival of those most congruous with them. All these slight variations of -function and structure, arising among the members of a species, serve as so -many experiments; the great majority of which fail, but a few of which -succeed. Just as each plant bears a multitude of seeds, out of which some -two or three happen to fulfil all the conditions required for reaching -maturity and continuing the race; so each species is ever producing -numerous slightly-modified forms, deviating in all directions from the -average, out of which most fit the surrounding conditions no better than -their parents, or not so well, but some few of which fit the conditions -better; and, doing so, are enabled the better to preserve themselves, and -to produce offspring similarly capable of preserving themselves. Among -animals the like process results in the like development of various -structures which cannot have been affected by the performance of -functions--their functions being purely passive. The thick shell of a -mollusk cannot have arisen from direct reactions of the organism against -the external actions to which it is exposed; but it is quite explicable as -an effect of the survival, generation after generation, of individuals -whose thicker coverings protected them against enemies. Similarly with such -dermal structure as that of the tortoise. Though we have evidence that the -skin, where it is continually exposed to pressure and friction, may -thicken, and so re-establish the equilibrium by opposing a greater inner -force to a greater outer force; yet we have no evidence that a coat of -armour like that of the tortoise can be so produced. Nor, indeed, are the -conditions under which alone its production in such a manner could be -accounted for, fulfilled; since the surface of the tortoise is not exposed -to greater pressure and friction than the surfaces of other creatures. This -massive carapace, and the strangely-adapted osseous frame-work which -supports it, are inexplicable as results of evolution, unless through the -process of natural selection. So, too, is it with the formation of -odoriferous glands in some mammals, or the growth of such excrescences as -those of the camel. Thus, in short, is it with all those organs of animals -which do not play active parts. - -Besides giving us explanations of structural characters that are otherwise -unaccountable, Mr. Darwin shows how natural selection explains peculiar -relations between individuals in certain species. Such facts as the -dimorphism of the primrose and other flowers, he proves to be in harmony -with his hypothesis, though stumbling-blocks to all other hypotheses. The -various differences which accompany difference of sex, sometimes slight, -sometimes very great, are similarly accounted for. As before suggested -(§ 79), natural selection appears capable of producing and maintaining the -right proportion of the sexes in each species; and it requires but to -contemplate the bearings of the argument, to see that the formation of -different sexes may itself have been determined in the same way. - -To convey here an adequate idea of Mr. Darwin's doctrine, throughout the -immense range of its applications, is of course impossible. The few -illustrations just given, are intended simply to remind the reader what Mr. -Darwin's hypothesis is, and what are the else insoluble problems which it -solves for us. - - -§ 166. But now, though it seems to me that we are thus supplied with a key -to phenomena which are multitudinous and varied beyond all conception; it -also seems to me that there is a moiety of the phenomena which this key -will not unlock. Mr. Darwin himself recognizes use and disuse of parts, as -causes of modifications in organisms; and does this, indeed, to a greater -extent than do some who accept his general conclusion. But I conceive that -he does not recognize them to a sufficient extent. While he shows that the -inheritance of changes of structure caused by changes of function, is -utterly insufficient to explain a great mass--probably the greater mass--of -morphological phenomena; I think he leaves unconsidered a mass of -morphological phenomena which are explicable as results of -functionally-produced modifications, and are not explicable as results of -natural selection. - -By induction, as well as by inference from the hypothesis of natural -selection, we know that there exists a balance among the powers of organs -which habitually act together--such proportions among them that no one has -any considerable excess of efficiency. We see, for example, that throughout -the vascular system there is maintained an equilibrium of the component -parts: in some cases, under continued excess of exertion, the heart gives -way, and we have enlargement; in other cases the large arteries give way, -and we have aneurisms; in other cases the minute blood-vessels give -way--now bursting, now becoming chronically congested. That is to say, in -the average constitution, no superfluous strength is possessed by any of -the appliances for circulating the blood. Take, again, a set of motor -organs. Great strain here causes the fibres of a muscle to tear. There the -muscle does not yield but the tendon snaps. Elsewhere neither muscle nor -tendon is damaged, but the bone breaks. Joining with these instances the -general fact that, under the same adverse conditions, different individuals -show their slight differences of constitution by going wrong some in one -way and some in another; and that even in the same individual, similar -adverse conditions will now affect one viscus and now another; it becomes -manifest that though there cannot be maintained an accurate balance among -the powers of the organs composing an organism, yet their excesses and -deficiencies of power are extremely slight. That they must be extremely -slight, is, as before said, a deduction from the hypothesis of natural -selection. Mr. Darwin himself argues "that natural selection is continually -trying to economise in every part of the organization. If under changed -conditions of life a structure before useful becomes less useful, any -diminution, however slight, in its development, will be seized on by -natural selection, for it will profit the individual not to have its -nutriment wasted in building up an useless structure." In other words, if -any muscle has more fibres than are required, or if a bone is stronger than -needful, no advantage results but rather a disadvantage--a disadvantage -which will decrease the chances of survival. Hence it follows that among -any organs which habitually act in concert, an increase of one can be of no -service unless there is a concomitant increase of the rest. The -co-operative parts must vary together; otherwise variation will be -detrimental. A stronger muscle must have a stronger bone to resist its -contractions; must have stronger correlated muscles and ligaments to secure -the neighbouring articulations; must have larger blood-vessels to bring it -supplies; must have a more massive nerve to transmit stimulus, and some -extra development of a nervous centre to supply the extra stimulus. The -question arises, then,--do variations of the appropriate kinds occur -simultaneously in all these co-operative parts? Have we any reason to think -that the parts spontaneously increase or decrease together? The assumption -that they do seems to me untenable; and its untenability will, I think, -become conspicuous if we take a case, and observe how extremely numerous -and involved are the variations which must be supposed to occur together. -In illustration of another point, we have already considered the -modifications required to accompany increased weight of the head (§ 155). -Instead of the bison, the moose deer, or the extinct Irish elk, will here -best serve our purpose. In this last species the male has -enormously-developed horns, used for purposes of offence and defence. These -horns, weighing upwards of a hundred-weight, are carried at great -mechanical disadvantage: supported as they are, along with the massive -skull which bears them, at the extremity of the outstretched neck. Further, -that these heavy horns may be of use in fighting, the supporting bones and -muscles must be strong enough, not simply to carry them, but to put them in -motion with the rapidity needed for giving blows. Let us, then, ask how, by -natural selection, this complex apparatus of bones and muscles can have -been developed, _pari passu_ with the horns? If we suppose the horns to -have been originally of like size with those borne by other kinds of deer; -and if we suppose that in some individual they became larger by spontaneous -variation; what would be the concomitant changes required to render their -greater size useful? Other things equal, the blow given by a larger horn -would be a blow given by a heavier mass moving at a smaller velocity: the -momentum would be the same as before; and the area of contact with the body -struck being somewhat increased, while the velocity was decreased, the -injury done would be less. That horns may become better weapons, the whole -apparatus concerned in moving them must be so strengthened as to impress -more force on them, and to bear the more violent reactions of the blows -given. The bones of the skull on which the horns are seated must be -thickened; otherwise they will break. The vertebræ of the neck must be -further developed; and unless the ligaments which hold together these -vertebræ, and the muscles which move them, are also enlarged, nothing will -be gained. Again the upper dorsal vertebræ and their spines must be -strengthened, that they may withstand the stronger contractions of the -neck-muscles; and like changes must be made on the scapular arch. Still -more must there be required a simultaneous development of the bones and -muscles of the fore-legs; since these extra growths in the horns, in the -skull, in the neck, in the shoulders, add to the burden they have to bear; -and without they are strengthened the creature will not only suffer from -loss of speed but will fail in fight. Hence, to make larger horns of use, -additional sizes must be acquired by numerous bones, muscles, and -ligaments, as well as by the blood-vessels and nerves on which their -actions depend. On calling to mind how the spraining of a single small -muscle in the foot incapacitates for walking, or how permanent weakness in -a knee-ligament will diminish the power of the leg, it will be seen that -unless all these many changes are simultaneously made, they may as well be -none of them made--or rather, they would better be none of them made; since -the enlargements of some parts, by putting greater strains on connected -parts, would render them relatively weaker if they remained unenlarged. Can -we with any propriety assume that these many enlargements duly proportioned -will be simultaneously effected by spontaneous variations? I think not. It -would be a strong supposition that the vertebræ and muscles of the neck -suddenly became bigger at the same time as the horns. It would be a still -stronger supposition that the upper dorsal vertebræ not only at the same -time became more massive, but appropriately altered their proportions, by -the development of their immense neural spines. And it would be an -assumption still more straining our powers of belief, that along with -heavier horns there should spontaneously take place the required -strengthenings in the bones, muscles, arteries, and nerves of the scapular -and the fore-legs. - -Besides the multiplicity of directly-coöperative organs, the multiplicity -of organs which do not coöperate, save in the degree implied by their -combination in the same organism, seems to me a further hindrance to the -development of special structures by natural selection alone. Where the -life is simple, or where circumstances render some one function supremely -important, survival of the fittest may readily bring about the appropriate -structural change, without aid from the transmission of functionally-caused -modifications. But in proportion as the life grows complex--in proportion -as a healthy existence cannot be secured by a large endowment of some one -power, but demands many powers; in the same proportion do there arise -obstacles to the increase of any particular power by "the preservation of -favoured races in the struggle for life." As fast as the faculties are -multiplied, so fast does it become possible for the several members of a -species to have various kinds of superiorities over one another. While one -saves its life by higher speed, another does the like by clearer vision, -another by keener scent, another by quicker hearing, another by greater -strength, another by unusual power of enduring cold or hunger, another by -special sagacity, another by special timidity, another by special courage; -and others by other bodily and mental attributes. Conditions being alike, -each of these life-saving attributes is likely to be transmitted to -posterity. But we may not assume that it will be increased in subsequent -generations by natural selection. Increase of it can result only if -individuals possessing average endowments of it are more frequently killed -off than individuals highly endowed with it; and this can happen only when -the attribute is one of greater importance, for the time being, than most -of the other attributes. If those members of the species which have but -ordinary shares of it, nevertheless survive by virtue of other -superiorities which they severally possess; then it is not easy to see how -this particular attribute can be developed by natural selection in -subsequent generations. The probability seems rather to be that, by -gamogenesis, this extra endowment will, on the average, be diminished in -posterity--just serving in the long run to make up for the deficient -endowments of those whose special powers lie in other directions; and so to -keep up the normal structure of the species. As fast as the number of -bodily and mental faculties increases, and as fast as maintenance of life -comes to depend less on the amount of any one and more on the combined -actions of all; so fast does the production of specialities of character by -natural selection alone, become difficult. Particularly does this seem to -be so with a species so multitudinous in its powers as mankind; and above -all does it seem to be so with such of the human powers as have but minor -shares in aiding the struggle for life--the æsthetic faculties, for -example. - -It by no means follows, however, that in cases of this kind, and cases of -the preceding kind, natural selection plays no part. Wherever it is not the -chief agent in working organic changes, it is still, very generally, a -secondary agent. The survival of the fittest must nearly always further the -production of modifications which produce fitness, whether they be -incidental modifications, or modifications caused by direct adaptation. -Evidently, those individuals whose constitutions have facilitated the -production in them of any structural change consequent on any functional -change demanded by some new external condition, will be the individuals -most likely to live and to leave descendants. There must be a natural -selection of functionally-acquired peculiarities, as well as of -spontaneously-acquired peculiarities; and hence such structural changes in -a species as result from changes of habit necessitated by changed -circumstances, natural selection will render more rapid than they would -otherwise be. - -There are, however, some modifications in the sizes and forms of parts, -which cannot have been aided by natural selection; but which must have -resulted wholly from the inheritance of functionally-caused alterations. -The dwindling of organs of which the undue sizes entail no appreciable -evils, furnishes the best evidence of this. Take, for an example, that -diminution of the jaws and teeth which characterizes the civilized races, -as contrasted with the savage races.[53] How can the civilized races have -been benefited in the struggle for life, by the slight decrease in these -comparatively-small bones? No functional superiority possessed by a small -jaw over a large jaw in civilized life, can be named as having caused the -more frequent survival of small-jawed individuals. The only advantage -accompanying smallness of jaw, is the advantage of economized nutrition; -and this cannot be great enough to further the preservation of those -distinguished by it. The decrease of weight in the jaw and co-operative -parts, which has arisen in the course of thousands of years, does not -amount to more than a few ounces. This decrease has to be divided among the -many generations which have lived and died in the interval. Let us admit -that the weight of these parts diminished to the extent of an ounce in a -single generation (which is a large admission); it still cannot be -contended that the having to carry an ounce less in weight, and to keep in -repair an ounce less of tissue, could sensibly affect any man's fate. And -if it never did this--nay, if it did not cause a _frequent_ survival of -small-jawed individuals where large-jawed individuals died; natural -selection could neither cause nor aid diminution of the jaw and its -appendages. Here, therefore, the decreased action which has accompanied the -growth of civilized habits (the use of tools and the disuse of coarse -food), must have been the sole cause at work. Through direct equilibration, -diminished external stress on these parts has resulted in diminution of the -internal forces by which this stress is met. From generation to generation, -this lessening of the parts consequent on functional decline has been -inherited. And since the survival of individuals must always have been -determined by more important structural traits, this trait can have neither -been facilitated nor retarded by natural selection. - - -§ 167. Returning from these extensive classes of facts for which Mr. -Darwin's hypothesis does not account, to the still more extensive classes -of facts for which it does account, and which are unaccountable on any -other hypothesis; let us consider in what way this hypothesis is -expressible in terms of the general doctrine of evolution. Already it has -been pointed out that the evolving of modified types by "natural selection -or the preservation of favoured races in the struggle for life," must be a -process of equilibration; since it results in the production of organisms -which are in equilibrium with their environments. At the outset of this -chapter, something was done towards showing how this continual survival of -the fittest may be understood as the progressive establishment of a balance -between inner and outer forces. Here, however, we must consider the matter -more closely. - -On previous occasions we have contemplated the assemblage of individuals -composing a species, as an aggregate in a state of moving equilibrium. We -have seen that its powers of multiplication give it an expansive energy -which is antagonized by other energies; and that through the rhythmical -variations in these two sets of energies there is maintained an oscillating -limit to its habitat, and an oscillating limit to its numbers. On another -occasion (§ 96) it was shown that the aggregate of individuals constituting -a species, has a kind of general life which, "like the life of an -individual, is maintained by the unequal and ever-varying actions of -incident forces on its different parts." We saw that "just as, in each -organism, incident forces constantly produce divergences from the mean -state in various directions, which are constantly balanced by opposite -divergences indirectly produced by other incident forces; and just as the -combination of rhythmical functions thus maintained, constitutes the life -of the organism; so, in a species there is, through gamogenesis, a -perpetual neutralization of those contrary deviations from the mean state, -which are caused in its different parts by different sets of incident -forces; and it is similarly by the rhythmical production and compensation -of these contrary deviations that the species continues to live." Hence, to -understand how a species is affected by causes which destroy some of its -units and favour the multiplication of others, we must consider it as a -whole whose parts are held together by complex forces that are ever -re-balancing themselves--a whole whose moving equilibrium is continually -disturbed and continually rectified. Thus much premised, let us next call -to mind how moving equilibria in general are changed. In the first place, a -new incident force falling on any part of an aggregate with balanced -motions, produces a new motion in the direction of least resistance. In the -second place, the new incident force is gradually used up in overcoming the -opposing forces, and when it is all expended the opposing forces produce a -recoil--a reverse deviation which counter-balances the original deviation. -Consequently, to consider whether the moving equilibrium of a species is -modified in the same way as moving equilibria in general, is to consider -whether, when exposed to a new force, a species yields in the direction of -least resistance; and whether, by its thus yielding, there is generated in -the species a compensating change in the opposite direction. We shall find -that it does both these things. - -For what, expressed in mechanical terms, is the effect wrought on a species -by some previously-unknown enemy, that kills such of its members as fail in -defending themselves? The disappearance of those individuals which meet the -destroying forces by the smallest preserving forces, is tantamount to the -yielding of the species as a whole at the places where the resistances are -the least. Or if by some general influence, such as alteration of climate, -the members of a species are subject to increase of external actions which -are ever tending to overthrow their equilibria, and which they are ever -counter-balancing by certain physiological actions, which are the first to -die? Those least able to generate the internal energies which antagonize -these external energies. If the change be an increase of the winter's cold, -then such members of the species as have unusual powers of getting food or -of digesting food, or such as are by their constitutional aptitude for -making fat, furnished with reserve stores of force, available in times of -scarcity, or such as have the thickest coats and so lose least heat by -radiation, survive; and their survival implies that in each of them the -moving equilibrium of functions presents such an adjustment of internal -forces, as prevents overthrow by the modified aggregate of external forces. -Conversely, the members which die are, other things equal, those deficient -in the power of meeting the new action by an equivalent counter-action. -Thus, in all cases, a species considered as an aggregate in a state of -moving equilibrium, has its state changed by the yielding of its -fluctuating mass wherever this mass is weakest in relation to the special -forces acting on it. The conclusion is, indeed, a truism. But now what must -follow from the destruction of the least-resisting individuals and survival -of the most-resisting individuals? On the moving equilibrium of the species -as a whole, existing from generation to generation, the effect of this -deviation from the mean state is to produce a compensating deviation. For -if all such as are deficient of power in a certain direction are destroyed, -what must be the effect on posterity? Had they lived and left offspring, -the next generation would have had the same average powers as preceding -generations: there would have been a like proportion of individuals less -endowed with the needful power, and individuals more endowed with it. But -the more-endowed individuals being alone left to continue the race, there -must result a new generation characterized by a larger average endowment of -this power. That is to say, on the moving equilibrium of a species, an -action producing change in a given direction is followed, in the next -generation, by a reaction producing an opposite change. Observe, too, that -these effects correspond in their degrees of violence. If the alteration -of some external factor is so great that it leaves alive only the few -individuals possessing extreme endowments of the power required to -antagonize it; then, in succeeding generations, there is a rapid -multiplication of individuals similarly possessing extreme endowments of -this power--the force impressed calls out an equivalent conflicting force. -Moreover, the change is temporary where the cause is temporary, and -permanent where the cause is permanent. All that are deficient in the -needful attribute having been killed off, and the survivors having the -needful attribute in a comparatively high degree, there will descend from -them, not only some possessing equal amounts of this attribute with -themselves, but also some possessing less amounts of it. If the destructive -agency has not continued in action, such less-endowed individuals will -multiply; and the species, after sundry oscillations, will return to its -previous mean state. But if this agency be a persistent one, such less -endowed individuals will be continually killed off, and eventually none but -highly-endowed individuals will be produced--a new moving equilibrium, -adapted to the new environing conditions, will result. - -It may be objected that this mode of expressing the facts does not include -the cases in which a species becomes modified in relation to surrounding -agencies of a passive kind--cases like that of a plant which acquires -hooked seed-vessels, by which it lays hold of the skins of passing animals, -and makes them the distributors of its seeds--cases in which the outer -agency has no direct tendency at first to affect the species, but in which -the species so alters itself as to take advantage of the outer agency. To -cases of this kind, however, the same mode of interpretation applies on -simply changing the terms. While, in the aggregate of influences amid which -a species exists, there are some which tend to overthrow the moving -equilibria of its members, there are others which facilitate the -maintenance of their moving equilibria, and some which are capable of -giving their moving equilibria increased stability: instance the spread -into their habitat of some new kind of prey, which is abundant at seasons -when other prey is scarce. Now what is the process by which the moving -equilibrium in any species becomes adapted to some additional external -factor furthering its maintenance? Instead of an increased resistance to be -met and counterbalanced, there is here a diminished resistance; and the -diminished resistance is equilibrated in the same way as the increased -resistance. As, in the one case, there is a more frequent survival of -individuals whose peculiarities enable them to resist the new adverse -factor; so, in the other case, there is a more frequent survival of -individuals whose peculiarities enable them to take advantage of the new -favourable factor. In each member of the species, the balance of functions -and correlated arrangement of structures, differ slightly from those -existing in other members. To say that among all its members, one is better -fitted than the rest to benefit by some before-unused agency in the -environment, is to say that its moving equilibrium is, in so far, more -stably adjusted to the sum of surrounding influences. And if, consequently, -this individual maintains its moving equilibrium when others fail, and has -offspring which do the like--that is, if individuals thus characterized -multiply and supplant the rest; there is, as before, a process which -effects equilibration between the organism and its environment, not -immediately but mediately, through the continuous intercourse between the -species as a whole and the environment. - - -§ 168. Thus we see that indirect equilibration does whatever direct -equilibration cannot do. All these processes by which organisms are -re-fitted to their ever-changing environments, must be equilibrations of -one kind or other. As authority for this conclusion, we have not simply the -universal truth that change of every order is towards equilibrium; but we -have also the truth that life itself is a moving equilibrium between inner -and outer actions--a continuous adjustment of internal relations to -external relations; or the maintenance of a balance between the forces to -which an organism is subject and the forces which it evolves. Hence all -changes which enable a species to live under altered conditions, are -changes towards equilibrium with the altered conditions; and therefore -those which do not come within the class of direct equilibrations, must -come within the class of indirect equilibrations. - -And now we reach an interpretation of Natural Selection regarded as a part -of Evolution at large. As understood in _First Principles_, Evolution is a -continuous redistribution of matter and motion; and a process of evolution -which is not expressible in terms of matter and motion has not been reduced -to its ultimate form. The conception of Natural Selection is manifestly one -not known to physical science: its terms are not of a kind physical science -can take cognisance of. But here we have found in what manner it may be -brought within the realm of physical science. Rejecting metaphor we see -that the process called Natural Selection is literally a survival of the -fittest; and the outcome of the above argument is that survival of the -fittest is a maintenance of the moving equilibrium of the functions in -presence of outer actions: implying the possession of an equilibrium which -is relatively stable in contrast with the unstable equilibria of those -which do not survive. - - - - -CHAPTER XIII. - -THE CO-OPERATION OF THE FACTORS. - - -§ 169. Thus the phenomena of Organic Evolution may be interpreted in the -same way as the phenomena of all other Evolution. Fully to see this, it -will be needful for us to contemplate in their _ensemble_, the several -processes separately described in the four preceding chapters. - -If the forces acting on any aggregate remain the same, the changes produced -by them will presently reach a limit, at which the outer forces are -balanced by the inner forces; and thereafter no further metamorphosis will -take place. Hence, that there may be continuous changes of structure in -organisms, there must be continuous changes in the incident forces. This -condition to the evolution of animal and vegetal forms, we find to be fully -satisfied. The astronomic, geologic, and meteorologic changes that have -been slowly but incessantly going on, and have been increasing in the -complexity of their combinations, have been perpetually altering the -circumstances of organisms; and organisms, becoming more numerous in their -kinds and higher in their kinds, have been perpetually altering one -another's circumstances. Thus, for those progressive modifications upon -modifications which organic evolution implies, we find a sufficient cause. -The increasing inner changes for which we thus find a cause in the -perpetual outer changes, conform, so far as we can trace them, to the -universal law of the instability of the homogeneous. In organisms, as in -all other things, the exposure of different parts to different kinds and -amounts of incident forces, has necessitated their differentiation; and, -for the like reason, aggregates of individuals have been lapsing into -varieties, and species, and genera, and orders. Further, in each type of -organism, as in the aggregate of types, the multiplication of effects has -continually aided this transition from a more homogeneous to a more -heterogeneous state. And yet again, that increasing segregation and -concomitant increasing definiteness, associated with the growing -heterogeneity of organisms, has been aided by the continual destruction of -those which expose themselves to aggregates of external actions markedly -incongruous with the aggregates of their internal actions, and the survival -of those subject only to comparatively small incongruities. Finally, we -have found that each change of structure, superposed on preceding changes, -has been a re-equilibration necessitated by the disturbance of a preceding -equilibrium. The maintenance of life being the maintenance of a balanced -combination of functions, it follows that individuals and species that have -continued to live, are individuals and species in which the balance of -functions has not been overthrown. Hence survival through successive -changes of conditions, implies successive adjustments of the balance to the -new conditions. - -The actions that are here specified in succession, are in reality -simultaneous; and they must be so conceived before organic evolution can be -rightly understood. Some aid towards so conceiving them will be given by -the annexed table, representing the co-operation of the factors. - - -§ 170. Respecting this co-operation, it remains only to point out the -respective shares of the factors in producing the total result; and the way -in which the proportions of their respective shares vary as evolution -progresses. - - - Astronomic } - changes } - } alter the } - Geologic } incidence } - changes } of inorganic } - } forces. } - Meteorologic } } - changes } } - } - } - } on each species: affecting - } | - } | - } | - } | - } | - Enemies } } | - Competitors } } | - } varying in } } | - Co-operators } number } } | - Prey } } alter the } | - } incidence } | - Enemies } } of organic } | - Competitors } } forces. } | - } varying in } | - Co-operators } kind } | - Prey } | - | - ----------------------------------------------------------------- - | - | { which, partially in the first - | { generation, and completely in - | { the course of generations, are - | { directly equilibrated with the - | { changed agencies. - | { immediately { - | { through their { which have their direct - | { functions; { equilibration with the changed - | { { agencies, aided by indirect - | { { equilibration, through the more - | { { frequent survival of those in - | { { which the direct equilibration - | { { is most rapid. - | { - | { its individuals, { { positively--by aiding the - | { { { multiplication of those whose - | { { { moving equilibria happen to be - | { { { most congruous with the - | { { mediately { changed agencies: thus, in the - | { { through the { course of generations, indirectly - | { { aggregate of { equilibrating certain individuals - | { { individuals; { with them. - --{ { - { { negatively--by killing those - { { whose moving equilibria are - { { most incongruous with the - { { changed agencies: thus, in - { { the course of generations, - { { indirectly equilibrating each - { { of its surviving individuals - { { with them. - { - { { by acting on it in some parts of the habitat - { { more than in others; and thus differentiating - { its aggregate { the species into local varieties. - { of individuals, { - { { and thus causing - { { differentiations of - { { the species into - { by acting differently { varieties, irrespective - { on slightly-unlike { of locality. - { individuals in the { - { same locality; { and thus causing - { modification of the - { species as a whole, - { by abstracting a - { certain class of - { its units. - -At first, changes in the amounts and combinations of inorganic forces, -astronomic, geologic, and meteorologic, were the only causes of the -successive modifications; and these changes have continued to be causes. -But as, through the diffusion of organisms and consequent differential -actions of inorganic forces, there arose unlikenesses among them, producing -varieties, species, genera, orders, classes, the actions of organisms on -one another became new sources of organic modifications. And as fast as -types have multiplied and become more complex, so fast have the mutual -actions of organisms come to be more influential factors in their -respective evolutions: eventually becoming the chief factors. - -Passing from the external causes of change to the internal processes of -change entailed by them, we see that these, too, have varied in their -proportions: that which was originally the most important and almost the -sole process, becoming gradually less important, if not at last the least -important. Always there must have been, and always there must continue to -be, a survival of the fittest; natural selection must have been in -operation at the outset, and can never cease to operate. While yet -organisms had small abilities to coordinate their actions, and adjust them -to environing actions, natural selection worked almost alone in moulding -and remoulding organisms into fitness for their changing environments; and -natural selection has remained almost the sole agency by which plants and -inferior orders of animals have been modified and developed. The -equilibration of organisms that are almost passive, is necessarily effected -indirectly, by the action of incident forces on the species as a whole. But -along with the evolution of organisms having some activity, there grows up -a kind of equilibration which is in part direct. In proportion as the -activity increases direct equilibration plays a more important part. Until, -when the nervo-muscular apparatus becomes greatly developed, and the power -of varying the actions to fit the varying requirements becomes -considerable, the share taken by direct equilibration rises into -co-ordinate importance or greater importance. As fast as essential -faculties multiply, and as fast as the number of organs which co-operate in -any given function increases, indirect equilibration through natural -selection becomes less and less capable of producing specific adaptations; -and remains capable only of maintaining the general fitness of constitution -to conditions. The production of adaptations by direct equilibration then -takes the first place: indirect equilibration serving to facilitate it. -Until at length, among the civilized human races, the equilibration becomes -mainly direct: the action of natural selection being limited to the -destruction of those who are constitutionally too feeble to live, even with -external aid. As the preservation of incapables is secured by our social -arrangements; and as very few save incarcerated criminals are prevented by -their inferiorities from leaving the average number of offspring; it -results that survival of the fittest can scarcely at all act in such way as -to produce specialities of nature, either bodily or mental. Here the -specialities of nature, chiefly mental, which we see produced, and which -are so rapidly produced that a few centuries show a considerable change, -must be ascribed almost wholly to direct equilibration.[54] - - - - -CHAPTER XIV. - -THE CONVERGENCE OF THE EVIDENCES. - - -§ 171. Of the three classes of evidences that have been assigned in proof -of Evolution, the _à priori_, which we took first, were partly negative, -partly positive. - -On considering the "General Aspects of the Special-creation hypothesis," we -discovered it to be worthless. Discredited by its origin, and wholly -without any basis of observed fact, we found that it was not even a -thinkable hypothesis; and, while thus intellectually illusive, it turned -out to have moral implications irreconcilable with the professed beliefs of -those who hold it. - -Contrariwise, the "General Aspects of the Evolution-hypothesis" begot the -stronger faith in it the more nearly they were considered. By its lineage -and its kindred, it was found to be as closely allied with the proved -truths of modern science, as is the antagonist hypothesis with the proved -errors of ancient ignorance. We saw that instead of being a mere -pseud-idea, it admits of elaboration into a definite conception: so showing -its legitimacy as an hypothesis. Instead of positing a purely fictitious -process, the process which it alleges proves to be one actually going on -around us. To which add that, morally considered, this hypothesis presents -no radical incongruities. - -Thus, even were we without further means of judging, there could be no -rational hesitation which of the two views should be entertained. - - -§ 172. Further means of judging, however, we found to be afforded by -bringing the two hypotheses face to face with the general truths -established by naturalists. These inductive evidences were dealt with in -four chapters. - -"The Arguments from Classification" were these. Organisms fall into groups -within groups; and this is the arrangement which we see results from -evolution, where it is known to take place. Of these groups within groups, -the great or primary ones are the most unlike, the sub-groups are less -unlike, the sub-sub-groups still less unlike, and so on; and this, too, is -a characteristic of groups demonstrably produced by evolution. Moreover, -indefiniteness of equivalence among the groups is common to those which we -know have been evolved, and those here supposed to have been evolved. And -then there is the further significant fact, that divergent groups are -allied through their lowest rather than their highest members. - -Of "the Arguments from Embryology," the first is that when developing -embryos are traced from their common starting point, and their divergences -and re-divergences symbolized by a genealogical tree, there is manifest a -general parallelism between the arrangement of its primary, secondary, and -tertiary branches, and the arrangement of the divisions and sub-divisions -of our classifications. Nor do the minor deviations from this general -parallelism, which look like difficulties, fail, on closer observation, to -furnish additional evidence; since those traits of a common ancestry which -embryology reveals, are, if modifications have resulted from changed -conditions, liable to be disguised in different ways and degrees in -different lines of descendants. - -We next considered "the Arguments from Morphology." Apart from those -kinships among organisms disclosed by their developmental changes, the -kinships which their adult forms show are profoundly significant. The -unities of type found under such different externals, are inexplicable -except as results of community of descent with non-community of -modification. Again, each organism analyzed apart, shows, in the likenesses -obscured by unlikenesses of its component parts, a peculiarity which can be -ascribed only to the formation of a more heterogeneous organism out of a -more homogeneous one. And once more, the existence of rudimentary organs, -homologous with organs that are developed in allied animals or plants, -while it admits of no other rational interpretation, is satisfactorily -interpreted by the hypothesis of evolution. - -Last of the inductive evidences, came "the Arguments from Distribution." -While the facts of distribution in Space are unaccountable as results of -designed adaptation of organisms to their habitats, they are accountable as -results of the competition of species, and the spread of the more fit into -the habitats of the less fit, followed by the changes which new conditions -induce. Though the facts of distribution in Time are so fragmentary that no -positive conclusion can be drawn, yet all of them are reconcilable with the -hypothesis of evolution, and some of them yield it strong support: -especially the near relationship existing between the living and extinct -types in each great geographical area. - -Thus of these four groups, each furnished several arguments which point to -the same conclusion; and the conclusion pointed to by the arguments of any -one group, is that pointed to by the arguments of every other group. This -coincidence of coincidences would give to the induction a very high degree -of probability, even were it not enforced by deduction. But the conclusion -deductively reached, is in harmony with the inductive conclusion. - - -§ 173. Passing from the evidence that evolution has taken place, to the -question--How has it taken place? we find in known agencies and known -processes, adequate causes of its phenomena. - -In astronomic, geologic, and meteorologic changes, ever in progress, ever -combining in new and more involved ways, we have a set of inorganic factors -to which all organisms are exposed; and in the varying and complicating -actions of organisms on one another, we have a set of organic factors that -alter with increasing rapidity. Thus, speaking generally, all members of -the Earth's Flora and Fauna experience perpetual re-arrangements of -external forces. - -Each organic aggregate, whether considered individually or as a -continuously-existing species, is modified afresh by each fresh -distribution of external forces. To its pre-existing differentiations new -differentiations are added; and thus that lapse to a more heterogeneous -state, which would have a fixed limit were the circumstances fixed, has its -limit perpetually removed by the perpetual change of the circumstances. - -These modifications upon modifications which result in evolution -structurally considered, are the accompaniments of those functional -alterations continually required to re-equilibrate inner with outer -actions. That moving equilibrium of inner actions corresponding with outer -actions, which constitutes the life of an organism, must either be -overthrown by a change in the outer actions, or must undergo perturbations -that cannot end until there is a re-adjusted balance of functions and -correlative adaptation of structures. - -But where the external changes are either such as are fatal when -experienced by the individuals, or such as act on the individuals in ways -that do not affect the equilibrium of their functions; then the -re-adjustment results through the effects produced on the species as a -whole--there is indirect equilibration. By the preservation in successive -generations of those whose moving equilibria are least at variance with the -requirements, there is produced a changed equilibrium completely in harmony -with the requirements. - - -§ 174. Even were this the whole of the evidence assignable for the belief -that organisms have been gradually evolved, it would have a warrant higher -than that of many beliefs which are regarded as established. But the -evidence is far from exhausted. - -At the outset it was remarked that the phenomena presented by the organic -world as a whole, cannot be properly dealt with apart from the phenomena -presented by each organism, in the course of its growth, development, and -decay. The interpretation of either implies interpretation of the other; -since the two are in reality parts of one process. Hence, the validity of -any hypothesis respecting the one class of phenomena, may be tested by its -congruity with phenomena of the other class. We are now about to pass to -the more special phenomena of development, as displayed in the structures -and functions of individual organisms. If the hypothesis that plants and -animals have been progressively evolved be true, it must furnish us with -keys to these phenomena. We shall find that it does this; and by doing it -gives numberless additional vouchers for its truth. - - - - -CHAPTER XIV^A. - -RECENT CRITICISMS AND HYPOTHESES. - - -§ 174a. Since the first edition of this work was published, and more -especially since the death of Mr. Darwin, an active discussion of the -Evolution hypothesis has led to some significant results. - -That organic evolution has been going on from the dawn of life down to the -present time, is now a belief almost universally accepted by zoologists and -botanists--"almost universally," I say, because the surviving influence of -Cuvier prevents acceptance of it by some of them in France. Omitting the -ideas of these, all biological interpretations, speculations, and -investigations, tacitly assume that organisms of every kind in every era -and in every region have come into existence by the process of descent with -modification. - -But while concerning the fact of evolution there is agreement, concerning -its causes there is disagreement. The ideas of naturalists have, in this -respect, undergone a differentiation increasingly pronounced; which has -ended in the production of two diametrically opposed beliefs. The cause -which Mr. Darwin first made conspicuous has come to be regarded by some as -the sole cause; while, on the part of others there has been a growing -recognition of the cause which he at first disregarded but afterwards -admitted. Prof. Weismann and his supporters contend that natural selection -suffices to explain everything. Contrariwise, among many who recognize the -inheritance of functionally-produced changes, there are a few, like the -Rev. Prof. Henslow, who regard it as the sole factor. - -The foregoing chapters imply that the beliefs of neither extreme are here -adopted. Agreeing with Mr. Darwin that both factors have been operative, I -hold that the inheritance of functionally-caused alterations has played a -larger part than he admitted even at the close of his life; and that, -coming more to the front as evolution has advanced, it has played the chief -part in producing the highest types. I am not now about to discuss afresh -these questions, but to deal with certain further questions. - -For while there has been taking place in the biological world the major -differentiation above indicated, there have been taking place certain minor -differentiations--there have been arising special views respecting the -process of organic evolution. Concerning each of these it is needful to say -something. - - -§ 174b. Among the implied controversies the most conspicuous one has -concerned the alleged process called by Prof. Weismann _Panmixia_--a -process which Dr. Romanes had foreshadowed under the name of "the Cessation -of Selection." Dr. Romanes says:--"At that time it appeared to me, as it -now appears to Weismann, entirely to supersede the necessity of supposing -that the effect of disuse is ever inherited in any degree at all."[55] The -alleged mode of action is exemplified by Prof. Weismann as follows:-- - - "A goose or a duck must possess strong powers of flight in the natural - state, but such powers are no longer necessary for obtaining food when it - is brought into the poultry-yard, so that a rigid selection of - individuals with well-developed wings, at once ceases among its - descendants. Hence in the course of generations, a deterioration of the - organs of flight must necessarily ensue, and the other members and organs - of the bird will be similarly affected."[56] - -Here, and throughout the arguments of those who accept the hypothesis of -Panmixia, there is an unwarranted assumption--nay, an assumption at -variance with the doctrine in support of which it is made. It is contended -that in such cases as the one given there will, apart from any effects of -disuse, be decrease in the disused organs because, not being kept by -Natural Selection up to the level of strength previously needed, they will -vary in the direction of decrease; and that variations in the direction of -decrease, occurring in some individuals, will, by interbreeding, produce an -average decrease throughout the species. But why will the disused organs -vary in the direction of decrease more than in the direction of increase? -The hypothesis of Natural Selection postulates indeterminate -variations--deviations no more in one direction than in the opposite -direction: implying that increases and decreases of size will occur to -equal extents and with equal frequencies. With any other assumption the -hypothesis lapses; for if the variations in one direction exceed those in -another the question arises--What makes them do this? And whatever makes -them do this becomes the essential cause of the modification: the selection -of favourable variations is tacitly admitted to be an insufficient -explanation. But if the hypothesis of Natural Selection itself implies the -occurrence of equal variations on all sides of the mean, how can Panmixia -produce decrease? _Plus_ deviations will cancel _minus_ deviations, and the -organ will remain where it was.[57] - -"But you have forgotten the tendency to economy of growth," will be the -reply--"you have forgotten that in Mr. Darwin's words 'natural selection is -continually trying to economize in every part of the organization;' and -that this is a constant cause favouring _minus_ variations." I have not -forgotten it; but have remembered it as showing how, to support the -hypothesis of Panmixia, there is invoked the aid of that very hypothesis -which it is to replace. For this principle of economy is but another aspect -of the principle of functionally-produced modifications. Nearly forty years -ago I contended that "the different parts of ... an individual organism -compete for nutriment; and severally obtain more or less of it according as -they are discharging more or less duty:"[58] the implication being that as -all other organs are demanding blood, decrease of duty in any one, -entailing decreased supply of blood, brings about decreased size. In other -words, the alleged economy is nothing else than the abstraction, by active -parts, of nutriment from an inactive part; and is merely another name for -functionally-produced decrease. So that if the variations are supposed to -take place predominantly in the direction of decrease, it can only be by -silently assuming the cause which is overtly denied. - -But now we come to the strange fact that the particular case in which -panmixia is assigned in disproof of alleged inheritance of -functionally-produced modifications, is a case in which it would be -inapplicable even were its assumption legitimate--the case of disused -organs in domestic animals. For since nutrition is here abundant, the -principle of economy under the form alleged does not come into play. -Contrariwise, there even occurs a partial re-development of rudimentary -organs: instances named by Mr. Darwin being the supplementary mammæ in -cows, fifth toes on the hind feet of dogs, spurs and comb in hens, and -canine teeth in mares. Now clearly, if organs disused for innumerable -generations may thus vary in the direction of increase, it must, _a -fortiori_, be so with recently disused organs, and there disappears all -plea (even the illegitimate plea) for assuming that in the wing of a wild -duck which has become domesticated, the _minus_ variations will exceed the -_plus_ variations: the hypothesis of panmixia loses its postulate. - -If it be said that Mr. Darwin's argument is based on the changed ratio -between the weights of leg-bones and wing-bones, and that this changed -ratio may result not from decrease of the wing-bones but from increase of -the leg-bones, then there comes a fatal reply. Such, increase cannot be -ascribed to selection of varieties, since there is no selective breeding to -obtain larger legs, and as it is not pretended that panmixia accounts for -increase the case is lost: there remains no cause for such increase save -increase of function. - - -§ 174c. The doctrine of determinate evolution or definitely-directed -evolution, which appears to be in one form or other entertained by sundry -naturalists, has been set forth by the late Prof. Eimer under the title -"Orthogenesis." A distinct statement of his conception is not easily made -for the reason that, as I think, the conception itself is indistinct. Here -are some extracts from a translation of his paper published at Chicago. Out -of these the reader may form a notion of the theory: - - "Orthogenesis shows that organisms develop in definite directions without - the least regard for utility through purely physiological causes as the - result of _organic growth_, as I term the process." - - "I am concerned in this paper with definitely directed evolution as the - cause of transmutation, and not with the effects of the use and activity - of organs which with Lamarck I adopted as the second main explanatory - cause thereof." - - "The causes of definitely directed evolution are contained, according to - my view, in the effects produced by outward circumstances and influences - such as climate and nutrition upon the constitution of a given organism." - - "At variance with all the facts of definitely directed evolution ... is - also the contention of my opponent [Weismann] ... that the variations - demonstrably oscillate to and fro in the most diverse directions about a - given zero-point. There is no oscillation in the direction of - development, but simply an advance forwards in a straight line with - occasional lateral divergences whereby the forkings of the ancestral tree - are produced."[59] - -These sentences contain one of those explanations which explain nothing; -for we are not enabled to see how the "outward circumstances and -influences" produce the effects ascribed to them. We are not shown in what -way they cause organic evolution in general, still less in what way they -cause the infinitely-varied forms in which organic evolution results. The -assertion that evolution takes definitely-directed lines is accompanied by -no indication of the reasons why particular lines are followed rather than -others. In short, we are simply taken a step back, and for further -interpretation referred to a cause said to be adequate, but the operations -of which we are to imagine as best we may. - -This is a re-introduction of supernaturalism under a disguise. It may pair -off with the conception made popular by the _Vestiges of the Natural -History of Creation_, in which it was contended that there exists a -persistent tendency towards the birth of a higher form of creature; or it -may be bracketed with the idea entertained by the late Prof. Owen, who -alleged an "ordained becoming" of living things. - - -§ 174d. An objection to the Darwinian doctrine which has risen into -prominence, is that Natural Selection does not explain that which it -professes to explain. In the words of Mr. J. T. Cunningham:-- - - "Everybody knows that the theory of natural selection was put forward by - Darwin as a theory of the origin of species, and yet it is only a theory - of the origin of adaptations. The question is: Are the differences - between species differences of adaptation? If so, then the origin of - species and the origin of adaptations are equivalent terms. But there is - scarcely a single instance in which a specific character has been shown - to be useful, to be adaptive."[60] - -To illustrate this last statement Mr. Cunningham names the plaice, -flounder, and dab as three flat fishes in which, along with the adaptive -characters related to the mode of life common to them all, each has -specific characters which are not adaptive. No evidence is forthcoming that -these in any way conduce to the welfare of the species. Two propositions -are here involved which should be separately dealt with. - -The first is that the adaptive modifications which survival of the fittest -is able to produce, do not become specific traits: they are traits separate -in kind from those which mark off groups proved to be specifically distinct -by their inability to breed together. Such evidence as we at present have -seems to warrant this statement. Out of the many varieties of dogs most, if -not all, have been rendered distinct by adaptive modifications, mostly -produced by selection. But, notwithstanding the immense divergences of -structure so produced, the varieties inter-breed. To this, however, it may -be replied that sufficient time has not elapsed--that the process by which -a structural adaptation so reacts on the constitution as to make it a -distinct one, possibly, or probably, takes many thousands of years. Let us -accept for the moment Lord Kelvin's low estimate of the geologic time -during which life has existed--one hundred million years. Suppose we divide -that time into as many parts as there are hours occupied in the development -of a human foetus. And suppose that during these hundred million years -there has been going on with some uniformity the evolution of the various -organic types now existing. Then the amount of change undergone by the -foetus in an hour, will be equivalent to the amount of change undergone by -an evolving organic form in fifteen thousand years. That is to say, during -general evolution it may have taken fifteen thousand years to establish, as -distinct, two species differing from one another no more than the foetus -differs from itself after the lapse of an hour. Hence, though we lack proof -that adaptive modifications become specific traits, it is quite possible -that they are in course of becoming specific traits. - -The converse proposition, that the traits by which species are ordinarily -distinguished are non-adaptive traits is well sustained; and the statement -that, if not themselves useful they are correlated with those which are -useful, is, to say the least, unproved. For the instances given by Mr. -Darwin of correlated traits are not those between adaptive traits and the -traits regarded as specific, but between traits none of which are specific; -as between skull and limbs in swine, tusks and bristles in swine, horns and -wool in sheep, beak and feet in pigeons. - -If we seek a clue in those processes by which correlations are brought -about--the physiological actions and reactions--we may at once see that any -organic modification, be it adaptive or not, must entail secondary -modifications throughout the rest of the organism, most of them insensible -but some of them sensible. The competition for blood among organs, referred -to above, necessitates that, other things remaining the same, the extra -growth of any one tells on all others, in variable degrees according to -conditions, and may cause appreciable diminutions of some. This is not all. -While the quantity of blood supplied to other organs is affected, its -quality also is in some cases affected. Each organ, or at any rate each -class of organs, has special nutrition--abstracts from the blood a -proportion of ingredients different from that abstracted by other organs or -classes of organs. Hence may result a deficiency or a surplus of some -element: instance the change in the blood which must be caused by growth of -a stag's antlers. Now if such effects are always produced, and if, further, -a change of general nutrition caused by a new food or by a difference of -ability to utilize certain components of food, similarly operates (instance -the above named correlation between horns and wool), then every -modification must entail throughout the organism multitudinous alterations -of structure. Such alterations will ordinarily be neither in themselves -useful nor necessarily correlated with those which are useful; since they -must arise as concomitants of any change, whether adaptive or not. There -will consequently arise the innumerable minute differences presented by -allied species in addition to the differences called specific. - -On joining with recognition of this general process a recognition of the -tendency towards localization of deposit, one possible origin of specific -marks is suggested. When in an organism the circulating fluids contain -useless matter, normal or abnormal, the excretion of it, once determined -towards a certain place, continues at that place. Trees furnish examples in -the casting out of gums and resins. Animal life yields evidence in gouty -concretions and such morbid products as tubercle. A place of enfeebled -nutrition is commonly chosen--not unfrequently a place where a local injury -has occurred. Now if we extend this principle, well recognized in -pathological processes, to physiological processes, we may infer that where -an adaptive modification has so reacted on the blood as to leave some -matter to be got rid of, the deposit of this, initiated at some place of -least resistance, may produce a local structure which eventually becomes a -species-mark. A relevant inquiry suggests itself--What proportion of -species-marks are formed out of inanimate tissue or tissue of low -vitality--tissue which, like hair, feathers, horns, teeth, is composed of -by-products unfit for carrying on vital actions. - - -§ 174e. In the days when, not having been better instructed by Mr. Darwin, -I believed that all changes of structure in organisms result from changes -of function, I held that the cause of such changes of function is -migration. Assuming as the antecedent of migration a great geologic change, -such as upheaval of the East Indian Archipelago step by step into a -continent, it was argued, in an essay I then wrote, that, subjected -primarily to new influences in its original habitat, each kind of plant and -animal would secondarily be subjected to the altered conditions consequent -on spreading over the upheaved regions. - - "Each species being distributed over an area of some extent, and tending - continually to colonize the new area exposed, its different members would - be subject to different sets of changes. Plants and animals spreading - towards the equator would not be affected in the same way with others - spreading from it. Those spreading towards the new shores would undergo - changes unlike the changes undergone by those spreading into the - mountains. Thus, each original race of organisms would become the root - from which diverged several races differing more or less from it and from - one another." - -It was further argued that, beyond modifications caused by change of -physical conditions and food, others would be caused by contact of the -Flora and Fauna of each island with the Floras and Faunas of other islands: -bringing experience of animals and plants before unknown.[61] - -While this conception was wrong in so far as it ascribed the production of -new species entirely to inheritance of functionally-wrought alterations -(thus failing to recognize Natural Selection, which was not yet -enunciated), it was right in so far as it ascribed organic changes to -changes of conditions. And it was, I think, also right in so far as it -implied that isolation is a condition precedent to such changes. Apparently -it did not occur to me as needful to specify this isolation as making -possible the differentiation of species; since it goes without saying that -members of a species spreading east, west, north, south, and forming groups -hundreds of miles apart, must, while breeding with those of the same group -be prevented from breeding with those of other groups--prevented from -having their locally-caused modifications mutually cancelled. - -The importance of isolation has of late been emphasized by Dr. Romanes and -others, who, to that isolation consequent on geographical diffusion, have -added that isolation which results from difference of station in the same -habitat, and also that due to differences in the breeding periods arising -in members of the same species. Doubtless in whatever way effected, the -isolation of a group subject to new conditions and in course of being -changed, is requisite as a means to permanent differentiation. Doubtless -also, as contended by Mr. Gulick and Dr. Romanes, there is a difference -between the case in which an entire species being subject to the same -conditions is throughout modified in character, thus illustrating what Mr. -Gulick calls "monotypic evolution," and the case in which different parts -of the species, leading different lives, will, if they are by any means -prevented from inter-breeding with other parts, form divergent varieties: -thus illustrating "polytypic evolution." - - -§ 174f. Beyond geographical and topographical isolation, there is an -isolation of another kind regarded by some as having had an important share -in organic evolution. Foreshadowed by Mr. Belt, subsequently enunciated by -Mr. Catchpool, fully thought out by Mr. Gulick, and more recently -elaborated by Dr. Romanes, "Physiological Selection" is held to account for -the genesis of marked varieties side by side with their parents. It is -contended that without the kind of isolation implied by it, variations will -be swamped by inter-crossing, and divergence prevented; but that by the aid -of this kind of isolation, a uniform species may be differentiated into two -or more species, though its members continue to live in the same area. - -Facts are assigned to show that slightly unlike varieties may become unable -to inter-breed either with the parent-species or with one another. This -mutual inferiority is not of the kind we might expect. We might reasonably -suppose that when varieties had diverged widely, crossing would be -impracticable, because their constitutions had become so far unlike as to -form an unworkable mixture. But there seems evidence that the infertility -arises long before such a cause could operate, and that instead of failure -to produce a workable constitution, there is failure to produce any -constitution at all--failure to fertilize. Some change in the sexual system -is suggested as accounting for this. That a minute difference in the -reproductive elements may suffice, plants prove by the fact that when two -members of slightly-divergent varieties are fertilized by each other's -pollen, the fertility is less than if each were fertilized by the pollen of -its own variety; and where the two kinds of pollen are both used, that -derived from members of the same variety is prepotent in its effect over -that derived from members of the other variety. - -The writers above named contend that variations must occur in the -reproductive organs as well as in other organs; that such variations may -produce relative infertility in particular directions; and that such -relative infertility may be the first step towards prevention of crossing -and establishment of isolation: so making possible the accumulation of such -differences as mark off new species. Without doubt we have here a -legitimate supposition and a legitimate inference. Necessarily there must -happen variations of the kind alleged, and considering how sensitive the -reproductive system is to occult influences (witness among ourselves the -frequent infertility of healthy people while feeble unhealthy ones are -fertile), it is reasonable to infer that minute and obscure alterations of -this kind may make slightly-different varieties unable to inter-breed. - -Granting that there goes on this "physiological selection," we must -recognize it as one among the causes by which isolation is produced, and -the differentiating influence of natural selection in the same locality -made possible. - - -§ 174g. The foregoing criticisms and hypotheses do not, however, affect in -any essential way the pre-existing conceptions. If, as in the foregoing -chapters, we interpret the facts in terms of that redistribution of matter -and motion constituting Evolution at large, we shall see that the general -theory, as previously held, remains outstanding. - -It is indisputable that to maintain its life an organism must maintain the -moving equilibrium of its functions in presence of environing actions. This -is a truism: overthrow of the equilibrium is death. It is a corollary that -when the environment is changed, the equilibrium of functions is disturbed, -and there must follow one of two results--either the equilibrium is -overthrown or it is re-adjusted: there is a re-equilibration. Only two -possible ways of effecting the re-adjustment exist--the direct and the -indirect. In the one case the changed outer action so alters the moving -equilibrium as to call forth an equivalent reaction which balances it. If -re-equilibration is not thus effected in the individual it is effected in -the succession of individuals. Either the species altogether disappears, or -else there disappear, generation after generation, those members of it the -equilibria of whose functions are least congruous with the changed actions -in the environment; and this is the survival of the fittest or natural -selection. - -If now we persist in thus contemplating the problem as a statico-dynamical -one, we shall see that much of the discussion commonly carried on is beside -the question. The centre around which the collision of arguments has taken -place, is the question of the formation of species. But here we see that -this question is a secondary and, in a sense, irrelevant one. We are -concerned with the production of evolving and diverging organic forms; and -whether these are or are not marked off by so-called specific traits, and -whether they will or will not breed together, matters little to the general -argument. If two divisions of a species, falling into unlike conditions and -becoming re-equilibrated with them, eventually acquire the differences of -nature called specific, this is but a collateral result. The _essential_ -result is the formation of divergent organic forms. The biologic -atmosphere, so to speak, has been vitiated by the conceptions of past -naturalists, with whom the identification and classification of species was -the be-all and end-all of their science, and who regarded the traits which -enabled them to mark off their specimens from one another, as the traits of -cardinal importance in Nature. But after ignoring these technical ideas it -becomes manifest that the distinctions, morphological or physiological, -taken as tests of species, are merely incidental phenomena. - -Moreover, on continuing thus to look at the facts, we shall better -understand the relation between adaptive and specific characters, and -between specific characters and those many small differences which always -accompany them. For during re-equilibration there must, beyond those -changes of structure required to balance outer actions by inner actions, be -numerous minor changes. In any complex moving equilibrium alterations of -larger elements inevitably cause alterations of elements immediately -dependent on them, and these again of others: the effects reverberate and -re-reverberate throughout the entire aggregate of actions down to the most -minute. Of resulting structural changes a few will be conspicuous, more -will be less conspicuous, and so on continuously multiplying in number and -decreasing in amount. - -Here seems a fit place for remarking that there are certain processes which -do not enter into these re-equilibrations but in a sense interfere with -them. One example must suffice. Among dogs may be observed the trick of -rolling on some mass having a strong animal smell: commonly a decaying -carcase. This trick has probably been derived from the trick of rolling on -the body of an animal caught and killed, and so gaining a tempting odour. A -male dog which first did this, and left a trail apt to be mistaken for that -of prey, would be more easily found by a female, and would be more likely -than others to leave posterity. Now such a trick could have no relation to -better maintenance of the moving equilibrium, and might very well arise in -a dog having no superiority. If it arose in one of the worst it would be -eliminated from the species, but if it arose in one of medium constitution, -fairly capable of self-preservation, it would tend to produce survival of -certain of the less fit rather than the fittest. Probably there are many -such minor traits which are in a sense accidental, and are neither adaptive -nor specific in the ordinary sense. - - -§ 174h. But now let it be confessed that though all phenomena of organic -evolution must fall within the lines above indicated, there remain many -unsolved problems. - -Take as an instance the descent of the testes in the _Mammalia_. Neither -direct nor indirect equilibration accounts for this. We cannot consider it -an adaptive change, since there seems no way in which the production of -sperm-cells, internally carried on in a bird, is made external by -adjustment to the changed requirements of mammalian life. Nor can we -ascribe it to survival of the fittest; for it is incredible that any mammal -was ever advantaged in the struggle for life by this changed position of -these organs. Contrariwise, the removal of them from a place of safety to a -place of danger, would seem to be negatived by natural selection. Nor can -we regard the transposition as a concomitant of re-equilibration; since it -can hardly be due to some change in the general physiological balance. - -An example of another order is furnished by the mason-wasp. Several -instincts, capacities, peculiarities, which are in a sense independent -though they cooperate to the same end, are here displayed. There is the -instinct to build a cell of grains of sand, and the ability to do this, -which though in a sense separate may be regarded as an accompaniment; and -there is the secretion of a cement--a physiological process not directly -connected with the psychological process. After oviposition there comes -into play the instinct to seek, carry home, and pack into the cell, the -small caterpillars, spiders, &c., which are to serve as food for the larva; -and then there is the instinct to sting each of them at a spot where the -injected hypnotic poison keeps the creature insensible though alive till it -is wanted. These cannot be regarded as parts of a whole developed in -simultaneous coordination. There is no direct connexion between the -building instinct and the hypnotizing instinct; still less between these -instincts and the associated appliances. What were the early stages they -passed through imagination fails to suggest. Their usefulness depends on -their combination; and this combination would seem to have been useless -until they had all reached something like their present completeness. Nor -can we in this case ascribe anything to the influence of teaching by -imitation, supposed to explain the doings of social insects; for the -mason-wasp is solitary. - -Thus the process of organic evolution is far from being fully understood. -We can only suppose that as there are devised by human beings many puzzles -apparently unanswerable till the answer is given, and many necromantic -tricks which seem impossible till the mode of performance is shown; so -there are apparently incomprehensible results which are really achieved by -natural processes. Or, otherwise, we must conclude that since Life itself -proves to be in its ultimate nature inconceivable, there is probably an -inconceivable element in its ultimate workings. - - -END OF VOL. I. - - - - -APPENDICES. - - - - -APPENDIX A. - -THE GENERAL LAW OF ANIMAL FERTILITY. - - -[_In the_ Westminster Review _for April, 1852, I published an essay under -the title "A Theory of Population deduced from the General Law of Animal -Fertility." That essay was the germ of Part VI of this work, "The Laws of -Multiplication," in which its essential theses are fully developed. When -developing them, I omitted some portions of the original essay--one which -was not directly relevant, and another which contained a speculation open -to criticism. As indicated in § 74f, I find that this speculation has an -unexpected congruity with recent results of inquiry. I therefore decide to -reproduce it here along with the definition of Life propounded in that -essay, which, though subsequently replaced by the definition elaborated in -Part I, contains an element of truth._] - -* * * * * - -Some clear idea of the nature of Life itself, must, indeed, form a needful -preliminary. We may be sure that a search for the influences determining -the maintenance and multiplication of living organisms, cannot be -successfully carried out unless we understand what is the peculiar property -of a living organism--what is the widest generalization of the phenomena -that indicate life. By way of preparation, therefore, for the Theory of -Population presently to be developed, we propose devoting a brief space to -this prior question. - -* * * * * - -Employing the term, then, in its usual sense, as applicable only to -organisms, Life may be defined as--_the co-ordination of actions_. The -growth of a crystal, which is the highest inorganic process we are -acquainted with, involves but one action--that of accretion. The growth of -a cell, which is the lowest organic process, involves two -actions--accretion and disintegration--repair and waste--assimilation and -oxidation. Wholly deprive a cell of oxygen, and it becomes inert--ceases to -manifest vital phenomena; or, as we say, dies. Give it no matter to -assimilate, and it wastes away and disappears, from continual oxidation. -Evidently, then, it is in the balance of these two actions that the life -consists. It is not in the assimilation alone; for the crystal assimilates: -neither is it in the oxidation alone; for oxidation is common to inorganic -matter: but it is in the joint maintenance of these--the _co-ordination_ of -them. So long as the two go on together, life continues: suspend either of -them, and the result is--death. - -The attribute which thus distinguishes the lowest organic from the highest -inorganic bodies, similarly distinguishes the higher organisms from the -lower ones. It is in the greater complexity of the co-ordination--that is, -in the greater number and variety of the co-ordinated actions--that every -advance in the scale of being essentially consists. And whether we regard -the numerous vital processes carried on in a creature of complex structure -as so many additional processes, or whether, more philosophically, we -regard them as subdivisions of the two fundamental ones--oxidation and -accretion--the co-ordination of them is still the life. Thus turning to -what is physiologically classified as the _vegetative system_, we see that -stomach, lungs, heart, liver, skin, and the rest, must work in concert. If -one of them does too much or too little--that is, if the co-ordination be -imperfect--the life is disturbed; and if one of them ceases to act--that -is, if the co-ordination be destroyed--the life is destroyed. So likewise -is it with the _animal system_, which indirectly assists in co-ordinating -the actions of the viscera by supplying food and oxygen. Its component -parts, the limbs, senses, and instruments of attack or defence must perform -their several offices in proper sequence; and further, must conjointly -minister to the periodic demands of the viscera, that these may in turn -supply blood. How completely the several attributes of animal life come -within the definition, we shall best see on going through them _seriatim_. - -Thus _Strength_ results from the co-ordination of actions; for it is -produced by the simultaneous contraction of many muscles and many fibres of -each muscle; and the strength is great in proportion to the number of these -acting together--that is, in proportion to the co-ordination. _Swiftness_ -also, depending partly on strength, but requiring also the rapid -alternation of movements, equally comes under the expression; seeing that, -other things equal, the more quickly sequent actions can be made to follow -each other, the more completely are they co-ordinated. So, too, is it with -_Agility_; the power of a chamois to spring with safety from crag to crag -implies accurate co-ordination in the movements of many different muscles, -and a due subordination of them all to the perceptions. The definition -similarly includes _Instinct_, which consists in the uniform succession of -certain actions or series of actions after certain sensations or groups of -sensations; and that which surprises us in instinct is the accuracy with -which these compound actions respond to these compound sensations; that -is--the completeness of their co-ordination. Thus, likewise, is it with -_Intelligence_, even in its highest manifestations. That which we call -rationality is the power to combine, or co-ordinate a great number and a -great variety of complex actions for the achievement of a desired result. -The husbandman has in the course of years, by drainage and manuring, to -bring his ground into a fertile state; in the autumn he must plough, -harrow, and sow, for his next year's crop; must subsequently hoe and weed, -keep out cattle, and scare away birds; when harvest comes, must adapt the -mode and time of getting in his produce to the weather and the labour -market; he must afterwards decide when, and where, and how to sell to the -best advantage; and must do all this that he may get food and clothing for -his family. By properly coordinating these various processes (each of which -involves many others)--by choosing right modes, right times, right -quantities, right qualities, and performing his acts in right order, he -attains his end. But if he have done too little of this, or too much of -that; or have done one thing when he should have done another--if his -proceedings have been badly co-ordinated--that is, if he have lacked -intelligence--he fails. - -We find, then, that _the co-ordination of actions_ is a definition of Life, -which includes alike its highest and its lowest manifestations; and not -only so, but expresses likewise the degree of Life, seeing that the Life is -high in proportion as the co-ordination is great. Proceeding upwards, from -the simplest organic cell in which there are but two interdependent -actions, on through the group in which many such cells are acting in -concert, on through the higher group in which some of these cells assume -mainly the respiratory and others the assimilative function--proceeding -still higher to organisms in which these two functions are subdivided into -many others, and in which some cells begin to act together as contractile -fibres; next to organisms in which the visceral division of labour is -carried yet further, and in which many contractile fibres act together as -muscles--ascending again to creatures that combine the movements of several -limbs and many bones and muscles in one action; and further, to creatures -in which complex impressions are followed by the complex acts we term -instinctive--and arriving finally at man, in whom not only are the separate -acts complex, but who achieves his ends by combining together an immense -number and variety of acts often extending through years--we see that the -progress is uniformly towards greater co-ordination of actions. Moreover, -this co-ordination of actions unconsciously constitutes the essence of our -common notion of life; for we shall find, on inquiry, that when we infer -the death of an animal, which does not move on being touched, we infer it -because we miss the usual co-ordination of a sensation and a motion: and we -shall also find, that the test by which we habitually rank creatures high -or low in the scale of vitality is the degree of co-ordination their -actions exhibit. - -* * * * * - -There remains but to notice the objection which possibly may be raised, -that the co-ordination of actions is not life, but the ability to maintain -life. Lack of space forbids going into this at length. It must suffice to -say, that life and the ability to maintain life will be found the same. We -perpetually expend the vitality we have that we may continue our vitality. -Our power to breathe a minute hence depends upon our breathing now. We must -digest during this week that we may have strength to digest next. That we -may get more food, we must use the force which the food we have eaten gives -us. Everywhere vigorous life is the strength, activity, and sagacity -whereby life is maintained; and equally in descending the scale of being, -or in watching the decline of an invalid, we see that the ebbing away of -life is the ebbing away of the ability to preserve life.[62] - -[Only on now coming to re-read the definition of Life enunciated at the -commencement of this essay with the arguments used in justification of it, -does it occur to me that its essential thought ought to have been -incorporated in the definition of Life given in Part I. The idea of -co-ordination is there implied in the idea of correspondence, but the idea -of co-ordination is so cardinal a one that it should be expressed not by -implication but overtly. It is too late to make the required amendment in -the proper place, for the first part of this work is already stereotyped -and printed. Being unable to do better I make the amendment here. The -formula as completed will run:--The definite combination of heterogeneous -changes, both simultaneous and successful, _co-ordinated into_ -correspondence with external co-existences and sequences.] - -* * * * * - -Ending here this preliminary dissertation, let us now proceed to our -special subject. - - -§ 1. On contemplating its general circumstances, we perceive that any race -of organisms is subject to two sets of conflicting influences. On the one -hand by natural death, by enemies, by lack of food, by atmospheric changes, -&c., it is constantly being destroyed. On the other hand, partly by the -strength, swiftness and sagacity of its members, and partly by their -fertility, it is constantly being maintained. These conflicting sets of -influences may be conveniently generalized as--the forces destructive of -race, and the forces preservative of race. - - -§ 2. Whilst any race continues to exist, the forces destructive of it and -the forces preservative of it must perpetually tend towards equilibrium. If -the forces destructive of it decrease, the race must gradually become more -numerous, until, either from lack of food or from increase of enemies, the -destroying forces again balance the preserving forces. If, reversely, the -forces destructive of it increase, then the race must diminish, until, -either from its food becoming relatively more abundant, or from its enemies -dying of hunger, the destroying forces sink to the level of the preserving -forces. Should the destroying forces be of a kind that cannot be thus met -(as great change of climate), the race, by becoming extinct, is removed out -of the category. Hence this is necessarily the _law of maintenance_ of all -races; seeing that when they cease to conform to it they cease to be. - -Now the forces preservative of race are two--ability in each member of the -race to preserve itself, and ability to produce other members--power to -maintain individual life, and power to propagate the species. These must -vary inversely. When, from lowness of organization, the ability to contend -with external dangers is small, there must be great fertility to compensate -for the consequent mortality; otherwise the race must die out. When, on the -contrary, high endowments give much capacity of self-preservation, there -needs a correspondingly low degree of fertility. Given the dangers to be -met as a constant quantity; then, as the ability of any species to meet -them must be a constant quantity too, and as this is made up of the two -factors--power to maintain individual life and power to multiply--these -cannot do other than vary inversely. - - -§ 3. To show that observed phenomena harmonise with this _à priori_ -principle seems scarcely needful But, though axiomatic in its character, -and therefore incapable of being rendered more certain, yet illustrations -of the conformity to it which nature everywhere exhibits, will facilitate -the general apprehension of it. - -In the vegetable kingdom we find that the species consisting of simple -cells, exhibit the highest reproductive power. The yeast fungus, which in a -few hours propagates itself throughout a large mass of wort, offers a -familiar example of the extreme rapidity with which these lowly organisms -multiply. In the _Protococcus nivalis_, a microscopic plant which in the -course of a night reddens many square miles of snow, we have a like -example; as also in the minute _Algæ_, which colour the waters of stagnant -pools. The sudden appearance of green films on damp decaying surfaces, the -spread of mould over stale food, and the rapid destruction of crops by -mildew, afford further instances. If we ascend a step to plants of -appreciable size, we still find that in proportion as the organization is -low the fertility is great. Thus of the common puff-ball, which is little -more than a mere aggregation of cells, Fries says, "in a single individual -of _Reticularia maxima_, I have counted (calculated?) 10,000,000 sporules." -From this point upwards, increase of bulk and greater complexity of -structure are still accompanied by diminished reproductive power; instance -the _Macrocystis pyrifera_, a gigantic sea-weed, which sometimes attains a -length of 1500 feet, of which Carpenter remarks, "This development of the -nutritive surface takes place at the expense of the fructifying apparatus, -which is here quite subordinate."[63] And when we arrive at the -highly-organized exogenous trees, we find that not only are they many years -before beginning to bear with any abundance, but that even then they -produce, at the outside, but a few thousand seeds in a twelvemonth. During -its centuries of existence, an oak does not develop as many acorns as a -fungus does spores in a single night. - -Still more clearly is this truth illustrated throughout the animal kingdom. -Though not so great as the fertility of the Protophyta, which, as Prof. -Henslow says, in some cases passes comprehension, the fertility of the -Protozoa is yet almost beyond belief. In the polygastric animalcules -spontaneous fission takes place so rapidly that "it has been calculated by -Prof. Ehrenberg that no fewer than 268 millions might be produced in a -month from a single _Paramecium_;"[64] and even this astonishing rate of -increase is far exceeded in another species, one individual of which, "only -to be perceived by means of a high magnifying power, is calculated to -generate 170 billions in four days."[65] Amongst the larger organisms -exhibiting this lowest mode of reproduction under a modified form--that of -gemmation--we see that, though not nearly so rapid as in the Infusoria, the -rate of multiplication is still extremely high. This fact is well -illustrated by the polypes; and in the apparent suddenness with which whole -districts are blighted by the Aphis (multiplying by internal gemmation), we -have a familiar instance of the startling results which the parthenogenetic -process can achieve. Where reproduction becomes occasional instead of -continuous, as it does amongst higher creatures, the fertility equally -bears an inverse ratio to the development. "The queen ant of the African -_Termites_ lays 80,000 eggs in twenty-four hours; and the common hairworm -(_Gordius_) as many as 8,000,000 in less than one day."[66] Amongst the -_Vertebrata_ the lowest are still the most prolific. "It has been -calculated," says Carpenter, "that above a million of eggs are produced at -once by a single codfish."[67] In the strong and sagacious shark -comparatively few are found. Still less fertile are the higher reptiles. -And amongst the Mammalia, beginning with small Rodents, which quickly reach -maturity, produce large litters, and several litters in the year; advancing -step by step to the higher mammals, some of which are long in attaining the -reproductive age, others of which produce but one litter in a year, others -but one young one at a time, others who unite these peculiarities; and -ending with the elephant and man, the least prolific of all, we find that -throughout this class, as throughout the rest, ability to multiply -decreases as ability to maintain individual life increases. - - -§ 4. The _à priori_ principle thus exemplified has an obverse of a like -axiomatic character. We have seen that for the continuance of any race of -organisms it is needful that the power of self-preservation and the power -of reproduction should vary inversely. - -We shall now see that, quite irrespective of such an end to be subserved, -these powers could not do otherwise than vary inversely. In the nature of -things species can subsist only by conforming to this law; and equally in -the nature of things they cannot help conforming to it. - -Reproduction, under all its forms, may be described as the separation of -portions of a parent plant or animal for the purpose of forming other -plants or animals. Whether it be by spontaneous fission, by gemmation, or -by gemmules; whether the detached products be bulbels, spores or seeds, -ovisacs, ova or spermatozoa; or however the process of multiplication be -modified, it essentially consists in the throwing off of parts of adult -organisms for the purpose of making new organisms. On the other hand, self -preservation is fundamentally a maintenance of the organism in undiminished -bulk. Amongst the lowest forms of life, aggregation of tissue is the only -mode in which the self-preserving power is shown. Even in the highest, -sustaining the body in its integrity is that in which self-preservation -most truly consists--is the end which the widest intelligence is indirectly -made to subserve. Whilst, on the one side, it cannot be denied that the -increase of tissue constituting growth is self-preservation both in essence -and in result; neither can it, on the other side, be denied that a -diminution of tissue, either from injury, disease, or old age, is in both -essence and result the reverse. - -Hence the maintenance of the individual and the propagation of the race -being respectively aggregative and separative, _necessarily_ vary -inversely. Every generative product is a deduction from the parental life; -and, as already pointed out, to diminish life is to diminish the ability to -preserve life. The portion thrown off is organised matter; vital force has -been expended in the organisation of it, and in the assimilation of its -component elements; which vital force, had no such portion been made and -thrown off, _would have been available for the preservation of the parent_. - -Neither of these forces, therefore, can increase, save at the expense of -the other. The one draws in and incorporates new material; the other throws -off material previously incorporated. The one adds to; the other takes -from. Using a convenient expression for describing the facts (though one -that must not be construed into an hypothesis), we may say that the force -which builds up and repairs the individual is an attractive force, whilst -that which throws off germs is a repulsive force. But whatever may turn out -to be the true nature of the two processes, it is clear that they are -mutually destructive; or, stating the proposition in its briefest -form--Individuation and Reproduction are antagonistic. - -Again, illustrating the abstract by reference to the concrete, let us now -trace throughout the organic world the various phases of this antagonism. - - -§ 5. All the lowest animal and vegetable forms--_Protozoa_ and -_Protophyta_--consist essentially of a single cell containing fluid, and -having usually a solid nucleus. This is true of the Infusoria, the simplest -Entozoa, and the microscopic Algæ and Fungi. The organisms so constituted -uniformly multiply by spontaneous fission. The nucleus, originally -spherical, becomes elongated, then constricted across its smallest -diameter, and ultimately separates, when "its divisions," says Prof. Owen, -describing the process in the Infusoria, "seem to repel each other to -positions equidistant from each other, and from the pole or end of the body -to which they are nearest. The influence of these distinct centres of -assimilation is to divert the flow of the plasmatic fluid from a common -course through the body of the polygastrian to two special courses about -those centres. So much of the primary developmental process is renewed, as -leads to the insulation of the sphere of the influence of each assimilative -centre from that of the other by the progressive formation of a double -party wall of integument, attended by progressive separation of one party -wall from the other, and by concomitant constriction of the body of the -polygastrian, until the vibratile action of the superficial cilia of each -separating moiety severs the narrowed neck of union, and they become two -distinct individuals."[68] Similar in its general view is Dr. Carpenter's -description of the multiplication of vegetable cells, which he says divide, -"in virtue, it may be surmised, of a sort of mutual repulsion between the -two halves of the endochrome (coloured cell-contents) which leads to their -spontaneous separation."[69] Under a modified form of this process, the -cell-contents, instead of undergoing bisection, divide into numerous parts, -each of which ultimately becomes a separate individual. In some of the Algæ -"a whole brood of young cells may thus be at once generated in the cavity -of the parent-cell, which subsequently bursts and sets them free."[70] The -_Achlya prolifera_ multiplies after this fashion. Amongst the Fungi, too, -the same mode of increase is exemplified by the _Protococcus nivalis_. And -"it would appear that certain Infusoria, especially the _Kolpodinæ_, -propagate by the breaking-up of their own mass into reproductive -particles."[71] - -Now in this fissiparous mode of multiplication, which "is amazingly -productive, and indeed surpasses in fertility any other with which we are -acquainted,"[72] we see most clearly the antagonism between individuation -and reproduction. We see that the reproductive process involves destruction -of the individual; for in becoming two, the parent fungus or polygastrian -must be held to lose its own proper existence; and when it breaks up into a -numerous progeny, does so still more completely. Moreover, this rapid mode -of multiplication not only destroys the individuals in whom it takes place, -but also involves that their individualities, whilst they continue, shall -be of the lowest kind. For assume a protozoon to be growing by imbibition -at a given rate, and it follows that the oftener it divides the smaller -must be the size it attains to; that is, the smaller the development of its -individuality. And a further manifestation of the same truth is seen in the -fact that the more frequent the spontaneous fission the shorter the -existence of each individual. So that alike by preventing anything beyond a -microscopic bulk being attained, by preventing the continuance of this in -its integrity beyond a few hours, and by being fatal when it occurs, this -most active mode of reproduction shows the strongest antagonism to -individual life. - - -§ 6. Whether or not we regard reproduction as resulting from a repulsive -force (and, as seen above, both Owen and Carpenter lean to some such view), -and whether or not we consider the formation of the individual as due to -the reverse of this--an attractive force--we cannot, on studying the -phenomena, help admitting that two opposite activities thus generalized are -at work; we cannot help admitting that the aggregative and separative -tendencies do in each case determine the respective developments of the -individual and the race. On ascending one degree in the scale of organic -life, we shall find this truth clearly exemplified. - -For if these single-celled organisms which multiply so rapidly be supposed -to lose some of their separative tendency, what must be the result? They -now not only divide frequently, but the divided portions fly apart. How, -then, will a diminution of this separative tendency first show itself? May -we not expect that it will show itself in the divided portions _not_ flying -apart, but remaining near each other, and forming a group? This we find in -nature to be the first step in advance. The lowest compound organisms are -"_simple aggregations of vesicles without any definite arrangement, -sometimes united, but capable of existing separately_."[73] In these cases, -"every component cell of the aggregate mass that springs from a single -germ, being capable of existing independently of the rest, may be regarded -as a distinct individual."[74] The several stages of this aggregation are -very clearly seen in both the animal and vegetable kingdoms. In the -_Hæmatococcus binalis_, the plant producing the reddish slime seen on damp -surfaces, not only does each of the cells retain its original sphericity, -but each is separated from its neighbour by a wide interval filled with -mucus; so that it is only as being diffused through a mass of mucus common -to them all, that these cells can be held to constitute one individual. We -find, too, that "the component cells, even in the highest Algæ, are -generally separated from each other by a large quantity of mucilaginous -intercellular substance."[75] And, again, the tissue of the simpler -Lichens, "in consequence of the very slight adhesion of its component -cells, is said to be pulverulent."[76] Similarly the Protozoa, by their -feeble union, constitute the organisms next above them. Amongst the -Polygastrica there are many cases "in which the individuals produced by -fission or gemmation do not become completely detached from each -other."[77] The _Ophrydium_, for instance, "exists under the form of a -motionless jelly-like mass ... made up of millions of distinct and similar -individuals imbedded in a gelatinous connecting substance;"[78] and again, -the _Uvella_, or "grape monad," consists of a cluster "which strongly -resembles a transparent mulberry rolling itself across the field of view by -the ciliary action of its component individuals."[79] The parenchyma of the -Sponge, too, is made up of cells "each of which has the character of a -distinct animalcule, having a certain power of spontaneous motion, -obtaining and assimilating its own food, and altogether living _by_ and -_for_ itself;" and so small is the cohesion of these individual cells, that -the tissue they constitute "drains away when the mass is removed from the -water, like white of egg."[80] - -Of course in proportion as the aggregate tendency leading to the formation -of these groups of monads is strong, we may expect that, other things -equal, the groups will be large. Proceeding upwards from the yeast fungus, -whose cells hold together in groups of four, five, and six,[81] there must -be found in each species of these composite organisms a size of group -determined by the strength of the aggregative tendency in that species. -Hence we may expect that, when this limit is passed, the group no longer -remains united, but divides. Such we find to be the fact. These groups of -cells undergo the same process that the cells themselves do. They increase -up to a certain point, and then multiply either by simple spontaneous -fission or by that modification of it called gemmation. The _Volvox -globator_, which is made up of a number of monads associated together in -the form of a hollow sphere, develops within itself a number of smaller -spheres similarly constituted; and after these, swimming freely in its -interior, have reached a certain size, the parent group of animalcules -bursts and sets the interior groups free. And here we may observe how this -compound individuality of the Volvox is destroyed in the act of -reproduction as the simple individuality of the monad is. Again, in the -higher forms of grouped cells, where something like organisation begins to -show itself, the aggregations are not only larger, but the separative -process, now carried on by the method of gemmation, no longer wholly -destroys the individual. And in fact, this gemmation may be regarded as the -form which spontaneous fission must assume in ceasing to be fatal; seeing -that gemmation essentially consists in the separation, not into halves, but -into a larger part and a smaller part; the larger part continuing to -represent the original individual. Thus in the common _Hydra_ or -fresh-water polype, "little bud-like processes are developed from the -external surface, which are soon observed to resemble the parent in -character, possessing a digestive sac, mouth, and tentacula; for a long -time, however, their cavity is connected with that of the parent; but at -last the communication is cut off, and the young polype quits its -attachment, and goes in quest of its own maintenance."[82] - - -§ 7. Progress from these forms of organisation to still higher forms is -similarly characterized by increase of the aggregative tendency or -diminution of the separative, and similarly exhibits the necessary -antagonism between the development of the individual and the increase of -the race. That process of grouping which constitutes the first step towards -the production of complex organisms, we shall now find repeated in the -formation of series of groups. Just as a diminution of the separative -tendency is shown in the aggregation of divided monads, so is a further -diminution of it shown in the aggregation of the divided groups of monads. -The first instance that occurs is afforded by the compound polypes. "Some -of the simpler forms of the composite _Hydroida_," says Carpenter, "may be -likened to a _Hydra_, whose gemmæ, instead of becoming detached, remain -permanently connected with the parent; and as these in their turn may -develop gemmæ from their own bodies, a structure of more or less -arborescent character may be produced."[83] A similar species of -combination is observable amongst the _Bryozoa_, and the compound -_Tunicata_. Every degree of union may be found amongst these associated -organisms; from the one extreme in which the individuals can exist as well -apart as together, to the other extreme in which the individuals are lost -in the general mass. Whilst each _Bryozoon_ is tolerably independent of its -neighbour, "in the compound _Hydroida_, the lives of the polypes are -subordinate to that of the polypdom."[84] Of the _Salpidæ_ and -_Pyrosomidæ_, Carpenter says:--"Although closely attached to one another, -these associated animals are capable of being separated by a smart shock -applied to the sides of the vessel in which they are swimming.... In other -species, however, the separate animals are imbedded in a gelatinous mass," -and in one kind "there is an absolute union between the vascular systems of -the different individuals."[85] - -In the same manner that with a given aggregative tendency there is a limit -to the size of groups, so is there a similarly-determined limit to the size -of series of groups; and that spontaneous fission which we have seen in -cells and groups of cells we here find repeated. In the lower _Annelida_, -for example, "after the number of segments in the body has been greatly -multiplied by gemmation, a separation of those of the posterior portion -begins to take place; a constriction forms itself about the beginning of -the posterior third of the body, in front of which the alimentary canal -undergoes a dilatation, whilst on the segment behind it a proboscis and -eyes are developed, so as to form the head of the young animal which is to -be budded off; and in due time, by the narrowing of the constriction, a -complete separation is effected."[86] Not unfrequently in the _Nais_ this -process is repeated in the young one before it becomes independent of the -parent. The higher _Annelida_ are distinguished by the greater number of -segments held in continuity; an obvious result of comparatively infrequent -fission. In the class _Myriapoda_, which stands next above, "there is no -known instance of multiplication by fission."[87] Yet even here the law may -be traced both in the number and structure of the segments. The length of -the body is still increased after birth "by gemmation from (or partial -fission of) the penultimate segment." The lower members of the class are -distinguished from the higher by the greater extent to which this gemmation -is carried. Moreover, the growing aggregative tendency is seen in the fact, -that each segment of the Julus "is formed by the coalescence of two -original segments,"[88] whilst in the _Scolopendridæ_, which are the -highest of this class, "the head, according to Mr. Newport, is composed of -eight segments, which are often consolidated into one piece;"[89] both of -which phenomena may be understood as arrests of that process of fission, -which, if allowed to go a little further, would have produced distinct -segments; and, if allowed to go further still, would have separated these -segments into groups. - - -§ 8. Remarking, first, how gradually this mode of multiplication -disappears--how there are some creatures that spontaneously divide or not -according to circumstances; others that divide when in danger (the several -parts being capable of growing into complete individuals); others which, -though not self-dividing, can live on in each half if artificially divided; -and others in which only one of the divided halves can live--how, again, in -the Crustaceans the power is limited to the reproduction of lost limbs; how -there are certain reptiles that can re-supply a lost tail, but only -imperfectly; and how amongst the higher _Vertebrata_ the ability to repair -small injuries is all that remains--remarking thus much, let us now, by way -of preparation for what is to follow, consider the significance of the -foregoing facts taken in connection with the definition of Life awhile -since given. - -This spontaneous fission, which we have seen to be, in all cases, more or -less destructive of individual life, is simply a cessation in the -co-ordination of actions. From the single cell, the halves of whose -nucleus, instead of continuing to act together, begin to repel each other, -fly apart, establish distinct centres of assimilation, and finally cause -the cell to divide; up to the Annelidan, whose string of segments -separates, after reaching a certain length; we everywhere see the -phenomenon to be fundamentally this. The tendency to separate is the -tendency not to act together, probably arising from inability to act -together any longer; and the process of separation is the process of -ceasing to act together. How truly non-co-ordination is the essence of the -matter will be seen on observing that fission takes place more or less -rapidly, according as the co-ordinating apparatus is less or more -developed. Thus, "the capability of spontaneous division is one of the most -distinctive attributes of the acrite type of structure;"[90] the acrite -type of structure being that in which the neurine or nervous matter is -supposed to be diffused through the tissues in a molecular state, and in -which, therefore, there exists no distinct nervous or co-ordinating system. -From this point upwards the gradual disappearance of spontaneous fission is -clearly related to the gradual appearance of nerves and ganglia--a fact -well exemplified by the several grades of _Annelida_ and _Myriapoda_. And -when we remember that in the embryotic development of these classes, the -nervous system does not make its appearance until after the rest of the -organism has made great progress, we may even suspect that that coalescence -of segments characteristic of the _Myriapoda_, exhibits the co-ordinating -power of the rapidly-growing nervous system overtaking and arresting the -separative tendency; and doing this most where it (the nervous system) is -most developed, namely, in the head. - -And here let us remark, in passing, how, from this point of view, we still -more clearly discern the antagonism of individuation and reproduction. We -before saw that the propagation of the race is at the expense of the -individual: in the above facts we may contemplate the obverse of this--may -see that the formation of the individual is at the expense of the race. -This combination of parts that are tending to separate and become distinct -beings--this union of many incipient minor individualities into one large -individuality--is an arrest of reproduction--a diminution in the number -produced. Either these units may part and lead independent lives, or they -may remain together and have their actions co-ordinated. Either they may, -by their diffusion, form a small, simple, and prolific race, or, by their -aggregation, a large, complex, and infertile one. But manifestly the -aggregation involves the infertility; and the fertility involves the -smallness. - - -§ 9. The ability to multiply by spontaneous fission, and the ability to -maintain individual life, are opposed in yet another mode. It is not in -respect of size only, but still more in respect of structure, that the -antagonism exists. - -Higher organisms are distinguished from lower ones partly by bulk, and -partly by complexity. This complexity essentially consists in the mutual -dependence of numerous different organs, each subserving the lives of the -rest, and each living by the help of the rest. Instead of being made up of -many like parts, performing like functions, as the Crinoid, the Star-fish, -or the Millipede, a vertebrate animal is made up of many unlike parts, -performing unlike functions. From that initial form of a compound organism, -in which a number of minor individuals are simply grouped together, we may, -more or less distinctly, trace not only the increasing closeness of their -union, and the gradual disappearance of their individualities in that of -the mass, but the gradual assumption by them of special duties. And this -"physiological division of labour," as it has been termed, has the same -effect as the division of labour amongst men. As the preservation of a -number of persons is better secured when, uniting into a society, they -severally undertake different kinds of work, than when they are separate -and each performs for himself every kind of work; so the preservation of a -congeries of parts, which, combining into one organism, respectively assume -nutrition, respiration, circulation, locomotion, as separate functions, is -better secured than when those parts are independent, and each fulfils for -itself all these functions. - -But the condition under which this increased ability to maintain life -becomes possible is, that the parts shall cease to separate. While they are -perpetually separating, it is clear that they cannot assume mutually -subservient duties. And it is further clear that the more the tendency to -separate diminishes, that is, the larger the groups that remain connected, -_the more minutely and perfectly can that subdivision of functions which we -call organization be carried out_. - -Thus we see that in its most active form the ability to multiply is -antagonistic to the ability to maintain individual life, not only as -preventing increase of bulk, but also as preventing organization--not only -as preventing homogeneous co-ordination, but as preventing heterogeneous -co-ordination. - - -§ 10. To establish the unbroken continuity of this law of fertility, it -will be needful, before tracing its results amongst the higher animals, to -explain in what manner spontaneous fission is now understood, and what the -cessation of it essentially means. Originally, naturalists supposed that -creatures which multiply by self-division, under any of its several forms, -continue so to multiply perpetually. In many cases, however, it has -latterly been shown that they do not do this; and it is now becoming a -received opinion that they do not, and cannot, do this, in any case. A -fertilised germ appears here, as amongst higher organisms, to be the point -of departure; and that constant formation of new tissue implied in the -production of a great number of individuals by fission, seems gradually to -exhaust the germinal capacity in the same way that the constant formation -of new tissue, during the development of a single mammal, exhausts it. The -phenomena classified by Steenstrup as "Alternate Generation," and since -generalised by Professor Owen in his work "On Parthenogenesis," illustrate -this. The egg of a _Medusa_ (jellyfish) develops into a polypoid animal -called the _Strobila_. This _Strobila_ lives as the polype does, and, like -it, multiplies rapidly by gemmation. After a great number of individuals -has been thus produced, and when, as we must suppose, the germinal capacity -is approaching exhaustion, each _Strobila_ begins to exhibit a series of -constrictions, giving it some resemblance to a rouleau of coin or a pile of -saucers. These constrictions deepen; the segments gradually develop -tentacula; the terminal segment finally separates itself, and swims away in -the form of a young _Medusa_; the other segments, in succession, do the -same; and from the eggs which these _Medusæ_ produce, other like series of -polypoid animals, multiplying by gemmation, originate. In the compound -Polypes, in the _Tunicata_, in the _Trematoda_, and in the Aphis, we find -repeated, under various modifications, the same phenomenon. - -Understanding then, this lowest and most rapid mode of multiplication to -consist essentially in the production of a great number of individuals from -a single germ--perceiving, further, that diminished activity of this mode -of multiplication consists essentially in the aggregation of the -germ-product into larger masses--and seeing, lastly, that the disappearance -of this mode of multiplication consists essentially in the aggregation of -the germ-product into _one_ mass--we shall be in a position to comprehend, -amongst the higher animals, that new aspect of the law, under which -increased individuation still involves diminished reproduction. Progressing -from those lowest forms of life in which a single ovum originates countless -organisms, through the successive stages in which the number of organisms -so originated becomes smaller and smaller; and finally arriving at a stage -in which one ovum produces but one organism; we have now, in our further -ascent, to observe the modified mode in which this same necessary -antagonism between the ability to multiply, and the ability to preserve -individual life, is exhibited. - - -§ 11. Throughout both the animal and vegetable kingdoms, generation is -effected "by the union of the contents of a 'sperm-cell' with those of a -'germ-cell;' the latter being that from within which the embryo is evolved, -whilst the former supplies some material or influence necessary to its -evolution."[91] Amongst the lowest vegetable organisms, as in the -_Desmideæ_, the _Diatomaceæ_, and other families of the inferior _Algæ_, -those cells do not appreciably differ; and the application to them of the -terms "sperm-cell" and "germ-cell" is hypothetical. From this point -upwards, however, distinctions become visible. As we advance to higher and -higher types of structure, marked differences arise in the character of -these cells, in the organs evolving them, and in the position of these -organs, which are finally located in separate sexes. Doubtless a separation -in the _functions_ of "sperm-cell" and "germ-cell" has simultaneously -arisen. That change from homogeneity of function to heterogeneity of -function which essentially constitutes progress in organization may be -assumed to take place here also; and, indeed, it is probable that the -distinction gradually established between these cells, in origin and -appearance, is merely significant of, and consequent upon, the distinction -that has arisen between them in constitution and office. Let us now inquire -in what this distinction consists. - -If the foundation of every new organism be laid by the combination of two -elements, we may reasonably suspect that these two elements are typical of -some two fundamental divisions of which the new organism is to consist. As -nothing in nature is without meaning and purpose, we may be sure that the -universality of this binary origin, signifies the universality of a binary -structure. The simplest and broadest division of which an organism is -capable must be that signified. What, then, must this division be? - -The proposed definition of organic life supplies an answer. If organic life -be the co-ordination of actions, then an organism may be primarily divided -into parts whose actions are co-ordinated, and parts which co-ordinate -them--organs which are made to work in concert, and the apparatus which -makes them so work--or, in other words, the assimilative, vascular, -excretory, and muscular systems on the one hand, and the nervous system on -the other. The justness of this classification will become further -apparent, when it is remembered that by the nervous system alone is the -individuality established. By it all parts are made one in purpose, instead -of separate; by it the organism is rendered a conscious whole--is enabled -to recognise its own extent and limits; and by it are all injuries -notified, repairs directed, and the general conservation secured. The more -the nervous system is developed, the more reciprocally subservient do the -components of the body become--the less can they bear separating. And that -which thus individuates many parts into one whole, must be considered as -more broadly distinguished from the parts individuated, than any of these -parts from each other. Further evidence in support of this position may be -drawn from the fact, that as we ascend in the scale of animal life, that -is, as the co-ordination of actions becomes greater, we find the -co-ordinating or nervous system becoming more and more definitely separated -from the rest; and in the vertebrate or highest type of structure we find -the division above insisted on distinctly marked. The co-ordinating parts -and the parts co-ordinated are placed on opposite sides of the vertebral -column. With the exception of a few ganglia, the whole of the nervous -masses are contained within the neural arches of the vertebræ; whilst all -the viscera and limbs are contained within, or appended to, the hæmal -arches--the terms neural and hæmal having, indeed, been chosen to express -this fundamental division. - -If, then, there be truth in the assumption that the two elements, which, by -their union, give origin to a new organism, typify the two essential -constituents of such new organism, we must infer that the sperm-cell and -germ-cell respectively consist of co-ordinating matter and matter to be -co-ordinated--neurine and nutriment. That apparent identity of sperm-cell -and germ-cell seen in the lowest forms of life may thus be understood as -significant to the fact that no extended co-ordination of actions exists in -the generative product--each cell being a separate individual; and the -dissimilarity seen in higher organic types may, conversely, be understood -as expressive of, and consequent upon, the increasing degree of -co-ordination exhibited.[92] - -That the sperm-cell and germ-cell are thus contrasted in nature and -function may further be suspected on considering the distinctive -characteristics of the sexes. Of the two elements they respectively -contribute to the formation of a fertile germ, it may be reasonably -supposed that each furnishes that which it possesses in greatest abundance -and can best spare. Well, in the greater size of the nervous centres in the -male, as well as in the fact that during famines men succumb sooner than -women, we see that in the male the co-ordinating system is relatively -predominant. From the same evidence, as well as from the greater abundance -of the cellular and adipose tissues in women, we may infer that the -nutritive system predominates in the female.[93] Here, then, is additional -support for the hypothesis that the sperm-cell, which is supplied by the -male, contains co-ordinating matter, and the germ-cell, which is supplied -by the female, contains matter to be co-ordinated. - -The same inference may, again, be drawn from a general view of the maternal -function. For if, as we see, it is the office of the mother to afford milk -to the infant, and during a previous period to afford blood to the foetus, -it becomes probable that during a yet earlier stage it is still the -function to supply nutriment, though in another form. Indeed when, -ascending gradually the scale of animal life, we perceive that this -supplying of milk, and before that of blood, is simply a continuation of -the previous process, we may be sure that, with Nature's usual consistency, -this process is essentially one from the beginning. - -Quite in harmony with this hypothesis concerning the respective natures of -the sperm-cell and germ-cell is a remark of Carpenter's on the same -point:-- - - "Looking," he says, "to the very equal mode in which the characters of - the two parents are mingled in _hybrid_ offspring, and to the certainty - that the _material_ conditions which determine the development of the - germ are almost exclusively female, it would seem probable that the - _dynamical_ conditions are, in great part, furnished by the male."[94] - - -§ 12. Could nothing but the foregoing indirect evidence be adduced in proof -of the proposition that the spermatozoon is essentially a neural element, -and the ovum essentially a hæmal element, we should scarcely claim for it -anything more than plausibility. On finding, however, that this indirect -evidence is merely introductory to evidence of a quite direct nature, its -significance will become apparent. Adding to their weight taken separately -the force of their mutual confirmation, these two series of proofs will be -seen to give the hypothesis a high degree of probability. The direct -evidence now to be considered is of several kinds. - -On referring to the description of the process of multiplication in monads, -quoted some pages back (§ 5), from Professor Owen, the reader will perceive -that it is by the pellucid nucleus that the growth and reproduction of -these single-celled creatures are regulated. The nucleus controls the -circulation of the plasmatic fluid; the fission of the nucleus is the first -step towards the formation of another cell; each half of the divided -nucleus establishes round itself an independent current; and, apparently, -it is by the repulsion of the nuclei that the separation into two -individuals is finally effected. All which facts, when generalised, imply -that the nucleus is the governing or _co-ordinating_ part. Now, Professor -Owen subsequently points out that the matter of the sperm-cell performs in -the fertilised germ-cell just this same function which the nucleus performs -in a single-celled animal. We find the absorption by a germ-cell of the -contents of a sperm-cell "followed by the appearance of a pellucid nucleus -in the centre of the opaque and altered germ-cell; we further see its -successive fissions governed by the preliminary division of the pellucid -centre;" and, led by these and other facts, Professor Owen thinks that "one -cannot reasonably suppose that the nature and properties of the nucleus of -the impregnated germ-cell and that of the monad can be different."[95] And -hence he further infers that "the nucleus of the monad is of a nature -similar to, if not identical with," the matter of the spermatozoon. But we -have seen that in the monad the nucleus is the co-ordinating part; and -hence to say that the sperm-cell is, in nature, identical with it, is to -say that the sperm-cell consists of co-ordinating matter. - -Chemical analysis affords further evidence, though, from the imperfect data -at present obtained, less conclusive evidence than could be wished. Partly -from the white and gray nervous substances having been analysed together -instead of separately, and partly from the difficulty of isolating the -efficient contents of the sperm-cells, a satisfactory comparison cannot be -made. Nevertheless, possessing in common, as they do, one element, by which -they are specially characterised, the analysis, as far as it goes, supports -our argument. The following table, which has been made up from data given -in the _Cyclopædia of Anatomy and Physiology, Art._ NERVOUS SYSTEM, gives -the proportion of this element in the brain in different conditions, and -shows how important is its presence. - - +-----------------------------+--------+-------+-------+--------+-------+ - | | In | In | In | In | In | - | |Infants.| Youth.|Adults.|Old Men.|Idiots.| - | +--------+-------+-------+--------+-------+ - | Solid constituents in a | | | | | | - | hundred parts of the brain | 17.21 | 25.74 | 27.49 | 26.15 | 29.07 | - | Of these solid constituents | | | | | | - | the phosphorus amounts to | 0.8 | 1.65 | 1.80 | 1.00 | 0.85 | - | Which gives a percentage of | | | | | | - | phosphorus in the solid | | | | | | - | constituents of | 4.65 | 6.41 | 6.54 | 3.82 | 2.92 | - +-----------------------------+--------+-------+-------+--------+-------+ - -This connection between the quantity of phosphorus present and the degree -of mental power exhibited, is sufficiently significant; and the fact that -in the same individual the varying degrees of cerebral activity are -indicated by the varying quantities of alkaline phosphates excreted by the -kidneys,[96] still more clearly shows the essentialness of phosphorus as a -constituent of nervous matter. Respecting the constitution of sperm-cells -chemists do not altogether agree. One thing, however, is certain--that they -contain unoxidized phosphorus; and also a fatty acid, that is not -improbably similar to the fatty acid contained in neurine.[97] In fact, -there would seem to be present the constituents of that oleophosphoric acid -which forms so distinctive an element of the brain. That a large quantity -of binoxide of protein is also present, may be ascribed to the fact that a -great part of the sperm-cell consists merely of the protective membrane and -its locomotive appendage; the really efficient portion being but the -central contents.[98] - -Evidence of a more conclusive nature--evidence, too, which will show in -what direction our argument tends--is seen in the marked antagonism of the -nervous and generative systems. Thus, the fact that intense mental -application, involving great waste of the nervous tissues, and a -corresponding consumption of nervous matter for their repair, is -accompanied by a cessation in the production of sperm-cells, gives strong -support to the hypothesis that the sperm-cells consist essentially of -neurine. And this becomes yet clearer on finding that the converse fact is -true--that undue production of sperm-cells involves cerebral inactivity. -The first result of a morbid excess in this direction is headache, which -may be taken to indicate that the brain is out of repair; this is followed -by stupidity; should the disorder continue, imbecility supervenes, ending -occasionally in insanity. - -That the sperm-cell is co-ordinating matter, and the germ-cell matter to be -co-ordinated, is, therefore, an hypothesis not only having much _à priori_ -probability, but one supported by numerous facts. - - -§ 13. This hypothesis alike explains, and is confirmed by, the truth, that -throughout the vertebrate tribes the degree of fertility varies inversely -as the development of the nervous system. - -The necessary antagonism of Individuation and Reproduction does indeed show -itself amongst the higher animals, in some degree in the manner hitherto -traced; namely, as determining the total bulk. Though the parts now thrown -off, being no longer segments or gemmæ, are not obvious diminutions of the -parent, yet they must be really such. Under the form of internal fission, -the separative tendency is as much opposed to the aggregative tendency as -ever; and, _other things equal_, the greater or less development of the -individual depends upon the less or greater production of new individuals -or germs of new individuals. As in groups of cells, and series of groups of -cells, we saw that there was in each species a limit, passing which, the -germ product would not remain united; so in each species of higher animal -there is a limit, passing which, the process of cell-multiplication results -in the throwing off of cells, instead of resulting in the formation of more -tissue. Hence, taking an average view, we see why the smaller animals so -soon arrive at a reproductive age, and produce large and frequent broods; -and why, conversely, increased size is accompanied by retarded and -diminished fertility. - -But, as above implied, it is not so much to the bulk of the body as a -whole, as to the bulk of the nervous system, that fertility stands related -amongst the higher animals. Probably, indeed, it stands thus related in all -cases; the difference simply arising from the fact, that whereas in the -lower organisms, where the nervous system is not concentrated, its bulk -varies as the bulk of the body, in the higher organisms it does not do so. -Be this as it may, however, we see clearly that, amongst the vertebrata, -the bodily development is not the determining circumstance. In a fish, a -reptile, a bird, and a mammal of the same weight, there is nothing like -equality of fecundity. Cattle and horses, arriving as they do so soon at a -reproductive age, are much more prolific than the human race, at the same -time that they are much larger. And whilst, again, the difference in size -between the elephant and man is far greater, their respective powers of -multiplication are less unlike. Looking in these cases at the nervous -systems, however, we find no such discrepancy. On learning that the average -ratio of the brain to the body is--in fishes, 1 to 5668; in reptiles, 1 to -1321; in birds, 1 to 212; and in mammals, 1 to 186;[99] their different -degrees of fecundity are accounted for. Though an ox will outweigh -half-a-dozen men, yet its brain and spinal cord are far less than those of -one man; and though in bodily development the elephant so immensely exceeds -the human being, yet the elephant's cerebro-spinal system is only thrice -the size attained by that of civilized men.[100] Unfortunately, it is -impossible to trace throughout the animal kingdom this inverse relationship -between the nervous and reproductive systems with any accuracy. Partly from -the fact that, in each case, the degree of fertility depends on three -variable elements--the age at which reproduction begins, the number -produced at a birth, and the frequency of the births; partly from the fact -that, in respect to most animals, these data are not satisfactorily -attainable, and that, when they are attainable, they are vitiated by the -influence of domesticity; and partly from the fact that no precise -measurement of the respective nervous systems has been made, we are unable -to draw any but general and somewhat vague comparisons. These, however, as -far as they go, are in our favour. Ascending from beings of the acrite -nerveless type, which are the most prolific of all, through the various -invertebrate sub-kingdoms, amongst which spontaneous fission disappears as -the nervous system becomes developed; passing again to the least nervous -and most fertile of the vertebrate series--Fishes, of which, too, the -comparatively large-brained cartilaginous kinds multiply much less rapidly -than the others; progressing through the more highly endowed and less -prolific Reptiles to the Mammalia, amongst which the Rodents, with their -unconvoluted brains, are noted for their fecundity; and ending with man and -the elephant, the least fertile and largest-brained of all--there seems to -be throughout a constant relationship between these attributes. - -And indeed, on turning back to our _à priori_ principle, no other -relationship appears possible. We found it to be the necessary law of -maintenance of races, that the ability to maintain individual life and the -ability to multiply vary inversely. But the ability to maintain individual -life _is in all cases measured by the development of the nervous system_. -If it be in good visceral organization that the power of self-preservation -is shown, this implies some corresponding nervous apparatus to secure -sufficient food. If it be in strength, there must be a provision of nerves -and nervous centres answering to the number and size of the muscles. If it -be in swiftness and agility, a proportionate development of the cerebellum -is presupposed. If it be in intelligence, this varies with the size of the -cerebrum. As in all cases co-ordination of actions constitutes the life, -or, what is the same thing, the ability to maintain life; and as throughout -the animal kingdom this co-ordination, under all its forms, is effected by -nervous agents of some kind or other; and as each of these nervous agents -performs but one function; it follows that in proportion to the number of -the actions co-ordinated must be the number of nervous agents. Hence the -nervous system becomes the universal measure of the degree of co-ordination -of actions; that is, of the life, or ability to maintain life. And if the -nervous system varies directly as the ability to maintain life, it _must_ -vary inversely as the ability to multiply.[101] - -And here, assuming the constitution of the sperm-cell above inferred to be -the true one, we see how the obverse _à priori_ principle is fulfilled. -Where, as amongst the lowest organisms, bulk is expressive of life, the -antagonism of individuation and reproduction was broadly exhibited in the -fact that the making of two or more new individuals was the _un_making of -the original individual. And now, amongst the higher organisms, where bulk -is no longer the measure of life, we see that this antagonism is between -the neural elements thrown off, and that internal neural mass whose bulk -_is_ the measure of life. The production of co-ordinating cells must be at -the expense of the co-ordinating apparatus; and the aggregation of the -co-ordinating apparatus must be at the expense of co-ordinating cells. How -the antagonism affects the female economy is not so clear. Possibly the -provision required to be made for supplying nervous as well as other -nutriment to the embryo, involves an arrest in the development of the -nervous system; and if so, probably this arrest takes place early in -proportion as the number of the coming offspring makes the required -provision great: or rather, to put the facts in their right sequence, an -early arrest renders the production of a numerous offspring possible. - - -§ 14. The law which we have thus traced throughout the animal kingdom, and -which must alike determine the different fertilities of different species, -and the variations of fertility in the same species, we have now to -consider in its application to mankind. - - [_The remainder of the essay, which as implied, deals with the - application of this general principle to the multiplication of the human - race, need not be here reproduced. The subject is treated in full in Part - VI._] - - - - -APPENDIX B. - -THE INADEQUACY OF NATURAL SELECTION, ETC., ETC. - - -[_In this Appendix are included four essays originally published in the_ -Contemporary Review _and subsequently republished as pamphlets. The first -appeared under the above title in February and March, 1893; the second in -May of that year under the title "Prof. Weismann's Theories;" the third in -December of that year under the title "A Rejoinder to Prof. Weismann;" and -the fourth in October, 1894, under the title "Weismannism Once More." As -these successive essays practically form parts of one whole, I have thought -it needless to keep them separate by repeating their titles, and have -simply marked them off from one another by the numbers I, II, III, IV. Of -course, as they are components of a controversy, some incompleteness arises -from the absence of the essays to which portions of them were replies; but -in each the course of the argument sufficiently indicates the -counter-arguments which were met._] - - -I. - -Students of psychology are familiar with the experiments of Weber on the -sense of touch. He found that different parts of the surface differ widely -in their ability to give information concerning the things touched. Some -parts, which yielded vivid sensations, yielded little or no knowledge of -the sizes or forms of the things exciting them; whereas other parts, from -which there came sensations much less acute, furnished clear impressions -respecting the tangible characters, even of relatively small objects. These -unlikenesses of tactual discriminativeness he ingeniously expressed by -actual measurements. Taking a pair of compasses, he found that if they were -closed so nearly that the points were less than one-twelfth of an inch -apart, the end of the forefinger could not perceive that there were two -points: the two points seemed one. But when the compasses were opened so -that the points were one-twelfth of an inch apart, then the end of the -forefinger distinguished the two points. At the same time, he found that -the compasses must be opened to the extent of two and a half inches, before -the middle of the back could distinguish between two points and one. That -is to say, as thus measured, the end of the forefinger has thirty times the -tactual discriminativeness which the middle of the back has. - -Between these extremes he found gradations. The inner surfaces of the -second joints of the fingers can distinguish separateness of positions only -half as well as the tip of the forefinger. The innermost joints are still -less discriminating, but have powers of discrimination equal to that of the -tip of the nose. The end of the great toe, the palm of the hand, and the -cheek, have alike one-fifth of the perceptiveness which the tip of the -forefinger has; and the lower part of the forehead has but one-half that -possessed by the cheek. The back of the hand and the crown of the head are -nearly alike in having but a fourteenth or a fifteenth of the ability to -perceive positions as distinct, which is possessed by the finger-end. The -thigh, near the knee, has rather less, and the breast less still; so that -the compasses must be opened more than an inch and a half before the breast -distinguishes the two points from one another. - -What is the meaning of these differences? How, in the course of evolution, -have they been established? If "natural selection," or survival of the -fittest, is the assigned cause, then it is required to show in what way -each of these degrees of endowment has advantaged the possessor to such -extent that not infrequently life has been directly or indirectly preserved -by it. We might reasonably assume that in the absence of some -differentiating process, all parts of the surface would have like powers of -perceiving relative positions. They cannot have become widely unlike in -perceptiveness without some cause. And if the cause alleged is natural -selection, then it is necessary to show that the greater degree of the -power possessed by this part than by that, has not only conduced to the -maintenance of life, but has conduced so much that an individual in whom a -variation has produced better adjustment to needs, thereby maintained life -when some others lost it; and that among the descendants inheriting this -variation, there was a derived advantage such as enabled them to multiply -more than the descendants of individuals not possessing it. Can this, or -anything like this, be shown? - -That the superior perceptiveness of the forefinger-tip has thus arisen, -might be contended with some apparent reason. Such perceptiveness is an -important aid to manipulation, and may have sometimes given a life-saving -advantage. In making arrows or fish-hooks, a savage possessing some extra -amount of it may have been thereby enabled to get food where another -failed. In civilized life, too, a sempstress with well-endowed finger-ends -might be expected to gain a better livelihood than one with finger-ends -which were obtuse; though this advantage would not be so great as appears. -I have found that two ladies whose finger-ends were covered with -glove-tips, reducing their sensitiveness from one-twelfth of an inch -between compass-points to one-seventh, lost nothing appreciable of their -quickness and goodness in sewing. An experience of my own here comes in -evidence. Towards the close of my salmon-fishing days I used to observe -what a bungler I had become in putting on and taking off artificial flies. -As the tactual discriminativeness of my finger-ends, recently tested, comes -up to the standard specified by Weber, it is clear that this decrease of -manipulative power, accompanying increase of age, was due to decrease in -the delicacy of muscular co-ordination and sense of pressure--not to -decrease of tactual discriminativeness. But not making much of these -criticisms, let us admit the conclusion that this high perceptive power -possessed by the forefinger-end may have arisen by survival of the fittest; -and let us limit the argument to the other differences. - -How about the back of the trunk and its face? Is any advantage derived from -possession of greater tactual discriminativeness by the last than the -first? The tip of the nose has more than three times the power of -distinguishing relative positions which the lower part of the forehead has. -Can this greater power be shown to have any advantage? The back of the hand -has scarcely more discriminative ability than the crown of the head, and -has only one-fourteenth of that which the finger-tip has. Why is this? -Advantage might occasionally be derived if the back of the hand could tell -us more than it does about the shapes of the surfaces touched. Why should -the thigh near the knee be twice as perceptive as the middle of the thigh? -And, last of all, why should the middle of the forearm, middle of the -thigh, middle of the back of the neck, and middle of the back, all stand on -the lowest level, as having but one-thirtieth of the perceptive power which -the tip of the forefinger has? To prove that these differences have arisen -by natural selection, it has to be shown that such small variation in one -of the parts as might occur in a generation--say one-tenth extra -amount--has yielded an appreciably greater power of self-preservation; and -that those inheriting it have continued to be so far advantaged as to -multiply more than those who, in other respects equal, were less endowed -with this trait. Does any one think he can show this? - -But if this distribution of tactual perceptiveness cannot be explained by -survival of the fittest, how can it be explained? The reply is that, if -there has been in operation a cause which it is now the fashion among -biologists to ignore or deny, these various differences are at once -accounted for. This cause is the inheritance of acquired characters. As a -preliminary to setting forth the argument showing this, I have made some -experiments. - -It is a current belief that the fingers of the blind, more practised in -tactual exploration than the fingers of those who can see, acquire greater -discriminativeness: especially the fingers of those blind who have been -taught to read from raised letters. Not wishing to trust to this current -belief, I recently tested two youths, one of fifteen and the other younger, -at the School for the Blind in Upper Avenue Road, and found the belief to -be correct. I found that instead of being unable to distinguish between -points of the compasses until they were opened to one-twelfth of an inch -apart, both of them could distinguish between points when only -one-fourteenth of an inch apart. They had thick and coarse skins; and -doubtless, had the intervening obstacle, so produced, been less, the -discriminative power would have been greater. It afterwards occurred to me -that a better test would be furnished by those whose finger-ends are -exercised in tactual perceptions, not occasionally, as by the blind in -reading, but all day long in pursuit of their occupations. The facts -answered expectation. Two skilled compositors, on whom I experimented, were -both able to distinguish between points when they were only one-seventeenth -of an inch apart. Thus we have clear proof that constant exercise of the -tactual nervous structure leads to further development.[102] - -Now if acquired structural traits are inheritable, the various contrasts -above set down are obvious consequences; for the gradations in tactual -perceptiveness correspond with the gradations in the tactual exercises of -the parts. Save by contact with clothes, which present only broad surfaces -having but slight and indefinite contrast, the trunk has scarcely any -converse with external bodies, and it has but small discriminative power; -but what discriminative power it has is greater on its face than on its -back, corresponding to the fact that the chest and abdomen are much more -frequently explored by the hands: this difference being probably in part -inherited from inferior creatures; for, as we may see in dogs and cats, the -belly is far more accessible to feet and tongue than the back. No less -obtuse than the back are the middle of the back of the neck, the middle of -the forearm, and the middle of the thigh; and these parts have but rare -experiences of irregular foreign bodies. The crown of the head is -occasionally felt by the fingers, as also the back of one hand by the -fingers of the other; but neither of these surfaces, which are only twice -as perceptive as the back, is used with any frequency for touching objects, -much less for examining them. The lower part of the forehead, though more -perceptive than the crown of the head, in correspondence with a somewhat -greater converse with the hands, is less than one-third as perceptive as -the tip of the nose; and manifestly, both in virtue of its relative -prominence, in virtue of its contacts with things smelt at, and in virtue -of its frequent acquaintance with the handkerchief, the tip of the nose has -far greater tactual experience. Passing to the inner surfaces of the hands, -which, taken as wholes, are more constantly occupied in touching than are -the back, breast, thigh, forearm, forehead, or back of the hand, Weber's -scale shows that they are much more perceptive, and that the degrees of -perceptiveness of different parts correspond with their tactual activities. -The palms have but one-fifth the perceptiveness possessed by the -forefinger-ends; the inner surfaces of the finger-joints next the palms -have but one-third; while the inner surfaces of the second joints have but -one-half. These abilities correspond with the facts that whereas the inner -parts of the hand are used only in grasping things, the tips of the fingers -come into play not only when things are grasped, but when such things, as -well as smaller things, are felt at or manipulated. It needs but to observe -the relative actions of these parts in writing, in sewing, in judging -textures, &c., to see that above all other parts the finger-ends, and -especially the forefinger-ends, have the most multiplied experiences. If, -then, it be that the extra perceptiveness acquired from actual tactual -activities, as in a compositor, is inheritable, these gradations of tactual -perceptiveness are explained. - -Doubtless some of those who remember Weber's results, have had on the tip -of the tongue the argument derived from the tip of the tongue. This part -exceeds all other parts in power of tactual discrimination: doubling, in -that respect, the power of the forefinger-tip. It can distinguish points -that are only one-twenty-fourth of an inch apart. Why this unparalleled -perceptiveness? If survival of the fittest be the ascribed cause, then it -has to be shown what the advantages achieved have been; and, further, that -those advantages have been sufficiently great to have had effects on the -maintenance of life. - -Besides tasting, there are two functions conducive to life, which the -tongue performs. It enables us to move about food during mastication, and -it enables us to make many of the articulations constituting speech. But -how does the extreme discriminativeness of the tongue-tip aid these -functions? The food is moved about, not by the tongue-tip, but by the body -of the tongue; and even were the tip largely employed in this process, it -would still have to be shown that its ability to distinguish between points -one-twenty-fourth of an inch apart, is of service to that end, which cannot -be shown. It may, indeed, be said that the tactual perceptiveness of the -tongue-tip serves for detection of foreign bodies in the food, as -plum-stones or as fish-bones. But such extreme perceptiveness is needless -for the purpose. A perceptiveness equal to that of the finger-ends would -suffice. And further, even were such extreme perceptiveness useful, it -could not have caused survival of individuals who possessed it in slightly -higher degrees than others. It needs but to observe a dog crunching small -bones, and swallowing with impunity the sharp-angled pieces, to see that -but a very small amount of mortality would be prevented. - -But what about speech? Well, neither here can there be shown any advantage -derived from this extreme perceptiveness. For making the _s_ and _z_, the -tongue has to be partially applied to a portion of the palate next the -teeth. Not only, however, must the contact be incomplete, but its place is -indefinite--may be half an inch further back. To make the _sh_ and _zh_, -the contact has to be made, not with the tip, but with the upper surface of -the tongue; and must be an incomplete contact. Though, for making the -liquids, the tip of the tongue and the sides of the tongue are used, yet -the requisite is not any exact adjustment of the tip, but an imperfect -contact with the palate. For the _th_, the tip is used along with the edges -of the tongue; but no perfect adjustment is required, either to the edges -of the teeth, or to the junction of the teeth with the palate, where the -sound may equally well be made. Though for the _t_ and _d_ complete contact -of the tip and edges of the tongue with the palate is required, yet the -place of contact is not definite, and the tip takes no more important share -in the action than the sides. Any one who observes the movements of his -tongue in speaking, will find that there occur no cases in which the -adjustments must have an exactness corresponding to the extreme power of -discrimination which the tip possesses: for speech, this endowment is -useless. Even were it useful, it could not be shown that it has been -developed by survival of the fittest; for though perfect articulation is an -aid, yet imperfect articulation has rarely such an effect as to impede a -man in the maintenance of his life. If he is a good workman, a German's -interchanges of _b's_ and _p's_ do not disadvantage him. A Frenchman who, -in place of the sound of _th_, always makes the sound of _z_, succeeds as a -teacher of music or dancing, no less than if he achieved the English -pronunciation. Nay, even such an imperfection of speech as that which -arises from cleft palate, does not prevent a man from getting on if he is -capable. True, it may go against him as a candidate for Parliament, or as -an "orator" of the unemployed (mostly not worth employing). But in the -struggle for life he is not hindered by the effect to the extent of being -less able than others to maintain himself and his offspring. Clearly, then, -even if this unparalleled perceptiveness of the tongue-tip is required for -perfect speech, such use is not sufficiently important to have been -developed by natural selection. - -How, then, is this remarkable trait of the tongue-tip to be accounted for? -Without difficulty, if there is inheritance of acquired characters. For the -tongue-tip has, above all other parts of the body, unceasing experiences of -small irregularities of surface. It is in contact with the teeth, and -either consciously or unconsciously is continually exploring them. There is -hardly a moment in which impressions of adjacent but different positions -are not being yielded to it by either the surfaces of the teeth or their -edges; and it is continually being moved about from some of them to others. -No advantage is gained. It is simply that the tongue's position renders -perpetual exploration almost inevitable; and by perpetual exploration is -developed this unique power of discrimination. Thus the law holds -throughout, from this highest degree of perceptiveness of the tongue-tip to -its lowest degree on the back of the trunk; and no other explanation of the -facts seems possible. - -"Yes, there is another explanation," I hear some one say: "they may be -explained by _panmixia_." Well, in the first place, as the explanation by -_panmixia_ implies that these gradations of perceptiveness have been -arrived at by the dwindling of nervous structures, there lies at the basis -of the explanation an unproved and improbable assumption; and, in the -second place, even were there no such difficulty, it may with certainty be -denied that _panmixia_ can furnish an explanation. Let us look at its -pretensions. - -* * * * * - -It was not without good reason that Bentham protested against metaphors. -Figures of speech in general, valuable as they are in poetry and rhetoric, -cannot be used without danger in science and philosophy. The title of Mr. -Darwin's great work furnishes us with an instance of the misleading effects -produced by them. It runs:--_The Origin of Species by means of Natural -Selection, or the Preservation of favoured Races in the Struggle for Life_. -Here are two figures of speech which conspire to produce an impression more -or less erroneous. The expression "natural selection" was chosen as serving -to indicate some parallelism with artificial selection--the selection -exercised by breeders. Now selection connotes volition, and thus gives to -the thoughts of readers a wrong bias. Some increase of this bias is -produced by the words in the second title, "favoured races;" for anything -which is favoured implies the existence of some agent conferring a favour. -I do not mean that Mr. Darwin himself failed to recognize the misleading -connotations of his words, or that he did not avoid being misled by them. -In chapter iv of the _Origin of Species_, he says that, considered -literally, "natural selection is a false term," and that the -personification of Nature is objectionable; but he thinks that readers, and -those who adopt his views, will soon learn to guard themselves against the -wrong implications. Here I venture to think that he was mistaken. For -thinking this, there is the reason that even his disciple, Mr. Wallace--no, -not his disciple, but his co-discoverer, ever to be honoured--has -apparently been influenced by them. When, for example, in combating a view -of mine, he says that "the very thing said to be impossible by variation -and natural selection has been again and again effected, by variation and -artificial selection," he seems clearly to imply that the processes are -analogous, and operate in the same way. Now this is untrue. They are -analogous only within certain narrow limits; and, in the great majority of -cases, natural selection is utterly incapable of doing that which -artificial selection does. - -To see this it needs only to de-personalise Nature, and to remember that, -as Mr. Darwin says, Nature is "only the aggregate action and product of -many natural laws [forces]." Observe its relative shortcomings. Artificial -selection can pick out a particular trait, and, regardless of other traits -of the individuals displaying it, can increase it by selective breeding in -successive generations. For, to the breeder or fancier, it matters little -whether such individuals are otherwise well constituted. They may be in -this or that way so unfit for carrying on the struggle for life, that were -they without human care, they would disappear forthwith. On the other hand, -if we regard Nature as that which it is, an assemblage of various forces, -inorganic and organic, some favourable to the maintenance of life and many -at variance with its maintenance--forces which operate blindly--we see that -there is no such selection of this or that trait; but that there is a -selection only of individuals which are, by the aggregate of their traits, -best fitted for living. And here I may note an advantage possessed by the -expression "survival of the fittest;" since this does not tend to raise the -thought of any one character which, more than others, is to be maintained -or increased; but tends rather to raise the thought of a general adaptation -for all purposes. It implies the process which Nature can alone carry -on--the leaving alive of those which are best able to utilize surrounding -aids to life, and best able to combat or avoid surrounding dangers. And -while this phrase covers the great mass of cases in which there are -preserved well-constituted individuals, it also covers those special cases -which are suggested by the phrase "natural selection," in which individuals -succeed beyond others in the struggle for life, by the help of particular -characters which conduce in important ways to prosperity and -multiplication. For now observe the fact which here chiefly concerns us, -that survival of the fittest can increase any serviceable trait, only if -that trait conduces to prosperity of the individual, or of posterity, or of -both, _in an important degree_. There can be no increase of any structure -by natural selection unless, amid all the slightly varying structures -constituting the organism, increase of this particular one is so -advantageous as to cause greater multiplication of the family in which it -arises than of other families. Variations which, though advantageous, fail -to do this, must disappear again. Let us take a case. - -Keenness of scent in a deer, by giving early notice of approaching enemies, -subserves life so greatly that, other things equal, an individual having it -in an unusual degree is more likely than others to survive; and, among -descendants, to leave some similarly endowed or more endowed, who again -transmit the variation with, in some cases, increase. Clearly this highly -useful power may be developed by natural selection. So also, for like -reasons, may quickness of vision and delicacy of hearing; though it may be -remarked in passing that since this extra sense-endowment, serving to give -early alarm, profits the herd as a whole, which takes the alarm from one -individual, selection of it is not so easy, unless it occurs in a -conquering stag. But now suppose that one member of the herd--perhaps -because of more efficient teeth, perhaps by greater muscularity of stomach, -perhaps by secretion of more appropriate gastric juices--is enabled to eat -and digest a not uncommon plant which the others refuse. This peculiarity -may, if food is scarce, conduce to better self-maintenance, and better -fostering of young if the individual is a hind. But unless this plant is -abundant, and the advantage consequently great, the advantages which other -members of the herd gain from other slight variations may be equivalent. -This one has unusual agility, and leaps a chasm which others balk at. That -one develops longer hair in winter, and resists the cold better. Another -has a skin less irritated by flies, and can graze without so much -interruption. Here is one which has an unusual power of detecting food -under the snow; and there is one which shows extra sagacity in the choice -of a shelter from wind and rain. That the variation giving ability to eat a -plant before unutilized, may become a trait of the herd, and eventually of -a variety, it is needful that the individual in which it occurs shall have -more descendants, or better descendants, or both, than have the various -other individuals severally having their small superiorities. If these -other individuals severally profit by their small superiorities, and -transmit them to equally large numbers of offspring, no increase of the -variation in question can take place: it must soon be cancelled. Whether in -the _Origin of Species_ Mr. Darwin has recognized this fact, I do not -remember, but he has certainly done it by implication in his _Animals and -Plants under Domestication_. Speaking of variations in domestic animals, he -there says that "any particular variation would generally be lost by -crossing, reversion, and the accidental destruction of the varying -individuals, unless carefully preserved by man." (Vol. II, p. 292.) That -which survival of the fittest does in cases like the one I have instanced, -is to keep all faculties up to the mark, by destroying such individuals as -have faculties in some respect below the mark; and it can produce -development of some one faculty only if that faculty is predominantly -important. It seems to me that many naturalists have practically lost sight -of this, and assume that natural selection will increase _any_ advantageous -trait. Certainly a view now held by some assumes as much. - -The consideration of this view, to which the foregoing paragraph is -introductory, may now be entered upon. This view concerns, not direct -selection, but what has been called, in questionable logic, "reversed -selection"--the selection which effects, not increase of an organ, but -decrease of it. For as, under some conditions, it is of advantage to an -individual and its descendants to have some structure of larger size, it -may be, under other conditions--namely, when the organ becomes useless--of -advantage to have it of smaller size; since, even if it is not in the way, -its weight and the cost of its nutrition are injurious taxes on the -organism. But now comes the truth to be emphasized. Just as direct -selection can increase an organ only in certain cases, so can reversed -selection decrease it only in certain cases. Like the increase produced by -a variation, the decrease produced by one must be such as will sensibly -conduce to preservation and multiplication. It is, for instance, -conceivable that were the long and massive tail of the kangaroo to become -useless (say by the forcing of the species into a mountainous and rocky -habitat filled with brushwood), a variation which considerably reduced the -tail might sensibly profit the individual in which it occurred; and, in -seasons when food was scarce, might cause survival when individuals with -large tails died. But the economy of nutrition must be considerable before -any such result could occur. Suppose that in this new habitat the kangaroo -had no enemies; and suppose that, consequently, quickness of hearing not -being called for, large ears gave no greater advantage than small ones. -Would an individual with smaller ears than usual, survive and propagate -better than other individuals, in consequence of the economy of nutrition -achieved? To suppose this is to suppose that the saving of a grain or two -of protein per day would determine the kangaroo's fate. - -Long ago I discussed this matter in the _Principles of Biology_ (§ 166), -taking as an instance the decrease of the jaw implied by the crowding of -the teeth, and now proved by measurement to have taken place. Here is the -passage:-- - - "No functional superiority possessed by a small jaw over a large jaw, in - civilized life, can be named as having caused the more frequent survival - of small-jawed individuals. The only advantage which smallness of jaw - might be supposed to give, is the advantage of economized nutrition; and - this could not be great enough to further the preservation of men - possessing it. The decrease of weight in the jaw and co-operative parts - that has arisen in the course of many thousands of years, does not amount - to more than a few ounces. This decrease has to be divided among the many - generations that have lived and died in the interval. Let us admit that - the weight of these parts diminished to the extent of an ounce in a - single generation (which is a large admission); it still cannot be - contended that the having to carry an ounce less in weight, or having to - keep in repair an ounce less of tissue, could sensibly affect any man's - fate. And if it never did this--nay, if it did not cause a _frequent_ - survival of small-jawed individuals where large-jawed individuals died, - natural selection could neither cause nor aid diminution of the jaw and - its appendages." - -When writing this passage in 1864, I never dreamt that a quarter of a -century later, the supposable cause of degeneration here examined and -excluded as impossible, would be enunciated as an actual cause and named -"reversed selection." - -One of the arguments used to show the adequacy of natural selection under -its direct or indirect form consists of a counter-argument to the effect -that inheritance of functionally-wrought changes, supposing it to be -operative, does not explain certain of the facts. This is alleged by Prof. -Weismann as a part justification for his doctrine of Panmixia. Concerning -the "blind fish and amphibia" found in dark places, which have but -rudimentary eyes "hidden under the skin," he argues that "it is difficult -to reconcile the facts of the case with the ordinary theory that the eyes -of these animals have simply degenerated through disuse." After giving -instances of rapid degeneration of disused organs, he argues that if "the -effects of disuse are so striking in a single life, we should certainly -expect, if such effects can be transmitted, that all traces of an eye would -soon disappear from a species which lives in the dark." Doubtless this is a -reasonable conclusion. To explain the facts on the hypothesis that acquired -characters are inheritable, seems very difficult. One possible explanation -may, indeed, be named. It appears to be a general law of organization that -structures are stable in proportion to their antiquity--that while organs -of relatively modern origin have but a comparatively superficial root in -the constitution, and readily disappear if the conditions do not favour -their maintenance, organs of ancient origin have deep-seated roots in the -constitution, and do not readily disappear. Having been early elements in -the type, and having continued to be reproduced as parts of it during a -period extending throughout many geological epochs, they are comparatively -persistent. Now the eye answers to this description as being a very early -organ. But waiving possible explanations, let us take the particular -instance cited by Prof. Weismann and see what is to be made of it. He -writes:-- - - "The caverns in Carniola and Carinthia, in which the blind _Proteus_ and - so many other blind animals live, belong geologically to the Jurassic - formation; and although we do not exactly know when for example the - _Proteus_ first entered them, the low organization of this amphibian - certainly indicates that it has been sheltered there for a very long - period of time, and that thousands of generations of this species have - succeeded one another in the caves. - - "Hence there is no reason to wonder at the extent to which the - degeneration of the eye has been already carried in the _Proteus_; even - if we assume that it is merely due to the cessation of the conserving - influence of natural selection."[103] - -Let me first note a strange oversight on the part of Prof. Weismann. He -points out that the caverns in question belong to the Jurassic formation: -apparently intending to imply that they have an antiquity related to that -of the formation. But there is no such relation, except that the caverns -cannot be older than the formation. They may have originated at any period -since the containing strata were deposited; and they may be therefore -relatively modern. But passing over this, and admitting that the _Proteus_ -has inhabited the caverns for an enormous period, what is to be said of the -fact that their eyes have not disappeared entirely, as Prof. Weismann -contends they should have done had the inheritance of the effects of disuse -been all along operative? There is a very sufficient answer--the -rudimentary eyes are not entirely useless. It seems that when the -underground streams it inhabits are unusually swollen, some individuals of -the species are carried out of the caverns into the open (being then -sometimes captured). It is also said that the creatures shun the light; -this trait being, I presume, observed when it is in captivity. Now -obviously, among individuals carried out into the open, those which remain -visible are apt to be carried off by enemies; whereas, those which, -appreciating the difference between light and darkness, shelter themselves -in dark places, survive. Hence the tendency of natural selection is to -prevent the decrease of the eyes beyond that point at which they can -distinguish between light and darkness. Thus the apparent anomaly is -explained. - -Let me suggest, as another possible reason for persistence of rudimentary -organs, that the principle of economy of growth will cause diminution of -them only in proportion as their constituents are of value for other uses -in the organism; and that in many cases their constituents are practically -valueless. Hence probably the reason why, in the case of stalk-eyed -crustaceans, the eye is gone but the pedicle remains, or to use Mr. -Darwin's simile, the telescope has disappeared but not its stand. - -* * * * * - -Along with that inadequacy of natural selection to explain changes of -structure which do not aid life in important ways, alleged in § 166 of _The -Principles of Biology_, a further inadequacy was alleged. It was contended -that the relative powers of co-operative parts cannot be adjusted solely by -survival of the fittest; and especially where the parts are numerous and -the co-operation complex. In illustration it was pointed out that immensely -developed horns, such as those of the extinct Irish elk, weighing over a -hundred-weight, could not, with the massive skull bearing them, be carried -at the extremity of the outstretched neck without many and great -modifications of adjacent bones and muscles of the neck and thorax; and -that without strengthening of the fore-legs, too, there would be failure -alike in fighting and in locomotion. And it was argued that while we cannot -assume spontaneous increase of all these parts proportionate to the -additional strains, we cannot suppose them to increase by variations, one -at once, without supposing the creature to be disadvantaged by the weight -and nutrition of parts that were for the time useless--parts, moreover, -which would revert to their original sizes before the other needful -variations occurred. - -When, in reply to me, it was contended that co-operative parts vary -together, I named facts conflicting with this assertion--the fact that the -blind cray-fish of the Kentucky caves have lost their eyes but not the -foot-stalks carrying them; the fact that the normal proportion between -tongue and beak in certain selected varieties of pigeons is lost; the fact -that lack of concomitance in decrease of jaws and teeth in sundry kinds of -pet dogs, has caused great crowding of the teeth ("The Factors of Organic -Evolution," _Essays_, i, 401-402). And I then argued that if co-operative -parts, small in number and so closely associated as these are, do not vary -together, it is unwarrantable to allege that co-operative parts which are -very numerous and remote from one another vary together. After making this -rejoinder I enforced my argument by a further example--that of the giraffe. -Tacitly recognizing the truth that the unusual structure of this creature -must have been, in its most conspicuous traits, the result of survival of -the fittest (since it is absurd to suppose that efforts to reach high -branches could lengthen the legs), I illustrated afresh the obstacles to -co-adaptation. Not dwelling on the objection that increase of any -components of the fore-quarters out of adjustment to the others, would -cause evil rather than good, I went on to argue that the co-adaptation of -parts required to make the giraffe's structure useful, is much greater than -at first appears. This animal has a grotesque gallop, necessitated by the -great difference in length between the fore and the hind limbs. I pointed -out that the mode of action of the hind limbs shows that the bones and -muscles have all been changed in their proportions and adjustments; and I -contended that, difficult as it is to believe that all parts of the -fore-quarters have been co-adapted by the appropriate variations, now of -this part now of that, it becomes impossible to believe that all the parts -in the hind-quarters have been simultaneously co-adapted to one another and -to all the parts of the fore-quarters: adding that want of co-adaptation, -even in a single muscle, would cause fatal results when high speed had to -be maintained while escaping from an enemy. - -Since this argument, repeated with this fresh illustration, was published -in 1886, I have met with nothing to be called a reply; and might, I think, -if convictions usually followed proofs, leave the matter as it stands. It -is true that, in his _Darwinism_, Mr. Wallace has adverted to my renewed -objection, and, as already said, contended that changes such as those -instanced can be effected by natural selection, since such changes can be -effected by artificial selection: a contention which, as I have pointed -out, assumes a parallelism that does not exist. But now, instead of -pursuing the argument further along the same line, let me take a somewhat -different line. - -If there occurs some change in an organ, say by increase of its size, which -adapts it better to the creature's needs, it is admitted that when, as -commonly happens, the use of the organ demands the co-operation of other -organs, the change in it will generally be of no service unless the -co-operative organs are changed. If, for instance, there takes place such a -modification of a rodent's tail as that which, by successive increases, -produces the trowel-shaped tail of the beaver, no advantage will be derived -unless there also take place certain modifications in the bulks and shapes -of the adjacent vertebræ and their attached muscles, as well as, probably, -in the hind limbs; enabling them to withstand the reactions of the blows -given by the tail. And the question is, by what process these many parts, -changed in different degrees, are co-adapted to the new -requirements--whether variation and natural selection alone can effect the -readjustment. There are three conceivable ways in which the parts may -simultaneously change:--(1) they may all increase or decrease together in -like degree; (2) they may all simultaneously increase or decrease -independently, so as not to maintain their previous proportions, or assume -any other special proportions; (3) they may vary in such ways and degrees -as to make them jointly serviceable for the new end. Let us consider -closely these several conceivabilities. - -And first of all, what are we to understand by co-operative parts? In a -general sense, all the organs of the body are co-operative parts, and are -respectively liable to be more or less changed by change in any one. In a -narrower sense, more directly relevant to the argument, we may, if we -choose to multiply difficulties, take the entire framework of bones and -muscles as formed of co-operative parts; for these are so related that any -considerable change in the actions of some entails change in the actions of -most others. It needs only to observe how, when putting out an effort, -there goes, along with a deep breath, an expansion of the chest and a -bracing up of the abdomen, to see that various muscles beyond those -directly concerned are strained along with them. Or, when suffering from -lumbago, an effort to lift a chair will cause an acute consciousness that -not the arms only are brought into action, but also the muscles of the -back. These cases show how the motor organs are so tied together that -altered actions of some implicate others quite remote from them. - -But without using the advantage which this interpretation of the words -would give, let us take, as co-operative organs, those which are obviously -such--the organs of locomotion. What, then, shall we say of the fore limbs -and hind limbs of terrestrial mammals, which co-operate closely and -perpetually? Do they vary together? If so, how have there been produced -such contrasted structures as that of the kangaroo, with its large hind -limbs and small fore limbs, and that of the giraffe, in which the hind -limbs are small and the fore limbs large--how does it happen that, -descending from the same primitive mammal, these creatures have diverged in -the proportions of their limbs in opposite directions? Take, again, the -articulate animals. Compare one of the lower types, with its rows of almost -equal-sized limbs, and one of the higher types, as a crab or a lobster, -with limbs some very small and some very large. How came this contrast to -arise in the course of evolution, if there was the equality of variation -supposed? - -But now let us narrow the meaning of the phrase still further, giving it a -more favourable interpretation. Instead of considering separate limbs as -co-operative, let us consider the component parts of the same limb as -co-operative, and ask what would result, from varying together. It would in -that case happen that, though the fore and hind limbs of a mammal might -become different in their sizes, they would not become different in their -structures. If so, how have there arisen the unlikenesses between the hind -legs of the kangaroo and those of the elephant? Or if this comparison is -objected to, because the creatures belong to the widely different divisions -of implacental and placental mammals, take the cases of the rabbit and the -elephant, both belonging to the last division. On the hypothesis of -evolution these are both derived from the same original form; but the -proportions of the parts have become so widely unlike that the -corresponding joints are scarcely recognized as such by the unobservant: at -what seem corresponding places the legs bend in opposite ways. Equally -marked, or more marked, is the parallel fact among the _Articulata_. Take -that limb of the lobster which bears the claw and compare it with the -corresponding limb in an inferior articulate animal, or the corresponding -limb of its near ally, the rock lobster, and it becomes obvious that the -component segments of the limb have come to bear to one another in the one -case, proportions immensely different from those they bear in the other -case. Undeniably, then, on contemplating the general facts of organic -structure, we see that the concomitant variations in the parts of limbs, -have not been of a kind to produce equal amounts of change in them, but -quite the opposite--have been everywhere producing inequalities. Moreover, -we are reminded that this production of inequalities among co-operative -parts, is an essential principle of development. Had it not been so, there -could not have been that progress from homogeneity of structure to -heterogeneity of structure which constitutes evolution. - -We pass now to the second supposition:--that the variations in co-operative -parts occur irregularly, or in such independent ways that they bear no -definite relations to one another--miscellaneously, let us say. This is the -supposition which best corresponds with the facts. Glances at the faces -around yield conspicuous proofs. Many of the muscles of the face and some -of the bones, are distinctly co-operative; and these respectively vary in -such ways as to produce in each person a different combination. What we see -in the face we have reason to believe holds in the limbs and in all other -parts. Indeed, it needs but to compare people whose arms are of the same -lengths, and observe how stumpy are the fingers of one and how slender -those of another; or it needs but to note the unlikenesses of gait of -passers-by, implying small unlikenesses of structure; to be convinced that -the relations among the variations of co-operative parts are anything but -fixed. And now, confining our attention to limbs, let us consider what must -happen if, by variations taking place miscellaneously, limbs have to be -partially changed from fitness for one function to fitness for another -function--have to be re-adapted. That the reader may fully comprehend the -argument, he must here have patience while a good many anatomical details -are set down. - -Let us suppose a species of quadruped of which the members have, for -immense past periods, been accustomed to locomotion over a relatively even -surface, as, for instance, the "prairie-dogs" of North America; and let us -suppose that increase of numbers has driven part of them into a region full -of obstacles to easy locomotion--covered, say, by the decaying stems of -fallen trees, such as one sees in portions of primeval forest. Ability to -leap must then become a useful trait; and, according to the hypothesis we -are considering, this ability will be produced by the selection of -favourable variations. What are the variations required? A leap is effected -chiefly by the bending of the hind limbs so as to make sharp angles at the -joints, and then suddenly straightening them; as any one may see on -watching a cat leap on to the table. The first required change, then, is -increase of the large extensor muscles, by which the hind limbs are -straightened. Their increases must be duly proportioned; for if those which -straightened one joint become much stronger than those which straightened -the other joint, the result must be collapse of the other joint when the -muscles are contracted together. But let us make a large admission, and -suppose these muscles to vary together; what further muscular change is -next required? In a plantigrade mammal the metatarsal bones chiefly bear -the reaction of the leap, though the toes may have a share. In a -digitigrade mammal, however, the toes form almost exclusively the fulcrum, -and if they are to bear the reaction of a higher leap, the flexor muscles -which depress and bend them must be proportionately enlarged: if not, the -leap will fail from want of a firm _point d'appui_. Tendons as well as -muscles must be modified; and, among others, the many tendons which go to -the digits and their phalanges. Stronger muscles and tendons imply greater -strains on the joints; and unless these are strengthened, one or other, -dislocation will be caused by a more vigorous spring. Not only the -articulations themselves must be so modified as to bear greater stress, but -also the numerous ligaments which hold the parts of each in place. Nor can -the bodies of the bones remain unstrengthened; for if they have no more -than the strengths needed for previous movements they will fail to bear -more violent movements. Thus, saying nothing of the required changes in the -pelvis, as well as in the nerves and blood-vessels, there are, counting -bones, muscles, tendons, ligaments, at least fifty different parts in each -hind leg which have to be enlarged. Moreover they have to be enlarged in -unlike degrees. The muscles and tendons of the outer toes, for example, -need not be added to so much as those of the median toes. Now, throughout -their successive stages of growth, all these parts have to be kept fairly -well balanced; as any one may infer on remembering sundry of the accidents -he has known. Among my own friends I could name one who, when playing -lawn-tennis, snapped the Achilles tendon; another who, while swinging his -children, tore some of the muscular fibres in the calf of his leg; another -who, in getting over a fence, tore a ligament of one knee. Such facts, -joined with every one's experience of sprains, show that during the extreme -exertions to which limbs are now and then subject, there is a giving way of -parts not quite up to the required level of strength. How, then, is this -balance to be maintained? Suppose the extensor muscles have all varied -appropriately; their variations are useless unless the other co-operative -parts have also varied appropriately. Worse than this. Saying nothing of -the disadvantage caused by extra weight and cost of nutrition, they will be -causes of mischief--causes of derangement to the rest by contracting with -undue force. And then, how long will it take for the rest to be brought -into adjustment? As Mr. Darwin says concerning domestic animals:--"Any -particular variation would generally be lost by crossing, reversion, &c. -... unless carefully preserved by man." In a state of nature, then, -favourable variations of these muscles would disappear again long before -one or a few of the co-operative parts could be appropriately varied, much -more before all of them could. - -With this insurmountable difficulty goes a difficulty still more -insurmountable--if the expression may be allowed. It is not a question of -increased sizes of parts only, but of altered shapes of parts, too. A -glance at the skeletons of mammals shows how unlike are the forms of the -corresponding bones of their limbs; and shows that they have been severally -re-moulded in each species to the different requirements entailed by its -different habits. The change from the structures of hind limbs fitted only -for walking and trotting to hind limbs fitted also for leaping, implies, -therefore, that, along with strengthenings of bones there must go -alterations in their forms. Now the fortuitous alterations of form which -may take place in any bone are countless. How long, then, will it be before -there takes place that particular alteration which will make the bone -fitter for its new action? And what is the probability that the many -required changes of shape, as well as of size, in bones will each of them -be effected before all the others are lost again? If the probabilities -against success are incalculable, when we take account only of changes in -the sizes of parts, what shall we say of their incalculableness when -differences of form also are taken into account? - -"Surely this piling up of difficulties has gone far enough"; the reader -will be inclined to say. By no means. There is a difficulty immeasurably -transcending those named. We have thus far omitted the second half of the -leap, and the provisions to be made for it. After ascent of the animal's -body comes descent; and the greater the force with which it is projected -up, the greater is the force with which it comes down. Hence, if the -supposed creature has undergone such changes in the hind limbs as will -enable them to propel it to a greater height, without having undergone any -changes in the fore limbs, the result will be that on its descent the fore -limbs will give way, and it will come down on its nose. The fore limbs, -then, have to be changed simultaneously with the hind. How changed? -Contrast the markedly bent hind limbs of a cat with its almost straight -fore limbs, or contrast the silence of the spring on to the table with the -thud which the fore paws make as it jumps off the table. See how unlike the -actions of the hind and fore limbs are, and how unlike their structures. In -what way, then, is the required co-adaptation to be effected? Even were it -a question of relative sizes only, there would be no answer; for facts -already given show that we may not assume simultaneous increases of size to -take place in the hind and fore limbs; and, indeed, a glance at the various -human races, which differ considerably in the ratios of their legs to their -arms, shows us this. But it is not simply a question of sizes. To bear the -increased shock of descent the fore limbs must be changed throughout in -their structures. Like those in the hind limbs, the changes must be of many -parts in many proportions; and they must be both in sizes and in shapes. -More than this. The scapular arch and its attached muscles must also be -strengthened and re-moulded. See, then, the total requirements. We must -suppose that by natural selection of miscellaneous variations, the parts of -the hind limbs will be co-adapted to one another, in sizes, shapes, and -ratios; that those of the fore limbs will undergo co-adaptation similar in -their complexity, but dissimilar in their kinds; and that the two sets of -co-adaptations will be effected _pari passu_. If, as may be held, the -probabilities are millions to one against the first set of changes being -achieved, then it may be held that the probabilities are billions to one -against the second being simultaneously achieved, in progressive adjustment -to the first. - -There remains only to notice the third conceivable mode of adjustment. It -may be imagined that though, by the natural selection of miscellaneous -variations, these adjustments cannot be effected, they may nevertheless be -made to take place appropriately. How made? To suppose them so made is to -suppose that the prescribed end is somewhere recognized; and that the -changes are step by step simultaneously proportioned for achieving it--is -to suppose a designed production of these changes. In such case, then, we -have to fall back in part upon the primitive hypothesis; and if we do this -in part, we may as well do it wholly--may as well avowedly return to the -doctrine of special creations. - -What, then, is the only defensible interpretation? If such modifications of -structure produced by modifications of function as we see take place in -each individual, are in any measure transmissible to descendants, then all -these co-adaptations, from the simplest up to the most complex, are -accounted for. In some cases this inheritance of acquired characters -suffices by itself to explain the facts; and in other cases it suffices -when taken in combination with the selection of favourable variations. An -example of the first class is furnished by the change just considered; and -an example of the second class is furnished by the case, before named, of -development in a deer's horns. If, by some extra massiveness spontaneously -arising, or by formation of an additional "point," an advantage is gained -either for attack or defence, then, if the increased muscularity and -strengthened structure of the neck and thorax, which wielding of these -somewhat heavier horns produces, are in a greater or less degree inherited, -and in several successive generations are by this process brought up to the -required extra strength, it becomes possible and advantageous for a further -increase of the horns to take place, and a further increase in the -apparatus for wielding them, and so on continuously. By such processes -only, in which each part gains strength in proportion to function, can -co-operative parts be kept in adjustment, and be re-adjusted to meet new -requirements. Close contemplation of the facts impresses me more strongly -than ever with the two alternatives--either there has been inheritance of -acquired characters, or there has been no evolution. - -This very pronounced opinion will be met, on the part of some, by a no less -pronounced demurrer, which involves a denial of possibility. It has been of -late asserted, and by many believed, that inheritance of acquired -characters cannot occur. Weismann, they say, has shown that there is early -established in the evolution of each organism such a distinctness between -those component units which carry on the individual life and those which -are devoted to maintenance of the species, that changes in the one cannot -affect the other. We will look closely into his doctrine. - -Basing his argument on the principle of the physiological division of -labour, and assuming that the primary division of labour is that between -such part of an organism as carries on individual life and such part as is -reserved for the production of other lives, Weismann, starting with "the -first multicellular organism," says that--"Hence the single group would -come to be divided into two groups of cells, which may be called somatic -and reproductive--the cells of the body as opposed to those which are -concerned with reproduction." (_Essays upon Heredity_, i, p. 27.) - -Though he admits that this differentiation "was not at first absolute, and -indeed is not always so to-day," yet he holds that the differentiation -eventually becomes absolute in the sense that the somatic cells, or those -which compose the body at large, come to have only a limited power of -cell-division, instead of an unlimited power which the reproductive cells -have; and also in the sense that eventually there ceases to be any -communication between the two further than that implied by the supplying of -nutriment to the reproductive cells by the somatic cells. The outcome of -this argument is that, in the absence of communication, changes induced in -the somatic cells, constituting the individual, cannot influence the -natures of the reproductive cells, and cannot therefore be transmitted to -posterity. Such is the theory. Now let us look at a few facts--some -familiar, some unfamiliar. - -His investigations led Pasteur to the positive conclusion that the silkworm -diseases are inherited. The transmission from parent to offspring resulted, -not through any contamination of the surface of the egg by the body of the -parent while being deposited, but resulted from infection of the egg -itself--intrusion of the parasitic organism. Generalized observations -concerning the disease called _pébrine_, enabled him to decide, by -inspection of the eggs, which were infected and which were not: certain -modifications of form distinguishing the diseased ones. More than this; the -infection was proved by microscopical examination of the contents of the -egg; in proof of which he quotes as follows from Dr. Carlo Vittadini:-- - - "Il résulte de mes recherches sur les graines, à l'époque où commence le - développement du germe, que les corpuscules, une fois apparus dans - l'oeuf, augmentent graduellement en nombre, à mesure que l'embryon se - développe; que, dans les derniers jours de l'incubation, l'oeuf en est - plein, au point de faire croire que la majeure partie des granules du - jaune se sont transformés en corpuscules. - - "Une autre observation importante est que l'embryon aussi est souillé de - corpuscules, et à un degré tel qu'on peut soupçonner que l'infection du - jaune tire son origine du germe lui-même; en d'autres termes que le germe - est primordialement infecté, et porte en lui-même ces corpuscules tout - comme les vers adultes, frappés du même mal."[104] - -Thus, then the substance of the egg and even its innermost vital part, is -permeable by a parasite sufficiently large to be microscopically visible. -It is also of course permeable by the invisible molecules of protein, out -of which its living tissues are formed, and by absorption of which they -subsequently grow. But, according to Weismann, it is _not_ permeable by -those invisible units of protoplasm out of which the vitally active tissues -of the parent are constituted: units composed, as we must assume, of -variously arranged molecules of protein. So that the big thing may pass, -and the little thing may pass, but the intermediate thing may not pass! - -A fact of kindred nature, unhappily more familiar, may be next brought in -evidence. It concerns the transmission of a disease not infrequent among -those of unregulated lives. The highest authority concerning this disease, -in its inherited form, is Mr. Jonathan Hutchinson; and the following are -extracts from a letter I have received from him, and which I publish with -his assent:-- - - "I do not think that there can be any reasonable doubt that a very large - majority of those who suffer from inherited syphilis take the taint from - the male parent.... It is the rule when a man marries who has no - remaining local lesion, but in whom the taint is not eradicated, for his - wife to remain apparently well, whilst her child may suffer. No doubt the - child infects its mother's blood, but this does not usually evoke any - obvious symptoms of syphilis.... I am sure I have seen hundreds of - syphilitic infants whose mothers had not, so far as I could ascertain, - ever displayed a single symptom." - -See, then, to what we are committed if we accept Weismann's hypothesis. We -must conclude, that whereas the reproductive cell may be effectually -invaded by an abnormal living element in the parental organism, those -normal living elements which constitute the vital protoplasm of the -parental organism, cannot invade it. Or if it be admitted that both -intrude, then the implication is that, whereas the abnormal element can so -modify the development as to cause changes of structure (as of the teeth), -the normal element can cause no changes of structure![105] - -We pass now to evidence not much known to the world at large, but widely -known in the biological world, though known in so incomplete a manner as to -be undervalued in it. Indeed, when I name it, probably many will vent a -mental pooh-pooh. The fact to which I refer is one of which record is -preserved in the museum of the College of Surgeons, in the shape of -paintings of a foal borne by a mare not quite thoroughbred, to a sire which -was thoroughbred--a foal which bears the markings of the quagga. The -history of this remarkable foal is given by the Earl of Morton, F.R.S., in -a letter to the President of the Royal Society (read November 23, 1820). In -it he states that wishing to domesticate the quagga, and having obtained a -male but not a female, he made an experiment. - - "I tried to breed from the male quagga and a young chestnut mare of - seven-eighths Arabian blood, and which had never been bred from; the - result was the production of a female hybrid, now five years old, and - bearing, both in her form and in her colour, very decided indications of - her mixed origin. I subsequently parted with the seven-eighths Arabian - mare to Sir Gore Ouseley, who has bred from her by a very fine black - Arabian horse. I yesterday morning examined the produce, namely, a - two-year-old filly and a year-old colt. They have the character of the - Arabian breed as decidedly as can be expected, where fifteen-sixteenths - of the blood are Arabian; and they are fine specimens of that breed; but - both in their colour and in the hair of their manes, they have a striking - resemblance to the quagga. Their colour is bay, marked more or less like - the quagga in a darker tint. Both are distinguished by the dark line - along the ridge of the back, the dark stripes across the forehead, and - the dark bars across the back part of the legs."[106] - -Lord Morton then names sundry further correspondences. Dr. Wollaston, at -that time President of the Royal Society, who had seen the animals, -testified to the correctness of his description, and, as shown by his -remarks, entertained no doubt about the alleged facts. But good reason for -doubt may be assigned. There naturally arises the question--How does it -happen that parallel results are not observed in other cases? If in any -progeny certain traits not belonging to the sire, but belonging to a sire -of preceding progeny, are reproduced, how is it that such anomalously -inherited traits are not observed in domestic animals, and indeed in -mankind? How is it that the children of a widow by a second husband do not -bear traceable resemblances to the first husband? To these questions -nothing like satisfactory replies seem forthcoming; and, in the absence of -replies, scepticism, if not disbelief, may be held reasonable. - -There is an explanation, however. Forty years ago I made acquaintance with -a fact which impressed me by its significant implications, and has, for -this reason I suppose, remained in my memory. It is set forth in the -_Journal of the Royal Agricultural Society_, Vol. XIV (1853), pp. 214 _et -seq._, and concerns certain results of crossing French and English breeds -of sheep. The writer of the translated paper, M. Malingie-Nouel, Director -of the Agricultural School of La Charmoise, states that when the French -breeds of sheep (in which were included "the _mongrel_ Merinos") were -crossed with an English breed, "the lambs present the following results. -Most of them resemble the mother more than the father; some show no trace -of the father." Joining the admission respecting the mongrels with the -facts subsequently stated, it is tolerably clear that the cases in which -the lambs bore no traces of the father were cases in which the mother was -of pure breed. Speaking of the results of these crossings in the second -generation, "having 75 per cent. of English blood," M. Nouel says:--"The -lambs thrive, wear a beautiful appearance, and complete the joy of the -breeder.... No sooner are the lambs weaned than their strength, their -vigour, and their beauty begin to decay.... At last the constitution gives -way ... he remains stunted for life:" the constitution being thus proved -unstable or unadapted to the requirements. How, then, did M. Nouel succeed -in obtaining a desirable combination of a fine English breed with the -relatively poor French breeds? - - He took an animal from "flocks originally sprung from a mixture of the - two distinct races that are established in those two provinces [Berry and - La Sologne]," and these he "united with animals of another mixed breed - ... which blended the Tourangelle and native Merino blood of" La Beauce - and Touraine, and obtained a mixture of all four races "without decided - character, without fixity ... but possessing the advantage of being used - to our climate and management." - - Putting one of these "mixed blood ewes to a pure New-Kent ram ... one - obtains a lamb containing fifty-hundredths of the purest and most ancient - English blood, with twelve and a half hundredths of four different French - races, which are individually lost in the preponderance of English blood, - and disappear almost entirely, leaving the improving type in the - ascendant.... All the lambs produced strikingly resembled each other, and - even Englishmen took them for animals of their own country." - -M. Nouel goes on to remark that when this derived breed was bred with -itself, the marks of the French breeds were lost. "Some slight traces" -could be detected by experts, but these "soon disappeared." - -Thus we get proof that relatively pure constitutions predominate in progeny -over much mixed constitutions. The reason is not difficult to see. Every -organism tends to become adapted to its conditions of life; and all the -structures of a species, accustomed through multitudinous generations to -the climate, food, and various influences of its locality, are moulded into -harmonious co-operation favourable to life in that locality: the result -being that in the development of each young individual, the tendencies -conspire to produce the fit organization. It is otherwise when the species -is removed to a habitat of different character, or when it is of mixed -breed. In the one case its organs, partially out of harmony with the -requirements of its new life, become partially out of harmony with one -another; since, while one influence, say of climate, is but little changed, -another influence, say of food, is much changed; and, consequently, the -perturbed relations of the organs interfere with their original stable -equilibrium. Still more in the other case is there a disturbance in -equilibrium. In a mongrel, the constitution derived from each source -repeats itself as far as possible. Hence a conflict of tendencies to evolve -two structures more or less unlike. The tendencies do not harmoniously -conspire, but produce partially incongruous sets of organs. And evidently -where the breed is one in which there are united the traits of various -lines of ancestry, there results an organization so full of small -incongruities of structure and action, that it has a much-diminished power -of maintaining its balance; and while it cannot withstand so well adverse -influences, it cannot so well hold its own in the offspring. Concerning -parents of pure and mixed breeds respectively, severally tending to -reproduce their own structures in progeny, we may therefore say, -figuratively, that the house divided against itself cannot withstand the -house of which the members are in concord. - -Now if this is shown to be the case with breeds the purest of which have -been adapted to their habitats and modes of life during some few hundred -years only, what shall we say when the question is of a breed which has had -a constant mode of life in the same locality for ten thousand years or -more, like the quagga? In this the stability of constitution must be such -as no domestic animal can approach. Relatively stable as may have been the -constitutions of Lord Morton's horses, as compared with the constitutions -of ordinary horses, yet, since Arab horses, even in their native country, -have probably in the course of successive conquests and migrations of -tribes become more or less mixed, and since they have been subject to the -conditions of domestic life, differing much from the conditions of their -original wild life, and since the English breed has undergone the -perturbing effects of change from the climate and food of the East to the -climate and food of the West, the organizations of the horse and mare in -question could have had nothing like that perfect balance produced in the -quagga by a hundred centuries of harmonious co-operation. Hence the result. -And hence at the same time the interpretation of the fact that analogous -phenomena are not obvious among most domestic animals, or among ourselves; -since both have relatively mixed, and generally extremely mixed, -constitutions, which, as we see in ourselves, have been made generation -after generation, not by the formation of a mean between two parents, but -by the jumbling of traits of the one with traits of the other; until there -exist no such conspiring tendencies among the parts as cause repetition of -combined details of structure in posterity. - -Expectation that scepticism might be felt respecting this alleged anomaly -presented by the quagga-marked foal, had led me to think over the matter; -and I had reached this interpretation before sending to the College of -Surgeons Museum (being unable to go myself) to obtain the particulars and -refer to the records. When there was brought to me a copy of the account as -set forth in the _Philosophical Transactions_, it was joined with the -information that there existed an appended account of pigs, in which a -parallel fact had been observed. To my immediate inquiry--"Was the male a -wild pig?" there came the reply--"I did not observe." Of course I forthwith -obtained the volume, and there found what I expected. It was contained in a -paper communicated by Dr. Wollaston from Daniel Giles, Esq., concerning his -"sow and her produce," which said that-- - - "she was one of a well-known black and white breed of Mr. Western, the - Member for Essex. About ten years since I put her to a boar of the wild - breed, and of a deep chestnut colour which I had just received from - Hatfield House, and which was soon afterwards drowned by accident. The - pigs produced (which were her first litter) partook in appearance of both - boar and sow, but in some the chestnut colour of the boar strongly - prevailed. - - "The sow was afterwards put to a boar of Mr. Western's breed (the wild - boar having been long dead). The produce was a litter of pigs, some of - which, we observed with much surprise, to be stained and clearly marked - with the chestnut colour which had prevailed in the former litter." - -Mr. Giles adds that in a second litter of pigs, the father of which was of -Mr. Western's breed, he and his bailiff believe there was a recurrence, in -some, of the chestnut colour, but admits that their "recollection is much -less perfect than I wish it to be." He also adds that, in the course of -many years' experience, he had never known the least appearance of the -chestnut colour in Mr. Western's breed. - -What are the probabilities that these two anomalous results should have -arisen, under these exceptional conditions, as a matter of chance? -Evidently the probabilities against such a coincidence are enormous. The -testimony is in both cases so good that, even apart from the coincidence, -it would be unreasonable to reject it; but the coincidence makes acceptance -of it imperative. There is mutual verification, at the same time that there -is a joint interpretation yielded of the strange phenomenon, and of its -non-occurrence under ordinary circumstances. - -And now, in presence of these facts, what are we to say? Simply that they -are fatal to Weismann's hypothesis. They show that there is none of the -alleged independence of the reproductive cells; but that the two sets of -cells are in close communion. They prove that while the reproductive cells -multiply and arrange themselves during the evolution of the embryo, some of -their germ-plasm passes into the mass of somatic cells constituting the -parental body, and becomes a permanent component of it. Further, they -necessitate the inference that this introduced germ-plasm, everywhere -diffused, is some of it included in the reproductive cells subsequently -formed. And if we thus get a demonstration that the somewhat different -units of a foreign germ-plasm permeating the organism, permeate also the -subsequently formed reproductive cells, and affect the structures of the -individuals arising from them, the implication is that the like happens -with those native units which have been made somewhat different by modified -functions: there must be a tendency to inheritance of acquired characters. - -One more step only has to be taken. It remains to ask what is the flaw in -the assumption with which Weismann's theory sets out. If, as we see, the -conclusions drawn from it do not correspond to the facts, then, either the -reasoning is invalid, or the original postulate is untrue. Leaving aside -all questions concerning the reasoning, it will suffice here to show the -untruth of the postulate. Had his work been written during the early years -of the cell-doctrine, the supposition that the multiplying cells of which -the _Metazoa_ and _Metaphyta_ are composed, become completely separate, -could not have been met by a reasonable scepticism; but now, not only is -scepticism justifiable, but denial is called for. Some dozen years ago it -was discovered that in many cases vegetal cells are connected with one -another by threads of protoplasm--threads which unite the internal -protoplasm of one cell with the internal protoplasms of cells around It is -as though the pseudopodia of imprisoned rhizopods were fused with the -pseudopodia of adjacent imprisoned rhizopods. We cannot reasonably suppose -that the continuous network of protoplasm thus constituted has been -produced after the cells have become adult. These protoplasmic connections -must have survived the process of fission. The implication is that the -cells forming the embryo-plant retained their protoplasmic connections -while they multiplied, and that such connections continued throughout all -subsequent multiplications--an implication which has, I believe, been -established by researches upon germinating palm-seeds. But now we come to a -verifying series of facts which the cell-structures of animals in their -early stages present. In his _Monograph of the Development of Peripatus -Capensis_, Mr. Adam Sedgwick, F.R.S., Reader in Animal Morphology at -Cambridge, writes as follows:-- - - "All the cells of the ovum, ectodermal as well as endodermal, are - connected together by a fine protoplasmic reticulum." (p. 41) - - "The continuity of the various cells of the segmenting ovum is primary, - and not secondary; _i. e._, in the cleavage the segments do not - completely separate from one another. But are we justified in speaking of - cells at all in this case? _The fully segmented ovum is a syncytium, and - there are not and have not been at any stage cell limits._" (p. 41) - - "It is becoming more and more clear every day that the cells composing - the tissues of animals are not isolated units, but that they are - connected with one another. I need only refer to the connection known to - exist between connective tissue cells, cartilage cells, epithelial cells, - &c. And not only may the cells of one tissue be continuous with each - other, but they may also be continuous with the cells of other tissues." - (pp. 47-8) - - "Finally, if the protoplasm of the body is primitively a syncytium, and - the ovum until maturity a part of that syncytium, the separation of the - generative products does not differ essentially from the internal - gemmation of a Protozoon, and the inheritance by the offspring of - peculiarities first appearing in the parent, though not explained, is - rendered less mysterious; for the protoplasm of the whole body being - continuous, change in the molecular constitution of any part of it would - naturally be expected to spread, in time, through the whole mass." (p. - 49) - -Mr. Sedgwick's subsequent investigations confirm these conclusions. In a -letter of December 27, 1892, passages which he allows me to publish run as -follows:-- - - "All the embryological studies that I have made since that to which you - refer confirm me more and more in the view that the connections between - the cells of adults are not secondary connections, but primary, dating - from the time when the embryo was a unicellular structure.... My own - investigations on this subject have been confined to the Arthropoda, - Elasmobranchii, and Aves. I have thoroughly examined the development of - at least one kind of each of these groups, and I have never been able to - detect a stage in which the cells were not continuous with each other; - and I have studied innumerable stages from the beginning of cleavage - onwards." - -So that the alleged independence of the reproductive cells does not exist. -The _soma_--to use Weismann's name for the aggregate of cells forming the -body--is, in the words of Mr. Sedgwick, "a continuous mass of vacuolated -protoplasm;" and the reproductive cells are nothing more than portions of -it separated some little time before they are required to perform their -functions. - -Thus the theory of Weismann is doubly disproved. Inductively we are shown -that there _does_ take place that communication of characters from the -somatic cells to the reproductive cells, which he says cannot take place; -and deductively we are shown that this communication is a natural sequence -of connections between the two which he ignores; his various conclusions -are deduced from a postulate which is untrue. - -* * * * * - -From the title of this essay, and from much of its contents, nine readers -out of ten will infer that it is directed against the views of Mr. Darwin. -They will be astonished on being told that, contrariwise, it is directed -against the views of those who, in a considerable measure, dissent from Mr. -Darwin. For the inheritance of acquired characters, which it is now the -fashion in the biological world to deny, was, by Mr. Darwin, fully -recognized and often insisted on. Such of the foregoing arguments as touch -Mr. Darwin's views, simply imply that the cause of evolution which at first -he thought unimportant, but the importance of which he increasingly -perceived as he grew older, is more important than he admitted, even at the -last. The neo-Darwinists, however, do not admit this cause at all. - -Let it not be supposed that this explanation implies any disapproval of the -dissentients, considered as such. Seeing how little regard for authority I -have myself usually shown, it would be absurd in me to reflect in any -degree upon those who have rejected certain of Mr. Darwin's teachings, for -reasons which they have held sufficient. But while their independence of -thought is to be applauded rather than blamed, it is, I think, to be -regretted that they have not guarded themselves against a long-standing -bias. It is a common trait of human nature to seek some excuse when found -in the wrong. Invaded self-esteem sets up a defence, and anything is made -to serve. Thus it happened that when geologists and biologists, previously -holding that all kinds of organisms arose by special creations, surrendered -to the battery opened upon them by _The Origin of Species_, they sought to -minimise their irrationality by pointing to irrationality on the other -side. "Well, at any rate, Lamarck was in the wrong." "It is clear that we -were right in rejecting _his_ doctrine." And so, by duly emphasizing the -fact that he overlooked "Natural Selection" as the chief cause, and by -showing how erroneous were some of his interpretations, they succeeded in -mitigating the sense of their own error. It is true their creed was that at -successive periods in the Earth's history, old Floras and Faunas had been -abolished and others introduced; just as though, to use Professor Huxley's -figure, the table had been now and again kicked over and a new pack of -cards brought out. And it is true that Lamarck, while he rejected this -absurd creed, assigned for the facts reasons some of which are absurd. But -in consequence of the feeling described, his defensible belief was -forgotten and only his indefensible ones remembered. This one-sided -estimate has become traditional; so that there is now often shown a subdued -contempt for those who suppose that there can be any truth in the -reasonings of a man whose general conception was partly sense, at a time -when the general conceptions of his contemporaries were wholly nonsense. -Hence results unfair treatment--hence result the different dealings with -the views of Lamarck and of Weismann. - -"Where are the facts proving the inheritance of acquired characters?" ask -those who deny it. Well, in the first place, there might be asked the -counter-question--Where are the facts which disprove it? Surely if not only -the general structures of organisms, but also many of the modifications -arising in them, are inheritable, the natural implication is that all -modifications are inheritable; and if any say that the inheritableness is -limited to those arising in a certain way, the _onus_ lies on them of -proving that those otherwise arising are not inheritable.[107] Leaving this -counter-question aside, however, it will suffice if we ask another -counter-question. It is asserted that the dwindling of organs from disuse -is due to the successive survivals in posterity of individuals in which the -organs have varied in the direction of decrease. Where now are the facts -supporting this assertion? Not one has been assigned or can be assigned. -Not a single case can be named in which _panmixia_ is a proved cause of -diminution. Even had the deductive argument for _panmixia_ been as valid as -we have found it to be invalid, there would still have been required, in -pursuance of scientific method, some verifying inductive evidence. Yet, -though not a shred of such evidence has been given, the doctrine is -accepted with acclamation, and adopted as part of current biological -theory. Articles are written and letters published in which it is assumed -that this mere speculation, justified by not a tittle of proof, displaces -large conclusions previously drawn. And then, passing into the outer world, -this unsupported belief affects opinions there too; so that we have -recently had a Right Honourable lecturer who, taking for granted its truth, -represents the inheritance of acquired characters as an exploded -hypothesis, and proceeds to give revised views of human affairs. - -Finally, there comes the reply that there _are_ facts proving the -inheritance of acquired characters. All those assigned by Mr. Darwin, -together with others such, remain outstanding when we find that the -interpretation by _panmixia_ is untenable. Indeed, even had that hypothesis -been tenable, it would have been inapplicable to these cases; since in -domestic animals, artificially fed and often overfed, the supposed -advantage from economy cannot be shown to tell; and since, in these cases, -individuals are not naturally selected during the struggle for life, in -which certain traits are advantageous, but are artificially selected by man -without regard to such traits. Should it be urged that the assigned facts -are not numerous, it may be replied that there are no persons whose -occupations and amusements incidentally bring out such facts; and that they -are probably as numerous as those which would have been available for Mr. -Darwin's hypothesis, had there been no breeders and fanciers and gardeners -who, in pursuit of their profits and hobbies, furnished him with evidence. -It may be added that the required facts are not likely to be numerous, if -biologists refuse to seek for them. - -See, then, how the case stands. Natural selection, or survival of the -fittest, is almost exclusively operative throughout the vegetal world and -throughout the lower animal world, characterized by relative passivity. But -with the ascent to higher types of animals, its effects are in increasing -degrees involved with those produced by inheritance of acquired characters; -until, in animals of complex structures, inheritance of acquired characters -becomes an important, if not the chief, cause of evolution. We have seen -that natural selection cannot work any changes in organisms save such as -conduce in considerable degrees, directly or indirectly, to the -multiplication of the stirp; whence failure to account for various changes -ascribed to it. And we have seen that it yields no explanation of the -co-adaptation of co-operative parts, even when the co-operation is -relatively simple, and still less when it is complex. On the other hand, we -see that if, along with the transmission of generic and specific -structures, there tend to be transmitted modifications arising in a certain -way, there is a strong _a priori_ probability that there tend to be -transmitted modifications arising in all ways. We have a number of facts -confirming this inference, and showing that acquired characters are -inherited--as large a number as can be expected, considering the difficulty -of observing them and the absence of search. And then to these facts may be -added the facts with which this essay set out, concerning the distribution -of tactual discriminativeness. While we saw that these are inexplicable by -survival of the fittest, we saw that they are clearly explicable as -resulting from the inheritance of acquired characters. And here let it be -added that this conclusion is conspicuously warranted by one of the methods -of inductive logic, known as the method of concomitant variations. For -throughout the whole series of gradations in perceptive power, we saw that -the amount of the effect is proportionate to the amount of the alleged -cause. - - -II. - -Apart from those more special theories of Professor Weismann I lately dealt -with, the wide acceptance of which by the biological world greatly -surprises me, there are certain more general theories of his--fundamental -theories--the acceptance of which surprises me still more. Of the two on -which rests the vast superstructure of his speculations, the first concerns -the distinction between the reproductive elements of each organism and the -non-reproductive elements. He says:-- - - "Let us now consider how it happened that the multicellular animals and - plants, which arose from unicellular forms of life, came to lose this - power of living for ever. - - "The answer to this question is closely bound up with the principle of - division of labour which appeared among multicellular organisms at a very - early stage.... - - "The first multicellular organism was probably a cluster of similar - cells, but these units soon lost their original homogeneity. As the - result of mere relative position, some of the cells were especially - fitted to provide for the nutrition of the colony, while others undertook - the work of reproduction." (_Essays upon Heredity_, i, p. 27) - -Here, then, we have the great principle of the division of labour, which is -the principle of all organization, taken as primarily illustrated in the -division between the reproductive cells and the non-reproductive or somatic -cells--the cells devoted to the continuance of the species, and the cells -which subserve the life of the individual. And the early separation of -reproductive cells from somatic cells, is alleged on the ground that this -primary division of labour is that which arises between elements devoted to -species-life and elements devoted to individual life. Let us not be content -with words but look at the facts. - -When Milne-Edwards first used the phrase "physiological division of -labour," he was obviously led to do so by perceiving the analogy between -the division of labour in a society, as described by political economists, -and the division of labour in an organism. Every one who reads has been -familiarized with the first as illustrated in the early stages, when men -were warriors while the cultivation and drudgery were done by slaves and -women; and as illustrated in the later stages, when not only are -agriculture and manufactures carried on by separate classes, but -agriculture is carried on by landlords, farmers, and labourers, while -manufactures, multitudinous in their kinds, severally involve the actions -of capitalists, overseers, workers, &c., and while the great function of -distribution is carried on by wholesale and retail dealers in different -commodities. Meanwhile students of biology, led by Milne-Edwards's phrase, -have come to recognize a parallel arrangement in a living creature; shown, -primarily, in the devoting of the outer parts to the general business of -obtaining food and escaping from enemies, while the inner parts are devoted -to the utilization of food, and supporting themselves and the outer parts; -and shown, secondarily, by the subdivision of these great functions into -those of various limbs and senses in the one case, and in the other case -into those of organs for digestion, respiration, circulation, excretion, -&c. But now let us ask what is the essential nature of this division of -labour. In both cases it is an _exchange of services_--an arrangement under -which, while one part devotes itself to one kind of action and yields -benefits to all the rest, all the rest, jointly and severally performing -their special actions, yield benefits to it in exchange. Otherwise -described, it is a system of _mutual_ dependence: A depends for its welfare -upon B, C, and D; B upon A, C, and D; and so with the rest: all depend upon -each and each upon all. Now let us apply this true conception of the -division of labour, to that which Professor Weismann calls a division of -labour. Where is the _exchange of services_ between somatic cells and -reproductive cells? There is none. The somatic cells render great services -to the reproductive cells, by furnishing them with materials for growth and -multiplication; but the reproductive cells render no services at all to the -somatic cells. If we look for the _mutual_ dependence we look in vain. We -find entire dependence on the one side and none on the other. Between the -parts devoted to individual life and the part devoted to species-life, -there is no division of labour whatever. The individual works for the -species; but the species works not for the individual. Whether at the stage -when the species is represented by reproductive cells, or at the stage when -it is represented by eggs, or at the stage when it is represented by young, -the parent does everything for it, and it does nothing for the parent. The -essential part of the conception is gone: there is no giving and receiving, -no exchange, no mutuality. - -But now suppose we pass over this fallacious interpretation, and grant -Professor Weismann his fundamental assumption and his fundamental -corollary. Suppose we grant that because the primary division of labour is -that between somatic cells and reproductive cells, these two groups are the -first to be differentiated. Having granted this corollary, let us compare -it with the facts. As the alleged primary division of labour is universal, -so the alleged primary differentiation should be universal too. Let us see -whether it is so. Already, in the paragraph from which I have quoted above, -a crack in the doctrine is admitted: it is said that "this differentiation -was not at first absolute, and indeed it is not always so to-day." And -then, on turning to page 74, we find that the crack has become a chasm. Of -the reproductive cells it is stated that--"In Vertebrata they do not become -distinct from the other cells of the body until the embryo is completely -formed." That is to say, in this large and most important division of the -animal kingdom, the implied universal law does not hold. Much more than -this is confessed. Lower down the page we read--"There may be in fact cases -in which such separation does not take place until after the animal is -completely formed, and others, as I believe that I have shown, in which it -first arises one or more generations later, viz., in the buds produced by -the parent." - -So that in other great divisions of the animal kingdom the alleged law is -broken; as among the _Coelenterata_ by the _Hydrozoa_, as among the -_Mollusca_ by the Ascidians, and as among the _Platyhelminthes_ by the -Trematode worms. - -Following this admission concerning the _Vertebrata_, come certain -sentences which I partially italicize:-- - - "Thus, as their development shows, a marked antithesis exists between the - substance of the undying reproductive cells and that of the perishable - body-cells. We cannot explain this fact except _by the supposition_ that - each reproductive cell potentially contains two kinds of substance, which - at a variable time after the commencement of embryonic development, - separate from one another, and finally produce two sharply contrasted - groups of cells." (p. 74) - -And a little lower down the page we meet with the lines:-- - - "_It is therefore quite conceivable_ that the reproductive cells might - separate from the somatic cells much later than in the examples mentioned - above, without changing the hereditary tendencies of which they are the - bearers." - -That is to say, it is "quite conceivable" that after sexless _Cercariæ_ -have gone on multiplying by internal gemmation for generations, the "two -kinds of substance" have, notwithstanding innumerable cell-divisions, -preserved their respective natures, and finally separate in such ways as to -produce reproductive cells. Here Professor Weismann does not, as in a case -before noted, assume something which it is "easy to imagine," but he -assumes something which it is difficult to imagine; and apparently thinks -that a scientific conclusion may be thereon safely based. - -* * * * * - -Associated with the assertion that the primary division of labour is -between the somatic cells and the reproductive cells, and associated with -the corollary that the primary differentiation is that which arises between -them, there goes another corollary. It is alleged that there exists a -fundamental distinction of nature between these two classes of cells. They -are described as respectively mortal and immortal, in the sense that those -of the one class are limited in their powers of multiplication, while those -of the other class are unlimited. And it is contended that this is due to -inherent unlikeness of nature. - -Before inquiring into the truth of this proposition, I may fitly remark -upon a preliminary proposition set down by Professor Weismann. Referring to -the hypothesis that death depends "upon causes which lie in the nature of -life itself," he says:-- - - "I do not however believe in the validity of this explanation: I consider - that death is not a primary necessity, but that it has been secondarily - acquired as an adaptation. I believe that life is endowed with a fixed - duration, not because it is contrary to its nature to be unlimited, but - because the unlimited existence of individuals would be a luxury without - any corresponding advantage." (p. 24) - -This last sentence has a teleological sound which would be appropriate did -it come from a theologian, but which seems strange as coming from a man of -science. Assuming, however, that the implication was not intended, I go on -to remark that Professor Weismann has apparently overlooked a universal law -of evolution--not organic only, but inorganic and super-organic--which -implies the necessity of death. The changes of every aggregate, no matter -of what kind, inevitably end in a state of equilibrium. Suns and planets -die, as well as organisms. The process of integration, which constitutes -the fundamental trait of all evolution, continues until it has brought -about a state which negatives further alterations, molar or molecular--a -state of balance among the forces of the aggregate and the forces which -oppose them.[108] In so far, therefore, as Professor Weismann's conclusions -imply the non-necessity of death, they cannot be sustained. - -But now let us consider the above-described antithesis between the immortal -_Protozoa_ and the mortal _Metazoa_. An essential part of the theory is -that the _Protozoa_ can go on dividing and subdividing without limit, so -long as the fit external conditions are maintained. But what is the -evidence for this? Even by Professor Weismann's own admission there is no -proof. On p. 285 he says:-- - - "I could only consent to adopt the hypothesis of rejuvenescence [achieved - by conjugation], if it were rendered absolutely certain that reproduction - by division could never under any circumstances persist indefinitely. But - this cannot be proved with any greater certainty than the converse - proposition, and hence, as far as direct proof is concerned, the facts - are equally uncertain on both sides." - -But this is an admission which seems to be entirely ignored when there is -alleged the contrast between the immortal _Protozoa_ and the mortal -_Metazoa_. Following Professor Weismann's method, it would be "easy to -imagine" that occasional conjugation is in all cases essential; and this -easily imagined conclusion might fitly be used to bar out his own. Indeed, -considering how commonly conjugation is observed, it may be held difficult -to imagine that it can in any cases be dispensed with. Apart from -imaginations of either kind, however, here is an acknowledgment that the -immortality of _Protozoa_ is not proved; that the allegation has no better -basis than the failure to observe cessation of fission; and that thus one -term of the above antithesis is not a fact, but is only an assumption. - -And now what about the other term of the antithesis--the alleged inherent -mortality of the somatic cells? This we shall, I think, find is no more -defensible than the other. Such plausibility as it possesses disappears -when, instead of contemplating the vast assemblage of familiar cases which -animals present, we contemplate certain less familiar and unfamiliar cases. -By these we are shown that the usual ending of multiplication among somatic -cells is due, not to an intrinsic cause, but to extrinsic causes. Let us, -however, first look at Professor Weismann's own statements:-- - - "I have endeavoured to explain death as the result of restriction in the - powers of reproduction possessed by the somatic cells, and I have - suggested that such restriction may conceivably follow from a limitation - in the number of cell-generations possible for the cells of each organ - and tissue." (p. 28) - - "The above-mentioned considerations show us that the degree of - reproductive activity present in the tissues is regulated by internal - causes while the natural death of an organism is the termination--the - hereditary limitation--of the process of cell-division, which began in - the segmentation of the ovum." (p. 30) - -Now, though, in the above extracts there is mention of "internal causes" -determining "the degree of reproductive activity" of tissue cells, and -though, on page 28, the "causes of the loss" of the power of unlimited -cell-production "must be sought outside the organism, that is to say, in -the external conditions of life," yet the doctrine is that somatic cells -have become constitutionally unfitted for continued cell-multiplication. - - "The somatic cells have lost this power to a gradually increasing extent, - so that at length they became restricted to a fixed, though perhaps very - large, number of cell-generations." (p. 28) - -Examination will soon disclose good reasons for denying this inherent -restriction. We will look at the various causes which affect their -multiplication, and usually put a stop to increase after a certain point is -reached. - -There is first the amount of vital capital given by the parent; partly in -the shape of a more or less developed structure, and partly in the shape of -bequeathed nutriment. Where this vital capital is small, and the young -creature, forthwith obliged to carry on physiological business for itself, -has to expend effort in obtaining materials for daily consumption as well -as for growth, a rigid restraint is put on that cell-multiplication -required for a large size. Clearly, the young elephant, starting with a big -and well-organized body, and supplied _gratis_ with milk during early -stages of growth, can begin physiological business on his own account on a -great scale; and by its large transactions his system is enabled to supply -nutriment to its multiplying somatic cells until they have formed a vast -aggregate--an aggregate such as it is impossible for a young mouse to -reach, obliged as it is to begin physiological business in a small way. -Then there is the character of the food in respect of its digestibility and -its nutritiveness. Here, that which the creature takes in requires much -grinding-up, or, when duly prepared, contains but a small amount of -available matter in comparison with the matter that has to be thrown away; -while there, the prey seized is almost pure nutriment, and requires but -little trituration. Hence, in some cases, an unprofitable physiological -business, and in other cases a profitable one; resulting in small or large -supplies to the multiplying somatic cells. Further, there has to be noted -the grade of visceral development, which, if low, yields only crude -nutriment slowly distributed, but which, if high, serves by its good -appliances for solution, depuration, absorption, and circulation, to yield -to the multiplying somatic cells a rich and pure blood. Then we come to an -all-important factor, the cost of obtaining food. Here large expenditure of -energy in locomotion is necessitated, and there but little--here great -efforts for small portions of food, and there small efforts for great -portions: again resulting in physiological poverty or physiological wealth. -Next, beyond the cost of nervo-muscular activities in foraging, there is -the cost of maintaining bodily heat. So much heat implies so much consumed -nutriment, and the loss by radiation or conduction, which has perpetually -to be made good, varies according to many circumstances--climate, medium -(as air or water), covering, size of body (small cooling relatively faster -than large); and in proportion to the cost of maintaining heat is the -abstraction from the supplies for cell-formation. Finally, there are three -all-important co-operative factors, or rather laws of factors, the effects -of which vary with the size of the animal. The first is that, while the -mass of the body varies as the cubes of its dimensions (_proportions_ being -supposed constant), the absorbing surface varies as the squares of its -dimensions; whence it results that, other things equal, increase of size -implies relative decrease of nutrition, and therefore increased obstacles -to cell-multiplication.[109] The second is a further sequence from these -laws--namely, that while the weight of the body increases as the cubes of -the dimensions, the sectional areas of its muscles and bones increase as -their squares; whence follows a decreasing power of resisting strains, and -a relative weakness of structure. This is implied in the ability of a small -animal to leap many times its own length, while a great animal, like the -elephant, cannot leap at all: its bones and muscles being unable to bear -the stress which would be required to propel its body through the air. What -increasing cost of keeping together the bodily fabric is thus entailed, we -cannot say; but that there is an increasing cost, which diminishes the -available, materials for increase of size, is beyond question.[110] And -then, in the third place, we have augmented expense of distribution of -nutriment. The greater the size becomes, the more force must be exerted to -send blood to the periphery; and this once more entails deduction from the -cell-forming matters. - -Here, then, we have nine factors, several of them involving subdivisions, -which co-operate in aiding or restraining cell-multiplication. They occur -in endlessly varied proportions and combinations; so that every species -differs more or less from every other in respect of their effects. But in -all of them the co-operation is such as eventually arrests that -multiplication of cells which causes further growth; continues thereafter -to entail slow decrease in cell-multiplication, accompanying decline of -vital activities; and eventually brings cell-multiplication to an end. Now -a recognized principle of reasoning--the Law of Parsimony--forbids the -assumption of more causes than are needful for explanation of phenomena; -and since, in all such living aggregates as those above supposed, the -causes named inevitably bring about arrest of cell-multiplication, it is -illegitimate to ascribe this arrest to some inherent property in the cells. -Inadequacy of the other causes must be shown before an inherent property -can be rightly assumed. - -For this conclusion we find ample justification when we contemplate types -of animals which lead lives that do not put such decided restraints on -cell-multiplication. First let us take an instance of the extent to which -(irrespective of natures of cells as reproductive or somatic) -cell-multiplication may go, where the conditions render nutrition easy and -reduce expenditure to a minimum. I refer to the case of the _Aphides_. -Though it is early in the season (March), the hothouses at Kew have -furnished a sufficient number of these to show that twelve of them weigh a -grain--a larger number than would be required were they full-sized. Citing -Professor Owen, who adopts the calculations of Tougard to the effect that -by agamic multiplication "a single impregnated ovum of _Aphis_ may give -rise, without fecundation, to a quintillion of _Aphides_," Professor Huxley -says:-- - - "I will assume that an Aphis weighs 1/1000 of a grain, which is certainly - vastly under the mark. A quintillion of _Aphides_ will, on this estimate, - weigh a quatrillion of grains. He is a very stout man who weighs two - million grains; consequently the tenth brood alone, if all its members - survive the perils to which they are exposed, contains more substance - than 500,000,000 stout men--to say the least, more than the whole - population of China!"[111] - -And had Professor Huxley taken the actual weight, one-twelfth of a grain, -the quintillion of _Aphides_ would evidently far outweigh the whole human -population of the globe: five billions of tons being the weight, as brought -out by my own calculation! Of course I do not cite this in proof of the -extent to which multiplication of somatic cells, descending from a single -ovum, may go; because it will be contended, with some reason, that each of -the sexless _Aphides_, viviparously produced, arose by fission of a cell -which had descended from the original reproductive cell. I cite it merely -to show that when the cell-products of a fertilized ovum are perpetually -divided and subdivided into small groups, distributed over an unlimited -nutritive area, so that they can get materials for growth at no cost, and -expend nothing appreciable in motion or maintenance of temperature, -cell-production may go on without limit. For the agamic multiplication of -_Aphides_ has been shown to continue for four years, and to all appearance -would be ceaseless were the temperature and supply of food continued -without break. But now let us pass to analogous illustrations of cause and -consequence, open to no criticism of the kind just indicated. They are -furnished by various kinds of _Entozoa_, of which take the _Trematoda_, -infesting molluscs and fishes. Of one of them we read:--"_Gyrodactylus_ -multiplies agamically by the development of a young Trematode within the -body, as a sort of internal bud. A second generation appears within the -first, and even a third within the second, before the young _Gyrodactylus_ -is born."[112] And the drawings of Steenstrup, in his _Alternation of -Generations_, show us, among creatures of this group, a sexless individual -the whole interior of which is transformed into smaller sexless -individuals, which severally, before or after their emergence, undergo -similar transformations--a multiplication of somatic cells without any sign -of reproductive cells. Under what circumstances do such modes of agamic -multiplication, variously modified among parasites, occur? They occur where -there is no expenditure whatever in motion or maintenance of temperature, -and where nutriment surrounds the body on all sides. Other instances are -furnished by groups in which, though the nutriment is not abundant, the -cost of living is almost unappreciable. Among the _Coelenterata_ there are -the Hydroid Polyps, simple and compound; and among the _Mollusca_ we have -various types of Ascidians, fixed and floating, _Botryllidæ_ and _Salpæ_. - -But now from these low animals in which sexless reproduction, and continued -multiplication of somatic cells, is common, and one class of which is named -"zoophytes," because its form of life simulates that of plants, let us pass -to plants themselves. In these there is no expenditure in effort, there is -no expenditure in maintaining temperature, and the food, some of it -supplied by the earth, is the rest of it supplied by a medium which -everywhere bathes the outer surface: the utilization of its contained -material being effected _gratis_ by the Sun's rays. Just as was to be -expected, we here find that agamogenesis may go on without end. Numerous -plants and trees are propagated to an unlimited extent by cuttings and -buds; and we have sundry plants which cannot be otherwise propagated. The -most familiar are the double roses of our gardens: these do not seed, and -yet have been distributed everywhere by grafts and buds. Hothouses furnish -many cases, as I learn from an authority second to none. Of "the whole host -of tropical orchids, for instance, not one per cent. has ever seeded, and -some have been a century under cultivation." Again, we have the _Acorus -calamus_, "that has hardly been known to seed anywhere, though it is found -wild all over the north temperate hemisphere." And then there is the -conspicuous and conclusive case of _Eloidea Canadensis_ (alias -_Anacharis_,) introduced no one knows how (probably with timber), and first -observed in 1847, in several places; and which, having since spread over -nearly all England, now everywhere infests ponds, canals, and slow rivers. -The plant is dioecious, and only the female exists here. Beyond all -question, therefore, this vast progeny of the first slip or fragment -introduced, sufficient to cover many square miles were it put together, is -constituted entirely of somatic cells. Hence, as far as we can judge, these -somatic cells are immortal in the sense given to the word by Professor -Weismann; and the evidence that they are so is immeasurably stronger than -the evidence which leads him to assert immortality for the -fissiparously-multiplying _Protozoa_. This endless multiplication of -somatic cells has been going on under the eyes of numerous observers for -forty odd years. What observer has watched for forty years to see whether -the fissiparous multiplication of _Protozoa_ does not cease? What observer -has watched for one year, or one month, or one week?[113] - -Even were not Professor Weismann's theory disposed of by this evidence, it -might be disposed of by a critical examination of his own evidence, using -his own tests. Clearly, if we are to measure relative mortalities, we must -assume the conditions to be the same and must use the same measure. Let us -do this with some appropriate animal--say Man, as the most open to -observation. The mortality of the somatic cells constituting the mass of -the human body, is, according to Professor Weismann, shown by the decline -and final cessation of cell-multiplication in its various organs. Suppose -we apply this test to all the organs: not to those only in which there -continually arise bile-cells, epithelium-cells, &c., but to those also in -which there arise reproductive cells. What do we find? That the -multiplication of these last comes to an end long before the multiplication -of the first. In a healthy woman, the cells which constitute the various -active tissues of the body, continue to grow and multiply for many years -after germ-cells have died out. If similarly measured, then, these cells of -the last class prove to be more mortal than those of the first. But -Professor Weismann uses a different measure for the two classes of cells. -Passing over the illegitimacy of this proceeding, let us accept his other -mode of measurement, and see what comes of it. As described by him, absence -of death among the _Protozoa_ is implied by that unceasing division and -subdivision of which they are said to be capable. Fission continued without -end, is the definition of the immortality he speaks of. Apply this -conception to the reproductive cells in a _Metazoon_. That the immense -majority of them do not multiply without end, we have already seen: with -very rare exceptions they die and disappear without result, and they cease -their multiplication while the body as a whole still lives. But what of -those extremely exceptional ones which, as being actually instrumental to -the maintenance of the species, are alone contemplated by Professor -Weismann? Do these continue their fissiparous multiplications without end? -By no means. The condition under which alone they preserve a qualified form -of existence, is that, instead of one becoming two, two become one. A -member of series A and a member of series B, coalesce; and so lose their -individualities. Now, obviously, if the immortality of a series is shown if -its members divide and subdivide perpetually, then the opposite of -immortality is shown when, instead of division, there is union. Each series -ends, and there is initiated a new series, differing more or less from -both. Thus the assertion that the reproductive cells are immortal, can be -defended only by changing the conception of immortality otherwise implied. - -Even apart from these last criticisms, however, we have clear disproof of -the alleged inherent difference between the two classes of cells. Among -animals, the multiplication of somatic cells is brought to an end by sundry -restraining conditions; but in various plants, where these restraining -conditions are absent, the multiplication is unlimited. It may, indeed, be -said that the alleged distinction should be reversed; since the fissiparous -multiplication of reproductive cells is necessarily interrupted from time -to time by coalescence, while that of the somatic cells may go on for a -century without being interrupted. - -* * * * * - -In the essay to which this is a postscript, conclusions were drawn from the -remarkable case of the horse and the quagga, there narrated, along with an -analogous case observed among pigs. These conclusions have since been -confirmed. I am much indebted to a distinguished correspondent who has -drawn my attention to verifying facts furnished by the offspring of whites -and negroes in the United States. Referring to information given him many -years ago, he says:--"It was to the effect that the children of white women -by a white father, had been _repeatedly_ observed to show traces of black -blood, in cases when the woman had previous connection with [_i. e._ a -child by] a negro." At the time I received this information, an American -was visiting me; and, on being appealed to, answered that in the United -States there was an established belief to this effect. Not wishing, -however, to depend upon hearsay, I at once wrote to America to make -inquiries. Professor Cope of Philadelphia has written to friends in the -South, but has not yet sent me the results. Professor Marsh, the -distinguished palæontologist, of Yale, New Haven, who is also collecting -evidence, sends a preliminary letter in which he says:--"I do not myself -know of such a case, but have heard many statements that make their -existence probable. One instance, in Connecticut, is vouched for so -strongly by an acquaintance of mine, that I have good reason to believe it -to be authentic." - -That cases of the kind should not be frequently seen in the North, -especially nowadays, is of course to be expected. The first of the above -quotations refers to facts observed in the South during slavery days; and -even then, the implied conditions were naturally very infrequent. Dr. W. J. -Youmans of New York has, on my behalf, interviewed several medical -professors, who, though they have not themselves met with instances, say -that the alleged result, described above, "is generally accepted as a -fact." But he gives me what I think must be regarded as authoritative -testimony. It is a quotation from the standard work of Professor Austin -Flint, and runs as follows:-- - - "A peculiar and, it seems to me, an inexplicable fact is, that previous - pregnancies have an influence upon offspring. This is well known to - breeders of animals. If pure-blooded mares or bitches have been once - covered by an inferior male, in subsequent fecundations the young are - likely to partake of the character of the first male, even if they be - afterwards bred with males of unimpeachable pedigree. What the mechanism - of the influence of the first conception is, it is impossible to say; but - the fact is incontestable. The same influence is observed in the human - subject. A woman may have, by a second husband, children who resemble a - former husband, and this is particularly well marked in certain instances - by the colour of the hair and eyes. A white woman who has had children by - a negro may subsequently bear children to a white man, these children - presenting some of the unmistakable peculiarities of the negro - race."[114] - -Dr. Youmans called on Professor Flint, who remembered "investigating the -subject at the time his larger work was written [the above is from an -abridgment], and said that he had never heard the statement questioned." - -Some days before I received this letter and its contained quotation, the -remembrance of a remark I heard many years ago concerning dogs, led to the -inquiry whether they furnished analogous evidence. It occurred to me that a -friend who is frequently appointed judge of animals at agricultural shows, -Mr. Fookes, of Fairfield, Pewsey, Wiltshire, might know something about the -matter. A letter to him brought various confirmatory statements. From one -"who had bred dogs for many years" he learnt that-- - - "It is a well known and admitted fact that if a bitch has two litters by - two different dogs, the character of the first father is sure to be - perpetuated in any litters she may afterwards have, no matter how - pure-bred a dog may be the begetter." - -After citing this testimony, Mr. Fookes goes on to give illustrations known -to himself. - - "A friend of mine near this had a very valuable Dachshund bitch, which - most unfortunately had a litter by a stray sheep-dog. The next year her - owner sent her on a visit to a pure Dachshund dog, but the produce took - quite as much of the first father as the second, and the next year he - sent her to another Dachshund with the same result. Another case:--A - friend of mine in Devizes had a litter of puppies, unsought for, by a - setter from a favourite pointer bitch, and after this she never bred any - true pointers, no matter of what the paternity was." - - [Since the publication of this article additional evidences have come to - hand. One is from the late Prof. Riley, State Entomologist at Washington, - who says that telegony is an "established principle among well-educated - farmers" in the United States, and who gives me a case in horse-breeding - to which he was himself witness. - - Mr. W. P. Smith, writing from Stoughton Grange, Guildford, but giving the - results of his experiences in America, says that "the fact of a previous - conception influencing subsequent offspring was so far recognised among - American cattle-breeders" that it was proposed to raise the rank of any - heifer that had borne a first calf by a thoroughbred bull, and though - this resolution when brought before one of the chief societies was not - carried, yet on all sides it was admitted that previous conceptions had - effects of the kind alleged. Mr. Smith in another letter says:--"When I - had a large mule and horse ranche in America I noticed that the foals of - mares by horse stallions had a mulish appearance in those cases where the - mare had previously given birth to a mule foal. Common heifers who have - had calves by a thoroughbred bull are apt thereafter to have well-bred - calves even from the veriest scrubs." - - Yet another very interesting piece of evidence is furnished by Mr. W. - Sedgwick, M.R.C.S., in an article on "The Influence of Heredity in - Disease," published in the _British Medical Journal_ for Feb. 22, 1896, - pp. 460-2. It concerns the transmission of a malformation known among - medical men as hypospadias. Referring to a man belonging to a family in - which this defect prevailed, he writes:--"The widow of the man from whom - these three generations of hypospadians were descended married again, - after an interval of eighteen months; and in this instance the second - husband was not only free from the defect, but there was no history of it - in his family. By this second marriage she had four hypospadiac sons and - four hypospadiac grandsons; whilst there were seven grandsons and three - great-grandsons who were not malformed."] - -Coming from remote places, from those who have no theory to support, and -who are some of them astonished by the unexpected phenomena, the agreement -dissipates all doubt. In four kinds of mammals, widely divergent in their -natures--man, horse, dog, and pig--we have this same seemingly-anomalous -kind of heredity, made visible under analogous conditions. We must take it -as a demonstrated fact that, during gestation, traits of constitution -inherited from the father produce effects upon the constitution of the -mother; and that these communicated effects are transmitted by her to -subsequent offspring. We are supplied with an absolute disproof of -Professor Weismann's doctrine that the reproductive cells are independent -of, and uninfluenced by, the somatic cells; and there disappears absolutely -the alleged obstacle to the transmission of acquired characters. - -* * * * * - -Notwithstanding experiences showing the futility of controversy for the -establishment of truth, I am tempted here to answer opponents at some -length. But even could the editor allow me the needful space, I should be -compelled, both by lack of time and by ill-health, to be brief. I must -content myself with noticing a few points which most nearly concern me. - -Referring to my argument respecting tactual discriminativeness, Mr. Wallace -thinks that I-- - - "afford a glaring example of taking the unessential in place of the - essential, and drawing conclusions from a partial and altogether - insufficient survey of the phenomena. For this 'tactual - discriminativeness,' which is alone dealt with by Mr. Spencer, forms the - least important, and probably only an incidental portion of the great - vital phenomenon of skin-sensitiveness, which is at once the watchman and - the shield of the organism against imminent external dangers." - (_Fortnightly Review_, April, 1893, p. 497) - -Here Mr. Wallace assumes it to be self-evident that skin-sensitiveness is -due to natural selection, and assumes that this must be admitted by me. He -supposes it is only the unequal distribution of skin-discriminativeness -which I contend is not thus accounted for. But I deny that either the -general sensitiveness or the special sensitiveness results from natural -selection; and I have years ago justified the first disbelief as I have -recently the second. In "The Factors of Organic Evolution" (_Essays_, -454-8), I have given various reasons for inferring that the genesis of the -nervous system cannot be due to survival of the fittest; but that it is due -to the direct effects of converse between the surface and the environment; -and that thus only is to be explained the strange fact that the nervous -centres are originally superficial, and migrate inwards during development. -These conclusions I have, in the essay Mr. Wallace criticizes, upheld by -the evidence which blind boys and skilled compositors furnish; proving, as -this does, that increased nervous development is peripherally initiated. -Mr. Wallace's belief that skin-sensitiveness arose by natural selection, is -unsupported by a single fact. He assumes that it _must_ have been so -produced because it is all-important to self-preservation. My belief that -it is directly initiated by converse with the environment, is supported by -facts; and I have given proof that the assigned cause is now in operation. -Am I called upon to abandon my own supported belief and accept Mr. -Wallace's unsupported belief? I think not. - -Referring to my argument concerning blind cave-animals, Professor -Lankester, in _Nature_ of February 23, 1893, writes:-- - - "Mr. Spencer shows that the saving of ponderable material in the - suppression of an eye is but a small economy: he loses sight of the fact, - however, that possibly, or even probably, the saving to the organism in - the reduction of an eye to a rudimentary state is not to be measured by - mere bulk, but by the non-expenditure of special materials and special - activities which are concerned in the production of an organ so peculiar - and elaborate as is the vertebrate eye." - -It seems to me that a supposition is here made to do duty as a fact; and -that I might with equal propriety say that "possibly, or even probably," -the vertebrate eye is physiologically cheap: its optical part, constituting -nearly its whole bulk, consisting of a low order of tissue. There is, -indeed, strong reason for considering it physiologically cheap. If any one -remembers how relatively enormous are the eyes of a fish just out of the -egg--a pair of eyes with a body and head attached; and if he then remembers -that every egg contains material for such a pair of eyes; he will see that -eye-material constitutes a very considerable part of the fish's roe; and -that, since the female fish provides this quantity every year, it cannot be -expensive. My argument against Weismann is strengthened rather than -weakened by contemplation of these facts. - -Professor Lankester asks my attention to a hypothesis of his own, published -in the _Encyclopædia Britannica_, concerning the production of blind -cave-animals. He thinks it can-- - - "be fully explained by natural selection acting on congenital fortuitous - variations. Many animals are thus born with distorted or defective eyes - whose parents have not had their eyes submitted to any peculiar - conditions. Supposing a number of some species of Arthropod or Fish to be - swept into a cavern or to be carried from less to greater depths in the - sea, those individuals with perfect eyes would follow the glimmer of - light and eventually escape to the outer air or the shallower depths, - leaving behind those with imperfect eyes to breed in the dark place. A - natural selection would thus be effected" in successive generations. - -First of all, I demur to the words "many animals." Under the abnormal -conditions of domestication, congenitally defective eyes may be not very -uncommon; but their occurrence under natural conditions is, I fancy, -extremely rare. Supposing, however, that in a shoal of young fish, there -occur some with eyes seriously defective. What will happen? Vision is -all-important to the young fish, both for obtaining food and for escaping -from enemies. This is implied by the immense development of eyes just -referred to; and the obvious conclusion to be drawn is that the partially -blind would disappear. Considering that out of the enormous number of young -fish hatched with perfect eyes, not one in a hundred reaches maturity, what -chance of surviving would there be for those with imperfect eyes? -Inevitably they would be starved or be snapped up. Hence the chances that a -matured or partially matured semi-blind fish, or rather two such, male and -female, would be swept into a cave and left behind are extremely remote. -Still more remote must the chances be in the case of cray-fish. Sheltering -themselves as these do under stones, in crevices, and in burrows which they -make in the banks, and able quickly to anchor themselves to weeds or sticks -by their claws, it seems scarcely supposable that any of them could be -carried into a cave by a flood. What, then, is the probability that there -will be two nearly blind ones, and that these will be thus carried? Then, -after this first extreme improbability, there comes a second, which we may, -I think, rather call an impossibility. How would it be possible for -creatures subject to so violent a change of habitat to survive? Surely -death would quickly follow the subjection to such utterly unlike conditions -and modes of life. The existence of these blind cave-animals can be -accounted for only by supposing that their remote ancestors began making -excursions into the cave, and, finding it profitable, extended them, -generation after generation, further in: undergoing the required -adaptations little by little.[115] - -Between Dr. Romanes and myself the first difference concerns the -interpretation of "Panmixia." Clearer conceptions of these matters would be -reached if, instead of thinking in abstract terms, the physiological -processes concerned were brought into the foreground. Beyond the production -of changes in the sizes of parts by the selection of fortuitously-arising -variations, I can see but one other cause for the production of them--the -competition among the parts for nutriment. This has the effect that active -parts are well-supplied and grow, while inactive parts are ill-supplied and -dwindle.[116] This competition is the cause of "economy of growth"; this is -the cause of decrease from disuse; and this is the only conceivable cause -of that decrease which Dr. Romanes contends follows the cessation of -selection. The three things are aspects of the same thing. And now, before -leaving this question, let me remark on the strange proposition which has -to be defended by those who deny the dwindling of organs from disuse. Their -proposition amounts to this:--that for a hundred generations an inactive -organ may be partially denuded of blood all through life, and yet in the -hundredth generation will be produced of just the same size as in the -first! - -There is one other passage in Dr. Romanes' criticism--that concerning the -influence of a previous sire on progeny--which calls for comment. He sets -down what he supposes Weismann will say in response to my argument. "First, -he may question the fact." Well, after the additional evidence given above, -I think he is not likely to do that; unless, indeed, it be that along with -readiness to base conclusions on things "it is easy to imagine" there goes -reluctance to accept testimony which it is difficult to doubt. Second, he -is supposed to reply that "the Germ-plasm of the first sire has in some way -or another become partly commingled with that of the immature ova"; and Dr. -Romanes goes on to describe how there may be millions of spermatozoa and -"thousands of millions" of their contained "ids" around the ovaries, to -which these secondary effects are due. But, on the one hand, he does not -explain why in such cases each subsequent ovum, as it becomes matured, is -not fertilized by the sperm-cells present, or their contained germ-plasm, -rendering all subsequent fecundations needless; and, on the other hand, he -does not explain why, if this does not happen, the potency of this -remaining germ-plasm is nevertheless such as to affect not only the next -succeeding offspring, but all subsequent offspring. The irreconcilability -of these two implications would, I think, sufficiently dispose of the -supposition, even had we not daily multitudinous proofs that the surface of -a mammalian ovarium is not a spermatheca. The third reply Dr. Romanes -urges, is the inconceivability of the process by which the germ-plasm of a -preceding male parent affects the constitution of the female and her -subsequent offspring. In response, I have to ask why he piles up a mountain -of difficulties based on the assumption that Mr. Darwin's explanation of -heredity by "Pangenesis" is the only available explanation preceding that -of Weismann? and why he presents these difficulties to me, more especially; -deliberately ignoring my own hypothesis of physiological units? It cannot -be that he is ignorant of this hypothesis, since the work in which it is -variously set forth (_Principles of Biology_, §§ 66-97) is one with which -he is well acquainted: witness his _Scientific Evidences of Organic -Evolution_; and he has had recent reminders of it in Weismann's -_Germ-plasm_, where it is repeatedly referred to. Why, then, does he assume -that I abandon my own hypothesis and adopt that of Darwin; thereby -entangling myself in difficulties which my own hypothesis avoids? If, as I -have argued, the germ-plasm consists of substantially similar units (having -only those minute differences expressive of individual and ancestral -differences of structure), none of the complicated requirements which Dr. -Romanes emphasizes exist; and the alleged inconceivability disappears. - -Here I must end: not intending to say more, unless for some very urgent -reason; and leaving others to carry on the discussion. I have, indeed, been -led to suspend for a short time my proper work, only by consciousness of -the transcendent importance of the question at issue. As I have before -contended, a right answer to the question whether acquired characters are -or are not inherited, underlies right beliefs, not only in Biology and -Psychology, but also in Education, Ethics, and Politics. - - -III. - -As a species of literature, controversy is characterised by a terrible -fertility. Each proposition becomes the parent of half a dozen; so that a -few replies and rejoinders produce an unmanageable population of issues, -old and new, which end in being a nuisance to everybody. Remembering this, -I shall refrain from dealing with all the points of Professor Weismann's -answer. I must limit myself to a part; and that there may be no suspicion -of a selection convenient to myself, I will take those contained in his -first article. - -Before dealing with his special arguments, let me say something about the -general mode of argument which Professor Weismann adopts. - -The title of his article is "The All-Sufficiency of Natural -Selection."[117] Very soon, however, as on p. 322, we come to the -admission, which he has himself italicised, "that _it is really very -difficult to imagine this process of natural selection in its details_; and -to this day it is impossible to demonstrate it in any one point." -Elsewhere, as on pp. 327 and 336 _à propos_ of other cases, there are like -admissions. But now if the sufficiency of an assigned cause cannot in any -case be demonstrated, and if it is "really very difficult to imagine" in -what way it has produced its alleged effects, what becomes of the -"all-sufficiency" of the cause? How can its all-sufficiency be alleged when -its action can neither be demonstrated nor easily imagined? Evidently to -fit Professor Weismann's argument the title of the article should have been -"The Doubtful Sufficiency of Natural Selection." - -Observe, again, how entirely opposite are the ways in which he treats his -own interpretation and the antagonist interpretation. He takes the problem -presented by certain beautifully adapted structures on the anterior legs of -"very many insects," which they use for cleansing their antennæ. These, he -argues, cannot have resulted from the inheritance of acquired characters; -since any supposed changes produced by function would be changes in the -chitinous exo-skeleton, which, being a dead substance, cannot have had its -changes transmitted. He then proceeds, very candidly, to point out the -extreme difficulties which lie in the way of supposing these structures to -have resulted from natural selection: admitting that an opponent might "say -that it was absurd" to assume that the successive small variations implied -were severally life-saving in their effects. Nevertheless, he holds it -unquestionable that natural selection has been the cause. See then the -difference. The supposition that the apparatus has been produced by the -inheritance of acquired characters is rejected _because_ it presents -insuperable difficulties. But the supposition that the apparatus has been -produced by natural selection is accepted, _though_ it presents insuperable -difficulties. If this mode of reasoning is allowable, no fair comparison -between diverse hypotheses can be made. - -With these remarks on Professor Weismann's method at large, let me now pass -to the particular arguments he uses, taking them _seriatim_. - -* * * * * - -The first case he deals with is that of the progressive degradation of the -human little toe. This he considers a good test case; and he proceeds to -discuss an assigned cause--the inherited and accumulated effects of -boot-pressure. Without much difficulty he shows that this interpretation is -inadequate; since fusion of the phalanges, which constitutes in part the -progressive degradation, is found among peoples who go barefoot, and has -been found also in Egyptian mummies. Having thus disposed of Mr. Buckman's -interpretation, Professor Weismann forthwith concludes that the ascription -of this anatomical change to the inheritance of acquired characters is -disposed of, and assumes, as the only other possible interpretation, a -dwindling "through panmixia": "the hereditary degeneration of the little -toe is thus quite simply explained from my standpoint." - -It is surprising that Professor Weismann should not have seen that there is -an explanation against which his criticism does not tell. If we go back to -the genesis of the human type from some lower type of _primates_, we see -that while the little toe has ceased to be of any use for climbing -purposes, it has not come into any considerable use for walking and -running. A glance at the feet of the sub-human _primates_ in general, shows -that the inner digits are, as compared with those of men, quite small, have -no such relative length and massiveness as the human great toes. Leaving -out the question of cause, it is manifest that the great toes have been -immensely developed, since there took place the change from arboreal habits -to terrestrial habits. A study of the mechanics of walking shows why this -has happened. Stability requires that the "line of direction" (the vertical -line let fall from the centre of gravity) shall fall within the base, and, -in walking, shall be brought at each step within the area of support, or so -near it that any tendency to fall may be checked at the next step. A -necessary result is that if, at each step, the chief stress of support is -thrown on the outer side of the foot, the body must be swayed so that the -"line of direction" may fall within the outer side of the foot, or close to -it; and when the next step is taken it must be similarly swayed in an -opposite way, so that the outer side of the other foot may bear the weight. -That is to say, the body must oscillate from side to side, or waddle. The -movements of a duck when walking or running show what happens when the -points of support are wide apart. Clearly this kind of movement conflicts -with efficient locomotion. There is a waste of muscular energy in making -these lateral movements, and they are at variance with the forward -movement. We may infer, then, that the developing man profited by throwing -the stress as much as possible on the inner sides of the feet; and was -especially led to do this when going fast, which enabled him to abridge the -oscillations: as indeed we now see in a drunken man. Thus there was thrown -a continually increasing stress upon the inner digits as they progressively -developed from the effects of use; until now that the inner digits, so -large compared with the others, bear the greater part of the weight, and -being relatively near one another, render needless any marked swayings from -side to side. But what has meanwhile happened to the outer digits? -Evidently as fast as the great toes have come more and more into play and -developed, the little toes have gone more and more out of play and have -been dwindling for--how long shall we say?--perhaps a hundred thousand -years. - -So far, then, am I from feeling that Professor Weismann has here raised a -difficulty in the way of the doctrine I hold, that I feel indebted to him -for having drawn attention to a very strong evidence in its support. This -modification in the form of the foot, which has occurred since arboreal -habits have given place to terrestrial habits, shows the effects of use and -disuse simultaneously. The inner digits have increased by use while the -outer digits have decreased by disuse. - -* * * * * - -Saying that he will not "pause to refute other apparent proofs of the -transmission of acquired characters," Professor Weismann proceeds to deal -with the argument which, with various illustrations, I have several times -urged--the argument that the natural selection of fortuitously-arising -variations cannot account for the adjustment of co-operative parts. Very -clearly and very fairly he summarises this argument as used in _The -Principles of Biology_ in 1864. Admitting that in this case there are -"enormous difficulties" in the way of any other interpretation than the -inheritance of acquired characters, Professor Weismann before proceeding to -assault this "last bulwark of the Lamarckian principle," premises that the -inheritance of acquired characters cannot be a cause of change because -inactive as well as active parts degenerate when they cease to be of use: -instancing the "skin and skin-armature of crabs and insects." On this I may -remark in the first place that an argument derived from degeneracy of -passive structures scarcely meets the case of development of active -structures; and I may remark in the second place that I have never dreamt -of denying the efficiency of natural selection as a cause of degeneracy in -passive structures when the degeneracy is such as aids the prosperity of -the stirp. - -Making this parenthetical reply to his parenthetical criticism I pass to -his discussion of this particular argument which he undertakes to dispose -of. - -His _cheval de bataille_ is furnished him by the social insects--not a -fresh one, however, as might be supposed from the way in which he mounts -it. From time to time it has carried other riders, who have couched their -lances with fatal effects as they supposed. But I hope to show that no one -of them has unhorsed an antagonist, and that Professor Weismann fails to do -this just as completely as his predecessors. I am, indeed, not sorry that -he has afforded me the opportunity of criticising the general discussion -concerning the peculiarities of these interesting creatures, which it has -often seemed to me sets out with illegitimate assumptions. The supposition -always is that the specialities of structures and instincts in the unlike -classes of their communities, have arisen during the period in which the -communities have existed in something like their present forms. This cannot -be. It is doubtless true that association without differentiations of -classes may pre-exist for co-operative purposes, as among wolves, and as -among various insects which swarm under certain circumstances. Hence we may -suppose that there arise in some cases permanent swarms--that survival of -the fittest will establish these constant swarms where they are -advantageous. But admitting this, we have also to admit a gradual rise of -the associated state out of the solitary state. Wasps and bees present us -with gradations. If, then, we are to understand how the organized societies -have arisen, either out of the solitary state or out of undifferentiated -swarms, we must assume that the differences of structure and instinct among -the members of them arose little by little, as the social organization -arose little by little. Fortunately we are able to trace the greater part -of the process in the annually-formed communities of the common wasp; and -we shall recognize in it an all-important factor (ignored by Professor -Weismann) to which the phenomena, or at any rate the greater part of them, -are due. - -But before describing the wasp's annual history, let me set down certain -observations made when, as a boy, I was given to angling, and, in July or -August, sometimes used for bait "wasp-grubs," as they were called. After -having had two or three days the combs or "cakes" of these, full of unfed -larvæ in all stages of growth, I often saw some of them devouring the edges -of their cells to satisfy their appetites; and saw others, probably the -most advanced in growth, which were spinning the little covering caps to -their cells, in preparation for assuming the pupa state. It is to be -inferred that if, after a certain stage of growth has been reached, the -food-supply becomes inadequate or is stopped altogether, the larva -undergoes its transformation prematurely; and, as we shall presently see, -this premature transformation has several natural sequences. - -Let us return now to the wasp's family history. In the spring, a queen-wasp -or mother-wasp which has survived the winter, begins to make a small nest -containing four or more cells in which she lays eggs, and as fast as she -builds additional cells, she lays an egg in each. Presently, to these -activities, is added the feeding of the larvæ: one result being that the -multiplication of larvæ involves a restriction of the food that can be -given to each. If we suppose that the mother-wasp rears no more larvæ than -she can fully feed, there will result queens or mothers like herself, -relatively few in number. But if we suppose that, laying more numerous eggs -she produces more larvæ than she can fully feed, the result will be that -when these have reached a certain stage of growth, inadequate supply of -food will be followed by premature retirement and transformation into pupæ. -What will be the characters of the developed insects? The first effect of -arrested nutrition will be smaller size. This we find. A second effect will -be defective development of parts that are latest formed and least -important for the survival of the individual. Hence we may look for -arrested development of the reproductive organs--non-essential to -individual life. And this expectation is in accord with what we see in -animal development at large; for (passing over entirely sexless -individuals) we see that though the reproductive organs may be marked out -early in the course of development, they are not made fit for action until -after the structures for carrying on individual life are nearly complete. -The implication is, then, that an inadequately-fed and small larva will -become a sterile imago. Having noted this, let us pass to a remarkable -concomitant. In the course of development, organs are formed not alone in -the order of their original succession, but partly in the order of -importance and the share they have to take in adult activities--a change of -order called by Haeckel "heterochrony." Hence the fact that we often see -the maternal instinct precede the sexual instinct. Every little girl with -her doll shows us that the one may become alive while the other remains -dormant. In the case of wasps, then, premature arrest of development may -result in incompleteness of the sexual traits, along with completeness of -the maternal traits. What happens? Leave out the laying of eggs, and the -energies of the mother-wasp are spent wholly in building cells and feeding -larvæ, and the worker-wasp forthwith begins to spend its life in building -cells and feeding larvæ. Thus interpreting the facts, we have no occasion -to assume any constitutional difference between the eggs of worker-wasps -and the eggs of queens; and that, their eggs are not different we see, -first, in the fact that occasionally the worker-wasp is fertile and lays -drone-producing eggs, and we see secondly that (if in this respect they are -like the bees, of which, however, we have no proof) the larva of a -worker-wasp can be changed into the larva of a queen-wasp by special -feeding. But be this as it may, we have good evidence that the feeding -determines everything. Says Dr. Ormerod, in his _British Social Wasps_:-- - - "When the swarm is strong and food plentiful ... the well fed larvæ - develop into females, full, large, and overflowing with fat. There are - all gradations of size, from the large fat female to the smallest - worker.... The larger the wasp, the larger and better developed, as the - rule, are the female organs, in all their details. In the largest wasps, - which are to be the queens of another year, the ovaries differ to all - appearances in nothing but their size from those of the larger worker - wasps.... Small feeble swarms produce few or no perfect females; but in - large strong swarms they are found by the score." (pp. 248-9) - -To this evidence add the further evidence that queens and workers pass -through certain parallel stages in respect of their maternal activities. At -first the queen, besides laying eggs, builds cells and feeds larvæ, but -after a time ceases to build cells, and feeds larvæ only, and eventually -doing neither one nor the other, only lays eggs, and is supplied with food -by the workers. So it is in part with the workers. While the members of -each successive brood, when in full vigour, build cells and feed larvæ, -by-and-by they cease to build cells, and only feed larvæ: the maternal -activities and instincts undergo analogous changes. In this case, then, we -are not obliged to assume that only by a process of natural selection can -the differences of structure and instinct between queens and workers be -produced. The only way in which natural selection here comes into play is -in the better survival of the families of those queens which made as many -cells, and laid as many eggs, as resulted in the best number of half-fed -larvæ, producing workers; since by a rapid multiplication of workers the -family is advantaged, and the ultimate production of more queens surviving -into the next year insured. - -The differentiation of classes does not go far among the wasps, because the -cycle of processes is limited to a year, or rather to the few months of the -summer. It goes further among the hive-bees, which, by storing food, -survive from one year into the next. Unlike the queen-wasp, the queen-bee -neither builds cells nor gathers food, but is fed by the workers: egg -laying has become her sole business. On the other hand the workers, -occupied exclusively in building and nursing, have the reproductive organs -more dwarfed than they are in wasps. Still we see that the worker-bee -occasionally lays drone-producing eggs, and that, by giving extra nutriment -and the required extra space, a worker-larva can be developed into a -queen-larva. In respect to the leading traits, therefore, the same -interpretation holds. Doubtless there are subsidiary instincts which are -apparently not thus interpretable. But before it can be assumed that an -interpretation of another kind is necessary, it must be shown that these -instincts cannot be traced back to those pre-social types and semi-social -types which must have preceded the social types we now see. For -unquestionably existing bees must have brought with them from the -pre-social state an extensive endowment of instincts, and, acquiring other -instincts during the unorganized social state, must have brought these into -the present organized social state. It is clear, for instance, that the -cell-building instinct in all its elaboration was mainly developed in the -pre-social stage; for the transition from species building solitary cells -to those building combs is traceable. We are similarly enabled to account -for swarming as being an inheritance from remote ancestral types. For just -in the same way that, with under-feeding of larvæ, there result individuals -with imperfectly developed reproductive systems, so there will result -individuals with imperfect sexual instincts; and just as the imperfect -reproductive system partially operates upon occasion, so will the imperfect -sexual instinct. Whence it will result that on the event which causes a -queen to undertake a nuptial flight which is effectual, the workers may -take abortive nuptial flights: so causing a swarm. - -And here, before going further, let us note an instructive class of facts -related to the class of facts above set forth. Summing up, in a chapter on -"The Determination of Sex," an induction from many cases, Professor Geddes -and Mr. Thompson remark that "such conditions as deficient or abnormal -food," and others causing "preponderance of waste over repair ... tend to -result in production of males;" while "abundant and rich nutrition" and -other conditions which "favour constructive processes ... result in the -production of females."[118] Among such evidences of this as immediately -concern us, are these:--J. H. Fabre found that in the nests of _Osmia -tricornis_, eggs at the bottom, first laid, and accompanied by much food, -produced females, while those at the top, last laid, and accompanied by -one-half or one-third the quantity of food, produced males,[119] Huber's -observations on egg-laying by the honey-bee, show that in the normal course -of things, the queen lays eggs of workers for eleven months, and only then -lays eggs of drones: that is, when declining nutrition or exhaustion has -set in. Further, we have the above-named fact, shown by wasps and bees, -that when workers lay eggs these produce drones only.[120] Special -evidence, harmonizing with general evidence, thus proves that among the -social insects the sex is determined by degree of nutrition while the egg -is being formed. See then how congruous this evidence is with the -conclusion above drawn; for it is proved that after an egg, predetermined -as a female, has been laid, the character of the produced insect as a -perfect female or imperfect female is determined by the nutrition of the -larva. _That is, one set of differences in structures and instincts is -determined by nutrition before the egg is laid, and a further set of -differences in structures and instincts is determined by nutrition after -the egg is laid._ - -We come now to the extreme case--that of the ants. Is it not probable that -the process of differentiation has been similar? There are sundry reasons -for thinking so. With ants as with wasps and bees--the workers occasionally -lay eggs; and an ant-community can, like a bee-community, when need be, -produce queens out of worker-larvæ: presumably in the same manner by extra -feeding. But here we have to add special evidence of great significance. -For observe that the very facts concerning ants, which Professor Weismann -names as exemplifying the formation of the worker type by selection, serve, -as in the case of wasps, to exemplify its formation by arrested nutrition. -He says that in several species the egg-tubes in the ovaries show -progressive decrease in number; and this, like the different degrees of -arrest in the ovaries of the worker-wasps, indicates arrest of -larva-feeding at different stages. He gives cases showing that, in -different degrees, the eyes of workers are less developed in the number of -their facets than those of the perfect insects; and he also refers to the -wings of workers as not being developed: remarking, however, that the -rudiments of their wings show that the ancestral forms had wings. Are not -these traits also results of arrested nutrition? Generally among insects -the larvæ are either blind or have but rudimentary eyes; that is to say, -visual organs are among the latest organs to arise in the genesis of the -perfect organism. Hence early arrest of nutrition will stop formation of -these, while various more ancient structures have become tolerably -complete. Similarly with wings. Wings are late organs in insect phylogeny, -and therefore will be among those most likely to abort where development is -prematurely arrested. And both these traits will, for the same reason, -naturally go along with arrested development of the reproductive system. -Even more significant, however, is some evidence assigned by Mr. Darwin -respecting the caste-gradations among the driver ants of West Africa. He -says:-- - - "But the most important fact for us is, that, though the workers can be - grouped into castes of different sizes, yet they graduate insensibly into - each other, as does the widely-different structure of their jaws."[121] - -"Graduate insensibly," he says; implying that there are very numerous -intermediate forms. This is exactly what is to be expected if arrest of -nutrition be the cause; for unless the ants have definite measures, -enabling them to stop feeding at just the same stages, it must happen that -the stoppage of feeding will be indefinite; and that, therefore, there will -be all gradations between the extreme forms--"insensible gradations," both -in size and in jaw-structure. - -In contrast with this interpretation, consider now that of Professor -Weismann. From whichever of the two possible suppositions he sets out, the -result is equally fatal. If he is consistent, he must say that each of -these intermediate forms of workers must have its special set of -"determinants," causing its special set of modifications of organs; for he -cannot assume that while perfect females and the extreme types of workers -have their different sets of determinants, the intermediate types of -workers have not. Hence we are introduced to the strange conclusion that -besides the markedly-distinguished sets of determinants there must be, to -produce these intermediate forms, many other sets slightly distinguished -from one another--a score or more kinds of germ-plasm in addition to the -four chief kinds. Next comes an introduction to the still stranger -conclusion, that these numerous kinds of germ-plasm, producing these -numerous intermediate forms, are not simply needless but injurious--produce -forms not well fitted for either of the functions discharged by the extreme -forms: the implication being that natural selection has originated these -disadvantageous forms! If to escape from this necessity for suicide, -Professor Weismann accepts the inference that the differences among these -numerous intermediate forms are caused by arrested feeding of the larvæ at -different stages, then he is bound to admit that the differences between -the extreme forms, and between these and perfect females, are similarly -caused. But if he does this, what becomes of his hypothesis that the -several castes are constitutionally distinct, and result from the operation -of natural selection? Observe, too, that his theory does not even allow him -to make this choice; for we have clear proof that unlikenesses among the -forms of the same species cannot be determined this way or that way by -differences of nutrition. English greyhounds and Scotch greyhounds do not -differ from one another so much as do the Amazon-workers from the inferior -workers, or the workers from the queens. But no matter how a pregnant -Scotch greyhound is fed, or her pups after they are born, they cannot be -changed into English greyhounds: the different germ-plasms assert -themselves spite of all treatment. But in these social insects the -different structures of queens and workers _are_ determinable by -differences of feeling. Therefore the production of their various castes -does not result from the natural selection of varying germ-plasm. - -Before dealing with Professor Weismann's crucial case--that co-adaptation -of parts, which, in the soldier-ants, has, he thinks, arisen without -inheritance of acquired characters--let me deal with an ancillary case -which he puts forward as explicable by "panmixia alone." This is the -"degeneration, in the warlike Amazon-ants, of the instinct to search for -food."[122] Let us first ask what have been the probable antecedents of -these Amazon-ants; for, as I have above said, it is absurd to speculate -about the structures and instincts the species possesses in its existing -organized social state without asking what structures and instincts it -brought with it from its original solitary state and its unorganized social -state. From the outset these ants were predatory. Some variety of them led -to swarm--probably at the sexual season--did not again disperse so soon as -other varieties. Those which thus kept together derived advantages from -making simultaneous attacks on prey, and prospered accordingly. Of -descendants the varieties which carried on longest the associated state -prospered most; until, at length, the associated state became permanent. -All which social progress took place while there existed only perfect males -and females. What was the next step? Ants utilize other insects, and, among -other ways of doing this, sometimes make their nests where there are useful -insects ready to be utilized. Giving an account of certain New Zealand -species of _Tetramorium_, Mr. W. W. Smith says they seek out underground -places where there are "root-feeding aphides and coccids," which they begin -to treat as domestic animals; and further he says that when, after the -pairing season, new nests are being formed, there are "a few ants of both -sexes ... from two up to eight or ten."[123] Carrying with us this fact as -a key, let us ask what habits will be fallen into by the conquering species -of ants. They, too, will seek places where there are creatures to be -utilized; and, finding it profitable, will invade the habitations not of -defenceless creatures only, but of creatures whose powers of defence are -inadequate--weaker species of their own order. A very small modification -will affiliate their habits on habits of their prototypes. Instead of being -supplied with sweet substance excreted by the aphides they are supplied -with sweet substance by the ants among which they parasitically settle -themselves. How easily the subjugated ants may fall into the habit of -feeding them, we shall see on remembering that already they feed not only -larvæ but adults--individuals bigger than themselves. And that attentions -kindred to these paid to parasitic ants may be established without -difficulty, is shown us by the small birds which continue to feed a young -cuckoo in their nest when it has outgrown them. This advanced form of -parasitism grew up while there were yet only perfect males and females, as -happens in the initial stage with these New Zealand ants. What further -modifications of habits were probably then acquired? From the practice of -settling themselves where there already exist colonies of aphides, which -they carry about to suitable places in the nest, like _Tetramorium_, other -ants pass to the practice of making excursions to get aphides, and putting -them in better feeding places where they become more productive of -saccharine matter. By a parallel step these soldier-ants pass from the -stage of settling themselves among other ants which feed them, to the stage -of fetching the pupæ of such ants to the nest: a transition like that which -occurs among slave-making human beings. Thus by processes analogous to -those we see going on, these communities of slave-making ants may be -formed. And since the transition from an unorganized social state to a -social state characterized by castes, must have been gradual, there must -have been a long interval during which the perfect males and females of -these conquering ants could acquire habits and transmit them to progeny. A -small modification accounts for that seemingly-strange habit which -Professor Weismann signalizes. For if, as is observed, those ants which -keep aphides solicit them to excrete a supply of ant-food by stroking them -with the antennæ, they come very near to doing that which Professor -Weismann says the soldier-ants do towards a worker--"they come to it and -beg for food:" the food being put into their mouths in this last case as -almost or quite in the first. And evidently this habit of passively -receiving food, continued through many generations of perfect males and -females, may result in such disuse of the power of self-feeding that this -is eventually lost. The behaviour of young birds, during, and after, their -nest-life, gives us the clue. For a week or more after they are full-grown -and fly about with their parents, they may be seen begging for food and -making no efforts to recognize and pick up food for themselves. If, -generation after generation, feeding of them in full measure continued, -they would not learn to feed themselves: the perceptions and instincts -implied in self-feeding would be later and later developed, until, with -entire disuse of them, they would disappear altogether by inheritance. Thus -self-feeding may readily have ceased among these soldier-ants before the -caste-organization arose among them. - -With this interpretation compare the interpretation of Professor Weismann. -I have before protested against arguing in abstracts without descending to -concretes. Here let us ask what are the particular changes which the -alleged explanation by survival of the fittest involves. Suppose we make -the very liberal supposition that an ant's central ganglion bears to its -body the same ratio as the human brain bears to the human body--say, -one-fortieth of its weight. Assuming this, what shall we assume to be the -weight of those ganglion-cells and fibres in which are localized the -perceptions of food and the suggestion to take it? Shall we say that these -amount to one-tenth of the central ganglion? This is a high estimate -considering all the impressions which this ganglion has to receive, and all -the operations which it has to direct. Still we will say one-tenth. Then it -follows that this portion of nervous substance is one-400th of the weight -of its body. By what series of variations shall we say that it is reduced -from full power to entire incapacity? Shall we say five? This is a small -number to assume. Nevertheless we will assume it. What results? That the -economy of nerve-substance achieved by each of these five variations will -amount to one-2000th of the entire mass. Making these highly favourable -assumptions, what follows:--The queen-ant lays eggs that give origin to -individuals in each of which there is achieved an economy in -nerve-substance of one-2000th of its weight; and the implication of the -hypothesis is that such an economy will so advantage this ant-community -that in the competition with other ant-communities it will conquer. For -here let me recall the truth before insisted upon, that natural selection -can operate only on those variations which appreciably benefit the stirp. -Bearing in mind this requirement, is any one now prepared to say that -survival of the fittest can cause this decline of the self-feeding -faculty?[124] - -Not limiting himself to the Darwinian interpretation, however, Professor -Weismann says that this degradation may be accounted for by "panmixia -alone." Here I will not discuss the adequacy of this supposed cause, but -will leave it to be dealt with by implication a few pages in advance, where -the general hypothesis of panmixia will be reconsidered. - -And now, at length, we are prepared for dealing with Professor Weismann's -crucial case--with his alleged disproof that co-adaptation of co-operative -parts results from inheritance of acquired characters, because in the case -of the Amazon-ants, it has arisen where the inheritance of acquired -characters is impossible. For after what has been said, it will be manifest -that the whole question is begged when it is assumed that this -co-adaptation has arisen since there existed among these ants an organized -social state. Unquestionably this organized social state pre-supposes a -series of modifications through which it has been reached. It follows, -then, that there can be no rational interpretation without a preceding -inquiry concerning that earlier state in which there were no castes, but -only males and females. What kinds of individuals were the ancestral -ants--at first solitary, and then semi-social? They must have had marked -powers of offence and defence. Of predacious creatures, it is the more -powerful which form societies, not the weaker. Instance human races. -Nations originate from the relatively warlike tribes, not from the -relatively peaceful tribes. Among the several types of individuals forming -the existing ant community, to which, then, did the ancestral ants bear the -greatest resemblance? They could not have been like the queens, for these, -now devoted to egg-laying, are unfitted for conquest. They could not have -been like the inferior class of workers, for these, too, are inadequately -armed and lack strength. Hence they must have been most like these -Amazon-ants or soldier-ants, which now make predatory excursions--which now -do, in fact, what their remote ancestors did. What follows? Their -co-adapted parts have not been produced by the selection of variations -within the ant-community, such as we now see it. They have been inherited -from the pre-social and early social types of ants, in which the -co-adaptation of parts had been effected by inheritance of acquired -characters. It is not that the soldier-ants have gained these traits; it is -that the other castes have lost them. Early arrest of development causes -absence of them in the inferior workers; and from the queens they have -slowly disappeared by inheritance of the effects of disuse. For, in -conformity with ordinary facts of development, we may conclude that in a -larva which is being so fed as that the development of the reproductive -organs is becoming pronounced, there will simultaneously commence arrest in -the development of those organs which are not to be used. There are -abundant proofs that along with rapid growth of some organs others abort. -And if these inferences are true, then Professor Weismann's argument falls -to the ground. Nay, it falls to the ground even if conclusions so definite -as these be not insisted upon; for before he can get a basis for his -argument he must give good reasons for concluding that these traits of the -Amazon-ants have _not_ been inherited from remote ancestors. - -One more step remains. Let us grant him his basis, and let us pass from the -above negative criticism to a positive criticism. As before, I decline to -follow the practice of talking in abstracts instead of in concretes, and -contend that, difficult as it may be to see how natural selection has in -all cases operated, we ought, at any rate, to trace out its operation -whenever we can, and see where the hypothesis lands us. According to -Professor Weismann's admission, for production of the Amazon-ant by natural -selection, "_many parts must have varied simultaneously and in harmony with -one another_;"[125] and he names as such, larger jaws, muscles to move -them, larger head, and thicker chitin for it, bigger nerves for the -muscles, bigger motor centres in the brain, and, for the support of the big -head, strengthening of the thorax, limbs, and skeleton generally. As he -admits, all these parts must have varied simultaneously in due proportion -to one another. What must have been the proximate causes of their -variations? They must have been variations in what he calls the -"determinants." He says:-- - - "We have, however, to deal with the transmission of parts which are - _variable_ and this necessitates the assumption that just as many - independent and variable parts exist in the germ-plasm as are present in - the fully formed organism."[126] - -Consequently to produce simultaneously these many variations of parts, -adjusted in their sizes and shapes, there must have simultaneously arisen a -set of corresponding variations in the "determinants" composing the -germ-plasm. What made them simultaneously vary in the requisite ways? -Professor Weismann will not say that there was somewhere a foregone -intention. This would imply supernatural agency. He makes no attempt to -assign a physical cause for these simultaneous appropriate variations in -the determinants: an adequate physical cause being inconceivable. What, -then, remains as the only possible interpretation? Nothing but _a -fortuitous concourse of variations_; reminding us of the old "fortuitous -concourse of atoms." Nay, indeed, it is the very same thing. For each of -the "determinants," made up of "biophors," and these again of -protein-molecules, and these again of simpler chemical molecules, must have -had its molecular constitution changed in the required way; and the -molecular constitutions of all the "determinants," severally modified -differently, but in adjustment to one another, must have been thus modified -by "a fortuitous concourse of atoms." Now if this is an allowable -supposition in respect of the "determinants," and the varying organs -arising from them, why is it not an allowable supposition in respect of the -organism as a whole? Why not assume "a fortuitous concourse of atoms" in -its broad, simple form? Nay, indeed, would not this be much the easier? For -observe, this co-adaptation of numerous co-operative parts is not achieved -by one set of variations, but is achieved gradually by a series of such -sets. That is to say, the "fortuitous concourse of atoms" must have -occurred time after time in appropriate ways. We have not one miracle, but -a series of miracles! - -* * * * * - -Of the two remaining points in Professor Weismann's first article which -demand notice, one concerns his reply to my argument drawn from the -distribution of tactual discriminativeness. In what way does he treat this -argument? He meets it by an argument derived from hypothetical -evidence--not actual evidence. Taking the case of the tongue-tip, I have -carefully inquired whether its extreme power of tactual discrimination can -give any life-saving advantage in moving about the food during mastication, -in detecting foreign bodies in it, or for purposes of speech; and have, I -think, shown that the ability to distinguish between points one -twenty-fourth of an inch apart is useless for such purposes. Professor -Weismann thinks he disposes of this by observing that among the apes the -tongue is used as an organ of touch. But surely a counter-argument -equivalent in weight to mine should have given a case in which power to -discriminate between points one twenty-fourth of an inch apart instead of -one-twentieth of an inch apart (a variation of one-sixth) had a life-saving -efficacy; or, at any rate, should have suggested such a case. Nothing of -the kind is done or even attempted. But now note that his reply, accepted -even as it stands, is suicidal. For what has the trusted process of -panmixia been doing ever since the human being began to evolve from the -ape? Why during thousands of generations has not the nervous structure -giving this extreme discriminativeness dwindled away? Even supposing it had -been proved of life-saving efficacy to our simian ancestors, it ought, -according to Professor Weismann's own hypothesis, to have disappeared in -us. Either there was none of the assumed special capacity in the ape's -tongue, in which case his reply fails, or panmixia has not operated, in -which case his theory of degeneracy fails. - -All this, however, is but preface to the chief answer. The argument drawn -from the case of the tongue-tip, with which alone Professor Weismann deals, -is but a small part of my argument, the remainder of which he does not -attempt to touch--does not even mention. Had I never referred to the -tongue-tip at all, the various contrasts in discriminativeness which I have -named, between the one extreme of the forefinger-tip and the other extreme -of the middle of the back, would have abundantly sufficed to establish my -case--would have sufficed to show the inadequacy of natural selection as a -key and the adequacy of the inheritance of acquired characters. - -It seems to me, then, that judgment must go against him by default. -Practically he leaves the matter standing just where it did.[127] - -The other remaining point concerns the vexed question of panmixia. -Confirming the statement of Dr. Romanes, Professor Weismann says that I -have misunderstood him. Already (_Contemporary Review_, May, 1893, p. 758, -and Reprint, p. 66) I have quoted passages which appeared to justify my -interpretation, arrived at after much seeking.[128] Already, too, in this -review (July, 1893, p. 54) I have said why I did not hit upon the -interpretation now said to be the true one: I never supposed that any one -would assume, without assigned cause, that (apart from the excluded -influence of disuse) the _minus_ variations of a disused organ are greater -than the _plus_ variations. This was a tacit challenge to produce reasons -for the assumption. Professor Weismann does not accept the challenge, but -simply says:--"In my opinion all organs are maintained at the height of -their development only through uninterrupted selection" (p. 332): in the -absence of which they decline. Now it is doubtless true that as a -naturalist he may claim for his "opinion" a relatively great weight. Still, -in pursuance of the methods of science, it seems to me that something more -than an opinion is required as the basis of a far-reaching theory.[129] - -Though the counter-opinion of one who is not a naturalist (as Professor -Weismann points out) may be of relatively small value, yet I must here -again give it, along with a final reason for it. And this reason shall be -exhibited, not in a qualitative form, but in a quantitative form. Let us -quantify the terms of the hypothesis by weights; and let us take as our -test case the rudimentary hind-limbs of the whale. Zoologists are agreed -that the whale has been evolved from a mammal which took to aquatic habits, -and that its disused hind-limbs have gradually disappeared. When they -ceased to be used in swimming, natural selection played a part--probably an -important part--in decreasing them; since, being then impediments to -movement through the water, they diminished the attainable speed. It may -be, too, that for a period after disappearance of the limbs beneath the -skin, survival of the fittest had still some effect. But during the latter -stages of the process it had no effect; since the rudiments caused no -inconvenience and entailed no appreciable cost. Here, therefore, the cause, -if Professor Weismann is right, must have been panmixia. Dr. Struthers, -Professor of Anatomy at Aberdeen, whose various publications show him to be -a high, if not the highest, authority on the anatomy of these great -cetaceans, has kindly taken much trouble in furnishing me with the needful -data, based upon direct weighing and measuring and estimation of specific -gravity. In the Black Whale (_Balænoptera borealis_) there are no rudiments -of hind-limbs whatever: rudiments of the pelvic bones only remain. A sample -of the Greenland Right Whale, estimated to weigh 44,800 lbs., had femurs -weighing together 3½ ozs.; while a sample of the Razor-back Whale -(_Balænoptera musculus_), 50 feet long, and estimated to weigh 56,000 lbs., -had rudimentary femurs weighing together one ounce; so that these vanishing -remnants of hind-limbs weighed but one-896,000th part of the animal. Now in -considering the alleged degeneration by panmixia, we have first to ask why -these femurs must be supposed to have varied in the direction of decrease -rather than in the direction of increase. During its evolution from the -original land-mammal, the whale has grown enormously, implying habitual -excess of nutrition. Alike in the embryo and in the growing animal, there -must have been a chronic plethora. Why, then, should we suppose these -rudiments to have become smaller? Why should they not have enlarged by -deposit in them of superfluous materials? But let us grant the unwarranted -assumption of predominant _minus_ variations. Let us say that the last -variation was a reduction of one-half--that in some individuals the joint -weight of the femurs was suddenly reduced from two ounces to one ounce--a -reduction of one-900,000th of the creature's weight. By inter-crossing with -those inheriting the variation, the reduction, or a part of the reduction, -was made a trait of the species. Now, in the first place, a necessary -implication is that this _minus_ variation was maintained in posterity. So -far from having reason to suppose this, we have reason to suppose the -contrary. As before quoted, Mr. Darwin says that "unless carefully -preserved by man," "any particular variation would generally be lost by -crossing, reversion, and the accidental destruction of the varying -individuals."[130] And Mr. Galton, in his essay on "Regression towards -Mediocrity,"[131] contends that not only do deviations of the whole -organism from the mean size tend to thus disappear, but that deviations in -its components do so. Hence the chances are against such _minus_ variation -being so preserved as to affect the species by panmixia. In the second -place, supposing it to be preserved, may we reasonably assume that, by -inter-crossing, this decrease, amounting to about a millionth part of the -creature's weight, will gradually affect the constitutions of all -Razor-back Whales distributed over the Arctic seas and the North Atlantic -Ocean, from Greenland to the Equator? Is this a credible conclusion? For -three reasons, then, the hypothesis must be rejected. - -Thus, the only reasonable interpretation is the inheritance of acquired -characters. If the effects of use and disuse, which are known causes of -change in each individual, influence succeeding individuals--if -functionally-produced modifications of structure are transmissible, as well -as modifications of structure otherwise arising--then this reduction of the -whale's hind limbs to minute rudiments is accounted for. The cause has been -unceasingly operative on all individuals of the species ever since the -transformation began. - -In one case see all. If this cause has thus operated on the limbs of the -whale, it has thus operated in all creatures on all parts having active -functions. - -* * * * * - -At the outset I intimated that I must limit my replies to those arguments -of Professor Weismann which are contained in his first article. That those -contained in his second might be dealt with no less effectually, did time -and space permit, is manifest to me; but about the probability of this the -reader must form his own judgment. My replies thus far may be summed up as -follows:-- - -Professor Weismann says he has disproved the conclusion that degeneration -of the little toe has resulted from inheritance of acquired characters. But -his reasoning fails against an interpretation he overlooks. A profound -modification of the hind limbs and their appendages must have taken place -during the transition from arboreal habits to terrestrial habits; and -dwindling of the little toe is an obvious consequence of disuse, at the -same time that enlargement of the great toe is an obvious consequence of -increased use. - -The entire argument based on the unlike forms and instincts presented by -castes of social insects is invalidated by an omission. Until probable -conclusions are reached respecting the characters which such insects -brought with them into the organized social state, no valid inferences can -be drawn respecting characters developed during that state. - -A further large error of interpretation is involved in the assumption that -the different caste-characters are transmitted to them in the eggs laid by -the mother insect. While we have evidence that the unlike structures of the -sexes are determined by nutrition of the germ before egg-laying, we have -evidence that the unlike structures of classes are caused by unlikenesses -of nutrition of the larvæ. That these varieties of forms do not result from -varieties of germ-plasms, is demonstrated by the fact that where there are -varieties of germ-plasms, as in varieties of the same species of mammal, no -deviations in feeding prevent display of their structural results. - -For such caste-modifications as those of the Amazon-ants, which are unable -to feed themselves, there is a feasible explanation other than Professor -Weismann's. The relation of common ants to their domestic animals--aphides -and coccids--which yield them food on solicitation, does not differ widely -from this relation between these Amazon-ants and their domestic -animals--the slave-ants. And the habit of being fed, contracted during the -first stages of their parasitic life, when there were perfect males and -females, may, during that stage, have become established by inheritance. -Meanwhile the opposed interpretation--that this incapacity has resulted -from the selection of those ant-communities the queens of which laid eggs -that had so varied as to entail this incapacity--implies that a scarcely -appreciable economy of nerve-matter advantaged the stirp so greatly as to -cause it to spread more than other stirps: an incredible supposition. - -As the outcome of these alternative interpretations we saw that the -argument respecting the co-adaptation of co-operative parts, which -Professor Weismann thinks is furnished to him by the Amazon-ants, -disappears. The ancestral ants were conquering ants. These founded the -communities; and hence those members of the present communities which are -most like them are the Amazon-ants. If so, the co-adaptation of the -co-operative parts was effected by inheritance during the solitary and -semi-social stages. Even were there no such solution, the opposed solution -will be unacceptable. These simultaneous appropriate variations of the -co-operative parts in sizes, shapes, and proportions, are supposed to be -effected by simultaneous variations in the "determinants" of the -germ-plasms; and in the absence of an assigned physical cause, this implies -a fortuitous concourse of appropriate variations, which carries us back to -a "fortuitous concourse of atoms." This may just as well be extended to the -entire organism. The old hypothesis of special creations is more consistent -and comprehensible. - -To rebut my inference drawn from the distribution of discriminativeness, -Professor Weismann uses not an argument but the blank form of an argument. -The ability to discriminate one twenty-fourth of an inch by the tongue-tip -_may_ have been useful to the ape: no conceivable use being even suggested. -And then the great body of my argument derived from the distribution of -discriminativeness over the skin, which amply suffices, is wholly ignored. - -The tacit challenge I gave to name some facts in support of the hypothesis -of panmixia--or even a solitary fact--is passed by. It remains a pure -speculation having no basis but Professor Weismann's "opinion." When from -the abstract statement of it we pass to a concrete test, in the case of the -whale, we find that it necessitates an unproved and improbable assumption -respecting _plus_ and _minus_ variations; that it ignores the unceasing -tendency to reversion; and that it implies an effect out of all proportion -to the cause. - -It is curious what entirely opposite conclusions men may draw from the same -evidence. Professor Weismann thinks he has shown that the "last bulwark of -the Lamarckian principle is untenable." Most readers will hold with me that -he is, to use the mildest word, premature in so thinking. Contrariwise my -impression is that he has not shown either this bulwark or any other -bulwark to be untenable; but rather that while his assault has failed it -has furnished opportunity for strengthening sundry of the bulwarks. - - -IV. - -Among those who follow a controversy to its close, not one in a hundred -turns back to its beginning to see whether its chief theses have been dealt -with. Very often the leading arguments of one disputant, seen by the other -to be unanswerable, are quietly ignored, and attention is concentrated on -subordinate arguments to which replies, actually or seemingly valid, can be -made. The original issue is thus commonly lost sight of. - -More than once I have pointed out that, as influencing men's views about -Education, Ethics, Sociology, and Politics, the question whether acquired -characters are inherited is the most important question before the -scientific world. Hence I cannot allow the discussion with Professor -Weismann to end in so futile a way as it will do if no summary of results -is made. Here, therefore, I propose to recapitulate the whole case in -brief. Primarily my purpose is to recall certain leading propositions -which, having been passed by unnoticed, remain outstanding. I will turn, in -the second place, to such propositions as have been dealt with; hoping to -show that the replies given are invalid, and consequently that these -propositions also remain outstanding. - -But something beyond a summing-up is intended. A few pages at the close -will be devoted to setting forth new evidence which has come to light since -the controversy commenced--evidence which many will think sufficient in -itself to warrant a positive conclusion. - -* * * * * - -The fact that the tip of the fore finger has thirty times the power of -discrimination possessed by the middle of the back, and that various -intermediate degrees of discriminative power are possessed by various parts -of the skin, was set down as a datum for my first argument. The causes -which might be assigned for these remarkable contrasts were carefully -examined under all their aspects. I showed in detail that the contrasts -could not in any way be accounted for by natural selection. I further -showed that no interpretation of them is afforded by the alleged process of -panmixia: this has no _locus standi_ in the case. Having proved -experimentally, that ability of the fingers to discriminate is increased by -practice, and having pointed out that gradations of discriminativeness in -different parts correspond with gradations in the activities of the parts -as used for tactual exploration, I argued that these contrasts have arisen -from the organized and inherited effects of tactual converse with -surrounding things, varying in its degrees according to the positions of -the parts--in other words, that they are due to the inheritance of acquired -characters. As a crowning proof I instanced the case of the tongue-tip, -which has twice the discriminativeness of the forefinger-tip: pointing out -that consciously, or semi-consciously, or unconsciously, the tongue-tip is -perpetually exploring the inner surfaces of the teeth. - -Singling out this last case, Professor Weismann made, or rather adopted -from Dr. Romanes, what professed to be a reply but was nothing more than -the blank form of a reply. It was said that though this extreme -discriminativeness of the tongue-tip is of no use to mankind, it may have -been of use to certain ancestral _primates_. No evidence of any such use -was given; no imaginable use was assigned. It was simply suggested that -there perhaps was a use. - -In my rejoinder, after indicating the illusory nature of this proceeding -(which is much like offering a cheque on a bank where no assets have been -deposited to meet it), I pointed out that had the evidence furnished by the -tongue tip never been mentioned, the evidence otherwise furnished amply -sufficed. I then drew attention to the fact that this evidence had been -passed over, and tacitly inquired why. - -No reply.[132] - -* * * * * - -In his essay on "The All-Sufficiency of Natural Selection," Professor -Weismann set out, not by answering one of the arguments I had used, but by -importing into the discussion an argument used by another writer, which it -was easy to meet. It had been contended that the smallness and deformity of -the little toe are consequent upon the effects of boot-pressure, inherited -from generation to generation. To this Professor Weismann made the -sufficient reply that the fusion of the phalanges and otherwise degraded -structure of the little toe, exist among peoples who go barefoot. - -In my "Rejoinder" I said that though the inheritance of acquired characters -does not explain this degradation in the way alleged, it explains it in a -way which Professor Weismann overlooks. The cause is one which has been -operating ever since the earliest anthropoid creatures began to decrease -their life in trees and increase their life on the earth's surface. The -mechanics of walking and running, in so far as they concern the question at -issue, were analyzed; and it was shown that effort is economized and -efficiency increased in proportion as the stress is thrown more and more on -the inner digits of the foot and less and less on the outer digits. So that -thus the foot furnishes us simultaneously with an instance of increase from -use and of decrease from disuse; a further disproof being yielded of the -allegation that co-operative parts vary together, since we have here -co-operative parts of which one grows while the other dwindles. - -I ended by pointing out that, so far from strengthening his own case, -Professor Weismann had, by bringing into the controversy this changed -structure of the foot, given occasion for strengthening the opposite case. - -No reply. - -* * * * * - -We come now to Professor Weismann's endeavour to disprove my second -thesis--that it is impossible to explain by natural selection alone the -co-adaptation of co-operative parts. It is thirty years since this was set -forth in _The Principles of Biology_. In § 166 I instanced the enormous -horns of the extinct Irish elk, and contended that in this, and in kindred -cases, where for the efficient use of some one enlarged part many other -parts have to be simultaneously enlarged, it is out of the question to -suppose that they can have all spontaneously varied in the required -proportions. In "The Factors of Organic Evolution," by way of enforcing -this argument, which had, so far as I know, never been met, I dwelt upon -the aberrant structure of the giraffe. And then, in the essay which -initiated this controversy, I brought forward yet a third case--that of an -animal which, previously accustomed only to walking, acquires the power of -leaping. - -In the first of his articles in the _Contemporary Review_ (September, -1893), Professor Weismann made no direct reply, but he made an indirect -reply. He did not attempt to show how there could have taken place in the -stag the "harmonious variation of the different parts that co-operate to -produce one physiological result" (p. 311); but he contended that such -harmonious variation _must_ have taken place, because the like has taken -place in "the neuters of state-forming insects"--"animal forms which do not -reproduce themselves, but are always propagated anew by parents which are -unlike them" (p. 313), and which therefore cannot have transmitted acquired -characters. Singling out those soldier-neuters which exist among certain -kinds of ants, he described (p. 318) the many co-ordinated parts required -to make their fighting organs efficient. He then argued that the required -simultaneous changes can "only have arisen by a selection of the -parent-ants dependent on the fact that those parents which produced the -best workers had always the best prospect of the persistence of their -colony. No other explanation is conceivable; _and it is just because no -other explanation is conceivable, that it is necessary for us to accept the -principle of natural selection_" (pp. 318-9). - -[This passage initiated a collateral controversy, which, as continually -happens, has greatly obscured the primary controversy. It became a question -whether these forms of neuter insects have arisen as Professor Weismann -assumes, or whether they have arisen from arrested development consequent -upon innutrition. To avoid entanglements I must for the present pass over -this collateral controversy, intending to resume it presently, when the -original issues have been dealt with.] - -No one will suspect me of thinking that the inconceivability of the -negation is not a valid criterion, since, in "The Universal Postulate," -published in the _Westminster Review_ in 1852 and afterwards in _The -Principles of Psychology_, I contended that it is the ultimate test of -truth. But then in every case there has to be determined the question--Is -the negation inconceivable; and in assuming that it is so in the case -named, lies the fallacy of the above-quoted passage. The three separate -ways in which I dealt with this position of Professor Weismann are as -follows:-- - -If we admit the assumption that the form of the soldier-ant has been -developed since the establishment of the organized ant-community in which -it exists, Professor Weismann's assertion that no other process than that -which he alleges is conceivable, is true. But I pointed out that this -assumption is inadmissible; and that no valid conclusion respecting the -genesis of the soldier-ant can be drawn without postulating either the -ascertained, or the probable, structure of those pre-social, or -semi-social, ants from which the organized social ants have descended. I -went on to contend that the pre-social type must have been a conquering -type, and that therefore in all probability the soldier-ants represent most -nearly the structures of those ancestral ants which existed when the -society had perfect males and females and could transmit acquired -characters, while the other members of the existing communities are -degraded forms of the type. - -No reply. - -A further argument I used was that where there exist different castes among -the neuter-ants, as those seen in the soldiers and workers of the Driver -ants of West Africa, "they graduate insensibly into each other" alike in -their sizes and in their structures; and that Professor Weismann's -hypothesis implies a special set of "determinants" for each intermediate -form. Or if he should say that the intermediate forms result from mixtures -of the determinants of the two extreme forms, there still remains the -further difficulty that natural selection has maintained, for innumerable -generations, these intermediate forms which are injurious deviations from -the useful extreme forms. - -No reply. - -One further reason--fatal it seems to me--was urged in bar of his -interpretation. No physical cause has been, or can be, assigned, why in the -germ-plasm of any particular queen-ant, the "determinants" initiating these -various co-operative organs, all simultaneously vary in fitting ways and -degrees, and still less why there occur such co-ordinated variations -generation after generation, until by their accumulated results these -efficient co-operative structures have been evolved. I pointed out that in -the absence of any assigned or assignable physical cause, it is necessary -to assume a fortuitous concurrence of favourable variations, which means "a -fortuitous concourse of atoms;" and that it would be just as rational, and -much more consistent, to assume that the structure of the entire organism -thus resulted. - -No reply. - -* * * * * - -It is reasonable to suspect that Professor Weismann recognized these -difficulties as insuperable, for, in his Romanes Lecture on "The Effect of -External Influences upon Development," instead of his previous indirect -reply, he makes a direct reply. Reverting to the stag and its enlarging -horns, he alleges a process by which, as he thinks, we may understand how, -by variation and selection, all the bones and muscles of the neck, of the -thorax, and of the fore-legs, are step by step adjusted in their sizes to -the increasing sizes of the horns. He ascribes this harmonization to the -internal struggle for nutriment, and that survival of the fittest which -takes place among the parts of an organism: a process which he calls -"_intra-individual_-selection, or more briefly--_intra-selection_" (p. 12). - - "Wilhelm Roux has given an explanation of the cause of these wonderfully - fine adaptations by applying the principle of selection to the parts of - the organism. Just as there is a struggle for survival among the - individuals of a species, and the fittest are victorious, so also do even - the smallest living particles contend with one another, and those that - succeed best in securing food and place grow and multiply rapidly, and so - displace those that are less suitably equipped" (p. 12).[133] - -That I do not explain as he does the co-adaptation of co-operative parts, -Professor Weismann ascribes to my having overlooked this "principle of -intra-selection"--an unlucky supposition, as we see. But I do not think -that when recognizing it a generation ago, I should have seen its relevancy -to the question at issue, had that issue then been raised, and I certainly -do not see it now. Full reproduction of Professor Weismann's explanation is -impracticable, for it occupies several pages, but here are the essential -sentences from it:-- - - "The great significance of intra-selection appears to me not to depend on - its producing structures that are directly transmissible,--it cannot do - that,--but rather consists in its causing a development of the - germ-structure, acquired by the selection of individuals, which will be - suitable to varying conditions.... We may therefore say that - intra-selection effects the adaptation of the individual to its chance - developmental conditions,--the suiting of the hereditary primary - constituents to fresh circumstances" (p. 16).... "But as the primary - variations in the phyletic metamorphosis occurred little by little, the - secondary adaptations would probably as a rule be able to keep pace with - them. Time would thus be gained till, in the course of generations, by - constant selection of those germs the primary constituents of which are - best suited to one another, the greatest possible degree of harmony may - be reached, and consequently a definitive metamorphosis of the species - involving all the parts of the individual may occur" (p. 19). - -The connecting sentences, along with those which precede and succeed, would -not, if quoted, give to the reader clearer conceptions than these by -themselves give. But when disentangled from Professor Weismann's involved -statements, the essential issues are, I think, clear enough. In the case of -the stag, that daily working together of the numerous nerves, muscles, and -bones concerned, by which they are adjusted to the carrying and using of -somewhat heavier horns, produces on them effects which, as I hold, are -inheritable, but which, as Professor Weismann holds, are not inheritable. -If they are not inheritable, what must happen? A fawn of the next -generation is born with no such adjustment of nerves, muscles and bones as -had been produced by greater exercise in the parent, and with no tendency -to such adjustment. Consequently if, in successive generations, the horns -go on enlarging, all these nerves, muscles, and bones, remaining of the -original sizes, become utterly inadequate. The result is loss of life: the -process of adaptation fails. "No," says Professor Weismann, "we must -conclude that the germ-plasm has varied in the needful manner." How so? The -process of "intra-individual selection," as he calls it, can have had no -effect, since the cells of the soma cannot influence the reproductive -cells. In what way, then, has the germ-plasm gained the characters required -for producing simultaneously all these modified co-operative parts. Well, -Professor Weismann tells us merely that we must suppose that the germ-plasm -acquires a certain sensitiveness such as gives it a proclivity to -development in the requisite ways. How is such proclivity obtainable? Only -by having a multitude of its "determinants" simultaneously changed in fit -modes. Emphasizing the fact that even a small failure in any one of the -co-operative parts may be fatal, as the sprain of an over-taxed muscle -shows us, I alleged that the chances are infinity to one against the -needful variations taking place at the same time. Divested of its -elaboration, its abstract words and technical phrases, the outcome of -Professor Weismann's explanation is that he accepts this, and asserts that -the infinitely improbable thing takes place! - -Either his argument is a disguised admission of the inheritableness of -acquired characters (the effects of "intra-selection") or else it is, as -before, the assumption of a fortuitous concourse of favourable variations -in the determinants--"a fortuitous concourse of atoms." - -* * * * * - -Leaving here this main issue, I return now to that collateral issue named -on a preceding page as being postponed--whether the neuters among social -insects result from specially modified germ-plasms or whether they result -from the treatment received during their larval stages. - -For the substantiation of his doctrine Professor Weismann is obliged to -adopt the first of these alternatives; and in his Romanes Lecture he found -it needful to deal with the evidence I brought in support of the second -alternative. He says that "poor feeding is not the _causa efficiens_ of -sterility among bees, but is merely the stimulus which _not only results in -the formation of rudimentary ovaries, but at the same time calls forth all -the other distinctive characters of the workers_" (pp. 29-30); and he says -this although he has in preceding lines admitted that it is "true of all -animals that they reproduce only feebly or not at all when badly and -insufficiently nourished:" a known cause being thus displaced by a supposed -cause. But Professor Weismann proceeds to justify his interpretation by -experimentally-obtained evidence. - -He "reared large numbers of the eggs of a female blow-fly"; the larvæ of -some he fed abundantly, but the larvæ of others sparingly; and eventually -he obtained, from the one set flies of full size, and from the other small -flies. Nevertheless the small flies were fertile, as well as the others. -Here, then, was proof that innutrition had not produced infertility; and he -contends that therefore among the neuter social insects, infertility has -not resulted from innutrition. The argument seems strong, and to many will -appear conclusive; but there are two differences which entirely vitiate the -comparison Professor Weismann institutes. - -One of them has been pointed out by Mr. Cunningham. In the case of the -blow-fly the food supplied to the larvæ though different in quantity was -the same in quality; in the case of the social insects the food supplied, -whether or not different in quantity, differs in quality. Among bees, -wasps, ants, &c., the larvæ of the reproductive forms are fed upon a more -nitrogenous food than are the larvæ of the workers; whereas the two sets of -larvæ of the blow-fly, as fed by Professor Weismann, were alike supplied -with highly nitrogenous food. Hence there did not exist the same cause for -non-development of the reproductive organs. Here, then, is one vitiation of -the supposed parallel. There is a second. - -While the development of an embryo follows in a rude way the phyletic -metamorphoses passed through by its ancestry, the order of development of -organs is often gradually modified by the needs of particular species: the -structures being developed in such order as conduces to self-sustentation -and the welfare of offspring. Among other results there arise differences -in the relative dates of maturity of the reproductive system and of the -other systems. It is clear, _à priori_, that it must be fatal to a species -if offspring are habitually produced before the conditions requisite for -their survival are fulfilled. And hence, if the life is a complex one, and -the care taken of offspring is great, reproduction must be much longer -delayed than where the life is simple and the care of offspring absent or -easy. The contrast between men and oxen sufficiently illustrates this -truth. Now the subordination of the order of development of parts to the -needs of the species, is conspicuously shown in the contrast between these -two kinds of insects which Professor Weismann compares as though their -requirements were similar. What happens with the blow fly? If it is able to -suck up some nutriment, to fly tolerably, and to scent out dead flesh, -various of its minor organs may be more or less imperfect without -appreciable detriment to the species: the eggs can be laid in a fit place, -and that is all that is wanted. Hence it profits the species to have the -reproductive system developed comparatively early--in advance, even, of -various less essential parts. Quite otherwise is it with social insects, -which take such remarkable care of their young; or rather to make the case -parallel--quite otherwise is it with those types from which the social -insects have descended, bringing into the social state their inherited -instincts and constitutions. Consider the doings of the mason-wasp, or -mason-bee, or those of the carpenter-bee. What, in these cases, must the -female do that she may rear members of the next generation? There is a fit -place for building or burrowing to be chosen; there is the collecting -together of grains of sand and cementing them into a strong and water-proof -cell, or there is the burrowing into wood and there building several cells; -there is the collecting of food to place along with the eggs deposited in -these cells, solitary or associated, including that intelligent choice of -small caterpillars which, discovered and carried home, are carefully packed -away and hypnotized by a sting, so that they may live until the growing -larva has need of them. For all these proceedings there have to be provided -the fit external organs--cutting instruments, &c., and the fit internal -organs--complicated nerve-centres in which are located these various -remarkable instincts, and ganglia by which these delicate operations have -to be guided. And these special structures have, some if not all of them, -to be made perfect and brought into efficient action before egg-laying -takes place. Ask what would happen if the reproductive system were active -in advance of these ancillary appliances. The eggs would have to be laid -without protection or food, and the species would forthwith disappear. And -if that full development of the reproductive organs which is marked by -their activity, is not needful until these ancillary organs have come into -play, the implication, in conformity with the general law above indicated, -is that the perfect development of the reproductive organs will take place -later than that of these ancillary organs, and that if innutrition checks -the general development, the reproductive organs will be those which -chiefly suffer. Hence, in the social types which have descended from these -solitary types, this order of evolution of parts will be inherited, and -will entail the results I have inferred. - -If only deductively reached, this conclusion would, I think, be fully -justified. But now observe that it is more than deductively reached. It is -established by observation. Professor Riley, Ph.D., late Government -Entomologist of the United States, in his annual address as President of -the Biological Society of Washington,[134] on January 29, 1894, said:-- - - "Among the more curious facts connected with these Termites, because of - their exceptional nature, is the late development of the internal sexual - organs in the reproductive forms." (p. 34.) - -Though what has been shown of the Termites has not been shown of the other -social insects, which belong to a different order, yet, considering the -analogies between their social states and between their constitutional -requirements, it is a fair inference that what holds in the one case holds -partially, if not fully, in the other. Should it be said that the larval -forms do not pass into the pupa state in the one case as they do in the -other, the answer is that this does not affect the principle. The larva -carries into the pupa state a fixed quantity of tissue-forming material for -the production of the imago. If the material is sufficient, then a complete -imago is formed. If it is not sufficient, then, while the earlier formed -organs are not affected by the deficiency, the deficiency is felt when the -latest formed organs come to be developed, and they are consequently -imperfect. - -Even if left without reply, Professor Weismann's interpretation commits him -to some insuperable difficulties, which I must now point out. -Unquestionably he has "the courage of his opinions;" and it is shown -throughout this collateral discussion as elsewhere. He is compelled by -accumulated evidence to admit "that there is only _one_ kind of egg from -which queens and workers as well as males arise."[135] But if the -production of one or other form from the same germ does not result from -speciality of feeding, what does it result from? Here is his reply:-- - - "We must rather suppose that the primary constituents of two distinct - reproductive systems--_e. g._ those of the queen and worker--are - contained in the germ-plasm of the egg."[136] - -"The courage of his opinions," which Professor Weismann shows in this -assumption, is, however, quite insufficient. For since he himself has just -admitted that there is only one kind of egg for queens, workers, and males, -he must at any rate assume three sets of "determinants." (I find that on a -subsequent page he does so.) But this is not enough, for there are, in many -cases, two if not more kinds of workers, which implies that four sets of -determinants must co-exist in the same egg. Even now we have not got to the -extent of the assumption required. In the address above referred to on -"Social Insects from Psychical and Evolutional Points of View," Professor -Riley gives us (p. 33) the-- - - -_Forms in a Termes Colony under Normal Conditions._ - - 1. Youngest larvæ. - / \ - / \ - / \ - 2. Larvæ [of those] unfit 3. Larvæ [that will be] fit - for reproduction. for reproduction. - / \ / \ - / \ / \ - 4. Larvæ of 5. Larvæ of 8. Nymphs of 9. Nymphs of 2nd - workers. soldiers. 1st form. form. - | | | - 6. Workers. 7. Soldiers. 10. Winged forms. - | - 11. True royal pairs. - -Hence as, in this family tree, the royal pair includes male and female, it -results that there are _five_ different adult forms (Grassi says there are -two others) arising from like eggs or larvæ; and Professor Weismann's -hypothesis becomes proportionately complicated. Let us observe what the -complications are. - -It often happens in controversy--metaphysical controversy more than any -other--that propositions are accepted without their terms having been -mentally represented. In public proceedings documents are often "taken as -read," sometimes with mischievous results; and in discussions propositions -are often taken as thought when they have not been thought and cannot be -thought. It sufficiently taxes imagination to assume, as Professor Weismann -does, that two sets of "ids" or of "determinants" in the same egg are, -throughout all the cell-divisions which end in the formation of the -_morula_, kept separate, so that they may subsequently energize -independently; or that if they are not thus kept separate, they have the -power of segregating in the required ways. But what are we to say when -three, four, and even five sets of "ids" or bundles of "determinants" are -present? How is dichotomous division to keep these sets distinct; or if -they are not kept distinct, what shall we say to the chaos which must arise -after many fissions, when each set in conflict with the others strives to -produce its particular structure? And how are the conquering determinants -to find they ways out of the _mêlée_ to the places where they are to fulfil -their organizing functions? Even were they all intelligent beings and each -had a map by which to guide his movements, the problem would be -sufficiently puzzling. Can we assume it to be solved by unconscious units? - -Thus even had Professor Weismann shown that the special structures of the -different individuals in an insect-community are not due to differences in -the nurtures they receive, which he has failed to do, he would still be met -by this difficulty in the way of his own view, in addition to the three -other insuperable difficulties grouped together in a preceding section. - -* * * * * - -The collateral issue, which has occupied the largest space in the -controversy, has, as commonly happens, begotten a second generation of -collateral issues. Some of these are embodied in the form of questions put -to me, which I must here answer, lest it should be supposed that they are -unanswerable and my view therefore untenable. - -In the notes he appends to his Romanes Lecture, Professor Weismann -writes:-- - - "One of the questions put to Spencer by Ball is quite sufficient to show - the utter weakness of the position of Lamarckism:--if their - characteristics did not arise among the workers themselves, but were - transmitted from the pre-social time, how does it happen that the queens - and drones of every generation can give anew to the workers the - characteristics which they themselves have long ago lost?" (p. 68). - -It is curious to see put forward in so triumphant a manner, by a professed -naturalist, a question so easily disposed of. I answer it by putting -another. How does it happen that among those moths of which the female has -but rudimentary wings, she continues to endow the males of her species with -wings? How does it happen, for example, that among the _Geometridæ_, the -peculiar structures and habits of which show that they have all descended -from a common ancestor, some species have winged females and some wingless -females; and that though they have lost the wings the ancestral females -had, these wingless females convey to the males the normal developments of -wings? Or, still better, how is it that in the _Psychidæ_ there are -apterous worm-like females, which lay eggs that bring forth winged males of -the ordinary imago form? If for males we read workers, the case is parallel -to the cases of those social insects, the queens of which bequeath -characteristics they have themselves lost. The ordinary facts of embryonic -evolution yield us analogies. What is the most common trait in the -development of the sexes? When the sexual organs of either become -pronounced, the incipient ancillary organs belonging to the opposite sex -cease to develop and remain rudiments, while the organs special to the sex, -essential and nonessential, become fully developed. What, then, must happen -with the queen-ant, which, through countless generations, has ceased to use -certain structures and has lost them from disuse? If one of the eggs which -she lays, capable, as Professor Weismann admits, of becoming queen, male, -or worker of one or other kind, does not at a certain stage begin actively -to develop its reproductive system, then those organs of the ancestral or -pre-social type which the queen has lost begin to develop, and a worker -results. - -Another difficulty in the way of my view, supposed to be fatal, is that -presented by the Honey-ants--aberrant members of certain ant-colonies which -develop so enormously the pouch into which the food is drawn, that the -abdomen becomes little else than a great bladder out of which the head, -thorax, and legs protrude. This, it is thought, cannot be accounted for -otherwise than as a consequence of specially endowed eggs, which it has -become profitable to the community for the queen to produce. But the -explanation fits in quite easily with the view I have set forth. No one -will deny that the taking in of food is the deepest of vital requirements, -and the correlative instinct a dominant one; nor will any one deny that the -instinct of feeding young is less deeply seated--comes later in order of -time. So, too, every one will admit that the worker-bee or worker-ant -before regurgitating food into the mouth of a larva must first of all take -it in. Hence, alike in order of time and necessity, it is to be assumed -that development of the nervous structures which guide self-nutrition, -precedes development of the nervous structures which guide the feeding of -larvæ. What, then, will in some cases happen, supposing there is an -arrested development consequent on innutrition? It will in some cases -happen that while the nervous centres prompting and regulating deglutition -are fully formed, the formation of those prompting and regulating the -regurgitation of the food into the mouths of larvæ are arrested. What will -be the consequence? The life of the worker is mainly passed in taking in -food and putting it out again. If the putting out is stopped its life will -be mainly passed in taking in food. The receptacle will go on enlarging and -it will eventually assume the monstrous form that we see.[137] - -Here, however, to exclude misinterpretations, let me explain. I by no means -deny that variation and selection have produced, in these -insect-communities, certain effects such as Mr. Darwin suggested. Doubtless -ant-queens vary; doubtless there are variations in their eggs; doubtless -differences of structure in the resulting progeny sometimes prove -advantageous to the stirp, and originate slight modifications of the -species. But such changes, legitimately to be assumed, are changes in -single parts--in single organs or portions of organs. Admission of this -does not involve admission that there can take place numerous correlated -variations in different and often remote parts, which must take place -simultaneously or else be useless. Assumption of this is what Professor -Weismann's argument requires, and assumption of this we have seen to be -absurd. - -Before leaving the general problem presented by the social insects, let me -remark that the various complexities of action not explained by inheritance -from pre-social or semi-social types, are probably due to accumulated and -transmitted knowledge. I recently read an account of the education of a -butterfly, carried to the extent that it became quite friendly with its -protector and would come to be fed. If a non-social and relatively -unintelligent insect is capable of thus far consciously adjusting its -actions, then it seems a reasonable supposition that in a community of -social insects there has arisen a mass of experience and usage into which -each new individual is initiated; just as happens among ourselves. We have -only to consider the chaos which would result were we suddenly bereft of -language, and if the young were left to grow up without precept and -example, to see that very probably the polity of an insect community is -made possible by the addition of intelligence to instinct, and the -transmission of information through sign-language. - -* * * * * - -There remains now the question of _panmixia_, which stands exactly where it -did when I published the "Rejoinder to Professor Weismann." - -After showing that the interpretation I put upon his view was justified by -certain passages quoted; and after pointing out that one of his adherents -had set forth the view which I combated--if not as his view yet as -supplementary to it; I went on to criticize the view as set forth afresh by -Professor Weismann himself. I showed that as thus set forth the actuality -of the supposed cause of decrease in disused organs, implies that _minus_ -variations habitually exceed _plus_ variations--in degree or in number, or -in both. Unless it can be proved that such an excess ordinarily occurs, the -hypothesis of _panmixia_ has no place; and I asked, where is the proof that -it occurs. - -No reply. - -Not content with this abstract form of the question I put it also in a -concrete form, and granted for the nonce Professor Weismann's assumption: -taking the case of the rudimentary hind limbs of the whale. I said that -though, during those early stages of decrease in which the disused limbs -were external, natural selection probably had a share in decreasing them, -since they were then impediments to locomotion, yet when they became -internal, and especially when they had dwindled to nothing but remnants of -the femurs, it is impossible to suppose that natural selection played any -part: no whale could have survived and initiated a more prosperous stirp in -virtue of the economy achieved by such a decrease. The operation of natural -selection being out of the question, I inquired whether such a decrease, -say of one-half when the femurs weighed a few ounces, occurring in one -individual, could be supposed in the ordinary course of reproduction to -affect the whole of the whale species inhabiting the Arctic Seas and the -North Atlantic Ocean; and so on with successive diminutions until the -rudiments had reached their present minuteness. I asked whether such an -interpretation could be rationally entertained. - -No reply. - -Now in the absence of replies to these two questions it seems to me that -the verdict must go against Professor Weismann by default. If he has to -surrender the hypothesis of _panmixia_, what results? All that evidence -collected by Mr. Darwin and others, regarded by them as proof of the -inheritance of acquired characters, which was cavalierly set aside on the -strength of this alleged process of panmixia, is reinstated. And this -reinstated evidence, joined with much evidence since furnished, suffices to -establish the repudiated interpretation. - -In the printed report of his Romanes Lecture, after fifty pages of -complicated speculations which we are expected to accept as proofs, -Professor Weismann ends by saying, in reference to the case of the neuter -insects:-- - - "This case is of additional interest, as it may serve to convince those - naturalists who are still inclined to maintain that acquired characters - are inherited, and to support the Lamarckian principle of development, - that their view cannot be the right one. It has not proved tenable in a - single instance" (p. 54). - -Most readers of the foregoing pages will think that since Professor -Weismann has left one after another of my chief theses without reply, this -is rather a strong assertion; and they will still further raise their -eyebrows on remembering that, as I have shown, where he has given answers -his answers are invalid. - -* * * * * - -And now we come to the additions which I indicated at the outset as having -to be made--certain evidences which have come to light since this -controversy commenced. - -When, by a remembered observation made in boyhood, joined with the familiar -fact that worker-larvæ can be changed into the larvæ of queens by feeding, -I was led to suggest that probably all the variations of form in the social -insects are consequent on differences of nurture, I was unaware that -observations and experiments were being made which have justified this -suggestion. Professor Grassi has recently published accounts of the -food-habits of two European species of Termites, shewing that the various -forms are due to feeding. He is known to be a most careful observer, and -some of the most curious of his facts are confirmed by the collection of -white ants exhibited by Dr. David Sharp, F.R.S., at the _soirée_ of the -Royal Society in May last. He has favoured me with the following account of -Grassi's results, which I publish with his assent:-- - - "There is great variety as to the constituents of the community and - economy of the species in White Ants. One of the simplest conditions - known is that studied by Grassi in the case of the European species - Calotermes flavicollis. In this species there is no worker caste; the - adult forms are only of two kinds, viz., soldiers, and the males and - females; the sexes are externally almost indistinguishable, and there are - males and females of soldiers as well as of the winged forms, though the - sexual organs do not undergo their full development in any soldier - whether male or female. - - "The soldier is not however a mere instance of simple arrested - development. It is true that there is in it arrested development of the - sexual organs, but this is accompanied by change of form of other - parts--changes so extreme that one would hardly suppose the soldier to - have any connection with either the young or the adult of the winged - forms. - - "Now according to Grassi the whole of the individuals when born are - undifferentiated forms (except as to sex), and each one is capable of - going on the natural course of development and thus becoming a winged - insect, or can be deviated from this course and made into a soldier; this - is accomplished by the White Ants by special courses of feeding. - - "The evidence given by Grassi is not conclusive as to the young being all - born alike; and it may be that there are some individuals born that could - not be deviated from the natural course and made into soldiers. But there - is one case which seems to show positively that the deviation Grassi - believes to occur is real, and not due to the selection by the ants of an - individual that though appearing to our eyes undifferentiated is not - really so. This is that an individual can be made into a soldier after it - has visibly undergone one half or more of the development into a winged - form. The Termites can in fact operate on an individual that has already - acquired the rudiments of wings and whose head is totally destitute of - any appearance of the shape of the armature peculiar to the soldier, and - can turn it into a soldier; the rudiments of the wings being in such a - case nearly entirely re-absorbed." - -Grassi has been for many years engaged in investigating these phenomena, -and there is no reason for rejecting his statement. We can scarcely avoid -accepting it, and if so, Professor Weismann's hypothesis is conclusively -disposed of. Were there different sets of "determinants" for the -soldier-form and for the winged sexual form, those "determinants" which had -gone a long way towards producing the winged sexual form, would inevitably -go on to complete that form, and could not have their proclivity changed by -feeding. - -[Yet more evidence to the like effect has since become known. At the -meeting of the Entomological Society, on March 14, 1894 (reported in -_Nature_, March 29):-- - - "Dr. D. Sharp, F.R.S., exhibited a collection of white ants (_Termites_), - formed by Mr. G. D. Haviland in Singapore, which comprised about twelve - species, of most of which the various forms were obtained. He said that - Prof. Grassi had recently made observations on the European species, and - had brought to light some important particulars; and also that in the - discussion that had recently been carried on between Mr. Herbert Spencer - and Prof. Weismann, the former had stated that in his opinion the - different forms of social insects were produced by nutrition. Prof. - Grassi's observations showed this view to be correct, and the specimens - now exhibited confirmed one of the most important points in his - observations. Dr. Sharp also stated that Mr. Haviland found in one nest - eleven neoteinic queens--that is to say, individuals having the - appearance of the queen in some respects, while in others they are still - immature." - -Another similarly conclusive verification I published in _Nature_ for -December 6, 1894, under the title "The Origin of Classes among the -'Parasol' Ants." The letter ran as follows:-- - - "Mr. J. H. Hart is Superintendent of the Royal Botanic Gardens in - Trinidad. He has sent me a copy of his report presented to the - Legislative Council in March, 1893, and has drawn my attention to certain - facts contained in it concerning the 'Parasol' ants--the leaf-cutting - ants which feed on the fungi developed in masses of the cut leaves - carried to their nests. Both Mr. Bates and Mr. Belt described these ants, - but described, it seems, different, though nearly allied, species, the - habits of which are partially unlike. As they are garden-pests, Mr. Hart - was led to examine into the development and social arrangements of these - ants; establishing, to that end, artificial nests, after the manner - adopted by Sir John Lubbock. Several of the facts set down have an - important bearing on a question now under discussion. The following - extracts, in which they are named, I abridge by omitting passages not - relevant to the issue:-- - - "'The history of my nests is as follows: Nos. 1 and 2 were both taken - (August 9) on the same day, while destroying nests in the Gardens, and - were portions of separate nests but of the same species. No. 3 was - procured on September 5, and is evidently a different although an allied - species to Nos. 1 and 2. - - "'Finding neither of my nests had a queen, I procured one from another - nest about to be destroyed, and placed it with No. 1 nest. It was - received by the workers, and at once attended by a numerous retinue in - royal style. On August 30 I removed the queen from No. 1 and placed it - with No. 2, when it was again received in a most loyal manner.... - - "'Ants taken from Nos. 1 and 2 and placed with No. 3 were immediately - destroyed by the latter, and even the soldiers of No. 3, as well as - workers or nurses, were destroyed when placed with Nos. 1 and 2. - - "'In nest No. 2, from which I removed the queen on August 30, there are - now in the pupa stage several queens and several males. The forms of ant - in nests Nos. 1 and 2 are as follows: (_a_) queen, (_b_) male (both - winged, but the queen loses its wings after marital flight), (_c_) large - workers, (_d_) small workers, and (_e_) nurses. In nest No. 3 I have not - yet seen the queen or male, but it possesses--(_a_) soldier, (_b_) larger - workers, (_c_) smaller workers, and (_d_) nurses; but these are different - in form to those of nests No. 1 and No. 2. Probably we might add a third - form of worker, as there are several sizes in the nest.... - - "'It is curious that in No. 1 nest, from which the queen was removed on - August 30, new queens and males are now being developed, while in No. 2 - nest, where the queen is at present, nothing but workers have been - brought out, and if a queen larva or pupa is placed there it is at once - destroyed, while worker larvæ or pupæ are amicably received. In No. 3 all - the eggs, larvæ, and pupæ collected with the nest have been hatched, and - no eggs have since made their appearance to date. There is no queen with - this nest.... On November 14 I attempted to prove by experiment how small - a number of "parasol" ants it required to form a new colony. I placed two - dozen of ants (one dozen workers and one dozen nurses) in two separate - nests, No. 4 and No. 5. With No. 4 I placed a few larvæ with a few rose - petals for them to manipulate. With No. 5 I gave a small piece of nest - covered with mycelium. On the 16th these nests were destroyed by small - foraging ants, known as the "sugar" or "meat" ant, and I had to remove - them and replace with a new colony. My notes on these are not - sufficiently lengthy to be of much importance. But I noted four eggs laid - on the 16th, or two days after being placed in their new quarters; no - queen being present. The experiment is being continued. I may mention - that in No. 4 nest, in which no fungus was present, the larvæ of all - sizes appeared to change into the pupæ stage at once for want of food [a - fact corresponding with the fact I have named as observed by myself sixty - years ago in the case of wasp larvæ]. The circumstance tends to show that - the development of the insect is influenced entirely by the feeding it - gets in the larva stage. - - "'In nest No. 2 before the introduction of a queen there were no eggs or - larvæ. The first worker was hatched on October 27, or fifty-seven days - afterwards, and a continual succession has since been maintained, but as - yet (November 19) no males or queens have made their appearance.' - - "In a letter accompanying the report, Mr. Hart says:-- - - "'Since these were published, my notes go to prove that ants can - practically manufacture at will, male, female, soldier, worker, or nurse. - Some of the workers are capable of laying eggs, and from these can be - produced all the various forms as well as from a queen's egg. - - "'There does not, however, appear to be any difference in the character - of the food; as I cannot find that the larger larvæ are fed with anything - different to that given to the smaller.' - - "These results were obtained before the recent discussion of the question - commenced, and joined with the other evidence entirely dispose of those - arguments which Prof. Weismann bases on facts furnished by the social - insects."] - -The other piece of additional evidence I have referred to, is furnished by -two papers contributed to _The Journal of Anatomy and Physiology_ for -October 1893 and April 1894, by R. Havelock Charles, M. D., &c. &c., -Professor of Anatomy in the Medical College, Lahore. These papers set forth -the differences between the leg-bones of Europeans and those of the Punjaub -people--differences caused by their respective habits of sitting in chairs -and squatting on the ground. He enumerates more than twenty such -differences, chiefly in the structures of the knee-joint and ankle-joint. -From the _résumé_ of his second paper I quote the following passages, which -sufficiently show the data and the inferences:-- - - "7. The habits as to sitting postures of Europeans differ from those of - their prehistoric ancestors, the Cave-dwellers, &c., who probably - squatted on the ground. - - "8. The sitting postures of Orientals are the same now as ever. They have - retained the habits of their ancestors. The Europeans have not done so. - - "9. Want of use would induce changes in form and size, and so, gradually, - small differences would be integrated till there would be total - disappearance of the markings on the European skeleton, as no advantage - would accrue to him from the possession of facets on his bones fitting - them for postures not practised by him. - - "10. The facets seen on the bones of the Panjabi infant or foetus have - been transmitted to it by the accumulation of peculiarities gained by - habit in the evolution of its racial type--in which an acquisition having - become a permanent possession, 'profitable to the individual under its - conditions of life,' is transmitted as a useful inheritance. - - "11. These markings are due to the influence of certain positions, which - are brought about by the use of groups of muscles, and they are the - definite results produced by actions of these muscles. - - "12. The effects of the use of the muscles mentioned in No. 11 are - transmitted to the offspring, for the markings are present in the - _foetus-in-utero_, in the child at birth, and in the infant. - - "13. The markings are instances of the transmission of acquired - characters, which heritage in the individual, function subsequently - develops." - -No other conclusion appears to me possible. _Panmixia_, even were it not -invalidated by its unwarranted assumption as above shown, would be out of -court: the case is not a case of either increase or decrease of size but of -numerous changes of form. Simultaneous variation of co-operative parts -cannot be alleged, since these co-operative parts have not changed in one -way but in various ways and degrees. And even were it permissible to -suppose that the required different variations had taken place -simultaneously, natural selection cannot be supposed to have operated. The -assumption would imply that in the struggle for existence, individuals of -the European races who were less capable than others of crouching and -squatting, gained by those minute changes of structure which incapacitated -them, such advantages that their stirps prevailed over other stirps--an -absurd supposition. - -And now I must once more point out that a grave responsibility rests on -biologists in respect of the general question; since wrong answers lead, -among other effects, to wrong beliefs about social affairs and to -disastrous social actions. In me this conviction has unceasingly -strengthened. Though _The Origin of Species_ proved to me that the -transmission of acquired characters cannot be the sole factor in organic -evolution, as I had assumed in _Social Statics_ and in _The Principles of -Biology_, published in pre-Darwinian days, yet I have never wavered in the -belief that it is a factor and an all-important factor. And I have felt -more and more that since all the higher sciences are dependent on the -science of life, and must have their conclusions vitiated if a fundamental -datum given to them by the teachers of this science is erroneous, it -behoves these teachers not to let an erroneous datum pass current: they are -called on to settle this vexed question one way or other. The times give -proof. The work of Mr. Benjamin Kidd on _Social Evolution_, which has been -so much lauded, takes Weismannism as one of its data; and if Weismannism be -untrue, the conclusions Mr. Kidd draws must be in large measure erroneous -and may prove mischievous. - - -POSTSCRIPT.--Since the foregoing pages have been put in type there has -appeared in _Natural Science_ for September, an abstract of certain parts -of a pamphlet by Professor Oscar Hertwig, setting forth facts directly -bearing on Professor Weismann's doctrine respecting the distinction between -reproductive cells and somatic cells. In _The Principles of Biology_, § 77, -I contended that reproductive cells differ from other cells composing the -organism, only in being unspecialized. And in support of the hypothesis -that tissue-cells in general have a reproductive potentiality, I instanced -the cases of the _Begonia phyllomaniaca_ and _Malaxis paludosa_. In the -thirty years which have since elapsed, many facts of like significance have -been brought to light, and various of these are given by Professor Hertwig. -Here are some of them:-- - - "Galls are produced under the stimulus of the insect almost anywhere on - the surface of a plant. Yet in most cases these galls, in a sense grown - at random on the surface of a plant, when placed in damp earth will give - rise to a young plant. In the hydroid _Tubularia mesembryanthemum_, when - the polyp heads are cut off, new heads arise. But if both head and root - be cut off, and the upper end be inserted in the mud, then from the - original upper end not head-polyps but root filaments will arise, while - from the original lower end not root filaments but head-polyps will - grow.... Driesch, by separating the first two and the first four - segmentation spheres of an _Echinus_ ovum, obtained two or four normal - plutei, respectively one half and a quarter of the normal size.... So, - also, in the case of _Amphioxus_, Wilson obtained a normal, but - proportionately diminished embryo with complete nervous system from a - separated sphere of a two- or four- or eight celled stage.... Chabry - obtained normal embryos in cases where some of the segmentation-spheres - had been artificially destroyed." - -These evidences, furnished by independent observers, unite in showing, -firstly, that all the multiplying cells of the developing embryo are alike; -and, secondly, that the soma-cells of the adult severally retain, in a -latent form, all the powers of the original embryo-cell. If these facts do -not disprove absolutely Professor Weismann's hypothesis, we may wonderingly -ask what facts would disprove it? - -Since Hertwig holds that all the cells forming an organism of any species -primarily consist of the same components, I at first thought that his -hypothesis was identical with my own hypothesis of "physiological units," -or, as I would now call them, constitutional units. It seems otherwise, -however; for he thinks that each cell contains "only those material -particles which are bearers of cell-properties," and that organs "are the -functions of cell-complexes." To this it may be replied that the ability to -form the appropriate cell-complexes, itself depends upon the constitutional -units contained in the cells. - - - - -APPENDIX C. - -THE INHERITANCE OF FUNCTIONALLY-WROUGHT MODIFICATIONS: A SUMMARY. - - -The assertion that changes of structure caused by changes of function are -transmitted to descendants is continually met by the question--Where is the -evidence? When some facts are assigned in proof, they are pooh-poohed as -insufficient. If after a time the question is raised afresh and other facts -are named, there is a like supercilious treatment of them. Successively -rejected in this way, the evidences do not accumulate in the minds of -opponents; and hence produce little or no effect. When they are brought -together, however, it turns out that they are numerous and weighty. We will -group them into negative and positive. - -* * * * * - -Negative evidence is furnished by those cases in which traits otherwise -inexplicable are explained if the structural effects of use and disuse are -transmitted. In the foregoing chapters and appendices three have been -given. - -(1) Co-adaptation of co-operative parts comes first. This has been -exemplified by the case of enlarged horns in a stag, by the case of an -animal led into the habit of leaping, and in the case of the giraffe (cited -in "The Factors of Organic Evolution"); and it has been shown that the -implied co-adaptations of parts cannot possibly have been effected by -natural selection. - -(2) The possession of unlike powers of discrimination by different parts of -the human skin, was named as a problem to be solved on the hypothesis of -natural selection or the hypothesis of panmixia; and it was shown that -neither of these can by any twisting yield a solution. But the facts -harmonize with the hypothesis that the effects of use are inherited. - -(3) Then come the cases of those rudimentary organs which, like the hind -limbs of the whale, have nearly disappeared. Dwindling by natural selection -is here out of the question; and dwindling by panmixia, even were its -assumptions valid, would be incredible. But as a sequence of disuse the -change is clearly explained. - -Failure to solve any _one_ of these three problems would, I think, alone -prove the Neo-Darwinian doctrines untenable; and the fact that we have -_three_ unsolved problems seems to me fatal. - -* * * * * - -From this negative evidence, turn now to the positive evidence. This falls -into several groups. - -There are first the facts collected by Mr. Darwin, implying -functionally-altered structures in domestic animals. The hypothesis of -panmixia is, as we have seen, out of court; and therefore Mr. Darwin's -groups of evidences are reinstated. There is the changed ratio of -wing-bones and leg-bones in the duck; there are the drooping ears of cats -in China, of horses in Russia, of sheep in Italy, of guinea-pigs in -Germany, of goats and cattle in India, of rabbits, pigs, and dogs in all -long-civilized countries. Though artificial selection has come into play -where drooping has become a curious trait (as in rabbits), and has probably -caused the greater size of ears which has in some cases gone along with -diminished muscular power over them; yet it could not have been the -initiator, and has not been operative on animals bred for profit. Again -there are the changes produced by climate; as instance, among plants, the -several varieties of maize established in Germany and transformed in the -course of a few generations. - -Facts of another class are yielded by the blind inhabitants of caverns. One -who studies the memoir by Mr. Packard on _The Cave Fauna of North America_, -&c., will be astonished at the variety of types in which degeneration or -loss of the eyes has become a concomitant of life passed in darkness. A -great increase in the force of this evidence will be recognized on learning -that absence or extreme imperfection of visual organs is found also in -creatures living in perpetual night at the bottoms of deep oceans. -Endeavours to account for these facts otherwise than by the effects of -disuse we have seen to be futile. - -Kindred evidence is yielded by decrease of the jaws in those races which -have had diminished use of them--mankind and certain domestic animals. -Relative smallness in the jaws of civilized men, manifest enough on -comparison, has been proved by direct measurement. In pet dogs--pugs, -household spaniels--we find associated the same cause with the same effect. -Though there has been artificial selection, yet this did not operate until -the diminution had become manifest. Moreover there has been diminution of -the other structures concerned in biting: there are smaller muscles, feeble -zygomata, and diminished areas for insertion of muscles--traits which -cannot have resulted from selection, since they are invisible in the living -animal. - -In abnormal vision produced by abnormal use of the eyes we have evidence of -another kind. That the Germans, among whom congenital short sight is -notoriously prevalent, have been made shortsighted by inheritance of -modifications due to continual reading of print requiring close attention, -is by some disputed. It is strange, however, that if there exists no causal -connexion between them, neither trait occurs without the other elsewhere. -But for the belief that there is a causal connexion we have the verifying -testimony of oculists. From Dr. Lindsay Johnson I have cited cases within -his professional experience of functionally-produced myopia transmitted to -children; and he asserts that other oculists have had like experiences. - -Development of the musical faculty in the successive members of families -from which the great composers have come, as well as in the civilized races -at large, is not to be explained by natural selection. Even when it is -great, the musical faculty has not a life-saving efficiency as compared -with the average of faculties; for the most highly gifted have commonly -passed less prosperous lives and left fewer offspring than have those -possessed of ordinary abilities. Still less can it be said that the musical -faculty in mankind at large has been developed by survival of the fittest. -No one will assert that men in general have been enabled to survive and -propagate in proportion as their musical appreciation was great. - -The transmission of nervous peculiarities functionally produced is alleged -by the highest authorities--Dr. Savage, president of the Neurological -Society, and Dr. Hughlings Jackson. The evidence they assign confirms, and -is confirmed by, that which the development of the musical faculty above -named supplies. - -Here, then, we have sundry groups of facts directly supporting the belief -that functionally-wrought modifications descend from parents to offspring. - -* * * * * - -Now let us consider the position of those Darwinians who dissent from -Darwin, and who make light of all this evidence. We might naturally suppose -that their own hypothesis is unassailable. Yet, strange to say, they admit -that there is no direct proof that any species has been established by -natural selection. The proof is inferential only. - -The certainty of an axiom does not give certainty to the deductions drawn -from it. That natural selection is, and always has been, operative is -incontestable. Obviously I should be the last person to deny that survival -of the fittest is a necessity: its negation is inconceivable. The -Neo-Darwinians, however, judging from their attitude, apparently assume -that firmness of the basis implies firmness of the superstructure. But -however high may be the probability of some of the conclusions drawn, none -of them can have more than probability; while some of them remain, and are -likely to remain, very questionable. Observe the difficulties. - -(1) The general argument proceeds upon the analogy between natural -selection and artificial selection. Yet all know that the first cannot do -what the last does. Natural selection can do nothing more than preserve -those of which the _aggregate_ characters are most favourable to life. It -cannot pick out those possessed of one particular favourable character, -unless this is of extreme importance. - -(2) In many cases a structure is of no service until it has reached a -certain development; and it remains to account for that increase of it by -natural selection which must be supposed to take place before it reaches -the stage of usefulness. - -(3) Advantageous variations, not preserved in nature as they are by the -breeder, are liable to be swamped by crossing or to disappear by atavism. - -Now whatever replies are made, their component propositions cannot be -necessary truths. So that the conclusion in each case, however reasonable, -cannot claim certainty: the fabric can have no stability like that of its -foundation. - -When to uncertainties in the arguments supporting the hypothesis we add its -inability to explain facts of cardinal significance, as proved above, there -is I think ground for asserting that natural selection is less clearly -shown to be a factor in the origination of species than is the inheritance -of functionally-wrought changes. - -* * * * * - -If, finally, it is said that the mode in which functionally-wrought -changes, especially in small parts, so affect the reproductive elements as -to repeat themselves in offspring, cannot be imagined--if it be held -inconceivable that those minute changes in the organs of vision which cause -myopia can be transmitted through the appropriately-modified sperm-cells or -germ-cells; then the reply is that the opposed hypothesis presents a -corresponding inconceivability. Grant that the habit of a pointer was -produced by selection of those in which an appropriate variation in the -nervous system had occurred; it is impossible to imagine how a -slightly-different arrangement of a few nerve-cells and fibres could be -conveyed by a spermatozoon. So too it is impossible to imagine how in a -spermatozoon there can be conveyed the 480,000 independent variables -required for the construction of a single peacock's feather, each having a -proclivity towards its proper place. Clearly the ultimate process by which -inheritance is effected in either case passes comprehension; and in this -respect neither hypothesis has an advantage over the other. - - - - -APPENDIX D. - -ON ALLEGED "SPONTANEOUS GENERATION," AND ON THE HYPOTHESIS OF PHYSIOLOGICAL -UNITS. - - -[_The following letter, originally written for publication in the_ North -American Review, _but declined by the Editor in pursuance of a general -rule, and eventually otherwise published in the United States, I have -thought well to append to this first volume of the_ Principles of Biology. -_I do this because the questions which it discusses are dealt with in this -volume; and because the further explanations it furnishes seem needful to -prevent misapprehensions._] - - -_The Editor of the North American Review._ - - SIR, - -It is in most cases unwise to notice adverse criticisms. Either they do not -admit of answers or the answers may be left to the penetration of readers. -When, however, a critic's allegations touch the fundamental propositions of -a book, and especially when they appear in a periodical having the position -of the _North American Review_, the case is altered. For these reasons the -article on "Philosophical Biology," published in your last number, demands -from me an attention which ordinary criticisms do not. - -It is the more needful for me to notice it, because its two leading -objections have the one an actual fairness and the other an apparent -fairness; and in the absence of explanations from me, they will be -considered as substantiated even by many, or perhaps most, of those who -have read the work itself--much more by those who have not read it. That to -prevent the spread of misapprehensions I ought to say something, is further -shown by the fact that the same two objections have already been made in -England--the one by Dr. Child, of Oxford, in his _Essays on Physiological -Subjects_, and the other by a writer in the _Westminster Review_ for July, -1865. - -* * * * * - -In the note to which your reviewer refers, I have, as he says, tacitly -repudiated the belief in "spontaneous generation;" and that I have done -this in such a way as to leave open the door for the interpretation given -by him is true. Indeed the fact that Dr. Child, whose criticism is a -sympathetic one, puts the same construction on this note, proves that your -reviewer has but drawn what seems to be a necessary inference. -Nevertheless, the inference is one which I did not intend to be drawn. - -In explanation, let me at the outset remark that I am placed at a -disadvantage in having had to omit that part of the System of Philosophy -which deals with Inorganic Evolution. In the original programme will be -found a parenthetic reference to this omitted part, which should, as there -stated, precede the _Principles of Biology_. Two volumes are missing. The -closing chapter of the second, were it written, would deal with the -evolution of organic matter--the step preceding the evolution of living -forms. Habitually carrying with me in thought the contents of this -unwritten chapter, I have, in some cases, expressed myself as though the -reader had it before him; and have thus rendered some of my statements -liable to misconstructions. Apart from this, however, the explanation of -the apparent inconsistency is very simple, if not very obvious. In the -first place, I do not believe in the "spontaneous generation" commonly -alleged, and referred to in the note; and so little have I associated in -thought this alleged "spontaneous generation" which I disbelieve, with the -generation by evolution which I do believe, that the repudiation of the one -never occurred to me as liable to be taken for repudiation of the other. -That creatures having _quite specific structures_ are evolved in the course -of a few hours, without antecedents calculated to determine their specific -forms, is to me incredible. Not only the established truths of Biology, but -the established truths of science in general, negative the supposition that -organisms having structures definite enough to identify them as belonging -to known genera and species, can be produced in the absence of germs -derived from antecedent organisms of the same genera and species. If there -can suddenly be imposed on simple protoplasm the organization which -constitutes it a _Paramoecium_, I see no reason why animals of greater -complexity, or indeed of any complexity, may not be constituted after the -same manner. In brief, I do not accept these alleged facts as exemplifying -Evolution, because they imply something immensely beyond that which -Evolution, as I understand it, can achieve. In the second place, my -disbelief extends not only to the alleged cases of "spontaneous -generation," but to every case akin to them. The very conception of -spontaneity is wholly incongruous with the conception of Evolution. For -this reason I regard as objectionable Mr. Darwin's phrase "spontaneous -variation" (as indeed he does himself); and I have sought to show that -there are always assignable causes of variation. No form of Evolution, -inorganic or organic, can be spontaneous; but in every instance the -antecedent forces must be adequate in their quantities, kinds, and -distributions, to work the observed effects. Neither the alleged cases of -"spontaneous generation," nor any imaginable cases in the least allied to -them, fulfil this requirement. - -If, accepting these alleged cases of "spontaneous generation," I had -assumed, as your reviewer seems to do, that the evolution of organic life -commenced in an analogous way; then, indeed, I should have left myself open -to a fatal criticism. This supposed "spontaneous generation" habitually -occurs in menstrua that contain either organic matter, or matter originally -derived from organisms; and such organic matter, proceeding in all known -cases from organisms of a higher kind, implies the pre-existence of such -higher organisms. By what kind of logic, then, is it inferrible that -organic life was initiated after a manner like that in which _Infusoria_ -are said to be now spontaneously generated? Where, before life commenced, -were the superior organisms from which these lowest organisms obtained -their organic matter? Without doubting that there are those who, as the -reviewer says, "can penetrate deeper than Mr. Spencer has done into the -idea of universal evolution," and who, as he contends, prove this by -accepting the doctrine of "spontaneous generation"; I nevertheless think -that I can penetrate deep enough to see that a tenable hypothesis -respecting the origin of organic life must be reached by some other clue -than that furnished by experiments on decoction of hay and extract of beef. - -From what I do not believe, let me now pass to what I do believe. Granting -that the formation of organic matter, and the evolution of life in its -lowest forms, may go on under existing cosmical conditions; but believing -it more likely that the formation of such matter and such forms, took place -at a time when the heat of the Earth's surface was falling through those -ranges of temperature at which the higher organic compounds are unstable; I -conceive that the moulding of such organic matter into the simplest types, -must have commenced with portions of protoplasm more minute, more -indefinite, and more inconstant in their characters, than the lowest -Rhizopods--less distinguishable from a mere fragment of albumen than even -the _Protogenes_ of Professor Haeckel. The evolution of specific shapes -must, like all other organic evolution, have resulted from the actions and -reactions between such incipient types and their environments, and the -continued survival of those which happened to have specialities best fitted -to the specialities of their environments. To reach by this process the -comparatively well-specialized forms of ordinary _Infusoria_, must, I -conceive, have taken an enormous period of time. - -To prevent, as far as may be, future misapprehension, let me elaborate this -conception so as to meet the particular objections raised. The reviewer -takes for granted that a "first organism" must be assumed by me, as it is -by himself. But the conception of a "first organism," in anything like the -current sense of the words, is wholly at variance with conception of -evolution; and scarcely less at variance with the facts revealed by the -microscope. The lowest living things are not properly speaking organisms at -all; for they have no distinctions of parts--no traces of organization. It -is almost a misuse of language to call them "forms" of life: not only are -their outlines, when distinguishable, too unspecific for description, but -they change from moment to moment and are never twice alike, either in two -individuals or in the same individual. Even the word "type" is applicable -in but a loose way; for there is little constancy in their generic -characters: according as the surrounding conditions determine, they undergo -transformations now of one kind and now of another. And the vagueness, the -inconstancy, the want of appreciable structure, displayed by the simplest -of living things as we now see them, are characters (or absences of -characters) which, on the hypothesis of Evolution, must have been still -more decided when, as at first, no "forms," no "types," no "specific -shapes," had been moulded. That "absolute commencement of organic life on -the globe," which the reviewer says I "cannot evade the admission of," I -distinctly deny. The affirmation of universal evolution is in itself the -negation of an "absolute commencement" of anything. Construed in terms of -evolution, every kind of being is conceived as a product of modifications -wrought by insensible gradations on a pre-existing kind of being; and this -holds as fully of the supposed "commencement of organic life" as of all -subsequent developments of organic life. It is no more needful to suppose -an "absolute commencement of organic life" or a "first organism," than it -is needful to suppose an absolute commencement of social life and a first -social organism. The assumption of such a necessity in this last case, made -by early speculators with their theories of "social contracts" and the -like, is disproved by the facts; and the facts, so far as they are -ascertained, disprove the assumption of such a necessity in the first case. -That organic matter was not produced all at once, but was reached through -steps, we are well warranted in believing by the experiences of chemists. -Organic matters are produced in the laboratory by what we may literally -call _artificial evolution_. Chemists find themselves unable to form these -complex combinations directly from their elements; but they succeed in -forming them indirectly, by successive modifications of simpler -combinations. In some binary compound, one element of which is present in -several equivalents, a change is made by substituting for one of these -equivalents an equivalent of some other element; so producing a ternary -compound. Then another of the equivalents is replaced, and so on. For -instance, beginning with ammonia, N H_{3}, a higher form is obtained by -replacing one of the atoms of hydrogen by an atom of methyl, so producing -methyl-amine, N (C H_{3} H_{2}); and then, under the further action of -methyl, ending in a further substitution, there is reached the still more -compound substance dimethyl-amine, N (C H_{3}) (C H_{3}) H. And in this -manner highly complex substances are eventually built up. Another -characteristic of their method is no less significant. Two complex -compounds are employed to generate, by their action upon one another, a -compound of still greater complexity: different heterogeneous molecules of -one stage, become parents of a molecule a stage higher in heterogeneity. -Thus, having built up acetic acid out of its elements, and having by the -process of substitution described above, changed the acetic acid into -propionic acid, and propionic into butyric, of which the formula is - - {C(CH_{3})(CH_{3})H} - {CO(HO) }; - -this complex compound, by operating on another complex compound, such as -the dimethyl-amine named above, generates one of still greater complexity, -butyrate of dimethyl-amine - - {C(CH)(CH_{3})H} N(CH_{3})(CH_{3})H. - {CO(HO) } - -See, then, the remarkable parallelism. The progress towards higher types of -organic molecules is effected by modifications upon modifications; as -throughout Evolution in general. Each of these modifications is a change of -the molecule into equilibrium with its environment--an adaptation, as it -were, to new surrounding conditions to which it is subjected; as throughout -Evolution in general. Larger, or more integrated, aggregates (for compound -molecules are such) are successively generated; as throughout Evolution in -general. More complex or heterogeneous aggregates are so made to arise, one -out of another; as throughout Evolution in general. A -geometrically-increasing multitude of these larger and more complex -aggregates so produced, at the same time results; as throughout Evolution -in general. And it is by the action of the successively higher forms on one -another, joined with the action of environing conditions, that the highest -forms are reached; as throughout Evolution in general. - -When we thus see the identity of method at the two extremes--when we see -that the general laws of evolution, as they are exemplified in known -organisms, have been unconsciously conformed to by chemists in the -artificial evolution of organic matter; we can scarcely doubt that these -laws were conformed to in the natural evolution of organic matter, and -afterwards in the evolution of the simplest organic forms. In the early -world, as in the modern laboratory, inferior types of organic substances, -by their mutual actions under fit conditions, evolved the superior types of -organic substances, ending in organizable protoplasm. And it can hardly be -doubted that the shaping of organizable protoplasm, which is a substance -modifiable in multitudinous ways with extreme facility, went on after the -same manner. As I learn from one of our first chemists, Prof. Frankland, -_protein_ is capable of existing under probably at least a thousand -isomeric forms; and, as we shall presently see, it is capable of forming, -with itself and other elements, substances yet more intricate in -composition, that are practically infinite in their varieties of kind. -Exposed to those innumerable modifications of conditions which the Earth's -surface afforded, here in amount of light, there in amount of heat, and -elsewhere in the mineral quality of its aqueous medium, this extremely -changeable substance must have undergone now one, now another, of its -countless metamorphoses. And to the mutual influences of its metamorphic -forms under favouring conditions, we may ascribe the production of the -still more composite, still more sensitive, still more variously-changeable -portions of organic matter, which, in masses more minute and simpler than -existing _Protozoa_, displayed actions verging little by little into those -called vital--actions which protein itself exhibits in a certain degree, -and which the lowest known living things exhibit only in a greater degree. -Thus, setting out with inductions from the experiences of organic chemists -at the one extreme, and with inductions from the observations of biologists -at the other extreme, we are enabled deductively to bridge the -interval--are enabled to conceive how organic compounds were evolved, and -how, by a continuance of the process, the nascent life displayed in these -became gradually more pronounced. And this it is which has to be explained, -and which the alleged cases of "spontaneous generation" would not, were -they substantiated, help us in the least to explain. - -It is thus manifest, I think, that I have not fallen into the alleged -inconsistency. Nevertheless, I admit that your reviewer was justified in -inferring this inconsistency; and I take blame to myself for not having -seen that the statement, as I have left it, is open to misconstruction. - -* * * * * - -I pass now to the second allegation--that in ascribing to certain specific -molecules, which I have called "physiological units," the aptitude to build -themselves into the structure of the organism to which they are peculiar, I -have abandoned my own principle, and have assumed something beyond the -re-distribution of Matter and Motion. As put by the reviewer, his case -appears to be well made out; and that he is not altogether unwarranted in -so putting it, may be admitted. Nevertheless, there does not in reality -exist the supposed incongruity. - -Before attempting to make clear the adequacy of the conception which I am -said to have tacitly abandoned as insufficient, let me remove that excess -of improbability the reviewer gives to it, by the extremely-restricted -meaning with which he uses the word mechanical. In discussing a proposition -of mine he says:-- - - "He then cites certain remarks of Mr. Paget on the permanent effects - wrought in the blood by the poison of scarlatina and small-pox, as - justifying the belief that such a 'power' exists, and attributes the - repair of a wasted tissue to 'forces analogous to those by which a - crystal reproduces its lost apex.' (Neither of which phenomena, however, - is explicable by mechanical causes.)" - -Were it not for the deliberation with which this last statement is made, I -should take it for a slip of the pen. As it is, however, I have no course -left but to suppose the reviewer unaware of the fact that molecular actions -of all kinds are now not only conceived as mechanical actions, but that -calculations based on this conception of them, bring out the results that -correspond with observation. There is no kind of re-arrangement among -molecules (crystallization being one) which the modern physicist does not -think of. and correctly reason upon, in terms of forces and motions like -those of sensible masses. Polarity is regarded as a resultant of such -forces and motions; and when, as happens in many cases, light changes the -molecular structure of a crystal, and alters its polarity, it does this by -impressing, in conformity with mechanical laws, new motions on the -constituent molecules. That the reviewer should present the mechanical -conception under so extremely limited a form, is the more surprising to me -because, at the outset of the very work he reviews, I have, in various -passages, based inferences on those immense extensions of it which he -ignores; indicating, for example, the interpretation it yields of the -inorganic chemical changes effected by heat, and the organic chemical -changes effected by light (_Principles of Biology_, § 13). - -Premising, then, that the ordinary idea of mechanical action must be -greatly expanded, let us enter upon the question at issue--the sufficiency -of the hypothesis that the structure of each organism is determined by the -polarities of the special molecules, or physiological units, peculiar to it -as a species, which necessitate tendencies towards special arrangements. My -proposition and the reviewer's criticism upon it, will be most conveniently -presented if I quote in full a passage of his from which I have already -extracted some expressions. He says:-- - - "It will be noticed, however, that Mr. Spencer attributes the possession - of these 'tendencies,' or 'proclivities,' to natural inheritance from - ancestral organisms; and it may be argued that he thus saves the - mechanist theory and his own consistency at the same time, inasmuch as he - derives even the 'tendencies' themselves ultimately from the environment. - To this we reply, that Mr. Spencer, who advocates the nebular hypothesis, - cannot evade the admission of an absolute commencement of organic life on - the globe, and that the 'formative tendencies,' without which he cannot - explain the evolution of a single individual, could not have been - inherited by the first organism. Besides, by his virtual denial of - spontaneous generation, he denies that the first organism was evolved out - of the inorganic world, and thus shuts himself off from the argument - (otherwise plausible) that its 'tendencies' were ultimately derived from - the environment." - -This assertion is already in great measure disposed of by what has been -said above. Holding that, though not "spontaneously generated," those -minute portions of protoplasm which first displayed in the feeblest degree -that changeability taken to imply life, were evolved, I am _not_ debarred -from the argument that the "tendencies" of the physiological units are -derived from the inherited effects of environing actions. If the conception -of a "first organism" were a necessary one, the reviewer's objection would -be valid. If there were an "absolute commencement" of life, a definite line -parting organic matter from the simplest living forms, I should be placed -in the predicament he describes. But as the doctrine of Evolution itself -tacitly negatives any such distinct separation; and as the negation is the -more confirmed by the facts the more we know of them; I do not feel that I -am entangled in the alleged difficulty. My reply might end here; but as the -hypothesis in question is one not easily conceived, and very apt to be -misunderstood, I will attempt a further elucidation of it. - -Much evidence now conspires to show that molecules of the substances we -call elementary are in reality compound; and that, by the combination of -these with one another, and re-combinations of the products, there are -formed systems of systems of molecules, unimaginable in their complexity. -Step by step as the aggregate molecules so resulting, grow larger and -increase in heterogeneity, they become more unstable, more readily -transformable by small forces, more capable of assuming various characters. -Those composing organic matter transcend all others in size and intricacy -of structure; and in them these resulting traits reach their extreme. As -implied by its name _protein_, the essential substance of which organisms -are built, is remarkable alike for the variety of its metamorphoses and the -facility with which it undergoes them: it changes from one to another of -its thousand isomeric forms on the slightest change of conditions. Now -there are facts warranting the belief that though these multitudinous -isomeric forms of protein will not unite directly with one another, yet -they admit of being linked together by other elements with which they -combine. And it is very significant that there are habitually present two -other elements, sulphur and phosphorus, which have quite special powers of -holding together many equivalents--the one being pentatomic and the other -hexatomic. So that it is a legitimate supposition (justified by analogies) -that an atom of sulphur may be a bond of union among half-a-dozen different -isomeric forms of protein; and similarly with phosphorus. A moment's -thought will show that, setting out with the thousand isomeric forms of -protein, this makes possible a number of these combinations almost passing -the power of figures to express. Molecules so produced, perhaps exceeding -in size and complexity those of protein as those of protein exceed those of -inorganic matter, may, I conceive, be the special units belonging to -special kinds of organisms. By their constitution they must have a -plasticity, or sensitiveness to modifying forces, far beyond that of -protein; and bearing in mind not only that their varieties are practically -infinite in number, but that closely allied forms of them, chemically -indifferent to one another as they must be, may coexist in the same -aggregate, we shall see that they are fitted for entering into unlimited -varieties of organic structures. - -The existence of such physiological units, peculiar to each species of -organism, is not unaccounted for. They are evolved simultaneously with the -evolution of the organisms they compose--they differentiate as fast as -these organisms differentiate; and are made multitudinous in kind by the -same actions which make the organism they compose multitudinous, in kind. -This conception is clearly representable in terms of the mechanical -hypothesis. Every physicist will endorse the proposition that in each -aggregate there tends to establish itself an equilibrium between the forces -exercised by all the units upon each and by each upon all. Even in masses -of substance so rigid as iron and glass, there goes on a molecular -re-arrangement, slow or rapid according as circumstances facilitate, which -ends only when there is a complete balance between the actions of the parts -on the whole and the actions of the whole on the parts: the implications -being that every change in the form or size of the whole, necessitates some -redistribution of the parts. And though in cases like these, there occurs -only a polar re-arrangement of the molecules, without changes in the -molecules themselves; yet where, as often happens, there is a passage from -the colloid to the crystalloid state, a change of constitution occurs in -the molecules themselves. These truths are not limited to inorganic matter: -they unquestionably hold of organic matter. As certainly as molecules of -alum have a form of equilibrium, the octahedron, into which they fall when -the temperature of their solvent allows them to aggregate, so certainly -must organic molecules of each kind, no matter how complex, have a form of -equilibrium in which, when they aggregate, their complex forces are -balanced--a form far less rigid and definite, for the reason that they have -far less definite polarities, are far more unstable, and have their -tendencies more easily modified by environing conditions. Equally certain -is it that the special molecules having a special organic structure as -their form of equilibrium, must be reacted upon by the total forces of this -organic structure; and that, if environing actions lead to any change in -this organic structure, these special molecules, or physiological units, -subject to a changed distribution of the total forces acting upon them will -undergo modification--modification which their extreme plasticity will -render easy. By this action and reaction I conceive the physiological units -peculiar to each kind of organism, to have been moulded along with the -organism itself. Setting out with the stage in which protein in minute -aggregates, took on those simplest differentiations which fitted it for -differently-conditioned parts of its medium, there must have unceasingly -gone on perpetual re-adjustments of balance between aggregates and their -units--actions and reactions of the two, in which the units tended ever to -establish the typical form produced by actions and reactions in all -antecedent generations, while the aggregate, if changed in form by change -of surrounding conditions, tended ever to impress on the units a -corresponding change of polarity, causing them in the next generation to -reproduce the changed form--their new form of equilibrium. - -This is the conception which I have sought to convey, though it seems -unsuccessfully, in the _Principles of Biology_; and which I have there used -to interpret the many involved and mysterious phenomena of Genesis, -Heredity, and Variation. In one respect only am I conscious of having so -inadequately explained myself, as to give occasion for a -misinterpretation--the one made by the _Westminster_ reviewer above -referred to. By him, as by your own critic, it is alleged that in the idea -of "inherent tendencies" I have introduced, under a disguise, the -conception of "the archæus, vital principle, _nisus formativus_, and so -on." This allegation is in part answered by the foregoing explanation. That -which I have here to add, and did not adequately explain in the _Principles -of Biology_, is that the proclivity of units of each order towards the -specific arrangement seen in the organism they form, is not to be -understood as resulting from their own structures and actions only; but as -the product of these and the environing forces to which they are exposed. -Organic evolution takes place only on condition that the masses of -protoplasm formed of the physiological units, and of the assimilable -materials out of which others like themselves are to be multiplied, are -subject to heat of a given degree--are subject, that is, to the unceasing -impacts of undulations of a certain strength and period; and, within -limits, the rapidity with which the physiological units pass from their -indefinite arrangement to the definite arrangement they presently assume, -is proportionate to the strengths of the ethereal undulations falling upon -them. In its complete form, then, the conception is that these specific -molecules, having the immense complexity above described, and having -correspondently complex polarities which cannot be mutually balanced by any -simple form of aggregation, have, for the form of aggregation in which all -their forces are equilibrated, the structure of the adult organism to which -they belong; and that they are compelled to fall into this structure by the -co-operation of the environing forces acting on them, and the forces they -exercise on one another--the environing forces being the source of the -_power_ which effects the re-arrangement, and the polarities of the -molecules determining the _direction_ in which that power is turned. Into -this conception there enters no trace of the hypothesis of an "archæus or -vital principle;" and the principles of molecular physics fully justify it. - -It is, however, objected that "the living body in its development presents -a long succession of _differing_ forms; a continued series of changes for -the whole length of which, according to Mr. Spencer's hypothesis, the -physiological units must have an 'inherent tendency.' Could we more truly -say of anything, 'it is unrepresentable in thought?'" I reply that if there -is taken into account an element here overlooked, the process will not be -found "unrepresentable in thought." This is the element of size or mass. To -satisfy or balance the polarities of each order of physiological units, not -only a certain structure of organism, but a certain size of organism is -needed; for the complexities of that adult structure in which the -physiological units are equilibrated, cannot be represented within the -small bulk of the embryo. In many minute organisms, where the whole mass of -physiological units required for the structure is present, the very thing -_does_ take place which it is above implied _ought_ to take place. The mass -builds itself directly into the complete form. This is so with _Acari_, and -among the nematoid _Entozoa_. But among higher animals such direct -transformations cannot happen. The mass of physiological units required to -produce the size as well as the structure that approximately equilibrates -them, is not all present, but has to be formed by successive -additions--additions which in viviparous animals are made by absorbing, and -transforming into these special molecules, the organizable materials -directly supplied by the parent, and which in oviparous animals are made by -doing the like with the organizable materials in the "food-yelk," deposited -by the parent in the same envelope with the germ. Hence it results that, -under such conditions, the physiological units which first aggregate into -the rudiment of the future organism, do not form a structure like that of -the adult organism, which, when of such small dimensions, does not -equilibrate them. They distribute themselves so as partly to satisfy the -chief among their complex polarities. The vaguely-differentiated mass thus -produced cannot, however, be in equilibrium. Each increment of -physiological units formed and integrated by it, changes the distribution -of forces; and this has a double effect. It tends to modify the -differentiations already made, bringing them a step nearer to the -equilibrating structure; and the physiological units next integrated, being -brought under the aggregate of polar forces exercised by the whole mass, -which now approaches a step nearer to that ultimate distribution of polar -forces which exists in the adult organism, are coerced more directly into -the typical structure. Thus there is necessitated a series of compromises. -Each successive form assumed is unstable and transitional: approach to the -typical structure going on hand in hand with approach to the typical bulk. - -Possibly I have not succeeded by this explanation, any more than by the -original explanation, in making this process "representable in thought." It -is manifestly untrue, however, that I have, as alleged, re-introduced under -a disguise the conception of a "vital principle." That I interpret -embryonic development in terms of Matter and Motion, cannot, I think, be -questioned. Whether the interpretation is adequate, must be a matter of -opinion; but it is clearly a matter of fact, that I have not fallen into -the inconsistency asserted by your reviewer. At the same time I willingly -admit that, in the absence of certain statements which I have now supplied, -he was not unwarranted in representing my conception in the way that he has -done. - - - ----- - -NOTES - - [1] Gross misrepresentations of this statement, which have been from time - to time made, oblige me, much against my will, to add here an - explanation of it. The last of these perversions, uttered in a - lecture delivered at Belfast by the Rev. Professor Watts, D.D., is - reported in the _Belfast Witness_ of December 18, 1874; just while a - third impression of this work is being printed from the plates. The - report commences as follows:--"Dr. Watts, after showing that on his - own confession Spencer was indebted for his facts to Huxley and - Hooker, who," &c., &c. - - Wishing in this, as in other cases, to acknowledge indebtedness when - conscious of it, I introduced the words referred to, in recognition - of the fact that I had repeatedly questioned the distinguished - specialists named, on matters beyond my knowledge, which were not - dealt with in the books at my command. Forgetting the habits of - antagonists, and especially theological antagonists, it never - occurred to me that my expression of thanks to my friends for - "information where my own was deficient," would be turned into the - sweeping statement that I was indebted to them for my facts. - - Had Professor Watts looked at the preface to the second volume (the - two having been published separately, as the prefaces imply), he - would have seen a second expression of my indebtedness "for their - valuable criticisms, and for the trouble they have taken in - _checking_ the numerous statements of fact on which the arguments - proceed"--no further indebtedness being named. A moment's comparison - of the two volumes in respect of their accumulations of facts, would - have shown him what kind of warrant there was for his interpretation. - - Doubtless the Rev. Professor was prompted to make this assertion by - the desire to discredit the work he was attacking; and having so good - an end in view, thought it needless to be particular about the means. - In the art of dealing with the language of opponents, Dr. Watts might - give lessons to Monsignor Capel and Archbishop Manning. - - _December 28th, 1874._ - - [2] In this passage as originally written (in 1862) they were described - as incondensible; since, though reduced to the density of liquids, - they had not been liquefied. - - [3] Here and hereafter the word "atom" signifies a unit of something - classed as an element, because thus far undecomposed by us. The word - must not be supposed to mean that which its derivation implies. In - all probability it is not a simple unit but a compound one. - - [4] The name hydro-carbons was here used when these pages were written, - thirty-four years ago. It was the name then current. In this case, as - in multitudinous other cases, the substitution of newer words and - phrases for older ones, is somewhat misleading. Putting the thoughts - of 1862 in the language of 1897 gives an illusive impression of - recency. - - [5] It will perhaps seem strange to class oxygen as a crystalloid. But - inasmuch as the crystalloids are distinguished from the colloids by - their atomic simplicity, and inasmuch as sundry gases are reducible - to a crystalline state, we are justified in so classing it. - - [6] The remark made by a critic to the effect that in a mammal higher - temperature diminishes the rate of molecular change in the tissues, - leads me to add that the exhalation I have alleged is prevented if - the heat rises above the range of variation normal to the organism; - since, then, unusually rapid pulsations with consequent inefficient - propulsion of the blood, cause a diminished rate of circulation. To - produce the effect referred to in the text, heat must be associated - with dryness; for otherwise evaporation is not aided. General - evidence supporting the statement I have made is furnished by the - fact that the hot and dry air of the eastern deserts is extremely - invigorating; by the fact that all the energetic and conquering races - of men have come from the hot and dry regions marked on the maps as - rainless; and by the fact that travellers in Africa comment on the - contrast between the inhabitants of the hot and dry regions - (relatively elevated) and those of the hot and moist regions: active - and inert respectively. - - [7] The increase of respiration found to result from the presence of - light, is probably an _indirect_ effect. It is most likely due to the - reception of more vivid impressions through the eyes, and to the - consequent nervous stimulation. Bright light is associated in our - experience with many of our greatest outdoor pleasures, and its - presence partially arouses the consciousness of them, with the - concomitant raised vital functions. - - [8] To exclude confusion it may be well here to say that the word "atom" - is, as before explained, used as the name for a unit of a substance - at present undecomposed; while the word "molecule" is used as the - name for a unit of a substance known to be compound. - - [9] On now returning to the subject after many years, I meet with some - evidence recently assigned, in a paper read before the Royal Society - by Mr. J. W. Pickering, D.Sc. (detailing results harmonizing with - those obtained by Prof. Grimaux), showing clearly how important an - agent in vital actions is this production of isomeric changes by - slight changes of conditions. Certain artificially produced - substances, simulating proteids in other of their characters and - reactions, were found to simulate them in coagulability by trifling - disturbances. "In the presence of a _trace of neutral salt_ they - coagulate on heating at temperatures very similar to proteid - solutions." And it is shown that by one of these factitious organic - colloids a like effect is produced in coagulating the blood, to that - "produced by the intravenous injection of a nucleoproteid." - - [10] After this long interval during which other subjects have occupied - me, I now find that the current view is similar to the view above set - forth, in so far that a small molecular disturbance is supposed - suddenly to initiate a great one, producing a change compared to an - explosion. But while, of two proposed interpretations, one is that - the fuse is nitrogenous and the charge a carbo-hydrate, the other is - that both are nitrogenous. The relative probabilities of these - alternative views will be considered in a subsequent chapter. - - [11] When writing this passage I omitted to observe the verification - yielded of the conclusion contained in § 15 concerning the part - played in the vital processes by the nitrogenous compounds. For these - vegeto-alkalies, minute quantities of which produce such great - effects in exalting the functions (_e. g._, a sixteenth of a grain of - strychnia is a dose), are all nitrogenous bodies, and, by - implication, relatively unstable bodies. The small amounts of - molecular change which take place in these small quantities of the - vegeto-alkalies when diffused through the system, initiate larger - amounts of molecular change in the nitrogenous elements of the - tissues. - - But the evidence furnished a generation ago by these vegeto-alkalies - has been greatly reinforced by far more striking evidence furnished - by other nitrogenous compounds--the various explosives. These, at the - same time that they produce by their sudden decompositions violent - effects outside the organism, also produce violent effects inside it: - a hundredth of a grain of nitro-glycerine being a sufficient dose. - Investigations made by Dr. J. B. Bradbury, and described by him in - the Bradshaw Lecture on "Some New Vaso-Dilators" (see _The Lancet_, - Nov. 16, 1895), details the effects of kindred - bodies--methyl-nitrate, glycol-dinitrate, erythrol-tetranitrate. The - first two, in common with nitro-glycerine, are stable only when cool - and in the dark--sunlight or warmth decomposes them, and they explode - by rapid heating or percussion. The fact which concerns us here is - that the least stable--glycol-dinitrate--has the most powerful and - rapid physiological effect, which is proportionately transient. In - one minute the blood-pressure is reduced by one-fourth and in four - minutes by nearly two-thirds: an effect which is dissipated in a - quarter of an hour. So that this excessively unstable compound, - decomposing in the body in a very short time, produces within that - short time a vast amount of molecular change: acting, as it seems, - not through the nervous system, but directly on the blood-vessels. - - [12] This interpretation is said to be disproved by the fact that the - carbo-hydrate contained in muscle amounts to only about 1.5 of the - total solids. I do not see how this statement is to be reconciled - with the statement cited three pages back from Professor Michael - Foster, that the deposits of glycogen contained in the liver and in - the muscles may be compared to the deposits in a central bank and - branch banks. - - [13] Before leaving the topic let me remark that the doctrine of - metabolism is at present in its inchoate stage, and that the - prevailing conclusions should be held tentatively. As showing this - need an anomalous fact may be named. It was long held that gelatine - is of small value as food, and though it is now recognized as - valuable because serving the same purposes as fats and - carbo-hydrates, it is still held to be valueless for structural - purposes (save for some inactive tissue); and this estimate agrees - with the fact that it is a relatively stable nitrogenous compound, - and therefore unfit for those functions performed by unstable - nitrogenous compounds in the muscular and other tissues. But if this - is true, it seems a necessary implication that such substances as - hair, wool, feathers, and all dermal growths chemically akin to - gelatine, and even more stable, ought to be equally innutritive or - more innutritive. In that case, however, what are we to say of the - larva of the clothes-moth, which subsists exclusively on one or other - of these substances, and out of it forms all those unstable - nitrogenous compounds needful for carrying on its life and developing - its tissues? Or again, how are we to understand the nutrition of the - book-worm, which, in the time-stained leaves through which it - burrows, finds no proteid save that contained in the dried-up size, - which is a form of gelatine; or, once more, in what form is the - requisite amount of nitrogenous substance obtained by the - coleopterous larva which eats holes in wood a century old? - - [14] This chapter and the following two chapters originally appeared in - Part III of the original edition of the _Principles of Psychology_ - (1855): forming a preliminary which, though indispensable to the - argument there developed, was somewhat parenthetical. Having now to - deal with the general science of Biology before the more special one - of Psychology, it becomes possible to transfer these chapters to - their proper place. - - [15] See _Westminster Review_ for April, 1852.--Art. IV. "A Theory of - Population." See Appendix A. - - [16] This paragraph replaces a sentence that, in _The Principles of - Psychology_, referred to a preceding chapter on "Method;" in which - the mode of procedure here indicated was set forth as a mode to be - systematically pursued in the choice of hypotheses. This chapter on - Method is now included, along with other matter, in a volume entitled - _Various Fragments_. - - [17] Speaking of "the general idea of _life_" M. Comte says:--"Cette idée - suppose, en effet, non-seulement celle d'un être organisé de manière - à comporter l'état vital, mais aussi celle, non moins indispensable, - d'un certain ensemble d'influences extérieures propres à son - accomplissement. Une telle harmonie entre l'être vivant et le - _milieu_ correspondant, caractérise evidemment la condition - fondamentale de la vie." Commenting on de Blainville's definition of - life, which he adopts, he says:--"Cette lumineuse définition ne me - paraît laisser rien d'important à désirer, si ce n'est une indication - plus directe et plus explicite de ces deux conditions fondamentales - co-relatives, nécessairement inséparables de l'état vivant, un - _organisme_ déterminé et un _milieu_ convenable." It is strange that - M. Comte should have thus recognized the necessity of a harmony - between an organism and its environment, as a _condition_ essential - to life, and should not have seen that the continuous maintenance of - such inner actions as will counterbalance outer actions, - _constitutes_ life. - - [When the original edition was published Dr. J. H. Bridges wrote to - me saying that in the _Politique Positive_, Comte had developed his - conception further. On p. 413, denying "le prétendu antagonisme des - corps vivants envers leurs milieux inorganiques," he says "au lieu de - ce conflit, on a reconnu bientôt que cette relation nécessaire - constitue une condition fondamentale de la vie réelle, dont la notion - systématique consiste dans une intime conciliation permanente entre - la spontanéité intérieure et la fatalité extérieure." Still, this - "conciliation _permanente_" seems to be a "_condition_" to life; not - that varying adjustment of changes which life consists in - maintaining. In presence of an ambiguity, the interpretation which - agrees with his previous statement must be chosen.] - - [18] In further elucidation of this general doctrine, see _First - Principles_, § 25. - - [19] In ordinary speech Development is often used as synonymous with - Growth. It hence seems needful to say that Development as here and - hereafter used, means _increase of structure_ and not _increase of - bulk_. It may be added that the word Evolution, comprehending growth - as well as Development, is to be reserved for occasions when both are - implied. - - [20] This paragraph originally formed part of a review-article on - "Transcendental Physiology," published in 1857. - - [21] When, in 1863, the preceding chapter was written, it had not occurred - to me that there needed an accompanying chapter treating of - Structure. The gap left by that oversight I now fill up. In doing - this there have been included certain statements which are tacitly - presupposed in the last chapter, and there may also be some which - overlap statements in the next chapter. I have not thought it needful - so to alter adjacent chapters as to remove these slight defects: the - duplicated ideas will bear re-emphasizing. - - [22] In connexion with this matter I add here a statement made by Prof. - Foster which it is difficult to understand: "Indeed it has been - observed that a dormouse actually gained in weight during a - hybernating period; it discharged during this period neither urine - nor fæces, and the gain in weight was the excess of oxygen taken in - over the carbonic acid given out." (_Text-book of Physiology_, 6th - ed., Part II, page 859.) - - [23] In the account of James Mitchell, a boy born blind and deaf, given by - James Wardrop, F.R.S. (Edin. 1813), it is said that he acquired a - "preternatural acuteness of touch and smell." The deaf Dr. Kitto - described himself as having an extremely strong visual memory: he - retained "a clear impression or image of everything at which he ever - looked." - - [24] Here, as in sundry places throughout this chapter, the necessities of - the argument have obliged me to forestall myself, by assuming the - conclusion reached in a subsequent chapter, that modifications of - structure produced by modifications of function are transmitted to - offspring. - - [25] Whether the _Volvox_ is to be classed as animal or vegetal is a - matter of dispute; but its similarity to the blastula stage of many - animals warrants the claim of the zoologists. - - [26] While the proof was in my hands there was published in _Science - Progress_ an essay by Dr. T. G. Brodie on "The Phosphorus-containing - Substances of the Cell." In this essay it is pointed out that - "nucleic acid is particularly characterized by its instability.... In - the process of purification it is extremely liable to decompose, with - the result that it loses a considerable part of its phosphorus. In - the second place it is most easily split up in another manner in - which it loses a considerable part of its nitrogen.... To avoid the - latter source of error he [Miescher] found that it was necessary to - keep the temperature of all solutions down to 0°C., the whole time of - the preparation." These facts tend strongly to verify the hypothesis - that the nucleus is a source of perpetual molecular disturbance--not - a regulating centre but a stimulating centre. - - [27] The writing of the above section reminded me of certain allied views - which I ventured to suggest nearly 50 years ago. They are contained - in the _Westminster Review_ for April, 1852, in an article entitled - "A Theory of Population deduced from the General Law of Animal - Fertility." It is there suggested that the "spermatozoon is - essentially a neural element, and the ovum essentially a hæmal - element," or, as otherwise stated, that the "sperm-cell is - co-ordinating matter and the germ-cell matter to be co-ordinated" - (pp. 490-493). And along with this proposition there is given some - chemical evidence tending to support it. Now if, in place of "neural" - and "hæmal," we say--the element that is most highly phosphorized and - the element that is phosphorized in a much smaller degree; or if, in - place of co-ordinating matter and matter to be co-ordinated, we - say--the matter which initiates action and the matter which is made - to act; there is disclosed a kinship between this early view and the - view just set forth. In the last part of this work, "Laws of - Multiplication," which is developed from the essay referred to, I - left out the portion containing the quoted sentences, and the - evidence supporting the conclusion drawn. Partly I omitted them - because the speculation did not form an essential link in the general - argument, and partly because I did not see how the suggested - interpretation could hold of plants as well as of animals. If, - however, the alleged greater staining capacity of the male generative - nucleus in plants implies, as in other cases, that the male cell has - a larger proportion of the phosphorized matter than the other - elements concerned, then the difficulty disappears. - - As, along with the idea just named, the dropped portion of the - original essay contains other ideas which seem to me worth - preserving, I have thought it as well to reproduce it, in company - with the chief part of the general argument as at first sketched out. - It will be found in Appendix A to this volume. - - [28] Unfortunately the word _heterogenesis_ has been already used as a - synonym for "spontaneous generation." Save by those few who believe - in "spontaneous generation," however, little objection will be felt - to using the word in a sense that seems much more appropriate. The - meaning above given to it covers both Metagenesis and - Parthenogenesis. - - [29] Prof. Huxley avoids this difficulty by making every kind of Genesis a - mode of development. His classification, which suggested the one - given above, is as follows:-- - - { Growth - { Continuous { - { { Metamorphosis - { - Development { - { { Metagenesis - { { Agamogenesis { - { Discontinuous { { Parthenogenesis - { Gamogenesis - - [30] The implication is that an essentially similar process occurs in - those fragments of leaves used for artificial propagation. Besides - the Begonias in general, I learn that various other plants are thus - multiplied--Citron and orange trees, _Hoya carnosa_, _Aucuba - japonica_, _Clianthus puniceus_, etc., etc. _Bryophyllum calicinum_, - _Rochea falcata_, and _Echeveria_. I also learn that the following - plants, among others, produce buds from their foliage - leaves:--_Cardamine pratensis_, _Nasturtium officinale_, _Roripa - palustris_, _Brassica oleracea_, _Arabis pumila_, _Chelidonium - majus_, _Nymphæa guianensis_, _Episcia bicolor_, _Chirita sivensis_, - _Pinguicula Backeri_, _Allium_, _Gagea_, _Tolmia_, _Fritillaria_, - _Ornithogalum_, etc. In _Cardamine_ and several others, a complete - miniature plant is at once produced; in other cases bulbils or - similar detachable buds. - - [31] Among various examples I have observed, the most remarkable were - among Foxgloves, growing in great numbers and of large size, in a - wood between Whatstandwell Bridge and Crich, in Derbyshire. In one - case the lowest flower on the stem contained, in place of a pistil, a - shoot or spike of flower-buds, similar in structure to the - embryo-buds of the main spike. I counted seventeen buds on it; of - which the first had three stamens, but was otherwise normal; the - second had three; the third, four; the fourth, four; &c. Another - plant, having more varied monstrosities, evinced excess of nutrition - with equal clearness. The following are the notes I took of its - structure:--1st, or lowest flower on the stem, very large; calyx - containing eight divisions, one partly transformed into a corolla, - and another transformed into a small bud with bract (this bud - consisted of a five-cleft calyx, four sessile anthers, a pistil, and - a rudimentary corolla); the corolla of the main flower, which was - complete, contained six stamens, three of them bearing anthers, two - others being flattened and coloured, and one rudimentary; there was - no pistil but, _in place of it_, a large bud, consisting of a - three-cleft calyx of which two divisions were tinted at the ends, an - imperfect corolla marked internally with the usual purple spots and - hairs, three anthers sessile on this mal-formed corolla, a pistil, a - seed vessel with ovules, and, growing to it, another bud of which the - structure was indistinct. 2nd flower, large; calyx of seven - divisions, one being transformed into a bud with bract, but much - smaller than the other; corolla large but cleft along the top; six - stamens with anthers, pistil, and seed-vessel. 3rd flower, large; - six-cleft calyx, cleft corolla, with six stamens, pistil, and - seed-vessel, with a second pistil half unfolded at its apex. 4th - flower, large; divided along the top, six stamens. 5th flower, large; - corolla divided into three parts, six stamens. 6th flower, large; - corolla cleft, calyx six cleft, the rest of the flower normal. 7th, - and all succeeding flowers, normal. - - While this chapter is under revision, another noteworthy illustration - has been furnished to me by a wall-trained pear tree which was - covered in the spring by luxuriant "foreright" shoots. As I learned - from the gardener, it was pruned just as the fruit was setting. A - large excess of sap was thus thrown into other branches, with the - result that in a number of them the young pears were made monstrous - by reversion. In some cases, instead of the dried up sepals at the - top of the pear, there were produced good sized leaves; and in other - cases the seed-bearing core of the pear was transformed into a growth - which protruded through the top of the pear in the shape of a new - shoot. - - [32] In partial verification, Mr. Tansley writes:--"Prof. Klebs of Basel - has shown that in _Hydrodictyon_, gametes can only be produced by the - cells of a net when these are above a certain size and age; and then - only under conditions unfavourable to growth, such as a feeble light - or poverty of nutritive inorganic salts or absence of oxygen, or a - low temperature in the water containing the plant. The presence of - organic substances, especially sugar, also acts as a stimulus to the - formation of gametes, and this is also the case in _Vaucheria_. Many - other _Algæ_ produce gametes mainly at the end of the vegetative - season, when food is certainly difficult to obtain in their natural - habitat, and we may well suppose that their assimilative power is - waning. Where, however, as is the case in _Vaucheria_, the plant - depends for propagation mainly on the production of fertilized eggs, - we find the sexual organs often produced in conditions very - favourable to vegetative growth, in opposition to those cases such as - _Hydrodictyon_, where the chief means of propagation is by zoospores. - So that side by side with, and to some extent obscuring, the - principle developed above we have a clear adaptation of the - production of reproductive cells to the special circumstances of the - case." - - [33] This establishment by survival of the fittest of reproductive - processes adapted to variable conditions, is indirectly elucidated by - the habits of salmon. As salmon thrive in the sea and fall out of - condition in fresh water (having during their sea-life not exercised - the art of catching fresh-water prey), the implication is that the - species would profit if all individuals ran up the rivers just before - spawning time in November. Why then do most of them run up during - many preceding months? Contemplation of the difficulties which lie in - the way to the spawning grounds, will, I think, suggest an - explanation. There are falls to be leaped and shallow rapids to be - ascended. These obstacles cannot be surmounted when the river is low. - A fish which starts early in the season has more chances of getting - up the falls and the rapids than one which starts later; and, out of - condition as it will be, may spawn, though not well. On the other - hand, one which starts in October, if floods occur appropriately, may - reach the upper waters and then spawn to great advantage; but in the - absence of adequate rains it may fail altogether to reach the - spawning grounds. Hence the species profits by an irregularity of - habits adapted to meet irregular contingencies. - - [34] I owe to Mr. (now Sir John) Lubbock an important confirmation of this - view. After stating his belief that between Crustaceans and Insects - there exists a physiological relation analogous to that which exists - between water vertebrata and land-vertebrata, he pointed out to me - that while among Insects there is a definite limit of growth, and an - accompanying definite commencement of reproduction, among - Crustaceans, where growth has no definite limit, there is no definite - relation between the commencement of reproduction and the decrease or - arrest of growth. - - [35] While this chapter is passing through the press, I learn from Mr. - White Cooper, that not only are near sight, long sight, dull sight, - and squinting, hereditary; but that a peculiarity of vision confined - to one eye is frequently transmitted: re-appearing in the same eye in - offspring. - - [36] An instance here occurs of the way in which those who are averse to a - conclusion will assign the most flimsy reasons for rejecting it. - Rather than admit that the eyes of these creatures living in darkness - have disappeared from lack of use, some contend that such creatures - would be liable to have their eyes injured by collisions with - objects, and that therefore natural selection would favour those - individuals in which the eyes had somewhat diminished and were least - liable to injury: the implication being that the immunity from the - inflammations due to injuries would be so important a factor in life - as to cause survival. And this is argued in presence of the fact that - one of the most conspicuous among these blind cave-animals is a - cray-fish, and that the cray-fish in its natural habitat is in the - habit of burrowing in the banks of rivers holes a foot or more deep, - and has its eyes exposed to all those possible blows and frictions - which the burrowing involves! - - [37] In addition to the numerous illustrations given by Mr. Sedgwick, here - is one which Colonel A. T. Fraser published in _Nature_ for Nov. 9, - 1893, concerning two Hindoo dwarfs:--"In speech and intelligence the - dwarfs were indistinguishable from ordinary natives of India. From an - interrogation of one of them, it appeared that he belonged to a - family all the male members of which have been dwarfs for several - generations. They marry ordinary native girls, and the female - children grow up like those of other people. The males, however, - though they develop at the normal rate until they reach the age of - six, then cease to grow, and become dwarfs." - - [38] This remarkable case appears to militate against the conclusion, - drawn a few pages back, that the increase of a peculiarity by - coincidence of "spontaneous variations" in successive generations, is - very improbable; and that the special superiorities of musical - composers cannot have thus arisen. The reply is that the extreme - frequency of the occurrence among so narrow a class as that of - musical composers, forbids the interpretation thus suggested. - - [39] I omitted to name here a cause which may be still more potent in - producing irregularity in the results of cousin-marriages. So far as - I can learn, no attempt has been made to distinguish between such - results as arise when the related parents from whom the cousins - descend are of the same sex and those which arise when they are of - different sexes. In the one case two sisters have children who - intermarry; and in the other case a brother and a sister have - children who intermarry. The marriages of cousins in these two cases - may be quite dissimilar in their results. If there is a tendency to - limitation of heredity by sex--if daughters usually inherit more from - the mother than sons do, while sons inherit more from the father than - from the mother, then two sisters will on the average of cases be - more alike in constitution than a sister and a brother. Consequently - the descendants of two sisters will differ less in their - constitutions than the descendants of a brother and a sister; and - marriage in the first case will be more likely to prove injurious - from absence of dissimilarity in the physiological units than - marriage in the second. My own small circle of friends furnishes - evidence tending to verify this conclusion. In one instance two - cousins who intermarried are children of two sisters, and they have - no offspring. In another the cousins who intermarried are children of - two brothers, and they have no offspring. In the third case the - cousins were descendants of two brothers and only one child resulted. - - [40] _A propos_ of this sentence one of my critics writes:--"I cannot find - in this book the statement as first made that the 'life of an - individual is maintained by the unequal and ever-varying actions of - incident forces on its different parts.' Recent physiological work - offers a startling example of the statement." - - To the question contained in the first sentence the answer is that I - have not made the statement in the above words, but that it is - implied in the chapter entitled "The Degree of Life varies as the - Degree of Correspondence," and more especially in § 36, which, - towards its close, definitely involves the statement. The verifying - evidence my critic gives me is this:-- - - "Prof. Sherrington has shown that if the sensory roots of the spinal - nerves are cut one by one there is at first no general effect - produced. That is to say, the remainder of the nervous system - continues to function as before. This condition (lack of general - effect) persists until about six pairs have been cut. With the - severance of the seventh pair, however, the whole central nervous - system ceases to function, so that stimulation of intact sensory - nerves produces no reflex action. After a variable period, but one of - many hours duration, the power of functioning is recovered. That is - to say, if the sensory impulses (from the skin, &c.) reaching the - central nervous system are rapidly reduced in amount, there comes a - point where those remaining do not suffice to keep the structure - 'awake.' After a time, however, it adjusts itself to work with the - diminished supply. Similarly Strumpell describes the case of a boy - 'whose sensory inlets were all paralyzed except one eye and one ear.' - When these were closed he instantly fell asleep." - - [41] Fifty years before the discovery of the Röntgen rays and those - habitually emanating from uranium, it had been observed by Moser that - under certain conditions the surfaces of metals receive permanent - impressions from appropriate objects placed upon them. Such facts - show that the molecules of substances propagate in all directions - special ethereal undulations determined by their special - constitutions. - - [42] This classification, and the three which follow it, I quote - (abridging some of them) from Prof. Agassiz's "Essay on - Classification." - - [43] For explanations, see "Illogical Geology," _Essays_, Vol. I. How much - we may be misled by assuming that because the remains of creatures of - high types have not been found in early strata, such creatures did - not exist when those strata were formed, has recently (1897) been - shown by the discovery of a fossil Sea-cow in the lower Miocene of - Hesse-Darmstadt. The skeleton of this creature proves that it - differed from such Sirenian mammals as the existing Manatee only in - very small particulars: further dwindling of disused parts being an - evident cause. The same is true as regards, now, we consider that - since the beginning of Miocene days this aberrant type of mammal has - not much increased its divergence from the ordinary mammalian type; - if we then consider how long it must have taken for this large - aquatic mammal (some eight or ten feet long) to be derived by - modification from a land-mammal; and if then we contemplate the - probable length of the period required for the evolution of that - land-mammal out of a pre-mammalian type; we seem carried back in - thought to a time preceding any of our geologic records. We are shown - that the process of organic evolution has most likely been far slower - than is commonly supposed. - - [44] Since this passage was written, in 1863, there has come to light much - more striking evidence of change from a more generalized to a less - generalized type during geologic time. In a lecture delivered by him - in 1876, Prof. Huxley gave an account of the successive modifications - of skeletal structure in animals allied to the horse. Beginning with - the _Orohippus_ of the Eocene formation, which had four complete toes - on the front limb and three toes on the hind limb, he pointed out the - successive steps by which in the _Mesohippus_, _Miohippus_, - _Protohippus_, and _Pliohippus_, there was a gradual approach to the - existing horse. - - [45] Several of the arguments used in this chapter and in that which - follows it, formed parts of an essay on "The Development Hypothesis," - originally published in 1852. - - [46] _Studies from the Morphological Laboratory in the University of - Cambridge_, vol. vi, p. 84. - - [47] _Ibid._, p. 81. - - [48] _Studies from the Morphological Laboratory in the University of - Cambridge_, vol. vi, p. 89. - - [49] Early in our friendship (about 1855) Prof. Huxley expressed to me his - conviction that all the higher articulate animals have twenty - segments or somites. That he adhered to this view in 1880, when his - work on _The Crayfish_ was published, is shown by his analysis there - given of the twenty segments existing in this fluviatile crustacean; - and adhesion to it had been previously shown in 1877, when his work - on _The Anatomy of Invertebrated Animals_ was published. On p. 398 of - that work he writes:--"In the abdomen there are, at most, eleven - somites, none of which, in the adult, bear ambulatory limbs. Thus, - assuming the existence of six somites in the head, the normal number - of somites in the body of insects will be twenty, as in the higher - _Crustacea_ and _Arachnida_." To this passage, however, he puts the - note:--"It is open to question whether the podical plates represent a - somite; and therefore it must be recollected that the total number of - somites, the existence of which can be actually demonstrated in - insects, is only seventeen, viz., four for the head, three for the - thorax, and ten for the abdomen." I have changed the number twenty, - which in the original edition occurred in the text, to the number - seventeen in deference to suggestions made to me; though I find in - Dr. Sharp's careful and elaborate work on the _Insecta_, that - Viallanes and Cholodkovsky agree with Huxley in believing that there - are six somites in the insect-head. The existence of a doubt on this - point, however, does not essentially affect the argument, since there - is agreement among morphologists respecting the _constancy_ of the - total number of somites in insects. - - [50] To avoid circumlocution I let these words stand, though they are not - truly descriptive; for the prosperity of imported species is largely, - if not mainly, caused by the absence of those natural enemies which - kept them down at home. - - [51] While these pages are passing through the press (in 1864), Dr. Hooker - has obliged me by pointing out that "plants afford many excellent - examples" of analogous transitions. He says that among true "water - plants," there are found, in the same species, varieties which have - some leaves submerged and some floating; other varieties in which - they are all floating; and other varieties in which they are all - submerged. Further, that many plants characterized by floating - leaves, and which have all their leaves floating when they grow in - deeper water, are found with partly aerial leaves when they grow in - shallower water; and that elsewhere they occur in almost dry soil - with all their leaves aerial. - - [52] It will be seen that the argument naturally leads up to this - expression--Survival of the Fittest--which was here used for the - first time. Two years later (July, 1866) Mr. A. R. Wallace wrote to - Mr. Darwin contending that it should be substituted for the - expression "Natural Selection." Mr. Darwin demurred to this proposal. - Among reasons for retaining his own expression he said that I had - myself, in many cases, preferred it--"continually using the words - Natural Selection." (_Life and Letters_, &c., vol. III, pp. 45-6.) - Mr. Darwin was quite right in his statement, but not right in the - motive he ascribed to me. My reason for frequently using the phrase - "Natural Selection," after the date at which the phrase "Survival of - the Fittest" was first used above, was that disuse of Mr. Darwin's - phrase would have seemed like an endeavour to keep out of sight my - own indebtedness to him, and the indebtedness of the world at large. - The implied feeling has led me ever since to use the expressions - Natural Selection and Survival of the Fittest with something like - equal frequency. - - [53] I am indebted to Mr. [now Sir W.] Flower for the opportunity of - examining the many skulls in the Museum of the College of Surgeons - for verification of this. Unfortunately the absence, in most cases, - of some or many teeth, prevented me from arriving at that specific - result which would have been given by weighing a number of the under - jaws in each race. Simple inspection, however, disclosed a - sufficiently-conspicuous difference. The under jaws of Australians - and Negroes, when collated with those of Englishmen, were visibly - larger, not only relatively but absolutely. One Australian jaw only - seemed about of the same size as an average English jaw; and this - (probably the jaw of a woman), belonging as it did to a smaller - skull, bore a greater ratio to the whole body of which it formed - part, than did an English jaw of the same actual size. In all the - other cases, the under jaws of these inferior races (containing - larger teeth than our own) were _absolutely_ more massive than our - own--often exceeding them in all dimensions; and _relatively_ to - their smaller skeletons were much more massive. Let me add that the - Australian and Negro jaws are thus strongly contrasted, not with all - British jaws, but only with the jaws of the civilized British. An - ancient British skull in the collection possesses a jaw almost or - quite as massive as those of the Australian skulls. All this is in - harmony with the alleged relation between greater size of jaws and - greater action of jaws, involved by the habits of savages. - - [In 1891 Mr. F. Howard Collins carefully investigated this matter: - measuring ten Australian, ten Ancient British, and ten recent English - skulls in the College of Surgeons Museum. The result proved an - absolute difference of the kind above indicated, and a far greater - relative difference. To ascertain this last a common standard of - comparison was established--an equal size of skull in all the cases; - and then when the relative masses or cubic sizes of the jaws were - calculated, the result which came out was this:--Australian jaw, - 1948; Ancient British jaw, 1135; Recent English jaw, 1030. "Hence," - in the words of Mr. Collins, "the mass of the Recent English jaw is, - roughly speaking, half that of the Australian relatively to that of - the skull, and a ninth less than that of the Ancient British." He - adds verifying evidence from witnesses who have no hypothesis to - support--members of the Odontological Society. The Vice-President, - Mr. Mummery, remarks of the Australians that "the jaw-bones are - powerfully developed, and large in proportion to the cranium."] - - [54] As bearing on the question of the varieties of Man, let me here refer - to a paper on "The Origin of the Human Races" read before the - Anthropological Society, March 1st, 1864, by Mr. Alfred Wallace. In - this paper, Mr. Wallace shows that along with the attainment of that - intelligence implied by the use of implements, clothing, &c., there - arises a tendency for modifications of brain to take the place of - modifications of body: still, however, regarding the natural - selection of spontaneous variations as the cause of the - modifications. But if the foregoing arguments be valid, natural - selection here plays but the secondary part of furthering the - adaptations otherwise caused. It is true that, as Mr. Wallace argues, - and as I have myself briefly indicated (see _Westminster Review_, for - April, 1852, pp. 496-501), the natural selection of races leads to - the survival of the more cerebrally-developed, while the less - cerebrally-developed disappear. But though natural selection acts - freely in the struggle of one society with another; yet, among the - units of each society, its action is so interfered with that there - remains no adequate cause for the acquirement of mental superiority - by one race over another, except the inheritance of - functionally-produced modifications. - - [55] _Darwin and after Darwin_, Part II, p. 99. - - [56] _Essays upon Heredity_, vol. i, p. 90. - - [57] In a letter published by Dr. Romanes in _Nature_, for April 26, 1894, - he alleges three reasons why "as soon as selection is withdrawn from - an organ the _minus_ variations of that organ outnumber the _plus_ - variations." The first is that "the survival-mean must descend to the - birth-mean." The interpretation of this is that if the members of a - species are on the average born with an organ of the required size, - and if they are exposed to natural selection, then those in which the - organ is relatively small will some of them die, and consequently the - mean size of the organ at adult age will be greater than at birth. - Contrariwise, if the organ becomes useless and natural selection does - not operate on it, this difference between the birth-mean and the - survival-mean disappears. Now here, again, the _plus_ variations and - their effects are ignored. Supposing the organ to be useful, it is - tacitly assumed that while _minus_ variations are injurious, _plus_ - variations are not injurious. This is untrue. Superfluous size of an - organ implies several evils:--Its original cost is greater than - requisite, and other organs suffer; the continuous cost of its - nutrition is unduly great, involving further injury; it adds - needlessly to the weight carried and so again is detrimental; and - there is in some cases yet a further mischief--it is in the way. - Clearly, then, those in which _plus_ variations of the organ have - occurred are likely to be killed off as well as those in which - _minus_ variations have occurred; and hence there is no proof that - the survival-mean will exceed the birth-mean. Moreover the assumption - has a fatal implication. To say that the survival-mean of an organ is - greater than the birth-mean is to say that the organ is greater _in - proportion to other organs_ than it was at birth. What happens if - instead of one organ we consider all the organs? If the survival-mean - of a particular organ is greater than its birth-mean, the survival - mean of each other organ must also be greater. Thus the proposition - is that every organ has become larger in relation to every other - organ!--a marvellous proposition. I need only add that Dr. Romanes' - inferences with respect to the two other causes--atavism and failing - heredity--are similarly vitiated by ignoring the plus variations and - their effects. - - [58] _Westminster Review_, January, 1860. See also _Essays, &c._, vol. i, - p. 290. - - [59] "On Orthogenesis and the Impotence of Natural Selection in - Species-Formation," pp. 2, 19, 22, 24. - - [60] Address to Plymouth Institution, at opening of Session 1895-6. - - [61] _Westminster Review_, April, 1857. "Progress: its Law and Cause." See - also _Essays_, vol. i. - - [62] It may be needful to remark, that by the proposed expression it is - intended to define--not Life in its essence; but, Life as manifested - to us--not Life as a _noumenon_: but, Life as a _phenomenon_. The - ultimate mystery is as great as ever: seeing that there remains - unsolved the question--What _determines_ the co-ordination of - actions? - - [63] _Prin. of Phys._, 2nd edit., p. 77. - - [64] _Ibid._, 3rd edit., p 249. - - [65] _Ibid._, p. 124. - - [66] Agassiz and Gould, p. 274. - - [67] _Prin. of Phys._, 3rd edit., p. 964. - - [68] "Parthenogenesis," p. 8. - - [69] _Prin. of Phys._, p. 92. - - [70] _Ibid._, p. 93. - - [71] _Ibid._, p. 917. - - [72] "A General Outline of the Animal Kingdom." By Prof. T. R. Jones, F. - G. S., p. 61. - - [73] Carpenter. - - [74] _Prin. of Phys._, p. 873. - - [75] _Ibid._, p. 203. - - [76] _Ibid._, p. 209. - - [77] _Ibid._, p. 249. - - [78] _Ibid._, p. 249. - - [79] _Ibid._, p. 250. - - [80] _Prin. of Phys._, p. 256. - - [81] _Ibid._, p. 212. - - [82] _Ibid._, p. 266. - - [83] _Prin. of. Phys._, p. 267. - - [84] _Ibid._, p. 276. - - [85] _Ibid._, 2nd edit., p. 115. - - [86] _Prin. of Phys._, p. 954. - - [87] _Ibid._, p. 958. - - [88] _Ibid._, p. 688. - - [89] _Ibid._, p. 958. - - [90] "A General Outline of the Animal Kingdom." By Professor T. R. Jones, - p. 61. - - [91] _Prin. of Phys._, p. 907. - - [92] Should it be objected that in the higher plants the sperm-cell and - germ-cell differ, though no distinct co-ordinating system exists, it - is replied that there _is_ co-ordination of actions, though of a - feeble kind, and that there must be some agency by which this is - carried on. - - [93] It is a significant fact that amongst the dioecious invertebrata, - where the nutritive system greatly exceeds the other systems in - development, the female is commonly the largest, and often greatly - so. In some of the Rotifera the male has no nutritive system at all. - See _Prin. of Phys._, p. 954. - - [94] _Prin. of Phys._, p. 908. - - [95] "Parthenogenesis," pp. 66, 67. - - [96] "Lectures on Animal Chemistry." By Dr. Bence Jones. _Medical - Times_, Sept. 13th, 1851. See also _Prin. of Phys._, p. 171. - - [97] _Cyclopædia of Anatomy and Physiology_, Vol. IV, p. 506. - - [98] From a remark of Drs. Wagner and Leuckart this chemical evidence - seems to have already suggested the idea that the sperm-cell becomes - "metamorphosed into the central parts of the nervous system." But - though they reject this assumption, and though the experiments of Mr. - Newport clearly render it untenable, yet none of the facts latterly - brought to light conflict with the hypothesis that the sperm-cell - contains unorganized co-ordinating matter. - - [99] Quain's _Elements of Anatomy_, p. 672. - -[100] The maximum weight of the horse's brain is 1 lb. 7 ozs.; the human - brain weighs 3 lbs., and occasionally as much as 4 lbs.; the brain of - a whale, 75 feet long, weighed 5 lbs. 5 ozs.; and the elephant's - brain reaches from 8 lbs. to 10 lbs. Of the whale's fertility we know - nothing; but the elephant's quite agrees with the hypothesis. The - elephant does not attain its full size until it is thirty years old, - from which we may infer that it arrives at a reproductive age later - than man does; its period of gestation is two years, and it produces - one at a birth. Evidently, therefore, it is much less prolific than - man. See Müller's _Physiology_ (Baly's translation), p. 815, and - Quain's _Elements of Anatomy_, p. 671. - -[101] That the size of the nervous system is the measure of the ability to - maintain life, is a proposition that must, however, be taken with - some qualifications. The ratio between the amounts of gray and white - matter present in each case is probably a circumstance of moment. - Moreover, the temperature of the blood may have a modifying - influence; seeing that small nervous centres exposed to rapid - oxidation will be equivalent to larger ones more slowly oxidized. - Indeed, we see amongst mankind, that though, in the main, size of - brain determines mental power, yet temperament exercises some - control. There is reason to think, too, that certain kinds of nervous - action involve greater consumption of nervous tissue than others; and - this will somewhat complicate the comparisons. Nevertheless, these - admissions do not affect the generalization as a whole, but merely - prepare us to meet with minor irregularities. - -[102] Let me here note in passing a highly significant implication. The - development of nervous structures which in such cases take place, - cannot be limited to the finger-ends. If we figure to ourselves the - separate sensitive areas which severally yield independent feelings, - as constituting a network (not, indeed, a network sharply marked out, - but probably one such that the ultimate fibrils in each area intrude - more or less into adjacent areas, so that the separations are - indefinite), it is manifest that when, with exercise, the structure - has become further elaborated, and the meshes of the network smaller, - there must be a multiplication of fibres communicating with the - central nervous system. If two adjacent areas were supplied by - branches of one fibre, the touching of either would yield to - consciousness the same sensation: there could be no discrimination - between points touching the two. That there may be discrimination, - there must be a distinct connection between each area and the tract - of grey matter which receives the impressions. Nay more, there must - be, in this central recipient-tract, an added number of the separate - elements which, by their excitements, yield separate feelings. So - that this increased power of tactual discrimination implies a - peripheral development, a multiplication of fibres in the - trunk-nerve, and a complication of the nerve-centre. It can scarcely - be doubted that analogous changes occur under analogous conditions - throughout all parts of the nervous system--not in its sensory - appliances only, but in all its higher co-ordinating appliances, up - to the highest. - -[103] _Essays upon Heredity_, p. 87. - -[104] _Les Maladies des Vers à soie_, par L. Pasteur, Vol. I, p. 39. - -[105] Curiously enough, Weismann refers to, and recognizes, syphilitic - infection of the reproductive cells. Dealing with Brown-Séquard's - cases of inherited epilepsy (concerning which, let me say, that I do - not commit myself to any derived conclusions), he says:--"In the case - of epilepsy, at any rate, it is easy to imagine [many of Weismann's - arguments are based on things 'it is easy to imagine'] that the - passage of some specific organism through the reproductive cells may - take place, as in the case of syphilis" (p. 82). Here is a sample of - his reasoning. It is well known that epilepsy is frequently caused by - some peripheral irritation (even by the lodging of a small foreign - body under the skin), and that, among peripheral irritations causing - it, imperfect healing is one. Yet though, in Brown-Séquard's cases, a - peripheral irritation caused in the parent by local injury was the - apparent origin, Weismann chooses gratuitously to assume that the - progeny were infected by "some specific organism," which produced the - epilepsy! And then though the epileptic virus, like the syphilitic - virus, makes itself at home in the egg, the parental protoplasm is - not admitted! - -[106] _Philosophical Transactions of the Royal Society for the Year 1821_, - Part I, pp. 20-24. - -[107] It will, I suppose, be said that the non-inheritance of mutilations - constitutes evidence of the kind here asked for. The first reply is - that the evidence is conflicting, as it may well be. It is forgotten - that to have valid evidence of non-inheritance of mutilations, it is - requisite that both parents shall have undergone mutilation, and that - this does not often happen. If they have not, then, assuming the - inheritableness of mutilations, there would, leaving out other - causes, be an equal tendency to appearance and non-appearance of the - mutilation in offspring. But there is another cause--the tendency to - reversion, which ever works in the direction of cancelling individual - characters by the return to ancestral characters. So that even were - the inheritance of mutilations to be expected (and for myself I may - say that its occurrence surprises me), it could not be reasonably - looked for as more than exceptional: there are two strong - countervailing tendencies. But now, in the second place, let it be - remarked that the inheritance or non-inheritance of mutilations is - beside the question. The question is whether modifications of parts - produced by modifications of functions are inheritable or not. And - then, by way of disproof of their inheritableness, we are referred to - cases in which the modifications of parts are not produced by - modifications of functions, but are otherwise produced! - -[108] See _First Principles_, Part II, Chap. XXII, "Equilibration." - -[109] _Principles of Biology_, § 46, (No. 8. April, 1863). - -[110] _Ibid._ This must not be understood as implying that while the mass - increases as the cubes, the _quantity of motion_ which can be - generated increases only as the squares; for this would not be true. - The quantity of motion is obviously measured, not by the sectional - areas of the muscles alone, but by these multiplied into their - lengths, and therefore increases as the cubes. But this admission - leaves untouched the conclusion that the ability to _bear stress_ - increases only as the squares; and thus limits the ability to - generate motion, by relative incoherence of materials. - -[111] _The Transactions of the Linnæan Society of London_, Vol. XXII, p. - 215. The estimate of Reaumur, cited by Kirby and Spence, is still - higher--"in five generations one Aphis may be the progenitor of - 5,904,900,000 descendants; and that it is supposed that in one year - there may be twenty generations." (_Introduction to Entomology_, Vol. - I, p. 175) - -[112] _A Manual of the Anatomy of Invertebrated Animals_, by T. H. Huxley, - p. 206. - -[113] Respecting the _Eloidea_ I learn that in 1879--thirty years after it - had become a pest--one solitary male plant was found in a pond near - Edinburgh; but "in an exhaustive inquiry on the plant made by Dr. - Groenland, of Copenhagen, he could find no trace of any male - specimens having been found in Europe other than the Scotch." In - waters from which the _Eloidea_ has disappeared, it seems to have - done so in consequence of the growth of an _Alga_, which has produced - turbid water unfavourable to it. That is to say, the decreased - multiplication of somatic cells in some cases, is not due to any - exhaustion, but is caused by the rise of enemies or adverse - conditions; as happens generally with introduced species of plants - and animals which multiply at first enormously, and then, without any - loss of reproductive power, begin to decrease under the antagonizing - influences which grow up. - -[114] _A Text Book of Human Physiology._ By Austin Flint, M.D., LL.D. - Fourth edition. New York: D. Appleton & Co. 1888. Page 797. - -[115] This supposition I find verified by Mr. A. S. Packard in his - elaborate monograph on "The Cave Fauna of North America, &c.," as - also in his article published in the _American Naturalist_, - September, 1888; for he there mentions "variations in _Pseudotremia - cavernarum_ and _Tomocerus plumbeus_, found living near the entrance - to caves in partial daylight." The facts, as accumulated by Mr. - Packard, furnished a much more complete answer to Prof. Lankester - than is above given, as, for example, the "blindness of _Neotoma_, or - the Wood-Rat of Mammoth Cave." It seems that there are also "cave - beetles, with or without rudimentary eyes," and "eyeless spiders" and - Myriapods. And there are insects, as some "species of Anophthalmus - and Adelops, whose larvæ are lacking in all traces of eyes and optic - nerves and lobes." These instances cannot be explained as sequences - of an inrush of water carrying with it the remote ancestors, some of - which did not find their way out; nor can others of them be explained - by supposing an inrush of air, which did the like. - -[116] See "Social Organism" in _Westminster Review_ for January, 1860; also - _Principles of Sociology_, § 247. - -[117] _Contemporary Review_, September, 1893. - -[118] _Evolution of Sex_, p. 50. - -[119] _Souvenirs Entomologiques_, 3^{me} Série, p. 328. - -[120] _Natural History of Bees_, new ed., p. 33. - -[121] _Origin of Species_, 6th ed., p. 232. - -[122] _Contemporary Review_, September, 1893, p. 333. - -[123] _The Entomologist's Monthly Magazine_, March, 1892, p. 61. - -[124] Perhaps it will be alleged that nerve-matter is costly, and that this - minute economy might be of importance. Anyone who thinks this will no - longer think it after contemplating a litter of half-a-dozen young - rabbits (in the wild rabbit the number varies from four to eight); - and on remembering that the nerve-matter contained in their brains - and spinal cords, as well as the materials for building up the bones, - muscles, and viscera of their bodies, has been supplied by the doe in - the space of a month; at the same time that she has sustained herself - and carried on her activities: all this being done on relatively poor - food. Nerve-matter cannot be so very costly then. - -[125] _Loc. cit._, p. 318. - -[126] _The Germ Plasm_, p. 54. - -[127] While Professor Weismann has not dealt with my argument derived from - the distribution of discriminativeness on the skin, it has been - criticized by Mr. McKeen Cattell, in the last number of _Mind_ - (October, 1893). His general argument, vitiated by extreme - misconceptions, I need not deal with. He says:--"Whether changes - acquired by the individual are hereditary, and if so to what extent, - is a question of great interest for ethics no less than for biology. - But Mr. Spencer's application of this doctrine to account for the - origin of species [!] simply begs the question. He assumes useful - variations [!]--whether of structure or habit is immaterial--without - attempting to explain their origin": two absolute misstatements in - two sentences! The only part of Mr. Cattell's criticism requiring - reply is that which concerns the "sensation-areas" on the skin. He - implies that since Weber, experimental psychologists have practically - set aside the theory of sensation areas: showing, among other things, - that relatively great accuracy of discrimination can be quickly - acquired by "increased interest and attention.... Practice for a few - minutes will double the accuracy of discrimination, and practice on - one side of the body is carried over to the other." To me it seems - manifest that "increased interest and attention" will not enable a - patient to discriminate two points where a few minutes before he - could perceive only one. That which he can really do in this short - time is to learn to discriminate between the _massiveness of a - sensation_ produced by two points and the massiveness of that - produced by one, and to _infer_ one point or two points accordingly. - Respecting the existence of sensation-areas marked off from one - another, I may, in the first place, remark that since the eye - originates as a dermal sac, and since its retina is a highly - developed part of the sensitive surface at large, and since the - discriminative power of the retina depends on the division of it into - numerous rods and cones, each of which gives a separate - sensation-area, it would be strange were the discriminative power of - the skin at large achieved by mechanism fundamentally different. In - the second place I may remark that if Mr. Cattell will refer to - Professor Gustav Retzius's _Biologische Untersuchungen_, New Series, - vol. iv (Stockholm, 1892), he will see elaborate diagrams of - superficial nerve-endings in various animals showing many degrees of - separateness. I guarded myself against being supposed to think that - the sensation-areas are sharply marked off from one another; and - suggested, contrariwise, that probably the branching - nerve-terminations intruded among the branches of adjacent - nerve-terminations. Here let me add that the intrusion may vary - greatly in extent; and that where the intruding fibres run far among - those of adjacent areas, the discriminativeness will be but small, - while it will be great in proportion as each set of branching fibres - is restricted more nearly to its own area. All the facts are - explicable on this supposition. - -[128] To save space and exclude needless complication I have omitted these - passages from the preceding divisions of this appendix. - -[129] Though Professor Weismann does not take up the challenge, Dr. Romanes - does. He says:--"When selection is withdrawn there will be no - excessive _plus_ variations, because so long as selection was present - the efficiency of the organ was maintained at its highest level: it - was only the _minus_ variations which were then eliminated" - (_Contemporary Review_, p. 611). In the first place, it seems to me - that the phrases used in this sentence beg the question. It says that - "the efficiency of the organ was maintained at its _highest_ level"; - which implies that the highest level (tacitly identified with the - greatest size) is the best and that the tendency is to fall below it. - This is the very thing I ask proof of. Suppose I invert the idea and - say that the organ is maintained at its right size by natural - selection, because this prevents increase beyond the size which is - best for the organism. Every organ should be in due proportion, and - the welfare of the creature as a whole is interfered with by excess - as well as by defect. It may be directly interfered with--as for - instance by too big an eyelid; and it may be indirectly interfered - with, where the organ is large, by needless weight and cost of - nutrition. In the second place the question which here concerns us is - not what natural selection will do with variations. We are concerned - with the previous question--What variations will arise? An organ - varies in all ways; and, unless reason to the contrary is shown, the - assumption must be that variations in the direction of increase are - as frequent and as great as those in the direction of decrease. Take - the case of the tongue. Certainly there are tongues inconveniently - large, and probably tongues inconveniently small. What reason have we - for assuming that the inconveniently small tongues occur more - frequently than the inconveniently large ones? None that I can see. - Dr. Romanes has not shown that when natural selection ceases to act - on an organ the _minus_ variations in each new generation will exceed - the _plus_ variations. But if they are equal the alleged process of - panmixia has no place. - -[130] _The Variation of Animals and Plants under Domestication_, vol. ii, - p. 292. - -[131] _Journal of the Anthropological Institute_ for 1885, p. 253. - -[132] In "The All-Sufficiency of Natural Selection" (_Contemporary Review_, - Sept., 1893, p. 311), Professor Weismann writes:--"I have ever - contended that the acceptance of a principle of explanation is - justified, if it can be shown that without it certain facts are - inexplicable." Unless, then, Prof. Weismann can show that the - distribution of discriminativeness is otherwise explicable, he is - bound to accept the explanation I have given, and admit the - inheritance of acquired characters. - -[133] Prof. Weismann is unaware that the view here ascribed to Roux, - writing in 1881, is of far earlier date. In the _Westminster Review_ - for January, 1860, in an essay on "The Social Organism," I - wrote:--"One more parallelism to be here noted, is that the different - parts of a social organism, like the different parts of an individual - organism, compete for nutriment; and severally obtain more or less of - it according as they are discharging more or less duty." (See also - _Essays_, i, 290.) And then, in 1876, in _The Principles of - Sociology_, vol. i, § 247, I amplified the statement thus:--"All - other organs, therefore, jointly and individually, compete for blood - with each organ ... local tissue-formation (which under normal - conditions measures the waste of tissue in discharging function) is - itself a cause of increased supply of materials ... the resulting - competition, not between units simply, but between organs, causes in - a society, as in a living body, high nutrition and growth of parts - called into greatest activity by the requirements of the rest." - Though I did not use the imposing phrase - "intra-individual-selection," the process described is the same. - -[134] _Proceedings of the Biological Society of Washington_, vol. ix. - -[135] Romanes Lecture, p. 29. - -[136] _Ibid._, p. 35. - -[137] This interpretation harmonizes with a fact which I learn from Prof. - Riley, that there are gradations in this development, and that in - some species the ordinary neuters swell their abdomens so greatly - with food that they can hardly get home. - - - - - - - -End of the Project Gutenberg EBook of The Principles of Biology, Volume 1 -(of 2), by Herbert Spencer - -*** END OF THIS PROJECT GUTENBERG EBOOK PRINCIPLES OF BIOLOGY, VOL 1 *** - -***** This file should be named 54612-8.txt or 54612-8.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/5/4/6/1/54612/ - -Produced by Keith Edkins, MFR, Adrian Mastronardi and the -Online Distributed Proofreading Team at http://www.pgdp.net -(This file was produced from images generously made -available by The Internet Archive/American Libraries.) - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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