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diff --git a/44582-h/44582-h.htm b/44582-h/44582-h.htm new file mode 100644 index 0000000..3d28b23 --- /dev/null +++ b/44582-h/44582-h.htm @@ -0,0 +1,13348 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" + "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> +<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> + <head> + <meta http-equiv="Content-Type" content="text/html;charset=UTF-8" /> + <meta http-equiv="Content-Style-Type" content="text/css" /> + <title> + The Project Gutenberg eBook of Problems of Genetics, + by William Bateson, M.A., F.R.S. + </title> + <link rel="coverpage" href="images/titlepage.jpg" /> + <style type="text/css"> + +body { + margin-left: 10%; + margin-right: 10%; +} + + h1,h2,h3 { + text-align: center; + clear: both; +} + +p { + margin-top: .51em; + text-align: justify; + margin-bottom: .49em; +} + +.indent {text-indent: 1.5em;} +.space-above { margin-top: 3em; } +.space-below { margin-bottom: 3em; } +.advert { margin: 5%; } + +hr.tb {width: 45%;} +hr.chap {width: 65%} +hr.full {width: 95%;} +hr.r5 {width: 5%; margin-top: 1em; margin-bottom: 1em;} + +table { + margin-left: auto; + margin-right: auto; +} + + .tdl {text-align: left;} + .tdr {text-align: right;} + .tdc {text-align: center;} + +.pagenum { /* visibility: hidden; */ + position: absolute; + left: 92%; + font-size: smaller; + text-align: right; +} + +.blockquot { + margin-left: 7%; + margin-right: 12%; +} + +.bb {border-bottom: solid 2px;} + +.bl {border-left: solid 2px;} + +.bt {border-top: solid 2px;} + +.br {border-right: solid 2px;} + +.center {text-align: center;} + +.smcap {font-variant: small-caps;} + +.u {text-decoration: underline;} + +.gesperrt +{ + letter-spacing: 0.2em; + margin-right: -0.2em; +} + +em.gesperrt +{ + font-style: normal; +} + +.figcenter { + margin: auto; + text-align: center; +} + +.footnotes {border: dashed 1px;} + +.footnote {margin-left: 10%; margin-right: 10%; font-size: 0.9em;} + +.footnote .label {position: absolute; right: 84%; text-align: right;} + +.fnanchor { + vertical-align: super; + font-size: .8em; + text-decoration: + none; +} + +.transnote {background-color: #E6E6FA; + color: black; + font-size:smaller; + padding:0.5em; + margin-bottom:5em; + font-family:sans-serif, serif; } + + </style> + </head> +<body> +<div>*** START OF THE PROJECT GUTENBERG EBOOK 44582 ***</div> + +<div class="figcenter" > + <img src="images/titlepage.jpg" alt="logo" width="500" height="778" /> +</div> +<hr class="chap" /> +<p class="space-above" style="font-size: 110%; text-align: center;"><b>YALE UNIVERSITY</b></p> +<p class= "space-below" style="font-size: 110%; text-align: center;"><b> + MRS. HEPSA ELY SILLIMAN MEMORIAL LECTURES</b></p> +<hr class="r5" /> +<h1>PROBLEMS OF GENETICS</h1> + +<div class="bt bl br bb"> +<p style="font-size: 150%; text-align: center;"><b>SILLIMAN MEMORIAL LECTURES</b></p> +<p style="font-size: 120%; text-align: center;"><b>PUBLISHED BY YALE UNIVERSITY PRESS</b></p> +<hr class="full" /> +<div class="advert"> +<p><b>ELECTRICITY AND MATTER.</b> <i>By</i> <span class="smcap">Joseph John Thomson,<br /> + d.sc., ll.d., ph.d., f.r.s.</span>, <i>Fellow of Trinity College,<br /> + Cambridge, Cavendish Professor of Experimental Physics, Cambridge</i>.<br /> +<i>Price $1.25 net; postage 10 cents extra.</i></p> + +<p><b>THE INTEGRATIVE ACTION OF THE NERVOUS SYSTEM.</b><br /> + <i>By</i> <span class="smcap">Charles S. Sherrington,<br /> + <span style="margin-left: 0.5em;">d.sc., m.d., hon. ll.d., tor., f.r.s.</span>,</span><br /> + <i>Holt Professor of Physiology in the University of Liverpool</i>.<br /> +<i>Price $3.50 net; postage 25 cents extra.</i></p> + +<p><b>RADIOACTIVE TRANSFORMATIONS</b>. <i>By</i> <span class="smcap">Ernest Rutherford,<br /> + d.sc., ll.d., f.r.s.</span>, <i>Macdonald Professor of Physics, McGill University</i>.<br /> +<i>Price $3.50 net; postage 22 cents extra.</i></p> + +<p><b>EXPERIMENTAL AND THEORETICAL APPLICATIONS OF<br /> +    THERMODYNAMICS TO CHEMISTRY</b>.<br /> + <i>By</i> <span class="smcap">Dr. Walther Nernst,</span><br /> + <i>Professor and Director of the Institute of Physical Chemistry + in the University of Berlin</i>.<br /> +<i>Price $1.25 net; postage 10 cents extra.</i></p> + +<p><b>THE PROBLEMS OF GENETICS</b>. <i>By</i> <span class="smcap">William Bateson, m.a.,<br /> + f.r.s.</span>, <i>Director of the John Innes Horticultural Institution,<br /> + Merton Park, Surrey, England</i>.<br /> +<i>Price $4.00 net; postage 25 cents extra.</i></p> + +<p><b>STELLAR MOTIONS</b>. <span class="smcap">With Special Reference to Motions<br /> + Determined by Means of the Spectrograph.</span> <i>By</i> <span class="smcap">William<br /> + Wallace Campbell, sc.d., ll.d.</span>,<br /> + <i>Director of the Lick Observatory, University of California</i>.<br /> +<i>Price $4.00 net; postage 30 cents extra.</i></p> + +<p><b>THEORIES OF SOLUTIONS</b>. <i>By</i> <span class="smcap">Svante August Arrhenius,<br /> + ph.d., sc.d., m.d.</span>, <i>Director of the Physico-Chemical Department<br /> + of the Nobel Institute, Stockholm, Sweden</i>.<br /> +<i>Price $2.25 net; postage 15 cents extra.</i></p> + +<p><b>IRRITABILITY</b>. <span class="smcap">A Physiological Analysis of the General<br /> + Effect of Stimuli in Living Substances</span>.<br /> + <i>By</i> <span class="smcap">Max Verworn</span>,<br /> + <i>Professor at Bonn Physiological Institute</i>.<br /> +<i>Price $3.50 net; postage 20 cents extra.</i></p> + +<p><b>THE EVOLUTION OF MODERN MEDICINE</b>.<br /> + <i>By</i> <span class="smcap">Sir William Osler, Bart., m.d., ll.d., sc.d.</span>,<br /> + <i>Regius Professor of Medicine, Oxford University</i>.<br /> +<i>Price $3.00 net; postage 40 cents extra.</i></p> +</div></div> + +<p style="font-size: 150%; text-align: center;"><b>PROBLEMS OF GENETICS</b></p> +<p style="font-size: 90%; text-align: center;">BY</p> +<p style="font-size: 150%; text-align: center;"><b><span class="smcap">William Bateson, m.a., f.r.s.</span></b></p> + +<p class="space-above" style="font-size: 90%; text-align: center;"> +DIRECTOR OF THE JOHN INNES HORTICULTURAL INSTITUTION, +HON. FELLOW OF ST. JOHN'S COLLEGE, CAMBRIDGE, +AND FORMERLY PROFESSOR OF BIOLOGY IN THE UNIVERSITY</p> + +<p style="font-size: 120%; text-align: center;"><i>WITH ILLUSTRATIONS</i></p> + +<div class="figcenter" > + <img src="images/i_004.jpg" alt="logo" width="200" height="206" /> +</div> + +<p class="space-above" style="font-size: 110%; text-align: center;"> +<span class="smcap">New Haven: Yale University Press</span><br /> +<span class="smcap">London: Humphrey Milford</span><br /> +<span class="smcap">Oxford University Press</span></p> + +<p class="space-above" style="font-size: 130%; text-align: center;"><b>MCMXIII</b></p> + +<hr class="r5" /> +<p class="center">Copyright, 1913<br /> +By <span class="smcap">Yale University</span></p> + +<p class="center space-above space-below">First printed August, 1913, 1000 copies</p> + +<hr class="chap" /> +<p style="font-size: 150%; text-align: center;"><b>THE SILLIMAN FOUNDATION</b></p> + +<p class="indent">In the year 1883 a legacy of about eighty-five thousand +dollars was left to the President and Fellows of Yale College +in the city of New Haven, to be held in trust, as a gift from her +children, in memory of their beloved and honored mother, Mrs. +Hepsa Ely Silliman.</p> + +<p class="indent">On this foundation Yale College was requested and directed +to establish an annual course of lectures designed to illustrate +the presence and providence, the wisdom and goodness of God, +as manifested in the natural and moral world. These were to be +designated as the Mrs. Hepsa Ely Silliman Memorial Lectures. +It was the belief of the testator that any orderly presentation +of the facts of nature or history contributed to the end of this +foundation more effectively than any attempt to emphasize the +elements of doctrine or of creed; and he therefore provided that +lectures on dogmatic or polemical theology should be excluded +from the scope of this foundation, and that the subjects should be +selected rather from the domains of natural science and history, +giving special prominence to astronomy, chemistry, geology, and anatomy.</p> + +<p class="indent">It was further directed that each annual course should +be made the basis of a volume to form part of a series constituting a +memorial to Mrs. Silliman. The memorial fund came into the +possession of the Corporation of Yale University in the year 1901; +and the present volume constitutes the fifth of the series of +memorial lectures.</p> + +<p><span class="pagenum"><a name="Page_i" id="Page_i">[Pg i]</a></span></p> +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_ii" id="Page_ii">[Pg ii]</a></span></p> + +<h2>PREFACE</h2> + +<p class="indent">This book gives the substance of a series of lectures delivered +in Yale University, where I had the privilege of holding the office +of Silliman Lecturer in 1907.</p> + +<p class="indent">The delay in publication was brought about by a variety of causes.</p> + +<p class="indent">Inasmuch as the purpose of the lectures is to discuss +some of the wider problems of biology in the light of knowledge acquired +by Mendelian methods of analysis, it was essential that a fairly +full account of the conclusions established by them should first +be undertaken and I therefore postponed the present work till +a book on Mendel's Principles had been completed.</p> + +<p class="indent">On attempting a more general discussion of the +bearing of the phenomena on the theory of Evolution, I found myself +continually hindered by the consciousness that such treatment +is premature, and by doubt whether it were not better that the +debate should for the present stand indefinitely adjourned. +That species have come into existence by an evolutionary +process no one seriously doubts; but few who are familiar with +the facts that genetic research has revealed are now inclined to +speculate as to the manner by which the process has been +accomplished. Our knowledge of the nature and properties of +living things is far too meagre to justify any such attempts. +Suggestions of course can be made: though, however, these +ideas may have a stimulating value in the lecture room, they +look weak and thin when set out in print. The work which may +one day give them a body has yet to be done.</p> + +<p class="indent">The development of negations is always an ungrateful +task apt to be postponed for the positive business of experiment. +Such work is happily now going forward in most of the centers +of scientific life. Of many of the subjects here treated we already +know more than we did in 1907. The delay in production has +made it possible to incorporate these new contributions.</p> + +<p class="indent">The book makes no pretence at being a treatise and the +<span class="pagenum"><a name="Page_iii" id="Page_iii">[Pg iii]</a></span> +number of illustrative cases has been kept within a moderate +compass. A good many of the examples have been chosen from +American natural history, as being appropriate to a book intended +primarily for American readers. The facts are largely +given on the authority of others, and I wish to express my +gratitude for the abundant assistance received from American +colleagues, especially from the staffs of the American Museum +in New York, and of the Boston Museum of Natural History. +In connexion with the particular subjects personal acknowledgments +are made.</p> + +<p class="indent">Dr. F. M. Chapman was so good as to supervise the preparation +of the coloured Plate of <i>Colaptes</i>, and to authorize the loan +of the Plate representing the various forms of <i>Helminthophila</i>, +which is taken from his <i>North American Warblers</i>.</p> + +<p class="indent">I am under obligation to Messrs. Macmillan & Co., +for permission to reproduce several figures from <i>Materials for the Study +of Variation</i>, illustrating subjects which I wished to treat in +new associations, and to M. Leduc for leave to use Fig. 9.</p> + +<p class="indent">In conclusion I thank my friends in Yale for the high +honour they did me by their invitation to contribute to the series of +Silliman Lectures, and for much kindness received during a +delightful sojourn in that genial home of learning. +<span class="pagenum"><a name="Page_iv" id="Page_iv">[Pg iv]</a></span></p> +<hr class="chap" /> +<p class="u" style="font-size: 150%; text-align: center;"><b>TABLE OF CONTENTS</b>.</p> + +<table border="0" style="max-width: 45em;" cellspacing="2" summary="TOC" cellpadding="0" > + <tbody><tr> + <td class="tdr"><b>CHAPTER</b></td> + <td class="tdl"> </td> + <td class="tdr"><b>PAGE</b></td> + </tr><tr> + <td class="tdr"><b>I.</b> </td> + <td class="tdl"><b><span class="smcap">Introductory. The Problem of Species and Variety</span></b> </td> + <td class="tdr"><a href="#Page_1"> 1</a></td> + </tr><tr> + <td class="tdr"><b>II.</b> </td> + <td class="tdl"><b><span class="smcap">Meristic Phenomena</span></b></td> + <td class="tdr"><a href="#Page_31"> 31</a></td> + </tr><tr> + <td class="tdr"><b>III.</b> </td> + <td class="tdl"><b><span class="smcap">Segmentation, Organic and Mechanical</span></b></td> + <td class="tdr"><a href="#Page_60"> 60</a></td> + </tr><tr> + <td class="tdr"><b>IV.</b> </td> + <td class="tdl"><b><span class="smcap">The Classification of Variation and the Nature</span></b></td> + <td class="tdr"> </td> + </tr><tr> + <td class="tdr"> </td> + <td class="tdl">  <b><span class="smcap">of Substantive Variation</span></b></td> + <td class="tdr"><a href="#Page_83"> 83</a></td> + </tr><tr> + <td class="tdr"> </td> + <td class="tdl"><b><span class="smcap">Note to Chapter IV</span></b></td> + <td class="tdr"><a href="#Page_94"> 94</a></td> + </tr><tr> + <td class="tdr"><b>V.</b> </td> + <td class="tdl"><b><span class="smcap">The Mutation Theory</span></b></td> + <td class="tdr"><a href="#Page_97"> 97</a></td> + </tr><tr> + <td class="tdr"> </td> + <td class="tdl"><b><span class="smcap">Note to Chapter V</span></b></td> + <td class="tdr"><a href="#Page_116">116</a></td> + </tr><tr> + <td class="tdr"><b>VI.</b> </td> + <td class="tdl"><b><span class="smcap">Variation and Locality</span></b></td> + <td class="tdr"><a href="#Page_118">118</a></td> + </tr><tr> + <td class="tdr"><b>VII.</b> </td> + <td class="tdl"><b><span class="smcap">Local Differentiation</span>—<i>continued</i>.</b></td> + <td class="tdr"> </td> + </tr><tr> + <td class="tdr"> </td> + <td class="tdl">  <b><span class="smcap">Overlapping Forms</span></b></td> + <td class="tdr"><a href="#Page_146">146</a></td> + </tr><tr> + <td class="tdr"><b>VIII.</b> </td> + <td class="tdl"><b><span class="smcap">Locally Differentiated Forms</span>—<i>continued</i>.</b></td> + <td class="tdr"> </td> + </tr><tr> + <td class="tdr"> </td> + <td class="tdl"><b>  <span class="smcap">Climatic Varieties</span></b></td> + <td class="tdr"><a href="#Page_164">164</a></td> + </tr><tr> + <td class="tdr"><b>IX.</b> </td> + <td class="tdl"><b><span class="smcap">The Effects of Changed Conditions</span></b></td> + <td class="tdr"><a href="#Page_187">187</a></td> + </tr><tr> + <td class="tdr"><b>X.</b> </td> + <td class="tdl"><b><span class="smcap">The Effects of Changed Conditions</span>—<i>continued</i>.</b></td> + <td class="tdr"> </td> + </tr><tr> + <td class="tdr"> </td> + <td class="tdl"><b>  <span class="smcap">The Causes of Genetic Variation</span></b></td> + <td class="tdr"><a href="#Page_213">213</a></td> + </tr><tr> + <td class="tdr"><b>XI.</b> </td> + <td class="tdl"><b><span class="smcap">The Sterility of Hybrids. Concluding Remarks</span></b></td> + <td class="tdr"><a href="#Page_234">234</a></td> + </tr><tr> + <td class="tdr"> </td> + <td class="tdl"><b><span class="smcap">Concluding Remarks</span></b></td> + <td class="tdr"><a href="#Page_248">248</a></td> + </tr><tr> + <td class="tdr"> </td> + <td class="tdl"><b><span class="smcap">Index of Subjects</span></b></td> + <td class="tdr"><a href="#Page_251">251</a></td> + </tr><tr> + <td class="tdr"> </td> + <td class="tdl"><b><span class="smcap">Index of Persons</span></b></td> + <td class="tdr"><a href="#Page_252">252</a></td> + </tr> + </tbody> +</table> + +<hr class="chap" /> +<p class="u" style="font-size: 150%; text-align: center;"><b>LIST OF ILLUSTRATIONS</b>.</p> + +<table border="0" style="max-width: 45em;" cellspacing="2" summary="LOC" cellpadding="0" > + <tbody><tr> + <td class="tdl"><b><span class="smcap">Figure</span></b></td> + <td class="tdl"> </td> + <td class="tdr"><b><span class="smcap">Page</span></b></td> + </tr><tr> + <td class="tdr"><b>1. </b></td> + <td class="tdl"> <b>Tusk of Indian Elephant.</b></td> + <td class="tdr"><a href="#Page_37"> 37</a></td> + </tr><tr> + <td class="tdr"><b>2. </b></td> + <td class="tdl"> <b>Jaws of Skates.</b></td> + <td class="tdr"><a href="#Page_38"> 38</a></td> + </tr><tr> + <td class="tdr"><b>3. </b></td> + <td class="tdl"> <b>Syndactyly of Human Hand and Foot.</b> </td> + <td class="tdr"><a href="#Page_47"> 47</a></td> + </tr><tr> + <td class="tdr"><b>4. </b></td> + <td class="tdl"> <b>Syndactyly of the Human Foot.</b></td> + <td class="tdr"><a href="#Page_48"> 48</a></td> + </tr><tr> + <td class="tdr"><b>5. </b></td> + <td class="tdl"> <b>Petiole of</b> <i>Begonia phyllomaniaca</i>.</td> + <td class="tdr"><a href="#Page_51"> 51</a></td> + </tr><tr> + <td class="tdr"><b>6. </b></td> + <td class="tdl"> <b>Feet of Polydactyle Cats.</b></td> + <td class="tdr"><a href="#Page_52"> 52</a></td> + </tr><tr> + <td class="tdr"><b>7.</b> & <b>8.</b></td> + <td class="tdl"> <b>Vertebrae of Python.</b></td> + <td class="tdr"><a href="#Page_61"> 61</a></td> + </tr><tr> + <td class="tdr"><b>9. </b></td> + <td class="tdl"> <b>Osmotic Growths.</b></td> + <td class="tdr"><a href="#Page_64"> 64</a></td> + </tr><tr> + <td class="tdr"><b>10. </b></td> + <td class="tdl"> <b>Leaf type in</b> <i>Primula sinensis</i>.</td> + <td class="tdr"><a href="#Page_70"> 70</a></td> + </tr><tr> + <td class="tdr"><b>11. </b></td> + <td class="tdl"> <b>Geometrical relations in</b> <i>Arthropoda</i>.</td> + <td class="tdr"><a href="#Page_73"> 73</a></td> + </tr><tr> + <td class="tdr"><b>12. </b></td> + <td class="tdl"> <b>Right claw of Lobster.</b></td> + <td class="tdr"><a href="#Page_76"> 76</a></td> + </tr><tr> + <td class="tdr"><b>13.</b> </td> + <td class="tdl"> <b>Forms of</b> <i>Aceras hircina</i>.</td> + <td class="tdr"><a href="#Page_124">124</a></td> + </tr><tr> + <td class="tdr"> </td> + <td class="tdl"> <b>Variation in Warblers.</b></td> + <td class="tdr"><a href="#Page_159">159</a></td> + </tr> + </tbody> +</table> +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_1" id="Page_1">[Pg 1]</a></span></p> + +<p style="font-size: 150%; text-align: center;"><b>PROBLEMS OF GENETICS</b></p> +<hr class="r5" /> +<h2>CHAPTER I</h2> +<h3><span class="smcap">Introductory</span></h3> + +<p class="indent">The purpose of these lectures is to discuss some of +the familiar phenomena of biology in the light of modern discoveries. +In the last decade of the nineteenth century many of us perceived +that if any serious advance was to be made with the group +of problems generally spoken of as the Theory of Evolution, +methods of investigation must be devised and applied of a kind +more direct and more penetrating than those which after the +general acceptance of the Darwinian views had been deemed +adequate. Such methods obviously were to be found in a +critical and exhaustive study of the facts of variation and heredity, +upon which all conceptions of evolution are based. To construct +a true synthetic theory of Evolution it was necessary that variation +and heredity instead of being merely postulated as axioms +should be minutely examined as phenomena. Such a study +Darwin himself had indeed tentatively begun, but work of a +more thorough and comprehensive quality was required. In +the conventional view which the orthodoxy of the day prescribed, +the terms variation and heredity stood for processes so vague +and indefinite that no analytical investigation of them could be +contemplated. So soon, however, as systematic inquiry into +the natural facts was begun it was at once found that the accepted +ideas of variation were unfounded. Variation was seen +very frequently to be a definite and specific phenomenon, affecting +different forms of life in different ways, but in all its +diversity showing manifold and often obvious indications of +regularity. This observation was not in its essence novel. +Several examples of definite variation had been well known to +<span class="pagenum"><a name="Page_2" id="Page_2">[Pg 2]</a></span> +Darwin and others, but many, especially Darwin himself in his +later years, had nevertheless been disposed to depreciate the +significance of such facts. They consequently then lapsed into +general disparagement. Upon more careful inquiry the abundance +of such phenomena proved to be far greater than was +currently supposed, and a discussion of their nature brought +into prominence a consideration of greater weight, namely +that the differences by which these definite or discontinuous +variations are constituted again and again approximate to and +are comparable with the class of differences by which species +are distinguished from each other.</p> + +<p class="indent">The interest of such observations could no longer be +denied. The more they were examined the more apparent it became that +by means of the facts of variation a new light was obtained on +the physiological composition and capabilities of living things. +Genetics thus cease to be merely a method of investigating +theories of evolution or of the origin of species but provide a +novel and hitherto untried instrument by which the nature of +the living organism may be explored. Just as in the study of +non-living matter science began by regarding the external +properties of weight, opacity, colour, hardness, mode of occurrence, +etc., noting only such evidences of chemical attributes +and powers as chance spontaneously revealed; and much later +proceeded to the discovery that these casual manifestations of +chemical properties, rightly interpreted, afford a key to the +intrinsic nature of the diversity of matter, so in biology, having +examined those features of living things which ordinary observations +can perceive, we come at last to realize that when studied +for their own sake the properties of living organisms in respect +of heredity and variation are indications of their inner nature +and provide evidences of that nature which can be obtained from +no other source.</p> + +<p class="indent">While such ideas were gradually forming in our minds, +came the rediscovery of Mendel's work. Investigations which before +had only been imagined as desirable now became easy to pursue, +and questions as to the genetic inter-relations and compositions +of varieties can now be definitely answered. Without prejudice +<span class="pagenum"><a name="Page_3" id="Page_3">[Pg 3]</a></span> +to what the future may disclose whether by way of limitation +or extension of Mendelian method, it can be declared with +confidence and certainty that we have now the means of beginning +an analysis of living organisms, and distinguishing many +of the units or factors which essentially determine and cause the +development of their several attributes.</p> + +<p class="indent">Briefly put, the essence of Mendelism lies in the +discovery of the existence of unit characters or factors. For an +account of the Mendelian method, how it is applied and what it has +already accomplished, reference must be made to other works.<a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a> +With this part of the subject I shall assume a sufficient acquaintance. +In these lectures I have rather set myself the task +of considering how certain problems appear when viewed from +the standpoint to which the application of these methods has +led us. It is indeed somewhat premature to discuss such questions. +The work of Mendelian analysis is progressing with +great rapidity and anything I can say may very soon be superseded +as out of date. Nevertheless a discussion of this kind +may be of at least temporary service in directing inquiry to the +points of special interest.</p> + +<h3><span class="smcap">The Problem of Species and Variety</span></h3> + +<p class="indent">Nowhere does our new knowledge of heredity and variation +apply more directly than to the problem what is a species and +what is a variety? I cannot assert that we are already in a +position to answer this important question, but as will presently +appear, our mode of attack and the answers we expect to receive +are not those that were contemplated by our predecessors. +If we glance at the history of the scientific conception of Species +we find many signs that it was not till comparatively recent +times that the definiteness of species became a strict canon of +the scientific faith and that attempts were made to give precise +limits to that conception. When the diversity of living things +began to be accurately studied in the sixteenth and seventeenth +<span class="pagenum"><a name="Page_4" id="Page_4">[Pg 4]</a></span> +centuries names were applied in the loosest fashion, and in giving +a name to an animal or a plant the naturalists of those times had +no ulterior intention. Names were bestowed on those creatures +about which the writer proposed to speak. When Gesner or Aldrovandi +refer to all the kinds of horses, unicorns, dogs, mermaids, +etc., which they had seen or read of, giving to each a descriptive +name, they do not mean to "elevate" each named kind to "specific +rank"; and if anyone had asked them what they meant by +a species, it is practically certain that they would have had not +the slightest idea what the question might imply, or any suspicion +that it raised a fundamental problem of nature.</p> + +<p class="indent">Spontaneous generation being a matter of daily observation, +then unquestioned, and supernatural events of all kinds being +commonly reported by many witnesses, transmutation of species +had no inherent improbability. Matthioli,<a name="FNanchor_2_2" id="FNanchor_2_2"></a><a href="#Footnote_2_2" class="fnanchor">[2]</a> for instance, did not +expect to be charged with heresy when he declared <i>Stirpium +mutatio</i> to be of ordinary occurrence. After giving instances +of induced modifications he wrote, "Tantum enim in plantis +naturae germanitas potest, ut non solum saepe praedictos +praestet effectus, sed etiam ut alteram in alteram stirpem facile +vertat, ut cassiam in cinnamomum, sisymbrium in mentham, +triticum in lolium, hordeum in avenam, et ocymum in serpyllum."</p> + +<p class="indent">I do not know who first emphasized the need for a +clear understanding of the sense in which the term species is to be +applied. In the second half of the seventeenth century Ray +shows some degree of concern on this matter. In the introduction +to the <i>Historia Plantarum</i>, 1686, he discusses some +of the difficulties and lays down the principle that varieties +which can be produced from the seed of the same plant are to +be regarded as belonging to one species, being, I believe, the +first to suggest this definition. That new species can come into +existence he denies as inconsistent with Genesis 2, in which it is +declared that God finished the work of Creation in six days. +Nevertheless he does not wholly discredit the possibility of a +"transmutation" of species, such that one species may as an +exceptional occurrence give rise by seed to another and nearly +<span class="pagenum"><a name="Page_5" id="Page_5">[Pg 5]</a></span> +allied species. Of such a phenomenon he gives illustrations the +authenticity of which he says he is, against his will, compelled to +admit. He adds that some might doubt whether in the cases +quoted the two forms concerned are really distinct species, but +the passage is none the less of value for it shews that the conception +of species as being distinct unchangeable entities was not +to Ray the dogma sacrosanct and unquestionable which it +afterwards became.<a name="FNanchor_3_3" id="FNanchor_3_3"></a><a href="#Footnote_3_3" class="fnanchor">[3]</a> +</p> + +<p class="indent">In the beginning of the eighteenth century Marchant,<a name="FNanchor_4_4" id="FNanchor_4_4"></a><a href="#Footnote_4_4" class="fnanchor">[4]</a> +having observed the sudden appearance of a lacinated variety of +<i>Mercurialis</i>, makes the suggestion that species in general may have +arisen by similar mutations. Indeed from various passages it is +manifest that to the authors of the seventeenth and early eighteenth +centuries species appeared simply as groups more or less +definite, the boundaries of which it was unnecessary to determine +with great exactitude. Such views were in accord with the +general scientific conception of the time. The mutability of +<span class="pagenum"><a name="Page_6" id="Page_6">[Pg 6]</a></span> +species is for example sometimes likened (see for instance Sharrock, +loc. cit.) to the metamorphoses of insects, and it is to be +remembered that the search for the Philosopher's Stone by which +the transmutation of metals was to be effected had only recently +fallen into discredit as a pursuit.</p> + +<p class="indent">The notion indeed of a peculiar, fixed meaning to be +attached to species as distinct from variety is I think but rarely to be +found categorically expressed in prae-Linnaean writings.</p> + +<p class="indent">But with the appearance of the <i>Systema Naturae</i> +a great change supervened. Linnaeus was before all a man of order. +Foreseeing the immense practical gain to science that must come +from a codification of nomenclature, he invented such a system.</p> + +<p class="indent">It is not in question that Linnaeus did great things for +us and made Natural History a manageable and accessible collection of +facts instead of a disorderly heap; but orderliness of mind has +another side, and inventors and interpreters of systems soon attribute +to them a force and a precision which in fact they have not.</p> + +<p class="indent">The systematist is primarily a giver of names, as +Ray with his broader views perceived. Linnaeus too in the exordium to +the <i>Systema Naturae</i> naively remarks, that he is setting out to +continue the work which Adam began in the Golden Age, to give +names to the living creatures. Naming however involves very +delicate processes of mind and of logic. Carried out by the light +of meagre and imperfect knowledge it entails all the mischievous +consequences of premature definition, and promotes facile +illusions of finality. So was it with the Linnaean system. An +interesting piece of biological history might be written respecting +the growth and gradual hardening of the conception of Species. +To readers of Linnaeus's own writings it is well known that his +views cannot be summarized in a few words. Expressed as they +were at various times during a long life and in various connexions, +they present those divers inconsistencies which commonly +reflect a mind retaining the power of development. Nothing +certainly could be clearer than the often quoted declaration of the +<i>Philosophia Botanica</i>, "Species tot numeramus quot diversae +formae in principio sunt creatae," with the associated passage +"Varietates sunt plantae ejusdem speciei mutatae a caussa +<span class="pagenum"><a name="Page_7" id="Page_7">[Pg 7]</a></span> +quacunque occasionali." Those sayings however do not stand +alone. In several places, notably in the famous dissertation +on the peloric <i>Linaria</i> he explicitly contemplates the possibility +that new species may arise by crossing, declaring nevertheless +that he thinks such an event to be improbable. In that essay +he refers to Marchant's observation on a laciniate <i>Mercurialis</i>, +but though he states clearly that that plant should only be +regarded as a variety of the normal, he does not express any +opinion that the contemporary genesis of new species must be an +impossibility. In the later dissertation on Hybrid Plants he +returns to the same topic. Again though he states the belief +that species cannot be generated by cross-breedings, he treats +the subject not as heretical absurdity but as one deserving +respectful consideration.</p> + +<p class="indent">The significance of the aphorisms that precede the +lectures on the Natural Orders is not easy to apprehend. These are +expressed with the utmost formality, and we cannot doubt that +in them we have Linnaeus's own words, though for the record +we are dependent on the transcripts of his pupils.</p> + +<p class="blockquot">The text of the first five is as follows:</p> + +<p class="blockquot">1. Creator T. O. in primordio vestiit Vegetabile <i>Medullare</i> +principiis constitutivis diversi <i>Corticalis</i> unde tot difformia +individua, quot <i>Ordines</i> Naturales prognata.</p> + +<p class="blockquot">2. <i>Classicas</i> has (1) plantas Omnipotens miscuit inter se, +unde tot <i>Genera</i> ordinum, quot inde plantae.</p> + +<p class="blockquot">3. <i>Genericas</i> has (2) miscuit Natura, unde tot <i>Species</i> congeneres +quot hodie existunt.</p> + +<p class="blockquot">4. <i>Species</i> has miscuit Casus, unde totidem quot passim +occurrunt, <i>Varietates</i>.</p> + +<p class="blockquot">5. Suadent haec (1-4) Creatoris leges a simplicibus ad +Composita.</p> + +<p class="blockquot">Naturae leges generationis in hybridis.</p> + +<p class="blockquot">Hominis leges ex observatis a posteriori.</p> + +<p class="indent">I am not clear as to the parts assigned in the first sentence +respectively to the "<i>Medulla</i>" and the "<i>Cortex</i>," beyond that +Linnaeus conceived that multiformity was first brought about +by diversity in the "<i>Cortex</i>." The passage is rendered still +<span class="pagenum"><a name="Page_8" id="Page_8">[Pg 8]</a></span> +more obscure if read in connection with the essay on "<i>Generatio +Ambigena</i>," where he expresses the conviction that the <i>Medulla</i> +is contributed by the mother, and the <i>Cortex</i> by the father, both +in plants and animals.<a name="FNanchor_5_5" id="FNanchor_5_5"></a><a href="#Footnote_5_5" class="fnanchor">[5]</a></p> + +<p class="indent">But however that may be, he regards this original diversity +as resulting in the constitution of the Natural Orders, each represented +by one individual.</p> + +<p class="indent">In the second aphorism the Omnipotent is represented as +creating the genera by intermixing the individual <i>plantae classicae</i>, +or prototypes of the Natural Orders.</p> + +<p class="indent">The third statement is the most remarkable, for in +it he declares that Species were formed by the act of Nature, who by +inter-mixing the genera produced <i>Species congeneres</i>, namely +species inside each genus, to the number which now exist. +Lastly, Chance or Accident, intermixing the species, produced +as many varieties as there are about us.</p> + +<p class="indent">Linnaeus thus evidently regarded the intermixing +of an originally limited number of types as the sufficient cause of +all subsequent diversity, and it is clear that he draws an antithesis +between <i>Creator</i>, <i>Natura</i>, and <i>Casus</i>, assigning to each +a special part in the operations. The acts resulting in the +formation of genera are obviously regarded as completed within +the days of the Creation, but the words do not definitely show +that the parts played by Nature and Chance were so limited.</p> + +<p class="indent">Recently also E. L. Greene<a name="FNanchor_6_6" id="FNanchor_6_6"></a><a href="#Footnote_6_6" class="fnanchor">[6]</a> +has called attention to some curious utterances buried in the <i>Species Plantarum</i>, +in which Linnaeus refers to intermediate and transitional species, using +language that even suggests evolutionary proclivities of a +modern kind, and it is not easy to interpret them otherwise.</p> + +<p class="indent">Whatever Linnaeus himself believed to be the truth, the +effect of his writings was to induce a conviction that the species +<span class="pagenum"><a name="Page_9" id="Page_9">[Pg 9]</a></span> +of animals and plants were immutably fixed. Linnaeus had +reduced the whole mass of names to order and the old fantastical +transformations with the growth of knowledge had lapsed into +discredit; the fixity of species was taken for granted, but not +till the overt proclamation of evolutionary doctrine by Lamarck +do we find the strenuous and passionate assertions of immutability +characteristic of the first half of the nineteenth century.</p> + +<p class="indent">It is not to be supposed that the champions of fixity +were unacquainted with varietal differences and with the problem +thus created, but in their view these difficulties were apparent +merely, and by sufficiently careful observation they supposed +that the critical and permanent distinctions of the true species +could be discovered, and the impermanent variations detected +and set aside.</p> + +<p class="indent">This at all events was the opinion formed by the great +body of naturalists at the end of the eighteenth and beginning of the +nineteenth centuries, and to all intents and purposes in spite +of the growth of evolutionary ideas, it remains the guiding +principle of systematists to the present day. There are 'good +species' and 'bad species' and the systematists of Europe and +America spend most of their time in making and debating them.</p> + +<p class="indent">In some of its aspects the problem of course confronted +earlier naturalists. Parkinson for instance (1640) in introducing his +treatment of <i>Hieracium</i> wrote, "To set forth the whole family of +the Hawkeweedes in due forme and order is such a world of +worke that I am in much doubt of mine own abilitie, it having +lyen heavie on his shoudiers that hath already waded through +them ... for such a multitude of varieties in forme pertaining +to one herbe is not to be found againe in <i>rerum natura</i> as I +thinke," and the same idea, that the difficulty lay rather in +man's imperfect powers of discrimination than in the nature of +the materials to be discriminated, is reflected in many treatises +early and late.</p> + +<p class="indent">It was however with the great ouburst of scientific +activity which followed Linnaeus that the difficulty became acute. +Simultaneously vast masses of new material were being collected +from all parts of the world into the museums, and the products +<span class="pagenum"><a name="Page_10" id="Page_10">[Pg 10]</a></span> +of the older countries were re-examined with a fresh zeal and on +a scale of quantity previously unattempted. But the problem +how to name the forms and where to draw lines, how much +should be included under one name and where a new name was +required, all this was felt, rather as a cataloguer's difficulty +than as a physiological problem. And so we still hear on the +one hand of the confusion caused by excessive "splitting" and +subdivisions, and on the other of the uncritical "lumpers" who +associate together under one name forms which another collector +or observer would like to see distinguished.</p> + +<p class="indent">In spite of Darwin's hopes, the acceptance of his views +has led to no real improvement—scarcely indeed to any change at +all in either the practice or aims of systematists. In a famous +passage in the <i>Origin</i> he confidently declares that when his +interpretation is generally adopted "Systematists will be able +to pursue their labours as at present; but they will not be incessantly +haunted by the shadowy doubt whether this or that +form be a true species. This, I feel sure, and I speak after +experience, will be no slight relief. The endless disputes whether +or not some fifty species of British brambles are good species +will cease." Those disputes nevertheless proceed almost exactly +as before. It is true that biologists in general do not, +as formerly, participate in these discussions because they have +abandoned systematics altogether; but those who are engaged +in the actual work of naming and cataloguing animals and +plants usually debate the old questions in the old way. There +is still the same divergence of opinion and of practice, some inclining +to make much of small differences, others to neglect them.</p> + +<p class="indent">Not only does the work of the systematists as a whole proceed +as if Darwin had never written but their attitude towards these +problems is but little changed. In support of this statement I may +refer to several British Museum Catalogues, much of the <i>Biologia +Centrali-Americana</i>, Ridgway's <i>Birds of North America</i>, the +<i>Fauna Hawaiensis</i>, indeed to almost any of the most important +systematic publications of England, America, or any other +country. These works are compiled by the most proficient +<span class="pagenum"><a name="Page_11" id="Page_11">[Pg 11]</a></span> +systematists of all countries in the several groups, but with +rare exceptions they show little misgiving as to the fundamental +reality of specific differences. That the systematists consider +the species-unit as of primary importance is shown by the +fact that the whole business of collection and distribution of +specimens is arranged with regard to it.</p> + +<p class="indent">Almost always the collections are arranged in such a +way that the phenomena of variation are masked. Forms intermediate +between two species are, if possible, sorted into separate boxes +under a third specific name. If a species is liable to be constantly +associated with a mutational form, the mutants are picked out, +regardless of the circumstances of their origin, from the samples +among which they were captured, and put apart under a special +name. Only by a minute study of the original labels of the +specimens and by redistributing them according to locality and +dates, can their natural relations be traced. The published +accounts of these collections often take no notice of variations, +others make them the subject of casual reference. Very few +indeed treat them as of much importance. From such indications +it is surely evident that the systematists attach to the +conception of species a significance altogether different from that +which Darwin contemplated.</p> + +<p class="indent">I am well aware that some very eminent systematists +regard the whole problem as solved. They hold as Darwin did that +specific diversity has no physiological foundation or causation +apart from fitness, and that species are impermanent groups, +the delimitations of which are ultimately determined by environmental +exigency or "fitness." The specific diversity of +living things is thus regarded as being something quite different +in nature from the specific diversity of inorganic substances. +In practice those who share these opinions are, as might be anticipated, +to be found among the 'lumpers' rather than among +the 'splitters.' In their work, certainly, the Darwinian theory +is actually followed as a guiding principle; unanalysed inter-gradations +of all kinds are accepted as impugning the integrity +of species; the underlying physiological problem is forgotten, +and while the product is almost valueless as a contribution to +<span class="pagenum"><a name="Page_12" id="Page_12">[Pg 12]</a></span> +biological research, I can scarcely suppose that it aids greatly +in the advances of other branches of our science.</p> + +<p class="indent">But why is it that, with these exceptions, the consequences +of the admittedly general acceptance of a theory of evolution +are so little reflected in the systematic treatment of living things? +Surely the reason is that though the systematist may be convinced +of the general truth of the evolution theory at large, he +is still of opinion that species are really distinct things. For +him there are still 'good' species and 'bad' species and his experience +tells him that the distinction between the two is not +simply a question of degree or a matter of opinion.</p> + +<p class="indent">To some it may seem that this is mere perversity, a refusal +to see obvious truth, a manifestation of the spirit of the collector +rather than of the naturalist. But while recognising that from +a magnification of the conception of species the systematists +are occasionally led into absurdity I do not think the grounds +for their belief have in recent times been examined with the +consideration they deserve. The phenomenon of specific +diversity is manifested to a similar degree by living things belonging +to all the great groups, from the highest to the lowest, +Vertebrates, Invertebrates, Protozoa, Vascular Plants, Algae, +and Bacteria, all present diversities of such a kind that among +them the existence of specific differences can on the whole be +recognised with a similar degree of success and with very similar +limitations. In all these groups there are many species quite +definite and unmistakable, and others practically indefinite. +The universal presence of specificity, as we may call it, similarly +limited and characterised, is one of its most remarkable +features. Not only is this specificity thus universally present +among the different forms of life, but it manifests itself in respect +of the most diverse characteristics which living things display. +Species may thus be distinguished by peculiarities of form, of +number, of geometrical arrangement, of chemical constitution +and properties, of sexual differentiation, of development, and of +many other properties. In any one or in several of these features +together, species may be found distinguished from other species. +It is also to be observed that the definiteness of these distinctions +<span class="pagenum"><a name="Page_13" id="Page_13">[Pg 13]</a></span> +has no essential dependence on the nature of the characteristic +which manifests them. It is for example sometimes said that +colour-distinctions are of small systematic importance, but every +systematist is familiar with examples (like that of the wild species +of <i>Gallus</i>) in which colours though complex, show very little +variation. On the other hand features of structure, sexual +differentiation, and other attributes which by our standards are +estimated as essential, may be declared to show much variation +or little, not according to any principle which can be detected, +but simply as the attention happens to be applied to one species +or group of species, or to another. In many groups of animals and +plants observers have hit upon characters which were for a time +thought to be finally diagnostic of species. The Lepidoptera and +Diptera for instance, have been re-classified according to their +neuration. Through a considerable range of forms determinations +may be easily made on these characters, but as is now well +known, neuration is no more immune from variation than any +other feature of organisation, and in some species great variability +is the rule. Again it was once believed by some that the +genitalia of the Lepidoptera provided a basis of final determination—with +a similar sequel. In some groups, for example the Lycaenidae, +or the Hesperidae, there are forms almost or quite indistinguishable +on external examination, but a glance at the +genitalia suffices to distinguish numerous species, while on the +contrary among Pieridae a great range of species show scarcely +any difference in these respects: and again in occasional species +the genitalia show very considerable variations.</p> + +<p class="indent">The proposition that animals and plants are on the whole +divisible into definite and recognisable species is an approximation +to the truth. Such a statement is readily defensible, +whereas to assert the contrary would be palpably absurd. For +example, a very competent authority lately wrote: "In the +whole Lepidopterous fauna of England there is no species of +really uncertain limits."<a name="FNanchor_7_7" id="FNanchor_7_7"></a><a href="#Footnote_7_7" class="fnanchor">[7]</a> +Others may be disposed to make +certain reservations, but such exceptions would be so few as +scarcely to impair the validity of the general statement. The +<span class="pagenum"><a name="Page_14" id="Page_14">[Pg 14]</a></span> +declaration might be extended to other orders and other lands.</p> + +<p class="indent">We know, of course, that the phenomenon of specific diversity +is complicated by local differentiation: that, in general, forms +which cannot disperse themselves freely exhibit a multitude of +local races, and that of these some are obviously adaptative, +and that a few even owe their peculiarity to direct environmental +effects. Every systematist also is perfectly aware that in dealing +with collections from little explored countries the occurrence +of polymorphism or even of sporadic variation may make the +practical business of distinguishing the species difficult and +perhaps for the time impossible; still, conceding that a great +part of the diversity is due to geographical differentiation, and +that some is sporadic variation, our experience of our own floras +and faunas encourages the belief that if we were thoroughly +familiar with these exotic productions it would usually be +possible to assign their specific limitations with an approach +to certainty.</p> + +<p class="indent">For apart from any question of the justice of these +wider inferences, if we examine the phenomenon of specificity as it +appears in those examples which are nearest to hand, surely we +find signs in plenty that specific distinction is no mere consequence +of Natural Selection. The strength of this proposition has +lain mainly in the appeal to ignorance. Steadily with the growth +of knowledge has its cogency diminished, and such a belief +could only have been formulated at a time when the facts of +variation were unknown.</p> + +<p class="indent">In Darwin's time no serious attempt had been made +to examine the manifestations of variability. A vast assemblage of +miscellaneous facts could formerly be adduced as seemingly +comparable illustrations of the phenomenon "Variation." +Time has shown this mass of evidence to be capable of analysis. +When first promulgated it produced the impression that variability +was a phenomenon generally distributed amongst living +things in such a way that the specific divisions must be arbitrary. +When this variability is sorted out, and is seen to be in part a +result of hybridisation, in part a consequence of the persistence +<span class="pagenum"><a name="Page_15" id="Page_15">[Pg 15]</a></span> +of hybrids by parthenogenetic reproduction, a polymorphism +due to the continued presence of individuals representing various +combinations of Mendelian allelomorphs, partly also the transient +effect of alteration in external circumstances, we see how +cautious we must be in drawing inferences as to the indefiniteness +of specific limits from a bare knowledge that intermediates exist. +Conversely, from the accident of collocation or from a misleading +resemblance in features we deem essential, forms genetically +distinct are often confounded together, and thus the divergence +of such forms in their other features, which we declare to be +non-essential, passes as an example of variation. Lastly, and +this is perhaps the most fertile of all the sources of confusion, +the impression of the indefiniteness of species is created by the +existence of numerous local forms, isolated geographically from +each other, forms whose differences may be referable to any one +of the categories I have enumerated.</p> + +<p class="indent">The advance has been from many sides. Something has +come from the work of systematists, something from cultural +experiments, something from the direct study of variation as it +appears in nature, but progress is especially due to experimental +investigation of heredity. From all these lines of inquiry we +get the same answer; that what the naturalists of fifty years +ago regarded as variation is not one phenomenon but many, +and that what they would have adduced as evidence against +the definiteness of species may not in fact be capable of this +construction at all.</p> + +<p class="indent">If we may once more introduce a physical analogy, +the distinctions with which the systematic naturalist is concerned in +the study of living things are as multifarious as those by which +chemists were confronted in the early days of their science. +Diversities due to mechanical mixtures, to allotropy, to differences +of temperature and pressure, or to degree of hydration, had all +to be severally distinguished before the essential diversity due +to variety of chemical constitution stood out clearly, and I +surmise that not till a stricter analysis of the diversities of animals +and plants has been made on a comprehensive scale, shall we +be in a position to declare with any confidence whether there is +<span class="pagenum"><a name="Page_16" id="Page_16">[Pg 16]</a></span> +or is not a natural and physiological distinction between species +and variety.</p> + +<p class="indent">As I have said above, it is in the cases nearest to +hand that the problem may be most effectively studied. Comparison +between forms from dissimilar situations contributes something; +but it is by a close examination of the behaviour, especially the +genetic behaviour, of familiar species when living in the presence +of their nearest allies that the most direct light on the problem +is to be obtained. I cannot understand the attitude of those who, +contemplating such facts as this examination elicits, can complacently +declare that specific difference is a mere question of +degree. With the spread of evolutionary ideas to speak much +of the fixity of species has become unfashionable, and yet how +striking and inscrutable are the manifestations of that fixity!</p> + +<p class="indent">Consider the group of species composing the <i>agrestis</i> +section of the genus <i>Veronica</i>, namely <i>Tournefortii</i>, +<i>agrestis</i>, and <i>polita</i>.</p> + +<p class="indent">These three grow side by side in my garden, as they +do in suitable situations over a vast area of the temperate regions. +I have for years noticed them with some care and become familiar +with their distinctions and resemblances. Never is there any +real doubt as to the identity of any plant. The species show +some variability, but I have never seen one which assumed any +of the distinguishing features of the others. A glance at the +fruits decides at once to which species a plant belongs. I find +it impossible to believe that the fixity of these distinctions is +directly dependent on their value as aids in the struggle for +existence. The mode of existence of the three forms in so far +as we can tell is closely similar. By whatever standard we reckon +systematic affinity I suppose we shall agree that these species +come very near indeed to each other. Bentham even takes +the view that <i>polita</i> is a mere variety of <i>agrestis</i>.</p> + +<p class="indent">Now in such cases as this it has been argued that the +specific features of the several types have been separately developed +in as many distinct localities, and that their present association +is due to subsequent redistribution. Of these Veronicas indeed +we know that one, <i>Tournefortii</i> (= <i>Buxbaumii</i>) is as a matter of fact +<span class="pagenum"><a name="Page_17" id="Page_17">[Pg 17]</a></span> +a recent introduction from the east.<a name="FNanchor_8_8" id="FNanchor_8_8"></a><a href="#Footnote_8_8" class="fnanchor">[8]</a> +But this course of argument leads to still further difficulties. +For if it is true that the peculiarities +of the several species have been perfected and preserved on +account of their survival-value to their possessors, it follows +that there must be many ways of attaining the same result. +But since sufficient adaptation may be ensured in so many ways, +the disappearance of the common parent of these forms is difficult +to understand. Obviously it must have been a plant +very similar in general construction to its modern representatives. +Like them it must have been an annual weed, with an organisation +conformable to that mode of life. Why then, after having been +duly perfected for that existence should it have been entirely +superseded in favour of a number of other distinct contrivances +for doing the same thing, and—if a gradual transition be predicated—not +only by them, but by each intermediate stage +between them and the original progenitor? Surely the obvious +inference from such facts is that the burden cast upon the theory +of gradual selection is far greater than it can bear; that adaptation +is not in practice a very close fit, and that the distinctions +between these several species of Veronica have not arisen on +account of their survival-value but rather because none of their +diversities was so damaging as to lead to the extermination of +its possessor. When we see these various Veronicas each rigidly +reproducing its parental type, all comfortably surviving in +competition with each other, are we not forced to the conclusion +that <i>tolerance</i> has as much to do with the diversity of species +as the stringency of Selection? Certainly these species owe their +continued existence to the fact that they are each good enough +to live, but how shall we refer the distinctions between them directly +or indirectly to the determination of Natural Selection? +<span class="pagenum"><a name="Page_18" id="Page_18">[Pg 18]</a></span></p> + +<p class="indent">The control of Selection is loose while the conformity to +specific distinction is often very strict and precise, and no less +so even when several closely related species co-exist in the same +area and in the same circumstances.</p> + +<p class="indent">The theory of Selection fails at exactly the point +where it was devised to help: <i>Specific</i> distinction.</p> + +<p class="indent">Let us examine a somewhat different set of facts in the +case of another pair of nearly allied species <i>Lychnis diurna</i> and +<i>vespertina</i>. The two plants have much in common. Both are +dioecious perennials, with somewhat similar flowers, the one +crimson, the other white. Each however has its peculiarities +which are discernible in almost any part of its structure, whether +flower, leaf, fruit or seed, distinctions which would enable a +person thoroughly familiar with the plants to determine at once +from which species even a small piece had been taken. There +is so much resemblance however as readily to support the surmise +that the two were mere varieties of one species. Bentham, +following Linnaeus, in fact actually makes this suggestion, +with what propriety we will afterwards consider. Now this case +is typical of many. The two forms have a wide distribution, +occurring sometimes separately, sometimes in juxtaposition. +<i>L. diurna</i> is a plant of hedgerows and sheltered situations. <i>L. +vespertina</i> is common in fields and open spaces, where <i>diurna</i> +is hardly ever found; but not rarely <i>vespertina</i> occurs in association +with <i>diurna</i> in the places which that plant frequents. In +this case I do not doubt that we have to do with organisms of +somewhat different aptitudes. That <i>L. vespertina</i> has powers +which <i>diurna</i> has not is shown very clearly by the fact that +<i>diurna</i> is sometimes entirely absent from areas where <i>vespertina</i> +can abound.<a name="FNanchor_9_9" id="FNanchor_9_9"></a><a href="#Footnote_9_9" class="fnanchor">[9]</a> +But in order to understand the true genetic +relations of the two plants to each other it is necessary to observe +their behaviour when they meet as they not unfrequently do. +<span class="pagenum"><a name="Page_19" id="Page_19">[Pg 19]</a></span> +If the <i>Lychnis</i> population of such a locality be examined it will +be found to consist of many undoubted and unmodified <i>diurna</i>, +a number—sometimes few, sometimes many—of similarly +unmodified <i>vespertina</i>, and an uncertain but usually rather small +proportion of plants obviously hybrids between the two. How +is it possible to reconcile these facts with the view that specific +distinction has no natural basis apart from environmental exigency?</p> + +<p class="indent">Darwinian orthodoxy suggests that by a gradual process +of Natural Selection either one of these two types was evolved from +the other, or both from a third type. I cannot imagine that +anyone familiar with the facts would propose the first hypothesis +in the case of <i>Lychnis</i>, nor can I conceive of any process, whether +gradual or sudden, by which <i>diurna</i> could have come out of +<i>vespertina</i>, or <i>vespertina</i> out of <i>diurna</i>. Both however may no +doubt have been derived from some original third type. It is +conceivable that <i>Lychnis macrocarpa</i> of Boissier, a native of +Southern Spain and Morocco, may be this original form. This +species is said to combine a white flower (like that of <i>L. vespertina</i>), +with capsule-teeth rolled back (like those of <i>diurna</i>).<a name="FNanchor_10_10" id="FNanchor_10_10"></a><a href="#Footnote_10_10" class="fnanchor">[10]</a> +But whatever the common progenitor may have been, if we are +to believe that these two species have been evolved from it by +a gradual process of Natural Selection based on adaptation, +enormous assumptions must be made regarding the special fitness +of these two forms and the special unfitness of the common +parent, and these assumptions must be specially invoked and +repeated for each several feature of structure or habits distinguishing +the three forms.</p> + +<p class="indent">Why, if the common parent was strong enough to live to +give rise to these two species, is it either altogether lost now, or at +least absent from the whole of Northern Europe? Its two +putative descendants, though so distinct from each other, are, +as we have seen, able often to occupy the same ground. If +they were gradually derived from a common progenitor—necessarily +very like themselves—can we believe that this original +<span class="pagenum"><a name="Page_20" id="Page_20">[Pg 20]</a></span> +form should always, in all the diversities of soil and situation +which they inhabit, be unable to exist? Some one may fancy +that the hybrids which are found in the situations occupied by +both forms are this original parental species. But nothing can +be more certain than that these plants are simply heterozygous +combinations made by the union of gametes bearing the characters +of <i>diurna</i> and <i>vespertina</i>.<a name="FNanchor_11_11" id="FNanchor_11_11"></a><a href="#Footnote_11_11" class="fnanchor">[11]</a> +For they may be reproduced exactly in F<sub>1</sub> or in +later generations of that cross when it is artificially +made; when bred from their families exhibit palpable phenomena +of segregation more or less complex; and usually, if perhaps not +always, they are partially sterile.<a name="FNanchor_12_12" id="FNanchor_12_12"></a><a href="#Footnote_12_12" class="fnanchor">[12]</a> +In a locality on the Norfolk coast that I know well, +there is a strip of rough ground chiefly +sand-bank, which runs along the shore. This ground is full of +<i>vespertina</i>. Not a hundred yards inland is a lane containing +<i>diurna</i>, and among the <i>vespertina</i> on the sand-bank are always +some of the hybrid form, doubtless the result of fertilisation +from the neighbouring <i>diurna</i> population. Seed saved from these +hybrids gave <i>vespertina</i> and hybrids again, having obviously been +fertilised by other <i>vespertina</i> or by other hybrids, and I have +no doubt that such hybrid plants if fertilised by <i>diurna</i> would +have shown some <i>diurna</i> offspring. The absence of <i>diurna</i> +in such localities may fairly be construed as an indication that +<i>diurna</i> is there at a real disadvantage in the competition for life.</p> + +<p class="indent">But if, admitting this, we proceed to consider how the special +aptitude of <i>vespertina</i> is constituted, or what it is that puts +<i>diurna</i> at a disadvantage, we find ourselves quite unable to +show the slightest connexion between the success of one or the +<span class="pagenum"><a name="Page_21" id="Page_21">[Pg 21]</a></span> +failure of the other on the one hand, and <i>the specific characteristics</i> +which distinguish the two forms on the other. The orthodox +Selectionist would, as usual, appeal to ignorance. We +ask what can <i>vespertina</i> gain by its white flowers, its more lanceolate +leaves, its grey seeds, its almost erect capsule-teeth, +its longer fruits, which <i>diurna</i> loses by reason of its red flowers, +more ovate leaves, dark seeds, capsule-teeth rolled back, and +shorter fruits? We are told that each of these things <i>may</i> +affect the viability of their possessors. We cannot assert that +this is untrue, but we should like to have evidence that it is true. +The same problem confronts us in thousands upon thousands +of examples, and as time goes on we begin to feel that speculative +appeals to ignorance, though dialectically admissible, provide +an insufficient basis for a proposition which, if granted, is to +become the foundation of a vast scheme of positive construction.</p> + +<p class="indent">One thing must be abundantly clear to all, that to +treat two forms so profoundly different as one, because intermediates of +unknown nature can be shown to exist between them, is a mere +shirking of the difficulties, and this course indeed creates artificial +obstacles in the way of those who are seeking to discover the +origin of organic diversity.</p> + +<p class="indent">In the enthusiasm with which evolutionary ideas were +received the specificity of living things was almost forgotten. +The exactitude with which the members of a species so often +conform in the diagnostic, specific features passed out of account; +and the scientific world by dwelling with a constant emphasis +on the fact of variability, persuaded itself readily that species +had after all been a mere figment of the human mind. +Without presuming to declare what future research only can +reveal, I anticipate that, when variation has been properly +examined and the several kinds of variability have been successfully +distinguished according to their respective natures, +the result will render the natural definiteness of species increasingly +apparent. Formerly in such a case as that of the two +<i>Lychnis</i> species, the series of "intermediates" was taken to be a +palpable proof that <i>vespertina</i> "graded" to <i>diurna</i>. It is this +fact, doubtless, upon which Bentham would have relied in suggesting +<span class="pagenum"><a name="Page_22" id="Page_22">[Pg 22]</a></span> +that both may be one species.<a name="FNanchor_13_13" id="FNanchor_13_13"></a><a href="#Footnote_13_13" class="fnanchor">[13]</a> +Genetic tests, though as yet imperfectly applied, make it almost certain +that these inter-grading forms are not in any true sense variations from either +species in the direction of the other, but combinations of elements +derived from both.</p> + +<p class="indent">The points in which very closely allied species are +distinguished from each other may be found in the most diverse +features of their organisation. Sometimes specific difference +is to be seen in a character which we can believe to be important +in the struggle, but at least as often it is some little detail that +we cannot but regard as trivial which suffices to differentiate +the two species. Even when the diagnostic point is of such a +nature that we can imagine it to make a serious difference in the +economy we are absolutely at a loss to suggest why this feature +should be a necessity to species A and unnecessary to species B +its nearest ally. The house sparrow (<i>Passer domesticus</i>) is in +general structure very like the tree sparrow (<i>P. montanus</i>). +They differ in small points of colour. For instance <i>montanus</i> +has a black patch on the cheek which is absent in <i>domesticus</i>. +The presence in the one species and the absence in the other +are equally definite, and in both cases we are equally unable to +suggest any consideration of utility in relation to these features. +The two species are distinguished also by a characteristic that +may well be supposed to be of great significance. In <i>domesticus</i> +the two sexes are strongly differentiated, the cock being more +ornate than the hen. On the other hand the two sexes in <i>montanus</i> +are alike, and, if we take a standard from <i>domesticus</i>, we +may fairly say that in <i>montanus</i> the hen has the colouration of +the male. It is not unreasonable to suppose that such a distinction +may betoken some great difference in physiological +economy, but the economical significance of this perhaps important +distinction is just as unaccountable as that of the seemingly +trivial but equally diagnostic colour-point. +<span class="pagenum"><a name="Page_23" id="Page_23">[Pg 23]</a></span></p> + +<p class="indent">I have spoken of the fixed characteristics of the two species. +If we turn to a very different feature, their respective liability +to albinistic variation, we find ourselves in precisely similar +difficulty. <i>Passer domesticus</i> is a species in which individuals +more or less pied occur with especial frequency, but in <i>P. montanus</i> +such variation is extremely rare if it occurs at all. The +writer of the section on Birds in the <i>Royal Natural History</i> +(III., 1894-5, p. 393) calls attention to this fact and remarks +that in that species he knows no such instance.</p> + +<p class="indent">The two species therefore, apart from any differences that we +can suppose to be related to their respective habits, are characterised +by small fixed distinctions in colour-markings, by a +striking difference in secondary sexual characters, and by a +difference in variability. In all these respects we can form +no surmise as to any economic reason why the one species +should be differentiated in the one way and the other in the other +way, and I believe it is mere self-deception which suggests the +hope that with fuller knowledge reasons of this nature would be +discovered.</p> + +<p class="indent">The two common British wasps, <i>Vespa vulgaris</i> and +<i>Vespa germanica</i>, are another pair of species closely allied although +sharply distinguished, which suggest similar reflexions. Both +usually make subterranean nests but of somewhat different +materials. <i>V. vulgaris</i> uses rotten wood from which the nest +derives a characteristic yellow colour, while <i>V. germanica</i> scrapes +off the weathered surfaces of palings and other exposed timber, +material which is converted into the grey walls of the nest. The +stalk by which the nest is suspended (usually to a root) in the +case of <i>germanica</i> passes freely through a hole in the external +envelope, but <i>vulgaris</i> unites this external wall solidly to the +stalk. In bodily appearance and structure the two species are +so much alike that they have often been confounded even by +naturalists, and to the untrained observer they are quite indistinguishable. +There are nevertheless small points of difference +which almost though not quite always suffice to distinguish the +two forms. For example the yellow part of the sinus of the eyes +is emarginate in <i>vulgaris</i> but not emarginate in <i>germanica</i>. <i>V. +vulgaris</i> often has black spots on the tibiae while in <i>germanica</i> the +<span class="pagenum"><a name="Page_24" id="Page_24">[Pg 24]</a></span> +tibiae are usually plain yellow. In both species there is a horizontal +yellow stripe on the thorax, but whereas in <i>vulgaris</i> this is +a plain narrow stripe, it is in <i>germanica</i> enlarged downwards in +the middle. These and other apparently trivial details of colouration, +though not absolutely constant, are yet so nearly constant +that irregularities in these respects are quite exceptional. Lastly +the genitalia of the males, though not very different, present +small structural points of distinction which are enough to distinguish +the two species at a glance.<a name="FNanchor_14_14" id="FNanchor_14_14"></a><a href="#Footnote_14_14" class="fnanchor">[14]</a> +</p> + +<p class="indent">In considering the meaning of the distinctions between +these two wasps we meet the old problem illustrated by the Sparrows. +The two species have somewhat different habits of life and we +should readily expect to find differences of bodily organisation +corresponding with the differences of habits. But is that what +we do find? Surely not. To suppose that there is a correspondence +between the little points of colour and structure which +we see and the respective modes of life of the two species is +perfectly gratuitous. We have no inkling of the nature of such +a correspondence, how it can be constituted, or in what it may +consist.</p> + +<p class="indent">Is it not time to abandon these fanciful expectations +which are never realised? Everywhere both among animals and plants +does the problem of specific difference reiterate itself in the same +form. In view of such facts as I have related and might indefinitely +multiply, the fixity of specific characters cannot readily +be held to be a measure of their economic importance to their +possessors. The incidence of specific fixity is arbitrary and +capricious, sometimes lighting on a feature or a property which +can be supposed to matter much, but as often is it attached to the +most trifling of superficial peculiarities.</p> + +<p class="indent">The incidence of <i>variability</i> is no less paradoxical, +and without investigation of the particular case no one can say what will be +<span class="pagenum"><a name="Page_25" id="Page_25">[Pg 25]</a></span> +found to show much or little variability. The very characteristic +which in one species may exhibit extreme variability +may in an allied species show extreme constancy. Illustrations +will occur to any naturalist, but nowhere is this truth more +strikingly presented than in the British Noctuid Moths. Many +are so variable that, in the common phrase, "scarcely two can +be found alike," while others show comparatively slight variation. +It need scarcely be remarked that, in the instances I have in +mind, the evidence of great variability is in no way due to the +abundance with which the particular species occurs, for common +species may show constancy, and less abundant species may show +great variability. The polymorphism seems to be now at least +a general property of the variable species, as the fixity is a +property of the fixed species. In illustration I may refer to the +following examples.</p> + +<p class="indent"><i>Dianthoecia capsincola</i> is a common and widely distributed +moth which feeds on <i>Lychnis</i>. It shows little variation. <i>Dianthoecia +carpophaga</i> is another species which feeds chiefly on +<i>Silene</i>. Its habits are very similar to those of <i>capsincola</i>. Like +that species it has a wide geographical range and is abundant +in its localities, but in contrast to the fixity of <i>capsincola</i>, <i>carpophaga</i> +exhibits a complex series of varieties. <i>Agrotis suffusa</i> +(= <i>ypsilon</i>) is a moth widely spread through the southern half +of England. It is very constant in colour and markings. <i>Agrotis +segetum</i> and <i>tritici</i> are excessively variable both in ground +colour and markings, being found in an immense profusion of +dissimilar forms throughout their distribution. Of these and +several other species of <i>Agrotis</i> there are many named varieties, +some of which have by various writers been regarded as specifically +distinct. Of the genus <i>Noctua</i> many species (e. g. <i>festiva</i>) +show a similar polymorphism, but <i>N. triangulum</i>, though showing +some variation in certain respects, is usually very constant to +its type, and the same is true of <i>N. umbrosa</i>.</p> + +<p class="indent">In several species of <i>Taeniocampa</i>, especially <i>instabilis</i>, +the multiplicity of forms is extreme, while <i>cruda</i> (= <i>pulverulenta</i>) +is a comparatively constant species. The genus <i>Plusia</i> contains +a number of constant species, but in <i>Plusia interrogationis</i> we +<span class="pagenum"><a name="Page_26" id="Page_26">[Pg 26]</a></span> +meet the fact that the central silvery mark undergoes endless +variation. "Truly no two are alike," says Mr. Tutt, "and to +look down a long series of <i>interrogationis</i> is something like looking +at a series of Chinese characters." In contrast to this we have +the fact that in <i>Plusia gamma</i> the very similar silvery mark is +by no means variable.</p> + +<p class="indent">I have taken this series of cases from the Noctuid moths, +but it would be as easy to illustrate the same proposition from +the Geometridae or the Micro-Lepidoptera.<a name="FNanchor_15_15" id="FNanchor_15_15"></a><a href="#Footnote_15_15" class="fnanchor">[15]</a> +I have a longseries of <i>Peronea cristana</i>, for example, which was given to +me by Mr. W. H. B. Fletcher, of Bognor. All were beaten out of the same +hedge, and their polymorphism is such that no one unaccustomed +to such examples could suppose that they belonged to a single +species. Another common form, <i>P. schalleriana</i>, which lives in +similar circumstances, exhibits comparatively slight variability.</p> + +<p class="indent">It should be expressly noted that the variation of +which I am speaking is a genuine polymorphism. Several of the species +enumerated exhibit also geographical variation, possessing definite +and often strikingly distinct races peculiar to certain +localities; but apart from the existence of such local differentiation, +stands out the fact upon which I would lay stress, that +some species are excessively variable while others are by comparison +constant, in circumstances that we may fairly regard as comparable.</p> + +<p class="indent">This fact is difficult to reconcile with the conventional +view that specific type is directly determined by Natural Selection +<span class="pagenum"><a name="Page_27" id="Page_27">[Pg 27]</a></span> +and that the precision with which a species conforms to its +pattern is an indication of the closeness of that control. Anyone +familiar with the characteristics of Moths will agree that the +Noctuids, Geometrids and Tortricids are creatures whose existence +depends in some degree on the success with which they can escape +detection by their enemies in the imaginal state. We are therefore +not surprised to find that some species of these orders +exhibit definite geographical variation in conformity with the +character of the ground, which may reasonably be supposed to +aid in their protection. If this were all, there would be nothing to +cause surprise. We might even be disposed to allow that variability +might contribute to the perpetuation of animals so situated, +on the principle that among a variety of surroundings +some would probably be in harmony with the objects on which +they rest. But we cannot admit the plausibility of an argument +which demands on the one hand that the extreme precision with +which species A adheres in the minutest details of its colour and +pattern to a certain type shall be ascribed to the protective fitness +of those details, and on the other hand that the abundant variability +of species B shall be ascribed to the same determination. +If it is absolutely necessary for A to conform to one type how +comes it that B may range through some twenty distinct forms, +any two of which differ more from each other than the regular +species of many other genera? The only reply I can conceive +is a suggestion that there <i>may</i> be some circumstance which +differentiates the various classes of cases, that the exigencies +of the fixed species <i>may</i> be different from those of the variable. +Those who make such appeals to ignorance do not always perhaps +realise whither this course of reasoning may lead. If admissible +here the same argument would lead us to suggest that because +albino moles have for an indefinite period occurred on a certain +land near Bath there may be something in the soil or in the +conditions of life near Bath which requires a proportion of albinos +in its mole population. Or again, because the butterfly <i>Thais +rumina</i> in one locality, Digne in the south of France, has a percentage +of individuals of the variety <i>Honoratii</i> (with certain +normally yellow spots on the hind wing coloured bright red) +<span class="pagenum"><a name="Page_28" id="Page_28">[Pg 28]</a></span> +and nowhere else throughout its distribution, that therefore we +may suggest that there is some difference in the condition of +life at Digne which makes the continuance of <i>Honoratii</i> there +possible and beneficial.</p> + +<p class="indent">A polymorphism offering a parallel to that of the +variable moths is afforded by the breeding plumage of the Ruff, the +male of <i>Machetes pugnax</i>. The variety of plumage which these +cocks exhibit is such that the statement that no two can be +found alike is only a venial exaggeration. Newton remarks<a name="FNanchor_16_16" id="FNanchor_16_16"></a><a href="#Footnote_16_16" class="fnanchor">[16]</a> +"that all this wonderful 'show' is the consequence of the polygamous +habit of the Ruff can scarcely be doubtful"; but even if +it be conceded that the great external differentiation of the +cocks may be a result of sexual selection, the problem of their +<i>polymorphism</i> remains unsolved, for, as we are well aware, +polygamy is not usually associated with polymorphism of the +male. The Black Cock (<i>Tetrao tetrix</i>), for example, is as polygamous +as the Ruff, but in that and countless other cases, both +sexes are constant to one type of plumage.</p> + +<p class="indent">When we thus compare the polymorphism of one species +with the fixity of another, and attempt to determine the causes which +have led to these extraordinary contrasts, two distinct lines of +argument are open to us. We may ascribe the difference either +to causes external to the organisms, primarily, that is to say, +to a difference in the exigencies of Adaptation under Natural +Selection; or on the other hand we may conceive the difference +as due to innate distinctions in the chemical and physiological +constitutions of the fixed and the variable respectively. There +is truth undoubtedly in both conceptions. If the mole were +physiologically incapable of producing an albino that variety +would not have come into being, and if the albino were totally +incapable of getting its living it would not be able to hold its +<span class="pagenum"><a name="Page_29" id="Page_29">[Pg 29]</a></span> +own. Were <i>Plotheia frontalis</i> constructed on a chemical plan +which admitted of no variation, the countless varieties would +not have been produced; and if one of its varieties had an overwhelming +success out of all proportion to that of the rest, then +the species would soon become monomorphic again. We +cannot declare that Natural Selection has no part in the determination +of fixity or variability; nevertheless looking at the whole +mass of fact which a study of the incidence of variation provides, +I incline to the view that the variability of polymorphic forms +should be regarded rather as a thing tolerated than as an element +contributing directly to their chances of life; and on the other +hand that the fixity of the monomorphic forms should be looked +upon not so much as a proof that Natural Selection controls +them with a greater stringency, but rather as evidence of a natural +and intrinsic stability of chemical constitution.</p> + +<p class="indent">Compare the condition of a variable form like the male +Ruff (or in a less degree the Red Grouse in both its sexes) with +that of the common Pheasant which is comparatively constant. +In the Pheasant no doubt variations do occur as in other wild +birds, but apart from the effects of mongrelisation the species +is unquestionably uniform. Could it seriously be proposed +that we should regard the constancy of the pheasant's plumage +in this country as depending on the special fitness of that type +of colouration? Even if the pheasant be not an alien in Western +Europe, it has certainly been protected for centuries, and for a +considerable period has existed in a state of semi-domestication. +Such conditions should give good opportunity for polymorphism +to be produced. In some coverts various aberrations do of +course occur and persist, yet there is nothing indicative of a +general relaxation of the fixity of the specific type, and the pheasant +remains substantially a fixed species.<a name="FNanchor_17_17" id="FNanchor_17_17"></a><a href="#Footnote_17_17" class="fnanchor">[17]</a> +The common pheasant (<i>Phasianus colchicus</i>) even shows little of that disposition to +<span class="pagenum"><a name="Page_30" id="Page_30">[Pg 30]</a></span> +form local races which appears in the species of Further India. +Are we not then on safer ground in regarding the fixity of our +species as a property inherent in its own nature and constitution? +Just as in ages of domestication no rose has ever given +off a blue variety so has the pheasant never broken out into the +polymorphism of the Ruff.</p> + +<p class="indent">As soon as it is realised how largely the phenomena +of variation and stability must be an index of the internal constitution +of organisms, and not mere consequences of their relations to +the outer world, such phenomena acquire a new and more profound significance.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_31" id="Page_31">[Pg 31]</a></span></p> + +<h2>CHAPTER II</h2> +<h3><span class="smcap">Meristic Phenomena</span></h3> + +<p class="indent">Twenty years ago in describing the facts of Variation, +argument was necessary to show that these phenomena had a special +value in the sciences of Zoology and Botany. This value is +now universally understood and appreciated. In spite however +of the general attention devoted to the study of Variation, and +the accumulation of material bearing on the problem, no satisfactory +or searching classification of the phenomena is possible. +The reason for this failure is that a real classification must +presuppose knowledge of the chemistry and physics of living things +which at present is quite beyond our reach.</p> + +<p class="indent">It is however becoming probable that if more knowledge +of the chemical and physical structure of organisms is to be attained, +the clue will be found through Genetics, and thus that +even in the uncoordinated accumulation of facts of Variation +we are providing the means of analysis applicable not only to +them, but to the problems of normality also.</p> + +<p class="indent">The only classification that we can yet institute +with any confidence among the phenomena of Variation is that which +distinguishes on the one hand variations in the processes of +division from variations in the nature of the substances divided.</p> + +<p class="indent">Variations in the processes of division are most often +made apparent by a change in the number of the parts, and are therefore +called <i>Meristic</i> Variations, while the changes in actual composition +of material are spoken of as <i>Substantive</i> Variations. The Meristic +Variations form on the whole a natural and fairly well +defined group, but the Substantive Variations are obviously +a heterogeneous assemblage.</p> + +<p class="indent">Though this distinction does not go very far, it is +useful, and in all probability fundamental. It is of value inasmuch as +it brings into prominence the distinct and peculiar part which +<span class="pagenum"><a name="Page_32" id="Page_32">[Pg 32]</a></span> +the process of division, or, more generally, repetition of parts, +plays in the constitution of the forms of living things.</p> + +<p class="indent">That there may be a real independence between the +Meristic and the Substantive phenomena is evident from the fact both +that Meristic changes may occur without Substantive Variation, +and that the substances composing an organism may change +without any perceptible alteration in its meristic structure. +When the distinction between these two classes of phenomena +is perceived it will be realised that the study of genetics has on +the one hand a physical, or perhaps more strictly a mechanical +aspect, which relates to the manner in which material is divided +and distributed; and also a chemical aspect, which relates to +the constitution of the materials themselves. Somewhat as +the philosophers of the seventeenth and eighteenth centuries +were awaiting both a chemical and a mechanical discovery which +should serve as a key to the problems of unorganised matter, +so have biologists been awaiting two several clues. In Mendelian +analysis we have now, it is true, something comparable with the +clue of chemistry, but there is still little prospect of penetrating +the obscurity which envelops the mechanical aspect of our phenomena. +To make clear the application of the terms chemical +and mechanical to the problem of Genetics the nature of that +problem must be more fully described. In its most concrete +form this problem is expressed in the question, how does a cell +divide? If the organism is unicellular, and the single cell is +the whole body, then the process of heredity is accomplished +in the single operation of cell-division. Similarly in animals and +plants whose bodies are made up of many cells, the whole process +of heredity is accomplished in the cell-divisions by which the +germ-cells are formed. When therefore we see a cell dividing, +we are witnessing the process by which the form and the properties +of the daughter-cells are determined.</p> + +<p class="indent">Now this process has the two aspects which I have +called mechanical and chemical. The term "<i>Entwicklungsmechanik</i>" +has familiarised us with the application of the word mechanics +to these processes, but on reflexion it will be seen that this comprehensive +term includes two sorts of events which are sometimes +<span class="pagenum"><a name="Page_33" id="Page_33">[Pg 33]</a></span> +readily distinguishable. There is the event by which the cell +<i>divides</i>, and the event by which the two halves or their descendants +are or may be <i>differentiated</i>. It is common knowledge that +in some cell-divisions two similar halves, indistinguishable in +appearance, properties, and subsequent fate, may be produced, +while in other divisions daughter-cells with distinct properties +and powers are formed. We cannot imagine but that in the +first case, when the resulting cells are identical, the division +is a mechanical process by which the mother-cell is simply +cut in two; while in order that two differentiated halves may be +produced, some event must have taken place by which a chemical +distinction between the two halves is effected.<a name="FNanchor_1_18" id="FNanchor_1_18"></a><a href="#Footnote_1_18" class="fnanchor">[1]</a> +In any ordinary Mendelian case we have a clear proof that such a +chemical difference may be established between germ-cells. The facts +of colour-inheritance for instance prove that germ-cells, otherwise +identical, may be formed <i>possessing</i> the chromogen-factor which +is necessary to the formation of colour in the flowers, or <i>destitute</i> +of that factor. Similarly the germ-cells may possess the ferment +which, by its action on the chromogenic substance, produces +the colour, or they may be without that ferment. The same +line of argument applied to a great range of cases. Nevertheless, +though differences in chemical properties are often thus constituted +by cell-divisions, and though we are thus able to make a +quasi-chemical analysis of the individual by determining and +enumerating these properties, yet it is evident that the distribution +of these factors is not itself a chemical process. This +is proved by the fact that similar divisions may be effected between +halves which are exactly alike, and also by the fact that +the numbers in which the various types of germ-cells are formed +negative any suggestion of valency between them. The recognition +of the unit-factors may lead—indeed must lead—to great +advances in chemical physiology which without that clue would +have been impossible, but in causation the chemical phenomena +of heredity must be regarded as secondary to the physical or +<span class="pagenum"><a name="Page_34" id="Page_34">[Pg 34]</a></span> +mechanical phenomena by which the cells and their constituents +are divided and separated. When therefore we speak of the +<i>essential</i> phenomena of heredity we mean the mechanics of division, +especially, though not, as we shall see, exclusively, of <i>cell</i>-division; +and in the relation between the two halves of the dividing +cell we have the problem presented in what seems to be its simplest form.</p> + +<p class="indent">In attempting to form some conception of the processes +by which bodily characteristics are transmitted, or—to avoid that +confusing metaphor of "transmission"—how it comes about +that the offspring can grow to resemble its parent, continuity of +the germ-substance which in some animals is a visible phenomenon,<a name="FNanchor_2_19" id="FNanchor_2_19"></a><a href="#Footnote_2_19" class="fnanchor">[2]</a> +gives at least apparent help. An egg for example on becoming +adult develops in certain parts a particular pigment. +The eggs of that adult when they reach the appropriate age develop +the same pigment. We have no clear picture of the +mechanism by which this process is effected, but when we realise +that the pigment results from the interaction of certain substances, +and that since all the eggs are in reality pieces of the same +material, it seems, unless we inquire closely, not unnatural that +the several pieces of the material should exhibit the same colours +at the same periods of their development. The continuity of +the material of the germs suggests that there is a continuity +of the materials from which the pigment is formed, and that +thus an actual bit of those substances passes into each egg +ready at the appropriate moment to generate the pigment. +The argument thus outlined applies to all <i>substantive</i> characteristics. +In each case we can imagine, if we will, the appearance +of that characteristic as due to the contribution of its rudiment +from the germ tissues.</p> + +<p class="indent">When we consider more critically it becomes evident +that the aid given by this mental picture is of very doubtful reality, +for even if it were true that any predestined particle actually +corresponding with the pigment-forming materials is definitely +<span class="pagenum"><a name="Page_35" id="Page_35">[Pg 35]</a></span> +passed on from germ to germ, yet the power of increase which +must be attributed to it remains so incomprehensible that the +mystery is hardly at all illuminated.</p> + +<p class="indent space-below">When however we pass from the substantive to the +meristic characters, the conception that the character depends on the +possession by the germ of a particle of a specific material becomes +even less plausible. Hardly by any effort of imagination can we +see any way by which the division of the vertebral column into <i>x</i> +segments or into <i>y</i> segments, or of a Medusa into 4 segments or +into 6, can be determined by the possession or by the want of a +material particle. The distinction must surely be of a different +order. If we are to look for a physical analogy at all we should +rather be led to suppose that these differences in segmental +numbers corresponded with changes in the amplitude or number +of dividing waves than with any change in the substance or +material divided.</p> + +<h3><span class="smcap">Phenomena of Division</span></h3> + +<p class="indent space-below">I have said that in the division of a cell we seem to +see the problem in its simplest form, but it is important to observe that +the problem of division may be presented by the bodies of animals +and plants in forms which are independent of the divisions between +cells. The existence of pattern implies a repetition of +parts, and repetition of parts when developed in a material +originally homogeneous can only be created by division. Cell-division +is probably only a special case of a process similar to +that by which the pattern of the skeleton is laid down in a unicellular +body such as that of a Radiolarian or Foraminiferan. +Attempts have lately been made to apply mathematical treatment +to problems of biology. It has sometimes seemed to +me that it is in the geometrical phenomena of life that the most +hopeful field for the introduction of mathematics will be found. +If anyone will compare one of our animal patterns, say that of a +zebra's hide, with patterns known to be of purely mechanical +production, he will need no argument to convince him that there +must be an essential similarity between the processes by which +the two kinds of patterns were made and that parts at least of +<span class="pagenum"><a name="Page_36" id="Page_36">[Pg 36]</a></span> +the analysis applicable to the mechanical patterns are applicable +to the zebra stripes also. Patterns mechanically produced are +of many and very diverse kinds. One of the most familiar +examples, and one presenting some especially striking analogies +to organic patterns, is that provided by the ripples of a mackerel +sky, or those made in a flat sandy beach by the wind or the ebbing +tide. With a little search we can find among the ripple-marks, and +in other patterns produced by simple physical means, the closest +parallels to all the phenomena of striping as we see them in our +animals. The forking of the stripes, the differentiation of two +"faces," the deflections round the limbs and so forth, which in the +body we know to be phenomena of division, are common both to +the mechanical and the animal patterns. We cannot tell what in +the zebra corresponds to the wind or the flow of the current, but +we can perceive that in the distribution of the pigments, that is +to say, of the chromogen-substances or of the ferments which +act upon them, a rhythmical disturbance has been set up which +has produced the pattern we see; and I think we are entitled to +the inference that in the formation of patterns in animals and +plants mechanical forces are operating which ought to be, and +will prove to be, capable of mathematical analysis. The comparison +between the striping of a living organism and the sand-ripples +will serve us yet a little farther, for a pattern may either +be formed by actual cell-divisions, and the distribution of differentiation +coincidently determined, or—as visibly in the pigmentation +of many animal and plant tissues—the pattern may +be laid down and the pigment (for example) distributed through +a tissue across or independently of the cell-divisions of the tissue. +Our tissues therefore are like a beach composed of sands of +different kinds, and different kinds of sands may show distinct +and interpenetrating ripples. When the essential analogy between +these various classes of phenomena is perceived, no one +will be astonished at, or reluctant to admit, the reality of discontinuity +in Variation, and if we are as far as ever from knowing +the actual causation of pattern we ought not to feel surprised that +it may arise suddenly or be suddenly modified in descent. Biologists +have felt it easier to conceive the evolution of a striped +animal like a zebra from a self-coloured type like a horse (or +of the self-coloured from the striped) as a process involving many +intergradational steps; but so far as the <i>pattern</i> is concerned, the +change may have been decided by a single event, just as the +multitudinous and ordered rippling of a beach may be created +or obliterated at one tide. +<span class="pagenum"><a name="Page_37" id="Page_37">[Pg 37]</a></span></p> + +<div class="figcenter" > + <img src="images/i_050.jpg" alt="Elephant Tusk" width="600" height="283" /> +</div> +<p class="space-below" style="font-size: 120%; text-align: center;"> + <b><span class="smcap">Fig. 1.</span> Tusk of Indian elephant, showing an abnormal segmentation.</b></p> +<p><span class="pagenum"><a name="Page_38" id="Page_38">[Pg 38]</a></span></p> + +<p class="indent">This point is well illustrated by the tusk of an Indian +elephant which I lately found in a London sale-room. This tusk is by +some unknown cause, presumably a chronic inflammation, +thrown up into thirteen well-marked ridges which closely simulate +a series of segments (Fig. 1). Whatever the cause the condition +shows how easily a normally unsegmented structure may be +converted into a series of repeated parts.</p> + +<p class="indent space-below">The spread of segmentation through tissues normally +unsegmented is very clearly exemplified in the skates' jaws shown in +in Fig. 2. The right side of the upper figure shows the normal +arrangement in the species <i>Rhinoptera jussieui</i>, but the structure +on the left side is very different. The probable relations of the +several rows of teeth to the normal rows is indicated by the lettering, +but it is evident that by the appearance of new planes +of division constituting separate centers of growth, the series has +been recast. The pattern of the left side is so definite that had +the variation affected the right side also, no systematist would +have hesitated to give the specimen a new specific name. The +other two drawings show similar variations of a less extensive +kind, the nature of which is explained by the lettering of the +rows of teeth.</p> + +<div class="figcenter" > + <img src="images/i_052.jpg" alt="Jaws of Skates" width="600" height="674" /> +</div> +<p class="space-below" style="font-size: 120%; text-align: center;"> + <b><span class="smcap">Fig. 2.</span> Jaws of Skates</b> (<i>Rhinoptera</i>) +<b>showing meristic variation.</b><br /> +(For a detailed discussion see <i>Materials for the Study of Variation</i>, p. 259.)</p> + +<p class="indent">This power to divide is a fundamental attribute of +life, and of that power cell-division is a special example. In regard to +almost all the chief vital phenomena we can say with truth that +science has made some progress. If I mention respiration, metabolism, +digestion, each of these words calls to mind something +more than a bare statement that such acts are performed by an +animal or a plant. Each stands for volumes of successful experiment +and research, But the expression cell-division, the +fundamental act which typifies the rest, and on which they all +depend, remains a bare name. We can see with the microscope +<span class="pagenum"><a name="Page_39" id="Page_39">[Pg 39]</a></span> +the outward symptoms of division, but we have no surmise as +to the nature of the process by which the division is begun or +accomplished. I know nothing which to a man well trained in +scientific knowledge and method brings so vivid a realisation +of our ignorance of the nature of life as the mystery of cell-division. +What is a living thing? The best answer in few words +that I know is one which my old teacher, Michael Foster, used +to give in his lectures introductory to biology. "A living thing +is a vortex of chemical and molecular change." This description +gives much, if not all, that is of the essence of life. The living +thing is unlike ordinary matter in the fact that, through it, matter +is always passing. Matter is essential to it; but, provided +that the flow in and out is unimpeded, the life-process can go +on so far as we know indefinitely. Yet the living "vortex" +differs from all others in the fact that it can divide and throw +off other "vortices," through which again matter continually swirls. +<span class="pagenum"><a name="Page_40" id="Page_40">[Pg 40]</a></span></p> + +<p class="indent">We may perhaps take the parallel a stage further. A simple +vortex, like a smoke-ring, if projected in a suitable way will twist +and form two rings. If each loop as it is formed could grow and +then twist again to form more loops, we should have a model +representing several of the essential features of living things.</p> + +<p class="indent">It is this power of spontaneous division which most +sharply distinguishes the living from the non-living. In the excellent +book dealing with the problems of development, lately published +by Mr. Jenkinson a special emphasis is very properly laid on the +distinction between the processes of division, and those of +differentiation. Too often in discussions of the developmental +processes the distinction is obscured. He regards differentiation +as the "central difficulty." "Growth and division of the nucleus +and the cells," he tells us, are side-issues. This view is quite +defensible, but I suspect that the division <i>is</i> the central +difficulty, and that if we could get a rationale of what is happening +in cell-division we should not be long before we had a clue to the +nature of differentiation. It may be self-deception, but I do +not feel it impossible to form some hypothesis as to the mode of +differentiation, but in no mood of freest speculation are we ever +able to form a guess as to the nature of the division. We see +differentiations occurring in the course of chemical action, in +some phenomena of vibration and so forth: but where do we +see anything like the spontaneous division of the living cell? +Excite a gold-leaf electroscope, and the leaves separate, but we +know that is because they were double before. In electrolysis +various substances separate out at the positive and negative +poles respectively. Now if in cell-division the two daughter-cells +<span class="pagenum"><a name="Page_41" id="Page_41">[Pg 41]</a></span> +were always dissimilar—that is to say, if differentiation +always occurred—we could conceive some rough comparison +with such dissociations. But we know the dissimilarity between +daughter-cells is not essential. In the reproduction of unicellular +organisms and many other cases, the products formed at the +two poles are, so far as we can tell, identical. Any assumption +to the contrary, if we were disposed to make it, would involve +us in difficulties still more serious. At any rate, therefore, if +differentiation be really the central difficulty in development, +it is division which is the essential problem of heredity.</p> + +<p class="indent">Sir George Darwin and Professor Jeans tell us that +"gravitational instability" consequent on the condensation of gases is +"the primary agent at work in the actual evolution of the universe," +which has led to the division of the heavenly bodies. The +greatest advance I can conceive in biology would be the discovery +of the nature of the instability which leads to the continual +division of the cell. When I look at a dividing cell I feel as an +astronomer might do if he beheld the formation of a double +star: that an original act of creation is taking place before me. +Enigmatical as the phenomenon seems, I am not without hope +that, if it were studied for its own sake, dissociated from the +complications which obscure it when regarded as a mere incident +in development, some hint as to the nature of division could be +found. It is I fear a problem rather for the physicist than for +the biologist. The sentiment may not be a popular one to utter +before an assembly of biologists, but looking at the truth impersonally +I suspect that when at length minds of first rate analytical +power are attracted to biological problems, some advance will +be made of the kind which we are awaiting.</p> + +<p class="indent">The study of the phenomena of bodily symmetry offers +perhaps the most hopeful point of attack. The essential fact +in reproduction is cell-division, and the essential basis of hereditary +resemblance is the symmetry of cell-division. The phenomena +of twinning provide a convincing demonstration that this +is so. By twinning we mean the production of equivalent structures +by division. The process is one which may affect the whole +body of an animal or plant, or certain of its parts. The term +<span class="pagenum"><a name="Page_42" id="Page_42">[Pg 42]</a></span> +twin as ordinarily used refers to the simultaneous birth of two +individuals. Those who are naturalists know that such twins +are of two kinds, (1) twins that are not more alike than any other +two members of the same family, and (2) twins that are so much +alike that even intimate friends mistake them. These latter +twins, except in imaginative literature, are always of the same sex.</p> + +<p class="indent">It is scarcely necessary for me to repeat the evidence +from which it has been concluded that without doubt such twins arise +by division of the same fertilised ovum. There is a perfect +series of gradations connecting them with the various forms of +double monsters united by homologous parts. They have been +shown several times to be enclosed in the same chorion, and the +proofs of experimental embryology show that in several animals +by the separation of the two first hemispheres of a dividing egg +twins can be produced. Lastly we have recently had the extraordinarily +interesting demonstration of Loeb, to which I may +specially refer. Herbst some years ago found that in sea water, +from which all lime salts had been removed, the segments of the +living egg fall apart as they are formed. Using this method +Loeb has shown that a temporary immersion in lime-free sea +water may result in the production of 90 per cent. of twins. +We are therefore safe in regarding the homologous or "identical" +twins as resulting from the divisions of one fertilised egg, while +the non-identical or "fraternal" twins, as they are called, arise +by the fertilisation of two separate ova.<a name="FNanchor_3_20" id="FNanchor_3_20"></a><a href="#Footnote_3_20" class="fnanchor">[3]</a> +<span class="pagenum"><a name="Page_43" id="Page_43">[Pg 43]</a></span></p> + +<p class="indent">In the resemblance of identical twins we have an extreme case +of hereditary likeness<a name="FNanchor_4_21" id="FNanchor_4_21"></a><a href="#Footnote_4_21" class="fnanchor">[4]</a> +and a proof, if any were needed, that the +cause of individual variation is to be sought in the differentiation +of germ-cells. The resemblance of identical twins depends on +two circumstances, First, since only two germ-cells take part +in their production, difference between the germ cells of the +same individual cannot affect them. Secondly the division of +the fertilised ovum, the process by which they became two instead +of one, must have been a symmetrical division. The +structure of twins raises however one extremely significant +difficulty, which as yet we cannot in any way explain. The +resemblance between twins is a phenomenon of symmetry, +like the resemblance between the two sides of a bilaterally symmetrical +body. Not only is the general resemblance readily +so interpreted, but we know also that in double monsters, namely +unseparated twins, various anatomical abnormalities shown by +the one half-body are frequently shown by the other half-also.<a name="FNanchor_5_22" id="FNanchor_5_22"></a><a href="#Footnote_5_22" class="fnanchor">[5]</a> +The two belong to one system of symmetry. How then +does it happen that the body of one of a pair of twins does not +show a transposition of viscera? We know that the relation of +right and left implies that the one should be the mirror-image of +the other. Such a relation of images may be maintained even +in minute details. For example if the same pattern of finger-print +is given by the fingers of the two hands, one is the reverse +of the other. In double monsters, namely unseparated twins, +there is evidence that an inversion of viscera does occur with +some frequency. Evidence from such cases is not so clear and +simple as might be expected, because as a matter of fact, the +heart and stomach, upon which the asymmetry of the viscera +chiefly depend, are usually common to the two bodies. Duplicity +generally affects either the anterior end alone, or the posterior +end alone. The division is generally <i>from the heart forwards</i>, +giving two heads and two pairs of anterior limbs on a common +trunk, or <i>from the heart backwards</i>, giving two pairs of posterior +limbs with the anterior body common. In either case, though +<span class="pagenum"><a name="Page_44" id="Page_44">[Pg 44]</a></span> +the bodies may be grouped in a common system of symmetry, +neither can be proved to show definite reversal of the parts. To +see that reversal recourse must be had to more extreme duplications, +such as the famous Siamese Twins. They, as a matter of +fact, were an excellent instance of the proposition that twins +are related as mirror-images, for both of them had eleven pairs +of ribs instead of the normal twelve, and one of them had a partial reversal +of viscera.<a name="FNanchor_6_23" id="FNanchor_6_23"></a><a href="#Footnote_6_23" class="fnanchor">[6]</a> +(Küchenmeister, <i>Verlagerung</i>, etc., p. 204.)</p> + +<p class="indent">If anyone could show how it is that neither of a +pair of twins has transposition of viscera the whole mystery of division +would, I expect, be greatly illuminated.<a name="FNanchor_7_24" id="FNanchor_7_24"></a><a href="#Footnote_7_24" class="fnanchor">[7]</a> +At present we have simply to accept the fact that twins, by virtue of +their detachment from each other, have the power of resuming the polarity +which is proper to any normal individual. It was nevertheless +with great interest that I read Wilder's recent observation<a name="FNanchor_8_25" id="FNanchor_8_25"></a><a href="#Footnote_8_25" class="fnanchor">[8]</a> +that occasionally in identical twins the finger-print of one or both +the index-fingers may be reversed, showing that there is after +all some truth in the notion that reversal should occur in them.</p> + +<p class="indent">There is another phenomenon by twinning which, if we +could understand it, might help. I refer to the free-martin, the subject +of one of John Hunter's masterpieces of anatomical description. +In horned cattle twin births are rare, and when twins of opposite +sexes are born, the male is perfect and normal, but the reproductive +<span class="pagenum"><a name="Page_45" id="Page_45">[Pg 45]</a></span> +organs of the female are deformed and sterile, being +known as a free-martin. The same thing occasionally happens +in sheep, suggesting that in sheep also twins may be formed by +the division of one ovum; for it is impossible to suppose that +mere development in juxtaposition can produce a change of this +character. I mention the free-martin because it raises a question +of absorbing interest. It is conceivable that we should interpret +it by reference to the phenomenon of gynandromorphism, seen +occasionally in insects, and also in birds as a great rarity. In +the gynandromorph one side of the body is male, the other female. +A bullfinch for instance has been described with a sharp line of +division down the breast between the red feathers of the cock +on one side and the brown feathers of the hen on the other. +(Poll, H., <i>SB. Ges. Nat. Fr.</i>, Berlin, 1909, p. 338.) In such cases +neither side is sexually perfect. If the halves of such a gynandromorph +came apart, perhaps one would be a free-martin.</p> + +<p class="indent">The behaviour of homologous twinning in heredity has been +little studied. It does not exist as a normal feature in any animal +which is amenable to experiment, and we cannot positively +assert that a comparable phenomenon exists in plants; for in +them—the Orange, for example—polyembryony may evidently +be produced by a parthenogenetic development of nucellar tissue. +It is possible that in Man twinning is due to a peculiarity of the +mother, not of the father. It may and not rarely does descend +from mother to daughter, but whether it can be passed on +through a male generation to a daughter again, there is not +sufficient evidence to show. The facts as far as they go are +consistent with the inference which may be drawn from Loeb's +experiment, that the twinning of a fertilized ovum may be determined +not by the germ-cells which united to form it, but by +the environment in which it begins to develop. The opinion that +twinning may descend through the male directly has been lately +expressed by Dr. J. Oliver in the <i>Eugenics Review</i> (1912), on the +evidence of cases in which twins had occurred among the relations +of fathers of twins, but I do not know of any comprehensive +collection of evidence bearing on the subject. +<span class="pagenum"><a name="Page_46" id="Page_46">[Pg 46]</a></span></p> + +<p class="indent">Besides twinning of the whole body a comparable duplicity +of various parts of the same body may occur. Such divisions +affect especially those organs which have an axis of bilateral +symmetry, such as the thumb, a cotyledon, a median petal, +the frond of a fern or the anal fin of a fish. From the little +yet known it is clear that the genetic analysis of these conditions +must be very difficult, but evidence of any kind regarding them +will be valuable. We want especially to know whether these +divisions are due to the <i>addition</i> of some factor or power which +enables the part to divide, or whether the division results from +the <i>absence</i> of something which in the normal body prevents +the part from dividing. Breeding experiments, so far as they +go, suggest that the less divided state is usually dominant to +the more divided.<a name="FNanchor_9_26" id="FNanchor_9_26"></a><a href="#Footnote_9_26" class="fnanchor">[9]</a> +The two-celled Tomato fruit is dominant to +the many-celled type. The Manx Cat's tail, with its suppression +of caudal segmentation is a partial dominant over the normal +tail. The tail of the Fowl in what is called the "Rumpless" +condition is at least superficially comparable with that of the +Manx Cat, and though the evidence is not wholly consistent, +Davenport obtained facts indicating that this suppressed condition +of the caudal vertebrae is an imperfect dominant.<a name="FNanchor_10_27" id="FNanchor_10_27"></a><a href="#Footnote_10_27" class="fnanchor">[10]</a> +</p> + +<p class="indent">Some evidence may also be derived from other examples +of differences which at first sight appear to be substantive though +they are more probably meristic in ultimate nature. The +distinction between the normal and the "Angora" hair of the +Rabbit is a case in point. We can scarcely doubt that one of +the essential differences between these two types is that in the +Angora coat the hair-follicles are more finely divided than they +are in the normal coat, and we know that the normal, or less-divided +condition, is dominant to the Angora, or more finely divided.</p> +<p><span class="pagenum"><a name="Page_47" id="Page_47">[Pg 47]</a></span></p> + +<div class="figcenter" > + <img src="images/i_060.jpg" alt="Elephant Tusk" width="500" height="602" /> +</div> +<p class="space-below" style="font-size: 120%;"> + <b><span class="smcap">Fig. 3.</span>  <i>I</i>, <i>II</i>, <i>III</i>,</b> +various degrees of syndactyly affecting the medius and annularis in the hand; +<b><i>IV</i></b>, syndactyly affecting the index and medius in the foot. (After Annandale.)</p> + +<p class="indent space-below">In the case of the solid-hoofed or "mule-footed" swine, the +evidence shows, as Spillman has lately pointed out,<a name="FNanchor_11_28" id="FNanchor_11_28"></a><a href="#Footnote_11_28" class="fnanchor">[11]</a> +that the condition behaves as a dominant. The essential feature of +this abnormality is that the digits III and IV are partially +united. The union is greatest peripherally. Sometimes the +third phalanges only are joined to form one bone, but the second +and even the first phalanges may also be compounded together. +Here the variation is obviously meristic and consists in a failure +to divide, the normal separation of the median digits of the foot being suppressed. +<span class="pagenum"><a name="Page_48" id="Page_48">[Pg 48]</a></span></p> + +<div class="figcenter" > + <img src="images/i_061.jpg" alt="Syndactyly in the Foot" width="600" height="315" /> +</div> +<p class="space-below" style="font-size: 120%;"> +<b><span class="smcap">Fig. 4.</span></b>  Case of complete syndactyly in the foot. <b><i>II</i></b> +and <b><i>III</i></b>, digit apparently representing the index and medius. +<b><i>c</i><sup>2</sup> + <i>c</i><sup>3</sup></b>, bone apparently representing the +middle and external cuneiform; <b><i>cb</i></b>, cuboid; <b><i>c</i><sup>1</sup></b>, +internal cuneiform. (After Gruber.)</p> + +<p class="indent">Webbing between the digits, in at least some of its +manifestations, is a variation of similar nature. The family recorded +by Newsholme<a name="FNanchor_12_29" id="FNanchor_12_29"></a><a href="#Footnote_12_29" class="fnanchor">[12]</a> +very clearly shows the dominance of this condition. +The case is morphologically of great interest and must +undoubtedly have a bearing on the problems of the mechanics +of Division. In discussing the phenomena of syndactylism +I pointed out some years ago that the digits most frequently +united in the human hand are III and IV, while in the foot, +union most frequently takes place between II and III.<a name="FNanchor_13_30" id="FNanchor_13_30"></a><a href="#Footnote_13_30" class="fnanchor">[13]</a> +In Newsholme's family the union was always between II and III +of the foot, except in the case of one male who had the digits +III and IV of the right <i>hand</i> alone webbed together. There +can be little doubt that the geometrical system on which the +foot is planned has an axis of symmetry passing between the +digits II and III, while the corresponding axis in the hand passes +between III and IV. Union between such digits may therefore +be regarded as comparable with any non-division or "coalescence" +of lateral structures in a middle line, and when as in these examples +<span class="pagenum"><a name="Page_49" id="Page_49">[Pg 49]</a></span> +such a condition is shown to be a dominant we cannot +avoid the inference that some concrete factor has the power of +suppressing or inhibiting this division. Figs. 3 and 4 illustrate +degrees of union between digits in the human hand and foot.</p> + +<p class="indent">It is not in question that various other forms of irregular +webbing and coalescence of digits exist, and respecting the genetic +behaviour of these practically nothing is as yet known. Such +a case is described by Walker,<a name="FNanchor_14_31" id="FNanchor_14_31"></a><a href="#Footnote_14_31" class="fnanchor">[14]</a> +in which the first and second +metacarpals of both feet were fused in mother and daughter, +and several more are found in literature. Contrasted with these +phenomena we have the curious fact that in the Pigeon, Staples-Browne +found webbing of the toes a <i>recessive</i> character. The +question thus arises whether this webbing is of the same nature +as that shown to be a dominant in Man, and indeed whether the +phenomenon in pigeons is really meristic at all. There is some +difference perceptible between the two conditions; for in Man +there is not so much a development of a special web-like skin +uniting the digits as a want of proper division between the digits +themselves, and in extreme cases two digits may be represented +by a single one. In the Pigeon I am not aware that a real +union of this kind has ever been observed, and though the web-like +skin may extend the whole length of the digits and be so narrow +as to prevent the spread of the toes, it may, I think, be maintained +that the unity of the digits is unimpaired. For the +present the nature of this variation in the pigeon's feet must be +regarded as doubtful, and we should note that if it is actually +an example of a more perfect division being dominant to a less +perfect division, the case is a marked exception to the general +rule that non-division is dominant to division.</p> + +<p class="indent">Reference must also be made to the phenomenon of fasciation +in the stems of plants. As Mendel showed in the case of <i>Pisum</i> +this condition is often a recessive. The appearances suggest +that the difference between a normal and a fasciated plant +consists in the inability of the fasciated plant to separate its +lateral branches. The nature of the condition is however very +<span class="pagenum"><a name="Page_50" id="Page_50">[Pg 50]</a></span> +obscure and it is equally likely that some multiplication of the +growing point is the essential phenomenon.<a name="FNanchor_15_32" id="FNanchor_15_32"></a><a href="#Footnote_15_32" class="fnanchor">[15]</a> +</p> + +<p>Stockard's interesting experiments<a name="FNanchor_16_33" id="FNanchor_16_33"></a><a href="#Footnote_16_33" class="fnanchor">[16]</a> +illustrate this question. He showed that by treating the embryos of a fish +(<i>Fundulus heteroclitus</i>) with a dilute solution of magnesium salts, +various cyclopian monstrosities were frequently produced. These have +been called cases of <i>fusion</i> of the optic vesicles. I would prefer +to regard them as cases of a division suppressed or restricted by +the control of the environment. Conversely, the splendid discovery +of Loeb, that an unfertilised egg will divide and develop +parthenogenetically without fertilisation, as a consequence of +exposure to various media, may be interpreted as suggesting that +the action of those media releases the strains already present +in the ovum, though I admit that an interpretation based on the +converse hypothesis, that the medium acts as a stimulus, is as +yet by no means excluded.</p> + +<p class="indent">In these cases we come nearest to the direct causation +or the direct inhibition of a division, but the meaning of the +evidence is still ambiguous. I incline to compare Loeb's parthenogenesis +with the development (and of course accompanying +cell-division) of dormant buds on stems which have been cut back.</p> + +<p class="indent">It is interesting to note that sometimes as an abnormality, +the faculty of division gets out of hand and runs a course apparently +uncontrolled. A remarkable instance of this condition +is seen in <i>Begonia</i> "<i>phyllomaniaca</i>", which breaks out into buds +at any point on the stem, petioles, or leaves, each bud having, +like other buds, the power of becoming a new plant if removed. +We would give much to know the genetic properties of <i>B. phyllomaniaca</i>, +and in conjunction with Mr. W. O. Backhouse I have +for some time been experimenting with this plant. It proved +totally sterile. Its own anthers produce no pollen, and all attempts +to fertilise it with other species failed though the pollen +of a great number of forms was tried. +<span class="pagenum"><a name="Page_51" id="Page_51">[Pg 51]</a></span></p> + +<p class="indent space-below">Recently however we have succeeded in making plants +which are in every respect <i>Begonia phyllomaniaca</i>, so far as the characters +of stems and leaves are concerned. These plants, of +which we have sixteen, were made by fertilising <i>B. heracleifolia</i> +with <i>B. polyantha</i>. They are all beginning to break out in +"phyllomania." As yet they have not flowered, but as they +agree in all details with <i>phyllomaniaca</i> there can be little doubt +that the original plant bearing that name was a hybrid similarly +produced. The production of "phyllomania" on a hybrid +Begonia has also been previously recorded by Duchartre.<a name="FNanchor_17_34" id="FNanchor_17_34"></a><a href="#Footnote_17_34" class="fnanchor">[17]</a> +In this case the cross was made between <i>B. incarnata</i> and <i>lucida</i>. +The synonymy of the last species is unfortunately obscure, and +I have not succeeded in repeating the experiment.</p> + +<div class="figcenter" > + <img src="images/i_064.jpg" alt="Petiole" width="600" height="223" /> +</div> +<p class="space-below" style="font-size: 120%; text-align: center;"> +<b><span class="smcap">Fig. 5.</span> Piece of petiole of</b> <i>Begonia phyllomaniaca</i>. +<b>The proximal end is to the right of the figure.</b></p> + +<p class="indent space-below">From these facts it seems practically certain that +the condition is one which is due to the meeting of complementary factors. +At first sight we may incline to think that the phyllomania is in +some way due to the sterility. This however cannot be seriously +maintained; for not only is sterility in plants not usually associated +with such manifestations, but we know a Begonia called +"Wilhelma" which is exactly <i>phyllomaniaca</i> and equally sterile, +though it has no trace of phyllomania. This plant arose in the +nurseries of MM. P. Bruant of Poitiers, and has generally been +described as a seedling of <i>phyllomaniaca</i>, but from the total +sterility of that form this account of its origin must be set aside. +<span class="pagenum"><a name="Page_52" id="Page_52">[Pg 52]</a></span></p> + +<div class="figcenter" > + <img src="images/i_065.jpg" alt="Petiole" width="600" height="560" /> +</div> +<p style="font-size: 120%;"> +<b><span class="smcap">Fig. 6.</span></b> Two right hind feet of polydactyle +cats. <b><i>II</i></b> shows the lowest development of the condition yet recorded. +The digit, <b><i>d</i><sup>1</sup></b>, which stands as hallux is +fully formed and has three phalanges. Both it and the digit marked +<b><i>d</i><sup>2</sup></b> are formed as <b><i>left</i></b> digits. In the +normal hind foot of the cat the hallux is represented by a rudiment only.</p> + +<p class="space-below" style="font-size: 120%;"><b><i>I</i></b> +shows a further development of the condition. In this foot +there are <i>six</i> digits. <b><i>d</i><sup>1</sup></b> has two phalanges, but both +it and <b><i>d</i><sup>2</sup></b> and <b><i>d</i><sup>3</sup></b> are shaped as left digits. +Thus <b><i>d</i><sup>3</sup></b>, which in the normal foot would be shaped as a right digit, +is transformed so as to look like a <b><i>left</i></b> digit.</p> + +<p class="indent">The phenomenon in this case can hardly be regarded +as due to the excitation of dormant buds, for it is apparent on +examination that the new growths are not placed in any fixed +geometrical relation to the original plant. They arise on the +petiole, for example, as small green outgrowths each of which +gradually becomes a tiny leaf. The attitude of these leaves is +quite indeterminate, and they may point in any direction, +<span class="pagenum"><a name="Page_53" id="Page_53">[Pg 53]</a></span> +some having their apices turned peripherally, some centrally, and +others in various oblique or transverse positions (Fig. 5). These +little leaves are thus comparable with seedlings, in that their +polarity is not related to, or consequent upon that of the parent +plant. They have in fact that "individuality," which we associate +with germinal reproduction.</p> + +<p class="indent">There are many curious phenomena seen in the behaviour +of parts normally repeated in bilateral symmetry which may some +day guide us towards an understanding of the mechanics of +division. A part like a hand, which needs the other hand to +complete its symmetry, cannot twin by mere division, yet by +proliferation and special modifications on the radial side of the +same limb, even a hand may be twinned. In the well known polydactyle +cats a change of this kind is very common and indeed +almost the rule. When extra digits appear at the inner (tibial) +side of the limb, they are shaped as digits of the other side, and +even the normal digit II (index) is usually converted into the +mirror-image of its normal self. The limb then develops a +new symmetry in itself. Nevertheless it is not easy to interpret +these facts as meaning that there has been some interruption +in the control which one side of the body exercises over the +other. The heredity of polydactylism is complex but there is +little doubt that the condition familiar in the Cat is a dominant. +In some human cases also the descent is that of a dominant, but +irregularities are so frequent that no general rule can yet be +perceived. The dominance of such a condition is an exception +to the principle that the less-divided is usually dominant to the +more-divided, a fact which probably should be interpreted as +meaning that divisions are of more than one kind.</p> + +<p class="indent">Among ordinary somatic divisions, whether of organs, +cells, or patterns of differentiation, the control of symmetry is usually +manifested. There is however one class of somatic differentiations +which are exceptionally interesting from the fact that they +may show a complete independence of such geometrical control. +The most familiar examples of these geometrically uncontrolled +Variations are to be seen in bud-sports. The normal differentiation +of the organs of a plant is arranged on a definite geometrical +<span class="pagenum"><a name="Page_54" id="Page_54">[Pg 54]</a></span> +system, which to those who have never given special +attention to such things before, will often seem surprisingly +precise. The arrangement of the leaves on uninjured, free-growing +shoots can generally be seen to follow a very definite +order, just as do the flowers or the parts of the flowers. If +however bud sports occur, then though the parts included in +the sports show all the geometrical peculiarities proper to the +sport-variety, yet the sporting-buds themselves are not related +to each other according to any geometrical plan.</p> + +<p class="indent">A very familiar illustration is provided by the +distribution of colour in those Carnations that are not self-coloured. +The pigment may, as in Picotees, be distributed peripherally with +great regularity to the edges of the petals; or, as in Bizarres and +Flakes, it may be scattered in radial sectors which show no +geometrical regularity. Now in this case the pigments are the +same in both types of flower, and the chemical factors concerned +in their production must surely be the same. The difference +must lie in the mechanical processes of distribution of the pigment. +In the Picotee we see the orderly differentiation which we +associate with normality; in the Bizarre we see the disorderly +differentiation characteristic of bud-sports. The distribution of +colour in this case lies outside the scheme of symmetry of the plant.</p> + +<p class="indent">Such a distribution is characteristic of bud-sports, +and of certain other differentiations in both plants and animals, which +I cannot on this occasion discuss. Now reflexion will show that +these facts have an intimate bearing on the mechanical problems +of heredity. For first in the bud-sports we are witnessing the +distribution of factors which distinguish genetic varieties. We +do not know the physical nature of those factors, but if we must +give them a name, I suppose we should call them "ferments" +exactly as Boyle did in 1666. He is discussing how it comes about +that a bud, budded on a stock, becomes a branch bearing the +fruit of its special kind. He notes that though the bud inserted +be "not so big oftentimes as a Pea," yet "whether by the help +of some peculiar kind of Strainer or by the Operation of some +powerful Ferment lodged in it, or by both these, or some other +<span class="pagenum"><a name="Page_55" id="Page_55">[Pg 55]</a></span> +cause," the sap is "so far changed as to constitute a Fruit quite +otherwise qualify'd."<a name="FNanchor_18_35" id="FNanchor_18_35"></a><a href="#Footnote_18_35" class="fnanchor">[18]</a> +We can add nothing to his speculation, +and we believe still that by a differential distribution of "ferments" +the sports are produced. All the factors are together +present in the normal parts; some are left out in the sport. +In an analogous case however, that of a variegated <i>Pelargonium</i> +which has green and also albino shoots, Baur proved that the +shoots pure in colour are also pure in their posterity. There +can be no doubt that the sports of Carnations, Azaleas, Chrysanthemums, +etc., would behave in the same way.</p> + +<p class="indent">The well-known Azaleas Perle de Ledeburg, President +Kerchove, and <i>Vervaeana</i> are familiar illustrations. Perle de +Ledeburg is predominantly white, but it has red streaks in some +of its flowers. It not very rarely gives off a self-red sport. This +is evidently due to the development of a bud in a red-bearing +area of the stem. The red in this plant is not under "geometrical +control." Many plants have white flowers with no markings, +but if the red markings are geometrically ordered differentiations, +no self-coloured sports are formed. The case of <i>Vervaeana</i> is a +good illustration of this proposition. It has white flowers with +red markings arranged in an orderly manner on the lower parts +of the petals, especially on the dorsal petals. This is one of the +Azaleas most liable to have red sports, and at first sight it might +seem that the sport represented the red of the central marks. +Examination however of a good many flowers shows that irregular +red streaks like those of Perle de Ledeburg occur, about as commonly +as in that variety. <i>Vervaeana</i> in fact is Perle de Ledeburg +with <i>definite</i> red markings added, and its red sports obviously are +those branches the germs of which came in a patch of the stem +bearing these red elements. That this is the true account is +rendered quite obvious by the fact that the red of the sport is a +colour somewhat different from that of the definite marks, and +that these marks are still present on the red ground of the sporting +flowers.</p> + +<p class="indent">It will be understood that these remarks apply to those +cases in which the production of sports is habitual or frequent, and +<span class="pagenum"><a name="Page_56" id="Page_56">[Pg 56]</a></span> +I imagine in all such examples it will be found that there are +indications of irregularity in the distribution of the differentiations +such as to justify the view that they are not under that +geometrical control which governs the normal differentiation +of the parts. The question next arises whether these considerations +apply also to the production of a bud-sport as a rare +exception, but by the nature of the case it is not possible to +say positively whether the appearance of an exceptional sport +is due to the unsuspected presence of a pre-existing fragment of +material having a special constitution, or to the origin, <i>de novo</i>, +of such a material. For instance one of the garden forms of +<i>Pelargonium</i> known as <i>altum</i> is liable perhaps once in some +hundreds of flowers to have one or two magenta petals. The +normal colour is a brilliant red; and as we may be fairly sure that +this red is recessive to magenta the interpretation would be +quite different according as the appearance of the magenta is +regarded as due to the presence of small areas endowed with +magentaness, or to the spontaneous generation of the factor +for that pigment. Either interpretation is possible on the facts, +but the view that the whole plant has in it scarce mosaic particles +of magenta seems on the whole more consistent with present knowledge.</p> + +<p class="indent">In <i>Pelargonium altum</i> the enzyme causing the magenta +colours must be distributed in very small areas, but a case in which the +magenta is similarly arranged in a much coarser patchwork +may be seen in the <i>Pelargonium</i> "Don Juan," which often bears +whole trusses or branches of red flowers upon plants having the +normal dominant magenta trusses. In most cases there is little +doubt that though the magenta flowered parts can "sport" to +red, the red parts could not produce the magenta flowers.</p> + +<p class="indent">The asymmetrical, or to speak more precisely, the disorderly, +mingling of the colours in the somatic parts is thus an indication +of a similarly disorderly mixing of the factors for those colours +in the germ-tissues, so that some of the gametes bear enough of +the colour-factors to make a self-coloured plant, while others +bear so little that the plant to which they give rise is a patchwork. +If this view is correct we may extend it so far as to consider +<span class="pagenum"><a name="Page_57" id="Page_57">[Pg 57]</a></span> +whether the fineness or coarseness of the mixture visible +in the flowers or leaves may not give an indication of the degree +to which the factors are subdivided among the germ-cells. We +know very little about the genetic properties of striped varieties. +In both <i>Antirrhinum</i> and <i>Mirabilis</i> it has been found that the +striped may occasionally and irregularly throw self-coloured +plants, and therefore the striping cannot be regarded simply as +a recessive character. On the other hand in <i>Primula Sinensis</i> +there are well-known flaked varieties which ordinarily at least +breed true. Whether these ever throw selfs I do not know, +but if they do it must be quite exceptionally. The power of +these flaked plants to breed true is, I suspect, connected with +the fact that in their flowers the coloured and white parts are +<i>intimately</i> mixed, this intimate mixture thus being an indication +of a similarly intimate mixture in the germ-cells. It would be +important to ascertain whether self-fertilised seed from the occasional +flowers in which the colour has run together to join a large patch gives +more self-coloured plants than the intimately flaked flowers do.</p> + +<p class="indent">The next fact may eventually prove of great importance. +We have seen that in bud-sports the differentiation is of the same +nature as that between pure types, and also that in the sporting +plant this differentiation is distributed without any reference +to the plant's axis, or any other consideration of symmetry. +Now among the germ-cells of a Mendelian hybrid exactly such +characters are being distributed allelomorphically, and there +again we have strong evidence for believing that the distribution +obeys no pattern. For example, we can in the case of seeds still +<i>in situ</i> perceive how the characters were distributed among the +germ-cells, and there is certainly no obvious pattern connecting +them, nor can we suppose that there is an actual pattern obscured.</p> + +<p class="indent">Of this one illustration is especially curious. Individual +plants of the same species are, as regards the decussations of +their leaves and in other respects, <i>either rights or lefts</i>. +The fact is not emphasized in modern botany and is in some danger of +being forgotten. When, as in the flowers of Arum, some <i>Gladioli</i>, +<i>Exacum</i>, <i>St. Paulia</i>, or the fruits of <i>Loasa</i>, rights and lefts occur +<span class="pagenum"><a name="Page_58" id="Page_58">[Pg 58]</a></span> +on the same stem, they come off alternately. But if, as in the +seedlings of Barley the twist of the first leaf be examined, it +will be seen to be either a right-or left-handed screw. An +ear of barley, say a two-row barley, is a definitely symmetrical +structure. The seeds stand in their envelopes back to back in +definite positions. Each has its organs placed in perfectly +definite places. <i>If these seeds were buds</i> their differentiations +would be grouped into a common plan. One might expect that +the differentiations of these embryos would still fall into the +pattern; but they do not, and so far as I have tested them, any +one may be a right or a left, just as each may carry any of the +Mendelian allelomorphs possessed by the parent plant, without +reference to the differentiation of any other seed. The fertilisation +may be responsible, but our experience of the allelomorphic +characters suggest that the irregularity is in the egg-cells +themselves.<a name="FNanchor_19_36" id="FNanchor_19_36"></a><a href="#Footnote_19_36" class="fnanchor">[19]</a> +</p> + +<p class="indent"><i>Germ cells thus differ from somatic cells in the fact +that their differentiations are outside the geometrical order which governs +the differentiation of the somatic cells.</i> I can think of possible +exceptions, but I have confidence that the rule is true and I +regard it as of great significance.</p> + +<p class="indent">The old riddle, what is an individual, finds at least a partial +solution in the reply that an individual is a group of parts differentiated +in a geometrically interdependent order. With the +germ-cell a new geometrical order, with independent polarity is +almost if not quite always, begun, and with this geometrical independence +the power of rejuvenescence may possibly be associated.</p> + +<p class="indent">The problems thus raised are unsolved, but they do +not look insoluble. The solution may be nearer than we have thought. +In a study of the geometry of differentiation, germinal and +somatic, there is a way of watching and perhaps analyzing what +may be distinguished as the mechanical phenomena of heredity. +If any one could in the cases of the Picotee and the Bizarre Carnation, +respectively, detect the real distinction between the two +<span class="pagenum"><a name="Page_59" id="Page_59">[Pg 59]</a></span> +types of distribution, he would make a most notable advance. +Any one acquainted with mechanical devices can construct a +model which will reproduce some of these distinctions more or less +faithfully. The point I would not lose sight of is that the analogy +with such models must for a long way be a true and valuable +guide. I trust that some one with the right intellectual equipment +will endeavor to follow this guide; and I am sanguine enough +to think that a comprehensive study of the geometrical phenomena +of differentiation will suggest to a penetrative mind that critical +experiment which may one day reveal the meaning of spontaneous +division, the mystery through which lies the road, perhaps the +most hopeful, to a knowledge of the nature of life.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_60" id="Page_60">[Pg 60]</a></span></p> +<h2>CHAPTER III</h2> +<h3><span class="smcap">Segmentation, Organic And Mechanical</span></h3> + +<p class="indent">Models may be and often have been devised imitating +some of the phenomena of division, but none of them have reproduced +the peculiarity which characterises divisions of living tissues, +that <i>the position of chemical differentiation</i> is <i>determined by +those divisions</i>. For example, models of segmentation, whether +radial or linear, may be made by the vibration of plates as in +the familiar Chladni figures of the physical laboratory, or by +the bowing of a tube dusted on the inside with lycopodium +powder, and in various other ways. The sand or the powder +will be heaped up in the nodes or regions of least movement, and +the patterns thus formed reproduce many of the geometrical +features of segmentation. But in the segmentations of living +things the nodes and internodes, once determined by the dividing +forces, would each become the seat of appropriate and distinct +chemical processes leading to the differentiation of the parts, +and the deposition of the bones, petals, spines, hairs, and other +organs in relation to the meristic ground-plan. The "ripples" of +meristic division not merely divide but differentiate, and when +a "ripple" forks the result is not merely a division but a reduplication +of the organ through which the fork runs. An +example illustrating such a consequence is that of the half-vertebrae +of the Python. On the left side the vertebra is single +(Fig. 7) and bears a single rib, but on the right side a division +has occurred with the result that two half-vertebrae, each +bearing a rib, are formed, one standing in succession to the +other. We cannot, indeed, imagine any operation of physiological +division carried out in such an organ as a vertebra, passing +through a plane at right angles to the long axis of the body, which +does not necessarily involve the further process of reduplication.</p> + +<p class="indent space-below">As the meristic system of distribution spreads through +the body, chemical differentiations follow in its track, with +<span class="pagenum"><a name="Page_61" id="Page_61">[Pg 61]</a></span> +segmentation and pattern as the visible result. Could we analyse +these simultaneous phenomena and show how it is that the places +of chemical differentiation are determined by the system of +division, progress would then be rapid. It is here that all +speculation fails.</p> + +<div class="figcenter" > + <img src="images/i_074.jpg" alt="" width="600" height="654" /> +</div> +<p class="space-below" style="font-size: 120%;"> +<b><span class="smcap">Figs.</span> 7 and 8.</b> Two examples of +imperfect division in the vertebræ of a python. <b><i>I</i></b>, the +vertebræ <b>147-150</b> from the right side, showing imperfect division between +the <b>148th</b> and <b>149th</b>. The condition on the left side of this +vertebra was the same. <b><i>II</i></b>, the dorsal surface of vertebræ +<b>165-167</b>. On the right side the <b>166th</b> is double and bears two ribs, +but on the left side it is normal and has one rib only.</p> + +<p class="indent">Many attempts have been made to interpret the processes +of division and repetition, in terms of mechanics, or at least to +refer them to their nearest mechanical analogies, so far with +<span class="pagenum"><a name="Page_62" id="Page_62">[Pg 62]</a></span> +little success. The problem is beset with difficulties as yet +insurmountable and of these one must be especially noticed. In +the living thing the process by which repetition and patterns +come into being consists partly in division but partly also in +growth. We have no means of studying the phenomena of +pattern-formation except in association with that of growth. +Growth soon ceases unless division takes place, and if growth is +impossible division soon ceases also. In consequence of this +fact that the final pattern is partly a product of growth, it can +never be used as unimpeachable evidence of the primary geometrical +relations of the members as laid down in the divisions.</p> + +<p class="indent">In the last chapter in referring to the problem of +repetition I introduced an analogy, comparing the patterns of the organic +world with those produced in unorganised materials by wave-motion. +In the preliminary stage of ignorance, having no more +trustworthy clue, I do not think it wholly unprofitable to consider +the applicability of this analogy somewhat more fully. It +possesses, as I hope to show, at least so much validity as to +encourage the belief that morphology may safely discard one +source of long-standing error and confusion.</p> + +<p class="indent">Those who have studied the structure of parts repeated +in series will have encountered the old morphological problem of +"Serial Homology," which has absorbed so much of the attention +of naturalists and especially of zoologists at various periods. +This problem includes two separate questions. The first of +these is the origin in evolution of the resemblance between two +organs occurring in a repeated series, of which the fore and hind +limbs of Vertebrates are the prerogative instance. From the +fact that these resemblances can be traced very far, often into +minute details of structure, many anatomists have inclined to +the opinion that the resemblance must originally have been still +more complete, and that the two limbs, for instance, must have +acquired their present forms by the differentiation of two identical +groups of parts.</p> + +<p class="indent">Similar questions arise whenever parts are repeated in +series, whether the series be linear or radial, and, though less obviously, +even when the repetition is bilateral only. In each such example +<span class="pagenum"><a name="Page_63" id="Page_63">[Pg 63]</a></span> +the question arises, is the resemblance between the parts the +remains of a still closer resemblance, or is differentiation original? +Sometimes the view that these parts have arisen by the differentiation +of a series of identical parts is plausible enough, +as for example when the peculiarities of various appendages of a +Decapod Crustacean are referred to modifications of the Phyllopod +series. In application to other cases however we soon meet +with difficulty, and the suggestion that the segments of a vertebrate +were originally all alike is seen at once to be absurd, for +the reason that a creature so constituted could not exist, and that, +differentiation of at least one anterior and one posterior segment, +is an essential condition of a viable organism consisting of parts +repeated in a linear series. Between these two terminal segments +it is possible to imagine the addition of one segment, or of a +series of approximately similar segments; but when once it is +realised that the terminals must have been differentiated from +the beginning, it will be seen that the problem of the origin of +the resemblance between segments is not rendered more comprehensible +by the suggestion that even the intervening members +were originally alike. Seeing indeed that some differentiation +must have existed primordially it is as easy to imagine that the +original body was composed of a series grading from the condition +of the anterior segment to that of the posterior, as any other +arrangement. The existence of a linear or successive series in +fact postulates a polarity of the whole, and in such a system the +conception of an ideal segment containing all the parts represented +in the others has manifestly no place. The introduction of that +conception though sanctioned by the great masters of comparative +anatomy, has, as I think, really delayed the progress of +a rational study of the phenomena of division. The same notion +has been applied to every class of repetition both in animals and +plants, generally with the same unhappy results. In the cruder +forms in which this doctrine was taught thirty years ago it is +now seldom expressed, but modified presentations of it still +survive and confuse our judgments. +<span class="pagenum"><a name="Page_64" id="Page_64">[Pg 64]</a></span></p> + +<p class="indent space-below">The process of repetition of parts in the bodies +of organisms is however a periodic phenomenon. This much, provided we +remain free from prejudice as to the nature and causation of the +period or rhythm, we may safely declare, and a comparison may +thus be instituted between the consequences of meristic repetition +in the bodies of living things and those repetitions which in +the inorganic world are due to rhythmical processes. Of such +processes there is a practically unlimited diversity and we have +nothing to indicate with which of them our repetitions should +rather be compared.</p> + +<div class="figcenter" > + <img src="images/i_077.jpg" alt="" width="600" height="665" /> +</div> +<p class="space-below" style="font-size: 120%;"> +<b><span class="smcap">Fig. 9.</span></b>  Osmotic growths simulating segmentation. (After Leduc.)</p> + +<p><span class="pagenum"><a name="Page_65" id="Page_65">[Pg 65]</a></span></p> + +<p class="indent">In some respects perhaps the best models of living organisms +yet made are the "osmotic growths" produced by Leduc.<a name="FNanchor_1_37" id="FNanchor_1_37"></a><a href="#Footnote_1_37" class="fnanchor">[1]</a> +These curious structures were formed by placing a fragment of +a salt, for instance calcium chloride, in a solution of some colloidal +substance. As the solid takes up water from the solution +a permeable pellicle or membrane is formed around it. The vesicle +thus enclosed grows by further absorption of water, often +extending in a linear direction, and in many examples this growth +occurs by a series of rhythmically interrupted extensions. Some +of the growths thus formed are remarkably like organic structures, +and might pass for a series of antennary segments or many other +organs consisting of a linear series of repeated parts. In admitting +the essential resemblance between these "osmotic growths" +and living bodies or their organs I lay less stress on the general +conformation of the growths, which often as Leduc points out, recall +the forms of fungi or hydroids, but rather on the fact that the +interruptions in the development of these systems are so closely +analogous to the segmentations or repetitions of parts characteristic +of living things (Fig. 9). In the same way I am less impressed +by Leduc's models of Karyokinesis, wonderful as they +nevertheless are, for the division is here imitated by putting +separate drops on the gelatine film. What we most want to know +is how in the living creature one drop becomes two. The models +of linear segmentation have the remarkable merit that they do in +some measure imitate the process of actual division or repetition. +So in a somewhat modified method Leduc, by causing the diffusion +of a solution in a gelatine film, produced rhythmical or periodic +precipitations strikingly reminiscent of various organic tissues, +for here also the process of periodic repetition is imitated with +success.</p> + +<p class="indent">It is a feature common to these and to all other +rhythmical repetitions produced by purely mechanical forces that there +is resemblance between the members of the series, and that this +similarity of conformation may be maintained in most complex +detail. When however in the mechanical series some of the +members differ from the rest we have no difficulty in recognising +<span class="pagenum"><a name="Page_66" id="Page_66">[Pg 66]</a></span> +that these differences—which correspond with the differentiations +of the organic series—are due to special heterogeneity in +the conditions or in the materials, and it never occurs to us to +suppose that all the members must have been primordially alike. +For example, in the case of ripple-marks on the sand, which I +choose as one of the most familiar and obvious illustrations +of a repeated series due to mechanical agencies, if we notice +one ripple different in form from those adjacent to it, we do +not suppose that this variation must have been brought about by +deformation of a ripple which was at first formed like the others, +but we ascribe it to a difference in the sand at that point, or to a +difference in the way in which the wind or the tide dealt with it. +We may press the analogy further by observing that in as much +as such a series of waves has a beginning and an end, it possesses +polarity like that of the various linear series of parts in organisms, +and even the formation of each member must influence the +shape of its successor. Since in an organism the beginning and +end of the series are always included, some differentiation among +the repetitions must be inevitable. If therefore it be conceded, +as I think it must, that segmentation and pattern are the consequence +of a periodic process we realize that it is at least as +easy to imagine the formation of such a series of parts having +family likeness combined with differentiation as it would be to +conceive of their arising primordially as a series of identical repetitions. +The suggestion that the likenesses which we now perceive +are the remains of a still more complete resemblance +is a substitution of a more complex conception for a simpler one.</p> + +<p class="indent">The other question raised by the problem of Serial +Homology is how far there is a correspondence between individual members +of series when the series differ from each other either in the +number of parts, or in the mode of distribution of differentiation +among them. Students, for example, of vertebrate morphology +debate whether the <i>n</i>th vertebra which carries the pelvic girdle +in Lizard A is individually homologous with the <i>n</i> + <i>x</i>th vertebra +which fulfils this function in Lizard B, or whether it is not more +truly homologous with the vertebra standing in the <i>n</i>th ordinal +position, though that vertebra in Lizard B is free. +<span class="pagenum"><a name="Page_67" id="Page_67">[Pg 67]</a></span></p> + +<p class="indent">In various and more complex aspects the same question +is debated in regard to the cranial and spinal nerves, the branches +of the aorta, the appendages of Arthropoda, and indeed in regard +to all such series of differentiated parts in linear or successive +repetition. Persons exercised with these problems +should before making up their minds consider how similar +questions would be answered in the case of any series of rhythmical +repetitions formed by mechanical agencies. In the case +of our illustration of the ripples in the sand, given the same forces +acting on the same materials in the same area, the number of +ripples produced will be the same, and the <i>n</i>th ripple counting +from the end of the series will stand in the same place whenever +the series is evoked. If any of the conditions be changed, the +number and shapes can be changed too, and a fresh "distribution +of differentiation" created. Stated in this form it is evident +that the considerations which would guide the judgment in the +case of the sand ripples are not essentially different from those +which govern the problem of individual homology in its application +to vertebrae, nerves, or digits.</p> + +<p class="indent">The fact that the unit of repetition is also the unit +of growth is the source of the obscurity which veils the process. When we +compare the skeleton of a long-tailed monkey with that of a +short-tailed or tailless ape we see at once how readily the additional +series of caudal segments may be described as a consequence +of the propagation of the "waves" of segmentation +beyond the point where they die out in the shorter column, and +we see that with an extension of the series of repetitions there is +growth and extension of material.</p> + +<p class="indent">The considerations which apply to this example will +be found operating in many cases of the variation of terminal members of +linear series. Some of these series, like the teeth of the dog, +end in a terminal member of a size greatly reduced below that of +the next to it. Even when there is thus a definite specialisation +of the last member of the series it not infrequently happens that +the addition, by variation, of a member beyond the normal +terminal, is accompanied by a very palpable increase in size of +the member which stands numerically in the place of the normal +<span class="pagenum"><a name="Page_68" id="Page_68">[Pg 68]</a></span> +terminal.<a name="FNanchor_2_38" id="FNanchor_2_38"></a><a href="#Footnote_2_38" class="fnanchor">[2]</a> +So also with variation in the number of ribs, when a +lumbar vertebra varies homoeotically into the likeness of the +last dorsal and bears a rib, the rib placed next in front of this, +which in the normal trunk is the last, shows a definite increase +in development.</p> + +<p class="indent">The consequences of such homoeoses are sometimes +very extensive, involving readjustments of differentiation affecting +a long series of members, as may easily be seen by comparing +the vertebral columns of several individual Sloths<a name="FNanchor_3_39" id="FNanchor_3_39"></a><a href="#Footnote_3_39" class="fnanchor">[3]</a> +(whether <i>Bradypus</i> or <i>Choloepus</i>) to take a specially striking example.</p> + +<p class="indent">It may be urged that no feature as yet enables us +to perceive wherein lies the primary distinction which determines such +variation, whether it is due to a difference in the dividing forces +or in the material to be divided. If for instance we were to +imitate such a series of segments by pressing hanging drops of +a viscous fluid out of a paint-tube by successive squeezes, the +number of times the tube is contracted before it is empty will give +the number of the segments, but their size may depend either +on the force of the contractions or on the capacity of the tube, +or on various other factors. Nevertheless in the case of the +variation of terminal members, whatever be the nature of the +rhythmical impulse which produces the series of organs, the elevation +of the normally terminal member in correspondence with +the addition of another is what we should expect.</p> + +<p class="indent">If the organism acquired its full size first and the +delimitation of the parts took place afterwards, there might be some hope +that the resemblance between living patterns and those mechanically +caused by wave-motion might be shown to be a consequence of +some real similarity of causation, but in view of the part played +by growth, appeal to these mechanical phenomena cannot be +declared to have more than illustrative value. Similarly in as +much as living patterns appear, and almost certainly do in reality +come into existence by a rhythmical process, comparisons of +these patterns with those developed in crystalline structures, and +in the various fields of force are, as it seems to me, inadmissible, +or at least inappropriate. +<span class="pagenum"><a name="Page_69" id="Page_69">[Pg 69]</a></span></p> + +<p class="indent">However their intermittence be determined, the +rhythms of division must be looked upon as the immediate source of +those geometrically ordered repetitions universally characteristic of +organic life. In the same category we may thus group the segmentation +of the Vertebrates and of the Arthropods, the concentric +growth of the Lamellibranch shells or of Fishes' scales, the +ripples on the horns of a goat, or the skeletons of the Foraminifera +or of the Heliozoa. In the case of plant-structures Church<a name="FNanchor_4_40" id="FNanchor_4_40"></a><a href="#Footnote_4_40" class="fnanchor">[4]</a> +has admirably shown, with an abundance of detail, how on analysis +the definiteness of phyllotaxis is an expression of such rhythm in +the division of the apical tissues, and how the spirals and "orthostichies" +displayed in the grown plant are its ultimate consequences. +The problem thus narrows itself down to the question +of the mode whereby these rhythms are determined.</p> + +<p class="indent">It is natural that we should incline to refer them to a +chemical source. If we think of the illustration just given, of the +segmentation of a viscous fluid into drops by successive contractions +of a soft-walled tube we can, I think, conceive of such rhythmic +contractions as due to summations of chemical stimuli, somewhat +as are the beats of the heart. But when we recognize the vast +diversity of materials the distribution of which is determined by +an ostensibly similar rhythmic process it seems hopeless to look +forward to a directly chemical solution. That the chemical +degradation of protoplasm or of materials which it contains is +the source of the energy used in the divisions cannot be in dispute, +but that these divisions can be themselves the manifestations +of chemical action seems in the highest degree improbable.</p> + +<p class="indent">We may therefore insist with some confidence on the +distinction between the Meristic and the substantive constitution +of organisms, between, that is to say, the system according to +which the materials are divided and the essential composition +of the materials, conscious of the fact that the energy of division +is supplied from the materials, and that in the ontogeny the +manner in which the divisions are effected must depend secondarily +on the nature of the substances to be divided. The +<span class="pagenum"><a name="Page_70" id="Page_70">[Pg 70]</a></span> +mechanical processes of division remain a distinguishable group of +phenomena, and variations in the substances to be distributed in +division may be independent of variations in the system by which +the distribution is effected.</p> + +<p class="indent space-below">Modern genetic analysis supplies many remarkable +examples of this distinction. When formerly we compared the leaves of a +normal palmatifid Chinese Primula with the pinnatifid leaves<a name="FNanchor_5_41" id="FNanchor_5_41"></a><a href="#Footnote_5_41" class="fnanchor">[5]</a> +of its fern-leaved variety we were quite unable to say whether the +difference between the two types of leaf was due to a difference +in the material cut up in the process of division or to a difference +in that process itself. Knowledge that the distinction is determined +by a single segregable factor tends to prove that the +critical difference is one of substance. So also in the Silky fowl +we know that the condition of its feathers is due to the absence +of some one factor present in the normal form. We may conceive +such differences as due to change of form in the successive +"waves" of division, but we cannot yet imagine segregation +otherwise than as acting by the removal or retention of a material +element. Future observation by some novel method may suggest +some other possibility, but such cases bring before us very clearly +the difficulties by which the problem is beset.</p> + +<div class="figcenter" > + <img src="images/i_084.jpg" alt="Primula Sinensis Leaves" width="600" height="389" /> +</div> +<p class="space-below" style="font-size: 120%; text-align: center;"> +<b><span class="smcap">Fig. 10.</span></b> The palm-and fern type of leaf in +<i>Primula Sinensis</i>.<br />   The palm is dominant and the fern is recessive.</p> + +<p class="indent">In another region of observation phenomena occur which +as it seems to me put it beyond question that the meristic forces are +essentially independent of the materials upon which they act, +save, in the remoter sense, in so far as these materials are the +sources of energy. The physiology of those regenerations and +repetitions which follow upon mutilation supplies a group of +facts which both stimulate and limit speculation. No satisfactory +interpretations of these extraordinary occurrences has +ever been found, but we already know enough to feel sure that +in them we are witnessing indications which should lead to the +discovery of the true mechanics of repetition and pattern. +The consequences of mutilation in causing new growth or perhaps +more strictly in enabling new growth to take place, are such that +they cannot be interpreted as responses to chemical stimuli in +<span class="pagenum"><a name="Page_71" id="Page_71">[Pg 71]</a></span> +any sense which the word chemical at present connotes. Powers +are released by mutilation of which in the normal conditions of +life no sign can be detected. All who have tried to analyse the +phenomena of regeneration are compelled to have recourse to the +metaphor of equilibrium, speaking of the normal body as in a state +of strain or tension (Morgan) which when disturbed by mutilation +results in new division and growth. The forces of division +are inacessible to ordinary means of stimulation. Applications, +for example, of heat or of electricity excite no responses of a +positive kind unless the stimuli are so violent as to bring about +actual destruction.<a name="FNanchor_6_42" id="FNanchor_6_42"></a><a href="#Footnote_6_42" class="fnanchor">[6]</a> +These agents do not, to use a loose expression, +come into touch with the meristic forces. Changes in the +chemical environment of cells may, as in the experiments of +Loeb and of Stockard produce definite effects, but the facts +suggest that these effects are due rather to alterations in the +living material than to influence exerted directly on the forces +of division themselves.</p> + +<p class="indent">By destruction of tissue however the forces both of +growth and of division also may often be called into action with a +resulting regeneration. Interruption of the solid connexion +between the parts may produce the same effects, as for example +when the new heads or tails grow on the divided edges of Planarians +(Morgan), or when from each half embryo partially separated +from its normally corresponding half, a new half is formed +with a twin monster as the result.</p> + +<p class="indent">Often classed with regenerations but in reality +quite distinct from them are those special and most interesting examples +where the growth of a <i>paired</i> structure is excited by a simple +<span class="pagenum"><a name="Page_72" id="Page_72">[Pg 72]</a></span> +wound. Some of the best known of these instances are presented +by the paired extra appendages of Insects and Crustacea. Some +years ago I made an examination of all the examples of such +monstrosities to which access was to be obtained, and it was with +no ordinary feeling of excitement that I found that these supernumerary +structures were commonly disposed on a recognizable +geometrical plan, having definite spatial relations both to each +other and to the normal limb from which they grew. The more +recent researches of Tornier<a name="FNanchor_7_43" id="FNanchor_7_43"></a><a href="#Footnote_7_43" class="fnanchor">[7]</a> +and especially his experiments on +the Frog have shown that a cut into the posterior limb-bud +induces the outgrowth of such a <i>pair</i> of limbs at the wounded +place. Few observations can compare with this in novelty or +significance; and though we cannot yet interpret these phenomena +or place them in their proper relations with normal occurrences, +we feel convinced that here is an observation which is no mere +isolated curiosity but a discovery destined to throw a new light +on biological mechanics. The supernumerary legs of the Frog +are evidently grouped in a system of symmetry similar to that +which those of the Arthropods exhibit, and though in Arthropods +paired repetitions have not been actually produced by injury +under experimental conditions we need now have no hesitation +in referring them to these causes as Przibram has done.</p> + +<p class="indent">At this point some of the special features of +the supernumerary appendages become important. First they may arise +at any point on the normal limb, being found in all situations +from the base to the apex. Nor are they limited as to the surface +from which they spring, arising sometimes from the dorsal, +anterior, ventral, or posterior surfaces, or at points intermediate +between these principal surfaces.</p> + +<p class="indent space-below">With rare and dubious exceptions, the parts which are +contained in these extra appendages are only those which lie <i>peripheral +to their point of origin</i>. Thus when the point of origin is +in the apical joint of the tarsus, the extra growth if completely +developed consists of a double tarsal apex bearing two pairs +of claws. If they arise from the tibia, two complete tarsi are +<span class="pagenum"><a name="Page_73" id="Page_73">[Pg 73]</a></span> +added. If they spring from the actual base of the appendage +then two complete appendages may be developed in addition to +the normal one. We must therefore conclude that in any point +on a normal appendage the power exists which, if released, may +produce a bud containing in it a paired set of the parts peripheral +to this point.</p> + +<div class="figcenter" > + <img src="images/i_088.jpg" alt="Arthropoda" width="600" height="590" /> +</div> +<p class="space-below" style="font-size: 120%;"> +<b><span class="smcap">Fig. 11.</span></b> Diagrams of +the geometrical relations which are generally exhibited by extra pairs of +appendages in Arthropoda. The sections are supposed to be those of the apex +of a tibia in a beetle. <b><i>A</i></b>, anterior, <b><i>P</i></b>, posterior, <b><i>D</i></b>, +dorsal, <b><i>V</i></b>, ventral. <b><i>M</i><sup>1</sup></b>, <b><i>M</i><sup>2</sup></b> are +the imaginary planes of reflexion. The shaded figure is in each case a limb +formed like that of the other side of the body, and the outer unshaded figures +are shaped like the normal for the side on which the appendages are. On the +several radii are shown the extra pairs in their several possible relations to +the normal from which they arise. The normal is drawn in thick lines in the center.</p> + +<p class="indent">Next the geometrical relations of the halves of the +supernumerary pair are determined by the position in which they stand +<span class="pagenum"><a name="Page_74" id="Page_74">[Pg 74]</a></span> +in regard to the original appendage. These relations are best +explained by the diagram (Fig. 11), from which it will be seen +that the two supernumerary appendages stand as images of each +other; and, of them, that which is adjacent to the normal appendage +forms an image of it. Thus if the supernumerary pair +arise from a point on the dorsal surface of the normal appendage, +the two <i>ventral</i> surfaces of the extra pair will face each other. +If they arise on the anterior surface of the normal appendage, +their morphologically posterior surfaces will be adjacent, and so on.</p> + +<p class="indent">These facts give us a view of the relations of the two +halves of a dividing bud very different from that which is to be derived +from the exclusive study of normal structures. Ordinary morphological +conceptions no longer apply. The distribution of the +parts shows that the bud or rudiment which becomes the supernumerary +pair may break or open out in various ways according +to its relations to the normal limb. Its planes of division are +decided by its geometrical relations to the normal body.</p> + +<p class="indent">Especially curious are some of the cases in which +the extra pair are imperfectly formed. The appearance produced is then +that of two limbs in various stages of coalescence, though in +reality of course they are stages of imperfect separation. The +plane of "coalescence" may fall anywhere, and the two appendages +may thus be compounded with each other much as an +object partially immersed in mercury "compounds" with its +optical image reflected from the surface.</p> + +<p class="indent">Supernumerary paired structures are not usually, if +ever, formed when an appendage is simply amputated. Cases occasionally +are seen which nevertheless seem to be of this nature. +Borradaile,<a name="FNanchor_8_44" id="FNanchor_8_44"></a><a href="#Footnote_8_44" class="fnanchor">[8]</a> +for example, described a crab (<i>Cancer pagurus</i>) +having in place of the right chela three <i>small</i> chelae arising from +a common base, where the appearances suggested that the three +reduced limbs replaced a single normal limb. From the details reported +however it seems still possible that one of the chelae +(that lettered F. I in Borradaile's figure) may be the normal +one, and the other two an extra pair. The chela which I suspect +to be the normal is in several respects deformed as well as being +<span class="pagenum"><a name="Page_75" id="Page_75">[Pg 75]</a></span> +reduced in size, and this deformity may perhaps have ensued as +a consequence of the same wound which excited the growth of the +extra pair. Its reduced size may be due to the same injury, +which may quite well have checked its growth to full proportions.</p> + +<p class="indent">Admitting doubt in these ambiguous cases it seems to +be a general rule that for the production of the extra pair the normal +limb should persist in connexion with the body. Moreover it is +practically certain that in no case can a <i>single</i>, viz. an unpaired, +duplicate of the normal appendage grow from it. Many examples +have been described as of this nature, but all of them may be with +confidence regarded as instances of a supernumerary pair in +which only the two morphologically anterior or the two morphologically +posterior surfaces are developed. We have thus +the paradox that a limb of one side of the body, say the right, +has in it the power to form a pair of limbs, right and left, as an +outgrowth of itself, but cannot form a second left limb alone.</p> + +<p class="indent">A very interesting question arises whether it is strictly +correct to describe the extra pair as a right and a left, or whether +they are not rather two lefts or two rights of which one is reversed. +This question did not occur to me when in former years I studied +these subjects. It was suggested to me by Dr. Przibram. +The answer might have an important bearing on biological +mechanics, but I know no evidence from which the point can +be determined with certainty. In order to decide this question +it would be necessary to have cases in which the paired repetition +affected a limb markedly differentiated on the two sides of the +body, and of course the development of the extra parts in order +to be decisive must be fairly complete. One example only is +known to me which at all satisfies these requirements, that of the +lobster's chela figured (after Van Beneden) in <i>Materials for the +Study of Variation</i>, p. 531, Fig. 184, III.</p> + +<p class="indent space-below">Here the drawing distinctly suggests that one of +the extra dactylopodites, namely that lettered R, is differentiated +as a left and not merely a reversed right. For the teeth on this +dactylopodite are those of a cutting claw, not of a crushing claw, +whereas the dactylopodites R' and L' bear crushing teeth. The +figure makes it fairly certain also that the limb affected was a +<span class="pagenum"><a name="Page_76" id="Page_76">[Pg 76]</a></span> +crushing claw. Accepting this interpretation, we reach the +remarkable conclusion that the bud of new growth consisted of +halves differentiated into cutter and crusher as the normal claws +are, and that the extra crusher is geometrically a left but physiologically +a right. Though shaped as a left in respect of the +direction in which it points, the extra crusher is really an optically +reversed right, while the dactylopodite R, which is +placed pointing like a right, is really a reversed left (Fig. 12).</p> + +<div class="figcenter" > + <img src="images/i_091.jpg" alt="Lobster Claw" width="600" height="550" /> +</div> +<p class="space-below" style="font-size: 120%;"> +<b><span class="smcap">Fig. 12.</span></b> Right claw of lobster bearing +a pair of extra dactylopodites (after van Beneden). The fine toothing on R suggests +that this is part of a cutting claw, though the limb bearing it is a crusher.</p> + +<p class="indent">If these indications are reliable<a name="FNanchor_9_45" id="FNanchor_9_45"></a><a href="#Footnote_9_45" class="fnanchor">[9]</a> +and are established by further +observation we shall be led to the conclusion that the bud which +becomes an extra pair of limbs does not merely contain the parts +proper to the side on which it grows, but is comparable with +the original zygotic cell, and consists not simply of two halves, +but of two halves differentiated as a right and a left like the two +halves of the normal body.</p> + +<p class="indent">Phenomena of this kind, evoked by mutilation or +injury, together with the cognate observations on regeneration throw +<span class="pagenum"><a name="Page_77" id="Page_77">[Pg 77]</a></span> +very curious lights on the nature of living things. To an understanding +of the nature of the mechanics of living matter and its +relation to matter at large they offer the most hopeful line of +approach. I allude especially to the examples in which it has +been established that the part which is produced after mutilation +is a structure different from that which was removed. The +term "regeneration" was introduced before such phenomena +were discovered, and though every one recognizes its inapplicability +to these remarkable cases, the word still misleads us by +presenting a wrong picture to the mind. The expression "heteromorphosis" +(Loeb) has been appropriately applied to various +phenomena of this kind, and Morgan has given the name "morphallaxis" +to another group of cases in which the renewal occurs +by the transformation of a previously existing part.<a name="FNanchor_10_46" id="FNanchor_10_46"></a><a href="#Footnote_10_46" class="fnanchor">[10]</a> +But we must continually remember that all these occurrences which +we know only as abnormalities and curiosities must in reality be +exemplifications of the normal mechanics of division and growth. +The conditions needed to call them forth are abnormal, but the +responses which the system makes are evidences of its normal +constitution. When therefore, for example, the posterior end +of a worm produces a reversed tail from its cut end we have a +proof that there must be in the normal body forces ready to +cause this outgrowth. The new structure is not an ill-shaped +head-end, for, as Morgan shows, the nephridial ducts have their +funnels perforating the segments in a reversed direction. The +"tension" of growth is actually reversed.<a name="FNanchor_11_47" id="FNanchor_11_47"></a><a href="#Footnote_11_47" class="fnanchor">[11]</a> +So also when in a Planarian amputation of the body immediately behind +the head leads to the formation of a new reversed head at the back of +the normal head, while amputation further back leads to the +regeneration of a new tail, these responses give indications +of forces normally present in the body of the Planarian. Such +facts open up a great field of speculation and research. Especially +important it would be to determine where the critical +region may be at which the one response is replaced by the +<span class="pagenum"><a name="Page_78" id="Page_78">[Pg 78]</a></span> +other. I suppose it is even possible that there is some neutral +zone in which neither kind of response is made.</p> + +<p class="indent">Physical parallels to the phenomena of regeneration are +not easy to find and we still cannot penetrate beyond the empirical +facts. Przibram has laid stress on the general resemblance +between the new growth of an amputated part in an animal and +the way in which a broken crystal repairs itself when placed in +the mother-solution. That the two processes have interesting +points of likeness cannot be denied. It must however never be +forgotten that there is one feature strongly distinguishing the +two; for I believe it is universally recognized by physicists that +all the phenomena of geometrical regularity which crystals +display are ultimately dependent on the forms of the particles +of the crystalline body. This cannot in any sense be supposed +to hold in regard to protoplasm or its constituents. The definiteness +of crystals is also an unlikely guide for the reason that +it is absolute and perfect, or in other words because this kind of +regularity cannot be disturbed at all without a change so great +that the substance itself is altered; whereas we know that the +forms of living things are capable of such changes, great and small, +that we must regard perfection of form, whether manifested in +symmetry or in number, as an ideal which will only be produced +in the absence of disturbance. The symmetry of the living +things is like the symmetry of the concentric waves in a pool +caused by a splash. Perfect circles are made only in the imaginary +case of mathematical uniformity, but the system maintains +an approximate symmetry though liable to manifold deformation.</p> + +<p class="indent">Since the geometrical order of the living body cannot +be a direct function of the materials it must be referred to some more +proximate control. In renewing a part the body must possess +the power of seizing particles of many dissimilar kinds, and whirl +them into their several and proper places. The action in renewal, +like that of original growth, may be compared—very +crudely—with the action of a separator which simultaneously +distributes a variety of heterogeneous materials in an orderly +fashion; but in the living body the thing distributed must rather +be the <i>appetency</i> for special materials, not the materials themselves. +<span class="pagenum"><a name="Page_79" id="Page_79">[Pg 79]</a></span></p> + +<p class="indent">If the analogy of crystals be set aside and we seek +for other parallels to regeneration there are none very obvious. I have +sometimes wondered whether it might not be possible to institute +a fruitful comparison between the renewal of parts and the reformation +of waves of certain classes after obliteration. In several +respects, as I have already said, some curious resemblances with +the repetitions formed by wave-motion are to be traced in our +organic phenomena, and though admitting that I cannot develop +these comparisons, I think nevertheless they may be worth +bearing in mind. When, after obliteration, an eddy in a stream, +or a ripple-mark (a more complex case of eddy-formation) in +blown sand is re-formed, we have an example in which pattern is +reconstituted and growth takes place not by virtue of the composition +of the materials—in this case the water or the sand—but +by the way in which they are acted upon by extraneous forces.</p> + +<p class="indent">A feature in the actual mode by which ripple-marks +are reconstituted may not be without interest in connexion with +our phenomena of regeneration. When, for example, the wind +is blowing steadily over a surface of fine, dry sand, the familiar +ripple-marks are formed by a heaping of the sand in lines transverse +to the direction of the wind. The heaping is due to the +formation of eddies corresponding with positions of instability. +When the wind is steady and the sand homogeneous, the distances +between the ripples, or wave-lengths, are sensibly equal. +If while the wind continues to blow, the ripples are obliterated +with a soft brush they will quickly be re-formed over the whole +area, but I have noticed that at first their wave-length is approximately +half that of the ripples in the undisturbed parts of +the system.<a name="FNanchor_12_48" id="FNanchor_12_48"></a><a href="#Footnote_12_48" class="fnanchor">[12]</a> +The normal wave-length is restored by the gradual +accentuation of alternate ripples. Of course the sand-ripples are +in reality slowly travelling forward in the direction towards +which the wind is blowing, and for this our living segmentations +afford no obvious parallel, but the appearances in the area of +<span class="pagenum"><a name="Page_80" id="Page_80">[Pg 80]</a></span> +reformation, and especially the forking of the old ridges where +they join the new ones, are curiously reminiscent of the irregularities +of segmentation seen in regenerated structures. The +value of the considerations adduced in the chapter is, I admit, +very small. The utmost that can be claimed for them is that +mechanical segmentations, like those seen in ripple-mark, or +in Leduc's osmotic growths, show how by the action of a continuous +force in one direction, repeated and serially homologous +divisions can be produced having features of similarity common +to those repetitions by which organic forms and patterns are +characterised. The analogy supplies a vicarious picture of the +phenomena which in default of one more true may in a slight +degree assist our thoughts. It suggests that the rhythms of +segmentation may be the consequence of a single force definite +in direction and continuously acting during the time of growth. +The polarity of the organism would thus be the expression of +the fact that this meristic force is definitely directed after it has +once been excited, and the reversal seen in some products of regeneration +suggest further that it is capable of being reflected. +This polarity cannot be a property of the material, as such, +but is determined by a force acting on that material, just as the +polarity of a magnet is not determined by the arrangement of its +particles, but by the direction in which the current flows.</p> + +<p class="indent">To some it may appear that even to embark on such discussions +as this is to enter into a perilous flirtation with vitalistic theories. +How, they may ask, can any force competent to produce chemical +and geometrical differentiation in the body be distinguished +from the "Entelechy" of Driesch? Let me admit that in this +reflexion there is one element of truth. If those who proclaim +a vitalistic faith intend thereby to affirm that in the processes +by which growth and division are effected in the body, a part is +played by an orderly force which we cannot <i>now</i> translate into +terms of any known mechanics, what observant man is not a +vitalist? Driesch's first volume, putting as it does into intelligible +language that positive deduction from the facts—especially +of regeneration—should carry a vivid realisation of this +truth to any mind. If after their existence is realised, it is +<span class="pagenum"><a name="Page_81" id="Page_81">[Pg 81]</a></span> +desired that these unknown forces of order should have a name, +and the word entelechy is proposed, the only objection I have to +make is that the adoption of a term from Aristotelian philosophy +carries a plain hint that we propose to relegate the future study +of the problem to metaphysic.</p> + +<p class="indent">From this implication the vitalist does not shrink. +But I cannot find in the facts yet known to us any justification of so +hopeless a course. It was but yesterday that the study of +<i>Entwicklungsmechanik</i> was begun, and if in our slight survey +we have not yet seen how the living machine is to be expressed +in terms of natural knowledge that is poor cause for despair. +Driesch sums up his argument thus:<a name="FNanchor_13_49" id="FNanchor_13_49"></a><a href="#Footnote_13_49" class="fnanchor">[13]</a> +</p> + +<p class="blockquot">"It seems to me that there is only one conclusion possible. +If we are going to explain what happens in our harmonious-equipotential +systems by the aid of causality based upon the +constellation of single chemical factors and events, there <i>must</i> +be some such thing as a machine. Now the assumption of the +existence of a machine proves to be absolutely absurd in the +light of the experimental facts. <i>Therefore there can be neither +any sort of a machine nor any sort of causality based upon constellation +underlying the differentiation of harmonious-equipotential systems.</i>"</p> + +<p class="blockquot">"For a machine, typical with regard to the three chief dimensions +of space, cannot remain itself if you remove parts of it +or if you rearrange its parts at will."</p> + +<p>To the last clause a note is added as follows:</p> + +<p class="blockquot">"The pressure experiments and the dislocation experiments +come into account here; for the sake of simplicity they have +not been alluded to in the main line of our argument."</p> + +<p class="indent">I doubt whether any man has sufficient knowledge of +all possible machines to give reality to this statement. In spite also +of the astonishing results of experiments in dislocation, doubt +may further be expressed as to whether they have been tried in +such variety or on such a scale as to justify the suggestion that +the living organism remains itself if its parts are rearranged at +<span class="pagenum"><a name="Page_82" id="Page_82">[Pg 82]</a></span> +will. All we know is that it can "remain itself" when much is +removed, and when much rearrangement has been affected, +which is a different thing altogether.</p> + +<p class="indent">I scarcely like to venture into a region of which my +ignorance is so profound, but remembering the powers of eddies to re-form +after partial obliteration or disturbance, I almost wonder whether +they are not essentially machines which remain themselves +when parts of them are removed.</p> + +<p class="indent">Real progress in this most obscure province is not likely +to be made till it attracts the attention of physicists; and though they +for long may have to forego the application of exact quantitative +methods, I confidently anticipate that careful comparison +between the phenomena of repetition formed in living organisms +and the various kinds of segmentation produced by mechanical +agencies would be productive of illuminating discoveries.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_83" id="Page_83">[Pg 83]</a></span></p> + +<h2>CHAPTER IV</h2> +<h3><span class="smcap">The Classification Of Variation And<br />The Nature Of Substantive Factors</span></h3> + +<p class="indent">We have now seen that among the normal physiological +processes the phenomena of division form a recognisable, and +in all likelihood a naturally distinct group. Variations in these +respects may thus be regarded as constituting a special class +among variations in general.</p> + +<p class="indent">The substantive variations have only one property in +common—the negative one that they are not Meristic. The work +of classifying them and distinguishing them according to their +several types demands a knowledge of the chemistry of life far +higher than that to which science has yet attained. In reference +to some of the simplest variations Garrod has introduced the +appropriate term "Chemical sports." The condition in man +known as Alkaptonuria in which the urine is red is due especially +to the absence of the enzyme which decomposes the excretory +substance, alkapton. The "chemical sport" here consists in the +inability to break up the benzene ring. The chemical feature +which distinguishes and is the proximate cause of several colour-varieties +can now in a few cases be declared. The work of Miss +Wheldale has shown that colour-varieties may be produced by +the absence of the chromogen compound the oxidation of which +gives rise to sap-colours, by differences in the completeness of +this process of oxidation, and by a process of reduction supervening +on or perhaps suppressing the oxidation. Some of these +processes moreover may be brought about by the combined action +of two bodies, the one an enzyme, for example an oxygenase, and +the other a substance regarded as a peroxide, contributing the +oxygen necessary for the oxidation to take place. Variation in +colour may thus be brought about by the addition or omission +of any one of the bodies concerned in the action. +<span class="pagenum"><a name="Page_84" id="Page_84">[Pg 84]</a></span></p> + +<p class="indent">Similar variations, or rather similar series of variations will +undoubtedly hereafter be identified in reference to all the various +kinds of chemical processes upon which the structure and functions +of living things depend. The identification of these processes +and of the bodies concerned in them will lead to a real +classification of Substantive Variations.</p> + +<p class="indent">To forecast the lines on which such classification +will proceed is to look too far ahead. We may nevertheless anticipate +with some confidence that future analysis will recognise among the +contributing elements, some which are intrinsic and inalienable, +and others which are extrinsic and superadded.</p> + +<p class="indent">We already know that there may be such interdependence +among the substantive characters that to disentangle them will +be a work of extreme difficulty. The mere fact that in our +estimation characters belong to distinct physiological systems is +no proof of their actual independence. In illustration may be +mentioned the sap-colour in Stocks and the development of +hoariness on the leaves and stems, which Miss Saunders's experiments +have shown to be intimately connected, so that in certain +varieties no hoariness is produced unless the elements for sap-colour +are already present in the individual plant.</p> + +<p class="indent">The first step in the classification of substantive +variations is therefore to determine which are due to the addition of +new elements or factors, and which are produced by the omission of +old ones. <i>A priori</i> there is no valid criterion by which this can +be known, and actual experiments in analytical breeding can +alone provide the knowledge required. Some very curious results +have by this method been obtained, which throw an altogether +unexpected light on these problems. For example, in order that +the remarkable development of mesoblastic black pigment characteristic +of the Silky Fowl should be developed, it is practically +certain that two distinct variations from such a type as <i>Gallus +bankiva</i> must have occurred. I assume, as is reasonable, that +<i>G. bankiva</i> has genetic properties similar to those of the Brown +Leghorn breed which has been used in the experiments which Mr. +Punnett and I have conducted. <i>Gallus bankiva</i> was not available +but the Brown Leghorn agrees with it very closely in colouration, +and probably in the general physiology of its pigmentation. +<span class="pagenum"><a name="Page_85" id="Page_85">[Pg 85]</a></span> +Setting aside the various structural differences between the two +breeds, the Silky is immediately distinguished from the Leghorn +by the fact that the skin of the whole body including that of the +face and comb appears to be of a deep purplish colour. The +face and comb of the Leghorn are red and the skin of the body +is whitish yellow. On examination it is found that the purple +colour of the Silky is in reality due to the distribution of a deep +black pigment in the mesoblastic membranes throughout the +body. The somatopleura, the pleura, <i>pia mater</i>, the dermis, and +in most organs the connective tissue and the sheaths of the blood-vessels, +are thus impregnated with black. No such pigmentation +exists in the Leghorn. As the result of an elaborate series of +experimental matings we have proved that the distinction between +the Leghorn and the Silky consists primarily in the fact +that the Silky possesses a pigment-producing factor, <i>P</i>, which is +not present in the Leghorn.</p> + +<p class="indent">This variation must undoubtedly have been one of <i>addition</i>. +But besides this there is another difference of an altogether dissimilar +nature; for the Brown Leghorn possesses a factor which +has the power of partially or completely restricting the operation +of the pigment-producing factor, <i>P</i>. Moreover in respect of +this pigment-restricting factor which we may call <i>D</i>, the sexes +of the Brown Leghorn differ, for the male is homozygous or <i>DD</i>, +but the female is heterozygous, <i>Dd</i>. Thus in order that the +black-skinned breed could be evolved from such a type as a +Brown Leghorn it must be necessary <i>both</i> that <i>P</i> should be added +and that <i>D</i> should drop out. We have not the faintest conception +of the process by which either of these events have come to +pass, but there is no reasonable doubt that in the evolution of +the Silky fowl they did actually happen.</p> + +<p class="indent">We may anticipate that numerous interdependences of this +kind will be discovered.</p> + +<p class="indent">Before any indisputable progress can be made with +the problem of evolution it is necessary that we should acquire some +real knowledge of the genesis of that class of phenomena which +formed the subject of the last chapter. So long as the process +of division remains entirely mysterious we can form no conception +<span class="pagenum"><a name="Page_86" id="Page_86">[Pg 86]</a></span> +even of the haziest sort as to the nature of living organisms, or +of the proximate causes which determine their forms, still less +can we attempt any answer to those remoter questions of origin +and destiny which form the subject of the philosopher's contemplation. +It is in no spirit of dogmatism that I have ventured +to indicate the direction in which I look for a solution, though +I have none to offer. It may well be that before any solution +is attained, our knowledge of the nature of unorganised matter +must first be increased. For a long time yet we may have to +halt, but we none the less do well to prepare ourselves to utilise +any means of advance that may be offered, by carefully reconnoitering +the ground we have to traverse. The real difficulty +which blocks our progress is ignorance of the nature of division, +or to use the more general term, of repetition.</p> + +<p class="indent">Let us turn to the more familiar problem of the +causes of variation. Now since variation consists as much in meristic +change as in alteration in substance or material, there is one +great range of problems of causation from which we are as yet +entirely cut off. We know nothing of the causation of division, +and we have scarcely an observation, experiment or surmise +touching the causes by which the meristic processes may be altered.</p> + +<p class="indent">Of the way in which variations in the substantive +composition of organisms are caused we have almost as little real evidence, +but we are beginning to know in what such variations must consist. +These changes must occur either by the addition or loss of factors.</p> + +<p class="indent">We must not lose sight of the fact that though the +factors operate by the production of enzymes, of bodies on which these +enzymes can act, and of intermediary substances necessary to +complete the enzyme-action, yet these bodies themselves can +scarcely be themselves genetic factors, but consequences of their +existence. What then are the factors themselves? Whence do +they come? How do they become integral parts of the organism? +Whence, for example, came the power which is present in a White +Leghorn of destroying—probably reducing—the pigment in its +feathers? That power is now a definite possession of the breed, +<span class="pagenum"><a name="Page_87" id="Page_87">[Pg 87]</a></span> +present in all its germ-cells, male and female, taking part in their +symmetrical divisions, and passed on equally to all as much as +is the protoplasm or any other attribute of the breed. From the +body of the bird the critical and efficient substance could in all +likelihood be isolated by suitable means, just as the glycogen +of the liver can be. But even when this extraction has been +accomplished and the reducing body isolated, we shall know no +more than we did before respecting the mode by which the power +to produce it was conferred on the fowl, any more than we know +how the walls of its blood-vessels acquired the power to form a +fibrin-ferment.</p> + +<p class="indent">It is when the scope of such considerations as this are +fully grasped that we realise the fatuousness of the conventional treatment +which the problem of the causes of variation commonly +receives. Environmental change, chemical injury, differences in +food supply, in temperature, in moisture, or the like have been +proposed as "causes." Admitting as we must do, that changes +may be produced—usually inhibitions of development—by +subjecting living things to changes in these respects, how can we +suppose it in the smallest degree likely that very precise, new, +and adaptative powers can be conferred on the germs by such +treatment? Reports of positive genetic consequences observed +comparable with those I have mentioned, become from time to +time current. We should I think regard them with the gravest +doubt. Few, so far as I am aware, have ever been confirmed, +though clear and repeated confirmation should be demanded +before we suffer ourselves at all to build upon such evidence. +In a subsequent chapter some of these cases will be considered in +detail.</p> + +<p class="indent">In no class of cases would the transmission of an acquired +character superficially appear so probable as in those where power +of resisting the attack of a pathogenic organism is acquired in +the lifetime of the zygote. The possession of such a power is +moreover a distinction comparable with those which differentiate +varieties and species. It is due to the development in the blood +of specific substances which pervade the whole fluid. This +development is exactly one of those "appropriate responses to +<span class="pagenum"><a name="Page_88" id="Page_88">[Pg 88]</a></span> +stimuli" which naturalists who incline to regard adaptation as +a direct consequence of an environmental influence might most +readily invoke as an illustration of their views. And yet all +evidence is definitely unfavourable to the suggestion of an +inheritance of the acquired power of resistance. Such change as +can be perceived in the virulence of the attacks on successive +generations may be most easily regarded as due to the extermination +of the more susceptible strains, and perhaps in some +measure to variation in the invading organisms themselves, an +"acquired character" of quite different import.</p> + +<p class="indent">The specific "anti-body" may have been produced in +response to the stimulus of disease, but the power to produce it +without this special stimulus is not included in the germ-cells +any more than a pigment. All that they bear is the <i>power to +produce</i> the anti-bodies when the stimulus is applied.</p> + +<p class="indent">If we could conceive of an organism like one of +those to which disease may be due becoming actually incorporated with +the system of its host, so as to form a constituent of its germ-cells +and to take part in the symmetry of their divisions, we should +have something analogous to the case of a species which acquires +a new factor and emits a dominant variety. When we see the +phenomenon in this light we realise the obscurity of the problem. +The appearance of recessive varieties is comparatively easy to +understand. All that is implied is the omission of a constituent. +How precisely the omission is effected we cannot suggest, but +it is not very difficult to suppose that by some mechanical fault +of cell-division a power may be lost. Such variation by unpacking, +or analysis of a previously existing complex, though unaccountable, +is not inconceivable. But whence come the new dominants? +Whether we imagine that they are created by some +rearrangement or other change internal to the organism, or +whether we try to conceive them as due to the assumption of +something from without we are confronted by equally hopeless +difficulty.</p> + +<p class="indent">The mystery of the origin of a dominant increases when +it is realised that there is scarcely any recent and authentic account +of such an event occurring under critical observation, which can +<span class="pagenum"><a name="Page_89" id="Page_89">[Pg 89]</a></span> +be taken as a basis for discussion. The literature of horticulture +for example abounds in cases alleged, but I do not think anyone +can produce an illustration quite free from doubt. Such evidence +is usually open to the suspicion that the plant was either introduced +by some accident, or that it arose from a cross with a pre-existing +dominant, or that it owed its origin to the meeting of +complementary factors. In medical literature almost alone however, +there are numerous records of the spontaneous origin of +various abnormal conditions in man which habitually behave +as dominants, and of the authenticity of some of these there +can be no doubt.</p> + +<p class="indent">When we know that such conditions as hereditary +cataract or various deformities of the fingers behave as dominants, +we recognize that those conditions must be due to the addition of +some element to the constitution of the normal man. In the +collections of pedigrees relating to such pathological dominants +there are usually to be found alleged instances of the origin of the +condition <i>de novo</i>. Not only do these records occur with such +frequency that they cannot be readily set aside as errors, but from +general considerations it must be obvious that as these malformations +are not common to normal humanity they must at +some moment of time have been introduced. The lay reader +may not be so much impressed with the difficulty as we are. He +is accustomed to regard the origin of <i>any</i> new character as +equally mysterious, but when once dominants are distinguished from +recessives the problem wears a new aspect. Thus the appearance +of high artistic gifts, whether as an attribute of a race or as a +sporadic event among the children of parents destitute of such +faculties, is not very surprising, for we feel fairly sure that the +faculty is a recessive, due to the loss of a controlling or inhibiting +factor; but the <i>de novo</i> origin of brachydactylous fingers in a +child of normal parents is of quite a different nature, and must +indicate the action of some new specific cause.</p> + +<p class="indent">Whether such evidence is applicable to the general +problem of evolution may with some plausibility be questioned; but +there is an obvious significance in the fact that it is among these +pathological occurrences that we meet with phenomena most +<span class="pagenum"><a name="Page_90" id="Page_90">[Pg 90]</a></span> +nearly resembling the spontaneous origin of dominant factors, +and I cannot see such pedigrees as these without recalling Virchow's +aphorism that every variation owes its origin to some +pathological accident. In the evolution of domestic poultry, +if <i>Gallus bankiva</i> be indeed the parent form of all our breeds, +at least some half dozen new factors must have been added +during the process. In <i>bankiva</i> there is, for example, no factor +for rose comb, pea comb, barring on the feathers, or for the +various dominant types of dark plumage. Whence came all +these? It is, I think, by no means impossible that some other +wild species now extinct did take part in the constitution of +domestic poultry. It seems indeed to me improbable that the +heavy breeds descend from <i>bankiva</i>. Both in regard to domestic +races of fowls, pigeons, and some other forms, the belief in origin +within the period of human civilization from one simple primitive +wild type seems on a balance of probabilities insecurely +founded, but allowing something for multiplicity of origin we +still fall far short of the requisite total of factors. Elements +exist in our domesticated breeds which we may feel with confidence +have come in since their captivity began. Such elements +in fowls are dominant whiteness, extra toe, feathered +leg, frizzling, etc., so that even hypothetical extension of the +range of origin is only a slight alleviation of the difficulty.</p> + +<p class="indent">Somehow or other, therefore, we must recognize that +dominant factors do arise. Whether they are created by internal change, +or whether, as seems to me not wholly beyond possibility, they +obtain entrance from without, there is no evidence to show. +If they were proved to enter from without, like pathogenic +organisms, we should have to account for the extraordinary +fact that they are distributed with fair constancy to half the +gametes of the heterozygote.</p> + +<p class="indent">In proportion as the nature of dominants grows more +clear so does it become increasingly difficult to make any plausible +suggestion as to their possible derivation. On the other hand +the origin of a recessive variety by the loss of a factor is a process +so readily imagined that our wonder is rather that the phenomenon +is not observed far more often. Some slip in the accurate +<span class="pagenum"><a name="Page_91" id="Page_91">[Pg 91]</a></span> +working of the mechanical process of division, and a factor +gets left out, the loss being attested by the appearance of a +recessive variety in some subsequent generation.</p> + +<p class="indent">Consistently with this presentation of the facts we find +that, as in our domesticated animals and plants, a diversity of recessives +may appear within a moderately short period, and that when +variations come they often do not come alone. Witness the +cultural history of the Sweet Pea, <i>Primula Sinensis</i>, <i>Primula +obconica</i>, <i>Nemesia strumosa</i> and many such examples in which +variation when it did come was abundant. The fact cannot +be too often emphasized that in the vast proportion of these +examples of substantive variation under domestication, as well +as of substantive variation in the natural state, the change +has come about by omission, not by addition. To take, for +example, the case of the Potato, in which so many spontaneous +bud-variations have been recorded, East after a careful study +of the evidence has lately declared his belief that all are of this +nature, and the opinion might be extended to many other groups +of cases whether of bud or seminal variation. Morgan draws +the same conclusion in reference to the many varieties he has +studied in <i>Drosophila</i>.</p> + +<p class="indent">In the Sweet Pea, a form which is beyond suspicion +of having been crossed with anything else, and has certainly produced +all the multitude of types which we now possess by variations +from one wild species, there is only one character of the modern +types which could, with any plausibility, be referred to a factor +not originally forming part of the constituents of the wild species. +This is the waved edge, so characteristic of the "Spencer" +varieties; for the cross between a smooth-edged and a waved +type gives an intermediate not unfrequently. Nevertheless +there is practically no doubt that this is merely an imperfection +in the dominance of the smooth edge, and we may feel sure +that any plant homozygous for smooth edge would show no wave +at all. Hence it is quite possible that even the appearance of +the original waved type, Countess Spencer, was due to the loss +of one of the factors for smooth edge at some time in the history +of the Sweet Pea. +<span class="pagenum"><a name="Page_92" id="Page_92">[Pg 92]</a></span></p> + +<p class="indent">In the case of the Chinese Primrose (<i>Primula Sinensis</i>) +one dominant factor has been introduced in modern times, probably +within the last six years at most. This is the factor which +causes suppression of the yellow eye, giving rise to the curious +type known as "Queen Alexandra." Mr. R. P. Gregory's +experiments proved that this was a very definite dominant, and +the element responsible for this development is undoubtedly an +addition to the original ingredient-properties, with which the +species was endowed. Unfortunately, as happens in almost every +case of the kind, the origin of this important novelty appears +to be lost. Its behaviour, however, when crossed with various +other types is that of a simple dominant giving an ordinary 3:1 +ratio. There is therefore no real doubt that it came into existence +by the definite addition of a new factor, for if it was simply +a case of the appearance of a new character made by combination +of two previously existing complementary factors we should +expect that when Queen Alexandra was self-fertilised a 9:7 ratio +would be a fairly common result, which is not in practice found.</p> + +<p class="indent">In <i>Oenothera</i> Gates<a name="FNanchor_1_50" id="FNanchor_1_50"></a><a href="#Footnote_1_50" class="fnanchor">[1]</a> +has observed the appearance, in a large sowing of +about 1,000 <i>Oenothera rubrinervis</i>, of a single individual +having considerably more red pigment in the calyx than is usual +in <i>rubrinervis</i>. The whole of the hypanthium in the flowers of +this plant was red instead of green as in <i>rubrinervis</i>, and the +whole of the sepals were red in the bud-stage, except for small +green areas at the base. This type behaved as a dominant over +<i>rubrinervis</i>, but so far a pure-breeding individual was not found. +Admittedly the variation of this plant from the type of <i>rubrinervis</i> +can be represented as one of degree, though there is a very +sensible gap in the series between the new form which Gates +names "<i>rubricalyx</i>" and the reddest <i>rubrinervis</i> seen in his +cultures. It must certainly be recognised as a new dominant. +Gates, rightly as I consider, regards the distinction between +<i>rubrinervis</i> and <i>rubricalyx</i> as a quantitative one, and the same +remark applies to certain other types differing in the amount of +anthocyanin which they produce. I do not understand the argument +<span class="pagenum"><a name="Page_93" id="Page_93">[Pg 93]</a></span> +which Gates introduces to the effect that the difference +between such quantitative types cannot be represented in terms +of presence and absence. We are quite accustomed to the fact +that in the rabbit self-colour segregates from the Dutch-marked +type. These two types differ in a manner which we may reasonably +regard as quantitative. It is no doubt possible that the +self-coloured type contains an ingredient which enables the colour +to spread over the whole body, but it is, I think, perhaps more +easy to regard the Dutch type as a form from which a part of +the colour is absent. It may be spoken of in terms I have used, +as a <i>subtraction-stage</i> in colour. Following a similar method we +may regard <i>rubricalyx</i> as an addition-stage in colour-variation. +The fact that crosses between <i>rubrinervis</i>, or <i>rubricalyx</i> and +<i>Lamarckiana</i> give a mixture of types in F<sub>1</sub>, does not I +think show, as Gates declares, that there is any system here at work to +which a factorial or Mendelian analysis does not apply; but that +question may be more fitly discussed in connexion with the other +problems raised by the behaviour of <i>Oenothera</i> species in their crosses.</p> + +<p class="indent">I do, however, feel that, interesting as this case must +be admitted to be, we cannot quite satisfactorily discuss it as an +illustration of the <i>de novo</i> origin of a dominant factor. The +difference between the novelty and the type is quantitative, and +it is not unreasonable to think of such a difference being brought +about by some "pathological accident" in a cell-division.</p> + +<p class="indent">Recognition of the distinction between dominant and +recessive characters has, it must be conceded, created a very serious +obstacle in the way of any rational and concrete theory of evolution. +While variations of all kinds could be regarded as manifestations +of some mysterious instability of organisms this difficulty +did not occur to the mind of evolutionists. To most of +those who have taken part in genetic analysis it has become a +permanent and continual obsession. With regard to the origin +of recessive variations, there is, as we have seen, no special +difficulty. They are negative and are due to absences, but as +soon as it is understood that dominants are caused by an addition +we are completely at a loss to account for their origin, for we +<span class="pagenum"><a name="Page_94" id="Page_94">[Pg 94]</a></span> +cannot surmise any source from which they may have been +derived. Just as when typhoid fever breaks out in his district +the medical officer of health knows for certain that the bacillus of +typhoid fever has by some means been brought into that district +so do we know that when first dominant white fowls arose in the +evolution of the domestic breeds, by some means the factor for +dominant whiteness got into a bird, or into at least one of its +germ-cells. Whence it came we cannot surmise.</p> + +<p class="indent">Whether we look to the outer world or to some rearrangement +within the organism itself, the prospect of finding a source of +such new elements is equally hopeless.</p> + +<p class="indent">Leaving this fundamental question aside as one which +it is as yet quite unprofitable to discuss, we are on safe ground in +foreseeing that the future classification of substantive variations, +which genetic research must before long make possible, will be +based on a reference to the modes of action of the several factors. +Some will be seen to produce their effects by oxidation, some by +reduction, some by generating substances of various types, +sugars, enzymes, activators, and so forth. It may thus be +anticipated that the relation of varieties to each other and to +types from which they are derived will be expressible in terms +of definite synthetical formulae. Clearly it will not for an indefinite +time be possible to do this in practice for more than a +few species and for characters especially amenable to experimental +tests, but as soon as the applicability of such treatment +is generally understood the influence on systematics must be +immediate and profound, for the nature of the problem will at +length be clear and, though the ideal may be unattainable, its +significance cannot be gainsaid.</p> + +<hr class="tab" /> +<p class="blockquot"><b><i>Note.</i></b>—With hesitation I allow this +chapter to appear in the form in which it was printed a year ago, but in +passing it for the press after that interval I feel it necessary to call +attention to a possible line of argument not hitherto introduced.</p> +<hr class="tab" /> + +<p class="indent">In all our discussions we have felt justified in declaring +that the dominance of any character indicates that some factor is +<span class="pagenum"><a name="Page_95" id="Page_95">[Pg 95]</a></span> +present which is responsible for the production of that character. +Where there is no definite dominance and the heterozygote is +of an intermediate nature we should be unable to declare on +which side the factor concerned was present and from which side +it was absent. The degree of dominance becomes thus the +deciding criterion by which we distinguish the existence of factors. +But it should be clearly realized that in any given case the argument +can with perfect logic be inverted. We already recognize +cases in which by the presence of an inhibiting factor a character +may be suppressed and purely as a matter of symbolical expression +we might apply the same conception of inhibition to any +example of factorial influence whatever. For instance we say +that in as much as two normal persons do not have brachydactylous +children, there must be some factor in these abnormal persons +which causes the modification. Our conclusion is based on the +observed fact that the modification is a dominant. But it may +be that normal persons are homozygous in respect of some factor +<i>N</i>, which prevents the appearance of brachydactyly, and that in +any one heterozygous, <i>Nn</i>, for this inhibiting factor, brachydactyly +can appear. Similarly the round pea we say contains +<i>R</i>, a factor which confers this property of roundness, without +which its seeds would be wrinkled. But here we know that the +wrinkled seed is in reality one having compound starch-grains, +and that the heterozygote, though outwardly round enough, is +intermediate in that starch-character. If we chose to say that +the compoundness of the grains is due to a factor <i>C</i> and that two +doses of it are needed to make the seed wrinkled, I know no +evidence by which such a thesis could be actually refuted. That +such reasoning is seemingly perverse must be conceded; but +when we consider the extraordinary difficulties which beset +any attempt to conceive the mode of origin of a new dominant +factor, we are bound to remember that there is this other line of +argument which avoids that difficulty altogether. In the case of +the "Alexandra"-eye in <i>Primula</i>, or the red calyx in Gates's +<i>Oenothera</i>, inverting the reasoning adopted in the text, we may +see that only the <i>Primula</i> homozygous for the yellow eye can +develop it and that two doses of the factor for the <i>rubrinervis</i> +calyx are required to prevent that part of the plant from being red. +<span class="pagenum"><a name="Page_96" id="Page_96">[Pg 96]</a></span></p> + +<p class="indent">We may proceed further and extend this mode of reasoning +to all cases of genetic variation, and thus conceive of all alike as +due to loss of factors present in the original complex. Until we +can recognize factors by means more direct than are provided by +a perception of their effects, this doubt cannot be positively +removed. For all practical purposes of symbolic expression we +may still continue to use in our analyses the modes of representation +hitherto adopted, but we must not, merely on the ground of +its apparent perversity, refuse to admit that the line of argument +here indicated may some day prove sound.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_97" id="Page_97">[Pg 97]</a></span></p> +<h2>CHAPTER V</h2> +<h3><span class="smcap">The Mutation Theory</span></h3> + +<p class="indent">When with the thoughts suggested in the last chapter +we contemplate the problem of Evolution at large the hope at the +present time of constructing even a mental picture of that process +grows weak almost to the point of vanishing. We are left +wondering that so lately men in general, whether scientific or +lay, were so easily satisfied. Our satisfaction, as we now see, +was chiefly founded on ignorance.</p> + +<p class="indent">Every specific evolutionary change must represent +a definite event in the construction of the living complex. That event +may be a disturbance in the meristic system, showing itself in +a change in the frequency of the repetitions or in the distribution +of differentiation among them, or again it may be a chemical +change, adding or removing some factor from the sum total.</p> + +<p class="indent">If an attempt be made to apply these conceptions to an +actual series of allied species the complexity of the problem is such that +the mind is appalled. Ideas which in the abstract are apprehended +and accepted with facility fade away before the concrete +case. It is easy to imagine how Man was evolved from an +<i>Amoeba</i>, but we cannot form a plausible guess as to how <i>Veronica +agrestis</i> and <i>Veronica polita</i> were evolved, either one from the +other, or both from a common form. We have not even an +inkling of the steps by which a Silver Wyandotte fowl descended +from <i>Gallus Bankiva</i>, and we can scarcely even believe that it +did. The Wyandotte has its enormous size, its rose comb, its +silver lacing, its tame spirit, and its high egg production. The +tameness and the high egg production are probably enough both +recessives, and though we cannot guess how the corresponding +dominant factors have got lost, it is not very difficult to imagine +that they were lost somehow. But the rose comb and the silver +colour are <i>dominants</i>. The heavy weight also appears in the +crosses with Leghorns, but we need not at once conclude that it +<span class="pagenum"><a name="Page_98" id="Page_98">[Pg 98]</a></span> +depends on a simple dominant factor, because the big size of +the crosses may be a consequence of the cross and may depend on +other elements.</p> + +<p class="indent">Now no wild fowl known to us has these qualities. +May we suppose that some extinct wild species had them? If so, may +we again make the same supposition in all similar cases? To do +so is little gain, for we are left with the further problem, whence +did those lost wild species acquire those dominants? Suppositions +of this kind help no more than did the once famous +conjecture as to the origin of living things—that perhaps they +came to earth on a meteorite. The unpacking of an original +complex, the loss of various elements, and the recombination of +pre-existing materials may all be invoked as sources of specific +diversity. Undoubtedly the range of possibilities thus opened +up is large. It will even cover an immense number of actual +examples which in practice pass as illustrations of specific +distinction. The Indian Rock pigeon which has a blue rump +may quite reasonably be regarded as a geographically separated +recessive form of our own <i>Columba livia</i>, for as Staples-Browne +has shown the white rump of <i>livia</i> is due to a dominant factor. +The various degrees to which the leaves of Indian Cottons are +incised have, as Leake says, been freely used as a means of +classification. The diversities thus caused are very remarkable, +and when taken together with diversities in habit, whether +sympodial or monopodial, the various combinations of points +of difference are sufficiently distinctive to justify any botanist +in making a considerable number of species by reference to them +alone. Nevertheless Leake's work goes far to prove that all of +these forms represent the re-combinations of a very small number +of factors. The classical example of <i>Primula Sinensis</i> and its +multiform races is in fact for a long way a true guide as to the +actual interrelations of the species which systematists have +made. That they did make them was due to no mistake in +judgment or in principle, but simply to the want of that extended +knowledge of the physiological nature of the specific +cases which we now know to be a prime necessity. +<span class="pagenum"><a name="Page_99" id="Page_99">[Pg 99]</a></span></p> + +<p class="indent">But will such analysis cover all or even most of the +ordinary cases of specific diversity between near allies? Postponing the +problem of the interrelations of the larger divisions as altogether +beyond present comprehension, can we suppose, that in general, +closely allied species and varieties represent the various consequences +of the presence or absence of allelomorphic factors +in their several combinations? The difficulty in making a +positive answer lies in the fact that in most of the examples in +which it has been possible to institute breeding experiments with +a view to testing the question, a greater or less sterility is +encountered. Where, however, no such sterility is met with, as +for instance in the crosses made by E. Baur among the species +of <i>Antirrhinum</i> there is every reason to think that the whole +mass of differences can and will eventually be expressed in terms +of ordinary Mendelian factors. Baur has for example crossed +species so unlike as <i>Antirrhinum majus</i> and <i>molle</i>, forms differing +from each other in almost every feature of organisation.<a name="FNanchor_1_51" id="FNanchor_1_51"></a><a href="#Footnote_1_51" class="fnanchor">[1]</a> +The F<sub>2</sub> generation from this cross presents an amazingly motley +array of types which might easily if met with in nature be described +as many distinct species. Yet all are fertile and there +is not the slightest difficulty in believing that they can all be +reduced to terms of factorial analysis.</p> + +<p class="indent">If allowance be made for the complicating effects of +sterility, is there anything which prevents us from supposing that such +good species as those of <i>Veronica</i> or of any other genus comprising +well-defined forms may not be similarly related? I do not know +any reason which can be pointed to as finally excluding such a +possibility. Nevertheless it has been urged with some plausibility +that good species are distinguished by <i>groups</i> of differentiating +characters, whereas if they were really related as the +terms of a Mendelian F<sub>2</sub> family are, we should expect to find +not groups of characters in association, but rather series of forms +corresponding to the presence and absence of the integral factors +composing the groups of characters. I am not well enough +versed in systematic work to be able to decide with confidence +how much weight should be attached to this consideration. Some +<span class="pagenum"><a name="Page_100" id="Page_100">[Pg 100]</a></span> +weight it certainly has, but I cannot yet regard it as forming a +fatal objection to the application of factorial conceptions on +the grand scale. It may be recalled that we are no longer under +any difficulty in supposing that differences of all classes may be +caused by the presence or absence of factors. It seemed at first +for example that such characters as those of leaf shape might +be too subtle and complex to be reducible to a limited number of +factors. But first the work of Gregory on <i>Primula Sinensis</i> +showed that several very distinct types of leaves were related +to each other in the simplest way. In that particular example, +intermediates are so rare as to be negligible, but subsequently +Shull dealing with such a complicated example as <i>Capsella</i>, and +Leake in regard to Cottons, both forms in which intergrades occur +in abundance, have shown that a simple factorial scheme is +applicable. We need not therefore, to take an extreme case, +doubt that if it were possible to examine the various forms of +fruit seen in the Squashes by really comprehensive breeding +tests, even this excessive polymorphism in respect of structural +features would be similarly reducible to factorial order.</p> + +<p class="indent">It must always be remembered also that in a vast number +of cases, nearly allied forms which are distinct, occupy distinct +ground. Moreover, by whatever of the many available mechanisms +that end be attained, it is clear that nature very often does +succeed in preventing intercrossing between distinct forms so +far that the occurrence of that phenomenon is a rarity under +natural conditions. The facts may, I think, fairly be summarized +in the statement that species are on the whole distinct and not +intergrading, and that the distinctions between them are usually +such as might be caused by the presence, absence, or inter-combination +of groups of Mendelian factors; but that they are so caused the evidence +is not yet sufficient to prove in more than a very few instances.</p> + +<p class="indent">The alternative, be it explicitly stated, is not to +return to the view formerly so widely held, that the distinctions between +species have arisen by the accumulation of minute or insensible +differences. The further we proceed with our analyses the more +inadequate and untenable does that conception of evolutionary +<span class="pagenum"><a name="Page_101" id="Page_101">[Pg 101]</a></span> +change become. If the differences between species have not +come about by the addition or loss of factors one at a time, then +we must suppose that the changes have been effected by even +larger steps, and variations including groups of characters, must +be invoked.</p> + +<p class="indent">That changes of this latter order are really those +by which species arise, is the view with which de Vries has now made +us familiar by his writings on the Mutation Theory. In so far as +mutations may consist in meristic changes of many kinds and +in the loss of factors it is unnecessary to repeat that we have +abundant evidence of their frequent occurrence. That they may +also more rarely occur by the addition of a factor we are, I think, +compelled to believe, though as yet the evidence is almost entirely +circumstantial rather than direct. The evidence for the +occurrence of those mutations of higher order, by which new +species characterized by several distinct features are created, +is far less strong, and after the best study of the records which +I have been able to make, I find myself unconvinced. The facts +alleged appear capable of other interpretations.</p> + +<p class="indent">The most famous and best studied examples are of course the +forms of <i>Oenothera</i> raised by de Vries from <i>Oenothera Lamarckiana</i> +in circumstances well known to all readers of genetic literature. +Whatever be the true significance of these extraordinary "mutations" +there can be no question about the great interest which +attaches to them, and the historical importance which they will +long preserve. Apart also from these considerations it is becoming +more and more evident that in their peculiarities they +provide illustrations of physiological phenomena of the highest +consequence in the study of genetics at large.</p> + +<p class="indent">De Vries found, as is well known, that <i>Oenothera Lamarckiana</i> +gives off plants unlike itself. These mutational forms are of +several distinct and recognizable types which recur, and several +of them breed true from their first appearance. The obvious +difficulty, which in my judgment should make us unwilling at +present to accept these occurrences as proof of the genesis of new +species by mutation, is that we have as yet no certainty that the +appearance of the new forms is not an effect of the recombination +<span class="pagenum"><a name="Page_102" id="Page_102">[Pg 102]</a></span> +of factors, such as is to be seen in so many generations of plants +derived from a cross involving many genetic elements. The +first question is what is <i>Oenothera Lamarckiana</i>? Is it itself a +plant of hybrid origin? To this fundamental question no satisfactory +answer has yet been given. All attempts to find it as a +wild plant in America have failed. It existed in Europe in the +latter half of the eighteenth century. Whence it came is still +uncertain, but the view that it came into existence in Europe and +perhaps in Paris, seems on the whole the most probable. The +question has been debated by Macdougal, Gates, and Davis. +From historical sources there is little expectation of further +light. Those who favour the notion of a hybrid origin look on +<i>Oenothera biennis</i> as one of the putative parents. It has been +conjectured that a species called <i>grandiflora</i> lately re-discovered +on the Alabama river was the other parent. Experiments have +been instituted by Davis to discover whether <i>Lamarckiana</i> can +be made artificially by crossing these two species. The results +so far have shown that while plants approximating in various +respects to <i>Lamarckiana</i> have thus been produced, none agree +exactly with that form. Davis, to whom reference should be +made for a full account of the present state of the enquiry, +points out that there are many strains of <i>biennis</i> in existence +and that it is by no means impossible that by using others of +these strains a still closer approximation can be made. None +of Davis's artificial productions as yet breed at all true, as +<i>Lamarckiana</i> on the whole does. In such a case, however, where +several characters are involved, this is perhaps hardly to be expected.</p> + +<p class="indent">One feature of the <i>Oenotheras</i> is very curious. +Not only <i>Lamarckiana</i>, but all the allied species so far as I am +aware, have a considerable proportion of bad and shrivelled pollen +grains. This is undoubtedly true of species living in the wild +state as well as of those in cultivation. I have had opportunities +of verifying this for myself in the United States. No one looking +at the pollen of an <i>Oenothera</i> would doubt that it was taken from +some hybrid plant exhibiting partial sterility. On the other +hand, it is difficult to suppose that numbers, perhaps all, of the +<span class="pagenum"><a name="Page_103" id="Page_103">[Pg 103]</a></span> +"species" of the genus are really hybrids, and many of them breed +substantially true. I regard this constant presence of bad +pollen grains as an indication that the genetic physiology of +<i>Oenothera</i> is in some way abnormal, and as we shall presently +see, there are several other signs which point in the same direction.</p> + +<p class="indent">Discussion of the whole series of phenomena is rendered +exceedingly difficult first, by reason of the actual nature of the +material. The characteristics of many of the types which de Vries +has named are evasive. A few of these types, for instance, <i>gigas</i>, +<i>nanella</i>, <i>albida</i>, <i>brevistylis</i>, and perhaps a few more are +evidently clear enough, but we have as yet no figures and descriptions +precise enough to enable a reader to appreciate exactly the peculiarities +of the vast number of forms which have now to be considered in any +attempt to gain a comprehensive view of the whole mass of facts. +It is also not in dispute that the forms are susceptible of great +variations due simply to soil and cultural influences.</p> + +<p class="indent">The fact that no Mendelian analysis has yet been found +applicable to this group of <i>Oenotheras</i> as a whole is perhaps largely +due to the fact that until recently such analysis has not been +seriously attempted. Following the system which he had +adopted before the rediscovery of Mendelism, or at all events, +before the development of that method of analysis, de Vries has +freely applied <i>names</i> to special combinations of characters and +has scarcely ever instituted a factorial analysis. Before we can +get much further this must be attempted. It may fail, but we +must know exactly where and how this failure comes about. +There are several indications that such a recognition of factorial +characters, could be carried some way. For example, the height, +the size of the flowers, the crinkling of the leaves, the brittleness +of the stems, perhaps even the red stripes on stems and fruits, +and many more, are all characters which may or may not depend +on distinct factors, but if such characters are really transmitted +in unresolved groups, the limitations of those groups should be +carefully determined. The free use of names for the several +forms, rather than for the characters, has greatly contributed +to deepen the obscurity which veils the whole subject. +<span class="pagenum"><a name="Page_104" id="Page_104">[Pg 104]</a></span></p> + +<p class="indent">I do not mean to suggest that these <i>Oenotheras</i> +follow a simple Mendelian system. All that we know of them goes to show +that there are curious complications involved. One of these, probably +the most important of all, has lately been recognized by +de Vries himself, namely, that in certain types the characters +borne by the female and the male germ-cells of the same plant +are demonstrably different. There can be little doubt that +further research will reveal cognate phenomena in many unsuspected +places. The first example in which such a state of things +was proved to exist is that of the Stocks investigated by Miss +Saunders.<a name="FNanchor_2_52" id="FNanchor_2_52"></a><a href="#Footnote_2_52" class="fnanchor">[2]</a> +By a long course of analysis she succeeded in establishing +in 1908 the fact that if a plant of <i>Matthiola</i> is of that +eversporting kind which gives a large proportion of double-flowered +plants among its offspring (produced by self-fertilisation), +then the egg-cells of such a plant are mixed in type, but +the pollen of the same plant is homogeneous. Some of the egg-cells +have in them the two factors for singleness, but some of +them are short of one or both of these factors. The pollen-grains, +however, are all recessives, containing neither of these +factors. The egg-cells, in other words, are mixed, "singles" and +"doubles," while the pollen-grains are all "doubles." The same +is true of the factor differentiating "white," or colourless plastids +from cream-coloured plastids in <i>Matthiola</i>, the egg-cells being +mixed "whites" and "creams," while the pollen-grains are all +"creams," viz: recessives. Later in the same year (1908) +de Vries<a name="FNanchor_3_53" id="FNanchor_3_53"></a><a href="#Footnote_3_53" class="fnanchor">[3]</a> +announced a remarkable case which will be discussed +in detail subsequently. It relates to certain <i>Oenotheras</i> heterozygous +for dwarfness, in which (p. 113) the ovules were mixed, +tails and dwarfs, while the pollen is all dwarf.</p> + +<p class="indent">Again in <i>Petunia</i> Miss Saunders's<a name="FNanchor_4_54" id="FNanchor_4_54"></a><a href="#Footnote_4_54" class="fnanchor">[4]</a> +work has shown that a somewhat similar state of things exists, but with +this remarkable difference, that though the egg-cells are mixed, singles and +doubles, the pollen-grains are all <i>singles</i>, viz: dominants. All +the <i>Petunias</i> yet examined have been in this condition, including +<span class="pagenum"><a name="Page_105" id="Page_105">[Pg 105]</a></span> +some which in botanic gardens pass for original species. Whether +actual wild plants from their native habitats are in the same +state, is not yet known, but it is by no means improbable. The +case may be compared with that of the moth <i>Abraxas grossulariata</i> +studied by Doncaster and Raynor, in which the females +are all heterozygous, or we may almost say "hybrids" of <i>grossulariata</i> +and the variety <i>lacticolor</i>. Similarly we may say that at +least garden Petunias are heterozygous in respect of singleness. +The proof of this is of course that when fertilised with the pollen +of doubles they throw a mixture of doubles and singles. The +statements which de Vries has published regarding the behaviour +of several of the <i>Oenotheras</i> go far to show that they must have +a somewhat similar organisation. On the present evidence it is +still quite impossible to construct a coherent scheme which will +represent all the phenomena in their interrelations, and among +the facts are several which, as will appear, seem mutually incompatible. +The first indication that the <i>Oenotheras</i> may have +either mixed ovules or mixed pollen appears in the fact that +<i>Lamarckiana</i> and several of its "mutants" used as males, with +several other forms as females, give a mixed offspring. For +example, de Vries (1907) found that</p> + +<p class="blockquot"><b> +<i>biennis</i> ♀ × <i>Lamarckiana</i> ♂<br /> +<i>biennis cruciata</i> ♀ × <i>Lamarckiana</i> ♂<br /> +<i>muricata</i> ♀ × <i>Lamarckiana</i> ♂<br /> +<i>biennis</i> ♀ × <i>rubrinervis</i> ♂<br /> +<i>biennis cruciata</i> ♀ × <i>rubrinervis</i> ♂<br /> +</b></p> + +<p>all give a mixture of two distinct types which he names <i>laeta</i> +and <i>velutina</i>, consisting of about equal numbers of each. On +account of the fact that the two forms are produced in association +de Vries has called these forms "twin hybrids," a designation +which is not fortunate, seeing that it is impossible to imagine +that any kind of twinning is concerned in their production. The +distinction between these two seems to be considerable, <i>laeta</i> +having leaves broader, bright green in colour, and flat, with +pollen scanty, while <i>velutina</i> has leaves narrower, grayish green, +more hairy, and furrow-shaped, with pollen abundant. +<span class="pagenum"><a name="Page_106" id="Page_106">[Pg 106]</a></span></p> + +<p class="indent">We next meet the remarkable fact that these two forms, +<i>laeta</i> and <i>velutina</i> breed true to their respective types, and do not +reproduce the parent-types among their offspring resulting from +self-fertilisation. This statement must be qualified in two +respects. When <i>muricata</i> ♂ is fertilised by <i>brevistylis</i> the forms +<i>laeta</i> and <i>velutina</i> are produced, but each of them subsequently +throws the short-styled form as a recessive (de Vries, 1907, +p. 406). It may be remembered that de Vries's previous publications +had already shown that the short style of <i>brevistylis</i>, +one of the <i>Lamarckiana</i> "mutants," behaves as a recessive +habitually (<i>Mutationstheorie</i>, II, p. 178, etc.).</p> + +<p class="indent">Also when <i>nanella</i>, the dwarf "mutant" of <i>Lamarckiana</i> is +used as male on <i>muricata</i> as female, <i>laeta</i> and <i>velutina</i> are produced, +but one only of these, namely, <i>velutina</i>, subsequently +throws dwarfs on self-fertilisation. The dwarfs thus thrown are +said to form about 50 per cent. of the families in which they +occur (de Vries, 1908, p. 668). The fact that the two forms, +<i>laeta</i> and <i>velutina</i>, are produced by many matings in which +<i>Lamarckiana</i> and its mutant <i>rubrinervis</i> are used as males is +confirmed abundantly by Honing, who has carried out extensive +researches on the subject. After carefully reading his paper, +I have failed to understand the main purport of the argument +respecting the "double nature" of <i>Lamarckiana</i> which he founds +on these results, but I gather that in some way <i>laeta</i> is shown to +partake especially of the nature of <i>Lamarckiana</i>, while <i>velutina</i> +is a form of <i>rubrinervis</i>. The paper contains many records which +will be of value in subsequent analysis of these forms.</p> + +<p class="indent">Before considering the possible meaning of these +facts we must have in our minds the next and most novel of the recent +extensions of knowledge as to the genetic properties of the +<i>Oenotheras</i>. In the previous statement we have been concerned +with the results of using either <i>Lamarckiana</i> itself or one of its +"mutants" <i>rubrinervis</i>, <i>brevistylis</i>, or <i>nanella</i> as male, on one of +the species <i>biennis</i> or <i>muricata</i>. The new experiments relate +to crosses between the two species <i>biennis</i> and <i>muricata</i> themselves. +<span class="pagenum"><a name="Page_107" id="Page_107">[Pg 107]</a></span></p> + +<p class="indent">De Vries found:</p> + +<p class="blockquot">1. That the reciprocal hybrids from these two species differed, + +<i>biennis</i> × <i>muricata</i> producing one type of F<sub>1</sub> and <i>muricata</i> × +<i>biennis</i> producing another. Each F<sub>1</sub> resembled the father more +than the mother.</p> + +<p class="blockquot">2. That each of the hybrids so produced breeds true on self-fertilisation.</p> + +<p class="blockquot">3. That if we speak of the hybrid from <i>biennis</i> × <i>muricata</i> +as <i>BM</i> and of the reciprocal as <i>MB</i>, then</p> + +<p class="center"><b><i>BM</i> × <i>MB</i></b></p> + +<p class="blockquot">gives exclusively offspring of <i>biennis</i> type but that</p> + +<p class="center"><b><i>MB</i> × <i>BM</i></b></p> + +<p class="blockquot">gives exclusively offspring of <i>muricata</i> type. +Evidently, apart from all controversy as to the significance of the "mutants" +of <i>Lamarckiana</i>, we have here a series of observations of the first importance.</p> + +<p class="indent">The fact that reciprocal crossings give constantly +distinct results must be taken to indicate that the male and female sides +of one, if not of both, of the parents are different in respect of +characters which they bear. This is de Vries's view, and he +concludes rightly, I think, that the evidence from all the experiments +shows that both <i>biennis</i> and <i>muricata</i> are in this condition, +having one set of characters represented in their pollen-grains +and another in their ovules. The plants breed true, but their +somatic structures are compounded of the two sets of elements +which pass into them from their maternal and paternal sides +respectively. This possibility that species may exist of which +the males really belong to one form and the females to another, +is one which it was evident from the first announcement of the +discovery of Mendelian segregation might be found realised in nature.<a name="FNanchor_5_55" id="FNanchor_5_55"></a><a href="#Footnote_5_55" class="fnanchor">[5]</a> +</p> + +<p class="indent"><i>Oe. biennis</i> and <i>muricata</i> were crossed +reciprocally with each other and with a number of other species, and the +behaviour of each, when used as mother, was consistently different from its +behaviour when used as father. De Vries is evidently justified +<span class="pagenum"><a name="Page_108" id="Page_108">[Pg 108]</a></span> +by the results of this series of experiments in stating that the +"Bild," as he terms it, or composition of the male and female +sides of these two species, <i>biennis</i> and <i>muricata</i>, are distinct. +On the evidence before us it is not, however, possible to form a +perfectly clear idea of each, and until details are published, a +reader without personal knowledge of the material cannot do +more than follow the general course of the argument. For fuller +comprehension a proper analysis of the characters with a clear +statement of how they are distributed among the several types +and crosses is absolutely necessary. According to de Vries the +female of <i>biennis</i> possesses a group of characters which he defines +as "<i>conica</i>" in allusion to the shape of the flower-buds. Besides +the conical buds, this group of features includes imperfect +development of wood, rendering the plant very liable to attacks +of <i>Botrytis</i>, and comparatively narrow leaves.</p> + +<p class="indent">The female of <i>muricata</i> carries a group of features which he +calls "<i>frigida</i>," and, though this is not quite explicitly stated in a +definition of that type, it is to be inferred<a name="FNanchor_6_56" id="FNanchor_6_56"></a><a href="#Footnote_6_56" class="fnanchor">[6]</a> +that its characteristics are regarded as greater height, strong development +of wood with comparative resistance to <i>Botrytis</i>, and broad leaves.</p> + +<p class="indent">The characters borne by the male parts of the two +species are in general those by which they are outwardly distinguished. +For example, the leaves of <i>Oe. biennis</i> are comparatively broad +and are bright green, while those of <i>muricata</i> are much narrower +and of a glaucous green, and I understand that de Vries regards +these properties as contributed by the male side in each case and +to be carried by the male cells of each species. The suggestion +as regards <i>biennis</i> and <i>muricata</i> comes near the conception often +expressed by naturalists in former times (<i>e. g.</i>, Linnaeus) and +not rarely entertained by breeders at the present day, that the +internal structure is contributed by the mother and the external +by the father.</p> + +<p class="indent">On the other hand, the offspring of each species when +used as mother is regarded as possessing in the main the features of +the maternal "Bild," but the matter is naturally complicated +by the introduction of features from the father's side, and it is +<span class="pagenum"><a name="Page_109" id="Page_109">[Pg 109]</a></span> +here especially that the account provided is at present unsatisfactory +and inconclusive. There seems, however, to be no serious +doubt that <i>biennis</i> and <i>muricata</i> each in their outward appearance +exhibit on the whole the features which their pollens respectively +carry, and that the features borne by their ovules are in many respects distinct.</p> + +<p class="indent">The <i>types</i> are thus "hybrids" which breed true. +The results of intercrossing them each way are again "hybrids" which breed +true. It will be remembered that on former occasions de Vries +has formulated a general rule that <i>species</i>-hybrids breed true, +but that the cross-breds raised by interbreeding <i>varieties</i> do not. +One of these very cases was quoted<a name="FNanchor_7_57" id="FNanchor_7_57"></a><a href="#Footnote_7_57" class="fnanchor">[7]</a> +as an illustration of thisprinciple, viz: <i>muricata</i> × <i>biennis</i>. +The grounds for this general statement have always appeared to me insufficient, +and with the further knowledge which the new evidence provides we are +encouraged to hope that when a proper factorial analysis of the +types is instituted we shall find that the phenomenon of a constant +hybrid will be readily brought into line with the systems +of descent already worked out for such cases as that of the Stocks, +and others already mentioned.</p> + +<p class="indent space-below">In further discussion of these facts de Vries makes a suggestion +which seems to me improbable. Since the egg-cells of <i>muricata</i>, +for instance, bear a certain group of features which are missing +on the male side, and conversely the pollen bears features absent +from the female side, he is inclined to regard the <i>bad pollen grains</i> +as the bearers of the missing elements of the male side and to +infer that there must similarly be defective ovules representing +the missing elements of the female side. No consideration is +adduced in support of this view beyond the simple fact that the +characters borne by male and female are dissimilar, whereas +it would be more in accord with preconception if the same sets +of combinations were represented in each—as in a normal +Mendelian case. There is as yet no instance in which the absence +of any particular class of gametes has been shown with any +plausibility to be due to defective viability, though there are, of +course, cases in which certain classes of zygotes do not survive +<span class="pagenum"><a name="Page_110" id="Page_110">[Pg 110]</a></span> +owing to defective constitution (<i>e. g.</i>, the albinos of <i>Antirrhinum</i> +studied by Baur, and the homozygous yellow mice). I am +rather inclined to suppose that in these examples of hybrids +breeding true we shall find a state of things comparable with +that to which we formerly applied the terms "coupling" and +"repulsion." In these cases certain of the possible combinations +of factors occur in the gametic series with special frequency, +being in excess, while the gametes representing other combinations +are comparatively few. In a recent paper on these cases +Professor Punnett and I have shown that these curious results +vary according to the manner in which the factors are grouped +in the parents. If <i>A</i> and <i>B</i> are two factors which exhibit these +phenomena we find that the gametic series of the double heterozygote +differs according as the combination is made by crossing +<i>AB × ab</i>, or by crossing <i>AB × aB</i>. In a normal Mendelian case +the F<sub>1</sub> form, <i>AaBb</i>, produces gametes <i>AB</i>, <i>Ab</i>, +<i>aB</i>, <i>ab</i>, in equal numbers; +but in these peculiar cases those gametes which contain</p> + +<table border="0" style="max-width: 45em;" cellspacing="2" summary="_" cellpadding="0" > + <tbody><tr> + <td class="tdc"> </td> + <td class="tdr"> </td> + <td class="tdc"> </td> + <td class="tdc"><b>Gametic</b></td> + <td class="tdc"><b>series</b></td> + <td class="tdc"> </td> + <td class="tdr"> <b># of gametes</b></td> + <td class="tdr"> <b># of zygotes</b></td> + </tr><tr> + <td class="tdc"> </td> + <td class="tdr"> </td> + <td class="tdr"><b><i>AB</i></b></td> + <td class="tdr"><b><i>Ab</i></b></td> + <td class="tdr"><b><i>aB</i></b></td> + <td class="tdr"><b><i>ab</i></b></td> + <td class="tdr"><b>in series</b> </td> + <td class="tdr"><b>in series</b> </td> + </tr><tr> + <td class="tdc"> </td> + <td class="tdr"> </td> + <td class="tdr">1</td> + <td class="tdr">(<i>n</i>-1)</td> + <td class="tdr">(<i>n</i>-1)</td> + <td class="tdr">1</td> + <td class="tdr"><i>2n</i></td> + <td class="tdr"><i>4n<sup>2</sup></i></td> + </tr><tr> + <td class="tdc"><i>Partial repulsion</i></td> + <td class="tdr"> { </td> + <td class="tdr">1</td> + <td class="tdr">31</td> + <td class="tdr">31</td> + <td class="tdr">1</td> + <td class="tdr">64</td> + <td class="tdr">4096</td> + </tr><tr> + <td class="tdc">from zygote</td> + <td class="tdr"> { </td> + <td class="tdr">1</td> + <td class="tdr">15</td> + <td class="tdr">15</td> + <td class="tdr">1</td> + <td class="tdr">32</td> + <td class="tdr">1024</td> + </tr><tr> + <td class="tdc">of form</td> + <td class="tdr"> { </td> + <td class="tdr">1</td> + <td class="tdr">7</td> + <td class="tdr">7</td> + <td class="tdr">1</td> + <td class="tdr">16</td> + <td class="tdr">256</td> + </tr><tr> + <td class="tdc"><b><i>Ab × aB</i></b></td> + <td class="tdr"> { </td> + <td class="tdr">1</td> + <td class="tdr">3</td> + <td class="tdr">3</td> + <td class="tdr">1</td> + <td class="tdr">8</td> + <td class="tdr">64</td> + </tr><tr> + <td class="tdc"> </td> + <td class="tdr"> </td> + <td class="tdr">1</td> + <td class="tdr">1</td> + <td class="tdr">1</td> + <td class="tdr">1</td> + <td class="tdr">4</td> + <td class="tdr">16</td> + </tr><tr> + <td class="tdc"> </td> + <td class="tdr"> { </td> + <td class="tdr">3</td> + <td class="tdr">1</td> + <td class="tdr">1</td> + <td class="tdr">3</td> + <td class="tdr">8</td> + <td class="tdr">64</td> + </tr><tr> + <td class="tdc"><i>Partial coupling</i></td> + <td class="tdr"> { </td> + <td class="tdr">7</td> + <td class="tdr">1</td> + <td class="tdr">1</td> + <td class="tdr">7</td> + <td class="tdr">16</td> + <td class="tdr">256</td> + </tr><tr> + <td class="tdc">from zygote</td> + <td class="tdr"> { </td> + <td class="tdr">15</td> + <td class="tdr">1</td> + <td class="tdr">1</td> + <td class="tdr">15</td> + <td class="tdr">32</td> + <td class="tdr">1024</td> + </tr><tr> + <td class="tdc">of form</td> + <td class="tdr"> { </td> + <td class="tdr">31</td> + <td class="tdr">1</td> + <td class="tdr">1</td> + <td class="tdr">31</td> + <td class="tdr">64</td> + <td class="tdr">4096</td> + </tr><tr> + <td class="tdc"><b><i>AB × ab</i></b></td> + <td class="tdr"> { </td> + <td class="tdr">63</td> + <td class="tdr">1</td> + <td class="tdr">1</td> + <td class="tdr">63</td> + <td class="tdr">128</td> + <td class="tdr">16384</td> + </tr><tr> + <td class="tdc"> </td> + <td class="tdr"> { </td> + <td class="tdr">(<i>n</i>-1)</td> + <td class="tdr">1</td> + <td class="tdr">1</td> + <td class="tdr"> (<i>n</i>-1)</td> + <td class="tdr"><i>2n</i></td> + <td class="tdr"><i>4n<sup>2</sup></i></td> + </tr><tr> + <td class="tdc"> </td> + <td class="tdr"> </td> + <td class="tdr"> </td> + <td class="tdr"> </td> + <td class="tdr"> </td> + <td class="tdr"> </td> + <td class="tdr"> </td> + <td class="tdr"> </td> + </tr><tr> + <td class="tdc"> </td> + <td class="tdr"> </td> + <td class="tdr"><b>Nature</b></td> + <td class="tdc"><b>of</b></td> + <td class="tdl"><b>zygotic</b></td> + <td class="tdc"><b>series</b></td> + <td class="tdc"> </td> + <td class="tdc"> </td> + </tr><tr> + <td class="tdc"> </td> + <td class="tdr"> </td> + <td class="tdr"><b><i>AB</i></b></td> + <td class="tdr"><b><i>Ab</i></b></td> + <td class="tdr"><b><i>aB</i></b></td> + <td class="tdr"><b><i>ab</i></b></td> + <td class="tdc"> </td> + <td class="tdc"> </td> + </tr><tr> + <td class="tdc"><i>Partial repulsion</i></td> + <td class="tdr"> { </td> + <td class="tdr"><i>2n<sup>2</sup></i>+1</td> + <td class="tdr"><i>n<sup>2</sup></i>-1</td> + <td class="tdr"><i>n<sup>2</sup></i>-1</td> + <td class="tdr">1</td> + <td class="tdr"> </td> + <td class="tdr"> </td> + </tr><tr> + <td class="tdc">from zygote</td> + <td class="tdr"> { </td> + <td class="tdr">2049</td> + <td class="tdr">1023</td> + <td class="tdr">1023</td> + <td class="tdr">1</td> + <td class="tdr"> </td> + <td class="tdr"> </td> + </tr><tr> + <td class="tdc">of form</td> + <td class="tdr"> { </td> + <td class="tdr">513</td> + <td class="tdr">255</td> + <td class="tdr">255</td> + <td class="tdr">1</td> + <td class="tdr"> </td> + <td class="tdr"> </td> + </tr><tr> + <td class="tdc"><b><i>Ab × aB</i></b></td> + <td class="tdr"> { </td> + <td class="tdr">33</td> + <td class="tdr">15</td> + <td class="tdr">15</td> + <td class="tdr">1</td> + <td class="tdr"> </td> + <td class="tdr"> </td> + </tr><tr> + <td class="tdc"> </td> + <td class="tdr"> </td> + <td class="tdr">9</td> + <td class="tdr">3</td> + <td class="tdr">3</td> + <td class="tdr">1</td> + <td class="tdr"> </td> + <td class="tdr"> </td> + </tr><tr> + <td class="tdc"> </td> + <td class="tdr"> { </td> + <td class="tdr">41</td> + <td class="tdr">7</td> + <td class="tdr">7</td> + <td class="tdr">9</td> + <td class="tdr"> </td> + <td class="tdr"> </td> + </tr><tr> + <td class="tdc"><i>Partial coupling</i></td> + <td class="tdr"> { </td> + <td class="tdr">177</td> + <td class="tdr">15</td> + <td class="tdr">15</td> + <td class="tdr">49</td> + <td class="tdr"> </td> + <td class="tdr"> </td> + </tr><tr> + <td class="tdc">from zygote</td> + <td class="tdr"> { </td> + <td class="tdr">737</td> + <td class="tdr">31</td> + <td class="tdr">31</td> + <td class="tdr">225</td> + <td class="tdr"> </td> + <td class="tdr"> </td> + </tr><tr> + <td class="tdc">of form</td> + <td class="tdr"> { </td> + <td class="tdr">3009</td> + <td class="tdr">63</td> + <td class="tdr">63</td> + <td class="tdr">961</td> + <td class="tdr"> </td> + <td class="tdr"> </td> + </tr><tr> + <td class="tdc"><b><i>AB × ab</i></b></td> + <td class="tdr"> { </td> + <td class="tdr">12161</td> + <td class="tdr">127</td> + <td class="tdr">127</td> + <td class="tdr">3969</td> + <td class="tdr"> </td> + <td class="tdr"> </td> + </tr><tr> + <td class="tdc"> </td> + <td class="tdr"> { </td> + <td class="tdr">(<i>3n<sup>2</sup></i>-(<i>2n</i>-1)</td> + <td class="tdr">(<i>2n</i>-1)</td> + <td class="tdr">(<i>2n</i>-1)</td> + <td class="tdr"> <i>n<sup>2</sup></i>-(<i>2n</i>-1)</td> + <td class="tdr"> </td> + <td class="tdr"> </td> + </tr> + </tbody> +</table> +<p class="space-above"><span class="pagenum"><a name="Page_111" id="Page_111">[Pg 111]</a></span> +the <i>parental combinations</i> are in excess. This excess almost +certainly follows the system indicated by the accompanying +table. In the general expressions <i>n</i> is half the number of gametes +required to express the whole system. Now if we imagine that +sex-factors are involved with the others concerned in such a relationship +as this we have a system of distribution approximating +to that found in <i>biennis</i> and <i>muricata</i>. The difference in +reciprocals is represented in a not improbable way. It cannot yet +be said that the rarer terms in the series are formed at all, and +perhaps they are not. As we pointed out in our discussion of +these phenomena, the peculiar distribution of factors in these +cases must be taken to mean that the planes of division at some +critical stage in the segregation are determined with reference +to the parental groups of factors, or in other words, that the +whole system has a polarity, and that the distribution of factors +with reference to this polarity differs according to the grouping +of factors in the gametes which united in fertilization to produce +the plant. Subsequent proliferation of cells representing certain +combinations would then lead to excess of the gametes bearing +them. It is on similar lines that I anticipate we shall hereafter +find the interpretation of the curious facts discovered by de Vries, +though it is evident that a long course of experiment and analysis +must be carried through before any certainty is reached. The +work must be begun by a careful study of the descent of some +single factor, for example, that causing the broader leaf of +<i>biennis</i>, and we may hope that the study of <i>Oenothera</i> +by proper analytical methods will no longer be deferred.</p> + +<p class="indent">We have now to return to the relations of <i>laeta</i> +and <i>velutina</i>. These two forms, it will be remembered are frequently +produced when <i>Lamarckiana</i> or one of its derivatives is used as male, +and the most unexpected feature in their behaviour is that <i>both +breed true as regards their essential characteristics, on +self-fertilisation</i>. If one only bred true the case might, in view +of the approximate numerical equality of the two types, be difficult +to interpret on ordinary lines, but as both breed true it must be +clear that some quite special system of segregation is at work. +What this may be cannot be detected on the evidence, but with +<span class="pagenum"><a name="Page_112" id="Page_112">[Pg 112]</a></span> +the results from the <i>biennis-muricata</i> experiments before us, +it is natural to suspect that we may here again have to recognise +a process of allocation of different factors to the male and female +sides in <i>laeta</i> and <i>velutina</i>. That some such system is in +operation becomes the more probable from the new fact which de Vries +states in describing the group of characters which he calls <i>conica</i>, +namely that this type is the same as that of <i>velutina</i>.</p> + +<p class="indent">There are many collateral observations recorded +both by de Vries and others which have a bearing on the problems, +but they do not yet fall into a coherent scheme. For example, we +cannot yet represent the formation of <i>laeta</i> and <i>velutina</i> from the +various species fertilised by <i>Lamarckiana</i> ♂. That this is not +due to any special property associated with the pollen of <i>Lamarckiana</i> +is shown by the fact that a species called <i>Hookeri</i> +gives <i>laeta</i> and <i>velutina</i> in both its reciprocal crosses with +<i>Lamarckiana</i> (de Vries, 1909, p. 3), and also by the similar fact that +<i>Lamarckiana</i> ♀ fertilised by the pollen of a peculiar race of +<i>biennis</i> named <i>biennis Chicago</i> throws the same types. Before +these very complicated phenomena can be usefully discussed +particulars must be provided as to the individuality of the various +plants used. This criticism applies to much of the work which +de Vries has lately published, for, as we now know familiarly, +plants to which the same name applies can be quite different in +genetic composition.</p> + +<p class="indent">Attention should also be called to one curiously paradoxical +series of results. When the dwarf "mutant" of <i>Lamarckiana</i> +which de Vries names "<i>nanella</i>" is used as father on <i>muricata</i>, +F<sub>1</sub> consists of <i>laeta</i> and <i>velutina</i> in approximately equal numbers. +Both forms breed true to their special characteristics, but +<i>velutina</i> throws dwarfs of its own type, while <i>laeta</i> does not +throw dwarfs. Subsequent investigation of the properties of +these types has led to some remarkable conclusions, and it was +in a study of these plants that de Vries first came upon the phenomena +of dissimilarity between the factors borne by the male +and female cells of the same plant, a condition which had been +recently detected in the Stocks as a result of Miss Saunders's +investigations. The details are very remarkable. We have +<span class="pagenum"><a name="Page_113" id="Page_113">[Pg 113]</a></span> +first the fact that <i>muricata</i> ♀ × dwarf <i>nanella</i> ♂ gives about +50 per cent. <i>laeta</i> and about 50 per cent. of <i>velutina</i>.</p> + +<p class="indent">As regards <i>Velutina</i> it was shown that:</p> + +<table border="0" style="max-width: 45em;" cellspacing="2" summary="_" cellpadding="0" > + <tbody><tr> + <td class="tdc"> </td> + <td class="tdr"> </td> + <td class="tdc"> </td> + <td class="tdc"><b>Talls,</b></td> + <td class="tdc"><b>Dwarfs,</b></td> + </tr><tr> + <td class="tdc"> </td> + <td class="tdr"> </td> + <td class="tdr"> </td> + <td class="tdc"><b>per cent.</b></td> + <td class="tdc"><b>per cent.</b></td> + </tr><tr> + <td class="tdr">1. </td> + <td class="tdr"> </td> + <td class="tdl"><i>Velutina</i> selfed gave</td> + <td class="tdc">38</td> + <td class="tdc">62</td> + </tr><tr> + <td class="tdc"> </td> + <td class="tdr"> { </td> + <td class="tdl"><i>Velutina</i> ♀ × dwarf <i>nanella</i> ♂ gave</td> + <td class="tdc">39</td> + <td class="tdc">61</td> + </tr><tr> + <td class="tdr">2. </td> + <td class="tdr"> { </td> + <td class="tdl">  do.   ×  do.    gave</td> + <td class="tdc">49</td> + <td class="tdc">51</td> + </tr><tr> + <td class="tdc"> </td> + <td class="tdr"> { </td> + <td class="tdl">  do.   ×  do. ♂ derived from <i>velutina</i> gave</td> + <td class="tdc">43</td> + <td class="tdc">57</td> + </tr><tr> + <td class="tdr">3. </td> + <td class="tdr"> </td> + <td class="tdl">Dwarfs  × <i>velutina</i> ♂ gave</td> + <td class="tdc">—</td> + <td class="tdc">all dwarfs</td> + </tr> + </tbody> +</table> + +<p class="space-above">The three experiments taken together prove, as de Vries +says, that the ovules of <i>velutina</i> are mixed, talls and dwarfs, and that +the pollen is all dwarf. The condition is almost the same as +that of the Stocks. It may be noted also that in the Stocks the +egg-cells of the "double" type are in excess, being approximately +9 to 7 of the "single" type, but de Vries regards the two types +in <i>velutina</i> as probably equal in number. The figures (169:231) +rather suggest some excess of the recessives, perhaps 9:7, and +the point would be worth a further investigation.</p> + +<p class="indent">As regards <i>laeta</i>, by self-fertilisation <i>no dwarfs were +produced</i>, but in all other respects it behaved almost exactly like <i>velutina</i>. +The ovules are evidently mixed talls and dwarfs, and whether +fertilised by dwarfs or by the pollen of <i>velutina</i>, which is already +proved to be all dwarf, the result was a steady 50 per cent. of +talls and 50 per cent. of dwarfs. The pollen of <i>laeta</i> used on +dwarfs gives nothing but dwarfs, and in three series of such experiments +226 dwarfs were produced.</p> + +<p class="indent">We are thus faced with this difficulty. Since the egg-cells +of <i>laeta</i> are evidently mixed, talls and dwarfs, and the pollen used +on dwarfs gives all dwarfs, why does not self-fertilisation give +a mixed result, talls and dwarfs, instead of <i>all talls</i>? De Vries +regards the result of self-fertilisation as showing the real nature +of the pollen, and declares it to be all talls, while he represents +the behaviour of the same pollen used on dwarfs by stating that +in these combinations the dwarf character dominates. This +does not seem to me a natural interpretation. I should regard +the pollen of <i>laeta</i> as identical with that of <i>velutina</i>, namely +dwarf, and I suspect the difficulty is really created by the behaviour of +<i>laeta</i> on self-fertilisation. Until a proper analysis is made in +<span class="pagenum"><a name="Page_114" id="Page_114">[Pg 114]</a></span> +which the identity of the different individuals used is recorded, +no further discussion is possible.<a name="FNanchor_8_58" id="FNanchor_8_58"></a><a href="#Footnote_8_58" class="fnanchor">[8]</a> +</p> + +<p class="indent">Other results of a complicated kind involving production +of <i>laeta</i> and <i>velutina</i> together with a third form have been +published by de Vries in his paper on "Triple Hybrids." To these also +the same criticism applies. Some of the observations seem capable +of simple factorial representation and others are conflicting.</p> + +<p class="indent">Taking the work on <i>Oenothera</i> as a whole we see +in it continually glimpses of order which further on are still blocked by +difficulties and apparent inconsistencies. Through such a stage +all the successful researches in complicated factorial analysis +have passed and I see no reason for supposing that with the +application of more stringent methods this more difficult set of +problems will be found incapable of similar solutions. To +return to the original question whether in <i>Oenothera</i> we can claim +to see a special contemporaneous output of new species in actual +process of creation, it will be obvious that while the interrelation +of the several types is still so little understood, such a claim has +no adequate support. It is true that many of the "mutants" +of <i>Lamarckiana</i> can well pass for species, but this is equally true +of many new combinations of pre-existing factors as we have +seen in <i>Primula Sinensis</i> and other cases. Still less can it be +admitted that these facts of uncertain import supply a justification +for the conception which has played a prominent part +in the scheme of the <i>Mutationstheorie</i>, namely that there are +special periods of Mutation, when the parent-species has peculiar +genetic properties. To conclude: The impression which the +evidence leaves most definitely on the mind is that further discussion +of the bearing which the <i>Oenotheras</i> may have on the +problem of evolution should be postponed until we have before +us the results of a searching analysis applied to a limited part of +the field. In such an analysis it is to be especially remembered +that we have now a new clue in the well-ascertained fact that the +genetic composition of the male and female germ-cells of the +<span class="pagenum"><a name="Page_115" id="Page_115">[Pg 115]</a></span> +same individual may be quite different. When with this possibility +in view the behaviour of the types is re-examined I +anticipate that many of the difficulties will be removed.</p> + +<p class="indent">Outside the evidence from <i>Oenothera</i>, which, as we +have seen, is still ambiguous, I know no considerable body of facts favourable +to that special view of Mutation which de Vries has promulgated. +Of variation, or if we will, Mutation, in respect of +some one character, or resulting from recombination, there is +proof in abundance; but of that simultaneous variation in several +independent respects to which de Vries especially attributes the +origin of new specific types I know only casual records which +have yet to undergo the process of criticism.</p> + +<hr class="tb" /> + +<p class="indent">Besides de Vries's "<i>Mutationstheorie</i>" and "Species +and Varieties" the chief publications relating to the subject of the behaviour +of <i>Oenothera</i> are the following: (Many other papers +relating especially to the cytology of the forms have appeared.)</p> + +<p> +<span style="margin-left: 4em;">Davis, B. M.</span><br /> +<span style="margin-left: 6em;">Genetical Studies on <i>Oenothera</i>, I. <i>Amer. Nat.</i>, XLIV, 1910, p. 108.</span><br /> +<span style="margin-left: 6em;">Genetical Studies on <i>Oenothera</i>, II. <i>Ibid.</i>, XLV, 1911, p. 193.</span> +<br /><br /> +<span style="margin-left: 4em;">Gates, R. R.</span><br /> +<span style="margin-left: 6em;">An Analytical Key to some of the Segregates of Oenothera.</span><br /> +<span style="margin-left: 7em;"><i>Twentieth Annual Report of the Missouri Botanical Garden</i>, 1909.</span><br /> +<span style="margin-left: 6em;">Studies on the Variability and Heritability of Pigmentation in <i>Oenothera</i>.</span><br /> +<span style="margin-left: 7em;"><i>Ztsch. f. Abstammungslehre</i>, 1911, IV, p. 337.</span> +<br /><br /> +<span style="margin-left: 4em;">Honing, J. A.</span><br /> +<span style="margin-left: 6em;">Die Doppelnatur der <i>Oenothera Lamarckiana</i>.</span><br /> +<span style="margin-left: 7em;"><i>Ztsch. f. Abstammungslehre</i>, 1911, IV, p. 227.</span> +<br /><br /> +<span style="margin-left: 4em;">Macdougal, D. T. (with A. M. Vail, G. H. Shull, and J. K. Small).</span><br /> +<span style="margin-left: 6em;">Mutants and Hybrids of the <i>Oenotheras</i>.</span><br /> +<span style="margin-left: 7em;"><i>Carnegie Institution's Publication</i>, No. 24, 1905.</span> +<br /><br /> +<span style="margin-left: 4em;">Macdougal, D. T., Vail, A. M., Shull, J. H.</span><br /> +<span style="margin-left: 6em;">Mutations, Variations and Relationships of the <i>Oenotheras</i>.</span><br /> +<span style="margin-left: 7em;"><i>Carnegie Institution's Publication</i>, No. 81, 1907.</span> +<br /><br /> +<span style="margin-left: 4em;">de Vries, H.</span><br /> +<span style="margin-left: 6em;">On Atavistic Variation in <i>Oenothera cruciata</i>.</span><br /> +<span style="margin-left: 7em;"><i>Bull. Torrey Club</i>, 1903, Vol. 30, p. 75.</span><br /> +<span style="margin-left: 6em;">On Twin Hybrids,</span><br /> +<span style="margin-left: 7em;"><i>Bot. Gaz.</i>, Vol. 44, 1907, p. 401.</span><br /> +<span style="margin-left: 4em;">Ueber die Zwillingsbastarde von <i>Oenothera nanella</i>.</span><br /> +<span style="margin-left: 6em;"><i>Ber. Deut. Bot. Ges.</i>, 1908, XXVI, <i>a</i>, p. 667.</span> +<br /><br /> +<span style="margin-left: 4em;">Bastarde von <i>Oenothera gigas</i>. <i>Ibid.</i>, p. 754.</span> +<br /><br /> +<span style="margin-left: 4em;">On Triple Hybrids. <i>Bot. Gaz.</i>, 1909, Vol. 47, p. 1.</span><br /> +<span style="margin-left: 4em;">Ueb. doppeltreziproke Bastarde von <i>Oenothera biennis</i> L. und <i>Oenothera muricata</i> L.</span><br /> +<span style="margin-left: 6em;"><i>Biol. Cbltt.</i>, 1911, XXXI, p. 97.</span> +<br /><br /> +<span style="margin-left: 4em;">Zeijlstra, H. H.</span><br /> +<span style="margin-left: 6em;"><i>Oenothera nanella</i> de Vries, eine krankhafte Pflanzenart.</span><br /> +<span style="margin-left: 7em;"><i>Biol. Cbltt.</i>, 1911, XXXI, p. 129.</span> +<span class="pagenum"><a name="Page_116" id="Page_116">[Pg 116]</a></span></p> + +<h3><span class="smcap">Note.</span></h3> + +<p class="indent">Since this chapter was written two contributions of special +importance have been made to the study of the <i>Oenothera</i> problems. +The first is that of Heribert-Nilsson.<a name="FNanchor_9_59" id="FNanchor_9_59"></a><a href="#Footnote_9_59" class="fnanchor">[9]</a> +The author begins by giving a critical account of the evidence for +de Vries's interpretation of the nature of the mutants. In general this +criticism pursues lines similar to those sketched in the foregoing chapter, +concluding, as I have done, that the chief reason why factorial +analysis has been declared to be inapplicable to the <i>Oenothera</i> +mutants is because no one has hitherto set about this analysis +in the right way. He has also himself made a valuable beginning +of such an analysis and gives good evidential reasons for the belief +that at least the red veining depends on a definite factor which +also influences the size of certain parts of the plant. He argues +further that many of the distinctions between the mutants are +quantitative in nature. With great plausibility he suggests that +the system of cumulative factors which Nilsson-Ehle discovered +in the case of wheat (subsequently traced by East in regard to maize) +may be operating also in these <i>Oenotheras</i>. According +to this system several factors having similar powers may coexist +in the same individual, and together produce a cumulative effect. +Scope would thus be given for the production of the curious and +seemingly irregular numbers so often recorded in the "mutating" families.</p> + +<p class="indent">Another remarkable observation relating to the crosses of +<i>muricata</i> and <i>biennis</i> has been published by Goldschmidt.<a name="FNanchor_10_60" id="FNanchor_10_60"></a><a href="#Footnote_10_60" class="fnanchor">[10]</a> +He finds that in the formation of this cross the female pronucleus +takes no part in the development of the zygotic cell, but that +when the male pronucleus enters, the female pronucleus is +pushed aside and degenerates. As de Vries observed, the reciprocal +hybrids are in each case very like the father ("<i>stark +patroklin</i>"), a consequence which finds a natural explanation in +the phenomenon witnessed by Goldschmidt. The results of +the subsequent matings can also be readily interpreted on the +same lines. Indications of maternal characters are nevertheless +<span class="pagenum"><a name="Page_117" id="Page_117">[Pg 117]</a></span> +mentioned by de Vries, and if Goldschmidt's account of the +cytology is confirmed, these must presumably be referred to the +influence of the maternal cytoplasm. Clearly this new work +opens up lines of exceptional interest. The interpretation I +have offered above must probably be reconsidered. The distinction +between the male and female cells of the types may no doubt be +ultimately factorial, but it is difficult to regard such a distinction +as created by a differential distribution of the ordinary factors.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_118" id="Page_118">[Pg 118]</a></span></p> + +<h2>CHAPTER VI</h2> + +<h3>Variation And Locality</h3> + +<p class="indent">In all discussions of the modes of Evolution the +phenomena of Geographical Distribution have been admitted to be of +paramount importance. First came the broad question, were the +facts of distribution consistent with the Doctrine of Descent? +I suppose all naturalists are now agreed that they are thus +consistent, and that though some very curious and as yet inexplicable +cases remain to be accounted for, the distribution of +animal and plant life on the face of the earth is much what we +might expect as a result of a process of descent with modification. +Passing from this general admission to the more particular question +whether the facts of distribution favour one special conception +of the mode of progress of evolution rather than another, +no agreement has yet been reached. One outstanding feature +is hardly in dispute, namely that prolonged isolation is generally +followed by greater or less change in the population isolated. +Groups of individuals which from various causes are debarred +from free intermixture with other groups almost always exhibit +peculiarities, but on the other hand, cosmopolitan types which +range over wide areas are on the whole uniform, or nearly so +throughout their distribution. Examples of these two categories +will be familiar to all naturalists. The barriers to intercourse +may be seas, deserts, prairies, mountain-chains, or circumstances +of a much less obvious character which isolate quite as effectually. +The local unit is not necessarily an island, a district, or an area +of special geological formation, but may, as every collector knows, +be a valley, a pond, a creek, a "bank" in the sea, a clump of +trees, a group of rocks in a bay, or a particular patch of ground +on a mountain side. All the great groups provide examples of +such specially isolated forms. The botanist knows them well; +the conchologist, the entomologist, the ornithologist and the +student of marine life are all equally aware that special varieties +<span class="pagenum"><a name="Page_119" id="Page_119">[Pg 119]</a></span> +or special species come from special places and from nowhere +else. In one remarkable case the season of appearance plainly +acts as the isolating barrier. <i>Tephrosia bistortata</i> is a small +Geometrid moth which has two broods, appearing in <i>March</i> +and <i>July</i> respectively. It is closely allied to <i>T. crepuscularia</i> +which emerges in <i>May</i> and <i>June</i>. From the fact that occasional +specimens cannot be quite certainly referred to one or other of +the two, many have held that the two are one species. Nevertheless, +in general they present distinctions which are plain +enough. Some localities have one form only, but in several +woods they co-exist. Experiment has shown that the two can +be crossed, and that the cross-breds can breed <i>inter se</i> and with +at least one of the parent stocks.<a name="FNanchor_1_61" id="FNanchor_1_61"></a><a href="#Footnote_1_61" class="fnanchor">[1]</a> +Some diminution in fertility +was observed, but perhaps not more than is commonly encountered +when wild forms are bred in captivity. In such a case it can +scarcely be doubted that the distinctness of the two forms in +the places where they co-exist is maintained by the seasonal isolation.</p> + +<p class="indent">Just as the consequences of isolation are to be seen in the +most different forms of life so may they also affect the most diverse +features of organisation, such as size, colour, sculpture, +shape, or number of parts. In the Sloth (<i>Choloepus</i>) the geographical +races differ in the number of cervical vertebrae—or +in other words, in the distribution of vertebral differentiation. +The geographical races of <i>Cistudo</i> differ in the number of claws +and phalanges.<a name="FNanchor_2_62" id="FNanchor_2_62"></a><a href="#Footnote_2_62" class="fnanchor">[2]</a> +</p> + +<p class="indent">In Shetland, the males of <i>Hepialus humuli</i> (the Ghost Moth) +are not sharply differentiated in colour from the females, as they +are elsewhere, but in varying degrees resemble them.<a name="FNanchor_3_63" id="FNanchor_3_63"></a><a href="#Footnote_3_63" class="fnanchor">[3]</a> +No such males are found in other localities, and even in the other Scottish +islands they are normal. In the island of Waigiu the converse +phenomenon has been observed in <i>Phalanger maculatus</i>. +<span class="pagenum"><a name="Page_120" id="Page_120">[Pg 120]</a></span> +Generally the male is spotted with white, and the female is without +spots, but in Waigiu the females are spotted like the males.<a name="FNanchor_4_64" id="FNanchor_4_64"></a><a href="#Footnote_4_64" class="fnanchor">[4]</a> +</p> + +<p class="indent">The following striking illustration was pointed out to +me by Dr. W. D. Miller. <i>Euphonia elegantissima</i> as it occurs in Mexico +and Central America has the two sexes very distinct from each +other. The male has the lower parts orange and the upper +parts a dark indigo blue, with a bright turquoise-blue head and +neck. The female, except for the head, is of a bright olive green. +A form in which the sexes are similarly differentiated exists in +Porto Rico and is known as <i>E. Sclateri</i>. But in many of the +other West Indian islands the representative "species" (<i>E. +flavifrons</i>) has the two sexes closely resembling the <i>female</i> of +<i>E. elegantissima</i>. This form is found in Antigua, Barbados, +St. Vincent, and Guadeloupe, from which localities the British +Museum has specimens. All three so-called species are very +much alike otherwise.</p> + +<p class="indent">In the genus <i>Pyrrhulagra</i> (<i>Loxigilla</i>) to which Mr. Outram +Bangs called my attention, several distinct and alternative possibilities +occur. The genus has many local species occurring +on the various West Indian islands. These species are characterized +by differences in size, colour, and the shape of the bill. +The colours have a narrow range, being black or greyish, with +or without chestnut marks about the head and throat. In +most of the islands the males are in general colour a full black, +and the females are distinctly grey. They are thus found in +San Domingo, Jamaica, Bahama, and most of the Lesser Antilles. +In Porto Rico we meet the peculiarity that the hens are almost +as black as the males (Ridgway describes the black of the hens +as slightly less intense). This form is called <i>portoricensis</i>. +A larger type, known as <i>grandis</i>, similarly coloured, inhabits +St. Kitt's. Then, on the contrary, in Barbados, <i>both sexes</i> are +a dull blackish grey, like the hens of the Lesser Antilles in general.</p> + +<p class="indent">The local species of <i>Agelaius</i> show similarly +capricious distinctions. <i>A. phoeniceus</i> is a widely spread species, found over +a great part of North America. The male is black with red-orange +<span class="pagenum"><a name="Page_121" id="Page_121">[Pg 121]</a></span> +bars on the wings, but the female is somewhat thrush-like in +colour. In the island of Porto Rico there is a form called <i>xanthomus</i>, +in which <i>both sexes</i> are like the males of the mainland. +A similar species called <i>humeralis</i>, also with both sexes male-like, +lives in Cuba. The island of Cuba, curiously enough, has also a +distinct species named <i>assimilis</i>, in which the female is a dull +black all over, though the male is like the mainland type.</p> + +<p class="indent">So also may local races differ in respect of variability. +<i>Argynnis paphia</i>, the Silver Washed Fritillary, through a great +part of its distribution has only one female form. In the English +New Forest a second female form, <i>valesina</i>, co-exists with the +ordinary <i>paphia</i> female. But in the southern valleys of the Alps +the <i>valesina</i> female is much the commoner of the two, and indeed +in some localities where the species is abundant, I have seen no +<i>paphia</i> females in many days collecting.</p> + +<p class="indent">The beetle <i>Gonioctena variabilis</i> furnishes an +illustration of a comparable phenomenon affecting the male sex. In 1894 and +1895 I studied the curious colour variations of this species especially +in the neighbourhood of Granada, and Mr. Doncaster +ten years later repeated the observations on the same ground, +and also collected the insect in other places in the south of Spain. +The distinctions are not easy to give in words and the reader is +referred to the colour plate accompanying my paper.<a name="FNanchor_5_65" id="FNanchor_5_65"></a><a href="#Footnote_5_65" class="fnanchor">[5]</a> +The essential fact is that the males commonly have the elytra <i>red +with black spots</i> and the females for the most part have greenish +grey elytra with black stripes. In some localities a large minority +of males closely resemble the female type, being identical in +colour and then only distinguishable by structural differences. +In two Granada localities I found the proportion of such males +quite different. In the Darro valley about 38 per cent. (in 718) +<span class="pagenum"><a name="Page_122" id="Page_122">[Pg 122]</a></span> +were of this feminine type, but on the hills some 300 feet +above only 19 per cent. (in 3,230) were like the females. At +Castillejo, not far from Toledo I found no such male in 75 specimens.</p> + +<p class="indent">Mr. Doncaster collected from several localities, +especially from two areas near Malaga, about 5 miles apart. In one of +these the female-like males were, as usual, in a minority, but +in the other these were actually in great excess, amounting to +about 81 per cent. in the 173 taken. Doncaster found a doubtful +indication that the composition of the population varies with +the season, which is quite possible, but it is most interesting +to note that in my chief locality after the lapse of ten years he +found the proportions very much the same as I had done at the +same season, for where I had 19 per cent. of the female-like males +his collecting gave 16 per cent. In other respects also, his statistics +corresponded very closely with mine.<a name="FNanchor_6_66" id="FNanchor_6_66"></a><a href="#Footnote_6_66" class="fnanchor">[6]</a> +</p> + +<p class="indent">The various forms of <i>Heliconius erato</i> are well known +to entomologists. They are strikingly distinguished by the colours +of the strong comb-like marking on the hind wing, which may be +red, yellow, green or blue. In various parts of the distribution +in South America sometimes two and sometimes three of these +distinct types co-exist.<a name="FNanchor_7_67" id="FNanchor_7_67"></a><a href="#Footnote_7_67" class="fnanchor">[7]</a> +</p> + +<p class="indent">The distribution of the varieties of <i>Noctua castanea</i> +typifies a large range of cases. The form which is reckoned the normal +of the species has red fore-wings. It is practically restricted to +Great Britain and Germany, according to Tutt. The other +common form, <i>neglecta</i>, has grey fore-wings, and in this pattern +it ranges through West Central Europe from North Italy to +Germany. In the British Isles it extends up to Orkney. In +Britain this grey form is by far the commoner, occurring +<span class="pagenum"><a name="Page_123" id="Page_123">[Pg 123]</a></span> +wherever the species is found. The red form is much scarcer in +England, and does not occur at all in many localities where the +grey form is common. Mr. Woodforde, from whom this account +is taken,<a name="FNanchor_8_68" id="FNanchor_8_68"></a><a href="#Footnote_8_68" class="fnanchor">[8]</a> +states that in August, 1899, he saw considerably over a +hundred of the grey in the New Forest at sugar, but only two +red ones. In Staffordshire however the red is proportionately +more numerous and he estimates them as 40 per cent. of the +population. Lastly a form has been taken in Staffordshire as a +rarity in which the red is replaced by yellow, and this has hitherto +been seen nowhere else. It is beyond our immediate purposes +to discuss the genetic relationships of such forms, but the details +of this case are interesting as making fairly clear the fact that +the distinctions between <i>castanea</i> and <i>neglecta</i> are due to +combinations of the presence of and absence of two pairs of factors, +of which one produces a red pigment in the ground colour of the +forewing and the other irrorates the same region with black +scales. Mr. Woodforde states that all intermediates exist, +and that in Staffordshire the greys always have a pinkish tinge. +The yellow is doubtless another recessive to the red.</p> + +<p class="indent">Species which are uniform in some localities may be +polymorphic in others. Such a phenomenon is well exemplified by +the orchid <i>Aceras hircina</i>. Of this species distinct varieties had +previously been known in Germany, but Gallé<a name="FNanchor_9_69" id="FNanchor_9_69"></a><a href="#Footnote_9_69" class="fnanchor">[9]</a> +has lately given a detailed account of a number of most diverse forms found +growing in a district of Eastern France. Without reference to his plates +it is impossible to give any adequate conception of the profusion +of types which the flowers of the species there assume. In some +the lip is elongated to many times its usual length, twisting +and dividing in a fashion suggesting some of the strangest of the +Tropical Orchids. In others the labellum and the lateral petals +are all comparatively short and wide (Fig. 13). Intermediates, +combining these qualities in various degrees, were abundant, and +the condition of the species, which was the only representative of +the genus in the locality, recalls the extreme polymorphism of +many of the Noctuid Moths. +<span class="pagenum"><a name="Page_124" id="Page_124">[Pg 124]</a></span></p> + +<div class="figcenter" > + <img src="images/i_139.jpg" alt="Aceras hircina" width="600" height="847" /> +</div> +<p class="space-below" style="font-size: 120%;"> +<b><span class="smcap">Fig. 13.</span></b> Various forms of <i>Aceras hircina</i>. +(After Gallé.) This figure only shows a few of the more striking forms +illustrated in Gallé's plates.</p> + +<p class="indent"><span class="pagenum"><a name="Page_125" id="Page_125">[Pg 125]</a></span> +Somewhat comparable variability has been seen in another +Orchid genus <i>Ophrys</i>. In Great Britain the species <i>apifera</i>, +<i>aranifera</i> and <i>muscifera</i> though variable are fairly distinct, +but Moggridge has published two series of plates<a name="FNanchor_10_70" id="FNanchor_10_70"></a><a href="#Footnote_10_70" class="fnanchor">[10]</a> +showing a very different state of things as regards the <i>Ophrys</i> population +of the Riviera. Here the outward diversity is such that the ordinary +specific names cannot be applied with any confidence and the +limits of the species are quite uncertain. It may well be supposed +that these Riviera plants are interbreeding, and indeed we may +safely assume that they are. It is, however, to be remembered +that Darwin showed <i>apifera</i> in this country to be habitually +self-fertilised, so that the different behaviour on the Riviera may +itself constitute a local peculiarity. Moreover it is to be gathered +from Moggridge's account that in the districts which he examined +the condition was not to be described by the statement that our +three types were there co-existing and hybridising, but rather +we should say that the population was polymorphic, containing +these three types amongst others. Conchologists are aware +that on the Dogger Bank <i>Modiola</i> attains a size unparalleled +elsewhere. The same is true of the sponges <i>Grantia compressa</i> +and <i>Grantia ciliata</i> in the estuary of the Orwell.<a name="FNanchor_11_71" id="FNanchor_11_71"></a><a href="#Footnote_11_71" class="fnanchor">[11]</a> +Conversely, as we know so well in the case of Man, dwarf races occur in +several special localities. Such examples may be multiplied indefinitely.</p> + +<p class="indent">The relation of local forms to species has often +been discussed from many points of view, but I know no treatment of +the subject clearer or more comprehensive than an excellent +account of some of the various manifestations of local differentiation +as they appear in Helicidæ published by Coutagne<a name="FNanchor_12_72" id="FNanchor_12_72"></a><a href="#Footnote_12_72" class="fnanchor">[12]</a> +and a reader interested in the problem which they raise would +<span class="pagenum"><a name="Page_126" id="Page_126">[Pg 126]</a></span> +do well to make himself acquainted with the original from +which the following notes are taken. He speaks for example +of <i>Helix lapicida</i>. This is on the whole a constant form ranging +up to the altitude of 1,300 m., common all over France except +at great heights and in the Olive regions where it is restricted +to moist places. Though subjected to such diverse conditions +it shows only trivial variations in colour and other respects +throughout its distribution, excepting that on both sides of the +Pyrenees it has a very distinct sporadic variety called <i>Andorrica</i> +or <i>microporus</i>. This variety occurs here and there, together +with the type-form sometimes in colonies (pp. 26-30 and 86).</p> + +<p class="indent"><i>Bulimus detritus</i> though more restricted in +geographical range is a much more variable form. It exhibits great +variations in colour, form, and size, and as Coutagne well insists, +these are independent of each other. Foreshadowing the methods of +factorial analysis he suggests that distinctions in each respect, +the "modes" as he calls them, should be denoted by a letter, +or if desired, by a name, and the several combinations of differences +might thus be most logically and usefully expressed. Of +such combinations he says there are at least 18, all of which can +be found. The whole possible series does not necessarily occur +in the same place, and various localities are characterised by +the presence or absence of certain of the combinations as Coutagne +calls them, and by the relative frequency with which they +occur. The ideas thus enunciated are much in advance of the +ordinary practice of systematists, who give names to forms which +are nothing but accidental combinations of factors, just as the +horticulturists for practical reasons give names to similar combinations, +which as we now know are merely specially noticeable +terms in a long series of possibilities. In each case it is rather +the <i>factors</i> which should be named than the forms which are +constituted by their casual collocation. In this special example +of <i>Bulimus detritus</i> the 18 forms are made by the combinations +of three pairs of independent factors. Besides these combinations +which may occur anywhere or almost anywhere in the distribution +there are two more distinct local forms, each of which +is regarded by Coutagne as probably constituting a fresh "mode," +perhaps compatible with the others. +<span class="pagenum"><a name="Page_127" id="Page_127">[Pg 127]</a></span></p> + +<p class="indent"><i>Helix striata</i> (Draparnauld)<a name="FNanchor_13_73" id="FNanchor_13_73"></a><a href="#Footnote_13_73" class="fnanchor">[13]</a> +is truly polymorphic; and its +various forms have been described under various specific names. +It abounds in the calcareous hills of Provence and Languedoc, +disappearing in the alluvial lowlands and equally in the upper +levels at about 800-1,000 m. From this district it extends +through regions of similar altitude over a great part of France +(details given).</p> + +<p class="indent">Locard in his monograph of this group, which he calls +collectively the group of <i>Helix Heripensis</i>, tabulates 27 distinct +named forms. The characteristics in which these forms differ +have been reckoned as 17, and as several of these vary in degree +of development, the number of modes may be increased to 109. +For practical purposes however Coutagne considers that the +various developments of 7 characteristics in their several combinations +are enough to express the various forms, and he gives +examples of this method of definition. As he observes, though +names may be required to define the modes, no one need be +alarmed at that, for the same names of modes will be applicable +to a great range of distinct species, and the formulae expressing +their combinations will replace the varietal names.</p> + +<p class="indent">This particular example of polymorphism is but little +limited by locality. Occasional colonies present some special physiognomy +which may in a given place seem almost invariable, though +in this very respect the colonies found elsewhere may be highly +variable, but such limitations are exceptional for <i>H. striata</i>.</p> + +<p class="indent">Some distinct and obvious susceptibilities to the +influence of soil and climate are however noticeable. For example on +siliceous ground the shells are thinner, while on calcareous soils +they are thicker; similarly those from the Northern districts +attain a larger size than those from further South. Moreover +those subjected to curtailed development, whether from drought, +heat or cold often show a shortening of the spire. In contrast +with this case Coutagne describes the varieties of <i>Helix caespitum</i>, +which he says are for the most part localised, quoting many illustrative cases.</p> + +<p class="indent">Another remarkable case in which locality plays a curious +part is provided by the two species <i>Helix trochoides</i> and +<span class="pagenum"><a name="Page_128" id="Page_128">[Pg 128]</a></span> +<i>pyramidata</i>. In France generally they are distinct enough from +each other, <i>trochoides</i> being smaller and having a characteristic +keel. Coutagne says that after having collected these species +from more than a score of localities he came upon a colony of +<i>trochoides</i> on the island of Pomègues in which the shells were +relatively enormous, most of them having only a slight keel, +and a few none at all. On the other hand he received a consignment +of <i>pyramidata</i> from four localities in Sicily, all small, +and one of them exactly like the <i>trochoides</i> from Pomègues. +Judging by the samples received from Sicily, <i>trochoides</i> is there +not more variable than it is in Provence, while the Sicilian +<i>pyramidata</i> is protean.</p> + +<p class="indent">The relations of the two species <i>Helix nemoralis</i> and +<i>hortensis</i> provide an illustration of another kind of manifestation of local +peculiarity. <i>H. hortensis</i> and <i>nemoralis</i> as usually met with, +are two very distinct forms. <i>H. hortensis</i> is smaller and duller, +and its peristome is white. <i>H. nemoralis</i> is larger and more shiny, +and its peristome is brown. In several anatomical points, +moreover, especially in the shape of the dart, there are great +differences. For a full account of these peculiarities of the two +forms and a discussion of their inter-relations the reader is referred +to the elaborate work of A. Lang<a name="FNanchor_14_74" id="FNanchor_14_74"></a><a href="#Footnote_14_74" class="fnanchor">[14]</a> +who has studied them extensively and has also succeeded in experimentally +raising hybrids between them. These hybrids were in a slight degree +fertile with both the parent species, but up to the time of publication +no young had been reared from hybrids <i>inter se</i>.</p> + +<p class="indent">Coutagne describes the result of collections made in 62 +French localities. Some had exclusively <i>hortensis</i>, some exclusively +<i>nemoralis</i>, and in some the two were found in association. +He gives details of five of these collections from which I +take the following summary of the more essential facts, omitting +much that is almost equally significant.</p> + +<p class="indent"><i>Locality A</i>, near Honfleur. Both forms present, each +sharply and normally distinguished, without any intermediates. They +<span class="pagenum"><a name="Page_129" id="Page_129">[Pg 129]</a></span> +are thus found in many places. Coutagne instances Müller's +observations in Denmark, his own series from the Jura, etc.</p> + +<p class="indent"><i>Locality B.</i> Vonges (Côte d'Or), 242 <i>hortensis</i> +taken at random, showed 128 with light peristomes (either more or less pinkish +or quite white) and 114 with dark <i>brown</i> peristomes; together +with 26 <i>nemoralis</i> all with the usual brown peristomes.</p> + +<p class="indent">Of the <i>hortensis</i> 50 were in ground-colour +<i>opalescens</i> and 1 <i>roseus</i>; and in shape 5 were <i>umbilicatus</i>.</p> + +<p class="indent"><i>Locality C</i>, about 3 kilometres from <i>B</i>. There were +found 35 <i>hortensis</i>, of which 20 had light peristomes and 15 brown; together +with 7 <i>nemoralis</i>.</p> + +<p class="indent">Of the <i>hortensis</i> none were <i>opalescens</i>; 18 were +<i>roseus</i> and none has the shape of <i>umbilicatus</i>.</p> + +<p class="indent"><i>Locality D</i>, about 1,200 metres from <i>B</i>. 147 +<i>hortensis</i>, of which 4 had light peristomes and 143 had brown. +No <i>nemoralis</i> were found.</p> + +<p class="indent">None of the <i>hortensis</i> were <i>opalescens</i> or +<i>roseus</i>, but 30 were <i>umbilicatus</i>.</p> + +<p class="indent">In these localities intermediates of every grade existed between +the well-characterised <i>opalescens</i>, <i>roseus</i>, or <i>umbilicatus</i>, +and the other forms, but there were no intergrades between the +other <i>nemoralis</i> and the smaller <i>hortensis</i>, about which there +was no hesitation. In the next locality a very different state +of things was found.</p> + +<p class="indent"><i>Locality E.</i> Banks of the Yvette at Orsay (Seine-et-Oise). +The actual numbers are not given, but we are told that 58 per +cent. were <i>hortensis</i>, 33 per cent. <i>nemoralis</i>, and 9 per cent. +intermediate. As at Honfleur, the <i>hortensis</i> had white peristomes, +and the <i>nemoralis</i> brown. Coutagne's visits to this locality were +in 1878 and 1880, and he calls attention to the fact that Pascal +found similar intermediates in the same neighbourhood in 1873.</p> + +<p class="indent">The two species, in Coutagne's view, when they +occur together, can generally be sorted from each other with perfect +confidence, and it is only in exceptional localities that these +intermediates occur. Whether they are hybrids, or whether +sometimes the species in their variations transgress their usual +limitations is regarded both by Coutagne and by Lang as a +<span class="pagenum"><a name="Page_130" id="Page_130">[Pg 130]</a></span> +question not yet answerable with certainty. Coutagne moreover +lays stress on the fact that although each species may be easily +known from the other <i>in its own district</i>, yet when shells from +different districts are brought together it is sometimes impossible +to sort them. He mentions an example of such casual intermixture +occurring under natural conditions on an island in the +Rhone, to which it may well be supposed that floods had brought +immigrants from miscellaneous localities. This population contained +a very large number of uncertain specimens, and as he +says, it was much as if he were to mix the shells from his 62 localities, +after which it would certainly be impossible to separate +the two species again.<a name="FNanchor_15_75" id="FNanchor_15_75"></a><a href="#Footnote_15_75" class="fnanchor">[15]</a> +</p> + +<p class="indent">Further evidence is given in the same treatise as to +other examples of polymorphism, especially in the genus <i>Anodonta</i>, +of which Locard made 251 species for France alone. Here again +are cases like those already given, and many forms or "modes" +are found restricted to special localities, while occasionally +in the same locality dissimilar forms are found, collectively +forming a colony, without intermediates.</p> + +<p class="indent">Taken as a whole the evidence shows the following conclusions +to be true. Local races, whether of animals or plants, may be +distinguished by characters which we are compelled to regard +as trivial, or again by features of such magnitude that if they +were known to us only as the characteristics of a uniform species +they would certainly be assumed without hesitation to be essential +for its maintenance. Local forms may be sharply differentiated +from the corresponding populations of other localities or they +may be connected with them by numbers of intermediates. +Not rarely also we find a fact which has always seemed to me of +special significance, that the peculiarity of the local population +or colony may show itself in a special liability to variation, and +this variability may show itself in one of many degrees, either +in the constant possession of a definite aberration, in a dimorphism, +or in an extreme polymorphism. +<span class="pagenum"><a name="Page_131" id="Page_131">[Pg 131]</a></span></p> + +<p class="indent">At this stage attention should be called to two points. +First, that when the details of the geographical distribution of +any variable species are studied in that thorough and minute +fashion which is necessary for any true knowledge of the interrelations +of the several forms, the conception of a species invented +by the popular expositions of Evolution under Selection is found +to be rarely if ever realised in nature.</p> + +<p class="indent">A species in this generalised sense is an aggregate +of individuals, none exactly alike, but varying round a normal type, +the characters of which are fixed in so far as they are adapted to +environmental exigency. In nature, however, the occurrence of +the varieties, and even the occurrence of the variability is +sporadic. In one place a population may be perfectly uniform. +In another it may be again uniform but distinct. In others +the two forms may occur together, sometimes with and sometimes +without intergrades. In some localities a sporadic variety +may be an element of the population, persisting through long +periods of time. In other localities there may be several such +aberrations occurring together which are absent elsewhere.</p> + +<p class="indent">Secondly, I would remind the reader that in the light +of genetic analysis we know that intergrades, when they do occur, cannot +be assumed to represent conditions through which the species +must pass or has passed on its way to the extreme and definite forms.</p> + +<p class="indent">Often, perhaps generally, they are nothing but heterozygous +forms, and often also they are conditions corresponding with the +presence of factors in their reduction-stages.</p> + +<p class="indent">A broad survey of the facts shows beyond question that +it is impossible to reconcile the mode of distribution of local forms +with any belief that they are on the whole adaptational. Their +peculiarities are occasionally the result of direct environmental +influence, as we shall hereafter notice in certain cases, but none +can attribute such sporadic and irregular phenomena to causes +uniformly acting.</p> + +<p class="indent">Writers on systematics, especially those of former generations +often conjecture or assert that local distinctions are caused by +"differences of climate, soil, food, etc.," in vague general terms. +It is usually safe to assume that these remarks do not represent +<span class="pagenum"><a name="Page_132" id="Page_132">[Pg 132]</a></span> +conclusions drawn from actual evidence, for only rarely can they +be translated into more precise language. So thoroughly have +the biological sciences become permeated with the belief that all +distinctions are dependent upon adaptation, that the mere +existence of definite distinctions is felt by many to be sufficient +ground to warrant an assumption that these distinctions are +directly or indirectly due to special local conditions. For +example, Dr. J. A. Allen, who has done so much careful and valuable +work in delimiting the local forms of the United States +fauna, writes of the Ground Squirrels (Tamias)<a name="FNanchor_16_76" id="FNanchor_16_76"></a><a href="#Footnote_16_76" class="fnanchor">[16]</a> +as follows:—</p> + +<p class="blockquot">"From the extreme susceptibility of this plastic +group to the influences of environment, it is one of the most instructive +and fascinating groups among North American mammals. No +one can doubt its comparatively recent differentiation from a +common stock, and its dispersion from some common centre. +Whether the type originated at some point in North America, +or in the Northern part of Eurasia, it is perhaps idle to speculate, +but that it has increased, multiplied, spread, and become differentiated +to a wonderful degree in North America is beyond +question; as it is found from the Arctic regions to the high +mountain ranges of Central Mexico, and has developed some +twenty to thirty very palpable local phases."</p> + +<p class="blockquot">"Some of them easily take rank as species, others +as subspecies. Probably a more striking illustration of evolution by +environment cannot be cited."</p> + +<p class="indent">He proceeds to point out that the habits of these +creatures are such as lead to isolation. This may well be admitted, and +indeed no exception can possibly be taken to the passage as a +whole, save in the one respect that there is no real proof that +the local diversity is due to "evolution by environment" or an +indication of "susceptibility to the influences of environment."</p> + +<p class="indent">Dr. Allen does indeed adduce the fact that California +"extending through 800 miles of latitude, with numerous sharply +contrasted physiographic regions, has apparently no less than +six strongly differentiated forms, while the region east of the +Rocky Mountains from a little below the northern boundary of +<span class="pagenum"><a name="Page_133" id="Page_133">[Pg 133]</a></span> +the United States northward to the limit of trees—a slightly +diversified region of at least ten times the area of California—has +only one"! But when one comes to ask how the various forms +are adaptational, and how the influences of environment have +led to their production, only conjectures of a preliminary and +tentative character could be expected in reply. Desert forms +are no doubt pallid as in so many instances, and forest forms are +more fully coloured, and we may readily enough accept such facts +as indications of a connection between bodily features and the +conditions of life, but further than that no one can go; so that +when we find size, length of ears or of tail, the number of dorsal +stripes, the pattern of the colours, not to speak of differences in +the pigments themselves, all exhibiting large modifications, we +cannot refer these peculiarities to the causation of environmental +difference, save as a simple expression of faith. I incline far +more to agree with Gulick who, after years of study of the local +variations of the Achatinellidae, came to the conclusion that it +was useless to expect that such local differentiation can be +referred to adaptation in any sense.<a name="FNanchor_17_77" id="FNanchor_17_77"></a><a href="#Footnote_17_77" class="fnanchor">[17]</a> +Even the most convinced Selectionist must hesitate before such facts as +those related by A. G. Mayer regarding the distribution of <i>Partula otaheitana</i>, +one of these Achatinellidae. The island of Tahiti has been scored +by erosion so that a series of separated valleys radiate to the coast. +From four successive valleys Mayer collected the species, and +found that in the first (Tipaerui) valley all the shells were +dextral (115, containing 73 young); in the second valley +(Fautaua) 54 per cent. of adults and 55.5 per cent. of the young +contained were sinistral; in the third valley (Hamuta) 69 per +cent. of adults and 73 per cent. of young contained in them were +sinistral; and lastly, in the fourth valley (Pirae) all the shells +(131, containing 62 young) were sinistral.<a name="FNanchor_18_78" id="FNanchor_18_78"></a><a href="#Footnote_18_78" class="fnanchor">[18]</a> +In connection with these observations I may mention the fact that in a certain +pond in the North of England<a name="FNanchor_19_79" id="FNanchor_19_79"></a><a href="#Footnote_19_79" class="fnanchor">[19]</a> +the sinistral form of <i>Limnaea peregra</i> +<span class="pagenum"><a name="Page_134" id="Page_134">[Pg 134]</a></span> +has been known to occur for about fifty years. Visiting +it lately I found the left-handed shells to be about 3 per cent. of +the population. The species is the commonest British freshwater +shell, but left-handed specimens are exceedingly rare. +Will anyone ask us to suppose that the persistence of a percentage +of this rarity in the same place is an indication of some specially +favouring circumstance in the waters of that pond? It is a +horse-pond to all appearances exactly like any other horse-pond; +and I believe that in perfect confidence we may accept the +suggestion of common sense, which teaches us that there is +nothing particular in the circumstances which either calls such +varieties into existence or contributes in any direct way to their +survival. Had the phenomenon of local variation been studied +in detail before Darwin wrote, the attempt to make selection +responsible for fixity wherever found, could never have been +made. The proposition that not only the definiteness of local +forms but their variability also is sporadic, can be established +by countless illustrations taken from any group of either the +animal or the vegetable kingdoms. Only exceptionally can the +fixed differences be even suspected of contributing to adaptation, +and sporadic variability, which is a no less positive fact, must +manifestly lie outside the range of such suspicions. It is open +to any one to suggest speculatively that the persistence of +special varieties or of special variability in special places is an +indication that in those places the conditions of life are such +that the forms in question are tolerated though elsewhere +the same types are exterminated; but that consideration, even +if it could be proved to be well founded, is not one which lends +much force to the thesis that definiteness of type is a consequence +of Natural Selection. On the contrary, recourse to such reasoning +implies the inevitable but very damaging admission that +the stringency of Selection is frequently so far relaxed that two +or more equally definite forms of the same species can persist +side by side. There is no doubt that this is the simple truth, +but when once that truth is perceived it is useless to invoke the +control of Selection as the factor to which definiteness of type +in general must be referred. +<span class="pagenum"><a name="Page_135" id="Page_135">[Pg 135]</a></span></p> + +<p class="indent">The genetic relations of local forms to each other +cannot in the absence of actual breeding experiments be often ascertained. +Standfuss formerly enunciated as a general principle that when +two forms co-exist in the same locality and are able to interbreed, +they do not produce intermediates; but that when the forms are +geographically separated as local races, crosses between them +result in a series of intermediates.<a name="FNanchor_20_80" id="FNanchor_20_80"></a><a href="#Footnote_20_80" class="fnanchor">[20]</a> +In this aphorism there is a good deal of truth, +but if in the light of Mendelian principles we +examine the two statements we see now that the first is in reality +only another way of saying that the distinctness of an aberrational +form co-existing with another is due to segregation, accompanied +by some degree of dominance of one type. Whether, however, +one geographically isolated race will give intermediates when +bred with another must depend entirely on the genetic physiology +of the special case, and no general rule can be laid down. It +may well be that, inasmuch as the distinctness of the variety is +maintained by isolation, the difference in factorial composition +between it and the representative form in another area is neither +simple nor sharp; but when two varieties co-exist, though interbreeding, +it is now clear that their differences must depend on +the segregation of simple factors. Plainly such aberrations may +in one place co-exist with another type, and elsewhere be separated +from it as local races.</p> + +<p class="indent">Excellent illustrations of these two stages in +evolution are provided by the melanic varieties of British Lepidoptera. +The fact that black or blackish varieties of many species especially +of Geometridae have come into existence in recent years is well +known to British collectors, and it is not in dispute that they +have in several instances replaced the older type more or less +completely in certain districts. In the year 1900 the Evolution +Committee of the Royal Society instituted a collective inquiry +as to the contemporary distribution of these dark varieties. As +the change had happened within living memory and had greatly +progressed in recent years it was hoped that a record of the +existing distribution would serve as a point of departure for +future comparison. The records thus obtained were tabulated +<span class="pagenum"><a name="Page_136" id="Page_136">[Pg 136]</a></span> +by Mr. L. Doncaster.<a name="FNanchor_21_81" id="FNanchor_21_81"></a><a href="#Footnote_21_81" class="fnanchor">[21]</a> +From that account and from the statements +in Barrett's British Lepidoptera<a name="FNanchor_22_82" id="FNanchor_22_82"></a><a href="#Footnote_22_82" class="fnanchor">[22]</a> +this description of some of the more notable cases is taken.</p> + +<p class="indent">The most striking and familiar case is that of <i>Amphidasys +betularia</i>, of which only the ordinary type was known in any +locality until about 1848-1850, when the totally black var. +<i>doubledayaria</i> first appeared in the neighbourhood of Manchester. +This black form was subsequently recorded in Huddersfield +between 1860 and 1870; Kendal about 1870; Cannock Chase, +1878; Berkshire, 1885; Norfolk, Essex and Cambridge about +1892; Suffolk, 1894; London, 1897. For the Southern Counties +of England, except in the London district, there are still very few +records. It cannot of course be asserted positively that the +variety spread from its place of first appearance into the other +localities, and that it did not arise <i>de novo</i> in them, but there +can be little doubt that the process was one of colonisation. +On the European Continent the first records are from Hanover +in 1884, Belgium 1886 and 1894, Crefeld 188-, Berlin 1903, +Dresden about the same date.</p> + +<p class="indent">As regards the increase of the variety we have the +fact that in Lancashire, Cheshire and the West Riding of Yorkshire the +black is now the prevalent form; and in some places, as for example, +Huddersfield, the black alone is now found, though it was unknown +there till between 1860 and 1870. About 1870 at Newport, +Monmouth, the two forms were in about equal numbers, +but a few years later the type had almost vanished. Similarly +in Crefeld, where the black form was still very rare in the eighties, +it now forms about 50 per cent. of the population. In the +London district the black remains scarce and at the date of the +report it was still very scarce. From Ireland there is only one +record and there are hardly any from Scotland.</p> + +<p class="indent"><i>Boarmia repandata</i> is another species which is behaving +in a somewhat similar way. Unlike <i>betularia</i>, however, the species +is a variable one, and has several colour-forms, amongst them +the banded var. <i>conversaria</i>, and many others. In addition +<span class="pagenum"><a name="Page_137" id="Page_137">[Pg 137]</a></span> +to these there is a black form in the North of England which +seems to be spreading. In Huddersfield the black was first +recorded in 1888, and in 1900 20-25 per cent. were black. At +Rotherham the black or very dark are now prevalent and have +increased in the last 15 years. From the Midlands, East Anglia +and Southern Counties the returns show only the light and +medium forms.</p> + +<p class="indent">Of <i>Odontoptera bidentata</i> several intergrading +dark forms exist, and these are found exclusively in the North and the +Midlands. Unicolorous blacks have been found recently in the Lancashire +mosses and at Wakefield. At Huddersfield 50 years ago the +light forms were prevalent, but now a rather dark brown, not +infrequently suffused with black, is the commonest. In Southern +Counties only light forms are known.</p> + +<p class="indent"><i>Phigalia pilosaria</i> in South England is always +light, but in the North the prevalent form is darker. About 35 years ago +a form with unicolorous sooty fore-wings and dull grey hind +wings was first seen in Yorkshire and a similar form is now taken +regularly in South Wales.</p> + +<p class="indent">In the following cases the dark varieties were +found originally only in the South.</p> + +<p class="indent"><i>Boarmia rhomboidaria</i> gave rise about 40 years ago +to a unicolorous smoky variety called <i>perfumaria</i>. This was at first +peculiar to the London district, but it has since been taken in +Birmingham and other large cities. More lately coal-black +specimens have been found at Norwich, and others similar but +hardly so dark were taken in the South of Scotland and at Cannock Chase.</p> + +<p class="indent"><i>Eupithecia rectangulata</i> is a similar case. +Formerly the light forms were prevalent but within sixty years they have +almost entirely been replaced in the South of London by a nearly black form.</p> + +<p class="indent"><i>Tephrosia</i> (<i>Boarmia</i>) <i>consortaria</i> and +<i>Tephrosia consonaria</i> are exceptionally interesting, for they have both +given off dark forms in the same wood near Maidstone, which is far from the +usual "centres of melanism." They were discovered in this +locality by Mr. E. Goodwin. That of <i>consortaria</i> is a dark +<span class="pagenum"><a name="Page_138" id="Page_138">[Pg 138]</a></span> +grey, but that of <i>consonaria</i> is a full black, and nothing like +either has been found anywhere else.</p> + +<p class="indent">These examples are all taken from the Geometridae but +others, though of a less conspicuous kind, could be given from +the Noctuidae or the Micro-Lepidoptera. <i>Acronycta psi</i>, for +instance, has a suffused form which is believed to be becoming +more frequent in the London district. <i>Polia chi</i> has two dark +forms, <i>olivacea</i>, a yellowish grey with dark markings, and <i>suffusa</i> +which is a darker, blackish-slate colour. Both occur in the North +of England, sometimes together, sometimes separately, or mixed +with the type and many intermediates. The distribution is +peculiarly irregular. At Huddersfield, where the very dark form +appeared suddenly about 1890, some 30 per cent. are said to be +now dark and about 6-7 per cent. very dark, but at Saddleworth, +12 miles away, only the pale forms occur.</p> + +<p class="indent">Several questions of interest arise in regard to this +evidence. This progressive Melanism has arisen in certain families only, +and may be confined to certain species only, within those families. +As in almost all other examples in which variation has been much +observed, its incidence is capricious and specific. A collateral +line of inquiry relates to the degree of discontinuity which the +variation manifests. Here again there is no rule. Generally +speaking, in <i>A. betularia</i>, to take the case most fully studied, the +variation is discontinuous. Real intermediates between <i>betularia</i> +and <i>doubledayaria</i> are in most localities absent or rare. +The black spots of <i>betularia</i> may often be larger or more numerous +than in the normal, but this variation has nothing to do with +<i>doubledayaria</i>, and is not an intermediate stage towards it, +though sometimes wrongly so described. <i>Doubledayaria</i> owes its +characteristic appearance to a factor which blurs the surface +of the wings with a layer of black. Sometimes this blurring is +slighter than in the real <i>doubledayaria</i>, and these forms are real +intermediates. Occasionally the fore-wings alone are thus blurred. +These intermediates are clearly due to reduction-stages of the +<i>doubledayaria</i> factor, and are related to it as a blue mouse is to +a black, or a dutch rabbit to a self-colour. It cannot positively +be asserted that the full <i>doubledayaria</i> existed before the +<span class="pagenum"><a name="Page_139" id="Page_139">[Pg 139]</a></span> +intermediate, but it almost certainly did. In certain places as for +instance in Belgium, there is evidence that intermediates have +at various times been fairly abundant, but they have never become +common, nor are they known to exist in the absence of +<i>doubledayaria</i>. When the black variety and the light type breed +together they do not usually have intermediates among their +offspring, and the evidence is consistent with the view that the +black is a complete dominant. The same is probably true of +<i>Tephrosia consonaria</i>.</p> + +<p class="indent">In some of the other species we know that the darkest +forms did not appear first. For example in <i>Phigalia pilosaria</i> and +<i>Boarmia rhomboidaria</i> dark forms existed and are believed to +have increased in number before the darkest made its appearance. +<i>Hybernia progemmaria</i> is said to have become darker gradually +both in Cheshire and in the West Riding, and a uniformly smoky +variety appeared in South Yorkshire less than 45 years ago which +has spread to neighbouring counties. The dark medium has +become the commonest form in Huddersfield district, where the +very dark variety is now about 20 per cent. of the population, +though the light form is still common.</p> + +<p class="indent">Taking the evidence together we find it consistent with +the view that dark forms have appeared sporadically, in some species +the very dark appearing first and intermediates later, in others +the moderately dark came first and the darkest later in time. It +is practically certain that the change has in general come about +not by a gradual change supervening on the population at large, +but by the sporadic appearance of dark specimens as a new element +in the population, and strains derived from these dark +individuals have gradually superseded the normal type more or +less completely.</p> + +<p class="indent">If it could be shown that these melanic novelties +had a definite advantage in the struggle for existence they would provide +an instance of evolution proceeding much in the way which +Darwin contemplated. The whole process would differ from +that conceived by him as the normal method of evolution only +in so far as the change has come about with great rapidity and +in some instances largely by the appearance and success of +<span class="pagenum"><a name="Page_140" id="Page_140">[Pg 140]</a></span> +discontinuous varieties. The question, however, must be asked +whether the dark form can reasonably be supposed to have +an advantage by reason of their darkness. Some naturalists +believe that the darkness of the colours does thus definitely contribute +to their protection by making the insects less conspicuous +and thus more likely to escape the search of birds. In support +of this view it may be pointed out that it is in the manufacturing +districts of Lancashire and Yorkshire, and again in the London +area that the melanics have attained their greatest development. +Consistently with this argument also, it is in the neighbourhood of +Crefeld and Essen, the black country of Germany, that they have +chiefly established themselves on the Continent, and <i>Phigalia +pilosaria</i> in the black form is now at home in South Wales. Thus +superficially regarded, the evidence looks rather strong, but it +is difficult to apply the reasoning in detail. We have first the +difficulty that the black form of <i>betularia</i> for instance has +established itself in thoroughly rural districts, notably near King's +Lynn in Norfolk, and in the neighbourhood of Kendal and +Windermere. The black form of <i>consonaria</i> and the dark +<i>consortaria</i> appeared in a wood near Maidstone, far from town +smoke, and the black <i>rhomboidaria</i> was first found at Norwich, +which, as towns go, is clean. Then again the spread of the +melanics is very irregular and unaccountable. The black <i>pilosaria</i> +is found both in the West Riding and in the Swansea +district, but not yet elsewhere. It rapidly increased at Huddersfield, +but made no noticeable progress at Sheffield though recorded +there for ten years. It is also a remarkable fact that no +similar melanic development has been observed in America, +and, so far as I am aware, comparable melanic varieties have not +appeared on the European continent except in the case of the +few sorts which possibly may have come from England.</p> + +<p class="indent">The whole subject is beset with complications. It must +not be forgotten that in a few species of moths there is an obvious +and recognised conformity between the colours of the perfect +insect and that of the soil on which they live, comparable with +that which is so striking in the case of some Oedipodidae and +other grasshoppers. Of this phenomenon the clearest example +<span class="pagenum"><a name="Page_141" id="Page_141">[Pg 141]</a></span> +is <i>Gnophos obscurata</i>, which is a most variable species with many +local forms. Of these a well-known dark variety lives on the +peaty heaths of the New Forest and other districts, but on the +chalk hills of Kent, Sussex and Surrey various light varieties +are found, of which one is a bright silvery white, very near in +colour to the colour of a chalky bank. This case does not seem +to be one of direct environmental action,<a name="FNanchor_23_83" id="FNanchor_23_83"></a><a href="#Footnote_23_83" class="fnanchor">[23]</a> +for Poulton found no change induced by rearing larvae among either +white or black surrounding objects. No one however can doubt that +there is some indirect connection between the colour of the +ground and that of the moths.</p> + +<p class="indent">To my mind there is a serious objection to the theory +of protective resemblance in application to such a case as that of the +<i>betularia</i> forms, which arises from the fact that the black +<i>doubledayaria</i> is a fairly conspicuous insect anywhere except +perhaps on actually black materials, which are not common in any +locality. Tree trunks and walls are dirty in smoky districts but +they are not often black, and I doubt whether in the neighbourhood +of Rotherham, for instance, which is one of the great +melanic centres, <i>doubledayaria</i> can be harder for a bird to find +than <i>betularia</i> would be. After all, too, many of the species +much affected are not urban insects. They live in country +places between the towns, and the general tone of these places +even in Lancashire and the West Riding is not very different +from that of similar places elsewhere. As against the objection +that the black varieties are much blacker than the case requires +it may be replied that we know nothing of the senses of birds, +and that perhaps to their eyes blackness does constitute a disguise +even though the surroundings are much less dark. This is +undeniable, but recourse to such an argument is dangerous; for +if the sight of the insect-eating birds is so dull that it does not +distinguish dark things from dingy grey, we cannot subsequently +regard the keen sight of birds as the sufficient control which has +led to the minute and detailed resemblance of many insects to +their surroundings. Those who see in such cases examples of +the omnipotence of Selection must frequently find themselves in this dilemma. +<span class="pagenum"><a name="Page_142" id="Page_142">[Pg 142]</a></span></p> + +<p class="indent">Taking the evidence as a whole, we may say that it +fairly suggests the existence of some connection between modern urban +developments and the appearance and rise of the melanic varieties. +More than that we cannot yet affirm. It is a subject +in which problems open up on every side, and all of them are +profitable subjects for investigation. Unhappily such animals +are difficult to rear successfully in captivity for many generations, +owing to their extreme liability to disease. Not the least interesting +feature of the melanics is the fact that the black varieties +provide about the best and clearest example of a new dominant +factor attaching itself to a wild species in recent times. +None of the cases are satisfactorily recorded or analysed as yet, +but the evidence is clear that <i>doubledayaria</i> is a dominant to its +type, and in several other dark varieties, though the pigment +deposited is not black, the records show that the increased +amount of the pigment almost certainly is due to a positive factor. +Of this, <i>Hemerophila abruptaria</i> is a good example.<a name="FNanchor_24_84" id="FNanchor_24_84"></a><a href="#Footnote_24_84" class="fnanchor">[24]</a> +There are some irregularities in the results, but taken together they +leave little doubt that the dark brown variety is a dominant and the +light, yellowish brown a recessive.</p> + +<p class="indent">A curious parallel to the rise of the melanic moths +in England is provided by the case of the Honey-creepers or Sugar-birds, +in certain West Indian islands.<a name="FNanchor_25_85" id="FNanchor_25_85"></a><a href="#Footnote_25_85" class="fnanchor">[25]</a> +These birds of the genus <i>Coereba</i> (<i>Certhiola</i>) range from Southern +Mexico to the Northern parts of South America and through the whole chain +of the West Indian islands and Bahamas except Cuba. There are numerous +local forms, and many of the islands have types peculiar to themselves, +as is usual in such cases. Some of the types or species +range through several islands, but according to Austin Clark<a name="FNanchor_26_86" id="FNanchor_26_86"></a><a href="#Footnote_26_86" class="fnanchor">[26]</a> +no island has more than one of them. Cory<a name="FNanchor_27_87" id="FNanchor_27_87"></a><a href="#Footnote_27_87" class="fnanchor">[27]</a> +reckoned twelve such species within the Antillean region. They are small birds +<span class="pagenum"><a name="Page_143" id="Page_143">[Pg 143]</a></span> +about the size of a nuthatch with a general colouring of black, +yellow, and white. From the island of St. Vincent the Smithsonian +Institution received in the late seventies of last century +several completely black specimens in addition to two of the usual +type of colouring. The black were described by W. N. Lawrence +as <i>atrata</i>, and those marked with the usual yellow and white +were called <i>saccharina</i>. The collector (Mr. F. A. Ober) reported +that the black form was common, and that the <i>saccharina</i> form +was rarer. Lawrence remarks, "Had there been only a single +example (of the black form) I should have considered it as probably +a case of abnormal colouring, but it seems to be a representative +form of the genus in this island."<a name="FNanchor_28_88" id="FNanchor_28_88"></a><a href="#Footnote_28_88" class="fnanchor">[28]</a> +There is of course no doubt of the correctness of the view taken by Austin Clark +that "<i>atrata</i>" is a black variety. The black bird is in every respect, +other than colour, identical with <i>saccharina</i>, and it is even possible +to detect a greenish colour in the areas which would normally be +yellow, showing plainly enough the yellow pigment obscured by the black.</p> + +<p class="indent">We have next the interesting fact that like our melanic +moths the dark form is replacing the "type." At the time of Ober's +visit the type was already in a minority, but now it is nearly +or perhaps actually extinct, though the black form is one of the +commonest birds on the island. Austin Clark found no specimen +when he collected there in 1903-4, though formerly it was not +uncommon in the vicinity of Kingston and in the immediate +windward district of St. Vincent.</p> + +<p class="indent">The Grenadines are geographically just south of +St. Vincent, though separated by a deep channel. In these islands no +black forms have yet been taken, but Grenada, the next island to the +south, has both normals and blacks. There are trifling differences +of size between the Grenada birds and those from St. +Vincent, the Grenada specimens being slightly smaller and for +this reason they have received distinct names, the form marked +with yellow and white being called <i>Godmani</i> (Cory) and the black, +<i>Wellsi</i> (Cory), but this merely introduces a useless complication. +<span class="pagenum"><a name="Page_144" id="Page_144">[Pg 144]</a></span> +There is evidence that in Grenada, as in St. Vincent, the black is +gradually ousting the original type, but the process has not gone +so far as in St. Vincent. Austin Clark very properly compares +this case of the Sugar-birds with that of <i>Papilio turnus</i>, which as +is well-known, has a black female in the southern parts of its distribution, +in addition to a female of the yellow type, but in the +Northern States the black female does not occur.</p> + +<p class="indent">During the present year P. R. Lowe, who lately studied +<i>Coerebas</i> on a large scale in the West Indies, has published an +important paper on the subject.<a name="FNanchor_29_89" id="FNanchor_29_89"></a><a href="#Footnote_29_89" class="fnanchor">[29]</a> +He calls attention to the fact that Cory recently found +a black form of <i>Coereba</i> on Los Roques Islands, +and he himself discovered another on the +Testigos Islands. Both localities are on the coast of Venezuela, +far from St. Vincent and Grenada. The whole problem is thus +further complicated by the fact that the black varieties have, +as we are almost driven to admit, arisen independently in remote +places. Improbable as this conclusion may be, it is still more +difficult to regard all the black forms as derived from one source. +For first, they present definite small differences from each other; +and secondly we have to remember a consideration of greater +importance, that the very fact that each island has its own type +must be accepted as proving that the localities are effectively +isolated from each other, and that migration must be a very rare event.</p> + +<p class="indent">The rarity of such illustrative cases is, I believe, +more apparent than real. It is probably due to the extreme reluctance +of systematists to admit that such things can be, and of course +to the almost complete absence of knowledge as to the genetic +behaviour of wild animals and plants. Only in such examples +as this of the <i>Coereba</i>, where colour constitutes the sole +difference, or that of the moths which have been minutely studied by +many collectors, does the significance of the facts appear. The +arrangement of catalogues and collections is such that much +practical difficulty of a quite unnecessary kind is introduced. For +example, in this very case of <i>Coereba</i>, I find the British Museum +has a fine series from Grenada including 3 normals and 11 black, +<span class="pagenum"><a name="Page_145" id="Page_145">[Pg 145]</a></span> +and also 16 blacks from St. Vincent. If the black specimens from +Grenada were put with the normals which are almost certainly +nothing but a recessive form of the same bird, the variation would +strike the eye on even a superficial glance at the drawer. But +following the notions so naively expressed in the passage quoted +above from W. N. Lawrence, the blacks from Grenada are put +apart together with the other blacks from St. Vincent, though +two of them were shot on the same date as one of the normals.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_146" id="Page_146">[Pg 146]</a></span></p> + +<h2>CHAPTER VII</h2> +<h3><span class="smcap">Local Differentiation.</span> <i>Continued</i></h3> + +<p style="font-size: 120%; text-align: center;"> +<b><span class="smcap">Overlapping Forms</span></b></p> + +<p class="indent">The facts of the distribution of local forms on the whole +are consistent with the view that these forms come into existence by the +sporadic appearance of varieties in a population, rather than by +transformation of the population as a whole. Of such sporadically +occurring varieties there are examples in great abundance, +though by the nature of the case it can be but rarely that we are +able to produce evidence of a previous type being actually superseded +by the variety. When the two forms are found co-existing +in the same area they are usually recorded as one species if intergrades +are observed, and as two species if the intergrades are +absent. On the other hand when two forms are found occupying +separate areas, when, that is, the process of replacement is completed +in one of the areas, then forthwith each is named separately +either as species or subspecies. Successive observations carried +out through considerable periods of time would be necessary to +establish beyond question that the history proceeds in one way +rather than another. Such continuity of observation has for +the most part never been attempted. The kind of information +wanted has indeed only been lately recognized, and really critical +collecting is a thing of only the last few decades. The methods of +the older collectors, who aimed at bringing together a few typical +specimens of all distinct forms, are of little service in this class +of inquiry, which is better promoted by the indiscriminate collection +of large numbers of common forms from many localities. +When this has been done on a comprehensive scale we shall be +in a position to form much more confident judgments as to the +general theory of evolution.</p> + +<p class="indent">Some little work of the kind has however been done and +the results are already of great value. Seeing that the differentiation +of local forms is only made possible by isolation, it necessarily +<span class="pagenum"><a name="Page_147" id="Page_147">[Pg 147]</a></span> +happens that the collector finds one form in one locality +and another in a distinct locality, and there is no evidence as to +the behaviour which the two representative species might exhibit +if they came into touch with each other. In the most familiar +examples of such distinction each inhabits an island, completely +occupying it to the exclusion of any other similar form. It can +only be when the two representative species occupy parts of a +continental area connected with each other by regions habitable +for the organism in question, that there is a chance of seeing the +two forms in contact. Often also, even where this condition is +satisfied, the habits, social organisation, or some other special +cause may act as a barrier which prevents the distinguishable +forms from ever coming into such complete contact as to interbreed +or to behave as a genetically continuous race. When +genetic continuity is ensured by a constant diffusion of the population +over the whole area which they inhabit there will manifestly +be no formation of local races. The practical uniformity, +for example, of so many species of birds which inhabit widely +extended ranges of Western Europe is doubtless maintained by +such constant diffusion. When, as in the case of the Falcons, +many localities have peculiar forms, the fact may be taken as +conclusive evidence that there is little or no diffusion; and when +we find in such a species as the Goldfinch that in spite of migratory +fluctuations there are nevertheless geographical races +fairly well differentiated, it may similarly be inferred that these +fluctuations habitually move up and down on paths which do +not intermingle. There are however a few examples of animals, +not given to much irregular wandering, which occupy a wide and +continuous range of diversified country and are differentiated as +local races in two or more districts, though the distinct races +meet in intervening areas. Of these the most notorious illustration +which has been investigated with any thoroughness is +that of the species of <i>Colaptes</i> (Woodpeckers) known in the United +States as Flickers. The study of the variations of these forms, +made by J. A. Allen<a name="FNanchor_1_90" id="FNanchor_1_90"></a><a href="#Footnote_1_90" class="fnanchor">[1]</a> +is an admirable piece of work, with which +<span class="pagenum"><a name="Page_148" id="Page_148">[Pg 148]</a></span> +every student of variation and evolutionary problems should +make himself familiar. The two forms with which we are most +concerned are known as <i>C. auratus</i> and <i>C. cafer</i>, and are very +strikingly different in appearance. In size, proportions, general +pattern of colouration, habits, and notes, the two are alike, but +they differ in the following seven respects as stated by Allen.</p> + +<table border="0" style="max-width: 45em;" cellspacing="2" summary="Woodpeckers" cellpadding="0" > + <tbody><tr> + <td class="tdr"> </td> + <td class="tdl">   <i>Auratus</i></td> + <td class="tdr"> </td> + <td class="tdl">   <i>Cafer</i></td> + </tr><tr> + <td class="tdr"> </td> + <td class="tdl"> </td> + <td class="tdr"> </td> + <td class="tdl"> </td> + </tr><tr> + <td class="tdr">1. </td> + <td class="tdl">Quills <i>yellow</i>.</td> + <td class="tdr">1. </td> + <td class="tdl">Quills <i>red</i>.</td> + </tr><tr> + <td class="tdr">2. </td> + <td class="tdl">Male with a <i>black</i> malar stripe.</td> + <td class="tdr">2. </td> + <td class="tdl">Male with a <i>red</i> malar stripe.</td> + </tr><tr> + <td class="tdr">3. </td> + <td class="tdl">Adult female with <i>no</i> malar stripe.</td> + <td class="tdr">3. </td> + <td class="tdl">Adult female with usually a <i>brown</i> malar stripe.</td> + </tr><tr> + <td class="tdr">4. </td> + <td class="tdl">A <i>scarlet</i> nuchal crescent in both sexes.</td> + <td class="tdr">4. </td> + <td class="tdl"><i>No</i> nuchal crescent in either sex.</td> + </tr><tr> + <td class="tdr">5. </td> + <td class="tdl">Throat and fore neck <i>brown</i>.</td> + <td class="tdr">5. </td> + <td class="tdl">Throat and fore neck <i>grey</i>.</td> + </tr><tr> + <td class="tdr">6. </td> + <td class="tdl">Whole top of head and hind neck <i>grey</i>.</td> + <td class="tdr">6. </td> + <td class="tdl">Whole top of neck and hind neck <i>brown</i>.</td> + </tr><tr> + <td class="tdr">7. </td> + <td class="tdl">General plumage with an <i>olivaceous</i> cast. </td> + <td class="tdr">7. </td> + <td class="tdl">General plumage with a <i>rufescent</i> cast.</td> + </tr> + </tbody> +</table> + +<p>These differences are illustrated in the accompanying coloured +plate, which has been most kindly prepared for me under the instructions +of Dr. F. M. Chapman of the American Museum of +Natural History. Before going further it is worth considering the +nature of these differences a little more closely. All but the last +are large differences which no one would overlook even in a hasty +glance at the birds. If the only distinction lay in the colour of the +quills we might feel fairly sure that <i>auratus</i> was a recessive form +of <i>cafer</i>, and so probably it is in this respect. Similarly the black +malar stripe of <i>auratus</i> is in all probability recessive to the red +malar stripe of <i>cafer</i> and I imagine the pigments concerned are +comparable with those in the Gouldian Finch (<i>Poephila gouldiae</i>) +of Australia. Both sexes in that species may have the head black, +red, or, less often, yellow, and though it is not any longer in +question that birds may breed in either plumage, I believe that +the young are always black-headed and I imagine that those +which become red-headed possess a dominant factor absent from +the permanently black-headed birds.<a name="FNanchor_2_91" id="FNanchor_2_91"></a><a href="#Footnote_2_91" class="fnanchor">[2]</a> +<span class="pagenum"><a name="Page_149" id="Page_149">[Pg 149]</a></span> +Yellow as a recessive form of a red is certainly very common, but red +and black as variants of the same pigment are less usual. In the Gouldian +Finch we seem to have a case where a pigment can assume all +three forms. It would be interesting to know whether the red +of the malar stripes in <i>Colaptes</i> is a pigment of the same nature +as the red of the quills. Both in <i>Colaptes</i> and in <i>Poephila +gouldiae</i> I have seen specimens intermediate between the black +and the red, and the appearance of the part affected was exactly +alike in the two cases, red feathers coming up among the black +ones, and many feathers containing both red and black pigments +mixed together.</p> + +<p class="indent">The development of the scarlet nuchal crescent in <i>auratus</i> +and the absence of this conspicuous mark in <i>cafer</i> constitute from +the physiological point of view the most remarkable pair of differences. +When the red crescent is not formed, the feathers +which would bear it are exactly like the rest, and no special +pigment is visible in them which one can regard as ready to be +modified into red. If the crescent is due to a factor it must +therefore be supposed that this factor has the power of modifying +the pigment of the neck in one special place alone. Dr. W. D. +Miller called my attention to the fact that a similar variation +occurs in another American woodpecker, the Sapsucker, +<i>Sphyropicus varius</i>.<a name="FNanchor_3_92" id="FNanchor_3_92"></a><a href="#Footnote_3_92" class="fnanchor">[3]</a> +</p> + +<p class="indent">I do not suggest that such variations are without parallel: +indeed in <i>P. gouldiae</i> the factor which turns the black of the head +into scarlet affects one special region of the black only, being +sharply distinct from the unmodified black of the throat. These +regions of the head are however often the seat of special colours +in birds.<a name="FNanchor_4_93" id="FNanchor_4_93"></a><a href="#Footnote_4_93" class="fnanchor">[4]</a> +So also may be instanced the variety of the Common +<span class="pagenum"><a name="Page_150" id="Page_150">[Pg 150]</a></span> +Guillemot (<i>Uria troile</i>) which has a white line round the eyes and +at the sides of the head where the normal has no such mark; but +this line is formed in a very special place, the groove joining the +eye to the ear, whereas the feathers of the nuchal crescent are +not ostensibly distinguished from those adjacent.<a name="FNanchor_5_94" id="FNanchor_5_94"></a><a href="#Footnote_5_94" class="fnanchor">[5]</a> +</p> + +<p class="indent">The transposition of the brown and the grey on the back +and front of the neck also constitutes a very remarkable difference. +If either grey or brown depends on a factor then it must be supposed +that <i>auratus</i> has one of these factors and <i>cafer</i> the other.</p> + +<p class="indent">From these several considerations it is quite clear that if +<i>auratus</i> and <i>cafer</i> are modifications of the same type produced +by presence or absence of factors, several independent elements +must be concerned, and to unravel their inter-relations would be +most difficult even if it were possible to breed the types under observation, +which is of course quite beyond present possibilities.</p> + +<p class="indent">The distribution of the two is as follows. On the east side +of the Continent <i>C. auratus</i>, relatively pure, occupies the whole of +Canada and the States from the North to Galveston. Westward +it extends across the whole continent in the more northern +region to Alaska, but in its pure form it only reaches down the +Pacific coast to about the northern border of British Columbia. +Its southern and western limit is thus roughly a line drawn from +north of Vancouver, southeast to North Dakota and then south +to Galveston. <i>C. cafer</i> in the comparatively pure form inhabits +Mexico, Arizona, California (except Lower California and the +opposite coast), central and western Nevada, Utah, Oregon, and +is bounded on the east by a line drawn from the Pacific south of +Washington, south and eastward through Colorado to the mouth +<span class="pagenum"><a name="Page_151" id="Page_151">[Pg 151]</a></span> +of the Rio Grande or the Gulf of Mexico. Between the two +lines thus roughly defined is a band of country about 1,200-1,300 +miles long and 300-400 miles wide, which contains some normal +birds of each type, but chiefly birds exhibiting the characters of +both, mixed together in various and irregular ways. Even in +the areas occupied by the pure forms occasional birds are recorded +with more or less indication of characteristics of the other +form, but within the area in which the two forms are conterminous, +the mixed birds are in the majority. The condition of these birds +of mixed character is described by Allen as follows:</p> + +<p class="blockquot">"As has been long known—indeed, as shown by Baird +in 1858—the 'intermediates' or 'hybrids' present ever-varying +combinations of the characters of the two birds, from individuals +of <i>C. auratus</i> presenting only the slightest traces of the characters +of <i>C. cafer</i>, or, conversely—individuals of <i>C. cafer</i> +presenting only the slightest traces of the characters of +<i>C. auratus</i>—to birds in which the characters of the two are about +equally blended. Thus we may have <i>C. auratus</i> with merely a few red +feathers in the black malar stripe, or with the quills merely +slightly flushed with orange, or <i>C. cafer</i> with either merely a few +black feathers in the red malar stripe, or a few red feathers at +the sides of the nape, or an incipient, barely traceable scarlet +nuchal crescent. Where the blending of the characters is more +strongly marked, the quills may be orange-yellow or orange-red, +or of any shade between yellow and red, with the other features +of the two birds about equally blended. But such examples are +exceptional, an unsymmetrical blending being the rule, the two +sides of the same bird being often unlike. The quills of the tail, +for example, may be part red and part yellow, the number of +yellow or red feathers varying in different individuals, and very +often in the opposite sides of the tail in the same bird. The +same irregularity occurs also, but apparently less frequently, +in the quills of the wings. In such cases the quills may be mostly +yellow with a few red or orange quills intermixed, or red with a +similar mixture of yellow. A bird may have the general colouration +of true <i>cafer</i> combined with a well-developed nuchal crescent, +or nearly pure <i>auratus</i> with the red malar stripes of a <i>cafer</i>. +<span class="pagenum"><a name="Page_152" id="Page_152">[Pg 152]</a></span> +Sometimes the body plumage is that of <i>C. auratus</i> with the head +nearly as in pure <i>cafer</i>, or exactly the reverse may occur. Or we +may have the general plumage as in <i>cafer</i> with the throat and +crown as in <i>auratus</i>, and the malar stripe either red or black, +or mixed red and black, and so on in almost endless variations, +it being rare to find, even in birds of the same nest, two individuals +alike in all their features of colouration. Usually the +first trace of <i>cafer</i> seen in <i>auratus</i> manifests itself as a mixture of +red in the black malar stripe, either as a few red feathers, or as a +tipping of the black feathers with red, or with merely the basal +portion of the feathers red. Sometimes, however, there is a +mixture of orange or reddish quills, while the malar stripe remains +normal. In <i>C. cafer</i> the traces of <i>auratus</i> are usually shown by a +tendency to an incipient nuchal crescent, represented often by +merely a few red-tipped feathers on the sides of the nape; at +other times by a slight mixture of black in the red malar stripe."</p> + +<p class="indent">Such a state of things accords very imperfectly with +expectations under any received theory of Evolution. As in some of +the instances discussed in the first chapter we have here two +fairly definite forms, nearly allied, which on any evolutionary +hypothesis must have been evolved either the one from the other, +or both from a third form at a time not very remote from the +present, as time must be measured in evolution. Yet though +intermediates exist in some quantity, no one can for a moment +suggest that they are that definite intermediate from which +<i>auratus</i> and <i>cafer</i> descend in common. One cannot imagine +that the immediate ancestor of these birds was a mosaic, made up of +asymmetrical patches of each sort: but that is what many of the +intermediates are. It is not much easier to suppose the ancestor +to have been a nondescript, with a compromise between the +developed characters of each, with quills buff, malar stripes +neither black nor red, with a trace of nuchal crescent, and so on. +Such Frankenstein-monsters have played, a considerable part in +the imaginations of evolutionary philosophers, but if it were +true that there was once a population of these monsters capable +of successful existence, surely they should now be found as a +population occupying the neutral zone between the two modern +<span class="pagenum"><a name="Page_153" id="Page_153">[Pg 153]</a></span> +forms. Yet, though much remains to be done in clearing up the +facts, one thing is certain, namely that the neutral zone has not a +definite and normally intermediate population, but on the contrary +it is peopled by fragments of the two definite types and +miscellaneous mongrels between them.</p> + +<p class="indent">On the other hand, one cannot readily suppose that +either form was the parent of the other. The process must have +involved both addition and loss of factors, for whatever hypothesis +be adopted, such changes must be supposed to have occurred. +A careful statistical tabulation of the way in which the characters +are distributed in the population of the mixed zone would be +of great value, and till that has been done there is little that can +be said with certainty as to the genetics of these characters. +In the collection of Dr. Bishop of New Haven I was very kindly +allowed to examine a sample, all taken at random, near together, +in Saskatchewan. There were females 4 adult, 2 young; males +4 adult and 5 young. This number, though of course insufficient, +is enough to give some guide as to the degree of definiteness which +the characters generally show in their variations. Of the 15 +birds, 8 had simply yellow quills; 2 had red; 1 was almost red +but had one yellow tail-quill; 3 were intermediate and 1 was +buff. As regards the malar patch, which can only be determined +properly in the adult males, 1 was red, 1 was approximately red, +2 intermediate. As to nuchal crescent 4 females had none, 2 +females very slight; 7 males had it, 1 had only a slight crescent, +and 1 had none. In point of quills therefore 10 were definite +out of 15; in point of crescent, 11 were definite out of 15; and in +point of malar patch 1 only was definite out of 4. The last is a +feature directly dependent on age and so counts for less, but as +regards the other two features there is some indication that the +factors show definiteness in their behaviour. It must be remembered +that we have no knowledge what the heterozygous +form may be, and in the case of red and yellow it is probably a +reddish buff. The patch-works are no doubt to be compared with +other well-known pied forms, and in these we must suppose the +active factor broken up, which it probably can be very easily. +The asymmetry, which Allen notices as so marked a feature, in the +<span class="pagenum"><a name="Page_154" id="Page_154">[Pg 154]</a></span> +distribution of the red and yellow quills of the tail especially, +recalls that of the black markings in the pied Canaries. As is well +known to students of variations <i>some</i> pigment-factors in <i>some</i> +animals are apparently uncontrolled by symmetry, while in other +specific cases symmetry is the rule. On the other hand the +blackness or redness of the malar patches is, I think, as a rule +nearly symmetrical. It should be mentioned that two of Dr. +Bishop's young birds belonged to the same nest, one a female +with <i>red</i> quills, the other a male with <i>yellow</i>. +Both are without crescent.</p> + +<p class="indent">As to the question whether certain combinations of characters +occur with special frequency, the evidence is insufficient to give a +definite answer. Among all the birds I have seen in America +or in England I have not yet found one having the malar patches +black without any nuchal crescent. Of Dr. Bishop's 8 adults +not one, however, showed the combination of the three chief +features normal for <i>auratus</i> or for <i>cafer</i>.</p> + +<p class="indent">Besides the two forms that we have hitherto considered, +several other local types exist, and these throw some further +light on the problem. Of these the most important in this +connexion is <i>chrysoides</i>, which inhabits the whole of southern +California and the mainland opposite. This remarkable form +is as Allen says, very different from <i>auratus</i> except that it has +the quills yellow like <i>auratus</i>, not red like <i>cafer</i>. So that +we find here in the extreme west of the whole distribution a type +agreeing in one of its chief features with the eastern type. Between +this and <i>cafer</i> intergrades have, according to Allen, not +been found. The relations of this <i>chrysoides</i> are, Allen thinks, +rather with <i>mexicanoides</i>, a southern, smaller race with colours +more intense, which inhabits Guatemala, but however that may +be, it must be regarded as a <i>cafer</i> which has lost its red quills. +The island of Guadeloupe off Lower California has an island +form. Beyond the other side of the continent there is also an +island form of <i>auratus</i>, inhabiting Cuba, so that clearly the yellow +quills can extend into the tropics.</p> + +<p class="indent">The above account is in many respects incomplete, but +it suffices to give an outline of the chief facts. The whole problem +<span class="pagenum"><a name="Page_155" id="Page_155">[Pg 155]</a></span> +is complicated by the undoubted effects of an uncertain amount +of migration, and in many, perhaps all, districts, the winter +population differs from the summer population of the same +localities. The existence of these seasonal ebbs and flows is +now well known to ornithologists, and most of the bird species +of temperate regions are subject to them.</p> + +<p class="indent">Difficult as it may be to conceive the actual process of +origin of the two types <i>auratus</i> and <i>cafer</i>, it is I think still +harder to suggest any possible circumstance which can have determined +their development as distinct races, or which can maintain +that distinctness when created. Some will no doubt be disposed +to appeal once more to our ignorance and suggest that if we only +knew more we should see that the yellow quills, the black +"moustache" and the red crescent, specially qualify <i>auratus</i> +for the north and eastern region, and the red quills, red "moustache" +and absence of crescent fit <i>cafer</i> to the conditions of +its homes. Each can judge for himself, but my own view is +that this is a vain delusion, and that to cherish it merely blunts +the receptivity of the mind, which if unoccupied with such fancies +would be more ready to perceive the truth when at last it shall +appear. Think of the range of conditions prevailing in the +country occupied by <i>auratus</i>—a triangle with its apex in Florida +and its base the whole Arctic region of North America. Is it +seriously suggested that there is some element common to the +"conditions" of such an area which demands a nuchal crescent +in the Flickers, though the birds of the <i>cafer</i> area, almost equally +varied, can dispense with the same character? Curiously enough, +the geographical variation of <i>Sphyropicus varius</i>, another though +a very different Woodpecker<a name="FNanchor_6_95" id="FNanchor_6_95"></a><a href="#Footnote_6_95" class="fnanchor">[6]</a> +shows that conversely the nuchal crescent can be dispensed with +in the Eastern form though it is assumed by the Western.<a name="FNanchor_7_96" id="FNanchor_7_96"></a><a href="#Footnote_7_96" class="fnanchor">[7]</a> +<span class="pagenum"><a name="Page_156" id="Page_156">[Pg 156]</a></span></p> + +<p class="indent">Allen points out the interesting additional fact that superposed +upon each of the two distinct forms, <i>auratus</i> and <i>cafer</i>, +are many geographical variations which can very naturally be +regarded as climatic. Each decreases in size from the North +southward, as so many species do.<a name="FNanchor_8_97" id="FNanchor_8_97"></a><a href="#Footnote_8_97" class="fnanchor">[8]</a> +They become paler in the arid plains, and show the +ordinary phases which are seen in other birds having +the same distribution. Such differences +we may well suppose to be determined directly or indirectly, by +environment, and we may anticipate with fuller knowledge it +will be possible to distinguish variations of this nature as in the +broad sense environmental, from the larger differences separating +the two main types of <i>Colaptes</i>, which I surmise are altogether +independent of such influences.</p> + +<p class="indent">It is generally supposed that phenomena like those now so +well established in the case of <i>Colaptes</i> are very exceptional, and +as has already been stated a number of circumstances must +combine in order that they may be produced. I suspect however +that the examples are more numerous than is commonly thought. +In all likelihood the three forms <i>Sphyropicus varius</i>, <i>nuchalis</i> +and <i>ruber</i> are in a very similar condition though the details +have not, so far as I know, been worked out. A complex example +which is closely parallel to the case of <i>Colaptes</i> was described by +F. M. Chapman<a name="FNanchor_9_98" id="FNanchor_9_98"></a><a href="#Footnote_9_98" class="fnanchor">[9]</a> +at the same date as Allen's work. This is the +case of <i>Quiscalus</i>, the Grackles, which in the North American +Continent have three fairly distinct forms which Chapman speaks +of as <i>Q. aeneus</i>, <i>Q. quiscula</i>, and <i>Q. quiscula aglaeus</i>. +The birds are all, so far as pigment is concerned, dark blackish brown, but +the head and mantle have superposed a metallic sheen of interference-colours +which in the various forms take different tints, +<span class="pagenum"><a name="Page_157" id="Page_157">[Pg 157]</a></span> +bluish green, bronze green, or bronze purple. The details are +complicated and difficult to appreciate without actual specimens, +but the two common types are sufficiently distinct. The birds +inhabit the whole area east of the Rockies, <i>quiscula aglaeus</i> +occupying Florida and the Southern States southwest of a band +of country about a hundred miles broad extending roughly from +Connecticut to the mouth of the Mississippi; and <i>aeneus</i> taking +the area north and west of this band. In discussing this case +Chapman expresses the same view as Allen does in the <i>Colaptes</i> +case, that there are two distinct populations, substantially fixed, +and that the band of country in which they meet each other +has a mongrel population, with no consistent type, but showing +miscellaneous combinations of the character of the two chief types.</p> + +<p class="indent">The warblers of the genus <i>Helminthophila</i> provide +another illustration which has points of special interest. The two chief +species are <i>H. pinus</i>, which has a yellow mantle and lower parts, +white bars on the wings, a black patch behind the eyes and a +broad black mark on the throat; and <i>H. chrysoptera</i> with dark grey +mantle and pale whitish grey lower parts, yellow bars on the +wings, and grey marks on cheeks and throat where <i>pinus</i> has +black. These two birds are exceeding distinct, and in addition +their songs are quite unlike. <i>H. pinus</i> ranges through the eastern +United States up to Connecticut and Iowa. <i>H. chrysoptera</i> is a +northern form extending down to Connecticut and New Jersey. +Both are migrants.</p> + +<p class="indent space-below">In these two States, where the two types overlap, +certain forms have been repeatedly found which have been described as +two distinct species, <i>Lawrencei</i> and <i>leucobronchialis</i>. Dr. L. B. +Bishop and Mr. Brewster showed me two long series of <i>Helminthophila</i> +containing various intergrades between the four +named kinds, and details regarding these may be found in +Chapman's <i>North American Warblers</i> and in Dr. Bishop's paper +in Auk, 1905, XXII. Though the characters evidently break +up to some extent, the series can be represented as due to +recombinations of definite factors more easily than the others +which I have described. The differentiating characters are: +<span class="pagenum"><a name="Page_158" id="Page_158">[Pg 158]</a></span></p> +<table border="0" style="max-width: 45em;" cellspacing="2" summary="Warblers" cellpadding="0" > + <tbody><tr> + <td class="tdr"> </td> + <td class="tdl">   <i>Pinus</i></td> + <td class="tdr"> </td> + <td class="tdl">   <i>Chrysoptera</i></td> + </tr><tr> + <td class="tdr"> </td> + <td class="tdl"> </td> + <td class="tdr"> </td> + <td class="tdl"> </td> + </tr><tr> + <td class="tdr">1. </td> + <td class="tdl">Mantle and lower parts <i>yellow</i> (<b>Y<sup>1</sup></b>). </td> + <td class="tdr">1. </td> + <td class="tdl">Mantle and lower parts <i>grey</i> (<b>y<sup>1</sup></b>).</td> + </tr><tr> + <td class="tdr">2. </td> + <td class="tdl">Wing-bars <i>white</i> (<b>y<sup>2</sup></b>).</td> + <td class="tdr">2. </td> + <td class="tdl">Wing-bars <i>yellow</i> (<b>Y<sup>2</sup></b>).</td> + </tr><tr> + <td class="tdr">3. </td> + <td class="tdl">Cheek and throat <i>not black</i> (<b>b</b>).</td> + <td class="tdr">3. </td> + <td class="tdl">Cheek and throat <i>black</i> (<b>B</b>).</td> + </tr> + </tbody> +</table> + +<p class="indent space-above">The grey pigment of the mantle is common to both, but is +masked by the yellow in <i>pinus</i>, the net result being an olive-green.<a name="FNanchor_10_99" id="FNanchor_10_99"></a><a href="#Footnote_10_99" class="fnanchor">[10]</a> +</p> + +<p class="indent">I am much indebted to Dr. F. M. Chapman for the loan +of the coloured plate in which these distinctions are shown. It first +appeared in his book, <i>North American Warblers</i>.</p> + +<p class="indent">We cannot tell whether <i>yellow</i> or <i>not-yellow</i> +is due to the presence of a factor, but we may suppose that one or other +gives the special colour to the parts. The black of character 3 +is no doubt a dominant. Thus <i>pinus</i> becomes Y<sup>1</sup>y<sup>2</sup>b and <i>chrysoptera</i> +in y<sup>1</sup>Y<sup>2</sup>B. The <i>Lawrencei</i> which has the underparts +<i>yellow</i>, wing-bars <i>white</i>, and <i>black</i> patches is Y<sup>1</sup>y<sup>2</sup>B and +<i>leucobronchialis</i> which has mantle and underparts <i>not-yellow</i>, wing-bars +<i>yellow</i> and <i>no black patches</i> is y<sup>1</sup>Y<sup>2</sup>b. This representation, +it should be clearly understood, is tentative and approximate +only. The characters are not really sharp, for there is much +grading; but allowing for the effects of heterozygosis and for some +actual breaking-up of factors I believe it gives a fairly correct +view of the case. In particular we can see how it meets the difficulty +which Chapman felt in accepting <i>leucobronchialis</i> as in +any sense derived from <i>pinus</i> which has a yellow breast, and +<i>chrysoptera</i> which has a black throat, seeing that <i>leucobronchialis</i> +has neither. We now recognize at once that this form could be +produced by ordinary re-combination of the absence of Y<sup>1</sup> with +the absence of B.</p> + +<p class="indent space-below">I note also with great interest that the modern +observers agree that the so-called hybrids may have the song either of +the one species, or of the other, or a song intermediate between +the two. It may also be added that these two types have several +times been seen, in the breeding season, paired with each other +or with one of the other combinations.</p> + +<p><span class="pagenum"><a name="Page_159" id="Page_159">[Pg 159]</a></span></p> +<table border="0" style="max-width: 45em;" cellspacing="2" summary="_" cellpadding="0" > + <tbody><tr> + <td class="tdr"><span class="smcap"><b>Fig. 1.</b> </span></td> + <td class="tdl"><i>Helminthophila pinus</i>, male.</td> + </tr><tr> + <td class="tdr"><span class="smcap"><b>Fig. 2.</b> </span></td> + <td class="tdl"><i>Helminthophila pinus</i>, female.</td> + </tr><tr> + <td class="tdr"><span class="smcap"><b>Fig. 3.</b> </span></td> + <td class="tdl">"Lawrence's Warbler," male; one of the integrading forms.</td> + </tr><tr> + <td class="tdr"><span class="smcap"><b>Fig. 4.</b> </span></td> + <td class="tdl">"Brewster's Warbler," male; another of the integrading forms.</td> + </tr><tr> + <td class="tdr"><span class="smcap"><b>Fig. 5.</b> </span></td> + <td class="tdl"><i>Helminthophila chrysoptera</i>, male.</td> + </tr><tr> + <td class="tdr"><span class="smcap"><b>Fig. 6.</b> </span></td> + <td class="tdl"><i>Helminthophila chrysoptera</i>, female.</td> + </tr> + </tbody> +</table> + +<div class="figcenter" > + <img src="images/i_175.jpg" alt="Warblers" width="600" height="941" /> +</div> + +<p class="indent space-above space-below">Allen<a name="FNanchor_11_100" id="FNanchor_11_100"></a><a href="#Footnote_11_100" class="fnanchor">[11]</a> +has described another excellent American example, +the Tits of the group <i>Baeolophus bicolor-atricristatus</i>. The form +<i>bicolor</i> belongs to the eastern States and ranges from the Atlantic +coast to the Great Plains, and <i>atricristatus</i>, of east Mexico, +extends from Vera Cruz to central Texas. In southern and +central Texas the breeding ranges adjoin, and in this country +various intermediates occur. The chief types differ in two main points.</p> + +<table border="0" style="max-width: 45em;" cellspacing="2" summary="Warblers" cellpadding="0" > + <tbody><tr> + <td class="tdl">   <i>B. bicolor</i></td> + <td class="tdl">   <i>B. atricristatus</i></td> + </tr><tr> + <td class="tdl"> </td> + <td class="tdl"> </td> + </tr><tr> + <td class="tdl">Forehead varies from <i>deep black</i></td> + <td class="tdl"> Forehead <i>white</i> to <i>buffish white</i>.</td> + </tr><tr> + <td class="tdl"> to <i>dull black</i>, suffused with <i>rusty brown</i>.</td> + <td class="tdl"> </td> + </tr><tr> + <td class="tdl">Crown and crest <i>grey</i>,</td> + <td class="tdl"> Crown and crest <i>black</i>, abruptly</td> + </tr><tr> + <td class="tdl"> slightly darker than the back.</td> + <td class="tdl">  contrasting with the back.</td> + </tr> + </tbody> +</table> + +<p class="space-above">The intergrades between the two have, as usual, +received specific names. A detailed description is given by Allen, from which +it appears that the gradation is very complete. In one case a +series of 16 adults were all intermediates. It is not stated whether +the collector took these at random, but from the local lists it is +clear that the types are found not far away from the place where +the intergrades were shot.</p> + +<p class="indent">Another very striking case is that of the Tanagers, of the +genus <i>Rhamphocoelus</i>. In this group there are several local forms +which are related to each other in remarkable ways. The forms +known as <i>passerinii</i> and <i>icteronotus</i> exhibit the clearest +phenomena of intergradation. The species <i>passerinii</i> has a +brilliant scarlet and black male, and it inhabits Honduras and Nicaragua. +Proceeding southwards along the isthmus we find next <i>costaricensis</i> +which has a male like that of <i>passerinii</i> (but a female +with more orange than the olive-grey female of <i>passerinii</i>). +Next we come to Panama which is occupied by <i>icteronotus</i>, +sharply distinguished from <i>passerinii</i> by the fact that the <i>scarlet +is replaced by lemon-yellow</i>. This same <i>icteronotus</i> occurs again +as a pure type in Ecuador and many other parts of South America; +but Colombia, <i>between Panama and Ecuador</i>, contains scarlets +like <i>passerinii</i>, yellows like <i>icteronotus</i>, and various intergrades +<span class="pagenum"><a name="Page_160" id="Page_160">[Pg 160]</a></span> +of several shades of orange. The <i>passerinii</i> males from Nicaragua +are indistinguishable from those of Colombia, and the +<i>icteronotus</i> of Ecuador are the same as those in Panama. The +orange intergrades, doubtless heterozygous forms, though collected +at the same locality (Medellin in Colombia) as several +pure yellows and pure scarlets, are in the British Museum series +sorted out as a separate species under the name <i>chrysonotus</i>! +Complications are introduced by the relations of these forms to +another named type, <i>flammigerus</i>, but we may for our purpose +leave that out of consideration, and say that the order of geographical +sequence from Honduras to Ecuador is (1) scarlet, (2) yellow, +(3) mixture of types, scarlet, yellow, orange, (4)yellow.</p> + +<p class="indent space-below">Similar examples exist in the birds of the old world, +but I do not know of any that have been studied so fully as those of +America. The best known is that of the two Rollers, <i>Coracias +indicus</i> which spreads from Asia Minor through Persia, Baluchistan, +the Indian Peninsula and Ceylon, and <i>affinis</i> which +ranges from Nepal, through Assam, Tenasserim and the Indo-Chinese +countries. The two types are very different and may be +distinguished as follows:</p> +<table border="0" style="max-width: 45em;" cellspacing="2" summary="Rollers" cellpadding="0" > + <tbody><tr> + <td class="tdl">   <i>C. indicus</i></td> + <td class="tdl">   <i>C. affinis</i></td> + </tr><tr> + <td class="tdl"> </td> + <td class="tdl"> </td> + </tr><tr> + <td class="tdl"><i>Mantle</i> drab brown-chestnut.</td> + <td class="tdl">Dark olive-green.</td> + </tr><tr> + <td class="tdl"><i>Breast</i> chestnut.</td> + <td class="tdl">Dull purple brown.</td> + </tr><tr> + <td class="tdl"><i>Throat</i> purplish, streaked with white. </td> + <td class="tdl">Purple, streaked with blue.</td> + </tr><tr> + <td class="tdl"><i>Upper tail-coverts</i> indigo.</td> + <td class="tdl">Turquoise.</td> + </tr> + </tbody> +</table> + +<p class="indent space-above">The wings are the same in both. In the provinces +of Nepal, Sikhim, and Darjiling the two species coexist, with the result +that intergrades have been frequently recorded. The line of +intergradation extends to the coast, and birds showing various +combinations of the two types from the Calcutta district exist +in collections.<a name="FNanchor_12_101" id="FNanchor_12_101"></a><a href="#Footnote_12_101" class="fnanchor">[12]</a> +The case is interesting inasmuch as like that of +<i>Quiscalus</i> it shows a series of combinations of various metallic +colours. Some of these are probably evoked by the development +of pigment behind striations or other interferences already existing, +but in the present state of knowledge it would be quite impossible +<span class="pagenum"><a name="Page_161" id="Page_161">[Pg 161]</a></span> +to suggest what the actual factors producing these appearances may be.</p> + +<p class="indent">There are, naturally, many other cases among birds +which are suspected of being in reality comparable, but in most of them +the evidence is still inadequate. Among Lepidoptera also there +are a few of these; perhaps the most striking is that of <i>Basilarchia +"proserpina."</i><a name="FNanchor_13_102" id="FNanchor_13_102"></a><a href="#Footnote_13_102" class="fnanchor">[13]</a> +The genus is well known to European collectors +under the name <i>Limenitis</i>, of which we in England have +one species, <i>L. sibylla</i>, the "White Admiral." A species very +like <i>sibylla</i> in general appearance is common in the northern +parts of the United States, ranging through Canada and Northern +New England, but rarely south of Boston. This species has +the conspicuous white bands across both wings like our <i>sibylla</i>.</p> + +<p class="indent">There is also a more Southern type known as <i>astyanax</i>, +which is very different in its appearance, being without the white +bands and having a broad irroration of blue scales on the posterior +border of the hind wings. The two are so distinct that one would +not be tempted to suspect any very close relation between them. +In its distribution <i>astyanax</i> is described by Field as replacing +arthemis south of latitude 42°. About Boston it is much more +common than <i>arthemis</i>.</p> + +<p class="indent">The two forms encroach but little on each other's territory, +but where they do coexist, a third form, known as <i>proserpina</i>, is +found which is almost intermediate, with the white bands much reduced. +There is now no doubt that this <i>proserpina</i> is a heterozygous +form, resulting from a combination of the characters of +<i>arthemis</i> and <i>astyanax</i>. Field succeeded in rearing a brood of 16 +from a <i>proserpina</i> mother caught wild which laid 31 eggs, and of +these, nine (five males, four females) resembled the mother, +being <i>proserpina</i>, and seven (four males, three females) were +<i>arthemis</i>. There can be no question therefore that the mother +had been fertilised by a male <i>arthemis</i> and that <i>no-white-band</i> +is a factor partially dominant over the <i>white band</i>. Another +point of interest which Field observed was that the <i>proserpina</i> +female refused to lay on birch, poplar or willow, but accepted +<span class="pagenum"><a name="Page_162" id="Page_162">[Pg 162]</a></span> +wild cherry (<i>Prunus serotina</i>) a species on which <i>astyanax</i> can +live, though that tree is not known to be eaten by <i>arthemis</i>. +Incidentally also the observations show that sterility cannot be +supposed to be the bar which maintains the distinctness of +<i>arthemis</i> and <i>astyanax</i>.</p> + +<p class="indent">In this connection <i>Papilio oregonia</i> and <i>bairdii</i> should be +mentioned.<a name="FNanchor_14_103" id="FNanchor_14_103"></a><a href="#Footnote_14_103" class="fnanchor">[14]</a> +<i>P. oregonia</i> is one of the numerous forms like +<i>machaon</i>, but rather paler. It is a northern insect, inhabiting +British Colombia east of the Cascade Range, and reaching to +Colorado. <i>P. bairdii</i> is a much darker butterfly, representing +the <i>asterias</i> group of the genus <i>Papilio</i>. Like <i>asterias</i> +it has the abdomen spotted at the sides, not banded as in the <i>machaon</i> +group. It belongs to Arizona and Utah extending into Colorado. +From Colorado the form <i>brucei</i> is described, more or less intermediate, +like <i>bairdii</i> but with the abdomen banded as in <i>oregonia</i>. +W. H. Edwards records the results of rearing the offspring of the +<i>bairdii</i>-like and of the <i>oregonia</i>-like mothers. Each was found +able to have offspring of both kinds, that is to say, <i>bairdii</i> +females gave both forms, and <i>oregonia</i> females gave both forms. +It is not possible to say which is dominant, since the fathers were +unknown. On general grounds one may expect that the <i>bairdii</i> +form will be found to dominate, but this is quite doubtful.</p> + +<p class="indent">From this particular discussion I omit reference to +those examples in which the permanently established types are obviously +associated with special conditions of life. Where considerable +climatic differences exist between localities, or when we +pass from South to North, or from the plains into Alpine levels +we often find that in correspondence with the change of climate +there is a change in the characteristics of a species common to +both. When I say "species" in such a connection I am obviously +using the term in the inclusive sense. Some would prefer to +say that in the two sets of conditions two <i>representative species</i> +exist. Whichever expression be preferred it is plain that such +examples present another phase of the problem we have been +just considering, and in them also we have an opportunity of +<span class="pagenum"><a name="Page_163" id="Page_163">[Pg 163]</a></span> +observing the consequences of the overlap of two closely related +types, but there are advantages in considering them separately. +In the examples hitherto given, with the possible exception of +the Papilios,<a name="FNanchor_15_104" id="FNanchor_15_104"></a><a href="#Footnote_15_104" class="fnanchor">[15]</a> +the two fixed types severally range over so extensive +a region that it may fairly be supposed that in the different +parts they are subject to considerable diversities of climate. +There is no outstanding difference that we know distinguishing +the habitats of the two forms; but in comparing Alpine with +Lowland forms, or essentially northern with essentially southern +forms we do know an external circumstance, temperature, that +may reasonably be supposed to have an influence, direct or indirect, +on the population.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_164" id="Page_164">[Pg 164]</a></span></p> +<h2>CHAPTER VIII</h2> + +<h3>LOCALLY DIFFERENTIATED FORMS. <i>Continued.</i></h3> +<p style="font-size: 120%; text-align: center;"> +<b><span class="smcap">Climatic Varieties</span></b></p> + +<p class="indent">In this chapter we will examine certain cases which +illustrate phenomena comparable with those just considered, though as +I have already indicated, they form to some extent a special +group. The outstanding fact that emerges prominently from +the study of the local forms is that when two definite types, +nearly allied, and capable of interbreeding with production of +fertile offspring, meet together in the region where their +distributions overlap, though intergrades are habitually found, +there is no normally or uniformly intermediate population occupying +the area of intergradation. Such phenomena as these +must, I think, be admitted to have great weight in any attempt +to construct a theory of evolution. True we must hesitate in +asserting their positive significance, but I see no escape from the +conclusion that they throw grave doubt on conventional views. +Again and again the same question presents itself. If <i>A</i> and +<i>B</i> lately emerged from a common form why is that common form +so utterly lost that it does not even maintain itself in the region +of overlapping? Almost equally difficult is it, in the cases which +I have numerated, to apply concrete suggestions based on any +factorial scheme. We may see that in <i>Heliconius erato</i> the type +with the red mark on the hind wing probably contains a dominant +factor, and that where the red mark is absent the metallic colours +are exposed; and that similarly the green metallic colour may have +another factor which distinguishes it from the blue. In this way +we can fairly easily represent the various types of <i>erato</i> on a +factorial system as the result of the various possible combinations +of two pairs of factors. But there we stop, and we are quite +unable to suggest any reason why one area should have the red +and the green type while another should have the blue also. So +again with <i>Colaptes</i> or the Warblers. By application of a +<span class="pagenum"><a name="Page_165" id="Page_165">[Pg 165]</a></span> +factorial system, admittedly in a somewhat lax fashion, the genetic +interrelations of the types can be represented; but how it comes +about that each type maintains a high degree of integrity in +its own region we can only imagine. Each has in actual fact +a stability which the intermediate forms have not, but we cannot +yet analyse the nature of that stability. Mendelian conceptions +show us how by segregation the integrity of the factors can be in +some degree maintained, but not why certain combinations of +factors should be exceptionally stable. All that is left us to +fall back on is the old unsatisfying suggestions that some combinations +<i>may</i> have greater viability than others, that there +<i>may</i> be a tendency for like to mate with like, and so forth.</p> + +<p class="indent">These difficulties acquire more than ordinary force +in those cases in which the two fixed types inhabit regions differing +in some respect so obvious and definite that we are compelled to +regard each type as climatic and as specially adapted to the +conditions. When for example an animal has a distinct type +never met with except in Arctic or Alpine conditions, and another +type proper to the plains and temperate regions, what are the +characteristics of the population of intermediate latitudes or +at intermediate levels? Some of the examples discussed in the +last chapter may be instances of this very nature, but even if +they are not, others are forthcoming which certainly are. The +evidence of these cases leads to the suspicion that with further +knowledge they will be found to consist of two classes, some in +which the observer as he passes from the one climate to the other +will find the intermediate area actually occupied by a population +of intermediate character, and others in which, though we may +presume the maintenance of intermediate conditions in the transitional +area, there is no definite transitional population. This +interrupted or discontinuous distribution seems, so far as I +have means of judging, to be by far the more common of the two. +I do not doubt that by sufficient search individuals representing +every or almost every transitional form can be found, but it is +apparently rare that <i>populations</i> corresponding to these several +grades can be seen. The question has in few if any cases been +studied with precision sufficient to provide a positive answer; +<span class="pagenum"><a name="Page_166" id="Page_166">[Pg 166]</a></span> +but I suspect that real and complete continuity, in the sense +thus defined, will only be found where the character of the local +populations depends <i>directly</i> on the conditions of life, and shows +an immediate response to changes in them apart from that postponed +response which we suppose to be achieved by selection. +Obviously the character must be one, like size for instance, +capable of sensibly complete gradation.</p> + +<p class="indent">The only example I have met with of the phenomenon +of anything like a complete intergradation between local types +really distinct in kind is that provided by the butterfly <i>Pararge +egeria</i>. It is well known to entomologists that this insect exists +in two very different types, a northern one, the "Speckled Wood" +of England, in which the spots are a pale whitish yellow, and a +southern type having the full fulvous colour that we know as +characteristic of <i>megaera</i>, the "Gatekeeper." It appears that +Linnaeus gave the name <i>egeria</i> to the southern type,<a name="FNanchor_1_105" id="FNanchor_1_105"></a><a href="#Footnote_1_105" class="fnanchor">[1]</a> +and our own is now called <i>egerides</i>. Broadly speaking, so far +as Great Britain, France, and the Spanish Peninsula are concerned, the +tawny-coloured <i>egeria</i> occupies Spain and western France up +to the latitude of Poitiers and the pale yellow <i>egerides</i> extends from +Scotland, where it has a scanty distribution, through southern +England, where in suitable localities it is common, and the north +of France to Paris.<a name="FNanchor_2_106" id="FNanchor_2_106"></a><a href="#Footnote_2_106" class="fnanchor">[2]</a> +The two types when placed side by side +are strikingly different from each other, and are an excellent +illustration of what is meant by climatic variation. The insect +is not a great traveller and probably scarcely ever wanders far +from its home. It should therefore be possible by collecting +from north to south to find out how the transition is effected, +whether suddenly or gradually. This at various times I have +endeavoured to do, but I am still without exact information as +to the population in certain critical areas. In addition to the +information derived from specimens which I have collected or +seen in the collections of others there is a good account of the +general distribution in Europe given by the Speyers,<a name="FNanchor_3_107" id="FNanchor_3_107"></a><a href="#Footnote_3_107" class="fnanchor">[3]</a> +<span class="pagenum"><a name="Page_167" id="Page_167">[Pg 167]</a></span> +who evidently paid more attention to the subject than most lepidopterists +have done, and many more recent records. In particular +Oberthür<a name="FNanchor_4_108" id="FNanchor_4_108"></a><a href="#Footnote_4_108" class="fnanchor">[4]</a> +has published many details as to the distribution in +western France and I am especially indebted to Mr. H. Rowland-Brown +for a long series of notes as to the distribution in France +generally, and to Mr. H. E. Page and Dr. T. A. Chapman, Mr. +Oberthür Prof. Arrigoni degli Oddi, Mr. H. Williams and other +correspondents, for showing me forms from many localities. The +butterfly is attached for the most part to woods of deciduous trees +and to country abounding in tall hedges or rough scrub. It is not +usually to be found in highly cultivated districts or in very dry +regions. Hence there is necessarily some want of continuity in +the distribution at the present time and I should think a mile or +two of arable land without big hedges would constitute a barrier +hardly ever passed. The larva feeds on several coarse grasses, +especially <i>Dactylis glomerata</i>. Barrett mentions also <i>Triticum +repens</i>. In this country the winter is usually passed in the larval +stage, but I have found that in captivity, at least, there is much +irregularity. The larvæ feed whenever the weather is not very +cold and may pupate, but if sharp cold comes on when they are pupating +or nearly full-grown they often get killed unless protected.</p> + +<p class="indent">Some writers speak of a difference between the early +and later broods, but I have never noticed this, and I do not think +that the general tone of the yellow is affected by the seasons +(see Tutt, <i>Ent. Rec.</i>, IX, 1897, p. 37).<a name="FNanchor_5_109" id="FNanchor_5_109"></a><a href="#Footnote_5_109" class="fnanchor">[5]</a> +</p> + +<p class="indent">Beginning at the south of Spain the thoroughly fulvous +type <i>egeria</i> is common at Gibraltar in the Cork woods, at Granada, +and doubtless generally. Lederer is said to have found only +this type in Spain (Speyer), and though I have no precise information +as to other places in the Peninsula north of Jaen I feel +tolerably sure that there is no change from south to north.<a name="FNanchor_6_110" id="FNanchor_6_110"></a><a href="#Footnote_6_110" class="fnanchor">[6]</a> +<span class="pagenum"><a name="Page_168" id="Page_168">[Pg 168]</a></span> +Immediately north of the Pyrenees we still meet <i>egeria</i> exclusively, +and up to Poitiers at least there is no noticeable change. But +somewhere between Poitiers and the bottom of the Loire valley +at Tours, the genuine southern type comes to an end, and the +whole population begins at the Loire to be of an intermediate +type, easy to distinguish both from <i>egeria</i> and from <i>egerides</i>. +As to the exact condition of the species in the fifty miles separating +St. Savin on the Vienne from places on the Loire I have no adequate +information. I have only one small sample from there, +but it does contain insects both of the southern and intermediate +types taken on the same day, in a wood near Preuilly. Oberthür +also states that at Nantes the true southern form exists in company +with the northern. From this I infer that the southern +form extends up the coast further than it does inland, but I +imagine the representative spoken of as northern would be of +usual Brittany or intermediate type.</p> + +<p class="indent">The Vienne river joins the Loire, so the true southern +type reaches over into the basin of the Loire. From the Loire (Tours, +Corméry) north to Calvados (Balleroy) only the intermediate +is found, so far as I know, and the same type extends over +Brittany.<a name="FNanchor_7_111" id="FNanchor_7_111"></a><a href="#Footnote_7_111" class="fnanchor">[7]</a> +In general, however, the woods near Paris have the +thoroughly northern type <i>egerides</i>, but at St. Germain-en-Laye +and at Etampes (Oberthür) the population approaches the intermediate type.</p> + +<p class="indent">On the whole the intermediate type is certainly less +homogeneous than either of the extremes, and females with the two +central spots either paler or more fulvous than the rest are not +uncommon, but I have never taken one on the Loire or in Brittany +which I should class with either of the extreme types.</p> + +<p class="indent">Before speaking of the distribution in other parts +of France and in Europe generally I will briefly state the results of my +breeding experiments. The work was done many years ago before +we had the Mendelian clue, and it is greatly to be hoped that +some one will find opportunities of repeating it. Crossing the +English and the thoroughly southern type the families produced +<span class="pagenum"><a name="Page_169" id="Page_169">[Pg 169]</a></span> +agree entirely with the intermediates of Brittany and the Loire. +Reciprocals are alike. Of F<sub>2</sub> I only succeeded in raising very +few and of those that I had (about 30) nearly all were intermediate +in character, though perhaps rather less uniform than F<sub>1</sub>. One +family alone, containing only 4 specimens, had one <i>egerides</i>, +and three fulvous intermediates. As the case stands alone I +hesitate whether or not to suppose it due to some mistake. Moreover +from F<sub>1</sub> crossed back with the respective parental types I +had fairly long series, especially from F<sub>1</sub> × the southern type, and +looking at these families I cannot see any clear evidence of segregation. +On the contrary, I think that though there are slight +irregularities, they would, taken as a whole, be classed as coming +between the intermediate type and the extreme form used as the +second parent. This at least is true when the second parent was +of the southern type.</p> + +<p class="indent">On this evidence I have regarded the case as one in +which there is no good evidence of segregation and as conforming most +nearly with the conventional view of gradual transition in response +to climatic influences. Such influence must however be +indirect; for I reared five generations of the northern type in +England, and these, though they included several abnormal-looking +specimens in the last generation and then died out, did +not show any noticeable change from the fulvous colour of the +wild type. Merrifield<a name="FNanchor_8_112" id="FNanchor_8_112"></a><a href="#Footnote_8_112" class="fnanchor">[8]</a> +also found that heat applied to pupae of the northern type +produced no approach to the southern type.</p> + +<p class="indent">Looking at the facts now in the light of more experience +it seems to me just possible that the case may be one in which, as +in Nilson-Ehle's Wheats, the dominant differs from the recessive +in having two pairs of factors with similar effects. The +fulvous type for example may have two or more elements in +separate pairs which together produce the full effect, and the +intermediate may have one of these. If this were so, some +segregation should of course eventually be observable, but the +proportion of the various fulvous and fulvous-intermediate +individuals would be large, and the reappearance of actual +<span class="pagenum"><a name="Page_170" id="Page_170">[Pg 170]</a></span> +representatives of the northern type might be rare. I admit that this +is a somewhat strained interpretation of the facts, and as yet it +is not entitled to serious consideration. Nevertheless I am led +to form some such expectation partly from the great difficulty +in the way of any other, partly from the evidence of the small +mixed sample found at Preuilly and partly from the statements +given by Oberthür. There are moreover other features in the +general distribution of the species which make it improbable +that the dependence on climate can after all be so close. Published +lists are unfortunately of little use in deciding which form +occurs at a particular place, because, since the name <i>Meone</i> has +ceased to be used for the southern form, there is no complete +unanimity among authors as to the application of the names +<i>egeria</i> and <i>egerides</i>, and unless more particulars are given, +either name may be used for either form. Besides this, difficulty arises +from the fact that the intermediate type is not generally distinguished +at all, and English collectors finding it, may easily +record it as the southern type. From Staudinger's note on the +distribution, I gather that he, on the contrary, reckoned the +intermediate with the northern type, as do the Speyers also. +The late Mr. J. W. Tutt was careful to distinguish the three +forms and has left several useful records. Easy therefore as it +might seem to be to make out the distribution of such a familiar +insect in its various modifications, there are serious practical +difficulties, and until long series are brought together with this +special object in view many obscurities will remain.</p> + +<p class="indent">With only the series from England, the west of France, +and Spain before one it would be easy to regard the successive series +of tones as a fair measure of climate; the brighter the colour, the +hotter might one expect the locality to be. Such rough correspondence +is often to be observed in butterflies and birds. It +becomes impossible to take these simple views in the light of +more complete knowledge. Beginning with France the fulvous +<i>egeria</i> occupies the lower valley of the Rhone, probably from well +above Lyon, though I have no exact information respecting the +country above Avignon. According to Speyer it also takes the +department of Lozère. The same authority says that Puy-de-Dôme +<span class="pagenum"><a name="Page_171" id="Page_171">[Pg 171]</a></span> +has "<i>egeria</i>," meaning perhaps the intermediate form, +with the fulvous form much less commonly. Next comes the +curious fact that though the Lower Rhone (Avignon, Tarascon, +Nîmes) has the true fulvous form, Hyères, Cannes, Grasse, Nice, +Digne, and Alassio have <i>the intermediate</i>. Savoy has the +intermediate (Chambéry) and even <i>egerides</i> perhaps, though +in the same latitude on the west of France there is nothing but +the fulvous type. At Chalseul and Besançon (Doubs) the ordinary +northern type is found. Switzerland generally, I believe, has the +northern type, but Staudinger gives <i>egeria</i> for Valais and the +intermediate occurs in Vaud.<a name="FNanchor_9_113" id="FNanchor_9_113"></a><a href="#Footnote_9_113" class="fnanchor">[9]</a> +The south side of the Alps has probably colonies of the +pale <i>egerides</i>, and of intermediates. Orta, with a +very hot summer, has the English type (Tutt, <i>Ent. +Rec.</i>, XII, 1900, p. 328). Locarno has the intermediate (<i>ibid.</i>, +XV, 1903, p. 321). North Italy in general and western Piedmont +have the intermediate; but further south <i>egeria</i> begins, +at what region I do not know. Speyer gives on his own authority +the remarkable statement that at Florence both extremes occur, +but chiefly intermediates between the two. Mr. R. Verity +however kindly informs me that in his experience this is not so, +and that neither the real southern type nor the northern occur +there. Sardinia, Sicily, Crete all have the southern type. +Greece probably has various types. Staudinger (<i>Hor. Ross.</i>, VII, +1870, p. 78) says intermediates resembling Nice types common +everywhere, but from "Greece" the British Museum has a series +that would pass for English specimens; and the same type occurs +near Constantinople. The island of Corfu has a pale intermediate, +distinct from <i>egerides</i> but approaching it. In Roumania +all three forms are recorded from various places: <i>egeria</i> in the +Dobrutscha; not quite typical (presumably an intermediate) +at Bukharest; intermediate in various mountainous localities +as well as in Macedonia and Dalmatia; but <i>egerides</i> in Azuga +at about 3,000 feet.<a name="FNanchor_10_114" id="FNanchor_10_114"></a><a href="#Footnote_10_114" class="fnanchor">[10]</a> +Hungary has the true <i>egerides</i> also. +(Cf. Caradja, <i>Deut. Ent. Zt.</i>, IX, p. 58.) Mathew records the same +<span class="pagenum"><a name="Page_172" id="Page_172">[Pg 172]</a></span> +from Gallipoli (<i>E. M. M.</i>, 1881, p. 95). Staudinger does not +distinguish the intermediates from the northern, but he gives +"<i>egerides</i>" for Armenia and Fergana (Central Asia). As against +the mere proximity of a great mountain chain being the influence +which keeps the Riviera population intermediate may be +mentioned the fact that the northern foothills of the Pyrenees +have the pure southern type, and the climate of Cambo must +surely be far cooler than that of Nice. The exact locality of +the Greek specimens is not given, but there can be no part of +Greece which is not much hotter in summer than Brittany, or +Calvados, which have the intermediate, not the English type.</p> + +<p class="indent">In face of these facts it can scarcely be maintained +that average temperature is the efficient cause of the particular tone +of colour which the butterfly shows in a given region. Nevertheless +it is clear that climate counts for much in determining +the distribution. It is noticeable that though the pale <i>egerides</i> +can be established in a warm climate we never find <i>egeria</i> in cold +climates, and even the intermediate is not found in places that +have a hard winter. I suspect that the distribution of the +broods through the year and the condition of the animal at the +onset of hard frost are features which really determine whether +a strain can live in a particular place or not. Though the truth +of the suggestion cannot be tested by experiments in captivity, +which at once introduce disturbances, I incline to the idea that +<i>egeria</i> has not got the right periodicity for northern climates. +If it could arrange its life so that the population consisted either +of young larvae, or perhaps of thoroughly formed pupae<a name="FNanchor_11_115" id="FNanchor_11_115"></a><a href="#Footnote_11_115" class="fnanchor">[11]</a> +at the onset of winter, it might, for any obvious reason to the +contrary, be able to live in England. It is irregularly "polyvoltine," +as the silk-worm breeders say, and as soon as a little +warmth encourages it, a new generation starts into being, which +if the frost comes at an untimely moment, is immediately +<span class="pagenum"><a name="Page_173" id="Page_173">[Pg 173]</a></span> +destroyed. Many species are continually throwing off individuals +which feed up fast<a name="FNanchor_12_116" id="FNanchor_12_116"></a><a href="#Footnote_12_116" class="fnanchor">[12]</a> +and emerge at once if the temperature permits, and +I imagine a species of Satyrid wholly or largely +represented by such individuals could scarcely survive in a +country which had a hard winter. For such a climate some +definite periodicity in the appearance of the broods may well be +indispensable. But assuming that <i>egeria</i> is cut off from cold +climates for such a reason, there is nothing yet to connect these +habits with the fulvous colour, and until breeding can be carried +out on a satisfactory scale there is no more to be said.</p> + +<p class="indent">From time to time records appear of individual specimens +more or less fulvous being caught in southern England, especially +in the New Forest.<a name="FNanchor_13_117" id="FNanchor_13_117"></a><a href="#Footnote_13_117" class="fnanchor">[13]</a> +It would be interesting to know what offspring such +individuals might produce. From the evidence +now given some notion both of the strength and the weakness +of the case considered as one of continuous climatic variation +can be formed. I know no other equally satisfactory. Whether +or not definite mixture of the intermediates with either of the +extremes will be proved to occur, the case differs materially from +those considered in the last chapter in the fact that at all events +there is no general overlapping of forms. In a species so little +given to wandering, overlapping could indeed scarcely be expected +to occur. It is this circumstance which makes the species +preeminently suitable as a subject for the study of climatic +influences, and I trust that entomologists with the right opportunities +may be disposed to explore the facts further.</p> + +<p class="indent">Just as many species, like <i>egeria</i>, have varieties +which can be regarded as adapted to northern and southern regions, so +there are also several which have lowland and Alpine forms quite +distinct from each other. Every such case presents an example +of the problem we have been considering. As the collector +passes from the plains to the Alpine region, how will he find the +<span class="pagenum"><a name="Page_174" id="Page_174">[Pg 174]</a></span> +transition from one form to the other effected? Does the lowland +form give place to the Alpine form suddenly, with a region +in which the two are mixed, or will he find a zone inhabited by +an intermediate population? I have spent a good deal of time +examining the facts in the case of <i>Pieris napi</i> and its Alpine +female variety <i>bryoniae</i>, and though there are many complications +which still have to be cleared up, no doubt is possible +as to the main lines of the answer. If in any valley in the Alps +inhabited by both <i>napi</i> and <i>bryoniae</i> the collector catches +every specimen he can, beginning at the bottom and working up to +7,000 feet, he will at first get nothing but <i>napi</i>. At about 2,500 +feet, he may catch an occasional <i>bryoniae</i> flying with the <i>napi</i>. +After 3,000 feet <i>napi</i> usually ceases, and only <i>bryoniae</i> are found. +As an exception a colony of <i>napi</i> may be met with at much +greater heights. I once found them in numbers at about 6,000 +feet.<a name="FNanchor_14_118" id="FNanchor_14_118"></a><a href="#Footnote_14_118" class="fnanchor">[14]</a> +Not only were they free from any trace of modification +in the direction of <i>bryoniae</i>, but they were of the thoroughly +southern type of <i>napi</i>, being a late brood of that large and very +pale kind (<i>meridionalis</i>) almost destitute both of dark veining +above and of green veining below, which are common on the +shores of Lago Maggiore and in other hot southern localities. +Not far off at the same level were typical <i>bryoniae</i> in fair +abundance. Occasionally an intermediate may be met with. I have +taken a few, for example, at Macugnaga and at Fobello. These, +however, in my experience are rarities in the Alps. Fleck<a name="FNanchor_15_119" id="FNanchor_15_119"></a><a href="#Footnote_15_119" class="fnanchor">[15]</a> +gives notes on the distribution in Roumania which shows the +same state of things. The lowland form is not transformed +though found at great heights, and at Azuga (nearly 3,000 feet) +<i>bryoniae</i> occurs with only occasional "<i>flavescens</i>," +viz., intermediates of the second brood.</p> + +<p class="indent">If this were all the evidence we should be satisfied +that the lowland and Alpine types keep practically distinct, overlapping +occasionally, but rarely interbreeding. The problem would +remain, how is the distinctness of the two types maintained in +the region of overlapping? Nowadays, I suppose, we should +<span class="pagenum"><a name="Page_175" id="Page_175">[Pg 175]</a></span> +incline to answer this question by reference to segregation, and +perhaps by an appeal to selective mating. The suggestion that +segregation does take place is certainly true to some extent. +There are, however, difficulties in the way, and the whole subject +is one of great complexity. My own experiments were made in +pre-Mendelian times and were not arranged with the simplicity +which we now know to be essential. The results are neither +extensive enough nor clear enough to settle the many collateral +questions which have to be considered, and the work ought to +be done again. Nevertheless, some notes of the observations +may have a suggestive value.</p> + +<p class="indent">When I began, I did not sufficiently appreciate that +the "<i>napi</i>" group, omitting the North American forms, and the +Asiatic representatives, has at least three chief types in western +Europe. The differences we have to deal with are manifested +by the females only, so in this account particulars as to the males +are omitted for the most part. These are (1) our own British +<i>napi</i>; (2) the form found in the south, from the Loire downwards, +and in the Italian Alps, which I think may be spoken of as +<i>meridionalis</i>; (3) <i>bryoniae</i>, which is a form clearly recognizable +in the <i>female</i> only, and is found only in the arctic regions and +in the Alps above 2,500 feet. The first two have several broods, +two, three, or more, according to opportunity, and the first +brood is different from the later ones. In <i>napi</i> the markings on +the upper surface are a dark grey but in <i>meridionalis</i> they are a +pale silvery grey and much less extensive. In the later broods +of <i>napi</i> there is much less general irroration of the veins, and the +spots stand out as more defined and blacker. These differences +vary greatly in degree of emphasis. In <i>meridionalis</i> the later +broods are entirely different from the first. Instead of having +silvery markings they have the ground colour quite white, with +the spots large and a full black. On the under side of the hind +wings the usual green veins are almost absent, and I have seen +individuals which could scarcely be distinguished from <i>rapae</i>. +To these later broods the term <i>napaeae</i> is sometimes applied, +but I here use <i>meridionalis</i> for the southern race in general as +applicable to all broods. +<span class="pagenum"><a name="Page_176" id="Page_176">[Pg 176]</a></span></p> + +<p class="indent">The female <i>bryoniae</i> is totally unlike the others. +The ground colour is a full yellow, and each nervure is thickly irrorated +with a brown pigment often spreading so far as to hide the ground +almost entirely in the fore-wings. The males corresponding with +these females are not certainly distinguishable from those of our +own <i>napi</i>. Both sexes have the green veining of the underside +of the hind wing fully developed, rather more than is usual in the +lowland races, but this is not really diagnostic of the variety. +The first serious difficulty arises in regard to the second brood +of <i>bryoniae</i>. It is stated that there is only one brood,<a name="FNanchor_16_120" id="FNanchor_16_120"></a><a href="#Footnote_16_120" class="fnanchor">[16]</a> +but I feel fairly sure that a second brood is sometimes produced, and +that the females with a yellow ground and diminished irroration of the +veins, not very uncommon in the Italian Alps in July to August, +are generally representatives of it. Such insects would of course +be classed with <i>bryoniae</i> in collections.</p> + +<p class="indent">My experiments began with eggs of true <i>bryoniae</i> +females caught at about 2,500 feet early in July. These emerged in +August-September as intermediates with yellow ground and +about half as much black on the upper surface as <i>bryoniae</i>. +They are exactly like the intermediates usually found in nature +and in the light of later experience I regard them as natural F<sub>1</sub> +forms, and I think the mothers had been fertilised by <i>napi</i> males, +though I admit that in view of the rarity of natural intermediates +there is a difficulty in this suggestion. Three of these females +were mated with males raised from thorough <i>meridionalis</i> +females, and three families were produced. Two of them +showed distinct evidence of segregation, some being yellow and +some white with various intergrades, some being no blacker than +<i>meridionalis</i> and some ranging up to a dark intermediate type. +Part emerged in the same autumn; and part overwintered, emerging +as the spring <i>meridionalis</i> or as the peculiar type which I +afterwards learnt to know as the spring F<sub>1</sub> form. The distinctions +were fairly sharp between the several forms. But the offspring +of the third female gave a series practically continuous from +<span class="pagenum"><a name="Page_177" id="Page_177">[Pg 177]</a></span> +<i>meridionalis</i> to the F<sub>1</sub> type. The work of subsequent years +gave results similarly irregular which could only be described +adequately at great length. The outcome may however be +summed up in the statement that there is evidence that both +the yellow ground and the dark veining are due to factors, but +that there are several of these and that imperfect segregation +is not uncommon, producing various reduction-stages. The +yellow ground may be due to one factor, and the several shades +may be the result of irregularities in dominance, but the black +markings when fully developed cannot I think be the result of +less than three factors, one for the basal darkening, one for general +irroration, and one for the margins. Probably also the enlargement +of the spots is produced by a fourth factor.</p> + +<p class="indent">There was not, in my experience any great difficulty +in getting the various forms to pair in captivity. Some attempts were +made to see whether individuals of either type selected mates +of their own type in preference to those of the other, but the +results were inconclusive. There were some indications of such +a preference; though, from the impossibility of judging how much +of this may be due to other circumstances, I could not come to +a positive conclusion on the rather meagre evidence.</p> + +<p class="indent">Recently Schima<a name="FNanchor_17_121" id="FNanchor_17_121"></a><a href="#Footnote_17_121" class="fnanchor">[17]</a> +has given a careful and detailed account +of all the forms found in Lower Austria which he enumerates +under 14 distinct varietal names. He gives full references to +previous accounts, especially to the beautiful plates lately published +by Roger Verity.<a name="FNanchor_18_122" id="FNanchor_18_122"></a><a href="#Footnote_18_122" class="fnanchor">[18]</a> +Examination of these and of my own +specimens strongly suggests that the several forms are due to +the recombination of the factors I have named. Among those +which I have bred are representatives of most if not all the types +enumerated by Schima in addition to other curious forms. For +example I have <i>bryoniae</i> markings on a ground practically white; +the dark veins with spots almost obsolete; <i>meridionalis</i> on a +yellow ground; the intermediate amount of black on a white +ground, etc. The last-named may occur wild and I have one +from Macugnaga as well as one given me by Mr. F. Gayner from +Lulea (Lapmark). +<span class="pagenum"><a name="Page_178" id="Page_178">[Pg 178]</a></span></p> + +<p class="indent">To obtain really exact knowledge of the number of +factors and their properties it would be necessary to repeat the work. +After the beginning, I made a mistake in using British <i>napi</i> +instead of <i>meridionalis</i> and the results were much confused +thereby. The contrast between <i>meridionalis</i> and the various +dark forms is much greater and classification of the types would +have been therefore easier. The British form is presumably +<i>meridionalis</i> plus the factor for the basal pigmentation. The +problem is greatly complicated by the differentiation of the +seasonal forms. The first point to be determined is whether +<i>bryoniae</i> is capable of producing a second brood when it is +thoroughly pure-bred, and whether such a second brood is, as I +suspect, normally intermediate in character.</p> + +<p class="indent">In the Alps generally there is no definitely intermediate +population; nor I believe, is any such population met with in +the north where the arctic <i>bryoniae</i> meets <i>napi</i>, but as +to this I have no precise information. One curious fact, however, must +be mentioned, namely that there is a population that can probably +be so described with fairness established at Mödling near +Vienna. This is not in any sense an Alpine locality, and does +not, as I am told, differ in any obvious way from the other suburbs +of Vienna. Dr. H. Przibram was so good as to send me a +set taken at this place, representing a second brood, and they +were decidedly heterogeneous, ranging from an intermediate +form such as <i>bryoniae</i> fertilised by <i>napi</i> usually produces, +to a light yellowish second-brood type with little dark pigment. +There are also two actual <i>bryoniae</i>. Whether true <i>napi</i> +also occur there I do not know, but I have no doubt they do. It +would be well worth while to investigate the Mödling population +statistically, and to breed from the intermediates which might +not impossibly prove to be heterozygotes. There are also records +of such intermediates being occasionally found in some parts of +Ireland, in the north of Scotland, and in south Wales,<a name="FNanchor_19_123" id="FNanchor_19_123"></a><a href="#Footnote_19_123" class="fnanchor">[19]</a> +but I do not know of any regular colony of these forms. We can scarcely +avoid the inference that one or more of the factors which make +up <i>bryoniae</i> may be carried by these intermediates. It is not +<span class="pagenum"><a name="Page_179" id="Page_179">[Pg 179]</a></span> +clear why their interbreeding does not produce actual <i>bryoniae</i> +occasionally. If this occurred, the probability is that the fact +would be known to collectors, at least in the British localities. +The absence of true <i>bryoniae</i> must, I think, be taken to mean +that some essential factor is absent from these intermediates.</p> + +<p class="indent">To sum up the evidence, the facts that are clear +may be thus enumerated:</p> + +<p class="blockquot">1. <i>Napi</i> and <i>bryoniae</i>, or in the +Italian Alps, <i>napaeae</i> and <i>bryoniae</i> frequently meet each other.</p> + +<p class="blockquot">2. They cross without difficulty, producing fertile offspring.</p> + +<p class="blockquot">3. But in the levels at which they overlap there is no +intermediate population, and only occasional intermediate individuals.</p> + +<p class="blockquot">4. In certain parts of the distribution of <i>napi</i> +similar intermediates sometimes occur, and at one place (Mödling) they are +so frequent as apparently to constitute a colony.</p> + +<p class="blockquot">5. As to the genetic relations of the two forms there +is no complete certainty. Indications of segregation have been observed +in some cases, but there are several factors concerned and +they are liable to some disintegration.</p> + +<p class="indent">Another form in which I tried to investigate the same +problem is <i>Coenonympha arcania</i>, which has one Alpine form known as +<i>darwiniana</i>, and another, <i>satyrion</i>. In calling <i>satyrion</i> +a form of <i>arcania</i> I follow Staudinger and other authorities, but I +have never been quite satisfied that it should be so regarded. The +differences between <i>arcania</i> and <i>darwiniana</i> are essentially +differences of degree; <i>C. arcania</i> occurs in places where there is +cover, and reaches up the valleys usually as high as the mixed +woods of deciduous trees, which is about 2,500 feet. The variety +<i>darwiniana</i>, on the contrary, is an insect of treeless hillsides, +and I regard it as a dwarf and possibly a stunted form. It would not +greatly surprise me to find that with the application of good +conditions <i>arcania</i> could be raised from <i>darwiniana</i> eggs, or that +if <i>arcania</i> larvae were starved they might give rise to <i>darwiniana</i> +butterflies. I have been unsuccessful in trying to rear the species, +having lost the larvae by disease. Usually one does not catch +<i>arcania</i> and <i>darwiniana</i> on the same ground, and as <i>Festuca ovina</i>—a +typically hill-side grass—is a common food-plant of <i>darwiniana</i> +<span class="pagenum"><a name="Page_180" id="Page_180">[Pg 180]</a></span> +there can be little doubt that <i>arcania</i> feeds on some other +grass, probably woodland species. Colonies of <i>arcania</i> of varying +size and brightness are commonly found, and though a sample of +<i>arcania</i>, finely grown, from a warm Italian wood, presents a +striking contrast with <i>darwiniana</i> from an Alpine pasture, one +certainly may get samples which fill all the gradations. Generally +the sample from a given locality is fairly homogeneous.</p> + +<p class="indent">Of <i>satyrion</i> I have little personal experience. I +only twice found it, namely at Zinal, and at Hallstatt in Austria, but it +occurs at Zermatt, Arolla, and in several Swiss localities above +5,000 feet, and I understand that it is the typical Alpine form in +the Engadine. With its darkened colour and reduced size it +might well be expected to be a still further stunted form of +<i>darwiniana</i>. Yet I have never found the one succeed to the +other at the higher levels. If <i>darwiniana</i> appears when Alpine +conditions are reached in a valley it will be met with up to the +highest level at which such butterflies live. Tutt was of opinion +that <i>satyrion</i> is a distinct species.<a name="FNanchor_20_124" id="FNanchor_20_124"></a><a href="#Footnote_20_124" class="fnanchor">[20]</a> +I once, at the top of the +Vorderrheinthal caught a sample of <i>darwiniana</i> a few of which (males) +were so dark and had the eye spots so poorly developed that they +looked like transitions to <i>satyrion</i>. Otherwise I never found +any such transitional forms and they are certainly exceptional. +There is further a record<a name="FNanchor_21_125" id="FNanchor_21_125"></a><a href="#Footnote_21_125" class="fnanchor">[21]</a> +of <i>satyrion</i> having been taken flying +with <i>arcania</i>. This was near Susa, at about 2,000 feet I infer. +Mr. H. E. Page has similar specimens from Caud and from St. +Anton (Arlberg). The females, however, both of mine and of +Mr. Page's samples are a pale brown, quite unlike the females +both of <i>arcania</i> and of the dark Zinal <i>satyrion</i>. The difficulty +thus raised has not I think yet been considered by the authorities, +and it is possible that the Alpine forms of <i>arcania</i> are in reality +three, not two.</p> + +<p class="indent">The evidence taken together suggests, I think, that <i>darwiniana</i> +is related to <i>arcania</i> much as so many of the Alpine varieties +<span class="pagenum"><a name="Page_181" id="Page_181">[Pg 181]</a></span> +of plants are to the well-developed individuals of the lower +levels. I do not anticipate that factorial differences will be +found in these insects, and it is by no means impossible that the +distinctions between them are the direct consequences of altered +conditions. The relations of <i>arcania</i> to <i>satyrion</i> are more +doubtful, and in that case a factorial difference may at least be suspected.</p> + +<p class="indent">The species of the genus <i>Setina</i> have Alpine forms +which agree in possessing a characteristic extension of the black pigment +to form radiating junctions between the spots on the wings. +Speyer, who discussed the interrelations of these forms in detail,<a name="FNanchor_22_126" id="FNanchor_22_126"></a><a href="#Footnote_22_126" class="fnanchor">[22]</a> +lays stress on the absence of genuine transitional forms between +<i>aurita</i> and the variety <i>ramosa</i>. Both are mountain insects but +<i>ramosa</i> extends to levels higher than that at which <i>aurita</i> +ceases, which is about 4,000 feet. The two forms are often found flying +together. Speyer says that his brother searched diligently for +transitional forms at the level of overlapping, but found none, +so that at least they may be regarded as rare. The variety +<i>ramosa</i> is not infrequent at much lower levels (<i>e. g.</i>, +Chiavenna, 1,020 feet; Reussthal, 1,500 feet) and extends as high as +the permanent snows. In the British Museum collection, however, +I have seen several that I should regard as transitional. Speyer +perhaps would have classed as <i>ramosa</i> all in which the spots of +the central field were united, and it is by no means unlikely that +breeding would prove such individuals to be heterozygous.<a name="FNanchor_23_127" id="FNanchor_23_127"></a><a href="#Footnote_23_127" class="fnanchor">[23]</a> +<span class="pagenum"><a name="Page_182" id="Page_182">[Pg 182]</a></span></p> + +<p class="indent">There can scarcely be a doubt that the distinction between +<i>aurita</i> and <i>ramosa</i> is factorial, the radiate <i>ramosa</i> probably +having the factor for striping. In support of this view may be mentioned +the observation of Boisduval,<a name="FNanchor_24_128" id="FNanchor_24_128"></a><a href="#Footnote_24_128" class="fnanchor">[24]</a> +respecting a gynandromorphous individual, which was <i>aurita</i> male on one +side, and <i>ramosa</i> female on the other. Speyer makes another excellent +comment. He points out that the simple notion that the radiation +is a mere extension of pigmentation consequent on the +climate of the higher levels, will not fit the facts very easily, +because the size of the spots varies greatly in <i>aurita</i> itself at +any level, and lowland specimens may actually have more black +confined to the spots alone than some <i>ramosa</i> possess on spots +and lines combined.<a name="FNanchor_25_129" id="FNanchor_25_129"></a><a href="#Footnote_25_129" class="fnanchor">[25]</a> +</p> + +<p class="indent">The two Salamanders, <i>S. maculosa</i> and its Alpine +form <i>atra</i>, might not improbably furnish evidence bearing on the same +problem. The two are of course very distinct, not merely in +colour (<i>maculosa</i> being spotted with yellow or orange while <i>atra</i> +is entirely black) but also in the mode of reproduction, a feature +to which reference will be made in the next chapter. I cannot, +however, find any evidence as to the overlapping of the two forms. +<i>S. atra</i> occurs from about 3,000 feet or somewhat less, and reaches +great elevations in the Eastern Alps, but I do not know if the +two forms ever occur in the same localities. Leydig,<a name="FNanchor_26_130" id="FNanchor_26_130"></a><a href="#Footnote_26_130" class="fnanchor">[26]</a> Boulenger,<a name="FNanchor_27_131" id="FNanchor_27_131"></a><a href="#Footnote_27_131" class="fnanchor">[27]</a> +and most modern authorities regard the two types as distinct +species, but they are in any case closely allied, and it would be of +interest to have exact knowledge of their geographical delimitations.</p> + +<p class="indent">The reader who has considered the cases adduced will +appreciate the difficulties which must be faced in any attempt to +<span class="pagenum"><a name="Page_183" id="Page_183">[Pg 183]</a></span> +account for the facts in a rational way. As always in a problem +of Evolution, two separate questions have to be answered. +First how did the form under consideration come into existence, +and secondly, how did it succeed in maintaining itself so as to +become a race? The evidence from the local forms, though very +far from giving complete answers to either of these questions +definitely refutes the popular notion that a new race comes into +existence by transformation of an older race. If a gradual mass-transformation +of this kind took place we should certainly expect +that when two types, nearly allied and capable of interbreeding, +overlap each other in their geographical distribution, a normally +intermediate population would exist. If each type can maintain +itself, and if each came into existence by gradual transformation, +then there must have been an intermediate capable of +existing and maintaining itself as a population; and if this had +ever been, surely in the region of overlapping, that intermediate +population should continue. Especially should such a population +be found when the two extreme types are adaptational forms and +the region of overlap is a region of intermediate conditions. +But of the examples we have examined there is only one, that of +<i>Pararge egeria</i> and <i>egerides</i>, which can at all be so interpreted, +and even in that case it is not impossible that more minute observation +would reveal discontinuity between the extremes +and the admittedly normal intermediate population. Granting +provisionally however that this example, as it stands, is consistent +with the conventional theory of evolution, I know not +where we should look for another case equally good. When the +distinctions are produced by direct influence of conditions operating +during the lifetime of the individuals, examples of intermediate +populations occupying the areas of intermediate conditions +can no doubt be produced. Many turf-like Alpine +plants, for instance, if protected from exposure and properly +nourished can grow as large as those of the same species found in +the valleys, and in the case of such quantitative effects, intermediate +conditions can doubtless produce intermediate characters.</p> + +<p class="indent">Even these examples however are not very abundant, +and often the intermediate locality has not a form intermediate +<span class="pagenum"><a name="Page_184" id="Page_184">[Pg 184]</a></span> +between those of the two extreme localities, but some third +form distinct from either. This is the case for instance in the +fauna of brackish waters. We are taught to believe that the +fresh water fauna was evolved from the marine fauna, which +it well may have been; but as students of Crustacea and Mollusca +know familiarly, the brackish water forms are not as a rule intermediates +between fresh water species and sea species, but more +usually they are special forms belonging to the brackish waters, +with the peculiar property that they can tolerate a great range of +conditions, and live without ostensible variation in waters of +most various compositions and densities, which very few marine +or fresh water species are able to do.</p> + +<p class="indent">Sometimes the distinction between local races, as in <i>Rhamphocoelus +passerinii</i> and <i>icteronotus</i> may be regarded with confidence +as due to one simple Mendelian factor possessed by one +race and absent from the other, but I think, more often, as in +<i>Colaptes</i> or in the varieties of <i>Pieris napi</i>, the existence of +several distinct factors is to be inferred. As we have seen, the races +of <i>Colaptes</i> show almost beyond doubt that in different areas at +least three distinct factorial combinations can be perpetuated as races.</p> + +<p class="indent">In the distribution of variability we find, I think, +some hint as to the steps by which the phenomena under consideration +have come to their present stage, and I am disposed to regard +the facts so well attested in the case of our own melanic moths +as a true indication of the process. Following this indication +we should regard the change in the character of a population +as beginning sporadically, by the appearance of varying individuals, +possibly only one varying individual, in, it may be, one +place only. As to <i>why</i> a variety should increase in numbers we +have nothing but mere speculation to offer, and for the present +we must simply recognise the fact that it may. That such survival +and replacement may reasonably be taken as an indication +that the replacing race has some superior power of holding its +own I am quite disposed to admit. Nevertheless it seems in +the highest degree unlikely that the outward and perceptible +character or characters which we recognise as differentiating the +<span class="pagenum"><a name="Page_185" id="Page_185">[Pg 185]</a></span> +race should be the actual features which contribute effectively +to that result.</p> + +<p class="indent">In discussions of geographical distribution in relation +to problems of origin it is generally said that very nearly allied +species usually occupy distinct areas, while other competent +observers state the exact contrary. Lately, for example, Dr. +R. G. Leavitt<a name="FNanchor_28_132" id="FNanchor_28_132"></a><a href="#Footnote_28_132" class="fnanchor">[28]</a> +has published an important collection of evidence +upholding the latter proposition, taken chiefly from the botanical +side, showing how in numerous genera two or more closely allied +species coexist, frequently without intermediates, in the same +localities, and may even be thus found in company throughout +their distribution. The difference of opinion evidently arises +from a confusion as to the sense in which the term "species" +is understood and applied. Leavitt, for example, is avowedly +following Jordan and, among moderns, Sargent, in applying +a close analysis, and denoting as species all forms which are +distinct and breed true. Against this use of the term I know +no valid objection<a name="FNanchor_29_133" id="FNanchor_29_133"></a><a href="#Footnote_29_133" class="fnanchor">[29]</a> +but it must be obvious that if others follow +a different practice confusion may result when observations are +summarised in general statements. We will consider this subject +again in another place, but here it may be sufficient to say that +there can scarcely now be a doubt that numbers of these associated +species, such as Jordan discriminated, represent various +combinations of the presence and absence of Mendelian factors. +This does not in any way weaken the argument which Leavitt +founds upon the facts, namely, that the observed distribution +of these forms is consistent with the supposition of an evolution +largely discontinuous.</p> + +<p class="indent">On the other hand, those who have come to the opinion +that nearly allied species generally occupy distinct ground are +presumably more impressed by the characters differentiating the +geographically distinct or adaptational races, seeing that genuine +intermediates between them are less commonly found. Those +geographical races may no doubt contain various differentiated +forms; but when all live together, occasional intermediates are +<span class="pagenum"><a name="Page_186" id="Page_186">[Pg 186]</a></span> +usually to be found even in the case of characters habitually +segregating. These segregating forms Jordan would certainly +have determined as species, and it must be conceded that no +physiological definition has yet been drawn which consistently +excludes them.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_187" id="Page_187">[Pg 187]</a></span></p> + +<h2>CHAPTER IX</h2> +<h3>THE EFFECTS OF CHANGED CONDITIONS: ADAPTATION</h3> + +<p class="indent">In the attempt to conceive a process by which +Evolution may have come about, the first phenomenon to be recognized +and accounted for is specific difference. With that recognition +the outline of the problem is defined. The second prerogative +fact is adaptation. Forms of life are <i>on the whole</i> divided into +species, and these species <i>on the whole</i> are adapted and fit the +places in which they live. To many students of Evolution, +adaptation has proved so much more interesting and impressive +than specific diversity that they have preferred it to the first +place in their considerations.</p> + +<p class="indent">Whether this is, as I believe, an inversion of the +logical order or not, there is one most serious practical objection to +such preference, that whereas specific diversity is a subject which +can be investigated both by the study of variation and by the +analytical apparatus which modern genetic science has developed, +we have no very effectual means of directly attacking the problems +of Adaptation.</p> + +<p class="indent">The absence of any definite progress in genetics in +the last century was in great measure due to the exclusive prominence +given to the problem of Adaptation. Almost all debates on +heredity centered in that part of the subject. No one disputes +that the adaptation of organisms to their surroundings is one +of the great problems of nature, but it is not the primary problem +of descent. Moreover, until the normal and undisturbed course +of descent under uniform conditions is ascertained with some +exactness, it is useless to attempt a survey of the consequences +of external interference; nor as a rule can it be even possible to +decide with much confidence whether such interferences have or +have not definite consequences. Those, for example, who debated +with enthusiasm whether acquired characters are or are +not transmitted were constantly engaged in discussing occurrences +<span class="pagenum"><a name="Page_188" id="Page_188">[Pg 188]</a></span> +which we now know to be ordinary features of descent +under uniform conditions, and the origin of variations which +were certainly not caused directly by circumstances at all. In +the absence of any factorial analysis, or of any conception of what +factorial composition means and implies, no one knew what +varieties might be expected from given parents. The appearance +of any recessive variety was claimed as a consequence of some +treatment which might have been applied to the parents. There +was no possible standard of evidence or means of controlling it, +and thus the discussion was singularly unfruitful. Before we can +tell how the course of descent has departed from the normal, we +must know what the normal would have been if we had let alone. +We are still far from having such knowledge in adequate measure, +but it does now exist in some degree, and we are steadily approaching +a position from which we shall be able to form fairly sound +estimates of the true significance of evidence for or against the +proposition that environmental treatment can produce positive +disturbances in the physiological course of descent.</p> + +<p class="indent">Thus described, the field for consideration is very +wide. Though the effects of changed conditions were especially studied +in the hope of solving the problem of adaptation by direct observation, +that, as all are now agreed, is but a part of a more +general question. We must ask not only do changed conditions +produce an <i>adaptative</i> response on the part of the offspring, but +whether they produce any response on the part of the offspring +at all. It is not in doubt that by violent means, such as starvation +or poisoning of the reproductive cells, effects of a kind, stunting +and deformity for instance, can be made evident, just as similar +effects may follow similar treatment during embryonic or larval +life. Apart from interferences of this class, are there any that +may be reasonably invoked as modifying the course of inheritance?</p> + +<p class="indent">No epitome of the older evidence for the inheritance +of adaptative changes is here required. That has often been collected, +especially by Weismann, who exposed its weaknesses so +thoroughly as to carry conviction to most minds, and showed +that whether the phenomenon occurs or not, no one can yet prove +<span class="pagenum"><a name="Page_189" id="Page_189">[Pg 189]</a></span> +that it does. Belief in these transmissions, after being almost +universally held, was with singular unanimity abandoned. This +change in opinion, though doing credit to the faith of the scientific +community in evidential reasoning, is the more remarkable +inasmuch as the strength of the idea was not derived from the +minute amounts of supposed facts now demolished. On the +contrary, it was really an instinctive deduction from a wide +superficial acquaintance with the properties of animals and +plants. They <i>can</i> accommodate themselves to circumstances. +They <i>do</i> make responses sometimes marvellously appropriate +to demands for which they can scarcely have been prepared. +What more natural than to suppose that the permanent adaptations +have been achieved by inherited summation of such responses? +No one had actually been driven to believe in the +inheritance of adaptative changes because bitches which had +been docked had been known to give birth to tailless puppies, +or because certain wheat in Norway was alleged to have become +acclimatized in a few generations. Evidence of this kind was +collected and produced rather as an ornamental appendix to a +proposition already accepted, and held to be plainly demonstrated +by the facts of nature. Looked at indeed in that preliminary +and uncritical way, the case is simply overwhelming. +Those who desire to see how strong it is should turn to Samuel +Butler's <i>Life and Habit</i>, and even if in reading they reiterate to +themselves that no experimental evidence exists in support of +the propositions advanced, the misgiving that none the less they +may be true is likely to remain. Making every deduction for +the fact that the wonders of adaptation have been grossly exaggerated, +and that marvels of fitness and correspondence between +means and ends have grown out of mere anthropomorphic +speculations, there is much more left to be accounted for than +can at all comfortably be accepted as the product of happy +accidents. So oppressive are these difficulties that we can scarcely +blame those who imagine that the study of heredity is primarily +directed to the problem of the transmission of acquired characters, +a preconception still almost universal among the laity. +<span class="pagenum"><a name="Page_190" id="Page_190">[Pg 190]</a></span></p> + +<p class="indent">But since the belief in transmission of acquired +adaptations arose from preconception rather than from evidence, it is +worth observing that, rightly considered, the probability should surely +be the other way. For the adaptations relate to every variety +of exigency. To supply themselves with food, to find it, to seize +and digest it, to protect themselves from predatory enemies +whether by offence or defence, to counter-balance the changes +of temperature, or pressure, to provide for mechanical strains, +to obtain immunity from poison and from invading organisms, +to bring the sexual elements into contact, to ensure the distribution +of the type; all these and many more are accomplished +by organisms in a thousand most diverse and alternative methods. +Those are the things that are hard to imagine as produced by +any concatenation of natural events; but the suggestions that +organisms had had from the beginning innate in them a power +of modifying themselves, their organs and their instincts so as +to meet these multifarious requirements does not materially +differ from the more overt appeals to supernatural intervention.</p> + +<p class="indent">The conception, originally introduced by Hering and +independently by S. Butler, that adaptation is a consequence or +product of accumulated <i>memory</i> was of late revived by Semon +and has been received with some approval, especially by F. +Darwin. I see nothing fantastic in the notion that memory +may be unconsciously preserved with the same continuity that +the protoplasmic basis of life possesses. That idea, though +purely speculative and, as yet, incapable of proof or disproof +contains nothing which our experience of matter or of life at all +refutes. On the contrary, we probably do well to retain the +suggestion as a clue that may some day be of service. But if +adaptation is to be the product of these accumulated experiences, +<i>they must in some way be translated into terms of physiological and +structural change</i>, a process frankly inconceivable.</p> + +<p class="indent">To attempt any representation of heredity as a +product of memory is, moreover, to substitute the more obscure for the +less. Both are now inscrutable; but while we may not unreasonably +aspire to analyse heredity into simpler components by ordinary +methods of research, the case of memory is altogether +different. Memory is a mystery as deep as any that even +<span class="pagenum"><a name="Page_191" id="Page_191">[Pg 191]</a></span> +psychology can propound. Philosophers might perhaps encourage +themselves to attack the problem of the nature of memory by +reflecting that after all the process may in some of its aspects +be comparable with that of inheritance, but the student of genetics, +as long as he can keep in close touch with a profitable basis +of material fact, will scarcely be tempted to look for inspiration +in psychical analogies.</p> + +<p class="indent">For a summary of the recent evidence I may refer the reader +to Semon's paper<a name="FNanchor_1_134" id="FNanchor_1_134"></a><a href="#Footnote_1_134" class="fnanchor">[1]</a> +where he will find a collection of these +observations described from the standpoint of a convinced believer. +At the outset one cannot help being struck by the fact that of +the instances alleged, very few, even if authentic, show the transmission +of acquired modifications which can in any sense be regarded +as adaptative, and many are examples not so much of a +transmission of characters produced in the parents as of variation +induced in the offspring as a consequence of treatment to which +the parents were submitted, the parents themselves remaining +apparently unmodified. No one questions the great importance +of evidence of this latter class as touching the problem of the +causes of variation, but it is not obvious why it is introduced in +support of the thesis that acquired characters are inherited.</p> + +<p class="indent">It is most difficult to form a clear judgment of the +value of the evidence as a whole. To doubt the validity of testimony +put forward by reputable authors is to incur a charge of obstinacy +or caprice; nevertheless in matters of this kind, where the alleged +phenomena are, if genuine, of such exceptional significance, belief +should only be extended to evidence after every possible source +of doubt has been excluded. We believe such things when we +must, but not before. At the very least we are entitled to require +that confirmatory evidence should be forthcoming from independent +witnesses. So far as I have seen, this requirement is +satisfied in scarcely any of the examples that have been lately +published, and until it is, judgment may reasonably be suspended. +<span class="pagenum"><a name="Page_192" id="Page_192">[Pg 192]</a></span></p> + +<p class="indent">In some cases, however, the facts are not doubtful. Standfuss, +by subjecting pupae of <i>Vanessa urticae</i> to cold, produced +the now well-known temperature-aberrations in which the dark +pigment is greatly extended. He put together in a breeding-cage +32 males and 10 females showing this modification in various +degrees. Two of these females died without leaving young. +Seven produced exclusively normal offspring. From the eighth +female 43 butterflies were bred, and of these there were four (all +males) which to a greater or less extent exhibited the aberrational +form.<a name="FNanchor_2_135" id="FNanchor_2_135"></a><a href="#Footnote_2_135" class="fnanchor">[2]</a> +The mother of this family was the most abnormal of the +10 females originally put in.</p> + +<p class="indent">Fischer's experiment with <i>Aretia caja</i> was on +similar lines. From pupae which had been frozen almost all the moths +which emerged showed aberrational markings. A pair of these mated +and produced 173 young which pupated. Those which emerged +early were all normal, but of those which emerged late, 17 had +in various degrees abnormal markings like those of the parents.<a name="FNanchor_3_136" id="FNanchor_3_136"></a><a href="#Footnote_3_136" class="fnanchor">[3]</a> +In neither of these examples is there any question as to the facts. +Both observers have great experience and give full details of their work.</p> + +<p class="indent">As regards <i>Vanessa urticae</i>, however, it must be +recalled that Fischer himself showed that in Nymphalids somewhat similar +aberrations could be produced both by heat and by cold, and +even by centrifuging the pupae. Frl. von Linden produced a +transitional form of the same aberration in <i>V. urticae</i> by the +action of carbonic acid gas.<a name="FNanchor_4_137" id="FNanchor_4_137"></a><a href="#Footnote_4_137" class="fnanchor">[4]</a> +It is highly probable that the appearance +is due to a morbid change, perhaps an arrest of development, +which may be brought about by a great diversity of causes. +In the experiments the cause probably was a diseased condition +of the tissues of the mother herself. She had been subjected to +freezing sufficiently severe to prevent the proper development of +the pigments and some of the ovarian cells presumably suffered +also. It will be observed that the only specimens which were +affected were the offspring of the most abnormal female, and of +them only four out of forty-three showed any change. +<span class="pagenum"><a name="Page_193" id="Page_193">[Pg 193]</a></span></p> + +<p class="indent">The same interpretation probably applies to the cases +in <i>Arctia caja</i>. In this species the markings are well known to +be liable to great variation. As Barrett says, even in nature +individuals are rarely quite alike, and an immense number of +strange forms occur in collections.<a name="FNanchor_5_138" id="FNanchor_5_138"></a><a href="#Footnote_5_138" class="fnanchor">[5]</a> +These are greatly sought after by some collectors, especially in England, +where they fetch high prices at auctions, and it is notorious that most of +them come from Lancashire and the West Riding of Yorkshire. It is commonly +supposed that the breeders of that district subject them to +abnormal conditions, and especially to unnatural feeding, but +I know no clear evidence that this is true. From whatever cause +it is certain that the natural pattern is, in some strains at all +events, very easily disturbed.</p> + +<p class="indent">The elaborate experiments of Schröder with +<i>Abraxas grossulariata</i> are difficult to follow and are complicated +by the fact that the series which was submitted to abnormal temperatures +was derived from an abnormal original pair. From the evidence +given it is not clear to me whether the temperature had a distinct +effect. This insect, like <i>Arctia caja</i>, produces an immense number +of variations (especially in the amount of the black pigment) +and as most of these are, I believe, reared in domestication for +sale, it is highly probable that the species is easily influenced +by cultural conditions.</p> + +<p class="indent">Schröder describes two other experiments which have +been accepted by Semon and other supporters of the view that acquired +characters are transmitted. In the first, <i>Phratora vitellinae</i>, a +phytophagous beetle living on the undersides of leaves, was used. +It naturally feeds on <i>Salix fragilis</i>, a species without a felt, or +tomentum, on the underside of the leaves. Larvae were transferred +to another willow (near <i>S. viminalis</i>) which has the undersides +of the leaves felted. The larvae took readily to the new +food, pushing the tomentum before them as they gnawed the +leaves. They came to maturity and when they were about to +lay their eggs they were given a free choice between <i>S. fragilis</i> +and the tomentose species. The greater number of ovipositions, +<span class="pagenum"><a name="Page_194" id="Page_194">[Pg 194]</a></span> +219, took place on <i>fragilis</i>, and there were 127 on the tomentose +bush, which we are told was six times as large as the <i>fragilis</i>. +The larvae from <i>fragilis</i> were next put on the tomentose species +and reared on it. When they became imagines they were similarly +given their choice, with the result that there were 104 +ovipositions on the tomentose species and only 83 on <i>fragilis</i>. +In the next generations there were 48 ovipositions on the tomentose +and 11 on <i>fragilis</i>. Finally the fourth generation made +15 ovipositions on the tomentose and none on <i>fragilis</i>.</p> + +<p class="indent">The difficulty about such experiments is obviously that +one has no assurance that the change of instinct, in so far as there +is any, may not be a mere consequence of the captivity. It +must, besides, be extremely difficult to arrange the experiment so +that there is really an equal choice between the two bushes, when +one stands beside the other. Przibram, in quoting this case, +considers that as the tomentose bush was about six times as +large as the <i>fragilis</i>, some indication of the relative attractiveness +of the two may be obtained by dividing the ovipositions on the +larger bush by six, but I imagine the matter must be much more complex.</p> + +<p class="indent">Schröder's second example is not more convincing, +in my opinion, though Semon regards it as one of the most important +pieces of evidence. It concerns a leaf-rolling moth, <i>Gracilaria +stigmatella</i>, the larva of which is said normally to make its house +by bending over the <i>tips</i> of the sallow leaves on which it feeds. +Schröder placed larvae on leaves from which the tips had been +cut, and these larvae made their houses by rolling over the <i>sides</i> +of the leaves. Their offspring were again fed on leaves without +tips, and as before, they rolled in the leaf-margins either on one +side or both. The offspring of this second generation were then +fed on entire leaves. There were 19 houses made by these (?19) +larvae, and of them 15 were normal, made by folding down the +tips of the leaves, while 4 were abnormal, made by rolling in the +leaf-margins. Schröder says that in nature he has only twice +seen abnormal houses; but it is clearly essential not only that the +frequency of such variability in nature should be thoroughly +examined, but also that we should know whether when the species +<span class="pagenum"><a name="Page_195" id="Page_195">[Pg 195]</a></span> +is bred in captivity these irregularities of behaviour do or do not +occur when the larvae are fed on uninjured leaves.</p> + +<p class="indent">The famous case of Schübeler's wheat is revived by Semon. +The story will be familiar to most readers of the literature of the +subject. Briefly it is that annuals, especially wheat and maize, +raised from seed in Central Europe take more time in coming +to maturity and ripening than similar plants raised in Norway, +where the summer days are much longer. The received account +is that he imported seed especially of maize and of wheat from +Central Europe to Norway and found that in successive years +the period of growth and ripening was increasingly reduced. +After two generations seed of the accelerated wheat was sent +back to Breslau where it was grown, and was found to ripen rather +more slowly than in Norway, but much more quickly than the +original stock had done. The facts recorded by Schübeler<a name="FNanchor_6_139" id="FNanchor_6_139"></a><a href="#Footnote_6_139" class="fnanchor">[6]</a> +are that he received seed from Eldena, which is on the Baltic near +Greifswald. The variety is described as "<i>100 tägiger Sommer +Weizen</i>," but no more exact record of its behaviour in Germany +is given. This wheat, grown at Christiania in 1857, took 103 +days to harvest. Its seed was again grown in Christiania in 1858, +and took 93 days, and sown again in 1859 it took only 75 days, 28 +days less than in the first year of cultivation in Norway. Seed of +the 1858 crop was sent to Breslau, and grown there by Roedelius +in 1859; it took 80 days. Evidently before such a record can be +used as proving an inheritance of acquired characters numbers of +particulars should be forthcoming. The view that Johannsen +has taken is that the result was probably due to unconscious +selection of the earlier individuals among a population consisting +of many types of various compositions. Some effect may no +doubt be ascribed to that cause, but I cannot think that alone +it would account for the results. My impression is rather that +they were produced by differences in the cultivation and especially +in the seasons. Research of an elaborate character would be +necessary in order to eliminate the various sources of error, and +nothing of the kind has been done; nor does Semon allude to these +difficulties in prominently adducing Schübeler's evidence. A +<span class="pagenum"><a name="Page_196" id="Page_196">[Pg 196]</a></span> +difference of even three weeks in time of harvesting may easily +be due to variation in the season. It would in any case be difficult +to analyse the meteorological conditions, and to decide how +much effect in postponing or accelerating the harvest might be +due to cold days, to cloudy days, to wet weather, to fluctuations +in average temperature, to hot days, and other such incidents +occurring at the different periods of growth, even if they were +specially watched while the experiments were in progress, and +at this distance of time such analysis is practically impossible. +Without careful simultaneous control-experiments this evidence +is almost worthless. The director of the Meteorological +Office<a name="FNanchor_7_140" id="FNanchor_7_140"></a><a href="#Footnote_7_140" class="fnanchor">[7]</a> +has, however, kindly sent me some details of the weather +at Breslau from 1857 to 1860, and I notice that as a matter of +fact July, 1859, was an exceptionally hot month, <i>having an average +of 2.67° C. above the mean</i> for the twenty years 1848-1867. June +in that year was slightly (0.31° C.) below the mean and May +slightly above it (0.18° C.). August was also abnormally hot, +2.35° C. above the average. The Breslau wheat was sown on +<i>May 19</i> and harvested on August 6. There was a cold spell from +May 11 to 14, which this wheat escaped, as it was sown on May +19. In the other years the cold spell came much later. These +elements of the weather may possibly have done something to +hurry the ripening in 1859. It unfortunate that we are not +told how long similar wheat from Breslau seed took to ripen in +that year.</p> + +<p class="indent">As regards the Norway cultivations we have the +average monthly temperatures recorded by Schübeler, though he does +not discuss them in connection with this special problem. It is +quite clear that 1857, in which the period was 103 days, was an +exceptionally cold summer, especially as regards the months of +June and July, but though there was, so far as the temperature +<span class="pagenum"><a name="Page_197" id="Page_197">[Pg 197]</a></span> +records go, no great difference between 1858 and 1859, the year +1859, in which the period of ripening was the shortest, was somewhat +colder in Norway than 1858. But we have the further +difficulty that there were ten days difference in sowing, for in +1858 the sowing was made on May 14, and in 1859 on May 24. +With all these possibilities uncontrolled, and indeed unconsidered, +I am surprised that Semon should claim these experiments as one +of the chief supports for his views.</p> + +<p class="indent">Schübeler's other allegations respecting the influence +of climate on plants grown in various places and especially at different +elevations in Norway have been destructively criticised by +Wille<a name="FNanchor_8_141" id="FNanchor_8_141"></a><a href="#Footnote_8_141" class="fnanchor">[8]</a> +to whose paper readers interested in the subject should refer.</p> + +<p class="indent">Before the appearance of Wille's criticisms Wettstein<a name="FNanchor_9_142" id="FNanchor_9_142"></a><a href="#Footnote_9_142" class="fnanchor">[9]</a> +made a favourable reference to Schübeler's work, accepting his +conclusion. He states also that he has himself made analogous +experiments with flax, finding that the length of the period of +development and a series of morphological characters show an +adaptation to local conditions, and that on transference of seed +to other conditions the previous effects are maintained. No +details, however, are given, and I do not know if anything more +on the subject has appeared since. The other examples cited +by Wettstein, such as the observations of Cieslar on forest-trees +and those of Jakowatz on gentians seem to me open to all the +usual objections applicable to evidence of this kind. Such work, +to be of any value for the purpose to which it is applied, must be +preceded by a study of the normal heredity and of the variations +of the species.</p> + +<p class="indent">Most of the recent writers (Semon, Przibram, etc.) on the +inheritance of acquired characters accept the story of Brown-Séquard's +guinea pigs, which are said to have inherited a liability +to peculiar epileptiform attacks induced in their parents by various +nervous lesions.</p> + +<p class="indent">The question has been often debated and several observers +have repeated the experiments with varying results, some failing +<span class="pagenum"><a name="Page_198" id="Page_198">[Pg 198]</a></span> +to confirm Brown-Séquard, others finding evidence which in +various degrees supported his conclusions. Recently a new and +especially valuable paper has been published by Mr. T. Graham +Brown<a name="FNanchor_10_143" id="FNanchor_10_143"></a><a href="#Footnote_10_143" class="fnanchor">[10]</a> +which goes far towards settling this outstanding question. +He states that "the Brown-Séquard phenomenon is nothing more +or less than a specific instance of the scratch-reflex," and it is +due to a raised excitability of the mechanism of this reflex. This +raised excitability is the character acquired as a consequence, +for instance, of the removal of part of one great sciatic nerve. +The nature of this raised excitability and its causation are discussed +and elucidated, but this part of the work is not essential +to the present consideration. Mr. Graham Brown in his summary +of conclusions remarks that it is very difficult to see how this +condition of raised excitability can be transmitted to the offspring, +and this comment which might be made in reference to any of +the alleged cases certainly applies with special cogency to the +present example.</p> + +<p class="indent">He then calls special attention to three observations:</p> + +<p class="blockquot">1. That guinea pigs which had a "trophic" change in the +foot, as a result of division of the great sciatic nerve, have repeatedly +been seen to nibble the feet of other guinea pigs which +had this change in the foot from the same causes.</p> + +<p class="blockquot">2. That accidental injury to the toes may be followed +by the Brown-Séquard phenomenon in an otherwise normal animal.</p> + +<p class="blockquot">3. That in several instances the young of guinea pigs +which exhibited the phenomenon have been noticed to have one or more +toes eaten off by the mother.</p> + +<p class="indent">Brown-Séquard noticed that almost all his animals +in which the great sciatic was divided acquired the "epilepsy" and +nibbled those parts of their feet in which sensation had been lost. +Of the offspring of such animals he found that a very small proportion +exhibited a malformation of the feet, and of these some +showed the "epilepsy." The proportion which showed the +"epilepsy" was one to two per cent. of the offspring.</p> + +<p class="indent">Morgan<a name="FNanchor_11_144" id="FNanchor_11_144"></a><a href="#Footnote_11_144" class="fnanchor">[11]</a> +is quoted by Graham Brown as having suggested +<span class="pagenum"><a name="Page_199" id="Page_199">[Pg 199]</a></span> +that the loss of toes in the offspring may have been due to mutilation +by the mother, following his experience in a case in which +the tails of mice in succeeding litters were thus devoured, and +there can be little doubt that in this suggestion lies the clue to +the explanation of the whole mystery. Graham Brown concludes +that it may be supposed with every degree of probability that +the "transmission" was due to injuries inflicted upon the young +by their parents. With this conclusion most people will now be +disposed to agree, and we may hope that we shall hear the last +of this curious myth—to the elucidation of which a vast +quantity of research has been devoted.</p> + +<p class="indent">The series of experiments made by Kammerer with +various Amphibia have attracted much attention and have been +acclaimed by Semon and other believers in the transmission of +acquired characters as giving proof of the truth of their views. +With respect to these observations the chief comment to be made +is that they are as yet unconfirmed. Many of the results that +are described, it is scarcely necessary to say, will strike most +readers as very improbable; but coming from a man of Dr. +Kammerer's wide experience, and accepted as they are by Dr. +Przibram, under whose auspices the work was done in the Biologische +Vesuchsanstalt at Vienna, the published accounts are +worthy of the most respectful attention.</p> + +<p class="indent">The evidence relates chiefly to three distinct groups of occurrences:</p> + +<p class="blockquot">1. Modification in <i>Alytes obstetricans</i>, the Midwife Toad, +affecting both the structure and the mode of reproduction, induced +by compulsory change of habits.</p> + +<p class="blockquot">2. Modification in the mode of reproduction of <i>Salamandra +atra</i> and <i>maculosa</i> induced by compulsory change of habits.</p> + +<p class="blockquot">3. Modification in the colour of <i>Salamandra maculosa</i> induced +by change in the colour of the soil on which the animals were kept.</p> + +<p class="indent">1. I will take first the case of <i>Alytes</i>,<a name="FNanchor_12_145" id="FNanchor_12_145"></a><a href="#Footnote_12_145" class="fnanchor">[12]</a> +because it is the most definite example, and because it is the case +<span class="pagenum"><a name="Page_200" id="Page_200">[Pg 200]</a></span> +which most readily admits of repetition and verification.</p> + +<p class="indent">The habits of <i>Alytes obstetricans</i> are well +known. The animals copulate on land. As the strings of eggs leave the +female they are entangled by the hind legs of the male, and being +adhesive they stick to him and undergo their development attached +to his back and legs. The number of eggs varies from 18 +to 86, a number much smaller than is usual in toads and frogs +which lay their eggs in water. The eggs are large and full of yolk.</p> + +<p class="indent">There are two breeding seasons, one about April and +the other about September, and a winter hibernation. Not only animals +brought in from outside, but their offspring reared in domestication +maintain these normal habits in confinement, if the temperature +does not exceed 17° C. (pp. 499 and 534).</p> + +<p class="indent">If, however, the temperature be artificially raised +and kept at 25-30° C., the males do not attach the eggs to themselves +when spawning occurs on land but let them lie. The adhesion +of the eggs is said to be hindered by the comparatively rapid +drying of their surfaces.</p> + +<p class="indent">More usually in the high temperatures the animals +<i>take to the water</i> and copulate there. The eggs are ejected into +the water, and as their gelatinous coverings immediately swell up, +they do not stick to the males.</p> + +<p class="indent">The offspring thus derived from the parents subjected +to heat for one breeding-period only, whether they were laid in water +or on land, did not show departures from the normal type.</p> + +<p class="indent">Kammerer states next, however, that in subsequent +breeding-periods the same parents frequently take to the water to +breed, though they have become quite accustomed to the heated +chamber; and furthermore that if such animals, having thus lost +their instinct to brood their young, be transferred to ordinary +temperatures they do not readily reassume their normal habits, +but for several breeding seasons—at least four—will take to +the water. These parents lay from 90 to 115 eggs, which are +small and contain little yolk, and the larvae, on hatching, breathe +with their embryonic gills until they are absorbed instead of +being broken off as normally. +<span class="pagenum"><a name="Page_201" id="Page_201">[Pg 201]</a></span></p> + +<p class="indent">The offspring thus abnormally developed when they +mature are said never to brood their eggs. If they are derived from +the earlier spawnings of their parents, before, that is to say, the +parents had been submitted to the changed conditions long enough +to transmit their effects, they lay on land; but if they are derived +from the later spawnings, they lay in the water. These changes +of habit are manifested without the continued application of +the abnormal experimental conditions, and, as I understand the +account, in normal conditions of temperature.</p> + +<p class="indent">If the abnormal experimental conditions are continued, +the toads always lay in water, and their eggs become progressively +smaller and more numerous. The larvae in the fourth generation +acquire three pairs of gills instead of one pair, and are in other +respects also different from the normal form.</p> + +<p class="indent">Respecting the <i>Alytes</i> bred in this way Kammerer makes +the very striking statement that <i>the males in the third generation</i> +(p. 535) <i>have roughened swellings on their thumbs and that in the +fourth generation</i> (pp. 516 and 535) <i>these swellings develop black +pigment</i>. Together with the appearance of this secondary sexual +character there is hypertrophy of the muscles of the fore-arm. +To my mind this is the critical observation. If it can be +substantiated it would go far towards proving Kammerer's case. +<i>Alytes</i>, among toads and frogs, is peculiar in that the males do +not develop these lumps in the breeding season, and the fact +may no doubt be taken to be correlated with the breeding habits, +copulation occurring on land and not in water as is usual with +Batrachians. It is to be expressly noticed that these lumps on +the thumbs or arms of male toads and frogs are not merely pigmented +swellings, but are pads bearing numerous minute horny +black spines, which are used in holding the females in the water. +The figures which Kammerer gives (Taf. XVI, figs. 26 and 26a) +are quite inadequate, and as they merely indicate a dark patch +on the thumbs it is not possible to form any opinion as to the +nature of the structure they represent.</p> + +<p class="indent">The systematists who have made a special study of Batrachia +appear to be agreed that <i>Alytes</i> in nature does not have these +structures; and when individuals possessing them can be +<span class="pagenum"><a name="Page_202" id="Page_202">[Pg 202]</a></span> +produced for inspection it will, I think be time to examine the evidence +for the inheritance of acquired characters more seriously. +I wrote to Dr. Kammerer in July, 1910, asking him for the loan +of such a specimen<a name="FNanchor_13_146" id="FNanchor_13_146"></a><a href="#Footnote_13_146" class="fnanchor">[13]</a> +and on visiting the Biologische Versuchsanstalt +in September of the same year I made the same request, but +hitherto none has been produced. In matters of this kind much +generally depends on interpretations made at the time of observation; +here, however, is an example which could readily +be attested by preserved material. I notice with some surprise +that in a later publication on the same subject no reference to the +development of these structures is made (see below).</p> + +<p class="indent">The statements here given represent but a small +part of Kammerer's papers on the subject. He gives much further +information as to the course of the experiments, especially in +regard to the fate of the eggs laid on land and the aberrations +induced in them by treatment. The ramifications of the experiments +are, however, very difficult to follow, and as I am not +sure that I have always understood them I must refer the reader +to the original.</p> + +<p class="indent">More recently Kammerer has published<a name="FNanchor_14_147" id="FNanchor_14_147"></a><a href="#Footnote_14_147" class="fnanchor">[14]</a> +a most curious account of experiments in crossing his modified and abnormal +<i>Alytes</i>, derived from the water-eggs, with normal individuals.</p> + +<p class="indent">In the first case the cross was made between a +<i>normal female</i> and an <i>abnormal male</i>. The offspring were normal +in their habits. In the next generation bred from these almost exactly +a quarter showed the abnormal instinct.</p> + +<p class="indent">The reciprocal cross was made between an <i>abnormal female</i> +and a <i>normal male</i>. In this case the offspring were abnormal in +their behaviour; but the second generation bred from them +showed three quarters abnormal and one quarter normal.</p> + +<p class="indent">Certain details as to numbers and sexes of the various +families bred in the course of this amazing experiment are given in a +<span class="pagenum"><a name="Page_203" id="Page_203">[Pg 203]</a></span> +subsequent publication.<a name="FNanchor_15_148" id="FNanchor_15_148"></a><a href="#Footnote_15_148" class="fnanchor">[15]</a> +This later paper goes somewhat fully +into the question of the difference in behaviour between the +normal and modified individuals, describing the ways in which +the males and females possessing the acquired character could be +recognised from the males and females which were normal, but +in this account I find no reference to the development of the +"<i>Brunftschwielen</i>"—the horny pads on the hands of the males. +As these structures would be of special value in such a diagnosis +the omission of any allusion to them calls for explanation. +Kammerer claims the evidence as proof of Mendelian segregation +in regard to an acquired character, the first example recorded. +Pending a repetition of the experiments there is no more to be said.</p> + +<p class="indent">2. <i>The Mode of Reproduction of Salamandra atra and maculosa.</i><a name="FNanchor_16_149" id="FNanchor_16_149"></a><a href="#Footnote_16_149" class="fnanchor">[16]</a> +mdash;<i>Salamandra maculosa</i>, the common lowland form, with yellow +bands or spots, deposits its young in water, generally as gill-bearing +tadpoles, with a wide, swimming tail, though occasionally +they are born still enclosed in the egg-capsule out of which they +soon hatch. Spawning extends over a considerable period, +often many weeks, and during the season one female may bear +more than 50 young.</p> + +<p class="indent"><i>S. atra</i>, the black Alpine form, produces its young +on land. They are born without gills, ready to breathe air, and with the +rounded tail of the adult. These differences may, as Kammerer +says, naturally be regarded as adaptations to the Alpine conditions. +Moreover, the female bears <i>only two</i> young in a season, +and this reduction in the number must be taken to be a consequence +or condition of viviparity. There are many eggs in the +ovary, but all except the two which are destined to develop +degenerate and form a yolk-material on which these two survivors feed.</p> + +<p class="indent">Kammerer gives a long account of the various conditions +to which he subjected both species. The treatment was complicated +<span class="pagenum"><a name="Page_204" id="Page_204">[Pg 204]</a></span> +in many ways, but the essential statements are, as regards +<i>S. maculosa</i>, that when no water was provided in which the young +might be born, they were dropped on land, larger and in a later +stage of development and of a darker colour than is normal; that +the larvae so born gradually diminished in number until only +two were deposited in each breeding-period; that dissection +showed that the other ova degenerated to form a yolk-material. +The larvae so produced reached maturity. The summary of +results describes their behaviour, stating that they produced:</p> + +<p class="indent">(<i>a</i>) <i>In water</i>, either (1) <i>very</i> advanced, +large-headed larvae 45 mm. long (instead of 25-30 mm.) with gills already +reduced, which had awkward, embryo-like movements, and in some few +days metamorphosed into small perfect salamanders; or (2) +moderately advanced, properly proportioned larvae, 40-41 mm. +long, provided with large gills of (at first) intrauterine character, +which were reduced during aquatic life.</p> + +<p class="indent">(<i>b</i>) <i>On land</i>, small (26 mm. long) larvae with +rudimentary gills, having the body rounded instead of being flattened from +above downwards, and an elongated narrow head, which were +unable to live in deep water. These larvae changed to the salamander +colour in 10-12 days, and after four weeks metamorphosed +into salamanders 29 mm. long.</p> + +<p class="indent">(<i>c</i>) In the foregoing cases the experimental +conditions were not continued, or in other words, basins of water were +provided in which they could spawn. But if the experimental conditions +are continued, these <i>Salamandra maculosa</i> which were born +newt-like (viz., not in a larval condition), are themselves newt-bearing +from the first time they give birth, using the dry land, +and bringing forth only two young, the normal number for the +births of <i>S. atra</i>. These young are 40-41 mm. long, and are +dark-coloured, resembling greatly the normal new-born <i>S. atra</i>.</p> + +<p class="indent">This epitome of the observations illustrating the +inheritance of acquired characters has been very widely quoted, and may +not unnaturally be taken to summarize a wide experience of +the modified animals. Reference to the details given in the +same paper shows that, as alleged, each of the four types of behaviour +enumerated was witnessed <i>once</i> only in the case of each +<span class="pagenum"><a name="Page_205" id="Page_205">[Pg 205]</a></span> +of four females, no two agreeing with each other. As to the +number of the males or their habits nothing is said. The first +female, <i>a</i> (1), bore five young; the second, <i>a</i> (2), bore two, of +which one was a partial albino; the third, <i>b</i>, produced four young; +and the fourth, <i>c</i>, two as already stated.</p> + +<p class="indent">In the case of <i>c</i> the details show that the female +gave birth immediately after being transferred from the open-air terrarium +to one indoors, which contained no basin of water. This is the +example of the consequences which follow on a continuance of +the experimental conditions.<a name="FNanchor_17_150" id="FNanchor_17_150"></a><a href="#Footnote_17_150" class="fnanchor">[17]</a> +</p> + +<p class="indent">As regards <i>S. atra</i> the converse is reported. +Various means were used to induce them to eject their young prematurely +in water, such as massaging the sides of the mothers, or raising +the temperature to 25° or 30° C., with various degrees of success. +But afterwards it was found that specimens collected wild at an +elevation of about 1,000 metres responded to much simpler +treatment, and gave birth prematurely in water when they were +kept in a large shallow basin of water not so deep but that they +could everywhere touch the bottom with their feet and keep their +heads above the surface. With specimens collected at higher +elevations this treatment was inoperative, and the suggestion is +made that <i>S. atra</i> at the lower confines of its habitat partakes +more of the nature of <i>maculosa</i> than do the individuals from +greater heights; for Kammerer argues that pools suitable for +breeding must be more uncommon at those elevations than they +are lower down.</p> + +<p class="indent">In the earlier paper<a name="FNanchor_18_151" id="FNanchor_18_151"></a><a href="#Footnote_18_151" class="fnanchor">[18]</a> +Kammerer states that newly caught +females of <i>S. atra</i> often give birth in the water, and show an +undoubted preference for doing so. He describes also how he +once saw several females, wild in their natural habitat, lay their +young in a rain-puddle at 1,800 metres elevation, but the larvae +thus born were fully formed. +<span class="pagenum"><a name="Page_206" id="Page_206">[Pg 206]</a></span></p> + +<p class="indent">When the deposition of the young as larvae has become +"habitual"<a name="FNanchor_19_152" id="FNanchor_19_152"></a><a href="#Footnote_19_152" class="fnanchor">[19]</a> +with <i>S. atra</i>, three to nine larvae may be produced +at one spawning period, from 35 to 45 mm. long, with gills at +most 8 mm. long, and a tail-fin 2-3 mm. broad. Such larvae +are generally coffee-brown, or grey (instead of black), and show +other minor differences.</p> + +<p class="indent">The summary states that when grown to maturity they +become in their turn larva-bearing, and go into the water to bring +forth. Their young are more than two (3 to 5 being the numbers +observed) with a length of 33-40 mm. or of 21-23 mm. at birth. +They are light grey, spotted (mottled with lighter and darker +colour), have relatively short gills (8 to 9 mm. at most) and a +broad tail-fin (3 mm. wide). At metamorphosis they are relatively +long (44 mm.) and one of them had some yellow pigment.</p> + +<p class="indent">Here again this summary is, as a matter of fact, +describing the behaviour of two mothers, of which one produced three, +and the other five young.</p> + +<p class="indent">To my mind these experiments suggest that the reproductive +habits of both species, if closely observed, will be found to be +subject to considerable variation, and I think it not impossible +that each species is, especially in confinement, capable of being a +good deal deflected from its normal behaviour. Moreover, there +seems to me no great improbability in the idea that there is an +interdependence between the number of young and the stage of +maturity in which they are born. But, at the same time, the case +as told by Kammerer strikes me as proving too much. If each +species is so sensitive to conditions that the normal procedure +is gravely modified in one generation, and if that modification +can reappear in a pronounced form in the next generation without +a renewal of the disturbing conditions, it becomes extremely +difficult to understand how the regularity which each species is +believed to display in nature can be maintained. Surely both +species might be expected to be in confusion. From a passage +in Kammerer's earlier paper (1904, p. 55) on the subject, I infer +that he also would expect considerable irregularity in the natural +behaviour, but that he has not investigated the point.<a name="FNanchor_20_153" id="FNanchor_20_153"></a><a href="#Footnote_20_153" class="fnanchor">[20]</a> +<span class="pagenum"><a name="Page_207" id="Page_207">[Pg 207]</a></span></p> + +<p class="indent">3. <i>Modification of the Colour of Salamandra maculosa +induced by Change in the Colour of the Soil on which the Animals were +kept.</i>—Kammerer speaks of this as the most convincing of all +his experiments on the transmission of acquired characters. So +far, however, no full account of them has been published.<a name="FNanchor_21_154" id="FNanchor_21_154"></a><a href="#Footnote_21_154" class="fnanchor">[21]</a> +The statement is that when salamanders are kept in yellow surroundings +the yellow markings gradually in the course of years increase +in amount relatively to the black ground colour. Conversely by +keeping the animals on black garden soil, the yellow may be +greatly diminished in quantity until it largely disappears. (The +account in <i>Natur</i> adds that very moist conditions also favour the +increase of yellow, and that with less moist conditions the yellow +diminishes.) From each kind, the (induced) yellower and the +(induced) blacker, a second generation was raised, on soil of +neutral colour, and each family was later divided into two parts, +half being put on black and half on yellow ground.</p> + +<p class="indent">As regards the offspring of those which had lived on <i>black</i> +soil no positive result had been reached up to the date of publication, +but it is stated that these young resembled their parents +in having the yellow distributed in <i>irregular spots</i>.</p> + +<p class="indent">As regards the offspring of those which had lived on +yellow soil the account follows up the story of that part of the offspring +which were put on yellow soil again. It is stated that these, though +derived from parents with irregular spots, <i>developed the yellow +as longitudinal bands</i>.</p> + +<p class="indent">This account is given with slight differences of +expression in the three places to which I have referred. On returning +from Vienna in 1910 I consulted Mr. G. A. Boulenger in reference to +the subject, and he very kindly showed me the fine series from +many localities in the British Museum, and pointed out that in +nature the colour-varieties can be grouped into two distinct types, +<span class="pagenum"><a name="Page_208" id="Page_208">[Pg 208]</a></span> +one in which the yellow of the body is irregularly distributed in +spots and one in which this yellow is arranged for the most part +in two longitudinal bands which may be continuous or interrupted. +<i>The spotted form is, as he showed me, an eastern variety, +and the striped form belongs to western Europe.</i> Mr. E. G. +Boulenger<a name="FNanchor_22_155" id="FNanchor_22_155"></a><a href="#Footnote_22_155" class="fnanchor">[22]</a> +has since published a careful account of the distribution +of the two forms. The spotted he regards as the typical +form, var. <i>typica</i>, and for the striped he uses the name var. +<i>taeniata</i>. The typical form occupies eastern Europe in general, +including Austria and Italy, extending as far west as parts of +eastern France. The var. <i>taeniata</i> is found all over France, +excepting parts of the eastern border, Belgium and western +Germany, Spain and Portugal. Of the very large series examined +there was only one specimen (Lausanne) which could not with +confidence be referred to one or other of the two varieties. +Mr. E. G. Boulenger points out that both varieties inhabit very +large areas, and live on soils of most different colours and +compositions. Both are liable to variations in the amount and the +shade of the yellow, but that any suggestion that <i>taeniata</i> belongs +especially to yellow soils and <i>typica</i> to black soils is altogether +inadmissible. He expresses surprise that Kammerer should not +allude to these peculiarities in the geographical distribution of +the two forms. He suggests further that it is more likely that +some mistake occurred in Kammerer's observations than that the +east European <i>typica</i> should, in the course of a generation, have +been transformed into the west European <i>taeniata</i> by the influence +of yellow clay soil.</p> + +<p class="indent">In his last paper on the subject Kammerer states incidentally<a name="FNanchor_23_156" id="FNanchor_23_156"></a><a href="#Footnote_23_156" class="fnanchor">[23]</a> +that he has found the <i>striped form recessive to the spotted</i>. No +evidence for this statement is given, and I have not found any +other reference to crosses effected between the two natural types. +If, however, this representation is correct, it is conceivable that +the production of <i>taeniata</i> from <i>typica</i> was in fact the re-appearance +of a recessive form. The plate which Kammerer gives +in illustration of his modified parent figures a single animal at +four stages, and though it is certainly more like the spotted than +<span class="pagenum"><a name="Page_209" id="Page_209">[Pg 209]</a></span> +the striped form, it has a certain suggestion of the striped arrangement, +such as I can well imagine being produced in the heterozygote.<a name="FNanchor_24_157" id="FNanchor_24_157"></a><a href="#Footnote_24_157" class="fnanchor">[24]</a> +</p> + +<p class="indent">In continuation<a name="FNanchor_25_158" id="FNanchor_25_158"></a><a href="#Footnote_25_158" class="fnanchor">[25]</a> +of the experiments on the colour of <i>S. maculosa</i> +Kammerer publishes an account of elaborate experiments +in grafting ovaries of the various forms, modified and unmodified, +into each other, and describes the offspring which followed. +Before pursuing this part of the inquiry I am disposed to wait +until the earlier steps have been made much more secure than +they yet are.</p> + +<p class="indent">More recently Kammerer has published similar statements +in regard to the inheritance of characters induced in various lizards +by keeping them in abnormal temperatures, high and low. The +changes induced affected in some species the colours, in others +the reproductive habits. Respecting these examples I feel the +same scepticism that I have indicated in regard to the others, +somewhat heightened by the fact that insufficient evidence is +given both regarding the behaviour of these various species in +captivity when not subjected to abnormal temperatures, and +in the wild state.</p> + +<p class="indent">Respecting this part of the evidence Mr. G. A. Boulenger has +lately published a criticism<a name="FNanchor_26_159" id="FNanchor_26_159"></a><a href="#Footnote_26_159" class="fnanchor">[26]</a> +from which I extract the following +passages. Referring to a previous note<a name="FNanchor_27_160" id="FNanchor_27_160"></a><a href="#Footnote_27_160" class="fnanchor">[27]</a> +on the question of the +melanism of the various insular forms of <i>Lacerta muralis</i> he +writes: "I also alluded (<i>l. c.</i>) to the theories that have been +propounded to explain the melanism of various insular forms. +This is a subject which has been lately taken up by Dr. Kammerer +at the Biologische Versuchsanstalt in Vienna, and he claims +to have produced nigrinos artificially by a very strong elevation +of the temperature, accompanied by extreme dryness. Dr. +Werner<a name="FNanchor_28_161" id="FNanchor_28_161"></a><a href="#Footnote_28_161" class="fnanchor">[28]</a> +has already opposed his own experiments to those of +Kammerer, artificial melanism having been produced by him in +<i>Lacerta oxycephala</i> by keeping two very light specimens from +<span class="pagenum"><a name="Page_210" id="Page_210">[Pg 210]</a></span> +Ragusa for a whole summer in very damp conditions. Neither is +Kammerer's theory in accordance with the distribution of the +black lizards, as pointed out by Werner. Kammerer also finds +that those forms which are known to produce melanic races in a +state of nature, lend themselves more readily than the others +to the success of his experiments. But he shows himself misinformed +when he states that the variety called <i>Lacerta fiumana</i> +belongs to the category of those of which black forms are not +known. He overlooks the fact, first pointed out by Scherer in +1904, and which I can confirm, that the black lizard from Melisello +near Lissa in the Adriatic is unquestionably derived from +the lizard from Lissa, which he correctly regards as not separable +from <i>L. fiumana</i>...."</p> + +<p class="indent">"Another colour modification which Dr. Kammerer states +that he obtained by raising the temperature is the assumption by +the female of the typical <i>Lacerta muralis</i> of the bright red colour +of the lower parts which often distinguishes the male from the +female, and which was not shown by the individuals of the latter +sex kept by him under normal conditions. He quotes various +authorities to show that the lower parts are never red in the +females, but he has omitted to consult others who say the contrary. +Thus Bedriaga (1878 and 1879) remarks that a so-called +var. <i>rubriventris</i> of the typical wall lizard has the lower parts red +in both sexes."<a name="FNanchor_29_162" id="FNanchor_29_162"></a><a href="#Footnote_29_162" class="fnanchor">[29]</a> +</p> + +<p class="indent">In reading such papers as those of Semon or Kammerer +the thought uppermost in my mind is that to multiply illustrations +of supposed transmission of acquired characters is of little use +until some one example has been thoroughly investigated. If +we had certain assurance that even a single unimpeachable case +could be repeated at will, the whole matter would assume a more +serious aspect. If, for instance, Kammerer were able to show us +<i>Alytes</i> males with horny pads on their hands, it would be +something tangible; still more, if the experiment were repeated by +others until no doubt remained that the offspring of <i>Alytes</i> which +had bred in water for some three generations did acquire these +<span class="pagenum"><a name="Page_211" id="Page_211">[Pg 211]</a></span> +pads and that they could transmit these novelties to descendants +raised in normal conditions. Till evidence of this kind is published +by at least two independent observers investigating similar +material, I find it easier to believe that mistakes of observation or +of interpretation have been made than that any genuine transmission +of acquired characters has been witnessed.</p> + +<p class="indent space-below">Meanwhile there is no denying that the origin of adaptational +features is a very grave difficulty. With the lapse of time since +evolutionary conceptions have become a universal subject of study +that difficulty has, so far as I see, been in nowise diminished. +But I find nothing in the evidence recently put forward which +justifies departure from the agnostic position which most of us +have felt obliged to assume.<a name="FNanchor_30_163" id="FNanchor_30_163"></a><a href="#Footnote_30_163" class="fnanchor">[30]</a> +</p> + +<hr class="chap" /> +<h3><span class="smcap">Appendix to Chapter IX.</span></h3> + +<p class="indent">Professor G. Klebs, as is well known to students of +evolutionary phenomena, has for several years been engaged in investigations +relating to the inheritance of acquired characters. In +his many publications on the subject the issue has always been +represented as more or less uncertain.</p> + +<p class="indent">Desiring to know how the matter now stands according +to Professor Klebs' present judgment I wrote to him asking him to +favour me with a brief general statement. This he most kindly +sent in a letter dated 8th July, 1912.</p> + +<p class="indent">As such a statement will be read with the greatest +interest by all who are watching the progress of these studies I obtained +permission to publish it as follows:</p> + +<p class="blockquot"> +<span style="margin-left: 38em;">8. Juli 1912</span><br /> + Ihre liebenswurdige Anfrage will ich sehr gern +beantworten, obwohl ich sie nicht so beantworten kann wie ich +erwünschte. Ihr Skepticismus in der Frage der Uebertragung erworbener +Charactere auf die Nachkommen ist nur zu berechtigt. Meine +Versuche mit Veronica sind <i>nicht</i> beweisend, da es mir bisher +nicht gelungen ist eine einigermasse konstante Varietät mit +verlaubten Inflorescenze zu erzeugen. In Bezug auf mein +Semper vivum bin ich allerdings noch heute der Meinung dass +<span class="pagenum"><a name="Page_212" id="Page_212">[Pg 212]</a></span> +die starke künstliche Veränderung der Blüte einen Einfluss auf +einzelnen Nachkommen gehabt hat. Ich habe seither nichts +darüber veröffentlicht: die Mehrzahl der anormalen gefüllten +Blüten war leider steril. Von einem weniger veränderten Exemplar +erhielt ich einige Sämlinge, aber sie haben noch nicht +geblüht. Es kann sich in diesem Falle nur um eine <i>Nachwirkung +in der ersten Generation</i> handeln, vergleichbar jenen Fällen in +denen Samen von Bäumen aus den hohen Alpen in der Ebene +gewisse Nachwirkungen zeigen. Aber es ist bisher kein sicherer. +Fall bekannt in den der kunstliche herbeigeführte Charakter +<i>mehrere Generationen hindurch unter der gewöhnlichen "normalen" +Bedingungen</i> übertragen worden ist.</p> + +<p class="blockquot"> Auf der andere Seite sind diese negativen Resultaten +nicht entscheidend. Denn wie wenig ist in dieser Beziehung überhaupt +ernstlich versucht worden! Und zweifellos geht die +Sache nicht so einfach.</p> + +<p class="blockquot"> Ich versuche es mit anderen Pflanzen weil ich der +Meinung bin dass es möglich sein müsse wenigstens solche neuen Varietäten +zu erzeugen, wie sie die Gartenvarietäten entsprechen.</p> + +<p class="blockquot"> Aber bis jetzt leider sind die Versuche nicht +gelungen, weder mir noch irgend einem anderen.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_213" id="Page_213">[Pg 213]</a></span></p> + +<h2>CHAPTER X</h2> +<h3><span class="smcap">Effects Of Changed Conditions</span> <i>Continued</i></h3> +<p style="font-size: 110%; text-align: center;"> + <b><span class="smcap">The Causes of Genetic Variation</span></b></p> + +<p class="indent">In the last chapter we examined some of the evidence +offered in support of the belief that adaptation in highly organised +forms is a consequence of the inheritance of adaptative changes induced +by the influence of external conditions. The state of knowledge +of this whole subject is, as I have said, most unsatisfactory, +chiefly for the reason that in none of the cases which are alleged +to show a positive result have two observers been over the +same ground, or as yet confirmed each other. In the wider +consideration respecting the causes of variation at large we find +ourselves still in the same difficulty. The study has thus far +proved sadly unfruitful. In spite of the considerable efforts +lately made by many observers to induce genetic variation in +highly organised plants or animals, and though successes have +occasionally been announced, I do not know a single case which +has been established and confirmed in such a way that we could +with confidence expect to witness the alleged phenomena if we +were to repeat the experiment. Abundant illustrations are +available in which individuals exposed to novel conditions manifest +considerable changes in characters or properties, but as yet +there is no certain means of determining that germ-cells of a new +type shall be formed.</p> + +<p class="indent">Of the direct effect of conditions the lower organisms, +especially bacteria, offer the best examples, the alterations of virulence +which can be produced in so many distinct ways being the most +striking and familiar. That attenuation of virulence can be +produced by high temperatures or by exposure to chemical +agents, and that this diminution in virulence may remain permanent +is, from our point of view, not surprising; but the fact that +in many cases the full virulence can by suitable cultivation be +<span class="pagenum"><a name="Page_214" id="Page_214">[Pg 214]</a></span> +restored is difficult to understand. Similar variations have been +observed in power of pigment production and other properties.</p> + +<p class="indent">These phenomena naturally raise the question whether any +cases of apparent loss of factors in higher forms may be comparable.</p> + +<p class="indent">The subject of variations in the lower organisms and +their dependence on conditions is a highly special one, and I have no +knowledge which can justify me in offering any discussion of +them, but I understand that hitherto little beyond empirical +recognition of the phenomena has been attempted. A useful +summary of observations made by many investigators was lately +published by Hans Pringsheim,<a name="FNanchor_1_164" id="FNanchor_1_164"></a><a href="#Footnote_1_164" class="fnanchor">[1]</a> +who enumerates the different +agencies which have been observed to produce modifications, +and the various ways in which these changes are manifested. +One of the most comprehensive studies of the subject from the +genetic point of view is that made by F. Wolf.<a name="FNanchor_2_165" id="FNanchor_2_165"></a><a href="#Footnote_2_165" class="fnanchor">[2]</a> +In his extensive cultivations of <i>Bacillus prodigiosus</i>, +<i>Staphylococcus pyogenes</i> and <i>Myxococcus</i> he succeeded in producing +many strains with modified properties. In most of these the modifications arose +in consequence of the application of high or low temperatures or of +the addition of various chemical substances to the culture-media. +Some of the variations, which are for the most part in the powers +of pigment-formation, persisted when the strains were returned +to normal conditions, and others did not. In reference especially +to the variations witnessed in the Cocci the reader should consult +the critical account of variation in that group published by the +Winslows,<a name="FNanchor_3_166" id="FNanchor_3_166"></a><a href="#Footnote_3_166" class="fnanchor">[3]</a> +where much information on the subject is to be +found. The authors attempted to determine the systematic +relationships of the several forms, as far as possible, by the +application of statistical methods. The result is interesting as +showing that the problem of species in its main features is presented +by these organisms in a form identical with that which +we know so well in the higher animals and plants, whatever +<span class="pagenum"><a name="Page_215" id="Page_215">[Pg 215]</a></span> +properties be selected as the diagnostic characters. There are +many types perfectly distinct and others which intergrade. +Some of the types change greatly with conditions while others do +not. This is exactly what we encounter whenever we study the +problem of species on an extended scale among the higher forms of life.</p> + +<p class="indent">There is now practically complete agreement among +bacteriologists that the observations made first by Massini on the +change in color of <i>Bacterium coli mutabile</i> grown in Endo's +medium, associated with the acquisition of the power to ferment +lactose, are perfectly reliable and free from possibilities of mistake. +The work has been extended and confirmed by many +workers, especially R. Müller, who finds that this bacterium can +similarly acquire and maintain the power to ferment other +sugars. A careful account of the whole subject written by Müller +for the information of biologists will be found in <i>Zts. für Abstammungsl.</i>, +VIII, 1912. After discussing the biological significance +of the facts, he concludes with a caution to the effect that bacteria +are so different from all other living things that generalizations +from their behavior must not be indiscriminately applied to animals and plants.</p> + +<p class="indent">In all work with this class of material there is +obviously danger of error through foreign infection of the cultures, +but there can be no doubt that though some of the "mutations" +recorded may be due to this cause, the majority of the instances +observed under stringent conditions are genuine.</p> + +<p class="indent">Another and equally serious difficulty besetting work +with bacteria and fungi cultivated from spores is that the appearance +of variation may in reality be due to the selection of a special +strain previously living masked among other strains. This +possibility must be remembered especially in those instances +which are claimed as exemplifying the effects of acclimatisation. +Manifestly this consideration can be urged with most force when +the strain which gave rise to the novelty was not raised from a +single individual spore. Moreover, when once the possibility of +spontaneous variation is admitted, it must be difficult to be quite +confident that any given variation observed is in reality due to +<span class="pagenum"><a name="Page_216" id="Page_216">[Pg 216]</a></span> +the novel conditions applied, and as I understand the evidence, +the appearance of the mutational forms does not with any +regularity follow upon the application of the changed conditions.</p> + +<p class="indent">Researches into the variation of these lower forms +will, no doubt, be continued on a comprehensive scale. So long as the +instances recorded are each isolated examples it is impossible to +know what value they possess. If they could be coordinated in +such a way as to provide some general conception of the types of +variation in properties to which bacteria, or any considerable +group of them, are habitually liable, the knowledge might +greatly advance the elucidation of genetic problems.</p> + +<p class="indent">Of mutational changes directly produced with regularity +in micro-organisms by treatment, the experiments with trypanosomes +provide some of the clearest examples. A summary of +the evidence was lately published by Dobell,<a name="FNanchor_4_167" id="FNanchor_4_167"></a><a href="#Footnote_4_167" class="fnanchor">[4]</a> +from which the present account is taken. The most definite fact of this +kind established is that certain dyes introduced into the blood of the +host have the effect of destroying the small organ known as the +"kinetonucleus" in the trypanosomes. The trypanosomes thus +altered continue to breed, and give rise to races destitute of +kinetonuclei. This observation was originally made by Werbitzki +and has been confirmed by several observers. The exact +way in which this alteration is effected in the trypanosomes is not +quite definitely made out, but there is good reason for supposing +that the dyes have a direct and specific action upon the kinetonucleus +itself, and circumstances make it improbable that in +some division a daughter-organism without that body is produced, +or that any selection of a pre-existing defective variety occurs.</p> + +<p class="indent">Ehrlich has suggested with great probability that the +dyes which possess this action owe it to the fact that they have the +particular chemical linkage which he calls "ortho-quinoid." In +outward respects, such as motility and general appearance, the +modified organisms are unchanged, but their virulence is diminished. +As regards the possibility of the defective strain +<span class="pagenum"><a name="Page_217" id="Page_217">[Pg 217]</a></span> +reacquiring the kinetonucleus, Werbitzki states that in one case +passage through 50 animals and treatment with dyes left the +strain unaltered; but that in another case at the sixteenth +passage 7 per cent. of the trypanosomes were found to have +re-acquired the organ, and in subsequent passages the percentage +increased, until at the twenty-seventh passage practically all had +re-acquired it. Kudicke, however, in similar experiments did not +succeed in causing re-acquisition by transplantation.</p> + +<p class="indent">By the action of various drugs and anti-bodies races +of trypanosomes resistant to those substances have been obtained. +These breed true, at least when kept in the same species of animal +in which the resistance was acquired. As to whether change of +virulence is produced by passage through certain animals or not, +there is as yet no general agreement.</p> + +<p class="indent">Other changes, especially in size and some points of +structure, are said to occur when certain trypanosomes proper to mammals +are passed through cold-blooded vertebrates (Wendelstadt and +Fellmer), and it is stated that these changes persist, but the +observations have not yet been confirmed.</p> + +<p class="indent">Experiments lately conducted by Woltereck with <i>Daphnia</i> +are interesting as having given a definite positive result, in so far, +at least, as the ova were affected by conditions before leaving the +bodies of the parent individuals. The observations relate to +the offspring resulting from <i>parthenogenetic</i> eggs. Females +bearing ephippia (fertilised eggs) were isolated until the ephippia +were dropped, and in this way the offspring of fertilisation were +excluded. Males, of course, appeared from time to time in the +cultures, but as fertilised eggs were rejected, their presence did +not disturb the result. The most remarkable observations +related to <i>Daphnia longispina</i>.</p> + +<p class="indent">This species as found in the lower lake at Lunz had +the front end of the body blunt and nearly round in profile; but on being +cultivated in a warm temperature and with abundant nourishment +the front end of the body became produced into an elongated +"helmet," as Woltereck calls it. Experiment showed that +the change was primarily due to the abundance of food, and owing +to temperature in a subordinate degree. +<span class="pagenum"><a name="Page_218" id="Page_218">[Pg 218]</a></span></p> + +<p class="indent">This distinction arose as soon as the species was taken +into the hothouse, but when the modified individuals were put back +into the original conditions, a lower temperature and scanty +food-supply, the next generation returned to their original form. +After being cultivated for two years and about 40 generations in +the more favourable conditions, when similarly put back into +the lower temperature with scanty food the <i>first generation</i> born +in these conditions was helmeted like the modified parents. +Woltereck is of opinion that the ova were still unformed at the +time the parents were put back, and the influence of the favourable +conditions upon the unformed ova he speaks of as a "prae-induction." +The effect never extended beyond the one generation, +after which the strain returned to its original state.</p> + +<p class="indent">The fact that the influence on the offspring was not +manifested at first led Woltereck to expect that by more prolonged +cultivation in the favourable conditions a further extension of +this influence would be produced, but this expectation was never +fulfilled, though the attempt was made again and again.</p> + +<p class="indent">Similar experiments were made with <i>Hyalodaphnia cucullata</i>, +which is far more sensitive to cultural influences, and in nature +manifests a considerable elongation of the helmet as a seasonal +modification, but the results were essentially the same as in the +preceding case, no modification extending beyond the first +generation born after the restoration to <i>normal conditions</i>.<a name="FNanchor_5_168" id="FNanchor_5_168"></a><a href="#Footnote_5_168" class="fnanchor">[5]</a> +</p> + +<p class="indent">The only criticism of these extremely interesting results +which suggests itself is that perhaps the original appearance of the +modification was not in reality due to an <i>accumulated</i> effect of +the conditions, but to some change in the conditions themselves +which was not noticed. It is difficult to see how length of +time or even the lapse of several generations could have so specific +an effect on the race. It is no doubt often vaguely supposed +by many that a long period of time may be necessary for the +effect of climate or of other environmental conditions to be +produced in an organism which does not thus respond at first. +I have never been able to see any reason for this opinion nor how +<span class="pagenum"><a name="Page_219" id="Page_219">[Pg 219]</a></span> +it is to be translated into terms of physiological fact, and I +imagine that in those cases in which the lapse of time is really +required for the production of an effect, the influence of the +prolongation is rather on the conditions than on the organisms. +The response of the organisms thus probably indicates not that +the creature is at length feeling the effects because of their +accumulated action on itself, but that the conditions have at +length ripened.</p> + +<p class="indent">As this sheet is passing through the press Agar has published<a name="FNanchor_6_169" id="FNanchor_6_169"></a><a href="#Footnote_6_169" class="fnanchor">[6]</a> +an abstract of evidence as to another comparable case in a parthenogenetic +strain in the daphnid, <i>Simocephalus vetulus</i>. When +fed on certain abnormal foods the shape of the body is changed, +the edges of the carapace being rolled backwards so as to expose +the appendages. The offspring of animals thus modified showed +similar modification in the first, and to a very slight degree, in +the second generation, though the original mothers were removed +to normal conditions before their eggs were laid. In the third +generation there was "a very pronounced reaction in the opposite +direction." Agar suggests that the change may be due to some +toxin-like substances, carried on passively by the egg into the +next generation, against which the protoplasm eventually produces +an anti-body.</p> + +<p class="indent">The experiments which have been in recent years regarded +by evolutionary writers as the most conclusive proof that direct +environmental action may produce germinal variation are those +of Professor W. L. Tower, of Chicago, on <i>Leptinotarsa</i>, the +potato beetles. This work has attained considerable celebrity +and has been generally accepted as making a definite extension +of knowledge. After frequently reading Tower's papers and +after having been privileged to see some of the experiments in +progress (in 1907) I am still in doubt as to the weight which +should be assigned to this contribution.</p> + +<p>The work is described in two chief publications, the first of +which appeared in 1906.<a name="FNanchor_7_170" id="FNanchor_7_170"></a><a href="#Footnote_7_170" class="fnanchor">[7]</a> +This treatise contains a vast amount +of information about numerous species and varieties of these +<span class="pagenum"><a name="Page_220" id="Page_220">[Pg 220]</a></span> +beetles which the author has observed and bred in many parts of +their distribution throughout the United States, Mexico and +Central America. The part of the book which has naturally +excited the greatest interest is that in which Tower states that +by subjecting the beetles to change in temperature and moisture, +he caused them to produce offspring quite unlike themselves, +which in several cases bred true.</p> + +<p class="indent">It is much to be regretted that the author did not +happen to become acquainted with Mendelian analysis at an earlier stage +in the investigation. The evidence might then have been +handled in a much more orderly and comprehensive way, and a +watch would have been kept for several possibilities of error.</p> + +<p class="indent">The headquarters of the genus is evidently as Tower +states, in Mexico and the adjoining countries. In this region there is +a great profusion of forms, some very local, some as for instance +the well-known <i>decemlineata</i>,<a name="FNanchor_8_171" id="FNanchor_8_171"></a><a href="#Footnote_8_171" class="fnanchor">[8]</a> +more widely spread. The distinctions are almost all found in peculiarities of +colour and pattern, and the limits of species are even more indefinable than +is usual in multiform animals. Tower arranges the various types +into seven groups of which the one most studied is that which +he calls the <i>lineata</i> group. To this group belong all the forms +to which reference is here made, and, as I understand, they differ +among themselves entirely in size, colour and pattern. There +is no suggestion of infertility in the crosses made between the +several forms of the <i>lineata</i> group; in fact they present, like many +Chrysomelidae, a good example of what most of us would now +call a polymorphic species, consisting of many types, some found +existing in the same locality, others being geographically isolated.</p> + +<p class="indent">A series of experiments was devoted to the attempt to +fix strains corresponding to the extremes of continuous variations. +For example, those with most black pigment and those with +least black taken from a population continuously varying in this +respect, were separately bred; but almost always the selection +led to no sensible change in the position of the mean of the +<span class="pagenum"><a name="Page_221" id="Page_221">[Pg 221]</a></span> +population. The variations in these cases were evidently fluctuational. +In some instances, however, real genetic differences +were met with, and strains exhibiting them were, as usual, +rapidly fixed.</p> + +<p class="indent">Tower points out that several of the varieties (or species, +as he prefers to call them) were obviously recessive to <i>decemlineata</i>. +This is most clearly demonstrated in the case of the form called +<i>pallida</i>, which is a pale depauperated-looking creature, with the +orange of the thorax almost white and the eyes devoid of pigment.<a name="FNanchor_9_172" id="FNanchor_9_172"></a><a href="#Footnote_9_172" class="fnanchor">[9]</a> +This form behaved as an ordinary Mendelian recessive, +breeding true whenever it appeared in the cultures, or when +individuals found wild were studied in captivity. A black form +which Tower names <i>melanicum</i> was similarly shown to be a +Mendelian recessive. Wild specimens of this variety of opposite +sexes were not found simultaneously in nature, and there was thus +no opportunity of breeding them together, but the hereditary +behaviour was seen in the F<sub>2</sub> generation from a <i>melanicum</i> found +coupled with <i>decemlineata</i>. Experiments also occurred giving +indication that a variety with the stripes anastomosing in pairs +(<i>tortuosa</i>), was another recessive, and that a variety—called +"<i>rubri-vittata</i>"—gave an intermediate F<sub>1</sub> with subsequent +segregation. All these are forms of <i>decemlineata</i> Stål.</p> + +<p class="indent">Similar observations were made regarding forms recessive +to <i>multitaeniata</i> Stål. Of these two were thrown by <i>multitaeniata</i> +itself, namely a form named by Stål <i>melanothorax</i>, and regarded +by him as a species, and one which Tower names <i>rubicunda</i> n. sp. +The facts proving the recessive behaviour of their several forms +will be found in the following places in Tower's book:</p> + +<p> +<span style="margin-left: 6em;"><i>pallida</i>,  pp. 273-278.</span><br /> +<span style="margin-left: 6em;"><i>melanicum</i>,  p. 279.</span><br /> +<span style="margin-left: 6em;"><i>tortuosa</i>,  p. 280.</span><br /> +<span style="margin-left: 6em;"><i>rubrivittata</i>,  pp. 280-281.</span><br /> +<span style="margin-left: 6em;"><i>melanothorax</i> and <i>rubicunda</i>,  pp. 283-285.</span> +<span class="pagenum"><a name="Page_222" id="Page_222">[Pg 222]</a></span></p> + +<p class="indent"> +Following this evidence of recessive nature of the six forms +enumerated, Tower describes experiments showing, as he believes, +that some of them may be caused to appear by applying special +treatment to the parents during the "growth and fertilisation" +(p. 287) of the eggs. The most striking example is that in which +4 males and 4 females of <i>decemlineata</i> were kept very hot (average +35° C.) and dry, and at low atmospheric pressure (19-21 inches). +The eggs laid were restored to natural conditions. These gave +506 larvae, from which emerged 14 normal, 82 <i>pallida</i> and 2 +"<i>immaculothorax</i>," viz., without pigment on the pronotum. +The account of the rest of the experiment is somewhat involved, +but I understand that the <i>pallida</i>, of which two only survived, +behaved as normal recessives when bred to the type: also that +the parents, after having laid the eggs whose history has been +given, were restored to normal conditions and laid 319 eggs which +gave 61 normals.</p> + +<p class="indent">In another case normal parents laid 409 eggs in the +hot and dry conditions, and on restoration to normal conditions, the +same parents laid 840 eggs. Then 409 eggs gave 64 adults as follows:</p> + +<table border="0" style="max-width: 45em;" cellspacing="2" summary="_" cellpadding="0" > + <tbody><tr> + <td class="tdl"> </td> + <td class="tdc"> </td> + <td class="tdc">Males</td> + <td class="tdc"> </td> + <td class="tdc">Females</td> + </tr><tr> + <td class="tdl"><i>decemlineata</i></td> + <td class="tdc"> </td> + <td class="tdc">12</td> + <td class="tdc"> </td> + <td class="tdc">8</td> + </tr><tr> + <td class="tdl"><i>pallida</i></td> + <td class="tdc"> </td> + <td class="tdc">10</td> + <td class="tdc"> </td> + <td class="tdc">13</td> + </tr><tr> + <td class="tdl"><i>immaculothorax</i></td> + <td class="tdc"> </td> + <td class="tdc">2</td> + <td class="tdc"> </td> + <td class="tdc">3</td> + </tr><tr> + <td class="tdl"><i>albida</i></td> + <td class="tdc"> </td> + <td class="tdc">9</td> + <td class="tdc"> </td> + <td class="tdc">7</td> + </tr><tr> + <td class="tdl"> </td> + <td class="tdc"> </td> + <td class="tdc">——</td> + <td class="tdc"> </td> + <td class="tdc">——</td> + </tr><tr> + <td class="tdl"> </td> + <td class="tdc"> </td> + <td class="tdc">33</td> + <td class="tdc"> </td> + <td class="tdc">31</td> + </tr> + </tbody> +</table> + +<p class="indent">The 840 eggs laid in normal conditions +gave 123 normal <i>decemlineata</i>.</p> + +<p class="indent">Similar experiments were made with <i>multitaeniata</i> and gave +comparable results, the two recessives (<i>melanothorax</i>, <i>rubicunda</i>) +being produced in large numbers when the parents were subjected +to heat, but in this case the atmosphere was kept <i>saturated</i> with +moisture, instead of dry, as in the previous instance. The same +parents transferred to normal conditions gave normals only.</p> + +<p class="indent">Lastly the form <i>undecimlineata</i> was exposed "to an +extreme stimulus of high temperature, 10° C. above the average," and a +dry atmosphere, with the result that from 190 eggs there emerged +11 beetles, all of the form <i>angustovittata</i> Jacoby, which subsequently +bred true to that type (see p. 295). +<span class="pagenum"><a name="Page_223" id="Page_223">[Pg 223]</a></span></p> + +<p class="indent">In the results of these experiments, as described, there +is one feature which I regard as quite unaccountable. Tower makes no +comment upon it. Indeed, from the general tenour of the paper, +I infer, not only that he does not perceive that he is recounting +anything contrary to usual experience, but rather that he regards +the result as conforming to expectations previously formed. +The point in question is the genetic behaviour of the dominant +normals produced under the abnormal conditions. These +normals were the result of the breeding of parents declared to be +at the same time giving off many recessive gametes. Some of +these normals must be expected therefore to be heterozygous +unless some selective fertilisation occurs. Nevertheless in every +case they and their offspring are reported to have continually +bred true. I allude especially to the tables given on pp. 288, 289, +292, and 293. Tower does not mention any misgiving about +this result, and I think he regards himself as recounting phenomena +in general harmony with the ideas of mutation expressed +by De Vries. This they may be; but to anyone familiar with +analytical breeding the course of these experiments must seem so +surprising as to call for most careful, independent confirmation.</p> + +<p class="indent">In 1910<a name="FNanchor_10_173" id="FNanchor_10_173"></a><a href="#Footnote_10_173" class="fnanchor">[10]</a> +Tower published an account of further experiments +with <i>Leptinotarsa</i>. The work described related to two subjects. +Crosses were made between three forms, <i>undecimlineata</i> Stål, +<i>signaticollis</i> Stål and "<i>diversa</i>" named by Tower as a new +species. The distinctions between these three depend partly on +characters of the adults and partly on those of the larvae. The +adults of <i>undecimlineata</i> and <i>diversa</i> have the elytra striped, but +the elytra of <i>signaticollis</i> are unstriped. The larvae of <i>signaticollis</i> +and of <i>diversa</i> are yellow, but those of <i>undecimlineata</i> +are white.<a name="FNanchor_11_174" id="FNanchor_11_174"></a><a href="#Footnote_11_174" class="fnanchor">[11]</a> +Moreover, in <i>signaticollis</i> and <i>diversa</i> the black increases +in the third stage of the larvae to form transverse bands +which are absent in <i>undecimlineata</i>. The general course of the +experiments shows that these differences may be approximately +<span class="pagenum"><a name="Page_224" id="Page_224">[Pg 224]</a></span> +represented as due to the action of three factors, any of which +may be independently present or absent. The stripings of the +elytra and of the larvae are each due to a separate factor. As +regards the distinction between the yellow and the white larvae +the evidence does not prove that there is decided dominance of +either colour and I infer that the heterozygotes are often intermediate.</p> + +<p class="indent">The chief contribution which this new paper claims to +make relates to differences in the results which ensue from crosses +effected between these three types at different average temperatures.</p> + +<p class="indent">We are first concerned with four experiments +which I number (1), (2), (3), (4):</p> + +<p class="indent">1. <i>Signaticollis</i> ♀ × <i>diversa</i> ♂ bred at +an average temperature of 80º F. by day and 75° F. by night, gave two groups in +about equal numbers. The first (49) was pure <i>signaticollis</i> and +bred true. The second (53) was of an intermediate type, which +on being bred together gave the typical Mendelian result—1 <i>sig.</i>: +2 <i>intermediate</i>: 1 <i>div</i>.</p> + +<p class="indent">2. Next, as the account originally stood in the published +paper, we are told that <i>sig</i> ♀ × <i>div</i> ♂ bred together at +a day-temp. average 75° F. and night average 50° F. gave an <i>intermediate</i> +only, which subsequently produced a normal 1:2:1 ratio. The two crosses were +repeated eleven times with identical results.</p> + +<p class="indent">In a further experiment (3) +<i>signaticollis</i> ♀ × <i>diversa</i> ♂ +were bred under the same conditions as those used in expt. (1). They +again gave <i>sig.</i> and intermediates as before in fairly equal numbers. +The <i>sig.</i> as before bred true, and the intermediate gave 1:2:1, +all exactly as in expt. (1).</p> + +<p class="indent">In expt. (4) <i>the same parents used</i> in (3) were again +mated under conditions of expt. (2) at the lower temperature, and this +time gave <i>signaticollis</i> exclusively, which bred true for four +generations. This experiment was repeated seven times with +uniform results.</p> + +<p class="indent">Diagrams are given representing all these histories in graphic fashion. +<span class="pagenum"><a name="Page_225" id="Page_225">[Pg 225]</a></span></p> + +<p class="indent">From these observations, Tower concludes that the +determination of dominance, and the ensuing type of behaviour, is clearly +a function of the conditions incident upon the combining germ plasms.</p> + +<p class="indent">It will be observed that expts. (1) and (3) gave +identical results but (2) and (4), though much the same conditions +were applied, are at variance, for (2) gave all intermediates, while +(4) gave all <i>signaticollis</i>. In <i>Amer. Nat.</i>, XLIV, 1910, p. 747, +Professor T. D. A. Cockerell commented on this paper of Tower's +and pointed out that there must be an error somewhere, for when +he discusses these experiments Tower speaks of (2) and (4) as +confirming each other. To this Tower replied<a name="FNanchor_12_175" id="FNanchor_12_175"></a><a href="#Footnote_12_175" class="fnanchor">[12]</a> +that there had been a mistake. He states that in preparing the paper +"certain minor experiments were taken from a larger series and combined +to illustrate a general point in the behaviour of alternative +characters in inheritance," and that expt. (2) was introduced +inadvertently in place of another which he desires to substitute. +In this, which I number (5), <i>signaticollis</i> ♀ × <i>diversa</i> +♂ from exactly the same stocks as those used in (1), were mated at +the lower temperatures specified for (2), day average 75° F., night +average 50° F. These gave all of the <i>signaticollis</i> type with a +narrow range of variability, which bred true, in some cases to F<sub>6</sub>. +Tower says he has repeated this experiment six times with identical results.</p> + +<p class="indent">Nevertheless he proceeds to say that the description +of expt. (2), which was repeated eleven times with identical results, was +correct "as far as given." That experiment was "from a second +series of cultures parallel to the one given, but in which there are +other factors involved, which in H. 410 [my (2)] are productive +of a typical Mendelian behaviour." He adds he does "not care +at this time to make any statement of what these factors are, +nor of their relations to the behaviours given in the H. 409, H. 411, +H. 409/11 series [my (1), (5) and (3)—(4)] which are the simplest +and most easily presented series obtained in the crossing of +<i>signaticollis</i> and <i>diversa</i>." +<span class="pagenum"><a name="Page_226" id="Page_226">[Pg 226]</a></span></p> + +<p class="indent">Professor Cockerell's intervention has thus elicited the fact +that we have as yet only a small selected part of the evidence +before us, even as concerning the effect of temperature on the +cross between <i>signaticollis</i> ♀ × <i>diversa</i> ♂. We learn that at +the lower temperatures the result was eleven times the expected +one, and six times an unexpected one; further, that we owe it to +the author's inadvertence that we have come to hear of the +expected result at all, and that though he knows the factors +which determine the discrepancy, he declines for the present to +name them. In these circumstances we can scarcely venture as +yet to estimate the significance of these records.</p> + +<p class="indent">The paper goes on to recount somewhat comparable, but +more complex instances in which the descent of the colour of adults and +of larvae was affected by temperature in crosses between <i>undecimlineata</i> +and <i>signaticollis</i>. As they stand the results are +very striking and unexpected, but I think, in view of what has +been admitted respecting the former part of the paper, full discussion +may be postponed till confirmation is forthcoming.</p> + +<p class="indent">One feature, however, calls for remark. This second +paper is written apparently without any reference to the discoveries +related by Tower in his previous book, to which no allusion is +made. This is most noticeable in the case of an experiment in +which (p. 296, H. 700A) <i>undecimlineata</i> ♀ (the dominant) +was mated to <i>signaticollis</i> ♂ with the result that all the offspring +were <i>undecimlineata</i> and bred true to that type (Parthenogenesis +was tested for, but never found to occur). This experiment was +made at a temperature averaging 95° F. ± 3.5° by day and 89° F. +± 4.8° by night, and in a humidity given as 84 per cent. by day +and 100 per cent. by night; but in the previous book (p. 294) we +are told that pure <i>undecimlineata</i> bred together "under an extreme +stimulus of high temperature, 10° C. above the average" +and a relative humidity of 40 per cent. gave 11 beetles only, +all <i>angustovittata</i>. But reference to the Plate 16, Fig. 2, shows +that <i>angustovittata</i> must be exceedingly like <i>signaticollis</i>, +having, like it, the elytral stripes obsolete, and if there is any marked +difference at all, it can only be in the larvae. It seems strange +that if <i>undecimlineata</i> really gives off ova of this recessive type at +high temperatures, the fact should not be alluded to in connection +<span class="pagenum"><a name="Page_227" id="Page_227">[Pg 227]</a></span> +with expt. H. 700A, where, as the father was <i>signaticollis</i>, having +the same recessive character, their appearance might have been +expected not to pass unobserved. The temperature in the +older experiment is, of course, not given with the great accuracy +used in the second, and it may have been higher still. The humidity +also was widely different. Still, in discussing the phenomena +we should expect some reference to the very remarkable and +closely cognate discovery which Tower himself had previously +reported in regard to the same species.<a name="FNanchor_13_176" id="FNanchor_13_176"></a><a href="#Footnote_13_176" class="fnanchor">[13]</a> +</p> + +<p class="indent">The hesitation which I had come to feel respecting +these two publications of Tower's has been, I confess, increased by the +appearance of a destructive criticism by Gortner<a name="FNanchor_14_177" id="FNanchor_14_177"></a><a href="#Footnote_14_177" class="fnanchor">[14]</a> +who has examined the parts of Chapter III of Tower's book, in which he +discusses at some length the chemistry of the pigments in <i>Leptinotarsa</i> +and other animals. As Gortner has shown, this discussion, +though offered with every show of confidence, exhibits +such elementary ignorance, both of the special subject and of +chemistry in general, that it cannot be taken into serious consideration.</p> + +<p class="indent">Some observations made by Dr. W. T. Macdougal<a name="FNanchor_15_178" id="FNanchor_15_178"></a><a href="#Footnote_15_178" class="fnanchor">[15]</a> +have also been interpreted as showing the actual causation of genetic +variation by chemical treatment. Of these perhaps the least open +to objection were the experiments with <i>Raimannia odorata</i>, a +Patagonian plant closely allied to <i>Oenothera</i>. The ovaries were +injected with various substances and from some of the seeds +which subsequently formed in them a remarkable new variety +was raised. This varying or mutational form was strikingly +different from the parental type, with which it was not connected +by any intergradational forms, and it bred true. It made +<span class="pagenum"><a name="Page_228" id="Page_228">[Pg 228]</a></span> +no rosette, growing to a much smaller size than the parent, and +was totally glabrous instead of being very hairy as the parental +type is. I was shown specimens of these plants by the kindness +of Dr. Britton in the Bronx Park Botanic Garden in 1907 and +can testify to their very remarkable peculiarities. They had a +somewhat weakly look, and might at first sight be thought to be +a pathological product, but they had bred true for several +generations. From the evidence, however, I am by no means +satisfied that their original appearance was a consequence of +the treatment applied. This treatment was of a most miscellaneous +description. Two of the mutants came from an ovary +which had been treated with a ten per cent. sugar solution. Ten +came from one into which a 0.1 per cent. solution of calcium +nitrate had been injected. One was from a capsule which "had +been exposed to the action of a radium pencil." Macdougal +speaks of these results as decisive, but clearly before such evidence +can be admitted even for consideration it must be shown by control +experiments that the individual plants which threw the +mutant were themselves breeding true in ordinary circumstances. +Nothing is more likely than that the mutant was an ordinary +recessive. I may add that Mr. R. H. Compton made a number +of experiments with <i>Raimannia odorata</i>, raised from seeds kindly +given me by Dr. Britton, injecting the ovaries with a variety of +substances, including those named by Macdougal; but though a +numerous progeny was raised from the ovaries treated, all were +normal. Macdougal relates also that some mutational forms +came from ovaries of <i>Oenothera Lamarckiana</i> exposed to radium +pencils, and also from <i>Oenothera biennis</i> injected with zinc sulphate +a peculiar mutant was raised, but taking into account the +frequency of these occurrences in those species, he very properly +regarded this evidence as of doubtful application. In a later +paper,<a name="FNanchor_16_179" id="FNanchor_16_179"></a><a href="#Footnote_16_179" class="fnanchor">[16]</a> +however, he has returned to the subject and affirms his +conviction that the appearance of a mutant among seedlings +raised from an ovary of <i>Oenothera biennis</i> treated with zinc +sulphate was really a consequence of the injection, saying that +<span class="pagenum"><a name="Page_229" id="Page_229">[Pg 229]</a></span> +the variation previously observed in the species was afterwards +shown to be due to fungoid disease. The circumstances to which +he mainly points in support of his view is that the mutation bred +true, but this is only evidence of its genetic distinctness, which +may, of course, be admitted by those who remain unconvinced +as to the original cause of its appearance. He adds that he is +making similar experiments with some twenty genera; but what +is more urgently needed is repeated confirmation of the original +observation. When it has been shown that this mutation can +be produced with any regularity from a plant which does not +otherwise produce it on normal self-fertilisation, the enquiry +may be profitably extended to other plants.</p> + +<p class="indent">A curious and novel experiment, which however, led +ultimately to a negative result, was made by F. Payne. Many discussions +have been held respecting the blindness of cave animals. +The phenomenon is one of the well-known difficulties, and most +of us would admit that the theory of evolution by the natural +selection of small differences does not offer a really satisfying account +of it. Those who believe in the causation of such modifications +by environmental influences and in their hereditary transmission +make, of course, the simple suggestion that the darkness +is the cause of the loss of sight, and that disuse has led to the +reduction of the visual organs. Payne bred <i>Drosophila ampelophila</i>, +the pomace-fly (which is easy to keep in confinement, fed +on fermenting bananas), for sixty-nine generations in darkness. +At the end of that period there was no perceptible change in the +structure of the eyes, or in any other respect. The number of +generations may possibly be regarded as insufficient to prove +anything, but comparing them, as he does, with the generations +of mankind, we see that they correspond with a period of about +two thousand years, an interval far longer than those which +many writers in particular cases have deemed sufficient.</p> + +<p class="indent">In his first paper Payne states that, though no structural +difference could be perceived, the flies which had been bred in +the dark reacted less readily to light than those which had been +reared under normal conditions, and he inclined to think that +the treatment had thus produced a definite effect. After more +<span class="pagenum"><a name="Page_230" id="Page_230">[Pg 230]</a></span> +careful tests, however, he withdrew this opinion. It proved +that both individual flies and individual groups of flies, both of +those bred in the light and of those bred in the dark, differed +greatly in their reactions, which were measured by counting the +time that it took for a fly to travel to the light end of a covered +tube, various sources of error being eliminated. He found further +that these differences of behaviour were not inherited in any +simple way, but he is disposed to attribute them to accidental +differences in the nature of the food, an account which seems +probable enough.<a name="FNanchor_17_180" id="FNanchor_17_180"></a><a href="#Footnote_17_180" class="fnanchor">[17]</a> +</p> + +<p class="indent">In several recent publications Blaringhem<a name="FNanchor_18_181" id="FNanchor_18_181"></a><a href="#Footnote_18_181" class="fnanchor">[18]</a> +has described the origin of many abnormal forms of plants, especially +of maize, which he attributes to various mutilations practised upon the +parents. Respecting these the same difficulty which has been +expressed in other cases reappears, that before drawing any +conclusion as to the value of such evidence we require to know +that the plants treated belong to a really pure line, which if +left to nature in the ordinary circumstances of its life in that +locality would have had normal offspring. Abnormalities abound +in the experience of everyone who examines pans of seedlings +of almost any species of plant, and in maize they are well known +to be exceptionally common. Some of those which we meet +with when we attempt to ripen maize in this country are very +similar to those which Blaringhem describes, consisting in irregularities +in the distribution of the sexes, in the shapes of the +panicles, etc. Many of these are doubtless imperfections of +development, due to the dullness of our climate, but others are +presumably genetic and would recur in the offspring however +treated. If some one working in a climate where maize could +be raised in perfection would repeat these experiments, and show +that a strain which was thoroughly reliable and normal in its +genetic behaviour did, after mutilation, throw the miscellaneous +types observed by Blaringhem, that would be evidence at least +that the development of the seed could be so influenced by +injury to the parental tissues that its properties were changed. +<span class="pagenum"><a name="Page_231" id="Page_231">[Pg 231]</a></span> +Such evidence could be used for what it is worth; but pending an +inquiry of this kind I am disposed to regard these observations of +variation following on parental injury as suggestive rather than convincing.</p> + +<p class="indent">Some evidence of a remarkably interesting kind has been +collected by J. H. Powers<a name="FNanchor_19_182" id="FNanchor_19_182"></a><a href="#Footnote_19_182" class="fnanchor">[19]</a> +respecting the structure and habits +of <i>Amblystoma tigrinum</i>, which led him to the conclusion that +striking differences in the form, anatomy, and developmental +processes could be effected directly by change in the conditions +of life. It is well known that a profusion of forms, distinct in +various degrees, is grouped round <i>Amblystoma tigrinum</i>. Some +of these are believed to be geographically isolated, others occur +together in the same waters, and, as usual, authorities have differed +greatly as to the number of names to be given. These forms +were studied in detail by Cope who described them in the <i>Batrachia +of North America</i>. The view which he inclined to take +was that the individual variations of <i>Amblystoma tigrinum</i> resulted +from variations in the time and completeness of the +metamorphosis, and these were regarded as due to external +causes, such as differences in season, temperature, and geographical +conditions. Powers, however, states that collecting +within a radius of six or eight miles he found almost if not quite +the whole "gamut of recorded variation in this species." Some, +however, as he states, occurred rarely except under experimental +conditions, but considerable differences in temperature were not +found necessary in producing them. Every year, he says, he +has been able to add to the number of peculiar types found in +the same small area in nature, until the amount of natural +variation at least equals that seen by Cope in the collections of +the National Museum and those of the Philadelphia Academy.</p> + +<p class="indent">Powers states that his observations by no means confirm +Cope's view that these differences are in the main referable to +variation in the completeness of metamorphosis, and on the +contrary, he regards metamorphosis as on the whole a levelling +process, tending to obliterate diversity. The enormous +<span class="pagenum"><a name="Page_232" id="Page_232">[Pg 232]</a></span> +differences in size and proportions which he describes can only be +appreciated by reference to his figures. They affect almost all +features of bodily organisation. These striking differences he +looks upon as brought about by differences in nutrition, "diversities +in habitual locomotion," and diversity in the age at which +metamorphosis occurs, and to sexual difference. Apart from +sexual difference he regards the chief distinctions, in brief, as +"acquired variations of the larva."</p> + +<p class="indent">As an example he gives the great elongation of some +of the forms as "due first to slow growth, second to the free-swimming +habit, third to the prolongation of larval life, and finally to the +assumption of sexual maturity as males," either in the branchiate +or non-branchiate condition. He describes the rapid growth of +some and the slow growth of others. A larva of intermediate +type may grow about a centimeter a month, but a rapidly growing +specimen may grow more than four times as much. The slower +rate of growth may, he says, be induced by winter feeding, and +other treatment.<a name="FNanchor_20_183" id="FNanchor_20_183"></a><a href="#Footnote_20_183" class="fnanchor">[20]</a> +</p> + +<p class="indent">When, however, he goes on to describe the influences +which he regards as exerted by the habit of freely swimming, I am led +to wonder whether after all in most of these illustrations, the +primary distinctions are not in reality genetic. "Specimens +raised in the same aquarium or in similar aquaria, side by side +with all conditions as uniform as it is possible to make them, +seldom fail to furnish striking examples of broad-headed, short-bodied, +and short-tailed types which are habitually found at the +bottom, while others, slender and elongated, are free swimmers, +and maintain themselves in almost as continual suspension and +motion as does a gold fish." Later, again, he writes, "Yet despite +the uniformity of these favourable conditions, the larvae soon +began to split up into two noticeably distinct groups, the one of +<span class="pagenum"><a name="Page_233" id="Page_233">[Pg 233]</a></span> +unusually compact proportions, the other of uniform intermediate +build, such is most commonly met with." It is to my mind +scarcely possible to resist the inference that, though there may be +definite responses to certain conditions, yet the chief distinctions +are genetic, and that it is these distinctions which confer the +power to respond. The parts respectively played by cause and +effect are always difficult to assign; but when it is stated that +"a weak-limbed, long-bodied and long-tailed animal becomes +well nigh perforce an undulatory swimmer, while the strong-limbed, +short-tailed, heavy-bodied specimen, when these characteristics +are rapidly forced upon it, is, under certain circumstances, +just as forcibly induced to become a crawler," we feel +how erroneous any estimates of causation are likely to be.</p> + +<p class="indent">One of the most remarkable and interesting sections +of Powers' paper is that in which he describes the differences in +bodily structure and habits which he attributes to cannibalism, +and the whole account of the phenomena should be read in the +original. It appears that there are two extremely distinct +types of larvae, those with narrow heads and slender bodies +which live for the most part on small Crustacea such as <i>Daphnias</i>, +and those with huge mouths and very wide heads, which disregard +such small animals altogether and live on amphibian +larvae, whether of their own or other species. As the illustrations +show, the differences between these two types are very +great, and the differences in instinct and behaviour are no less. +The cannibals take no heed of the pelagic crustacea, lying sluggishly +at the bottom, rousing themselves immediately to a +violent attack on the larger living things which approach them. +Nothing but the most incontrovertible evidence based on abundant +control experiments should convince us that such differences +are not primarily genetic, and in the present state of knowledge +I incline to think that the families really consist of individuals +which are ready to assume the cannibal habit if opportunity +offers, and others which are congenitally incapable of it. It may +readily be that if all chance of cannibal diet be excluded, the +full development of the wide head and mouth, or the other +peculiarities, would never become pronounced, but I doubt whether +such change could be induced in any individual taken at random.</p> + +<hr class="chap" /> +<p><span class="pagenum"><a name="Page_234" id="Page_234">[Pg 234]</a></span></p> + +<h2>CHAPTER XI.</h2> +<h3><span class="smcap">Sterility of Hybrids. Concluding Remarks.</span></h3> + +<p class="indent">When we consider the bearing of recent discoveries on +those comprehensive schemes of evolution with which we were formerly +satisfied, we find that certain details of the process are more easy +to imagine. We readily now understand how varieties once +formed, can persist, but at the same time difficulties hitherto +faced with complacency become formidable in the light of the +new knowledge. So generally is this admitted by those familiar +with modern genetic research that most are rightly inclined to +postpone the discussion. The premisses, indeed, on which such +a discussion must be based are almost wholly wanting.</p> + +<p class="indent">The difficulties to which I chiefly refer are not those + created by the phenomena of adaptation, though they are serious enough. +In treating of that subject I have felt obliged to express scepticism +as to the validity of nearly all the new evidence for the +transmission of acquired characters. At the present time the +utmost we are bound to accept is the proof that (1) in some +parthenogenetic forms variations, or perhaps we may say malformations, +produced in response to special conditions, recur in +one or perhaps two generations asexually produced after removal +to other conditions. (2) That violent maltreatment may in rare +instances so affect the germ-cells contained in the parents as to +cause the individuals resulting from the fertilisation of those +cells to exhibit an arrest of development similar to that which +their parents underwent.</p> + +<p class="indent">I do not doubt that evidence of this type will be +greatly extended. As a contribution to genetic physiology these facts +are very important and interesting, but I cannot think that any +one, on reflexion, will feel encouraged by such indications to +revive old beliefs in the direct origin of adaptations.</p> + +<p class="indent">In these respects we are simply left where we +were. The force of objections based upon the existence of adaptative +<span class="pagenum"><a name="Page_235" id="Page_235">[Pg 235]</a></span> +mechanisms is no greater than it has always been. On the contrary +the fact that variations can now so generally be recognized +as definite is some alleviation of the difficulty. We can moreover +disabuse ourselves of the notion that for all characters which are +definite or fixed, some utilitarian rationale may be presumed. +Upon that point the study of variation has provided a perfectly +clear answer.</p> + +<p class="indent">In frankly recognizing that the fixity of characters in +general need not connote usefulness to their possessors we deliver ourselves +of a distracting pre-occupation and prepare our minds for +an investigation of the properties of living organisms in the +same spirit as that in which the chemist and the physicist +examine the properties of unorganized materials. The creature +persists not merely by virtue of its characteristics but in spite +of them, and the fact of its persistence proves no more than +that on the whole the balance of its properties leaves something +in its favour.</p> + +<p class="indent">It may be noted by the way that the fact that the +structures of living things are on the whole adaptative was not always +obvious. Though to naturalists of this generation it is a truism, +we have only to turn to Buffon to find that in his philosophy of +nature it played no essential part. The passage in which Buffon +describes what he regards as the forlorn and degraded condition +of the Woodpecker is well known. We have come to think of +the Woodpecker as a capital example of adaptation to the mode +of life; but Buffon after enumerating the hard features of the +bird's existence, forced to earn its living by piercing the bark of +trees in an attitude of perpetual constraint, remarks<a name="FNanchor_1_184" id="FNanchor_1_184"></a><a href="#Footnote_1_184" class="fnanchor">[1]</a> +"Tel est l'instinct étroit et grossier d'un oiseau borné a une vie triste +et chétive. Il a reçu de la Nature des organes et des instrumens +appropriés a cette destinée <i>ou plutôt il tient cette destinée même +des organes avec lesquels il est né</i>" (my italics). His reflexions +on the Stilt (<i>Himantopus</i>) read even more strangely to us, +accustomed as we are to see in the prodigious length and thinness +of the shanks and in the other features of its organisation palpable +adaptations to a wading life. For Buffon, however, this +<span class="pagenum"><a name="Page_236" id="Page_236">[Pg 236]</a></span> +curious bird seemed a poor, neglected production, extravagant +in its disproportions, one of the misfits of creation, left as a +shadow in the picture composed of nature's more successful +efforts.<a name="FNanchor_2_185" id="FNanchor_2_185"></a><a href="#Footnote_2_185" class="fnanchor">[2]</a> This theme he develops at some length, being evidently +well pleased with the idea.</p> + +<p class="indent">Our way of regarding these things is doubtless sounder +and more fruitful than Buffon's, but it is well to remember that what +seems so obvious to us looked quite differently to other excellent +observers; and stupid as it may have been to have overlooked +plain examples of adaptation, it is a far worse mistake to see +adaptation everywhere. I do not seek to minimise the real +and permanent difficulty which the existence of adaptations +creates, but by the suggestion that all normal specific differences +are adaptational that difficulty was quite gratuitously increased.</p> + +<p class="indent">In these respects it may be claimed that progress has +been made, even if that progress seem outwardly of small account.</p> + +<p class="indent">But all constructive theories of evolution have been +built on the understanding that what we know of the relation of varieties +to species justifies the assumption that the one phenomenon is a +phase of the other, and that each species arises or has arisen +from another species either by one or several genetic steps. In +the varieties we have accustomed ourselves to think that we see +those steps. We still know little enough of the mode of occurrence +of variation, but we do begin to know something, and if we +ask ourselves whether our knowledge, such as it is, conforms at +all readily with our former expectations, we cannot with any +confidence assert that it does. Among the plants and animals +genetically investigated are many illustrations of very striking +and distinct varieties. Many of these might readily enough be +accepted as species by even the most exacting systematists, and +not a few have been so treated in classification; but when we +have examined their relationship to each other we feel not merely +that they are not species in any strict sense but that the distinctions +they present cannot be regarded as stages in the direction +of specific difference. Complete fertility of the results of +inter-crossing is and I think must rightly be regarded as +<span class="pagenum"><a name="Page_237" id="Page_237">[Pg 237]</a></span> +inconsistent with actual specific difference; and of variations leading +to that consequence no clear indication has yet been found. +As an example of possible exceptions mention should perhaps be +made of the case of a giant form of <i>Primula sinensis</i> investigated +by Keeble.<a name="FNanchor_3_186" id="FNanchor_3_186"></a><a href="#Footnote_3_186" class="fnanchor">[3]</a> +It arose from a "Star" Primula of normal size, +and though fertile with its own pollen all attempts to fertilise +it with the pollen of other forms failed. Miss Pellew, who did +these fertilisations, tells me that very extensive trials were made, +and repeated in several seasons. Ultimately two plants were +raised from it fertilised with a plant of the strain from which it +sprang, and these proved sterile. In the light of modern experience +the significance of such isolated instances is doubtful.</p> + +<p class="indent">All the strains known as "Giants" are, as Messrs. Sutton +have always found, more or less sterile, and their sterility is +presumably due to some negative defect.</p> + +<p class="indent">In regard to the fertility of Primula species there +are several paradoxes. For example the long-styled varieties, apart from +giants, are fertile with their own pollen, and for many years +short-styled plants have not been used in most strains. Auriculas +and Polyanthuses, on the contrary, are generally if not always +bred from short-styled plants, as the florists have decided that +the long-styled are inadmissible. Mr. R. P. Gregory tells me +that, though most strains of <i>P. sinensis</i> give seed enough +when only long-styled plants are used, he finds nevertheless +that when a "legitimate" union is made the amount of seed +usually increases much as Darwin observed. Darwin's statement +that plants of "illegitimate" origin are less fertile than the +"legitimately" raised plants is also in general confirmed by his +experience. To this rule there were some marked exceptions in +strains derived from <i>long</i>-styled plants, which though illegitimate +showed a high degree of fertility, but illegitimate unions between +<i>short</i>-styled plants always produced comparatively sterile offspring. +I have no records of the behavior of Auriculas and +Polyanthuses. It would be interesting to know whether among +them pure strains of short-styled plants (dominants) have +appeared, and, if so, how their fertility is affected. Without +<span class="pagenum"><a name="Page_238" id="Page_238">[Pg 238]</a></span> +much more critical data I suppose no one would nowadays be +inclined to follow Darwin in instituting a comparison between +the sterility of hybrids and that of illegitimately raised plants of +heterostyle species.<a name="FNanchor_4_187" id="FNanchor_4_187"></a><a href="#Footnote_4_187" class="fnanchor">[4]</a> +It is even difficult to imagine any essential +resemblance between these two phenomena, nor has evidence +ever been produced to show that illegitimately raised plants +have bad pollen grains, which is the usual symptom of sterility +in hybrid plants and the consequence, as we believe, of failure +of some essential division in the process of maturation.</p> + +<p class="indent">The difficulty that we have no knowledge of the contemporary +origin of forms, from a common stock, which when crossed together +give a sterile product, is one of the objections constantly and +prominently adduced from the time of the first promulgation of +evolutionary ideas. In the light of recent work the objection +has gathered strength. Why, if we are able to produce instances +of variation colourably simulating specific difference in almost +all other respects, do we never find an original appearance of this +most widely spread of all specific characteristics? No doubt all +breeders know that sterile animals and plants occasionally appear +in their cultures, but it is more in accordance with probability +that the sterility in these sporadic instances should be regarded +as due to defect than that it should be thought comparable +with that of the sterile hybrids. For their sterility must, by all +analogy with results elsewhere seen, be attributed not to the +absence of something, but to the presence and operation of +complementary factors leading to the production of inhibition +of division; and consistently with that interpretation, we find +that when from a partially sterile hybrid comparatively fertile +offspring can be raised, their comparative fertility continues in +the posterity generally if not always without diminution. The +distinction between these several kinds of sterility was of course +not understood in Darwin's time. The comparison, for example, +which he instituted<a name="FNanchor_5_188" id="FNanchor_5_188"></a><a href="#Footnote_5_188" class="fnanchor">[5]</a> +between the sterility of "contabescent" +anthers and that of hybrids no longer holds, for at least in those +cases in which the nature of contabescent anthers have been +genetically investigated (Sweet Pea, <i>Tropaeolum</i>) they proved +<span class="pagenum"><a name="Page_239" id="Page_239">[Pg 239]</a></span> +to be a simple recessive character. Nor can we now easily +suppose that the attempt there made by Darwin to suggest +resemblance between the sterility produced by unnatural conditions +and that of hybrids has any physiological justification.</p> + +<p class="indent">In regarding the power to produce a sterile or partially +sterile hybrid as a distinction in kind, of a nature other than +those which we perceive among our varieties, I am aware that +I am laying stress on an impression which may hereafter prove +false. The distinction nevertheless is so striking and so continually +before the eyes of a practical breeder that he can +scarcely avoid the inference that when he meets a considerable +degree of sterility in a cross-bred he is dealing with something +belonging to a distinct category, and not merely a varietal feature +of an exceptional kind.</p> + +<p class="indent">Besides the sterility of hybrids appeal has often been +made to the phenomenon of incompatibility, in its several stages of +completeness, as distinguishing species. No one doubts that +incompatibility may arise from a variety of causes of most diverse +degrees of importance, but though sometimes referred to as an +extreme case of interspecific sterility, it is really a very different +matter. In regard to one phase of this incompatibility, that +associated with self-sterility, some progress has been made, and +we are not wholly without experimental evidence of its being +within the range of contemporary variation.</p> + +<p class="indent">Given the outline of Mendelian teaching as to +gametic differentiation and the classification of individuals in a mixed +population, it seemed highly probable that what we call self-sterility +must mean that the species really consisted of <i>classes</i>, +some of which are capable of interbreeding with others while +others are not. According to the received account every individual, +though incapable of fertilising itself, was supposed to be +able both to fertilise and to be fertilised by any other individual. +This notion has always seemed to me a self-evident absurdity, +for it would imply that there can be as many categories as +individuals. Such experiments, however, as I made did certainly +give results consistent with that belief. I first tried +Cinerarias, which are usually self-sterile, but I found no +<span class="pagenum"><a name="Page_240" id="Page_240">[Pg 240]</a></span> +incompatible pairs of plants. Whether I was deceived by the +consequences of apogamy, or whether the pollen of certain plants +may belong to more than one class I do not know. The results +were confused in various ways. Usually the self-fertilised plants +set little or nothing, and cross-fertilised they set fully with such +uniformity that the few failures could plausibly be attributed to +mistakes in manipulation or to other extraneous causes. Later +de Vries announced<a name="FNanchor_6_189" id="FNanchor_6_189"></a><a href="#Footnote_6_189" class="fnanchor">[6]</a> +(without giving particulars) that he had +proved the existence of such classes in <i>Linaria vulgaris</i>; but on +making experiments with that species I again got no positive +results, and I came to the conclusion that in spite of inherent +improbability the conventional belief must be substantially true. +At last, however, the work of Correns, lately published,<a name="FNanchor_7_190" id="FNanchor_7_190"></a><a href="#Footnote_7_190" class="fnanchor">[7]</a> +does definitely show that in one species, <i>Cardamine pratensis</i>, +classes of individuals exist such that individuals of the same class are +incapable of fertilising themselves or each other, but fertilisation +made between the classes is usually completely effective. Many +complications were encountered and some contradictory evidence +is recorded, but the general bearing of the results was positive +and indubitable.</p> + +<p class="indent">We know far too little of this phenomenon as yet to +be able to understand its significance, but I suppose we may anticipate +with some confidence that it will be found to be a manifestation +of dissimilarity between the male and female gametes of the +same individual, comparable with that first seen in the Stocks +(<i>Matthiola</i>) which throw doubles—a state of things in all likelihood +to be found widely spread among hermaphrodite organisms. +Whether the incompatibility between species is to be associated +with that of the self-steriles also cannot be positively asserted, +though it seems not unreasonable to expect that such an association +will be discovered.</p> + +<p class="indent">The case of the apple and the pear is an impressive +illustration of this possibility. The two species are of course exceedingly +alike in all outward respects, but nevertheless the pollen of each +is entirely without effect on the other. Presumably we should +<span class="pagenum"><a name="Page_241" id="Page_241">[Pg 241]</a></span> +interpret this fact as meaning not so much that the apple and +the pear are in reality very wide apart, but rather that either, +each is lacking in one of two complementary elements, or that +each possesses a factor with an inhibitory effect. Their incompatibility +may well be of the same nature as that of the classes +in <i>Cardamine pratensis</i>.</p> + +<p class="indent">Returning now to the problem of inter-specific sterility; +we note, as I have said, the absence of contemporary evidence that +variation can confer on a variety the power to form a sterile +hybrid with the parent species. The considerations based on +this want of evidence have for a long while been familiar to all +who have discussed evolutionary theories, and it is worth observing +the exact reason why the difficulty strikes us now with a new +and special force. In pre-Mendelian times all that was known +was that some forms could freely interbreed without diminution +of fertility in the product, while others could not. But now we +find that, by virtue of segregation, from one and the same pair +of parents, or even, in the case of hermaphrodites, from one and +the same individual, offspring commonly arises showing among +themselves exactly such differences as distinguish species—and +very good species too. This we see happening again and again. +But to forms capable of arising as brethren in one family the +title species has never been meant to apply, and if we are going +to use the term in application to fraternal groups we must +definitely recognise that by "specific" difference is to be understood +simply <i>difference</i>, without any immediate or even ulterior +physiological limitation whatever. Naturally, therefore, we begin +to think of the appearance of sterility in crosses as something +apart, and as a manifestation which distinguishes certain kinds +of unions in a very special way.</p> + +<p class="indent">I am perfectly aware that there are gradations in the +sterility of hybrids as in every other characteristic upon which it has +been proposed to base specific definitions; but, as also so often happens +in the matter of defining intergrading categories, the difficulty +in practice is not often such as to lead to actual ambiguity. I +am speaking of course of those examples which are amenable to +genetic experiment. As to the rest there is complete and permanent +<span class="pagenum"><a name="Page_242" id="Page_242">[Pg 242]</a></span> +uncertainty. But the experience of the practical breeder +does, I think, on the whole, support the contention to which +systematists have so steadily clung under all the assaults of +evolutionary philosophers, that, though we cannot strictly define +species, they yet have properties which varieties have not, and +that the distinction is not merely a matter of degree.</p> + +<p class="indent">The first step is to discover the nature of the factors +which by their complementary action inhibit the critical divisions and so +cause the sterility of the hybrid. Thus expressed, we see the +problem of inter-specific sterility in its right place; and the +question why we do not now find contemporary instances of +varieties lately arisen in domestication, which when crossed back +with their parents, or with their coderivatives, can produce +sterile products, is perceived to be only a special case of a problem +which in its more general form is that of the origin of new and +additional factors.</p> + +<p class="indent">For the requisite evidence no comprehensive search +has been made, but perhaps it will yet be found. All that we can say +at the present time is that the incidence both of hybrid sterility, +and of incompatibility also, is most capricious; and provided +that two forms have such features in common that a cross between +them seems not altogether out of the question, no one can predict +without experiment whether such a cross is feasible, and if +feasible whether the product will be fertile, or sterile more or +less completely. For instance, though probably all the British +and some Foreign Finches (Fringillidae) have been crossed +together, and some of these crosses, as for instance, the various +Canary-mules have been made in thousands, I believe no quite +clear example of a fertile hybrid can be produced. Many species +of Anatidae cross readily and produce fertile hybrids: others give +results uniformly sterile. Though most of the Equidae can be +crossed and some of the hybrids are among the commonest of +domesticated animals there is no certain record of a fertile mule. +Among the Canidae the dogs, wolves and jackals all give fertile +hybrids, but there is no clearly authenticated instance of a cross +between any of these forms and the European fox. In spite of +their close anatomical resemblance it is doubtful if the rabbit +<span class="pagenum"><a name="Page_243" id="Page_243">[Pg 243]</a></span> +and the hare have ever interbred. Many of the wild species of +<i>Bos</i> have been crossed and recrossed both with each other and +with many domesticated races, but I understand that no cross +with the Indian buffalo (<i>Bos bubalus</i>) has yet been successful +even in producing a live calf.<a name="FNanchor_8_191" id="FNanchor_8_191"></a><a href="#Footnote_8_191" class="fnanchor">[8]</a> +In the genus <i>Primula</i> many +hybrids are known and several of them occur in nature, but +hitherto no certain hybrid between <i>P. sinensis</i> and any other +species has been made, in spite of repeated attempts.</p> + +<p class="indent">In <i>Nicotiana</i> many—doubtless all—the various +forms of <i>N. tabacum</i> can be crossed together without diminution of fertility, +though some are very distinct in appearance, but crosses between +<i>tabacum</i> and <i>sylvestris</i> are highly sterile (in my experience totally +sterile<a name="FNanchor_9_192" id="FNanchor_9_192"></a><a href="#Footnote_9_192" class="fnanchor">[9]</a>), +though the distinctions between them are not to outward +observation nearly so great as those which can be found between +the various races of <i>Primula sinensis</i>.</p> + +<p class="indent">Recently some remarkable experiments bearing closely on +these questions have been published by F. Rosen.<a name="FNanchor_10_193" id="FNanchor_10_193"></a><a href="#Footnote_10_193" class="fnanchor">[10]</a> +They concern the forms of <i>Erophila (Draba) verna</i>, celebrated in +the history of evolutionary theory as the plants especially chosen by +Alexis Jordan for the exposition of his views on these subjects.</p> + +<p class="indent">The "species" contains a profusion of forms dissimilar +in many structural characters, such as the size and shape of leaves, +flowers, fruits, etc. Of these forms many grow in association. +Jordan found, on experiment, that each, to the number of some +two hundred, bred true, and that therefore, the conventional +assumption that polymorphism of this kind must mean great +contemporary variability had no foundation in fact. So far +<span class="pagenum"><a name="Page_244" id="Page_244">[Pg 244]</a></span> +indeed is the evidence from favouring the belief that such forms +are in any way transitional or indeterminate, that, as is well +known, Jordan used it with every plausibility to support the +doctrine of the fixity of species. To certain aspects of Jordan's +work we will return later in this chapter, but the matter is in +the present connection of especial interest for the reason that +Rosen has lately found by experiment that some of these presumably +very closely allied forms, crossed together, gave hybrids +more or less sterile. In the case of the offspring of one pair of +forms only (<i>E. cochleata</i> and <i>stricta</i>) was the fertility +undiminished, and the various degrees of sterility found in the +other crosses ranged up to the extreme infertility of the hybrids +between <i>E. stricta</i> × <i>elata</i>. From this cross ten plants were +bred. Of these the four strongest were chosen to breed from, +but two of the four proved totally sterile; one had only bad seeds; +and from the fourth a single seedling was raised which in its +turn proved to be sterile. From the less sterile hybrids F<sub>2</sub> +families were raised, with the usual experience that in this and +subsequent generations the sterility diminished among extracted +forms, new and true-breeding types with complete fertility being +thus derived from the original cross.<a name="FNanchor_11_194" id="FNanchor_11_194"></a><a href="#Footnote_11_194" class="fnanchor">[11]</a> +</p> + +<p class="indent">The production of sterility as a consequence of crossing +plants so nearly approaching each other as these <i>Erophila</i> +"species" do is not a little interesting, and the fact well exemplifies +the futility of the various attempts to frame general +expressions as to specific properties or behaviour. Commenting +on his results Rosen argues that the polymorphic group commonly +called by systematists <i>Erophila (Draba) verna</i> may now be +regarded as having arisen by crossing, as did his own types mentioned +above. The question, however, <i>what</i> species were the +original progenitors of the group cannot be answered. Rosen +considers that no form which he knows satisfies the requirements, +<span class="pagenum"><a name="Page_245" id="Page_245">[Pg 245]</a></span> +and that it or they must be supposed to be lost. This conclusion +will recall the similar problem raised by the <i>Oenothera</i> mutants +(Chap. V); and unsatisfactory as it may be to have recourse +to such hypotheses we must remember the possibility that as a +consequence of hybridisation, subsequent segregation and recombination +of factors, species may have thus actually, as we +may say, exploded, and left nothing but a polymorphic group of +miscellaneous types to represent them in posterity. If this way +of regarding the phenomena be a true one, the sterility now seen +when some of the group are re-crossed, becomes analogous to +that "reversion or crossing" which we now so well understand +to be a consequence of the recombination of characters separated +at some previous point in the history of descent. In the partial +sterility of the contemporary hybrid we see this character reappearing, +formed now as it was on the occasion of the original +cross, by the meeting of complementary factors.</p> + +<p class="indent">Another case that may be mentioned in this connection is +that of the crosses between various culinary peas (<i>Pisum sativum</i>) +and a peculiar form found by Mr. Arthur Sutton growing ostensibly +in a wild state in Palestine. This Palestine Pea is low +growing, rarely reaching 18 inches. It is in general appearance +like a small and poorly grown field pea. The stems are thin and +rather hard. The most obvious differences which distinguish +this from other field peas are the marked serration of the stipules, +and the development of pith in the pods. Such pith is often +present in the pods of peas more or less, but in the Palestines it +is so strongly developed as almost to form a lomentum. Curiously +enough, though the flowers are purple much as those of +ordinary field peas, there is no coloured spot in the axils. On +the other hand, the stems have coloured stripes running up +from the axils. Though this plant differs so little from domesticated +peas, all crosses with them either failed, or produced +hybrids quite or almost quite sterile. This was Mr. Sutton's +experience, and on repeating the experiments with material +kindly given by him I found the same result.<a name="FNanchor_12_195" id="FNanchor_12_195"></a><a href="#Footnote_12_195" class="fnanchor">[12]</a> +<span class="pagenum"><a name="Page_246" id="Page_246">[Pg 246]</a></span></p> + +<p class="indent">In a large series of crosses some seeds died or gave +rise to feeble plants. Of the plants which lived, few gave any seed. +The seed, however, that was obtained from F<sub>1</sub> plants grew well +enough, and the F<sub>2</sub> plants proved, as often in such cases, fertile. +In these, indeed, no sign of sterility was noticeable. The experiment +is being repeated in various ways, for, as the genetic +behaviour of peas is comparatively well known, the subject is an +exceptionally favourable one for these investigations.</p> + +<p class="indent">Such an example shows the confusion produced the +moment we attempt to harmonize conceptions of specific difference +with results attained by experimental methods. It has been usual +to regard the field pea (<i>P. arvense</i>) as a species distinct from +the edible pea (<i>P. sativum</i>). De Candolle and others regard the +field pea as derived from a form wild in Italy, but the origin of +the edible pea is considered to be unknown. From breeding +experiments we find no sterility whatever in the crosses between +the various <i>arvense</i> and <i>sativum</i> types, nor in the crosses +made between them and several other peculiar types from various +countries; whereas this Palestine Pea, which only differs from a +small <i>arvense</i> in what might have been thought trivial characters,<a name="FNanchor_13_196" id="FNanchor_13_196"></a><a href="#Footnote_13_196" class="fnanchor">[13]</a> +either fails to cross altogether or gives a sterile product, whatever +type be chosen as the other parent.</p> + +<p class="indent">Examples of this kind have at least the merit that +they lead to more precise delimitations of the problem. We are confronted +with two distinct alternatives.</p> + +<p class="indent">1. We may apply the term Species promiscuously to +all distinct forms. If we do so it must be clearly understood that +we cannot even rule out the several combinations of "presences +and absences" represented by the various types whether wild or +domesticated. For we may feel perfectly assured that at least +all the <i>arvense</i> and all the <i>sativum</i> types yet subjected to +experimental tests are on precisely the same level in this respect. There +is no distinction, logical or physiological, to be drawn between +them. Some contain more factors, and others contain fewer. +In some the re-combinations have been brought about by natural +variation or crossing, while the same consequences in the others +have resulted from man's interference. +<span class="pagenum"><a name="Page_247" id="Page_247">[Pg 247]</a></span></p> + +<p class="indent">2. We may follow the conventions of systematists and +distinguish the outstanding or conspicuous forms such as <i>arvense</i>, +<i>quadratum</i>, <i>sativum</i> and perhaps a few more as species, and +leave the rest unheeded. If this course is followed it must be clearly +understood and permitted as a piece of pure pragmatism, deliberately +adopted for the convenience of cataloguers and collectors, +without regard to any natural fact or system whatsoever.</p> + +<p class="indent">But while following either the one plan or the other +we shall be still awaiting the answer, which only genetic experiment +can provide, to the question whether among the various types +there are some which differ from the rest in a peculiar way: +whether by having groups of characters linked together in +especially durable combinations, or by possessing ingredients +which cause greater or less disturbance in the processes of cell-division, +and especially in the processes of gametic maturation, +when they are united by fertilisation with complementary ingredients.</p> + +<p class="indent">Before any but the vaguest ideas regarding the nature +and significance of inter-specific sterility can be formed, a vast +amount of detailed work must be done. Sterility as a result of +crossing, as well as that which is alleged sometimes to arise in +consequence of changed conditions, is at best a negative characteristic, +and there are endless opportunities for mistake and misinterpretation +in studying features of this kind. No one, I +suppose, would now feel any great confidence in most of the data +which from time to time are resuscitated for the purpose of such +discussions. Even the best collections of evidence, such as those +given by Darwin in <i>Forms of Flowers</i>, cannot be regarded as +critical when judged by present-day standards. Nothing short +of the most familiar acquaintance with the habitual behaviour +of individuals, and of strains kept under constant scrutiny for +several years would enable the experimenter to form reliable +judgments as to the value to be attached to observations of +this class.</p> + +<p class="indent">The admission must, however, be faced that nothing in +recent work materially tends to diminish the surprise which has always +been felt at the absence of sterility in the crosses between +<span class="pagenum"><a name="Page_248" id="Page_248">[Pg 248]</a></span> +co-derivatives. We should expect such groups of forms to behave +like the <i>Erophila</i> types, and frequently to produce sterile +products on crossing. Whatever be the explanation, the fact remains +that such evidence is wanting almost completely. In spite of +all that we know of variability nothing readily comparable with +the power to produce a sterile hybrid on crossing with a near +ally, has yet been observed spontaneously arising, though that +characteristic of specificity is one of the most widely distributed +in nature. It may be that the lacuna in our evidence is due +merely to want of attention to this special aspect of genetic +inquiry, and on the whole that is the most acceptable view which +can be proposed. But seeing that naturalists are more and +more driven to believe the domesticated animals and plants to be +poly-phyletic in origin—the descendants, that is to say, of several +wild forms—the difficulty is proportionately greater than it was +formerly, when variation spontaneously occurring was regarded +as a sufficient account of their diversity.</p> + +<hr class="chap" /> +<h2><span class="smcap">Concluding Remarks.</span></h2> + +<p class="indent">The many converging lines of evidence point so clearly to +the central fact of the origin of the forms of life by an evolutionary +process that we are compelled to accept this deduction, but as +to almost all the essential features, whether of cause or mode, +by which specific diversity has become what we perceive it to +be, we have to confess an ignorance nearly total. The transformation +of masses of population by imperceptible steps guided +by selection, is, as most of us now see, so inapplicable to the +facts, whether of variation or of specificity, that we can only +marvel both at the want of penetration displayed by the advocates +of such a proposition, and at the forensic skill by which it +was made to appear acceptable even for a time.</p> + +<p class="indent">In place of this doctrine we have little teaching of a positive +kind to offer. We have direct perception that new forms of life +may arise sporadically, and that they differ from their progenitors +quite sufficiently to pass for species. By the success and maintenance +of such sporadically arising forms, moreover, there is +no reasonable doubt that innumerable strains, whether in isolation +<span class="pagenum"><a name="Page_249" id="Page_249">[Pg 249]</a></span> +or in community with their co-derivatives, have as a fact +arisen, which now pass in the lists of systematists as species. For +an excellent account of typical illustrations I would refer the +reader to the book lately published by R. E. Lloyd<a name="FNanchor_14_197" id="FNanchor_14_197"></a><a href="#Footnote_14_197" class="fnanchor">[14]</a> +on the rat-population of India. The observations there recorded are typical +of the state of things disclosed whenever the variations of large +numbers of individuals are closely investigated, whether in +domestication or in natural conditions.</p> + +<p class="indent">Guided by such clues we may get a good way into +the problem. We see the origin of colourable species in abundance. +Then, however, doubt arises whether though these new forms +are as good species as many which are accepted as such by even +cautious systematists, there may not be a stricter physiological +sense in which the term species can be consistently used, which +would exclude the whole mass of these <i>petites espèces</i>.</p> + +<p class="indent">If further we find that we have, with certain somewhat +doubtful exceptions, never seen the contemporary origin of a +dominant factor, or of inter-racial sterility between indubitable +co-derivatives, it needs no elaboration of argument to show that +the root of the matter has not been reached.</p> + +<p class="indent">Examination of the inter-relations of unquestionably distinct +species nearly allied, such as the two common species of <i>Lychnis</i>, +leads to the same disquieting conclusion, and the best suggestion +we can make as to their origin is that <i>conceivably</i> they may have +arisen as two re-combinations of factors brought together by the +crossing of parent species, one or both of which must be supposed +to be lost.</p> + +<p class="indent">All this is, as need hardly be said, an unsatisfying +conclusion. To those permanently engaged in systematics it may well bring +despair. The best course for them is once for all to recognise +that whether or no specific distinction may prove hereafter to +have any actual physiological meaning, it is impossible for the +systematist with the means at his disposal to form a judgment of +value in any given case. Their business is purely that of the +cataloguer, and beyond that they cannot go. They will serve +science best by giving names freely and by describing everything +<span class="pagenum"><a name="Page_250" id="Page_250">[Pg 250]</a></span> +to which their successors may possibly want to refer, and generally +by subdividing their material into as many species as +they can induce any responsible society or journal to publish. +Between Jordan with his 200 odd species for <i>Erophila</i>, and +Grenier and Godron with one, there is no hesitation possible. +Jordan's view, as he again and again declares with vehemence, is +at least a view of natural facts, whereas the collective species is a +mere abstraction, convenient indeed for librarians and beginners, +but an insidious misrepresentation of natural truth, perhaps +more than any other the source of the plausible fallacies regarding +evolution that have so long obstructed progress.</p> + +<p class="indent">Nevertheless though we have been compelled to retreat +from the speculative position to which scientific opinion had rashly +advanced, the prospect of permanent progress is greatly better +than it was. With the development of genetic research clear +conceptions have at length been formed of the kind of knowledge +required and of the methods by which it is to be attained. If +we no longer see how varieties give rise to species, we may feel +confident that a minute study of genetic physiology of varieties +and species is the necessary beginning of any critical perception +of their inter-relations. It is little more than a century since +no valid distinction between a mechanical mixture and a chemical +combination could be perceived, and in regard to the forms of +life we may well be in a somewhat similar confusion.</p> + +<p class="indent">As yet the genetic behaviour of animals and plants +has only been sampled. When the work has been done on a scale so +large as to provide generalisations, we may be in a position to +declare whether specific difference is or is not a physiological reality. +<span class="pagenum"><a name="Page_251" id="Page_251">[Pg 251]</a></span></p> + +<hr class="chap" /> +<h2>INDEX OF SUBJECTS</h2> + +<table border="0" style="max-width: 45em;" cellspacing="2" summary="Subject Index" cellpadding="0" > + <tbody><tr> + <td class="tdr"> </td> + <td class="tdr"><b>PAGE</b></td> + </tr><tr> + <td class="tdl">Abraxa grossulariata,</td> + <td class="tdr"><a href="#Page_105">105</a>, + <a href="#Page_193">193</a></td> + </tr><tr> + <td class="tdl">Aceras hircina, local variability,</td> + <td class="tdr"><a href="#Page_123">123</a></td> + </tr><tr> + <td class="tdl">Achatinellidae, local forms of,</td> + <td class="tdr"><a href="#Page_133">133</a></td> + </tr><tr> + <td class="tdl">Acquired characters, inheritance of,</td> + <td class="tdr"><a href="#Page_188">188 et seq.</a>, + <a href="#Page_217">217</a>, + <a href="#Page_233">233</a></td> + </tr><tr> + <td class="tdl">Acronycta psi, melanic,</td> + <td class="tdr"><a href="#Page_138">138</a></td> + </tr><tr> + <td class="tdl">Adaptation, problem of,</td> + <td class="tdr"><a href="#Page_187">187</a>, + <a href="#Page_234">234</a></td> + </tr><tr> + <td class="tdl">Agelaius, local forms,</td> + <td class="tdr"><a href="#Page_120">120</a></td> + </tr><tr> + <td class="tdl">Agrotis, fixed and variable species,</td> + <td class="tdr"><a href="#Page_25"> 25</a></td> + </tr><tr> + <td class="tdl">Alkaptonuria,</td> + <td class="tdr"><a href="#Page_83"> 83</a></td> + </tr><tr> + <td class="tdl">Alpine Plants, growing larger, if protected,</td> + <td class="tdr"><a href="#Page_183">183</a></td> + </tr><tr> + <td class="tdl">Alpine Varieties,</td> + <td class="tdr"><a href="#Page_165">165</a></td> + </tr><tr> + <td class="tdl">Alytes obstetricans, Kammerer's experiments on,</td> + <td class="tdr"><a href="#Page_199">199</a>, + <a href="#Page_210">210</a></td> + </tr><tr> + <td class="tdl">Amblystoma, races of,</td> + <td class="tdr"><a href="#Page_230">230</a></td> + </tr><tr> + <td class="tdl">Amphidasys betularia, melanic form,</td> + <td class="tdr"><a href="#Page_136">136</a>, + <a href="#Page_138">138</a></td> + </tr><tr> + <td class="tdl"> dimorphic larvae,</td> + <td class="tdr"><a href="#Page_141">141</a></td> + </tr><tr> + <td class="tdl">Anodonta, polymorphism of,</td> + <td class="tdr"><a href="#Page_130">130</a></td> + </tr><tr> + <td class="tdl">Antirrhinum, striped,</td> + <td class="tdr"><a href="#Page_57"> 57</a></td> + </tr><tr> + <td class="tdl"> species-hybrids,</td> + <td class="tdr"><a href="#Page_99"> 99</a></td> + </tr><tr> + <td class="tdl"> albinos,</td> + <td class="tdr"><a href="#Page_110">110</a></td> + </tr><tr> + <td class="tdl">Apple, will not cross with pear,</td> + <td class="tdr"><a href="#Page_239">239</a></td> + </tr><tr> + <td class="tdl">Arctia caja, effects of temperature,</td> + <td class="tdr"><a href="#Page_192">192</a></td> + </tr><tr> + <td class="tdl"> larval variation in,</td> + <td class="tdr"><a href="#Page_231">231</a></td> + </tr><tr> + <td class="tdl">Arctic varieties,</td> + <td class="tdr"><a href="#Page_165">165</a></td> + </tr><tr> + <td class="tdl">Argynnis paphia and valesina in Italy,</td> + <td class="tdr"><a href="#Page_121">121</a></td> + </tr><tr> + <td class="tdl">Armadillo, polyembryony,</td> + <td class="tdr"><a href="#Page_42"> 42</a></td> + </tr><tr> + <td class="tdl">Artistic faculty,</td> + <td class="tdr"><a href="#Page_89"> 89</a></td> + </tr><tr> + <td class="tdl">Arum, rights and lefts,</td> + <td class="tdr"><a href="#Page_57"> 57</a></td> + </tr><tr> + <td class="tdl">Auriculas, short-styled selected,</td> + <td class="tdr"><a href="#Page_236">236</a></td> + </tr><tr> + <td class="tdl">Axis of symmetry in hand and foot,</td> + <td class="tdr"><a href="#Page_48"> 48</a></td> + </tr><tr> + <td class="tdl">Axolotl, alleged effect of conditions,</td> + <td class="tdr"><a href="#Page_230">230</a></td> + </tr><tr> + <td class="tdl">Azalea, bud-sports,</td> + <td class="tdr"><a href="#Page_55"> 55</a></td> + </tr><tr> + <td class="tdl"><br />Bacillus anthracis, unsegmented form,</td> + <td class="tdr"><br /><a href="#Page_71"> 71</a></td> + </tr><tr> + <td class="tdl">Bacillus prodigiosus, variation in,</td> + <td class="tdr"><a href="#Page_213">213</a></td> + </tr><tr> + <td class="tdl">Bacteria, variation in,</td> + <td class="tdr"><a href="#Page_212">212</a></td> + </tr><tr> + <td class="tdl">Bacterium coli, variation in,</td> + <td class="tdr"><a href="#Page_214">214</a></td> + </tr><tr> + <td class="tdl">Baeolophus, geographical races of,</td> + <td class="tdr"><a href="#Page_159">159</a></td> + </tr><tr> + <td class="tdl">Barley, right and left-handed,</td> + <td class="tdr"><a href="#Page_58"> 58</a></td> + </tr><tr> + <td class="tdl">Basilarchia, geographical races of,</td> + <td class="tdr"><a href="#Page_161">161</a></td> + </tr><tr> + <td class="tdl">Begonia phyllomaniaca,</td> + <td class="tdr"><a href="#Page_50"> 50</a></td> + </tr><tr> + <td class="tdl"> hybrids,</td> + <td class="tdr"><a href="#Page_51"> 51</a></td> + </tr><tr> + <td class="tdl">Bizarre Carnation, genetics of,</td> + <td class="tdr"><a href="#Page_54"> 54</a></td> + </tr><tr> + <td class="tdl">Black, as a variation from red,</td> + <td class="tdr"><a href="#Page_148">148</a></td> + </tr><tr> + <td class="tdl">Blackbird, varying,</td> + <td class="tdr"><a href="#Page_150">150</a></td> + </tr><tr> + <td class="tdl">Black Cock, fixity of,</td> + <td class="tdr"><a href="#Page_28"> 28</a></td> + </tr><tr> + <td class="tdl">Boarmia repandata, melanic form,</td> + <td class="tdr"><a href="#Page_136">136</a></td> + </tr><tr> + <td class="tdl"> rhomboidaria,</td> + <td class="tdr"><a href="#Page_137">137</a>, + <a href="#Page_139">139</a></td> + </tr><tr> + <td class="tdl">Botrytis susceptibility to,</td> + <td class="tdr"><a href="#Page_108">108</a></td> + </tr><tr> + <td class="tdl">Bovidae, hybrid,</td> + <td class="tdr"><a href="#Page_242">242</a></td> + </tr><tr> + <td class="tdl">Brachydactyly,</td> + <td class="tdr"><a href="#Page_89"> 89</a>, + <a href="#Page_95"> 95</a></td> + </tr><tr> + <td class="tdl">Bradypus, vertebral variation,</td> + <td class="tdr"><a href="#Page_68"> 68</a></td> + </tr><tr> + <td class="tdl">Bud-sports geometrically irregular,</td> + <td class="tdr"><a href="#Page_54">54-57</a></td> + </tr><tr> + <td class="tdl">Buffalo, attempts to hybridize,</td> + <td class="tdr"><a href="#Page_242">242</a></td> + </tr><tr> + <td class="tdl">Bullfinch, gynandromorph,</td> + <td class="tdr"><a href="#Page_45"> 45</a></td> + </tr><tr> + <td class="tdl">Bulimus detritus, local variation of,</td> + <td class="tdr"><a href="#Page_126">126</a></td> + </tr><tr> + <td class="tdl"><br />Canary, asymmetrical markings in,</td> + <td class="tdr"><br /><a href="#Page_154">154</a></td> + </tr><tr> + <td class="tdl">Canidae, hybrid,</td> + <td class="tdr"><a href="#Page_241">241</a></td> + </tr><tr> + <td class="tdl">Capsella,</td> + <td class="tdr"><a href="#Page_100">100</a></td> + </tr><tr> + <td class="tdl">Cardamine pratensis,</td> + <td class="tdr"><a href="#Page_239">239</a></td> + </tr><tr> + <td class="tdl">Cat, Polydactylism,</td> + <td class="tdr"><a href="#Page_53"> 53</a></td> + </tr><tr> + <td class="tdl">Carnation, Picotees and bizarres compared,</td> + <td class="tdr"><a href="#Page_54"> 54</a>, + <a href="#Page_58"> 58</a></td> + </tr><tr> + <td class="tdl">Cataract, hereditary,</td> + <td class="tdr"><a href="#Page_89"> 89</a></td> + </tr><tr> + <td class="tdl">Certhiola, melanic,</td> + <td class="tdr"><a href="#Page_142">142</a></td> + </tr><tr> + <td class="tdl">Chladni figures,</td> + <td class="tdr"><a href="#Page_60"> 60</a></td> + </tr><tr> + <td class="tdl">Choloepus, vertebral variation in,</td> + <td class="tdr"><a href="#Page_68"> 68</a></td> + </tr><tr> + <td class="tdl"> local variation in,</td> + <td class="tdr"><a href="#Page_119">119</a></td> + </tr><tr> + <td class="tdl">Cinerarias, self-sterility in,</td> + <td class="tdr"><a href="#Page_238">238</a></td> + </tr><tr> + <td class="tdl">Cistudo, local variation in,</td> + <td class="tdr"><a href="#Page_119">119</a></td> + </tr><tr> + <td class="tdl">Climatic varieties,</td> + <td class="tdr"><a href="#Page_164">164</a></td> + </tr><tr> + <td class="tdl">Coccaceae, variation in,</td> + <td class="tdr"><a href="#Page_213">213</a></td> + </tr><tr> + <td class="tdl">Coenonympha arcania, climatic forms of,</td> + <td class="tdr"><a href="#Page_179">179</a></td> + </tr><tr> + <td class="tdl"> satyrion,</td> + <td class="tdr"><a href="#Page_180">180</a></td> + </tr><tr> + <td class="tdl">Coereba, melanic,</td> + <td class="tdr"><a href="#Page_142">142</a></td> + </tr><tr> + <td class="tdl">Colaptes, geographical races,</td> + <td class="tdr"><a href="#Page_147">147 et seq.</a></td> + </tr><tr> + <td class="tdl"> chrysoides,</td> + <td class="tdr"><a href="#Page_154">154</a></td> + </tr><tr> + <td class="tdl">Colloids, growth in,</td> + <td class="tdr"><a href="#Page_65"> 65</a></td> + </tr><tr> + <td class="tdl">Colorado beetles, experiments on,</td> + <td class="tdr"><a href="#Page_218">218</a></td> + </tr><tr> + <td class="tdl">Colour blindness in twins,</td> + <td class="tdr"><a href="#Page_44"> 44</a></td> + </tr><tr> + <td class="tdl">Continuous variation, possible example of,</td> + <td class="tdr"><a href="#Page_173">173</a></td> + </tr><tr> + <td class="tdl">Coracias, geographical races of,</td> + <td class="tdr"><a href="#Page_160">160</a></td> + </tr><tr> + <td class="tdl">Cotton, genetics of,</td> + <td class="tdr"><a href="#Page_98"> 98</a>, + <a href="#Page_100">100</a></td> + </tr><tr> + <td class="tdl">Coupling,</td> + <td class="tdr"><a href="#Page_110">110</a></td> + </tr><tr> + <td class="tdl">Crab, extra claws,</td> + <td class="tdr"><a href="#Page_74"> 74</a></td> + </tr><tr> + <td class="tdl">Crustacean appendages and Serial Homology,</td> + <td class="tdr"><a href="#Page_63"> 63</a></td> + </tr><tr> + <td class="tdl">Crystals, analogy with,</td> + <td class="tdr"><a href="#Page_78"> 78</a></td> + </tr><tr> + <td class="tdl">Cyclopian monsters, artificial,</td> + <td class="tdr"><a href="#Page_50"> 50</a></td> + </tr><tr> + <td class="tdl"><br />Daphnia, changed by environment,</td> + <td class="tdr"><br /><a href="#Page_217">217</a></td> + </tr><tr> + <td class="tdl">Dasypus, polyembryony,</td> + <td class="tdr"><a href="#Page_42"> 42</a></td> + </tr><tr> + <td class="tdl">Dianthoecia, fixed and variable species,</td> + <td class="tdr"><a href="#Page_25"> 25</a></td> + </tr><tr> + <td class="tdl">Disease-resistance,</td> + <td class="tdr"><a href="#Page_87"> 87</a></td> + </tr><tr> + <td class="tdl">Division, power of,</td> + <td class="tdr"> </td> + </tr><tr> + <td class="tdl"> a fundamental attribute of living things,</td> + <td class="tdr"><a href="#Page_38"> 38</a></td> + </tr><tr> + <td class="tdl"> Genetics of,</td> + <td class="tdr"><a href="#Page_46"> 46</a>, + <a href="#Page_50"> 50</a></td> + </tr><tr> + <td class="tdl">Dogger Bank, large varieties on,</td> + <td class="tdr"><a href="#Page_125">125</a></td> + </tr><tr> + <td class="tdl">Dogs, hybrid,</td> + <td class="tdr"><a href="#Page_241">241</a></td> + </tr><tr> + <td class="tdl">Dominance, nature of,</td> + <td class="tdr"><a href="#Page_95"> 95</a></td> + </tr><tr> + <td class="tdl">Dominants, origin of new,</td> + <td class="tdr"><a href="#Page_88"> 88</a>, + <a href="#Page_90"> 90</a>, + <a href="#Page_95"> 95</a></td> + </tr><tr> + <td class="tdl">Double monsters,</td> + <td class="tdr"><a href="#Page_42"> 42</a></td> + </tr><tr> + <td class="tdl">Draba, experiments with,</td> + <td class="tdr"><a href="#Page_242">242</a></td> + </tr><tr> + <td class="tdl">Drosophila,</td> + <td class="tdr"><a href="#Page_91"> 91</a></td> + </tr><tr> + <td class="tdl"> Payne's experiments on,</td> + <td class="tdr"><a href="#Page_228">228</a></td> + </tr><tr> + <td class="tdl"><br />Earthworm, regeneration,</td> + <td class="tdr"><br /><a href="#Page_77"> 77</a></td> + </tr><tr> + <td class="tdl">Elephant, tusk segmented,</td> + <td class="tdr"><a href="#Page_38"> 38</a></td> + </tr><tr> + <td class="tdl">Entelechy,</td> + <td class="tdr"><a href="#Page_80"> 80</a></td> + </tr><tr> + <td class="tdl">Environmental treatment, effects of,</td> + <td class="tdr"><a href="#Page_188">188 et seq.</a></td> + </tr><tr> + <td class="tdl">Enzymes and genetic factors,</td> + <td class="tdr"><a href="#Page_86"> 86</a></td> + </tr><tr> + <td class="tdl">Epilepsy, inheritance of traumatic,</td> + <td class="tdr"><a href="#Page_197">197</a></td> + </tr><tr> + <td class="tdl">Equidae, sterility of hybrid,</td> + <td class="tdr"><a href="#Page_241">241</a></td> + </tr><tr> + <td class="tdl">Erophila, experiments with,</td> + <td class="tdr"><a href="#Page_242">242</a></td> + </tr><tr> + <td class="tdl"> species,</td> + <td class="tdr"><a href="#Page_249">249</a></td> + </tr><tr> + <td class="tdl">Exacum, right and left,</td> + <td class="tdr"><a href="#Page_57"> 57</a></td> + </tr><tr> + <td class="tdl">Euphonia elegantissima, local forms,</td> + <td class="tdr"><a href="#Page_120">120</a></td> + </tr><tr> + <td class="tdl">Eupithecia rectangulata, melanic form,</td> + <td class="tdr"><a href="#Page_137">137</a></td> + </tr><tr> + <td class="tdl"><br />Factors, new,</td> + <td class="tdr"><br /><a href="#Page_88"> 88</a></td> + </tr><tr> + <td class="tdl"> loss of,</td> + <td class="tdr"><a href="#Page_96"> 96</a></td> + </tr><tr> + <td class="tdl">Factorial representation of varieties,</td> + <td class="tdr"><a href="#Page_158">158</a>, + <a href="#Page_165">165</a></td> + </tr><tr> + <td class="tdl">Falcons, geographical races,</td> + <td class="tdr"><a href="#Page_147">147</a></td> + </tr><tr> + <td class="tdl">Fasciation,</td> + <td class="tdr"><a href="#Page_49"> 49</a></td> + </tr><tr> + <td class="tdl">Ferments, Boyle on,</td> + <td class="tdr"><a href="#Page_54"> 54</a></td> + </tr><tr> + <td class="tdl">Finger-prints of twins,</td> + <td class="tdr"><a href="#Page_44"> 44</a></td> + </tr><tr> + <td class="tdl">Fixity and Variability in species,</td> + <td class="tdr"><a href="#Page_25"> 25</a></td> + </tr><tr> + <td class="tdl">Flax, climatic experiments,</td> + <td class="tdr"><a href="#Page_197">197</a></td> + </tr><tr> + <td class="tdl">Fowl, Silky,</td> + <td class="tdr"><a href="#Page_84"> 84</a></td> + </tr><tr> + <td class="tdl"> Leghorn,</td> + <td class="tdr"><a href="#Page_85"> 85</a>, + <a href="#Page_90"> 90</a></td> + </tr><tr> + <td class="tdl"> Dominant white,</td> + <td class="tdr"><a href="#Page_94"> 94</a></td> + </tr><tr> + <td class="tdl"> Wyandotte,</td> + <td class="tdr"><a href="#Page_97"> 97</a></td> + </tr><tr> + <td class="tdl"> Rumpless,</td> + <td class="tdr"><a href="#Page_46"> 46</a></td> + </tr><tr> + <td class="tdl">Foxes, incompatibility with dogs,</td> + <td class="tdr"><a href="#Page_241">241</a></td> + </tr><tr> + <td class="tdl">Free-martin,</td> + <td class="tdr"><a href="#Page_44"> 44</a></td> + </tr><tr> + <td class="tdl">Fringillidae, sterility of hybrid,</td> + <td class="tdr"><a href="#Page_241">241</a></td> + </tr><tr> + <td class="tdl">Fundulus, cyclopian,</td> + <td class="tdr"><a href="#Page_50"> 50</a></td> + </tr><tr> + <td class="tdl"><br />Gallus, invariability of wild species,</td> + <td class="tdr"><br /><a href="#Page_13"> 13</a></td> + </tr><tr> + <td class="tdl"> and origin of poultry,</td> + <td class="tdr"><a href="#Page_90"> 90</a>, + <a href="#Page_97"> 97</a></td> + </tr><tr> + <td class="tdl">Genitalia, a basis for classification in insects,</td> + <td class="tdr"><a href="#Page_13"> 13</a></td> + </tr><tr> + <td class="tdl">Gentians, climatic experiments,</td> + <td class="tdr"><a href="#Page_197">197</a></td> + </tr><tr> + <td class="tdl">Geometrical structure and differentiation,</td> + <td class="tdr"><a href="#Page_54"> 54</a>, + <a href="#Page_56"> 56</a></td> + </tr><tr> + <td class="tdl">Geometrical distinction between germ-cells and somatic cells,</td> + <td class="tdr"><a href="#Page_58"> 58</a></td> + </tr><tr> + <td class="tdl">Gladiolus, right and left,</td> + <td class="tdr"><a href="#Page_57"> 57</a></td> + </tr><tr> + <td class="tdl">Gnophus obscurata, protective colouring,</td> + <td class="tdr"><a href="#Page_141">141</a></td> + </tr><tr> + <td class="tdl">Goldfinch, geographical races,</td> + <td class="tdr"><a href="#Page_147">147</a></td> + </tr><tr> + <td class="tdl">Gonioctena variabilis, variation in sexes of,</td> + <td class="tdr"><a href="#Page_121">121</a></td> + </tr><tr> + <td class="tdl">Gouldian Finch, polymorphism,</td> + <td class="tdr"><a href="#Page_148">148-149</a></td> + </tr><tr> + <td class="tdl">Gracilaria stigmatella, experiments on,</td> + <td class="tdr"><a href="#Page_193">193</a></td> + </tr><tr> + <td class="tdl">Grantia, large varieties of,</td> + <td class="tdr"><a href="#Page_125">125</a></td> + </tr><tr> + <td class="tdl">Ground-Squirrels, local forms of,</td> + <td class="tdr"><a href="#Page_132">132</a></td> + </tr><tr> + <td class="tdl">Grouse, red, variation,</td> + <td class="tdr"><a href="#Page_29"> 29</a></td> + </tr><tr> + <td class="tdl">Guillemot, Ringed,</td> + <td class="tdr"><a href="#Page_150">150</a></td> + </tr><tr> + <td class="tdl">Guinea-pig, Brown-Séquard's experiments on,</td> + <td class="tdr"><a href="#Page_198">198</a></td> + </tr><tr> + <td class="tdl">Gynandromorphs,</td> + <td class="tdr"><a href="#Page_45"> 45</a></td> + </tr><tr> + <td class="tdl"><br />Heliconius erato, forms of,</td> + <td class="tdr"><br /><a href="#Page_122">122</a>, + <a href="#Page_164">164</a></td> + </tr><tr> + <td class="tdl">Helix lapicida, local variation of,</td> + <td class="tdr"><a href="#Page_126">126</a></td> + </tr><tr> + <td class="tdl"> striata,</td> + <td class="tdr"><a href="#Page_127">127</a></td> + </tr><tr> + <td class="tdl"> Heripensis,</td> + <td class="tdr"><a href="#Page_127">127</a></td> + </tr><tr> + <td class="tdl"> Caespitum,</td> + <td class="tdr"><a href="#Page_127">127</a></td> + </tr><tr> + <td class="tdl"> trochoides,</td> + <td class="tdr"><a href="#Page_127">127</a></td> + </tr><tr> + <td class="tdl"> nemoralis and hortensis,</td> + <td class="tdr"><a href="#Page_128">128</a></td> + </tr><tr> + <td class="tdl">Helminthophila, geographical races of,</td> + <td class="tdr"><a href="#Page_157">157</a></td> + </tr><tr> + <td class="tdl">Hemerophila abruptaria, melanic,</td> + <td class="tdr"><a href="#Page_142">142</a></td> + </tr><tr> + <td class="tdl">Hepialus humuli, in Shetland,</td> + <td class="tdr"><a href="#Page_119">119</a></td> + </tr><tr> + <td class="tdl">Heterostyle plants,</td> + <td class="tdr"><a href="#Page_236">236</a></td> + </tr><tr> + <td class="tdl">Hieracium,</td> + <td class="tdr"><a href="#Page_9"> 9</a></td> + </tr><tr> + <td class="tdl">Himantopus,</td> + <td class="tdr"><a href="#Page_234">234</a></td> + </tr><tr> + <td class="tdl">Homoeosis,</td> + <td class="tdr"><a href="#Page_68"> 68</a></td> + </tr><tr> + <td class="tdl">Hybernia progemmaria,</td> + <td class="tdr"><a href="#Page_139">139</a></td> + </tr><tr> + <td class="tdl">Hybrids, sterility of,</td> + <td class="tdr"><a href="#Page_233">233 et seq.</a></td> + </tr><tr> + <td class="tdl"><br />Incompatibility between certain allied species,</td> + <td class="tdr"><br /><a href="#Page_239">239</a></td> + </tr><tr> + <td class="tdl">Individual, geometrical independence of,</td> + <td class="tdr"><a href="#Page_58"> 58</a></td> + </tr><tr> + <td class="tdl">Inhibiting Factors,</td> + <td class="tdr"><a href="#Page_95"> 95</a></td> + </tr><tr> + <td class="tdl">Intermediates, nature of,</td> + <td class="tdr"><a href="#Page_131">131</a>, + <a href="#Page_135">135</a></td> + </tr><tr> + <td class="tdl">Isolation, consequences of,</td> + <td class="tdr"><a href="#Page_118">118</a></td> + </tr><tr> + <td class="tdl"><br />Lacerta muralis, Kammerer's experiments on,</td> + <td class="tdr"><br /><a href="#Page_209">209</a></td> + </tr><tr> + <td class="tdl"> fiumana,</td> + <td class="tdr"><a href="#Page_210">210</a></td> + </tr><tr> + <td class="tdl">Leptinotarsa, Power's experiments on,</td> + <td class="tdr"><a href="#Page_218">218</a></td> + </tr><tr> + <td class="tdl">Limbs, extra, in pairs,</td> + <td class="tdr"><a href="#Page_72"> 72</a></td> + </tr><tr> + <td class="tdl">Limnaea, sinistral,</td> + <td class="tdr"><a href="#Page_134">134</a></td> + </tr><tr> + <td class="tdl">Linaria vulgaris, self-sterility,</td> + <td class="tdr"><a href="#Page_239">239</a></td> + </tr><tr> + <td class="tdl">Loasa fruits, right and left,</td> + <td class="tdr"><a href="#Page_57"> 57</a></td> + </tr><tr> + <td class="tdl">Lobster, extra claws,</td> + <td class="tdr"><a href="#Page_76"> 76</a></td> + </tr><tr> + <td class="tdl">Locality, variation connected with,</td> + <td class="tdr"><a href="#Page_14"> 14</a>, + <a href="#Page_118">118</a>, + <a href="#Page_146">146 et seq.</a>, + <a href="#Page_208">208</a></td> + </tr><tr> + <td class="tdl">Lumbricus, regeneration,</td> + <td class="tdr"><a href="#Page_77"> 77</a></td> + </tr><tr> + <td class="tdl">Lychnis dioica and vespertina, inter-relations of,</td> + <td class="tdr"><a href="#Page_18"> 18</a></td> + </tr><tr> + <td class="tdl"> macrocarpa, possibly a common parent of,</td> + <td class="tdr"><a href="#Page_19"> 19</a></td> + </tr><tr> + <td class="tdl"><br />Machetes pugnax, polymorphism of male,</td> + <td class="tdr"><br /><a href="#Page_28"> 28</a></td> + </tr><tr> + <td class="tdl">Maize, Blaringhem's experiments on,</td> + <td class="tdr"><a href="#Page_229">229</a></td> + </tr><tr> + <td class="tdl">Maize, cumulative factors in,</td> + <td class="tdr"><a href="#Page_116">116</a></td> + </tr><tr> + <td class="tdl">Malformations, dominants, arising de novo,</td> + <td class="tdr"><a href="#Page_89"> 89</a></td> + </tr><tr> + <td class="tdl">Manx Cat, heredity,</td> + <td class="tdr"><a href="#Page_46"> 46</a></td> + </tr><tr> + <td class="tdl">Matthiola,</td> + <td class="tdr"><a href="#Page_84"> 84</a>, + <a href="#Page_104">104</a>, + <a href="#Page_113">113</a></td> + </tr><tr> + <td class="tdl">Melanic varieties,</td> + <td class="tdr"><a href="#Page_135">135 et seq.</a></td> + </tr><tr> + <td class="tdl">Memory, analogy with heredity,</td> + <td class="tdr"><a href="#Page_190">190</a></td> + </tr><tr> + <td class="tdl">Meristic variation,</td> + <td class="tdr"><a href="#Page_69"> 69</a>, + <a href="#Page_83"> 83</a>, + <a href="#Page_86"> 86</a></td> + </tr><tr> + <td class="tdl">Mirabilis, striped,</td> + <td class="tdr"><a href="#Page_57"> 57</a></td> + </tr><tr> + <td class="tdl">Models of segmentation,</td> + <td class="tdr"><a href="#Page_59"> 59</a>, + <a href="#Page_60"> 60</a></td> + </tr><tr> + <td class="tdl">"Modes," Coutagne's conception of,</td> + <td class="tdr"><a href="#Page_126">126</a></td> + </tr><tr> + <td class="tdl">Mödling, peculiar race of <i>Pieris napi</i> at,</td> + <td class="tdr"><a href="#Page_178">178</a></td> + </tr><tr> + <td class="tdl">Mole, albino,</td> + <td class="tdr"><a href="#Page_27">27-28</a></td> + </tr><tr> + <td class="tdl">Mule, Linnaeus on,</td> + <td class="tdr"><a href="#Page_8"> 8</a></td> + </tr><tr> + <td class="tdl">Mutation, Matthioli on,</td> + <td class="tdr"><a href="#Page_4"> 4</a></td> + </tr><tr> + <td class="tdl"> in Mercurialis,</td> + <td class="tdr"><a href="#Page_5"> 5</a></td> + </tr><tr> + <td class="tdl"> in Kales,</td> + <td class="tdr"><a href="#Page_5"> 5</a></td> + </tr><tr> + <td class="tdl"> alleged in bulbs,</td> + <td class="tdr"><a href="#Page_5"> 5</a></td> + </tr><tr> + <td class="tdl"> Theory,</td> + <td class="tdr"><a href="#Page_97"> 97</a></td> + </tr><tr> + <td class="tdl"> periods of,</td> + <td class="tdr"><a href="#Page_114">114</a></td> + </tr><tr> + <td class="tdl"> in Bacteria,</td> + <td class="tdr"><a href="#Page_214">214</a></td> + </tr><tr> + <td class="tdl">Mutilation, consequences of,</td> + <td class="tdr"><a href="#Page_71"> 71</a></td> + </tr><tr> + <td class="tdl"> alleged effect of, on offspring,</td> + <td class="tdr"><a href="#Page_229">229</a></td> + </tr><tr> + <td class="tdl">Myxococcus, variation in,</td> + <td class="tdr"><a href="#Page_213">213</a></td> + </tr><tr> + <td class="tdl"><br />Narwhal, asymmetry of tusks,</td> + <td class="tdr"><br /><a href="#Page_44"> 44</a></td> + </tr><tr> + <td class="tdl">Nemesia strumosa,</td> + <td class="tdr"><a href="#Page_91"> 91</a></td> + </tr><tr> + <td class="tdl">Neuration, a basis for classification,</td> + <td class="tdr"><a href="#Page_13"> 13</a></td> + </tr><tr> + <td class="tdl">Nicotiana, sterility of hybrid,</td> + <td class="tdr"><a href="#Page_242">242</a></td> + </tr><tr> + <td class="tdl">Nightjars, varying,</td> + <td class="tdr"><a href="#Page_150">150</a></td> + </tr><tr> + <td class="tdl">Noctuidae, fixity and variability,</td> + <td class="tdr"><a href="#Page_25"> 25</a></td> + </tr><tr> + <td class="tdl">Noctua, polymorphic and fixed species,</td> + <td class="tdr"><a href="#Page_25"> 25</a></td> + </tr><tr> + <td class="tdl">Noctua castanea, local forms of,</td> + <td class="tdr"><a href="#Page_122">122</a></td> + </tr><tr> + <td class="tdl">Nomenclature, future of,</td> + <td class="tdr"><a href="#Page_94"> 94</a>, + <a href="#Page_245">245</a></td> + </tr><tr> + <td class="tdl">Notonecta, variations of,</td> + <td class="tdr"><a href="#Page_130">130</a></td> + </tr><tr> + <td class="tdl"><br />Odontoptera bidentata, melanic form,</td> + <td class="tdr"><br /><a href="#Page_137">137</a></td> + </tr><tr> + <td class="tdl">Oedipodidae, protectively coloured,</td> + <td class="tdr"><a href="#Page_140">140</a></td> + </tr><tr> + <td class="tdl">Oenothera, new dominant in,</td> + <td class="tdr"><a href="#Page_92"> 92</a></td> + </tr><tr> + <td class="tdl"> rubricalyx and rubrinervis,</td> + <td class="tdr"><a href="#Page_92"> 92</a>, + <a href="#Page_95"> 95</a></td> + </tr><tr> + <td class="tdl"> Lamarckiana,</td> + <td class="tdr"><a href="#Page_92"> 92</a>, + <a href="#Page_101">101</a></td> + </tr><tr> + <td class="tdl"> origin of,</td> + <td class="tdr"><a href="#Page_102">102</a>, + <a href="#Page_244">244</a></td> + </tr><tr> + <td class="tdl"> has bad pollen-grains,</td> + <td class="tdr"><a href="#Page_102">102</a></td> + </tr><tr> + <td class="tdl"> factorial analysis of,</td> + <td class="tdr"><a href="#Page_103">103</a></td> + </tr><tr> + <td class="tdl"> pollen and egg-cells genetically dissimilar,</td> + <td class="tdr"><a href="#Page_104">104</a></td> + </tr><tr> + <td class="tdl">Oenothera, "twin hybrids",</td> + <td class="tdr"><a href="#Page_105">105</a></td> + </tr><tr> + <td class="tdl"> laeta and velutina,</td> + <td class="tdr"><a href="#Page_105">105</a></td> + </tr><tr> + <td class="tdl"> reciprocal crosses in,</td> + <td class="tdr"><a href="#Page_105">105 et seq.</a></td> + </tr><tr> + <td class="tdl"> possible coupling in,</td> + <td class="tdr"><a href="#Page_111">111</a></td> + </tr><tr> + <td class="tdl"> dwarfs,</td> + <td class="tdr"><a href="#Page_112">112</a>, + <a href="#Page_114">114</a></td> + </tr><tr> + <td class="tdl"> "Triple hybrids",</td> + <td class="tdr"><a href="#Page_114">114</a></td> + </tr><tr> + <td class="tdl"> alleged variation due to treatment,</td> + <td class="tdr"><a href="#Page_227">227</a></td> + </tr><tr> + <td class="tdl">Ophrys, local variability,</td> + <td class="tdr"><a href="#Page_125">125</a></td> + </tr><tr> + <td class="tdl">Orange, polyembryony,</td> + <td class="tdr"><a href="#Page_45"> 45</a></td> + </tr><tr> + <td class="tdl">Osmotic growth,</td> + <td class="tdr"><a href="#Page_65"> 65</a></td> + </tr><tr> + <td class="tdl">Overlapping forms,</td> + <td class="tdr"><a href="#Page_146">146</a>, + <a href="#Page_174">174</a></td> + </tr><tr> + <td class="tdl"><br />Papilio, geographical races of,</td> + <td class="tdr"><br /><a href="#Page_162">162</a></td> + </tr><tr> + <td class="tdl">Papilio turnus, variation of,</td> + <td class="tdr"><a href="#Page_144">144</a></td> + </tr><tr> + <td class="tdl">Pararge egeria, geographical forms,</td> + <td class="tdr"><a href="#Page_166">166 et seq.</a></td> + </tr><tr> + <td class="tdl">Parthenogenesis,</td> + <td class="tdr"><a href="#Page_50"> 50</a></td> + </tr><tr> + <td class="tdl">Partula, local forms of,</td> + <td class="tdr"><a href="#Page_133">133</a></td> + </tr><tr> + <td class="tdl">Passer domesticus and montanus, distinctions,</td> + <td class="tdr"><a href="#Page_22"> 22</a></td> + </tr><tr> + <td class="tdl">Pea, round and wrinkled,</td> + <td class="tdr"><a href="#Page_95"> 95</a></td> + </tr><tr> + <td class="tdl">Pear, will not cross with apple,</td> + <td class="tdr"><a href="#Page_239">239</a></td> + </tr><tr> + <td class="tdl">Pelargonium, variegated,</td> + <td class="tdr"><a href="#Page_55"> 55</a></td> + </tr><tr> + <td class="tdl"> bud-sports,</td> + <td class="tdr"><a href="#Page_56"> 56</a></td> + </tr><tr> + <td class="tdl">Periodic phenomena in structure,</td> + <td class="tdr"><a href="#Page_63"> 63</a></td> + </tr><tr> + <td class="tdl">Peronea, fixed and variable species,</td> + <td class="tdr"><a href="#Page_26"> 26</a></td> + </tr><tr> + <td class="tdl">"Petites espèces",</td> + <td class="tdr"><a href="#Page_248">248</a></td> + </tr><tr> + <td class="tdl">Petunia, double,</td> + <td class="tdr"><a href="#Page_104">104</a></td> + </tr><tr> + <td class="tdl">Phalanger maculatus, local variation,</td> + <td class="tdr"><a href="#Page_119">119</a></td> + </tr><tr> + <td class="tdl">Pheasant, fixity of,</td> + <td class="tdr"><a href="#Page_29"> 29</a></td> + </tr><tr> + <td class="tdl">Phigalia pilosaria, melanic,</td> + <td class="tdr"><a href="#Page_139">139-140</a></td> + </tr><tr> + <td class="tdl">Phratora vitellinae, experiments on,</td> + <td class="tdr"><a href="#Page_193">193</a></td> + </tr><tr> + <td class="tdl">Phyllotaxis,</td> + <td class="tdr"><a href="#Page_69"> 69</a></td> + </tr><tr> + <td class="tdl">Pied varieties common in Passer domesticus unknown in Montanus,</td> + <td class="tdr"><a href="#Page_23"> 23</a></td> + </tr><tr> + <td class="tdl">Pieris napi and bryoniae,</td> + <td class="tdr"><a href="#Page_174">174</a></td> + </tr><tr> + <td class="tdl">Pig, mule-footed,</td> + <td class="tdr"><a href="#Page_46"> 46</a></td> + </tr><tr> + <td class="tdl">Pigeon, web-footed,</td> + <td class="tdr"><a href="#Page_46"> 46</a>, + <a href="#Page_49"> 49</a></td> + </tr><tr> + <td class="tdl"> Indian Rock, a recessive form,</td> + <td class="tdr"><a href="#Page_98"> 98</a></td> + </tr><tr> + <td class="tdl">Pigments, nature of,</td> + <td class="tdr"><a href="#Page_83"> 83</a></td> + </tr><tr> + <td class="tdl">Pisum humile, hybrids with culinary peas,</td> + <td class="tdr"><a href="#Page_244">244</a></td> + </tr><tr> + <td class="tdl"> species,</td> + <td class="tdr"><a href="#Page_246">246</a></td> + </tr><tr> + <td class="tdl">Planarian, regeneration of,</td> + <td class="tdr"><a href="#Page_71"> 71</a>, + <a href="#Page_77"> 77</a></td> + </tr><tr> + <td class="tdl">Plotheia frontalis, polymorphic,</td> + <td class="tdr"><a href="#Page_26"> 26</a>, + <a href="#Page_29"> 29</a></td> + </tr><tr> + <td class="tdl">Plusia, fixity and variation in,</td> + <td class="tdr"><a href="#Page_26"> 26</a></td> + </tr><tr> + <td class="tdl">Poephila gouldiae, variation of,</td> + <td class="tdr"><a href="#Page_148">148-149</a></td> + </tr><tr> + <td class="tdl">Polarity of individual,</td> + <td class="tdr"><a href="#Page_44"> 44</a></td> + </tr><tr> + <td class="tdl">Polia chi, melanic,</td> + <td class="tdr"><a href="#Page_138">138</a></td> + </tr><tr> + <td class="tdl">Polyanthus, short-styled selected,</td> + <td class="tdr"><a href="#Page_236">236</a></td> + </tr><tr> + <td class="tdl">Polydactylism in Cat,</td> + <td class="tdr"><a href="#Page_52">52-53</a></td> + </tr><tr> + <td class="tdl">Polyembryony,</td> + <td class="tdr"><a href="#Page_45"> 45</a></td> + </tr><tr> + <td class="tdl">Potato, variation in,</td> + <td class="tdr"><a href="#Page_91"> 91</a></td> + </tr><tr> + <td class="tdl">Poultry, evolution of,</td> + <td class="tdr"><a href="#Page_90"> 90</a></td> + </tr><tr> + <td class="tdl">Primula obconica,</td> + <td class="tdr"><a href="#Page_91"> 91</a></td> + </tr><tr> + <td class="tdl">Primula sinensis, flaked,</td> + <td class="tdr"><a href="#Page_57"> 57</a></td> + </tr><tr> + <td class="tdl"> Leaf-shapes,</td> + <td class="tdr"><a href="#Page_70"> 70</a></td> + </tr><tr> + <td class="tdl"> new dominant in,</td> + <td class="tdr"><a href="#Page_92"> 92</a></td> + </tr><tr> + <td class="tdl"> sterility in,</td> + <td class="tdr"><a href="#Page_236">236</a></td> + </tr><tr> + <td class="tdl"> "Giants",</td> + <td class="tdr"><a href="#Page_236">236</a></td> + </tr><tr> + <td class="tdl">Primula, species-hybrids,</td> + <td class="tdr"><a href="#Page_242">242</a></td> + </tr><tr> + <td class="tdl">Protective coloration,</td> + <td class="tdr"><a href="#Page_140">140</a></td> + </tr><tr> + <td class="tdl">Pyrrhulagra, local forms,</td> + <td class="tdr"><a href="#Page_120">120</a></td> + </tr><tr> + <td class="tdl">Python, twin-vertebrae,</td> + <td class="tdr"><a href="#Page_60"> 60</a></td> + </tr><tr> + <td class="tdl"><br />Quiscalus, geographical races of,</td> + <td class="tdr"><br /><a href="#Page_156">156</a></td> + </tr><tr> + <td class="tdl"><br />Rabbit, Angora,</td> + <td class="tdr"><br /><a href="#Page_46"> 46</a></td> + </tr><tr> + <td class="tdl"> colours of,</td> + <td class="tdr"><a href="#Page_93"> 93</a></td> + </tr><tr> + <td class="tdl"> Incompatibility with hare,</td> + <td class="tdr"><a href="#Page_242">242</a></td> + </tr><tr> + <td class="tdl">Raimannia odorata, Macdougal's experiments on,</td> + <td class="tdr"><a href="#Page_226">226</a></td> + </tr><tr> + <td class="tdl">Rats, Variation in,</td> + <td class="tdr"><a href="#Page_248">248</a></td> + </tr><tr> + <td class="tdl">Recessives, origin of,</td> + <td class="tdr"><a href="#Page_90"> 90</a></td> + </tr><tr> + <td class="tdl">Reciprocal crosses, giving distinct results,</td> + <td class="tdr"><a href="#Page_105">105 et seq.</a></td> + </tr><tr> + <td class="tdl">Regeneration,</td> + <td class="tdr"><a href="#Page_70"> 70</a></td> + </tr><tr> + <td class="tdl">Repulsion,</td> + <td class="tdr"><a href="#Page_110">110</a></td> + </tr><tr> + <td class="tdl">Reversal on Regeneration,</td> + <td class="tdr"><a href="#Page_77"> 77</a></td> + </tr><tr> + <td class="tdl">Rhamphocoelus, geographical forms,</td> + <td class="tdr"><a href="#Page_159">159</a>, + <a href="#Page_184">184</a></td> + </tr><tr> + <td class="tdl">Rhinoptera, variation in jaws of,</td> + <td class="tdr"><a href="#Page_38"> 38</a></td> + </tr><tr> + <td class="tdl">Rhythm in repetition,</td> + <td class="tdr"><a href="#Page_69"> 69</a></td> + </tr><tr> + <td class="tdl">Ribs, variation of,</td> + <td class="tdr"><a href="#Page_68"> 68</a></td> + </tr><tr> + <td class="tdl">Rights and Lefts,</td> + <td class="tdr"><a href="#Page_57">57-58</a></td> + </tr><tr> + <td class="tdl">Ripples, analogous to segments,</td> + <td class="tdr"><a href="#Page_60"> 60</a>, + <a href="#Page_66"> 66-67</a></td> + </tr><tr> + <td class="tdl"> regeneration of,</td> + <td class="tdr"><a href="#Page_79"> 79</a></td> + </tr><tr> + <td class="tdl">Rollers, geographical races of,</td> + <td class="tdr"><a href="#Page_160">160</a></td> + </tr><tr> + <td class="tdl">Ruff, polymorphism of male,</td> + <td class="tdr"><a href="#Page_28"> 28</a></td> + </tr><tr> + <td class="tdl"><br />Salamandra, maculosa and atra,</td> + <td class="tdr"><br /><a href="#Page_182">182</a>, + <a href="#Page_199">199</a>, + <a href="#Page_203">203</a></td> + </tr><tr> + <td class="tdl"> spotted and striped,</td> + <td class="tdr"><a href="#Page_207">207</a></td> + </tr><tr> + <td class="tdl"> geographical variation of,</td> + <td class="tdr"><a href="#Page_208">208</a></td> + </tr><tr> + <td class="tdl">Segmentation, nature of,</td> + <td class="tdr"><a href="#Page_63"> 63</a></td> + </tr><tr> + <td class="tdl"> simulated mechanically,</td> + <td class="tdr"><a href="#Page_64"> 64</a></td> + </tr><tr> + <td class="tdl"> compared with rippling,</td> + <td class="tdr"><a href="#Page_65"> 65</a></td> + </tr><tr> + <td class="tdl"> analogies with,</td> + <td class="tdr"><a href="#Page_68"> 68</a></td> + </tr><tr> + <td class="tdl">Segmentation of normally unsegmented structures,</td> + <td class="tdr"><a href="#Page_38"> 38</a></td> + </tr><tr> + <td class="tdl">Selection, Natural, an insufficient cause of definiteness of types,</td> + <td class="tdr"><a href="#Page_17"> 17</a>, + <a href="#Page_134">134</a>, + <a href="#Page_142">142</a></td> + </tr><tr> + <td class="tdl">Sempervivum,</td> + <td class="tdr"><a href="#Page_250">250</a></td> + </tr><tr> + <td class="tdl">Serial Homology, the true nature of,</td> + <td class="tdr"><a href="#Page_62"> 62</a>, + <a href="#Page_66"> 66</a></td> + </tr><tr> + <td class="tdl">Setina, Alpine varieties,</td> + <td class="tdr"><a href="#Page_181">181</a></td> + </tr><tr> + <td class="tdl">Sex of Twins,</td> + <td class="tdr"><a href="#Page_44"> 44</a></td> + </tr><tr> + <td class="tdl">Sex-factors, possible coupling of,</td> + <td class="tdr"><a href="#Page_111">111</a></td> + </tr><tr> + <td class="tdl">Sexual characters, variation in,</td> + <td class="tdr"><a href="#Page_119">119 et seq.</a></td> + </tr><tr> + <td class="tdl">Siamese twins,</td> + <td class="tdr"><a href="#Page_44"> 44</a></td> + </tr><tr> + <td class="tdl">Silky Fowl,</td> + <td class="tdr"><a href="#Page_84">84-85</a></td> + </tr><tr> + <td class="tdl">Simocephalus, changed by environment,</td> + <td class="tdr"><a href="#Page_218">218</a></td> + </tr><tr> + <td class="tdl">Sinistral forms,</td> + <td class="tdr"><a href="#Page_33">33-34</a></td> + </tr><tr> + <td class="tdl">Situs transversus,</td> + <td class="tdr"><a href="#Page_43"> 43</a></td> + </tr><tr> + <td class="tdl">Skate's jaws, variation in,</td> + <td class="tdr"><a href="#Page_38"> 38</a></td> + </tr><tr> + <td class="tdl">Sloths, vertebral variation,</td> + <td class="tdr"><a href="#Page_68"> 68</a></td> + </tr><tr> + <td class="tdl">Species, conceptions of,</td> + <td class="tdr"><a href="#Page_3"> 3</a>, + <a href="#Page_94"> 94</a>, + <a href="#Page_99"> 99</a>, + <a href="#Page_240">240</a>, + <a href="#Page_245">245</a></td> + </tr><tr> + <td class="tdl"> allied, distribution of,</td> + <td class="tdr"><a href="#Page_185">185</a></td> + </tr><tr> + <td class="tdl"> alternative uses of the term,</td> + <td class="tdr"><a href="#Page_245">245</a></td> + </tr><tr> + <td class="tdl">Specific difference, universality of,</td> + <td class="tdr"><a href="#Page_12"> 12</a></td> + </tr><tr> + <td class="tdl"> of organisms compared with those of inorganic materials,</td> + <td class="tdr"><a href="#Page_15"> 15</a></td> + </tr><tr> + <td class="tdl"> failure of theory of Selection to explain,</td> + <td class="tdr"><a href="#Page_18"> 18</a>, + <a href="#Page_134">134</a>, + <a href="#Page_247">247</a></td> + </tr><tr> + <td class="tdl">Sphyropicus varius,</td> + <td class="tdr"><a href="#Page_149">149</a>, + <a href="#Page_156">156</a></td> + </tr><tr> + <td class="tdl">Spilosoma lubricipeda, varieties of,</td> + <td class="tdr"><a href="#Page_181">181</a></td> + </tr><tr> + <td class="tdl"> Zatima, Heligoland form,</td> + <td class="tdr"><a href="#Page_181">181</a></td> + </tr><tr> + <td class="tdl">Spinal nerves, segmentation of,</td> + <td class="tdr"><a href="#Page_67"> 67</a></td> + </tr><tr> + <td class="tdl">Sporadic variation,</td> + <td class="tdr"><a href="#Page_131">131</a>, + <a href="#Page_134">134</a>, + <a href="#Page_248">248</a></td> + </tr><tr> + <td class="tdl">Squashes, polymorphism of,</td> + <td class="tdr"><a href="#Page_100">100</a></td> + </tr><tr> + <td class="tdl">Staphylococcus pyogenes, variation in,</td> + <td class="tdr"><a href="#Page_213">213</a></td> + </tr><tr> + <td class="tdl">Sterility of hybrids, in general,</td> + <td class="tdr"><a href="#Page_233">233</a></td> + </tr><tr> + <td class="tdl"> in Lychnis hybrids,</td> + <td class="tdr"><a href="#Page_20">20 et seq.</a></td> + </tr><tr> + <td class="tdl"> in crossing forms of Draba,</td> + <td class="tdr"><a href="#Page_243">243</a></td> + </tr><tr> + <td class="tdl"> Significance of,</td> + <td class="tdr"><a href="#Page_244">244</a></td> + </tr><tr> + <td class="tdl"> Self,</td> + <td class="tdr"><a href="#Page_238">238</a></td> + </tr><tr> + <td class="tdl">Stilt,</td> + <td class="tdr"><a href="#Page_234">234</a></td> + </tr><tr> + <td class="tdl">Stocks,</td> + <td class="tdr"><a href="#Page_84"> 84</a>, + <a href="#Page_104">104</a>, + <a href="#Page_113">113</a></td> + </tr><tr> + <td class="tdl">Striped varieties,</td> + <td class="tdr"><a href="#Page_57"> 57</a></td> + </tr><tr> + <td class="tdl">Substantive variation,</td> + <td class="tdr"><a href="#Page_84"> 84</a></td> + </tr><tr> + <td class="tdl">Subtraction-stages,</td> + <td class="tdr"><a href="#Page_93"> 93</a></td> + </tr><tr> + <td class="tdl">Supernumerary limbs,</td> + <td class="tdr"><a href="#Page_72">72-76</a></td> + </tr><tr> + <td class="tdl">Sweet pea, variation of,</td> + <td class="tdr"><a href="#Page_91"> 91</a></td> + </tr><tr> + <td class="tdl"> sterile anthers in,</td> + <td class="tdr"><a href="#Page_237">237</a></td> + </tr><tr> + <td class="tdl">Symmetry compared with heredity,</td> + <td class="tdr"><a href="#Page_41"> 41</a></td> + </tr><tr> + <td class="tdl">Symmetry of body approximate,</td> + <td class="tdr"><a href="#Page_78"> 78</a></td> + </tr><tr> + <td class="tdl">Syndactyly,</td> + <td class="tdr"><a href="#Page_47"> 47</a></td> + </tr><tr> + <td class="tdl"> in foot,</td> + <td class="tdr"><a href="#Page_48"> 48</a></td> + </tr><tr> + <td class="tdl">Synthetic formulae, in nomenclature,</td> + <td class="tdr"><a href="#Page_94"> 94</a></td> + </tr><tr> + <td class="tdl"><br />Taeniocampa, fixed and variable species,</td> + <td class="tdr"><br /><a href="#Page_25"> 25</a></td> + </tr><tr> + <td class="tdl">Tamias, local forms of,</td> + <td class="tdr"><a href="#Page_132">132</a></td> + </tr><tr> + <td class="tdl">Tanagers, geographical races of,</td> + <td class="tdr"><a href="#Page_159">159</a></td> + </tr><tr> + <td class="tdl">Teeth, variation in,</td> + <td class="tdr"><a href="#Page_39"> 39</a>, + <a href="#Page_67"> 67</a></td> + </tr><tr> + <td class="tdl">Tephrosia consortaria and consonaria,</td> + <td class="tdr"><a href="#Page_137">137</a>, + <a href="#Page_139">139</a>, + <a href="#Page_140">140</a></td> + </tr><tr> + <td class="tdl">Tephrosia species, separated by season,</td> + <td class="tdr"><a href="#Page_119">119</a></td> + </tr><tr> + <td class="tdl">Terminal members, variation of,</td> + <td class="tdr"><a href="#Page_68"> 68</a></td> + </tr><tr> + <td class="tdl">Thais rumina, local variation in,</td> + <td class="tdr"><a href="#Page_27"> 27</a></td> + </tr><tr> + <td class="tdl">Tolerance, persistence of diversity due to,</td> + <td class="tdr"><a href="#Page_17"> 17</a>, + <a href="#Page_134">134</a></td> + </tr><tr> + <td class="tdl">Tomato, number of cells in fruit,</td> + <td class="tdr"><a href="#Page_46"> 46</a></td> + </tr><tr> + <td class="tdl">Transitional populations, rarity of,</td> + <td class="tdr"><a href="#Page_165">165</a></td> + </tr><tr> + <td class="tdl"> an example,</td> + <td class="tdr"><a href="#Page_178">178</a></td> + </tr><tr> + <td class="tdl">Tropaeolum, sterile anthers in,</td> + <td class="tdr"><a href="#Page_237">237</a></td> + </tr><tr> + <td class="tdl">Trypanosomes, variation in,</td> + <td class="tdr"><a href="#Page_215">215</a></td> + </tr><tr> + <td class="tdl">Tusk, of Elephant, segmented,</td> + <td class="tdr"><a href="#Page_38"> 38</a></td> + </tr><tr> + <td class="tdl"> of Narwhal,</td> + <td class="tdr"><a href="#Page_44"> 44</a></td> + </tr><tr> + <td class="tdl">Twinning,</td> + <td class="tdr"><a href="#Page_41"> 41</a>, + <a href="#Page_44"> 44</a>, + <a href="#Page_71"> 71</a></td> + </tr><tr> + <td class="tdl"> heredity of,</td> + <td class="tdr"><a href="#Page_45"> 45</a></td> + </tr><tr> + <td class="tdl"> in organs,</td> + <td class="tdr"><a href="#Page_46"> 46</a></td> + </tr><tr> + <td class="tdl"><br />Uria troile, variety of,</td> + <td class="tdr"><br /><a href="#Page_150">150</a></td> + </tr><tr> + <td class="tdl"><br />Vanessa urticae, effects of temperature,</td> + <td class="tdr"><br /><a href="#Page_191">191</a></td> + </tr><tr> + <td class="tdl">Variation, a medley of phenomena,</td> + <td class="tdr"><a href="#Page_14">14-15</a></td> + </tr><tr> + <td class="tdl"> sporadic,</td> + <td class="tdr"><a href="#Page_131">131</a>, + <a href="#Page_134">134</a></td> + </tr><tr> + <td class="tdl"> and locality,</td> + <td class="tdr"><a href="#Page_118">118</a></td> + </tr><tr> + <td class="tdl"> Causes of genetic,</td> + <td class="tdr"><a href="#Page_86"> 86</a>, + <a href="#Page_87"> 87</a>, + <a href="#Page_131">131</a>, + <a href="#Page_212">212</a></td> + </tr><tr> + <td class="tdl"> Substantive and meristic,</td> + <td class="tdr"><a href="#Page_83"> 83</a></td> + </tr><tr> + <td class="tdl">Veronica, specific difference in,</td> + <td class="tdr"><a href="#Page_16"> 16</a></td> + </tr><tr> + <td class="tdl"> intermediates between species,</td> + <td class="tdr"><a href="#Page_17"> 17</a></td> + </tr><tr> + <td class="tdl">Vertebrae, division in,</td> + <td class="tdr"><a href="#Page_60">60-61</a></td> + </tr><tr> + <td class="tdl"> homologies of,</td> + <td class="tdr"><a href="#Page_66"> 66</a></td> + </tr><tr> + <td class="tdl">Vespa, specific difference in,</td> + <td class="tdr"><a href="#Page_23"> 23</a></td> + </tr><tr> + <td class="tdl">Vortex, living organism compared with,</td> + <td class="tdr"><a href="#Page_40"> 40</a></td> + </tr><tr> + <td class="tdl"><br />Wave-motion compared with repetition of parts,</td> + <td class="tdr"><br /><a href="#Page_62"> 62</a>, + <a href="#Page_67"> 67</a>, + <a href="#Page_79"> 79</a></td> + </tr><tr> + <td class="tdl">Wheat, cumulative factors in,</td> + <td class="tdr"><a href="#Page_116">116</a></td> + </tr><tr> + <td class="tdl"> climatic experiments on,</td> + <td class="tdr"><a href="#Page_195">195</a></td> + </tr><tr> + <td class="tdl">Woodpecker,</td> + <td class="tdr"><a href="#Page_234">234</a></td> + </tr><tr> + <td class="tdl"><br />Zebra, pattern of stripes compared with ripples,</td> + <td class="tdr"><br /><a href="#Page_38"> 38</a></td> + </tr> + </tbody> +</table> +<p><span class="pagenum"><a name="Page_252" id="Page_252">[Pg 252]</a></span></p> +<hr class="chap" /> +<h2>INDEX OF PERSONS</h2> + +<table border="0" style="max-width: 45em;" cellspacing="2" summary="Person Index" cellpadding="0" > + <tbody><tr> + <td class="tdr"> </td> + <td class="tdr"><b>PAGE</b></td> + </tr><tr> + <td class="tdl">Ackermann,</td> + <td class="tdr"><a href="#Page_242">242</a></td> + </tr><tr> + <td class="tdl">Agar,</td> + <td class="tdr"><a href="#Page_218">218</a></td> + </tr><tr> + <td class="tdl">Allen, J. A.,</td> + <td class="tdr"><a href="#Page_132">132</a>, + <a href="#Page_147">147</a>, + <a href="#Page_159">159</a></td> + </tr><tr> + <td class="tdl">Annandale,</td> + <td class="tdr"><a href="#Page_47"> 47</a></td> + </tr><tr> + <td class="tdl">Arrigoni degli Oddi,</td> + <td class="tdr"><a href="#Page_167">167</a></td> + </tr><tr> + <td class="tdl"><br />Backhouse,</td> + <td class="tdr"><br /><a href="#Page_50"> 50</a></td> + </tr><tr> + <td class="tdl">Baker, G. T.,</td> + <td class="tdr"><a href="#Page_166">166</a></td> + </tr><tr> + <td class="tdl">Bangs, Outram,</td> + <td class="tdr"><a href="#Page_120">120</a>, + <a href="#Page_142">142</a>, + <a href="#Page_155">155</a></td> + </tr><tr> + <td class="tdl">Barrett,</td> + <td class="tdr"><a href="#Page_26"> 26</a>, + <a href="#Page_136">136</a>, + <a href="#Page_167">167</a>, + <a href="#Page_173">173</a>, + <a href="#Page_178">178</a>, + <a href="#Page_193">193</a></td> + </tr><tr> + <td class="tdl">Baur, E.,</td> + <td class="tdr"><a href="#Page_55"> 55</a>, + <a href="#Page_99"> 99</a></td> + </tr><tr> + <td class="tdl">Baur, G.,</td> + <td class="tdr"><a href="#Page_119">119</a></td> + </tr><tr> + <td class="tdl">Beneden, van,</td> + <td class="tdr"><a href="#Page_75"> 75</a></td> + </tr><tr> + <td class="tdl">Bentham, on species of Veronica,</td> + <td class="tdr"><a href="#Page_16"> 16</a></td> + </tr><tr> + <td class="tdl"> Lychnis,</td> + <td class="tdr"><a href="#Page_21"> 21</a></td> + </tr><tr> + <td class="tdl"> Primula,</td> + <td class="tdr"><a href="#Page_22"> 22</a></td> + </tr><tr> + <td class="tdl">Bernadin,</td> + <td class="tdr"><a href="#Page_42"> 42</a></td> + </tr><tr> + <td class="tdl">Bishop, L. B.,</td> + <td class="tdr"><a href="#Page_153">153</a>, + <a href="#Page_157">157</a></td> + </tr><tr> + <td class="tdl">Blaringhem,</td> + <td class="tdr"><a href="#Page_229">229</a></td> + </tr><tr> + <td class="tdl">Bobart,</td> + <td class="tdr"><a href="#Page_5"> 5</a></td> + </tr><tr> + <td class="tdl">Boisduval,</td> + <td class="tdr"><a href="#Page_182">182</a></td> + </tr><tr> + <td class="tdl">Boissier,</td> + <td class="tdr"><a href="#Page_19"> 19</a></td> + </tr><tr> + <td class="tdl">Borradaile,</td> + <td class="tdr"><a href="#Page_74">74-75</a></td> + </tr><tr> + <td class="tdl">Boulenger, E. G.,</td> + <td class="tdr"><a href="#Page_208">208</a></td> + </tr><tr> + <td class="tdl">Boulenger, G. A.,</td> + <td class="tdr"><a href="#Page_182">182</a>, + <a href="#Page_207">207</a>, + <a href="#Page_209">209</a></td> + </tr><tr> + <td class="tdl">Boyle,</td> + <td class="tdr"><a href="#Page_5"> 5</a>, + <a href="#Page_54"> 54</a></td> + </tr><tr> + <td class="tdl">Brewster, W.,</td> + <td class="tdr"><a href="#Page_149">149-150</a></td> + </tr><tr> + <td class="tdl">Britton,</td> + <td class="tdr"><a href="#Page_227">227</a></td> + </tr><tr> + <td class="tdl">Brown, T. Graham,</td> + <td class="tdr"><a href="#Page_198">198</a></td> + </tr><tr> + <td class="tdl">Brown-Séquard,</td> + <td class="tdr"><a href="#Page_197">197 et seq.</a></td> + </tr><tr> + <td class="tdl">Bruant, P.,</td> + <td class="tdr"><a href="#Page_51"> 51</a></td> + </tr><tr> + <td class="tdl">Buffon,</td> + <td class="tdr"><a href="#Page_234">234</a></td> + </tr><tr> + <td class="tdl">Butler, S.,</td> + <td class="tdr"><a href="#Page_189">189-190</a></td> + </tr><tr> + <td class="tdl"><br />Candolle, de,</td> + <td class="tdr"><br /><a href="#Page_245">245</a></td> + </tr><tr> + <td class="tdl">Carpenter, J. H.,</td> + <td class="tdr"><a href="#Page_172">172</a></td> + </tr><tr> + <td class="tdl">Chapman, F. M.,</td> + <td class="tdr"><a href="#Page_148">148</a>, + <a href="#Page_156">156-158</a></td> + </tr><tr> + <td class="tdl">Chapman, T. A.,</td> + <td class="tdr"><a href="#Page_13"> 13</a>, + <a href="#Page_167">167</a>, + <a href="#Page_182">182</a>, + <a href="#Page_231">231</a></td> + </tr><tr> + <td class="tdl">Church, A. H.,</td> + <td class="tdr"><a href="#Page_69"> 69</a></td> + </tr><tr> + <td class="tdl">Cieslar,</td> + <td class="tdr"><a href="#Page_197">197</a></td> + </tr><tr> + <td class="tdl">Clark, Austin,</td> + <td class="tdr"><a href="#Page_142">142</a>, + <a href="#Page_144">144</a></td> + </tr><tr> + <td class="tdl">Cockayne, E. A.,</td> + <td class="tdr"><a href="#Page_43"> 43</a></td> + </tr><tr> + <td class="tdl">Cockerell, T. D. A.,</td> + <td class="tdr"><a href="#Page_224">224</a></td> + </tr><tr> + <td class="tdl">Compton, R. H.,</td> + <td class="tdr"><a href="#Page_50"> 50</a>, + <a href="#Page_58"> 58</a>, + <a href="#Page_227">227</a></td> + </tr><tr> + <td class="tdl">Cope,</td> + <td class="tdr"><a href="#Page_230">230</a></td> + </tr><tr> + <td class="tdl">Cory,</td> + <td class="tdr"><a href="#Page_142">142</a></td> + </tr><tr> + <td class="tdl">Correns,</td> + <td class="tdr"><a href="#Page_239">239</a></td> + </tr><tr> + <td class="tdl">Coutagne,</td> + <td class="tdr"><a href="#Page_125">125 et seq.</a></td> + </tr><tr> + <td class="tdl"><br />Darwin, on Variation,</td> + <td class="tdr"><br /><a href="#Page_1"> 1</a>, + <a href="#Page_2"> 2</a></td> + </tr><tr> + <td class="tdl"> Systematics,</td> + <td class="tdr"><a href="#Page_10"> 10</a></td> + </tr><tr> + <td class="tdl"> Selection,</td> + <td class="tdr"><a href="#Page_134">134</a>, + <a href="#Page_139">139</a></td> + </tr><tr> + <td class="tdl"> Heterostyle plants,</td> + <td class="tdr"><a href="#Page_236">236-237</a></td> + </tr><tr> + <td class="tdl">Darwin, F.,</td> + <td class="tdr"><a href="#Page_190">190</a></td> + </tr><tr> + <td class="tdl">Darwin, Sir G.,</td> + <td class="tdr"><a href="#Page_41"> 41</a></td> + </tr><tr> + <td class="tdl">Davenport,</td> + <td class="tdr"><a href="#Page_46"> 46</a></td> + </tr><tr> + <td class="tdl">Davis, H. M.,</td> + <td class="tdr"><a href="#Page_102">102</a></td> + </tr><tr> + <td class="tdl">Delcourt,</td> + <td class="tdr"><a href="#Page_130">130</a></td> + </tr><tr> + <td class="tdl">Deschange,</td> + <td class="tdr"><a href="#Page_181">181</a></td> + </tr><tr> + <td class="tdl">Dobell,</td> + <td class="tdr"><a href="#Page_215">215</a></td> + </tr><tr> + <td class="tdl">Doncaster,</td> + <td class="tdr"><a href="#Page_105">105</a>, + <a href="#Page_121">121</a>, + <a href="#Page_136">136</a></td> + </tr><tr> + <td class="tdl">Driesch,</td> + <td class="tdr"><a href="#Page_80">80-81</a></td> + </tr><tr> + <td class="tdl">Duchartre,</td> + <td class="tdr"><a href="#Page_51"> 51</a></td> + </tr><tr> + <td class="tdl"><br />East,</td> + <td class="tdr"><br /><a href="#Page_91"> 91</a>, + <a href="#Page_116">116</a></td> + </tr><tr> + <td class="tdl">Edwards, W. H.,</td> + <td class="tdr"><a href="#Page_162">162</a></td> + </tr><tr> + <td class="tdl">Ehrlich,</td> + <td class="tdr"><a href="#Page_215">215</a></td> + </tr><tr> + <td class="tdl"><br />Fellmer,</td> + <td class="tdr"><br /><a href="#Page_215">215</a></td> + </tr><tr> + <td class="tdl">Field, W. L. W.,</td> + <td class="tdr"><a href="#Page_161">161</a></td> + </tr><tr> + <td class="tdl">Fischer, E.,</td> + <td class="tdr"><a href="#Page_192">192</a></td> + </tr><tr> + <td class="tdl">Fleck,</td> + <td class="tdr"><a href="#Page_171">171</a>, + <a href="#Page_174">174</a></td> + </tr><tr> + <td class="tdl">Fletcher, W. H. B.,</td> + <td class="tdr"><a href="#Page_26"> 26</a>, + <a href="#Page_181">181</a></td> + </tr><tr> + <td class="tdl">Foster, Sir N.,</td> + <td class="tdr"><a href="#Page_39"> 39</a></td> + </tr><tr> + <td class="tdl"><br />Gallé,</td> + <td class="tdr"><br /><a href="#Page_123">123</a></td> + </tr><tr> + <td class="tdl">Garrod,</td> + <td class="tdr"><a href="#Page_83"> 83</a></td> + </tr><tr> + <td class="tdl">Gates,</td> + <td class="tdr"><a href="#Page_92"> 92</a>, + <a href="#Page_95"> 95</a>, + <a href="#Page_102">102</a></td> + </tr><tr> + <td class="tdl">Gayner, F.,</td> + <td class="tdr"><a href="#Page_177">177</a></td> + </tr><tr> + <td class="tdl">Godron,</td> + <td class="tdr"><a href="#Page_249">249</a></td> + </tr><tr> + <td class="tdl">Gold, E.,</td> + <td class="tdr"><a href="#Page_196">196</a></td> + </tr><tr> + <td class="tdl">Goldschmidt,</td> + <td class="tdr"><a href="#Page_116">116</a></td> + </tr><tr> + <td class="tdl">Goodwin, E.,</td> + <td class="tdr"><a href="#Page_137">137</a></td> + </tr><tr> + <td class="tdl">Gortner,</td> + <td class="tdr"><a href="#Page_226">226</a></td> + </tr><tr> + <td class="tdl">Greene, E. L.,</td> + <td class="tdr"><a href="#Page_8"> 8</a></td> + </tr><tr> + <td class="tdl">Gregory, R. P.,</td> + <td class="tdr"><a href="#Page_92"> 92</a>, + <a href="#Page_100">100</a>, + <a href="#Page_236">236</a></td> + </tr><tr> + <td class="tdl">Grenier,</td> + <td class="tdr"><a href="#Page_249">249</a></td> + </tr><tr> + <td class="tdl">Grover,</td> + <td class="tdr"><a href="#Page_173">173</a></td> + </tr><tr> + <td class="tdl">Gruber,</td> + <td class="tdr"><a href="#Page_48"> 48</a></td> + </tr><tr> + <td class="tdl">Gulick,</td> + <td class="tdr"><a href="#Page_119">119</a>, + <a href="#Page_133">133</a></td> + </tr><tr> + <td class="tdl"><br />Hamling,</td> + <td class="tdr"><br /><a href="#Page_142">142</a></td> + </tr><tr> + <td class="tdl">Hampson, Sir G.,</td> + <td class="tdr"><a href="#Page_26"> 26</a></td> + </tr><tr> + <td class="tdl">Harris,</td> + <td class="tdr"><a href="#Page_142">142</a></td> + </tr><tr> + <td class="tdl">Hartlaub,</td> + <td class="tdr"><a href="#Page_182">182</a></td> + </tr><tr> + <td class="tdl">Herbst,</td> + <td class="tdr"><a href="#Page_42"> 42</a></td> + </tr><tr> + <td class="tdl">Heribert-Nilsson,</td> + <td class="tdr"><a href="#Page_116">116</a></td> + </tr><tr> + <td class="tdl">Hewett,</td> + <td class="tdr"><a href="#Page_182">182</a></td> + </tr><tr> + <td class="tdl">Honing,</td> + <td class="tdr"><a href="#Page_105">105</a></td> + </tr><tr> + <td class="tdl">Hunter, John,</td> + <td class="tdr"><a href="#Page_44"> 44</a></td> + </tr><tr> + <td class="tdl"><br />Jakowatz,</td> + <td class="tdr"><br /><a href="#Page_197">197</a></td> + </tr><tr> + <td class="tdl">Janet,</td> + <td class="tdr"><a href="#Page_24"> 24</a></td> + </tr><tr> + <td class="tdl">Jeans,</td> + <td class="tdr"><a href="#Page_41"> 41</a></td> + </tr><tr> + <td class="tdl">Jenkinson,</td> + <td class="tdr"><a href="#Page_40"> 40</a></td> + </tr><tr> + <td class="tdl">Jentink,</td> + <td class="tdr"><a href="#Page_120">120</a></td> + </tr><tr> + <td class="tdl">Johannsen,</td> + <td class="tdr"><a href="#Page_195">195</a></td> + </tr><tr> + <td class="tdl">Jordan,</td> + <td class="tdr"><a href="#Page_185">185</a>, + <a href="#Page_242">242</a>, + <a href="#Page_249">249</a></td> + </tr><tr> + <td class="tdl"><br />Kammere,</td> + <td class="tdr"><br /><a href="#Page_199">199 et seq.</a></td> + </tr><tr> + <td class="tdl">Keeble,</td> + <td class="tdr"><a href="#Page_236">236</a></td> + </tr><tr> + <td class="tdl">Klebs,</td> + <td class="tdr"><a href="#Page_250">250</a></td> + </tr><tr> + <td class="tdl">Krancher,</td> + <td class="tdr"><a href="#Page_182">182</a></td> + </tr><tr> + <td class="tdl">Küchenmeister,</td> + <td class="tdr"><a href="#Page_44"> 44</a></td> + </tr><tr> + <td class="tdl">Kudicke,</td> + <td class="tdr"><a href="#Page_215">215</a></td> + </tr><tr> + <td class="tdl"><br />Lamarck,</td> + <td class="tdr"><br /><a href="#Page_9"> 9</a></td> + </tr><tr> + <td class="tdl">Lang, A.,</td> + <td class="tdr"><a href="#Page_128">128</a></td> + </tr><tr> + <td class="tdl">Lawrence, W. N.,</td> + <td class="tdr"><a href="#Page_142">142</a>, + <a href="#Page_145">145</a></td> + </tr><tr> + <td class="tdl">Leake, H. Martin,</td> + <td class="tdr"><a href="#Page_98"> 98</a>, + <a href="#Page_100">100</a></td> + </tr><tr> + <td class="tdl">Leavitt,</td> + <td class="tdr"><a href="#Page_185">185</a></td> + </tr><tr> + <td class="tdl">Lecoq,</td> + <td class="tdr"><a href="#Page_99"> 99</a></td> + </tr><tr> + <td class="tdl">Lederer,</td> + <td class="tdr"><a href="#Page_167">167</a></td> + </tr><tr> + <td class="tdl">Leduc,</td> + <td class="tdr"><a href="#Page_64"> 64-65</a>, + <a href="#Page_80"> 80</a></td> + </tr><tr> + <td class="tdl">Leydig,</td> + <td class="tdr"><a href="#Page_182">182</a></td> + </tr><tr> + <td class="tdl">Linden, M. von,</td> + <td class="tdr"><a href="#Page_192">192</a></td> + </tr><tr> + <td class="tdl">Linnaeus,</td> + <td class="tdr"><a href="#Page_6">6-8</a></td> + </tr><tr> + <td class="tdl">Lloyd, R. E.,</td> + <td class="tdr"><a href="#Page_248">248</a></td> + </tr><tr> + <td class="tdl">Locard,</td> + <td class="tdr"><a href="#Page_130">130</a></td> + </tr><tr> + <td class="tdl">Lock, R. H.,</td> + <td class="tdr"><a href="#Page_242">242</a>, + <a href="#Page_244">244</a></td> + </tr><tr> + <td class="tdl">Loeb,</td> + <td class="tdr"><a href="#Page_42"> 42</a>, + <a href="#Page_45"> 45</a>, + <a href="#Page_50"> 50</a>, + <a href="#Page_71"> 71</a>, + <a href="#Page_77"> 77</a></td> + </tr><tr> + <td class="tdl">Lotsy,</td> + <td class="tdr"><a href="#Page_99"> 99</a></td> + </tr><tr> + <td class="tdl">Lowe, P. R.,</td> + <td class="tdr"><a href="#Page_143">143</a></td> + </tr><tr> + <td class="tdl"><br />Macdougal, W. T.,</td> + <td class="tdr"><br /><a href="#Page_102">102</a>, + <a href="#Page_226">226</a></td> + </tr><tr> + <td class="tdl">Marchant,</td> + <td class="tdr"><a href="#Page_7"> 7</a></td> + </tr><tr> + <td class="tdl">Mathew,</td> + <td class="tdr"><a href="#Page_171">171</a></td> + </tr><tr> + <td class="tdl">Matthioli,</td> + <td class="tdr"><a href="#Page_4"> 4</a></td> + </tr><tr> + <td class="tdl">Mayer, A. G.,</td> + <td class="tdr"><a href="#Page_133">133</a></td> + </tr><tr> + <td class="tdl">Mendel, Rediscovery of,</td> + <td class="tdr"><a href="#Page_2"> 2</a></td> + </tr><tr> + <td class="tdl"> On Fasciation,</td> + <td class="tdr"><a href="#Page_49"> 49</a></td> + </tr><tr> + <td class="tdl">Merrifield,</td> + <td class="tdr"><a href="#Page_169">169</a>, + <a href="#Page_172">172</a></td> + </tr><tr> + <td class="tdl">Miller, W. D.,</td> + <td class="tdr"><a href="#Page_120">120</a>, + <a href="#Page_149">149</a></td> + </tr><tr> + <td class="tdl">Morgan,</td> + <td class="tdr"><a href="#Page_71"> 71</a>, + <a href="#Page_77"> 77</a>, + <a href="#Page_91"> 91</a>, + <a href="#Page_198">198</a></td> + </tr><tr> + <td class="tdl">Moggridge,</td> + <td class="tdr"><a href="#Page_125">125</a></td> + </tr><tr> + <td class="tdl"><br />Nathusius, S.,</td> + <td class="tdr"><br /><a href="#Page_242">242</a></td> + </tr><tr> + <td class="tdl">Nettleship,</td> + <td class="tdr"><a href="#Page_44"> 44</a></td> + </tr><tr> + <td class="tdl">Newman, H. H.,</td> + <td class="tdr"><a href="#Page_42"> 42</a></td> + </tr><tr> + <td class="tdl">Newsholme,</td> + <td class="tdr"><a href="#Page_48"> 48</a></td> + </tr><tr> + <td class="tdl">Nilsson-Ehle,</td> + <td class="tdr"><a href="#Page_116">116</a>, + <a href="#Page_169">169</a></td> + </tr><tr> + <td class="tdl">Norman, A. M.,</td> + <td class="tdr"><a href="#Page_125">125</a>, + <a href="#Page_156">156</a></td> + </tr><tr> + <td class="tdl"><br />Ober,</td> + <td class="tdr"><br /><a href="#Page_142">142</a></td> + </tr><tr> + <td class="tdl">Oberthür,</td> + <td class="tdr"><a href="#Page_168">168</a>, + <a href="#Page_170">170</a>, + <a href="#Page_193">193</a></td> + </tr><tr> + <td class="tdl">Oliver, J.,</td> + <td class="tdr"><a href="#Page_45"> 45</a></td> + </tr><tr> + <td class="tdl"><br />Page, H. E.,</td> + <td class="tdr"><br /><a href="#Page_167">167</a>, + <a href="#Page_180">180</a></td> + </tr><tr> + <td class="tdl">Patterson, J. T.,</td> + <td class="tdr"><a href="#Page_42"> 42</a></td> + </tr><tr> + <td class="tdl">Payne, F.,</td> + <td class="tdr"><a href="#Page_229">229</a></td> + </tr><tr> + <td class="tdl">Pellew,</td> + <td class="tdr"><a href="#Page_236">236</a></td> + </tr><tr> + <td class="tdl">Poll,</td> + <td class="tdr"><a href="#Page_45"> 45</a></td> + </tr><tr> + <td class="tdl">Porritt,</td> + <td class="tdr"><a href="#Page_136">136</a></td> + </tr><tr> + <td class="tdl">Poulton,</td> + <td class="tdr"><a href="#Page_141">141</a></td> + </tr><tr> + <td class="tdl">Powers, J. H.,</td> + <td class="tdr"><a href="#Page_230">230</a></td> + </tr><tr> + <td class="tdl">Pringsheim, H.,</td> + <td class="tdr"><a href="#Page_213">213</a></td> + </tr><tr> + <td class="tdl">Przibram,</td> + <td class="tdr"><a href="#Page_72"> 72</a>, + <a href="#Page_78"> 78</a>, + <a href="#Page_178">178</a>, + <a href="#Page_194">194</a>, + <a href="#Page_197">197</a>, + <a href="#Page_199">199</a></td> + </tr><tr> + <td class="tdl">Punnett,</td> + <td class="tdr"><a href="#Page_110">110</a></td> + </tr><tr> + <td class="tdl"><br />Ray,</td> + <td class="tdr"><br /><a href="#Page_4">4-5</a></td> + </tr><tr> + <td class="tdl">Raynor,</td> + <td class="tdr"><a href="#Page_105">105</a></td> + </tr><tr> + <td class="tdl">Ridgway,</td> + <td class="tdr"><a href="#Page_10"> 10</a>, + <a href="#Page_120">120</a></td> + </tr><tr> + <td class="tdl">Roedelius,</td> + <td class="tdr"><a href="#Page_195">195</a></td> + </tr><tr> + <td class="tdl">Rolfe,</td> + <td class="tdr"><a href="#Page_20"> 20</a></td> + </tr><tr> + <td class="tdl">Rosen, F.,</td> + <td class="tdr"><a href="#Page_242">242</a></td> + </tr><tr> + <td class="tdl">Rosner,</td> + <td class="tdr"><a href="#Page_42"> 42</a></td> + </tr><tr> + <td class="tdl">Rowland-Brown, H.,</td> + <td class="tdr"><a href="#Page_167">167</a>, + <a href="#Page_180">180</a></td> + </tr><tr> + <td class="tdl"><br />Sargent,</td> + <td class="tdr"><br /><a href="#Page_185">185</a></td> + </tr><tr> + <td class="tdl">Saunders, E. R.,</td> + <td class="tdr"><a href="#Page_84"> 84</a>, + <a href="#Page_104">104</a>, + <a href="#Page_112">112</a></td> + </tr><tr> + <td class="tdl">Schima,</td> + <td class="tdr"><a href="#Page_177">177</a></td> + </tr><tr> + <td class="tdl">Schröder,</td> + <td class="tdr"><a href="#Page_193">193-194</a></td> + </tr><tr> + <td class="tdl">Schübeler,</td> + <td class="tdr"><a href="#Page_195">195</a></td> + </tr><tr> + <td class="tdl">Semon, R.,</td> + <td class="tdr"><a href="#Page_190">190 et seq.</a></td> + </tr><tr> + <td class="tdl">Sharrock,</td> + <td class="tdr"><a href="#Page_5"> 5</a></td> + </tr><tr> + <td class="tdl">Shull,</td> + <td class="tdr"><a href="#Page_100">100</a></td> + </tr><tr> + <td class="tdl">Speyer, A.,</td> + <td class="tdr"><a href="#Page_166">166</a>, + <a href="#Page_170">170</a>, + <a href="#Page_181">181</a></td> + </tr><tr> + <td class="tdl">Spillman,</td> + <td class="tdr"><a href="#Page_47"> 47</a></td> + </tr><tr> + <td class="tdl">Standfuss,</td> + <td class="tdr"><a href="#Page_135">135</a>, + <a href="#Page_181">181</a>, + <a href="#Page_191">191</a></td> + </tr><tr> + <td class="tdl">Staples-Browne,</td> + <td class="tdr"><a href="#Page_49"> 49</a>, + <a href="#Page_98"> 98</a></td> + </tr><tr> + <td class="tdl">Staudinger,</td> + <td class="tdr"><a href="#Page_170">170</a>, + <a href="#Page_179">179</a></td> + </tr><tr> + <td class="tdl">Stockard,</td> + <td class="tdr"><a href="#Page_50"> 50</a>, + <a href="#Page_71"> 71</a></td> + </tr><tr> + <td class="tdl">Sutton,</td> + <td class="tdr"><a href="#Page_236">236</a>, + <a href="#Page_244">244</a></td> + </tr><tr> + <td class="tdl"><br />Tornier,</td> + <td class="tdr"><br /><a href="#Page_72"> 72</a></td> + </tr><tr> + <td class="tdl">Tower, W. L.,</td> + <td class="tdr"><a href="#Page_218">218-226</a></td> + </tr><tr> + <td class="tdl">Trechmann,</td> + <td class="tdr"><a href="#Page_133">133</a></td> + </tr><tr> + <td class="tdl">Tugwell,</td> + <td class="tdr"><a href="#Page_181">181</a></td> + </tr><tr> + <td class="tdl">Tutt, J. W. On Definiteness of Species,</td> + <td class="tdr"><a href="#Page_13"> 13</a></td> + </tr><tr> + <td class="tdl"> On Plusia interrogationis,</td> + <td class="tdr"><a href="#Page_26"> 26</a></td> + </tr><tr> + <td class="tdl"> On Tephrosia,</td> + <td class="tdr"><a href="#Page_119">119</a></td> + </tr><tr> + <td class="tdl"> On N. castanea,</td> + <td class="tdr"><a href="#Page_122">122</a></td> + </tr><tr> + <td class="tdl"> On Pararge egeria,</td> + <td class="tdr"><a href="#Page_167">167 et seq.</a></td> + </tr><tr> + <td class="tdl"><br />Verity, R.,</td> + <td class="tdr"><br /><a href="#Page_171">171</a>, + <a href="#Page_177">177</a></td> + </tr><tr> + <td class="tdl">Vries, H. de,</td> + <td class="tdr"><a href="#Page_101">101-115</a>, + <a href="#Page_222">222</a>, + <a href="#Page_239">239</a></td> + </tr><tr> + <td class="tdl"><br />Walker, G,</td> + <td class="tdr"><br /><a href="#Page_49"> 49</a></td> + </tr><tr> + <td class="tdl">Weir, Jenner,</td> + <td class="tdr"><a href="#Page_119">119</a></td> + </tr><tr> + <td class="tdl">Weismann,</td> + <td class="tdr"><a href="#Page_176">176</a>, + <a href="#Page_188">188</a></td> + </tr><tr> + <td class="tdl">Wendelstadt,</td> + <td class="tdr"><a href="#Page_215">215</a></td> + </tr><tr> + <td class="tdl">Werbitzki,</td> + <td class="tdr"><a href="#Page_215">215</a></td> + </tr><tr> + <td class="tdl">Werner,</td> + <td class="tdr"><a href="#Page_209">209</a></td> + </tr><tr> + <td class="tdl">Wettstein,</td> + <td class="tdr"><a href="#Page_197">197</a></td> + </tr><tr> + <td class="tdl">Wheeler, G.,</td> + <td class="tdr"><a href="#Page_168">168</a>, + <a href="#Page_171">171</a></td> + </tr><tr> + <td class="tdl">Wheldale,</td> + <td class="tdr"><a href="#Page_83"> 83</a></td> + </tr><tr> + <td class="tdl">Wilder,</td> + <td class="tdr"><a href="#Page_44"> 44</a></td> + </tr><tr> + <td class="tdl">Wille,</td> + <td class="tdr"><a href="#Page_197">197</a></td> + </tr><tr> + <td class="tdl">Williams, H.,</td> + <td class="tdr"><a href="#Page_167">167</a>, + <a href="#Page_172">172</a></td> + </tr><tr> + <td class="tdl">Windle, B. C. A.,</td> + <td class="tdr"><a href="#Page_43"> 43</a></td> + </tr><tr> + <td class="tdl">Winslow,</td> + <td class="tdr"><a href="#Page_213">213</a></td> + </tr><tr> + <td class="tdl">Wolf, F.,</td> + <td class="tdr"><a href="#Page_213">213</a></td> + </tr><tr> + <td class="tdl">Woodforde,</td> + <td class="tdr"><a href="#Page_123">123</a></td> + </tr><tr> + <td class="tdl">Woltereck,</td> + <td class="tdr"><a href="#Page_215">215</a></td> + </tr><tr> + <td class="tdl"><br />Zeijlstra,</td> + <td class="tdr"><br /><a href="#Page_114">114</a></td> + </tr> + </tbody> +</table> +<hr class="chap space-below" /> +<div class="transnote"> +<p style="font-size: 150%; text-align: center;"><i>Transcriber's Notes</i>:</p> +<p class="indent"> + The illustrations have been moved so that they do not break up + paragraphs and so that they are next to the text they illustrate.</p> +<p class="indent"> + Errors in punctuation and inconsistent hyphenation were not corrected + unless otherwise noted.</p> +</div> +<hr class="chap" /> +<h2 class="u">FOOTNOTES:</h2> + +<div class="footnotes"><h3>FOOTNOTES: CHAPTER I.</h3> + +<div class="footnote"><p><a name="Footnote_1_1" id="Footnote_1_1"></a> +<a href="#FNanchor_1_1"><span class="label">[1]</span></a> +In <i>Mendel's Principles of Heredity</i> (Cambridge University Press, 1909) +I have dealt with this subject, giving an account of the principal +facts discovered up to the beginning of 1909.</p></div> + +<div class="footnote"><p><a name="Footnote_2_2" id="Footnote_2_2"></a> +<a href="#FNanchor_2_2"><span class="label">[2]</span></a> +Matthioli Opera, Ed. 1598, p. 8, originally published 1565.</p></div> + +<div class="footnote"><p><a name="Footnote_3_3" id="Footnote_3_3"></a> +<a href="#FNanchor_3_3"><span class="label">[3]</span></a> +Ray's instances relate to Kales, and in most of these examples we can +see that there was no question of mutation or transmutation at all, but that the +occurrence was due either to mistake or to cross-fertilisation. Sharrock, to whom +Ray refers, was inclined to discredit stories of transmutation, but he has also this +passage (<i>History of the Propagation and Improvement of Vegetables by the +Concurrence of Art and Nature</i>, Oxford, 1660, p. 29):</p> +<p>"It is indeed grown to be a great question, whether the transmutation of a +species be possible either in the vegetable, Animal, or Minerall Kingdome. For the +possibility of it in the vegetable; I have heard <i>Mr. Bobart</i> and his <i>Son</i> often report it, +and proffer to make oath that the Crocus and Gladiolus, as likewise the Leucoium, +and Hyacinths by a long standing without replanting have in his garden changed +from one kind to the other: and for satisfaction about the curiosity in the presence +of <i>Mr. Boyle</i> I tooke up some bulbs of the very numericall roots whereof the relation +was made, though the alteration was perfected before, where we saw the +diverse bulbs growing as it were on the same stoole, close together, but no bulb +half of the one kind, and the other half of the other: But the changetime being past +it was reason we should believe the report of good artists in matters of their own +faculty."</p> +<p>Robert Sharrock was a fellow of New College, Oxford. Both the Bobarts were +professional botanists, the father was author of a Catalogue of the plants in the +Hortus Medicus at Oxford, and the son was afterwards Curator of the Oxford Garden.</p></div> + +<div class="footnote"><p><a name="Footnote_4_4" id="Footnote_4_4"></a> +<a href="#FNanchor_4_4"><span class="label">[4]</span></a> +<i>Mém. Ac. roy. des Sci.</i> for 1719 (1721), p. 59.</p></div> + +<div class="footnote"><p><a name="Footnote_5_5" id="Footnote_5_5"></a> +<a href="#FNanchor_5_5"><span class="label">[5]</span></a> +<i>Amoen. Acad.</i>, 1789, vol. 6. I do not know whether attention has been called +to the curious mistake which Linnaeus makes in the course of this argument. He +cites the differences between the Mule and the Hinny in illustration of his thesis, +pointing out that the Mule is externally more like a horse and the Hinny more like +an ass. This, he says, is because the Mule has the horse for a father, and the +Hinny the ass, thus inverting the actual facts!</p></div> + +<div class="footnote"><p><a name="Footnote_6_6" id="Footnote_6_6"></a> +<a href="#FNanchor_6_6"><span class="label">[6]</span></a> +<i>Proc. Washington Ac. Sci.</i>, 1909, XI, pp. 17-26.</p></div> + +<div class="footnote"><p><a name="Footnote_7_7" id="Footnote_7_7"></a> +<a href="#FNanchor_7_7"><span class="label">[7]</span></a> +J. W. Tutt, in <i>Ent. Rec.</i>, 1909, XXI, p. 185.</p></div> + +<div class="footnote"><p><a name="Footnote_8_8" id="Footnote_8_8"></a> +<a href="#FNanchor_8_8"><span class="label">[8]</span></a> +E. Lehmann (<i>Bull. l'Herb. Boissier</i>, Ser. 2, VIII, 1908, p. 229) has published +an admirable paper on the interrelationships of these species and has instituted +cultural experiments which will probably much elucidate the nature of their specific +distinctness. As regards the existence of intermediate forms he comes to the conclusion +that two only can be so regarded. The first was described by Kuntze from +specimens found on a flower-pot on board a Caspian steamer, from which Lehmann +proposes the new specific name <i>Siaretensis</i>. This comes between <i>polita</i> +and <i>filiformis</i>, a close ally of <i>Tournefortii</i>. The other, which combines +some of the features of both <i>polita</i> and <i>Tournefortii</i>, was found in the +province of Asterabad.</p></div> + +<div class="footnote"><p><a name="Footnote_9_9" id="Footnote_9_9"></a> +<a href="#FNanchor_9_9"><span class="label">[9]</span></a> +In Cambridgeshire for example <i>vespertina</i> is common but <i>diurna</i> is absent. +Whether this absence is connected with the general presence of chalk I cannot say. +When introduced artificially <i>diurna</i> establishes itself, for a time at least, +without any apparent difficulty and occasionally escapes from the garden on to the +neighbouring roadside.</p></div> + +<div class="footnote"><p><a name="Footnote_10_10" id="Footnote_10_10"></a> +<a href="#FNanchor_10_10"><span class="label">[10]</span></a> +Conceivably however it may be a segregated combination. For an account +of this plant see Boissier, <i>Voy. Bot. Midi de l'Espagne</i>, 1839, II, 722.</p></div> + +<div class="footnote"><p><a name="Footnote_11_11" id="Footnote_11_11"></a> +<a href="#FNanchor_11_11"><span class="label">[11]</span></a> +A discussion of this subject with references to literature is given by Rolfe, +in an excellent paper on "Hybridisation viewed from the standpoint of Systematic +Botany" (<i>Jour. R. Hort. Soc.</i>, XXIV, 1900, p. 197). He concludes: "The simple +fact is that the two plants (<i>L. diurna</i> and <i>vespertina</i>) are thoroughly distinct +in numerous particulars, and affect such different habitats that in some localities +one or the other of them is completely wanting. But when their stations are +adjacent they hybridise together very readily, and it is here that these intermediate +forms occur which have puzzled botanists so much." The same paper contains +valuable information concerning several cognate illustrations.</p></div> + +<div class="footnote"><p><a name="Footnote_12_12" id="Footnote_12_12"></a> +<a href="#FNanchor_12_12"><span class="label">[12]</span></a> +In only two cases have I seen such plants (both females) completely sterile.</p></div> + +<div class="footnote"><p><a name="Footnote_13_13" id="Footnote_13_13"></a> +<a href="#FNanchor_13_13"><span class="label">[13]</span></a> +As is well known, in an even more notorious example, he proposed to unite +<i>Primula vulgaris</i>, <i>P. elatior</i>, and <i>P. acaulis</i>, +similarly relying on the existence of "intermediates," +which we now well know to be mongrels between the species.</p></div> + +<div class="footnote"><p><a name="Footnote_14_14" id="Footnote_14_14"></a> +<a href="#FNanchor_14_14"><span class="label">[14]</span></a> +For an account of the distinctions between <i>Vespa vulgaris</i> and <i>germanica</i> +see Ch. Janet, <i>Études sur les Fourmis, les Guêpes et les Abeilles</i>, 11<sup>e</sup>, +Note. Sur <i>Vespa germanica</i> et <i>V. vulgaris</i>. Limoges (Ducourtieux), +1895; and R. du Buysson, Monographie des Guêpes, <i>Ann. Soc. Ent. France</i>, +1903, Vol. LXXII, p. 603, Pl. VIII.</p></div> + +<div class="footnote"><p><a name="Footnote_15_15" id="Footnote_15_15"></a> +<a href="#FNanchor_15_15"><span class="label">[15]</span></a> +The statements made above are for the most part taken from Barrett, C. G., +<i>Lepidoptera of the British Islands</i>, and from Tutt, J. W., <i>The British Noctuae and +their Varieties</i>. The reader who is unfamiliar with the amazing polymorphism +exhibited by some of these moths should if possible take an opportunity of looking +over a long series in a collection, or, if that be impossible, refer to the admirable +coloured plates published by Barrett. It may not be superfluous to observe that +plenty of similar examples are known in other countries. For instance <i>Plotheia +frontalis</i>, a Noctuid which often abounds in Ceylon, shows an equally bewildering +wealth of forms. If a dozen specimens of such a species were to be brought home +from some little known country, each individual would almost certainly be described +as the type of a distinct species. (See the coloured plate published by Sir G. Hampson, +Cat. Brit. Mus., Heterocera, Vol. IX.)</p></div> + +<div class="footnote"><p><a name="Footnote_16_16" id="Footnote_16_16"></a> +<a href="#FNanchor_16_16"><span class="label">[16]</span></a> +<i>Dict. of Birds</i>, p. 800. It would be interesting and profitable to attempt in +a long series of Ruffs to determine the Mendelian factors which by their combinations +give rise to this complex assemblage of varietal forms. A few such factors both of +colour and pattern can be at once distinguished, and it is noticeable that some of +the resulting types of barring, spangling and penciling show a perceptible correspondence +with some of the types of colouration found in the breeds of domestic fowls.</p></div> + +<div class="footnote"><p><a name="Footnote_17_17" id="Footnote_17_17"></a> +<a href="#FNanchor_17_17"><span class="label">[17]</span></a> +Howard Saunders (<i>Illust. Manual of British Birds</i>, 1899, p. 499) states that +there is evidence that the pheasant had become naturalized in the south of England +before the Norman invasion. He adds, "little, if any, deviation from the typical +<i>P. colchicus</i> took place up to the end of last century, when the introduction of +the Chinese Ring-necked <i>P. torquatus</i> commenced, which has left almost indelible +marks, especially with regard to the characteristic white collar."</p></div> + +<hr class="chap" /> +<h3>FOOTNOTES: CHAPTER II.</h3> + +<div class="footnote"><p><a name="Footnote_1_18" id="Footnote_1_18"></a> +<a href="#FNanchor_1_18"><span class="label">[1]</span></a> +In saying this we make no assumption as to the particular cell-division at +which differentiation occurs. This may be one of the maturation-divisions, or it +may perhaps be much earlier.</p></div> + +<div class="footnote"><p><a name="Footnote_2_19" id="Footnote_2_19"></a> +<a href="#FNanchor_2_19"><span class="label">[2]</span></a> +From the recent discoveries of Erwin Baur we are led to surmise that in the +flowering plants the sub-epidermal layer, or some of its elements, may legitimately +be regarded as a similar germ-substance, continuous in Weismann's sense.</p></div> + +<div class="footnote"><p><a name="Footnote_3_20" id="Footnote_3_20"></a> +<a href="#FNanchor_3_20"><span class="label">[3]</span></a> +These fraternal twins, which show no special resemblance to each other, +are like the multiple births of other animals, and there is no disposition for them to +be of the same sex. In the sheep, for example, statistics show that the frequency +of pairs of twins, male and female, is approximately double that of the frequency +of pairs, both male or both female, as it should be if the sex-distribution were fortuitous. +For instance Bernadin (<i>La Bergerie de Rambouillet</i>, 1890, p. 100) gives +the following figures for twin-lambs in Merinos: both male, 87; both female, 83; +sexes mixed, 187. The 9-banded Armadillo (<i>Dasypus novemcinctus</i>), in which +the young born in one litter are said to be always of one sex, is the only known +exception in Vertebrates, and is presumably a genuine case of normal polyembryony +(see especially, Rosner, <i>Bull. Ac. Soc. Cracovie</i>, 1901, p. 443, and Newman and +Patterson, <i>Biol. Bull.</i>, XVII, 1909, p. 181), and an important paper lately published +by H. H. Newman and J. T. Patterson, <i>Jour. Morph.</i>, 1911, XXII, p. 855.</p></div> + +<div class="footnote"><p><a name="Footnote_4_21" id="Footnote_4_21"></a> +<a href="#FNanchor_4_21"><span class="label">[4]</span></a> +A good collection of evidence as to disease in homologous twins was lately +published by E. A. Cockayne, <i>Brit. Jour. Child. Diseases</i>, Nov., 1911.</p></div> + +<div class="footnote"><p><a name="Footnote_5_22" id="Footnote_5_22"></a> +<a href="#FNanchor_5_22"><span class="label">[5]</span></a> +Cp. Windle, B. C. A., <i>Jour. Anal. Phys.</i>, XXVI, p. 295.</p></div> + +<div class="footnote"><p><a name="Footnote_6_23" id="Footnote_6_23"></a> +<a href="#FNanchor_6_23"><span class="label">[6]</span></a> +Mr. E. Nettleship tells me that in the course of collecting pedigrees of families +containing colour-blind members he has discovered two cases (shortly to be published) +of pairs of twins, which on account of their very close resemblances must +be deemed homologous, one of each pair being colour-blind and the other normal. +Such a distinction between closely similar twins is most curious and unexpected.</p></div> + +<div class="footnote"><p><a name="Footnote_7_24" id="Footnote_7_24"></a> +<a href="#FNanchor_7_24"><span class="label">[7]</span></a> +Another paradoxical phenomenon of the same nature occurs in the Narwhal +The males normally have the <i>left</i> tusk alone developed, the corresponding right +tusk remaining as an undeveloped rudiment in its socket. The left tusk is a +left-handed screw. Occasionally the right tusk is also developed and grows to +the same length as that of the left side, but in such specimens the right tusk is +also a left-hand screw like the tusk of the other side, instead of being reversed +as we should certainly have expected. It need scarcely be remarked that in the +case of the horns of antelopes, and in other examples of spiral organs arranged in +pairs, that of one side of the body is the mirror image of that on the other side. +The Narwhal's tusks in being both twisted in the same direction are thus highly +anomalous, and are comparable with pairs of twins.</p></div> + +<div class="footnote"><p><a name="Footnote_8_25" id="Footnote_8_25"></a> +<a href="#FNanchor_8_25"><span class="label">[8]</span></a> +Wilder, H. H., <i>Amer. Jour. Anat.</i>, 1904, III, p. 452.</p></div> + +<div class="footnote"><p><a name="Footnote_9_26" id="Footnote_9_26"></a> +<a href="#FNanchor_9_26"><span class="label">[9]</span></a> +Polydactylism which is often a dominant and the web-foot of Pigeons which +is recessive should be remembered as possible exceptions (see p. 49).</p></div> + +<div class="footnote"><p><a name="Footnote_10_27" id="Footnote_10_27"></a> +<a href="#FNanchor_10_27"><span class="label">[10]</span></a> +Davenport inclined at first to regard rumplessness as a recessive, but in his +latest publication on the subject he definitely concludes that it is an imperfect +dominant. This conclusion accords well with evidence quoted by Darwin (<i>An. +and Plts.</i>, II, ed. 2, p. 4) that rumpless fowls may throw tailed offspring. +(<i>Amer. Nat.</i>, 1910, XLIV, p. 134.)</p></div> + +<div class="footnote"><p><a name="Footnote_11_28" id="Footnote_11_28"></a> +<a href="#FNanchor_11_28"><span class="label">[11]</span></a> +Spillman, W. J., <i>Amer. Breeders Mag.</i>, 1910, I, p. 178.</p></div> + +<div class="footnote"><p><a name="Footnote_12_29" id="Footnote_12_29"></a> +<a href="#FNanchor_12_29"><span class="label">[12]</span></a> +Newsholme, <i>Lancet</i>, December 10, 1910, p. 1690.</p></div> + +<div class="footnote"><p><a name="Footnote_13_30" id="Footnote_13_30"></a> +<a href="#FNanchor_13_30"><span class="label">[13]</span></a> +<i>Materials for the Study of Variation</i>, 1894, p. 358.</p></div> + +<div class="footnote"><p><a name="Footnote_14_31" id="Footnote_14_31"></a> +<a href="#FNanchor_14_31"><span class="label">[14]</span></a> +Walker, G., <i>Johns Hopkins Hospital Bulletin</i>, XII, 1901, p. 129.</p></div> + +<div class="footnote"><p><a name="Footnote_15_32" id="Footnote_15_32"></a> +<a href="#FNanchor_15_32"><span class="label">[15]</span></a> +Cp. R. H. Compton, <i>New Phytologist</i>, 1911, p. 249.</p></div> + +<div class="footnote"><p><a name="Footnote_16_33" id="Footnote_16_33"></a> +<a href="#FNanchor_16_33"><span class="label">[16]</span></a> +<i>Arch. f. Entwickelungsmech.</i>, 1907, XXIII, p. 249.</p></div> + +<div class="footnote"><p><a name="Footnote_17_34" id="Footnote_17_34"></a> +<a href="#FNanchor_17_34"><span class="label">[17]</span></a> +Bull. Soc. Bot. de France, xxxiv, 1887, p. 182.</p></div> + +<div class="footnote"><p><a name="Footnote_18_35" id="Footnote_18_35"></a> +<a href="#FNanchor_18_35"><span class="label">[18]</span></a> +R. Boyle, <i>The Origine of Formes and Qualities</i>, Oxford, 1666.</p></div> + +<div class="footnote"><p><a name="Footnote_19_36" id="Footnote_19_36"></a> +<a href="#FNanchor_19_36"><span class="label">[19]</span></a> +Remarkable experiments on this question have lately been carried out by +R. H. Compton (<i>Camb. Phil. Soc.</i>, XV, 1910, p. 495), showing that in a certain +Barley, "Plumage Corn," the average ratio of left to right is about 1.5. A fuller +paper has since been published by Compton, <i>Jour. Genetics</i>, 1912, II, I, p. 53.</p></div> + +<hr class="chap" /> +<h3>FOOTNOTES: CHAPTER III.</h3> + +<div class="footnote"><p><a name="Footnote_1_37" id="Footnote_1_37"></a> +<a href="#FNanchor_1_37"><span class="label">[1]</span></a> +Stéphane Leduc, <i>Théorie Physico-Chymique de la Vie</i>, Paris, 1910.</p></div> + +<div class="footnote"><p><a name="Footnote_2_38" id="Footnote_2_38"></a> +<a href="#FNanchor_2_38"><span class="label">[2]</span></a> +<i>Materials for the Study of Variation</i>, No. 249, p. 217; and p. 272.</p></div> + +<div class="footnote"><p><a name="Footnote_3_39" id="Footnote_3_39"></a> +<a href="#FNanchor_3_39"><span class="label">[3]</span></a> +<i>Materials</i>, p. 118.</p></div> + +<div class="footnote"><p><a name="Footnote_4_40" id="Footnote_4_40"></a> +<a href="#FNanchor_4_40"><span class="label">[4]</span></a> +Church, A. H., <i>On the Relation of Phyllotaxis to Mechanical Laws</i>, London, 1904.</p></div> + +<div class="footnote"><p><a name="Footnote_5_41" id="Footnote_5_41"></a> +<a href="#FNanchor_5_41"><span class="label">[5]</span></a> +It is a question whether the dominance of the palmatifid leaf +over the pinnatifid is not really an example of the dominance of a lower +number of segmentations over a higher. From the uncertainty whether two +given leaves of two separate plants are actually comparable one cannot +institute quite satisfactory numerical comparisons, but I think the view +that the "Fern" leaf has more lobes than an otherwise similar "Palm" leaf +may be fairly maintained. If this be admitted, the "Palm" leaf represents +the dominant low number and its round shape is a consequence of the greater +powers of growth which are so often possessed by the members of a shorter series.</p></div> + +<div class="footnote"><p><a name="Footnote_6_42" id="Footnote_6_42"></a> +<a href="#FNanchor_6_42"><span class="label">[6]</span></a> +It is perhaps of importance to remember that in certain species of bacteria +(e. g. <i>Bacillus Anthracis</i>) division may cease where the organism is +cultivated under certain artificial conditions though growth continues. +In this way very long unsegmented threads are produced.</p></div> + +<div class="footnote"><p><a name="Footnote_7_43" id="Footnote_7_43"></a> +<a href="#FNanchor_7_43"><span class="label">[7]</span></a> +<i>Arch. f. Entwm.</i>, XX, 1905, p. 76; +<i>Sitzungsb. d. Ges. Naturf.</i>, Berlin, 1907, p. 41, etc.</p></div> + +<div class="footnote"><p><a name="Footnote_8_44" id="Footnote_8_44"></a> +<a href="#FNanchor_8_44"><span class="label">[8]</span></a> +Borradaile, L. A., <i>Jour. Marine Zool.</i>, 1897, No. 8.</p></div> + +<div class="footnote"><p><a name="Footnote_9_45" id="Footnote_9_45"></a> +<a href="#FNanchor_9_45"><span class="label">[9]</span></a> +Dr. Przibram, I should mention, concludes that on the whole the facts are +against this interpretation, but as more evidence is certainly required, +I call attention to the possibility.</p></div> + +<div class="footnote"><p><a name="Footnote_10_46" id="Footnote_10_46"></a> +<a href="#FNanchor_10_46"><span class="label">[10]</span></a> + Morgan, T. H., <i>Regeneration</i>, 1901.</p></div> + +<div class="footnote"><p><a name="Footnote_11_47" id="Footnote_11_47"></a> +<a href="#FNanchor_11_47"><span class="label">[11]</span></a> +It would be interesting to know whether growth continues at the original +posterior end after the new "posterior" end has been formed in front.</p></div> + +<div class="footnote"><p><a name="Footnote_12_48" id="Footnote_12_48"></a> +<a href="#FNanchor_12_48"><span class="label">[12]</span></a> +In the actual case observed, the ripples unsmoothed had a wave-length of +about 2-1/2 inches; and when the new ones were first formed, there were +about 30 ridges in the length originally traversed by 15 or 16.</p></div> + +<div class="footnote"><p><a name="Footnote_13_49" id="Footnote_13_49"></a> +<a href="#FNanchor_13_49"><span class="label">[13]</span></a> +<i>The Science and Philosophy of the Organism</i>; +Gifford Lectures, 1907. London, 1908, p. 141.</p></div> + +<hr class="chap" /> +<h3>FOOTNOTES: CHAPTER IV.</h3> + +<div class="footnote"><p><a name="Footnote_1_50" id="Footnote_1_50"></a> +<a href="#FNanchor_1_50"><span class="label">[1]</span></a> +Gates, R. R., <i>Zts. f. Abstammungslehre</i>, 1911, IV, pp. 341 and 361.</p></div> + +<hr class="chap" /> +<h3>FOOTNOTES: CHAPTER V.</h3> + +<div class="footnote"><p><a name="Footnote_1_51" id="Footnote_1_51"></a> +<a href="#FNanchor_1_51"><span class="label">[1]</span></a> +See Lotsy and Baur, Rep. Genetics Conf., Paris, 1911, pp. 416-426. +Compare Lecoq on <i>Mirabilis jalapa</i> × <i>longiflora</i>, +Fécondation des Végétaux, 1862, p. 311.</p></div> + +<div class="footnote"><p><a name="Footnote_2_52" id="Footnote_2_52"></a> +<a href="#FNanchor_2_52"><span class="label">[2]</span></a> +<i>Rep. Evol. Ctee. R. S.</i>, IV, 1908, p. 38.</p></div> + +<div class="footnote"><p><a name="Footnote_3_53" id="Footnote_3_53"></a> +<a href="#FNanchor_3_53"><span class="label">[3]</span></a> +<i>Ber. Deut. Bot. Ges.</i>, 1908, XXVI, <i>a</i>, p. 672.</p></div> + +<div class="footnote"><p><a name="Footnote_4_54" id="Footnote_4_54"></a> +<a href="#FNanchor_4_54"><span class="label">[4]</span></a> +<i>Jour. Genetics</i>, 1, 1910, p. 57.</p></div> + +<div class="footnote"><p><a name="Footnote_5_55" id="Footnote_5_55"></a> +<a href="#FNanchor_5_55"><span class="label">[5]</span></a> +In Rep. 1 to Evol. Committee, 1902, p. 132, attention was called +to this possibility, though of course at that date it was in sexual animals alone +that it was supposed to exist. It had not occurred to me that even a hermaphrodite +plant might be in this condition.</p></div> + +<div class="footnote"><p><a name="Footnote_6_56" id="Footnote_6_56"></a> +<a href="#FNanchor_6_56"><span class="label">[6]</span></a> +From the description of the offspring of <i>muricata</i> used as mother.</p></div> + +<div class="footnote"><p><a name="Footnote_7_57" id="Footnote_7_57"></a> +<a href="#FNanchor_7_57"><span class="label">[7]</span></a> +de Vries, <i>Species and Varieties</i>, 1905, p. 259.</p></div> + +<div class="footnote"><p><a name="Footnote_8_58" id="Footnote_8_58"></a> +<a href="#FNanchor_8_58"><span class="label">[8]</span></a> +Zeijlstra in a recent paper announces that many <i>nanella</i> +plants are the subject of a bacterial disease to which he attributes their +dwarfness. I gather that this does not apply to all <i>nanella</i> plants and +that some are dwarfs apart from disease. The matter may no doubt be further +complicated from this cause.</p></div> + +<div class="footnote"><p><a name="Footnote_9_59" id="Footnote_9_59"></a> +<a href="#FNanchor_9_59"><span class="label">[9]</span></a> +<i>Zts. f. Abstamm.</i>, 1912, VIII.</p></div> + +<div class="footnote"><p><a name="Footnote_10_60" id="Footnote_10_60"></a> +<a href="#FNanchor_10_60"><span class="label">[10]</span></a> +<i>Arch. f. Zellforschung</i>, 1912, IX, p. 331.</p></div> + +<hr class="chap" /> +<h3>FOOTNOTES: CHAPTER VI.</h3> + +<div class="footnote"><p><a name="Footnote_1_61" id="Footnote_1_61"></a> +<a href="#FNanchor_1_61"><span class="label">[1]</span></a> +For the evidence see Tutt, J. W., <i>Trans. Ent. Soc.</i>, 1898, p. 17. +Compare the remarkable case given by Gulick +(<i>Evolution Racial and Habitudinal</i>, p. 123) of the two races of +<i>Cicada</i>, which are separated by reason of their life-cycles, +one having a period of 13, the other 17 years.</p></div> + +<div class="footnote"><p><a name="Footnote_2_62" id="Footnote_2_62"></a> +<a href="#FNanchor_2_62"><span class="label">[2]</span></a> +For references see <i>Materials</i>, p. 396, and also G. Baur, +<i>Amer. Nat.</i>, 1893, July, p. 677.</p></div> + +<div class="footnote"><p><a name="Footnote_3_63" id="Footnote_3_63"></a> +<a href="#FNanchor_3_63"><span class="label">[3]</span></a> +Jenner Weir, <i>Entomologist</i>, 1880, XIII, p. 251.</p></div> + +<div class="footnote"><p><a name="Footnote_4_64" id="Footnote_4_64"></a> +<a href="#FNanchor_4_64"><span class="label">[4]</span></a> +Jentink, <i>Notes Leyden Mus.</i>, 1885, VII, p. 111. +Specimens illustrating this peculiarity are in the British Museum.</p></div> + +<div class="footnote"><p><a name="Footnote_5_65" id="Footnote_5_65"></a> +<a href="#FNanchor_5_65"><span class="label">[5]</span></a> +<i>Proc. Zool. Soc.</i>, 1895, p. 850. Plate. Many points beyond that mentioned +above are involved in this remarkable case. For example, not only are there males +like females, but a small proportion of females resemble the ordinary male type. +The stripes are not merely the spots produced, for they occupy different anatomical +positions. The spots almost always go with a black ventral surface, but the striped +forms nearly always have that region testaceous. <i>Spartium retama</i>, the food-plant, +will not grow in England, but if it could be naturalised in America the whole problem +might be investigated there and results of exceptional interest would almost certainly +be attained.</p></div> + +<div class="footnote"><p><a name="Footnote_6_66" id="Footnote_6_66"></a> +<a href="#FNanchor_6_66"><span class="label">[6]</span></a> +Doncaster, L., <i>Proc. Zool. Soc.</i>, 1905, II, p. 528.</p></div> + +<div class="footnote"><p><a name="Footnote_7_67" id="Footnote_7_67"></a> +<a href="#FNanchor_7_67"><span class="label">[7]</span></a> +I am not aware that the details of this striking case have ever been worked +out. It should be noted that the green and blue forms are not due to simple modification +of the red pigment; for these colours, due to interference, fork over the +area occupied by the red lines. The distinctions between these forms cannot +therefore be simply chemical, as we may suppose them to be, for instance, in the +case of many red and yellow forms, and the genetic relationships of the <i>Heliconid</i> +varieties would raise many novel problems and be well worth studying experimentally.</p></div> + +<div class="footnote"><p><a name="Footnote_8_68" id="Footnote_8_68"></a> +<a href="#FNanchor_8_68"><span class="label">[8]</span></a> +Woodeforde, F. C., <i>Trans. North Staffordshire Field Club</i>, XXXV, 1901, Plate.</p></div> + +<div class="footnote"><p><a name="Footnote_9_69" id="Footnote_9_69"></a> +<a href="#FNanchor_9_69"><span class="label">[9]</span></a> +E. Gallé, <i>Compte Rendus du Congres Internat. de Bot. a l'Expos. Univ.</i>, +1900, p. 112.</p></div> + +<div class="footnote"><p><a name="Footnote_10_70" id="Footnote_10_70"></a> +<a href="#FNanchor_10_70"><span class="label">[10]</span></a> +Flora of Mentone, 1864-8, <i>Nova Acta Acad. Caes.</i>, XXXV, 1869.</p></div> + +<div class="footnote"><p><a name="Footnote_11_71" id="Footnote_11_71"></a> +<a href="#FNanchor_11_71"><span class="label">[11]</span></a> +I owe these facts to Canon A. M. Norman, who showed me illustrative +specimens. They were originally described by Bowerbank (<i>Monogr. Brit. Spongiadae</i>, +vol. II, pp. 18 and XX; vol. III, Pls. I and III). A specimen of <i>G. compressa</i> +measured 5 inches, with a greatest width of 3-1/4 in. <i>G. ciliata</i> was found measuring +3 in. long and 3/4 in. wide. These dimensions are many times those of normal +specimens.</p></div> + +<div class="footnote"><p><a name="Footnote_12_72" id="Footnote_12_72"></a> +<a href="#FNanchor_12_72"><span class="label">[12]</span></a> +Coutagne, G., <i>Recherches sur le Polymorphisme des Mollusques de France</i>, +<i>Annales Soc. d'Agric. Sci. et Industr. Lyon</i>, 1895.</p></div> + +<div class="footnote"><p><a name="Footnote_13_73" id="Footnote_13_73"></a> +<a href="#FNanchor_13_73"><span class="label">[13]</span></a> +As to the synonymy and references see Coutagne, p. 45.</p></div> + +<div class="footnote"><p><a name="Footnote_14_74" id="Footnote_14_74"></a> +<a href="#FNanchor_14_74"><span class="label">[14]</span></a> +A. Lang, <i>Die Bastarde von H. hortensis Muller H. nemoralis L.</i> Jena, G. Fischer, 1908; +with a fine coloured plate showing the varieties of the species and their hybrids.</p></div> + +<div class="footnote"><p><a name="Footnote_15_75" id="Footnote_15_75"></a> +<a href="#FNanchor_15_75"><span class="label">[15]</span></a> +With this evidence compare that given by A. Delcourt in +his valuable papers lately published relating to the variations of +<i>Notonecta</i>. See especially <i>Bull. Sci. Fr. Belg.</i>, 1909, +XLIII, p. 443; and <i>C. R. Soc. Biol.</i>, 1909, LXVI, p. 589.</p></div> + +<div class="footnote"><p><a name="Footnote_16_76" id="Footnote_16_76"></a> +<a href="#FNanchor_16_76"><span class="label">[16]</span></a> +Allen, J. A., <i>Bull. Amer. Mus. N. H.</i>, III, 1891, pp. 51-54.</p></div> + +<div class="footnote"><p><a name="Footnote_17_77" id="Footnote_17_77"></a> +<a href="#FNanchor_17_77"><span class="label">[17]</span></a> +J. T. Gulick, <i>Evolution, Racial and Habitudinal</i>, +Carnegie Institution, Publication No. 25, 1905.</p></div> + +<div class="footnote"><p><a name="Footnote_18_78" id="Footnote_18_78"></a> +<a href="#FNanchor_18_78"><span class="label">[18]</span></a> +A. G. Mayer, <i>Mem. Mus. Comp. Anat. Harvard</i>, Vol. XXVI, 1902, p. 117. +From the tables given I cannot ascertain the actual numbers from the two +intermediate valleys, but they were considerable.</p></div> + +<div class="footnote"><p><a name="Footnote_19_79" id="Footnote_19_79"></a> +<a href="#FNanchor_19_79"><span class="label">[19]</span></a> +To which I was very kindly guided by Mr. C. T. Trechmann.</p></div> + +<div class="footnote"><p><a name="Footnote_20_80" id="Footnote_20_80"></a> +<a href="#FNanchor_20_80"><span class="label">[20]</span></a> +Standfuss, <i>Handbuch d. paläarkt Gross-schmet</i>, 1896, p. 321.</p></div> + +<div class="footnote"><p><a name="Footnote_21_81" id="Footnote_21_81"></a> +<a href="#FNanchor_21_81"><span class="label">[21]</span></a> +<i>Ent. Rec.</i>, XVIII, No. 7, 1906.</p></div> + +<div class="footnote"><p><a name="Footnote_22_82" id="Footnote_22_82"></a> +<a href="#FNanchor_22_82"><span class="label">[22]</span></a> +This evidence was largely collected by Mr. G. T. Porritt, +who has given much attention to the subject.</p></div> + +<div class="footnote"><p><a name="Footnote_23_83" id="Footnote_23_83"></a> +<a href="#FNanchor_23_83"><span class="label">[23]</span></a> +Such direct action has of course been proved to occur in the +case of several dimorphic larvae (<i>e. g.</i>, <i>A. betularia</i>, +itself) and pupae.</p></div> + +<div class="footnote"><p><a name="Footnote_24_84" id="Footnote_24_84"></a> +<a href="#FNanchor_24_84"><span class="label">[24]</span></a> +See Harris, <i>Proc. Ent. Soc. London</i>, 1904, p. lxxii, and 1905, p. lxiii; +also Hamling, <i>Trans. City of London Ent. Soc.</i>, 1905, p. 5.</p></div> + +<div class="footnote"><p><a name="Footnote_25_85" id="Footnote_25_85"></a> +<a href="#FNanchor_25_85"><span class="label">[25]</span></a> +I am indebted to Mr. Outram Bangs of the Harvard Museum +for calling my attention to this remarkable case.</p></div> + +<div class="footnote"><p><a name="Footnote_26_86" id="Footnote_26_86"></a> +<a href="#FNanchor_26_86"><span class="label">[26]</span></a> +<i>Auk</i>, 1889, VI, p. 219.</p></div> + +<div class="footnote"><p><a name="Footnote_27_87" id="Footnote_27_87"></a> +<a href="#FNanchor_27_87"><span class="label">[27]</span></a> +<i>Ann. N. Y. Acad. Sci.</i>, 1878, I, p. 149.</p></div> + +<div class="footnote"><p><a name="Footnote_28_88" id="Footnote_28_88"></a> +<a href="#FNanchor_28_88"><span class="label">[28]</span></a> +<i>Ann. N. Y. Acad. Sci.</i>, 1878, I, p. 149.</p></div> + +<div class="footnote"><p><a name="Footnote_29_89" id="Footnote_29_89"></a> +<a href="#FNanchor_29_89"><span class="label">[29]</span></a> +<i>Ibid</i>, 1912, pp. 523-8.</p></div> + +<hr class="chap" /> +<h3>FOOTNOTES: CHAPTER VII.</h3> + +<div class="footnote"><p><a name="Footnote_1_90" id="Footnote_1_90"></a> +<a href="#FNanchor_1_90"><span class="label">[1]</span></a> +J. A. Allen, <i>The North American Species of the Genus Colaptes, +Considered with Special Reference to the Relationships of C. auratus and +C. cafer</i>. Bull. Am. Mus. Nat. Hist., IV, 1892.</p></div> + +<div class="footnote"><p><a name="Footnote_2_91" id="Footnote_2_91"></a> +<a href="#FNanchor_2_91"><span class="label">[2]</span></a> +For a case in which a red-headed female × a black-headed male +gave a black-headed female and a red-headed male, see <i>Avian Mag.</i>, +N. S., IV, pp. 49 and 329</p></div> + +<div class="footnote"><p><a name="Footnote_3_92" id="Footnote_3_92"></a> +<a href="#FNanchor_3_92"><span class="label">[3]</span></a> +The other variations of this bird are also interesting and important. The +normal male has a red head and a red throat. The female has a red head and a +white throat, but varieties of the female are known with a black head, thus again +illustrating the change from black to red. It should be noted that this is not a +mere retention of a juvenile character, but, as the birds mature, the red feathers +come up, or as an exception, the black. There is also a western species, <i>ruber</i>, +in which both sexes have a great extension of red, and are alike. The male of +<i>nuchalis</i> intergrades with this type, but the female does not.</p></div> + +<div class="footnote"><p><a name="Footnote_4_93" id="Footnote_4_93"></a> +<a href="#FNanchor_4_93"><span class="label">[4]</span></a> +Dr. W. Brewster, for example, has a remarkable specimen +of the Teal (<i>Nettion carolinense</i>) with a white collar strongly +developed at the front and sides of the neck, in a place where the +normal has no such mark.</p></div> + +<div class="footnote"><p><a name="Footnote_5_94" id="Footnote_5_94"></a> +<a href="#FNanchor_5_94"><span class="label">[5]</span></a> +This variety is spoken of as the Ringed Guillemot and is sometimes regarded +as a distinct species to which the name <i>ringvia</i> was given by Brünnich. In support +of this view Dr. William Brewster, to whom I am indebted for much assistance +in regard to the variation of birds, called my attention to observations of his +own and also of Maynard's, that the ringed birds were sometimes mated together, +though in a small minority (see Brewster, <i>Proc. Boston Soc. N. H.</i>, XXII, 1883, p. +410). It would however be possible to produce many instances of varieties mated +together though surrounded by a typical population (<i>e. g.</i>, two varying Blackbirds, +<i>Zoologist</i>, p. 2765; two varying Nightjars, <i>ibid.</i>, p. 5278). I am inclined to +believe that in nature matings between brothers and sisters are frequent in many +species of animals, and that the production of sporadically varying colonies is +thus greatly assisted.</p></div> + +<div class="footnote"><p><a name="Footnote_6_95" id="Footnote_6_95"></a> +<a href="#FNanchor_6_95"><span class="label">[6]</span></a> +The Sap-suckers feed on trees and somewhat resemble our Spotted Woodpeckers +in general appearance. <i>Colaptes</i> feeds on the ground and corresponds +perhaps rather with the European Green Woodpecker.</p></div> + +<div class="footnote"><p><a name="Footnote_7_96" id="Footnote_7_96"></a> +<a href="#FNanchor_7_96"><span class="label">[7]</span></a> +For an introduction to this example I am indebted to Mr. W. D. Miller of +the American Museum of Natural History. Some account of the facts is given by +Baird, Brewer, and Ridgway (<i>A Hist. of N. Amer. Birds</i>. 1874, II, pp. 540, 544, +etc.). <i>S. varius</i> occupies the whole country in suitable places from the Atlantic +to the eastern slopes of the Rockies, and all Mexico to Guatemala. <i>S. nuchalis</i> was +first known from the Southern Rockies only, but many were afterwards taken in +Utah. <i>S. ruber</i> is restricted to the Pacific coast. In Ridgway's opinion all three +are geographical forms of one species. In <i>ruber</i> the sexes are alike having both a +great extension of the red in the throat, and a red crescent. The male of <i>nuchalis</i> +grades to the <i>ruber</i> form, but the female does not. This female has some red in +the throat like the male of <i>varius</i>, whereas the female of <i>varius</i> has +a whitish throat.</p></div> + +<div class="footnote"><p><a name="Footnote_8_97" id="Footnote_8_97"></a> +<a href="#FNanchor_8_97"><span class="label">[8]</span></a> +Not only vertebrates but the marine Crustacea and Mollusca illustrate +this curious "principle" of variation, as Canon Norman formerly pointed out to me +with abundant illustrations. There are of course cases to the contrary also.</p></div> + +<div class="footnote"><p><a name="Footnote_9_98" id="Footnote_9_98"></a> +<a href="#FNanchor_9_98"><span class="label">[9]</span></a> +Chapman, F. M., <i>Bull. Amer. Mus.</i>, IV, 1892, p. 1; +see also Ridgway, <i>Birds of North and Middle America</i>, 1902, Part II, p. 214.</p></div> + +<div class="footnote"><p><a name="Footnote_10_99" id="Footnote_10_99"></a> +<a href="#FNanchor_10_99"><span class="label">[10]</span></a> +It would aid greatly in factorial analysis if the descriptive term "green" +could be avoided in application to cases where the green effect is due only to a mixture +of black and yellow pigments. The absence of yellow is the sole difference +between the mantle and underparts of <i>pinus</i> and <i>chrysoptera</i>.</p></div> + +<div class="footnote"><p><a name="Footnote_11_100" id="Footnote_11_100"></a> +<a href="#FNanchor_11_100"><span class="label">[11]</span></a> +<i>Bull. Amer. Mus. Nat. Hist.</i>, XXIII, 1907, p. 467.</p></div> + +<div class="footnote"><p><a name="Footnote_12_101" id="Footnote_12_101"></a> +<a href="#FNanchor_12_101"><span class="label">[12]</span></a> +References on this subject will be found in +<i>Brit. Mus. Cat. Birds</i>, XVII, p. 13.</p></div> + +<div class="footnote"><p><a name="Footnote_13_102" id="Footnote_13_102"></a> +<a href="#FNanchor_13_102"><span class="label">[13]</span></a> +For these facts I am indebted to Mr. W. L. W. Field, +who has lately published an account of his observations and experiments. +See especially, <i>Psyche</i>, 1910, XVII, No. 3, where full references +to previous publications are given.</p></div> + +<div class="footnote"><p><a name="Footnote_14_103" id="Footnote_14_103"></a> +<a href="#FNanchor_14_103"><span class="label">[14]</span></a> +For the facts and further references see W. H. Edwards, +<i>Butterflies of N. America</i>, 2d series, Papilio VII and X; +3d series, 1897, Papilio IV, <i>Can. Entom.</i>, 1895, XXVII, p. 239.</p></div> + +<div class="footnote"><p><a name="Footnote_15_104" id="Footnote_15_104"></a> +<a href="#FNanchor_15_104"><span class="label">[15]</span></a> +I think this case is fairly included because the <i>machaon</i> +type is so widespread that it cannot be regarded as a product of a Northern +climate, nor can <i>asterias</i> be claimed as especially a warm country form, +seeing that <i>brevicauda</i>, which is scarcely distinguishable from +<i>asterias</i>, inhabits Newfoundland (having a curious phase there in which +the yellow is largely replaced by red).</p></div> + +<hr class="chap" /> +<h3>FOOTNOTES: CHAPTER VIII.</h3> + +<div class="footnote"><p><a name="Footnote_1_105" id="Footnote_1_105"></a> +<a href="#FNanchor_1_105"><span class="label">[1]</span></a> +Often referred to by older writers as <i>Meone</i>, Esper's name.</p></div> + +<div class="footnote"><p><a name="Footnote_2_106" id="Footnote_2_106"></a> +<a href="#FNanchor_2_106"><span class="label">[2]</span></a> +There are also two distinct island forms, unlike the European, +<i>Xiphia</i> of Madeira, and a smaller variety, <i>Xiphioides</i> of Canary. +See especially, Baker, G. T., <i>Trans. Ent. Soc. London</i>, 1891, p. 292.</p></div> + +<div class="footnote"><p><a name="Footnote_3_107" id="Footnote_3_107"></a> +<a href="#FNanchor_3_107"><span class="label">[3]</span></a> +Speyer, Adolf, and August. <i>Verbreitung der Schmetterlinge</i>, 1858, I, p. 217.</p></div> + +<div class="footnote"><p><a name="Footnote_4_108" id="Footnote_4_108"></a> +<a href="#FNanchor_4_108"><span class="label">[4]</span></a> +<i>Lepid. Comparée</i>, fsc. III, p. 372.</p></div> + +<div class="footnote"><p><a name="Footnote_5_109" id="Footnote_5_109"></a> +<a href="#FNanchor_5_109"><span class="label">[5]</span></a> +Mr. Rowland-Brown has called my attention to a statement by Dr. Vaillantin +(<i>Petites Nouv. Ent.</i>, II, 235) that in Indre-et-Cher the first brood is of the +northern type and the second of the southern. My experience is that in captivity these +distinctions do not occur, and I have true <i>egeria</i> as first brood from Vienne and +as the late brood from the Landes. I never collected in Indre-et-Cher.</p></div> + +<div class="footnote"><p><a name="Footnote_6_110" id="Footnote_6_110"></a> +<a href="#FNanchor_6_110"><span class="label">[6]</span></a> +I have since seen true <i>egeria</i> from Ferrol in the +extreme northwest, which was in Mr. Tutt's collection.</p></div> + +<div class="footnote"><p><a name="Footnote_7_111" id="Footnote_7_111"></a> +<a href="#FNanchor_7_111"><span class="label">[7]</span></a> +Mr. G. Wheeler kindly showed me a series identical with +this type, from Guernsey, and others from near Laon.</p></div> + +<div class="footnote"><p><a name="Footnote_8_112" id="Footnote_8_112"></a> +<a href="#FNanchor_8_112"><span class="label">[8]</span></a> +<i>Ent. Rec.</i>, V, 1894, p. 134.</p></div> + +<div class="footnote"><p><a name="Footnote_9_113" id="Footnote_9_113"></a> +<a href="#FNanchor_9_113"><span class="label">[9]</span></a> +Mr. Wheeler has some pale but rather worn specimens +from the Rhone Valley at Vernayaz.</p></div> + +<div class="footnote"><p><a name="Footnote_10_114" id="Footnote_10_114"></a> +<a href="#FNanchor_10_114"><span class="label">[10]</span></a> +See Fleck, E., Die Macrolep. Rumäniens, +<i>Bul. Soc. Sciinte</i>, VIII, 1899, p. 720.</p></div> + +<div class="footnote"><p><a name="Footnote_11_115" id="Footnote_11_115"></a> +<a href="#FNanchor_11_115"><span class="label">[11]</span></a> +My experience agrees with that of Mr. H. Williams (<i>Ent. Rec.</i>, VIII, 1896, p. 181) +that pupae, well-formed, can stand considerable frost; but I used to find +that half-grown larvae usually died if unprotected, and I believe that larvae which +attempted to pupate in warm autumn weather and then got caught by frosts, +always died. Small larvae which can creep into shelter at the bottom of the plants +survived, and I expect that in the north the winter is usually passed in that state +(see also Merrifield, F., <i>Ent. Rec.</i>, VIII, 1896, p. 168, and +Carpenter, J. H., <i>ibid.</i>).</p></div> + +<div class="footnote"><p><a name="Footnote_12_116" id="Footnote_12_116"></a> +<a href="#FNanchor_12_116"><span class="label">[12]</span></a> +Some most unlikely species do this. I once had a larva +of <i>Parnassius delius</i>, found at about 5,500 feet, which emerged +late in the autumn (in October I believe), a season at which it must +have perished in its own country.</p></div> + +<div class="footnote"><p><a name="Footnote_13_117" id="Footnote_13_117"></a> +<a href="#FNanchor_13_117"><span class="label">[13]</span></a> +See, for examples, Barrett, G. C., <i>Lepidoptera of the Brit. Islands</i>, +I, 1893, p. 229; also Grover, W., <i>Ent. Rec.</i>, IX, 1897, p. 314; Williams, H., +<i>Proc. Ent. Soc.</i>, 1898, who reared several specimens from the New Forest +which would pass for Bretons, though the rest of the family were true <i>egerides</i>.</p></div> + +<div class="footnote"><p><a name="Footnote_14_118" id="Footnote_14_118"></a> +<a href="#FNanchor_14_118"><span class="label">[14]</span></a> +Above the Tosa falls.</p></div> + +<div class="footnote"><p><a name="Footnote_15_119" id="Footnote_15_119"></a> +<a href="#FNanchor_15_119"><span class="label">[15]</span></a> +<i>Bul. Soc. Sciinte</i>, VIII, 1899, p. 691.</p></div> + +<div class="footnote"><p><a name="Footnote_16_120" id="Footnote_16_120"></a> +<a href="#FNanchor_16_120"><span class="label">[16]</span></a> +The fact that Weismann by heating pupæ obtained only one +autumn specimen seems to me to show rather that a second brood can be +produced than that it cannot, which is the inference usually drawn.</p></div> + +<div class="footnote"><p><a name="Footnote_17_121" id="Footnote_17_121"></a> +<a href="#FNanchor_17_121"><span class="label">[17]</span></a> +Schima, K., <i>Verh. Zool. bot. Ges. Wien</i>, LX, 1910, p. 268.</p></div> + +<div class="footnote"><p><a name="Footnote_18_122" id="Footnote_18_122"></a> +<a href="#FNanchor_18_122"><span class="label">[18]</span></a> +<i>Rhopalocera Palaearctica</i>, Florence, 1905-11, especially Pl. XXXII.</p></div> + +<div class="footnote"><p><a name="Footnote_19_123" id="Footnote_19_123"></a> +<a href="#FNanchor_19_123"><span class="label">[19]</span></a> +See figures in Barrett, G. C., <i>Lepidoptera of Brit. Islands</i>, I, pt. 3, p. 25.</p></div> + +<div class="footnote"><p><a name="Footnote_20_124" id="Footnote_20_124"></a> +<a href="#FNanchor_20_124"><span class="label">[20]</span></a> +Tutt, J. W., <i>Ent. Rec.</i>, XVIII, 1905, p. 5. +In the same place he states that on the Mendel Pass <i>arcania</i> +"runs into" <i>darwiniana</i> and that in the Tyrolean localities the +transition is especially evident. Wheeler (<i>ibid.</i>, XIII, 1901, p. 121) +expresses the contrary opinion, that <i>satyrion</i> does grade to <i>arcania</i>.</p></div> + +<div class="footnote"><p><a name="Footnote_21_125" id="Footnote_21_125"></a> +<a href="#FNanchor_21_125"><span class="label">[21]</span></a> +H. Rowland-Brown, <i>Ent. Rec.</i>, XI, 1899, p. 293.</p></div> + +<div class="footnote"><p><a name="Footnote_22_126" id="Footnote_22_126"></a> +<a href="#FNanchor_22_126"><span class="label">[22]</span></a> +Speyer, Stettiner, <i>Ent. Ztg.</i>, XXXI, 1870, p. 63.</p></div> + +<div class="footnote"><p><a name="Footnote_23_127" id="Footnote_23_127"></a> +<a href="#FNanchor_23_127"><span class="label">[23]</span></a> +In regard to the closely analogous case of <i>Spilosoma lubricipeda</i>, Standfuss +makes a similar statement. He bred the type on a large scale with the radiate form +which he calls <i>intermedia</i>, and says that in four years of miscellaneous crossing he +never obtained really transitional forms. Nevertheless after examining large series, +especially those of Mr. W. H. B. Fletcher, I came to the conclusion that several +might be so classed, but I am quite prepared to find that such specimens are heterozygous. +(See Standfuss, <i>Handb. d. Gross-Schmet.</i>, 1896, p. 307.) It is by no means +unlikely that various dark forms of <i>lubricipeda</i> correspond with a progressive series +of factorial additions. Many of the stages have been named, and of these the most +definite are the <i>intermedia</i> of Standfuss (probably = <i>eboraci</i> of Tugwell) and the +very dark <i>Zatima</i> of Heligoland, in which only the thorax, the nervures and a small +field in the fore-wings remain yellow. A form was bred by Deschange from <i>Zatima</i> +in which even the field in the forewing is obliterated. The exact circumstances in +which <i>Zatima</i> occurs in Heligoland would be worthy of special investigation, for the +normal <i>lubricipeda</i> is also found on the island. For references as to the British +occurrences see especially, Hewett, W., <i>Naturalist</i>, 1894, p. 353. As to <i>Zatima</i> see +especially Krancher, <i>Soc. Ent.</i>, II, 1887-8, p. 26. I am indebted to Dr. Hartlaub +for information as to the Heligoland types.</p></div> + +<div class="footnote"><p><a name="Footnote_24_128" id="Footnote_24_128"></a> +<a href="#FNanchor_24_128"><span class="label">[24]</span></a> +Boisduval, <i>Bull. Soc. Ent. Fr.</i>, III, 1834, p. 5.</p></div> + +<div class="footnote"><p><a name="Footnote_25_129" id="Footnote_25_129"></a> +<a href="#FNanchor_25_129"><span class="label">[25]</span></a> +The systematics of <i>Setina</i> have been much controverted, +but no one I believe doubts that <i>aurita</i> and <i>ramosa</i> are forms +of one species. See also Chapman, A. T., <i>Ent. Rec.</i>, XIII, 1901, p. 139.</p></div> + +<div class="footnote"><p><a name="Footnote_26_130" id="Footnote_26_130"></a> +<a href="#FNanchor_26_130"><span class="label">[26]</span></a> +<i>Arch. Naturg.</i>, 33, 1867, p. 116.</p></div> + +<div class="footnote"><p><a name="Footnote_27_131" id="Footnote_27_131"></a> +<a href="#FNanchor_27_131"><span class="label">[27]</span></a> +<i>Brit. Mus. Cat., Batrachia Gradientia</i>, 1882.</p></div> + +<div class="footnote"><p><a name="Footnote_28_132" id="Footnote_28_132"></a> +<a href="#FNanchor_28_132"><span class="label">[28]</span></a> +The Geographical Distribution of nearly related Species. +<i>Amer. Nat.</i>, XLI. 1907, p. 207.</p></div> + +<div class="footnote"><p><a name="Footnote_29_133" id="Footnote_29_133"></a> +<a href="#FNanchor_29_133"><span class="label">[29]</span></a> +See later, p. 242.</p></div> + +<hr class="chap" /> +<h3>FOOTNOTES: CHAPTER IX.</h3> + +<div class="footnote"><p><a name="Footnote_1_134" id="Footnote_1_134"></a> +<a href="#FNanchor_1_134"><span class="label">[1]</span></a> +Semon, R., Der Stand der Frage nach der Vererbung +erworbener Eigenschaften, published in <i>Fortschr. der naturw. +Forschung.</i>, Bd. 11, 1910.</p></div> + +<div class="footnote"><p><a name="Footnote_2_135" id="Footnote_2_135"></a> +<a href="#FNanchor_2_135"><span class="label">[2]</span></a> +Standfuss, M., <i>Denks. Schweiz. naturf. Ges.</i>, XXXVI, 1898, p. 32.</p></div> + +<div class="footnote"><p><a name="Footnote_3_136" id="Footnote_3_136"></a> +<a href="#FNanchor_3_136"><span class="label">[3]</span></a> +Fischer, E., <i>Allg. Ztschr. f. Entomologie</i>, Bd. VI, 1901.</p></div> + +<div class="footnote"><p><a name="Footnote_4_137" id="Footnote_4_137"></a> +<a href="#FNanchor_4_137"><span class="label">[4]</span></a> +Out of 12 pupae treated 8 died and of the 4 survivors, one only was affected. +See M. v. Linden, <i>Archiv. Rassen. u. Gesells.</i>, 1904, I.</p></div> + +<div class="footnote"><p><a name="Footnote_5_138" id="Footnote_5_138"></a> +<a href="#FNanchor_5_138"><span class="label">[5]</span></a> +For illustrations see <i>Oberthur's Études d'Entom.</i>, 1896, +where many of these curious aberrations are represented; +also Barrett, <i>Lepid. Brit. Islands</i>, II, pp. 71 and 72.</p></div> + +<div class="footnote"><p><a name="Footnote_6_139" id="Footnote_6_139"></a> +<a href="#FNanchor_6_139"><span class="label">[6]</span></a> +Schübeler, F. C., <i>Die Culturpflanzen Norwegens</i>, +1862, especially pp. 24 and 28.</p></div> + +<div class="footnote"><p><a name="Footnote_7_140" id="Footnote_7_140"></a> +<a href="#FNanchor_7_140"><span class="label">[7]</span></a> +I am obliged to him and to Dr. E. Gold for much trouble +taken to answer my questions. Some idea of the kind of weather indicated +by an average of 2.76° C. above the mean may be got from a comparison +with the year 1911, which most people will remember as one of the hottest +summers they have known. The July of that year was in east and southeast +England about 4° F. above the mean but 2.67 C. means about 4.8° F. above +the mean. At Greenwich July, 1859, was about 6.5° F. above the average.</p></div> + +<div class="footnote"><p><a name="Footnote_8_141" id="Footnote_8_141"></a> +<a href="#FNanchor_8_141"><span class="label">[8]</span></a> +Wille, N., <i>Biol. Cbltt.</i>, XXV, 1905, p. 521.</p></div> + +<div class="footnote"><p><a name="Footnote_9_142" id="Footnote_9_142"></a> +<a href="#FNanchor_9_142"><span class="label">[9]</span></a> +Wettstein, R. von. <i>Der Neo-marckismus u. seine +Beziehungen zum Darwinismus</i>, Jena, 1903.</p></div> + +<div class="footnote"><p><a name="Footnote_10_143" id="Footnote_10_143"></a> +<a href="#FNanchor_10_143"><span class="label">[10]</span></a> +T. Graham Brown, <i>Proc. Roy. Soc.</i>, 1912, vol. 84, B, p. 555. +This paper gives full reference to the previous literature of the subject.</p></div> + +<div class="footnote"><p><a name="Footnote_11_144" id="Footnote_11_144"></a> +<a href="#FNanchor_11_144"><span class="label">[11]</span></a> +Morgan, T. H., <i>Evolution and Adaptation</i>, New York, 1903.</p></div> + +<div class="footnote"><p><a name="Footnote_12_145" id="Footnote_12_145"></a> +<a href="#FNanchor_12_145"><span class="label">[12]</span></a> +Kammerer's chief paper on this subject is in <i>Arch. f. Entwm.</i>, +1909, XXVIII, p. 447, and it is to this that the paginal references in the present +text relate. His previous paper appeared, <i>ibid.</i>, 1906, XXII, p. 48. +An account of his further experiments with <i>Alytes</i> is given in +<i>Natur</i>, 1909-10, Heft 6, p. 95.</p></div> + +<div class="footnote"><p><a name="Footnote_13_146" id="Footnote_13_146"></a> +<a href="#FNanchor_13_146"><span class="label">[13]</span></a> +In reply to my letter Dr. Kammerer who was then away from home very +kindly replied that he was not quite sure whether he had killed specimens of <i>Alytes</i> +with "<i>Brunftschwielen</i>" or whether he only had living males of the fourth generation, +but that he would send illustrative material.</p></div> + +<div class="footnote"><p><a name="Footnote_14_147" id="Footnote_14_147"></a> +<a href="#FNanchor_14_147"><span class="label">[14]</span></a> +Kammerer, P., <i> Natur</i>, 12 December, 1909, Heft 6, p. 95, repeated in +<i>12 Flugschrift d. Deutsch Ges. f. Züchtungskunde</i>, Berlin, 1910.</p></div> + +<div class="footnote"><p><a name="Footnote_15_148" id="Footnote_15_148"></a> +<a href="#FNanchor_15_148"><span class="label">[15]</span></a> +<i>Festschrift zum Andenken an Gregor Mendel</i>, being +vol. XLIX of the <i>Verh. Naturf. Ver. in Brünn</i>, 1911, p. 98.</p></div> + +<div class="footnote"><p><a name="Footnote_16_149" id="Footnote_16_149"></a> +<a href="#FNanchor_16_149"><span class="label">[16]</span></a> +Kammerer's chief papers on this subject are <i>Archiv fur Entwm.</i>, XVII, +1904, and <i>ibid.</i>, XXV, 1907. An epitome of results is also given by him in +<i>12 Flugschrift d. Deutsch. Ges. f. Züchtungskunde</i>, Berlin, 1910.</p></div> + +<div class="footnote"><p><a name="Footnote_17_150" id="Footnote_17_150"></a> +<a href="#FNanchor_17_150"><span class="label">[17]</span></a> +"<i>Bei Fortdauer der Versuchsbedingungen sind als Vollmolche +geborene Salamandra maculosa</i> gleich bei der ersten Geburt <i>abermals +voll molchgebärend</i>, benutzen zum Geburtsakt das trockene Land, +und zwar unter Erreichung der (bei <i>Salamandra atra</i> normalen) +<i>Embryonen-Zweizahl</i>," Kammerer, 1907, p. 49.</p></div> + +<div class="footnote"><p><a name="Footnote_18_151" id="Footnote_18_151"></a> +<a href="#FNanchor_18_151"><span class="label">[18]</span></a> 1904, p. 56.</p></div> + +<div class="footnote"><p><a name="Footnote_19_152" id="Footnote_19_152"></a> +<a href="#FNanchor_19_152"><span class="label">[19]</span></a> +Throughout Kammerer's papers this is used almost as a technical term. +It means, I presume, that the feature was manifested more than once.</p></div> + +<div class="footnote"><p><a name="Footnote_20_153" id="Footnote_20_153"></a> +<a href="#FNanchor_20_153"><span class="label">[20]</span></a> +It should be stated that the papers contain a quantity of detail, especially +descriptive of the state of the larvae, which I have not attempted to represent, but +the account here given contains all that seemed essential to an understanding of +the more important features of the account.</p></div> + +<div class="footnote"><p><a name="Footnote_21_154" id="Footnote_21_154"></a> +<a href="#FNanchor_21_154"><span class="label">[21]</span></a> +The first appeared in <i>Natur</i>, 1909-10, Heft 6, p. 94; +and the second, which contains coloured plates of the animals, in the lecture +already referred to, <i>12 Flugschr. d. Deut. Ges. f. Züchtungkunde</i>, +Berlin, 1910, p. 26. In the paper in <i>Mendel Festschrift</i>, 1911, +the subject is continued, but no more is added as to this part of the experiment.</p></div> + +<div class="footnote"><p><a name="Footnote_22_155" id="Footnote_22_155"></a> +<a href="#FNanchor_22_155"><span class="label">[22]</span></a> +E. G. Boulenger, <i>Proc. Zool. Soc.</i>, 1911, p. 323.</p></div> + +<div class="footnote"><p><a name="Footnote_23_156" id="Footnote_23_156"></a> +<a href="#FNanchor_23_156"><span class="label">[23]</span></a> +<i>Mendel Festschrift</i>, 1911, p. 84.</p></div> + +<div class="footnote"><p><a name="Footnote_24_157" id="Footnote_24_157"></a> +<a href="#FNanchor_24_157"><span class="label">[24]</span></a> +<i>12 Flugschrift. Deut. Ges. Züchtungskunde</i>, 1910, Fig. 15, <i>P. Reihe</i>.</p></div> + +<div class="footnote"><p><a name="Footnote_25_158" id="Footnote_25_158"></a> +<a href="#FNanchor_25_158"><span class="label">[25]</span></a> +<i>Mendel Festschrift</i>, 1911, p. 83.</p></div> + +<div class="footnote"><p><a name="Footnote_26_159" id="Footnote_26_159"></a> +<a href="#FNanchor_26_159"><span class="label">[26]</span></a> +Field, 1912, 30 March.</p></div> + +<div class="footnote"><p><a name="Footnote_27_160" id="Footnote_27_160"></a> +<a href="#FNanchor_27_160"><span class="label">[27]</span></a> +<i>Ibid.</i>, 1904, p. 863.</p></div> + +<div class="footnote"><p><a name="Footnote_28_161" id="Footnote_28_161"></a> +<a href="#FNanchor_28_161"><span class="label">[28]</span></a> +<i>Mitth. Naturw. Ver. a. d. Univ. Wien</i>, 1908, p. 53.</p></div> + +<div class="footnote"><p><a name="Footnote_29_162" id="Footnote_29_162"></a> +<a href="#FNanchor_29_162"><span class="label">[29]</span></a> +As to the variations of <i>Lacerta muralis</i> in Western Europe and +North Africa see Boulenger, G. A., <i>Trans. Zool. Soc.</i>, 1905, vol. XVII, p. 351.</p></div> + +<div class="footnote"><p><a name="Footnote_30_163" id="Footnote_30_163"></a> +<a href="#FNanchor_30_163"><span class="label">[30]</span></a> +As to the experiments of Klebs relating to the +transmission of acquired characters, see Appendix.</p></div> + +<hr class="chap" /> +<h3>FOOTNOTES: CHAPTER X.</h3> + +<div class="footnote"><p><a name="Footnote_1_164" id="Footnote_1_164"></a> +<a href="#FNanchor_1_164"><span class="label">[1]</span></a> +Pringsheim, H., <i>Die Variabilität niederer Organismen</i>, Berlin, 1910.</p></div> + +<div class="footnote"><p><a name="Footnote_2_165" id="Footnote_2_165"></a> +<a href="#FNanchor_2_165"><span class="label">[2]</span></a> + F. Wolf, Modifikationen u. Mutationen von Bakterien, +<i>Zts. F. indukt. Abstam. u. Vererbungslehre</i>, II, 1909, p. 90.</p></div> + +<div class="footnote"><p><a name="Footnote_3_166" id="Footnote_3_166"></a> +<a href="#FNanchor_3_166"><span class="label">[3]</span></a> +Winslow, C. E. A. and A. R.,<i>Systematic Relationships of the Coccaceae</i>. +New York. 1909.</p></div> + +<div class="footnote"><p><a name="Footnote_4_167" id="Footnote_4_167"></a> +<a href="#FNanchor_4_167"><span class="label">[4]</span></a> +C. C. Dobell, <i>Jour. Genetics</i>, 1912, II, p. 201, +where full references are given.<br /> +Still more recently the same author has contributed an excellent summary of +the evidence relating to bacteria (<i>ibid.</i>, II. 1913, p. 325).</p></div> + +<div class="footnote"><p><a name="Footnote_5_168" id="Footnote_5_168"></a> +<a href="#FNanchor_5_168"><span class="label">[5]</span></a> +See Woltereck, <i>Verh. d. Deut. Zool. Ges.</i>, 1909, p. 110; +and 1911, p. 142. This is a subject which can only be +properly appreciated on reference to the original papers. +Several complications are involved to which I have not here alluded.</p></div> + +<div class="footnote"><p><a name="Footnote_6_169" id="Footnote_6_169"></a> +<a href="#FNanchor_6_169"><span class="label">[6]</span></a> +<i>Proc. Roy. Soc.</i>, B, Vol. 86, 1913, p. 113.</p></div> + +<div class="footnote"><p><a name="Footnote_7_170" id="Footnote_7_170"></a> +<a href="#FNanchor_7_170"><span class="label">[7]</span></a> +<i>An Investigation of Evolution in Chrysomelid Beetles +of the Genus Leptinotarsa</i>, Carnegie Publications, 1906, No. 48.</p></div> + +<div class="footnote"><p><a name="Footnote_8_171" id="Footnote_8_171"></a> +<a href="#FNanchor_8_171"><span class="label">[8]</span></a> +This is the famous Colorado beetle or potato-bug, which has +caused such serious destruction in potato crops. There seems to be no doubt +that this insect, formerly unknown in the eastern States, made its way east +along the mining trails when the west was opened up.</p></div> + +<div class="footnote"><p><a name="Footnote_9_172" id="Footnote_9_172"></a> +<a href="#FNanchor_9_172"><span class="label">[9]</span></a> +This is indicated in the coloured plate, but I have not found +any explicit statement to this effect in the text, and am not sure +if the absence of pigment was regarded as complete.</p></div> + +<div class="footnote"><p><a name="Footnote_10_173" id="Footnote_10_173"></a> +<a href="#FNanchor_10_173"><span class="label">[10]</span></a> +<i>Biol. Bull.</i>, XVIII, 1910, p. 285.</p></div> + +<div class="footnote"><p><a name="Footnote_11_174" id="Footnote_11_174"></a> +<a href="#FNanchor_11_174"><span class="label">[11]</span></a> +This description does not quite agree with the representation of the larvae +in Pl. 17 of the book <i>Evolution in the Genus Leptinotarsa</i> for there the larva of +<i>undecimlineata</i> is shown as white in the second stage, but yellowish in the third +stage; perhaps there is an error in printing.</p></div> + +<div class="footnote"><p><a name="Footnote_12_175" id="Footnote_12_175"></a> +<a href="#FNanchor_12_175"><span class="label">[12]</span></a> +<i>Biol. Bull.</i>, XX, 1910, p. 67.</p></div> + +<div class="footnote"><p><a name="Footnote_13_176" id="Footnote_13_176"></a> +<a href="#FNanchor_13_176"><span class="label">[13]</span></a> +As to the interrelations of these three forms, Tower states (1906, p. 18) that +<i>angustovittata</i>, which he reared from <i>undecimlineata</i>, is intermediate between it +and <i>signaticollis</i>. Compare Stål, "<i>Monogr. des Chrysomélides</i>," 1862, p. 163; and +Jacoby, <i>Biol. Centr. Amer. Celeopt.</i>, vi, Pt. 1, p. 234, Pl. xiii, fig. 20; Tab. 41, fig. +15; <i>ibid.</i>, Suppl., p. 253. All these forms are evidently very closely related, and +the delimitation of species is quite arbitrary. Jacoby indeed suggests that +<i>undecimlineata</i> may be a variety of <i>decemlineata</i>.</p></div> + +<div class="footnote"><p><a name="Footnote_14_177" id="Footnote_14_177"></a> +<a href="#FNanchor_14_177"><span class="label">[14]</span></a> +Gortner, <i>Amer. Nat.</i>, Dec., 1911, XLV, p. 743.</p></div> + +<div class="footnote"><p><a name="Footnote_15_178" id="Footnote_15_178"></a> +<a href="#FNanchor_15_178"><span class="label">[15]</span></a> +<i>Mutations, Variations, and Relationships of the Oenotheras</i>, +Carnegie Institution Publication No. 81, 1907, pp. 61-64.</p></div> + +<div class="footnote"><p><a name="Footnote_16_179" id="Footnote_16_179"></a> +<a href="#FNanchor_16_179"><span class="label">[16]</span></a> +Macdougal, D. T., "Alterations in Heredity induced by Ovarial Treatments", +<i>Bot. Gaz.</i>, vol. 51, 1911, p. 241.</p></div> + +<div class="footnote"><p><a name="Footnote_17_180" id="Footnote_17_180"></a> +<a href="#FNanchor_17_180"><span class="label">[17]</span></a> +Payne, Fernandus, <i>Biol. Bull.</i>, XVIII, 1910, +p. 188, and <i>ibid.</i>, XXI, 1911, p. 297.</p></div> + +<div class="footnote"><p><a name="Footnote_18_181" id="Footnote_18_181"></a> +<a href="#FNanchor_18_181"><span class="label">[18]</span></a> +See especially, <i>Mutation et Traumatismes</i>, Paris, Felix Alcan, 1908.</p></div> + +<div class="footnote"><p><a name="Footnote_19_182" id="Footnote_19_182"></a> +<a href="#FNanchor_19_182"><span class="label">[19]</span></a> +J. H. Powers, "Morphological Variation and its Causes in +<i>Amblystoma tigrinum</i>." <i>Studies from the Zoological Laboratory. +</i> The University of Nebraska, No. 71, 1907.</p></div> + +<div class="footnote"><p><a name="Footnote_20_183" id="Footnote_20_183"></a> +<a href="#FNanchor_20_183"><span class="label">[20]</span></a> +In connexion with this case I would refer the reader to some remarkable +observations of Dr. T. A. Chapman on various types of larvae which he reared +from the moth <i>Arctia caja</i> (<i>Ent. Rec.</i>, IV, 1893, p. 265, and following parts). +From a single mother he raised a great diversity of forms, some which fed up rapidly +and passed through their development without assuming certain stages, and others +which were, as he called them, "laggards," moulting more times than their brethren +and developing at a much slower rate. It is greatly to be hoped that such a case +may be critically investigated by analytical breeding.</p></div> + +<hr class="chap" /> +<h3>FOOTNOTES: CHAPTER X.</h3> + +<div class="footnote"><p><a name="Footnote_1_184" id="Footnote_1_184"></a> +<a href="#FNanchor_1_184"><span class="label">[1]</span></a> +Buffon, <i>Hist. Nat.</i>, Oiseaux, 1780, VII, p. 3.</p></div> + +<div class="footnote"><p><a name="Footnote_2_185" id="Footnote_2_185"></a> +<a href="#FNanchor_2_185"><span class="label">[2]</span></a> +Ibid., VIII, p. 115.</p></div> + +<div class="footnote"><p><a name="Footnote_3_186" id="Footnote_3_186"></a> +<a href="#FNanchor_3_186"><span class="label">[3]</span></a> +Keeble, <i>Jour. Gen.</i>, 1912, II, p. 173.</p></div> + +<div class="footnote"><p><a name="Footnote_4_187" id="Footnote_4_187"></a> +<a href="#FNanchor_4_187"><span class="label">[4]</span></a> +<i>Animals and Plants</i>, ed. 1, 1868, II, pp. 180-5.</p></div> + +<div class="footnote"><p><a name="Footnote_5_188" id="Footnote_5_188"></a> +<a href="#FNanchor_5_188"><span class="label">[5]</span></a> +<i>Animals and Plants</i>, ed. 1, 1868, II, p. 165.</p></div> + +<div class="footnote"><p><a name="Footnote_6_189" id="Footnote_6_189"></a> +<a href="#FNanchor_6_189"><span class="label">[6]</span></a> +<i>Species and Varieties</i>, 1905, p. 471.</p></div> + +<div class="footnote"><p><a name="Footnote_7_190" id="Footnote_7_190"></a> +<a href="#FNanchor_7_190"><span class="label">[7]</span></a> +Correns, <i>Festschr. med.-nat. Ges. zur 84 Versamml. Deutsch. +Naturf. u. Aertze. Münster i. W.</i>, 1912.</p></div> + +<div class="footnote"><p><a name="Footnote_8_191" id="Footnote_8_191"></a> +<a href="#FNanchor_8_191"><span class="label">[8]</span></a> +This is a case of a somewhat different order and I mention it partly for that +reason as an illustration of the complexity which such negative instances may +present. The difficulty is that though the buffalo and the zebu can breed together, +the foetus is too large to be born alive. (See Ackermann <i>Ber. d. Ver. f. Naturk.</i>, +Kassel, 1898, p. 69. Prof. S. Nathusius, of Halle, who has great experience in +crossing Bovidae, tells me that he has always failed to cross the buffalo with +other species.)</p></div> + +<div class="footnote"><p><a name="Footnote_9_192" id="Footnote_9_192"></a> +<a href="#FNanchor_9_192"><span class="label">[9]</span></a> +In a paper to be published in the Report of the Genetic Conference, Paris, +1911, Bellair states that he obtained some partially fertile hybrids in the cross +<i>N. sylvestris</i> × <i>tabacum</i>. As to the various degrees of sterility in +hybrids between <i>Nicotiana</i> species +see Lock, R. H., <i>Ann. Roy. Bot. Gardens</i>. Peradeniya, IV, 1909, p. 195.</p></div> + +<div class="footnote"><p><a name="Footnote_10_193" id="Footnote_10_193"></a> +<a href="#FNanchor_10_193"><span class="label">[10]</span></a> +<i>Beitrage zur Biol. der Pflanzen.</i>, X, 1911, p. 379.</p></div> + +<div class="footnote"><p><a name="Footnote_11_194" id="Footnote_11_194"></a> +<a href="#FNanchor_11_194"><span class="label">[11]</span></a> +One very peculiar feature was observed, namely, that all the new forms in F<sub>2</sub> +which were bred from came true. As I understand, this statement applied to five +such new types, and they were represented by 76 individuals in F<sub>3</sub>, but further +details on this point are desirable. Another curious fact was observed, namely +that one of the F<sub>1</sub> forms (<i>cochleata</i> × <i>radiata</i>) when fertilised by +<i>cochleata</i> gave a highly polymorphic family, but fertilised by <i>radiata</i> +the resulting offspring were almost uniform.</p></div> + +<div class="footnote"><p><a name="Footnote_12_195" id="Footnote_12_195"></a> +<a href="#FNanchor_12_195"><span class="label">[12]</span></a> +I also had a few F<sub>1</sub> seeds given me by Mr. R. H. Lock.</p></div> + +<div class="footnote"><p><a name="Footnote_13_196" id="Footnote_13_196"></a> +<a href="#FNanchor_13_196"><span class="label">[13]</span></a> +In a paper about to appear in <i>Jour. Linn. Soc.</i> Mr. A. W. Sutton identifies +this Palestine pea as <i>Pisum humile</i> of Boissier and Noé.</p></div> + +<div class="footnote"><p><a name="Footnote_14_197" id="Footnote_14_197"></a> +<a href="#FNanchor_14_197"><span class="label">[14]</span></a> +Lloyd, R. E., <i>The Growth of Groups in the Animal Kingdom</i>, London, 1912.</p></div> + +</div> + +<div>*** END OF THE PROJECT GUTENBERG EBOOK 44582 ***</div> +</body> +</html> diff --git a/44582-h/images/i_004.jpg b/44582-h/images/i_004.jpg Binary files differnew file mode 100644 index 0000000..907fad6 --- /dev/null +++ b/44582-h/images/i_004.jpg diff --git a/44582-h/images/i_050.jpg b/44582-h/images/i_050.jpg Binary files differnew file mode 100644 index 0000000..9345647 --- /dev/null +++ b/44582-h/images/i_050.jpg diff --git a/44582-h/images/i_052.jpg b/44582-h/images/i_052.jpg Binary files differnew file mode 100644 index 0000000..3e49808 --- /dev/null +++ b/44582-h/images/i_052.jpg diff --git a/44582-h/images/i_060.jpg b/44582-h/images/i_060.jpg Binary files differnew file mode 100644 index 0000000..0b0c200 --- /dev/null +++ b/44582-h/images/i_060.jpg diff --git a/44582-h/images/i_061.jpg b/44582-h/images/i_061.jpg Binary files differnew file mode 100644 index 0000000..fbe510f --- /dev/null +++ b/44582-h/images/i_061.jpg diff --git a/44582-h/images/i_064.jpg b/44582-h/images/i_064.jpg Binary files differnew file mode 100644 index 0000000..a5f445f --- /dev/null +++ b/44582-h/images/i_064.jpg diff --git a/44582-h/images/i_065.jpg b/44582-h/images/i_065.jpg Binary files differnew file mode 100644 index 0000000..c3dd369 --- /dev/null +++ b/44582-h/images/i_065.jpg diff --git a/44582-h/images/i_074.jpg b/44582-h/images/i_074.jpg Binary files differnew file mode 100644 index 0000000..ee9a817 --- /dev/null +++ b/44582-h/images/i_074.jpg diff --git a/44582-h/images/i_077.jpg b/44582-h/images/i_077.jpg Binary files differnew file mode 100644 index 0000000..b7f9931 --- /dev/null +++ b/44582-h/images/i_077.jpg diff --git a/44582-h/images/i_084.jpg b/44582-h/images/i_084.jpg Binary files differnew file mode 100644 index 0000000..2862ec7 --- /dev/null +++ b/44582-h/images/i_084.jpg diff --git a/44582-h/images/i_088.jpg b/44582-h/images/i_088.jpg Binary files differnew file mode 100644 index 0000000..3202ef2 --- /dev/null +++ b/44582-h/images/i_088.jpg diff --git a/44582-h/images/i_091.jpg b/44582-h/images/i_091.jpg Binary files differnew file mode 100644 index 0000000..639357f --- /dev/null +++ b/44582-h/images/i_091.jpg diff --git a/44582-h/images/i_139.jpg b/44582-h/images/i_139.jpg Binary files differnew file mode 100644 index 0000000..2671954 --- /dev/null +++ b/44582-h/images/i_139.jpg diff --git a/44582-h/images/i_175.jpg b/44582-h/images/i_175.jpg Binary files differnew file mode 100644 index 0000000..5936bad --- /dev/null +++ b/44582-h/images/i_175.jpg diff --git a/44582-h/images/titlepage.jpg b/44582-h/images/titlepage.jpg Binary files differnew file mode 100644 index 0000000..ef2c515 --- /dev/null +++ b/44582-h/images/titlepage.jpg |
