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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 <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you -are not located in the United States, you will have to check the laws of the -country where you are located before using this eBook. -</div> - -<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: The Evolution Theory, Vol. 2 of 2</div> - -<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Author: August Weismann</div> - -<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Translator: J. Arthur Thomson and Margaret R. Thomson</div> - -<div style='display:block; margin:1em 0'>Release Date: April 10, 2021 [eBook #65049]</div> - -<div style='display:block; margin:1em 0'>Language: English</div> - -<div style='display:block; margin:1em 0'>Character set encoding: UTF-8</div> - -<div style='display:block; margin-left:2em; text-indent:-2em'>Produced by: Constanze Hofmann, Alan, Marilynda Fraser-Cunliffe and the Online Distributed Proofreading Team at https://www.pgdp.net (This book was produced from images made available by the HathiTrust Digital Library.)</div> - -<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK THE EVOLUTION THEORY, VOL. 2 OF 2 ***</div> - - - - - -<h1> -<span class="more">THE</span><br /> -EVOLUTION THEORY</h1> - -<p class="c"><b>VOLUME II</b> -</p> - -<div class="figcenter"> -<img src="images/cover.jpg" alt="" /> -</div> - -<p class="c p6"> -<span class="xlarge">THE</span><br /> -<span class="xxxlarge gesperrt">EVOLUTION THEORY</span></p> - - -<p class="c p2"> -BY</p> - - - -<p class="c xlarge"> -<span class="smcap">Dr.</span> AUGUST WEISMANN</p> - -<p class="c more"> -PROFESSOR OF ZOOLOGY IN THE UNIVERSITY OF FREIBURG IN BREISGAU</p> - -<p class="c p4"> -TRANSLATED WITH THE AUTHOR'S CO-OPERATION</p> - -<p class="c half"> -BY</p> - -<p class="c large"> -J. ARTHUR THOMSON</p> - -<p class="c half"> -REGIUS PROFESSOR OF NATURAL HISTORY IN THE UNIVERSITY OF ABERDEEN</p> - -<p class="c half p2"> -AND</p> - -<p class="c large"> -MARGARET R. THOMSON</p> - -<p class="c p4"> -ILLUSTRATED</p> - -<p class="c p2"> -IN TWO VOLUMES</p> - -<p class="c"> -VOL. II</p> - -<p class="c xlarge p4"> -LONDON<br /> -EDWARD ARNOLD</p> - -<p class="c"> -41 & 43 MADDOX STREET, BOND STREET, W.</p> - -<p class="c"> -1904</p> - -<p class="c"> -<i>All rights reserved</i> -</p> - -<hr class="full" /> - -<div class="chapter"> -<p class="ph2">CONTENTS</p> -</div> - - -<table> - -<tr><td class="tdr"><span class="half">LECTURE</span></td> - <td class="tdl"></td> - <td class="tdr"><span class="half">PAGE</span></td></tr> - -<tr><td class="tdr">XX.</td> - <td class="tdl"><span class="smcap">Regeneration</span></td> - <td class="tdr"><a href="#LECTURE_XX">1</a></td></tr> - -<tr><td class="tdr">XXI.</td> - <td class="tdl"><span class="smcap">Regeneration</span> (<i>continued</i>)</td> - <td class="tdr"><a href="#LECTURE_XXI">23</a></td></tr> - -<tr><td class="tdr">XXII.</td> - <td class="tdl"><span class="smcap">Share of the Parents in the Building up of the - Offspring</span></td> - <td class="tdr"><a href="#LECTURE_XXII">37</a></td></tr> - -<tr><td class="tdrt">XXIII.</td> - <td class="tdl"><span class="smcap">Examination of the Hypothesis of the Transmissibility<br /> - of Functional Modifications</span></td> - <td class="tdrb"><a href="#LECTURE_XXIII">62</a></td></tr> - -<tr><td class="tdrt">XXIV.</td> - <td class="tdl"><span class="smcap">Objections to the Thesis that Functional Modifications<br /> -are not transmitted</span></td> - <td class="tdrb"><a href="#LECTURE_XXIV">80</a></td></tr> - -<tr><td class="tdrt">XXV.</td> - <td class="tdl"><span class="smcap">Germinal Selection</span></td> - <td class="tdrb"><a href="#LECTURE_XXV">113</a></td></tr> - -<tr><td class="tdrt">XXVI.</td> - <td class="tdl"><span class="smcap">Germinal Selection</span> (<i>continued</i>)</td> - <td class="tdrb"><a href="#LECTURE_XXVI">136</a></td></tr> - -<tr><td class="tdrt">XXVII.</td> - <td class="tdl"><span class="smcap">The Biogenetic Law</span></td> - <td class="tdrb"><a href="#LECTURE_XXVII">159</a></td></tr> - -<tr><td class="tdrt">XXVIII.</td> - <td class="tdl"><span class="smcap">The General Significance of Amphimixis</span></td> - <td class="tdrb"><a href="#LECTURE_XXVIII">192</a></td></tr> - -<tr><td class="tdrt">XXIX.</td> - <td class="tdl"><span class="smcap">The General Significance of Amphimixis</span> (<i>continued</i>)</td> - <td class="tdrb"><a href="#LECTURE_XXIX">210</a></td></tr> - -<tr><td class="tdrt">XXX.</td> - <td class="tdl"><span class="smcap">In-breeding, Parthenogenesis, Asexual Reproduction,<br /> - and their Consequences</span></td> - <td class="tdrb"><a href="#LECTURE_XXX">238</a></td></tr> - -<tr><td class="tdrt">XXXI.</td> - <td class="tdl"><span class="smcap">The Influences of Environment</span></td> - <td class="tdrb"><a href="#LECTURE_XXXI">265</a></td></tr> - -<tr><td class="tdrt">XXXII.</td> - <td class="tdl"><span class="smcap">Influence of Isolation on the Formation of - Species</span></td> - <td class="tdrb"><a href="#LECTURE_XXXII">280</a></td></tr> - -<tr><td class="tdrt">XXXIII.</td> - <td class="tdl"><span class="smcap">Origin of the Specific Type</span></td> - <td class="tdrb"><a href="#LECTURE_XXXIII">299</a></td></tr> - -<tr><td class="tdrt">XXXIV.</td> - <td class="tdl"><span class="smcap">Origin of the Specific Type</span> (<i>continued</i>)</td> - <td class="tdrb"><a href="#LECTURE_XXXIV">330</a></td></tr> - -<tr><td class="tdrt">XXXV.</td> - <td class="tdl"><span class="smcap">The Origin and the Extinction of Species</span></td> - <td class="tdrb"><a href="#LECTURE_XXXV">346</a></td></tr> - -<tr><td class="tdrt">XXXVI.</td> - <td class="tdl"><span class="smcap">Spontaneous Generation and Evolution: Conclusion</span></td> - <td class="tdrb"><a href="#LECTURE_XXXVI">364</a></td></tr> - -<tr><td class="tdrt">INDEX</td> - <td class="tdl"></td> - <td class="tdrb"><a href="#INDEX">397</a></td></tr> - -</table> - - - - - -<hr class="full" /> - -<div class="chapter"> -<p class="ph2">LIST OF ILLUSTRATIONS</p> -</div> - - -<table> - -<tr><td class="tdc"><span class="half">FIGURE</span></td> - <td class="tdl"></td> - <td class="tdr"><span class="half">PAGE</span></td></tr> - -<tr><td class="tdr">35 <i>B</i> (repeated).</td> - <td class="tdl"><i>Hydra viridis</i>, the Green Freshwater Polyp.</td> - <td class="tdr"><a href="#ff1">4</a></td></tr> - -<tr><td class="tdr">96.</td> - <td class="tdl">A Planarian cut transversely into nine pieces</td> - <td class="tdr"><a href="#ff2">6</a></td></tr> - -<tr><td class="tdr">97.</td> - <td class="tdl">A Planarian which has been divided into two by a longitudinal cut</td> - <td class="tdr"><a href="#ff3">14</a></td></tr> - -<tr><td class="tdr">98.</td> - <td class="tdl">The leg of a Crab, adapted for self-mutilation or autotomy</td> - <td class="tdr"><a href="#ff4">17</a></td></tr> - -<tr><td class="tdr">99.</td> - <td class="tdl">Regeneration of the lens in a Newt's eye</td> - <td class="tdr"><a href="#ff5">21</a></td></tr> - -<tr><td class="tdr">100.</td> - <td class="tdl">Regeneration of Planarians</td> - <td class="tdr"><a href="#ff6">25</a></td></tr> - -<tr><td class="tdr">101.</td> - <td class="tdl">A Starfish arm</td> - <td class="tdr"><a href="#ff7">27</a></td></tr> - -<tr><td class="tdr">76 (repeated).</td> - <td class="tdl">Diagram of the maturation divisions of the ovum</td> - <td class="tdr"><a href="#ff8">39</a></td></tr> - -<tr><td class="tdr">82 (repeated).</td> - <td class="tdl">Fertilization in the Lily</td> - <td class="tdr"><a href="#ff9">59</a></td></tr> - -<tr><td class="tdr">91 (repeated).</td> - <td class="tdl">Hind-leg of a Grasshopper</td> - <td class="tdr"><a href="#ff10">83</a></td></tr> - -<tr><td class="tdr">102.</td> - <td class="tdl">Brush and comb on the leg of a Bee</td> - <td class="tdr"><a href="#ff11">84</a></td></tr> - -<tr><td class="tdr">103.</td> - <td class="tdl">Claw on the leg of a 'Beach-fly'</td> - <td class="tdr"><a href="#ff12">85</a></td></tr> - -<tr><td class="tdr">104.</td> - <td class="tdl">Digging leg of the Mole-cricket</td> - <td class="tdr"><a href="#ff13">86</a></td></tr> - -<tr><td class="tdr">105.</td> - <td class="tdl">Ovary of a fertile Queen-Ant and ovaries of a Worker</td> - <td class="tdr"><a href="#ff14">91</a></td></tr> - -<tr><td class="tdr">106.</td> - <td class="tdl">Three Workers of the same species of Indian Ant</td> - <td class="tdr"><a href="#ff15">97</a></td></tr> - -<tr><td class="tdr">107.</td> - <td class="tdl"><i>A</i>, <i>B</i>. Larva of a Caddis-fly</td> - <td class="tdr"><a href="#ff16">105</a></td></tr> - -<tr><td class="tdr">107 <i>C</i>.</td> - <td class="tdl">Leptocephalus stage of an American Eel</td> - <td class="tdr"><a href="#ff17">133</a></td></tr> - -<tr><td class="tdr">108.</td> - <td class="tdl">Nauplius larva of one of the lower Crustaceans</td> - <td class="tdr"><a href="#ff18">161</a></td></tr> - -<tr><td class="tdr">109 <i>A</i>, <i>B</i>.</td> - <td class="tdl">Metamorphosis of one of the higher Crustacea, a Shrimp</td> - <td class="tdr"><a href="#ff19">162</a></td></tr> - -<tr><td class="tdr">109 <i>C</i>.</td> - <td class="tdl">Second Zoæa stage</td> - <td class="tdr"><a href="#ff20">163</a></td></tr> - -<tr><td class="tdr">109 <i>D</i>, <i>E</i>.</td> - <td class="tdl">Mysis-stage and fully-formed Shrimp</td> - <td class="tdr"><a href="#ff21">164</a></td></tr> - -<tr><td class="tdr">70 (repeated).</td> - <td class="tdl">Daphnella</td> - <td class="tdr"><a href="#ff22">166</a></td></tr> - -<tr><td class="tdrt">110.</td> - <td class="tdl">The largest of the Daphnids (<i>Leptodora hyalina</i>), with summer ova<br /> - beneath the shell</td> - <td class="tdrb"><a href="#ff23">166</a></td></tr> - -<tr><td class="tdr">111.</td> - <td class="tdl">Nauplius larva from the winter egg of <i>Leptodora hyalina</i></td> - <td class="tdr"><a href="#ff24">167</a></td></tr> - -<tr><td class="tdr">112.</td> - <td class="tdl">Development of the parasitic Crustacean <i>Sacculina carcini</i></td> - <td class="tdr"><a href="#ff25">168</a>, <a href="#ff41">242</a></td></tr> - -<tr><td class="tdr">113.</td> - <td class="tdl">The two sexes of the parasitic Crustacean <i>Chondracanthus gibbosus</i></td> - <td class="tdr"><a href="#ff26">170</a></td></tr> - -<tr><td class="tdr">114.</td> - <td class="tdl">Zoæa-larva of a Crab</td> - <td class="tdr"><a href="#ff27">171</a></td></tr> - -<tr><td class="tdrt">115.</td> - <td class="tdl">Caterpillar of the Humming-bird Hawk-moth <i>Macroglossa stellatarum</i></td> - <td class="tdrb"><a href="#ff28">178</a></td></tr> - -<tr><td class="tdr">3 (repeated).</td> - <td class="tdl">Full-grown caterpillar of the Eyed Hawk-moth</td> - <td class="tdr"><a href="#ff29">178</a></td></tr> - -<tr><td class="tdr">4 (repeated).</td> - <td class="tdl">Full-grown caterpillar of the Eyed Hawk-moth</td> - <td class="tdr"><a href="#ff30">179</a></td></tr> - -<tr><td class="tdr">8 (repeated).</td> - <td class="tdl">Caterpillars of the Buckthorn Hawk-moth</td> - <td class="tdr"><a href="#ff31">179</a></td></tr> - -<tr><td class="tdrt">116.</td> - <td class="tdl">Development of the eye-spots in the caterpillar of the Elephant<br /> - Hawk-moth <i>Chærocampa elpenor</i></td> - <td class="tdrb"><a href="#ff32">180</a></td></tr> - -<tr><td class="tdr">117.</td> - <td class="tdl">Caterpillar of the Bed-straw Hawk-moth <i>Deilephila galii</i></td> - <td class="tdr"><a href="#ff33">181</a></td></tr> - -<tr><td class="tdrt">118.</td> - <td class="tdl">Two stages in the life-history of the Spurge Hawk-moth <i>Deilephila - euphorbiæ</i></td> - <td class="tdrb"><a href="#ff34">182</a></td></tr> - -<tr><td class="tdr">119.</td> - <td class="tdl">Caterpillar of the Poplar Hawk-moth <i>Smerinthus populi</i></td> - <td class="tdr"><a href="#ff35">184</a></td></tr> - -<tr><td class="tdr">120.</td> - <td class="tdl"><i>A</i>, Symmetrical, and <i>B</i>, asymmetrical curve of frequency</td> - <td class="tdr"><a href="#ff36">207</a></td></tr> - -<tr><td class="tdr">121.</td> - <td class="tdl">Life-cycle of <i>Coccidium lithobii</i></td> - <td class="tdr"><a href="#ff37">214</a></td></tr> - -<tr><td class="tdr">122.</td> - <td class="tdl">Conjugation of a Coccidium (<i>Adelea ovata</i>)</td> - <td class="tdr"><a href="#ff38">216</a></td></tr> - -<tr><td class="tdr">123.</td> - <td class="tdl">Conjugation of <i>Coccidium proprium</i></td> - <td class="tdr"><a href="#ff39">218</a></td></tr> - -<tr><td class="tdr">79 (repeated).</td> - <td class="tdl">The two maturation divisions of the 'drone eggs'</td> - <td class="tdr"><a href="#ff40">236</a></td></tr> - -<tr><td class="tdr">124.</td> - <td class="tdl">Alternation of generations in a Gall-wasp</td> - <td class="tdr"><a href="#ff42">245</a></td></tr> - -<tr><td class="tdr">125.</td> - <td class="tdl">The two kinds of galls formed by the species</td> - <td class="tdr"><a href="#ff43">246</a></td></tr> - -<tr><td class="tdrt">126.</td> - <td class="tdl">Ovipositor and ovum of the two generations of the same species<br /> - of Gall-wasp</td> - <td class="tdrb"><a href="#ff44">247</a></td></tr> - -<tr><td class="tdrt">127.</td> - <td class="tdl">Life-cycle of the Vine-pest (<i>Phylloxera vastatrix</i></td> - <td class="tdrb"><a href="#ff45">249</a></td></tr> - -<tr><td class="tdrt">128.</td> - <td class="tdl">Heterostylism</td> - <td class="tdrb"><a href="#ff46">254</a></td></tr> - -<tr><td class="tdrt">38 (repeated).</td> - <td class="tdl">A fragment of a Lichen</td> - <td class="tdrb"><a href="#ff47">261</a></td></tr> - -<tr><td class="tdrt">129.</td> - <td class="tdl">Aberration of <i>Arctia caja</i>, produced by low temperature</td> - <td class="tdrb"><a href="#ff48">276</a></td></tr> - -<tr><td class="tdrt">130.</td> - <td class="tdl">Skeleton of a Greenland Whale, with the contour of the body</td> - <td class="tdrb"><a href="#ff49">313</a></td></tr> - -<tr><td class="tdrt">131.</td> - <td class="tdl">Peridineæ: species of <i>Ceratium</i></td> - <td class="tdrb"><a href="#ff50">325</a></td></tr> - -</table> - -<hr class="full" /> - -<div class="chapter"> -<p><span class="pagenum"><a id="Page_1"></a>[Pg 1]</span></p> -<h2 class="nobreak" id="LECTURE_XX">LECTURE XX</h2> -</div> - -<p class="c">REGENERATION</p> - -<div class="blockquot"> - -<p>Budding and division—Every theory of regeneration in the meantime only -provisional, a mere 'portmanteau theory'—Regeneration not a primary character—Volvox—Hydra—Vital -affinities—Planarians—Heteromorphoses—Enemies of Hydroid-colonies—Regeneration -in Plants—In Amphibians—In Earthworms—Different degrees -of regenerative capacity according to the liability of the part to injury—Different results -of longitudinal halving in Earthworms and in Planarians—Regeneration in Birds—The -disappearance of the power of regeneration is very slow—Morgan's experiments -on Hermit-crabs—Autotomy in Crustaceans and Insects—Regeneration of the lens in -Triton.</p></div> - - -<p><span class="smcap">We</span> have endeavoured to explain the handing on of the complement -of heritable qualities from one generation to another as due to -a continuity of the germ-plasm, and we assumed that the germ-cells -never arise except from cells in the 'germ-track'; that is, from cells -which are equipped, from the fertilized egg-cell onwards, with a complete -sample of slumbering germ-plasm, and are thereby enabled -to become germ-cells, and, subsequently, new individuals, in which -the aggregate of inherited primary constituents implied in the germ-plasm -can again attain to development.</p> - -<p>We have now to consider other cases of inheritance in relation to -the same problem—the origin of their hereditary equipment.</p> - -<p>We know, of course, that new individuals may arise apart from -germ-cells, that, in many of the lower animals and in plants, they -may arise by budding and fission.</p> - -<p>For both these cases the germ-plasm theory will suffice, with -a somewhat modified form of the same assumption which we made in -regard to the formation of germ-cells. The origin of a new individual -by budding seems often, indeed, to proceed from any set of somatic -cells in the mother animal; but somatic cells, if they contain solely -the determinants controlling themselves, cannot possibly give rise to -a complete new individual, since this presupposes the presence of <i>all</i> -the determinants of the species. But as these determinants cannot -be formed <i>de novo</i>, the budding cells must contain in addition to the -usual controlling somatic determinants, idioplasm in a latent, inactive -state, which only becomes active under certain internal or external -influences, and then gives rise to the formation of a bud. The source<span class="pagenum"><a id="Page_2"></a>[Pg 2]</span> -of this accessory idioplasm must, however, be looked for only in the -egg-cell.</p> - -<p>In plants this bud-idioplasm must be complete germ-plasm, -because the budding starts only from one kind of cell, the cambium-cells; -but in animals in which—as it seems—it always proceeds from -at least two different kinds of cells—those of the ectoderm and those -of the endoderm—the matter is more complex. In this case these -two kinds of cells will contain as bud-idioplasm two different groups -of determinants, which mutually complete each other and form -perfect germ-plasm, and only the co-operation of these two sets will -give rise to the formation of a bud. I will not, however, go further -into detail in regard to these relations, for the theory can do nothing -more here than formulate what has been observed; it is hardly in -a position to help us to a better understanding of the facts.</p> - -<p>The case is not much clearer in regard to the processes which -lead to the replacing of lost parts. The manifold phenomena of -regeneration can also be brought into harmony with the theory, if we -attribute to those cells from which the replacing or entire reconstruction -of the lost part arises an 'accessory-idioplasm,' which, at least, -contains the determinants indispensable to the building up of the part. -It is possible that the assumed accessory idioplasm frequently contains -a much larger complex of determinants, and that it depends on the -liberating stimuli which, and how many of these, will become active.</p> - -<p>If we take a survey of regenerative phenomena in the animal -kingdom, it strikes us at once that the capacity is very different in -different species, extraordinarily great in some and very slight in -others. In general it is greater in lower animals than in higher, but, -nevertheless, the degree of differentiation cannot be the only factor -that determines the capacity for regeneration. That unicellular -organisms can completely replace lost parts, that even a piece of an -infusorian can reconstruct the whole animal if only the piece contain -a part of the nucleus, we have already seen when discussing the significance -of the nuclear substance. In this case the nucleus must -contain the complete germ-plasm, that is, the collective determinants -of the species, and these induce the reconstruction of the lost part, -though they do so in a way that is still entirely obscure to us. In -the meantime, our interpretation will not carry us further, either here -or in regard to any other order of vital phenomena. To go further -would be little short of propounding a causal theory of life itself; it -would mean having a complete and real 'explanation' of what 'life' is. -As yet no one has been able to claim this position. We can see the -different stages through which every organism passes, and that they arise<span class="pagenum"><a id="Page_3"></a>[Pg 3]</span> -one out of the other; we can even penetrate down to the succession of -those delicate and marvellously complex processes which effect nuclear -and cell-division; but we are still far from being able to deduce, -except quite empirically, from the present state of a cell what the -succeeding one will be, that is, from being able to understand the -succession of events as a necessary nexus which could be predicted. -How a biophor comes to develop from itself the phenomena of life is -quite unknown to us; we know neither the interaction of the ultimate -material particles nor the forces which bring it about; we cannot -tell what moves the hordes of different kinds of biophors to range -themselves together in a particular order, what molecular displacements -and variations arise from this, or what influence the external -world has, and so forth. We see only the visible outcome of an endless -number of invisible movements—growth, division, multiplication, -reconstruction, and differentiation.</p> - -<p>As long as we are so far from an understanding of life no theory -of regeneration can be anything more than a 'portmanteau theory,' as -Delage once expressed himself in relation to the whole theory of -inheritance, a theory which is like a portmanteau in that one can -only take out of it what has previously been put in. If we wish to -explain the renewal of the aboral band of cilia in a Stentor, we first -pack our trunk, in this case the nucleus of the Infusorian, with the -determinants of the ciliated region, and then think of these as being -liberated by the stimulus of wounding, and being brought to and -arranged in the proper place by unknown forces to reconstruct the -ciliary region in some unknown way. No one could be more clearly -aware than I am that this is not an exhaustive causal explanation of -the process itself. Nevertheless, it is not quite without value, inasmuch -as it allows us at least to bring the facts together in rational -order—in this case the dependence of the faculty of regeneration on -the presence of nuclear substance—under a formula which we can use -provisionally, that is, with which we can raise new questions. As -soon as we ascend higher in the series of organisms the theory gains -a greater value, for, while we leave altogether out of account any -answer to the ultimate question, and thus renounce for the present -the attempt to find out how the determinants set to work to call to -life the parts which they control, we are brought face to face with -other, in a sense, preliminary questions which we <i>can</i> solve, and the -solution of which seems to me at least not entirely without value.</p> - -<p>The first of these questions runs thus: Is the power of regeneration -a fundamental, primary character of every living being in the -sense that it is present everywhere in equal strength, independently<span class="pagenum"><a id="Page_4"></a>[Pg 4]</span> -of external conditions, and thus is an inevitable outcome of the -primary characters of the living substance? Or is it, though -primaeval in its beginnings, a phenomenon of adaptation, which -depends on a special mechanism, and does not occur everywhere in -equal extent and potency?</p> - -<p>We have already become acquainted with some facts which must -incline us to the latter view. The globular Alga-colonies of <i>Volvox</i> -(<a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#f67">Fig. 63</a>) consist of two kinds of cells, -of which only one kind, the reproductive -cells, possess the power of reproducing -the whole, the others, the flagellate, or, -as we called them, somatic cells, being -only able to produce their like, but never -the whole.</p> - -<p>New investigations which have been -carried out by Dr. Otto Hübner in my -Institute have placed these facts beyond -doubt. We may conclude that, in -this case, a disintegration of the germ-plasm -has taken place during ontogeny, -by means of differential cell-division, so -that only the reproductive cells receive -the complete germ-plasm, while the -somatic cells receive only the determinants -necessary to their own specific -differentiation, the somatic determinants.</p> - -<p>In this case regeneration and reproduction -coincide; there is no regeneration -except the origin of a new individual -from a reproductive cell.</p> - -<div class="figleft" id="ff1"> -<img src="images/ff1.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 35</span> <i>B</i> (repeated). <i>Hydra viridis</i>,<br /> -the Green Freshwater Polyp.<br /> -Section through the body-wall,<br /> -somewhere in the direction of <i>ov</i><br /> -in Fig. 35 <i>A</i>. <i>Eiz</i>, the ovum lying in<br /> -the ectoderm (<i>ect</i>), and including<br /> -zoochlorellæ (<i>schl</i>) which have immigrated<br /> -from the endoderm (<i>ent</i>)<br /> -through the supporting lamella<br /> -(<i>st</i>). After Hamann.</p> -</div> - -<p>Let us now ascend to the lowest of -the Metazoa, for instance, the freshwater -polyp, Hydra (<a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#f39">Fig. 35 <i>A</i></a>), and we find a -high degree of regenerative capacity in -the restricted sense, for, in addition to the -power of producing germ-cells, that is, cells which, when two combine -in amphimixis, give rise again to a new animal, almost any part of -the polyp can regrow a whole animal. Not only has Hydra been cut -in from two to twenty different pieces, but it has even been chopped -up into innumerable fragments, and yet each of these, under favourable -circumstances, was able to grow again into a complete animal. -Nevertheless, we are not justified in concluding that every cell<span class="pagenum"><a id="Page_5"></a>[Pg 5]</span> -possesses the power of reproducing the whole. If, with the help of -a bristle, we turn one of these polyps outside in like the finger -of a glove, and then prevent it turning right again by sticking the -bristle transversely through it, it does not live, but soon dies, -obviously because the cells of the two layers of the body, ectoderm -and endoderm, cannot mutually replace each other, and cannot -mutually produce each other. The inner layer, now turned outwards, -cannot resist the influence of the water, and the outer layer, now -turned inwards, cannot effect digestion; in short, one cannot be transformed -into the other, and we must therefore conclude that both are -specialized, that they no longer contain the complete germ-plasm, but -only the specific determinants of ectoderm and endoderm respectively.</p> - -<p>The animal's high regenerative capacity must therefore depend -on the fact that certain cells of the ectoderm are equipped with the -complete determinant-complex of the ectoderm, in the form of an -inactive accessory idioplasm, which is excited to regenerative activity -by the stimulus of wounding, and that, in the same way, the cells of -the endoderm are equipped with the whole determinant-complex of -the endoderm. It need not be decided whether all or only many -of the cells, perhaps the younger ones, are thus adapted for regeneration; -in any case a great many of them must be distributed throughout -the whole body, with perhaps the exception of the tentacles, -which are by themselves unable to reproduce the whole animal. -When the animal is mutilated, the cells of both layers, equipped with -their respective determinant-aggregates, co-operate in reproducing the -whole from a part.</p> - -<p>It is true that even with these assumptions we only reach the -threshold of a real explanation. For, given that all the determinants -of the species must be present in a fragment, we are not in a position -to show how these set about reconstructing the animal in its integrity, -and the most that we can say is, that it must depend on the specific -kind of stimulus to which each of the cells is exposed through its -direct and more remote environment, which determinants are to be -first liberated, and therefore which parts are to be reconstructed.</p> - -<p>That there are at work regulative forces, such as we were already -compelled to assume in regard to the division and regeneration of -unicellular organisms, as to the nature of which we cannot yet make -any definite statement, but which we may call 'polarities,' or, as -I prefer to say, 'affinities,' is shown by countless experiments which -have been made, particularly with the freshwater polyp. Thus Rand -cut off the anterior end of the polyp with its circle of tentacles, and -the excised disk of living substance lengthened in a transverse<span class="pagenum"><a id="Page_6"></a>[Pg 6]</span> -direction, so that half the tentacles came to lie to the right, the other -half to the left, while the body developed between these two groups, -so that they became further and further separated from each other, -till finally the original transverse axis of the animal became the -longitudinal axis. One group of tentacles survived and surrounded -the new mouth, while the other at the opposite aboral pole, the new -foot, died off. This total change of structure in the polyp, as to the -arrangement of its main parts, points to unknown forces, which -cannot depend on the determinants as such, but on the vital -characters of the living parts, and on the interactions of these with -one another.</p> - -<div class="figcenter" id="ff2"> -<img src="images/ff2.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 96.</span> A Planarian cut transversely into nine pieces. The regeneration -of seven of these into entire animals is shown. After Morgan.</p> -</div> - -<p>The same holds true of all the lower Metazoa that have highly -developed regenerative capacity, not only of polyps, but of worms -such as the Planarians. Through the experiments of Loeb, Morgan, -Voigt, Bickford, and others, we know that these animals respond to -almost every mutilation by complete reconstruction, that they may, for -instance, as is indicated in Fig. 96, be cut transversely into nine or ten -pieces with the result that each of these pieces grows again to a whole -animal, unless external influences are unfavourable and prevent it.</p> - -<p>Something similar happens if the head be cut off a Tubularia-polyp, -it forms a new head with proboscis and tentacles. It does so, -at least, if the stalk of the polyp be left in the normal position; but<span class="pagenum"><a id="Page_7"></a>[Pg 7]</span> -if it be stuck into the sand in the reverse position a head arises at -the end which is uppermost, where the roots arose previously, and the -previous head-end now sends out roots. By suspending a beheaded -stalk horizontally in the water a head can be caused to develop at -each end of the stalk, so that we must assume that every part of the -polyp is, under some circumstances, capable of developing a head, and -that it must be 'circumstances'—in this case gravity, contact with -earth or with water, and the mutual influence of the parts of the -animal upon each other—which decide what is to be produced. -Loeb, who was the first to observe this form of regeneration, called it -heteromorphosis, to express the fact that particular parts of the -animal might be produced at quite different places from those -originally intended for them.</p> - -<p>It would certainly be erroneous to range these cases of heteromorphosis -against the determinant theory, but they certainly do not -afford any special evidence of its validity as an interpretation, for all -that we can say here again is that all, or at least many, cells of the -animal must contain the full determinant-complex of the ectoderm, -and others those of the endoderm, and that particular groups of -determinants become active when they are affected by certain external -or internal liberating stimuli. In regard to such animals the theory -is hardly more convincing than the rival theory, that the faculty of -regeneration is a general property of living substance, which does not -attain to equally full expression everywhere, because it is met by ever-increasing -difficulties involved in the increasing complexity of structure. -The validity of the theory only begins to be seen when we -deal with cases where it is demonstrable that every part cannot bring -forth every other, where the power of regeneration is limited, and -occurs only in definite parts in a definite degree, and can only start -from particular parts. Here the assumption of a general and primary -regenerative capacity fails. Any one who insists, as O. Hertwig does, -that the idioplasm in all cells of the body is the same, can always -plead that, in the cases in which regeneration does not occur, the -fault lies, not in the regenerative capacity, but in the absence of the -adequate liberating stimuli, and at first sight it does seem as if this -position were unassailable. We shall find, however, that there are -facts which make Hertwig's interpretation quite untenable.</p> - -<p>My own view is that the regenerative capacity is not something -primary, but rather an adaptation to the organism's susceptibility -to injury, that is, a power which occurs in organisms in varying -degrees, proportionate to the degree and frequency of their liability -to injury. Regeneration prevents the injured animal from perishing,<span class="pagenum"><a id="Page_8"></a>[Pg 8]</span> -or from living on in a mutilated state, and in this lies an advantage -for the maintenance of the species, which is the greater the more -frequently injuries occur in the species, and the more they menace -its life directly or indirectly. A certain degree of regenerative -capacity is thus indispensable to all multicellular animals, even to -the highest among them. We ourselves, for instance, could not -escape the numerous dangers of infection by bacilli and other micro-organisms -if our protective outer skin did not possess the faculty -of regeneration, at least so far that it can close up a wound and -fill up with cicatrice-tissue a place where a piece of skin has been -excised. Obviously, then, the mechanism which evokes regeneration -must have been preserved in some degree and in some parts at every -stage of the phyletic development, and must have been strengthened -or weakened according to the needs of the relevant organism, being -concentrated in certain parts which were much exposed to injury -and withdrawn from other rarely threatened parts. Thus the great -diversity which we can now observe in the strength and localization -of the regenerative capacity has been brought about. But all this -can only be regarded as adaptation.</p> - -<p>I should like to submit a few examples to show that the -regenerative capacity is by no means uniformly distributed, and -that, as far as we can see, it is greater or less in correspondence -with the needs of the animal, both in regard to the whole and to -particular parts.</p> - -<p>It must first be pointed out that those lower Metazoa, like the -Hydroid polyps in particular, which are endowed with such a high -and general power of regeneration, do actually require this for -their safety; they are not only soft, easily injured and torn, but -they are most severely decimated by many enemies. In the beginning -of May I found on the walls of the harbour at Marseilles whole -forests of polyp-stocks of the genera <i>Campanularia</i>, <i>Gonothyræa</i>, and -<i>Obelia</i>, all large and splendidly developed, with thousands of individual -polyps and medusoids, but in a very short time the great -majority of the polyps were eaten up by little spectre-shrimps -(Caprellids) and other crustaceans, worms, and numerous other -enemies, and towards the end of May it was no longer possible to -find a fine well-grown colony. It must therefore be of decisive -importance for these species if the stems and branches, which are -spared because protected by horny tubes, possess the faculty of -transforming their simple soft parts into polyp-heads, or of giving -off buds which become polyps, or even of growing a new stock from -the twigs which have been half-eaten and bitten loose from the<span class="pagenum"><a id="Page_9"></a>[Pg 9]</span> -stock and have fallen to the ground. If, finally, a torn-off polyp-stalk -(of <i>Tubularia</i>) falls to the ground with the wrong side up, the end -which is now the lower will send out roots, and the end now uppermost -will give off a new head. This also appears to us adaptive, and -does not surprise us, since we have been long accustomed to recognize -that what is adapted to an end will realize this if it be possible at all. -Think again of the innumerable adaptations in colour and form which -we discussed in the earlier lectures. I hope later to be able to show -in more detail how it comes to pass that necessity gives rise to -adaptation. In regard to the case of the polyps, we can understand -that, as far as a high degree of regeneration and budding was possible -in these animals at all, it could not but be developed. Regeneration -and budding complete each other in this case, for the former brings -about in the individual 'person' what the latter does in the colony, -namely, a <i>Restitutio in integrum</i>. It is readily intelligible that the -former was not difficult to establish where the latter—the capacity -of budding—was already in existence.</p> - -<p>It seems at first sight very striking that the higher plants, which -all depend upon budding, and which form plant-colonies (corms) -in the same sense as the polyps form animal-colonies, only possess -the faculty of true regeneration in a very low degree, although they -are extremely liable to injury.</p> - -<p>We see from this that the two capacities are not co-extensive, -that germ-plasm may be contained in numerous cells of the body -in a latent state, and yet that regeneration of each and every detailed -defect may not be possible. This is the case in the higher plants -in regard to most of their parts. A leaf in which a hole has been -cut does not close the hole with new cell-material; a fern frond from -which some of the pinnules have been cut off does not grow new ones, -but remains mutilated. Even leaves which, if laid on damp earth, -readily give off buds which grow to new plants, as the Begonias do, -do not replace a piece cut out of the leaf; they are not at all adapted -to regeneration.</p> - -<p>From the standpoint of utility this is readily intelligible. It -was, so to speak, not worth Nature's while to make such adaptations -in the case of leaves or blossoms, partly because these are very -transient structures, and partly because they are rapidly and easily -replaceable by the development of others of the same kind. Moreover, -the leaf in which we have cut a hole continues to function, -but the polyp whose mouth and tentacles we have cut off could no -longer take nourishment unless it were adapted for regeneration. -But that this adaptation <i>could</i> have been made in the case of plants<span class="pagenum"><a id="Page_10"></a>[Pg 10]</span> -is proved by the root-tips which are formed anew when they are -injured, and the closing of wounds on the stem by a 'callus.'</p> - -<p>I shall return to plants when we are dealing with the mechanism -of regeneration, but I must now direct more attention to animals, -inquiring further into the question as to whether the faculty of -regeneration is correlated with the degree of liability to injury to -which the animal is exposed, and with the biological importance -of the injured part, for this must be the case if regeneration be -really regulated by adaptation.</p> - -<p>Hardly any other vertebrate has attained such celebrity on -account of its high regenerative capacity as the water-newt, species -of the genus <i>Triton</i>. It can regrow not only its tail, but the legs -and their parts if they are cut off. Spallanzani saw the legs grow six -times, after he had cut them off six times. In the blind newt (<i>Proteus</i>) -of the Krainer caves, a near relative of the common newt, the leg -regenerated only after a year and a half, although the animal stands -on a lower stage of organization than the newt, and thus should -rather replace lost parts more easily. But Proteus lives sheltered -from danger in dark, still caves, while Triton is exposed to numerous -enemies which bite off pieces from its tail or legs; and the legs are -its chief means of locomotion, without which it would have difficulty -in procuring food. It is different with the elongated eel-like newt -of the marshes of South Carolina, <i>Siren lacertina</i>. This animal -moves by wriggling its very muscular trunk, after the manner of -an eel, and in consequence of the disuse of its hind legs it has almost -completely lost them. Even the fore-legs have become small and -weak, and possess only two toes, and these do not regrow if they are -bitten off, or only do so very slowly.</p> - -<p>Earthworms are exposed to much persecution; not only birds, -such as blackbirds and some woodpeckers, but, above all, the moles -prey upon them, and Dahl has shown that moles often lay up stores -of worms in winter which they have half crippled by a bite, while -even Réaumur knew that moles frequently only half devoured earthworms. -It was thus an obvious advantage to earthworms that a part -of the animal should be able to regrow a whole, and accordingly we -find a fairly well-developed regenerative capacity among them. But -it varies greatly in the different species, and it would be interesting -if we knew the conditions of life well enough to be able to decide -whether the faculty of regeneration rises and falls in proportion -to the dangers to which the species is exposed. Unfortunately we -are far from this as yet; we only know that, in the common -earthworms of the genera <i>Lumbricus</i> and <i>Allolobophora</i>, the faculty<span class="pagenum"><a id="Page_11"></a>[Pg 11]</span> -of regeneration is still very limited, for at most two worms, and -sometimes only one, can develop from an animal cut into two -pieces. Cutting into a greater number of pieces does not yield -a larger number of worms, but usually only one, and often none -at all.</p> - -<p>This corresponds to the behaviour of their enemies, which may -often bite off a piece or tear it away when the worm attempts to -escape, but never cut it up into pieces. The regenerative capacity -is more highly developed in the genus <i>Allurus</i>, more highly still -in the worms of the genus <i>Criodrilus</i> which lives in the mud at -the bottom of lakes, and most highly of all in the genus <i>Lumbriculus</i> -which lives at the bottom of small ponds. Long ago Bonnet cut up -a specimen of <i>Lumbriculus</i> into twenty-six pieces, of about two -millimetres in length, and he observed most of these grow to complete -worms again. His experiments have often been repeated in recent -times, and have been extended and made more precise in many ways. -Von Bülow was able to get whole animals from pieces consisting of -from four to five somatic segments, and with eight or nine segments -he almost invariably succeeded. A <i>Lumbriculus</i> which he had cut -into fourteen pieces, one of which only measured 3.5 mm. in length, -gave rise to thirteen complete worms with head and tail; only one -piece perished.</p> - -<p>These worms have little enemies with sharp jaws which may -gnaw at them behind or before but cannot swallow them whole. -Lyonet, famous for his analytic dissection of the wood-caterpillar -(<i>Cossus ligniperda</i>), observed when he was feeding the larvæ of -dragon-flies with these Lumbriculid worms that 'the anterior end -of some whose posterior end had been gnawed away by the larvæ -continued to live on the ground.' We can thus understand why -a high power of regeneration is of use to these worms, and at the -same time why it is advantageous to them to contract so that -they break in pieces on very slight irritation, but to this we shall -refer again.</p> - -<p>The very diverse potency of the faculty of regeneration in -animals belonging to the same small group, and nearly, if not quite, -at the same level of organization, seems to show clearly that we -have here to do with adaptation to different conditions of life, -although we cannot demonstrate this in detail. It would certainly -be erroneous to regard the conditions of life as uniform, since the -worms in question not only live in different places—in the earth, -in mud, or in water—and are thus exposed to different enemies, and -since they may also be quite different in regard to size and speed,<span class="pagenum"><a id="Page_12"></a>[Pg 12]</span> -in means of defence, and possibly also of defiance, as is indeed in some -measure demonstrable.</p> - -<p>We meet with the same thing in a group of still smaller worms, -Rösel's 'water-snakelets,' species of the genus <i>Nais</i>. These, too, -behave in a variety of ways in the matter of regeneration, for -while many species, such as <i>Nais proboscidea</i> and <i>Nais serpentina</i> -will, if cut into two or three pieces, become two or three worms -respectively, Bonnet expressly mentions an unnamed species of <i>Nais</i> -which does not bear cutting up at all, and even dies if its head -be cut off.</p> - -<p>Thus neither the degree of organization nor the relationship -alone determines the strength of the regenerative capacity. And -as nearly related species may behave quite differently in this respect, -so also do the different parts of one and the same animal; and here, -too, the strength of the capacity seems to depend on the more frequent -or rarer injury of the relevant part and on its importance in the -maintenance of life. Let us take a few examples.</p> - -<p>Parts which, in the natural life of the animal, are never injured, -show in many cases no power of regeneration. This is so in regard -to the internal parts of the newt, whose regenerative capacity is -otherwise so high. I cut half or nearly the whole of a lung away -from newts anæsthetized with ether; the wound closed, <i>but no -renewal of the organ took place</i>. The same thing happened when -a piece of the spermatic duct or of the oviduct was cut away. It -is true that the kidney enlarges in higher animals when a piece has -been cut out, by the proliferation of the remaining tissues, but that is -a mere physiological substitution, evoked by the increased functional -stimulus, due to the accumulation in the blood of the constituents -of the urine. Such substitution depends on the growth of parts -already existing, and it occurs in man when one kidney is removed, -for the other, as is well known, may then grow to double its normal -size. This is mere hypertrophy of the part that is left, it is not -regeneration in the morphological sense, and it is not comparable -to the re-formation of a cut-off leg in the salamander, or of a head in -the worm, where the growth is not a mere increase of the remaining -stump, but a new formation. It would be regeneration if a new -kidney developed from the remnants of the kidney-tissue, or, in -the liver, if new lobes grew in place of those which were cut off. -But neither of these things happens, and, as far as I am aware, -nothing of the kind has ever been observed, nothing more than -new formation of liver-cells through increase of existing ones; that, -however, is not regeneration in the morphological sense.</p> -<p><span class="pagenum"><a id="Page_13"></a>[Pg 13]</span></p> - -<p>I have referred to the slight power of regeneration possessed by -the blind Proteus in regard to its legs or tail, and I connected this -with the absence of enemies in its thinly peopled cave-habitat. But -the same animal can regenerate its gills when these are bitten -off, and this is probably associated with the habit that Proteus -has, in common with other newts with external gills, of nibbling -at its neighbour's gills. Thus, the power of regenerating the -gills was retained even when the animals migrated to the quiet -caves of Krain, and were thus secured from the attacks of other -enemies.</p> - -<p>In lizards, a leg which has been cut off does not grow again, but -an amputated tail does, and this has quite a definite biological reason, -since the active little animal will seldom be caught by the foot by -any pursuer, but may easily be caught by the tail, which is far -behind. Thus the tail is adapted not only for regeneration, but -also for 'autotomy' that is, for breaking off easily when it is caught -hold of.</p> - -<p>We have already seen that some segmented worms have a very -high regenerative capacity; yet every part cannot produce every -other, and while, in <i>Lumbriculus</i>, any piece of from five to nine -segments is able to grow a new head or tail, neither ten nor twenty -nor all the segments together, if they are <i>halved longitudinally</i>, can -reproduce the other half, and the cause of this inability does not lie in -the fact that the animal is thereby hindered from taking food, for even -the transversely cut pieces do not feed until they have grown a new -head and tail. The reason must lie in the fact that the primary constituents -for this kind of regeneration are wanting, and they are so -because a longitudinal splitting of this cylindrical and relatively thin -animal never occurs under natural conditions, and thus could not be -provided against by Nature<a id="FNanchor_1" href="#Footnote_1" class="fnanchor">[1]</a>.</p> - -<div class="footnote"> - -<p><a id="Footnote_1" href="#FNanchor_1" class="label">[1]</a> Morgan maintains that this statement is incorrect, and that <i>Lumbriculus</i> is -capable of lateral regeneration. But if we look into the matter more closely we find -that all he says is, that small gaps made by cutting a piece out of one side are filled -up again, while the cut pieces perish. If the whole animal be halved, according to -Morgan, both halves die, or if a 'very long piece' be cut out of one side, not only this -piece dies, but also 'the remaining piece.' There is thus, as I have said, an essential -difference between the regenerative capacity of <i>Lumbriculus</i> and that of <i>Planaria</i>.</p> - -</div> - -<p>That regeneration of this kind could have been arranged for if it -had been useful we learn from the Planarians among the flat worms, -in which every piece cut out of the body, large or very small, from the -middle, from the left side, or from the right side of the animal, grows -into a complete Planarian. The animal can be halved longitudinally, as -in Fig. 97, and each half will grow to a whole. This again is quite -intelligible from the biological point of view, for these flat, soft,<span class="pagenum"><a id="Page_14"></a>[Pg 14]</span> -and easily torn animals are exposed to all sorts of injuries, and are, -in point of fact, frequently mutilated by enemies which are unable to -swallow them whole. Von Graaf not infrequently found examples of -marine Planarians (<i>Macrostomum</i>) which lacked 'a part of the -posterior end or the whole tail region as far as the food-canal,' and of -species of <i>Monotus</i> he found 'very often' in May specimens with the -posterior end split or broken off. Probably the persecutors of these -flat-worms are some species of Crustacean, but, at any rate, so much -is proved, that the Planarians have abundant opportunities of making -use of their faculty of regeneration, and that the species gains an -advantage from it in respect to its preservation.</p> - -<div class="figleft" id="ff3"> -<img src="images/ff3.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 97.</span> <i>A</i>, a Planarian,<br /> -which has been divided into<br /> -two by a longitudinal cut.<br /> -Each half can grow into<br /> -an entire animal. <i>B</i>, the<br /> -left half at the beginning<br /> -of the regenerative process.<br /> -<i>C</i>, the same completed. After<br /> -Morgan.</p> -</div> - -<p>In contrast to this, worms which live within other animals, -and are thus secure from mutilation, such as -the familiar round-worms (<i>Nematoda</i>), have -no power of regeneration at all, and do not -survive either longitudinal or transverse -division.</p> - -<p>Until recently birds were regarded as -possessing a very low degree of regenerative -capacity, and, as a matter of fact, they cannot -replace a leg or a wing wholly or in part; -but, what is otherwise unheard of among -higher vertebrates, they can renew the whole -anterior portion of the skeleton of the face, -the bill, and can indeed almost reconstruct -it with new bones and horny parts. Von -Kennel communicated a case of this kind in -regard to a stork, and for a long time this -remained an isolated case, but a few years -ago Bordage showed that, in the cocks which -are used in the Island of Bourbon for the -favourite sport of cock-fighting, the bill is -regularly renewed when it has been broken -off or shattered. Quite recently Barfurth gave an account of a case -of complete renewal of a broken bill in a parrot. Yet it should -not astonish us that the bill in birds has such a high regenerative -power, for of all parts in a bird it is the one that is most readily -injured; with it the bird defends itself against its enemies and its -rivals, masters its prey, and tears it to pieces, pecks holes in trees -(woodpecker), or climbs (parrot), or digs and burrows in the ground, -or builds its nest, and so on. That the faculty of regeneration -could be developed to so high a degree in relation to this<span class="pagenum"><a id="Page_15"></a>[Pg 15]</span> -particular part of the body, while the rest of the very important -but rarely injured parts do not possess it at all, again points to -the conclusion that the faculty of regeneration has an adaptive -character.</p> - -<p>It does not affect matters to discover cases in which we cannot -recognize this relation between the regenerative capacity of a part -and its importance or its liability to injury. Such instances do not -lessen the convincingness of the positive cases, since we do not know -the exact conditions which may lead to the increase of regenerative -capacity in a part, and, above all, since we do not know the rate at -which such an increase may take place. If adaptation in general -depends upon processes of selection, these processes must also be able -to give rise to an increase in the power of regeneration. On the -other hand, it by no means follows that the disappearance of a faculty -of regeneration which was once present in a part, but which has -become superfluous in the course of time, must take place immediately -through natural selection. For it is the very essence of natural -selection that it only furthers what is useful, and only removes what -is injurious; over what is indifferent it has no power at all. Thus it -follows that the faculty of regeneration, when it has once been present -in a part, cannot be set aside by natural selection (personal selection), -for it is in no way injurious to its possessor. If it gradually decreases -and becomes extinct notwithstanding this, when it is of no further -use, as seems to be to some extent the case in regard to the legs and -tail of the blind Proteus, that must depend on other processes, -on those which generally bring about the gradual disappearance -of disused parts or capacities. We shall attempt to probe to the -roots of these processes later on; for the present let it suffice us to -know that, according to our experience, they go on with exceeding -slowness, and that it has taken whole geological periods to eliminate -the legs of the snake-ancestors so completely as has been done from -the structure of most of our modern snakes, while the Proteus which -migrated into the caves of Krain as far back as the Cretaceous period -is indeed blind, but still retains its eyes under the skin, though in -a degenerate condition.</p> - -<p>Since the degeneration of disused parts and capacities goes on so -slowly it need not surprise us that we meet many parts which still -possess regenerative capacity, although they are protected from injury. -Thus Morgan found that, in the hermit-crab, the limbs which are -protected within the mollusc shell were quite as ready to regrow as -those which are actually used for walking, and thus are exposed to -possibility of attack, but this proves nothing against the conclusion<span class="pagenum"><a id="Page_16"></a>[Pg 16]</span> -we drew from the facts cited above, according to which the faculty -of regeneration comes under the law of adaptation. For the -disappearance of this faculty must take place very much more <i>slowly -than its growth</i>. For instance, the development of the tail-fin of the -whale has long been an accomplished fact, while the hind-legs of this -colossal mammal, which were rendered useless by the development of -the tail-fin, still lie concealed in a rudimentary state within the -muscles of the trunk. Yet these limbs must have lost their significance -for the animal exactly at the time that the tail-fin became more -powerful. Thus the retrogression must have taken place more slowly -than the progressive transformation.</p> - -<p>It is clear, then, that the faculty of regeneration is not a primary -character of living beings occurring uniformly in all species of -equally high organization and in all parts of an animal in the same -degree; it is a power which occurs in animals of equal complexity in -as varying degrees as in their parts, and which is manifestly -regulated by adaptation. Between parts with the faculty of regeneration -and parts without it there must be an essential difference; -there must be present in the former something that is wanting in the -latter, and, according to our theory, this is the equipment with -regeneration-determinants, that is, with the determinants of the parts -which are to be reconstructed.</p> - -<p>If this be really so it should be capable of proof, at least in so -far that we should be able to establish that the power of completing -or re-forming a damaged or lost part is a limited one, localized in -certain parts and cell-layers. This can be actually proved, as may be -seen from numerous cases in which the faculty of regeneration is -associated with autotomy, that is, with the power of breaking off -or dropping off a part of the body. Even in worms we find this -power, as we mentioned before in speaking of the high regenerative -capacity of <i>Lumbriculus</i>. This worm reproduces in summer by what -is called 'schizogony,' that is, by breaking into two, three, or more -pieces, and it does not seem to require a very strong stimulus, such as -pressure of the end of the worm by the jaws of an insect larva, to -start this rupture; it often follows from quite insignificant friction -on the ground. Certainly the power of regeneration is so great in this -animal that it is out of the question to talk of localizing the primary -constituents of regeneration; almost every broken surface is capable -of regeneration.</p> - -<p>But this localization is well illustrated in Insects and Crustaceans, -which possess the power of self-amputation in their appendages, -especially in their legs. As far back as 1826 MacCullock observed<span class="pagenum"><a id="Page_17"></a>[Pg 17]</span> -this remarkable power in crabs, and described the mechanism on -which it depends. When the leg is irritated, for instance when it is -pinched at the tip and held fast, it breaks off at a particular place. -This line of breakage lies in the middle of the short second joint -(Fig. 98, <i>A</i> and <i>B</i>, <i>s</i>), just between the insertions of the muscles -(<i>me</i>, <i>mf</i>, <i>m</i>) which extend from this line towards the extremity of -the limb and in the opposite direction towards the body-wall. -Between these muscle-attachments the external skeleton is thin and -brittle, and forms a suture, <i>s</i>, which -breaks through when the animal contracts -the muscles of the leg convulsively, -and thus presses the lower protuberance -(<i>a</i>) against a projection (<i>b</i>) of the first -upper joint. Crabs require to make -a very considerable muscular exertion -before they can throw off the limb, and -therefore they can only do it when they -are in full vigour.</p> - -<div class="figright" id="ff4"> -<img src="images/ff4.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 98.</span> The leg of a Crab,<br /> -adapted for self-mutilation or autotomy.<br /> -<i>A</i>, the first three joints -of the limb, <i>I</i>, <i>II</i>, <i>III</i>. <i>s</i>, the<br /> -suture, that is, a thin area on<br /> -the second joint which is predisposed<br /> -to breakage. <i>mf</i>, flexor<br /> -muscle, <i>me</i>, extensor muscle, both<br /> -inserted at the suture. <i>B</i>, the<br /> -entire leg with its six joints and<br /> -with the suture (<i>s</i>). Slightly enlarged.<br /> -After MacCullock.</p> -</div> - -<p>We have here a quite definite -structural adaptation of the parts to -a danger which often recurs—that of -falling entirely into the power of an -enemy which has seized the leg. By -a sudden violent throwing-off of the -leg the crab escapes from this danger. -Quite similar adaptations are found -among certain insects, such as the -walking-stick insects or Phasmids, in -which the mechanism is much the same, -and lies at an almost exactly corresponding -place, namely, at the line -where the second and third joints of -the leg, the 'trochanter' and the 'femur' -meet. In this case the advantage of the arrangement is not merely -that the animals are thus enabled to escape from enemies; it is -useful in another connexion, for a knowledge of which we have to -thank Bordage. This naturalist observed that the Phasmids not -infrequently perished at one of their numerous moultings, by remaining -partially fixed in the discarded husk. Of 100 Phasmids nine -died in this way, twenty-two got free with the loss of one or more -legs, and only sixty-nine survived the moult without any loss -at all.</p> - -<p><span class="pagenum"><a id="Page_18"></a>[Pg 18]</span></p> - -<p>That the moulting or ecdysis of insects is often hazardous may -be observed in our own country, and it is familiar to every one who -has reared caterpillars. These, too, often fail to get clear of their -'cast' cuticle, and they perish unless artificial aid is given to them. I -have never observed any autotomy in them, but in the Phasmids -it seems to be a much-used 'device' and is therefore of great importance -in the persistence of the species.</p> - -<p>Limbs which are thus thrown off by autotomy regenerate again -from the place at which they broke off, that is from the 'suture.' It -had been noticed even by the earlier observers (e.g. Goodsir) that there -was a jelly-like mass of cells within the joint, and that the development -of the new limb started from this. It might be supposed that -the regeneration-primordium is present in the rest of the leg also, but -that is not the case, for the animal responds to the tearing off of one -joint or of a smaller number than to the suture, not by regenerating -the torn part directly, but by amputating the whole of the leg up to -the suture, and then from this the regeneration of the whole leg takes -place. In the Phasmids the case is similar, but with the difference -that regeneration is possible from three places, from the tarsal joints, -from the lower third of the tibia, and finally, from the suture between -the femur and the trochanter. There is thus a regeneration-primordium -(<i>Anlage</i>) at the beginning of the tarsal joints, another in the -tibia, and a third in the 'suture' and the first must be equipped, -as we should express it, with the determinants of the five tarsal joints, -the second with those for the lower end of the tibia as well, and the -third with all the determinants of the whole leg, from the 'suture' -downwards.</p> - -<p>In any case, regeneration is here associated with definite localized -pieces of tissue, and is not a general character of all the cells of the -leg, and, as it obviously runs parallel at the same time with another -adaptation—that of autotomy—there can be no doubt that it too is -dominated by the principle of selection, and that it can not only be -increased, but that it can be concentrated at particular places and -removed from others. But this is only possible if it be bound up with -material particles which may be present in or absent from a tissue, -and which are therefore a supplement to the ordinary essential -constituents of the living cells, although they do not themselves -belong to the essential organization.</p> - -<p>I might cite many more examples of localization of regenerative -capacity, but will confine myself to one other, which seems to me -particularly instructive, because it was first interpreted as an indication -of the existence of an adaptive principle in the organism,<span class="pagenum"><a id="Page_19"></a>[Pg 19]</span> -a principle which always creates what is useful. I refer to the -regeneration of the lens in the newt's larva.</p> - -<p>G. Wolff, an obstinate opponent of the theory of selection, -attempted to solve the same problem as I had before me in my -experiments on the regeneration of the internal organs of newts, that -is, he tried to answer the question whether organs which are never -exposed to injury or to complete removal in the conditions of natural -life, and which could not therefore have been influenced in this -direction by the processes of selection, are nevertheless capable of -regeneration. He extirpated the lens from the eye of Triton larvæ, -and saw that in a short time it was formed anew, and from this he -concluded that there was here 'a new adaptiveness appearing for the -first time,' and that therefore adaptive forces must be dominant within -the organism. The current theory of the 'mechanical' origin of -vital adjustments seemed to some to be shaken by this, and the -proclamation of the old 'vital force' seemed imminent. And in -truth, if the body were really able to replace, after artificial injury, -parts which are never liable to injury in natural conditions, and to -do so in a most beautiful and appropriate manner, then there would -be nothing for it but at least to regard the faculty of regeneration as -a primary power of living creatures, and to think of the organism as -like a crystal, which invariably completes itself if it be damaged -in any part. But we have to ask whether this is really the case.</p> - -<p>What makes the regeneration of the lens seem particularly surprising -is the fact that in the fully formed animal it must arise in -a manner different from that in which it develops in the embryo, that -is, it must be formed from different cell-material. In the embryo it -arises by the proliferation and invagination of the epidermic layer of -cells to meet the so-called 'primary' optic vesicle growing out from -the brain—a mode of development which cannot of course be repeated -under the altered conditions in the fully developed animal. The -reconstruction of the organ must therefore take place in a different -way, and if the organism were really able, the very first time the lens -was removed, to react in a manner so perfectly adapted to the end, -and so to inspire certain cells, which had till then had a different -function, that they could put together a lens of flawless beauty and -transparency, we should have reason to suspect that nearly all our -previous conceptions were erroneous, and to fall back upon a belief in -a <i>spiritus rector</i> in the organism.</p> - -<p>But the excision of the lens in these experiments was not by any -means an unprecedented occurrence! It is true enough that newts in -their pools are not liable to an operation for cataract, but it does not<span class="pagenum"><a id="Page_20"></a>[Pg 20]</span> -follow that the lens is never liable to injury, and could not therefore -be adapted for regeneration. It can be bitten out along with the rest of -the eye by water-beetles or other enemies, and as far back as the time -of Bonnet and Blumenbach (1781) it was known that the eye of the -newt would renew itself if it were cut out, given that a small portion -of the bulb was left. But if this were removed the possibility of -regeneration was at an end. Thus, before the first artificial excision -of the lens, a regeneration-mechanism must have existed, by means of -which the eye with its lens was reconstructed, and this depends on -the characters of the cells of the eye itself—it is localized in the eye, -and without the presence of a piece of eye-tissue no regeneration can -take place. Is it then so especially remarkable that the lens should -be renewed when it is artificially removed without the rest of the -eye? The mechanism for its renewal is there, and is roused to -activity whether the lens alone or other parts of the eye also be -removed. We do not need, therefore, to assume the existence of -a purposeful or adaptive force; it is more to the point to inquire -where the regeneration-mechanism which suggests this inference is to -be found.</p> - -<p>A definite answer to this is given in a detailed experimental -work recently published by Fischel. It corroborates what Wolff had -already found, that the substance of the new lens develops from -cells which cover the posterior surface of the iris, that is, from cells of -the retinal layer of the eye. First, the margin of the pupil begins to -react to the stimulus of the injury (extraction of the lens); its cells -enlarge, become clear, while previously they were filled with dark -pigment, and finally they proliferate. They thus form a cell-vesicle -similar to the ectoderm-vesicle from which the lens arises in the -embryo, and into this the already mentioned retina-cells from the -posterior wall of the iris grow, elongate, and arrange themselves to -form the so-called 'lens-fibres,' on whose form, arrangement, and -transparency the function of the lens depends. This is marvellous -enough, but not more marvellous than that a whole foot should grow -on the cut stump of a newt's leg, or that a whole eye should arise -from a residual fragment. Here, again, we do not know the processes -which cause the arrangement of the cells and their often manifold -locally-conditioned differentiations, in short, we do not know the -<i>essential nature of regeneration</i>. But, in the meantime, we can -endeavour to find out which cell-groups regeneration is bound up with -in particular cases, so as to know where the vital particles, the 'determinants,' -which condition regeneration, are placed by nature.</p> - -<div class="figcenter" id="ff5"> -<img src="images/ff5.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 99.</span> Regeneration of the lens in the Newt's eye. <i>A</i>, section through -the iris (<i>J</i>); from its margin and posterior (retinal) surface the primordium -of a new lens (<i>L</i>) has developed after the artificial removal of the old one. -<i>B</i>, section through the eye after duplicated regeneration of the lens (<i>L</i>) from -two areas of the iris. <i>Gl</i>, vitreous humour. <i>J</i>, iris. <i>C</i>, cornea. <i>R</i>, retina. -After Fischel.</p> -</div> - -<p>In this case there can be no doubt on that point: they are the<span class="pagenum"><a id="Page_21"></a>[Pg 21]</span> -cells on the posterior wall and the margin of the iris. And it is -certainly not the <i>absence</i> of the lens which gives rise to its renewal, -as would necessarily be the case if it were due to the dominance of an -adaptive force. If the lens, instead of being excised, be simply -pressed back into the vitreous humour occupying the cavity of the -eye, a new lens is developed all the same from the irritated margin of -the pupil. And if by chance this margin has been irritated in two -places while extraction of the lens was being performed, then two -small lenses will develop (Fig. 99, <i>B</i>). Indeed, several may begin to -develop at the posterior wall of the iris, although they do not attain -to full development; mechanical irritation of any part of this cell-layer -is responded to by the formation of lenses. This surely disposes -of the 'mystical nimbus' which would dazzle us with a new force of -life, always creating what is appropriate. We have before us an -adaptation to the liability of newts' eyes to injury, which, like all -adaptations, is only relatively perfect, since under the usual conditions -of eye injury it gives rise to a usable lens, but under unusual conditions -to unsuitable structures. It is exactly the same as in the case of -animal instincts, which are all 'calculated' for the <i>ordinary</i> conditions -of life, but, under unusual conditions, may operate in a manner quite -unsuited to the necessary end. The ant-lion has the instinct to -bore backwards into the sand, and he makes the same backward-pressing -movements when placed on a glass plate into which he cannot -force the tip of the abdomen. The same is true of the mole-cricket, -which makes its usual digging movements with the forelegs even on -a plate of glass. The wall-bee roofs over her cell when she has laid<span class="pagenum"><a id="Page_22"></a>[Pg 22]</span> -an egg in it, but she does so even if the egg be taken out beforehand, -or if a hole be made in the bottom of the cell, so that the honey which -is to serve the larva for food when it emerges from the egg runs -out (Fabre). Her instinct is calculated for filling the cell <i>once</i> with -honey, and <i>once</i> laying an egg in it, because such disturbances as we -may cause artificially do not occur or occur very rarely in natural -conditions. There are countless facts of this kind, for every instinct -and every adaptation can, in certain circumstances, go astray and -become inappropriate. This should be considered by those who still -persist in opposing the theory of selection, for herein lies one of the -most convincing proofs of its correctness. Adaptations can only arise -in reference to the majority of occurrences, and variations which are -only useful in an individual case must, according to the principle, -disappear again. Adaptation always means the establishment of what -is appropriate in an average number of cases.</p> - -<p>Therefore the inappropriate reaction of the margin of the iris to -an artificial double stimulus affords additional reason for regarding -regeneration as an adaptive phenomenon. If it were the outcome of -an adaptive force it could never be inappropriate; and if it were the -operation of a general and primary power of the organism it would be -exhibited by the nearly-related frog as well as by the newt. But, in -the frog, extraction of the lens gives rise only to a sac-like proliferation -of the cells of the iris margin, which form no transparent lens, -but an opaque cluster of cells, which destroys vision altogether. It -appears, therefore, that the frog no longer requires the power its -ancestors possessed of regenerating a lost lens.</p> -<hr class="full" /> - -<div class="chapter"> -<p><span class="pagenum"><a id="Page_23"></a>[Pg 23]</span></p> - -<h2 class="nobreak" id="LECTURE_XXI">LECTURE XXI</h2> -</div> - -<p class="c">REGENERATION (<i>continued</i>)</p> - -<div class="blockquot"> - -<p>Phyletic origin of the regenerative capacity—The liberating stimuli of regeneration—Production -of extra heads and tails in Planarians (Voigt)—Regeneration in the -Starfish—Atavistic regeneration in Insects and Crustaceans—Progressive regeneration—Regeneration -has its roots in the differentiation of organisms—The nuclear substance -of unicellular organisms is the first organ for regeneration—The ultimate roots of -regeneration.</p></div> - - -<p><span class="smcap">In</span> the previous lecture we have considered many different forms -of regeneration, and have recognized them as adaptive phenomena; -we have now to inquire how such regeneration-adaptations have -arisen, and this is a very difficult question even in general, while in -particular cases it is often quite unanswerable at present. In regard -to the case last discussed, the regeneration of the lens in the eye of -Triton, our hypotheses would require to reach back to the time of the -primitive vertebrates with an unpaired eye, for the lens of the paired -vertebrate eye, from Mammals down to the lowest Fishes, does not -arise in embryonic development from the retinal cells, but always -from the corneal epithelium, as the elaborate researches of Rabl have -recently shown. It is true that the unpaired parietal eye of some -reptiles forms its lens from the cells of the retinal layer, but it would -be difficult to demonstrate the possibility of a genetic connexion -between it and paired eyes, and in the meantime we must refrain -from elaborating a hypothesis as to the origin of the marvellous -faculty the retinal cells possess of transforming themselves into -lens-fibres.</p> - -<p>But it is easier to form some sort of picture of the origin and -adaptation of the faculty of regeneration in general.</p> - -<p>We saw that the power of regenerating a part can be localized, -and that it does not belong to all the cells of the body, but only to -some of them, and we have to ask how and by what steps it has been -imparted to these. The faculty depends on the possession of a -regeneration-primordium (<i>Anlage</i>), and this again, in our mode of -expression, consists of a definite complex of determinants, and as -determinants are the products of an evolution, and thus are vital -units which have arisen historically, they can nowhere suddenly<span class="pagenum"><a id="Page_24"></a>[Pg 24]</span> -originate anew in a species, but must be derived directly or indirectly -from the sole basis which, in each species, forms the starting-point of -the individual—that is to say, in the Metazoa, from the germ-plasm -of the ovum. From it the determinant-complex of every regeneration-rudiment -mast in the ultimate instance be derived.</p> - -<p>We may think of the matter thus: all the determinants of the -germ-plasm vary, grow slowly or quickly, and in certain circumstances -may be doubled. In this way there arise what we may -call 'supernumerary' determinants, which are not required in the -primary building up of the body from the ovum, and which may -remain in an inactive state in the nuclei of certain cells, ready to -become active under certain circumstances and to produce anew the -part which they control. Such regeneration-idioplasm will at first -come to lie in the younger cells of the determinate organ, but it is -conceivable that under the influence of selection it may be gradually -shifted to other cells of a later developmental origin, or, conversely, -to others in a less external position, so that, for instance, the regeneration-rudiment -for the finger of a newt may be contained not merely -in the cells of the hand, but in those of the fore-arm or even of the -upper arm.</p> - -<p>But all such segregation of determinant-groups cannot have taken -place, as we might perhaps be inclined to think, at the periphery in -the organ itself during its development; it must take place in the -germ-plasm of the ovum, for otherwise it could not be transmissible, -and could not be directed and modified by the processes of selection, -as is actually the case, as I shall show in more detail later on.</p> - -<p>I have already pointed out the importance of the rôle played -by liberating stimuli in regeneration, and not only of extra-organismal -stimuli, such as gravity, but above all of intra-organismal stimuli -that is, the influences exerted in a mysterious manner by other parts -of the animal on the parts which are in process of regeneration. It is -a great merit of the modern tendency in evolution theory that it has -demonstrated the importance of such internal influences. Although -we are still far from being able to define the manner in which these -influences operate, we may say so much, that it depends essentially -on the nature and extent of the loss which parts are reproduced by -the regenerating cells, and, also, on the position and direction of the -injured surface from which the regeneration starts. The influences, -still quite beyond our comprehension, which are exerted on the -regenerating part by the uninjured parts constitute the liberating -stimuli, which evoke the activity of one or other of the determinants -contained in the regeneration-idioplasm.</p> - -<p><span class="pagenum"><a id="Page_25"></a>[Pg 25]</span></p> - -<div class="figcenter" id="ff6"> -<img src="images/ff6.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 100.</span> Regeneration of Planarians. <i>A</i>, an animal divided into three -parts by two oblique cuts. <i>B</i>, the fragments(<i>a</i>, <i>b</i>, <i>c</i>) in process of regeneration. -<i>C</i>, an animal with various oblique incisions in the margin of the body, which -have induced the new formation of heads (<i>k</i>), of tails (<i>s</i>), and pharynx (<i>ph</i>). -<i>A</i> and <i>B</i> after Morgan; <i>C</i> after Walter Voigt.</p> -</div> - -<p>Walter Voigt has shown, by a series of most interesting experiments, -that it is possible not only to cause the development of a new -head in Planarians by cutting them, in which case a tail may grow -from the anterior portion and a head from the posterior portion, but -it is also possible in an intact animal, that is, one with both head and -tail, to cause the production of a second head, or a second tail, or both -at once, at any part of the body margin at will, according to the -direction of the cut. If the margin of the body be cut obliquely -forwards (Fig. 100, <i>A</i>) a supernumerary tail arises (<i>C</i>, <i>s</i>), if it be cut -obliquely backwards a supernumerary head arises (<i>C</i>, <i>k</i>), and in this -way several heads and several tails may be produced in the same -animal. It is obvious, then, that the interaction, in the first place, of -the cells of the cut surface, but probably also of the deeper-lying cells, -decides which determinants are to come into action, those of the head -or those of the tail, but both must be present at every part of the cut. -How far below the cut surface the cells take part in this determination -we cannot make out, but that it cannot be due to the -co-operation of all parts is clear in this case at least, since the animal -still possesses its original head and tail. The extra heads and tails -thus produced prove, at any rate, that there can be no question here -of the expression of an adaptive principle, a <i>spiritus rector</i>, or a vital -force, which always creates what is good, but that it is rather a -purely mechanical process, which takes its course quite independently<span class="pagenum"><a id="Page_26"></a>[Pg 26]</span> -of what is useful or disadvantageous, and that it must take this course -according to the given regeneration-mechanism and the stimulus -supplied in the special case. It cannot be supposed that these -supernumerary heads and tails are purposeful, but who would expect -an adaptive reaction from the animal in a case like this, since cuts of -the kind which we make artificially, and <i>must keep open artificially</i> -if the deformities are to develop, hardly occur in nature, and, if they -did occur, would very quickly close up again? Adaptations can only -develop in response to conditions which occur and recur in a majority -of cases, and when they have a useful, that is, species-preserving -result. The adaptiveness of the organism is blind, it does not see the -individual case, it only takes into account the cases in the mass, and -acts as it must after the mechanism has once been evolved. The case -is the same as that of 'aberrant' or mistaken instincts, whose origin -by means of selection is the more clearly proved, since we must -recognize such an instinct as a pure mechanism and not as the -outcome of purposeful forces.</p> - -<p>In the regeneration of Planarians we must think of the regeneration-idioplasm -as containing the full complex of the collective -determinants of the three germinal layers, and possibly we must -add to this cells with the complete germ-plasm for giving rise to -the reproductive cells. But when the amputated tail of the newt -is regenerated, or its leg, or the arm of a starfish, or the bill of a bird, -we have no ground for assuming that the cells, from which regeneration -starts, contain the whole germ-plasm, since the determinants of -the replaceable parts suffice to explain the facts. We must even -dispute the possibility of the presence of the whole germ-plasm in -this case, because the faculty of regeneration of the relevant cells is -really no longer a general one, but is limited to the reproduction of -a particular part. This is seen in the fact that, in the starfish, whose -high regenerative capacity is well known, the central disk of the body -may indeed give rise to new arms<a id="FNanchor_2" href="#Footnote_2" class="fnanchor">[2]</a>; but an excised arm, to which no -part of the disk adheres, is in most starfishes unable to give rise to -the body. Thus the arm does not contain in its cells the determinants -of the disk, but the latter contains those of the arm. We are not -surprised that the amputated tail of the salamander does not reproduce<span class="pagenum"><a id="Page_27"></a>[Pg 27]</span> -the whole animal, but this can only be because the impelling forces -to the regeneration of the whole animal are wanting, that is, that -the cut surface only contains the determinants of the tail and not the -complete germ-plasm. It might be objected here that the tail-piece -is too small to give rise to the whole body, but in <i>Planaria</i> it is -only very diminutive heads and tails which grow from the artificial -incisions, and the same is true of starfishes when only a single arm -and a small piece of the disk have been left. Notwithstanding the -small amount of living substance at their -disposal, and although they are at first unable -to take nourishment, they send out -very small new arms (Fig. 101), close up the -wounded surface, and, after reconstruction -of the mouth and stomach, begin to feed -anew. The new arms may then grow to the -normal size.</p> - -<div class="footnote"> - -<p><a id="Footnote_2" href="#FNanchor_2" class="label">[2]</a> I see now that there are contradictory statements in regard to this case. -Possibly these depend on the different behaviour of different species, and this on the -varying frequency of mutilation. Starfishes which live on the shore between the -rocks, for instance on the movable stones of a breakwater, are very frequently -mutilated; in some places it is rare to find a specimen without traces of former -wounds. H. D. King counted among 1,914 specimens of <i>Asterias vulgaris</i> 206 in the act -of regenerating a part, that is, 10.76 per cent. In the case of the starfishes from deep -water this cause of injury does not of course exist.</p> - -</div> - -<div class="figright" id="ff7"> -<img src="images/ff7.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 101.</span> A starfish arm,<br /> -growing four new arms;<br /> -the so-called 'comet-form.'<br /> -After Haeckel.</p> -</div> - -<p>We must therefore assume that, in many -cases, the regeneration-primordium consists of -cells which only contain a definite complex -of determinants in the form of latent regeneration-idioplasm, -as, for instance, certain cells of -the tail of Triton contain the determinants -of the tail, certain cells of its leg the determinants -of the leg, and so on. In many -cases we can speak even more precisely, and -determine from which cells the nerve-centres, -from which the muscles, and from which the -missing section of the food-canal will be -formed, as was recently shown by Franz von -Wagner in regard to the worm <i>Lumbriculus</i>, -whose regenerative capacity is so extraordinarily -high. We must then attribute to each of -the relevant cells an equipment of regeneration-idioplasm, -which includes only the relevant complex of determinants.</p> - -<p>I need not here go further into detail, but I should still like to -show that, in reality, as I assumed in regard to the regenerative -capacity of a part, the root of the regeneration-idioplasm lies in the -germ-plasm, that it is present there as an independent determinant-group, -and, like every other bodily rudiment (<i>Anlage</i>), must be -handed on from generation to generation. This assumption is -necessary, as has been already indicated, on the ground that the -faculty of regeneration is hereditary, and hereditarily variable, on<span class="pagenum"><a id="Page_28"></a>[Pg 28]</span> -the same ground, therefore, as that on which the whole determinant -theory is based. The regeneration-determinants must be contained -<i>as such</i> in the germ-plasm, otherwise a twofold phyletic development -could not have occurred, as it actually has, in many parts. The tail -of the lizard is adapted for autotomy; it breaks off when it is held -by the tip, and this depends on a special adaptation of the vertebræ, -which are very brittle in a definite plane from the seventh onwards. -This is thus a very effective adaptation to persecution by enemies. -The tail which has been seized remains with the pursuer, but the -lizard itself escapes, and the tail grows again. But this regeneration -does not take place in the same way as in the embryo; no new -vertebræ are formed, but only a 'cartilaginous-tube,' a new structure, -a substitute for the vertebral column; the spinal cord with its nerves -is not regenerated either, and the arrangement of the scales is -somewhat different.</p> - -<p>This last point, in particular, indicates that the determinants of -the regeneration-rudiment may pursue an independent phylogenetic -path of their own, for this scale arrangement of the regenerated tail -is an atavistic one, that is, it corresponds to a more primitive mode of -scale arrangement in these Saurians. We know quite a number of -cases similar to this. It not infrequently happens that cut-off parts -regenerate, but that they do so not in the modern form, but in one -that is in all probability phyletically older. Thus the legs of -various Orthoptera, as of the cockroaches and grasshoppers, regenerate -readily, but with a tarsus composed of four joints instead of five<a id="FNanchor_3" href="#Footnote_3" class="fnanchor">[3]</a>, -and the long-fingered claws of a shrimp (<i>Atyoida potimirim</i>) is -replaced by the older short-fingered type of claw, while in the -Axolotl an atavistic five-fingered hand grows instead of the amputated -four-fingered one.</p> - -<div class="footnote"> - -<p><a id="Footnote_3" href="#FNanchor_3" class="label">[3]</a> New investigations, specially directed to this point, by R. Godelmann, have -shown that 'in the great majority of cases' the regenerated legs of a Phasmid -(<i>Bacillus rossii</i>) exhibit a four-jointed tarsus; but the regeneration of five joints also -occurs, though only after autotomy, and only in seven out of fifty cases (<i>Archiv für -Entwicklungsmechanik</i>, Bd. xii, Heft 2, July 1901). The regeneration-rudiment in this -species seems to be in process of advancing slowly to the five-jointed type.</p> - -</div> - -<p>This last case shows that it is not merely a lesser power of -growth that accounts for the difference between the regenerated part -and the original, for here more is regenerated than was previously -present. There remains nothing for it but the assumption that the -regeneration-determinants have remained at a lower phyletic level, -while the determinants which direct embryogenesis have varied, -and either developed further or retrogressed. It is easy to understand -that the regeneration-rudiment must vary phyletically much<span class="pagenum"><a id="Page_29"></a>[Pg 29]</span> -more slowly than the parts which evolved in the ordinary way -and much more slowly than the determinants of these parts, for -natural selection means a selection of the fittest, and the speed with -which the establishment of a variation is attained depends, <i>ceteris -paribus</i>, on the number of individuals that are exposed to selection -with respect to the varying part. If in a species of a million -living at the same time nine-tenths perish by accident, there will -remain only 100,000 from which to select the 1,000 which we will -assume constitute the normal number of the species. The more of -these 100,000 which possess the useful variation the higher will be -the percentage of the normally surviving 1,000 possessing it, and -the more rapidly will the useful variation increase. But when it -is a question of the variation of the regeneration-primordium, the -selection will take place not among all the 100,000 individuals which -chance has spared, but only among those of them which have lost -a limb by accident, and thus are in a position to regenerate it more -or less completely. If we assume that this takes place in 10 per cent. -of cases, then selection for the improvement of the regeneration-apparatus -will only take place among 1,000 individuals, and thus the -process of modification of the regeneration-primordium must go on -very much more slowly than that of the limb itself.</p> - -<p>I do not see how the opponents of the germ-plasm theory can -explain these facts at all, for the appeal to external influences is -here entirely futile, and that to internal liberating stimuli does not -suffice, since these must be different after a part has been cut off from -what they were when the limb developed normally, and also different -from those which prevailed at the normal origin of the limb in -ancestral forms. The four-jointed tarsus of the ancestors of our -cockroaches did not arise as a result of amputation. We cannot -therefore avoid referring the processes of regeneration to particular -'regeneration-determinants,' which are contained in the germ-plasm -and are handed on in ontogeny with the other determinants from -cell-division to cell-division, till ultimately they reach the cells which -are to respond, or may have to respond, to the stimulus of injury by -some expression of their regenerative capacity. As these determinants, -as has been shown, can often only be very slightly subject -to the influence of selection processes, they will, in many respects, lag -behind in the phyletic development, and will tend to belong to an -ancestral type of the relevant part. They will often remain for a long -time at this ancestral level, and they will always adapt themselves to -new requirements more slowly than the parts which arise in the -normal way, and the determinants representing these in the germ.<span class="pagenum"><a id="Page_30"></a>[Pg 30]</span> -But the regeneration-determinants <i>are</i> variable, and, indeed, are so -hereditarily, and independently of the structure of the normal parts. -They thus follow their own path of phyletic development, and this -one fact is enough to secure a preference for the germ-plasm theory -above others that have hitherto been suggested. None of these has -even attempted an explanation of this fact; the tendency has rather -been to call it in question. This, however, can be done at most only -in regard to the explanation of the regenerations as atavistic, certainly -not in regard to the progressive variations of the regenerated -part, such as have been established by Leydig and Fraisse in regard -to the lizard's tail. It may be doubted whether the most primitive -insects had only four tarsal joints, but there is no disputing the -kainogenetic deviation of the lizard's-tail.</p> - -<p>I have interpreted the regenerative capacity as secondary and -acquired, not as a primary power of all living substance, and I should -like to substantiate this in another way.</p> - -<p>Let us go back to the simplest organism conceivable, which must -have represented the beginning of life on our earth, and we see that -this need not have possessed any special power of regeneration, -because, for an organism without differentiation of parts, growth is -equivalent to regeneration. But growth is the direct outcome of one -of the primary characters of the living substance, the capacity of -assimilation. This cannot be an adaptive phenomenon, nor can it -have arisen through selection, because selection presupposes reproduction, -and reproduction is only a periodic form of growth; but -growth follows directly from assimilation. The fundamental characters -of the living substance, above all the dissimilation and assimilation -which condition metabolism, must have been in existence from the -first when living substance arose, and must depend on its unique -chemico-physical composition. But the faculty of regeneration could -only be acquired when organisms became qualitatively differentiated, -so that each part was no longer like every other part or like the -whole. As soon as this stage was reached the faculty of regeneration -would necessarily be developed, if further multiplication was to take -place. For when each fragment could no longer become a whole by -simply growing, some arrangement had to be made by which each -fragment should receive, in the form of primary constituents, what it -lacked to make up the whole. We do not know the first beginning -of this adaptation, but, in its further development, it appears in the -form of 'nuclear substance,' enclosed in the nucleus of the cell, and, -as is well known, it is now to be found in all unicellular organisms. -That the nucleus there precedes regeneration in the sense that without<span class="pagenum"><a id="Page_31"></a>[Pg 31]</span> -a piece of it the cell-soma is not able to complete itself alone, we have -already seen, and the explanation of this fact has always seemed to -me to be that invisibly minute vital units relating to the regeneration -of the injured part leave the nucleus and evoke the development of -the missing parts by laws and forces still unknown to us. Loeb has -recently claimed that the nucleus is the cell's organ of oxidation; but -if that be true it would still not exclude the possibility that the -nucleus is also and primarily a storehouse of the material bearers of -the primary constituents of a species. It must be regarded as such -when we call to mind the phenomena of amphimixis in its twofold -aspects as conjugation and as fertilization, and its obvious outcome -among higher organisms where it implies the mingling of the -parental qualities.</p> - -<p>Thus the 'nuclear substance' of unicellular organisms is for us -the first demonstrable organ of regeneration, and first of all for -normal regeneration, which takes place at every reproduction, for -instance, of an Infusorian. For we have already seen that, in the -transverse division of a trumpet animalcule (<i>Stentor</i>), the anterior -part must develop the posterior half anew, while the posterior half -must develop the much more complex anterior half, with mouth -region and spiral bands of cilia. But as soon as the arrangement for -normal reproduction was elaborated, as soon as the nucleus was -present, as a depôt of 'primary constituents,' this implied the possibility -of regeneration in exceptional cases, that is, after injury. -The mechanism was already there, and it came into operation as soon -as a part of the animal was missing.</p> - -<p>It is in the first nucleus, therefore, that we have to look for the -source of all regenerative capacity, both in unicellular and multicellular -organisms. But with the origin of the latter a limitation -took place, either quite at the beginning or a little later, for each -nucleus of the cell-colony no longer contained the whole complex of -'primary constituents' or determinants of the species, but, in many -cases, only the reproductive cell possessed them. As soon as this began -to develop into a whole by cell-division the determinant-complex was -segregated. Thus the first cell-colonies with two kinds of cells arose, -as we have seen in the case of <i>Volvox</i>—the reproductive cells with -a complete equipment for regeneration in their nucleus, and the -somatic cells with a limited equipment for regeneration in their -nuclei. The somatic cell could no longer give rise anew to the whole -organism, but could only reproduce itself or its like.</p> - -<p>But as many of the lower Metazoa and Metaphyta possess <i>the -power of budding</i>, that is, are able not only to produce a new indi<span class="pagenum"><a id="Page_32"></a>[Pg 32]</span>vidual -from definite cells—the reproductive cells—with or without -sexual differentiation, but from other cell-groups also, these must -contain the whole complex of determinants appertaining to the -reconstruction of the organism, and we have to ask how this is -reconcilable with the differentiation of a multicellular organism, -whose different kinds of cells depend, according to our interpretation, -on the fact that they are controlled by different determinants.</p> - -<p>Obviously, there is only one way out of this difficulty, and it -is the one we have already indicated, that although the diffuse -regenerative capacity which we have just alluded to occurs in species -which exhibit gemmation, this does not exclude the control of a cell -by a specific determinant; other determinants may be contained in -the cell, in a state, however, in which they do not affect it, that is, -in an inactive or latent state.</p> - -<p>Thus we arrive in this way also at our earlier assumption that -an inactive accessory-idioplasm is given to all, or at least to many -cell-generations. Only among plants must this necessarily be complete -germ-plasm, and among the lower plant-forms, as in <i>Caulerpa</i> among -the Algæ, in <i>Marchantia</i> among Liverworts, it must be assumed to -be present in nearly all the cells, according to the experiments -in regeneration made by Reinke and Vöchting. But in multicellular -animals which develop from two different germinal layers equipped -with a different complex of determinants budding arises from a -combination of at least two different kinds of cells, and we must -only ascribe to each of these its own peculiar determinant-complex -as regeneration-idioplasm. Higher plants show us that well-marked -power of budding is not necessarily associated with a high regenerative -capacity, the histologically specialized cells among them will contain -no inactive germ-plasm, because they do not need it. But in animals -the power of budding is probably always combined with high regenerative -capacity, as is shown by the Polyps and Medusoids above -all, and in a different way by the Ctenophores, which exhibit no -budding and at the same time a very slight regenerative capacity, -although they possess an organization scarcely higher than that of -the Hydromedusæ. In the Ctenophores each of the first segmentation-cells, -when artificially separated, yields only a half-embryo, and we -may conclude from this that it contains no complete germ-plasm in an -inactive state, or at least very little, and certainly not a sufficient -quantity to make it readily regenerative.</p> - -<p>Undoubtedly, however, the regenerative capacity occurs apart -from the capacity for budding, yet this in no way contradicts the -theory. As we have seen, a high regenerative capacity is to be<span class="pagenum"><a id="Page_33"></a>[Pg 33]</span> -found among many animals which occur only as 'persons' and not -as colonies or stocks, but only in those which are readily liable to -injury, and only in the manner conditioned by their injury. In the -higher Metazoa the regenerative power becomes more and more limited, -and in the Mammals it sinks to a mere closing up of wounds.</p> - -<p>If we take a survey of the assumptions we have been compelled -to make from the standpoint of the theory to explain the development -of germ-cells, budding, and regeneration, it would seem as if it -were contradictory to assume that, on the one hand, complete germ-plasm -should be given to certain cell-series as inactive accessory idioplasm, -and, on the other, that very numerous cells, at least in the lower -Metazoa, should have received the idioplasm of budding, and still more -numerous cells that of regeneration. But it is obvious that among the -lower Metazoa the idioplasm of budding and the idioplasm of regeneration -are equivalent; the same idioplasm, which, when liberated by -stimuli unknown to us, co-operates from two or three germinal layers -in the formation of a bud, effects, in response to the known stimulus of -injury, the regeneration of the mutilated part. But germ-cells can never -arise in the Metazoa from the partial budding-idioplasm or regeneration-idioplasm, -because this is not complete germ-plasm, and because it can -only give rise to budding or regeneration through the co-operation of -two or more kinds of cells, while germ-cells always originate from <i>one</i> -cell and never arise from the fusion of cells. Germ-cells can thus only -arise from the cells of the germ-track, and in no other way, no matter -whether the germ-track lie in the ectoderm, as in the Hydromedusæ, or -in the endoderm, as in true jellyfishes (Acalephæ) and the Ctenophores, -or in the mesoderm, as in many higher groups of animals. It is only -apparently that these cells belong to one particular layer, for in reality -they are unique in kind, and they are simply assisted in their development -by one or other cell-layer, from which they not infrequently -emancipate themselves, as happens so notably in the Hydromedusæ. -As we have already said, it is only among plants that we must -think of budding as arising from cells which contain complete germ-plasm, -for here there are no 'germinal layers' corresponding to those -of animal development, and the cells of 'the growing point' must be -equipped with the complete germ-plasm. The plant, like the Hydroid -stock and the Siphonophore colony, is saved from death, in spite of the -frequent loss of its members, mainly by the fact that it is capable of -producing, at almost any part above the ground, buds which develop -into new shoots, with leaves and the like. This makes a power of -regeneration on the part of the individual leaves and flower-parts -superfluous, but at the same time it implies that an enormous number<span class="pagenum"><a id="Page_34"></a>[Pg 34]</span> -of cells must be distributed over the whole surface of the plant, -each of which can in certain circumstances become the starting-point of -a bud. That is to say, each must contain, in a latent state, the complete -germ-plasm which is necessary for the production of an entire plant.</p> - -<p>We must therefore assume that, in the higher colony-forming -plants, germ-plasm is contained in a great many cells, perhaps -in all which are not histologically differentiated, and sometimes -even in those which are so, as, for instance, in the leaves of Begonias. -I suppose, therefore, that in the higher plants the process -of development implies a segregation of the determinant-complexes of -the germ-plasm, but that this takes place at a late stage, and that -in a much higher degree than among animals the individual or the -'person' carries with it germ-plasm in a latent state. To this must -be attributed the fact that the plant is not only able to make good its -losses in twigs and branches by sending out new shoots, but that -cuttings, that is, detached shoots, are also able to take root, and in -general to give rise to what is necessary to complete themselves -according to the position of the part in question. In the ontogeny -of animals, too, we must assume that it requires a liberating stimulus -to rouse the determinants to activity, that this stimulus is to be -sought for in the influence exercised by the constitution of the cell -on the idioplasm contained within it, and that this constitution in -its turn is subject to influences from external conditions, including -the cell-soma itself. We may therefore suppose that, among plants also, -the germ-plasm latent in numerous cells only becomes active in whole or -in part according to the influences exerted on it by the state of the cell -at the moment; but this varies with external circumstances, according -to whether the cell is exposed to light or lies under ground, according -as it is influenced by gravity, by moisture, chemical stimuli, and so on.</p> - -<p>It might be objected to this that it would be simpler not to -assume a segregation of the germ-plasm into determinant-complexes -at all in order to explain the process of development, but rather to -credit each cell with a complete equipment of germ-plasm from the -beginning to the end of the ontogeny, and to attribute the differences -in the cells, which condition the structure of the plant and its -differentiation, solely to the different influences, external and internal, -to which the cell is exposed, and which rouse some determinants to -activity at one part and others at another. Perhaps the botanists -would be more readily reconciled to this idea, but it seems to me that -there are two points which tell against the possibility of its being -correct. In the first place, it is far from being established that every -cell in the higher plants is capable of giving rise, under favourable<span class="pagenum"><a id="Page_35"></a>[Pg 35]</span> -conditions, to a whole new plant; every tree and every higher plant -has a multitude of cells in its leaves, its flowers, and so on, which -cannot do this, which are in fact differentiated in one particular -direction, that is, they contain only one kind of determinants, like the -histologically differentiated cells of the tissues of the human body. -Secondly, there are other organisms besides plants, and a theory of -development cannot be based on the phenomena to be observed -among plants alone, any more than a theory of heredity can. There -are obvious differences in the processes of life among plants as -contrasted with those among animals, but it is improbable that -there is any thoroughly fundamental difference. It is, however, -indubitable that the cells forming the tissues of higher animals, the -nerve, muscle, and glandular cells, are really differentiated in one -direction, and are quite incapable, under any circumstances whatever, -of growing into an entire organism, and even from this alone we might -conclude that they contain only one primordium or determinant. Are -we then to assume that the vascular cells, epidermis-cells, wood-cells, -and so on, of the higher plants, which are also differentiated in one -direction, do nevertheless contain the complete germ-plasm? I do not -see any ground for such an assumption.</p> - -<p>To conclude what can be said on the subject of regeneration we -must return to the question of an ultimate explanation of this marvellous -phenomenon. I have declined to attempt any explanation at all, -because I do not consider it possible to give a sufficient one as yet, -but I should like at least to give an indication as to the direction -in which we must look for it.</p> - -<p>We assumed that there is a regeneration-idioplasm, and therefore -that there are 'primary constituents' at certain positions in the -body, but how does it happen that these are able to build up the lost -parts in the proper situation and detail? A theoretical formula -might well be thought out, according to which the determinants of -successive parts would become active successively, and would thus -liberate one another in an appropriate order of sequence, but there -would not be much gained by this, especially as what we already -know in regard to the regrowth of the legs and toes in Triton does -not harmonize with such an assumption. It appears to me more -important—though even here we must still be very vague as to details—to -recognize that, in all vital units, there are forces at work -which we do not yet know clearly, which bind the parts of each unit -to one another in a particular order and relation. We were obliged -to assume such forces even in regard to the lowest units, the biophors, -since otherwise they could not be capable of multiplication by<span class="pagenum"><a id="Page_36"></a>[Pg 36]</span> -division, on which all organic growth depends, unless we are to -assume, as Nägeli did, a continual <i>generatio æquivoca</i> of the specific -kinds of biophors (his 'micellæ'). But we shall see later, when we -come to speak of spontaneous generation, that we cannot acquiesce -in such an assumption. If, then, we cannot conceive of a power -of division arising from within and depending solely on growth -by means of assimilation, without such attractive and repellent -forces or 'vital affinities' the internal parts would necessarily fall -into disorder at every division. It seems to me therefore that such -'affinities' must be operative at <i>all stages</i> in the life of the vital -units, not only in biophors, but also in the cell, and in the 'person' -as well as in determinant and id. It is true that 'persons' no longer -generally possess the power of multiplying by division, but in plants -and lower animals many do possess it; and the power of giving rise -anew to certain parts is obviously a part of that power of doubling -the whole by division. The ultimate roots of regeneration, then, -must lie in these 'affinities' between the parts, which preside over -their arrangement and are able to maintain it and to give rise to it -anew. In this respect the organism appears to us like a crystal -whose broken points always complete themselves again from the -mother-lye after the same system of crystallization, obviously in -this case too as a result of certain internal directive forces, polarities, -which here again we are unable precisely to define. But the difference -between the organism and the crystal does not—as people have -been hitherto inclined to believe—lie only in the fact that the crystal -requires the mother-lye to complete itself, while the vital unit itself -procures the material for its further growth; it lies also in the fact -that such regeneration is not possible in every organism and at every -place, but that <i>special</i> 'primary constituents' are necessary, without -which the relevant part cannot arise. The indispensableness of these -primary constituents, the determinants, seems to me to depend on the -fact that the new structure cannot be built up simply by procuring -organic material, but <i>that specially hewn stones, different in every case, -are necessary</i>, which can only be supplied in virtue of an historical -transmission, or, to abandon the metaphor, because the vital units -of which the organ is to be reconstructed possess a specific character -and have a long history behind them; thus they can only arise from -such vital units as have been handed on through generations, that -is, from the determinants. But these primary constituents are given -to the different forms of life in very varying degrees and in very -unequal distribution, and as far as we can see according to their -suitability to an end.</p> -<hr class="full" /> - -<div class="chapter"> -<p><span class="pagenum"><a id="Page_37"></a>[Pg 37]</span></p> - -<h2 class="nobreak" id="LECTURE_XXII">LECTURE XXII</h2> -</div> - -<p class="c">SHARE OF THE PARENTS IN THE BUILDING UP<br /> -OF THE OFFSPRING</p> - -<div class="blockquot"> - -<p>The ids are 'ancestral plasms'—The reducing division brings about a diversity of -germ-plasm in the germ-cells—Bolles Lee's 'Neotaxis' even in the primordial germ-cells—Häcker's -observations on the persistent distinctness of the maternal and paternal -chromosomes—Identical twins—The individuality is determined at fertilization—Unequal -share of the ids in the determination of the offspring—Preponderance of -one parent in the composition of the offspring—Certain ids of the ancestors remain -unchanged in the germ-plasm of the descendants—Struggle of the Biophors—Alternation -of the hereditary sequences in the parts of the child—Reversion—Datura-hybrids—Zebra-striping -in the horse—Three-toed horses—New experiments in hybridization -among plants by Correns and De Vries—Xenia.</p></div> - - -<p><span class="smcap">As</span> far as the phenomena of regeneration and budding are concerned, -we have not been able to do much more than bring them -under a formula, which harmonizes with the germ-plasm theory. -But the case is different with the actual phenomena of inheritance -in the restricted sense, for instance, with regard to the transmission -of individual peculiarities <i>from parent to child</i>. Here the theory -really increases our insight and lets us penetrate deeper into the -causes of the phenomena; it is here no longer a mere 'portmanteau-theory.'</p> - -<p>We are well aware, especially from observation on ourselves, that -is, on Man, that the children of a pair often resemble one another -but are never alike, and that one child frequently resembles one -parent, another the other, while a third may exhibit a mingling -of both parents. How does this come about? Since the germinal -substance of both parents is derived from that of the ovum, from -which they themselves have arisen—and must therefore be the -same in all the germ-cells to which they give rise—new determinants -cannot be added, and old ones cannot be dropped out, and variation -of the determinants, the possibility of which is granted, would still -not directly bring about the familiar mingling of resemblances to -the two parents, but would at most give rise to something new and -strange.</p> - -<p>Here the theory helps to elucidate matters. We found ourselves -obliged to assume that the germ-plasm is composed of ids, that is,<span class="pagenum"><a id="Page_38"></a>[Pg 38]</span> -of equivalent portions of germ-plasm, each of which contains all -the kinds of determinants appertaining to the building up of -an individual, but each of these kinds in a particular individual -form. I have already called these ids 'ancestral plasms,' and -the term is appropriate, in so far that in every fertilization an -equal number of ids from the father and from the mother are -united in the ovum, so that the child is built up of the ids of his -two nearest 'ancestors.' But as the ids of the parents are derived -from those of the grandparents, and these again from those of the -great-grandparents, the ids are in truth the idioplasm of the -ancestors.</p> - -<p>The expression, however, has been very frequently misunderstood, -as if it were intended to mean that the ids retained <i>unchanged</i> -for all time the character of their respective ancestors, and I have -even been credited with supposing that our own ids still consist -of the determinant-complexes of our fish-like or even Amœba-like -ancestors. But in reality no id exactly or completely corresponds -to the type, that is, to the whole being of any one of the ancestors -in whose germ-plasm it was formerly contained, for each of the -ancestors had many ids in his germ-plasm, and his entire constitution -was not determined by any one of these alone, but by the co-operation -of them all. The individual arising from a germ-cell must necessarily -be the result of all the ids which make up his germ-plasm, but -undoubtedly the share taken by some of them may be much stronger -than that taken by others. It is also clear that, if we leave out -of account any possible variation on the part of the ids, each of them -belongs, not to one ancestor only, but to a whole series of ancestors, -and must have taken part in their development, so that it is not -the idioplasm of any particular ancestor, but only ancestral plasm -in the general sense. In this sense we may quite well retain the -designation, 'ancestral plasm,' for the id.</p> - -<p>Thus, according to our view, the germ-plasm consists of ids, -each of which contains all the determinants of the whole ontogeny, -but usually in individually different quality.</p> - -<p>Returning for a moment to the processes by which the reduction -of the chromosomes, that is, of the nuclear rods of germ-plasm in -the ovum and sperm-cell is brought about, we recall the fact that -this happens at the last two divisions of the germ-cell, the so-called -'maturing divisions.' In these the nuclear substance, as we have -seen, is divided between the two daughter-nuclei in a manner quite -different from the usual one, for a longitudinal splitting of the rods, -bands, or spheres in the equatorial plane of the nucleus does not<span class="pagenum"><a id="Page_39"></a>[Pg 39]</span> -take place, but half the number of rods move into the right and -half into the left daughter-nucleus without previous division, so -that in each daughter-nucleus the number of rods is reduced to -half (Fig. 76).</p> - -<div class="figcenter" id="ff8"> -<img src="images/ff8.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 76.</span> Diagram of the maturation divisions of the ovum. <i>A</i>, primitive -germ-cell. <i>B</i>, mother-egg-cell, which has grown and has doubled the number -of its chromosomes. <i>C</i>, first maturation division. <i>D</i>, immediately thereafter; -<i>Rk</i> 1, the first directive cell or polar body. <i>E</i>, the second maturation spindle -has been formed; the first polar body has divided into two (2 and 3); the -four chromosomes remaining in the ovum lie in the second directive spindle. -<i>F</i>, immediately after the second maturation division; 1, the mature ovum; -2, 3, and 4, the three polar cells, each of these four cells containing two -chromosomes.</p> -</div> - -<p>Although the distribution of the rods in this manner takes -place twice in succession, the normal number is not, as we have -already seen, reduced to a quarter, because, long before the occurrence -of the first maturing division, a duplication of the rods by means -of longitudinal division had taken place, and thus the first division -differs from an ordinary division in that the splitting of the rods -does not take place during the process of dividing but long beforehand. -Only the second maturing division differs from all other -nuclear divisions known to us, since it is not associated with any -splitting of the rods at all, but conveys half of the existing rods<span class="pagenum"><a id="Page_40"></a>[Pg 40]</span> -into each daughter-nucleus. It is the time reducing division, through -which the number of the rods is reduced to one half<a id="FNanchor_4" href="#Footnote_4" class="fnanchor">[4]</a>.</p> - -<div class="footnote"> - -<p><a id="Footnote_4" href="#FNanchor_4" class="label">[4]</a> Recent investigations have shown that the reduction of the chromosomes does -not always take place exactly in accordance with the scheme here indicated, but that -it differs from it in many cases. But as investigations on this point are as yet by no -means complete, I need not go into the question further; the ultimate result is the -same in any case.</p> - -</div> - -<p>This numerical reduction must, however, have other consequences; -it must make the germ-cells of the same individual qualitatively -unlike, that is, in relation to their value in inheritance. Let us assume -only four chromosomes of the rod-form ('idants') as the nuclear -elements of a species, two of which, <i>A</i> and <i>B</i>, come from the mother, and -other two, <i>C</i> and <i>D</i>, from the father, the last maturing division -may, as far as we can see, result either in removing the combination -<i>A</i> and <i>B</i> from <i>C</i> and <i>D</i>, or <i>A</i> and <i>C</i> from <i>B</i> and <i>D</i>, or <i>A</i> and <i>D</i> from <i>B</i> and -<i>C</i>; there is thus a possibility of one of six different combinations of rods -in any one germ-cell. What is the same thing, six different kinds of -germ-cells differing in their hereditary primary constituents may <i>be -developed in the same</i> individual. As this new combination, or, as we -may call it, neotaxis of the germ-plasm elements, takes place in female -as well as in male individuals, there is a possibility that, in fertilization, -6 × 6 = 36 individuals with different primary constituents may -arise from the germ-cells of the same two parents. Of course the -number of possible combinations increases very considerably in proportion -to the normal number of rods, for with eight of these it comes -up to 70, and with sixteen to 12,870; the number of individuals differing -in their inherited primary constituents would thus be enormous, for -each of the 70 or of the 12,870 different hereditary minglings of the -ovum could combine in amphimixis with 70 or 12,870 different -sperm-cells, so that 70 × 70 and 12,870 × 12,870 offspring individually -different in their primary constituents might arise from the same two -parents. In Man there are said to be sixteen nuclear rods; so that in -his case the last-mentioned number of parental hereditary minglings -might occur. This may seem a disproportionately high number as -compared with the small number of children of a human pair, but we -must not judge from the case of Man alone, and in plants and animals, -which we have already discussed, the number of descendants is very -much larger, and is often enormous. We saw what significance this -apparent extravagance on the part of nature has, for without it adaptation -to changed conditions of life would not be possible, since, if only -so many were born as could attain to reproduction, no selection of the -fittest could take place. The same would be the case if all the young -of a species were alike, and even if all the descendants of a single pair<span class="pagenum"><a id="Page_41"></a>[Pg 41]</span> -were alike, effective selection would be excluded, since only as many -individualities could be selected as there were pairs of parents. It is -easy to understand that selection works more effectively the larger -the number of descendants of a species and the more they differ from -each other. The chance that the best possible combination of characters -will occur is thereby increased.</p> - -<p>Although we cannot calculate how many individuals of different -combinations of characters natural selection requires to work upon in -order to direct the evolution of the species<a id="FNanchor_5" href="#Footnote_5" class="fnanchor">[5]</a>, we can understand that -only as large a choice as possible can secure that the best possible -adaptations of all parts and organs are brought about and maintained. -Precisely in the fact that in every generation such an -enormous superfluity of individuals is produced lies the possibility -of such intensive processes of selection as must continually take place, -if the adaptation of all parts is to be explained. For if among the -thousands of descendants of a fertile species each hundred were alike -among themselves, these hundreds would have, as far as natural -selection was concerned, only the value of a single variant. But such -an all-round adaptation as actually exists in the structure of species -requires as many variants as possible; it requires that each individual -should be a <i>peculiar complex of hereditary characters</i>; that is, that -all the fertilized germ-cells of a pair should possess an individually -well-marked character.</p> - -<div class="footnote"> - -<p><a id="Footnote_5" href="#FNanchor_5" class="label">[5]</a> For this reason I have left the number of id-combinations given above unaltered, -though, according to the most recent researches into the processes of maturation, they -are probably too high, since every conceivable combination does not actually -occur. We are here concerned less with the exact number than with the -principle.</p> - -</div> - -<p>The justification of this postulate becomes all the clearer if we -take into consideration the male germ-cells as well as the female. -Let us think of the enormous number of sperm-cells which are -produced by many animals, and indeed by the highest of them—an -almost incalculably large number which certainly goes far beyond -millions. Let us assume that in Man there may be 12,870 million -spermatozoa, then, with sixteen ids, and with an equally frequent -occurrence of all possible combinations of germ-plasm—there would -be 12,870—there would be a million of each type containing -identical germ-plasm. The danger that several ova would be -fertilized by identical sperm-cells would be by no means small.</p> - -<p>It cannot, therefore, surprise us that other means have been -employed by Nature to secure re-groupings of the ids. The simplest -means would be, if before each division of the primitive germ-cells -the nuclear rods were to divide, and if the split halves were<span class="pagenum"><a id="Page_42"></a>[Pg 42]</span> -irregularly intermingled, then at the formation of the next nuclear -spindle an entirely new arrangement of the halves would result. But -in animals, at least, this is certainly not the case; the processes of -reduction are restricted to the maturing divisions.</p> - -<p>Years ago Ischikawa observed that, in the conjugation of -<i>Noctiluca</i>, the nuclei of the two animals become closely apposed, but -that they do not fuse, although they behave like a single nucleus in -the division which follows. In this case <i>paternal and maternal -nuclear substance remain separate</i> (<a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#f87">Fig. 83, vol. i. p. 317</a>). The -same phenomenon has since been repeatedly observed in many-celled -animals, first by Häcker, then by Rückert in the Copepods, and afterwards -by Conklin in the eggs of a Gastropod (<i>Crepidula</i>). But all -these observations referred only to the earlier stages of ovum-segmentation -up to twenty-nine cells, and it could not be affirmed that the -distinctiveness of the paternal and maternal chromosomes lasted till -farther on into the ontogeny. Professor Häcker now informs me, -however, that he has been able to trace this separateness in -a Copepod (<i>Canthocamptus</i>) not only from the beginning of segmentation -on to the primitive genital cell, but also through the -divisions of this up to the mother-egg-cell<a id="FNanchor_6" href="#Footnote_6" class="fnanchor">[6]</a>. Thus we may now -assume that the paternal and maternal hereditary bodies remain -distinct, not only for a time, but throughout the whole development, -a fact which confirms our assumption of the independence of the -nuclear rods, notwithstanding their apparent breaking-up in the -nuclear reticulum of the 'resting' nucleus. This new knowledge -throws fresh light in another direction; it proves to us that the -remarkable and complicated processes which go on in the nucleus -during the maturing divisions have really the significance which I -long ago ascribed to them<a id="FNanchor_7" href="#Footnote_7" class="fnanchor">[7]</a>, that of effecting the maximum diversity -of intermingling of the paternal and maternal hereditary elements. -For Häcker has shown that during the second maturation-division -the paternal and maternal chromosomes are no longer united each in a -special group, but occur scattered about in the nucleus, and subsequently -come together again to form two differently combined groups.</p> - -<div class="footnote"> - -<p><a id="Footnote_6" href="#FNanchor_6" class="label">[6]</a> Since this was written Häcker has published his results. See <i>Anatom. Anzeiger</i> xv -(1902), p. 440.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_7" href="#FNanchor_7" class="label">[7]</a> See my essay, <i>Amphimixis</i>, Jena, 1891.</p> - -</div> - -<p>If this were not so, if the maternal and paternal chromosomes -remained separate, then the reducing division would cause only one of -these groups to reach each of the germ-cells, and thus each mature -ovum or sperm-cell would contain either only paternal or only -maternal hereditary bodies. But this would make a reversion to<span class="pagenum"><a id="Page_43"></a>[Pg 43]</span> -more than three generations back impossible, and as such reversions -undoubtedly occur, we must conclude that manifold new combinations -of the paternal and maternal chromosomes take place. This obviously -happens during the maturation-divisions, at least in the Metazoa.</p> - -<p>The more numerous the rods or the free individual ids in -a species are, the more numerous are the possible combinations. -Whether all the mathematically possible combinations actually occur -is a different question, which I should not like to answer in the -affirmative just yet; but in any case the <i>actual</i> number of combinations -in a species with many nuclear elements will be greater than -in one with few, and in this respect those species in which the ids -occur as independent granules will have an advantage over those in -which they are combined into rods or bands (idants). These latter, -however, afford us a better possibility of deducing the new combinations -of the ids, although the idants themselves are not outwardly -distinguishable from each other.</p> - -<p>I must refrain from going into these highly interesting processes -in more detail just now. So much is certain, that Nature makes use -of <i>various means</i> to bring about the re-combination, and at the same -time the reduction of the ids during the two 'reducing divisions.' -This is proved by the fact recently established by Montgomery, that -in many animal groups reduction results from the <i>first</i> maturing -division. Whether it operates at this stage with rings, bands, double -rods, X-shaped structures, groups of four (tetrads), and so on, all this -serves the same end, the more or less thoroughgoing re-arrangement of -the hereditary vital units. I am convinced that new investigations -into these processes, if they were undertaken from this point of view, -would lead to very important results<a id="FNanchor_8" href="#Footnote_8" class="fnanchor">[8]</a>. It would be important to find -out how great the variations are which thus arise, for it is very -probable that they differ in degree in the different animal-groups. -Even the combination of the ids into rods (idants) indicates that some -species may be more conservative than others in maintaining their -id-combinations, and that there will be among them a greater tenacity -in the hereditary combinations of characters (i.e. of the 'type' of the<span class="pagenum"><a id="Page_44"></a>[Pg 44]</span> -parents). If we should succeed in penetrating more deeply into -these processes we should probably also understand why in certain -human families the hereditary characters are transmitted more purely -and more tenaciously than those of other families with which they -have mingled, and so on. It may well be that the persistence of -character is due to the fact that ids which have once combined into -rods hold firmly together, for it seems to me in no way impossible -that individual differences should occur even in these most delicate -processes.</p> - -<div class="footnote"> - -<p><a id="Footnote_8" href="#FNanchor_8" class="label">[8]</a> Since this was written for the first edition observations of this process have been -considerably increased, and discussions as to the exact interpretation of these are in -full tide; we are surrounded by a wealth of new observations, facts, and explanations, -without having attained to a consistent and unified theory. Several naturalists, such -as Boveri, Häcker, Wilson, and others, have attempted interpretations, but these are -in many points contradictory to one another. It is therefore impossible to enter into -the question in detail here; further light from new observations must be awaited. So -much we may say, however, that it is not chance alone which presides over the -re-arrangement of the chromosomes during the reducing divisions; <i>affinities</i> play a part -also; there are stronger or weaker attractions between the chromosomes, which aid in -determining their relative position to one another.</p> - -</div> - -<p>But let us leave these more intimate questions out of account -altogether, and turn our attention to the more obvious and less delicate -phenomena, and we find that the re-arrangement of ids (Neotaxis) -which we have just discussed affords a simple explanation of the -generally observed phenomenon of the differences between individuals! -Each individual is different from every other, not in the case of Man -alone, but in all species in which we can judge of differences, and this -is true not only of descendants of different parents, but even of those -of the same parents.</p> - -<p>Of course the differences between two brothers or two sisters do -not depend entirely on the hereditary basis, but in part also on external -conditions which have affected them from embryonic development -onwards. Let us suppose that of two brothers who have sprung from -identical germ-cells one becomes a sailor, the other a tailor; it would -not surprise us to find them very different in their fiftieth year, one -weather-beaten and tanned, the other pale; one muscular, straight, -and vigorous, the other weakly and of bent carriage. The same -primary constituents develop differently according to the conditions -to which they are exposed. But the two brothers will still -resemble each other in the features of the face, colour of hair, form -of eyes, stature and proportion of limbs, perhaps even in a birthmark, -more than any other human beings of their own or any other -family, and this resemblance will depend upon the identity of the -hereditary primary constituents, on the similar id-combination of the -germ-plasm.</p> - -<p>Man himself affords a particularly good example in favour of this -interpretation in the case of so-called 'identical twins.' It is well -known that there are two kinds of twins, those that are not strikingly -alike, and often very different, and those that are alike to the extent -of being mistakable for one another. Among the latter the resemblance -may go so far that the parents find it necessary to mark the -children by some outward sign, so that they may not be continually -confused. We have now every reason to believe that twins of the<span class="pagenum"><a id="Page_45"></a>[Pg 45]</span> -former kind are derived from two different ova, and that those of the -latter kind arise from a single ovum, which, after fertilization, has -divided into two ova. This not infrequently occurs in fishes and -other animals, and we can bring it about artificially in a number -of species by experimentally separating the two first blastomeres.</p> - -<p>We have here, then, a case of absolute identity of the germ-plasm -in two individuals, for the id-combination of the two ova derived -from the same process of fertilization must be exactly the same. -That in such a case, notwithstanding the inevitable differences of -external influences to which the twins are exposed from intra-uterine -life onwards, such a high degree of resemblance should arise is a fact -of great theoretical importance. From the basis of the germ-plasm -theory we can very well understand it, for, according to the theory, -only precisely similar combinations of ids can give rise to identical -individuals.</p> - -<p>But we learn more than this from the occurrence of identical twins. -They prove above all that the whole future individual is determined at -fertilization, or, to express it theoretically, that the id-composition of -the germ-plasm is decisive for the whole ontogeny. It might have -been supposed that the combination of ids could change again during -development, and that a greater multiplication of some than of others -might take place at certain stages of development, or through certain -chance external influences. It might have been thought that there was -a struggle among the ids in the sense that some of them were suppressed -and set aside. All such suppositions break down in face of -the fact of identical twins, which teaches us that identical germ-plasm -evokes an ontogeny which runs its course as regularly as two chronometers, -which are constructed and regulated alike.</p> - -<p>But when I say that a struggle of the ids, in the sense of a material -setting aside of some of them, cannot take place, I by no means intend -to maintain that the influence which each individual id exerts on the -course of development may not be disproportionate to that exerted -by others, and, under some circumstances, very disproportionate -indeed. I must refrain from entering into this subject in detail -now, but I should like to give at least an indication of what I -mean.</p> - -<p>If the germ-plasm consists of ids, these ids collectively must -determine the structure, the whole individuality—let us say, -briefly, the 'type' of the offspring; it is the resultant of all the -different impelling forces which are contained in the different ids. -If these were all equally strong, and all operating in the same -direction, they would necessarily all have the same share in the<span class="pagenum"><a id="Page_46"></a>[Pg 46]</span> -resultant of development, the 'type' of the child. But this is not the -case.</p> - -<p>Numerous experiments on the hybridization of two species of -plant have taught us that the descendants of such hybridization -usually maintain a medium between the ancestral species; but it is -not always the case, for in many hybrids the character of one -species, whether paternal or maternal, preponderates in the young -plant.</p> - -<p>We recognize the same thing still more clearly in Man, whose -children by no means always maintain a medium between the -characters of the two parents, but frequently resemble one—the -father or the mother—much more strongly than the other.</p> - -<p>How can this fact be theoretically explained? Must we ascribe -to the ids of the father or of the mother a greater determining -power? Without excluding such an assumption as on <i>a priori</i> -grounds inadmissible, I am inclined to believe that we do not -require it to explain <i>this</i> phenomenon. For, if we take our stand -simply on the fact of the preponderance of one parent, it follows -directly from this that not all the ids control the type of the -child, let the cause of the non-co-operation of some of them be what -it may. But if in this case only a portion of the ids contained in -the germ-plasm controls the type, this combination of ids suffices -to make the child resemble one parent, the father, for instance, -and consequently half the number of ids is sufficient in some circumstances -to determine the child—taking for granted that the -one-sidedness of the inheritance is complete, which never actually -happens. But the half number of ids can only suffice if it includes -the same combinations of ids which have determined the type in -the case of the father; as soon as one or more ids of this particular -combination are replaced by others the paternal germ-plasm -alone is not enough to call forth complete resemblance in the child.</p> - -<p>But, at the reduction, a change of arrangement of ids takes place, -and a new combination arises, and thus each germ-cell receives its -particular group of ids. It may thus happen that, in one particular -sperm-cell, exactly the same group of ids is contained as that which -determined the type of the father, and that the same is true of a particular -egg-cell in regard to the type of the mother. Let us now -assume that a sperm-cell and an egg-cell meet, which contain both -those groups of ids which had determined the type of the father and -of the mother; if the determining power of the maternal and paternal -ids were equal a child would result which would maintain the medium -between father and mother.</p> - -<p><span class="pagenum"><a id="Page_47"></a>[Pg 47]</span></p> - -<p>As is well known this does happen not infrequently, although -it is difficult or impossible to demonstrate it precisely. In plant-hybrids -proof is easier, and it has been established that by far the -greater number of hybrids maintain a medium between the characters -of the two ancestral species. This proves that our assumption of -equal strength of the ids of both species must be correct in general, -for we know definitely in this case, as I shall show later, that the -paternal and the maternal ids are equivalent as regards the characters -of the species. This is the case, for instance, with the hybrids between -the two species of tobacco-plant, <i>Nicotiana rustica</i> and <i>N. paniculata</i>, -which were reared by Kölreuter as far back as the eighteenth century, -and which then, as now, maintained a fairly exact medium between -the two ancestral species, and did so in all the individuals. Both -species thus strive to stamp their own character on the young plant, -and in both the hereditary power is equally great; in both it is -contained in the same number of ids, that is, in the half, for both -kinds of sex-cell have undergone reducing division. We have here, -then, strict proof that the half number of ids suffices to reproduce -in the offspring the type of the species, or, more generally, of the -parents.</p> - -<p>If we apply these results to the inheritance of individual -differences in Man, we may say, that those germ-cells, to which at -the reducing division the same combination of ids has been handed -on as that which already determined the type of the parent, will -endeavour to impress this image again on the child. If a female -cell of this kind combine with a male which likewise contains the -facies-combination of the parent, in this case the father, the same -thing will happen which we described in the case of the plant-hybrids, -that is, a medium form between the type of the two parents will arise.</p> - -<p>Not infrequently, however, there is a marked preponderance of -the one parent in the type of the child, and we have to inquire -whether the theory gives us any help with regard to such a case.</p> - -<p>One might be inclined to assume a difference in the determining -power of the paternal and maternal ids, but if we cannot show to -what extent and for what reason this power may be different such -an assumption remains rather an evasion than an explanation. -Moreover, it would not always apply to the conditions in Man, for -if, for instance, the ids of a particular mother were in general stronger -than those of the father, all the children of the pair in question -would necessarily take after the mother; but it happens not infrequently -that one child resembles the father preponderantly, and -another the mother. Moreover, the ids pass continually from the<span class="pagenum"><a id="Page_48"></a>[Pg 48]</span> -male to the female individual, and conversely, by virtue of the -continuity of the germ-plasm, so that the idea that sex can have -anything to do with the relative strength of the ids is altogether -erroneous.</p> - -<p>But, as I have already said, unilateral inheritance occurs even -in the mingling of species-characters, and most clearly in the case -of plant-hybrids. Thus, for instance, hybrids between the two species -of pink, <i>Dianthus barbatus</i> and <i>Dianthus deltoides</i>, resemble the -latter species much more closely than the former, and the hybrid -between the two species of foxglove which are wild in Germany, -<i>Digitalis purpurea</i> and <i>Digitalis lutea</i>, is much more like the latter -than like the former.</p> - -<p>It might be reasonably asked whether, in these crossings, the -normal number of ids in one species is not greater than in the other. -We know that, among animals at least, differences in the normal -number of chromosomes occur even in very nearly related species. -It is not impossible that this, in many cases, is really the cause of -the diversity of transmitting power in different species. Nevertheless, -we cannot rest satisfied with this, for, in the first place, -this cause could not apply to the apparent unilateral inheritance -from one parent in Man, since the normal number of ids, as far -as we know, is strictly maintained in the same species, and second, -this would not explain certain phenomena of inheritance in plant-hybrids.</p> - -<p>It happens not only frequently, but usually, that the different parts -of the hybrid take after one or other parent in different degree, and -this is the case also with children. In the hybrid between the two -species of tobacco-plant, <i>Nicotiana rustica</i> and <i>N. paniculata</i>, which -I have already given as an example of a medium form between -the two parents, such diversities occur regularly in all the hybrid -individuals. Thus the corolla-tube of the hybrid is nearer <i>N. paniculata</i> -in regard to its length, but nearer <i>N. rustica</i> in regard to its -breadth. Many hybrids suggest one parent-form in the leaves, -the other in the blossoms. In the same way in the child the form -of eye may be that of the father, the colour of the iris that of -the mother, the nose maternal, the mouth paternal—in short, the -preponderance in heredity swings hither and thither from part to -part, from organ to organ, from character to character, and this -is even the rule though the oscillations may not always be apparent -and are often invisible.</p> - -<p>If we think of the proposition we arrived at earlier, and which -was proved chiefly by the case of identical twins, that the facies<span class="pagenum"><a id="Page_49"></a>[Pg 49]</span> -or 'type' of the descendant is determined at fertilization, we may -be inclined to regard such an oscillation of the hereditary tendencies -as almost impossible, for it means that, with the given mingling of -parental germ-plasms, the potency of inheritance from the two parents -in every part of the offspring is determined once for all in advance. -But the case of identical twins corroborates these oscillations, for -in them, too, the father predominates in one part, the mother in -another, and it proves, at the same time, that these oscillations -do not depend on any chances whatever in development, but that -they are exactly predetermined in the mingling of the hereditary -substances in the germ-plasm of the fertilized ovum, and are strictly -adhered to throughout development.</p> - -<p>This fact can only be explained thus: the primary constituents -of the different parts and characters of the body are contained in -the parental germ-plasm in varying degrees of hereditary or transmissive -strength, and this can be understood very well from our -point of view without putting anything new into the 'portmanteau' -of our theory (Delage).</p> - -<p>But I must digress a little in order to make this plain.</p> - -<p>When, in speaking of plant-hybrids, I said that the collective -ids of the germ-plasm of a species must be equivalent in regard -to the characters of the species, I did not speak quite precisely; -in the majority of ids, in many cases in an overwhelming majority, -this must be the case, but not actually in all, at least not on the -assumption we make that the transformation of species is accomplished -under the control of natural selection.</p> - -<p>Let us recall what we have already established in regard to the -evolutionary power of natural selection, namely, that the changes -which it controls can never transcend the range of their utility, -and it will be clear to us that, of the many ids which make up the -germ-plasm of the species, only so many will be modified as are -necessary to evoke the character which has varied. Just as the -protective resemblance of an insect to a leaf may be raised to a very -high pitch, but can never become perfect, because an imperfect -resemblance is already sufficient to deceive the persecutor, and the -selective process comes to a standstill because individuals which -possessed a still greater resemblance to a leaf would be no better -protected from destruction than the others, so in the modification -of a species the whole of the ids need not at once be modified, if -a majority is sufficient to stamp the great majority of individuals -with the desired variation. But it may happen that, at the reduction -of ids during the development of the germ-cells, an id-combination<span class="pagenum"><a id="Page_50"></a>[Pg 50]</span> -with wholly or almost wholly unchanged ids may come together in -one germ-cell, and if another sperm-cell of this kind meets with an -egg-cell similarly constituted, an individual of the old species must -arise. But this must—on our assumption—be at a disadvantage as -compared with the transformed individuals in the struggle for existence, -and will perish in it, and therefore the number of unmodified -ids in the germ-plasm of the species will gradually diminish. It is -obvious, however, that this will take place very slowly, as we may -conclude from the phenomena of reversion, of which I shall have to -speak later on.</p> - -<p>But what is true of the ids is true also of their constituent parts, -the determinants, and that—if I mistake not—is fundamental in the -interpretation of the alternation of hereditary succession in the parts -of the child.</p> - -<p>According to our theory, the ids do not collectively exert -a controlling influence on the cells, not even on the germ-cells, whose -histological differentiation into spermatic or egg-cells can only depend -on control through specific sex-cell determinants. It is the different -determinants of the ids that control; transformations of the species -will, it is true, depend on transformations of the ids, but this need -not necessarily consist in a variation of <i>all</i> the determinants of the -id. If, for instance, two species of butterfly, <i>Lycæna agestis</i> in -Germany and <i>Lycæna artaxerxes</i> in Scotland, only differ from each -other in that the black spot in the middle of the wing in <i>L. agestis</i> -is milk-white in <i>L. artaxerxes</i>, no other determinants in the id of the -germ-plasm can be different except those which control this particular -spot. In a majority of the ids in <i>L. artaxerxes</i> the determinants of -this spot must have been modified, let us say, to the production of -'milk-white.' This majority will increase very slowly if the white -colour has no pronounced advantage for the persistence of the -species, but it will increase gradually, as we have already seen, though -extremely slowly, through the elimination of those individuals whose -germ-plasm at the reducing division has chanced to receive a majority -of ids with the old, unmodified determinants, and which have therefore -reverted to the ancestral form. This will happen whenever the -new character has any use, however small, in maintaining the species.</p> - -<p>But in most modifications of species quite a number of parts and -characters have undergone variation either simultaneously or in rapid -succession; in many cases nearly all the details of structure, and -therefore almost all the determinants of the germ-plasm, must have -varied. We must not, however, assume that all the equivalent -determinants, for instance, all the determinants <i>K</i> in all the ids,<span class="pagenum"><a id="Page_51"></a>[Pg 51]</span> -have varied<a id="FNanchor_9" href="#Footnote_9" class="fnanchor">[9]</a>, and above all we must not take for granted that -the determinants of different characters or parts of the body, for -instance, the determinants <i>L</i>, <i>M</i>, or <i>N</i>, must all undergo variation -in an equal number of ids. It will depend on two factors whether -a new character is implicit, in the form of varied determinants, in -a small or in a very large majority of ids: first, on the relative age -of the character, and, secondly, on its value in relation to the -persistence of the species. The more important a character is for -the species the more frequently is it decisive for the life or death -of the individual, and the more sharply will individuals not possessing -it be eliminated, and the more rapidly, therefore, will those whose -germ-plasm still contains a majority of the unvaried determinants -of this character tend to disappear. In this way these determinants -will tend to sink down from generation to generation to an ever -smaller minority in the germ-plasm of those that survive.</p> - -<div class="footnote"> - -<p><a id="Footnote_9" href="#FNanchor_9" class="label">[9]</a> By equivalent or homologous determinants I mean the determinants of different -ids which determine <i>similar parts</i>, e.g. the scales of that wing-spot in <i>Lycæna agestis</i> -which is alluded to above, and to which we must refer again in more detail.</p> - -</div> - -<p>Thus in the ids of any species which has been in some way -transformed—and that is as much as to say, in every species—the -equivalent or homologous determinants are modified in a very varied -percentage. A very modern and at the same time not very important -character <i>K´</i> will only be contained in a small majority of ids, while -in the remainder the original homologous ancestral determinant <i>K</i> -is contained; an older but not very much more important character -<i>M´</i> must have its determinants in a larger majority in the ids, while -a character <i>V´</i> of decisive importance for the preservation of the -species, if it has been in existence long enough, must be represented -in almost all the ids, so that the homologous unvaried determinants -of the ancestral species <i>V</i> can only have persisted in an id here -and there.</p> - -<p>If this argument be correct, many phenomena of inheritance -become intelligible, especially the variability in the expression of -the inheritance in the parts of the offspring, which is more or less -rigidly predetermined at fertilization. For the germ-plasm thus -contains in advance every kind of determinant in diverse <i>nuances</i>, -and in definite numerical proportions. In a plant <i>N´</i>, for instance, -<i>Ba´</i> may be the determinant of the modern leaf-form, and may -occur in twenty-two out of twenty-four ids of the germ-plasm, -while the two remaining ids still contain unmodified the old leaf-form -determinants <i>Ba</i>, which the ancestral form <i>N</i> possessed. But -the flower of <i>N´</i> may be of still more recent origin, and contain the<span class="pagenum"><a id="Page_52"></a>[Pg 52]</span> -modern flower determinants <i>Bl´</i> only in sixteen out of twenty-four -ids, while in the other eight the old flower-determinant <i>Bl</i> of the -ancestral form has persisted. Let us now suppose that another -nearly related species <i>P´</i> has, conversely, a recently changed leaf-form -but a very ancient form of flower, so that the former is -represented only in sixteen ids by the determinants of the leaf <i>ba´</i> -and the latter in twenty-two ids by the flower-determinants <i>bl´</i>: -it is obvious that when the two species are crossed, notwithstanding -the equal number of ids in the germ-plasm, the leaves of the hybrid -will resemble more closely those of the ancestral form <i>N</i>, and the -flowers those of the form <i>P</i>; it is even conceivable that in such -a case the numerically preponderating leaf-determinants <i>N</i>, and -the equally preponderating flower-determinants of <i>P</i> may form -a close phalanx, so to speak, against the much less numerous -homologous determinants of the other species, and that against -this power working in a definite direction the others can make no -headway and are simply condemned to inaction.</p> - -<p>How we may or can picture this as occurring is a question -which of course admits only of being answered very hypothetically, -and it leads us, moreover, into the region of the fundamental phenomena -of life, with the interpretation of which we are not here -concerned. For the present we have assumed that life is a chemico-physical -phenomenon, and we have postponed the deeper explanation -of it to the remote future, that we may confine ourselves in the -meantime to the solution of the problem of inheritance on the basis -of the forces resident in the vital elements. But we may, nevertheless, -make the supposition that a kind of struggle between the different -kinds of biophors may take place within the cell, if the homologous -determinants of all the ids for the control of the cell have entered -into it.</p> - -<p>In many cases this struggle will be decided by the numerical -preponderance of one kind of determinant over the other, but it is -certainly conceivable that dynamic differences may also have something -to do with it.</p> - -<p>Let us, however, abstain from trying to penetrate further into -the obscurity of these processes, and let us content ourselves with -establishing that the preponderance of one parent in some or many -parts of the child may be almost if not quite complete, and that this -compels us to assume that the hereditary substance of the other -parent is in such cases rendered inoperative—for we know it is -present—since the ids of both parents all go through the whole -ontogeny, and are contained in every somatic cell.</p> - -<p><span class="pagenum"><a id="Page_53"></a>[Pg 53]</span></p> - -<p>Upon this struggle between homologous determinants depends -the possibility of the entire suppression or inhibition of the influence -of one parent, the whole diversity in the mingling of paternal and -maternal character in the body of the offspring. It is in this -that we must seek the explanation of the fact that not only whole -bodily parts of the child, such as arms, legs, the nature of the skin, -the form of the skull, may take after sometimes the father, sometimes -the mother wholly or predominantly, but that the small separate subdivisions -of a complex organ may sometimes turn out more maternal, -sometimes more paternal. Thus intelligence from the mother and -will from the father, musical talent from the father and a talent -for drawing from the mother, may be inherited by the same child. -I do not doubt that genius depends in great part on a happy -combination of such mental endowments of the ancestors in one -child. Of course something more is necessary, namely, the strengthening -of certain of these hereditary endowments, but of this we shall -speak later.</p> - -<p>It is, however, not only the immediate ancestors, that is to -say, the parents, that have to be taken account of in this mingling -of hereditary contributions, but also those more remote. Not a few -characters in the child do not occur in either parent, but were present -in the grandparents, and their reappearance is called 'atavism' or -'reversion.' Let us consider this phenomenon in more detail, and -try to find out whether and how far it can be interpreted by -means of our theory.</p> - -<p>The simplest and clearest cases are again found among plant-hybrids. -It may happen, for instance, that a hybrid between two -species, when dusted with its own pollen, gives rise to descendants, -some of which resemble only one of the ancestral forms: thus we -have reversion to one of the grandparents. The explanation of this -lies in the different modes in which the reducing divisions are -effected; if they take place in such a manner that all the paternal -ids of the hybrid are separated from the maternal ids, then the -result is germ-cells which are like those of the grandparents, that -is, those of the parent species, and these, if they happen to combine -in amphimixis, must give rise to a pure seedling of one or other -of the two ancestral species. This case occurs less rarely than was -formerly supposed, and than it could do if absolutely free combination -of the idants took place at reduction. If combination were quite -unrestricted, all other possible combinations would be likely to occur -as frequently as these. But recent experiments have shown that, -in many plant-hybrids, the germ-cells of the hybrids which are<span class="pagenum"><a id="Page_54"></a>[Pg 54]</span> -fertilized by their own pollen are either purely paternal or purely -maternal. There cannot, therefore, be free combination of the -idants at the reducing divisions; the idants of the two parent-forms -separate from one another and do not combine. It is doubtful, -however, whether the same thing occurs within a race, for instance, -in the case of reproduction within a human race.</p> - -<p>In Man reversion to a grandparent occurs not infrequently, and -we may explain it thus: the id-group which controlled the type of -the grandfather was also contained in the germ-cell which gave rise -to the existence of the father, but it did not dominate the type in -that case because a more powerful id-group was opposed to it in -the germ-cell of the grandmother. When, later on, at the reducing -divisions of the germ-cells of the father, this id-group again arrived -in the sperm-cells of the father, it would predominately control -the type of the child, that is, of the third generation, provided -that the egg-cell with which it combined contained a weaker -id-group.</p> - -<p>In the case of ordinary plant-hybrids what are designated reversions -can only be called so in a wide sense, for the ancestral characters -are contained <i>visibly</i> in the parent, although mingled with those of -the other parent. In human families, however, there are undoubted -cases in which one or more characters of the grandparent reappear -in the child which were not in any visible way expressed in the -parent, and must therefore have been contained in the parent's -germ-plasm in a latent state. And there are both in animals and -plants reversions to ancestors lying much further back, to characters -and groups of characters which have not been visible for many -generations, and the occurrence of these can only be explained on -the assumption that certain groups of ancestral determinants have -been carried on in the germ-plasm in too small a number to be able -to give rise ordinarily to the relevant character. Such isolated -determinants may, however, in certain circumstances be strengthened -by the amphimixis of two germ-cells both containing small groups -of them, and thus augmented they may gain a controlling influence. -In this case the chances of the reducing divisions have a part to -play, since they bring together the old unvaried ancestral determinants -which, as we have seen, may persist in the germ-plasm of any species -through a long series of generations. This would, of course, only -suffice to bring about a reversion if the determinants of the ancestral -species were still contained in the germ-plasm in comparative abundance. -If this is no longer the case, something more is necessary, and -that is the relative weakness of the more modern determinants.</p> - -<p><span class="pagenum"><a id="Page_55"></a>[Pg 55]</span></p> - -<p>If two white-flowered species of thorn-apple, <i>Datura ferox</i> and -<i>Datura lævis</i>, be crossed, there arises a hybrid with bluish violet -flowers and brown stalks instead of green. This was interpreted -by Darwin as a reversion to violet-flowering ancestral species on -both sides, for there are even now a great number of species of -<i>Datura</i> with violet flowers and brown stalks. When the two -white forms are crossed the reversion takes place every time, not -merely in some cases, and we may conclude from this that in both -these species there is still such a strong admixture of the same -unvaried ancestral ids that they always excel the ids of the two -modern species crossed, in strength though certainly not in number. -And this superiority must again depend on the fact that similar -determinants of the same part are cumulative in their effect, while -dissimilars are not.</p> - -<p>For this reason reversions to remote ancestors occur readily when -species and breeds are crossed, while they are rare in the normal inbreeding -of a species. The reversions of the breeds of pigeon to their -wild ancestral form, the slaty-blue rock-pigeon, never result, as -Darwin showed, and as we have already noticed, from pure breeding of -one race, but only when two or more breeds are repeatedly crossed -with one another. Even then it occurs by no means always, but only -now and again. The germ-plasm of the breeds must therefore still -contain ids of the rock-pigeon, but in a small number, varying from -individual to individual. If by fortunate reducing divisions and the -meeting together of a sperm-cell rich in ancestral ids with a similarly -endowed egg-cell the number of ancestral ids be raised to such a point -that it exceeds the number of modern-breed ids contained in each one -of the conjugating germ-cells, the ancestral ids control the development -and reversion occurs, for the ancestral ids together have a cumulative -effect while the ids of the two parent breeds are different and -therefore, as far as they are so, cannot be co-operative in their -influence. But it must be understood that they need not be different -as far as <i>all</i> their determinants are concerned, but usually only as -regards some groups, and thus it happens that reversion does not -occur in regard to all, but only in regard to particular characters—thus -in the <i>Datura</i> hybrids, chiefly in regard to the colour of the -flowers and the stem, and in the hybrids between different breeds -of pigeon mainly in regard to the colour and marking of the -plumage.</p> - -<p>The reversions of the horse and ass to striped ancestors, which -Darwin has made famous, go much further back into the ancestral -history of the species, for while we know the ancestral form of the<span class="pagenum"><a id="Page_56"></a>[Pg 56]</span> -domestic pigeon in the still living rock-dove (<i>Columba livia</i>), the -ancestral form common to the horse and the ass is extinct, and we -can only suppose that it was striped like a zebra, because such striping -occasionally occurs in pure horses and pure asses, at least in their -youth, although now only on the legs, and because this striping is -often more marked in the hybrid between the horse and the ass, the -mule. In Italy, where one sees hundreds of mules, the striping is not -exactly frequent, but it may occur in about two per cent., while in -America it is said to be much more frequent. The germ-plasm of the -horse and the ass must therefore contain, in varying numbers, ids -whose skin-colour determinants represent in part still unmodified -ancestral characters. When two germ-cells chance to meet in fertilization, -both of which have received, through a favourable reducing -division, a relatively large number of such ids, a relative majority of -these in the fertilized ovum is opposed to the dissimilar and therefore -mutually neutralizing homologous determinants of horse and ass, and -reversion to the ancestral form occurs.</p> - -<p>These cases of reversion are enough to show us that the old -unmodified ancestral determinants may persist in the germ-plasm -through long series of generations. But an even deeper glimpse into -the dim ancient history of our modern species of horses is afforded by -the occurrence of three-toed horses, references to a small number of -which the palæontologist Marsh was able to discover in literature, -and one of which he was able to observe in life. Julius Caesar -possessed a horse whose three-toed feet represented a reversion to the -horses of Tertiary times, <i>Mesohippus</i>, <i>Miohippus</i>, and <i>Protohippus</i> or -<i>Hipparion</i>; for all these genera possessed, in addition to the strong -middle toe, two weaker and shorter lateral toes.</p> - -<p>In the germ-plasm of our modern horses there must still persist -in certain ids the determinants of the ancestral foot, which, after a long -succession of favourable reducing divisions combined with favourable -chances of fertilization, may come to be in a majority, and may thus -be able to induce a reappearance of a character which has long been -hidden under the surface of the present-day type of the species.</p> - -<p>I do not propose to enter further on the discussion of the -phenomena of inheritance. A more detailed investigation of the -phenomena of reversion is to be found in '<i>The Germ-plasm</i>' published -ten years ago; and the discussion could not be resumed here without -a critical consideration of a relatively large series of newly acquired -facts not always harmonizing, and, as yet, not even fully available. -The year 1900 has given us the investigations of three botanists, -De Vries, Correns, and Tschermak, who have sought by experiments<span class="pagenum"><a id="Page_57"></a>[Pg 57]</span> -in hybridization between different sorts of peas, beans, maize, and -other plants, to throw light on the phenomena of inheritance, and -thus on the actual processes which occur in the germ-plasm at the -reducing division. This led to the discovery that similar experiments -had been published as far back as 1866 by the Abbot of Brünn, -Gregor Mendel, and that these had been formulated as a law which is -now called Mendel's law. Correns showed, however, that this law, -though correct in certain cases, did not by any means hold good in all, -and we must thus postpone the working of this new material into -our theory until a very much wider basis of facts has been supplied -by the botanists. There is less to be hoped for from the zoologists in -regard to this problem owing to the almost insuperable difficulties in -the way of a long series of experiments in hybridization in animals. -I myself have repeatedly attempted experiments in this direction, and -have always had to abandon them, either because the crossing -succeeded too rarely, or because the hybrids did not reproduce among -themselves, or did so defectively, or because the distinguishing -characters of the crossed breeds proved insufficiently tenacious or -diagnostic. But it would be a fine task for zoological gardens to -undertake such experiments from the point of view of the germ-plasm -theory, and their success would afford material for the criticism of -the theory, the more valuable because it is apparent from the experiments -on plants that the processes of heredity are manifold, and are -far from being uniform in different domains<a id="FNanchor_10" href="#Footnote_10" class="fnanchor">[10]</a>.</p> - -<div class="footnote"> - -<p><a id="Footnote_10" href="#FNanchor_10" class="label">[10]</a> Castle and Allen have recently published the results of experiments in crossing -white mice with grey, and these confirm Mendel's Law.</p> - -</div> - -<p>I have assumed for my theory that the reducing division took -place according to the laws of chance, and that thus every combination of -ids occurred with equal frequency. This assumption seems to be confirmed, -by the experiments of the botanists I have mentioned, only in -so far that in the crossing of hybrids with one another every combination -of distinctive characters occurred with equal frequency. -But, on the other hand, the splitting of the germ-plasm at the reducing -division seems, as I said before, in many cases to take place in -such a way that the id-groups of the two parents are discretely -separated from one another; this was so in the stocks, peas, beans, -and other hybrids. But even if this were always the case in these, -we could hardly infer that it must be the same everywhere; we -should rather expect that the relationship of the two parents and -their ids would bring into play the finer attractions and repulsions -between the ids of the germ-plasm, and would thus determine their -arrangement and grouping. Further investigations may clear up this<span class="pagenum"><a id="Page_58"></a>[Pg 58]</span> -point; in the meantime we can only say that already—even among -hybrids—many deviations from Mendel's Law have been established, -for instance, by Bateson and Saunders (1902).</p> - -<p>Before I conclude this lecture I should like to refer briefly to -a phenomenon which Darwin was acquainted with and sought to -explain through his theory of Pangenesis, but which at a later date -was regarded as not sufficiently authenticated to justify any attempt -at a theoretical explanation, since it seemed to contradict all our -conceptions of hereditary substance and its operations. I refer to -the phenomenon to which the botanists have given the pretty name -of Xenia (guest-gifts), and which consists in the fact that in crosses -of two different plants the characters of the male may appear not -only in the young plant but even in the seed, so that a transference -of paternal characters seems to take place from the pollen-tube to -the mother, to the 'tissue of the maternal ovary.' In heads of yellow-grained -maize (<i>Zea</i>) it is said that, after dusting with pollen from -a blue-seeded variety, blue seeds appear among the yellow, and -similar observations on other cultivated plants have been on record -for more than half a century. Thus dusting the stigma of green -varieties of grape with the pollen of a dark blue kind is said -frequently to give rise to dark blue fruits.</p> - -<p>Darwin accepted these observations as correct, and endeavoured -to explain them as due to a migration of his 'gemmules' from the -fertilized ovum into the surrounding tissue of the mother-plant. -His explanation was not correct, we can say with confidence now, -but he was right so far, for the phenomena of Xenia do occur; they -are not illusory as most modern botanists seem inclined to believe. -I myself was at first inclined to wait for further facts in proof that -the phenomena of Xenia really occurred before attempting to bring -them into harmony with my theory, and this will not be found fault -with when it is remembered that these cases of Xenia seem to stand -in direct contradiction to the fundamental postulates of the germ-plasm -theory. For this depends essentially on a definite stable -structure of the germ-substance, which lies within the nucleus in -the form of chromosomes, and which cannot pass from one cell to -another in any other way than by cell-division and division of the -nucleus; how then could it pass from the fertilized ovum to the cells -of the endosperm which do not derive their origin from it at all, -but from other cells of the embryo-sac? In point of fact some -of my opponents have cited Xenia as an actual refutation of my -theory.</p> - -<div class="figcenter" id="ff9"> -<img src="images/ff9.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 82.</span> Fertilization in the Lily (<i>Lilium martagon</i>), after Guignard. <i>A</i>, the -embryo-sac before fertilization; <i>sy</i>, synergidæ; <i>eiz</i>, ovum; <i>op</i> and <i>up</i>, upper and -lower 'polar nuclei'; <i>ap</i>, antipodal cells. <i>B</i>, the upper part of the embryo-sac, -into which the pollen-tube (<i>pschl</i>) has penetrated with the male sex-nucleus -(♂<i>k</i>) and its centrosphere; below that is the ovum-nucleus (♀<i>k</i>) with its (also -doubled) centrosphere (<i>csph</i>). <i>C</i>, remains of the pollen-tube (<i>pschl</i>); the two -sex-nuclei are closely apposed. Highly magnified.</p> -</div> - -<p>That cases of Xenia really do occur is now established by the -<span class="pagenum"><a id="Page_59"></a>[Pg 59]</span>comprehensive and at the same time exceedingly careful experiments -recently made by C. Correns with <i>Zea Mais</i>; it is only necessary to -look through the beautiful figures with which his work is adorned -to be convinced that heads of maize whose blossoms have been dusted -with the pollen of a different kind produce more or less numerous seeds -of the paternal kind, usually mingled with those of the maternal. -Thus heads of the variety <i>Zea alba</i> resulting from fertilization with -<i>Z. cyanea</i> exhibited a majority of white grains, but among them -a smaller number of blue; and the converse experiment, of dusting -<i>Z. cyanea</i> with the pollen of <i>Z. alba</i>, yielded heads in which a minority -of white grains appears among a majority of blue. But it is always -only the nutritive layer surrounding the embryo—the endosperm—which -exhibits the character of the paternal species, and even the capsule -surrounding the seed shows nothing of it, but is purely maternal. -Thus the heads of different species with pale-yellow capsule, when -dusted with the pollen of <i>Z. rubra</i>, never have red seeds like those of -<i>Z. rubra</i>, but always seeds with a pale-yellow skin, while, in the converse -experiment, dusting of the red-skinned species <i>Z. rubra</i> with -pollen from <i>Z. vulgata</i>, all the seeds are red, like those of the maternal -species, and the influence of the paternal species only shows when the -strong red skin has been removed, so that the intense yellow colour -of the endosperm, which in the pure maternal species is white, is -exposed to view. Thus the mysterious influence of the pollen never<span class="pagenum"><a id="Page_60"></a>[Pg 60]</span> -goes beyond the endosperm, and the riddle of this influence is solved -in the most unexpected manner, indeed was solved even before Correns -had securely established the genuine occurrence of Xenia. The explanation -is due to recent disclosures in regard to the processes of -fertilization in flowering plants.</p> - -<p>It had long been known that the pollen-tube contains not merely -one generative nucleus but two, which arise from one by division. -But what had till recently remained unknown was that not only one -of these penetrated into the embryo-sac to enter into amphimixis with -the egg-cell, but that the other also makes its way in, and there fuses -with the two nuclei which had long been designated the upper and -lower polar-nuclei (<a href="#ff9">Fig. 82, op. cit.</a>). Nawaschin and Guignard -demonstrated that these two nuclei fuse <i>with the second male</i> nucleus; -thus two acts of fertilization are accomplished in the embryo-sac, and -one of these gives rise to the embryo, while the second becomes -nothing less than the endosperm, the nutritive layer which surrounds -the embryo, whose origin from the polar nuclei had been previously -recognized.</p> - -<p>Thus the riddle of Xenia is essentially solved. We understand -how paternal primary constituents may find their way into the -endosperm, and indeed must do so regularly; we understand also -how the paternal influence never goes beyond the endosperm. The -riddle is thus not only solved, but at the same time the view which -assumes a fixed germ-plasm, and believes it to lie in the nuclear -substance of the germ-cells, receives further confirmation, if it -should need any, for if facts which are apparently contradictory to -a theory can be naturally brought into harmony with it, this affords -a stronger argument for the correctness of the theory than the power -of explaining the facts which were used in building it up.</p> - -<p>There is much more to be said in regard to Xenia, and I am -sure that much that is of interest will be brought to light by deeper -investigation; theoretical difficulties will still have to be overcome, -and I have already pointed out one of these in my '<i>Germ-plasm</i>,' but -I must here rest satisfied with what has been already said.</p> - -<p>We have now passed in review and attempted to fit into the -theory a sufficiently large number of the phenomena of heredity for -the purpose of these lectures. Although, as is natural, much of this -must remain hypothetical, we may accept the following series of -propositions as well founded: there is a hereditary substance, the -germ-plasm; it is contained in very minimal quantity in the germ-cells, -and there in the chromosomes of the nucleus; it consists of -primary constituents or determinants, which in their diverse arrange<span class="pagenum"><a id="Page_61"></a>[Pg 61]</span>ment -beside or upon one another form an extremely complex structure, -the id. Ids and determinants are living vital unities. Each nucleus -contains several, often many, ids, and the number of ids varies with -the species and is constant for each. The ids of the germ-plasm of -each species have had a historical development, and are derived from -the germ-plasm of the preceding lineage of species; therefore ids -can never arise anew but only through multiplication of already -existing ids.</p> - -<p>And now, equipped with this knowledge, let us return to the -point from which we started, and inquire whether the Lamarckian -principle of evolution, the inheritance of functional modifications, -must be accepted or rejected.</p> -<hr class="full" /> - -<div class="chapter"> -<p><span class="pagenum"><a id="Page_62"></a>[Pg 62]</span></p> - -<h2 class="nobreak" id="LECTURE_XXIII">LECTURE XXIII</h2> -</div> - -<p class="c">EXAMINATION OF THE HYPOTHESIS OF THE<br /> -TRANSMISSIBILITY OF FUNCTIONAL MODIFICATIONS</p> - -<div class="blockquot"> - -<p>Darwin's Pangenesis—Alleged proofs of functional inheritance—Mutilations not -transmissible—Brown-Séquard's experiments on Epilepsy in guinea-pigs—Confusion of -infection of the germ with inheritance, Pebrine, Syphilis, and Alcoholism—Does the -interpretation of the facts require the assumption of the transmission of functional -modifications?—Origin of instincts—The untaught pointer—Vom Rath's and Morgan's -views—Attachment of the dog to his master—Fearlessness of sea-birds and seals on -lonely islands—Flies and butterflies—Instincts exercised only once in the course of -a lifetime.</p></div> - - -<p><span class="smcap">As</span> I have already said in an earlier lecture, Darwin adhered to -Lamarck's assumption of the transmission of functional adaptations, -and perhaps the easiest way to make clear the theoretical difficulties -which stand in the way of such an assumption is to show how -Darwin sought to present this principle as theoretically conceivable -and possible.</p> - -<p>Darwin was the first to think out a theory of heredity which -was worthy of the name of theory, for it was not merely an idea -hastily suggested, but an attempt, though only in outline, at elaborating -a definite hypothesis. His theory of 'Pangenesis' assumes that cells -give rise to special gemmules which are infinitesimally minute, and -of which each cell brings forth countless hosts in the course of its -existence. Each of these gemmules can give rise to a cell similar -to the one in which it was itself produced, but it cannot do this at -all times, but only under definite circumstances, namely, when it -reaches 'those cells which precede in order of development' those -that it has to give rise to. Darwin calls this the 'elective affinity' -of each gemmule for this particular kind of cell. Thus, from the -beginning of development there arises in every cell a host of gemmules, -each of which virtually represents a specific cell. These gemmules, -however, do not remain where they originated, but migrate from -their place of origin into the blood-stream, and are carried by it -in myriads to all parts of the body. Thus they reach also the ovaries -and testes and the germ-cells lying within these, penetrate into them, -and there accumulate, so that the germ-cells, in the course of life, -come to contain gemmules from all the kinds of cells which have -appeared in the organism, and, at the same time, all the variations<span class="pagenum"><a id="Page_63"></a>[Pg 63]</span> -which any part may have undergone, whether due to external or -internal influences, or through use and disuse.</p> - -<p>In this manner Darwin sought to attribute to the germ-cells -the power of giving rise, in the course of their development, to the -same variations as the individual had acquired during its lifetime -in consequence of external conditions or functional influences.</p> - -<p>I abstain from analysing the assumptions here made; their -improbability and their contradictions to established facts are so great -that it is not necessary to emphasize them; the theory shows plainly -that it is necessary to have recourse to very improbable assumptions, -if an attempt is to be made to find a theoretical basis for the transmission -of acquired (somatogenic) characters. Even when Darwin -formulated his theory of Pangenesis his assumptions were hardly -reconcileable with what was known of cell-multiplication; now they -are above all irreconcileable with the fact that the germ-substance -never arises anew, but is always derived from the preceding generation—that -is, with the continuity of the germ-plasm.</p> - -<p>If we were now to try to think out a theoretical justification we -should require to assume that the conditions of all the parts of the -body at every moment, or at least at every period of life, were -reflected in the corresponding primary constituents of the germ-plasm -and thus in the germ-cells. But, as these primary constituents -are quite different from the parts themselves, they would require to -vary in quite a different way from that in which the finished parts -had varied; which is very like supposing that an English telegram -to China is there received in the Chinese language.</p> - -<p>In spite of this almost insuperable theoretical obstacle, various -authors have worked out the idea that the nervous system, which -connects all parts of the body with the brain and thus also with each -other, communicates these conditions to the reproductive organs, and -that thus variations may arise in the germ-cells corresponding to -those which have taken place in remote parts of the body.</p> - -<p>Even supposing it were proved that every germ-cell in ovary -or testis was associated with a nerve-fibre, what could be transmitted -to it by the nerves, except a stronger or weaker nerve-current? -There is no such thing as <i>qualitative</i> differences in the current; how -then could the primary constituents of the germ be influenced by -the nerve-current, either individually or in groups, in harmony with -the organs and parts of the body corresponding to them, much less -be caused to vary in a similar manner? Or are we to imagine that -a particular nerve-path leads to every one of the countless primary -constituents? Or does it make matters more intelligible if we assume<span class="pagenum"><a id="Page_64"></a>[Pg 64]</span> -that the germ-plasm is without primary constituents, and suppose -that, after each functional variation of a part, telegraphic notice -is sent to the germ-plasm by way of the brain as to how it has to -alter its 'physico-chemical constitution,' so that the descendants may -receive some benefit from the acquired improvement?</p> - -<p>I am not of the number of those who believe that we already -know all, or at least nearly all, that is essential, but am rather -convinced that whole regions of phenomena are still sealed to us, -and I consider it probable that the nervous system in particular is -not yet exhaustively known to us, either in regard to its functioning -or in regard to its finest structural architecture, although I gratefully -recognize the advances in this domain that the last decades have -brought about. In any case, such assumptions as I have just -indicated, or similar ones, seem to me quite too improbable to furnish -any foothold for progress. Yet we must always remain conscious -that we cannot decide as to the possibility or impossibility of any -biological process whatever from a purely theoretical standpoint, -because we can only guess at, not discern, the fundamental nature -of biological processes. At the close of this lecture I shall return -to the question of the theoretical conceivability of an inheritance of -functional adaptations; but first of all we must consider the facts -and be guided by them alone. If they prove, or even make it seem -probable, that such inheritance exists, then it must be possible, and -our task is no longer to deny it, but to find out how it can come about.</p> - -<p>Let us therefore investigate the question whether an inheritance -of acquired characters, that is, in the first place, of functional adaptations, -is demonstrable from experience. We shall speak later on of the -effect of climatic and similar influences in causing variation; the case -in regard to them is quite different, because they undoubtedly affect -not only the parts of the body but the germ-cells as well.</p> - -<p>When we inquire into the facts which have been brought forward -by the modern adherents of the Lamarckian principle as proofs of -the inheritance of acquired characters in this restricted sense, we -shall find that none of them can withstand criticism.</p> - -<p>First, there are the numerous reputed cases of the inheritance -of mutilations and losses of whole parts of the body.</p> - -<p>It is not without interest to note here how opinion in regard -to this point has altered in the course of the debate.</p> - -<p>At the beginning of the discussion they were all brought forward -as evidence of undoubted value for the Lamarckian principle.</p> - -<p>At the Naturalists' Congress in Wiesbaden in 1887, kittens with -only stumps of tails were exhibited, and they were said to have<span class="pagenum"><a id="Page_65"></a>[Pg 65]</span> -inherited this peculiarity from their mother, whose tail, it was -asserted, had been accidentally amputated. The newspapers reported -that the case excited great interest, and biologists of the standing -of Rudolf Virchow declared it to be noteworthy, and regarded it -as a proof, if all the details of it were correct. From many sides -similar cases were brought forward, intended to prove that the -amputation of the tail in cats and dogs could give rise to hereditary -degeneration of this part; even students' fencing-scars were said -to have been occasionally transmitted to their sons (happily not to -the daughters); a mutilated or torn ear-lobe in the mother was said -to have given rise to deformity of the ear in a son; an injury to -a father's eye was said to have caused complete degeneration of the -eyes in his children; and deformity of a father's thumb, due to frostbite, -was said to have produced misshapen thumbs in the children -and grandchildren. A multitude of cases of this kind are to be found -in the older textbooks of physiology by Burdach, and above all by -Blumenbach, and the majority have no more than an anecdotal value, -for they are not only related without any adequate guarantee, but -even without the details indispensable to criticism.</p> - -<p>As far back as the eighteenth century the great philosopher -Kant, and in our own day the anatomist Wilhelm His, gave their -verdict decidedly against such allegations, and absolutely denied any -inheritance of mutilations; and now, after a decade or more of lively -debate over the pros and cons, combined with detailed anatomical -investigations, careful testing of individual cases, and experiment, -we are in a position to give a decided negative and say <i>there is no -inheritance of mutilations</i>.</p> - -<p>Let me briefly explain how this result has been reached.</p> - -<p>In the first place, the assertion that congenital stump-tails in dogs -and cats depended on inherited mutilation proved to be unfounded. -In none of the cases of stump-tails brought forward could it even -be proved that the tail of the relevant parent had been torn or cut -off, much less that the occurrence, in parents or grandparents, of short -tails from internal causes was excluded. At the same time anatomical -investigation of such stump-tails as occur in cats in the Isle of Man, -and in many Japanese cats, and are frequently found in the most -diverse breeds of dogs, showed that these had, in their structure, -nothing in common with the remains of a tail that had been cut off, -but were spontaneous degenerations of the whole tail, and are thus -deformed tails, not shortened ones (Bonnet).</p> - -<p>Experiments on mice also showed that the cutting off of the -tail, even when performed on both parents, does not bring about<span class="pagenum"><a id="Page_66"></a>[Pg 66]</span> -the slightest diminution in the length of tail in the descendants. -I have myself instituted experiments of this kind, and carried them -out through twenty-two successive generations, without any positive -result. Corroborative results of these experiments on mice have been -communicated by Ritzema Bos and, independently, by Rosenthal, -and a corresponding series of experiments on rats, which these two -investigators carried out, yielded the same negative results.</p> - -<p>When we remember that all the cases which have been brought -forward in support of an inheritance of mutilations refer to a <i>single</i> -injury to one parent, while, in the experiments, the same mutilation -was inflicted on both parents through numerous generations, we must -regard these experiments as a proof that all earlier statements were -based either on a fallacy or on fortuitous coincidence. This conclusion -is confirmed by all that we know otherwise of the effects of -oft-repeated mutilations, as for instance the well-known mutilations -and distortions which many peoples have practised for long, sometimes -inconceivably long, ages on their children, especially circumcision, the -breaking of the incisors, the boring of holes in lip, ear, or nose, and -so forth. No child of any of these races has ever been brought -into the world with one of these marks: they have to be re-impressed -on every generation.</p> - -<p>The experience of breeders agrees with this, and they therefore, -as Wilckens remarks, have long regarded the non-inheritance of -mutilations as an established fact. Thus there are breeds of sheep in -which, for purely practical reasons, the tails have been curtailed quite -regularly for about a century (Kühn); but no sheep with a stump-tail -has ever been born in this breed. This is all the more important -because there are other breeds of sheep (fat-rumped sheep) in which -the lack of the tail is a breed character; it is thus not the case that -there is anything in the intrinsic nature of the tail of the sheep to -prevent it becoming rudimentary. The artificially rounded ear of -fox-terriers, too, though cut for generations, never occurs hereditarily. -Mr. Postans of Eastbourne informs me that the cocks which are to be -used for cock-fighting are docked of their combs and wattles beforehand, -and that this had been done for at least a century, but that no -fighting cock without comb and wattles has been reared. In the -same way various breeds of dog, such as the spaniels, have had their -tails cut to half their length regularly and in both sexes for more -than a century, yet in this case there is no hereditary diminution of -the length of tail. Deformed stump-tails do indeed occur in most -breeds of dog, but, as I said before, their anatomical character is quite -different from that of artificially shortened tails, moreover they may<span class="pagenum"><a id="Page_67"></a>[Pg 67]</span> -occur in breeds whose tails have not suffered from the fashion of -docking, as, for instance, in the Dachshund.</p> - -<p>We may therefore affirm that an inheritance of artificially produced -defects and mutilations is quite unproved, and in no way bears -out the supposed inheritance of functional changes.</p> - -<p>This is now admitted by the great majority of the adherents of -the Lamarckian principle, and we may now regard this kind of 'proof' -as disposed of.</p> - -<p>In addition to the above, various sets of facts have been brought -forward as proofs, and in particular the much discussed experiments -of Brown-Séquard on guinea-pigs, from which it was inferred that -epilepsy artificially induced could be transmitted. But these experiments -do not really prove anything in regard to the question at issue, -because epileptic-like convulsions may have very various causes, and -these are, for the most part, quite unknown. Since artificial epilepsy can -be induced in guinea-pigs by the most diverse injuries to the central -or peripheral parts of the nervous system, this of itself points to the -fact that it is not a question of the mere lesion of anatomical structure, -I mean, of the breaking of the continuity of a definite part, and of -its transmission. The result would, in any case, differ according -to whether certain centres of the brain, or half the spinal cord, or -the main nerve-trunks were cut through. There must, therefore, be -something more needed to produce the appearance of epilepsy—some -morbid process which may arise at different parts of the nervous -system, and be continued from them to the brain-centres. This is -corroborated by the fact that it takes at least fourteen days, and -often from six to eight weeks, for epilepsy to develop after the -operation, and that in many cases it does not develop at all. I have -made the suggestion that, during or after the operation, some kind of -pathogenic micro-organism might easily reach the wounded parts, -and there excite inflammation, which may extend centripetally to the -brain. Similar processes have been observed in connexion with lymph-vessels, -and why should they not occur in connexion with nerves?</p> - -<p>It has been objected to this that the guinea-pig's epilepsy may be -produced by blows on the skull, and also by a destructive compression -of the <i>nervus ischiadicus</i> through the skin, and that in both cases the -epilepsy may reappear in the following generation; and this, it is supposed, -shows that the intrusion of microbes is excluded. If this were so -beyond a doubt, and if we could exclude the possibility that there -were previously various microbes within the body, which could only -penetrate into the nervous substance after the cutting or destruction -of the neurilemma, nothing would be gained that would in any way<span class="pagenum"><a id="Page_68"></a>[Pg 68]</span> -support the Lamarckian principle. One could only say: Certain -injuries to the nervous system give rise secondarily in guinea-pigs -to morbid phenomena like epilepsy, and all sorts of functional disturbances -of the nervous system often appear in the next generation, -including in rare cases even the phenomena of epileptic convulsions. -That this is a case of the transmission of an acquired anatomical -modification brought about by the injury is not only unproved, but -is decidedly negatived, for the injuries themselves are never transmitted. -Thus what is transmitted must be quite different from what -was acquired, for no one has ever detected in the offspring the lesion of -the nerve-trunk which was cut through in the parent, or any other -result except the disease to which the original injury gives rise. -Moreover, the inheritance of these morbid phenomena has been again -brought into dispute quite recently owing to the investigations of such -experts in nervous diseases as Sommer and Binswanger, and the correctness -of Brown-Séquard's results, which have dragged through the -literature of the subject for so long, has been emphatically denied<a id="FNanchor_11" href="#Footnote_11" class="fnanchor">[11]</a>.</p> - -<div class="footnote"> - -<p><a id="Footnote_11" href="#FNanchor_11" class="label">[11]</a> See H. E. Ziegler's report in <i>Zool. Centralblatt</i>, 1900, Nos. 12 and 13.</p> - -</div> - -<p>Clearly formulated problems, like that of the inheritance of -acquired characters, should not be confused by bringing into them -phenomena whose causes are quite unknown. What do we know of -the real causes of those central brain-irritations which give rise to the -phenomena of epilepsy? It is certain enough that there are diseases -which are acquired and are yet 'inherited,' but that has nothing to -do with the Lamarckian principle, because it is a question of <i>infection</i> -of the germ, not of a definite variation in the constitution of the -germ. We know this with certainty in regard to the so-called -Pebrine, the silkworm disease which wrought such devastation in -its time; the germs of the pebrine organism have been demonstrated -<i>in the egg</i> of the silk-moth; they multiply, not at once but later, in the -young caterpillar, and it is the half-grown caterpillar, or even the -moth, that succumbs to the disease.</p> - -<p>Whether in this case also the disease germs are transmitted -through the male sex-cells is not proved, as far as I am aware, but -that this can happen is shown by the transmission of syphilis from -father to child. That in this case, also, the exciting cause of the -disease is a micro-organism cannot be doubted, although it has not yet -been proved. Thus even the minute spermatozoon of Man can contain -microbes, and transmit them to the germ of a new individual.</p> - -<p>This discussion of scientific questions ought not to be brought -down to the level of a play upon words, by bringing forward cases -like the above as evidence for the inheritance of 'acquired characters,'<span class="pagenum"><a id="Page_69"></a>[Pg 69]</span> -as was done, for instance, by M. Nussbaum, who cited as a proof of -this the migration of the alga-cells which live in the endoderm of the -green freshwater Hydra into the ovum, which is originally colourless, -and originates in the ectoderm of the animal (<a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#f39">Fig. 35<i>B</i>, p. 169, vol. i</a>). -It seems to me better to make a precise distinction between the transmission -of extraneous micro-organisms through the germ-cells and the -handing on of the germ-plasm with the characters inherent in its -structure. Only the latter is inheritance in the strict scientific sense, -the former is infection of the germ.</p> - -<p>Still less than the cases of inherited traumatic epilepsy can the -morbid constitution of the children of drunkards be regarded as a -proof of the inheritance of somatogenic characters, though this has -often been maintained. I will not lay any stress on the fact that the -allegation itself is, according to the most competent observers, such as -Dr. Thomas Morton<a id="FNanchor_12" href="#Footnote_12" class="fnanchor">[12]</a>, far from being established. But even if it -were quite certain that the numerous diseases of the nervous system, -amounting sometimes to mania, which are frequently observed in the -children of drunkards, were really <i>caused</i> by the drinking of the -parents, it ought not to be overlooked that we have here to do not -with the hereditary transmission of somatic variations, but of variations -<i>directly</i> induced in the germ-plasm of the reproductive cells, for -these are exposed to the influence of the alcohol circulating in the -blood, just as any other part of the body is. That by this means -variations in the germ-plasm can be brought about, and that these -may lead to morbid conditions in the children cannot be denied, and -ought not on <i>a priori</i> grounds to be called in question. For we are -acquainted with many other influences—climatic, for instance—which -directly affect and cause variation in the germ-plasm. Whether -this is so in the case of drunkenness, and in what manner it comes -about, whether through direct action of the alcohol, or through -infection of the germ with some microbe, we must leave to the -future to decide; the whole question is out of place here; it can in no -way help us to clear up the problem with which we are now occupied.</p> - -<div class="footnote"> - -<p><a id="Footnote_12" href="#FNanchor_12" class="label">[12]</a> Morton, 'The Problem of Heredity in Reference to Inebriety,' Proceed. Soc. for -the Study of Inebriety, No. 42, Nov. 1894.</p> - -</div> - -<p>But even if there were not a trace of proof of the transmissibility -of functional modifications, that alone would not justify us in concluding -that the transmission is impossible, for many things may -happen that we are not in a position to prove at present. If it could -be shown that there was a whole group of phenomena that could not -be explained in any other way than on the hypothesis of such -inheritance, then we should be obliged to assume that it really<span class="pagenum"><a id="Page_70"></a>[Pg 70]</span> -occurred, although it was not demonstrable, and, indeed, not even -theoretically conceivable. This is the standpoint of the adherents of -the Lamarckian principle at present.</p> - -<p>They say there are a great number of transformations which are -simply and easily explained, if we regard them as the effects of -inherited use or disuse, but which admit only of a strained explanation, -and sometimes of none at all, on the basis of natural selection, and -these are not a few isolated cases, but whole categories of them.</p> - -<p>I will submit a few of these, and show at the same time why I -cannot regard them as convincing, even if it be the case that we are -not at present in a position to explain them without the aid of the -Lamarckian principle. But let me hasten to add that it is my belief -that we can do this, although certainly not without first giving -a somewhat extended application to the principle of selection.</p> - -<p>It has often been maintained that the existence of animal -instincts is in itself enough to prove that the Lamarckian principle -is operative. In one of the earlier lectures I showed that at least the -greater number of instincts must have originated in purely reflex -actions, and therefore, like these actions themselves, can only be -explained through natural selection. A reflex action, such as coughing, -sneezing, shutting of the eyelids, and so on, differs from an -instinctive action in the lesser complexity and shorter duration of the -series of movements liberated by a sense-impression, and also in that -it does not require to enter into consciousness at all; but no very precise -boundary can be drawn between the two, and, in any case, both -depend, as we have already seen, on a quite analogous anatomical -basis. It is only a difference in degree whether, at the sight of -a rapidly approaching object, the muscles of the eyelids contract, and -by shutting the lids, protect the eye, or whether the fly, which we -intend to seize with our hand, is impelled by the sight of the rapidly -approaching shadow of the hand to fly quickly up. The action of the -fly may be regarded as reflex, or equally well as instinctive. But -there is also only a difference in degree, not in kind, between this -simple action and the complex and protracted behaviour of a mason-bee, -the sight of whose colony impels her to fly out and fetch clay, with -it gradually to build a neat cell, to fill this with honey, to lay an egg -in it, and finally to furnish the cell with a roof of clay. Since all reflex -mechanisms, and all the natural instincts of animals, contribute to the -maintenance of the species, and are therefore useful, their origins -must be referable to natural selection, and we have only to ask -whether they must be referred to it always, and to it alone.</p> - -<p>It cannot be doubted that, in Man, and in the higher animals<span class="pagenum"><a id="Page_71"></a>[Pg 71]</span> -voluntary actions which are often repeated gradually acquire the -character of instinctive actions. The individual movements pertaining -to the particular action are no longer each guided by the will, but -a single exercise of will is enough to liberate the whole complex -action, such as writing, speaking, walking, or the playing of a whole -piece of music; frequently the will-impulse may be absent altogether, -and the action be set going simply by an adequate external stimulus, -as in the case of sleep-walking, which is observed in fatigued children -and soldiers, and in somnambulists. The external stimulus is transmitted -to the proper group of muscles as unfailingly as in the case -of true instincts, and this happens not only in regard to actions which, -like walking, are essential to the life of the species, but also in regard -to those which have arisen from chance habits or exercises. Often -a short practice is sufficient to make an action in this sense instinctive, -and the complexity of the instinct-mechanism gained by such practice -is often astounding. Under some circumstances a person may play -a piece on the piano from the score, and yet be thinking intently -of other things, and be quite unconscious of what is played. In -the same way it may happen that a person dominated by violent -emotion, when trying to free himself from it by reading, may read -a whole page, line by line, without understanding in the least what -has been read. In the last case it is not directly demonstrable that -the reader has made all the complex delicate eye-movements which -would be liberated by the sight of the words, but in the case of -playing, the listeners can perceive that the piece is correctly played, -and thus that the stimulus exercised by each note on the retina -of the eye is translated into the complex muscular movement of -arm and finger, corresponding both to the pitch and the duration -of the note, and to the simultaneousness of several notes.</p> - -<p>In all these cases it is probably not always quite new paths -which are established in the brain, but use is made of particular -tracks in the innumerable nerve-paths already existing in the nerve-cells -(neurons) which are 'more thoroughly trodden' by practice, -so that the distribution of the nerve-current takes place more easily -along them than along others<a id="FNanchor_13" href="#Footnote_13" class="fnanchor">[13]</a>. This much-used metaphor does -not indicate the actual structural changes which have taken place, -but it serves at least to indicate that we have to do with material -changes in the ultimate living elements of the nerve-substance (nerve-biophors) -whether these changes be in position or in quality. Now,<span class="pagenum"><a id="Page_72"></a>[Pg 72]</span> -if such brain-structures and mechanisms acquired through exercise -in the individual life could be transmitted, new instincts would -certainly arise in this way, and many naturalists hold this view still.</p> - -<div class="footnote"> - -<p><a id="Footnote_13" href="#FNanchor_13" class="label">[13]</a> This, however, is by no means intended to cast doubt on the possibility that quite -new paths may arise during the individual life, as is made probable by the recent -investigations of Apáthy, Bethe, and others.</p> - -</div> - -<p>If the inheritance of acquired characters had already been proved -in other ways, we could not refuse to admit that it might play a part -in the higher animals in the modification and new formation of -instincts. We should then have to admit that habits can be -inherited, and that instincts actually are or may be, as they have -often been said to be, inherited habits. But to make the converse -conclusion, and to infer from the result of the brain-exercise in -the individual life and its similarity to inborn instincts that the -latter also depend on inherited exercise, and that there must therefore -be inheritance of acquired characters, is hardly admissible.</p> - -<p>It might be all very well if there were no other explanation! -But as instincts depend on material brain-mechanisms which are -variable, like every other part of the body, and as, furthermore, -they are essential to the existence of the species, and, down to the -minutest detail, are adapted to the circumstances of life, there is -no obstacle in the way of referring their origin and transformation -to processes of selection.</p> - -<p>It has been asserted that the results of training, for instance -in dogs, can be inherited, since the untaught young pointer points -at the game, and the young sheep-dog runs round and barks at -the flock of sheep without biting them. It is, however, often forgotten -that, not only have these breeds arisen under the influence -of artificial selection by Man, but that they are even now strictly -selected. My colleague and friend, Dr. Otto vom Rath, who unhappily -died all too soon for Science, and who was not only a capable investigator, -but an experienced sportsman, told me that huntsmen distinguish -very carefully between the better and the inferior young in a litter, -and that by no means every whelp of a pair of pointers can be used -for hunting game-birds. Lloyd Morgan points out the same thing, -and he is undoubtedly a competent judge in the domain of instinct; -he confirms the statement that the pointer 'often points at the -quarry, it may be a lark's nest, without instruction,' but he says -at the same time, that the power is inborn in very varying degrees, -and that, in his opinion, selection undoubtedly plays a part.</p> - -<p>It must not, therefore, be believed that the habit of the pointer -depends on training; it is only strengthened in each individual by -training, but it depends on an innate predisposition to creep up to -the game, and is thus a form of the hunting instinct. Man has taken -advantage of this, and has increased it, but has certainly not ingrafted<span class="pagenum"><a id="Page_73"></a>[Pg 73]</span> -it into the breed by whipping. And something similar will be found -to be true in all cases of so-called inheritance of the effects of training. -It must not be forgotten what astounding results can be achieved -in the individual by training. The elephant is the best example of -this, for it only exceptionally breeds in captivity, and all the thousands -of 'domesticated' elephants in India are tamed wild elephants. Yet -they are as gentle and docile as the horse, which has been domesticated -for thousands of years; they perform all kinds of tasks with the -greatest patience and carefulness, in many cases without being -under constant superintendence. They are indeed animals of great -intelligence; they understand what is required of them, and they -accommodate themselves readily to new conditions of life.</p> - -<p>The attachment of the dog to its master and to Man generally -has often been cited as a proof of the origin of a new instinct by -the inheritance of acquired habitude; but the dog is a sociable animal -even in a wild state, and by living in co-operative association with -Man it has transferred its sociable affections to him. We find exactly -the same thing in the elephant which has been caught wild and -tamed. It is particularly emphasized by those who have accompanied -animal transports in Africa that the young elephants are wild and -malicious towards the blacks who teased and maltreated them, but -complaisant and harmless towards the whites who treated them -kindly. The attachment of elephants to their keepers and to -every one who shows them kindness is familiar enough; it does not -depend on a newly acquired impulse, but on the sociable impulse -inherent in the species, which, in the wild state, causes them to live -in fairly large companies, and on their inoffensive, timid, and, we -may almost say, affectionate disposition.</p> - -<p>Of course it is easy enough to give an imaginative theoretical -interpretation of the origin of a new instinct from a newly acquired -habit. We have often heard that sailors have found the birds in -distant uninhabited islands quite free from fear; they let themselves -be struck down with cudgels without attempting to escape. -The extermination of the Dodo three centuries ago is a well-known -example of this. Chun, in his magnificent work on the -German Deep Sea Expedition of 1898, has recently communicated -numerous interesting examples of the indifference of birds towards -Man when they have not learned what his presence means: thus -the sea-birds of Kerguelen, penguins, cormorants, gulls, 'kelp-pigeons' -(<i>Chionis</i>), and others, behaved towards Man very much like -the tame geese of our poultry yards. Even enormous mammals -like the 'sea-elephant,' a seal with a proboscis-like prolongation of<span class="pagenum"><a id="Page_74"></a>[Pg 74]</span> -the nose, neither attempted to escape nor showed any hostility to -man, but quietly let itself be caught. Similar tales were told by -Steller in 1799, after he had been obliged to pass a winter with -his sailors on an island in the Behring Straits. The numerous -gigantic sea-cows (<i>Rhytina stelleri</i>) which lived there were so confiding -that they allowed the boat to come quite up to them, and -the sailors were able to kill many of them from time to time, -using their flesh for food. But towards the end of the winter -the animals began to be shy, and, in the following winter, when -other sailors to the polar regions endeavoured to hunt them too, it -was very difficult to secure them; they had recognized man as an -enemy, and fled from him when they saw him from afar. Thus -the same individuals which had earlier carelessly allowed man to -come up to them now avoided him as an enemy. <i>This was not -instinct, it was a behaviour controlled by the will and founded on -experience.</i> But it would soon become 'instinctive' if the meeting -with the enemy were often repeated, just like the winding-up of -a watch, which is often done at a wrong time, for instance, on -changing clothes during the day, and thus without reflection. It -is quite easy to conceive that if the material brain-adaptation which -causes flight without reflection at the sight of man were transmissible, -the flight-instinct might become a congenital instinct in the species -in question. But this assumption is unfounded; for, as is shown -by the case of the sea-cow, we do not require it where the animal -is of sufficient intelligence to perform by its own discernment the -action necessary to its existence. The action may thus become -'instinctive' through exercise and imitation in the <i>individual</i> life, -without however attaining to transmissibility.</p> - -<p>But in many cases this is not enough, namely, in all cases in -which the degree of intelligence is not sufficiently high, or where -the flight movement must follow so rapidly that it would be too -late if it had to be regulated by the will, as, for instance, the shutting -of the lids when the eye is threatened, or the flight of the fly or the -butterfly when an enemy approaches. Both fly and butterfly would -be lost in every case if they had voluntarily to set the flight-movement -going after they became conscious of danger. If they had -first of all to find out from whom danger threatened no individual -would escape an early death, and the species would die out. But -they possess an instinct which impels them to fly up with lightning -speed, and in an opposite direction, whenever they have a visual -perception of the rapid approach of any object of whatever nature. -For this reason they are difficult to catch. I once watched the<span class="pagenum"><a id="Page_75"></a>[Pg 75]</span> -play of a cat, ordinarily very clever at catching, as she attempted -to seize a peacock-butterfly (<i>Vanessa Io</i>), which settled several -times on the ground in front of her. Quietly and slowly she crept -within springing distance, but even during the spring the butterfly -flew up just before her nose and escaped every time, and the cat gave -it up after three attempts.</p> - -<p>In this case the beginning of the action cannot lie in a voluntary -action, for the insect cannot know what it means to be caught and -killed, and the same is true of innumerable still lower animal forms, -the hermit-crabs and the Serpulids, which withdraw with lightning -speed into their houses, and so forth. It seems to me important -theoretically, that the same action can be liberated at one time -by the will, at another by the inborn instinct-mechanism. In both -cases quite similar association-changes in the nerve-centres must -lie at the root of the animal's action, but in the first case these -are developed only in the course of the individual life by exercise, -while in the second they are inborn. In the former, they are confined -to the individual, and must be acquired in each generation by imitation -of older individuals (tradition) and by inference from experience, -in the latter they are inherited as a stable character of the species.</p> - -<p>It has been maintained by many that the origin of instincts -through processes of selection is not conceivable, because it is -improbable that the appropriate variations in the nervous system, -which are necessary for the selective establishment of the relevant -brain-mechanism, should occur fortuitously. But this is an objection -directed against the principle of selection itself, and one which points, -I think, to an incompleteness in it, as it was understood by Darwin -and Wallace. The same objection can be made to every adaptation -of an organ through natural selection; it is always doubtful whether -the useful variations will present themselves, as long as they are due -solely to chance, as the discoverers of the selective principle assumed. -We shall attempt later to fill up this gap in the theory, but, in the -meantime, I should like to point out that the process of selection -offers the only possible explanation of the origin of instincts, since -their origin through modifications of voluntary actions into instinctive -actions, with subsequent transmission of the instinct-mechanism -due to exercise in the individual life, has been shown to be untenable.</p> - -<p>If any one is still unconvinced of this, I can only refer to the -cases we have already discussed of instincts which are only exercised -once in a lifetime, since, in these, the only factor that can transform -a voluntary action into an instinctive one is absent, namely, the -frequent repetition of the action. In this case, if any explanation<span class="pagenum"><a id="Page_76"></a>[Pg 76]</span> -is to be attempted at all, it can only be through natural selection, -and as we have assumed once for all that our world does admit of -explanation, we may say, <i>these instincts have arisen through natural -selection</i>.</p> - -<p>Even though it may be difficult to think out in detail the process -of the gradual origin of such an instinctive activity, exercised once -in a lifetime, such as that, for instance, which impels the caterpillar -to spin its intricate cocoon, which it makes only once, without ever -having seen one, and thus without being able to imitate the actions -which produce it, we must not push aside the only conceivable -solution of the problem on that account, for then we should have to -renounce all hope of a scientific interpretation of the phenomenon. -We may ask, however, whether there is not something lacking in our -present conception of natural selection, and how it comes about that -useful variations always crop up and are able to increase.</p> - -<p>But if we must explain, through natural selection, such complex -series of actions as are necessary to the making of the cocoon of the -silkworm or of the Saturnia moth (<i>Saturnia carpini</i>), what reason -have we for not referring other instincts also to selection, even if -they be repeated several times, or often, in the course of a lifetime? -It is illogical to drag in any other factor, if this one, which has been -proved to operate, is sufficient for an explanation.</p> - -<p>Thus, as far as instincts are concerned, there is no necessity -to make the assumption of an inheritance of functional changes, -any more than there is in regard to any purely morphological -modifications. As the instincts only exercised once show us that -even very complicated impulses may arise without any inheritance -of habit, that is, without inheritance of functional modification, so -there are among purely morphological characters not a few which, -though effective, are purely passive, which are of use to the organism -only through their existence, and not through any real activity, so -that they cannot be referred to exercise, and therefore cannot be due -to the transmission of the results of exercise. And, if this be the -case, then transformations of the most diverse parts may take place -without the inheritance of acquired characters, that is, of functional -modifications, and there is no reason for dragging in an unproved -mode of inheritance to explain a process which can quite well be -explained without it. For if any part whatever can be transformed -solely through natural selection, why, since there is general variability -of all parts, should this be confined to the passive organs alone, when -the active ones are equally variable, and equally important in the -struggle for existence?</p> - -<p><span class="pagenum"><a id="Page_77"></a>[Pg 77]</span></p> - -<p>There are, indeed, many of these passive parts among animals; -I need only recall the coloration of animals, the whole set of skeletal -parts, so diversely formed, of the Arthropods, the legs, wings, antennæ, -spines, hairs, claws, and so on, none of which can be changed by the -inherited results of exercise, because they are no longer capable of -modification by exercise; they are ready before they are used; they -come into use only after they have been hardened by exposure to the -air, and are no longer plastic; they are at most capable of being used -up or mutilated. Finally, even so convinced an advocate of the -Lamarckian principle as Herbert Spencer has stated that among -plants the great majority of characters and distinctive features -cannot be explained by it, but only through the principle of selection; -all the diverse protective arrangements of individual parts, -like thorns, bristles, hairs, the felt-hairs of certain leaves, the -shells of nuts, the fat and oil in seeds, the varied arrangements for -the dispersal of seeds, and so on, all operate by their presence -alone, not through any real activity which causes them to vary, -and the results of which might be transmitted. An acacia covered -all over with thorns seldom requires to use its weapons even once, -and if a hungry ruminant does prick itself on the thorns it is -only a few of these which are thus 'exercised,' the rest remain -untouched.</p> - -<p>But since all these parts have originated notwithstanding their -passivity, there must be a principle which evokes them in relation -to the necessities of the conditions of life, and this can only be -natural selection, that is, the self-regulation of variations in reference -to utility. And if there is this principle, we require no other to -explain what is already explained.</p> - -<p>I can quite well understand, however, that many naturalists, -and especially palæontologists, find it difficult to accept this conclusion. -If we think only of those parts that actively function, and -thus change by reason of their function, being strengthened by use -and weakened and diminished in size by disuse, and if, further, we -follow these parts through the history of whole geological epochs, -we may certainly get the impression that the exercise of the parts -has directly caused their phyletic evolution. The direction prescribed -by utility in the course of the individual life and in the phylogeny -is the same, and the intra-selection, that is, the selection of tissues -within the individual animal, leads towards the same improvements -as the selection of 'persons.' Thus it appears as if the phyletic -variations followed those of the individual life, while in reality -the reverse is true; the changes arising from variations in the<span class="pagenum"><a id="Page_78"></a>[Pg 78]</span> -<i>germ are primary, and they determine the course of phylogenesis</i>, -while the tissue-selection in the individual life only elaborates and -improves, according to the demands made upon it, the material -afforded by the primordial equipment of the germ.</p> - -<p>The American palæontologist, Osborn, cites the case of the -horse's feet as an example in support of his view that modification -brought about by use in the individual life must be transmitted -in order that the phyletic transformations may be brought about, -but this example is perhaps the best that could be chosen to prove -the contrary. He supposes that, in every young horse, the means -of locomotion are improved at every step, so to speak, through the -contact with the ground, and I am quite willing to admit that -this is so. But that only proves that, even now, an elaboration -and improvement of the equipment which the germ affords is -indispensable, as it has been at all times and in all animals, and -thus that, notwithstanding the enormous number of generations -which our modern horse has behind it, the functional acquirements -of the individual have not yet been impressed upon the germ. Why -not? Because the horse becomes perfect without this, and there -was no reason why personal selection should perfect the primary -constituents of the germ still further, since the finishing touch -of perfection through use is readily afforded by the conditions of -each individual life.</p> - -<p>Moreover, when Osborn, Cope, and other palæontologists emphasize -that, in phyletic evolutionary series, <i>definite paths of evolutionary</i> -change are adhered to, and are not deviated from either to right -or to left, they are undoubtedly right, but the conclusion which -they draw is not justifiable, whether they assume with Nägeli that -there is a power of development, a principle of perfecting, or whether, -as Osborn does, they assume the transmission of the modifications -brought about through use in the individual life. There remains -a third possibility, that the quiet and constant evolution in a definite -direction is guided by selection, and as, in passively useful parts, that -principle alone is admissible, I see no justification for assuming it -to be inoperative in regard to those which are actively functional. -All these variations which have led up, for instance, to the modern -form of the horse's foot are useful; if they were not, they could -not have been produced either by use or by disuse in the individual -life.</p> - -<p>At the same time, here again, we are justified in inquiring -whether the assumption of 'chance' germinal variations, which we -have hitherto made with Darwin and Wallace, affords a sufficient<span class="pagenum"><a id="Page_79"></a>[Pg 79]</span> -basis for selection. Osborn says very neatly in this connexion, 'We -see with Weismann and Galton the element of chance; but the dice -appear to be loaded, and in the long run turn "sixes" up. Here arises -the question: What loads the dice?'</p> - -<p>Until recently we might have answered, 'external conditions'; -it is they that load the dice one-sidedly, and condition that the same -straight path of phylogenesis is adhered to, and exactly the same -direction of variations is preferred and maintained. It has to be -asked, however, whether this answer, which is certainly not absolutely -incorrect, is sufficient by itself, whether the dice are not falsified and -one-sidedly loaded in another sense, so that they always throw a -preponderating number of the useful variations. We shall attempt -very soon to solve this problem, but in the meantime I must refer -to another argument in favour of assuming the Lamarckian principle, -perhaps the most important and it may be thought the most difficult -of all to refute, the so-called co-adaptation of the parts of an organism, -that is, the fitting together of many individual organs for a common -purposeful functioning.</p> -<hr class="full" /> - -<div class="chapter"> -<p><span class="pagenum"><a id="Page_80"></a>[Pg 80]</span></p> - -<h2 class="nobreak" id="LECTURE_XXIV">LECTURE XXIV</h2> -</div> - -<p class="c">OBJECTIONS TO THE THESIS THAT FUNCTIONAL<br /> -MODIFICATIONS ARE NOT TRANSMITTED</p> - -<div class="blockquot"> - -<p>Giant stag as an example of co-adaptation or 'harmonious adaptation'—This occurs -even in passively functioning parts—Skeleton of Arthropods—Stridulating organ of -ants and crickets—Limbs of the mole-cricket—Wing-venation—Colorations which -form mimetic pictures—Harmonious adaptations in worker-bees and ants—Degeneration -of their wings and ovaries—The quality of food acts as a liberating stimulus—Vom -Rath's case of drones fed with royal food—Transition-forms between females and -workers—Wasmann's explanation of these—The Amazon ants—Two kinds of workers—Appendix: -Zehnder on the case of ants—On the skeleton of Arthropods—Hering's -interpretation of Ehrlich's Ricin experiments—Hering's position in regard to the -transmission of functional modifications.</p></div> - - -<p><span class="smcap">It</span> was Herbert Spencer, the English philosopher, who first -brought the argument of co-adaptation into the field against my -view of the non-inheritance of functionally acquired modifications. -He pointed out that many, if not, indeed, most modifications of bodily -parts, to be effective, implied further changes, often very numerous, in -other parts, and these latter must therefore have changed <i>simultaneously</i> -with the part which was being changed under the control of natural -selection; this, however, is only conceivable as due to an inheritance -of the changes caused by use, since a simultaneous alteration of so many -parts through natural selection would be impossible. If, for instance, -the antlers of our modern stag were to grow to the size of those of the -Giant Stag of the Irish peat-bogs, which measured over ten feet across -from tip to tip, this would mean—as has already been shown—a -simultaneous thickening of the skull, and to bear the heavy burden, -a strengthening of the <i>ligamentum nuchæ</i>, of the muscles of the neck and -back, of the bones of the legs and their muscles, and, finally, of all the -nerves supplying the muscles; and how could all this happen simultaneously -with, and in exact proportion to the growth of the antlers, -if it depended—as natural selection assumes—on chance variations of -all these parts? What if the appropriately favourable variation in -one of these organs did not occur? A harmonious variation of all -the parts—bones, muscles, nerves, ligaments—which unite in a common -activity, is an inadmissible assumption, because, in many cases, -such co-operating groups of organs have in the course of evolution<span class="pagenum"><a id="Page_81"></a>[Pg 81]</span> -developed in opposite directions. In the giraffe, for instance, the -fore-legs are longer than the hind-legs, which is the reverse of what -obtains in the majority of ruminants; in the kangaroo the hind-legs, -on the contrary, have developed to a disproportionate size, while the -fore-legs have degenerated into relatively small grasping arms. -Co-operating parts, like the fore and hind limbs, may thus follow -opposite paths of evolution; their variations need not always be -directed to the same end.</p> - -<p>The difficulty presented by these so-called co-adaptations or harmonious -correlations cannot be denied, and we must also admit that, -if the results of exercise were inherited, the explanation of the phenomenon -would, in many cases—but not, indeed, in all—be easy, -because the adaptation of the secondarily varying parts in each -individual life would correspond exactly to the altered function of -the part, and would be transmitted to the descendants, and in them -would again be subject to such a degree of variation, according to the -principle of histonal selection, as might be conditioned by the further -progress of the primary variation. The simplicity of the explanation -is striking, if only it were at the same time correct! But there are -whole series of facts, or rather of groups of facts, which prove that -the causes of co-adaptation do not lie in the inheritance of functional -modifications, and this must be recognized, even though we may not -yet be in a position to state the causes of co-adaptation, and to say -whether natural selection suffices to explain it or not.</p> - -<p>I must first point out that co-adaptations occur not only in -<i>actively, but also in passively functioning parts</i>. Very numerous -instructive examples are to be found among the Arthropods, -whose whole skeleton belongs to this category. It has been -objected that this is not wholly passive, but that, like the bones of -vertebrates, it is stimulated by the contraction of the muscles and -incited to functional reaction, and that it thickens at places where -strong muscles are inserted, and becomes or remains thin where it is -not exposed to any strain from the muscles. But this is not the case, -for the chitinous skeleton can only offer resistance to the muscular -contractions when it is no longer soft, as it is immediately after it is -secreted. As soon as it has become hard, it can no longer be altered, -and can at most be worn away externally by long use. The proof -of this lies in the necessity for moulting, which is indispensable -to all Arthropods as long as they continue to grow, but does not -occur later. Every one who has followed the growth of an insect or -a crustacean knows well that the moultings or ecdyses are often -accompanied by great changes, and hardly ever occur without some<span class="pagenum"><a id="Page_82"></a>[Pg 82]</span> -slight changes in the form of the body, especially of the limbs, with -their teeth, bristles, spines, and so on. These new or transformed -parts are formed before the throwing-off of the old chitinous shell, -and under its protection, and they are brought about by an elaboration -or transformation of the living soft matrix of the skeleton, the hypodermis, -which consists of cells, and is the true skin. They must thus -have arisen in the ancestors of our modern Arthropods in the same -way, that is, not by a gradual modification <i>during</i> use, but by a slight -sudden transformation before use. The steps in the transformation -may have been very small, a bristle may have become a little longer -in the second stage of life than it was in the first, or instead of five -bristles a particular spot may bear six in the second or third stage of -life; but the variations in the phyletic development must always be -caused by germ-variations which effect from within the variation in -the relevant stage of development. But the part which has varied -can only function after it has become firm and immodifiable.</p> - -<p>If these circumstances be kept clearly in mind, they furnish -a quite overwhelming mass of proof against the views of the -Lamarckians.</p> - -<p>Furthermore, it is not even true that the thickest parts of the -external skeleton are those at which the muscles are inserted. The -wing-covers of beetles offer the best proof to the contrary, for there -are no muscles at all in them, yet they are, in many species, the -hardest and thickest part of the whole chitinous coat of mail. The -reason is not far to seek; they protect the wings and the soft -skin of the back, which lies concealed beneath them, and the muscles -are inserted in this!—a relation which can be explained only by -its suitability to the end, and not as due to any direct effect.</p> - -<p>When we remember the origin—which we have just described—of -the external skeleton from the soft layer of cells underneath it, the -thickness of the chitinous skeleton, which is very different at different -places in the same animal, but always adapted to its end, furnishes -a case of co-adaptation in parts which have a purely passive function. -The thickened part cannot be due to the insertion of a muscle, but it is -always there in advance, from internal causes, so that the muscle finds -sufficient resistance. Close to it there may lie, perhaps, the edge of -a segment, and at this spot the chitinous skeleton becomes almost -suddenly thinned to a joint membrane capable of being bent or folded, -not <i>because</i> there was no pull from the muscles at this spot, but in -order that the two segments may be connected movably. Thus, -nowhere in the whole body of the Arthropod can the adaptation of the -skeleton, in regard to thickness and power of resistance, be regulated<span class="pagenum"><a id="Page_83"></a>[Pg 83]</span> -by function itself, but only by processes of selection which imparted to -each spot the thickness it required, in order to be effective in its -function, whether that be offering resistance to the strain of the -muscles, or giving suppleness to a joint, or affording the necessary -hardness for biting the prey, or for boring into wood or earth, or -merely for protecting the animal from external injuries.</p> - -<div class="figright" id="ff10"> -<img src="images/ff10.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 91</span> (repeated). Hind-leg of a<br /> -Grasshopper (<i>Stenobothrus protorma</i>), after<br /> -Graber. <i>fe</i>, femur. <i>ti</i>, tibia. <i>ta</i>, tarsal<br /> -joints. <i>schr</i>, the stridulating ridge.</p> -</div> - -<p>There are, however, many individual functions of the Arthropods -the exercise of which depends on the simultaneous change of several -skeletal parts; as, for instance, many of the 'singing' or vocal -apparatuses in insects. In quite recent times such vocal organs have -been discovered in ants, in which they consist of a small striated -region on the surface of the third abdominal segment, and a sharp -ridge on the segment in front; the -latter is rubbed against the former -by the movements of the two segments. -Quite a similar 'stridulating -organ' has long been known in -the bee-ant (<i>Mutilla</i>), and the -whistling sound produced by it is -easily heard by our ears; moreover -August Forel has heard it in the large -wood-ant (<i>Camponotus ligniperdus</i>), -and has described it as an 'alarm-signal,' -which the animals give each -other on the approach of danger—an -observation which has recently -been confirmed by Wasmann and -extended by Robert Wroughton in -regard to Indian ants. All these -arrangements for producing sound depend always on two organs, of -which one resembles the bow, the other the strings of a violin; the -one is of no value without the other, and they must therefore have -developed simultaneously, yet they cannot have arisen through -use, and the inheritance of the results of use, because they are both -dead chitinous parts, which are never strengthened by rubbing against -each other with the movements of the abdomen, but are rather worn -away.</p> - -<p>The same is true of the chirping organs of grasshoppers, -beetles, and crickets; in all cases they consist of two different parts, -which together produce a sound, and which therefore must have -arisen simultaneously, and the origin of which cannot be referred to -the inheritance of the results of exercise, but rather to selection. It<span class="pagenum"><a id="Page_84"></a>[Pg 84]</span> -is thus possible that co-adaptation of at least two parts may take -place even when the hypothetical Lamarckian principle is altogether -excluded.</p> - -<p>When I say that we have here a case of two parts adapted to -each other, that is, strictly speaking, understating the case, for, in -the crickets and locusts, for instance, there is a whole series of peg-like -chitinous papillæ (<a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#f90">Fig. 86</a>), the so-called 'bridge,' each of which -must have arisen by itself through variation of the corresponding spot -of skin. At least I can see no ground -for the assumption that the chitinous -surfaces on which the 'bridge' is now -placed would necessarily, from internal -reasons, have varied precisely in -the line of the bridge as it has done.</p> - -<div class="figleft" id="ff11"> -<img src="images/ff11.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 102.</span> Brush and comb on the<br /> -leg of a Bee (<i>Nomada</i>). <i>tib</i>, end of<br /> -the tibia. <i>t<sup>1</sup></i>, first tarsal joint with the<br /> -brush and its comb (<i>tak</i>). Between<br /> -these and the tibial spine (<i>tisp</i>) with<br /> -its lappet (<i>L</i>) the cross-section of an<br /> -antenna (<i>At</i>) is indicated. Drawn from<br /> -a preparation by Dr. Petrunkewitsch.</p> -</div> - -<p>Instructive examples of the co-adaptation -of several parts to a -common action in organs which are -not subject to the Lamarckian principle -are afforded by the diverse -arrangements for cleaning the antennæ -the bearers of the smelling-organ -which are so important to the -life of insects (Fig. 102). Here even -the adaptation of an indented area -on the tibia of the anterior leg to -the cylindrical form of the antenna -which passes through it, is sometimes -so striking (Fig. 102, <i>tak</i>) that it -might be thought that it must have -arisen through a gradual wearing -out; yet this is impossible, since we -have to do with hard dead chitinous -surfaces, and moreover not with a -solid mass, like a hone, which is worn -down by the knife, but with a hollow, thin-walled tube. In ants, bees, -and ichneumon-flies this minute, semi-circular indentation contains -small, pointed, triangular saw-teeth, closely set like those of a comb (<i>tak</i>), -and the apparatus is made usable by the fact that a firm spine (<i>tisp</i>), fused -to the end of the tibia, overhangs the notch and presses the antenna -towards it. In many species this spine is double, or it is furnished -with a thin comb or lappet (Fig. 102, <i>L</i>), or with rows of teeth, or -with short bristles; in short, it may be equipped in the most different<span class="pagenum"><a id="Page_85"></a>[Pg 85]</span> -ways. Not infrequently, as in wasps of the genera <i>Sphex</i>, <i>Scolia</i>, -<i>Ammophila</i>, the spine itself is also bent in a semicircle on the surface -directed towards the notch, and this may be effected in very different -ways, either by a bending of the whole thickness of the spine, or by -the presence of a comb which is concave on its inner surface. I -should never come to an end if I were to enumerate all the remarkable -details which may be found in the two main parts of this apparatus, and -which show very clearly how essential a co-operation of the two is in -fulfilling the function of cleaning the antennæ. This fitting together -of the two main parts cannot have been brought about in accordance -with the Lamarckian principle; the adaptation must therefore have -come about in some other way.</p> - -<p>The same thing is shown by the legs and other appendages of -insects and crustaceans, -which are adapted for -the most diverse functions, -and the individual -sections of which must -be correlated if the function -is to be possible. -Let us consider only -the claw structures in -crustaceans and scorpions. -Here, too, it -seems as if the outgrowth -of the last joint -of the leg, which functions -as the arm of the -claw, must have arisen -as a direct effect of use, through the pressure of an object held -fast by the last joint, the movable half of the claw. Frequently, -moreover, tooth-like protuberances occur on the fixed blade of the -claw (Fig. 103). But how could these have arisen as a direct effect -of pressure, since they are always preformed during the soft state of -the appendage <i>before use</i>, and are only made use of after it is fully -hardened. The soft crustaceans, the so-called 'butter-crabs' which -have just cast their shells, creep carefully away and avoid using their -limbs until they have become hard again. Here, too, we have the -co-adaptation of two parts which vary independently, and which cannot -be affected by the Lamarckian principle.</p> - -<div class="figright" id="ff12"> -<img src="images/ff12.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 103.</span> Claw (<i>Sch</i>) on the leg of a 'Beach-fly,' an<br /> -Amphipod Crustacean (<i>Orchestia</i>). <i>I</i>, <i>II</i>, the two first<br /> -joints. <i>uA</i>, the lower blade of the claw, a non-mobile<br /> -prolongation of the penultimate joint. <i>oA</i>, the upper<br /> -blade of the claw, the movable last joint; the tubercles<br /> -and indentations of the two blades fit one another. After<br /> -F. Müller.</p> -</div> - -<p>But the appendages furnish more complex examples of mutual -adaptation. Thus the individual sections of the anterior leg of the<span class="pagenum"><a id="Page_86"></a>[Pg 86]</span> -mole-cricket (<i>Gryllotalpa</i>) have varied greatly, yet quite differently, -and the whole together forms a most effective digging-tool. With -it the animal digs out the earth before it to right and to left, and to -do this it makes with both legs simultaneous outward movements, -which are otherwise quite unusual among insects, and does so with -such strength that Rösel von Rosenhof saw two bodies each weighing -three pounds pushed away in this manner. In this case four chief -parts of the leg (Fig. 104), the coxa (<i>cox</i>), the femur (<i>fe</i>), the tibia (<i>tib</i>), -and the tarsi (<i>tars</i>) are so adapted to each other in form, joints, thickness -of skeleton, and size, that they cannot have varied otherwise than in -relation to each other, but each piece has done so in an individual -manner. Most remarkable of all is the short -broad tibia, equipped with four large, hard teeth, -which has to perform the digging in the ground -after the manner of a spade, while the disproportionately -thin and weak tarsal joints, the last -of which bears two perfectly straight spines -instead of claws, are directed upwards, and do -not touch the ground, being no longer used for -walking. Rösel supposed, probably correctly, -that they are used for cleaning the spade when -it becomes clogged up with earth, since the -animal cannot clean it with its mouth. These -quite unusually formed parts of the limb cannot -have become what they are as the direct results -of use, because, for one thing, it would have been -not their broad surfaces, but their narrow edges, -which would most easily cut through the earth, -that would have been directed outwards. The -peculiar curving, first concave, then convex, of -the outer surface of the digging foot is exactly what is best adapted -for cutting into the earth and for the pushing aside which follows, -but it is not what it would have become if the chitin-wall had yielded -to the pressure of the earth and adapted itself to it. But, as we are -again dealing with the chitinous skeleton, there can be no question of -the direct effect of use, and, it seems to me, it must be admitted that -here we have a case of co-adaptation of at least seven different parts, -which have varied independently of each other, without any -assistance from the Lamarckian principle.</p> - -<div class="figleft" id="ff13"> -<img src="images/ff13.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 104.</span> Digging leg<br /> -of the Mole-cricket<br /> -(<i>Gryllotalpa</i>). <i>cox</i>, coxa<br /> -attaching the limb to<br /> -the thorax. <i>fe</i>, the short<br /> -broad femur. <i>tib</i>, the<br /> -tibia forming a broad<br /> -spade with six large<br /> -sharp teeth. <i>tars</i>, the<br /> -tarsal joints, which are<br /> -turned upwards and<br /> -cannot be used in locomotion.<br /> -After Rösel.</p> -</div> - -<p>But much more complicated cases than this might be cited, if -we were in a position to estimate exactly the functional value of the -individual parts of the wing-venation in the different insects, for<span class="pagenum"><a id="Page_87"></a>[Pg 87]</span> -it is well known that this venation serves the systematist as a basis -for the definition of genera, especially in Lepidoptera and Hymenoptera. -That is to say, it varies from genus to genus in a characteristic -manner, obviously corresponding to the differences in the wing-form, -and in the flight itself. But, unfortunately, we are still far from -being able to make more than quite general hypotheses as to the -meaning of the lengthening and strengthening, or conversely, the -degeneration or elimination, of this or that vein. From extreme cases, -however, as for instance the rich venation in good fliers with large -wings, and the scanty venation in poor fliers with small wings, we -learn at least so much, that the degree and even the manner of -venation bears a definite relation to the function of the wing, and -this we might have assumed. But these wing-veins, in as far as -they serve as a support for the weak wing-membrane, are purely -chitinous structures, skeletal parts which are not even renewed -from time to time like the skeletal parts of the leg and many -other parts of the insect. As they are laid down at first in the -pupa as soft strings of cells, so they remain, and they only begin -to be used when they are completely hardened. <i>They can therefore -never have been caused to vary through use in the course of the -phyletic development of species and genera</i>, and the Lamarckian -principle can have no part in their transformations. But if they -follow the most subtle changes, which we cannot precisely demonstrate, -of the whole wing-surface and in the mode of flight, as a man -is followed by his shadow, there must be some other principle which -adapts the organ to its function, and which is able continually to -adapt the large number of individual wing-veins in the manner -most advantageous for the general function. Here, therefore, we -have a state of matters exactly corresponding to that obtaining in -the transformation of actively functioning parts which form a system -with common co-operative action, as, for instance, in the case we first -discussed, that of the stag's antlers.</p> - -<p>Other even more complicated examples of harmonious adaptation -of passively functioning parts are afforded by the markings of animals, -such as those of the butterfly's wing. The colours have only a passive -rôle, whether they be due to pigments alone, or to structure, or to -both combined. When the coloration of a surface undergoes adaptive -variation, this cannot be due to any action of the colour, but must -depend on adaptation through selection. Yet it is well known that -there are many butterfly-wings whose surfaces exhibit different -colours and different shades of colour on their different parts, and -that in such a way that together they form a picture, that of a leaf,<span class="pagenum"><a id="Page_88"></a>[Pg 88]</span> -a piece of bark, a stone overgrown with lichen, an eye, and so on. -In such a case the individual colour-spots stand in a particular, -indirect relation to each other; although they are independent of -each other in their variation, they are not indifferent and due to -chance, for together they produce a common picture; this is harmonious -adaptation of many parts, where the Lamarckian principle -is absolutely excluded.</p> - -<p>It may, perhaps, be objected that this mimetic picture does not -arise all at once, but very slowly in the course of long series of -generations, and, indeed, of species. This must of course be so; -the simple beginnings are complicated and perfected through the -course of long ages. This is implied in the principle of selection -as we understand it. But does any one suppose that the gigantic -antlers of the giant-stag were developed in a few generations? In -this case, too, must not numerous races have succeeded each other -before the primitive antlers attained this enormous size? If this -must be assumed there was abundance of time for the adaptation, -through <i>germinal</i> variations, of the secondarily varying parts, the -muscles, tendons, nerves, and bones, for all these parts function -actively, and can without difficulty meet, in the individual life, the -increased claims made upon them by a slight increase in the size -of the antlers. For the certain and indubitable consequence of -exercise, of increased use, is the strengthening of the functioning -parts.</p> - -<p>Thus the appropriate germinal variation of the secondarily -varying parts may be delayed for a little without the individual -being any the less effective, or being obliged to succumb in the -struggle for existence. I do not, however, assert for a moment -that the whole explanation of the phenomena of co-adaptation is -included in this; on the contrary, I hope soon to be able to show -that we may in such cases assume a preponderance of variational -tendencies in a favourable direction, and that there is thus an -indirect connexion between the utility of a variation and its actual -occurrence. In the first place, however, I must refer to the other -group of facts which I have indicated, which show, likewise, that -the simultaneous co-adaptation of different parts may arise in -certain circumstances, although the Lamarckian principle be excluded. -These are the facts presented to us by the sterile forms of those -insects, which, like bees, termites, and ants, live together in large -societies.</p> - -<p>Ants and bees are of special interest to us in this connexion, -because they have long been carefully watched by a number of<span class="pagenum"><a id="Page_89"></a>[Pg 89]</span> -distinguished naturalists, and most of their vital functions have -been precisely studied. Ever since the days of 'Old Peter Huber' in -Geneva there have again and again been excellent observers who -have devoted almost the whole of their life-work and talents to the -more complete study of these wonderful animals. These insects are -of interest to us here, because, in the course of the social life, a type -of individual has arisen which diverges in structure in many parts -of the body from both the male and the female, although it is sterile -and does not reproduce, or does so in so few instances that the fact -is of no moment in considering the origin of the present bodily -structure. As is well known, these neuters, or better, workers, -are, among ants and bees, females which differ from the true females -not only in their smaller size and their infertility, but in many other -points as well. Among ants, for instance, they are absolutely wingless, -and at the same time they have a much smaller and differently -formed thorax and a larger head. But the most striking point is -the difference in their instincts, for while the females, concerned only -with reproduction, pair and lay eggs, it is the workers who feed -and clean the helpless emerging larvæ, and put them in places of -safety, who carry the pupæ into the warm sunshine, and afterwards -back again to the sheltered nest, who make this nest itself, and keep -it in order, after having collected or prepared the material for it; -it is they alone who defend the colony against the attacks of enemies, -who undertake predatory expeditions, attacking the nests of other -ants, and engaging in obstinate combats with them.</p> - -<p>How can all these peculiarities have arisen, since the workers -do not reproduce, or do so only exceptionally, and, in any case, are -incapable of pairing, and therefore—among bees at least—only produce -male offspring? Obviously it cannot have been through the -transmission of the effects of use and disuse, since they leave no -offspring to which anything could be transmitted.</p> - -<p>Herbert Spencer has attempted to maintain the position that -the characters of the workers of to-day already existed in the pre-social -state, that is, before the ants began to form colonies, and that, -therefore, they have not been newly evolved but only preserved. -But, even if this be conceded in regard to the care of the brood -and the building instinct, so much remains that could not have -existed at that stage, that the problem of the origin of these new -characters remains unsolved. The wings, for instance, among ants, -can only have been lost when females appeared which did not reproduce, -for the pairing of ants is associated with a nuptial flight high -in the air. The wings are not merely absent in the workers, they<span class="pagenum"><a id="Page_90"></a>[Pg 90]</span> -do not even develop in the pupæ; they are, as Dewitz showed, -present even now in the larva in the form of imaginal disks, but -from the pupa-stage onwards they degenerate, and the segments -of the thorax to which they are attached likewise appear small and -modified. A variation of the germ-plasm must therefore have taken -place, and to this is due the fact that the wing-primordia no longer -develop, and that the thorax has a different development from what -it had at the time when the animals were still fertile.</p> - -<p>It has indeed been said that there is no need for assuming -a variation of the germ-plasm, since the degeneration of the wing -might be produced by inferior nourishment. This opinion is based -on the fact that, among bees, the workers do actually arise from -female larvæ which have received a meagre diet poor in nitrogenous -elements, while the same female larvæ supplied with an abundant -diet rich in nitrogen develop into queens.</p> - -<p>But even though we may assume that there is a similar difference -in the mode of feeding among most ants, because the workers are -considerably smaller than the fertile females, it would be quite -erroneous to conclude that the difference between the two types -rests solely on the effect of differences in diet. The elimination -of an individual organ has never yet been determined by bad and -scanty nourishment; it is the whole animal with all its parts that -degenerates and becomes small and weakly. Often as caterpillars -of different species have been placed on starvation diet, whether -for experimental purposes or to procure very small butterflies, it -has never yet happened that a single organ, such as antenna, leg, -or wing, has thereby been eliminated or caused to degenerate. I have -myself instituted many such experiments with the maggots of the -blue-bottle fly, by supplying them from their earliest youth with just -as little food as possible without actually starving them to death, yet -never have these larvæ given rise to flies in which the wings were -absent or rudimentary.</p> - -<p>Nor did these starved flies ever exhibit degenerate ovaries; -they were always completely developed and equipped with the -full number of ovarian-tubes. It was to decide this particular point -that these experiments were instituted, for my opponents maintained -that degeneration of the ovaries was a direct result of inferior -nourishment. But that is not the case. Special investigations in -regard to ants, undertaken at my request by Miss Elizabeth Bickford, -showed that the anatomical results reached by earlier investigators, -like Adlerz and Lespès, in regard to the degeneration of the ovaries -in workers, were absolutely correct, and that the 'degeneration'<span class="pagenum"><a id="Page_91"></a>[Pg 91]</span> -consists not merely in the fact that the ovarian-tubes and ovum-primordia -remain small, but also in a diminution of the <i>number</i> -of ovarian-tubes (Fig. 105); the workers have always fewer ovarian-tubes -than the females of the same species, and—what is of especial -importance—the reduction in the number of ovarian-tubes has been -effected to a different extent in different species of ants. In the red -wood-ant (<i>Formica rufa</i>) the workers still possess from twelve to -sixteen ovarian-tubes; in the meadow-ant (<i>Formica pratensis</i>) only -eight, six, or four; in <i>Lasius fuliginosus</i> there are usually only -two (one on either side); and in the little turf-ant (<i>Tetramorium -cæspitum</i>) there are none -at all. We have here, therefore, -a phylogenetic process -of degeneration, which has -reached different degrees -in the different species, and -has only been completed -in one (<i>Tetramorium</i>). The -case stands as I previously -stated it: 'The elimination -of a typical organ is not -an ontogenetic process, but -a phylogenetic one,' it depends -not upon 'the mere -influences of nutrition -which affect the development -of the individual, but -always on variations in the -germ-plasm, which, to all -appearance, can only come about in the course of a long series of -generations'<a id="FNanchor_14" href="#Footnote_14" class="fnanchor">[14]</a>.</p> - -<div class="footnote"> - -<p><a id="Footnote_14" href="#FNanchor_14" class="label">[14]</a> <i>Aeussere Einflüsse als Entwickelungsreize</i>, Jena, 1894.</p> - -</div> - -<div class="figright" id="ff14"> -<img src="images/ff14.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 105.</span> Ovary of a fertile Queen-Ant and<br /> -ovaries of a Worker. <i>Od</i>, oviduct. <i>A</i>, one ovary<br /> -of <i>Myrmica lævinodis</i> with many ovarian-tubes,<br /> -in each of which there is an almost ripe egg<br /> -(<i>Ei</i>) and a younger egg (<i>Ei´</i>). <i>B</i>, the ovaries of a<br /> -Worker of <i>Lasius fuliginosus</i>; each ovary has only<br /> -one ovarian-tube, and no ripening egg-cells. After<br /> -Elizabeth Bickford.</p> -</div> - -<p>Against this proposition an observation by O. vom Rath has -been cited. According to it, three drone larvæ which had been -accidentally fed by the workers with royal food exhibited striking -retrogressive peculiarities in their sexual organs. The testes contained -only immature sperms (just before emergence from the pupa), and -the copulatory organ was entirely wanting. That a certain degree -of fatty degeneration of the testes should be caused by the 'unusual -fattening' is not surprising, but it seems to me very questionable -whether the absence of the copulatory organ can be referred to -the abnormal diet; it ought to be definitely decided, by the investiga<span class="pagenum"><a id="Page_92"></a>[Pg 92]</span>tion -of numerous cases, whether some abnormal peculiarity in the -constitution of the germ-plasm in these eggs was not the true cause. -Hitherto, unfortunately, I have not been able to procure the fresh -material necessary to decide this point<a id="FNanchor_15" href="#Footnote_15" class="fnanchor">[15]</a>. From all this it must be -evident that we are not justified in regarding either the absence -of wings or the degeneration of the ovaries as a direct result of -the inferior nourishment supplied to the workers in the larval state: -but should any one still have doubt on this point I may mention that, -among our indigenous ants, there are two species in which the workers -are just as large as the fertile females, and that in tropical America -a species (<i>Myrmecocystus megalocola</i>) occurs in which the workers -are larger than the true females; this must mean that they have -received more food than the females, though perhaps not the same -mixture of food.</p> - -<div class="footnote"> - -<p><a id="Footnote_15" href="#FNanchor_15" class="label">[15]</a> Since completing my manuscript I note that the point was settled three years -ago, when Koshewnikow had the opportunity of investigating drone-pupæ which were -abnormally reared in royal cells, and therefore fed with royal food. He found their -sexual organs perfectly normal, and agrees with me that the abnormalities in Vom Rath's -case must have been due to some other cause. (See the report by Von Adelung on the -Russian paper in <i>Zool. Centralblatt</i>, Sept. 10, 1901.)</p> - -</div> - -<p>From all the facts we have discussed we can confidently conclude -that the differences in structure, which distinguish the workers from -the true females, do not depend upon the influence, in the individual -lifetime, of a poorer diet, but upon variation in the primary constituents -of the germ; we must conceive of the germ-plasm of ants -as containing, in addition to male and female ids, special ids of -workers, in which the determinants of wing and ovary are degenerate -in some degree, while the determinants of other parts, such as the -brain, are more highly developed. The manner of feeding, however, -and perhaps the mingling with the food of a special secretion of the -salivary glands, acts as a stimulus which determines whether one -kind of id or another is to be liberated, that is, to become active and -to enter on the path of development.</p> - -<p>A proof of this view is to be found, it seems to me, in the -existence of transition forms between workers and true females, -which was first brought to general knowledge by Forel. Perhaps -it would be better to call these 'mongrel forms' for their various -parts do not maintain a medium between the two types, but many -parts follow the type of the worker, and others that of the true -female. Thus Forel twice found a nest of the red wood-ant which -contained a large number of these mixed forms, all of which possessed -the small head and large curved thorax of the queen, but otherwise -resembled the workers in size and appearance, and also in the<span class="pagenum"><a id="Page_93"></a>[Pg 93]</span> -degeneration of the ovaries. Many of them were very small, only -5 mm. in length, and had probably received very little food, and, -according to the theory of direct influence, these should have been -pure workers. That they possessed the head and thorax of a queen -is a proof that the characters of both forms of individual were -present in the germ-plasm as primary constituents, or indeed entire -ids. In normal circumstances only one kind of these ids would have -become active, either the worker-id or the queen-id, but in abnormal -circumstances they might both be liberated to activity simultaneously, -and then they would stamp one part of the body with the character of -a queen, another with that of a worker. Forel observed one of these -nests in two successive years, and both times found the mixed forms -in large numbers<a id="FNanchor_16" href="#Footnote_16" class="fnanchor">[16]</a>. In the second year he found a great number -of newly-emerged individuals of this type. I have already inferred -from this observation that the mixed forms were probably in both -years the offspring of the same mother, and this may well have been -the case. My further conclusion, that the mixed forms must be due -to some abnormality in the constitution of the germ-plasm of the -maternal eggs, no longer appears to me so convincing as it did -formerly, because, in the interval, we have learnt, through that -indefatigable investigator of ants, Pater Wasmann, that there is -another possible explanation of these mixed forms; it, too, is based -upon a hypothesis, but it is so interesting that I must briefly outline -it to you.</p> - -<div class="footnote"> - -<p><a id="Footnote_16" href="#FNanchor_16" class="label">[16]</a> There are different kinds of 'mixed forms' among ants, which may owe their -origin to a variety of conditions, as Forel, Wasmann, and Emery have shown in -detail.</p> - -</div> - -<p>Like Forel and myself, Pater Wasmann had supposed that the -reason of this kind of mixed form (the so-called pseudogynous worker) -lay in an abnormality of the constitution of the germ-plasm, but he now -regards it as the result of a change in the mode of rearing instituted by -the workers with respect to the constitutionally female or queen larvæ, -because there was a scarcity of workers. The hypothesis sounds very -daring, but it is well founded, at least in so far that there really is -a reason why a scarcity of females must occur at certain times in -some colonies of ants, and this might certainly determine the workers -in charge of the larvæ to feed females with worker food, so as to rear -them to render the necessary assistance.</p> - -<p>This reason lies in the occasional presence of a parasitic beetle, -<i>Lomechusa strumosa</i>, whose larvæ, curiously enough, are cared for and -fed by the ants as though they were their own, and in return they -eat up the larvæ of the ants, often destroying them in large numbers.<span class="pagenum"><a id="Page_94"></a>[Pg 94]</span> -Wasmann informs us that the parasitic larvæ grow up just at the -time at which the ants are rearing their workers, and it is these, -therefore, which fall victims to the Lomechusa-larvæ, and the result -is that a scarcity of young workers must soon make itself felt. The -workers seek to make this good by rearing as workers all the larvæ -previously destined for queens. But this only succeeds partially, -because the development towards true females has already begun; -thus mixed forms arise.</p> - -<p>This explanation would be rather in the air if we did not -know that, among bees, such changes in the manner of rearing are by -no means uncommon. Indeed they occur regularly when the queen -of a hive perishes and no more 'female' eggs are in store; young -worker larvæ are then fed with royal food, and these develop into -queens. There can thus be no doubt that these insects have it in -their power to liberate to activity either the female ids or the worker -ids by a specific mode of feeding, and there is nothing contrary to -reason in admitting the possibility of an alternation of this influence in -the course of development, for something analogous occurs in regard -to secondary sexual characters, as, for instance, the appearance of -male decorative colours in ducks that have become sterile.</p> - -<p>But this change in the mode of rearing bee-larvæ gives rise to -pure queens and not to mixed forms, and we must therefore regard it -as undecided whether Wasmann's explanation is correct in this case, -and whether an abnormality in the constitution of the germ-plasm -may not be the true cause of this or other kinds of mixed forms -among ants. In any case the 'Lomechusa hypothesis' rests upon the -assumption of different kinds of ids in the germ-plasm, as Pater -Wasmann expressly states, and the differences between the worker and -queen-ants have their cause in this, and not directly in the kind of -larval food.</p> - -<p>If there were not different ids corresponding to the different -kinds of individuals in the germ-plasm a kind of polymorphism -might indeed have arisen in the colony through differences in -nutrition, but it could not have been of the kind we now see—that -is, a sharply defined differentiation of persons, in adaptation to their -different functions. This presupposes elements in the germ which -can vary slowly and consistently in a definite direction without -causing any change in the rest of the germ.</p> - -<p>This state of affairs gives to the phyletic evolution of the workers -a great theoretical significance, for it proves that positive as well as -negative variations of the most diverse parts of the body, that simultaneous -and correlative variations of many parts, can take place in the<span class="pagenum"><a id="Page_95"></a>[Pg 95]</span> -course of the phylogeny, without the co-operation of the Lamarckian -factor. I have not hitherto laid any special emphasis upon the degree -of differences occurring between workers and queens; but I must -now add that this may far exceed the degree that we are familiar -with in our common indigenous ants, both in regard to instinct and -to bodily form. Even in the red Amazon ant of Western Switzerland, -<i>Polyergus rufescens</i>, we find quite a new instinct<a id="FNanchor_17" href="#Footnote_17" class="fnanchor">[17]</a>, that of carrying off -the pupæ of other species of ants, not to devour, but to introduce them -to their own nest and thus secure 'slaves.' For these workers of -a strange species, which emerge in a strange nest, naturally regard the -place of their birth as their home, and do there what instinct -impels them, and what they would have done in the nest of their -parents: they feed the larvæ, fetch food, collect building material, -and so on. The domestic activity of the workers of the master-species -thus becomes superfluous, and they have ceased to exercise it, -and have now entirely lost the power of caring for their brood, searching -for food, and keeping up the nest. They have even forgotten how -to take food themselves, because they are always fed by the 'slaves.' -Forel informs us—and I have myself repeated the experiment—that -<i>Polyergus</i> workers, which are shut up with a drop of honey on the -floor of their prison, will leave it, their favourite food, untouched, -and finally starve, unless one of their 'slaves' be shut up with them. -As soon as this happens, and the slave perceives the honey, it partakes -of it, and then the 'mistress' comes and strokes the 'slave' with her -antennæ to signify her desires, whereupon the 'slave' proceeds to feed -her from its own crop.</p> - -<div class="footnote"> - -<p><a id="Footnote_17" href="#FNanchor_17" class="label">[17]</a> 'New' in this sense, that the instinct is not exhibited by most worker-ants, that -it did not occur in the primaeval ancestors of modern ants. It is, however, exhibited -by a number of modern forms, and even by some German species.</p> - -</div> - -<p>But while the <i>Polyergus</i> workers have forgotten their domestic -habits, and have even ceased to be able to recognize their food, -remarkable changes have taken place in their jaws; these have lost -the blunt teeth on the inner margin, which, in other species, serve for -masticating the food, for seizing building material, and for other -domestic occupations, and have become sharp weapons, bent in the -form of a sabre, very well suited for piercing the head of an enemy, -but also well adapted for carrying off the pupæ, because they can -seize them without doing them any injury.</p> - -<p>No one will doubt that the predatory expeditions of the Amazon -ants, and the slave-making habit, can only have developed after the -habit of living in large companies had long existed, and this case -proves that variations of instinct, as well as of bodily structure, can<span class="pagenum"><a id="Page_96"></a>[Pg 96]</span> -take place even after the workers have long been sterile. The case is the -more instructive that it <i>seems</i> as if it were due to the transmission of -a newly acquired and inherited habit of life, while in point of fact -these Amazon-workers can transmit nothing, because they bear no -offspring. But if old instincts can be lost, and new ones acquired, -when all possibility of inheritance is excluded, we see that Nature -has no need of the Lamarckian factor of modification for her -transformations and new adaptations.</p> - -<p>If we wish to understand clearly that, in these changes, we have -to do not merely with the alteration of a single part, but of many -parts which all work together, we have only to think of the still more -striking physical modifications which have taken place in many -tropical ants, and which have led to a dimorphism of the workers. -In many species, certainly, the only difference is in size, so that one -can distinguish between large workers and small, and the former are -sometimes five times as big as the latter. But even in the South -European <i>Pheidole megalocephala</i>, which is abundant in Italy, the -larger workers are also different in structure from the smaller, for -they have an enormous head with powerful jaws. They are usually -known as 'soldiers,' and are entrusted with the defence of the colony. -Emery directly observed in regard to <i>Colobopsis truncata</i>, an ant -which lives in the trunks of trees, that the soldiers, with their -enormous heads, occupied all the entrances to the nest, ready to seize -any intruder with their powerful jaws. In the Sauba ant (<i>Œcodoma -cephalotes</i>) Bates described three different types of worker, differing in -size, and although he was not able to determine with certainty what the -particular function of each was, there can be no doubt that they have -special offices, and that the differences in their structure are adaptations -to the differences in their functions. The same is true of the -Indian ant, <i>Pheidologeton diversus</i>, depicted in Fig. 106, whose three -forms of workers I owe to the kindness of Professor August Forel.</p> - -<p>If the increase in the size of the head and jaws must bring with -it an increase in the thickness of the skeleton of these parts, as well as -a strengthening of the musculature of the head, it follows that the -strain on the body must be greater, just as in the case of the increase in -the weight of the stag's antlers, so that the skeleton of the thorax must -likewise have become thicker and heavier, the muscles and nerves -of the legs stronger, the articulations of the joints capable of greater -resistance; in short, a whole series of variations of other parts must -have taken place simultaneously, if the primary variation was to be -of use, and not to lead to the destruction of its possessor. Here again -we have a proof that the co-adaptation of many parts can take place<span class="pagenum"><a id="Page_97"></a>[Pg 97]</span> -without any intervention of the Lamarckian principle, and that there -must be some other factor which brings this about.</p> - -<div class="figcenter" id="ff15"> -<img src="images/ff15.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 106.</span> Three workers of the same species of Indian Ant (<i>Pheidologeton -diversus</i>), drawn from specimens supplied by Prof. August Forel. <i>A</i>, the largest, -<i>B</i>, the intermediate, <i>C</i>, the smallest form.</p> -</div> - -<p>Where, then, shall we look for this other factor, if not in the -processes of selection, in the selecting of the most suitable variations -among all those which occur? We are confronted with the alternative -of either working out a sufficient explanation with this factor, or of -giving up the attempt at explanation altogether. Yet the application -of the principle of selection in relation to the neuters of colony-forming -insects is by no means simple, for, as the workers are -sterile, a modification of them through processes of breeding cannot -begin directly with themselves. The workers which exhibit the most -suitable variations cannot be selected for breeding, but only their -parents, the sexual animals, and these according to whether they -produce better workers or worse. This is how Darwin looked at the -matter, and his view receives support from one peculiarity in the -composition of these animal colonies, whose significance becomes -apparent in relation to this problem. It has long been known that -in a bee-hive there are from 10,000 to 20,000 workers, but only one -true female, the so-called queen, and the meaning of this remarkable -arrangement probably is, that the adaptation of the workers through -natural selection becomes much more easily possible, <i>since the whole -number are the children of a single pair</i>. It is not the individual<span class="pagenum"><a id="Page_98"></a>[Pg 98]</span> -workers, but the whole colony, that is, the whole progeny of the -queen, which is selected, according to the greater or less degree of -effectiveness displayed by the workers. Strictly speaking, it is the -single queen that is selected in relation to her power of producing -superior or inferior workers. A colony whose queen was unsatisfactory -in this respect could not hold its own in the struggle for existence, -and only the best colonies and the best hives would survive, that is, -through their descendants. If the hive contained a hundred queens -instead of a single queen the process of selection would be much -more complex and less clear, and it is even quite conceivable that the -production of specially modified workers, adapted to their functions, -or of two or three different kinds of workers, would not have been -possible at all. For it would not have helped much if one out of -a hundred females had produced workers of better structure; only -a majority of such females could give the colony any advantage as -compared with other colonies.</p> - -<p>It has not been definitely established whether, among ants, a -single female is in all cases the founder of the whole colony, but it is -certain that there are only a few females. In the tropical Termites -we know that the ovaries of the female attain to such a colossal size -that one female must certainly suffice for the necessities of the largest -colonies. Grassi has shown, indeed, that, as far as the South European -Termites are concerned, not only are there several females present, but -that even the workers frequently reproduce; but the Termites in general -are inhabitants of warm countries, and the few European species -probably hardly represent the original composition of these animal -colonies. But of the tropical species, which have as yet not been -sufficiently studied, we know at least the extraordinary dimensions -of the body, and the corresponding fertility of the queens, and -we conclude from this that only a few can be present in each -termitary<a id="FNanchor_18" href="#Footnote_18" class="fnanchor">[18]</a>.</p> - -<div class="footnote"> - -<p><a id="Footnote_18" href="#FNanchor_18" class="label">[18]</a> Ingwe Sjostedt has recently established in Africa that it is usually a single queen -and a single king that found a termitary (<i>Schwed. Akad. Abh.</i> Bd. 34, 1902).</p> - -</div> - -<p>Now that we have discussed all these facts it will not be out of -place to summarize the results, in as far as they have any relation to -the acceptance or rejection of the theory of the inheritance of acquired -characters.</p> - -<p>No <i>direct</i> proof of such transmission could be found; on the -contrary, it has been shown that all that has hitherto been advanced -as such will not stand the test of close examination; an inheritance -of wounds and mutilations does not exist, the transmission of traumatically -induced epilepsy is not only doubtful as regards its causes, but<span class="pagenum"><a id="Page_99"></a>[Pg 99]</span> -cannot even be considered as the transmission of a particular morphological -lesion.</p> - -<p>We may regard as <i>indirect</i> proofs such facts as can only be -explained on the assumption of this mode of inheritance, and in this -connexion our opponents have cited especially the correspondence -between modifications acquired through use in the individual lifetime, -and worked out through histonal selection, with the phyletic -transformations of the same parts. But it has been shown that -a number of parts which do not function actively at all, but only -passively, and thus cannot be caused to change through use, like the -hard skeletal parts of the Arthropods, vary phyletically in the same -certain and direct course as those which function actively, so that we -have every ground for assuming that there are other factors operating -in the transformation of the active as well as of the passive parts. -Finally, we discussed the last and strongest argument which has been -put forward in favour of the Lamarckian principle, that of co-adaptation, -that is, the simultaneous adaptation of many parts -co-operating in a common action, and we were able to controvert this -altogether by showing that exactly similar phenomena of co-adaptation -occur in systems of passively functioning parts, and further, that -they occur also among the workers of ants and bees, that is, in -animals which do not reproduce, and which, therefore, cannot transmit -the acquired results of exercise during their life.</p> - -<p>We therefore reject—and are compelled to reject—the Lamarckian -principle, not only on the ground that it cannot be proved correct, but -also because the phenomena, to explain which it is used, occur also -under circumstances which absolutely exclude any possibility of the -co-operation of this principle.</p> - - -<p class="c"><i>Supplementary note on the Transmissibility of Acquired<br /> -Functional Modifications.</i></p> - -<p>I cannot conclude this section without some reference to the -utterances of some naturalists who have quite recently attempted to -represent the inheritance of functional modifications as a conceivable -and even a necessary assumption.</p> - -<p>I may name first Ludwig Zehnder, a physicist who has wandered -into the domain of biology. In regard to the very facts which I have -adduced as evidence <i>against</i> the existence of such inheritance, he has -endeavoured to show how we might conceive of them as having by -this very means arisen<a id="FNanchor_19" href="#Footnote_19" class="fnanchor">[19]</a>.</p> - -<div class="footnote"> - -<p><a id="Footnote_19" href="#FNanchor_19" class="label">[19]</a> Zehnder, <i>Die Entstehung des Lebens</i>, Freiburg-i.-Br., 1899.</p> - -</div> - -<p>He deals with the case of ants, that is to say, with the differen<span class="pagenum"><a id="Page_100"></a>[Pg 100]</span>tiation -of the sterile workers into several castes, in the following -interesting manner.</p> - -<p>The task of the workers is to procure all the food necessary for -all the individuals of the colony in quantity and quality corresponding -to the demand; failing this the whole colony would perish. Now the -different persons of the colony need different food, according to their -constitution and their functions. Soldiers, for instance, are more -powerful than ordinary workers, since they are adapted for fighting, -and they therefore require a different kind of food from the -weaker workers who are adapted only for other duties. Since the -soldiers have evolved from the latter by selection, what we may, for -the sake of brevity, call the soldier-food in the common stores of the -ants would be drawn upon more lavishly than before, and would -therefore disappear more quickly, and, whenever this occurred, those -workers which had already brought in this kind of food would be -impelled to bring more and more to satisfy the demand for it. But in -order to do this they would require to exert themselves more, and would -therefore require a larger quantity of food—not of course soldier-food, -but the particular kind which their particular qualities demanded. -Probably this second importation of food was undertaken by a second -kind of worker, for, according to Zehnder, each worker does not carry -all the kinds of food; they are divided into legions, each of which has -its particular task of food-collecting to fulfil.</p> - -<p>In the end the storehouse of the ant-colony must contain -a provision in which the different kinds of food are in exact proportion -to the necessities of the different kinds of persons in the colony. It -must alter in its composition again as soon as, in the course of time, -one or other kind of person acquires new characters, for these presuppose -a new kind of diet.</p> - -<p>But how are these acquired characters to be transmitted since -neither soldiers nor workers reproduce? Zehnder answers this by -pointing out that the sexual animals eat <i>all</i> the kinds of nourishment -which are accumulated in the stores, that is to say, all the different -kinds of food exactly in the proportion in which they have been -imported—the proportion in which the different kinds of persons are -represented in the colony. Thus the kinds of nourishment which -caused the appearance of the newly acquired characters in the non-sexual -animals also reached the sexual animals and their sex-cells, -and there gave rise to substances which evoke the relevant qualities -in their descendants, for instance, in the soldiers, or in the still more -modified workers, and so on; and thus we have an 'inheritance of -acquired characters.'</p> - -<p><span class="pagenum"><a id="Page_101"></a>[Pg 101]</span></p> - -<p>This is certainly ingeniously and cleverly thought out, and it reads -even better and more smoothly in the original than in my brief -summary, but it will hardly be regarded as a refutation of my -position; the hypotheses are all too daring for that. We have no -knowledge that particular modifications in form can be produced and -conditioned by particular kinds of food, and, indeed, the contrary has -been proved, namely, that the two or three different castes of polymorphic -species have precisely similar diet. I need only recall the -six forms of female in <i>Papilio merope</i>, of which at least three have -been obtained from the same set of eggs, and by feeding with the -same plant.</p> - -<p>It is true that there are ants which lay in stores of nourishment, -but these consist, for the most part, of one kind of seeds, or of honey, -not of different substances, and we have no knowledge that the -different persons use different food, or even that there is any diversity -in the mode of feeding the helpless larvæ. The feeding in some -species takes place from mouth to mouth, and therefore cannot be -precisely investigated, and we can only suppose from analogy with -bees that the larvæ of the males and females frequently receive not -only more abundant, but qualitatively different food. They are fed -from the crop unless the food consists of the pith of a tree in which -the larvæ are imbedded, as Dahl informs us is the case with some -tropical ants of the Bismarck Archipelago.</p> - -<p>But even if we assume that the soldiers take different food from -the ordinary workers, and different again from that of the sexual -animals, is it by virtue of the quality of their food that they have -become what they are? Have our breeds of pigeons or hens been -produced by different diet, or do we know anything in the whole -range of animal life of such a parallelism between food and bodily -structure as Zehnder here assumes? And if, in reality, let us say, the -breeds of pigeon had arisen through specific dieting, and we were to -feed one pair with the specific food-stuffs of three different breeds, -would the descendants of this pair exhibit the form of these three -breeds? Or would they exhibit them in precisely the proportion in -which the food-stuffs had been mixed? It seems to me that Zehnder's -assumptions diverge so far from what we are accustomed to regard -as solid ground in biology that they hardly require consideration, and -yet he not only uses them for the explanation of the case of the ants, -but bases upon them the whole of his theory of the inheritance of -acquired characters.</p> - -<p>He considers that the results of use (that is, increased function) -are generally transmitted, because the increase in the organ which is<span class="pagenum"><a id="Page_102"></a>[Pg 102]</span> -functioning more strongly changes the composition of the blood, by -withdrawing from it in a greater degree the specific substances which -the organ in question—a muscle, for instance—requires for its activity. -All parts of the animal are thereby affected and modified, but especially -those smallest vital units or 'fistellæ' (corresponding to biophors) which -preside over digestion, and of which there are several sorts. Among -them those work most arduously which have to produce the specific -substances which serve for the nutrition of the muscles with increased -function, because these are needed in larger quantities. This kind of -digestive 'fistella' therefore multiplies, while other kinds, whose -products are not required and therefore not used up, cease to be so -active, diminish in number, and in course of time disappear. In this -way the composition of the blood is altered, and with it to a greater -or less degree all the characters of the whole organism. Of course -the reproductive cells are also under the influence of this change in -the composition of the blood, because the different nutritive substances -are accumulated within them in an altered proportion corresponding to -the changed composition of the blood, the nutritive substances for the -muscles with increased function being contained in it in a larger -quantity, and thus the greater development of the muscle will repeat -itself in the progeny, that is to say, <i>the acquired character is transmitted</i>.</p> - -<p>It is obvious that this is precisely the same line of argument as -that used in reference to the origin of the worker and soldier ants. -The different kinds of 'digestive fistellæ' correspond to the different -food-carrying workers, and the blood to the assumed storehouse from -which soldiers and workers select the food suitable for their respective -needs, while the sex-cells in the one case, the sexual animals in the -other, partake of all kinds exactly in the proportion in which they are -stored, and thus the organ which functions most vigorously must be -stronger in the offspring.</p> - -<p>How the minute quantity of nutritive material contained in the -ovum, still less in the sperm, is to effect the strengthening of the particular -muscles in the descendants is not stated; moreover, such minimal -quantities of food must soon be exhausted, and cannot possibly -increase. It would seem as if the muscles could not even begin by -being stronger, much less that they should remain so, if they were not -exercised equally vigorously by the descendants. If the specific -nutritive stuffs were 'fistellæ,' that is to say, were living units capable -of multiplication, one could understand it. But there can, of course, -be no possibility of a production of living units through digestion; -that can only give rise to digested substances. Or if the alteration in the<span class="pagenum"><a id="Page_103"></a>[Pg 103]</span> -composition of the blood produced in the determinant system of the -germ-plasm just those variations requisite to bring about a strengthening -of the muscular system, it would remain to be shown how this -could happen, for the gist of the problem lies in this. For muscles -do not lie in the germ-plasm as miniature models of the subsequent -muscular system, and even if they did, would not all the muscles, and -not merely those which were no longer exercised, decrease hereditarily -when a particular group, like the muscles of the ear in man, -degenerates? Zehnder replies to this with the hypothesis that the -muscles are not all chemically alike, but that each possesses a -particular chemical formula, though they may all be very similar, and -that, therefore, the nutritive materials required by each must be -slightly different. In that case there would require to be, in the ovum -and sperm of man, in order that functional modifications might be -transmitted, as many special nutritive substances as there are muscles, -and, in addition to these, innumerable hosts of other kinds of specific -nutritive substances for all the other parts of the body, since all of -them can be strengthened by exercise and weakened by disuse. And -even if we suppose that all these millions of specific nutritive substances -are accommodated within the germ-cells, as Zehnder's theory -requires, they could not perform what Zehnder ascribes to them, for, -as we have already said, they cannot multiply in the manner of living -units, and so control the growing organism. The different specific -nutritive materials contained in the blood are just as powerless to -perform the task ascribed to them by Zehnder as the specific kinds of -food in the hypothetical storehouse of the ants are to give rise to the -different persons of the ant-colony.</p> - -<p>Zehnder also attempts to overthrow the arguments against the -Lamarckian principle which I based on the skeleton of Arthropods.</p> - -<p>It does not seem to him probable that the chitinous coat of mail -can be an absolutely dead structure, and he supposes that very delicate -nerve-fibrils penetrate into all its most minute parts, and so are -stimulated by 'every pressure and every strain' exerted on the -chitinous skeleton. They 'work' when they are stimulated, and in -doing so they use up 'their specific food-stuffs.' At places which are -frequently stimulated the corresponding nerves develop more than -elsewhere. The necessary specific food-stuffs for these particular -nerves therefore increase proportionately within the body, and -also in the reproductive cells. Accordingly, in the germ-cells there -is an increase of the aforesaid nervous substances, which in the -offspring become associated with the relevant part of the chitinous -covering, and induce in development the secretion of chitin at this<span class="pagenum"><a id="Page_104"></a>[Pg 104]</span> -part. At this particular spot, then, the chitin will be specially -thick.</p> - -<p>This clearly implies that each particular part of the skin has its -specific nutritive substances, necessitated by the nerves which traverse -it! Thus there must be as many nerve-nutritive substances as there -are skin-nerves, specific chemical combinations for every part of the -body which is capable of heritable variation. This is so extraordinarily -improbable that I need say nothing more about it. If -the Lamarckian principle requires this kind of hypothesis to bolster -it up, it is undoubtedly doomed.</p> - -<p>If we disregard altogether the positive aspect of Zehnder's -hypothesis, and assume that the skin-nerves are really stimulated -through the chitin by every strain and pressure to which a spot -of skin is exposed, and that they cause a correspondingly greater -secretion of chitin, which would then, according to the Lamarckian -principle, be hereditary, does this harmonize with what actually -occurs in the development of the skeleton as we know it in the -case of Insects and Crustaceans? Not at all! Can we suppose that -the carapace of a crab or the enormously hard wing-covers of a water-beetle -are exposed to a continual pounding and pressing and pushing? -Exactly the contrary is the case. Every assailant takes care not to -grasp the animal where it is so well protected, and seeks out the -most vulnerable parts for its attacks. It may be answered that, -while this is certainly the case now, the animals were badly protected -when the ancestral forms were evolving. But that they could not -have become hard by dint of being frequently bitten or otherwise -wounded should be obvious from the fact that the whole of the -wing-covers and the whole of the carapace is uniformly covered -with thick chitin, while each wound would only stimulate particular -spots; and we should also have to admit that, since these parts of -the skin which are now so well protected are no longer seized and -stimulated, they would long ago have become thin again, according -to the principle of the degeneration of parts no longer used, or, in -this case, no longer stimulated. But there is no need for wasting -time over such quibbles, since there is a fact which absolutely contradicts -Zehnder's hypothesis. I mean the degeneration of the chitinous -skeleton in those Crustaceans and Insects which protect the abdomen -within a shelter like the hermit-crabs, the caddis-flies (Phryganidæ), -(Fig. 107) and the sack-carrying caterpillars of the Psychidæ among -Lepidoptera. The hermit-crabs, as is well known, squeeze their -abdomen into a usually spirally-coiled Gasteropod shell, and they -always choose houses which are wide enough to conceal the whole<span class="pagenum"><a id="Page_105"></a>[Pg 105]</span> -body up to the hard claws when necessity arises. In this case there -is surely a continual pressure on the abdomen, which, being soft, -must be squeezed very tightly every time the animal retreats into -its shell. One of my opponents has described the disappearance of -the tough integumentary skeleton from the abdomen of these animals -as an inherited result of this pressure, and another regards it as the -inherited result of the degeneration of the muscles in this part of -the body. But, according to Zehnder, this continuous pressure, and -the frequent rubbing up and down of the abdomen on the inner -surface of the Gasteropod's shell, would undoubtedly have a stimulating -effect on the skin-nerves, and would therefore bring about -a thickening of the chitinous cuticle. In regard to the larval -Phryganidæ and Psychidæ, the case would be the same, though -perhaps hardly to the same degree, for while these larvæ make -their own houses, and will therefore at least make them big enough -to begin with, the pressure and friction must increase with the -growth of the animal.</p> - -<div class="figcenter" id="ff16"> -<img src="images/ff16.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 107.</span> Larva of a Caddis-fly, after Rösel. <i>A</i>, removed from its case, -showing the hooks (<i>h</i>) which attach it thereto, and the whitish abdomen, -covered only by a thin cuticle. <i>B</i>, the same larva, moving about with its case.</p> -</div> - -<p>If the regulation of the strength of the integumentary skeleton -be referred to selection, we see at once why carapace and wing-covers -should be of equal thickness throughout their whole extent, and why -they do not disappear, although they do not function actively, and -are less stimulated than any other parts of the skeleton; and we -also understand why the abdomen of hermit-crabs and of larval -Phryganidæ and Psychidæ has become soft, whether it be exposed -to pressure or friction in a greater or a less degree. It no longer -requires to be hard, because it is protected by the house, and in<span class="pagenum"><a id="Page_106"></a>[Pg 106]</span> -the case of the Pagurids it must not be hard because it could not -then be readily squeezed into the hard-walled and narrow recesses -of the Gasteropod shell; in this case there has therefore been positive -selection. I have not yet referred to the fact that the chitinous -covering is certainly not living, though it is not exactly dead; it -is a secretion of the epidermic cells, not a tissue, and we cannot -suppose that there are any nerve-endings in it. It is almost superfluous -to say that the fact that the skin is cast is in itself enough to -make such an assumption untenable, for the whole of the assumed -delicate nervous network would be shed at every moult and torn -away from the nerves which lead to it. As far as my knowledge -goes, nothing of this kind occurs anywhere in the whole range of -the animal kingdom.</p> - -<p>Even if we assume, for the benefit of Zehnder's hypothesis, that -although there are no nerves in the chitin itself yet irritations -affecting the chitinous coat may be transmitted through it to the -delicate nerve-endings lying beneath it, this should take place in -a greater degree at the thin places of the skeleton than <i>at the thick -parts</i>! But this interpretation is again fallacious, for we see that -the tactile organs of Arthropods always break through the chitinous -cuticle and protrude beyond it in the form of setæ.</p> - -<p>Of the many other opponents of my views in regard to the -transmissibility of acquired functional modifications, I need only -deal in detail with Oscar Hertwig.</p> - -<p>He seeks for direct proofs of an inheritance of acquired characters, -and believes that he has found these in the hereditary transmission -of acquired immunity from certain diseases. He reminds us of -Ehrlich's well-known experiments on mice with ricin and abrin.</p> - -<p>Even small doses of these two poisons kill mice, but they are -tolerated in very minute doses, and if their administration be continued -for some time in such minute doses, the animals gradually -acquire a high degree of insensitiveness to these poisons; they -become immune to ricin and abrin.</p> - -<p>This immunity is transmitted from mother to young, but it only -lasts for a short time, about six to eight weeks after birth. Yet -this is regarded by Hertwig as an illustration of the transmission -of an acquired character, as an acquired modification of the cells -of the body, for he explains the immunity on the assumption that -all the cells of the body undergo a particular variation due to the -influence of the poison, and are thus, to a certain extent, modified -in their nature, and that the ovum also undergoes this variation -and transmits it to the young animal. The immunization might<span class="pagenum"><a id="Page_107"></a>[Pg 107]</span> -certainly come under the category of functional modifications, and -it might be thought that we have here a case of transmission of such -an acquired character.</p> - -<p>Against this, however, we have to put the fact that <i>the acquired -immunity is not transmitted from the father to the descendants</i>. -Hertwig attempts to explain this by saying that the short duration -of the experiments has only allowed the poison to affect the cell-substance -(cytoplasm) and not the nucleus, that is, the hereditary -substance of the sperm-cells, an assumption which has little probability -considering the intimate nutritive relations between the cell-nucleus -and the cytoplasm. I should be rather inclined to conclude -from the difference in the transmitting power of the sperm and of -the ovum, that this 'inheritance of immunity' does not depend, as -Hertwig supposes, on a modification of the cells to 'ricin immunity,' -but, as Ehrlich and the bacteriologists believe, on the production of -so-called 'anti-toxins,' and that these anti-toxins are handed on to -the embryo not by the ovum itself, but by the interchange of blood -between mother and offspring which lasts throughout the whole -embryonic period. It is then self-evident why no transmission of -immunity through the father occurs.</p> - -<p>But it would lead me too far were I to attempt to refute all -the attempts that have been made to interpret individual cases as -due to the inheritance of acquired characters. I should, however, -like to say something as to the theoretical possibility of such an -assumption.</p> - -<p>When we try to conceive how experiences and their consequences -can be entailed, how new acquirements of the 'personal part can have -representative effects on the germinal part,' we find ourselves confronted -with almost, if not entirely, insuperable difficulties. How -could it happen that the constant exercise of memory throughout -a lifetime, as, for instance, in the case of an actor, could influence -the germ-cells in such a way that in the offspring the same brain-cells -which preside over memory will likewise be more highly -developed—that is, capable of greater functional activity? We know -what Zehnder's answer to such a question would be; he would make -the blood the intermediary between the brain-cells and the germ-cells, -but we have seen that specific food-stuffs for each specific cell-group -cannot be assumed, and that, even if they could, they would not -meet the necessities of the case. Yet every one who does not regard -the germ-plasm as composed of determinants is constrained to make -some such assumption. But if we take our stand upon the theory -of determinants, it would be necessary to a transmission of acquired<span class="pagenum"><a id="Page_108"></a>[Pg 108]</span> -strength of memory that the states of these brain-cells should be -communicated by the telegraphic path of the nerve-cells to the -germ-cells, and should there modify only the determinants of the -brain-cells, and should do so in such a way that, in the subsequent -development of an embryo from the germ-cell, the corresponding -brain-cells should turn out to be capable of increased functional -activity. But as the determinants are not miniature brain-cells, but -only groups of biophors of unknown constitution, and are assuredly -different from those cells; as they are not 'seed-grains' of the brain-cells, -but only living germ-units which, in co-operation with the rest -exercise a decisive influence on the memory-cells of the brain, I can only -compare the assumption of the transmission of the results of memory-exercise -to the telegraphing of a poem, which is handed in in German, -but at the place of arrival appears on the paper translated into -Chinese.</p> - -<p>Nevertheless, as I have said before, I do not disagree with those -who say, with Oscar Hertwig, that the impossibility of forming -a conception of the physiological nexus involved in the assumed -transmission does not <i>ipso facto</i> constrain us to conclude that the -transmission does not occur. I cannot, however, agree with Hertwig -that the case is exactly the same as in the 'converse process,' that is, -'in the development of the given invisible primary constituents in -the inheritance of the cell into the visible characters of the personal -part.' Certainly no one can state with any definiteness how the germ -goes to work, so that from it there arises an eye or a brain with its -millionfold intricacies of nerve-paths, but although the process cannot -be understood in detail, it can in principle, and this is just what -is impossible in regard to the communication of functional modifications -to the germ. Moreover, in addition to this, there is the very -important difference that, in the one case, we know with certainty -that the process actually takes place, although we cannot understand -its mechanical sequence in detail, while in the other we cannot even -prove that the supposed process is a real one at all. From the -fact that we are unable to form clear conceptions of a hypothetical -process, we are not justified, it seems to me, in assuming it to be real, -even though we are aware of many other processes in nature which -we are unable to understand.</p> - -<p>Nor does Hertwig take up this position, for he is at pains to -show the mechanical possibility of the process of inheritance which -he assumes, and he bases this upon the suggestions made by Hering -in his famous work <i>Ueber das Gedächtniss als eine allgemeine -Function der organisirten Materie</i> [<i>On memory as a general<span class="pagenum"><a id="Page_109"></a>[Pg 109]</span> -function of organized matter</i>], 1870. As this essay probably contains -the best that can be said in favour of a transmission of functional -modifications, and as it also includes some indisputable truths, we may -consider it in some detail.</p> - -<p>Hering is undoubtedly right in regarding 'the phenomena of -consciousness as functions of the material changes of organic substance, -and conversely.' That is, he believes that every sensation, -every perception, every act of will arises from material changes in -the relevant nerve-substances. But we know that 'whole groups -of impressions, which our brain has received through the sense-organs, -are stored up in it, as if resting, and below the margin of -consciousness, to be reproduced when occasion arises, in correct order -of space and time, and with such vividness, that we may be deceived -into regarding as a present reality what has long ceased to be present.' -There must therefore remain in the nerve-substance a 'material -impact,' a modification of the molecular or atomic structure, which -enables it 'to ring out to-day the note that it gave forth yesterday -if only it be rightly struck.'</p> - -<p>Hering attributes a similar power of memory and reproduction -to the germ-substance; he believes that he is justified in making -the assumption that acquired characters can be inherited, although -he admits that it 'appears to him puzzling in the highest degree' -how characters which developed in the most diverse organs of the -mother-being can exert any influence on the germ. That he may -be able to assume this he points to the interconnexion of all organs -by means of the nervous system; it is this that makes it possible -that 'the fate of one reverberates in the other, and that, when -excitement takes place at any point, some echo of it, however dull, -penetrates to the remotest parts.' To the delicate-winged communication -by means of the nervous system, which unites all parts -among themselves, must be added the general communication by -means of the circulation of the fluids of the body. According to -Hering's view, the germ experiences, in some degree, in itself all that -befalls the rest of the organs and parts of the organism, and these -experiences stamp themselves more or less upon its substance, just as -sense-impressions or perceptions stamp themselves upon the nerve-substance -of the brain, and these experiences are reproduced during -the development of the germ, just as the brain brings memory-pictures -back to consciousness. He says, 'If something in the mother-organism -has so changed its nature, through long habit or exercise repeated -a thousand times, that the germ-cell resting in it is also penetrated -by it in however weakened a fashion, when the latter begins a new<span class="pagenum"><a id="Page_110"></a>[Pg 110]</span> -existence, expands, and increases to a new being whose individual -parts are still itself and flesh of its flesh, it reproduces what it -experienced as part of a great whole. This is just as wonderful as -when an old man suddenly remembers his earliest childhood, but it -is not more wonderful than this.'</p> - -<p>But I think it is more wonderful. There exist demonstrably in -the brain thousands upon thousands of nerve-elements, whose activity -is a definite and limited one, because each particular visual impression, -for instance, only excites to activity certain definite nerve-elements, -and can leave memory-pictures in these alone. According to my conception -of it, the germ-plasm is quite as complex in its composition, -and does not consist of homogeneous elements, but of innumerable -different kinds, which are not related to the parts of the complete -organism indiscriminately, but only to particular parts. But is it -allowable to assume that there are invisible nerve-connexions, -not only to every germ-cell, but also within the germ-plasm, to -every determinant, like the nerve-paths which lead from the eye -to the nerve-cells in the optic-area of the brain? For if it -were otherwise, how could we conceive of the modification of an -organ—as, for instance, the ear-muscles in Man—communicating -itself to the precise determinants of these muscles in the germ-plasm? -I have often been met with the reproach that my conception -of the composition of the germ-plasm is much too complex—but the -complexity of Hering's suggestion seems to me to go a long way -beyond mine.</p> - -<p>Hering's ideas, which are not only ingenious but very stimulating, -might be accepted as the first indication of an understanding -of the assumed inheritance of functional modifications, if it could be -proved that such inheritance is a fact; but, as we have seen, that is -not the case. The assumption might be permitted, perhaps, if it -could be shown that certain groups of phenomena left no other possibility -of explanation open except this assumption, but that also, as far -as I can see, is not the case. Of course, others hold a different opinion, -but chiefly because they have rejected without much reflection the -sole explanation which presents itself for numerous phenomena—I -mean the processes which we are about to study under the name of -'germinal selection.' But, in any case, Hering's ideas seem to me very -valuable, because they make it apparent that, however much we know -of the organism, we only know it in a general way, and that numberless -delicate processes go on in it which leave no trace for our microscope, -and that we can only recognize the final results of numerous invisible -and often, in their subtlety, also unimaginable factors. This ought to<span class="pagenum"><a id="Page_111"></a>[Pg 111]</span> -be taken to heart, especially by all those who speak of simplicity in -reference to the germ-plasm. So much at least is certain: If there -were any inheritance of functional modifications, we should have -another proof that the germ-plasm is composed of determinants, for -without them there could be no possibility that the 'experiences' of -an individual organ would be transmitted to the germ in the way that -the Lamarckian principle implies. Something, and that something -material, must be modified in the germ-plasm if the vigorous use of -a group of muscles, or of a gland, or of a nerve-cell, is to be communicated -to the germ, and not to the whole germ-plasm, but only -to so much of it as is necessary to cause variation in the corresponding -group of cells in the child. It may perhaps be said that this still -does not necessitate the assumption of special determinants for these -cell-groups, and that one might, with Herbert Spencer, conceive of the -germ-plasm as consisting of homogeneous units which vary in the -development in accordance with the diverse regularly alternating -influences to which it is exposed from step to step, and that, therefore, -in each of these units of very complex structure only a single molecule, -or perhaps only a single atom, would need to vary in order that, in -the course of development, the resulting cell-group should appear in -the rudiment in somewhat altered strength.</p> - -<p>But I do not believe that a chemical molecule, still less an atom, -is sufficient for this, for reasons which I have already stated—yet we -need not go into this now, but rather deduce the consequences of this -admission. It follows that the 'unit' is made up of numerous 'molecules' -or 'atoms,' of which each, by dint of changes it has undergone, -causes particular parts of the body to vary in a definite manner; -in other words, we have here again a theory of determinants, only -they are on a much smaller scale, since each invisible little vital -particle or 'unit' contains all the determinants within itself, while in -my theory it is only the id, that is, the visible chromosome, which -includes the determinant complex. Such a theory would be far from -a simplification of mine, it would rather complicate it enormously, -and that without anything being gained. At most it would be made -more evident how inconceivably complex the nerve-paths must be -which lead from the part that has been modified by exercise to the -germ-plasm, and must also lead to all the innumerable 'molecules or -atoms' of the individual 'units.' But even on my theory of the composition -of the ids as aggregates of living determinants, such nervous -transmission of qualities would be a monstrosity which no one would -accept, and I think on this account that my argument as to the -impossibility of conceiving of the transmission of the modifications of<span class="pagenum"><a id="Page_112"></a>[Pg 112]</span> -the personal part to the germinal part retains its force, notwithstanding -Hering's interesting analogy.</p> - -<p>If the transmission of functional modifications were an indisputable -fact, I repeat, we should have to give in, and then we might regard -the 'memory of organized material' as affording a hint of the possibility -of the unimaginable process. But as long as the occurrence of -this transmission cannot be proved either directly or indirectly, such -a vague possibility of explanation need not induce us to assume -an unproved process.</p> -<hr class="full" /> - -<div class="chapter"> -<p><span class="pagenum"><a id="Page_113"></a>[Pg 113]</span></p> - -<h2 class="nobreak" id="LECTURE_XXV">LECTURE XXV</h2> -</div> - -<p class="c">GERMINAL SELECTION</p> - -<div class="blockquot"> - -<p>On what does disappearance after disuse depend, if not on the Lamarckian principle?—Panmixia—Romanes—Fluctuations -in the determinant-system of the germ-plasm -due to unequal nutrition—Persistence of germinal variations in a definite direction—The -disappearance of non-functioning parts—Preponderance of minus germinal variations—Law -of the retrogression of useless parts—Variation in an upward direction—Artificial -selection—Influence of the multiplicity of ids and of sexual reproduction—Personal -selection depends on the removal of certain id-variants—Range of influence -of germinal selection—Self-regulation of the germ-plasm, which is striving towards -stability—Ascending variation-tendencies may persist to excess—Origin of secondary -sexual characters—Significance of purely morphological characters—The markings of -butterflies.</p></div> - - -<p><span class="smcap">Now</span> that we have recognized that the assumption of a transmission -of functional modifications is not justifiable, let us discuss -some of the many phenomena to explain which many people believe -the Lamarckian principle to be indispensable, and let us inquire -whether we are in a position to give any other explanation of these. -How has it come about that the effects of use and disuse <i>appear</i> to be -inherited? Can we find a sufficient explanation in the principle of -selection, and in the natural selection of Darwin and Wallace?</p> - -<p>The answer to these two questions will be most quickly found -if we begin by seeking for an explanation of the disappearance of -a part when it ceases to be exercised.</p> - -<p>That this cannot lie in the Lamarckian principle we have already -learnt from the fact that passively functioning parts, such as -superfluous wing-veins, also disappear, and that the loss of the wings -and degeneration of the ovaries has taken place in worker ants, which -can transmit nothing because they do not reproduce.</p> - -<p>We might be inclined to regard this gradual disappearance and -ultimate elimination of a disused organ as a direct gain, on the -ground that the economy of material and space thus effected may be -of decided advantage to the individual animal and thereby also for the -maintenance of the species, and that those animals would have an -advantage in the struggle for existence in which the superfluous -organ was reduced to the smallest expression. But that would be -far from supplying us with a sufficient explanation of the phenomenon; -the individual variations in the size of an organ which is in<span class="pagenum"><a id="Page_114"></a>[Pg 114]</span> -process of degenerating are, even in extreme cases, far too slight to -have any selection-value, and I cannot call to mind a single case in -which the contrary could be assumed with any degree of probability. -What advantage can a newt or a crustacean living in darkness derive -from the fact that its eye is smaller and more degenerate by one -degree of variation than those of its co-partners in the struggle for -existence? Or, to use Herbert Spencer's striking illustration, how -could the balance between life and death, in the case of a colossus like -the Greenland whale, be turned one way or another by the difference -of a few inches in the length of the hind-leg, as compared with his -fellows, in whom the reduction of the hind-limb may not have gone -quite so far? Such a slight economy of material is as nothing -compared with the thousands of hundredweights the animal weighs. -As long as the limbs protrude beyond the surface of the trunk they -may prove an obstacle to rapid swimming, although that could hardly -make much difference, but as soon as the phyletic evolution had proceeded -so far that they were reduced to the extent of sinking beneath -the surface, they would no longer be a hindrance in swimming, and -their further reduction to their modern state of great degeneration -and absolute concealment within the flesh of the animal cannot be -referred even to negative selection.</p> - -<p>Years ago I endeavoured to explain the degeneration of disused -parts in terms of a process which I called Panmixia. Natural selection -not only effects adaptations, it also maintains the organ at the pitch -of perfection it has reached by a continual elimination of those -individuals in which the organ in question is less perfect. The longer -this conservative process of selection continues, the greater must be -the constancy of the organ produced by it, and deviations from the -perfect organ will be of less and less frequent occurrence as time -goes on.</p> - -<p>Now if this conservative action of natural selection secures the -maintenance of the parts and organs of a species at their maximum -of perfection, it follows that these will <i>fall below this maximum as -soon as the selection ceases to operate</i>. And it does cease as soon as an -organ ceases to be of use to its species, like the eye to the species of -crustacean which descends into the dark depths of our lakes, or to the -abyssal zones of the ocean, or into a subterranean cave-system. In -this case all selection of individuals ceases as far as the eye is -concerned; it has no importance in deciding survival in the struggle -for existence, because no individual is at a disadvantage through its -inferior eyes, for instance, by being in any way hindered in procuring -its food. Those with inferior organs of vision will, <i>ceteris paribus</i>,<span class="pagenum"><a id="Page_115"></a>[Pg 115]</span> -produce as good offspring as those with better eyes, and the consequence -of this must be that there will be a general deterioration of -eyes, because the bad ones can be transmitted as well as the good, -and thus the selection of good eyes is made impossible.</p> - -<p>The mixture thus arising may be compared to a fine wine to -which a litre of vinegar has been added; the whole cask is ruined -because the vinegar mingles with every drop of the wine. As -deviations from the normal are always occurring in every part of -every species, and among them some that lessen the value of the -organ, rarely perhaps at first, but after a time in every generation, -a sinking of the organ from the highest point of possible perfection -becomes inevitable as soon as the organ becomes superfluous. The -functional uselessness of the organ must go on increasing the longer -it is disused, as will readily be admitted if it be remembered that -only the most perfect adaptation of all the separate parts of an organ -can maintain its functional capacity, that all the parts of an organ -are subject to variation, and that every deviation from the optimum -implies a further deterioration of the whole. An eye, for instance, -can no longer vary in the direction of 'better' if it has already -reached the highest possible point of perfection; every further -variation must deteriorate it.</p> - -<p>Romanes gave expression to this idea, that the cessation of -natural selection alone must cause the degeneration of a part, a decade -before I did, but neither he nor the scientific world of his time -attached great importance to it, and it was forgotten again. This -was intelligible enough, for, at that time, the validity of the -Lamarckian principle had not been called in question, and therefore the -need for some other principle to explain the disappearance of disused -parts had not begun to be felt.</p> - -<p>I found myself in quite a different position. As my doubts -regarding the Lamarckian principle grew greater and greater, I was -obliged to seek for some other factor in modification, which should be -sufficient to effect the degeneration of a disused part, and for a time -I thought I had found this in panmixia, that is, in the mingling of -all together, well and less well equipped alike. This factor does -certainly operate, but the more I thought over it the clearer it -became to me that there must be some other factor at work as well, -for while panmixia might explain the deterioration of an organ, it -could not explain its decrease in size, its gradual wearing away, and -ultimate total disappearance. Yet this is the path followed, slowly -indeed, but quite surely, by all organs which have become useless. -If panmixia alone guided the deterioration of the organ, and it was<span class="pagenum"><a id="Page_116"></a>[Pg 116]</span> -thus only chance variations which were inherited through panmixia -and gradually diffused over the whole species, how could it come -about that all the variations were in the direction of smaller size? -Yet this is obviously the case. Why should no variations in the -direction of larger size occur? And if this were so, why should -a useless organ not be maintained at its original size, if it be admitted -that an increase in size would be prevented by natural selection? -But this never occurs, and diminution in size is so absolutely the -rule that the idea of a 'vestigial or rudimentary' organ suggests -a 'small organ' almost more than an 'imperfect' one.</p> - -<p>There must then be something else at work which causes the -minus-variations in a disused organ to preponderate persistently and -permanently over the plus-variations, and this something can lie -nowhere else than where the roots of all hereditary variations are to -be found—in the germ-plasm. This train of thought leads us to the -discovery of a process which we must call selection between the -elements of the germ-plasm, or, as I have named it shortly, <i>Germinal -Selection</i>.</p> - -<p>If the substance of the germ-plasm is—as we assumed—composed -of heterogeneous living particles, which have dissimilar rôles in the -building up of the organism, there must of necessity be among them -a definite labile state of equilibrium, which cannot be disturbed -without modifying in some way the structure of the organism itself -which arises from the germ-plasm. But if our further view be -correct, that these individual and different living units of the germ-plasm -are 'determinants,' that is, are the primary constituents of -particular parts of the organism, in the sense that these parts could -not arise if their determinants were absent from the germ-plasm, and -that they would be different if the determinants were differently -composed, we can draw far-reaching deductions.</p> - -<p>It is true that we cannot learn <i>anything directly</i> in regard to -the intimate structure of the germ-plasm, and even in regard to the -vital processes going on within it we can only guess a very little, -but so much we may say—that its living parts are nourished, and -that they multiply. But it follows from this that nourishment in -a dissolved state must penetrate between its vital particles, and that -whether the determinants grow, and at what rate they do so, depends -mainly on the amount of nourishment which reaches them. As long -as the germ-cells multiply by division the determinants have no other -function but to grow; a part of their substance undergoes oxidation -and thereby yields the supply of energy necessary to assimilation, -that is, to the formation of new living substance.</p> - -<p><span class="pagenum"><a id="Page_117"></a>[Pg 117]</span></p> - -<p>If each kind of determinant always secured the same quantity -of nourishment, all would grow in the same degree, that is, in exact -proportion to their power of assimilation. But we know that in -less minute conditions which we can observe more directly, there -is nowhere absolute equality, that all vital processes are subject to -fluctuations; any little obstacles in the current of the nutritive fluid, -or in its composition, may cause poorer nutrition of one part, better -of another. We may therefore assume that there are similar -irregularities and differences in the minute and unobservable -conditions of the germ-plasm likewise, and the result must be -a slight shifting of the position of equilibrium as regards size and -strength in the determinant system; for the less well-nourished -determinants will grow more slowly, will fail to attain to the size and -strength of their neighbours, and will multiply more slowly.</p> - -<p>But the vigour of growth does not depend only on the influence -of nourishment; one cell grows quickly, another slowly in the same -nutritive fluid; it depends in great part on the cell's power of -assimilation. In the same way the assimilating power of the -determinants and their affinity for nourishment will vary with -their constitution, and a weaker determinant will remain smaller -than a stronger one, even when the stream of nourishment is the -same.</p> - -<p>It seems to me that it is upon the unequal nutrition of the -determinants conditioned by the chances of the food-supply that -individual hereditary variability ultimately depends. If, for instance, -the determinant <i>A</i> receives poorer nourishment at a particular time -than the determinant <i>B</i>, it will grow more slowly, remain weaker, and -then, when the germ-cells develop into an animal, the part to which -it gives rise will be weaker than it usually is in other individuals.</p> - -<p>These primary inequalities in the equipment of the determinants -which are caused by a passing inequality in the food-stream are, -of course, so slight that we are unable to observe their consequences. -They must persist for a considerable time before they become -observable, but they may persist for a long time, and their effect -must then mount up, because every diminution in the strength of -the determinant also signifies a lessened power of assimilation, and -growth becomes slower for the twofold reason that passive and -active nutrition decrease at the same time. In the less minute -conditions observable in the histological elements of the body we -know that function strengthens the organ, and that disuse weakens -it, and we are justified in applying this proposition also to these -more intimate conditions and minuter vital units. Thus, in the<span class="pagenum"><a id="Page_118"></a>[Pg 118]</span> -course of the multiplication of the germ-cells, the less vigorously -working determinant, <i>A</i>, will gradually, but very slowly, become -weaker, that is, of diminished power of assimilation, presupposing -of course that the intra-germinal food-stream does not become -stronger again at the same place—a possibility to which I shall -subsequently refer. But while one determinant may be slowly -becoming weaker, its neighbour, on the other hand, may be varying -on an ascending scale, just because the former is, on account of its -diminished power of assimilation, no longer able to exhaust -completely the food-stream which flows to it.</p> - -<p>The determinants are thus in constant motion, here ascending, -there descending, and it is in these fluctuations of the equilibrium -of the determinant-system that I see the roots of all hereditary variation, -while in the fact that the variation-directions of particular -determinants must continue the same without limit as long as they -meet with no obstacle lies the possibility of the adaptation of the -organism to changing conditions, the increase and transformation -of one part, the degeneration and disappearance of another, in short, -the processes of natural selection. The reason why such variation -movements must continue until they meet some resistance is that -every chance upward or downward movement—due, that is, to mere -passive fluctuation in the food-supply, at the same time strengthens -or weakens the determinant, and makes it either more or less capable -of attracting nourishment to itself; in the former case an increasingly -strong stream of food will be directed towards it, in the latter more -and more of the available food-supply will be withdrawn from it -by its neighbour-determinants on all sides; in the former the -determinant will go on increasing in strength as long as it can go -on attracting more nourishment, in the latter it will continue to -become weaker until it disappears altogether. To the ascending -progression, as is evident, there are limits set, not only by the -amount of food which can circulate through the whole id, but also -by the neighbour determinants, which will sooner or later resist -the withdrawal of nourishment from them; but for the descending -progression there are no limits except total disappearance, and this -is actually reached in all cases in which the determinants are related -to a part which has become useless. But both these movements, -the upward and the downward alike, are quite independent of natural -selection, i.e. of personal selection; they are processes of a unique kind -which run their course purely in accordance with intra-germinal laws. -Whether a determinant 'ascends' or 'descends' depends solely upon -the play of forces within the germ-plasm, not upon whether the<span class="pagenum"><a id="Page_119"></a>[Pg 119]</span> -direction of the variation in question is useful or prejudicial, or -on whether the organ in question, the determinate, is of value or -otherwise. In this fact lies the great importance of this play of -forces within the germ-plasm, that it gives rise to variations quite -independently of the relations of the organism to the external world. -In many cases, of course, personal selection intervenes, but even then -it cannot directly effect the rising or falling of <i>the individual</i> -determinants—these are processes quite outside of its influence—but -it can, by eliminating the bearers of unfavourably varying -determinants, set a limit to further advance in such directions. -This we shall consider in more detail later on. Personal selection -operates by removing unfavourably varying individuals from the -genealogical tree of the species, but at the same time the determinants -which are varying unfavourably are also removed, and their variation -is thus put a stop to for all time.</p> - -<p>I have called these processes which are ceaselessly going on -within the germ-plasm, Germinal Selection, because they are -analogous to those processes of selection which we already know -in connexion with the larger vital units, cells, cell-groups and -persons. If the germ-plasm be a system of determinants, then -the same laws of struggle for existence in regard to food and -multiplication must hold sway among its parts which hold sway -between all systems of vital units—among the biophors which form -the protoplasm of the cell-body, among the cells of a tissue, among -the tissues of an organ, among the organs themselves, as well as -among the individuals of a species and between species which compete -with one another.</p> - -<p>If this be the case, we have here ready to hand the explanation of -every heritable variation of a part, ascending and descending alike. -Let us consider for a little the latter category—that is, the disappearance -of functionless <i>or useless organs</i>. It is clear that, from the -moment in the life of a species that an organ, <i>N</i>, becomes useless, -natural selection withdraws her hand from it; individuals with better -or worse organs <i>N</i> are now equally capable of life and struggle, the -state of panmixia is entered upon, and the organ <i>N</i> of necessity falls -somewhat below its previously attained degree of perfection.</p> - -<p>That this must be so will be admitted when it is remembered that -each organ of a species is only maintained at its highest level because -personal selection keeps ceaseless watch over it, and sets aside all the -less favourable variations by eliminating the individuals which -exhibit them. But this is no longer the case with a useless organ. -When a weaker variant of a disused organ arises through the intra<span class="pagenum"><a id="Page_120"></a>[Pg 120]</span>-germinal -fluctuations of nutrition, this is transmitted to the -descendants just as well as the normally developed organ, and in -the course of generations will be inherited by a greater and greater -number of individuals, and must ultimately be inherited by all in -some degree or other. The objection has been urged from many -sides that variations upwards would be quite as likely to arise as -those downwards, but this is an error. Even if, at the beginning, -the minus-variations were rarer than the plus-variations, in the -course of generations the minus ones would preponderate because -ascending variations of disused organs are not indifferent for the -organism but injurious to it. Perhaps an increase in the size of the -organ itself would do no harm, but in that of its determinant -it certainly would, because an ascending determinant requires more -nourishment than previously, and withdraws it from its surroundings, -and thus from the determinants in its immediate neighbourhood; -but these are those of functioning and indispensable parts. Individuals -in whose germ-plasm the determinants of disused organs ascend, and -thereby depress the determinants of organs which are still active, -are subject to personal selection, and are eliminated. There thus -remain only those with descending determinants; in other words, -the chance of variants in the direction of weakness in useless determinants -far outweighs that of variants in the direction of increased -strength; the latter will soon cease to occur at all, for as soon -as a determinant has fallen a little below its normal level, it finds -itself upon an inclined plane, along which it glides very slowly -but steadily downwards. This might be disputed if it could be -maintained that, at every stage of the descent, a change of direction was -possible. But this probably takes place rarely and only in the case -of individual ids, and will therefore not be permanent because -in general the stronger neighbour determinants will possess themselves -of the superfluous nourishment, and a lasting ascent will thus -be impossible to the weakened determinant. This is precisely what -I have called Germinal Selection. The determinant whose assimilating -power is weakened by ever so little is continually being robbed -by its neighbours of a part of the nourishment which flows towards -it, and must consequently become further weakened. As no more -help will be given to it by natural selection, since the organ is no -longer of any value to the species, the better among the weakened -determinants of <i>N</i> are never selected out, and they must gradually give -way in the struggle with the neighbouring determinants which are -necessary to the species, becoming gradually weaker and ultimately -disappearing.</p> - -<p><span class="pagenum"><a id="Page_121"></a>[Pg 121]</span></p> - -<p>This process can, of course, no more be proved mathematically -than any other biological processes. No one who is unwilling to -accept germinal selection can be compelled to do so, as he might be to -accept the Pythagorean propositions. It is not built up from beneath -upon axioms, but is an attempt at an explanation of a fact established -by observation—the disappearance of disused parts. But when once -the inheritance of functional modifications has been demonstrated -to be a fallacy, and when it has been shown that, even with the -assumption of such inheritance, the disappearance of parts which are -only <i>passively</i> useful, and of any parts whatever in sterile animal forms, -remains unexplained, he who rejects germinal selection must renounce -all attempt at explanation. It is the same as in the case of personal -selection. No one can demonstrate mathematically that any variation -possesses selection value, but whoever rejects personal selection gives -up hope of explaining adaptations, for these cannot be referred to -purely internal forces of development.</p> - -<p>The total disappearance of a part which has become useless takes -place with exceeding slowness; the whales, which have existed as such -since the beginning of the tertiary period, have even now not -completely lost their hind-limbs, but carry them about with them -as rudiments in the muscular mass of the trunk, and the birds, which -are even older, still show in their embryonic primordia the five fingers -of their reptilian forefathers, although even their bird-ancestors of the -Jurassic period, if we may argue from <i>Archæopteryx</i>, had only three -fingers like our modern birds. A long series of similar examples -might be given, and modern embryology in particular has contributed -much that, like this example of birds' fingers, points to a certain -orderliness in the disappearance of the individual parts of an organ -which has become superfluous. Parts which, in the complete animal, -have disappeared without leaving a trace, appear again in each -embryonic primordium, and disappear in the course of the ontogeny. -Speaking metaphorically, we might express this on the basis of the -determinant theory, by saying that the determinants, as they become -weaker, can only control an increasingly short period of the whole -ontogeny of the organ, so that ultimately nothing more than -its first beginning comes into existence. But this is only a -metaphor; we cannot tell what really happens as long as we are -ignorant of the physiological rôle of the determinants, and even of -the laws governing the degeneration of a useless organ. In respect -of the latter, much might still be achieved if comparative anatomy -and embryology were studied with this definite end in view, and -perhaps we should even be able to draw more definite conclusions<span class="pagenum"><a id="Page_122"></a>[Pg 122]</span> -in regard to the composition and activity of the determinants in -the germ.</p> - -<p>In the meantime we must be content with the knowledge that, -on the determinant hypothesis, the disappearance of organs which have -become useless may be regarded as a process of intra-selection going -on between the 'primary constituents' (<i>Anlagen</i>) of the germ, and -depending on the same principle of the 'struggle of parts' which -William Roux introduced into science with such brilliant results. -If a struggle for food and space actually takes place, then every -passive weakening must lead to a permanent condition of weakness -and a lasting and irretrievable diminution in the size and strength -of the primary constituent concerned, unless personal selection -intervenes, and choosing out the strongest among these weakened -primary constituents, raises them again to their former level. But -this never happens when the organ has become useless.</p> - -<p>This explains why not only parts with active function, like limbs, -muscles, tendons, nerves, and glands, disappear when they cease to -function, but also passive parts like the colouring of the external -surfaces of animals, the lifeless skeletal parts of Arthropods and the -exact adaptation of their thickness to the dwindling function, the -disappearance of superfluous wing-veins, and of the hard chitinous -covering of the abdomen when it is concealed in a protecting house, -as in the case of hermit-crabs, Phryganidæ, and Psychidæ. Here too -we find a sufficient explanation of the fact that parts which have -become functionless, such as the wings of ants, can disappear even in -the case of sterile workers.</p> - -<hr class="tb" /> - -<p>The principle of germinal selection, however, can only be understood -in its full significance if we take the positive aspect also into -consideration. We had reached the conclusion that because of the -fluctuations of the food-supply one set of the homologous determinants -represented in the various ids may vary in a minus direction, and -another set in a plus direction, and that this direction will be adhered -to as long as no intra-germinal obstacles come in the way. As long -as this does not happen the determinant concerned will pursue the -path of variation it has once struck out, and indeed the tendency -will be strengthened, because every passive variation, upwards or -downwards, results in a strengthening or weakening of the determinant's -power of assimilation.</p> - -<p>Let us take a case of positive variation of the determinants of an -organ <i>N</i>, which would be more useful to the species if it were more -highly developed than it had previously been. The variation in an<span class="pagenum"><a id="Page_123"></a>[Pg 123]</span> -upward direction is at first purely passive, having arisen from -fluctuations in the food-supply, but it soon becomes active, since -the determinants that have become stronger will have a stronger -affinity for food and will attract more and more of the available -supply. The increased food-stream is thus maintained, and its gradual -result is such a strengthening of the determinants in the course -of generations of germ-cells, that the parts controlled by these -determinants—the determinates—must enter on a path of plus-variations. -If to this there be added personal selection, either natural -or artificial, any fluctuations of this primary constituent towards the -minus side will be effectually prevented, the direction of variation -will remain positive, and the continued intervention of personal selection -may raise its development to its possible maximum, that is, so far -that further development in the same direction would not make -for greater fitness, and personal selection must call a halt. This will -always happen as soon as further increase of the organ would be -prejudicial to the living power of the whole, and when the harmony -of the bodily parts would thereby be permanently disturbed.</p> - -<p>That variation in an upward direction really can persist for -a long time is shown by artificial selection as practised by Man in -regard to his domesticated animals and cultivated plants. At first -general variability, or at least variability in many directions, sets in -as a result of the greatly altered conditions of life; the ordinary -fluctuations of the determinants are intensified by the greater fluctuations -in the nutritive stream, and it becomes possible for Man -consciously or unconsciously to select for breeding whatever he -prefers among the chance variations that arise in individual parts -or in whole complexes of parts, and he may thus give rise to -a long-continued, often apparently unlimited, augmentation of variations -in the same direction, although he cannot exercise <i>any direct</i> -influence upon the germ-plasm or its determinants. When a determinant -has assumed a certain variation-direction it will follow it -up of itself, and selection can do nothing more than secure it a free -course by setting aside variations in other directions by means of -the elimination of those that exhibit them.</p> - -<p>That artificial selection can cause the increase of a part has long -been established, but in what way this is possible, and how it can -be theoretically explained has hitherto been very obscure, for even -if we take the favourable case that both parents possess the desired -variation, it cannot be supposed that the characters of the parents are, -so to speak, added together in the child; all we can say is that -the probability that the children will also exhibit the character in<span class="pagenum"><a id="Page_124"></a>[Pg 124]</span> -question—for instance, a long or crooked nose—becomes greater. -Certainly an increase of the character may result if in both parents -the determinants <i>K</i> are present in excess as compared with the heterodynamous -determinants <i>K´</i> and <i>K´´</i>, for in that case there is an -increased probability that, through reducing divisions and amphimixis, -there will again be a preponderance of the determinants <i>K</i> composing -the germ-plasm of the child, and further, that these determinants <i>K</i> -will dominate strongly as compared with the few <i>K´</i>'s. It may thus -happen that the long nose of the two parents will give rise to a still -longer nose in the child, or that parents of considerable bodily size -may have still bigger children, but such increase would be confined to -one generation, and would not lead to a permanent increase of the -character; permanent increase cannot depend merely on the number -of the determinants <i>K</i> and on their supremacy over their converse, -the determinants <i>K´</i>; it must also depend on their own variation, -and this again can depend only on germinal selection and not upon -personal selection, although the former can be materially assisted by -the latter.</p> - -<p>That inheritance from both parents is only a secondary consideration -in regard to the increase of a part by artificial selection -is made evident by the fact that <i>many secondary sexual characters</i> -have been modified, although the breeder selected only in regard to -one parent. Nevertheless in this very domain the greatest results -have been achieved; witness the Japanese breed of cocks with tail-feathers -six feet long. This astonishing result has been reached by -the strictest selection of the cocks in which the feathers were a little -longer than those of other cocks, and the increase in the length of -feathers depended—according to our theory—simply on the fact that, -by the selection of the determinants which were already varying in -the direction of increased length, this process of increase was guarded -from interruption by chance unfavourable conditions of nutrition. -The continuance of variation in the upward direction in which it had -already started is not effected directly by personal selection, but is so -indirectly, for without this constant fresh intervention of selection -the increase would be apt to come to a standstill, or the variation -might even take a contrary direction. There are two other factors -operative to which we have not yet given sufficient attention. They -are, the multiplicity of the ids in every germ-plasm, and sexual reproduction.</p> - -<p>If—as we must assume—each germ-plasm is made up of several -or many ids, there must be several or many determinants of each part -of the organism, for each id contains potentially the whole organism,<span class="pagenum"><a id="Page_125"></a>[Pg 125]</span> -though with some individuality of expression. The child is thus not -determined by the determinants of a single id, but by those of many -ids, and the variations of any part of the body do not depend on the -variations of a single determinant <i>X</i>, but on the co-operation of all -the determinants <i>X</i> which are contained in the collective ids of the -relevant germ-plasm. Thus it is only when a majority of the -determinants have varied upwards or downwards that they dominate -collectively the development of the part <i>X´</i> and cause it to be larger -or smaller.</p> - -<p>We have assumed passive fluctuations in nutrition to be the first -cause in individual variation, and it is obvious that the action of this -first cause of dissimilarity must be greatly restricted by the multiplicity -of the ids and the corresponding homologous determinants. -For although passive fluctuations in nutrition should occur continually -in the case of all determinants, this would not imply that they -would follow the same direction in all the determinants <i>X</i> of all -ids, for some determinants <i>X</i> might vary upwards, and others -downwards, and these might counteract each other in ontogeny; so -that in many cases the fluctuations of the individual determinants -will not be felt in their products at all. But since there are—as we -shall see later—only two directions of variation, upwards and -downwards, plus and minus, it must also sometimes happen that -a majority take one direction, and this affords the basis on which -germinal selection can build further, and on which it is materially -supported by reducing division and the subsequent amphimixis.</p> - -<p>For reducing division removes half of the ids and thus of -the determinants from the mature germ-cell, and according as chance -leaves together or separates a majority of <i>X</i>-determinants varying -in the same direction, this particular germ-cell will contain the -primary constituents of a plus- or of a minus-variation of <i>X</i>, and it -is possible that the presence of a majority or a minority may be -entirely due to the reduction. The germ-plasm of the parent may -contain, for instance, the determinant <i>X</i> in its twenty ids 12 times -in minus-variation form, 8 times in plus-variation form; and the -reducing division, according to our view, may separate these into two -groups of which one contains eight plus- and two minus-variations, -the other ten minus-variations, or the one six plus- and four minus-variations, -the other two plus- and eight minus-variations, and so on. -Now every germ-cell which contains a majority of plus- or minus-variations—and -this must be the case with most of them—may unite, -if it attains to amphimixis, with a germ-cell which also contains -a majority of plus or minus <i>X</i>-determinants, and if similar majorities<span class="pagenum"><a id="Page_126"></a>[Pg 126]</span> -let us say plus—meet together, the plus-variation of <i>X</i> must be all the -more sharply emphasized in the child.</p> - -<p>Thus, although the individual determinants <i>X</i> may not be incited -to further variation by their co-operation with others varying in the -same direction, the collective effect of the plus-determinants will be -greater, and adherence to the same direction of variation in the -following generation will be assured, for if in the germ-plasm of the -parent there be, for instance, sixteen out of twenty determinants -possessing the plus-variation, a minus-majority can no longer result -from reducing division.</p> - -<p>It is upon this that the operation of natural selection, that is, -personal selection, must depend—that the germ-plasms in which the -favourable variation-direction is in the majority are selected for -breeding, for it is this and nothing else that natural selection does -when it selects the individuals which possess the preferred variations. -The ascending process is thus considerably advanced, because the -opposing determinants are more and more eliminated from the germ-plasm, -till the preferred variations of <i>X</i> are left, and among these, as -ascent in the direction begun continues, the opposing variations are -again set aside by germinal selection, and so on. Reducing divisions -and amphimixis are thus powerful factors in furthering the transformations -of the forms of life, although they are not the ultimate -causes of these.</p> - -<p>Now that we have made ourselves familiar with the idea of -germinal selection we shall attempt to gain clearness as to what it can -do, and how far the sphere of its influence extends, and, in particular, -whether it can effect lasting transformations of species without the -co-operation of personal selection, and what kind of variations we may -ascribe to it alone.</p> - -<p>First, I must return for a moment to the question we have -already briefly discussed—whether the variation of a determinant -upwards or downwards must so continue without limit. We might -be inclined to think that the great constancy which many species -exhibit was a plain contradiction of this, for if every minute variation -of a determinant necessarily persisted without limit in the same -direction, we should expect to find all the parts of the organism in -a state of continual unrest, some varying upwards, some downwards, -always ready to break the type of the species. Must there not be -some internal self-regulation of the germ-plasm which makes it -impossible that every variation which crops up can persist unlimitedly? -Must there not be some kind of automatic control on the part -of the germ-plasm, which is always striving to re-establish the state<span class="pagenum"><a id="Page_127"></a>[Pg 127]</span> -of equilibrium that has once been attained by the determinant system -whenever it is disturbed?</p> - -<p>It is difficult to give any confident answer to this question. We -cannot reach clearness on this point through our present knowledge of -the germ-plasm, because we possess no insight into its structure; we -can only draw conclusions as to the processes in the germ-plasm from -the observed phenomena of variation and inheritance. But two facts -stand in direct antithesis to one another, first, the high power of -adaptation possessed by all species, and the undoubted occurrence of -unrestricted persistence in a given direction of variation, as seen in -artificial selection, and in the disappearance of parts which have -ceased to function; and, secondly, the great constancy of old-established -species which do indeed always exhibit a certain degree of -individual variability, but without showing marked deviations as -a frequent occurrence or in all possible directions, as they certainly -would if every determinant favoured by a chance increase in the -nutritive stream necessarily and irresistibly went on varying further -in the same direction. Or can the constancy of such species be maintained -solely by means of personal selection, which is continually -setting aside all the determinants which rise above the selection-value -by eliminating their possessors? I was for long satisfied that this -was the true solution of the difficulty, and even now I do not doubt -that personal selection does, in point of fact, maintain the constancy of -the species at a certain level, but I do not believe that this is sufficient, -but rather that it is necessary to recognize an equalizing influence -due to germinal selection, and to attribute to this a share in maintaining -the constancy of a species which has long been well adapted. -I am led to this assumption chiefly by the phenomena of variation in -Man, for we find in him a thousand kinds of minute hereditary -individual variations, of which not one is likely to attain to selection -value. Of course the constant recurrence of reducing divisions -prevents any particular id which contains a varying determinant from -being inherited through many generations; for so many ids are being -continually removed from the genealogical tree by the constant -rejection of the half of all ids of every germ-plasm, that only a small -part of the ancestral id remains in the grandchild, great-grandchild, -and so on. Certainly some of the ids of the ancestors compose the -germ-plasm of the descendants, and if all the determinants of one -of these ids had begun to vary persistently upwards or downwards in an -ancestor, then all the determinants of the relative id in the descendants -would possess the variation in an intensified degree; and however -slowly the variation advanced it would attain selection-value in some<span class="pagenum"><a id="Page_128"></a>[Pg 128]</span> -one or other of the descendants, and would thus break the previously -stable type of the most perfectly adapted species. The descendant in -question would then succumb in the struggle for existence. But as -the number of the determinants in the germ-plasm is probably much -greater than that of the descendants of one generation, every descendant -would in the course of time deviate unfavourably in some one character -from the type of the species, and then either all the descendants would be -eliminated or the type would become unstable. But neither of these -things happens, and there are undoubtedly species which remain -constant for long periods of time, therefore the assumption must be -false and every variation of a determinant does not of necessity -go on in the same direction without limit.</p> - -<p>I therefore suppose that although slight variations are ceaselessly -taking place upwards or downwards in all determinants, even in -constant species, the majority of these turn again in the other direction -before they have attained to any important degree of increase, at -least in the germ-plasm of all species which have had a definitely -established equilibrium for thousands of generations. In such a germ-plasm, -or to speak more precisely, in the id of such a germ-plasm, -marked fluctuations in the nutritive stream will not be likely to -occur as long as the external conditions are unchanged, but slight -fluctuations, which will not be wanting even here, may often alternate -and turn in an opposite direction, and thus the upward movement of -a determinant may be transformed into a downward one. Every -determinant is surrounded by several others, and we can imagine that -the regular nutritive stream which we have assumed may be partially -dammed up by a slight enlargement of the determinant, and that this -will drive the surplus back again. But however we may picture -these conditions, which are for all time outside of the sphere of observation, -the assumption of a self-regulation of the germ-plasm, up to -a certain degree, cannot be regarded as inconceivable or unphysiological.</p> - -<p>But there are limits to this self-regulation; as soon as the -increase or decrease of a determinant attains a certain degree, as soon -as it has got beyond the first slight deviation, it overcomes all -obstacles, and goes on increasing in the direction in which it has -started. This must happen even in the case of old and constant -species, and frequently enough to admit of an apparent capacity for -adaptation in all directions. Every part of a species can vary beyond -the usual individual fluctuations, and as this is possible only by means -of intra-germinal processes, we must assume that even in the case of -germ-plasms which have long remained in a state of stable equili<span class="pagenum"><a id="Page_129"></a>[Pg 129]</span>brium -there may occasionally be marked fluctuations in the nutritive -stream, and thus more than usually pronounced variations of the determinants -affected by it will occur. These yield the material for new -adaptations if they are in the direction of fitness, or they are eliminated -either by the chances of reducing division or by personal selection if -new adaptations are not required.</p> - -<p>The old-established hereditary equilibrium of the germ-plasm -must be most easily disturbed when the species is in some way -brought into new conditions of existence, as, for instance, when plants -or animals are domesticated, and when in consequence, as we have -already assumed, the nutritive currents within the id gradually alter, -quantitatively and qualitatively; and on this account alone certain -kinds of determinants are favoured, while others are at a disadvantage. -In this way there arises the intensified general variability of domesticated -animals and cultivated plants which has been known since the -time of Darwin. Something analogous to this must occur in natural -conditions, though more slowly, when a species is subjected to a -change of climatic conditions, but we shall discuss this later on in -more detail.</p> - -<p>We have thus arrived at the idea that the slight variations of the -determinants may be counteracted whether they be directed upwards -or downwards, and that in the case of so-called constant species -they do frequently equalize themselves; but that more marked -variations, produced by more pronounced nutritive fluctuations, may -in a sense go on without limit, and then can only be restricted -and controlled by personal selection, that is, by the removal of the -ids concerned from the genealogical lineage of the species.</p> - -<p>In one direction variation can be proved to go on without limit, -and that is downwards, as is proved by the fact of the disappearance -of <i>disused organs</i>, for here we have a variation-direction, which -has been followed to its utmost limit, and which is completely independent -of personal selection; it proceeds quite <i>uninterfered with</i> by -personal selection, and is left entirely to itself. It is a significant fact -that the disappearance of the individual parts of a larger organ, according -to all the data that are as yet available, proceeds at a very -<i>unequal rate</i>, so that it evidently depends to a great extent on chance -whether a disused part begins to degenerate sooner or later. Thus in -one of the Crustaceans living in the darkness of the caves of North -America the optic lobes and optic nerves have disappeared, while the -retina of the eye, the lens, and the pigment have been retained, and -in others the reverse has taken place, and the nerve-centres have persisted -while the parts of the eye have been lost (Packard). Variations<span class="pagenum"><a id="Page_130"></a>[Pg 130]</span> -of the relevant determinants towards the minus direction may thus -occur, sometimes sooner, sometimes later; but when once they have -started they proceed irresistibly, though with exceeding slowness.</p> - -<p>But variation in an upward direction also, when it has once been -set a-going, may in many cases go on unchecked until limits are set to -it by personal selection, when the excess of the organ would disturb -the harmony of the parts, or in any other way lessen the individual's -chances of survival in the struggle for existence. This is proved -especially by the phenomena of artificial selection, for almost all the -parts of fowls and pigeons have been caused to vary to excess by -breeding, and must thus have been, so to speak, capable of unlimited -increase; and yet, as we have seen, personal selection cannot directly -cause progress in any direction of variation; it can only secure a free -course by excluding from breeding the bearers of variations with an -opposite tendency. The beards of hens, the tail-feathers of the long-tailed -domestic cocks, the long and short, straight and curved bills of -pigeons, the enormously long ruffled feathers of the Jacobin, the multiplication -of the tail-feathers in the fan-tail, and innumerable other -breed-characters of these playthings of the breeder, prove that when -variation-tendencies of any part are once present, that is, when -they have arisen through germinal selection, they apparently go on -unchecked until their further development would permanently and -irretrievably destroy the harmony of the parts. As soon as this is -threatened the breed loses its power of survival, and Darwin in his -time cited the case of many extremely short-billed breeds of pigeon, -which require the aid of the breeder before they can emerge from the -hard-shelled egg, because their short and soft bills no longer allow -them to break their way out. Here the correlation between the -hardness of the egg-shell and that of the pigeon's bill has been -disturbed, and the breed can now only be kept in existence by -artificial aid.</p> - -<p>There must be a possibility of something similar occurring in -natural conditions, and when it does the species concerned must die -out. But in the majority of cases the self-regulation which is afforded -by personal selection will be enough to force back an organ which is -in the act of increasing out of due proportion to within its proper -limits. The bearers of such excessively increased determinants -succumb in the struggle for existence, and the determinants are -thus removed from the genealogical lineage of the species.</p> - -<p>Having now established the fact that determinants can continue -their direction of variation without limit because of internal, that is -intra-germinal, reasons, we have come nearer an understanding of<span class="pagenum"><a id="Page_131"></a>[Pg 131]</span> -many secondary sexual characters, whose resemblance to the excessive -developments artificially produced in our domestic poultry is so very -striking. Here, too, we shall have to regard germinal selection as the -root of the variations of plumage and other distinguishing characters, -which have evolved by intra-germinal augmentation into the magnificently -coloured crests, tufts, and collars, into the long or graduated, -multiplied or erectile tail-feathers of the birds of Paradise, pheasants, -and humming-birds. The conception of sexual selection formulated -by Darwin will be so far modified, that we are no longer -compelled to regard every minute step in this cumulative process as -due to the selection of the males by the females. A preference of the -finest males may still take place, and is probably general, since only -thus could the distinguishing male characters become common property, -that is, be transmitted to all or the majority of the ids -of the germ-plasm, but the increase of the individual determinants -which are in the act of varying goes on in each individual id, quite -independently of this personal selection.</p> - -<p>As it is not a single id with its determinant <i>a</i> in ascending variation -that controls the organ <i>A</i>, but as it always requires a majority of -the ids <i>a</i>, this must be secured here by personal selection just as it is -in ordinary natural selection. If the handsomest males are the successful -competitors, then a majority of the transformed ids <i>a´</i> will -be transmitted to a number of their descendants, and the oftener this -happens the larger will the majority be, and the less becomes the -danger that it will be dispersed again by reducing division and amphimixis. -Personal selection is thus in no way rendered superfluous by -germinal selection, only it does not produce the augmentation of the -distinguishing characters, but is chiefly instrumental in fixing them in -the germ-plasm; it collects, so to speak, only the favourably varying -ids, and, where complex variations depending on the proper variation -of many ids are concerned, it combines these. How very great the -influence of personal selection may be in this case of secondary sexual -characters we see clearly from the soberly coloured mates of the -brilliant males, for here natural selection has been operative in -conserving the coloration inherited from remote ancestry.</p> - -<p>But if the question be asked, how <i>the first majority</i> of determinants -varying in the same direction is brought about, there are two -possibilities: first, by chance, and secondly, by influences which cause -particular determinants of all the ids to vary in almost exactly the -same manner. We shall find illustrations of the latter among climatic -varieties; but the cases of the first kind are the more important, for they -form the foundation and the starting-point for processes of selection<span class="pagenum"><a id="Page_132"></a>[Pg 132]</span> -of a higher order, for personal selection. It might seem perplexing -that processes of such importance should depend ultimately upon -chance; but when we remember that there are only two directions of -variation, namely a plus direction or a minus direction, we recognize -that the chance of a majority in one direction or another is much -greater than that of absolute equilibrium between the two, and there -is therefore a very strong probability that in many individuals of the -species either the upward or the downward movement of a determinant -<i>A</i> will preponderate.</p> - -<p>Now as such variation movements, when they are of a certain -strength, increase automatically, we can easily see that they must -gradually attain to a level at which they acquire selection value, and -how then, by personal selection, the ids with favourably varying -determinants may be collected together.</p> - -<p>Of course it is not possible to state positively the time at which -in individual cases a variation acquires a biological significance, that -is, selection value. We can only say in a general way that, as soon as it -attains this, personal selection either in a positive or a negative sense -<i>must</i> intervene; an injurious variation tends to the elimination of its -possessor, a useful one increases the probability of its survival.</p> - -<p>There must, however, be for every variation a stage of development -in which it has as yet no decisive biological importance, and this -stage need not by any means be so insignificant that we cannot see it, -or can hardly do so: in other words, there are characters which have -arisen through germinal selection, which are of purely 'morphological -importance.'</p> - -<p>It has often been disputed whether there can be any such thing -as 'purely morphological characters,' which are indifferent as far as -the existence of the species is concerned. This question used to be an -important one, because the sphere of operation, and therefore the -importance of the Darwin-Wallace selection—personal selection—depends -on the answer, since this mode of selection only begins when -a character has some biological importance. But as soon as we take -germinal selection into consideration the question loses its importance, -because we now know that every variation is indifferent to begin with, -but every one can, under favourable circumstances, be increased to such -a pitch that it attains biological importance, and that personal selection -then takes over the task of carrying it on, either in a positive or -a negative sense. We may therefore leave this disputed point alone -just now, for while germinal selection seems still far from being -generally recognized, we have to remember that we are not at all in -a position to judge with any certainty as to the biological value<span class="pagenum"><a id="Page_133"></a>[Pg 133]</span> -of a character. What labour and painstaking investigation it has -cost to give a verdict as to this even in a few instances! Innumerable -characters appear indifferent, and are nevertheless adaptations. Darwin -in his day pointed out the need for caution in this matter, referring to -the case of animal coloration as an example; very little attention had -been directed to it for a long time because it had been believed to be -without significance. And how many diverse kinds of characters -among animals and plants, which had likewise been regarded as -'purely morphological,' have on more careful investigation shown -themselves of very great biological importance. I need only refer to -the shape, position, hair-arrangement, colour, and lustre of flowers, -and their relation to cross-fertilization by means of insects, or to the -thickness and shape of the leaves of tropical trees with their coating -of wax and their gutter-like outlets for carrying off the tropical rain -which falls in terrible downpour (Haberlandt, Schimper), or to the -limp, perpendicular drooping of the tufts of the young and tender -leaves of the same trees, which also secures protection from being -battered and torn by the rain.</p> - -<div class="figcenter" id="ff17"> -<img src="images/ff17.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 107</span>, <i>C.</i> Leptocephalus stage of an American Eel, with seven pigment -spots, of which three are on the left (<i>l</i>) and four on the right (<i>r</i>) side. After -Eigenmann.</p> -</div> - -<p>There are even characters the biological use of which is unknown -to us, but in regard to which we can affirm that they have a use. -Thus Eigenmann described the larva of an American eel, which differs -from other so-called 'Leptocephali' in that a row of seven black spots -runs along its side. Apparently all these lie upon the side turned towards -us, but in reality they are distributed on both sides, three lying on the -left and four on the right, and so arranged that they look like a single -row of spots at regular intervals, for the flat little fish is absolutely -transparent. The habits of this larva are not yet known, but we may -conclude that this appearance of a simple row of spots must have some -value for the animal, for such a significant asymmetry could not have -arisen for purely internal reasons (Fig. 107, <i>C</i>). It is possible that the -fish is thus made to resemble parts of some marine alga, and that it is -thereby protected from many enemies; that there is not a complete<span class="pagenum"><a id="Page_134"></a>[Pg 134]</span> -row upon each side may depend upon the fact that the two rows -would be visible at the same time, and that they would blur each -other in the eyes of the swimming enemy, and so destroy the -resemblance of the picture to its unknown model.</p> - -<p>But it cannot be denied that there are characters which have -no special biological significance. There are doubtless many such -characters, which stand beyond the threshold of good or bad, and -which are therefore not affected by personal selection; it is difficult -and often impossible to point these out with certainty. The shape of -the human nose and of the human ear, the colour of the hair and of -the iris, may be such indifferent characters whose peculiarities are to -be referred solely to germinal selection. On the other hand, I would -not venture to assert that the gay colouring and the complex markings -on the wings of our modern Lepidoptera are always and in all cases -unimportant, even when we cannot interpret their details either as -protective, or as a sign of nauseousness, or as mimetic. The usually -very exact similarity of the colour pattern in the individuals of each -species seems to point to the intervention of personal selection in some -form or other, for in what other way could such a large majority of -variations in the same direction have developed in the germ-plasm -as this constancy of the character indicates.</p> - -<p>We know, of course, that the colours of butterflies and moths can -be caused to vary through external and especially climatic influences, -but this would only account for simple modifications of colour, and not -for the origin of the complex colour patterns that actually occur. I -therefore believe with Darwin that sexual selection has had much to -do with this by giving a slight preference to the variations produced -by spontaneous germinal selection, and thus preventing the majority -of varied ids once acquired from being scattered again, but always -collecting more of them, and so securing free play for the increase -of the new character through intra-germinal processes. In this -way have arisen not only the brilliance of our Lycænidæ and of -the large Morphidæ of South America, but also many of the coloured -spots, streaks, bands, eyes, and other components which have gradually -in the course of time evolved into the complex colour pattern of many -of our modern butterflies. I should like to remind any one who doubts -this of a fact which corroborates the view that personal selection has -co-operated in the production of these colours—I refer to the inconspicuous -colouring of the females of many of these brilliant males—while -in contradistinction to these cases there are other species in -which both sexes are alike brilliant, so that it is impossible that mere -spontaneous germinal selection can have determined that the females,<span class="pagenum"><a id="Page_135"></a>[Pg 135]</span> -because of their femaleness, should vary in a different manner from -the males.</p> - -<p>But while I believe that sexual selection in particular has had -much to do with producing the colours of Lepidoptera, the basis of -all these colour variations must still be looked for in germinal -selection, and we shall see later on how it is possible to think of -the diversified and often relatively abrupt transformations of marking -as the resultant of the co-operation of climatic influences with germinal -selection.</p> - -<p>Of course there must also be unimportant changes in butterfly-markings -which depend solely on the internal play of forces in the -determinant system, and to this must be referred the markings of -many of the 'variable' species whose variations are mere fluctuations -in the details of marking, which have therefore caused much trouble -to the systematists. Truly unimportant variations will rarely or -never combine into a 'constant' form, and the fact that there -are species which are 'variable' in such a high degree is enough -to make us refer their variations to their lack of importance, for -if they possessed any biological value the less valuable among them -would gradually be removed by selection. Perhaps the variable -species of certain moths like <i>Arctia caja</i>, and especially <i>Arctia -plantaginis</i>, the little 'bear' of the Alps and Apennines, must -be reckoned among these. But from the fact that there are such -fluctuations in the markings of Lepidoptera, it seems to me that -we must conclude that species which show a high degree of constancy -in their markings have been influenced by selection, or -by climatic influences which turned the play of forces within the -determinant system in the same direction in all individuals. All -these considerations and conclusions are quite sound and serviceable -theoretically, but they are difficult to apply to individual cases, and -where this is attempted it must be with the greatest caution, and, -if possible, on a basis of investigations specially undertaken for the -purpose; for how should we know whether a species which to-day -is highly variable may not a geological epoch later become a very -constant one? We must in any case assume that marked fluctuations -of characters are associated with many transformations.</p> -<hr class="full" /> - -<div class="chapter"> -<p><span class="pagenum"><a id="Page_136"></a>[Pg 136]</span></p> - -<h2 class="nobreak" id="LECTURE_XXVI">LECTURE XXVI</h2> -</div> - -<p class="c">GERMINAL SELECTION (<i>continued</i>)</p> - -<div class="blockquot"> - -<p>Germinal selection, spontaneous and induced—Climatic forms of <i>Polyommatus phlæas</i>—Deformities—Excessive -augmentation of variations—Can it lead to the elimination -of a species?—Saltatory variations, copper-beech, weeping trees—Origin of sexual -distinguishing characters—Formation of breeds among domesticated animals—Degenerate -jaws—Human teeth—Short-sightedness—Milk-glands—Small hands and -feet—Ascending variation—Talents, intellect—Combination of mental endowments—The -ultimate roots of heritable variation—There are only plus- and minus-variations—Relations -of the determinants to their determinates—The play of forces in the -determinant system of the id—Germinal selection inhibited by personal selection—Objection -on the score of the minuteness of the substance of the germ-plasm.</p></div> - - -<p><span class="smcap">Hitherto</span> we have derived the variations of the determinants -of the germ-plasm, upon which we based the process of germinal -selection, from <i>chance local</i> fluctuations in nutrition, such as must -occur in an individual id, independently of the nutrition of the other -ids of the same germ-plasm. But there are doubtless also influences -which set up similar nutritive changes in <i>all ids</i>, and by which, -therefore, all homologous determinants, in as far as they are sensitive -to the nutritive change in question, are affected in the same manner. -To this category belong changes in the external conditions of life, -and particularly climatic changes. It is, then, germinal selection -alone which brings about the presence of a majority of ids with -determinants varying in the same direction, and personal selection -has no part in the transformation of the species. Many years ago -I instituted experiments with a small butterfly, <i>Pararga egeria</i>, -and these showed that a heightened temperature so influenced the -pupæ of this form that the butterflies emerged with a different -and deeper yellow ground-colour, similar to that of the long-known -southern variety <i>Meione</i>. More thoroughly decisive, however, were -the experiments on <i>Polyommatus phlæas</i>, the small 'fire-butterfly,' -which were carried on in the eighties by Merrifield in England and -by myself almost at the same time. I shall discuss these later in -more detail, and will only say here that this butterfly, whose range -extends from Lapland to Sicily, occurs in two forms, the southern -distinguished by a 'dusting' of deep black from the northern, in -which the wing-surfaces are of a pure red-gold. The experiments<span class="pagenum"><a id="Page_137"></a>[Pg 137]</span> -showed that the southern form can be artificially produced by -warmth, and the interpretation must be that the direct influence -of higher temperature affects the quality of the nutritive fluids in -the germ-plasm, and thereby at the same time the determinants of -one or more kinds of wing-scales are caused to vary in all the ids -in the same direction, in such a fashion that they give rise to -black scales instead of the former red-gold ones. It is thus certain -that there are external influences which cause particular determinants -to vary in a particular manner. I call this form of germinal variation -'induced' germinal selection, and contrast it with 'spontaneous' -selection, which is caused, not by extra-germinal influences, but by -the chances of the intra-germinal nutritive conditions, and which -will, therefore, not readily occur at the same time in all the ids of -a germ-plasm, and so will not give rise to variation of the same -kind in the homologous determinants of all the ids.</p> - -<p>The two processes must also be distinguished from each other -in their relation to personal selection, for induced germinal selection -will go on increasing until the maximum of variation corresponding -to the nature of the external influences and of the determinants -concerned is reached. Since <i>all</i> the ids are equally affected and -caused to vary in the same way, personal selection has nothing to -take hold of, and the variation might go on intensifying even if it -should become biologically prejudicial. But it is quite otherwise -with spontaneous germinal selection, which has its roots not in all, -but only in a majority of the ids. Here the variation may go on -increasing by germinal selection alone, but only until it acquires -a positive or negative biological value, that is, until it becomes -advantageous or prejudicial to the life of the individual; then -personal selection intervenes and decides whether it is to go on -increasing or not. Spontaneous germinal selection can therefore -only lead to the general variation of a whole species when it is -supplemented by some external factor such as, especially, the utility -of the variation.</p> - -<p>This does not imply, however, that indifferent variations of large -amount could not arise through spontaneous germinal selection, but -they would remain confined to a small number of individuals, and -would sooner or later disappear again. The congenital deformities -of Man may in part fall under this category. If, for instance, -certain determinants are, by reason of specially favourable local -nutritive conditions, maintained for a long time in progressive variation, -they will become so strong that the part which they determine -will turn out excessive, perhaps double. Hereditary polydactylism<span class="pagenum"><a id="Page_138"></a>[Pg 138]</span> -in Man may perhaps be explained on this principle, and I had already -referred it to the more rapid growth and duplication of certain -determinants of the germ even before formulating the idea of -germinal selection. In this I was at one with the pathologist -Ernst Ziegler, who had designated polydactylism as a germ-variation, -and in contrast to others had not interpreted it in an atavistic -sense, as a reversion to unknown six-fingered ancestors. All excessive -or defective hereditary malformations may be referred to germinal -selection alone, that is, to the long-continued progressive or -regressive variation of particular determinant-groups in a majority -of ids.</p> - -<p>The fact that, as far as our experience goes, superfluous fingers -are never inherited for more than five generations may be simply -explained, for there has been no reason for the intervention of -personal selection, either in the negative sense, for the six-fingered -state does not threaten life, nor in the positive, since it is not of -advantage. The deformity depends on spontaneous germinal variation, -which must have taken place in a majority of ids or it would -not have become manifest. But such a majority of 'polydactylous' -ids is liable to become scattered again in every new descendant, -and to be reduced again into a minority which can no longer make -itself felt by the chances of reducing division and the admixture -of normal ids in amphimixis. A polydactylous race of men could -only arise through the assistance of personal selection; in that case -there would doubtless be just as much chance of success in breeding -a six-fingered race as there was in breeding the crooked-legged -Ancon sheep from a single ram which was malformed in this -manner. Without a gradual setting aside of the germs with normal -ids, that is, without personal selection, such spontaneous deformities, -and indeed all <i>spontaneous</i> variations, must fail of attaining to -permanent mastery.</p> - -<p>This must frequently be the case in free nature also, but we -shall have to investigate later on, in the section devoted to the -formation of species, whether external circumstances (inbreeding) -may not also occur which make it possible for spontaneous variations -to become constant breed-characters, even although they remain -neither good nor bad, and are thus not subject to the action of -personal selection.</p> - -<p>In general, however, amphigony with its reduction of the ids -and its constant mingling of strange ids will form the corrective -to the deviations which may arise through the processes of selection -within the id, and which lead to excessive or superfluous development<span class="pagenum"><a id="Page_139"></a>[Pg 139]</span> -of certain structures, to a complete disturbance of the harmony of -the parts, and ultimately to the elimination of the species.</p> - -<p>It must be admitted, however, that Emery was probably right -when he directed attention to the possibility of a 'conflict between -germinal and personal selection.' It is quite conceivable that in -cases of useful variations, that is, of adaptations, the processes of -selection within the germ-plasm may lead to excessive developments, -which personal selection cannot control, because, on account of their -earlier usefulness, they have in the course of a series of generations -and species become fixed not only in a majority of ids, but in almost -all the ids of the collective germ-plasm of the species. In this case -a reversal must be difficult and slow, for the gathering together of -ids with relatively weaker determinants can only take place slowly, -and it is questionable whether the species would survive long enough -for the slow process to take effect. But, apart from the question -of time, such a reduction of an excessive development would sometimes -be quite impossible, for the simple reason that there is nothing -for personal selection to take hold of.</p> - -<p>Döderlein has pointed out that many characters go on increasing -through whole series of extinct species, and ultimately grow to such -excess that they bring about the destruction of the species, as, for -instance, the antlers of the giant stag or the sabre-like teeth of -certain carnivores in the diluvial period. I shall have to discuss this -in more detail in speaking of the extinction of species; it is enough -to say here that such long-continued augmentations in the same -direction can never be referred <i>solely</i> to germinal selection, since it is -hardly conceivable that a species—much less a whole series of species—should -arise with injurious characters; they would have become -extinct while they were still in process of arising. Although we see -that the Irish stag, with his enormous antlers over ten feet across from -tip to tip, was heavily burdened, we are hardly justified in concluding -that the size and weight of the burden on his head tended to his -destruction from the first—for in that case the species would never -have developed at all—but it may well be that at some time or other -the life-conditions of the species altered in such a manner that the -heavy antlers became fatal to it. In this case the variation-direction -which had gained the mastery in all ids could no longer be sufficiently -held in check by personal selection, because the variations in the -contrary direction would be much too slight to attain to selective -value. Sudden, or at least rapidly occurring changes in the conditions -of life, such as the appearance of a powerful enemy, exclude all chance -of adaptation by the slow operation of personal selection.</p> - -<p><span class="pagenum"><a id="Page_140"></a>[Pg 140]</span></p> - -<p>If we look into the matter more carefully, we see that it is not -strictly true to say that germinal selection alone brings about the -extinction of a species by cumulative augmentation of structures which -are already excessive; <i>it is the incapacity of personal selection to keep -pace with the more rapid changes in the conditions of life and to -reduce excessive developments to any considerable extent in a short -time</i>. This would always be possible in a long time, for the determinants -of the excessive organ <i>E</i> can never be equally strong in all -the ids; they always fluctuate about a mean, however high this -mean may be. Here again it must still be possible that reducing-divisions -and amphimixis may lead to the formation of majorities -of ids with weaker <i>E</i>-determinants, and if sufficient time be allowed, -artificial selection could, by consistently selecting the individuals -with, let us say, weaker antlers, give rise to a descending variation-movement. -There are no variation-movements which cannot be -checked; every direction can be reversed, but time and something -to take hold of must be granted. That was wanting in the case -of the giant stag, for it would not have been saved even if its antlers -had at once become a couple of feet shorter, and germinal selection -can hardly make so much difference as that.</p> - -<p>Analogous to hereditary deformities, and of special interest in -connexion with the processes within the germ-plasm, are '<i>sports</i>' -variations of considerable magnitude which suddenly appear without -our being able to see any definite external reason for them. I have -already discussed these in detail in my <i>Germ-plasm</i>, and have shown -how simply these apparently capricious phenomena of heredity can -be understood <i>in principle</i> from the standpoint of the germ-plasm -theory.</p> - -<p>The chances of the transmission of the saltatory variation will -be greater or less according to whether the variation of the relevant -determinants involves a bare majority of ids or a large majority, for -the more ids that have varied, the greater is the probability that the -majority will be maintained throughout the course of ensuing reducing -divisions and amphimixis, that is, that the seeds of the plant will -reproduce the variation, and will not revert to the ancestral form. -Although one of the most satisfactory results of the id-theory lies -precisely in the interpretation of these conditions, I do not wish -to enter into the matter here, but will refer to the details in my -<i>Germ-plasm</i>, published in 1894, which I consider valid still. At -that time I had not formulated the idea of germinal selection, but -the explanation of the occurrence of such sport-variations which -I gave was based upon the assumption of nutritive fluctuations in the<span class="pagenum"><a id="Page_141"></a>[Pg 141]</span> -germ-plasm, which gave rise to variations in certain determinants. -There was still lacking the recognition that the direction of variation -once taken must be adhered to until resistance was met with, and -that the determinants stand in nutritive correlation with one another, -so that changes in one determinant must re-act upon the neighbouring -ones, as I shall explain more fully afterwards. I also showed from -definite cases that such sports, though they are sudden—'saltatory'—in -their mode of occurrence, are long being prepared for by intimate -processes in the germ-plasm. This 'invisible prelude' of variation -depends on germinal selection. When a wild plant is sown in garden-ground -it does not require to vary at once; several, even many, -generations may succeed each other which show no sports; suddenly, -however, sports appear, at first singly, then, perhaps, in considerable -numbers. It is not, however, by any means always the case that -considerable numbers occur, for some varieties of our garden flowers -have arisen only once, and then have been propagated by seed; and -such saltatory sports in plants which are raised from seed are usually -constant in their seed, and if they are fertilized with their own pollen -they breed true—a proof that the same variations must have taken -place in the relevant determinants in a large majority of ids.</p> - -<p>In animals, it would appear, such saltatory variations occur -much more rarely than in plants; the case examined in detail by -Darwin of the 'black-shouldered peacock' which suddenly appeared -in a poultry-yard is an example of this kind. Much more numerous, -however, are the instances among plants, and especially among plants -which are under cultivation. This indicates that we have here to do -with the effect of external conditions, of nutritive influences which -cause the slow variation of certain determinants, sometimes abetting -and sometimes checking. As soon as a majority of ids varied in this -way comes to lie in a seed, a sport springs up suddenly and apparently -discontinuously—a plant with differently coloured or shaped petals -or leaves, with double flowers, with degenerate stamens, or with some -other distinguishing mark, and these new characters persist if the -variety is propagated without inter-crossing.</p> - -<p>But it happens sometimes, though more rarely, that not the whole -plant but individual shoots may exhibit the variation. To this class -belong the 'bud-variations' of our forest trees, the copper-beeches, -copper-oaks, and copper-hazels, the various fasciated varieties of -oak, beech, maple, and birch, and the 'weeping trees'; also the -numerous varieties of potato, plantain, and sugar-cane. It seems that -only a few of these breed true when reproduced from seed, or in other -words, they usually exhibit reversions to the ancestral form: on the<span class="pagenum"><a id="Page_142"></a>[Pg 142]</span> -other hand, in the weeping oak for instance, nearly all the seedlings -exhibit the character of the new variety, though 'in varying degrees.' -The records as to the transmissibility of bud-variations through seed -are probably not all to be relied upon, and new investigations are -much to be desired, but the fact that in many cases they may be -propagated not only by means of layers and cuttings but by seed -also, is most important in our present discussion, for it proves that -here too the varied determinants must be contained in a majority of -ids. As it is only a single shoot that exhibits the saltatory variation, -only the germ-plasm which was contained in the cells of this one -shoot can have varied, and it must have done so in so many ids that -the variation prevailed and found expression. But that, in this case -also, the variation does not appear in all, but only in a small majority -of ids, is proved by the frequent reversion of bud-varieties to the -ancestral form. I have already reported a case of this kind shown -to me by Professor Strasburger in the Botanic Gardens in Bonn, -where a hornbeam with deeply indented 'oak-leaves' had one branch -which bore quite normal hornbeam leaves. In my own garden there -is an oak shrub of the 'fern-leaved' variety, whose branches bear -some leaves of the ordinary form; variegated maples with almost -white leaves often exhibit in individual branches a reversion to the -fresh green leaves of the ancestral form. We see from this that what -is so energetically disputed by many must in reality occur—namely, -differential or non-equivalent nuclear division—for otherwise it would -be unintelligible how the ids of the new variety, if they once attain -a majority in the tree, could give place in an individual branch to -a majority of the ancestral ids. Only differential nuclear division, -in the manner of a reducing division, can be the cause of this. Of -course this implies only a dissimilar or differential distribution of the -ids between the two daughter-nuclei, not a splitting up of the individual -ids into non-equivalents.</p> - -<p>That in free Nature bud-variations left to themselves can ever -become permanent varieties is probably an unlikely assumption, -because of the inconstancy of their seeds which only breed true in -rare cases; nor is it likely that such variations as the copper-beech, -the weeping ash, and so on could hold their own in the struggle for -existence with the older species; but there is certainly nothing -to prevent our assuming that, in certain circumstances, saltatory -variations, when they have a germinal origin, may become persistent -varieties and may even lead to a splitting of the species. This may -happen, for instance, when the variations remain outside the limits -of good and bad, and thus are neither of advantage to the existence<span class="pagenum"><a id="Page_143"></a>[Pg 143]</span> -of the species nor a drawback thereto. In the next chapter we shall -discuss the influence of isolation upon the formation of species, and -it will be seen that in certain conditions even indifferent variations -may be preserved, and that saltatory variations, as for instance in the -evolution of species of land-snails or butterflies, may have materially -contributed to bring this about.</p> - -<p>I should like to emphasize still more the part played by saltatory -variations arising from germinal selection in the origin of secondary -sexual characters. As soon as personal selection, whether sexual -or ordinary, prefers as useful in any sense a saltatory variation, it is -not only preserved and becomes a character of a variety, but it may -increase, and we have to ask whether such sudden variations are -frequently of a useful kind, especially when not individual characters -alone, but whole combinations of them are implicated. If we may -judge from the sports of the flowers and the leaves of plants, transformations -useful to the species as a whole rarely occur suddenly, that -is, they occur only in a few out of very numerous sports; they are -much more frequently indifferent, although quite visible and often -conspicuous variations.</p> - -<p>For this reason I am disposed to attribute to saltatory variations -a considerable share in the production of distinctive sexual characters. -From saltatory variations in flowers, fruits, and leaves we know that -these may be conspicuous enough even on their first appearance, and -so we are justified in finding in such variations the first beginnings -of many of the decorative distinguishing characters which occur in the -males of so many animals, especially butterflies and birds. As soon -as it is admitted that variations of considerable amount, which have -been slowly prepared in the germ-plasm by means of germinal selection, -can suddenly attain to expression, one of the objections against -sexual selection is disposed of, for conspicuous variations are necessary -for the operation of this kind of selection, since the changes in question -must attract the attention of the females if they are to be preferred. -Without such preference, even though it be not quite strict and consistent, -a long-continued augmentation of the decorative characters -is inconceivable.</p> - -<p>But as intra-germinal disturbances of the position of equilibrium -in the determinant system is at the root of the saltatory variations of -our cultivated plants, it must also have played a large share in the -evolution of breeds among our domesticated animals, which is therefore -by no means wholly due to artificial selection operating upon the -variation of individual characters. In all breeds in the formation of -which the production of more than a single definite character was<span class="pagenum"><a id="Page_144"></a>[Pg 144]</span> -concerned, as, for instance, in the broad-nosed breeds of dog—bull-dog -and pug-dog—we may refer the peculiar variation of many parts to -disturbances of the equilibrium of the determinant system, which -bring to light, not suddenly as in the case of saltatory variations, but -gradually and increasingly, the curious complex of characters. -Darwin referred such transformations of the whole animal facies, -where a single varying character is deliberately selected, to correlation, -and by this he understood the mutual influence of the parts of -an animal upon one another. Such correlation certainly exists, as we -have already seen in discussing histonal selection, but here we have -rather to do with the correlation of the parts of the germ-plasm, with -the effects of germinal selection, which, affected by the artificial -selection of particular characters, gradually brings about a more -marked disturbance in the whole determinant system.</p> - -<p>In the evolution of our breeds of domesticated animals, germinal -selection in the negative sense must also have played a part—I mean -through the weakening and degeneration of individual determinants. -Only in this way, it seems to me, can we explain the tameness of our -domestic animals, dogs, cats, horses, &c., in which all the instincts -of wildness, fleeing from Man, the inclination to bite, and to attack, -have at least partly disappeared. It is, of course, very difficult to -estimate how much of this is to be ascribed to acquired habitude -during the individual lifetime. The case of the elephant might be -cited in evidence of tameness which arises in the individual lifetime, -for all tame elephants are caught wild, but it seems that captured -young beasts of prey, such as the fox, wolf, and wild cat, not to speak -of lions and tigers, never attain to the degree of tameness exhibited -by many of our domesticated dogs and cats. The very considerable -differences in the degree of tameness of dogs and cats go to show that -the case is one of instincts varying in different degree.</p> - -<p>If this be so, then the instinct of wildness, if I may express -myself so for the sake of brevity, has degenerated in consequence of -its superfluity, and through the process of germinal selection, which -allowed the determinants of the brain-parts concerned to set out on -a path of downward variation upon which they met with no -resistance on the part of personal selection.</p> - -<p>Herbert Spencer adduced against my position the case of the -reduction in the size of the jaws in many breeds of dog, especially in -pugs and other lap-dogs, which he regarded as evidence of the -inheritance of acquired characters. But this and analogous cases of -the degeneration of an organ during a long period in which the -animal had been withdrawn from the conditions of natural life is<span class="pagenum"><a id="Page_145"></a>[Pg 145]</span> -intelligible enough on the assumption of persistent germinal selection -aided by panmixia. The jaws and teeth in these spoilt pets no longer -require to be maintained at the level of strength and sharpness -essential to their ancestors which depended on these characters, and -so they fell below it, became smaller and weaker, but could not -disappear altogether, for the process of degeneration was brought, or -is being brought, to a standstill by the intervention of personal -selection.</p> - -<p>Even the lower jaw in Man is declared by many authors to be -degenerate. Collins found that the lower jaw of the modern Englishman -was one-ninth smaller than that of the ancient Briton, and one-half -smaller than that of the Australians; Flower showed that we are a -microdont race like the Egyptians, while the Chinese, Indians, Malays, -and Negroes are mesodont, and the Andamanese, Melanese, Australians, -and Tasmanians are macrodont. This does not of itself imply that -we exhibit a degeneration of dentition, though this conclusion is -hinted at by other facts, such as the variability of the wisdom-teeth. -It need not surprise us, indeed, that a retrogressive variation tendency -should have started in this case, for, with higher culture and more -refined methods of eating, the claims which personal selection was -obliged to make on the dentition have been greatly diminished, and -germinal selection would thus intervene.</p> - -<p>Every one knows how the quality of human teeth has deteriorated -with culture, and this not in the higher classes only, but even -among the peasantry, as Ammon has observed. The time is past -when raw flesh was a dainty, and when bad teeth meant poor -nutrition, if not actual starvation. Even nowadays famine plays a -terrible and periodically recurrent rôle as an eliminator among some -negroid races.</p> - -<p>Many other organs in man have been reduced from their former -pitch of perfection through culture, and some of them are still in -process of dwindling. When I formulated the idea of panmixia and -applied it to explain cases which had previously been referred to the -inheritance of the results of disuse, I regarded the short-sightedness of -civilized Man from this point of view. My opinion aroused lively -opposition at the time, especially on the part of oculists, who very -emphatically referred the phenomenon to the inheritance of acquired -shortsight, and indeed regarded it as a proof of the transmission -of functional modifications.</p> - -<p>But, apart from the fact that the assumption of this mode of -inheritance must now be regarded not only as unproved, but as -contradicted by reliable data, panmixia, in conjunction with the<span class="pagenum"><a id="Page_146"></a>[Pg 146]</span> -ceaseless fluctuations within the germ-plasm—germinal selection—affords -a better explanation than the other theory was ever in -a position to offer. At that time I pointed out that the survival of -the individual among civilized races had not for a very long time -depended on the perfection of his eyesight, as it does for instance in -the case of a hunting or warlike Indian, or of a beast of prey, or of -a herbivore persecuted by the beast of prey. And this is by no means -due solely to the invention of spectacles, but in a much greater degree -to the fact that every man no longer has to do everything, so that -numerous possibilities of gaining a livelihood remain open to the less -sharp-sighted; that is, the division of labour in human society has -made the survival of the short-sighted quite feasible. As soon as this -division of labour reached such a degree that the founding of a family -offered no greater difficulty to the short-sighted individual than to -one with normal sight, short-sightedness could no longer be eliminated; -and partly because of the mingling with normal sight, but partly -also because of the never-failing minus-fluctuations of the germ-plasm -determinants concerned, a variation in a downward direction was -bound to set in, and will continue until a limit is set to it by personal -selection. Meantime, we are obviously still in the midst of the process -of eye-deterioration; and the resistance to it is somewhat inhibited -in its operation, because although individuals with extremely bad -sight are for the most part hindered from gaining an independent -livelihood and having a family, this is certainly, thanks to our -mistaken humanity, not always the case. There are even instances of -marriage between two blind persons!</p> - -<p>As yet, however, the deterioration of eyes has not advanced very -far; not nearly all families are affected by it, and even in Germany, -the land of the 'longest school form' and of the greatest number of -spectacle-wearers, short-sight is still usually acquired by individuals, -although there must frequently be a more or less marked predisposition -to it. It is a common objection to this view that in England, -France, and Italy the percentage of short-sighted individuals is much -lower, and, in point of fact, one sees far fewer people wearing -spectacles in those countries. This, however, does not prove that -a similar deterioration of eyes has not begun there also, for how could -the small inherited beginnings be detected if they were not accentuated -by the spoiling of the eyesight in the lifetime of the individual by -much reading of bad print, and by writing with bent head, as is -still too often the case in many German schools.</p> - -<p>That our interpretation, through panmixia on a basis of -germinal selection, is the correct one, we infer also from the fact that<span class="pagenum"><a id="Page_147"></a>[Pg 147]</span> -short-sightedness has been proved to be a frequent character even -among our domesticated animals, such as the dog and the horse. -These animals receive protection and maintenance from Man, and -their survival and reproduction no longer depend on the acuteness of -their sight, and thus the eye has fallen from its original perfection, -just as in Man, although in this case reading and writing play no part.</p> - -<p>A whole series of similar slight deteriorations of individual -organs and systems of organs might be enumerated, all of which have -appeared in consequence of long and intensive culture in Man. All -these must depend upon germinal selection, on a gradually progressive -weakening of the determinant-groups concerned, under the -conditions of panmixia, that is, in the absence of positive selection.</p> - -<p>To these must be added the deterioration of the mammary-glands -and breasts, and the inability to suckle the offspring which results -chiefly from this. Here we have a variational tendency which could -not appear in a people at a lower stage of culture, and it has not -become general in the lower classes of society among ourselves.</p> - -<p>The muscular weakness of the higher classes is another case in -point, and all gymnastics and sports will be of no avail as long as a -relative weakness of the muscles is not a hindrance to gaining a livelihood, -and having a family. Even universal conscription will do -nothing to check this falling off of the bodily strength. Certainly -military service strengthens thousands, and hundreds of thousands of -individuals, but it does not prevent the weaklings from multiplying, -and thus reproducing the race-deterioration. But it would indeed be -well if only those who had gone through a term of military service -were allowed to beget children.</p> - -<p>It is only among the peasantry, inasmuch as they really work -and do not merely look on as proprietors of the ground, that such -a deterioration of the general muscular strength could not become the -permanent variational tendency of the determinants concerned, because -among genuine peasants bodily strength is a condition of having and -supporting a family—at least on an average.</p> - -<p>The diminution in the firmness and thickness of the bones in the -higher classes, and many another mark of civilization, must be looked -at from the point of view of panmixia and germinal selection; -perhaps also the smaller hands and feet which frequently occur along -with a more graceful general build in the higher ranks of European -peoples. It would certainly not be surprising if in families which -usually intermarry, and which in no way depend for their material -subsistence on the possession of large and powerful hands and feet or -bones generally, a downward variation of the relevant germ-deter<span class="pagenum"><a id="Page_148"></a>[Pg 148]</span>minants -should have developed, but this could never overstep -a certain limit, because it would then be prejudicial even in civilized -life. That we must be very careful not to regard large hands and -feet as the direct result of hard physical toil was brought home to me -by an observation of Strasburger's. He was particularly struck by the -fact that the peasants of the high Tatra (Carpathians) were distinguished -by the smallness of their hands and feet.</p> - -<p>But while civilization has excited numerous downward variations -in the germ, it has, on the other hand, been the cause of numerous -hereditary improvements—variations in an upward direction. This -opens up new ground, for hitherto we have been confronted with the -alternative of either accepting the inheritance of acquired characters, -and on this basis referring the talents and mental endowments of -civilized Man to exercise continued throughout many generations, or of -admitting an increase of mental powers only in as far as they possess -'selection value,' that is, as they may be decisive in the struggle for -existence. To these mental qualities belong cleverness and ingenuity -in all directions, courage, endurance, power of combination, inventive -power, with its roots in imagination and fertility of ideas, as well as -desire for achievement, and industry. Throughout the long history of -human civilization these mental qualities must have increased through -the struggle for existence, but how have the specific talents such as -those exhibited in music, painting, and mathematics come into -existence? And how have the moral virtues of civilized Man been -evolved, and particularly unselfishness? For it can hardly be -maintained of any of these endowments that they possess selection-value -for the individual.</p> - -<p>It is not my intention to discuss these questions in detail; they -are too many-sided and of too much importance to be treated of -merely in passing; moreover, I gave expression years ago to my -views on this subject by dealing with one example—the musical -sense in Man. I do not believe that the musical sense had its -beginnings in Man, or that it has materially increased since the days -of primitive Man, but in conjunction with the higher psychical life -of civilized peoples its expressions and applications have risen to -a higher level. It is, so to speak, an instrument which has been -transmitted to us from our animal ancestors, and on which we have -learnt to play better the more our mind has developed; it is an -unintended 'accessory effect' of the extremely fine and highly -developed organs of hearing with their nerve-centres which our -animal ancestors acquired in the struggle for existence, and which -played a much more important rôle in the preservation of life in<span class="pagenum"><a id="Page_149"></a>[Pg 149]</span> -their case than it does in ours. The musical sense may be compared -to the hand, which was developed even among the apes, but which -civilized Man in modern times no longer uses merely to perform its -original function, grasping, but also for many other purposes, such as -writing and playing the piano. And just as the hand did not -originate through the necessities of the piano, neither did the -extremely delicate sense of hearing of the higher animals develop -for the sake of music, but rather that they might recognize their -enemies, friends, and prey, in darkness and mist, in the forest, on -the heath, and at great distances.</p> - -<p>The case is probably the same with the rest of the special -psychical endowments or talents. I do not of course maintain that -they, like the musical sense, did not at some time play a rôle in the -struggle for existence and survival, and therefore could not increase, -but the increase was certainly not continuous, but much -interrupted, so that it would extend only to small groups of -descendants, and therefore could only contribute very slowly to the -elevation of the psychic capacities of a whole people. But in certain -individuals and families such augmentations would certainly take place -through germinal selection, and it seems to me probable that these -would never be wholly lost again, even if they appeared to be so, -but would be handed on, in id-minorities, through the chain of -generations, and would slightly raise the average of the talent in -question, and might even, under favourable circumstances, combine -in the development of a genius. We know how strongly hereditary -such specific talents are; let us suppose that the determinants of, say, -the musical sense have, by the intra-germinal chances of nutrition, -been started on a path of ascending variation; they will continue -in this path until a halt is called from some quarter or other. This -can only happen if, in the reducing division, or in amphimixis, the -highly developed musical determinants are wholly or partly eliminated, -or are reduced to a minority. As long as this does not happen -the ascending variation will go on, and then we may have the birth -of a Mozart or of a Beethoven. Personal selection will not interfere -either in a positive or a negative sense, since high development of the -musical sense has no effect either in advancing or retarding the -struggle for existence; the increase will therefore go on until the -large majority of highly developed musical determinants, which we -must assume in the case of a musical genius, is reduced, or even -transformed into a minority, through unfavourable reducing divisions -of the germ-cells, and by association with the germ-cells of less -musical mates.</p> - -<p><span class="pagenum"><a id="Page_150"></a>[Pg 150]</span></p> - -<p>The fact that highly developed specific talents have never been -known to be inherited through more than seven generations is quite -in keeping with this view. But even this persistence has been -observed only in the case of musical talent, and the long continuance -of the inherited talent may well be due, as Francis Galton suggests in -his famous statistical investigations into the phenomena of inheritance, -to the fact that musical men do not readily choose wives who are -absolutely lacking in this talent. It would be easy to rear an -exceedingly highly gifted musical group of families within the -German nation, if we could secure that only the highly-gifted musically -should unite in marriage—that is, if personal selection could play its -part. In another more general domain of mental endowment a case -of this kind has been recorded, for Galton tells us of three highly -gifted English families which intermarried for ten generations, and -in that time scarcely produced a descendant who did not deserve -to be called a distinguished man in some direction or other.</p> - -<p>Of course, such continued persistence, through a long series of -generations, of a high general mental level is more possible than the -transmission and increase of a specific talent, for in the former case -it is a question of a mixture of different high mental endowments, -of which not all need be developed in every individual, and yet the -individual need not fall to mediocrity if he possesses a combination -of other qualities. But in musical talent, on the other hand, the -falling from the height once attained takes place as soon as this one -character is no longer represented in a sufficiently strong majority -of determinants. Of course it would be a mistake to believe that -the talent of a Sebastian Bach or a Beethoven depended solely on the -highly developed musical sense; in them, as in all great artists, many -highly developed mental qualities must have combined with the -musical sense; a simpleton could never have written the Mass in -B minor or the Passion of St. Matthew even if he had possessed the -musical genius of Sebastian Bach. In this fact lies a further reason -why genius is seldom found at the same pitch in two successive -generations; the combination of mental characters always varies -from father to son, and slight displacements may give rise to very -great differences in relation to the manifestations of the specific -talent. Under certain circumstances, the weak development of -a single trait of character, as, for instance, power of action, or the -excessive development of another, such as indecision or desultoriness, -may so nullify the existing favourable combinations of mental -characters, such as, let us say, musical sense, inventive talent, depth -of feeling, &c., that they bear no fruit worth mentioning. And since<span class="pagenum"><a id="Page_151"></a>[Pg 151]</span> -as we have already seen, the different mental qualities of the parents -are to a certain extent separately transmitted, that is, since they may -appear in the children in the most diverse combinations, we should -rather be surprised that pronounced talent in a specific direction can -persist in a family for two and a half centuries than that it should -do so very rarely. For reducing division is always combining the -existing mental qualities anew, and amphimixis is adding fresh ones -to them.</p> - -<p>Thus germinal selection, that is, the free, spontaneous, but -definitely directed variation of individual groups of determinants, is -at the root of those striking individual peculiarities which we call -specific talents; but it can attain to the highest level only rarely and -in isolated cases, because these talents are not favoured by personal -selection, and therefore the excessively highly developed determinants -upon which they depend may be dispersed in the course of generations; -they may sink to smaller majorities, or even to minorities, in -which case they will no longer manifest themselves in visible mental -qualities.</p> - -<p>We deduced the process of germinal selection on the basis of the -assumption that the nutrition of all the parts and particles of the -body, therefore also of the determinants and biophors of the germ-plasm, -is subject to fluctuations. We regarded the resulting variations -of these last and smallest units of the germ-plasm as the ultimate -source of all hereditary variation, and therefore the basis of all the -transformations which the organic world has undergone in the course -of ages and is undergoing still.</p> - -<p>We have still to inquire whether we can give any more precise -account of the nature of these units of the germ-plasm. If I mistake -not, we may say at least so much, that all variations are, in ultimate -instance, quantitative, and that they depend on the increase or -decrease of the vital particles, or their constituents, the molecules. -For this reason I have hitherto always spoken of only two directions -of variation—a plus or a minus direction from the average. -What appears to us a qualitative variation is, in reality, nothing -more than a greater or a less, a different mingling of the constituents -which make up a higher unit, an unequal increase or decrease of -these constituents, the lower units. We speak of the simple growth -of a cell when its mass increases without any alteration in its -composition, that is, when the proportion of the component parts and -chemical combinations remains unchanged; but the cell changes its -<i>constitution</i> when this proportion is disturbed, when, for instance, -the red pigment-granules which were formerly present but scarcely<span class="pagenum"><a id="Page_152"></a>[Pg 152]</span> -visible increase so that the cell looks red. If there had previously -been no red granules present, they might have arisen through the -breaking up of certain other particles—of protoplasm, for instance, in -the course of metabolism, so that, among other substances, red -granules of uric acid or some other red stuff were produced. In -this case also the qualitative change would depend on an increase -or decrease of certain simpler molecules and atoms constituting the -protoplasm-molecule. Thus, in ultimate instance, all variations depend -upon quantitative changes of the constituents of which the varying -part is composed.</p> - -<p>It might be objected to this argument that chemistry has made -us acquainted with isomeric combinations whose qualitative differences -do not depend upon a different <i>number</i> of the molecules composing -them, but upon their different arrangement; it might be supposed -that something similar would occur also in morphological relations. -And, in point of fact, this seems to be the case. We may, for instance, -imagine one hundred hairs as being at one time equally distributed -on the back of a beetle, and at another standing close together and -forming a kind of brush, but although this brush would be a new -character of the beetle, yet its development would depend upon -quantitative differences, namely, on the fact that the same skin-area, -which in the first case bore perhaps only one hair, had in the second -case a hundred. The quantity of hair cells has notably increased -upon this small area. In the same way the characteristic striping of -the zebra depends not on a qualitative change in the skin as a whole, -but upon an increased deposit of black pigment in particular cells of -the skin, therefore on a quantitative change. In relation to the -whole animal it is a qualitative variation, as contrasted, for instance, -with the horse, but in respect of the constituent parts which give -rise to the qualitative variation it is purely quantitative. The -character of the whole edifice is changed when the proportion of -the stones of which it consists are altered.</p> - -<p>Thus the determinants of the germ may not only become larger -or smaller as a whole, but some kinds of the biophors of which they -are made up may increase more than others, under definite altered -conditions, and in that case the determinants themselves will vary -qualitatively, so that, from the changing numerical proportions of -the different kinds of biophors, a variation of the characters of the -determinants can arise, and consequently also qualitative variations -of the organs controlled by the determinants—the determinates. -But, since nothing living can be thought of as invariable, the biophors -themselves may, on account of nutritive fluctuations, grow unequally,<span class="pagenum"><a id="Page_153"></a>[Pg 153]</span> -and thereby vary in their qualities. To follow this out in greater -detail and attempt to guess at the play of forces within the minutest -life-complexes would at present only be giving the rein to imagination, -but in principle no objection can be made to the assumption that -every element of life down to the very lowest and smallest can, by -reason of inequalities in its nutrition, be not only started on an -ascending or descending movement of uniform growth, but can also -be caused to vary <i>qualitatively</i>, that is, in its characters, because its -component parts change their proportions.</p> - -<p>Of course we know nothing definite or precise with regard to the -units of the germ-plasm, and we cannot tell what is necessary in order -that a determinant shall determine a part of the developing body -in this way or in that; thus we have no definite idea of the relations -subsisting between the variations of the determinants and those of -their determinates, but we know at least so much, that hereditary -variation of a part is only possible when a corresponding particle -in the germ-plasm varies; and we may at least assume that these -correspond to each other so far, that a greater development of the -one implies a greater development of the other, and that a reversal -of these relations is impossible. If the determinant <i>X</i> disappears -from the germ-plasm the determinate <i>X´</i> disappears from the soma. -It is therefore justifiable to infer from the degree of development -of an organ the strength of its determinant, and to assume that plus- and -minus-variations in both are correspondingly large.</p> - -<p>But in addition to the fluctuations in the equilibrium of the -germ-plasm which lie at the root of all hereditary variation, we have -to take into account something which we have already touched upon -briefly—the correlation of the determinants, the influencing of one -determinant by those round about it. I have spoken for the sake -of brevity of 'the determinant' of a part, although all the large and -more important parts must certainly be thought of as represented -by several or many, if not, indeed, by whole groups of determinants. -Although it is quite out of our power to follow the complex processes -of the mutual influences of the determinants upon each other, we can -say this at least, <i>that these influences must exist</i>, and we have here -a faint indication of what must occur in the case of spontaneous -variations within the germ-plasm. We must, in the first place, think -of the individual determinants as arranged in groups, so that, for -instance, the determinants of the right and left half of the body lie -together, and therefore are frequently affected together by influences -which cause variation, so that both vary in the same direction at -the same time. In point of fact, analogous deformities, such as<span class="pagenum"><a id="Page_154"></a>[Pg 154]</span> -polydactylism of both right and left hands, and even of hands and -feet at once, do actually occur. That the right and left hands, the fore- and -hind-limbs, are represented in the germ by particular determinants, -may be inferred from their frequently different phyletic -evolution into different forms of hand and foot, e.g. into flipper and -rudimentary hind-leg in the whale, as well as from the cases of -particulate inheritance, which are rare, but which undoubtedly do -occur, such as when, in Man, there is a maternal blue eye on one side -of the head and a paternal brown eye on the other. But almost more -striking than the differences between these homologous or homotypic -parts are their points of resemblance, and these may probably be in part -referred to their disposition side by side and common history in the -germ-substance, although a far larger proportion of them are probably -due to their adaptation to similar functions, and are therefore to -be regarded as a phenomenon of convergence within the same -organism.</p> - -<p>We have already seen that the first increase in the growth of -one determinant means a withdrawal of nourishment, however slight, -from its neighbours; this can, of course, be equalized again if the -claims on the common nutritive stream from another quarter are -at the same time diminished; but it is possible that the claims from -another quarter may also be increased, and the withdrawal will then -be more marked, and the determinants being thus injured from two -directions at once will sink downwards with greater rapidity. But -it is also conceivable that the majority of determinants of a part may -vary upwards, and, by their combined increased power of assimilation, -direct towards themselves such a greatly increased stream of nourishment -that the whole organ—for instance, a particular feather in a bird—varies -in an upward direction, and becomes larger and larger, as we -see in the case of many decorative feathers; or that certain determinants -vary only as far as some of their biophors are concerned, -and similarly for their determinates, as when a group of scales on -a butterfly's wing that had previously been black turn out a brilliant -blue. It can probably also happen that such variations within the -determinants are transmitted to neighbouring determinants because -the nutritive conditions which caused the first to vary have extended -to those about them. The increase of brightly coloured spots in birds -and butterflies gives us ground for concluding that there are processes -of this kind within the germ-plasm.</p> - -<p>I will refrain from following this idea into greater detail, and -translating the observable relations and variations of the fully-formed -parts of the body into the language of the germ-plasm; but so much<span class="pagenum"><a id="Page_155"></a>[Pg 155]</span> -may be taken as certain, that multitudinous inter-relations and -influences exist between the elements of the germ-plasm, and that -one variation brings another in its train, so that—usually at a very -slow rate, that is, in the course of generations and of species-forming, -definite variations occur from purely intra-germinal reasons—variations -which as far as they remain outside the limits of good or bad -may of themselves change the character of a species, but which when -they are seized upon by personal selection may, by sifting and combination -of the ids, be led on to still higher development.</p> - -<p>If we consider further that the variation of a part must depend -not only on the quality of the external stimulus but also upon the -constitution, the reacting power of the part, we shall understand that -similar nutritive variations may cause two different determinants -to vary in different ways, and when we reflect that every nutritive -change must extend from the point from which it started with -diminishing strength in a particular direction, we have a further -factor in the variation of determinants and one which influences even -similar determinants differently.</p> - -<p>Finally, if we remember that determinants of different constitution -will also extract different ingredients from the nutritive stream -and thus set up in it different kinds of chemical change, thus causing -an altered supply of nutritive substances to flow to the neighbour -determinants, we get some insight into a very complex and delicate -but perfectly definite set of processes, into a mechanism which we can -certainly only guess at, but whose results lie plainly before us in the -spontaneous variations of the organism. We understand in principle -the possibility of saltatory variation, as a more or less widespread, -more or less marked disturbance of the species-type in this or that -group of characters, and we may acknowledge that those 'kaleidoscopic -variations' which Eimer supposed to be the sole basis of the transformation -of species, and which have been brought to the foreground -again quite recently by De Vries<a id="FNanchor_20" href="#Footnote_20" class="fnanchor">[20]</a>, are probably factors in transmutation -operative within a limited sphere.</p> - -<div class="footnote"> - -<p><a id="Footnote_20" href="#FNanchor_20" class="label">[20]</a> See end of chap. xxxiii.</p> - -</div> - -<p>But we must think of all these struggles and mutual influencings -as taking place on the smallest possible scale, so that it is only by long -summation that they can produce any visible effect, and we must never -forget the essential significance of the plurality of ids, for these -'spontaneous' variations may take place in a different and quite -independent manner in each individual id. If this were not so no -intervention of personal selection would be possible, natural selection -would not exist, and the adaptation of the organism from the single<span class="pagenum"><a id="Page_156"></a>[Pg 156]</span> -cell up to the whole would remain wholly unexplained. The whole -crop of spontaneous germ-variations, whenever it ceases to be -'indifferent,' and becomes either 'good' or 'bad,' comes under the -shears of personal selection and under its almost sovereign sway.</p> - -<p>On the other hand, the sudden first appearance of a saltatory -variation takes place quite independently of personal selection, -depending on similar variations in a number of ids, which remain -latent until they have by the process of reducing division which -precedes amphimixis, chanced to attain a majority. In sudden bud-variations -we may perhaps suppose that reducing division occurring -in some still unverified abnormal manner is the reason why the -germinal variation suddenly makes itself visible—a supposition previously -suggested as the explanation of the reversion of these sports.</p> - -<p>The rarity of bud-variation is thus explained, while the greater -frequency of saltatory variations in plants propagated by seed may -be accounted for by the regular occurrence of reducing division -in sexual reproduction. But that the same or similar variations -may occur in several, it may be in many, ids at the same time must -depend upon similar general influences which affect the plant as -a whole, as happens through cultivation, manuring, and so on. I shall -return to this when discussing the influence of the environment.</p> - -<p>In some quarters this whole conception of germinal selection has -been characterized as the merest figment of imagination, condemned -on this ground alone, that it is based on the differences in nutrition -between such extremely minute quantities of substance as the -chromosomes of nuclear substance within the germ-cell. The quantity -of substance is certainly minute, but it needs nutriment none the -less, and can we believe that the stream of nourishment for all -the invisibly minute vital elements is exactly alike? It may be -admitted that the nourishment outside the ids is usually abundant, -although undoubtedly fluctuations occur in it also, but it certainly -does not follow from this that every vital unit within the id is -similarly disposed in relation to the nutritive supply, or has food -in equal quantities at its command, or even that each has as much -as it can ever need. To make an assertion like this seems to me -much the same as if an inhabitant of the moon, looking at this earth -through an excellent telescope and clearly descrying the city of -Berlin with its thronging crowds and its railways bringing in the -necessaries of life from every side, should conclude from this abundant -provision that the greatest superfluity prevailed within the town, -and that every one of its inhabitants had as much to live upon -as he could possibly require.</p> - -<p><span class="pagenum"><a id="Page_157"></a>[Pg 157]</span></p> - -<p>We certainly ought not to conclude from the fact that we -cannot see into the structure and requirements and methods of -nutrition of a very minute mass of substance that its nutrition -cannot be unequal, and that it cannot, by its inequalities, give rise -to very material differences, especially when we are dealing with -a substance to which we must attribute an extraordinarily complex -organization built up of enormous numbers of extremely minute -particles. That this complexity is undeniable is now admitted by -many who formerly thought it possible to believe in the simple -structure of the germ-substance. How complex not only the germ-substance -but every cell of a higher organism is in its structure, -and how far below the limits of visibility its differentiations and -arrangements reach, is pressed upon our attention by the most recent -histological researches, such as those we owe to Heidenhain, Boveri, -and many others. The whole scientific world was amazed when -it came to know the mysterious nuclear spindle in the seventies, -and since then this has been quite thrown into the shade by the -discovery of the centrosphere, the centrosome, and more recently -even the centriole, and now we believe that these marvellous centres -of force may, or must, possess their own dividing apparatus! In -the face of discoveries like these no one is likely to be able to persist -in recognizing as existing only what is disclosed or even hinted at by -the most powerful lenses; no one can any longer doubt that far below -the limit of visibility organization is still at the basis of life, and that -it is dominated by orderly forces. To me, at least, it seems more -cogent to argue from the phenomena of heredity and variation to -an enormous mass of minute vital units crowded together in the -narrow space of the id, than to argue from the calculated size of -atoms and molecules to the number which we are justified in assuming -to be present in an id. In my book on the germ-plasm I made -a calculation of this kind, and I arrived at figures which seemed -rather too small for the requirements of the germ-plasm theory. -This has been regarded as a proof that I disregard the facts for -the sake of my theory, but it should rather be asked whether the -size of the atoms and molecules is a fact, and not rather the very -questionable result of an uncertain method of calculation. Undoubtedly -modern chemistry has established the <i>relative</i> weight-proportions -of the atoms and molecules with admirable precision, -but it can make only very uncertain statements in regard to the -<i>absolute</i> size of the ultimate particles. It is therefore admissible -to assume that these have a still greater degree of minuteness when -the facts in another domain of science require this.</p> - -<p><span class="pagenum"><a id="Page_158"></a>[Pg 158]</span></p> - -<p>We <i>must</i> assume determinants, and consequently the germ-plasm -must have room for these; the variations of species can only be -explained through variations of the germ-plasm, for these alone give -rise to hereditary variation. It is upon this foundation that my -germinal selection is built up; whether I have in the main reached -the truth the future will show: but that I have not exhausted this -new domain, but only opened it up, I am very well aware.</p> -<hr class="full" /> - -<div class="chapter"> -<p><span class="pagenum"><a id="Page_159"></a>[Pg 159]</span></p> - -<h2 class="nobreak" id="LECTURE_XXVII">LECTURE XXVII</h2> -</div> - -<p class="c">THE BIOGENETIC LAW</p> - -<div class="blockquot"> - -<p>Fritz Müller's ideas—Development of the Crustaceans—Of the Daphnidæ—Of -Sacculina—Of parasitic Copepods—Larvæ of the higher Crustaceans—Change of -phyletic stages in Ontogeny—Haeckel's <i>Fundamental Biogenetic Law</i>—Palingenesis and -Cœnogenesis—Variation of phyletic forms by interpolation in a lengthened Ontogeny—Justification -of deductions from Ontogeny to Phylogeny—Würtemberger's series -of Ammonites—Phylogeny of the markings in the caterpillars of the Sphingidæ—Condensation -of Phylogeny in Ontogeny—Example from the Crustaceans—Disappearance -of useless parts—The variation of homologous parts, according to Emery—Germ-plasmic -correlations—Harmony with the theory of determinants—Multiplication of the -determinants in the course of the phylogeny.</p></div> - - -<p><span class="smcap">What</span> I propose to discuss in this lecture should have been -considered at an earlier stage, if we had pledged ourselves to adhere -strictly to the historical sequence of scientific discovery, for the -phenomena which we are about to deal with attained recognition -shortly after the revival of the evolution idea, and indeed they -formed the first important discovery which was made on the basis -of the Darwinian Doctrine of Descent. I have introduced them at -this stage because they have to do with phenomena of inheritance -and modifications of these, the understanding of which—in as far -as we can as yet speak of understanding at all—is only possible -on the basis of a theory of inheritance. Therefore, in order to -examine these phenomena and their causes, it was necessary first -to submit a theory of heredity, as I have done in the germ-plasm -theory. We have to treat of the connexion between the <i>development</i> -of many-celled individuals and the <i>evolution</i> of the species, between -germinal history and racial history, or, as we say with Haeckel, -between ontogeny and phylogeny.</p> - -<p>Long before Darwin's day individual naturalists had observed -that certain stages in the development of the higher vertebrates, such -as birds and mammals, showed a likeness to fishes, and they had spoken -of a fish-like stage of the bird-embryo. The 'Natural Philosophers' -of the beginning of the nineteenth century, Oken, Treviranus, Meckel, -and others, had, on the basis of the transmutation theory of the time, -gone much further, and had professed to recognize in the embryonic -history of Man, for example, a repetition of the different animal<span class="pagenum"><a id="Page_160"></a>[Pg 160]</span> -stages, from polyp and worm up to insect and mollusc. But von Baer -afterwards showed that such resemblances are never between different -types, but only between representatives of the same general type, -e.g. that of Vertebrata; and Johannes Müller maintained, from the -standpoint of the old Creation theory, that an 'expression of the most -general and simple plan of the Vertebrates' recurred in the development -of higher Vertebrates, giving as an instance that, at a certain -stage of embryogenesis, even in Man, gill-arches were laid down and -were subsequently absorbed. But why this 'plan' should have been -carried out where it was afterwards to be departed from remained -quite unintelligible.</p> - -<p>An answer to this question only became possible with the revival -of the Theory of Descent, and the first to throw light in this direction -was Fritz Müller, who, in his work <i>Für Darwin</i>, published in 1864, -interpreted the developmental history of the individual, 'the ontogeny,' -as a shortened and simplified repetition, a recapitulation, so to speak, -of the racial history of the species, the 'phylogeny.' But at the same -time he recognized quite clearly—what indeed was plain to all eyes—that -the 'racial history' cannot be simply read out of the 'germinal -history,' but that the phylogeny is often 'blurred,' on the one hand -by the fusing and shortening of its stages, since development is -always 'striking out' a more direct course from the egg to the perfect -animal, while, on the other hand, it is frequently 'falsified' by the -struggle for existence which the free-living larvæ have to maintain.</p> - -<p>For the establishment of these views Fritz Müller relied chiefly -upon larvæ, and in particular upon those of Crustaceans, and the -facts, which were in part new and in part interpreted in a new -manner, were so striking that it was impossible to deny their -importance. In particular, he drew attention to the fact that in -several of the lower orders of Crustaceans the most diverse species -have a similar form when they leave the egg, all of them being small, -unsegmented larvæ, with a frontal eye and a helmet-like upper lip, -and with three pairs of appendages, the two posterior pairs being two-branched -swimming-legs beset with bristles. In the size and form -of the body, and especially of the chitinous carapace, these larvæ -differ in the various systematic groups; thus, for instance, the larvæ -of the Copepods are simply oval, while those of the Cirrhipedes are -produced anteriorly into two horn-like processes, and so on, but in -essentials they are all alike, and for a long time these larval forms -had been distinguished by the special name of 'Nauplius' (<a href="#ff19">Fig. 109</a>).</p> - -<p>The development of the perfect animal begins with the longitudinal -growth of the Nauplius; the posterior end lengthens and<span class="pagenum"><a id="Page_161"></a>[Pg 161]</span> -becomes segmented, between the anterior portion and the tail more -segments are interpolated, and on these new pairs of limbs may grow. -The number of these segments and limbs varies according to the group -to which the animal belongs. Thus the body of the perfect animal in -the little Cyprids always consists of eight segments, seven of which -bear a pair of limbs apiece; in the Branchiopods, on the other hand, the -number of segments varies from twenty to sixty, with ten to over -forty pairs of legs; in the Daphnids or water-fleas there are about ten -segments, with seven to ten pairs of limbs, and in the Copepods about -seventeen segments with eleven pairs of limbs. The difference -between the orders depends not only upon the differences in the -number of segments and limbs, but quite as much upon the form and -development of the segments, and above all of the limbs, and in this -connexion it is worthy of note that the additional limbs which grow -out usually appear at first as biramose swimming-legs, and are -subsequently modified in form. Thus the pairs of jaws, three in -number, which appear in the Copepods are developed from such -swimming-legs, and so also is the second pair of antennæ in the -Copepods and the jaws of the Branchiopods, Cirrhipedes, &c.</p> - -<div class="figcenter" id="ff18"> -<img src="images/ff18.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 108.</span> Nauplius larva of one of the lower Crustaceans. After Fritz -Müller. <i>Au</i>, the frontal eye; <i>I</i>, first pair of limbs, corresponding to the future -antennæ; <i>II</i> and <i>III</i>, two biramose swimming appendages.</p> -</div> - -<p>If then we have before us in the 'germinal history' (ontogeny) -a fairly precise repetition of the 'racial history' (phylogeny), we may -deduce from this that the primitive forms of the Crustacean race -were animals which consisted of few segments, and that from these, -in the course of the earth's history, the very diverse modern groups -of Crustaceans have arisen, by the addition of new segments, and the -adaptation of the limbs upon them, which were at first biramose<span class="pagenum"><a id="Page_162"></a>[Pg 162]</span> -swimming-legs, to different kinds of functions, one becoming an -antenna, another a jaw or a swimming-arm, a third, fourth, fifth, and -so on, a jumping-leg, a copulatory organ, an egg-bearer, a gill-bearer, -or a tail-fin.</p> - -<div class="figcenter" id="ff19"> -<img src="images/ff19.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 109.</span> Metamorphosis of one of the higher Crustacea, a Shrimp -(<i>Peneus potimirim</i>), after Fritz Müller. <i>A</i>, the nauplius larva with the three -pairs of appendages: <i>I</i>, the antennæ; <i>II</i> and <i>III</i>, the biramose swimming-feet. -<i>Au</i>, the single eye. <i>B</i>, first Zoæa stage, with six pairs of appendages -(<i>I-VI</i>). <i>Skn</i>, area where new segments are being formed.</p> -</div> - -<p>That the development has in general followed those lines is made -clear chiefly by the fact that the members of all these different orders -of Crustaceans still arise from nauplius larvæ, even in those cases in -which the perfect animal possesses a structure differing widely from -the usual Crustacean form. <i>All</i> Crustaceans arise from the <i>nauplius -form</i>, even those of the higher orders, though they may not arise from -a nauplius <i>larva</i>. But this very circumstance, that in most of the -higher and many of the lower Crustaceans, the young animal, when -it emerges from the egg, already possesses more numerous segments -and limbs than a nauplius larva, again points to the connexion -between phylogeny and ontogeny, for in these cases the nauplius -stage <i>is gone through within the ovum</i>. The whole difference between -this and the forms we considered first lies in the fact that, in the -latter, the development is greatly shortened, condensed, as we might<span class="pagenum"><a id="Page_163"></a>[Pg 163]</span> -say, so that the nauplius stage forms a part of the <i>embryonic</i> development, -and that new segments and limbs develop in the embryo -nauplius within the egg, so that -the young animal leaves the egg -in a more advanced state, nearer -to that of the perfect animal, to -which it can, therefore, attain in -a shorter time.</p> - -<div class="figright" id="ff20"> -<img src="images/ff20.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 109.</span> <i>C</i>, second Zoæa stage. The<br /> -thorax is now divided into cephalothorax<br /> -(<i>Cph</i>) and abdomen (<i>Abd</i>); seven pairs of<br /> -appendages are developed, and five more<br /> -(<i>VIII-XII</i>) are beginning to appear. <i>Au</i>,<br /> -paired eyes.</p> -</div> - -<p>We should expect that this -shortening of the larval period -would be associated with a prolongation -of embryogenesis, especially -in those Crustaceans -which possess a large number of -segments and limbs, that is—in -the higher forms—and in the -main this is the case. But there -are exceptions in two directions; -in the first place there are some, -even among the lower Crustaceans, -which leave the egg not -as a nauplius but in the perfect -form of the adult, and secondly, -there are, among the higher -Crustaceans, certain species which -emerge from the egg not in the -more mature form but still in the -primitive nauplius form. Fritz -Müller was the first to furnish -an example of this last case, a -Brazilian shrimp, <i>Peneus potimirim</i>. -Like the lowest -Copepods or Branchiopods, this -species, which belongs to the -highest order of Crustaceans, -goes through the whole long -development, from the nauplius -through a series of higher larval -forms up to the perfect animal, -and all <i>outside of the egg</i>, as -an independent free-swimming larva (Fig. 109, <i>A-E</i>). This is in sharp -contrast to its near relative, the freshwater crayfish, which goes<span class="pagenum"><a id="Page_164"></a>[Pg 164]</span> -through this whole development within the egg, and emerges -perfectly formed.</p> - -<p>We see from this example that it is not some inward necessity -which thus, in the higher and more complicated organism, contracts -the ontogeny into the embryonic state, but that this depends upon -external adaptive factors. Here again we have adaptation, mainly to -the conditions of larval life. The elimination of the larvæ by enemies, -for instance, will, other things being equal, be so much the more -incisive the longer the larval development is protracted, but in that -case the general ratio of elimination of the species, and the degree of -fertility the species must possess in order to hold its own in the -struggle for existence, will also play a part in determining the mode<span class="pagenum"><a id="Page_165"></a>[Pg 165]</span> -of development. For the higher the ratio of elimination the more -eggs the female must produce, and the more eggs that have to be -produced the smaller will be the quantity of nutritive material for the -building up of the young embryo which each egg can be furnished -with. I know of no records in regard to the eggs of that Brazilian -shrimp in which embryonic development ends with the nauplius -stage, but we shall certainly not be wrong in predicting that the eggs -in this case will be very small and very numerous, in contrast to -those of the freshwater crayfish, which are large and, as compared -with others known to us, not very numerous.</p> - -<p>It is a point of undeniable theoretical significance which the life-histories -of these Crustaceans disclose, that embryogenesis is not condensed -according to hidden internal laws when the structure increases -in complexity, but that the condensation of the ontogenetic stages -depends upon adaptation, and may be quite different in nearly related -species. It shows us anew that all biological occurrences are dominated -by the process of selection.</p> - -<div class="figcenter" id="ff21"> -<img src="images/ff21.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 109.</span> <i>D</i>, Mysis-stage. Thirteen pairs of appendages are now formed: -<i>I</i> and <i>II</i>, antennæ; <i>III</i>, mandibles; <i>IV</i> and <i>V</i>, maxillæ; <i>VI-XIII</i>, swimming -appendages with one branch or with two. <i>Abd</i>, abdomen. <i>Sfl</i>, tail-fin. -<i>E</i>, the fully-formed Shrimp, with thirteen pairs of appendages on the -cephalothorax (<i>Cph</i>); <i>I</i> and <i>II</i>, the two pairs of antennæ; then follow the -maxillæ and maxillipedes (<i>III-VIII</i>), the last of which is visible in the figure, -and the five pairs of walking-legs (<i>IX-XIII</i>) of which the third bears a long -chela. On the abdomen there are now six pairs of appendages (<i>XIV-XIX</i>).</p> -</div> - -<p>I have already mentioned that exceptions to the usual mode of -development occur even among the lower Crustaceans, and I was -thinking at the time of the Daphnids, which leave the egg as fully -formed little animals, already equipped with all their segments and -limbs. The nauplius stage is passed through in the egg, and it is -an interesting indication that the ancestors of the modern species -were in the way of moulting, that this embryo nauplius moults within -the egg by forming a fine cuticle which is shed after a time. -If it be asked why there should be direct development in the case of -these small and not very complex water-fleas, while related species, -the Branchiopods, which are much richer in segments and in limbs, -should emerge from the egg in the form of a nauplius, and then pass -through a longer larval period, we may answer that the reason -probably lies in the fact that, in the former case, very few eggs are -produced, sometimes only one, often two, seldom more than a dozen, -that these eggs can thus be relatively well equipped with yolk, -and that the formation of the little body which bears only from -seven to nine pairs of limbs can be easily completed within this egg. -Other things being equal, the direct development would always be an -advantage, because reproduction can begin sooner in the young generation -and the number of individuals will thus increase more rapidly. -And this is of particular importance in the case of the water-fleas.</p> - -<p>But if it be asked, further, why so few eggs are produced in this -case, and whether these animals have no enemies, we must answer that, -on the contrary, they are preyed upon and eaten in thousands by<span class="pagenum"><a id="Page_166"></a>[Pg 166]</span> -fishes and other freshwater animals, but that the drawback of the scanty -production of eggs is counteracted on the one hand by their habit of -reproducing parthenogenetically for the greater part of the year, and -on the other hand by their habit of concealing the eggs in a special -brood-chamber. This is the case not only in the summer eggs, to -which nourishment is conveyed in the brood-chamber -from the blood of the mother (Fig. -70), but also in the winter or 'lasting' eggs, -which receive within the chamber a protecting -covering (the shell or ephippium).</p> - -<div class="figleft" id="ff22"> -<img src="images/ff22.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 70</span> (repeated). Daphnella.<br /> -<i>A</i>, summer ovum, with<br /> -an oil-globule (<i>Oe</i>). <i>B</i>, winter<br /> -ovum.</p> -</div> - -<p>In almost all the Daphnids the winter egg -develops into a perfect animal just like that -to which the summer egg gives rise, although -it no longer receives any nourishment after it -passes into the brood-chamber. But it receives -a larger supply of yolk on this account, so -that the nutritive provision within the egg is sufficient to develop the -perfect animal. There is only one exception to this, and it is of special -theoretical interest, because it shows more plainly than any other -fact that the greater or less degree of condensation in the ontogeny -depends upon the combined effect of the external conditions of life. -The largest of the Daphnidæ, <i>Leptodora hyalina</i>, a beautifully -transparent inhabitant of our lakes, which measures about a centimetre -in length (Fig. 110), also emerges from the summer egg as a perfect -animal, but from the winter egg, which floats freely in the water and -has only a small provision of yolk, it emerges as a nauplius, which<span class="pagenum"><a id="Page_167"></a>[Pg 167]</span> -then undergoes larval metamorphosis before it becomes a perfect -animal (Fig. 111).</p> - -<div class="figcenter" id="ff23"> -<img src="images/ff23.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 110.</span> The largest of the Daphnids (<i>Leptodora hyalina</i>), with summer -ova (<i>Ei</i>) beneath the shell (<i>Sch</i>). <i>I-IX</i>, the appendages. <i>II</i>, the oars (second -antennæ) which always remain biramose in Daphnids. <i>sb</i>, setæ. <i>ov</i>, ovaries. -<i>Schl</i>, œsophagus. <i>Ma</i>, stomach. <i>a</i>, anus. <i>H</i>, heart. <i>Au</i>, eye. <i>nG</i>, natural size.</p> -</div> - -<p>Fritz Müller concluded from the repetition of the nauplius form -in all orders of Crustaceans that the primitive form of the Crustacean -must have been a nauplius, and that from it all the modern Crustaceans -must have evolved phyletically by the addition of segments varying in -number and differentiation. Now, however, it is doubted whether -there ever were nauplioid types capable of reproduction. But even if -the nauplii only <i>represent what have been the larval</i> forms from very -early times, they are equally important in illustrating the relations -between ontogeny and phylogeny; they at any rate represent the -primitive pre-cambrian larval form from which all modern Crustaceans -are derived. This is borne out not only by the facts to which we have -already referred, but also by those Crustacean-groups which have -diverged far from the usual Crustacean habit and type.</p> - -<div class="figright" id="ff24"> -<img src="images/ff24.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 111.</span> Nauplius larva from the<br /> -winter egg of <i>Leptodora hyalina</i>; after<br /> -Sars.</p> -</div> - -<p>Thus the sessile Cirrhipedes, with their mollusc-like shells, their -soft, unsegmented bodies, degenerate -heads, and their twelve vibratile food-wafting -limbs, emerge from the egg as -nauplius larvæ. But the remarkable -parasites on the shore-crabs and the -hermit-crab deviate much further from -the type of the rest of the Crustaceans, -for they hang like a sac or formless -sausage-like soft mass to the abdomen -of their host, growing into it by -fine, pale, root-like threads, through -which they suck up the blood of their hosts (Fig. 112, <i>C. Sacc.</i>). They -possess neither head, nor thorax, nor abdomen, not even an indication -of segmentation, no limbs of any kind, neither antennæ, nor mouth -parts, nor swimming-legs. Nevertheless they are Crustaceans; indeed, -we can say with certainty that they belong to the order of Cirrhipedes, -for they leave the egg in the form of a nauplius larva (<i>A</i>), -with 'horns' on their carapace which no other forms except themselves -and the Cirrhipedes possess. That they are of the same stock -as these is also proved by their further development, for the nauplius -grows first, just as in the case of the Cirrhipedes proper, into a 'Cypris-like -larva' (<i>B</i>), so called because it bears a certain resemblance to the -Ostracods of the genus Cypris, and only from this point do their paths -of development diverge. The Cypris-like larva of the true Cirrhipedes -settles down somewhere, attached by its antennæ; it grows, and its -body becomes that of the perfect Cirrhipede; but the Cypris-like<span class="pagenum"><a id="Page_168"></a>[Pg 168]</span> -larva of the Sacculinæ bores its way into the inside of a crab or -hermit-crab, at the same time losing its limbs, segmentation, and its -chitinous covering; and within the body of its host it is transformed -into the sac-like organism we have already described. After a time it -emerges again on the surface, and remains attached to the abdomen of -its host (Fig. 112, <i>C. Sacc.</i>), drawing its nourishment from the blood -which it sucks up by means of its numerous delicate roots (<i>W</i>, <i>W</i>).</p> - -<div class="figcenter" id="ff25" > -<img src="images/ff25.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 112.</span> Development of the parasitic Crustacean <i>Sacculina carcini</i>, after -Delage. <i>A</i>, Nauplius stage. <i>Au</i>, eye. <i>I</i>, <i>II</i>, <i>III</i>, the three pairs of appendages. -<i>B</i>, Cypris-stage. <i>VI</i>-<i>XI</i>, the swimming appendages. <i>C</i>, mature animal (<i>Sacc</i>), -attached to its host, the shore-crab (<i>Carcinus mænus</i>), with a feltwork of fine -root-processes enveloping the crab's viscera. <i>s</i>, stalk. <i>Sacc</i>, body of the -parasite. <i>oe</i>, aperture of the brood-cavity. <i>Abd</i>, abdomen of the crab with -the anus (<i>a</i>).</p> -</div> - -<p>From all this we may conclude that certain Cirrhipedes in times -long past adopted a parasitic habit in the Cypris-larva stage, and that -they gradually underwent adaptations to this mode of life, and that -these went further and further, until the animal was transformed -into the singular creature which we now see in the sexually mature -form.</p> - -<p>The same is the case with the numerous fish-parasites of the -order Copepoda. They all leave the egg as nauplius larvæ, however -greatly they may be modified later on by adaptation to a parasitic -habit, and in them we can still observe, in the fully developed -animals, a whole series of grades of transformation. Thus many -genera, like <i>Ergasilus</i>, are distinguished from the free-swimming -Copepods only by the modification of their jaws into piercing and -sucking organs, and of a single pair of antennæ into hooks, by means -of which they attach themselves to the fish on which they feed. In -other genera the degeneration and modification go further; the -antennæ, the eye, and the appendages degenerate more or less, and<span class="pagenum"><a id="Page_169"></a>[Pg 169]</span> -very remarkable attaching organs are sometimes developed, in the -form of hooks or of knobbed pincers, or of actual suckers. In several -types the degeneration and modification go so far that the segmentation -of the body disappears, and the animal looks more like an -intestinal worm than like a Crustacean (<i>Lernæocera</i> and others). In -all these forms adapted to a parasitic mode of life it is always only -the mature animal which has been transformed in this manner, for -previously it has gone through a series of stages which are quite -similar to those of the free-swimming Copepods, beginning with the -nauplius, and ending with the so-called Cyclops stage, that is, a larval -form which possesses antennæ, eyes, and swimming-legs similar to our -freshwater Copepods of the genus <i>Cyclops</i>.</p> - -<p>Here again we see in the ontogeny the repetition of a series of -phyletic stages before the mature form is assumed. Why these -stages should have persisted it is easy enough to understand, for how -could an animal which emerged from the egg as a worm-shaped -<i>Lernæocera</i> find a fresh fish which would serve it as host? Yet these -parasites could not possibly go on preying upon the same fish generation -after generation. To secure the existence of the species it was -therefore indispensable that the faculty of swimming should be -retained at least in the young stages; in other words, that the free-swimming -ancestral stages should be preserved in the ontogeny. In -all these cases it is therefore beyond doubt that the germinal history -recapitulates a series of stages comparable to those of the racial -history, although not quite unchanged but adapted to the modern -conditions of life, for instance in having shorter antennæ, smaller -eyes, and with four instead of the usual five swimming-legs. The -search for a host does not seem to last long, for fishes are usually -found in large numbers together, and thus the young parasitic -Crustacean does not require to make a long journey before it finds -a refuge.</p> - -<p>It is noteworthy that the males of parasitic Crustaceans are not -only much smaller than the females (Fig. 113), but that they are also -much less modified, and resemble the ancestral free-swimming -Copepods to a much greater degree. They usually possess small but -well-developed swimming-legs, and by means of these they seek out -the female, dying after fertilization is accomplished. They are thus -not sessile parasites at all, and have therefore to go through the -stages of the free-swimming Copepods much more completely than -the females, whose task is to accumulate within themselves from the -blood of the fish as much material as possible for the forming of the -eggs, and to produce the largest possible number of these. These<span class="pagenum"><a id="Page_170"></a>[Pg 170]</span> -therefore greatly surpass the free-swimming Copepods in fertility, as -is evidenced by the enormous egg-sacs they bear at the posterior end -of the body (Fig. 113, <i>ei</i>).</p> - -<p>Even among the higher Crustaceans, the so-called Malacostraca, -the germinal history not infrequently exhibits more or less of the -racial history in distinct recapitulation.</p> - -<div class="figleft" id="ff26"> -<img src="images/ff26.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 113.</span> The two<br /> -sexes of the parasitic<br /> -Crustacean <i>Chondracanthus<br /> -gibbosus</i>, enlarged<br /> -about six times; after<br /> -Claus. The main figure<br /> -is that of the female,<br /> -whose body bears quaint<br /> -blunt processes. At its<br /> -genital aperture (♂) a<br /> -dwarf male is situated.<br /> -<i>F</i> and <i>F´</i>, the two pairs<br /> -of appendages. <i>ei</i>, the<br /> -long egg-sacs, portions<br /> -of which have been cut<br /> -off in the figure.</p> -</div> - -<p>It is true however, as we have already shown, that there are only -a few of the higher Crustaceans which emerge -from the egg in the form of a nauplius; in -most of them this stage has been shunted backwards -in the ontogeny, and most of the crabs -and hermit-crabs leave the egg in a higher larval -form, that of the so-called Zoæa (Fig. 114). This -term is applied to a larva which already exhibits -two main divisions of the body, a head and -thorax portion (cephalothorax, <i>Cph</i>) and an -abdomen (<i>abd</i>). The cephalothorax is frequently -equipped with remarkable long spines (<i>st</i>), and -it always bears from five to eight pairs of limbs, -anteriorly the antennæ (<i>I</i> and <i>II</i>), then the -mandibles (<i>III</i>), further back swimming-legs (<i>IV</i>, -<i>V</i>), and behind these can be recognized the -primordia of the other legs (<i>VI</i>-<i>XIII</i>), which -will grow freely out later on. Large facetted -and stalked eyes (<i>Au</i>) are borne on the head. -This Zoæa form is not now found as a mature -Crustacean form, so we cannot maintain with -any confidence that it lived as a mature animal -at an earlier period of the earth's history, but -a second still more complex larval form of the -higher Crustaceans is preserved for us in a group -of marine Crustaceans, the Schizopods. These -are Crustaceans which, though small, approach -in external appearance our freshwater crayfish, -only they have, instead of the ten walking-legs, -biramose swimming-legs, by means of which they move freely in the -water. The number of these branched legs is even greater than ten, -there are sixteen of them (<a href="#ff21">Fig. 109</a> <i>D</i>, p. 164, <i>VI</i>-<i>XIII</i>). In the -aquaria of the Zoological Station at Naples one may often see these -dainty little creatures swimming about in large companies. Here -they are of interest to us chiefly because their structure occurs -in the ontogeny of the highest Crustaceans, the Decapods; that is,<span class="pagenum"><a id="Page_171"></a>[Pg 171]</span> -the phyletic stage represented by the Schizopods appears as an -ontogenetic stage, just before the final metamorphosis of the larva -to the perfect animal. This is the case in most of the marine -Decapods, in those forms which do not go through the whole course -of their development within the egg, but emerge as Zoæa larvæ, -or even, as in <i>Peneus potimirim</i>, as nauplii. In the last-named -species (<a href="#ff21">Fig. 109</a>) the ontogeny contains at least three stages which -must have lived, perhaps not as mature forms, but as primitive larval -forms, for unthinkable ages—the stage of the nauplus (<a href="#ff21">Fig. 109</a> <i>A</i>), -that of the Zoæa (<a href="#ff21">Fig. 109</a> <i>B</i> and <i>C</i>), and that of the Schizopod -(<a href="#ff21">Fig. 109</a>, <i>D</i>); from this last the fully -developed Decapod Crustacean arises -(<a href="#ff21">Fig. 109</a>, <i>E</i>).</p> - -<p>We are, therefore, justified in -saying that here the racial evolution -is recapitulated in the individual development, -although condensed and -shortened in proportion as more numerous -stages of the phyletic development -are gone through within the egg, for -there the different stages can succeed -each other more rapidly and directly -than in a metamorphosis of the free-swimming -larvæ, since these must -procure their own material for their -further growth and their metamorphosis, -while the yolk of the egg -supplies a store of material which is -sufficient for the production of a whole -series of successive stages.</p> - -<div class="figright" id="ff27"> -<img src="images/ff27.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 114.</span> Zoæa-larva of a Crab,<br /> -after R. Hertwig. <i>I</i>-<i>V</i>, the already<br /> -functional anterior appendages—antennæ,<br /> -mandibles, and swimming-legs.<br /> -<i>VI</i>-<i>XIII</i>, rudiments of the<br /> -posterior appendages of the cephalothorax<br /> -(<i>Cph</i>). <i>Abd</i>, the abdomen.<br /> -<i>st</i>, spine of the carapace. <i>Au</i>, eye.<br /> -<i>H</i>, heart.</p> -</div> - -<p>For this reason it inevitably resulted that the sharply defined -characters of the phyletic stages were more and more lost as -soon as they were transferred from larval stages to stages in -embryogenesis. For, in the first place, these sharply defined characters, -such as the spines of the Zoæa larva, or the swimming bristles of -the 'oars,' or the shape of thorax or abdomen characteristic of certain -species, are adapted to a free life, and would be valueless in an -embryonic stage; and secondly, in the transference of the free larval -stages to embryonic development the greatest possible condensation -and abbreviation of the stages must have been striven for, which -could only come about by a continual mutual adaptation of the -embryonic parts to one another, involving the suppression of every<span class="pagenum"><a id="Page_172"></a>[Pg 172]</span>thing -superfluous. Otherwise the transference of the free stages to -the embryogenesis would have brought no advantage, but rather -a most prejudicial protracting of the development.</p> - -<p>We must not, therefore, expect to find the stages of the -phylogeny occurring unaltered in every ontogeny in the way we have -found the nauplius, Zoæa, or Mysis stages in the larval development of -the Decapods. I have noticed already that in the water-fleas -(Daphnidæ) and other Crustaceans without metamorphosis the -nauplius stage is still passed through, but within the egg, and as an -embryonic stage, and this is quite true, but nevertheless it would -hardly do to liberate a nauplius like this from its shell and place it in -the water, for the influence of the water upon the delicate embryonic -cells of its body would soon cause it to swell, and would destroy -it utterly. And, even apart from this, it has no hard and resistant -chitinous covering, no fully-developed appendages, but only the -stump-like blunt beginnings of these without swimming-bristles and -without muscles capable of function, so that it could not even move. -Nevertheless it is a nauplius with all its typical distinctive characters, -only it is not a perfect nauplius capable of life, but rather a 'schema' -of one, which must be retained in the embryogenesis that it may give -rise to the later stages.</p> - -<p>Shall we therefore say that the statement that phylogeny -repeats itself in ontogeny is false, that the nauplius stage within the -embryo is not a true nauplius at all? That would be pushing precision -beyond reasonable limits, and would obscure our insight into -the causal connexion between phylogeny and ontogeny, which, as we -have seen, undoubtedly exists.</p> - -<p>A few years after the appearance of Fritz Müller's work <i>Für -Darwin</i>, Haeckel elaborated Müller's idea, and applied it in a much -more comprehensive manner. He formulated it under the name of -'the fundamental biogenetic law,' and then he used this 'law' to -deduce from the ontogeny of animals, and more particularly of Man, -the paths of evolution along which our modern species have passed in -the course of the earth's history. In doing so the greatest caution -was necessary, since ontogeny is not an actual unaltered recapitulation -of the phylogeny, but an 'abridged' and in most cases—in my own -belief, in all cases—<i>a greatly modified recapitulation</i>. Therefore we -cannot simply accept each ontogenetic stage as an ancestral stage, but -must take into consideration all the facts supplied to us by other -departments of biological inquiry which afford help in the decision of -such questions, especially those brought to light by comparative -morphology and by the whole range of comparative embryology.</p> - -<p><span class="pagenum"><a id="Page_173"></a>[Pg 173]</span></p> - -<p>Haeckel was quite well aware of this difficulty, and repeatedly -emphasized it by laying stress on the fact that a 'blurring' of the -phyletic stages of development had arisen through the abridgement of -the phylogeny in the ontogeny, and a 'falsification' of it through the -secondary adaptation of individual ontogenetic stages to new conditions -of life. He therefore distinguished between 'Palingenesis,' -that is, simple though abridged repetition of the ancestral history, and -'Cœnogenesis,' that is, modification of the racial history by later -adaptation of a few or many stages to new conditions of life. As an -example of cœnogenetic modification, I may cite the pupæ of butterflies. -Since these can neither feed nor move from one spot, they can -at no time have been mature forms, and cannot, therefore, represent -independent ancestors of our modern Lepidoptera; they have -originated through the constantly increasing difference between the -structure of the caterpillar and that of the moth or butterfly. -Originally, that is, among the oldest flying insects, the mature animal -could be gradually prepared within the larva as it grew, so that finally -nothing was necessary but a single moult to set free the wings, -which had in the meantime been growing underneath the skin, and to -allow the perfect insect to emerge, complete in all its parts. This is -the case even now with the grasshoppers and crickets. In these -forms the larval mode of life differs very little, if at all, from that of -the perfect insect, and the main difference between the two is the -absence of wings in the larva. But when the perfect insect adapted -itself to conditions of life quite different from the larval conditions, as -was the case with the nectar-sucking bees and butterflies adapted -entirely for flight, while the larvæ were still adapted exclusively to an -abundant diet of leaves and other parts of plants, and to a very -inactive life upon plants, the two stages of development ultimately -diverged so widely in structure that the transition from one to the -other could no longer be made at a single moulting, and a period of -rest had to be interpolated, in order that the transformation of the -body could take place. In this way arose the stage of the resting and -fasting pupa, a 'cœnogenetic' modification of the last larval stage, -<i>not a recapitulation of an ancestral form</i>, but a stage which has -been interpolated, or better, has 'interpolated itself' into the ontogeny -on account of the widely different adaptations of the early and the -final stages.</p> - -<p>This is a perfectly clear idea, and Haeckel's distinction between -palingenesis and cœnogenesis is undoubtedly justified.</p> - -<p>But it is quite a different matter to be able to decide whether -a particular stage or organ has arisen palingenetically or cœnogeneti<span class="pagenum"><a id="Page_174"></a>[Pg 174]</span>cally -with the same certainty as in the case of the insect-pupa, or -even with any degree of probability, and we must admit that in very -many cases, perhaps even in most cases, it is impossible. This is so -chiefly because pure palingenesis is hardly likely to occur now; the -ancestral stages were bound to be modified in any case if they were to -be compressed into the ever-shortening ontogeny of later descendants, -and particularly so if they were to be shunted back into embryogenesis. -In the latter case they would not only be materially -shortened, and, as I have already shown, modified by the mutual -adaptations of the different developing parts, but time-displacements -of embryonic parts and organs would be necessary, as has been very -clearly proved by the excellent recent investigations, which we owe in -particular to Oppel, Mehnert, and Keibel. A shunting forward or -backward of the individual organs takes place—conditioned apparently -by the decreasing or increasing importance of the organ in the -finished state; for in the course of the phylogeny everything may vary, -and not only may a new, somewhat modified, and often more complex -stage be added on at the end of the ontogeny, but each one of the -preceding stages may vary independently, whenever this is required -by a change in its relations to the other stages or organs. Adaptation -is effected at every stage and for every part by the process of selection, -for all parts of the same rank are ceaselessly struggling with one -another, from the lowest vital units, the biophors, up to the highest, -the persons. If we reflect that, in the course of the phylogeny of -every series of species, a number of organs always become superfluous -and begin to disappear in consequence, we can understand what great -changes must take place gradually as such a series of phyletic stages -is compressed into the ontogeny, for all organs which are no longer -used are gradually shifted further and further back in the ontogeny -till ultimately they disappear from it altogether. But, while the -primary constituents of these 'vestiges' play their part in ontogeny -for a shorter and shorter time, new acquisitions are being more and -more highly developed, and thus, in the course of the phylogeny, -numerous time-displacements of the parts and organs in ontogeny -must result, so that ultimately it is impossible to compare a particular -stage in the embryogenesis of a species with a particular ancestral -form. <i>Only the stages of individual organs can be thus compared -and parallelized.</i></p> - -<p>But we must not on that account 'empty out the child with the -bath,' and conclude that there is no such thing as a 'biogenetic law' -or recapitulation of the phylogeny in the ontogeny. Not only is there -such a recapitulation, but—as F. Müller and Haeckel have already<span class="pagenum"><a id="Page_175"></a>[Pg 175]</span> -said—ontogeny is nothing but a recapitulation of the phylogeny, -only with innumerable subtractions and interpolations, additions and -displacements of the organ-stages both in time and place. It would -be a great mistake to conclude from the fact of these manifold -alterations that the whole proposition of the recapitulation of the -phylogeny in the ontogeny is erroneous, or at least valueless. If its -only use were to enable us to read the racial history of a species out -of its germinal history, it is intelligible enough that we might be led -to give it up in despair, but I think that the main thing is to get -some insight into the history of the ontogeny, and there can be no -doubt that this can have been built up on no other foundation than -upon the racial history. What is new could only have arisen from -what was already in existence, and everything in ontogeny, not only -the palingenetic stages which still represent in some measure the -facies of fully-formed ancestral stages, but also the cœnogenetic -stages, like the pupa-stage we have already discussed, have arisen -historically, nothing <i>de novo</i>, but all in connexion with what was -already present. But what was first present was in all cases the -stages of the ancestral forms.</p> - -<p>It is undoubtedly of the greatest value to be able to penetrate -more and more deeply into embryonic development, and to discover -more precisely the changes that have taken place throughout its -course in the originally existing material of ancestral forms. But -it must not be forgotten that, all transformations notwithstanding, -so much of the racial history is still very plainly indicated in the -germinal history, that this must always remain for us a most important -source from which to draw conclusions in regard to the phyletic -development of any animal group. I admit that these conclusions -have sometimes been drawn with too great confidence, but even if we -cannot regard as well founded Haeckel's view that in the ontogeny -of Man there are fourteen different ancestral stages recognizable, -a protist stage, a gastræa stage, a prochordate, an acranial, a cyclostome, -a fish-stage, and so on, we must recognize that the unicellular -stage of ontogeny, with which even now the development of every -human being begins, undoubtedly repeats the facies of an ancestor, -although greatly altered; for we must be descended from unicellular -organisms. The essential part of this ancestral stage is thus preserved -in the ontogeny, and only what is special and in some measure -due to chance, that is, to adaptation to special conditions of existence, -has been modified.</p> - -<p>It has been supposed that the proposition that phylogeny is -recapitulated in the ontogeny is disproved, because the ontogenetic<span class="pagenum"><a id="Page_176"></a>[Pg 176]</span> -stage must always contain within it the primordia of the later stages -which have been added since the corresponding phylogenetic stage. -It is certain that the egg-cell or the sperm-cell of Man contains, -though in a form not recognizable by us, all the determinants of the -perfect human body, but this neither affects its nature as a cell nor its -particular form as ovum or spermatozoon. It is essentials that are -important in this comparison, not accessories. Neither can I agree -with Hensen's argument when he says that the 'recapitulation-idea' -is erroneous, because the actual course of ontogeny is the 'best and -only possible one,' which, apart from previous history altogether, -must of necessity be followed. Certainly the actual course is the -best, and under the given circumstances the only possible one, but -that does not exclude recapitulation, on the contrary it implies it, for -ontogeny could at no time have arisen from a <i>tabula rasa</i>, but only -from what was historically existent.</p> - -<p>I do not propose to examine each of Haeckel's ancestral stages -in Man's pedigree, or to estimate the degree of probability with which -they may be deduced from the ontogeny; but that Man's ancestry -does, in a general way, include such a series of phyletic stages may -be admitted, even if we grant that many of these stages are now -no longer represented in the ontogeny as stages of the developing organism -as a whole, but only by stages of individual organs or group of -organs. Thus it may be disputed whether there is still a fish-stage in -human development, but it cannot be disputed that the rudiments of -'gill-arches' and 'gill-clefts,' which are peculiar to one stage of -human ontogeny, give us every ground for concluding that we -possessed fish-like ancestors.</p> - -<p>As we now know that the history of a given mode of embryogenesis -has involved numerous time-displacements of the organ-rudiments, -we must attach all the more weight to the developmental -history of the individual parts and characters, in which the phylogeny -can often be read more clearly than in the stages of the organism -as a whole, and we can probably find out important laws in this way.</p> - -<p>As far back as 1873 Würtemberger investigated the fossil -ammonites with special reference to this point. He was concerned -even more at that time with finding proofs of the theory of descent -in general, and this was the first case in which any one succeeded -in demonstrating phyletic transformation-series of species, deposited -one above the other in a corresponding series of geological strata, -and connected by transition forms lying between these. In studying -this interesting material, of which many examples were at his disposal, -Würtemberger proved that the variations which had taken place<span class="pagenum"><a id="Page_177"></a>[Pg 177]</span> -in the spirally coiled shell in the course of ages appeared first on the -last whorl, and then subsequently extended to the one before this, -and thence to the still younger whorls of the shell. Meanwhile the -last whorl not infrequently exhibited another new character. Thus, -for instance, protuberances on the shell were shifted in the course -of the phylogeny from the last convolution to the second last, and -later to the third last, and so on, while at the same time the last -convolution showed the protuberance changed into spines. In other -words, the new phyletic acquirements first appeared in the mature -animal (in the last-formed whorl or chamber of the shell), but were -subsequently shifted back in the ontogeny to younger stages in -proportion as new transformations of the mature animal appeared. -Thus there was, so to speak, a retraction of the phyletic acquisitions -of the mature animal deeper and deeper into the germinal history -of the species.</p> - -<p>About the same time—in the seventies—I obtained similar -results from living species when I was attempting to work out the -ontogeny of the markings on the external skin of the caterpillars -of certain butterflies, and I should like to submit a short account -of these.</p> - -<p>In one of the early lectures we discussed the protective and -defensive colours of caterpillars in general, and those of caterpillars -of the Sphingidæ in particular. I showed that those naked caterpillars -which live on plants among the grass, or on the grass itself, -are often not only green, like fresh grass-stalks, or yellowish-grey, -like dry ones, but all the larger forms also exhibit light, usually -white, longitudinal lines, which, by mimicking the sharp light reflections -on the grass-stems, heighten the protective resemblance.</p> - -<p>We also spoke of the light transverse stripes, often marked with -pink or lilac-blue, of many of the large green caterpillars which live -on trees and bushes, and whose likeness to the leaves is heightened -by this imitation of the lateral veining of a leaf; and finally we -mentioned the warning coloration indicative of unpleasant or nauseous -taste, among which must be classed not only vivid contrasts of colour, -but also specially conspicuous elements of colour such as light ring-spots -upon a dark ground. These different colour schemes which -protect the caterpillars from their enemies are usually only to be -found in the adolescent caterpillar, not in the very small one which -has just emerged from the egg, and the development of the markings -in the individual life clearly shows that the phylogeny of the markings -is more or less obviously contained in the ontogeny.</p> - -<p>There are three different schemes of marking which occur in the<span class="pagenum"><a id="Page_178"></a>[Pg 178]</span> -caterpillars of hawk-moths or Sphingidæ—longitudinal striping, -obliquely transverse striping, and spots. Longitudinal striping pure -and unmixed is now found only in a few species, for instance -in the caterpillar of the <i>Macroglossa stellatarum</i> (Fig. 115), in -which a white longitudinal line, beginning at the tip of the tail, -runs up each side of the body to the head as a 'sub-dorsal stripe' -(<i>sbd</i>). These, with other two similar stripes, effectively secure -the fairly large caterpillar -from discovery when it is -among grass and herbs.</p> - -<div class="figleft" id="ff28"> -<img src="images/ff28.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 115.</span> Caterpillar of the Humming bird<br /> -Hawk-moth, <i>Macroglossa stellatarum</i>. <i>sbd</i>, the sub-dorsal<br /> -line.</p> -</div> - -<p>Transverse striping occurs -as the sole mode of marking -in species which live on bushes -and trees whose leaves have -strong lateral veins, such as -willows, poplars, oaks, privet, syringa, and so on, and these markings -associated with the leaf-green of their colouring protect them most -effectively from discovery.</p> - -<p>The third scheme of marking, namely by spots, occurs in various -forms in species of the genera <i>Deilephila</i> and <i>Chærocampa</i>, and it -varies in its biological significance; in many species the spots serve -as a warning colour, by making the caterpillar conspicuous and easily -seen from a distance (<i>Deilephila galii</i>, <a href="#ff33">Fig. 117</a>); in others they -imitate the eyes of a larger animal, and have a 'terrifying' effect, -as we have already said (Fig. 4); in still other and rarer cases they -heighten the resemblance -of the caterpillar to its -food-plant by mimicking -parts of it, as, for instance, -the red berries of the -buckthorn (<i>Deilephila -hippophaës</i>, Fig. 8, <i>r</i>).</p> - -<div class="figleft" id="ff29"> -<img src="images/ff29.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 3</span> (repeated). Full-grown caterpillar of the<br /> -Eyed Hawk-moth, <i>Smerinthus ocellatus</i>. <i>sb</i>, the sub-dorsal<br /> -stripe.</p> -</div> - -<p>Thus all three modes -of marking possess a biological value, and protect the soft and -easily wounded animal in some way, and, in the case of at least -two of them, it is clear that they must have arisen at the very -end of the caterpillar's development, since they can only be effective -as the animal is approaching full size, and would be valueless -in the very young caterpillar. The transverse striping only makes -the caterpillar like a leaf when the stripes bear about the same -relation to each other as those on the leaf, and eye-spots can only -scare away lizards and birds when they are of a certain size. Only<span class="pagenum"><a id="Page_179"></a>[Pg 179]</span> -longitudinal striping is effective as a protection in the case of young -caterpillars, supposing, that is, that they live in or on the grass (Fig. 116).</p> - -<div class="figcenter" id="ff30"> -<img src="images/ff30.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 4</span> (repeated). Full-grown caterpillar of the Elephant Hawk-moth, -<i>Chærocampa elpenor</i>, in its 'terrifying attitude.'</p> -</div> - -<div class="figcenter" id="ff31"> -<img src="images/ff31.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 8</span> (repeated). Caterpillars of the Buckthorn Hawk-moth, <i>Deilephila -hippophaës</i>. <i>A</i>, Stage III. <i>B</i>, Stage V. <i>r</i>, annular spots.</p> -</div> - -<p>Let us consider the ontogeny of these different forms of markings, -beginning with the eye-spots. It appears that these develop from -a sub-dorsal stripe, which appears in the young caterpillar in the -second stage of its life, and from it, in the course of the further -development, two pairs of large eye-spots are formed. Even in the -young caterpillar, scarcely one centimetre in length (Fig. 116), it can -be observed that the fine, white sub-dorsal line takes a slight curve -upwards on the fourth and fifth segments (<i>C</i>), and on the lower edge -of these curves a black line is laid down (<i>D</i>). This is then continued -to the upper side (<i>E</i>), and encloses the piece of the sub-dorsal stripe -(<i>F</i> and <i>G</i>), and thus there arises a white-centred, black-framed spot -which only requires to grow and to differentiate a blackish shadow-centre, -the pupil (<i>G</i>), to give the impression of a large eye. This -occurs as the caterpillar goes on growing, and after the fourth moult -or ecdysis the eyes have already some effect, as the animal is six centimetres -in length, but they become even more perfect in the fifth and -last stage. During this development of the eye-spot the sub-dorsal -stripe disappears completely from the greater part of the caterpillar, -persisting only on the first three segments (Fig. 116, <i>B-F</i>).</p> - -<p><span class="pagenum"><a id="Page_180"></a>[Pg 180]</span></p> - -<div class="figcenter" id="ff32"> -<img src="images/ff32.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 116.</span> Development of the eye-spots in the caterpillar of the Elephant -Hawk-moth (<i>Chærocampa elpenor</i>). <i>A</i>, Stage I, still without marking, simply -green. <i>B</i>, Stage II, with sub-dorsal stripe (<i>sbd</i>). <i>C</i>, sub-dorsal line somewhat -later, with the first hint of the eye-spot (<i>Au</i>) on segments 4 and 5. <i>D</i>, eye-spots -in Stage III of the caterpillar, somewhat further developed than in <i>E</i>, -the third stage. <i>F</i>, Stage IV. <i>G</i>, the anterior eye-spot at the same stage.</p> -</div> - -<p>When we consider that this stripe in the little caterpillar -a centimetre long, which lives on the large leaves of the vine, or on -the obliquely ribbed willow-herb (<i>Epilobium hirsutum</i>), is quite -without protective value, its occurrence at that stage can only be -regarded as a phyletic reminiscence due to the fact that the ancestors -of these species of <i>Chærocampa</i> possessed longitudinal stripes in -the adult state, probably because at that time they lived on plants -among the grass, and that, later, when the species changed their -habitat to plants with broad leaves which had arisen in the meantime, -eye-spots were developed in addition to the green or brown protective -colouring which they retained. Thus the modern development of -these spots mirrors their phyletic evolution very faithfully; on the -two segments there were formed, from pieces of the sub-dorsal line, -first white spots ringed round with black, then unmistakeable eyes -with pupils (<i>C</i>, <i>D</i>, <i>G</i>). This transformation can only have begun in -the fairly well-grown caterpillar, because it was only of any use to it; -but later on it was shunted further back in the ontogeny, from the<span class="pagenum"><a id="Page_181"></a>[Pg 181]</span> -sixth and fifth to the fourth and third caterpillar stage, not in its -complete development, but in more and more incipient form; and -nowadays the first traces of eyes, as we have already seen, are visible -in the course of the second stage. The marking of the more remote -ancestors, the longitudinal striping, is now lost in proportion as the -eye-spots develop, perhaps because the former would take away from -the full effect of the latter. The longitudinal stripes are still quite -plainly visible on the first three segments, but these segments are -drawn in and are scarcely noticeable when the caterpillar assumes -a defiant attitude (Fig. 4).</p> - -<p>In the case of marking with ring-spots, which is found especially -in species of the genus <i>Deilephila</i>, the ontogeny discloses that it has -developed phyletically from the sub-dorsal stripe; in the young stage -of this caterpillar also, the -sole marking is longitudinal -striping; in <i>Deilephila zygophylli</i>, -from the steppes of -Southern Russia, this persists -apparently through all the -stages, but in the others it -disappears almost completely -in the later stages, but only -on the segments on which the -spot-marking has developed -from it. This happens in a -manner similar to that in which -the eye-spot in <i>Chærocampa</i> -arises, a piece of the white sub-dorsal stripe is enclosed above and below -by a semicircle of black, and later these semicircles unite, and cut -off the portion of the sub-dorsal line, and form a black spot with a -light centre within which a red spot frequently appears (Fig. 117, <i>A</i>).</p> - -<div class="figright" id="ff33"> -<img src="images/ff33.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 117.</span> Caterpillar of the Bed-straw Hawk-moth<br /> -(<i>Deilephila galii</i>). <i>A</i>, Stage IV, sub-dorsal<br /> -stripe still distinct, the annular spots are still<br /> -incompletely enclosed in it. <i>B</i>, fully-formed<br /> -caterpillar without trace of a sub-dorsal stripe,<br /> -but with ten annular spots.</p> -</div> - -<p>In most species these ring-spots occur on many segments (10-12) -(Fig. 117, <i>B</i>), and in cases where they are of importance in making -the caterpillar conspicuous and easily seen they sometimes form -a double row. But we know one species, <i>Deilephila hippophaës</i>, in -which only a single ring-spot exists, and it is a large brick-red spot -on the second last segment, mimicking the red berry of the buckthorn -(Fig. 8, <i>A</i> and <i>B</i>, <i>r</i>). But individuals also occur in which there are, -on the five or six segments in front, smaller ring-spots which become -less distinct the further forward they are, and in most caterpillars it -is possible, on careful examination, to recognize little red dots on the -faded sub-dorsal stripes of these segments (Fig. 8, <i>B</i>). We might be<span class="pagenum"><a id="Page_182"></a>[Pg 182]</span> -disposed to think, on this account, that the ancestors of <i>D. hippophaës</i> -bore rings on all the segments, and that these had gradually become -vestigial on the majority of them, because they had lost their earlier -biological importance, and now, by adaptation to the buckthorn, could -only be of use on the second last. But when we take the ontogeny -also into account we find in the young caterpillar only a simple -sub-dorsal line, upon which, in the third stage, the red spot of the -tail-horn segment appears (Fig. 8, <i>A</i>).</p> - -<p>No spots ever occur on the other segments at this stage; they -only appear in the last stage, but as they may be entirely wanting, -they must have arisen as the result of internal laws of correlation, -that is, they must be recapitulations of the hindmost spots which -arose in the phylogeny through natural selection. We may conclude -this, at least, if we believe in the truth of the fundamental proposition -of the biogenetic law, and admit that there is in the ontogeny some -more or less distinct recapitulation of the phylogeny.</p> - -<div class="figcenter" id="ff34"> -<img src="images/ff34.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 118.</span> Two stages in the life-history of the Spurge Hawk-moth (<i>Deilephila -euphorbiæ</i>). <i>A</i>, first stage, the caterpillar dark blackish-green, without -marking. <i>B</i>, second stage, the row of spots is distinctly connected by a light -streak, the vestige of the sub-dorsal stripe.</p> -</div> - -<p>This proposition may be recognized as true in the case of <i>Deilephila</i> -also, if we compare the different species with one another as regards -their ontogeny. We find here too that not only the sub-dorsal, that -is, the phyletically oldest marking of the Sphingid caterpillars, occurs -everywhere in the young stages, but also that it is being shunted -back to younger and younger stages, in proportion to the degree of -the development of the spot-marking reached in the full-grown -caterpillar. Thus, for instance, in the caterpillar of <i>Deilephila -euphorbiæ</i> the highest form of spot-marking is reached, and in this -species the sub-dorsal line is no longer the sole marking element at<span class="pagenum"><a id="Page_183"></a>[Pg 183]</span> -any stage. Leaving out of the question the absolutely unmarked -little caterpillar which emerges from the egg (Fig. 118, <i>A</i>), there -appears at once in the second stage a series of ring-spots connected -by a fine white sub-dorsal line (Fig. 118, <i>B</i>). In the following -stage, the third, this sub-dorsal line disappears without leaving -a trace, and there remains only the spot-marking, which is subsequently -duplicated.</p> - -<p>Let us compare with this the ontogeny of the bed-straw hawk-moth, -<i>Deilephila galii</i> (Fig. 117). The full-grown caterpillar possesses -only a single row of ring-spots (<i>B</i>), and accordingly the young -stages of the caterpillar up to the fourth show a distinct sub-dorsal -line (<i>A</i>), although spots are seen upon it. A still earlier -phyletic stage of development is illustrated by <i>Deilephila livornica</i>, -in which the ring-spots are all connected by the sub-dorsal line.</p> - -<p>It can thus hardly be doubted that the biogenetic law is guiding -us aright when we conclude from a comparison of the ontogeny of -the different species of <i>Deilephila</i>, that the oldest ancestors of the -genus possessed only the longitudinal stripes, and that from these -small pieces were cut off as ring-spots, and that these were gradually -perfected and ultimately duplicated, while at the same time the -original marking, the longitudinal stripe, was shunted back further -and further in the young stages, until it finally disappeared -altogether.</p> - -<p>Let us now refer for a moment to the third form of marking -in the caterpillars of the Sphingidæ—transverse striping. This has -not arisen out of the sub-dorsal line, but quite independently and at -a later date. This is proved with great certainty by the ontogeny -of species of the genus <i>Smerinthus</i>. The full-grown, and usually also -the young caterpillars, of these species have quite regularly the seven -broad oblique stripes which run in the direction of the tail-horn -at equal intervals on the lateral surfaces of the body (<a href="#ff29">Fig. 3</a>). -They are absent only from the three anterior segments, and upon -these a part of the older marking, the sub-dorsal stripe, has persisted. -But we find this fully developed in the youngest stages of other -species. In <i>Smerinthus populi</i>, the little caterpillar, which has no -markings at all when it leaves the egg, very soon shows the white -sub-dorsal line, and simultaneously with it the seven transverse -stripes, which cut obliquely through it; in the older caterpillars the -sub-dorsal then disappears (Fig. 119).</p> - -<p>When I was investigating these matters at the beginning of the -seventies I did not succeed in procuring eggs of the species of the -genus <i>Sphinx</i>, which likewise almost all exhibit the oblique striping<span class="pagenum"><a id="Page_184"></a>[Pg 184]</span> -in their full-grown stages. But from what I knew of the ontogeny -of <i>Smerinthus</i> species I was able to predict that, among the young -stages of <i>Sphinx</i>, there must be some with sub-dorsal lines. This -was confirmed later, for Poulton found in <i>Sphinx convolvuli</i> that -in the first stage there are no oblique stripes, but only the sub-dorsal -stripe, while in <i>Sphinx ligustri</i> both kinds of marking were present -at the same time.</p> - -<div class="figcenter" id="ff35"> -<img src="images/ff35.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 119.</span> Caterpillar of <i>Smerinthus populi</i>, the Poplar Hawk-moth, at the -end of the first stage, showing both the complete sub-dorsal stripe and the -oblique stripes.</p> -</div> - -<p>From all these facts, which I have summarized as briefly as -possible, we see that the older phyletic characters are gradually -crowded by the newer into ever-younger stages in the ontogeny, until -ultimately they disappear altogether. We have now to ask to what -this phenomenon is due; is it a simple crowding out of the old and less -advantageous by the new and better characters as a result of natural -selection, or is there some other factor at work? It is clear in regard -to these forms of marking that they can have been developed at first -only in the almost full-grown larva by natural selection, because they -are of use only there, and that, at the same time, the old marking -must have been set aside through the influence of the same factor, in -as far as it prejudiced the effect of the new adaptation. This seems -to be indicated by the persistence of the sub-dorsal line on those -segments which are drawn in when <i>Chærocampa</i> assumes a terrifying -attitude, or which do not bear oblique stripes in the leaf-like caterpillars, -e.g. the three anterior segments in the species of <i>Sphinx</i> and -<i>Smerinthus</i>. When newly acquired schemes of marking like the eye-spots -of <i>Chærocampa</i> are transmitted from the last stage to the stage -before, this can be explained by following the same train of thought, -for the caterpillar is already of sufficient size to be able to inspire -terror with its eyes; but in still younger stages the spots would -not be likely to have that effect, and yet they occur in quite small -animals (20 mm.). More obvious still is the uselessness of the oblique -striping in the young stages of the <i>Sphinx</i> and <i>Smerinthus</i> caterpillars, -for in the earliest stages of life the caterpillars are much too -small to look like a leaf, and the oblique stripes stand much closer<span class="pagenum"><a id="Page_185"></a>[Pg 185]</span> -together than the lateral ribs of any leaf. Moreover, the little green -caterpillars require no further protection when they sit on the under -side of a leaf; they might then very easily be mistaken <i>in toto</i> for -a leaf-rib. <i>Thus it is certainly not natural selection which effects the -shunting back of the new characters.</i> Nor can this be caused by the -fact that the new character can only be developed gradually and -in several stages, for the oblique striping at any rate arises in the -ontogeny all at once. There must therefore be some mechanical -factor in development to which is due the fact that characters -acquired in the later stages are gradually transferred to the younger -stages. But this shifting backwards can be checked by the agency -of natural selection as soon as it becomes disadvantageous for the -stage concerned.</p> - -<p>It is in this way that I explain the fact that the majority of the -caterpillars of the Sphingidæ are absolutely without markings when -they emerge from the egg. Thus, for instance, the caterpillars of <i>Chærocampa</i> -(Fig. 116, <i>A</i>), of <i>Macroglossa</i> (Fig. 115), and of <i>Deilephila</i> (Fig. -118, <i>A</i>), as well as those of the <i>Smerinthus</i> species, are at first without -stripe or mark of any kind; they are of a pale green colour, almost -transparent, and very difficult to recognize when they sit upon a leaf. -How very greatly the different stages <i>can be</i> independently adapted -to the different conditions of their life, when that is necessary for -the preservation of the species, is shown in the most striking manner -by many species. Thus the little green caterpillar of <i>Aglia tau</i>, -when it leaves the egg, bears five remarkable reddish rod-like thorns, -which in form and colour resemble the bud-scales of the young -beech-buds among which they live, and which disappear later on; -the full-grown caterpillar shows nothing of these, but is leaf-green, -marked with oblique stripes. Even if the use of these reddish -thorns be other than I have indicated, we have in any case to deal -with a special adaptation of <i>one</i>, and that the first caterpillar-stage, -and what can happen at this stage is possible also at every other. -Nor is it only animals which undergo metamorphosis that can exhibit -independent phyletic variation at every stage, but those also with -direct development, and indeed, in the case of these, we may assume -adaptation of this kind at almost every stage in the history of the -organs, as we have already seen, because the great abridgement of the -phylogeny into the ontogeny necessitates a very precise mutual -adaptation of the organ-rudiments and of the diverse rates of -development.</p> - -<p>We have thus been led by the facts discussed—and numerous -others from other groups in the animal kingdom might be ranked<span class="pagenum"><a id="Page_186"></a>[Pg 186]</span> -along with them—to two main propositions, which express the relation -of phylogeny to ontogeny. The first and fundamental proposition -is the one already formulated. The ontogeny arises from -the phylogeny by a condensation of its stages, which may be varied, -shortened, thrown out, or compressed by the interpolation of new -stages. The second proposition refers to individual parts, and may -run as follows: As each stage can undergo new adaptations by itself, -so can every part, every organ; such new adaptations very often -show a tendency to be transferred to the immediately antecedent -stage in ontogeny.</p> - -<p>It is not my intention to formulate the laws of ontogeny just -now, otherwise many others might be added to these, such as that of -the regular transference of characters acquired at one end of a segmented -animal to the other segments: I must confine myself here -to bringing the two main propositions into harmony with the principles -of our theory of heredity.</p> - -<p>How phylogeny is condensed in ontogeny can be understood -readily enough in a general way, although we cannot profess to have -any insight into the detailed processes. The continuity of the germ-plasm -brings about inheritance, in that it is continually handing -over to the germ-plasm of the next generation the determinant-complex -of the preceding one. Every new adaptation at any stage -whatever depends on the variation of particular determinants within -the germ-plasm, and this in its turn depends on germinal selection, -that is, on the struggle of the different determinant-variants among -themselves, and on the variation in a definite direction which arises -from this, as we have already shown. A new kind of determinant -can never arise of itself, but always only from already existing -determinants, and through variation of these. But as spontaneous -variation never causes all the homologous determinants of a germ-plasm -to vary in quite the same way, but only a majority of them, -there always remains a minority of the old determinants, which may, -under certain circumstances, predominate again, as is proved by the -aberrations in <i>Vanessa</i> species due to cold, and by many other kinds -of reversion.</p> - -<p>But it is not this variation which leads to the prolongation of -ontogeny, and the repetition of the phyletic stages within it. In -this case it is rather that a new character takes the place of an old -one, not that it is added to it. A black spot may arise instead of -a red one, but not first a black spot and then a red one. Of course -we still know far too little in regard to the intimate succession of -events in the stages of ontogeny to be able to say definitely that, in<span class="pagenum"><a id="Page_187"></a>[Pg 187]</span> -such apparently simple transformations, the older stage does not, in -every ontogeny, precede the more recent one as a preparation for it, -though it may be only for a brief and transient period.</p> - -<p>It is certain, however, that variations such as the addition of -a new stage in ontogeny are undergone, and that this implies the -occurrence of something really quite new. Therefore such a new -stage can arise only from the germ-plasm, by the duplication, and in -part variation, of the determinants of the preceding stage. If, for -instance, the body of a Crustacean be lengthened by a segment, this -must be due to a process of this kind, and in such a case it is -intelligible enough that the new segment can be formed in the -ontogeny only after the development of the older preceding one, for -its determinants come from that, and are from the beginning so -arranged that they are only liberated to activity by the formation -of the preceding segment.</p> - -<p>Now, if in the course of the phylogeny numerous new segments -were added to the body of the Crustacean, the ontogeny would be -materially prolonged, and condensation would become necessary in -the interests of species-preservation. To bring this condensation -about, whole series of segments which were added successively in the -phylogeny succeeded each other with gradually increasing rapidity -in the ontogeny, until finally they appeared <i>simultaneously</i>: the -determinants of the segments <i>n</i>, <i>n</i> + 1, <i>n</i> + 2, ... <i>n</i> + <i>x</i> varied in regard -to their liberating stimuli, and were roused to activity no longer -successively, but simultaneously, in the cell complexes controlled by -them. We have thus recapitulation, but with abridgement and compression, -of the phyletic stages in the ontogeny. Thus in the nauplius -of <i>Leptodora</i> we see the rudiments of five of the pairs of legs of the -subsequent thorax (<a href="#ff24">Fig. 111</a>, <i>IV-VIII</i>), and in the Zoæa larva the -rudiments of six thoracic legs may be seen behind the already -developed swimming-leg (<a href="#ff27">Fig. 114</a>, <i>VI-XIII</i>).</p> - -<p>But in the course of the phylogeny a segment may also become -superfluous, and we know that it then degenerates and is ultimately -eliminated altogether. Thus in a parasitic Isopod, which lives -within other Crustaceans, a segment of the thorax is wanting in the -relatively well-developed larva, and in the Caprellidæ among the -Amphipod Crustaceans the whole abdomen of from six to seven segments -has degenerated to a narrow, rudimentary structure. In such -cases the gradual degeneration of the relative determinants has preceded -step for step the degeneration of the part itself, and when this -is complete the ontogeny shows nothing of what was previously -present, and so we may speak of a 'falsification' of the phylogeny.<span class="pagenum"><a id="Page_188"></a>[Pg 188]</span> -But that the complete disappearance of the determinants only comes -about with extreme slowness, so that whole geological periods are -sometimes not enough for its accomplishment, we have already learnt -from our study of rudimentary organs, instances of which can be -demonstrated in every higher animal, bearing witness to the -presence of the relevant organs or structures in the ancestors of -the species.</p> - -<p>We can infer with certainty, from the observational data at -our disposal, that the disappearance of useless parts is regulated -by definite laws; but it is too soon to attempt to formulate these -laws, or even to trace them back to their mechanical causes. As we -have already said, a much more comprehensive collection of facts, and -above all one which has been made on a definite plan, is a necessary -preliminary condition to this. But so much at least we may gather -from the facts before us, that the degeneration of an organ begins at -the final stage, and is transferred gradually backwards into the -embryogenesis. Thus the two fingers of birds which have disappeared -since Cretaceous times are still indicated in every bird-embryo, -though they subsequently degenerate. In various mammals 'pre-lacteal -tooth-germs' have been demonstrated in the jaws of embryos, -which show us that not only did ancestors exist whose dentition was -the modern 'milk-teeth,' but that still more remote ancestors possessed -another set of teeth, which was crowded out by the 'milk-teeth'; -thus the teeth of the ancestors of the modern right whale (<i>Balæna -mysticetus</i>) are only represented in the embryo of to-day in the form -of dental pits. And, as we saw already, the Os centrale so characteristic -of the wrist of lower vertebrates only appears in Man at a very -early embryonic stage, and disappears again as such in the further -course of the embryogenesis.</p> - -<p>We may perhaps give a preliminary statement of this law as -follows: It is impossible that any part or organ should be removed -suddenly from the ontogeny without bringing the whole into disorder, -and the least serious disturbance of the course of development will -undoubtedly be caused if the final stage of the part in question -become rudimentary first. Only after this has happened, and the -neighbouring parts have adapted themselves to the disappearance, -can this extend to the stages immediately preceding it, so that these -too degenerate, and allow the surrounding parts to adapt themselves. -The further back into the ontogeny the disappearance extends the -greater will be the number of other structures affected in some way -or other by the degeneration, and these must not all be brought -suddenly into new conditions, else the whole course of development<span class="pagenum"><a id="Page_189"></a>[Pg 189]</span> -would suffer. Thus at first only those determinants may disappear—and -can disappear according to the laws of germinal selection—which -control the final form of the useless organ, then those just preceding -them, which controlled, let us say, its size, and thus more and more of -the previously active determinants disappear, and hand in hand with -this disappearance there is variation of all the parts correlated with -the dwindling condition of the organ, so that their own development -and that of the animal as a whole suffers no injury. If it were -otherwise, if when a part became useless its collective determinants -were all to disappear at the same time, the whole ontogeny would -totter, in fact it would be much as if a man who wished to remove -the breadth of a window from a house standing on pillars were to -begin by taking away the foundation pillar.</p> - -<p>It is, of course, to be understood that these processes go on so -exceedingly slowly that personal selection takes a share in them, at -least at the beginning. Later on, the further degeneration of a useless -organ or rudiment has no effect on the individual's power of life, and -therefore depends solely upon the struggle of the parts within the -germ-plasm (germinal selection).</p> - -<p>If we could see the determinants, and recognize directly their -arrangement in the germ-plasm and their importance in ontogeny, we -should doubtless understand many of the phenomena of ontogeny and -their relation to phylogeny which must otherwise remain a riddle, or -demand accessory hypotheses for their interpretation. Several years -ago Emery rightly pointed out that the phenomena of the variation -of homologous parts might be inferred by reasoning from the germ-plasm -theory. If one hand has six fingers instead of five, it not -infrequently happens that the other also exhibits a superfluity of -fingers, and sometimes the foot does so too. The phyletic modification -of the limbs in the Ungulates has taken place with striking uniformity -in the fore and hind extremities; no animal has ever been one-hoofed -in front and two-hoofed behind. Although I might suggest that this -primarily depends on adaptation to different conditions of the ground, -and that the Artiodactyls were evolved in relation to the soft marshy -soil of the forest, and the Perissodactyls for the steppes, it cannot be -denied that germinal conditions may have co-operated in bringing -about this uniformity of the direction of variation, especially as the -whole structure of the fore- and hind-limbs exhibits such marked -similarity. Emery is inclined to refer this to 'germ-plasmic correlations,' -and we have assumed from the very first that the different -determinants and groups of determinants do indeed stand in definite -and close relations to one another. But it seems to me premature to<span class="pagenum"><a id="Page_190"></a>[Pg 190]</span> -say anything more precise and definite than that in the meantime. -I should like, however, to say that determinants or groups of determinants -which had in old ancestral germ-plasms to give rise to a series -of quite similar structures by multiplication during the ontogeny, and -therefore only needed to be present <i>singly</i> in the germ-plasm, would, -in later descendants, have to shift their multiplication back into -the germ-plasm itself, if necessity required that the homologous parts -which they controlled should become <i>different</i> from each other. Then -the previously single group of determinants in the germ-plasm would -have to become multiple. But as new determinants can only arise from -those which already exist, these new ones must have had their place -beside the old, and would therefore probably be exposed to any intra-germinal -causes of variation in common with them—that is to say, -they will tend to vary even later in a similar manner. For instance, -we might think of the segments of primitive Annelids, which in form -and contents are for the most part alike, as arising from one germ-rudiment, -from which, when, in the higher Annelids, the various -regions of the body had to take a different form, several primary constituents -of the germ-plasm separated themselves off; and in a similar -way the much higher and more complex differentiation of the somatic -segments in the Crustaceans must have been brought about. Thus -we understand how the determinant groups of the germ-plasm -multiplied according to the need for increasing differentiation, but -remained in intimate relation, which exposed them in some measure -to a common fate, that is, to common modifying influences, and in -many cases determined them to similar variation.</p> - -<p>But we cannot see directly into the germ-plasm, and are therefore -thrown back on the inductions we can make from the facts -presented to us by the phenomena of visible living organisms. As -yet the material for such inductions is scanty, because it has been -got together haphazard, and not collected on a definite plan. I -therefore refrain for the present from attempting any further elaboration -of my germ-plasm theory. It is only when an abundance of -observation material, collected according to a definite plan, lies at our -disposal that anything more in regard to the intimate structure of -the germ-plasm, or the mutual influences and relations of its determinants -and its modification in the course of phylogeny can be -deduced with any certainty. Meanwhile, we must content ourselves -with having, through the hypothesis of determinants, made intelligible -at least the one fundamental fact, how it is possible that in the course -of the phylogeny single parts and single stages can be thrown out or -interpolated, or even only caused to vary, without giving rise to<span class="pagenum"><a id="Page_191"></a>[Pg 191]</span> -variation in all the rest of the parts and stages of the animal. -A theory of epigenesis cannot do this, for, if no representative -particles were contained in the germ-plasm, then every variation of -it would affect the whole course of development and every part of the -organism, and variations of individual parts arising from the germ -would be impossible.</p> -<hr class="full" /> - -<div class="chapter"> -<p><span class="pagenum"><a id="Page_192"></a>[Pg 192]</span></p> - -<h2 class="nobreak" id="LECTURE_XXVIII">LECTURE XXVIII</h2> -</div> - -<p class="c">THE GENERAL SIGNIFICANCE OF AMPHIMIXIS</p> - -<div class="blockquot"> - -<p>Twofold import of amphimixis—It conditions the continual changing of individuality—Analogy -from game of cards—The germ-plasm is at once variable and -persistent—The two roots of individual variation: germinal selection and new -combinations of the ids—'Harmonious' adaptation conditions amphimixis—Difference -between adaptation and mere variation—Is a 'direct' use of amphimixis to be insisted -upon?—Ceaseless intervention of personal selection in the lineage of the germ-plasm—Far-reaching -effects of personal selection—Fixing of the arrangements for amphimixis -in the course of generations of species—Increase of the constancy of a character with its -duration—Characters in the same species variable in different degrees—The upper and -under surfaces of Kallima—Wild plants brought under cultivation do not at first vary—Amphimixis -very ancient, therefore very firmly established—Does amphimixis bring -about equalization (Hatschek, Haycraft, Quetelet)?—Galton's frequency curves—Ammon's -free scope for variations—De Vries' asymetrical curves of frequency.</p></div> - - -<p><span class="smcap">We</span> have already made ourselves familiar with the process which -in unicellular organisms is called conjugation and in multicellular -organisms fertilization, and we have seen that its most obvious -significance lay in the fact that through it the germ-plasms of two -individuals are united. Since, according to our view, this germ-plasm -or idioplasm is the bearer of the hereditary tendencies of the -organism concerned, the mingling or amphimixis of two germ-plasms -brings together the hereditary tendencies of two individuals, and the -organism whose development is derived from this mingled germ-plasm -must therefore exhibit traits of both parents, and must to -a certain extent be made up of the traits of both. This is one result -attained by amphimixis.</p> - -<p>But we went further than this, and saw that there is a second -result implied in amphimixis, namely, that the individual character -of the germ-plasm is being continually altered by new combinations -of the ids contained in it. We inferred from what I believe to be the -demonstrated hypothesis that the germ-plasm is composed of ids, that -its reduction to half the original mass must mean a reduction of the -ids to half the number, and as the ids contain primary constituents -which are individually different, this must effect a new arrangement, -a new mingling of these individual peculiarities. The reduction of the -germ-plasm to half, that is, the diminution of the number of its ids -to half, is a phenomenon generally associated with amphimixis, and<span class="pagenum"><a id="Page_193"></a>[Pg 193]</span> -has been established in the case of all animals which have hitherto -been investigated, and of all the most carefully studied plants, and -finally, it has been shown to be very probable in unicellular organisms, -for the processes of conjugation in Infusorians and many other -Protozoa include phenomena very similar to those of reducing division -in the higher animals. The prediction made on theoretical grounds -has here been verified by observation, and it is obvious that the -assumption of ids, that is, of units in the germ-plasm which are handed -on from one generation to the succeeding one, involves a reduction -of their number in each amphimixis. Without this the number of ids -would be doubled at each amphimixis, and would therefore gradually -amount to something enormous. We see therefore why this normally -recurrent reduction of ids before each amphimixis was established -in the course of evolution, and we see that it inevitably involves that -a new combination of ids should be associated with each amphimixis.</p> - -<p>If nothing persists unless it be purposeful, that is, necessary, what -is the meaning of the fact that arrangements for amphimixis occur -over almost the whole known domain of life, from the very simple -organisms up to the highest, in unicellular and multicellular organisms, -in plants and animals alike? Why is it that this arrangement has -been departed from only in a few small groups of forms, while it occurs -everywhere else, in almost every generation, so indissolubly associated -with reproduction that it has even been regarded—with a lack -of clearness—as itself a form of reproduction, and is even now -generally called 'sexual reproduction'? And why is it that in many -organisms, especially lowly ones, it is not associated with every -reproduction, though it recurs at regular or irregular intervals? -Such a universal arrangement must undoubtedly be of fundamental -importance, and we have to ask wherein this importance lies. That -is the problem to the solution of which we must now apply -ourselves.</p> - -<p>So much we may say at once: The significance of amphimixis -cannot be that of making multiplication possible, for multiplication -may be effected without amphimixis in the most diverse ways—by -division of the organism into two or more, by budding, and even -by the production of unicellular germs. Even though these last are -usually in various ways so organized that they must undergo amphimixis -before they can develop into new organisms, yet there are -numerous germ-cells which are not subject to this condition (e.g. -spores), and there are—as we have seen—many germ-cells, adapted -for amphimixis, which always, or in certain generations, or even -only occasionally, emancipate themselves from this condition under<span class="pagenum"><a id="Page_194"></a>[Pg 194]</span> -certain external influences: I refer to egg-cells which develop -parthenogenetically.</p> - -<p>If amphimixis is not a universal preliminary condition of reproduction, -wherein lies the necessity for its general occurrence among -living organisms?</p> - -<p>We have already learned that there are two results produced -without exception by amphimixis; one of these is the antecedent -reduction of the original number of ids by one half, and the consequent -new combination of ids which results from this; the other -is the union of two such halved germ-plasms from two different -individuals. The first we may, with Hartog, compare to the removal -of half of a pack of cards previously mixed, the second to the combination -of two such halves from different packs. The first process -brings nothing new into the complex of primary constituents, but -rather removes a part—larger or smaller—of its characters: not -necessarily exactly half of these, since each individual kind of id may -be represented by doubles or multiples. But the reduction simplifies -the composition of the germ-plasm, and might by itself, through the -struggle of the ids in ontogeny, lead to a resultant different from -the parent, that is, to a new individuality. Through the second -process, however, new individual traits are of necessity added, and -make the resultants still more markedly diverse, that is, if the ids -of both parents attain to expression in the struggle of ontogeny, and -this, as we have already seen, is usually the case, though not always -and certainly not always in all parts. Thus amphimixis, together -with the preparatory reduction of the ids, secures the constant -recurrence of individual peculiarities through the ceaseless new -combinations of individual characters already existing in the -species.</p> - -<p>When sixteen years ago I first inquired into the actual and -ultimate significance of sexual reproduction, I thought I had found -it in this ceaseless production of new individualities. This seemed -to me a sufficient reason for the introduction of amphimixis into -nature, since the difference between individuals is the basis of the -process of selection, and thus the basis of all the transformations -of organisms, which we may refer to natural or sexual selection. -Now these differences of selection-value are—as I believed then, and -do still—not only by far the most frequent organic changes, but also -the most important, since they not only initiate, but control new lines -of evolution. Therefore I still regard amphimixis as the means -by which a continual new combination of variations is effected -a process without which the evolution of this world of organisms<span class="pagenum"><a id="Page_195"></a>[Pg 195]</span> -so endlessly diverse in form and so inconceivably complex, could -not have taken place.</p> - -<p>But I do not regard this amphimixis as the real root of variation -itself, for that must depend not on a mere exchange of ids, but -rather upon a variation of the ids. The ids of a worm of the primitive -world could not without variation now make up the germ-plasm -of an elephant, even if it be true that mammals are descended from -worms. The ids must have been meanwhile transformed times without -number by the modification, degeneration, and new formation of determinants. -Amphimixis, that is, the union of two germ-plasms, does not -of itself cause variation of the determinants, it only arranges the -ids (the ancestral plasms) in ever-new combinations. If the origin -of variation were limited to that alone, a transmutation of species -and genera would only be possible on a very limited scale; there could -at most be a narrow circle of variations, just as in the example -already given of the packs of cards; even if the taking away and -mixing up of the halves were repeated a thousand times, a definite -though undoubtedly large number of card-combinations would in the -long run recur again and again. But the case is different with the germ-plasm -and amphimixis, where there is an infinitely more varied series -of results, because the individual cards—the ids—are variable, even -between one time of sifting and shuffling and another, and therefore -infinitely productive of variety in the course of numerous repetitions -of the shuffling.</p> - -<p>I have been frequently and persistently credited with maintaining -that the germ-plasm is invariable—a misunderstanding of my -position, due perhaps to a somewhat too brief and terse statement -which I made at an earlier period (1886). I had described the germ-plasm -as 'a substance of great power of persistence,' and as varying -with difficulty and slowly, basing this statement upon the age-long -persistence of many species in which the specific constitution -of the germ-plasm must have remained unchanged. The idea of -'germinal selection,' of a ceaseless struggle between the 'primary -constituents' of the germ, and of the resulting continual slight and -invisible rising and falling of individual characters, had not yet -dawned upon me, nor had I at that time formulated the conception -of 'determinants.' I was even doubtful at that time whether development, -heredity, and variation were not interpretable on the assumption -of an undifferentiated substance without primary constituents. But -at no time was I unaware that the whole phyletic evolution of the -organic world is only conceivable on the assumption of continual -variation of the germ-plasm, that it actually depends upon this, even<span class="pagenum"><a id="Page_196"></a>[Pg 196]</span> -if these variations come about with exceeding slowness, and are thus -in a certain sense 'difficult.'</p> - -<p>Now that I understand these processes more clearly, my opinion -is that the roots of all heritable variation lie in the germ-plasm, and -furthermore, that the determinants are continually oscillating hither -and thither in response to very minute nutritive changes, and are -readily compelled to <i>variation in a definite direction</i>, which may -ultimately lead to considerable variations in the structure of the -species, if they are favoured by personal selection, or at least if they -are not suppressed by it as prejudicial. But selection is continually -keeping watch over both kinds of variation, and if the conditions -of life do not further the variation or do not even allow it to persist, -selection eliminates everything that lessens the purity of the specific -type, everything that transgresses the limits of utility, or that might -endanger the existence of the species. Thus we understand how the -germ-plasm may be variable, and yet at the same time remain -unvaried for thousands of years, how it is ready and able to furnish -any variation that is possible in a species if that is required by -external circumstances, and yet is able to preserve the characters -of the species in almost absolute constancy through whole geological -ages; in short, how it can be at once readily variable and yet slow -to vary.</p> - -<p>The importance which amphimixis thus has in connexion with the -adaptation of organisms lies, if I mistake not, in the necessity for -co-adaptation, that is, in the fact that in almost all adaptations it is -not merely a question of the variation of a single determinant, but of -the correlated variations of many—often very numerous—determinants, -of 'harmonious adaptation,' as we have already said. Many-sided -adaptation of this kind seems to me impossible without a continually -recurrent sifting and recombining of the germ-plasms, and this can -only be effected by amphimixis.</p> - -<p>It may be objected that, apart from amphimixis, variation can -be brought about in many parts of an organism, as in purely asexual -reproduction. A plant, for instance, may vary when it is transferred -to a strange soil or climate; and even in that case the variations -seem to be harmonious, at least the harmony of the parts is so far -maintained that the plant continues to flourish, at any rate under -cultivation. A plant species may be incited by abundant nourishment -to gigantic growth, and caused to vary in many of its parts, and -the abundant food may even directly affect the germ-plasm so that -all or some of these variations may become hereditary; and yet this -is far from being a case of adaptation, it is merely a case of<span class="pagenum"><a id="Page_197"></a>[Pg 197]</span> -simultaneous variations, and it is questionable whether they -will make the continued existence of the plant under the -new conditions possible or not. It might easily happen, for instance, -that the plant, though it became larger and bore more -abundant blossoms, would be sterile, and therefore unfitted for -continued existence in a natural state. Variations are not necessarily -adaptations; the latter can never come about solely through direct -influence upon the germ-plasm. What direct influence upon the -germ-plasm could, for instance, make the hind-legs of a mammal -long and strong and the fore-legs short and weak? Obviously -neither an increase nor decrease in the food-supply, nor a higher or -lower temperature—in short, no direct influence, because all these -affect the germ-plasm as a whole, and therefore cannot possibly -influence two homologous groups of determinants in opposite -directions.</p> - -<p>This, it seems to me, is only possible when amphimixis brings -about in one individual a favourable coincidence of the chance -germinal variations of the determinants of the fore- and hind-limbs; -and just as it is with the two variations in this simple hypothetical -case, so it will be in the actual processes of adaptation where there are -involved numerous—we know not how numerous—variations essential -to a 'harmonious adaptation.'</p> - -<p>It need not be objected that the very number of variations -necessary to a 'harmonious adaptation' makes its occurrence impracticable; -for it is <i>the complete</i> harmony of the parts that makes -the adaptation, and without this the individual was only imperfectly -adapted, and therefore incapable of survival. It is certainly not -mathematically demonstrable that this is the case, but as the whole -process of transformation which makes an old adaptation into a new -one begins with minimal fluctuations of the determinants, which -must first be brought by germinal selection to the level of selection-value, -and must then be subject to personal selection, so the whole process -goes on so gradually and by such small steps that the harmony of the -parts is maintained by functional adaptation during the individual -life in a great number of individuals. But these are just the -individuals which survive in the struggle for existence, and at the -same time possess at every stage of the process <i>the best combination -of favourably varying determinants</i>. As these favourable -variations are, in consequence of germinal selection, not mere isolated -variations of fluctuating importance, but variations in a <i>definite -direction</i>, the whole process of variation must persist in every single -part in the direction imposed upon it by personal selection. But<span class="pagenum"><a id="Page_198"></a>[Pg 198]</span> -since at every reducing division the ids of the germ-cells are brought -down to half their number, a possibility is offered for gradually -removing the unfavourable ids from the germ-plasm of the species, -since the descendants resulting from the most unfavourable id-combinations -always perish, and so from generation to generation the germ-plasm -gets rid of its unfavourably varying ids, and the most propitious -combinations afforded by amphimixis are preserved, till ultimately -there remain only those combinations which are varying appropriately, -or at least only those in which the appropriately varying determinants -are in the majority, and so have controlling influence.</p> - -<p>Logically this deduction is undoubtedly indisputable, from the -standpoint of the germ-plasm theory; but whether it may be regarded -as a sufficient reason for the introduction of amphimixis, and for -its extremely tenacious persistence throughout the course of the -long and intricate phylogeny, cannot be maintained without special -investigation.</p> - -<p>Against my position the objection has often been urged that an -arrangement cannot arise or be maintained through natural selection -unless it is <i>of direct use</i> to the individual in which it occurs. Sexual -reproduction cannot therefore have been established simply because -it advances, or even because it makes possible the adaptations of -species, for these adaptations only came about occasionally, perhaps -once in a thousand generations or even less frequently; thus the -intervening generations could derive no advantage of any kind from -the arrangement in question, and therefore, according to the law of -the degeneration of unused characters, it must have long since been lost. -I mentioned this objection before, but was obliged to postpone -a detailed consideration of it until we had discussed germinal -selection.</p> - -<p>We admit, of course, that characters are only preserved intact -as long as they are of advantage sufficient to turn the scale in favour -of their possessors, and that they begin to fall from their height of -perfection when that is no longer the case; we admit also that new -adaptations are not continually necessary, but are so only at intervals -of long series of generations, and yet the objection cited seems to me -baseless.</p> - -<p>Leaving out of account, for the moment, the first introduction of -amphimixis, let us deal with it as an existing occurrence, for the -tenacious persistence of which we wish to find reasons.</p> - -<p>Is it really the case that amphimixis is only of importance in -connexion with the new adaptation of a species, and that it has nothing -to do with the persistence of the species in the state of adaptation<span class="pagenum"><a id="Page_199"></a>[Pg 199]</span> -already attained? According to the conception of the processes -within the germ-plasm which we have already stated, it is impossible -that this should be the case, for continual slight fluctuations are -occurring in the determinants in consequence of the fluctuations of -the nutritive stream, and these slight variations, plus or minus, do -not in many cases equalize one another or counteract one another -by turning again in a contrary direction; they go on increasing in -the direction in which they have begun. It is only when personal -selection opposes them that they come to a standstill, and this can -only happen when they attain to selection-value, that is to say, when -they reach a level at which they become disadvantageous in the -struggle of persons. But as germinal variations of this kind are -continually occurring, personal selection must keep continual watch -over them, and eradicate them as soon as they have attained selection-value.</p> - -<p>Therefore, when a species is most perfectly adapted to its -conditions, it would of necessity begin to degenerate if personal -selection were not continually guarding it, and setting aside everything -that is in excess or deficient as soon as it begins to be -prejudicial. But the adaptation of a species does not depend upon -<i>one</i> character persisting at its normal level, but on the persistence of -very many, and many of these vary simultaneously upwards or -downwards, and reach the limit of selection-value at one time or -another. If there were no amphimixis, then either all individuals -with any excessive variant would be at once eliminated, or the species -would go on deteriorating until this excessive variant was so -numerously and strongly represented in all its individuals that it -would perish through degeneration. But even in the first of these -cases the species would drift towards the fate of extinction, because -excessive variations do occur even in every asexual generation, and -would appear in an increasingly large number of determinants if -there were no possibility of rejecting them and eliminating them -from the lineage of the species.</p> - -<p>This is made possible through the periodic intervention of -amphimixis; it is actually effected thereby; and in this way alone -the species is kept at its high-water mark of adaptation. It is not -necessary to assume that every single determinant which is varying -in an unfavourable direction is at once eliminated as soon as it -becomes prejudicial, that is, reaches negative selection-value, or—to -make use of an expression introduced by Ammon—as soon as it -oversteps the boundaries of the 'playground of variations,' the limits -within which variations are neither favourable nor unfavourable. But<span class="pagenum"><a id="Page_200"></a>[Pg 200]</span> -<i>in the course of generations</i> they are unfailingly eliminated, especially -when a large number of unfavourably varying determinants are -coincident in the germ-plasm. Then the individuals which arise -from a germ-plasm thus composed must perish in the struggle for -existence, and thus the id-combinations with excessive determinants -are eliminated from the germinal constitution of the species. As -this is repeated as often as excesses of the ids occur, the species -is kept pure.</p> - -<p>It might be objected that, through such a continual weeding-out -of rebellious determinants, the germ-plasm would become so -constant in its constitution that it would ultimately be secure from -all such aberrations of it on the part of its determinants, and therefore -would in time become quite incapable of diverging from its proper -path at all, and would thus no longer require this continual correction -through amphimixis.</p> - -<p>I do not wish to contradict this conclusion; indeed, I believe -that the constitution of the species becomes more and more constant -in the way I have indicated, and that an ever more perfect and stable -equilibrium of the whole determinant system is thus brought about. -It follows that in the course of generations the diverse determinants -of the germ-plasm will vary within a progressively shortened radius, -and will thus more and more rarely overstep the limits of the -'variation-playground'—and yet I still believe that this justifiable -conclusion tells in favour of my interpretation of the utility of the -persistence of amphigony once introduced.</p> - -<p>Let it be remarked, in the first place, that it is by no means -essential to the preservation of a useful institution that it should -practically justify its utility in <i>every</i> generation. Although, for -instance, the warm winter coat of a species of mammal may be -necessary to its survival, it does not disappear at once when a winter -happens to occur which is so warm that even individuals with poor -pellage can survive. Indeed, several such mild winters might occur -in succession, in which there was no weeding-out of the individuals -with poor fur, and yet the thickness of the winter fur of the species -would not become less fixed, just because this character no longer -varies perceptibly in an old-established species which has long been -perfectly adapted, and it could only be brought into a state of marked -fluctuation very slowly through direct influence on the germ-plasm, -or through panmixia. But exactly the same thing is true in regard -to the determinants of the reproductive cells, in respect of their -adaptation to amphimixis, only very much more emphatically.</p> - -<p>Before going further, I should like to show that the conclusion<span class="pagenum"><a id="Page_201"></a>[Pg 201]</span> -we have just deduced from the theory, namely, that the equilibrium -of the determinant system of a species increases in stability with the -duration of its persistence, holds good not only for the whole system, -but for its individual parts, that is, for the individual characters and -adaptations. Experience teaches that characters are the more exactly -and constantly transmitted the older they are; generic characters are -more constant than species-characters, order-characters more persistent -than family-characters—this is implied even in their name. But -we are able to show even in relation to the characters of a species -that those which have been fixed for a long time are most precisely -and purely transmitted; that is, that their determinants are least -inclined to overstep the limits of the 'variation-playground' either -in an upward or downward direction.</p> - -<p>Two groups of facts prove this: first the observed fact that the -very different degree of variability which the different species exhibit -is by no means common to all the characters of the species in the -same measure; for individual characters may be variable or constant -in very different degrees.</p> - -<p>Many years ago<a id="FNanchor_21" href="#Footnote_21" class="fnanchor">[21]</a> I drew attention to the fact that the different -stages in the life-history of insects, especially of Lepidoptera, might -be variable in quite different degrees. Thus, for instance, the caterpillar -might be very variable, and yet the butterfly which arises from -it might be extremely constant. I concluded from this—what -probably no one now will dispute—that the various stages may vary -phyletically independently of one another, that, for instance, the -caterpillar may adapt itself to a new manner of life, a new food-plant, -a new means of defence, while the butterfly, unaffected by this, goes -on quietly as it was before. Every new adaptation necessarily -implies variability, and so the stage which is in process of transformation -must have its period of variability, which gradually returns -again to greater constancy, and this the more completely the longer -the series of generations through which the weeding out of the less -well-adapted has endured.</p> - -<div class="footnote"> - -<p><a id="Footnote_21" href="#FNanchor_21" class="label">[21]</a> <i>Studien zur Descendenztheorie</i>, Leipzig, 1876.</p> - -</div> - -<p>But it is not only the individual stages of development that may -be unequally variable; the same is true of the characters of a species -which occur simultaneously. The most striking example of this -known to me is the leaf-butterfly, which I have already mentioned -many times in the course of these lectures—the Indian <i>Kallima -paralecta</i>. In this species the brown and red upper surface is almost -alike in colour and marking in all individuals, but the under surface, -the colour and marking of which is so deceptively mimetic of a leaf,<span class="pagenum"><a id="Page_202"></a>[Pg 202]</span> -is variable to such a degree that it is difficult, among a large number -of specimens, to find even a few which are as like one another as are -the members of species in which the under side is constant. It need -not be urged that this is due to the complexity of the marking on -the under side. In many of our indigenous butterflies the under side -is just as complex in coloration and marking, and nevertheless it -is very constant, being almost identical in all individuals, as for -instance in <i>Vanessa cardui</i>. In <i>Kallima</i> the great variability of -the under surface certainly depends not merely on the fact that -the mimetic character has been only recently acquired (phyletically -speaking), but chiefly on the fact that the dead leaves to which they -approximate are themselves very diverse in appearance, for many -are dry, others moist and covered with mould, and that the adaptations -have therefore gone in different directions, and as yet, at least, have -neither combined to form a single constant type, nor diverged to form -two or three distinct types. The various 'leaf-pictures' seem equally -effective in concealing the insects from their enemies, and thus there -is still a continual crossing and mingling of the different essays at -leaf-picturing.</p> - -<p>A second group of facts, which indicates that old-established -characters have less tendency to overstep the limits of the neutral -'variation-playground,' is to be found in the experience of breeders, -and especially that of gardeners who have brought wild plants under -cultivation in order to procure varieties.</p> - -<p>It has been proved that the wild plants often exhibit no hereditary -variations for a long series of generations, notwithstanding the greatly -altered conditions of life, but that then a moment comes in which -isolated variations crop up, which may then be intensified by the -manipulations of the breeder to form sport-species with large -conspicuously coloured flowers, or with some other distinctive -character. Darwin called this a shattering of the constitution of -the plant; but the stable and slowly varying 'constitution' simply -means that in old-established and well-adapted species the determinants -possess only a very restricted 'variation-playground,' and -because of their firmly based harmonious correlation are not easily -and never very quickly induced to overstep its limits in any marked -degree.</p> - -<p>Let us now apply all this to the institution of amphimixis and -amphigony, and it is immediately obvious that these determinants -of the germ-plasm which control the characters relating to sexual -reproduction must be <i>more stable and less variable than all others -which a species possesses, for they are infinitely older</i>. They are<span class="pagenum"><a id="Page_203"></a>[Pg 203]</span> -older than all species-characters, older than the characters of the -genus, of the family, of the class, and indeed of the whole series -or phylum to which a higher animal, a vertebrate, for instance, -belongs. We cannot wonder, therefore, that amphigony has persisted -through hundreds and thousands of generations, even if it had not -been reinforced in the germ-plasm during this period by selection. -We should rather wonder that an institution so primaeval, and so -firmly engrained in the germ-plasm, can ever be departed from, even -when its abandonment is to the advantage of the species, as has -happened in parthenogenesis.</p> - -<p>I have entered upon this long discussion because I believe that -we require to appreciate this power of persistence on the part of the -sexual determinants before we can explain the general occurrence of -amphigony. The occurrence of pure parthenogenesis, unaccompanied -by any degeneration of the species, can hardly be understood except -on the assumption that the constancy of the species, when it has once -been attained, may be preserved without the continual intervention of -amphimixis. How long it can be preserved is another question, which -it is difficult or impossible to answer, since species exhibiting <i>pure</i> -parthenogenesis are rare, and since we cannot tell with certainty how -long it is since amphimixis ceased to occur in them. Generally -speaking, the answer in regard to the few species which have to be -taken into account in this connexion would be 'not long,' but whether -this 'not long' signifies hundreds of generations or thousands of -generations we must leave undecided. So much only we can say, that -in all species of animals in which the male sex has quite died out or -has dwindled to a minimal remnant, there are as yet no traces of -degeneration to be found, and that even organs which have fallen into -disuse and become functionless because amphigony has disappeared, -are nevertheless in several cases retained in perfect completeness. I -shall return to this subject later on, but in the meantime I wish -to work out our conception of the actual efficacy of amphigony or -ordinary sexual reproduction, and thereby increase our understanding -of its significance and power of persistence.</p> - -<p>We have seen that amphigony not only renders possible the novel -'harmonious adaptations' which are continually required, but that it -also leads, by a continual crossing of individuals, simultaneously with -the elimination of the less fit, to a gradually increasing constancy of -the species. This has been regarded by some writers as its sole effect; -thus recently by Hatschek, whose view has already been refuted.</p> - -<p>Haycraft also finds the significance of amphigony simply in the -equalizing or neutralizing of individual differences which it effects.<span class="pagenum"><a id="Page_204"></a>[Pg 204]</span> -Quetelet and Galton have attempted to show that intercrossing leads -to a mean which then remains constant. Haycraft supposes that -a species can only remain constant if its individuals are being continually -intercrossed, and that otherwise they would diverge and take -different forms, because the 'protoplasm' has within itself the tendency -to continual variation. The transformation of species is effected by -means of this variation tendency, and the persistency and constancy -of species which are already adapted to the conditions of their life -are secured by the constant intercrossing of the individuals, and the -consequent neutralization of individual peculiarities.</p> - -<p>Although the cases already mentioned in which great constancy -of species is associated with purely parthenogenetic reproduction do not -tell in favour of the accuracy of the view just stated, yet the fundamental -idea, that amphigony is an essential factor in the maintenance -and even in the evolution of species, is undoubtedly sound. -We should certainly find neither genera nor species in Nature if -amphigony did not exist; but we cannot simply suppose that -amphigony and variation are, so to speak, antipodal forces, the -former of which secures the constancy of the species, the latter its -transformation. In my opinion, at all events, there is no such thing -as a 'tendency' of the protoplasm to vary, although there is a constant -fluctuation of the characters—dependent on the imperfect equality of -the external influences, especially of nutrition. This certainly results, -as far as it takes place within the germ-plasm, in a continual upward -and downward variation of the hereditary tendencies, and it would -lead to increasing dissimilarity of the individuals were it not that -amphigony is continually equalizing the differences by a constantly -repeated mingling of individuals. Quetelet and Galton have shown -that the tendency of this mingling is towards the establishment of -a mean; the characters of Man, such as bodily size, fluctuate about -a mean, which at the same time shows the maximum of frequency; -and the frequency curve of the various bodily sizes assumes a perfectly -symmetrical form, so that the average size is the most frequent, and -deviations from it upwards or downwards occur more rarely in proportion -to the amount of deviation, the largest and the smallest sizes -occurring least frequently.</p> - -<p>Thus an equalizing of variations by means of amphimixis really -exists, and the question we have to ask is, How does it come about? -The case is assuredly not the same as that in which equal quantities -of red and white wine are mixed to make a so-called 'Schiller.' This -is proved even by the fact that the mixture may turn out quite -different even when the wines—the two parents—are alike: for the<span class="pagenum"><a id="Page_205"></a>[Pg 205]</span> -children of a pair are often dissimilar. And while the 'Schiller' -cannot be separated again into red wine and white, this happens often -in sexual reproduction, and sometimes to such an extent that the -grandchild exactly resembles one or other of the grand-parents, as is -most clearly proved in the case of plant-hybrids.</p> - -<p>There is thus a deep-seated difference, depending on the fact -that what is mingled in amphigony is not simple but composite, not -a simple uniform developmental tendency associated with a simple -and definite substance, but a combination of several or many -developmental tendencies, associated with several equivalent but different -material units. These units are the ids or ancestral plasms, and -we have seen how they are not only halved by reducing division, -but are also arranged in new combinations in amphimixis.</p> - -<p>These ids differ very little within the same germ-plasm; in species -which have long been established the majority probably only differ -in correspondence to the individual differences of the fully-formed -organisms, but they are only absolutely alike in the case of two ids -which have been formed by the division of a mother-id. Let us disregard -this for the moment, and assume that all the ids of a germ-plasm -are different: the germ-plasm of a father, <i>A</i>, will be composed of ids -<i>A</i> 1-100, that of the mother, <i>B</i>, of the ids <i>B</i> 1-100. But in each -mature germ-cell of these two parents only fifty ids are contained, -and if we assume that the mingling of the ids is controlled solely by -chance, then in the various germ-cells <i>A</i> × <i>B</i> the most diverse combinations -of ids may be contained; for instance, <i>A</i> 1, 3, 5, 7, 9, 11, -...to 99, or <i>A</i> 1-10 and 20-30, and 40-50, and so on, and similarly -in the germ-cells <i>B</i>. If all germ-cells produced by <i>A</i> and by <i>B</i> -attained to development, or even if all the ova succeeded, the thousand -or hundred thousand children of this pair would necessarily exhibit -every possible mingling of their characters, and each in the same -number according to the rules of probability calculations. But it is well -known <i>that this does not happen</i>; of the thousands of human ova, for -instance, which come to maturity in the course of the life of a female -individual more than ten rarely develop, and more than thirty never, -and these are determined solely by chance and quite independently of -the mixture of ids which they contain. It is thus purely a matter of -chance which of the complexes of primary constituents contained in -the germ-plasm of an individual are transmitted to descendants, and -it is also purely a matter of chance which combination of ids comes -to be developed. Therefore we may say that no regular neutralizing -of contrasts, either in the primary constituents of the parents or as -regards the differences in their characters, can occur. In one case there is<span class="pagenum"><a id="Page_206"></a>[Pg 206]</span> -a blended inheritance; in another the child takes after the father or -after the mother; in a third, and this probably occurs most frequently, -the child resembles the father in some characters and the mother in -others.</p> - -<p>But how then does Galton's curve of frequency of variations -come about? Why does the mean of any character occur by far the -most frequently, and why does the frequency of a variation diminish -regularly in proportion to its approximation to either extreme? -To this it is answered: Because the process of mingling through -amphigony goes on through numerous generations, and thus an -elimination of chance, and the establishment of an average, must -be brought about.</p> - -<p>But this does not quite suffice to explain matters, for experience -shows that asymmetrical frequency-curves of variations also occur, -even in species with sexual reproduction. As De Vries has recently -shown, there are also 'half-Galton curves,' that is, curves which suddenly -break off at their highest point. We must conclude from this -that the frequency of the different variations depends not only on their -degree, but also on the greater or less facility with which they -arise from the constitution of the species.</p> - -<p>This consideration can be readily elucidated with the help of -Ammon's exposition, and especially of his graphic representation of the -'playground of variations.' If we think of the indifferent variations -occurring in any character of a species as arranged in a series ascending -from the smallest to the largest, this line may be regarded as the -abscissal-axis, and from it ordinates may be drawn which express the -frequency of the variation in question by the differences in their -length. If the tips of these ordinates be united, we have the curve -of frequency (Fig. 120, <i>A</i>), which according to Galton ought to be -symmetrical, and in most cases really is so. Ammon calls the space -between the smallest and the largest variations the 'variation-playground,' -that is, the playground within which all variations are equally -advantageous to the species. This is not co-extensive with the variation-area, -for there may be more marked deviations below the beginning -or above the upper end of the variation-playground, but these, being -disadvantageous, fall under the shears of personal selection. The -variation-playground may also be called the area of indulgence of -variation, because the variations falling within it are spared from the -eliminating activity of selection, or the variation-area of survivors, -because on an average only those survive whose variations do not -overstep the limits of this area.</p> - -<div class="figcenter" id="ff36"> -<img src="images/ff36.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 120.</span> <i>A</i>, symmetrical, and <i>B</i>, asymmetrical curve of frequency; after -Ammon. <i>U</i>, minimal, <i>O</i>, maximal limit of individual variation. <i>U-O</i>, the -'variation-playground.' <i>M</i>, the mean of variation. <i>H</i>, the greatest frequency -or mode of variation.</p> -</div> - -<p>This implies that variations below <i>U</i> (the lower limit of the -<span class="pagenum"><a id="Page_207"></a>[Pg 207]</span>area of exemption) and above <i>O</i> (the upper limit) can occur, but do -not survive and leave descendants, and we can therefore easily -understand why characters, of which different degrees arise with -equal ease from the constitution of the species, must gradually -develop a symmetrical curve of frequency because of the constant -crossing. Obviously those individuals which stand just upon the -borders of admissible variation will, other conditions being equal, -leave behind them fewer descendants than those which approximate -to the middle of the area of exemption; for as the characters concerned -can vary in the offspring in both directions, there will always -be at the lower end some of the descendants of a pair which will fall -below the limits of exemption, and at the upper end some which will -rise above it. This will happen even when pairing takes place -between parents at the middle or at the other end of the abscissa, -for there are always cases of the preponderance of one parent in -heredity. A higher percentage of the descendants of individuals -on the borderline will therefore be eliminated, and their frequency -<i>must therefore be less</i>. Even if at the beginning of the series of -observations a condition obtained in which all the ordinates -of the area of exemption were equally high, those nearest the -boundaries would of necessity very soon become lower, and this in -proportion to their distance from the boundary, and the frequency-curve, -which at first would be a straight line (according to our -assumption, which of course does not tally with natural conditions),<span class="pagenum"><a id="Page_208"></a>[Pg 208]</span> -would become a symmetrical curve, highest in the middle and falling -equally at either side.</p> - -<p>Ammon has worked out the hypotheses on which the curve of -frequency would become asymmetrical. Firstly, when the fertility -is greater towards the upper or lower limit of the area of exemption; -secondly, when germinal selection forces the variation in a particular -direction, upwards or downwards; and thirdly, 'when natural selection -intervenes diversely at the upper or lower limit.' Of these three -possibilities the first two must be acknowledged as quite probable, -but the third, it seems to me, could only cause a temporary asymmetry -of the curve, lasting, that is, only until a state of equilibrium -has again been reached; but that may in certain conditions take -a long time.</p> - -<p>Asymmetrical curves of frequency (Fig. 120, <i>B</i>) therefore arise, -for instance, when the intra-germinal conditions (the 'constitution -of the species') more easily and therefore more frequently produce -extreme variations. In this case the area of exemption can only -extend on one side, and must remain in this state. In <i>Caltha palustris</i>, -the marsh marigold, we may find, according to De Vries, among -a hundred flowers, those with five, six, seven, and eight petals, in -the following proportions:—</p> - -<table> - -<tr><td class="tdl">Petals</td> - <td class="tdr">5</td> - <td class="tdr">6</td> - <td class="tdr">7</td> - <td class="tdr">8</td></tr> - -<tr><td class="tdl">Number of flowers</td> - <td class="tdrp">72</td> - <td class="tdrp">21</td> - <td class="tdrp">6</td> - <td class="tdrp">1</td></tr> - -</table> - -<p>and thus there is an asymmetrical curve of frequency. But if we -take the whole area of variation as the area of exemption, that is, -if we assume that it is indifferent for the species whether the flowers -have five, six, seven, or eight petals, the preponderance of the five-petalled -flowers may have its reason in the fact that it is much easier -for five than six or more petals to be produced because of the internal -structure of the whole plant.</p> - -<p>In this case the maximum of frequency lies at the lower limit -of variation, but it may also lie at the upper. Thus, according to -De Vries, the blossoms of <i>Weigelia</i> vary, in regard to the number of -their petal-tips, in the following manner. Six-tipped corollas were not -found, and among 1,145 flowers there were the following proportions:—</p> - -<table> - -<tr><td class="tdl">Tips of the corolla</td> - <td class="tdr">3</td> - <td class="tdr">4</td> - <td class="tdr">5</td></tr> - -<tr><td class="tdl">Number of flowers</td> - <td class="tdrp">61</td> - <td class="tdrp">196</td> - <td class="tdrp">888</td></tr> - -</table> - -<p>It is thus clear that amphimixis is an essential factor in the -fixing of forms, but that it certainly does not of itself determine these, -and that it is not always the average of the variations that is the -most frequent, but that the form of the curve of frequency is<span class="pagenum"><a id="Page_209"></a>[Pg 209]</span> -determined by other factors also, namely, by germinal and personal -selection and by the directive control which these exert on variations.</p> - -<p>The equalizing effect of amphigony may perhaps be expressed -thus: In the case of every new adaptation there is at first a large -area of variation, but this gradually decreases owing to a continual -restriction on the part of natural selection, until ultimately—when -the highest degree of constancy of the character or species has been -attained—it only extends very little beyond the 'adaptation-playground' -or the 'area of exemption.'</p> - -<p>One of the effects of amphimixis is thus to bring about an -increasing restriction of the area of variation, or, as we usually say, -a constancy of the facies of a given form, a condensation into -a species. How far this result is necessary or useful, and therefore -how far it may be regarded as accounting for the persistence of -amphimixis, we shall discuss in the chapter on the formation of -species. My own view is that even the fact that new adaptations -are rendered possible through amphimixis and amphigony, the -mode of reproduction associated with it, affords in itself a sufficient -reason why amphimixis should have been retained when once it -had been introduced.</p> -<hr class="full" /> - -<div class="chapter"> -<p><span class="pagenum"><a id="Page_210"></a>[Pg 210]</span></p> - -<h2 class="nobreak" id="LECTURE_XXIX">LECTURE XXIX</h2> -</div> - -<p class="c">THE GENERAL SIGNIFICANCE OF AMPHIMIXIS (<i>continued</i>)</p> - -<div class="blockquot"> - -<p>Association of amphimixis with reproduction—Origin of amphimixis—Its lowest -forms—Amphimixis in Coccidia—Chromosomes in unicellular organisms—<i>Coccidium -proprium</i>—'Amœba-nests' as a preliminary stage to amphimixis—Plastogamy of the -Myxomycetes—Result: a strengthening of the power of adaptation—Strengthening of -the power of assimilation—Use of complete amphimixis—Proof of its constant efficacy -to be found in the rudimentary organs of Man—Allogamy—Means taken to prevent the -mingling of nearly related forms—Amphimixis is not a 'formative' stimulus—Attraction -of the germ-cells—Effects of inbreeding compared with those of parthenogenesis—Nathusius's -case of injurious inbreeding—Hindrances to fertilization in the crossing of -species—Probable reason for the injurious effect of inbreeding.</p></div> - - -<p><span class="smcap">We</span> have endeavoured to understand why amphimixis should -have been established among the processes of life, and we have now -to turn to the question when and how, that is, in what form, it was -first introduced. But first I should like to refer for a little to the -association of amphimixis with reproduction, which we find in all -multicellular organisms, and among the higher types so unexceptionally -that, until not very long ago, amphimixis and reproduction -were looked on as one and the same thing, and all multiplication was -believed to be associated with 'fertilization.' We have seen that this -is not the case, that on the other hand the two processes are quite -distinct, and may be called contrasts rather than equivalents, for -reproduction always means an increase in the number of individuals, -while amphimixis implies—originally at least—their diminution by -a half.</p> - -<p>Accordingly we found that, in unicellular organisms, amphimixis -is not associated with reproduction, but interpolated between the -divisions, and not even in such a manner that amphimixis precedes -every multiplication by division, but so that the conjugation of two -animals occurs only from time to time, after numerous divisions, -sometimes hundreds, have occurred. It is obvious that this must -be so, since, if amphimixis occurred regularly between every two -divisions, no increase in the number of individuals would be brought -about, at least if the fusion of the two conjugating individuals were -complete.</p> - -<p><span class="pagenum"><a id="Page_211"></a>[Pg 211]</span></p> - -<p>Why, then, is there such an intimate, and in the case of the -higher types, such an indissoluble, association between reproduction -and amphimixis that 'fertilization' appears to be a <i>sine qua non</i> of -reproduction, and not very long ago seemed to us to be the 'quickening -of the ovum,' the 'burning spark' which causes the powder-barrel -to explode?</p> - -<p>The reason of this is not difficult to discover; it lies in the -structure of multicellular animals, and in their differentiation according -to the principle of division of labour, for since only particular -cells are capable of reproduction, that is, of giving rise to the whole, -it is in these necessarily that the process of amphimixis has to occur -if its significance lies in its effects on the succeeding generations. It -is true that in the lowest multicellular organisms, such as the species -of <i>Volvox</i>, there are, in addition to the sex-cells, other reproductive -cells quite similar to the ova, whose development into a new colony -takes place without amphimixis, but the higher we ascend in the -animal and plant series the rarer are these 'asexual' germ-cells or -'spores,' and in the highest animal types they are entirely absent and -reproduction occurs only by means of the 'sex-cells.'</p> - -<p>I am inclined to look for the cause of this striking phenomenon -mainly in the fact that, if amphimixis had to be retained, this was -effected with increasingly great difficulty the more highly and complexly -differentiated the organisms became, and that more complicated -adaptations were therefore necessary in order that the union of the -two germ-cells might be rendered possible at all. There is first of -all the separation into two kinds of sex-cells, whose far-reaching -differentiations and precise adaptations to the most minute conditions -we have already discussed; then follow the innumerable adaptations -to bring about the meeting of the sex-cells, the arrangements for -copulation, and, finally, the instincts which draw the two sexes -together, the means of attraction which are employed, whether -decorative colours or attractive shapes, stimulating odours or musical -notes, in short, all the diverse and intricate arrangements, which seem -to be more subtly elaborated the higher the organism stands upon -the ladder of life. When we call to mind that sexual differentiations -finally go so far that they dominate the whole organism, alike in its -external appearance and in its internal nature, its feelings, inclinations, -instincts, its will and ability, as well as its structure down to the -finest nerve-elements, we can understand that a mode of reproduction -which demands such a composite disposition of details, involving -a moulding of the whole organism, so to speak, from birth till death, -must of necessity remain the only one, and that there was no room<span class="pagenum"><a id="Page_212"></a>[Pg 212]</span> -for the persistence of any essentially different mode of reproduction -with quite different adaptations. Or, to speak metaphorically, the -power of adaptation which is innate in the organism so exhausted -itself in the establishment of this marvellous amphimixis adjustment -that the possibility of any other was totally excluded.</p> - -<p>It is true that it is only among the Vertebrates that we find -'the reproductive apparatus' so highly developed, but even among -Molluscs and Arthropods 'sexual' reproduction, that is, reproduction -associated with amphimixis, is the prevailing mode. In these, indeed, -parthenogenesis does occasionally occur, that is to say, sexually -differentiated female germ-cells are, by means of some slight variations -in the maturation of the egg, rendered capable of developing -without previous amphimixis, but this happens only in quite special -cases as an adaptation to quite special circumstances, and can only be -regarded as a temporary cessation of the association between reproduction -and amphimixis. In some cases it is a moiety of the ova -adapted for amphimixis which develop parthenogenetically, as it -is the same sexually differentiated animals, true females, which -produce both sorts, and this is often true to some extent when the -differentiation in the direction of parthenogenesis has advanced -further, and the ova have been separated into those requiring fertilization -and those which are parthenogenetic (e.g. the winter and -the summer eggs of the Daphnidæ). Parthenogenesis is not asexual -but unisexual reproduction, a mode of multiplication which shows -us that even in highly differentiated animals the apparently indissoluble -association between reproduction and amphimixis can be -dissolved if circumstances require it.</p> - -<p>But if amphimixis had to be retained in the higher animal forms—and -we have seen reasons why this must be—it could only be -effected by means of unicellular germs, for amphimixis is in essence -a fusion of nuclei, and this is the reason why 'vegetative' reproduction, -so-called, becomes less and less prominent in animals at least, -and above the level of the Arthropods disappears almost entirely.</p> - -<p>Let us now return to the question we asked at the beginning—When -and in what form was amphimixis first introduced into the -world of organisms? The best way to answer this is by observation. -We must turn to the lowest forms which now exhibit it, and -see whether it occurs in them in a simpler form, so that we may draw -conclusions as to its origin and its primitive significance, for it would -be possible, <i>a priori</i>, that this was something different from what -it is now in the relatively higher organisms, and that a change -of function has gradually come about.</p> - -<p><span class="pagenum"><a id="Page_213"></a>[Pg 213]</span></p> - -<p>Assuredly the whole intricate complex of adaptations which -is now exhibited on the conjugation of the two sex-cells in animals -and plants, the differentiation of two kinds of 'sexually' antagonistic -cells, with all their special adaptations, the reduction of the chromosomes, -the institution of the karyo-kinetic apparatus, together with the -centrospheres and so on, cannot possibly have arisen all at once by -fortuitous variation, but can only have arisen gradually, step by step, -and as the result of 'innumerable external and internal influences.' -But why should not these arrangements, nowadays so complex, have -had a simple beginning? Why might not this beginning have been -the simple union of the protoplasmic bodies of two non-nucleated -Monera; followed, after the origin of nuclear substances, by the union -of these, and, finally, after the differentiation of a nucleus with -a definite number of chromosomes, with a dividing apparatus, with -a membrane, and so on, by complete amphimixis as we now know it? -And how many transition stages may not be added to fill up the -gaps between these three main stages?</p> - -<p>But how much we can actually prove in regard to these conceivable -preliminary stages of amphimixis is another matter. If we -take a survey of the observations that have been made up till now, -we are confronted at first by the undoubtedly striking fact that very -little is known about it as yet, for in fact the whole process is gone -through even in quite lowly forms of life in a manner very similar to -that in the higher forms. Amphimixis has been shown to be widespread -even among unicellular organisms, yet not in an <i>essentially</i> -simpler mode than among multicellulars. We have seen that even in -ciliated Infusorians reducing division obtains, and that of the four -nuclei which arise from twofold division of the original nucleus -three break up again, and only the fourth, by a further division, -separates into a male and a female pronucleus, 'which then complete -the amphimixis with the corresponding pro-nuclei of another -animal' (compare <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#f89">Fig. 85</a>, 4-7, vol. i. p. 321). This, and the existence -of a dividing apparatus and of chromosomes, make the process appear -very little less complicated than the fertilization of higher animals. -The case is similar even in much lower unicellular organisms, such as -<i>Noctiluca</i> (<a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#f87">Fig. 83</a>, vol. i. p. 317). In this form and in Rhizopods it is -true that reducing divisions have not yet been made out, but their -occurrence in the lower Algæ (<i>Basidiobolus</i>), and above all in those -simple unicellular organisms which give rise to malaria, and their -allies, which live as 'Coccidia' in the blood-cells and intestinal cells -of animals, leads us to expect that they may prove to be of general -occurrence among unicellulars.</p> - -<p><span class="pagenum"><a id="Page_214"></a>[Pg 214]</span></p> - -<p>In the Coccidia, which are extremely simple unicellular organisms, -equipped, however, with a nucleus, the adaptations relating to amphimixis -are more extensive and more complex than in the Rhizopods. -For while in the latter the two conjugating cells are absolutely alike -in external appearance, in the former the male cell is distinct from -the female, and indeed the differences are as marked as those that -usually occur in multicellular animals.</p> - -<div class="figcenter" id="ff37"> -<img src="images/ff37.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 121.</span> Life-cycle of <i>Coccidium lithobii</i>, a cell-parasite of the centipede -<i>Lithobius</i>; after Schaudinn. 1, a 'sporozoite'; 2, the same penetrating into an -intestinal epithelial cell; 3, the same growing into a 'schizont' capable of -division; 4, the same dividing, and 5, breaking up into numerous pieces which -separate from the 'residual body' in the centre, and either, as in 1, migrate -into epithelial cells and repeat the history, or pass on to the phase of sexual -reproduction. In the latter case, after eliminating a portion of the nucleus -(reduction) in 6 and 6 <i>a</i>, they form the 'macrogamete' (the ovum); or within -the mother-cell they produce microgametes (or sperm-cells), 7 and 7 <i>a</i>. The -penetration of a sperm-cell into an egg-cell (amphimixis) is shown in 8, the -fertilized egg-cell (9) becomes the so-called oocyst or permanent spore, from -which by repeated division (10 and 11), new sporozoites, as in 1, arise, and -begin the cycle afresh.</p> -</div> - -<p>We owe our present knowledge of these processes especially to -Schuberg, Schaudinn, and Siedlecki, and, because of their theoretical -importance, I should like to summarize the essential points.</p> - -<p>One of these Coccidia lives in the intestinal cells of a small -centipede, <i>Lithobius</i>; in Fig. 121 the parasite is shown as a so-called -'Sporozoite,' that is, as a minute sickle-shaped cell, which at first -moves freely about the intestine of the host (1), but then soon -penetrates into an epithelial cell (2). There it grows to a spherical<span class="pagenum"><a id="Page_215"></a>[Pg 215]</span> -shape (3), and then, after having devoured the cell, it gives rise, by -a peculiar process of division (Schizogony), to a number of very -minute nucleated pieces, again sickle-shaped, the Schizonts, each -of which bores its way into an epithelial cell as in 2, and follows -the same path of development, so that a large number of cells in the -intestine of the same host are attacked in this manner. But there -is still another mode of reproduction, with which amphimixis is -associated, which leads directly to the formation of 'lasting' germs -which are enclosed in a capsule or cyst, reach the exterior with the -excrement of the host, and thus spread the infection to other centipedes. -The Schizonts which take this course develop into so-called macro-gametes -and microgametes, the former being the female, the latter the -male germ-cells. Then follows the penetration of a male gamete, -actively motile because of its two flagella, into the female gamete (8). -Amphimixis is accomplished, and the product of the fusion of the -two sex-cells (9) surrounds itself with a thinner cyst, within which it -multiplies by twofold division into four cells (10). These are the -'lasting' spores, which may dry up within the voided excrement -of the centipede (11), and if they be eaten by another animal of the -species, they infect it, for the sporozoites which have been formed by -the previous divisions creep out, and in form 1 begin the life-history -anew.</p> - -<p>We have thus an alternation of four generations which are all -unicellular, and of which one series (1-5) shows multiplication by -fission, while the other (6-11) includes, besides multiplication by -fission and as a condition of this, the process of amphimixis. Amphimixis -<i>must</i> occur in order that the formation of 'lasting' spores and -new sporozoites may result. We have thus a regular alternation of -'asexual' and 'sexual' reproduction, and the latter shows great -resemblance to that of multicellular organisms. The macrogamete -corresponds to the ovum, the microgametes to the spermatozoa, and -they resemble these also in their greater numbers and in their -structure.</p> - -<p>But the resemblance goes even further. The ovum is much larger -than the sperm-cell, and undergoes a kind of reduction of its nuclear -substance; shortly before fertilization the ovum-nucleus ('the -germinal vesicle') comes to the surface—just as in the case of animal -ova—bursts, and extrudes a part of its substance in the form of -a sphere (Fig. 121, 6 and 7). A reduction of the nuclear substance in -the male cell has not been demonstrated in all cases, but in one of the -<i>Lithobius</i>-Coccidia, <i>Adelea ovata</i>, the relatively large microgamete -(the sperm-cell, Fig. 122, <i>Mi</i>) places itself close to one pole of the<span class="pagenum"><a id="Page_216"></a>[Pg 216]</span> -female macrogamete (the egg-cell) and then divides twice in succession, -so that four small cells arise (Fig. 122, <i>A-C</i>); of these only one -penetrates into the egg-cell (<i>D</i>, ♂<i>K</i>) and unites with it, the other three -come to nought (<i>D</i>, <i>Mi</i>). What a surprising resemblance this bears -to the twofold division of the mother sperm-cell in multicellular -animals, through which the number of chromosomes is reduced to -half! In the conjugation itself the thread-like chromosomes of the -female nucleus are plainly recognizable, while those of the male -remain coiled up (Fig. 122, <i>D</i>).</p> - -<p>That the nuclear substance can be separated into chromosomes -(ids) even in lowly unicellular organisms was probably first demonstrated -by R. Hertwig for <i>Actinosphærium</i>, a Heliozoon or freshwater -sun-animalcule, then by Lauterborn in regard to Diatoms, by Blochmann -for an indigenous Rhizopod, <i>Euglypha</i>, and by Ishikawa for -the marine <i>Noctiluca</i>. Fresh cases have been added in the last -decade, so that we can now say that a considerable number of unicellulars, -from the ciliated Infusorians and lower Algæ down to the -Coccidia and Diatoms, exhibit a germ-plasm composed of ids. These -structures behave in the same way as those in higher organisms, and -Berger was able to demonstrate, in 1900, in the case of a Radiolarian, -their multiplication by spontaneous splitting.</p> - -<div class="figcenter" id="ff38"> -<img src="images/ff38.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 122.</span> Conjugation of a Coccidium (<i>Adelea ovata</i>), after Schaudinn and -Siedlecki. <i>A</i>, the microgamete (sperm-cell) (<i>Mi</i>) has become closely apposed -to the macrogamete (<i>Ma</i>). <i>B</i>, the reduction division of the nucleus of the -macrogamete has been effected; <i>Rk</i>, directive corpuscles. In the microgamete -the first division of the nucleus has begun. <i>C</i>, four nuclei in the microgamete, -of which three come to nought. <i>D</i>, the fourth microgamete-nucleus (♂<i>K</i>) has -become apposed to the nucleus of the ovum, in which distinct chromosomes -are seen.</p> -</div> - -<p>From our point of view all this cannot surprise us, since all -these organisms, though only single cells, possess great complexity of -structure; we need only call to mind the extremely fine differentiation -of structure in numerous ciliated Infusorians, such as <i>Stentor</i>, which -has already been mentioned, or the bell-animalcule (<i>Vorticella</i>) with -its long and peculiarly ciliated gullet, its retractile ciliated disk, its -muscular or myophane layer, its spirally retractile stalk with the<span class="pagenum"><a id="Page_217"></a>[Pg 217]</span> -ribbon-like, rapidly acting muscular axis; or the regular geometrically -constructed flinty skeleton of the Radiolarians, with their radially -disposed sword-like or rod-like needles and their complex interlacing -lattice-work shells. In the latter case the complexity of the living -substance becomes visible only through its product, the shell, for the -protoplasm itself does not show any visible intricacy, and the same is -true of the Coccidium whose life-history we have just been tracing, -for in each of its stages it seems to be of very simple organization, -though the succession of numerous different forms shows that its -germ-substance must be composed of numerous determinants.</p> - -<p>We cannot doubt, however, that, in all unicellular organisms, -the protoplasm can be hardly less complicated as regards its minute -invisible structure, since otherwise it would be impossible that the -delicate vital processes which we observe in them should run their -course. In this I agree, at least in principle, with the beautiful -picture drawn by Ludwig Zehnder in his recent book<a id="FNanchor_22" href="#Footnote_22" class="fnanchor">[22]</a> already -mentioned, though he reached it in quite a different way, namely, by -a purely synthetic method. He made the daring attempt to build up -the organic world from below, starting from atoms and molecules, -and ascending from these to the lowest vital units, our biophors, -to which he attributes a tubular shape and therefore calls fistellæ. -He imagines the cell to be made up of a large number, perhaps millions, -of different kinds of fistellæ, of which one presides over the power of -turgidity, another over endosmosis, a third over contraction, a fourth -over the conduction of stimuli, &c., so that there results a high degree -of cellular complexity, a composition out of numerous kinds of -biophors arranged on a definite architectural plan. All this corresponds -perfectly with the views I have so long championed, and -which alone make the existence of a nucleus intelligible, if it is composed—as -I assume—essentially of an accumulation of determinants, -that is, of hereditary substances. And that such a high degree of -complexity of structure is not a mere fanciful picture we see -occasionally even in the case of unicellular organisms. Thus, for -instance, in <i>Coccidium proprium</i>, parasitic in the newt (<i>Triton</i>), the -macrogamete or egg-cell (Fig. 123, <i>Ma</i>) before fertilization by the -sperm-cell or microgamete (Fig. 123, <i>Mi</i>) surrounds itself with a capsule, -at one pole of which a minute opening, the micropyle, remains -for the entrance of the male cell. This proves, it seems to me, that -this particular spot of the capsule is hereditarily determined, just as -much and just as definitely as the ray of the flint-skeleton of a Radiolarian. -But if any spot of the capsule can vary by itself alone, may<span class="pagenum"><a id="Page_218"></a>[Pg 218]</span> -not numerous other points in the animal also be hereditarily determinable? -With such complexity of the invisible structure it would -not greatly surprise us if we should find amphimixis occurring in all -unicellular organisms, and in many of them at a high level of elaboration. -These apparently lowly and simple organisms are obviously very -far from being the lowliest and simplest, as we shall discover later -in a different connexion. But that amphimixis is found as a periodically -recurring process even among these, must depend upon the -fact that here too the preservation of the best-adapted structure, as -well as adaptability to new conditions, requires that the best variants -of many different parts of the cell should be brought together, and -since the hereditary substance lies in the ids of the nucleus, the union -of the ids of two unicellulars will make harmonious and many-sided -adaptation materially easier. It will thus give an advantage in the -struggle for existence, and we may therefore expect to find that the -nuclear substance in all unicellular organism is made up of ids.</p> - -<div class="footnote"> - -<p><a id="Footnote_22" href="#FNanchor_22" class="label">[22]</a> Zehnder, <i>Die Entstehung des Lebens</i>, Freiburg-i.-Br., 1899.</p> - -</div> - -<div class="figcenter" id="ff39"> -<img src="images/ff39.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 123.</span> Conjugation of <i>Coccidium proprium</i>, a cellular parasite of the -newt (<i>Triton</i>), after Siedlecki. <i>A</i>, a microgamete (<i>Mi</i>) in the act of penetrating -the shell of a macrogamete (<i>Ma</i>) through the micropyle. <i>B</i>, the male and -the female nuclear constituents are uniting (♂ <i>chr</i> and ♀ <i>chr</i>).</p> -</div> - -<p>The observations hitherto made do not, however, appear to bear -this out, for in the lower Flagellata and Algæ the nuclear substance -does indeed consist of chromatin, but—as far as it can be made -out—of a compact unarranged mass of it. But even though deeper -investigations should succeed in demonstrating chromosomes in many -of these, the nucleus <i>must have arisen at some time</i>, and we must -assume that it did so through a more intimate union of previously -loose aggregates of determinants, which were gradually arranged -and bound together by the combining forces (affinities) we have<span class="pagenum"><a id="Page_219"></a>[Pg 219]</span> -assumed to obtain among them, thus giving rise to the first chromosomes -or ids which were complete in themselves. Then came the multiplication -of these ids by the process of division, and only then was the -state arrived at from which amphimixis, as we now know it, could -have arisen, namely, the existence of a considerable number of identical -ids, half of which could be exchanged for the identical ids of another -individual in conjugation.</p> - -<p>But as to our question, In what organisms did amphimixis first -arise, and how? there seems, from what we have already learned -with regard to the Coccidia, little prospect of our being able to give a -definite answer, for if amphimixis occurs even in these lowly organisms, -and occurs, too, in the same manner as in the higher unicellular -organisms, and not very much more simply than among the highest -multicellular organisms, we may conclude that the preliminary stages -will now be very difficult or impossible to detect, either because they -are extinct, or because they occur only in ultra-microscopic organisms.</p> - -<p>Nevertheless there do appear to be preliminary stages, and they -are exactly those which we should have assumed if we had been -obliged to construct them theoretically.</p> - -<p>The first phenomenon of this kind is the mere juxtaposition of -two or more unicellular organisms, without the occurrence of fusion. -This was probably first observed by Gruber in Amœbæ, and it was -theoretically interpreted at a later date by Rhumbler. As many as fifty -Amœbæ gather together to form a 'nest,' and remain closely apposed -to each other for a fortnight. Although no fusion took place, and there -were no visible results of this juxtaposition, it may be concluded that -the animals had some sort of attractive effect upon each other, and it -may be supposed that some sort of advantage must have been -associated with this state of quiet, close apposition against one another. -Cytotropism, the mutual attraction of similar cells, which Wilhelm -Roux first observed in the segmentation-cells of the frog's egg, seems -to occur also in unicellular organisms, and this may help us to understand -how a fusion of cell-bodies may have come about.</p> - -<p>Fusion of this kind was demonstrated in the Myxomycetes -almost forty years ago by De Bary, and it has been observed more -recently in various unicellular organisms, especially in Rhizopods and -in Heliozoa. These last often place themselves close together in pairs, -threes, or even more at a time, and then the delicate cell-bodies -coalesce, though no fusion of the nuclei takes place. With Hartog, -we call this process 'Plastogamy,' but we cannot agree with that -observer when he regards the importance of the process as consisting -in the fact that the nuclei thus come into contact with fresh<span class="pagenum"><a id="Page_220"></a>[Pg 220]</span> -cell-substance, after having been surrounded for a very long time -with the same cytoplasm. If this were the import of amphimixis, -then an <i>exchange of nuclei</i> would take place, and this we find nowhere -even among the lowest forms of life, for everywhere there is a union -of the nuclear substance of two individuals. But this is by the way! -Further cases of plastogamy have been observed in many of the limy-shelled -Rhizopods. A union of this kind does not usually lead to any -visible consequences, but in some Foraminifera a group of young -animals is developed within the cell-bodies by the division of the -nuclei and the cell-body; thus multiplication follows the fusion just -as in perfect amphimixis, and we may therefore assume that there -is a causal connexion between the two. In the slime-fungi, too, -the union of several amœba-like cells into a multi-nucleated plasmodium -is followed later by the development of numerous encapsuled -spores, but only after the plasmodium, which to begin with is microscopically -small, has grown to a macroscopically visible, reticulated -mass (<i>Æthalium</i>) sometimes a foot in extent. In this case the fungus, -creeping slowly over its foundation of decaying substance, takes up -nourishment from it, and it is not possible to tell whether the union -of the amœbæ yields any further advantage than that of facilitating -the spreading over large uneven surfaces, and through this, later, -the development of large fruit-bodies. But in the case of the Foraminifera -the plastogamy has obviously another effect, unknown and -mysterious, which as yet no one has ever been able to define precisely. -Words like 'stimulus to growth,' 'stimulating of the metabolism,' and -even 'rejuvenation,' give no insight into what happens, but that -something happens, that through the fusion of two or more unicellulars -a stimulus is exerted, which reveals itself later in increased -rapidity of growth, we may, and indeed must assume, because this -process has become a permanent arrangement in so many unicellular -organisms. Only what is useful survives, and the uniting individuals -must derive some advantage from the process of fusion, and it remains -to be seen whether we can find out with any clearness what this -advantage may be.</p> - -<p>Till within a few decades ago it was believed that in this process -one individual devoured the other, but this can now no longer be maintained. -If any one still seriously considers this possible, Schaudinn's -observations would convince him of his error, for in <i>Trichosphærium</i>, -a marine, many-nucleated Rhizopod, he observed, on the one hand, -the union of two or more animals, i. e. plastogamy, and on the other -hand, the swallowing and digesting of a smaller member of the species -by a larger one—two processes which are absolutely different, for in<span class="pagenum"><a id="Page_221"></a>[Pg 221]</span> -the first case the cell-bodies of the two animals remain intact, while -an animal that is eaten becomes surrounded by a food-vacuole, and is -dissolved and digested within it. In the former case the vital units -(biophors) of each animal obviously remain intact and capable of -function; in the second, those of the over-mastered animal are at -once dissolved and chemically broken up; as biophors, therefore, -they cease to exist. Whether one or the other process takes place may -perhaps depend on whether the two animals differ greatly in size, so -that the smaller can be quite surrounded by the larger.</p> - -<p>In a former lecture I have emphatically expressed my dissent from -the view which interprets amphimixis as a process of rejuvenation, -meaning thereby a necessary renewal of life, and I need not go into -this again in detail: for that the metabolism can continue through -uncounted generations without being artificially stimulated—that is, -in any other way than by nutrition—is proved by all those lowly -organisms which exhibit neither plastogamy nor complete amphimixis, -and also by the occurrence of purely parthenogenetic reproduction, -&c. In what, then, can the advantage lie which the conjugating -unicellular organisms derive from conjugation? Obviously not in that -they impart to each other what each already possessed, but only in the -communication of something special and individual, something that -was peculiar to each, and becomes common to both.</p> - -<p>Haberlandt believed that the development of auxo-spores in -Diatoms pointed towards the processes which form the deepest roots -of amphimixis. As is well known, the hard and unyielding flinty -shell of these lowly Algæ involves a diminution of the organism -at every division, so that the Diatoms become smaller and smaller -as they go on multiplying, and if that went on without limit they -would come rapidly to extinction. But a corrective is supplied in the -periodical occurrence of conjugation of two organisms which have -already materially diminished in size, and this is followed by the -growth of the two fused individuals to the original normal size of -the species.</p> - -<p>It is, of course, obvious that in this case the union of two -organisms which have become too small may be of advantage -in bringing them back to the requisite normal size; but this is an -isolated special case, which certainly does not justify our regarding -conjugation as a means whereby diminished bodily size may be brought -back to its normal proportions. By far the greater number of unicellular -organisms are not permanently diminished in size by division, -and even in the Diatoms the mass of the two fused individuals does -not amount to the normal size of the species, so that even in this<span class="pagenum"><a id="Page_222"></a>[Pg 222]</span> -case there must be growth subsequent to the conjugation before the -normal is re-attained. It may be doubted, therefore, whether the -increase in mass is, even in the case cited, the essential event in -conjugation, and whether there are not other effects which we cannot -clearly recognize. Here, too, there must be differences between the -two conjugating individuals, as we have just seen, for if they only -communicated something similar to each other, the result would be an -increase only in their mass, not in their qualities.</p> - -<p>Although we cannot demonstrate differences of this kind in the -case of the lowly organisms with which we are now dealing, we may -assume their existence from analogy with the higher organisms. -We know, especially through G. Jäger, that in Man every individual -has a specific exhalation, his particular odour, and that in the secretions -of his glands there are incalculably minute differences in chemical -composition, which justify the conclusion that the living substance -of the secreting cells themselves exhibits such differences, and that -all the various kinds of cells in an individual are not absolutely -identical with the corresponding cells of another individual, but that -they are distinguished from them by minute yet constant chemical -differences. The assumption that differences of this kind exist even -in unicellulars, and in all lowly organisms generally, is not a merely -fanciful one, but has much probability.</p> - -<p>How far the combination of these individual differences of -chemical, and at the same time vital, organization is able to quicken, -to strengthen the metabolism, to bring about 'physiological regeneration,' -or whatever we may choose to call it, we do not yet understand. -It has been said that in plastogamy an exchange of 'substances' takes -place; that each gives to the other the substances which it possesses -and the other lacks, and that this causes an increase of vital energy. -But it is unlikely that we have here to do merely with chemical -substances, although these, of course, as the material basis of all vital -processes, are indispensable; it seems to me more probable that the -vital units (biophors) themselves in their specific individuality must -play the chief part. But even this is saying very little, for we have -not yet reached an understanding of these processes, and if we were -not forced by the fact of plastogamy to the conclusion that this union -must have some use, no one would have been likely to postulate -it as useful, still less as necessary. It has, of course, been frequently -suggested that multiplication by fission, if long-continued, results -in 'exhaustion,' and that this is corrected by amphimixis, but who -can tell why this 'exhaustion' might not be remedied, and even more -effectually remedied, by a fresh supply of fuel, that is, of food? One<span class="pagenum"><a id="Page_223"></a>[Pg 223]</span> -might have thought that the vital processes would be thus more -readily recuperated than by the co-operative combination of two -already 'exhausted' cells. Two exhausted horses may perhaps be able -to pull the load that one of them was no longer equal to, but in the -case we are considering it is the combined burdens of two units that -have to be borne, although each was no longer equal to its own share! -That is more than we can understand.</p> - -<p>Zehnder has recently defined the effect of amphimixis as -a 'strengthening of the power of adaptation,' and he infers that the -'digestive fistellæ' (Biophors) of two individuals, which have somewhat -different powers of digestion, are, when they combine, able -to assimilate more kinds of food than either was able to assimilate -by itself. But I confess that I do not see how an advantage for the -whole would be gained through this alone, since half of the digestive -biophors would have to work for the nutrition of the mass of the -individual <i>A</i>, the other half of the differently constituted biophors -for that of the individual <i>B</i>, and the nutritive capacity would thus -remain exactly what it was before conjugation. Nevertheless I believe -that Zehnder was right in his supposition that conjugation is concerned -with strengthening the power of adaptation, and I have long -maintained and defended this interpretation with regard to true -amphimixis in nucleated organisms. In these cases it is quite obvious -that the communication of fresh ids to the germ-plasm implies an -augmentation of the variational tendencies, and thus an increase -of the power of adaptation. Under certain circumstances this may -be of <i>direct</i> advantage to the individual which results from the -amphimixis, but in most cases the advantage will be only an indirect -one, which may not necessarily be apparent in the lifetime of this -one individual, but may become so only in the course of generations -and with the aid of selection. For amphimixis must bring together -favourable as well as unfavourable variations, and the advantage -it has for the species lies simply in the fact that the latter are weeded -out in the struggle for existence, and that by repetition of the process -the unfavourable variational tendencies are gradually eliminated more -and more completely from the germ-plasm of the species.</p> - -<p>But this cannot have been the efficient cause in the introduction -of amphimixis into the series of vital phenomena; the reason for this -must be found in some <i>direct</i> advantage, such as that it improved and -increased the assimilating power, the growth, and the multiplication -of the particular individual, so that it gained an advantage over -individuals which had not entered into conjugation. This advantage -must exist, at least in the lower forms of conjugation, in pure plasto<span class="pagenum"><a id="Page_224"></a>[Pg 224]</span>gamy, -i. e. in the mere coalescence of the protoplasmic bodies. But, -as it seems to me, we have not yet clearly recognized what the -advantage precisely is; we do not yet see how such a mingling -or combination of two plasms should every time be of advantage -for the combined conjugate. If we assume with Zehnder that two -kinds of 'nutritive' biophors are brought together which differ slightly -from each other in digestive capacity, three cases may occur. Either -the food <i>a</i>, adequate for the animal <i>A</i>, is just as abundant as the -food <i>b</i>, suitable for the animal <i>B</i>, and then half the conjugated animal -will be nourished by means of the biophors <i>a</i>, the other half by means -of the biophors <i>b</i>, and the state of matters is the same as it was -before conjugation; or the food <i>b</i> is more abundant than the food <i>a</i>, -or conversely, and then the biophors <i>b</i> will have to take the larger -share in the nourishment of the conjugate <i>A</i> + <i>B</i>, and they will -therefore multiply more rapidly and the biophors <i>a</i> will decrease -relatively in number. Nutrition and growth will then go on more -slowly for a time, but will soon attain to their former intensity. The -combined individual <i>A</i> + <i>B</i> has then certainly gained an advantage -over the isolated animal <i>A</i>, and the living substance of <i>A</i> which, -if left to itself would probably have perished, can continue to live -in combination with <i>B</i>. But in that case it is not obvious where -the advantage in the union can lie, as far as <i>B</i> is concerned. An -advantage to <i>B</i> only results if there be a combination not of <i>one</i> kind -of biophor only, but of several or many kinds of biophors. If for -instance <i>A</i>, whose digestive biophors were weak, brought with it into -the partnership 'secretory' or nervous biophors stronger than those of -<i>B</i>, then there would be an advantage for both in the combination, -and it is thus that, in the meantime, I interpret the direct benefit -which results from pure plastogamy. This benefit must be the more -important and far-reaching the longer multiplication by fission continues -without the occurrence of conjugation.</p> - -<p>We thus reach what is perhaps a not wholly unsatisfactory conception -of amphimixis, in so far at least that we do not require -to assume that there has been a fundamental change in its significance -between its expression in the lowest organisms and in the higher and -even highest forms. Everywhere it is the same advantage: an increase -in the power of adaptation; but it sometimes finds expression directly -in the product of conjugation, sometimes only indirectly, sooner or -later, among the descendants of the product.</p> - -<p>How far below the Myxomycetes pure plastogamy reaches we do -not know; whether it also occurs among non-nucleated organisms -(Haeckel's Monera) we cannot tell from experience, since these<span class="pagenum"><a id="Page_225"></a>[Pg 225]</span> -assumed organisms have not yet been observed with certainty. -Perhaps they all lie below the limits of visibility, and then we could -never do more than <i>suppose</i> that plastogamic processes occur among -them. Logically and purely theoretically we may <i>suppose</i> that amphimixis -occurred first between the plasmic bodies of non-nucleated -Monera, then between the cell-bodies of true cells, and finally between -the nuclei of cells.</p> - -<p>Let us hold fast to what we have found to be probable, namely, -that the fusion of individually different simple organisms must or may -bring about a direct advantage—a stimulation of the metabolism, and -at the same time an improvement of the constitution in different -directions, and let us go on to the consideration of cell-fusion combined -with nuclear fusion, or complete amphimixis. In this something -is added which we can recognize as an important advantage, namely, -the combination of two hereditary substances, and thus the union -of two variation-complexes which, according to our view, is necessary -if transformation of species is to take place. In mere plastogamy -such a union of two hereditary masses could only take place in Monera, -not in nucleated organisms. If then there are really unicellular -organisms which exhibit plastogamy without karyogamy (certain -Foraminifera), we have a further proof that these processes of plasmic -fusion imply direct advantage, which is distinct from the indirect -advantage lying in the mingling of two different hereditary contributions, -since in these cases of plastogamy there is no demonstrable -mingling of hereditary bodies, no karyogamy.</p> - -<p>But as soon as karyogamy or nuclear fusion was associated with -mere plastogamy, complete amphimixis could never be lost again, -because it alone made it possible that there should be harmonious -transformation and adaptation in organisms which were becoming -ever more complex; the primary effect of the mingling would be more -and more transcended, since, without amphimixis, transmutation with -harmonious adaptation in all directions would be less and less possible -as organisms became more complex in structure. I have already -referred to the manifold details in the structure and development -of the lowest organisms which make this conclusion appear luminous -to us, but we can also infer the necessity for an unceasingly active -selection, from a quite different set of facts, namely, from what we -know of rudimentary organs in Man.</p> - -<p>We may regard Mankind as a species which has its local races -and sub-races, but which is fixed in its essential characters, and only -fluctuates hither and thither in individual variation in each sub-race, -just like any other modern mammal, such as the marmot or the hare.<span class="pagenum"><a id="Page_226"></a>[Pg 226]</span> -Nevertheless we know that Man, as regards certain fairly numerous -parts, is continually and persistently varying in a definite direction. -Wiedersheim, in his book <i>On the Structure of Man</i><a id="FNanchor_23" href="#Footnote_23" class="fnanchor">[23]</a>, enumerates a long -series of parts and organs of the human body, which are in process -of gradual degeneration, and of which it may be predicted that they -will disappear from the human structure since they have lost functional -significance. Among these dwindling structures are the two last ribs, -the eleventh and twelfth, while the thirteenth has already disappeared, -and only occurs exceptionally as a small vestige in the adult human -being of to-day. The series includes also the seventh cervical rib, -the <i>os centrale</i> of the wrist, the wisdom teeth, and the vermiform -appendix of the intestine. The last is much larger in many mammals, -and represents an important part of the digestive apparatus, but -in Man it has dwindled to an unimportant appendage, which is -a source of danger when foreign bodies (cherry stones and such like) -lodge in it and set up inflammation. The variations in its length -warrant us in concluding that it is still in process of degeneration; -its average length is about 8½ cm., but it varies from 2 cm. to 23 cm. -in length, and in about 25 per cent. of cases a partial or entire closing -up of its opening into the intestine may be observed.</p> - -<div class="footnote"> - -<p><a id="Footnote_23" href="#FNanchor_23" class="label">[23]</a> <i>Ueber den Bau des Menschen</i>, 2nd ed., Freiburg-i.-Br., 1893. Trans. London, 1896.</p> - -</div> - -<p>Wiedersheim enumerates nearly a hundred parts thus in process -of degeneration: this means that nearly a hundred structures in Man -are at the present time in process of variation, and this could not be -so unless amphimixis were continually mingling the hereditary contributions -anew from generation to generation, so that the minus-variations -of the parts in question, starting from the germ-plasm in -which they arose at one time as chance variations, and confirmed in -their direction by means of germinal selection, are gradually being -transmitted to all the germ-plasms of the species. We thus see that -even in a period of species-life, which we may fairly call a period of -constancy, variations of a phyletic kind are continually in process, -which could not become general without the co-operation of amphimixis.</p> - -<p>Now, we have already seen that personal selection plays no part, -or, at least, no important part in such degenerations, because the -variations which are here concerned do not usually attain to selection -value, but it is just such variations proceeding with infinite slowness -that occur in functionally important organs likewise, and in the progressive -advance of which personal selection and mutual adaptation -probably play a part, so that in this way we can understand why the -preservation of amphigony by natural selection must be effected. It -is impossible—for obvious reasons—to name particular instances with<span class="pagenum"><a id="Page_227"></a>[Pg 227]</span> -certainty, as we can do in the case of the rudimentary organs, but -even on general considerations we might expect that among the -incipient variations of the determinants of the germ-plasm there -would be some which were in an ascending direction, and that among -these there would be some which, advanced by germinal selection, -would go on ascending until they attained selection value. Wiedersheim -reckons, for instance, the gradually increasing differentiation of -the cortical zone of the human brain among the parts which are still -in process of ascending variation, and he is probably right in -doing so.</p> - -<p>But if variations, so slow as to be unnoticeable, are still of -abundant occurrence in Man, we have no reason to doubt that similar -processes are going on in other animals; among the higher Vertebrates -at least there is hardly a species which does not exhibit regressive -variations even now, and in many cases progressive variations also -are occurring, although we cannot give definite proofs of this.</p> - -<p>The appearance of fixity which most species have is, therefore, -illusory; in reality they exhibit a slow flux, gradually setting aside -the superfluities they received from their ancestors, perfecting -the important parts to more precise adaptation and greater -functional capacity, and at the same time endeavouring to maintain -all the parts in constant harmony. We can understand that as long -as this process of gradual perfecting goes on, amphimixis will not -readily be given up. Those that retain it must always, in the long -run, have the preference. Moreover, as we have seen, <i>it cannot be -given up</i>, when it has existed through æons, because of the power of -persistence which the germ-plasm has gradually acquired in the -course of such a long hereditary succession. It could only be given -up if an advantage decisive as to survival were associated with its -abandonment, such as can be actually recognized in most cases of -parthenogenesis, among animals at least.</p> - -<p>In my opinion this indirect effect of amphimixis, that is, the -increasing of the possibilities of adaptation by new combinations of -individual variational tendencies, is the main one, while the direct -nutritive effect of the two germ-cells upon one another is quite -subsidiary. In this opinion I find myself in opposition to the views -of many if not most naturalists, who assume that amphimixis has -a direct, sometimes, indeed, only a direct effect, and believe that they -can prove it by facts.</p> - -<p>In support of this position it has been pointed out that allogamy, -that is, the mingling of individuals of different ancestry, occurs -even among lowly unicellulars, and then higher up among most<span class="pagenum"><a id="Page_228"></a>[Pg 228]</span> -organisms; but the question has not been asked whether this mutual -attraction of the unlike really expresses a primary characteristic of -organisms, and may not possibly be a secondary acquisition adapted -to ensure the occurrence of amphimixis. If we examine the facts we -find that even in the lowly Algæ, such as <i>Pandorina</i> and <i>Ulothrix</i>, -only the migratory cells or swarm-spores of different cell-colonies -conjugate with one another, but not those of the same lineage, and -this phenomenon may be observed in many unicellular plants and -animals. We are justified in concluding from this that a fairly large -degree of difference between the conjugating gametes secures the best -results, whether this result is to be looked for in a 'rejuvenation' or in an -increased adaptive capacity; but it is erroneous to regard the stronger -attraction between individuals of different descent as a direct outcome -of this. To me, at least, it seems to be an adaptive arrangement. -The whole of the long and complex phylogenetic history of the sex-cells, -the gametes, shows clearly that we have here to deal with -a succession of adaptations, and that the degree of attraction which -obtains between gametes has gradually been increased and specialized -in the course of the phylogeny. I need only briefly recall what we -have discussed in a former lecture, that at first the copulating cells -were exactly alike in appearance and size, that then one kind of cell -became rather larger than the other, and that only gametes which were -thus different in size were mutually attractive—the micro-gametes and -the macro-gametes, or male and female germ-cells; we have seen that -these differences between the two became more and more accentuated, -that the female cell continued to grow larger than the male, and to -accumulate more and more nutritive material for the building up of -the young organism which arises from its union with the male cell, -and that the male cells became smaller, but more numerous, as was -essential if their chances of finding the often remote female cell were -not to disappear altogether. And besides, there are all the innumerable -adaptations of the egg-cell to the countless special circumstances -which obtain in the different groups, and the innumerable varieties -in the form of the sperm-cell, with all its delicate and complicated -adaptations to the special conditions under which the egg-cell can be -reached and fertilized in this or that group of organisms. Of a truth, -he is past helping who does not regard with wonder and admiration -the adaptations which have been worked out in this connexion -in the course of evolution! But if all these details are adaptations, -so is the beginning of the whole process of differentiation; allogamy, -the attraction of conjugating cells of different lineage, is not a primary -outcome of individual diversity; gametes of different descent did not<span class="pagenum"><a id="Page_229"></a>[Pg 229]</span> -strongly attract each other of themselves, but they were equipped -with a strong power of mutual attraction, because the union of very -different individualities was the more advantageous.</p> - -<p>This is an important distinction, for the adaptation to allogamy is -widely distributed, and its latest manifestations have frequently been -misunderstood in the same way as its beginnings. The widespread -occurrence of allogamy has been interpreted as evidence in favour of -the rejuvenation theory, and the endeavour on Nature's part to secure -the union of the unlike has been associated with the hypothetical -'rejuvenating' power of amphimixis, and regarded as a direct and -inevitable outcome of this. That this view is erroneous we shall see -even more clearly from what follows.</p> - -<p>As among unicellular Algæ it is frequently only gametes of -different lineage which conjugate, so among animals and plants there -are numerous cases in which the union of nearly related gametes is -more or less strictly excluded, both by the prevention of self-fertilization -(autogamy) in hermaphrodites, or by the prevention of inbreeding, -that is, the continued pairing of near relatives. Now all the -preventive measures which effect this are of a secondary nature; they -are adaptations which result from the advantage involved in the -mingling of unrelated germ-plasms, even though it sometimes seems -as if they were an outcome of the primary nature of the -germ-cells.</p> - -<p>The primary result of the mutual chemical influence of the two -germ-cells upon one another is—apart from the impulse to development -which the centrosphere of the sperm-cell supplies—as far as -I see, only the more favourable or the more unfavourable mingling of -the biophor- or determinant-variants, and the resulting increase or -decrease in adaptive capacity, which leads to the better thriving of -the offspring, or conversely to its degeneration. Everything else is -secondary and depends upon adaptation, effected in very diverse ways, -to secure the most favourable mingling of the germ-plasms for the -particular species concerned. Undoubtedly the parental ids united -through amphimixis have an effect upon each other, since throughout -the building up of the organism of the child the homologous determinants -struggle with one another for food, but they do not affect -each other in the way that many prominent physiological and medical -writers suppose, namely, that the union of the parental germ-plasms -sets up a 'formative stimulus' which 'advances' or even 'greatly -advances' the process of development in the egg.</p> - -<p>Parthenogenetic development goes on just as rapidly, sometimes -even more rapidly than that of the fertilized ova of the same species!<span class="pagenum"><a id="Page_230"></a>[Pg 230]</span> -How can the supposed 'formative stimulus' be so entirely dispensed -with in this case?</p> - -<p>Of course I am well aware that the two kinds of germ-cells -have a strong attraction for each other, and that the protoplasm -of the ovum actually exhibits tremulous movement when the -spermatozoon penetrates through the micropyle. I myself observed -this in the case of the lamprey (<i>Petromyzon</i>) when Calberla instituted -his investigations on the fertilization of that animal, but has -that anything to do with a formative stimulus? Is it anything -more than the result of the chemotactic stimulus exerted by the -substance of the ovum upon that of the spermatozoon and conversely? -And have we any ground for seeing anything more in this than an -adaptation of the sex-cells to the necessity of mutually finding each -other out and thereafter combining? Two quite different things are -often confused with one another in this connexion: the mutual -attraction of the two kinds of sex-cells which tends to secure their -union, and the results of this union. A more exact distinction -is necessary between the effects and the advantages which allogamy -brings in its train and the means by which it is secured in the -different species.</p> - -<p>If amphimixis really set up a 'formative' stimulus, and if the -amount of this was regulated by the differences between the two -parental germ-plasms, then parthenogenesis, which implies the entire -absence of the mingling of two parental cells, would necessarily be even -less advantageous than amphimixis between near relatives; but this -is not the case. Continued inbreeding leads in many cases to the -degeneration of the descendants, and particularly to lessened fertility -and even to complete sterility. Thus in my prolonged breeding -experiments with white mice, which were later carried on by G. von -Guaita, strict inbreeding, effected throughout twenty-nine generations, -resulted in a gradually diminishing fertility, and similar observations -have been made by Ritzema Bos and others. But why does not the -same thing happen in pure parthenogenesis? My experiments in -breeding parthenogenetic Ostracods (<i>Cypris reptans</i>) shows that these -crustaceans, in the course of the eighty generations which I have -observed till now<a id="FNanchor_24" href="#Footnote_24" class="fnanchor">[24]</a>, have lost nothing of their prolific fertility and -vital power; and the same is true in free nature of the rose-gall wasp -(<i>Rhodites rosæ</i>), which enjoys the greatest fertility notwithstanding -its purely parthenogenetic reproduction, the females not infrequently<span class="pagenum"><a id="Page_231"></a>[Pg 231]</span> -laying a hundred eggs in a single bud. How does it happen that -'the mutual influence of two different hereditary substances which -so powerfully promotes individual development' can be here altogether -dispensed with? Only because it does not really exist, except in the -imagination of my opponents, still influenced by the old dynamic -theory of fertilization.</p> - -<div class="footnote"> - -<p><a id="Footnote_24" href="#FNanchor_24" class="label">[24]</a> The cultures were begun in 1884 and are still continued (March 6, 1902), still -multiplying as abundantly as at the outset. I reckon that there are on an average five -generations in a year, which means about eighty generations in sixteen years.</p> - -</div> - -<p>But it may be asked, whence come the injurious results of -inbreeding, if not from the union of two nearly related germ-plasms? -They certainly do arise from that cause, but it is not through -a 'formative stimulus,' <i>too slight</i> in this case, exercising a direct -formative chemical effect upon the two hereditary substances, but -through the indirect influences exerted by these <i>too similar</i> hereditary -contributions during the development of the new individual. Lest -it be imagined that I am tilting against windmills, I will refer to one -of the numerous examples of the evil effects of inbreeding which -have been submitted to me as specially corroborative of the conception -of amphimixis as a 'formative stimulus' whose strength depends upon -the difference between the germ-substances. The renowned breeder, -Nathusius, allowed the progeny of a sow of the large Yorkshire breed, -imported from England when with young, to reproduce by inbreeding -for three generations. The result was unfavourable, for the young -were weakly in constitution and were not prolific. One of the last -female animals, for instance, when paired with its own uncle—known -to be fertile with sows of a different breed—produced a -litter of six, and a second litter of five weakly piglings. But when -Nathusius paired the same sow with a boar of a small black breed, -which boar had begotten seven to nine young when paired with sows -of his own breed, the sow of the large Yorkshire breed produced in the -first litter twenty-one and in the second eighteen piglings.</p> - -<p>How could this really remarkable difference in the fertility of the -sow in question be the result of a formative stimulus, exercised by the -sperm-cells of the unrelated boar upon the ova of the female animal? -If the progeny of the sow had been more fertile than herself, then -we should have been at least logically justified in concluding that -this was the case, but it is not intelligible that the egg-cells of this -mother sow should be increased twice or three times because they -were fertilized by a new kind of sperm as they glided from the ovary. -The number of ova which are liberated from the ovary depends in the -first instance upon the number of mature ova contained in it; and -unless we are to make the highly improbable assumption that the -crossing with the strange boar had as an immediate result the -maturing of a large number of ova, we must look elsewhere than<span class="pagenum"><a id="Page_232"></a>[Pg 232]</span> -in the ovary of the animal for the cause of this sudden fertility, -possibly in chance circumstances which we are unaware of and which -make the ovary occasionally more productive, possibly however in the -fact that inbreeding may have brought about various slight structural -variations in the animal, and among these some which made the -fertilization of the abundantly produced ova by the sperm of the -related boar less easy, and caused it to fail more frequently. As will -be readily understood, I cannot say anything definite on this point, -but we know that very slight variations in the sperm-cell or the ovum -may make fertilization difficult, or may even prevent it. I need only -remind you of the interesting experiments in hybridization which -Pflüger and Born made with Batrachians nearly thirty years ago, -which showed that in two nearly related species of frog the ova of the -species A were frequently fertilized by the sperms of the species B, -but not conversely, the ova of the species B by the sperms of A. This -is the case, for instance, with the green edible frog (<i>Rana esculenta</i>), -and the brown grass-frog (<i>Rana fusca</i>), and the reason of this dissimilarity -in the effectiveness of the sperm lies simply in 'rough -mechanical conditions,' in the width of the micropyle of the ovum, -and the thickness of the head of the spermatozoon. If each species -possesses a micropyle which is exactly wide enough to admit of the -passage of the spermatozoon of its own species, another species will -only be able to fertilize these eggs if the head of its spermatozoon -be not larger than that of the first species. Thus, as experiment -has proved, the spermatozoa of <i>Rana fusca</i> fertilize the ova of almost -all other related species, for they have the thinnest head and it is at -the same time very pointed. In this case, therefore, it depends upon -the microscopic structure of the ovum whether fertilization can take -place or not, and we can imagine that similar or perhaps other -minute variations had taken place in the ova in the case of Nathusius's -sow, and that these made it difficult for the sperms of boars of -the same family to effect fertilization. These variations may have -arisen as a result of the continued inbreeding, because the same ids -were constantly being brought together in the fertilized ova, and -thus any unfavourable directions of variations which existed were -strengthened.</p> - -<p>It seems to me that in this way alone can the injurious effect -of inbreeding be made intelligible. From both parents identical ids -meet in the fertilized ovum, in greater numbers the longer inbreeding -continues, for at the maturation of every germ-cell the number -of different ids is diminished by a few, and their number must therefore -gradually decrease, and it is conceivable that ultimately it may<span class="pagenum"><a id="Page_233"></a>[Pg 233]</span> -sink to one kind of id, that is, that the germ-plasm may then consist -entirely of identical ids. If chance variations of certain determinants -in unfavourable directions occur in some of the ids composing the -germ-plasm, these are brought together in the offspring from both -the maternal and the paternal side, and will occur in an increasing -number of ids the longer the inbreeding has gone on, that is, the -smaller the number of different ids has become. The unfavourable -variation-tendency is therefore persisted in, and its influence upon -the development of a new descendant will be the greater the larger -the number of identical ids with these unfavourable variations. It is -obvious that the crossing of an animal, which is thus, so to speak, -degenerating slightly, with a member of an unrelated family must -immediately have a good effect upon the descendants, for in this -way quite different ids with other variations of their determinants -are introduced into the inbred germ-plasm which had become too -monotonous.</p> - -<p>From this theoretical interpretation of the injurious consequences -of inbreeding we may at once infer that not every inbreeding -necessarily implies degeneration, for the occurrence of unfavourable -variational tendencies in the germ-plasm is presupposed as the -starting-point of degeneration, and if these do not exist there can be -no degeneration. This harmonizes with the fact that the evil effects -of inbreeding are observed <i>to vary greatly in amount, and may not -occur at all</i>. But they are greatest in breeds artificially selected -by man, which have long been under unnatural, directly influential -conditions, and are also removed from the purifying influence of -natural selection. In such cases, therefore, there is every probability -that diverse unfavourable variational tendencies in the determinants -will occur.</p> - -<p>But how are we to understand the fact that pure parthenogenesis -may last through innumerable generations, and yet no degeneration -sets in? I believe very simply. In this case too, the same ids which -were peculiar to the mother of the race are contained in the descendants, -but they do not diminish in number, for in pure and normal -parthenogenesis, such as that of <i>Cypris reptans</i>, the second maturation-division -of the ovum does not take place, and this is precisely the -nuclear division which effects reduction. In addition, the introduction -of identical ids, which must take place in the case of inbreeding -at every amphimixis, does not occur, and, what is certainly of great -importance, all these cases are old species, living under natural -conditions—the same conditions under which they lived as amphigonous -species, and not newly formed breeds under artificial<span class="pagenum"><a id="Page_234"></a>[Pg 234]</span> -conditions, as has probably always been the case in the experiments -in inbreeding.</p> - -<p>It is true that even in old species, living in a state of nature, -unfavourable variations may arise in the germ-plasm, and may go on -increasing during purely parthenogenetic multiplication, for the ids -with unfavourably varying determinants will no longer be set aside -by means of reducing division. But those individuals in which the -unfavourable variational tendency increases until it has attained -selection-value will be subject to selection and will be gradually -eliminated; indeed, the weeding out of the inferior individuals will -be more drastic here than where amphigony obtains, because in this -case all the offspring of one mother are nearly alike, so that the whole -progeny is exterminated if the mother varies unfavourably.</p> - -<p>On the other hand, a transformation in a favourable direction, -an adaptation to new conditions of life, as far at least as that implies -the simultaneous variation and harmonious co-adaptation of many -parts, cannot, as far as I can see, be effected in the course of purely -parthenogenetic reproduction, nor can a degeneration of complicated -parts which have become superfluous. For both these changes, in my -opinion, require that the ids of the germ-plasm should be frequently -mingled afresh, since apart from this there cannot be a harmonious -readjustment of complicated structures, nor can a uniform degeneration -affecting all parts set in. As an example of this last case -we may take that organ which became functionless in the purely -parthenogenetic species of Ostracods when amphigonous reproduction -was given up—the sperm-pocket or receptaculum of the female. -All these species still possess an unaltered receptaculum seminis, -a large pear-shaped bladder with a long, narrow, spirally coiled -entrance-duct, very well adapted for allowing the enormous spermatozoa -of the males to make their way in singly, and to arrange themselves -within the receptacle side by side in the most beautiful order, like -a long ribbon, and finally to migrate out again singly to fertilize the -liberated ova. In <i>Cypris reptans</i> and several other species, however, -no males have been found in any of the places which have been -carefully searched, and the receptaculum of the female is always -found to be empty. Nevertheless it shows no hint of degeneration. -It is possible enough that, as in <i>Apus cancriformis</i>, which is of similar -habit, the males have become extinct in most colonies of these species, -but that nevertheless they do occur here and there from time to time -in the area inhabited by the species, and if this should prove to be -the case, it would confirm the conclusion, which is very probable -on other grounds, that the pure parthenogenesis of these species<span class="pagenum"><a id="Page_235"></a>[Pg 235]</span> -has not existed in most of their habitats for a long time, speaking -phylogenetically. For this reason we must not over-estimate the -significance of the complete persistence of the receptaculum even with -exclusively parthenogenetic reproduction. It proves, however, that -degeneration of a superfluous organ does not necessarily set in even -after hundreds of generations, and in this fact there is certainly -a corroboration of the view that it is 'chance' germinal variations -which give the impulse to degeneration. These first induce a downgrade -variation through germinal selection, and this, if it concerns -an organ of no importance to the survival of the species, is not -hindered in its progress by personal selection. Whether degeneration of -the receptaculum would have occurred in these parthenogenetic species -if they had retained even a periodic sexual reproduction, as is actually -the case in the generations of the alternately parthenogenetic and -sexually reproducing Aphides, we cannot decide, since we know -nothing in either case as to the length of time that parthenogenesis -has prevailed among them, nor have we any method of computing the -number of generations that must elapse before a superfluous organ -begins to vacillate. We only know that the parthenogenetic generations -of Aphides no longer possess a receptaculum, while other forms -with alternating bi-sexual and parthenogenetic modes of reproduction, -which are in this respect possibly more modern, e. g. some of the gall-wasps, -possess one similar to that of the Ostracods.</p> - -<div class="figcenter" id="ff40"> -<img src="images/ff40.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 79</span> (repeated). The two maturation divisions of the 'drone eggs' -(unfertilized eggs) of the bee, after Petrunkewitsch. <i>Rsp</i> 1, the first directive -spindle. <i>K</i> 1 and <i>K</i> 2, the two daughter-nuclei of the same. <i>Rsp</i> 2, the second -directive spindle. <i>K</i> 3 and <i>K</i> 4, the two daughter-nuclei. In the next stage -<i>K</i> 2 and <i>K</i> 3 unite to form the primitive sex-nucleus. Highly magnified.</p> -</div> - -<p>I must refer to one other case of parthenogenesis, since it has -been hitherto regarded as a formidable puzzle for the germ-plasm -theory, and has only recently found its solution, I mean the facultative -parthenogenesis of the queen-bee. As the 'male' eggs of the bee -remain unfertilized, and yet undergo two reducing divisions, which -must diminish the number of ids in the ovum-nucleus by a half, -the number of ids in the germ-plasm of the bee must be steadily -decreasing, and this state of things has therefore been regarded by -some English biologists as convincing evidence of the untenability -of the conception of ids and of the whole germ-plasm theory. -Apparently, indeed, it is contradictory to the theory, and we must -inquire whether the contradiction is merely an <i>apparent</i> one, disappearing -when the facts are more precisely known. It was -mainly on this ground that I instituted the researches carried out -by Dr. Petrunkewitsch, the results of which I have already in part -communicated in a former lecture. These results confirmed the -previous conclusions that the 'male' eggs of the queen-bee remain -unfertilized, that two reducing divisions occur, and that in consequence -the ovum-nucleus only contains half the normal number of chromosomes. -<span class="pagenum"><a id="Page_236"></a>[Pg 236]</span>That these increase again by division to the normal number -does not save the theory, for only <i>identical</i> ids can arise in this way, -while the significance of the multiplicity of the ids lies mainly in -their difference. The halving of the number of ids in each 'male' -ovum would necessarily lead, if not to a permanent diminution in -the number of ids, at least to a monotony of the germ-plasm, since -the number of <i>different</i> ids would be steadily decreasing and the -number of <i>identical</i> ids as steadily increasing. This too would be -a contradiction of the theory. But Dr. Petrunkewitsch's investigations -have shown that, of the four nuclei which are formed by the -two reducing divisions, the two middle ones (Fig. 79, <i>K</i> 2 and <i>K</i> 3) -recombine with one another, and fuse into a single nucleus, and that -from this copulation-nucleus in the course of development the primitive -germ-cells of the embryo arise. Now all the ids which were originally -present in the nucleus of the immature ovum may be reunited in -this 'polar copulation-nucleus' if the two nuclei <i>K</i> 2 and <i>K</i> 3 turned -towards each other in Fig. 79 contain different ids. That this is -the case cannot of course be seen from the ids themselves, but it -seems to me extremely probable, since it is dissimilar poles of the -two nuclear spindles which here unite, namely, the lower pole -(daughter-nucleus) of the upper spindle and the upper pole of the -lower spindle. In the first directive or polar spindle there lay -thirty-two chromosomes, which had increased by duplication from -sixteen, and of these sixteen passed over into the first polar nucleus, -while sixteen formed the basis of the second directive spindle. These<span class="pagenum"><a id="Page_237"></a>[Pg 237]</span> -two sets of sixteen chromosomes must have been quite similar, since -the two sets arose by division of the sixteen mother-chromosomes. -Let us call the chromosomes <i>a</i>, <i>b</i>, <i>c</i>, <i>d-q</i>, then similar sets of chromosomes -must have been contained in the two nuclear spindle figures -depicted in Fig. 79 at the beginning of the division, and eight of these -went to each daughter-nucleus. Now, if <i>a-k</i> migrated to the upper -pole of the spindle and <i>l-q</i> to the lower pole, then the union of <i>K</i> 2 -with <i>K</i> 3 would bring together again all the ids that had before been -present. In consideration of this I predicted to Dr. Petrunkewitsch -that this copulation-product might be the basis of the formation -of the germ-cells in the drone-bee, and his painstaking and difficult -researches have confirmed this prediction, strange though it may -seem, that the male germ-cells have a different origin from the -female germ-cells. But this discovery gives a strong support to the -germ-plasm theory. It may, of course, be objected that the assumed -regular distribution of the ids in the two daughter-nuclei cannot -be proved, but we know already that this dividing apparatus does -<i>very exact</i> work, and we are at liberty to assume it in an even higher -degree. Moreover, what other interpretation of the unexpected -development of the germ-cells discovered by Petrunkewitsch could -be given if this had to be rejected? A clearer proof of the individual -differences of ids and of their essential importance could not -be desired, than lies in the fact that in the 'male' eggs of the queen-bee -a different and novel mode of germ-cell formation is instituted, -after half the ids have been irrecoverably withdrawn from the -ovum-nucleus. We see from this that for individual development -a duplication of individual ids may suffice, but that for the further -development of the species a retention of the diversity of the ids -is important.</p> -<hr class="full" /> - -<div class="chapter"> -<p><span class="pagenum"><a id="Page_238"></a>[Pg 238]</span></p> - -<h2 class="nobreak" id="LECTURE_XXX">LECTURE XXX</h2> -</div> - -<p class="c">INBREEDING, PARTHENOGENESIS, ASEXUAL REPRODUCTION,<br /> -AND THEIR CONSEQUENCES</p> - -<div class="blockquot"> - -<p>The separation of the sexes exists even among the Protozoa—Conditions determining -the occurrence of Hermaphroditism—Tape-worms, Cirrhipeds—Primordial males—Advantages -of parthenogenesis—Alternation with bi-sexual generations—In Gall-wasps—In -Aphides—Cross-fertilization secured in plants—Self-fertilization is avoided -whenever possible—The mechanism of fertilization and the mingling of germ-plasms -must be clearly distinguished from one another—Cases of persistent self-fertilization—The -effects of inbreeding compared with those of parthenogenesis—The effect of purely -asexual reproduction—In sea-wracks—In lichens and fungi—In cultivated plants—Degeneration -of the sex-organs—Summary.</p></div> - - -<p><span class="smcap">We</span> have seen that continued inbreeding must make the germ-plasm -monotonous, and therefore unplastic as regards the requirements -of adaptation. Accordingly, we found that the gametes of many -unicellulars are so constituted that they only possess a power of -attraction for gametes of a different lineage, not for those of their -own stock. Among multicellular organisms the most intense mode -of inbreeding is to be found in the uninterrupted self-fertilization of -hermaphrodites: in such cases the monotony of the germ-plasm -must reach extreme expression more readily than in the case of -ordinary inbreeding. We can thus understand why, in the scale -of organisms, there is such an early occurrence of gonochorism, the -separation of the species into male and female individuals. Even -among unicellular plants or Protophytes this occurs occasionally, as -it does in the Vorticellids among Infusorians.</p> - -<p>In the Metazoa and Metaphyta the separation of the sexes finds -emphatic expression; it is absent from no important group, and -in many, such as, for instance, among the Vertebrates, it has become -the absolutely normal condition, with hardly any exception. But -in many divisions of the animal and plant kingdoms hermaphroditism -also plays an important part, as, for instance, in terrestrial snails and -in flowering plants.</p> - -<p>Obviously the sexual adaptations of a species are definitely -related to the conditions of its life, and, though Nature's endeavour -to prevent inbreeding and to secure cross-fertilization is evidenced -by the occurrence of separate sexes in such a multitude of forms<span class="pagenum"><a id="Page_239"></a>[Pg 239]</span> -yet in many cases gonochorism has been relinquished, and always -where this was necessitated by the conditions of life to which the -group concerned was subject. In such a case inbreeding is regulated -as far as possible, for instance, by an arrangement which ensures -that individuals shall be crossed at least from time to time. But -cases of exclusive and constant self-fertilization do also seem to -occur, and even these may be brought into harmony with our -conception, according to which cross-fertilization is an advantage, -but only an advantage which must be weighed against others, and -which may eventually be given up in favour of greater advantages. -This occurrence of persistent autogamy can no more be reconciled -with the rejuvenation theory than can continuous parthenogenesis, -because, according to this theory, the mingling of different individuals -is a <i>sine qua non</i>, for the continued life of the species.</p> - -<p>It is impossible for me here to discuss in detail all the deviations -from pure gonochorism or bi-sexuality which occur in nature, but -I must at least attempt to take a general survey, and to arrange -the chief phenomena of these various modes of 'sexual reproduction' -in an orderly scheme. I must take a survey of both plants and -animals, but I shall give the precedence to animals, as being to me -more familiar ground.</p> - -<p>Where do we find, in the animal kingdom, that Nature has -departed from gonochorism, from the separation of the sexes, and -for what reasons was this departure necessary? And further, what -means does Nature take to compensate for this renunciation of the -simplest method of securing the continual cross-fertilization of -individuals?</p> - -<p>Let us glance over the animal kingdom with special reference -to these questions: we find that hermaphroditism prevails chiefly -among species which at maturity have lost their power of free -locomotion, and have become sedentary, such as oysters, barnacles -among Crustaceans, the Bryozoa, and the sea-squirts (Ascidians) -which are fixed to the rocks at the bottom of the sea. For forms -such as these it must often have been advantageous that each -individual could function both as male and as female, especially -when it was capable of self-fertilization, since individuals which -settled down singly, or in very small numbers together, would not -be lost as regards the persistence of the species. The continuance -of the species is thus better secured than it would be by separation -of the sexes, because in the latter case it might frequently have -happened that the animals which had settled beside each other by -chance were of the same sex, and would therefore remain unfertile.<span class="pagenum"><a id="Page_240"></a>[Pg 240]</span> -But many of these species do not fertilize themselves, but fertilize -each other mutually; and this, too, carries a great advantage with -it, because in sedentary animals the sperms will fertilize twice as -many individuals, if each contains eggs, than if half were exclusively -male. It is thus to some extent an economy of sperms, but at the -same time also of ova, which is effected by hermaphroditism: the -result is that these valuable products are wasted as little as possible. -On this account we find that not only sedentary, but also sluggish, -slow-moving animals are equipped with male and female organs of -reproduction, as, for instance, all our terrestrial snails. They fertilize -each other mutually: when two meet it is always as males and -females, and notwithstanding the sluggishness of movement, it -is not likely to happen that a snail does not attain to reproduction -because it has not found a mate. The same is true of the earthworms, -which are likewise not adapted for making long journeys -in search of the opposite sex; they, and the leeches also, function -as male and female simultaneously, while their nearest relatives, -the marine Chætopods, are of separate sexes, which may be associated -with their much greater power of free movement in the water.</p> - -<p>In these cases self-fertilization is often absolutely excluded; it -may be physically impossible, and hermaphroditism therefore secures -cross-fertilization in such cases just as effectively as if the sexes were -separate. Similarly, in many hermaphrodite flowers, as we have -already seen, the pollen is so constituted and so placed within the -flower that it cannot of itself make its way to the stigma. In oysters, -for instance, the young animal is male, and liberates into the water -an enormous quantity of minute spermatozoa, and therewith fertilizes -the older individuals, functioning only as females, which have grown -upon the same bank. At a later stage of its development the oyster -which was male becomes female, and produces only ova. This state -of affairs, of which I shall shortly mention another case, has been -called <i>temporary</i> hermaphroditism. In this case not only is self-fertilization -excluded, but close inbreeding also, since it is always -a young generation functioning simultaneously as males that mingles -with an older generation which has become female.</p> - -<p>It is quite otherwise with parasites which live singly within -the body of a host: for these it was indispensably necessary that -they should not only produce both kinds of germ-cells, but that they -should unite the two kinds in fertilization, and they therefore possess -the power of self-fertilization. Thus, in the urinary bladder of the -frog, there occurs a flat-worm (<i>Polystomum integerrimum</i>) which -possesses special organs for pairing with another individual, but<span class="pagenum"><a id="Page_241"></a>[Pg 241]</span> -which is also capable of self-fertilization when, as frequently occurs, -it has no companion in its place of abode. But this self-fertilization -is always liable to be interrupted by cross-fertilization, for not infrequently -there are two, three, or even four such parasites within -the bladder of a single frog.</p> - -<p>In the tape-worms, too, cross-fertilization is not excluded, for -there are often two or more of these animals together in the intestine -of a host at the same time. But even where there is only one, self-fertilization -on the part of the joints, that is, the sexual individuals, -is prevented, and by the same device, metaphorically speaking, as in -the case of the oyster, for in each joint the male elements mature -first and the female elements afterwards. In certain parasitic Isopods -of the genus <i>Anilocra</i> and related forms close inbreeding is -prevented in the same way—by a difference in the period at which -the two sets of gonads in the hermaphrodite individual become -mature (dichogamy).</p> - -<p>This is secured in a different way in Crustaceans which have -grown to maturity in a sedentary state, like the Cirrhipeds. These -animals, known as 'acorn-shells' and 'barnacles,' are sedentary, -sometimes on rocks and stones, sometimes on a movable object, the -keel of a ship, floating pieces of wood, cork, or cane, or sometimes -attached to turtles or whales, and although they generally occur -in great numbers together, they are probably only able to fertilize -each other occasionally, and are therefore essentially dependent upon -self-fertilization. But Charles Darwin discovered long ago that -many of them, notwithstanding their hermaphroditism, have males -which are small, dwarf-like, and very mobile organisms, destined -only for a very brief life. These seemed quite superfluous in -association with hermaphrodite animals, and they have therefore -long been regarded as vestigial males, as the last remnant, so to -speak, of a past stage of the modern Cirrhipeds, in which the sexes -were separate. It is obvious, however, that we must now attribute -to them a deeper significance, for these so-called 'primordial males,' -although extremely transitory creatures without mouth or intestine, -represent a means of securing the cross-fertilization of the species. -What importance nature attaches to their preservation is shown -especially by the parasitic Cirrhipeds which have been so carefully -studied by Fritz Müller and Yves Delage—those sac-like Rhizocephalidæ -or root-crustaceans which are altogether disfigured by -parasitism. The fully developed animals are hermaphrodite and live -partly in, partly upon crabs and hermit-crabs (Fig. 112, <i>C</i>, <i>Sacc</i>). -These hermaphrodites indeed fertilize themselves, but in their youth<span class="pagenum"><a id="Page_242"></a>[Pg 242]</span> -they are of distinct sexes, and the females are so constituted that -they lay eggs for the first time just when the males of the current -year are appearing. Thus the first batch of eggs liberated by the -females are fertilized by the minute free-swimming 'primordial -males,' but after that the females themselves develop testes, and then -fertilize themselves; the males die very soon after copulation, and only -appear the following year in a new generation. They are therefore -far from being mere historic reminiscences, vestiges of the early history -of the modern species, for they are the instruments of a regular -cross-fertilization of the species, and therefore of a constant mingling -of new ids in the germ-plasm. This is not the place to discuss the -marvellous life-history of these parasites in detail; I can only say -that when we inquire into the whole story, and appreciate the -difficulties associated with the persistence of these 'primordial males,' -we can no longer doubt that crossing is an indispensable feature of -amphimixis—a feature which must at least occasionally occur if -amphimixis is to retain its significance. This is shown, it seems to -me, especially by these numerous instances of what we may call -compulsory retention of ephemeral males in hermaphrodite, self-fertilizing -animals; it follows also from the theory, for with continued -self-fertilization all the ids in the germ-plasm of an individual would -tend to become identical, and the mingling of two germ-plasms which -contained <i>identical</i> ids would, at least according to the germ-plasm -theory, have no meaning at all.</p> - -<div class="figcenter" id="ff41"> -<img src="images/ff41.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 112</span> (repeated). Development of the parasitic Crustacean <i>Sacculina -carcini</i>, after Delage. <i>A</i>, Nauplius stage. <i>Au</i>, eye. <i>I</i>, <i>II</i>, <i>III</i>, the three pairs -of appendages. <i>B</i>, Cypris stage. <i>VI-XI</i>, the swimming appendages. <i>C</i>, mature -animal (<i>Sacc</i>), attached to its host, the shore-crab (<i>Carcinus mænas</i>), with a -feltwork of fine root-processes enveloping the crab's viscera. <i>S</i>, stalk. <i>Sacc</i>, body -of the parasite. <i>oe</i>, aperture of the brood-cavity. <i>Abd</i>, abdomen of the crab -with the anus (<i>a</i>).</p> -</div> - -<p><span class="pagenum"><a id="Page_243"></a>[Pg 243]</span></p> - -<p>Thus we see that in the animal kingdom hermaphroditism is -always associated with cross-fertilization in some way or other, even -though the latter may occur rarely, being usually periodically -interpolated, and thus bringing new ids into the germ-plasm which is -rapidly becoming monotonous or uniform. Adaptations quite analogous -to these are found in relation to parthenogenesis, and it will repay us -to give a brief summary of these.</p> - -<p>Parthenogenesis effects a very considerable increase in the -fertility of a species, and in this increase the reason for its introduction -among natural phenomena obviously lies. By the occurrence of -parthenogenesis, the number of ova produced by a particular colony -of animals may be doubled, because each individual is a female, and as -the multiplication increases in geometrical ratio a few parthenogenetic -generations result in a number of descendants enormously in excess of -those produced by bi-sexual reproduction. We can therefore understand -why parthenogenesis should obtain among animals whose -conditions of life are favourable only for a short time, and are then -uncertain and dangerous for a long period. This is the case with the -water-fleas, the Daphnids (see Figs. <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#f61">57</a> and <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#f62">58</a>), whose habitats—pools, -ponds, and marshes—often dry up altogether in summer, or -freeze in winter, so that it becomes almost if not quite impossible for -the colonies to go on living, and the preservation of the species can -only be secured by the production of hard-shelled 'lasting' eggs, -which sink to the bottom, dry up in the mud, or become frozen, or at -least remain latent in a sort of slumber. As soon as the favourable -conditions reappear, young animals which emerge from the eggs -are all females and reproduce parthenogenetically, so that after a few -days there is a numerous progeny swimming freely about, which in -their turn are all females, and reproduce after the same manner. In -many Daphnids this goes on for a series of generations, and there -thus arises an enormous number of animals, which may fill a marsh -so densely that, by drawing a fine net a few times through the water, -one can draw out a veritable animal soup. In our ponds and lakes -these little Crustaceans form the fundamental food of numerous fishes. -But notwithstanding the enormous havoc wrought among them by -enemies, large numbers remain at the end of a favourable season, and -these produce the lasting eggs, <i>after fertilization</i>. For shortly before -the end of the season males appear among the progeny of the -hitherto purely parthenogenetic females. Although each female will -only produce a few of these 'lasting' eggs, which require fertilization -and are richly supplied with yolk, the whole number in each colony -is a very large one, because the number of individuals is very large;<span class="pagenum"><a id="Page_244"></a>[Pg 244]</span> -and it must be so, since the eggs, though secure against cold and -desiccation, are very imperfectly protected against the numerous -enemies which may do them injury.</p> - -<p>Of course the number of individuals which form a colony may -vary greatly in the different species, and the same is true of the -number of parthenogenetic generations which precede the bi-sexual -generation. I have already shown in detail that this depends -precisely on the average duration of the favourable conditions, so that, -for instance, a species which lives in large lake-basins will produce -many purely parthenogenetic generations before the bi-sexual one, -which only appears towards autumn, while species which live in -quickly-drying pools have only a few parthenogenetic generations, -and the true puddle-dwellers give rise to males and sexual females -along with the parthenogenetic females as early as the second -generation.</p> - -<p>We thus find in the Daphnids an alternation, regulated and made -normal by natural selection, of purely parthenogenetic with bi-sexual -generations, and the result is that the uniformity of the germ-plasm, -which is the necessary consequence of pure parthenogenesis, is interrupted -after a longer or shorter series of generations by the occurrence -of amphimixis. That the number of parthenogenetic generations may -be so varied, though with a definite norm for each species, indicates -again that amphimixis is not an absolute condition of the maintenance -of life, not an indispensable rejuvenation, designed to counteract -the exhaustion of vital force—whether this be meant in a transcendental -sense or otherwise—but that it is an important advantage -calculated to keep the species at its highest level, and that its influence -appears whether it occurs in the species regularly, or frequently, or -only rarely.</p> - -<p>This kind of alternation of generations, that is, the alternation -between unisexual (female) and bi-sexual generations, has been called -heterogony. In the Daphnids, certainly, a difference in form between -the parthenogenetic and the bi-sexual generation does not exist, for -the same females which produce eggs requiring fertilization can also -produce parthenogenetic ova, although these are very different from -each other, as we have already seen. The difference between generations, -therefore, does not lie in their structure, but in their tendency -to parthenogenetic or to amphigonous reproduction, and in the absence -or presence of male individuals. There are, however, other cases of -alternation of generations in which the different generations diverge -from each other in structure. One of the most remarkable of these is -that of the gall-wasps (Cynipidæ). In many of these little Hymeno<span class="pagenum"><a id="Page_245"></a>[Pg 245]</span>ptera, -which form galls on leaves, blossoms, buds, and roots, especially -of the oak, two generations occur annually, one in summer, the other -in early spring, or even in the middle of winter. The latter consists -of females only and reproduces parthenogenetically. We can readily -understand this from the point of view of adaptation to particular -conditions, since the young wasps which emerge from their galls in -winter, or in the middle of a raw spring, are exposed to many -dangers and are terribly decimated before they can succeed in laying -their eggs in the proper place on the plant. Moreover, much precious -time would be lost by the mutual search of the sexes for each other,—a -search which would often be entirely without result. Thus the -wingless female of <i>Biorhiza renum</i> (Fig. 124, <i>A</i>), which is not unlike -a plump ant, attempts, without taking food, and often interrupted -by a spell of cold or a snowstorm, to reach a neighbouring oak-shrub, -creeps up on it, and lays its eggs in the heart of a winter bud, whose -hard protecting scales it laboriously perforates by means of its short, -thick, sharp ovipositor.</p> - -<div class="figcenter" id="ff42"> -<img src="images/ff42.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 124.</span> Alternation of generations in a Gall-wasp. <i>A</i>, winter generation -(<i>Biorhiza renum</i>). <i>B</i> and <i>C</i>, summer generations (<i>Trigonaspis crustalis</i>). -<i>B</i>, male. <i>C</i>, female. After Adler.</p> -</div> - -<p>After it has succeeded in sinking its ovipositor into the heart of -the bud, it goes on working for hours, piercing the delicate tissue -with a multitude of fine canals, one close beside the other, and then -deposits an egg in each of these. The whole detailed piece of work -requires, according to Adler, uninterrupted active exertion for about -three days, even though in the end only two buds may be filled with -eggs. If at every egg-laying the arrival of a male had to be waited -for, an even larger number of females would fall victims to the unfavourable -weather and other dangers, while at the same time the -number of emerging females could be only half as large as it is. -It is obvious that in this case parthenogenesis is of very great -advantage.</p> - -<p><span class="pagenum"><a id="Page_246"></a>[Pg 246]</span></p> - -<p>In summer the climatic conditions are incomparably more -favourable for the gall-wasps, and accordingly we find that the -summer generation is bi-sexual, but, strangely enough, is so different -from the winter generation that the relationship of the two forms -was for a long time overlooked. The antennæ, the legs, and particularly -the ovipositor, the whole shape of the animal, its size, the length -of the abdomen, the structure of the thorax, and many other points -are so different that as long as the structural features afforded the -only criterion of relationship, the systematists quite naturally placed -the winter and summer forms in different genera. It was only when -Dr. H. Adler succeeded in breeding the one form from the other that -people were convinced that such marked -differences in structure could be found -within the same life-cycle.</p> - -<div class="figleft" id="ff43"> -<img src="images/ff43.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 125.</span> The two kinds of<br /> -Galls formed by the species. <i>A</i>,<br /> -the many-chambered galls produced<br /> -by the parthenogenetic<br /> -winter form, <i>Biorhiza renum</i>. <i>B</i>,<br /> -those produced on oak-leaves by<br /> -<i>Trigonaspis crustalis</i>, the bi-sexual<br /> -form. After Adler.</p> -</div> - -<p>But we see here quite clearly why the -two generations had to become so different; -simply because the winter generation had -to adapt itself to different conditions from -the summer generation, above all as to -the laying of its eggs within the tissues -of a plant of a different constitution. In -our example, the winter form <i>Biorhiza -renum</i> pierces the terminal buds of the -oak, and lays in each of them a large -number of eggs, sometimes as many as -300, so that a very large gall is formed, -in which a great many larvæ can find -food, and grow on to the pupa-stage. -From this spongy gall, something like -an inverted onion in shape, and about -the size of a walnut (Fig. 125, <i>A</i>), there -emerge in July the slender, delicately formed male and female -gall-wasps which were long known as <i>Trigonaspis crustalis</i>. Both -males and females are winged, and fly rapidly about in the air -(Fig. 124, <i>B</i> and <i>C</i>). The sexes pair, and the females lay their eggs -in the cell-layers on the under side of an oak-leaf, on which arise -small, wart-like, kidney-shaped galls (Fig. 125, <i>B</i>) which fall to the -ground in autumn, and from which there emerge, in the middle of -winter, the plump, wingless females, to which, as we have already seen, -the name <i>Biorhiza renum</i> was given.</p> - -<p>One generation, therefore, lays its eggs in the parenchyma of -tender leaves, and has only to pierce through a thin layer of plant-<span class="pagenum"><a id="Page_247"></a>[Pg 247]</span>tissue, -while the other must penetrate deep down into the hard winter -bud, to be able to deposit its eggs in the proper place, and we therefore -find that in the two kinds of female the ovipositor differs in -length, thickness, and general structure, and so also does the whole -complex apparatus by which the ovipositor is moved. But these -differences are associated with the form of the abdomen, in which the -ovipositor lies, and with the strength and shape of the legs, which -must be shorter and stronger when the boring has to be performed -through a hard plant-tissue or to a considerable depth. We can -readily understand how numerous must be the secondary variations -which a transformation of -the ovipositor brings in its -train when we compare the -ovipositor apparatus in the -two generations of one of -these species (Fig. 126).</p> - -<div class="figright" id="ff44"> -<img src="images/ff44.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 126.</span> Ovipositor and ovum of the two<br /> -generations of the same species of Gall-wasp.<br /> -<i>A</i>, those of the winter form, <i>Neuroterus læviusculus</i>.<br /> -<i>B</i>, those of the summer-form, <i>Spathegaster albipes</i>.<br /> -<i>st</i>, ovipositor. <i>ei</i>, ovum. Similarly magnified.<br /> -After Adler.</p> -</div> - -<p>Figure 126 shows the -ovipositor of another gall-wasp, -of which the winter -form, <i>Neuroterus læviusculus</i>, -also perforates the hard -winter buds of the oak, while -the summer form, <i>Spathegaster -albipes</i>, lays its eggs in the -tender young leaves of the -same tree. The ovipositor of -the former is thin and long, -that of the latter short and -strong (Fig. 126, <i>A</i> and <i>B</i>), -and corresponding also to -the depth at which the egg -must be sunk, or, so to speak, -sown in the tissue of the plant, the egg of the summer generation -differs from that of the winter generation by having a much -shorter stalk (Fig. 126, <i>ei</i>). These little wasps thus afford a beautiful -example of the way in which even marked changes in the conditions -of life of a generation may be associated with transformations -in bodily structure, and we understand how it was possible that -by means of processes of selection the generations which alternate -periodically in the year should come to diverge very considerably -in structure. The example may also serve to illustrate how diverse -are the harmonious co-adaptations which such transformations require,<span class="pagenum"><a id="Page_248"></a>[Pg 248]</span> -and how necessary, therefore, the continual re-combination of the -ids of the germ-plasm by means of amphimixis must be. We -understand why bi-sexual reproduction was only abandoned in -one generation, and that the one in which parthenogenesis was of -considerable advantage. But such transformations must have come -about with extreme slowness, since they were the result of climatic -changes which only come about very gradually. We thus come -again to the same conclusion to which we were led by our study -of vestigial organs in Man, that numerous species which appear -to be at a standstill are continually working towards their own -improvement. But for this amphimixis is essential; consequently -the descendants which have arisen through amphimixis, and whose -ancestors have arisen in the same way, have an advantage over those -of parthenogenetic origin. On the whole, at least, this must be so; -in special cases it may be otherwise, namely, when the advantage -offered by parthenogenesis in respect to the maintenance of the -species preponderates over the advantage which amphimixis implies -as regards possibilities of transformation.</p> - -<p>As far as we have seen from the case of the gall-wasps, the -absence of amphimixis in every second generation implies no disadvantage -in regard to the capability for transformation which the -species exhibits. As to whether any disadvantage would ensue if the -number of parthenogenetic generations in the life-cycle were greater -we can only guess, since no case is known which enables us to decide -this point, <i>pro</i> or <i>con</i>, with any certainty. The heterogony of the -plant-lice, the Aphides, and their relatives might be cited as against -the probability, for in this case a long series of parthenogenetic -generations often alternates with a single bi-sexual one, but the -difference in structure is not so great in this case, although it does -exist, and moreover we can quite well assume that the adaptation to -parthenogenesis was effected at the beginning of heterogony, when it -still consisted of a cycle of only two generations, and that further -virgin generations were interpolated subsequently.</p> - -<p>This assumption is supported by the fact that in some species of -our indigenous Ostracods, in <i>Cypris vidua</i> and <i>Candona candens</i>, in -contrast to the Daphnids, several bi-sexual generations alternate with -one parthenogenetic generation. But in this case again there is no -difference whatever in the structure of the two generations, the -parthenogenetic generation being distinguished from the bi-sexual -generation simply by the absence of males.</p> - -<p>The alternation of generations in the plant-lice is particularly -instructive, because it emphatically indicates how much Nature is<span class="pagenum"><a id="Page_249"></a>[Pg 249]</span> -concerned with the retention of amphimixis, and how little mere -multiplication has to do with this. This is especially striking in the -case of the bark-lice; for instance, in their notorious representative, the -vine-pest, <i>Phylloxera vastatrix</i>.</p> - -<div class="figcenter" id="ff45"> -<img src="images/ff45.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 127.</span> Life-cycle of the Vine-pest (<i>Phylloxera vastatrix</i>), after Leuckart -and Nitsche, and Ritter and Rübsamen. <i>A</i>, the fertilized ovum. <i>B</i>, the -resulting apterous and parthenogenetic Phylloxera. <i>C</i>, its eggs, from which, as -the uppermost arrow indicates, there may arise similar apterous, parthenogenetic -forms, or, as the horizontal arrow indicates, winged forms (<i>D</i>), which -produce 'female' and 'male' ova (<i>E<sup>1</sup></i> and <i>E<sup>2</sup></i>); from these the sexual generation -arises, the female (<i>F<sup>1</sup></i>) and the male (<i>F<sup>2</sup></i>); the former lays the fertilized -ovum (<i>A</i>).</p> -</div> - -<p>As in all plant-lice, the advantage for the sake of which sexual -reproduction was given up depends upon the fact that a practically -unlimited food supply is at the disposal of these parasites of the vine, -which can be made full use of during the proper season, and which, -since every animal is female and produces eggs, results in an -enormous increase in the number of individuals, and thus secures -the continuance of the species. These insects emerge in spring from -small fertilized eggs, which have lain dormant throughout the winter -(Fig. 127, <i>A</i>), and they develop rapidly into wingless females (<i>B</i>), -which, sucking the juice of the vine, multiply by producing large -numbers of little white eggs (<i>C</i>). These develop without fertilization -into similar wingless females. Several generations of females succeed -each other, but then, usually from August onwards, differently formed -winged females (<i>D</i>) make their appearance, and these, flying from -plant to plant, effect the distribution of the species. But these, too,<span class="pagenum"><a id="Page_250"></a>[Pg 250]</span> -lay parthenogenetic eggs (<i>E<sup>1</sup></i> and <i>E<sup>2</sup></i>), and from these there emerge, -late in autumn, the members of the single bi-sexual generation, -males and females (<i>F<sup>1</sup></i> and <i>F<sup>2</sup></i>), both very minute and wingless, -without a piercing proboscis, and thus incapable of taking food. -These pair, and the female lays a single egg (<i>A</i>) under the bark of the -vine, from which the leaves are now falling; this egg survives the -winter, and from it in the following April or May there emerges once -more a parthenogenetic female.</p> - -<p>It could hardly be more plainly shown than it is by this case -that the importance of amphimixis is something quite apart from -reproduction and multiplication, for here the number of individuals is -not only not increased by amphimixis, but is materially diminished, -being indeed lessened by a half. By the retention of amphimixis, -the species gains in this case no advantage <i>except</i> the mingling of two -germ-plasms.</p> - -<p>Something similar occurs in plants which exhibit alternation of -generations, for instance the ferns, in which the sexual generation, the -so-called prothallium or prothallus, contributes nothing to the <i>multiplication</i> -of the plant, since only a single egg-cell is developed; and the -same is true of the mosses. In both cases <i>multiplication</i> depends -solely on the asexual generation, which, as the so-called 'moss-fruit' or -'fern-plant proper,' produces an enormous number of spores, in addition -to multiplying by runners.</p> - -<p>To sum up: we have seen that self-fertilization does occur in -hermaphrodite animals, where otherwise the species would be in -danger of extinction, but this is never the <i>sole</i> and exclusive mode of -fertilization<a id="FNanchor_25" href="#Footnote_25" class="fnanchor">[25]</a>, for hermaphrodite species have always the possibility -of securing inter-crossing of individuals, and that in various ways, -whether by the intervention of 'primordial males' or by an occasional -or a periodic alternation of self-fertilization and mutual fertilization. -Pure parthenogenesis enduring through innumerable generations does -appear to occur, but in most cases unisexual generations alternate with -bi-sexual, so that a stereotyping of the germ-plasm with complete -uniformity of ids is obviated.</p> - -<div class="footnote"> - -<p><a id="Footnote_25" href="#FNanchor_25" class="label">[25]</a> As to the cases Maupas has brought into notice, of permanent and apparently -exclusive self-fertilization in Rhabditidæ (round worms), it seems fair to say that they -have not been as yet sufficiently investigated to admit of a secure appreciation of their -value in their theoretical bearings. Cf. <i>Arch. Zool. Exper.</i>, 3rd ser., vol. viii, 1900.</p> - -</div> - -<p>We must now briefly consider the higher plants with reference to -the maintenance of diversity in the germ-plasm through crossing.</p> - -<p>We saw in an earlier lecture that most flowers are hermaphrodite, -but that they do not fertilize themselves, and are adapted for crossing,<span class="pagenum"><a id="Page_251"></a>[Pg 251]</span> -since the pollen of one flower is carried by insects to the pistil of -another, which cannot be reached by its own pollen, either because it -ripens too early or too late, or because the stigma, notwithstanding -its proximity, is so placed as to be out of reach of the pollen from -the adjacent stamens. I showed, following the fundamental investigations -of Sprengel, Charles Darwin, Hermann Müller, and other -successors of Darwin, that the flowers may in a sense be regarded -as the resultants of the insect-visits, since all their accessory -adaptations—large coloured petals, fragrance, nectar, and even little -minutiæ of colour and markings (honey-guides)—as well as their -detailed shape, as seen in 'landing stages,' corolla tubes, and so on, are -only intelligible when we refer their existence to natural selection. -We assume that each of these adaptations secured some advantage -for the species concerned, and that therefore their first beginnings as -slight germinal varieties were accepted, and were brought gradually -to their full expression by the united operation of germinal and -personal selection. This at least is how we should express ourselves -now that we have become acquainted with the factor of germinal -selection. The advantage secured by every such improvement in -a flower's means of attracting insects is obvious, as soon as it is -established that cross-fertilization is more advantageous for the species -than self-fertilization.</p> - -<p>We have discussed this already; we saw that experiments instituted -by Darwin proved that seedlings which had arisen through -cross-fertilization were superior to those arising through self-fertilization, -and that in many cases the mother-plant itself produced fewer -seeds when self-fertilized than when cross-fertilized. This discovery -afforded an explanation of the cross-fertilization of flowers by -insects which Sprengel had previously observed. We understand how -the flowers must have become so adapted through processes of selection -that they were unable to fertilize themselves, but attracted insects, -and, so to speak, compelled these to dust them with pollen from -another plant of the same species. We also understand how self-fertilization -remained possible for many flowers in the event of cross-fertilization -through insects not being effected, since after a certain -period of waiting, a curvature of the stamens or the pistil may take -place and lead to the stigma being dusted with the pollen of the same -flower. Obviously the development of <i>fewer</i> seeds is preferable to -complete sterility. It is a well-known fact that peculiar inconspicuous -and closed flowers, designed solely for self-fertilization, may occur -along with the open flowers, as in the case of the so-called cleistogamous -flowers of the violet (<i>Viola</i>) and the little dead-nettle (<i>Lamium<span class="pagenum"><a id="Page_252"></a>[Pg 252]</span> -amplexicaule</i>), and the phyletic origin of these becomes intelligible -as soon as it is established that cross-fertilization is more advantageous -than self-fertilization.</p> - -<p>Now, however, it seems as if the fundamental proposition of this -theory of flowers will have to be rejected. Not only do the cleistogamous -flowers just mentioned exhibit a great fertility, not at all less -than that of the open flowers of the same species which are adapted -for cross-fertilization, but there is a small number of plants which -produce seeds by <i>self-fertilization alone</i>. Thus in <i>Myrmecodia</i> cross-fertilization -is absolutely prevented by the fact that the flowers never -open, and according to Charles Darwin <i>Ophrys apifera</i> also reproduces -by self-fertilization alone, and is nevertheless a thoroughly -vigorous plant. There are several other cases of this sort, and particularly -among the orchids, though the whole of the structure of -their flowers is specially adapted for pollination by insects. Many -of them are only rarely visited by insects, some not at all, we know -not why, but it is readily intelligible that in such cases they should -have adapted themselves to self-fertilization wherever that was possible. -For this <i>no great</i> variation was necessary; it was enough that -the pollinia, which formerly only became detached from their attachment -at a touch or a push from an insect, should free themselves -spontaneously. And this, according to Darwin, is what happens, for -instance, in <i>Ophrys scolopax</i>, which at Cannes is frequently self-fertilizing. -For the development of seed, however, it is not enough -that the pollen should reach the stigma; the pollen-grain has to send -out its tube and penetrate into the ovary, and in many orchids this -does not happen; they are infertile with their own pollen. Various -other plants are also non-fertile with their own pollen, for instance the -common corydalis, <i>Corydalis cava</i>, or the meadow cuckoo-flower, -<i>Cardamine pratensis</i> (Hildebrand).</p> - -<p>How are we to reconcile these apparently absolutely contradictory -facts? On the one hand, the innumerable devices for securing -crossing lead us to conclude that it is necessary, or at least advantageous, -and on the other we find a small number of plants which -reproduce continually by self-fertilization and yet remain strong and -vigorous. And again there are many plants which yield seed when -fertilized with their own pollen, and others which remain absolutely -sterile in the same circumstances, yielding no seed or very little, and -there is indeed one on which its own pollen has the effect of a poison, -for if it reaches the stigma the flower dies. If there is anything -injurious in self-fertilization (Darwin), we can understand that it will -be avoided, but how can it be continued so long in many cases, and<span class="pagenum"><a id="Page_253"></a>[Pg 253]</span> -even become in others the exclusive method of fertilization without -visible evil results?</p> - -<p>It seems to me that in these facts, established by observation, the -results of two quite different processes have been confused, and that -we can only gain clearness by studying them apart from one another; -I mean the processes involved in the mechanism of fertilization and -those involved in the mingling of the germ-plasms.</p> - -<p>In many cases self-fertilization is said to yield less seed and -weaker seedlings. Let us for the present take this statement as the -basis of our consideration; it does not seem to me conceivable, though -here I am not in agreement with views that have been expressed -by others, that both effects should depend upon the same causes, for -the smaller number of seeds cannot possibly depend upon the mingling -of the two parental germ-plasms, and thus not upon the process -of amphimixis itself, since the effect of the mingling does not -make itself felt until the organism of the offspring is being built up. -Of course the plant seed is the embryo of the young plant, but it will -hardly be thought probable that its development could be absolutely -prevented by the too close relationship of the two germ-cells, and thus -the number of the developing seeds cannot depend on the quality -of the ids co-operating in the segmentation-nucleus, but presumably -on the number of ova awaiting fertilization in the ovary, which are -reached by a pollen-tube and then by a paternal sex-nucleus. This -again will depend upon the impelling and attracting forces of the -pollen-grain on the one hand, and of the stigma and 'embryo-sac' -of the flower on the other. In other words, the fertility of a flower -with its own pollen will depend upon whether the two products of the -flower are adapted for mutual co-operation, and in what degree they are -so. We are here dealing not with the <i>primary</i> reactions of the germ-plasms, -which are as they are and cannot be varied, but with secondary -relations, which may be thus or thus—in short, with <i>adaptations</i>.</p> - -<p>By what adaptations the pollen of a flower can be made ineffective -for that flower is a question which we must leave the botanists to -answer; in any case it must have been possible, and we see clearly -that it depends upon adaptation when we consider the numerous -stages which occur—from the rare case of the actually poisonous -influence of self-pollination already noticed, to complete sterility, -and from lessened fertility to greater or even perfect fertility. It is -possible that chemical products, secretions of the stigma or the pollen-grain, -or the so-called synergid-cells, have to do with this, or that the -size and therewith the penetrating power of the pollen-cell in self-fertilization -stand in inverse ratio to the length of the pistil, as has<span class="pagenum"><a id="Page_254"></a>[Pg 254]</span> -been proved in regard to heterostylism by Strasburger; but in any -case it was possible for Nature, by means of slight variations in the -characters of the male and female parts of the flower, to diminish the -certainty of the meeting of the two germ-cells, even to the total exclusion -of the possibility of any union of these.</p> - -<p>If, then, self-fertilization had to be guarded against or at least -rendered difficult because its consequences were injurious, all variations -pointing in the direction of safeguarding would necessarily be preserved -and increased. In many cases variations in the structure -of the flower were sufficient; but when, as in <i>Corydalis cava</i>, the pollen -could not readily be prevented from falling upon the stigma, the -pollen might be made sterile as far as its own flower was concerned -by a process of selection, in which on an average those plants would -remain successful which produced the largest number of cross-fertilized -seeds, and in this case those which did so were those whose pollen -reacted most feebly to the stimulus of their own stigma.</p> - -<div class="figcenter" id="ff46"> -<img src="images/ff46.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 128.</span> Heterostylism (<i>Primula sinensis</i>), after Noll. Two heterostylic -flowers from different plants. <i>L</i>, the long-styled form. <i>K</i>, the short-styled -form. <i>G</i>, style. <i>S</i>, anthers. <i>P</i>, <i>p</i>, pollen-grains. <i>N</i>, <i>n</i>, stigmatic papillæ of -the long-styled and short-styled forms respectively. <i>P</i>, <i>p</i>, <i>N</i>, <i>n</i>, magnified -110 times.</p> -</div> - -<p>That self-sterility in all these different degrees is not a primary -character of the species, but an adaptation to the advantages of cross-fertilization, -is apparent—if indeed it seems doubtful to any one—especially -from cases of heterostylism. I refer to the dimorphism -and trimorphism which Darwin discovered in many flowers, and -which shows itself in the fact that flowers otherwise almost exactly -alike, as, for instance, primroses, may exhibit a long style in some -individuals, and in others a short one (Fig. 128). At the same -time, there is a difference in the position of the stamens, which are -placed higher up in flowers with short styles, and much lower down<span class="pagenum"><a id="Page_255"></a>[Pg 255]</span> -in those with long styles. Experiments have proved that the dusting of -the stigma has the best results if pollen from the short-styled reaches -the stigma of the long-styled form, or if pollen from the long-styled -form reaches the stigma of the short-styled. Thus we have again -to deal with an arrangement for crossing, an adaptation to the -advantages of cross-fertilization, and we can in this case see the reason -why the pollen has a different effect upon the two stigmas; the pollen-grains -of the flowers with short style are larger than those of the -flowers with long style, and as the length of the pollen-tube that can -be sent out must depend upon the mass of protoplasm within the -pollen-grain, it follows that the smaller pollen-grains will send out too -short a tube to reach through the long style to the embryo-sac. In -addition to this there is a difference in the papillæ of the stigmas, and -it is possible that these may form an obstacle to the penetrating of -pollen from a similar type. The process of selection which gives rise -to such arrangements as we find in Primulas may easily be imagined, -as soon as we are able to assume that cross-fertilization is more -advantageous than self-fertilization as regards progeny, that is, as -regards the continuance of the species.</p> - -<p>We have already seen that uninterrupted self-fertilization is -unknown among animals, but that it is not even very rare among -plants, and this emphatically corroborates our previous conclusion, -that the reason for which amphimixis was introduced as a normal -event in nature is not to be sought for in the necessity for a renewing -of life, or 'rejuvenation.' It cannot be a necessity, but only an -advantage, which can in certain circumstances be dispensed with.</p> - -<p>Although it is obvious enough that continued inbreeding in its -most extreme form, self-fertilization, does not imply an absolute -abandonment of amphimixis, the adherents of the rejuvenescence -theory have regarded the unfavourable consequences of pure inbreeding -as a confirmation of their assumption, according to which amphimixis -is indispensable to the continuance of the life of the species, -and it is therefore an important fact, if it can be proved, that continued -self-fertilization can occur persistently, among plants at least, -and yet not cause any injurious results to the species.</p> - -<p>But how can this fact be understood from our point of view? -How does it happen that crossing is striven after in so many different -ways and yet so often given up again, and continued self-fertilization -resorted to?</p> - -<p>To this it may be answered, in the first place, that it is not, as -far as we can see, for <i>internal</i> reasons that persistent self-fertilization -becomes the rule; there is no peculiar condition of the germ-plasm<span class="pagenum"><a id="Page_256"></a>[Pg 256]</span> -which makes it disadvantageous or superfluous that the diversity -of the id-combinations should be maintained; self-fertilization is due -to <i>external</i> influences which bring it about that the plant has only -the alternative of producing no seeds at all or of producing them -by self-fertilization. In this connexion Darwin's experiments with -orchids are particularly noteworthy.</p> - -<p>In this very diversified order of plants there are numerous species -whose flowers are infertile with their own pollen, although it does -not reach the stigma in natural conditions, and therefore there was -no necessity—as far as we can see—for guarding against self-fertilization -by 'self-sterility.' These flowers are thus doubly adapted, so to -speak, for crossing by means of insects. But as regards many of -these, as well as many other modern orchids, insect-visits are very -rare, and in some cases do not occur at all, and therefore these species -cannot produce seed or can do so only exceptionally.</p> - -<p>This is true of most of the Epidendra of South America, and -of <i>Coryanthus triloba</i> of New Zealand, two hundred blossoms of which -only yielded five seed-capsules, and also of our <i>Ophrys muscifera</i> and -<i>O. aranifera</i>, the latter of which yielded only a single seed-capsule -from 3,000 flowers gathered in Liguria. We might expect that the -species in question must have become very rare, but this is not always -the case, since each of these capsules contains an enormous number -of seeds, sometimes many thousands. As soon as the visits of insects -cease altogether, the species must die out in the particular locality -concerned, unless it can revert to self-pollination and self-fertility. -There is a whole series of species in which the stigma of the flower -is sensitive to its own pollen, and in many of these an adaptation -to self-fertilization has actually been effected, for the pollinia detach -themselves from their anthers at maturity and fall upon the stigma. -I have already mentioned <i>Ophrys apifera</i>, which, according to Charles -Darwin, is no longer visited by insects, although its flowers still possess -the structure required for insect-fertilization. This species has saved -itself from extinction by the normal occurrence of self-fertilization.</p> - -<p>This seems to me noteworthy in two respects. In the first place, -it shows that pure self-fertilization need not necessarily result in -a weakening of the species, and secondly, it affords a clear instance -of a species being transformed in one minute character only, all the -other characters remaining unaltered. In this case it was only the -pollinia that required to vary a little in their mode of attachment and -maturation, in order to effect the transformation of the flower for -self-fertilization, and in point of fact that is all that has varied. The -case is not relevant to our investigation at this moment, but cases<span class="pagenum"><a id="Page_257"></a>[Pg 257]</span> -of the kind can so rarely be clearly demonstrated that I cannot lose -the opportunity of calling attention to it. The germ-plasm of this -<i>Ophrys</i> must have varied at an earlier stage, for otherwise the detachment -of the pollinia would not have become normal and hereditary, -but it can only have varied to the extent that the structure of this -one small part of the flower was affected by the variation; something -must have varied in the germ-plasm that had no influence upon the -other parts of the flower, that is, solely the <i>determinants</i> of the pollinia.</p> - -<p>Let us return after this digression to our previous train of -thought; we have to inquire how we can interpret the fact of continued -self-fertilization without any visible injurious results to the -species. If cross-fertilization be a material advantage as regards the -continuance of the species, how can it be transformed into its opposite -without evil effects? And there are no visible evil effects in <i>Ophrys -apifera</i>. It is indeed not so abundant as <i>Ophrys muscifera</i>, or other -allied species, but it certainly does not follow from that that it is on -the way to extinction; certainly no decrease either of vigour of growth -or of fertility can be observed.</p> - -<p>If we inquire from the standpoint of our theory, how the composition -of the germ-plasm must have altered through continual -inbreeding, we have already found the answer—that through the -reduction of the number of ids at the maturation of every germ-cell -the diversity of the germ-plasm would gradually be lessened, that the -number of different ids would thereby be lessened possibly even to the -identity of the whole of the ids.</p> - -<p>The consequences of such extreme uniformity of the germ-plasm -would not, according to our theory, necessarily be that the species -would be incapable of continued existence, but it would be that the -species would become incapable of adaptations in many directions. -Adaptations in one direction, such, for instance, as the variation in the -mode of attachment and detachment of the pollinia of an Orchid, -would still be possible. Thus a species which has long been perfectly -adapted will be able to make the transition to inbreeding without -injury to its chances of continued existence, if it be compelled by -circumstances to do so. Species, on the other hand, which are still -undergoing considerable transformations in many directions must be -exposed by these to the danger of degeneration, just as happens in -the artificial experiments with domesticated animals, whose secret -weaknesses are greatly exaggerated by inbreeding.</p> - -<p>We might be inclined to regard the effects of inbreeding as -similar to those of parthenogenesis; they are certainly analogous, for -both modes of reproduction must lead to a certain degree of uniformity<span class="pagenum"><a id="Page_258"></a>[Pg 258]</span> -in the germ-plasm. But there seems to me to be a difference and -one which is not without importance.</p> - -<p>In parthenogenesis no amphimixis occurs, but neither does any -reduction of the number of the ids to one-half; all the ids present at -the beginning of parthenogenesis are retained; they are only no -longer mingled with strange ids. In inbreeding both amphimixis -and reduction take place, but the former soon ceases to convey any -really strange ids to the germ-plasm, but only the same as those -which it already contains, so that a rapidly increasing monotony -of the germ-plasm must result. To this must be added the possibility -that among the few ids which now—many times repeated—form the -germ-plasm, some must occur which exhibit unfavourable variational -tendencies in one or many determinants, and then the same thing -will occur which usually occurs in experimental inbreeding of -domesticated animals, namely, <i>degeneration of the progeny</i>. In -parthenogenesis the case is otherwise; unfavourable variational -tendencies, as soon as they attain selection-value, are, so to speak, -eliminated root and branch, because the individuals which exhibit -them, and their whole lineage, are exterminated, without their having -any effect upon the other collateral lines of descent. A purely -parthenogenetic species will, therefore, not degenerate as long as -individuals of normal constitution are present, for these reproduce -with perfect purity. But if in later generations unfavourable -variational tendencies crop up in the germ-plasm through germinal -selection, the process of personal selection will be reinforced on these -or on their descendants, and it is conceivable, and even probable, that -in perfectly adapted species parthenogenesis may last for a very long -time without doing any injury to the constitution of the species.</p> - -<p>The same is true of purely asexual reproduction, to the -investigation of which we shall now turn.</p> - -<p>Let us leave out of account the simplest animals (Monera) without -amphimixis, which we have already discussed. In simple animals -reproduction by budding or by fission is frequent, or it occurs in -alternation with sexual reproduction; in higher animals, Arthropods, -Mollusca, and Vertebrates, asexual reproduction is wholly absent. -In plants it plays an enormously greater part, and what is called -'vegetative reproduction,' which is purely asexual without any -amphimixis, is to be found in all groups of plants, especially in the -form of budding and spore-formation, besides which there is multiplication -by runners, rhizomes, tubers, bulbs, and bulbils. In most -cases there is, in addition to the purely asexual reproduction, so-called -sexual reproduction associated with amphimixis, and often the sexual<span class="pagenum"><a id="Page_259"></a>[Pg 259]</span> -and asexual generations alternate with each other, so that 'alternation -of generations' occurs, as is common in lower animals, especially -polyps, medusæ, and worms.</p> - -<p>But it sometimes happens among plants that the sexual reproduction -is absent, and that a species reproduces by the asexual mode -only, and this is the case which we must now consider more closely.</p> - -<p>Let us first of all seek to gain clearness as to the composition of -the germ-plasm in the case of purely asexual multiplication, and -what conclusions may be drawn from this, and then let us compare -these with the known observational data, and it will be apparent that in -individuals which have arisen by budding the complete germ-plasm -of the species must be contained; the number of ids will not only -remain the same in the bud as it was in the mother plant, but the -number of <i>different ids</i> will not be diminished. The case is analogous -to that of pure parthenogenesis, in which the absence of the second -maturation-division of the ovum allows the germ-plasm to retain -the full complement of ids. Charles Darwin held that purely asexual -multiplication was 'closely analogous to long-continued self-fertilization,' -yet, as we have seen, according to our theory there must be a not -inconsiderable difference between the two processes, depending on the -fact that in exclusive self-fertilization the number of different ids is -continually decreasing, while in purely asexual reproduction the -germ-plasm loses nothing of the diversity of its ids. If, therefore, the -germ-plasm in purely asexual reproduction no longer receives fresh -ids through amphimixis, it at least loses none of those it formerly -possessed. Although we cannot consider it adapted for entering upon -new adaptations in many directions, yet we may expect that the -species will continue to reproduce unchanged for longer than in the -case of exclusive self-fertilization, the more so since all unfavourable -variational tendencies which crop up are eliminated as soon as they -attain to selection-value, and, as in the case of parthenogenesis, they -are eliminated without being mingled with other lines of descent.</p> - -<p>Let us take, for instance, the purely asexual reproduction which -obtains in Algæ of the genus <i>Laminaria</i>, in regard to which it is -stated that it multiplies only through asexual swarm-spores. There -are quite a number of species of this large tangle, and if it should be -established that in all these the spore-cells really do not conjugate, -then the case would prove that the species of a genus can maintain -a well-defined existence for a long time after amphimixis has been -given up. But this would not be a proof of the possibility of <i>species-formation</i>, -for that the ancestral forms of the Laminarians must have -possessed amphigony may be assumed, since their nearest relatives<span class="pagenum"><a id="Page_260"></a>[Pg 260]</span> -exhibit it still. It cannot be proved, but there seems nothing against -the assumption that these tangles have existed for a long time under -uniform conditions, and have become adapted to these with a high -degree of constancy.</p> - -<p>The conditions are similar in the marine Algæ of the genus -<i>Caulerpa</i>, the nearest relatives of which reproduce sexually, though -they themselves, as far as is known, reproduce only by spores.</p> - -<p>In the Lichens, which represent, as we have already seen, a life-partnership -between Fungi and Algæ, amphimixis appears not to -occur at all; the unicellular Alga reproduces by cell-division, the -Fungus by producing a great number of swarm-spores, which do not -conjugate with one another. As far as the Alga is concerned we -might perhaps suppose that the simplicity of its structure makes -it possible for it to dispense with a constant recombination of its -few characters to bring about the most favourable composition in -its idioplasm; in support of this we may note that even the life-long -combination with the Fungus has caused no visible variation in the -Alga, as we must conclude from the fact that these Algæ can also live -independently, and that the same species of Alga may combine with -several different Fungi to form different species of lichen, just as -the same Fungus may also form part of several species of lichen. -We might also imagine that we have here no more than a direct -influence of the Alga and Fungus upon one another, and that there -is no adaptation to the new conditions of life at all, yet that can -hardly be seriously maintained in regard to species which live under -such definite and diverse conditions. It now seems to be established—contrary -to the older statements—that the lichen-fungus only -reproduces asexually, and in face of this it seems to me that nothing -remains except to make the assumption that lichens formerly -possessed sexual reproduction, but that they have lost it, though -whether all have done so is, perhaps, not yet quite certain.</p> - -<p>The same assumption must be made in regard to the Basidiomycetes -among the Fungi, and for most of the Ascomycetes, for -in these groups of Fungi sexual reproduction has only been demonstrated -'with certainty in a few genera.' That in these cases also -there has been a degeneration of amphigony, until it has completely -disappeared, seems probable from the two other groups of Fungi, the -Zygomycetes and Oomycetes, since in these 'a reduction of sexuality -amounting in some cases to complete disappearance' can be demonstrated -even in existing forms. But whether it may be assumed that -the Fungi which are now asexual are no longer capable of new -adaptations, and whether their parasitic habit may be regarded<span class="pagenum"><a id="Page_261"></a>[Pg 261]</span> -as making up in some way for the lack of the remingling of the -germ-plasm, as the botanist Möbius supposes, I am not able to decide. -It is obvious that data in regard to amphimixis among the Fungi are -still incomplete, and recent investigations lead us to suspect that -sexual mingling may not be absent, but only disguised. Dangeard, -Harold Wager, and others have observed that a fusion of nuclei -precedes the formation of spores, and this may be regarded as -amphimixis, although the conjugating nuclei belong to cells of the -same plant and sometimes even to the same cell. But although we -are here dealing with a set of facts which cannot yet be satisfactorily -formulated in terms of our theory, it is nevertheless not contradictory -to it that amphimixis should be wholly absent in the higher Fungi. -But the fact would be contradictory to the unadulterated rejuvenescence-theory, -for if amphimixis were really a condition of the -continuance of life, no species—as we have already said—could -continue to exist without it for countless generations.</p> - -<div class="figcenter" id="ff47"> -<img src="images/ff47.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 38</span> (repeated). A fragment of a Lichen (<i>Ephebe kerneri</i>), magnified 450 -times. <i>a</i>, the green alga-cells. <i>P</i>, the fungoid filaments. After Kerner.</p> -</div> - -<p>The same argument holds true for the higher plants, which have -become purely asexual under the influence of cultivation. I refer to -many of the well-marked varieties of our cultivated plants which -multiply exclusively, or almost exclusively, by means of tubers and -slips, as is the case with the potato, the manioc, the sugar-cane, the -arrowroot-plant (<i>Maranta arundinacea</i>), and others. All these facts -can easily be reconciled with our interpretation of the meaning of -amphimixis, although the attempt to range them as evidence against -our theory has more than once been made. We have thus arrived at -the conclusion that while many-sided adaptations, that is, variations<span class="pagenum"><a id="Page_262"></a>[Pg 262]</span> -which transform the plant in accordance with the indirect influences -of new conditions of life, cannot be brought about without a persistent -mingling of germ-plasms, simple modifications may readily appear -although amphimixis is altogether absent. If a wild plant be -permanently transferred to a well-manured culture-bed, it is probable -that certain changes will occur in it, either gradually or at once. -But these are not adaptations; they are, so to speak, direct reactions -of the organism which do not even require selection to make them -increase, but depend upon the influencing of certain determinants -of the germ-plasm, and which, like all germinal variations, will follow -their course steadily until a halt is called either by germinal or by -personal selection. When a given plant is exposed to these new and -artificial conditions, the changes in question make their appearance -sooner or later, and follow their course, and go on increasing as long -as that is compatible with the harmony of the structure and functioning -of the plant, this depending, as in all individual development, -on the struggle between the parts, that is to say, on histonal -selection. Only in this respect is the utility or injuriousness of -the change of importance, for personal selection, the struggle between -individuals, does not affect plants which are under cultivation.</p> - -<p>That such modifications may increase and may persist through -many generations, even with asexual multiplication, depends upon the -fact that the budding cells contain germ-plasm, as well as the germ-cells, -and if particular determinants of the germ-plasm in general are -caused to vary by these new influences, the variation may be transmitted -from bud to bud, from shoot to shoot, and so go on increasing -as long as the new conditions persist, as well as in amphigonic (bisexual) -reproduction, where they are transmitted from germ-cell to -germ-cell. It is not inconceivable that an individual adaptation, that -is to say a useful adjustment, might be effected in the course of asexual -reproduction, although it is improbable that direct influences would -give rise to just those changes which would be useful under the new -conditions. But there are a number of cases which have been interpreted -in this way. In several of the cultivated plants named, the -reproductive organs have themselves degenerated, either only the -male, or only the female, or both at the same time; and some observers, -accepting the hypothesis of an inheritance of functional modifications, -have regarded this as the direct result of disuse during the long period -of asexual reproduction.</p> - -<p>Leaving out of account this erroneous presupposition, we may ask -how asexual reproduction, such as that of the potato by tubers instead -of by seed, which has gone on exclusively for several centuries, could<span class="pagenum"><a id="Page_263"></a>[Pg 263]</span> -exercise any influence upon the flowers and seed-forming of this -species? In point of fact it has exercised none in most potatoes, for -the flowers and seeds are just as fertile now as they were when the -potato was first discovered.</p> - -<p>Whether the pollen of a flower is utilized in one or other of its -thousands of pollen-grains by reaching the stigma of another plant of -the same species, or whether all the pollen-grains are uselessly scattered -abroad, cannot possibly affect the flower so as to cause degeneration; -the theory of disuse cannot be applied in this case. What is true of -the potato holds good also of the manioc (<i>Manihot utilissima</i>), but, -on the other hand, many of the best varieties of common fruits—pears, -figs, grapes, pine-apples, and bananas—are seedless. In <i>Maranta -arundinacea</i> 'the whole wonderful structure of the flower has persisted, -but the pollen-grains, that is the germ-cells, are wanting.' -Whether this implies a permanent degeneration of the sexual organs, -that is to say, one that is embodied in the primary constituents of the -species, or whether it is only the result of over-abundant nourishment, -or of other causes in the circumstances affecting the particular plant, -can only be decided by experiment. Probably both occur. The -common ivy, for instance, does not now blossom in the northern parts -of Sweden and Russia, but it does so still in the southern provinces. -If plants were brought to us from the most northerly zone of distribution, -they would in all probability flower and bear fruit with us, and -in that case the absence of bloom in these plants must have been -a direct effect of the cold climate. But it is quite conceivable that -cultivated plants have in many cases become hereditarily infertile, -when they are constantly propagated only by means of buds, layering, -and so on, not however because of any direct effect of this mode of -propagation, but through chance germinal variations. For in regard -to many of them man has lost all interest in the flowers and fruit, -as, for instance, in the case of the potato; in other cases he is even -interested in procuring seedless fruits.</p> - -<p>In the first case he will quite readily make use of plants with -imperfect flowers for propagating, if they are otherwise fit and exhibit -what he wants in other respects; in the second case, he will give -a preference to individuals with seedless fruits, and thus increase and -strengthen the tendency to degeneration of the seeds in the race -concerned.</p> - -<p>All these cases are quite in harmony with our conception of -amphimixis, which, now that we have investigated the facts throughout -the animate kingdom, we may sum up in the following propositions. -In the whole organic world, from unicellular organisms up to the<span class="pagenum"><a id="Page_264"></a>[Pg 264]</span> -highest plants and animals, amphimixis now means an augmentation -of the organism's power of adaptation to the conditions of its life, -since it is only through amphimixis that simultaneous harmonious -adaptation of many parts becomes possible. It effects this by the -mingling and constant recombination of the germ-plasm ids of different -individuals, and thus gives the selection-processes the chance of favouring -advantageous variational tendencies and eliminating those which -are unfavourable, as well as of collecting and combining all the variations -which are necessary for the further evolution of the species. -This indirect influence of amphimixis on the capacity of organisms -for surviving and being transformed is the fundamental reason for its -general introduction and for its persistence through the whole known -realm of organisms from unicellulars upwards.</p> - -<p>The reason for its <i>first</i> introduction among the lower forms of life -must have been a direct effect which had a favourable influence on the -metabolism, and this is so far coincident with the subsequent import -of amphimixis, inasmuch as it may be regarded not only as a heightening -of the power of adaptation, but as an immediate and direct -increase and extension of the power of assimilation. In any case, -amphimixis is not necessary to the actual preservation of life itself, -but it does bring about a wealth and diversity of organic architecture -which without it would have been unattainable.</p> - -<p>If amphimixis has been abandoned in the course of phylogeny -by isolated groups of organisms, this has happened because other -advantages accrued to them in consequence, which gave them greater -security in the struggle for existence; but it must be admitted that -they thereby lost their perfect power of adaptation, and that they -have thus bartered their future for the temporary securing of their -existence.</p> - -<p>In addition to this variational influence, amphimixis has also -played a part in the evolution of sharply defined organic types, especially -of specific types; but of this we shall have more to say later on.</p> -<hr class="full" /> - -<div class="chapter"> -<p><span class="pagenum"><a id="Page_265"></a>[Pg 265]</span></p> - -<h2 class="nobreak" id="LECTURE_XXXI">LECTURE XXXI</h2> -</div> - -<p class="c">THE INFLUENCES OF ENVIRONMENT</p> - -<div class="blockquot"> - -<p>Different modes and grades of selection—Changes due to the influences of environment—Superfluity -and lack of food—The horses and cattle of the Falkland Islands—Angora -animals—Protection against cold in Arctic and marine mammals—Plant-galls—Nägeli's -<i>Hieracium</i> experiments—Experiments with <i>Polyommatus phlæas</i>—Artificially -produced <i>Vanessa</i>-aberrations—Vöchting's experiments on the influence of light in the -production of flower-forms—Heliotropism and other tropisms—Primary and secondary -reactions of organisms—Herbst's 'lithium larvæ'—Schmankewitsch's experiments -with <i>Artemia</i>—Poulton's caterpillars with facultative colour adaptation—Colour-change -in fishes, chamæleon, &c.—Actual scope of those influences which directly produce -organic changes.</p></div> - - -<p><span class="smcap">Through</span> a long series of lectures we have devoted our attention to -those phenomena which bear some relation to the processes of selection; -we have attempted to gain clearness in regard to the modes and stages -of these, and we reached the result that all variations which have -taken place in organisms since the first appearance of living matter -are directed by processes of selection, that is, their direction and -duration are determined by these processes, although they may have -their roots in external influences. But it is not to be supposed that -this guidance is due solely to that one kind of selection which, with -Darwin and Wallace, we designate 'natural selection'; on the contrary, -we must regard this as only one of the different modes of the processes -of selection, necessarily occurring between all living units which are -equivalent to one another, and which, therefore, must maintain a continual -struggle with one another for space and food. If the expression -'natural selection' were not already so firmly fixed in its meaning, -I should propose that it should be employed in the most general sense -for all the processes of selection collectively, but we must keep to its -original meaning and use it only for personal selection.</p> - -<p>We have seen that processes of selection take place even between -the elements of the germ-plasm in all organisms which possess a germ-plasm -as distinguished from the mass of the body, and that through -these processes there arise those hereditary individual variations -which, under some circumstances, form the basis of transformations -in the species.</p> - -<p>Obviously this may come about in a twofold manner: firstly,<span class="pagenum"><a id="Page_266"></a>[Pg 266]</span> -a variation movement originating in the germ-plasm may go on -increasing till it attains to selection-value, and then 'personal selection' -steps in, and seeks to make it the common property of the -species. But it is obviously also conceivable that variational tendencies -arising in the germ-plasm may never attain to selection-value -at all, and then in most cases they will only continue to exist through -a longer or shorter series of generations as individual distinguishing -characters, without being transmitted to a larger number of individuals -or becoming a constant character of the species. Their persistence -will depend essentially on the chance of mingling with other individuals, -and on the halving of the germ-plasm which precedes sexual -reproduction. Sooner or later these individual peculiarities disappear -again, as may often be observed in the case of abnormalities or morbid -tendencies in man, in as far as these do not weaken vitality. In the -latter case they attain selection-value, though only negatively.</p> - -<p>But even quite indifferent germinal variations, which neither raise -nor lower the individual's power of survival, may, under some circumstances, -increase and lead to permanent variations of all the individuals -of a species, and this happens when they are conditioned by external -influences which affect all the individuals of a species, or of the particular -colony concerned, and it is this kind of organismal change which -we shall now study for a little in detail.</p> - -<p>The ordinary never-ceasing, always active germinal selection -depends, we must assume, upon intra-germinal fluctuations of nutrition, -or inequalities in the nutritive stream which circulates within -the germ-plasm. The variations which it produces may, therefore, be -different in each individual, since these fluctuations are a matter of -chance and may affect the determinants <i>A</i> in one individual and the -determinants <i>B</i>, <i>C</i>, or <i>X</i> in another, or alternating groups of these. -Or it may be that the homologous determinants <i>A</i> may vary in a plus -direction in one individual, and in a minus direction in another, while -in a third they may remain unchanged, and although the same direction -of variation of a determinant <i>N</i> may occur in many individuals, it will -certainly not do so in all, and still less will it occur in all along with -the same combination of fluctuations in the rest of the determinants. -It is only if this occurs that the variation can become a specific -character.</p> - -<p>We might expect on <i>a priori</i> grounds that not only the chance -fluctuations of nutrition within the germ-plasm would cause its elements -to vary in this or that direction, but that there would also be influences -of a more general kind, especially those of nutrition and climate, which -would in the first place affect the body as a whole, but with it also the<span class="pagenum"><a id="Page_267"></a>[Pg 267]</span> -germ-plasm, and which would therefore bring about variations, either -in all or only in certain determinants. In this case all the individuals -would vary in the same way, because all would be similarly affected -by the same causes of change.</p> - -<p>This is actually the case; it is indubitable that external influences, -such as those emanating from the environment or media in which -species live, are able to cause direct variation of the germ-plasm, -that is, permanent, because hereditary variations. We have already -referred to this process and called it 'induced germinal selection.'</p> - -<p>That such influences of environment may bring about changes -in <i>individual</i> organisms is obvious enough; that, for instance, good -nutrition makes the body strong and vigorous, that too abundant -food makes it fat and causes degeneration, that insufficient food lessens -its stamina and vigour, are well-known facts. We have to inquire, -on the one hand, to what extent such influences are able to cause -changes in the individual body in the course of a lifetime, and, on -the other hand, more particularly, how far such changes or modifications -of the soma can call forth corresponding variations in the -determinant system of the germ-cells, and whether and under what -circumstances they may be transmitted; for where this is not the -case there can be no permanent hereditary variation of the whole -species, and the variation will only persist as long as the conditions -which gave rise to it endure, and will disappear again with these.</p> - -<p>The influence of nutrition as a cause of variation has often been -over-estimated. The old statement which has gone the round of the -textbooks since the time of John Hunter, that the stomach of -carnivores may be transformed by vegetable diet into a herbivore -stomach, is absolutely unproved. Brandes at least, who not only -subjected all the statements in the literature on this point to a critical -investigation, but also instituted experiments of his own, regards the -statement as altogether unfounded. All the 'cases' cited, in which -the stomach of a gull or of an owl fed on grain became transformed -into an organ with stronger muscles and covered with horny plates, -depend, according to Brandes, upon inexact observation. There can -therefore be no question of any inheritance of this fictitious stomach-transformation, -and the idea that such a fundamental histological -adaptation as the alleged transformation of the stomach of the grain-eating -bird should arise as a direct effect of the food is wholly without -foundation.</p> - -<p>But it is quite otherwise with purely quantitative differences in -nutrition. That meagre diet influences individuals unfavourably is -indubitable, and we are certainly justified in considering whether<span class="pagenum"><a id="Page_268"></a>[Pg 268]</span> -this may not have an effect on the germ-cells, and one which will -correspond to the changes induced on the body, so that if the poor -nutrition should last through many generations an hereditary degeneration -of the species would occur, which would not at once -disappear though the animals were transferred to more favourable -conditions.</p> - -<p>We certainly know nothing of how far the minuteness of the -determinants of the germ-plasm, the whole quantity of the germ-plasm, -or the reduced size of the germ-cell, may bear an internal -relation to the smallness of the animal which develops therefrom, -but it surely cannot be regarded as absurd to suppose that there is -some such relation. There are no experiments known to me which -prove that meagre diet brings about a progressive decrease in the -size of the body. Carl von Voit has observed that dogs of the same -litter grew to very different sizes of body according as they received -abundant or scanty food, but it would be difficult to make animals -small through scantiness of food and at the same time to keep them -capable of reproduction, and thus proofs of the inheritance of the -dwarfing are lacking. Moreover, the experiments which Nature -herself has made are never quite convincing, because we never can -definitely exclude the indirect effect of altered circumstances. The -case of the feral horses of the Falkland Islands, so often cited since -the time of Darwin, which have become small 'through the damp -climate and scanty food,' seems to me, of all known cases of the kind, the -one we should most readily attribute to the direct effect of continued -scanty diet; but even here we cannot altogether exclude the possibility -of the co-operation of adaptations of some kind to the very peculiar -conditions of life in these islands, as far as the feral horses are -concerned. I have not been able to find any record of more modern -exact investigations either regarding these feral horses, or in regard -to the others which are reared in the Falklands under conditions of -domestication. Darwin himself, however, in the Journal of his famous -voyage tells us much that is interesting in regard to the mammals of -the Falkland Islands. Cattle and horses were brought there in -1764 by the French, and have increased greatly in numbers since -that time; they roam about wild in large herds, and the cattle are -strikingly large and strong, while the horses both wild and tame are -rather small, and have lost so much of their original strength that -they cannot be used for catching wild cattle with the lasso, and -horses have to be imported from La Plata for this purpose. From -this contrast between the horses and the cattle we may at least -conclude that it cannot be 'scanty food' alone which causes the<span class="pagenum"><a id="Page_269"></a>[Pg 269]</span> -horses to become smaller, but that the climatic conditions as a whole -are concerned in the matter. Whether the total amount of variation -which has taken place in the horses which have lived wild there -for a hundred years would take place in the course of a single life, or -whether it is a cumulative phenomenon, has still to be decided.</p> - -<p>Similar statements, for the most part still more uncertain, are made -in regard to changes in the hair of goats, sheep, cattle, cats, and sheep-dogs, -which are referred to climatic influence. The raw climate of -many highlands, like Tibet and Angora, is said to have directly -produced the long and fine-haired breeds. But there is a lack of -proof that adaptation or artificial selection did not also play a part, -and the fact that similar long-haired breeds have arisen among -rabbits and guinea-pigs in quite different places and under quite -different climatic conditions, but under the directing care of man, -speaks in favour of our supposition. But, on the other hand, it does -not seem impossible that the climate may have a variational influence -upon certain determinants of the germ-plasm, for we have already -seen that the influence of cultivation may incite plants and animals -to hereditary variations, and that slowly increasing disturbances -in the equilibrium of the determinant system may thereby be produced, -which may suddenly find marked expression as 'mutations.' -But there is little probability that <i>adaptations</i>, that is, transformations -corresponding to the altered climate, can arise in this way. The -thick fur of the Arctic mammals is assuredly not a direct effect of -the cold, although it has developed in all Arctic animals, not only -in the modern polar bears, foxes, and hares of the polar regions, but -also in the shaggy-haired mammoth of diluvial Siberia, whose tropical -relatives of to-day, the elephants, have an almost naked skin. -Another interesting case, recently brought to light, shows that -a group of animals which, in correspondence with their otherwise -exclusively tropical distribution, have only a moderately developed -coat of hair, may, on migrating to a cold country, grow as good a fur -as the members of other families. I refer to one of the higher apes, -<i>Rhinopithecus roxellanæ</i>, which live in companies in the forest on -the high mountains of Tibet, notwithstanding that the snow lies -there for six months<a id="FNanchor_26" href="#Footnote_26" class="fnanchor">[26]</a>.</p> - -<div class="footnote"> - -<p><a id="Footnote_26" href="#FNanchor_26" class="label">[26]</a> See Milne-Edwards, <i>Recherches pour servir à l'histoire nat. d. mammifères</i>, Paris, -1868-74.</p> - -</div> - -<p>But we should assuredly make a mistake if we were to regard -the thick fur of these apes as a direct reaction of the organism to -the cold. We see at once that this cannot be the case if we compare -them with marine mammals, which differ just as much from one<span class="pagenum"><a id="Page_270"></a>[Pg 270]</span> -another in this respect and yet are exposed to the same low temperature. -The whale and the dolphin are quite naked, absolutely hairless, -but the seals possess a thick hairy coat. This striking difference is -obviously connected with the mode of life; the whales remain always -in the water, the seals leave it often and therefore require the hairy -coat, especially in colder climates, since otherwise they would be too -rapidly cooled by the evaporation of the water from their bodies. For -the whales, on the other hand, even a very thick hairy coat would not -have sufficed as a protection against cold, since water is a much -better conductor of heat than air, and so it was necessary for them -to become enveloped with the well-known thick layer of blubber, -a deposit of fat lying under the skin, and this—after it was -once developed—made the hairy coat superfluous, so that it disappeared. -The seals certainly also possess a layer of fat under the -skin, but it is only in the largest of them that it affords sufficient -protection against the cooling effect of evaporation when they go -upon land or on the ice, and it is therefore only in these larger ones -that the hairy coat has markedly degenerated, as, for instance, in the -walrus and the sea-lion; in all the smaller seals, in which the mass -of the body is much less, the hairy coat is necessarily very thick -and protected from soaking by being very oily, because the layer of -fat under the skin would not be sufficient to prevent excessive -cooling when on land. But the thick coat of hair is no more <i>produced</i> -by the cold than is the layer of fat. As Kükenthal has shown, all -these characters are adaptations, and may depend here as elsewhere -upon natural selection and upon the 'fluctuating' variations of the -germ-plasm upon which that process is based. They are directed -by personal selection because there is the need for them, and they are -produced and augmented by germinal selection.</p> - -<p>In all these cases the <i>direct</i> effect of external influences has -nothing to do with the matter, but in other cases that alone brings -about the whole change, which is then limited to the individual and -does not affect the species as a whole at all.</p> - -<p>Plant-galls afford striking illustration of the extraordinary changes -that may be brought about in an organism or in its parts by external -influence in the course of the individual life. All possibility of -adaptation on the part of the plant is excluded in this case. The gall -can only depend upon the direct influence of a stimulus, which is -exercised by the young animal, the larva, upon the cells which surround -it; and yet these cells vary to a considerable extent, become filled -with starch or form a woody layer, secrete special substances, such as -tannic acid, in large quantities, or develop hairs, moss-like growths,<span class="pagenum"><a id="Page_271"></a>[Pg 271]</span> -pigments, and so on, which do not otherwise occur in that particular -part of the plant. Since Adler and Beyerinck have proved that it is not -a poison conveyed by the mother animal into the leaf or bud when -laying the eggs, which gives rise to the gall-formation, the matter has -become rather clearer. We can now understand that different stimuli -in succession affect the cells which enclose the larva, and that the -ordered succession of these and the exactly graded stimulation incite -the cells to activity in various ways, whether to mere growth and -multiplication in a given direction, or to the secretion of tannic acid, -or to the formation of wood, or to the deposition of reserve material, -and so on. Even the feeble movements of the young larva may -form a stimulus that increases with its growth; then the movements -made by the larva in feeding, and not least the different -secretions emanating from the salivary glands of the animal, which -must contain some substances capable of acting as stimuli and probably -changing in character as time goes on. All these factors must act as -specific stimuli to the plant-cells, influencing and modifying their -processes of growth and metabolism in one direction or another. In -principle at least, if not in detail, we understand the possibility that -through the ordered succession and exact balancing of these different -cell-stimuli the really marvellous structure of the gall may be brought -about as the product of the direct influence, exercised only once, -of the gall-insect upon the plant's parts. But the animal's power of -exercising such a succession of finely graded stimuli upon the plant -must be referred to long-continued processes of selection, and the -structure of the gall, which is adapted to its purpose down to -the minutest details, can thus be understood. The assumption of -substances which can act even in minute quantities as specific cell-stimuli, -which we require to make in this attempt to explain galls, -is no longer without corroboration since we find analogies in the -Iodothyrin of Baumann, the specific secretions of the thymus and the -supra-renal bodies in the higher animals, not to speak of the 'anti-toxins' -of the pathogenic bacteria, which are only known by their -effects.</p> - -<p>The case of plant-galls is thus of great theoretical interest -because we can exclude all preparation of the plant-cells for the -stimuli exercised by the animal, since the gall is quite useless for -the plant, though many have endeavoured to discover some utility. -We have therefore here a clear case of modification due to the effect, -exercised once only, of external influences, an adaptation of the -animal to the mode of reaction of particular plant-tissues.</p> - -<p>It might be supposed that if any inheritance of somatogenic<span class="pagenum"><a id="Page_272"></a>[Pg 272]</span> -modifications, any transmission of the acquirements of the personal -part to the germinal part, were possible at all, it would occur in this -case, for many species of gall-insects attack plants, particularly oaks, -in great numbers every year. It has actually been maintained -that galls may arise spontaneously, that is without the presence -of a gall-insect. But no proof of this has ever been found, and the -fact that no one has paid any attention to the assertion probably -implies an unconscious condemnation of the hypothesis of the transmissibility -of acquired characters.</p> - -<p>It has been proved by Nägeli's often discussed experiments on -hawkweeds (<i>Hieracium</i>) that much less specialized external influences -can give rise to changes which are not hereditary. The Alpine -species of hawkweed varied considerably in their whole habit in the -rich soil of the Botanic Gardens at Munich, but their descendants, -when transferred to a poor flinty soil, returned to the habit of the -Alpine species. The changes which occurred in garden soil were -therefore somatic and, as I have called them, 'transient,' and they -did not depend upon variations of the germ-plasm. It may be -objected in regard to these experiments that they were not continued -long enough to prove that hereditary variations would not also have -cropped up in consequence of the altered conditions. But in any -case they prove that marked changes in the whole body of the plant -may occur without any obvious variation of the germ-plasm. This -does not mean, however, that the possibility of variations of the germ-plasm -through such direct external influences is disputed. We must -assume the occurrence of these on <i>a priori</i> grounds, if we refer—as -we have done—individual hereditary variation to fluctuations in the -nutrition of the individual determinants of the germ-plasm. It is -probable that many general nutritive variations or climatic factors -affect the germ-plasm as well as the soma, and it is by no means -inconceivable that it is not all, but only certain definite determinants -that are caused to vary.</p> - -<p>A proof of this may be found in the results of experiments made -upon the little red-gold fire-butterfly (<i>Polyommatus phlæas</i>), to which -I have briefly referred in a former lecture. This little diurnal -butterfly of the family Lycænidæ has a wide distribution and occurs -in two climatic varieties. In the far north and also in the whole -of Germany the upper surface is red-gold with a narrow black outer -margin, but in the south of Europe the red-gold has been almost -crowded out by the black. I reared caterpillars in Germany from -eggs of <i>P. phlæas</i> found at Naples and exposed them directly after -they had entered on the pupa-stage to a relatively low temperature<span class="pagenum"><a id="Page_273"></a>[Pg 273]</span> -(10° C.). Butterflies emerged which were not quite so black as those -of Naples, but considerably darker than the German form. Conversely, -German pupæ were exposed to greater warmth (38° C.), and -these gave rise to butterflies which were rather less fiery gold and -considerably blacker than the ordinary German form. If I had to -repeat these experiments I should use a much lower temperature -in the case of the cold experiments, because we now know from the -experiments of Standfuss, E. Fischer, and Bachmetjeff, that most -of the pupæ of diurnal butterflies can stand a temperature below -zero for a considerable time; probably the results would be even more -marked then.</p> - -<p>But even from the results of my former experiments we are -justified in concluding that the blackening of the upper surface -of the wing is really the direct result of the increased temperature -during pupahood, and that the pure red-gold results from the -lowered temperature. Similar experiments made by Merrifield with -English <i>Phlæas</i> pupæ agree exactly with mine. But we may conclude -further from these experiments that both warmth and cold -only give rise to slight variations in the individual pupæ, and that -the pure red-gold of the northern form and the black of the southern -are the result of a long process of inheritance and accumulation, -in which the germ-plasm has been caused to vary in as far as the -relevant determinants are concerned, so that these yield the respective -northern and southern forms even in less extreme temperatures.</p> - -<p>As it is to be assumed that these determinants are present not -only in the primordium of the wing in the pupa, but also in the -germ-cells, both must be affected by the varying temperature, and, -in accordance with the continuity of the germ-plasm, each variation -of these determinants, however slight, would be continued in the -next generation. It is thus intelligible that somatic variations like -the blackening of the wings through warmth appear to be directly -inherited and accumulate in the course of generations; in reality, however, -it is not the somatic change itself which is transmitted, but -the corresponding variation evoked by the same external influence -in the relevant determinants of the germ-plasm within the germ-cells, -in other words, in the determinants of the following generation.</p> - -<p>This interpretation of these experiments, which I offered some -years ago, has been confirmed in several ways in regard to various -other diurnal Lepidoptera. By employing a temperature as low -as 8° C. in the case of fresh pupæ of various species of <i>Vanessa</i> -Standfuss and Merrifield, and especially E. Fischer, succeeded in -getting great deviations in the marking and colour of the full-grown<span class="pagenum"><a id="Page_274"></a>[Pg 274]</span> -insects,—so-called aberrations, such as had previously been found only -very rarely and singly under natural conditions. The deviations -from the normal must undoubtedly be ascribed to the effect of cold, but -it does not follow that they are new forms which have suddenly sprung -into existence, as many have assumed without further experiment. -Dixey, on the other hand, has attempted to establish, by a comparison -of the different species of <i>Vanessa</i>, the phyletic development of their -markings, and has found that these aberrations due to cold are more -or less complete reversions to earlier phyletic stages. As regards the -common small painted lady (<i>Vanessa cardui</i>), the small tortoise-shell -butterfly (<i>Vanessa urticæ</i>), the 'Admiral' (<i>Vanessa atalanta</i>), the -peacock (<i>Vanessa io</i>), and the large tortoise-shell (<i>Vanessa -polychioros</i>), I can agree with this interpretation, and I do so the -more readily because some years ago I suggested that the alternation -of differently coloured generations of seasonally dimorphic Lepidoptera -might be considered as a reversion. But this by no means -excludes the possibility that other than atavistic aberrations may -be produced by cold or heat. There is nothing against this theoretically. -Yet we must not, without due consideration, compare these -abruptly occurring variations to the sport-varieties of plants which -we have already discussed; there is an important difference between -the two sets of cases. In the Lepidoptera a single interference, -lasting only for a short time, modifies the wing-marking, but in the -plant varieties the visible appearance of the variation is preceded -by a long period of preparatory change within the germ-plasm. -This period required for the external influences to take effect was -already recognized by Darwin, and it has recently been named by -De Vries the 'premutation period.'</p> - -<p>We may explain these remarkable aberrations theoretically in -the following way: The determinants of the wing-scales in the wing-primordium -of the young pupa are influenced by the cold in different -ways, some kinds of determinants being strengthened by it, others -markedly weakened, even crippled so to speak, and in this way one -colour-area spreads itself out more than is normal on the surface -of the wing, and another less, while a third is suppressed altogether. -That this disturbance of the equilibrium between the determinants -leads usually to the development of a phyletically older marking -pattern leads us to the conclusion that in the germ-plasm of the -modern species of <i>Vanessa</i> a certain number of determinants of -the ancestors must be contained in addition to the modern ones. -We might even inquire whether these were not better able to endure -cold than their modern descendants, since their original possessors,<span class="pagenum"><a id="Page_275"></a>[Pg 275]</span> -the old species of the Ice age, were accustomed to greater cold, but -this idea is contradicted by the experiments of E. Fischer, which go -to show that the same aberrations are evoked by abnormally high -temperature. That the old ancestral determinants are present in -<i>different</i> numbers in the germ-plasm of the modern species, I am -inclined to infer from the fact that among a large number of -experiments made by me in the course of several years the aberrations -have always occurred in very different numbers in the different -broods, although the greatest care was taken to have the conditions -as nearly alike as possible; absolutely alike, of course, they never -can be.</p> - -<p>But it would lead me too far if I were to enter on a detailed -discussion of these cases, which have not yet been fully worked up; -only one thing more need be mentioned, that is, that the aberrations -induced by cold are to a certain extent transmissible. Standfuss -first succeeded in making some aberrant specimens of <i>Vanessa urticæ</i> -reproduce, and from their eggs he procured butterflies which showed -a much slighter deviation from the normal, which however was still -so decided that it could not be regarded as due to chance. I myself -succeeded in doing the same, but the deviation in this case was -much slighter. But that these observed cases are rightly referred -to the cold to which their parents had been subjected is proved by -other observations recently published by E. Fischer. These refer -to one of the Bombycidæ (<i>Arctia caja</i>), which flies by day, and accordingly -has a gay and very definite marking and coloration. A large -number of pupæ were exposed to cold at 8° C., and some of these -resulted in striking and very dark aberrant forms (Fig. 129, <i>A</i>). -A pair of these yielded fertilized eggs; in the progeny, which were -reared at a normal temperature, there were among the much more -numerous normal forms a few (17) which exhibited the aberration -of the parents, though to a considerably less degree (Fig. 129, <i>B</i>).</p> - -<p>This shows that the cold had affected not only the wing-primordia -of the parental pupæ, but the germ-plasm as well, and at the same -time that this latter variation was less marked than that of the -determinants of the wing-rudiments. This gives rise <i>to an appearance</i> -of the transmission of acquired characters.</p> - -<p>In the case of many of these cold-aberrations in Lepidoptera the -cold gives rise to variations, but does so not by creating anything -new, but by giving the predominance to primary constituents which -have long been present, but are usually suppressed, and so it is also -among the plants. I have in mind, for instance, the interesting -experiments of Vöchting on the influence of light in the production<span class="pagenum"><a id="Page_276"></a>[Pg 276]</span> -of flowers in phanerogams. These showed that the common balsam -(<i>Impatiens noli me tangere</i>) produces its familiar open flowers in -a strong light, but in weak light only bears small, closed, so-called -'cleistogamous' flowers. But it would be utterly erroneous to suppose -that the strong or weak light is the real cause, the <i>causa materialis</i>, -of these two forms of flowers: the degree of illumination is merely the -stimulus which provokes one or other of the primary constituents -to development, both kinds being present in the constitution of the -plant. As has long been known, the balsam normally possesses two -kinds of flowers, and the slumbering primary constituents of these -are so arranged that the open flowers develop where there is a prospect -of insect visits and cross-fertilization, that is, in sunny weather or -in a strong light, while closed and inconspicuous flowers adapted for -self-fertilization develop in weak light, that is, in shady places and -in concealed parts of the plant, where insect visits are not to be -expected.</p> - -<div class="figcenter" id="ff48"> -<img src="images/ff48.jpg" alt="" /> -<p class="caption"><span class="smcap">Fig. 129.</span> <i>A</i>, an aberration of <i>Arctia caja</i>, produced by low temperature. -<i>B</i>, the most divergent member of its progeny. After E. Fischer.</p> -</div> - -<p>Among plants we find thousands of instances of such reactions -of the organism to external stimulus—reactions which are not of -a primary nature, that is, are not the inevitable consequences of the -plant's constitution, but which depend upon adaptations of the special -constitution of a species or group of species to the specific conditions -of its life. To this category belong all the phenomena of heliotropism, -geotropism, and chemotropism, which have been discovered by the<span class="pagenum"><a id="Page_277"></a>[Pg 277]</span> -numerous and excellent observations of the plant physiologists. That -all these are adaptations and secondary reactions to stimuli is proved -by the fact that the same stimuli affect the homologous parts of -different species in very different, and often in opposite ways. For -instance, while the green shoots of most plants turn towards the light, -being positively heliotropic, the climbing shoots of the ivy and the -gourd are negatively heliotropic, which is an adaptation to climbing. -In this case the reason of the difference in the mode of reaction must -lie in the difference of constitution of the cellular substance of the -shoot, and since this may differentiate so very diversely in its relation -to light, the power of reaction which plant substance in general has -to light must not be regarded as a primary character, like the specific -gravity of a metal or the chemical affinities of oxygen and hydrogen, -but as adaptations of the living and varying substance to the special -conditions of life. And the origin of these adaptations must depend -upon processes of selection, and on these alone. This is just the -difference between living and non-living matter,—that the former -is variable to a high degree, the latter is not; it is the fundamental -difference upon which the whole possibility of the origin of an animate -world depends.</p> - -<p>Among animals also we must distinguish between the direct -effects of external influence to which the organism is not already -adapted, and those reactions which imply a previously established -adjustment to the stimulus. That is, we must distinguish between -primary and secondary reactions.</p> - -<p>For instance, Herbst made artificial sea-water in which the -sodium was partially replaced by lithium, and the eggs of sea-urchins -developed in this artificial sea-water into very divergent larvæ of -peculiar structure. We have here a primary reaction of the organism -to changed conditions of life—not an adaptation, not a prepared -reaction. Accordingly these 'lithium larvæ' eventually perished.</p> - -<p>The increasing blackness of <i>Polyommatus phlæas</i>, which we have -already discussed, must also be regarded as a primary reaction, but -not so the variations—often misinterpreted—of those species of -<i>Artemia</i> which live in the brine-pools of the Crimea, in regard -to which Schmankewitsch showed that, when the amount of salt -in the water is diminished, they undergo certain changes which bring -them nearer to the fresh-water form <i>Branchipus</i>, while when the salt -is increased in amount they vary in the contrary direction. Probably -these are adaptations to the periodically changing salinity of their -habitat.</p> - -<p>There can be no doubt of this in the case of the caterpillars<span class="pagenum"><a id="Page_278"></a>[Pg 278]</span> -of different families, in regard to which Poulton showed that in their -early youth they possess the power of adapting themselves exactly -to the colour of their chance surroundings. It is obvious that the -protection which the caterpillar would gain from being coloured -<i>approximately</i> like its surroundings would be insufficient, for instance -because the surroundings may be very diverse, since the species lives -upon different, variously coloured plants and plant-parts. Thus a -facultative adaptation arose. Selection gave rise to an extraordinarily -specialized susceptibility on the part of the different cell elements -of the skin to differences of light, and the result of this is that the -skin of the caterpillar invariably takes on the colouring which is -reflected upon it in the first few days of its life from the plants and -plant-parts by which it is surrounded. Thus the caterpillars of one -of the Geometridæ, <i>Amphidasis betularia</i>, take on the colours of the -twig between and upon which they sit, and they can be made black, -brown, white, or light green quite independently of their food, according -to the colour of the twigs (or paper) among which they are reared.</p> - -<p>Colour-change in fishes, Amphibians, Reptiles, and Cephalopods, -depends upon much more complex adaptations. In their case a reflex-mechanism -is present which conducts the light-stimulus affecting -the eye to the brain, and there excites certain nerves of the skin; -these in their turn cause the movable cells of the skin which -condition the colouring to change and rearrange themselves in -the manner necessary to bring about the harmonization of colour. -On this depends the colour-change of the famous chamæleon, and also -the scarcely less striking case of the tree-frog, which is light green -when it sits on trees, but dark brown when it is kept in the dark. -All these are secondary reactions of the organism in which the -external stimulus is, so to speak, made use of to liberate adaptive -variations, either permanently or transitorily. In the caterpillars -colour-changes are permanent, that is, it is only the young caterpillar -which takes on the colour of its surroundings; later it does not change, -even when it is exposed to different light, or intentionally placed upon -a food-plant of a different colour. In fishes, frogs, and cuttlefishes, -on the contrary, the reaction of the colour-cells to light only lasts -a little longer than the light-stimulus, and it changes with it. The -purposiveness of this difference of reaction is obvious.</p> - -<p>We cannot say to what degree the direct influence of external -conditions is effectively operative on the germ-plasm, or how far, by -persistently repeated slight changes, the determinants and the parts -of the body determined by them may be made to vary in the course -of generations; that is to say, how large a part this direct influence<span class="pagenum"><a id="Page_279"></a>[Pg 279]</span> -of climate and food may play in the transmutation of species. We -can give no answer from experience, because there is an entire lack -of perfectly satisfactory and clear experiments; we only know in -a few cases how great the variations are which can be brought about -in the body during the individual life by means of any of these -factors. In most cases it is uncertain whether actually hereditary -effects play any part, that is, whether the germ-plasm itself is affected. -But if we wish to be theoretically clear as to how far direct climatic -effects may go, we may say this, that they may operate as long as they -cause no disturbance in the life of the species concerned, for at the -moment that such a direct effect begins to be prejudicial to the species -personal selection will step in, and, by preferring the individuals which -react least strongly to the climatic stimulus, will inhibit the variation. -If in any case this should be physically impossible, the species would -die out in the climate in question. That a species of plant or animal -has climatic limits indicates that individuals which go beyond these -are exposed to influences which make life impossible and which natural -selection is unable to neutralize. We are here brought face to face -with one of the limits to the scope of natural selection. There is no -doubt that the influences of the environment must always have -a powerful effect upon the soma of the individual, but we have seen, -in the case of Alpine plants and of galls, how very far this effect may -go without leaving any trace in the germ-plasm.</p> -<hr class="full" /> - -<div class="chapter"> -<p><span class="pagenum"><a id="Page_280"></a>[Pg 280]</span></p> - -<h2 class="nobreak" id="LECTURE_XXXII">LECTURE XXXII</h2> -</div> - -<p class="c">INFLUENCE OF ISOLATION ON THE FORMATION<br /> -OF SPECIES</p> - -<div class="blockquot"> - -<p>Introduction—Isolated regions are rich in endemic species—Is isolation a condition -in the origin of species?—Moriz Wagner, Romanes—'Amiktic' local forms, the -butterflies of Sardinia, of the Alps, and of the Arctic zone—Periods of constancy and -periods of variation in species—Amixia furthered by germinal selection—The thrushes -of the Galapagos Islands—The intervention of sexual selection—Humming-birds—Central -American thrushes—Weaver-birds of South Africa—Papilionidæ of the Malay -Archipelago—Natural selection and isolation—Snails of the Sandwich Islands—Influences -of variational periods—Comparison with the edible snail and with the snail -fauna of Ireland and England—Changed conditions do not always give rise to variation—Summary.</p></div> - - -<p><span class="smcap">In</span> an earlier lecture I endeavoured to show, by means of Darwinian -arguments and examples, how important for every species, in -relation to its transmutation, is the companionship of the other -species which live with it in the same area. We saw that the -'conditions of life' operated as a determining factor in the composition -of an animal and plant association quite as momentously as -any climatic conditions whatsoever, and, indeed, Darwin rated the -influences of vital association even more highly, and attributed to -them an even greater power of evoking adaptation than he granted -to the physical conditions of life.</p> - -<p>We are, therefore, prepared to recognize that even the transference -of a species to a different fauna or flora may cause it to vary, -and this occurs when the species gradually extends the area of its -distribution, so that it penetrates into regions which contain a -materially different association of forms of life. But these migrations -are not necessarily only gradual, that is, due to the slow -extension of the original area of distribution in the course of generations -as the species increases in numbers; they may also occur suddenly, -when isolated individuals or small companies of a species -transcend in some unusual manner the natural boundaries of the old area, -and reach some distant new region in which they are able to thrive.</p> - -<p>Species-colonies of this kind may be due to the agency of -man, who has spread many of his domesticated animals and plants -widely over the earth, but who has also intentionally or unintentionally -forced many wild animals and plants from their original<span class="pagenum"><a id="Page_281"></a>[Pg 281]</span> -area to distant parts of the earth, as, for instance, when the English -humble-bees were imported into New Zealand with a view to -securing the fertilization of the clover; but such colonies also -occur in thousands of cases independently of man's agency, and -the means by which they are brought about are very diverse. Little -singing-birds are sometimes driven astray by storms, and carried far -away across the sea, to find, if fortune favours them, a new home -on some remote oceanic island; fresh-water snails, which have just -emerged from the egg, creep on to the broad, webbed feet or among -the plumage of a wild duck or some other migratory bird, and are -carried by it far over land and sea, and finally deposited in a distant -marsh or lake. This must happen not infrequently, as is evidenced by -the wide distribution of our Central European fresh-water snails -towards the north and south. But terrestrial snails can also, though -more rarely, be borne in passive migration far beyond limits which -are apparently impassable, as is evidenced by the presence of land-snails -on remote oceanic islands.</p> - -<p>The Sandwich Islands are more than 4,000 kilometres from the -continent of America; they originated as volcanoes in the midst of -the Pacific Ocean; and yet they possess a rich fauna of terrestrial -snails, the beginnings of which can only have reached them by the -chance importation of individual snails carried by strayed land-birds. -Charles Darwin was the first who attempted to investigate the -problem of the colonization of oceanic islands by animal inhabitants, -and the chapter in <i>The Origin of Species</i> which deals with the -geographical distribution of animals and plants still forms the basis -of all the investigations directed towards this point. We learn from -these that many land-animals, of which one would not expect it on -<i>a priori</i> grounds, may be carried away by chance over the ocean, -either, as in the case of butterflies and other flying insects, and of -birds and bats, by being driven out of their course by the wind, or by -being concealed—either as eggs or as fully-formed animals—in the -clefts of driftwood, where they can resist for a considerable time the -usually destructive influence of salt water. Thus eggs of some of the -lowest Crustacea (Daphnidæ), which are contained in large numbers in -the mud of fresh water, may be transported with some of the mud on -the feet of birds, and this may happen also to encysted infusorians and -other unicellulars, and to the much more highly organized Rotifers as -well. In all these cases, and in many others, it may happen occasionally -that single individuals, or a few at a time, may be carried far afield, -and may reach regions from which their fellows of the same species -are entirely excluded. If they thrive there they may establish<span class="pagenum"><a id="Page_282"></a>[Pg 282]</span> -colonies which will gradually spread all over the isolated area as far -as it affords favourable conditions of life.</p> - -<p>But oceanic islands are not the only cases of isolated regions; -mountains and mountain-ranges which rise in the midst of a plain -also form isolation-areas for mountain-dwelling plants or animals -which have not much power of migrating. In the same way marine -animals may be completely isolated from each other by land-barriers, -as the inhabitants of the Red Sea, for instance, are from those of the -Mediterranean, as has been clearly expounded by Darwin. The -idea of an isolated region is always a relative one, and the region -which seems absolutely insular for a terrestrial snail is not so at all -for a strong-flying sea-bird. There is no such thing as <i>absolute</i> -isolation of any existing colony, for otherwise the colony could never -have reached the region; but the degree of isolation may be <i>absolute</i> -as far as the time of our observation is concerned, if the transportation -of the species concerned occurs so rarely that we cannot -observe it in centuries, or perhaps in thousands of years, or if the -extension of its range could only take place through climatic or -geological changes, such as a subsidence of land-barriers between -previously separated portions of the sea, or, in the case of land -animals such as snails, the elevation of the sea-floor and the filling -up of arms of the sea which had separated two land-areas. But -even the transportation of a species by the accidental means already -indicated will occur so rarely, if the isolated insular region is very -distant, that the isolation of a colony by such a chance may be -regarded as almost absolute as far as the members of the same species -in the original habitat are concerned.</p> - -<p>If we examine one of these insular regions with reference to the -animal inhabitants which live upon it in isolation, we are confronted -with the surprising fact that it harbours numerous so-called endemic -species, that is to say, species which occur nowhere else upon the -earth, and that these species are the more numerous the further -the island is removed from the nearest area of related species. It -looks at first sight quite as if isolation alone were a direct cause of -the transformation of species.</p> - -<p>The facts which seem to point in this direction are so numerous -that I can only select a few of them. The Sandwich Islands, to which -we have already referred, possess eighteen endemic land-birds, and no -fewer than 400 endemic terrestrial snails, all belonging to the family -group of Achatinellinæ, which occurs there alone.</p> - -<p>The Galapagos Islands lie 1,000 kilometres distant from the coast -of South America, and they too harbour twenty-one endemic species of<span class="pagenum"><a id="Page_283"></a>[Pg 283]</span> -land-birds, among them a duck, a buzzard, and about a dozen different -but nearly related mocking-birds, each of which is found in only one -or two of the fifteen islands. The group of islands also possesses -peculiar reptiles, and they take their name from the gigantic land-tortoises, -sometimes 400 kilogrammes in weight, which in Tertiary -times inhabited also the continent of South America, but are now -found in the Galapagos Islands only. The islands also possess -endemic lizards of the genus <i>Tropidurus</i>, and although the lizards -can no more have been transported across the ocean than the tortoises, -but corroborate the conclusion drawn from geological data, that the -islands were still connected with the mainland in Tertiary times, -the occurrence of a particular species of <i>Tropidurus</i> upon almost -every one of the fifteen islands testifies anew to the mysterious -influence of isolation, for most of these islands are quite isolated -regions for the different species of lizard, even more than for the -mocking-birds, which have also split up into a series of species.</p> - -<p>We are thus led to the hypothesis, which was first introduced -into the Evolution Theory by Darwin, that the prevention of constant -crossing of an isolated colony with the others of the same species -from the original habitat favours the origin of new endemic species, -and his conclusion is confirmed when we learn that islands like the -Galapagos group possess twenty-one endemic land-birds, but only two -endemic sea-birds out of eleven, for the latter traverse great stretches -of sea, and crossings with others of the same species on the neighbouring -continental coasts will often take place. The Bermuda Islands -also afford a proof that the development of endemic species is -prevented by regular crossing with other members of the species -from the original habitat, for although they are 1,200 kilometres -distant from the continent of North America—that is, further than the -Galapagos Islands from South America—they possess no endemic -species of bird, and we may undoubtedly associate this with the fact -that the migratory birds from the continent visit the Bermudas every -year.</p> - -<p>Madeira also confirms our conclusion, for only one of the -ninety-nine species of bird occurring there can be regarded as endemic, -and it has often been observed that birds from the neighbouring -African mainland (only 240 kilometres distant) are driven across -to Madeira. Terrestrial snails, on the other hand, will seldom be -carried to Madeira by birds, and accordingly we find there an -extraordinary number of endemic terrestrial snails, namely, 109 species.</p> - -<p>Although these and similar facts indicate strongly that isolation -favours the evolution of new species, it would be erroneous to imagine<span class="pagenum"><a id="Page_284"></a>[Pg 284]</span> -that every isolation of a species-colony conditions its transmutation -to a new species, or, as has been maintained, first by Moritz Wagner, -and later by Gulick and by Dixon, that isolation is a necessary preliminary -to the variation of species—that not selection but isolation -alone renders the transmutation of a species possible, and thus admits -of its segregation into several different groups of forms. Romanes -went so far as to regard the natural selection of Darwin and Wallace -as a sub-species of isolation, and isolation in its diverse forms he -regarded as the sole factor in the formation of species. He assumed -that it was only by the segregation of individuals which did not -vary that the constant reversion to the ancestral species could -be prevented, and he regarded the process of selection as essentially -resulting in the 'isolation' of the fittest through the elimination -of the less-fit. The idea is correct in so far, that selection undoubtedly -aids the favourable variation to conquest over the old forms, precisely -because the latter, being less favourably placed in the struggle for -existence, are gradually more completely overcome and weeded out, -so that a constant mingling of the new forms with the old is prevented, -just as it is by isolation of locality. Obviously the new and fitter -forms could not become dominant, could not even become permanent, -if they were always being mingled again with the old. But whether -it serves any useful purpose to bring this under the category of -'isolation,' and to say that mingling with the ancestral form during -transmutation is prevented by natural selection, in that favourably -varying individuals <i>are isolated</i> by their superiority from the inferior -ones, that is, the non-varying individuals which are doomed to -elimination, is somewhat doubtful. For my part, I should prefer -to retain the original meaning of the word, and to call 'isolation' the -separation of a species-colony by spatial barriers.</p> - -<p>Whether this factor by itself prevents the mingling with the -ancestral form as effectually as selection does, and whether isolation -alone and by itself can lead to the evolution of new forms, or perhaps -must lead to them, must now be investigated.</p> - -<p>I look at this question from exactly the same point of view as -I did nearly thirty years ago, when in a short paper<a id="FNanchor_27" href="#Footnote_27" class="fnanchor">[27]</a> I endeavoured -to show that, under favourable circumstances, an individual variation -of a species may become the origin of a local variety if it finds itself -in an isolated region. Suppose an island had no diurnal butterflies, -until one day a fertilized female of a species from the continent was -driven thither, found suitable conditions of life, laid its eggs, and -became the founder of a colony; the prevention of constant crossing<span class="pagenum"><a id="Page_285"></a>[Pg 285]</span> -between this colony and the ancestral continental species would not in -itself be any reason why the colony should develop into a variety. But -suppose that the foundress of the colony diverged in some unimportant -detail of colouring, such as may at any time arise through germinal -selection, from the ancestral species; then this variation would be -transmitted to a portion of her progeny, and there would thus be -a possibility that a variety should establish itself upon the island -which would be the mean of the characters of the surviving progeny. -The greater the divergence was in the first progeny of the mother-colonist, -and the stronger this variational tendency was, the greater -also would be the chance that it would be transmitted further -and become a characteristic aberration from the marking of the -original species. I then designated this effect of isolation as due to -<i>amixia</i>, that is, to the mere prevention of crossing with the members -of the same species in the original habitat.</p> - -<div class="footnote"> - -<p><a id="Footnote_27" href="#FNanchor_27" class="label">[27]</a> <i>Ueber den Einfluss der Isolirung auf die Artbildung</i>, Leipzig, 1872.</p> - -</div> - -<p>We have examples of this from the Mediterranean islands, -Sardinia and Corsica, which possess in common nine endemic varieties -of butterflies, most of which diverge from the species of the continent -in a quite inconsiderable degree, though quite definitely and constantly. -Thus there flies in these islands a variety (<i>Vanessa ichnusa</i>) of our -common little <i>Vanessa urticæ</i> in which the two black spots on the -anterior wing exhibited by the original species are wanting. The -large tortoise-shell (<i>Vanessa polychloros</i>) also occurs there, but it -has not varied and still exhibits the black spots. Our little indigenous -butterfly (<i>Pararga megæra</i>), which is abundant on warm, stony -slopes, quarries, and roads, flies about in Sardinia, but as a variety -(<i>tigelius</i>), which is distinguished from the original species by the -absence of a black curved line on the posterior wings.</p> - -<p>That of two nearly related and similarly marked species, like the -large and small tortoise-shell, one should remain unvaried, while the -other has become a variety, shows us that amixia alone does not necessarily -lead to the evolution of varieties in every case. It might of -course be objected that one species may have migrated to the islands -at a much earlier period than the other, and that it might be a direct -effect of the climate which found expression in this way. But we -have other similar cases in which one of two species has varied in an -isolated region, while the other has not, and in regard to which we -can prove definitely that both were isolated at the same time.</p> - -<p>An instance of this kind is to be found in Arctic and Alpine -Lepidoptera, which inhabited the plains of Europe during the Glacial -period, and subsequently, when the climate became milder again, -migrated some to the north into countries within the Arctic zone, and<span class="pagenum"><a id="Page_286"></a>[Pg 286]</span> -some to the south to the Alps to escape in their heights from the increasing -warmth. There are many diurnal Lepidoptera which now belong -to both regions, and of these some have remained exactly alike, so that -the Arctic form cannot be distinguished from the Alpine form; others -show slight differences, so that we can distinguish an Arctic and -an Alpine variety. To the former category belong, for instance, -<i>Lycæna donzelii</i> and <i>Lycæna pheretes</i>, <i>Argynnis pales</i>, <i>Erebia manto</i>, -and others; to the second category belong, for instance, <i>Lycæna -orbitulus</i>, Prun., <i>Lycæna optilete</i>, <i>Argynnis thore</i>, and some species -of the genus <i>Erebia</i>.</p> - -<p>This cannot be an instance of the direct effect of general climatic -influences, for in that case all the nearly related species of a genus -would have varied or not varied; nor have we to do with adaptations, for -the differences in marking are seen on the upper surfaces of the wing, -which do not exhibit protective colouring, at least in these Lepidoptera. -It can only have been the prevention of crossing that has fixed the -existing variational tendencies in the isolated colonies—variations -which would have been swamped and obliterated if there had been -constant crossing with all the rest of the members of the species.</p> - -<p>But there is another factor to be considered. Those Alpine -Lepidoptera, for instance, which have not remained exactly the same -in the far north, have formed local varieties in the rest of the area of -their distribution also, while species which have remained quite alike -in isolated regions, such as the Alps and the north, exhibit no aberrations -in other isolated regions, such as the Pyrenees, in Labrador, or -in the Altai. Thus one species must have had a tendency in the -Glacial period to form local varieties, and the other had not; and -I have already attempted to explain this on the hypothesis that the -former at the time of their migration and segregation into different -colonies were at a period of dominant variability, the latter at a -period of relatively great constancy. Leaving aside the question of -the causes of this phenomenon, we may take it as certain that there -are very variable and very constant species, and it is obvious that -colonies which are founded by a very variable species can hardly ever -remain exactly identical with the ancestral species; and that several -of them will turn out differently, even granting that the conditions of -life be exactly the same, for no colony will contain all the variants of -the species in the same proportion, but at most only a few of them, -and the result of mingling these must ultimately result in the development -of a somewhat different constant form in each colonial area.</p> - -<p>If we were to try to imitate this 'amixia' artificially we should -only require to take at random from the streets of a large town<span class="pagenum"><a id="Page_287"></a>[Pg 287]</span> -a number of pregnant bitches, and place each of them upon an island -not previously inhabited by dogs, and then a different breed of dog -would arise upon each of these islands, even if the conditions of life -were exactly similar. But if, instead of these variable bitches, the -females of a Russian wolf were placed on the islands, the developing -wolf-colonies would differ as little from the ancestral species as the -various Russian wolves do from one another—similar climate and -similar conditions of life being presupposed.</p> - -<p>There is thus an evolution of varieties due to amixia alone, and -we shall not depreciate the significance of this if we consider that -individual variations are the outcome of the fluctuations in the equilibrium -of the determinant system of the germ-plasm, to which it is -always more or less subject, and that variations of the germ-plasm, -whether towards plus or minus, bear within themselves the tendency -to go on increasing in the direction in which they have begun, and to -become definite variational tendencies. In isolated regions such variational -tendencies must continue undisturbed for a long period, because -they run less risk of being suppressed by mingling with markedly -divergent germ-plasms.</p> - -<p>The probability that variational tendencies set up in some ids of -the germ-plasm by germinal selection will persist and increase is -obviously greater the more the germ-plasms combining in amphimixis -resemble each other. For instance, let us call the varying determinants -<i>Dv</i>, and assume as a favourable case that these are represented -in three-fourths of all the ids in the fertilized eggs of a butterfly-female -which has been driven astray on to an island, that is, that they -are present in twelve out of sixteen ids; then of 100 offspring of the -first generation it is possible that seventy-five or more will contain -the determinants <i>Dv</i>, some of them in a smaller number of ids, some -in a great number than the mother, according as the reducing division -has turned out. If the pairing of the second generation be favourable—and -this again is purely a matter of chance—a third generation -must arise which would contain the variants <i>Dv</i> throughout, and thus -the fixation of this particular variation on this particular island would -be begun. In other words, the possibility would arise, that, if individuals -with a majority of <i>Dv</i> ids predominated, they would gradually -come to be the only ones, since by continual crossing with the minority -which possessed only the determinants <i>D</i>, they would mingle the -varied ids with those of the descendants of these last, till ultimately -germ-plasm with only the old ids would no longer occur.</p> - -<p>In following out this process it is not necessary to assume that -the first immigrant possessed the variation <i>visibly</i>; if determinants<span class="pagenum"><a id="Page_288"></a>[Pg 288]</span> -varying in a particular direction occurred in the majority of its ids, -these would, as a consequence of persistent germinal selection, go on -varying gradually until the externally visible variation appeared. -This would not have appeared at all if the animal concerned had -remained in the original habitat of its species, for there it would -have been surrounded by normal germ-plasms, and its direct -descendants, even if they had been as favourably situated for the -origin of variations as we have assumed, would not have reproduced -only among themselves, and therefore even in the next generation the -number of <i>Dv</i> ids would have diminished.</p> - -<p>Obviously it is to a certain extent a matter of chance whether in -the isolated descendants the variation or the normal form remains the -victor, for it depends on the number of <i>Dv</i> ids originally present in -the fertilized eggs, then on the chances of reducing divisions, and -finally on the chance which brings together for pairing individuals in -which the similarly varied ids preponderate. The probability of the -conquest of the variation will depend in the main on the strength of -the majority of the varied ids in the fertilized eggs of the parents; -if this be an overwhelming majority, then the chances of favourable -reducing divisions and pairings will also be great. The origin of -a pure amixia variety will thus depend upon the fact that <i>the same</i> -variational tendency <i>Dv</i> was present in a large number of the ids of -the ancestral germ-plasm. We need not wonder therefore that of the -numerous diurnal butterflies of Corsica and Sardinia only eight have -developed into endemic, probably 'amiktic,' varieties.</p> - -<p>But since we know that so many species in oceanic islands and -other isolated regions are endemic or autochthonous, i.e. of local origin, -there must obviously be some other factor in their evolution in -addition to the mere prevention of crossing with unvaried individuals -of the same species. The variational tendencies which have arisen in -the germ-plasm through germinal selection may—as we have already -seen—gain the ascendancy in various ways; first, by being favoured by -the climatic influences, then by being taken under the protection of personal -selection, whether in the form of natural or of sexual selection.</p> - -<p>As the inhabitants of insular areas are not infrequently subject -to special climatic conditions, we may assume at the outset that many -of the 'endemic' species are climatic varieties, but in many cases this -explanation is insufficient. For instance, special local forms of mocking-bird -live on several of the Galapagos Islands, but this cannot -depend upon differences of climate, for the islands are only a few -kilometres apart, and resemble one another as regards the conditions -of life which they present. But as the differences between these local<span class="pagenum"><a id="Page_289"></a>[Pg 289]</span> -forms show themselves especially in the male sex, as colour variations -of certain parts of the plumage, we must take account of -sexual selection, which, though with its basis in germinal selection, -has in many islands followed a path of its own. Sexual selection -operates especially in the case of sporadically occurring characters -which are in any way conspicuous. But it is just such variations as -these that are called into existence by germinal selection, whenever -it is allowed to continue its course undisturbed through a long series -of generations. Characters of this kind, such, for instance, as feathers -of abnormal structure or colour in a bird, or new colour spots in -a butterfly, make their appearance when a group of determinants has -been able to go on varying in the same direction for a long time unimpeded, -that is, without being eliminated as injurious by natural -selection or obliterated by crossing. This is very likely to happen in -the case of an isolated area, and as soon as the conspicuous character -thus brought about makes its appearance, sexual selection takes control -of it, and ensures that all the individuals, that is, all the germ-plasms -which possess it, have the preference in reproduction.</p> - -<p>I believe, therefore, that a large number of the endemic species of -birds and butterflies in isolated regions result from amixia based upon -germinal selection, whose results have been emphasized by sexual -selection. Experience corroborates this, as far as I can see, for many -of the endemic species of birds in the Galapagos and other islands -differ from one another solely or mainly in their colouring, and in -many it is especially the males which differ greatly.</p> - -<p>As to the humming-birds we may say, without going into details -regarding their sexual characters and their distribution, that the -many endemic species which inhabit the Alpine regions of isolated -South American volcanic mountains differ from one another chiefly in -the males and in the secondary sexual characters of these. The -family of humming-birds is characteristically Neotropical, that is, it -has its centre in the Tropics of the New World, and by far the greater -number of humming-bird species—there are about a hundred and fifty—occur -there only, while a few occur as migrants north of the Tropical -zone, and visit the United States as far north as Washington and -New York. We know that many of the most beautiful species have -quite a small area of distribution, that many are restricted to a single -volcanic mountain, living in the forests which clothe its sides. These -species are isolated there, for they do not migrate; apparently they -cannot endure the climate of the plains, but remain always in their -mountain forests. Without doubt they originated there, chiefly, I am -inclined to think, through the variation of the males due to sexual<span class="pagenum"><a id="Page_290"></a>[Pg 290]</span> -selection. Any one who has seen Gould's magnificent collection of -humming-birds in the British Museum in London knows what -a surprising diversity of red, green, and blue metallic brilliance these -birds display, what contrasts are to be found in the diverse colour-schemes, -and what differences they exhibit in the length and form of -the feathers of the head, of the neck, of the breast, and especially of -the tail. There are wedge-shaped, evenly truncate, and deeply forked -tails, some with single long, barbless feathers, and so on. All these -characters are confined to the males, and are at most only hinted at -in the female; in no species does the female even remotely approach -the male in brilliance or decorativeness of plumage.</p> - -<p>I do not believe that so many species with very divergent plumage in -the males could have developed if they had all lived together on a large -connected area. But here, distributed over a large number of isolated -mountain forests, the decorative colouring or the distinctive shape which -chances to arise through germinal selection on any of these terrestrial -islands can go on increasing, undisturbed by crossing with individuals -of the ancestral species, and furthered, moreover, by sexual selection.</p> - -<p>In this way, if I mistake not, numerous new species have arisen -as a result of isolation, and it is quite intelligible that several new -species may have arisen from one and the same ancestral species, as -we may see from the nearly related yet constantly different species -of mocking-bird on the different islands of the Galapagos group.</p> - -<p>A number of similar examples might be given from among -birds. Thus Dixon calls attention to the species of the thrush genus -<i>Catharus</i>, twelve of which live in the mountain forests of Mexico -and of South America as far as Bolivia, all differing only slightly from -one another and all locally separated. They came from the plains, -migrated to the highlands, were isolated there, and then no longer -varied together all in the same direction, but each isolated group -evolved in a different direction according to the occurrence of chance -germinal variations: one developed a chestnut-brown head, another -a slate-grey mantle, a third a brown-red mantle, and so on. From -what we have already seen in regard to the importance of sexual -selection in evolving the plumage of birds, it is probable that this -factor has been operative in this case also.</p> - -<p>Another example is afforded by the weaver-birds (<i>Ploceus</i>) of -South Africa, those ingenious singing-birds resembling blackbirds -in size and form, whose pouch-shaped nests, hanging freely from -a branch, usually over the water, and with their little openings on -the under side, are excellently protected from almost every form -of persecution. These birds have in South Africa split up into twenty<span class="pagenum"><a id="Page_291"></a>[Pg 291]</span> -or more species, but the areas of each are not sharply isolated, and -the division into species cannot, therefore, be due to isolation. But -it is not difficult to guess upon what it depends, when we know that -the males alone are of a beautiful yellow and black colour, while the -females are of a greenish protective colouring all over.</p> - -<p>Thus, in my opinion, sexual selection plays a part more or less -important in the origin of the numerous endemic species of diurnal -Lepidoptera which are characteristic especially of the islands of the -Malay Archipelago, and which make the Lepidopteran fauna there -so rich in individuality. A large number, indeed the majority of -the types of Papilionidæ, have a peculiar species, a local form, on -most of the larger islands, which is sharply and definitely distinguished -from those of the other islands, usually in both sexes, but most -markedly in the much more brilliantly coloured males.</p> - -<p>Thus each of these types forms a group of species, each of which -is restricted to a particular locality, and has usually originated where -we now find it, although of course the diffusion of one of these -large strong-flying insects from one island to the other is in no way -excluded. As an example we may take the <i>Priamus</i> group, the -blackish yellow <i>Helena</i> group, the blue <i>Ulyssus</i> group, and the predominantly -green <i>Peranthus</i> group.</p> - -<p>If we inquire into the causes of this divergence of forms and -their condensation into numerous species, we shall find that their -roots lie in this case, as in that of all transformations, in germinal -selection and the variational tendencies resulting therefrom, but we -must <i>regard their fixation us the result of isolation</i>, which prevented -the variational tendencies which happened to develop on any one -island from being neutralized and swamped by mingling with the -variations of other islands. But that sexual selection took control -of these striking colour-variations and increased them still further -is obvious from the rarely absent dimorphism of the sexes. Even -if the females do not consciously select mates from among the males, -they will more readily accept as a mate the one among several suitors -which excites them most strongly. And that will be the one which -exhibits the most brilliant colours or exhales the most agreeable -perfume, for we know from their behaviour in regard to flowers how -sensitive butterflies are to both these influences.</p> - -<p>Although isolation has an important rôle in the formation of all -these species, it seems to me an exaggeration to maintain, as many -naturalists do, that the splitting up of a species is impossible without -isolation. Certainly the splitting up of species is, in numerous cases, -facilitated by isolation, and indeed could only have been brought<span class="pagenum"><a id="Page_292"></a>[Pg 292]</span> -about in its present precision by that means, but it is underestimating -the power of natural selection not to credit it with being able to adapt -a species on one and the same area to <i>different</i> conditions of life, -and we shall return to this point later on in a different connexion. -But in the meantime it must suffice to point out that the polymorphism -of the social insects affords a proof that a species may -break up into several forms in the same area through the operation -of natural selection alone.</p> - -<p>I am therefore of opinion, with Darwin and Wallace, that -adaptation to new conditions of life has, along with isolation, had -a material share in the evolution of the large number of endemic -species of snail on the oceanic islands. This brings us to the co-operation -of natural selection and isolation. If, thousands of years -ago, by one of the rarest chances, an <i>Achatina</i>-like snail was carried -by birds to the Sandwich Islands, it would spread slowly, at first -unvaried, from the spot where it arrived over the whole of the -snailless island. But during this process of diffusion it would frequently -come in contact with conditions of life which would not -prevent it from penetrating further, but to which it was imperfectly -adapted, and in such places a process of transformation would begin, -which would consist in the fostering of favourably varying individuals, -and which would run its course quietly by means of personal -selection, based upon the never-ceasing germinal selection, and -unhindered by any occasional intrusion of still unvaried members -of the species from the original settlement on the island. But these -new conditions were not merely different from those of the ancestral -country; the island region itself presented very diverse conditions, -to which the snail immigrant had to adapt itself in the course of -time, as far as its constitution allowed. Terrestrial snails are almost -all limited to quite definite localities with quite definite combinations -of conditions; none of our indigenous species occurs everywhere, -but one species frequents the woods, another the fields; one lives on -the mountains, another in the valleys; one on gneiss soil, another -on limy soil, a third on rich humus, a fourth on poor river-sand; -one in clefts and hollows among damp moss, and another -in hot, dry banks of loess, and so on. Although we cannot see in -the least from the structure of the animal why this or that spot -should be the only suitable one for this or that species, we may say -with certainty that each species remains permanently in a particular -place because its body is most exactly adapted to the conditions -of life there, and therefore it remains victorious in the competition -with other species in that particular spot.</p> - -<p><span class="pagenum"><a id="Page_293"></a>[Pg 293]</span></p> - -<p>In this way the immigrants to the Sandwich Islands must have -adapted themselves in the course of time to their increasingly -specialized habitats, and in doing so have divided up into increasingly -numerous forms, varieties, and species, and indeed into several genera.</p> - -<p>But this alone is not sufficient to explain the facts. According -to Gulick's valuable researches there live on one little island of the -Sandwich group no fewer than 200 species of Achatinellidæ, with -600-700 varieties! This remarkable splitting up of an immigrant -species is regarded by him as a result of the isolation of each -individual species and variety, and I do not doubt that this is correct -as far as a portion of these forms is concerned, and that isolation -plays a certain part in regard to them all. Gulick, who lived a long -time upon the island, attempts to prove that the habitats of all -these nearly related varieties and species are really isolated as far -as terrestrial snails are concerned; that intermingling of the snails -of one valley with those of a neighbouring one is excluded, and that -the varieties of the species diverge more markedly from one another -in proportion as their habitats are distant. On the other hand, -species of different genera of Achatinellidæ often live together on -the same area; but they do not intermingle.</p> - -<p>Although Gulick's statements are worthy of all confidence, and -though his conclusions have great value as contributions to the -theory of evolution, I do not think that he has exhausted the problem -of the causes of this remarkable wealth of forms among the terrestrial -snails of oceanic islands. It is not that I doubt the relative and -temporary isolation of the snail-colonies at numerous localities in -the island of Oahu. But why have we not the same phenomenon -in Germany, in England or Ireland? Gulick anticipates this objection -by pointing out the peculiar habits of the Oahu snails. Many of -the species there are purely arboreal animals, living upon trees and -never leaving them, even during the breeding season, or in order -to deposit eggs, for they bring forth their young alive. Active -migration from forest to forest seems excluded by the fact that on -the crests of the mountains there is a less dense forest of different -kinds of trees, and dry sunny air, which could not be endured by -the species of <i>Achatinella</i> and <i>Bulimella</i>, which love the moist shades -of the tropical forests. Active migration over the open grass-land -at the mouths of the valleys is also excluded.</p> - -<p>It must be admitted that the isolation of these forest snails -in their valleys is for the time being very complete, and that intermingling -of two colonies which live in neighbouring valleys <i>does -not occur by active migration, within the span of one or several -<span class="pagenum"><a id="Page_294"></a>[Pg 294]</span>human generations</i>. It will also be admitted that our terrestrial -snails in Central Europe are much less isolated in their different -areas, that, for instance, they could get from one side of a mountain -to the other by active migration; but we must nevertheless repeat the -question: how does it happen that in Oahu every forest, every mountain-crest, -and so on, has its own variety or species, while our snails are -distributed over wide stretches of country, frequently without even -developing sharply defined local varieties? The large vineyard or -edible snail (<i>Helix pomatia</i>) occurs from England to Turkey, that -is, over a distance of about 3,000 kilometres, and within this region -it is found in many places which might quite as well be considered -isolated as adjacent forest valleys in Oahu. It occurs also on the -islands of the Channel and of the Irish Sea, and lives there without -intermingling with the members of the species on the mainland. -But even on the Continent itself it would be possible to name -hundreds of places in which they are just as well protected from -intermingling with those of other areas as they are in Oahu. There -too the snails must <i>somehow</i> have reached their present habitat -some time or other, perhaps rather in an indirect way, by means -of other animals; but this is true also of the snails of a continent, -as we shall show more precisely later on. In the meantime let us -assume that this is so, and that the vineyard snail (<i>Helix pomatia</i>), or -some other widely distributed snail, is relatively isolated. <i>Why then -have not hundreds of well-marked varieties evolved—a special one for -each of the isolated areas?</i></p> - -<p>Obviously there must have been something in operation in the -Sandwich Islands which is absent from the continental habitats of -<i>Helix pomatia</i>, for this species shows fluctuations only in size, but is -otherwise the same everywhere, and the few local varieties of it which -occur are unimportant. I am inclined to believe that this 'something' -depends on two factors, and especially on the fact that the -immigrant snail enters upon a period of variability. This will be -brought about in the first place by the fact that the climate and other -changes in the conditions of life will call forth a gradually cumulative -disturbance in the equilibrium of the determinant system, and thus -a variability in various directions and in various combinations of -characters. To this must be added the operation of natural selection, -which attempts to adapt the immigrant to many new spheres of life, -and thus increases in diverse ways the variational tendencies afforded -by germinal selection. These two co-operating factors bring the -species into a state of flux or lability, just as a species becomes -more variable under domestication, likewise as a direct effect of<span class="pagenum"><a id="Page_295"></a>[Pg 295]</span> -change of food and other conditions, such as the consciously or -unconsciously exercised processes of selection. It follows from this that, -in the gradual diffusion of snails all over the island, similar localities -would almost never be colonized by exactly similar immigrants, but -by individuals containing a different combination of the existing -variations, so that in the course of time different <i>constant forms</i> would -be evolved through amixia in relatively isolated localities.</p> - -<p>But everything would be different in the diffusion of a new -species of snail in a region which was already fully or at least -abundantly occupied by snail-species. Let us leave out of account -altogether the first factor in variation, the changed climate, and we -see that a species in such circumstances would have no cause for -variation, because it would find no area unoccupied outside of the -sphere to which it was best adapted; it would therefore not be -impelled to adapt itself to any other, and in most cases could not do -so, because in each it would have to compete with another species -superior to it because already adapted.</p> - -<p>The case would be the same if an island were suddenly peopled -with the whole snail-fauna of a neighbouring continent, with which -a land connexion had arisen. If the island had previously been free -from snails, all the species of the mainland would be able to exist -there in so far as they were able to find suitable conditions of life, -but each species would speedily take complete possession of the area -peculiarly suited to it, so that none of their fellow migrants would -be impelled, or would even find it possible, to adapt themselves to -new conditions and thus to become variable and split up into varieties. -If Ireland were at present free from snails, and if a land connexion -between it and England came about, then the snail-fauna of England -would probably migrate quite unvaried to Ireland, and in point of -fact the snail-fauna of the two islands, which were formerly connected, -is almost the same. For the same reason the fauna of -England, as far as terrestrial snails are concerned, is almost the same -as that of Germany.</p> - -<p>On the other hand, it may be almost regarded as a law that an -individual migrant to virgin territory must become variable. This -could not be better illustrated than by the geographical distribution -of terrestrial snails, which emphasizes the fact that a striking wealth -of endemic species is to be found on all oceanic islands. Moreover, the -fact that the number of these endemic species is greater in proportion -to the distance of the island from the continent, indicates that the -variability sets in more intensively and lasts longer in proportion to -the small number of species which become immigrants in the island,<span class="pagenum"><a id="Page_296"></a>[Pg 296]</span> -and in proportion to the number of unoccupied areas which are open -to the descendants of the immigrant species. This is undoubtedly the -reason why the Sandwich Islands do not possess <i>a single species</i> which -occurs elsewhere, and the segregation of the unknown ancestral form -into many species and several (four) sub-genera is also to be interpreted -in the same way. There was probably in this case only one -immigrant species, which found a free field, and adapted itself in its -descendants to all the conditions of snail-life which obtained there, -and in doing so split up into numerous and somewhat markedly -divergent forms. But the number of different forms is much greater -than the number of distinctive habitats, as Gulick indicates and substantiates -in detail, for similar areas, if they are relatively isolated from -one another, are inhabited not by the same forms, but by different -though nearly related varieties, and this depends on the fact that from -the species which was in process of varying a different combination of -variations would be sent out at different periods, and the temporary -isolation would result in the evolution of special local varieties.</p> - -<p>But I do not believe that this would continue for all time. I -rather think that these—let us say—representative varieties would -diminish in numbers in the course of a long period. For the isolation -of single valley-slopes or of particular woods is not permanent, individuals -are liable to be carried from one to another in the course of -centuries as they were at the beginning of the colonization of the -isolated woods; forests are cleared or displaced by geological changes, -connexions are formed between places, which were formerly separated, -and in the course of another geological period the number of representative -varieties, and probably even of species, will have diminished -considerably,—the former will have been fused together, the latter -in part eliminated. Even now Gulick speaks regretfully of the -decimation of rare local forms by their chief enemies, the mice.</p> - -<p>But even if the number of endemic forms in insular regions -diminishes from the time when they were first fully taken possession -of, it nevertheless remains a very high one, for even now Madeira -possesses 104 endemic terrestrial snails, the Philippines have more -than the whole of India, and the Antilles as many as the whole -American continent.</p> - -<p>Many naturalists believe that each isolated variety must diverge -further and further from its nearest relatives as time goes on. -Although I entirely admit that this is possible, for I have -endeavoured to show that variational tendencies which have once -arisen in the germ-plasm go on in the same direction until they are -brought to a full stop in some way or other, yet I cannot admit that<span class="pagenum"><a id="Page_297"></a>[Pg 297]</span> -this must always be so. The species which has been carried to -a strange area need not always contain particular variational tendencies -in its germ-plasm, and need not in every case be impelled to -such variations by the influence of new conditions. We know species -which have made their way into new regions, and, without varying at -all, have held their own with, or even proved superior to, the species -which were already settled there. Many cases of this kind are -known, both among plants and animals; these have been brought -by man, intentionally or by chance, from one continent to another, -and have established themselves and spread over the new area. I -need only recall the evening primrose (<i>Œnothera biennis</i><a id="FNanchor_28" href="#Footnote_28" class="fnanchor">[28]</a>), whose -fatherland is Virginia, but whose beautiful big yellow blossoms now -display themselves beside nearly every river in Germany, having -migrated stream-upwards along the gravelly soil; or the troublesome -weed (<i>Erigeron canadense</i>), which is now scarcely less common in our -gardens than in those of Canada; or the sparrow (<i>Passer domesticus</i>), -which was introduced into the United States to destroy the caterpillars, -but which preferred instead to plunder the rich stores of corn, -and in consequence of these favourable conditions increased to such -an extent that it has now become a veritable pest, all imaginable -means for its extirpation having been tried—as yet, however, with no -great results.</p> - -<div class="footnote"> - -<p><a id="Footnote_28" href="#FNanchor_28" class="label">[28]</a> This was written before the appearance of the researches which De Vries has -made on the variations of <i>Œnothera</i> in Europe. Thus the illustration may not be quite -apposite, for it seems to remain undetermined whether the 'mutations' which occur -in Holland do not also occasionally appear in America. See end of lecture xxxiii.</p> - -</div> - -<p>In all these cases the migration is certainly of recent date, and -it is quite possible that, when a longer time has elapsed, some -variations will take place in the new home, but in any case these -instances prove that an immigrant species can spread over its new -area without immediately varying.</p> - -<p>Similarly, it must be admitted that species which have belonged -to two continents ever since Tertiary times need not have diverged -since that time, and we know, for instance, thirty-two species of -nocturnal Lepidoptera which are common to North America and to -Europe and yet exhibit no differences, while twenty-seven other -nocturnal Lepidoptera are, according to Grote, represented in America -by 'vicarious' species, that is, by species which have varied slightly -in one or other of the two areas, perhaps in both.</p> - -<p>To sum up: we must undoubtedly admit that isolation has -a considerable influence in the evolution of species, though only in -association with selection in its various grades and modes, especially<span class="pagenum"><a id="Page_298"></a>[Pg 298]</span> -germinal selection, natural selection, and sexual selection. We can -say generally that each grade and mode of selection will more readily -lead to the transformation if it be combined with isolation. Thus -germinal selection may call forth slight divergences in colour and -marking, which will be permanent if the individuals concerned are in -an isolated region. In isolation these variations will increase undisturbed, -and in some circumstances will be intensified by sexual -selection, so that the male sex will vary alone in the first place, -though the female may follow, so that ultimately the whole species -will be transformed. Finally, the most marked effect of isolation is -seen when individual members of a species are transferred to virgin -territory which offers unoccupied areas, suitable not to one particular -species alone, but to many nearly related species, so that the immigrant -colony can adapt itself to all the different possibilities of life, -and develop into a whole circle of species. But we saw that such an -aftergrowth of new forms, whether varieties, species, or even genera, -may far exceed the number of different kinds of localities, if there be -relative isolation between the different groups of immigrants within -the insular region, as happens in the case of slow-moving animals like -the terrestrial snails, or of small singing-birds, to which each island of -a little archipelago is a relatively isolated region (Galapagos).</p> - -<p>We may thus fully recognize the importance of local isolation -without regarding the absence of crossing with the members of the -species in the original habitat as the sole cause of species-formation, -without setting 'isolation' in the place of the processes of selection. -These last, taken in the wide sense, always remain the indispensable -basis of all transformations, but they certainly do not operate -only in the form of personal selection, but, wherever indifferent -characters are concerned, in that of germinal selection. Here, too, we -see the possibility of reconciliation with those naturalists who regard -transformations as primarily dependent upon internal forces of -development. The fact is that <i>all variations depend upon internal -causes</i>, and their course must be guided by forces which work in an -orderly way. But the actual co-operation of all these forces and -variations is not predetermined, but depends to a certain extent upon -chance, for of the possible modes of evolution the one which gains the -upper hand in the play of forces at the moment is alone followed, the -better are everywhere preferred, from the most minute vital units of -the germ-plasm, up to the struggle between individuals and between -species.</p> -<hr class="full" /> - -<div class="chapter"> -<p><span class="pagenum"><a id="Page_299"></a>[Pg 299]</span></p> - -<h2 class="nobreak" id="LECTURE_XXXIII">LECTURE XXXIII</h2> -</div> - -<p class="c">ORIGIN OF THE SPECIFIC TYPE</p> - -<div class="blockquot"> - -<p>Transition species of Celebes snails, according to Sarasin—Possible variations in -the shell due to nutrition—Natural selection plays a part—Germinal selection—Temporary -transitions between species—The fresh-water snails of Steinheim—How do -sharply-defined species arise?—Nägeli's Developmental Force—The species a complex -of adaptations—Adaptive differences between species—Adaptive nature of specific -characters—The case of Cetaceans—Of birds—Additional note: the observations and -theories of De Vries.</p></div> - - -<p><span class="smcap">Our</span> study of the influence which geographical isolation may -have in transforming old and giving rise to new forms of life has -led us naturally to a much more important problem, that of the -origin of species as more or less sharply defined groups of forms, -and I wish to make the transition to this problem by discussing -another case of species-splitting effected in association with, or, as -is usually said, <i>through</i> isolation. The naturalists Paul and Fritz -Sarasin, well known through their excellent studies on many components -of the tropical fauna, have published in their latest work -interesting discoveries in regard to the terrestrial snails of Celebes. -These observations show that on this island a great transformation -of snails has taken place, even since the later Tertiary period. A large -number of new species of snail have arisen on this island since that -time, and this, as the authors show to be probable, in association with -the receding of the sea, that is, with the elevation of the island further -out of the water, and thus with the increase of its surface. The -modern terrestrial snails show chains of forms connected in many -ways so that a series of species is connected by transition forms, and -therefore does not really consist of separate species at all, although -the extremes would seem to be separate species if they were -studied by themselves without taking the transition forms into -account. The state of things is exactly as if a Tertiary snail had -spread from any small area over the whole island, and had been -transformed slowly and in a definite direction in accordance with its -distance from its starting-point. It is thus that we must interpret -this discovery; we have here, beside each other in space, and indeed -often disposed along geographical lines, the individual stages of a -phyletic process of transformation, which has reached different levels<span class="pagenum"><a id="Page_300"></a>[Pg 300]</span> -at different places. One of the longest of these chains of forms is -that of <i>Nanina cincta</i>, which runs across the island from east to -west, and, beginning with the smallest and most delicate forms, ascends -through many intermediate stages to the giant form <i>N. limbifera</i>. -Such chains of forms have been previously recognized; thus Kobelt -described one in the case of the Sicilian land-snails of the genus -<i>Iberus</i>, and other cases are recorded in literature, but in all instances -they refer to areas which must be regarded as isolated for the snails, -and which have been colonized from a single starting-point.</p> - -<p>We have now to inquire whether and how we can explain the -origin of these chains of forms. The cousins Sarasin tell us how -they at first attempted to refer the differences between the individual -links of such a chain to the diverse influence of the external conditions -of life, but in vain; neither the height above sea-level nor -the character of the soil was sufficient, and natural selection was -no more so; 'for why should a high <i>Obba</i>-form twisted like a beehive -be either better or worse equipped for the struggle for existence -than a smaller and flatter one?' It is true that we do not -understand why, but this does not seem to me any reason to doubt -that natural selection should be regarded as one of the causes of -the divergence of these species, for we could not answer the same -question in regard to any of the other structural differences between -two species of snail, for the simple reason that we have far too little -knowledge of the biological value of the parts of a snail. Or could -any one tell of what use it would be to a snail-species to have -the horns slightly longer, the foot somewhat narrower, the radula -beset with rather larger or more numerous teeth? We might indeed -imagine many ways in which it might be of advantage, but we are -not in a position to say definitely why, for instance, longer horns -should be better for one species than for another, and yet we do -not believe that the structure of snails is less well adapted to the -life of each species than that of any other animals. The snail's -structure is certainly built up of hundreds and thousands of adaptations, -like that of every other animal species, but while in many -others we can, at least in part, recognize the adaptations as such, -we cannot do so at all in regard to the snail. Simroth has pointed -out that the spiral asymmetrical shell bears a relation to the one-sided -opening of the genital organs, but that only states the general -reason for the coiling of the shell. In studying the differences in the -shell one is apt to think of its external appearance alone, of the protection -which it affords to the soft internal organs of the easily -wounded animal; perhaps also of the distribution of weight, which<span class="pagenum"><a id="Page_301"></a>[Pg 301]</span> -must be different in a high tower-like structure and a low flat spiral; -possibly, too, of the varied obstacles and resistance the snail has to -encounter in creeping into clefts and holes, or among a tangle of -plants, according to the form of its shell; but is it not also conceivable -that the form of the shell has been determined by its contents? As -Rudolph Leuckart taught, the snail may be regarded as composed -of two parts, one of which is formed by the head and foot, the other -by the so-called 'visceral sac': the former may be called the animal -half, because it chiefly contains the dominant organs—the nerve-centres, -almost the whole mass of muscle, and the sense-organs; -the latter the vegetative half, since it contains the main mass of the -nutritive and reproductive systems—the stomach and intestine, the -large liver, the heart, the kidneys, the reproductive organs, and so on. -The vegetative half of the animal is always concealed within the -shell; would not therefore any great variation in the size of liver, -stomach, intestine, and so on, bring with it a variation in the size -and form of the shell, as well as in the expansion or contraction -of its coils? And might not such variations become necessary -because of some change in the food-supply? It is only a supposition, -but it seems to me very probable that becoming accustomed to a new -diet, less easily broken up and dissolved and of diminished nutritive -value, would cause modification not only of the radula and jaw-plate, -but also of the stomach and the liver, the intestine and the kidneys, -whose activity is closely associated. The stomach must become more -voluminous, the liver which yields the digestive fluid must become -more massive, and so forth. I will not follow this hypothetical -example further, for I merely wished to recall the fact that the snail -shell, to the form of which no biological significance can be commonly -attributed, is actually a sort of external cast of the visceral sac, and -consequently dependent on the variations to which that is liable in -accordance with the conditions of its life. To give precise proofs -for such processes is certainly not yet possible, for we do not even -know with certainty what the diet of the various species of snail -is, much less the difference between the modes of nutrition in two -varieties, or the nutritive value of the materials used, or the -changes in secretion, absorption, assimilation, and excretion which -must be brought about by these differences. But we can at least -see that variations in nutrition must be enough in themselves to -give rise to new adaptations in the size, constitution, and mutual -adaptation of the internal vegetative organs, and we cannot overlook -the possibility that the form and size of the vegetative half, and -therefore the form and size of its secretion, the shell, may also be<span class="pagenum"><a id="Page_302"></a>[Pg 302]</span> -caused to vary<a id="FNanchor_29" href="#Footnote_29" class="fnanchor">[29]</a>. The fact that we cannot recognize, for instance, -the beehive shape of an <i>Obba</i> as an adaptation, is thus no proof -that it is not one. But let us assume for the moment that it is not, -and that it cannot be referred to natural selection any more than the -other variations in the Celebes chain of forms, and we may further -admit that they cannot be referred to sexual selection, still less to -some 'inherent principle of perfecting,' not only because there is -no question of perfecting in the matter, but because such a mystical -principle is outside of the scope of natural history and its principles -of interpretation.</p> - -<div class="footnote"> - -<p><a id="Footnote_29" href="#FNanchor_29" class="label">[29]</a> That this suggestion was not unjustified is evident from a recent contribution -by Simroth ('Ueber die Raublungenschnecken,' <i>Naturwissenschaftliche Wochenschrift</i>, -December 8 and 15, 1901). In this paper the author, who is an expert as regards -the biology of Gastropods, shows that a change of diet may evoke many kinds of -changes in the structure of the food-canal, which may indirectly compel changes in -the shell. Thus in a small indigenous snail, <i>Daudebardia</i>, the pharynx has grown so -enormously in thickness and length in adaptation to the predatory mode of life, that -the head and the anterior part of the body can no longer be retracted within the -shelter of the shell. For this reason, and also because of the snail's habit of following -earthworms into their burrows, the shell has been shunted far back and obliquely -downwards. It has at the same time markedly changed in its shape, as may still be -verified by comparing the form of the shell in the young stages with that of the adult.</p> - -</div> - -<p>But that transformations in a definite direction can and must -arise from fresh disturbances in the equilibrium of the determinant -system, that is from germinal selection, we have already shown.</p> - -<p>Even if the changes of form with which we are here dealing -had really no biological importance, they might quite well have been -brought about by germinal selection, and only one thing remains -obscure, that is, why the different stages on the path of distribution -of a species are at a different level of evolution and not all at the -same level. Why have not all been transformed? Why have some -colonies remained near the ancestral form, while others have varied -only a little, and others again a great deal? This cannot be -explained by any assumption of an internal power of development, -and the explanation can only be found in germinal selection associated -with isolation, since the internal processes in the germ-plasm can -quite well run a different course in different colonies. Nevertheless -I am inclined to infer from these differences in the individual -colonies of these chains of forms, that natural selection in the -accepted sense has also played a part in the evolution of these -snail varieties.</p> - -<p>Such series of forms are especially interesting, because they -show us the process of species-formation in its different stages -beside each other in space, and thus simultaneously. They represent, -so to speak, a horizontal branch of the genealogical tree of the species, -as the Sarasins well express it, that is, a series of species arising from<span class="pagenum"><a id="Page_303"></a>[Pg 303]</span> -each other, which do not break off, but are all capable of life at the -same time, and so exist simultaneously on different areas; they are -species adapted to different localities, not to different times. The -same is true of the snails of other isolated regions, except that the -chains of forms are usually not simple, but split up into several -chains of forms arising from one ancestral form, and under certain -circumstances each of these may break up into two or more diverging -series. The great number of related species in Madeira, or in the -Sandwich Islands, compels us to this assumption, although the branching -of the genealogical trees can no longer be demonstrated with -certainty.</p> - -<p>This splitting up of forms into several series on a varied insular -region shows us once more that it is germinal selection alone which -forms the basis of all transformations, and that there is not, as earlier -naturalists, especially the botanists Nägeli and Askenasy, maintained, -any peculiar impelling Force of Development innate in organisms. -If there were such a force, a species would be obliged to go on continuously -in the same direction, exactly like the Sarasins' chains of forms, -but no breaking of the species into one or many forms could occur. -But this breaking up into series is easy to understand when we take -germinal selection into consideration, for the germ-plasm contains -many ids and determinants, and each of these can enter upon new -variations, so that one colony can vary in this direction, another in -that, and a great diversity of forms living in isolation must, or at -least may be the result, as we see in the case of the Sandwich Islands.</p> - -<p>Let us delay a moment over the Sarasins' case of the Celebes -snails. We are dealing here with series of forms in regard to which -the ordinary conception of species fails us, for they contain varieties -whose extremes are as far apart as distinct species usually are, which -are not, however, distinct, since they are connected with one another -by one and often by several transition forms. Thus we can only -break them up into two or more 'species' by an arbitrary division -at one place or other. The phenomenon itself is not new to us; we -have seen that even Lamarck and Treviranus made use of similar -series of forms, connected by transition stages, in their attack upon -the old theory of creation, and sought to prove by means of these -that the idea of species is an artificial one, read into nature by man, -and not innate in nature, and that the forms of life were only -apparently fixed and sharply defined, being in reality in process of -slow transformation. Such beautiful and convincing examples as we -now possess were not available at that time, but it might even then be -said that it was the easier to make a new species the fewer examples<span class="pagenum"><a id="Page_304"></a>[Pg 304]</span> -one had to deal with, and the more difficult the more numerous these -became, because with the number of individuals, especially if they -come from a wide area, the number and diversity of the divergences -increases also, so that in many cases, as in that of the Celebes snails, -it becomes impossible to draw a line between the different species.</p> - -<p>There are, however, many animal and plant forms which do not -show such marked divergences, but rather exhibit a great harmony -of individuals even in detail, and the conception of species is more -readily applied to these. It would certainly be foolish to give it up, -since we should then lose all possibility of arriving at any sort of -orientation among the enormous wealth of forms in nature. But at -the same time we must not forget that these 'typical' species only -appear so to our short-sighted vision—short-sighted as far as time is -concerned—and that they are connected from long-past times with -'species' which lived at an earlier date, by just such transition stages -as connect the Celebes species of to-day, which are all living at the -same time. The world of life on the earth only presents at any given -time a 'cross-section of its genealogical tree,' and according as its -branches grow out vertically or horizontally we receive an impression -of typical, sharply defined species or of circles or chains of forms. -In the first case the evolution of new species was associated with the -dying out of the horizontal branches, and the end-twigs of the branch -stand beside each other now apparently isolated and sharply defined; -in the other case only a portion of the ancestral species has been -transmuted, and the other part continues to live alongside of the -species derived from it, and perhaps repeats the process of giving -off a varied race of descendants.</p> - -<p>The last thirty years have yielded much palæontological evidence -of the successive stages of species-transformation. In quietly deposited -horizontal strata of the earth's crust, lying one above another, -the whole phyletic history of a group of snail-species has repeatedly -been found in historic order, the oldest in the deepest layer, the -youngest in the uppermost, and the numerous and often very divergent -'species' of a particular deposit are connected by transition forms -in the intermediate strata. From the point of view of time, therefore, -these are not 'typical' species, but circles of forms in a state of -variability.</p> - -<p>The most beautiful of such cases are the <i>Planorbis</i> species from -the small lacustrine deposits of Steinheim in Swabia, the <i>Paludina</i> -strata of Slavonia, and various groups of Ammonites.</p> - -<p>These cases have been described and discussed so often that -I need only refer to their most essential features.</p> - -<p><span class="pagenum"><a id="Page_305"></a>[Pg 305]</span></p> - -<p>The <i>Planorbis</i> strata of Steinheim were first investigated, from -the point of view of the theory of descent, by Hilgendorf (1866). He -described nineteen different varieties, which, as they are all connected -in chronological succession with each other, he grouped together -under the name of <i>Planorbis multiformis</i>. These little freshwater -snails are found in millions in the strata of the former lake-basin of -Steinheim, and they are arranged in so orderly and regular a manner -that two observers, working independently and at different times, -succeeded in building up the genealogical tree in almost the same -way. According to Alpheus Hyatt, the later investigator, all the -forms are derived from one ancestral form, <i>Planorbis lævis</i>, from -which four different series have descended, one of them splitting up -again into three subordinate series.</p> - -<p>All the individual members of these series are connected by -intermediate forms in such a manner that a long period of constancy -of forms seems to be succeeded by a shorter period of transformation, -from which again a relatively constant form arises.</p> - -<p>We see, therefore, that the idea of species is fully justified in -a certain sense; we find indeed at certain times a breaking up of -the fixed specific type, the species becomes variable, but soon the -medley of forms clears up again and a new constant form arises—<i>a -new species</i>, which remains the same for a long series of generations, -until ultimately it too begins to waver, and is transformed once -more. But if we were to place side by side the cross-sections of this -genealogical tree at different levels, we should only see several well-defined -species between which no intermediate forms could be recognized; -these would only be found in the intermediate strata.</p> - -<p>The problem we have now to discuss is, how it comes about -that relatively sharply defined species exist which are connected with -ancestral forms further back, but which form among themselves an -exclusive, more or less homogeneous, host of individuals. How does -it happen that we everywhere find a specific type, and not an endless -number of individual forms connected with one another in all directions?</p> - -<p>This would require no further explanation if a phyletic evolutionary -force impelled the forms of life to vary in a definite manner, -and thus to become transmuted into new forms in the course of -generations. In that case the whole genealogical tree of the organisms -on the earth must have been potentially contained in the lowest -moneron, so that, given time and the most indispensable general -conditions of existence, the living world just as we know it must have -resulted. Nägeli was the first to express this view, and he followed -it out consistently, not even hesitating to deny the existence of all<span class="pagenum"><a id="Page_306"></a>[Pg 306]</span> -processes of selection, and to represent the whole of evolution as -a process conditioned by this phyletic force, which would have given -rise to the world of organisms which has actually arisen, even if the -conditions of life at the different periods of the earth's history had -been other than they were. I have always combated this idea, without -however overlooking that it is based upon facts which—at that -time at any rate—gave it a certain justification. We cannot pass -it by without giving some other interpretation of the facts. Following -Nägeli, the botanist Askenasy championed this view of 'variation -in a different direction,' which gives rise to new forms; and in more -recent times Romanes, Henslow, and Eimer expressed similar views, -and—although they did not actually dispute the existence of processes -of selection—they attributed a much less important rôle to -them, and referred the phyletic genealogical tree of organisms in the -main to other and internal causes.</p> - -<p>Like Nägeli himself, his followers have laid stress upon the -fact that natural selection cannot be the cause of the evolution and -succession of particular species, because the differences which separate -species from species are not of an adaptive nature, and therefore -cannot depend upon selection; but if the step from one species to the -next succeeding one does not depend upon adaptation, then the -greater steps to genera, families, and orders cannot be referred to it -either, since these can only be thought of as depending upon a long-continued -splitting up of species. Genera, families, and all higher -groups must be recognized as conventional categories, not as real -divisions existing in nature itself. Even Treviranus and Lamarck -maintained that the differences between genera depended just as -much upon our estimate, our intellectual convenience, as do the -differences between species. All forms were originally connected, -though they may not be so now, and if the species are really not -distinguished by adaptive characters, then neither are any other -grades of our classificatory system, neither order nor classes, since -they all depend originally on the transmutation of species. It was -therefore quite consistent of Nägeli to seek the mainspring of organic -evolution, not in adaptation, but in an unknown evolutionary force. Thus -he refused to recognize adaptation as a consequence of selection, but regarded -it, as Lamarck had done, as the direct effect of external conditions, -and as an entirely subordinate factor in the transmutation of forms.</p> - -<p>Nägeli and his modern successors conceive of phyletic evolution -as depending upon definitely directed variation, resulting from internal -causes and occurring at definite times, which of necessity causes -the existing form to be transformed into a new one. To them the<span class="pagenum"><a id="Page_307"></a>[Pg 307]</span> -species appears, so to speak, as a vital crystallization, or to use Herbert -Spencer's phraseology, as an equilibrium of living matter, which -becomes displaced from time to time, and passes over into a new state -of equilibrium, being transmuted into a new species, something like -the pictures in a kaleidoscope. The species is thus something conditioned -from within, which must be as it is and could not be -otherwise, just like a crystal which crystallizes in one particular -system and not in another; it must be just thus or it could not be -at all. From the point of view of this theory it would be easy to -understand that the thousands or millions of individuals composing -a species all agree in essentials—that a specific type exists.</p> - -<p>But this conception can hardly be entirely correct, although -there is some truth at its foundation, namely, that germinal variations -which arise independently are the basal roots of all transmutation. -But the species is not simply the result of these internal processes, it -is not even mainly so; it is not the result of an internal, definitely -directed developmental force, even if we attempt to think out such -a force in a purely scientific or mechanical, instead of a mystical, -sense. It seems clear to me that the species is not a life-crystal in the -sense that it must, like a rock-crystal, take form in a particular way -and in no other for purely internal reasons and by virtue of its -physical constitution; the species is essentially a complex of adaptations, -of modern adaptations which have been recently acquired, -and of inherited adaptations handed down from long ago—a complex -which might quite well have been other than it is, and indeed must -have been different if it had originated under the influence of other -conditions of life.</p> - -<p>But of course species are not exclusively complicated systems -of adaptations, for they are at the same time 'variation-complexes,' -the individual components of which are not all adaptive, since they -do not all reach the limits of the useful or the injurious. All transformations -arise from a basis of spontaneous chance variations, just -as all forest plants grow from the soil of the forest, but do not all -grow into trees, the adaptive forms which determine the essential -character of the forest; for many species remain small and low, like -the mosses, grasses, and herbs; and these too have a share, though -a subordinate one, in determining the character of the forest, which -depends definitely, though only partially, on the loftier growths.</p> - -<p>According to my view all adaptation depends on an alteration in -the equilibrium of the determinant system, such as must arise from intra-germinal -or even general fluctuations in the nutritive supply, affecting -larger or smaller groups of determinants and causing variation in them<span class="pagenum"><a id="Page_308"></a>[Pg 308]</span> -to a greater or less degree. And these variations may be in a quite definite -direction, persisted in for internal reasons, as we have already seen -in the section dealing with germinal selection. These variations are the -building-stones out of which, under the guidance of personal selection, -a new specific type, that is, a new complex of adaptations, can be -established. In this type many indifferent characters are involved, -which are just as constant characters of the species as the adaptations.</p> - -<p>The opponents of the selection theory have often urged against -it this constancy of indifferent characters, but as soon as we cease -to restrict the principle of selection to 'persons,' and extend it also -to the lower categories of vital units, the occurrence of indifferent -characters is easily understood. To illustrate characters of this kind, -Henslow has recently called attention to the species of gentian, whose -flowers have a corona split into five tips in some species and into six -or seven in others, and we cannot possibly ascribe any biological -significance to these specific characters. It is quite possible that they -possess none; but did not even Darwin express his belief that many -peculiarities of form 'are to be attributed to the laws of growth, and -to the mutual influence of parts,' forces which he rightly refrained -from including under 'natural selection' in his sense of the word, but -which we now regard as an expression of intra-selection or of histonal -selection? It is this, in our opinion, which brings about the co-adaptation -of the parts to form a harmonious whole, which admits of the -primary adaptations to the conditions of life being followed or accompanied -by correlative secondary variations, and which plays an -important part in directing the course of every individual development, -and is therefore uninterruptedly active within the organism. -We cannot analyse the factors precisely enough to be able to demonstrate -in an individual case why the corona should be divided into -four in one species of gentian and into five in another, but we can -understand in principle that all adaptations of a species which are not -primary are determined by the compelling influence of intra-selection. -And we need not now rest content even with that, for we know that -this intra-selection—as we have already seen—is active within the -germ-plasm, and it is only a logical consequence of the principle of -germinal selection to suppose that variations of definite determinants -due to personal selection may in the germ-plasm itself give rise to -correlative variations in determinants next to them or related to -them in any way, and that these may possess the same stability as -the primary variation. This seems to me a sufficient reason why -biologically unimportant characters may become constant characters -of the species. Correlation is not effected only in the perfect<span class="pagenum"><a id="Page_309"></a>[Pg 309]</span> -organism; it exists at every period of its life, from the germ till -death, and what it brings about is quite as inevitable as what is -evoked through adaptation by means of personal selection.</p> - -<p>We can thus also understand that indifferent characters may -be contained not only in individual ids of the germ-plasm, but also -coincidentally in a great majority of them, as soon as we think of them -as dependent upon the characters established through personal selection, -for these must be contained in a majority of the ids.</p> - -<p>But there is still another reason why indifferent characters should -become stable, and that is the effect of general variational influences -on all the individuals of a species, as, for instance, in many climatic -varieties, and probably also in many cultivated varieties.</p> - -<p>But even when we have fully recognized that, from the arcana of -the germ-plasm, new minimal variations are continually cropping up, -which are biologically indifferent, and nevertheless become variational -tendencies, and may increase even to the extent of causing <i>visible</i> -differences, and that therefore varieties of snails or of butterflies, or -of any animal or plant whatever, may originate through germinal -selection alone, it cannot for a moment be supposed that the transmutation -of species depends upon this process exclusively or even -preponderantly. This was Nägeli's mistake, and that of his followers -as well, that he ascribed to his 'principle of perfecting' the essential -rôle in directing the whole movement of evolution, while the general -structure of all species shows us that they are, so to speak, built up of -adaptations. But adaptations could not be—or could only be fortuitously -and exceptionally—the <i>direct</i> result of an internal power of -development, since the very essence of adaptational changes is that they -are variations which bring the organism into harmony with the -conditions of its life. We are therefore forced either to underestimate -greatly the part played by adaptation in every organism—and that is -what Nägeli did—or to leave the standpoint of natural science -altogether, and assume a transcendental force which varied and -adapted the species of organisms <i>pari passu</i> with the changes in the -conditions of life during the geological evolution of our earth. This -would be a sort of pre-established harmony, through which the two -clocks of evolution—that of the earth and that of organisms—kept -exact time, although they had quite different and independent works!</p> - -<p>But that the determining significance of adaptations in organic -forms is underestimated even now is evidenced by the continually -repeated statement that species differ, not in their adaptive characters, -but in purely morphological characters, whereas it is obvious that -we are far from being able to estimate the functions of a part<span class="pagenum"><a id="Page_310"></a>[Pg 310]</span> -with sufficient precision to be able to say definitely whether the -differences between two nearly allied species are or are not adaptations -to different conditions. The same is true with regard to the -other side of the problem—the conditions of life. These are often -to all appearances identical in two allied species, but even where -they are visibly different it is often difficult to assert that the -differences between the two species can be interpreted with certainty -as adaptations to the specific conditions of life. At an -earlier stage we discussed the protective coloration of butterflies, and -we saw that the forest butterflies of the Tropics frequently mimicked -a dry leaf on their under surfaces. In the various regions of the -extensive forest districts of the Orinoco and the Amazon in South -America there are fifty species of the genus <i>Anæa</i> alone, and in the -resting pose all these bear a most deceptive resemblance to a leaf, yet -each of them differs from the rest in the mingling of its colours, its -brilliance, and usually in markings when these are present. If we -wished to be able to decide whether these specific differences were of -an adaptive nature or not, we should first of all require to know in -what kind of forest two neighbouring species lived, and in what -places, among what sort of leaves, they were in the habit of settling. -Even then we should at best only know whether the species <i>A</i> was -better protected, as far as our own eyes were concerned, among the -leaves of the forest <i>A´</i> than the species <i>B</i>, and conversely; but we -could not tell whether they <i>required</i> this protection, or whether the -species <i>A</i>, if transferred to the forest <i>B´</i>, would be more frequently -discovered and destroyed by its enemies than in its own forest-home, -and that alone could prove the difference to be biologically important, -that is, to have selection-value. The difficulty, indeed the impossibility, -of arriving at such decisions can perhaps be better illustrated by -an example from our indigenous fauna. No one doubts that the upper -surface of the anterior wing in the so-called banner-moth (<i>Catocala</i>) -possesses a very effective protective colouring; by day the moths rest -with wings spread out flat upon tree trunks, wooden fences, walls, &c., -and they are so excellently suited to their environment that they are -usually overlooked both by man and animals. But each of the twelve -German species of <i>Catocala</i> has a special protective colouring; in <i>Catocala -fraxini</i> it is a light grey, in <i>Catocala nupta</i>, a dark ash-grey, in -<i>Catocala elocata</i> rather a yellowish-brown grey, in <i>Catocala sponsa</i> an -olive brown, in <i>Catocala promissa</i> a mingling of whitish-grey and olive -brown, and so on. All these colourings are protective; but could any -even of our most experienced and sharp-sighted entomologists prove -that each of these different shades of colour depends upon adaptation<span class="pagenum"><a id="Page_311"></a>[Pg 311]</span> -to the usual resting-place of the particular species to which it belongs? -And yet it is on <i>a priori</i> grounds highly probable that this is the -case. But even this would by no means dispose of the whole problem, -for each of these protective colour schemes is composed of several, -often many, tints; it must be so if they are to fulfil their end at all, -for a uniformly coloured wing would contrast with the bark of every -tree and with every wooden fence. The wing-surface must therefore -bear on a lighter background a number of lines and streaks varying -from brown to black, and usually running zigzag across the wing; -beside these are spots of lighter colour, which complete the deceptive -picture. This 'marking' of the wing is similar in all twelve species, -and yet in each it is different in detail. It is constant in each, and -thus is a specific character. But who would venture to undertake the -task of proving that each of these streaks, spots, zigzag lines, &c., is -or is not adaptive—that the details are necessary adaptations to the -resting-place which had become habitual to the species, or, on the -other hand, simply expressions of the variational tendencies of the -elements of marking, depending upon germinal selection? This would -be an impossible task, and yet we are here dealing with a character -which, <i>as a whole</i>, is undoubtedly adaptive; in many of the differences -between other species even that is not certain.</p> - -<p>It seems to me, therefore, hardly reasonable to talk of the 'insufficiency -of natural selection' because we are not able to demonstrate -that the minutiæ of specific characters are adaptational results. -Personal selection intervenes whenever the variations produced by -germinal selection attain to selection-value; and whether we can -determine the exact point at which this takes place in individual -cases is, as I have said before, theoretically quite indifferent.</p> - -<p>Moreover, there are cases in which we <i>can</i> prove that specific -differences are of an adaptive nature. When, of two nearly related -species of frog, the spermatozoon of one possesses a thick head and that -of the other a thin head, and when at the same time the micropyle -through which alone the spermatozoon can make its way into the -ovum is wide in the first species and narrow in the second, we have -before us a specific character which is obviously adaptive.</p> - -<p>In order to gain clearness as to the significance of natural -selection in the restricted sense, that is of personal selection, it seems -to me much more important to study the different groups of animals -and plants with special reference to what they undoubtedly exhibit -in the way of adaptation. For that reason I discussed different -groups of adaptations in detail in some of the preceding lectures, -although, or rather because, they all teach us that every part of every<span class="pagenum"><a id="Page_312"></a>[Pg 312]</span> -species, whether animal or plant, even every secretion, and indeed -every habit, every inherited instinct, is subject to adaptation to the -conditions of life. It seems difficult to refuse to admit that this is the -natural impression which this study conveys, and it is strengthened -as our knowledge increases; <i>that every essential part of a species is -not merely regulated by natural selection, but is originally produced -by it</i>, if not in the species under consideration at the time, then in -some ancestral species; and, further, that every part can adjust itself -in a high degree to the need for adaptation. It was not without -a purpose that I discussed the phenomenon of mimicry so fully, for it, -above all others, teaches us how great a power of adaptation the -organism possesses, and what insignificant and small parts may be -transformed, in a remarkable degree, in accordance with some actual -need. We saw that a butterfly might assume a colouring which -diverged entirely from that of its nearest relatives, but which caused -it to resemble an immune species of a different family, and thereby -protected it more effectively from persecution. Such a case can no -more be due to a dominating phyletic force than to a chance and -sudden displacement of the state of equilibrium of the determinant -system; it can depend only on natural selection, that is, on a sifting -out of the diverse variations offered by germinal selection, and the -unhampered expression and augmentation of those favoured.</p> - -<p>But it is not only these minute variations, insignificant in relation to -the whole structure of the animal, which can be determined by natural -selection. The same applies to the phyletic evolution as a whole; even -that is not directed by the assumed internal principle of development.</p> - -<p>Adaptations, from their very nature, can only depend upon -selection, and not upon an internal principle of evolution, since that -could take no account whatever of external circumstances, but would -cause variations in the organism altogether independently of these. -Thus, in considering the origin of any of the larger groups of animals, -we may exclude a phyletic power as the guide of its evolution as soon -as we can prove that all its essential structural relations, as far as -they diverge from those of nearly related groups, are adaptations. -We may not be able to do this for nearly all of the animal groups, -and it will hardly be possible in regard to a single group of plants, -because our insight into the <i>biological</i> significance of characters, which -means more than the functional significance of the individual parts, -and their correlation as parts of a whole, is seldom sufficiently -intimate or thorough. But among animals we can do this in regard -to some groups; one of these is the order of whales or Cetaceans.</p> - -<div class="figright" id="ff49"> -<img src="images/ff49.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 130.</span> Skeleton of a Greenland<br /> -Whale with the contour of the body.<br /> -<i>Ok</i>, upper jaw. <i>Uk</i>, lower jaw. <i>Sch</i>,<br /> -shoulder-blade. <i>OA</i>, upper arm.<br /> -<i>UA</i>, bones of fore-arm. <i>H</i>, hand.<br /> -<i>Br</i>, vestige of the pelvis. <i>Fr</i>, vestige<br /> -of the femur. <i>Tr</i>, vestige of the lower<br /> -part of the leg. After Claus.</p> -</div> - -<p>Cetaceans, as is well known, belong to the Mammalia, that is to -<span class="pagenum"><a id="Page_313"></a>[Pg 313]</span>say, to a class whose structure was built for life on the land. The -ancestors of Cetaceans were similar -to the other mammals, and possessed -a coat of hair and four legs, and a -body the mass of which was so distributed -that it could be borne by -those four legs. But all the modern -Cetaceans live in the sea, and they -have therefore entirely changed their -bodily form; they have become spindle-shaped -like fishes, well adapted for -cleaving the water, but incapable of -moving upon land. At the same time, -their hind-legs have completely disappeared, -and can now be demonstrated -only as rudiments within the mass of -muscle (Fig. 130, <i>Br</i>, <i>Tr</i>, <i>Fr</i>), while -the fore-legs have been transformed -into flippers, in which, however, the -whole inherited, but greatly shortened, -skeleton of the mammalian arm is -concealed (<i>OA</i>, <i>UA</i>, <i>H</i>). The skin has -lost its covering of hair so completely -that in some cases no traces of it are -demonstrable except in the embryo. -All these changes are adaptations to an -aquatic life, and could not have been -produced independently of the influence -of external conditions. But -there is much more than this. A -thick layer of blubber under the -skin gives this warm-blooded animal -an effective protection against being -cooled down by the surrounding water, -and at the same time gives it the -appropriate specific gravity for life in -the sea; an enormous tail-fin similar -to that of fishes, but placed horizontally, -forms the chief organ of -locomotion, and for this reason the -hind-legs became superfluous and degenerated. -Similarly, the muscles of<span class="pagenum"><a id="Page_314"></a>[Pg 314]</span> -the ear have also disappeared, for the hearing organ of this aquatic type -is no longer suited for receiving the sound-waves through an air-containing -trumpet, but receives them by a shorter route from the -surrounding water, directly through the bones of the skull. Remarkable -changes in the respiratory and circulatory organs make prolonged -submersion possible, and the displacement of the external nares from -the snout to the forehead enables the animal to draw breath when it -comes up from the depths to a frequently stormy surface. It would -take a long time to enumerate all that can be recognized as adaptive -in these remarkable aquatic mammals to a life in what to their -ancestors would have been a strange and hostile element. Let us -study particularly the case of the whalebone whales, for instance the -Greenland whale, and we are at once struck by the enormous size of -the head, which makes up about a third of the whole body (Fig. 130). -Can this, which has such an important effect in determining the -whole type of animal, be an outcome of some internal power of -development? By no means! It is rather an adaptation to the -mode of nutrition peculiar to this swimming mammal, for it does not, -like dolphins and toothed whales, feed on large fishes and Cephalopods, -but on minute delicate molluscs—Pteropods and pelagic Gastropods, -on Salpæ, and the like, which often cover the surface of the -Arctic Ocean in endless shoals, sometimes extending for many miles. -To enable the whale to sustain life on such minute morsels it was -necessary that it should be able to swallow enormous quantities; -teeth were therefore useless, and they have become rudimentary, -and can only be demonstrated in the embryo as rudiments (dental -germs) in the jaw; but in place of these there hang from the roof -of the mouth-cavity great plates of 'whalebone,' a quite peculiar -product of the mucous membrane of the mouth, the ends of which are -frayed into fibres, and form a sieve-net for catching the little animals -which are engulfed with the sea-water. The mouth-cavity itself has -become enormous, so that great quantities of water at a time can be -strained through the net of whalebone-plates.</p> - -<p>When I mention that peculiar changes have occurred also in the -internal organs, that the lungs have elongated longitudinally and thus -enable the animal more readily to lie in the water in a horizontal -position, that peculiar arrangements exist within the nostrils and the -larynx which enable the animal to breathe and swallow simultaneously, -and that the diaphragm lies almost horizontally because of the length -of the lung, I think I have said enough to indicate that not only does -almost everything about the whale diverge from the usual mammalian -type, but that all these deviations are adaptations to an aquatic life. If<span class="pagenum"><a id="Page_315"></a>[Pg 315]</span> -everything that is characteristic, that is, typical either of the order or -of the family to which animals belong, depends upon adaptation, what -room is left for the activity of an internal power of evolution? How -much is left of the whale when the adaptations are subtracted? -Nothing more than the general scheme of a mammal; but this was -implicit in their ancestors before the whales originated at all. But -if what makes whales what they are, that is, the whole 'scheme' of -a whale, has originated through adaptation, then the hypothetical -evolutionary power—wherever its seat may be—has had no share -in the origin of this group of animals.</p> - -<p>I said all this more than ten years ago, but the idea of an -internal directive evolutionary force is firmly rooted in many minds, -and new modifications of the idea are always cropping up, and of -these the most dangerous seem to me to be those which are not clear -in themselves, but suppose that the use of a shibboleth like 'organic -growth' means anything. That organic growth is at the base of the -phyletic evolution of organisms may be maintained from any scientific -standpoint whatsoever, from ours as well as from Nägeli's, for no one -is so extreme and one-sided as to regard the process of evolution as -due <i>solely</i> to internal or <i>solely</i> to external factors. The process may -thus always be compared to the growth of a plant, which likewise -depends on both internal and external influences. But that is saying -very little; we have still to show how much and how little is effected -by these internal and external factors, what their nature precisely -is, and what relation they bear to one another. There is thus a great -difference between believing, with Nägeli, that 'the animal and plant -kingdoms must have become very much what they actually are, even -had there been on the earth no adaptation to new conditions and no -competition in the struggle for existence,' and sharply emphasizing, -in accordance with the facts just discussed, that, in any case, a whole -order of mammals—the Cetaceans—could never have arisen at all if -there had been no adaptation.</p> - -<p>The same thing could be proved in regard to the class of Birds, for -in them too we are able to recognize so many adaptive features, that -we may say everything about them that makes them birds depends -upon adaptation to aerial life, from the articulations of the backbone -to the structure of the skull and the existence of a bill; from the -transformation of the fore-limbs into wings, and of the hind-limbs to -very original organs of locomotion on land or swimming organs in -water, to the structure of the bones, the position, size, and number -of the internal organs, down even to the microscopic structure of -numerous tissues and parts. What could be more characteristic of<span class="pagenum"><a id="Page_316"></a>[Pg 316]</span> -a class of animals than feathers are of birds? They alone are -enough to distinguish the class from all other living classes; an -animal with feathers can now be nothing but a bird, and yet the -feather is a skin-structure which has arisen through adaptation, -a reptilian scale which has been so transformed that an organ -of flight could develop from its anterior extremity. We find it thus -even in the two impressions of the primitive bird <i>Archcæopteryx</i>, -which have been preserved for us in the Solenhofen slate since the -Jurassic period in the history of the earth. And into what detail -does adaptation go in the case of the feathers! Is not the whole -structure, with its quill, shaft, and vane, precisely adapted to its -function, although that is purely passive? What I have just said -of the whole class of Birds holds true for this individual structure, -the feather; everything about it is adaptation, and indeed illustrates -adaptation in two directions, for in the first place the feathers, by -spreading a broad, light, and yet resistant surface with which to beat -the air, act as organs of flight, while they are also the most effective -warmth-retaining covering conceivable. In both these directions -their achievements border on the marvellous. I need only recall the -most recent discovery in this domain, the proof recently given by the -Viennese physiologist, Sigmund Exner, that the feathers become -positively electric in their superficial layer, and negatively electric -in their deeper layer, whenever they rub against one another and -strike the air. But they are rubbed whenever the bird flies or moves, -and the consequence of the contrast in the electric charging of the -two layers is that the covering feathers are closely apposed over the -down-feathers, while, on the other hand, the similar charging of the -down-feathers makes them mutually repel each other, with the result -that a layer of air is retained between them, and thus there is -between the skin and the covering feathers a loose thicket of feathers -uniformly penetrated by air—the most effective warmth-preserver -imaginable. The electric characters of the feathers—and the same is -true of the hairs of animals—are thus not indifferent characters, but -with an appreciable biological importance, and the same is true of the -almost microscopical series of little hooklets which attach the barbs -of the covering feathers to one another, and thus form a relatively -firm but exceedingly light wing-surface which offers a strong resistance -to the air. But as we must regard these hooklets as adaptations, -so must we also regard the electrical characters of the feathers, and -we must think of them as having arisen through natural selection, as -Exner himself has insisted.</p> - -<p>If we are able to recognize all the more prominent features<span class="pagenum"><a id="Page_317"></a>[Pg 317]</span> -of the organization in Cetaceans and Birds as due to adaptation, we -must conclude that, in the rest of the great groups of the animal -kingdom, the main and essential parts of the structure are adaptations -to the conditions of life, even although the relations between external -circumstances and internal organization are not so readily recognizable. -For if there were an internal evolutionary force at all, we should -be able to recognize its operation in the origin of the races of Cetaceans -and Birds; but if there be no such power, then even in cases -where the conditions of life are not so conspicuously divergent as in -Cetaceans and Birds, we must refer the typical structure of the group -to adaptation. Thus everything about organisms depends upon -adaptation, not only the main features of the organization, but the -little details in as far as they possess selection-value; it is only what -lies below this level that is determined by internal factors alone, -by germinal selection; but this is not an imperative force in the -sense in which the term is used by Nägeli and his successors, for it is -capable of being guided; it does not necessarily lead to an invariable -and predetermined goal, but it can be directed according to circumstances -into many different paths. But it is precisely this that -constitutes the main problem of the evolution theory—how development -due to internal causes can, at the same time, bring about -adaptation to external circumstances.</p> - -<p>This lecture had been transcribed so far, and was ready for the -press, when I received the first volume of a new work by De Vries, -in which that distinguished botanist develops new views in regard to -the transformation of species, based upon numerous experiments, -carried on for many years on the variation of plants. As not only -his views, but the interesting facts he sets forth, seem to contradict -the conclusions as to the transmutation of organisms which I have -been endeavouring to establish, I cannot refrain from saying something -on the subject.</p> - -<p>De Vries does not believe that the transformation of species can -depend on the cumulative summation of minute 'individual' variations; -he distinguishes between 'variations' and 'mutations,' and -attributes only to the latter the power of changing the character -of a species. He regards the former as mere fluctuating deviations -which may be increased by artificial selection, and may even, with -difficulty, if carefully and purely bred for a long period, be made -use of to give character to a new breed, but which play no part at all -in the natural course of phylogeny. As regards phylogeny, he maintains -that only 'mutations' have any influence, that is, the larger -or smaller saltatory variations which crop up suddenly and which<span class="pagenum"><a id="Page_318"></a>[Pg 318]</span> -have from the very first a tendency to be purely transmitted, that -is, to breed true.</p> - -<p>The facts upon which these views are mainly based are observations -on and breeding experiments with a species of evening primrose -(<i>Œnothera</i>) which was found in quantities on a fallow potato-field at -Hilversum in Holland. It had been cultivated previously in a neighbouring -garden, and had sown itself thence in the field. The numerous -specimens of this <i>Œnothera lamarckiana</i> growing there were in a -state of marked 'fluctuating' variability, but in addition there grew -among them two strongly divergent forms which must have arisen -from the others, and which led De Vries to bring the parent stock -under cultivation, in the hope that it would yield new forms, in the -Botanical Gardens at Amsterdam. This hope was fulfilled; in the -second cultivated generation there were, among the 15,000 plants, ten -which represented two divergent forms, and in the succeeding generations -these forms were repeated several times and in many cases, and -five other new forms cropped up, most of them in several specimens -and in different generations of the original stock. All these new -forms, which De Vries calls 'elementary species,' breed true, that is to -say, when they are fertilized with their own pollen they yield seed -which gives rise to the same 'elementary species.' The differences -between the new forms are usually manifold, and of the same kind as -those between the 'elementary' species of the wild Linnæan species. -But, according to De Vries, what we have been accustomed since the -time of Linné to call a 'species' is a collective category, whose -components are these 'elementary' species which De Vries has -observed in his experiments with <i>Œnothera</i>. In other species, such -as <i>Viola tricolor</i> and <i>Draba verna</i>, true-breeding varieties have long -been known to botanists, and these have been studied carefully and -tested experimentally, especially by A. Jordan, and more recently by -De Bary.</p> - -<p>All 'species,' according to the Linnæan conception, consist, De -Vries maintains, of a larger or smaller number (in <i>Draba</i> there are -two hundred) of these 'elementary' species, and these arise, as is -proved by the case of <i>Œnothera</i>, by saltatory or discontinuous 'variations' -which occur periodically and suddenly break up a species into -many new species, because the variations of the germ-plasm, which -are for a time merely latent, suddenly find expression in the descendants -of one individual or another. According to this view, species -must be the outcome of purely internal causes of development, which -reveal themselves as 'mutations,' that is as saltatory variations, which -are stable and transmissible from the very first, and among which the<span class="pagenum"><a id="Page_319"></a>[Pg 319]</span> -struggle for existence decides which shall survive and which shall be -eliminated. For the mutations themselves occur in no particular -direction; they are sometimes advantageous, sometimes indifferent, -sometimes even injurious (for instance, when one sex is left out), and -so it is always only a fraction of the mutations, often only a few, -which prove themselves capable of permanent existence. Thus -'species do not arise through the struggle for existence, but they -are eliminated by it' (p. 150); natural selection does nothing more -than weed out what is unfit for existence, it does not exercise any -selective, in the sense of directive, influence on the survivors. A difference -in the nature of variations was previously maintained by -the American palæontologist Scott, though for different reasons and -also with a different meaning. He believed that variations in -a definite direction were necessary to explain the direct course of -development which many animal groups, such as the horses and the -ruminants, have actually followed, and which he thought could not be -ascribed to cumulative adaptation to the conditions of life. The -'mutations' of De Vries are not distinguished from the 'fluctuating' -variations by following a definite direction, but in that they are -strictly heritable, that they 'breed true.' It is true that 'fluctuating' -individual differences are also transmissible, and can be increased by -artificial selection, but they lack one thing that would make them -component parts of a natural species, namely, constancy; they do -not breed true, and are therefore never independent of selection, but -require to be continually selected out afresh in order that they may -be kept pure. They form 'breeds,' not species, and if left to themselves -they soon revert to the characters of the parent species, as is -well known of the numerous 'ennobled races' among our cereals. -De Vries therefore denies absolutely that a new species could be -developed by natural selection from 'fluctuating' variations, and not -alone because there is no constancy of character, but also because the -capacity of the character for being increased is very limited. Usually -nothing more can be achieved than doubling of the original character, -and then progress becomes more difficult and finally ceases altogether.</p> - -<p>These are incisive conclusions, based upon an imposing array of -weighty facts. I readily admit that I have rarely read a scientific -book with as much interest as De Vries's <i>Mutationstheorie</i>. Nevertheless -I believe that one might be carried away too far by De Vries, -for he obviously overestimates the value of his facts, interesting and -important as these undoubtedly are, and under the influence of what -is new he overlooks what lies before him—the other aspect of the -transmutation of species, to which the attention of most observers<span class="pagenum"><a id="Page_320"></a>[Pg 320]</span> -since Darwin and Wallace has been almost exclusively devoted—I -mean the origin of adaptations. Not that he does not mention these, -he assumes in regard to his mutations 'a selection working in a constant -direction,' and seeks to interpret them in terms of it, but as the -mutations occur from purely internal reasons—I mean without any -connexion with the necessity for a new adaptation—and occur only in -a small percentage of individuals, and in no definite direction, they -cannot possibly suffice to explain <i>adaptation</i>, which seems to dominate -the whole organic world. But this is precisely the point at which -many botanists cease to understand the zoologists, because among -plants there are fewer adaptations than among animals; or, in any -case, adaptations in plants are not so readily demonstrated as among -animals, which not infrequently seem to us to be entirely built up -of adaptations.</p> - -<p>In this book, and in this chapter itself, I have discussed adaptations -and their origin so much already that I need only refer to these -pages for convincing evidence that we cannot think of them as being -brought about by the accumulation and augmentation of individually -occurring saltatory 'mutations.' Not even if we assume that the -leaps of mutation can be increased in the course of generations; in -short, even if we say that mutations are all those variations which -breed true and lead to the development of species, while variations -are those which do not. This would only be playing with words, so -let us say that the fluctuating variations are really different in their -nature, that is, in their causes, from mutations. De Vries lays great -stress on the fact that these two kinds of variations must be sharply -distinguished from one another, and this may have been useful or -necessary for the first investigation of the facts before him, for we -must first analyse and then recombine, but that variations and -mutations are in reality different in nature can assuredly not be -assumed, since innumerable adaptations can only have arisen through -the augmentation of individual variations. These must therefore be -able to become 'pure breeding,' even although they may not have -done so in the cases of artificial selection which have hitherto been -observed. How is it possible that chance mutations, in no particular -direction, occurring only rarely and in a small percentage of individuals, -can explain the origin of the leaf-marking of a <i>Kallima</i> or -an <i>Anæa</i>—the shifting of the original wing-nervures to form leaf-veins, -and the exact correlation of these veins across the surfaces of -both pairs of wings? And even if we were to admit that a mutation -might have occurred which caused the veins of the anterior and -posterior wings to meet exactly by chance, that would still not be<span class="pagenum"><a id="Page_321"></a>[Pg 321]</span> -a leaf-adaptation, for there would still be wanting the instinct which -compels the butterfly, when it settles down, to hold the wings in such -a position that the two pictures on the anterior and posterior wings -fit into each other. Correlated mutations of the nervous system -suited to this end are required, but that is too much to attribute to -happy chance! The same holds true in regard to the whole leaf-picture -on the two wings, for it could not possibly have arisen as a whole -by a sudden mutation. The whole litany of objections which have -been urged throughout several decades against the Darwin-Wallace -theory of natural selection, which were based on the improbability -that chance variations not in a definite direction should yield suitable -material for the necessary adaptations, may be urged much more -strongly against mutations, which make their appearance in much -smaller numbers and with less diversity.</p> - -<p>But it is—as we have already seen—in regard to the necessity -which exists almost everywhere for the co-adaptation of numerous -variations of the most different parts, that the 'mutation theory' breaks -down utterly. The kaleidoscopic picture, the mutation, is implicit -from the first, and must be accepted or rejected just as it is in the -struggle for existence; but harmonious adaptation requires a gradual, -simultaneous, or successive purposive variation of all the parts concerned, -and this can be secured only through the fluctuating variations -which are always occurring, and are increased by germinal selection -and guided by personal selection.</p> - -<p>Many naturalists, and especially many botanists, regard adaptation -as something secondary, something given to species by the way, -to improve the conditions of their existence, but not affecting their -nature—comparable perhaps to the clothing worn by man to protect -himself from cold; but that is hardly the real state of the matter.</p> - -<p>The deep-sea expedition conducted by Chun in 1898 and 1899 -made many interesting discoveries in regard to animals living in the -depths of the ocean, all of which exhibit peculiar adaptations to -the special conditions of their life, and especially to the darkness of -the great depths. One of the most striking of these discoveries was -that of the luminous organs which are found not in all but in a great -many animals living on the bottom of the abyssal area, and also among -the animals occurring at various levels above the floor of the abyss. -These are sometimes glands which secrete a luminous substance, but -sometimes complex organs, 'lanterns' which are controlled by the will of -the animal, and suddenly evolve a beam of light and project it in -a particular direction, like an electric searchlight. These organs have -a most complex structure, composed of nerves and lenses, which focus<span class="pagenum"><a id="Page_322"></a>[Pg 322]</span> -the light, and on the whole are not unlike eyes. That this sort of -structure should have arisen all at once through a 'mutation' is inconceivable; -it can have originated only from simple beginnings by -a gradual increase of its structure along with continual strict selection -among the variations which cropped up. They all depend upon complicated -'harmonious' adaptation, and cannot possibly have been -derived from mutations, that is, from ready-made structural 'constellations,' -unless we are to call in the aid of the miraculous. But -lanterns of this kind are found in many different kinds of animals—in -Schizopod Crustaceans, in shrimps, in fishes of different genera and -families. Many fishes have long rows of luminous organs on the sides -and on the belly, and these probably serve to light up the sea-floor -and facilitate the finding of food; in others the luminous organs are -placed upon the snout just above the wide voracious mouth, and in -that position they have undoubtedly the significance attributed to -them by Chun, namely, that they attract small animals, just as the -electric lamps allure all sorts of nocturnal animals, and especially -insects, in large numbers to their destruction. But not fishes only, -but molluscs, e.g. the Cephalopods of the great depths, have developed -luminous organs, and one species of Cephalopod has about twenty -large luminous organs, like gleaming jewels, ultramarine, ruby-red, -sky-blue and silvery, while in another the whole surface of the belly -is dotted over with little pearl-like luminous organs. Even if we -cannot be quite clear as to the special use of these lanterns of deep-sea -animals, there can be no doubt that they are adaptations to the darkness -of the great depths, and when we find the <i>same</i> adaptations (in -a physiological sense) in many animals belonging to the most diverse -groups, there is no possibility of referring them to sudden mutations -which have arisen all at once in these groups with no relation to -utility, and yet have not occurred in any animals living in the -light. Only 'variations' progressing and combining in the direction of -utility can give us the key to an explanation of the origin of such -structures.</p> - -<p>The same is true of the eyes of deep-sea animals. It was believed -at one time that all the inhabitants of dark regions had lost their eyes. -This is the case with many cave animals and the inhabitants of the -lightless depths of our lakes, but in the abyssal zone of the sea it is -only some fishes and Crustaceans whose eyes have degenerated to the -vanishing point. Moreover, the disappearance apparently occurs in -species which are restricted to the ocean-floor in their search for food, -which therefore can make more use of their tactile organs than of -their eyes, for while the ocean-floor undoubtedly contains over wide<span class="pagenum"><a id="Page_323"></a>[Pg 323]</span> -areas an abundance of food for these mud-eaters, it is only partly -illuminated, that is, only in places where there are luminous animals -such as polyp-colonies, &c. The fact that so many of the animals -of the great depths are luminous obviously conditions, not that most -of the immigrants into the abyssal zone should lose their eyes as -useless, but that they should adapt them to the light which is very -weak in comparison with that of the superficial layers. The eyes of -deep-sea fishes, for instance, are either enormously large, and therefore -suited for perceiving the faint light of the depths, or they have varied -in another and very characteristic manner: they have become -elongated into a cylinder, which projects far beyond the level of the -head. It looks almost as if the animals were looking through an -opera-glass, and Chun has called these eyes 'telescope-eyes.' A. Brauer -has recently shown what far-reaching variations of the original eye -of fishes were necessary in order to transform it into an organ for -seeing in the dark. These variations, however, have occurred in the -eyes of the most diverse animals in the deep sea, and not only do -different families of deep-sea fishes possess 'telescope-eyes,' but Crustaceans -and Cephalopods as well. Even our owls possess quite a -similar structure, although it does not project beyond the head in the -same way. Here again we have to deal with the phenomenon which -Oscar Schmidt in his time called <i>convergence</i>, that is, corresponding -adaptations to similar conditions in animal forms not genealogically -connected with one another. These telescope-eyes are not all descended -from one species which chanced in one of the 'mutation-periods' suddenly -to produce this combination of harmonious adaptations, but -they have risen independently through variation progressing step by -step in the direction of the required end, that is to say, through -natural selection based upon germinal selection. Only thus can their -origin be understood.</p> - -<p>But what is time of eyes adapted to darkness is true in some -measure of all eyes, for the eyes of animals are not mere decorative -points which might be present or absent; they cannot have arisen -in any animal whatever through sudden mutation—they have been -laboriously acquired with difficulty, by the slow increase of gradually -perfecting adaptations; they are parts which bear the most precise -internal correlation with the whole organization of the animal, and -which can only cease to exist when they become superfluous. Thus -the origin of eyes seems to me only conceivable on the basis of -germinal selection controlled towards what is purposeful by natural -selection, that is to say, on a basis of fluctuating variation, and not -through chance.</p> - -<p><span class="pagenum"><a id="Page_324"></a>[Pg 324]</span></p> - -<p>This is the case with all adaptations. Just as the eyes of animals -are adaptations which utilize the light-waves in the interest of the -organism and its survival, the same is true of all the sense-organs, -tactile organs, smelling and tracking organs, organs of hearing, and -so on. The animal cannot do without these; first the lower sense-organs -arose and then the higher; the increasingly high organization -of the animal conditioned this, and a multicellular animal without -sensory structures is inconceivable. The same may be said of the -nervous system as a whole, whose function it is to translate into action -the stimuli received through the sense-organs, whether directly or by -means of intervening nerve-cells, which form central organs of ever-increasing -complexity of composition. As telescope-eyes have evolved -in some groups of deep-sea animals, independently of one another, -and certainly not through the fortuitous occurrence of a mutation, -but under the compulsion of necessity in competition, so all the organs -we have just named, the whole nervous system with all its sense-organs, -must have arisen through the same factors of evolution in -numerous independent genealogical lines. And it must not be -supposed that this is all; what is true of the sense-organs—that they -are necessities—is undoubtedly true also of all parts and organs of -the animal body, both as a whole and in every detail. It cannot be -demonstrated in all cases, but it is nevertheless certain that this -applies also to all the organs of movement, digestion, and reproduction, -to all animal groups and also to the differences between them, even -although these may not always be obvious adaptations to the conditions -of life. What part is left for mutation to play if almost -everything is an adaptation? Possibly the specific differences; and -these in point of fact cannot in many cases be interpreted with -certainty as adaptations, though this can hardly be taken as a proof -that they are not. Possibly also the geometrical skeletons of many -unicellulars, in which again we cannot recognize any definite relation -to the mode of life. It is easy enough to conceive of the wondrously -regular and often very complex siliceous skeleton of the Radiolarians -or Diatoms as due to saltatory mutations, and 'leaps' of considerable -magnitude must certainly have been necessary to produce some of the -manifold transformations here as everywhere else. But whether -these are or are not without importance for the life of the organisms, -we are in the meantime quite unable to decide. Here too it is well to be -cautious in concluding that these organic 'crystallizations' are without -importance, and therefore to infer that they have arisen suddenly -from purely internal causes. One of the experts on Diatoms, F. Schütt, -has shown us that differences in length in the skeletal process of<span class="pagenum"><a id="Page_325"></a>[Pg 325]</span> -the Peridineæ have a definite relation to their power of floating in -the sea-water, that the long skeletal arms or horns which these -microscopic vegetable organisms extend -into the surrounding water form a -float-apparatus, for their friction against -the particles of the water prevents -sinking and enables them to float for -a considerable time at approximately -the same level. These skeletal forms -are thus adaptations, and Chun has -recently been able to corroborate the -conclusion that this adaptation is exactly -regulated, for the length of these horns -varies with the specific gravity of the -different ocean-currents, species with -'monstrously long' horns occurring, for -instance, in the Gulf of Guinea, which -is distinguished by its low salinity -and high temperature (Fig. 131, <i>A</i>), -while in the equatorial currents with -higher salinity and cooler water, and -thus a higher specific gravity, there is -a predominance of species of Peridineæ -with 'very short' processes and relatively -undeveloped float-apparatus (Fig. 131, -<i>B</i>). It could be seen clearly in the -course of the voyage that the long-armed -Peridineæ became more abundant -as the ship passed from the North -Equatorial current into the Gulf of -Guinea, and that by and by they held -the field altogether, but later, when the -'Valdivia' entered into the South Equatorial -current, they disappeared 'all at -once.' Thus in this case, in which the -veil over the relations between form -and function in unicellular organisms -has been lifted a little, we recognize -that the smallest parts of the cell-body -obey the laws of adaptation, and consistent thinking must lead us -to the conviction that even in the most lowly organisms the whole -structure in all its essential features depends upon adaptation.</p> - -<div class="figright" id="ff50"> -<img src="images/ff50.jpg" alt="" /> -<p class="caption1"><span class="smcap">Fig. 131.</span> Peridineæ: species of<br /> -<i>Ceratium</i>. <i>A</i>, from the Gulf of<br /> -Guinea. <i>B</i>, from the South Equatorial<br /> -currents. After Chun.</p> -</div> - -<p><span class="pagenum"><a id="Page_326"></a>[Pg 326]</span></p> - -<p>If the horns of the Peridineæ grow to twelve times the usual -length in adaptation to life in sea-water with a salinity increased to -the extent of .002 per cent., then undoubtedly not only the protoplasmic -particles of the body which form the horns, but all the rest -as well, may be capable of adaptation; and if the <i>Peridinium</i> protoplasm -has this power of adapting itself to the external conditions, -then the capacity for adaptation must be a general character of -all unicellular organisms, or rather of all living substance. As will -be seen later on, we shall be brought to the same conclusion by -different lines of evidence. But a recognition of this must greatly -restrict the sphere of operation which we can attribute to saltatory -mutations in the sense in which the term is used by De Vries, for -adaptations from their very nature cannot arise suddenly, but must -originate gradually and step by step, from 'variations' which combine -with one another in a definite direction under the influence -of the indirect, that is, selective influence of the conditions.</p> - -<p>According to the theory of De Vries it seems as if 'variations,' -augmented by selection, could never become constant, and that even -the degree to which they can be augmented is very limited. As far -as this last point is concerned, De Vries seems to me to overlook the -fact that every increase in a character must have limits set by the -harmony of the parts, which cannot be exceeded unless other parts -are being varied at the same time. Artificial selection, in fact, -in many cases reaches a limit which it cannot pass, because it has -no control over the unknown other parts which ought to be varied, -in order that the character desired may be increased still further. -Natural selection would in many cases be able to accomplish this, -provided that the variation is useful. But of what use is it to the -beetroot when its sugar-content is doubled, or to the Anderbeck -oats to be highly prized by man? And yet many individual -characters have been very considerably increased in domesticated -animals by selection: of these we need only call to mind the Japanese -cock with tail-feathers twelve feet long.</p> - -<p>But undoubtedly these artificial variations do not usually 'breed -true' in the sense that De Vries's mutations of <i>Œnothera lamarckiana</i> -did, that is to say, they only transmit their characters in purity with -the continual co-operation of artificial selection. This at least appears -to be the case, according to De Vries, in the ennobled cereal races, -which, if cultivated in quantities, rapidly degenerate. In many -animal breeds, however, this is not the case to the same degree; many, -indeed the majority, of the most distinct races of pigeon breed true, -and only degenerate when they are crossed with others.</p> - -<p><span class="pagenum"><a id="Page_327"></a>[Pg 327]</span></p> - -<p>De Vries regards it as a mistake to believe that artificial selection, -persevered in for a long time, will succeed in producing a breed which -will—as he expresses it—be independent of further selection and will -maintain itself in purity. Experience cannot decide this, as we have -not command over the unlimited time necessary for selection, but -theoretically it is quite intelligible that a variation which had arisen -through selection would be more apt to breed true the longer selection -was practised, and there is nothing to prevent it becoming ultimately -quite as constant as a natural species. For, at the beginning of -breeding, we must assume that the variation is contained in only -a small number of ids; as the number of generations mounts up, more -and more numerous ids with this variation will go to make up the -germ-plasm, and the more the breed-ids preponderate the less likelihood -will there be that a reversion to the parent-form will be brought -about by the chances of reducing-division and amphimixis. That -most if not all breeds of pigeon still contain ids of the ancestral form -in the germ-plasm, although probably only a small number of them, -we see from the occasional reversion to the rock-dove which occurs -when species are repeatedly crossed, but that ancestral ids may also -be contained in the germ-plasm of long-established natural species is -shown by the occurrence of zebra-striping in horse-hybrids. We can -understand why these ancestral ids should not have been removed -long ago from the germ-plasm by natural selection, since they are not -injurious and may remain, so to speak, undetected. It is only when -they have an injurious effect by endangering the purity of the -new species-type that they can and must be eliminated by natural -selection, and this does not cease to operate, as the human breeder does, -but continues without pause or break.</p> - -<p>I therefore regard it as a mistake on the part of De Vries to -exclude fluctuating variation from a share in the transformation of -organisms. Indeed, I believe that it plays the largest part, because -adaptations cannot arise from mutations, or can only do so exceptionally, -and because whole families, orders, and even classes are -based on adaptations, especially as regards their chief characters. -I need only recall the various families of parasitic Crustaceans, the -Cetaceans, the birds, and the bats. None of these groups can have -arisen through saltatory, perhaps even retrogressive, 'mutation': they -can only have arisen through variation in a definite direction, which -we can think of only as due to the selection of the fluctuations of the -determinants of the germ-plasm which are continually presenting -themselves.</p> - -<p>The difference between 'fluctuating' variability and 'mutation'<span class="pagenum"><a id="Page_328"></a>[Pg 328]</span> -seems to me to lie in this: that the former has always its basis only -in a small majority of the ids of the germ-plasm, while the mutation -must be present in most of the ids if it is to be stably transmitted -from the very first. How that comes about we cannot tell, but we -may suppose that similar influences causing variation within the -germ-plasm may bring about variation of many ids in the same -direction. I need only recall what I have already said as to the -origin of saltatory variations, such as the copper-beech and similar -cases. The experiments made by De Vries seem to me to give -a weighty support to my interpretation of these phenomena. De -Vries himself distinguishes a 'pre-mutation period,' just as I have -assumed that the variations which spring suddenly into expression have -been in course of preparation within the germ-plasm by means of -germinal selection for a long time beforehand. At first perhaps only -in a few ids, but afterwards in many, a new state of equilibrium of -the determinant-system would be established, which would remain -invisible until the chance of reducing division and amphimixis gave -predominance to a decided majority of the 'mutation-ids.' In the -experiments made by De Vries the same seven new 'species' were -produced repeatedly and independently of one another in different -generations of <i>Œnothera lamarckiana</i>, and we thus see that the same -constellations (states of equilibrium) had developed in many specimens -of the parent plant, and that it depended on the proportion in which -the ids containing these were represented in the seed whether one or -another of the new 'species' was produced.</p> - -<p>My interpretation, according to which a larger or smaller number -of ids were the bearers of the new forms, receives further support -from the experiments, for the new species did not always breed true. -Thus De Vries found one species, <i>Œnothera scintillans</i>, which only -yielded 35-40 per cent. of heirs, or in another group about 70 per -cent.; the other descendants belonged to the forms <i>lamarckiana</i> or -<i>oblonga</i>, but the number of pure heirs could be increased by -selection!</p> - -<p>I cannot devote sufficient space to go fully into these very -interesting experiments; but one point must still be referred to: -the parent form, <i>Œnothera lamarckiana</i>, was very variable from the -beginning, that is, it exhibited a high degree of fluctuating variability. -This tells in favour, on the one hand, of a deep-rooted connexion -between 'variation' and 'mutation,' and, on the other hand, it -indicates that saltatory variation may be excited by transference to -changed conditions of life—as Darwin in his day supposed, and as -I have endeavoured to show in the foregoing discussion. De Vries<span class="pagenum"><a id="Page_329"></a>[Pg 329]</span> -assumes mutation-periods, I believe rightly; but they are not periods -prescribed, so to speak, from within, as those who believe in a -'phyletic force' must suppose; they are caused by the influences of the -environment which affect the nutritive stream within the germ-plasm, -and which, increasing latently, bring about in part mere variability, -in part mutations, just as I have indicated in the section on Isolation -(vol. ii. p. <a href="#Page_280">280</a>), and indeed, in one of my earliest contributions to the -theory of descent<a id="FNanchor_30" href="#Footnote_30" class="fnanchor">[30]</a>. In that essay I suggested the conclusion that -periods of constancy alternate with periods of variability, basing my -opinion on general considerations, and on Hilgendorf's study of the -Steinheim snail-shells. According to De Vries's <i>Œnothera</i> experiments -we may assume that periods of increased variability may lead to the -marked variations sometimes affecting several characters simultaneously, -and occurring in many ids, which have hitherto been called -'saltatory' variations, and which we should perhaps do well to call -in future, with De Vries, <i>mutations</i>.</p> - -<div class="footnote"> - -<p><a id="Footnote_30" href="#FNanchor_30" class="label">[30]</a> <i>Ueber den Einfluss der Isolirung auf die Artbildung</i>, Leipzig, 1872, p. 51.</p> - -</div> - -<p>We cannot yet determine how far the influence of such mutations -reaches. I think it is plain that De Vries himself overestimated it, -but how many of the species-types which we find to-day depend upon -mere mutation can only be decided with any certainty after further -investigations. For the present it is well to be clear as to the validity -of the general conclusion, that all 'complex,' and especially all -'harmonious,' adaptation, must depend, not upon 'mutation,' but -only upon 'variation' guided by selection. As species are essentially -complexes of adaptation, originating from a basis of previous complexes -of adaptation, there remains, as far as I can see, only the small field of -indifferent characters to be determined by mutations, unless indeed -we are to include under the term 'mutation' all variations which gain -stability, but this would be merely a play upon words. In my opinion -<i>there is no definable boundary line between variation and mutation, -and the difference between these two phenomena depends solely on the -number—larger or smaller—of ids which have varied in the same -direction</i>.</p> -<hr class="full" /> - -<div class="chapter"> -<p><span class="pagenum"><a id="Page_330"></a>[Pg 330]</span></p> - -<h2 class="nobreak" id="LECTURE_XXXIV">LECTURE XXXIV</h2> -</div> - -<p class="c">ORIGIN OF THE SPECIFIC TYPE (<i>continued</i>)</p> - -<div class="blockquot"> - -<p>Illustration of phyletic evolution by an analogy—Reconciliation of Nägeli and -Darwin—Unity of the specific type furthered by climatic variation—By natural -selection: illustration from aquatic animals—Direct path of evolution—Natural selection -works in association with amphigony—Influence of isolation in defining the -specific type—Duration of the periods of constancy—The Siberian pine-jays—Species -are, so to speak, 'variable crystals'—Gradual increase of constancy and decrease of -reversions—Physiological segregation of species through mutual sterility—Romanes's -physiological selection—Breeds of domestic animals mutually fertile, presumably -therefore 'amiktic' species also—Mutual fertility in plant species—Mutual sterility -certainly not a condition of the splitting up of species—Splitting up of species without -amphigony—Lichens—Splitting up of species apart from isolation and mutual sterility, -<i>Lepus variabilis</i>.</p></div> - - -<p><span class="smcap">Our</span> train of thought in the last lecture brought us back again -to the so-called 'indifferent' characters, whose occurrence is so often -used as evidence against natural selection, as a proof that evolution -is guided essentially by internal forces alone. But this objection was -based on a fallacy, for the fact that the first small variations are due -to internal processes in the germ-plasm does not imply that the whole -further course of evolution is determined by these alone, any more -than the fact that a sleigh requires a push to start it on its descent -down an inclined plane would imply that its rapid descent is due only -to the force of the push, and not at the same time to the attraction -of gravity. But the analogy is not quite sound, for the processes -within the germ-plasm which condition and direct variation do not -merely give them the first shove off; they are associated with every -onward step in the evolutionary path of the species, they impel it -further and further, and without these continual impulses the progressive -movement would cease altogether. We have seen that, for -internal reasons, germinal selection continues to impel the varying -determinants further along the path on which they have started, and -thus gives them cumulative strength, and that it is in this way that -adaptations to the conditions of life are brought about. The evolution -of the character of a species may thus be compared to the course of -a sleigh upon a level snow-surface which it could traverse in any -direction, but it is moved only by the impulses received through -germinal selection. The conditions of life to which the varying parts<span class="pagenum"><a id="Page_331"></a>[Pg 331]</span> -have to adapt themselves may be thought of as the distant goal, -and the processes in the germ-plasm which give rise to variation in -a definite direction may be compared to numerous human beings -scattered irregularly over the surface of the snow. If the sleigh -receives from one of these a push which chances to be in the direction -of the goal, it rushes on towards this and ultimately reaches it if the -person pushing continues to push in the same direction. So far, then, -it seems as if the transformation of the part concerned depended upon -germinal selection alone, but we must not forget that the germ-plasm -does not contain only one determinant for every part of the body, but -as many determinants as there are ids. We must therefore increase -the number of our sleighs, and now it is obvious that the pushers of -the sleighs, that is, germinal selection, may push one sleigh on toward -the goal, but others in the opposite or in any other direction. If we -assume that all the sleighs which have taken a wrong direction must -reach dangerous ground, and ultimately plunge into an abyss, but -that from a neighbouring point sleighs were being dispatched to -replace all that came to grief, that these in their turn might attempt -to reach the goal, it would ultimately come about that the requisite -number of sleighs would arrive at the goal—that is to say, that the -new adaptation would be attained.</p> - -<p>The abysses represent the elimination of the less favourable -variational tendencies, and the constant replacing of sleighs represents -the intermingling of fresh ids through amphimixis. If all the sleighs -run in the wrong direction they all come to grief, that is, the individual -concerned is eliminated with all the ids of its germ-plasm—it disappears -altogether from the ranks of the species. But if only -a portion of them run in the right direction, care is taken that in -following generations, that is, in the continuation of the sleigh-race, -this portion combines with those of another group which are also -running in the right direction—that is, with the half number of ids -from another germ-plasm in amphimixis.</p> - -<p>It is not possible to follow the analogy further, but perhaps it -may serve to illustrate how germinal selection may be the only -impelling force in the organisms, and yet only a small part of its -results are determined by itself, and by far the larger part by external -conditions. We understand how a variation in a definite direction -can exist, and yet it is not that which creates species, genera, orders, -and classes; it is the selection and combination of the variational -tendencies by the conditions of life, which occurs at every step. There -was no variational tendency leading from terrestrial mammals to -Cetaceans, but there was a variational tendency moving the nostrils<span class="pagenum"><a id="Page_332"></a>[Pg 332]</span> -upwards towards the forehead, the hind-limbs towards diminution, -the lungs towards lengthening, the tail towards broadening out. But -each of these variational tendencies was always only one of several -possibilities, and that the particular path which led towards the -'goal' was followed was due to the fact that the others plunged into -the abyss to which the wrong paths led, that is to say, they were -weeded out by selection. Thus germinal selection offers a possibility -of reconciliation between Nägeli's and Darwin's interpretations, which -seem so directly contradictory; for the former referred everything -to the hypothesis of an internal evolutionary force, the latter rejected -this, and regarded natural selection as the main, if not the exclusive -factor in evolution. The internal struggles for food, which we have -assumed as occurring in the germ-plasm, represent an internal force, -though not in the sense of Nägeli, who thought of determining -influence operative from first to last, but still an impelling force, -which determines the direction of variation for the individual determinants, -and must therefore do the same for the whole evolution -up to a certain point; for it is only the <i>possible</i> variations of the -determinants in a germ-plasm which can be chosen, selected, combined, -and increased by natural selection, and every germ-plasm cannot give -rise to all sorts of variations; the determinants contained in it condition -what is possible and what is not, and this is an important -limitation to the efficacy of natural selection, and to a certain extent -also implies a guiding and determining power on the part of the -internal mainspring, to wit, <i>germinal selection</i>.</p> - -<p>The essential difference between Darwin's view of the transformation -of forms and my own lies in the fact that Darwin conceived -of natural selection as working only with variations which are not -only due to chance themselves, but the intensification of which also -depends in its turn solely upon natural selection, while, according -to my view, natural selection works with variational tendencies which -become intensified through internal causes, and are simply accumulated -by natural selection in an ever-growing majority of ids in a germ-plasm -through the selection of individuals.</p> - -<p>This affects our view of the establishment of a specific type -in so far that my intra-germinal variational tendencies are not -necessarily, and not always due to chance, though they are so in most -cases. If certain determinants are impelled to vary in a particular -direction through climatic or any other influences, as we have seen -to be the case, for instance, with the climatic varieties of many -Lepidoptera, then the corresponding determinants in all the individuals -must vary in the same direction, and thus all the individuals of the<span class="pagenum"><a id="Page_333"></a>[Pg 333]</span> -species which are subject to the same influence must undergo the -same variation. Transformations of this kind have exactly the -appearance of resulting from 'an internal evolutionary force,' such as -Nägeli assumed, and the unity of the specific type will not be disturbed -by them.</p> - -<p>Nor will this occur, as far as I can see, if the transformation -of a species depends solely upon new adaptations and their internal -consequences, for if a particular organism has to adapt itself to special -new conditions, it will usually be able to do so only in <i>one</i> way, and -thus natural selection will always allow <i>the same</i> suitable variational -tendencies to survive and reproduce, so that the unity of the specific -type will not be permanently disturbed in this way either. The more -advantageous the new conditions of life prove, and the more diverse -the ways in which they can be utilized, the more rapidly will the -species first adapted to them multiply, and the more will their -descendants be impelled to adapt themselves specially to the <i>different</i> -possibilities of utilizing the new situation, and thus, from a parent -species adapted in general to the new conditions, there arise forms -adapted to its more detailed possibilities. I must refer again to the -previous instance of the Cetaceans which originated from vegetarian -littoral, or fluviatile mammals, and have evolved since the Triassic -period into a very considerable number of species-groups. All are -alike in their <i>general</i> adaptation, and these adaptations to the conditions -of life of aquatic animals—the fish-like form, the flippers, the -peculiarities of the respiratory organs and the organs of hearing—when -once acquired would not and could not be lost again; but each -of the modern groups of whales has its particular sphere of life, -which it effectively exploits by means of subordinate adaptations. -Thus there are the dolphins with their bill-like jaws and the two -rows of conical teeth, their active temperament, rapid movements, and -diet of fish; and the whalebone whales with their enormous gape, -the sieve-apparatus of whalebone-plates, and a diet of small molluscs -and the like. But each of these groups has split up into species, and -if we again regard the principle of adaptation as determining and -directing evolution, we are no nearer being able to prove the assumption -in regard to individual cases, for we know too little about the -conditions of life to be able to demonstrate that the peculiarities -of structure are actually adaptations to these. Theoretically, however, -it is quite easy to suppose that adaptation to a particular sphere of life -was the guiding factor in their evolution, and if this be so—as we have -already proved that it is in regard to the two chief groups and the -whole class—then the harmony of the structure must be due solely<span class="pagenum"><a id="Page_334"></a>[Pg 334]</span> -to the continued selection of the fittest. We require no further principle -of explanation for the establishment of a specific type.</p> - -<p>This 'type' is thus not reached by any indefinite varying of -the parent form in all directions, but in general it is reached by the -most direct and shortest way. The parent form must indeed have -become to some extent fluctuating, since not only the variational -tendencies 'aiming at the goal,' but others as well, must have emerged -in the germ-plasm; but gradually these others would occur less and less -frequently, being always weeded out afresh by selection, until the -great majority of the individuals would follow the same path of -evolution, under the guidance of germinal selection, which continues -to work in the direction that has once been taken. After a short -period of variation, which need not, of course, involve the whole -organism, but may refer only to certain parts of it, a steady direct progress -in the direction of the 'goal,' that is, of perfect adaptation, will set -in, as we have seen in the case of the <i>Planorbis</i> snails of Steinheim.</p> - -<p>We must not forget, however, that natural selection works -essentially upon a basis of sexual reproduction, which with its reduction -of the ids and its continually repeated mingling of germ-plasms, -combines the existing variational tendencies, and thus diffuses them -more and more uniformly among the individuals of a whole area -of occupation. Sexual reproduction, continual intermingling of the -individuals selected for breeding, is thus a very effective and important, -if not an indispensable, factor in the evolution of the specific type.</p> - -<p>But it is not only in the case of species transformations due -to new adaptations that sexual mingling operates; it does so also -in the case of variations due to purely intra-germinal causes. We -have already seen in discussing Isolation that isolated colonies may -come to have a peculiar character somewhat different from that of the -parent form, because they were dominated by some germinal variational -tendency which occurred only rarely in the home of the parent form, -and therefore never found expression there. On the isolated area -this would indeed be mingled with the rest of the existing germinal -variational tendencies, but the result of this mingling would be -different, and the further development of the tendency in question -would probably not be suppressed.</p> - -<p>We need not wonder, therefore, that specific types occur in such -varying degrees of definiteness. If a species is distributed over -a wide connected territory, sporadically, not uniformly, it will depend -partly upon the mutual degree of isolation of the sporadic areas -whether the individual colonies will exhibit the same specific type -or will diverge from one another. If the animal in question is a slow-<span class="pagenum"><a id="Page_335"></a>[Pg 335]</span>moving -one like a snail, the intermingling from neighbouring sporadic -colonies will be much slower than in the case of a resident bird such -as a woodpecker. Many interesting results would undoubtedly be -gained if the numerous careful investigations into the geographical -distributions of species and their local races were studied with -special reference to this point, and much light would undoubtedly -be thrown upon the evolution of the specific type. But it would -be absolutely necessary to study carefully all the biological relations -of the animals concerned, to trace back the history of the species -as far as possible, and to decide the period of immigration, the mode -and direction of distribution, and so on.</p> - -<p>Nothing shows more plainly the enormous duration of the period -of constancy in species than the wide distribution of the same specific -type on scattered areas or even over different areas absolutely isolated -from one another. If, as we saw, the same diurnal butterflies live -in the Alps and the far North, they must have remained unvaried -since the Glacial period, for it was the close of that period that brought -them to their present habitats, and while other diurnal butterflies -now living on the Alps differ from their relatives in the Arctic zone -(Lapland, Siberia, and Labrador) in some unimportant spot or line, -and must therefore have diverged from one another in the course -of the long period since the Glacial epoch, they have done so only -to a minimal degree, and in characters which possibly depend solely -upon germinal selection and can hardly be regarded as adaptations.</p> - -<p>I should like, however, to cite one of the few cases known to me -in which a slight deviation from the specific type undoubtedly -depending upon adaptation has occurred on an isolated region. The -nut-jay (<i>Caryocatactes nucifraga</i>) lives not only upon our Alps and in -the Black Forest, but also in the forests of Siberia, and the birds -there differ from those with us in small peculiarities of the bill, which -is longer and thinner in them, shorter and more powerful in ours. -Ornithologists associate this difference with the fact that in this -country the birds feed chiefly on hard hazel nuts, which they break -open with their bill, and on acorns, beechmast, and, in the Alps, on -cembra-cones, while in Siberia, where there are no hazel nuts, -they feed chiefly upon the seeds of the Siberian cedar, which are -concealed deep down in the cones. Thus we find that in Siberia -the bill is slender, and that the upper jaw protrudes awl-like -beyond the lower, for about 2.5 mm., and probably serves chiefly -to pick out the cedar nuts from behind the cone-scales. In the -Alps the birds (var. <i>pachyrhynchus</i>) break up the whole cone of -the cembra-pine with their thick, hard bill, and in the Upper<span class="pagenum"><a id="Page_336"></a>[Pg 336]</span> -Engadine, where the nut-jay is abundant, I have often seen the -ground underneath the cembra-pines covered with the débris of -its meal. In addition to these differences between the two races, -the Alpine form is stronger in build, the Siberian form is daintier; -in the former the white terminal band on the tail is narrow (about -18 mm.), in the Siberian form it is broader (about 27 mm.).</p> - -<p>Such cases of variation of individual parts in different areas -seem to me very important theoretically, because they furnish us -with an answer to the view which represents the species as a 'life-crystal,' -which must be as it is or not at all, and which therefore -cannot vary as regards its individual parts. The case of the nut-jay -has the further interest that it is one of the few in which we find -the new adaptation of a single character without variation of most -of the other characters.</p> - -<p>It is only in an essentially different sense that we can compare -the species, like any other vital unit, to a crystal, in so far as its -parts are harmoniously related one to another, or, as I expressed -myself years ago, are in a state of equilibrium, which must be -brought about by means of intra-selection. This analogy, however, -only applies to the actual adjustment of the parts to a whole, and -not to their casual adjustment. Species are variable crystals; the -constancy of a species in all its parts must be regarded as something -quite relative, which may vary at any time, and which is sure to -vary at some time in the course of a long period. But the longer -the adaptation of a species to new conditions persists the more -constant, <i>ceteris paribus</i>, and the more slowly variable will it become, -and this for two reasons: first, because the determinants which are -varying in a suitable direction are being more and more strictly -selected, more and more precisely adapted, and are thus becoming -more like each other; and secondly, because, according to our theory, -the homologous determinants of all the ids do not vary in the -required direction, and a portion of the unvaried ancestral ids is -always carried on through the course of the phylogeny, and only -gradually set aside by the chances of reducing division. But the -more completely these unvaried ids are eliminated from the germ-plasm -the less likely will they be to find expression in reversions -or in impurities of the new specific characters. I may recall the -reversions of the various breeds of pigeons to the rock-dove, those -of the white species of <i>Datura</i> to the blue form, and the <i>Hipparion</i>-like -three-toed horse of Julius Cæsar, and so on. The unvaried -ancestral ids, which in these cases find only quite exceptional -expression, will make the new 'specific character' fluctuating, as<span class="pagenum"><a id="Page_337"></a>[Pg 337]</span> -long as they are contained in the germ-plasm in considerable numbers, -but they must become more and more infrequent in the germ-plasm -as successive generations are passed through the sieve of natural -selection, and the oftener these germ-plasms, to which the chances -of reducing division and amphimixis have assigned a majority of -the old determinants, are expelled from the ranks of the species -by personal selection. The oftener this has occurred in a species -the less frequently will it recur, and the more constant, <i>ceteris paribus</i>, -will the 'type' of the species become.</p> - -<p>If we add to this idea the fact that adaptations take place very -slowly, and that every variation of the germ-plasm in an appropriate -direction has time to spread over countless hosts of individuals, we -gain some idea of the way in which new adaptations gradually bring -about the evolution of a more and more sharply-defined specific type.</p> - -<p>So far, however, we have only explained the morphological -aspect of the problem of the nature of species, but there is also -a physiological side, and for a long time this played an important -part in the definition of the conception of species. Until the time -of Darwin it was regarded as certain that species do not intermingle -in the natural state, and that, though they could be crossed in rare -cases, the progeny would be infertile.</p> - -<p>Although we now know that these statements are only relatively -correct, and that in particular there are many higher plants which -yield perfectly fertile hybrids, it is nevertheless a striking phenomenon -that among the higher animals, mammals, and birds the old law -holds good, and hybrids between two species are very rarely fertile. -The two products of crossing between the horse and the ass, the mule -and the hinny, are never fertile <i>inter se</i>, and very rarely with a member -of the parent stock.</p> - -<p>We have to ask, therefore, what is the reason of this mutual -sterility of species; whether it is a necessary outcome of the morphological -differences between the species, or only a chance accessory -phenomenon, or perhaps an absolutely necessary preliminary condition -to the establishment of species.</p> - -<p>The last was the view held by Romanes. He believed that -a species could only divide into two when it was separated into -isolated groups either geographically or physiologically, that is, when -sexual segregation in some form is established within the species, -so that all the individuals can no longer pair with one another, but -groups arise which are mutually sterile. It is only subsequently, he -maintained, that these groups come to differ from one another in -structure. To this hypothetical process he gave the name of 'physio<span class="pagenum"><a id="Page_338"></a>[Pg 338]</span>logical -selection.' This view depends—it seems to me—upon an -underestimate of the power of natural selection. Romanes believed -that when a species began to split up, even the adaptive variations -would always disappear again because of the continual crossing, -and that only geographical isolation or sexual alienation, that is, -physiological selection, would be able to prevent this. But even -the fact that there are dimorphic and polymorphic species proves -sufficiently that adaptation to two or even several sets of conditions -can go on on the same area. In many ants we find many kinds -of individuals—the two sexual forms, the workers, and the soldiers, -and these last are undoubtedly distinguished by adaptive characters -which must be referred to selection. The same is true of the -caterpillars, whose coloration is adapted to their surroundings -in two different ways. If the individuals of one and the same -species can be broken up into two or more different forms and -combinations of adaptations, while they are mingling uninterruptedly -with one another, natural selection must undoubtedly be able, -notwithstanding the continual intermingling of divergent types, to -discriminate between them and to separate them sharply from one -another. Assuredly then a species can not only exhibit uniform -variation on a single area, but may also split up into two without -the aid of physiological selection. Theoretically it is indisputable -that of two varieties which are both equally well suited to the -struggle for existence, a mixed form arising through crossing may -not be able to survive. Let us recall, for instance, the caterpillars, -of which some individuals are green and some brown, and let us -assume that the brown colour is as effective a protection as the -green, then the two forms would occur with equal frequency; but -though a mixed hybrid form which was adapted neither to the green -leaves nor to the brown might occasionally crop up, it would always -be eliminated. It would occur because the butterflies themselves are -alike, whether they owe their origin to green caterpillars or to brown, -and thus at first, at least, all sexual combinations would be equally -probable.</p> - -<p>I do not believe therefore in a 'physiological selection,' in -Romanes's sense, as an indispensable preliminary condition to the -splitting of species, but it is a different question whether the mutual -sterility so frequently observable between species has not conversely -been produced by natural selection in order to facilitate the separation -of incipient species. For there can be no doubt that the process of -separating two new forms, or even of separating one new form from -an old one, would be rendered materially easier if sexual antipathy or<span class="pagenum"><a id="Page_339"></a>[Pg 339]</span> -diminished fertility of the crossings could be established simultaneously -with the other variations. This would be useful, since pure and -well-defined variations would be better adapted to their life-conditions -than hybrids, and would become increasingly so in the course of -generations. But as soon as it is useful it must actually come about, -if that is possible at all. It may be, however, as we have said before, -that the two divergent forms depend merely upon quantitative -variations of the already existing characters; sexual attraction, -whether it depends upon very delicate chemical substances, or on -odours, or on mutual complementary tensions unknown to us, will -always fluctuate upwards and downwards, and plus or minus determinants, -which lie at the root of these unknown characters in -the germ-plasm, must continually present themselves and form the -starting-point for selection-processes of a germinal and personal -kind, which may bring about sexual antipathy and mutual sterility -between the varieties. I therefore consider Romanes's idea correct -in so far that separation between species is in many cases accompanied -by increasing sexual antipathy and mutual sterility. While Romanes -supposed that 'natural selection could in no case have been the -cause' of the sterility, I believe, on the contrary, that it could only -have been produced by natural selection; it arises simply, as all -adaptations do, through personal selection on a basis of germinal -selection, and it is not a preliminary condition of the separation -of species, but an adaptation for the purpose of making as pure -and clean a separation as possible. It is obviously an advantage -for both the divergent tendencies of variation that they should -intermingle as little as possible. This is corroborated by the fact -that by no means all the marked divergences of species are accompanied -by sexual alienation, and that the mutual sterility so frequently -seen is not an inevitable accompaniment of differences in the rest -of the organism.</p> - -<p>That this is not the case is very clearly proved by our domesticated -animals. The differences in structure between the various breeds -of pigeon and poultry are very great, and breeds of dog also diverge -from one another very markedly, especially in shape and size of -body. Yet all these are fertile with one another, and they yield -fertile offspring. But they are products of artificial selection by -man, and he has no interest in making them mutually sterile, so -that they have not been selected with a view to sexual alienation, -but in reference to the other characters. The segregation of animal -species into several sub-species on the same area is probably usually -accompanied by sexual antipathy, since in this case it would be<span class="pagenum"><a id="Page_340"></a>[Pg 340]</span> -useful although not indispensable. But the matter is different in the -case of the transformation of a colony upon a geographically isolated -region. 'Amiktic' forms, such as <i>Vanessa ichnusa</i> of Corsica, -are hardly likely to be sexually alienated from the parent form; -we have here to do only with the preponderance of a fortuitous -and biologically valueless variation and its consequent elevation to -the rank of a variety. The new form was not an adaptation, but -only a variation, and as it was of no use, it was not in a position -to incite any process of selection favouring its advancement.</p> - -<p>But even adaptive transformations on isolated regions from -which the parent species is excluded are not likely to develop -rapidly any sexual antipathy as regards the parent stock, and -I should not be surprised if experiments showed that there is perfect -mutual fertility between, for instance, many of the species of -<i>Achatinella</i> on the Sandwich Islands or of <i>Nanina</i> on Celebes, or -between the species of thrushes on the different islands of the -Galapagos Archipelago, or between these and the ancestral species -on the adjacent continent, if that species is still in existence. For -there was no reason why sexual antipathy to the parent form -should have developed in any of these adaptation forms which -have arisen in isolation, and therefore it has probably not been -evolved.</p> - -<p>That our view of the mutual sterility between species, as an -adaptation to the utility of precise species-limitation, is the correct -one is evidenced not only by our domesticated races, but even more -clearly by plants, in regard to which it is particularly plain that the -sexual relations between two species are adaptational. We have -already seen in what a striking way the sensitiveness of the stigma -of a flower is regulated in reference to pollen from the same plant, -that some species are not fertilizable by their own pollen at all, that -others yield very little seed when self-fertilization is effected, and -that others again are quite fertile—as much so as with the pollen -of another plant of the same species. We regarded these gradations -of sexual sensitiveness as adaptations to the perfectly or only -moderately well-assured visits of insects, or to their entire absence. -I wish to cite these cases as well as the heterostylism of some flowers -as evidence in support of the conception of the mutual sterility of -species which I have just outlined. But this only in passing. The -point to which I chiefly wish to direct attention is the mutual -fertility of many plant-species. In lower as well as in higher plants -fertile hybrids occur not infrequently under natural conditions, and -cultivated hybrids, such as the new <i>Medicago media</i>, a form made by<span class="pagenum"><a id="Page_341"></a>[Pg 341]</span> -mingling two species of clover, may go on reproducing with its own -kind for a considerable time. A number of Phanerogams yield fertile -hybrids, and in Orchids even species of different genera have been -crossed and have yielded offspring which was in some cases successfully -crossed with a third genus.</p> - -<p>If these facts prove anything it is that the factors which -determine the mutual sterility of species are quite distinct from -their morphological differences, in other words, from the diagnostic -characters of the specific type. For a long time the verdict on this -matter was too entirely based on observations made on animals, among -which mutual sterility arises relatively easily, even where it was -not intended (<i>sit venia verbo!</i>). Even the pairing, but still more the -period of maturity, the relations of maturity in ovum and sperm, and -even the most minute details in the structure of the sperm-cell, the -egg-shell, envelope, &c., have to be taken into account, and these may -bring about mutual or, as Born has shown, one-sided sterility. We -know, through the researches of Strasburger, that a great many -Phanerogams, when pollinated artificially from widely separated -species of different genera and families, will at least allow the pollen-tube -to penetrate down to the ovule, and that in many cases -amphimixis actually results. It follows that we must not lay too -great stress upon the mutual sterility which occurs almost without -exception among the higher animals, but must turn to the plants with -greater confidence.</p> - -<p>Among plants there is very widely distributed mutual fertility -between species. I doubt, however, whether the observations on this -point are sufficient to warrant any certain conclusion in regard to the -importance of the phenomena in the formation of species. At least it -is not easy to see why the mutual sterility of many species of plants -should not have been necessary or useful in separating species, and -why it was not therefore evolved. We may point to the fact that -animals can move from place to place as the chief reason, and this -factor does undoubtedly play a part, but the widespread crossing -of plants by insects makes up to some extent, as far as sexual -intermingling is concerned, for their inability to move from place to -place. I do not know whether the species of orchid which are fertile -with one another belong to different countries, so that we may -assume that they originated in isolation, or whether fertile orchids -from the same area are fertilized by different insects and are thus -sexually isolated. This and many other things must be taken into -consideration. Probably these relations have not yet been adequately -investigated; probably what is known by some experts has not yet<span class="pagenum"><a id="Page_342"></a>[Pg 342]</span> -been made available to all. Future investigations and studies must -throw more light upon the problem.</p> - -<p>In any case, however, we can see from the frequency of mutual -fertility among plants that mutual sterility is not a <i>conditio sine qua -non</i> of the splitting up of species, and we must beware of laying too -great stress upon it even among animals. Germinal selection is -a process which not only forms the basis of all personal selection, but -which is also able to give rise of itself, without the usual aid of sexual -intermingling, to a new specific type. And we cannot with any -confidence dispute that, even without amphigony, a certain degree of -personal selection may not ensue solely on the basis of the favourable -variational departures originating in the germ-plasm. It would be -premature to express any definite views on this point as yet, but the -diverse cases of purely asexual or parthenogenetic reproduction -in groups of plants rich in species make this hypothesis seem -probable.</p> - -<p>The most remarkable example of this is probably to be found in -the Lichens, the symbiotic nature of which we have already discussed, -in which—now at least—neither the Fungus nor the Alga associated -with it is known to exhibit sexual reproduction. If this is really -the case, then the existence of numerous and well-marked species -of lichens leads us to the hypothesis just expressed, and we must -suppose that the unity of the specific type is attained in this case -solely by a continual sifting of the useful from the useless variations -of the determinants, and through purely germinal intensification of -the surviving variational tendencies.</p> - -<p>Of course it is possible that the mutual adaptation of the Algæ -and Fungi in the evolution of species of Lichens took place very long -ago, at a time when sexual reproduction still existed, at least in one -of the associated organisms, the Fungus. The Ascomycetes, to which -most of the Lichens belong, do not at present usually exhibit the -process of amphimixis, as I have already noted; but it may perhaps -be still possible to decide whether they must have exhibited it, or at -least could have exhibited it at an earlier stage in their evolution. -As the group of Thallophytes is a very ancient one, it is not inconceivable -that the modern species of Lichens have existed for a long -time, and that they had their origin in the remote past with the -assistance of amphimixis.</p> - -<p>Nor need it be objected to this supposition that it has been found -possible to <i>make new Lichens</i> by bringing together Fungi and Algæ -which had not previously been associated with one another; for in -the first place both were already adapted to partnership with other<span class="pagenum"><a id="Page_343"></a>[Pg 343]</span> -species, and, moreover, so far no one has succeeded in rearing these -artificial lichens for any length of time, still less in seeing them -evolve into specific forms persistent in natural conditions.</p> - -<p>But if this supposition should prove to be not only improbable, -but actually erroneous, then the existence of Lichens would afford -a clear proof that the 'type' of the species does not depend essentially -upon the constant intermingling of individuals, but upon a process -which we may best designate <i>uniformity of adaptation</i>. We have -simply to suppose that under similar external influences similar -variational tendencies were started by germinal selection in all the -individuals of the two parent species of a lichen, and set a-going -by germinal selection, just as a warmer climate gives rise to a black -variety in the butterfly <i>Polyommatus phlæas</i>, because similar determinants -of the germ-plasm of all the individuals were impelled to -vary in the same manner and direction. This would then give rise -to quite definite variations, and since only the suitable variational -tendencies could survive, primitive though never complicated adaptations -would arise. But we cannot assume that the lichens are not -adapted to the conditions of their life as well as all other organisms. -We cannot judge how far even their shape is to be regarded as an -adaptation, whether the formation of encrusting growths, of tree-like -forms, of cup or bush-lichens, may not be regarded as adaptations -towards a full utilization of the conditions of their life—but even -if this is not the case, the formation of soredia remains an undoubted -adaptation to the symbiosis of those lichens which exhibit them. -The soredia cannot depend upon the direct effect of the conditions -of life, for they are reproductive bodies which did not exist before -the existence of the lichen, and only originated to facilitate their -distribution.</p> - -<p>Thus there is still a great deal that is doubtful in our theories as -to the transformations of organisms, and much remains still to be -done. But even though we may doubt whether adaptations could -come about in multicellular organisms without amphigony, we may -be quite certain of the converse, that is, that the specific type can -be changed in every individual feature by natural selection on -the basis of amphigony, even as regards invisible features which only -express themselves in altered periods of growth. Even when there -is no isolation whatever and no mutual sterility, and when a mobile -species is uniformly distributed over a large area, a splitting up into -races in regard to one particular character may occur, <i>simply through -adaptation</i> to the spatially different climatic conditions of the area -inhabited.</p> - -<p><span class="pagenum"><a id="Page_344"></a>[Pg 344]</span></p> - -<p>Early in these lectures we discussed the twofold protective value -of the coloration of the 'variable hare' (<i>Lepus variabilis</i>), which -is distributed over the Arctic zone of the Old and New World, and -also occurs in the higher regions of the Alps. Wherever there is -a sharp contrast between winter and summer the variable hare -exhibits the same specific type, being brown in summer and white in -winter, but in regard to this very character of colour-change it forms -races to some extent, for it is white for a longer or shorter time according -to the length of the winter—in Greenland for the whole twelve -months of the year, in Northern Norway only for eight or nine -months, in the Alps for six or seven months, but in the south of -Sweden and in Ireland not at all. There it remains brown in winter -like our common hare (<i>Lepus timidus</i>). This is not a question of the -direct effect of cold; if it were the species would become white in -Southern Sweden also, for there is no lack of severe cold there, but -the ground is not so uninterruptedly covered with snow, and so the -white colour of the hare would be as often, probably oftener, a danger -than a safeguard, and the more primitive double coloration has -therefore been done away with by natural selection. The change -of colour is thus hereditarily fixed, as is proved by the fact that -the Alpine hare, if caught and kept in the valleys below, puts on -a white dress at the usual time, which the common hare never does.</p> - -<p>As in Southern Sweden the winter coloration has been wholly -eliminated, so, conversely, from there to the Arctic zone the summer -colouring has been more and more crowded out, and in the Farthest -North it has totally disappeared from the characters of the species. -We thus see that wherever the species lives the double colouring is regulated, -as regards the duration of the winter coat, in exact harmony with -the external conditions. There is a pure white, a pure brown, and -a colour-changing race, and the latter is subdivided into two—one -wearing the winter dress for six, the other for eight months. -Probably these could be still further subdivided, if the different -regions of the Scandinavian Peninsula were investigated individually -from south to north. That the duration of the winter dress has its -roots in the germ-plasm, and does not depend solely on the earlier or -later period at which the cold sets in, is made clear by the two -extreme forms, the white and the brown <i>Lepus variabilis</i>, as well as -by the behaviour of captive animals. The familiar case of Ross's -lemming, which remained brown in the warm cabin, and then -suddenly became white when it was exposed to the cold of winter, -only shows that the cold acts as a liberating stimulus. The preparatory -changes in the pellage are already present, and the stimulus<span class="pagenum"><a id="Page_345"></a>[Pg 345]</span> -of cold brings them rapidly to a climax. Here, therefore, the necessary -variations of the relevant germinal parts must have continually -presented themselves for selection, which is intelligible enough, since -it is merely a question of plus- or minus-variations. The fact that the -six-months' dress can be transformed into an eight-months' dress -must have its cause in some minute biological units of the germ-plasm; -the determinants of the fur must be able to vary in such a way -that a longer or shorter duration of the winter's coat is the result. -The possibility of the whole variation depends upon the continual -fluctuations of all determinants, now towards plus, now towards -minus, and the necessity and inevitableness of each adaptation to the -duration of the winter lies in the unceasing personal selection—the -inexorable preferring of the better adapted.</p> -<hr class="full" /> - -<div class="chapter"> -<p><span class="pagenum"><a id="Page_346"></a>[Pg 346]</span></p> - -<h2 class="nobreak" id="LECTURE_XXXV">LECTURE XXXV</h2> -</div> - -<p class="c">THE ORIGIN AND THE EXTINCTION OF SPECIES</p> - -<p>Adaptation does not depend upon chance—The case of eyes—Of leaf-mimicry—All -persistent change depends ultimately on selection—Mutual sterility without great -significance—Relative isolation (<i>Lepus variabilis</i>)—Influence of hybridization—Decadence -of species—Differences in the duration of decadence—Natural death of individuals—Extinction -due to excessive variability (Emery)?—<i>Machairodus</i> as interpreted by -Brandes—Lower types more capable of adaptation than higher—Flightless birds—Disturbance -of insular fauna and flora by cultivation—The big game of Central Europe.</p> - - -<p><span class="smcap">In</span> the polar hare we have a case in which the adaptations to the -life conditions both of time and space are recognizable as the effect -of definite causes, and thus as a necessity; but the same must be true -everywhere even in regard to the most complex adaptations which -seem to depend entirely upon chance; everywhere adaptation results -of necessity—if it is possible at all with the given organization of the -species—as certainly as the adaptation dress of the hare depends -on the length of the winter, and in point of fact not less certainly -than the blue colour of starch on the addition of iodine. The most -delicate adaptations of the vertebrate eye to the task set for it by -life in various groups have been gradually brought about as the -necessary results of definite causes, just in the same way as the -complex protective markings and colouring on the wing of the -<i>Kallima</i> and other leaf-mimicking butterflies.</p> - -<p>That adaptations can be regarded as mechanically necessitated -is due to the fact that in every process of adaptation the same -direction of variation on the part of the determinants concerned is -guaranteed, since personal selection eliminates those which vary in -a wrong direction, so that only those varying in a suitable direction -survive, and they then continue to vary in the same direction. But -the greatest difference between our conception of natural selection -and that of Darwin lies in this: that Darwin regarded its intervention -as dependent upon chance, while we consider it as necessary and -conditioned by the upward and downward intra-germinal fluctuation -of the determinants. Appropriate variational tendencies not only -<i>may</i> present themselves, they <i>must</i> do so, if the germ-plasm contains -determinants at all by whose fluctuations in a plus or minus direction -the appropriate variation is attainable.</p> - -<p><span class="pagenum"><a id="Page_347"></a>[Pg 347]</span></p> - -<p>That a horse should grow wings is beyond the limits of the -possibilities of equine variation—there are no determinants which -could present variations directed towards this goal; but that any -multicellular animal which lives in the light should develop eyes lies -within the variational possibilities of its ectoderm determinants, and -in point of fact almost all such animals do possess eyes, and eyes, too, -whose functional capacity may be increased in any direction, and -which are adaptable and modifiable in any manner in accordance with -the requirements of the case. As soon as the determinants of the -most primitive eye came into existence, they formed the fundamental -material by whose plus- or minus-variations all the marvellous eye -structures might be brought about, which we find in the different -groups of the Metazoa, from a mere spot sensitive to light to a -shadowy perception of a moving body, and from that again to the -distinct recognition of a clear image, which we are aware of in our -own eyes. And what wonderful special adaptations of the eye to -near and to distant vision, to vision in the dusk and at night, or in -the great ocean-depths, to recognition of mere movement or the -focussing of a clear image, have been interpolated in the course of -this evolution!</p> - -<p>All such adaptations are possible, because they can proceed -from variations of determinants which are in existence; and in the -same way it is possible, at every stage of the evolution of organisms, -for eyes to degenerate again, whether they have been high up or low -down in the scale of gradations of this perhaps the most delicate -of all our sense-organs. As soon as a species migrated permanently -from the light into perfect darkness its eyes began to degenerate. -We know blind flat worms, blind water-fleas and Isopods, also blind -insects and higher Crustaceans, and even blind fishes and amphibians, -the eyes of which are now to be found at very different levels of -degeneration, as Eigenmann has recently shown in regard to several -species of cave-dwelling salamanders of the State of Ohio. In all -these cases it is only necessary for the determinants of the eye to -continue to vary in the minus direction, and the disappearance of the -eye must be gradually brought about.</p> - -<p>We must picture upward development in quite a similar way. -The forest butterflies of the Tropics could not possibly all have their -under surfaces coloured like a leaf if the protective pattern depended -solely upon the chance of a useful variation presenting itself. It -always presented itself through the fluctuations of the determinants, -and thus the appropriate colourings were not merely able to develop, -but of necessity did so in gradually increasing perfection. If chance<span class="pagenum"><a id="Page_348"></a>[Pg 348]</span> -played any part in the matter, it would be quite unintelligible why -the protective colouring should occur only where it acts as a -protection, and why, for instance, it should not appear sometimes -upon the upper surface of the butterfly wing, or upon the posterior -wings which are covered when the butterfly is at rest. We have -already studied in detail the precision with which the coloration is -localized on minute points and corners of the wing: this can only -be understood if natural selection works with the certainty of -a perfect mechanism. Chance only comes into the matter in so far -as it depends upon chance whether the relevant determinants in -one id or another are to vary in the direction of plus or minus; but as -the germ-plasm contains many ids, and chance may decide it differently -in each of these, the presence of a majority of determinants varying -in a desirable direction does not depend upon chance, for if they are -not contained in one individual they are in another. It is only -necessary that they should be present in some, and that these should -be selected for reproduction.</p> - -<p>We must therefore regard natural selection, that is to say, -<i>personal selection</i>, as a mechanical process of development, which -begins with the same certainty and works 'in a straight line' towards -its 'goal,' just as any principle of development might be supposed to -do. Fundamentally it is after all a purely internal force which gives -rise to evolution, the power of the most minute vital units to vary -under changing influences, and it is only the guidance of evolution -along particular paths that is essentially left to personal selection, -which brings together what is useful and thus determines the -direction of further evolution. If we bear in mind that even the -minutest variations of the biophors and determinants express nothing -more or less than reactions to changed external conditions in the -direction of adaptation, and that the same is true of each of the -higher categories of vital units, whether they be called cell, tissue, -organ, person, or corm, we see that the whole evolution of the forms of -life upon the earth depends upon adaptations following each other in -unbroken succession, and fitting into each other in the most complex -way. The whole evolution is made possible by the power of variation -of the living units of every grade, and called forth and directed -by the ceaseless changes of the external influences. I said years ago -that <i>everything</i> in organic evolution depended upon selection, for every -lasting change in a vital unit means adaptation to changed external -influences, and implies a preference in favour of the parts of the -unit concerned, which are thereby more fitly disposed.</p> - -<p>In this sense we can also say that the species is a complex of<span class="pagenum"><a id="Page_349"></a>[Pg 349]</span> -adaptations, for we have seen that it depends upon the co-operation -of different grades of selective processes, that in many cases it is -produced solely by germinal selection, but that in very many more -personal selection plays the chief part, whether in bringing about -sexual adaptations, or adaptations to the conditions of existence.</p> - -<p>When we have thus recognized that the origin of a variation in -a definite direction results as inevitably when it is called forth by the -indirect influence of conditions, that is, through the need for a new -adaptation, as when it is induced in the germ-plasm by direct causes such -as those of climate, we shall not be disposed to estimate very highly the -part played by mutual sterility in the origin of species. We shall -rather be inclined to assign it a rôle at a later stage, after the -separation of the forms has taken place, and this view is supported by -the fact of the mutual sterility of most nearly related species, and by -the theoretical consideration that the frequency of hybrids, even if -these are always eliminated in the struggle for existence, must signify -a loss for both the parent species. But no certain conclusion can be -based upon either of these arguments—not upon the theoretical one, -because here again we are unable to estimate the extent of this loss; -and not upon the argument from fact, because the results of experiments -in crossing animals have generally been overestimated, since -we are apt to regard the most nearly related animals that are at our -disposal as being very closely related. Thus, for instance, horse and -ass, horse and zebra are undoubtedly rightly included within the -same genus, but the fact that there are several species of zebra in -Africa gives us an idea of the number of transition stages that may -have existed between the horse and the zebra. Entomologists have -sometimes reared hybrids between the most nearly related indigenous -species of hawk-moth of the genus <i>Smerinthus</i>—hybrids of <i>Smerinthus -ocellata</i>, the eyed hawk-moth, and <i>Smerinthus populi</i>, the poplar -hawk-moth. I have myself made many experiments of this kind, and -have often succeeded in getting the two species to pair and even to -deposit eggs, but I have never seen a caterpillar emerge from them. -The hybrids do occur, however, and they have been repeatedly -obtained by Standfuss. In external appearance they are intermediate -between the parent forms, but with marked divergences, thus, for -instance, the beautiful blue eye on the posterior wing of <i>S. ocellata</i> -(<a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#f5">Fig. 5</a>, vol. i. p. 69) may have almost disappeared or be only indicated. -They are sterile. But we know three species of <i>Smerinthus</i> in North -America, which are all much nearer to <i>S. ocellata</i> than <i>S. populi</i> is, -for they all possess the eye-spot referred to, although it is less well -developed. The proof that the most nearly related species do not<span class="pagenum"><a id="Page_350"></a>[Pg 350]</span> -yield fertile descendants should be sought for by crossing <i>Smerinthus -ocellata</i> with one of these American species if it is to have any -decisive value.</p> - -<p>Experiments of the same kind have been made by Standfuss with -different species of indigenous <i>Saturnia</i>, and these have shown not -only that crossing is possible, but that the hybrids are fertile in their -turn. These results are to be valued the more highly because it is -well known that Lepidoptera, and even the usually prolific silk-moths, -do not readily reproduce in captivity, even within the same species. -We have in <i>Saturnia pyri</i>, <i>spini</i>, and <i>carpini</i> three well-marked -distinct species with no intermediate forms in nature, and with quite -different colouring in the caterpillars. That these should have been -successfully combined in a triple hybrid proves at least that sexual -alienation cannot have advanced far in this case.</p> - -<p>We must beware, however, of attributing too much to the constant -mutual crossing which occurs in a species living on a connected -area and of regarding its influence as irresistible. Undoubtedly it -must go far towards securing the uniformity of individuals, but not -only is it unable to achieve this, but it cannot successfully resist the -stronger influences making for variation which may be exerted upon -a part of the area of the species. We have already seen that it is -quite erroneous to suppose that every new adaptation must be lost -sight of again because of the continual crossing with other members -of the species upon the same area. Other things being equal, this -depends entirely upon the importance of the adaptation in question. -Just as climatic influences may be so strong that they entirely overcome -the influence of crossing, and give rise to a local race notwithstanding -imperfect geographical isolation, so the same may happen -in the case of adaptations. It is quite conceivable that the polar -hare of Scandinavia may have evolved a whole series of races, each of -which is adapted to the duration of the snow in its geographical -range, although a crossing of these quick-footed animals must frequently -occur in the course of time, even as regards forms from widely -separated areas, and although the whole region is inhabited without -a break by the species, so that a 'mingling' of the hares of all regions -from south to north, and conversely, may take place, and indeed must -be continually taking place, though of course very slowly.</p> - -<p>It is precisely this extreme slowness with which the intermingling -of racial characters take place that seems to me essential -for the production of local or, as in this case, regional races. It is not -difficult to calculate the rate of 'blood-distribution' if we assume that -the conditions for a rapid dissemination are as favourable as possible.<span class="pagenum"><a id="Page_351"></a>[Pg 351]</span> -Let us assume that it takes place along a certain line—in this case -from south to north—and that the numerical strength of the species -remains constant, each pair of hares yielding a pair of surviving -offspring, which will attain to reproduction. Let us suppose that one -of these hares moves his home northwards to the extent of his range, -that is, as far as a hare is accustomed to range from his head -quarters, and that he pairs with one of the descendants on the next -stretch.</p> - -<p>Let us further suppose that this stretch is ten kilometres in -extent, and that the change of quarters take place once in each -year, then the blood of a South Scandinavian hare would have -extended ten kilometres further north in ten years, and in a -hundred years 100 kilometres; it would not, however, be quite -pure, but mixed and thinned by crossing with a hundred mates -of different individual bloods, that is, thinned to the extent of -2 to the 100<sup>th</sup> power, that is, to less than a millionth part. Thus -even with these much too favourable assumptions the influence of -a region of hares 100 kilometres distant would be actually <i>nil</i> upon the -inhabitants of a region which was in process of new adaptation. That -the assumptions are too favourable is quite obvious, since every -surviving hare would not be likely to move his home, and probably -the majority would remain in the old quarters and find mates there. -The blood-mingling would therefore take place much more rarely, -perhaps only once in ten years, and the wandering descendants of the -second generation might move southward, and so neutralize the previous -blood-mingling, and so on. But let us keep to our favourable -assumptions, and attempt to determine how strong the assimilating -influence of the blood-mingling from south to north would be upon -a point <i>A</i>. The blood of the nearest stretch diluted to a half would -affect the inhabitants of <i>A</i> once in each year; the second stretch would -only contribute blood of ¼ strength, the third of 1/8, the fourth of 1/16, -and the blood of the tenth would be diluted to 1/1024. A region <i>B</i>, -extending over twenty such stretches, or 200 kilometres, would thus -shelter within it a hare population of which the centre would only -be influenced from the periphery in vanishing proportions. If the -winter were of equal length over the whole area of <i>B</i>, all the inhabitants -would be tending to vary the period for which the winter dress -was worn in correspondence with the length of the winter, and the -centre of the region would be the less impeded in this process because -the more peripheral areas would also be approximating to the same -adaptation. But since even the admixture of 1/32 of strange blood -could have no hindering influence upon a variation, there would<span class="pagenum"><a id="Page_352"></a>[Pg 352]</span> -remain a region of 2 × 5 = 10 stretches upon which the influence of the -non-varying regions would be without effect. There would therefore -arise a new race in relation to the duration of the winter dress, and -this would not cease abruptly, but would gradually pass over into the -neighbouring regions, which however would be pure at their centre, -just as is probably the case in reality, if we regard <i>B</i> as any point in -the line of distribution from south to north.</p> - -<p>The harmony of the individuals within a species will therefore -depend in part upon the mingling of hereditary primary constituents -associated with reproduction, but in greater part upon adaptation to -the same conditions; it is <i>a similarity of adaptation</i>, and the strongest -influence which sexual reproduction exerts lies not in the mingling -of these hereditary constituents alone, but above all in the reduction -in the germ-plasm of the two parental hereditary contributions—a -reduction which results from and through the sexual intermingling. -It is only this that prevents these primary constituents from varying -at too unequal a rate in the transformations of species, and causes -them ultimately to resemble each other closely again.</p> - -<p>But while mutual sterility is not an absolutely necessary condition -in the separation of species, it would be going too far in the -opposite direction to regard mutual fertility as something general, -or to attribute to it a rôle in the origination of new species.</p> - -<p>Certain botanists, like Kerner von Marilaun, regard the mingling -of species as a means of forming new species with better adaptations; -they suppose that fertile hybrids may, in certain circumstances, crowd -out the parent species, and themselves become new species. It will be -admitted that such cases do occur, that, for instance, in the north of -Europe the hybrid between the large and the small water-lily, -<i>Nuphar luteum</i> and <i>Nuphar pumilum</i>, to which the name <i>Nuphar -intermedium</i> has been given, has driven both the parent species from -the field, because its seeds mature earlier, and it is therefore better -adapted to the short vegetative period of the north, but nevertheless -we must maintain that the evolution of species on the whole does not -take place through hybridization. Such cases are probably nothing -more than rare exceptions. This is corroborated by the entire insignificance -of hybridization in animals, among which species appear in -the same way as they do in plants, and where the mingling of two -species occurs only sporadically and in a few species, never to any -very great extent.</p> - -<p>If species are complexes of adaptations, based in each case on -the given physical constitution of the parent species, then we can -readily understand the fact that they are in our experience not fixed<span class="pagenum"><a id="Page_353"></a>[Pg 353]</span> -or eternal, but that they change in the course of the earth's history. -The numerous fossil remains in the various strata of the earth's crust -prove that this is true in a high degree, that in almost every one of -the more important geological strata new species occur, and that not -only species and genera, but families, orders, indeed whole classes of -animals, which lived at one time, have now completely disappeared -from the face of the earth. We can understand this phenomenon -when we reflect that the conditions of life have also been slowly -changing through the course of the earth's history, so that the -old species had only the alternative of dying out, or of becoming -transformed into new species.</p> - -<p>But simple as this conclusion is, it can hardly be deduced with -certainty from the occurrence and succession of the fossil species -alone. For instance, we should strive in vain to recognize the cause -which led one of those regularly arranged snail-species of the -Steinheim lake basin to become transformed into one or two new -species at a particular time, or to find the cause which moved those -curious tripartite Crustaceans of primitive times, the Trilobites, which -peopled the Silurian seas with such a wealth of forms, to become -suddenly scarce towards the end of the Silurian period, and to disappear -altogether in the succeeding period, the Devonian. The -famous geologist Neumayr sought to refer this striking phenomenon -to the fact that just at that time the Cephalopods, 'the most formidable -and savage marauders among the invertebrate marine fauna,' -gained the ascendancy, and it is quite possible that he was right -in his surmise, but who is to prove it? Can we decide even in the -case of animals now living whether the losses inflicted on a much -persecuted species by an abundant and greedy persecutor exceed the -numbers of progeny, and are therefore driving the species gradually -towards extermination? Probable as such a supposition appears, it -cannot be accepted as proven.</p> - -<p>Since in many cases of the extinction of great animal-groups we -cannot even prove that there was a simultaneous ascendancy of powerful -enemies, other factors must be discovered to which the apparently -sudden disappearance may be attributed. Many naturalists have -tried to guess at internal reasons for extinction, and have adopted the -theory—associated with the tendency to assume mystical principles of -evolution—that species in dying out are obeying an internal necessity, -as if their birth and death were predestined, as it is in the case of -multicellular individuals, as if there were a <i>physiological death of the -species</i> as there is of the multicellular individual.</p> - -<p>Neumayr showed, however, that the facts of palæontology afford<span class="pagenum"><a id="Page_354"></a>[Pg 354]</span> -no support for this view. I need not repeat his arguments, but will -simply refer to his clear and concise exposition of the problem. It is -obvious that our theory of the extinction of species as due to external -causes cannot be rejected on the ground that our knowledge of the -struggle that species had to maintain for their existence in past times -is even mere imperfect than our knowledge of the struggle nowadays, -and that we are frequently unable to judge of it at all. But the facts -of geology are of value in another, quite different way. They reveal -such an extraordinary dissimilarity in the duration of species, and -also of the great groups of organisms, that the dissimilarity of itself -is sufficient to prevent our regarding the extinction of species as -regulated by <i>internal</i> causes. Certain genera of Echinoderms, such -as starfish (<i>Astropecten</i>), lived in the Silurian times, and they are -represented nowadays in our seas by a number of species: and in the -same way the Cephalopod genus <i>Nautilus</i> has maintained itself -among the living all through the enormous period from the Silurian -sea to our own day. Formerly the Nautilids formed a predatory -horde that peopled the seas, and, as we have seen, we may perhaps -attribute to their dominance the disappearance of an order of -Crustaceans, the Trilobites, which were equally abundant at that -period. Now only a single species of nautilus (<i>Nautilus pompilius</i>) -lives on the coral reefs of the southern seas. Similarly, the genus -<i>Lingula</i> of the nearly extinct class of Brachiopods, somewhat mussel-like -sessile marine animals, has been preserved from the grey dawn -of primitive times, with its records in the oldest deposits, and is -represented in the living world of to-day by the so-called 'barnacle-goose' -mussel, <i>Lingula anatina</i>.</p> - -<p>On the other hand, we know of numerous species which lasted -for quite a short time, such as, for instance, the individual members -of the series of Steinheim <i>Planorbis</i> species, or of the Slavonic -<i>Paludina</i> species. Not infrequently, too, genera make their appearance -and disappear again within the period of one and the same -geological stratum.</p> - -<p>These facts not only tell against an unknown vitalistic principle -of evolution, but in general against the idea of the determination -of the great paths of evolution by purely internal causes. If there -were a principle of evolution the dissimilarity in the duration of life -could not be so excessive; if there were a 'senile stage' of species -and a natural death of species comparable to the natural death of -individuals, it would not have been possible for most of the Nautilidæ -to have been restricted to the Silurian epoch, and yet for one species -to have continued to live till now; and if there were a 'tendency'<span class="pagenum"><a id="Page_355"></a>[Pg 355]</span> -of species to vary persistently onwards, and to 'become further and -further removed from the primitive type,' as has been maintained, -then such ancient and primitive Cephalopod forms like the <i>Nautilus</i>-species -could not have persisted until now, but must long ago have -Wen transmuted into higher forms. The converse, however, is conceivable -enough, namely, that the great mass of the species of a group -such as the Nautilidæ were crowded out by superior rivals in the -struggle for existence, but that certain species were able to survive -on specially protected or otherwise favoured areas. We have a fine -example of this in the few still living species of the otherwise -extinct class of Ganoid fishes. During the Primary and Secondary -epochs these Ganoids peopled all the seas, but at the boundary -between the Cretaceous and the Tertiary period they retrograded -considerably, simultaneously with the great development of bony -fishes or Teleosteans, and now they are only represented by a dozen -species distributed over the earth, and most of these are purely river -forms, while the others at least ascend the rivers during the spawning -season to secure the safety of their progeny. For the rivers are -sheltered areas as compared with the seas, and large fishes like the -Ganoids will be able there to hold their own in the struggle better -than they could in the incomparably more abundantly peopled sea.</p> - -<p>Thus I can only regard it as playing with ideas to speak of birth, -blossoming, standstill, decay, and death of species in any other than -a figurative sense. Undoubtedly the life of the species may be -compared with that of the individual, and if the comparison be used -only to make clear the difference between the causes of the two kinds -of phenomena, there can be no objection to it, only we must beware -of thinking we have explained anything we do not know by comparing -it with something else that is also unknown.</p> - -<p>We have already shown that the natural death of multicellular -organisms is a phenomenon which first made its appearance with the -separation of the organism into somatic or body cells and reproductive -or germ cells, and that death is not an inevitable Nemesis of every life, -for unicellular organisms do not necessarily die, though they may -be killed by violence. These unicellular organisms have thus no -natural death, and we have to explain its occurrence among multicellular -organisms as an adaptation to the cellular differentiation, -which makes the unlimited continuance of the life of the whole -organism unnecessary and purposeless, and even prejudicial to the -continuance of the species. For the species it is enough if the germ-cells -alone retain the potential immortality of the unicellulars, while, -on the other hand, the high differentiation of the somatic cells necessarily -<span class="pagenum"><a id="Page_356"></a>[Pg 356]</span>involves that they should wear themselves away in the -performance of their functions, and so become subject to death, or at -least that they should undergo such changes that they are no longer -capable of functioning properly, so that thus the organism as a whole -loses the power of life.</p> - -<p>There can be no doubt whatever that death is virtually implied -in the very constitution of a multicellular organism, and is thus, -so to speak, a foreseen occurrence, the inevitable end of a development -which begins with the egg-cell and reaches its highest point with the -liberation of the germ-cells, that is, with reproduction, and then -enters on a longer or shorter period of decadence, leading to the -natural death of the individual.</p> - -<p>It is only by straining the analogy that this course of development -can be compared with the origin and transformation or extinction -of species. Not even the entirely external analogy of the -blossoming from a small beginning and the subsequent decay is -always correct; for in the fresh-water snails of Steinheim, at any rate, -almost the whole of the members of the species underwent a -transformation at a particular time, and became a new species, -which was after a long time retransformed without any appreciable -decrease in the number of individuals being observable. To speak -of a 'senile stage' of the species, of a stiffening of its form, of an -incapacity for further transformation, is to indulge in a play of fancy -quite inadmissible in the domain of natural science.</p> - -<p>It is admitted, however, that there is a correct idea at the base -of all this, for many species have not passed over into new forms, but -have simply died out because they were unable to adapt themselves -to changed conditions. This did not happen because they had -become incapable of variation, but because they could not produce -variations of sufficient magnitude, or variations of the kind required -to enable the species to remain an active competitor in the struggle -for existence.</p> - -<p>It obviously depends upon the coincidence of manifold circumstances, -whether an adaptation can be successfully effected or not. -Above all, it must be able to keep pace with the changes in the -conditions of life, for if these advance at a more rapid rate the -organisms will succumb in the midst of the attempt at adaptation. -It is probably in this way that the striking disappearance of the -Trilobites is to be explained, as Neumayr has pointed out, for the -Nautilidæ, a new group of enemies, multiplied so quickly at their -expense that they had not time to evolve any effective means of -protection. It cannot be maintained for a moment that every species<span class="pagenum"><a id="Page_357"></a>[Pg 357]</span> -is able to protect itself against extermination by any other; the -increased fertility, the increased rapidity of locomotion, the increased -intelligence and similar qualities, may all be insufficient, and then -extinction follows; not, however, because the species has become -'senile,' but because the variations possible to its organization do not -suffice to maintain it in the struggle.</p> - -<p>In discussing germinal selection I mentioned the view expressed -by Emery, that excessive variation in the same direction from intra-germinal -causes has not rarely been the cause of the extinction of -species. I also mentioned the very similar view of Döderlein, who -could not refer at that time to germinal selection, but assumed internal -compelling forces, which pressed a variation irresistibly forward in -the direction in which it had started, even beyond the bounds of what -is useful for the desired end, and which might thus bring about the -extinction of the species. I cannot entirely agree with these views, as -I have already indicated, because I do not believe that the impulse -to variation can ever become irresistible and uncontrollable. If it -could, then we should not see, as we do, innumerable cases in which -the augmentation or diminution of a part has gone on precisely to the -point at which it ceases to be purposeful. Even the degeneration of -organs only proceeds as far as is necessary to accomplish a particular -end, as we see plainly from the parasitic Crustaceans of different -orders. In many of these parasitic forms the swimming legs degenerate, -but in the female only, because these attach themselves by -suckers or in some other manner to their host, so that they cannot -let go again. But the males need their swimming legs to seek out the -females. The females too require them in their youth, in order to seek -out the fish from which they are to obtain their food-supply, and thus -the degeneration of the swimming legs has come to a full stop exactly -at the point where they cease to be of use; they develop in early -youth and degenerate later, when the animal becomes sessile. In -accordance with the law of biogenesis we may say that while the -degeneration is complete in the final stages of ontogeny, its retrogression -was not continued back to the germ, but only to the young -stages. From this it follows that the progress of a variation may -at any time have a goal fixed for it, and we have seen that this -is possible by means of personal selection, which accumulates the -never-failing fluctuations of the variation in the direction of plus -or of minus. In the individual id a determinant <i>X</i> may perhaps -decrease and possibly also increase without limit, although we -have no certain knowledge in regard to the latter point, but as -this determinant is contained in all the ids, there are always plus<span class="pagenum"><a id="Page_358"></a>[Pg 358]</span> -and minus fluctuations by means of which personal selection can -operate.</p> - -<p>But of course it requires a certain amount of time for this, and -in the fact that this time is often not available lies, I think, the -reason why excessive differentiations have often led to the extinction -of a species, not because the increase of the excessive organ must -go on irresistibly, but because changes in the conditions have made -the exuberant organ inappropriate, and it could not degenerate quickly -enough to save the species from extinction.</p> - -<p>Brandes has recently given a beautiful illustration of this by -associating the existence of the remarkable sabre-toothed tigers -(<i>Machairodus</i>) with enormously long canine teeth, which lived in the -Diluvial period in South America, with the gigantic Armadillos which -lived there at the same time, whose bony armature two yards in -height now excites our admiration. He rightly points out that the -dentition of <i>Machairodus neogæus</i> is by no means a typically perfect -dentition for a beast of prey, like that of the Indian tiger or the lion; -as far as incisors and molars are concerned it was much less effective -than that of these predatory animals, and the great length of the -dagger-like flattened canines, which protruded far beyond the mouth, -entirely prevented the bringing together of the teeth of the upper and -lower jaw after the fashion of a pair of pincers. He rightly infers -from this that this dentition was adapted to a specialized mode of -nutrition, and he regards the great mailed Armadillos, such as the -three-yards-long heavy Glyptodont of the Pampas, as the victims into -which they were wont to thrust their sabre-teeth in the region of the -unprotected neck, and thus to master the almost invulnerable creature, -which was invincible as far as all other predatory animals were concerned. -Thus the remarkable dentition is explained on the one hand, -and on the other the amazing extent and hardness of the victim's -coat of mail. Thus, too, we can understand why there should have -been at that time a whole series of cat-like animals with sabre-like -teeth, in which the length and sharpness of the teeth increased with -the bodily size, for these predatory animals corresponded to a whole -series of Armadillos, whose size was increasing, as was also the strength -of their armour.</p> - -<p>Of course this interpretation is hypothetical, but it contains -much internal probability, so that it may be taken as a good illustration -of the reciprocal increase of adaptations between two animal -groups. We understand now why, on the one hand, this colossal -tortoise-like armour should have developed in a mammal, and, on the -other hand, why these enormously long sabre-teeth should have been<span class="pagenum"><a id="Page_359"></a>[Pg 359]</span> -evolved; we also understand—and this is the point with which we -are here chiefly concerned—why these two 'excessive' developments -should ultimately lead to the destruction of their possessors. For -a long period the Armadillos were able to save themselves from extermination -by increasing their bodily size and the strength of their -armour, and they thus saved themselves from persecution on the part -of beasts of prey with smaller and weaker teeth. But the predatory -animals followed suit and lengthened their teeth and increased their -bodily size, until ultimately even the strongest armour of the victim -afforded no efficient protection, and the mighty Glyptodonts were by -degrees utterly exterminated. But then the death-knell of the -<i>Machairodus</i> had also sounded, for he was so exactly adapted to this -one kind of diet that he could no longer overpower other victims and -feed on their flesh; the sabre-teeth prevented him from tearing his -prey like other predatory animals, he could probably only suck them.</p> - -<p>Even if this is a supposititious case, it serves to show that it was -not an internal principle of variation that caused the teeth of these -carnivores and the armour of their victims to increase so unlimitedly; -it was the necessity of adaptation. They did not perish because -armour and teeth increased so excessively, but because neither of -these adaptations could be neutralized all at once, and small variations -were of no use to them in their final struggle for survival.</p> - -<p>In a certain sense we may say that simpler, more lowly organisms -are more capable of adaptation than those which are highly differentiated -and adapted to specialized conditions in all parts of their -bodies, since from the former much that is new may arise in the -course of time, while very little and nothing very novel can spring -from the latter. From the simplest Protozoan the whole world of -unicellular organisms could arise, and also the much more diverse -Metazoa; from the lower marine worms there could arise not only -many kinds of higher marine worms—the segmented worms or -Annelids—but also quite new groups of animals, the Arthropods and -the Vertebrates. It is hardly likely that a new class of animals -will evolve from our modern birds, because these are already so -perfectly adapted to their aerial life that they could hardly adapt -themselves to life on land or in the water sufficiently well to be able -to hold their own in regard to all the possibilities of life with the rest -of the dwellers on land or in water. We do indeed know of birds -which have returned entirely to a purely terrestrial life—the ostriches, -for instance—and of others which have adapted themselves to -a purely aquatic life, such as the penguins, but these are small -groups of species, and are hardly likely to increase. On the contrary,<span class="pagenum"><a id="Page_360"></a>[Pg 360]</span> -we can prove that many have already succumbed in the struggle -with man, and we anticipate the extermination of others. But the -reason why they are so readily exterminated obviously lies in the fact -that they have surrendered the advantage given to them by their bird-nature, -by adapting themselves to terrestrial life, and that they are -not able to regain it, at least not in the short time that is at their -disposal if they are to be saved from extermination. The best -example of this is the Dodo (<i>Didus ineptus</i>). This remarkable-looking -bird, of about the size of a swan, lived in flocks upon the island of -Mauritius until about the end of the seventeenth century. It had -small wings with short quills which were useless for flight. As it -could neither escape by flight nor through the water, and could only -move clumsily and awkwardly upon land with its short legs and -heavy body, it was hopelessly doomed as soon as a stronger enemy -made his appearance. It fell a victim to the sailors who first landed -on the island and clubbed it with sticks in huge numbers. Until that -event it was without doubt excellently adapted to life on that fertile -island, for on a volcanic island in the middle of the ocean there were -no large enemies, and it was therefore not dependent on the power of -flight for safety, and could pick up abundant food from the ground. -But when man suddenly appeared on the scene and began to persecute -it, it was not the 'senile rigidity' of its organization that -prevented it from making use of its wings again; it was the slowness -of variation and consequently of selection, which is common to all -species, which impelled it to extinction. The same fate will probably -overtake the Kiwi of New Zealand (<i>Apteryx australis</i>) in the near -future, for though it has so far escaped the arrows of the aborigines, -it is not likely in its wingless condition to be able to hold out long -against European guns, unless close times and preserved forests are -instituted for it, as they have been for our chamois.</p> - -<p>Even sadder from the biologist's point of view than such extermination -of individual species through the vandalism and greed of our -own race is the disturbance of whole societies of animals and plants -by man that is going on or has been accomplished on most of the -oceanic islands, and we must briefly notice these cases while we are dealing -with the decadence of species. I refer to the crowding out of the -usually endemic animal and plant population on such islands through -cultivation. The first step in this work of 'cultivation' is always the -cutting down of the forests which for thousands of years have clothed -these islands as with a mantle of green, have regulated their rainfall, -secured their fertility, and allowed a medley of indigenous animals, -usually peculiar to the spot, to arise. We have already spoken of<span class="pagenum"><a id="Page_361"></a>[Pg 361]</span> -St. Helena. The original and remarkable fauna and flora of this -island had for the most part disappeared 200 years ago, through the -cutting down of trees in the forests, and these were later wholly -destroyed by the introduction of goats, which devoured all the young -trees as they grew. But with the forests most of the indigenous -insects and birds were doomed to destruction, so that now there is -not an indigenous bird or butterfly to be found there; only a few -terrestrial snails and beetles of the original fauna still survive.</p> - -<p>But it is not only on islands that a large number of species have -been decimated or entirely exterminated by deforestation, by the -introduction of plants cultivated by man and of the 'weeds' -associated with these, and by the importation of domesticated animals. -In Central Europe not only have all the larger beasts of prey, like -the bear, the lynx, and the wolf, almost completely disappeared, but -the reindeer, the bison, the wild ox (Aurochs), and the elk have been -exterminated as wild animals, and in North America the buffalo will -soon only exist in preserved herds, if that is not already the case. Here, -of course, the direct interference of the all-too-powerful enemy, man, -has played the largest part in causing the disappearance of the species -referred to, but the process may give us an idea of the way in which a -superior animal enemy may be able gradually to exterminate a weaker -species where there is no attainable or even conceivable variation -which might preserve them from such a fate. Several of the mammals -which I have mentioned are not yet entirely exterminated; even the -Aurochs perhaps still exists in the pure white herds preserved in -some British parks; but there are more instances than that of the -Dodo of the utter extermination of a species through human agency -within historic times. It may be doubtful whether the sea-otter -(<i>Enhydris marina</i>) has not been already quite exterminated because -of its precious fur, but it is quite certain that the huge sea-cow -(<i>Rhytina stelleri</i>), which lived in large numbers in the Behring Straits -at the end of the eighteenth and the beginning of the nineteenth -centuries, was completely exterminated by sailors within a few -decades.</p> - -<p>We may therefore gain from what is going on before our eyes, -so to speak, some sort of idea of the way in which the extermination -of species may go on even independently of man at the present time, -and how it must have gone on also in past ages of the earth's history. -Migrations of species have taken place ceaselessly, although very -slowly, for every species is endeavouring slowly to extend its range -and to take possession of new territories, and thus the fauna and -flora of any region must have changed in the course of time, new<span class="pagenum"><a id="Page_362"></a>[Pg 362]</span> -species must have settled in it from time to time, and the conditions -of life must have changed, and in many cases this must have led -to the extermination of species, in the same way, though not so -quickly, as human interference now brings about their doom.</p> - -<p>This is true for plants as for animals. A good example, not -indeed of complete extermination, but of very considerable diminution -in the numbers of individuals of a plant-species by the advent of -a species of mammal, is communicated to us by Chun in regard -to Kerguelen Land. A flowering plant, the Kerguelen cabbage -(<i>Pringlea antiscorbutica</i>), has been greatly reduced in numbers since -the thoughtless introduction of rabbits to this uninhabited island -(1874). While, in 1840, Captain Ross used this plant in great -quantities as a preventative against scurvy in his crew, and even -carried away stores to last for months, the Valdivia Expedition in -1898 found rabbits in abundance, but the Kerguelen cabbage had -been entirely exterminated at every spot accessible to these prolific -and voracious rodents. It was only found growing upon perpendicular -cliffs or upon the islands lying out in the fiords.</p> - -<p>An avoidance of the threatened destruction of a species by -its adaptation to the new circumstances can only be possible when the -changes occur very slowly, and will therefore be more likely to -be achieved in the case of physical changes in the conditions of -life, such as climatic changes, a change in the mutual relations of land -and sea, and so on. But it appears that even climatic changes do not -evoke any variation and new adaptation as long as the species can -avoid the changes by migrating. The often quoted case of Alpine -and Arctic plants proves this at any rate, that those species which -inhabited the plateaus and highlands of Europe did not all vary -to suit the change when a warmer climate prevailed, but that in -part at least they followed the climate to which they were already -adapted, that is, that they migrated towards the north on the one -hand and higher up the Alps on the other. It cannot be denied -that many of the insects and plants did adapt themselves at that time -to the warmer climate, and became the modern species which now -inhabit the plains, for many related species occur on the Alps and in -the plains, but apparently many others simply made their escape from -a climate which no longer suited their requirements. Thus, as far -as I am aware, there is no species of <i>Primula</i> in South or Central -Germany which could be derived from the beautiful red <i>Primula -farinosa</i> of the Alps, but this species occurs also upon the old -glacier-soil at the northern base of the Alps, and in similar soil -again in the north of Germany and on the meadows of Holstein.<span class="pagenum"><a id="Page_363"></a>[Pg 363]</span> -Similar examples might be cited in regard to the Alpine-Arctic -butterflies.</p> - -<p>It is intelligible enough that we are still very far from being able -to give a precise account of the main changes in the plant and animal -world during the history of the earth in regard to the special causes -which have produced them. Possibly the future will throw more -light upon this subject by extending our knowledge of the fossil -remains of all countries. But so much at least we can say at present, -that there is no reason to refer the dying out of the earlier forms -to anything else than the changes in the conditions of life, the -struggle for existence, and the limitation of the power of transformation -and adaptation due to the organization which the species has already -attained; there is no trace of any such thing as a phyletic principle -of life in the vitalistic sense, as far as the decadence of species is -concerned.</p> -<hr class="full" /> - -<div class="chapter"> -<p><span class="pagenum"><a id="Page_364"></a>[Pg 364]</span></p> - -<h2 class="nobreak" id="LECTURE_XXXVI">LECTURE XXXVI</h2> -</div> - -<p class="c">SPONTANEOUS GENERATION AND EVOLUTION:<br /> -CONCLUSION</p> - -<div class="blockquot"> - -<p>Spontaneous generation—Experimental tests impossible—Only the lowest and -smallest forms of life can be referred to spontaneous generation—Chemical postulates -for spontaneous generation—Empedocles modernized—The locality of spontaneous -generation—Progress of organization—Direct and indirect influences causing variation—The -various modes of selection—Everything depends upon selection—Sinking from -heights of organization already attained—Paths of evolution—The forces effecting it—Plasticity -of living matter—Predetermination of the animate world—Many-sided -adaptation of each group—Aquatic mammals and insects, parasites—Nägeli's variation -in a definite direction—Analogy of the traveller—Genealogical trees—The diversity of -forms of life is unlimited—The origin of the purposeful apart from purposive forces -working towards an end—The limits of knowledge—Limitation of the human intelligence -by selection—Human genius—Conclusion.</p></div> - - -<p><span class="smcap">We</span> have now reached the end of our studies, and they have -given us satisfaction, at least in so far that they have brought us -certainty in regard to the chief and fundamental question which can -be asked in reference to the origin of the modern animate world -of organisms. There remains no doubt in our minds that the theory -of descent is justified; we know, just as surely as that the earth goes -round the sun, that the living world upon our earth was not created -all at once and in the state in which we know it, but that it has -gradually evolved through what, to our human estimate, seem enormously -long periods of time. This conclusion is now firmly established -and will never again become doubtful. The assumption, too, -that the more lowly organisms formed the beginnings of life, and -that an ascent has taken place from the lowest to the higher and -highest, has become to our minds a probability verging upon certainty. -But there remains one point which we have not yet touched upon—the -problem of the origin of these first organisms.</p> - -<p>There are only two possibilities: either that they have been borne -to our earth from outside, from somewhere else in the universe, or that -they have originated upon our earth itself through what is called -'spontaneous generation'—<i>generatio spontanea</i>.</p> - -<p>The idea that very lowly living organisms might have been -concealed within the clefts and crevices of meteorites, and might thus<span class="pagenum"><a id="Page_365"></a>[Pg 365]</span> -have fallen upon our earth and so have formed the first germs of life, -was first formulated by that chemical genius, Justus Liebig. It seems -certain that the state of glowing heat in which meteorites are, when -they come into our atmosphere, only affects the outer crust of these -cosmic fragments, and that living germs, which might be concealed in -the depths of their crevices and fissures, might therefore remain alive, -but nevertheless it is undoubtedly impossible that any germ should -reach us alive in this way, because it could neither endure the -excessive cold nor the absolute desiccation to which it would be -exposed in cosmic space, which contains absolutely no water. This -could not be endured even for a few days, much less for immeasurable -periods of time.</p> - -<p>But we have to take account, too, of an entirely general reason, -which lies in the fact that all life is transient, that it can be -annihilated, and is not merely mortal! Everything that is distinctively -organic may be destroyed to the extent of becoming -inorganic. Not only may the phenomena of life disappear, and the -living body as such cease to be, but the organic compounds which -form the physical basis of all life are ceaselessly breaking up, and -they fall back by stages to the level of the inorganic. It seems to me -that we must necessarily conclude from this that the basis of Liebig's -idea was incorrect, that is, the assumption that 'organic substances -are everlasting and have existed from the first just in the same way -as inorganic substances.' This is obviously not the case, for a thing -that has an end cannot be everlasting; it must have had a beginning -too, and consequently organic combinations are not everlasting, but -are transitory; they come and go, they arise wherever the conditions -suitable for them occur, and they break up into simpler combinations -when these conditions cease to be present. It is only the elements -which are eternal, not their combinations, for these are subject to -more or less rapid continual change, whether they have arisen outside -of organisms or within them.</p> - -<p>It seems to me that these considerations destroy the foundations -of the hypothesis of the cosmic origin of life on our earth; in any -case they leave the hypothesis without great significance; for if we -could even admit the possibility of a transference of living organisms -from space, the question would only be pushed a little further back -by the assumption, and not solved, for the organisms thus brought in -must have had their origin on some other planet, since they are, <i>ex -hypothesi</i>, not everlasting.</p> - -<p>Thus we are directed to our earth itself as the place of the origin -of the tellurian world of life, and I see no possibility of avoiding<span class="pagenum"><a id="Page_366"></a>[Pg 366]</span> -the assumption of <i>spontaneous generation</i>. It is for me a logical -necessity.</p> - -<p>Even about the middle of the nineteenth century there was acute -discussion in regard to the occurrence of spontaneous generation. In -the French Academy especially Pouchet brought forward arguments -in favour of it, and Pasteur against it. Pouchet observed that living -organisms made their appearance in infusions of hay and other -vegetable material in which any possible living germs had presumably -been destroyed by prolonged boiling. Living organisms, Algæ, and -Infusorians appeared, notwithstanding the fact that the glass bottles -in which they were kept were hermetically sealed. But Pasteur -showed that the air contains numerous living germs of lowly -organisms in its so-called motes, and that, if these were first removed, -Pouchet's infusion would not exhibit any signs of life. He caused -the air, which was continually passed through the tubes, to stream -first along the heated barrel of a gun, and so destroyed these germs, -and no organisms were obtained in the infusions. He showed that -the air is teeming with germs by an experiment with boiled infusions -which were allowed to lie undisturbed for a considerable time in -bottles with open necks, one on the roof of the Institute at Paris, the -other on the top of the Puy de Dôme in Auvergne, which was at that -time still the highest mountain in France. In the Parisian experiment, -organisms appeared in the bottles in a very few days, while in those -exposed to the pure air at the mountain-top none were seen, even -after months had elapsed.</p> - -<p>Strangely enough, these and similar experiments were at the time -regarded as conclusive proof against the existence of spontaneous -generation, though it is obvious enough that the first living being on -this earth cannot have sprung from hay, or from any other organic substance, -since that would presuppose what we are attempting to explain. -After the fiery earth had so far cooled that its outermost layer had -hardened to a firm crust, and after water had condensed to a liquid -form, there could at first only have been inorganic substances in -existence. In order to prove spontaneous generation, therefore, it -would be necessary to try to find out from what mingling of inorganic -combinations organisms could arise; to prove that spontaneous -generation could never have been possible is out of the question.</p> - -<p>It would be impossible to prove by experiment that spontaneous -generation could <i>never</i> have taken place; because each negative experiment -would only prove that life does not arise <i>under the conditions -of the experiment</i>. But this by no means excludes the possibility that -it might arise under other conditions.</p> - -<p><span class="pagenum"><a id="Page_367"></a>[Pg 367]</span></p> - -<p>Up till now all attempts to discover these conditions have been -futile, and I do not believe that they will ever be successful, not -because the conditions must be so peculiar in nature that we cannot -reproduce them, but above all, because we should not be able to -perceive the results of a successful experiment. I shall be able to -prove this convincingly without difficulty.</p> - -<p>If we ask ourselves the question how the living beings which -might have arisen through spontaneous generation must be constituted, -and on the other hand, in regard to what kinds of living forms -we can maintain with certainty that they <i>could not</i> have arisen thus, -it is obvious that we must place on the latter list all organisms which -presuppose the existence of others, from which they have been -derived. But to this category belong all the organisms which possess -a germ-plasm, an idioplasm that we conceive of as composed of primary -constituents (<i>Anlagen</i>) which have gradually been evolved and -accumulated through a long series of ancestors. Thus not only all -multicellular animals and plants which reproduce by means of germ-cells, -buds, and so forth, but also all unicellular organisms, must be -placed in this class. For these last—as we have seen—possess in -their nucleus a substance made up of primary constituents, without -which the mutilated body is unable to make good its loss, in short, an -idioplasm. That this plays the same rôle in unicellular as in multicellular -organisms we can infer with the greatest certainty from the -process of amphimixis, which runs its course in an analogous way in -both cases.</p> - -<p>Thus, even though we did not know what Ehrenberg demonstrated -in the third decade of last century, that Infusorians in an -encapsuled state can be blown about everywhere, and can even be -carried across the ocean in the dust of the trade-winds, to re-awaken -to life wherever they fall into fresh water, we should still not have -remained at the standpoint of Leuwenhoek, who regarded Infusorians -as having arisen through spontaneous generation. They cannot arise -in this way, nor can they have done so at any time, because they -contain a substance made up of primary constituents, which can only -be of historic origin, and cannot therefore have arisen suddenly after -the manner of a chemical combination.</p> - -<p>The same is true of all the unicellular organisms, even of those -which are much more simple in structure than the Infusorians, whose -differentiation into cortical and medullary substances, oral and anal -openings, complex arrangements of cilia and much else, betokens -a high degree of differentiation in the cell. But even the Amœba is -only apparently simple, for otherwise it could not send out processes<span class="pagenum"><a id="Page_368"></a>[Pg 368]</span> -and retract them again, creep in a particular direction, encyst itself, -and so on, for all this presupposes a differentiation of its particles in -different directions, and a definite arrangement of them; and there is -in addition the marvellous dividing-apparatus of the nucleus which -is not wanting even in the Amœba. All this again points to -a historic evolution, a gradual acquiring and an orderly arrangement -of differentiations, and such an organism cannot have arisen suddenly -like a crystal or a chemical combination.</p> - -<p>Thus we are driven back to the lowest known organisms, and -the question now before us is whether these smallest living organisms, -which are only visible under the highest powers of the microscope, -may be referred to spontaneous generation. But here too the answer -is, No; for although there is no nucleus to be found, and no substance -which we can affirm with any certainty to be composed of primary -constituents or idioplasm, we do find distinct traces of a previous -history, and not the absolutely simple structure of homogeneous living -particles, unarranged in any orderly way, which is all that could be -derived from spontaneous generation. It has been shown quite -recently that the typhus bacillus possesses an extremely delicate -much-branched tuft of flagella, which gives it a tremulous motion, -and in the cholera bacillus cortical and medullary substances can be -distinguished. Thus even here there is differentiation according to -the principle of division of labour, and how numerous must be -the minute vital particles of which a substance consists when it can -form such fine threads as the flagella just mentioned! Nägeli, who -elaborated an analogous train of thought in regard to spontaneous -generation, calculated the number of these smallest vital particles (his -'micellæ') which must be contained in a 'moneron' of 0.6 mm. -diameter, if we take its dry substance at 10 per cent., and he arrived -at the amazing figure of 100 billions of vital particles. Even if -we suppose the diameter of such an organism to be 0.0006 mm., it -would still be composed, according to this calculation, of a million -of these vital particles.</p> - -<p>We have reached, in the course of these lectures, the conviction -that minute living units form the basis of all organisms, namely, -our 'life-bearers' or 'biophors.' These must be present in countless -multitudes, and in a great number of varieties in the different forms -of life, but all agree in this, that they are simple, that is, they are not -composed in their turn of living particles, but only of molecules, -whose chemical constitution, combination, and arrangement are such -as to give rise to the phenomena of life. But they may vary, and -on this power depends the possibility of their differentiation, which<span class="pagenum"><a id="Page_369"></a>[Pg 369]</span> -has taken place in more and more diverse ways in the course of -phylogeny. They, too, arise in the existing organism, like all vital -units, only by multiplication of the biophors already present, but they -do not necessarily presuppose a historic origin; it is conceivable of -them, at least as far as their first and simplest forms are concerned, -that they may have arisen some time or other through spontaneous -generation. In regard to them alone is the possibility of origin -through purely chemico-physical causes, without the co-operation of -life already existing, admissible. It is only in regard to them that -spontaneous generation is not inconceivable.</p> - -<p>We must, therefore, assume that, at some time or other in the -history of the earth, the conditions necessary to the development of -these invisible little living particles must have existed, and that the -whole subsequent development of the organic world must have depended -upon an aggregation of these biophors into larger complexes, -and upon their differentiation within these complexes.</p> - -<p>We shall never be able, then, directly to observe spontaneous -generation, for the simple reason that the smallest and lowest living -particles which could arise through it, the Biophoridæ, are so extremely -far below the limits of visibility, that there is no hope of our -ever being able to perceive them, even if we should succeed in -producing them by spontaneous generation.</p> - -<p>I do not propose to discuss the chemical problem raised by the -possible occurrence of spontaneous generation. We have already -seen that dead protoplasm, in addition to water, salts, phosphorus, -sulphur, and some other elements, chiefly and invariably contains -albumen; an albuminoid substance must, therefore, have arisen from -inorganic combinations. No one will maintain that this is impossible, -for we continually see albuminoid substances produced in plants from -inorganic substances, compounds of carbon and nitrogen; but under -what conditions this would be possible in free nature, that is, outside -of organisms, cannot as yet be determined. Possibly we may some -time succeed in procuring albumen from inorganic substances in the -laboratory, and if that happens the theory of spontaneous generation -will rest upon a firmer basis, but it will not have been experimentally -proved even then. For while dead albumen is certainly nearly allied -to living matter, it is precisely <i>life</i> that it lacks, and as yet we -do not know what kinds of chemical difference prevail between the -dead proteid and the living; indeed we must honestly confess that it -is a mere assumption when we take for granted that there are only -chemico-physical differences between the two. It cannot be proved, -in the meantime, that there is not another unknown power in the<span class="pagenum"><a id="Page_370"></a>[Pg 370]</span> -living protoplasm, a 'vitalistic principle,' a 'life-force,' on the activity -of which these specific phenomena of life, and particularly the continually -repeated alternation of disruption and reconstruction of the -living substance, dissimilation and assimilation, growth and multiplication, -depend. It is just as difficult to prove the converse, that it -is impossible that chemico-physical forces alone should have called -forth life in a chemical substance of very special composition. -Although no one has ever succeeded, in spite of many attempts, in -thinking out a combination of chemical substances which—as this -wonderful living substance does—on the one hand undergoes combustion -with oxygen and, on the other hand, renews itself again with -'nutritive' material, yet we cannot infer from this the impossibility -of a purely chemico-physical basis of life, but must rather hold fast -to it until it is shown that it is not sufficient to explain the facts, -thus following the fundamental rule that natural science must not -assume unknown forces until the known ones are <i>proved</i> insufficient. -If we were to do otherwise we should have to renounce all hope of -ever penetrating deeper into the phenomena. And we have no need -to do this, for in a general way we can quite well believe that an -organic substance of exactly proportioned composition exists, in which -the fundamental phenomena of all life—combustion with simultaneous -renewal—must take place under certain conditions by virtue of its -composition.</p> - -<p>How, and under what external conditions, such a substance -first arose upon the earth, from and of what materials it was formed, -cannot be answered with any certainty in the meantime. Who knows -whether the fantastic ideas of Empedocles in an altered form would -not be justified here? I mean that, at the time of the first origin of -life, the conditions necessary for many kinds of complex chemical -combinations may have been present simultaneously on the earth, -and that, out of a manifold variety of such substances, only those -survived which possessed that marvellous composition which conditioned -their continual combustion, but also their ceaseless reconstruction -by multiplication. According to Empedocles, there arose -from chaos only parts of animals—heads without bodies, arms without -trunks, eyes without faces, and so on—and these whirled about in wild -confusion and flew together as chance directed them. But those -only survived which had united rightly with others so as to form -a whole, capable of life. Translated into the language of our time, -that would mean what I have just said—that, of a large number -of organic combinations which arose, only a few, perhaps one, would -possess the marvellously adjusted composition which resulted in life,<span class="pagenum"><a id="Page_371"></a>[Pg 371]</span> -and with it self-maintenance and multiplication; and that would be -the first instance of selection!</p> - -<p>But let us leave these imaginings, and wait to see whether the -chemists will not possibly be able to furnish us with a starting-point -for a more concrete picture of the first origin of life. In the meantime, -we must confess that we find ourselves confronted with deep darkness.</p> - -<p>The question as to the 'Where' of spontaneous generation must -also be left without any definite answer. Some have supposed that -life began in the depths of the sea, others on the shore, and others in -the air. But who is to divine this, when we cannot even name -theoretically the conditions and the materials out of which albuminoid-like -substances might be built up in the laboratory? Nägeli's -hypothesis still seems to me to have the greatest probability. -According to his theory, the first living particles originated not in -a free mass of water, but in the reticulated superficial layer of a fine -porous substance (clay or sand), where the molecular forces of solid, -fluid, and gaseous bodies were able to co-operate.</p> - -<p>Only so much is certain, that wherever life may first have -arisen upon this earth, it can have done so only in the form of the -very simple and very minute vital units, which even now we only -infer to be parts of the living body, but which must first have arisen -as independent organisms, the 'Biophoridæ.' As these, according to -our theory, possessed the character of life, they must have possessed -above all the capacity of assimilating in the sense in which the -plants assimilate, that is, of renewing their bodily substance continually -from inorganic substances, of growing, and of reproducing. -They need not on that account have possessed the chemical constitution -of chlorophyll, although the capacity of assimilation in green -plants depends upon this substance, for we know colourless fungi, -which, notwithstanding the absence of chlorophyll, are able to build -up the substance of their body from compounds of carbon and nitrogen.</p> - -<p>The first advance to a higher stage of life must have been brought -about by multiplication, since accumulations of Biophoridæ, unintegrated -but connected masses, would be formed.</p> - -<p>In this way the threshold of microscopical visibility would -gradually be reached and crossed, but—to argue from the modern -Baccilli—long before that time a differentiation of the biophors on -the principle of division of labour would have taken place within -a colony of Biophoridæ. This first step towards higher organization -must probably have taken enormous periods of time, for before any -differentiation could occur and bring any advantage the unintegrated -aggregates of Biophors must first have become orderly, and have<span class="pagenum"><a id="Page_372"></a>[Pg 372]</span> -formed themselves into a stable association with definite form and -definite structure, somewhat analogous to the spherical cell-colonies -of <i>Magosphæra</i> or <i>Pandorina</i>. Only then was the further step made -of a differentiation of the individual biophors forming the colony, and -this is comparable to the species of <i>Volvox</i> among the lower Algæ. -The gradual ascent of these colonies of biophors must, then, be -referred to the principles to which we attribute the ascent of the -higher forms of life to ever-higher and ever-new differentiations; -the principles of division of labour and selection.</p> - -<p>These differentiated colonies of biophors have brought us nearer -to the lowest known organisms, among which there are some whose -existence we can only infer from their pathological effects, since -we have not been able to make them visible. The bacillus of measles -has never yet been seen, but we cannot doubt its existence, and -we must assume that there are bacilli of such exceeding smallness -that we shall never be able to see them, even with the most improved -methods of staining and the strongest lenses.</p> - -<p>These non-nucleated Monera lead on to the stage of nucleus-formation, -and this at once implies the cell. As, on our view, the -nucleus is primarily a storehouse of 'primary constituents' (<i>Anlagen</i>), -its origin must have begun at the moment at which the differentiation -of the cell-body reached such a degree of differentiation of its parts -that a mechanical division into two halves was no longer possible, -and that the two products of division, if they were each to develop -to a new and intact whole, required a reserve of primordia (<i>Anlagen</i>) -to give rise to the missing parts. As this higher differentiation -would bring about a superiority over the lower forms of life, in that -they would make possible the utilization of new conditions of life, -but on the other hand could only survive if the differentiation of -a reserve of primary constituents, that is, a nucleus, were introduced -at the same time, the development of the nucleus can be ranged under -the principle of utility to which we traced back the evolution of -all higher and more differentiated forms of life. But it would -scarcely be profitable to try to follow out in detail the first steps -in the progress of organization under the control of selective processes, -since we know far too little about the life of the simplest organisms -to be able to judge how far their differentiations are of use in improving -their capacity for life.</p> - -<p>That would be a bold undertaking even in regard to unicellular -organisms, and it is only in the case of multicellular organisms that -we can speak with greater certainty and really recognize the changing -of the external conditions, in the most general and comprehensive<span class="pagenum"><a id="Page_373"></a>[Pg 373]</span> -sense, as the fundamental cause of the lasting variations of organic -forms. We can here distinguish with certainty between the direct -and the indirect effect of external influences, and we see how these -sources of variation interact upon each other. The lowest and deepest -root of variation is without doubt the direct effect of changed conditions. -Without this the indirect effect would have had no lever with -which to work, for the primitive beginnings of variation would -be absent, and an accumulation of these through personal selection -could not take place. It is a primitive character of living substance -to be variable, that is, to be able to respond to some extent to changed -external conditions, and to vary in accordance with them, or—as we -might also say—to be able to exist in many very similar but not -identical combinations of substances, and we must imagine that even -the first biophors which arose through spontaneous generation were -different according to the conditions under which, and the substances -from which, they originated. And from each of these slightly different -beginnings there must, in the course of multiplication by fission, have -been produced a whole genealogical tree of divergent variations of -the primitive Biophoridæ, since it is inconceivable that all the -descendants would remain constantly under the same conditions of -life under which they originated. For every persistent change in -the conditions of existence, and especially of nutrition, must have -involved a variation in the constitution of the organism, whose vital -processes, and especially the repair of its body, depended on these -conditions.</p> - -<p>But the external influences to which the descendants of a -particular form of life were subject never remained permanently -the same. Not only did the surface of the earth and its climatic -conditions change in the course of time with the cooling of the earth, -but mountains arose and were levelled again, old land-surfaces sank -out of sight or emerged again, and so on; all that, of course, played -its part in the transformation of the forms of life, but did so to any -considerable extent only at a later stage, when there were already -highly differentiated organisms. These unknown primitive beginnings -of life must have been forced to diverge into different variations -through the different conditions of the same place in which they -lived.</p> - -<p>Let us think of the simplest microscopic Monera on the mud of -the sea-coast, equipped with the faculty of plant-like assimilation, -and we shall see that their unlimited multiplication would cause -differences in nutrition, for those lying uppermost would be in -a stronger light than those below, and would, therefore, be better<span class="pagenum"><a id="Page_374"></a>[Pg 374]</span> -nourished, and, consequently, would transmit the variations thus -induced to their progeny which arose by fission. Thus it is conceivable -that even the more or less favourable position as regards light would -bring about the origin of two different races from the same parent -form, and as it is conceivable in the case of light, so is it also in -regard to all the influences which cause variation in the organism.</p> - -<p>We have already seen that variations in the lowest (non-nucleated) -forms of life caused by the direct influence of the vital processes may -be directly transmitted to the descendants, but that in all those whose -bodies have already differentiated into a germ- or idioplasmic-substance, -in contrast to a somatic substance in the more restricted sense, -this hereditary transmission is only possible in the case of the -variations of the germ-plasm, and <i>hereditary</i> variations of the species -can only arise by the circuitous route of influencing the germ-plasm. -The body (soma) can be caused to change by external influences, -by the use or disuse of an organ, but variations of this kind are not -transmitted; they do not become a lasting possession of the species, -but cease with the individual; they are transient changes.</p> - -<p>Thus it was only through those external influences—including -those from the soma of the organism itself—which affected the germ-substance, -either as a whole or in certain of its primary constituents, -that hereditarily transmissible variations of the organism arose, and -we have already discussed in detail how particular variational -tendencies may arise through the struggle of the parts within the -germ-plasm, which may give an advantage to certain groups of -primary constituents. And these tendencies are of themselves -sufficient to cause the specific type to vary further and further in -given directions.</p> - -<p>Nevertheless, the infinite diversity of the forms of life could -never have been brought about in this way alone, if there had not -been another—the <i>indirect</i>—effect of the changeful external influences.</p> - -<p>This is due to the fact that the variations of direct origin sooner -or later obtain an influence in determining the viability of their -possessors, either increasing or diminishing it. It is this, in association -with the unlimited multiplication of individuals, which gives a basis -to the principle of transformation, which it is the immortal merit -of Charles Darwin and Alfred Russel Wallace to have introduced -into science: <i>the principle of selection</i>. We have seen that this -principle may have a much more comprehensive meaning than was -attributed to it by either of these two naturalists; that there is -not merely a struggle between individuals which brings about their -adaptation to their environment, by preserving those which vary in<span class="pagenum"><a id="Page_375"></a>[Pg 375]</span> -the most favourable way and rejecting those which vary unfavourably, -but that there is an analogous struggle between the parts of these -individuals, which, as Wilhelm Roux showed, effects the adaptation -of the parts to their functions, and that this struggle must be assumed -to occur even between the determinants and biophors of the germ-plasm. -There is thus a germinal selection, a competition between the -smaller and larger particles of the germ-plasm for space and food, -and that it is through this struggle that there arise those definitely -and purposefully directed variations of the individual, which are -transmissible because they have their seat in the immortal germ-plasm, -and without which an adaptation of individuals in the sense -and to the extent in which we actually observe it would be altogether -inconceivable. I have endeavoured to show that the whole evolution -of the living world is guided essentially by processes of selection, -in as far as adaptations of the parts to one another, and of the whole -to the conditions of life, cannot be conceived of as possible except -through these, and that all fluctuations of the organism, from the -very lowest up to the highest, are forced into particular paths by this -principle, by 'the survival of the fittest.' This ends the whole -dispute as to whether there are indifferent 'characters' which have -no influence on the existence of the species, for even the characters -most indifferent for the 'person' would not exist unless the -germinal constituents (determinants) which condition them had -been victorious in the struggle for existence over others of their -kind, and even the 'indifferent' characters, which depend solely upon -climatic or other external influences, owe their existence to processes -of germinal selection, for those elements of the determinants concerned -were victorious which throve best under such influences. But should -variations thus produced by external influence increase so far that -they become prejudicial to the survival of their bearers, then they -are either set aside by personal selection or, if that be no longer -possible, they lead to the extinction of the species. Thus the multitude -of small individual variations, which probably occur in every species, -but which strike us most in Man—the differences in the development -of mouth, nose, and eyes, in the hair, in the colour of skin, &c., -as far as they are without significance in the struggle for existence—depend -upon processes of germinal selection, which permitted the -greater development of one group of determinants, or of one kind -of biophor in one case, of another in another. The proportionate -strength of the elements of the germ-plasm is not readily lost at -once, but is handed on to successive generations, and thus even these -'indifferent' characters are transmitted.</p> - -<p><span class="pagenum"><a id="Page_376"></a>[Pg 376]</span></p> - -<p>It is obvious that, if the principle of selection operates in nature -at all, it must do so wherever living units struggle together for the -same requirements of life, for space and food, and these units need -not be persons, but may represent every category of vital units, from -the smallest invisible units up to the largest. For in all these cases -the conditions of the selection-process are given: individual variability, -nutrition, and multiplication, transmission of the advantage attained, -and, on the other hand, limitation of the conditions of existence—especially -food and space. The resulting struggle for existence must, -in every category of vital units, be most acute between the individual -members of each category, as Darwin emphasized in the case of -species from the very first, and persistent variations of a species of -living units can only be brought about by this kind of struggle. -Strictly speaking, therefore, we should distinguish as many kinds of -selection-processes as there are categories of living units, and these -could not be sharply separated from one another, apart from the fact -that we have to infer many of them, and cannot recognize their -gradations. Here, as everywhere else, we must break up the -continuity of nature into artificial groups, and it seems best to -assume and distinguish between four main grades of selective -processes corresponding to the most outstanding and significant -categories of vital units, namely: Germinal, Histonal, Personal, and -Cormal Selection.</p> - -<p>Histonal Selection includes all the processes of selection which -take place between the elements of the body (soma), as distinguished -from the germ-plasm, of the Metazoa and Metaphyta, not only between -the 'tissues' in the stricter sense, but also between the parts of the -tissues, that is, the lower vital units of which they are composed, -and which Wilhelm Roux, when he published his <i>Kampf der Teile</i> -('Struggle of the Parts'), called 'molecules.' It occurs between all the -parts of the tissues down to the lowest vital units, the biophors. -We must also reckon under histonal selection the processes of selection -which take place between the elements of the simplest organisms, -and through which these have gradually attained to greater -complexity of structure and increased functional capacity. As long -as no special hereditary substance had been differentiated, variations -which arose in the simplest organisms through selection-processes of -this kind were necessarily transmitted to the descendants, but after -this differentiation had taken place this could no longer occur—'acquired' -modifications of the soma were no longer transmitted, and -the importance of histonal selection was limited to the individual. -But this form of selection must be of the greatest importance in<span class="pagenum"><a id="Page_377"></a>[Pg 377]</span> -regard to the adaptations of the parts which develop from the ovum, -especially during the course of development, and it is also indispensable -all through life in maintaining the equilibrium of the -parts, and their adaptation to the varying degree of function -required from them (use and disuse). But its influence does not -reach directly beyond the life of the individual, since it can only -give rise to 'transient' modifications, that is, to changes which cease -with the individual life.</p> - -<p>In contrast to this is Germinal Selection, which depends upon the -struggle of the parts of the germ-plasm, and thus only occurs in -organisms with differentiation of somatoplasm and germ-plasm, -especially in all Metazoa and Metaphyta—forming in these -the basis of all hereditary variations. But not every individual -variation to which germinal selection gives rise persists and spreads -gradually over the whole species, for, apart from the cases we have -already mentioned, in which indifferent variations favoured by external -circumstances gain the victory, this happens only if the variations -in question are of use to their bearer, the individual. Any variation -whatever may arise in a particular individual purely through -germinal selection, but it is only the higher form of selection—Personal -Selection—that decides whether the variation is to persist -and to spread to many descendants so that it ultimately becomes -the common property of the species. Germinal and personal selection -are thus continually interacting, so that germinal selection continually -presents hereditary variations, and personal selection rejects those -that are detrimental and accepts those that are useful. I will not -repeat any exposition of the marvellous way in which personal -selection reacts upon germinal selection, and prevents it from -continuing to offer unfavourable variations, and compels it to give -rise to what is favourable in ever-increasing potency. Although it -apparently selects only the best-adapted <i>persons</i> for breeding, it -really selects the favourable id-combinations of the germ-plasm, that -is, those which contain the greatest number of favourably varying -determinants. We saw that this depends upon the multiplicity of -ids in the germ-plasm, since every primary constituent of the body -is represented in the germ-plasm, not once only, but many times, -and it is always half of the homologous determinants contained in -the germ-plasm of an individual which reach each of its germ-cells, -always, moreover, in a different combination. Thus, with the -rejection of an individual by personal selection, a particular combination -of ids, a particular kind of germ-plasm is in reality removed, and thus -prevented from having any further influence upon the evolution of<span class="pagenum"><a id="Page_378"></a>[Pg 378]</span> -the species. By this means germinal selection itself is ultimately -influenced, because only those ids remain unrejected in the germ-plasm -whose determinants are varying in directions useful to the species. -Thus there comes about what until recently was believed to be -impossible: the conditions of life give rise to useful directions of -variation, not directly, certainly, but indirectly.</p> - -<p>We may distinguish as a fourth grade of selection Cormal -Selection, that is, the process of selection which effects the adaptation -of animal and plant stocks or corms, and which depends on the -struggle of the colonies among themselves. This differs from personal -selection only in that it decides, not the fitness of the individual -person, but that of the stock as a whole. It is a matter of -indifference whether the stocks concerned are stocks in the actual -material sense, or only in the metaphorical sense of sharing the -common life of a large family separated by division of labour. In -both cases, in the polyp-stock as well as in the termite or ant-colony, -the collective germ-plasm, with all its different personal forms, is what -is rejected or accepted. The distinction between this cormal selection -and personal selection is, therefore, no very deep one, because here -too it is in the long run the two sexual animals which are selected, -not indeed only in reference to their visible features, but also in -reference to their invisible characters, those, namely, which determine -in their germ-plasm the constitution of their neuter progeny -or, in the case of polyps, their asexually reproducing descendants.</p> - -<p>We venture to maintain that everything in the world of -organisms that has permanence and significance depends upon -adaptation, and has arisen through a sifting of the variations which -presented themselves, that is, through selection. Everything is -adaptation, from the smallest and simplest up to the largest and -most complex, for if it were not it could not endure, but would -perish. The principle which Empedocles announced, in his own -peculiar and fantastic way, is the dominating one, and I must insist -upon what has so often been objected to as an exaggeration—that -everything depends upon adaptation and is governed by processes of -selection. From the first beginnings of life, up to its highest point, -only what is purposeful has arisen, because the living units at every -grade are continually being sifted according to their utility, and the -ceaseless struggle for existence is continually producing and favouring -the fittest. Upon this depends not only the infinite diversity of the -forms of life, but also, and chiefly, the closely associated progress of -organization.</p> - -<p>It cannot be proved in regard to each individual case, but it can<span class="pagenum"><a id="Page_379"></a>[Pg 379]</span> -be shown in the main that attaining a higher stage in organization -also implies a predominance in the struggle for existence, because it -opens up new possibilities of life, adaptations to situations not -previously utilizable, sources of food, or places of refuge. Thus -a number of the lower vertebrates ascended from the water to the -land, and adapted themselves to life on dry land or in the air, -first as clumsily moving salamanders, but later as actively leaping -frogs; thus, too, other descendants of the fishes gained a sufficient -carrying power of limbs to raise the lightened body from the ground, -and so attained to the rapid walk of the lizards, the lightning-like -leaps of the arboreal agamas, the brief swooping of the flying-dragons, -and ultimately the continuous flight which we find in the flying -Saurians and the primitive birds of the Jurassic period, and in the -birds and bats of our own day.</p> - -<p>It is obvious that each of these groups, as it originated, conquered -a new domain of life, and in many cases this was such a vast one, -and contained so many special possibilities, that numerous subordinate -adaptations took place, and the group broke up into many species -and genera, even into families and orders. All this did not come -about because of some definitely directed principle of evolution of -a mysterious nature, which impelled them to vary in this direction -and in no other, but solely through the rivalry of all the forms of -life and living units, with their enormous and ceaseless multiplication, -in the struggle for existence. They were, and they are still, forced -to adapt themselves to every new possibility of life attainable to -them; they are able to do this because of the power of the lowest -vital units of the germ to develop numerous variations; and they are -obliged to do it because, of the endless number of descendants from -every grade of vital unit, it is only the fittest which survive.</p> - -<p>Thus higher types branched off from the lower from time to -time, although the parent type did not necessarily disappear; indeed -it could not have disappeared as long as the conditions of its life -endured; it was only the superfluous members of the parent form -that adapted themselves to new conditions, and as, in many cases, -these required a higher organization, there arose a semblance of -general upward development which simulated a principle of evolution -always upwards. But we know that, at many points on this long -road, there were stations where individual groups stopped short and -dropped back again to lower stages of organization. This kind -of retreat was almost invariably caused by a parasitic habit of life, -and in many cases this degeneration has gone so far that it is difficult -to recognize the relationship of the parasite to the free-living ances<span class="pagenum"><a id="Page_380"></a>[Pg 380]</span>tors -and nearest relatives. Many parasitic Crustaceans, such as the -Rhizocephalids, lack almost all the typical characteristics of the -crustacean body, and dispense not only with segmentation, with -head and limbs, but also with stomach and intestine. As we have -seen, they feed like the lower fungi, by sucking up the juices of -their hosts, by means of root-like outgrowths from the place where -the mouth used to be. Nevertheless, their relationship to the Cirrhipedes -can be proved from their larval stages. There are, however, -parasites in the kidneys of cuttlefish—the Dicyemidæ—in regard to -which naturalists are even now undecided whether they ought to -form a lowly class by themselves between unicellular animals and -Metazoa, or whether they have degenerated, by reason of their -parasitism, from the flat worms to a simplicity of structure elsewhere -unknown. They consist only of a few external cells, which enclose -a single large internal cell, possess no organs of any kind, neither -mouth nor intestine, neither nervous system nor special reproductive -organs. But although degeneration cannot be proved in this case, -it can be in hundreds of other cases with absolute certainty, as, -for instance, in the Crustacea belonging to the order of Copepods, -which are parasitic upon fishes, in which we find all possible stages -of degeneration, according to the degree of parasitism, that is, to -the greater or less degree of dependence upon the host; for organs -degenerate and disappear in exact proportion to the need for them, -and they thus show us that degeneration also is under the domination -of adaptation.</p> - -<p>Thus retrogressive evolution also is based upon the power of the -living units to respond to changing influences by variation, and upon -the survival of the fittest.</p> - -<p>The roots of all the transformations of organisms, then, lie in -changes of external conditions. Let us suppose for a moment that -these might have remained absolutely alike from the epoch of -spontaneous generation onwards, then no variation of any kind and -no evolution would have taken place. But as this is inconceivable, -since even the mere growth of the first living substance must have -exposed the different kinds of biophors composing it to different -influences, variation was inevitable, and so also was its result—the -evolution of an animate world of organisms.</p> - -<p>External influences had a twofold effect at every stage upon -every grade of vital unit, namely, that of directly causing variation -and that of selecting or eliminating. Not only the biophors, but -every stage of their combinations, the histological elements, chlorophyll -bodies, muscle-disks, cells, organs, individuals, and colonies, can<span class="pagenum"><a id="Page_381"></a>[Pg 381]</span> -not only be caused to vary by the external influences to which they -are subjected, but can be guided by these along particular paths of -variation, so that among the variations which crop up some are -better adapted to the conditions than others, and these thrive better, -and thus alone form the basis of further evolution. In this way -definite tendencies of evolution are produced, which do not move -blindly and rigidly onwards like a locomotive which is bound once -for all to the railroad, but rather in exact response to the external -conditions, like an untrammelled pedestrian who makes his way, over -hill and dale, wherever it suits him best.</p> - -<p>The ultimate forces operative in bringing about this many-sided -evolution are the known—and although we do not recognize it as yet, -perhaps the unknown—chemico-physical forces which certainly work -only according to laws; and that they are able to accomplish such -marvellous results is due to the fact that they are associated in -peculiar and often very complex different kinds of combinations, -and thus conform to the same sort of regulated arrangements as -those which condition the operations of any machine made by man. -All complex effects depend upon a co-operation of forces. This is -seen, to begin with, in the chemical combinations whose characters -depend entirely upon the number and arrangement of the elementary -substances of which they consist; the atoms of carbon, hydrogen, and -oxygen, which compose sugar, can also combine to form carbonic acid -gas and water, or alcohol and carbonic acid gas; and the same thing -is true if we ascend from the most complex but still inanimate organic -molecules to those chemical combinations which, in a still higher -form, condition the phenomena of life, to the lowest living units, -the biophors. Not only do these last differ in having life, but -they themselves may appear in numerous combinations, and can -combine among themselves to form higher units, whose characters -and effectiveness will depend upon these combinations. Just as -man may adjust various metallic structures, such as wheels, plates, -cylinders, and mainsprings in the combination which we call a watch, -and which measures time for us, so the biophors of different kinds -in the living body may form combinations of a second, third, &c., -degree, which perform the different functions essential to life, and -by virtue of their specific, definite combination of elementary forces.</p> - -<p>But if it be asked, what replaces human intelligence in these -purposeful combinations of primary forces, we can only answer -that there is here a self-regulation depending upon the characters -of the primary vital parts, and this means that these last are caused -to vary by external influences and are selected by external influences,<span class="pagenum"><a id="Page_382"></a>[Pg 382]</span> -that is, are chosen for survival or excluded from it. Thus combinations -of living units must always result which are appropriate to -the situation at the moment, for no others can survive, although, -as we have seen, they must arise. This is our view of the causes -of the evolution of the world of organisms; the living substance -may be compared to a plastic mass which is poured out over a wide -plain, and in its ceaseless flowing adapts itself to every unevenness, -flows into every hole, covers every stone or post, leaving an exact -model of it, and all this simply by virtue of its constitution, which -is at first fluid and then becomes solid, and of the form of the surface -over which it flows.</p> - -<p>But it is not merely the surface in our analogy which determines -the form of the organic world; we must take account not only of the -external conditions of existence, but also of the constitution of the flowing -mass, the living substance itself, at every stage of its evolution. -The combination of living units which forms the organism is different -at each stage, and it is upon this that its further evolution depends; -this difference determines what its further evolution <i>may</i> be, but -the conditions of life determine what it <i>must</i> be in a particular case. -Thus, in a certain sense, it was with the first biophors, originating -through spontaneous generation, that the whole of the organic world -was determined, for their origin involved not only the physical constitution -by which the variations of the organism were limited, but also -the external conditions, with their changes up till now, to which -organisms had to adapt themselves. There can be no doubt that -on another planet with other conditions of life other organisms would -have arisen, and would have succeeded each other in diverse series. -On the planet Mars, for instance, with its entirely different conditions as -regards the proportions in weight and volume of the chemical elements -and their combinations, living substance, if it could arise at all, would -occur in a different chemical composition, and thus be equipped with -different characters, and without doubt also with quite different -possibilities of further development and transformation. The highly -evolved world of organisms which we may suppose to exist upon Mars, -chiefly on the ground of the presence of the remarkable straight -canals discovered by Schiaparelli, must therefore be thought of as -very different from the terrestrial living world.</p> - -<p>But upon the earth things could not have been very different -from what they actually are, even if we allow a good deal to chance -and assume that the form of seas and continents might have been -quite different, the folding of the surface into mountains and valleys, -and the formation of rents and fissures, with the volcanoes that burst<span class="pagenum"><a id="Page_383"></a>[Pg 383]</span> -from them, need not have turned out exactly as it has done. In that -case many species would never have arisen, but others would have -taken their place; on the whole, the same types of species-groups -would have succeeded each other in the history of the earth. Let -us suppose that the Sandwich Islands, like many other submarine -volcanoes, had never risen above the surface of the sea, then the -endemic species of snails, birds, and plants which now live there -could not have arisen, and if the volcanic group of the Galapagos -Islands had arisen from the sea not in their actual situation, but -forty degrees further south or north, or 1,000 kilometres further -west, then it would have received other colonists, and probably fewer -of them, and a different company of endemic species would be found -there now. But there would be terrestrial snails and land-birds none -the less, and on the whole we may say that both the extinct and the -living groups of organisms would have arisen even with different -formations of land and sea, of heights and depths, of climatic changes, -of elevations and depressions of the earth's crust, at least in so far -as they are adaptations to the more general conditions of life and not -to specialized ones. The great adaptation to swimming in the sea, -for instance, must have taken place in any case; swimming worms, -swimming polyps (Medusæ), swimming vertebrates, would have arisen; -terrestrial animals would have evolved also, on the one hand from -an ancestry of worms in the form of jointed animals and land or -freshwater worms, and again from an ancestry of fishes. Aerial -animals would also undoubtedly have evolved even if the lands -had been quite differently formed and bounded, and I know of no -reason why the adaptation to flight should not have been attempted -in as many different ways as it has actually been by so many different -groups—the insects, the reptiles (the flying Saurians of the Jurassic -period), the extinct <i>Archæopteryx</i>, the birds, and the bats among -mammals.</p> - -<p>We can trace plainly in every group the attempt not only to -spread itself out as far as possible over as much of the surface of -the earth as is accessible to it, but also to adapt itself to all possible -conditions of life, as far as the capacity for adaptation suffices. This -is very obvious from the fact that such varied groups have striven -to rise from life on the earth to life in the air, and have succeeded -more or less perfectly, and we can see the same thing in all manner -of groups. Almost everywhere we find species and groups of species -which emancipate themselves from the general conditions of life -in their class, and adapt themselves to very different conditions, to -which the structure of the class as a whole does not seem in the least<span class="pagenum"><a id="Page_384"></a>[Pg 384]</span> -suited. Thus the mammals are lung-breathers, and their extremities -are obviously adapted for locomotion on the solid earth, yet several -groups have returned to aquatic life, as, for instance, the family of -otters and the orders of seals and whales. Thus among insects which -are adapted for direct air-breathing, certain families and stages of -development have returned to aquatic life, and have developed -breathing-tubes by means of which they can suck in air from the -surface of the water into their tracheal system, or so-called tracheal -gills, into which the air from the water diffuses. But the most convincing -proof of the organism's power of adaptation is to be found -in the fact that the possibility of living parasitically within other -animals is taken advantage of in the fullest manner, and by the -most diverse groups, and that their bodies exhibit the most marvellous -and far-reaching adaptations to the special conditions prevailing -within the bodies of other animals. We have already referred to -the high degree reached by these adaptive changes, how the parasite -may depart entirely from the type of its family or order, so that -its relationship is difficult to recognize. Not only have numerous -species of flat worms and round worms done this, but we find -numerous parasites among the great class of Crustaceans; there -are some among spiders, insects, medusoids, and snails, and there -are even isolated cases among fishes.</p> - -<p>If we consider the number of obstacles that have to be overcome -in existence within other animals, and how difficult and how much -a matter of chance it must be even to reach to such a place as, for -instance, the intestine, the liver, the lungs, or even the brain or the -blood of another animal, and when, on the other hand, we know how -exactly things are now regulated for every parasitic species so that -its existence is secured notwithstanding its dependence upon chance, -we must undoubtedly form a high estimate of the plasticity of the -forms of life and their adaptability. And this impression will only -be strengthened when we remember that the majority of internal -parasites do not pass directly from one host to another, but do so only -through their descendants, and that these descendants, too, must -undergo the most far-reaching and often unexpected adaptations in -relation to their distribution, their penetration into a new host, and -their migrations and change of form within it, if the existence of the -species is to be secured.</p> - -<p>We are tempted to study these relations more closely; but it is -now time to sum up, and we must no longer lose ourselves in wealth -of detail. Moreover, the life-history of many parasites, and of the -tape-worm in particular, is widely known, and any one can easily fill<span class="pagenum"><a id="Page_385"></a>[Pg 385]</span> -up the story, of which we have given a mere outline. I simply wish to -point out that in parasitic animals there is a vast range of forms of life -in which the most precise adaptation to the conditions occurs in almost -every organ, and certainly at every stage of life, in the most conspicuous -and distinct manner. In the earlier part of these lectures we -gained from the study of the diverse protective means by which plants -and animals secure their existence the impression that whatever is -suited to its end (<i>Das Zweckmässige</i>) does not depend upon chance for -its origin, but that every adaptation which lies at all within the -possibilities of a species will arise if there is any occasion for it. -This impression is notably strengthened when we think of the life-history -of parasites, and we shall find that our view of adaptations as -arising, not through the selection of indefinite variations, but through -that of variations in a definite direction, will be confirmed. Adaptations -so diverse, and succeeding one another in such an unfailing order as -those in the life-history of a tape-worm, a liver-fluke, or a <i>Sacculina</i>, -cannot possibly depend upon pure chance.</p> - -<p>Nevertheless, chance does play a part in adaptations and species-transformations, -and that not only in relation to the fundamental -processes within the germ-plasm, but also in connexion with the higher -stages of the processes of selection, as I have already briefly indicated. -After the publication of my hypothesis of germinal selection it was -triumphantly pointed out that I had at last been obliged to admit -a phyletic evolutionary force, the 'definitely directed' variation of -Nägeli and Askenazy. This reproach—if to allow oneself to be convinced -be a reproach—is based upon a serious misunderstanding. -My 'variation in a definite direction' does not refer to the evolution -of the organic world as a whole. I do not suppose, as Nägeli did, -that this would have turned out essentially as it has actually done, -even although the conditions of life or their succession upon the earth -had been totally different; I believe that the organic world, its classes -and orders, its families and species, would have differed from those -that have actually existed, both in succession and appearance, in proportion -as the conditions of life were different. My 'variation in -a definite direction' is not predetermined from the beginning, is not, -so to speak, exclusive, but is many-sided; each determinant of a -germ-plasm may vary in a plus or minus direction, and may continue -under certain circumstances in the direction once begun, but its components, -the different biophors, may do the same, and so likewise may -the groups, larger and smaller, of biophors which form the primordia -(<i>Anlagen</i>) of the organs within the germ-plasm. Thus an enormously -large number of variational tendencies is available for every part of<span class="pagenum"><a id="Page_386"></a>[Pg 386]</span> -the complete organism, and as soon as a variation would be of -advantage it arises—given that it is within the possibilities of the -physical constitution of the species. It occurs because its potentialities -are already present, but it persists and follows a definite course -because this is the one that is favoured. In other words, it is -primarily fixed by germinal selection alone, but is then preferred by -personal selection above the variants running parallel with it. In my -opinion the definite direction of the chance germinal variations is -determined only by the advantage which it affords to the species with -regard to its capacity for existence. But according to Nägeli the -direction of a variation is quite independent of its utility, which may -or may not exist. From Nägeli's point of view we could never understand -the all-prevailing adaptation, but if the utility of a variant is -itself sufficient to raise it to the level of a persistent variational -tendency, then we understand it.</p> - -<p>Years ago (1883) I compared the species to a wanderer who has -before him a vast immeasurable land, through which he is at liberty -to choose whatever path he prefers, and in which he may sojourn -wherever and for as long as he pleases. But although he may go or -stay entirely of his own free will, yet at all times his going or staying -will be determined—it must be so and cannot be otherwise—by -two factors: first, by the paths available at each place—the variations -which crop up—and secondly, by the prospects each of these available -paths open up to him. He is striving after a restful place of -abode which shall afford him comfortable subsistence, his former -home having been spoilt for him by increasing expensiveness or too -great competition. Even the direction of his first journey will not -depend upon chance, since of the many paths available he will, and -must, choose that which leads to a habitable and not too crowded -spot. If this has been reached—that is to say, if the species has -adapted itself to the new conditions—the colonist sets up his abode -there, and remains as long as a comfortable existence and a competence -are secure; but if these fail him, if grain becomes scarce, or -if prices rise, or if a dangerous epidemic breaks out, then he makes -up his mind to wander anew, and once more he will choose, among -the many available paths, that which offers him the prospect of the -speediest and most certain exit from the threatened region, and leads -him to another where he may live without risk. There, too, he will -remain as long as he is comfortable and not exposed to want or -danger, for the species as a whole only becomes transformed when it -must. And so it will go on <i>ad infinitum</i>; the traveller will, when he is -scared away from one dwelling-place, be able to continue his journey in<span class="pagenum"><a id="Page_387"></a>[Pg 387]</span> -many directions, but he will always select the one path which offers -him the best prospects of a comfortable settlement, and will follow it -only to the nearest suitable place of abode, and never further. The -transformation of a species only goes on until it has again completely -adapted itself. In this way he will in the course of years have -traversed a large number of different places which, taken together, -may lie in a strange and unintelligible course, but this course has -nevertheless not arisen through a mere whim, but through the twofold -necessity of starting from a given spot—that in which he had -previously lived—the constitution of the species, and secondly of -choosing the most promising among the many available paths.</p> - -<p>But chance does play a part in determining the route of the -traveller, for on it depends the nature of the conditions in the -surroundings of his previous dwelling-place, when he is forced to -make another move; for these conditions change, colonies are extended -or depopulated, a town previously cheap becomes dear, -competition increases or decreases, disease breaks out or disappears; -in short, the chances of a pleasureable sojourn in a particular place -may alter and determine the wanderer who is on the point of leaving -his place of abode to take a different direction from that which he -would probably have chosen, say, ten years earlier.</p> - -<p>The analogy might be carried further, as, for instance, to illustrate -the possibility of a splitting up of the species; we may suppose -that instead of one wanderer there is a pair, who found a family -at their first halting-place. Children and grandchildren grow up in -numbers and food becomes scarce. One part of the descendants still -finds enough to live upon, but the rest set out to look for a new -habitation. In this case, too, many paths, sidewards or backwards, -stand open to the wanderers, but only those paths will be actually -and successfully followed by any company of them which will lead to -a habitable place where settlement is possible. If some of the descendants -follow paths with no such prospect they will soon turn back -or will succumb to the perils of the journey.</p> - -<p>It seems to me that the contrast between this and Nägeli's view -of the transmutation of species is obvious enough. According to him -the wanderer is not free to choose his path, but goes on and on along -a definite railway-line that only diverges here and there, and it -cannot be foreseen whether the track leads to paradisaic dwellings or -to barren wastes—the travellers must just make the best of what -they find. They carry a marvellous travelling outfit with them—a -sort of <i>Tischlein, deck' dich</i>—the Lamarckian principle, but the -magic power of this is very doubtful, and it will hardly suffice to<span class="pagenum"><a id="Page_388"></a>[Pg 388]</span> -guard them against the heat of the deserts, the frost of the Arctic -regions, or the malaria of the marshes into which their locomotive -blindly carries them.</p> - -<p>According to my view, the traveler—that is, the species—has -always a large choice of paths, and is able, even while he is on the -way, to discern whether he has chosen a right or a wrong one; moreover, -in most cases, one or, it may be, a number of the paths lead to -the desired dwelling-place. But it also undoubtedly happens that, -after long wandering and when many regions have been traversed, -a company may finally arrive at a place which is quite habitable and -inviting at first sight, but which is surrounded on several sides by the -sea or by a rushing stream. As long as the soil remains fertile and -the climate healthy all goes well, but when matters change in this -respect, and perhaps the only way back lies through marshes and -desert land and is therefore impassable, then the colony will gradually -die out—that is the death of the species.</p> - -<p>But let us now leave our parable and inquire what paths the -organic world has actually taken in its transformations, in what -succession the individual forms of life have evolved from one another; -in short, how the actual genealogical tree of this earth's animate -population is really constructed in detail. To this I can only reply -that we have many well-grounded suppositions, but only real certainty -in regard to isolated cases. Thus the genealogical tree of the horse -has been traced far back, and a great deal is known of the phylogeny -of several Gastropods and Cephalopods, but in regard to the genealogical -tree of organisms as a whole we can only make guesses, many -of which are probable, but are never quite certain. The palæontological -records which the earth's crust has preserved for us for all the -ages are much too incomplete to admit of any certainty. Many -naturalists, notably Ernst Haeckel, have done good service in this -direction, for from what we know of palæontology, embryology, and -morphology, they have constructed genealogical trees of the different -groups of organisms, which are intended to show us the actual succession -of animal and plant forms. But, interesting as these attempts -are, they cannot for the most part be anything more than guesswork, -and I need not, therefore, state or discuss them here in any -detail, since they can afford us no aid in regard to the problem of the -origin of species with which these lectures are concerned. In regard -to the animal world at least—and the case of plants is probably very -similar—the record of fossil forms fails us at an early stage. Thus -the oldest and deepest strata in which fossils can be demonstrated, -the Cambrian formation, already contains Crustaceans, animals at<span class="pagenum"><a id="Page_389"></a>[Pg 389]</span> -a relatively high stage of organization, which must have been preceded -by a very long series of ancestors of which no trace has been -preserved. The whole basal portion of the animal genealogical tree, -from the lowest forms of life at least up to these primitive Crustaceans, -the Trilobites, lies buried in the deepest sedimentary rocks -raised from the sea-floor, the crystalline schists, in which it is -unrecognizable. Enormous pressure and, probably also, high temperature -have destroyed the solid parts as far as there were any, and -the soft parts have only left an occasional impression even in the -higher strata.</p> - -<p>Thus enormous periods of time must have elapsed from the -beginning of life to the laying down of that deepest 'Palæozoic' -formation, the Cambrian, for not only does the whole chain which -leads from the Biophoridæ to the origin of the first unicellulars fall -within this period, as well as the evolution of these unicellulars themselves -into their different classes, and their integration into the first -multicellulars, but also the evolution of these last into all the main -branches of the animal kingdom as it is now, into Sponges, Starfishes, -and their allies, Molluscs, Brachiopods, and Crustaceans, for all these -branches appear even in the Cambrian formation, and we may -conclude that the worms also, most of which are soft and not likely to -be preserved, were abundantly present at that time, since jointed -animals like the Crustaceans can only have arisen from worms. -Moreover, we have every reason for the assumption that Cœlenterates -also, that is to say polyps and medusoids, lived in the Cambrian seas, -because their near relatives with a solid skeleton, the corals, are -represented in the formation next above, the Silurian. The same -is true of the fishes, of which the first undoubtedly recognizable -remains, the spines of sharks, have been found in the Silurian. -These two presuppose a long preparatory history, and thus we come -to the conclusion already stated, that all the branches of the animal -kingdom were already in existence when the earth's crust shut up -within itself the first records available for us of the ancestors of our -modern world of organisms.</p> - -<p>Of course at that time the higher branches had only been -represented by their lower classes, and this is true especially of -vertebrates, so that, from the laying down of the Cambrian strata -to the modern world of organisms, a very considerable increase of -complexity in structure and an infinite diversifying of new groups -must have taken place. Amphibians do not appear to have been -present in Cambrian times; reptiles are represented in the Carboniferous -strata, but only appear in abundance in Secondary times;<span class="pagenum"><a id="Page_390"></a>[Pg 390]</span> -birds appear first in the Jurassic, but in a very different guise -(<i>Archæopteryx</i>) from the modern forms, covered indeed with feathers, -but still possessing a reptilian tail; later they occur as toothed birds -in the Cretaceous, and in Tertiary times they have their present form. -The development of mammals must have run almost parallel with -that of birds, that is, from the beginning of Secondary times onwards, -and their highest and last member appears, as far as is known to -research, only in post-Glacial times, in the Diluvial deposits.</p> - -<p>To the types which have arisen since the Cambrian period -belongs the class of Insects with its twelve orders and its enormous -wealth of known species, now reckoned at 200,000. They are -demonstrable first in the Devonian, and then in the Carboniferous -period, in forms, just as our theory requires, with <i>biting</i> mouth-organs; -it is not until the Cretaceous strata that insects with purely -suctorial mouth-organs—bees and butterflies—occur, as it was also at -that time that the flowers, which have evolved in mutual adaptation -with insects, first appeared.</p> - -<p>The number of fossil species hitherto described is reckoned at -about 80,000—certainly only a mere fragment of the wealth of forms -of life which have arisen on our earth throughout this long period, -and which must have passed away again; for very few <i>species</i> outlive -a geological epoch, and even genera appear only for a longer or -shorter time, and then disappear for ever. But even of many of the -older classes, such, for instance, as the Cystoids among the Echinoderms -of the Silurian seas, no living representative remains; and in -the same way, the Ichthyosaurs or fish-lizards of the Secondary times -have completely disappeared from our modern fauna, and many other -animal types, like the class of Brachiopods and the hard-scaled Ganoid -fishes, have almost died out and are represented only by a few -species in specially sheltered places, such as the great depths of the -sea, or in rivers.</p> - -<p>Thus an incredible wealth of animal and plant species was -potentially contained in these simplest and lowest 'Biophorids' which -lay far below the limits of microscopic visibility—an indefinitely -greater wealth than has actually arisen, for that is only a small part -of what was possible, and of what would have arisen had the changes -of life-conditions and life-possibilities followed a different course. -The greater the complexity of the structure of an organism is, the -more numerous are the parts of it which are capable of variation, -and the different directions in which it can adapt itself to new conditions; -and it will hardly be disputed that <i>potentially</i> the first -Biophorids contained an absolutely inexhaustible wealth of forms<span class="pagenum"><a id="Page_391"></a>[Pg 391]</span> -of life, and not merely those which have actually been evolved. If -this were not so, Man could not still call forth new animal and plant -forms, as he is continually doing among our domesticated animals and -cultivated plants, just as the chemist is continually 'creating' new -combinations in the laboratory which have probably never yet -occurred or been formed on the earth. But just as the chemist does -not really 'create' these combinations, but only brings the necessary -elements and their forces together in such a combination that they -must unite to form the desired new body, so the breeder only guides -the variational tendencies contained in the germ-plasm, and consciously -combines them to procure a new race. And what the breeder -does within the narrow limits of human power is being accomplished -in free nature, through the conditions which allow only what is fit -to survive and reproduce, and thus bring about the wonderful result—as -though it were guided by a superior intelligence—the adaptation -of species to their environment.</p> - -<p>Thus in our time the great riddle has been solved—the riddle -of the origin of what is suited to its purpose, without the co-operation -of purposive forces. Although we cannot demonstrate and follow out -the particular processes of transformation and adaptation in all their -phases with mathematical certainty, we can understand the principle, -and we see the factors through the co-operation of which the result -must be brought about. It has lately become the fashion, at least -among the younger school of biologists, to attach small value to natural -selection, if not, indeed, to regard it as a superseded formula; -mathematical proofs are demanded or, at any rate, desired. I do not -believe that we shall ever arrive at giving such proofs, but we shall -undoubtedly succeed in clearing up much that now remains obscure, -and in essentially modifying and correcting many of the theories -we have formed in regard to this question. But what has been -already gained must certainly be regarded as an enormous advance -on the knowledge of fifty years ago. We now <i>know</i> that the modern -world of organisms has been evolved, and we can form an idea, though -still only an imperfect one, how and through the co-operation of what -factors it could and must have evolved.</p> - -<p>When I say <i>must</i>, this refers only to the course of evolution from -a given beginning; but as to this beginning itself, the spontaneous -generation of the lowest Biophorids from inorganic material, we are far -from having understood it as a necessary outcome of its causes. And -if we have assumed it as a reasonable postulate, we by no means seek -to conceal that this assumption is far from implying an understanding -of what the process of biogenesis was. I do not merely mean that<span class="pagenum"><a id="Page_392"></a>[Pg 392]</span> -we do not know under what external conditions the origin of living -matter, even in the smallest quantity, can take place; I mean, -especially, that we do not understand how this one substance -should suddenly reveal qualities which have never been detected -in any other chemical combination whatever—the circulation of -matter, metabolism, growth, sensation, will, and movement. But -we may confidently say that we shall never be able fully to understand -these specific phenomena of life, as indeed how should we, -since nothing analogous to them is known to us, and since understanding -always presupposes a comparison with something known. -Even although we assume that we might succeed in understanding -the mere chemistry of life, as is not inconceivable, I mean the <i>perpetuum -mobile</i> of dissimilation and assimilation, the so-called 'animal' functions -of the living substance would remain uncomprehended: Sensation, Will, -Thought. We understand in some measure how the kidneys secrete -urine, or the liver bile; we can also—given the sensitiveness to stimulus -of the living substance—understand how a sense-impression may be -conveyed by the nerves to the brain, carried along certain reflex -paths to motor nerves and give rise to movement of the muscles, but -how the activity of certain brain-elements can give rise to a thought -<i>which cannot be compared with anything material</i>, which is nevertheless -able to react upon the material parts of our body, and, as -Will, to give rise to movement—that we attempt in vain to understand. -Of course the dependence of thinking and willing upon a material -substratum is clear enough, and it can be demonstrated with certainty -in many directions, and thus materialism is so far justified in drawing -parallels between the brain and thought on the one hand, and the -kidneys and urine on the other, but this is by no means to say that -we have understood how Thought and Will have come to be. In -recent times it has often been pointed out that the physical functions -of the body increase very gradually with the successive stages of the -organization, and from the lowest beginnings ascend slowly to the -intelligence of Man, in exact correspondence with the height of -organization that has been reached by the species; that they begin -so imperceptibly among the lower animal forms that we cannot -tell exactly where the beginning is; and it has been rightly concluded -from this that the elements of the Psyche do not originate in the -histological parts of the nervous system, but are peculiar to all living -matter, and it has further been inferred that even inorganic material -may contain them, although in an unrecognizable expression, and that -their emergence in living matter is, so to speak, only a phenomenon -of summation. If we are right in our assumption of a spontaneous<span class="pagenum"><a id="Page_393"></a>[Pg 393]</span> -generation it can hardly be otherwise, but saying this does not mean -that we have understood Spirit, but at most secures us the advantage -and the right of looking at this world, as far as we know it, as a unity. -This is the standpoint of Monism.</p> - -<p>The psychical phenomena, which we know from ourselves, and -can assume among animals with greater certainty the nearer they -stand to us, occupy a domain by themselves, and such a vast and -complex one that there can be no question of bringing it within the -scope of our present studies, and the same is true of the phyletic -development of Man. But we must at least take up a position in -regard to these problems, and there can be no question that Man -has evolved from animal ancestors, whose nearest relatives were the -Anthropoid Apes. Not many years ago bony remains of a human -skeleton, or at least of some form very near to modern Man, were -found in the Diluvial deposits of Java, and this has been designated -<i>Pithecanthropus erectus</i>, and perhaps rightly regarded as a transition -form between Apes and Man. It is possible that more may yet be -discovered; but even if that is not so, the conclusion that Man had -his origin from animal forefathers must be regarded as inevitable -and fully established. We do not draw conclusions with our eyes, -but with our reasoning powers, and if the whole of the rest of living -nature proclaims with one accord from all sides the evolution of the -world of organisms, we cannot assume that the process stopped short -of Man. But it follows also that the <i>factors</i> which brought about -the development of Man from his Simian ancestry must be the same -as those which have brought about the whole of evolution: change -of external influences in its direct and indirect effects, and, besides this, -germinal variational tendencies and their selection. And in this connexion -I should like to draw attention to a point which has, perhaps, -as yet received too little attention.</p> - -<p>Selection only gives rise to what is suited to its end; <i>beyond that -it can call forth nothing</i>, as we have already emphasized on several -occasions. I need only recall the protective leaf-marking of butterflies, -which is never a botanically exact copy of a leaf, with all its lateral -veins, but is comparable rather to an impressionist painting, in which -it is not the reproduction of every detail that is of importance, but -the total impression which it makes at a certain distance. If we -apply this to the organs and capacities of Man, we shall only expect -to find these developed as far as their development is of value for the -preservation of his existence and no further. But this may perhaps -seem a contradiction of what observation teaches us, that, for instance, -our eyes can see to the infinite distance of the fixed stars, although<span class="pagenum"><a id="Page_394"></a>[Pg 394]</span> -this can be of no importance in relation to the struggle for existence. -But this intensity of the power of vision has obviously not been -acquired for the investigation of the starry heavens, but was of -the greatest value in securing the existence of many of our animal -ancestors, and was not less important for our own. In the same -way our finely evolved musical ear might be regarded as a perfecting -of the hearing apparatus far beyond the degree necessary to existence, -but this is not really the case: our musical ear, too, has been inherited -from our animal ancestors, and to them, as to primitive Man, it -was a necessity of existence. It was quite necessary for the animals -to distinguish the higher and lower notes of a long scale, sharply -and certainly, in order to be able to evade an approaching enemy, -or to recognize prey from afar. That we are able to make music -is, so to speak, only an unintentional accessory power of the hearing -organs, which were originally developed only for the preservation -of existence, just as the human hand did not become what it is -<i>in order to play the piano</i>, but to touch and seize, to make tools, -and so on.</p> - -<p><i>Must this, then, be true also of the human mind?</i> Can it, too, -only be developed as far as its development is of advantage to Man's -power of survival? I believe that this is certainly the case in -a general way; the intellectual powers which are the common -property of the human race will never rise beyond these limits, -but this is not to say that certain individuals may not be more -highly endowed. The possibility of a higher development of certain -mental powers or of their combinations—whether it be intelligence, will, -feeling, inventive power, or a talent for mathematics, music or painting—may -be inferred with certainty from our own principles; for not -only may the variational tendencies of individual groups of determinants -in the germ-plasm be continued for a series of generations -without becoming injurious, that is to say, without being put a stop -to by personal selection, but sexual intermingling always opens up the -possibility that some predominantly developed intellectual tendencies -(<i>Anlagen</i>) may combine in one way or another, and so give rise to -individuals of great mental superiority, in whatever direction. In -this way, it seems to me, the geniuses of humanity have arisen—a -Plato, a Shakespeare, a Goethe, a Beethoven. But they do not last; -they do not transmit their greatness; if they leave descendants at all, -these never inherit the <i>whole</i> greatness of their father, and we can -easily understand this, since the greatness does not depend upon -a single character, but upon a particular combination of many high -mental qualities (<i>Anlagen</i>). Geniuses, therefore, probably never raise<span class="pagenum"><a id="Page_395"></a>[Pg 395]</span> -the average of the race through their descendants; they raise the -intellectual average only through their own performances, by increasing -the knowledge and power handed on by tradition from generation -to generation. But the raising of the average of mental capacity, -which has undoubtedly taken place to a considerable degree from the -Australasian aborigines to the civilized peoples of antiquity and -of our own day, can only depend on the struggle for existence -between individuals and races.</p> - -<p>But if the human mind has been raised to its present level -through the same slow process of selection by means of which all -evolution has been directed and raised to the height necessary -for the 'desired end,' we must see in this a definite indication that -even the greatest mind among us can never see beyond the conditions -which limit our capacity for existence, and that now and for all time -we cannot hope to understand what is supernatural. We can recognize -the stars in the heavens, it is true, and after thousands of years -of work we have succeeded in determining their distance, their size, -and gravity, as well as their movements and the materials of which -they are composed, but we have been able to do all this with -a thinking power created for the conditions of human existence upon -the earth, that is to say, developed by them, just as we do not only -grasp with our hands, but may also play the piano with them. -But all that involves a higher thinking power that would enable -us to recognize the pseudo-ideas of everlastingness and infinity, the -limits of causality, in short, all that we do not know but regard -as at best a riddle, will always remain sealed to us, because our -intelligence did not, and does not, require this power to maintain -our capacity for existence.</p> - -<p>I say this in particular to those who imagine they have summed -up the whole situation when they admit that much is still lacking -to complete knowledge, say, to a true understanding of the powers -of Nature or of the Psyche, but who do not feel that in spite of -all our very considerably increased knowledge we stand before the -world as a whole as before a great riddle. But I say it also to those -who fear that the doctrine of evolution will be the overthrow of their -faith. Let them not forget that truth can only be harmful, and may -even be destructive, when we have only half grasped it, or when -we try to evade it. If we follow it unafraid, we shall come now -and in the future to the conclusion that a limit is set to our knowledge -by our own minds, and that beyond this limit begins the region -of faith, and this each must fashion for himself as suits his nature. -In regard to ultimate things Goethe has given us the true formula,<span class="pagenum"><a id="Page_396"></a>[Pg 396]</span> -when the 'Nature-spirit' calls to Faust, 'Du gleichst dem Geist, den -Du begreifst, nicht mir!' For all time Man must repeat this to -himself, but the need for an ethical view of the world, a religion, -will remain, though even this must change in its expression according -to the advance of our knowledge of the world.</p> - -<p>But we must not conclude these lectures in a spirit of mere -resignation. Although we must content ourselves without being able -to penetrate the arcana of this wonderful world, we must remain -conscious, at the same time, that these unfathomable depths exist, -and that we may 'still verehren was unerforschlich ist' (Goethe). But -the other half of the world, I mean the part which is accessible -to us, discloses to us such an inexhaustible wealth of phenomena, -and such a deep and unfailing enjoyment in its beauty and the -harmonious interaction of the innumerable wheels of its marvellous -mechanism, that the investigation of it is quite worthy to fill our -lives. And we need have no fear that there will ever be any lack -of new questions and new problems to solve. Even if Mankind -could continue for centuries quietly working on in the manifold and -restless manner that has, for the first time in the history of human -thought, characterized the century just gone, each new solution would -raise new questions above and below, in the immeasurable space -of the firmament, as in the world of microscopical or ultramicroscopical -minuteness, new insight would be gained, new satisfaction won, -and our enthusiasm over the marvel of this world-mechanism, so -extraordinarily complex yet so beautifully simple in its operation, -will never be extinguished, but will always flame up anew to warm -and illumine our lives.</p> -<hr class="full" /> - -<div class="chapter"> -<p><span class="pagenum"><a id="Page_397"></a>[Pg 397]</span></p> - -<h2 class="nobreak" id="INDEX">INDEX</h2> -</div> - -<p class="c">[References to vol. ii have the volume prefixed.]</p> - -<ul class="index"> -<li class="ifrst">Accessory idioplasm, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_383">383</a>.</li> - -<li class="indx">Acræides, immunity of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_100">100</a>.</li> - -<li class="indx">Adaptation, in leaf butterflies, ii. <a href="#Page_346">346</a>;</li> -<li class="isub1">of the sperm-cells to fertilization, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_278">278</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_279">279</a>;</li> -<li class="isub1">facultative, ii. <a href="#Page_278">278</a>;</li> -<li class="isub1">functional, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_244">244</a>;</li> -<li class="isub1">harmonious, ii. <a href="#Page_80">80</a>, <a href="#Page_197">197</a>;</li> -<li class="isub1">not chance but necessity, ii. <a href="#Page_346">346</a>;</li> -<li class="isub1">all evolution depends upon, ii. <a href="#Page_347">347</a>.</li> - -<li class="indx">Affinities, vital, within the 'person,' ii. <a href="#Page_36">36</a>;</li> -<li class="isub1">within the id, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_374">374</a>.</li> - -<li class="indx">Agassiz, L., immutability of the species, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_16">16</a>.</li> - -<li class="indx">Alcoholism, ii. <a href="#Page_68">68</a>.</li> - -<li class="indx">Aldrovandi, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_13">13</a>.</li> - -<li class="indx">Amixia, ii. <a href="#Page_285">285</a>, <a href="#Page_286">286</a>.</li> - -<li class="indx">Ammon, O., the variation-playground, ii. <a href="#Page_199">199</a>, <a href="#Page_202">202</a>.</li> - -<li class="indx">Amœba 'nests,' ii. <a href="#Page_219">219</a>.</li> - -<li class="indx">Amphigony, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_267">267</a>;</li> -<li class="isub1">as a factor in maintaining species, ii. <a href="#Page_204">204</a>.</li> - -<li class="indx">Amphimixis, general significance of, ii. <a href="#Page_192">192</a>;</li> -<li class="isub1">antiquity of, ii. <a href="#Page_202">202</a>;</li> -<li class="isub1">Ammon's playground of variations, ii. <a href="#Page_206">206</a>;</li> -<li class="isub1">Amœba 'nests' as a preliminary stage, ii. <a href="#Page_219">219</a>;</li> -<li class="isub1">beginnings of, ii. <a href="#Page_213">213</a>;</li> -<li class="isub1">parthenogenesis as self-fertilization, ii. <a href="#Page_233">233</a>;</li> -<li class="isub1">in Coccidium, ii. <a href="#Page_214">214</a>, <a href="#Page_216">216</a>;</li> -<li class="isub1">chromosomes in Protozoa, ii. <a href="#Page_216">216</a>;</li> -<li class="isub1">the 'cycle' idea, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_326">326</a>;</li> -<li class="isub1">increased stability due to, ii. <a href="#Page_200">200</a>;</li> -<li class="isub1">continued inbreeding, ii. <a href="#Page_231">231</a>;</li> -<li class="isub1">'formative' stimulus, ii. <a href="#Page_229">229</a>;</li> -<li class="isub1">Galton's curves of frequency, ii. <a href="#Page_206">206</a>;</li> -<li class="isub1">in relation to rudimentary organs, ii. <a href="#Page_226">226</a>;</li> -<li class="isub1">immediate consequences of, ii. <a href="#Page_224">224</a>;</li> -<li class="isub1">plastogamy as a preliminary stage of, ii. <a href="#Page_222">222</a>;</li> -<li class="isub1">alters individuality, ii. <a href="#Page_192">192</a>;</li> -<li class="isub1">and natural death, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_335">335</a>;</li> -<li class="isub1">direct advantages of, ii. <a href="#Page_198">198</a>;</li> -<li class="isub1">origin of, ii. <a href="#Page_211">211</a>;</li> -<li class="isub1">association of, with reproduction, ii. <a href="#Page_210">210</a>;</li> -<li class="isub1">increases power of adaptation, ii. <a href="#Page_223">223</a>;</li> -<li class="isub1">preliminary stages of, ii. <a href="#Page_213">213</a>;</li> -<li class="isub1">not a rejuvenescence in the sense of preserving life, ii. <a href="#Page_221">221</a>.</li> - -<li class="indx">Ancestral plasm, ids of, ii. <a href="#Page_38">38</a>.</li> - -<li class="indx">Ants, several kinds of ids in the germ-plasm of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_390">390</a>;</li> -<li class="isub1">harmonious adaptation of sterile forms, ii. <a href="#Page_89">89</a>;</li> -<li class="isub1">degeneration of wings and ovaries in the workers, ii. <a href="#Page_90">90</a>;</li> -<li class="isub1">transition forms between females and workers, ii. <a href="#Page_92">92</a>;</li> -<li class="isub1">Wasmann's explanation of these, ii. <a href="#Page_93">93</a>;</li> -<li class="isub1"><i>Polyergus rufescens</i>, ii. <a href="#Page_95">95</a>;</li> -<li class="isub1">dimorphism of workers, ii. <a href="#Page_96">96</a>;</li> -<li class="isub1">number of queens, ii. <a href="#Page_98">98</a>.</li> - -<li class="indx">Apes, furred, in Tibet, ii. <a href="#Page_269">269</a>.</li> - -<li class="indx">Arctic animals, sympathetic colouring in, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_62">62</a>.</li> - -<li class="indx">Aristotle, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_10">10</a>.</li> - -<li class="indx">Assimilation, ii. <a href="#Page_371">371</a>.</li> - -<li class="indx">Auerbach, spindle-figure of the dividing cell-nucleus, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_289">289</a>.</li> - -<li class="indx">Autotomy, self-amputation, ii. <a href="#Page_18">18</a>.</li> - - -<li class="ifrst">Baer, K. E. von, development of the chick in the egg, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_25">25</a>.</li> - -<li class="indx">Barfurth, on the segmentation of the egg in the sea-urchin, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_408">408</a>.</li> - -<li class="indx">Bates, discovery of mimicry, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_91">91</a>;</li> -<li class="isub1">on the Sauba ant, ii. <a href="#Page_96">96</a>.</li> - -<li class="indx">Beccari, <i>Amblyornis inornata</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_223">223</a>.</li> - -<li class="indx">Bees, harmonious adaptation in the workers, ii. <a href="#Page_89">89</a>;</li> -<li class="isub1">influence of nutrition on the degeneration of the ovaries, ii. <a href="#Page_92">92</a>;</li> -<li class="isub1">importance of the fact that there is only one queen, ii. <a href="#Page_97">97</a>.</li> - -<li class="indx">Belt, plants and ants, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_171">171</a>.</li> - -<li class="indx">Beneden, E. van, fertilization of the ovum of <i>Ascaris</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_295">295</a>;</li> -<li class="isub1">deutoplasm, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_282">282</a>;</li> -<li class="isub1">theory of mitotic cell-division, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_291">291</a>.</li> - -<li class="indx">Bickford, Elizabeth, experiments on regeneration, ii. <a href="#Page_90">90</a>.</li> - -<li class="indx">Binswanger, on artificial epilepsy in guinea-pigs, ii. <a href="#Page_68">68</a>.</li> - -<li class="indx">Biogenetic Law, Fritz Müller's view, ii. <a href="#Page_160">160</a>;</li> -<li class="isub1">crustacean larvæ, ii. <a href="#Page_161">161</a>;</li> -<li class="isub1">Haeckel's views, ii. <a href="#Page_173">173</a>;</li> -<li class="isub1">markings of the caterpillars of the Sphingidæ, ii. <a href="#Page_177">177</a>;</li> -<li class="isub1">shunting back of the stages in the ontogeny, ii. <a href="#Page_177">177</a>.</li> - -<li class="indx">Biophors, the smallest vital units, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_369">369</a>;</li> -<li class="isub1">struggle of the, ii. <a href="#Page_52">52</a>;</li> -<li class="isub1">spontaneous generation of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_369">369</a>.</li> - -<li class="indx">Birds, adaptation in, ii. <a href="#Page_315">315</a>.</li> - -<li class="indx">Blochmann, on the directive corpuscles in parthenogenetic ova, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_304">304</a>;</li> -<li class="isub1">on the development of the ovum of the bee, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_336">336</a>;</li> -<li class="isub1">on chromosomes in unicellulars, ii. <a href="#Page_217">217</a>.</li> - -<li class="indx">Blumenbach, 'nisus formativus,' <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_352">352</a>;</li> -<li class="isub1">inheritance of mutilations, ii. <a href="#Page_66">66</a>.</li> - -<li class="indx">Bois-Reymond, doubts as to the inheritance of functional modifications, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_242">242</a>.</li> - -<li class="indx">Bonnet, preformation theory, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_350">350</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_351">351</a>.</li> - -<li class="indx">Bordage, regeneration, ii. <a href="#Page_20">20</a>.</li> - -<li class="indx">Borgert, proof of the splitting of the chromosomes in the division of unicellulars, ii. <a href="#Page_216">216</a>.</li> - -<li class="indx">Boveri, fertilization of non-nucleated pieces of ovum with nucleus of another species, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_341">341</a>.</li> - -<li class="indx">Brandes, on the extinction of <i>Machairodus</i> species and the giant armadillos, ii. <a href="#Page_358">358</a>, <a href="#Page_359">359</a>;</li> -<li class="isub1">on the supposed transformation of the stomach in birds as a result of nutrition, <a href="#Page_267">267</a>.</li> - -<li class="indx">Brown-Séquard, artificial epilepsy in guinea-pigs, ii. <a href="#Page_67">67</a>.</li> - -<li class="indx">Brücke, Ernst, organization of the living substance, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_368">368</a>.</li> - -<li class="indx">Budding and division, ii. <a href="#Page_1">1</a>.</li> - -<li class="indx">Bütschli, theories of amphimixis, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_330">330</a>;</li> -<li class="isub1">discovery of the spindle-figure in nuclear division, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_289">289</a>.</li> - -<li class="indx">Burdach, inheritance of mutilations, ii. <a href="#Page_65">65</a>.</li> - -<li class="indx">Buttel-Reepen, Hugo von, on fertilization in the bee ovum, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_306">306</a>.</li> - -<li class="indx">Butterflies, their enemies, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_98">98</a>;</li> -<li class="isub1">aggressive colourings, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_68">68</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_70">70</a>;</li> -<li class="isub1">aberrations due to cold, ii. <a href="#Page_274">274</a>;</li> -<li class="isub1">transmissibility of these, <a href="#Page_275">275</a>;</li> -<li class="isub1">endemic species, <a href="#Page_285">285</a>;</li> -<li class="isub1">polar and Alpine species, <a href="#Page_285">285</a>;</li> -<li class="isub1">species of the Malay region, <a href="#Page_291">291</a>.</li> - -<li class="indx">Butterflies, protective coloration in, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_74">74</a>.</li> - - -<li class="ifrst">Cænogenesis, ii. <a href="#Page_173">173</a>.</li> - -<li class="indx">Calkins, conjugation of infusorians, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_329">329</a>.</li> - -<li class="indx">Caterpillars, protective coloration in, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_67">67</a>.</li> - -<li class="indx"><i>Catocala</i>, adaptive coloration in the various species, ii. <a href="#Page_310">310</a>.</li> - -<li class="indx">Cell-division, integral and differential, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_374">374</a>;</li> -<li class="isub1">differential in Ctenophores, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_408">408</a>;</li> -<li class="isub1">proofs of differential, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_377">377</a>.</li> - -<li class="indx">Centrospheres, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_289">289</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_309">309</a>.</li> - -<li class="indx">Ceratium, ii. <a href="#Page_326">326</a>.</li> - -<li class="indx">Chance, elimination sometimes due to, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_44">44</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_47">47</a>.</li> - -<li class="indx">Characters, purely morphological, ii. <a href="#Page_133">133</a>.</li> - -<li class="indx">Child, determination of, at fertilization, ii. <a href="#Page_46">46</a>.</li> - -<li class="indx">Chromatin, the hereditary substance, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_287">287</a>;</li> -<li class="isub1">grounds for the belief, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_337">337-43</a>.</li> - -<li class="indx">Chromosomes, their occurrence in unicellulars, ii. <a href="#Page_217">217</a>;</li> -<li class="isub1">simple and plurivalent (-idants), <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_349">349</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_350">350</a>;</li> -<li class="isub1">individuality of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_349">349</a>;</li> -<li class="isub1">number of, in different species, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_291">291</a>;</li> -<li class="isub1">indications of complexity of their structure, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_292">292</a>;</li> -<li class="isub1">reasons for their existence, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_303">303</a>.</li> - -<li class="indx">Chun, segmentation of the ovum in Ctenophores, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_408">408</a>;</li> -<li class="isub1">Kerguelen cabbage and rabbits, ii. <a href="#Page_362">362</a>;</li> -<li class="isub1">deep-sea investigation, ii. <a href="#Page_322">322</a>.</li> - -<li class="indx">Cirrhipeds, ii. <a href="#Page_241">241</a>.</li> - -<li class="indx">Climate, influence of, in causing variation, ii. <a href="#Page_269">269</a>.</li> - -<li class="indx">Climatic varieties, ii. <a href="#Page_269">269</a>, <a href="#Page_272">272</a>.</li> - -<li class="indx">Coadaptation, ii. <a href="#Page_80">80</a>;</li> -<li class="isub1">in crustaceans, ii. <a href="#Page_81">81</a>;</li> -<li class="isub1">in the markings of butterflies, ii. <a href="#Page_87">87</a>;</li> -<li class="isub1">in the forelegs of the mole-cricket, ii. <a href="#Page_86">86</a>.</li> - -<li class="indx">Cold aberrations in butterflies, transmissibility of, ii. <a href="#Page_275">275</a>.</li> - -<li class="indx">Coloration, animal, its biological import, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_58">58</a>;</li> -<li class="isub1">sympathetic in butterflies, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_74">74</a>;</li> -<li class="isub1">in moths, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_76">76</a>;</li> -<li class="isub1">of animals in green surrounding, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_64">64</a>;</li> -<li class="isub1">of eggs, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_60">60</a>;</li> -<li class="isub1">of nocturnal animals, of polar animals, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_64">64</a>;</li> -<li class="isub1">water animals, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_63">63</a>.</li> - -<li class="indx">Coloration, shunting backwards of, in the ontogeny, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_73">73</a>.</li> - -<li class="indx">Colour-adaptation, double, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_64">64</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_73">73</a>;</li> -<li class="isub1">colour change in fishes, amphibians, reptiles and Cephalopoda, ii. <a href="#Page_278">278</a>.</li> - -<li class="indx">Combinations of determinants, ii. <a href="#Page_40">40</a>.</li> - -<li class="indx">Conjugation, in Protozoa, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_317">317</a>;</li> -<li class="isub1">in <i>Paramæcium</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_319">319</a>.</li> - -<li class="indx">Conklin, on the behaviour of the centrosphere in the ovum of <i>Crepidula</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_309">309</a>, ii. <a href="#Page_41">41</a>.</li> - -<li class="indx">Connective tissue of vertebrates, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_386">386</a>.</li> - -<li class="indx">Constancy and variability, periods of, ii. <a href="#Page_294">294</a>, <a href="#Page_295">295</a>;</li> -<li class="isub1">degree of constancy of a character increases with its age, ii. <a href="#Page_200">200</a>.</li> - -<li class="indx">Convergence, ii. <a href="#Page_323">323</a>.</li> - -<li class="indx">Cope, supposed palæontological proofs for the Lamarckian principle, ii. <a href="#Page_77">77</a>.</li> - -<li class="indx">Copernicus, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_13">13</a>.</li> - -<li class="indx">Copulation of <i>Coccidium proprium</i>, ii. <a href="#Page_217">217</a>.</li> - -<li class="indx">Correlation of the parts of the body, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_41">41</a>;</li> -<li class="isub1">of determinants of the germ-plasm, ii. <a href="#Page_153">153</a>.</li> - -<li class="indx">Correns on Xenia, ii. <a href="#Page_59">59</a>.</li> - -<li class="indx">Corsica, endemic butterflies of, ii. <a href="#Page_285">285</a>.</li> - -<li class="indx">Crampton, segmentation in a marine snail, <i>Ilyanassa</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_409">409</a>.</li> - -<li class="indx">Crystal animals, sympathetic colouring, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_63">63</a>.</li> - -<li class="indx">Cultivated plants, asexual reproduction in, ii. <a href="#Page_261">261</a>.</li> - -<li class="indx">Cuvier, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_16">16</a>;</li> -<li class="isub1">his dispute with St.-Hilaire, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_24">24</a>.</li> - - -<li class="ifrst">Dahl, the ants of the Bismarck Archipelago, ii. <a href="#Page_101">101</a>.</li> - -<li class="indx">Danaides, immune butterflies, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_94">94</a>.</li> - -<li class="indx"><i>Danais erippus</i> and <i>Limenitis archippus</i> (mimicry), <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_113">113</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_114">114</a>.</li> - -<li class="indx">Darwin, Charles, first appearance of <i>The Origin of Species</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_28">28</a>;</li> -<li class="isub1">story of his life, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_29">29</a>.</li> - -<li class="indx">Darwin, Erasmus, theory of evolution, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_17">17</a>.</li> - -<li class="indx">Darwin and Nägeli, ii. <a href="#Page_322">322</a>.</li> - -<li class="indx">Darwinian theory, dependence of the frequency of species on enemies, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_47">47</a>;</li> -<li class="isub1">on external circumstances, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_45">45</a>;</li> -<li class="isub1">correlation of parts, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_41">41</a>;</li> -<li class="isub1">races of pigeons, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_34">34</a>;</li> -<li class="isub1">of domesticated animals, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_31">31</a>;</li> -<li class="isub1">geometrical ratio of increase, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_46">46</a>;</li> -<li class="isub1">struggle for existence, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_47">47</a>;</li> -<li class="isub1">struggle between individuals of the same species, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_52">52</a>;</li> -<li class="isub1">artificial selection, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_39">39</a>;</li> -<li class="isub1">natural selection, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_42">42</a>;</li> -<li class="isub1">affects all parts and stages, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_54">54</a>;</li> -<li class="isub1">variation, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_43">43</a>;</li> -<li class="isub1">summary, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_55">55</a>;</li> -<li class="isub1">origin of flowers, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_182">182</a>;</li> -<li class="isub1">pangenesis, ii. <a href="#Page_62">62</a>.</li> - -<li class="indx">Death, natural, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_260">260</a>.</li> - -<li class="indx">Degeneration of a typical organ not an ontogenetic but a phylogenetic process, ii. <a href="#Page_91">91</a>;</li> -<li class="isub1">of disused parts, ii. <a href="#Page_116">116</a>.</li> - -<li class="indx">Delage, the germ-substance, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_401">401</a>;</li> -<li class="isub1">'a portmanteau theory,' ii. <a href="#Page_3">3</a>;</li> -<li class="isub1">experiments with sea-urchins, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_342">342</a>.</li> - -<li class="indx">Desert animals, sympathetic colouring in, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_62">62</a>.</li> - -<li class="indx">Determinants, active and passive state, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_380">380</a>;</li> -<li class="isub1">controlling the cells, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_381">381</a>;</li> -<li class="isub1">proofs of their existence, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_361">361</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_371">371</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_408">408</a>;</li> -<li class="isub1">in limbs of Arthropods, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_361">361</a>;</li> -<li class="isub1">liberation of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_382">382</a>;</li> -<li class="isub1">size and number, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_369">369</a>.</li> - -<li class="indx">Determinates, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_355">355</a>.</li> - -<li class="indx">Deutoplasm, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_280">280</a>.</li> - -<li class="indx">Dewitz, degeneration of wings in the ontogeny of worker-ants, ii. <a href="#Page_90">90</a>.</li> - -<li class="indx">Diatoms, ii. <a href="#Page_324">324</a>.</li> - -<li class="indx">Dimorphism, sexual, its idioplasmatic cause, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_388">388</a>.</li> - -<li class="indx">Disappearance of disused parts, ii. <a href="#Page_135">135</a>;</li> -<li class="isub1">unequal rate of, ii. <a href="#Page_129">129</a>.</li> - -<li class="indx">Dividing apparatus of the ovum, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_288">288</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_308">308</a>.</li> - -<li class="indx">Division, proof of differential nuclear division (<i>Phylloxera</i>), <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_377">377</a>;</li> -<li class="isub1">multiplication by division, ii. <a href="#Page_1">1</a>.</li> - -<li class="indx">Dixon, isolation as a condition of species formation, ii. <a href="#Page_284">284</a>.</li> - -<li class="indx">Döderlein, increase of characters in diluvial forms, ii. <a href="#Page_139">139</a>.</li> - -<li class="indx">Dog, breeds of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_31">31</a>;</li> -<li class="isub1">attachment to man, ii. <a href="#Page_73">73</a>.</li> - -<li class="indx">Driesch, 'prospective' importance of a cell, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_378">378</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_408">408</a>.</li> - -<li class="indx">Dzierzon, discovery of parthenogenesis in bees, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_303">303</a>.</li> - - -<li class="ifrst">Echinoderms, mesoderm cells of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_386">386</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_387">387</a>.</li> - -<li class="indx">Ectocarpus, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_334">334</a>.</li> - -<li class="indx">Egg-cell, form and structure, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_280">280</a>;</li> -<li class="isub1">its migrations, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_281">281</a>.</li> - -<li class="indx">Ehrlich, experiments with ricin and abrin, ii. <a href="#Page_106">106</a>.</li> - -<li class="indx">Eigenmann, on blind cave-salamanders, ii. <a href="#Page_347">347</a>;</li> -<li class="isub1">on species of Leptocephalus, ii. <a href="#Page_133">133</a>.</li> - -<li class="indx">Eisig, on symbiosis, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_162">162</a>.</li> - -<li class="indx">Elimination, ratio of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_47">47</a>.</li> - -<li class="indx"><i>Elymnias</i>, a genus of mimetic butterflies, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_103">103</a>.</li> - -<li class="indx">Emery, on extinction of species, ii. <a href="#Page_357">357</a>;</li> -<li class="isub1">on <i>Colobopsis truncata</i>, ii. <a href="#Page_96">96</a>;</li> -<li class="isub1">on germinal selection, ii. <a href="#Page_139">139</a>;</li> -<li class="isub1">'mixed' forms in ants, ii. <a href="#Page_93">93</a>;</li> -<li class="isub1">variation of homologous parts, ii. <a href="#Page_189">189</a>.</li> - -<li class="indx">Empedocles, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_9">9</a>;</li> -<li class="isub1">ii. <a href="#Page_370">370</a>, <a href="#Page_378">378</a>.</li> - -<li class="indx">Endemic species, ii. <a href="#Page_283">283</a>.</li> - -<li class="indx">Endres, 'prospective' significance of the blastomeres of the ovum of the frog, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_407">407</a>.</li> - -<li class="indx">Epigenesis and evolution, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_350">350</a>.</li> - -<li class="indx">Epilepsy, artificial, in guinea-pigs, ii. <a href="#Page_67">67</a>.</li> - -<li class="indx">Equilibrium between species of a region, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_49">49</a>.</li> - -<li class="indx">Evolution, phyletic, ii. <a href="#Page_332">332</a>;</li> -<li class="isub1">paths of, ii. <a href="#Page_381">381</a>;</li> -<li class="isub1">forces of, ii. <a href="#Page_381">381</a>;</li> -<li class="isub1">mechanism of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_353">353</a>;</li> -<li class="isub1">facts of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_406">406</a>.</li> - -<li class="indx">Evolution, progressive, attempt of species to extend its range, ii. <a href="#Page_383">383</a>;</li> -<li class="isub1">unlimited diversity of forms of life, ii. <a href="#Page_391">391</a>;</li> -<li class="isub1">parable of the traveller, ii. <a href="#Page_386">386</a>.</li> - -<li class="indx">Evolution theory, general meaning of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_6">6</a>;</li> -<li class="isub1">'prospective' import of the cell, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_378">378</a>.</li> - -<li class="indx">Exner, electric adaptation of the fur of mammals and feathers of birds, ii. <a href="#Page_316">316</a>;</li> -<li class="isub1">vision of insects, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_216">216</a>.</li> - -<li class="indx">Eye-spots, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_69">69</a>;</li> -<li class="isub1">ii. <a href="#Page_179">179</a>.</li> - - -<li class="ifrst">Falkland Islands, influence of climate on cattle and horses, ii. <a href="#Page_268">268</a>.</li> - -<li class="indx">Feathers, regarded as an adaptation, ii. <a href="#Page_316">316</a>.</li> - -<li class="indx">Fertilization, process of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_286">286</a>;</li> -<li class="isub1">in lichens, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_313">313</a>;</li> -<li class="isub1">in <i>Ascaris</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_296">296</a>;</li> -<li class="isub1">in the sea-urchin ovum, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_293">293</a>;</li> -<li class="isub1">in Phanerogams, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_313">313</a>;</li> -<li class="isub1">in higher plants, ii. <a href="#Page_251">251</a>;</li> -<li class="isub1">importance of the chromatin, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_290">290</a>;</li> -<li class="isub1">conjugation, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_317">317</a>;</li> -<li class="isub1">the centrosphere the dividing apparatus of the cell, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_289">289</a>;</li> -<li class="isub1">chromatin the hereditary substance, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_287">287</a>;</li> -<li class="isub1">differentiation of individuals among the Protozoa, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_322">322</a>;</li> -<li class="isub1">number of chromosomes reduced to half, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_297">297</a>;</li> -<li class="isub1">rôle of the centrosphere, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_308">308</a>;</li> -<li class="isub1">summary of process of fertilization, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_343">343</a>.</li> - -<li class="indx">Fischel, segmentation, of the Ctenophore ovum, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_408">408</a>;</li> -<li class="isub1">regeneration of the lens in Triton, ii. <a href="#Page_20">20</a>.</li> - -<li class="indx">Fischer, E., experiments with butterfly pupæ in low temperature, ii. <a href="#Page_275">275</a>.</li> - -<li class="indx">Flowers, origin of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_179">179</a>;</li> -<li class="isub1">adaptation to insects, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_189">189</a>;</li> -<li class="isub1">in <i>Aristolochia</i>, <i>Pinguicula</i>, and <i>Daphne</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_186">186</a>;</li> -<li class="isub1">colour as an attraction to insects, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_195">195</a>;</li> -<li class="isub1">collecting apparatus of bee, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_193">193</a>;</li> -<li class="isub1">cross-fertilization, means for securing, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_182">182</a>;</li> -<li class="isub1">in <i>Salvia</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_183">183</a>;</li> -<li class="isub1">lousewort, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_184">184</a>;</li> -<li class="isub1">flowers adapted to fly-visits, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_185">185</a>;</li> -<li class="isub1">orchids, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_187">187</a>;</li> -<li class="isub1">deceptive flowers, <i>Cypripedium</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_200">200</a>;</li> -<li class="isub1">fertilization of Yucca, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_202">202</a>;</li> -<li class="isub1">imperfection of adaptation a proof of origin through selection, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_204">204</a>;</li> -<li class="isub1">mouth-parts of insects, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_189">189</a>;</li> -<li class="isub1">bee, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_172">172</a>;</li> -<li class="isub1">butterfly, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_193">193</a>;</li> -<li class="isub1">cockroach, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_191">191</a>;</li> -<li class="isub1">wind-pollination, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_182">182</a>.</li> - -<li class="indx">Forel, Auguste, alarm-signals in ants, ii. <a href="#Page_83">83</a>.</li> - -<li class="indx">Fraisse, on regeneration, ii. <a href="#Page_30">30</a>.</li> - -<li class="indx">Function, passively functioning parts in relation to the Lamarckian principle, ii. <a href="#Page_77">77</a>;</li> -<li class="isub1">harmonious adaptation in these, ii. <a href="#Page_81">81</a>.</li> - -<li class="indx">Fungi, reproduction of, ii. <a href="#Page_267">267</a>.</li> - -<li class="indx">Fur of mammals, adaptation to the conditions of life, ii. <a href="#Page_269">269</a>.</li> - - -<li class="ifrst">Galapagos Islands, fauna of, ii. <a href="#Page_283">283</a>, <a href="#Page_292">292</a>.</li> - -<li class="indx">Galileo, Galilei, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_13">13</a>.</li> - -<li class="indx">Galls, plant, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_385">385</a>;</li> -<li class="isub1">ii. <a href="#Page_271">271</a>.</li> - -<li class="indx">Gall-wasps, reproduction of, ii. <a href="#Page_245">245</a>.</li> - -<li class="indx">Galton, Francis, on continuity of the germ-plasm, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_411">411</a>;</li> -<li class="isub1">on inheritance of talents, ii. <a href="#Page_150">150</a>;</li> -<li class="isub1">curves of frequency, ii. <a href="#Page_206">206</a>;</li> -<li class="isub1">doubt of the Lamarckian principle, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_242">242</a>.</li> - -<li class="indx">Genius, human, ii. <a href="#Page_394">394</a>.</li> - -<li class="indx">Germ-cells, and somatic cells, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_411">411</a>;</li> -<li class="isub1">development of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_410">410</a>;</li> -<li class="isub1">their mutual attraction, ii. <a href="#Page_230">230</a>.</li> - -<li class="indx">Germinal infection, ii. <a href="#Page_69">69</a>.</li> - -<li class="indx">Germinal Selection, ii. <a href="#Page_113">113</a>;</li> -<li class="isub1">influenced by personal selection, ii. <a href="#Page_155">155</a>;</li> -<li class="isub1">relation of determinants to determinates, ii. <a href="#Page_153">153</a>;</li> -<li class="isub1">combination of mental gifts, ii. <a href="#Page_150">150</a>;</li> -<li class="isub1">influence of amphimixis, ii. <a href="#Page_125">125</a>;</li> -<li class="isub1">influence of the multiplicity of ids, ii. <a href="#Page_124">124</a>;</li> -<li class="isub1">objections on the score of smallness of the substance of the germ-plasm, ii. <a href="#Page_156">156</a>;</li> -<li class="isub1">degeneration of a species through cultivation, ii. <a href="#Page_144">144</a>;</li> -<li class="isub1">there are only plus and minus variations, ii. <a href="#Page_151">151</a>;</li> -<li class="isub1">excessive increase of variations, ii. <a href="#Page_139">139</a>;</li> -<li class="isub1">basis of sexual characters, ii. <a href="#Page_135">135</a>;</li> -<li class="isub1">its sphere of operation, ii. <a href="#Page_127">127</a>;</li> -<li class="isub1">small hands and feet in the higher classes, ii. <a href="#Page_147">147</a>;</li> -<li class="isub1">climatic forms, ii. <a href="#Page_134">134</a>;</li> -<li class="isub1">bud-variations, ii. <a href="#Page_141">141</a>;</li> -<li class="isub1">play of forces in the determinant system, ii. <a href="#Page_154">154</a>;</li> -<li class="isub1">artificial selection, ii. <a href="#Page_123">123</a>;</li> -<li class="isub1">short-sight, ii. <a href="#Page_146">146</a>;</li> -<li class="isub1">milk-glands, ii. <a href="#Page_147">147</a>;</li> -<li class="isub1">deformities, ii. <a href="#Page_137">137</a>;</li> -<li class="isub1">muscular weakness in the higher classes of men, ii. <a href="#Page_147">147</a>;</li> -<li class="isub1">positive variation, ii. <a href="#Page_122">122</a>;</li> -<li class="isub1">regulated by personal selection, ii. <a href="#Page_131">131</a>;</li> -<li class="isub1">source of purely morphological characters, ii. <a href="#Page_132">132</a>;</li> -<li class="isub1">disappearance of disused parts, ii. <a href="#Page_119">119</a>, <a href="#Page_129">129</a>;</li> -<li class="isub1">self-regulation of the germ-plasm, ii. <a href="#Page_128">128</a>;</li> -<li class="isub1">specific talents, ii. <a href="#Page_149">149</a>;</li> -<li class="isub1">sport-variations, ii. <a href="#Page_140">140</a>;</li> -<li class="isub1">spontaneous and induced, ii. <a href="#Page_137">137</a>;</li> -<li class="isub1">excessive increase of a variation tendency, ii. <a href="#Page_130">130</a>;</li> -<li class="isub1">preponderance of panmixia, ii. <a href="#Page_120">120</a>;</li> -<li class="isub1">origin of secondary sexual characters, ii. <a href="#Page_143">143</a>.</li> - -<li class="indx">Germinal vesicle, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_295">295</a>.</li> - -<li class="indx">Germ-plasm, conception of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_410">410</a>;</li> -<li class="isub1">continuity of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_411">411</a>;</li> -<li class="isub1">at once variable and persistent, ii. <a href="#Page_220">220</a>;</li> -<li class="isub1">disintegration of, in ontogeny, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_379">379</a>;</li> -<li class="isub1">nutritive variations within the, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_379">379</a>;</li> -<li class="isub1">structure of the, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_373">373</a>;</li> -<li class="isub1">variation of, due to environment, ii. <a href="#Page_267">267</a>;</li> -<li class="isub1">to nutrition, ii. <a href="#Page_268">268</a>.</li> - -<li class="indx">Germ-plasm theory, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_345">345</a>;</li> -<li class="isub1">accessory idioplasm, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_383">383</a>;</li> -<li class="isub1">active and passive state of determinants, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_379">379</a>;</li> -<li class="isub1">connective tissue-cells, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_386">386</a>;</li> -<li class="isub1">determinants and determinates, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_355">355</a>;</li> -<li class="isub1">lithium-larvæ, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_383">383</a>;</li> -<li class="isub1">ids, conception of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_349">349</a>;</li> -<li class="isub1">idants, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_349">349</a>;</li> -<li class="isub1">male end female ids, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_389">389</a>;</li> -<li class="isub1">mesoderm cells of sea-urchin, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_387">387</a>;</li> -<li class="isub1">plant-galls, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_385">385</a>;</li> -<li class="isub1">polymorphism, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_390">390</a>;</li> -<li class="isub1">proofs of existence of determinants (<i>Lycæna agestis</i>, insect metamorphosis, &c.), <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_356">356</a>;</li> -<li class="isub1">sexual dimorphism, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_388">388</a>.</li> - -<li class="indx">Germ-tracks, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_411">411</a>.</li> - -<li class="indx">Gesner's <i>Book of Animals</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_13">13</a>.</li> - -<li class="indx">Godelmann, regeneration of Phasmids, ii. <a href="#Page_28">28</a> <i>n.</i></li> - -<li class="indx">Goebel, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_269">269</a>.</li> - -<li class="indx">Goethe, archetypal animal and plant, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_18">18</a>.</li> - -<li class="indx">Green animals, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_64">64</a>.</li> - -<li class="indx">Gruber, A., regeneration experiments on the Protozoa, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_340">340</a>.</li> - -<li class="indx">Guignard, fertilization of Phanerogams, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_315">315</a>.</li> - -<li class="indx">Gulick, snails in the Sandwich Islands. ii. <a href="#Page_329">329</a>.</li> - - -<li class="ifrst">Haase, Erich, on Pharmacopagæ, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_101">101</a>;</li> -<li class="isub1">on mimicry, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_104">104</a>.</li> - -<li class="indx">Haberlandt, protection of leaves, ii. <a href="#Page_133">133</a>;</li> -<li class="isub1">Auxo-spores, ii. <a href="#Page_221">221</a>.</li> - -<li class="indx">Haeckel, Ernst, fundamental biogenetic law, ii. <a href="#Page_173">173</a>;</li> -<li class="isub1">monogony and amphigony, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_267">267</a>;</li> -<li class="isub1">palingenesis and cœnogenesis, ii. <a href="#Page_173">173</a>;</li> -<li class="isub1">genealogical trees, ii. <a href="#Page_388">388</a>.</li> - -<li class="indx">Häcker, Valentin, importance of the nucleolus, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_287">287</a>;</li> -<li class="isub1">separateness of paternal and maternal nuclear substance during development, ii. <a href="#Page_42">42</a>;</li> -<li class="isub1">process of nuclear division, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_291">291</a>.</li> - -<li class="indx">Hahnel, observations on the enemies of butterflies, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_154">154</a>;</li> -<li class="isub1">lizards and birds as enemies of butterflies, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_97">97</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_98">98</a>.</li> - -<li class="indx">Haller, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_267">267</a>.</li> - -<li class="indx">Harmony, pre-established, apparently existing in development, ii. <a href="#Page_309">309</a>.</li> - -<li class="indx">Hartog, views on amphimixis, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_334">334</a>;</li> -<li class="isub1">ii. <a href="#Page_194">194</a>.</li> - -<li class="indx">Haycraft, on the equalizing effect of amphigony, ii. <a href="#Page_203">203</a>.</li> - -<li class="indx">Heidenhain, theory of mitotic division, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_291">291</a>.</li> - -<li class="indx">Heider, on the intimate processes of segmentation of the ovum, 'regulation' and 'mosaic' ova, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_409">409</a>.</li> - -<li class="indx">Heliconiidæ, first example of immune butterflies, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_91">91</a>.</li> - -<li class="indx">Henslow, on purely morphological specific differences, ii. <a href="#Page_308">308</a>.</li> - -<li class="indx">Herbst, lithium-larvæ, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_383">383</a>;</li> -<li class="isub1">ii. <a href="#Page_277">277</a>.</li> - -<li class="indx">Hereditary sequence, alternation of, ii. <a href="#Page_50">50</a>.</li> - -<li class="indx">Hering, his reasons for assuming the inheritance of functional modifications, ii. <a href="#Page_110">110</a>.</li> - -<li class="indx">Hermaphroditism in flowers, ii. <a href="#Page_250">250</a>;</li> -<li class="isub1">in animals, ii. <a href="#Page_239">239</a>;</li> -<li class="isub1">advantages of, ii. <a href="#Page_239">239</a>.</li> - -<li class="indx">Herrich-Schäfer, on mimicry, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_105">105</a>.</li> - -<li class="indx">Hertwig, O., fertilization of sea-urchin eggs, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_293">293</a>;</li> -<li class="isub1">theory of development, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_354">354</a>;</li> -<li class="isub1">differential cell-division, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_376">376</a>;</li> -<li class="isub1">inheritance of functional modifications, ii. <a href="#Page_106">106</a>;</li> -<li class="isub1">maturation divisions of the sperm-cells, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_300">300</a>.</li> - -<li class="indx">Hertwig, R., chromosomes in Actinosphærium, ii. <a href="#Page_216">216</a>.</li> - -<li class="indx">Heterogony, ii. <a href="#Page_244">244</a>.</li> - -<li class="indx">Heteromorphosis, Loeb on, ii. <a href="#Page_7">7</a>.</li> - -<li class="indx">Heterostylism, ii. <a href="#Page_254">254</a>.</li> - -<li class="indx">Heterotopia, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_365">365</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_367">367</a>.</li> - -<li class="indx">Hirasé, fertilization of Phanerogams, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_313">313</a>.</li> - -<li class="indx">Histonal selection, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_240">240</a>;</li> -<li class="isub1">and personal selection, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_280">280</a>.</li> - -<li class="indx">Hübner, O., experiments on regeneration in <i>Volvox</i>, ii. <a href="#Page_4">4</a>.</li> - -<li class="indx">Humming-birds, species fixed by isolation, ii. <a href="#Page_290">290</a>.</li> - -<li class="indx">Hyatt, Alpheus, the snail-strata of Steinheim, ii. <a href="#Page_305">305</a>.</li> - -<li class="indx">Hybrids, ii. <a href="#Page_60">60</a>;</li> -<li class="isub1">of pigeons, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_34">34</a>;</li> -<li class="isub1">plant, ii. <a href="#Page_57">57</a>.</li> - -<li class="indx">Hydra, regeneration in, ii. <a href="#Page_4">4</a>.</li> - -<li class="indx">Hydroid polyps, development of germ-cells in, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_411">411</a>.</li> - - -<li class="ifrst">Idants, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_349">349</a>.</li> - -<li class="indx">Ids, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_349">349</a>;</li> -<li class="isub1">male and female, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_389">389</a>;</li> -<li class="isub1">mimicry a proof of the existence of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_390">390</a>.</li> - -<li class="indx">Immortality, potential, of the Protozoa, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_260">260</a>.</li> - -<li class="indx">Immunity of butterflies, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_99">99</a>.</li> - -<li class="indx">Imperfection of adaptation, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_203">203</a>.</li> - -<li class="indx">Inbreeding, evil consequences of, ii. <a href="#Page_231">231</a>.</li> - -<li class="indx">Infection of the germ, ii. <a href="#Page_69">69</a>.</li> - -<li class="indx">Infusorians, experiments of Maupas on, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_328">328</a>;</li> -<li class="isub1">Calkins on, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_329">329</a>;</li> -<li class="isub1">differentiation of nucleus into macro-and micro-nucleus a means of compelling conjugation, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_334">334</a>.</li> - -<li class="indx">Inheritance, of acquired characters, ii. <a href="#Page_62">62</a> (<i>see</i> also Lamarckian principle);</li> -<li class="isub1">of functional modifications, ii. <a href="#Page_64">64</a>;</li> -<li class="isub1">of mutilations disproved, ii. <a href="#Page_65">65</a>;</li> -<li class="isub1">from parent to child, ii. <a href="#Page_38">38</a>;</li> -<li class="isub1">hereditary substance, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_288">288</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_341">341</a>;</li> -<li class="isub1">preponderance of one parent, ii. <a href="#Page_47">47</a>;</li> -<li class="isub1">alternation in ontogeny, ii. <a href="#Page_48">48</a>.</li> - -<li class="indx">Instinct, <a href="#Page_141">141</a>, ii. <a href="#Page_70">70</a>;</li> -<li class="isub1">will and, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_152">152</a>.</li> - -<li class="indx">Instincts, aberrant, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_149">149</a>;</li> -<li class="isub1">attachment of dog, ii. <a href="#Page_73">73</a>;</li> -<li class="isub1">change of, in <i>Eristalis</i>, &c., <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_150">150</a>;</li> -<li class="isub1">egg-laying of butterfly, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_159">159</a>;</li> -<li class="isub1">exercised only once, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_155">155</a>;</li> -<li class="isub2">ii. <a href="#Page_75">75</a>;</li> -<li class="isub1">'feigning death,' <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_145">145</a>;</li> -<li class="isub1">imperfectly adapted, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_152">152</a>;</li> -<li class="isub1">inheritance of, ii. <a href="#Page_72">72</a>;</li> -<li class="isub1">masking of crabs, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_145">145</a>;</li> -<li class="isub1">material basis of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_142">142</a>;</li> -<li class="isub1">monophagy of caterpillars, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_146">146</a>;</li> -<li class="isub1">new in domesticated animals, ii. <a href="#Page_73">73</a>;</li> -<li class="isub1">nutritive, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_146">146</a>;</li> -<li class="isub1">in Ephemerids and sea-cucumbers, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_148">148</a>;</li> -<li class="isub1">in predatory fishes, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_149">149</a>;</li> -<li class="isub1">origin of, ii. <a href="#Page_70">70</a>;</li> -<li class="isub1">pupation of butterflies, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_156">156</a>;</li> -<li class="isub1">self-preservation of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_144">144</a>;</li> -<li class="isub1">wild animals on lonely islands, ii. <a href="#Page_73">73</a>.</li> - -<li class="indx">Intra-selection (histonal selection), <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_240">240</a>.</li> - -<li class="indx">Ischikawa, on chromosomes in unicellulars, ii. <a href="#Page_216">216</a>;</li> -<li class="isub1">on the conjugation of <i>Noctiluca</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_317">317</a>;</li> -<li class="isub2">ii. <a href="#Page_42">42</a>.</li> - -<li class="indx">Island faunas, ii. <a href="#Page_283">283</a>.</li> - -<li class="indx">Isolated regions, ii. <a href="#Page_284">284</a>.</li> - -<li class="indx">Isolation, favours species-formation, ii. <a href="#Page_383">383</a>;</li> -<li class="isub1">relative, ii. <a href="#Page_350">350</a>;</li> -<li class="isub1">snails on the Sandwich Islands, ii. <a href="#Page_292">292</a>.</li> - - -<li class="ifrst">Jäger, G., on the continuity of the germ-plasm, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_411">411</a>.</li> - -<li class="indx">Japanese cock, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_356">356</a>.</li> - - -<li class="ifrst">Kaleidoscope, transformation resembles a, ii. <a href="#Page_307">307</a>.</li> - -<li class="indx">Kallima, mimicry of leaf, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_83">83</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_236">236</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_237">237</a>.</li> - -<li class="indx">Karyokinesis, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_290">290</a>.</li> - -<li class="indx">Kathariner, birds as enemies of butterflies, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_97">97</a>.</li> - -<li class="indx">Kennel, birds as enemies of butterflies, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_97">97</a>.</li> - -<li class="indx">Kerner von Marilaun, Alpine plants, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_122">122</a>;</li> -<li class="isub1">influence of hybridization on the formation of new species, ii. <a href="#Page_352">352</a>.</li> - -<li class="indx">Knowledge, limits of, ii. <a href="#Page_392">392</a>.</li> - -<li class="indx">Köhler, on scent-scales in the Lycænidæ, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_370">370</a>.</li> - -<li class="indx">Koshewnikow, on the influence of royal food on drone-larvæ, ii. <a href="#Page_92">92</a>.</li> - -<li class="indx">Kükenthal, on the fur of aquatic mammals, ii. <a href="#Page_270">270</a>.</li> - - -<li class="ifrst">Lamarck, theory of development, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_21">21</a>;</li> -<li class="isub1">on limits of genera and species, ii. <a href="#Page_306">306</a>.</li> - -<li class="indx">Lamarckian principle, ii. <a href="#Page_62">62</a>;</li> -<li class="isub1">Lamarck regarded inheritance of functional modifications as a matter of course, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_241">241</a>;</li> -<li class="isub1">cleaning apparatus of bees, ii. <a href="#Page_84">84</a>;</li> -<li class="isub1">claw of crustacean, ii. <a href="#Page_85">85</a>;</li> -<li class="isub1">Darwin's attitude to, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_242">242</a>;</li> -<li class="isub1">facts (foreleg of mole, cricket, &c.), ii. <a href="#Page_86">86</a>;</li> -<li class="isub1">Galton's attitude to, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_242">242</a>;</li> -<li class="isub1">Hering's view, ii. <a href="#Page_109">109</a>;</li> -<li class="isub1">O. Hertwig's view, ii. <a href="#Page_106">106</a>;</li> -<li class="isub1">neuters among ants and bees, ii. <a href="#Page_89">89</a>;</li> -<li class="isub1">phyletic development, ii. <a href="#Page_77">77</a>;</li> -<li class="isub1">skeleton of Arthropods, ii. <a href="#Page_82">82</a>;</li> -<li class="isub1">stridulating organs, ii. <a href="#Page_83">83</a>;</li> -<li class="isub1">theoretical impossibility of, ii. <a href="#Page_107">107</a>;</li> -<li class="isub1">variation of passive parts, ii. <a href="#Page_77">77</a>;</li> -<li class="isub1">venation of butterfly's wing, ii. <a href="#Page_87">87</a>;</li> -<li class="isub1">Zehnder's defence of, ii. <a href="#Page_99">99</a>.</li> - -<li class="indx"><i>Lathræa</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_135">135</a>.</li> - -<li class="indx">Lauterborn, on amphimixis in diatoms, ii. <a href="#Page_216">216</a>.</li> - -<li class="indx">Leaf-imitation, in Locustidæ, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_88">88</a>;</li> -<li class="isub1">in moths, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_87">87</a>;</li> -<li class="isub1">in butterflies, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_83">83</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_357">357-61</a>;</li> -<li class="isub1">in Anæa species, ii. <a href="#Page_310">310</a>.</li> - -<li class="indx"><i>Lepus variabilis</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_62">62</a>;</li> -<li class="isub1">ii. <a href="#Page_344">344</a>, <a href="#Page_350">350</a>.</li> - -<li class="indx">Leeuwenhoek, first use of the microscope, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_14">14</a>.</li> - -<li class="indx">Leuckart, <i>Trichosomum crassicauda</i>, with dwarf males, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_227">227</a>;</li> -<li class="isub1">structure of snails, ii. <a href="#Page_301">301</a>.</li> - -<li class="indx">Leuckart and von Siebold, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_333">333</a>.</li> - -<li class="indx">Leydig, regeneration of the lizard's tail, ii. <a href="#Page_30">30</a>.</li> - -<li class="indx">Liberation of the determinants in ontogeny, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_382">382-6</a>;</li> -<li class="isub1">quality of nutrition as a liberating stimulus in bees and ants, ii. <a href="#Page_92">92</a>.</li> - -<li class="indx">Liebig, theory of the origin of life, ii. <a href="#Page_365">365</a>.</li> - -<li class="indx">Limits of knowledge determined by selection, ii. <a href="#Page_394">394</a>.</li> - -<li class="indx">Linné, conception of species, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_14">14</a>.</li> - -<li class="indx">Lloyd Morgan, artificially induced instincts, ii. <a href="#Page_72">72</a>.</li> - -<li class="indx">Loeb, experiments on regeneration, ii. <a href="#Page_6">6</a>, <a href="#Page_7">7</a>;</li> -<li class="isub1">the cell-nucleus as an organ for oxidation, ii. <a href="#Page_31">31</a>.</li> - -<li class="indx">Luminous organs in deep-sea animals, ii. <a href="#Page_321">321</a>.</li> - - -<li class="ifrst">MacCullock, autotomy, ii. <a href="#Page_19">19</a>.</li> - -<li class="indx"><i>Machairodus</i>, ii. <a href="#Page_358">358</a>.</li> - -<li class="indx">Mammals, adaptation to aquatic life, ii. <a href="#Page_333">333</a>.</li> - -<li class="indx">Maturation divisions, ii. <a href="#Page_40">40</a>;</li> -<li class="isub1">in plants, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_315">315</a>;</li> -<li class="isub1">in the ovum, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_298">298</a>;</li> -<li class="isub1">in the sperm, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_301">301</a>;</li> -<li class="isub1">influence of, ii. <a href="#Page_44">44</a>.</li> - -<li class="indx">Maupas, intimate processes of conjugation, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_319">319</a>;</li> -<li class="isub1">conjugation of Infusorians, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_329">329</a>.</li> - -<li class="indx">Medium, influence of, ii. <a href="#Page_267">267</a>.</li> - -<li class="indx">Mendel's Law, ii. <a href="#Page_57">57</a>.</li> - -<li class="indx">Merogony, fertilization of non-nucleated pieces of ovum, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_343">343</a>.</li> - -<li class="indx">Merrifield, temperature-experiments with <i>Polyommatus phlæas</i>, ii. <a href="#Page_273">273</a>;</li> -<li class="isub1">cold experiments with <i>Vanessa</i>, ii. <a href="#Page_274">274</a>.</li> - -<li class="indx">Meyer, Hermann, architecture of the bone spongiosa, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_246">246</a>.</li> - -<li class="indx">Mimicry, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_91">91</a>;</li> -<li class="isub1">in beetles, bees, ants, &c., <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_116">116</a>;</li> -<li class="isub1">in butterflies does not affect caterpillar or pupa, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_104">104</a>;</li> -<li class="isub1">in both sexes, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_96">96</a>;</li> -<li class="isub1">in vertebrates, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_117">117</a>;</li> -<li class="isub1">degree of resemblance to model, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_104">104</a>;</li> -<li class="isub1"><i>Elymnias undularis</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_106">106</a>;</li> -<li class="isub1"><i>Papilio merope</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_108">108</a>;</li> -<li class="isub1"><i>P. turnus</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_110">110</a>;</li> -<li class="isub1">same effect produced in different ways, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_105">105</a>;</li> -<li class="isub1">several imitators of one immune species, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_101">101</a>;</li> -<li class="isub1">species of genera which need protection imitate different immune models, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_102">102</a>;</li> -<li class="isub1">'rings' of mimetic species, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_112">112</a>;</li> -<li class="isub1">rarity of mimetic species, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_108">108</a>;</li> -<li class="isub1">wide divergence of mimetic species from their congeners, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_115">115</a>.</li> - -<li class="indx">Mitosis, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_288">288</a>.</li> - -<li class="indx">Möbius, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_296">296</a>.</li> - -<li class="indx">Monism, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_393">393</a>.</li> - -<li class="indx">Monogony, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_266">266</a>.</li> - -<li class="indx">Montgomery, on reduction of the chromosomes, ii. <a href="#Page_43">43</a>.</li> - -<li class="indx">Morgan, experiments on regeneration, ii. <a href="#Page_15">15</a>.</li> - -<li class="indx">Morphological characters, dependent on germinal selection, ii. <a href="#Page_132">132</a>;</li> -<li class="isub1">discussion as to indifferent characters, ii. <a href="#Page_132">132</a>, <a href="#Page_309">309</a>.</li> - -<li class="indx">Mortality of multicellular organisms, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_260">260</a>;</li> -<li class="isub1">causes of this, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_263">263</a>.</li> - -<li class="indx">Morton, Thomas, on degeneration in the children of alcoholics, ii. <a href="#Page_69">69</a>.</li> - -<li class="indx">Moths, protective coloration in, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_80">80</a>.</li> - -<li class="indx">Müller, Fritz, scent-scales, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_217">217</a>;</li> -<li class="isub1">on mimicry, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_111">111</a>;</li> -<li class="isub1">plants and ants, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_171">171</a>;</li> -<li class="isub1">relation between ontogeny and phylogeny, ii. <a href="#Page_160">160</a>.</li> - -<li class="indx">Müller, Johannes, the vision of insects, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_216">216</a>.</li> - -<li class="indx">Musical sense in man, ii. <a href="#Page_148">148</a>.</li> - -<li class="indx">Mutation theory of de Vries, ii. <a href="#Page_317">317</a>.</li> - -<li class="indx">Mutilations, supposed inheritance of, ii. <a href="#Page_65">65</a>.</li> - -<li class="indx">Mutual sterility, of no great importance in connexion with lasting variation, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_349">349</a>.</li> - - -<li class="ifrst">Nägeli, Carl von, on the definite directions of variations, ii. <a href="#Page_306">306</a>, <a href="#Page_385">385</a>;</li> -<li class="isub1">objection to origin of flowers through selection, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_198">198</a>;</li> -<li class="isub1">on the difference in size between egg and sperm, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_337">337</a>;</li> -<li class="isub1">his <i>Hieracium</i> experiments, ii. <a href="#Page_272">272</a>;</li> -<li class="isub1">Nägeli's view and Darwin's reconciled through germinal selection, ii. <a href="#Page_334">334</a>;</li> -<li class="isub1">number of smallest vital units in a 'moneron,' ii. <a href="#Page_368">368</a>.</li> - -<li class="indx">Nathusius, inbreeding experiments, ii. <a href="#Page_231">231</a>.</li> - -<li class="indx">Natural Selection, not directly observable, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_58">58</a>;</li> -<li class="isub1">under the influence of isolation, ii. <a href="#Page_292">292</a>.</li> - -<li class="indx">Neo-Lamarckism, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_243">243</a>.</li> - -<li class="indx">Neotaxis, ii. <a href="#Page_40">40</a>.</li> - -<li class="indx">Nerve-tracks in relation to instincts, ii. <a href="#Page_71">71</a>.</li> - -<li class="indx">Normal number of a species, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_45">45</a>.</li> - -<li class="indx"><i>Notodonta</i>, protective coloration in, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_80">80</a>.</li> - -<li class="indx">Nuclear division, process of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_289">289</a>;</li> -<li class="isub1">integral and differential, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_374">374</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_377">377</a>.</li> - -<li class="indx">Nussbaum, M., regeneration-experiments in Protozoa, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_340">340</a>;</li> -<li class="isub1">on the continuity of the germ-cells, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_411">411</a>;</li> -<li class="isub1">infection of the ovum in Hydra, ii. <a href="#Page_68">68</a>.</li> - -<li class="indx">Nutrition, influence of, on variation, ii. <a href="#Page_267">267</a>;</li> -<li class="isub1">relation between nutrition and the number in a species, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_45">45</a>.</li> - - -<li class="ifrst">Oken's 'Naturphilosophie,' <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_21">21</a>.</li> - -<li class="indx">Omnipotence of selection, ii. <a href="#Page_348">348</a>.</li> - -<li class="indx">Ontogenesis, relation to phylogenesis, ii. <a href="#Page_159">159</a>;</li> -<li class="isub1">shunting back of the phyletic stages in embryogenesis, ii. <a href="#Page_176">176</a>;</li> -<li class="isub1">condensation of phylogeny in ontogeny, ii. <a href="#Page_186">186</a>.</li> - -<li class="indx">Orchids, fertilization of, ii. <a href="#Page_256">256</a>.</li> - -<li class="indx">Organs, rudimentary, ii. <a href="#Page_226">226</a>.</li> - -<li class="indx">Origin of flowers, <i>see</i> Flowers.</li> - -<li class="indx">Osborn, supposed palæontological proofs for the Lamarckian principle, ii. <a href="#Page_77">77</a>.</li> - -<li class="indx">Ovaries, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_282">282</a>.</li> - -<li class="indx">Ovogenic determinants, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_388">388</a>.</li> - -<li class="indx">Ovum, maturation of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_295">295</a>.</li> - - -<li class="ifrst">Packard, disappearance of useless parts, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_129">129</a>.</li> - -<li class="indx">Palingenesis, ii. <a href="#Page_173">173</a>.</li> - -<li class="indx"><i>Pandorina</i>, reproduction of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_257">257</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_293">293</a>.</li> - -<li class="indx">Pangenesis, ii. <a href="#Page_62">62</a>.</li> - -<li class="indx">Panmixia, ii. <a href="#Page_114">114</a>.</li> - -<li class="indx"><i>Papilio meriones</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_108">108</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_427">427</a>;</li> -<li class="isub1"><i>P. turnus</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_110">110</a>.</li> - -<li class="indx">Parasites, power of adaptation in, ii. <a href="#Page_384">384</a>.</li> - -<li class="indx">Parthenogenesis, discovery of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_303">303</a>;</li> -<li class="isub1">exceptional and artificial, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_307">307</a>;</li> -<li class="isub1">facultative in bees, ii. <a href="#Page_235">235</a>;</li> -<li class="isub1">receptaculum seminis in Cypris-species without males, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_326">326</a>, ii. <a href="#Page_234">234</a>;</li> -<li class="isub1">advantages of, ii. <a href="#Page_243">243</a>;</li> -<li class="isub1">its effects compared with those of inbreeding, ii. <a href="#Page_233">233</a>;</li> -<li class="isub1">alternation of, with bisexual generations (heterogony), ii. <a href="#Page_243">243</a>.</li> - -<li class="indx">Personal selection, indirect effects of, ii. <a href="#Page_200">200</a>.</li> - -<li class="indx">Petrunkewitsch, A., maturing divisions in the ovum of the bee, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_306">306</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_336">336</a>.</li> - -<li class="indx">Pfeffer, rôle of malic acid in the fertilization of ferns, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_273">273</a>.</li> - -<li class="indx">Pflüger and Born, experiments in hybridization, ii. <a href="#Page_232">232</a>.</li> - -<li class="indx">Phasmids, regeneration in, ii. <a href="#Page_17">17</a>.</li> - -<li class="indx"><i>Phylloxera</i>, reproduction in, ii. <a href="#Page_249">249</a>.</li> - -<li class="indx">Phylogenetic variation of butterfly and caterpillar independent of each other, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_362">362</a>.</li> - -<li class="indx">Phylogeny, condensation of, in ontogeny, ii. <a href="#Page_186">186</a>.</li> - -<li class="indx"><i>Physiologus</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_11">11</a>.</li> - -<li class="indx">Pictet, turban eyes in male Ephemerids, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_229">229</a>.</li> - -<li class="indx">Pigeons, breeds of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_34">34</a>.</li> - -<li class="indx">Plants, fertilization of the higher, ii. <a href="#Page_250">250</a>;</li> -<li class="isub1">carnivorous, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_132">132</a>;</li> -<li class="isub1"><i>Aldrovandia</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_138">138</a>;</li> -<li class="isub1"><i>Dionæa</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_138">138</a>;</li> -<li class="isub1"><i>Drosera</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_136">136</a>;</li> -<li class="isub1"><i>Lathræa</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_135">135</a>;</li> -<li class="isub1"><i>Nepenthes</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_134">134</a>;</li> -<li class="isub1"><i>Pinguicula</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_135">135</a>;</li> -<li class="isub1"><i>Utricularia</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_133">133</a>.</li> - -<li class="indx">Plant-galls, ii. <a href="#Page_270">270</a>.</li> - -<li class="indx">Plastogamy a preliminary stage to fertilization, ii. <a href="#Page_220">220</a>.</li> - -<li class="indx">Pliny, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_11">11</a>.</li> - -<li class="indx">Polar bodies, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_294">294</a>.</li> - -<li class="indx">Polymorphism, its idioplasmic roots, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_390">390</a>.</li> - -<li class="indx"><i>Polyommatus phlæas</i>, dimorphism of caterpillars, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_363">363</a>;</li> -<li class="isub1">climatic varieties, ii. <a href="#Page_272">272</a>.</li> - -<li class="indx">Postgeneration (Roux), <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_407">407</a>.</li> - -<li class="indx">Pouchet, spontaneous generation, ii. <a href="#Page_366">366</a>.</li> - -<li class="indx">Poulton, on facultative colour adaptation in caterpillars, ii. <a href="#Page_278">278</a>;</li> -<li class="isub1">on mimicry, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_105">105</a>.</li> - -<li class="indx">Prediction on the basis of the evolution theory, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_3">3</a>.</li> - -<li class="indx">Preformation and Epigenesis, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_351">351</a>.</li> - -<li class="indx">Primordial males among Cirrhipeds, ii. <a href="#Page_242">242</a>.</li> - -<li class="indx">Protective arrangements in plants, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_119">119</a>;</li> -<li class="isub1">Alpine plants, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_126">126</a>;</li> -<li class="isub1">chemical substances, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_128">128</a>;</li> -<li class="isub1">ethereal oils, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_128">128</a>;</li> -<li class="isub1">hairs, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_122">122</a>;</li> -<li class="isub1">poisons, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_120">120</a>;</li> -<li class="isub1">Raphides, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_129">129</a>;</li> -<li class="isub1">'Prigana scrub,' <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_126">126</a>;</li> -<li class="isub1">against small enemies, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_127">127</a>;</li> -<li class="isub1">Tragacanth, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_124">124</a>.</li> - -<li class="indx">Protective colouring, rôle of light in, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_78">78</a>;</li> -<li class="isub1"><i>Kallima</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_83">83</a>;</li> -<li class="isub1"><i>Notodonta</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_80">80</a>;</li> -<li class="isub1"><i>Xylina</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_82">82</a>.</li> - -<li class="indx">Protective marking in caterpillars, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_67">67</a>.</li> - -<li class="indx">Protozoa, chromosomes in, ii. <a href="#Page_216">216</a>.</li> - - -<li class="ifrst">Quetelet, amphigony preserves the mean of the species, ii. <a href="#Page_204">204</a>.</li> - - -<li class="ifrst">Races, development of, depending on adaptation, ii. <a href="#Page_335">335</a>;</li> -<li class="isub1">dependent on germinal selection, ii. <a href="#Page_144">144</a>.</li> - -<li class="indx">Radiolarians, skeleton of, ii. <a href="#Page_324">324</a>.</li> - -<li class="indx">Rand, experiments on regeneration in Hydra, ii. <a href="#Page_5">5</a>.</li> - -<li class="indx">Rath, O. von, on the influence of royal food on drone-larvæ, ii. <a href="#Page_91">91</a>.</li> - -<li class="indx">Ray, John, conception of 'species,' <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_14">14</a>.</li> - -<li class="indx">Reactions, primary and secondary, ii. <a href="#Page_277">277</a>.</li> - -<li class="indx">Reducing divisions, <i>see</i> Maturation divisions.</li> - -<li class="indx">Regeneration, ii. <a href="#Page_1">1</a>;</li> -<li class="isub1">atavistic, ii. <a href="#Page_30">30</a>;</li> -<li class="isub1">autotomy, ii. <a href="#Page_16">16</a>;</li> -<li class="isub1">in birds, ii. <a href="#Page_14">14</a>;</li> -<li class="isub1">in Hydra, ii. <a href="#Page_4">4</a>;</li> -<li class="isub1">in Hydroid polyps, ii. <a href="#Page_9">9</a>;</li> -<li class="isub1">in plants, ii. <a href="#Page_9">9</a>, <a href="#Page_32">32</a>;</li> -<li class="isub1">in Planarians, ii. <a href="#Page_6">6</a>, <a href="#Page_13">13</a>;</li> -<li class="isub1">in starfishes, ii. <a href="#Page_30">30</a>;</li> -<li class="isub1">in Vertebrates, ii. <a href="#Page_10">10</a>;</li> -<li class="isub1">of the lens in Triton, ii. <a href="#Page_19">19</a>;</li> -<li class="isub1">a phenomenon of adaptation, ii. <a href="#Page_9">9</a>;</li> -<li class="isub1">nuclear substance the first organ of, ii. <a href="#Page_31">31</a>;</li> -<li class="isub1">phyletic origin of, ii. <a href="#Page_23">23</a>;</li> -<li class="isub1">disappearance of the power of, ii. <a href="#Page_16">16</a>;</li> -<li class="isub1">and budding, ii. <a href="#Page_31">31</a>;</li> -<li class="isub1">relation of, to liability of part to injury, ii. <a href="#Page_7">7</a>;</li> -<li class="isub1">not always purposive, ii. <a href="#Page_25">25</a>.</li> - -<li class="indx">Reinke, objections to the 'machine theory' of life, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_402">402</a>;</li> -<li class="isub1">on regeneration, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_32">32</a>.</li> - -<li class="indx">Rejuvenescence, theory of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_325">325-8</a>.</li> - -<li class="indx">Reproduction, adaptation of the germ-cells, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_277">277</a>;</li> -<li class="isub1">asexual, ii. <a href="#Page_259">259</a>;</li> -<li class="isub1">structure of the ovum, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_280">280</a>;</li> -<li class="isub1">of the bird's egg, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_285">285</a>;</li> -<li class="isub1">zoosperm, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_273">273</a>;</li> -<li class="isub1">in Amœbæ, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_253">253</a>;</li> -<li class="isub1">in Infusorians, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_254">254</a>;</li> -<li class="isub1">in <i>Pandorina morum</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_257">257</a>, <a href="#Page_269">269</a>;</li> -<li class="isub1">in fungi, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_267">267</a>;</li> -<li class="isub1">by means of germ-cells, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_266">266</a>;</li> -<li class="isub1">differentiation of germ-cells into male and female, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_267">267</a>;</li> -<li class="isub1">by division, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_264">264</a>;</li> -<li class="isub1">two kinds of eggs in same species, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_282">282</a>;</li> -<li class="isub1">nutritive ovum cells, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_283">283</a>;</li> -<li class="isub1">introduction of death into the living world, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_261">261</a>;</li> -<li class="isub1">contrast between reproductive and body cells in the Metazoa, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_256">256</a>;</li> -<li class="isub1">budding and division in the Metazoa, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_264">264</a>;</li> -<li class="isub1">potential immortality of the Protozoa, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_260">260</a>;</li> -<li class="isub1">sperm and ovum in Algæ, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_272">272</a>;</li> -<li class="isub1">in <i>Volvox</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_265">265</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_271">271</a>;</li> -<li class="isub1">zoosperms of Ostracods, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_275">275</a>;</li> -<li class="isub1">different kinds of spermatozoa, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_278">278</a>.</li> - -<li class="indx">Reproductive cells, development of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_410">410</a>;</li> -<li class="isub1">in Diptera, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_411">411</a>;</li> -<li class="isub1">in Hydroid polyps, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_413">413</a>.</li> - -<li class="indx">Reversion, ii. <a href="#Page_53">53</a>;</li> -<li class="isub1">in doves, ii. <a href="#Page_55">55</a>;</li> -<li class="isub1">in the horse, ii. <a href="#Page_55">55</a>.</li> - -<li class="indx">Riley, fertilization of the Yucca by a moth, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_202">202</a>.</li> - -<li class="indx">Ritzema Bos, experiments on mice, ii. <a href="#Page_65">65</a>, <a href="#Page_66">66</a>.</li> - -<li class="indx">Romanes, isolation theory, ii. <a href="#Page_284">284</a>;</li> -<li class="isub1">physiological selection, ii. <a href="#Page_337">337</a>;</li> -<li class="isub1">panmixia, ii. <a href="#Page_115">115</a>.</li> - -<li class="indx">Rosenthal, experiments with mice, ii. <a href="#Page_65">65</a>, <a href="#Page_66">66</a>.</li> - -<li class="indx">Roux, Wilhelm, Mosaic theory, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_379">379</a>;</li> -<li class="isub1">struggle of the parts, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_244">244</a>;</li> -<li class="isub1">postgeneration, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_407">407</a>.</li> - -<li class="indx">Rückert, the nuclear substances in Copepods, ii. <a href="#Page_42">42</a>.</li> - -<li class="indx">Rudimentary organs in man, ii. <a href="#Page_226">226</a>.</li> - - -<li class="ifrst">St.-Hilaire, unity of type, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_18">18</a>.</li> - -<li class="indx">Samassa, segmentation of the frog's egg, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_407">407</a>.</li> - -<li class="indx">Sarasin, snails of Celebes, ii. <a href="#Page_299">299</a>.</li> - -<li class="indx"><i>Saturnia</i>, pupation of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_158">158</a>.</li> - -<li class="indx">Schaudinn, fertilization in Coccidia, ii. <a href="#Page_214">214</a>;</li> -<li class="isub1">maturing division in Sun-animalcule, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_318">318</a>.</li> - -<li class="indx">Schimper, plants and ants, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_171">171</a>.</li> - -<li class="indx">Schleiden and Schwann, discovery of the cell, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_26">26</a>.</li> - -<li class="indx">Schmankewitsch, experiments with <i>Artemia</i>, ii. <a href="#Page_277">277</a>.</li> - -<li class="indx">Schmidt, Oscar, ii. <a href="#Page_324">324</a>.</li> - -<li class="indx">Schneider, discovery of the 'spindle-figure' of nuclear division, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_289">289</a>.</li> - -<li class="indx">Schütt, Diatoms, ii. <a href="#Page_325">325</a>.</li> - -<li class="indx">Schwarz, Ostracods, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_276">276</a>.</li> - -<li class="indx">Segmentation-cells in animal ova, their prospective importance, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_406">406</a>.</li> - -<li class="indx">Seitz, a case of mimicry, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_114">114</a>.</li> - -<li class="indx">Selection-processes, grades of, ii. <a href="#Page_265">265</a>;</li> -<li class="isub1">evolution guided by, ii. <a href="#Page_298">298</a>.</li> - -<li class="indx">Selection, sexual, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_210">210-39</a>;</li> -<li class="isub1">absence of secondary sexual characters in the lower animals, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_231">231</a>;</li> -<li class="isub1">adaptations for seizing the females, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_229">229</a>;</li> -<li class="isub1">choice on the part of the females, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_214">214</a>;</li> -<li class="isub1">odours and scent-scales, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_217">217</a>;</li> -<li class="isub1">song of cicadas and birds, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_221">221</a>;</li> -<li class="isub1">superfluity of males, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_213">213</a>;</li> -<li class="isub1">weapons for the struggle for mates, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_228">228</a>;</li> -<li class="isub1">summary, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_238">238</a>.</li> - -<li class="indx">Selection value, ii. <a href="#Page_132">132</a>, <a href="#Page_311">311</a>.</li> - -<li class="indx">Self-fertilization in plants, ii. <a href="#Page_252">252</a>;</li> -<li class="isub1">continued influence of, ii. <a href="#Page_257">257</a>;</li> -<li class="isub1">alternation of self- with cross-fertilization, ii. <a href="#Page_241">241</a>.</li> - -<li class="indx">Self-preservation, instinct of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_144">144</a>.</li> - -<li class="indx">Sex-cells, mutual attraction of, ii. <a href="#Page_228">228</a>.</li> - -<li class="indx">Sex, determination of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_377">377</a>; ii. <a href="#Page_44">44</a>.</li> - -<li class="indx">Sexual characters, secondary, have their roots in germinal selection, ii. <a href="#Page_130">130</a>, <a href="#Page_143">143</a>, <a href="#Page_289">289-91</a>, <a href="#Page_378">378</a>.</li> - -<li class="indx">Sexual selection, <i>see</i> Selection, sexual.</li> - -<li class="indx">Sexual selection through isolation, ii. <a href="#Page_289">289</a>.</li> - -<li class="indx">Short-sight, ii. <a href="#Page_146">146</a>.</li> - -<li class="indx">Siedlecky, copulation in <i>Coccidium proprium</i>, ii. <a href="#Page_218">218</a>.</li> - -<li class="indx">Simroth, ii. <a href="#Page_302">302</a>.</li> - -<li class="indx">Slevogt, on birds as enemies of butterflies, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_97">97</a>.</li> - -<li class="indx">Sluiter, on symbiosis, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_167">167</a>.</li> - -<li class="indx">Smerinthus, markings of the caterpillars, ii. <a href="#Page_177">177</a>, <a href="#Page_184">184</a>.</li> - -<li class="indx">Snail-strata of Steinheim, ii. <a href="#Page_305">305</a>.</li> - -<li class="indx">Sommer, on artificial epilepsy in guinea-pigs, ii. <a href="#Page_68">68</a>.</li> - -<li class="indx">Special investigation, period of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_25">25</a>.</li> - -<li class="indx">Species, the, a complex of adaptations and variations, ii. <a href="#Page_307">307</a>.</li> - -<li class="indx">Species-colonies, ii. <a href="#Page_280">280</a>.</li> - -<li class="indx">Species, extinction of, ii. <a href="#Page_357">357</a>;</li> -<li class="isub1">dying out of the large animals of Central Europe, ii. <a href="#Page_361">361</a>;</li> -<li class="isub1">extinction due to cultivation, ii. <a href="#Page_360">360</a>;</li> -<li class="isub1">to unlimited variation, ii. <a href="#Page_357">357</a>;</li> -<li class="isub1"><i>Machairodus</i>, ii. <a href="#Page_358">358</a>;</li> -<li class="isub1">lower types more capable of adaptation than higher, ii. <a href="#Page_359">359</a>;</li> -<li class="isub1">extinction of flightless birds, ii. <a href="#Page_360">360</a>.</li> - -<li class="indx">Species-formation, ii. <a href="#Page_299">299</a>;</li> -<li class="isub1">favoured by isolation, ii. <a href="#Page_284">284</a>;</li> -<li class="isub1">snails of Celebes, ii. <a href="#Page_219">219</a>;</li> -<li class="isub1">without amphigony in lichens, ii. <a href="#Page_343">343</a>;</li> -<li class="isub1">without isolation in <i>Lepus variabilis</i>, ii. <a href="#Page_344">344</a>;</li> -<li class="isub1">Peridineæ, ii. <a href="#Page_325">325</a>;</li> -<li class="isub1">protective coloration in butterflies, ii. <a href="#Page_310">310</a>;</li> -<li class="isub1">the Steinheim snail-strata, ii. <a href="#Page_315">315</a>;</li> -<li class="isub1">telescope eyes in deep-sea animals, ii. <a href="#Page_323">323</a>;</li> -<li class="isub1">typical species, ii. <a href="#Page_304">304</a>;</li> -<li class="isub1">variation in definite directions, ii. <a href="#Page_306">306</a>;</li> -<li class="isub1">the bird as a complex of adaptations, ii. <a href="#Page_316">316</a>;</li> -<li class="isub1">the whale as a complex of adaptations, ii. <a href="#Page_313">313</a>;</li> -<li class="isub1">mutual fertility between many plant-species, ii. <a href="#Page_340">340</a>.</li> - -<li class="indx">Species, variable and constant, ii. <a href="#Page_286">286</a>.</li> - -<li class="indx">Specific type, its occurrence favoured by germinal variation, ii. <a href="#Page_333">333</a>, <a href="#Page_334">334</a>;</li> -<li class="isub1">by natural selection, ii. <a href="#Page_334">334</a>;</li> -<li class="isub1">origin of the, ii. <a href="#Page_299">299</a>, <a href="#Page_332">332-5</a>.</li> - -<li class="indx">Spencer, Herbert, germinal substance composed of homogeneous particles, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_355">355</a>;</li> -<li class="isub1">on 'units,' the smallest vital particles, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_369">369</a>;</li> -<li class="isub1">protective adaptations in plants to be referred to selection, ii. <a href="#Page_77">77</a>.</li> - -<li class="indx">Spermaries, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_282">282</a>.</li> - -<li class="indx">Spermatozoa, <i>see</i> Zoosperms.</li> - -<li class="indx">Sperm-cells, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_272">272</a>.</li> - -<li class="indx">Spermogenic determinants, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_388">388</a>.</li> - -<li class="indx">Sphingidæ, caterpillars of the, biological value of their markings, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_73">73</a>;</li> -<li class="isub1">ontogeny and phylogeny of the markings, ii. <a href="#Page_177">177</a>.</li> - -<li class="indx"><i>Sphinx convolvuli</i>, double adaptation of the caterpillar, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_71">71</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_72">72</a>;</li> -<li class="isub1"><i>S. euphorbiæ</i>, var. <i>Nicæa</i>, purely local form of caterpillar, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_362">362</a>.</li> - -<li class="indx">Spontaneous generation, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_410">410</a>;</li> -<li class="isub1">conditions necessary, ii. <a href="#Page_370">370</a>;</li> -<li class="isub1">only possible as regards invisible minute organisms, ii. <a href="#Page_369">369</a>;</li> -<li class="isub1">the 'where' of, ii. <a href="#Page_371">371</a>;</li> -<li class="isub1">impossibility of proving or disproving it experimentally, ii. <a href="#Page_366">366</a>.</li> - -<li class="indx">Sprengel, fertilization of flowers, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_180">180</a>.</li> - -<li class="indx">Standfuss, cold experiments with butterfly pupæ, ii. <a href="#Page_275">275</a>.</li> - -<li class="indx">Steinheim snail-strata, ii. <a href="#Page_305">305</a>.</li> - -<li class="indx">Steller's sea-cow (<i>Rhytina stelleri</i>), ii. <a href="#Page_74">74</a>.</li> - -<li class="indx">Stick-insects, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_88">88</a>.</li> - -<li class="indx">Strasburger, fertilization of Phanerogams, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_314">314</a>.</li> - -<li class="indx">Stuhlmann, zoosperms in Ostracods, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_276">276</a>.</li> - -<li class="indx">Swammerdam, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_14">14</a>.</li> - -<li class="indx">Symbiosis, candelabra trees and ants, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_171">171</a>;</li> -<li class="isub1">hermit-crabs and Hydroid polyps, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_163">163</a>;</li> -<li class="isub1">hermit-crabs and sea-anemones, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_162">162</a>;</li> -<li class="isub1">origin of symbiosis, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_176">176</a>;</li> -<li class="isub1">lichens, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_173">173</a>;</li> -<li class="isub1">fishes and sea-anemones, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_167">167</a>;</li> -<li class="isub1">green Amœbæ, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_170">170</a>;</li> -<li class="isub1">green fresh-water polyp (<i>Hydra viridis</i>), <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_168">168</a>;</li> -<li class="isub1"><i>Nostoc</i> and <i>Azolla</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_177">177</a>;</li> -<li class="isub1">sea-anemones and yellow Algæ, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_171">171</a>;</li> -<li class="isub1">root-fungi, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_175">175</a>.</li> - - -<li class="ifrst">Talents, specific, of man referred to germinal selection, ii. <a href="#Page_149">149</a>;</li> -<li class="isub1">depend on a combination of mental gifts, ii. <a href="#Page_150">150</a>.</li> - -<li class="indx">Tichomiroff, artificial parthenogenesis, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_307">307</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_333">333</a>.</li> - -<li class="indx">Thorn-bugs, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_89">89</a>.</li> - -<li class="indx">Transparent winged butterflies, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_106">106</a>.</li> - -<li class="indx">Treviranus, as founder of the evolution theory, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_18">18</a>;</li> -<li class="isub1">on generic differences, ii. <a href="#Page_306">306</a>.</li> - -<li class="indx">Trimen, observations on the immunity of the Acræidæ, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_100">100</a>.</li> - -<li class="indx">Tropism in plants, ii. <a href="#Page_276">276</a>.</li> - -<li class="indx">Twins, identical, ii. <a href="#Page_44">44</a>.</li> - - -<li class="ifrst"><i>Vanessa</i>, endemic species of, with protective colouring, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_75">75</a>.</li> - -<li class="indx">Variability, fluctuating, ii. <a href="#Page_327">327</a>.</li> - -<li class="indx">Variation, all ultimately quantitative, ii. <a href="#Page_151">151</a>;</li> -<li class="isub1">in a definite direction, ii. <a href="#Page_118">118</a>;</li> -<li class="isub1">double roots of, ii. <a href="#Page_195">195</a>;</li> -<li class="isub1">ascending, ii. <a href="#Page_122">122</a>;</li> -<li class="isub1">sports or saltatory variations, ii. <a href="#Page_140">140</a>;</li> -<li class="isub1">roots of hereditary, ii. <a href="#Page_118">118</a>.</li> - -<li class="indx">Variation of individual characters, ii. <a href="#Page_336">336</a>;</li> -<li class="isub1">not always due to adaptation, ii. <a href="#Page_197">197</a>.</li> - -<li class="indx">Variation, periods of, ii. <a href="#Page_294">294</a>.</li> - -<li class="indx">Vital force, ii. <a href="#Page_369">369</a>.</li> - -<li class="indx">Vitalism, ii. <a href="#Page_369">369</a>.</li> - -<li class="indx">Virchow, Rudolf, on the inheritance of mutilations, ii. <a href="#Page_65">65</a>.</li> - -<li class="indx">Vöchting, influence of light on the production of flowers, ii. <a href="#Page_276">276</a>;</li> -<li class="isub1">on regeneration, ii. <a href="#Page_32">32</a>.</li> - -<li class="indx">Voigt, Walter, experiments in regeneration, ii. <a href="#Page_6">6</a>;</li> -<li class="isub1">on Planarians, ii. <a href="#Page_25">25</a>.</li> - -<li class="indx">Voit, Carl von, influence of nutrition on bodily size, ii. <a href="#Page_268">268</a>.</li> - -<li class="indx">Volvocineæ, reproduction in, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_257">257</a>.</li> - -<li class="indx">Vries, de, asymmetrical curves of frequency, ii. <a href="#Page_234">234</a>;</li> -<li class="isub1">theory of mutations, ii. <a href="#Page_317">317</a>;</li> -<li class="isub1">Pangen theory, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_380">380</a>.</li> - - -<li class="ifrst">Wagner, Franz von, regeneration in <i>Lumbriculus</i>, ii. <a href="#Page_27">27</a>.</li> - -<li class="indx">Wagner, Moriz, on the influence of isolation, ii. <a href="#Page_284">284</a>.</li> - -<li class="indx">Wahl, Bruno, on the development of <i>Eristalis</i>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_399">399</a>.</li> - -<li class="indx">Wallace, on the immunity of Heliconiidæ, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_99">99</a>;</li> -<li class="isub1">on the causes of the coloration of butterflies, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_211">211</a>.</li> - -<li class="indx">Wasmann, Erich, on transition forms in ants, ii. <a href="#Page_93">93</a>;</li> -<li class="isub1">on sounds produced by ants, ii. <a href="#Page_83">83</a>.</li> - -<li class="indx">Weaver birds, ii. <a href="#Page_290">290</a>.</li> - -<li class="indx">Whales, their origin through adaptation, ii. <a href="#Page_313">313</a>.</li> - -<li class="indx">Wheeler, rôle of the centrosphere in the ovum, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_309">309</a>.</li> - -<li class="indx">Wiedersheim, rudimentary organs in man, ii. <a href="#Page_226">226</a>.</li> - -<li class="indx">Wiesner, the smallest vital particles, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_369">369</a>.</li> - -<li class="indx">Wing-primordia in insects, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_364">364</a>.</li> - -<li class="indx">Winkler, Hans, experiments on artificial parthenogenesis, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_307">307</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_333">333</a>;</li> -<li class="isub1">on merogony, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_343">343</a>.</li> - -<li class="indx">Wolff, G., regeneration of the lens in Triton, ii. <a href="#Page_19">19</a>.</li> - -<li class="indx">Wolff, K. v., the founder of the epigenetic theory of evolution, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_352">352</a>.</li> - -<li class="indx">Wroughton, Robert, production of sounds by Indian ants, ii. <a href="#Page_95">95</a>.</li> - -<li class="indx">Würtemberger, form-series of ammonites, ii. <a href="#Page_176">176</a>.</li> - - -<li class="ifrst">Xenia, ii. <a href="#Page_58">58</a>.</li> - -<li class="indx"><i>Xylina</i>, protective colouring of, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_82">82</a>.</li> - - -<li class="ifrst">Yolk of egg, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_282">282</a>.</li> - - -<li class="ifrst">Zehnder, the living substance made up of fistellæ, ii. <a href="#Page_217">217</a>;</li> -<li class="isub1">polymorphism in ants, ii. <a href="#Page_99">99</a>;</li> -<li class="isub1">on the Lamarckian principle, ii. <a href="#Page_99">99-106</a>;</li> -<li class="isub1">on the skeleton of Arthropods, ii. <a href="#Page_103">103</a>;</li> -<li class="isub1">effect of amphimixis, ii. <a href="#Page_223">223</a>.</li> - -<li class="indx">Ziegler, Ernst, on deformities, ii. <a href="#Page_138">138</a>.</li> - -<li class="indx">Ziegler, H. E., experiments on merogony in sea-urchin ova, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_342">342</a>.</li> - -<li class="indx">Zoja, experiments with the ova of Medusæ, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_407">407</a>.</li> - -<li class="indx">Zoosperms, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_273">273</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_278">278</a>, <a href="http://www.gutenberg.org/files/64227/64227-h/64227-h.htm#Page_279">279</a>.</li> -</ul> - - -<hr class="full2" /> - - - -<p class="c more"> -OXFORD: HORACE HART<br /> -PRINTER TO THE UNIVERSITY -</p> - -<p class="c xxxlarge"><span class="smcap">Standard Scientific Works.</span></p> - -<hr class="ad" /> - -<p class="c xxlarge">HABIT AND INSTINCT:</p> - -<p class="c"><i>A STUDY IN HEREDITY</i>.</p> - -<p class="c"><span class="smcap">By Professor</span> C. 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