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diff --git a/old/63540-0.txt b/old/63540-0.txt deleted file mode 100644 index 4b85476..0000000 --- a/old/63540-0.txt +++ /dev/null @@ -1,16695 +0,0 @@ -The Project Gutenberg eBook, Evolution and Adaptation, by Thomas Hunt -Morgan - - -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: Evolution and Adaptation - - -Author: Thomas Hunt Morgan - - - -Release Date: October 24, 2020 [eBook #63540] - -Language: English - -Character set encoding: UTF-8 - - -***START OF THE PROJECT GUTENBERG EBOOK EVOLUTION AND ADAPTATION*** - - -E-text prepared by Turgut Dincer, Barry Abrahamsen, and the Online -Distributed Proofreading Team (http://www.pgdp.net) from page images -generously made available by Internet Archive (https://archive.org) - - - -Note: Project Gutenberg also has an HTML version of this - file which includes the original illustrations. - See 63540-h.htm or 63540-h.zip: - (http://www.gutenberg.org/files/63540/63540-h/63540-h.htm) - or - (http://www.gutenberg.org/files/63540/63540-h.zip) - - - Images of the original pages are available through - Internet Archive. See - https://archive.org/details/evolutionadaptat00morg - - -Transcriber’s note: - - Text that was in italics is enclosed by underscores - (_italics_). - - Text that was in bold face is enclosed by by equal - signs (=bold=). - - A caret character is used to denote superscription. A - single character following the caret is superscripted - (example: J^1). - - - - - -EVOLUTION AND ADAPTATION - - ------------------------------------------------------------------------- - - -[Illustration: Publisher's logo (The Macmillan Company)] - - ------------------------------------------------------------------------- - - -EVOLUTION AND ADAPTATION - -by - -THOMAS HUNT MORGAN, Ph.D. - - - - - - -New York -The Macmillan Company -London: Macmillan & Co., Ltd. -1908 - -All rights reserved - -Copyright, 1903, -by The Macmillan Company. - -Set up and electrotyped. Published October, 1903. Reprinted January, -1908. - -Norwood Press -J. S. Cushing Co.—Berwick & Smith Co. -Norwood, Mass., U.S.A. - - ------------------------------------------------------------------------- - - - - - TO - - Professor William Keith Brooks - - - AS A TOKEN OF SINCERE ADMIRATION AND RESPECT - - - - ------------------------------------------------------------------------- - - - - - PREFACE - - -The adaptation of animals and plants to the conditions under which they -live has always excited the interest, and also the imagination, of -philosophers and scientists; for this relation between the organism and -its environment is one of the most characteristic features of living -things. The question at once suggests itself: How has such a relation -been brought about? Is it due to something inherent in the living matter -itself, or is it something that has been, as it were, superimposed upon -it? An example may make my meaning clearer. No one will suppose that -there is anything inherent in iron and other metals that would cause -them to produce an engine if left to themselves. The particular -arrangement of the pieces has been superimposed upon the metals, so that -they now fulfil a purpose, or use. Have the materials of which organisms -are composed been given a definite arrangement, so that they fulfil the -purpose of maintaining the existence of the organism; and if so, how has -this been accomplished? It is the object of the following pages to -discuss this question in all its bearings, and to give, as far as -possible, an idea of the present state of biological thought concerning -the problem. I trust that the reader will not be disappointed if he -finds in the sequel that many of the most fundamental questions in -regard to adaptation are still unsettled. - -In attempting to state the problem as clearly as possible, I fear that -it may appear that at times I have “taken sides,” when I should only -have been justified in stating the different aspects of the question. -But this will do little harm provided the issue has been sharply drawn. -Indeed, it seems to me that the only scientific value, that a discussion -of what the French call “les grands problèmes de la Biologie” has, is to -get a clearer understanding of the relation of what is known to what is -unknown or only surmised. - -In some quarters speculation concerning the origin of the adaptation of -living things is frowned upon, but I have failed to observe that the -critics themselves refrain entirely from theorizing. They shut one door -only to open another, which also leads out into the dark. To deny the -right to speculative thought would be to deny the right to use one of -the best tools of research. - -Yet it must be admitted that all speculation is not equally valuable. -The advance of science in the last hundred years has shown that the kind -of speculation that has real worth is that which leads the way to -further research and possible discovery. Speculation that leads to this -end must be recognized as legitimate. It becomes useless when it deals -with problems that cannot be put to the actual test of observation or -experiment. It is in this spirit that I have approached the topics -discussed in the following pages. - -The unsophisticated man believes that all other animals exist to -minister to his welfare; and from this point of view their adaptations -are thought of solely in their relation to himself. A step in advance -was taken when the idea was conceived that adaptations are for the good -of the organisms themselves. It seemed a further advance when the -conclusion was reached that the _origin_ of adaptations could be -accounted for, as the result of the benefit that they conferred on their -possessor. This view was the outcome of the acceptation of the theory of -evolution, combined with Darwin’s theory of natural selection. It is the -view held by most biologists at the present time; but I venture to -prophesy that if any one will undertake to question modern zoologists -and botanists concerning their relation to the Darwinian theory, he will -find that, while professing _in a general way_ to hold this theory, most -biologists have many reservations and doubts, which they either keep to -themselves or, at any rate, do not allow to interfere either with their -teaching of the Darwinian doctrine or with the applications that they -may make of it in their writings. The claim of the opponents of the -theory that Darwinism has become a dogma contains more truth than the -nominal followers of this school find pleasant to hear; but let us not, -therefore, too hastily conclude that Darwin’s theory is without value in -relation to one side of the problem of adaptation; for, while we can -profitably reject, as I believe, much of the theory of natural -selection, and more especially the idea that adaptations have arisen -because of their usefulness, yet the fact that living things must be -adapted more or less well to their environment in order to remain in -existence may, after all, account for the widespread _occurrence_ of -adaptation in animals and plants. It is this point of view that will be -developed in the following pages. - -I am fully aware of the danger in attempting to cover so wide a field as -that of “Evolution and Adaptation,” and I cannot hope to escape the -criticism that is certain to be directed against a specialist who -ventures nowadays beyond the immediate field of his own researches; yet, -in my own defence, I may state that the whole point of view underlying -the position here taken is the immediate outcome of my work on -regeneration. One of the general questions that I have always kept -before me in my study of regenerative phenomena is how such a useful -acquirement as the power to replace lost parts has arisen, and whether -the Darwinian hypothesis is adequate to explain the result. The -conclusion that I have reached is that the theory is entirely inadequate -to account for the _origin_ of the power to regenerate; and it seemed to -me, therefore, desirable to reëxamine the whole question of adaptation, -for might it not prove true here, also, that the theory of natural -selection was inapplicable? This was my starting-point. The results of -my examination are given in the following pages. - -I am deeply indebted to Professor G. H. Parker and to Professor E. G. -Conklin for advice and friendly criticism; and in connection with the -revision of the proof I am under many obligations to Professor Joseph W. -Warren and to Professor E. A. Andrews. Without their generous help I -should scarcely have ventured into a field so full of pitfalls. - - -Bryn Mawr, Penn., June 10, 1903. - - ------------------------------------------------------------------------- - - - - - CONTENTS - - - CHAPTER I - - PAGE - - The Problem of Adaptation 1 - - – Structural Adaptations 1 - - – Adaptations for the Good of the 19 - Species - - – Organs of Little Use to the 22 - Individual - - – Changes in the Organism that are 25 - of No Use to the Individual or to - the Race - - – Comparison with Inorganic 26 - Phenomena - - - CHAPTER II - - The Theory of Evolution 30 - - – Evidence in Favor of the 32 - Transmutation Theory - - – – Evidence from Classification 32 - and from Comparative Anatomy - - – – The Geological Evidence 39 - - – – Evidence from Direct 43 - Observation and Experiment - - – – Modern Criticism of the Theory 44 - of Evolution - - - CHAPTER III - - The Theory of Evolution (continued) 58 - – The Evidence from Embryology 58 - – – The Recapitulation Theory 58 - – Conclusions 84 - - - CHAPTER IV - - Darwin’s Theories of Artificial and 91 - of Natural Selection - - – The Principle of Selection 91 - - – Variation and Competition in 104 - Nature - - – The Theory of Natural Selection 116 - - - CHAPTER V - - The Theory of Natural Selection 129 - (continued) - - – Objections to the Theory of 129 - Natural Selection - - – Sterility between Species 147 - - – Weismann’s Germinal Selection 154 - - - CHAPTER VI - - Darwin’s Theory of Sexual Selection 167 - - – Sexual Selection 167 - - – General Criticism of the Theory 213 - of Sexual Selection - - - CHAPTER VII - - The Inheritance of Acquired 222 - Characters - - – Lamarck’s Theory 222 - - – Darwin’s Hypothesis of Pangenesis 233 - - – The Neo-Lamarckian School 240 - - - CHAPTER VIII - - Continuous and Discontinuous 261 - Variation and Heredity - - – Continuous Variation 261 - - – Heredity and Continuous Variation 270 - - – Discontinuous Variation 272 - - – Mendel’s Law 278 - - – The Mutation Theory of De Vries 287 - - – Conclusions 297 - - - CHAPTER IX - - Evolution as the Result of External 300 - and Internal Factors - - – The Effect of External Influences 300 - - – Responsive Changes in the 319 - Organism that adapt it to the New - Environment - - – Nägeli’s Perfecting Principle 325 - - - CHAPTER X - - The Origin of the Different Kinds 340 - of Adaptations - - – Form and Symmetry 340 - - – Mutual Adaptation of Colonial 350 - Forms - - – Degeneration 352 - - – Protective Coloration 357 - - – Sexual Dimorphism and Trimorphism 360 - - – Length of Life as an Adaptation 370 - - – Organs of Extreme Perfection 371 - - – Secondary Sexual Organs as 372 - Adaptations - - – Individual Adjustments as 375 - Adaptations - - – Color Changes as Individual 375 - Adaptations - - – Increase of Organs through Use 376 - and Decrease through Disuse - - – Reactions of the Organism to 377 - Poisons, etc. - - – Regeneration 379 - - - CHAPTER XI - - Tropisms and Instincts as 382 - Adaptations - - - CHAPTER XII - - Sex as an Adaptation 414 - - – The Different Kinds of Sexual 414 - Individuals - - – The Determination of Sex 422 - - – Sex as a Phenomenon of Adaptation 439 - - - CHAPTER XIII - - Summary and General Conclusions 452 - - - INDEX 465 - ------------------------------------------------------------------------- - - - - - EVOLUTION AND ADAPTATION - - - - - CHAPTER I - - THE PROBLEM OF ADAPTATION - - -Between an organism and its environment there takes place a constant -interchange of energy and of material. This is, in general, also true -for all bodies whether living or lifeless; but in the living organism -this relation is a peculiar one; first, because the plant or the animal -is so constructed that it is suited to a particular set of physical -conditions, and, second, because it may so respond to a change in the -outer world that it further adjusts itself to changing conditions, -_i.e._ the response may be of such a kind that it better insures the -existence of the individual, or of the race. The two ideas contained in -the foregoing statement cover, in a general way, what we mean by the -adaptation of living things. The following examples will serve to -illustrate some of the very diverse phenomena that are generally -included under this head. - - - Structural Adaptations - -The most striking cases of adaptations are those in which a special, in -the sense of an unusual, relation exists between the individual and its -surroundings. For example, the foreleg of the mole is admirably suited -for digging underground. A similar modification is found in an entirely -different group of the animal kingdom, namely, in the mole-cricket, in -which the first legs are also well suited for digging. By their use the -mole-cricket makes a burrow near the surface of the ground, similar to, -but of course much smaller than, that made by the mole. In both of these -cases the adaptation is the more obvious, because, while the leg of the -mole is formed on the same general plan as that of other vertebrates, -and the leg of the mole-cricket has the same fundamental structure as -that of other insects, yet in both cases the details of structure and -the general proportions have been so altered, that the leg is fitted for -entirely different purposes from that to which the legs of other -vertebrates and of other insects are put. The wing of the bat is another -excellent case of a special adaptation. It is a modified fore-limb -having a strong membrane stretched between the fingers, which are -greatly elongated. Here we find a structure, which in other mammals is -used as an organ for supporting the body, and for progression on the -ground, changed into one for flying in the air. - -The tails of mammals show a number of different adaptations. The tail is -prehensile in some of the monkeys; and not only can the monkey direct -its tail toward a branch in order to grasp it, but the tail can be -wrapped around the branch and hold on so firmly that the monkey can -swing freely, hanging by its tail alone. The animal has thus a sort of -fifth hand, one as it were in the middle line of the body, which can be -used as a hold-fast, while the fingered hands are put to other uses. In -the squirrels the bushy tail serves as a protection during the winter -for those parts of the body not so thickly covered by hair. The tail of -the horse is used to brush away the flies that settle on the hind parts -of the body. In other mammals, the dog, the cat, and the rat, for -example, the tail is of less obvious use, although the suggestion has -been made that it may serve as a sort of rudder when the animal is -running rapidly. In several other cases, as in the rabbit and in the -higher apes, the tail is very short, and is of no apparent use; and in -man it has completely disappeared. - -A peculiar case of adaptation is the so-called basket on the third pair -of legs of the worker honey-bee. A depression of the outer surface of -the tibia is arched over by stiff hairs. The pollen collected from the -stamens of flowers is stowed away in this receptacle by means of the -other pairs of legs. The structure is unique, and is not found in any -other insects except the bees. It is, moreover, present only in the -worker bees, and is absent in the queen and the males. - -The preceding cases, in which the adapted parts are used for the -ordinary purposes of life of the individual, are not essentially -different from the cases in which the organ is used to protect the -animal from its enemies. The bad taste of certain insects is supposed to -protect them from being eaten by birds. Cases like this of passive -protection grade off in turn into those in which, by some reflex or -voluntary act, the animal protects itself. The bad-smelling horns of the -caterpillar of the black swallow-tailed butterfly (_Papilio polyxenes_) -are thrust out when the animal is touched, and it is believed that they -serve to protect the caterpillar from attack. The fœtid secretion of the -glands of the skunk is believed to serve as a protection to the animal, -although the presence of the nauseous odor may lead finally to the -extermination of the skunk by man. The sting of bees and of wasps serves -to protect the individual from attack. The sting was originally an -ovipositor, and used in laying the eggs. It has, secondarily, been -changed into an organ of offence. - -The special instincts and reflex acts furnish a striking group of -adaptations. The building of the spider’s web is one of the most -remarkable cases of this kind. The construction of the web cannot be the -result of imitation, since, in many instances, the young are born in the -spring of the year following the death of the parents. Each species of -spider has its own type of web, and each web has as characteristic a -form as has the spider itself. It is also important to find that a -certain type of web may be characteristic of an entire family of -spiders. Since, in many cases, the web is the means of securing the -insects used for food, it fulfils a purpose necessary for the welfare of -the spider. - -The making of the nests by birds appears to be also in large part an -instinctive act; although some writers are inclined to think that memory -of the nest in which the young birds lived plays a part in their -actions, and imitation of the old birds at the time of nest-building -may, perhaps, also enter into the result. It has been stated that the -first nest built by young birds is less perfect than that built by older -birds, but this may be due to the bird’s learning something themselves -in building their nests, _i.e._ to the perfecting of the instinct in the -individual that makes use of it. In any case much remains that must be -purely instinctive. The construction of the comb by bees appears to be -largely, perhaps entirely, an instinctive act. That this is the case was -shown by isolating young workers as soon as they emerged from the cell, -and before they could have had any experience in seeing comb built. When -given some wax they set to work to make a comb, and made the -characteristic six-sided structures like those made by the bees in a -hive. The formation of so remarkable a structure as the comb is worthy -of admiration, for, with the greatest economy of material, a most -perfect storeroom for the preservation of the honey is secured. This -adaptation appears almost in the nature of foresight, for the store of -honey is used not only to feed the young, but may be drawn on by the -bees themselves in time of need. It is true that a comparison with other -kinds of bees makes it probable that the comb was first made for the -eggs and larvæ, and only later became used as a storehouse, but so far -as its form is concerned there is the same economy of constructive -materials in either case. - -The behavior of young birds, more especially those that take care of -themselves from the moment they leave the egg, furnishes a number of -cases of instincts that are protective. If, for example, a flock of -young pheasants is suddenly disturbed, the birds at once squat down on -the ground, and remain perfectly quiet until the danger is past. Their -resemblance to the ground is so perfect that they are almost invisible -so long as they remain quiet. If, instead of remaining still, they were -to attempt to run away when disturbed, they would be much more easily -seen. - -Certain solitary wasps (_Ammophila_) have the habit of stinging -caterpillars and spiders, and dragging them to their nests, where they -are stored away for the future use of the young that hatch from the eggs -laid by the wasp on the body of the prey. As a result of the sting which -the wasp administers to the caterpillar, the latter is paralyzed, and -cannot escape from the hole in which it is stored, where it serves as -food for the young wasp that emerges from the egg. It was originally -claimed by Forel that the wasp stings the caterpillar in such a way that -the central nervous system is always pierced, and many subsequent -naturalists have marvelled at the perfection of such a wonderful -instinct. But the recent results of the Peckhams have made it clear that -the act of the wasp is not carried out with the precision previously -supposed, although it is true that the wasp pierces the caterpillar on -the lower surface where the ventral chain of ganglia lies. The habit of -this wasp is not very dissimilar from that shown by many other kinds of -wasps that sting their captive in order to quiet it. We need not imagine -in this case that the act carries with it the consciousness that the -caterpillar, quieted in this way, will be unable to escape before the -young wasps have hatched. - -The resemblance in color of many animals to their natural backgrounds -has in recent years excited the interest and imagination of many -naturalists. The name of protective coloration has been given to this -group of phenomena. The following cases which have less the appearance -of purely imaginative writing may serve by way of illustration. A -striking example is that of the ptarmigan which has a pure white coat in -winter, and a brown coat in summer. The white winter plumage renders the -animal less conspicuous against the background of snow, while in summer -the plumage is said to closely resemble the lichen-covered ground on -which the bird rests. The snowy owl is a northern bird, whose color is -supposed to make it less conspicuous, and may serve either as a -protection against enemies, or may allow the owl to approach its prey -unseen. It should not pass unnoticed, however, that there are white -birds in other parts of the world, where their white color cannot be of -any use to them as a protection. The white cockatoos, for example, are -tropical birds, living amongst green foliage, where their color must -make them conspicuous, rather than the reverse. - -The polar bear is the only member of the family that is white, and while -this can scarcely be said to protect it from enemies, because it is -improbable that it has anything to fear from the other animals of the -ice-fields, yet it may be claimed that the color is an adaptation to -allow the animal to approach unseen its prey. - -In the desert many animals are sand-colored, as seen for instance in the -tawny color of the lion, the giraffe, the antelopes, and of many birds -that live on or near the ground. - -It has been pointed out that in the tropics and temperate zones there -are many greenish and yellowish birds whose colors harmonize with the -green and yellow of the trees amongst which they live; but on the other -hand we must not forget that in all climes there are numbers of birds -brilliantly colored, and many of these do not appear to be protected in -any special way. The tanagers, humming-birds, parrots, Chinese -pheasants, birds of paradise, etc., are extremely conspicuous, and so -far as we can see they must be much exposed on account of the color of -their plumage. Whether, therefore, we are justified in picking out -certain cases as examples of adaptation, because of an agreement in -color between the organism and its surroundings, and in neglecting all -others, is, as has been already said, a point to be further examined. - -Not only among mammals and birds have many cases of protective -coloration been described by writers dealing with this subject, but in -nearly every group of the animal kingdom similar cases have been -recognized. The green and brown color of lizards may protect them, the -green color of many frogs is supposed to conceal them as they sit -amongst the plants on the edge of a stream or pond. The gray-brown color -of the toad has been described as a resemblance to the dry ground, while -the brilliant green of several tree-frogs conceals them very effectively -amongst the leaves. Many fishes are brilliantly colored, and it has even -been suggested that those living amongst corals and sea-anemonies have -acquired their colors as a protection, but Darwin states that they -appeared to him very conspicuous even in their highly colored -environment. - -Amongst insects innumerable cases of adaptive coloration have been -described. In fact this is the favorite group for illustrating the -marvels of protective coloration. A few examples will here serve our -purpose. The oft-cited case of the butterfly _Kallima_ is, apparently, a -striking instance of protective resemblance. When at rest the wings are -held together over the back, as in nearly all butterflies, so that only -the under surface is exposed. This surface has an unquestionably close -resemblance to a brown leaf. It is said on no less authority than that -of Wallace that when this butterfly alights on a bush it is almost -impossible to distinguish between it and a dead leaf. The special point -in the resemblance to which attention is most often called is the -distinct line running obliquely across the wings which looks like the -midrib of a leaf. Whether the need of such a close resemblance to a leaf -is requisite for the life of this butterfly, we do not know, of course, -and so long as we do not have this information there is danger that the -case may prove too much, for, if it should turn out that this remarkable -case is accidental the view in regard to the resemblance may be -endangered. - -Amongst caterpillars there are many cases of remarkable resemblances in -color between the animal and its surroundings. The green color of many -of those forms that remain on the leaves of the food-plant during the -day will give, even to the most casual observer, the impression that the -color is for the purpose of concealment; and that it does serve to -conceal the animal there can be no doubt. But even from the point of -view of those who maintain that this color has been acquired because of -its protective value it must be admitted that the color is insufficient, -because some of these same green caterpillars are marvellously armed -with an array of spines which are also supposed to be a protection -against enemies. Equally well protected are the brown and mottled -geometrid caterpillars. These have, moreover, the striking and unusual -habit of fixing themselves by the posterior pairs of false legs, and -standing still and rigid in an oblique position on the twigs to which -they are affixed. So close is their resemblance to a short twig, that -even when their exact position is known it is very difficult to -distinguish them. - -Grasshoppers that alight on the ground are, in many cases, so similar to -the surface of the ground that unless their exact location is known they -easily escape attention, while the green color of the katydid, a member -of the same group of orthoptera, protects it from view in the green -foliage of the trees where it lives. The veinlike wings certainly -suggest a resemblance to a leaf, but whether there is any necessity for -so close an imitation may be questioned. - -There can be little doubt in some of these cases that the color of the -animal may be a protection to it, but as has been hinted already, it is -another question whether it acquired these colors because of their -usefulness. Nevertheless, if the color is useful to its possessor, it is -an adaptation in our sense of the word, without regard to the way in -which it has been acquired. Even, for instance, if the resemblance were -purely the outcome of chance in the sense that the color appeared -without relation to the surroundings, it would still be an adaptation if -it were of use to the animal under the ordinary conditions of life. - -In the lower groups numerous cases in which animals resemble their -surroundings could be given. Such cases are known in crustacea, worms, -mollusks, hydroids, etc., and the possible value of these resemblances -may be admitted in many instances. - -It is rather curious that so few cases of adaptive coloration have been -described for plants. No one supposes that the slate color of the lichen -is connected with the color of the rocks on which it grows, in the sense -that the resemblance is of any use to the lichen. Nor does the color of -the marine red algæ serve in any way to protect the plants so far as is -known. The green color of nearly all the higher plants is obviously -connected with the substance, chlorophyl, that is essential for the -processes of assimilation, and has no relation to external objects. But -when we come to the colors of flowers we meet with curious cases of -adaptation, at least according to the generally accepted point of view. -For it is believed by many naturalists that the color of the corolla of -flowering plants is connected with the visits of insects to the flowers, -and these visits are in many cases essential for the cross-fertilization -of the flowers. This adaptation is one useful to the species, rather -than the individual, and belongs to another category. - -The leaf of the Venus’s fly-trap, which suddenly closes together from -the sides when a fly or other light body comes to rest on it, is -certainly a remarkable adaptation. A copious secretion of a digestive -fluid is poured out on the surface of the leaf, and the products of -digestion are absorbed. There can be no question that this contrivance -is of some use to the plant. In other insectivorous plants, the pitcher -plants, the leaves are transformed into pitchers. In Nepenthes a -digestive fluid is secreted from the walls. A line of glands secreting a -sweet fluid serves to attract insects to the top of the pitcher, whence -they may wander or fall into the fluid inside, and there being drowned, -they are digested. A lidlike cover projecting over the opening of the -pitcher is supposed to be of use to keep out the rain. - -In _Utricularia_, a submerged water-plant, the tips of the leaves are -changed into small bladders, each having a small entrance closed by an -elastic valve opening inwards. Small snails and crustaceans can pass -into this opening, to which they are guided by small outgrowths; but -once in the cup they cannot get out again, and, in fact, small animals -are generally found in the bladders where they die and their substance -is absorbed by forked hairs projecting into the interior of the bladder. - -The cactus is a plant that is well suited to a dry climate. Its leaves -have completely disappeared, and the stem has become swollen into a -water-reservoir. “It has been estimated that the amount of water -evaporated by a melon cactus is reduced to one six-hundredth of that -given off by any equally heavy climbing-plant.” - - -[Illustration: - - Fig. 1.—The fertilization of _Aristolochia Clematitis_. - A, portion of stem with flowers in axil of leaf in different stages. - B and C, longitudinal sections of two flowers, before and after - fertilization. (After Sachs.)] - - -Sachs gives the following account of the fertilization process in -_Aristolochia Clematitis_, which he refers to as a conspicuous and -peculiar adaptation. In Figure 1 A a group of flowers is shown, and in -Figure 1 B and C a single flower is split open to show the interior. In -B a small fly has entered, and has brought in upon its back some pollen -that has stuck to it in another flower. The fly has entered through the -long neck which is beset with hairs which are turned inwards so that the -fly can enter but cannot get out. In roaming about, the pollen that is -sticking to its back will be rubbed against the stigmatic surface. “As -soon as this has taken place the anthers, which have been closed -hitherto, dehisc and become freely accessible,” as a result in the -change in the stigma and of the collapse of the hairs at the base of the -enlargement which has widened. The fly can now crawl under the anthers, -and, if it does so, new pollen may stick to its back. At this time the -hairs in the throat dry up, and the fly can leave its prison house, -Figure 1 C. If the fly now enters another flower this is fertilized by -repeating the process. The unfertilized flowers stand erect with widely -open mouths. As soon as they have been fertilized they bend down, as -seen in Figure 1 A, and at the same time the terminal flap bends over -the open mouth of the throat, “stopping the entrance to the flies, which -have now nothing more to do here.” - - - Adjustments of the Individual to Changes in the Environment - -The most familiar cases of adjustments of the individual to the -environment are those that we recognize in our own bodies. After violent -exercise we breathe more rapidly, and take deeper inspirations. Since -during exercise our blood loses more oxygen and takes in more carbon -dioxide from the muscles, it is clear that one result of more rapid -breathing is to get more oxygen into the blood and more carbon dioxide -out of it. The process of sweating, that also follows exercise, may be -also looked upon as an adaptive process, since by evaporation the skin -is kept cooler, and, in consequence, the blood, which at this time flows -in larger quantities to the skin, is cooled also. - -More permanent adaptive changes than these also take place as the result -of prolonged use of certain parts. If the muscles work against powerful -resistance, they become larger after several days or weeks, and are -capable of doing more work than at first. Conversely, when any group of -muscles is not used, it becomes smaller than the normal and capable of -doing less work. It would be a nice point to decide whether this latter -change is also an adaptation. If so it is one in a somewhat different -sense from that usually employed. The result is of no direct advantage -to the animal, except possibly in saving a certain amount of food, but -since the same change will take place when an abundance of food is -consumed, the result is, under these conditions, of no use. - -The thickening of the skin on those parts of the body where continued -pressure is brought to bear on it is a change in a useful direction. The -thickening on the soles of the feet and on the palms of the hands is a -case in point. Not only is the skin thicker at birth in these parts, but -it becomes thicker through use. In other parts of the body also, the -skin hardens and becomes thicker if pressure is brought to bear on it. -We may regard this as a general property of the skin, which is present -even in those parts where, under ordinary circumstances, it can rarely -or never be brought into use. - -Even as complicated and as much used an organ as the eye can become -adaptively improved. It is said that the lateral region of the field of -vision can be trained to perceive more accurately; and every one who has -used a microscope is familiar with the fact that if one eye is -habitually used it becomes capable of seeing more distinctly and better -than the other eye. This seems to be due, in part at least, to the -greater contraction of the iris. - -Another phenomenon, which, I think, must be looked upon as an -adaptation, is the immunity to certain poisons that can be gradually -brought about by slowly increasing the amount introduced into the body. -Nicotine is a most virulent poison, and yet by slowly increasing the -dose an animal can be brought into a condition in which an amount of -nicotine, fatal to an ordinary individual, can be administered without -any ill effects at all resulting. - -The same phenomenon has been observed in the case of other poisons, not -only in case of other alkaloids, such as morphine and cocaine, but also -in the case of caffein, alcohol, and even arsenic. There is a curious -phenomenon in regard to arsenic, which appears to be well established, -viz., that a person who has gradually increased the dose to an amount -great enough to kill ten ordinary men, will die if he suddenly ceases -altogether to take arsenic. He can, however, be gradually brought back -to a condition in which arsenic is not necessary for his existence, if -the dose is gradually decreased. It is a curious case of adaptation that -we meet with here, since the man becomes so thoroughly adjusted to a -poison that if he is suddenly brought back to the normal condition of -the race he will die. - -Immunity to the poison of venomous snakes can also be acquired by slowly -increasing the amount given to an animal. It is possible to make a -person so immune to the poison of venomous snakes that he would become, -in a sense, adapted to live amongst them without danger to himself. It -is to be noted, moreover, that this result could be reached only by -quite artificial means, for, under natural conditions it is -inconceivable that the nicely graded series of doses of increasing -strength necessary to bring about the immunity could ever be acquired. -Hence we find here a case of response in an adaptive direction that -could not have been the outcome of experience in the past. It is -important to emphasize this capacity of organisms to adapt themselves to -certain conditions entirely new to them. - -These cases lead at once to cases of immunity to certain bacterial -diseases. An animal may become immune to a particular disease in several -ways. First, by having the disease itself, which renders it immune for a -longer or a shorter period afterwards; or, second, by having a mild form -of the disease as in the case of smallpox, where immunity is brought -about by vaccination, _i.e._ by giving the individual a mild form of -smallpox; or, third, by introducing into the blood an antidote, in the -form, for example, of antitoxin, which has been made by another animal -itself immune to the disease. The first two classes of immunity may be -looked upon as adaptations which are of the highest importance to the -organism; the last case can scarcely be looked upon as an adaptive -process, since the injurious effect of the poison may as well be -neutralized outside of the body by mixing it with the antitoxin. We may -suppose, then, that in the body a similar process goes on, so that the -animal itself takes no active part in the result. - -When we consider that there are a number of bacterial diseases, in each -of which a different poison is made by the bacteria, we cannot but ask -ourselves if the animal really makes a counter-poison for each disease, -or whether a single substance may not be manufactured that counteracts -all alike? That the latter is not the case is shown by the fact that an -animal made immune to one disease is not immune to others. When we -recall that the animal has also the capacity to react in one way or -another to a large number of organic and inorganic poisons, to which it -or its ancestors can have had little or no previous experience, we may -well marvel at this wonderful regulative power. - -The healing of wounds, which takes place in all animals, forms another -class of adaptive processes. The immense usefulness of this power is -obvious when it is remembered how exposed most animals are to injuries. -By repairing the injury the animal can better carry on its normal -functions. Moreover, the presence of the wound would give injurious -bacteria a ready means of entering the body. In fact, an intact skin is -one of the best preventives to the entrance of bacteria. - -Not only have most organisms the power of repairing injuries, but many -animals have also the closely related power of regenerating new parts if -the old ones are lost. If a crab loses its leg, a new one is -regenerated. If a fresh-water worm (_Lumbriculus_) is cut into pieces, -each piece makes a new head at its anterior end and a new tail at the -posterior end. In this way as many new worms are produced as there are -pieces. And while in a strict sense it cannot be claimed that this power -of regeneration is of any use to the original worm, since the original -worm, as such, no longer exists, yet since it has not died but has -simply changed over into several new worms, the process is of use -inasmuch as by this means the pieces can remain in existence. - -We need not discuss here the relative importance to different animals of -this power of regeneration, but it may be stated, that, while in some -cases it may be necessary to replace the lost part if the animal is to -remain in existence, as when a new head is formed on an earthworm after -the old one was cut off, in other cases the replacement of the lost part -appears to be of minor importance, as in the case of the leg of the -crab. While we are not, for the moment, concerned with the relative -importance of the different adaptations, this question is one of much -importance in other connections and will be considered later. - -The protective coloration of some animals, which is the direct result of -a change in color of the animal in response to the surroundings, -furnishes us with some most striking cases of adaptive coloration. A -change of this sort has been recorded in a number of fishes, more -especially in the flounders. The individuals found living on a dark -background are darker than those living on a lighter background; and -when the color of the background is changed it has been observed that -the color of the fish also changes in the same direction. I have -observed a change of this sort from dark to light, or from light to -dark, in the common minnow (_Fundulus_) in accordance with a change of -its background, and the same sort of change appears to take place in -many other fishes. - -The change from green to brown and from brown to green in certain tree -frogs and in the lizard (_Anolis_), which is popularly supposed to take -place according to whether the background is green or brown, is not -after all, it appears, connected with the color of the background, but -depends on certain other responses of the animals that have not yet been -satisfactorily made out. If it be claimed that in summer the animal -would generally be warm, and therefore, often green, and that this color -would protect it at this time of year when the surroundings are green, -and in winter brown, when this color is the prevailing one in temperate -regions, then it might appear that the change is of use to the animal; -but if it is true that the same change takes place in some of the -lizards that live in the tropics, where the prevailing color is always -green, it would appear that the result may have no direct relation with -the surroundings. It has been shown in a number of well-authenticated -cases that the pupæ of certain butterflies vary in color within certain -limits in response to the color of the background. When the caterpillar -fixes itself to some surface, and there throws off the outer skin, and -acquires a new one, the color of the latter is influenced by the -background. The result is a better protection to the pupa. The change is -not brought about through the ocelli or eyes, but through the general -surface of the skin, for the same change takes place when the eyes have -been previously covered with a dark pigment. - -The growth of plants toward the light may be looked upon as an adaptive -process, since only in the light can they find the conditions necessary -for their life. The extraordinary elongation of shoots and young plants -when grown in the dark may also be considered an adaptation for finding -the light, since in this way a plant, deeply embedded in the ground, may -ultimately reach the surface. Thus while the actual process of -elongation in the dark is not in itself of any use, yet under the -ordinary conditions of its life, this response may be of great benefit -to the plant. - -The closing together of the leaves of some plants has been supposed -to protect them from too rapid radiation of heat, and incidentally -this purpose may be fulfilled; but since some tropical plants also -close their leaves during the night, it can hardly be maintained -that the closing has been acquired for this purpose. It has been -suggested that the opening of certain flowers under certain -conditions of light is connected with the visits of insects that -bring about cross-fertilization. - -The preceding examples will suffice to give a general idea of what is -meant by adaptation in organisms. That the term includes a large number -of phenomena of very different kinds is apparent. When we have examined -these phenomena further we shall find, I think, that it will be -necessary to put some of them into different categories and treat them -differently. It is probably incorrect to suppose that all processes -useful to the organism have been acquired in the same way, nevertheless, -for the present the term adaptation is sufficiently general, even if -vague, to cover these different groups of cases. - -It may be asked, in what respects are these structures and processes of -adaptation different from the ordinary structures and changes that go on -in the organism? Why is the leg of the mole more of an adaptation than -that of a dog? The one is of as much use as the other to its possessor. -What reason can we give for citing the poison of the snake, and not -mentioning in the same connection the other glands of the body? In fact, -the poison gland of the snake is supposed to be a modified superior -labial gland. Why, in short, are not the processes of digestion, -excretion, secretion, the beating of the heart, the ordinary reflex acts -of the nervous system, and the action of the sense-organs, as truly -adaptations as the special cases that have been selected for -illustration. The answer is simply that we are more impressed by those -cases of adaptation that are more unusual, as when an animal departs in -the use of certain structures from the rest of the group to which it -belongs. For example, if all mammals lived underground, ourselves -included, and the fore-legs or arms were used for burrowing, we should -not think this unusual; but if we found an animal using all four legs to -support the body and for purposes of progression, we should, most -likely, think this was an excellent illustration of adaptation. - -In other instances the condition is somewhat different. The color of -certain animals may unquestionably be of use to them in concealing them -from their enemies. In other cases the color may not serve this purpose, -or any purpose at all. Thus while in the former case we speak of the -color as an adaptation to the surroundings, in the latter we do not -think of it as having any connection at all with the environment. Even -in the same animal the color of different parts of the body may appear -under this twofold relation. For example, the green color of the skin of -the frog renders it less conspicuous amongst the green plants on the -edge of the stream, but the brilliant orange and black pigment in the -body-cavity cannot be regarded as of any use to the animal. - - - Adaptations for the Good of the Species - -Aside from the class of adaptations that are for the good of the -individual, there is another class connected solely with the -preservation of the race. The organs for reproduction are the most -important examples of this kind. These organs are of no use to the -individual for maintaining its own existence, and, in fact, their -presence may even be deleterious to the animal. The instincts connected -with the use of these organs may lead inevitably to the death of the -individual, as in the case of the California salmon, which, on entering -fresh water in order to deposit its eggs, dies after performing this -act. - -The presence of the organs of reproduction in the individual is -obviously connected with the propagation of other individuals. Indeed in -many organisms the life of the individual appears to have for its -purpose the continuation of the race. In a large number of animals the -individual dies after it has deposited its eggs. The most striking case -is that of the May-flies, whose life, as mature individuals, may last -for only a few hours. The eggs are set free by the bursting of the -abdomen, and the insect dies. The male bee also dies after union with -the queen. In some annelids, the body is also said to burst when the -eggs are set free; and in other forms those parts of the body containing -the eggs break off, and, after setting free the eggs, die. These are -extreme cases of what is seen in many animals, namely the replacement of -the old individuals by a new generation; and while in general there is -only a loose connection between the death of the individual and the -consummation of its reproductive power, yet the two run a course so -nearly parallel that several writers have attempted to explain this -connection as one of racial adaptation. - -It has also been pointed out that in those higher animals that take care -of their young after birth, the life of the individual does not end with -the period of birth of the young, but extends at least throughout the -time necessary to care for the young. It has even been suggested that -this lengthening of the life period has been acquired on account of its -use to the species. When, however, as in the case of the vertebrates, -the young are born at intervals either in great numbers at a birth, as -in fishes and amphibia, or in lots of twos, threes, or fours, as in many -birds and mammals, or even only one at a time, as in a few birds and in -man, it will be evident that the relation cannot be so simple as has -been supposed. It cannot be assumed in these forms that the end of the -life of the individual is in any way connected with the ripening of the -last eggs, for, on the contrary, hundreds, or even many thousands, of -potential eggs may be present in the ovaries when the animal is -overtaken by old age, and its power of reproduction lost. - -In regard to several of the lower animals, we find, in a number of cases -where there are accurate data, that the individual goes on year after -year producing young. Whether they ever grow old, in the sense of losing -their power of reproduction, has not been definitely determined, but -there is, so far as I know, no evidence to show that such a process -takes place, and these animals appear to have the power of reproducing -themselves indefinitely. - -The phenomenon of old age (apart from its possible connection with the -cessation of the power of reproduction), which leads to the death of the -individual, has been looked upon by a few writers as an adaptation of -the individual for the good of the species. It has been pointed out by -these writers that the longer an individual lives, the more likely it is -to become damaged, and if along with this its powers of reproduction -diminish, as compared with younger individuals, then it stands in the -way and takes food that might be used by other, younger individuals, -that are better able to carry on the propagation of the race. It is -assumed, therefore, that the life of the individual has been shortened -for the benefit of the race. Whether such a thing is probable is a -question that will also be discussed later. We are chiefly concerned -here only in recording the different groups of phenomena that have been -regarded by biologists as adaptations. - -The so-called secondary sexual characters such as the brighter colors of -the males, ornaments of different kinds, crests, color-pattern, tail -feathers, etc., organs of offence and of defence used in fighting -members of the same species, present a rather unique group of -adaptations. These characters are supposed to be of use to the -individual in conquering its rivals, or in attracting the females. They -may be considered as useful to the individual in allowing it to -propagate at the expense of its rivals, but whether the race is thereby -benefited is a question that will be carefully considered later. - -The colors of flowers, that is supposed to attract insects, have been -already mentioned. The sweet fluid, or nectar, secreted by many flowers -is sought by insects, which on entering the flowers bring about -cross-fertilization. Thus while the nectar seems to be of no immediate -service to the plant itself, it is useful to the species in bringing -about the fertilization of the flowers. The odors of flowers also serve -to attract insects, and their presence is one of the means by which -insects find the flowers. This also is of advantage to the race. - - - Organs of Little Use to the Individual - -In every organism there are parts of the body whose presence cannot be -of vital importance to the individual. We may leave out of consideration -the reproductive organs, since their presence, as has just been stated, -is connected with the continuation of the race. The rudimentary organs, -so-called, furnish many examples of structures whose presence may be of -little or of no use to the individual; in fact, as in the case of the -appendix in man, the organs may be a source of great danger to the -individual. In this respect the organism is a structure not perfectly -adapted to its conditions of life, since it contains within itself parts -that are of little or of no use, which may even lead to its destruction, -and may often expose it to unnecessary danger. Nevertheless such parts -are surprisingly infrequent, and their presence is usually accounted for -on the supposition that in the past these organs have been of use, and -have only secondarily come to play an insignificant part in the -functions of the organism. Another example of the same thing is found in -the rudimentary eyes of animals living in the dark, such as the mole and -several cave animals, fishes, amphibia, and insects. - -There are still other organs, which cannot be looked upon as -rudimentary, yet whose presence can scarcely be considered as essential -to the life of the individual. It is with this class that we are here -chiefly concerned. For instance, the electric organs in some of the rays -and fish can hardly protect the animal from enemies, even when as highly -developed as in the torpedo; and we do not know of any other essential -service that they can perform. Whether the same may be also said of the -phosphorescent organs of many animals is perhaps open in some cases to -doubt, but there can be little question that the light produced by most -of the small marine organisms, such as noctiluca, jellyfish, -ctenophores, copepods, pyrosoma, etc., cannot be of use to these animals -in protecting them from attack. In the case of certain bacteria it seems -quite evident that the production of light can be of no use as such to -them. The production of light may be only a sort of by-product of -changes going on in the organism, and have no relation to outside -conditions. In certain cases, as in the glowworm, it has been supposed -that the display may serve to bring the sexes together; but since the -phosphorescent organs are also present in the larval stages of the -glowworm, and since even the egg itself is said to be phosphorescent, it -is improbable, in these stages at least, that the presence of the light -is of service to the organism. - -It has been pointed out that the colors of certain animals may serve to -conceal them and may be regarded as an adaptation; but it is also true -that in many cases the color of the whole animal or the color of special -parts can be of little if any direct use. While it is difficult to show -that the wonderful patterns and magnificent coloration of many of the -larger animals are not of service to the animal, however sceptical we -may be on the subject, yet in the case of many microscopical forms that -are equally brilliantly colored there can be little doubt that the -coloration can be of no special service to them. If it be admitted that -in these small forms the color and the color patterns are not -protective, we should at least be on our guard in ascribing off-hand to -larger forms a protective value in their coloration, unless there is -actual proof that it serves some purpose. - -We also see in other cases that the presence of color need not be -connected with any use that it bears as such to the animal. For -instance, the beautiful colors on the inside of the shells of many -marine snails and of bivalve mollusks, can be of no use to the animal -that makes the shell, because as long as the animal is alive this color -cannot be seen from the outside. This being the case let us not jump too -readily to the conclusion that when other shells are colored on the -outer surface that this must be of use to the mollusk. - -In regard to the colors of plants, there are many cases of brilliant -coloration, which so far as we can see can be of no service to the -organism. In such forms as the lichens and the toadstools, many of which -are brilliantly colored, it is very doubtful if the color, as such, is -of any use to the plant. The splendid coloring of the leaves in the -autumn is certainly of no service to the trees. - -It should not pass unnoticed in this connection that the stems and the -trunks of shrubs and of trees and also many kinds of fruits and nuts are -sometimes highly colored. It is true that some of the latter have been -supposed to owe their color to its usefulness in attracting birds and -other animals which, feeding on the fruit, swallow the seeds, and these, -passing through the digestive tract and falling to the ground, may -germinate. The dissemination of the seeds of such plants is supposed to -be brought about in this way; and since they may be widely disseminated -it may be supposed that it is an advantage to the plant to have -attracted the attention of the fruit-eating birds. On the other hand one -of the most brilliantly colored seeds, the acorn, is too large to pass -through the digestive tracts of birds, and is, in fact, ground to pieces -in the gizzard, and in the case of several mammals that feed on the -acorns, the acorn is crushed by the teeth. It would seem, therefore, -that its coloration is injurious to it rather than the reverse, as it -leads to its destruction. It has been suggested by Darwin that since the -acorns are for a time stored up in the crop of the bird, the passenger -pigeon for example, and since the birds may be caught by hawks and -killed, the seeds in the crop thus become scattered. Consequently it may -be, after all, of use to the oak to produce colored acorns that attract -the attention of these pigeons. This suggestion seems too far-fetched to -consider seriously. In the case of the horse-chestnut the rich brown -color is equally conspicuous, but the nut is too large to be swallowed -by any of the ordinary seed-feeding birds or mammals. Shall we try to -account for its color on the grounds of the poisonous character of the -seed? Has it been acquired as a warning to those animals that have eaten -it once, and been made sick or have died in consequence? I confess to a -personal repugnance to imaginative explanations of this sort, that have -no facts of experience to support them. - - - Changes in the Organism that are of No Use to the Individual or to the - Race - -As an example of a change in the organism that is of no use to it may be -cited the case of the turning white of the hair in old age in man and in -several other mammals. The absorption of bone at the angle of the chin -in man, is another case of a change of no immediate use to the -individual. We also find in many other changes that accompany old age, -processes going on that are of no use to the organism, and which may, in -the end, be the cause of its death. Such changes, for instance, as the -loss of the vigor of the muscles, and of the nervous system, the -weakening of the heart, and partial failure of many of the organs to -carry out their functions. These changes lead sooner or later to the -death of the animal, in consequence of the breaking down of some one -essential organ, or to disease getting an easier foothold in the body. -We have already discussed the possible relation of death as an -adaptation, but the changes just mentioned take place independently of -their relation to the death of the organism as a whole, and show that -some of the normal organic processes are not for the good of the -individual or of the race. In fact, the perversions of some of the most -deeply seated instincts of the species, as in infanticide, while the -outcome of definite processes in the organism, are of obvious -disadvantage to the individual, and the perversion of so deeply seated a -process as the maternal instinct, leading to the destruction of the -young, is manifestly disadvantageous to the race. As soon, however, as -we enter the field of so-called abnormal developments, the adaptive -relation of the organism to its environment is very obscure; and yet, as -in the case of adaptation to poisons, we see that we cannot draw any -sharp line between what we call normal and what we call abnormal -development. - - - Comparison with Inorganic Phenomena - -The preceding examples and discussion give some idea of what is meant by -adaptation in living things. In what respects, it may be asked, do these -adaptations differ from inorganic phenomena? The first group of -inorganic bodies that challenges comparison are machines. These are so -constructed that they may be said to accomplish a definite purpose, and -the question arises whether this purpose can be profitably compared with -the purposefulness of the structure and response of organisms. That the -two cannot be profitably compared is seen at once, when we recall the -fact that the activity of the machine is of no use to it, in the sense -of preserving its integrity. The object of the machine is, in fact, to -perform some useful purpose for the organism that built it, namely, for -man. Furthermore, the activity of the machine only serves to wear it -out, and, therefore, its actions do not assist in preserving its -integrity as do some, at least, of the activities of an animal. It is -true, of course, that in a mechanical sense every action of the organism -leads also to a breaking down of its structure in the same way that a -machine is also worn out by use; but the organism possesses another -property that is absent in the machine, namely, the power of repairing -the loss that it sustains. - -One of the most characteristic features of the organism is its power of -self-adjustment, or of regulation, by which it adapts itself to changes -in the environment in such a way that its integrity is maintained. Most -machines have no such regulative power, although, in a sense, the -fly-wheel of an engine regulates the speed, and a water-bath, with a -thermostat, regulates itself to a fixed temperature; but even this -comparison lacks one of the essential features of the regulation seen in -organisms, namely, in that the regulation does not protect the machine -from injury. It may be claimed, however, that the safety valve of an -engine does fulfil this purpose, since it may prevent the engine from -exploding. Here, in fact, we do find better grounds for comparison, but, -when we take into account the relation of the regulations in the -organism to all the other properties of the organism, we see that this -comparison is not very significant. The most essential difference -between a machine and an organism is the power of reproduction possessed -by the latter, which is absent in all machines. Here, however, we meet -with a somewhat paradoxical relation, since the reproductive power of -organisms cannot be looked upon as an adaptation for the continuation of -the individual, but rather for the preservation of a series of -individuals. Hence, in this respect also, we cannot profitably compare -the individual with a machine, but if we make any comparison we should -compare all the individuals that have come from a single one with a -machine. In this sense the power of reproduction is a sort of racial -regulation. A comparison of this sort is obviously empty of real -significance. - -The regenerative power of the organism, by means of which it may replace -a lost part, or by means of which a piece may become a new whole, is -also something not present in machines. - -In using a machine for comparison we should not leave out of sight the -fact that machines are themselves the work of organisms, and have been -made for some purpose useful to the organism. They may perform the same -purpose for which we would use our own hands, for they differ from parts -of the body mainly in that they are made of different compounds having -different properties, as the above comparisons have shown. But the -regulations of the machine have been added to it by man on account of -their usefulness to himself, and are not properties of the material of -which the machine itself is composed. This shows, I think, the -inappropriateness of making any comparison between these two entirely -different things. - -If, then, we find the comparison between machines and organisms -unprofitable, can we find any other things in inorganic nature that can -be better compared with the phenomenon of adaptation of the organism? -The following phenomena have been made the subject of comparison from -time to time. The bendings, which are gradually made by rivers often -lead to a meeting of the loops, so that a direct, new communication is -established, and the course of the river is straightened out. The water -takes, therefore, a more direct course to the sea. It cannot be said, -however, to be of any advantage to the river to straighten its course. -Again, a glacier moulds itself to its bed, and gradually moves around -obstacles to a lower level, but this adaptation of the glacier to the -form of its surroundings cannot be said to be of advantage to the -glacier. On the contrary, the glacier reaches so much the sooner a lower -level where it is melted. - -The unusual case of a solid being lighter than the liquid from which it -forms, as seen in the case of ice, has been looked upon as a useful -arrangement, since were the reverse the case all rivers and ponds would -become solid in winter in cold climates, and the polar regions would -become one solid block of ice. But no one will suppose for a moment that -there is any relation between the anomalous condition of the lightness -of ice, and its relation to the winter freezing of streams, ponds, etc. -It has even been suggested that this property of ice was given to it in -order that the animals living in the water might not be killed, which -would be the case if the ice sank to the bottom, but such a method of -interpreting physical phenomena would scarcely commend itself to a -physicist. - -The formation of a covering of oxide over the surface of a piece of iron -delays the further process of oxidation, but who will imagine that this -property of iron has been acquired in order to prevent the iron from -being destroyed by oxygen? - -If a piece is broken from a crystal, and the crystal is suspended in a -saturated solution of the same substance, new material is deposited over -its whole surface, and, as it grows larger, the broken side is completed -and the crystal assumes its characteristic form. But of what advantage -is it to the crystal whether it is complete or incomplete? In the case -of an animal it is of some importance to be able to complete itself -after injury, because it can then better obtain the food necessary to -keep it alive, or it can better escape its enemies; but this is not the -case with the crystal. - -In conclusion, therefore, it is obvious that the adaptations of -organisms are something peculiar to living things, and their obvious -purpose is to maintain the integrity of the individual, or that of the -species to which the individual belongs. We are, therefore, confronted -with the question as to how this peculiarity has come to be associated -with the material out of which living things are made. In subsequent -chapters this will be fully discussed, but before we take up this topic, -it will be necessary to reach some understanding in regard to the theory -of evolution, for the whole subsequent issue will turn upon the question -of the origin of the forms of animals and plants living at the present -time. - - ------------------------------------------------------------------------- - - - - - CHAPTER II - - THE THEORY OF EVOLUTION - - -One of the most important considerations in connection with the problem -of adaptation is that in all animals and plants the individuals sooner -or later perish and new generations take their places. Each new -individual is formed, in most cases, by the union of two germ-cells -derived one from each parent. As a result of this process of -intermixing, carried on from generation to generation, all the -individuals would tend to become alike, unless something else should -come in to affect the result. - -So far as our actual experience reaches, we find that the succeeding -generations of individuals resemble each other. It is true that no two -individuals are absolutely alike, but if a sufficiently large number are -examined at a given time, they will show about the same variations in -about the same proportionate numbers. Such a group of similar forms, -repeating itself in each generation, is the unit of the systematists, -and is called a species. - -It has been said that within each species the individuals differ more or -less from each other, but our experience teaches that in each generation -the same kinds of variations occur, and, moreover, that from any one -individual there may arise in the next generation any one of the -characteristic variations. Certain limitations will have to be made in -regard to this statement, but for the present it will suffice. The Law -of Biogenesis states that each living thing arises from another living -thing; that there is no life without antecedent life, _i.e._ spontaneous -generation does not occur. The law is not concerned with the likeness or -unlikeness of the different individuals that descend from each other. -The theory of evolution includes the same idea, but in addition it has -come to mean nowadays, that there have been changes, as the succeeding -generations have arisen. The transmutation theory, and even the descent -theory, have come to mean nearly the same thing as the theory of -evolution. It is unfortunate that one of these terms cannot be used to -signify simply the repetition, generation after generation, of groups of -similar individuals. The theory of descent might be used to convey only -this idea, but unfortunately it too has come to include also the idea of -change. I shall attempt nevertheless to discriminate between the descent -and the transmutation theory, and use the term _descent theory_ when I -do not wish to convey the idea of change, and _transmutation theory_ -when I do wish to emphasize this idea. - -On the transmutation theory it is assumed that a group (species) may -give rise to one or more groups of forms differing from their ancestors; -the original group being now replaced by its new kinds of offspring, or -the old and the new may remain in existence at the same time. This -process repeating itself, each or some of the new groups giving rise in -turn to one or more new species, there will be produced a larger group -of species having certain similar characters which are due to their -common descent. Such a group of species is called a genus. The -resemblances of these species is accounted for by their common descent; -but their differences must be due to those factors that have caused them -to depart from the original type. We may now proceed to consider the -evidence on which this idea of transmutation rests. - - - Evidence in Favor of the Transmutation Theory - - EVIDENCE FROM CLASSIFICATION AND FROM COMPARATIVE ANATOMY - -It does not require any special study to see that there are certain -groups of animals and of plants that are more like each other than they -are like the members of any other group. It is obvious to every one that -the group known as mammals has a combination of characters not found in -any other group; such, for instance, as a covering of hair, mammary -glands that furnish milk to the young, and a number of other less -distinctive features. These and other common characteristics lead us to -put the mammals into a single class. The birds, again, have certain -common characters such as feathers, a beak without teeth, the -development of a shell around the egg, etc., and on account of these -resemblances we put them into another class. Everywhere in the animal -and plant kingdoms we find large groups of similar forms, such as the -butterflies, the beetles, the annelidan worms, the corals, the snails, -the starfishes, etc. - -Within each of these groups we find smaller groups, in each of which -there are again forms more like each other than like those of other -groups. We may call these smaller groups families. Within the families -we find smaller groups, that are more like each other than like any -other groups in the same family, and these we put into genera. Within -the genus we find smaller groups following the same rule, and these are -the species. Here we seem to have reached a limit in many cases, for we -do not always find within the species groups of individuals more like -each other than like other groups. Although we find certain differences -between the individuals of a species, yet the differences are often -inconstant in the sense that amongst the descendants of any individual -there may appear any one of the other variations. If this were the whole -truth, it would seem that we had here reached the limits of -classification, the species being the unit. This, however, is far from -being the case, for, in many species we find smaller groups, often -confined to special localities. These groups are called varieties. - -In some cases it appears, especially in plants, these smaller groups of -varieties resemble in many ways the groups of species in other forms, -since they breed true to their kind, even under changed conditions. They -have been recognized as “smaller species” by a number of botanists. - -In this connection a point must be brought up that has played an -important rôle in all discussion as to what limits can be set to a -species. As a rule it is found that two distinct species cannot be made -to cross with each other, _i.e._ the eggs of an individual of one -species cannot be fertilized by spermatozoa derived from individuals of -another species; or, at least, if fertilization takes place the embryo -does not develop. In some cases, however, it has been found possible to -cross-fertilize two distinct species, although the offspring is itself -more or less infertile. Even this distinction, however, does not hold -absolutely, for, in a few cases, the offspring of the cross is fertile. -It cannot be maintained, therefore, that this test of infertility -between species invariably holds, although in a negative sense the test -may apply, for if two different forms are infertile, _inter se_, the -result shows that they are distinct species. If they cross they may or -may not be good species, and some other test must be used to decide -their relation. - -We should always keep in mind the fact that the individual is the only -reality with which we have to deal, and that the arrangement of these -into species, genera, families, etc., is only a scheme invented by man -for purposes of classification. Thus there is no such thing in nature as -a species, except as a concept of a group of forms more or less alike. -In nature there are no genera, families, orders, etc. These are -inventions of man for purposes of classification. - -Having discovered that it is possible to arrange animals and plants in -groups within groups, the question arises as to the meaning of this -relation. Have these facts any other significance than that of a -classification of geometric figures, or of crystals according to the -relations of their axes, or of bodies as to whether they are solids, -liquids, or gases, or even whether they are red, white, or blue? - -If we accept the transmutation view, we can offer an explanation of the -grouping of living things. According to the transmutation theory, the -grouping of living things is due to their common descent, and the -greater or less extent to which the different forms have diverged from -each other. It is the belief in this principle that makes the -classification of the biologist appear to be of a different order from -that in any other science; and it is this principle that appears to give -us an insight into a large number of phenomena. - -For example, if, as assumed in the theory, a group of individuals -(species) breaks up into two groups, each of these may be supposed to -inherit a large number of common characteristics from their ancestors. -These characters are, of course, the resemblances, and from them we -conclude that the species are related and, therefore, we put them into -the same genus. The differences, as has been said, between the species -must be explained in some other way; but the principle of classification -with which we are here concerned is based simply on the resemblances, -and takes no account of the differences between species. - -In this argument it has been tacitly assumed that the transformation of -one species into another, or into more than one, takes place by adding -one or more new characters to those already present, or by changing over -a few characters without altering others. But when we come to examine -any two species whatsoever, we find that they differ, not only in one or -in a few characters, but in a large number of points; perhaps in every -single character. It is true that sometimes the differences are so small -that it is difficult to distinguish between two forms, but even in such -cases the differences, although small, may be as numerous as when they -are more conspicuous. If, then, this is what we really find when we -carefully examine species of animals or of plants, what is meant when we -claim that our classification is based on the characters common to all -of the forms that have descended from the same ancestor? We shall find, -if we press this point that, in one sense, there is no absolute basis of -this sort for our classification, and that we have an unreal system. - -If this is admitted, does our boasted system of classification, based as -it is on the principle of descent, give us anything fundamentally -different from an artificial classification? A few illustrations may -make clearer the discussion that follows. If, for example, we take a -definition of the group of vertebrates we read: “The group of craniate -vertebrates includes those animals known as Fishes, Amphibians, -Reptiles, Birds, and Mammals; or in other words, Vertebrates with a -skull, a highly complex brain, a heart of three or four chambers, and -red blood corpuscles.” If we attempt to analyze this definition, we find -it stated that the skull is a characteristic of all vertebrates, but if -we ask what this thing is that is called skull, we find not only that it -is something different in different groups, being cartilaginous in -sharks, and composed of bones in mammals, but that it is not even -identical in any two species of vertebrates. If we try to define it as a -case of harder material around the brain, then it is not something -peculiar to the vertebrates, since the brain of the squid is also -encased in a cartilaginous skull. What has been said of the skull may be -said in substance of the brain, of the heart, and even of the red blood -corpuscles. - -If we select another group, we find that the birds present a sharply -defined class with very definite characters. The definition of the group -runs as follows: “Birds are characterized by the presence of feathers, -their fore-limbs are used for flight, the breast-bone is large and -serves for the attachment of the muscles that move the wings; outgrowths -from the lungs extend throughout the body and even into the bones and -serve as air sacs which make the body more buoyant. Only one aortic arch -is present, the right, and the right ovary and oviduct are not -developed. The eyes are large and well developed. Teeth are absent. We -have here a series of strongly marked characteristics such as -distinguish hardly any other class. Moreover, the organization of -existing birds is, in its essential features, singularly uniform; the -entire class presenting less diversity of structure than many orders of -Fishes, Amphibians, and Reptiles.”[1] The feathers are the most unique -features of birds, and are not found in any other group of the animal -kingdom; moreover the plan on which they are formed is essentially the -same throughout the group, yet in no two species are the feathers -identical, but differ not only in form and proportions, but even in the -character of the barbs and hooks for holding the vane together. The -modification of the fore-limbs for flight is another characteristic -feature; yet in some birds, as the ostrich and kiwi, although the wing -has the same general plan as in other birds, it is not used for flight. -In the latter it is so small that it does not project beyond the -feathers, and in some birds, as in the penguins, the wings are used only -as organs for swimming. - -Footnote 1: - - Parker and Haswell: “Text Book of Zoology.” - -In spite of these differences we have no difficulty in recognizing -throughout the group of birds a similarity of plan or structure, -modified though it be in a thousand different ways. - -Enough has been said to illustrate what is meant by the similarities of -organisms on which we base our system of classification. When we -conclude from the statement that all vertebrates have a skull that they -owe this to a common descent, we do not mean that a particular structure -has been handed down as a sort of entailed heirloom, but that the -descendants have followed the same plan of structure as that of their -ancestors, and have the brain enclosed in a covering of harder material, -although this material may not have exactly the same form, or be made of -the same substance in all cases. Furthermore while we may recognize that -the cartilaginous skull of the shark is simpler in structure than that -of the cartilaginous-bony skull of the frog, and that the skull of the -frog is simpler than that of the rabbit, yet we should not be justified -in stating, except in a metaphorical sense, that something has been -added to the skull of the shark to make that of the frog, and something -to the latter to make that of the rabbit. On the contrary, while -something may have been added, and the plan made more complicated, the -skull has also been changed throughout in every single part. - -There is another point of some importance to be taken into account in -this connection; namely, that each new generation begins life as a -single cell or egg. The egg does not contain any preformed adult -structures that it hands down unaltered, but it is so constructed that, -under constant conditions, the same, or nearly the same, kind of -structure is produced. Should something affect the egg, we can imagine -that it might form a new combination on the same general plan as that of -the old, yet one that differed from the original in every detail of its -structure. It is this idea, I believe, that lies at the base of the -transmutation theory. On some such assumption as this, and on this -alone, can we bring the theory of transmutation into harmony with the -facts of observation. - -What has been said in regard to individuals as a whole may be repeated -also in respect to the study of the single organs. Selecting any one -group of the animal or plant kingdom, we find the same organ, or the -same combination of organs present in whole groups of forms. We can -often arrange these organs in definite series passing from the simple to -the complex, or, in case of degeneration, in the reverse order. However -convenient it may be to study the structure of organisms from this point -of view, the artificiality of the procedure will be obvious, since here -also the organs of any two species do not differ from each other in only -one point, but in many, perhaps in all. Therefore to arrange or to -compare them according to any one scheme gives only an incomplete idea -of their structure. We should apply here the same point of view that we -used above in forming a conception of the meaning of the zoological and -botanical systems. We must admit that our scheme is only an ideal, which -corresponds to nothing real in nature, but is an abstraction based on -the results of our experience. It might be a pleasing fancy to imagine -that this ideal scheme corresponds to the plan of structure or of -organization that is in every egg, and furnishes the basis for all the -variations that have come or may come into existence; but we should find -no justification whatsoever for believing that our fiction corresponds -to any such real thing. - -To sum up the discussion: we find that the resemblances of animals and -plants can be accounted for on the transmutation theory, not in the way -commonly implied, but in a somewhat different sense. We have found that -the resemblances between the different members of a group are only of a -very general sort, and the structures are not identically the same in -any two species—in fact, perhaps in no two individuals. This conclusion, -however, does not stand in contradiction to the transmutation -hypothesis, because, since each individual begins as an egg which is not -a replica of the original adult from which it is derived, there can be -no identity, but at most a very close similarity. Admitting, then, that -our scheme is an ideal one, we can claim, nevertheless, that on this -basis the facts of classification find a legitimate explanation in the -transmutation theory. - - - THE GEOLOGICAL EVIDENCE - -On the theory of descent, as well as on the theory of transmutation, the -ancestors of all present forms are supposed to have lived at some time -in the past on the surface of the earth. If, therefore, their remains -should have been preserved, we should expect on the descent theory to -find some, at least, of these remains to be like present forms, while on -the transmutation theory we should expect to find most, if not all, of -the ancestral forms to be different from the present ones. - -The evidence shows that fossil forms are practically all different from -living forms, and the older they are the greater the difference from -present forms. In general, therefore, it may be said that the evidence -is in favor of the transmutation theory. It can scarcely be claimed that -the evidence is absolutely conclusive, however probable it may appear, -for the problem is complicated in a number of ways. - -In the first place, there is convincing evidence that some forms have -been entirely exterminated. Other groups have very few living -representatives, as is the case in the group containing nautilus, and in -that of the crinoids. It is therefore always possible that a given -fossil form may represent an extinct line, and may be only indirectly -connected with forms alive at the present time. Again the historical -record is so broken and incomplete in all but a few cases that its -interpretation is largely a question of probability. We can easily -conceive that it would be only in very exceptional cases that successive -generations of the same form would be buried one above the other, so -that we should find the series unbroken. This is evident not only -because the conditions that were at one time favorable for the -preservation of organic remains might not be favorable at another time, -but also because if the conditions remained the same the organisms -themselves might also remain unchanged. A new form, in fact, would be, -_ex hypothese_, better suited to live in a different environment, and -consequently we should not expect always to find its remains in the same -place as that occupied by the parent species. This possibility of -migration of new forms into a new locality makes the interpretation of -the geological record extremely hazardous. - -Nevertheless, if the evolution of the entire animal and plant kingdoms -had taken place within the period between the first deposits of -stratified rocks and the present time, we might still have expected to -find, despite the imperfections of the record, sufficient evidence to -show how the present groups have arisen, and how they are related to one -another. But, unfortunately, at the period when the history of the rocks -begins, nearly all the large groups of animals were in existence, and -some of them, indeed, as the trilobites and the brachiopods, appear to -have reached the zenith of their development. - -On the other hand, the subdivisions of the group of vertebrates have -evolved during the period known to us. It is true that the group was -already formed when our knowledge of it begins, but, from the fishes -onwards, the history of the vertebrates is recorded in the rocks. The -highest group of all, the mammals, has arisen within relatively modern -times. The correctness of the transmutation theory could be as well -established by a single group of geological remains as by the entire -animal kingdom. Let us, therefore, examine how far the theory is -substantiated by the paleontological record of the vertebrates. We find -that the earliest vertebrates were fishes, and these were followed -successively by the amphibians, reptiles, birds, and mammals, one of the -last species of all to appear being man himself. There can be little -doubt that this series, with certain limitations to be spoken of in a -moment, represents a progressive series beginning with the simpler forms -and ending with the more complicated. Even did we not know this -geological sequence we would conclude, from the anatomical evidence -alone, that the progression had been in some such order as the -geological record shows. The limitation referred to above is this: that -while the mammals arose later than the birds, we need not suppose that -the mammals arose from the birds, and not even perhaps from the -reptiles, or at least not from reptiles like those living at the present -day. The mammals may in fact, as some anatomists believe, have come -direct from amphibian-like forms. If this is the case, we find the -amphibians giving rise on one hand to reptiles and these to birds, and -on the other hand to mammals. - -This case illustrates how careful we should be in interpreting the -record, since two or more separate branches or orders may arise -independently from the same lower group. If the mammals arose from the -amphibians later than did the reptiles, it would be easy to make the -mistake, if the record was incomplete at this stage, of supposing that -the mammals had come directly from the reptiles. - -That the birds arose as an offshoot from reptile-like forms is not only -probable on anatomical grounds, but the geological record has furnished -us with forms like archæopteryx, which in many ways appears to stand -midway between the reptiles and birds. This fossil, archæopteryx, has a -bird-like form with feathered wings, and at the same time has a beak -with reptilian teeth, and a long, feathered tail with a core of -vertebræ. - -From another point of view we see how difficult may be the -interpretation of the geological record, when we recall that throughout -the entire period of evolution of the vertebrates the fishes, -amphibians, reptiles, and birds remained still in existence, although -they, or some of them, may have at one time given origin to new forms. -In fact, all these groups are alive and in a flourishing condition at -the present time. The fact illustrates another point of importance, -namely, that we must not infer that because a group gives rise to a -higher one, that it itself goes out of existence, being exterminated by -the new form. There may be in fact no relation whatsoever between the -birth of a new group and the extermination of an old one. - -On the transmutation theory we should expect to find not only a sequence -of forms, beginning with the simplest and culminating with the more -complex, but also, in the beginning of each new group, forms more or -less intermediate in structure. It is claimed by all paleontologists -that such forms are really found. For example, transitional forms -between the fishes and the amphibia are found in the group of dipnoans, -or lung-fishes, a few of which have survived to the present day. There -are many fossil forms that have characters between those of amphibians -and reptiles, which if not the immediate ancestors of the reptiles, yet -show that at the time when this group is supposed to have arisen -intermediate forms were in existence. The famous archæopteryx remains -have been already referred to above, and it appears in this case that we -have not only an intermediate form, but possibly a transitional one. In -the group of mammals we find that the first forms to appear were the -marsupials, which are undoubtedly primitive members of the group. - -The most convincing evidence of transmutation is found in certain series -of forms that appear quite complete. The evolution of the horse series -is the most often cited. As this case will be discussed a little later, -we need not go into it fully here. It will suffice to point out that a -continuous series of forms has been found, that connect the living -horses having a single toe through three-toed, with the five-toed -horses. Moreover, and this is important, this series shows a -transformation not only in one set of structures, but in all other -structures. The fossil horses with three toes are found in the higher -geological layers, and those with more toes in the deeper layers -progressively. In some cases, at least, the fossils have been found in -the same part of the world, so that there is less risk of arranging them -arbitrarily in a series to fit in with the theory. - - - EVIDENCE FROM DIRECT OBSERVATION AND EXPERIMENT - -Within the period of human history we do not know of a single instance -of the transformation of one species into another one, if we apply the -most rigid and extreme tests used to distinguish wild species from each -other.[2] It may be claimed that the theory of descent is lacking, -therefore, in the most essential feature that it needs to place the -theory on a scientific basis. This must be admitted. On the other hand, -the absence of direct observation is not fatal to the hypothesis, for -several reasons. In the first place, it is only within the last few -hundred years that an accurate record of wild animals and plants has -been kept, so that we do not know except for this period whether any new -species have appeared. Again, the chance of observing the change might -not be very great, especially if the change were sudden. We would simply -find a new species, and could not state where it had come from. If, on -the other hand, the change were very slow, it might extend over so many -years that the period would be beyond the life of an individual man. In -only a few cases has it been possible to compare ancient pictures of -animals and plants with their prototypes living at the present time, and -it has turned out in all cases that they are the same. But these have -been almost entirely domesticated forms, where, even if a change had -been found, it might have been ascribed to other factors. In other -cases, as in the mummified remains of a few Egyptian wild animals (which -have also been found to be exactly like the same animals living at the -present day), it was pointed out by Geoffroy Saint-Hilaire that, since -the conditions of the Egyptian climate are the same to-day as they were -two thousand years ago, there is no reason to expect any change would -have taken place. But waiving this assumption, we should not forget that -the theory of evolution does not postulate that a change must take place -in the course of time, but only that it may take place sometimes. - -Footnote 2: - - The transformation of “smaller species,” described by De Vries, will - be described in a later chapter. - -The position that we have here taken in regard to the lack of evidence -as to the transformation of species is, perhaps, extreme, for, as will -be shown in some detail in later chapters, there is abundant evidence -proving that species have been seen to change greatly when the -conditions surrounding them have been changed; but never, as has been -stated, so far, or rather in such a way, that an actual new species that -is infertile with the original form has been produced. Whether, after -all, these changes due to a change in the environment are of the kind -that makes new species, is also a question to be discussed later. - -The experimental evidence, in favor of the transformation of species, -relates almost entirely to domesticated forms, and in this case the -conscious agency of man seems, in some cases, to have played an -important part; but here, even with the aid of the factor of isolation, -it cannot be claimed that a single new species has been produced, -although great changes in form have been effected. It is clear, -therefore, that we must, at present, rely on other data, less -satisfactory in all respects, to establish the probability of the theory -of transformation. - - - MODERN CRITICISM OF THE THEORY OF EVOLUTION - -Throughout the whole of the nineteenth century a steady fire of -criticism was directed against the theory of evolution; the names of -Cuvier and of Louis Agassiz stand out preëminent in this connection, yet -the theory has claimed an ever increasing number of adherents, until at -the present time it is rare to find a biologist who does not accept in -one form or another the general principle involved in the theory. The -storm of criticism aroused by the publication of Darwin’s “Origin of -Species,” was directed more against the doctrine of evolution than -against Darwin’s argument for natural selection. The ground has been -gone over so often that there would be little interest in going over it -again. It will be more profitable to turn our attention to the latest -attack on the theory from the ranks of the zoologists themselves. - -Fleischmann, in his recent book, “Die Descendenztheorie,” has made a new -assault on the theory of evolution from the three standpoints of -paleontology, comparative anatomy, and embryology. His general method is -to try to show that the recognized leaders in these different branches -of biology have been led to express essentially different views on the -same questions, or rather have compromised the doctrine by the examples -they have given to illustrate it. Fleischmann is fond of bringing -together the antiquated and generally exaggerated views of writers like -Haeckel, and contrasting them with more recent views on the same -subject, without making sufficient allowances for the advances in -knowledge that have taken place. He selects from each field a few -specific examples, by means of which he illustrates the weakness, and -even, as he believes, the falsity of the deductions drawn for the -particular case. For example, the plan of structure of the vertebrates -is dealt with in the following way: In this group the limbs, consisting -typically of a pair of fore-legs and a pair of hind-legs, appear under -the form of cylindrical outgrowths of the body. In the salamander, in -the turtle, in the dog, the cylindrical legs, supporting the body and -serving to support it above the ground, are used also for progression. -The general purpose to which the limbs are put as organs of locomotion -has not interfered with an astonishing number of varieties of structure, -adapted to different conditions of existence, such as the short legs -used for creeping in salamanders, lizards, turtles, crocodiles; the long -and thin legs of good runners, as the hoofed animals; the mobile legs of -the apes used for climbing; and the parachute legs of some squirrels -used for soaring. Even more striking is the great variety of hands and -feet, as seen in the flat, hairy foot of the bear; the fore-foot of the -armadillos, carrying long, sickle-shaped claws; the digging foot of the -mole; the plump foot of the elephant, ending in a broad, flat pad with -nails around the border, and without division into fingers; the hand of -man and of the apes ending with fine and delicate fingers for grasping. -To have discovered a general plan of structure running through such a -great variety of forms was proclaimed a triumph of anatomical study.[3] - -Footnote 3: - - This paragraph is a free translation of Fleischmann’s text. - -A study of the bony structure of the limb shows that typically it -consists of a single proximal bone (the humerus in the upper arm, the -femur in the thigh), followed by two bones running parallel to each -other (the radius and ulna in the arm and the tibia and fibula in the -shank); these are succeeded in the arm by the two series of carpal -bones, and in the leg by the two series of tarsal bones, and these are -followed in each by five longer bones (the metacarpals and metatarsals), -and these again by the series of long bones that lie in the fingers and -toes. Despite the manifold variety of forms, Fleischmann admits that -both the hind- and the fore-limbs are constructed on the same plan -throughout the vertebrates. Even forms like the camel, in which there -are fewer terminal bones, may be brought into the same category by -supposing a reduction of the bones to have taken place, so that three of -the digits have been lost. In the leg of the pig and of the reindeer, -even a greater reduction may be supposed to have taken place. -Fleischmann points out that these facts were supposed to be in full -harmony with the theory of descent. - -The analysis of the origin of the foot of the horse gave even better -evidence, it was claimed, in favor of the theory. The foot consists of a -single series of bones corresponding to the middle finger and toe. When, -as sometimes happens, individual horses are found in which in addition -to the single middle finger two smaller lateral fingers with small hoofs -appear, the followers of the descent theory rejoiced to be able to bring -this forward as a confirmation of their doctrine. The occurrence was -explained as a sporadic return to an ancestral form. The naïve -exposition of the laws of inheritance that were supposed to control such -phenomena was accepted without question. And when finally a large number -of fossil remains were found by paleontologists,—remains showing a -gradual increase in the middle finger, and a decrease in size of the -lateral fingers,—it was supposed that the proof was complete; and -anatomists even went so far as to hold that the original ancestor of the -horse was a five-fingered animal. - -This same law of type of structure was found to extend to the entire -vertebrate series, and the only plausible explanation appeared to be -that adopted by Darwin and his followers, namely, that the resemblance -is the result of the blood-relationship of the different forms. But a -simple comparison of the skeleton of the limbs if carried out without -theoretical prejudice would show, Fleischmann thinks, that there is only -a common style, or plan of structure, for the vertebrates. This -anatomical result has about the same value as the knowledge of the -different styles of historical architecture—that, for instance, all -large churches of the Gothic period have certain general principles in -common. The believers in the theory of descent have, however, he thinks, -gone beyond the facts, and have concluded that the common plan in -animals is the consequence of a common descent. “I cannot see the -necessity for such a conclusion, and I certainly should unhesitatingly -deny that the common plan of the Gothic churches depended on a common -architect. The illustration is, however, not perfect, because the -influence of the mediæval school of stone-cutters on its wandering -apprentices is well known.” - -Fleischmann adds that if the descent theory is true we should expect to -find that if a common plan of structure is present in one set of organs, -as the limbs, it should be present in all other organs as well, but he -does not add that this is generally the case. - -The weakness of Fleischmann’s argument is so apparent that we need not -attempt an elaborate refutation. When he says there is no absolute proof -that the common plan of structure must be the result of -blood-relationship, he is not bringing a fatal argument against the -theory of descent, for no one but an enthusiast sees anything more in -the explanation than a very probable theory that appears to account for -the facts. To demand an absolute proof for the theory is to ask for more -than any reasonable advocate of the descent theory claims for it. As I -have tried to show in the preceding pages, the evidence in favor of the -theory of descent is not absolutely demonstrative, but the theory is the -most satisfactory one that has as yet been advanced to account for the -facts. Fleischmann’s reference to the common plan of structure of the -Gothic churches is not very fortunate for his purpose, since he admits -himself that this may be the result of a common tradition handed down -from man to man, a sort of continuity that is not very dissimilar in -principle from that implied in the descent theory; in the latter the -continuity of substance taking the place of the tradition in the other. -Had the plan for each, or even for many of the churches, originated -independently in the mind of each architect, then the similarity in -style would have to be accounted for by a different sort of principle -from that involved in the theory of descent; but as a matter of fact the -historical evidence makes it probable that similar types of architecture -are largely the result of imitation and tradition. Certain variations -may have been added by each architect, but it is just the similarity of -type or plan that is generally supposed to be the outcome of a common -tradition. - -Fleischmann’s attempt in the following chapter to belittle Gegenbaur’s -theory of the origin of the five-fingered type of hand from a fin, like -that of a fish, need not detain us, since this theory is obviously only -a special application which like any other may be wrong, without in the -least injuring the general principle of descent. That all phylogenetic -questions are hazardous and difficult is only too obvious to any one -familiar with the literature of the last thirty years. - -Fleischmann devotes a long chapter to the geological evidences in -connection with the evolution of the horse, and attempts to throw -ridicule on the conclusions of the paleontologists by emphasizing the -differences of opinion that have been advanced in regard to the descent -of this form. After pointing out that the horse, and its few living -relatives, the ass and the zebra, are unique in the mammalian series in -possessing a single digit, he shows that by the discovery of the fossil -horses the group has been simply enlarged, and now includes horses with -one, three, and five toes. The discovery of the fossil forms was -interpreted by the advocates of the descent theory as a demonstration of -the theory. The series was arranged by paleontologists so that the -five-toed form came first, then those with three and one toe, the last -represented by the living horses. But the matter was not so simple, -Fleischmann points out, as it appeared to be to the earlier writers, for -example to Haeckel, Huxley, Leidy, Cope, Marsh. Different authors came -to express different opinions in regard to the genealogical connection -between the fossil forms. Several writers have tried to show that the -present genus, Equus, has not had a single line of descent, but have -supposed that the European horses and the original American horses had -different lines of ancestry, which may have united only far back in the -genus Epihippus. Fleischmann points out that the arrangement of the -series is open to the criticism that it is arbitrary, and that we could -equally well make up an analogous series beginning with the -five-fingered hand of man, then that of the dog with the thumb -incompletely developed, then the four-fingered hind-foot of the pig -without a big toe and with a weak second and fifth digit, then the foot -of the camel with only two toes, and lastly the foot of the horse with -only one toe. It sounds strange that Fleischmann should make such a -trivial reply as this, and deliberately ignore the all-important -evidence with which he is, of course, as is every zoologist, perfectly -conversant. Not only are there a hundred other points of agreement in -the horse series, but also the geological sequence of the strata, in -which some at least of the series have been found, shows that the -arrangement is not arbitrary, as he implies. - -Fleischmann then proceeds to point out that when the evidence from other -parts of the anatomy is taken into account, it becomes evident that all -the known fossil remains of horses cannot be arranged in a single line, -but that there are at least three families or groups recognizable. Many -of these forms are known only from fragments of their skeletons—a few -teeth, for instance, in the case of Merohippus, which on this evidence -alone has been placed at the uniting point of two series. At present -about eight different species of living horses are recognized by -zoologists, and paleontological evidence shows only that many other -species have been in existence, and that even three- and one-toed forms -lived together at the same time. - -Fleischmann also enters a protest against the ordinary arrangement of -the fossil genera Eo-, Oro-, Meso-, Merohippus in a series, for these -names stand not for single species, but for groups containing no less -than six species under Protohippus, fourteen under Equus, twelve under -Mesohippus, and twenty under Hipparion. Fleischmann concludes: “The -descent of the horses has not been made out with the precision of an -accurate proof, and it will require a great deal of work before we get -an exact and thorough knowledge of the fossil forms. What a striking -contrast is found on examination between the actual facts and the crude -hopes of the apostles of the descent theory!...” - -In so far as this criticism of Fleischmann’s applies to the difficulties -of determining the past history of the horse, it may be granted that he -has scored a point against those who have pretended that the evidence is -simple and conclusive; but we should not fail to remember that this -difficulty has been felt by paleontologists themselves, who have been -the first to call attention to the complexity of the problem, and to the -difficulties of finding out the actual ancestors of the living -representative of the series. And while we may admit that the early -enthusiasts exaggerated, unintentionally, the importance of the few -forms known to them, and went too far in supposing that they had found -the actual series of ancestors of living horses, yet we need not let -this blind us to the importance of the facts themselves. Despite the -fact that it may be difficult and, perhaps, in most cases, impossible, -to arrange the fossil forms in their relations to one another and to -living forms, yet on an unprejudiced view it will be clear, I think, -that so far as the evidence goes it is in full harmony with the theory -of descent. This is especially evident if we turn our attention to a -part of the subject that is almost entirely ignored by Fleischmann, and -yet is of fundamental importance in judging of the result. The series of -forms beginning with the five-toed horses and ending with those having a -single toe has not been brought together haphazard, as Fleischmann’s -comparison might lead one to suppose, but the five-fingered forms are -those from the older rocks, and the three-toed forms from more recent -layers. The value of this kind of evidence might have been open to -greater doubt had the series been made up of forms found scattered over -the whole world, for it is well known how difficult it is to compare in -point of time the rocks of different continents. But in certain parts of -the world, especially in North America, series of fossil horses have -been found in sedimentary deposits that appear to be perfectly -continuous. This series, by itself, and without regard to the point as -to whether in other parts of the world other series may exist, shows -exactly those results which the theory of descent postulates, and we -find here, in all probability, a direct line of descent. While it may be -freely admitted that no such series can demonstrate the theory of -descent with absolute certainty, yet it would be folly to disregard -evidence as clear as this. - -In regard to the other point raised by Fleischmann concerning the large -number of species of fossil horses that have existed in past times, it -is obvious that while this greatly increases the difficulty of the -paleontologist it is not an objection to the descent theory. In fact, -our experience with living species would lead us to expect that many -types have been represented at each geological period by a number of -related species that may have inhabited the same country. On the descent -theory, one species only in each geological period could have been in -the line of descent of the present species of horse. The difficulty of -determining which species (if there were several living in a given -epoch) is the ancestor of the horse is increased, but this is not in -itself an objection to the theory. - -The descent of birds from flying reptiles is used by Fleischmann as -another point of attack on the transmutation theory. The theory -postulates that the birds have come from ancestors whose fore-legs have -been changed into highly specialized wings. The long vertebrated tail of -the ancestral form is supposed to have become very short, and long -feathers to have grown out from its stump which act as a rudder during -flight. Flying reptiles with winged fore-legs and a long vertebrated -tail have been actually found as fossil remains, as seen in the -pterodactyls and in the famous archæopteryx. The latter, which is -generally regarded either as the immediate ancestor of living birds, or -at least as a closely similar form, possessed a fore-leg having three -fingers ending in claws, and feathers on the forearm similar to those of -modern birds. It had a long tail, like that of a lizard, but with -well-developed feathers along its sides. It had pointed teeth in the -horn-covered jaws. Fleischmann proceeds to point out that the -resemblance of the hand of archæopteryx to that of the reptiles is not -very close, for two fingers are absent as in modern birds. The typical -form of the foot is that of the bird, and is not the simple reptilian -type of structure. Feathers and not scales cover the body, and give no -clew as to how the feathers of birds have arisen. He concludes, -therefore, that archæopteryx, having many true bird-like characters, -such as feathers, union of bones in the foot, etc., has other characters -not possessed by living birds, namely, a long, vertebrated tail, a flat -breastbone, biconcave vertebræ, etc. Therefore, it cannot be regarded as -an intermediate form. Fleischmann does not point out that it is just -these characters that would be postulated on the descent theory for the -ancestor of the birds, if the latter arose from reptiles. Even if it -should turn out that archæopteryx is not the immediate forefather of -living birds, yet the discovery that a form really existed intermediate -in many characters between the reptiles and the birds is a gain for the -transmutation theory. It is from a group having such characters that the -theory postulates that the birds have been evolved, and to have -discovered a member of such a group speaks directly and unmistakably in -favor of the probability of the transmutation theory. - -Fleischmann again fails to point out that the geological period in which -the remains of archæopteryx were found, is the one just before that in -which the modern group of birds appeared, and, therefore, exactly the -one in which the theory demands the presence of intermediate forms. This -fact adds important evidence to the view that looks upon archæopteryx as -a form belonging to a group from which living birds have arisen. That a -number of recent paleontologists believe archæopteryx to belong to the -group of birds, rather than to the reptiles, or to an intermediate -group, does not in the least lessen its importance, as Fleischmann -pretends it does, as a form possessing a number of reptilian characters, -such as the transmutation theory postulates for the early ancestors of -the birds. - -The origin of the mammalian phylum serves as the text for another attack -on the transmutation theory. Fleischmann points out that the discovery -of the monotremes, including the forms ornithorhynchus and echidna, was -hailed at first as a demonstration of the supposed descent of the -mammals from a reptilian ancestor. The special points of resemblance -between ornithorhynchus and reptiles and birds are the complete fusion -of the skull bones, the great development of the vertebræ of the neck -region, certain similarities in the shoulder girdle, the paired oviducts -opening independently into the last part of the digestive tract -(cloaca), and the presence of a parchment-like shell around the large, -yolk-bearing egg. These are all points of resemblance to reptiles and -birds, and were interpreted as intermediate stages between the latter -groups and the group of mammals. In addition to these intermediate -characters, ornithorhynchus possesses some distinctive, mammalian -features—mammary glands and hair, for instance. Fleischmann takes the -ground, in this case, that there are so many points of difference -between the monotremes and the higher mammals, that it is impossible to -see how from forms like these the higher groups could have arisen, and -that ornithorhynchus cannot be placed as an intermediate form, a link -between saurians and mammals, as the followers of the transmutation -theory maintain. He shows, giving citations, that anatomists themselves -are by no means in accord as to the exact position of ornithorhynchus in -relation to the higher forms. - -In reply to this criticism, the same answer made above for archæopteryx -may be repeated here, namely, that because certain optimists have -declared the monotremes to be connecting forms, it does not follow that -the descent theory is untrue, and not even that these forms do not give -support to the theory, if in a less direct way. I doubt if any living -zoologist regards either ornithorhynchus or echidna as the ancestral -form from which the mammals have arisen. But on the other hand it may be -well not to forget that these two forms possess many characters -intermediate between those of mammals and reptiles, and it is from a -group having such intermediate characters that we should expect the -mammals to have arisen. These forms show, if they show nothing else, -that it is possible for a species to combine some of the characters of -the reptiles with those of the mammals; and the transmutation theory -does no more than postulate the existence at one time of such a group, -the different species of which may have differed in a number of points -from the two existing genera of monotremes. - -The origin of lung-bearing vertebrates from fishlike ancestors, in which -the swim-bladder has been changed into lungs, has been pointed to by the -advocates of the transmutation theory as receiving confirmation in the -existence of animals like those in the group of dipnoan fishes. In these -animals both gills and a swim-bladder, that can be used as a lung, are -present; and through some such intermediate forms it is generally -supposed that the lung-bearing animals have arisen. Fleischmann argues, -however, that, on account of certain trivial differences in the position -of the duct of the swim-bladder in living species, the supposed -comparison is not to the point; but the issue thus raised is too -unimportant to merit further discussion. Leaving aside also some even -more doubtful criticisms which are made by Fleischmann, and which might -be added to indefinitely without doing more than showing the credulity -of some of the more ardent followers of the transmutation theory, or -else the uncertainty of some of the special applications of the theory, -let us pass to Fleischmann’s criticism of the problem of development.[4] - -Footnote 4: - - The long argument of Fleischmann in regard to the origin of the - fresh-water snails, as illustrated by the planorbis series, and also - the origin of the nautiloid group, has been recently dealt with fully - by Plate, and, therefore, need not be considered here. - -With fine scorn Fleischmann points to the crudity of the ideas of Oken -and of Haeckel in regard to the embryology (or the ontogeny) repeating -the ancestral history (or the phylogeny). We may consider briefly (since -we devote the next chapter almost entirely to the same topic) the -exceptions to this supposed recapitulation, which Fleischmann has -brought together. The young of beetles, flies, and butterflies creep out -of the egg as small worm-like forms of apparently simple organization. -They have a long body, composed of a series of rings; the head is small -and lacks the feelers, and often the faceted eyes. The wings are absent, -and the legs are short. At first sight the larva appears to resemble a -worm, and this led Oken to conclude that the insects appear first in the -form of their ancestors, the segmented worms. If we examine the -structure of the larva more carefully, we shall find that there are a -great many differences between it and the segmented worms; and that even -the youngest larva is indeed a typical insect. The tracheæ, so -characteristic of the group of insects, are present, the structure of -the digestive tract with its Malpighian tubes, the form of the heart, -the structure of the head, as well as the blastema of the reproductive -organs, show in the youngest larva the type of the insects. In other -words the body of the caterpillar is formed on exactly the same -fundamental plan as that of the butterfly. - -In regard to the larval forms of other groups we find the same -relations, as, for example, in the amphibians. The young of salamanders, -toads, and frogs leave the egg not in the completed form, but as small -tadpoles adapted to life in the water. A certain resemblance to fish -cannot be denied. They possess a broad tail, gills (rich in blood -vessels) on each side of the neck, and limbs are absent for a long time. -These are characters similar to those of fish, but a more careful -anatomical examination destroys the apparent resemblance. The -superficial resemblances are due to adaptation to the same external -conditions. - -Fleischmann ridicules the idea that the young chick resembles at any -stage an adult, ancestral animal; the presence of an open digestive -tract shows how absurd such an idea is. The obvious contradiction is -explained away by embryologists, by supposing that the ancestral adult -stages have been crowded together in order to shorten the period of -development; and that, in addition, larval characters and provisional -organs have appeared in the embryo itself, which confuse and crowd out -the ancestral stages. - -In regard to the presence of gill-slits in the embryo of the higher -vertebrates, in the chick, and in man, for example, Fleischmann says: “I -cannot see how it can be shown by exact proof that the gill-slits of the -embryos of the higher vertebrates that remain small and finally -disappear could once have had the power of growing into functional -slits.” With this trite comment the subject is dismissed. - -On the whole, Fleischmann’s attack cannot be regarded as having -seriously weakened the theory of evolution. He has done, nevertheless, -good service in recalling the fact that, however probable the theory may -appear, the evidence is indirect and exact proof is still wanting. -Moreover, as I shall attempt to point out in the next chapter, we are -far from having arrived at a satisfactory idea of how the process has -really taken place. - - ------------------------------------------------------------------------- - - - - - CHAPTER III - - THE THEORY OF EVOLUTION (Continued) - - The Evidence from Embryology - - THE RECAPITULATION THEORY - - -At the close of the eighteenth, and more definitely at the beginning of -the nineteenth, century a number of naturalists called attention to the -remarkable resemblance between the embryos of higher animals and the -adult forms of lower animals. This idea was destined to play an -important rôle as one of the most convincing proofs of the theory of -evolution, and it is interesting to examine, in the first place, the -evidence that suggested to these earlier writers the theory that the -embryos of the higher forms pass through the adult stages of the lower -animals. - -The first definite reference[5] to the recapitulation view that I have -been able to find is that of Kielmeyer in 1793, which was inspired, he -says, by the resemblance of the tadpole of the frog to an adult fish.[6] -This suggested that the embryo of higher forms corresponds to the adult -stages of lower ones. He adds that man and birds are in their first -stages plantlike. - -Footnote 5: - - The earlier references of a few embryologists are too vague to have - any bearing on the subject. - -Footnote 6: - - Autenrieth in 1797 makes the briefest possible reference to some such - principle in speaking of the way in which the nose of the embryo - closes. - -Oken in 1805 gave the following fantastic account of this relation: -“Each animal ‘metamorphoses itself’ through all animal forms. The frog -appears first under the form of a mollusk in order to pass from this -stage to a higher one. The tadpole stage is a true snail; it has gills -which hang free at the sides of the body as is the case in _Unio -pictorum_. It has even a byssus, as in Mytilus, in order to cling to the -grass. The tail is nothing else than the foot of the snail. The -metamorphosis of an insect is a repetition of the whole class, -scolopendra, oniscus, julus, spider, crab.” - -Walther, in 1808, said: “The human fœtus passes through its -metamorphosis in the cavity of the uterus in such a way that it repeats -all classes of animals, but, remaining permanently in none, develops -more and more into the innate human form. First the embryo has the form -of a worm. It reaches the insect stage just before its metamorphosis. -The origin of the liver, the appearance of the different secretions, -etc., show clearly an advance from the class of the worm into that of -the mollusk.” - -Meckel first in 1808, again in 1811, and more fully in 1821 made much -more definite comparisons between the embryos of higher forms and the -adult stages of lower groups. He held that the embryo of higher forms, -before reaching its complete development, passes through many stages -that correspond to those at which the lower animals appear to be checked -through their whole life. In fact the embryos of higher animals, the -mammals, and especially man, correspond in the form of their organs, in -their number, position, and proportionate size to those of the animals -standing below them. The skin is at first, and for a considerable period -of embryonic life, soft, smooth, hairless, as in the zoophytes, medusæ, -many worms, mollusks, fishes, and even in the lower amphibians. Then -comes a period in which it becomes thicker and hairy, when it -corresponds to the skin of the higher animals. It should be especially -noted here, that the fœtus of the negro is more hairy than that of the -European. - -The muscular system of the embryo, owing to its lack of union in the -ventral wall, corresponds to the muscles of the shelled, headless -mollusks, whose mantle is open in the same region. Meckel compares the -bones of the higher vertebrates with the simpler bones of the lower -forms, and even with the cartilages of the cephalopod. He points out -that in the early human embryo the nerve cord extends the whole length -of the spinal canal. He compares the simple heart of the embryo with -that of worms, and a later stage, when two chambers are present, with -that of the gasteropod mollusk. The circulation of the blood in the -placenta recalls, he says, the circulation in the skin of the lower -animals. The lobulated form of the kidney in the human embryo is -compared with the adult condition in the fishes and amphibians. The -internal position of the reproductive organs in the higher mammals -recalls the permanent position of these organs in the lower animals. The -posterior end of the body of the human embryo extends backwards as a -tail which later disappears. - -Some of these comparisons of Meckel sound very absurd to us nowadays, -especially his comparison between the embryos of the higher vertebrates, -and the adults of worms, crustaceans, spiders, snails, bivalve mollusks, -cephalopods, etc. On the other hand, many of these comparisons are the -same as those that are to be found in modern text-books on embryology; -and we may do well to ask ourselves whether these may not sound equally -absurd a hundred years hence. Why do some of Meckel’s comparisons seem -so naïve, while others have a distinctly modern flavor? In a word, can -we justify the present belief of some embryologists that the embryos of -higher forms repeat the adult stages of lower members of the same group? -It is important to observe that up to this time the comparison had -always been made between the embryo of the higher form and the adult -forms of existing lower animals. The theory of evolution had, so far, -had no influence on the interpretation that was later given to this -resemblance. - -Von Baer opposed the theory of recapitulation that had become current -when he wrote in 1828. According to Von Baer, the more nearly related -two animals are, or rather the more nearly similar two forms are (since -Von Baer did not accept the idea of evolution), the more nearly alike is -their development, and so much longer in their development do they -follow in the same path. For example two similar species of pigeons will -follow the same method of development up to almost the last stage of -their formation. The embryos of these two forms will be practically -identical until each produces the special characters of its own species. -On the other hand two animals belonging to different families of the -same phylum will have only the earlier stages in common. Thus, a bird -and a mammal will have the first stages similar, or identical, and then -diverge, the mammal adding the higher characters of its group. The -resemblance is between corresponding embryonic stages and not between -the embryo of the mammal and the adult form of a lower group. - -Von Baer was also careful to compare embryos of the same phylum with -each other, and states explicitly that there are no grounds for -comparison between embryos of different groups.[7] - -Footnote 7: - - In one place Von Baer raises the question whether the egg may not be a - form common to all the phyla. - -We shall return again to Von Baer’s interpretation and then discuss its -value from our present point of view. - -Despite the different interpretation that Von Baer gave to this doctrine -of resemblance the older view of recapitulation continued to dominate -the thoughts of embryologists throughout the whole of the nineteenth -century. - -Louis Agassiz, in the Lowell Lectures of 1848, proposed for the first -time the theory that the embryo of higher forms resembled not so much -lower adult animals living at the present time, as those that lived in -past times. Since Agassiz himself did not accept the theory of -evolution, the interpretation that he gave to the recapitulation theory -did not have the importance that it was destined to have when the -animals that lived in the past came to be looked upon as the ancestors -of existing animals.[8] But with the acceptation of the theory of -evolution, which was largely the outcome of the publication of Darwin’s -“Origin of Species” in 1859, this new interpretation immediately -blossomed forth. In fact, it became almost a part of the new theory to -believe that the embryo of higher forms recapitulated the series of -ancestral adult forms through which the species had passed. The one -addition of any importance to the theory that was added by the Darwinian -school was that the history of the past, as exemplified by the embryonic -development, is often falsified. - -Footnote 8: - - Carl Vogt in 1842 suggested that fossil species, in their historical - succession, pass through changes similar to those which the embryos of - living forms undergo. - -Let us return once more to the facts and see which of them are regarded -at present as demanding an explanation. These facts are not very -numerous and yet sufficiently apparent to attract attention at once when -known. - -The most interesting case, and the one that has most often attracted -attention, is the occurrence of gill-clefts in the embryos of reptiles, -birds, and mammals. These appear on each side of the neck in the very -early embryo. Each is formed by a vertical pouch, that grows out from -the wall of the pharynx until it meets the skin, and, fusing with the -latter, the walls of the pouch separate, and a cleft is formed. This -vertical cleft, placing the cavity of the pharynx in communication with -the outside, is the gill-slit. Similar openings in adult fishes put the -pharynx in communication with the exterior, so that water taken through -the mouth passes out at the sides of the neck between the gill filaments -that border the gill-slits. In this way the blood is aerated. The number -of gill-slits that are found in the embryos of different groups of -higher vertebrates, and the number that open to the exterior are -variable; but the number of gill-openings that are present in the adults -of lower vertebrates is also variable. No one who has studied the method -of development of the gill-slits in the lower and higher vertebrates -will doubt for a moment that some kind of relation must subsist between -these structures. - -In the lowest adult form of the vertebrates, amphioxus, the gill-system -is used largely as a sieve for procuring food, partly also, perhaps, for -respiration. In the sharks, bony fishes, and lower amphibians, water is -taken in through the mouth, and passes through the gill-slits to the -exterior. As it goes through the slits it passes over the gills, that -stand like fringes on the sides of the slits. The blood that passes in -large quantities through the gills is aerated in this way. In the -embryos of the higher vertebrates the gill-slits may appear even before -the mouth has opened, but in no case is there a passage of water through -the gill-slits, nor is the blood aerated in the gill-region, although it -passes through this part on its way from the heart to the dorsal side of -the digestive tract. It is quite certain that the gill-system of the -embryo performs no respiratory function.[9] - -Footnote 9: - - This statement is not intended to prejudice the question as to whether - the presence of the gill-slits and arches may be essential to the - formation of other organs. - -In the higher amphibians, the frogs for example, we find an interesting -transition. The young embryo, when it emerges from the egg-membranes, -bears three pairs of external gills that project from the gill-arches -into the surrounding water. Later these are absorbed, and a new system -of internal gills, like those of fishes, develops on the gill-arches. -These are used throughout the tadpole stage for respiratory purposes. -When the tadpole is about to leave the water to become a frog, the -internal gills are also absorbed and the gill-clefts close. Lungs then -develop which become the permanent organs of respiration. - -There are two points to be noticed in this connection. First, the -external gills, which are the first to develop, do not seem to -correspond to any permanent adult stage of a lower group. Second, the -transition from the tadpole to the frog can only be used by way of -analogy of what is supposed to have taken place ancestrally in the -reptiles, birds, and mammals, since no one will maintain that the frogs -represent a group transitional between the amphibians and the higher -forms. However, since the salamanders also have gills and gill-slits in -the young stages, and lose them when they leave the water to become -adult land forms, this group will better serve to illustrate how the -gill-system has been lost in the higher forms. Not that in this case -either, need we suppose that the forms living to-day represent -ancestral, transitional forms, but only that they indicate how such a -remarkable change from a gill-breathing form, living in the water, might -become transformed into a lung-breathing land form. Such a change is -supposed to have taken place when the ancestors of the reptiles and the -mammals left the water to take up their abode on the land. - -The point to which I wish to draw especial attention in this connection -is that in the higher forms the gill-slits appear at a very early stage; -in fact, as early in the mammal as in the salamander or the fish, so -that if we suppose their appearance in the mammal is a repetition of the -adult amphibian stage, then, since this stage appears as early in the -development of the mammal as in the amphibians themselves, the -conclusion is somewhat paradoxical. - -The history of the notochord in the vertebrate series gives an -interesting parallel. In amphioxus it is a tough and firm cord that -extends from end to end of the body. On each side of it lie the plates -of muscles. It appears at a very early stage of development as a fold of -the upper wall of the digestive tract. In the cartilaginous fishes the -notochord also appears at a very early stage, and also from the dorsal -wall of the digestive tract. In later embryonic stages it becomes -surrounded by a cartilaginous sheath, or tube, which then segments into -blocks, the vertebræ. The notochord becomes partially obliterated as the -centra of the vertebræ are formed, but traces of it are present even in -adult stages. In the lower amphibians the notochord arises also at an -early stage over and perhaps, in part, from the dorsal wall of the -digestive tract. It is later almost entirely obliterated by the -development of the vertebræ. These vertebræ first appear as a -membraneous tube which breaks up into cartilaginous blocks, and these -are the structures around and in which the bone develops to form the -permanent vertebræ. - -In higher forms, reptiles, birds, and mammals, the notochord also -appears at the very beginning of the development, but it is not certain -that we can call the material out of which it forms the dorsal wall of -the archenteron (the amphibians giving, perhaps, intermediate stages). -It becomes surrounded by continuous tissue which breaks up into blocks, -and these become the bases of the vertebræ. The notochord becomes so -nearly obliterated in later stages that only the barest traces of it are -left either in the spaces between, or in, the vertebræ. - -In this series we see the higher forms passing through stages similar at -first to those through which the lower forms pass; and it is especially -worthy of note that the embryo mammal begins to produce its notochord at -the very beginning of its development, at a stage, in fact, so far as -comparison is possible, as early as that at which the notochord of -amphioxus develops. - -The development of the skull gives a somewhat similar case. The skulls -of sharks and skates are entirely cartilaginous and imperfectly enclose -the brain. The ganoids have added to the cartilaginous skull certain -plates in the dermal layer of the skin. In the higher forms we find the -skull composed of two sets of bones, one set developing from the -cartilage of the first-formed cranium, and the other having a more -superficial origin; the latter are called the membrane bones, and are -supposed to correspond to the dermal plates of the ganoids. - -In the development of the kidneys, or nephridia, we find, perhaps, -another parallel, although, owing to recent discoveries, we must be very -cautious in our interpretation. As yet, nothing corresponding to the -nephridia of amphioxus has been discovered in the other vertebrates. Our -comparison must begin, therefore, higher up in the series. In the sharks -and bony fishes the nephridia lie at the anterior end of the -body-cavity. In the amphibia there is present in the young tadpole a -pair of nephridial organs, the head-kidneys, also in the anterior end of -the body-cavity. Later these are replaced by another organ, the -permanent mid-kidney, that develops behind the head-kidney. In reptiles, -birds, and mammals a third nephridial organ, the hind-kidney, develops -later than and posterior to the mid-kidney, and becomes the permanent -organ of excretion. Thus in the development of the nephridial system in -the higher forms we find the same sequence, more or less, that is found -in the series of adult forms mentioned above. The anterior end of the -kidney develops first, then the middle part, and then the most -posterior. The anterior part disappears in the amphibians, the anterior -and the middle parts in the birds and mammals, so that in the latter -groups the permanent kidney is the hind-kidney alone. - -The formation of the heart is supposed to offer certain parallels. -Amphioxus is without a definite heart, but there is a ventral blood -vessel beneath the pharynx, which sends blood to the gill-system. This -blood vessel corresponds in position to the heart of other vertebrates. -In sharks we find a thick-walled muscular tube below the pharynx; the -blood enters at its posterior end, flows forward and out at the anterior -end into a blood vessel that sends smaller vessels up through the -gill-arches to the dorsal side. - -In the amphibia the heart is a tube, so twisted on itself that the -original posterior end is carried forward to the anterior end, and this -part, the auricle, is divided lengthwise by a partition into a right and -a left side. In the reptiles the ventricle is also partially separated -into two chambers, completely so in the crocodiles. In birds and mammals -the auricular and ventricular septa are complete in the adult, and the -ventral aorta that carries the blood forward from the heart is -completely divided into two vessels, one of which now carries blood to -the lungs. When we examine the development of the heart of a mammal, or -of a bird, we find something like a parallel series of stages, -apparently resembling conditions found in the different groups just -described. The heart is, at first, a straight tube, it then bends on -itself, and a constriction separates the auricular part from the -ventricular, and another the ventricular from the ventral aorta. -Vertical longitudinal partitions then arise, one of which separates the -auricle into two parts, and another the ventricle into two parts, and a -third divides the primitive aorta into two parts. In the early stages -all the blood passes from the single ventral aorta through the -gill-arches to the dorsal side, and it is only after the appearance of -the lung-system that the gill-system is largely obliterated. - -We find here, then, a sort of parallel, provided we do not inquire too -particularly into details. This comparison may be justified, at least so -far that the circulation is at first through the arches and is later -partially replaced by the double circulation, the systemic and the -pulmonary. - -A few other cases may also be added. The proverbial absence of teeth in -birds applies only to the adult condition, for, as first shown by -Geoffroy Saint-Hilaire, four thickenings, or ridges, develop in the -mouth of the embryo; two in the upper, two in the lower, jaw. These -ridges appear to correspond to those of reptiles and mammals, from which -the teeth develop. It may be said, therefore, that the rudiments of -teeth appear in the embryo of the bird. This might be interpreted to -mean that the embryo repeats the ancestral reptilian stage, or, perhaps, -the ancestral avian stage that had teeth in the beak; but since only the -beginnings of teeth appear, and not the fully formed structures, this -interpretation would clearly overshoot the mark. - -The embryo of the baleen whale has teeth that do not break through the -gums and are later absorbed. Since the ancestors of this whale probably -had teeth, as have other whales at the present time, the appearance of -teeth in the embryo has been interpreted as a repetition of the original -condition. Some of the ant-eaters are also toothless, but teeth appear -in the embryo and are lost later. In the ruminants that lack teeth in -the front part of the upper jaw, _e.g._ the cow and the sheep, teeth -develop in the embryo which are subsequently lost. - -One interpretation of these facts is that the ancestral adult condition -is repeated by the embryo, but as I have pointed out above in the cases -of the teeth in whales, since the teeth do not reach the adult form, and -do not even break through the gums in some forms, it is obviously -stretching a point to claim that an adult condition is repeated. -Moreover, in the case of the birds only the dental ridges appear, and it -is manifestly absurd to claim in this case that the ancestral adult -condition of the reptiles is repeated. - -That a supposed ancestral stage may be entirely lost in the embryo of -higher forms is beautifully shown in the development of some of the -snakes. The snakes are probably derived from lizardlike ancestors, which -had four legs, yet in the development the rudiments of legs do not -appear, and this is the more surprising since a few snakes have small -rudimentary legs. In these, of course, the rudiments of legs must appear -in the embryo, but in the legless forms even the beginnings of the legs -have been lost, or at any rate very nearly so. - -Outside the group of vertebrates there are also many cases that have -been interpreted as embryonic repetitions of ancestral stages, but a -brief examination will suffice to show that many of these cases are -doubtful, and others little less than fanciful. A few illustrations will -serve our purpose. The most interesting case is that given by the -history of the nauplius theory. - -The free-living larva of the lower crustaceans—water-fleas, barnacles, -copepods, ostracods—emerges from the egg as a small, flattened oval form -with three pairs of appendages. This larva, known as the nauplius, -occurs also in some of the higher crustaceans, not often, it is true, as -a free form, as in penæus, but as an embryonic stage. The occurrence of -this six-legged form throughout the group was interpreted by the -propounders of the nauplius theory as evidence sufficient to establish -the view that it represented the ancestor of the whole group of -Crustacea, which ancestor is, therefore, repeated as an embryonic form. -This hypothesis was accepted by a large number of eminent embryologists. -The history of the collapse of the theory is instructive. - -It had also been found in one of the groups of higher crustaceans, the -decapods, containing the crayfish, lobster, and crabs, that another -characteristic larval form was repeated in many cases. This larva is -known as the zoëa. It has a body made up of a fused head and thorax -carrying seven pairs of appendages and of a segmented abdomen of six -segments. The same kind of evidence that justified the formulation of -the nauplius theory would lead us to infer that the zoëa is the ancestor -of the decapods. The later development of the zoëa shows, however, that -it cannot be such an ancestral form, for, in order to reach the full -number of segments characteristic of the decapods, new segments are -intercalated between the cephalothorax and abdomen. In fact, in many -zoëas this intercalated region is already in existence in a rudimentary -condition, and small appendages may even be present. A study of the -comparative anatomy of the crustaceans leaves no grounds for supposing -that the decapods with their twenty-one segments have been evolved from -a thirteen-segmented form like the zoëa by the intercalation of eight -segments in the middle of the body. It follows, if this be admitted, and -it is generally admitted now, that the zoëa does not represent an -original ancestral form at all, but a highly modified new form, as new, -perhaps, as the group of decapods itself. We are forced to conclude, -then, that the presence of a larval form throughout an entire group -cannot be accepted as evidence that it represents an ancestral stage. We -can account for the presence of the zoëa, however, by making a single -supposition, namely, that the ancestor from which the group of decapod -has evolved had a larva like the zoëa, and that this larval form has -been handed down to all of the descendants. - -The fate of the zoëa theory cast a shadow over the nauplius theory, -since the two rested on the same sort of evidence. The outcome was, in -fact, that the nauplius theory was also abandoned, and this was seen to -be the more necessary, since a study of the internal anatomy of the -lowest group of crustaceans, the phyllopods, showed that they have -probably come directly from many segmented, annelidian ancestors. The -presence of the nauplius is now generally accounted for by supposing -that it was a larval form of the ancestor from which the group of -crustaceans arose. - -The most extreme, and in many ways the most uncritical, application of -the recapitulation theory was that made by Haeckel, more especially his -attempt to reduce all the higher animals to an ancestral double-walled -sac with an opening at one end,—the gastræa. He dignified the -recapitulation theory with an appellation of his own, “The Biogenetic -Law.” Haeckel’s fanciful and extreme application of the older -recapitulation theory has probably done more to bring the theory into -disrepute amongst embryologists than the criticisms of the opponents of -the theory. - -In one of the recognized masterpieces of embryological literature, His’s -“Unsere Körperform,” we find the strongest protest that has yet been -made against the Haeckelian pretension that the phylogenetic history is -the “cause” of the ontogenetic series. His writes: “In the entire series -of forms which a developing organism runs through, each form is the -necessary antecedent step of the following. If the embryo is to reach -the complicated end-forms, it must pass, step by step, through the -simpler ones. Each step of the series is the physiological consequence -of the preceding stage and the necessary condition for the following. -Jumps, or short cuts, of the developmental process, are unknown in the -physiological process of development. If embryonic forms are the -inevitable precedents of the mature forms, because the more complicated -forms must pass through the simpler ones, we can understand the fact -that paleontological forms are so often like the embryonic forms of -to-day. The paleontological forms are embryonal, because they have -remained at the lower stage of development, and the present embryos must -pass also through lower stages in order to reach the higher. But it is -by no means necessary for the later, higher forms to pass through -embryonal forms because their ancestors have once existed in this -condition. To take a special case, suppose in the course of generations -a species has increased its length of life gradually from one, two, -three years to eighty years. The last animal would have had ancestors -that lived for one year, two years, three years, etc., up to eighty -years. But who would claim that because the final eighty-year species -must pass necessarily through one, two, three years, etc., that it does -so because its ancestors lived one year, two years, three years, etc.? -The descent theory is correct so far as it maintains that older, simpler -forms have been the forefathers of later complicated forms. In this case -the resemblance of the older, simpler forms to the embryos of later -forms is explained without assuming any law of inheritance whatsoever. -The same resemblance between the older and simpler adult forms, and the -present embryonic forms would even remain intelligible were there no -relation at all between them.” - -Interesting and important as is this idea of His, it will not, I think, -be considered by most embryologists as giving an adequate explanation of -many facts that we now possess. It expresses, no doubt, a part of the -truth but not the whole truth. - -We come now to a consideration of certain recently ascertained facts -that put, as I shall try to show, the whole question of embryonic -repetition in a new light. - -A minute and accurate study of the early stages of division or cleavage -of the egg of annelids has shown a remarkable agreement throughout the -group. The work of E. B. Wilson on nereis, and on a number of other -forms, as well as the subsequent work of Mead, Child, and Treadwell on -other annelids, has shown resemblances in a large number of details, -involving some very complicated processes.[10] - -Footnote 10: - - On the other hand it should not pass unnoticed that Eisigh as shown in - one form (in which, however, the eggs are under special conditions - being closely packed together) that the usual type of cleavage is - altered. - -Not only is the same method of cleavage found in most annelids, but the -same identical form of division is also present in many of the mollusks, -as shown especially by the work of Conklin, Lillie, and Holmes. This -resemblance has been discussed at some length by those who have worked -out these results in the two groups. The general conclusion reached by -them is that the only possible interpretation of the phenomenon is that -some sort of genetic connection must exist between the different forms; -and while not explicitly stated, yet there is not much doubt that some -at least of these authors have had in mind the view that the annelids -and mollusks are descended from common ancestors whose eggs segmented as -do those of most of the mollusks and annelids of the present day. This -conclusion is, I believe, of more far-reaching importance than has been -supposed, and may furnish the key that will unlock the whole question of -the resemblance of embryos to supposed ancestral forms. It is a most -fortunate circumstance that in the case of this cell lineage the facts -are of such a kind as to preclude the possibility that the stages in -common could ever have been ancestral adult stages. If this be granted -then only two interpretations are possible: the results are due either -to a coincidence, or to a common embryonic form that is repeated in the -embryo of many of the descendants. That the similarity is not due to a -coincidence is made probable from the number and the complexities of the -cleavage stages. - -I believe that we can extend this same interpretation to all other cases -of embryonic resemblance. It will explain the occurrence of gill-slits -in the embryo of the bird, and the presence of a notochord in the higher -forms in exactly the same way as the cleavage stages are explained. But -how, it may be asked, can we explain the apparent resemblance between -the embryo of the higher form and the adult of lower groups. The answer -is that this resemblance is deceptive, and in so far as there is a -resemblance it depends on the resemblance of the adult of the lower form -to its own embryonic stages with which we can really make a comparison. -The gill-slits of the embryo of the chick are to be compared, not with -those of the adult fish, but with those of the embryo of the fish. It is -a significant fact, in this connection, that the gill-slits appear as -early in the embryo of the fish as they do in the bird! The notochord of -the embryo bird is comparable with that of the embryo of amphioxus, and -not with the persistent notochord in the adult amphioxus. Here also it -is of the first importance to find that the notochord appears both in -the embryo bird and in amphioxus at the very beginning of the -development. The embryo bird is not fishlike except in so far as there -are certain organs in the embryo fish that are retained in the adult -form. The embryo bird bears the same relation to the embryo fish that -the early segmentation stages of the mollusk bear to the early -segmentation stages of the annelid. There are certain obvious -resemblances between this view and that of Von Baer, but there are also -some fundamental differences between the two conceptions. - -Von Baer thought that within each group the embryonic development is the -same up to a certain point. He supposed that the characters of the group -are the first to appear, then those of the order, class, family, genus, -and, finally, of the species. He supposed that two similar species would -follow the same method of development until the very last stage was -reached, when each would then add the final touches that give the -individual its specific character. We may call this the theory of -embryonic parallelism. Here there is an important difference between my -view and that of Von Baer, for I should not expect to find the two -embryos of any two species identical at any stage of their development, -but at most there might exist a close resemblance between them. - -Von Baer’s statement appears to be erroneous from a modern point of view -in the following respects. We know that in certain large groups some -forms develop in a very different way from that followed by other -members of the group, as shown by the cephalopods, for instance, in the -group of mollusks. Again, it is entirely arbitrary to assume that the -group-characters are the first to appear, and then successively those of -the order, family, genus, species. Finally, as has been said above, we -do not find the early embryos of a group identical; for with a -sufficient knowledge of the development it is always possible to -distinguish between the embryos of different species, as well as between -the adults, only it is more difficult to do so, because the embryonic -forms are simpler. The most fundamental difference between the view of -Von Baer and modern views is due to our acceptation of the theory of -evolution which seems to make it possible to get a deeper insight into -the meaning of the repetition, that carries us far ahead of Von Baer’s -position. For with the acceptance of this doctrine we have an -interpretation of how it is possible for the embryonic stages of most -members of a group to have the same form, although they are not -identical. There has been a continuous, although divergent, stream of -living material, carrying along with it the substance out of which the -similar embryonic forms are made. As the stream of embryonic material -divided into different paths it has also changed many of the details, -sometimes even all; but nevertheless it has often retained the same -general method of development that is associated with its particular -composition. We find the likeness, in the sense of similarity of plan, -accounted for by the inheritance of the same sort of substance; the -differences in the development must be accounted for in some other way. - -Among modern writers Hurst alone has advanced a view that is similar in -several respects to that which I have here defended. It may be well to -give his statement, since it brings out certain points of resemblance -with, as well as certain differences from, my own view.[11] He says: -“Direct observation has shown that, when an animal species _varies_ -(_i.e._ _becomes_ unlike what it was before) in adult structure, those -stages in the development which are nearest the adult undergo a similar, -but usually smaller, change. This is shown in domestic species by the -observations of Darwin, and the result is in exact harmony with the -well-known law of Von Baer, which refers to natural species, both nearly -related and widely dissimilar. Von Baer’s observations as well as -Darwin’s, and as well as those of every student who has ever compared -the embryos of two vertebrate species, may be summarized as follows:— - -Footnote 11: - - Hurst, C. H., “Biological Theories, III,” “The Recapitulation Theory,” - _Natural Science_, Vol. ii., 1893. - -“Animals which, though related, are very similar in the adult state, -resemble each other more closely in early stages of development, often, -indeed, so closely as to be indistinguishable in those early stages. As -development proceeds _in such species_, the differences between the two -embryos compared become more and more pronounced.” On this point, which -is an essential one, I cannot agree with Hurst; for I do not think that -the facts show that the early stages of two related forms are -necessarily more and more alike the farther back we go. The resemblance -that is sometimes so striking in the earlier stages is due to the fewer -points there are for comparison, and to the less development of the -parts then present. Hurst continues: “If similar comparisons could be -instituted between the ancestral species and its much modified -descendants, there is no reason for doubting that a similar result would -be reached. This, indeed, has been done in the case of some breeds of -pigeons, which we have excellent reasons for believing to be descended -from _Columba livia_. True, _C. livia_ is not a very remote ancestor, -but I do not think that will vitiate the argument. Let me quote Darwin -verbatim: ‘As we have conclusive evidence that the breeds of the pigeon -are descended from a single wild species, I have compared the young -within twelve hours after being hatched; I have carefully measured the -proportions (but will not here give the details) of the beak, width of -mouth, length of nostril, and of eyelid, size of feet, and length of leg -in the wild, parent species, in pouters, fantails, runts, barbs, -dragons, carriers, and tumblers. Now some of these birds when mature -differ in so extraordinary a manner in the length and form of the beak, -and in other characters, that they would certainly have been ranked as -distinct genera if found in a state of nature. But when the nestling -birds of these several breeds were placed in a row, though most of them -could just be distinguished, the proportional differences in the above -specified points were incomparably less than in the full-grown birds. -Some characteristic points of difference—for instance, that of the width -of the mouth—could hardly be detected in the young. But there was one -remarkable exception to this rule, for the young of the short-faced -tumbler differed from the young of the wild-rock pigeon, and of the -other breeds in almost exactly the same proportions as in the adult -state.’” - -Hurst concludes that: “The more the adult structure comes to be unlike -the adult structure of the ancestors, the more do the late stages of -development undergo a modification of the same kind. This is not mere -dogma, but it is a simple paraphrase of Von Baer’s law. It is proved -true not only by the observations of Von Baer and of Darwin, already -referred to, but by the direct observation of every one who takes the -trouble to compare the embryos of any two vertebrates, provided only he -will be content to see what actually lies before him and not the -phantasms which the recapitulation theory may have printed on his -imagination.” - -The growth of the antlers of stags is cited by Hurst in order to -illustrate that what has been interpreted as a recapitulation may have a -different interpretation. “Each stag develops a new pair of antlers in -each successive year, and each pair of antlers is larger than the pair -produced in the previous year. This yearly increase in the size of the -antlers has been put forward as an example of an ontogenetic record of -past evolution. I, however, deny that it is such a record.” - -“The series of ancestors may have possessed larger antlers in each -generation than in the generation before it. It is not an occasional -accidental parallelism between the ontogeny and the phylogeny which I -deny, but the causal relation between the two. Had the ancestors had -larger antlers than the existing ones, there is no justification for the -assumption that existing stags would acquire antlers of which each pair, -in later years, would be smaller than those of the previous year.” - -Hurst concludes: “There are many breeds of hornless sheep, but they do -not bear large horns in early years and then shed them. If a rudiment -ever appears in the embryo of such sheep, its growth is very early -arrested.” The case of the appendix in man might have been cited here as -a case in point. It is supposed to have been larger in the ancestors of -man, but we do not find it appearing full size in the embryo and later -becoming rudimentary. The preceding statements will show that, while -Hurst’s view is similar in some respects to my own, yet it differs in -one fundamental respect from it, and in this regard he approaches more -nearly to the theory of Von Baer. - -Hertwig has recently raised some new points of issue in regard to the -recapitulation theory, and since he may appear to have penetrated -farther than most other embryologists of the present time, it will be -necessary to examine his view somewhat carefully. He speaks of the -germ-cell (egg, or spermatozoön) as a species-cell, because it contains, -in its finer organization, the essential features of the species to -which it belongs. There are as many of these kinds of cells as there are -different kinds of animals and plants. Since the bodies of the higher -animals have developed from these species-cells, so the latter must have -passed in their phylogeny through a corresponding development from a -simple to a more and more complex cell-structure. “Our doctrine is, that -the species-cell, even as the adult, many-celled representative of the -species, has passed through a progressive, and, indeed, in general a -corresponding development in the course of phylogeny. This view appears -to stand in contradiction to the biogenetic law. According to the -formula that Haeckel has maintained, the germ development is an epitome -of the genealogy; or the ontogeny is a recapitulation of the phylogeny; -or, more fully, the series of forms through which the individual -organism passes during its development from the egg-cell to the finished -condition is a short, compressed repetition of the longer series of -forms which the forefathers of the same organism, or the stem-form of -the species, has passed through, from the earliest appearance of -organisms to the present time.” “Haeckel admits that the parallel may be -obliterated, since much may be absent in the ontogeny that formerly -existed in the phylogeny. If the ontogeny were complete, we could trace -the whole ancestry.” Hertwig states further, that “The theory of -biogenesis[12] makes it necessary to change Haeckel’s expression of the -biogenetic law, so that a contradiction contained in it may be removed. -We must drop the expression ‘repetition of the form of extinct -forefathers,’ and put in its place the repetition of forms which are -necessary for organic development, and lead from the simple to the -complex. This conception may be illustrated by the egg-cell.” - -Footnote 12: - - This term, by which Hertwig designates a particular view of his own, - has been already preoccupied in a much wider sense by Huxley to mean - that all life comes from preëxisting life. Hertwig means by the theory - of biogenesis that as the egg develops there is a constant interchange - between itself and its surroundings. - -Since each organism begins its life as an egg we must not suppose that -the primitive conditions of the time, when only single-celled amœbas -existed on our planet, are repeated. The egg-cell of a living mammal is -not, according to Hertwig’s hypothesis, an indifferent structure without -much specialization like an amœba, but is an extraordinarily complex -end-product of a long historical process, which the organized substance -has passed through. If the egg of a mammal is different from that of a -reptile, or of an amphibian, because in its organization it contains the -basis of a mammal, just so much more must it be different from the -hypothetical one-celled amœba, which has no other characteristics than -those that go to make up an amœba. Expressed more generally, the -developmental process in the many-celled organisms begins, not where it -began in primitive times, but as the representation of the highest point -which the organization has at present reached. The development commences -with the egg, because it is the elemental and fundamental form in which -organic life is represented in connection with the reproductive process, -and also because it contains in itself the properties of the species in -its primordia. - -“The egg-cell of the present time, and its one-celled predecessor in the -phylogenetic history, the amœba, are only comparable in so far as they -fall under the common definition of the cell, but beyond this they are -extraordinarily different from each other.” - -“The phyletic series must be divided into two different kinds of -processes:—First. The evolution of the species-cell, which is a steady -advance from a simple to a complex organization. Second. The -periodically repeated development of the many-celled individual out of -the single cell, representative of the species (or the individual -ontogeny), which in general follows the same rules as the preceding -ontogeny, but is each time somewhat modified according to the amount to -which the species-cell has itself been changed in the phylogeny. Similar -restricting and explanatory additions to the biogenetic law, like those -stated here for the one-celled stage, must be made in other directions. -Undoubtedly there exists in a certain sense a parallel between the -phylogenetic, and the ontogenetic, development. - -“On the basis of the general developmental hypothesis on which we stand, -all forms which in the chain of ancestors were end-products of the -individual development are now passed through by their descendants as -embryonic stages, and so in a certain degree are recapitulated. We also -admit that the embryonic forms of higher animals have many points of -comparison with the mature forms of related groups standing lower in the -system. - -“Nevertheless, a deeper insight into the conditions relating to these -resemblances shows that there are very important differences that should -not be overlooked. Three points need to be mentioned: 1. The -cell-material which in the ancestral chain gives the basis for each -ontogenetic process is each time a different material as far as concerns -its finer organization and primordia. Indeed, the differences become -greater the farther apart the links of the original chain become. This -thought may be formulated in another way: The same ontogenetic stages -that repeat themselves periodically in the course of the phylogeny -always contain at bottom a somewhat different cell-material. From this -the second rule follows as a consequence. 2. Between the mature end-form -of an ancestor and the corresponding embryonic form of a widely remote -descendant (let us say between the phylogenetic gastræa and the -embryonic gastrula stage of a living mammal, according to the -terminology of Haeckel) there exists an important difference, namely, -that the latter is supplied with numerous primordia which are absent in -the other, and which force it to proceed to the realization of its -developmental process. The gastrula, therefore, as the bearer of -important latent forces, is an entirely different thing from the -gastræa, which has already reached the goal of its development. 3. In -the third place, at each stage of the ontogeny outer and inner factors -are at work, in fact even more intensely than in the fully formed -organism. Each smallest change that acts anew in this way at the -beginning of the ontogeny can start an impulse leading to more extensive -changes in later stages. Thus the presence of yolk and its method of -distribution in the egg alone suffice to bring about important changes -in the cleavage, and in the formation of the germ-layers, the blastula, -and gastrula stages,” etc. “Moreover, the embryo may adapt itself to -special conditions of embryonic life, and produce organs of an ephemeral -nature like the amnion, chorion, and placenta.” - -“A comparison of ontogenetic with antecedent phylogenetic stages must -always keep in view the fact that the action of external and internal -factors has brought about considerable changes in the ontogenetic -system, and, indeed, in a generally advancing direction, so that in -reality a later condition can never correspond to a preceding one.” - -Hertwig sums up his conclusion in the statement that ontogenetic stages -give us, therefore, a greatly changed picture of the phylogenetic series -of adult ancestors. “The two correspond not according to their actual -contents but only as to their form.” Hertwig also repeats His’s idea, -that the reason that certain kinds of form repeat themselves in the -development of animals with a great constancy depends principally on -this, that they supply the necessary conditions under which alone the -following higher stage of the ontogeny can be formed. The development, -for instance, begins with the division of the egg, because this is the -only way that a one-celled condition can give rise to a many-celled -form. Again, the organs can be formed only when groups of cells have -made a closer union with one another. Thus the gastrula must begin with -the antecedent blastula, etc. Definite forms are, despite all modifying -influences, held to firmly, because by their presence the complicated -end-stages can be reached in the simplest and most suitable way. - -Thus Hertwig adopts here a little from one doctrine and there a little -from another, and between his attempt to reinstate the old biogenetic -law of Haeckel, and to adopt a more modern point of view, he brings -together a rather curious collection of statements which are not any too -well coördinated. Take, for example, his description of the relation -between Haeckel’s gastræa and the embryonic gastrula stage. The latter -he maintains is a repetition of the other, but only in form, not in -actual contents. And in another connection we are told that the cause of -this repetition is that the gastrula is the simplest way in which the -later stages can be reached, and, therefore, it has been retained. It -seems to me that Hertwig has undertaken an unnecessary and impossible -task when he attempts to adjust the old recapitulation theory to more -modern standards. His statement that the egg is entirely different from -its amœba prototype is, of course, only the view generally held by all -embryologists. His mystical statement that the embryonic form _repeats -the ancestral adult stage in its form, but not in its contents_, will -scarcely recommend itself as a model of clear thinking. Can we be asked -to believe for instance that a young chick repeats the ancestral adult -fish form but not the contents of the fish? - -In conclusion, then, it seems to me that _the idea that adult ancestral -stages have been pushed back into the embryo, and that the embryo -recapitulates in part these ancestral adult stages is in principle -false_. The resemblance between the embryos of higher forms and the -adults of lower forms is due, as I have tried to show, to the presence -in the embryos of the lower groups of certain organs that remain in the -adult forms of this group. It is only the embryonic stages of the two -groups that we are justified in comparing; and their resemblances are -explained on the assumption that there has been an ancestral adult form -having these embryonic stages in its development and these stages have -been handed down to the divergent lines of its descendants. - -Since we have come to associate with the name of the recapitulation -theory the idea of the recurrence of an ancestral adult form, it may be -better to find a substitute for this term. I suggest, therefore, for the -view, that the embryos of the higher group repeat the modified form of -the embryos of the lower groups, the term, the theory of embryonic -repetition, or, more briefly, the repetition theory. - - - Conclusions - -In the light of the preceding discussion concerning the evidence in -favor of the transmutation theory, we may now proceed to sum up our -general conclusions, and at the same time discuss some further -possibilities in regard to the descent theory. - -The most widely accepted view in regard to the theory of organic -evolution is that which looks upon the resemblances between the members -of a group as due to their common descent from one original species that -has broken up, as it were, into a number of new forms. Strictly applied, -this means that all the vertebrates have come from one original species, -all the mollusks from another, the echinoderms from a third, etc. Even -farther back there may have been a common ancestral species for any two -of the large groups, as, for example, the annelids and the mollusks; and -if the relationship of all the many-celled forms be looked upon as -probable, then they too have originated from one ancestral species. - -Many zoologists appear to hesitate to apply strictly this fundamental -idea contained in the transmutation theory, because, perhaps, they feel -that it does not fit in with their general experience of living forms. -Yet there can be no doubt that it is the primary conception of the -transmutation theory. This is, however, not the whole question, for we -must further consider the number of individuals of a species that are -involved. - -In some species there are smaller groups of individuals that are more -like one another than like other individuals of the same species. Such -groups are called varieties, and are often associated with certain -localities, or with a special environment. In the latter case they are -called local varieties. Some of these appear to breed true, not only -when kept under the same conditions, but even when transferred to a new -environment. Others change with the environment. It is not improbable -that the varieties are of a different kind in these two cases, as shown -by their different behavior when put under new and different -surroundings. The variety that owes its peculiarities, not to the -immediate environment, but to some internal condition independent of the -surroundings, is recognized by some biologists as a smaller species. -Such species appear to be commoner in plants than in animals, although -it is possible that this only means that more cases have been found by -the botanists, owing to the greater ease with which plants can be -handled. These smaller species, in contradistinction to the ordinary -Linnæan species, differ from the latter in the smaller amount of -differences between the groups, and probably also in that they freely -interbreed, and leave fertile descendants; but whether this is only on -account of the smaller differences between them than between larger -species, or because of some more fundamental difference in the kind of -variation that gives rise to these two kinds of groups, we do not know. - -These smaller species, or constant varieties, as we may call them, may -be looked upon as incipient Linnæan species, which, by further -variations of the same, or of other sorts, may end by giving rise to -true species. A genus composed of several species might be formed in -this way, and then, if each species again broke up into a number of new -groups, each such group would now be recognized as a genus, and the -group of genera would form a family, etc. The process continuing, a -whole class, or order, or even phylum, might be the result of this -process that began in a single species. - -But we must look still farther, and inquire whether the start was made -from a single individual, that began to vary, or from a number of -individuals, or even from all the individuals, of a species. If we -suppose the result to depend on some external cause that affects all the -individuals of a species alike, then it might appear that the species, -or at least as many individuals of a species as are affected, will give -the starting-point for the new group. But if the new variation arises -not directly as a response to some change in the surroundings, then it -might appear in one or in a few individuals at a time. Let us consider -what the results might be under these two heads. - -If amongst the descendants of a single individual a new form or a number -of new forms were to arise, then, if they represented only a variety, -they would cross with the other forms like the parent species; and, -under these conditions, it is generally assumed that the new variety -would be swamped. If, however, the new forms have the value of new -species, then, _ex hypothese_, they are no longer fertile with the -original forms, and might perpetuate themselves by self-fertilization, -as would be possible in some of the higher plants, and in those animals -that are bisexual. But as a rule even bisexual forms are not -self-fertilized, and, therefore, unless a number of offspring arose from -the same form the chance of propagation would be small. - -If, however, a number of new forms appeared at the same time and left a -number of descendants, then the probability that the new group might -perpetuate itself is greater, and the chance that such a group would -arise is in proportion to the number of individuals that varied in the -same direction simultaneously. In this case the new species has not come -from a single individual or even from a pair of individuals, but from a -number of individuals that have varied more or less in the same -direction. - -This point of view puts the descent theory in a somewhat unforeseen -light, for we cannot assume in such a case that the similarities of the -members of even the same species are due to direct descent from an -original ancestor, because there are supposed to have been a number of -ancestors that have all changed in the same direction. The question is -further complicated by the fact that the new individuals begin to -interbreed, so that their descendants come to have, after a time, the -common blood, so to speak, of all the new forms. If with each union -there is a blending of the substances of the individuals, there will -result in the end a common substance representing the commingled racial -germ-plasm. - -A new starting-point is then reached, and new species may continue to be -formed out of this homogeneous material. Thus, in a sense, we have -reached a position which, although it appears at first quite different -from the ordinary view, yet, after all, gives us the same standpoint as -that assumed by the transmutation theory; for, while the latter assumes -that the resemblances of the members of a group are due to descent from -the same original form, and often by implication from a single -individual, we have here reached the conclusion that it is only a -common, commingled germ-plasm that is the common inheritance. - -When we examine almost any group of living animals or plants, whether -they are low or high in organization, we find that it is composed of a -great many different species, and so far as geology gives any answer, we -find that this must have been true in the past also. Why, then, do we -suppose that all the members of the higher groups have come from a -single original species or variety? Why may not all, or many, of the -similar species of the lower group have changed into the species of the -higher group,—species for species? If this happened, the resemblance of -the new species of the group could be accounted for on the supposition -that their ancestors were also like one another. The likeness would not -be due, then, to a common descent, and it would be false to attempt to -explain their likeness as due to a common inheritance. But before going -farther, it may be well to inquire to what the resemblances of the -individuals of the original species were due; for, if they have come -from an older group that has given rise to divergent lines of descent, -then we are only removing the explanation one step farther back. If this -original group has come from numerous species of a still older group, -and this, in turn, from an older one still, then we must go back to the -first forms of life that appeared on the globe, and suppose that the -individuals of these primitive forms are the originals of the species -that we find living to-day. For instance, it is thinkable that each -species of vertebrate arose from a single group of the earliest forms of -life that appeared on the surface of the earth. If this were the case, -there must have been as many different kinds of species of the original -group as there are species alive at the present time, and throughout all -the past. This view finds no support from our knowledge of fossil -remains, and, although it may be admitted that this knowledge is very -incomplete, yet, if the process of evolution had taken place as sketched -out above, we should expect, at least, to have found some traces of it -amongst fossil forms. Since this question is an historical one, we can, -at best, only expect to decide which of all the possible suggestions is -the more probable. - -We conclude, then, that it is more probable that the vertebrates, the -mollusks, the insects, the crustaceans, the annelids, the cœlenterates, -and the sponges, etc., have come each from a single original species. -Their resemblances are due to a common inheritance from a common -ancestral species. Even if it be probable that at the time when the -group of vertebrates arose from a single species, there were in -existence other closely related species, yet we must suppose, if we -adhere to our point of view, that these other related species have had -nothing to do with the group of vertebrates, but that they have died -out. Moreover, we must suppose that each order, each class of -vertebrate, has come from a single original species; each family has had -a similar origin, as well as each genus, but, of course, at different -periods of time. Let us not shrink from carrying this principle to its -most extreme point, for, unless the principle is absolutely true, then -our much boasted explanation of the resemblances of forms in the same -group will be thrown into hopeless confusion. - -Let us ask another question in this connection. If a single species gave -rise to a group of new species that represented the first vertebrates, -they would have formed the first genus; and if the descendants of these -diverged again so that new genera were formed, then a group which we -should call a family would have been formed. - -As the divergence went on, an _order_ would be developed, and then a -_class_, and then a _phylum_. The common characters possessed by the -members of this phylum would have been present in the original species -that began to diverge. Hence, we find the definition of the phylum -containing only those points that are the features possessed by all of -the descendants, and in the same way we should try to construct the -definition of each of the subordinate groups. This is the ideal of the -principle of classification based on the theory of descent with -divergence. If we admit the possibility of the other view that I have -mentioned above, or of any other of the numerous possibilities that will -readily suggest themselves, then we must be prepared to give up some of -the most attractive features of the explanation of resemblance as due to -descent. - -That all biologists believe strictly in divergent descent, to the -exclusion of any other processes, is not the case. And, as I have said -before, since we are dealing with an historical question, it would be -very unwise, in our present ignorance on many points, to pretend that we -have any direct proof of the explanation that we find generally given to -account for the resemblances of the species of a group to each other. At -most we can claim that it is the simplest point of view, and that most -biologists believe it to be also the most probable. It has been -suggested that, in some cases, the new forms that arise from two or more -species run a parallel course. If the original forms from which they -came were very much alike, it would soon be impossible to say what the -parentage of a particular form was; that is, to which of the two -original forms it belonged. It has also been suggested that even a -convergence has at times taken place, so that the descendants of -different species have become more alike than the original forms, _at -least in some one or more respects_. This last limitation is the saving -clause, for species differ in so many points that, even when they -converge in a few, it is unlikely that they will do so in all, and, -therefore, the deception may be discovered by the acute observer. One -famous paleontologist has gone so far even as to suppose that a species -may change its generic characters, so that it goes over bodily into a -new genus without losing its specific characters. If such things do -occur, then our classifications may well be the laughing-stock of -Nature. - - ------------------------------------------------------------------------- - - - - - CHAPTER IV - - DARWIN’S THEORIES OF ARTIFICIAL AND OF NATURAL SELECTION - - The Principle of Selection - - -Darwin’s theory of natural selection is preëminently a theory of -adaptation. It appears, in fact, better suited to explain this -phenomenon than that of the “origin of species.” Darwin prepared his -reader for the ideas contained in the theory of natural selection by a -brief consideration of the results of artificial selection; and since -the key to the situation is, I believe, to be found in just this -supposed resemblance, we cannot do better than examine the theories in -the order followed by Darwin himself. - -One of the means by which the artificial races of animals and plants -have been formed by man is selection. The breeder picks out individuals -having a certain peculiarity, and allows them to breed together. He -hopes to find among the offspring, not only individuals like the parent -forms, but also some that have the special peculiarity even more -strongly developed. If such are found, they are isolated and allowed to -breed, and in the next generation it is hoped to find one or more new -individuals that show still more developed the special character that is -sought. This process, repeated through a number of generations, is -supposed to have led to the formation of many of our various forms of -domesticated animals and plants. - -This heaping up as a result of the union of similar individuals cannot -for a moment be supposed to be the outcome of the addition of the two -variations to each other. Such an idea is counter to all the most -familiar facts of inheritance. For instance, when two similar forms -unite, we do not find that the young show all the characters of the -mother plus all those of the father, _i.e._ each peculiarity that is the -same in both, increased twofold. On the contrary, the young are in the -vast majority of cases not essentially different from either parent. - -A more thorough examination of the facts shows that the problem is by no -means so simple as the preceding general statement might lead one to -suppose, for our experience shows that it is not always possible to -increase all variations by selection, and, furthermore, there is very -soon found a limit, even in favorable cases, to the extent to which the -process can be carried. The most important point appears to be the -nature of the variations themselves which may arise from different -causes, and which have different values in relation to the possibility -of their continuation. - -We may begin, therefore, by following Darwin in his analysis of -variation, as given in the opening chapter of the “Origin of Species.” -He thinks that the great amount of variation shown by domesticated -animals and plants is due, in the first place, to the new conditions of -life to which they are exposed, and also to the lack of uniformity of -these conditions. Darwin thinks, also, that there is some probability -that this variability is due, in part, to an excess of food. “It seems -clear that organic beings must be exposed during several generations to -new conditions to cause any great amount of variation, and that when the -organization has once begun to vary, it generally continues varying for -many generations. No case is on record of a variable organism ceasing to -vary under cultivation. Our oldest cultivated plants, such as wheat, -still yield new varieties; our oldest domesticated animals are still -capable of rapid improvement or modification.” In this statement of -Darwin, full of significance, we must be careful to notice that he does -not mean to imply, when he states that an organism that has once begun -to vary continues to vary for many generations, that this continuous -variation is always in the same direction, but only that new -combinations, scattering in all directions, continue to appear. - -The nature of the organism seemed to Darwin to be a more important -factor in the origin of new variations than the external conditions, -“for nearly similar variations sometimes arise under, as far as we can -judge, dissimilar conditions; and, on the other hand, dissimilar -variations arise under conditions which appear to be nearly uniform.” -The following statement is important in connection with the origin of -“definite” variations. “Each of the endless variations which we see in -the plumage of our fowls must have had some efficient cause; and if the -same causes were to act uniformly during a long series of generations on -many individuals, all probably would be modified in the same direction.” -Here we find an explicit statement in regard to the accumulation of -variation in a given direction as the result of an external agent, but -Darwin hastens to add: “Indefinite variability is a much more common -result of changed conditions than definite variability, and has probably -played a more important part in the formation of our domestic races. We -see indefinite variability in the endless slight peculiarities which -distinguish the individuals of the same species, and which cannot be -accounted for by inheritance from either parent or from some more remote -ancestor. Even strongly marked differences occasionally appear in the -young of the same litter, and in seedlings from the same seed capsule. -At long intervals of time, out of millions of individuals reared in the -same country and fed on nearly the same food, deviations of structure so -strongly pronounced as to deserve to be called monstrosities arise; but -monstrosities cannot be separated by any distinct line from slighter -variations.” - -Another cause of variation, Darwin believes, is in the inherited effect -of “habit and of the use and disuse of parts,” or what is generally -known as the Lamarckian factor of heredity. Darwin believes that changes -in the body of the parent, that are the result of the use or of the -disuse of a part, may be transmitted to the descendants, and cites a -number of cases which he credits to this process. As we shall deal more -fully with this topic in another chapter, we may treat it here quite -briefly. As an example of the inheritance of disuse, Darwin gives the -following case: “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 this change may be -safely attributed to the domestic duck flying much less and walking more -than its wild parents.” The great and inherited development of the -udders of cows and of goats in countries where they are habitually -milked, in comparison with these organs in other countries, is given as -another instance of the effect of use. “Not one of our domestic animals -can be named that in some country has not drooping ears, and the view -has been suggested that the drooping is due to the disuse of the muscles -of the ears from the animals being seldom much alarmed.” - -It need scarcely be pointed out here, that, in the first case given, -those ducks would have been most likely to remain in confinement that -had less well-developed wings, and hence at the start artificial -selection may have served to bring about the result. The great -development of the udders of cows and of goats is obviously connected -with the greater milk-giving qualities of these animals, which may have -been selected for this purpose. - -Another “law” of variation recognized by Darwin is what is called -correlated variation. For example, it has been found that cats which are -entirely white and have blue eyes are generally deaf, and this is stated -to be confined to the males. The teeth of hairless dogs are imperfect; -pigeons with feathered feet have skin between the outer toes, and those -with short beaks have small feet, and _vice versa_. - -Another source of variation is that of reversion, or the reappearance in -the offspring of characters once possessed by the ancestors. Finally, -Darwin thinks that a source of variation is to be found in modifications -due to the influence of a previous union with another male, or, as it is -generally called, telegony. As an example Darwin cites the famous case -of Lord Morton’s mare. “A nearly purely bred Arabian chestnut mare bore -a hybrid to a quagga. She subsequently produced two colts by a black -Arabian horse. These colts were partially dun-colored and were striped -on the legs more plainly than the real hybrid or even than the -quagga.”[13] This case, however, is not above suspicion, since it is -well known that stripes often appear on young horses, and the careful -analysis made later by Ewart, as well as his other experiments on the -possibility of the transmission of influences of this sort, puts the -whole matter in a very dubious light. - -These citations show that Darwin recognized quite a number of sources of -variation, and, although he freely admits that “our ignorance of the -laws of variation is profound,” yet some at least of these sources of -variation are very questionable. Be this as it may, it is important to -emphasize that Darwin recognized two main sources of variation,—one of -which is the indefinite, or fluctuating, variability that appears -constantly in domesticated animals and plants, and the other, definite -variability, or a change in a definite direction, that can often be -traced to the direct action of the environment on the parent or on its -reproductive cells. It is the former, _i.e._ the fluctuating -variability, that, according to Darwin, has been used by the breeder to -produce most of our domestic races. In regard to the other source of -variation, the definite kind, we must analyze the facts more closely. - -Footnote 13: - - “Animals and Plants under Domestication,” Chap. IX. - -A definite change in the surroundings might bring about a definite -change in the next generation, because the new condition acts either on -the developing organism, or on the egg itself from which the individual -develops. The distinction may be one of importance, for, if the new -condition only effects the developing organism directly, then, when the -influence is removed, there should be a return to the former condition; -but if the egg itself is affected, so that it is fundamentally changed, -then the effect might persist even if the animal were returned to its -former environment. More important still is Darwin’s recognition of the -cumulative effect in a given direction of external influences, for a new -variation, that was slight at first, might, through prolonged action, -continue to become more developed without any other processes affecting -the organism. - -From the Darwinian point of view, however, the all-important source for -the origin of new forms is the fluctuating variation, which is made use -of both in the process of artificial and of natural selection. We may -now proceed to inquire how this is supposed to take place. - -It has been stated that, by means of artificial selection, Darwin -believes the breeder has produced the greater number of domesticated -animals and plants. The most important question is what sort of -variations he has made use of in order to produce his result. Has he -made use of the fluctuating variations, or of the definite ones? It is -difficult, if not impossible, to answer this question in most cases, -because the breeder does not always distinguish between the two. There -can be little question, however, that he may sometimes have made use of -the definite kinds, whether these are the outcome of external or of -internal influences. The question has been seriously raised only in -recent years, and we are still uncertain how far we can accumulate and -fix a variation that is of the fluctuating kind. In a few cases it has -been found that the upper limit is soon reached, as shown by De Vries’s -experiments with clover, and it is always possible that a definite -variation of the right sort may arise at any stage of the process. If -this should occur, then a new standard is introduced from which, as from -a new base, variations fluctuating in the desired direction may be -selected. - -This question, before all others, ought to be settled before we begin to -speculate further as to what selection is able to accomplish. - -Darwin’s theory is often stated in such a general way that it would be -applicable to either sort of variation; but if definite variation can go -on accumulating without selection, then possibly we could account for -evolution without supposing any other process to intervene. Under these -circumstances all that could be claimed for selection would be the -destruction of those variations incapable of living, or of competing -with other forms. Hence the process of selection would have an entirely -negative value. - -The way in which domesticated animals and plants have originated is -explained by Darwin in the following significant passage:— - -“Let us now briefly consider the steps by which domestic races have been -produced, either from one or from several allied species. Some effect -may be attributed to the direct and definite action of the external -conditions of life, and some to habit; but he would be a bold man who -would account by such agencies for the differences between a dray- and -race-horse, a greyhound and bloodhound, a carrier and tumbler pigeon. -One of the most remarkable features in our domesticated races is that we -see in them adaptation, not indeed to the animal’s or plant’s own good, -but to man’s use or fancy. Some variations useful to him have probably -arisen suddenly, or by one step; many botanists, for instance, believe -that the fuller’s-teasel, with its hooks, which cannot be rivalled by -any mechanical contrivance, is only a variety of the wild Dipsacus; and -this amount of change may have suddenly arisen in a seedling. So it has -probably been with the turnspit dog; and this is known to have been the -case with the ancon sheep. But when we compare the dray-horse and -race-horse, the dromedary and camel, the various breeds of sheep fitted -either for cultivated land or mountain pasture, with the wool of one -breed good for one purpose, and that of another breed for another -purpose; when we compare the many breeds of dogs, each good for man in -different ways; when we compare the game-cock, so pertinacious in -battle, with other breeds so little quarrelsome, with ‘everlasting -layers’ which never desire to sit, and with the bantam so small and -elegant; when we compare the host of agricultural, culinary, orchard, -and flower-garden races of plants, most useful to man at different -seasons and for different purposes, or so beautiful in his eyes, we -must, I think, look further than to mere variability. We cannot suppose -that all the breeds were suddenly produced as perfect and as useful as -we now see them; indeed, in many cases, we know that this has not been -their history. The key is man’s power of accumulative selection: nature -gives successive variations; man adds them up in certain directions -useful to him. In this sense he may be said to have made for himself -useful breeds.” - -Darwin also gives the following striking examples, which make probable -the view that domestic forms have really been made by man selecting -those variations that are useful to him:— - -“In regard to plants, there is another means of observing the -accumulated effects of selection—namely, by comparing the diversity of -flowers in the different varieties of the same species in the -flower-garden; the diversity of leaves, pods, or tubers, or whatever -part is valued, in the kitchen-garden, in comparison with the flowers of -the same varieties; and the diversity of fruit of the same species in -the orchard, in comparison with the leaves and flowers of the same set -of varieties. See how different the leaves of the cabbage are, and how -extremely alike the flowers; how unlike the flowers of the heartsease -are, and how alike the leaves; how much the fruit of the different kinds -of gooseberries differ in size, color, shape, and hairiness, and yet the -flowers present very slight differences. It is not that the varieties -which differ largely in some one point do not differ at all in other -points; this is hardly ever,—I speak after careful observation,—perhaps -never, the case. The law of correlated variation, the importance of -which should never be overlooked, will insure some differences; but, as -a general rule, it cannot be doubted that the continued selection of -slight variations, either in the leaves, the flowers, or the fruit, will -produce races differing from each other chiefly in these characters.” - -Exception may perhaps be taken to the concluding sentence, for, -interesting as the facts here recorded certainly are, it does not -necessarily follow that all domestic products have arisen “by the -continued selection of slight variations,” however probable the -conclusion may appear. Darwin also believes that a process of -“unconscious selection” has given even more important “results than -methodical selection.” By unconscious selection is meant the outcome of -“every one trying to possess and breed from best individual animals.” -“Thus a man who intends keeping pointers naturally tries to get as good -dogs as he can, and afterwards breeds from his own best dogs, but he has -no wish, or expectation of permanently altering the breed. Nevertheless -we may infer that this process, continued during centuries, would -improve and modify any breed.... There is reason to believe that the -King Charles spaniel has been unconsciously modified to a large extent -since the time of that monarch.” - -The enormous length of time required to produce new species by the -selection of fluctuating variations is everywhere admitted by Darwin; -nowhere perhaps more strikingly than in the following statement: “If it -has taken centuries or thousands of years to improve or modify most of -our plants up to their present standard of usefulness to man, we can -understand how it is that neither Australia, the Cape of Good Hope, nor -any other region inhabited by quite uncivilized man has afforded us a -single plant worth culture. It is not that these countries, so rich in -species, do not by a strange chance possess the aboriginal stocks of any -useful plants, but that the native plants have not been improved by -continued selection up to a standard of perfection comparable with that -acquired by the plants in countries anciently civilized.” - -In reply to this, it may be said that if the selection of fluctuating -variations leads to an accumulation in the given direction, it is not -apparent why it should take thousands of years to produce a new race, or -require such a high degree of skill as Darwin supposes the breeder to -possess. - -The conditions favorable to artificial selection are, according to -Darwin: 1. The possession of a large number of individuals, for in this -way the chance of the desired variation appearing is increased. 2. -Prevention of intercrossing, such as results when the land is enclosed, -so that new forms may be kept apart. 3. Changed conditions, as -introducing variability. 4. The intercrossing of aboriginally distinct -species. 5. The intercrossing of new breeds, “but the importance of -intercrossing has been much exaggerated.” 6. In plants propagation of -bud variations by means of cuttings. The chapter concludes with the -statement, “Over all these causes of Change, the accumulative action of -Selection, whether applied methodically and quickly, or unconsciously -and slowly, but more efficiently, seems to have been the predominant -Power.” - -Variability, Darwin says, is governed by many unknown laws, and the -final result is “infinitely complex.” If this is so, we may at least -hesitate before we accept the statement that selection of fluctuating -variations has been the only principle that has brought about these -results. This is a most important point, for, as we shall see, the -central question in the theory of natural selection has come to be -whether by the accumulation of fluctuating variations a new species -could ever be produced. If it be admitted that the evidence from -artificial selection is far from convincing, in showing that selection -of fluctuating variations could have been the main source, even in the -formation of new races, we need not be prejudiced in favor of such a -process, when we come to examine the formation of species in nature. - -There are still other questions raised in this same chapter that demand -serious consideration. Darwin writes as follows:— - -“When we look to the hereditary varieties or races of our domestic -animals and plants, and compare them with closely allied species, we -generally perceive in each domestic race, as already remarked, less -uniformity of character than in true species. Domestic races often have -a somewhat monstrous character; by which I mean, that, although -differing from each other, and from other species of the same genus, in -several trifling respects, they often differ in an extreme degree in -some one part, both when compared one with another, and more especially -when compared with the species under nature to which they are nearest -allied. With these exceptions (and with that of the perfect fertility of -varieties when crossed,—a subject hereafter to be discussed), domestic -races of the same species differ from each other in the same manner as -do the closely allied species of the same genus in a state of nature, -but the differences in most cases are less in degree. This must be -admitted as true, for the domestic races of many animals and plants have -been ranked by some competent judges as the descendants of aboriginally -distinct species, and by other competent judges as mere varieties. If -any well-marked distinction existed between a domestic race and a -species, this source of doubt would not so perpetually recur.” - -The point here raised in regard to the systematic value of the new forms -is the question that first demands our attention. We must exclude all -those cases in which several original species have been blended to make -a new form, because the results are too complicated to make use of at -present. The domesticated races of dogs appear to have had such a -multiple origin, the origin of horses is in doubt; but the domesticated -pigeons, ducks, rabbits, and fowls are supposed, by Darwin, to have come -each from one original wild species. The great variety of the domestic -pigeons gives perhaps the most striking illustration of changes that -have taken place under domestication; and Darwin lays great stress on -the evidence from this source. - -It seems probable in this case, (1) that all the different races of -pigeons have come from one original species; (2) that the structural -differences are in some respects as great as those recognized by -systematists as specifically distinct; (3) that the different races -breed true to their kind; (4) that the result has been reached mainly by -selecting and isolating variations that have appeared under -domestication, and that probably some, at least, of these variations -were fluctuating ones. - -Does not this grant all that Darwin contends for? In one sense, yes; in -another, no! The results appear to show that by artificial selection of -some kind a group of new forms may be produced that in many respects -resemble a natural family, or a genus; but if this is to be interpreted -to mean that the result is the same as that by which natural groups have -arisen, then I think that there are good reasons for dissenting from -such a conclusion. Moreover, we must not grant too readily that the -different races of pigeons have arisen by the selection of _fluctuating -variations_ alone, for this is not established with any great degree of -probability by the evidence. - -In regard to the first point we find that one of the most striking -differences between species in nature is their infertility, and the -infertility of their offspring when intercrossed. This is a very general -rule, so far as we know. In regard to the different races of -domesticated forms, the most significant fact is that, no matter how -different they may be, they are perfectly fertile _inter se_. In this -respect, as well as in others, there are important differences between -domesticated races and wild species. The further difference, that has -been pointed out by a number of writers, should also not pass unnoticed, -namely, that the domestic forms differ from each other in the extreme -development of some one character, and not in a large number of less -conspicuous characters, as is the case in wild species. - -These considerations show that, interesting and suggestive as are the -facts of artificial selection, they fail to demonstrate the main point -for which they are used by Darwin. With the most rigorous attention to -the process of artificial selection, new species comparable in all -respects to wild ones have not been formed, even in those cases in which -the variation has been carried farthest (where the history of the forms -is most completely known). - -There is another point on which emphasis should be laid. If by selecting -the most extreme forms in each generation and breeding from them the -standard can be raised, it might appear that we could go on indefinitely -in the same direction, and produce, for instance, pigeons with legs five -metres long, and with necks of corresponding length. But experience has -shown that this cannot be done. As Darwin frequently remarks, the -breeder is entirely helpless until the desired variation appears. It -seems possible, by selecting the more extreme of the fluctuating -variations in each generation, that a higher plane of variation is -established, and even that more extreme forms are likely to arise for a -few generations; but, even if this is the case, a limit is soon reached -beyond which it is impossible to go. - -The facts of observation show, that when a new variety appears its -descendants are more likely, on the average, to produce proportionately -more individuals that show the same variation, and some even that may go -still farther in the same direction. If these latter are chosen to be -the parents of the next generation, then once more the offspring may -show the same advance; but little by little the advance slows down, -until before very long it may cease altogether. Unless, then, a new kind -of variation appears, or a new standard of variation develops of a -different kind, the result of selection of fluctuating variations has -reached its limit. Our experience seems, therefore, to teach us that -selection of fluctuating variations leads us to only a certain point, -and then stops in this direction. We get no evidence from the facts in -favor of the view that the process, if carried on for a long time, could -ever produce such great changes, or the kind of changes, as those seen -in wild animals and plants. - - - Variation and Competition in Nature - -Darwin rests his theory on the small individual variations which occur -in nature, as the following quotation shows:— - -“It may be doubted whether sudden and considerable deviations of -structure such as we occasionally see in our domestic productions, more -especially with plants, are ever permanently propagated in a state of -nature. Almost every part of every organic being is so beautifully -related to its complex conditions of life that it seems as improbable -that any part should have been suddenly produced perfect, as that a -complex machine should have been invented by man in a perfect state. -Under domestication monstrosities sometimes occur which resemble normal -structures in widely different animals. Thus pigs have occasionally been -born with a sort of proboscis, and if any wild species of the same genus -had naturally possessed a proboscis, it might have been argued that this -had appeared as a monstrosity; but I have as yet failed to find, after -diligent search, cases of monstrosities resembling normal structures in -nearly allied forms, and these alone bear on the question. If monstrous -forms of this kind ever do appear in a state of nature and are capable -of reproduction (which is not always the case), as they occur rarely and -singly, their preservation would depend on unusually favorable -circumstances. They would, also, during the first and succeeding -generations cross with the ordinary form, and thus their abnormal -character would almost inevitably be lost.” - -It is clear that Darwin does not think that the sudden and large -variations that sometimes occur furnish the basis for natural selection, -and the final statement in the last citation (which was added in later -editions of the “Origin of Species”), to the effect that if such -monstrous variations appeared as single or occasional variations they -would be lost by intercrossing implies that, in general, single -variations would likewise be lost unless they appeared in a sufficient -number of individuals to maintain themselves against the swamping -effects of intercrossing. - -It is necessary to quote again, in order to show that, in some cases at -least, Darwin believed selection plays little or no part in the origin -and maintenance of certain peculiarities that are of no use to the -species. “There is one point connected with individual differences, -which is extremely perplexing: I refer to those genera which have been -called protean or ‘polymorphic,’ in which the species present an -inordinate amount of variation. With respect to many of these forms, -hardly two naturalists agree, whether to rank them as species or as -varieties. We may instance Rubus, Rosa, and Hieracium amongst plants, -several genera of insects and of Brachiopod shells. In most polymorphic -genera some of the species have fixed and definite characters. Genera -which are polymorphic in one country seem to be, with a few exceptions, -polymorphic in other countries, and likewise, judging from Brachiopod -shells, at former periods of time. These facts are very perplexing, for -they seem to show that this kind of variability is independent of the -conditions of life. I am inclined to suspect that we see, at least in -some of these polymorphic genera, variations which are of no service or -disservice to the species, and which consequently have not been seized -on by selection to act on and accumulate, in the same manner as man -accumulates in any given direction individual differences in his -domesticated productions. These individual differences generally affect -what naturalists consider unimportant parts; but I could show by a long -catalogue of facts, that parts which must be called important, whether -viewed under a physiological or classificatory point of view, sometimes -vary in the individuals of the same species. I am convinced that the -most experienced naturalist would be surprised at the number of cases of -variability, even in important parts of structure, which he could -collect on good authority, as I have collected, during a course of -years.” - -After pointing out that naturalists have no definite standard to -determine whether a group of individuals is a variety or a species, -Darwin makes the highly important admissions contained in the following -paragraph: “Hence, I look at individual differences, though of small -interest to the systematist, as of the highest importance for us, as -being the first steps toward such slight varieties as are barely thought -worth recording in works on natural history. And I look at varieties -which are in any degree more distinct and permanent, as steps toward -more strongly marked and permanent varieties; and at the latter, as -leading to subspecies, and then to species. The passage from one stage -of difference to another may, in many cases, be the simple result of the -nature of the organism and of the different physical conditions to which -it has long been exposed; but with respect to the more important and -adaptive characters, the passage from one stage of difference to another -may be safely attributed to the cumulative action of natural selection, -hereafter to be explained, and to the effects of the increased use or -disuse of parts. A well-marked variety may therefore be called an -incipient species; but whether this belief is justifiable must be judged -by the weight of the various facts and considerations to be given -throughout this work.” - -In this paragraph attention should be called especially, first, to the -statement in respect to the origin of varieties, which are said to arise -through individual differences. It is not clear whether these -differences are supposed to have appeared first in one, or in a few -individuals, or in large numbers at the same time. Again, especial note -should be made of the striking admission, that the passage from one -stage to another may, in many cases, be the simple result of the nature -of the organism and of the physical conditions surrounding it; but with -respect to the more important and adaptive differences, natural -selection “may safely” be supposed to have intervened. Is it to be -wondered at that Darwin’s critics have sometimes accused him of playing -fast and loose with the origin of varieties? And since this question is -fundamental for the theory of natural selection, it is much to be -regretted that Darwin leaves the matter in such a hazy condition. It may -be said that, at the time when he wrote, he made the best of the -evidence in regard to the origin of varieties. Be this as it may, a -theory standing on no better foundations than this is not likely to be -found satisfactory at the present time. - -We come now to the most important chapters, the third and the fourth, of -the “Origin of Species,” dealing with “the struggle for existence,” -“natural selection,” or the “survival of the fittest.” Behind these -fatal phrases, which have become almost household words, lurk many -dangers for the unwary. - -“It has been seen in the last chapter that amongst organic beings in a -state of nature there is some individual variability: indeed I am not -aware that this has ever been disputed. It is immaterial for us whether -a multitude of doubtful forms be called species or subspecies or -varieties; what rank, for instance, the two or three hundred doubtful -forms of British plants are entitled to hold, if the existence of any -well-marked varieties be admitted. But the mere existence of individual -variability and of some few well-marked varieties, though necessary as -the foundation for the work, helps us but little in understanding how -species arise in nature. How have all those exquisite adaptions of one -part of the organization to another part, and to the conditions of life, -and of one organic being to another being, been perfected? We see these -beautiful coadaptions most plainly in the woodpecker and the mistletoe; -and only a little less plainly in the humblest parasite which clings to -the hairs of a quadruped or feathers of a bird; in the structure of the -beetle which dives through the water; in the plumed seed which is wafted -by the gentlest breeze; in short, we see beautiful adaptions everywhere -and in every part of the organic world. - -“Again, it may be asked, how is it that varieties, which I have called -incipient species, become ultimately converted into good and distinct -species, which in most cases obviously differ from each other far more -than do the varieties of the same species? How do those groups of -species, which constitute what are called distinct genera, and which -differ from each other more than do the species of the same genus, -arise? All these results, as we shall more fully see in the next -chapter, follow from the struggle for life. Owing to this struggle, -variations, however slight and from whatever cause proceeding, if they -be in any degree profitable to the individuals of a species, in their -infinitely complex relations to other organic beings and to their -physical conditions of life, will tend to the preservation of such -individuals, and will generally be inherited by the offspring. The -offspring, also, will thus have a better chance of surviving, for, of -the many individuals of any species which are periodically born, but a -small number can survive. I have called this principle, by which each -slight variation, if useful, is preserved, by the term Natural -Selection, in order to mark its relation to man’s power of selection. -But the expression often used by Mr. Herbert Spencer of the Survival of -the Fittest is more accurate, and is sometimes equally convenient. We -have seen that man by selection can certainly produce great results, and -can adapt organic beings to his own uses, through the accumulation of -slight but useful variations, given to him by the hand of Nature. But -Natural Selection, as we shall hereafter see, is a power incessantly -ready for action, and is as immeasurably superior to man’s feeble -efforts, as the works of Nature are to those of Art.” - -Darwin gives the following explicit statement of the way in which he -intends the term “struggle for existence” to be understood: “I should -premise that I use this term in a large and metaphorical sense, -including dependence of one being on another, and including (which is -more important) not only the life of the individual, but success in -leaving progeny. Two canine animals, in time of dearth, may be truly -said to struggle with each other which shall get food and live. But a -plant on the edge of a desert is said to struggle for life against the -drought, though more properly it should be said to be dependent on the -moisture. A plant which actually produces a thousand seeds of which only -one on an average comes to maturity may be more truly said to struggle -with the plants of the same and other kinds which already clothe the -ground. The mistletoe is dependent on the apple, and a few other trees, -but can only in a far-fetched sense be said to struggle with these -trees, for if too many of these parasites grow on the same tree, it -languishes and dies. But several seedling mistletoes, growing close -together on the same branch, may more truly be said to struggle with -each other. As the mistletoe is disseminated by birds, its existence -depends on them, and it may metaphorically be said to struggle with -other fruit-bearing plants, in tempting the birds to devour and thus -disseminate its seeds. In these several senses, which pass into each -other, I use for convenience’ sake the general term ‘Struggle for -Existence.’” - -A number of writers have objected to the general and often vague way in -which Darwin makes use of this phrase; but it does not seem to me that -this is a serious objection, provided we are on our guard as to what the -outcome will be in each case. In each instance we must consider the -question on its own merits, and if it is found convenient to have a -sufficiently general and non-committal term, such as the “struggle for -existence,” to include all cases, I see no serious objection to the use -of such an expression, although it is true the outcome has been that it -has become a catchword, that is used too often by those who have no -knowledge of its contents. - -Were it not that each animal and plant gives birth, on an average, to -more than two offspring, the species would soon become exterminated by -accidents, etc. We find in some of the lower animals, and in some of the -higher plants, that thousands and even millions of eggs are produced by -a single individual in the course of its life. A single nematode may lay -sixty million eggs, and a tapeworm one thousand million. A starfish may -produce about thirty-nine million eggs, a salmon may contain fifteen -thousand, and a large shad as many as one hundred thousand. The queen of -a termite nest is said to lay eighty thousand eggs a day. - -In the higher vertebrates the number of young is considerably less, but -since the young stages are passed within the body of the parent, -proportionately more of them reach maturity, so that even in man the -population may be doubled in twenty-five years, and in the elephant, -slowest breeder of all animals, Darwin has calculated that, if it begins -breeding when about thirty years old and goes on until ninety years, -bringing forth six young in the interval, after 750 years there will be -nearly nineteen million elephants alive which have descended from the -first pair. - -Obviously, then, if all the descendants of all the individuals of a -species were to remain alive, the world would be over-crowded in a very -short time, and the want of room would in itself lead to the destruction -of countless individuals, if for no other reason than lack of food. We -can easily carry out on a small scale an experiment that shows how the -overstocking, resulting from favorable conditions, comes about, and how -it checks itself. If we make a meat broth suitable for the life of a -particular bacterium, and sow in the broth a very few individuals, we -find in the course of several days the fluid swarming with the -descendants of the original individuals. Thus it has been shown that, if -we start with a few hundred bacteria, there will be five thousand after -twenty-four hours, and twenty thousand, forty-eight hours later; and -after four days they are beyond calculation. - -Cohn found that a single bacterium produces two individuals in one hour, -and four in two hours, and if they continue to multiply at this rate -there will be produced at the end of three days 4,772 billions of -descendants. If these are reduced to weight, they would weigh -seventy-five hundred tons. Thus when the conditions are favorable, -bacteria are able to increase at such an enormous rate that they could -cover the surface of the earth in a very few days. The reason that they -do not go on increasing at this rate is that they soon exhaust the food -supply, and the rate of increase slows down, and will finally cease -altogether. If the bacteria were dependent on a continuous supply of -food, they would perish after the supply had been exhausted, so that the -rapid rate of multiplication would serve only to bring the career of the -organism to an untimely end. If the weaker individuals were to die -first, the products of their disintegration might serve to nourish the -stronger individuals; hunger coming on again, the next weakest might -die; and the same process continuing, we might imagine that the bacteria -were finally reduced to a single one which would then die in turn for -lack of food. Like a starving shipload of men, reduced by hunger to -cannibalism, the life of some and finally of the last individual might -be prolonged in the hope of rescue, but if this did not arrive, the last -and perhaps the strongest individual would perish. But this is not what -we find occurring in these lower organisms, for, as a rule, they -gradually cease to increase when the food supply becomes lessened, and -their activities slow down. Finally, when the food is gone, they pass -into a resting stage, in which condition they can remain dormant for a -long time, even for years. If they should again find themselves in -favorable surroundings, they become active, and begin once more their -round of multiplication. We cannot follow the individuals in such a -culture of bacteria, but there is nothing to be seen that suggests a -struggle for existence, if this idea conveys the impression of the -destruction of certain individuals by competition with others. In fact, -the results are in some respects exactly the reverse. Millions of -individuals are present at the time when the food supply becomes -exhausted, and they all pass into a protected resting stage. - -The enormous rate of increase in this case finds its counterpart in -higher animals when the food supply, or the absence of enemies, allows a -species to multiply at its maximum rate of increase. The introduction of -rabbits into Australia was followed by an enormous increase in a few -years, and the introduction of the English sparrow into the United -States has had a similar result. But in no country can such a process -continue beyond a certain point, because, in the first place, the -scarcity of food will begin to keep the birth-rate down, and in the -second place, the increase in numbers may lead to an increase in the -number of its enemies, or even induce other forms to feed on it. -Crowding will also give an opportunity for the spread of disease, which -again may check the increase. Sooner or later a sort of ever shifting -balance will be reached for each species, and after this, if the -conditions remain the same, the number of individuals will keep -approximately constant. - -Darwin admits that the “causes which check the natural tendency of each -species to increase are most obscure.” “We know not exactly what the -checks are even in a single instance.” This admission may well put us on -our guard against a too ready acceptation of a theory in which the whole -issue turns on just this very point, namely, the nature of the checks to -increase. Darwin gives the following general cases to show what some of -the checks to increase are. He states that eggs and very young animals -and seeds suffer more than the adults; that “the amount of food for each -species of course gives the extreme limit to which each can increase; -but very frequently it is not the obtaining food, but the serving as -prey to other animals which determines the average numbers of a species. -Thus, there seems to be little doubt that the stock of partridges, -grouse, and hares on any large estate depends largely on the destruction -of the vermin.” “On the other hand, in some cases, as with the elephant, -none are destroyed by beasts of prey; for even the tiger in India most -rarely dares to attack a young elephant protected by its dam.” “Climate -plays an important part in determining the average number of a species, -and periodical seasons of extreme cold or drought seem to be the most -effective of all checks.” “The action of climate seems at first sight to -be quite independent of the struggle for existence; but in so far as -climate acts in reducing food, it brings on the most severe struggle -between the individuals, whether of the same, or of distinct species -which subsist on the same kind of food.” - -We need not follow Darwin through his account of how complex are the -relations of all animals and plants to one another in the struggle for -existence, for, if true, it only goes to show more plainly how -impossible it is to establish any safe scientific hypothesis, where the -conditions are so complex and so impossible to estimate. To show that -the young Scotch fir in an enclosed pasture is kept down by the browsing -of the cattle, and in other parts of the world, Paraguay for instance, -the number of cattle is determined by insects, and that the increase of -these flies is _probably_ habitually checked by other insects, leads to -a bewilderingly complex set of conditions. We cannot do better than to -quote Darwin’s conclusion: “Hence, if certain insectivorous birds were -to decrease in Paraguay, the parasitic insects would probably increase; -and this would lessen the number of the navel-frequenting flies—then -cattle and horses would become feral, and this would certainly greatly -alter (as indeed I have observed in parts of South America) the -vegetation: this again would largely affect the insects; and this, as we -have just seen in Staffordshire, the insectivorous birds, and so onwards -in ever increasing circles of complexity. Not that under nature the -relations will ever be as simple as this. Battle within battle must be -continually recurring with varying success; and yet in the long run the -forces are so nicely balanced, that the face of nature remains for long -periods of time uniform, though assuredly the merest trifle would give -the victory to one organic being over another. Nevertheless, so profound -is our ignorance, and so high our presumption, that we marvel when we -hear of the extinction of an organic being; and as we do not see the -cause, we invoke cataclysms to desolate the world, or invent laws on the -duration of the forms of life!” - -The effect of the struggle for existence in determining _the -distribution of species_ is well illustrated in the following cases:— - -“As the species of the same genus usually have, though by no means -invariably, much similarity in habits and constitution, and always in -structure, the struggle will generally be more severe between them, if -they come into competition with each other, than between the species of -distinct genera. We see this in the recent extension over parts of the -United States of one species of swallow having caused the decrease of -another species. The recent increase of the missel-thrush in parts of -Scotland has caused the decrease of the song-thrush. How frequently we -hear of one species of rat taking the place of another species under the -most different climates! In Russia the small Asiatic cockroach has -everywhere driven before it its great congener. In Australia the -imported hive-bee is rapidly exterminating the small, stingless native -bee. One species of charlock has been known to supplant another species; -and so in other cases. We can dimly see why the competition should be -most severe between allied forms, which fill nearly the same place in -the economy of nature; but probably in no one case could we precisely -say why one species has been victorious over another in the great battle -of life.” - -All this goes to show, if it really shows anything at all, that the -distribution of a species is determined, in part, by its relation to -other animals and plants—a truism that is recognized by every -naturalist. The statement has no necessary bearing on the origin of new -species through competition, as the incautious reader might infer. Not -that I mean in any way to imply that Darwin intended to produce this -effect on the reader; but Darwin is not always careful to discriminate -as to the full bearing of the interesting illustrations with which his -book so richly abounds. - -At the end of his treatment of the subject, Darwin emphasizes once more -how little we know about the subject of the struggle for existence. - -“It is good thus to try in imagination to give to any one species an -advantage over another. Probably in no single instance should we know -what to do. This ought to convince us of our ignorance on the mutual -relations of all organic beings; a conviction as necessary, as it is -difficult, to acquire. All that we can do, is to keep steadily in mind -that each organic being is striving to increase in a geometrical ratio; -that each at some period of its life, during some season of the year, -during each generation or at intervals, has to struggle for life and to -suffer great destruction. When we reflect on this struggle, we may -console ourselves with the full belief, that the war of nature is not -incessant, that no fear is felt, that death is generally prompt, and -that the vigorous, the healthy, and the happy survive and multiply.” - -The kindliness of heart that prompted the concluding sentence may arouse -our admiration for the humanity of the writer, but need not, therefore, -dull our criticism of his theory. For whether no fear is felt, and -whether death is prompt or slow, has no bearing on the question at -issue—except as it prepares the gentle reader to accept the dreadful -calamity of nature, pictured in this battle for existence, and make more -contented with their lot “the vigorous, the healthy, and the happy.” - - - The Theory of Natural Selection - -We have already anticipated, to some extent, Darwin’s conclusion in -regard to the outcome of the competition of animals and plants. This -result is supposed to lead to the survival of the fittest. The -competition is carried out by nature, who is personified as selecting -those forms for further experiments that have won in the struggle for -existence. - -“Can the principle of selection, which we have seen is so potent in the -hands of man, apply under Nature? I think we shall see that it can act -most efficiently. Let the endless number of slight variations and -individual differences occurring in our domestic productions, and, in a -lesser degree, in those under Nature, be borne in mind; as well as the -strength of the hereditary tendency. Can it, then, be thought -improbable, seeing that variations useful to man have undoubtedly -occurred, that other variations useful in some way to each being in the -great and complex battle for life, should occur in the course of many -successive generations? If such do occur can we doubt (remembering how -many more individuals are born than can possibly survive) that -individuals having any advantage, however slight, over others, would -have the best chance of surviving and of procreating their kind? On the -other hand, we may feel sure that any variation in the least degree -injurious would be rigidly destroyed.” - -The process of natural selection is defined as follows, “The -preservation of favorable individual differences and variations and the -destruction of those that are injurious I have called Natural Selection -or the Survival of the Fittest.” And immediately there follows the -significant statement, that, “Variations neither useful nor injurious -would not be affected by natural selection, and would be left either a -fluctuating element, as perhaps we see in certain polymorphic species, -or would ultimately become fixed, owing to the nature of the organism -and the nature of the conditions.” It will be seen from this quotation, -as well as from others already given, that Darwin leaves many structures -outside of the pale of natural selection, and uses his theory to explain -only those cases that are of sufficient use to be decisive in the life -and death struggle of the individuals with each other and with the -surrounding conditions. - -Darwin states that we can best understand “the probable course of -natural selection by taking the case of a country undergoing some slight -physical change, for instance, of climate. The proportional numbers of -its inhabitants will almost immediately undergo a change, and some -species will probably become extinct. We may conclude, from what we have -seen of the intimate and complex manner in which the inhabitants of each -country are bound together, that any change in the numerical proportions -of the inhabitants, independency of the change of climate itself, would -seriously affect the others.... In such cases, slight modifications, -which in any way favored the individuals of any species, by better -adapting them to their altered conditions, would tend to be preserved; -and natural selection would have free scope for the work of -improvement.” - -The first half of the first of these two quotations seems so plausible, -that without further thought we may be tempted to give a ready assent to -the second, yet the whole issue is contained in this statement. In the -abstract, it undoubtedly appears true that any slightly useful -modification might tend to be preserved. Whether it will, in reality, be -preserved must depend on many things that should be taken into account. -This question will come up later for further consideration; but it -should be pointed out here, that, even assuming that one or more -individuals happen to possess a favorable variation, it by no means -follows that natural selection would have free scope for the work of -improvement, because the question of the inheritance of this variation, -and of its accumulation and building up through successive generations, -must be determined before we can be expected to give assent to this -argument, that appears so attractive when stated in an abstract and -vague way. - -Darwin again makes the statement that under the term _variation_ it must -never be forgotten that mere individual differences are meant. “As a man -can produce a great result with his domestic animals and plants by -adding up in any given direction individual differences, so could -natural selection, but far more easily from having incomparably longer -time for action.” Too much emphasis cannot be laid on the fact that -Darwin believed that selection takes place amongst the small individual -differences that we find in animals and plants. Some of his followers, -as we shall see, are apt to put into the background this fundamental -conception of Darwin’s view. His constant comparison between the results -of artificial and natural selection leaves no room for doubt as to his -meaning. Darwin himself seems, at times, not unconscious of the weakness -of this comparison. He says: “How fleeting are the wishes and efforts of -man! how short his time! and consequently how poor will be his results, -compared with those accumulated by Nature during whole geological -periods. Can we wonder then that Nature’s productions should be far -‘truer’ in character than man’s productions; that they should be -infinitely better adapted to the most complex conditions of life, and -should plainly bear the stamp of far higher workmanship?” We should not -lose sight of the fact that even after the most rigorous selective -process has been brought to bear on organisms, namely, by isolation -under domestication, we do not apparently find ourselves gradually -approaching nearer and nearer to the formation of new species, but we -find, on the contrary, that we have produced something quite different. -In the light of this truth, the relation between the two selective -theories may appear quite different from the interpretation that Darwin -gives of it. We may well doubt whether nature does select so much better -than does man, and whether she has ever _made_ new species in this way. - -We come now to a point that touches the theory of natural selection in a -very vital spot. - -“It may be well here to remark that with all beings there must be much -fortuitous destruction, which can have little or no influence on the -course of natural selection. For instance, a vast number of eggs or -seeds are annually devoured, and these could be modified through natural -selection only if they varied in some manner which protected them from -their enemies. Yet many of these eggs or seeds would perhaps, if not -destroyed, have yielded individuals better adapted to their conditions -of life than any of those which happened to survive. So again a vast -number of mature animals and plants, whether or not they be the best -adapted to their conditions, must be annually destroyed by accidental -causes, which would not be in the least degree mitigated by certain -changes of structure or constitution which would in other ways be -beneficial to the species. But let the destruction of the adults be ever -so heavy, if the number which can exist in any district be not wholly -kept down by such causes,—or again let the destruction of eggs or seeds -be so great that only a hundredth or a thousandth part are -developed,—yet of those which do survive, the best adapted individuals, -supposing that there is any variability in a favorable direction, will -tend to propagate their kind in larger numbers than the less well -adapted. If the numbers be wholly kept down by the causes just -indicated, as will often have been the case, natural selection will be -powerless in certain beneficial directions; but this is no valid -objection to its efficiency at other times and in other ways; for we are -far from having any reason to suppose that many species ever undergo -modification and improvement at the same time in the same area.” - -Some of the admissions made in this paragraph have an important bearing -on the theory of natural selection. Far from supposing that fortuitous -destruction would have no influence on the course of natural selection, -it can be shown that it would have a most disastrous effect. In many -cases the destruction comes in the form of a catastrophe to the -individuals, so that small differences in structure, whether -advantageous or not, are utterly unavailing. Our experience shows us -that a destruction of this sort is going on around us all the time, and -accounts in large part for the way in which the majority of animals and -plants are destroyed. Unless, for example, a seed happen to fall on a -place suitable for its growth, it will perish without respect to a -slight advantage it may have over other seeds of its kind. Of the -thousands of eggs laid by one starfish, chance alone will decide whether -one or another embryo is destroyed by larger animals, or if they escape -this danger, the majority of them may be carried out to sea, where it -will not be of the least avail if one individual has a slight advantage -over the others. Darwin admits this, but adds that, if only a thousandth -part is developed, yet of those that do survive the best adapted -individuals will tend to propagate their kind in larger numbers than the -less well adapted. The argument is not, however, so simple as it appears -to be on the surface. I pass over, for the present, the apparent -inconsequence in this statement that the best adapted individuals will -tend to propagate their kind in larger numbers. It is not by any means -certain that this is the case. Darwin’s meaning is, however, fairly -clear, and can be interpreted to mean this: after the fortuitous -destruction has finished, there will be a further competition of the -survivors amongst themselves and with the surrounding conditions. In -this higher competition, which is less severe, small individual -differences suffice to determine the survival of certain individuals. -These are, therefore, selected. - -In this argument it is assumed that a second competition takes place -after the first destruction of individuals has occurred, and this -presupposes that more individuals reach maturity than there is room for -in the economy of nature. But we do not know to what extent this takes -place. If only as many mature as can survive, then the second -competition does not take place. If, on the other hand, fewer mature -than there is room for, then again competition does not take place. And -if at all times selection is not rigorously carried out, everything may -be lost that has been so laboriously gained. We see then that the result -that Darwin imagines would take place, can be carried out only when more -individuals reach maturity than there is room for (if it is a case of -competition with one another), or that escape their enemies (if it is a -question of competition with other forms). - -It is instructive to consider some of the examples that Darwin has given -to illustrate how the process of natural selection is carried out. The -first example is the imaginary case of a species of wolf, the -individuals of which secure their prey sometimes by craft, sometimes by -strength, and sometimes by fleetness. If the prey captured by the first -two methods should fail, then all the wolves would be obliged to capture -their food by fleetness, and consequently the fleetest alone would -survive. “I can see no more reason to doubt that this would be the -result than that man should improve the fleetness of his greyhounds.” -But even if the fleetness of the race could be kept up in this way, it -does not follow that a new species of wolf would be formed in -consequence, as Darwin implies. His own comment on this illustration is, -perhaps, the best criticism that can be made. - -“It should be observed that, in the above illustration, I speak of the -slimmest individual wolves, and not of any single strongly marked -variation having been preserved. In former editions of this work I -sometimes spoke as if this latter alternative had frequently occurred. I -saw the great importance of individual differences, and this led me -fully to discuss the results of unconscious selection by man, which -depends on the preservation of all the more or less valuable -individuals, and on the destruction of the worst. I saw, also, that the -preservation in a state of nature of any occasional deviation of -structure, such as a monstrosity, would be a rare event; and that, if at -first preserved, it would generally be lost by subsequent intercrossing -with ordinary individuals. Nevertheless, until reading an able and -valuable article in the _North British Review_ (1867), I did not -appreciate how rarely single variations, whether slight or strongly -marked, could be perpetuated. The author takes the case of a pair of -animals, producing during their lifetime two hundred offspring, of -which, from various causes of destruction, only two on an average -survive to procreate their kind. This is rather an extreme estimate for -most of the higher animals, but by no means so for many of the lower -organisms. He then shows that if a single individual were born, which -varied in some manner, giving it twice as good a chance of life as that -of the other individuals, yet the chances would be strongly against its -survival. Supposing it to survive and to breed, and that half its young -inherited the favourable variation; still, as the reviewer goes on to -show, the young would have only a slightly better chance of surviving -and breeding; and this chance would go on decreasing in the succeeding -generations. The justice of these remarks cannot, I think, be disputed. -If, for instance, a bird of some kind could procure its food more easily -by having its beak curved, and if one were born with its beak strongly -curved, and which consequently flourished, nevertheless there would be a -very poor chance of this one individual perpetuating its kind to the -exclusion of the common form; but there can hardly be a doubt, judging -by what we see taking place under domestication, that this result would -follow from the preservation during many generations of a large number -of individuals with more or less strongly curved beaks, and from the -destruction of a still larger number with the straightest beaks.” - -There then follows what, I believe, is one of the most significant -admissions in the “Origin of Species”:— - -“It should not, however, be overlooked that certain rather strongly -marked variations, which no one would rank as mere individual -differences, frequently recur owing to a similar organization being -similarly acted on—of which fact numerous instances could be given with -our domestic productions. In such cases, if the varying individual did -not actually transmit to its offspring its newly acquired character, it -would undoubtedly transmit to them, as long as the existing conditions -remained the same, a still stronger tendency to vary in the same manner. -There can also be little doubt that the tendency to vary in the same -manner has often been so strong that all the individuals of the same -species have been similarly modified without the aid of any form of -selection. Or only a third, fifth, or tenth part of the individuals may -have been thus affected, of which fact several instances could be given. -Thus Graba estimates that about one-fifth of the guillemots in the Faroe -Islands consist of a variety so well marked, that it was formerly ranked -as a distinct species under the name of _Uria lacrymans_. In cases of -this kind, if the variation were of a beneficial nature, the original -form would soon be supplanted by the modified form, through the survival -of the fittest.” - -Do not the admissions in this paragraph almost amount to a withdrawal of -much that has preceded in regard to the survival of fluctuating, -individual differences? In the last edition, from which we have just -quoted, Darwin, in response to the criticisms which his book met, -inserted here and there statements that are in many ways in -contradiction to the statements in the first edition, and yet the -earlier statements have been allowed to stand for the most part. - -The next example is also worthy of careful examination, since it appears -to prove too much:— - -“It may be worth while to give another and more complex illustration of -the action of natural selection. Certain plants excrete sweet juice, -apparently for the sake of eliminating something injurious from the sap: -this is effected, for instance, by glands at the base of the stipules in -some Leguminosæ, and at the backs of the leaves of the common laurel. -This juice, though small in quantity, is greedily sought by insects; but -their visits do not in any way benefit the plant. Now, let us suppose -that the juice or nectar was excreted from the inside of the flowers of -a certain number of plants of any species. Insects in seeking the nectar -would get dusted with pollen, and would often transport it from one -flower to another. The flowers of two distinct individuals of the same -species would thus get crossed; the act of crossing, as can be fully -proved, gives rise to vigorous seedlings, which consequently would have -the best chance of flourishing and surviving. The plants which produced -flowers with the largest glands or nectaries, excreting most nectar, -would oftenest be visited by insects, and would oftenest be crossed; and -so in the long run would gain the upper hand and form a local variety.” - -The reader will notice that the sweet juice or nectar secreted by -certain plants is supposed to have first appeared independently of the -action of natural selection. Why then account for its presence in -flowers as the outcome of an entirely different process? If the nectar -is eagerly sought for by insects, without the plant benefiting in any -way by their visitations, why give a different explanation of its origin -in flowers where it is of benefit to the plant? - -Darwin carries his illustration further: “When our plant, by the above -process long continued, had been rendered highly attractive to insects, -they would unintentionally, on their part, regularly carry pollen from -flower to flower; and that they do this effectually, I could easily show -by many striking facts. I will give only one, as likewise illustrating -one step in the separation of the sexes of plants.... As soon as the -plant had been rendered so highly attractive to insects that pollen was -regularly carried from flower to flower, another process might commence. -No naturalist doubts the advantage of what has been called the -‘physiological division of labour’; hence we may believe that it would -be advantageous to a plant to produce stamens alone in one flower or on -one whole plant, and pistils alone in another flower or on another -plant. In plants under culture and placed under new conditions of life, -sometimes the male organs and sometimes the female organs become more or -less impotent; now if we suppose this to occur in ever so slight a -degree under nature, then, as pollen is already carried regularly from -flower to flower, and as a more complete separation of the sexes of our -plant would be advantageous on the principle of the division of labour, -individuals with this tendency more and more increased would be -continually favoured or selected, until at last a complete separation of -the sexes might be effected. It would take up too much space to show the -various steps, through dimorphism and other means, by which the -separation of the sexes in plants of various kinds is apparently now in -progress; but I may add that some of the species of holly in North -America are, according to Asa Gray, in an exactly intermediate -condition, or, as he expresses it, are more or less diœciously -polygamous.” - -From this it will be seen that Darwin supposes that the separation of -the sexes in some of the higher plants has been brought about by natural -selection. Despite the supposed advantage of the so-called “division of -labor,” one may, I venture to suggest, be sceptical as to whether the -separation of the sexes can be explained in this way. The whole case is -largely supposititious, since in most of the higher hermaphroditic -plants and in nearly all hermaphroditic animals the sexual products -ripen at different times in the same individual. Hence there is no basis -for the assumption that unless the sexes are separated there will be -self-fertilization. Shall we assume that this difference in time of -ripening of the two kinds of sex-cells is also the outcome of natural -selection, and that there has existed an earlier stage in all animals -and plants, that now have different times for the ripening of their -sexual elements, a time when these products ripened simultaneously? I -doubt if even a Darwinian would give such loose rein to his fancy. - -But this is not yet the whole story that Darwin has made out in this -connection, for he continues:— - -“Let us now turn to the nectar-feeding insects; we may suppose the -plant, of which we have been slowly increasing the nectar by continued -selection, to be a common plant; and that certain insects depended in -main part on its nectar for food. I could give many facts showing how -anxious bees are to save time: for instance, their habit of cutting -holes and sucking the nectar at the bases of certain flowers, which with -a very little more trouble, they can enter by the mouth. Bearing such -facts in mind, it may be believed that under certain circumstances -individual differences in the curvature or length of the proboscis, -etc., too slight to be appreciated by us, might profit a bee or other -insect, so that certain individuals would be able to obtain their food -more quickly than others; and thus the communities to which they -belonged would flourish and throw off many swarms inheriting the same -peculiarities.” - -Aside from the general criticism that will suggest itself here also, it -should be pointed out that even if “certain individuals” of the bees had -slightly longer proboscides, this would, in the case of the hive-bees at -least, be of no avail, since they do not reproduce, and hence leave no -descendants with longer mouth-parts. Of course, it may be replied that -those colonies in which the queens produce more of the long-proboscis -kind of worker would have an advantage over other colonies not having so -many individuals of this sort. It would then be a competition of one -colony with another, as Darwin supposes to take place in colonial forms. -But whether slight differences of this sort would lead to the -elimination of the least well-endowed colonies is entirely a matter of -speculation. Since there are flowers with corolla-tubes of all lengths, -we can readily suppose that if one kind of flower excluded individuals -of certain colonies, they would search elsewhere for their nectar rather -than perish. While different races might arise in this way, the process -would not be the survival of the fittest, but a process of adaptation to -a new environment. - -We come now to a topic on which Darwin lays much stress: the divergence -of character. He tries to show how the “lesser differences between the -varieties become augmented into the greater differences between -species.” - -“Mere chance, as we may call it, might cause one variety to differ in -some character from its parents, and the offspring of this variety again -to differ from its parent in the very same character and in a greater -degree; but this alone would never account for so habitual and large a -degree of difference as that between the species of the same genus. As -has always been my practice, I have sought light on this head from our -domestic productions.” - -Then, after pointing out that under domestication two different races, -the race-horse and the dray-horse, for instance, might arise by -selecting different sorts of variations, Darwin inquires:— - -“But how, it may be asked, can any analogous principle apply in nature? -I believe it can and does apply most efficiently (though it was a long -time before I saw how), from the simple circumstance that the more -diversified the descendants from any one species become in structure, -constitution, and habits, by so much will they be better enabled to -seize on many and widely diversified places in the polity of nature, and -so be enabled to increase in numbers.” - -Here we touch on one of the fundamental principles of the doctrine of -evolution. It is intimated that the new form of animal or plant first -appears (without regard to any kind of selection), and then finds that -place in nature where it can remain in existence and propagate its kind. -Darwin refers here, of course, only to the less extensive variations, -the individual or fluctuating kind; but as we shall discuss at greater -length in another place, this same process, if extended to other kinds -of variation, may give us an explanation of evolution without -competition, or selection, or destruction of the individuals of the same -kind taking place at all. - - ------------------------------------------------------------------------- - - - - - CHAPTER V - - THE THEORY OF NATURAL SELECTION (_Continued_) - - Objections to the Theory of Natural Selection - - -Although in the preceding chapter a number of criticisms have been made -of the special parts of the theory of natural selection, there still -remain to be considered some further objections that have been made -since the first publication of the theory. It is a fortunate -circumstance from every point of view that Darwin himself was able in -the later editions of the “Origin of Species” to reply to those -criticisms that he thought of sufficient importance. He says:— - -“Long before the reader has arrived at this part of my work, a crowd of -difficulties will have occurred to him. Some of them are so serious that -to this day I can hardly reflect on them without being in some degree -staggered; but, to the best of my judgment, the greater number are only -apparent, and those that are real are not, I think, fatal to the -theory.” - -The first difficulty is this: “Why, if species have descended from other -species by fine gradations, do we not everywhere see innumerable -transitional forms? Why is not all nature in confusion, instead of the -species being, as we see them, well defined?” - -The answer that Darwin gives is, that by competition the new form will -crowd out its own less-improved parent form, and other less-favored -forms. But is this a sufficient or satisfactory answer? If we recall -what Darwin has said on the advantage that those forms will have, in -which a great number of new variations appear to fit them to the great -diversity of natural conditions, and if we recall the gradations that -exist in external conditions, I think we shall find that Darwin’s reply -fails to give a satisfactory answer to the question. - -It is well known, and Darwin himself has commented on it, that the same -species often remains constant under very diverse external conditions, -both inorganic and organic. Hence I think the explanation fails, in so -far as it is based on the accumulation by selection of small individual -variations that are supposed to give the individuals some slight -advantage under each set of external conditions. Darwin admits that -“this difficulty for a long time quite confounded me. But I think it can -be in large part explained.” The first explanation that is offered is -that areas now continuous may not have been so in the past. This may be -true in places, but the great continents have had continuous areas for a -long time, and Darwin frankly acknowledges that he “will pass over this -way of explaining the difficulty.” The second attempt is based on the -supposed narrowness of the area, where two species, descended from a -common parent, overlap. In this region the change is often very abrupt, -and Darwin adds:— - -“To those who look at climate and the physical conditions of life as the -all-important elements of distribution, these facts ought to cause -surprise, as climate and height or depth graduate away insensibly. But -when we bear in mind that almost every species, even in its metropolis, -would increase immensely in numbers, were it not for other competing -species; that nearly all either prey on or serve as prey for others; in -short, that each organic being is either directly or indirectly related -in the most important manner to other organic beings,—we see that the -range of the inhabitants of any country by no means exclusively depends -on insensibly changing physical conditions, but in a large part on the -presence of other species, on which it lives, or by which it is -destroyed, or with which it comes into competition; and as these species -are already defined objects, not blending one into another by insensible -gradations, the range of any one species, depending as it does on the -range of others, will tend to be sharply defined.” - -Here we have a _petitio principii_. The sharp definition of species, -that we started out to account for, is explained by the sharp definition -of other species! - -A third part of the explanation is that, owing to the relative fewness -of individuals at the confines of the range during the fluctuations of -their enemies, or of their prey, or in the nature of the seasons, they -would be extremely liable to utter extermination. If this were really -the case, then new species themselves which, on the theory, are at first -few in numbers ought to be exterminated. On the whole, then, it does not -appear that Darwin has been very successful in his attempt to meet this -objection to the theory. - -Darwin tries to meet the objection, that organs of extreme perfection -and complication cannot be accounted for by natural selection, as -follows:— - -“To suppose that the eye with all its inimitable contrivances for -adjusting the focus to different distances, for admitting different -amounts of light, and for the correction of spherical and chromatic -aberration, could have been formed by natural selection, seems, I freely -confess, absurd in the highest degree.” - -The following sketch that Darwin gives to show how he imagined the -vertebrate eye to have been formed is very instructive, as illustrating -how he supposed that natural selection acts:— - -“If we must compare the eye to an optical instrument, we ought in -imagination to take a thick layer of transparent tissue, with spaces -filled with fluid, and with a nerve sensitive to light beneath, and then -suppose every part of this layer to be continually changing slowly in -density, so as to separate into layers of different densities and -thicknesses, placed at different distances from each other, and with the -surfaces of each layer slowly changing in form. Further we must suppose -that there is a power, represented by natural selection or the survival -of the fittest, always intently watching each slight alteration in the -transparent layers; and carefully preserving each which, under varied -circumstances, in any way or in any degree, tends to produce a -distincter image. We must suppose each new state of the instrument to be -multiplied by the million; each to be preserved until a better one is -produced, and then the old ones to be all destroyed. In living bodies, -variation will cause the slight alterations, generation will multiply -them almost infinitely, and natural selection will pick out with -unerring skill each improvement. Let this process go on for millions of -years; and during each year on millions of individuals of many kinds; -and may we not believe that a living optical instrument might thus be -formed as superior to one of glass, as the works of the Creator are to -those of man.” - -We may conclude in Darwin’s own words:— - -“To arrive, however, at a just conclusion regarding the formation of the -eye, with all its marvellous yet not absolutely perfect characters, it -is indispensable that the reason should conquer the imagination; but I -have felt the difficulty far too keenly to be surprised at others -hesitating to extend the principle of natural selection to so startling -a length.” - -The electric organs, present in several fish, offer a case of special -difficulty to the selection theory. When well developed, as in the -Torpedo and in Gymnotus, it is conceivable that it may serve as an organ -of defence, but in other forms the shock is so weak that it is not to be -supposed that it can have any such function. Romanes, who in many ways -was one of the stanchest followers of Darwin, admits that, so far as he -can see, the evolution of the electric organs cannot be explained by the -selection theory. Darwin offers no explanation, but bases his defence on -the grounds that we do not know of what use this organ can be to the -animal. - -Darwin also refers to the phosphorescent, or luminous, organs as a -supposed case of difficulty for his theory. - -“The luminous organs which occur in a few insects, belonging to widely -different families, and which are situated in different parts of the -body, offer, under our present state of ignorance, a difficulty almost -exactly parallel with that of the electric organs.” - -In this case also, as in that of the electric organs, the structures -appear in entirely different parts of the body of the insect in -different species, so that their occurrence in this group cannot be -accounted for on a common descent. In whatever way they have arisen, -they must have evolved independently in different species. Darwin -advances no explanation of the origin of the luminous organs, but states -that they “offer under our present state of ignorance a difficulty -almost exactly parallel with that of the electric organs.” It will be -noticed that the difficulty referred to rests on the assumption that -since the organs are well developed they must have some important use! - -We may next consider “organs of little apparent importance as affected -by natural selection.” Darwin says:— - -“As natural selection acts by life and death,—by the survival of the -fittest, and by the destruction of the less well-fitted individuals,—I -have sometimes felt great difficulty in understanding the origin or -formation of parts of little importance; almost as great, though of a -very different kind, as in the case of the most perfect and complex -organs.” - -His answers to this difficulty are: (1) we are too ignorant “in regard -to the whole economy of any one organic being to say what slight -modifications would be of importance or not,”—thus such apparently -trifling characters as the down on fruit, or the colors of the skin and -hair of quadrupeds, which from being correlated with constitutional -differences or from determining the attacks of insects might be acted on -by natural selection; (2) organs now of trifling importance have in some -cases been of high importance to an early progenitor; (3) the changed -conditions of life may account for some of the useless organs; (4) -reversion accounts for others; (5) the complex laws of growth account -for still others, such as correlation, compensation of the pressure of -one part on another, etc.; (6) the action of sexual selection is -responsible for many characters not to be explained by natural -selection. Admitting that there may be cases that can be accounted for -on one or the other of these six possibilities, yet there can be no -doubt that there are still a considerable number of specific characters -that cannot be explained in any of these ways. I do not think that -Darwin has by any means met this objection, even if all these six -possibilities be admitted as generally valid. - -Amongst the “miscellaneous objections” to his theory that Darwin -considers we may select the most important cases. The following -paragraph has been sometimes quoted by later writers to show that Darwin -saw, to a certain extent, the insufficiency of fluctuating variations as -a basis for selection. What he calls here “spontaneous variability” -refers to sudden and extensive variations, or what we may call -discontinuous variations. “In the earlier editions of this work I -underrated, as it now seems probable, the frequency and importance of -modifications due to spontaneous variability. But it is impossible to -attribute to this cause the innumerable structures which are so well -adapted to the habits of life of each species. I can no more believe in -this, that the well-adapted form of a race-horse or greyhound, which -before the principle of selection by man was well understood, excited so -much surprise in the minds of the older naturalists, can thus be -explained.” - -Darwin appears to mean by the latter part of this statement, that he -cannot believe that such sudden and great variations as have caused a -peach tree to produce nectarines can account for the wonderful -adaptations of organisms; but it is not really necessary to suppose that -this would often occur, for the same result could be reached by several -stages, even if the discontinuous variations had been small, and had -appeared in many individuals simultaneously. After showing that in a -number of flowers, especially of the Compositæ and Umbelliferæ, the -individual flowers in the closely crowded heads are sometimes formed on -a different type, Darwin concludes: “In these several cases, with the -exception of that of the well-developed ray-florets, which are of -service in making the flowers conspicuous to insects, natural selection -cannot, as far as we can judge, have come into play, or only in a quite -subordinate manner. All these modifications follow from the relative -position and interaction of the parts; and it can hardly be doubted that -if all the flowers and leaves on the same plant had been subjected to -the same external and internal condition, as are the flowers and leaves -in certain positions, all would have been modified in the same manner.” - -Further on we meet with the following remarkable statement: “But when, -from the nature of the organism and of the conditions, modifications -have been induced which are unimportant for the welfare of the species, -they may be, and apparently often have been, transmitted in nearly the -same state to numerous, otherwise modified, descendants. It cannot have -been of much importance to the greater number of mammals, birds, or -reptiles, whether they were clothed with hair, feathers, or scales; yet -hair has been transmitted to almost all mammals, feathers to all birds, -and scales to all true reptiles. A structure, whatever it may be, which -is common to many allied forms, is ranked by us as of high systematic -importance, and consequently is often assumed to be of high vital -importance to the species. Thus, as I am inclined to believe, -morphological differences, which we consider as important,—such as the -arrangement of the leaves, the divisions of the flower or of the -ovarium, the position of the ovules, etc.,—first appeared in many cases -as fluctuating variations, which sooner or later became constant through -the nature of the organism and of the surrounding conditions, as well as -through the intercrossing of distinct individuals, but not through -natural selection; for as these morphological characters do not affect -the welfare of the species, any slight deviations in them could not have -been governed or accumulated through this latter agency. It is a strange -result which we thus arrive at, namely, that characters of slight vital -importance to the species are the most important to the systematist; -but, as we shall hereafter see when we treat of the genetic principle of -classification, this is by no means so paradoxical as it may at first -appear.” - -If all this be granted, it is once more evident that the only variations -that come under the action of selection are the limited number that are -of vital importance to the organism. How little the theory of natural -selection can be used to explain the origin of species will be apparent -from the above quotation. This is, of course, not an argument against -the theory itself, which would still be one of vast importance if it -explained adaptive characters alone; but enough has been said, I think, -to show that it is improbable that the origin of adaptive and -non-adaptive characters are to be explained by entirely different -principles. - -In reply to a criticism of Mivart, Darwin makes the further admission as -to the insufficiency of the theory of natural selection: “When -discussing special cases, Mr. Mivart passes over the effects of the -increased use and disuse of parts, which I have always maintained to be -highly important, and have treated in my ‘Variation under Domestication’ -at greater length than, as I believe, any other writer. He likewise -often assumes that I attribute nothing to variation, independent of -natural selection, whereas in the work just referred to I have collected -a greater number of well-established cases than is to be found in any -other work known to me.” If this is admitted, and if it can be shown -that the evidence in favor of the inheritance of acquired characters is -very doubtful at best, may we not conclude that Mivart’s criticisms have -sometimes hit the mark? - -The following objection appears to be a veritable stumbling-block to the -theory. Flatfishes and soles lie on one side, and do not stand in a -vertical position as do other fish. Some species lie on one side and -some on the other, and some species contain both right-sided and -left-sided individuals. In connection with this unusual habit we find a -striking change in the structure. The eye that would be on the under -side has shifted, so that it has come to lie on the upper side of the -head, _i.e._ both eyes lie on the same side,—a condition found in no -other vertebrate. As a result of the shifting of the eye, the bones of -the skull have also become profoundly modified. The young fish that -emerge from the egg swim at first upright, as do ordinary fish, and only -after they have led a free existence for some time do they turn to one -side and sink to the bottom. Unless the under eye moved to the upper -side it would be of no use to the flatfish, and might even be a source -of injury. Mivart points out that a sudden, spontaneous transformation -in the position of eye is hardly conceivable, and to this Darwin, of -course, assents. Mivart adds: “If the transit was gradual, then how such -transit of one eye a minute fraction of the journey towards the other -side of the head could benefit the individual is, indeed, far from -clear. It seems even that such an incipient transformation must rather -have been injurious.” Darwin’s reply is characteristic:— - -“We thus see that the first stages of the transit of the eye from one -side of the head to the other, which Mr. Mivart considers would be -injurious, may be attributed to the habit, no doubt beneficial to the -individual and to the species, of endeavoring to look upwards with both -eyes, whilst resting on one side at the bottom. We may also attribute to -the inherited effects of use the fact of the mouth in several kinds of -flatfish being bent towards the lower surface, with the jaw-bones -stronger and more effective on this, the eyeless side of the head, than -on the other side, for the sake, as Dr. Traquair supposes, of feeding -with ease on the ground. Disuse, on the other hand, will account for the -less developed condition of the whole inferior half of the body, -including the lateral fins; though Yarrell thinks that the reduced size -of these fins is advantageous to the fish, as ‘there is so much less -room for their action, than with the larger fins above.’ Perhaps the -lesser number of teeth in the proportion of four to seven in the upper -halves of the two jaws of the plaice, to twenty-five to thirty in the -lower halves, may likewise be accounted for by disuse. From the -colorless state of the ventral surface of most fishes and of many other -animals, we may reasonably suppose that the absence of color in flatfish -on the side, whether it be the right or left, which is undermost, is due -to the exclusion of light.” - -By falling back on the theory of inheritance of acquired characters -Darwin tacitly admits the incompetence of natural selection to explain -the evolution of the flatfish. If the latter theory prove incorrect, it -must then be admitted that the evolution of the flatfishes cannot be -accounted for by either of the two main theories on which Darwin relies. - -Mivart further points out that the beginning stages of the mammary -glands cannot be explained by Darwin’s theory. To which Darwin replies, -that an American naturalist, Mr. Lockwood, believes from what he has -seen of the development of the young of the pipe-fish (Hippocampus) that -“they are nourished by a secretion from the cutaneous glands of the sac” -in which the young are enclosed. This can scarcely be said to be a -satisfactory reply; for, if it is true that this is the case for the -pipe-fish,—and I cannot find on inquiry that this statement has been -confirmed,—it is still rather speculative to suppose that the ancestral -mammals nourished their young by secreting a fluid into the marsupial -sac around the embryos. - -Darwin deals with instincts of animals in the same way as he deals with -their structures. After pointing out that instincts are variable, and -that the variations are hereditary, he proceeds to show how selection -may act by picking out those individuals possessing the more favorable -instincts. In other words, the theory of natural selection is applied to -functions, as well as to structure. Darwin makes use here also of the -Lamarckian factor of inheritance, and concludes that “in most cases -habit and selection have probably both occurred.” - -A few examples will sufficiently serve to illustrate Darwin’s meaning. -The first case given is that of the cuckoo, which lays its eggs in the -nests of other birds, where they are hatched and the young reared by -their foster-parents. The starting-point for such a perversion of the -ordinary habits of birds is to be found, he thinks, in the occasional -deposition of eggs in the nests of other birds, which has at times been -observed for a number of species. For instance, this has been seen in -the American cuckoo, which ordinarily builds a nest of its own. It is -recorded and believed to be true that the young English cuckoo, when -only two or three days old, ejects from the nest the offspring of its -foster-parents, and this “strange and odious instinct” is supposed by -Darwin to have been acquired in order that the young cuckoo might get -more food, and that the young bird has acquired during successive -generations the strength and structure necessary for the work of -ejection. This is of course largely speculative, and it is by no means -obvious that it was a greater benefit to the cuckoo to have other birds -rear its young than to do so itself. We can equally well imagine, since -this is the turn the argument takes, that the occasional instinct to -deposit eggs in the nests of other birds would be disadvantageous, and -could not have been acquired by the selection of a fluctuating instinct -of this sort. We have no right to assume, that because a new habit has -been acquired, that it is a more advantageous one than the one that has -been lost. All that we can legitimately infer is, that, although the -normal instinct has been changed into another, the race has still been -able to remain in existence. The same conclusion applies to the case of -_Molothrus bonariensis_, cited by Darwin, and is here even more -obvious:— - -“Some species of Molothrus, a widely distinct genus of American birds, -allied to our starlings, have parasitic habits like those of the cuckoo; -and the species present an interesting gradation in the perfection of -their instincts. The sexes of _Molothrus badius_ are stated by an -excellent observer, Mr. Hudson, sometimes to live promiscuously together -in flocks, and sometimes to pair. They either build a nest of their own, -or seize on one belonging to some other bird, occasionally throwing out -the nestlings of the stranger. They either lay their eggs in the nest -thus appropriated, or oddly enough build one for themselves on the top -of it. They usually sit on their own eggs and rear their own young; but -Mr. Hudson says it is probable that they are occasionally parasitic, for -he has seen the young of this species following old birds of a distinct -kind and clamoring to be fed by them. The parasitic habits of another -species of Molothrus, the _M. bonariensis_, are much more highly -developed than those of the last, but are still far from perfect. This -bird, as far as is known, invariably lays its eggs in the nest of -strangers; but it is remarkable that several together sometimes commence -to build an irregular untidy nest of their own, placed in singularly -ill-adapted situations, as on the leaves of a large thistle. They never, -however, as far as Mr. Hudson has ascertained, complete a nest for -themselves. They often lay so many eggs—from fifteen to twenty—in the -same foster-nest, that few or none can possibly be hatched. They have, -moreover, the extraordinary habit of pecking holes in the eggs, whether -of their own species or of their foster-parents, which they find in the -appropriated nests. They drop also many eggs on the bare ground, which -are thus wasted.” - -Can we possibly be expected to believe that it has been to the advantage -of this species to give up its original regular method of incubating its -own eggs, and acquire such a haphazard, new method? Does not the -explanation prove too much, rather than give support to Darwin’s -hypothesis? Is it not better to conclude, that despite the disadvantages -entailed by a change in the original instincts, the species is still -able to remain in existence? - -Darwin points out, in the case of the slave-making ants, that the -slave-making instinct may have arisen in the first instance by ants -carrying pupæ, that they have captured, into their own nests. Later this -habit might become fixed, and, finally, after passing through several -stages of development, the ants might become absolutely dependent on -their slaves. It is also supposed that those colonies in which this -instinct was better developed would survive in competition with other -colonies of the same species on account of the supposed advantage of -owning slaves. In this way natural selection steps in and perfects the -process. - -It is far from proven, or even made probable, that a species of ant that -becomes gradually dependent on its slaves is more likely to survive than -other colonies that are not so dependent. All we can be certain of is -that with slaves they have still been able to maintain their own. -Moreover, we must not forget that it is not enough to show that a -particular habit might be useful to a species, but it should also be -shown that it is of sufficient importance, at every stage of its -evolution, to give a decisive advantage in the “struggle for existence.” -For unless a life and death struggle takes place between the different -colonies, natural selection is powerless to bring about its supposed -results. And who will be bold enough to affirm that the presence of -slaves in a nest will give victory to that colony in competition with -its neighbors? Has the history of mankind taught us that the -slave-making countries have exterminated the countries without slaves? -Is the question so simple as this? May not the degeneration of the -masters more than compensate for the acquirement of slaves, and may not -the loss of life in obtaining slaves more than counterbalance the -advantage of the slaves after they are captured? In the face of these -possibilities it is not surprising to find that Darwin, when summing up -the chapter, makes the following admission: “I do not pretend that the -facts in this chapter strengthen in any degree my theory; but none of -the cases of difficulty, to the best of my judgment, annihilate it.” -Darwin, with his usual frankness, adds:— - -“No doubt many instincts of very difficult explanation could be opposed -to the theory of natural selection,—cases, in which we cannot see how an -instinct could have originated; cases, in which no intermediate -gradations are known to exist; cases of instincts of such trifling -importance, that they could hardly have been acted on by natural -selection; cases of instincts almost identically the same in animals so -remote in the scale of nature, that we cannot account for their -similarity by inheritance from a common progenitor, and consequently -must believe that they were independently acquired through natural -selection. I will not here enter on these several cases, but will -confine myself to one special difficulty, which at first appeared to me -insuperable, and actually fatal to the whole theory. I allude to the -neuters or sterile females in insect communities; for these neuters -often differ widely in instinct and in structure from both the males and -fertile females, and yet, from being sterile, they cannot propagate -their kind. - -“The subject well deserves to be discussed at great length, but I will -here take only a single case, that of working or sterile ants. How the -workers have been rendered sterile is a difficulty; but not much greater -than that of any other striking modification of structure; for it can be -shown that some insects and other articulate animals in a state of -nature occasionally become sterile; and if such insects had been social, -and it had been profitable to the community that a number should have -been annually born capable of work, but incapable of procreation, I can -see no especial difficulty in this having been effected through natural -selection. But I must pass over this preliminary difficulty. The great -difficulty lies in the working ants differing widely from both the males -and the fertile females in structure, as in the shape of the thorax, and -in being destitute of wings and sometimes of eyes, and in instinct. As -far as instinct alone is concerned, the wonderful difference in this -respect between the workers and the perfect females, would have been -better exemplified by the hive-bee. If a working ant or other neuter -insect had been an ordinary animal, I should have unhesitatingly assumed -that all its characters had been slowly acquired through natural -selection; namely, by individuals having been born with slight -profitable modifications, which were inherited by the offspring; and -that these again varied and again were selected, and so onwards. But -with the working ant we have an insect differing greatly from its -parents, yet absolutely sterile; so that it could never have transmitted -successively acquired modifications of structure or instinct to its -progeny. It may well be asked, how is it possible to reconcile this case -with the theory of natural selection?” - -Darwin’s answer is that the differences of structure are correlated with -certain ages and with the two sexes, but this is obviously only shifting -the difficulty, not meeting it. He concludes, “I can see no great -difficulty in any character becoming correlated with the sterile -condition of certain members of the insect communities, the difficulty -lies in understanding how such correlated modifications of structure -could have been slowly accumulated by natural selection.” “This -difficulty, though appearing insuperable, is lessened, or, as I believe, -disappears, when it is remembered that selection may be applied to the -family, as well as to the individual, and may thus give the desired -end.” - -Darwin did not fail to see that there is a further difficulty even -greater than the one just mentioned. He says: “But we have not as yet -touched on the acme of the difficulty; namely, the fact that the neuters -of several ants differ, not only from the fertile females and males, but -from each other, sometimes to an almost incredible degree, and are thus -divided into two or even three castes. The castes, moreover, do not -commonly graduate into each other, but are perfectly well defined; being -as distinct from each other as are any two species of the same genus, or -rather as any two genera of the same family. Thus in Eciton, there are -working and soldier neuters, with jaws and instincts extraordinarily -different: in Cryptocerus, the workers of one caste alone carry a -wonderful sort of shield on their heads, the use of which is quite -unknown: in the Mexican Myrmecocystus, the workers of one caste never -leave the nest; they are fed by the workers of another caste, and they -have an enormously developed abdomen which secretes a sort of honey, -supplying the place of that excreted by the aphides, or the domestic -cattle as they may be called, which our European ants guard and -imprison.” - -“It will indeed be thought that I have an overweening confidence in the -principle of natural selection, when I do not admit that such wonderful -and well-established facts at once annihilate the theory. In the simpler -case of neuter insects all of one caste, which, as I believe, have been -rendered different from the fertile males and females through natural -selection, we may conclude from the analogy of ordinary variations, that -the successive, slight, profitable modifications did not first arise in -all the neuters in the same nest, but in some few alone; and that by the -survival of the communities with females which produced most neuters -having the advantageous modification, all the neuters ultimately came to -be thus characterized. According to this view we ought occasionally to -find in the same nest neuter insects, presenting gradations of -structure; and this we do find, even not rarely, considering how few -neuter insects out of Europe have been carefully examined.” - -From this the conclusion is reached:— - -“With these facts before me, I believe that natural selection, by acting -on the fertile ants or parents, could form a species which should -regularly produce neuters, all of large size with one form of jaw, or -all of small size with widely different jaws; or lastly, and this is the -greatest difficulty, one set of workers of one size and structure, and -simultaneously another set of workers of a different size and -structure;— a graduated series having first been formed, as in the case -of the driver ant, and then the extreme forms having been produced in -greater and greater numbers, through the survival of the parents which -generated them, until none with an intermediate structure were produced. - -“I have now explained how, as I believe, the wonderful fact of two -distinctly defined castes of sterile workers existing in the same nest, -both widely different from each other and from their parents, has -originated. We can see how useful their production may have been to a -social community of ants, on the same principle that the division of -labor is useful to civilized man. Ants, however, work by inherited -instincts and by inherited organs or tools, whilst man works by acquired -knowledge and manufactured instruments. But I must confess, that, with -all my faith in natural selection, I should never have anticipated that -this principle could have been efficient in so high a degree, had not -the case of these neuter insects led me to this conclusion. I have, -therefore, discussed this case, at some little but wholly insufficient -length, in order to show the power of natural selection, and likewise -because this is by far the most serious special difficulty which my -theory has encountered. The case, also, is very interesting, as it -proves that with animals, as with plants, any amount of modification may -be effected by the accumulation of numerous, slight, spontaneous -variations, which are in any way profitable, without exercise or habit -having been brought into play. For peculiar habits confined to the -workers or sterile females, however long they might be followed, could -not possibly affect the males and fertile females, which alone leave -descendants. I am surprised that no one has hitherto advanced this -demonstrative case of neuter insects, against the well-known doctrine of -inherited habit, as advanced by Lamarck.” - -We may dissent at once from Darwin’s statement which, he thinks, “proves -that any amount of modification may be affected by the accumulation of -numerous slight variations which are in any way profitable without -exercise or habit having been brought into play”; we may dissent if for -no other reason than that this begs the whole point at issue, and is not -proven. It does not follow because in some colonies all intermediate -stages of neuters exist, that in other colonies, where no such -intermediate stages are present, these have been slowly weeded out by -natural selection, causing to disappear all colonies slightly below the -mark. It is this that begs the question. Because we can imagine that -intermediate stages between the different castes may have been present, -it neither follows that such fluctuating variations have been the basis -for the evolution of the more sharply defined types, nor that the -imagined advantage of such a change would have led through competition -to the extermination of the other colonies. However much we may admire -the skill with which Darwin tried to meet this difficulty, let us not -put down the results to the good of the theory, but rather repeat once -more Darwin’s own words at the end of this chapter, to the effect that -the facts do not strengthen the theory. - - - Sterility between Species - -The care with which Darwin examined every bearing of his theory is -nowhere better exemplified than in his treatment of the question of -sterility between the individuals of different species. It would be so -obviously to the advantage of the selection theory if it were true that -sterility between species had been acquired by selection in order to -prevent intercrossing, that it would have been easy for a less cautious -thinker to have fallen into the error of supposing that sterility might -have been acquired in this way. Tempting as such a view appears, Darwin -was not caught by the specious argument, as the opening sentence in the -chapter of hybridism shows:— - -“The view commonly entertained by naturalists is that species, when -intercrossed, have been specially endowed with sterility, in order to -prevent their confusion. This view certainly seems at first highly -probable, for species living together could hardly have been kept -distinct had they been capable of freely crossing. The subject is in -many ways important for us, more especially as the sterility of species -when first crossed, and that of their hybrid offspring, cannot have been -acquired, as I shall show, by the preservation of successive profitable -degrees of sterility. It is an incidental result of differences in the -reproductive systems of the parent species.” - -In dealing with this subject Darwin points out that we must be careful -to distinguish between “the sterility of species when first crossed, and -the sterility of hybrids produced from them.” In the former case, the -reproductive organs of each individual are in a perfectly normal -condition, while hybrids appear to be generally impotent owing to some -imperfection in the reproductive organs themselves. They are not -perfectly fertile, as a rule, either with each other, or with either of -the parent forms. - -In striking contrast to the sterility between species is the fertility -of varieties. If, as Darwin believes, varieties are incipient species, -we should certainly expect to find them becoming less and less fertile -with other fraternal varieties, or with the parent forms in proportion -as they become more different. Yet experience appears to teach exactly -the opposite; but the question is not a simple one, and the results are -not so conclusive as appears at first sight. Let us first see how Darwin -met this obvious contradiction to his view. - -In the first place, he points out that all species are not infertile -when crossed with other species. The sterility of various species, when -crossed, is so different in degree, and graduates away so insensibly, -and the fertility of pure species is so easily affected by various -circumstances, that it is most difficult to say where perfect fertility -ends and sterility begins. “It can thus be shown that neither sterility -nor fertility afford any certain distinction between species and -varieties.” Darwin cites several cases in plants in which crosses -between species have been successfully accomplished. The following -remarkable results are also recorded: “Individual plants in certain -species of Lobelia, Verbascum, and Passiflora can easily be fertilized -by pollen from a distinct species, but not by pollen from the same -plant, though this pollen can be proved to be perfectly sound by -fertilizing other plants or species. In the genus Hippeastrum, in -Corydalis as shown by Professor Hildebrand, in various orchids as shown -by Mr. Scott and Fritz Müller, all the individuals are in this peculiar -condition. So that with some species, certain abnormal individuals, and -in other species all the individuals, can actually be hybridized much -more readily than they can be fertilized by pollen from the same -individual plant!”[14] - -Footnote 14: - - A somewhat parallel case has recently been discovered by Castle for - the hermaphroditic ascidian _Ciona intestinalis_. In this case the - spermatozoa of any individual fail to fertilize the eggs of the same - individual, although they will fertilize the eggs of any other - individual. - -In regard to animals, Darwin concludes that “if the genera of animals -are as distinct from each other as are the genera of plants, then we may -infer that animals more widely distinct in the scale of nature can be -crossed more easily than in the case of plants; but the hybrids -themselves are, I think, more sterile.” - -The most significant fact in this connection is that the more widely -different two species are, so that they are placed in different -families, so much the less probable is it that cross-fertilization will -produce any result. From this condition of infertility there may be -traced a gradation between less different forms of the same genus to -almost complete, or even complete, fertility between closely similar -species. Darwin further points out that: “The hybrids raised from two -species which are very difficult to cross, and which rarely produce any -offspring, are generally very sterile; but the parallelism between the -difficulty of making a first cross, and the sterility of the hybrids -thus produced—two classes of facts which are generally confounded -together—is by no means strict. There are many cases, in which two pure -species, as in the genus Verbascum, can be united with unusual facility, -and produce numerous hybrid offspring, yet these hybrids are remarkably -sterile. On the other hand, there are species which can be crossed very -rarely, or with extreme difficulty, but the hybrids, when at last -produced, are very fertile. Even within the limits of the same genus, -for instance in Dianthus, these two opposite cases occur.” - -In regard to reciprocal crosses Darwin makes the following important -statements: “The diversity of the result in reciprocal crosses between -the same two species was long ago observed by Kölreuter. To give an -instance: _Mirabilis jalapa_ can easily be fertilized by the pollen of -_M. longiflora_, and the hybrids thus produced are sufficiently fertile; -but Kölreuter tried more than two hundred times, during eight following -years, to fertilize reciprocally _M. longiflora_ with the pollen of _M. -jalapa_, and utterly failed.” - -A formal interpretation of this difference can be easily imagined. The -infertility in one direction may be due to some physical difficulty met -with in penetrating the stigma, or style. For instance, the tissue in -one species may be too compact, or the style too long. Pflüger, who -carried out a large number of experiments by cross-fertilizing different -species of frogs, reached the conclusion that the spermatozoa having -small and pointed heads could cross-fertilize more kinds of eggs, than -could the spermatozoa with large blunt heads. This is probably due to -the ability of the smaller spermatozoa to penetrate the jelly around the -eggs, or the pores in the surface of the egg itself. But there are also -other sides to this question, as recent results have shown, for, even if -a foreign spermatozoon can enter an egg, it does not follow that the -development of the egg will take place. Here the difficulty is due to -some obscure processes in the egg itself. Now that we know more of the -nicely balanced combinations that take place during fertilization of the -egg, and during the process of cell division, we can easily see that if -the processes were in the least different in the two species it might be -impossible to combine them in a single act. - -“Now do these complex and singular rules indicate that species have been -endowed with sterility simply to prevent their becoming confounded in -nature? I think not. For why should the sterility be so extremely -different in degree, when various species are crossed, all of which we -must suppose it would be equally important to keep from blending -together?” - -“The foregoing rules and facts, on the other hand, appear to me clearly -to indicate that the sterility both of first crosses and of hybrids is -simply incidental or dependent on unknown differences in their -reproductive systems; the differences being of so peculiar and limited a -nature, that, in reciprocal crosses between the same two species, the -male sexual element of the one will often freely act on the female -sexual element of the other, but not in a reversed direction.” - -Does Darwin give here a satisfactory answer to the difficulty that he -started out to explain away? On the whole, the reader will admit, I -think, that he has fairly met the situation, in so far as he has shown -that there is no absolute line of demarcation between the power of -intercrossing of varieties and races, and of species. It is also -_extremely important to have found that the difficulties increase, so to -speak, even beyond the limits of the species_; since species, belonging -to different genera, are as a rule more difficult to intercross than -when they belong to the same genus. The further question, as to whether -there are differences in respect to the power of intercrossing between -different kinds of varieties, such as those dependent on selection of -fluctuating variations, of local conditions, of mutations, etc., is far -from being settled at the present time. - -That this property of species is useful to them, in the somewhat unusual -sense that it keeps them from freely mingling with other species, is -true; but, as has been said, this would be a rather peculiar kind of -adaptation. If, however, it be claimed that this property is useful to -species, as Darwin himself claims, then, as he also points out, it is a -useful acquirement that cannot have arisen through natural selection. It -is not difficult to show why this must be so. If two varieties were to -some extent at the start less fertile, _inter se_, than with their own -kind, the only way in which they could become more infertile through -selection would be by selecting those individuals in each generation -that are still more infertile, but the forms of this sort would, _ex -hypothese_, become less numerous than the descendants of each species -itself, which would, therefore, supplant the less fertile ones. - -Darwin’s own statement in regard to this point is as follows:— - -“At one time it appeared to me probable, as it has to others, that the -sterility of first crosses and of hybrids might have been slowly -acquired through the natural selection of slightly lessened degrees of -fertility, which, like any other variation, spontaneously appeared in -certain individuals of one variety when crossed with those of another -variety. For it would clearly be advantageous to two varieties or -incipient species, if they could be kept from blending, on the same -principle that, when man is selecting at the same time two varieties, it -is necessary that he should keep them separate. - -“In considering the probability of natural selection having come into -action, in rendering species mutually sterile, the greatest difficulty -will be found to lie in the existence of many graduated steps from -slightly lessened fertility to absolute sterility. It may be admitted -that it would profit an incipient species, if it were rendered in some -slight degree sterile when crossed with its parent form or with some -other variety; for thus fewer bastardized and deteriorated offspring -would be produced to commingle their blood with the new species in -process of formation. But he who will take the trouble to reflect on the -steps by which this first degree of sterility could be increased through -natural selection to that high degree which is common with so many -species, and which is universal with species which have been -differentiated to a generic or family rank, will find the subject -extraordinarily complex. After mature reflection it seems to me that -this could not have been effected through natural selection. Take the -case of any two species which, when crossed, produced few and sterile -offspring; now, what is there which could favor the survival of those -individuals which happened to be endowed in a slightly higher degree -with mutual infertility, and which thus approached by one small step -toward absolute sterility? Yet an advance of this kind, if the theory of -natural selection be brought to bear, must have incessantly occurred -with many species, for a multitude are mutually quite barren.” - -Darwin points out the interesting parallel existing between the results -of intercrossing, and those of grafting together parts of different -species. - -“As the capacity of one plant to be grafted or budded on another is -unimportant for their welfare in a state of nature, I presume that no -one will suppose that this capacity is a _specially_ endowed quality, -but will admit that it is incidental on differences in the laws of -growth of the two plants. We can sometimes see the reason why one tree -will not take on another, from differences in their rate of growth, in -the hardness of their wood, in the period of the flow or nature of their -sap, etc.; but in a multitude of cases we can assign no reason whatever. -Great diversity in the size of two plants, one being woody and the other -herbaceous, one being evergreen and the other deciduous, and adapted to -widely different climates, do not always prevent the two grafting -together. As in hybridization, so with grafting, the capacity is limited -by systematic affinity, for no one has been able to graft together trees -belonging to quite distinct families; and, on the other hand, closely -allied species, and varieties of the same species, can usually, but not -invariably, be grafted with ease. But this capacity, as in -hybridization, is by no means absolutely governed by systematic -affinity. Although many distinct genera within the same family have been -grafted together, in other cases species of the same genus will not take -on each other. The pear can be grafted far more readily on the quince, -which is ranked as a distant genus, than on the apple, which is a member -of the same genus. Even different varieties of the pear take with -different degrees of facility on the quince; so do different varieties -of the apricot and peach on certain varieties of the plum.” - -“We thus see, that although there is a clear and great difference -between the mere adhesion of grafted stocks, and the union of the male -and female elements in the act of reproduction, yet that there is a rude -degree of parallelism in the results of grafting and of crossing of -distinct species. And we must look at the curious and complex laws -governing the facility with which trees can be grafted on each other as -incidental on unknown differences in their vegetative systems, so I -believe that the still more complex laws governing the facility of first -crosses are incidental on unknown differences in their reproductive -systems.... The facts by no means seem to indicate that the greater or -lesser difficulty of either grafting or crossing various species has -been a special endowment; although in the case of crossing, the -difficulty is as important for the endurance and stability of specific -forms, as in the case of grafting it is unimportant for their welfare.” - - - Weismann’s Germinal Selection - -We cannot do better, in bringing this long criticism of the Darwinian -theory to an end, than by considering the way in which Weismann has -attempted in his paper on “Germinal Selection” to solve one of the -“patent contradictions” of the selection theory. He calls attention, in -doing so, to what he regards as a vital weakness of the theory in the -form in which it was left by Darwin himself. Weismann says:— - -“The basal idea of the essay—the existence of Germinal Selection—was -propounded by me some time since,[15] but it is here for the first time -fully set forth and tentatively shown to be the necessary complement of -the process of selection. Knowing this factor, we remove, it seems to -me, the patent contradiction of the assumption that the general fitness -of organisms, or the adaptations _necessary_ to their existence, are -produced by _accidental_ variations—a contradiction which formed a -serious stumbling-block to the theory of selection. Though still -assuming that the _primary_ variations are ‘accidental,’ I yet hope to -have demonstrated that an interior mechanism exists which compels them -to go on increasing in a definite direction, the moment selection -intervenes. _Definitely directed variation exists_, but not predestined -variation, running on independently of the life conditions of the -organism, as Nägeli, to mention the most extreme advocate of this -doctrine, has assumed; on the contrary, the variation is such as is -elicited and controlled by those conditions themselves, though -indirectly.” - -Footnote 15: - - _Neue Gedanken zur Vererbungsfrage, eine Antwort an Herbert Spencer_, - Jena, 1895. - -“The real aim of the present essay is to rehabilitate the principle of -selection. If I should succeed in reinstating this principle in its -emperilled rights, it would be a source of extreme satisfaction to me; -for I am so thoroughly convinced of its indispensability as to believe -that its demolition would be synonymous with the renunciation of all -inquiry concerning the causal relation of vital phenomena. If we could -understand the adaptations of nature, whose number is infinite, only -upon the assumption of a teleological principle, then, I think, there -would be little inducement to trouble ourselves about the causal -connection of the stages of ontogenesis, for no good reason would exist -for excluding teleological principles from this field. Their -introduction, however, is the ruin of science.”[16] - -Footnote 16: - - Translated by J. McCormack. The Open Court Publishing Company. The - following quotations are also taken from this translation. - -Weismann states that those critics who maintain that selection cannot -create, but only reject, “fail to see that precisely through this -rejection its creative efficacy is asserted.” There is raised here, -though not for the first time, a point that is of no small importance -for both Darwinians and anti-Darwinians to consider; for, without -further examination, it is by no means self-evident, as Weismann -implies, that by exterminating all variations that are below the average -the standard of successive generations could ever be raised beyond the -most extreme fluctuating variation. At least this appears to be the case -if individual, fluctuating variations be the sort selected, and it is to -this kind of variation to which Weismann presumably refers. Without -discussing this point here, let us examine further what Weismann has to -say. He thinks that while in each form there may be a very large number -of possible variations, yet there are also impossible variations as -well, which do not appear. “The cogency, the irresistible cogency as I -take it, of the principle of selection is precisely its capacity of -explaining why fit structures always arise, and this certainly is the -great problem of life.” Weismann points out that it is a remarkable fact -that to-day, after science has been in possession of this principle for -something over thirty years, “during which time she has busily occupied -herself with its scope, the estimation in which the theory is held -should be on the decline.” “It would be easy to enumerate a long list of -living writers who assign to it a subordinate part only in evolution, or -none at all.” “Even Huxley implicitly, yet distinctly, intimated a doubt -regarding the principle of selection when he said: ‘Even if the -Darwinian hypothesis were swept away, evolution would still stand where -it is.’ Therefore he, too, regarded it as not impossible that this -hypothesis should disappear from among the great explanatory principles -by which we seek to approach nearer to the secrets of nature.” - -Weismann is not, however, of this opinion, and believes that the present -depression is only transient, because it is only a reaction against a -theory that had been exalted to the highest pinnacle. He thinks that the -principle of selection is not overestimated, but that naturalists -imagined too quickly that they understood its workings. “On the -contrary, the deeper they penetrated into its workings the clearer it -appeared that something was lacking, that the action of the principle, -though upon the whole clear and representable, yet when carefully looked -into encountered numerous difficulties, which were formidable, for the -reason that we were unsuccessful in tracing out the actual details of -the individual process, and, therefore, in _fixing_ the phenomenon as it -actually occurred. We can state in no single case how great a variation -must be to have selective value, nor how frequently it must occur to -acquire stability. We do not know when and whether a desired useful -variation really occurs, nor on what its appearance depends; and we have -no means of ascertaining the space of time required for the fulfilment -of the selective processes of nature, and hence cannot calculate the -exact number of such processes that do and can take place at the same -time in the same species. Yet all this is necessary if we wish to follow -out the precise details of a given case. - -“But perhaps the most discouraging circumstance of all is, that we can -assert in scarcely a single actual instance in nature whether an -observed variation is useful or not—a drawback that I distinctly -emphasized some time ago. Nor is there much hope of betterment in this -respect, for think how impossible it would be for us to observe all the -individuals of a species in all their acts of life, be their habitat -ever so limited—and to observe all this with a precision enabling us to -say that this or that variation possessed selective value, that is, was -a decisive factor in determining the existence of the species.” - -“And thus it is everywhere. Even in the most indubitable cases of -adaptation as, for instance, in that of the striking protective coloring -of many butterflies, the sole ground of inference that the species on -the whole is adequately adapted to its conditions of life, is the simple -fact that the species is, to all appearances, preserved undiminished, -but the inference is not at all permissible that just this protective -coloring has selective value for the species, that is, if it were -lacking, the species would necessarily have perished.” - -Few opponents of Darwinism could give a more pessimistic account of the -accomplishments of the theory of natural selection than this, by one of -the leaders of the modern school: “Discouraging, therefore, as it may be -that the control of nature in her minutest details is here gainsaid us, -yet it were equivalent to sacrificing the gold to the dross, if simply -from our inability to follow out the details of the individual case we -should renounce altogether the principle of selection, or should -proclaim it as only subsidiary, on the ground that we believe the -protective coloring of the butterfly is not a protective coloring, but a -combination of colors inevitably resulting from internal causes. The -protective coloring remains a protective coloring whether at the time in -question it is or is not necessary for the species; and it arose as -protective coloring—arose not because it was a constitutional necessity -of the animal’s organism that here a red and there a white, black, or -yellow spot should be produced, but because it was advantageous, because -it was necessary for the animal. There is only one explanation possible -for such patent adaptations, and that is selection. What is more, no -other natural way of their originating is conceivable, for we have no -right to assume teleological forces in the domain of natural phenomena.” - -Weismann states that he does not accept Eimers’s view that the markings -of the wings of the butterflies of the genus Papilio are due to a -process of evolution in a direct line, independent of external causes. - -“On the contrary, I believe it can be clearly proved that the wing of -the butterfly is a tablet on which Nature has inscribed everything she -has deemed advantageous to the preservation and welfare of her -creatures, and nothing else; or, to abandon the simile, that these color -patterns have not proceeded from inward evolutional forces but are the -result of selection. At least in all places where we do understand their -biological significance these patterns are constituted and distributed -over the wing exactly as utility would require.” - -Again: “I should be far from maintaining that the markings arose -unconformably to law. Here, as elsewhere, the dominance of law is -certain. But I take it, that the laws involved, that is, the -physiological conditions of the variation, here are without exception -subservient to the ends of a higher power—utility; and that it is -utility primarily that determines the kind of colors, spots, streaks, -and bands that shall originate, as also their place and mode of -disposition. The laws come into consideration only to the extent of -conditioning the quality of the constructive materials—the variations, -out of which selection fashions the designs in question. And this also -is subject to important restrictions, as will appear in the sequel.” -This conclusion contains all that the most ardent Darwinian could ask. - -He rejects the idea that internal laws alone could have produced the -result, because:— - -“If internal laws controlled the markings on butterflies’ wings, we -should expect that some general rule could be established, requiring -that the upper and under surfaces of the wings should be alike or that -they should be different, or that the fore wings should be colored the -same as or differently from the hind wings, etc. But in reality all -possible kinds of combinations occur simultaneously, and no rule holds -throughout. Or, it might be supposed that bright colors should occur -only on the upper surface or only on the under surface, or on the fore -wings or only on the hind wings. But the fact is they occur -indiscriminately, now here, now there, and no one method of appearance -is uniform throughout all the species. But the fitness of the various -distributions of colors is apparent, and the moment we apply the -principle of utility we know why in the diurnal butterflies the upper -surface alone is usually variegated and the under surface protectively -colored, or why in the nocturnal butterflies the fore wings have the -appearance of bark, of old wood, or of a leaf, whilst the hind wings, -which are covered when resting, alone are brilliantly colored. On this -theory we also understand the exceptions to these rules. We comprehend -why Danaids, Heliconids, Euploids, and Acracids, in fact all diurnal -butterflies offensive to the taste and smell, are mostly brightly marked -and equally so on both surfaces, whilst all species not thus exempt from -persecution have the protective coloring on the under surface and are -frequently quite differently colored there from what they are on the -upper. - -“In any event, the supposed formative laws are not obligatory. -Dispensations from them can be issued and are issued _whenever utility -requires it_.” - -Dispensations from the laws of growth! Does not a philosophy of this -sort seem to carry us back into the dark ages? Is this the best that the -Darwinian school can do to protect itself against the difficulties into -which its chief disciple confesses it has fallen? - -Weismann lays great emphasis on the case of the Indian leaf-butterfly, -_Kallima inachis_; and points out that the leaf markings are executed -“in absolute independence of the other uniformities governing the wing.” - -“The venation of the wing is utterly ignored by the leaf markings, and -its surface is treated as a _tabula rasa_ upon which anything -conceivable can be drawn. In other words, we are presented here with a -_bilaterally symmetrical_ figure engraved on a surface which is -essentially _radially symmetrical_ in its divisions. - -“I lay unusual stress upon this point because it shows that we are -dealing here with one of those cases which cannot be explained by -mechanical, that is, by natural means, unless natural selection actually -exists and is actually competent to create new properties; for the -Lamarckian principle is excluded here _ab initio_, seeing that we are -dealing with a formation which is only passive in its effects: the leaf -markings are effectual simply by their existence and not by any function -which they perform; they are present in flight as well as at rest, -during the absence of a danger, as well as during the approach of an -enemy. - -“Nor are we helped here by the assumption of _purely internal motive -forces_, which Nägeli, Askenasy, and others have put forward as -supplying a _mechanical_ force of evolution. It is impossible to regard -the coincidence of an Indian butterfly with the leaf of a tree now -growing in an Indian forest as fortuitous, as a _lusus naturæ_. Assuming -this seemingly mechanical force, therefore, we should be led back -inevitably to a teleological principle which produces adaptive -characters and which must have deposited the directive principle in the -very first germ of terrestrial organisms, so that after untold ages at a -definite time and place the illusive leaf markings should be developed. -The assumption of preëstablished harmony between the evolution of the -ancestral line of the tree with its prefigurative leaf, and that of the -butterfly with its imitating wing, is absolutely necessary here, as I -pointed out many years ago, but as is constantly forgotten by the -promulgators of the theory of internal evolutionary forces.” - -Weismann concludes, therefore, that for his present purpose it suffices -to show “that cases exist wherein all natural explanations except that -of selection fail us,” and he then proceeds to point out that even the -natural selection of Darwin and of Wallace also fail to give us a -reasonable explanation of how, for example, the markings on the wings of -the Kallima butterfly have come about. The main reason that he gives to -show that this is the case rests on the difficulty of the assumption -that the right variations should always be present in the right place. -Here “is the insurmountable barrier for the explanatory power of the -principle [natural selection] for who, or what, is to be our guarantee -that the dark scales shall appear at the exact spots on the wing where -the midrib of the leaf must grow? And that later dark scales shall -appear at the exact spots to which the midrib must be prolonged? And -that still later such dark scales shall appear at the places whence the -lateral ribs start, and that here also a definite acute angle shall be -preserved.” Thus the philosopher in his closet multiplies and magnifies -the difficulties for which he is about to offer a panacea. Had the same -amount of labor been spent in testing whether the life of this butterfly -is so closely dependent on the exact imitation of the leaf, we might -have been spared the pains of this elaborate exordium. There are at -least some grounds for suspicion that the whole case of Kallima is “made -up.” If this should prove true, it will be a bad day for the Darwinians, -unless they fall back on Weismann’s statement that their theory is -insufficient to prove a single case! - -Weismann has used Kallima only as the most instructive illustration. The -objections that are here evident are found not only in the cases of -protective coloration, but “are applicable in all cases where the -process of selection is concerned. Take, for example, the case of -instincts that are called into action only once in life, as the pupal -performances of insects, the fabrication of cocoons, etc. How is it that -the useful variations were always present here?” Weismann concludes that -“something is still wanting to the selection theory of Darwin and -Wallace, which it is obligatory on us to discover, if we possibly can, -and without which selection as yet offers no complete explanation of the -phyletic processes of transformation.” Weismann’s first step in the -solution of the difficulty is contained in the following statement:— - -“My inference is a very simple one: if we are forced by the facts on all -hands to the assumption that the useful variations which render -selection possible are always present, then, _some profound connection -must exist between the utility of a variation and its actual -appearance_, or, in other words, _the direction of the variation of a -part must be determined by utility_, and we shall have to see whether -facts exist that confirm our conjecture.” - -Weismann finds the solution in the method by which the breeder has -obtained his results in artificial selection. For instance, the -long-tailed variety of the domestic cock of Japan owes its existence, it -is claimed, to skilful selection, and not at all to the circumstance -that, at some period of the race’s history, a cock with tail-feathers -six feet in length suddenly and spasmodically appeared. - -Weismann continues: “Now what does this mean? Simply that the hereditary -diathesis, the germinal constitution (the _Anlage_) of the breed was -changed in the respect in question, and our conclusion from this and -numerous similar facts of artificial selection runs as follows: _by the -selection alone of the plus or minus variations of a character is the -constant modification of that character in the plus or minus direction -determined_. Obviously the hereditary _diminution_ of a part is also -effected by the simple selection of the individuals in each generation -possessing the smallest parts, as is proved, for example, by the tiny -bills and feet of numerous breeds of doves. We may assert, therefore, in -general terms: a definitely directed progressive variation of a given -part is produced by continued selection in that definite direction. This -is no hypothesis, but a direct inference from the facts and may also be -expressed as follows: _by a selection of the kind referred to the germ -is progressively modified in a manner corresponding with the production -of a definitely directed progressive variation of the part_.” - -So far there is nothing essentially new offered, since Darwin often -tacitly recognized that the standard of variation could be raised in -this way, and in some places he has made definite statements that this -will take place. Weismann thinks that after each selection, fluctuation -will then occur around a higher average (mode). He says “that this is a -fact,” and is proved by the case of the Japanese cock. It need scarcely -be pointed out that it is an assumption, based on what is supposed to -have taken place in this bird, and is not a “fact.” - -Weismann continues: “But the question remains, _why_ is this the fact?” -He believes his hypothesis of the existence of determinants in the germ -gives a satisfactory answer to this “why.” “According to this theory -every independent and hereditarily variable part is represented in the -germ by a _determinant_, whose size and power of assimilation -corresponds to the size and vigor of the part. These determinants -multiply as do all vital units by growth and division, and necessarily -they increase rapidly in every individual, and the more rapidly the -greater the quantity of the germinal cells the individual produces. And -since there is no more reason for excluding irregularities of passive -nutrition, and of the supply of nutriment in these minute, -microscopically invisible parts, than there is in the larger visible -parts of the cells, tissues, and organs, consequently the descendants of -a determinant can never all be exactly alike in size and capacity of -assimilation, but they will oscillate in this respect to and fro about -the maternal determinant as about their zero point, and will be partly -greater, partly smaller, and partly of the same size as that. In these -oscillations, now, the material for further selection is presented, and -in the inevitable fluctuations of the nutrient supply, I see the reason -why every step attained immediately becomes the zero point of new -fluctuations, and consequently why the size of a part can be augmented -or diminished by selection without limit, solely by the displacement of -the zero point of variation as the result of selection.” - -The best illustration of this process of germinal selection is found, -Weismann believes, in the case of the degeneration of organs. “For in -most retrogressive processes _active_ selection in Darwin’s sense plays -no part, and advocates of the Lamarckian principle, as above remarked, -have rightly denied that active selection, that is, the selection of -individuals possessing the useless organ in its most reduced state, is -sufficient to explain the process of degeneration. I, for my part, have -never assumed this, and have on this very account enunciated the -_principle of panmixia_. Now, although this, as I have still no reason -for doubting, is a perfectly correct principle, which really does have -an essential and indispensable share in the process of retrogression, -still it is not _alone_ sufficient for a full explanation of the -phenomena. My opponents, in advancing this objection, were right, to the -extent indicated, and as I expressly acknowledge, although they were -unable to substitute anything positive in its stead or to render my -explanation complete. The very fact of the cessation of control over the -organ is sufficient to explain its _degeneration_, that is, its -deterioration, the disharmony of its parts, but not the fact which -actually and always occurs where an organ has become useless—viz., _its -gradual and unceasing diminution continuing for thousands and thousands -of years and culminating in its final and absolute effacement_.” - -If then neither selection of persons nor the cessation of personal -selection can explain the phenomenon, we must look elsewhere for the -answer. This Weismann finds in the application of Roux’s hypothesis of -the struggle of the parts to obtain nourishment. - -“The production of the long tail-feathers of the Japanese cock does not -repose solely on the displacement directly effected by personal -selection, of the zero point of variation upward, but that _it is also -fostered and strengthened by germinal selection_. Were that not so, the -phenomena of the transmutation of species, in so far as fresh growth and -the enlargement and complication of organs already present are -concerned, _would not be a whit more intelligible than they were -before_.” - -Thus Weismann has piled up one hypothesis on another as though he could -save the integrity of the theory of natural selection by adding new -speculative matter to it. The most unfortunate feature is that the new -speculation is skilfully removed from the field of verification, and -invisible germs whose sole functions are those which Weismann’s -imagination bestows on them, are brought forward as though they could -supply the deficiencies of Darwin’s theory. This is, indeed, the old -method of the philosophizers of nature. An imaginary system has been -invented which attempts to explain all difficulties, and if it fails, -then new inventions are to be thought of. Thus we see where the theory -of the selection of fluctuating germs has led one of the most widely -known disciples of the Darwinian theory. - -The worst feature of the situation is not so much that Weismann has -advanced new hypotheses unsupported by experimental evidence, but that -the speculation is of such a kind that it is, from its very nature, -unverifiable, and therefore useless. Weismann is mistaken when he -assumes that many zoologists object to his methods because they are -largely speculative. The real reason is that the speculation is so often -of a kind that cannot be tested by observation or by experiment. - - ------------------------------------------------------------------------- - - - - - CHAPTER VI - - DARWIN’S THEORY OF SEXUAL SELECTION - - Sexual Selection - - -The theory of sexual selection was formulated by Darwin, even in the -first edition of the “Origin of Species,” but was developed at much -greater length in “The Descent of Man.” “This form of selection depends, -not on a struggle for existence in relation to other organic beings or -to external conditions, but on a struggle between the individuals of one -sex, generally the males, for the possession of the other sex. The -result is not death to the unsuccessful competitor, but few or no -offspring. Sexual selection is, therefore, less rigorous than natural -selection. Generally the most vigorous males, those which are best -fitted for their place in nature, will leave most progeny. But in many -cases victory depends, not so much on general vigor, as on having -special weapons, confined to the male sex. A hornless stag or spurless -cock would have a poor chance of leaving numerous offspring. Sexual -selection, by always allowing the victor to breed, might surely give -indomitable courage, length to the spur, and strength to the wing to -strike in the spurred leg in nearly the same manner as the brutal -cock-fighter by the careful selection of his best cocks.” It is -important to notice that the theory of sexual selection is admittedly an -extension of the selection principle into a new field. Having accounted -for domesticated animals and plants by artificial selection, and for the -adaptations of wild species by natural selection, there remained only to -account for the secondary sexual differences between the sexes by the -principle of sexual selection. - -There are two ways in which Darwin supposes sexual selection to act: (1) -through competition of the individuals of the same sex with each -other,—the strongest or best-equipped for fighting or for finding the -individuals of the other sex gaining an advantage; (2) through selection -by the individuals of one sex of certain preferred individuals of the -other sex. - -The first category is natural selection applied to the members of one -sex in competition with each other, although the result does not lead to -the death of the unsuccessful individual, but excludes it from leaving -progeny. In the second category a new element is introduced, namely, the -_selective power_ of the individuals of one sex, usually the female. It -is this part that adds a distinctly new element to Darwin’s other two -theories of selection, and it is this part that we naturally think of as -the theory of sexual selection _par excellence_. Darwin makes, however, -no sharp distinction between these two sides of his theory, but includes -both under the heading of sexual selection. - -In order to get the theory itself before us in as concrete form as -possible, let us examine some of the cases that Darwin has given to show -how he supposes the process to be carried out. - -“There are many other structures and instincts which must have been -developed through sexual selection—such as the weapons of offence and -the means of defence of the males for fighting with and driving away -their rivals—their courage and pugnacity—their various ornaments—their -contrivances for producing vocal or instrumental music—and their glands -for emitting odors, most of these latter structures serving only to -allure or excite the female. It is clear that these characters are the -result of sexual and not of ordinary selection, since unarmed, -unornamented, or unattractive males would succeed equally well in the -battle for life and in leaving a numerous progeny, but for the presence -of better-endowed males. We may infer that this would be the case, -because the females, which are unarmed and unornamented, are able to -survive and procreate their kind. Secondary sexual characters of the -kind just referred to will be fully discussed in the following chapters, -as being in many respects interesting, but especially as depending on -the will, choice, and rivalry of the individuals of either sex. When we -behold two males fighting for the possession of the female, or several -male birds displaying their gorgeous plumage, and performing strange -antics before an assembled body of females, we cannot doubt that, though -led by instinct, they know what they are about, and consciously exert -their mental and bodily powers.” - -This general statement gives an idea of the class of phenomena that -Darwin proposes to explain by the theory of sexual selection. The close -resemblance between this process and that of artificial selection may be -gathered from the following statement:— - -“Just as man can improve the breed of his game-cocks by the selection of -those birds which are victorious in the cockpit, so it appears that the -strongest and most vigorous males, or those provided with the best -weapons, have prevailed under nature, and have led to the improvement of -the natural breed or species. A slight degree of variability leading to -some advantage, however slight, in reiterated deadly contests would -suffice for the work of sexual selection; and it is certain that -secondary sexual characters are eminently variable. Just as man can give -beauty, according to his standard of taste, to his male poultry, or more -strictly can modify the beauty originally acquired by the parent -species, can give to the Sebright bantam a new and elegant plumage, an -erect and peculiar carriage—so it appears that female birds in a state -of nature have, by a long selection of the more attractive males, added -to their beauty or other attractive qualities. No doubt this implies -powers of discrimination and taste on the part of the female which will -at first appear extremely improbable; but by the facts to be adduced -hereafter, I hope to be able to show that the females actually have -these powers. When, however, it is said that the lower animals have a -sense of beauty, it must not be supposed that such sense is comparable -with that of a cultivated man, with his multiform and complex associated -ideas. A more just comparison would be between the taste for the -beautiful in animals, and that in the lowest savages, who admire and -deck themselves with any brilliant, glittering, or curious object.” - -Darwin did not close his eyes to the difficulties which the theory had -to contend against. One of the most formidable of these objections is -described in the following words: “Our difficulty in regard to sexual -selection lies in understanding how it is that the males which conquer -other males, or those which prove the most attractive to the females, -leave a greater number of offspring to inherit their superiority than -their beaten and less attractive rivals. Unless this result does follow, -the characters which give to certain males an advantage over others -could not be perfected and augmented through sexual selection. When the -sexes exist in exactly equal numbers, the worst-endowed males will -(except where polygamy prevails) ultimately find females, and leave as -many offspring, as well fitted for their general habits of life, as the -best-endowed males. From various facts and considerations, I formerly -inferred that with most animals, in which secondary sexual characters -are well developed, the males considerably exceeded the females in -number; but this is not by any means always true. If the males were to -the females as two to one, or as three to two, or even in a somewhat -lower ratio, the whole affair would be simple; for the better-armed or -more attractive males would leave the largest number of offspring. But -after investigating, as far as possible, the numerical proportion of the -sexes, I do not believe that any great inequality in number commonly -exists. In most cases sexual selection appears to have been effective in -the following manner. - -“Let us take any species, a bird for instance, and divide the females -inhabiting a district into two equal bodies, the one consisting of the -more vigorous and better-nourished individuals, and the other of the -less vigorous and healthy. The former, there can be little doubt, would -be ready to breed in the spring before the others; and this is the -opinion of Mr. Jenner Weir, who has carefully attended to the habits of -birds during many years. There can also be no doubt that the most -vigorous, best-nourished and earliest breeders would on an average -succeed in rearing the largest number of fine offspring. The males, as -we have seen, are generally ready to breed before the females; the -strongest, and with some species the best-armed of the males, drive away -the weaker; and the former would then unite with the more vigorous and -better-nourished females, because they are the first to breed. Such -vigorous pairs would surely rear a larger number of offspring than the -retarded females, which would be compelled to unite with the conquered -and less powerful males, supposing the sexes to be numerically equal; -and this is all that is wanted to add, in the course of successive -generations, to the size, strength and courage of the males, or to -improve their weapons.” - -I shall comment later on the points here raised, but we should not let -this opportunity pass without noticing, that even if the pairing were to -follow according to the method here imagined, still the argument breaks -down at the critical point, for there is no evidence that the more -precocious females would rear a larger number of offspring than the more -normal females, or even those that breed somewhat later. - -The greater eagerness of the males which has been observed in so many -different classes of animals is accounted for as follows:— - -“But it is difficult to understand why the males of species, of which -the progenitors were primordially free, should invariably have acquired -the habit of approaching the females, instead of being approached by -them. But in all cases, in order that the males should seek efficiently, -it would be necessary that they should be endowed with strong passions; -and the acquirement of such passions would naturally follow from the -more eager leaving a larger number of offspring than the less eager.” - -Thus we are led to the rather complex conclusion, that the more eager -males will leave more descendants, and those that are better endowed -with ornaments will be the ones selected. But unless it can be shown -that there is some connection between greater eagerness and better -ornamentation, it might often occur that the less ornamented were the -more eager individuals, in which case there would be an apparent -conflict between the two acquirements. - -After giving some cases of the greater variability of the males, in -respect to characters that are not connected with sexual selection, and -presumably not the result of any kind of selection, Darwin concludes: -“Through the action of sexual and natural selection male animals have -been rendered in very many instances widely different from their -females; but independently of selection the two sexes, from differing -constitutionally, tend to vary in a somewhat different manner. The -female has to expend much organic matter in the formation of her ova, -whereas the male expends much force in fierce contests with his rivals, -in wandering about in search of the female, in exerting his voice, -pouring out odoriferous secretions, etc.: and this expenditure is -generally concentrated within a short period. The great vigor of the -male during the season of love seems often to intensify his colors, -independently of any marked difference from the female. In mankind, and -even as low down in the organic scale as in the Lepidoptera, the -temperature of the body is higher in the male than in the female, -accompanied in the case of man by a slower pulse. On the whole, the -expenditure of matter and force by the two sexes is probably nearly -equal, though effected in very different ways and at different rates.” - -Again: “From the causes just specified, the two sexes can hardly fail to -differ somewhat in constitution, at least during the breeding season; -and although they may be subjected to exactly the same conditions, they -will tend to vary in a different manner. If such variations are of no -service to either sex, they will not be accumulated and increased by -sexual or natural selection. Nevertheless, they may become permanent if -the exciting cause acts permanently; and in accordance with a frequent -form of inheritance they may be transmitted to that sex alone in which -they first appeared. In this case, the two sexes will come to present -permanent, yet unimportant, differences of character. For instance, Mr. -Allen shows that with a large number of birds inhabiting the northern -and southern United States, the specimens from the south are -darker-colored than those from the north; and this seems to be the -direct result of the difference in temperature, light, etc., between the -two regions. Now, in some few cases, the two sexes of the same species -appear to have been differently affected; in the _Agelæus phœniceus_ the -males have had their colors greatly intensified in the south; whereas -with _Cardinalis virginianus_ it is the females which have been thus -affected: with _Quiscalus major_ the females have been rendered -extremely variable in tint, whilst the males remain nearly uniform.” - -The admissions contained in this statement would seem to jeopardize the -entire question, for, if it is admitted that, on account of the -difference in the constitution of the two sexes, the influence of the -surrounding conditions would produce a different effect on them, it -would seem that there is no need whatsoever for the theory of sexual -selection. What Darwin is probably attempting to show is that the -material for the further action of sexual selection is already given; -but the question may well be asked, if the external conditions have done -so much, why may they not have gone farther and produced the entire -result? - -Darwin makes the following suggestion to account for those cases in -which the female is the more highly colored:— - -“A few exceptional cases occur in various classes of animals, in which -the females instead of the males have acquired well-pronounced secondary -sexual characters, such as brighter colors, greater size, strength, or -pugnacity. With birds there has sometimes been a complete transposition -of the ordinary characters proper to each sex; the females having become -the more eager in courtship, the males remaining comparatively passive, -but apparently selecting the more attractive females, as we may infer -from the results. Certain hen birds have thus been rendered more highly -colored or otherwise ornamented, as well as more powerful and pugnacious -than the cocks; these characters being transmitted to the female -offspring alone.” - -Then follows immediately the discussion as to whether a double process -of sexual selection may not be supposed to go on at the same time. “It -may be suggested that in some cases a double process of selection has -been carried on; that the males have selected the more attractive -females, and the latter the more attractive males. This process, -however, though it might lead to the modification of both sexes, would -not make the one sex different from the other, unless indeed their -tastes for the beautiful differed; but this is a supposition too -improbable to be worth considering in the case of any animal, excepting -man. There are, however, many animals in which the sexes resemble each -other, both being furnished with the same ornaments, which analogy would -lead us to attribute to the agency of sexual selection. In such cases it -may be suggested with more plausibility, that there has been a double or -mutual process of sexual selection; the more vigorous and precocious -females selecting the more attractive and vigorous males, the latter -rejecting all except the more attractive females. But from what we know -of the habits of animals, this view is hardly probable, for the male is -generally eager to pair with any female. It is more probable that the -ornaments common to both sexes were acquired by one sex, generally the -male, and then transmitted to the offspring of both sexes. If, indeed, -during a lengthened period the males of any species were greatly to -exceed the females in number, and then during another lengthened period, -but under different conditions, the reverse were to occur, a double but -not simultaneous process of sexual selection might easily be carried on, -by which the two sexes might be rendered widely different.” - -The improbability of such a process is so manifest that the suggestion -can scarcely be looked upon as anything more than pure speculation. We -shall have occasion later to return to the same subject, and point out -its bearing more explicitly. - -Nearly the whole animal kingdom is passed in review by Darwin from the -point of view of the sexual selection theory. There is brought together -a large number of extremely interesting facts, and if the theory did no -more than merely hold them together, it has served, in this respect, a -useful end. We may select some of the most instructive cases by way of -illustrating the theory. - -In many of the lower animals in which the sexes are separated, and these -alone, of course, can be supposed to come within the range of the -theory, there are no striking differences between the sexes, in regard -to ornamentation, although in other respects differences may exist. - -“Moreover it is almost certain that these animals have too imperfect -senses and much too low mental powers, to appreciate each other’s beauty -or other attractions, or to feel rivalry. - -“Hence in these classes or subkingdoms, such as the Protozoa, -Cœlenterata, Echinodermata, Scolecida, secondary sexual characters, of -the kind which we have to consider, do not occur; and this fact agrees -with the belief that such characters in the higher classes have been -acquired through sexual selection, which depends on the will, desire, -and choice of either sex.” - -There are some cases, however, in which animals low in the scale show a -difference in the ornamentation of the two sexes. A few cases have been -recorded in the roundworms, where different shades of the same tint -distinguish the sexes. In the annelids the sexes are sometimes so -different, that, as Darwin remarks, they have been placed in different -genera and even families, “yet the differences do not seem to be of the -kind which can be safely attributed to sexual selection.” In regard to -the nemertian worms, although they “vie in variety and beauty of -coloring with any other group in the invertebrate series,” yet McIntosh -states that he “cannot discover that these colors are of any service.” -In the copepods, belonging to the group of lower Crustacea, Darwin -excludes those cases in which the males alone “are furnished with -perfect swimming legs, antennæ, and sense-organs; the females being -destitute of these organs, with their bodies often consisting of a mere -distorted mass,” because these extraordinary differences between the two -sexes are no doubt related to their widely different habits of life. -Nevertheless, it is important to observe that such extreme differences -may exist in cases where sexual selection cannot come in, because of the -absence of eyes in the female. - -In regard to another copepod, Saphirina, Darwin points out that the -males are furnished with minute scales, which exhibit beautiful changing -colors, and these are absent in the females; yet he states that it would -be extremely rash to conclude that these curious organs serve to attract -the females. Differences in the sexes are also found in one species of -Squilla, and a species of Gelasimus. In the latter case Darwin thinks -that the difference is probably due to sexual selection. In addition to -these cases, recorded by Darwin, there may be added the two remarkable -cases, shown in our Figure 2 A, B, of _Calocalanus pavo_, the female of -which has a gorgeous tail worthy of a peacock, and of _Calocalanus -plumulosus_, in which one of the setæ of the tail is drawn out into a -long featherlike structure. In the former, the male is much more -modestly adorned, as shown in Figure 2 C; in the latter species the male -is unknown. - - -[Illustration: - - Fig. 2.—A male of the copepod, _Calocalanus plumulosus_. - B and C, a male and a female of _Calocalanus pavo_. (After - Giesbrecht.)] - - -In spiders, where as a rule the sexes do not differ much from each other -in color, the males are often of a darker shade than the females. “In -some species, however, the difference is conspicuous; thus the female of -_Sparassus smaragdulus_ is dullish green, whilst the adult male has the -abdomen of a fine yellow with three longitudinal stripes of rich red.” -Darwin believes that sexual selection must take place in this group, -because Canestrini has observed that the males fight for the possession -of the females. He has also stated that the males pay court to the -female, and that she rejects some of the males who court her, and -sometimes devours them, until finally one is chosen. Darwin believed, on -this evidence, that the difference in color of the sexes had been -acquired by sexual selection, “though we have here not the best kind of -evidence—the display by the male of his ornaments.” This evidence has, -however, now been supplied through the interesting observations of Mr. -and Mrs. Peckham. These accurate observers have studied the courtship of -the male, and observed that during the process, he twists and turns his -body in such a way as to show to best advantage his colors to the -female. From their account this certainly appears to be the result of -his movements, but whether this is really the case, and whether the -female makes any choice amongst her suitors, according to whether they -are more or less brilliantly marked, we are absolutely ignorant. The -following account given by Darwin should not pass unnoticed:— - -“The male is generally much smaller than the female, sometimes to an -extraordinary degree, and he is forced to be extremely cautious in -making his advances, as the female often carries her coyness to a -dangerous pitch. De Geer saw a male that ‘in the midst of his -preparatory caresses was seized by the object of his attentions, -enveloped by her in a web and then devoured, a sight which, as he adds, -filled him with horror and indignation.’ The Rev. O. P. Cambridge -accounts in the following manner for the extreme smallness of the male -in the genus Nephila. ‘M. Vinson gives a graphic account of the agile -way in which the diminutive male escapes from the ferocity of the -female, by gliding about and playing hide and seek over her body and -along her gigantic limbs: in such a pursuit it is evident that the -chances of escape would be in favor of the smallest males, while the -larger ones would fall early victims; thus gradually a diminutive race -of males would be selected, until at last they would dwindle to the -smallest possible size compatible with the exercise of their generative -functions,—in fact probably to the size we now see them, _i.e._ so small -as to be a sort of parasite upon the female, and either beneath her -notice, or too agile and too small for her to catch without great -difficulty.’” - -It is certainly surprising to find Darwin ascribing even this difference -in size between the sexes to the action of selection. Is it not a little -ludicrous to suppose that the females have reduced the males to a size -too small for them to catch? - -There are many cases known in the animal kingdom where there is a -difference in size between the two sexes, especially in the group of -insects; but I doubt very much if they are to be accounted for as the -result of sexual selection. In some of these cases Darwin accounts for -the larger size of the female, on account of the large number of eggs -which she has to carry. In other insects where the male is larger, as in -the stag-beetle, the size is ascribed to the conflicts of the males, -leading to the survival of the larger individuals. In still other cases, -where the males are larger, but do not fight, an explanation is -admittedly wanting; but it is suggested that here there would be no -necessity for the males to be smaller than the females in order to -mature before them (as is supposed to happen in other species), for in -these cases the individuals are not short-lived, and there would be -ample time for pairing. Again, although the males of nearly all bees are -smaller than the females, yet the reverse is true in those forms in -which the females are fertilized during the marriage flight. The -explanation offered is that in these forms the male carries the female, -and this is assumed to require greater size on his part. This loose way -of guessing, as to a possible explanation, is characteristic of the -whole hypothesis of sexual selection. First one, and then another, guess -is made as to the causes of the differences between the sexes. It is not -shown in a single one of the instances that the postulated cause has -really had anything to do with the differences in question; and the -attempt to show that the theory is probable, by pointing out the large -number of cases which it appears to account for, is weakened to a very -great degree by the number of exceptional cases, for which an equally -ready explanation of a different kind is forthcoming. This way of giving -loose rein to the imagination has been the bane of the method that has -followed hard on the track of Darwin’s hypothesis, and for which his -example has been in no small measure responsible. Thus, in the case just -quoted, there are no less than four distinct conjectures made to account -for the differences in size between the sexes, and each guess involves -an entirely different set of processes. Considering the complicated -relation of the life of organisms, it may be doubted if any of the -imagined processes could bring about the result, and certainly not a -single one has been shown to be a real, or a sufficient, cause in the -evolutionary process. Neither the actuality of the postulated causes, -nor their application to a particular case, has been shown to exist. - -In the Diptera, or flies, Wallace records one interesting case of sexual -difference in the genus Elaphomyia of New Guinea, in which the males are -furnished with horns, which the females lack. Darwin writes:— - -“The horns spring from beneath the eyes, and curiously resemble those of -a stag, being either branched or palmated. In one of the species, they -equal the whole body in length. They might be thought to be adapted for -fighting, but as in one species they are of a beautiful pink color, -edged with black, with a pale central stripe, and as these insects have -altogether a very elegant appearance, it is perhaps more probable that -they serve as ornaments.” - -Presumably, therefore, Darwin means these colored horns have been -acquired by sexual selection. - -In the Hemiptera, or bugs, both sexes of some species are “beautifully -colored,” and as the members of the group are often unpalatable to other -animals, the color in this case is supposed to act as a warning signal. - -In other cases it is stated, however, that “a small pink and green -species” could hardly be distinguished from the buds on the trunks of -the lime trees which this insect frequents. In this case the color -appears “to be directly protective.” Thus without any means of forming a -correct judgment, the color of one animal is supposed to be the result -of natural selection, since it resembles its surroundings, but of sexual -selection if the color is present or more pronounced in one sex. Where -neither view can easily be applied, the color is ascribed in a general -way to the nature of the organism. - -In respect to the group of Hymenoptera, or bees, Darwin records the -following cases:— - -“In this order slight differences in color, according to sex, are -common, but conspicuous differences are rare except in the family of -bees; yet both sexes of certain groups are so brilliantly colored—for -instance in Chrysis, in which vermilion and metallic greens prevail—that -we are tempted to attribute the result to sexual selection. In the -Ichneumonidæ, according to Mr. Walsh, the males are almost universally -lighter-colored than the females. On the other hand, in the -Tenthredinidæ the males are generally darker than the females. In the -Siricidæ the sexes frequently differ; thus the male of _Sirex juvencus_ -is banded with orange, whilst the female is dark purple; but it is -difficult to say which sex is the more ornamented.” - -In other families of bees, differences in the color of the sexes have -been recorded, and since the males have been seen fighting for the -possession(?) of the females, and since bees are known to recognize -differences in color, Darwin believes that:— - -“In some species the more beautiful males appear to have been selected -by the females; and in others the more beautiful females by the males. -Consequently in certain genera, the males of the several species differ -much in appearance, whilst the females are almost indistinguishable; in -other genera the reverse occurs. H. Müller believes that the colors -gained by one sex through sexual selection have often been transferred -in a variable degree to the other sex, just as the pollen-collecting -apparatus of the female has often been transferred to the male, to whom -it is absolutely useless.” - -Although in beetles the sexes are generally colored alike, yet in some -of the longicorns there are exceptions to the rule. “Most of these -insects are large and splendidly colored. The males in the genus -Pyrodes, which I saw in Mr. Bates’s collection, are generally redder but -rather duller than the females, the latter being colored of a more or -less splendid golden-green. On the other hand, in one species the male -is golden-green, the female being richly tinted with red and purple. In -the genus Esmeralda the sexes differ so greatly in color that they have -been ranked as distinct species; in one species both are of a beautiful -shining green, but the male has a red thorax. On the whole, as far as I -could judge, the females of those Prionidæ, in which the sexes differ, -are colored more richly than the males, and this does not accord with -the common rule in regard to color, when acquired through sexual -selection.” - -The great horns that rise from the heads of many male beetles are very -striking cases of sexual difference, and Darwin compares them to the -horns of stags and of the rhinoceros. They “are wonderful from their -size and shapes.” Darwin offers the following conjecture as to their -meaning: “The extraordinary size of the horns, and their widely -different structure in closely allied forms, indicate that they have -been formed for some purpose; but their excessive variability in the -males of the same species leads to the inference that this purpose -cannot be of a definite nature. The horns do not show marks of friction, -as if used for any ordinary work. Some authors suppose that as the males -wander about much more than the females, they require horns as a defence -against their enemies; but as the horns are often blunt, they do not -seem well adapted for defence. The most obvious conjecture is that they -are used by the males for fighting together; but the males have never -been observed to fight; nor could Mr. Bates, after a careful examination -of numerous species, find any sufficient evidence, in their mutilated or -broken condition, of their having been thus used. If the males had been -habitual fighters, the size of their bodies would probably have been -increased through sexual selection, so as to have exceeded that of the -females; but Mr. Bates, after comparing the two sexes in above a hundred -species of the Copridæ, did not find any marked difference in this -respect amongst well-developed individuals. In Lethrus, moreover, a -beetle belonging to the same great division of the lamellicorns, the -males are known to fight, but are not provided with horns, though their -mandibles are much larger than those of the female.” - -“The conclusion that the horns have been acquired as ornaments is that -which best agrees with the fact of their having been so immensely, yet -not fixedly, developed,—as shown by their extreme variability in the -same species, and by their extreme diversity in closely allied species. -This view will at first appear extremely improbable; but we shall -hereafter find with many animals standing much higher in the scale, -namely fishes, amphibians, reptiles and birds, that various kinds of -crests, knobs, horns and combs have been developed apparently for this -sole purpose.” - -It is asking a great deal to suppose that animals, so dull and sluggish -as these beetles, are endowed with a sufficient æsthetic discrimination -to select in each generation those males whose horns are a little longer -than the average. The resemblance of the horns to those of stags is, as -Darwin points out, obvious, but in the latter case also it remains to be -proven that they are the result of sexual selection, as Darwin believes -to be the case; but the evidence for this belief is not much better, as -we shall see in the case of the antlers of deer, than it is in these -beetles. - -In regard to butterflies, the males and females are both often equally -brilliantly colored; in other species the differences in the sexes are -very striking. Darwin states:— - -“Even within the same genus we often find species presenting -extraordinary differences between the sexes, whilst others have their -sexes closely alike.” The fine colors of the wings of many moths are -also supposed by Darwin to have arisen through sexual selection, -although the colors are usually on the lower wings, which are covered -during the day by the less ornamented upper wings. It is assumed that, -since the moths often begin to fly at dusk, their colors might at this -time be seen and appreciated by the other sex. It should not be -overlooked, however, that, in the case of some of the most highly -colored moths, it is known that the males find the females through the -sense of smell. Moreover, although moths are often finely colored, -Darwin points out that “it is a singular fact that no British moths -which are brilliantly colored, and, as far as I can discover, hardly any -foreign species, differ much in color according to sex; though this is -the case with many brilliant butterflies.” - -Yet Darwin does not hesitate to conclude: “From the several foregoing -facts it is impossible to admit that the brilliant colors of -butterflies, and of some few moths, have commonly been acquired for the -sake of protection. We have seen that their colors and elegant patterns -are arranged and exhibited as if for display. Hence I am led to believe -that the females prefer or are most excited by the more brilliant males; -for on any other supposition the males would, as far as we can see, be -ornamented to no purpose. We know that ants and certain lamellicorn -beetles are capable of feeling an attachment for each other, and that -ants recognize their fellows after an interval of several months. Hence -there is no abstract improbability in the Lepidoptera, which probably -stand nearly or quite as high in the scale as these insects, having -sufficient mental capacity to admire bright colors. They certainly -discover flowers by color.” - -So far as the evidence of ants having an attachment for each other is -concerned, we may eliminate this part of the argument, since the -evidence on which the statement is based is now regarded as only showing -that ants recognize each other by their sense of smell, which resides in -the antennæ. Hence the so-called fondling means only that the ants are -trying by smell to determine the odor of the other individual. - -Darwin points out a number of cases in which the females are more -brightly colored than the males, and for such cases he reverses the -process of selection, supposing that the males have been discriminating, -and have not “gladly accepted any female.” No explanation is offered to -account for this reversal of instinct, in fact, no evidence to show that -such a reversal really exists. Darwin points out that in most cases the -male insect carries the female during the period of union, while in two -species of butterflies, _Colias edusa_ and _hyale_, the females carry -the males, and the females are here the more highly colored. He suggests -that since in this case “the females take the more active part in the -final marriage ceremony, so we may suppose that they likewise do so in -the wooing; and in this case we can understand how it is that they have -been rendered the more beautiful.” - -A most significant fact in connection with the difference in sexual -coloration of butterflies did not escape Darwin’s attention. - -“Whilst reflecting on the beauty of many butterflies, it occurred to me -that some caterpillars were splendidly colored; and as sexual selection -could not possibly have here acted, it appeared rash to attribute the -beauty of the mature insect to this agency, unless the bright colors of -their larvæ could be somehow explained. In the first place, it may be -observed that the colors of caterpillars do not stand in any close -correlation with those of the mature insect. Secondly, their bright -colors do not serve in any ordinary manner as a protection. Mr. Bates -informs me, as an instance of this, that the most conspicuous -caterpillar which he ever beheld (that of a Sphinx) lived on the large -green leaves of a tree on the open llanos of South America; it was about -four inches in length, transversely banded with black and yellow, and -with its head, legs, and tail of a bright red. Hence it caught the eye -of any one who passed by, even at the distance of many yards, and no -doubt that of every passing bird.” - -Darwin applied to Wallace for a solution of this difficulty, and -received the reply that he “thought it probable that conspicuously -colored caterpillars were protected by having a nauseous taste; but as -their skin is extremely tender, and as their intestines readily protrude -from a wound, a slight peck from the beak of a bird would be as fatal to -them as if they had been devoured. Hence, as Mr. Wallace remarks, -‘distastefulness alone would be insufficient to protect a caterpillar -unless some outward sign indicated to its would-be destroyer that its -prey was a disgusting morsel.’ Under these circumstances it would be -highly advantageous to a caterpillar to be instantaneously and certainly -recognized as unpalatable by all birds and other animals. Thus the most -gaudy colors would be serviceable, and might have been gained by -variation and the survival of the most easily recognized individuals.” - -It need scarcely be pointed out that an occasional peck can scarcely be -supposed to have led to the splendid development of color shown by some -caterpillars, and Darwin confesses that at first sight this hypothesis -appears bold, but nevertheless he believes that it will be found to be -true. He adds, “We cannot, however, at present thus explain the elegant -diversity in the colors of many caterpillars.” - -A most important fact in this connection should not be overlooked, -namely, that in the caterpillar stage the sexual organs are so little -developed that it is generally impossible at this time to distinguish -between the sexes, unless a microscopic examination is made. This gives -us, perhaps, a clew as to the difference between the mature sexual -forms. These differences are connected with difference of sex itself. -This conclusion also fits in well with the fact that during the period -when the sexual organs are at the height of their development the -individuals are most brilliantly colored. The primary cause of the -brilliant color of many animals concerns us here only secondarily, for, -since it is known that many of the lower forms are no less brilliantly -and elaborately colored than are the sexes of the higher forms, it is -not surprising that the sexes themselves sometimes differ in this -respect. - -Organs for producing sounds of different sorts are present in some -insects, and these organs Darwin includes under the head of secondary -sexual organs. In the group of Hemiptera, or bugs, the cicadas are the -most familiar species that produce sounds. The noise is made by the -males; the females are quite mute. - -“With respect to the object of the music, Dr. Hartman, in speaking of -the _Cicada septemdecim_ of the United States, says, ‘the drums are now -(June 6th and 7th, 1851) heard in all directions. This I believe to be -the marital summons from the males. Standing in thick chestnut sprouts -about as high as my head, where hundreds were around me, I observed the -females coming around the drumming males.’ He adds, ‘this season -(August, 1868) a dwarf pear-tree in my garden produced about fifty larvæ -of _C. pruinosa_; and I several times noticed the females to alight near -a male while he was uttering his clanging notes.’ Fritz Müller writes to -me from S. Brazil that he has often listened to a musical contest -between two or three males of a species with a particularly loud voice, -seated at a considerable distance from each other: as soon as one had -finished his song, another immediately began, and then another. As there -is so much rivalry between the males, it is probable that the females -not only find them by their sounds, but that, like female birds, they -are excited or allured by the male with the most attractive voice.” - -In the flies the following cases are given by Darwin:— - -“That the males of some Diptera fight together is certain; for Professor -Westwood has several times seen this with the Tipulæ. The males of other -Diptera apparently try to win the females by their music: H. Müller -watched for some time two males of an Eristalis courting a female; they -hovered above her, and flew from side to side, making a high humming -noise at the same time. Gnats and mosquitoes (Culicidæ) also seem to -attract each other by humming; and Professor Mayer has recently -ascertained that the hairs on the antennæ of the male vibrate in unison -with the notes of a tuning-fork, within the range of the sounds emitted -by the female.” - -In the crickets, grasshoppers, and locusts, the males “are remarkable -for their musical powers”; and it is generally assumed that the sounds -serve to call or to excite the female. In these forms the noise is made -by rubbing the wings over each other or the legs against the -wing-covers. - -In some of these forms both sexes have stridulating organs, and in one -case they differ to a certain extent from each other. “Hence we cannot -suppose that they have been transferred from the male to the female, as -appears to have been the case with the secondary sexual characters of -many other animals. They must have been independently developed in the -two sexes, which no doubt mutually call to each other during the season -of love.” - -Some beetles also possess rasping organs in different parts of the body, -but they cannot produce much noise by this means. - -“We thus see that in the different coleopterous families the -stridulating organs are wonderfully diversified in position, but not -much in structure. Within the same family some species are provided with -these organs, and others are destitute of them. This diversity is -intelligible, if we suppose that originally various beetles made a -shuffling or hissing noise by the rubbing together of any hard and rough -parts of their bodies, which happened to be in contact; and that from -the noise thus produced being in some way useful, the rough surfaces -were gradually developed into regular stridulating organs. Some beetles -as they move, now produce, either intentionally or unintentionally, a -shuffling noise, without possessing any proper organs for the purpose.” - -Darwin says that he expected from analogy to find in this group also -differences in the sexes, but none such were found, although in some -cases the males alone possess certain characters or have them more -highly developed. - -It is important not to forget, when considering all questions connected -with sexual selection, that in order for the result to be successful it -is not only necessary that the female respond to the noises and music of -the other sex, but that she choose the suitor that makes the greatest, -or the most pleasing, noise. If the stridulating organs are only used by -the animals in finding each other, then the case might be considered as -coming under the head of natural selection. If this be granted, then it -may be claimed, and apparently Darwin is inclined to adopt this view, -that those males that make the most noise will be more likely to be -heard, and possibly approached. They will, therefore, be more likely to -leave descendants. We have already considered this question when dealing -with the theory of natural selection in the preceding chapter and need -not go over the ground again. This much may, however, be said again, -that even if it is probable that these organs are of use to the animals -in finding each other, and this seems not improbable, it does not follow -that the organs have been acquired through selection for this purpose. - -Darwin finds his best examples of secondary sexual characters in the -group of vertebrates, and since in this group the intelligence is of a -higher order than in the other groups, the argument that the female -chooses the more pleasing suitor is made to appear more plausible. - -The elongation of the lower jaw that occurs in a few fishes at the -breeding season is regarded as a secondary sexual character. On the -other hand, Darwin recognizes the following difficulty in regard to the -size of the males:— - -“In regard to size, M. Carbonnier maintains that the female of almost -all fishes is larger than the male; and Dr. Günther does not know of a -single instance in which the male is actually larger than the female. -With some cyprinodonts the male is not even half as large. As in many -kinds of fishes the males habitually fight together, it is surprising -that they have not generally become larger and stronger than the females -through the effects of sexual selection. The males suffer from their -small size, for, according to M. Carbonnier, they are liable to be -devoured by the females of their own species when carnivorous, and no -doubt by other species. Increased size must be in some manner of more -importance to the females, than strength and size are to the males for -fighting with other males; and this perhaps is to allow of the -production of a vast number of ova.” - -The last sentence implies that this particular case is to be explained -by the females becoming larger on account of the number of eggs that -they are to produce. But why was not the same explanation offered in the -case of the spiders? It is this uncertain way of applying any -explanation that suggests itself, that puts the whole method in an -unfortunate light. - -In many species of fish the males are brighter in color than the -females. In the case of _Callionymus lyra_, Darwin states:— - -“When fresh caught from the sea the body is yellow of various shades, -striped and spotted with vivid blue on the head; the dorsal fins are -pale brown with dark longitudinal bands, the ventral, caudal, and anal -fins being bluish black. The female, or sordid dragonet, was considered -by Linnæus, and by many subsequent naturalists, as a distinct species; -it is of a dingy reddish brown, with the dorsal fin brown and the other -fins white. The sexes differ also in the proportional size of the head -and mouth, and in the position of the eyes; but the most striking -difference is the extraordinary elongation in the male of the dorsal -fin. Mr. W. Saville Kent remarks that this ‘singular appendage appears -from my observations of the species in confinement, to be subservient to -the same end as the wattles, crests, and other abnormal adjuncts of the -male in gallinaceous birds, for the purpose of fascinating their -mates.’” - -In the case of another fish, _Cottus scorpius_, there is also a great -difference between the sexes, and here the males become very brilliant -only at the breeding season. In other fishes, in which the sexes are -colored alike, the males may become more brilliant during the breeding -season. This, too, is explained by Darwin on the assumption that those -males that have varied at the breeding season, so as to become more -brightly colored, have been chosen in preference to the other males. - -A few cases are cited in which it has been observed that the males -appear to exhibit themselves before the females, as in the following -case of the Chinese Macropus:— - -“The males are most beautifully colored, more so than the females. -During the breeding season they contend for the possession of the -females; and, in the act of courtship, expand their fins, which are -spotted and ornamented with brightly colored rays, in the same manner, -according to M. Carbonnier, as the peacock. They then also bound about -the females with much vivacity, and appear by ‘l’étalage de leurs vives -couleurs chercher à attirer l’attention des femelles, lesquelles ne -paraissaient indifférentes à ce manége, elles nageaient avec une molle -lenteur vers les mâles et semblaient se complaire dans leur voisinage.’” - -In this connection Darwin makes the following general statement:— - -“The males sedulously court the females, and in one case, as we have -seen, take pains in displaying their beauty before them. Can it be -believed that they would thus act to no purpose during their courtship? -And this would be the case, unless the females exert some choice and -select those males which please or excite them most. If the female -exerts such choice, all the above facts on the ornamentation of the -males become at once intelligible by the aid of sexual selection.” - -While it may readily be granted that display of the male may have for -its purpose the excitement of the female, it is another question as to -whether she will be more excited by the more beautiful suitor. The -attentions of the male may be supposed to have a purpose, even if the -female does not choose the more beautiful of her suitors. It is this -last proposition, so necessary for the theory of sexual selection, that -seems improbable. But even if it were probable, there are, as we shall -see, other difficulties to be overcome before we should be justified in -accepting Darwin’s statement quoted above, concerning the results of -sexual selection. - -In regard to those species of fish in which both sexes are equally -ornamented, Darwin returns once more to his hypothesis that the color of -the male, acquired through sexual selection, may be transmitted to the -other sex, and then, as if in doubt on this point, he adds, that it may -be the result of the “nature of the tissues and of the surrounding -conditions.” He even makes the suggestion, somewhat further on, that the -colors may be warning, although it is confessedly unknown that these -fish are distasteful to fish-devouring animals. - -In amphibians the crest on the back of the male triton, which becomes -colored along its edge, is described as a secondary sexual character. -The vocal sacs, present in some species of frogs, are found sometimes in -both sexes, but more highly developed in the males. In other species, as -in the toad, it is the male alone that sings. In the reptiles we find -that the two sexes of the turtles are colored alike, and this holds also -for the crocodiles. Some male turtles make sounds at the breeding -season, and the same is true for the crocodiles, the males of which are -said to make a “prodigous display.” In snakes the males are smaller, as -a rule, than the females, and the colors are more strongly pronounced, -and although some snakes are very brilliantly colored, Darwin puts this -down either to protective coloration, or to mimicry of other kinds of -snakes. But surely the extremely brilliant colors of many snakes cannot -be accounted for in any of these ways. The cause of the color of the -venomous kinds, that are supposed to be imitated by the others, “remains -to be explained and this may perhaps be sexual selection.” - -“It does not, however, follow because snakes have some reasoning power, -strong passions and mutual affection, that they should likewise be -endowed with sufficient taste to admire brilliant colors in their -partners, so as to lead to the adornment of the species through sexual -selection. Nevertheless, it is difficult to account in any other manner -for the extreme beauty of certain species; for instance, of the -coral-snakes of South America, which are of a rich red with black and -yellow transverse bands.” - -In lizards the erectile crests of the male _Anolis_, the brilliant -throat patches of _Sitaria minor_, which is colored blue, black, and -red, the skinny appendages present on the throat of the little lizards -of the genus Draco, which in the beauty of their colors baffle -description, are given as cases of sexual adornment. In the last case -cited the ornaments are present, however, in both sexes. The remarkable -horns in the males of different species of chameleons are imagined to -have been acquired through the battle of the males with each other. - -In the group of birds we find some of the most striking cases of -secondary sexual differences. The spurs, combs, wattles, horns, -air-filled sacs, topknots, feathers with naked shafts, plumes, and -greatly elongated feathers are all secondary sexual characters. The -songs of the males, the rattling together of the quills of the peacock, -the drumming of the grouse, and the booming sounds made by the night -jars while on the wing, are further examples of secondary sexual -differences. The odor of the male of the Australian musk duck is also -put in the same category. - -The pugnacity of many male birds is well known, and it is imagined that -one of the results of the competition of the individuals of the same sex -with each other has led to the development of the organs of defence and -offence. The males that have been successful in these battles are then -supposed to mate with the best females. In this way those secondary -sexual differences, connected with the encounters of the males, are -supposed to have been formed. Darwin states in this connection:— - -“Even with the most pugnacious species it is probable that the pairing -does not depend exclusively on the mere strength and courage of the -male; for such males are generally decorated with various ornaments, -which often become more brilliant during the breeding season, and which -are sedulously displayed before the females. The males also endeavor to -charm or excite their mates by love-notes, songs, and antics; and the -courtship is, in many instances, a prolonged affair. Hence it is not -probable that the females are indifferent to the charms of the opposite -sex, or that they are invariably compelled to yield to the victorious -males.” - -Thus a double process of selection is imagined to take place; one, the -outcome of a competition of the males with each other, and the other, -through a choice of the more successful males by the females, the more -beautiful being supposed to be chosen. - -It may be well not to lose sight of the fact that unless the selection -is severe in each generation, its good effects will be lost, as has been -stated in connection with the theory of natural selection. Still more -important is the consideration that unless the same variations appear at -the same time, in many of the surviving males, the results will be lost -through crossing. These statements will show that the difficulties of -the theory are by no means small, and when we are asked to believe -further that another process still has been superimposed on this one, -namely, the selection of the more beautiful males by the females, we can -appreciate how great are the difficulties that must be overcome in order -that the process may be carried out. - -The love-antics and dances of male birds at the breeding season furnish -many curious data. The phenomena are imagined by Darwin to be connected -with sexual selection, for in the dances the males are supposed to -exhibit their ornaments to the females who are imagined to choose the -suitor that is most to their taste. - -Hudson, who has studied the habits of birds in the field, asks some very -pertinent questions in connection with their performances of different -kinds. “What relation that we can see or imagine to the passion of love -and the business of courtship have these dancing and vocal performances -in nine cases out of ten? In such cases, for instance, as that of the -scissortail tyrant-bird, and its pyrotechnic displays, when a number of -couples leave their nests containing eggs and young to join in a wild -aërial dance; the mad exhibitions of ypecahas and ibises and the -jacana’s beautiful exhibition of grouped wings; the triplet dances of -the spur-winged lapwing, to perform which two birds already mated are -compelled to call in a third bird to complete the set; the harmonious -duets of the oven-birds and the duets and choruses of nearly all the -wood-hewers, and the wing-slapping aërial displays of the whistling -widgeons,—will it be seriously contended that the female of this species -makes choice of the male able to administer the most vigorous and -artistic slaps?” - -“The believer in the theory would put all these cases lightly aside to -cite the case of the male cow-bird practising antics before the female, -and drawing a wide circle of melody around her, etc.... And this was in -substance what Darwin did.” “How unfair the argument is based on these -carefully selected cases gathered from all regions of the globe and -often not properly reported is seen when we turn to the book of nature -and closely consider the habits and actions of all the species -inhabiting any _one_ district.” Hudson concludes that he is convinced -that any one who will note the actions of animals for himself will reach -the conviction, that “conscious sexual selection on the part of the -female is not the cause of music and dancing performances in birds, nor -of the brighter colors and ornaments that distinguish the male.” - -The differences in color in the sexes of birds are classified by Darwin -as follows: (1) when the males are ornamented exclusively or in a much -higher degree than the females; (2) when both sexes are highly -ornamented; (3) when the female is more brightly colored. A few examples -of each sort may be chosen for illustration. - -“In regard to color, hardly anything need here be said, for every one -knows how splendid are the tints of many birds, and how harmoniously -they are combined. The colors are often metallic and iridescent. -Circular spots are sometimes surrounded by one or more differently -shaded zones, and are thus converted into ocelli. Nor need much be said -on the wonderful difference between the sexes of many birds. The common -peacock offers a striking instance. Female birds of paradise are -obscurely colored and destitute of all ornaments, whilst the males are -probably the most highly decorated of all birds, and in so many -different ways, that they must be seen to be appreciated. The elongated -and golden-orange plumes which spring from beneath the wings of the -_Paradisea apoda_, when vertically erected and made to vibrate, are -described as forming a sort of halo, in the centre of which the head -‘looks like a little emerald sun, with its rays formed by the two -plumes.’” - -Male humming-birds are almost as splendidly colored as are the birds of -paradise, some having the feathers modified in a truly extraordinary -way. “Almost every part of their plumage has been taken advantage of, -and modified; and the modifications have been carried, as Mr. Gould -showed me, to a wonderful extreme in some species belonging to nearly -every subgroup. Such cases are curiously like those which we see in our -fancy breeds, reared by man for the sake of ornament: certain -individuals originally varied in one character, and other individuals of -the same species in other characters; and these have been seized on by -man and much augmented—as shown by the tail of the fantail pigeon, the -hood of the jacobin, the beak and wattle of the carrier, and so forth. -The sole difference between these cases is that in the one the result is -due to man’s selection, whilst in the other, as with humming-birds, -birds of paradise, etc., it is due to the selection by the females of -the more beautiful males.” - -A remarkable bird of South America, the bell-bird, has a peculiar note -that “can be distinguished at the distance of nearly three miles and -astonishes every one who hears it.... The male is pure white, whilst the -female is dusky-green; and white is a very rare color in terrestrial -species of moderate size and inoffensive habits. The male, also, as -described by Waterton, has a spiral tube, nearly three inches in length, -which rises from the base of the beak. It is jet-black, dotted over with -minute downy feathers. This tube can be inflated with air, through a -communication with the palate; and when not inflated hangs down on one -side. The genus consists of four species, the males of which are very -distinct, whilst the females, as described by Mr. Sclater in a very -interesting paper, closely resemble each other, thus offering an -excellent instance of the common rule that within the same group the -males differ much more from each other than do the females. In a second -species (_C. nudicollis_) the male is likewise snow-white, with the -exception of a large space of naked skin on the throat and round the -eyes, which during the breeding season is of a fine green color. In a -third species (_C. tricarunculatus_) the head and neck alone of the male -are white, the rest of the body being chestnut-brown, and the male of -this species is provided with three filamentous projections half as long -as the body—one rising from the base of the beak, and the two others -from the corners of the mouth.” - -The most familiar case of sexual difference amongst North American birds -is that of the scarlet tanager, in which the male is scarlet with -jet-black wings, while the female is an inconspicuous yellow-green -color. Amongst domesticated animals the peafowl shows the most beautiful -case of sexual differences. The magnificent tail of the male can be -lifted up, so as to be seen to best advantage when the male faces the -observer. Moreover the wild form, living in the forests of India, has -the same gorgeous train. - -The male Argus pheasant has a remarkable series of spots, or ocelli, on -the secondary wing-covers. They are concealed until the male displays -them before the female. Darwin states that, while it may seem incredible -that such elegant shading and exquisite patterns could have been the -outcome of the taste of the female, yet the extraordinary attitude -assumed by the male during courtship appears entirely purposeless, -unless it be supposed that he is attempting to charm the female by a -display of his ornamentation. - -Let us pass to the second class of cases, in which both sexes are -similarly and brightly colored, and in which the young have a plumage -different from the adults. For example, the male and the female of the -splendid scarlet ibis are alike, whilst the young are brown. The males -and females of many finely colored herons are ornamented alike, and this -plumage, Darwin admits, has a nuptial character. He even tries to -explain this by the curious assumption, that while the color has been -acquired through the selection of the males by the females, the results -attained in this way have been transmitted to both sexes. We find here -another example of the method so often employed by Darwin. When he meets -with facts that are not in conformity with the theory, he proceeds to -make a new assumption without establishing its validity. Thus, to assume -that in all cases where the sexes are colored differently, the -characters acquired by the males have been transmitted only to the same -sex, and in those cases where the sexes are colored alike the -transmission has been to both sexes, is most arbitrary. - -In other cases, which are commoner than the last, the male and female -have the same color, and the young in their first plumage resemble the -adults. Darwin admits that here the facts are so complex that his -conclusions are doubtful. The following account of the tree-sparrow -shows how vague are the principles involved in the entire discussion in -relation to transmission:— - -“Now with the tree-sparrow (_P. montanus_) both sexes and the young -closely resemble the male of the house-sparrow; so that they have all -been modified in the same manner, and all depart from the typical -coloring of their early progenitor. This may have been effected by a -male ancestor of the tree-sparrow having varied, firstly, when nearly -mature; or secondly, whilst quite young, and by having in either case -transmitted his modified plumage to the females and the young; or, -thirdly, he may have varied when adult and transmitted his plumage to -both adult sexes, and, owing to the failure of the law of inheritance at -corresponding ages, at some subsequent period to his young.” - -The further admissions made in the following quotation are also -significant:— - -“The plumage of certain birds goes on increasing in beauty during many -years after they are fully mature; this is the case with the train of -the peacock, with some of the birds of paradise, and with the crest and -plumes of certain herons, for instance, the _Ardea ludovicana_. But it -is doubtful whether the continued development of such feathers is the -result of the selection of successive beneficial variations (though this -is the most probable view with birds of paradise) or merely of -continuous growth. Most fishes continue increasing in size, as long as -they are in good health and have plenty of food; and a somewhat similar -law may prevail with the plumes of birds.” - -We need not follow Darwin through his discussion of those cases in which -the adults have a winter and a summer dress and the young resemble the -one or the other in plumage, or are different from either. The -discussion of these cases, confessedly very complex, adds nothing to our -understanding of the theory, and little but conjecture is offered to -account for the facts. - -The extreme to which even conjecture can be carried may be gathered from -the following quotation, taken from the section dealing with cases in -which the young in their first plumage differ from each other according -to sex, the young males resembling more or less closely the adult males, -and the young females more or less closely the adult females: - -“Two humming-birds belonging to the genus Eustephanus, both beautifully -colored, inhabit the small island of Juan Fernandez, and have always -been ranked as specifically distinct. But it has lately been ascertained -that the one which is of a rich chestnut-brown color with a golden-red -head, is the male, whilst the other, which is elegantly variegated with -green and white with a metallic-green head, is the female. Now the young -from the first somewhat resemble the adults of the corresponding sex, -the resemblance gradually becoming more and more complete. - -“In considering this last case, if as before we take the plumage of the -young as our guide, it would appear that both sexes have been rendered -beautiful independently; and not that one sex has partially transferred -its beauty to the other. The male apparently has acquired his bright -colors through sexual selection in the same manner as, for instance, the -peacock or pheasant in our first class of cases; and the female in the -same manner as the female Rhynchæa or Turnix in our second class of -cases. But there is much difficulty in understanding how this could have -been effected at the same time with the two sexes of the same species. -Mr. Salvin states, as we have seen in the eighth chapter, that with -certain humming-birds the males greatly exceed the females in number, -whilst with other species inhabiting the same country the females -greatly exceed the males. If, then, we might assume that during some -former lengthened period the males of the Juan Fernandez species had -greatly exceeded the females in number, but that during another -lengthened period the females had far exceeded the males, we could -understand how the males at one time, and the females at another, might -have been rendered beautiful by the selection of the brighter-colored -individuals of either sex; both sexes transmitting their characters to -their young at a rather earlier age than usual. Whether this is the true -explanation I will not pretend to say; but the case is too remarkable to -be passed over without notice.” - -The third group of cases include those in which the females are more -brightly colored, or more ornamented, than the males. These cases are -rare, and the differences between the sexes are never so great as when -the male is the more highly colored. Wallace thinks that since in these -cases the male incubates the eggs his less conspicuous colors have been -acquired through natural selection. In the genus Turnix the female is -larger than the male, and lacks the black on the throat and neck, and -the plumage as a whole is lighter than that of the male. The natives -assert that the females after laying their eggs associate in flocks, and -leave the males to do the incubating; and from other evidence Darwin -thinks that this is true. - -In three species of painted snipe the females “are not only larger but -much more richly colored than the males,” and the trachea is more -convoluted in some species. “There is also reason to believe that the -male undertakes the duty of incubation.” In the dotterel plover the -female is larger and somewhat more strongly colored. The males take at -least a share in the incubation. In the common cassowary the female is -larger and the skin of the head more brightly colored than in the male. -The female is pugnacious during the breeding season and the male sits on -the eggs. The female emu is large and has a crest. She is more -courageous and pugilistic and makes a deep, hollow, guttural boom. The -male is more docile and can only hiss or croak. He not only incubates -the eggs, but defends the young against their own mother. “So that with -this emu we have a complete reversal not only of the parental and -incubating instincts, but of the usual moral qualities of the two sexes; -the females being savage, quarrelsome, and noisy, the males gentle and -good. The case is very different with the African ostrich, for the male -is somewhat larger than the female and has finer plumes with more -strongly contrasted colors; nevertheless he undertakes the whole duty of -incubation.” - -Darwin attempts to explain these reversals of instincts on the -assumption that the males have turned the tables on the females, and -have themselves done the selecting; and incidentally, it may be pointed -out in passing, they have had to pay the penalty by incubating the eggs. - -In the group of mammals, Darwin thinks that the male wins the female by -conquering other males rather than by charming her through his display. -The males, even when unarmed, engage in desperate conflicts with each -other, and sometimes kill, but more often only wound, their fellows. The -secondary sexual characters of the males have been acquired, therefore, -by natural selection applied to one sex, and less frequently through the -choice of the female. Since we are here more especially concerned with -the latter class of phenomena, we may examine only a few cases under the -first head. - -The horns of stags are used by them in their conflicts with each other; -the tusks of the elephant make this animal the most dangerous in the -world, when in must. The horns of bulls, the canine teeth of many -mammals, the tusks of the walrus, are further examples of organs which -have been, according to Darwin, acquired through the competitions of the -males with each other. - -The voices of mammals are used for various purposes, “as a signal of -danger, as a call from one member of the troup to another, and from the -mother to her lost offspring, or from the latter for protection.” - -“Almost all male animals use their voices much more during the rutting -season than at any other time; and some, as the giraffe and porcupine, -are said to be completely mute excepting at this season. As the throats -(_i.e._ the larynx and thyroid bodies) of stags periodically become -enlarged at the beginning of the breeding season, it might be thought -that their powerful voices must be somehow of high importance to them; -but this is very doubtful. From information given to me by two -experienced observers, Mr. McNeill and Sir P. Egerton, it seems that -young stags under three years old do not roar or bellow; and that the -old ones begin bellowing at the commencement of the breeding season, at -first only occasionally and moderately, whilst they restlessly wander -about in search of the females. Their battles are prefaced by loud and -prolonged bellowing, but during the actual conflict they are silent. -Animals of all kinds which habitually use their voices utter various -noises under any strong emotion, as when enraged and preparing to fight; -but this may merely be the result of nervous excitement, which leads to -the spasmodic contraction of almost all the muscles of the body, as when -a man grinds his teeth and clenches his fists in rage or agony. No doubt -stags challenge each other to mortal combat by bellowing; but those with -the more powerful voices, unless at the same time the stronger, -better-armed, and more courageous, would not gain any advantage over -their rivals.” - -“Some writers suggest that the bellowing serves as a call to the female; -but the experienced observers above quoted inform me that female deer do -not search for the male, though the males search eagerly for the -females, as indeed might be expected from what we know of the habits of -other male quadrupeds. The voice of the female, on the other hand, -quickly brings to her one or more stags, as is well known to the hunters -who in wild countries imitate her cry. - -“As the case stands, the loud voice of the stag during the breeding -season does not seem to be of any special service to him, either during -his courtship or battles, or in any other way. But may we not believe -that the frequent use of the voice, under the strong excitement of love, -jealousy, and rage, continued during many generations, may at last have -produced an inherited effect on the vocal organs of the stag, as well as -of other male animals? This appears to me, in our present state of -knowledge, the most probable view.” - -Here once more we find that Darwin makes use, as a sort of last resort, -of the principle of the inheritance of acquired characters. As long as -the theory of selection, in any of its forms, appears to offer a -satisfactory solution, we find the facts used in support of this theory, -but as soon as a difficulty arises the Lamarckian theory is brought to -the front. It is this shifting, as we have already more than once -pointed out, that shows how little real basis there is for the theory of -sexual selection. - -The male gorilla has a tremendous voice, and he has, as has also the -orang, a laryngeal sac. One species of gibbon has the power of producing -a correct octave of musical notes. - -“The vocal organs of the American _Mycetes caraya_ are one-third larger -in the male than in the female, and are wonderfully powerful. These -monkeys in warm weather make the forests resound at morning and evening -with their overwhelming voices. The males begin the dreadful concert, -and often continue it during many hours, the females sometimes joining -in with their less powerful voices. An excellent observer, Rengger, -could not perceive that they were excited to begin by any special cause; -he thinks that, like many birds, they delight in their own music, and -try to excel each other. Whether most of the foregoing monkeys have -acquired their powerful voices in order to beat their rivals and charm -the females—or whether the vocal organs have been strengthened and -enlarged through the inherited effects of long-continued use without any -particular good being thus gained—I will not pretend to say; but the -former view, at least in the case of the _Hylobates agilis_, seems the -most probable.” - -The odor of some mammals is confined to, or more developed, in the -males; but in some forms, as in the skunk, it is present in both sexes. -In the shrew mice, abdominal scent glands are present, but since these -mice are rejected by birds of prey, their glands probably serve to -protect them; “nevertheless the glands become enlarged in the males -during the breeding season.” In many other quadrupeds the scent glands -are of the same size in both sexes, and their function is unknown. - -“In other species the glands are confined to the males, or are more -developed than in the females; and they almost always become more active -during the rutting season. At this period the glands on the sides of the -face of the male elephant enlarge, and emit a secretion having a strong -musky odor. The males, and rarely the females, of many kinds of bats -have glands and protrudable sacs situated in various parts; and it is -believed that these are odoriferous. - -“The rank effluvium of the male goat is well known, and that of certain -male deer is wonderfully strong and persistent. Besides the general -odor, permeating the whole body of certain ruminants (for instance, _Bos -moschatus_) in the breeding season, many deer, antelopes, sheep, and -goats, possess odoriferous glands in various situations, more especially -on their faces. The so-called tear-sacs, or suborbital pits, come under -this head. These glands secrete a semifluid fetid matter which is -sometimes so copious as to stain the whole face, as I have myself seen -in an antelope. They are ‘usually larger in the male than in the female, -and their development is checked by castration.’ According to Desmarest -they are altogether absent in the female of _Antilope subgutturosa_. -Hence, there can be no doubt that they stand in close relation with the -reproductive functions. They are also sometimes present, and sometimes -absent, in nearly allied forms. In the adult male musk-deer (_Moschus -moschiferus_), a naked space round the tail is bedewed with an -odoriferous fluid, whilst in the adult female and in the male until two -years old, this space is covered with hair, and is not odoriferous.” -Darwin believes in these cases that the odor serves to attract the -females. He admits that here, “active and long-continued use cannot have -come into play as in the case of the vocal organs.” He concludes, -therefore, that “the odor emitted must be of considerable importance to -the male, inasmuch as large and complex glands, furnished with muscles -for everting the sac, and for closing or opening the orifice, have in -some cases been developed. The development of these organs is -intelligible through sexual selection, if the most odoriferous males are -the most successful in winning the females, and in leaving offspring to -inherit their gradually perfected glands and colors.” - -There is sometimes a difference in the mammals in the hair of the two -sexes both in amount and in color. In some species of goats the males -have a beard, in others it is present in both sexes. The bull, but not -the cow, has curly hair on the forehead. In some monkeys the beard is -confined to the male, as in the orang; in other species it is only -larger in the males. - -“The males of various members of the ox family (Bovidæ), and of certain -antelopes, are furnished with a dewlap, or great fold of skin on the -neck, which is much less developed in the female. - -“Now, what must we conclude with respect to such sexual differences as -these? No one will pretend that the beards of certain male goats, or the -dewlap of the bull, or the crests of hair along the backs of certain -male antelopes, are of any use to them in their ordinary habits. - -“Must we attribute all these appendages of hair or skin to mere -purposeless variability in the male? It cannot be denied that this is -possible; for in many domesticated quadrupeds, certain characters, -apparently not derived through reversion from any wild parent form, are -confined to the males, or are more developed in them than in the -females—for instance, the hump on the male zebu cattle of India, the -tail of fat-tailed rams, the arched outline of the forehead in the males -of several breeds of sheep, and, lastly, the mane, the long hairs on the -hind-legs, and the dewlap of the male of the Berbura goat.” - -In these cases and in others that Darwin cites, which seem clearly to -indicate that some of these secondary sexual characters are not the -result of sexual selection, he concludes, “that they must be due to -simple variability, together with sexually limited inheritance. - -“Hence it appears reasonable to extend this same view to all analogous -cases with animals in a state of nature. Nevertheless I cannot persuade -myself that it generally holds good, as in the case of the extraordinary -development of hair on the throat and fore-legs of the male Ammotragus, -or in that of the immense beard of the male Pithecia. Such study as I -have been able to give to nature makes me believe that parts or organs -which are highly developed, were acquired at some period for a special -purpose. With those antelopes in which the adult male is more strongly -colored than the female, and with those monkeys in which the hair on the -face is elegantly arranged and colored in a diversified manner, it seems -probable that the crests and tufts of hair were gained as ornaments; and -this I know is the opinion of some naturalists. If this be correct, -there can be little doubt that they were gained, or at least modified -through sexual selection; but how far the same view may be extended to -other mammals is doubtful.” - -The astonishing colors in some of the monkeys cannot be passed over -without comment. - -“In the beautiful _Cercopithecus diana_, the head of the adult male is -of an intense black, whilst that of the female is dark gray; in the -former the fur between the thighs is of an elegant fawn-color, in the -latter it is paler. - -“In the _Cercopithecus cynosurus_ and _griseoviridis_ one part of the -body, which is confined to the male sex, is of the most brilliant blue -or green, and contrasts strikingly with the naked skin on the hinder -part of the body, which is vivid red. - -“Lastly, in the baboon family, the adult male of _Cynocephalus -hamadryas_ differs from the female not only by his immense mane, but -slightly in the color of the hair and of the naked callosities. In the -drill (_C. leucophæus_) the females and young are much paler-colored, -with less green, than the adult males. No other member in the whole -class of mammals is colored in so extraordinary a manner as the adult -male mandrill (_C. mormon_). The face at this age becomes of a fine -blue, with the ridge and tip of the nose of the most brilliant red. -According to some authors, the face is also marked with whitish stripes, -and is shaded in parts with black, but the colors appear to be variable. -On the forehead there is a crest of hair, and on the chin a yellow -beard. ‘Toutes les parties supérieures de leurs cuisses et le grand -espace nu de leurs fesses sont également colorés du rouge le plus vif, -avec un mélange de bleu qui ne manque réellement pas d’élégance.’ When -the animal is excited all the naked parts become much more vividly -tinted.” - -Darwin sums up the evidence in regard to the differences in color -between the male and female in the following statement:— - -“I have now given all the cases known to me of a difference in color -between the sexes of mammals. Some of these may be the result of -variations confined to one sex and transmitted to the same sex, without -any good being gained, and therefore without the aid of selection. We -have instances of this with our domesticated animals, as in the males of -certain cats being rusty-red, whilst the females are tortoise-shell -colored. Analogous cases occur in nature: Mr. Bartlett has seen many -black varieties of the jaguar, leopard, vulpine phalanger, and wombat; -and he is certain that all or nearly all these animals, were males. On -the other hand, with wolves, foxes, and apparently American squirrels, -both sexes are occasionally born black. Hence it is quite possible that -with some mammals a difference in color between the sexes, especially -when this is congenital, may simply be the result, without the aid of -selection, of the occurrence of one or more variations, which from the -first were sexually limited in their transmission. Nevertheless it is -improbable that the diversified, vivid, and contrasted colors of certain -quadrupeds, for instance, of the above monkeys and antelopes, can thus -be accounted for.” - -Finally, the case of man must be considered from the point of view of -sexual selection, for Darwin claims that man has acquired a number of -his secondary sexual characters in this way. For instance, the beard is -an excellent case of a secondary sexual character. Darwin’s -interpretation is that the beard has been retained, or even developed, -through the selection by the females of those males that had this -outgrowth best developed. Conversely, the absence of hair on the face of -the female is supposed by Darwin to have been brought about by men -selecting those women having less hair on their faces. The greater -intellect, energy, courage, pugnacity, and size of man are the outcome -of the competition of the males with each other, since the individual -excelling in these qualities will be able to select the most desirable -wife, or wives, and it is assumed will, therefore, leave more -descendants. The standard of beauty has been kept up by men selecting -the most beautiful women in each generation (the fate of the other -married women is ignored), and this beauty is supposed to have been -transmitted primarily to their daughters, but also to their sons. - -Although all these forms of selection are imagined to be acting in man, -either alternately or simultaneously, yet Darwin recognizes in man a -number of checks to the action of sexual selection: amongst savages, the -so-called communal marriages; second, infanticide, generally of the -young females, which appears in some races to be practised to an -astonishing degree; third, early betrothals; fourth, the holding of -women as slaves. - -When we recall that selection to be effective can only be carried out -under very exacting conditions, we cannot but be appalled at the demands -made here on our credulity. The choice of the women has produced the -beard of man, the choice of man the absence of a beard in women; the -competition of the males with each other is leading at the same time to -the development of at least half a dozen qualities that are supposed to -be male specialities, and while all this is going on the results are -being checked sometimes by one means, sometimes by another. Moreover, -even this is not all that we are asked to accept, for there are several -other qualities of the male that are put down as secondary sexual -characters. For example, let us examine what Darwin has to say in regard -to the development of the voice, and of singing in man. - -In man the vocal cords are about a third longer than in woman and his -voice deeper. Emasculation arrests the development of the vocal -apparatus, and the voice remains like that of a woman. This difference -between the sexes, Darwin thinks, is due probably to long-continued use -by the male “under the excitement of love, rage, and jealousy.” In other -words, an appeal is again made to the Lamarckian theory, and in this -case to explain the origin of an organ that conforms to all the -requirements of the secondary sexual characters. - -“The capacity and love for singing, or music, though not a sexual -character in man,” in the sense of being confined to one sex, yet is -supposed to have arisen through sexual selection in the following way: -“Human song is generally admitted to be the basis or origin of -instrumental music. As neither the enjoyment nor the capacity of -producing musical notes are faculties of the least use to man in -reference to his daily habits of life, they must be ranked amongst the -most mysterious with which he is endowed.” - -Man is supposed to have possessed this faculty of song from a very -remote time, and even the most savage races make musical sounds, -although we do not enjoy their music, or they ours. - -“We see that the musical faculties, which are not wholly deficient in -any race, are capable of prompt and high development, for Hottentots and -Negroes have become excellent musicians, although in their native -countries they rarely practise anything that we should consider music. -Hence the capacity for high musical development, which the savage races -of man possess, may be due either to the practice by our semi-human -progenitors of some rude form of music, or simply to their having -acquired the proper vocal organs for a different purpose. But in this -latter case we must assume, as in the above instance of parrots, and as -seems to occur with many animals, that they already possessed some sense -of melody.” - -Darwin sums up the evidence in the two following statements, the -insufficiency of which to explain the phenomena is I think only too -obvious: “All these facts in respect to music and impassioned speech -become intelligible to a certain extent, if we assume that musical tones -and rhythm were used by our half-human ancestors, during the season of -courtship, when animals of all kinds are excited not only by love, but -by the strong passions of jealousy, rivalry, and triumph. From the -deeply laid principle of inherited associations, musical tones in this -case would be likely to call up vaguely and indefinitely the strong -emotions of a long past age.” Thus the difficulty is shifted to the -shoulders of our long-lost savage ancestors; or even, in fact, to our -simian forefathers, as the following paragraph indicates:— - -“As the males of several quadrumanous animals have their vocal organs -much more developed than in the females, and as a gibbon, one of the -anthropomorphous apes, pours forth a whole octave of musical notes and -may be said to sing, it appears probable that the progenitors of man, -either the males or females or both sexes, before acquiring the power of -expressing their mutual love in articulate language, endeavored to charm -each other with musical notes and rhythm. So little is known about the -use of the voice by the Quadrumana during the season of love, that we -have no means of judging whether the habit of singing was first acquired -by our male or female ancestors. Women are generally thought to possess -sweeter voices than men, and as far as this serves as any guide, we may -infer that they first acquired musical powers in order to attract the -other sex. But if so, this must have occurred long ago, before our -ancestors had become sufficiently human to treat and value their women -merely as useful slaves. The impassioned orator, bard, or musician, when -with his varied tones and cadences he excites the strongest emotions in -his hearers, little suspects that he uses the same means by which his -half-human ancestors long ago aroused each other’s ardent passions -during their courtship and rivalry.” - -We have now examined in some detail the evidence that Darwin has brought -forward in support of his hypothesis of sexual selection. A running -comment has been made while considering the individual cases, but it may -be well to sum up the matter by briefly indicating the reasons why the -hypothesis seems incompetent to explain the facts. - - - General Criticism of the Theory of Sexual Selection - -1. Some of the objections that apply to the theory of natural selection -apply also with equal force to the theory of sexual selection in so far -as the results in both cases are supposed to be the outcome of the -selection of individual, or fluctuating, variations. If these variations -appear in only a few individuals, their perpetuation is not possible, -since they will soon disappear through crossing. It would be, of course, -preposterous to suppose that at any one time only those few individuals -pair and leave descendants that have the secondary sexual characters -developed to the highest point, but if something of this sort does not -occur, the extreme of fluctuating variations cannot be maintained. Even -if half of the individuals are selected in each generation, the -accumulation of a variation in a given direction could not go very far. -The assumption, however, that only half of all the individuals that -reach maturity breed, and that all of these are chosen on account of the -special development of their secondary sexual characters, seems -preposterous. Furthermore, if it is assumed that the high development of -the new character appears in a large number of individuals, then it is -not improbable that its continued appearance might be accounted for -without bringing in, at all, the hypothesis of sexual selection. - -2. But even supposing that the females select the most beautiful males, -then, since in the vast majority of higher animals the males and the -females are in equal numbers, the others will also be able to unite with -each other in pairs after this first selection has taken place. Nothing -will therefore be gained in the next generation. It is interesting to -see how Darwin attempts to meet this argument. He tries to show in the -case of birds, that there are always unpaired individuals, but since the -few facts that he has been able to collect show that there are as many -additional females as males, the argument proves too much. A few species -are polygamous, one male having a number of female birds; but on this -basis we can only account, at best, for the development through -competition of the organs of offence and defence used to keep away the -weaker males. Yet it is just amongst these birds that we often find the -ornamental characters well developed. In fact, since all the females in -such cases are selected, and since they will transmit the characters of -all the males, it is evident that the secondary sexual characters could -not be formed in the way imagined. - -3. If the female fails to select only the more ornamental males, no -result will follow. It has not been shown that she is capable of making -such a choice, and in the lower forms particularly, it does not seem -probable that this is done. The argument that Darwin often employs, -namely, that unless she does select, the display of the males before her -is meaningless, is not to the point. So far as we can detect the “cause” -of the display of the male, it appears to be due to his own excitement; -and even if we go so far as to admit that the “purpose” is to attract -the other sex, it still does not in the least follow that the most -ornamental male is selected, and unless this occurs the display has no -bearing on the hypothesis of sexual selection. - -4. The two forms of sexual selection, namely, competition of the males -with one another (really one form of natural selection), and the -selection of the most ornamental or gifted individuals, are both used by -Darwin to explain secondary sexual characters, the one for organs of -offence and defence, and the other for ornamental characters. If we -fully appreciate the difficulties that any theory of selection meets -with, we shall realize how extraordinarily complex the action must be, -when two such processes are carried out at the same time, or even during -alternating periods. - -5. It has been objected to Darwin’s theory of sexual selection, that he -suddenly reverses its mode of action to explain those cases in which the -female is the stronger and more ornamented sex; but if, as Darwin shows, -the instincts of the male have also changed, and have become more like -those of the female, I can see no inherent difficulty in this way of -applying the theory. A much more serious objection, it seems to me, is -that the male is supposed to select the female for one set of -characteristics, and the female to select the male for another set. It -sounds a little strange to suppose that women have caused the beard of -man to develop by selecting the best-bearded individuals, and the -compliment has been returned by the males selecting the females that -have the least amount of beard. It is also assumed that the results of -the selection are transmitted to one sex only. Unless, in fact, the -character in question were from the beginning peculiar to only one sex -as to its inheritance, the two sexes might go on forever selecting at -cross-purposes, and the result would be nothing. - -6. The development, or the presence, of the æsthetic feeling in the -selecting sex is not accounted for on the theory. There is just as much -need to explain why the females are gifted with an appreciation of the -beautiful, as that the beautiful colors develop in the males. Shall we -assume that still another process of selection is going on, as a result -of which those females are selected by the males that appreciate their -unusual beauty, or that those females whose taste has soared a little -higher than that of the average (a variation of this sort having -appeared) select males to correspond, and thus the two continue heaping -up the ornaments on one side and the appreciation of these ornaments on -the other? No doubt an interesting fiction could be built up along these -lines, but would any one believe it, and, if he did, could he prove it? - -Darwin assumes that the appreciation on the part of the female is always -present, and he thus simplifies, in appearance, the problem, but he -leaves half of it unexplained. - -7. It has been pointed out, that it is important to distinguish between -the possible excitement of the female by the display or antics of the -male, and the selection of the more beautiful or agile performer. Darwin -himself records a few cases, which plainly show that the more beautiful -is not always the more successful. It has also been suggested that the -battles of the males are sometimes sham performances, and even when they -are real, if the less vigorous do not remain to be destroyed but run -away, they live to find mates of their own. In fact, the conduct of the -males at the breeding season appears to be much more the outcome of -their own excitement than an attempt to attract the females. - -8. There is another side to the question, the importance of which is so -great, that it is surprising that Darwin has not taken any notice of it. -If, in order to bring about, or even maintain, the results of sexual -selection, such a tremendous elimination of individuals must take place, -it is surprising that natural selection would not counteract this by -destroying those species in which a process, so useless for the welfare -of the species, is going on. It is curious that this has not been -realized by those who believe in both of these two hypotheses. - -9. What has just been said applies also with almost equal force to the -development of such structures as the horns of deer, bison, antelopes, -and the brilliant colors of many insects and birds. If in nature, -competition between species takes place on the scale that the Darwinian -theory of natural selection postulates, such forms, if they are much -exposed, would be needlessly reduced in numbers in the process of -acquiring these structures. So many individuals would have been at such -a disadvantage in breeding, that if competition is as severe as the -theory of natural selection postulates, these species could hardly be -expected to compete successfully with other species in which sexual -selection was not taking place. - -10. Darwin admits that, in certain cases, external conditions may have -acted directly to produce the colors in certain forms, and if these were -not injurious he thinks they might have become constant. Such cases are -left unexplained in the sense that they are not supposed to be -adaptations to anything in particular. That colors produced in this way -might afterward be found useful, irrespective of how they arose, is -admitted as one of the ways in which sexual differences may have arisen. - -11. It is baffling to find Darwin resorting to the Lamarckian -explanation in those cases in which the improbability of the hypothesis -of sexual selection is manifest. If either principle is true, we should -expect it to apply to all phenomena of the same sort; yet Darwin makes -use of the Lamarckian principle, in the hypothesis of sexual selection, -only when difficulties arise. - -12. In attempting to explain the development of the musical sense in -man, it is clear that the hypothesis of sexual selection fails to give a -satisfactory explanation. To suppose that the genius of a Beethoven or -of a Mozart could have been the result of a process of sexual selection -is too absurd to discuss. Neither the power of appreciation nor of -expression in music could possibly have been the outcome of such a -process, and it does not materially help the problem to refer it back to -a troop of monkeys making the woods hideous with their cries. - -We come now to some of the special cases to which Darwin’s hypothesis -has been applied. - -13. In one case at least, it is stated that a bird living on the ground -might have acquired the color of the upper surface of the body through -natural selection, while the under surface of the males of the same -species might have become ornamented through the action of sexual -selection. Thus in one and the same individual the two processes are -supposed to have been at work, and it does not lessen the difficulty -very much by supposing the two processes to have been carried out at -different times, because it is evident that what had been gained at one -time by one process might become lost while the color of certain parts -was being acquired through the other process. - -14. Darwin points out that “the plumage of certain birds goes on -increasing in beauty during many years after they are fully mature,” as -in the peacock, and in some of the birds of paradise, and with the -plumes and crests of some herons. This is explained as possibly merely -the result of “continued growth.” The improbability of selection is -manifest in these cases, but if “continued growth” can accomplish this -much, why may not the whole process be also the outcome of such growth? -At any rate, whatever the explanation is, it is important to find a case -of a secondary sexual character that the hypothesis obviously is -insufficient to explain. - -15. It is admitted in a number of cases, as in the stag for instance, -that, although the larynx of the male is enlarged, this is not, in all -probability, the outcome of sexual selection, but in other forms this -same enlargement is ascribed to the selection process. - -16. It is admitted that in none of the highly colored British moths is -there much difference according to sex, although when a difference of -color is found in butterflies this is put down to the action of sexual -selection. If such wonderful colors as those of moths can arise without -the action of selection, why make a special explanation for those cases -in which this difference is associated with sex? - -17. It is well known that birds sing at other times of the year than at -the breeding season, and an attempt is made to account for this in that -birds take pleasure in practising those instincts that they make use of -at other times, as the cat plays with the captive mouse. Does not this -suggest that, if they had certain instincts, they would be more likely -to employ them at the times when their vitality or excitement is at its -highest without regard to the way in which they have come by them? - -18. The color of the iris of the eyes of many species of hornbills is -said to be an intense crimson in the males, and white in the females. In -the male condor the eye is yellowish brown, and in the female a bright -red. Darwin admits that it is doubtful if this difference is the result -of sexual selection, since in the latter case the lining of the mouth is -black in the males, and flesh-colored in the females, which does not -affect the external beauty. Yet if these colors were more extensive and -on the exterior, there can be little doubt that they would have been -explained as due to sexual selection. - -19. When the females in certain species of birds differ more from each -other than they do from their respective males, the case is compared to -“those inexplicable ones, which occur independently of man’s selection -in certain sub-breeds of the game-fowl, in which the females are very -different, whilst the males can hardly be distinguished.” Here then is a -case of difference in color associated with sex, but not the outcome of -sexual selection. - -20. The long hairs on the throat of the stag are said possibly to be of -use to him when hunted, since the dogs generally seize him by the -throat, “but it is not probable that the hairs were specially developed -for this purpose; otherwise the young and the females would have been -equally protected.” Here also is a sexual difference that can scarcely -be ascribed to selection. - -Some cases of differences in color between the sexes “may be the result -of variations confined to one sex, and transmitted to the same sex -without any good being gained, and, therefore, without the aid of -selection. We have instances of this with our domesticated animals, as -in the males of certain cats being rusty-red while the females are -tortoise-shell colored. Analogous cases occur in nature: Mr. Bartlett -has seen many black varieties of the jaguar, leopard, vulpine phalanger, -and wombat; and he is certain that all or nearly all of these animals -were males.” If changes of this sort occur, associated with one sex, why -is there any need of a special explanation in other cases of difference? - - * * * * * - -In the light of the many difficulties that the theory of sexual -selection meets with, I think we shall be justified in rejecting it as -an explanation of the secondary sexual differences amongst animals. -Other attempts to explain these differences have been equally -unsuccessful. Thus Wallace accounts for them as due to the excessive -vigor of the male, but Darwin’s reply to Wallace appears to show that -this is not the cause of the difference. He points out that, while the -hypothesis might appear plausible in the case of color, it is not so -evident in the case of other secondary sexual characters, such, for -instance, as the musical apparatus of the males of certain insects, and -the difference in the size of the larynx of certain birds and mammals. - -Darwin’s theory served to draw attention to a large number of most -interesting differences between the sexes, and, even if it prove to be a -fiction, it has done much good in bringing before us an array of -important facts in regard to differences in secondary sexual characters. -More than this I do not believe it has done. The theory meets with fatal -objections at every turn. - -In a later chapter the question will be more fully discussed as to the -sense in which these secondary sexual differences may be looked upon as -adaptations. - - ------------------------------------------------------------------------- - - - - - CHAPTER VII - - THE INHERITANCE OF ACQUIRED CHARACTERS AS A FACTOR IN EVOLUTION - - Lamarck’s Theory - - -One of the most striking and peculiar characteristics of living things -is that through use a part is able to carry out a particular function -better than before, and in some cases the use of the part leads to its -increase in size. Conversely, disuse leads to the decrease of a part in -size. We are perfectly familiar with this process in ourselves as -applied to our nervous system and muscles. - -It is not surprising that the idea should have arisen that, if the -results of the use of a part are inherited by the next generation, the -adaptation of organisms might be explained in this way. The presence of -the organs of touch, in those parts of the body that are more likely to -come into contact with foreign bodies, offers a striking parallel to the -perfecting of the sensation of touch that can be brought about through -the use of any part. The development of eyes only on the exposed parts -of the body, as on the tentacles of the sedentary annelids, or along the -margin of the mantle of a bivalve mollusk, suggests that there may be -some direct connection between their presence in these regions and the -effect of light on the parts. In fact, ever since the time of Lamarck, -there have been many zoologists who have claimed that many of the -adaptations of organisms have arisen in this way, that is, through the -inheritance of the characters acquired through use. In general this -theory is summed up in the phrase, “the inheritance of acquired -characters.” - -This view is prominently associated with the name of Lamarck, who held, -however, a different view in regard to the origin of some of the other -structures of the organism. Moreover, Erasmus Darwin, even before -Lamarck, had suggested the principle of the inheritance of acquired -characters. - -As has just been said, Lamarck held that the inheritance of acquired -characters was only one of the ways in which animals have become -changed, and he clearly stated that in the case of all plants and of -some of the lower animals the change (evolution) which he supposed them -to undergo was due to the general influence of the environment. Since -plants and the lower animals (as he supposed) have no central nervous -system, or at least no such well-defined nervous system as have the -higher animals, Lamarck thought that they could not have evolved in the -same way as have the higher animals. We now know that, so far as the -lower animals, at least, are concerned, there was no need for such a -distinction, since many of their responses are like those of the higher -animals. This distinction that Lamarck made is responsible, no doubt, -for a misconception that was long held in regard to a part of his views. -It is often stated that he supposed the desire of the animal for a -particular part has led to the development of that part; while in -reality he only maintained the desire to use a particular organ to -fulfil some want led to its better development through exercise, and the -result was inherited. Lamarck also supposed that the _decrease_ in use -of a part which leads to its decrease in size accounts for the -degeneration of organs. - -Lamarck first advanced his theory in 1801, when he cited the following -examples in its favor. A bird, driven through want to the water to find -its food, will separate its toes when they strike the water. The skin -uniting the bases of the toes will be stretched in consequence, and in -this way the broad membrane between the toes of ducks and geese has been -acquired. The toes of a bird that is in the habit of perching on a tree -become elongated in consequence of becoming stretched, hence has arisen -the foot with the long toes characteristic of arboreal birds. - -Shore-birds, “which do not care to swim,” but must approach the water in -order to obtain food, will be in danger of sinking into the mud, “but, -wishing to act so that their body shall not fall into the liquid, they -will contract the habit of extending and lengthening their legs.” Hence -have arisen the stiltlike legs of shore-birds. - -These ideas were more fully elaborated in the following year. He added -the further examples: Our dray-horses have arisen through the use to -which they have been put, and the race-horse also, which has been used -in a different way. Cultivated plants, on the contrary, are the result -of the new environment to which they have been subjected. - -In the “Philosophic Zoologique,” published in 1809, Lamarck has much -more fully developed his theory. Here he combats strenuously the idea -that species are fixed. His point of view may be judged by the following -propositions, which he believes can be established:— - -1. That all organized bodies of our globe are veritable productions of -nature, which she has successively produced in the course of a long -time. - -2. That in her progress nature began, and begins still every day, to -produce the simplest organisms, and that she still produces directly the -same primitive kinds of organizations. This process has been called -spontaneous generation. - -3. That the first beginning of animals and of plants takes place in -favorable localities and under favorable circumstances. An organic -movement having once established their production, they have of -necessity gradually developed their organs, and have become diversified -in the course of time. - -4. That the power of growth of each part of the body being inherited as -a consequence of the first effect of life, different modes of -multiplication and of regeneration have arisen, and these have been -conserved. - -5. That with the aid of sufficient time and of favorable circumstances -the changes that have taken place on the surface of the globe have -called forth new structures and new habits, and in consequence have -modified the organs of the body, and made animals and plants such as we -see them at the present day. - -6. Finally, as a result of these changes that living bodies have been -forced to undergo, species have been formed, but these species have only -a relative constancy, and are not as ancient as is nature herself. If -the environment remains the same, species also remain the same, as is -exemplified by the animals living at present in Egypt, which are exactly -like those living there in ancient times. - -Lamarck concludes that the appearance of stability is always mistaken by -the layman for the reality, because, in general, every one judges things -relatively to himself. In fact, species are not absolutely constant, but -are so only temporarily. “The influence of the environment is continuous -and always active, but its effects may only be recognized after a long -time.” The irregularity and the complexity of the organization of -animals is the outcome of the infinitely diversified circumstances to -which they have been subjected. These changes, Lamarck claims, do not -directly cause modifications in the form of animals,[17] but bring about -changes in their needs, and changes in their needs bring about changes -in their actions. If the needs remain the same, the acquired actions -become habits. These habitual actions lead to the use of certain parts -in preference to others, and this in turn to an alteration in form and -structure. The individuals so changed breed together and leave -descendants that inherit the acquired modification. - -Footnote 17: - - This is clearly meant to be applied only in the case of higher - animals. - -Curiously enough, Lamarck follows up this argument by citing some cases -amongst plants that have been changed directly by the action of the -environment. He says that since plants have no motions they have -consequently no habits, but they are developed by changes in their -nutrition, etc., and this brings about the superiority of some of the -vital movements over others. - -Amongst domestic animals Lamarck cites the case of the dog, that has -come from a wild form like the wolf, but having been carried into -different countries has acquired different and new habits, and this has -led to the formation of new races, such as the bulldog, greyhound, -pug-dog, spaniel, etc. - -Lamarck’s argument shifts so often back and forth from animals to -plants, that it is clear that in his own mind he did not see any -important difference between the action of the environment on plants, -and the use of the organs of the animal. He gives in this same -connection his oft-quoted summary of what he calls the two laws of -nature “which observation always establishes.” - -First Law. In every animal, that has not passed beyond the term of its -development, the frequent and sustained use of any organ strengthens it, -develops it, increases its size, and gives it strength proportionate to -the length of time of its employment. On the other hand, the continued -lack of use of the same organ sensibly weakens it; it deteriorates, and -its faculties diminish progressively until at last it disappears. - -Second Law. Nature preserves everything that she has caused the -individual to acquire or to lose by the influence of the circumstances -to which the race has been for a long time exposed, and consequently by -the influence of the predominant use of certain organs (or in -consequence of its continued disuse). She does this by the generation of -new individuals which are produced with the newly acquired organs. This -occurs, provided that the acquired changes were common to the two sexes, -or to the individuals that produced the new forms. - -These laws are, Lamarck says, fundamental truths which cannot be -misunderstood except by those who have never observed or followed nature -in her operations. He insists that it is a mistake to suppose that the -parts are responsible for the functions, for it is easy to demonstrate -that it is the needs and uses of the organs that have caused the parts -to develop. - -If it is supposed, he continues, that these laws are hypothetical, they -may be demonstrated by the following facts: The adult baleen whale is -without teeth, although in the fœtus teeth are present, concealed in the -jaws. The loss of the teeth is the result of the whale swallowing its -food without first masticating it. The ant-eater is also without teeth, -and has also the habit of swallowing its food without chewing it. The -mole has very small eyes, and this is the result of its having made very -little use of them, since its habits are subterranean. Another animal, -the aspalax, has only the rudiments of eyes, and has almost completely -lost the power of sight. This animal also lives underground like the -mole. - -Proteus, an aquatic salamander living in deep caves, has only -rudimentary eyes. In these latter cases it is the disuse of the eye that -has led to its degeneration. This is proven, Lamarck adds, by the fact -that the organs of hearing are never in this condition, because sound -vibrations penetrate everywhere, even into the densest bodies. - -It is a part of the plan of organization of the reptiles that they have -four legs; but the snakes, although belonging to this group, have no -legs. This absence of legs is explained by their having acquired the -habit of gliding over the ground, and of concealing themselves in the -grass. Owing to their repeated effort to elongate themselves, in order -to pass through narrow spaces, their bodies have become drawn out. Under -these circumstances legs would be useless, since long ones would -interfere with their motion, and short ones could not move their long -bodies. Since the plan of organization limits the snakes to only four -legs, and since this number would be useless, they have disappeared. - -Many insects are destitute of wings, although wings are a part of the -plan of organization of this group. They are absent only in those forms -whose habits render wings useless, consequently they have disappeared -through disuse. - -The preceding cases are those in which the disuse of an organ has led to -its degeneration. The following cases are cited to show that by use an -organ increases in size. The formation of the web in the feet of -water-birds has already been given as a character which Lamarck supposes -to have been acquired through use; also the case of shore-birds, which, -by an effort to elongate their legs, have actually made them so in the -course of time. The necks of water-birds are also long on account of -their having been stretched in the efforts to catch fish. The long -tongues of the ant-eater, of the woodpecker, and of humming-birds are -the result of use, and the long, forked tongue of serpents has come from -their using their tongue to feel objects in front of them. - -Fishes that have acquired the habit of living in shallow water, -flounders, soles, etc., have been forced to swim on their sides in order -to approach nearer to the shore. Since more light comes from above than -from below, the eye on the under side, straining to turn to the light, -has finally migrated to the upper side. - -The habit of eating great quantities of food, which distends the -digestive organs, has caused the bodies of herbivorous quadrupeds to -become large, as seen in the elephant, the rhinoceros, oxen, horses, and -buffaloes. The habit of standing for a long time on their feet has -caused some animals to develop hard, thick hoofs. Herbivorous animals, -that inhabit countries where they are constantly subjected to attack, as -deer and antelopes for example, are forced to escape by rapid flight, -and in consequence their bodies have become slenderer and their legs -thinner. The horns, antlers, and protuberances that many of these -animals possess are the results of their butting each other when -angered. - -“The long neck and the form of the giraffe offer a curious case. We know -that the giraffe is the tallest of all animals. It inhabits the centre -of Africa, living in those localities where the earth is nearly always -dry and without herbage. It is obliged to browse on the foliage of -trees, and this leads to its stretching continually upwards. As a result -of this habit, carried on for a long time, in all the individuals of the -race, the anterior limbs have become longer than the posterior, and its -neck has also lengthened, so that the giraffe without rising on its -hind-legs stretches up its neck and can reach to the height of six -metres.” - -The curved claws of the carnivora have arisen from the necessity of -grasping their prey. The power of retracting the claws has also been -acquired by the effort to draw them in when running over hard ground. -The abdominal pouch of the kangaroo, in which the young are carried, -opens anteriorly, and this has led to the animal standing erect so that -its young are not injured. In consequence, the fore-legs have become -shorter through disuse, and the hind-legs have become stronger through -use. The tail, which is also used as a support, has become enormously -thick at its base. - -The sloth has been compelled to seek refuge in the trees, and has taken -up its abode permanently there, feeding on leaves. Its movements are -limited to those involved in crawling along the limbs in order to reach -the leaves. After feeding it remains inactive and sluggish, these habits -being provoked by the heat of the climate. The results of its mode of -life have been to cause the arms to become elongated due to the habit of -the sloth of grasping the limbs of the tree; the claws of the fingers -and toes have also become long and hooked in order to retain their hold. -The digits that do not make any individual movements have lost the power -to do so, and have become fused, and can only be bent in and -straightened out. The thighs, being bent out to clasp the larger -branches, have caused the pelvis to widen, and, in consequence, the -cotyloid cavities have become directed backward. Many of the bones of -the skeleton have become fused, as a result of the immobility of the -animal. - -Lamarck says, that “Nature, in producing, successively, all the species -of animals, beginning with the most imperfect, or the most simple, and -terminating with the most perfect, has gradually complicated their -organization. These animals becoming scattered throughout the habitable -regions of the globe each species has received from the influences of -its surroundings its present habits, and the modifications of the parts -the use of which we recognize.” - -Such are Lamarck’s views and a fairly complete statement of the facts -from which he draws his conclusions. His illustrations appear naïve, and -often not a little ludicrous, but it must be admitted that, despite -their absurdities, his theory appears in some cases to account -wonderfully well for the facts. The long legs of wading birds, the long -neck and disproportionately long fore-legs of the giraffe, the structure -of the sloth, and particularly the degeneration of the eyes of animals -living in the dark, seem to find a simple explanation in the principle -of the inheritance of acquired characters. But the crucial point of the -entire theory is passed over in silence, or rather is taken for granted -by Lamarck, namely, the inheritance in the offspring of the characters -acquired through use or disuse in the parent. He does not even discuss -this topic, but in several places states unreservedly that the increase -or decrease of a part reappears in the next generation. It is here that -Lamarck’s theory has been attacked in more modern times, for as soon as -experimental proof was demanded to show that the results of use or of -disuse of an organ is inherited, no such proof was forthcoming. Yet the -theory is one that has the great merit of being capable of experimental -test, and it is astonishing to find that, with the immense amount that -has been written by his followers, so few attempts have been made to -give the theory a thorough test. The few results that have been obtained -are not, however, favorable to the theory, but almost the only attempts -at experiment that have been made in this direction have been those of -mutilating certain parts; and were it not for popular belief to the -effect that such mutilations are inherited, one would least expect to -get evidence for or against the theory in this direction. Lamarck -himself believed that the changes were slowly acquired, and I think -modern Lamarckians are justified in claiming that the validity of the -theory can only be tested by experiments in which the organism is -subjected to influences extending over a considerable period, although -Lamarck appears to have believed that the first results may appear quite -soon. Before expressing any opinion in regard to the probability of the -theory, let us examine what the followers of Lamarck have contributed in -the way of evidence to the theory, rather than the applications that -they have made of the theory. We shall also find it profitable to -consider some of the modern criticism, to which the theory has been -subjected. - -Despite the contempt with which Darwin referred to Lamarck’s theory, he -himself, as we have seen, often made use of the principle of the -inheritance of acquired characters, and even employed the same -illustrations cited by Lamarck. Darwin seems to have misunderstood -Lamarck’s view, and to have accepted the current opinion that Lamarck -supposed an animal acquired a new organ by desiring or needing it. -Darwin says, “Heaven forefend me from Lamarck’s nonsense of a tendency -to progressive adaptation from the slow willing of the animals.” Darwin -speaks of Lamarck as stating that animals will that the egg shall be a -particular form so as to become attached to particular objects. -Lamarck’s latest biographer, Packard, says he is unable to find any -statements of this sort in Lamarck’s writings. - -The following cases that Darwin tried to explain through the inheritance -of acquired characters are exactly like those to which Lamarck applied -his theory. The bones of the wing of the domestic duck weigh less than -those of the wild duck, and the bones of the leg more. Darwin believes -this is due to the effects of the inheritance of acquired characters. -The drooping ears of many domestic mammals are also explained by him as -a result of disuse—“the animals being seldom much alarmed.” In speaking -of the male of the beetle, _Onites apelles_, Darwin quotes Kirby to the -effect that the tarsi are so habitually lost that the species has been -described without this part of the foot. In the sacred beetle of Egypt -the tarsus is totally absent. Hence he concludes that the absence of -tarsi in the sacred beetle, and the rudimentary condition of the tarsus -in others, is probably the result of disuse, rather than a case of -inheritance of a mutilation. Darwin grants that “the evidence that -accidental mutilations can be inherited is at present not decisive, but -the remarkable case observed by Brown-Séquard in guinea-pigs of the -inherited effects of operations should make us cautious in denying this -tendency.” - -The wingless condition of several insects inhabiting oceanic islands has -come about, Darwin thinks, through disuse. The ostrich also, owing to -its increase in size, made less use of its wings and more use of its -legs, with the result that its wings degenerated and its legs got -stronger. The rudimentary condition of the eyes of the mole is the -result of disuse, “aided perhaps by natural selection.” Many of the -animals inhabiting the caves of Kentucky and of Carniola are blind, and -this is ascribed to disuse. “As it is difficult to imagine that the -eyes, though useless, could be in any way injurious to animals living in -darkness, their loss may be attributed to disuse.” The long neck of the -giraffe Darwin attributes partly to natural selection and partly to use. - -These references will suffice to show that Darwin is in full accord with -the main argument of Lamarck. In fact, the curious hypothesis of -pangenesis that Darwin advanced was invented partly to account for the -inheritance of acquired characters. Despite the hesitancy that Darwin -himself felt in advancing this view, and contrary to Huxley’s advice, he -at last published his provisional hypothesis of pangenesis in the -twenty-seventh chapter of his “Animals and Plants under Domestication.” - - - Darwin’s Hypothesis of Pangenesis - -The study of bud variation, of the various forms of inheritance, and of -reproduction and of the causes of variation, led him, Darwin says, to -the belief that these subjects stand in some sort of relation to each -other. He says: “I have been led, or rather forced, to form a view which -to a certain extent connects these facts by a tangible method. Every one -would wish to explain to himself, even in an imperfect manner, how it is -possible for a character possessed by some remote ancestor suddenly to -reappear in the offspring; how the effects of increased or decreased use -of a limb can be transmitted to the child; how the male sexual element -can act not solely on the ovules, but occasionally on the mother form; -how a hybrid can be produced by the union of the cellular tissue of two -plants independently of the organs of generation; how a limb can be -reproduced on the exact line of amputation, with neither too much nor -too little added; how the same organism may be produced by such widely -different processes, as budding and true seminal generation; and, -lastly, how of two allied forms, one passes in the course of its -development through the most complex metamorphoses, and the other does -not do so, though when mature both are alike in every detail of -structure. I am aware that my view is merely a provisional hypothesis or -speculation; but, until a better one be advanced, it will serve to bring -together a multitude of facts which are at present left disconnected by -any efficient cause.” - -In presenting the hypothesis of pangenesis Darwin begins by enumerating -the different kinds of sexual and asexual processes of reproduction, for -which he hopes to offer a provisional explanation. Here we find -mentioned various methods of budding and self-division, regeneration, -parthenogenesis, sexual reproduction, and the inheritance of acquired -characters. It is with the last only that we are here chiefly concerned; -in fact, the need of an hypothesis _of this sort_ to explain the other -kinds of inheritance is by no means evident. There are, however, two -other phenomena, besides that of the supposed inheritance of acquired -characters, to which the hypothesis of pangenesis might appear to apply -specially, namely, the effect of foreign pollen on the tissues of the -mother plant, and the supposed influence of the union with the first -male on the subsequent young (telegony). It is, however, far from being -shown that any influence of this latter kind really occurs, despite the -fact that it is generally believed in by breeders. - -It is important to observe that Darwin proposes to explain on the -hypothesis of pangenesis, not only the inheritance of characters -acquired through use, but also the decrease of structures through -disuse; and this applies, not only to the structure, but to function as -well, as when the intelligence of the dog is explained through his -association with man, and the tameness of the domestic rabbits through -their long confinement. In the following quotation these points are -referred to: “How can the use or disuse of a particular limb or of the -brain affect a small aggregate of reproductive cells, seated in a -distant part of the body, in such a manner that the being developed from -these cells inherits the characters of either one or both parents? Even -an imperfect answer to this question would be satisfactory.” - -Coming now to the theory, we find that it consists of one chief -assumption and several minor ones. “It is universally admitted that the -cells or units of the body increase by self-division or proliferation, -retaining the same nature, and that they ultimately become converted -into the various tissues and substances of the body. But besides this -means of increase I assume that the units throw off minute granules -which are dispersed throughout the whole system; that these, when -supplied with proper nutriment, multiply by self-division, and are -ultimately developed into units like those from which they were -originally derived. These granules may be called gemmules. They are -collected from all parts of the system to constitute the sexual -elements, and their development in the next generation forms a new -being; but they are likewise capable of transmission in a dormant state -to future generations, and may then be developed.... Gemmules are -supposed to be thrown off by every unit, not only during the adult -state, but during each stage of development of every organism; but not -necessarily during the continued existence of the same unit. Lastly, I -assume that the gemmules in their dormant state have a mutual affinity -for each other, leading to their aggregation into buds, or into the -sexual elements. Hence, it is not the reproductive organs, or buds, -which generate new organisms, but the units of which each individual is -composed. These assumptions constitute the provisional hypothesis which -I have called Pangenesis.” - -It will be noticed that the first assumption is that the cells throw off -minute gemmules or granules. The second assumption is that these are -collected in the reproductive organs, or in buds, or in regenerating -parts; the third assumption is that the gemmules may lie dormant through -several generations; the fourth, that the development of the -reproductive cells is not so much the development of the cell itself, -but of the gemmules that have collected in it. The fifth assumption is -that the gemmules are thrown off at all stages of development; the -sixth, that in their dormant state they have a mutual affinity for each -other; the seventh, that there may be a sort of continual competition in -the germ-cells between the original gemmules and the new ones, and, -according to which win, the old or the new form develops. Thus we see on -closer analysis that the pangenesis hypothesis is made up of a goodly -number of different assumptions. At least half a dozen imaginary -properties are ascribed to the imaginary gemmules, and these attributes -are all essential to the working of the hypothesis. - -Some of the more obvious objections to the hypothesis have been stated -by Darwin himself. Such, for instance, as our ignorance at what stage in -their history the body-cells are capable of throwing off gemmules, and -whether they collect only at certain times in the reproductive organs, -as the increased flow of blood to these organs at certain seasons might -seem to indicate. Nor have we any evidence that they are carried by the -blood at all. The experiment of Galton, of transfusing the blood of one -animal into another, and finding that this produced no effect on the -young that were born later, might be interpreted to mean that gemmules -are not transported by the blood; but this kind of experiment is -inconclusive, especially in the light of recent results on the effect of -the blood of one animal on that of another. - -A part of the evidence on which Darwin relied to support his theory has -been shown to be incorrect by later work. Thus the assumption that more -than a single pollen grain, or more than one spermatozoon, is necessary -in some cases for fertilization, is certainly wrong. In most cases, in -fact, the entrance of more than one spermatozoon into the egg is -disastrous to the development. The cases referred to by Darwin can -probably be explained by the difficulty that some of the pollen grains, -or spermatozoa, may have in penetrating the egg, or to the immaturity or -impotence of some of the male germ-cells, and not to the need of more -than one to accomplish the true fertilization. - -Darwin’s idea that the small number of gemmules in the unfertilized egg -may account for the lack of power of such eggs to develop until they are -fertilized, has been shown to be incorrect by recent results in -experimental embryology. We now know that many different kinds of -stimuli have the power to start the development of the egg. Moreover, we -also know that if a single spermatozoon is supplied with a piece of -egg-protoplasm without a nucleus, it suffices to cause this piece of -protoplasm to develop. - -In the case of regeneration, which Darwin also tries to explain on the -pangenesis hypothesis, we find that there is no need at all for an -hypothesis of this sort; and there are a number of facts in connection -with regeneration that are not in harmony with the hypothesis. For -instance, when a part is cut off, the same part is regenerated; but -under these circumstances it cannot be imagined that the part removed -supplies the gemmules for the new part. Darwin tries to meet this -objection by the assumption that every part of the body contains -gemmules from every other part. But it has been shown that if a limb of -the newt is completely extirpated, a new limb does not regenerate; and -there is no reason why it should not do so on Darwin’s assumption that -germs of the limb exist throughout the body. - -The best-authenticated cases of the influence of the male on the tissues -of the female are those in plants, where one species, or variety, is -fertilized by another. Thus, if the orange is fertilized by the pollen -of the lemon, the fruit may have the color and flavor of the lemon. Now -the fruit is a product of the tissues of the ovary of the female, and -not a part of the seedling that develops in the fruit from the -cross-fertilized egg-cell. Analogous cases are recorded for the bean, -whose pods may have their color influenced by fertilizing the flower -with pollen of another variety having pods of a different color. In -these cases we do not know whether the color of the fruit is influenced -directly by the foreign pollen, or whether the influence is through the -embryo that develops from the egg-cell. The action may appear to be the -same, however, in either case; but because it seems probable here that -there is some sort of influence of one tissue on another, let us not too -readily conclude that this is brought about through any such imaginary -bodies as gemmules. It may be directly caused, for instance, by some -chemical substance produced in the young hybrid plant. If this is the -case, the result would not be different in kind from that of certain -flowers whose color may be influenced by certain chemical substances in -the soil. - -In the cases amongst animals, where the maternal tissues are believed to -be influenced by a previous union with the male, as in the oft-cited -case of Lord Morton’s mare, a reëxamination of the evidence by Ewart has -shown that the case is not demonstrated, and not even probable. Several -years ago I tried to test this view in the case of mice. A white mouse -was first bred to a dark male house-mouse, and the next time to a white -mouse, but none of the offspring from the second union showed any trace -of black. If the spermatozoa of the dark mouse are hypodermically -injected into the body-cavity of the female, the subsequent young from a -white male show no evidence that the male cells have had any influence -on the ovary. - -The following facts, spoken of by Darwin himself, are not in favor of -his hypothesis of pangenesis: “But it appears at first sight a fatal -objection to our hypothesis that a part of an organ may be removed -during several successive generations, and if the operation be not -followed by disease, the lost part reappears in the offspring. Dogs and -horses formerly had their tails docked during many generations without -any inherited effect; although, as we have seen, there is some reason to -believe that the tailless conditions of certain sheep-dogs is due to -such inheritance.” The answer that Darwin gives is that the gemmules -themselves, that were once derived from the part, are still present in -other parts of the body, and it is from these that the organs in the -next generation may be derived. But Darwin fails to point out that, if -this were the case, it must also be true for those cases in which an -organ is no longer used. Its decrease in size in successive generations -cannot be due to its disuse, for the rest of the body would supply the -necessary gemmules to keep it at its full state of development. Thus, in -trying to meet an obvious objection to his hypothesis, Darwin brings -forward a new view that is fatal to another part of his hypothesis. - -The following cases, also given by Darwin, are admitted by him to be -inexplicable on his hypothesis: “With respect to variations due to -reversion, there is a similar difference between plants propagated from -buds and seeds. Many varieties can be propagated securely by buds, but -generally or invariably revert to their parent forms by seed. So, also, -hybridized plants can be multiplied to any extent by buds, but are -continually liable to reversion by seed,—that is, to the loss of their -hybrid or intermediate character. I can offer no satisfactory -explanation of these facts. Plants with variegated leaves, phloxes with -striped flowers, barberries with seedless fruit, can all be securely -propagated by buds taken from the stem or branches; but buds from the -roots of these plants almost invariably lose their character and revert -to their former condition. This latter fact is also inexplicable, unless -buds developed from the roots are as distinct from those on the stem, as -is one bud on the stem from another, and we know that these latter -behave like independent organisms.” As Darwin here states, these facts -appear to be directly contradictory to his hypothesis, and he makes no -effort to account for them. - -The entire question of the possibility of the inheritance of acquired -characters is itself at present far from being on a satisfactory basis, -as we shall try to show; and Darwin’s attempt at an explanation, in his -chapter on pangenesis, does not put the matter in a much more -satisfactory condition. - - - The Neo-Lamarckian School - -Let us now turn our attention to a school that has grown up in modern -times, the members of which call themselves Neo-Lamarckians. Let us see -if they have supplied the essential evidence that is required to -establish the Lamarckian view, namely, that characters acquired by the -individual are transmitted to the offspring. - -Lamarck’s views were adopted by Herbert Spencer, and play an important -rôle in his “Principles of Biology” (1866-1871), and even a more -conspicuous part in his later writings. In the former he cites, amongst -other cases, that of “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.” If tricks like this are -inheritable is it not surprising that more puppies do not stand on their -hind-legs? - -The larger hands of the laboring classes in England are supposed to be -inherited by their children, and the smaller hands of the leisure -classes are supposed to be the result of the disuse of the hands by -their ancestors; but even if these statements in regard to size are -true, there are many other conceivable causes that may have led to this -result. - -Short-sightedness appears more often, it is said, in those classes of -society that make most use of their eyes in reading and in writing; but -if we ask for experimental evidence to show that this is due to -inheritance, and not due to the children spoiling their eyes at school, -there is none forthcoming. The problem is by no means so simple as the -uninitiated may be led to believe. - -Spencer thinks that “some of the best illustrations of functional -heredity are furnished by mental characteristics.” He cites the musical -faculty as one that could not have been acquired by natural selection, -and must have arisen through the inheritance of acquired modifications. -The explanation offered is “that the habitual association of certain -cadences of speech with certain emotions has clearly established in the -race an organized and inherited connection between such cadences and -such emotions, ... and that by the continued hearing and practice of -melody there has been gained and transmitted an increasing musical -sensibility.” But a statement that the results have been acquired in -this way does not supply the proof which the theory is in need of; -neither does it follow that, because the results cannot be explained by -the theory of natural selection, therefore, they must be explained by -the Lamarckian theory. - -The clearest proofs that Spencer finds of the inheritance of acquired -characters are in the well-known experiments of Brown-Séquard. These -experiments will be more fully discussed below. Amongst the other morbid -processes that Spencer thinks furnish evidence in favor of this view, -are cases of a tendency to gout, the occurrence of mental tricks, -musical prodigies, liability to consumption, in all of which cases the -fundamental distinction between the inheritance of an acquired character -and the inherited tendency toward a particular malady is totally -ignored. - -Twenty-seven years later (in 1893) Spencer took up the open challenge of -the anti-Lamarckian writers, and by bringing forward a number of new -_arguments_ attempted to reinstate the principle of the inheritance of -acquired characters. His first illustration is drawn from the -distribution of the sense of touch in different parts of our bodies. -Weber’s experiments have shown that if the sharp points of a pair of -compasses are applied to the tips of the forefingers, the sensation of -two separate points is given when the points are only one-twelfth of an -inch apart, and if the points are moved nearer together, they give the -sensation of only one point. The inner surfaces of the second joints of -the fingers can only distinguish two points when they are one-sixth of -an inch apart. The innermost joints are less discriminating, and are -about equal in the power of discrimination to the tip of the nose. The -end of the big toe, the palm of the hand, and the cheek discriminate -only about one-fifth as well as do the tips of the fingers. The back of -the hand and the top of the head distinguish only about one-fifteenth as -well as the finger-tips. The front of the thigh, near the knee, is -somewhat less sensitive than the back of the hand. On the breast the -points of the compasses must be separated by more than an inch and a -half in order to give two sensations. In the middle of the back the -points must be separated by two and a half inches, or more, in order to -give two separate impressions. - -What is the meaning of these differences, Spencer asks. If natural -selection has brought about the result, then it must be shown that -“these degrees of endowment have advantaged the possessor to such an -extent that not infrequently life has been directly or indirectly -preserved by it.” He asks if this, or anything approaching this, result -could have occurred. - -That the superior perceptiveness of the forefinger-tip might have arisen -through selection is admitted by Spencer, but how could this have been -the case, he asks, for the middle of the back, and for the face? The tip -of the nose has three times more power of discrimination than the lower -part of the forehead. Why should the front of the thigh near the knee be -twice as perceptive as in the middle of the thigh; and why should the -middle of the back and of the neck and the middle of the forearm and of -the thigh stand at such low levels? Is it possible, Spencer asks again, -that natural selection has determined these relations, and if not, how -can they be explained? His reply is that the differences can all be -accounted for on the theory of the inheritance of use, for it is evident -that “these gradations in tactile perceptiveness correspond with the -gradations in the tactual exercise of the parts.” Except from contact -with the clothing the body receives hardly any touch sensations from -outside, and this accounts for its small power of discrimination. The -greater sensitiveness of the chest and abdomen, as compared with the -back, is due to these regions being more frequently touched by the -hands, and is also owing to inheritance from more remote ancestors, in -which the lower surface of the body was more likely to have come in -contact with foreign objects than was the back. The middle of the -forearm and of the thigh are also less exposed than the knee and the -hand, and have correspondingly the power of tactile discrimination less -well developed. - -Weber showed that the tip of the tongue is more sensitive than any other -part of the body, for it can distinguish between two points only one -twenty-fourth of an inch apart. Obviously, Spencer says, natural -selection cannot account for such extreme delicacy of touch, because, -even if it were useful for the tongue to distinguish objects by touch, -this power could never be of vital importance to the animal. It cannot -even be supposed that such delicacy is necessary for the power of -speech. - -The sensitiveness of the tongue can be accounted for, however, Spencer -claims, as the result of the constant use of the tongue in exploring the -cavity of the mouth. It is continually moving about, and touching now -one part, and now another, of the mouth cavity. “No advantage is gained. -It is simply that the tongue’s position renders perpetual exploration -almost inevitable.” No other explanation of the facts seemed possible to -Spencer. - -Two questions will at once suggest themselves. First, can it be shown -that the sensitiveness to touch in various parts of the body is the -result of individual experience? Have we learned to discriminate in -those parts of the body that are most often brought into contact with -surrounding objects? Even the power of discrimination in the tips of the -fingers can be improved, as Spencer himself has shown, in the case of -the blind, and of skilled compositors. Can we account in this way for -the power of discrimination in various parts of the body? In other -words, if, beginning in infancy, the middle of the back constantly came -into contact with surrounding objects, would this region become as -sensitive as the tips of the fingers? The experiment has not, of course, -been carried out, but it is not probable that it would succeed. I -venture this opinion on the ground of the relative number of the nerves -and of the organs of touch on the back, as compared with those of the -finger-tips. But, it will be asked, will not the number of the -sense-organs become greater if a part is continually used by the -individual? It is improbable that much improvement could be brought -about in this way. The improvement that takes place through experience -is probably not so much the result of the development of more -sense-organs, as of better discrimination in the sensation, because the -increased power can be very quickly acquired. - -An examination of the relative abundance of touch-spots in the skin -shows that they are much more numerous in regions of greater -sensitiveness. The following table, taken from Sherrington’s account of -sense-organs in Schaefer’s “Textbook of Physiology,” gives the smallest -distance that two points, simultaneously applied, can be recognized as -such (and not simply as one impression) in different regions. - - - Mm. - - Tip of tongue 1.1 - - Volar surface of 2.3 - ungual phalanx of - finger - - Red surface of lip 4.5 - - Volar face of second 4.5 - phalanx - - Dorsal face of third 6.8 - phalanx - - Side of tongue 9.0 - - Third line of tongue, 9.0 - 27 mm. from tip - - Plantar face of ungual 11.3 - phalanx of first toe - - Palm 11.3 - - Back of second phalanx 11.3 - of finger - - Forehead 22.6 - - Back of ankle 22.6 - - Back of hand 31.6 - - Forearm, leg 40.6 - - Dorsum of foot 40.6 - - Outer sternum 45.1 - - Back of neck 54.1 - - Middle of back 67.1 - - Upper arm, thigh 67.1 - - -The great difference in the sensitiveness of the skin in the different -regions is very striking, and if, as seems probable, about the same -proportionate difference is found at birth, then the degree of -sensibility of the different regions is inborn, and is not the result of -each individual experience. Until it can be shown that more of the -sense-organs develop in any special part, as the result of the increased -use of the part, we have no real basis on which to establish, even as -probable, the Lamarckian view. - -But, after all, is the distribution of the sense-organs exactly that -which we should expect on the Lamarckian theory? Has not Spencer taken -too much for granted in this direction? The lower part of the forearm -(represented by 15) we should expect to be more sensitive than the -protected surface of the eyelid (11.3), but this is not the case. The -forehead (22.6) is much less sensitive than the forearm, and only half -as sensitive as the eyelid. The knee (36.1) is still less sensitive than -any of these other parts, and this does not in the least accord with the -theory, since in its constant moving forward it must be continually -coming into contact with foreign bodies. The fact that the back is as -insensitive as the upper arm (67.7) can hardly be accredited in favor of -the theory. The great difference between the lower third of the forearm -on the ulnar surface (15) and the upper arm (67.7) seems out of all -proportion to what we should expect on the theory. And is it not a -little odd that the end of the nose should be so highly sensitive? - -There is another point that we cannot afford to neglect in this -connection. It is known that in addition to touch-spots there are warm -and cold spots in the skin, which produce, when touched, the sensation -of warmth, or of cold, respectively, and not the sensation of touch. The -degree of sensitiveness of different regions of the body throws an -interesting side-light on Spencer’s argument. - -The warm spots are much fewer than the cold spots. The spots are -arranged in short lines radiating from centres which coincide with -hairs. The number of these spots varies a good deal, even in the same -region of the skin. If the sensitiveness of the skin is tested, the -following results will be obtained. The list includes twelve grades of -sensitiveness, beginning with the places giving the lowest maximum of -intensity. About one hundred square areas were tested in each region. - - - COLD SENSATIONS - - 1. Tips of fingers and toes, malleoli, ankle. - 2. Other parts of digits, tip of nose, olecranon. - 3. Glabella, chin, palm, gums. - 4. Occiput, patella, wrist. - 5. Clavicle, neck, forehead, tongue. - 6. Buttocks, upper eyelid. - 7. Lower eyelid, popliteal space, sole, cheek. - 8. Inner aspect of thigh, arm above elbow. - 9. The intercostal spaces along axillary line. - 10. Mammary areola. - 11. Nipple, flank. - 12. Certain areas of the loins and abdomen. - - - WARMTH SENSATIONS - - 0. Lower gum, mucosa of cheek, cornea. - 1. Tips of fingers and toes, cavity of mouth, conjunctiva, and - patella. - 2. Remaining surface of digits, middle of forehead, olecranon. - 3. Glabella, chin, clavicle. - 4. Palm, buttock, popliteal space. - 5. Neck. - 6. Back. - 7. Lower eyelid, cheek. - 8. Nipple, loin. - - -These two tables show the great differences in the range of -sensitiveness to cold and to warmth in different parts of the body. I -doubt if any one will attempt to show that these differences of range of -sensation can be accounted for either by natural selection or by the -Lamarckian hypothesis. - -Of course, it does not necessarily follow that, because this is true for -the warm and cold spots, that it must also be true for the tactile -organs; but I think that the fact of such a great difference in the -responsiveness to cold and to warmth in different parts of the body -should put us on our guard against a too ready acceptation of Spencer’s -argument. More especially is this seen to be necessary, when, as has -been shown above, the distribution of the touch-organs themselves by no -means closely corresponds to what we should expect, if they have -developed in response to contact, as Spencer maintains. - -The other main argument advanced by Spencer to fortify the theory of the -inheritance of acquired characters, and at the same time to show the -inadequacy of the theory of natural selection, is based on the idea of -what he calls the “coöperation of the parts” that is required in order -to carry out any special act. Spencer contends that “the relative powers -of coöperative parts cannot be adjusted solely by the survival of the -fittest, and especially where the parts are numerous and the coöperation -complex.” - -Spencer illustrates his point by the case of the extinct Irish elk, -whose immensely developed horns weighed over a hundredweight. The horns, -together with the massive skull, could not have been supported by the -outstretched neck without many and great changes of the muscles and -bones of the neck and of the fore-part of the body. Unless, for -instance, the fore-legs had been also strengthened, there would be -failure in fighting and in locomotion. Since “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 the parts that were for a time useless,—parts, moreover, -which would revert to their original sizes before the other needful -variations occurred.” - -The answer made to this argument was that coördinating parts vary -together. In reply to which Spencer points to the following cases, which -show that this is not so: The blind crayfish in the Kentucky caves have -lost their eyes, but not the stalks that carry them. Again, the normal -relation between the length of tongue and of beak in some varieties of -pigeons is lost. The greater decrease in the jaws in some species of pet -dogs than of the number of their teeth has caused the teeth to become -crowded.[18] “I then argued that if coöperative parts, small in number, -and so closely associated as these are, do not vary together, it is -unwarrantable to allege that coöperative parts, which are very numerous -and remote from one another, vary together.” Spencer puts himself here -into the position of seriously maintaining that, because some -coöperative parts do not vary together, therefore no coöperative parts -have ever done so, and he has taken this position in the face of some -well-known cases in which certain parts have been found to vary -together. - -Footnote 18: - - It is curious that Spencer does not see that this case is as much - against his point as in favor of it, since the _unused_ teeth did not - also degenerate. - -In this same connection Spencer brings up the familiar _pièce de -résistance_ of the Lamarckian school, the giraffe. He recognizes that -the chief traits in the structure of this animal are the result of -natural selection, since its efforts to reach higher branches could not -be the cause of the lengthening of the legs. But “the coadaptation of -the parts, required to make the giraffe’s structure useful, is much -greater than at first appears.” For example, the bones and the muscles -of the hind-legs have been also altered, and Spencer argues that it is -“impossible to believe” that all parts of the hind-quarters could have -been coadapted to one another, and to all parts of the fore-quarters. A -lack of coadaptation of a single muscle “would cause fatal results when -high speed had to be maintained while escaping from an enemy.” - -Spencer claims that, since 1886, when he first published this argument, -nothing like an adequate response has been made; and I think he might -have added that an adequate answer is not likely to be forthcoming, -since nothing short of a demonstration of how the giraffe really evolved -is likely to be considered as sufficient. Wallace’s reply, that the -changes in question could have been brought about by natural selection, -since similar changes have been brought about by artificial selection, -is regarded as inadequate by Spencer, since it assumes a parallel which -does not exist. Nevertheless, Wallace’s reply contains, in my opinion, -the kernel of the explanation, in so far as it assumes that congenital -variation[19] may suffice to account for the origin of a form even as -bizarre as that of the giraffe. The ancon ram and the turnspit dog were -marked departures from the normal types, and yet their parts were -sufficiently coördinated for them to carry out the usual modes of -progression. It would not have been difficult, if we adopted Spencer’s -mode of arguing, to show that these new forms could not possibly have -arisen as the result of congenital variations. - -Footnote 19: - - Wallace assumes fluctuating variation to suffice, but in this I cannot - agree with him. - -Again, it might be argued that the large, powerful dray-horse could not -have arisen through a series of variations from the ordinary horse, -because, even if variations in the right direction occurred in the -fore-quarters, it is unlikely that similar variations would occur in the -hind-quarters, etc. Yet the feat has been accomplished, and while it is -difficult to prove that the inheritance of acquired characters has not -had a hand in the process, it is improbable that this has been the case, -but rather that artificial selection of some kind of variations has been -the factor at work. - -So long as the Lamarckian theory is supported by arguments like these, -it can never hope to be established with anything more than a certain -degree of probability. If it is correct, then its demonstration must -come from experiment. This brings us to a consideration of the -experimental evidence which has been supposed by some writers to give -conclusive proof of the validity of the theory. - -The best direct evidence in favor of the Lamarckian argument is that -furnished by the experiments of Brown-Séquard. He found, as the result -of injury to the nervous system of guinea-pigs, that epilepsy appeared -in the adult animal, and that young born from these epileptic parents -became also epileptic. Still more important was his discovery that, -after an operation on the nerves, as a result of which certain organs, -the ear or the leg, for instance, are affected, the same affection -appears in the young born from such parents. These results of -Brown-Séquard have been vouched for by two of his assistants, and his -results in regard to the inheritance of epilepsy have been confirmed by -Obersteiner, and by Luciani on dogs. Equally important is their later -confirmation, as far as the main facts go, by Romanes. - -Brown-Séquard gives the following summary of his results. I follow -Romanes’ translation in his book on “Darwin and after Darwin,” where -there is also given a careful analysis of Brown-Séquard’s results, as -well as the outcome of the experiments of Romanes himself. The summary -is as follows:— - -1. “Appearance of epilepsy in animals born of parents which had been -rendered epileptic by an injury to the spinal cord. - -2. Appearance of epilepsy also in animals born of parents which had been -rendered epileptic by section of the sciatic nerve. - -3. A change in the shape of the ear in animals born of parents in which -such a change was the effect of a division of the cervical sympathetic -nerve. - -4. Partial closure of the eyelids in animals born of parents in which -that state of the eyelids had been caused either by section of the -cervical sympathetic nerve, or the removal of the superior cervical -ganglion. - -5. Exophthalmia in animals born of parents in which an injury to the -restiform body had produced that protrusion of the eyeball. This -interesting fact I have witnessed a good many times, and seen the -transmission of the morbid state of the eye continue through four -generations. In these animals modified by heredity, the two eyes -generally protruded, although in the parents usually only one showed -exophthalmia, the lesion having been made in most cases only on one of -the corpora restiformia. - -6. Hæmatoma and dry gangrene of the ears in animals born of parents in -which these ear alterations had been caused by an injury to the -restiform body near the nib of the calamus. - -7. Absence of two toes out of the three of the hind-leg, and sometimes -of the three, in animals whose parents had eaten up their hind-leg toes, -which had become anæsthetic from a section of the sciatic nerve alone, -or of that nerve and also of the crural. Sometimes, instead of complete -absence of the toes, only a part of one or two or three was missing in -the young, although in the parent not only the toes, but the whole foot -was absent (partly eaten off, partly destroyed by inflammation, -ulceration, or gangrene). - -8. Appearance of various morbid states of the skin and hair of the neck -and face in animals born of parents having had similar alterations in -the same parts as effects of an injury to the sciatic nerve.” - -Romanes, who later went over the same ground, in part under the -immediate direction of Brown-Séquard himself, has made some important -observations in regard to these results, many of which he was able to -confirm. - -He did not repeat the experiment of cutting the cord, but he found that, -to produce epilepsy, it was only necessary to cut the sciatic nerve. The -“epileptiform habit” does not appear in the animal until some time after -the operation; it lasts for some weeks or months, and then disappears. -The attacks are not brought on spontaneously, but by “irritating a small -area of the skin behind the ear on the same side of the body as that on -which the sciatic nerve had been divided.” The attack lasts for only a -few minutes, and during it the animal is convulsed and unconscious. -Romanes thinks that the injury to the sciatic nerve, or to the spinal -cord, produces some sort of a change in the cerebral centres, “and that -it is this change—whatever it is, and in whatever part of the brain it -takes place—which causes the remarkable phenomena in question.” - -In regard to Brown-Séquard’s statements, made in the 3d and the 4th -paragraphs, in respect to the results of the operation of cutting the -cervical sympathetic, Romanes had not confirmed the results when his -manuscript went to press; but soon afterward, after Romanes’ death, a -note was printed in _Nature_ by Dr. Hill, announcing that two -guinea-pigs from Romanes’ experiment had been born, “both of which -exhibited a well-marked droop of the upper eyelid. These guinea-pigs -were the offspring of a male and female in both of which I had produced -for Dr. Romanes, some months earlier, a droop of the left upper eyelid -by division of the left cervical sympathetic nerve. This result is a -corroboration of the series of Brown-Séquard experiments on the -inheritance of acquired characters.” - -Romanes states that he also found that injury to a particular spot of -the restiform bodies is quickly followed by a protrusion of the eye on -the same side, and further, that he had “also had many cases in which -some of the progeny of parents thus affected have shown considerable -protrusion of the eyeballs of both sides, and this seemingly abnormal -protrusion has occasionally been transmitted to the next generation. -Nevertheless, I am far from satisfied that this latter fact is anything -more than an accidental coincidence.” This reservation is made on the -ground that the protrusion in the young is never so great as in the -parents, and also because there is amongst guinea-pigs a considerable -amount of individual variation in the degree of prominence of the -eyeballs. Romanes, while unwilling to deny that an “obviously abnormal -amount of protrusion, due to the operation, may be inherited in lesser -degree,” is also unwilling to affirm so important a conclusion on the -basis of these experiments alone. - -In regard to Brown-Séquard’s 6th statement, Romanes found after injury -to the restiform body that hæmatoma and dry gangrene may supervene, -either several weeks after the operation, or at any subsequent time, -even many months afterward. The disease usually affects the upper parts -of both ears, and may then gradually extend downward until nearly the -whole ear is involved. “As regards the progeny of animals thus affected -in some cases, but by no means in all, a similarly morbid state of the -ears may arise apparently at any time in the life history of the -individual. But I have observed that in cases where two or more -individuals _of the same litter_ develop this diseased condition, they -usually do so at about the same time, even though this may be months -after birth, and therefore after the animals are fully grown.” Moreover, -the morbid process never extends so far in the young as it does in the -parents, and “it almost always affects the middle third of the ear.” -Several of the progeny from this first generation, which had apparently -inherited the disease, but had not themselves been directly operated -upon, showed a portion of the ear consumed apparently by the same -disease. Romanes then gives the following significant analysis of this -result. Since a different part of the ear of the progeny is affected, -and also a “very much less quantity thereof,” it might seem that the -result was due either to a mere coincidence, or to the transmission of -microbes. But he goes on to say, that he fairly well excluded both of -these possibilities, for, in the first place, he has never observed “the -very peculiar process in the ears, or in any other parts of guinea-pigs -which have neither themselves had the restiform bodies injured, nor been -born of parents thus mutilated.” In regard to microbes, Romanes tried to -infect the ears of normal guinea-pigs by first scarifying these parts, -and then rubbing them with the diseased surfaces of the ears of affected -guinea-pigs. In not a single case was the disease produced. - -Romanes concludes that these “results in large measure corroborate the -statements of Brown-Séquard; and it is only fair to add that he told me -they were the results which he had himself obtained most frequently, but -that he had also met with many cases where the diseased condition of the -ears in parents affected the same parts in their progeny and also -occurred in more equal degrees.” - -We come now to the remarkable conclusion given in Brown-Séquard’s 7th -statement, in regard to the absence of toes in animals whose parents had -eaten off their own hind toes and even parts of their legs. Romanes got -neuroses in the animals operated upon, and found that the toes might be -eaten off; but none of the young showed any defect in these parts. -Furthermore, Romanes repeated the same operation upon the descendants -through six successive generations, so as to produce, if possible, a -cumulative effect, but no inheritance of the mutilation was observed. -“On the other hand, Brown-Séquard informed me that he had observed this -inherited absence of toes only in about one or two per cent of cases.” -It is possible, therefore, Romanes adds, that his own experiments were -not sufficiently numerous to have obtained such cases. - -In this connection I may give an account of some observations that I -made while carrying out some experiments in telegony with mice. I found -in one litter of mice that when the young came out of the nest they were -tailless. The same thing happened again when the second litter was -produced, but this time I made my observations sooner, and examined the -young mice immediately after birth. I found that the mother had bitten -off, and presumably eaten, the tails of her offspring at the time of -birth. Had I been carrying on a series of experiments to see if, when -the tails of the parents were cut off, the young inherit the defect, I -might have been led into the error of supposing that I had found such a -case in these mice. If this idiosyncrasy of the mother had reappeared in -any of her descendants, the tails might have disappeared in succeeding -generations. This perversion of the maternal instincts is not difficult -to understand, when we recall that the female mouse bites off the -navel-string of each of her young as they are born, and at the same time -eats the afterbirth. Her instinct was carried further in this case, and -the projecting tail was also removed. - -Is it not possible that something of this sort took place in -Brown-Séquard’s experiment? The fact that the adults had eaten off their -own feet might be brought forward to indicate the possibility of a -perverted instinct in this case also. At least my observation shows a -possible source of error that must be guarded against in future work on -this subject. - -In regard to the 8th statement of Brown-Séquard, as to various morbid -states of the skin, Romanes did not test this, because the facts which -it alleges did not seem of a sufficiently definite character. - -These experiments of Brown-Séquard, and of those who have repeated them, -may appear to give a brilliant experimental confirmation of the -Lamarckian position; yet I think, if I were a Lamarckian, I should feel -very uncomfortable to have the best evidence in support of the theory -come from this source, because there are a number of facts in the -results that make them appear as though they might, after all, be the -outcome of a transmitted disease, as Weismann claims, rather than the -inheritance of an acquired character. Until we know more of the -pathology of epilepsy, it may be well not to lay too great emphasis on -these experiments. It should not be overlooked that during the long time -that the embryo is nourished in the uterus of the mother, there is ample -opportunity given for the transmission of material, or possibly even of -bacteria. If it should prove true that epilepsy is due to some substance -present in the nervous system, such substances could get there during -the uterine life of the embryo. Even if this were the case, it may be -claimed that it does not give an explanation of the local reappearance -of the disease in the offspring. But here also we must be on our guard, -for it is possible that only certain regions of the body are susceptible -to a given disease; and it has by no means been shown that the local -defect itself is inherited, but only the disease. Romanes insists that a -very special operation is necessary to bring about certain forms of -transmission. - -It is well also to keep in mind the fact, that if this sort of effect is -inherited, then we must be prepared to accept as a possibility that -other kinds of injury to the parent may be transmitted to the offspring. -It would be of great disadvantage to animals if they were to inherit the -injuries that their parents have suffered in the course of their lives. -In fact, we might expect to find many plants and animals born in a -dreadful state of mutilation as a result of inheritances of this sort. -Thus, while the Lamarckians try to show that, on their principle, -characters for the good of the species may be acquired, they must also -be prepared, if they accept this kind of evidence, to grant that immense -harm may also result from its action. I do not urge this as an argument -against the theory itself, but point it out simply as one of the -consequences of the theory. - -It has been shown quite recently, by Charrin, Delamare, and Moussu, that -when, after the operation of laparotomy on a pregnant rabbit or -guinea-pig, the kidney or the liver has become diseased, the offspring -sometimes show similar affections in the corresponding organs (kidney or -liver). The result is due, the authors think, to some substance set free -from the diseased kidney of the parent that affects the kidney of the -young in the uterus. By injecting into the blood of a pregnant animal -fresh extracts from the kidney of another animal, the authors believe -that the kidney of the young are also affected. It will be observed that -this transmission of an acquired character appears to be different from -that of transmission through the egg; for it is the developing, or -developed organ itself, that is acted upon. The results throw an -interesting light on the cases of epilepsy described by Brown-Séquard, -since they show that the diseased condition of the parent may be -transmitted to the later embryonic stages. May not, therefore, -Brown-Séquard’s results be also explained as due to direct transmission -from the organs of the parent to the similar organs of the young in the -uterus? - -There is another series of experiments of a different sort that has been -used as an argument in favor of the Lamarckian view. These are the -results that Cunningham has obtained on young flatfish. He put the very -young fish, while still bilaterally symmetrical (in which stage the -pigment is equally developed on both sides of the body) into aquaria -lighted from below. He found that when the young fish begins to undergo -its metamorphosis, the pigment gradually disappears on one side, as it -would have done under normal conditions, _i.e._ when they are lighted -from above. If, however, the fish are kept for some time longer, lighted -from below, the pigment begins to come back again. “The first fact -proves that the disappearance of the pigment-cells from the lower side -in the metamorphosis is an hereditary character, and not a change -produced in each individual by the withdrawal of the lower side from the -action of light. On the other hand, the experiments show that the -absence of pigment-cells from the lower side throughout life is due to -the fact that light does not act upon that side, for, when it is allowed -to act, pigment-cells appear. It seems to me that the only reasonable -conclusion from these facts is, that the disappearance of pigment-cells -was originally due to the absence of light, and that the change has now -become hereditary. The pigment-cells produced by the action of light on -the lower side are in all respects similar to those normally present on -the upper side of the fish. If the disappearance of the pigment-cells -were due entirely to a variation of the germ-plasm, no external -influence could cause them to reappear, and, on the other hand, if there -were no hereditary tendency, the coloration of the lower side of the -flatfish when exposed would be rapid and complete.”[20] - -Footnote 20: - - _Natural Science_, October, 1893. - -This evidence might be convincing were it not weakened by two or three -assumptions. In the first place, it is not shown that if the loss of -color on the lower side had been the result of the inheritance of an -acquired character that the results seen in Cunningham’s experiment -would follow as a consequence. Thus one of the starting-points of the -argument really begs the whole question. In the second place, it is -unproven that, had the loss of color of the lower side been the result -of a variation of the germ-plasm, no external influence could cause it -to reappear. In this connection there is another fact that has a bearing -on the point here raised. In some species of flatfish the right side is -turned down, and in other species the left. Occasionally an individual -is found in a right-sided species that is left-sided, and in such cases -the color is also reversed. Now, to explain this in the way suggested by -Cunningham, we should be obliged to assume that some of the ancestors -acquired the loss of pigment on one side of the body, and others on the -other side according to which side was turned down. This supposition -might be appealed to to give us an explanation of the occasional -reversal of the symmetry as a rare occurrence at the present time; but -the argument is so transparently improbable that, I believe, the -Lamarckian school would hesitate to make use of it, yet, in principle, -it is about the same as that Cunningham has followed above. - -If, on the other hand, we suppose the difference in color of the two -sides to have been the result of a germ-variation, we need only suppose -that this was of such a kind that the color of the under side is only in -a latent condition, and if an external factor can cause a reaction to -take place on the light side, it is not surprising that this should call -forth the latent color patterns. The result can be given at least a -formal explanation on the theory that the original change was a -germ-variation. - -We come now to the evidence derived from paleontology. A number of -evolutionists, more especially of the American school, have tried to -show that the evolution of a number of groups can best be accounted for -on the theory of the inheritance of acquired characters. A point that we -must always bear in mind is that evolution in a direct line need not -necessarily be the outcome of Lamarckian factors. Some of our leading -paleontologists, Cope, Hyatt, Scott, Osborn, have been strongly -impressed by the paleontological evidence in favor of the view that -evolution has often been in direct lines; and some, at least, of these -investigators have been led to conclude that only the Lamarckian factor -of the inheritance of acquired characters can give a sufficient -explanation of the facts. Paleontologists have been much impressed by -the fact that evolution has been along the lines which we might imagine -that it would follow if the effects of use and of disuse are inherited. -There is, however, no proof that this is the case, although there are a -number of instances to which this mode of explanation appears to give -the readiest solution. But, as has been said before, it is not this kind -of evidence that the theory is in need of, since Lamarck himself gave an -ample supply of illustrations. What we need is clear evidence that this -sort of inheritance is possible, and, from the very nature of the case, -it is just this evidence that fossil remains can never supply. - -The same criticism may be made of the work of Ryder, Packard, Dali, -Jackson, Eimer, Cunningham, Semper, De Varigny, and others of the -Lamarckian school. Despite the large number of cases that they have -collected, which appear to them to be most easily explained on the -assumption of the inheritance of acquired characters, the proof that -such inheritance is possible is not forthcoming. Why not then spend a -small part of the energy, that has been used to expound the theory, in -demonstrating that such a thing is really possible? One of the chief -virtues of the Lamarckian theory is that it is capable of experimental -verification or contradiction, and who can be expected to furnish such -proof if not the Neo-Lamarckians? - -We may fairly sum up our position in regard to the theory of the -inheritance of acquired characters in the verdict of “not proven.” I am -not sure that we should not be justified at present in claiming that the -theory is unnecessary and even improbable. - - ------------------------------------------------------------------------- - - - - - CHAPTER VIII - - CONTINUOUS AND DISCONTINUOUS VARIATION AND HEREDITY - - -The two terms _continuous_ and _discontinuous variation_ refer to the -succession or inheritance of the variations rather than to the actual -conditions amongst a group of individuals living at the same time; but -this distinction has only a subordinate value. The term _fluctuating_, -or _individual variation_, expresses more nearly the conditions of the -individuals of a species at any one time, and the continuation of this -sort of difference is the continuous variation spoken of above. The -discontinuous variations are probably of the same nature as those that -have been called mutations, and what Darwin sometimes called sports, or -single variations, or definite variations. - - - Continuous Variation - -If we examine a number of individuals of the same species, we find that -no two of them are exactly alike in all particulars. If, however, we -arrange them according to some one character, for example, according to -the height, we find that there is a gradation more or less perfect from -one end of the series to the other. Thus, if we were to take at random a -hundred men, and stand them in line arranged according to their height, -the tops of their heads, if joined, would form a nearly continuous line; -the line will, of course, incline downward from the tallest to the -shortest man. This illustrates individual variation. An arrangement of -this kind fails to bring out one of the most important facts connected -with individual differences. If the line is more carefully examined, it -will be found that somewhere near the middle the men are much more -nearly of the same height, or rather there are more men having about the -same height than there are near the ends of the line. Another -arrangement will bring this out better. If we stand in a line all the -men from 60 to 61.9 inches, and in another parallel line all those -between 62 and 63.9, then those between 64 and 65.9, then between 66 and -67.9 inches in__ height, etc., it will be found that there are more men -in some of these lines than in others. The longest line will be that -containing the men of about 65 inches; the two lines formed out of men -on each side of this one will contain somewhat fewer men, and the next -ones fewer still, and so on. If we looked at our new group of men from -above, we should have a figure triangular in outline, the so-called -frequency polygon, Figure 3 B. With a larger amount of data of this sort -it is possible to construct a curve, the curve of frequency, Figure 3 A. -In order to obtain this curve of frequency, it is of course not -necessary to actually put the individuals in line, but the curve can be -drawn on paper from the measurements. We sort out the measurements into -classes as in the case given above. The classes are laid off at regular -intervals along a base-line by placing points at definite intervals. -Perpendiculars are then erected at each point, the height of each being -proportional to the frequency with which each class occurs. If now we -join the tops of these perpendiculars, the curve of frequency is the -result. - - -[Illustration: - - Fig. 3.—Curves of frequency, etc. - A, normal curve. - B, showing the method of arranging individuals in lines containing - similar kinds of individuals. - C, curve that is skew to the right. - D, polygon of frequencies of horns of rhinoceros beetles. - (After Davenport.)] - -“In arranging the individuals it will be found, as has been said, that -certain groups contain more individuals. They will form the longest -line. This value that occurs with the greatest frequency is called the -mode. The position of this modal class in the polygon is one of the -points of importance, and the spread of the polygon at its base is -another. A polygon with a low mode and a broad range means great -variability. The range may, however, be much affected by a single -individual standing far removed from the rest, so that a polygon -containing such an individual might appear to show greater variation -than really exists. Therefore we need a measure of variability that -shall take into account the departures of all the individuals from the -mode. One such measure is the arithmetical average of all the departures -from the mean in both directions; and this measure has been widely -employed. At present another method is preferred, namely, the square -root of the squared departures. This measure is called the standard -deviation. The standard deviation is of great importance, because it is -the index of variability.”[21] - -Footnote 21: - - Davenport, C. B., “The Statistical Study of Biological Problems,” - _Popular Science Monthly_, September, 1900. - -Of the different kinds of polygons there are two main sorts, the simple -and the complex. The former have only a single mode, the latter have -more than one mode. Some simple polygons lie symmetrically on each side -of the mode, Figure 3 A; others are unsymmetrical or skew, Figure 3 B. -The skew polygon generally extends out on one side farther than on the -other. It has been suggested that when a polygon is symmetrical the -species is not changing, and when skew that the species is evolving in -the direction of the longer base. This assumes that the sort of -variation measured by these curves is of the kind of which evolution is -made up, but this is a question that we must further consider. How far -the change indicated by the skew curve may be carried is also another -point for further examination. - -A complex polygon of variation, Figure 3 D, has been sometimes -interpreted to mean that two subgroups exist in a species, as is well -shown in the case of the rhinoceros beetle described by Bateson. Two -kinds of male individuals exist, some with long horns, others with short -horns; each with a mode of its own, the two polygons overlapping. Other -complex polygons may be due to changes occurring at different times in -the life of the individual, as old age, for example. - -If, instead of examining the variations of the individuals of the race, -we study the variations in the different organs of the same individual, -we find in many cases that certain organs vary together. Thus the right -and the left leg nearly always vary in the same direction, also the -first joints of the index and middle fingers, and the stature and the -forearm. On the other hand, the length of the clavicle and that of the -humerus do not vary together to the same extent; and the breadth and -height of the skull even less so. - - - ════════════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════ - No. of Veins│ 10│ 11│ 12│ 13│ 14│ 15│ 16│ 17│ 18│ 19│ 20│ 21│ 22 - ────────────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼──── - First Tree │ —│ —│ —│ —│ —│ 1│ 4│ 7│ 9│ 4│ 1│ —│ — - Second Tree │ —│ —│ —│ 3│ 4│ 9│ 8│ 2│ —│ —│ —│ —│ — - ════════════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════ - - -We may also study those cases in which a particular organ is repeated a -number of times in the same individual, as are the leaves of a tree. If -the leaves of the same tree are examined in respect, for example, to the -number of veins that each contains, we find that the number varies, and -that the results give a variation polygon exactly like that when -different individuals are compared with one another. Let us take the -illustration given by Pearson. He counted the veins on each side of the -midrib of the leaves of the beech. If a number of leaves be collected -from one tree, and the same number from another, and if all those having -fifteen veins are put in one vertical column, and all those with sixteen -in another, as shown in the following table, it will be found that each -tree has a mode of its own. Thus in the first tree the mode is -represented by nine individuals having eighteen veins, and in the second -by nine individuals having fifteen veins. So far as this character is -concerned we might have interchanged certain of the individual leaves, -but we could not have interchanged the two series. They are _individual_ -to the two trees. Now in what does this individuality consist? Clearly -there are most leaves in one tree with eighteen ribs, and most in the -other with fifteen ribs. - -If we contrast these results with those obtained by picking at random a -large number of leaves from different beech trees, we have no longer -types of individuals, but racial characters. Pearson has given the -following table to illustrate these points: - - - Frequency of Different Types of Beech Leaves - - ════════════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════ - No. of Veins│ 10│ 11│ 12│ 13│ 14│ 15│ 16│ 17│ 18│ 19│ 20│ 21│ 22 - ────────────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼──── - Frequency │ 1│ 7│ 34│ 110│ 318│ 479│ 595│ 516│ 307│ 181│ 36│ 15│ 1 - ════════════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════ - - -Thus the mode for beech trees in general is sixteen; but, as shown in -the other table, this mode does not correspond with either of the two -individual modes here ascertained. The illustration shows that the -racial mode may differ from the individual mode. There are also cases -known in which the mode of a group of individuals living in one locality -is different from that of another group living in another locality. This -difference may be a constant one from year to year, although so slight, -that unless actual measurements are made, the difference cannot be -detected, because of the overlapping of the individuals from different -localities. If evolution took place by slow changes of this sort, it -might be possible to detect its action, even when very slow, by means of -measurements made on a large number of individuals. At least this has -been suggested by those who believe new species may result from changes -of this sort. - -There is some evidence showing that by selecting particular individuals -of a series, and breeding from them, the mode may be changed in the -direction of selection. Thus it has been stated by Davenport that the -descendants of twelve- and thirteen-rayed daisies give a polygon with a -skewness of +1.92; while the descendants of twenty-one-rayed plants give -a polygon with a skewness of -.13. - -Pearson has described very concisely the possibilities involved in the -selective action of the environment. He states that if we examine the -frequency distribution of a set of organisms that have just become -mature, and later make a similar examination on the same number of -individuals (but not the same individuals) during the period of -reproduction, we shall probably find that a change has taken place which -may have been due to selection of some sort. The same thing might be -found in the next generation, and, if it did, this would indicate that -“selection does not necessarily mean a permanent or a progressive -change.” The selection in this imaginary case would be purely periodic -and suffice only to maintain a given race under given conditions. “Each -new adolescent generation is not the product of the entire preceding -generation, but only of selected individuals. This is certainly the case -for civilized man, in which case twenty-six per cent of the married -population produce fifty per cent of the next generation.” - -Pearson believes that “if a race has been long under the same -environment it is probable that only periodic selection is at work, -maintaining its stability. Change the environment and a secular change -takes place, the deviations from the mode previously destroyed giving -the requisite material.” “Clearly periods of rapidly changing -environment, of great climatological and geological change, are likely -to be associated with most marked secular selection. To show that there -is little or no change year by year in the types of rabbit and wild -poppy in our English fields, or of daphnia in our English ponds, is to -put forward no great argument for the inefficiency of natural selection. -Take the rabbit to Australia, the wild poppy to the Cape, the daphnia -into the laboratory, and change their temperature, their food supply, -and the chemical constituents of water and air, and then the existence -of no secular selection would indeed be a valid argument against the -Darwinian theory of evolution.” In regard to the last point, it should -be noted that, even if under the changed conditions a change in the mode -took place, as Pearson assumes, it does not follow necessarily that -selection has had anything to do with it, but the environment may have -directly changed the forms. Furthermore, and this is the essential -point, even if selection does act to the extent of changing the mode, we -should not be justified in concluding that this sort of change could go -on increasing as long as the selection lasts. All that might happen -would be to keep the species up to the highest point to which -fluctuating variation can be held. This need not lead to the formation -of new species, or direct the course of evolution. - -Pearson points out further that, even if we suppose that a secular -change is produced in a new environment, we cannot explain how species -may break up into two or more races that are relatively infertile. -Suppose two groups of individuals, subjected to different environments, -become isolated geographically. Two local races will be produced. -“Isolation may account for the origin of local races, but never for the -origin of species unless it is accompanied by a differential fertility.” -In other words, Pearson thinks that, unless the reproductive organs are -correlated with other organs, in such a way that as these organs change -the interracial fertility of the germ-cells is altered, so that in the -two changed groups the individuals are no longer interfertile, new -species cannot be accounted for, since their mutual infertility is one -of their most characteristic features. “Without a barrier to -intercrossing during differentiation the origin of species seems -inexplicable.” - -We need not discuss the various suggestions that have been made to -explain this difficulty, none of which, as Pearson points out, have been -satisfactory. He himself believes that a process of segregation of like -individuals must occur, during the incipient stages at least, in the -formation of species. Afterwards a correlation may exist between the new -organs and the germ-cells, of such a sort that a relative or an absolute -sterility between the incipient species is attained. After this -condition has been reached the two new species may freely intermix -without a return to the primitive type, since they are no longer fertile -_inter se_. It seems to me, also, that this would be an essential -requisite if we assume that species are slowly formed out of races from -individual differences, as Pearson supposes to be the case. There are, -however, other possibilities that Pearson does not take into account, -namely, that from the very beginning the change may be so great that the -new form is not fertile with the original one; and there is also another -possibility as well, that, although the new and the old forms are -fertile, the hybrids may be like one or the other parent, as in several -cases to be given later. Not that I mean to say that in either of these -two ways can we really offer a solution of the question of infertility, -for, from the evidence that we possess, it appears improbable that the -infertility of species _inter se_ has been the outcome of either of -these causes. - -In support of his main thesis Pearson gives certain data in respect to -preferential mating in the human race. By this is meant that selection -of certain types of individuals is more likely to take place, and also -that the fertility of certain types of individuals is greater than that -of other types. The calculations are based on stature, color of hair, -and of eyes. The results appear to show in all cases examined that there -is a slight tendency to form new races as the result of the more -frequent selection of certain kinds of individuals. But even if this is -the case, what more do the results show than that local races may be -formed,—races having a certain mode for height, for color of eyes or of -hair? That changes of this kind can be brought about we knew already -without any elaborate measurements, yet we should not conclude from this -that new species will be formed by a continuation of the process. - -Pearson writes: “As to the problem of evolution itself we are learning -to see it under a new light. Natural selection, combined with sexual -selection [by which Pearson means segregation of certain types through -individual selection] and heredity, is actually at work changing types. -We have quantitative evidence of its effects in many directions.” Yes! -but no evidence that selection of this sort can do anything more than -keep up the type to the upper limit attained in each generation by -fluctuating variations. Pearson adds, “Variations do not occur -accidentally, or in isolated instances; autogamic and assortive mating -are realities, and the problem of the near future is not whether -Darwinism is a reality, but what is quantitively the rate at which it is -working and has worked.” This statement expresses no more than Pearson’s -conviction that the process of evolution has taken place by means of -selection. He ignores other possibilities, which if established may put -the whole question in a very different light. - - - Heredity and Continuous Variation - -It has been to a certain extent assumed in the preceding pages that both -parents are alike, or, if different, that they have an equal influence -on the offspring. This may be true in many cases for certain -characteristics. Thus a son from a tall father and a short mother may be -intermediate in height, or if the father is white and the mother black, -the children are mulattoes. But other characters rarely or never blend. -In such cases the offspring is more like one or the other parent, in -which case the inheritance is said to be exclusive. Thus if one parent -has blue eyes and the other black, some of the children may have black -eyes and others blue. There are also cases of particular inheritance -where there may be patches of color, some like the color of one parent, -some like that of the other parent. The latter two kinds of inheritance -will be more especially considered in the subsequent part of this -chapter; for the present we are here chiefly concerned with blended -characters. - -How much in such cases does each parent contribute to the offspring? -This has been expressed by Galton in his law of ancestral heredity. This -law takes into account not only the two parents, but also the four -grandparents, and the eight great-grandparents, etc. There will be 1024 -in the tenth generation. These 1024 individuals may be taken as a fair -sample of the general population, provided there has not been much -interbreeding. Are we then to look upon the individual as the fused or -blended product of the population a few generations back? If this were -true, should we not expect to find all the individuals of a community -very much alike, except for the fluctuating variations close around the -mode? - -As a result of his studies on the stature of man, and on the coat color -of the Basset hounds, Galton has shown that the inheritance from the -parents can be represented by the fraction 1/2; that is one-half of the -peculiarities of the individual comes from the two parents. The four -grandparents together count for 1/4 of the total inheritance, the -great-grandparents 1/8, and so on, giving the series 1/2, 1/4, 1/8. -Pearson, taking certain other points into consideration, believes the -following series more fully represents the inheritance from the -ancestors, .3, .15, .075, .0375, etc. He concludes that, “if Darwinism -be the true view of evolution, _i.e._ if we are to describe evolution by -natural selection combined with heredity, then the law which gives us -definitely and concisely the type of the offspring in terms of the -ancestral peculiarities is at once the foundation stone of biology and -the basis upon which heredity becomes an exact branch of science.” - -The preceding statements give some idea of what would occur in a -community in which no selection was taking place. The results will be -quite different, although the same general law of inheritance will hold, -if selection takes place in each generation. If, for instance, selection -takes place, the offspring after four generations will have .93 of the -selected character, and without further selection will not regress, but -breed true to this type.[22] “After six generations of selection the -offspring will, selection being suspended, breed true to under two per -cent divergence from the previously selected type.” - -Footnote 22: - - In this statement the earlier ancestors are assumed to be identical - with the general type of the population. - -If, however, we do not assume that the ancestors were mediocre, it is -found that after six generations of selection the offspring will breed -true to the selected type within one per cent of its value. Thus, if -selection were to act on a race of men having a mode of 5 feet 9 inches, -and the 6-foot men were selected in each generation, then in six -generations this type would be permanently established, and this change -could be effected in two hundred years.[23] - -Footnote 23: - - Quoted from Pearson’s “Grammar of Science.” - -Thus we have exact data as to what will happen on the average when -blended, fluctuating variations are selected. Important as such data -must always be to give us accurate information as to what will occur if -things are left to “chance” variations, yet if it should prove true that -evolution has not been the outcome of chance, then the method is -entirely useless to determine how evolution has occurred. - -More important than a knowledge of what, according to the theory of -chances, fluctuating variations will do, will be information that would -tell us what changes will take place in each individual. In this field -we may hope to obtain data no less quantitative than those of chance -variations, but of a different kind. A study of some of the results of -discontinuous variation will show my meaning more clearly. - - - Discontinuous Variation - -Galton, in his book on “Natural Inheritance,” points out that “the -theory of natural selection might dispense with a restriction for which -it is difficult to see either the need or the justification, namely, -that the course of evolution always proceeds by steps that are severally -minute and that become effective only through accumulation.” An apparent -reason, it is suggested, for this common belief “is founded on the fact -that whenever search is made for intermediate forms between widely -divergent varieties, whether they are of plants or of animals, of -weapons or utensils, of customs, religion, or language, or of any other -product of evolution, a long and orderly series can usually be made out, -each member of which differs in an almost imperceptible degree from the -adjacent specimens. But it does not at all follow because these -intermediate forms have been found to exist, that they were the very -stages that were passed through in the course of evolution. Counter -evidence exists in abundance, not only of the appearance of considerable -sports, but of their remarkable stability in hereditary transmission.” -Comparing such an apparently continuous series with machines, Galton -concludes, “If, however, all the variations of any machine that had ever -been invented were selected and arranged in a museum, each would differ -so little from its neighbors as to suggest the fallacious inference that -the successive inventions of that machine had progressed by means of a -very large number of hardly discernible steps.” - -Bateson, also, in his “Materials for the Study of Variation,” speaks of -the two possible ways in which variations may arise. He points out that -it has been tacitly assumed that the transitions have been continuous, -and that this assumption has introduced many gratuitous difficulties. -Chief of these is the difficulty that in their initial and imperfect -stages many variations would be useless. “Of the objections that have -been brought against the Theory of Natural Selection, this is by far the -most serious.” He continues: “The same objection may be expressed in a -form which is more correct and comprehensive. We have seen that the -differences between species on the whole are Specific, and are -differences of kind forming a discontinuous Series, while the -diversities of environment to which they are subject are, on the whole, -differences of degree, and form a continuous Series; it is, therefore, -hard to see how the environmental differences can thus be made in any -sense the directing cause of Specific differences, which by the Theory -of Natural Selection they should be. This objection of course includes -that of the utility of minimal Variations.” - -“Now the strength of this objection lies wholly in the supposed -continuity of the process of Variation. We see all organized nature -arranged in a discontinuous series of groups differing from each other -by differences which are Specific; on the other hand, we see the diverse -environments to which these forms are subject passing insensibly into -each other. We must admit, then, that if the steps by which the diverse -forms of life have varied from each other have been insensible,—if, in -fact, the forms ever made up a continuous series,—these forms cannot -have been broken into a discontinuous series of groups by a continuous -environment, whether acting directly as Lamarck would have, or as -selective agent as Darwin would have. This supposition has been -generally made and admitted, but in the absence of evidence as to -Variation it is nevertheless a gratuitous assumption, and, as a matter -of fact, when the evidence as to Variation is studied, it will be found -to be in a great measure unfounded.” - -There is a fair number of cases on record in which discontinuous -variations have been seen to take place. Darwin himself has given a -number of excellent examples, and Bateson, in the volume referred to -above, has brought together a large and valuable collection of facts of -this kind. - -Some of the most remarkable of these instances have been already -referred to and need only be mentioned here. The black-shouldered -peacock, the ancon ram, the turnspit dog, the merino sheep, tailless and -hornless animals, are all cases in point. In several of these it has -been discovered that the young inherit the peculiarities of their -parents if the new variations are bred together; and what is more -striking, if the new variation is crossed with the parent form, the -young are like one or the other parent, and not intermediate in -character. This latter point raises a question of fundamental importance -in connection with the origin of species. - -Darwin states that he knows of _no cases in which, when different -species or even strongly marked varieties are crossed, the hybrids are -like one form or the other_. They show, he believes, always a blending -of the peculiarities of the two parents. He then makes the following -significant statement: “All the characters above enumerated which are -transmitted in a perfect state to some of the offspring and not to -others—such as distinct colors, nakedness of skin, smoothness of leaves, -absence of horns or tail, additional toes, pelorism, dwarfed structure, -etc., have all been known to appear suddenly in individual animals or -plants. From this fact, and from the several slight, aggregated -differences which distinguish domestic races and species from each -other, not being liable to this peculiar form of transmission, we may -conclude that it is in some way connected with the sudden appearance of -the characters in question.” - -Darwin has, incidentally, raised here a question of the most -far-reaching import. If it should prove true, as he believes, that -inheritance of this kind of discontinuous variation is also -discontinuous, and that we do not get the same result when distinct -species are intercrossed, or even when well-marked domestic races are -interbred, then he has, indeed, placed a great obstacle in the path of -those who have tried to show that new species have arisen through -discontinuous variation of this sort. - -If wild species, when crossed, give almost invariably intermediate -forms, then it may appear that we are going against the only evidence -that we can hope to obtain if we claim that discontinuous variation, of -the kind that sports are made of, has supplied the material for -evolution. If, furthermore, when distinct races of domesticated animals -are crossed, we do not get discontinuous inheritance, it might, perhaps, -with justness be claimed that this instance is paralleled by what takes -place when wild species are crossed. And if domesticated forms have been -largely the result of the selection of fluctuating variations, as Darwin -believes, then a strong case is apparently made out in favor of Darwin’s -view that continuous variation has given the material for the process of -evolution in nature. Whether selection or some other factor has directed -the formation of the new species would not, of course, be shown, nor -would it make any difference in the present connection. - -Before we attempt to reach a conclusion on this point let us analyze the -facts somewhat more closely. - -In the first place, a number of these cases of discontinuous variation -are of the nature of abnormalities. The appearance of extra fingers or -toes in man and other mammals is an example of this sort. This -abnormality is, if inherited at all, inherited completely; that is, if -present the extra digit is perfect, and never appears in an intermediate -condition, even when one of the parents was without it. The most obvious -interpretation of this fact is that when the material out of which the -fingers are to develop is divided up, or separated into its component -parts, one more part than usual is laid down. Similarly, when a flower -belonging to the triradiate type gives rise to a quadriradiate form,—as -sometimes occurs,—the new variation seems to depend simply on the -material being subdivided once more than usual; perhaps because a little -more of it is present, or because it has a somewhat different shape. My -reasons for making a surmise of this sort are based on certain -experimental facts in connection with the regeneration of animals. It -has been shown in several cases that it is possible to produce more than -the normal number of parts by simply dividing the material so that each -part becomes more or less a new whole, and the total number of parts -into which the material becomes subdivided is increased. It seems not -improbable that phenomena of this sort have occurred in the course of -evolution, although it is, of course, possible that those characters -that define species do not belong to this class of variation. To take an -example. There are nine neck-vertebræ in some birds, but in the swan the -number is twenty-five. We cannot suppose that the ancestor of the swan -gradually added enough materially to make up one new vertebra and then -another, but at least one new whole vertebra was added at a time; and we -know several cases in which the number of vertebræ in the neck has -suddenly been increased by the addition of one more than normal, and the -new vertebra is perfectly formed from the first. - -In cases of this sort we can easily understand that the inheritance must -be either of one kind or the other, since intermediate conditions are -impossible, when it comes to the question of one or not one; but if one -individual had one and another six vertebræ, then it would be -theoretically possible for the hybrid to have three. - -This brings us to a question that should have been spoken of before in -regard to the inheritance of discontinuous variation. It sometimes -occurs that a variation, which appears in other respects to be -discontinuous, is inherited in a blended form. Thus the two kinds of -variation may not always be so sharply separated as one might be led to -believe. There may be two different kinds of discontinuous variation in -respect to inheritance, or there may be variations that are only to a -greater or a less extent inherited discontinuously; and it seems not -improbable that both kinds occur. - -This diversion may not appear to have brought us any nearer to the -solution of the difficulty that Darwin’s statement has emphasized, -except in so far as it may show that the lines are not so sharply drawn -as may have seemed to be the case. The solution of the difficulty is, I -believe, as follows:— - -_The discontinuity referred to by Darwin relates to cases in which only -a single step (or mutation) has been taken, and it is a question of -inheritance of one or not one. If, however, six successive steps should -be taken in the same direction, then when such a form is crossed with -the original form, the hybrid may inherit only three of the steps and -stand exactly midway between the parent forms; or it may inherit four, -or five, or three, or two steps and stand correspondingly nearer to the -one_ _or to the other parent. Thus while it may not be possible to halve -a single step (hence one-sided inheritance), yet when more than one step -has been taken the inheritance may be divided. There is every evidence -that most of the Linnæan (wild) species that Darwin refers to have -diverged from the parent form, and from each other, by a number of -successive steps; hence on crossing, the hybrid often stands somewhere -between the two parent forms. On this basis not only can we meet -Darwin’s objection, but the point of view gives an interesting insight -into the problem of inheritance and the formation of species._ - -The whole question of inheritance has assumed a new aspect; first on -account of the work of De Vries in regard to the appearance of -discontinuous variation in plants; and secondly, on account of the -remarkable discoveries of Gregor Mendel as to the laws of inheritance of -discontinuous variations. Mendel’s work, although done in 1865, was long -neglected, and its importance has only been appreciated in the last few -years. We shall take up Mendel’s work first, and then that of De Vries. - - - Mendel’s Law[24] - -Footnote 24: - - Bateson, in his book on “Mendel’s Principles of Heredity,” has given - an admirable presentation of Mendel’s results. I have relied largely - on this in my account. - -The importance of Mendel’s results and their wide application is -apparent from the results in recent years of De Vries, Correns, -Tschermak, Bateson, Castle, and others. Mendel carried out his -experiments on the pea, _Pisum sativum_. Twenty-two varieties were used, -which had been proven by experiment to be pure breeds. When crossed they -gave perfectly fertile offspring. Whether they all have the value of -varieties of a single species, or are different subspecies, or even -independent species, is of little consequence so far as Mendel’s -experiments are concerned. The flower of the pea is especially suitable -for experiments of this kind. It cannot be accidentally fertilized by -foreign pollen, because the reproductive organs are inclosed in the keel -of the flower, and, as a rule, the anthers burst and cover the stigma of -the same flower with its own pollen before the flower opens. In order to -cross-fertilize the plants it is necessary to open the young buds before -the anthers are mature and carefully remove all the anthers. Foreign -pollen may be then, or later, introduced. - -The principle involved in Mendel’s law may be first stated in a -theoretical case, from which a certain complication that appears in the -actual results may be removed. - -If _A_ represent a variety having a certain character, and _B_ another -variety in which the same character is different, let us say in color, -and if these two individuals, one of each kind, are crossed, the hybrid -may be represented by _H_. If a number of these hybrids are bred -together, their descendants will be of three kinds; some will be like -the grandparent, _A_, in regard to the special character that we are -following, some will be like the other grandparent, _B_, and others will -be like the hybrid parent, _H_. Moreover, there will be twice as many -with the character _H_, as with _A_, or with _B_. - - - A B - ↘ ↙ - H - ↙|↘ - A | B - ↙ | ↘ -A | B - H - ↙|↘ - A | B - ↙ | ↘ -A | B - H - ↙|↘ - A | B - H - - -If now we proceed to let these _A_’s breed together, it will be found -that their descendants are all _A_, forever. If the _B_’s are bred -together they produce only _B_’s. But when the _H_’s are bred together -they give rise to _H_’s, _A_’s, and _B_’s, as shown in the accompanying -diagram. In each generation, the _A_’s will also breed true, the _B_’s -true, but the _H_’s will give rise to the three kinds again, and always -in the same proportion. - -Thus it is seen that the hybrid individuals continue to give off the -pure original forms, in regard to the special character under -consideration. The numerical relation between the numbers is also a -striking fact. Its explanation is, however, quite simple, and will be -given later. - -In the actual experiment the results appear somewhat more complicated -because the hybrid cannot be distinguished from one of the original -parents, but the results really conform exactly to the imaginary case -given above. The accompanying diagram will make clearer the account that -follows. - - - A B - ↘ ↙ - ↘ ↙ - A(B) - ↙ | ↘ - ↙ | ↘ -A | B - | - A(B) - ↙ | ↘ - ↙ | ↘ -A | B - | - A(B) - ↙ | ↘ - ↙ | ↘ -A | B - | - A(B) - - -The hybrid, _A_(_B_), produced by crossing _A_ and _B_ is like _A_ so -far as the special character that we will consider is concerned. In -reality the character that _A_ stands for is only dominant, that is, it -has been inherited discontinuously, while the other character, -represented by _B_, is latent, or recessive as Mendel calls it. -Therefore, in the table, it is included in parentheses. If the hybrids, -represented by this form _A_(_B_), are bred together, there are produced -two kinds of individuals, _A_’s and _B_’s, of which there are three -times as many _A_’s as _B_’s. It has been found, however, that some of -these _A_’s are pure forms, as indicated by the _A_ on the left in our -table, while the others, as shown by their subsequent history, are -hybrids, _A_(_B_). There are also twice as many of these _A_(_B_)’s as -of the pure _A_’s (or of the _B_’s). Thus the results are really the -same as in our imaginary case, only obscured by the fact that the _A_’s -and the _A_(_B_)’s are exactly alike to us in respect to the character -chosen. We see also why there appear to be three times as many _A_’s as -_B_’s. In reality the results are 1 _A_, 2 _A_(_B_), 1 _B_. - -In subsequent generations the results are the same as in this one, the -_A_’s giving rise only to _A_, the _B_’s to _B_, and the _A_(_B_)’s -continuing to split up into the three forms, as shown in our diagram. -Mendel found the same law to hold for all the characters he examined, -including such different ones as the form of the seed, color of -seed-albumen, coloring of seed-coat, form of the ripe pods, position of -flowers, and length of stem. - -Mendel also carried out a series of experiments in which several -differentiating characters are associated. In the first experiment the -parental plants (varieties) differed in the form of the seed and in the -color of the albumen. The two characters of the seed plant are -designated by the capital letters _A_ and _B_; and of the pollen plant -by small _a_ and _b_. The hybrids will be, of course, combinations of -these, although only certain characters may dominate. Thus in the -experiments, the parents are _AB_ (seed plant) and _ab_ (pollen plant), -with the following seed characters:— - - - Seed parent {A form round Pollen parent {a form angular - _AB_ {B albumen yellow _ab_ {b albumen green - - -When these two forms were crossed the seeds appeared round and yellow -like those of the parent, _AB_, _i.e._ these two characters dominated in -the hybrid. - -The seeds were sown, and in turn yielded plants which when -self-fertilized gave four kinds of seeds (which frequently all appeared -in the same pod). Thus 556 seeds were produced by 15 plants, having the -following characters:— - - - _AB_ 315 round and yellow - _Ab_ 101 angular and yellow - _aB_ 108 round and green - _ab_ 32 angular and green - - -These figures stand almost in the relation of 9 : 3 : 3 : 1. - -These seeds were sown again in the following year and gave:— - -From the round yellow seeds:— - - _AB_ 38 round and yellow seeds - _ABb_ 65 round yellow and green seeds - _AaB_ 60 round yellow and angular yellow seeds - _AaBb_ 138 round yellow and green, angular yellow and green - seeds - -From the angular yellow seeds:— - - _aB_ 28 angular yellow seeds - _aBb_ 68 angular yellow and green seeds - -From the round green seeds:— - - _Ab_ 35 round green seeds - _Aab_ 67 round angular seeds - -From the angular green seeds:— - - _ab_ 30 angular green seeds - -Thus there were 9 different kinds of seeds produced. There had been -separated out at this time 38 individuals like the parent seed plant, -_AB_, and 30 like the parent pollen plant, _ab_. Since these had come -from similar seeds of the preceding generation they may be looked upon -as pure at this time. The forms _Ab_ and _aB_ are also constant forms -which do not subsequently vary. The remainder are still mixed or hybrid -in character. By successive self-fertilizations it is possible gradually -to separate out from these the pure types of which they are compounded. - -Without going into further detail it may be stated that the offspring of -the parent hybrids, having two pairs of differentiating characters, are -represented by the series:— - - _AB_ _Ab_ _aB_ _ab_ 2_ABb_ 2_aBb_ 2_Aab_ 2_ABa_ 2_AaBb_ - -This series is really a combination of the two series:— - - _A_ + 2_Aa_ + _a_ - _B_ + 2_Bb_ + _b_ - -Mendel even went farther, and used two parent varieties having three -differentiating characters, as follows:— - - _ABC seed parent_ _abc pollen plant_ - - { A form round { a form angular - - { B albumen yellow { b albumen green - - { C seed-coat grey { c seed-coat white - brown - -The results, as may be imagined, were quite complex, but can be -expressed by combining these series:— - - _A_ + 2_Aa_ + _a_ - _B_ + 2_Bb_ + _b_ - _C_ + 2_Cc_ + _c_ - -In regard to the two latter experiments, in which two and three -characters respectively were used, it is interesting to point out that -the form of the hybrid more nearly approaches “to that one of the -parental plants which possesses the greatest number of dominant -characters.” If, for instance, the seed plant has short stem, terminal -white flowers, and simply inflated pods; the pollen plant, on the other -hand, a long stem, violet-red flowers distributed along the stem, and -constricted pods,—then the hybrid resembles the seed parent only in the -form of the pod; in its other characters it agrees with the pollen -plant. From this we may conclude that, if two varieties differing in a -large number of characters are crossed, the hybrid might get some of its -dominant characters from one parent, and other dominant characters from -the other parent, so that, unless the individual characters themselves -were studied, it might appear that the hybrids are intermediate between -the two parents, while in reality they are only combinations of the -dominant characters of the two forms. But even this is not the whole -question. - -Mendel points out that, from knowing the characters of the two parent -forms (or varieties), one could not prophesy what the hybrid would be -like without making the actual trial. Which of the characters of the two -parent forms will be the dominant ones, and which recessive, can only be -determined by experiment. Moreover, the hybrid characters are something -peculiar to the hybrid itself, and to itself alone, and not simply the -combination of the characters of the two forms. Thus in one case a -hybrid from a tall and a short variety of pea was even taller than the -taller parent variety. Bateson lays much emphasis on this point, -believing it to be an important consideration in all questions relating -to hybridization and inheritance. - -The theoretical interpretation that Mendel has put upon his results is -so extremely simple that there can be little doubt that he has hit on -the real explanation. The results can be accounted for if we suppose -that the hybrid produces egg-cells and pollen-cells, each of which is -the bearer of only one of the alternative characters, dominant or -recessive as the case may be. If this is the case, and if on an average -there are the same number of egg-cells and pollen-cells, having one or -the other of these kinds of characters, then on a random assortment -meeting of egg-cells and pollen-cells, Mendel’s law would follow. For, -25 per cent of dominant pollen grains would meet with 25 per cent -dominant egg-cells; 25 per cent recessive pollen grains would meet with -25 per cent recessive egg-cells; while the remaining 50 per cent of each -kind would meet each other. Or, as Mendel showed by the following -scheme:— - - A A a a - | / | - | × | - | / | - A A a a - -Or more simply by this scheme:— - - A a - | /| - | × | - |/ - A a - -Mendel’s results have received confirmation by a number of more recent -workers, and while in some cases the results appear to be complicated by -other factors, yet there can remain little doubt that Mendel has -discovered one of the fundamental laws of heredity. - -It has been found that there are some cases in which the sort of -inheritance postulated by Mendel’s law does not seem to hold, and, in -fact, Mendel himself spoke of such cases. He found that some kinds of -hybrids do not break up in later generations into the parent forms. He -also points out that in cases of discontinuity the variations in each -character must be separately regarded. In most experiments in crossing, -forms are chosen which differ from each other in a multitude of -characters, some of which are continuous and others discontinuous, some -capable of blending with their contraries while others are not. The -observer in attempting to discover any regularity is confused by the -complications thus introduced. Mendel’s law could only appear in such -cases by the use of an overwhelming number of examples which are beyond -the possibilities of experiment.[25] - -Footnote 25: - - This statement is largely taken from Bateson’s book. - -Let us now examine the bearing of these discoveries on the questions of -variation which were raised in the preceding pages. It should be pointed -out, however, that it would be premature to do more than indicate, in -the most general way, the application of these conclusions. The chief -value of Mendel’s results lies in their relation to the theory of -inheritance rather than to that of evolution. - -In the first place, Mendel’s results indicate that we cannot make any -such sharp distinction as Darwin does between the results of inheritance -of discontinuous and of continuous variations. As Mendel’s results show, -it is the separate characters that must be considered in each case, and -not simply the sum total of characters. - -The more general objection that Darwin has made may appear to hold, -nevertheless. He thinks that the evolution of animals and plants cannot -rest primarily on the appearance of discontinuous variations, because -they occur rarely and would be swamped by intercrossing. If Mendel’s law -applies to such cases, that is, if a cross were made between such a -sport and the original form, the hybrid in this case, if -self-fertilized, would begin to split up into the two original forms. -But, on the other hand, it could very rarely happen that the hybrid did -fertilize its own eggs, and, unless this occurred, the hybrid, by -crossing with the parent forms in each generation, would soon lose all -its characters inherited from its “sport” ancestor. Unless, therefore, -other individuals gave rise to sports at the same time, there would be -little chance of producing new species in this way. We see then that -discontinuity in itself, unless it involved infertility with the parent -species, of which there is no evidence, cannot be made the basis for a -theory of evolution, any more than can individual differences, for the -swamping effect of intercrossing would in both cases soon obliterate the -new form. If, however, a species begins to give rise to a large number -of individuals of the same kind through a process of discontinuous -variation, then it may happen that a new form may establish itself, -either because it is adapted to live under conditions somewhat different -from the parent form, so that the dangers of intercrossing are lessened, -or because the new form may absorb the old one. It is also clear, from -what has gone before, that the new form can only cease to be fertile -with the parent form, or with its sister forms, after it has undergone -such a number of changes that it is no longer able to combine the -differences in a new individual. This result will depend both on the -kinds of the new characters, as well as the amounts of their difference. -This brings us to a consideration of the results of De Vries, who has -studied the first steps in the formation of new species in the -“mutations” of the evening primrose. - - - The Mutation Theory of De Vries - -De Vries defines the mutation theory as the conception that “the -characters of the organism are made up of elements (‘Einheiten’) that -are sharply separated from each other. These elements can be combined in -groups, and in related species the same combinations of elements recur. -Transitional forms like those that are so common in the external -features of animals and plants do not exist between the elements -themselves, any more than they do between the elements of the chemist.” - -This principle leads, De Vries says, in the domain of the descent theory -to the conception that species have arisen from each other, not -continuously, but by steps. Each new step results from a new combination -as compared with the old one, and the new forms are thereby completely -and sharply separated from the species from which they have come. The -new species is all at once there; it has arisen from the parent form -without visible preparation and without transitional steps. - -The mutation theory stands in sharp contrast to the selection theory. -The latter uses as its starting-point the common form of variability -known as individual or fluctuating variation; but according to the -mutation theory there are two kinds of variation that are entirely -different from each other. “The fluctuating variation can, as I hope to -show, not overstep the bounds of the species, even after the most -prolonged selection,—much less can this kind of variation lead to the -production of new, constant characters.” Each peculiarity of the -organism has arisen from a preceding one, not through the common form of -variation, but through a sudden change that may be quite small but is -perfectly definite. This kind of variability that produces new species, -De Vries calls mutability; the change itself he calls a mutation. The -best-known examples of mutations are those which Darwin called “single -variations” or “sports.” - -De Vries recognizes the following kinds of variation:— - -First, the polymorphic forms of the systematists. The ordinary groups -which, following Linnæus, we call species, are according to De Vries -collective groups, which are the outcome of mutations. Many such Linnæan -species include small series of related forms, and sometimes even large -numbers of such forms. These are as distinctly and completely separated -from each other as are the best species. Generally these small groups -are called varieties, or subspecies,—varieties when they are separated -by a single striking character, subspecies when they differ in the -totality of their characters, in the so-called habitus. - -These groups have already been recognized by some investigators as -elementary species, and have been given corresponding binary names. Thus -there are recognized two hundred elementary species of the form formerly -called _Draba verna_. - -When brought under cultivation these elementary species are constant in -character and transmit their peculiarities truly. They are not local -races in the sense that they are the outcome in each generation of -special external conditions. Many other Linnæan species are in this -respect like _Draba verna_, and most varieties, De Vries thinks, are -really elementary species. - -Second, the polymorphism due to intercrossing is the outcome of -different combinations of hereditary qualities. There are here, De Vries -says, two important classes of facts to be kept strictly -apart,—scientific experiment, and the results of the gardener and of the -cultivator. The experimenter chooses for crossing, species as little -variable as possible; the gardener and cultivator on the other hand -prefer to cross forms of which one at least is variable, because the -variations may be transmitted to the hybrid, and in this way a new form -be produced. - -New elementary characters arise in experiments in crossing only through -variability, not through crossing itself. - -Third, variability in the ordinary sense, that is, individual -variability, includes those differences between the individual organs -that follow Quetelet’s theory of chance. This kind of variability is -characterized by its presence at all times, in all groups of -individuals. - -De Vries recalls Galton’s apt comparison between variability and a -polyhedron which can roll from one face to another. When it comes to -rest on any particular face, it is in stable equilibrium. Small -vibrations or disturbances may make it oscillate, but it returns always -to the same face. These oscillations are like the fluctuating -variations. A greater disturbance may cause the polyhedron to roll over -on to a new face, where it comes to rest again, only showing the ever -present fluctuations around its new centre. The new position corresponds -to a mutation. It may appear from our familiarity with the great changes -that we associate with the idea of discontinuous variability, that a -mutation must also involve a considerable change. Such, however, De -Vries says, is not the case. In fact, numerous mutations are smaller -than the extremes of fluctuating variation. For example, the different -elementary species of _Draba verna_ are less different from each other -than the forms of leaves on a tree. The essential differences between -the two kinds of variation is that the mutation is constant, while the -continuous variation fluctuates back and forth. - -The following example is given by De Vries to illustrate the general -point of view in regard to varieties and species. The species _Oxalis -corniculata_ is a “collective” species that lives in New Zealand. It has -been described as having seven well-characterized varieties which do not -live together or have intermediate forms. If we knew only this group, -there would be no question that there are seven good species. But in -other countries intermediate forms exist, which exactly bridge over the -differences between the seven New Zealand forms. For this reason all the -forms have been united in a single species. - -Another example is that of the fern, _Lomaria procera_, from New -Zealand, Australia, South Africa, and South America. If the forms from -only one country be considered, they appear to be different species; but -if all the forms from the different parts of the world be taken into -account, they constitute a connected group, and are united into one -large species. - -It will be seen, therefore, that the limits of a collective species are -determined solely by the deficiencies in the genealogical tree of the -elementary species. If all the elementary species in one country were -destroyed, then the forms living in other countries that had been -previously held together because of those which have now been destroyed, -would, after the destruction, become true species. In other words: “The -Linnæan species are formed by the disappearance of other elementary -species, which at first connected all forms. This mode of origin is a -purely historical process, and can never become the subject of -experimental investigation.” Spencer’s famous expression, the “survival -of the fittest,” is incomplete, and should read the “survival of the -fittest species.” It is, therefore, not the study of Linnæan species -that has a physiological interest, but it is the study of the elementary -species of which the Linnæan species are made up, that furnishes the -all-important problem for experimental study. - -De Vries gives a critical analysis of a number of cases in which new -races have been formed under domestication. He shows very convincingly -that, whenever the result has been the outcome of the selection of -fluctuating variations, the product that is formed can only be kept to -its highest point of development by the most rigid and ever watchful -care. If selection ceases for only a few generations, the new form sinks -back at once to its original level. Many of our cultivated plants have -really arisen, not by selection of this sort, but by mutations; and -there are a number of recorded cases where the first and sudden -appearance of a new form has been observed. In such cases as these there -is no need for selection, for if left to themselves there is no return -to the original form. If, however, after a new mutation has appeared in -this way, we subject its fluctuating variations to selection, we can -keep the new form up to its most extreme limit, but can do nothing more. - -Another means, frequently employed, by which new varieties have been -formed is by bringing together different elementary species under -cultivation. For instance, there are a large number of wild elementary -species of apples, and De Vries believes that our different races of -apples owe their origin in part to these different wild forms. Crossing, -cultivation, and selection have done the rest. - -De Vries points out some of the inconsistencies of those who have -attempted to discriminate between varieties and species. The only rule -that can be adhered to is that a variety differs from a species to which -it belongs in only one or in a few characters. Most so-called varieties -in nature are really elementary species, which differ from their nearest -relatives, not in one character only, but in nearly all their -characters. There is no ground, De Vries states, for believing them to -be varieties. If it is found inconvenient to rank them under the names -of the old Linnæan species, it will be better, perhaps, to treat them as -subspecies, but De Vries prefers to call them elementary species. - -In regard to the distribution of species in nature, it may be generally -stated that the larger the geographical domain so much the larger is the -number of elementary species. They are found to be heaped up in the -centre of their area of distribution, but are more scattered at the -periphery. - -In any one locality each Linnæan species has as a rule only one or a few -elementary species. The larger the area the more numerous the forms. -From France alone Jordan had brought together in his garden 50 -elementary species of _Draba verna_. From England, Italy, and Austria -there could be added 150 more. This polymorphism is, De Vries thinks, a -general phenomenon, although the number of forms is seldom so great as -in this case. - -Amongst animals this great variety of forms is not often met with, yet -amongst the mammalia and birds of North America there are many cases of -local forms or races, some of which at least are probably mutations. -This can only be proven, however, by actually transferring the forms to -new localities in order to find out if they retain their original -characters, or become changed into another form. It seems not improbable -that many of the forms are not the outcome of the external conditions -under which the animal now lives, but would perpetuate themselves in a -new environment. - -From the evidence that his results have given, De Vries believes it is -probable that mutation has occurred in all directions. In the same way -that Darwin supposed that individual or fluctuating variations are -scattering, so also De Vries believes that the new forms that arise -through mutation are scattering. On this point it seems to me that De -Vries may be too much prejudiced by his results with the evening -primrose. If, as he supposes, many forms, generally ranked as varieties, -are really elementary species, it seems more probable that the mutation -of a form may often be limited to the production of one or of only a -very few new forms. The single variations, or sports, point even more -strongly in favor of this interpretation. Moreover, the general problem -of evolution from a purely theoretical point of view is very much -simplified, if we assume that the kinds of mutating forms may often be -very limited, and that mutations may often continue to occur in a direct -line. On this last point, De Vries argues that the evidence from -paleontology cannot be trusted, for all that we can conclude from fossil -remains is that certain mutations have dominated, and have been -sufficiently abundant to leave a record. In other words, the conditions -may have been such that only certain forms could find a foothold. - -De Vries asks whether there are for each species periods of mutation -when many and great changes take place, and periods when relatively -little change occurs. The evidence upon which to form an opinion is -scanty, but De Vries is inclined to think that such periods do occur. It -is at least certain from our experience that there are long periods when -we do not see new forms arising, while at other times, although we know -very few of them, epidemics of change may take place. The mutative -period which De Vries found in the evening primrose is the best-known -example of such a period of active mutation. Equally important for the -descent theory is the idea that the same mutation may appear time after -time. There is good evidence to show that this really occurs, and in -consequence the chances for the perpetuation of such a form are greatly -increased. Delbœuf, who advocated this idea of the repeated reappearance -of a new form, has also attempted to show that if this occurs the new -form may become established without selection of any kind taking -place,—the time required depending upon the frequency with which the new -form appears. This law of Delbœuf, De Vries believes, is correct from -the point of view of the mutation theory. It explains, in a very simple -way, the existence of numerous species-characters that are entirely -useless, such, for instance, as exist between the different elementary -species of _Draba verna_. “According to the selection theory only useful -characters can survive; according to the mutation theory, useless -characters also may survive, and even those that may be hurtful to a -small degree.” - -We may now proceed to examine the evidence from which De Vries has been -led to the general conclusions given in the preceding pages. De Vries -found at Hilversam, near Amsterdam, a locality where a number of plants -of the evening primrose, _Œnothera lamarckiana_, grow in large numbers. -This plant is an American form that has been imported into Europe. It -often escapes from cultivation, as is the case at Hilversam, where for -ten years it had been growing wild. Its rapid increase in numbers in the -course of a few years may be one of the causes that has led to the -appearance of a mutation period. The escaped plants showed fluctuating -variations in nearly all of their organs. They also had produced a -number of abnormal forms. Some of the plants came to maturity in one -year, others in two, or in rare cases in three, years. - -A year after the first finding of these plants De Vries observed two -well-characterized forms, which he at once recognized as new elementary -species. One of these was _O. brevistylis_, which occurred only as -female plants. The other new species was a smooth-leafed form with a -more beautiful foliage than _O. lamarckiana_. This is _O. lævifola_. It -was found that both of these new forms bred true from self-fertilized -seeds. At first only a few specimens were found, each form in a -particular part of the field, which looks as though each might have come -from the seeds of a single plant. - - - ŒNOTHERA LAMARCKIANA - - Elementary Species - - ===============+======+======+======+======+========+========+====+======== - GENERATION |GIGAS |ALBIDA|OBLON-|RUBRI-|LAMARCK-|NANNELLA|LATA|SCINTIL- - | | |GATA |NERVIS| IANA | | | LANS - ===============+======+======+======+======+========+========+====+======== - |8 Gener. | - VIII | 1899 | 5 1 0 1700 21 1 - |annual | ‘———————————v—————————’ - | | - |7 Gener. | - VII | 1898 | 9 0 3000 11 - |annual | ‘———————v——————’ - | | - |6 Gener. | - VI | 1897 | 11 29 3 1800 9 5 1 - |annual | ‘———————————v—————————————’ - | | - |5 Gener. | - V | 1896 | 25 135 20 8000 49 142 6 - |annual | ‘———————————v—————————————’ - | | - |4 Gener. | - IV | 1895 | 1 15 176 8 14000 60 73 1 - |annual | ‘—————————————v—————————————’ - | | - |3 Gener. | - III | 1890-91 | 1 10000 3 3 - |biennial | ‘———v——————————’ - | | - |2 Gener. | - II | 1888-89 | 15000 5 5 - |biennial | ‘—v——————————’ - | | - |1 Gener. | - I | 1886-87 | 9 - |biennial | - =====+=========+=========================================================== - - -These two new forms, as well as the common _O. lamarckiana_, were -collected, and from these plants there have arisen the three groups or -families of elementary species that De Vries has studied. In his garden -other new forms also arose from those that had been brought under -cultivation. The largest group and the most important one is that from -the original _O. lamarckiana_ form. The accompanying table shows the -mutations that arose between 1887 and 1899 from these plants. The seeds -were selected in each case from self-fertilized plants of the -_lamarckiana_ form, so that the new plants appearing in each horizontal -line are the descendants in each generation of _lamarckiana_ parents. It -will be observed that the species, _O. oblongata_, appeared again and -again in considerable numbers, and the same is true for several of the -other forms also. Only the two species, _O. gigas_ and _O. scintillans_, -appeared very rarely. - -Thus De Vries had, in his seven generations, about fifty thousand -plants, and about eight hundred of these were mutations. When the -flowers of the new forms were artificially fertilized with pollen from -the flowers on the same plant, or of the same kind of plant, they gave -rise to forms like themselves, thus showing that they are true -elementary species.[26] It is also a point of some interest to observe -that all these forms differed from each other in a large number of -particulars. - -Footnote 26: - - _O. lata_ is always female, and cannot, therefore, be self-fertilized. - When crossed with _O. lamarckiana_ there is produced fifteen to twenty - per cent of pure _lata_ individuals. - -Only one form, _O. scintillans_, that appeared eight times, is not -constant as are the other species. When self-fertilized its seeds -produce always three other forms, _O. scintillans_, _O. oblongata_, and -_O. lamarckiana_. It differs in this respect from all the other -elementary species, which mutate not more than once in ten thousand -individuals. - -From the seeds of one of the new forms, _O. lævifolia_, collected in the -field, plants were reared, some of which were _O. lamarckiana_ and -others _O. lævifolia_. They were allowed to grow together, and their -descendants gave rise to the same forms found in the _lamarckiana_ -family, described above, namely, _O. lata_, _elliptica_, _nannella_, -_rubrinervis_, and also two new species, _O. spatulata_ and -_leptocarpa_. - -In the _lata_ family, only female flowers are produced, and, therefore, -in order to obtain seeds they were fertilized with pollen from other -species. Here also appeared some of the new species, already mentioned, -namely, _albida_, _nannella_, _lata_, _oblongata_, _rubrinervis_, and -also two new species, _elliptica_ and _subovata_. - -De Vries also watched the field from which the original forms were -obtained, and found there many of the new species that appeared under -cultivation. These were found, however, only as weak young plants that -rarely flowered. Five of the new forms were seen either in the Hilversam -field, or else raised from seeds that had been collected there. These -facts show that the new species are not due to cultivation, and that -they arise year after year from the seeds of the parent form, _O. -lamarckiana_. - - - Conclusions - -From the evidence given in the preceding pages it appears that the line -between fluctuating variations and mutations may be sharply drawn. If we -assume that mutations have furnished the material for the process of -evolution, the whole problem appears in a different light from that in -which it was placed by Darwin when he assumed that the fluctuating -variations are the kind which give the material for evolution. - -From the point of view of the mutation theory, species are no longer -looked upon as having been slowly built up through the selection of -individual variations, but the elementary species, at least, appear at a -single advance, and fully formed. This need not necessarily mean that -great changes have suddenly taken place, and in this respect the -mutation theory is in accord with Darwin’s view that _extreme_ forms -that rarely appear, “sports,” have not furnished the material for the -process of evolution. - -As De Vries has pointed out, each mutation may be different from the -parent form in only a slight degree for each point, although all the -points may be different. The most unique feature of these mutations is -the constancy with which the new form is inherited. It is this fact, not -previously fully appreciated, that De Vries’s work has brought -prominently into the foreground. There is another point of great -interest in this connection. Many of the groups that Darwin recognized -as varieties correspond to the elementary species of De Vries. These -varieties, Darwin thought, are the first stages in the formations of -species, and, in fact, cannot be separated from species in most cases. -The main difference between the selection theory and the mutation theory -is that the one supposes these varieties to arise through selection of -individual variations, the other supposes that they have arisen -spontaneously and at once from the original form. The development of -these varieties into new species is again supposed, on the Darwinian -theory, to be the result of further selection, on the mutation theory, -the result of the appearance of new mutations. - -In consequence of this difference in the two theories, it will not be -difficult to show that the mutation theory escapes some of the gravest -difficulties that the Darwinian theory has encountered. Some of the -advantages of the mutation theory may be briefly mentioned here. - -1. Since the mutations appear fully formed from the beginning, there is -no difficulty in accounting for the incipient stages in the development -of an organ, and since the organ may persist, even when it has no value -to the race, it may become further developed by later mutations and may -come to have finally an important relation to the life of the -individual. - -2. The new mutations may appear in large numbers, and of the different -kinds those will persist that can get a foothold. On account of the -large number of times that the same mutations appear, the danger of -becoming swamped through crossing with the original form will be -lessened in proportion to the number of new individuals that arise. - -3. If the time of reaching maturity in the new form is different from -that in the parent forms, then the new species will be kept from -crossing with the parent form, and since this new character will be -present from the beginning, the new form will have much better chances -of surviving than if a difference in time of reaching maturity had to be -gradually acquired. - -4. The new species that appear may be in some cases already adapted to -live, in a different environment from that occupied by the parent form; -and if so, it will be isolated from the beginning, which will be an -advantage in avoiding the bad effects of intercrossing. - -5. It is well known that the differences between related species -consists largely in differences of unimportant organs, and this is in -harmony with the mutation theory, but one of the real difficulties of -the selection theory. - -6. Useless or even slightly injurious characters may appear as -mutations, and if they do not seriously affect the perpetuation of the -race, they may persist. - -In Chapters X and XI, an attempt will be made to point out in detail the -advantages which the mutation theory has over the Darwinian theory. - - ------------------------------------------------------------------------- - - - - - CHAPTER IX - - EVOLUTION AS THE RESULT OF EXTERNAL AND INTERNAL FACTORS - - -We come now to a consideration of other theories that have been advanced -to account for the evolution of new forms; and in so far as these new -forms are adapted to their environment, the theories will bear directly -on the question of the origin of adaptive variations. One school of -transformationists has made the external world and the changes taking -place in it the source of new variations. Another school believes that -the changes arise within the organism itself. We may examine these two -points of view in turn. - - - The Effect of External Influences - -We have already seen that Lamarck held as a part of his doctrine of -transformation that the changes in the external world, the environment, -bring about, directly, changes in the organism, and he believed that all -plants and many of the lower animals have evolved as the result of a -reaction of this sort. This idea did not originate with Lamarck, -however, since before him Buffon had advanced the same hypothesis, and -there cannot be much doubt that Lamarck borrowed from his patron, -Buffon, this part of his theory of evolution. - -This idea of the influence of the external world as a factor inducing -changes in the organism has come, however, to be associated especially -with the name of Geoffroy Saint-Hilaire, whose period of activity, -although overlapping, came after that of Lamarck. The central idea of -Geoffroy’s view was that species of animals and plants undergo change as -the environment changes; and it is important to note, in passing, that -he did not suppose that these changes were always for the benefit of the -individual, _i.e._ they were not always adaptive. If they were not, the -forms became extinct. So long as the conditions remain constant, the -species remains constant; and he found an answer in this to Cuvier’s -argument, in respect to the similarity between the animals living at -present in Egypt and those discovered embalmed along with mummies at -least two thousand years old. Geoffroy Saint-Hilaire said, that since -the climatic conditions of Egypt had remained exactly the same during -all these years, the animals of Egypt would also have remained -unchanged. - -Geoffroy’s views were largely influenced by his studies in systematic -zoology and by his conception of a unity of plan running through the -entire animal kingdom. His study of embryology and paleontology had led -him to believe that present forms have descended from other organisms -living in the past, and in this connection his discovery of teeth in the -jaws of the embryo of the baleen whale and also his discovery of the -embryonic dental ridges in the upper and in the lower jaws of birds, -were used with effect in supporting the theory of change or evolution. -Lastly, his remarkable work in the study of abnormal forms prepared the -way for his conception of sudden and great changes, which he believed -organisms capable of undergoing. He went so far in fact, in one -instance, as to suppose that it was not impossible that a bird might -have issued fully equipped from the egg of a crocodile. Such an extreme -statement, which seems to us nowadays only laughable, need not prejudice -us against the more moderate parts of his speculation. - -His study of the fossil gavials found near Caen led him to believe that -they are quite distinct from living crocodiles. He asked whether these -old forms may not represent a link in the chain that connects, without -interruption, the older inhabitants of the earth with animals living at -the present time. Without positively affirming that this is the case, he -did not hesitate to state that a transformation of this sort seemed -possible to him. He said: “I think that the process of respiration -constitutes an acquirement so important in the ‘disposition’ of the -forms of animals, that it is not at all necessary to suppose that the -surrounding respiratory gases become modified quickly and in large -amount in order that the animal may become slowly modified. The -prolonged action of time would ordinarily suffice, but if combined with -a cataclysm, the result would be so much the better.” - -He supposed that in the course of time respiration becomes difficult and -finally impossible as far as certain systems of organs are concerned. -The necessity then arises and creates another arrangement, perfecting or -altering the existing structures. Modifications, fortunate or fatal, are -created which through propagation are continued, and which, if -fortunate, influence all the rest of the organization. But if the -modifications are injurious to the animals in which they have appeared, -the animals cease to exist, and are replaced by others having a -different form, and one suited to the new circumstances. - -The comparison between the stages of development of the individual and -the evolution of the species was strongly impressed on the mind of -Geoffroy. He says: “We see, each year, the spectacle of the -transformation in organization from one class into another. A batrachian -is at first a fish under the name of a tadpole, then a reptile -(amphibian) under that of a frog.” “The development, or the result of -the transformation, is brought about by the combined action of light and -of oxygen; and the change in the body of the animal takes place by the -production of new blood-vessels, whose development follows the law of -the balancing of organs, in the sense, that if the circulating fluids -precipitate themselves into new channels there remains less in the old -vessels.” By preventing tadpoles from leaving the water, Geoffroy claims -that it has been shown that they can be prevented from changing into -frogs. The main point that Geoffroy attempts to establish is no doubt -fairly clear, but the way in which he supposes the change to be effected -is not so clear, and his ideas as to the way in which new change may be -perpetuated in the next generation are, from our more modern point of -view, extremely hazy. It is perhaps not altogether fair to judge his -view from the standpoint of the origin of adaptive structures, but -rather as an attempt to explain the causes that have brought about the -evolution of the organic world. - -During the remainder of the nineteenth century there accumulated a large -number of facts in relation to the action of the external conditions in -bringing about changes in animals and plants. Much of this evidence is -of importance in dealing with the question of the origin of organic -adaptation. - -The first class of facts in this connection is that of geographical -variation in animals and plants. It will be impossible here to do more -than select some of the most important cases. De Varigny, in his book on -“Experimental Evolution,” has brought together a large number of facts -of this kind, and from his account the following illustrations have been -selected. He says: “When the small brown honey-bee from High Burgundy is -transported into Bresse—although not very distant—it soon becomes larger -and assumes a yellow color; this happens even in the second generation.” -It is also pointed out that the roots of the beet, carrot, and radish -are colorless in their wild natural state, but when brought under -cultivation they become red, yellow, etc. Vilmorin has noted that the -red, yellow, and violet colors of carrots appear only some time after -the wild forms have been brought under cultivation. Moquin-Tandon has -seen “gentians which are blue in valleys become white on mountains.” -Other cases also are on record in which the colors of a plant are -dependent on external conditions. - -The sizes of plants and animals are also often directly traceable to -certain external conditions; the change is generally connected with the -amount of food obtainable. “Generally speaking,” De Varigny says, -“insular animals are smaller than their continental congeners. In the -Canary Islands the oxen of one of the smallest islands are smaller than -those on the others, although all belong to the same breed, and the -horses are also smaller, and the indigenous inhabitants are in the same -case, although belonging to a tall race. It would seem that in Malta -elephants were very small,—fossil elephants, of course,—and that during -the Roman period the island was noted for a dwarf breed of dogs, which -was named after its birthplace, according to Strabo. In Corsica, also, -horses and oxen are very small, and _Cervus corsicanus_, the indigenous -deer, is quite reduced in dimensions; ... and lastly, the small -dimensions of the Falkland horses—imported from Spain in 1764—are -familiar to all. The dwarf rabbits of Porto Santo described by Darwin -may also be cited as a case in point.” - -These facts, interesting as they are, will, no doubt, have to be more -carefully examined before the evidence can have great value, for it is -not clear what factor or factors have produced the decrease in size of -these animals. - -The following cases show more clearly the immediate effect of the -environment: “Many animals, when transferred to warm climates, lose -their wool, or their hairy covering is much reduced. In some parts of -the warmer regions of the earth, sheep have no wool, but merely hairs -like those of dogs. Similarly, as Roulin notices, poultry have, in -Columbia, lost their feathers, and while the young are at first covered -with a black and delicate down, they lose it in great part as they grow, -and the adult fowls nearly realize Plato’s realistic description of -man—a biped without feathers. Conversely, many animals when transferred -from warm to cold climates acquire a thicker covering; dogs and horses, -for instance, becoming covered with wool.” - -A number of kinds of snails that were supposed to belong to different -species have been found, on further examination, to be only varieties -due to the environment. “Locard has discovered through experiments that -_L. turgida_ and _elophila_ are mere varieties—due to environment—of the -common _Lymnæa stagnalis_.” He says, “These are not new species, but -merely common aspects of a common type, which is capable of modification -and of adaptation according to the nature of the media in which it has -to live.” It has also been shown by Bateson that similar changes occur -in _Cardium edule_, and other lamellibranchs are known to vary according -to the nature of the water in which they live. - -In regard to plants, the influence of the environment has long been -known to produce an effect on the form, color, etc., of the individuals. -“The common dandelion (_Taraxacum densleonis_) has in dry soil leaves -which are much more irregular and incised, while they are hardly dentate -in marshy stations, where it is called _Taraxacum palustre_. - -“Individuals growing near the seashore differ markedly from those -growing far inland. Similarly, species such as some Ranunculi, which can -live under water as well as in air, exhibit marked differences when -considered in their different stations, as is well known to all. These -differences may be important enough to induce botanists to believe in -the existence of two different species when there is only one.” - -An interesting case is that of _Daphnia rectirostris_, a small -crustacean living sometimes in fresh water, at other times in water -containing salt and also in salt lakes. There are two forms, -corresponding to the conditions under which they live, and it is said -that the differences are of a kind that suffice to separate species from -each other. In another crustacean, _Branchipus ferox_, the form differs -in a number of points, according to whether it lives in salt or in fresh -water. Schmankewitsch says that, had he not found all transitional -forms, and observed the transformation in cultures, he would have -regarded the two forms as separate species. The oft-quoted case of -Artemia furnishes a very striking example of the influence of the -environment. _Artemia salina_ lives in water whose concentration varies -between 5 and 12 degrees of saltness. When the amount of salt is -increased to 12 degrees, the animal shows certain characteristics like -those of _Artemia milhausenii_, which may live in water having 24 to 25 -degrees of saltness. The form _A. salina_ may be further completely -changed into that of _A. milhausenii_ by increasing the amount of salt -to the latter amount. - -Among domesticated animals and plants—a few instances of which have been -already referred to—we find a large number of cases in which a change in -the environment produces definite changes in the organism. Darwin has -made a most valuable collection of facts of this kind in his “Animals -and Plants under Domestication.” He believes that domesticated forms are -much more variable than wild ones, and that this is due, in part, to -their being protected from competition, and to their having been removed -from their natural conditions and even from their native country. “In -conformity with this, all our domesticated productions without exception -vary far more than natural species. The hive-bee, which feeds itself, -and follows in most respects its natural habits of life, is the least -variable of all domesticated animals.... Hardly a single plant can be -named, which has long been cultivated and propagated by seed, that is -not highly variable.” “Bud-variation ... shows us that variability may -be quite independent of seminal reproduction, and likewise of reversion -to long-lost ancestral characters. No one will maintain that the sudden -appearance of a moss-rose on a Provence rose is a return to a former -state, ... nor can the appearance of nectarines on peach trees be -accounted for on the principle of reversion.” It is said that -bud-variations are also much more frequent on cultivated than on wild -plants. - -Darwin adds: “These general considerations alone render it probable that -variability of every kind is directly or indirectly caused by changed -conditions of life. Or to put the case under another point of view, if -it were possible to expose all the individuals of a species during many -generations to absolutely uniform conditions of life, there would be no -variability.” - -In some cases it has been observed that, in passing from one part of a -continent to another, many or all of the forms of the same group and -even of different groups change in the same way. Allen’s account of the -variations in North American birds and mammals furnishes a number of -striking examples of this kind of change. He finds that, as a rule, the -birds and mammals of North America increase in size from the south -northward. This is true, not only for the individuals of the same -species, but generally the largest species of each genus are in the -north. There are some exceptions, however, in which the increase in size -is in the opposite direction. The explanation of this is that the -largest individuals are almost invariably found in the region where the -group to which the species belongs receives its greatest numerical -development. This Allen interprets as the hypothetical “centre of -distribution of the species,” which is in most cases doubtless also its -original centre of dispersal. If the species has arisen in the north, -then the northern forms are the largest; but if it arose in the south, -the reverse is the case. Thus, most of the species of North America that -live north of Mexico are supposed to have had a northern origin, as -shown by the circumpolar distribution of some of them and by the -relationship of others to Old World species; and in these the largest -individuals of the species of a genus are northern. Conversely, in the -exceptional cases of increase in size toward the south, it can be shown -that the forms have probably had a southern origin. - -The Canidæ (wolves and foxes) have their largest representatives, the -world over, in the north. “In North America the family is represented by -six species, the smallest of which (speaking generally) are southern and -the largest northern.” The three species that have the widest ranges -(the gray wolf, the common fox, and the gray fox) show the most marked -differences in size. The skull, for instance, of “the common wolf is -fully one-fifth larger in the northern parts of British America and -Alaska than it is in northern Mexico, where it finds the southern limit -of its habitat. Between the largest northern skull and the largest -southern skull there is a difference of about thirty-five per cent of -the mean size. Specimens from the intermediate region show a gradual -intergradation between the extremes, although many of the examples from -the upper Missouri country are nearly as large as those from the extreme -north.” The common fox is about one-tenth larger, on the average, in -Alaska than it is in New England. The gray fox, whose habitat extends -from Pennsylvania southward to Yucatan, has an average length of skull -of about five inches in the north, and less than four in Central -America—about ten per cent difference. - -The Felidæ, or cats, “reach their greatest development as respects both -the number and the size of the species in the intertropical regions. -This family has sent a single typical representative, the panther -(_Felis concolor_), north of Mexico, and this ranges only to about the -northern boundary of the United States. The other North American -representatives of the family are the lynxes, which in some of their -varieties range from Alaska to Mexico.” Although they vary greatly in -different localities in color and in length and texture of pelage, they -do not vary as to the size of their skulls. On the other hand the -panther (and the ocelots) greatly increases in size southward, “or -toward the metropolis of the family.” - -Other carnivora that increase in size northward are the badger, the -marten, the fisher, the wolverine, and the ermine, which are all -northern types. - -Deer are also larger in the north; in the Virginia deer the annually -deciduous antlers of immense size reach their greatest development in -the north. The northern race of flying squirrels is one-half larger than -the southern, “yet the two extremes are found to pass so gradually one -into the other, that it is hardly possible to define even a southern and -a northern geographical race.” The species ranges from the arctic -regions to Central America. - -In birds also similar relations exist, but there is less often an -increase in size northward. In species whose breeding station covers a -wide range of latitude, the northern birds are not only smaller, but -have quite different colors, as is markedly the case in the common -quail, the meadow-lark, the purple grackle, the red-winged blackbird, -the flicker, the towhee bunting, the Carolina dove, and in numerous -other species. The same difference is also quite apparent in the blue -jay, the crow, in most of the woodpeckers, in the titmice, numerous -sparrows, and several warblers and thrushes. The variation often amounts -to from ten to fifteen per cent of the average size of the species. - -Allen also states that certain parts of the animal may vary -proportionately more than the general size, there being an apparent -tendency for peripheral parts to enlarge toward the warmer regions, -_i.e._ toward the south. “In mammals which have the external ears -largely developed—as in the wolves, foxes, some of the deer, and -especially the hares—the larger size of this organ in southern as -compared with northern individuals of the same species, is often -strikingly apparent.” It is even more apparent in species inhabiting -open plains. The ears of the gray rabbit of the plains of western -Arizona are twice the size of those of the Eastern states. - -In birds the bill especially, but also the claws and tail, is larger in -the south. In passing from New England southward to Florida the bill in -slender-billed forms becomes larger, longer, more attenuated, and more -decurved; while in short-billed forms the southern individuals have -thicker and larger bills, although the birds themselves are smaller. - -The remarkable changes and gradations of color in birds in different -parts of North America are very instructive, and the important results -obtained by American ornithologists form an interesting chapter in -zoology. The evidence would convince the most sceptical of the -difficulty of distinguishing between Linnæan species. It is not -surprising to find in this connection a leading ornithologist -exclaiming, “if there really are such things as species.” The -differences here noted are mainly from east to west. We may briefly -review here a few striking cases selected from Coues’s “Key to North -American Birds.” - -The flicker, or golden-winged woodpecker (_Colaptes auratus_), has a -wide distribution in eastern North America. It is replaced in western -North America (from the Rocky Mountains to the Pacific) by _C. -mexicanus_. In the intermediate regions, Missouri and the Rocky Mountain -region, the characters of the two are blended in every conceivable -degree in different specimens. “Perhaps it is a hybrid, and perhaps it -is a transitional form, and doubtless there are no such things as -species in Nature.... In the west you will find specimens _auratus_ on -one side of the body, _mexicanus_ on the other.” There is a third form, -_C. chrysoides_, with the wings and tail as in _auratus_, and the head -as in _mexicanus_, that lives in the valley of the Colorado River, Lower -California, and southward. - -In regard to the song-sparrow (_Melospiza_), Coues writes: “The type of -the genus is the familiar and beloved song-sparrow, a bird of constant -characters in the east, but in the west is split into numerous -geographical races, some of them looking so different from typical -_fasciata_ that they have been considered as distinct species, and even -placed in other genera. This differentiation affects not only their -color, but the size, relative proportions of parts, and particularly the -shape of the bill; and it is sometimes so great, as in the case of _M. -cinerea_, that less dissimilar looking birds are commonly assigned to -different genera. Nevertheless the gradation is complete, and affected -by imperceptible degrees.... The several degrees of likeness and -unlikeness may be thrown into true relief better by some such -expressions as the following, than by formal antithetical phrases: (1) -The common eastern bird commonly modified in the interior into the -duller colored (2) _fallax_. This in the Pacific watershed, more -decidedly modified by deeper coloration,—broader black streaks in (3) -_hermanni_, with its diminutive local race (4) _samuelis_, and more -ruddy shades in (5) _guttata_ northward, increasing in intensity with -increased size in (6) _rafina_. Then the remarkable (7) _cinerea_, -insulated much further apart than any of the others. A former American -school would probably have made four ‘good species,’ (1) _fasciata_, (2) -_samuelis_, (3) _rafina_, (4) _cinerea_.” - -Somewhat similar relations are found in three other genera of finches. -Thus Passerella is “imperfectly differentiated”; Junco is represented by -one eastern species, but in the west the stock splits up into numerous -forms, “all of which intergrade with each other and with the eastern -bird. Almost all late writers have taken a hand at Junco, shuffling them -about in the vain attempt to decide which are ‘species’ and which -‘varieties.’ All are either or both, as we may elect to consider them.” -In the distribution of the genus Pipilo similar relations are found. -There is an eastern form much more distinct from the western forms than -these are from each other. - -Finally may be mentioned the curious variations in screech-owls of the -genus Scops. This owl has two strikingly different plumages—a mottled -gray and a reddish brown, which, although very distinct when fully -developed, yet “are entirely independent of age, season, or sex.” There -is an eastern form, _Scops asio_, that extends west to the Rocky -Mountains. There is a northwestern form, _S. kennicotti_, which in its -red phase is quite different from _S. asio_, but in its gray plumage is -very similar. The California form, _S. benderii_, is not known to have a -red phase, and the gray phase is quite different from that of _S. asio_, -but like the last form. The Colorado form, _S. maxwellæ_, has no red -phase, “but on the contrary the whole plumage is very pale, almost as if -bleached, the difference evident in the nestlings even.” The Texas form, -_S. maselli_, has both phases, and is very similar to _S. asio_. The -Florida form is smaller and colored like _S. asio_. The red phase is the -frequent, if not the usual, one. The flammulated form, _S. fiammula_, is -“a very _small species_, with much the general aspect of an ungrown _S. -asio_.” This is the southwestern form, easily distinguished on account -of its small size and color from the other forms. - -These examples might be greatly increased, but they will suffice, I -think, to convince one of the difficulty of giving a sharp definition to -“species.” The facts speak strongly in favor of the transmutation -theory, and show us how a species may become separated under different -conditions into a number of new forms, which would be counted as new -different species, if the intermediate forms were exterminated. - -In discussing the nature of the changes that bring about variability, -Darwin remarks: “From a remote period to the present day, under climates -and circumstances as different as it is possible to conceive, organic -beings of all kinds, when domesticated or cultivated, have varied. We -see this with the many domestic races of quadrupeds and birds belonging -to different orders, with goldfish and silkworms, with plants of many -kinds, raised in various quarters of the world. In the deserts of -northern Africa the date-palm has yielded thirty-eight varieties; in the -fertile plains of India it is notorious how many varieties of rice and -of a host of other plants exist; in a single Polynesian island, -twenty-four varieties of the breadfruit, the same number of the banana, -and twenty-two varieties of the arum, are cultivated by the natives. The -mulberry tree of India and Europe has yielded many varieties serving as -food for the silkworm; and in China sixty-three varieties of the bamboo -are used for various domestic purposes. These facts, and innumerable -others which could be added, indicate that a change of almost any kind -in the conditions of life suffices to cause variability—different -changes acting on different organisms.” - -Darwin thinks that a change in climate alone is not one of the potent -causes of variability, because the native country of a plant, where it -has been longest cultivated, is where it has oftenest given rise to the -greatest number of varieties. He thinks it also doubtful that a change -in food is an important source of variability, since the domestic pigeon -has varied more than any other species of fowl, yet the food has been -always nearly the same. This is also true for cattle and sheep, whose -food is probably much less varied in kind than in the wild species. - -Another point of interest is raised by Darwin. He thinks, as do others -also, that the influence of a change in the conditions is cumulative, in -the sense that it may not appear until the species has been subjected to -it for several generations. Darwin states that universal experience -shows that when new plants are first introduced into gardens they do not -vary, but after several generations they will begin to vary to a greater -or less extent. In a few cases, as in that of the dahlia, the zinnia, -the Swan River daisy, and the Scotch rose, it is known that the new -variations only appeared after a time. The following statement by Salter -is then quoted, “Every one knows that the chief difficulty is in -breaking through the original form and color of the species, and every -one will be on the lookout for any natural sport, either from seed or -branch; that being once obtained, however trifling the change may be, -the result depends on himself.” Jonghe is also quoted to the effect that -“there is another principle, namely, that the more a type has entered -into a state of variation, the greater is the tendency to continue doing -so, and the more it has varied from the original type, the more is it -disposed to vary still further.” Darwin also quotes with approval the -opinion of the most celebrated horticulturist of France, Vilmorin, who -maintained that “when any particular variation is desired, the first -step is to get the plant to vary in any manner whatever, and to go on -selecting the most variable individuals, even though they vary in the -wrong direction; for the fixed character of the species being once -broken, the desired variation will sooner or later appear.” - -Darwin also cites a few cases where animals have changed quite quickly -when brought under domestication. Turkeys raised from the eggs of wild -species lose their metallic tints, and become spotted with white in the -third generation. Wild ducks lose their true plumage after a few -generations. “The white collar around the neck of the mallard becomes -much broader and more irregular, and white feathers appear in the -duckling’s wings. They increase also in size of body.” In these cases it -appears that several generations were necessary in order to bring about -a marked change in the original type, but the Australian dingoes, bred -in the Zoological Gardens, produced puppies which were in the first -generation marked with white and other colors. - -The following cases from De Varigny are also very striking. The dwarf -trees from Japan, for the most part conifers, which may be a hundred -years old and not be more than three feet high, are in part the result -“of mechanical processes which prevent the spreading of the branches, -and in part of a starving process which consists in cutting most roots -and in keeping the plant in poor soil.” - -As an example of the sudden appearance of a new variation the following -case is interesting. A variety of begonia is recorded as having appeared -quite suddenly at a number of places at the same time. In another case a -narcissus which had met with adverse circumstances, and had then been -supplied with a chemical manure in some quantity, began to bear double -flowers. - -Amongst animals the following cases of the appearance of sudden -variations are pointed out by De Varigny. “In Paraguay, during the last -century (1770), a bull was born without horns, although his ancestry was -well provided with these appendages, and his progeny was also hornless, -although at first he was mated with horned cows. If the horned and the -hornless were met in fossil state, we would certainly wonder at not -finding specimens provided with semi-degenerate horns, and representing -the link between both, and if we were told that the hornless variety may -have arisen suddenly, we should not believe it and we should be wrong. -In South America also, between the sixteenth and eighteenth centuries -the niata breed of oxen sprang into life, and this breed of bulldog oxen -has thriven and become a new race. So in the San Paulo provinces of -Brazil, a new breed of oxen suddenly appeared which was provided with -truly enormous horns, the breed of franqueiros, as they are called. The -mauchamp breed of sheep owes its origin to a single lamb that was born -in 1828 from merino parents, but whose wool, instead of being curly like -that of its parents, remained quite smooth. This sudden variation is -often met with, and in France has been noticed in different herds.” - -The ancon race of sheep originated in 1791 from a ram born in -Massachusetts having short crooked legs and a long back. From this one -ram by crossing, at first with common sheep, the ancon race has been -produced. “When crossed with other breeds the offspring, with rare -exception, instead of being intermediate in character, perfectly -resemble either parent; even one of twins has resembled one parent and -the second the other.” - -Two especially remarkable cases remain to be described. These are the -Porto Santo rabbit and the japanned peacock. Darwin has given a full -account of both of these cases. “The rabbits which have become feral on -the island of Porto Santo, near Madeira, deserve a fuller account. In -1418 or 1419 J. Gonzales Zarco happened to have a female rabbit on board -which had produced young during the voyage, and he turned them all out -on the island. These animals soon increased so rapidly that they became -a nuisance, and actually caused the abandonment of the settlement. -Thirty-seven years subsequently, Cada Mosto describes them as -innumerable; nor is this surprising, as the island was not inhabited by -any beast of prey, or by any terrestrial mammal. We do not know the -character of the mother rabbit; but it was probably the common domestic -kind. The Spanish peninsula, whence Zarco sailed, is known to have -abounded with the common wild species at the most remote historical -period; and as these rabbits were taken on board for food, it is -improbable that they should have been of any peculiar breed. That the -breed was well domesticated is shown by the doe having littered during -the voyage. Mr. Wollaston, at my request, brought two of these feral -rabbits in spirits of wine; and, subsequently, Mr. W. Haywood sent home -three more specimens in brine and two alive. These seven specimens, -though caught at different periods, closely resemble each other. They -were full-grown, as shown, by the state of their bones. Although the -conditions of life in Porto Santo are evidently highly favorable to -rabbits, as proven by their extraordinarily rapid increase, yet they -differ conspicuously in their small size from the wild English -rabbit.... In color the Porto Santo rabbit differs considerably from the -common rabbit; the upper surface is redder, and is rarely interspersed -with any black or black-tipped hairs. The throat and certain parts of -the under surface, instead of being pure white, are generally gray or -leaden color. But the most remarkable difference is in the ears and -tail. I have examined many fresh English rabbits, and the large -collection of skins in the British Museum from various countries, and -all have the upper surface of the tail and the tips of the ears clothed -with blackish gray fur; and this is given in most works as one of the -specific characters of the rabbit. Now in the seven Porto Santo rabbits -the upper surface of the tail was reddish brown, and the tips of the -ears had no trace of the black edging. But here we meet with a singular -circumstance: in June, 1861, I examined two of these rabbits recently -sent to the Zoological Gardens and their tails and ears were colored as -just described; but when one of their dead bodies was sent to me in -February, 1863, the ears were plainly edged, and the upper surface of -the tail was covered with blackish gray fur, and the whole body was much -less red; so that under the English climate this individual rabbit had -recovered the proper color of its fur in rather less than four years.” - -Another striking case of sudden variation is found in the peacock. It is -all the more remarkable because this bird has hardly varied at all under -domestication, and is almost exactly like the wild species living in -India to-day. Darwin states: “There is one strange fact with respect to -the peacock, namely, the occasional appearance in England of the -‘japanned’ or ‘black-shouldered’ kind. This form has lately been named, -on the high authority of Mr. Slater, as a distinct species, viz. _Pavo -nigripennis_, which he believes will hereafter be found wild in some -country, but not in India, where it is certainly unknown. The males of -these japanned birds differ conspicuously from the common peacock in the -color of their secondary wing-feathers, scapulars, wing-coverts, and -thighs, and are, I think, more beautiful; they are rather smaller than -the common sort, and are always beaten by them in their battles, as I -hear from the Hon. A. S. G. Canning. The females are much paler-colored -than those of the common kind. Both sexes, as Mr. Canning informs me, -are white when they leave the egg, and they differ from the young of the -white variety only in having a peculiar pinkish tinge on their wings. -These japanned birds, though appearing suddenly in flocks of the common -kind, propagate their kind quite truly.” - -In two cases, in which these birds had appeared quite suddenly in flocks -of the ordinary kind, it is recorded that “though a smaller and weaker -bird, it increased to the extinction of the previously existing breed.” -Here we have certainly a remarkable case of a new species suddenly -appearing and replacing the ordinary form, although the birds are -smaller, and _are beaten in their battles_. - -Darwin has given an admirably clear statement of his opinion as to the -_causes of variability_ in the opening paragraph of his chapter dealing -with this topic in his “Animals and Plants.” Some authors, he says, -“look at variability as a necessary contingent on reproduction, and as -much an original law as growth or inheritance. Others have of late -encouraged, perhaps unintentionally, this view by speaking of -inheritance and variability as equal and antagonistic principles. Pallas -maintained, and he has had some followers, that variability depends -exclusively on the crossing of primordially distinct forms. Other -authors attribute variability to an excess of food, and with animals, to -an excess relatively to the amount of exercise taken, or again, to the -effects of a more genial climate. That these causes are all effective is -highly probable. But we must, I think, take a broader view, and conclude -that organic beings, when subjected during several generations to any -change whatever in their condition, tend to vary; the kind of variation -which ensues depending in most cases in a far higher degree on the -nature of the constitution of the being, than on the nature of the -changed conditions.” - -Most naturalists will agree, in all probability, with this conclusion of -Darwin’s. The examples cited in the preceding pages have shown that -there are several ways in which the organisms may respond to the -environment. In some cases it appears to affect all the individuals in -the same way; in other cases it appears to cause them to fluctuate in -many directions; and in still other cases, without any recognizable -change in the external conditions, new forms may suddenly appear, often -of a perfectly definite type, that depart widely from the parent form. - -For the theory of evolution it is a point of the first importance to -determine which of these modes of variation has supplied the basis for -evolution. Moreover, we are here especially concerned with the question -of how adaptive variations arise. Without attempting to decide for the -present between these different kinds of variability, let us examine -certain cases in which an immediate and adaptive response to the -environment has been described as taking place. - - -Responsive Changes in the Organism that adapt it to the New Environment - -There is some experimental evidence showing that sometimes organisms -respond directly and adaptively to certain changes in the environment. -Few as the facts are, they require very careful consideration in our -present examination. The most striking, perhaps, is the acclimatization -to different temperatures. It has been found that while few active -organisms can withstand a temperature over 45 degrees C., and that for -very many 40 degrees is a fatal point, yet, on the other hand, there are -organisms that live in certain hot springs where the temperature is very -high. Thus, to give a few examples, there are some of the lower plants, -nostocs and protococcus forms, that live in the geysers of California at -a temperature of 93 degrees C., or nearly that of boiling water. -Leptothrix is found in the Carlsbad springs, that have a temperature of -44 to 54 degrees. Oscillaria have been found in the Yellowstone Park in -water between 54 and 68 degrees, and in the hot springs in the -Philippines at 71 degrees, and on Ischia at 85 degrees, and in Iceland -at 98 degrees. - -It is probable from recent observations of Setchel that most of the -temperatures are too high, since he finds that the water at the edge of -hot springs is many degrees lower than that in the middle parts. - -The snail, _Physa acuta_, has been found in France living at a -temperature of 35 to 36 degrees; another snail, Paludina, at Abano, -Padua, at 50 degrees. Rotifers have been found at Carlsbad at 45 to 54 -degrees; Anguillidæ at Ischia at 81 degrees; _Cypris balnearia_, a -crustacean at Hammam-Meckhoutin, at 81 degrees; frogs at the baths of -“Pise” at 38 degrees. - -Now, there can be little doubt that these forms have had ancestors that -were like the other members of the group, and would have been killed had -they been put at once into water of these high temperatures, therefore -it seems highly probable that these forms have become specially adapted -to live in these warm waters. It is, therefore, interesting to find that -it has been possible to acclimatize animals experimentally to a -temperature much above that which would be fatal to them if subjected -directly to it. Dutrochet (in 1817) found that if the plant, nitella, -was put into water at 27 degrees, the currents in the protoplasm were -stopped, but soon began again. If put now into water at 34 degrees they -again stopped moving, but in a quarter of an hour began once more. If -then put into water at 40 degrees the currents again slowed down, but -began again later. - -Dallinger (in 1880) made a most remarkable series of experiments on -flagellate protozoans. He kept them in a warm oven, beginning at first -at a temperature of 16.6 degrees C. “He employed the first four months -in raising the temperature 5.5 degrees. This, however, was not -necessary, since the rise to 21 degrees can be made rapidly, but for -success in higher temperatures it is best to proceed slowly from the -beginning. When the temperature had been raised to 23 degrees, the -organisms began dying, but soon ceased, and after two months the -temperature was raised half a degree more, and eventually to 25.5 -degrees. Here the organisms began to succumb again, and it was necessary -repeatedly to lower the temperature slightly, and then to advance it to -25.5 degrees, until, after several weeks, unfavorable appearances -ceased. For eight months the temperature could not be raised from this -_stationary point_ a quarter of a degree without unfavorable -appearances. During several years, proceeding by slow stages, Dallinger -succeeded in raising the organisms up to a temperature of 70 degrees C., -at which the experiment was ended by an accident.”[27] - -Footnote 27: - - Quoted from Davenport’s “Experimental Morphology.” - -Davenport and Castle carried out a series of experiments on the egg of -the toad, in which they tried to acclimatize the eggs to a temperature -higher than normal. Recently laid eggs were used; one lot kept at a -temperature of 15 degrees C., the other at 24-25 degrees C. Both lots -developed normally. At the end of four weeks the temperature point at -which the tadpoles were killed was determined. Those reared at a -temperature of 15 degrees C. died at 41 degrees C., or below; those -reared at 24-25 degrees C. sustained a temperature 10 degrees higher; no -tadpole dying in this set under 43 degrees C. “This increased capacity -for resistance was not produced by the dying off of the less resistant -individuals, for no death occurred in these experiments during the -gradual elevation of the temperatures in the cultures.” The increased -resistance was due, therefore, to a change in the protoplasm of the -individuals. It was also determined that the acquired resistance was -only very gradually lost (after seventeen days’ sojourn in cooler -water). The explanation of this result may be due, in part, to the -protoplasm containing less water at higher temperatures, for it is known -that while the white of egg (albumen) coagulates at 56 degrees C. in -aqueous solution; with only 18 per cent of water it coagulates between -80 degrees and 90 degrees C.; and with 6 per cent, at 145 degrees C.; -and without water between 100 degrees and 170 degrees C. - -It has long been known that organisms in the dry condition resist a much -higher temperature. The damp uredospore is killed at 58.5 degrees to 60 -degrees C.; but dry spores withstand 128 degrees C. It is also known -that organisms may become acclimatized to cold through loss of water, -but we lack exact experimental data to show to what extent this can be -carried. - -There are also some experiments that go to show that animals may become -attuned to certain amounts of light, but the facts in this connection -will be described in another chapter. - -Some important results have been obtained by accustoming organisms to -solutions containing various amounts of salts. A number of cases of this -sort are given by De Varigny. It has been found that littoral marine -animals that live where the water may become diluted by the rain, or by -rivers, survive better when put into fresh water than do animals living -farther from the shore. Thus the oyster, the mussel, and the snail, -Patella, withstand immersion in fresh water better than other animals -that live farther out at sea. The reverse is also true; fresh-water -forms, such as Lymnæa, Physa, Paludina, and others may be slowly -acclimatized to water containing more salt. The forms mentioned above -could be brought by degrees into water containing 4 per cent of salt, -which would have killed the animals if they had been brought suddenly -into it. Similar results have been obtained for amœba. - -It has been shown that certain rotifers and tardigrades, and also some -unicellular animals, that live in pools and ponds that are liable to -become dry, withstand desiccation, while other members of the same -groups, living in the sea, do not possess this power of resistance. -Cases of this sort are usually explained as cases of adaptation, but it -has not been shown experimentally that resistance to drying can be -acquired by a process of acclimatization to this condition. The case is -also in some respects different from the preceding, since intermediate -conditions are less likely to be met with, or to be of sufficiently long -duration for the animal to become acclimatized to them. It seems more -probable, in such cases, that these forms have been able to live in such -precarious conditions from the beginning because they could resist the -effects of drying, not that they have slowly acquired this power. -Finally, there must be discussed the question of the acclimatization to -poisons, to which an individual may be rendered partially immune. The -point of special importance in this connection is that the animal may be -said to respond adaptively to a large number of substances, which it has -never met before in its individual history, or to which its ancestors -have never been subjected. It may become slowly adapted to many -different kinds of injurious substances. These cases are amongst the -most important adaptive individual responses with which we are familiar, -and the point cannot be too much emphasized that organisms have this -latent capacity without ever having had an opportunity to acquire it -through experience. - -The preceding groups of phenomena, included under the general heading of -individual acclimatization, have one striking thing in common, namely, -that a physiological adaptation is brought about without a corresponding -change in form, although we must suppose that the structure has been -altered in certain respects at least. The form of the individual remains -the same as before, but so far as its powers of resistance are concerned -it is a very different being. - -In regard to the perpetuation of the advantages gained by means of this -power of adaptation, it is clear in those cases in which the young are -nourished during their embryonic life by the mother, that, in this way, -the young may be rendered immune to a certain extent, and there are -instances of this sort recorded, especially in the case of some -bacterial diseases. Whether this power can also be transmitted through -the egg, in those instances in which the egg itself is set free and -development takes place outside the body, has not been shown. In any -case, the effect appears not to be a permanent one and will wear off -when the particular poison no longer acts. It is improbable, therefore, -that any permanent contribution to the race could be gained in this way. -Adaptations of this sort, while of the highest importance to the -individual, can have produced little direct effect on the evolution of -new forms, although it may have been often of paramount importance to -the individuals to be able to adapt themselves, or rather to become able -to resist the effect of injurious substances. The important fact in this -connection is the wonderful latent power possessed by all animals. So -many, and of such different kinds, are the substances to which they may -become immune, that it is inconceivable that this property of the -organism could ever have been acquired through experience, no matter how -probable it may be made to appear that this might have occurred in -certain cases of fatal bacterial diseases. And if not, in so many other -cases, why invent a special explanation for the few cases? - -We may defer the general discussion of the rôle that external factors -have played in the adaptation of organisms, until we have examined some -of the theories which attribute changes to internal factors. The idea -that something innate in the living substance itself has served as the -basis for evolution has given rise to a number of different hypotheses. -That of the botanist Nägeli is one of the most elaborately worked out -theories of this sort that has been proposed, and may be examined by way -of illustration. - - - Nägeli’s Perfecting Principle - -Nägeli used the term _completing principle_ (“Vervollkommungsprincip”) -to express a tendency toward perfection and specialization. -Short-sighted writers, he says, have pretended to see in the use of this -principle something mystical, but on the contrary it is intended that -the term shall be employed in a purely physical sense. It represents the -law of inertia in the organic realm. Once set in motion, the -developmental process cannot stand still, but must advance in its own -direction. Perfection, or completion, means nothing else than the -advance to complicated structure, “but since persons are likely to -attach more meaning to the word _perfection_ than is intended, it would -perhaps be better to replace it with the less objectionable word -_progression_.” - -Nägeli says that Darwin, having in view only the condition of -adaptation, designates that as more complete which gives its possessor -an advantage in the battle for existence. Nägeli claims that this is not -the only criterion that applies to organisms, and it leaves out the most -important part of the phenomenon. There are two kinds of completeness -which we should keep distinctly apart: (1) the completeness of -organization characterized by the complication of the structure and the -most far-reaching specialization of the parts; (2) the completeness of -the adaptation, present at each stage in the organization, which -consists in the most advantageous development of the organism (under -existing conditions) that is possible with a given complication of -structure and a given division of functions. - -The first of these conceptions Nägeli always calls “completeness” -(Vollkommenheit), for want of a simpler and better expression; the -second he calls adaptation. By way of illustrating the difference -between the two, the following examples may be given. The unicellular -plants and the moulds are excellently adapted each to its conditions of -life, but they are much less complete in structure than an apple tree, -or a grape vine. The rotifers and the leeches are well adapted to their -station, but in completeness of structure they are much simpler than the -vertebrates. - -If we consider only organization and division of labor as the work of -the completing principle, and leave for the moment adaptation out of -account, we may form the following picture of the rise of the organic -world. From the inorganic world there arose the simplest organic being -thinkable, being little more than a drop of substance. If this underwent -any change at all, it would have been necessarily in the direction of -greater complication of structure; and this would constitute the first -step in the upward direction. In this way Nägeli imagines the process -once begun would continue. When the movement has reached a certain -point, it must continue in the same direction. The organic kingdom -consists, therefore, of many treelike branches, which have had a common -starting-point. Not only does he suppose that organisms were once -spontaneously generated, and began their first upward course of -development, but the process has been repeated over and over again, and -each time new series have been started on the upward course. The organic -kingdom is made up, therefore, of all degrees of organization, and all -these have had their origins in the series of past forms that arose and -began their upward course at different times in the past. Those that are -the highest forms at the present time represent the oldest series that -successfully developed; the lowest forms living at the present time are -the last that have appeared on the scene of action. - -Organisms, as has been said, are distinguished from one another, not -only in that one is simpler and another more complicated, but also in -that those standing at the same stage of organization are unequally -differentiated in their functions and in their structure, which is -connected primarily with certain external relations which Nägeli calls -adaptations. - -Adaptation appears at each stage of the organization, which stage is, -for a given environment, the most advantageous expression of the main -type that was itself produced by internal causes. For this condition of -adaptation, a sufficient cause is demanded, and this is, as Nägeli tries -to show later, the result of the inherited response to the environment. -In many cases this cause will continue to act until complete adaptation -is gained; in other cases, the external conditions give a direction -only, and the organism itself continues the movement to its more perfect -condition. - -The difference between the conception of the organic kingdom as the -outcome of mechanical causes on the one hand, or of competition and -extermination on the other hand, can be best brought out, Nägeli thinks, -by the following comparison of the two respective methods of action. -There might have been no competition, and no consequent extermination in -the plant kingdom, if from the beginning the surface of the earth had -continually grown larger in proportion as living things increased in -numbers, and if animals had not appeared to destroy the plants. Under -these conditions each germ could then have found room and food, and have -unfolded itself without hinderance. If now, as is assumed to be the case -on the Darwinian theory, individual variations had been in all -directions, the developmental movement could not have gone beyond its -own beginnings, and the first-formed plants would have remained swinging -now on one side and now on another of the point first reached. The whole -plant kingdom would have remained in its entirety at its first stage of -evolution, that is, it would never have advanced beyond the stage of a -naked drop of plasma with or without a membrane. But, according to the -further Darwinian conception, competition, leading to extermination, is -capable of bringing such a condition to a higher stage of development, -since it is assumed that those individuals which vary in a beneficial -direction would have an advantage over those that have not taken such a -step, or have made a step backward. - -If, on the other hand, under the above-mentioned conditions of -unrestricted development, without competition, variations were -determined by “_mechanical principles_,” then, according to Nägeli’s -view, all plant forms that now exist would still have evolved, and would -be found living at the present time, but along with all those that now -exist there would be still other forms in countless numbers. These would -represent those forms which have been suppressed. On Nägeli’s view -competition and suppression do not produce new forms, but only weed out -the intermediate forms. He says without competition the plant kingdom -would be like the Milky Way; in consequence of competition the plant -kingdom is like the firmament studded with bright stars. - -The plant kingdom may also be compared to a branched tree, the ends of -whose branches represent living species. This tree has an inordinate -power of growth, and if left to itself it would produce an impenetrable -tangle of interwoven branches. The gardener prevents this crowding by -cutting away some of the parts, and thus gives to the tree distinct -branches and twigs. The tree would be the same without the watchful -trimming of the gardener, but without definite form. - -Nägeli states: “From my earlier researches I believe that the external -influences are small in comparison to the internal ones. I shall speak -here only of the influences of climate and of food, which are generally -described as the causes of change, without however any one’s having -really determined whether or not a definite result can be brought about -by these factors. Later I shall speak of a special class of external -influences which, according to my view, bring forth beyond a doubt -adaptive changes.” - -The external influence of climate and of food act only as transitory -factors. A rich food supply produces fat, lack of food leads to -leanness, a warm summer makes a plant more aromatic, and its fruit -sweeter; a cold year means less odor and sour fruit. Of two similar -seeds the one sown in rich soil will produce a plant with many branches -and abundance of flowers; the other, planted in sandy soil, will produce -a plant without branches, with few flowers, and with small leaves. The -seeds from these two plants will behave in exactly the same way; they -have inherited none of the differences of their parents. Influences of -this sort, even if extending over many generations, have no permanent -effect. Alpine plants that have lived since the ice age under the same -conditions, and have the characters of true high-mountain plants, lose -these characters completely during the first summer, if transplanted to -the plains. Moreover, it makes no difference whether the seed or the -whole plant itself be transferred. In place of the dwarfed, unbranched -growth, and the reduced number of organs, the plant when transferred to -the plains shoots up in height, branches strongly, and produces numerous -leaves and flowers. The plants retain their new characters as long as -they live in the plain without any other new variation being observed in -them. - -Other characteristics also, which arise from different kinds of external -influences due to different localities, such as dampness and shade, a -swampy region, or different geological substrata, last only so long as -the external conditions last. - -These transient peculiarities make up the characters of local varieties. -That they have no permanency is intelligible, since they exhibit no new -characters, but the change consists mainly in the over- or -under-development of those peculiarities that are dependent on external -influences. The effect of these influences may be compared to an elastic -rod, which, however much it may be distorted by external circumstances, -returns again to its original form as soon as released. - -Besides these temporary changes, due to external influences, there are -many cases known in which the same plant lives under very diverse -conditions and yet remains exactly the same. For example, the species of -_Rhododendron ferragineum_ lives on archæan mountains and especially -where the soil is poor in calcium. Another species, _Rhododendron -hirsutum_ is found especially on soil rich in calcium. The difference in -the two species has been supposed to depend on differences in the soil, -and if so, we would imagine that, if transplanted for a long time, the -one should change in the direction of the other. Yet it is known that -the rusty rhododendron may be found in all sorts of localities, even on -dry, sunny, calcareous rocks of the Apennines and of the Jura, and -despite its residence in these localities, since the glacial epoch, no -change whatever has taken place. - -Single varieties of the large and variable genus of _Hieracium_ have -lived since the glacial period in the high regions of the Alps, -Carpathians, and in the far north, and also in the plains of different -geological formations, but these varieties have remained exactly the -same, although on all sides there are transitional forms leading from -these to other varieties. - -Some parasitic species also furnish excellent illustrations of the same -principle. Besides the several species of Orobanchia and of the -parasitic moulds, the mistletoe deserves special mention. It lives on -both birch and apple trees and on both presents exactly the same -appearance; and even if it is true that mistletoe growing on conifers -presents certain small deviations in its character, it is still doubtful -whether, if transferred to the birch or apple tree, it would not lose -these differences, thus indicating that they are not permanent. - -It is a fact of general observation that, on the one hand, the same -variety occurs in different localities and under different surroundings, -and, on the other hand, that slightly different varieties live together -in the same place and therefore under the same external conditions. It -is evident, then, that food conditions have neither originated the -differences nor kept them up. The rarer cases in which in different -localities different varieties exist show nothing, because competition -and suppression keep certain varieties from developing where it would be -possible otherwise for them to exist. - -Nägeli says his conclusion may be tested from another point of view. If -food conditions, as is generally supposed, have a definite, _i.e._ a -permanent, effect on the organism, then all organisms living under the -same conditions should show the same characters. Indeed, it has been -claimed in some instances that this is actually the case. Thus it is -stated that dry localities cause plants to become hairy, and that -absence of hairiness is met with in shady localities. This may apply to -certain species, but in other cases exactly the reverse is true, and -even the same species behaves differently in different regions, as in -_Hieracium_. And so it is with all characteristics which are ascribed to -external influences. As soon as it is supposed a discovery has been made -in this direction, we may rest assured that in other cases the reverse -will be found to hold. We have had, in respect to the influence of the -outer world on organisms, the same experience as with the rules for the -weather,—when we come to examine the facts critically there are found to -be as many exceptions as confirmations of the rule. - -If climatic influence has a definite effect, the entire flora of a -special locality ought to have the same peculiarities, but this stands -in contradiction to all the results of experience. The character of the -vegetation is not determined by the environment of the plants but by -their prehistoric origin, and as the result of competition. Nägeli -concludes his discussion with the statement that all of our experience -goes to show that the effects of external influences (climate and food) -appear at once, and their results last only as long as the influences -themselves last, and are then lost, leaving nothing permanent behind. -This is true even when the external influences have lasted for a long -time,—since the glacial epoch, for instance. We find, he claims, nothing -that supports the view that such influences are inherited. - -If we next examine the question of changes from _internal causes_, -Nägeli claims that here also observation and research fail to show the -origin of a new species, or even of a new variety from external causes. -In the organic world little change has taken place, he believes, since -the glacial epoch. Many varieties have even remained the same throughout -the whole intervening time; and while it cannot be doubted that new -varieties have also been formed, yet the cause of their origin cannot be -empirically demonstrated. The permanent, hereditary characters, of whose -origin we know something from experience, belong to the individual -changes which have appeared under cultivation in the formation of -domestic races. These are for the most part the result of crossing. So -far as we have any definite information as to the origin of the changes, -they are the result of inner, and never of external, causes. We -recognize that this must be the case, since under the same external -conditions individuals behave differently—in the same flower-bud some -seeds give rise to plants like the parent, others to altered ones. The -strawberry with a single leaflet, instead of three, arose in the last -century in a single individual amongst many other ordinary plants. From -the ten seeds of a pear Van Mons obtained as many different kinds of -pears. The most conclusive proof of the action of inner causes is most -clearly seen when the branches of the same plant differ. In Geneva a -horse-chestnut bore a branch with “filled” flowers, and from this -branch, by means of cuttings, this variation has been carried over all -Europe. In the Botanic Garden at Munich there is a beech with small -divided leaves; but one of its branches produces the common broad -undivided leaves. Many such examples have been recorded which can only -be explained by assuming that a cell, or a group of cells, like those -from which the other branches arose, have become changed in some unknown -way as the result of inner causes. The properties that are permanent and -inherited are contained in the idioplasm, which the parent transmits to -its offspring. A cause that permanently transforms the organism must -also transform the idioplasm. How powerless, in comparison to internal -causes, the external causes are is shown most conclusively in grafting. -The graft, although it receives its nourishment through the stock, which -may be another species, remains itself unchanged. - -Nägeli makes the following interesting comparison between the -development of the individual from an egg, and the evolution, or -development, of the phylum. No one will doubt that the egg during the -entire time of its process of transformation is guided by internal -factors. Each successive stage follows with mechanical necessity from -the preceding. If an animal can develop from inner causes from a drop of -plasma, why should not the entire evolutionary process have also been -the outcome of developmental inner causes? He admits that there is a -difference in the two cases in that the plasma that forms the egg has -come from another animal, and contains all the properties of the -individual in a primordial condition. In the other case we must suppose -that the original drop of plasma did not contain at first the primordium -of definite structures, but only the ability to form such. Logically the -difference is unimportant. The main point is that in the primordium of -the germ a special peculiarity of the substance is present which by -forming new substances grows, and changes as it grows, and the one -change of necessity excites the next until finally a highly organized -being is the result. - -Nägeli discusses a question in this connection, which, he says, has been -unnecessarily confused in the descent theory. Since we are entirely in -the dark as to how much time has been required for the formation of -phyla, so also are we ignorant as to how long it may have taken for each -step in advance. We may err equally in ascribing too much and too little -time to the process. It is, moreover, not necessary that for every step -the same amount of time should have been required. On the contrary, the -probability is that recognizable changes may at times follow each other -rapidly, and then for a time come to a standstill,—just as in the -development of the individual there are periods of more rapid and others -of less rapid change. - -A more difficult problem than that relating to the sort of changes the -external influences bring about in the organism, is the question as to -how they effect the organism, or how they act on it mechanically. This, -as is well known, was answered by Darwin, who regards all organization -as a problem of adaptation: only those chance variations surviving which -are capable of existence, the others being destroyed. On this theory -external influences have only a negative or a passive action, namely, in -setting aside the unadapted individuals. Nägeli, on the other hand, -looks upon some kinds of external conditions as directly giving rise to -the adaptive characters of the organism. This is accomplished, he -supposes, in the following ways: two kinds of influence are recognized; -_the direct action_, which, as in inorganic nature, comes to an end when -the external influences come to an end, as when cold diminishes the -chemical actions in the plant; and _the indirect action_, generally -known as a stimulus, which starts a series of molecular motions, -invisible to us, but which we recognize only in their effects. Very -often the stimulus starts only a reflex action, usually at the place of -application. - -A stimulus acting for but a short time produces no lasting effect on the -idioplasm. A person stung by a wasp suffers no permanent effect from the -injury. But if a stimulus acts for a long time, and through a large -number of generations, then it may, even if of small strength, so change -the _idioplasm_, that a tendency or disposition capable of being seen -may be the result. This appears to be the case in regard to the action -of light, which causes certain parts of the plant to turn toward it and -others away from it; also for the action of gravity, which determines -the downward direction of the roots. It may be claimed, perhaps, that -these are the results of direct influence and not of an internal -response, but this is not the case; for some plants act in exactly the -opposite way, and send a stem downward, as in the case of the -cleistogamous flowers of _Cardamine chenopodifolia_; and other plants -turn away from the light. This means that the idioplasm behaves -differently in different plants in response to the same stimulus. - -Concerning the more visible effects of adaptation, Nägeli states that in -regard to some of them there can be no question as to how they must have -arisen. Protection against cold, by the formation of a thick coat of -hair, is the direct result of the action of the cold on the skin of the -animal. The different weapons of offence and of defence, horns, spurs, -tusks, etc., have arisen, he maintains, through stimulus to those parts -of the body where these structures arise. - -The causes of the other adaptations, especially of those occurring in -plants, are less obvious. Land plants protect themselves from drying by -forming a layer of cork over the surface. The most primitive plants were -water plants, which acclimated themselves little by little to moist, and -then to dry, air. When they first emerged from the water the drying -acted as a stimulus on the surface, and caused it to harden in the same -way as a drop of glue hardens. This hardening in turn acted as a -stimulus, causing a chemical transformation of the surface into a corky -substance. This effect was inherited, and in this way the power to form -cork originated. - -Land plants have, in addition to the soft parts, the hard bast and wood -which serves the mechanical purpose of supporting the soft tissues and -protecting them from being injured. The arrangement of the hard parts is -such as to suggest that they are the result of the action of pressures -and tensions on the plant, for the strongest cells are found where there -is most need for them. It is easy to imagine, Nägeli adds, that this -important arrangement of the tissues is the result of external forces -which brought about the result in these parts. - -Nägeli accounts for the origin of twining plants as follows. Being -overshadowed by other plants, the stem will grow rapidly in the damp -air. Coming in contact with the stems of other plants, the delicate stem -is stimulated on one side, and grows around the point of contact. This -tendency becomes inherited, and the habit to twine is ultimately -established. - -The difference in the two sides of leaves is explained by Nägeli as the -result of the difference in the illumination of the two sides. This -influence of light on the leaf has been inherited. The formation of the -tubular corolla that is seen in many plants visited by insects is -explained as the result of the stimulus produced by the insects in -looking for the pollen. The increase in the length of the proboscis of -the insect is the result of the animal straining to reach the bottom of -the ever elongating tube of the corolla. “The tubular corolla and the -proboscis of the insect appear as though made for each other. Both have -slowly developed to their present condition, the long tube from a short -tube and the long proboscis from a short one.” Thus, by purely -Lamarckian principles, Nägeli attempts to account for many of the -adaptations between the organism and the outer world. But if this takes -place, where is there left any room for the action for his so-called -perfecting principle? Nägeli proceeds to show how he supposes that the -two work together. - -As a result of inner causes the organism would pass through a series of -perfectly definite stages, J, J^1, J^2. But if, at any stage, external -influences produced an effect on the organism so that the arrangement of -the idioplasm changes in response, a new adaptation is produced. In this -way new characters, not inherent in the idioplasm, may be added, and old -ones be changed or lost. “In order not to be misunderstood in regard to -the completing or perfecting principle I will add, that I ascribe to it -no determinate action in the organism, neither in producing the long -neck of the giraffe, nor the prehensile tail of the ape, neither the -claws of the crab, nor the decoration of the bird of paradise. These -structures are the outcome of both factors. I cannot picture to myself -how external causes alone, and just as little how internal causes alone, -could have changed a monad into a man.” But Nägeli goes on to say, that -if at any stage of organization one of the two causes should cease to -act, the other could only produce certain limited results. Thus, if -external causes alone acted, the organization would remain at the same -stage of completeness, but might become adapted to all kinds of external -conditions—a worm, for instance, would not develop into a fish, but -would remain a worm forever, although it might change its worm structure -in many ways in response to external stimuli. If, on the other hand, -only the completing principle acted, then without changing its -adaptations the number of the cells and the size of the organs might be -increased, and functions that were formerly united might become -separated. Thus, without altering the character of the organism, a more -highly developed (in the sense of being more specialized) organism would -appear. - -Nägeli, as we have just seen, has attempted to build up a conception of -nature based on two assumptions, neither of which has been demonstrated -to be an actual principle of development. His hypothesis appears, -therefore, entirely arbitrary and speculative to a high degree. Even if -it were conceivable that two such principles as these control the -evolution of organisms, it still requires a good deal of imagination to -conceive how the two go on working together. Moreover, it is highly -probable that whole groups have evolved in the direction of greater -simplification, as seen especially in the case of those groups that have -become degenerate. To what principle can we refer processes of this -sort? - -It is certainly a strange conclusion this, at which Nägeli finally -arrives, for, after strenuously combating the idea that the external -factors of climate and of food have influence in producing new species, -he does not hesitate to ascribe all sorts of imaginary influences to -other external causes. The apparent contradiction is due, perhaps, to -the fact that his experience with actual species led him to deny that -the direct action of the environment produces permanent changes, while -in theory he saw the necessity of adding to his perfecting principle -some other factor to explain the adaptations of the new forms produced -by inner causes. Nägeli seems to have felt strongly the impossibility of -explaining the process of evolution and of adaptation as the outcome of -the selection of chance variations, now in this direction, now in that. -He seems to have felt that there must be something within the organism -that is driving it ever upward, and he attempts to avoid the -teleological element, which such a conception is almost certain to -introduce, by postulating the inheritance of the effects of -long-continued action of the environment, in so far as certain factors -in the environment produce a response in the organism. Nevertheless, -this combination is not one that is likely to commend itself, aside from -the fact that the assumptions have no evidence to support them. Despite -Nägeli’s protest that his principles are purely physical, and that there -is nothing mystical in his point of view, it must be admitted that his -conception, as a whole, is so vague and difficult in its application -that it probably deserves the neglect which it generally receives. - -Nägeli’s wide experience with living plants convinced him that there is -something in the organism over and beyond the influence of the external -world that causes organisms to change; and we cannot afford, I think, to -despise his judgment on this point, although we need not follow him to -the length of supposing that this internal influence is a “force” -driving the organism forward in the direction of ever greater -complexity. A more moderate estimate would be that the organism often -changes through influences that appear to us to be internal, and while -some of the changes are merely fluctuating or chance variations, there -are others that appear to be more limited in number, but perfectly -definite and permanent in character. It is the latter, which, I believe, -we can safely accredit to internal factors, and which may be compared to -Nägeli’s internal causes, but this is far from assuming that these -changes are in the direction of greater completeness or perfection, or -that evolution would take place independently of the action of external -agencies. - - ------------------------------------------------------------------------- - - - - - CHAPTER X - - THE ORIGIN OF THE DIFFERENT KINDS OF ADAPTATIONS - - -In the present chapter we may first consider, from the point of view of -discontinuous variations as contrasted with the theory of the selection -of individual variations, the structural adaptations of animals and -plants, _i.e._ those cases in which the organism has a definite form -that adapts it to live in a particular environment. In the second place, -we may consider those adaptations that are the result of the adjustment -of each individual to its surroundings. In subsequent chapters the -adaptations connected with the responses of the nervous system and with -the process of sexual reproduction will be considered. - -It should be stated here, at the outset, that the term _mutation_ will -be used in the following chapters in a very general way, and it is not -intended that the word shall convey only the idea which De Vries -attaches to it; it is used rather as synonymous with _discontinuous and -also definite variation_ of all kinds. The term will be used to include -“the single variations” of Darwin, “sports,” and even orthogenic -variation, if this has been definite or discontinuous. - - - Form and Symmetry - -Almost without exception, animals and plants have definite and -characteristic forms. In other words, they are not amorphous masses of -substance. The members of each species conform, more or less, to a sort -of ideal type. Our first problem is to examine in what sense the form -itself may be looked upon as an adaptation to the surroundings. - -It is a well-recognized fact that the forms of many animals appear to -stand in a definite relation to the environment. For instance, animals -that move in definite directions in relation to their structure have the -anterior and the posterior ends quite different, and it is evident that -these ends stand in quite different relations to surrounding objects; -while, on the other hand, the two sides of the body which are, as a -rule, subjected to the same influences are nearly exactly alike. The -dorsal and the ventral surfaces of the body are generally exposed to -very different external conditions, and are quite different in -structure. - -The relation is so obvious in most cases that it might lead one quite -readily to conclude that the form of the animal had been moulded by its -surroundings. Yet this first impression probably gives an entirely wrong -conception of how such a relation has been acquired. Before we attempt -to discuss this question, let us examine some typical examples. - -A radial type of structure is often found in fixed forms, and in some -floating forms, like the jellyfish. In a fixed form, a sea-anemone, for -instance, the conditions around the free end and the fixed end of the -body are entirely different, and we find that these two ends are also -different. The free end contains the special sense-organs, the mouth, -tentacles, etc.; while the fixed end contains the organ for attachment. -It is evident that the free end is exposed to the same conditions in all -directions, and it may seem probable that this will account for the -radial symmetry of the anemone. There are also a few free forms, the -sea-urchin for instance, that have a radial symmetry. Whether their -ancestors were fixed forms, for which there is some evidence, we do not -know definitely; but, even if this is true, it does not affect the main -point, namely, that, although at present free to move, the sea-urchin is -radially symmetrical. But when we examine its method of locomotion, we -find that it moves indifferently in any direction over a solid surface; -that is, it keeps its oral face against a solid object, and moves over -the surface in any direction. Under these circumstances the same -external conditions will act equally upon all sides of the body. In -contrast to these common sea-urchins, there are two other related -groups, in which, although traces of a well-marked radial symmetry are -found, the external form has been so changed that a secondary bilateral -form has been superimposed on it. These are the groups of the -clypeasters and the spatangoids, and it is generally supposed that their -forefathers were radially symmetrical forms like the ordinary forms of -sea-urchins. These bilateral forms move in the direction of their plane -of symmetry, but we have no means of knowing whether they first became -bilateral and, in consequence, now move in the direction of the median -plane, or whether they acquired the habit of moving in one direction, -and in consequence acquired a bilateral symmetry. It seems more probable -that the form changed first, for otherwise it is difficult to see why a -change of movement in one direction should ever have taken place. - -The radially symmetrical form is characteristic of many flowers that -stand on the ends of their stalks. They also will be subjected to -similar external influences in all directions. Many flowers, on the -other hand, are bilaterally symmetrical. Some of these forms are of such -a sort that they are generally interpreted as having been acquired in -connection with the visits of insects. Be this as it may, it is still -not clear why, if the flowers are terminal, insects should not approach -them equally from every direction. If the flowers are not terminal, as, -in fact, many of them are not, their relation to the surroundings is -bilateral with respect to internal as well as to external conditions. -The former, rather than the latter, may have produced the bilateral form -of the flower. Here also we meet with the problem as to whether the -flowers, being lateral in position, have assumed a bilateral form -because their internal relations were bilateral; or whether an external -relation, for example, the visits of insects, has been the principle -cause of their becoming bilateral. - - -[Illustration: - - Fig. 4.—A, right and left claws of lobster; - B, of the fiddler-crab; and - C, of Alpheus.] - -In some bilateral forms the right and left sides may be unsymmetrical in -certain organs. Right and left handedness in man is the most familiar -example, although the structural difference on which this rests is not -very obvious. More striking is the difference in the two big claws of -the lobster (Fig. 4 A). One of the two claws is flat and has a fine -saw-toothed edge. The other is thicker and has rounded knobs instead of -teeth. It is said that these two claws are used by the lobster for -different purposes,—the heavy one for crushing and for holding on, and -the narrower for cutting up the food. If this is true, then we find a -symmetrical organism becoming unsymmetrical, and in consequence it takes -advantage of its asymmetry by using its right and left claws for -different purposes. - -More striking still is the difference in the size of the right and left -claws in a related form, Alpheus—a crayfish-like form that lives in the -sea. With the larger claw (Fig. 4 C) it makes a clicking sound that can -be heard for a long distance. In some of the crabs the difference in the -size of the two claws is enormous, as in the male fiddler-crab, for -example (Fig. 4 B). One of the claws is so big and unwieldy that it must -put the animal at a distinct disadvantage. Its use is unknown, although -it has been suggested that it is a secondary sexual character. - -The asymmetry of the body of the snail is very conspicuous, at least so -far as certain organs are concerned. The foot on which the animal crawls -and the head have preserved their bilaterality; but the visceral mass of -the animal, contained in the spirally wound shell, lying on the middle -of the upper surface of the foot, is twisted into a spiral form. Many of -the organs of one side of the body are atrophied. The gill, the kidney, -the reproductive organ, and one of the auricles of the heart have -completely, or almost completely, disappeared. The cause of this loss -seems to be connected with the spiral twist of the visceral mass. One of -the consequences of the twisting has been to bring the organs of the -left side of the body around the posterior end until they come to lie on -the right side, the organs of the original right side being carried -forward and there atrophying. - -There is another remarkable fact connected with the asymmetry of the -snail. In some species, _Helix pomatia_, for example, the twist has been -toward the right, _i.e._ in the direction which the hands of a watch -follow when the face is turned upward toward the observer. Individuals -twisted in this direction are called dextral. Occasionally there is -found an individual with the spiral in the opposite direction -(sinistral), and in this the conditions of the internal organs are -exactly reversed. It is the left set of organs that is now atrophied, -and the right set that is functional. Such changes appear suddenly. -Organs of one side of the body that have not been functional for many -generations may become fully developed. Moreover, Lang has shown that -when a sinistral form breeds with a normal dextral form, or even when -sinistral forms are bred with each other, the young are practically all -of the ordinary type. - -An attempt has been made to connect these facts with the mode of -development of the mollusks. It is known that the eggs of a number of -gasteropod mollusks segment in a perfectly definite manner. A sort of -spiral cleavage is followed by the formation of a large mesodermal cell -from the left posterior yolk-cell. From this mesodermal cell nearly all -the mesodermal organs of the body are formed. Thus it may appear that -the spiral form of the snail is connected with the spiral form of the -cleavage. In a few species of marine and fresh-water snails the cleavage -spiral is reversed, and the mesoderm arises from the right posterior -yolk-cell. It has been shown in several cases that the snail coming from -such an egg is twisted in the reverse direction from that of ordinary -snails. - -It has been suggested, therefore, that the occasional sinistral -individual of Helix arises from an egg cleaving in the reverse -direction, and there is nothing improbable in an assumption of this -kind. No attempt has been made as yet to explain why, in some cases, the -cleavage spiral is turned in one direction, and in other cases in the -reverse direction; but even leaving this unaccounted for, the assumption -of the unusual form of Helix being the result of a reversal of the -cleavage throws some light as to how it is possible for the complete -reversal of the organs of the adult to arise. If it is assumed that in -the early embryo the cells on each side of the median line are alike, -and at this time capable of forming adult structures, a simple change of -the spiral from right to left might determine on which side of the -middle line the mesodermal cell would lie, and its presence on one side -rather than on the other might determine which side of the embryo would -develop, and which would not. This possibility removes much of the -mystery which may appear to surround a sudden change of this sort. - -It seems to me that we shall not go far wrong if we assume that it is -largely a matter of indifference whether an individual snail is a -right-handed or a left-handed form, as far as its relation to the -environment is concerned. One form would have as good a chance for -existing as the other. If this is granted, we may conclude that, while -in most species a perfectly definite type is found, a right or a left -spiral, yet neither the one nor the other has been acquired on account -of its relation to the environment. This conclusion does not, of course, -commit us in any way as to whether the spiral form of the visceral mass -has been acquired in relation to the environment, but only to the view -that, if a spiral form is to be produced, it is indifferent which way it -turns. From the evolutionary point of view this conclusion is of some -importance, since it indicates that one of the alternatives has been -adopted and has become practically constant in most cases without -selection having had anything to do with it. - -Somewhat similar conditions are found in the flounders and soles. As is -well known, these fishes lie upon one side of the body on the bottom of -the ocean. Some species, with the rarest exceptions to be mentioned in a -moment, lie always on the right side, others on the left side. A few -species are indifferently right or left. At rare intervals a left-sided -form is found in a right-sided species, and conversely, a right-sided -form in a left-sided species. In such cases the reversed type is as -perfectly developed in all respects as the normal form, but with a -complete reversal of its right and left sides. - -When the young flounders leave the egg, they swim in an upright -position, as do ordinary fishes, with both sides equally developed. -There cannot be any doubt that the ancestors of these fish were -bilaterally symmetrical. Therefore, within the group, both right-handed -and left-handed forms have appeared. It seems to me highly improbable -that if a right-handed form had been slowly evolved through the -selection of favorable variations in this direction, the end result -could be suddenly reversed, and a perfect left-sided form appear. -Moreover, as has been pointed out, the intermediate stages would have -been at a great disadvantage as compared with the parent, and this would -lead to their extermination on the selection theory. If, however, we -suppose that a variation of this sort appeared at once, and was fixed,—a -mutation in other words,—and that whether or not it had an advantage -over the parent form, it could still continue to exist, and propagate -its kind, then we avoid the chief difficulty of the selection theory. -Moreover, we can imagine, at least, that if this variation appeared in -the germ and was, in its essential nature, something like the relation -seen in the snail, the occasional reversal of the relations of the parts -presents no great difficulty. - -In this same connection may be mentioned a curious fact first discovered -by Przibram and later confirmed by others. If the leg carrying the large -claw of a crustacean be removed, then, at the next moult, the leg of the -other side that had been the smaller first leg becomes the new big one; -and the new leg that has regenerated from the place where the big one -was cut off becomes the smaller one. - -Wilson has suggested that both claws in the young crustacean have the -power to become either sort. We do not know what decides the matter in -the adult, after the removal of one of the claws. Some slight difference -may turn the balance one way or the other, so that the smaller claw -grows into the larger one. At any rate, there is seen a latent power -like that in the egg of the snail. Zeleny has found a similar relation -to exist for the big and the little opercula of the marine worm, -Hydroides. - -Let us consider now the more general questions involved in these -symmetrical and asymmetrical relations between the organism and its -environment. In what sense, it may be asked, is the symmetry of a form -an adaptation to its environment? That the kind of symmetry gives to the -animal in many cases a certain advantage in relation to its environment -is so evident that I think it will not be questioned. The main question -is how this relation is supposed to have been attained. Three points of -view suggest themselves: First, that the form has resulted directly from -the action of the environment upon the organism. This is the Lamarckian -point of view, which we rejected as improbable. Second, that the form -has been slowly acquired by selecting those individual variations that -best suited it to a given set of surrounding conditions. This is the -Darwinian view, which we also reject. The third, that the origin of the -form has had nothing to do with the environment, but appeared -independently of it. Having, however, appeared, it has been able to -perpetuate itself under certain conditions. - -It should be pointed out that the Darwinian view does not suppose that -the environment actually produces any of the new variations which it -selects after they have appeared, but in so far as the environment -selects individual differences it is supposed to determine the direction -in which evolution takes place. On the theory that evolution has taken -place independently of selection, this latter is not supposed to be the -case; the finished products, so to speak, are offered to the -environment; and if they pass muster, even ever so badly, they may -continue to propagate themselves. - -The asymmetrical form of certain animals living in a symmetrical -environment might be used as an argument to show that the relation of -symmetry between an animal and its environment can easily be overstepped -without danger. The enormous claw of the fiddler-crab must throw the -animal out of all symmetrical relation with its environment, and yet the -species flourishes. The snail carries around a spiral hump that is -entirely out of symmetrical relation with the surroundings of a snail. - -These facts, few though they are, yet suffice to show, I believe, that -the relation of symmetry between the organism and its environment may -be, and is no doubt in many cases, more perfect than the requirements of -the situation demand. The fact that animals made unsymmetrical through -injuries (as when a crab loses several legs on one side, or a worm its -head) can still remain in existence in their natural environment, is in -favor of the view that I have just stated. By this I do not mean to -maintain that a symmetrical form does not have, on the whole, an -advantage over the same form rendered asymmetrical, but that this -relation need not have in all forms a selective value, and if not, then -it cannot be the outcome of a process of natural selection. - -To sum up: it appears probable that the laws determining the symmetry of -a form are the outcome of internal factors, and are not the result -either of the direct action of the environment, or of a selective -process. The finished products and not the different imperfect stages in -such a process, are what the inner organization offers to the -environment. While the symmetry or asymmetry may be one of the numerous -conditions which determine whether a form can persist or not, yet we -find that the symmetrical relations may be in some cases more perfect -than the environment actually demands; and in other cases, although the -form may place the organism at a certain disadvantage, it may still be -able to exist in certain localities. - - - Mutual Adaptation of Colonial Forms - -In the white ants, true ants, and bees, we find certain individuals of -the community specialized in such a way that their modifications stand -in certain useful relations to other members of the community. Amongst -the bees, the workers collect the food, make the comb, and look after -the young. The queen does little more than lay eggs, and the drone’s -only function is to fertilize the queen. In the true ants there are, -besides the workers and the queen and the males, the soldier caste. -These have large thick heads and large strong jaws. On the Darwinian -theory it is assumed that this caste must have an important rôle to -play, for otherwise their presence as a distinct group of forms cannot -be accounted for; but I do not believe it is necessary to find an excuse -for their existence in their supposed utility. From the point of view of -the mutation theory, their real value may be very small, but so long as -their actual presence is not entirely fatal to the community they may be -endured. - -In regard to these forms, Sharp writes:[28] “The soldiers are not alike -in any two species of Termitidæ, so far as we know, and it seems -impossible to ascribe the differences that exist between the soldiers of -different species of Termitidæ to special adaptations for the work they -have to perform.” “On the whole, it would be more correct to say that -the soldiers are very dissimilar in spite of their having to perform -similar work, than to state that they are dissimilar in conformity with -the different tasks they carry on.” The soldiers have the same instincts -as the workers, and do the same kinds of things to a certain extent. -“The soldiers are not such effective combatants as the workers are.” -Statements such as these indicate very strongly that the origin of this -caste can have very little to do with its importance as a specialized -part of the community. - -Footnote 28: - - “The Cambridge Natural History,” Vol. V, 1895. - -The differences between the castes have gone so far in some of these -groups that the majority of the members of the community have even lost -the power to reproduce their kind, and this function has devolved upon -the queen, whose sole duty is to reproduce the different castes of which -the community is composed. This specialization carries with it the idea -of the individuals being adapted to each other, so that, taken all -together, they form a whole, capable of maintaining and reproducing -itself. It does not seem that we must necessarily look upon this union -as the result of competition leading to a death struggle between -different colonies, so that only those have survived in each generation -that carried the work of specialization one step farther. All that is -required is to suppose that such specialization has appeared in a group -of forms living together, and the group has been able to perpetuate -itself. We do not find that all other members of the two great groups to -which the white ants and true ants belong have been crowded out because -these colonial forms have been evolved. Neither need we suppose that -during the evolution of these colonial species there has been a death -struggle accompanying each stage in the evolution. If the members of a -colonial group began to give rise to different forms through mutations, -and if it happened that some of the combinations formed in this way were -capable of living together, and perpetuating the group, this is all that -is required for such a condition to persist. - -The relation of the parents to the offspring presents in some groups a -somewhat parallel case to that of these colonial forms. Not only are -some of the fundamental instincts of the parents changed, but structures -may be present in the parents whose only use is in connection with the -young. The marsupial pouch of the kangaroo, in which the immature young -are carried and suckled, is a case in point, and the mammary glands of -the Mammalia furnish another illustration. - -Adaptations of these kinds are clearly connected with the perpetuation -of the race. In the case of the mammals the young are born so immature -that they are dependent on the parental organs, just spoken of, for -their existence. Could we follow this relation through its evolutionary -stages, it would no doubt furnish us with important data, but -unfortunately we can do no more than guess how this relation became -established. The changes in the young and in the parent may have been -intimately connected at each stage, or more or less independent. If we -suppose the mammary glands to have appeared first, they might have been -utilized by the young in order to procure food. Their presence would -then make it possible for the young to be born in an immature condition, -as is the case with the young of many of the mammals. But this is pure -guessing, and until we know more of the actual process of evolution in -this case, it is unprofitable to speculate. - - - Degeneration - -In almost every group of the animal kingdom there are forms that are -recognized as degenerate. This degeneration is usually associated with -the habitat of the animal. In many cases it can be shown with much -probability that these degenerate forms have descended from members of -the group that are not degenerate. We find there is a loss of those -organs that are not useful to the organism in its new environment. The -degeneration may involve nearly the whole organization (except as a rule -the reproductive system), as seen in the tapeworm, or only certain -organs of the body, as the eyes in cave animals. A few examples will -bring the main facts before us. - -A parasitic existence is nearly always associated with degeneration. -Under these conditions, food can generally be obtained without -difficulty, at the expense of the host, and apparently associated with -this there is a degeneration, and even a complete loss of so important -an organ as the digestive tract. Thus the tapeworm has lost all traces -of its digestive tract, absorbing the already digested matter of its -host through its body wall. Some of the roundworms, that live in the -alimentary tracts of other animals, may have their digestive organs -reduced. In Trichina, this degeneration has gone so far that the -digestive tract is represented, in part, by a single line of endoderm -cells, pierced by a cavity. The digestive organs are also absent in -certain male rotifers, which are parasitic on the females, and these -organs are also very degenerate in the male of _Bonellia_, a gephyrean -worm. A parasitic snail, _Entoscolax ludwigii_, has its digestive -apparatus reduced to a sucking tube ending in a blind sac. The rest of -the tract has completely degenerated. The remarkable parasitic -crustacean, _Sacculina carcini_, looks like a tumor attached to the -under surface of the abdomen of a crab. It has neither mouth nor -digestive tract, and absorbs nourishment from the crab through rootlike -outgrowths that penetrate the body. From its development alone we know -that it is a degenerate barnacle. - -There seems to be in all these cases an apparent connection between the -absence of the digestive tract and the presence of an abundant supply of -food, that has already been partly digested by the host. Put in a -different way, we may say that the presence of this food has furnished -the environment in which an animal may live that has a rudimentary -digestive tract. - -An interesting case of degeneration is found in the rudimentary mouth -parts of the insects known as May-flies, or ephemerids. Some of these -species live in the adult condition for only a few hours, only long -enough to unite and deposit their eggs. In the adult stage the insects -do not take any food. In this case the degeneration is obviously not -connected with the presence of food, but apparently with the shortness -of the adult life. - -One of the most familiar cases of degeneration is blindness, associated -with life in the dark. The most striking cases are those of cave -animals, but this is only an extreme example of what is found everywhere -amongst animals that live concealed during the day under stones, etc. -The blind fish and the blind crayfish of the Mammoth Cave, the blind -proteus of the caves of Carniola, the blind mole that burrows -underground, the blind larvæ of many insects that live in the dark, are -examples most often cited. Some nocturnal animals, like the earthworm, -have no eyes, although they are still able to distinguish light; and -some of the deep-sea animals, that live below the depth to which light -penetrates, have degenerate eyes. The workers of some ants, that remain -in the nests, are blind, but the males and the queens of these forms -have well-developed eyes, although the eyes may be of use to them at -only one short period of their life, namely, at the time of the marriage -flight. This fact is significant and is underestimated by those who -believe that disuse accounts for the degeneration of organs. - -The wings of the ostrich and of the kiwi are rudimentary structures no -longer used for flight, and many insects, belonging to several different -orders, have lost their wings, as seen in fleas, some kinds of bugs, and -moths, and even in some grasshoppers. - -A curious case of degeneration is found in the abdomen of the hermit -crab, which is protected by the appropriated shell of a snail. The -appendages of one side of the abdomen have nearly disappeared in the -male, although in the female the abdominal appendages are used to carry -the eggs as in other decapod crustaceans. The abdomen, instead of being -covered by a hard cuticle, as in other members of this group, is soft -and unprotected except by the shell of the snail. - -Cases of these kinds could be added to almost indefinitely, and the -explanation of these degenerate structures has been a source of -contention amongst zoologists for a long time. The most obvious -interpretation is that the degeneration has been the result of disuse. -But as I have already discussed this question, and given my reasons for -regarding it as improbable that degeneration has arisen in this way, we -need not further consider this point here. - -The selectionists have offered several suggestions to account for -degeneration. In fact, this has been one of the difficulties that has -given them most concern. They have suggested, for example, that when an -organ is no longer of use to its possessor it would become a source of -danger, and hence would be removed through natural selection. They have -also suggested that since such organs draw on the general food supply -they would place their possessor at a disadvantage, and hence would be -removed. Weismann has attempted to meet the difficulty by his theory of -“Panmixia,” or universal crossing, by which means the useless structures -are imagined to be eliminated. - -These attempts will suffice to point out the straits to which the -Darwinians have found themselves reduced, and we have by no means -exhausted the list of suggestions that have been made. Let us see, if, -on any other view, we can avoid some of the difficulties that the -selection theory has encountered. - -In the first place we shall be justified, I think, in eliminating -competition as a factor in the process, since the admission that an -organ has become useless carries with it the idea that it has no longer -a selective value. If, in its useless condition, it is no longer greatly -injurious, as is probably, though not necessarily always, the case, then -selection cannot enter into the problem. If in parasitism we assume that -an animal finds a lodgement in another animal, where it is able to -exist, we may have the first stage of the process introduced at once. If -under these conditions a mutation appeared, involving some of the organs -that are no longer essential to the life of the individual in its new -environment, the new mutation may persist. We need not suppose that the -original form becomes crowded out, but only that a more degenerate form -has come into existence. As a matter of fact we find in most groups, in -which degenerate forms exist, a number of different stages in the -degeneration in different species. Mutation after mutation might follow -until many of the original organs have disappeared. The connection that -appears to exist between the degeneration of a special part and the -environment in which the animal lives finds its explanation simply in -the fact that the environment makes possible the existence of that sort -of mutation in it. We do not know, as yet, whether through mutative -changes an organ can completely disappear, although this seems probable -from the fact that in a few cases mutations are known to have arisen in -which a given part is entirely functionless. If we could assume that, a -mutation in the direction of degeneration being once established, -further mutations in the same direction would probably occur, the -problem would be much simplified; but we lack data, at present, to -establish this view. - -In the case of blind animals it seems probable that the transition has -taken place in such forms as had already established themselves in -places more or less removed from the light. Such forms as had the habit -of hiding away under stones, or in the ground, living partly in and -partly out of the light, might, if a mutation appeared of such a sort -that amongst other changes the eyes were less developed, still be -capable of leading an existence in the dark, while it might be -impossible for them to exist any longer with weakened vision in the -light. If such a process took place, the habitat of the new form would -be limited, or in other words it would be confined to the locality to -which it finds itself adapted; not that it has become adapted to the -environment through competition with the original species, or, in fact, -with any other. - -Thus, from the point of view that is here taken, an animal does not -become degenerate because it becomes parasitic, but the environment -being given, some forms have found their way there; in fact, we may -almost say, have been forced there, for these degenerate forms can only -exist under such conditions. - -In conclusion, this much at least can be claimed for the mutation -theory; that it meets with no serious difficulty in connection with the -phenomena of degeneration. It meets with no difficulty, because it makes -no pretence to explain the origin of adaptations, but can account for -the occurrence of degenerate forms, if it is admitted that these appear -as mutations, or as definite variations. Let us, however, not close our -eyes to the fact that there is still much to be explained in respect to -the degeneration of animals and plants. It is far from my purpose to -apply the mutation theory to all adaptations; in fact, it will not be -difficult to show that there are many adaptations whose existence can -have nothing directly to do with the mutation theory. - - - Protective Coloration - -That many species of animals are protected by their resemblance to their -environment no one will probably deny. That we are ignorant in all cases -as to how far this protection is necessary for the maintenance of the -species must be admitted. That some of the resemblances that have been -pointed out have been given fictitious value, I believe very probable. - -Resemblance in color between the organism and its environment has given -to the modern selectionist some of his most valuable arguments, but we -should be on our guard against supposing that, because an animal may be -protected by its color, the color has been acquired on this account. On -the supposition that the animal has become adapted by degrees, and -through selection, we meet with all the objections that have been urged, -in general, against the theory of natural selection. But if we assume -here also that mutations have occurred without relation to the -environment, and, having once appeared, determined in some cases the -distribution of the species, we have at least a simple hypothesis that -appears to explain the facts. If it be claimed that the resemblance is, -in some cases, too close for us to suppose that it has arisen -independently of the environment, it may be pointed out that it has not -been shown that such a close resemblance is at all necessary for the -continued existence of the species, and hence the argument is likely to -prove too much. For instance, the most remarkable case of resemblance is -that of Kallima, but in the light of a recent statement by Dean it may -be seriously asked whether there is absolute need of such a close -resemblance to a leaf. Even if it be admitted that to a certain extent -the butterfly is at times protected by its resemblance to a leaf, it is -not improbable that it could exist almost equally well without such a -close resemblance. If this is true, natural selection could never have -brought about such a close imitation of a leaf. Cases like these of -over-adaptation are not unaccountable on the theory of mutation, for on -this view the adaptation may be far ahead of what the actual -requirements for protection demand. We meet occasionally, I think, -throughout the living world with resemblances that can have no such -interpretation, and a number of the kinds of adaptations to be described -in this chapter show the same relation. - -Some of the cases of mimicry appear also to fall under this head; -although I do not doubt that many so-called cases of mimicry are purely -imaginary, in the sense that the resemblance has not been acquired on -account of its relation to the animal imitated. There is no need to -question that in some cases animals may be protected by their -resemblance to other animals, but it does not follow, despite the -vigorous assertions of some modern Darwinians, that this imitation has -been the result of selection. Until it can be shown that the imitating -species is dependent on its close imitation for its existence, the -evidence is unconvincing; and even if, in some cases, this should prove -to be the case, it does not follow that natural selection has brought -about the result, or even that it is the most plausible explanation that -we have to account for the results. The mutation theory gives, in such -cases, an equally good explanation, and at the same time avoids some of -the difficulties that appear fatal to the selection theory. - -What has been said against the theory of mimicry might be repeated in -much stronger terms against the hypothesis of warning colors. - -It seems to me, in this connection, that the imagination of the -selectionist has sometimes been allowed to “run wild”; and while it may -be true that in some cases the colors may serve as a signal to the -possible enemies of the animal, it seems strange that it has been -thought necessary to explain the origin of such colors as the result of -natural selection. Indeed, some of these warning colors appear -unnecessarily conspicuous for the purpose they have to perform. In other -words, it does not seem plausible that an animal already protected -should need to be so conspicuous. If we stop for a moment to consider -what an enormous amount of destruction must have occurred, according to -Darwin’s theory, in order to bring this warning coloration to its -supposed state of perfection, we may well hesitate before committing -ourselves to such an extreme view. - -That gaudy colors have appeared or been present in animals that are -protected in other ways is not improbable, when we consider the rôle -that color plays everywhere in nature. That the presence of such colors -may, to a certain limited extent, protect its possessor may be admitted -without in any degree supposing that natural selection has directed the -evolution of such color, or that it has been acquired through a life and -death struggle of the individuals of the species. - - - Sexual Dimorphism[29] and Trimorphism - -Footnote 29: - - This term is used here in the sense employed by Darwin. The same term - is sometimes used for those cases in which the male departs very - greatly from the female in form. - -It has been found in a few species of animals and plants that two or -more forms of one sex may exist, and here we find a condition that -appears to be far more readily explained on the mutation theory than on -any other. The most important cases, perhaps, are those in plants, but -there are also similar cases known amongst animals, and these will be -given first. - -There is a North American butterfly, _Papilio turnus_, that appears -under at least two forms. In the eastern United States the male has -yellow wings with black stripes. There are two kinds of females, one of -which resembles the male except that she has also an orange “eye-spot”; -the other female is much blacker, and this variety is found particularly -in the south and west. The species is dimorphic, therefore, mainly in -the latter regions. - -The cases of seasonal dimorphism offer somewhat similar illustrations. -The European butterfly, _Vanessa levana-prorsa_, has a spring generation -(_levana_) with a yellow and black pattern on the upper surface of the -wings. The summer generation (_prorsa_) has black wings “with a broad -white transverse band, and delicate yellow lines running parallel to the -margins.” These two types are sharply separated, and their differences -in color do not appear to be associated with any special protection that -it confers on the bearer. These facts in regard to Vanessa seem to -indicate that differences may arise that are perfectly well marked and -sharply defined, which yet appear to be without any useful significance. - -We meet with cases in which the same animal has at different times of -year different colors, as seen in the summer and winter plumage of the -ptarmigan. There is no direct evidence to show how this seasonable -change has been brought about; but from the facts in regard to Vanessa -we can see that it might have been at least possible for the white -winter plumage, for instance, to have appeared without respect to any -advantage it conferred on the animal, but after it had appeared it may -have been to a certain degree useful to its possessor. - - -[Illustration: - - Fig. 5.—A, long-styled, and - B, short-styled, forms of _Primula veris_. - C, D, E, the three forms of the trimorphic flower of _Lythrum - salicaria_, with petals and calyx removed on near side. (After - Darwin.)] - -Amongst plants there are some very interesting cases of dimorphism and -trimorphism in the structure of the flowers. Darwin has studied some of -these cases with great care, and has made out some important points in -regard to their powers of cross-fertilization.[30] The common European -cowslip, _Primula veris_, var. _officinalis_, is found under two forms, -Figure 5 A and B, which are about equally abundant. In one the style is -long so that the stigma borne on its end comes to the top of the tube of -the corolla. The stamens in this form stand about halfway up the tube. -This is called the long-styled form. The other kind, known as the -short-styled form, has a style only half as long as the tube of the -corolla, and the stamens are attached around the upper end of the tube -near its opening. In other words, the position of the end of the style -(the stigma) and that of the stamens is exactly reversed in the two -forms. The corolla is also somewhat differently shaped in the two forms, -and the expanded part of the tube above the stamens is larger in the -long-styled than in the short-styled form. Another difference is found -in the stigma, which is globular in the long-styled, and depressed on -its top in the short-styled, form. The papillæ on the former are twice -as long as those on the short-styled form. The most important difference -is found in the size of the pollen grains. These are larger in the -long-styled form, being in the two cases in the proportion of 100 to 67. -The shape of the grains is also different. Furthermore, the long-styled -form tends to flower before the other kind, but the short-styled form -produces more seeds. The ovules in the long-styled form, even when -unfertilized, are considerably larger than those of the short-styled, -and this, Darwin suggests, may be connected with the fact that fewer -seeds are produced, since there is less room for them. The important -point for our present consideration is that intermediate forms do not -exist, although there are fluctuating variations about the two types. -Moreover, the two kinds of flowers never appear on the same plant. - -Darwin tried the effect of fertilizing the long-styled flowers with the -pollen from the same flower or from other long-styled flowers. Unions of -this sort he calls illegitimate, for reasons that will appear later. He -also fertilized the long-styled flowers with pollen from short-styled -forms. A union of this sort is called legitimate. Conversely, the -short-styled forms were fertilized with their own pollen or with that -from another short-styled form. This is also an illegitimate union. -Short-styled forms fertilized with pollen from long-styled forms give -again legitimate unions. - -Footnote 30: - - Many of the facts as to the occurrence of these cases were known - before Darwin worked on them; but very little had been ascertained in - regard to the sexual relation between the dimorphic and trimorphic - forms, and it was here that Darwin obtained his most interesting - results. - -The outcome of these different crossings are most curious. In the table, -page 364, the results of the four combinations are given. It will be -seen at once that the legitimate unions give more capsules, and the -seeds weigh more, than in the illegitimate unions. - -The behavior of the offspring from seeds of legitimate and -illegitimate origin is even more astonishing. Darwin found in _Primula -veris_ (the form just described) that the seeds from the short-styled -form fertilized with pollen from the same form germinated so badly -that he obtained only 14 plants, of which 9 were short-styled and 5 -long-styled. The long-styled form fertilized with its own-styled -pollen produced “in the first generation 3 long-styled plants. From -their seed 53 long-styled grandchildren were produced; from their seed -4 long-styled great-grandchildren; from their seed 20 long-styled -great-great-grandchildren; and lastly, from their seed 8 long-styled -and 2 short-styled great-great-great-grandchildren.” - - - ══════════════╤═════════╤═════════╤═════════╤═════════╤═════════ - │Number of│ │ Maximum │ Minimum │ Average - Nature of │ Flowers │Number of│of Seeds │of Seeds │ No. of - Union │Fertilized│ Seed │ in any │ in any │Seeds per - │ │Capsules │ one │ one │ Capsule - │ │ │ Capsule │ Capsule │ - ──────────────┼─────────┼─────────┼─────────┼─────────┼───────── - Long-styled │ 10 │ 6 │ 62 │ 34 │ 46.5 - form by │ │ │ │ │ - pollen of │ │ │ │ │ - short-styled│ │ │ │ │ - form: │ │ │ │ │ - Legitimate │ │ │ │ │ - union. │ │ │ │ │ - ──────────────┼─────────┼─────────┼─────────┼─────────┼───────── - Long-styled │ 20 │ 4 │ 49 │ 2 │ 27.7 - form by │ │ │ │ │ - own-form │ │ │ │ │ - pollen: │ │ │ │ │ - Illegitimate│ │ │ │ │ - union. │ │ │ │ │ - ──────────────┼─────────┼─────────┼─────────┼─────────┼───────── - Short-styled │ 10 │ 8 │ 61 │ 37 │ 47.7 - form by │ │ │ │ │ - pollen of │ │ │ │ │ - long-styled │ │ │ │ │ - form: │ │ │ │ │ - Legitimate │ │ │ │ │ - union. │ │ │ │ │ - ──────────────┼─────────┼─────────┼─────────┼─────────┼───────── - Short-styled │ 17 │ 3 │ 19 │ 6 │ 12.1 - form by │ │ │ │ │ - own-form │ │ │ │ │ - pollen: │ │ │ │ │ - Illegitimate│ │ │ │ │ - union. │ │ │ │ │ - ──────────────┼─────────┼─────────┼─────────┼─────────┼───────── - The two │ 20 │ 14 │ 62 │ 37 │ 47.1 - legitimate │ │ │ │ │ - unions │ │ │ │ │ - together. │ │ │ │ │ - ──────────────┼─────────┼─────────┼─────────┼─────────┼───────── - The two │ 30 │ 7 │ 49 │ 2 │ 35.5 - illegitimate│ │ │ │ │ - unions │ │ │ │ │ - together. │ │ │ │ │ - ══════════════╧═════════╧═════════╧═════════╧═════════╧═════════ - - -From other long-styled plants, fertilized with their own-form pollen, 72 -plants were raised, which were made up of 68 long-styled and 4 -short-styled. In all, 162 illegitimate unions of this sort produced 156 -long-styled and 6 short-styled plants. It is evident from these results -that the long-form pistils, fertilized with pollen from flowers of the -same pistil-form (from other individuals as a rule), tend to produce the -same form as their parents, although occasionally the other form. The -fertility of these plants from an illegitimate union is found to be very -low. Darwin observed that sometimes the male and female organs of these -plants were in a very deteriorated condition. It is interesting to -notice, in this connection, that in another species, _Primula sinensis_, -illegitimate plants from long-styled parents were vigorous, but the -flowers were small and more like the wild form. They were, however, -perfectly fertile. - -Illegitimate plants from short-styled parents were dwarfed in stature, -and often had a weakly constitution. They were not very fertile _inter -se_, and remarkably infertile when legitimately fertilized. This kind of -result, where a difference in the power of mutual intercrossing exists -between two forms, recalls in many ways the difference in the results of -crossing of different species of animals and plants, especially those -cases in which a cross can be made in one way more successfully than in -the other. - -The heterostyled trimorphic plants, of which _Lythrum salicaria_, Figure -5 C, D, E, may be taken as an example, are even more remarkable. There -are three different kinds of flowers: in one the pistil is long and -there is a medium and a short set of stamens; in another the pistil is -of intermediate length and there is a long set and a short set of -stamens; in the third kind the pistil is short, and there is a medium -and a long set of stamens. There are possible only six sorts of -legitimate unions between these three sets of flowers. No less than -twelve kinds of illegitimate unions may occur. In regard to the -difference in the sizes of the pollen grains, those from the long-styled -form are the largest, from the mid-styled form next, and from the -short-styled form the smallest. The extreme difference is as 100 to 60. -“Nothing shows more clearly the extraordinary complexity of the -reproductive system of this plant than the necessity of making eighteen -distinct unions in order to ascertain the relative fertilizing power of -the three forms.” Darwin tried the effect of each of these combinations, -making 223 unions in all. The results are surprising. Comparing the -outcome of the six legitimate unions with the twelve illegitimate ones, -the following results were obtained:— - - - ═══════════════╤══════════╤══════════╤══════════╤══════════ - │Number of │ │ │ Average - │ Flowers │Number of │ Average │ No. of - Nature of Union│Fertilized│ Capsules │ No. of │Seeds per - │ │ Produced │Seeds per │ Flower - │ │ │ Capsule │Fertilized - ───────────────┼──────────┼──────────┼──────────┼────────── - The 6 │ 75 │ 56 │ 96.29 │ 71.89 - legitimate │ │ │ │ - unions │ │ │ │ - ───────────────┼──────────┼──────────┼──────────┼────────── - The 12 │ 146 │ 36 │ 44.72 │ 11.03 - illegitimate │ │ │ │ - unions │ │ │ │ - ═══════════════╧══════════╧══════════╧══════════╧══════════ - - -This table shows that the fertility of the legitimate to that of the -illegitimate is as 100 to 33, as judged by the flowers that produced -capsules; and as 100 to 46 as judged by the average number of seeds per -capsule. It is evident, therefore, that “it is only the pollen from the -longest stamens that can fully fertilize the longest pistil; only that -from the mid-length stamens, the mid-length pistil; and only that from -the shortest stamens, the shortest pistil.” - -Darwin tries to connect this fact with the visits of insects to the -flowers. He says: “And now we can comprehend the meaning of the almost -exact correspondence in length between the pistil in each form and of a -set of six stamens in two of the other forms; for the stigma of each -form is thus rubbed against that part of the insect’s body which becomes -charged with the proper pollen.” A further conclusion that Darwin draws -is “that the greater the inequality in length between the pistil and the -set of stamens, the pollen of which is employed for its fertilization, -by so much is the sterility the more increased.” Darwin also makes the -following significant comment on the problem here involved: “The -correspondence in length between the pistil in each form, and a set of -stamens in the other two forms, is probably the direct result of -adaptation, as it is of the highest service to the species by leading to -full and legitimate fertilization.” He points out, on the other hand, -that the increased sterility of the illegitimate unions, in proportion -to the inequality in length between the pistil and the stamens employed, -can be of no service at all. Neither can this relation have any -connection with the facility for self-fertilization. “We are led, -therefore, to conclude that the rule of increased sterility in -accordance with increased inequality in length between the pistils and -stamens is a purposeless result, incidental on those changes through -which the species has passed in acquiring certain characters fitted to -insure the legitimate fertilization of the three flowers.” - -In regard to the plants that were raised from the seeds from legitimate -and illegitimate unions, Darwin found in Lythrum that of twelve -illegitimate unions two were completely barren, and nearly all showed -lessened fertility; only one approached complete fertility. Darwin lays -much emphasis on the close resemblance in the sterility of the -illegitimate unions, and the sterility of different species when -crossed. In both cases every degree of sterility is met with, “from very -slightly lessened fertility to absolute barrenness.” The importance of -this comparison cannot, I think, be overestimated, for, if admitted, it -indicates clearly that the infertility between species cannot be used as -a criterion of their distinctness, because here, in individuals -belonging to the same species, we find sterility between pistils and -stamens of different lengths. If, as I shall urge below, we must -consider these different forms of Primula the results of a mutation, and -not the outcome of selection as Darwin supposed, then this relation in -regard to infertility becomes a point of great interest. - -This brings us to the central point of our examination of these cases of -dimorphism and trimorphism. How have these forms arisen? Darwin tries to -account for them as follows: Since heterostyled plants occur in fourteen -different families of plants, it is probable that this condition has -been acquired independently in each family, and “that it can be acquired -without any great difficulty.” The first step in the process he imagines -to have been due to great variability in the length of the pistil and -stamens, or of the pistil alone. Flowers in which there is a great deal -of variation of this sort are known. “As most plants are occasionally -cross-fertilized by the aid of insects, we may assume that this was the -case with our supposed varying plant; but that it would have been -beneficial to it to have been more regularly cross-fertilized.” “This -would have been better accomplished if the stigma and the stamens stood -at the same level; but as the stamens and pistil are supposed to have -varied much in length, and to be still varying, it might well happen -that they could be reduced much more easily through natural selection -into two sets of different lengths in different individuals than all to -the same length and level in all individuals.” By means of these -assumptions, improbable as they may appear, Darwin tries to explain -these cases of dimorphism. But when we attempt to apply the same -argument to the trimorphic forms, it is manifestly absurd to pretend -that three such sharply defined types could ever have been formed as the -result of natural selection. But we have not even yet touched the chief -difficulty, as Darwin himself points out. “The essential character of a -heterostyled plant is that an individual of one form cannot fully -fertilize, or be fertilized by, an individual of the same form, but only -by one belonging to another form.” This result Darwin admits cannot be -explained by the selection theory, for, as he says, “How can it be any -advantage to a plant to be sterile with half of its brethren, that is, -with the individuals belonging to the same form?” He concludes that this -sterility between the individuals of the same form is an incidental and -purposeless result. “Inner constitutional differences” between the -individuals is the only suggestion that is offered to account for the -phenomenon. In other words, it is clearly apparent that the attempt to -apply the theory of selection has here broken down, and it is a -fortunate circumstance that the Lamarckian theory cannot here be brought -to the rescue, as it so often is in Darwin’s writings, when the theory -of natural selection fails to give a sufficient explanation. - -On the other hand, this is one of the cases that seem to fit in -excellently with the mutation theory, for if these two forms of the -primrose should appear, as mutations, and if, as is the case, they do -not blend when crossed, but are equally inherited, they would both -continue to exist as we find them to-day. Whether the similar forms were -infertile with each other would be determined at the outset by the -nature of the individual variation, and if, despite this obvious -disadvantage, the forms could still continue to propagate themselves, -the new dimorphic form would remain in existence. Darwin cannot explain -the origin of dimorphic forms and trimorphic forms unless he can show -that there is some advantage in having two forms, and as we have seen, -he fails completely to show that there is an advantage. On the other -hand, the result might have been reached on the mutation theory, even if -the dimorphic and trimorphic forms were placed at a greater disadvantage -than were the parent forms. In such a case fewer individuals might -appear, or find a foothold; but as long as the race could be kept up the -new forms would remain in existence. Thus, while no attempt is made to -explain what has always been, and may possibly long remain, inexplicable -to us, namely, the origin of the new form itself, yet granting that such -new forms may sometimes appear spontaneously, they may be able to -establish themselves, regardless of whether they are a little more or a -little less well adapted to the environment than were their parent -forms. If it should appear that the question is begged by the assumption -that mutations such as these may appear (at one step or by a series of -steps is immaterial), it should not be forgotten that the whole -Darwinian theory itself also rests on the spontaneous appearance of -fluctuating variations, whose origin it does not pretend to explain. In -this respect both theories are on the same footing, but where the -Darwinian theory meets with difficulties at every turn by assuming that -new forms are built up through the action of selection, the mutation -theory escapes most of these difficulties, because it applies no such -rigid test as that of selection to account for the presence of new -forms. - - - Length of Life as an Adaptation - -It has been pointed out in the first chapter that the length of life of -the individual has been supposed by some of the most enthusiastic -followers of Darwin to be determined by the relation of the individual -to the species as a whole. In other words, the doctrine of utility has -been applied here also, on the ground that it would be detrimental to -the species to have part of the individuals live on to a time when they -can no longer propagate the race or protect the young. It is assumed -that those varieties or groups of individuals (unfortunately not sharply -defined) would have the best chance to survive in which the parent forms -died as soon as they had lost the power to produce new individuals. -Sometimes interwoven with this idea there is another, namely, that -_death itself_ has been acquired because it was more profitable to -supplant the old and the injured individuals by new ones, than to have -the old forms survive, and thus deprive the reproducing individuals of -some of the common food supply. - -This insidious form that the selection theory has taken in the hands of -its would-be advocates only serves to show to what extremes its -disciples are willing to push it. On the whole it would be folly to -pursue such a will-o’-the-wisp, when the theory can be examined in much -more tangible examples. If in these cases it can be shown to be -improbable, the remaining superstructure of quasi-mystical hypothesis -will fall without more ado. - -That the problem of the length of life may be a real one for -physiological investigation will be granted, no doubt, without -discussion, and that in some cases the length of life and the coming to -maturity of the germ-cell may be, in some way, physiologically connected -seems not improbable; but that this relation has been regulated by the -competition of species with each other can scarcely be seriously -maintained. I will not pretend to say whether the mutation theory can or -cannot be made to appear to give the semblance of an explanation of the -length of life in each species, but it seems to me fairly certain that -this is one of the questions which we are not yet in a position to -attempt to consider on any theory of evolution. - - - Organs of Extreme Perfection - -It has often been pointed out that certain organs may be more perfectly -developed than the requirements of the surroundings strictly demand. At -least we have no good reasons to suppose in some cases that constant -selection is keeping certain organs at the highest possible point of -development, yet, on the Darwinian theory, as soon as selection ceases -to be operative the level of perfection must sink to that which the -exigencies of the situation demand. The problem may be expressed in a -different way. Does the animal or plant ever possess organs that are -more perfectly adapted than the absolute requirements demand? If such -organs are the result of fluctuating variations, they will be unable to -maintain themselves in subsequent generations without a constant process -of selection going on. If, on the other hand, the organs have arisen as -mutations, they may become permanently established without respect to -the degree of perfection of their adaptation. We can see, therefore, -that cases of extreme perfection meet with no difficulty on the mutation -theory, while they have proven one of the stumbling-blocks to the -selection theory. - -There are, in fact, many structures in the animal and plant kingdoms -that appear to be more perfect than the requirements seem to demand. The -exact symmetry of many forms appears in some cases to be unnecessarily -perfect. The perfection of the hand of man, the development of his vocal -organs, and certain qualities of his brain, as his musical and -mathematical powers, seem to go beyond the required limits. It is not, -of course, that these things may not be of some use, but that their -development appears to have gone beyond what selection requires of these -parts. - -Closely related to this group of phenomena are those cases in which -certain organs are well developed, but which can scarcely be of use to -the animal in proportion to their elaboration. The electric organs of -several fishes and skates are excellent examples of this sort of -structures. The phosphorescent organs do not appear, in some forms at -least, to be useful in proportion to their development. The selection -theory fails completely to explain the building up of organs of this -kind, but on the mutation theory there is no difficulty at all in -accounting for the presence of even highly developed organs that are of -little or of no use to the individual. If the organs appeared in the -first place as mutations, and their presence was not injurious to the -extent of interfering seriously with the existence and propagation of -the new form, this new form may remain in existence, and if the -mutations continued in the same direction, the organs might become more -perfect, and highly developed. The whole class of secondary sexual -organs may belong to this category, but a discussion of these organs -will be deferred to the following section. - - - Secondary Sexual Organs as Adaptations - -In the sixth chapter we have examined at some length Darwin’s -interpretation of the secondary sexual characters. His explanation has -been found insufficient in many cases to account for the conditions. -That these organs do play in some cases a role in the relation of the -sexes to each other may be freely admitted. In other words, in some -animals the organs in one sex appear in the light of adaptations to -certain instincts in the other sex. It would, perhaps, appear to -simplify the problem to deny outright that any such relation exists; but -I think, in the light of the evidence that we have, this procedure would -be like that of the proverbial ostrich, which is supposed to stick its -head in the sand in order to escape an anticipated danger. If we assumed -this agnostic position, we might attempt to account for the appearance -of secondary sexual organs as mutations that had appeared in one sex, -and had no immediate connection with the other sex; and, so long as -these organs were not directly and seriously injurious, we might assume -that the animals in which such structures had appeared might be able to -exist. But, on the other hand, I think that an examination of the -evidence will show that this way out of the difficulty is not very -satisfactory, for the organs in question appear, in some cases at least, -to be closely connected with certain definite responses in the other -sex. Moreover, as Darwin has so insistently pointed out, the action of -the males is of such a sort that it is evidently associated with the -presence of the secondary sexual organs which they often display before -the other sex. Furthermore, the greater and often exclusive development -of these organs during the sexual period distinctly points to them as in -some way connected with the relation of the sexes to each other. And -finally, there is a small, although not entirely convincing, body of -evidence, indicating that the female is influenced by the action of the -male; but I do not think that this evidence shows that she selects one -individual at the expense of all other rivals. We meet here with a -problem that is as profoundly interesting as it is obscure. In fact, if -we admit that this relation exists we have a double set of conditions to -deal with: first, the development in the males of certain secondary -sexual organs; and secondly, the instinct to display these organs. The -supposed influence of the display on the female may also have to be -taken into account, although, for all we know to the contrary, the same -results might follow were there no secondary sexual character at all, as -is, in fact, the case in most animals. - -I have a strong suspicion that much that has been written on this -subject is imaginative, and in large part fictitious; so that it may, -after all, be the wisest course not to attempt to explain how this -relation has arisen until we have a more definite conception of what we -are really called upon to explain. For example, when we see a gorgeously -bedecked male displaying himself before a female, we feel that his -finery must have been acquired for this very purpose. On the other hand, -when we see an unornamented male also making definite movements before -the female, we do not feel called upon to explain the origin of his -colors. Now, it is not improbable that the ornaments of the first -individual have not been acquired in order to display them before the -female, and this view seems to me the more probable. From this -standpoint our problem is at least much simplified. What we need to -account for is only that the male is excited to undergo certain -movements in the presence of the female, and possibly that the female -may be influenced by the result. That this view is the more profitable -is indicated by the occurrence of secondary sexual characters in the -lower forms, as in the insects and crustaceans, in which it appears -almost inconceivable that the ornamentation could have been acquired in -connection with the æsthetic taste of the other sex. It does not seem to -me that the conditions in the higher animals call for any other -explanation than that which applies to these lower forms. - -My position may be summed up in the statement, that, while in some cases -there appears to be a connection between the presence in one sex of -secondary sexual organs and their effect on the other sex, yet their -origin cannot be explained on account of this connection. - - - Individual Adjustments as Adaptations - -As pointed out in the first chapter, there is a group of adaptations, -obviously including several quite different kinds of phenomena, that can -at least be conveniently brought together under the general rubric of -individual adjustments or regulations. A few examples of these will -serve to show in what sense they may be looked upon as adaptations, and -how they may be regarded from the evolutionary point of view. - - - Color Changes as Individual Adaptations - -The change in color of certain fish in response to the color of the -background, the change in color of some chrysalides also in response to -their surroundings, appears to be of some use to the animals in -protecting them from their enemies. The change in color from green to -brown and from brown back to green in several lizards and in some tree -frogs is popularly supposed to be in response to the color of the -surroundings, but a more searching examination has shown that, in some -cases at least, the response has nothing to do with the color of the -background. - -In the first cases mentioned above, in which the response appears to be -of some advantage to the animal, the question may be asked, how have -such responses arisen? The selection theory assumes that those animals -that responded at first to a slight degree in a favorable direction have -escaped, and this process being repeated, the power to change has been -gradually built up. The mutation theory will also account for the result -by assuming the response to have appeared as a new quality, but it has -been preserved, not because it has been of vital importance to its -possessor, but simply because the species possessing it has been able to -survive, perhaps in some cases even more easily, although this is not -essential. Even if the change were of no direct benefit, or even -injurious to a slight degree, it might have been retained, as appears in -fact to be the case in the change of color of the green lizards. - - - Increase of Organs through Use and Decrease through Disuse - -We meet here with one of the most characteristic and unique features of -living things as contrasted with non-living things. We shall have to -dismiss at once the idea that we can explain this attribute of organisms -by either the selection or the mutation theory; for we find animals -possessing this power that could never be supposed to have acquired it -by any experience to which they have been subjected; and since it -appears to be so universally present, we cannot account for it as a -chance mutation that may have appeared in each species. No doubt Wolff -had responses of this kind in view when he made the rather sweeping -statement that purposeful adaptation is the most characteristic feature -of living things. The statement appears to contain a large amount of -truth, if confined to the present group of phenomena. - -This power of self-regulation may confer a great benefit on its -possessor. The increase in the size and strength of the muscles through -use may give the animal just those qualities that make its existence -easier. The increase in the power of vision, or at least of visual -discrimination through use, of the power of smell and of taste, of -hearing and of touch, are familiar examples of this phenomenon. - -However much we may be tempted to speculate as to how this property of -the animal may have been acquired, we lack the evidence which would -justify us in formulating even a working hypothesis. It may be that when -we come to know more of what the process of contraction of the muscle -involves, the possibility of its development as a consequence of its use -may be found to be a very simple phenomenon that requires no special -explanation at all to account for its existence in the individual, -further than that the muscles are of such a kind that this is a -necessary physical result of their action. But until we know more of the -physiology involved in the process, it is idle to speculate about the -origin of the phenomenon. - - - Reactions of the Organism to Poisons, etc. - -In this case also we meet with a number of responses for whose origin we -can give not the shadow of an explanation. On the other hand, the cases -are significant in so far as a number of them show quite clearly that -the response cannot have been acquired through the experience of the -organism, or the selection of those individuals that have best resisted -the particular poison. This is true, because in a number of cases the -poison is a substance that the animal cannot possibly have met with -during the ordinary course of its life, or of that of its ancestors. It -may be argued, it is true, that in the case of the poisons produced by -certain bacteria the power of resistance has been acquired through the -survival of the less susceptible, or more resistant, individuals. -Improbable as this may be in some cases, it does not, even if it were -true, alter the real issue, for it can be shown, as has just been said, -that the same power of responding adaptively is sometimes shown in cases -of poisons that are new to the animal. - -There is no question that different individuals respond in very -different degrees to these poisonous substances, and it is easy to -imagine in the case of contagious diseases that a sort of selective -process might go on that would bring the race up to the highest point to -which fluctuating variations could be carried, even to complete -immunity; but even if this were the case, it seems to be true that the -moment the selection stopped the race would sink back to the former -condition. - -All this touches only indirectly the main point that we have under -consideration, namely, the existence of this power of resistance in -cases where it cannot have been the result of any educative process. -Since the responses to new poisons do not appear to be in principle -different from the responses to those to which the organism may have -possibly been subjected at times in the past, we shall probably not go -far wrong if we treat all cases on the same general footing. Whether the -power of adaptation to certain substances, such as nicotine, morphine, -cocaine, arsenic, alcohol, etc., is brought about by the formation of a -counter-substance is as yet unproven. And while it seems not improbable -that in some of these instances it may turn out that this is the case, -especially for poisons of plant origin, it is better to suspend judgment -on this point until each case has been established. - -In recent years it has been shown that the animal body has the power of -making counter-substances when a very large number of different kinds of -things are introduced into the blood. We seem to be here on the -threshold of a field for discovery which may, if opened up, give us an -insight into some of the most remarkable phenomena of adaptation shown -by living things. - -It has already been pointed out that it appears to be almost a _reductio -ad absurdum_ to speak of animals adapting themselves to poisonous -substances. It is curious, too, that in man at least the use of these -substances may arouse a craving for the poison, or at any rate the -individual may become so dependent on the poison that the depression -following its disuse may lead to a desire for a repetition of the dose. -The two questions that are raised here must be kept apart, for the -adaptation of the individual to the poison and the so-called craving for -it may depend on quite different factors. Nevertheless, it seems to be -true in the case of morphine and of arsenic, and probably for some other -substances as well, that if their use is suddenly stopped the individual -may die in consequence. In this respect the organism behaves exactly as -it does to an environment to which it has become adapted. - - - Regeneration - -Many animals are able to replace lost parts, and all of them can heal -wounds and mend injuries. This power is obviously of great advantage to -them, and it has been supposed by Darwin, and more especially by his -followers, that the power has been acquired through natural selection. -It is not difficult to show that regeneration could not, in many cases, -and presumably in none, have been acquired in this way. Since I have -treated this subject at some length recently in my book on -“Regeneration,” I shall attempt to do no more here than indicate the -outline of the argument. - -The Darwinians believe that, if some individuals of a species have the -power to replace a part that is lost better than have other individuals, -it would follow that those would survive that regenerate best, and in -this way after a time the power to regenerate perfectly would be -acquired. - -But the matter is by no means so simple as may appear from this -statement. In the first place, it is a matter of common observation that -all the individuals of a species are never injured in the same part of -the body at the same time. In those cases in which it is known that a -special part is often injured, an examination has shown that there are -not more than ten per cent of individuals that are injured at any one -time, and in the case of the vast majority of animals this estimate is -much too great. Thus there will be very little chance for competition of -the injured individuals in each generation with each other, and the -effects that are imagined to be gained as a result would be entirely -lost by crossing with the uninjured individuals. But it is not necessary -to consider this possibility, since there is another fact that shows at -once that the power to regenerate could not have been gained through -selection. The number of uninjured individuals in each generation will -be much greater than the injured ones, and these will have so great an -advantage over the injured individuals that, if competition approached -the degree assumed by the selectionists, the injured individuals should -be exterminated. A slight advantage gained through better powers of -regeneration would be of little avail in competition, as compared with -the competition with the uninjured individuals. Since selection is -powerless to accomplish its end without competition, and since with -competition all the injured individuals would be eliminated, it is clear -that an appeal cannot be made to selection to explain the power of -regeneration. - -In many cases the power of regeneration could not have been slowly -acquired through selection, since the intermediate steps would be of no -use. Unless, for example, a limb regenerated from the beginning almost -completely, the result would be of no use to the animal. If the limb did -regenerate completely the first time it was injured, then the selection -hypothesis becomes superfluous. - -There are also a few cases known in which a process of regeneration -takes place that is of no use to the animal. If, for instance, the -earthworm (_Allolobophora fœtida_) be cut in two in the middle, the -posterior piece regenerates at its anterior cut end, not a head, but a -tail. Not by the widest stretch of the imagination can such a result be -accounted for on the selection theory. Again, we find the reverse case, -as it were, in certain planarians. If the head of _Planaria lugubris_ is -cut off just behind the eyes, there develops at the cut surface of this -head-piece another head turned in the opposite direction. Here again we -have the regeneration of a perfect structure, but one that is entirely -useless to the individual. The development of an antenna in place of an -eye in the shrimp, when the eye stalk is cut off near its base, is -another instance of the occurrence of a perfectly constant process, but -one that is of no use to the organism. - -When we recall that in some organisms regeneration takes place in almost -every part of the body, it does not seem possible that this power could -have been acquired by selection. And when we find that many internal -organs regenerate, that can rarely or never be injured without the -animal perishing, it seems impossible that this can be ascribed to the -principle of natural selection. - -It has also been found that if the first two cells of the egg of a -number of animals, jellyfish, sea-urchins, salamanders, etc., be -separated, each will produce an entire animal. In some of these cases it -is inconceivable that the process could ever have been acquired through -selection, because the cells themselves can be separated only by very -special and artificial means. - -These, and other reasons, indicate with certainty that regeneration -cannot be explained by the theory of natural selection. - - ------------------------------------------------------------------------- - - - - - CHAPTER XI - - TROPISMS AND INSTINCTS AS ADAPTATIONS - - -Of the different kinds of adaptation none are more remarkable than those -connected with the immediate responses of organisms to external agents. -These responses are usually thought of as associated with the nervous -system; and while in the higher forms the nervous system plays an -important role in the reaction, yet in many cases it is little more than -the shortest path between the point stimulated and the muscles that -contract; and in the lower animals, where we find just as definite -responses, there may be no distinct nervous system, as in the protozoa, -for instance. - -Many of the so-called instincts of animals have been shown in recent -years to be little more than direct responses to external agents. Many -of these instincts are for the good of the individual, and must be -looked upon as adaptations. For example: if a frog is placed in a jar of -water, and the temperature of the water lowered, the frog will remain at -the top until the water reaches 8 degrees C., when it will dive down to -the bottom of the jar; and, if the temperature is further lowered, it -will remain there until the water becomes warmer again, when it will -come to the surface again. It is clear that, under the ordinary -conditions of life of the frog, this reaction is useful to it, since it -leads the animal to go to the bottom of the pond on the approach of cold -weather, and thus to avoid being frozen at the surface. - -Another illustration of an instinct that is a simple response to light -is shown by the earthworm. During the day the worm remains in its -burrow, but on dark nights it comes out of its hole, and lies stretched -out on the surface of the ground. It procures its food at this time, and -the union of the individuals takes place. In the early morning the worm -retires into its burrow. - -This habit of the earthworm is the direct result of its reaction to -light. It crawls away from ordinary light as bright as that of diffuse -daylight, and, indeed, from light very much fainter than that of -daylight. If, however, the light be decreased to a certain point, the -worm will then turn and crawl toward the source of light. This lower -limit has been found by Adams to be about that of .001 candle-metre. -This corresponds to the amount of light of a dark night, and gives an -explanation of why the worm leaves its burrow only at night, and also -why it crawls back on the approach of dawn. It is also obvious that this -response is useful to the animal, for if it left the burrow during the -day, it would quickly fall a prey to birds. - -The blow-fly lays its eggs on decaying meat, on which the larvæ feed. -The fly is drawn to the meat by its sense of smell, a simple and direct -response to a chemical compound given off by the meat. The maggot that -lives in the decaying meat is also attracted by the same odor, as Loeb -has shown, and will not leave the meat, or even a spot on a piece of -glass that has been smeared with the juice of the meat, so long as the -odor remains. Here again the life of the race depends on the proper -response to an external agent, and the case is all the more interesting, -since the response of the fly to the meat is of no immediate use to the -fly itself, but to the maggot that hatches from the egg of the fly. - -The movement toward or from a stimulating agent is, in some cases, -brought about in the following way. Suppose an earthworm is lying in -complete darkness, and light be thrown upon it from one side. The worm -turns its head, as it thrusts it forward, to the side away from the -light; and as it again moves forward, it continues to bend its head away -from the light, until it is crawling directly away from the source. When -the light first strikes the worm, the two sides will be differently -illuminated. This causes a bending of the head, as it stretches forward, -toward the side of less illumination, and the bending is due to a -stronger contraction of some of the muscles on the less illuminated -side; at least the reaction appears to be due to a simple response of -this kind. When the body has been so far turned that the two sides are -equally illuminated, the muscles of the two sides will contract equally, -and the movement will be straight forward and away from the light. If -the reaction is as simple as this (which is in principle the explanation -advanced by Loeb), the result is a simple reflex act, and need not -involve any consciousness or intentional action on the part of the worm -to crawl away from the light. In fact, the same reaction takes place -when the brain is removed, not so quickly or definitely, it is true, but -this may be due to the removal of the anterior segments of the worm, in -which part the skin appears to be more sensitive to light than -elsewhere. - -Another factor that plays an important rôle in the habits of the -earthworm is the response to contact,—the so-called stereotropism. If, -in crawling over a flat surface, the worm comes in contact with a -crevice, it will crawl along it, and refuse to leave until the end is -reached. The contact holds the worm as strongly as though it were -actually pulled into the crevice. It can be forced to leave a crevice -only by strong sunlight, and then it does not do so at once. If the worm -crawls into a small glass tube, it is also held there by its response to -contact, and the smaller the tube, the more difficult is it to make the -worm leave by throwing strong sunlight upon it. - -Loeb has found that when winged aphids, the sexual forms, are collected -in a tube, and the tube is kept in a room, the aphids crawl toward the -light. This happens in ordinary diffuse light, as well as in lamplight. -It is stated that the animals orientate themselves towards the light -more quickly when it is strong than when it is weak. They turn their -bodies toward the light, and then move forward in the direction from -which the rays come. It can be shown by a simple experiment that the -aphids are turned by the _direction_ of the light, and not by its -intensity. If they are placed in a tube, and the tube laid obliquely -before a window in such a way that the direct sunlight falls only on the -inner end of the tube, the aphids will, if started at the inner end of -the tube, first crawl toward the outer surface of the tube, and then -wander along this wall, passing out of the region of sunlight into the -end of the tube nearest the window, where they come to rest at the end. -They have moved constantly towards the direction from which the rays -come, passing, as it were, from ray to ray, but each time toward a ray -nearer the source of the light. - -If the tube be turned toward the window, and the window end be covered -with blue glass, the aphids crawl into this end of the tube, as they -would have done had the tube been uncovered. If, on the other hand, the -end of the tube be covered with red glass, they do not crawl into the -part of the tube that is covered, unless they are very sensitive to -light. Even in the latter case they may remain scattered in the red -part, and do not all accumulate at the end, as they do when blue glass -is used. In other words, while they respond to blue as they do to -ordinary light, they behave toward red as they do towards a very faint -light. - -In diffuse daylight the aphids, as has been said, crawl toward the -light, but if they come suddenly into the sunlight they begin to fly. -Thus they remain on the food-plant until the sun strikes it, and then -they fly away. - -The aphid also shows another response; it is negatively geotropic, -_i.e._ it tends to crawl upward against gravity. If placed on an -inclined, or on a vertical, surface, it will crawl upward. Such an -experiment is best made in the dark, since in the light the aphid also -responds to the light. If put on a window it crawls upward never -downward. - -Aphids are also sensitive to heat. If they are placed in a darkened tube -and put near a stove, they crawl away from the warmer end; but if they -are acted upon by the light at the same time, they will be more strongly -attracted by the light than repulsed by the heat. We thus see that there -are at least three external agents that determine the movements of this -animal, and its ordinary behavior is determined by a combination of -these, or by that one that acts so strongly as to overpower the others. - -The swarming of the male and female ants is also largely directed by the -influence of light. Loeb observed that when the direct sunlight fell -full upon a nest in a wall the sexual forms emerged, and then flew away. -Other nests in the ground were affected earlier in the day, because the -sun reached them first. These ants, when tested, were found to respond -to light in the same way as do the aphids. The wingless forms, or worker -ants, do not show this response, and the winged forms soon lose their -strong response to light after they have left the nest. Thus we see that -the heliotropism is here connected with a certain stage in the -development of the individual; and this is useful to the species, as it -leads the winged queens and males to leave the nest, and form new -colonies. Even the loss of response that takes place later may be looked -upon as beneficial to the species, since the queens do not leave the -nest after they have once established it. - -It is familiar to every one that many of the night-flying insects are -attracted to a lamplight, and since those that fly most rapidly may be -actually carried into the flame before they can turn aside, it may seem -that such a response is worse than useless to them. The result must be -considered, however, in connection with other conditions of their life. -The following experiments carried out by Loeb on moths show some of the -responses of these insects to light. - -Night-flying moths were placed in a box and exposed in a room to -ordinary light. As twilight approached the moths became active and began -to fly always toward the window side of the box. They were positively -heliotropic to light of this intensity. If let out of the case, they -flew toward the window, where they remained even during the whole of the -next day, fully exposed to light. If the moth is disturbed in the -daytime, so that it flies, it goes always toward the light, and never -away from it. These facts show that the moth is always positively -heliotropic, and also that the flight toward the lamp is a natural -response, misapplied in this case. That the moths do not fly by day is -due to another factor, namely, the alternation in the degree of their -sensitiveness at different times. But this condition alone does not seem -to account fully for all the facts. - -If the moths are given the alternative of flying toward the evening -light, or toward the lamp, they always go toward the brighter light. -Thus if, when they swarm at dusk, they are set free in the middle of the -room, at the back of which a lamp is burning, the moths fly toward the -window. If, however, they are set free within a metre of the lamp, they -fly toward it. - -The explanation that Loeb offers of the habit of these moths to fly only -in the evening is, that, although they are at all times positively -heliotropic, they respond to light only in the evening. In other words, -it is assumed that there is a periodic change in their sensitiveness to -light, which corresponds with the change from day to night. Loeb says -that, just as certain flowers open only at night, and others only during -the day, so do moths become more responsive in the evening, and -butterflies during the day. Both moths and butterflies are positively -heliotropic, and the sensitiveness of moths to light may be even greater -in the evening than is that of butterflies, for the light of the evening -to which the moth reacts is less than the minimal to which the butterfly -responds. - -Moths appear to pass into a sort of sleep during the day, while -butterflies are quiescent only at night. The periodicity of the sleeping -time continues, at least for several days, when the insects are kept in -the dark. For instance, moths kept in the dark become restless as the -evening approaches, as Réaumur observed long ago. It has been found in -plants that this sort of periodicity may continue for several days, but -gradually disappears if the plants are kept in the dark. By using -artificial light, and exposing the plants to it during the night, and -putting them in the dark during the day, a new periodicity, alternating -with the former one, may be induced; and this will continue for some -days if the plants are then kept continually in the dark. - -Loeb tried the experiment of exposing the quiescent moths suddenly to a -lower intensity of light, in order to see if they would respond equally -well at any time of day. It was found that if the change was made in the -forenoon, between six o’clock and noon, it was not possible to awaken -the moths by a sudden decrease in the intensity of the light. But it was -possible to do so in the afternoon, long before the appearance of dusk. -It appears, therefore, that in this species, _Sphinx euphorbiæ_, it is -possible to influence the period of awakening by decreasing the -intensity of light, but this can be done only near the natural period of -awakening. It seems to me that this awaking of a positively heliotropic -animal by decreasing the light needs to be further investigated. - -The day butterflies are also positively heliotropic. Butterflies of the -species _Papilio machaon_, that have been raised from the pupa, remain -quietly on the window in the diffuse daylight of a bright day. They can -be carried around on the finger without leaving it, but the moment they -come into the direct rays of the sun they fly away. - -Butterflies that have just emerged from their pupa case exhibit a marked -negative geotropic reaction, and this appears to be connected with the -necessity of unfolding their wings at this time. Loeb says that the same -cause that determines the direction of the falling stone and the paths -of the planets, namely, gravity, also directs the actions of the -butterfly that has just left its pupa case. The geotropic response is -especially strong at first. The animal wanders around until it reaches a -vertical wall, which it immediately ascends, straight upward, and -remains hanging at the top until its wings have unfolded. A similar -response occurs in the final stage of the larva of the May-fly, which -leaves the water and crawls up a blade of grass, or other vertical -support, and there, bursting the pupa skin, it dries its wings and flies -away. That this is a reaction to gravity and not to light is shown by -Loeb’s observation, that their empty skins are sometimes observed under -a bridge where the light does not come from above. “This observation on -the larva of the May-fly contradicts the assumption that the ‘purpose’ -of the geotropic response of the butterfly is that it may the better -unfold its new wings, for in the ephemerid larva the negative geotropism -appears at a time when no wings are present.” On the other hand, it -should not be overlooked that the reaction is important for the May-fly -larva in other ways, because it leads the larva to leave the water at -the right period, and come out into the air, where the flying insect can -more safely emerge. - -It is not without interest to find that caterpillars exhibit some of the -same reaction shown by butterflies. Loeb has made numerous experiments -with the caterpillars of _Porthesia chrysorrhœa_. The caterpillars of -this moth collect together in the autumn and spin a web or nest in which -they pass the winter. If they are taken from the nest and brought into a -warm room, they will orientate themselves to the light, and also crawl -toward it. If placed in a tube, they crawl to the upper side of the -glass and then along this side toward the light. If a covering is placed -over the end of the tube that is turned toward the window, the -caterpillars will crawl only as far as the edge of the cloth. They also -react negatively to gravity. If kept in a dark room, they will crawl -upward to the top of the receptacle in which they are enclosed. If -subjected to the influences of both light and gravity, they respond more -strongly to the light. The caterpillars also show a contact reaction. -They tend to collect on convex sides or on corners and angles of solid -bodies. They may even pile up one on top of the other in response to -this reaction; the convex side of a quiescent animal acting on another -animal crawling over it as any convex surface would do and holding the -animal fast. - -These three kinds of reactions determine the instincts of these -caterpillars. In the spring, when they become warm, they leave the nest. -Positive heliotropism and negative geotropism compel them to crawl -upward to the tops of the branches of the trees, and there the contact -reaction with the small buds holds them fast in this place. That they -are not attracted to the end of the branches by the food that they find -there is shown by placing buds in the bottom of the tubes in which the -caterpillars are contained. The caterpillars remain at the top of the -tube, although food is within easy reach. If, however, they are placed -directly on the buds, the contact reaction will hold them there, and -they will not crawl farther upward. Curiously enough, as soon as the -caterpillars have fed and the time for shedding approaches, the -responsiveness to light and to gravity decreases, and at the time of -shedding they do not respond at all to these agents. These same -caterpillars react also to warmth above a certain point. In a dark tube -placed near a stove, the caterpillars collect at the end farthest away -from the source of the heat. They react to light best at a temperature -between 20 and 30 degrees C., and above this temperature point they -become restless and wander about. - -The very close connection between the reactions of this caterpillar and -its mode of life is perfectly obvious. The entire series of changes -seems to have for its “purpose” the survival of the individual by -bringing it to the place where it will find its food. It may seem -natural to conclude that these responses have been acquired for this -very purpose, but let us not too quickly jump at this obvious conclusion -until the whole subject has been more fully examined. - -The upward and downward movements of some pelagic animals have been -shown to depend on certain tropic responses. Every student of marine -zoology is familiar with the fact that many animals come to the surface -at night, and go down at the approach of daylight. It has been shown -that this migration is due largely to a response to light. Light can -penetrate to only about four hundred metres in sea-water, and there is -complete darkness below this level. It has been shown that the swimming -larvæ of one of the barnacles is positively heliotropic in a weak light, -but negatively heliotropic in a stronger light. Animals having responses -like these will come to the surface as the light fades away in the -evening and remain there until the light becomes too bright in the -following morning. They will then become negatively heliotropic and -begin to go down. When they reach a level where the intensity of the -light is such that they become positively heliotropic, they will turn -and start upward again. Thus during the day they will keep below the -surface, remaining in the region where they change from positive to -negative, and _vice versa_. - -It would not be difficult to imagine that this upward and downward -migration of pelagic animals is useful to them, but, on the other hand, -it may be equally well imagined that the response may be injurious to -them. Thus it might be supposed that certain forms could procure their -food by coming to the surface at night, and avoid their enemies by going -down during the day. But it is difficult to see why organisms that serve -as prey should not have acquired exactly the opposite tropisms in order -to escape. - -Some of these marine forms are also geotropic. Loeb has determined that -“the same circumstances that make the animals negatively heliotropic -also make them positively geotropic, and _vice versa_.” It was found, -for instance, that the larva of the marine worm Polygordius is -negatively geotropic at a low temperature, while at a higher temperature -it is positively geotropic. This response would drive the animals upward -when the water becomes too cold, and back again if the surface water -becomes too warm; but whether the response is so adjusted that the -animals keep, as far as possible, in water of that temperature that is -best for their development, we do not know. We can easily imagine that -within wide limits this is the case. - -The change from positive to negative can also be brought about in other -ways. One of the most striking cases of this sort is that described by -Towle in one of the small crustaceans, _Cypridopsis vidua_. It was found -that after an animal had been picked up in a pipette its response was -always positive; that is, it swam toward the light, no matter what its -previous condition had been. The disturbance caused by picking the -animal up induced always a positive response towards light. If the light -were moved, the Cypridopsis followed the light. In this way it could be -kept positive for some time, but if it came to rest, or if it came into -contact with the sides or end of the trough, it became, after a short -time, negatively heliotropic, and remained negative as long as it could -be kept in motion, without being disturbed, or coming into contact with -a solid object. If when positive it were allowed to reach the glass at -the end of the trough, it would swim about there, knocking against the -glass, and then soon turn and swim away from the light. If the light -were shifted while the negative animal was in the middle of the trough, -it would turn and swim directly away, as before, from the source of -light. It could be kept in this negative state as long as it did not -come into contact with the ends. - -It appears that the positive condition in Cypridopsis is of short -duration, and ceases after a while either as a response to contact or -without any observable external factor causing the change. - -This crustacean lives at the bottom of pools, amongst water-plants, and -here also, no doubt, the same change from one to the other reaction -takes place. What possible advantage it may be to the animal to be kept -continually changing in this way is not at all obvious, nor, in fact, -are we obliged to assume that this reaction may be of any special use to -it. Indeed, it is far from obvious how the change that causes the animal -to swim toward the light when it is disturbed could be of the least -advantage to it. - -In another crustacean, one of the marine copepods, _Labidocera æstiva_, -it has been shown by Parker that the male and female react in a somewhat -different way both to light and to gravity. The females are strongly -negatively geotropic, and this sends them up to the top of the water. -The males are very slightly negatively geotropic. The females are -strongly positively heliotropic toward light of low intensity; the males -show the same response to a less degree. To strong light the females are -negative and the males are indifferent. On the other hand, the males are -attracted to the females, probably in response to some chemical -substance diffusing from the females, since the males show the same -reaction when the females are enclosed in an opaque tube through whose -ends a diffusion of substances may take place. This crustacean frequents -the surface of the ocean from sunset to sunrise. During the day it -retires to deeper water. Its migrations can be explained as follows: The -females come to the surface at night, because they are positively -heliotropic to weak light, and also because they are negatively -geotropic. They go down during the day, because they react to bright -light more strongly than to gravity. The males follow the females, -largely because they react positively chemotactically toward the -females. - -Some other animals respond in a somewhat different way to light, as -shown by the fresh-water planarians. These animals remain during the day -under stones, where the amount of light is relatively less than outside. -If they are placed in a dish in the light in front of a window, they -crawl away from the light, but when they reach the back of the dish they -do not come to rest, but continue to crawl around the sides of the dish -even toward the light. The light makes the worms restless, and while -they show a negative response as long as they are perfectly free to move -away from the light, they will not come to rest when they come to the -back of the dish if they are there still in the light, because the -irritating action of the light on them is stronger than its directive -action. If, however, in crawling about they come accidentally into a -place less bright than that in which they have been, they stop, and will -not leave this somewhat darker spot for a brighter one, although they -might leave the newly found spot for one still less bright. - -At night the planarians come out and wander around, which increases -their chance of finding food, although it would not be strictly correct -to say that they come out in search of food. If, however, food is placed -near them, a piece of a worm, for example, they will turn toward it, -being directed apparently by a sense of smell, or rather of taste. - -The heliotropic responses of the planarians appear to be of use to them, -causing them to hide away in the daytime, and to come out only after -dark, when their motions will not discover them to possible enemies. But -some of the planarians are protected in other ways, so that they will -not be eaten by fish, probably owing to a bad taste; so that it is not -so apparent that they are in real need of the protection that their -heliotropic response brings to them. Their turning towards their food -is, however, beyond question of great advantage to them, for in this way -they can find food that they cannot detect in any other way. - -The unicellular plants were amongst the first organisms whose tropic -responses were studied, and the classical work of Strasburger gave the -impetus to much of the later work. In recent years the unicellular -animals, the protozoans, have been carefully studied, more especially by -Jennings. His results show that the reactions in these animals are -different in some important respects from those met with in higher -forms. For instance, most of the free-swimming infusoria are -unsymmetrical, as are also many of the flagellate forms, and as they -move forward they rotate freely on a longitudinal axis. It is therefore -impossible that they could orientate themselves as do the higher animals -that have been described above, and we should not expect these Protozoa -to react in the same way. In fact, Jennings shows that they exhibit a -different mode of response. Paramœcium offers a typical case. As it -moves forward it rotates toward the aboral side of the body. As a result -of the asymmetry of the body, the path followed, as it revolves on its -own axis, is that of a spiral. Did the animal not rotate, as it swims -forward, its asymmetrical form would cause it to move in a circle, but -its rotation causes, as has been said, the course to be that of a -spiral, and the general direction of movement is forward.[31] The -rotation of a paramœcium on its axis is in turn caused by the oblique -stroke of the cilia that cover the surface of the body. Their action -when reversed causes the animal to rotate backward. - -Footnote 31: - - The same result is attained by a bullet that is caused by the rifling - to rotate as it moves forward. - -If a drop of weak acid be put into the water in which the paramœcia are -swimming,—for instance, in the water between a cover-slip and a -slide,—it will be found, after a time, that many individuals have -collected in the drop. It was at first supposed that the paramœcia are -attracted by the diffusion of the acid in the water, and turn toward the -source of the chemical stimulus; but Jennings has shown that this is not -the way in which the aggregation is brought about. If the individuals -are watched, it will be found that they swim forward in a spiral path -without regard to the position of the drop of acid. If one happens, by -chance, to run into the drop, there is no reaction as it enters, but -when it reaches the other side of the drop, and comes into contact with -the water on this side, it suddenly reacts. It stops, backs into the -middle of the drop, rotates somewhat toward the aboral side (_i.e._ away -from the vestibule), and then starts forward again, only to repeat the -action on coming into contact with the edge of the drop again. The -paramœcium has been caught in a veritable trap. All paramœcia that -chance to swim into the drop will also be caught, until finally a large -number will accumulate in the region. The result shows, that, in passing -from ordinary water into a weak acid, no reaction takes place; but -having once entered the acid, the animal reacts on coming into contact -with the water again. - -On the other hand, there are some substances to which the paramœcium may -be said to be negatively chemotropic. If a drop of a weak alkaline -solution be put into water in which paramœcium is swimming, an -individual that happens to run against it reacts at once. It stops -instantly, backs off, revolving in the opposite direction, turns -somewhat to one side, and swims forward again. The chances are that it -will again hit the drop, in which case it repeats the same reaction, -turning again to one side. If it continues to react in this way, it -will, in the course of time, turn so far that when it swims forward it -will miss the edge of the drop, and then continue on its way. If an -individual were put into an alkaline drop, it would leave it, because it -would not react when it passed from inside the drop into the surrounding -water. - -Unicellular animals react to other things besides differences in the -chemical composition of different parts of a solution. In many cases -they react to light, swimming toward or away from it according to -whether they are positively or negatively heliotropic. If they are -positively heliotropic, and while swimming run into a shadow, they react -as they would on coming into contact with a drop of acid. Since they -rotate as they swim forward, we cannot explain their orientation as in -the case of other animals that hold a fixed vertical position. If we -assume that the two ends of the body are differently affected by the -light, for which there is some evidence, we can perhaps in this way -account for their turning toward, or away from, the source of light. - -Changes in the osmotic pressure of the different parts of the fluid, -mechanical stimulation produced by jarring, extremes of heat and of -cold, all cause this same characteristic reaction in Paramœcium; and -this accounts for their behavior toward these agents that are so -different in other respects. - -Paramœcia, as well as other protozoans, show a contact response. They -fix themselves to certain kinds of solid bodies. If, for example, a -small bit of bacterial slime is put into the water, the paramœcia -collect around it in crowds, and eat the bacteria; but they will collect -in the same way around almost any solid. On coming in contact with -bodies having a certain physical texture, the cilia covering the -paramœcium stop moving, only those in the oral groove continuing to -strike backward. The animal comes to rest, pressed against the solid -body. If one or more paramœcia remain in the same place, they set free -carbon dioxide, as a result of their respiratory processes. There is -formed around them a region containing more of this acid than does the -surrounding water. If other moving paramœcia swim, by chance, into this -region, they are caught, and as a result an accumulation of individuals -will take place. The more that collect the larger will the area become, -and thus large numbers may be ultimately entrapped in a region where -there is formed a substance that, from analogy with other animals, we -should expect to be injurious. - -The question as to how far these responses of the unicellular forms are -of advantage to them is difficult to decide, for while, as in the above -case, the response appears to be injurious rather than useful, yet under -other conditions the same response may be eminently advantageous. In -other cases, as when the paramœcia back away, and then swim forward -again, only to repeat the process, the act appears to be such a stupid -way of avoiding an obstacle that the reaction hardly appears to us in -the light of a very perfect adaptation. If we saw a higher animal trying -to get around a wall by butting its head into it until the end was -finally reached, we should probably not look upon that animal as well -adapted for avoiding obstacles. - -Bacteria, which are generally looked upon as unicellular plants, appear, -despite the earlier statements to the contrary, to react in much the -same way as do the protozoans, according to the recent work of Rothert, -and of Jennings and Crosby. The bacteria do not seem to turn toward or -away from chemical substances, but they collect in regions containing -certain substances in much the same way as do the protozoans. The -collecting of bacteria in regions where oxygen is present has been known -for some time, but it appears from more recent results that they are not -attracted toward the oxygen, but by accidentally swimming into a region -containing more oxygen they are held there in the same way as is -paramœcium in a drop of acid. On the other hand bacteria do not enter a -drop of salt solution, or of acids, or of alkalies. They react -negatively to all such substances. Some kinds of bacteria have a -flagellum at each end, and swim indifferently in either direction. If -they meet with something that stimulates them, as they move forward, -they swim away in the opposite direction, and continue to move in the -new direction until something causes again a reversal of their movement. -In this respect their mode of reaction seems of greater advantage than -that followed by paramœcium. - -Another instinct, that appears to be due to a tropic response, is the -definite time of day at which some marine animals deposit their eggs. -The primitive fish, Amphioxus, sets free its eggs and sperm only in the -late afternoon. A jellyfish, Gonionema, also lays its eggs as the light -begins to grow less in the late afternoon, and in this case it has been -found that the process can be hastened if the animals are placed in the -dark some hours before their regular time of laying. There is no -evidence that this habit is of any advantage to the animal. We may -imagine, if we like, that the early stages may meet with less risk at -night, but this is not probable, for it is at this time that countless -marine organisms come to the surface, and it would seem that the chance -of the eggs being destroyed would then be much greater. It is more -probable that the response is of no immediate advantage to the animals -that exhibit it, although in particular cases it may happen to be so. - -This response recalls the diurnal opening and closing of certain -flowers. The flowers of the night-blooming cereus open only in the dusk -of evening, and then emit their strong fragrance. Other flowers open -only in the daytime, and some only in bright sunlight. It is sometimes -pointed out that it is of advantage to some of these flowers to open at -a certain time, since the particular insects that are best suited to -fertilize them may then be abroad. This may often be the case, but we -cannot but suspect that in other cases it may be a matter of little -importance. In special instances it may be that the time of opening of -the flowers is of importance to the species; but even if this is so, -there is no need to assume that the response has been gradually acquired -for this particular purpose. If it were characteristic of a new form to -open at a particular time, and there were insects in search of food at -this time that would be likely to fertilize the plant, then the plant -would be capable of existing; but this is quite different from supposing -that the plant developed this particular response, because this was the -most advantageous time of day for the fertilization of its flowers. - -We can apply this same point of view, I believe, to many of the -remarkable series of tropisms shown by plants, whose whole existence in -some cases is closely connected with definite reactions to their -environment. Let us examine some of these cases. - -When a seed germinates, the young stem is negatively geotropic, and, in -consequence, as it elongates it turns upward towards the light that is -necessary for its later growth. The root, on the contrary, is positively -geotropic, and, in consequence, it is carried downward in the ground. -Both responses are in this case of the highest importance to the -seedling, for in this way its principal organs are carried into that -environment to which they are especially adapted. It matters very little -how the seed lies in the ground, since the stem when it emerges will -grow upward and the root downward. The young stem, when it emerges from -the soil, will turn toward the light if the illumination comes from one -side, and this also may often be of advantage to the plant, since it -turns toward the source from which it gets its energy. The leaves also -turn their broad surfaces toward the light, and as a result they are -able to make use of a greater amount of the energy of the sunlight. The -turning is due to one side of the stem growing more slowly than the -opposite side, and it is true, in general, that plants grow faster at -night than in the daylight. Very bright light will in some cases -actually stop all growth for a time. Thus we see that this bending of -the stem toward the light and the turning of the leaves to face the -light are only parts of the general relation of the whole plant toward -the light. - -Negative heliotropism is much less frequent in plants. It has been -observed in aërial roots, in many roots that are ordinarily buried in -the ground, in anchoring tendrils that serve as holdfasts, and even in -the stems of certain climbers. In all of these cases, and more -especially in the case of the climbers, the reaction is obviously of -advantage to the plant; and it is significant to find, in plants that -climb by tendrils carrying adhering disks, that there is a reversal of -the ordinary heliotropism shown by homologous organs in other plants. -There is an obvious adaptation in the behavior of the tendril, since its -growth away from the more illuminated side is just the sort of reaction -that is likely to bring it into contact with a solid body. - -In this connection it is important to observe that these reactions to -light are perfectly definite, being either positive or negative under -given conditions, and therefore there is at present nothing to indicate -that there has been a gradual transformation from positive to negative, -or _vice versa_. It seems to me much more probable that when the -structural change took place, that converted the plant into a climber, -there appeared a new heliotropic response associated with the other -change. In other words, both appeared together in the new organ, and -neither was gradually acquired by picking out fluctuating variations. - -The leaves of plants also show a sort of transverse heliotropic -response. It has been found, for example, that the leaves of Malva will -turn completely over if illuminated by a mirror from below. A curious -case of change of heliotropism is found in the flower stalks of Linaria. -They are at first positively heliotropic, but after the flower has been -fertilized the stalk becomes negatively heliotropic. As the stalks -continue to grow longer, they push the fruits into the crevices of the -rocks on which the plants grow, and in this way insure the lodgement of -the seeds. Here we have an excellent example showing that the negative -heliotropism of the flower stalk could scarcely have been acquired by -slight changes in the final direction, for only the complete change is -useful to the plant. Intermediate steps would have no special value. - -As has been pointed out in the case of the seedling plant, the main stem -responds positively and the roots negatively to gravity. In addition to -this, the lateral position taken by the lateral roots and branches and -by underground stems are also, in part, due to a geotropic response. In -this case also the effect is produced by the increased growth on the -upper side when the response is positive, and on the lower when it is -negative. Leaves also assume a transverse position in response to the -action of gravity, or at least they make a definite angle with the -direction of its action. - -The most striking case of geotropic response is seen in plants that -climb up the stems of other plants. The twining around the support is -the result of a geotropic response of the sides of the stem. The young -seedling plant stands at first erect. As its end grows it begins to -curve to one side in an oblique position, and this is due to an increase -in growth on one side of the apex of the shoot. As a result the stem -bends toward the other side. Not only does the end “sweep round in a -circle like the hands of a watch,” but it rotates on its long axis as it -revolves. As a result of this rotation “the part of the stem subjected -to the action of the lateral geotropism is constantly changing; and the -revolving movement once begun, must continue, as no position of -equilibrium can be attained.” This movement will carry the end around -any support, not too thick, that the stem touches. - -Most climbers turn to the left, _i.e._ against the hands of a watch, -others are dextral, and a few climb either way.[32] Strasburger states -that whenever any external force, or substance, is important to the -vital activity of the plant or any of its organs, there will also be -found to be developed a corresponding irritability to their influence. -Roots in dry soil are diverted to more favorable positions by the -presence of greater quantities of moisture. This may, I venture to -suggest, be putting the cart before the horse. The plant may be only -able to exist whose responses are suited to certain external conditions, -and these determine the limits of distribution of the plant or the -places in which it is found. - -Footnote 32: - - These cases recall the spiral growth of the shell of the snail, but - the spiral in the latter is due to some other factor. - -A number of plants climb in a different way, and show another sort of -tropism. Those that climb by means of tendrils twist their tendrils -about any support that they happen to come in contact with, and thus the -plant is able to lift its weak stem, step by step, into the air. The -twining of the tendrils is due to contact, which causes a cessation of -growth at the points of contact. The growth of the opposite side -continues, and thus the tendril bends about its support. In the grape -and in ampelopsis the tendril is a modified branch. The stalk of the -leaves in a few plants, as in Lophospermum, act as tendrils. Other -climbers are able to ascend vertical walls owing to the presence of -disks, whose secretions hold the tendril firmly against the support, as -in ampelopsis. - -It is interesting to find in practically all these cases that, whatever -the stimulus may be, the results are reached in the same way, namely, by -one part growing faster than another. The fact of importance in this -connection is that the plant is so constructed that the response is -often beneficial to the organism. - -Before leaving this subject there is one set of responses to be referred -to that is not the result of growth. Certain movements are brought about -by the change in the turgidity of certain organs. The small lateral -leaflets of _Desmodium gyrans_ make circling movements in one to three -minutes. No apparent benefit results from their action. The terminal -leaflets of _Trifolium pratense_ oscillate in periods of two to four -hours, but do so only in the dark; in the light the leaflets assume a -rigid position. There is nothing in the process to suggest that the -movement is useful to the plant, and yet it appears to be as definite as -are those cases in which the response is of vital importance. Had these -movements been of use, their origin would, no doubt, have been explained -because of their usefulness, and the conclusion would have been wrong. - -The leaves of the Mimosa respond, when touched, and it cannot be -supposed that this is of any great advantage to the plant. The sleep -movements of many plants are also due to the effect of light. In some -cases the leaflets are brought together with their upper surfaces in -contact with one another; in other cases the lower surfaces are brought -together. Darwin supposed that these sleep movements served to protect -the leaves from a too rapid loss of heat through radiation, but it has -been pointed out that tropical plants exhibit the same responses. We -have here another admirable instance of the danger of concluding that -because we can imagine an advantage of a certain change, that the change -has, therefore, been acquired because of the advantage. In the Mimosa -not only do the leaflets close together, but the whole leaf drops down -if the stimulus is strong. Other plants also show in a less degree the -same movements, Robinia and Oxalis for instance, and certainly in these -latter the result does not appear to be of any advantage to the plants. - -The preceding account of some of the tropisms in animals and plants will -serve to give an idea of how certain movements are direct responses to -the environment. Some of the reactions appear to be necessary for the -life of the individual, others seem to be of less importance, and a few -of no use at all. Yet the latter appear to be as definite and -well-marked as are the useful responses. I think the conviction will -impress itself on any one who examines critically the facts, that we are -not warranted in applying one explanation to those responses that are of -use, and another to those that are of little or of no value. Inasmuch as -the Darwinian theory fails to account for the origin of organs of little -or of no value, it is doubtful if it is needed to explain the origin of -the useful responses. If, on the other hand, we assume that the _origin_ -of the responses has nothing to do with their value to the organism, we -meet with no difficulty in those cases in which the response is of -little or of no use to the organism. That great numbers of responses are -of benefit to the organism that exhibits them can be accounted for on -the grounds that those new species, that have appeared, that have useful -responses, are more likely, in the long run, to survive, than are those -that do not respond adaptively. - -We may now examine some of the more complicated responses and instincts, -more especially those of the higher animals. Some of these are pure -tropisms, _i.e._ definite responses or reactions to an external exciting -agent; others may be, in part, the result of individual experience, -involving memory; others, combinations of the two; and still others may -depend on a more complex reaction in the central nervous system of the -animal. These cases can be best understood by means of a few -illustrations. - -As an example of a simple action may be cited a well-known reflex after -cutting the nerve-cord of the frog, or after destroying the brain. If -the frog is held up, and its side tickled, the leg is drawn up to rub -the place touched. To accomplish this requires a beautifully adjusted -system of movements, yet the act seems to be a direct reflex, involving -only the spinal cord. - -An example of a somewhat more complex reflex is the biting off of the -navel-string by the mother in rodents and other mammals; an act -eminently useful to the young animal, although of no importance to the -mother herself. The protection of the young by their parents from the -attacks of other animals appears to be a somewhat complex instinct, and -it is interesting to note that the protection is extended to the young -only so long as they are in need of it, and as soon as they are able to -shift for themselves the maternal protection is withdrawn. - -The instinct of the young chick to seize in its beak any small moving -object is a simple and useful reflex action, but if the object should -happen to be a bee which stings the chick, another bee or similar insect -will not be seized. Here we see that a reflex has been changed, and -changed with amazing quickness. Moreover, the chick has learnt to -associate this experience with a particular sort of moving object. It is -this power to benefit by the result of a brief experience that is one of -the most advantageous properties of the organism. - -Young chicks first show a drinking reflex if by chance their beaks are -wet by water. At once the head is lifted up, and the drop of water -passes down the throat. In this way the chick first learns the meaning -of water, and no doubt soon comes to associate it with its own condition -of thirst. The sight of water produces no effect on the inexperienced -chick, and it may even stand with its feet in the water without -drinking; but as soon as it touches, by chance, the water with its beak, -the reflex, or rather the set of reflexes is started. - -A more complicated instinct is that shown by the spider in making its -web. In some cases the young are born from eggs laid in the preceding -summer, and can have had, therefore, no experience of what a web is -like; and yet, when they come to build this wonderfully complex -structure, they do so in a manner that is strictly characteristic of the -species. - -The formation of the comb by bees, in which process, with a minimum of -wax, they secure a maximum number of small storehouses in which to keep -their honey and rear their young, is often cited as a remarkable case of -adaptation. - -There has been some discussion as to whether birds build their nests in -imitation of the nest in which they were reared, or whether they do so -independently of any such experience. There can be no doubt, however, -that in some birds neither memory nor imitation can play any important -part in the result, and that they build their nests as instinctively as -spiders make webs. - -These instincts of spiders, bees, and birds appear to be more complex -than the reflexes and tropisms that were first described. Whether they -are really so, or only combinations of simple responses, we do not yet -know. That they have come suddenly into existence as we now find them -does not seem probable, but this does not mean that they must have been -slowly acquired as the result of selection. The mutation theory also -assumes that the steps of advance may have been small. - -Our account may be concluded with the recital of some instincts, chosen -almost at random, that serve to show some other adaptations which are -the result of these inborn responses. - -It is known that ants travel long distances from their nests, and yet -return with unerring accuracy. It has been shown that they are able to -do this through a marvellous sense of smell. The track left by the ant, -as it leaves the nest, serves as a trail in returning to the -starting-point. Moreover, it appears that the ant can pick out her own -trail, even when it has been crossed by that of other ants. This means -that she can distinguish the odor of her own trail from that of other -members of the colony. The sense-organs by means of which the odor is -detected lie in the antennæ. This fact accounts for certain actions of -ants that have been described as showing that they have an affection for -each other. Two ants, meeting, pat each other with their antennæ. In -this way they are quickly able to distinguish members of their own nest -from those of other nests. If they are of the same nest, they separate -quietly; if of other nests, they may fight. If an ant from one nest is -put into another nest, it is instantly attacked and killed—an act that -appears to be injurious rather than useful, for the ant might become a -valuable member of the new colony. If, however, an ant is first immersed -in the blood of a member of the community into which she is to be -introduced, she will not be attacked, and may soon become a part of the -new community. By her baptism of blood she has no doubt acquired -temporarily the odor of the new nest, and by the time that this has worn -off she will have acquired this odor by association, and become thereby -a member of another colony. - -Numerous stories have been related of cases in which an ant, having -found food, returns to the nest with as much of it as she can carry, and -when she comes out again brings with her a number of other ants. This -has been interpreted to mean that in some mysterious way the ant -communicates her discovery to her fellow-ants. A simpler explanation is -probably more correct. The odor of the food, or of the trail, serves as -a stimulus to other ants, that follow to the place where the first ant -goes for a new supply of the food. The fact that the first individual -returns to the supply of food seems to indicate that the ant has memory, -and this is obviously of advantage to her and to the whole colony. - -The peculiar habits of some of the solitary wasps, of stinging the -caterpillar or other insect which they store up as food for their young, -is often quoted as a wonderful case of adaptive instinct. The poison -that is injected into the wound paralyzes the caterpillar, but as a rule -does not kill it, so that it remains motionless, but in a fresh state to -serve as food for the young that hatch from the egg of the wasp. A -careful study of this instinct by Mr. and Mrs. Peckham has shown -convincingly that the act is not carried out with the precision formerly -supposed. It had been claimed that the sting is thrust into the -caterpillar on the lower side, a ventral ganglion being pierced, the -poison acting with almost instantaneous effect. But it may be questioned -whether this is really necessary, and whether the same end might not be -gained, although not quite so instantaneously, if the caterpillar were -pierced in almost any other part of the body. Can we be seriously asked -to believe that this instinct has been perfected by the destruction of -those individuals (or of their descendants) that have not pierced the -caterpillar in exactly the middle of a segment of the anterior ventral -surface? It seems to me that the argument proves too much from the -selectionist’s point of view. If the wasp pierced the caterpillar in the -middle of its back, we should have passed over the act without comment; -but since the injection is usually made on the ventral side, and since -we know that the nervous system lies in this position, it has been -assumed that the act is carried out in this way, in order that the -poison may penetrate the nervous system more quickly. Yet a fuller -knowledge may show that there is really no necessity for such precision. - -A curious response is the so-called death-feigning instinct shown by a -number of animals, especially by certain insects, but even by some -mammals and birds. Certain insects, if touched, draw in their legs, let -go their hold, and fall to the ground, if they happen to be on a plant. -It is not unusual to meet with the statement that this habit has been -acquired because it is useful to the insect, since it may often escape -in this way from an enemy. This does not appear on closer examination to -be always the case, and sometimes as much harm as good may result, or -what is more probable, neither much advantage, nor disadvantage, is the -outcome. This can, of course, only be determined in each particular case -from a knowledge of the whole life of a species and of the enemies that -are likely to injure it. - -Hudson has recorded[33] a number of cases of this death-feigning -instinct in higher animals, and attributes it to violent emotion, or -fear, that produces a sort of swoon. He describes the gaucho boys’ -method, in La Plata, of catching the silver-bill by throwing a stick or -a stone at it, and then rushing toward the bird, “when it sits perfectly -still, disabled by fear, and allows itself to be taken.” He also states -that one of the foxes (_Canis azaræ_) and one of the opossums -(_Didelphys azaræ_) “are strangely subject to the death-simulating -swoon.” - -Footnote 33: - - “The Naturalist in La Plata.” - -Hudson remarks that it seems strange that animals so well prepared to -defend themselves should possess this “safeguard.” When caught or run -down by dogs, the fox fights savagely at first, but after a time its -efforts stop, it relaxes, and it drops to the ground. The animal appears -dead, and Hudson states that the dogs are “constantly taken in by it.” -He has seen the gauchos try the most barbarous tricks on a captive fox -in this condition, and, despite the mutilations to which it was -subjected, it did not wince. If, however, the observer draws a little -away from the animal, “a slight opening of the eye may be detected, and -finally, when left to himself, he does not recover and start up like an -animal that has been stunned, but cautiously raises his head at first -and only gets up when his foes are at a safe distance.” Hudson, coming -once suddenly upon a young fox, saw it swoon at his approach, and -although it was lashed with a whip it did not move. - -The common partridge of the pampas of La Plata (_Hothura maculosa_) -shows this death-feigning instinct in a very marked degree. “When -captured, after a few violent struggles to escape, it drops its head, -gasps two or three times, and to all appearance dies.” But if it is -released it is off in an instant. The animal is excessively timid, and -if frightened, may actually die simply from terror. If they are chased, -and can find no thicket or burrow into which to escape, “they actually -drop down dead on the plain. Probably when they feign death in their -captor’s hand they are in reality very near to death.” - -In this latter instance it must appear very improbable that we are -dealing with an instinct that has been built up by slow degrees on -account of the benefit accruing at each stage to the individual. In -fact, it appears that the instinct is in this case of really no use at -all to the animal, for there can scarcely be any question of an escape -by this action. Yet so far as we can judge it is the same instinct shown -by other animals, and it is not logical to account for its origin in one -case on the grounds of its usefulness, when we cannot apply the -explanation in the other cases. If this be admitted, we have another -illustration of the importance of keeping apart the origin of an -instinct or of a structure and the fact of its usefulness or -non-usefulness to the organism. Thus under certain conditions this -death-feigning instinct might really be of use to the animal, while -under other conditions and in other animals it may be of no advantage at -all, and in still other conditions it may be a positive injury to its -possessor. Perhaps we need not go outside of our own experience to find -a parallel case, for the state of fright into which imminent danger may -throw an individual may deprive him for the moment of the proper use of -those very mental qualities of which he stands in this crisis in -greatest need. - -The peculiar behavior of cattle caused by the smell of blood is another -case of an instinct whose usefulness to its possessors is far from -apparent. It is known that cattle and horses and several wild animals -become violently excited by the smell of blood. Hudson gives a vivid -account of a scene witnessed by himself, the animals congregating, “and -moving around in a dense mass, bellowing continually.” Those animals -that forced their way into the centre of the mass where the blood was -“pawed the earth and dug it up with their horns, and trampled each other -down in their frantic excitement.” - -This action leads us to a consideration of the behavior of animals -toward companions in distress. “Herbivorous animals at such times will -trample and gore the distressed one to death. In the case of wolves and -other savage-tempered carnivorous species the distressed fellow is -frequently torn to pieces and devoured on the spot.” If any one will be -bold enough to claim in this case that this habit has been acquired -because of advantage to the pack, _i.e._ if it be imagined that the pack -gains more by feeding on a weak member than by letting him take his -chances of recovery, it may be pointed out in reply that cattle also -destroy their weak or injured, but do not devour them, and the same -statement holds for birds, where the same instinct has often been -observed. Romanes has suggested that the instinct of destroying the weak -or injured members is of use because such members are a source of danger -to the rest of the herd; but Hudson points out that it is not so much -the weak and sickly members of the herd that are attacked in this way, -as those that are injured, and concludes, “the instinct is not only -useless, but actually detrimental.” He suggests that these “wild -abnormal movements of social animals” are a sort of aberration, so “that -in turning against a distressed fellow they oppose themselves to the law -of being.” Yet whether we gain anything by calling this action aberrant -or abnormal, the important fact remains that it is a definite response -under certain external conditions, and is shown by all the individuals -of the species. - -The preceding illustrations of reactions that go to make up the -so-called instincts of animals may be separated into those that are -essential to the life of the individual or of the race, those that are -of some apparent use, although not absolutely essential, and a few of no -use at all, and fewer still that appear to be even injurious. If the -latter reactions take place only rarely, as appears often to be the -case, they are not sufficiently harmful to cause the destruction of the -race. The evidence points to the conclusion, I believe, that the origin -of these tropisms and instincts cannot be accounted for on the ground of -their benefit to the individual or to the race; and it does not seem -reasonable to make up one explanation for the origin of those that are -essential, and another for those that are of little use or even of no -use at all. - -From what has been already said more than once, while discussing each -particular case, the simplest course appears to be in all instances to -look upon these instincts as having appeared independently of the use to -which they may be put, and not as having been built up by selection of -the individual variations that happen to give an organism some advantage -over its fellows in a life and death struggle. It appears reasonable to -deal with the origin of tropisms and instincts in general in the same -way as in dealing with structures; for, after all, the tropism is only -the outcome of some material or structural basis in the organism. - -No attempt has been made here to interpret the more complex reactions of -the nervous system, for until we can get some insight into the meaning -of the simpler processes, we are on safer ground in dealing with these -first. - - ------------------------------------------------------------------------- - - - - - CHAPTER XII - - SEX AS AN ADAPTATION - - -In what sense may the separation of all the individuals of a species -into two kinds of individuals, male and female, be called an adaptation? -Does any advantage result to the species that would not come from a -non-sexual method of reproduction? Many attempts have been made to -answer these questions, but with what success I shall now try to show. - -There are four principal questions that must be considered:— - -I. The different kinds of sexual individuals in the animal and plant -kingdoms. - -II. The historical question as to the evolution of separate sexes. - -III. The factors that determine the sex in each individual developing -from an egg. - -IV. The question as to whether any advantage is gained by having each -new individual produced by the union of two germ-cells, or by having the -germ-cells carried by two kinds of individuals. - -While our main problem is concerned with the last of these topics, yet -there would be little hope of giving a complete answer to it unless we -could get some answer to the first three questions. - - - The Different Kinds of Sexual Individuals - -Amongst the unicellular animals and plants the fusion of two (or more) -individuals into a single one is generally regarded as the simplest, and -possibly also the most primitive, method of sexual reproduction. Two -amœbas, or amœba-like bodies, thus flow together, as it were, to produce -a new individual. - -In the more highly specialized unicellular animals, the processes are -different. Thus in vorticella, a small, active individual unites with a -larger fixed individual. The protoplasm fuses into a common mass, and a -very complicated series of changes is passed through by the nucleus. In -paramœcium, a free-swimming form very much like vorticella, two -individuals that are alike unite only temporarily, and after an -interchange of nuclear material they separate. - -In the lower plants, and more especially in some of the simple -aggregates or colonial forms, there are found a number of stages between -species in which the uniting individuals are alike, and those in which -they are different. There are several species whose individuals appear -to be exactly alike; and other species in which the only apparent -difference between the individuals that fuse together is one of size; -and still other species in which there are larger resting or passive -individuals, and smaller active individuals that unite with the larger -ones. In several of the higher groups, including the green algæ and -seaweeds, we find similar series, which give evidence of having arisen -independently of each other. If we are really justified in arranging the -members of these groups in series, beginning with the simpler cases and -ending with those showing a complete differentiation into two kinds of -germ-cells, we seem to get some light as to the way in which the change -has come about. It should not be forgotten, however, that it does not -follow because we can arrange such a series without any large gaps in -its continuity, that the more complex conditions have been gradually -formed in exactly this way from the simplest conditions. - -So far we have spoken mainly of those cases in which the forms are -unicellular, or of many-celled species in which all the cells of the -individual resolve themselves into one or the other kind of germ-cells. -This occurs, however, only in the lowest forms. A step higher we find -that only a part of the cells of the colony are set aside for purposes -of reproduction. The cells surrounding these germ-cells may form -distinct organs, which may show certain differences according to whether -they contain male or female germ-cells. When these two kinds of cells -are produced by two separate individuals, the individuals themselves may -be different in other parts of the body, as well as in the reproductive -organs. - -When this condition is reached, we have individuals that we call males -and females, because, although they do not themselves unite to form new -individuals, they produce one or the other kind of germ-cell. It is the -germ-cells alone that now combine to form the new individual. - -Amongst living groups of animals we find no such complete series of -forms as exist in plants, and the transition from the one-celled to the -many-celled forms is also more abrupt. On the other hand, we find an -astonishing variety of ways in which the reproduction is accomplished, -and several ways in which the germ-cells are carried by the sexual -individuals. Let us examine some of the more typical conditions under -the following headings: (1) sexes separate; (2) sexes united in the same -individual; (3) parthenogenetic forms; (4) exceptional methods of -propagation. - -1. _Sexes Separate; Unisexual Forms._[34]—Although the animals with -which we are more familiar have the sexes separate, this is far from -being universal amongst animals and plants; and, in fact, can scarcely -be said to be even the rule. When the sexes are separate they may be -externally alike, and this is especially true for those species that do -not unite, but set free their eggs and spermatozoa in the water, as -fish, frogs, corals, starfish, jellyfish, and many other forms. In other -animals there are sometimes other secondary differences in the sexes -besides those connected with the organs of reproduction. Such -differences are found, as we have seen, in insects, in some spiders, -crustaceans, and in many birds and mammals. In a few cases the -difference between the sexes is very great, especially when the female -is parasitic and the male free, as in some of the crustaceans. In some -other cases the male is parasitic on the female. Thus in Bonellia the -male is microscopic in size, being in length only one-hundredth part of -the female. In _Hydatina senta_ the male is only about a third as large -as the female. It has no digestive tract, and lives only a few days. In -another rotifer the males are mere sacs enclosing the male reproductive -organs. - -Footnote 34: - - Geddes and Thompson’s “The Evolution of Sex” has been freely used in - the preparation of this part of this chapter. - -2. _Hermaphroditic Forms._—There are many species of animals and plants -in which each individual contains both the male and the female organs of -reproduction, and there are whole groups in which only these -hermaphroditic forms occur. Thus in the ctenophors the eggs develop -along one side of each radial canal and spermatozoa along the other. The -group of flatworms is almost exclusively hermaphroditic. The earthworms -and the leeches have only these bisexual forms, and in the mollusks, -while a few groups have separate sexes, yet certain groups of -gasteropods and of bivalve forms are entirely hermaphroditic. - -In the common garden snail, although there are two sets of sexual ducts -closely united, yet from the same reproductive sac both eggs and sperm -are produced. The barnacles and the ascidians are for the most part -hermaphroditic forms. Many other examples might be cited, but these will -suffice to show that it is by no means unusual in the animal kingdom for -the same individual to produce both male and female germ-cells. However, -one of the most striking facts in this connection is that -self-fertilization seldom takes place, so that the result is the same in -certain respects as though separate sexes existed. This point will come -up later for further consideration. - -3. _Parthenogenetic Reproduction._—It has long been known that, in some -cases, eggs that are not fertilized will begin to develop and may even -produce new individuals. Tichomiroff showed that by rubbing with a brush -the unfertilized eggs of the silkworm moth, a larger percentage would -produce caterpillars than if they were not rubbed. During the last few -years it has been shown that the development of a non-fertilized egg may -be started in a number of ways. Such, for example, as by certain -solutions of salt or of sugar, by subjecting the eggs to cold, or by -simply shaking them. - -There are certain groups of animals in which the males appear only at -regular (in others at irregular) intervals. In their absence the females -produce eggs that develop without being fertilized, _i.e._ -parthenogenetically. The following examples will serve to show some of -the principal ways in which this “virgin reproduction” takes place. In -the group of rotifers the males are generally smaller than the females -and are usually also degenerate. In some species, although degenerate -males are present, they are unnecessary, since parthenogenesis is the -rule. In still other species no males exist and the eggs develop, -therefore, without being fertilized. In some of the lower crustaceans -parthenogenesis occurs in varying degrees. In Apus males may be entirely -absent at times in certain localities, and at other times a few, or even -very many, males may appear. Some species of ostracod crustaceans seem -to be purely parthenogenetic; others reproduce by means of fertilized -eggs; and others by an alternation of the two processes. The crustaceans -of the genus _Daphnia_ produce two kinds of eggs. The summer eggs are -small, and have a thin shell. These eggs develop without being -fertilized, but in the autumn both male and female individuals develop -from these unfertilized eggs, and the eggs of the female, the so-called -winter eggs, are fertilized. These are also larger than the summer eggs, -have thicker shells, and are much more resistant to unfavorable -conditions. They give rise in the following spring to females only, and -these are the parthenogenetic individuals that continue to produce -during the summer new parthenogenetic eggs. - -It is within the group of insects that some of the most remarkable cases -of parthenogenesis that we know are found. In the moth, _Psyche helix_, -only females are present, as a rule, but rarely males have been found. -In another moth, _Solenobia trinquetrella_, the female reproduces by -parthenogenesis, but at times males appear and may then be even more -numerous than the females. In the gall-wasps parthenogenetic generations -may alternate with a sexual generation, and it is interesting to note -that the sexual and the parthenogenetic generations are so different -that they were supposed to belong to separate species, until it was -found that they were only alternate generations of the same species. - -The aphids or plant-lice reproduce during the summer by parthenogenesis, -but in the autumn winged males and females appear, and fertilized winter -eggs are produced. From these eggs there develop, in the following -spring, the wingless parthenogenetic summer forms, which produce the -successive generations of the wingless forms. As many as fourteen summer -broods may be produced. By keeping the aphids in a warm temperature and -supplying them with plenty of moist food, it has been possible to -continue the parthenogenetic reproduction of the wingless forms for -years. As many as fifty successive broods have been produced in this -way. It has not been entirely determined whether it is the temperature -or a change in the amount, or kind, of food that causes the appearance -of the winged males and females, although it seems fairly certain that -diminution in the food, or in the amount of water contained in it, is -the chief cause of the change. - -In the honey-bee the remarkable fact has been well established that -fertilized eggs give rise only to females (queens and workers), while -unfertilized eggs develop into males. Whether a fertilized egg becomes a -queen or a worker (sterile female) depends solely on the kind of food -that is given to the young larva, and this is determined, in a sense, -entirely by the bees themselves. - -In plants also there are many cases of parthenogenesis known. Some -species of Chara when kept under certain conditions produce only female -organs, and seem to produce new plants parthenogenetically. In this case -it appears that the same conditions that caused the plants to produce -only female organs may also lead to the development of the egg-cells -without fertilization. In fact it is only by a combination of this kind -that parthenogenesis could arise. The result is similar when the eggs of -insects produce only females whose eggs are capable of parthenogenetic -development. If a case should arise in which only females appeared whose -eggs did not possess the power of parthenogenetic development, the -species would die out. - -In the green alga, Spirogyra, it has been found that if conjugation of -two cells is prevented, a single cell may become a parthenogenetic cell. -In a number of parasitic fungi the male organs appear to be degenerate, -and from the female organs parthenogenetic development takes place. A -small number of flowering plants are also capable of parthenogenetic -reproduction. - -There is a peculiarity in the development of the parthenogenetic eggs of -animals that will be more fully discussed later, but may be mentioned -here. Ordinarily an egg that becomes fertilized gives off two polar -bodies, but in a number of cases in which parthenogenetic development -occurs it has been found that only one polar body is given off. It is -supposed that in such cases one polar body is retained, and that it -plays the same part as the entrance of the spermatozoon of the male. - -4. _Exceptional Cases._—Occasionally in a species that is unisexual an -individual is found that is bisexual. The male of the toad, _Pelobates -fuscus_, has frequently a rudimentary ovary in front of the testis. The -same thing has been found in several species of fish. In Serranus, a -testis is present in the wall of the ovary, and the eggs are said to be -fertilized by the spermatozoa of the same individual. In frogs it has -been occasionally found that ovary and testis may be associated in the -same individual, or a testis may be present on one side, and a testis -with an anterior ovarian portion on the other. Cases like these lead up -to those in which the body itself may also show a mosaic of -sex-characters, and it is noticeable that when this occurs there is -nearly always a change in the reproductive organs also. Thus butterflies -have been found with the wings and the body of one side colored like the -male and the other side like the female. Similar cases have also been -found in bees and ants. Bees have been found with the anterior part of -the body of one sex and posterior part of another! - -The preceding cases illustrate, in different ways, the fact that in the -same individual both kinds of reproductive organs may suddenly appear, -although it is the rule in such species that only one set develops. -Conversely, there are cases known, especially amongst plants, in which -individuals, that usually produce male and female organs (or more -strictly spores of two kinds from which these organs develop), produce -under special conditions only one or the other kind. Facts like these -have led to the belief that each individual is potentially bisexual, but -in all unisexual forms one sex predominates, and the other remains -latent. This idea has been the starting-point for nearly all modern -theories of sex. - -An excellent illustration of this theory is found in those cases in -which the same individual may be male at one time and female at another. -For instance, it is said that in one of the species of starfish -(_Asterina gibbosa_) the individuals at Roscoff are males for one or two -years, and then become females. At Banyuls they are males for the first -two or three years, and then become females; while at Naples some are -always males, others females, some hermaphrodites, others transitional -as in the cases just given. In one of the isopod crustaceans, -_Angiostomum_, the young individuals are males and the older females. In -_Myzostomum glabrum_ the young animal is at first hermaphroditic, then -there is a functional male condition, followed by a hermaphroditic -condition, and finally a functional female phase, during which the male -reproductive organs disappear. - -The flowers of most of the flowering plants have both stamens and -pistils, which contain the two kinds of spores out of which the male and -female germ-cells are formed. The stamens become mature before the -pistils, as a rule, but in some cases the reverse is the case. This -difference in the time of ripening of the two organs is often spoken of -as an adaptation which prevents self-fertilization. The latter is -supposed to be less advantageous than cross-fertilization. This question -will be more fully considered later. - -Before we come to an examination of the question of the adaptations -involved in the cases in which the sexes are separate, and the different -times at which the sex-cells are ripened, it will be profitable first to -examine the question as to what determines in the egg or young whether a -male or a female or a hermaphroditic form shall arise. - - - The Determination of Sex - -A large number of views have been advanced as to what determines whether -an egg will give rise to a male or to a female individual. The central -question is whether the fertilized egg has its sex already determined, -or whether it is indifferent; and if the latter, what external factor or -factors determine the sex of the embryo. Let us first examine the view -that some external factor determines the sex of the individual, and then -the evidence pointing in the opposite direction. Among the different -causes suggested as determining the sex of the embryo, that of the -condition of the egg itself at the time of fertilization has been -imagined to be an important factor in the result. Another similar view -holds that the condition of the spermatozoon plays the same rôle. For -instance, it has been suggested that if the egg is fertilized soon after -it leaves the ovary, it produces a female, but if the fertilization is -delayed, a male is produced. It has also been suggested that the -relative age of the male and the female parents produces an effect in -determining the sex of the young. There is no satisfactory evidence, -however, showing that this is really the case. - -Another view suggested is that the sex is determined by the more -vigorous parent; but again there is no proof that this is the case, and -it would be a difficult point to establish, since as Geddes and Thompson -point out, what is meant by greater vigor is capable of many -interpretations. Somewhat similar is the idea that if the conditions are -favorable, the embryo develops further, as it were, and becomes a male; -but there are several facts indicating that this view is untenable. - -Düsing maintains that several of these factors may play a part in -determining the sex of the embryo, and if this be true, the problem -becomes a very complex one. He also suggests that there are -self-regulative influences of such a kind that, when one sex becomes -less numerous, the conditions imposed in consequence on the other sex -are such as to bring the number back to the normal condition; but this -idea is far from being established. The fact that in some species there -are generally more individuals of one sex than of the other shows that -this balance is not equally adjusted in such forms. - -Of far greater value than these speculations as to the origin of sex are -the experiments that appear to show that nutrition is an important -factor in determining sex. Some of the earlier experiments in this -direction are those of Born and of Yung. By feeding one set of tadpoles -with beef, Yung found the percentage of females that developed to be -greatly increased, and a similar increase was observed when the tadpoles -were fed on the flesh of fish. An even greater effect was produced by -using the flesh of frogs, the percentage rising to 92 females in every -hundred. These results have been given a different interpretation by -Pflüger and by others, and, as will be pointed out later, there is a -possible source of error that may invalidate them. - -Somewhat similar results have been obtained by Nussbaum for one of the -rotifers. He found that if the rotifer is abundantly fed in early life, -it produces _female eggs_, that is, larger eggs that become females; -while if sparingly fed, it produces only small eggs, from which males -develop. It has been claimed also in mammals, and even in man, that sex -is to some extent determined by the nourishment of the individual. - -Some experiments made by Mrs. Treat with caterpillars seemed to show -that if the caterpillars were well nourished more female moths were -produced, and if starved before pupation more males emerged. But Riley -has pointed out that since the larger female caterpillars require more -food they will starve sooner than the males, and, in consequence, it may -appear that proportionately more male butterflies are born when the -caterpillars are subjected to a starvation diet. This point of view is -important in putting us on our guard against hastily supposing that food -may directly determine sex. Unless the entire number of individuals -present at the beginning of the experiment is taken into account, the -results may be misleading, because the conditions may be more fatal to -one sex than to the other. - -In some of the hymenopterous insects, the bees for example, it has been -discovered that the sex of the embryo is determined by the entrance, or -lack of entrance, of the spermatozoon. In the honey-bee all the -fertilized eggs produce females and the unfertilized eggs males. The -same relation is probably true also in the case of ants and of wasps. In -the saw-flies, the conditions are very remarkable. Sharp gives the -following account of some of these forms:[35]—“It is a rule in this -family that males are very much less numerous than females, and there -are some species in which no males have been discovered. This would not -be of itself evidence of the occurrence of parthenogenesis, but this has -been placed beyond doubt by taking females bred in confinement, -obtaining unfertilized eggs from them, and rearing the larvæ produced -from the eggs. This has been done by numerous observers with curious -results. In many cases the parthenogenetic progeny, or a portion of it, -dies without attaining full maturity. This may or may not be due to -constitutional weakness, arising from the parthenogenetic state. -Cameron, who has made extensive observations on this subject, thinks -that the parthenogenesis does involve constitutional weakness, fewer of -the parthenogenetic young reaching maturity. This, he suggests, may be -compensated for—when the parthenogenetic progeny is all of the female -sex—by the fact that all those that grow up are producers of eggs. In -many cases the parthenogenetic young of Tenthredinidæ are of the male -sex, and sometimes the abnormal progeny is of both sexes. In the case of -one species—the common currant-fly, _Nematus ribesii_—the -parthenogenetic progeny is nearly, but not quite always, entirely of the -male sex; this has been ascertained again and again, and it is -impossible to suggest in these cases any advantage to the species to -compensate for constitutional parthenogenetic weakness. On the whole, it -appears most probable that the parthenogenesis, and the special sex -produced by it, whether male or female, are due to physiological -conditions of which we know little, and that the species continues in -spite of the parthenogenesis rather than profits by it. It is worthy of -remark that one of the species in which parthenogenesis with the -production of males occurs—_Nematus ribesii_—is perhaps the most -abundant of saw-flies.” - -Footnote 35: - - “The Cambridge Natural History,” Vol. V, “Insects,” by David Sharp. - -It has been pointed out that in a number of species of animals and -plants only parthenogenetic females are present at certain times. In a -sense this means a preponderance of one sex, but since the eggs are -adapted only to this kind of development, it may be claimed that the -conditions in such cases are somewhat different from those in which eggs -that would be normally fertilized may develop in the absence of -fertilization. Nevertheless, it is generally supposed that the actual -state of affairs is about the same. It is usually assumed, and no doubt -with much probability, that these parthenogenetic forms have evolved -from a group which originally had both male and female forms. One of the -most striking facts in this connection is that in the groups to which -these parthenogenetic species belong there are, as a rule, other species -with occasional parthenogenesis, and in some of these the males are also -fewer in number than the females. - -In the aphids, the parthenogenetic eggs give rise during the summer to -parthenogenetic females, but in the autumn the parthenogenetic eggs give -rise without fertilization both to males and to females. It appears, -therefore, that we can form no general rule as to a relation between -fertilization and the determination of sex. While in certain cases, as -in the bees, there appears to be a direct connection between these two, -in other cases, as in that of the aphids just mentioned, there is no -such relation apparent. - -Geddes and Thompson have advocated a view in regard to sex which at best -can only serve as a sort of analogy under which the two forms of sex may -be considered, rather than as a legitimate explanation of the phenomenon -of sex. They rest their view on the idea that living material is -continually breaking down and building up. An animal in which there is -an excess of the breaking-down process is a male, and one that is more -constructive is a female. Furthermore, whichever process is in the -excess during development determines the sex of the individual. Thus, if -conditions are very favorable, there will be more females produced; but -if, on the other hand, there is an excess of the breaking-down process, -males are produced. So far, the process is conceived as a purely -physiological one, but to this the authors then apply the selection -hypothesis, which, they suppose, acts as a sort of break or regulation -of the physiological processes, or in other words as a directive agent. -They state: “Yet the sexual dimorphism, in the main, and in detail, has -an adaptive significance, also securing the advantages of -cross-fertilization and the like, and is, therefore, to some extent the -result of the continual action of natural selection, though this may, of -course, check variation in one form as well as favor it in another.” -Disregarding this last addition, with which Geddes and Thompson think it -necessary to burden their theory, let us return to the physiological -side of the hypothesis. Their idea appears to me a sort of symbolism -rather than a scientific attempt to explain sex. If their view had a -real value, it ought to be possible to determine the sex of the -developing organism with precision by regulating the conditions of its -growth, and yet we cannot do this, nor do the authors make any claim of -being able to do so. The hypothesis lacks the only support that can give -it scientific standing, the proof of experiment. - -There have been made, from time to time, a number of attempts to show -that the sex of the embryo is predetermined in the egg, and is not -determined later by external circumstances. In recent years this view -has come more to the front, despite the apparent experimental evidence -which seemed in one or two cases to point to the opposite view. One of -the most complete analyses of the question is that of Cuénot, who has -attempted to show that the sex of the embryo is determined in the egg, -before or at the time of fertilization. He has also examined critically -the evidence that appeared to show that external conditions, acting on -the embryo, may determine the sex, and has pointed out some possible -sources of error that had been overlooked. The best-known case is that -of the tadpole of the frog, but Cuénot shows not only that there are -chances of error in this experiment as carried out, but also, by his own -experiments and observations, that the facts themselves are not above -suspicion. He points out that at the age at which some of the tadpoles -were when the examination was made, it was not always possible to tell -definitely the sex of the individual, and least of all by means of the -size alone of the reproductive organs, as was supposed, in one case at -least, to be sufficient. In his own experiments he did not find an -excess of one sex over the other as a result of feeding. - -Cuénot points out that Brocadello found that the larger eggs laid by the -silkworm give rise to from 88 to 95 per cent of females, and the small -eggs to from 88 to 92 per cent of males. Joseph has confirmed this for -_Ocneria dispar_, and Cuénot himself also reached this conclusion. -Korschelt found that the large eggs of Dinophilus produced females and -the small ones males. Cuénot experimented with three species of flies, -and found that when the maggots were well nourished the number of the -individuals of the two sexes was about equal, and when poorly nourished -there were a few more females in two cases, and in another about the -same number of males and females. - -It has been claimed that the condition of nourishment of the mother may -determine the number of eggs of a particular sex, but Cuénot found, in -three species of flies which he raised, that there was a slight response -in the opposite direction. He concludes that the condition of the mother -is not a factor in the determination of sex. - -The first egg of the two laid in each set by the pigeon is said, as a -rule, to produce a male, and the second a female. Both Flourens and -Cuénot found this to be the case in the few instances that they -examined, but Cuénot has shown that this does not always happen. Even -when this occurs, it has not been determined whether the result depends -on something in the egg itself, that causes a male egg to be set free -first, or on some external condition that determines that the first egg -shall become a male. It has been claimed that the age of the -spermatozoon might in this and in other cases determine the result; but -Gerbe has shown that if the domestic hen is isolated for fifteen days -after union with the male, she will continue to produce fertile eggs -from which both sexes are produced, without showing any relation between -the time the eggs are laid and the particular sex that develops. - -Cuénot does not discuss whether sex is determined by the nucleus or by -the protoplasm, but if, as he thinks probable, the size of the egg is a -determining factor, it would appear that the protoplasm must be the -chief agent. Even if this were the case it would still be possible that -the size of the egg itself might be connected with some action on the -part of the nucleus. If, as seems probable, identical twins come from -halves of the same egg, then, since they are of the same sex, the -absolute amount of protoplasm cannot be a factor in sex determination. - - -[Illustration: - - Fig. 6.—Diagram showing the maturation of the egg.] - -As a basis for the discussion that follows, certain processes that take -place during the maturation divisions of the egg and of the spermatozoon -must be briefly noticed. After the egg leaves the ovary it extrudes a -minute body called the first polar body (Fig. 6 B, C, D). This process -of extrusion is really a cell division accompanied by the regular -mitotic division of the nucleus; but since one of the products of the -division, the polar body, is extremely small, the meaning of the process -was not at first understood. The half of the nucleus, that remains in -the egg, divides again, and one of its halves is thrown out into a -second polar body (Fig. 6 E, F, G)). Meanwhile, the first polar body has -divided into two equal parts, so that we find now three polar bodies and -the egg (Fig. 6 G)). A strictly analogous process takes place in the -formation of the spermatozoa (Fig. 7 B-F). The mother-cell of the -spermatozoon divides into two parts, which are equal in this case (Fig. -7 B-D). Each of these then divides again (Fig. 7 E, F), producing four -cells that are comparable to the three polar bodies and the mature egg. -Each of the four becomes a functional spermatozoon (Fig. 7 G, H). Thus -while in the maturation of the egg only the egg itself is capable of -development, in the case of the male cells all four products of the two -maturation divisions are functional. - - -[Illustration: - - Fig. 7.—Diagram showing the maturation of the spermatozoon.] - - -Now, in certain cases of parthenogenesis, it has been found that one of -the polar bodies may not be given off, but, remaining in the egg, its -nucleus reunites with the egg nucleus, and thus takes the place of the -spermatozoon, which does exactly the same thing when it fertilizes the -egg, _i.e._ the nucleus of the spermatozoon unites with the nucleus of -the egg. This fact in regard to the action of the polar body in -fertilization is not as surprising as appears at first sight, for if -each of the polar bodies is equivalent to a spermatozoon, the -fertilization of the egg by one of its own polar bodies conforms to -theory. - -There is a considerable body of evidence showing that in many eggs at -one of the two maturation divisions the chromatin rods derived from the -nucleus are divided crosswise (Fig. 6 B, C). The same thing occurs at -one of the two divisions in the formation of the spermatozoon (Fig. 7 B, -C). At the other division to form the other polar body (or the other -sperm-cell) the chromatin rods appear to be split lengthwise, as in -ordinary cell division (Fig. 6 E, F, G). In recent years the -_cross-division_ of the chromatin rods has attracted a great deal of -notice, and Weismann in particular drew attention to the possible -importance of this kind of division. - -There is another fact that gives this division especial significance. It -has been discovered that the number of chromosomes that appears in each -dividing cell of the organism is a constant number, but it has also been -discovered that the egg, before extruding its polar bodies, and the -mother-cell of the spermatozoon (Figs. 6, 7 B), contain exactly half of -the number of chromosomes that are characteristic of the body-cells of -the same animal (Figs. 6, 7 A). Now there is good evidence to show that -the reduction in number is due to the chromosomes uniting sometimes end -to end in pairs, as shown in Figures A and B. Furthermore, it has been -suggested that at one of the maturation divisions, when the chromosomes -divide crosswise, the united chromosomes are separated (Figs. 6, 7 B, -C), so that one remains in the egg and the other goes out into the polar -body. The same thing is supposed to occur at one of the maturation -divisions of the sperm mother-cell. A further consideration of capital -importance in this connection has been advocated by Montgomery and by -Sutton, namely, that, when the chromosomes unite in pairs, a chromosome -from one parent unites with one from the other parent. Consequently at -one of the two reduction divisions maternal and paternal chromosomes may -separate again, some to go to one cell, some to the other. - -When the spermatozoon enters the egg it brings into the egg as many new -chromosomes as the egg itself possesses at this time, and the two -nuclei, uniting into a single one, furnish the total number of -chromosomes characteristic of the animal that develops from the egg. At -first the chromosomes that are brought in by the spermatozoon lie at one -side of the fused nucleus, and those from the egg itself at the other -side. This arrangement appears, however, in some cases at least, to be -lost later. At every division of the nucleus, each chromosome divides -and sends a half to each of the daughter-nuclei. Thus every cell in the -body contains as many paternal as maternal chromosomes. This statement -also applies to the first cells that go into the reproductive organs, -some of which become the mother-cells of the germ-cells. Later, however, -in the history of the germ-cells,—just before the maturation -divisions,—these chromosomes are supposed to unite in pairs, end to end, -as explained above, to give the reduced number. Later there follows the -separation of these paired chromosomes at one of the two maturation -divisions. If at this time all the paternal chromosomes should pass to -one pole, and all the maternal to the other, the germ-cell ceases to be -mixed, and becomes purely paternal or maternal. If this ever occurs, the -problem of heredity may become simplified, and even the question of sex -may be indirectly involved; but it has not been established that, when -the reduced number of chromosomes is formed, there is a strict union -between the paternal and maternal chromosomes, and if not, the -subsequent separation is probably not along these lines. If, however, -the chromosomes contain different qualities, as Boveri believes, there -may be two kinds of eggs, and two kinds of spermatozoa in regard to -_each particular character_. It is this last assumption only that is -made in Mendel’s theory of the purity of the germ-cells. - -Several attempts have been made at different times to connect the facts -in regard to the extrusion of the polar bodies with those involved in -the determination of sex. Minot suggested, in 1877, that the egg ejects -by means of the polar bodies its male elements, which are again received -in the fertilization of the egg by the spermatozoon. The same idea has -also been expressed by others. It has been objected to this view that -one polar body ought to suffice, and that no similar throwing out of -part of its substance is found in the process of formation of the -spermatozoon, which should, on the hypothesis, throw out its female -elements. It would seem, on first thought, that this view might find -support in the idea expressed above, namely, that in one of the polar -bodies half of the chromosomes pass out, so that there is conceivably a -separation of the maternal from the paternal. If this were the case also -in the spermatozoa, then two of each four would be paternal and two -maternal. This is, however, a very different thing from supposing them -to be male and female, for it by no means follows, because the -chromosomes correspond to those of the father or of the mother in the -sum of their characters, that they are, therefore, also male or female -in regard to sex. - -It has been pointed out already, that in most parthenogenetic eggs only -one polar body is extruded. There are, it is true, a few apparent -exceptions to this rule, but in most cases it is certain that only one -is extruded. In several cases the beginning of the formation of the -second maturation division of the nucleus takes place, but after the -chromosomes have divided they come together again in the nucleus. If -each polar body be interpreted as equivalent to a spermatozoon, then -this result is rather a process of self-fertilization than true -parthenogenesis. It is, nevertheless, true that in some cases -development seems to go on after both polar bodies have been extruded. -Moreover, it has been found possible to cause the eggs of the sea-urchin -to begin their development by artificial solutions after they have -extruded both polar bodies. A single spermatozoon may also produce an -embryo if it enters a piece of egg-protoplasm without a nucleus. The -last instance is a case of male parthenogenesis, and if the theory of -the equivalency of spermatozoon and egg be correct, this is what should -occur. - -Quite recently, Cuénot, Beard, Castle, and Lenhossek have contended that -the differentiation of sex is the outcome of internal factors. They -think that the view that sex is determined by external agents is -fundamentally erroneous. The fallacies that have given rise to this -conception, Castle points out, are, first, that in animals that -reproduce sometimes by parthenogenesis and sometimes by fertilized eggs, -the former process is favored by good nutrition and the latter by poor -nutrition. This only means, in reality, Castle thinks, that -parthenogenetic reproduction is favored by external conditions, and this -kind of reproduction, he thinks, is a thing _sui generis_, and not to be -compared to the formation of more females in the sexual forms of -reproduction. There is no proof, however, that this is anything more -than a superficial distinction, and it ignores the fact that in ordinary -cases the females sometimes lay parthenogenetic eggs which differ, as -far as we can see, from eggs that are destined to be fertilized in no -important respect. More significant, it seems to me, is the fact that -only parthenogenetic females develop the following spring from the -fertilized eggs of the last generation of the autumn series, whose -origin is described to be due to lack of food. We find, in the case of -aphids, that unfertilized parthenogenetic eggs and also fertilized eggs -give rise to females only, while a change in the amount of food causes -the parthenogenetic eggs to give rise both to males and to females. This -point is not, I think, fully met by Castle, for even if the change in -food does not, as he claims, cause only one sex to appear, yet lack of -food does seem to account for the appearance of the males at least. - -The other fallacy, mentioned by Cuénot, is that the excess of males that -has been observed when the food supply is limited is due to the early -death of a larger percentage of females, which require more food, but -this still fails to account for the excess of females when more food is -given, provided Yung’s experiments on tadpoles are correct. It may be, -however, in the light of Pflüger’s results, that there has been some -mistake in the experiments themselves. - -We may now proceed to examine Castle’s argument, attempting to show in -what way sex is predetermined in the embryo. His hypothesis rests on the -three following premises: “(1) the idea of Darwin, that in animals and -plants of either sex the characters of the opposite sex are latent; (2) -the idea of Mendel, that in the formation of the gametes [germ-cells] of -hybrids a segregation of the parental characters takes place, and when -in fertilization different segregated characters meet, one will -dominate, the other become latent or recessive; (3) the idea of -Weismann, that in the maturation of egg and spermatozoon a segregation -is attended by a visible reduction in the number of chromosomes in the -germinal nuclei.” - -Expressed in a somewhat more general way, Castle suggests that each egg -and each spermatozoon is either a male or a female germ-cell (and not a -mixture of the two), and when a female egg is fertilized by a male -spermatozoon, or _vice versa_, the individual is a sexual hybrid with -one sex dominating and the other latent. The assumption that there are -two kinds of eggs, male and female, and two kinds of spermatozoa, male -and female, is not supported by any direct or experimental evidence. -Moreover, in order to carry out the hypothesis, it is necessary to make -the further assumption that a female egg can only be fertilized by a -male spermatozoon, and a male egg by a female spermatozoon. While such a -view is contrary to all our previous ideas, yet it must be admitted that -there are no facts which disprove directly that such a selection on the -part of the germ-cells takes place. If these two suppositions be -granted, then Castle’s hypothesis is as follows:— - -In order that half of the individuals shall become males and half -females it is necessary to assume that in some individuals the male -element dominates and in others the female, and since each fertilized -egg contains both male and female elements, it is necessary to assume -that either the egg or the spermatozoon contains the dominating element. - -Castle supposes that in hermaphroditic organisms the two characters -“exist in the balanced relationship in which they were received from the -parents,” but, as has just been stated, in unisexual forms one or the -other sex dominates, except of course in those rare cases, as in the -bees and ants, where half of the body may bear the characters of one -sex, and the other half that of the other sex. - -In parthenogenetic species the female character is supposed to be -uniformly stronger, so that it dominates in every contest, “for the -fertilized egg in such species develops invariably into a female.” Under -certain circumstances, as Castle points out, the parthenogenetic female -produces both males and females, and this is also true in the occasional -development of the unfertilized egg of the silkworm moth, and of the -gypsy moth, in which both male and female individuals are produced by -parthenogenesis. These facts show that even in unfertilized eggs both -sexes are potentially present; but this might be interpreted to mean -that some eggs are male and some female, rather than that each egg has -the possibility of both kinds of development. If, however, one polar -body is retained in these parthenogenetic eggs, then _ex hypothese_ each -egg would contain the potentialities of both sexes (if the polar body -were of the opposite sex character). It seems necessary to make this -assumption because in some parthenogenetic forms males and females may -be produced later by each individual, as in the aphids, and this could -not occur if we assume that some parthenogenetic eggs are purely male -and some female. - -Castle assumes, in fact, that in animals like daphnids and rotifers one -polar body only is extruded, and the other (the second) is retained in -the egg, and hence the potentiality of producing males is present. In -the honey-bee, on the contrary, Castle assumes that both polar bodies -are extruded in the unfertilized egg (and there are some observations -that support this idea), and since only males are produced from these, -he believes it is the female element that has been sent out into the -second polar body. This hypothesis is necessary, because Castle assumes -that when both elements are present in the bee’s eggs, the female -element dominates. “Hence, if the egg which has formed two polar cells -develops without fertilization, it must develop into a male. But if such -an egg is fertilized, it invariably forms a parthenogenetic female ♀ -(♂), that is, an individual in which the male character is recessive. -Accordingly the _functional_ spermatozoon must in such cases invariably -bear the _female character_, and this is invariably dominant over the -male character when the two meet in fertilization.” - -If it should prove generally true that the size of the egg is one of the -factors determining the sex, we have still the further question to -consider as to whether some eggs are bigger because they are already -female, or whether all eggs that go beyond a certain size are females, -and all those that fail to reach this are males. If this is the case, an -animal might produce more females if the external conditions were -favorable to the growth of the eggs, and if in some cases these large -eggs were capable of developing, parthenogenetic races might become -established. Should, however, the conditions for nutrition become less -favorable, some of the eggs might fall below the former size and produce -males. It is not apparent, however, why all the fertilized autumn eggs -of the aphids should give rise to females, for although these eggs are -known to be larger than the summer eggs, yet they are produced under -unfavorable conditions. - -The preceding discussion will show how far we still are from knowing -what factors determine sex. Castle’s argument well illustrates how many -assumptions must be made in order to make possible the view that sex is -a predetermined quality of each germ-cell. Even if these assumptions -were admissible, we still return to the old idea that the _fertilized -egg_ has both possibilities, and something determines which shall -dominate. Until we have ascertained definitely by experimental work -whether the sex in some forms can be determined by external conditions, -it is almost worthless to speculate further. Whatever decision is -reached, the conclusion will have an immediate bearing on the question -to be next discussed. Meanwhile, we can at least examine some of the -theories that have been advanced as to what advantage, if any, has been -gained by having the individuals of many classes divided into two kinds, -male and female. - - - Sex as a Phenomenon of Adaptation - -Of what advantage is it to have the individuals of many species -separated into males and females? It is obviously a disadvantage from -the point of view of propagation to have half of the individuals -incapable of producing young, and the other half also incapable of doing -so, as a rule, unless the eggs are fertilized by the other sex. Is there -any compensation gained because each new individual arises from two -parents instead of from one? Many answers have been attempted to these -questions. - -At the outset it should be recognized that we are by no means forced to -assume, as is so often done, that because there is this separation of -the sexes it must have arisen on account of its advantage to the -species. Whether the result may be of some benefit regardless of how it -arose, may be an entirely different question. It would be extremely -difficult to weigh the relative advantages (if there are any) and -disadvantages (that are obvious as pointed out above), nor is it -probable that in this way we can hope to get a final answer to our -problem. We may begin by examining some of the modern hypotheses that -have been advanced in this connection. - -Darwin has brought together a large number of facts which appear to show -the beneficial effects of the union of germ-cells from two different -individuals. Conversely, it is very generally believed, both by breeders -and by some experimenters, that self-fertilization in the case of -hermaphroditic forms leads, in many cases, though apparently not in all, -to the production of less vigorous offspring. Darwin’s general position -is that it is an advantage to the offspring to have been derived from -two parents rather than to have come from the union of the germ-cells of -the same individual, and he sees, in the manifold contrivances in -hermaphroditic animals and plants to insure cross-fertilization, an -adaptation for this purpose. - -This question of whether self-fertilization is less advantageous than -cross-fertilization is, however, a different question from that of -whether _non-sexual_ methods of reproduction are less advantageous than -sexual ones. Since some plants, like the banana, have been propagated -for a very long time solely by non-sexual methods without any obvious -detriment to them, it is at first sight not easy to see what other -advantage could be gained by the sexual method. The case of the banana -shows that some forms do not require a sexual method of propagation. -Other forms, however, are so constituted, as we find them, that they -cannot reproduce at the present time except by the sexual method. In -other words, the latter are now adapted, as it were, to the sexual -method, and there is no longer any choice between the two methods. The -question of whether a non-sexual form might do better if it had another -method of propagation is not, perhaps, a profitable question to discuss. - -What we really need to know is whether or not the sexual method was once -acquired, because it was an advantage to a particular organism, or to -the species to reproduce in this way. It is assumed by many writers that -this was the case, but whether they have sufficient ground for forming -such an opinion is our chief concern here. On the other hand, it is -conceivable, at least, that if the sexual method once became -established, it might continue without respect to any superiority it -gave over other methods, and might finally become a necessary condition -for the propagation of particular species. Thus the method would become -essential to propagation without respect to whether the species lost -more than it gained. Whichever way the balance should turn, it might -make little difference, so long as the species was still able to -propagate itself. - -Brooks made the interesting and ingenious suggestion that the separation -of the sexes has been brought about as a sort of specialization of the -individuals in two directions. The male cells are supposed to accumulate -the newly acquired characters, and represent, therefore, the progressive -element in evolution. The female cells are the conservative element, -holding on to what has been gained in the past. It does not seem -probable, in the light of more recent work, that this is the function of -the two sexes, and it is unlikely that we could account for the origin -of the two sexes through the supposed advantage that such a -specialization might bring about. A number of writers, Galton, Van -Beneden, Bütschli, Maupas, and others, have looked at the process of -sexual reproduction as a sort of renewal of youth, or rejuvenescence of -the individuals. There is certainly a good deal in the process to -suggest that something of this sort takes place, although we must be on -our guard against assuming that the rejuvenescence is anything more than -the fulfilment of a necessary stage in the life history. Weismann has -ridiculed this suggestion on the ground that it is inconceivable that -two organisms, decrepit with old age, could renew their youth by -uniting. Two spent rockets, he says, cannot be imagined to form a new -one by combining. There is apparent soundness in this argument, if the -implication is taken in a narrow physical sense. If, on the other hand, -the egg is so constituted that at a certain stage in its development an -outside change is required to introduce a new phase, then the conception -of rejuvenescence does not appear in quite so absurd a light. - -This hypothesis of rejuvenescence is based mainly on certain processes -that take place in the life history of some of the unicellular animals. -Let us now see what this evidence is. The results of certain experiments -carried out by Maupas on some of the ciliate protozoans have been -fruitful in arousing discussion as to the ultimate meaning of the sexual -process. Maupas’ experiments consisted in isolating single individuals, -and in following the history of the descendants that were produced -non-sexually by division. He found that the descendants of an individual -kept on dividing, but showed no tendency to unite with each other. After -a large number of generations had been passed through (in _Stylonychia -pustulata_, between 128 and 175; in _Leucophys patula_, 300 to 450; and -in _Onychodromus grandis_, 140 to 230 generations), the division began -to slow down, and finally came to a standstill. Maupas found that if he -took one of these run-down individuals, and placed it with another in -the same condition from another culture, that had had a different -parentage, the two would unite and the so-called process of conjugation -take place. This process consists for the species used, in the temporary -union and partial fusion of the protoplasm of the two individuals, of an -interchange of micronuclei, and of a fusion, in each individual, of the -micronucleus received from the other individual with one of its own. The -individuals then separate, and a new nucleus (or nuclei) is formed out -of the fused pair. - -The individuals in question, in which this interchange of micronuclei -has taken place, undergo a change, and behave differently from what they -did before. They feed, become larger and less vacuolated, and are more -active. They soon begin once more to divide. Maupas found that an -individual that has conjugated will run through a new cycle of -divisions, which will, however, after a time also slow down, unless -conjugation with another individual having a different history takes -place. If conjugation is prevented, the individual will die after a -time. These results seemed to show that the division phase of the life -history cannot go on indefinitely, and that through conjugation the -individual is again brought back to the starting-point. - -Quite recently Calkins has carried out a somewhat similar series of -experiments, which have an important bearing on the interpretation of -Maupas’ results. The experiment of isolating an individual and tracing -the career of its descendants was repeated with the following results: -two series were started, the original forms coming from different -localities. Of their eight descendants four of each were isolated. The -remaining four of each set were kept together as stock material. The -rate of division was taken as the measure of vitality. The animals -divided more or less regularly from February to July. After each -division (or sometimes after two divisions) the individuals were -separated. About the 30th of July the paramœcia began to die “at an -alarming rate, indicating that a period of depression had apparently set -in, or degeneration in Maupas’ sense.” Up to this time the animals had -been living in hay infusion, renewed every few days, from which they -obtained the bacteria on which they feed. Calkins tried the effect of -putting the weakened paramœcia into a new environment. Infusion of -vegetables gave no good results, but meat infusions proved successful. -“The first experiment with the latter was with teased liver, which was -added to the usual hay infusion. The result was very gratifying, for the -organisms began immediately to grow and to divide, the rate of division -rising from five to nine divisions in successive ten-day periods.” This -beneficial effect was not lasting, however, and after ten days the -paramœcia began to die off faster than before, and the renewed -application of the liver extract failed to revive them. A number of -other extracts were then tried without effect. Finally they were -transferred to the clear extract of lean beef in tap water. The effect -of this medium was interesting, for, although it restored the weakened -vitality, there was no rapid increase in the rate of division, as when -first treated with the teased liver. The infusoria were, however, now -large and vigorous, and did not die unless transferred from the beef -medium to the usual hay infusion. “When this was attempted, they would -become abnormally active and would finally die. The division rate -gradually increased during the month of August until, in the last ten -days, they averaged six generations. Finally, in September, the attempts -to get them back on the old diet of hay infusion were successful, and -then the division rate went up at once to twelve times in ten days, and -a month later they were dividing at the rate of fifty times a month.” - -“These cultures went on well until December, when the paramœcia began to -die again. They were saved once more with the beef extract, and when -returned later to the hay infusion continued through another cycle of -almost three months. Some of these were treated, once a week for -twenty-four hours, with the beef extract, and while the two sets ran a -parallel course at first, those kept continuously in the hay infusion -died after a time, but those that had been put once a week into the beef -extract (which had been stopped, however, in March) continued their high -rate of division throughout the period of decline of their sister cells, -and did not show signs of diminished vitality until the first period in -June.” At this time their rate of division increased rapidly. They were -put back into the beef extract, but it failed now to have a beneficial -effect, and the animals continued to die at a rapid rate. To judge from -the appearance of the organisms the new decline was due to a different -cause; for, while in the former periods the food vacuoles contained -undigested food, at this period the interior was free from food masses. -The protoplasm became granular and different from that of a healthy -individual. None of the former remedies were now of any avail. “When the -last of the _B_-series stock had died in the five hundred and seventieth -generation (June 16th), it looked as though the cultures were about to -come to an end.” Extract of the brain and of the pancreas were then -tried. To this a favorable response took place at once. The organisms -became normal in appearance and began to divide. After forty-eight -hours’ treatment they were returned to the usual hay infusion. Here they -continued to multiply and reached on June 28th the six hundred and -sixty-fifth generation. - -There can be no doubt that the periods of depression that appear in -these infusoria kept in cultures can be successfully passed if the -animals are introduced into a new environment. Without a change of this -sort they will die. Calkins thinks that the effect is produced, not by -the new kind of food that is supplied, but by the presence of certain -chemical compounds. The beef extract “does not have a direct stimulating -effect upon the digestive process and upon division, for, while the -organisms are immersed in it, there is a very slow division rate; when -transferred again to the hay infusion, however, they divide more rapidly -than before.” - -This brings us back to the idea of the “renewal of youth” through -conjugation. Maupas claimed that union of individuals having the same -immediate descent is profitless. Calkins suggests that this is due to -the similarity in the chemical composition of the protoplasm of the two -individuals. When in nature two individuals that have lived under -somewhat different conditions conjugate, the result should be -beneficial, since there takes place the commingling of different -protoplasms. - -Calkins’s work has shown that by means of certain substances much the -same effect can be produced as that which is supposed to follow from the -conjugation of two unrelated individuals. The presumption, therefore, is -in favor of the view that the two results may be brought about in the -same way, although we should be careful against a too ready acceptation -of this plausible argument; for we have ample evidence to show that many -closely similar (if not identical) responses of organisms may be brought -about by very different agencies. The experiments seem to indicate that -paramœcium might go on indefinitely reproducing by division, provided -its environment is changed from time to time. If this is true, it is -conceivable that the same thing is accomplished through conjugation. In -the light of this possible interpretation much of the mystery connected -with the term _rejuvenescence_ is removed, for we see that there is -nothing in the process itself except that it brings the organism into a -new relation with other substances. Difficult as it assuredly is to -understand how this benefits the animal, the experimental fact shows, -nevertheless, that such a change is for its good. That there is really -nothing in the process of conjugation itself apart from the difference -in the constitution of the conjugating individuals is shown by the -result that the union of individuals having the same history and kept -under the same conditions is of no benefit. - -Can we apply this same conception to the process of fertilization in the -higher animals and plants? Is the substance of which their bodies are -made of such a sort that it cannot go on living indefinitely under the -same conditions, but must at times be supplied with a new environment? -If this could be established, we could see the advantage of sexual -reproduction over the non-sexual method. It would be extremely rash at -present to make a generalization of this kind, for there are many forms -known in which the only method of propagation that exists is the -non-sexual one. In other words, there are no grounds for the assumption -that this is a necessary condition for all kinds of protoplasm, but only -for certain kinds. - -In the insects, crustaceans, rotifers, and in some plants there are a -few species whose egg develops without fertilization. This makes it -appear probable that the particular kind of protoplasm of these animals -does not absolutely require union from time to time with the protoplasm -of another individual having a somewhat different constitution. - -There is also an interesting parallel between the effects of solutions -on the protozoans in Calkins’s experiments and certain results that have -been obtained in artificial parthenogenesis. It has been stated, that by -brushing the unfertilized eggs of the silkworm moth a larger percentage -will develop parthenogenetically; and more recently it has been shown by -Matthews that by agitation of the water in which the unfertilized eggs -of the starfish have been placed many of them will begin their -development. It was first shown by Richard Hertwig that by putting the -unfertilized eggs of the sea-urchin in strychnine solutions, they will -begin to segment, and I obtained the same results much better by placing -the eggs in solutions of magnesium chloride. Loeb then succeeded in -carrying the development to a later stage by using a different strength -of the same solution, as well as by other solutions. Under the most -favorable circumstances some of the eggs may produce larvæ that seem -normal in all respects, but whether they can develop into adult -sea-urchins has not yet been shown. - -These results indicate that one at least of the factors of fertilization -is the stimulus given to the egg. On the other hand, the lack of vigor -shown by many eggs that have been artificially fertilized indicates that -some other result is also accomplished by the normal method of -fertilization that is here absent. This may mean no more than that as -yet we have not found all the conditions necessary to supply the place -of the spermatozoon. - -In our study of the phenomena of adaptation we have found that sometimes -the adaptation is for the benefit of the individual and at other times -for the benefit of the species. May it not be true also that the process -of sexual reproduction has more to do with a benefit conferred on the -race rather than on the individual? In fact, Weismann has elaborated a -view based on the conception that the process of sexual reproduction is -beneficial to the race rather than to the individual. His idea, however, -is not so much that the result is of direct benefit to a particular -species, as it is advantageous to the formation of new species from the -original one. In a sense this amounts, perhaps, to nearly the same -thing, but in another sense the idea involves a somewhat different point -of view. - -According to his view “the deeper significance of conjugation” and of -sexual reproduction is concerned “with the mingling of the hereditary -tendencies of two individuals.” In this way, through the different -combinations that are formed, variations which he supposes are -indispensable for the action of natural selection originate. The purpose -of the sexual process is solely, according to Weismann, to supply the -variations for natural selection. If it be asked how this process has -been acquired for the purpose of supplying natural selection with the -material on which it can work, we find the following reply given by -Weismann. “But if amphimixis [by which he means the union of sex-cells -from different individuals] is not absolutely necessary, the rarity of -purely parthenogenetic reproduction shows that it must have a widespread -and deep significance. Its benefits are not to be sought in the single -individual; for organisms can arise by agamic methods, without thereby -suffering any loss of vital energy; amphimixis must rather be -advantageous for the maintenance and modification of species. As soon as -we admit that amphimixis confers some such benefits, it is clear that -the latter must be augmented, as the method appears more frequently in -the course of generations; hence we are led to inquire how nature can -best have undertaken to give this amphimixis the widest possible range -in the organic world.” Nature, Weismann says, could find no more -effectual means of bringing about the union of the sexual cells than by -rendering them incapable of developing alone. “The male germ-cells, -being specially adapted for seeking and entering the ovum, are, as a -rule, so ill provided with nutriment that their unaided development into -an individual would be impossible; but with the ovum it is otherwise, -and accordingly the ‘reduction division’ removes half the germ-plasm and -the power of developing is withdrawn.” It can scarcely be claimed, in -the light of more recent discoveries, that the reduction division takes -place in order to prevent the development of the ovum, for how then -could we explain the corresponding division of the male germ-cells? - -Whatever means has been employed to bring about the process of sexual -reproduction, the guiding principle is supposed by Weismann to be -natural selection as stated in the following paragraph: “If we regard -amphimixis as an adaptation of the highest importance, the phenomenon -can be explained in a simple way. I only assume that amphimixis is of -advantage in the phyletic development of life, and furthermore that it -is beneficial in maintaining the level of adaptation, which has been -once attained, in every single organism; for this is as dependent upon -the continuous activity of natural selection as the coming of new -species. According to the frequency with which amphimixis recurs in the -life of a species, is the efficiency with which the species is -maintained; since so much the more easily will it adapt itself to new -conditions of life, and thus become modified.” - -Thus we reach the somewhat startling conclusion that through natural -selection the germ-cells and their protozoan prototypes have been -rendered incapable, through natural selection, of reproducing by -non-sexual methods, in order that variations may be supplied for the -farther action of this same process of natural selection. The -speculation has the appearance of arguing in a circle, although if it -were worth the attempt an ingenious mind might perhaps succeed in -showing that such a thing is not logically inconceivable. - -It seems strange that a claim of this sort should have been made, when -it is so apparent that the most immediate effect of intercrossing is to -swamp all variations that depart from the average. Even if it were true -that new combinations of characters would arise through the union of the -germ-cells of two different animals, it is certainly true that in the -case of fluctuating variations this new combination would be lost by -later crossing with average individuals. Moreover, it is well known that -variations occur amongst forms that are produced asexually. On the -whole, it does not seem to be a satisfactory solution of the problem to -assume that sexual reproduction has been acquired in order to supply -natural selection with material on which it may work. - -Our examination of the suggestions that have been made and of the -speculation indulged in, as to what benefit the process of sexual -reproduction confers on the animals and plants that make use of this -method of propagation, has failed to show convincingly that any -advantage to the individual or to the species is the outcome. This may -mean, either that there is no advantage, or that we have as yet failed -to understand the meaning of the phenomenon. The only light that has -been thrown on the question is that a certain amount of renewed vigor is -a consequence of this process, but we cannot explain how this takes -place. There is also the suggestion that the union of different cells -produces the same beneficial effect as a change in the conditions of -life produces on the organism. The bad effects of close interbreeding -that seem sometimes to follow is explicable on this view. This, it seems -to me, is the most plausible solution of the question that has been -advanced; but, even if this should prove to be the correct view, we need -not assume that the process has been acquired on account of this -advantage, for there is nothing to show that it has been acquired in -this way. - - ------------------------------------------------------------------------- - - - - - CHAPTER XIII - - SUMMARY AND GENERAL CONCLUSIONS - - -The question of the origin of the adaptations with which the last three -chapters have so largely dealt is one of the most difficult problems in -the whole range of biology, and yet it is one whose immense interest has -tempted philosophers in the past, and will no doubt continue to excite -the imagination of biologists for many years to come. No pretence has -been made in the preceding pages to account for the cause of a single -useful variation. We have examined the evidence, and from this we -believe the assumption justified that such variations do sometimes -appear. The more fundamental question as to the origin of these -variations has not been taken up, except in those cases in which the -adaptive response appeared directly in connection with a known external -cause. But these kinds of responses do not appear to have been the -source of the other adaptations of the organic world. Our discussion has -been largely confined to the problem of the widespread occurrence of -adaptation in living things, and to the most probable kinds of known -variations that could have given rise to these adaptations. But, to -repeat, we have made no attempt to account for the causes or the origin -of the different kinds of variation. - -Nägeli, in speaking of the methods of the earlier theorists in Germany, -remarks with much acumen: “We might have expected that after the period -of the Nature-philosophizers, which in Germany crippled the best forces -that might have been used for the advance of the science, we should have -learnt something from experience, and have carefully guarded the field -of real scientific work from philosophical speculation. But the outcome -has shown that, in general, the philosophical, philological, and -æsthetic expression always gets the upper hand, and a fundamental and -exact treatment of scientific questions remains limited to a small -circle. The public at large always shows a distinct preference for the -so-called idealistic, poetic, and speculative modes of expression.” The -truth of this statement can scarcely be doubted when in our own time we -have seen more than once the same method employed with great public -applause. Nowhere is this more apparent than in the writings of many of -the followers of Darwin in respect to the adaptations of living things. -To imagine that a particular organ is useful to its possessor, and to -account for its origin because of the imagined benefit conferred, is the -general procedure of the followers of this school. Although protests -have from time to time been raised against this unwarrantable way of -settling the matter, they have been largely ignored and forgotten. The -fallacy of the argument has, for example, been admirably pointed out by -Bateson in the following statement:[36] “In examining cases of variation -I have not thought it necessary to speculate on the usefulness or -harmfulness of the variations described. For reasons given in Section II -such speculation, whether applied to normal structures or to variation, -is barren and profitless. If any one is curious on these questions of -Adaptation, he may easily thus exercise his imagination. In any case of -Variation there are a hundred ways in which it may be beneficial or -detrimental. For instance, if the ‘hairy’ variety of the moor-hen became -established on an island, as many strange varieties have been, I do not -doubt that ingenious persons would invite us to see how the hairiness -fitted the bird in some special way for life in that island in -particular. Their contention would be hard to deny, for on this class of -speculation the only limitations are those of the ingenuity of the -author. While the only test of utility is the success of the organism, -even this does not indicate the utility of one part of the economy, but -rather the net fitness of the whole.” Keeping in mind the admonitions -contained in the two preceding quotations, let us pass in review and -attempt to analyze more fully the different points that have been -considered in the preceding chapters. - -Footnote 36: - - “Materials for the Study of Variation.” - -It has been pointed out that the evidence in favor of the theory of -evolution appears to establish this theory with great probability, -although a closer examination shows that we are almost completely in the -dark as to how the process has come about. For example, we have not yet -been able to determine whether the great groups of animals and plants -owe their resemblance to descent from a single original species or from -a large number of species. The former view is more plausible, because on -it we appear to be furnished with a better explanation of resemblances -as due to divergence of character. Yet even here a closer scrutiny of -the homologies of comparative anatomy shows that this explanation may be -more apparent than real. If discontinuous variation represents the steps -by which evolution has taken place, the artificiality of the explanation -is apparent, at least to a certain degree. - -Admitting that the theory of evolution is the most probable view that we -have to account for the facts, we next meet with two further -questions,—the origin of species and the meaning of adaptation. These -are two separate and distinct questions, and not one and the same as the -Darwinian theory claims. The fact that all organisms are more or less -adapted to live in some environment appears from our examination to have -no direct connection with the origin of the adaptation, for, in the -first place, it seems probable that, in general, organisms do not -respond adaptively to the environment and produce new species in this -way; and, in the second place, there is no evidence to show that -variation from internal causes is so regulated that only adaptive -structures arise (although only adaptive ones may survive). - -Our general conclusion is then as follows: A species does not arise from -another one because it is better adapted. Selection, in other words, -does not account for the origin of new species; and adaptation cannot be -taken as the measure of a species. - -It may sound like a commonplace to state that only those individuals -survive and propagate themselves that can find some place in nature -where they can exist and leave descendants; and yet this statement may -contain all that it is necessary to assume, in order to account for the -fact that organisms are, on the whole, adapted. Let us see how this view -differs from the Darwinian statement of the origination of new forms -through a process of natural selection. - -According to Darwin’s view of the origin of species, each new species is -gradually formed out of an older one, because of the advantage that the -new individual may have over the parent form. Each step forward is -acquired, because it better adapts the organism to the old, or to a new -set of conditions. In contrast to this, I have urged that the formation -of the new species is, as a rule, quite independent of its adaptive -value in regard to the parent species. But after it has appeared, its -survival will depend upon whether it can find a place in nature where it -can exist and leave descendants. If it should be well adapted to an -environment, it will be represented in it by a large number of -individuals. If it is poorly adapted, it may only barely succeed in -existing, and leave correspondingly fewer descendants. If its -adaptiveness falls below a certain point, it can never get a permanent -foothold, however often it may appear. Thus the test of survival -determines which species can remain in existence, and which cannot, but -new species are not manufactured in this way. How far subsequent -variations may be supposed to be determined by the survival of certain -species and the destruction of others will be discussed presently. - -The difference between the two points of view that we are contrasting -can be best brought out by considering the two other kinds of selection -which Darwin supposes to have been at work; namely, artificial and -sexual selection. - -Darwin thinks that the results of artificial selection are brought about -by the breeder picking out fluctuating variations. It appears that he -has probably overestimated the extent to which this process can be -carried; for while there can be no doubt that a certain standard, or -fixity of type, can be obtained by selecting fluctuating variations, yet -it now seems quite certain that the extent to which this can be carried -is very limited. It appears that other factors have also played an -important rôle; amongst these the occasional appearance of discontinuous -variation, also the bringing under cultivation of the numerous “smaller -species” of De Vries, or the so-called “single variations” of Darwin. -Further, the effects of intercrossing in all combinations of the above -forms of variations, followed by the selection of certain of the new -forms obtained, has been largely employed, and also the direct influence -of food and of other external conditions, which may be necessary to keep -the race up to a certain standard, have played a part in some cases. The -outcome is, therefore, by no means so simple as one might infer from -Darwin’s treatment of the subject in his “Origin of Species.” For these -reasons, as well as for others that have been given, it will be evident -that the process of artificial selection cannot be expected to give a -very clear idea of how natural selection could act. - -It is, however, the process of sexual selection that brings out in the -strongest contrast the difference between Darwin’s main idea of natural -selection and the law of the survival of species. In sexual selection -the competition is supposed to be always between the individuals of the -same species and of the same sex. There can be no doubt in one’s mind, -after reading “The Descent of Man,” that Darwin held firmly to the -belief that the individual differences, or fluctuating variations, -furnish the material for selection. In this way it could never happen -that two competing species could exterminate each other, because in the -one the males were better adorned, or killed each other off on a larger -scale, owing to the presence of special weapons of warfare. It is clear -that on the law of the survival of species, secondary sexual characters -cannot be supposed to have evolved because of their value. Their origin -is totally inexplicable on this view. In fact, the presence of the -ornaments must be in some cases injurious to the existence of the -species. The interpretation of this means, I think, that individual -competition cannot be as severe as Darwin believed, and cannot lead to -the results that he imagined it does. For this reason it seemed -important to make as careful an examination of the claims of the theory -of sexual selection as possible, and I hope that the outcome of the -examination has shown quite definitely that the theory is incompetent to -account for the facts that it claims to explain. It is certain in this -case that we are dealing with a phenomenon that must be studied quite -apart from any selective value that the secondary sexual organs may -have. If this is granted, it will be seen that there is here a wide -field for experimental investigation that is practically untouched. - -It is evident that the first step that will clear the way to a fuller -understanding of the problem of evolution must be a more thorough -examination of the question of variation. Darwin himself fully -appreciated this fact, yet until within the last fifteen years the study -of variation has been largely neglected. With a fuller knowledge of the -nature of fluctuating variation as the outcome of the studies of Galton, -Pearson, De Vries, and others, and with a fuller knowledge of the -possibilities of discontinuous variation as emphasized by Bateson and by -De Vries, and, further, with a better knowledge of some of the laws of -inheritance in these cases, we have begun to get a different conception -of how evolution has come about. It may be well, therefore, to go once -more over the main points in regard to the different kinds of variation. - -While it has been found that no two individuals of a species are exactly -alike, yet, taken as a group, the variations appear as though they -followed the law of chance. The descendants of the group show the same -differences. Thus the group _as a whole_ appears constant, while the -individuals fluctuate continually in all directions. This is what we -understand by fluctuating variation. If the external conditions are -changed, a new “_mode_” may appear, but the change is generally only a -temporary one, and lasts only as long as the new conditions remain. -Thus, while the direct influence of the environment may show for a time, -the result is transient. Even if it were permanent, there is no evidence -that the adaptation of organisms could be accounted for in this way -unless the response were useful. It appears that this sometimes really -occurs, especially in responses to temperature, to moisture, to the -amount of salts in solution, to poisonous substances, etc. In this way, -one kind of adaptation is brought about, but there is no evidence that -the great number of structural adaptations have thus arisen. - -The Lamarckian principle of the inheritance of acquired characters has -also been supposed by many writers to be an important source of adaptive -variation. An examination of this theory is not found to inspire -confidence. We have, therefore, eliminated this hypothesis on the ground -that it lacks evidence in its favor, and also because it appears -improbable that in this way many of the adaptations in organisms could -have been acquired. - -Finally, there is the group of discontinuous variations. Of these there -may be several kinds, and there is some evidence showing that there are -such. For the present we may include all the different sorts under the -term _mutation_, meaning that the new character or group of characters -suddenly appears, and is inherited in its new form. From the results of -De Vries it appears that mutations are sometimes scattering, at least in -the case of the evening primrose. From such scattering mutations, the -smaller species or varieties (in so far as these do not depend on local -conditions) arise. There is here an important point of agreement with -Darwin’s idea in regard to evolution, inasmuch as he supposed that -varieties are incipient species. Our point of view is different, -however, in that we do not suppose these varieties (mutations) to have -been gradually formed out of fluctuating variations by a process of -selection, but to have arisen at once by a single mutation. It also -appears that in some cases a single new mutation may develop in a -species. We may suppose that the new form might in such a case supplant -the parent species by absorbing it, or both may go on living side by -side, as will be more likely the case if they are adapted to somewhat -different conditions. - -A number of writers have supposed that evolution marches steadily -forward toward its final goal, which may even lead in some cases to the -final but inevitable destruction of the species. By certain writers this -view has been called orthogenesis, although at other times the idea is -not so much that there is advance in a straight line, as advance in all -directions. This appears to be Nägeli’s view. It gives a splendid -picture of the organic world, as irresistibly marching toward its -goal,—a relentless process in some cases, leading to final annihilation, -a beneficent process in other cases, leading to the fullest perfection -of form of which the type is capable. Compared with the vacillating -progress which is supposed to be the outcome of individual selection, -this view of progression has a grandeur that appeals directly to the -imagination. We must be guided, however, by evidence, rather than by -sentiment. The case will, moreover, bear closer scrutiny. If evolution -has indeed taken place by the survival of a series of mutations, whose -origin has no connection with their value, does not this in the end -amount to nearly the same thing as maintaining that evolution of -organisms has been a steady progress forward,—a progress undirected by -external forces, but the outcome of internal development? Admitting that -innumerable creations have been lopped off, because they could find no -foothold, yet, as Nägeli points out, the result is that, instead of a -dense tangle of forms, there has been left relatively few that have been -found capable of existing,—those that have found some place in which -they can live and leave progeny. From this point of view it may appear, -at first thought, that the idea of evolution through mutations involves -a fundamentally different view from that of the Darwinian school of -selection; but in so far as selection also depends on the spontaneous -appearance of fluctuating variations, the same point of view is to some -extent involved,—only the steps are supposed to be smaller. This point -is usually ignored and passed over in silence by the Darwinians, but, as -Wigand has pointed out, it makes very little difference whether the -stages in the process of evolution are imagined to be very small or -somewhat larger, so long as they are spontaneous. Selection does not do -more than determine the survival of what is offered to it, and does not -create anything new. - -It is true that if the fluctuating variations that are selected be -connected by very slight differences with an almost continuous series of -other forms, and if little by little such a series be advanced in a -given direction by selection, we get the idea of a continuity, whose -advance is determined by selection. It is this conception that appears -to give the theory of natural selection a creative power, which in -reality it does not possess, and certainly not in the modified form in -which the theory was finally left by Darwin. For Darwin found himself -forced to admit that, unless a very considerable number of individuals -varied at the same time and in the same direction, the formation of new -species could not take place, and this idea of many individuals varying -at the same time, and in the same direction, at once involves the -conception that evolution moves forward by some force residing in the -organism, driving it forwards or backwards. Instability comes, perhaps, -nearer to expressing this idea than any other term, and yet to evolve -from a protozoan to a man implies the idea of something more than simple -unstableness. - -The idea that Weismann has touched upon in this connection, namely, that -the survival of a given form determines the future course of evolution -for that form, is very plausible, and also fits in well with the results -of our experience in the field of the inheritance of variations. We see -new variations or mutations departing in some or in many characters from -the original type, apparently by new combinations or perturbations of -those already present. We never expect to see a bird emerge from the egg -of an alligator. Thus it appears that by the survival of certain forms -the future course of evolution is determined in so far as the new types -of mutation are thereby limited. Weismann means, however, that in this -way new plus or minus steps will be indefinitely determined amongst the -new fluctuating variations, but this statement is contradicted by our -experience of the results of artificial selection. The upper limit does -not keep on pushing out indefinitely in the direction determined by the -first selection, but is soon brought to a standstill. So that, as far as -Weismann’s hypothesis is concerned, the idea appears to have no special -value. On the other hand, this idea may be fruitful if applied to -mutations, but here unfortunately we have not sufficient experience to -guide us, and we do not know definitely whether a new character that -appears as a mutation will be more likely, in subsequent mutations, to -go on increasing in some of the descendants. Thus, while the mutation -theory must assume that some new characters will go on heaping up, we -lack the experimental evidence to show that this really occurs. It would -be also equally important to determine, whether, if after several -mutations have successively appeared in the same direction, there would -be an established tendency to go on in the same direction in some of the -future mutations. But here again we must wait until we have more data -before we attempt to build up a theory on such a basis. - -The attacks on the Darwinian school by the followers of the modern -school of experimentalists are with few exceptions based on the -assumption that the natural selectionists pretend that their principle -is a sort of creative force,—a factor in evolution in the sense of being -an active agent. This assumption of the selectionists has led many of -them to ignore a fundamental weakness of their theory, namely, the -origin of the variations themselves, although Darwin did not overlook or -ignore this side of the problem, or fail to realize its importance, as -have some of his more ardent, but less critical, followers. They have -contented themselves, as a rule, with pointing out that certain -structures are useful, and this has seemed to them sufficient proof that -the structures must have been acquired because of their value. In -contrast to this complacency of the selectionists, we find here and -there naturalists who have, from time to time, insisted that the -scientific problem of evolution is not to be found in the principle of -selection, but in the origin of the variations themselves. It will be -clear, from what has been said, that this is our position also, and for -us adaptation itself does not appear to be any more a problem that can -be examined by scientific methods, than the lack of adaptation. The -causes of the change of whatever kind should be our immediate quest. The -destruction of the unfit, because they can find no place where they can -exist, does not explain the origin of the fit. - -Over and beyond the primary question of the _origin_ of the adaptive, or -non-adaptive, structure is the fact that we find that the great majority -of animals and plants show distinct evidence of being suited or adapted -to live in a special environment, _i.e._ their structure and their -responses are such that they can live and leave descendants behind them. -I can see but two ways in which to account for this condition, either -(1) teleologically, by assuming that only adaptive variations arise, or -(2) by the survival of only those mutations that are sufficiently -adapted to get a foothold. Against the former view is to be urged that -the evidence shows quite clearly that variations (mutations) arise that -are not adaptive. On the latter view the dual nature of the problem that -we have to deal with becomes evident, for we assume that, while the -origin of the adaptive structures must be due to purely physical -principles in the widest sense, yet whether an organism that arises in -this way shall persist depends on whether it can find a suitable -environment. This latter is in one sense selection, although the word -has come to have a different significance, and, therefore, I prefer to -use the term _survival of species_. - -The origin of a new form and its survival after it has appeared have -been often confused by the Darwinian school and have given the critics -of this school a fair chance for ridiculing the selection theory. The -Darwinian school has supposed that it could explain the origin of -adaptations on the basis of their usefulness. In this it seems to me -they are wrong. Their opponents, on the other hand, have, I believe, -gone too far when they state that the present condition of animals and -plants can be explained without applying the test of survival, or in a -broad sense the principle of selection amongst species. - -It will be clear, therefore, in spite of the criticism that I have not -hesitated to apply to many of the phases of the selection theory, -especially in relation to the selection of the individuals of a species, -that I am not unappreciative of the great value of that part of Darwin’s -idea which claims that the _condition_ of the organic world, as we find -it, cannot be accounted for entirely without applying the principle of -selection in one form or another. This idea will remain, I think, a most -important contribution to the theory of evolution. We may sum up our -position categorically in the following statements: - -Animals and plants are not changed in this or in that part in order to -become better adjusted to a given environment, as the Darwinian theory -postulates. Species exist that are in some respects very poorly adapted -to the environment in which they must live. If competition were as -severe as the selection theory assumes, this imperfection would not -exist. - -In other cases a structure may be more perfect than the requirements of -selection demand. We must admit, therefore, that we cannot measure the -organic world by the measure of utility alone. If it be granted that -selection is not a moulding force in the organic world, we can more -easily understand how both less perfection and greater perfection may be -present than the demands of survival require. - -If we suppose that new mutations and “definitely” inherited variations -suddenly appear, some of which will find an environment to which they -are more or less well fitted, we can see how evolution may have gone on -without assuming new species have been formed through a process of -competition. Nature’s supreme test is survival. She makes new forms to -bring them to this test through mutation, and does not remodel old forms -through a process of individual selection. - - ------------------------------------------------------------------------- - - - - - INDEX - - - Acclimatization, 319. - Acorn, 24. - Acracids, 160. - Adaptation, definition of, 1. - Adjustments, individual, 12. - Agassiz, 1, 44, 61. - Agelæus, 173. - Alcohol, 13. - Algæ, red, 9. - Alkaloids, 13. - Allen, 173, 307-310. - Allolobophora, 380. - Alpheus, 344. - Ammophila, 5. - Ammotragus, 208. - Ampelopsis, 403. - Amphimixis, 448-449. - Amphioxus, 399. - Ancon race, 315-316. - Angiostomum, 422. - Anguillidæ, 320. - Annelids, 19, 20. - Anolis, 10, 194. - Ant-eater, 227, 228. - Antelope, 6, 206, 208. - Antitoxin, 14. - Ants, 141-146, 354, 386, 407. - Aphids, 384-386, 419, 426. - Apus, 418. - Archæopteryx, 41, 42, 53, 54. - Ardea, 200. - Argus pheasant, 199. - Aristolochia Clematitis, 10, 11, 12. - Arsenic, 13. - Artemia, 306. - Ascidians, 417. - Askenasy, 161. - Aspalax, 227. - Asterina, 421-422. - Autenrieth, 58. - - - Baboon, 209. - Bacteria, 14, 15, 111, 398. - Baer, Von, 60, 61, 74, 75. - Bamboo, 313. - Barnacles, 417. - Bartlett, 209, 220. - Bat, 2. - Bates, 183, 186. - Bateson, 273, 278, 453. - Beard, 210, 211, 216. - Beard, J., 435. - Bee, 2, 3, 19, 143, 179, 303, 350, 406, 420, 421, 425, 438. - Beethoven, 218. - Beetles, 182, 183, 189. - Bell-bird, 198. - Beneden, Van, 441. - Berbura goat, 208. - Biogenetic Law, 71. - Birds, 6, 407; - definition of group, 36; - evolution of, 41; - instincts of young, 4; - nest, 4; - of paradise, 6; - teeth of, 301; - variation in, 309-312. - Blind animals, 354. - Blow-fly, 383. - Bonellia, 353, 417. - Born, 424. - Bos, 206. - Boveri, 433. - Bovidæ, 207. - Branchipus, 306. - Brocadello, 428. - Brooks, 441. - Brown-Séquard, 232, 241, 250-257. - Buffon, 300. - Bull, 207, 315. - Bütschli, 441. - Butterfly, 3, 184, 389. - - - Cactus, 10. - Caffein, 13. - California salmon, 19. - Calkins, 443-447. - Callionymus, 191. - Calocalanus, 177. - Cameron, 425. - Canestrini, 178. - Canidæ, 308. - Canis, 410. - Carbonnier, 190, 192. - Cardamine, 335. - Cardinalis, 173. - Cardium, 305. - Cassowary, 202. - Castle, 148, 321, 435, 438. - Caterpillar, 5, 8, 186. - Cattle, 411. - Cats, 209. - Cercopithecus, 208. - Cervus, 304. - Chara, 420. - Charrin, 257. - Chick, 57, 406. - Child, 72. - Chinese pheasants, 6. - Chlorophyl, 9. - Cicadas, 187, 188. - Ciona, 148. - Classification, 31-36. - Classification, scheme of, 38. - Cockatoo, 6. - Colaptes, 310. - Colias, 185. - Colonial forms, 127. - Color, 18, 19, 24, 133, 375. - Coloration, 5, 6, 7, 23, 357-360. - Columba livia, 76. - Comb of bees, 4. - Communal marriages, 210. - Competition, 104, 112, 119, 120, 121, 122, 123. - Compositæ, 130. - Cones, 310. - Conklin, 72. - Cope, 49, 259. - Copridæ, 183. - Coral-snakes, 194. - Correlated variation, 94. - Correlation, 134. - Cottus, 191. - Crab, 15, 248, 344, 354. - Crickets, 188. - Crocodiles, 193. - Crosby, 398. - Cross-fertilization, 21. - Crossing of species, 148, 149, 150. - Crystal, 29. - Cryptocerus, 144. - Ctenophors, 417. - Cuckoo, 139, 140, 141. - Cuénot, 427-428, 435. - Culicidæ, 188. - Cunningham, 257-260. - Cuvier, 44, 301. - Cynocephalus, 209. - Cypridopsis, 392-393. - Cyprinodonts, 190. - Cypris, 320. - - - Dall, 260. - Dallinger, 320. - Danaids, 160. - Dances, 195. - Daphnia, 305, 418. - Darwin, C., numerous references throughout the text. - Darwin, Erasmus, 223. - Date-palm, 313. - Davenport, 264, 266, 321. - Dean, 358. - Death, 370. - Death, feigning, 410, 411. - Deer, 309. - Degeneration, 165. - Delamare, 257. - Descent theory, 31-35. - Desmarest, 206. - Desmodium, 403. - Dianthus, 149. - Didelphys, 410. - Dimorphism, 360. - Dingoes, 314. - Dinophilus, 428. - Diptera, 180, 188. - Divergence of character, 127, 128. - Dog, 226. - Draba, 288, 289, 290, 292, 294. - Draco, 194. - Dragonet, 191. - Drill, 209. - Ducks, 94, 314. - Düsing, 423. - Dutrochet, 320. - - - Earthworm, 380, 382, 383, 384, 417. - Echidna, 54. - Eciton, 144. - Egerton, 204. - Egg, 429-430, 432. - Eggs, number of, 19. - Egypt, animals of, 225. - Egyptian remains of animals, 43, 44. - Eimer, 158, 260. - Eisig, 72. - Electric organs, 22, 132, 372. - Elephant, 110-111, 206, 304. - Emu, 202. - Entoscolax, 353. - Epihippus, 50. - Equus, 50. - Eristales, 188. - Esmeralda, 182. - Euploids, 160. - Eustephanus, 201. - Evolution, 29. - Ewart, 238. - Exercise, 12. - External conditions, 130. - Eye, 13, 131, 132. - Eye, evolution of, 131, 132. - Eye, of flatfish, 137. - - - Fayal Islands, 124. - Felidæ, 308. - Felis, 308. - Fish, change of color, 16. - Fishes, 7. - Fishes, secondary sexual character of, 190. - Flatfish, 137, 138. - Flatworms, 417. - Fleischmann, 45-57. - Flounders, 228, 346, 347. - Flowers, 9, 17, 21, 342, 399, 422, 429. - Fly, 428. - Foot of horse, 47. - Forel, 5. - Fossil horses, 52. - Foxes, 210, 410. - Franqueiros cattle, 315. - Frogs, 193, 320, 382, 405. - Frogs, cross-fertilization, 150. - Fruit, down of, 133. - Fundulus, 16. - - - Galton, 236, 270-272, 289, 441. - Gavials, 301. - Geddes and Thompson, 417, 423, 426, 427. - Geer, De, 178. - Gegenbaur, 49. - Gelasimus, 177. - Geoffroy St.-Hilaire, 44, 67, 300-303. - Geological evidence, 39. - Gerbe, 429. - Germinal selection, 154. - Gibbon, 213. - Gill-clefts, 62, 63, 64, 73. - Giraffe, 6, 203, 229, 248-249. - Glacier, 28. - Glowworm, 23. - Goat, 206, 208. - Gonionema, 399. - Gorilla, 205. - Gothic period, 47, 48. - Gould, 197. - Graba, 124, 125. - Grafting, 153. - Grasshoppers, 8, 188. - Gray, 126. - Greyhound, 134. - Growth of plants, 17. - Guillemots, 124. - Guinea-pigs, 232. - Günther, 190. - Gymnotus, 132. - - - Haeckel, 48, 49, 56, 70, 71, 79, 80, 82. - Hartman, 187. - Heart, 66, 67. - Heliconids, 160. - Helix, 344, 345-346. - Hemiptera, 181. - Heredity, 270. - Hermaphroditic animals, 126. - Hertwig, O., 78, 79, 80, 81, 82, 83. - Hertwig, R., 447. - Hieracium, 330, 331. - Hildebrand, 148. - Hill, 252. - Hipparion, 51. - Hippeastrum, 148. - His, 71, 72. - Holmes, 72. - Hornbills, 219. - Horns, 229. - Horse, 42, 224. - Horse-chestnut, 24. - Hothura, 410. - Hottentots, 212. - Hudson, 140, 195, 409-412. - Humming-birds, 6, 197, 228. - Hurst, 75, 76, 77, 78. - Huxley, 49, 156, 233. - Hyatt, 259. - Hybrids, 149, 239. - Hydatina, 417. - Hydroides, 348. - Hylobates, 205. - Hymenoptera, 181. - - - Ice, 28. - Ichneumonidæ, 181. - Idioplasm, 335. - Immunity, 13. - India cattle, 208. - Infanticide, 25. - Inorganic adaptations, 26. - Insectivorous plants, 10. - Insects, coloration of, 7; - wingless, 228. - Instinct, 25, 139, 140, 141. - Irish elk, 247. - - - Jackson, 260. - Jaguar, 209. - Japanese cock, 163. - Jennings, 395. - Jonghe, 314. - Jordan, 292. - Joseph, 428. - Junco, 311. - - - Kallima, 7, 161, 162, 358. - Kangaroo, 229, 351. - Katydid, 8. - Kent, W. Saville, 191. - Kidneys, 66. - Kielmeyer, 58. - Kirby, 232. - Kiwi, 354. - Kölreuter, 149. - Korschelt, 428. - - - Labidocera, 393. - Lamarck, 146, 222-233. - Lamarckian factor, 94, 205, 211, 222, 458. - Lang, 345. - Law of Biogenesis, 30. - Leaf, resemblance to, 7. - Leaves, closing of, 11. - Leeches, 417. - Leguminosæ, 124. - Leidy, 46. - Length of life, 20. - Lenhossek, 435. - Leopard, 209. - Lepidoptera, 172. - Leptothrix, 320. - Leucophys, 442. - Lichen, 9. - Lillie, 72. - Limbs of vertebrates, 46. - Linaria, 401. - Linnæan species, 83. - Linnæus, 191. - Lion, 6. - Lizards, 7, 16, 17, 193. - Lobelia, 148. - Lobster, 343. - Lockwood, 138. - Locusts, 188. - Loeb, 383-392, 447. - Lomaria, 290. - Lowell lectures, 61. - Lumbriculus, 15. - Luminous organs, 133. - Lymnæa, 305, 322. - Lythrum, 363-370. - - - Machines, 26, 27, 28. - McIntosh, 176. - McNeill, 204. - Macropus, 192. - Malva, 401. - Mammalia, origin, 54, 202. - Man, 210. - Marsh, 49. - Matthews, 447. - Mauchamp, 315. - Maupas, 441, 442, 445. - May-flies, 19, 353, 389. - Mead, 72. - Meckel, 59, 60. - Melanism, 209. - Melospiza, 311. - Mendel, 278-286, 433, 436. - Mesohippus, 51. - Mimosa, 404. - Minnow, 16. - Minot, 433. - Mirabilis, 149, 150. - Mivart, 136, 137. - Mole, 1, 2, 18, 227. - Mole-cricket, 1, 2. - Molothrus, 140. - Monkeys, 207, 208. - Mons, Van, 332. - Montgomery, 432. - Moor-hen, 453. - Moquin-Tandon, 303. - Morton, Lord, 238. - Moschus, 206. - Moths, 184, 387, 388. - Moussu, 257. - Mozart, 218. - Mulberry, 313. - Müller, 182, 188. - Müller, Fritz, 148. - Muscles, 12. - Mycetes, 205. - Myzostomum, 422. - - - Nägeli, 161, 325-339, 459. - Natural selection, 104-107, 108, 109, 110, etc.; - definition of, 117. - Nauplius, 69. - Nectar, 124. - Nectar-feeding insects, 126, 127. - Nectarines, 134. - Negroes, 212. - Nematode, number of eggs, 110. - Nematus, 425. - Nemertian worms, 176. - Neo-Lamarckians, 240, 259-260. - Nepenthes, 10. - Nephela, 178. - Nest of birds, 4, 407-408. - Neuters, 142. - Nicotine, 13. - Nostocs, 320. - Notochord, 64, 65. - Nussbaum, 424. - - - Ocneria, 428. - Œnothera, 294-297. - Oken, 56, 58. - Old age, 21, 25. - Onites, 232. - Onychodromus, 442. - Opossum, 410. - Organs of little use, 22. - “Origin of Species,” 129. - Ornithorynchus, 54. - Orobanchia, 330. - Osborn, 259. - Oscillaria, 320. - Ostrich, 203, 354. - Oxalis, 290, 404. - Oxen, 304. - Oxide, 29. - - - Packard, 231, 260. - Paludina, 320, 322. - Pangenesis, 233-240. - Papilio, 158, 360, 388; - polyxenes, 3. - Paradisea, 197. - Paramæcium, 395-398, 442-447. - Parasitism, 352-353. - Parker, 393. - Parrots, 6. - Partridge, 410. - Passerella, 311. - Passiflora, 148. - Pavo, 317. - Peach, 134. - Peacock, 200, 317-318. - Peafowl, 198. - Pearson, 265, 267, 268-270. - Peas, 281-286. - Peckham, 178, 408. - Pelobates, 421. - Pflüger, 424, 430. - Phosphorescent organs, 22, 133. - Physa, 320, 322. - Pigeons, selection in, 102. - Pipilo, 311. - Pisum, 278. - Pithecia, 208. - Planaria, 380. - Planarians, 394. - Plants, 403, 415; - color of, 24; - influence of light, 17. - Plato, 304. - Plover, 202. - Poisons, 13, 14, 15, 18, 20, 377. - Polar bear, 6. - Pollen, 2, 125. - Polygon, 262. - Porthesia, 389. - Primula, 361-365. - Prionidæ, 182. - Probosces of insects, 127. - Protective coloration, 5, 6, 16, 158, 159. - Proteus, 227. - Protohippus, 51. - Przibram, 347. - Psyche, 419. - Ptarmigan, 5. - Pyrodes, 182. - - - Quetelet, 289. - Quiscalus, major, 173. - - - Rabbit, Porto Santo, 316-317. - Rabbits, 304. - Rabbits in Australia, 112. - Race-horse, 134. - Ranunculus, 305. - Ray-florets, 135. - Rays, electric organs of, 22. - Réaumur, 388. - Recapitulation theory, 58-83. - Reduction division, 432-433. - Regeneration, 15, 16, 27, 379. - Regulations, 27, 28. - Reproductive organs, 19. - Reptiles, fossil, 52, 53. - Rengger, 205. - Rhododendron, 330. - Rhynchæa, 201. - Riley, 424. - Rivers, 28. - Robinia, 404. - Romanes, 132, 250-256, 412. - Rose, 307. - Rothert, 398. - Rotifers, 118, 353, 424. - Roulin, 304. - Roundworms, 176, 353. - Rudimentary organs, 22. - Ryder, 260. - - - Sacculina, 353. - Sachs, 10. - Salmon, 19. - Salter, 314. - Salvin, 201. - Saphirina, 176. - Savages, 210. - Saw-flies, 425. - Scarlet tanager, 198. - Schaefer, 244. - Sclater, 198. - Scops, 312. - Scott, 148, 259. - Sea-anemone, 341. - Sea-urchin, 341. - Secondary sexual characters, 21. - Selection, 116. - Selection, artificial, 91, 92, 96, 97, 98. - Self-fertilization, 126. - Semper, 260. - Setchel, 320. - Sexual characters, secondary, 372-374. - Sexual selection, 167. - Sharp, 350, 425. - Sheep, 208. - Sherrington, 244. - Shrew mice, 206. - Silkworm, 428, 447. - Silver-bill, 410. - Sirex, 181. - Siricidæ, 181. - Sitaria, 194. - Skin, thickening of, 12, 13. - Skull, 37, 65. - Skunk, 3. - Slaves of ants, 141. - Sleep in plants, 404. - Sloth, 229. - Snail, 417. - Snails, color of, 23. - Snakes, 14, 193-194, 227. - Snowy owl, 6. - Solenobia, 419. - Soles, 137, 228. - Sparassus, 178. - Sparrow, 200; - English, 112. - Species, 31, 32, 33; - adaptation for good of, 19; - sharp separation of, 131. - Spencer, 240-246, 247, 290. - Spermatozoa, 150, 430-433. - Sphinx, 186, 388. - Spiders, 177-178, 179, 406; - web, 3. - Spirogyra, 420. - Spontaneous variability, 134. - Spores, 322. - Squilla, 177. - Squirrels, 210. - Stag-beetle, 179. - Stags, 203-204, 219. - Sterility, 147-152. - Strasburger, 395. - Stridulating organs, 188, 189. - Struggle for existence, 109, 110. - Stylonychia, 442. - Survival of the fittest, 107, 108, 109, 117. - Sutton, 432. - Swallow, 115. - Sweating, 12. - - - Tadpole, 321, 428. - Tail, 2. - Tanager, 6. - Tapeworm, 353; - number of eggs, 110. - Taraxacum, 305. - Tear-sacs, 206. - Teeth, bird’s, 67, 68. - Telegony, 95, 234, 237, 238, 239. - Tenthredinidæ, 181, 425. - Termite, number of eggs, 110. - Termitidæ, 350. - Thrush, 115. - Tipulæ, 188. - Toad, 7. - Torpedo, 132. - Towle, 392. - Transitional forms, 42. - Transmutation theory, 31, 34. - Traquair, 138. - Treadwell, 72. - Treat, 424. - Tree-frogs, 7. - Trichina, 353. - Trifolium, 404. - Triton, 193. - Turkeys, 314. - Turnix, 201, 202. - Turtles, 193. - - - Umbelliferæ, 135. - Uria lacrymans, 124. - Utricularia, 10, - - - Vanessa, 360. - Variability, 92, 93, 95, 96, 318-319. - Variation, 261, 340. - Variation, fluctuating, 100, 118, 123. - Variation under domestication, 136. - Varieties, 106, 107, 148. - Varigny, De, 303-306, 314-315, 322. - Venus fly-trap, 9. - Verbascum, 148, 149. - Vertebrates, evolution of, 40, 45. - Vilmorin, 303, 314. - Vinson, 178. - Vries, De, 97, 278, 289-298, 340. - Vulpine, 209. - - - Wallace, 7, 162, 186, 202, 221, 249. - Walrus, 203. - Walsh, 181. - Walther, 59. - Wasp, 3, 5, 408, 409. - Waterton, 198. - Web, spider’s, 3, 4. - Weir, 171. - Weismann, 154-166, 441, 448-450. - Westwood, 188. - Whale, 227, 301. - Wilson, E. B., 72. - Wing of bat, 2. - Wolf, 308, 376. - Wolves, 412. - Women, 210. - Woodpecker, 228. - Wounds, healing of, 15. - - - Yarrell, 138. - Yung, 424, 436. - - - Zebu cattle, 208. - Zeleny, 348. - Zoea, 69, 70. - - - - ------------------------------------------------------------------------- - - - - -Transcriber’s note: - - ○ Missing or obscured punctuation was silently corrected. - - ○ Typographical errors were silently corrected. - - ○ Inconsistent spelling and hyphenation were made consistent only - when a predominant form was found in this book. - - ○ The cover image was created by the transcriber and is placed in - the public domain. - - ○ Some character-based illusrations were re-drawn using different - characters. - - - -***END OF THE PROJECT GUTENBERG EBOOK EVOLUTION AND ADAPTATION*** - - -******* This file should be named 63540-0.txt or 63540-0.zip ******* - - -This and all associated files of various formats will be found in: -http://www.gutenberg.org/dirs/6/3/5/4/63540 - - -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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