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vertical-align: middle } + + hr.pg { width: 100%; + margin-top: 3em; + margin-bottom: 0em; + margin-left: auto; + margin-right: auto; + height: 4px; + border-width: 4px 0 0 0; /* remove all borders except the top one */ + border-style: solid; + border-color: #000000; + clear: both; } + </style> +</head> +<body> +<h1>The Project Gutenberg eBook, Mendelism, by Reginald Crundall Punnett</h1> +<pre> +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at <a href = "http://www.gutenberg.org">www.gutenberg.org</a></pre> +<p>Title: Mendelism</p> +<p> Third Edition</p> +<p>Author: Reginald Crundall Punnett</p> +<p>Release Date: May 18, 2009 [eBook #28775]</p> +<p>Language: English</p> +<p>Character set encoding: ISO-8859-1</p> +<p>***START OF THE PROJECT GUTENBERG EBOOK MENDELISM***</p> +<p> </p> +<h3>E-text prepared by Paul Hollander, Malcolm Farmer, Keith Edkins,<br /> + and the Project Gutenberg Online Distributed Proofreading Team<br /> + (http://www.pgdp.net)</h3> +<p> </p> +<table border="0" cellpadding="10" style="background-color: #ccccff;"> +<tr> +<td style="width:25%; vertical-align:top"> +Transcriber's note: +</td> +<td> + + <p>A few typographical errors have been corrected. They appear in the + text <span class="correction" title="explanation will pop up">like + this</span>, and the explanation will appear when the mouse pointer is + moved over the marked passage.</p> + + <p>Fig. 8 has been re-mastered to match the text (the Black boxes were + shown as Albino and the heterozygous Albinos as Black).</p> + +</td> +</tr> +</table> +<p> </p> +<hr class="pg" /> +<p> </p> + + <div class="figcenter" style="width:60%;"> + <a href="images/001.jpg"><img style="width:100%" src="images/001t.jpg" + alt="Gregor Mendel." title="Gregor Mendel." /></a> + </div> +<h1>MENDELISM</h1> + + <p> </p> + +<p class="cenhead">BY</p> + +<h2>R. C. PUNNETT</h2> + +<p class="cenhead">FELLOW OF GONVILLE AND CAIUS COLLEGE</p> + +<p class="cenhead">PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF CAMBRIDGE</p> + + <p> </p> + +<h3><i>THIRD EDITION<br /> +ENTIRELY REWRITTEN AND MUCH ENLARGED</i></h3> + +<p> </p> +<p> </p> + +<h3>New York</h3> + +<h3>THE MACMILLAN COMPANY</h3> + +<h3>1911</h3> + +<p class="cenhead"><i>All rights reserved</i></p> + +<hr class="short" /> + +<p class="cenhead"><span class="sc">Copyright, 1911,</span></p> + +<p class="cenhead"><span class="sc">By</span> THE MACMILLAN COMPANY.</p> + +<hr class="short" /> + +<p class="cenhead">Set up and electrotyped. Published May, 1911.</p> + + <p> </p> + +<p class="cenhead">Norwood Press<br /> +J. S. Cushing Co.—Berwick & Smith Co.<br /> +Norwood, Mass., U.S.A.</p> + +<hr class="full" /> + +<p><!-- Page v --><span class="pagenum"><a name="pagev"></a>{v}</span></p> + +<h3>PREFACE</h3> + + <p>A few years ago I published a short sketch of Mendel's discovery in + heredity, and of some of the recent experiments which had arisen from it. + Since then progress in these studies has been rapid, and the present + account, though bearing the same title, has been completely rewritten. A + number of illustrations have been added, and here I may acknowledge my + indebtedness to Miss Wheldale for the two coloured plates of sweet peas, + to the Hon. Walter Rothschild for the butterflies figured on Plate VI., + to Professor Wood for photographs of sheep, and to Dr. Drinkwater for the + figures of human hands. To my former publishers also, Messrs. Bowes and + Bowes, I wish to express my thanks for the courtesy with which they + acquiesced in my desire that the present edition should be published + elsewhere.</p> + + <p>As the book is intended to appeal to a wide audience, I have not + attempted to give more experimental instances than were necessary to + illustrate the story, nor have I burdened it with bibliographical + reference. The reader who desires further information may be referred to + Mr. Bateson's indispensable Volume on <i>Mendel's <!-- Page vi --><span + class="pagenum"><a name="pagevi"></a>{vi}</span>Principles of + Heredity</i> (Cambridge, 1909), where a full account of these matters is + readily accessible. Neither have I alluded to recent cytological work in + so far as it may bear upon our problems. Many of the facts connected with + the division of the chromosomes are striking and suggestive, but while so + much difference of opinion exists as to their interpretation they are + hardly suited for popular treatment.</p> + + <p>In choosing typical examples to illustrate the growth of our ideas it + was natural that I should give the preference to those with which I was + most familiar. For this reason the book is in some measure a record of + the work accomplished by the Cambridge School of Genetics, and it is not + unfair to say that under the leadership of William Bateson the + contributions of this school have been second to none. But it should not + be forgotten that workers in other European countries, and especially in + America, have amassed a large and valuable body of evidence with which it + is impossible to deal in a small volume of this scope.</p> + + <p>It is not long since the English language was enriched by two new + words—Eugenics and Genetics—and their similarity of origin + has sometimes led to confusion between them on the part of those who are + innocent of Greek. Genetics is the term applied to the experimental study + of heredity and variation in animals and plants, and the main concern of + its students is the establishing of law and order among the phenomena + <!-- Page vii --><span class="pagenum"><a + name="pagevii"></a>{vii}</span>there encountered. Eugenics, on the other + hand, deals with the improvement of the human race under existing + conditions of law and sentiment. The Eugenist has to take into account + the religious and social beliefs and prejudices of mankind. Other issues + are involved besides the purely biological one, though as time goes on it + is coming to be more clearly recognised that the Eugenic ideal is sharply + circumscribed by the facts of heredity and variation, and by the laws + which govern the transmission of qualities in living things. What these + facts, what these laws are, in so far as we at present know them, I have + endeavoured to indicate in the following pages; for I feel convinced that + if the Eugenist is to achieve anything solid it is upon them that he must + primarily build. Little enough material, it is true, exists at present, + but that we now see to be largely a question of time and means. Whatever + be the outcome, whatever the form of the structure which is eventually to + emerge, we owe it first of all to Mendel that the foundations can be well + and truly laid.</p> + + <p class="author">R. C. P.</p> + + <p class="address"><span class="sc">Cambridge</span>, <i>March, 1911</i>.</p> + +<hr class="full" /> + +<p><!-- Page ix --><span class="pagenum"><a name="pageix"></a>{ix}</span></p> + +<h3>CONTENTS</h3> + +<table class="nobctr" summary="Contents." title="Contents."> +<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER I</td></tr> +<tr><td class="spacsingle"> </td><td class="spacsingle" align="right"> <span class="scac">PAGE</span></td></tr> + +<tr><td class="spacsingle"> <span class="sc">The Problem</span> </td><td class="spacsingle" align="right"> <a href="#page1">1</a></td></tr> + +<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER II</td></tr> + +<tr><td class="spacsingle"> <span class="sc">Historical</span> </td><td class="spacsingle" align="right"> <a href="#page8">8</a></td></tr> + +<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER III</td></tr> + +<tr><td class="spacsingle"> <span class="sc">Mendel's Work</span> </td><td class="spacsingle" align="right"> <a href="#page17">17</a></td></tr> + +<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER IV</td></tr> + +<tr><td class="spacsingle"> <span class="sc">The Presence and Absence Theory</span> </td><td class="spacsingle" align="right"> <a href="#page29">29</a></td></tr> + +<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER V</td></tr> + +<tr><td class="spacsingle"> <span class="sc">Interaction of Factors</span> </td><td class="spacsingle" align="right"> <a href="#page42">42</a></td></tr> + +<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER VI</td></tr> + +<tr><td class="spacsingle"> <span class="sc">Reversion</span> </td><td class="spacsingle" align="right"> <a href="#page59">59</a></td></tr> + +<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER VII</td></tr> + +<tr><td class="spacsingle"> <span class="sc">Dominance</span> </td><td class="spacsingle" align="right"> <a href="#page68">68</a></td></tr> + +<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> +<!-- Page x --><span class="pagenum"><a name="pagex"></a>{x}</span> +CHAPTER VIII</td></tr> + +<tr><td class="spacsingle"> <span class="sc">Wild Forms and Domestic Varieties</span> </td><td class="spacsingle" align="right"> <a href="#page79">79</a></td></tr> + +<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER IX</td></tr> + +<tr><td class="spacsingle"> <span class="sc">Repulsion and Coupling of Factors</span> </td><td class="spacsingle" align="right"> <a href="#page88">88</a></td></tr> + +<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER X</td></tr> + +<tr><td class="spacsingle"> <span class="sc">Sex</span> </td><td class="spacsingle" align="right"> <a href="#page99">99</a></td></tr> + +<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER XI</td></tr> + +<tr><td class="spacsingle"> <span class="sc">Sex</span> (<i>continued</i>) </td><td class="spacsingle" align="right"> <a href="#page115">115</a></td></tr> + +<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER XII</td></tr> + +<tr><td class="spacsingle"> <span class="sc">Intermediates</span> </td><td class="spacsingle" align="right"> <a href="#page125">125</a></td></tr> + +<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER XIII</td></tr> + +<tr><td class="spacsingle"> <span class="sc">Variation and Evolution</span> </td><td class="spacsingle" align="right"> <a href="#page135">135</a></td></tr> + +<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER XIV</td></tr> + +<tr><td class="spacsingle"> <span class="sc">Economical</span> </td><td class="spacsingle" align="right"> <a href="#page153">153</a></td></tr> + +<tr><td colspan="2" align="center" style="padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER XV</td></tr> + +<tr><td class="spacsingle"> <span class="sc">Man</span> </td><td class="spacsingle" align="right"> <a href="#page170">170</a></td></tr> +<tr><td class="spacsingle"> </td></tr> +<tr><td class="spacsingle"> APPENDIX </td><td class="spacsingle" align="right"> <a href="#page187">187</a></td></tr> +<tr><td class="spacsingle"> </td></tr> +<tr><td class="spacsingle"> INDEX </td><td class="spacsingle" align="right"> <a href="#page191">191</a></td></tr> +</table> + + <p> </p> + +<hr class="full" /> + +<p><!-- Page xi --><span class="pagenum"><a name="pagexi"></a>{xi}</span></p> + +<h3>ILLUSTRATIONS</h3> + +<table class="nobctr" summary="Illustrations." title="Illustrations."> +<tr><td colspan="4" align="center">PLATES</td></tr> + +<tr><td class="qspcsingle" align="right"> <span class="scac">PLATE</span> </td><td colspan="2"> </td><td class="nspac" align="right"> <span class="scac">PAGE</span></td></tr> + +<tr><td class="qspcsingle" align="right"> </td><td class="qspcsingle"> Gregor Mendel </td><td class="nspac" colspan="2" align="right"><i>Frontispiece</i></td></tr> + +<tr><td class="qspcsingle" align="right"> I. </td><td class="qspcsingle"> Rabbits </td><td class="nspac" align="center"> <i>To face</i> </td><td class="nspac" align="right"> <a href="#page60">60</a></td></tr> + +<tr><td class="qspcsingle" align="right"> II. </td><td class="qspcsingle"> Sweet Peas </td><td class="nspac" align="center"> " </td><td class="nspac" align="right"> <a href="#page64">64</a></td></tr> + +<tr><td class="qspcsingle" align="right"> III. </td><td class="qspcsingle"> Sheep </td><td class="nspac" align="center"> " </td><td class="nspac" align="right"> <a href="#page78">78</a></td></tr> + +<tr><td class="qspcsingle" align="right"> IV. </td><td class="qspcsingle"> Sweet Peas </td><td class="nspac" align="center"> " </td><td class="nspac" align="right"> <a href="#page80">80</a></td></tr> + +<tr><td class="qspcsingle" align="right"> V. </td><td class="qspcsingle"> Fowls </td><td class="nspac" align="center"> " </td><td class="nspac" align="right"> <a href="#page107">107</a></td></tr> + +<tr><td class="qspcsingle" align="right"> VI. </td><td class="qspcsingle"> Butterflies </td><td class="nspac" align="center"> " </td><td class="nspac" align="right"> <a href="#page146">146</a></td></tr> + +<tr><td colspan="4" align="center"> <br />FIGURES IN TEXT</td></tr> + +<tr><td class="qspcsingle" align="right"> <span class="scac">FIG.</span></td></tr> + +<tr><td class="qspcsingle" align="right"> 1. </td><td class="qspcsingle" colspan="2"> Scheme of Inheritance in simple Mendelian Case </td><td class="nspac" align="right"> <a href="#page21">21</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 2. </td><td class="qspcsingle" colspan="2"> Feathers of Silky and Common Fowl </td><td class="nspac" align="right"> <a href="#page30">30</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 3. </td><td class="qspcsingle" colspan="2"> Single and Double Primulas </td><td class="nspac" align="right"> <a href="#page31">31</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 4. </td><td class="qspcsingle" colspan="2"> Fowls' Combs </td><td class="nspac" align="right"> <a href="#page32">32</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 5. </td><td class="qspcsingle" colspan="2"> Diagram of Inheritance of Fowls' Combs </td><td class="nspac" align="right"> <a href="#page37">37</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 6. </td><td class="qspcsingle" colspan="2"> Fowls' Combs </td><td class="nspac" align="right"> <a href="#page39">39</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 7. </td><td class="qspcsingle" colspan="2"> Diagram of F<sub>2</sub> Generation resulting from Cross between two White Sweet Peas </td><td class="nspac" align="right"> <a href="#page46">46</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 8. </td><td class="qspcsingle" colspan="2"> Diagram illustrating 9 : 3 : 4 Ratio in Mice </td><td class="nspac" align="right"> <a href="#page52">52</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 9. </td><td class="qspcsingle" colspan="2"> Sections of Primulas </td><td class="nspac" align="right"> <a href="#page55">55</a></td></tr> + +<tr><td class="qspcsingle" align="right"> +<!-- Page xii --><span class="pagenum"><a name="pagexii"></a>{xii}</span> +10. </td><td class="qspcsingle" colspan="2"> Small and Large-eyed Primulas </td><td class="nspac" align="right"> <a href="#page55">55</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 11. </td><td class="qspcsingle" colspan="2"> Diagram illustrating Reversion in Pigeons </td><td class="nspac" align="right"> <a href="#page67">67</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 12. </td><td class="qspcsingle" colspan="2"> <i>Primula sinensis</i> × <i>Primula stellata</i> </td><td class="nspac" align="right"> <a href="#page68">68</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 13. </td><td class="qspcsingle" colspan="2"> Diagram illustrating Cross between Dominant and Recessive White Fowls </td><td class="nspac" align="right"> <a href="#page72">72</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 14. </td><td class="qspcsingle" colspan="2"> Bearded and Beardless Wheat </td><td class="nspac" align="right"> <a href="#page75">75</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 15. </td><td class="qspcsingle" colspan="2"> Feet of Fowls </td><td class="nspac" align="right"> <a href="#page76">76</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 16. </td><td class="qspcsingle" colspan="2"> Scheme of Inheritance of Horns in Sheep </td><td class="nspac" align="right"> <a href="#page76">76</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 17. </td><td class="qspcsingle" colspan="2"> <i>Abraxas grossulariata</i> and var. <i>lacticolor</i> </td><td class="nspac" align="right"> <a href="#page99">99</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 18. </td><td class="qspcsingle" colspan="2"> Scheme of Inheritance in <i>Abraxas</i> </td><td class="nspac" align="right"> <a href="#page102">102</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 19. </td><td class="qspcsingle" colspan="2"> Scheme of Inheritance of Silky Hen × Brown Leghorn Cock </td><td class="nspac" align="right"> <a href="#page105">105</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 20. </td><td class="qspcsingle" colspan="2"> Scheme of Inheritance of Brown Leghorn Hen × Silky Cock </td><td class="nspac" align="right"> <a href="#page106">106</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 21. </td><td class="qspcsingle" colspan="2"> Scheme of F<sub>1</sub> (ex Brown Leghorn × Silky Cock) crossed with pure Brown Leghorn </td><td class="nspac" align="right"> <a href="#page107">107</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 22. </td><td class="qspcsingle" colspan="2"> Scheme for Silky Hen × Brown Leghorn Cock </td><td class="nspac" align="right"> <a href="#page108">108</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 23. </td><td class="qspcsingle" colspan="2"> Scheme for Brown Leghorn Hen × Silky Cock </td><td class="nspac" align="right"> <a href="#page109">109</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 24. </td><td class="qspcsingle" colspan="2"> Diagram illustrating Nature of Offspring from Brown Leghorn Hen × F<sub>1</sub> Cock </td><td class="nspac" align="right"> <a href="#page111">111</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 25. </td><td class="qspcsingle" colspan="2"> Scheme to illustrate Heterozygous Nature of Brown Leghorn Hen </td><td class="nspac" align="right"> <a href="#page111">111</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 26. </td><td class="qspcsingle" colspan="2"> Scheme of Inheritance of Colour-blindness </td><td class="nspac" align="right"> <a href="#page117">117</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 27. </td><td class="qspcsingle" colspan="2"> Single and Double Stocks </td><td class="nspac" align="right"> <a href="#page122">122</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 28. </td><td class="qspcsingle" colspan="2"> F<sub>2</sub> Generation ex Silky Hen × Brown Leghorn Cock </td><td class="nspac" align="right"> <a href="#page127">127</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 29. </td><td class="qspcsingle" colspan="2"> Pedigree of Eurasian Family </td><td class="nspac" align="right"> <a href="#page131">131</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 30. </td><td class="qspcsingle" colspan="2"> Curve illustrating Influence of Selection </td><td class="nspac" align="right"> <a href="#page159">159</a></td></tr> + +<tr><td class="qspcsingle" align="right"> +<!-- Page xiii --><span class="pagenum"><a name="pagexiii"></a>{xiii}</span> +31. </td><td class="qspcsingle" colspan="2"> Curve illustrating Conception of pure Lines </td><td class="nspac" align="right"> <a href="#page162">162</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 32. </td><td class="qspcsingle" colspan="2"> Brachydactylous and Normal Hands </td><td class="nspac" align="right"> <a href="#page170">170</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 33. </td><td class="qspcsingle" colspan="2"> Radiograph of Brachydactylous Hand </td><td class="nspac" align="right"> <a href="#page170">170</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 34. </td><td class="qspcsingle" colspan="2"> Pedigree of Brachydactylous Family </td><td class="nspac" align="right"> <a href="#page173">173</a></td></tr> + +<tr><td class="qspcsingle" align="right"> 35. </td><td class="qspcsingle" colspan="2"> Pedigree of Hæmophilic Family </td><td class="nspac" align="right"> <a href="#page175">175</a></td></tr> +</table> + + <p> </p> + +<hr class="full" /> + +<p><!-- Page xiv --><span class="pagenum"><a name="pagexiv"></a>{xiv}</span></p> + +<blockquote class="b1n"> + + <p>For although it be a more new and difficult way, to find out the + nature of things, by the things themselves; then by reading of Books, to + take our knowledge upon trust from the opinions of Philosophers: yet must + it needs be confessed, that the former is much more open, and lesse + fraudulent, especially in the Secrets relating to <i>Natural + Philosophy</i>.</p> + + <div class="poem"> + <div class="stanza"> + <p><span class="sc">William Harvey</span>,</p> + <p><i>Anatomical Exercitations</i>, 1653.</p> + </div> + </div> + +</blockquote> + +<hr class="full" /> + +<p><!-- Page 1 --><span class="pagenum"><a name="page1"></a>{1}</span></p> + +<h3>CHAPTER I</h3> + +<p class="cenhead">THE PROBLEM</p> + + <p>A curious thing in the history of human thought so far as literature + reveals it to us is the strange lack of interest shown in one of the most + interesting of all human relationships. Few if any of the more primitive + peoples seem to have attempted to define the part played by either parent + in the formation of the offspring, or to have assigned peculiar powers of + transmission to them, even in the vaguest way. For ages man must have + been more or less consciously improving his domesticated races of animals + and plants, yet it is not until the time of Aristotle that we have clear + evidence of any hypothesis to account for these phenomena of heredity. + The production of offspring by man was then held to be similar to the + production of a crop from seed. The seed came from the man, the woman + provided the soil. This remained the generally accepted view for many + centuries, and it was not until the recognition of woman as more than a + passive agent that the physical basis of heredity became established. + That recognition was effected by the microscope, for only with its advent + was actual <!-- Page 2 --><span class="pagenum"><a + name="page2"></a>{2}</span>observation of the minute sexual cells made + possible. After more than a hundred years of conflict lasting until the + end of the eighteenth century, scientific men settled down to the view + that each of the sexes makes a definite material contribution to the + offspring produced by their joint efforts. Among animals the female + contributes the ovum and the male the spermatozoon; among plants the + corresponding cells are the ovules and pollen grains.</p> + + <p>As a general rule it may be stated that the reproductive cells + produced by the female are relatively large and without the power of + independent movement. In addition to the actual living substance which is + to take part in the formation of a new individual, the ova are more or + less heavily loaded with the yolk substance that is to provide for the + nutrition of the developing embryo during the early stages of its + existence. The size of the ova varies enormously in different animals. In + birds and reptiles where the contents of the egg form the sole resources + of the developing young they are very large in comparison with the size + of the animal which lays them. In mammals, on the other hand, where the + young are parasitic upon the mother during the earlier stages of their + growth, the eggs are minute and only contain the small amount of yolk + that enables them to reach the stage at which they develop the processes + for attaching themselves to the wall of the maternal uterus. But whatever + the differences in the size and appearance of the ova produced by + different <!-- Page 3 --><span class="pagenum"><a + name="page3"></a>{3}</span>animals, they are all comparable in that each + is a distinct and separate sexual cell which, as a rule, is unable to + develop into a new individual of its species unless it is fertilised by + union with a sexual cell produced by the male.</p> + + <p>The male sexual cells are always of microscopic size and are produced + in the generative gland or testis in exceedingly large numbers. In + addition to their minuter size they differ from the ova in their power of + active movement. Animals present various mechanisms by which the sexual + elements may be brought into juxtaposition, but in all cases some + distance must be traversed in a fluid or semifluid medium (frequently + within the body of the female parent) before the necessary fusion can + occur. To accomplish this latter end of its journey the spermatozoon is + endowed with some form of motile apparatus, and this frequently takes the + form of a long flagellum, or whip-like process, by the lashing of which + the little creature propels itself much as a tadpole with its tail.</p> + + <p>In plants as in animals the female cells or ovules are larger than the + pollen grains, though the disparity in size is not nearly so marked. + Still they are always relatively minute cells since the circumstances of + their development as parasites upon the mother plant render it + unnecessary for them to possess any great supply of food yolk. The ovules + are found surrounded by maternal tissue in the ovary, but through the + stigma and down the pistil a <!-- Page 4 --><span class="pagenum"><a + name="page4"></a>{4}</span>potential passage is left for the male cell. + The majority of flowers are hermaphrodite, and in many cases they are + also self-fertilising. The anthers burst and the contained pollen grains + are then shed upon the stigma. When this happens, the pollen cell slips + through a little hole in its coat and bores its way down the pistil to + reach an ovule in the ovary. Complete fusion occurs, and the minute + embryo of a new plant immediately results. But for some time it is + incapable of leading a separate existence, and, like the embryo mammal, + it lives as a parasite upon its parent. By the parent it is provided with + a protective wrapping, the seed coat, and beneath this the little embryo + swells until it reaches a certain size, when as a ripe seed it severs its + connection with the maternal organism. It is important to realise that + the seed of a plant is not a sexual cell but a young individual which, + except for the coat that it wears, belongs entirely to the next + generation. It is with annual plants in some respects as with many + butterflies. During one summer they are initiated by the union of two + sexual cells and pass through certain stages of larval + development—the butterfly as a caterpillar, the plant as a parasite + upon its mother. As the summer draws to a close each passes into a + resting-stage against the winter cold—the butterfly as a pupa and + the plant as a seed, with the difference that while the caterpillar + provides its own coat, that of the plant is provided by its mother. With + the advent of spring both butterfly and <!-- Page 5 --><span + class="pagenum"><a name="page5"></a>{5}</span>plant emerge, become + mature, and themselves ripen germ cells which give rise to a new + generation.</p> + + <p>Whatever the details of development, one cardinal fact is clear. + Except for the relatively rare instances of parthenogenesis a new + individual, whether plant or animal, arises as the joint product of two + sexual cells derived from individuals of different sexes. Such sexual + cells, whether ovules or ova, spermatozoa or pollen grains, are known by + the general term of <b>gametes</b>, or marrying cells, and the individual + formed by the fusion or yoking together of two gametes is spoken of as a + <b>zygote</b>. Since a zygote arises from the yoking together of two + separate gametes, the individual so formed must be regarded throughout + its life as a double structure in which the components brought in by each + of the gametes remain intimately fused in a form of partnership. But when + the zygote in its turn comes to form gametes, the partnership is broken + and the process is reversed. The component parts of the dual structure + are resolved, with the formation of a set of single structures, the + gametes.</p> + + <p>The life cycle of a species from among the higher plants or animals + may be regarded as falling into three periods: (1) a period of isolation + in the form of gametes, each a living unit incapable of further + development without intimate association with another produced by the + opposite sex; (2) a period of association in which two gametes become + yoked together into a zygote and react upon one <!-- Page 6 --><span + class="pagenum"><a name="page6"></a>{6}</span>another to give rise by a + process of cell division to what we ordinarily term an individual with + all its various attributes and properties; and (3) a period of + dissociation when the single structured gametes separate out from that + portion of the double structured zygote which constitutes its generative + gland. What is the relation between gamete and zygote, between zygote and + gamete? how are the properties of the zygote represented in the gamete, + and in what manner are they distributed from the one to the + other?—these are questions which serve to indicate the nature of + the problem underlying the process of heredity.</p> + + <p>Owing to their peculiar power of growth and the relatively large size + to which they attain, many of the properties of zygotes are appreciable + by observation. The colour of an animal or of a flower, the shape of a + seed, or the pattern on the wings of a moth are all zygotic properties, + and all capable of direct estimation. It is otherwise with the properties + of gametes. While the difference between a black and a white fowl is + sufficiently obvious, no one by inspection can tell the difference + between the egg that will hatch into a black and that which will hatch + into a white. Nor from a mass of pollen grains can any one to-day pick + out those that will produce white from those that will produce coloured + flowers. Nevertheless, we know that in spite of apparent similarity there + must exist fundamental differences among the gametes, even <!-- Page 7 + --><span class="pagenum"><a name="page7"></a>{7}</span>among those that + spring from the same individual. At present our only way of appreciating + those differences is to observe the properties of the zygotes which they + form. And as it takes two gametes to form a zygote, we are in the + position of attempting to decide the properties of two unknowns from one + known. Fortunately the problem is not entirely one of simple mathematics. + It can be attacked by the experimental method, and with what measure of + success will appear in the following pages.</p> + +<hr class="full" /> + +<p><!-- Page 8 --><span class="pagenum"><a name="page8"></a>{8}</span></p> + +<h3>CHAPTER II</h3> + +<p class="cenhead">HISTORICAL</p> + + <p>To Gregor Mendel, monk and abbot, belongs the credit of founding the + modern science of heredity. Through him there was brought into these + problems an entirely new idea, an entirely fresh conception of the nature + of living things. Born in 1822 of Austro-Silesian parentage, he early + entered the monastery of Brünn, and there in the seclusion of the + cloister garden he carried out with the common pea the series of + experiments which has since become so famous. In 1865 after eight years' + work he published the results of his experiments in the <i>Proceedings of + the Natural History Society of Brünn</i>, in a brief paper of some forty + pages. But brief as it is the importance of the results and the lucidity + of the exposition will always give it high rank among the classics of + biological literature. For thirty-five years Mendel's paper remained + unknown, and it was not until 1900 that it was simultaneously discovered + by several distinguished botanists. The causes of this curious neglect + are not altogether without interest. Hybridisation experiments before + Mendel there had been in plenty. The classificatory work of <!-- Page 9 + --><span class="pagenum"><a name="page9"></a>{9}</span>Linnaeus in the + latter half of the eighteenth century had given a definite significance + to the word species, and scientific men began to turn their attention to + attempting to discover how species were related to one another. And one + obvious way of attacking the problem was to cross different species + together and see what happened. This was largely done during the earlier + half of the nineteenth century, though such work was almost entirely + confined to the botanists. Apart from the fact that plants lend + themselves to hybridisation work more readily than animals, there was + probably another reason why zoologists neglected this form of + investigation. The field of zoology is a wider one than that of botany, + presenting a far greater variety of type and structure. Partly owing to + their importance in the study of medicine, and partly owing to their + smaller numbers, the anatomy of the vegetable was far better known than + that of the animal kingdom. It is, therefore, not surprising that the + earlier part of the nineteenth century found the zoologists, under the + influence of Cuvier and his pupils, devoting their entire energies to + describing the anatomy of the new forms of animal life which careful + search at home and fresh voyages of discovery abroad were continually + bringing to light. During this period the zoologist had little + inclination or inducement to carry on those investigations in + hybridisation which were occupying the attention of some botanists. Nor + did the efforts of the botanists afford much <!-- Page 10 --><span + class="pagenum"><a name="page10"></a>{10}</span>encouragement to such + work, for in spite of the labour devoted to these experiments, the + results offered but a confused tangle of facts, contributing in no + apparent way to the solution of the problem for which they had been + undertaken. After half a century of experimental hybridisation the + determination of the relation of species and varieties to one another + seemed as remote as ever. Then in 1859 came the <i>Origin of Species</i>, + in which Darwin presented to the world a consistent theory to account for + the manner in which one species might have arisen from another by a + process of gradual evolution. Briefly put, that theory was as follows: In + any species of plant or animal the reproductive capacity tends to outrun + the available food supply, and the resulting competition leads to an + inevitable struggle for existence. Of all the individuals born, only a + portion, and that often a very small one, can survive to produce + offspring. According to Darwin's theory, the nature of the surviving + portion is not determined by chance alone. No two individuals of a + species are precisely alike, and among the variations that occur some + enable their possessors to cope more successfully with the competitive + conditions under which they exist. In comparison with their less favoured + brethren they have a better chance of surviving in the struggle for + existence and consequently of leaving offspring. The argument is + completed by the further assumption of a principle of heredity, in virtue + of which offspring tend to <!-- Page 11 --><span class="pagenum"><a + name="page11"></a>{11}</span>resemble their parents more than other + members of the species. Parents possessing a favourable variation tend to + transmit that variation to their offspring, to some in greater, to others + in less degree. Those possessing it in greater degree will again have a + better chance of survival, and will transmit the favourable variation in + even greater degree to some of their offspring. A competitive struggle + for existence working in combination with certain principles of variation + and heredity results in a slow and continuous transformation of species + through the operation of a process which Darwin termed natural + selection.</p> + + <p>The coherence and simplicity of the theory, supported as it was by the + great array of facts which Darwin had patiently marshalled together, + rapidly gained the enthusiastic support of the great majority of + biologists. The problem of the relation of species at last appeared to be + solved, and for the next forty years zoologists and botanists were busily + engaged in classifying by the light of Darwin's theory the great masses + of anatomical facts which had already accumulated and in adding and + classifying fresh ones. The study of comparative anatomy and embryology + received a new stimulus, for with the acceptance of the theory of descent + with modification it became incumbent upon the biologist to demonstrate + the manner in which animals and plants differing widely in structure and + appearance could be conceivably related to one another. Thenceforward the + energies of both <!-- Page 12 --><span class="pagenum"><a + name="page12"></a>{12}</span>botanists and zoologists have been devoted + to the construction of hypothetical pedigrees suggesting the various + tracks of evolution by which one group of animals or plants may have + arisen from another through a long continued process of natural + selection. The result of such work on the whole may be said to have shown + that the diverse forms under which living things exist to-day, and have + existed in the past so far as palaeontology can tell us, are consistent + with the view that they are all related by the community of descent which + the accepted theory of evolution demands, though as to the exact course + of descent for any particular group of animals there is often + considerable diversity of opinion. It is obvious that all this work has + little or nothing to do with the manner in which species are formed. + Indeed, the effect of Darwin's <i>Origin of Species</i> was to divert + attention from the way in which species originate. At the time that it + was put forward his explanation appeared so satisfying that biologists + accepted the notions of variation and heredity there set forth and ceased + to take any further interest in the work of the hybridisers. Had Mendel's + paper appeared a dozen years earlier it is difficult to believe that it + could have failed to attract the attention it deserved. Coming as it did + a few years after the publication of Darwin's great work, it found men's + minds set at rest on the problems that he raised and their thoughts and + energies directed to other matters. <!-- Page 13 --><span + class="pagenum"><a name="page13"></a>{13}</span></p> + + <p>Nevertheless one interesting and noteworthy attempt to give greater + precision to the term heredity was made about this time. Francis Galton, + a cousin of Darwin, working upon data relating to the breeding of Basset + hounds, found that he could express on a definite statistical scheme the + proportion in which the different colours appeared in successive + generations. Every individual was conceived of as possessing a definite + heritage which might be expressed as unity. Of this, ½ was on the average + derived from the two parents (<i>i.e.</i> ¼ from each parent), ¼ from the + four grandparents, ⅛ from the eight great-grandparents, and so on. + <i>The Law of Ancestral Heredity</i>, as it was termed, expresses with + fair accuracy some of the statistical phenomena relating to the + transmission of characters in a mixed population. But the problem of the + way in which characters are distributed from gamete to zygote and from + zygote to gamete remained as before. Heredity is essentially a + physiological problem, and though statistics may be suggestive in the + initiation of experiment, it is upon the basis of experimental fact that + progress must ultimately rest. For this reason, in spite of its ingenuity + and originality, Galton's theory and the subsequent statistical work that + has been founded upon it failed to give us any deeper insight into the + nature of the hereditary process.</p> + + <p>While Galton was working in England the German zoologist August + Weismann was elaborating the complicated <!-- Page 14 --><span + class="pagenum"><a name="page14"></a>{14}</span>theory of heredity which + eventually appeared in his work on <i>The Germplasm</i> (1885), a book + which will be remembered for one notable contribution to the subject. + Until the publication of Weismann's work it had been generally accepted + that the modifications brought about in the individual during its + lifetime, through the varying conditions of nutrition and environment, + could be transmitted to the offspring. In this biologists were but + following Darwin, who held that the changes in the parent resulting from + increased use or disuse of any part or organ were passed on to the + children. Weismann's theory involved the conception of a sharp cleavage + between the general body tissues or somatoplasm and the reproductive + glands or germplasm. The individual was merely a carrier for the + essential germplasm whose properties had been determined long before he + was capable of leading a separate existence. As this conception ran + counter to the possibility of the inheritance of "acquired characters," + Weismann challenged the evidence upon which it rested and showed that it + broke down wherever it was critically examined. By thus compelling + biologists to revise their ideas as to the inherited effects of use and + disuse, Weismann rendered a valuable service to the study of genetics and + did much to clear the way for subsequent research.</p> + + <p>A further important step was taken in 1895, when Bateson once more + drew attention to the problem of the origin <!-- Page 15 --><span + class="pagenum"><a name="page15"></a>{15}</span>of species, and + questioned whether the accepted ideas of variation and heredity were + after all in consonance with the facts. Speaking generally, species do + not grade gradually from one to the other, but the differences between + them are sharp and specific. Whence comes this prevalence of + discontinuity if the process by which they have arisen is one of + accumulation of minute and almost imperceptible differences? Why are not + intermediates of all sorts more abundantly produced in nature than is + actually known to be the case? Bateson saw that if we are ever to answer + this question we must have more definite knowledge of the nature of + variation and of the nature of the hereditary process by which these + variations are transmitted. And the best way to obtain that knowledge was + to let the dead alone and to return to the study of the living. It was + true that the past record of experimental breeding had been mainly one of + disappointment. It was true also that there was no tangible clue by which + experiments might be directed in the present. Nevertheless in this kind + of work alone there seemed any promise of ultimate success.</p> + + <p>A few years later appeared the first volume of de Vries' remarkable + book on <i>The Mutation Theory</i>. From a prolonged study of the evening + primrose (<i>Oenothera</i>) de Vries concluded that new varieties + suddenly arose from older ones by sudden sharp steps or mutations, and + not by any process involving the gradual accumulation of minute <!-- Page + 16 --><span class="pagenum"><a name="page16"></a>{16}</span>differences. + The number of striking cases from among widely different plants which he + was able to bring forward went far to convincing biologists that + discontinuity in variation was a more widespread phenomenon than had + hitherto been suspected, and not a few began to question whether the + account of the mode of evolution so generally accepted for forty years + was after all the true account. Such in brief was the outlook in the + central problem of biology at the time of the rediscovery of Mendel's + work.</p> + +<hr class="full" /> + +<p><!-- Page 17 --><span class="pagenum"><a name="page17"></a>{17}</span></p> + +<h3>CHAPTER III</h3> + +<p class="cenhead">MENDEL'S WORK</p> + + <p>The task that Mendel set before himself was to gain some clear + conception of the manner in which the definite and fixed varieties found + within a species are related to one another, and he realised at the + outset that the best chance of success lay in working with material of + such a nature as to reduce the problem to its simplest terms. He decided + that the plant with which he was to work must be normally + self-fertilising and unlikely to be crossed through the interference of + insects, while at the same time it must possess definite fixed varieties + which bred true to type. In the common pea (<i>Pisum sativum</i>) he + found the plant he sought. A hardy annual, prolific, easily worked, + <i>Pisum</i> has a further advantage in that the insects which normally + visit flowers are unable to gather pollen from it and so to bring about + cross fertilisation. At the same time it exists in a number of strains + presenting well-marked and fixed differences. The flowers may be purple, + or red, or white; the plants may be tall or dwarf; the ripe seeds may be + yellow or green, round or wrinkled—such are a few of the characters + in which the various races of peas differ from one another. <!-- Page 18 + --><span class="pagenum"><a name="page18"></a>{18}</span></p> + + <p>In planning his crossing experiments Mendel adopted an attitude which + marked him off sharply from the earlier hybridisers. He realised that + their failure to elucidate any general principle of heredity from the + results of cross fertilisation was due to their not having concentrated + upon particular characters or traced them carefully through a sequence of + generations. That source of failure he was careful to avoid, and + throughout his experiments he crossed plants presenting sharply + contrasted characters, and devoted his efforts to observing the behaviour + of these characters in successive generations. Thus in one series of + experiments he concentrated his attention on the transmission of the + characters tallness and dwarfness, neglecting in so far as these + experiments were concerned any other characters in which the parent + plants might differ from one another. For this purpose he chose two + strains of peas, one of about 6 feet in height, and another of about 1½ + feet. Previous testing had shown that each strain bred true to its + peculiar height. These two strains were artificially crossed<a + name="footnotetag1" href="#footnote1"><sup>[1]</sup></a> with one + another, and it was found to make no difference which was used as the + pollen parent and which was used as the ovule parent. In either case the + result was the same. The result of crossing tall with dwarf was in every + case nothing but talls, as tall or even a little taller than the tall + parent. For this reason Mendel termed tallness the <b>dominant</b> and + <!-- Page 19 --><span class="pagenum"><a + name="page19"></a>{19}</span>dwarfness the <b>recessive</b> character. + The next stage was to collect and sow the seeds of these tall hybrids. + Such seeds in the following year gave rise to a mixed generation + consisting of talls and dwarfs <i>but no intermediates</i>. By raising a + considerable number of such plants Mendel was able to establish the fact + that the number of talls which occurred in this generation was almost + exactly three times as great as the number of the dwarfs. As in the + previous year, seed were carefully collected from this, the second hybrid + generation, and in every case <i>the seeds from each individual plant + were harvested separately and separately sown in the following year</i>. + By this respect for the individuality of the different plants, however + closely they resembled one another, Mendel found the clue that had eluded + the efforts of all his predecessors. The seeds collected from the dwarf + recessives bred true, giving nothing but dwarfs. And this was true for + every dwarf tested. But with the talls it was quite otherwise. Although + indistinguishable in appearance, some of them bred true, while others + behaved like the original tall hybrids, giving a generation consisting of + talls and dwarfs in the proportion of three of <!-- Page 20 --><span + class="pagenum"><a name="page20"></a>{20}</span>the former to one of the + latter. Counting showed that the number of the talls which gave dwarfs + was double that of the talls which bred true.</p> + + <div class="figcenter" style="width:32%;"> + <a href="images/031.png"><img style="width:100%" src="images/031.png" + alt="Generations of cross of tall and dwarf peas." title="Generations of cross of tall and dwarf peas." /></a> + </div> + <p>If we denote a dwarf plant as D, a true breeding tall plant as T, and + a tall which gives both talls and dwarfs in the ratio 3 : 1 as T(D), the + result of these experiments may be briefly summarised in the foregoing + scheme.<a name="footnotetag2" href="#footnote2"><sup>[2]</sup></a></p> + + <p>Mendel experimented with other pairs of contrasted characters and + found that in every instance they followed the same scheme of + inheritance. Thus coloured flowers were dominant to white, in the ripe + seeds yellow was dominant to green, and round shape was dominant to + wrinkled, and so on. In every case where the inheritance of an + alternative pair of characters was concerned the effect of the cross in + successive generations was to produce three and only three different + sorts of individuals, viz. dominants which bred true, dominants which + gave both dominant and recessive offspring in the ratio 3 : 1, and + recessives which always bred true. Having determined a general scheme of + inheritance which experiment showed to hold good for each of the seven + pairs of alternative characters with which he worked, Mendel set himself + to providing a theoretical interpretation of this scheme which, as he + clearly realised, must be in terms of germ cells. He <!-- Page 21 + --><span class="pagenum"><a name="page21"></a>{21}</span>conceived of the + gametes as bearers of something capable of giving rise to the characters + of the plant, but he regarded any individual gamete as being able to + carry one and one only of any alternative pair of characters. A given + gamete could carry tallness <i>or</i> dwarfness, but not both. The two + were mutually exclusive so far as the gamete was concerned. It must be + pure for one or the other of such a pair, and this conception of the + purity of the gametes is the most essential part of Mendel's theory.</p> + + <div class="figright" style="width:31%;"> + <a href="images/033.png"><img style="width:100%" src="images/033.png" + alt="Fig. 1. Scheme of inheritance for cross of tall and dwarf peas." title="Fig. 1. Scheme of inheritance for cross of tall and dwarf peas." /></a> + <span class="sc">Fig. 1.</span> + + <p class="poem">Scheme of inheritance in the cross of tall with dwarf + pea. Gametes represented by small and zygotes by larger circles.</p> + </div> + + <p>We may now proceed with the help of the accompanying scheme (Fig. 1) + to deduce the results that should flow from Mendel's conception of the + nature of the gametes, and to see how far they are in accordance with the + facts. Since the original tall plant belonged to a strain which bred + true, all the gametes produced by it must bear the tall character. + Similarly all the gametes of the original dwarf plant must bear the dwarf + character. A cross between these two means the union of <!-- Page 22 + --><span class="pagenum"><a name="page22"></a>{22}</span>a gamete + containing tallness with one bearing dwarfness. Owing to the completely + dominant nature of the tall character, such a plant is in appearance + indistinguishable from the pure tall, but it differs markedly from it in + the nature of the gametes to which it gives rise. When the formation of + the gametes occurs, the elements representing dwarfness and tallness + <b>segregate</b> from one another, so that half of the gametes produced + contain the one, and half contain the other of these two elements. For on + hypothesis every gamete must be pure for one or other of these two + characters. And this is true for the ovules as well as for the pollen + grains. Such hybrid F<sub>1</sub> plants, therefore, must produce a + series of ovules consisting of those bearing tallness and those bearing + dwarfness, and must produce them in equal numbers. And similarly for the + pollen grains. We may now calculate what should happen when such a series + of pollen grains meets such a series of ovules, <i>i.e.</i> the nature of + the generation that should be produced when the hybrid is allowed to + fertilise itself. Let us suppose that there are 4<i>x</i> ovules so that + 2<i>x</i> are "tall" and 2<i>x</i> are "dwarf." These are brought in + contact with a mass of pollen grains of which half are "tall" and half + are "dwarf." It is obvious that a "tall" ovule has an equal chance of + being fertilised by a "tall" or a "dwarf" pollen grain. Hence of our + 2<i>x</i> "tall" ovules, <i>x</i> will be fertilised by "tall" pollen + grains and <i>x</i> will be fertilised by "dwarf" pollen grains. The + former must give rise to tall <!-- Page 23 --><span class="pagenum"><a + name="page23"></a>{23}</span>plants, and since the dwarf character has + been entirely eliminated from them they must in the future breed true. + The latter must also give rise to tall plants, but since they carry also + the recessive dwarf character they must when bred from produce both tails + and dwarfs. Each of the 2<i>x</i> dwarf ovules, again, has an equal + chance of being fertilised by a "tall" or by a "dwarf" pollen grain. + Hence <i>x</i> will give rise to tall plants carrying the recessive dwarf + character, while <i>x</i> will produce plants from which the tall + character has been eliminated, <i>i.e.</i> to pure recessive dwarfs. + Consequently from the 4<i>x</i> ovules of the self-fertilised hybrid we + ought to obtain 3<i>x</i> tall and <i>x</i> dwarf plants. And of the + 3<i>x</i> talls <i>x</i> should breed true to tallness, while the + remaining 2<i>x</i>, having been formed like the original hybrid by the + union of a "tall" and a "dwarf" gamete, ought to behave like it when bred + from and give talls and dwarfs in the ratio 3 : 1. Now this is precisely + the result actually obtained by experiment (cf. p. <a + href="#page17">17</a>), and the close accord of the experimental results + with those deduced on the assumption of the purity of the gametes as + enunciated by Mendel affords the strongest of arguments for regarding the + nature of the gametes and their relation to the characters of the zygotes + in the way that he has done.</p> + + <p>It is possible to put the theory to a further test. The explanation of + the 3 : 1 ratio of dominants and recessives in the F<sub>2</sub> + generation is regarded as due to the F<sub>1</sub> individuals producing + equal numbers of gametes bearing the <!-- Page 24 --><span + class="pagenum"><a name="page24"></a>{24}</span>dominant and recessive + elements respectively. If now the F<sub>1</sub> plant be crossed with the + pure recessive, we are bringing together a series of gametes consisting + of equal numbers of dominants and recessives with a series consisting + solely of recessives. We ought from such a cross to obtain equal numbers + of dominant and recessive individuals, and further, the dominants so + produced ought all to give both dominants and recessives in the ratio + 3 : 1 when they themselves are bred from. Both of these expectations were + amply confirmed by experiment, and crossing with the recessive is now a + recognised way of testing whether a plant or animal bearing a dominant + character is a pure dominant, or an impure dominant which is carrying the + recessive character. In the former case the offspring will be all of the + dominant form, while in the latter they will consist on the average of + equal numbers of dominants and recessives.</p> + + <p>So far we have been concerned with the results obtained when two + individuals differing in a single pair of characters are crossed together + and with the interpretation of those results. But Mendel also used plants + which differed in more than a single pair of differentiating characters. + In such cases he found that each pair of characters followed the same + definite rule, but that the inheritance of each pair was absolutely + independent of the other. Thus, for example, when a tall plant bearing + coloured flowers was crossed with a dwarf plant <!-- Page 25 --><span + class="pagenum"><a name="page25"></a>{25}</span>bearing white flowers the + resulting hybrid was a tall plant with coloured flowers. For coloured + flowers are dominant to white, and tallness is dominant to dwarfness. In + the succeeding generation there are plants with coloured flowers and + plants with white flowers in the proportion of 3 : 1, and at the same + time tall plants and dwarf plants in the same proportion. Hence the + chances that a tall plant will have coloured flowers are three times as + great as its chance of having white flowers. And this is also true for + the dwarf plants. As the result of this cross, therefore, we should + expect an F<sub>2</sub> generation consisting of four classes, viz. + coloured talls, white talls, coloured dwarfs, and white dwarfs, and we + should further expect these four forms to appear in the ratio of 9 + coloured talls, 3 white talls, 3 coloured dwarfs, and 1 white dwarf. For + this is the only ratio which satisfies the conditions that the talls + should be to the dwarfs as 3 : 1, and at the same time the coloured + should be to the whites as 3 : 1. And these are the proportions that + Mendel found to obtain actually in his experiments. Put in a more general + form, it may be stated that when two individuals are crossed which differ + in two pairs of differentiating characters the hybrids (F<sub>1</sub>) + are all of the same form, exhibiting the dominant character of each of + the two pairs, while the F<sub>2</sub> generation produced by such + hybrids consists on the average of 9 showing both dominants, 3 showing + one dominant and one recessive, <!-- Page 26 --><span class="pagenum"><a + name="page26"></a>{26}</span>3 showing the other dominant and the other + recessive, and 1 showing both recessive characters. And, as Mendel + pointed out, the principle may be extended indefinitely. If, for example, + the parents differ in three pair of characters <i>A</i>, <i>B</i>, and + <i>C</i>, respectively dominant to <i>a</i>, <i>b</i>, and <i>c</i>, the + F<sub>1</sub> individuals will be all of the form <i>ABC</i>, while the + F<sub>2</sub> generation will consists of 27 <i>ABC</i>, 9 <i>ABc</i>, 9 + <i>AbC</i>, 9 <i>aBC</i>, 3 <i>Abc</i>, 3 <i>aBc</i>, 3 <i>abC</i>, and 1 + <i>abc</i>. When individuals differing in a number of alternative + characters are crossed together, the hybrid generation, provided that the + original parents were of pure strains, consists of plants of the same + form; but when these are bred from a redistribution of the various + characters occurs. That redistribution follows the same definite rule for + each character, and if the constitution of the original parents be known, + the nature of the F<sub>2</sub> generation, <i>i.e.</i> the number of + possible forms and the proportions in which they occur, can be readily + calculated. Moreover, as Mendel showed, we can calculate also the chances + of any given form breeding true. To this point, however, we shall return + later.</p> + + <p>Of Mendel's experiments with beans it is sufficient to say here that + they corroborated his more ample work with peas. He is also known to have + made experiments with many other plants, and a few of his results are + incidentally given in his series of letters to Nägeli the botanist. To + the breeding and crossing of bees he also devoted much <!-- Page 27 + --><span class="pagenum"><a name="page27"></a>{27}</span>time and + attention, but unhappily the record of these experiments appears to have + been lost. The only other published work that we possess dealing with + heredity is a brief paper on some crossing experiments with the hawkweeds + (<i>Hieracium</i>), a genus that he chose for working with because of the + enormous number of forms under which it naturally exists. By crossing + together the more distinct varieties, he evidently hoped to produce some + of these numerous wild forms, and so throw light upon their origin and + nature. In this hope he was disappointed. Owing in part to the great + technical difficulties attending the cross fertilisation of these flowers + he succeeded in obtaining very few hybrids. Moreover, the behaviour of + those which he did obtain was quite contrary to what he had found in the + peas. Instead of giving a variety of forms in the F<sub>2</sub> + generation, they bred true and continued to do so as long as they were + kept under observation. More recent research has shown that this is due + to a peculiar form of parthenogenesis (cf. p. <a + href="#page135">135</a>), and not to any failure of the characters to + separate clearly from one another in the gametes. Mendel, however, could + not have known of this, and his inability to discover in <i>Hieracium</i> + any indication of the rule which he had found to hold good for both peas + and beans must have been a source of considerable disappointment. Whether + for this reason, or owing to the utter neglect of his work by the + scientific world, Mendel gave up his experimental <!-- Page 28 --><span + class="pagenum"><a name="page28"></a>{28}</span>researches during the + latter part of his life. His closing years were shadowed with ill-health + and embittered by a controversy with the Government on a question of the + rights of his monastery. He died of Bright's disease in 1884.</p> + +<blockquote class="b1n"> + + <p><i>Note.</i>—Shortly after the discovery of Mendel's paper a + need was felt for terms of a general nature to express the constitution + of individuals in respect of inherited characters, and Bateson + accordingly proposed the words homozygote and heterozygote. An individual + is said to be homozygous for a given character when it has been formed by + two gametes each bearing the character, and all the gametes of a + homozygote bear the character in respect of which it is homozygous. When, + however, the zygote is formed by two gametes of which one bears the given + character while the other does not, it is said to be heterozygous for the + character in question, and only half the gametes produced by such a + heterozygote bear the character. An individual may be homozygous for one + or more characters, and at the same time may be heterozygous for + others.</p> + +</blockquote> + +<hr class="full" /> + +<p><!-- Page 29 --><span class="pagenum"><a name="page29"></a>{29}</span></p> + +<h3>CHAPTER IV</h3> + +<p class="cenhead">THE PRESENCE AND ABSENCE THEORY</p> + + <p>It was fortunate for the development of biological science that the + rediscovery of Mendel's work found a small group of biologists deeply + interested in the problems of heredity, and themselves engaged in + experimental breeding. To these men the extraordinary significance of the + discovery was at once apparent. From their experiments, undertaken in + ignorance of Mendel's paper, de Vries, Correns, and Tschermak were able + to confirm his results in peas and other plants, while Bateson was the + first to demonstrate their application to animals. Thenceforward the + record has been one of steady progress, and the result of ten years' work + has been to establish more and more firmly the fundamental nature of + Mendel's discovery. The scheme of inheritance, which he was the first to + enunciate, has been found to hold good for such diverse things as height, + hairiness, and flower colour and flower form in plants, the shape of + pollen grains, and the structure of fruits; while among animals the coat + colour of mammals, the form of the feathers and of the comb in poultry, + the waltzing habit of Japanese mice, and eye <!-- Page 30 --><span + class="pagenum"><a name="page30"></a>{30}</span>colour in man are but a + few examples of the diversity of characters which all follow the same law + of transmission. And as time went on many cases which at first seemed to + fall without the scheme have been gradually brought into line in the + light of fuller knowledge. Some of these will be dealt with in the + succeeding chapters of this book. Meanwhile we may concern ourselves with + the single modification of Mendel's original views which has arisen out + of more ample knowledge.</p> + + <div class="figcenter" style="width:35%;"> + <a href="images/042.jpg"><img style="width:100%" src="images/042t.jpg" + alt="Fig. 2. Feathers of an ordinary and a silky fowl." title="Fig. 2. Feathers of an ordinary and a silky fowl." /></a> + <span class="sc">Fig. 2.</span> + + <p class="poem">A wing feather and a contour feather of an ordinary and + a silky fowl. The peculiar ragged appearance of the silky feathers is + due to the absence of the little hooks or barbules which hold the barbs + together. The silky condition is recessive.</p> + </div> + + <p>As we have already seen, Mendel considered that in the gamete there + was either a definite something <!-- Page 31 --><span class="pagenum"><a + name="page31"></a>{31}</span>corresponding to the dominant character or a + definite something corresponding to the recessive character, and that + these somethings whatever they were could not coexist in any single + gamete. For these somethings we shall in future use the term + <b>factor</b>. The factor, then, is what corresponds in the gamete to the + <b>unit-character</b> that appears in some shape or other in the + development of the zygote. Tallness in the pea is a unit-character, and + the gametes in which it is represented are said to contain the factor for + tallness. Beyond their existence in the gamete and their mode of + transmission we make no suggestion as to the nature of these factors.</p> + + <div class="figcenter" style="width:50%;"> + <a href="images/043.jpg"><img style="width:100%" src="images/043t.jpg" + alt="Fig. 3. Two double and an ordinary single primula flower." title="Fig. 3. Two double and an ordinary single primula flower." /></a> + <span class="sc">Fig. 3.</span> + + <p class="cenhead">Two double and an ordinary single primula flower. + This form of double is recessive to the single.</p> + </div> + +<p><!-- Page 32 --><span class="pagenum"><a name="page32"></a>{32}</span></p> + + <div class="figright" style="width:47%;"> + <a href="images/044.png"><img style="width:100%" src="images/044.png" + alt="Fig. 4. Fowls' combs." title="Fig. 4. Fowls' combs." /></a> + <span class="sc">Fig. 4.</span> + + <p class="cenhead">Fowls' combs. A, pea; B, rose; C, single; D, + walnut.</p> + </div> + + <p>On Mendel's view there was a factor corresponding to the dominant + character and another factor corresponding to the recessive character of + each alternative pair of unit-characters, and the characters were + alternative because no gamete could carry more than one of the two + factors belonging to the alternative pair. On the other hand, Mendel + supposed that it always carried either one or the other of such a pair. + As experimental work proceeded, <!-- Page 33 --><span class="pagenum"><a + name="page33"></a>{33}</span>it soon became clear that there were cases + which could not be expressed in terms of this conception. The nature of + the difficulty and the way in which it was met will perhaps be best + understood by considering a set of experiments in which it occurred. Many + of the different breeds of poultry are characterised by a particular form + of comb, and in certain cases the inheritance of these has been carefully + worked out. It was shown that the rose comb (Fig. 4, B) with its + flattened papillated upper surface and backwardly projecting pike was + dominant in the ordinary way to the deeply serrated high single comb + (Fig. 4, C) which is characteristic of the Mediterranean races. + Experiment also showed that the pea comb (Fig. 4, A), a form with a low + central and two well-developed lateral ridges, such as is found in Indian + game, behaves as a simple dominant to the single comb. The interesting + question arose as to what would happen when the rose and the pea, two + forms each dominant to the same third form, were mated together. It + seemed reasonable to suppose that things which were alternative to the + same thing would be alternative to one another—that either rose or + pea would dominate in the hybrids, and that the F<sub>2</sub> generation + would consist of dominants and recessives in the ratio 3 : 1. The result + of the experiment was, however, very different. The cross rose × pea led + to the production of a comb quite unlike either of them. This, the + so-called walnut comb (Fig. 4, D), <!-- Page 34 --><span + class="pagenum"><a name="page34"></a>{34}</span>from its resemblance to + the half of a walnut, is a type of comb which is normally characteristic + of the Malay fowl. Moreover, when these F<sub>1</sub> birds were bred + together, a further unlooked-for result was obtained. As was expected, + there appeared in the F<sub>2</sub> generation the three forms walnut, + rose, and pea. But there also appeared a definite proportion of + single-combed birds, and among many hundreds of chickens bred in this way + the proportions in which the four forms walnut, rose, pea, and single + appeared was 9 : 3 : 3 : 1. <span class="figleft" style="width:25%;"><a + href="images/046.png"><img style="width:100%" src="images/046.png" + alt="Generations of Rose × Pea cross." title="Generations of Rose × Pea cross." + /></a></span> Now this, as Mendel showed, is the ratio found in an + F<sub>2</sub> generation when the original parents differ in two pairs of + alternative characters, and from the proportions in which the different + forms of comb occur we must infer that the walnut contains both + dominants, the rose and the pea one dominant each, while the single is + pure for both recessive characters. This accorded with subsequent + breeding experiments, for the singles bred perfectly true as soon as they + had once made their appearance. So far the case is clear. The difficulty + comes when we attempt to define these two pairs of characters. How are we + to express the fact that while single behaves as a simple recessive to + either pure rose, or to pure pea, it can yet appear in F<sub>2</sub> from + a cross <!-- Page 35 --><span class="pagenum"><a + name="page35"></a>{35}</span>between these two pure forms, though neither + of them should, on Mendel's view, contain the single? An explanation + which covers the facts in a simple way is that which has been termed the + "Presence and Absence" theory. On this theory the dominant character of + an alternative pair owes its dominance to the presence of a factor which + is absent in the recessive. The tall pea is tall owing to the presence in + it of the factor for tallness, but in the absence of this factor the pea + remains a dwarf. All peas are dwarf, but the tall is a dwarf plus a + factor which turns it into a tall. Instead of the characters of an + alternative pair being due to two separate factors, we now regard them as + the expression of the only two possible states of a single factor, viz. + its presence or its absence. The conception will probably become clearer + if we follow its application in detail to the case of the fowl's combs. + In this case we are concerned with the transmission of the two factors, + rose (<i>R</i>) and pea (<i>P</i>), the presence of each of which is + alternative to its absence. The rose-combed bird contains the factor for + rose but not that for pea, and the pea-combed bird contains the factor + for pea but not that for rose. When both factors are present in a bird, + as in the hybrid made by crossing rose with pea, the result is a walnut. + For convenience of argument we may denote the presence of a given factor + by a capital letter and its absence by the corresponding small letter. + The use of the small letter is merely a symbolic way of intimating <!-- + Page 36 --><span class="pagenum"><a name="page36"></a>{36}</span>that a + particular factor is absent in a gamete or zygote. Represented thus the + zygotic constitution of a pure rose-combed bird is <i>RRpp</i>; for it + has been formed by the union of two gametes both of which contained + <i>R</i> but not <i>P</i>. Similarly we may denote the pure pea-combed + bird as <i>rrPP</i>. On crossing the rose with the pea union occurs + between a gamete <i>Rp</i> and a gamete <i>rP</i>, resulting in the + formation of a heterozygote of the constitution <i>RrPp</i>. The use of + the small letters here informs us that such a zygote contains only a + single dose of each of the factors <i>R</i> and <i>P</i>, although, of + course, it is possible for a zygote, if made in a suitable way, to have a + double dose of any factor. Now when such a bird comes to form gametes a + separation takes place between the part of the zygotic cell containing + <i>R</i> and the part which does not contain it (<i>r</i>). Half of its + gametes, therefore, will contain <i>R</i> and the other half will be + without it (<i>r</i>). Similarly half of its gametes will contain + <i>P</i> and the other half will be without it (<i>p</i>). It is obvious + that the chances of <i>R</i> being distributed to a gamete with or + without <i>P</i> are equal. Hence the gametes containing <i>R</i> will be + of two sorts, <span class="correction" title="Original reads `PR'." + ><i>RP</i></span> and <i>Rp</i>, and these will be produced in equal + numbers. Similarly the gametes without <i>R</i> will also be of two + sorts, <i>rP</i> and <i>rp</i>, and these, again, will be produced in + equal numbers. Each of the hybrid walnut-combed birds, therefore, gives + rise to a series consisting of equal numbers of gametes of the four + different types <i>RP</i>, <i>Rp</i>, <i>rP</i>, and <i>rp</i>; and the + breeding <!-- Page 37 --><span class="pagenum"><a + name="page37"></a>{37}</span>together of such F<sub>1</sub> birds means + the bringing together of two such series of gametes. When this happens an + ovum of any one of the four types has an equal chance of being fertilised + by a spermatozoon of any one of the four types. A convenient and simple + method of demonstrating what happens under such circumstances is the + method sometimes termed the "chessboard" method. For two series each + consisting of four different types of gamete we require a square divided + up into 16 parts. The four terms of the gametic series are first written + horizontally across the four sets of four squares, so that the series is + repeated four times. It is then written vertically four times, care being + taken to keep to the same order. In this simple mechanical way all the + possible combinations are represented and in their proper proportions. + <span class="figright" style="width:31%;"><a href="images/049.png"><img + style="width:100%" src="images/049.png" alt="Fig. 5. Combs in F2." + title="Fig. 5. Combs in F2." /></a><span class="sc">Fig.</span> 5. <br + />Diagram to illustrate the nature of the F<sub>2</sub> generation from + the cross of rose comb × pea comb.</span> Fig. 5 shows the result of + applying this method to our series <i>RP</i>, <i>Rp</i>, <i>rP</i>, + <i>rp</i>, and the 16 squares represent the different kinds of zygotes + formed and the proportions in which they occur. As <!-- Page 38 --><span + class="pagenum"><a name="page38"></a>{38}</span>the figure shows, 9 + zygotes contain both <i>R</i> and <i>P</i>, having a double or a single + dose of either or both of these factors. Such birds must be all walnut + combed. Three out of the 16 zygotes contain <i>R</i> but not <i>P</i>, + and these must be rose-combed birds. Three, again, contain <i>P</i> but + not <i>R</i> and must be pea-combed birds. Finally one out of the 16 + contains neither <i>R</i> nor <i>P</i>. It cannot be rose—it cannot + be pea. It must, therefore, be something else. As a matter of fact it is + single. Why it should be single and not something else follows from what + we already know about the behaviour of these various forms of comb. For + rose is dominant to single; therefore on the Presence and Absence theory + a rose is a single plus a factor which turns the single into a rose. If + we could remove the "rose" factor from a rose-combed bird the underlying + single would come into view. Similarly a pea comb is a single plus a + factor which turns the single into a pea, and a walnut is a single which + possesses two additional modifying factors. Singleness, in fact, + underlies all these combs, and if we write their zygotic constitution in + full we must denote a walnut as <i>RRPPSS</i>, a rose as <i>RRppSS</i>, a + pea as <i>rrPPSS</i>, and a single as <i>rrppSS</i>. The crossing of rose + with pea results in a reshuffling of the factors concerned, and in + accordance with the principle of segregation some zygotes are formed in + which neither of the modifying factors <i>R</i> and <i>P</i> are present, + and the single character can then become manifest. <!-- Page 39 --><span + class="pagenum"><a name="page39"></a>{39}</span></p> + + <p>The Presence and Absence theory is to-day generally accepted by + students of these matters. Not only does it afford a simple explanation + of the remarkable fact that in all cases of Mendelian inheritance we + should be able to express our unit-characters in terms of alternative + pairs, but, as we shall have occasion to refer to later, it suggests a + clue as to the course by which the various domesticated varieties of + plants and animals have arisen from their wild prototypes.</p> + + <div class="figcenter" style="width:39%;"> + <a href="images/052.png"><img style="width:100%" src="images/052.png" + alt="Fig. 6. Fowls' combs, rose and Breda." title="Fig. 6. Fowls' combs, rose and Breda." /></a> + <span class="sc">Fig. 6.</span> + + <p class="poem">Fowls' combs. A and B, F<sub>1</sub> hen from rose × + Breda; C, an F<sub>1</sub> cock from the cross of single × Breda; D, + head of Breda cock.</p> + </div> + + <p>Before leaving this topic we may draw attention to some experiments + which offer a pretty confirmation of the view that the rose comb is a + single to which a modifying factor for roseness has been added. It was + argued that if we could find a type of comb in which the factor for + singleness was absent, then on crossing such a comb with a rose we ought, + if singleness really underlies rose, to obtain some single combs in + F<sub>2</sub> from such a cross. Such a comb we had the good fortune to + find in the Breda fowl, a breed largely used in Holland. This fowl is + usually spoken of as combless, for the place of the comb is taken by a + covering of short bristlelike feathers (Fig. 6, D). In reality it + possesses the vestige of a comb in the form of two minute lateral knobs + of comb tissue. Characteristic also of this breed is the high development + of the horny nostrils, a feature probably correlated with the almost + complete absence of comb. The first step in the experiment was to prove + the absence of the factor for singleness in the Breda. <!-- Page 40 + --><span class="pagenum"><a name="page40"></a>{40}</span>On crossing + Breda with single the F<sub>1</sub> birds exhibit a large comb of the + form of a double single comb in which the two portions are united + anteriorly, but diverge from one another towards the back of the head + (Fig. 6, C). The Breda contains an element of duplicity which is dominant + to the simplicity of the ordinary single comb. But it cannot contain the + factor for the single comb, because as soon as that is put into it by + crossing with a single the comb <!-- Page 41 --><span class="pagenum"><a + name="page41"></a>{41}</span><span class="figright" style="width:36%;"><a + href="images/053.png"><img style="width:100%" src="images/053.png" + alt="Breda and rose." title="Breda and rose." /></a></span>assumes a + large size, and is totally distinct in appearance from its almost + complete absence in the pure Breda. Now when the Breda is crossed with + the rose duplicity is dominant to simplicity, and rose is dominant to + lack of comb, and the F<sub>1</sub> generation consists of birds + possessing duplex rose combs (Fig. 6, A and B). On breeding such birds + together we obtain a generation consisting of Bredas, duplex roses, + roses, duplex singles, and singles. From our previous experiment we know + that the singles could not have come from the Breda, since a Breda comb + to which the factor for single has been added no longer remains a Breda. + Therefore it must have come from the rose, thus confirming our view that + the rose is in reality a single comb which contains in addition a + dominant modifying factor (<i>R</i>) whose presence turns it into a rose. + We shall take it, therefore, that there is good experimental evidence for + the Presence and Absence theory, and we shall express in terms of it the + various cases which come up for discussion in succeeding chapters.</p> + +<hr class="full" /> + +<p><!-- Page 42 --><span class="pagenum"><a name="page42"></a>{42}</span></p> + +<h3>CHAPTER V</h3> + +<p class="cenhead">INTERACTION OF FACTORS</p> + + <p>We have now reached a point at which it is possible to formulate a + definite conception of the living organism. A plant or animal is a living + entity whose properties may in large measure be expressed in terms of + unit-characters, and it is the possession of a greater or lesser number + of such unit-characters renders it possible for us to draw sharp + distinctions between one individual and another. These unit-characters + are represented by definite factors in the gamete which in the process of + heredity behave as indivisible entities, and are distributed according to + a definite scheme. The factor for this or that unit-character is either + present in the gamete or it is not present. It must be there in its + entirety or completely absent. Such at any rate is the view to which + recent experiment has led us. But as to the nature of these factors, the + conditions under which they exist in the gamete, and the manner in which + they produce their specific effects in the zygote, we are at present + almost completely in the dark.</p> + + <p>The case of the fowls' combs opens up the important question of the + extent to which the various factors can <!-- Page 43 --><span + class="pagenum"><a name="page43"></a>{43}</span>influence one another in + the zygote. The rose and the pea factors are separate entities, and each + when present alone produces a perfectly distinct and characteristic + effect upon the single comb, turning it into a rose or a pea as the case + may be. But when both are present in the same zygote their combined + effect is to produce the walnut comb, a comb which is quite distinct from + either and in no sense intermediate between them. The question of the + influence of factors upon one another did not present itself to Mendel + because he worked with characters which affected different parts of the + plant. It was unlikely that the factor which led to the production of + colour in the flower would affect the shape of the pod, or that the + height of the plant would be influenced by the presence or absence of the + factor that determined the shape of the ripe seed. But when several + factors can modify the same structure it is reasonable to suppose that + they will influence one another in the effects which their simultaneous + presence has upon the zygote. By themselves the pea and the rose factors + each produce a definite modification of the single comb, but when both + are present in the zygote, whether as a single or double dose, the + modification that results is quite different to that produced by either + when present alone. Thus we are led to the conception of characters which + depend for their manifestation on more than one factor in the zygote, and + in the present chapter we may consider a few of the <!-- Page 44 --><span + class="pagenum"><a name="page44"></a>{44}</span>phenomena which result + from such interaction between separate and distinct factors.</p> + + <div class="figright" style="width:22%;"> + <a href="images/056.png"><img style="width:100%" src="images/056.png" + alt="Generation of cross of red and white sweet peas." title="Generation of cross of red and white sweet peas." /></a> + </div> + <p>One of the most interesting and instructive cases in which the + interaction between separate factors has been demonstrated is a case in + the sweet pea. All white sweet peas breed true to whiteness. And + generally speaking the result of crossing different whites is to produce + nothing but whites, whether in F<sub>1</sub> or in succeeding + generations. But there are certain strains of white sweet peas which when + crossed together produce only coloured flowers. The colour may be + different in different cases, though for our present purpose we may take + a case in which the colour is red. When such reds are allowed to + self-fertilise themselves in the normal way and the seeds sown, the + resulting F<sub>2</sub> generation consists of reds and whites, the + former being rather more numerous than the latter in the proportion of + 9 : 7. The raising of a further generation from the seeds of these + F<sub>2</sub> plants shows that the whites always breed true to + whiteness, but that different reds may behave differently. Some breed + true, others give reds and whites in the ratio 3 : 1, while others, + again, give reds and whites in the ratio 9 : 7. As in the case of the + fowls' combs, this case may be interpreted in terms of the presence and + absence of two factors. <!-- Page 45 --><span class="pagenum"><a + name="page45"></a>{45}</span><span class="figleft" style="width:33%;"><a + href="images/057.png"><img style="width:100%" src="images/057.png" + alt="Factors in red and white sweet peas." title="Factors in red and white sweet peas." + /></a></span>Red in the sweet pea results from the interaction of two + factors, and unless these are both present the red colour cannot appear. + Each of the white parents carried one of the two factors whose + interaction is necessary for the production of the red colour, and as a + cross between them brings these two complementary factors together the + F<sub>1</sub> plants must all be red. As this case is of considerable + importance for the proper understanding of much that is to follow, and as + it has been completely worked out, we shall consider it in some detail. + Denoting these two colour factors by <i>A</i> and <i>B</i> respectively + we may proceed to follow out the consequences of this cross. Since all + the F<sub>1</sub> plants were red the constitution of the parental whites + must have been <i>AAbb</i> and <i>aaBB</i> respectively, and their + gametes consequently <i>Ab</i> and <i>aB</i>. The constitution of the + F<sub>1</sub> plants must, therefore, be <i>AaBb</i>. Such a plant being + heterozygous for two factors produces a series of gametes of the four + kinds <i>AB</i>, <i>Ab</i>, <i>aB</i>, <i>ab</i>, and produces them in + equal numbers (cf. p. <a href="#page36">36</a>). To obtain the various + types of zygotes which are produced when such <!-- Page 46 --><span + class="pagenum"><a name="page46"></a>{46}</span><span class="figright" + style="width:29%;"><a href="images/058.png"><img style="width:100%" + src="images/058.png" alt="Fig. 7. Scheme of inheritance for red and white + sweet peas." title="Fig. 7. Scheme of inheritance for red and white sweet peas." + /></a><span class="sc">Fig. 7.</span><br />Diagram to illustrate the + nature of the F<sub>2</sub> generation from the two white sweet peas + which give a coloured F<sub>1</sub>.</span>a series of pollen grains + meets a similar series of ovules we may make use of the same "chessboard" + system which we have already adopted in the case of the fowls' combs. An + examination of this figure (Fig. 7) shows that 9 out of the 16 squares + contain both <i>A</i> and <i>B</i>, while 7 contain either <i>A</i> or + <i>B</i> alone, or neither. In other words, on this view of the nature of + the two white sweet peas we should in the F<sub>2</sub> generation look + for the appearance of coloured and white flowers in the ratio 9 : 7. And + this, as we have already seen, is what was actually found by experiment. + Further examination of the figure shows that the coloured plants are not + all of the same constitution, but are of four kinds with respect to their + zygotic constitution, viz. <i>AABB</i>, <i>AABb</i>, <i>AaBB</i>, and + <i>AaBb</i>. Since <i>AABB</i> is homozygous for both <i>A</i> and + <i>B</i>, all the gametes which it produces must contain both of these + factors, and such a plant must therefore breed true to the red colour. A + plant of the <!-- Page 47 --><span class="pagenum"><a + name="page47"></a>{47}</span>constitution <i>AABb</i> is homozygous for + the factor <i>A</i>, but heterozygous for <i>B</i>. All of its gametes + will contain <i>A</i>, but only one-half of them will contain <i>B</i>, + <i>i.e.</i> it produces equal numbers of gametes <i>AB</i> and <i>Ab</i>. + Two such series of gametes coming together must give a generation + consisting of <i>x</i> <i>AABB</i>, 2<i>x</i> <i>AABb</i>, and <i>x</i> + <i>AAbb</i>, that is, reds and whites in the ratio 3 : 1. Lastly the red + zygotes of the constitution <i>AaBb</i> have the same constitution as the + original red made from the two whites, and must therefore when bred from + give reds and whites in the ratio 9 : 7. The existence of all these three + sorts of reds was demonstrated by experiment, and the proportions in + which they were met with tallied with the theoretical explanation.</p> + + <p>The theory was further tested by an examination into the properties of + the various F<sub>2</sub> whites which come from a coloured plant that + has itself been produced by the mating of two whites. As Fig. 7 shows, + these are, in respect of their constitution, of five different kinds, + viz. <i>AAbb</i>, <i>Aabb</i>, <i>aaBB</i>, <i>aaBb</i>, and <i>aabb</i>. + Since none of them produce anything but whites on self-fertilisation it + was found necessary to test their properties in another way, and the + method adopted was that of crossing them together. It is obvious that + when this is done we should expect different results in different cases. + Thus the cross between two whites of the constitution <i>AAbb</i> and + <i>aaBB</i> should give nothing but coloured plants; for these two whites + are of <!-- Page 48 --><span class="pagenum"><a + name="page48"></a>{48}</span>the same constitution as the original two + whites from which the experiment started. On the other hand, the cross + between a white of the constitution <i>aabb</i> and any other white can + never give anything but whites. For no white contains both <i>A</i> and + <i>B</i>, or it would not be white, and a plant of the constitution + <i>aabb</i> cannot supply the complementary factor necessary for the + production of colour. Again, two whites of the constitution <i>Aabb</i> + and <i>aaBb</i> when crossed should give both coloured and white flowers, + the latter being three times as numerous as the former. Without going + into further detail it may be stated that the results of a long series of + crosses between the various F<sub>2</sub> whites accorded closely with + the theoretical explanation.</p> + + <p>From the evidence afforded by this exhaustive set of experiments it is + impossible to resist the deduction that the appearance of colour in the + sweet pea depends upon the interaction of two factors which are + independently transmitted according to the ordinary scheme of Mendelian + inheritance. What these factors are is still an open question. Recent + evidence of a chemical nature indicates that colour in a flower is due to + the interaction of two definitive substances: (1) a colourless + "chromogen," or colour basis; and (2) a ferment which behaves as an + activator of the chromogen, and by inducing some process of oxidation, + leads to the formation of a coloured substance. But whether these two + bodies exist as such <!-- Page 49 --><span class="pagenum"><a + name="page49"></a>{49}</span>in the gametes or whether in some other form + we have as yet no means of deciding.</p> + + <p>Since the elucidation of the nature of colour in the sweet pea + phenomena of a similar kind have been witnessed in other plants, notably + in stocks, snapdragons, and orchids. Nor is this class of phenomena + confined to plants. In the course of a series of experiments upon the + plumage colour of poultry, indications were obtained that different white + breeds did not always owe their whiteness to the same cause. Crosses were + accordingly made between the white Silky fowl and a pure white strain + derived from the white Dorking. Each of these had been previously shown + to behave as a simple recessive to colour. When the two were crossed only + fully coloured birds resulted. From analogy with the case of the sweet + pea it was anticipated that such F<sub>1</sub> coloured birds when bred + together would produce an F<sub>2</sub> generation consisting of coloured + and white birds in the ratio 9 : 7, and when the experiment was made this + was actually shown to be the case. With the growth of knowledge it is + probable that further striking parallels of this nature between the plant + and animal worlds will be met with.</p> + + <p>Before quitting the subject of these experiments attention may be + drawn to the fact that the 9 : 7 ratio is in reality a 9 : 3 : 3 : 1 + ratio in which the last three terms are indistinguishable owing to the + special circumstances that neither factor can produce a visible effect + without <!-- Page 50 --><span class="pagenum"><a + name="page50"></a>{50}</span>the co-operation of the other. And we may + further emphasise the fact that although the two factors thus interact + upon one another they are nevertheless transmitted quite independently + and in accordance with the ordinary Mendelian scheme.</p> + + <div class="figright" style="width:24%;"> + <a href="images/062.png"><img style="width:100%" src="images/062.png" + alt="Coat colours of mice." title="Coat colours of mice." /></a> + </div> + <p>One of the earliest sets of experiments demonstrating the interaction + of separate factors was that made by the French zoologist Cuénot on the + coat colours of mice. It was shown that in certain cases agouti, which is + the colour of the ordinary wild grey mouse, behaves as a dominant to the + albino variety, <i>i.e.</i> the F<sub>2</sub> generation from such a + cross consists of agoutis and albinos in the ratio 3 : 1. But in other + cases the cross between albino and agouti gave a different result. In the + F<sub>1</sub> generation appeared only agoutis as before, but the + F<sub>2</sub> generation consisted of three distinct types, viz. agoutis, + albinos, <i>and blacks</i>. Whence the sudden appearance of the new type? + The answer is a simple one. The albino parent was really a black. But it + lacked the factor without which the colour is unable to develop, and + consequently it remained an albino. If we denote this factor by <i>C</i>, + then the constitution of an albino must be <i>cc</i>, while that of a + coloured animal may be <i>CC</i> or <i>Cc</i>, according as to whether it + breeds true to colour or can <!-- Page 51 --><span class="pagenum"><a + name="page51"></a>{51}</span>throw albinos. Agouti was previously known + to be a simple dominant to black, <i>i.e.</i> an agouti is a black rabbit + plus an additional greying factor which modifies the black into agouti. + This factor we will denote by <i>G</i>, and we will use <i>B</i> for the + black factor. Our original agouti and albino parents we may therefore + regard as in constitution <i>GGCCBB</i> and <i>ggccBB</i> respectively. + Both of the parents are homozygous for black. The gametes produced by the + two parents are <i>GCB</i>, and <i>gcB</i>, and the constitution of the + F<sub>1</sub> animals must be <i>GgCcBB</i>. Being heterozygous for two + factors they will produce four kinds of gametes in equal numbers, viz. + <i>GCB</i>, <i>GcB</i>, <i>gCB</i>, and <i>gcB</i>. The results of the + mating of two such similar series of gametes when the F<sub>1</sub> + animals are bred together we may determine by the usual "chessboard" + method (Fig. 8). Out of the 16 squares 9 contain both <span + class="correction" title="Original reads (small) `c'."><i>C</i></span> + and <i>G</i> in addition to <i>B</i>. Such animals must be agoutis. Three + squares contain <i>C</i> but not <i>G</i>. Such animals must be coloured, + but as they do not contain the modifying agouti factor their colour will + be black. The remaining four squares do not contain <i>C</i>, and in the + absence of this colour-developing factor they must all be albinos. Theory + demands that the three classes agouti, black, and albino should appear in + F<sub>2</sub> in the ratio 9 : 3 : 4; experiment has shown that these are + the only classes that appear, and that the proportions in which they are + produced accord closely with the theoretical expectation. Put briefly, + then, the explanation <!-- Page 52 --><span class="pagenum"><a + name="page52"></a>{52}</span><span class="figright" style="width:30%;"><a + href="images/064.png"><img style="width:100%" src="images/064.png" + alt="Fig. 8. Scheme of inheritance for agouti and black mice." + title="Fig. 8. Scheme of inheritance for agouti and black mice." + /></a><span class="sc">Fig. 8.</span><br />Diagram to illustrate the + nature of the F<sub>2</sub> generation which may arise from the mating of + agouti with albino in mice or rabbits.</span>of this case is that all the + animals are black, and that we are dealing with the presence and absence + of two factors, a colour developer (<i>C</i>), and a colour modifier + (<i>G</i>), both acting, as it were, upon a substratum of black. The + F<sub>2</sub> generation really consists of the four classes agoutis, + blacks, albino agoutis, and albino blacks in the ratio 9 : 3 : 3 : 1. But + since in the absence of the colour developer <i>C</i> the colour modifier + <i>G</i> can produce no visible result, the last two classes of the ratio + are indistinguishable, and our F<sub>2</sub> generation comes to consist + of three classes in the ratio 9 : 3 : 4, instead of four classes in the + ratio 9 : 3 : 3 : 1. This explanation was further tested by experiments + with the albinos. In an F<sub>2</sub> family of this nature there ought + to be three kinds, viz. albinos homozygous for <i>G</i> (<i>GGccBB</i>), + albinos heterozygous for <i>G</i> (<i>GgccBB</i>), and albinos without + <i>G</i> (<i>ggccBB</i>). These albinos are, as it were, like + photographic plates exposed but undeveloped. <!-- Page 53 --><span + class="pagenum"><a name="page53"></a>{53}</span>Their potentialities may + be quite different, although they all look alike, but this can only be + tested by treating them with a colour developer. In the case of the mice + and rabbits the potentiality for which we wish to test is the presence or + absence of the factor <i>G</i>, and in order to develop the colour we + must introduce the factor <i>C</i>. Our developer, therefore, must + contain <i>C</i> but not <i>G</i>. In other words, it must be a + homozygous black mouse or rabbit, <i>ggCCBB</i>. Since such an animal is + pure for <i>C</i> it must, when mated with any of the albinos, produce + only coloured offspring. And since it does not contain <i>G</i> the + appearance of agoutis among its offspring must be attributed to the + presence of <i>G</i> in the albino. Tested in this way the F<sub>2</sub> + albinos were proved, as was expected, to be of three kinds: (1) those + which gave only agouti, <i>i.e.</i> which were homozygous for <i>G</i>; + (2) those which gave agoutis and blacks in approximately equal numbers, + <i>i.e.</i> which were heterozygous for <i>G</i>; and (3) those which + gave only blacks, and therefore did not contain <i>G</i>.</p> + + <p>Though albinos, whether mice, rabbits, rats, or other animals, breed + true to albinism, and though albinism behaves as a simple recessive to + colour, yet albinos may be of many different sorts. There are in fact + just as many kinds of albinos as there are coloured forms—neither + more nor less. And all these different kinds of albinos may breed + together, transmitting the various colour factors according to the + Mendelian scheme of inheritance, <!-- Page 54 --><span class="pagenum"><a + name="page54"></a>{54}</span>and yet the visible result will be nothing + but albinos. Under the mask of albinism is all the while occurring that + segregation of the different colour factors which would result in all the + varieties of coloured forms, if only the essential factor for colour + development were present. But put in the developer by crossing with a + pure coloured form and their variety of constitution can then at last + become manifest.</p> + + <p>So far we have dealt with cases in which the production of a character + is dependent upon the interaction of two factors. But it may be that some + characters require the simultaneous presence of a greater number of + factors for their manifestation, and the experiments of Miss Saunders + have shown that there is a character in stocks which is unable to appear + except through the interaction of three distinct factors. Coloured stocks + may be either hoary, with the leaves and stem covered by small hairs, or + they may lack the hairy covering, in which case they are termed glabrous. + Hoariness is dominant to glabrousness; that is to say, there is a + definite factor which can turn the glabrous into a hoary plant when it is + present. But in families where coloured and white stocks occur the white + are always glabrous, while the coloured plants may or may not be hoary. + Now colour in the stock as in the sweet pea has been proved to be + dependent upon the interaction of two separate factors. Hence hoariness + depends upon three separate factors, and a stock cannot be hoary unless + <!-- Page 55 --><span class="pagenum"><a name="page55"></a>{55}</span>it + contains the hoary factor in addition to the two colour factors. It + requires the presence of all these three factors to produce the hoary + character, though how this comes about we have not at present the least + idea.</p> + + <div class="figcenter" style="width:49%;"> + <a href="images/067.png"><img style="width:100%" src="images/067.png" + alt="Fig. 9. Sections of primula flowers." title="Fig. 9. Sections of primula flowers." /></a> + <span class="sc">Fig. 9.</span> + + <p class="poem">Sections of primula flowers. The anthers are shown as + black. A, "pin" form with long style and anthers set low down; B, + "thrum" form with short style and anthers set higher up; C, homostyle + form with anthers set low down as in "pin," but with short style. This + form only occurs with the large eye.</p> + </div> + + <div class="figcenter" style="width:50%;"> + <a href="images/068.jpg"><img style="width:100%" src="images/068t.jpg" + alt="Fig. 10. Two primula flowers." title="Fig. 10. Two primula flowers." /></a> + <span class="sc">Fig. 10.</span> + + <p class="cenhead">Two primula flowers showing the extent of the small + and of the large eye.</p> + </div> + + <p>A somewhat different and less usual form of interaction between + factors may be illustrated by a case in primulas recently worked out by + Bateson and Gregory. Like the common primrose, the primula exhibits both + pin-eyed and thrum-eyed varieties. In the former the style is long, and + the centre of the eye is formed by the end of the stigma which more or + less plugs up the opening of the corolla (cf. Fig. 9, A); in the latter + the style is short and hidden by the four anthers which spring from + higher up in the corolla and form the centre of the eye (cf. Fig. 9, B). + The greater part of the "eye" is formed by the greenish-yellow patches on + each petal just at the opening <!-- Page 56 --><span class="pagenum"><a + name="page56"></a>{56}</span>of the corolla. In most primulas the eye is + small, but there are some in which it is large and extends as a flush + over a considerable part of the petals (Fig. 10). Experiments showed that + these two pairs of characters behave in simple Mendelian fashion, short + style ( = "thrum") being dominant to long style (= "pin") and small eye + dominant to large. Besides the normal long and short styled forms, there + occurs a third form, which has been termed homostyle. In this form the + anthers are placed low down in the corolla tube as they are in the + long-styled form, but the style remains short instead of reaching up to + the corolla opening (Fig. 9, C). In the course of their experiments + Bateson and Gregory crossed a large-eyed homostyle plant with a + small-eyed thrum ( = short style). The F<sub>1</sub> plants were all + short styled with small eyes. <!-- Page 57 --><span class="pagenum"><a + name="page57"></a>{57}</span><span class="figright" style="width:35%;"><a + href="images/069.png"><img style="width:100%" src="images/069.png" + alt="Generations of primulas." title="Generations of primulas." + /></a></span>On self-fertilisation these gave an F<sub>2</sub> generation + consisting of four types, viz. short styled with small eyes, short styled + with large eyes, <i>long styled</i> with small eyes, and + <i>homostyled</i> with large eyes. The notable feature of this generation + is the appearance of long-styled plants, which, however, occur only in + association with the small eye. The proportions in which these four types + appeared shows that the presence or absence of but two factors is + concerned, and at the same time provides the key to the nature of the + homostyled plants. These are potentially long styled, and the position of + the anthers is that of normal long-styled plants, but owing to some + interaction between the factors the style itself is unable to reach its + full development unless the factor for the small eye is present. For this + reason long-styled plants with the large eye are always of the homostyle + form. What the connecting-link between these apparently unrelated + structures may be we cannot yet picture to ourselves, any more than we + can picture the relation between flower <!-- Page 58 --><span + class="pagenum"><a name="page58"></a>{58}</span>colour and hairiness in + stocks. It is evident, however, that the conception of the interaction of + factors, besides clearing up much that is paradoxical in heredity, + promises to indicate lines of research which may lead to valuable + extensions in our knowledge of the way in which the various parts of the + living organism are related to one another.</p> + +<hr class="full" /> + +<p><!-- Page 59 --><span class="pagenum"><a name="page59"></a>{59}</span></p> + +<h3>CHAPTER VI</h3> + +<p class="cenhead">REVERSION</p> + + <p>As soon as the idea was grasped that characters in plants and animals + might be due to the interaction of complementary factors, it became + evident that this threw clear light upon the hitherto puzzling phenomenon + of reversion. We have already seen that in certain cases the cross + between a black mouse or rabbit and an albino, each belonging to true + breeding strains, might produce nothing but agoutis. In other words, the + cross between the black and the white in certain instances results in a + complete reversion to the wild grey form. Expressed in Mendelian terms, + the production of the agouti was the necessary consequence of the meeting + of the factors <i>C</i> and <i>G</i> in the same zygote. As soon as they + are brought together, no matter in what way, the reversion is bound to + occur. Reversion, therefore, in such cases we may regard as the bringing + together of complementary factors which had somehow in the course of + evolution become separated from one another. In the simplest cases, such + as that of the black and the white rabbit, only two factors are + concerned, and one of them is brought in from each of the <!-- Page 60 + --><span class="pagenum"><a name="page60"></a>{60}</span>two parents. But + in other cases the nature of the reversion may be more complicated owing + to a larger number of factors being concerned, though the general + principle remains the same. Careful breeding from the reversions will + enable us in each case to determine the number and nature of the factors + concerned, and in illustration of this we may take another example from + rabbits. The Himalayan rabbit is a well-known breed. In appearance it is + a white rabbit with pink eyes, but the ears, paws, and nose are black + (Pl. I., 2). The Dutch rabbit is another well-known breed. Generally + speaking, the anterior portion of the body is white, and the posterior + part coloured. Anteriorly, however, the eyes are surrounded by coloured + patches extending up to the ears, which are entirely coloured. At the + same time the hind paws are white (cf. Pl. I., 1). Dutch rabbits exist in + many varieties of colour, though in each one of these the distribution of + colour and white shows the same relations. In the experiments about to be + described a yellow Dutch rabbit was crossed with a Himalaya. The result + was a reversion to the wild agouti colour (Pl. I., 3). Some of the + F<sub>1</sub> individuals showed white patches, while others were + self-coloured. On breeding from the F<sub>1</sub> animals a series of + coloured forms resulted in F<sub>2</sub>. These were agoutis, blacks, + yellows, and sooty yellows, the so-called tortoise shells of the fancy + (Pl. I., 4-7).</p> + + <div class="figcenter" style="width:75%;"> + <a href="images/073.jpg"><img style="width:100%" src="images/073t.jpg" + alt="Plate I." title="Plate I." /></a> + <span class="sc">Plate</span> I. + + <p class="cenhead">1, Yellow Dutch Rabbit; 2, Himalayan; 3, Agouti ( = + grey) F<sub>1</sub> reversion; 4-8, F<sub>2</sub> types, viz.: 4, + Agouti; 5, Yellow; 6, Black; 7, Tortoiseshell; 8, Himalayan.</p> + </div> + +<p><!-- Page 61 --><span class="pagenum"><a name="page61"></a>{61}</span></p> + + <div class="figright" style="width:35%;"> + <a href="images/074.png"><img style="width:100%" src="images/074.png" + alt="Generations of rabbits." title="Generations of rabbits." /></a> + </div> + <p>In addition to these appeared Himalayans with either black points or + with lighter brownish ones, and the proportions in which they came showed + the Himalayan character to be a simple recessive. A certain number of the + coloured forms exhibited the Dutch marking to a greater or less extent, + but as its inheritance in this set of experiments is complicated and has + not yet been worked out, we may for the present neglect it and confine + our attention to the coloured types and to the Himalayans. The proportion + in which the four coloured types appeared in F<sub>2</sub> was very + nearly 9 agoutis, 3 blacks, 3 yellows, and 1 tortoiseshell. Evidently we + are here dealing with two factors: (1) the grey factor (G), which + modifies black into agouti, or tortoiseshell into yellow; and (2) an + intensifying factor (<i>I</i>), which intensifies yellow into agouti and + tortoiseshell into black. It may be mentioned here that other experiments + confirmed the view that the yellow rabbit is a dilute agouti, and the + tortoiseshell a dilute black. The Himalayan pattern behaves as a + recessive to self-colour. It is a self-coloured black rabbit lacking a + factor that allows the colour to develop except in the points. That + factor we may denote <!-- Page 62 --><span class="pagenum"><a + name="page62"></a>{62}</span>by <i>X</i>, and as far as it is concerned + the Himalayan is constitutionally <i>xx</i>. The Himalayan contains the + intensifying factor, for such pigment as it possesses in the points is + full coloured. At the same time it is black, <i>i.e.</i> lacking in the + factor <i>G</i>. With regard to these three factors, therefore, the + constitution of the Himalayan is <i>ggIIxx</i>. The last character which + we have to consider in this cross is the Dutch character. This was found + by Hurst to behave as a recessive to self-colour (<i>S</i>), and for our + present purpose we will regard it as differing from a self-coloured + rabbit in the lack of this factor.<a name="footnotetag3" + href="#footnote3"><sup>[3]</sup></a> The Himalayan is really a + self-coloured animal, which, however, is unable to show itself as a full + black owing to its not possessing the factor <i>X</i>. The results of + breeding experiments then suggest that we may denote the Himalayan by the + formula <i>ggIIxxSS</i> and the yellow Dutch by <i>GGiiXXss</i>. Each + lacks two of the factors upon the full complement of which the agouti + colour depends. By crossing them the complete series <i>GIXS</i> is + brought into the same zygote, and the result is a reversion to the colour + of the wild rabbit.</p> + + <div class="figright" style="width:35%;"> + <a href="images/076.png"><img style="width:100%" src="images/076.png" + alt="Generations of Bush and Cupid sweet peas." title="Generations of Bush and Cupid sweet peas." /></a> + </div> + <p>Most of the instances of reversion yet worked out are those in which + colour characters are concerned. The sweet pea, however, supplies us with + a good example of reversion in structural characters. A dwarf variety + known as the "Cupid" has been extensively grown for <!-- Page 63 --><span + class="pagenum"><a name="page63"></a>{63}</span>some years. In these + little plants the internodes are very short and the stems are few in + number, and attain to a length of only 9-10 inches. In course of growth + they diverge from one another, and come to lie prostrate on the ground + (Pl. II., 2). Curiously enough, although the whole plant is dwarfed in + other respects, this does not seem to affect the size of the flower, + which is that of a normal sweet pea. Another though less-known variety is + the "Bush" sweet pea. Its name is derived from its habit of growth. The + numerous stems do not diverge from one another, but all grow up side by + side, giving the plant the appearance of a compact bush (Pl. II., 1). + Under ordinary conditions it attains a height of 3½-4 feet. A number of + crosses were made between the Bush and Cupid varieties, with the somewhat + unexpected result that in every instance the F<sub>1</sub> plants showed + complete reversion to the size and habit of the ordinary tall sweet pea + (Pl. II., 3), which is the form of the wild plant as it occurs in Sicily + to-day. The F<sub>2</sub> generation from these reversionary talls + consisted of four different types, viz. <!-- Page 64 --><span + class="pagenum"><a name="page64"></a>{64}</span>talls, bushes, Cupids of + the procumbent type like the original Cupid parent, and Cupids with the + compact upright Bush habit (Pl. II., 4). These four types appeared in the + ratio 9 : 3 : 3 : 1, and this, of course, provided the clue to the nature + of the case. The characters concerned are (1) long internode of stem + between the leaves which is dominant to short internode, and (2) the + creeping procumbent habit which is dominant to the erect bush-like habit. + Of these characters length of internode was carried by the Bush, and the + procumbent habit by the original Cupid parent. The bringing of them + together by the cross resulted in a procumbent plant with long + internodes. This is the ordinary tall sweet pea of the wild Sicilian + type, reversion here, again, being due to the bringing together of two + complementary factors which had somehow become separated in the course of + evolution.</p> + + <p>To this interpretation it may be objected that the ordinary sweet pea + is a plant of upright habit. This, however, is not true. It only appears + so because the conventional way of growing it is to train it up sticks. + In reality it is of procumbent habit, with divergent stems like the + ordinary Cupid, a fact which can easily be observed by anyone who will + watch them grow without the artificial aid of prepared supports.</p> + + <div class="figcenter" style="width:50%;"> + <a href="images/078.jpg"><img style="width:100%" src="images/078t.jpg" + alt="Plate II." title="Plate II." /></a> + <span class="sc">Plate II.</span> + + <p class="cenhead">1, Bush Sweet Pea; 2, Cupid Sweet Pea; 3, + F<sub>1</sub> reversionary Tall; 4, Erect Cupid Sweet Pea; 5, Purple + Invincible; 6, Painted Lady; 7, Duke of Westminster (hooded + standard).</p> + </div> + +<p><!-- Page 65 --><span class="pagenum"><a name="page65"></a>{65}</span></p> + + <p>The cases of reversion with which we have so far dealt have been cases + in which the reversion occurs as an immediate result of a cross, + <i>i.e.</i> in the F<sub>1</sub> generation. This is perhaps the + commonest mode of reversion, but instances are known in which the + reversion that occurs when two pure types are crossed does not appear + until the F<sub>2</sub> generation. Such a case we have already met with + in the fowls' combs. It will be remembered that the cross between pure + pea and pure rose gave walnut combs in F<sub>1</sub>, while in the + F<sub>2</sub> generation a definite proportion, 1 in 16, of single combs + appeared (cf. p. <a href="#page32">32</a>). Now the single comb is the + form that is found in the wild jungle fowl, which is generally regarded + as the ancestor of the domestic breeds. If this is so, we have a case of + reversion in F<sub>2</sub>; and this in the <i>absence</i> of the two + factors brought together by the rose-comb and pea-comb parents. Instead + of the reversion being due to the bringing together of two complementary + factors, we must regard it here as due to the association of two + complementary absences. To this question, however, we shall revert later + in discussing the origin of domesticated varieties.</p> + + <div class="figright" style="width:39%;"> + <a href="images/080a.png"><img style="width:100%" src="images/080a.png" + alt="Darwin's case of reversion in pigeons." title="Darwin's case of reversion in pigeons." /></a> + </div> + <p>There is one other instance of reversion to which we must allude. This + is Darwin's famous case of the occasional appearance of pigeons reverting + to the wild blue rock (<i>Columba livia</i>), when certain domesticated + races are crossed together.<a name="footnotetag4" + href="#footnote4"><sup>[4]</sup></a> As is well known, Darwin made use of + this as an argument for regarding all the domesticated varieties as + having arisen from the same wild species. The original experiment is + somewhat complicated, and is shown in the accompanying scheme. + Essentially it lay in <!-- Page 66 --><span class="pagenum"><a + name="page66"></a>{66}</span>following the results flowing from crosses + between blacks and whites. Experiments recently made by Staples-Browne + have shown that this case of reversion also can be readily interpreted in + Mendelian terms. In these experiments the cross was made between black + barbs and white fantails. <span class="figleft" style="width:40%;"><a + href="images/080b.png"><img style="width:100%" src="images/080b.png" + alt="Reversion in pigeons." title="Reversion in pigeons." + /></a></span>The F<sub>1</sub> birds were all black with some white + splashes, evidently due to a separate factor introduced by the fantail. + On breeding these blacks together they gave an F<sub>2</sub> generation, + consisting of blacks (with or without white splashes), blues (with or + without white splashes), and whites in the ratio 9 : 3 : 4. The factors + concerned are colour (<i>C</i>), in the absence of <!-- Page 67 --><span + class="pagenum"><a name="page67"></a>{67}</span>which a bird is white, + and a black modifier (<i>B</i>), in the absence of which a coloured bird + is blue. The original black barb contained both of these factors, being + in constitution <i>CCBB</i>. The fantail, however, contained neither, and + was constitutionally <i>ccbb</i>. The F<sub>1</sub> birds produced by + crossing were in constitution <i>CcBb</i>, and being heterozygous for two + factors produced in equal numbers the four sorts of gametes <i>CB</i>, + <i>Cb</i>, <i>cB</i>, <i>cb</i>. The results of two such series of + gametes being brought together are shown in the usual way in Fig. 11. A + blue is a bird containing the colour factor but lacking the black + modifier, <i>i.e.</i> of the constitution <i>CCbb</i>, or <i>Ccbb</i>, + and such birds as the figure shows appear in the F<sub>2</sub> generation + on the average three times out of sixteen. Reversion here comes about in + F<sub>2</sub>, when the redistribution of the factors leads to the + formation of zygotes containing one of the two factors but not the + other.</p> + + <div class="figcenter" style="width:30%;"> + <a href="images/081.png"><img style="width:100%" src="images/081.png" + alt="Fig. 11. Scheme of inheritance for reversion in pigeons." title="Fig. 11. Scheme of inheritance for reversion in pigeons." /></a> + <span class="sc">Fig. 11.</span> + + <p class="poem">Diagram to illustrate the appearance of the + reversionary blue pigeon in F<sub>2</sub> from the cross of black with + white.</p> + </div> + +<hr class="full" /> + +<p><!-- Page 68 --><span class="pagenum"><a name="page68"></a>{68}</span></p> + +<h3>CHAPTER VII</h3> + +<p class="cenhead">DOMINANCE</p> + + <div class="figcenter" style="width:50%;"> + <a href="images/083.jpg"><img style="width:100%" src="images/083t.jpg" + alt="Fig. 11. Primula flowers." title="Fig. 11. Primula flowers." /></a> + <span class="sc">Fig. 12.</span> + + <p class="cenhead">Primula flowers to illustrate the intermediate + nature of the F<sub>1</sub> flower when <i>sinensis</i> is crossed with + <i>stellata</i>.</p> + </div> + + <div class="figright" style="width:40%;"> + <a href="images/084a.png"><img style="width:100%" src="images/084a.png" + alt="Generations of Primula Sinensis × Stellata." title="Generations of Primula Sinensis × Stellata." /></a> + </div> + <p>In the cases which we have hitherto considered the presence of a + factor produces its full effect whether it is introduced by both of the + gametes which go to form the zygote, or by one of them alone. The + heterozygous tall pea or the heterozygous rose-combed fowl cannot be + distinguished from the homozygous form by mere inspection, however close. + Breeding tests alone can decide which is the heterozygous and which the + homozygous form. Though this is true for the majority of characters yet + investigated, there are cases known in which the heterozygous form + differs in appearance from either parent. Among plants such a case has + been met with in the primula. The ordinary Chinese primula (<i>P. + sinensis</i>) (Fig. 12) has large rather wavy petals much crenated at the + edges. In the Star Primula (<i>P. stellata</i>) the flowers are much + smaller, while the petals are flat and present only a terminal notch + instead of the numerous crenations of <i>P. sinensis</i>. The + heterozygote produced by crossing these forms is intermediate in size and + appearance. When self-fertilised such plants behave in simple Mendelian + fashion, <!-- Page 69 --><span class="pagenum"><a + name="page69"></a>{69}</span>giving a generation consisting of + <i>sinensis</i>, intermediates, and <i>stellata</i> in the ratio + 1 : 2 : 1. Subsequent breeding from these plants showed that both the + <i>sinensis</i> and <i>stellata</i> which appeared in the F<sub>2</sub> + generation bred true, while the intermediates always gave all three forms + again in the same proportion. But though there is no dominance of the + character of either parent in such a case as this, the Mendelian + principle of segregation could hardly have a better illustration.</p> + +<p><!-- Page 70 --><span class="pagenum"><a name="page70"></a>{70}</span></p> + + <div class="figright" style="width:38%;"> + <a href="images/084b.png"><img style="width:100%" src="images/084b.png" + alt="Generations of Blue Andalusian fowl." title="Generations of Blue Andalusian fowl." /></a> + </div> + <p>Among birds a case of similar nature is that of the Blue Andalusian + fowl. Fanciers have long recognised the difficulty of getting this + variety to breed true. Of a slaty blue colour itself with darker hackles + and with black lacing on the feathers of the breast, it always throws + "wasters" of two kinds, viz. blacks, and whites splashed with black. + Careful breeding from the blues shows that the three sorts are always + produced in the same definite <!-- Page 71 --><span class="pagenum"><a + name="page71"></a>{71}</span>proportions, viz., one black, two blues, one + splashed white. This at once suggests that the black and the splashed + white are the two homozygous forms, and that the blues are heterozygous, + <i>i.e.</i>, producing equal numbers of "black" and "white splashed" + gametes. The view was tested by breeding the "wasters" + together—black with black, and splashed white with splashed + white—and it was found that each bred true to its respective type. + But when the black and the splashed white were crossed they gave, as was + expected, nothing but blues. In other words, we have the seeming paradox + of the black and the splashed white producing twice as many blues as do + the blues when bred together. The black and the splashed white "wasters" + are in reality the pure breeds, while the "pure" Blue Andalusian is a + mongrel which no amount of selection will ever be able to fix.</p> + + <p>In such cases as this it is obvious that we cannot speak of dominance. + And with the disappearance of this phenomenon we lose one criterion for + determining which of the two parent forms possesses the additional + factor. Are we, for example, to regard the black Andalusian as a splashed + white to which has been added a double dose of a colour-intensifying + factor, or are we to consider the white splashed bird as a black which is + unable to show its true pigmentation owing to the possession of some + inhibiting factor which prevents the manifestation of the black. Either + interpretation fits the facts equally well, <!-- Page 72 --><span + class="pagenum"><a name="page72"></a>{72}</span>and until further + experiments have been devised and carried out it is not possible to + decide which is the correct view.</p> + + <p>Besides these comparatively rare cases where the heterozygote cannot + be said to bear a closer resemblance to one parent more than to the + other, there are cases in which it is often possible to draw a visible + distinction between the heterozygote and the pure dominant. There are + certain white breeds of poultry, notably the White Leghorn, in which the + white behaves as a dominant to colour. But the heterozygous whites made + by crossing the dominant white birds with a pure coloured form (such as + the Brown Leghorn) almost invariably show a few coloured feathers or + "ticks" in their plumage. The dominance of white is not quite complete, + and renders it possible to distinguish the pure from the impure dominant + without recourse to breeding experiments.</p> + + <div class="figright" style="width:31%;"> + <a href="images/088.png"><img style="width:100%" src="images/088.png" + alt="Fig. 13. Scheme of inheritance for dominant and recessive white fowls." title="Fig. 13. Scheme of inheritance for dominant and recessive white fowls." /></a> + <span class="sc">Fig. 13.</span> + + <p class="poem">Diagram to illustrate the nature of the F<sub>2</sub> + generation from the cross between dominant white and recessive white + fowls.</p> + </div> + + <p>This case of the dominant white fowl opens up another interesting + problem in connection with dominance. By accepting the "Presence and + Absence" hypothesis we are committed to the view that the dominant form + possesses an extra factor as compared with the recessive. The natural way + of looking at this case of the fowl is to regard white as the absence of + colour. But were this so, colour should be dominant to white, which is + not the case. We are therefore forced to suppose that the absence of + colour in this instance is due to the presence of a factor whose <!-- + Page 73 --><span class="pagenum"><a name="page73"></a>{73}</span>property + is to inhibit the production of colour in what would otherwise be a pure + coloured bird. On this view the dominant white fowl is a coloured bird + plus a factor which inhibits the development of the colour. The view can + be put to the test of experiment. We have already seen that there are + other white fowls in which white is recessive to colour, and that the + whiteness of such birds is due to the fact that they lack a factor for + the development of colour. If we denote this factor by <i>C</i> and our + postulated inhibitor factor in the dominant white bird by <i>I</i>, then + we must write the constitution of the recessive white as <i>ccii</i>, and + the dominant white as <i>CCII</i>. We may now work out the results we + ought to obtain when a cross is made between these two pure white breeds. + The constitution of the F<sub>1</sub> bird must be <i>CcIi</i>. Such + birds being heterozygous for the inhibitor factor, should be whites + showing some coloured "ticks." Being heterozygous for both of the two + factors <i>C</i> and <i>I</i>, they will produce in equal numbers the + four different sorts of gametes <i>CI</i>, <i>Ci</i>, <i>cI</i>, + <i>ci</i>. The result of bringing two such similar series of gametes + together is shown in Fig. 13. Out of the sixteen squares, twelve contain + <i>I</i>; these will be white birds either with or without a few coloured + ticks. Three contain <i>C</i> but not <i>I</i>: these must be coloured + birds. One contains neither <i>C</i> nor <i>I</i>; this must be a white. + From such a mating we ought, therefore, to obtain both white and coloured + birds in the ratio 13 : 3. The results thus theoretically <!-- Page 74 + --><span class="pagenum"><a name="page74"></a>{74}</span>deduced were + found to accord with the actual facts of experiment. The F<sub>1</sub> + birds were all "ticked" whites, and in the F<sub>2</sub> generation came + white and coloured birds in the expected ratio. There seems, therefore, + little reason to doubt that the dominant white is a coloured bird in + which the absence of colour is due to the action of a colour-inhibiting + factor, though as to the nature of that factor we can at present make no + surmise. It is probable that other facts, which at first sight do not + appear to be in agreement with the "Presence and Absence" hypothesis, + will eventually be brought into line through the action of inhibitor + factors. Such a case, for instance, is that of bearded and beardless + wheats. Though the beard is obviously the additional character, the + bearded condition is recessive to the beardless. Probably we ought to + regard the beardless as a bearded wheat in which there is an inhibitor + that stops the beard from growing. It is not unlikely that as time goes + on we shall <!-- Page 75 --><span class="pagenum"><a + name="page75"></a>{75}</span>find many more such cases of the action of + inhibitor factors, and we must be prepared to find that the same visible + effect may be produced either by the addition or by the omission of a + factor. The dominant and recessive white poultry are indistinguishable in + appearance. Yet the one contains a factor more and the other a factor + less than the coloured bird.</p> + + <div class="figcenter" style="width:30%;"> + <a href="images/089.jpg"><img style="width:100%" src="images/089t.jpg" + alt="Fig. 14. Ears of beardless and bearded wheat." title="Fig. 14. Ears of beardless and bearded wheat." /></a> + <span class="sc">Fig. 14.</span> + + <p class="poem">Ears of beardless and bearded wheat. The beardless + condition is dominant to the bearded.</p> + </div> + +<p><!-- Page 76 --><span class="pagenum"><a name="page76"></a>{76}</span></p> + + <p>A phenomenon sometimes termed irregularity of dominance has been + investigated in a few cases. In certain breeds of poultry such as + Dorkings there occurs an extra toe directed backwards like the hallux + (cf. Fig. 15). In some families this character behaves as an ordinary + dominant to the normal, giving the expected 3 : 1 ratio in F<sub>2</sub>. + But in other families similarly bred the proportions of birds with and + without the extra toe appear to be unusual. It has been shown that in + such a family some of the birds without the extra toe may nevertheless + transmit the peculiarity when mated with birds belonging to strains in + which the extra toe never occurs. Though the external appearance of the + bird generally affords some indication of the nature of the gametes which + it is carrying, this is not always the case. Nevertheless we have reason + to suppose that the character segregates in the gametes, though the + nature of these cannot always be decided from the appearance of the bird + which bears them.</p> + + <div class="figcenter" style="width:46%;"> + <a href="images/091.png"><img style="width:100%" src="images/091.png" + alt="Fig. 15. Fowls' feet." title="Fig. 15. Fowls' feet." /></a> + <span class="sc">Fig. 15.</span> + + <p class="cenhead">Fowls' feet. On the right a normal and on the left + one with an extra toe.</p> + </div> + + <div class="figright" style="width:44%;"> + <a href="images/092.png"><img style="width:100%" src="images/092.png" + alt="Fig. 16. Scheme of inheritance of horns in sheep." title="Fig. 16. Scheme of inheritance of horns in sheep." /></a> + <span class="sc">Fig. 16.</span> + + <p class="poem">Scheme to illustrate the inheritance of horns in sheep. + Heterozygous males shown dark with a white spot, heterozygous females + light with a dark spot in the centre.</p> + </div> + + <p>There are cases in which an apparent irregularity of dominance has + been shown to depend upon another character, as in the experiments with + sheep carried out by Professor Wood. In these experiments two breeds were + crossed, of which one, the Dorset, is horned in both sexes, while the + other, the Suffolk, is without horns in either sex. Whichever way the + cross was made the resulting F<sub>1</sub> generation was similar; the + rams were horned, and <!-- Page 77 --><span class="pagenum"><a + name="page77"></a>{77}</span>the ewes were hornless. In the F<sub>2</sub> + generation raised from these F<sub>1</sub> animals both horned and + hornless types appeared in both sexes but in very different proportions. + While the horned rams were about three times as numerous as the hornless, + this relation was reversed among the females, in which the horned formed + only about one-quarter of the total. The simplest explanation of this + interesting case is to suppose that the dominance of the horned character + depends upon the sex of the animal—that it is dominant in the male + but recessive in the female. A pretty experiment was devised for putting + this view to the test. If it is true, equal numbers of gametes with and + without the horned factor must be produced by the F<sub>1</sub> ewes, + while the factor should be lacking in all the gametes of the hornless + F<sub>2</sub> rams. A <!-- Page 78 --><span class="pagenum"><a + name="page78"></a>{78}</span>hornless ram, therefore, put to a flock of + F<sub>1</sub> ewes should give rise to equal numbers of zygotes which are + heterozygous for the horned character, and of zygotes in which it is + completely absent. And since the heterozygous males are horned, while the + heterozgyous females are hornless, we should expect from this mating + equal numbers of horned and hornless rams, but only hornless ewes. The + result of the experiment confirmed this expectation. Of the ram lambs 9 + were horned and 8 were hornless, while all the 11 ewe lambs were + completely destitute of horns.</p> + + <div class="figcenter" style="width:60%;"> + <a href="images/093.jpg"><img style="width:100%" src="images/093t.jpg" + alt="Plate III." title="Plate III." /></a> + <span class="sc">Plate III.</span> + + <p class="cenhead">Sheep</p> + </div> + +<hr class="full" /> + +<p><!-- Page 79 --><span class="pagenum"><a name="page79"></a>{79}</span></p> + +<h3>CHAPTER VIII</h3> + +<p class="cenhead">WILD FORMS AND DOMESTIC VARIETIES</p> + + <p>In discussing the phenomena of reversion we have seen that in most + cases such reversion occurs when the two varieties which are crossed each + contain certain factors lacking in the other, of which the full + complement is necessary for the production of the reversionary wild form. + This at once suggests the idea that the various domestic forms of animals + and plants have arisen by the omission from time to time of this factor + or of that. In some cases we have clear evidence that this is the most + natural interpretation of the relation between the cultivated and the + wild forms. Probably the species in which it is most evident is the sweet + pea (<i>Lathyrus odoratus</i>). We have already seen reason to suppose + that as regards certain structural features the Bush variety is a wild + lacking the factor for the procumbent habit, that the Cupid is a wild + without the factor for the long inter-node, and that the Bush Cupid is a + wild minus both these factors. Nor is the evidence less clear for the + many colour varieties. In illustration we may consider in more detail a + case in which the cross between two whites resulted <!-- Page 80 --><span + class="pagenum"><a name="page80"></a>{80}</span>in a complete reversion + to the purple colour characteristic of the wild Sicilian form (Pl. IV.). + In this particular instance subsequent breeding from the purples resulted + in the production of six different colour forms in addition to whites. + The proportion of the coloured forms to the whites was 9 : 7 (cf. p. <a + href="#page44">44</a>), but it is with the relation of the six coloured + forms that we are concerned here. Of these six forms three were purples + and three were reds. The three purple forms were (1) the wild bicolor + purple with blue wings known in cultivation as the Purple Invincible (Pl. + IV., 4); (2) a deep purple with purple wings (Pl. IV., 5); and (3) a very + dilute purple known as the Picotee (Pl. IV., 6). Corresponding to these + three purple forms were three reds: (1) a bicolor red known as Painted + Lady (Pl. IV., 7); (2) a deep red with red wings known as Miss Hunt (Pl. + IV., 8); and (3) a very pale red which we have termed Tinged White<a + name="footnotetag5" href="#footnote5"><sup>[5]</sup></a> (Pl. IV., 9). In + the F<sub>2</sub> generation the total number of purples bore to the + total number of reds the ratio 3 : 1, and this ratio was maintained for + each of the corresponding classes. Purple, therefore, is dominant to red, + and each of the three classes of red differs from its corresponding + purple in not possessing the blue factor (<i>B</i>) which turns it into + purple.</p> + + <div class="figcenter" style="width:60%;"> + <a href="images/096.jpg"><img style="width:100%" src="images/096t.jpg" + alt="Plate IV." title="Plate IV." /></a> + <span class="sc">Plate IV.</span> + + <p class="cenhead">1, 2, Emily Henderson; 3, F<sub>1</sub> reversionary + Purple; 4-10, Various F<sub>2</sub> forms: 4, Purple; 5, Deep Purple; + 6, Picotee; 7, Painted Lady; 8, Miss Hunt; 9, Tinged White; 10, + White.</p> + </div> + +<p><!-- Page 81 --><span class="pagenum"><a name="page81"></a>{81}</span></p> + + <p>Again, the proportion in which the three classes of purples appeared + was 9 bicolors, 3 deep purples, 4 picotees. We are, therefore, concerned + here with the operation of two factors: (1) a light wing factor, which + renders the bicolor dominant to the dark winged form; and (2) a factor + for intense colour, which occurs in the bicolor and in the deep purple, + but is lacking in the dilute picotee. And here it should be mentioned + that these conclusions rest upon an exhaustive set of experiments + involving the breeding of many thousands of plants. In this cross, + therefore, we are concerned with the presence or absence of five factors, + which we may denote as follows:—</p> + + <div class="poem"> + <div class="stanza"> + <p>A colour base, <i>R</i>.</p> + <p>A colour developer, <i>C</i>.</p> + <p>A purple factor, <i>B</i>.</p> + <p>A light wing factor, <i>L</i>.</p> + <p>A factor for intense colour, <i>I</i>.</p> + </div> + </div> + + <p>On this notation our six coloured forms are:—</p> + +<table class="nob" summary="Factors of sweet peas." title="Factors of sweet peas."> +<tr><td class="spacsingle"> (1) Purple bicolor </td><td class="spacsingle"> <i>CRBLI</i>.<a name="footnotetag6" href="#footnote6"><sup>[6]</sup></a></td></tr> +<tr><td class="spacsingle"> (2) Deep purple </td><td class="spacsingle"> <i>CRBlI</i>.</td></tr> +<tr><td class="spacsingle"> (3) Picotee </td><td class="spacsingle"> <i>CRBLi</i> or <i>CRBli</i>.</td></tr> +<tr><td class="spacsingle"> (4) Red bicolor ( = Painted Lady) </td><td class="spacsingle"> <i>CRbLI</i>.</td></tr> +<tr><td class="spacsingle"> (5) Deep red ( = Miss Hunt) </td><td class="spacsingle"> <i>CRblI</i>.</td></tr> +<tr><td class="spacsingle"> (6) Tinged white </td><td class="spacsingle"> <i>CRbLi</i> or <i>CRbli</i>.</td></tr> +</table> + + <p>It will be noticed in this series that the various coloured <!-- Page + 82 --><span class="pagenum"><a name="page82"></a>{82}</span>forms can be + expressed by the omission of one or more factors from the purple bicolor + of the wild type. With the complete omission of each factor a new colour + type results, and it is difficult to resist the inference that the + various cultivated forms of the sweet pea have arisen from the wild by + some process of this kind. Such a view tallies with what we know of the + behaviour of the wild form when crossed by any of the garden varieties. + Wherever such crossing has been made the form of the hybrid has been that + of the wild, thus supporting the view that the wild contains a complete + set of all the differentiating factors which are to be found in the sweet + pea.</p> + + <p>Moreover, this view is in harmony with such historical evidence as is + to be gleaned from botanical literature, and from old seedsmen's + catalogues. The wild sweet pea first reached England in 1699, having been + sent from Sicily by the monk Franciscus Cupani as a present to a certain + Dr. Uvedale in the county of Middlesex. Somewhat later we hear of two new + varieties, the red bicolor, or Painted Lady, and the white, each of which + may be regarded as having "sported" from the wild purple by the omission + of the purple factor, or of one of the two colour factors. In 1793 we + find a seedsman offering also what he called black and scarlet varieties. + It is probable that these were our deep purple and Miss Hunt varieties, + and that somewhere about this time the factor for the <!-- Page 83 + --><span class="pagenum"><a name="page83"></a>{83}</span>light wing + (<i>L</i>) was dropped out in certain plants. In 1860 we have evidence + that the pale purple or Picotee, and with it doubtless the Tinged White, + had come into existence. This time it was the factor for intense colour + which had dropped out. And so the story goes on until the present day, + and it is now possible to express by the same simple method the relation + of the modern shades, of purple and reds, of blues and pinks, of hooded + and wavy standards, to one another and to the original wild form. The + constitution of many of these has now been worked out, and to-day it + would be a simple though perhaps tedious task to denote all the different + varieties by a series of letters indicating the factors which they + contain, instead of by the present system of calling them after kings and + queens, and famous generals, and ladies more or less well known.</p> + + <p>From what we know of the history of the various strains of sweet peas + one thing stands out clearly. The new character does not arise from a + pre-existing variety by any process of gradual selection, conscious or + otherwise. It turns up suddenly complete in itself, and thereafter it can + be associated by crossing with other existing characters to produce a + gamut of new varieties. If, for example, the character of hooding in the + standard (cf. Pl. II., 7) suddenly turned up in such a family as that + shown on Plate IV. we should be able to get a hooded form corresponding + to each of the forms with the erect <!-- Page 84 --><span + class="pagenum"><a name="page84"></a>{84}</span>standard; in other words, + the arrival of the new form would give us the possibility of fourteen + varieties instead of seven. As we know, the hooded character already + exists. It is recessive to the erect standard, and we have reason to + suppose that it arose as a sudden sport by the omission of the factor in + whose presence the standard assumes the erect shape characteristic of the + wild flower. It is largely by keeping his eyes open and seizing upon such + sports for crossing purposes that the horticulturist "improves" the + plants with which he deals. How these sports or <i>mutations</i> come + about we can now surmise. They must owe their origin to a disturbance in + the processes of cell division through which the gametes originate. At + some stage or other the normal equal distribution of the various factors + is upset, and some of the gametes receive a factor less than others. From + the union of two such gametes, provided that they are still capable of + fertilisation, comes the zygote which in course of growth develops the + new character.</p> + + <p>Why these mutations arise: what leads to the surmised unequal division + of the gametes: of this we know practically nothing. Nor until we can + induce the production of mutations at will are we likely to understand + the conditions which govern their formation. Nevertheless there are + already hints scattered about the recent literature of experimental + biology which lead us to hope that we may know more of these matters in + the future. <!-- Page 85 --><span class="pagenum"><a + name="page85"></a>{85}</span></p> + + <p>In respect of the evolution of its now multitudinous varieties, the + story of the sweet pea is clear and straightforward. These have all + arisen from the wild by a process of continuous loss. Everything was + there in the beginning, and as the wild plant parted with factor after + factor there came into being the long series of derived forms. Exquisite + as are the results of civilization, it is by the degradation of the wild + that they have been brought about. How far are we justified in regarding + this as a picture of the manner in which evolution works?</p> + + <p>There are certainly other species in which we must suppose that this + is the way that the various domesticated forms have arisen. Such, for + example, is the case in the rabbit, where most of the colour varieties + are recessive to the wild agouti form. Such also is the case in the rat, + where the black and albino varieties and the various pattern forms are + also recessive to the wild agouti type. And with the exception of a + certain yellow variety to which we shall refer later, such is also the + case with the many fancy varieties of mice.</p> + + <p>Nevertheless there are other cases in which we must suppose evolution + to have proceeded by the interpolation of characters. In discussing + reversion on crossing, we have already seen that this may not occur until + the F<sub>2</sub> generation, as, for example, in the instance of the + fowls' combs (cf. p. <a href="#page65">65</a>). The reversion to the + single comb occurred as the result of the removal of the two factors <!-- + Page 86 --><span class="pagenum"><a name="page86"></a>{86}</span>for rose + and pea. These two domesticated varieties must be regarded as each + possessing an additional factor in comparison with the wild single-combed + bird. During the evolution of the fowl, these two factors must be + conceived of as having been interpolated in some way. And the same holds + good for the inhibitory factor on which, as we have seen, the dominant + white character of certain poultry depends. In pigeons, too, if we regard + the blue rock as the ancestor of the domesticated breeds, we must suppose + that an additional melanic factor has arisen at some stage. For we have + already seen that black is dominant to blue, and the characters of + F<sub>1</sub>, together with the greater number of blacks than blues in + F<sub>2</sub>, negatives the possibility that we are here dealing with an + inhibitory factor. The hornless or polled condition of cattle, again, is + dominant to the horned condition, and if, as seems reasonable, we regard + the original ancestors of domestic cattle as having been horned, we have + here again the interpolation of an inhibitory factor somewhere in the + course of evolution.</p> + + <p>On the whole, therefore, we must be prepared to admit that the + evolution of domestic varieties may come about by a process of addition + of factors in some cases and of subtraction in others. It may be that + what we term additional factors fall into distinct categories from the + rest. So far, experiment seems to show that they are either of the nature + of melanic factors, or of inhibitory <!-- Page 87 --><span + class="pagenum"><a name="page87"></a>{87}</span>factors, or of + reduplication factors as in the case of the fowls' combs. But while the + data remain so scanty, speculation in these matters is too hazardous to + be profitable.</p> + +<hr class="full" /> + +<p><!-- Page 88 --><span class="pagenum"><a name="page88"></a>{88}</span></p> + +<h3>CHAPTER IX</h3> + +<p class="cenhead">REPULSION AND COUPLING OF FACTORS</p> + + <p>Although different factors may act together to produce specific + results in the zygote through their interaction, yet in all the cases we + have hitherto considered the heredity of each of the different factors is + entirely independent. The interaction of the factors affects the + characters of the zygote, but makes no difference to the distribution of + the separate factors, which is always in strict accordance with the + ordinary Mendelian scheme. Each factor in this respect behaves as though + the other were not present.</p> + + <p>A few cases have been worked out in which the distribution of the + different factors to the gametes is affected by their simultaneous + presence in the zygote. And the influence which they are able to exert + upon one another in such cases is of two kinds. They may repel one + another, refusing, as it were, to enter into the same zygote, or they may + attract one another, and, becoming linked together, pass into the same + gamete, as it were by preference. For the moment we may consider these + two sets of phenomena apart. <!-- Page 89 --><span class="pagenum"><a + name="page89"></a>{89}</span></p> + + <p>One of the best illustrations of repulsion between factors occurs in + the sweet pea. We have already seen that the loss of the blue or purple + factor (<i>B</i>) from the wild bicolor results in the formation of the + red bicolor known as Painted Lady (Pl. IV., 7). Further, we have seen + that the hooded standard is recessive to the ordinary erect standard. The + omission of the factor for the erect standard (<i>E</i>) from the purple + bicolor (Pl. II., 5) results in a hooded purple known as Duke of + Westminster (Pl. II., 7). And here it should be mentioned that in the + corresponding hooded forms the difference in colour between the wings and + standard is not nearly so marked as in the forms with the erect standard, + but the difference in structure appears to affect the colour, which + becomes nearly uniform. This may be readily seen by comparing the picture + of the purple bicolor on Plate II. with that of the Duke of Westminster + flower.</p> + + <p>Now when a Duke of Westminster is mated with a Painted Lady the factor + for erect standard (<i>E</i>) is brought in by the red, and that for blue + (<i>B</i>) by the Duke, and the offspring are consequently all purple + bicolors. Purples so formed are all heterozygous for these two factors, + and were the case a simple one, such as those which have already been + discussed, we should expect the F<sub>2</sub> generation to consist of + the four forms: erect purple, hooded purple, erect red, and hooded red in + the ratio 9 : 3 : 3 : 1. Such, however, is not the case. The + F<sub>2</sub> generation <!-- Page 90 --><span class="pagenum"><a + name="page90"></a>{90}</span>actually consists of only three forms, viz. + erect red, erect purple, and hooded purple, and the ratio in which these + three forms occur is 1 : 2 : 1. No hooded red has been known to occur in + such a family. Moreover further breeding shows that while the erect reds + and the hooded purples always breed true, the erect purples in such + families <i>never</i> breed true, but always behave like the original + F<sub>1</sub> plant, giving the three forms again in the ratio 1 : 2 : 1. + Yet we know that there is no difficulty in getting purple bicolors to + breed true from other families; and we know also that hooded red sweet + peas exist in other strains.</p> + + <div class="figcenter" style="width:36%;"> + <a href="images/106.png"><img style="width:100%" src="images/106.png" + alt="Generations of Painted Lady × Duke of Westminster cross." title="Generations of Painted Lady × Duke of Westminster cross." /></a> + </div> + <div class="figright" style="width:36%;"> + <a href="images/107.png"><img style="width:100%" src="images/107.png" + alt="Factors in Painted Lady × Duke of Westminster cross." title="Factors in Painted Lady × Duke of Westminster cross." /></a> + </div> + <p>On the assumption that there exists a repulsion between the factors + for erect standard and blue in a plant which is heterozygous for both, + this peculiar case receives a simple explanation. The constitutions of + the erect red and the hooded purple are <i>EEbb</i> and <i>eeBB</i> + respectively and that of the F<sub>1</sub> erect purple is <i>EeBb</i>. + Now let us suppose that in such a zygote there exists a repulsion <!-- + Page 91 --><span class="pagenum"><a name="page91"></a>{91}</span>between + <i>E</i> and <i>B</i>, such that when the plant forms gametes these two + factors will not go into the same gamete. On this view it can only form + two kinds of gametes, viz. <i>Eb</i> and <i>eB</i>, and these, of course, + will be formed in equal numbers. Such a plant on self-fertilisation must + give the zygotic series <i>EEbb</i> + 2 <i>EeBb</i> + <i>eeBB</i>, + <i>i.e.</i> 1 erect red, 2 erect purples, and 1 hooded purple. And + because the erect reds and the hooded purples are respectively homozygous + for <i>E</i> and <i>B</i>, they must thenceforward breed true. The erect + purples, on the other hand, being always formed by the union of a gamete + <i>Eb</i> with a gamete <i>eB</i>, are always heterozygous for both of + these factors. They can, consequently, never breed true, but must always + give erect reds, erect purples, and hooded purples in the ratio + 1 : 2 : 1. The experimental facts are readily explained on the assumption + of repulsion between the two <!-- Page 92 --><span class="pagenum"><a + name="page92"></a>{92}</span>factors <i>B</i> and <i>E</i> during the + formation of the gametes in a plant which is heterozygous for both.</p> + + <p>Other similar cases of factorial repulsion have been demonstrated in + the sweet pea, and two of these are also concerned with the two factors + with which we have just been dealing. Two distinct varieties of pollen + grains occur in this species, viz. the ordinary oblong form and a rather + smaller rounded grain. The former is dominant to the latter.<a + name="footnotetag7" href="#footnote7"><sup>[7]</sup></a> When a cross is + made between a purple with round pollen and a red with long pollen the + F<sub>1</sub> plant is a long pollened purple. But the F<sub>2</sub> + generation consists of purples with round pollen, purples with long + pollen, and reds with long pollen in the ratio 1 : 2 : 1. No red with + round pollen appears in F<sub>2</sub> owing to repulsion between the + factors for purple (<i>B</i>) and for long pollen (<i>L</i>). Similarly + plants produced by crossing a red hooded long with a red round having an + erect standard give in F<sub>1</sub> long pollened reds with an erect + standard, and these in F<sub>2</sub> produce the three types, round + pollened erect, long pollened erect, and long pollened hooded, in the + ratio 1 : 2 : 1. The repulsion here is between the long pollen factor + (<i>L</i>) and the factor for the erect standard (<i>E</i>).</p> + +<p><!-- Page 93 --><span class="pagenum"><a name="page93"></a>{93}</span></p> + + <p>Yet another similar case is known in which we are concerned with quite + different factors. In some sweet peas the axils whence the leaves and + flower stalks spring from the main stem are of a deep red colour. In + others they are green. The dark pigmented axil is dominant to the light + one. Again, in some sweet peas the anthers are sterile, setting no + pollen, and this condition is recessive to the ordinary fertile + condition. When a sterile plant with a dark axil is crossed by a fertile + plant with a light axil, the F<sub>1</sub> plants are all fertile with + dark axils. But such plants in F<sub>2</sub> give fertiles with light + axils, fertiles with dark axils, and steriles with dark axils in the + ratio 1 : 2 : 1. No light axilled steriles appear from such a cross owing + to the repulsion between the factor for dark axil (<i>D</i>) and that for + the fertile anther (<i>F</i>).</p> + + <p>These four cases have already been found in the sweet pea, and similar + phenomena have been met with by Gregory in primulas. To certain seemingly + analogous cases in animals where sex is concerned we shall refer + later.</p> + + <p>Now all of these four cases present a common feature which probably + has not escaped the attention of the reader. In all of them <i>the + original cross was such as to introduce one of the repelling factors with + each of the two parents</i>. If we denote our two factors by <i>A</i> and + <i>B</i>, the crosses have always been of the nature <i>AAbb</i> × + <i>aaBB</i>. Let us now consider what happens when both of the <!-- Page + 94 --><span class="pagenum"><a name="page94"></a>{94}</span>factors, + which in these cases repel one another, are introduced by one of the + parents, and neither by the other parent. And in particular we will take + the case in which we are concerned with purple and red flower colour, and + with long and round pollen, <i>i.e.</i> with the factors <i>B</i> and + <i>L</i>. When a purple long (<i>BBLL</i>) is crossed with a red round + (<i>bbll</i>) the F<sub>1</sub> (<i>BbLl</i>) is a purple with long + pollen, identical in appearance with that produced by crossing the long + pollened red with the round pollened purple. But the nature of the + F<sub>2</sub> generation is in some respects very different. The ratio of + purples to reds and of longs to rounds is in each case 3 : 1, as before. + But instead of an association between the red and the long pollen + characters the reverse is the case. The long pollen character is now + associated with purple and the round pollen with red. The association, + however, is not quite complete, and the examination of a large quantity + of similarly bred material shows that the purple longs are about twelve + times as numerous as the purple rounds, while the red rounds are rather + more than three times as many as the red longs. Now this peculiar result + could be brought about if the gametic series produced by the + F<sub>1</sub> plant consisted of 7 <i>BL</i> + 1 <i>Bl</i> + 1 <i>bL</i> + + 7 <i>bl</i> out of every 16 gametes. Fertilization between two such + similar series of 16 gametes would result in 256 plants, of which 177 + would be purple longs, 15 purple rounds, 15 red longs, and 49 red + rounds—a proportion of the four different kinds very close to <!-- + Page 95 --><span class="pagenum"><a name="page95"></a>{95}</span>that + actually found by experiment. It will be noticed that in the whole family + the purples are to the reds as 3 : 1, and the longs are also three times + as numerous as the rounds. The peculiarity of the case lies in the + distribution of these two characters with regard to one another. In some + way or other the factors for blue and for long pollen become linked + together in the cell divisions that give rise to the gametes, but the + linking is not complete. This holds good for all the four cases in which + repulsion between the factors occurs when one of the two factors is + introduced by each of the parents. <i>When both of the factors are + brought into the cross by the same parent we get coupling between them + instead of repulsion.</i> The phenomena of repulsion and coupling between + separate factors are intimately related, though hitherto we have not been + able to suggest why this should be so.</p> + + <p>Nor for the present can we suggest why certain factors should be + linked together in the peculiar way that we have reason to suppose that + they are during the process of the formation of the gametes. Nevertheless + the phenomena are very definite, and it is not unlikely that a further + study of them may throw important light on the architecture of the living + cell.</p> + +<p class="cenhead">APPENDIX TO CHAPTER IX</p> + + <p>As it is possible that some readers may care, in spite of its + complexity, to enter rather more fully into the peculiar phenomenon <!-- + Page 96 --><span class="pagenum"><a name="page96"></a>{96}</span>of the + coupling of characters, I have brought together some further data in this + Appendix. In the case we have already considered, where the factors for + blue colour and long pollen are concerned, we have been led to suppose + that the gametes produced by the heterozygous plant are of the nature 7 + <i>BL</i> : 1 <i>Bl</i> : 1 <i>bL</i> : 7 <i>bl</i>. Such a series of + ovules fertilised by a similar series of pollen grains will give a + generation of the following composition:—</p> + +<table class="nobctr" summary="Factors of blue colour and pollen." title="Factors of blue colour and pollen."> +<tr><td class="qspcsingle"> 49 <i>BBLL</i> </td><td class="qspcsingle"> + 7 <i>BBLl</i> </td><td class="qspcsingle"> + 7 <i>BbLL</i> </td><td class="qspcsingle"> + 49 </td><td class="qspcsingle"> <i>BbLl</i> </td><td class="qspcsingle"> + <i>BBll</i> </td><td class="qspcsingle"> + 7 <i>Bbll</i> </td><td class="qspcsingle"> + <i>bbLL</i> </td><td class="qspcsingle"> + 7 <i>bbLl</i> </td><td class="qspcsingle"> + 49 <i>bbll</i></td></tr> +<tr><td class="qspcsingle"> </td><td class="qspcsingle"> + 7 <i>BBLl</i> </td><td class="qspcsingle"> + 7 <i>BbLL</i> </td><td class="qspcsingle"> + </td><td class="qspcsingle"> <i>BbLl</i> </td><td class="qspcsingle"> </td><td class="qspcsingle"> + 7 <i>Bbll</i> </td><td class="qspcsingle"> </td><td class="qspcsingle"> + 7 <i>bbLl</i></td></tr> +<tr><td class="qspcsingle"> </td><td class="qspcsingle"> </td><td class="qspcsingle"> </td><td class="qspcsingle"> + </td><td class="qspcsingle"> <i>BbLl</i></td></tr> +<tr><td class="qspcsingle"> </td><td class="qspcsingle"> </td><td class="qspcsingle"> </td><td class="qspcsingle"> + 49 </td><td class="qspcsingle"> <i>BbLl</i></td></tr> +<tr><td colspan="5" align="center"><a href="images/$ubrace.png"><img src="images/$ubrace.png" class="middle" style="height:2ex; width:14em" alt="brace" /></a></td> + <td colspan="2" align="center"><a href="images/$ubrace.png"><img src="images/$ubrace.png" class="middle" style="height:2ex; width:5em" alt="brace" /></a></td> +<td colspan="2" align="center"><a href="images/$ubrace.png"><img src="images/$ubrace.png" class="middle" style="height:2ex; width:5em" alt="brace" /></a></td> +<td colspan="1" align="center"><a href="images/$ubrace.png"><img src="images/$ubrace.png" class="middle" style="height:2ex; width:3em" alt="brace" /></a></td></tr> +<tr><td colspan="5" align="center">177 purple, long</td> + <td colspan="2" align="center">15 purple, round</td> +<td colspan="2" align="center">15 red, long</td> +<td colspan="1" align="center">49 red, round</td></tr> +</table> + + <p>and as this theoretical result fits closely with the actual figures + obtained by experiment we have reason for supposing that the heterozygous + plant produces a series of gametes in which the factors are coupled in + this way. The intensity of the coupling, however, varies in different + cases. Where we are dealing with another, viz. fertility (<i>F</i>) and + the dark axil (<i>D</i>), the experimental numbers accord with the view + that the gametic series is here 15 <i>FD</i> : 1 <i>Fd</i> : 1 + <i>fD</i> : 15 <i>fd</i>. The coupling is in this instance more intense. + In the case of the erect standard (<i>E</i>) and blueness (<i>B</i>) the + coupling is even more intense, and the experimental evidence available at + present points to the gametic series here being 63 <i>Eb</i> : 1 + <i>EB</i> : 1 <i>eB</i> : 63 <i>eb</i>. There is evidence also for + supposing that the intensity of the coupling may vary in different + families for the same pair of factors. The coupling between blue and long + pollen is generally on the 7 : 1 : 1 : 7 <!-- Page 97 --><span + class="pagenum"><a name="page97"></a>{97}</span>basis, but in some cases + it may be on the 15 : 1 : 1 : 15 basis. But though the intensity of the + coupling may vary it varies in an orderly way. If <i>A</i> and <i>B</i> + are the two factors concerned, the results obtained in F<sub>2</sub> are + explicable on the assumption that the ratio of the four sorts of gametes + produced is a term of the series—</p> + +<table class="nobctr" summary="Factors of blue colour and pollen." title="Factors of blue colour and pollen."> +<tr><td class="qspcsingle" align="right"> 3 <i>AB</i> + </td><td class="qspcsingle" align="right"> <i>Ab</i> + </td><td class="qspcsingle" align="right"> <i>aB</i> + </td><td class="qspcsingle" align="right"> 3 </td><td class="qspcsingle"> <i>ab</i></td></tr> +<tr><td class="qspcsingle" align="right"> 7 <i>AB</i> + </td><td class="qspcsingle" align="right"> <i>Ab</i> + </td><td class="qspcsingle" align="right"> <i>aB</i> + </td><td class="qspcsingle" align="right"> 7 </td><td class="qspcsingle"> <i>ab</i></td></tr> +<tr><td class="qspcsingle" align="right"> 15 <i>AB</i> + </td><td class="qspcsingle" align="right"> <i>Ab</i> + </td><td class="qspcsingle" align="right"> <i>aB</i> + </td><td class="qspcsingle" align="right"> 15 </td><td class="qspcsingle"> <i>ab</i>, etc., etc.</td></tr> +</table> + + <p>In such a series the number of gametes containing <i>A</i> is equal to + the number lacking <i>A</i>, and the same is true for <i>B</i>. + Consequently the number of zygotes formed containing <i>A</i> is three + times as great as the number of zygotes which do not contain <i>A</i>; + and similarly for <i>B</i>. The proportion of dominants to recessives in + each case is 3 : 1. It is only in the distribution of the characters with + relation to one another that these cases differ from a simple Mendelian + case.</p> + + <p>As the study of these series presents another feature of some + interest, we may consider it in a little more detail. In the accompanying + table are set out the results produced by these different series of + gametes. The series marked by an asterisk have already been demonstrated + experimentally. The first term in the series, <!-- Page 98 --><span + class="pagenum"><a name="page98"></a>{98}</span>in which all the four + kinds of gametes are produced in equal numbers is, of course, that of a + simple Mendelian case where no coupling occurs.</p> + +<table class="allbctr" summary="Gametic series." title="Gametic series."> +<tr><td class="allb" align="center"> No. of<br />Gametes<br />in series. +</td><td class="allb" align="center"> Distribution of<br />Factors in Gametic<br />Series +</td><td class="allb" align="center"> No. of<br />Zygotes<br />produced. +</td><td class="allb" colspan="4" align="center"> Form of F<sub>2</sub> Generation.</td></tr> +<tr><td class="vertbsing" align="right" valign="bottom"> </td><td class="vertbsing" align="center" valign="bottom"> AB. Ab. aB. ab. </td><td class="vertbsing" align="right" valign="bottom"> </td><td class="qspcsingle" align="right" valign="bottom"> AB. </td><td class="qspcsingle" align="right" valign="bottom"> Ab.</td><td class="qspcsingle" align="right" valign="bottom"> aB.</td><td class="qspcsingle" align="right" valign="bottom"> ab. </td></tr> +<tr><td class="vertbsing" align="right" valign="bottom"> 4 </td><td class="vertbsing" align="center" valign="bottom"> 1 : 1 : 1 : 1 </td><td class="vertbsing" align="right" valign="bottom"> 16 </td><td class="qspcsingle" align="right" valign="bottom"> 9 </td><td class="qspcsingle" align="right" valign="bottom"> 3 </td><td class="qspcsingle" align="right" valign="bottom"> 3 </td><td class="qspcsingle" align="right" valign="bottom"> 1 </td></tr> +<tr><td class="vertbsing" align="right" valign="bottom"> 8 </td><td class="vertbsing" align="center" valign="bottom"> 3 : 1 : 1 : 3 </td><td class="vertbsing" align="right" valign="bottom"> 64 </td><td class="qspcsingle" align="right" valign="bottom"> 49 </td><td class="qspcsingle" align="right" valign="bottom"> 7 </td><td class="qspcsingle" align="right" valign="bottom"> 7 </td><td class="qspcsingle" align="right" valign="bottom"> 9 </td></tr> +<tr><td class="vertbsing" align="right" valign="bottom"> 16 </td><td class="vertbsing" align="center" valign="bottom"> 7 : 1 : 1 : 7 </td><td class="vertbsing" align="right" valign="bottom"> 256 </td><td class="qspcsingle" align="right" valign="bottom"> 177 </td><td class="qspcsingle" align="right" valign="bottom"> 15 </td><td class="qspcsingle" align="right" valign="bottom"> 15 </td><td class="qspcsingle" align="right" valign="bottom"> 49* </td></tr> +<tr><td class="vertbsing" align="right" valign="bottom"> 32 </td><td class="vertbsing" align="center" valign="bottom"> 15 : 1 : 1 : 15 </td><td class="vertbsing" align="right" valign="bottom"> 1024 </td><td class="qspcsingle" align="right" valign="bottom"> 737 </td><td class="qspcsingle" align="right" valign="bottom"> 31 </td><td class="qspcsingle" align="right" valign="bottom"> 31 </td><td class="qspcsingle" align="right" valign="bottom"> 225* </td></tr> +<tr><td class="vertbsing" align="right" valign="bottom"> 64 </td><td class="vertbsing" align="center" valign="bottom"> 31 : 1 : 1 : 31 </td><td class="vertbsing" align="right" valign="bottom"> 4096 </td><td class="qspcsingle" align="right" valign="bottom"> 3009 </td><td class="qspcsingle" align="right" valign="bottom"> 63 </td><td class="qspcsingle" align="right" valign="bottom"> 63 </td><td class="qspcsingle" align="right" valign="bottom"> 961 </td></tr> +<tr><td class="vertbsing" align="right" valign="bottom"> 128 </td><td class="vertbsing" align="center" valign="bottom"> 63 : 1 : 1 : 63 </td><td class="vertbsing" align="right" valign="bottom"> 16384 </td><td class="qspcsingle" align="right" valign="bottom"> 12161 </td><td class="qspcsingle" align="right" valign="bottom"> 127 </td><td class="qspcsingle" align="right" valign="bottom"> 127 </td><td class="qspcsingle" align="right" valign="bottom"> 3969* </td></tr> +<tr><td class="vertbsing" align="right" valign="bottom"> 2<i>n</i> </td><td class="vertbsing" align="center" valign="bottom"> (<i>n</i>-1) : 1 : 1 : (<i>n</i>-1) </td><td class="vertbsing" align="right" valign="bottom"> 4<i>n</i><sup>2</sup> </td><td class="qspcsingle" align="right" valign="bottom"> 3<i>n</i><sup>2</sup>-(2<i>n</i>-1) </td><td class="qspcsingle" align="right" valign="bottom"> (2<i>n</i>-1) </td><td class="qspcsingle" align="right" valign="bottom"> (2<i>n</i>-1) </td><td class="qspcsingle" align="right" valign="bottom"> <i>n</i><sup>2</sup>-(2<i>n</i>-1)</td></tr> +</table> + + <p>Now, as the table shows, it is possible to express the gametic series + by a general formula (<i>n</i> + 1) <i>AB</i> + <i>Ab</i> + <i>aB</i> + + (<i>n</i> - 1) <i>ab</i>, where 2<i>n</i> is the total number of the + gametes in the series. A plant producing such a series of gametes gives + rise to a family of zygotes in which 3<i>n</i><sup>2</sup> - (2<i>n</i> - + 1) show both of the dominant characters and <i>n</i><sup>2</sup> - + (2<i>n</i> - 1) show both of the recessive characters, while the number + of the two classes which each show one of the two dominants is (2<i>n</i> + - 1). When in such a series the coupling becomes closer the value of + <i>n</i> increases, but in comparison with <i>n</i><sup>2</sup> its value + becomes less and less. The larger <i>n</i> becomes the more negligible is + its value relatively to <i>n</i><sup>2</sup>. If, therefore, the coupling + were very close, the series 3<i>n</i><sup>2</sup> - (2<i>n</i> - 1) : + (2<i>n</i> - 1) : (2<i>n</i> - 1) : <i>n</i><sup>2</sup> - (2<i>n</i> - + 1) would approximate more and more to the series 3<i>n</i><sup>2</sup> : + <i>n</i><sup>2</sup>, <i>i.e.</i> to a simple 3 : 1 ratio. Though the + point is probably of more theoretical than practical interest, it is not + impossible that some of the cases which have hitherto been regarded as + following a simple 3 : 1 ratio will turn out on further analysis to + belong to this more complicated scheme.</p> + +<hr class="full" /> + +<p><!-- Page 99 --><span class="pagenum"><a name="page99"></a>{99}</span></p> + +<h3>CHAPTER X</h3> + +<p class="cenhead">SEX</p> + + <div class="figcenter" style="width:60%;"> + <a href="images/116.jpg"><img style="width:100%" src="images/116t.jpg" + alt="Fig. 17. Abraxas grossulariata varieties." title="Fig. 17. Abraxas grossulariata varieties." /></a> + <span class="sc">Fig.</span> 17. + + <p class="cenhead"><i>Abraxas grossulariata</i>, the common currant + moth, and (on the right) its paler lacticolor variety.</p> + </div> + + <p>In their simplest expression the phenomena exhibited by Mendelian + characters are sharp and clean cut. Clean cut and sharp also are the + phenomena of sex. It was natural, therefore, that a comparison should + have been early instituted between these two sets of phenomena. As a + general rule, the cross between a male and a female results in the + production of the two sexes in approximately equal numbers. The cross + between a heterozygous dominant and a recessive also leads to equal + numbers of recessives and of heterozygous dominants. Is it not, + therefore, possible that one of the sexes is heterozygous for a factor + which is lacking in the other, and that the presence or absence of this + factor determines the sex of the zygote? The results of some recent + experiments would appear to justify this interpretation, at any rate in + particular cases. Of these, the simplest is that of the common currant + moth (<i>Abraxas grossulariata</i>), of which there exists a pale variety + (Fig. 17) known as <i>lacticolor</i>. The experiments of Doncaster and + Raynor showed that the variety behaved as a simple recessive to the + normal form. But the distribution of the dominants and <!-- Page 100 + --><span class="pagenum"><a name="page100"></a>{100}</span>recessives + <span class="figright" style="width:52%;"><a href="images/116.png"><img + style="width:100%" src="images/116.png" alt="Results of crosses in + Abraxas grossulariata." title="Results of crosses in Abraxas grossulariata." + /></a></span>with regard to the sexes was peculiar. The original cross + was between a <i>lacticolor</i> female and a normal male. All the + F<sub>1</sub> moths of both sexes were of the normal <i>grossulariata</i> + type. The F<sub>1</sub> insects were then paired together and gave a + generation consisting of 3 normals : 1 <i>lacticolor</i>. But all the + <i>lacticolor</i> were females, and all the males were of the normal + pattern. It was, however, found possible to obtain the <i>lacticolor + male</i> by mating a <i>lacticolor</i> female with the F<sub>1</sub> + male. The family resulting from this cross consisted of normal males and + normal females, <i>lacticolor</i> males and <i>lacticolor</i> females, + and the <!-- Page 101 --><span class="pagenum"><a + name="page101"></a>{101}</span>four sorts were produced in approximately + equal numbers. In such a family there was no special association of + either of the two colour varieties with one sex rather than the other. + But the reverse cross, F<sub>1</sub> female by <i>lacticolor</i> male, + gave a very different result. As in the previous cross such families + contained equal numbers of the normal form and of the recessive variety. + But all of the normal <i>grossulariata</i> were males, while all the + <i>lacticolor</i> were females. Now this seemingly complex collection of + facts is readily explained if we make the following three + assumptions:—</p> + + <p>(1) The <i>grossulariata</i> character (<i>G</i>) is dominant to the + lacticolor character (<i>g</i>). This is obviously justified by the + experiments, for, leaving the sex distribution out of account, we get the + expected 3 : 1 ratio from F<sub>1</sub> × F<sub>1</sub>, and also the + expected ratio of equality when the heterozygote is crossed with the + recessive.</p> + + <p>(2) The female is heterozygous for a dominant factor (<i>F</i>) which + is lacking in the male. The constitution of a female is consequently + <i>Ff</i>, and of a male <i>ff</i>. This assumption is in harmony with + the fact that the sexes are produced in approximately equal numbers.</p> + + <p>(3) There exists repulsion between the factors <i>G</i> and <i>F</i> + in a zygote which is heterozygous for them both. Such zygotes + (<i>FfGg</i>) must always be females, and on this assumption will produce + gametes <i>Fg</i> and <i>fG</i> in equal numbers. <!-- Page 102 --><span + class="pagenum"><a name="page102"></a>{102}</span></p> + + <div class="figcenter" style="width:54%;"> + <a href="images/118.png"><img style="width:100%" src="images/118.png" + alt="Fig. 18. Scheme of inheritance for Abraxas grossulariata." title="Fig. 18. Scheme of inheritance for Abraxas grossulariata." /></a> + <span class="sc">Fig.</span> 18. + + <p class="poem">Scheme of inheritance in the F<sub>1</sub> and + F<sub>2</sub> generations resulting from the cross of <i>lacticolor</i> + female with <i>grossulariata</i> male. The character of each individual + is represented by the sex signs in brackets, the black being + <i>grossulariata</i> in appearance and the light ones + <i>lacticolor</i>.</p> + </div> + + <p>We may now construct a scheme for comparison with that on page <a + href="#page100">100</a> to show how these assumptions explain the + experimental results. The original parents were <i>lacticolor</i> female + and <i>grossulariata</i> male, which on our assumptions must be + <i>Ffgg</i> and <i>ffGG</i> respectively in constitution. Since the + female is always heterozygous for <i>F</i>, her gametes must be of two + kinds, viz. <i>Fg</i> and <i>fg</i>, while those of the pure + <i>grossulariata</i> male must be all <i>fG</i>. When an ovum <i>Fg</i> + is fertilised by a spermatozoon <i>fG</i>, the resulting zygote, + <i>FfGg</i>, is heterozygous for both <i>F</i> and <i>G</i>, and in + appearance is a female <i>grossulariata</i>. The zygote resulting from + the fertilisation of an ovum <i>fg</i> by a spermatozoon <i>fG</i> is + heterozygous for <i>G</i>, but does not contain <i>F</i>, and therefore + is a male <i>grossulariata</i>. Such a male being in constitution <!-- + Page 103 --><span class="pagenum"><a + name="page103"></a>{103}</span><i>ffGg</i> must produce gametes of two + kinds, <i>fG</i> and <i>fg</i>, in equal numbers. And since we are + assuming repulsion between <i>F</i> and <i>G</i>, the F<sub>1</sub> + female being in constitution <i>FfGg</i>, must produce equal numbers of + gametes <i>Fg</i> and <i>fG</i>. For on our assumption <i>F</i> and + <i>G</i> cannot enter into the same gamete. The series of gametes + produced by the F<sub>1</sub> moths, therefore, are <i>fG</i>, <i>fg</i> + by the male and <i>Fg</i>, <i>fG</i> by the female. The resulting + F<sub>2</sub> generation consequently consists of the four classes of + zygotes <i>Ffgg</i>, <i>FfGg</i>, <i>ffGg</i>, and <i>ffGG</i> in equal + numbers. In other words, the sexes are produced in equal numbers, the + proportion of normal grossulariata to <i>lacticolor</i> is 3 : 1, and all + of the <i>lacticolor</i> are females; that is to say, the results worked + out on our assumptions accord with those actually produced by experiment. + We may now turn to the results which should be obtained by crossing the + F<sub>1</sub> moths with the <i>lacticolor</i> variety. And first we will + take the cross <i>lacticolor</i> female × F<sub>1</sub> male. The gametes + produced by the lacticolor female we have already seen to be <i>Fg</i> + and <i>fg</i>, while those produced by the F<sub>1</sub> male are + <i>fG</i> and <i>fg</i>. The bringing together of these two series of + gametes must result in equal numbers of the four kinds of zygotes + <i>FfGg</i>, <i>Ffgg</i>, <i>ffGg</i>, and <i>ffgg</i>, <i>i.e.</i> of + female <i>grossulariata</i> and <i>lacticolor</i>, and of male + <i>grossulariata</i> and <i>lacticolor</i> in equal numbers. Here, again, + the calculated results accord with those of experiment. Lastly, we may + examine what should happen when the F<sub>1</sub> female is crossed with + the <i>lacticolor</i> <!-- Page 104 --><span class="pagenum"><a + name="page104"></a>{104}</span>male. The F<sub>1</sub> female, owing to + the repulsion between <i>F</i> and <i>G</i>, produces only the two kinds + of ova <i>Fg</i> and <i>fG</i>, and produces them in equal numbers. Since + the <i>lacticolor</i> male can contain neither <i>F</i> nor <i>G</i>, all + of its spermatozoa must be <i>fg</i>. The results of such a cross, + therefore, should be to produce equal numbers of the two kinds of zygote + <i>Ffgg</i> and <i>ffGg</i>, <i>i.e.</i> of <i>lacticolor</i> females and + of <i>grossulariata</i> males. And this, as we have already seen, is the + actual result of such a cross.</p> + + <p>Before leaving the currant moth we may allude to an interesting + discovery which arose out of these experiments. The <i>lacticolor</i> + variety in Great Britain is a southern form and is not known to occur in + Scotland. Matings were made between wild Scotch females and + <i>lacticolor</i> males. The families resulting from such matings were + precisely the same as those from <i>lacticolor</i> males and + F<sub>1</sub> females, viz. <i>grossulariata</i> males and + <i>lacticolor</i> females only. We are, therefore, forced to regard the + constitution of the wild <i>grossulariata</i> female as identical with + that of the F<sub>1</sub> female, <i>i.e.</i> as heterozygous for the + <i>grossulariata</i> factor as well as for the factor for femaleness. + Though from a region where <i>lacticolor</i> is unknown, the "pure" wild + <i>grossulariata</i> female is nevertheless a permanent mongrel, but it + can never reveal its true colours unless it is mated with a male which is + either heterozygous for <i>G</i> or pure <i>lacticolor</i>. And as all + the wild northern males are <!-- Page 105 --><span class="pagenum"><a + name="page105"></a>{105}</span>pure for the <i>grossulariata</i> + character this can never happen in a state of nature.</p> + + <div class="figright" style="width:28%;"> + <a href="images/122a.png"><img style="width:100%" src="images/122a.png" + alt="Fig. 19. Results of crossing Silky hen × Brown Leghorn cock." title="Fig. 19. Results of crossing Silky hen × Brown Leghorn cock." /></a> + <span class="sc">Fig. 19.</span> + + <p class="poem">Scheme illustrating the result of crossing a Silky hen + with a Brown Leghorn cock. Black sex signs denote deeply pigmented + birds, and light sex signs those without pigmentation. The light signs + with a black dot in the centre denote birds with a small amount of + pigment.</p> + </div> + + <p>An essential feature of the case of the currant moth lies in the + different results given by reciprocal crosses. <i>Lacticolor</i> female × + <i>grossulariata</i> male gives <i>grossulariata</i> alone of both sexes. + But <i>grossulariata</i> female × <i>lacticolor</i> male gives only + <i>grossulariata</i> males and <i>lacticolor</i> females. Such a + difference between reciprocal crosses has also been found in other + animals, and the experimental results, though sometimes more complicated, + are explicable on the same lines. An interesting case in which three + factors are concerned has been recently worked out in poultry. The Silky + breed of fowls is characterised among other peculiarities by a remarkable + abundance of melanic pigment. The skin is dull black, while the comb and + wattles are of a deep purple colour contrasting sharply with the white + plumage (Pl. V., 3). Dissection shows that this black pigment is widely + spread throughout the body, being especially marked in such membranes as + the mesenteries, the periosteum, and the pia mater surrounding the brain. + It also occurs in the connective tissues among the muscles. In the Brown + Leghorn, on the other hand, this pigment is not found. Reciprocal crosses + between these two breeds gave a remarkable difference in result. A cross + between the Silky hen and the Brown Leghorn cock produced F<sub>1</sub> + birds in which both sexes exhibited only traces of the pigment. On casual + observation they might have <!-- Page 106 --><span class="pagenum"><a + name="page106"></a>{106}</span>passed for unpigmented birds, for with the + exception of an occasional fleck of pigment their skin, comb and wattles + were as clear as in the Brown Leghorn (Pl. V., 1 and 4). Dissection + revealed the presence of a slight amount of internal pigment. Such birds + bred together gave some offspring with the full pigmentation of the + Silky, some without any pigment, and others showing different degrees of + pigment. None of the F<sub>2</sub> male birds, however, showed the full + deep pigmentation of the Silky.</p> + + <div class="figright" style="width:33%;"> + <a href="images/122b.png"><img style="width:100%" src="images/122b.png" + alt="Fig. 20. Results of crossing Brown Leghorn hen × Silky cock." title="Fig. 20. Results of crossing Brown Leghorn hen × Silky cock." /></a> + <span class="sc">Fig. 20.</span> + + <p class="cenhead">Scheme illustrating the result of crossing a Brown + Leghorn hen with a Silky cock (cf. Fig. 19).</p> + </div> + + <p>When, however, the cross was made the other way, viz. Brown Leghorn + hen × Silky cock, the result was different. While the F<sub>1</sub> male + birds were almost destitute of pigment as in the previous cross, the + F<sub>1</sub> hens, on the other hand, were nearly as deeply pigmented as + the pure Silky <!-- Page 107 --><span class="pagenum"><a + name="page107"></a>{107}</span>(Pl. V., 2). The male Silky transmitted + the pigmentation, but only to his daughters. Such birds bred together + gave an F<sub>2</sub> generation containing chicks with the full deep + pigment, chicks without pigment, and chicks with various grades of + pigmentation, all the different kinds in both sexes.</p> + + <div class="figleft" style="width:48%;"> + <a href="images/123.png"><img style="width:100%" src="images/123.png" + alt="Fig. 21. Result of crossing F_1 birds with Brown Leghorn." title="Fig. 21. Result of crossing F_1 birds with Brown Leghorn." /></a> + <span class="sc">Fig. 21.</span> + + <p class="cenhead">Scheme to illustrate the result of crossing + F<sub>1</sub> birds (<i>e.g.</i> Brown Leghorn × Silky) with the pure + Brown Leghorn.</p> + </div> + + <p>In analysing this complicated case many other different crosses were + made, but for the present it will be sufficient to mention but one of + these, viz. that between the F<sub>1</sub> birds and the pure Brown + Leghorn. The cross between the F<sub>1</sub> hen and the Brown Leghorn + cock produced only birds with a slight amount of pigment and birds + without pigment. And this was true for both the deeply pigmented and the + slightly pigmented types of F<sub>1</sub> hen. But when the F<sub>1</sub> + cock was mated to a Brown Leghorn hen, a definite proportion of the + chicks, one in eight, was deeply pigmented, and <i>these deeply pigmented + birds were always females</i> (cf. Fig. 21). And in this respect all the + F<sub>1</sub> males behaved alike, whether they were from the Silky hen + or from the Silky cock. We have, therefore, the paradox that the + F<sub>1</sub> hen, though herself deeply pigmented, cannot transmit this + condition to any of her offspring when she is mated to the unpigmented + Brown Leghorn, but that, when similarly mated, the F<sub>1</sub> cock can + transmit this pigmented condition to a quarter of his female offspring + though he himself is almost devoid of pigment.</p> + + <div class="figcenter" style="width:60%;"> + <a href="images/124.jpg"><img style="width:100%" src="images/124t.jpg" + alt="Plate V." title="Plate V." /></a> + <span class="sc">Plate V.</span> + + <p class="cenhead">1, 2, F<sub>1</sub> Cock and Hen, ex Brown Leghorn + Hen × Silky Cock; 3, Silky Cock; 4, Hen ex Silky Hen × Brown Leghorn + Cock.</p> + </div> + +<p><!-- Page 108 --><span class="pagenum"><a name="page108"></a>{108}</span></p> + + <div class="figright" style="width:28%;"> + <a href="images/126.png"><img style="width:100%" src="images/126.png" + alt="Fig. 22. Scheme of inheritance for Silky hen × Brown Leghorn cock." title="Fig. 22. Scheme of inheritance for Silky hen × Brown Leghorn cock." /></a> + <span class="sc">Fig. 22.</span> + + <p class="poem">Scheme to illustrate the nature of the F<sub>1</sub> + generation from the Silky hen and Brown Leghorn cock (cf. Fig. 23).</p> + </div> + + <p>Now all these apparently complicated results, as well as many others + to which we have not alluded, can be expressed by the following simple + scheme. There are three factors affecting pigment, viz. (1) a + pigmentation factor (<i>P</i>); (2) a factor which inhibits the + production of pigment (<i>I</i>); and (3) a factor for femaleness + (<i>F</i>), for which the female birds are heterozygous, but which is not + present in the males. Further, we make the assumptions (<i>a</i>) that + there is repulsion between <i>F</i> and <i>I</i> in the female zygote + (<i>FfIi</i>), and (<i>b</i>) that the male Brown Leghorn is homozygous + for the inhibitor factor (<i>I</i>), but that the hen Brown Leghorn is + always heterozygous for this factor just in the same way as the female of + the currant moth is always heterozygous for the <i>grossulariata</i> + factor. We may now proceed to show how this explanation fits the + experimental facts which we have given.</p> + + <p>The Silky is pure for the pigmentation factor, but does not contain + the inhibitor factor. The Brown Leghorn, on the other hand, contains the + inhibitor factor, but not the <!-- Page 109 --><span class="pagenum"><a + name="page109"></a>{109}</span>pigmentation factor. In crossing a Silky + hen with a Brown Leghorn cock we are mating two birds of the constitution + <i>FfPPii</i> and <i>ffppII</i>, and all the F<sub>1</sub> birds are + consequently heterozygous for both <i>P</i> and <i>I</i>. In such birds + the pigment is almost but not completely suppressed, and as both sexes + are of the same constitution with regard to these two factors they are + both of similar appearance.</p> + + <div class="figleft" style="width:27%;"> + <a href="images/127.png"><img style="width:100%" src="images/127.png" + alt="Fig. 23. Scheme of inheritance for Brown Leghorn hen × Silky cock." title="Fig. 23. Scheme of inheritance for Brown Leghorn hen × Silky cock." /></a> + <span class="sc">Fig. 23.</span> + + <p class="poem">Scheme to illustrate the nature of the F<sub>1</sub> + generation from the Brown Leghorn hen and Silky cock (cf. Fig. 22).</p> + </div> + + <p>In the reciprocal cross, on the other hand, we are mating a Silky male + (<i>ffPPii</i>) with a Brown Leghorn hen which on our assumption is + heterozygous for the inhibitor factor (<i>I</i>), and in constitution + therefore is <i>FfppIi</i>. Owing to the repulsion between <i>F</i> and + <i>I</i> the gametes produced by such a bird are <i>Fpi</i> and + <i>fpI</i> in equal numbers. All the gametes produced by the Silky cock + are <i>fPi</i>. Hence the constitution of the F<sub>1</sub> male birds + produced by this cross is <i>ffPpIi</i> as before, but the female birds + must be all of the constitution <i>FfPpii</i>. The Silky cock transmits + the fully pigmented condition to his daughters, because the gametes of + the Brown Leghorn hen which contain the factor for femaleness do not + contain the <!-- Page 110 --><span class="pagenum"><a + name="page110"></a>{110}</span>inhibitory factor owing to the repulsion + between these factors. The nature of the F<sub>2</sub> generation in each + case is in harmony with the above scheme. As, however, it serves to + illustrate certain points in connection with intermediate forms we shall + postpone further consideration of it till we discuss these matters, and + for the present shall limit ourselves to the explanation of the different + behaviour of the F<sub>1</sub> males and females when crossed with the + Brown Leghorn. And, first, the cross of Brown Leghorn female by + F<sub>1</sub> male. The Brown Leghorn hen is on our hypothesis + <i>FfppIi</i>, and produces gametes <i>Fpi</i> and <i>fpI</i>. The + F<sub>1</sub> cock is on our hypothesis <i>ffPpIi</i>, and produces in + equal numbers the four kinds of gametes <i>fPI</i>, <i>fPi</i>, + <i>fpI</i>, <i>fpi</i>. The result of the meeting of these two series of + gametes is given in Fig. 24. Of the eight different kinds of zygote + formed only one contains <i>P</i> in the absence of <i>I</i>, and this is + a female. The result, as we have already seen, is in accordance with the + experimental facts.</p> + + <div class="figright" style="width:30%;"> + <a href="images/128.png"><img style="width:100%" src="images/128.png" + alt="Fig. 24. Scheme of inheritance for Brown Leghorn hen × F_1 cock." title="Fig. 24. Scheme of inheritance for Brown Leghorn hen × F_1 cock." /></a> + <span class="sc">Fig.</span> 24. + + <p class="poem">Diagram showing the nature of the offspring from a + Brown Leghorn hen and an F<sub>1</sub> cock bred from Silky hen × Brown + Leghorn cock, or <i>vice versa</i>.</p> + </div> + + <p>On the other hand, the Brown Leghorn cock is on our hypothesis + <i>ffppII</i>. All his gametes consequently contain the inhibitor factor, + and when he is mated with an F<sub>1</sub> <!-- Page 111 --><span + class="pagenum"><a name="page111"></a>{111}</span>hen all the zygotes + produced must contain <i>I</i>. None of his offspring, therefore, can be + fully pigmented, for this condition only occurs in the absence of the + inhibitor factor among zygotes which are either homozygous or + heterozygous for <i>P</i>.</p> + + <div class="figleft" style="width:50%;"> + <a href="images/129.png"><img style="width:100%" src="images/129.png" + alt="Fig. 25. Scheme showing the heterozygous nature of the pure Brown Leghorn hen." title="Fig. 25. Scheme showing the heterozygous nature of the pure Brown Leghorn hen." /></a> + <span class="sc">Fig. 25.</span> + + <p class="cenhead">Scheme to illustrate the heterozygous nature of the + pure Brown Leghorn hen. For explanation see text.</p> + </div> + + <p>The interpretation of this case turns upon the constitution of the + Brown Leghorn hen, upon her heterozygous condition with regard to the two + factors <i>F</i> and <i>I</i>, and upon the repulsion that occurs between + them when the gametes are formed. Through an independent set of + experiments this view of the nature of the Brown Leghorn hen has been + confirmed in an interesting way. There are fowls which possess neither + the factor for pigment nor the inhibitory factor, which are in + constitution <i>ppii</i>. Such birds when crossed with the Silky give + dark pigmented birds of both sexes in F<sub>1</sub>, and the + F<sub>2</sub> generation consists of pigmented and unpigmented in the + ratio 3 : 1. Now a cock of such a strain crossed with a Brown Leghorn hen + should give only completely unpigmented birds. But if, as we have + supposed, the Brown Leghorn hen is producing gametes <i>Fpi</i> and + <i>fpI</i>, the male birds produced by such a cross should be + heterozygous for <i>I</i>, <!-- Page 112 --><span class="pagenum"><a + name="page112"></a>{112}</span><i>i.e.</i> in constitution <i>ffppIi</i>, + while the hen birds, though identical in appearance so far as absence of + pigmentation goes, should not contain this factor but should be + constitutionally <i>Ffppii</i>. Crossed with the pure Silky, the + F<sub>1</sub> birds of opposite sexes should give an entirely different + result. For while the hens should give only deeply pigmented birds of + both sexes, the cocks should give equal numbers of deeply pigmented and + slightly pigmented birds (cf. Fig. 25). These were the results which the + experiment actually gave, thus affording strong confirmation of the view + which we have been led to take of the Brown Leghorn hen. Essentially the + poultry case is that of the currant moth. It differs in that the factor + which <!-- Page 113 --><span class="pagenum"><a + name="page113"></a>{113}</span>repels femaleness produces no visible + effect, and its presence or absence can only be determined by the + introduction of a third factor, that for pigmentation.</p> + + <p>This conception of the nature of the Brown Leghorn hen leads to a + curious paradox. We have stated that the Silky cock transmits the + pigmented condition, but transmits it to his daughters only. Apparently + the case is one of unequal transmission by the father. Actually, as our + analysis has shown, it is one of unequal transmission by the mother, the + father's contribution to the offspring being identical for each sex. The + mother transmits to the daughters her dominant quality of femaleness, but + to balance this, as it were, she transmits to her sons another quality + which her daughters do not receive. It is a matter of common experience + among human families that in respect to particular qualities the sons + tend to resemble their mothers more than the daughters do, and it is not + improbable that such observations have a real foundation for which the + clue may be provided by the Brown Leghorn hen.</p> + + <p>Nor is this the only reflection that the Brown Leghorn suggests. Owing + to the repulsion between the factors for femaleness and for pigment + inhibition, it is impossible by any form of mating to make a hen which is + homozygous for the inhibitor factor. She has bartered away for femaleness + the possibility of ever receiving a double dose of this factor. We know + that in some cases, as, for example, <!-- Page 114 --><span + class="pagenum"><a name="page114"></a>{114}</span>that of the blue + Andalusian fowl, the qualities of the individual are markedly different + according as to whether he or she has received a single or a double dose + of a given factor. It is not inconceivable that some of the qualities in + which a man differs from a woman are founded upon a distinction of this + nature. Certain qualities of intellect, for example, may depend upon the + existence in the individual of a double dose of some factor which is + repelled by femaleness. If this is so, and if woman is bent upon + achieving the results which such qualities of intellect imply, it is not + education or training that will help her. Her problem is to get the + factor on which the quality depends into an ovum that carries also the + factor for femaleness.</p> + +<hr class="full" /> + +<p><!-- Page 115 --><span class="pagenum"><a name="page115"></a>{115}</span></p> + +<h3>CHAPTER XI</h3> + +<p class="cenhead">SEX (<i>continued</i>)</p> + + <p>The cases which we have considered in the last chapter belong to a + group in which the peculiarities of inheritance are most easily explained + by supposing that the female is heterozygous for some factor that is not + found in the male. Femaleness is an additional character superposed upon + a basis of maleness, and as we imagine that there is a separate factor + for each the full constitutional formula for a female is <i>FfMM</i>, and + for a male <i>ffMM</i>. Both sexes are homozygous for the male element, + and the difference between them is due to the presence or absence of the + female element <i>F</i>.</p> + + <p>There are, however, other cases for which the explanation will not + suffice, but can be best interpreted on the view that the male is + heterozygous for a factor which is not found in the female. Such a case + is that recently described by Morgan in America for the pomace fly + (<i>Drosophila ampelophila</i>). Normally this little insect has a red + eye, but white eyed individuals are known to occur as rare sports. Red + eye is dominant to white. In their relation to sex the eye colours of the + pomace fly <!-- Page 116 --><span class="pagenum"><a + name="page116"></a>{116}</span>are inherited on the same lines as the + <i>grossulariata</i> and <i>lacticolor</i> patterns of the currant moth, + but with one essential difference. The factor which repels the red-eye + factor is in this case to be found in the male, and here consequently it + is the male which must be regarded as heterozygous for a sex factor that + is lacking in the female.</p> + + <div class="figright" style="width:23%;"> + <a href="images/133.png"><img style="width:100%" src="images/133.png" + alt="Fertilisations in Drosophila." title="Fertilisations in Drosophila." /></a> + </div> + <p>In order to bring these cases and others into line an interesting + suggestion has recently been put forward by Bateson. On this suggestion + each sex is heterozygous for its own sex factor only, and does not + contain the factor proper to the opposite sex. The male is of the + constitution, <i>Mmff</i> and the female <i>Ffmm</i>. Each sex produces + two sorts of gametes, <i>Mf</i> and <i>mf</i> in the case of the male, + and <i>Fm</i>, <i>fm</i> in that of the female. But on this view a + further supposition is necessary. If each of the two kinds of spermatozoa + were capable of fertilising each of the two kinds of ova, we should get + individuals of the constitution <i>MmFf</i> and <i>mmff</i>, as well as + the normal males and females, <i>Mmff</i> and <i>Ffmm</i>. As the facts + of ordinary bisexual reproduction afford us no grounds for assuming the + existence of these two classes of individuals, whatever they may be, we + must suppose that fertilisation. is productive only between the + spermatozoa carrying <i>M</i> and the ova without <i>F</i>, or between + the spermatozoa <!-- Page 117 --><span class="pagenum"><a + name="page117"></a>{117}</span>without <i>M</i> and the ova containing + <i>F</i>. In other words we must on this view suppose that fertilisations + between certain forms of gametes, even if they can occur, are incapable + of giving rise to zygotes with the capacity for further development. If + we admit this supposition, the scheme just given will cover such cases as + those of the currant moth and the fowl, equally as well as that of the + pomace fly. In the former there is repulsion between either the + <i>grossulariata</i> factor and <i>F</i>, or else between the pigment + inhibitor factor and <i>F</i>, while in the latter there is repulsion + between the factor for red eye and <i>M</i>.</p> + + <div class="figright" style="width:20%;"> + <a href="images/134.png"><img style="width:100%" src="images/134.png" + alt="Fig. 26. Scheme of probable mode of inheritance of colour-blindness." title="Fig. 26. Scheme of probable mode of inheritance of colour-blindness." /></a> + <span class="sc">Fig. 26.</span> + + <p class="poem">Scheme to illustrate the probable mode of inheritance + of colour-blindness. The dark signs represent affected individuals. A + black dot in the centre denotes an unaffected female who is capable of + transmitting the condition to her sons.</p> + </div> + + <p>Whatever the merits or demerits of such a scheme it certainly does + offer an explanation of a peculiar form of sex limited inheritance in + man. It has long been a matter of common knowledge that colour-blindness + is much more common among men than among women, and also that unaffected + women can transmit it to their sons. At first sight the case is not + unlike that of the sheep, where the horned character is apparently + dominant in the male but recessive in the female. The hypothesis that the + colour-blind condition is due to the presence of an extra factor as + compared with the normal, and that a single dose of it will produce <!-- + Page 118 --><span class="pagenum"><a + name="page118"></a>{118}</span>colour-blindness in the male but not in + the female, will cover a good many of the observed facts (cf. Fig. 26). + Moreover, it serves to explain the remarkable fact that <i>all</i> the + sons of colour-blind women are also colour-blind. For a woman cannot be + colour-blind unless she is homozygous for the colour-blind factor, in + which case all her children must get a single dose of it even if she + marries a normal male. And this is sufficient to produce colour-blindness + in the male, though not in the female.</p> + + <p>But there is one notable difference in this case as compared with that + of the sheep. When crossed with pure hornless ewes the heterozygous + horned ram transmits the horned character to half his male offspring (cf. + p. <a href="#page71">71</a>). But the heterozygous colour-blind man does + not behave altogether like a sheep, for he apparently does not transmit + the colour-blind condition to any of his male offspring. If, however, we + suppose that the colour-blind factor is repelled by the factor for + maleness, the amended scheme will cover the observed facts. For, denoting + the colour-blind factor by <i>X</i>, the gametes produced by the + colour-blind male are of two sorts only, viz. <i>Mfx</i> and <i>mfX</i>. + If he marries a normal woman (<i>Ffmmxx</i>), the spermatozoa <i>Mfx</i> + unite with ova <i>fmx</i> to give normal males, while the spermatozoa + <i>mfX</i> unite with ova <i>Fmx</i> to give females which are + heterozygous for the colour-blind factor. These daughters are themselves + normal, but transmit the condition to about half their sons. <!-- Page + 119 --><span class="pagenum"><a name="page119"></a>{119}</span></p> + + <p>The attempt to discover a simple explanation of the nature of sex has + led us to assume that certain combinations between gametes are incapable + of giving rise to zygotes which can develop further. In the various cases + hitherto considered there is no reason to suppose that anything of the + sort occurs, or that the different gametes are otherwise than completely + fertile one with another. One peculiar case, however, has been known for + several years in which some of the gametes are apparently incapable of + uniting to produce offspring. Yellow in the mouse is dominant to agouti, + but hitherto a homozygous yellow has never been met with. The yellows + from families where only yellows and agoutis occur produce, when bred + together, yellows and agoutis in the ratio 2 : 1. If it were an ordinary + Mendelian case the ratio should be 3 : 1, and one out of every three + yellows so bred should be homozygous and give only yellows when crossed + with agouti. But Cuénot and others have shown that <i>all</i> of the + yellows are heterozygous, and when crossed with agoutis give both yellows + and agoutis. We are led, therefore, to suppose that an ovum carrying the + yellow factor is unproductive if fertilised by a spermatozoon which also + bears this factor. In this way alone does it seem possible to explain the + deficiency of yellows and the absence of homozygous ones in the families + arising from the mating of yellows together. At present, however, it + remains the only definite instance among animals in which we have <!-- + Page 120 --><span class="pagenum"><a + name="page120"></a>{120}</span>grounds for assuming that anything in the + nature of unproductive fertilisation takes place.<a name="footnotetag8" + href="#footnote8"><sup>[8]</sup></a></p> + + <p>If we turn from animals to plants we find a more complicated state of + affairs. Generally speaking, the higher plants are hermaphrodite, both + ovules and pollen grains occurring on the same flower. Some plants, + however like most animals, are of separate sexes, a single plant bearing + only male or female flowers. In other plants the separate flowers are + either male or female, though both are borne on the same individual. In + others, again, the conditions are even more complex, for the same plant + may bear flowers of three kinds, viz. male, female, and hermaphrodite. Or + it may be that these three forms occur in the same species but in + different individuals—female and hermaphrodites in one species; + males, females, and hermaphrodites in another. One case, however, must be + mentioned as it suggests a possibility which we have not hitherto + encountered. In the common English bryony (<i>Bryonia dioica</i>) the + sexes are separate, some plants having only male and others only female + flowers. In another European species, <i>B. alba</i>, both male and + female flowers occur on the same plant. Correns crossed these two species + reciprocally, and also fertilised <i>B. dioica</i> by its own male with + the following results:—</p> + +<p><!-- Page 121 --><span class="pagenum"><a name="page121"></a>{121}</span></p> + +<table class="nobctr" summary="Bryonia dioica × B. alba." title="Bryonia dioica × B. alba."> +<tr><td class="qspcsingle"> dioica </td><td class="qspcsingle"> ♀ </td><td class="qspcsingle"> × dioica </td><td class="qspcsingle"> ♂ gave </td><td class="qspcsingle"> ♀ ♀ and ♂ ♂</td></tr> + +<tr><td class="qspcsingle"> " </td><td class="qspcsingle"> </td><td class="qspcsingle"> × alba </td><td class="qspcsingle"> ♂ " </td><td class="qspcsingle"> ♀ ♀ only</td></tr> + +<tr><td class="qspcsingle"> alba </td><td class="qspcsingle"> ♀ </td><td class="qspcsingle"> × dioica </td><td class="qspcsingle"> ♂ " </td><td class="qspcsingle"> ♀ ♀ and ♂ ♂.</td></tr> +</table> + + <p>The point of chief interest lies in the striking difference shown by + the reciprocal crosses between <i>dioica</i> and <i>alba</i>. Males + appear when <i>alba</i> is used as the female parent but not when the + female <i>dioica</i> is crossed by male <i>alba</i>. It is possible to + suggest more than one scheme to cover these facts, but we may confine + ourselves here to that which seems most in accord with the general trend + of other cases. We will suppose that in <i>dioica</i> femaleness is + dominant to maleness, and that the female is heterozygous for this + additional factor. In this species, then, the female produces equal + numbers of ovules with and without the female factor, while this factor + is absent in all the pollen grains. <i>Alba</i> ♀ × <i>dioica</i> + ♂ gives the same result as <i>dioica</i> ♀ × <i>dioica</i> + ♂, and we must therefore suppose that alba produces male and + female ovules in equal numbers. <i>Alba</i> ♂ x <i>dioica</i> + ♀, however, gives nothing but females. Unless, therefore, we + assume that there is selective fertilisation we must suppose that all the + pollen grains of alba carry the female factor—in other words, that + so far as the sex factors are concerned there is a difference between the + ovules and pollen grains borne by the same plant. Unfortunately further + investigation of this case is rendered impossible owing to the complete + sterility of the F<sub>1</sub> plants. <!-- Page 122 --><span + class="pagenum"><a name="page122"></a>{122}</span></p> + + <div class="figcenter" style="width:40%;"> + <a href="images/140.jpg"><img style="width:100%" src="images/140t.jpg" + alt="Fig. 27. Single and double stocks." title="Fig. 27. Single and double stocks." /></a> + <span class="sc">Fig. 27.</span> + + <p class="cenhead">Single and double stocks raised from the same single + parent.</p> + </div> + + <p>That the possibility of a difference between the ovules and pollen + grains of the same individual must be taken into account in future work + there is evidence from quite a different source. The double stock is an + old horticultural favourite, and for centuries it has been known that of + itself it sets no seed, but must be raised from special strains of the + single variety. "You must understand withall," wrote John Parkinson of + his gilloflowers,<a name="footnotetag9" + href="#footnote9"><sup>[9]</sup></a> "that those plants that beare double + flowers, doe beare <span class="figright" style="width:44%;"><a + href="images/141.png"><img style="width:100%" src="images/141.png" + alt="Crosses of single and double stocks." title="Crosses of single and double stocks." + /></a></span>no seed at all ... but the onely way to have double flowers + any yeare is to save the seedes of those plants of this kinde that beare + single flowers, for from that seede will rise some that will beare + single, and some double flowers." With regard to the nature of these + double-throwing strains of singles, Miss Saunders has recently brought + out some interesting facts. She crossed the double-throwing singles with + pure singles belonging to strains in which doubles never occur. The cross + was made both ways, and in both cases all the F<sub>1</sub> plants were + single. A distinction, however, appeared when a further generation was + raised from the F<sub>1</sub> plants. All the F<sub>1</sub> plants from + the pollen of the double-throwing single behaved like double-throwing + singles, but of the F<sub>1</sub> plants from the ovules of the double + throwers some behaved as double throwers, and some as pure singles. We + are led to infer, therefore, that the ovules and pollen grains <!-- Page + 123 --><span class="pagenum"><a name="page123"></a>{123}</span>of the + double throwers, though both produced by the same plant, differ in their + relation to the factor (or factors) for doubleness. Doubleness is + apparently carried by all the pollen grains of such plants, but only by + some of the ovules. Though the nature of doubleness in stocks is not yet + clearly understood, the facts discovered by Miss Saunders suggest + strongly that the ovules and pollen grains of the same plant may differ + in their transmitting properties, probably owing to some process of + segregation in the growing plant which leads to an unequal distribution + of some or other factors to the cells which give rise to the ovules as + compared with those from which <!-- Page 124 --><span class="pagenum"><a + name="page124"></a>{124}</span>the pollen grains eventually spring. + Whether this may turn out to be the true account or not, the possibility + must not be overlooked in future work.</p> + + <p>From all this it is clear enough that there is much to be done before + the problem of sex is solved even so far as the biologist can ever expect + to solve it. The possibilities are many, and many a fresh set of facts is + needed before we can hope to decide among them. Yet the occasional + glimpses of clear-cut and orderly phenomena, which Mendelian spectacles + have already enabled us to catch, offer a fair hope that some day they + may all be brought into focus, and assigned their proper places in a + general scheme which shall embrace them all. Then, though not till then, + will the problem of the nature of sex pass from the hands of the + biologist into those of the physicist and the chemist.</p> + +<hr class="full" /> + +<p><!-- Page 125 --><span class="pagenum"><a name="page125"></a>{125}</span></p> + +<h3>CHAPTER XII</h3> + +<p class="cenhead">INTERMEDIATES</p> + + <p>So far as we have gone we have found it possible to express the + various characters of animals and plants in terms of definite factors + which are carried by the gametes, and are distributed according to a + definite scheme. Whatever may be the nature of these factors it is + possible for purposes of analysis to treat them as indivisible entities + which may or may not be present in any given gamete. When the factor is + present it is present as a whole. The visible properties developed by a + zygote in the course of its growth depend upon the nature and variety of + the factors carried in by the two gametes which went to its making, and + to a less degree upon whether each factor was brought in by both gametes + or by one only. If the given factor is brought in by one gamete only, the + resulting heterozygote may be more or less intermediate between the + homozygous form with a double dose of the factor and the homozygous form + which is entirely destitute of the factor. Cases in point are those of + the primula flowers and the Andalusian fowls. Nevertheless these + intermediates produce only pure gametes, as is <!-- Page 126 --><span + class="pagenum"><a name="page126"></a>{126}</span>shown by the fact that + the pure parental types appear in a certain proportion of their + offspring. In such cases as these there is but a single type of + intermediate, and the simple ratio in which this and the two homozygous + forms appear renders the interpretation obvious. But the nature of the + F<sub>2</sub> generation may be much more complex, and, where we are + dealing with factors which interact upon one another, may even present + the appearance of a series of intermediate forms grading from the + condition found in one of the original parents to that which occurred in + the other. As an illustration we may consider the cross between the Brown + Leghorn and Silky fowls which we have already dealt with in connection + with the inheritance of sex. The offspring of a Silky hen mated with a + Brown Leghorn are in both sexes birds with but a trace of the Silky + pigmentation. But when such birds are bred together they produce a + generation consisting of chicks as deeply pigmented as the original Silky + parent, chicks devoid of pigment like the Brown Leghorn, and chicks in + which the pigmentation shows itself in a variety of intermediate stages. + Indeed from a hundred chicks bred in this way it would be possible to + pick out a number of individuals and arrange them in an apparently + continuous series of gradually increasing pigmentation, with the + completely unpigmented at one end and the most deeply pigmented at the + other. Nevertheless, the case is one in which complete segregation of the + different factors takes <!-- Page 127 --><span class="pagenum"><a + name="page127"></a>{127}</span><span class="figright" + style="width:30%;"><a href="images/144.png"><img style="width:100%" + src="images/144.png" alt="Fig. 28. Scheme of inheritance for Silky hen × + Brown Leghorn cock." title="Fig. 28. Scheme of inheritance for Silky hen × Brown Leghorn cock." + /></a> <span class="sc">Fig. 28.</span><br />Diagram to illustrate the + nature and composition of the F<sub>2</sub> generations arising from the + cross of Silky hen with Brown Leghorn cock.</span>place, and the + apparently continuous series of intermediates is the result of the + interaction of the different factors upon one another. The constitution + of the F<sub>1</sub> ♂ is a <i>ffPpIi</i>, and such a bird + produces in equal numbers the four sorts of gametes <i>fPI</i>, + <i>fPi</i>, <i>fpI</i>, <i>fpi</i>. The constitution of the F<sub>1</sub> + ♀ in this case is <i>FfPpIi</i>. Owing to the repulsion between + <i>F</i> and <i>I</i> she produces the four kinds of gametes <i>FPi</i>, + <i>Fpi</i>, <i>fPI</i>, <i>fpi</i>, and produces them in equal numbers. + The result of bringing two such series of gametes together is shown in + Fig. 28. Out of the sixteen types of zygote formed one (<i>FfPPii</i>) is + homozygous for the pigmentation factor, and does not contain the + inhibitor factor. Such a bird is as deeply pigmented as the pure Silky + parent. Two, again, contain a single dose of <i>P</i> in the absence of + <i>I</i>. These are nearly as dark as the pure Silky. Four zygotes are + destitute of <i>P</i>, though they may or may not contain <i>I</i>. These + birds are completely devoid of pigment like the Brown Leghorn. The + remaining nine zygotes show <!-- Page 128 --><span class="pagenum"><a + name="page128"></a>{128}</span>various combinations of the two factors + <i>P</i> and <i>I</i>, being either <i>PPIi</i>, <i>PPII</i>, + <i>PpII</i>, or <i>PpIi</i>, and in each of these cases the pigment is + more or less intense according to the constitution of the bird. Thus a + bird of the constitution <i>PPIi</i> approaches in pigmentation a bird of + the constitution <i>Ppii</i>, while a bird of the constitution + <i>PpII</i> has but little more pigment than the unpigmented bird. In + this way we have seven distinct grades of pigmentation, and the series is + further complicated by the fact that these various grades exhibit a + rather different amount of pigmentation according as they occur in a male + or a female bird, for, generally speaking, the female of a given grade + exhibits rather more pigment than the corresponding male. The examination + of a number of birds bred in this way might quite well suggest that in + this case we were dealing with a character which could break up, as it + were, to give a continuous series of intergrading forms between the two + extremes. With the constant handling of large numbers it becomes possible + to recognise most of the different grades, though even so it is possible + to make mistakes. Nevertheless, as breeding tests have amply shown, we + are dealing with but two interacting factors which segregate cleanly from + one another according to the strict Mendelian rule. The approach to + continuity in variation exhibited by the F<sub>2</sub> generation depends + upon the fact that these two factors interact upon one another, and to + different degrees according as the zygote is for one <!-- Page 129 + --><span class="pagenum"><a name="page129"></a>{129}</span>or other or + both of them in a homozygous or a heterozygous state. Moreover, certain + of these intermediates will breed true to an intermediate condition of + the pigmentation. A male of the constitution <i>ffPPII</i> when bred with + females of the constitution <i>FfPPIi</i> will produce only males like + itself and females like the maternal parent. We have dealt with this case + in some detail, because the existence of families showing a series of + intermediate stages between two characters has sometimes been brought + forward in opposition to the view that the characters of organisms depend + upon specific factors which are transmitted according to the Mendelian + rule. But, as this case from poultry shows clearly, neither the existence + of such a continuous series of intermediates, nor the fact that some of + them may breed true to the intermediate condition, are incompatible with + the Mendelian principle of segregation.</p> + + <p>In connection with intermediates a more cogent objection to the + Mendelian view is the case of the first cross between two definite + varieties thenceforward breeding true. The case that will naturally occur + to the reader is that of the mulatto, which results from the cross + between the negro and the white. According to general opinion, these + mulattos, of intermediate pigmentation, continue to produce mulattos. + Unfortunately this interesting case has never been critically + investigated, and the statement that the mulatto breeds true rests almost + entirely upon <!-- Page 130 --><span class="pagenum"><a + name="page130"></a>{130}</span>information that is general and often + vague. It may be that the inheritance of skin pigmentation in this + instance is a genuine exception to the normal rule, but at the same time + it must not be forgotten that it may be one in which several interacting + factors are concerned, and that the pure white and the pure black are the + result of combinations which from their rarity are apt to be overlooked. + But until we are in possession of accurate information it is impossible + to pronounce definitely upon the nature of the inheritance in this + case.</p> + + <div class="figright" style="width:42%;"> + <a href="images/147.png"><img style="width:100%" src="images/147.png" + alt="Fig. 29. Pedigree of a family which originated from a cross between a Hindu and a European." title="Fig. 29. Pedigree of a family which originated from a cross between a Hindu and a European." /></a> + <span class="sc">Fig. 29.</span> + + <p class="poem">Pedigree of a family which originated from a cross + between a Hindu and a European. Black signs denote individuals as dark + as average Hindus. Plain signs denote quite-fair members, while those + with a dot in the centre are intermediate.</p> + </div> + +<p><!-- Page 131 --><span class="pagenum"><a name="page131"></a>{131}</span></p> + + <p>On the other hand, from the cross between the darkly pigmented Eastern + races and the white segregation seems to occur in subsequent generations. + Families are to be found in which one parent is a pure white, while the + other has arisen from the cross between the dark and light in the first + or some subsequent generation. Such families may contain children + indistinguishable from pure blonds as well as children of very dark and + of intermediate shades. As an example, I may give the following pedigree, + which was kindly communicated to me by an Anglo-Indian friend (Fig. 29). + The family had resided in England for several generations, so that in + this case there was no question of a further admixture of black. Most + noticeable is the family produced by a very dark lady who had married a + white man. Some of the children were intermediate in colour, but two were + fair whites and two were dark as dark Hindus. This sharp segregation or + splitting out of blacks and whites in addition to intermediates strongly + suggests that the nature of the inheritance is Mendelian, though it may + be complicated by the existence of several factors which may also react + upon one another. Nor must it be forgotten that in so far as these + different factors are concerned the whites themselves may differ in + constitution without showing any trace of it in their appearance. Before + the case can be regarded as settled all these different possibilities + will have to be definitely tested. With the dark Eastern races as with + the negro we cannot <!-- Page 132 --><span class="pagenum"><a + name="page132"></a>{132}</span>hope to come to any conclusion until we + have evidence collected by critical and competent observers.</p> + + <p>Though for the present we must regard the case of the negro as not + proven, there are nevertheless two others in which the heredity would + appear not to follow the Mendelian rule. Castle in America crossed the + lop-eared rabbit with the normal form, and found that the F<sub>1</sub> + animals were intermediate with respect to their ears. And subsequent + experiment showed that, on the whole, they bred true to this intermediate + condition. The other case relates to Lepidoptera. The speckled wood + butterfly (<i>Pararge egeria</i>) has a southern form which differs from + the northern one in the greater brightness and depth of its yellow-brown + markings. The northern form is generally distinguished as var. + <i>egeriades</i>. Bateson crossed the southern form from the south of + France with the paler British form, and found that the offspring were + more or less intermediate in colour, and that in subsequent generations + the parental types did not recur. These cases at present stand alone. It + is possible that further research may reveal complications which mask or + interfere with an underlying process of segregation. Or it may be that + segregation does not occur owing to some definite physiological reason + which at present we do not understand.</p> + + <p>And here it is impossible not to recall Mendel's own experiences with + the Hawkweeds (<i>Hieracium</i>). This <!-- Page 133 --><span + class="pagenum"><a name="page133"></a>{133}</span>genus of plants + exhibits an extraordinary profusion of forms differing from one another + sometimes in a single feature, sometimes in several. The question as to + how far these numerous forms were to be classified as distinct species, + how far as varieties, and how far as products of chance hybridisation, + was even at that time a source of keen controversy among botanists. There + is little doubt that Mendel undertook his experiments on the Hawkweeds in + the hope that the conception of unit-characters so brilliantly + demonstrated for the pea would serve to explain the great profusion of + forms among the Hieraciums. Owing to the minute size of their florets, + these plants offer very considerable technical difficulties in the way of + cross fertilisation. By dint of great perseverance and labour, however, + Mendel succeeded in obtaining a few crosses between different forms. + These hybrids were reared and a further generation produced from them, + and, no doubt somewhat to Mendel's chagrin, every one of them proved to + breed true. There was a complete absence of that segregation of + characters which he had shown to exist in peas and beans, and had + probably looked forward with some confidence to finding in + <i>Hieracium</i>. More than thirty years passed before the matter was + cleared up. To-day we know that the peculiar behaviour of the hybrid + Hieraciums is due to the fact that they normally produce seed by a + peculiar process of parthenogenesis. It is possible to take an unopened + flower and to shear off with a <!-- Page 134 --><span class="pagenum"><a + name="page134"></a>{134}</span>razor all the male organs together with + the stigmata through which the pollen reaches the ovules. The flower, + nevertheless, sets perfectly good seed. But the cells from which the + seeds develop are not of the same nature as the normal ovules of a plant. + They are not gametes but retain the double structure of the maternal + cells. They are rather to be regarded as of the nature of buds which + early become detached from the parent stock to lead an independent + existence, and, like buds, they reproduce exactly the maternal + characteristics. The discovery of the true nature of this case was only + rendered possible by the development of the study of cytology, and it was + not given to Mendel to live long enough to learn why his hybrid + Hieraciums all bred true.</p> + +<hr class="full" /> + +<p><!-- Page 135 --><span class="pagenum"><a name="page135"></a>{135}</span></p> + +<h3>CHAPTER XIII</h3> + +<p class="cenhead">VARIATION AND EVOLUTION</p> + + <p>Through the facts of heredity we have reached a new conception of the + individual. Hitherto we have been accustomed to distinguish between the + members of a family of rabbits like that illustrated on Plate I. by + assigning to each an individuality, and by making use of certain external + features, such as the coat colour or the markings, as convenient outward + signs to express our idea that the individuality of these different + animals is different. Apart from this, our notions as to what constituted + the individuality in each case were at best but vague. Mendelian analysis + has placed in our hands a more precise method of estimating and + expressing the variations that are to be found between one individual and + another. Instead of looking at the individual as a whole, which is in + some vague way endowed with an individuality marking it off from its + fellows, we now regard it as an organism built up of definite characters + superimposed on a basis beyond which for the moment our analysis will not + take us. We have begun to realise that each individual has a definite + architecture, and that this architecture depends <!-- Page 136 --><span + class="pagenum"><a name="page136"></a>{136}</span>primarily upon the + number and variety of the factors that existed in the two gametes that + went to its building. Now most species exhibit considerable variation and + exist in a number, often very large, of more or less well-defined + varieties. How far can this great variety be explained in terms of a + comparatively small number of factors if the number of possible forms + depends upon the number of the factors which may be present or + absent?</p> + + <p>In the simple case where the homozygous and heterozygous conditions + are indistinguishable in appearance the number of possible forms is 2, + raised to the power of the number of factors concerned. Thus where one + factor is concerned there are only 2<sup>1</sup> = 2 possible forms, + where ten factors are concerned there are 2<sup>10</sup> = 1024 possible + forms differing from one another in at most ten and at least one + character. Where the factors interact upon one another this number will, + of course, be considerably increased. If the heterozygous form is + different in appearance from the homozygous form, there are three + possible forms connected with each factor; for ten such factors the + possible number of individuals would be 3<sup>10</sup> = 59,049; for + twenty such factors the possible number of different individuals would be + 3<sup>20</sup> = 3,486,784,401. The presence or absence of a + comparatively small number of factors in a species carries with it the + possibility of an enormous range of individual variation. But every one + of these individuals has a perfectly definite constitution which can <!-- + Page 137 --><span class="pagenum"><a name="page137"></a>{137}</span>be + determined in each case by the ordinary methods of Mendelian analysis. + For in every instance the variation depends upon the presence or absence + of definite factors carried in by the gametes from whose union the + individual results. And as these factors separate out cleanly in the + gametes which the individual forms, such variations as depend upon them + are transmitted strictly according to the Mendelian scheme. Provided that + the constitution of the gametes is unchanged, the heredity of such + variation is independent of any change in the conditions of nutrition or + environment which may operate upon the individual producing the + gametes.</p> + + <p>But, as everybody knows, an individual organism, whether plant or + animal, reacts, and often reacts markedly, to the environmental + conditions under which its life is passed. More especially is this to be + seen where such characters as size or weight are concerned. More sunlight + or a richer soil may mean stronger growth in a plant, better nutrition + may result in a finer animal, superior education may lead to a more + intelligent man. But although the changed conditions produce a direct + effect upon the individual, we have no indisputable evidence that such + alterations are connected with alterations in the nature of the gametes + which the individual produces. And without this such variations cannot be + perpetuated through heredity, but the conditions which produce the effect + must always be renewed in each <!-- Page 138 --><span class="pagenum"><a + name="page138"></a>{138}</span>successive generation. We are led, + therefore, to the conclusion that two sorts of variations exist, those + which are due to the presence of specific factors in the organism and + those which are due to the direct effect of the environment during its + lifetime. The former are known as <i>mutations</i>, and are inherited + according to the Mendelian scheme; the latter have been termed + <i>fluctuations</i>, and at present we have no valid reason for supposing + that they are ever inherited. For though instances may be found in which + effects produced during the lifetime of the individual would appear to + affect the offspring, this is not necessarily due to heredity. Thus + plants which are poorly nourished and grown under adverse conditions may + set seed from which come plants that are smaller than the normal although + grown under most favorable conditions. It is natural to attribute the + smaller size of the offspring to the conditions under which the parents + were grown, and there is no doubt that we should be quite right in doing + so. Nevertheless, it need have nothing to do with heredity. As we have + already pointed out, the seed is a larval plant which draws its + nourishment from the mother. The size of the offspring is affected + because the poorly nourished parent offered a bad environment to the + young plant, and not because the gametes of the parent were changed + through the adverse conditions under which it grew. The parent in this + case is not only the producer of gametes, but also a part of the + environment of the young <!-- Page 139 --><span class="pagenum"><a + name="page139"></a>{139}</span>plant, and it is in this latter capacity + that it affects its offspring. Wherever, as in plants and mammals, the + organism is parasitic upon the mother during its earlier stages, the + state of nutrition of the latter will almost certainly react upon it, and + in this way a semblance of transmitted weakness or vigour is brought + about. Such a connection between mother and offspring is purely one of + environment, and it cannot be too strongly emphasised that it has nothing + to do with the ordinary process of heredity.</p> + + <p>The distinction between these two kinds of variation, so entirely + different in their causation, renders it possible to obtain a clearer + view of the process of evolution than that recently prevalent. As Darwin + long ago realised, any theory of evolution must be based upon the facts + of heredity and variation. Evolution only comes about through the + survival of certain variations and the elimination of others. But to be + of any moment in evolutionary change a variation must be inherited. And + to be inherited it must be represented in the gametes. This, as we have + seen, is the case for those variations which we have termed mutations. + For the inheritance of fluctuations, on the other hand, of the variations + which result from the direct action of the environment upon the + individual, there is no indisputable evidence. Consequently we have no + reason for regarding them as playing any part in the production of that + succession of temporarily stable forms which we term evolution. In <!-- + Page 140 --><span class="pagenum"><a name="page140"></a>{140}</span>the + light of our present knowledge we must regard the mutation as the basis + of evolution—as the material upon which natural selection works. + For it is the only form of variation of whose heredity we have any + certain knowledge.</p> + + <p>It is evident that this view of the process of evolution is in some + respects at variance with that generally held during the past half + century. There we were given the conception of an abstract type + representing the species, and from it most of the individuals diverged in + various directions, though, generally speaking, only to a very small + extent. It was assumed that any variation, however small, might have a + selection value, that is to say, could be transmitted to the offspring. + Some of these would possess it in a less and some in a greater degree + than the parent. If the variation were a useful one, those possessing to + a rather greater extent would be favoured through the action of natural + selection at the expense of their less fortunate brethren, and would + leave a greater number of offspring, of whom some possessed it in an even + more marked degree than themselves. And so it would go on. The process + was a cumulative one. The slightest variation in a favourable direction + gave natural selection a starting-point to work on. Through the continued + action of natural selection on each successive generation the useful + variation was gradually worked up, until at last it reached the magnitude + of a specific <!-- Page 141 --><span class="pagenum"><a + name="page141"></a>{141}</span>distinction. Were it possible in such a + case to have all the forms before us, they would present the appearance + of a long series imperceptibly grading from one extreme to the other.</p> + + <p>Upon this view are made two assumptions not unnatural in the absence + of any exact knowledge of the nature of heredity and variation. It was + assumed, in the first place that variation was a continuous process, and, + second, that any variation could be transmitted to the offspring. Both of + these assumptions have since been shown to be unjustified. Even before + Mendel's work became known Bateson had begun to call attention to the + prevalence of discontinuity in variation, and a few years later this was + emphasised by the Dutch botanist Hugo de Vries in his great work on + <i>The Mutation Theory</i>. The ferment of new ideas was already working + in the solution, and under the stimulus of Mendel's work they have + rapidly crystallised out. With the advent of heredity as a definite + science we have been led to revise our views as to the nature of + variation, and consequently in some respects as to the trend of + evolution. Heritable variation has a definite basis in the gamete, and it + is to the gamete, therefore, not to the individual, that we must look for + the initiation of this process. Somewhere or other in the course of their + production is added or removed the factor upon whose removal or addition + the new variation owes its existence. The new variation springs into + being by a <!-- Page 142 --><span class="pagenum"><a + name="page142"></a>{142}</span>sudden step, not by a process of gradual + and almost imperceptible augmentation. It is not continuous but + discontinuous, because it is based upon the presence or absence of some + definite factor or factors—upon discontinuity in the gametes from + which it sprang. Once formed, its continued existence is subject to the + arbitrament of natural selection. If of value in the struggle for + existence natural selection will decide that those who possess it shall + have a better chance of survival and of leaving offspring than those who + do not possess it. If it is harmful to the individual natural selection + will soon bring about its elimination. But if the new variation is + neither harmful nor useful there seems no reason why it should not + persist.</p> + + <p>In this way we avoid a difficulty that beset the older view. For on + that view no new character could be developed except by the piling up of + minute variations through the action of natural selection. Consequently + any character found in animals and plants must be supposed to be of some + definite use to the individual. Otherwise it could not have developed + through the action of natural selection. But there are plenty of + characters to which it is exceedingly difficult to ascribe any utility, + and the ingenuity of the supporters of this view has often been severely + taxed to account for their existence. On the more modern view this + difficulty is avoided. The origin of a new variation is independent of + natural <!-- Page 143 --><span class="pagenum"><a + name="page143"></a>{143}</span>selection, and provided that it is not + directly harmful there is no reason why it should not persist. In this + way we are released from the burden of discovering a utilitarian motive + behind all the multitudinous characters of living organisms. For we now + recognise that the function of natural selection is selection and not + creation. It has nothing to do with the formation of the new variation. + It merely decides whether it is to survive or to be eliminated.</p> + + <p>One of the arguments made use of by supporters of the older view is + that drawn from the study of adaptation. Animals and plants are as a rule + remarkably well adapted to living the life which their surroundings + impose upon them, and in some cases this adaptation is exceedingly + striking. Especially is this so in the many instances of what is called + protective coloration, where the animal comes to resemble its + surroundings so closely that it may reasonably be supposed to cheat even + the keenest sighted enemy. Surely, we are told, such perfect adaptation + could hardly have arisen through the mere survival of chance sports. + Surely there must be some guiding hand moulding the species into the + required shape. The argument is an old one. For John Ray that guiding + hand was the superior wisdom of the Creator: for the modern Darwinian it + is Natural Selection controlling the direction of variation. Mendelism + certainly offers no suggestion of any such controlling force. It + interprets the <!-- Page 144 --><span class="pagenum"><a + name="page144"></a>{144}</span>variations of living forms in terms of + definite physiological factors, and the diversity of animal and plant + life is due to the gain or loss of these factors, to the origination of + new ones, or to fresh combinations among those already in existence. Nor + is there any valid reason against the supposition that even the most + remarkable cases of resemblance, such as that of the leaf insect, may + have arisen through a process of mutation. Experience with domestic + plants and animals shows that the most bizarre forms may arise as sports + and perpetuate themselves. Were such forms, arising under natural + conditions, to be favoured by natural selection owing to a resemblance to + something in their environment we should obtain a striking case of + protective adaptation. And here it must not be forgotten that those + striking cases to which our attention is generally called are but a very + small minority of the existing forms of life.</p> + + <p>For that special group of adaptation phenomena classed under the head + of Mimicry, Mendelism seems to offer an interpretation simpler than that + at present in vogue. This perhaps may be more clearly expressed by taking + a specific case. There is in Africa a genus of Danaine butterflies known + as <i>Amauris</i>, and there are reasons for considering that the group + to which it belongs possesses properties which render it unpalatable to + vertebrate enemies such as birds or monkeys. In the same region is also + found the genus <i>Euralia</i> belonging to the entirely <!-- Page 145 + --><span class="pagenum"><a name="page145"></a>{145}</span>different + family of the Nymphalidae, to which there is no evidence for assigning + the disagreeable properties of the Danaines. Now the different species of + <i>Euralia</i> show remarkably close resemblances to the species of + <i>Amauris</i>, which are found flying in the same region, and it is + supposed that by "mimicking" the unpalatable forms they impose upon their + enemies and thereby acquire immunity from attack. The point at issue is + the way in which this seemingly purposeful resemblance has been brought + about.</p> + + <p>One of the species of <i>Euralia</i> occurs in two very distinct forms + (Pl. VI.), which were previously regarded as separate species under the + names <i>E. wahlbergi</i> and <i>E. mima</i>. These two forms + respectively resemble <i>Amauris dominicanus</i> and <i>A. echeria</i>. + For purposes of argument we will assume <i>A. echeria</i> to be the more + recent form of the two. On the modern Darwinian view certain individuals + of <i>A. dominicanus</i> gradually diverged from the <i>dominicanus</i> + type and eventually reached the <i>echeria</i> type, though why this + should have happened does not appear to be clear. At the same time those + specimens which tended to vary in the direction of <i>A. echeria</i> in + places where this species was more abundant than <i>A. dominicanus</i> + were encouraged by natural selection, and under its guiding hand the form + <i>mima</i> eventually arose from <i>wahlbergi</i>.</p> + + <p>According to Mendelian views, on the other hand, <!-- Page 146 + --><span class="pagenum"><a name="page146"></a>{146}</span><i>A. + echeria</i> arose suddenly from <i>A. dominicanus</i> (or <i>vice + versa</i>), and similarly <i>mima</i> arose suddenly from + <i>wahlbergi</i>. If <i>mima</i> occurred where <i>A. echeria</i> was + common and <i>A. dominicanus</i> was rare, its resemblance to the more + plentiful distasteful form would give it the advantage over + <i>wahlbergi</i> and allow it to establish itself in place of the latter. + On the modern Darwinian view natural selection gradually shapes + <i>wahlbergi</i> into the <i>mima</i> form owing to the presence of <i>A. + echeria</i>; on the Mendelian view natural selection merely conserves the + <i>mima</i> form when once it has arisen. Now this case of mimicry is one + of especial interest, because we have experimental evidence that the + relation between <i>mima</i> and <i>wahlbergi</i> is a simple Mendelian + one, though at present it is uncertain which is the dominant and which + the recessive form. The two have been proved to occur in families bred + from the same female without the occurrence of any intermediates, and the + fact that the two segregate cleanly is strong evidence in favour of the + Mendelian view. On this view the genera <i>Amauris</i> and <i>Euralia</i> + contain a similar set of pattern factors, and the conditions, whatever + they may be, which bring about mutation in the former lead to the + production of a similar mutation in the latter. Of the different forms of + <i>Euralia</i> produced in any region that one has the best chance of + survival, through the operation of natural selection, which resembles the + most plentiful <i>Amauris</i> form. Mimetic resemblance is a true + phenomenon, but natural selection plays the part of a conservative, not + of a formative agent.</p> + + <div class="figcenter" style="width:75%;"> + <a href="images/164.jpg"><img style="width:100%" src="images/164t.jpg" + alt="Plate VI. Mimicry by Euralia sp." title="Plate VI. Mimicry by Euralia sp." /></a> + <span class="sc">Plate VI.</span> + </div> + +<p><!-- Page 147 --><span class="pagenum"><a name="page147"></a>{147}</span></p> + + <p>It is interesting to recall that in earlier years Darwin was inclined + to ascribe more importance to "sports" as opposed to continuous minute + variation, and to consider that they might play a not inconsiderable part + in the formation of new varieties in nature. This view, however, he gave + up later, because he thought that the relatively rare sport or mutation + would rapidly disappear through the swamping effects of crossing with the + more abundant normal form, and so, even though favoured by natural + selection, would never succeed in establishing itself. Mendel's discovery + has eliminated this difficulty. For suppose that the sport differed from + the normal in the loss of a factor and were recessive. When mated with + the normal this character would seem to disappear, though, of course, + half of the gametes of its progeny would bear it. By continual crossing + with normals a small proportion of heterozygotes would eventually be + scattered among the population, and as soon as any two of these mated + together the recessive sport would appear in one quarter of their + offspring.</p> + + <p>A suggestive contribution to this subject was recently made by G. H. + Hardy. Considering the distribution of a single factor in a mixed + population consisting of the heterozygous and the two homozygous forms he + showed that such a population breeding at random rapidly fell into a <!-- + Page 148 --><span class="pagenum"><a + name="page148"></a>{148}</span>stable condition with regard to the + proportion of these three forms, whatever may have been the proportion of + the three forms to start with. Let us suppose for instance, that the + population consists of <i>p</i> homozygotes of one kind, <i>r</i> + homozygotes of the other kind, and 2 <i>q</i> heterozygotes. Hardy + pointed out that, other things being equal, such a population would be in + equilibrium for this particular factor so long as the condition + <i>q</i><sup>2</sup> = <i>pr</i> was fulfilled. If the condition is + fulfilled to start with, the population remains in equilibrium. If the + condition is not fulfilled to start with, Hardy showed that a position of + equilibrium becomes established after a single generation, and that this + position is thereafter maintained. The proportions of the three classes + which satisfy the equation <i>q</i><sup>2</sup> = <i>pr</i> are + exceedingly numerous, and populations in which they existed in the + proportions shown in the appended table would remain in stable + equilibrium generation after generation:—</p> + +<table class="nobctr" summary="Stable populations." title="Stable populations."> +<tr><td class="spacsingle" align="center"> <i>p.</i></td><td class="spacsingle" align="center"> <i>2q.</i> </td><td class="spacsingle" align="center"> <i>r.</i></td></tr> +<tr><td class="spacsingle" align="center"> 1 </td><td class="spacsingle" align="center"> 2 </td><td class="spacsingle" align="center"> 1</td></tr> +<tr><td class="spacsingle" align="center"> 1 </td><td class="spacsingle" align="center"> 4 </td><td class="spacsingle" align="center"> 4</td></tr> +<tr><td class="spacsingle" align="center"> 1 </td><td class="spacsingle" align="center"> 6 </td><td class="spacsingle" align="center"> 9</td></tr> +<tr><td class="spacsingle" align="center"> 1 </td><td class="spacsingle" align="center"> 8 </td><td class="spacsingle" align="center"> 16</td></tr> +<tr><td class="spacsingle" align="center"> 1 </td><td class="spacsingle" align="center"> 20,000 </td><td class="spacsingle" align="center"> 100,000,000</td></tr> +<tr><td class="spacsingle" align="center"> 1 </td><td class="spacsingle" align="center"> 2<i>n</i> </td><td class="spacsingle" align="center"> <i>n</i><sup>2</sup></td></tr> +</table> + + <p>This, of course, assumes that all three classes are equally fertile, + and that no form of selection is taking place to the <!-- Page 149 + --><span class="pagenum"><a name="page149"></a>{149}</span>benefit of one + class more than of another. Moreover, it makes no difference whether + <i>p</i> represents the homozygous dominants or whether it stands for the + recessives. A population containing a very small proportion of dominants + and one containing a similar proportion of recessives are equally stable. + The term dominant is in some respects apt to be misleading, for a + dominant character cannot in virtue of its dominance establish itself at + the expense of a recessive one. Brown eyes in man are dominant to blue, + but there is no reason to suppose that as years go on the population of + these islands will become increasingly brown eyed. Given equality of + conditions both are on an equal footing. If, however, either dominant or + recessive be favoured by selection the conditions are altered, and it can + be shown that even a small advantage possessed by the one will rapidly + lead to the elimination of the other. Even with but a 5 per cent + selection advantage in its favour it can be shown that a rare sport will + oust the normal form in a few hundred generations. In this way we are + freed from a difficulty inherent in the older view that varieties arose + through a long-continued process involving the accumulation of very + slight variations. On that view the establishing of a new type was of + necessity a very long and tedious business, involving many thousands of + generations. For this reason the biologist has been accustomed to demand + a very large supply of time, often a great deal more than the physicist + is <!-- Page 150 --><span class="pagenum"><a + name="page150"></a>{150}</span>disposed to grant, and this has sometimes + led him to expostulate with the latter for cutting off the supply. On the + newer views, however, this difficulty need not arise, for we realise that + the origin and establishing of a new form may be a very much more rapid + process than has hitherto been deemed possible.</p> + + <p>One last question with regard to evolution. How far does Mendelism + help us in connection with the problem of the origin of species? Among + the plants and animals with which we have dealt we have been able to show + that distinct differences, often considerable, in colour, size, and + structure, may be interpreted in terms of Mendelian factors. It is not + unlikely that most of the various characters which the systematist uses + to mark off one species from another, the so-called specific characters, + are of this nature. They serve as convenient labels, but are not + essential to the conception of species. A systematist who defined the + wild sweet pea could hardly fail to include in his definition such + characters as the procumbent habit, the tendrils, the form of the pollen, + the shape of the flower, and its purple colour. Yet all these and other + characters have been proved to depend upon the presence of definite + factors which can be removed by appropriate crossing. By this means we + can produce a small plant a few inches in height with an erect habit of + growth, without tendrils, with round instead of oblong pollen, and with + colourless deformed flowers quite different <!-- Page 151 --><span + class="pagenum"><a name="page151"></a>{151}</span>in appearance from + those of the wild form. Such a plant would breed perfectly true, and a + botanist to whom it was presented, if ignorant of its origin, might + easily relegate it to a different genus. Nevertheless, though so widely + divergent in structure, such a plant must yet be regarded as belonging to + the species <i>Lathyrus odoratus</i>. For it still remains fertile with + the many different varieties of sweet pea. It is not visible attributes + that constitute the essential difference between one species and another. + The essential difference, whatever it may be, is that underlying the + phenomenon of sterility. The visible attributes are those made use of by + the systematist in cataloguing the different forms of animal and plant + life, for he has no other choice. But it must not be forgotten that they + are often misleading. Until they were bred together <i>Euralia + wahlbergi</i> and <i>E. mima</i> were regarded as perfectly valid + species, and there is little doubt that numbers of recognised species + will eventually fall to the ground in the same way as soon as we are in a + position to apply the test of breeding. Mendelism has helped us to + realise that specific characters may be but incidental to a + species—that the true criterion of what constitutes a species is + sterility, and that particular form of sterility which prevents two + healthy gametes on uniting from producing a zygote with normal powers of + growth and reproduction. For there are forms of sterility which are + purely mechanical. The pollen of <i>Mirabilis jalapa</i> cannot fertilise + <i>M.</i> <!-- Page 152 --><span class="pagenum"><a + name="page152"></a>{152}</span><i>longiflora</i>, because the pollen + tubes of the former are not long enough to penetrate down to the ovules + of the latter. Hybrids can nevertheless be obtained from the reciprocal + cross. Nor should we expect offspring from a St. Bernard and a toy + terrier without recourse to artificial fertilisation. Or sterility may be + due to pathological causes which prevent the gametes from meeting one + another in a healthy state. But in most cases it is probable that the + sterility is due to some other cause. It is not inconceivable that + definite differences in chemical composition render the protoplasm of one + species toxic to the gametes of the other, and if this is so it is not + impossible that we may some day be able to express these differences in + terms of Mendelian factors. The very nature of the case makes it one of + extreme difficulty for experimental investigation. At any rate, we + realise more clearly than before that the problem of species is not one + that can be resolved by the study of morphology or of systematics. It is + a problem in physiology.</p> + +<hr class="full" /> + +<p><!-- Page 153 --><span class="pagenum"><a name="page153"></a>{153}</span></p> + +<h3>CHAPTER XIV</h3> + +<p class="cenhead">ECONOMICAL</p> + + <p>Since heredity lies at the basis of the breeder's work, it is evident + that any contribution to a more exact knowledge of this subject must + prove of service to him, and there is no doubt that he will be able to + profit by Mendelian knowledge in the conduct of his operations. Indeed, + as we shall see later, these ideas have already led to striking results + in the raising of new and more profitable varieties. In the first place, + heredity is a question of individuals. Identity of appearance is no sure + guide to reproductive qualities. Two individuals similarly bred and + indistinguishable in outward form may nevertheless behave entirely + differently when bred from. Take, for instance, the family of sweet peas + shown on Plate IV. The F<sub>2</sub> generation here consists of seven + distinct types, three sorts of purples, three sorts of reds, and whites. + Let us suppose that our object is to obtain a true breeding strain of the + pale purple picotee form. Now from the proportions in which they come we + know that the dilute colour is due to the absence of the factor which + intensifies the colour. Consequently the picotee cannot throw the <!-- + Page 154 --><span class="pagenum"><a name="page154"></a>{154}</span>two + deeper shades of red or purple. But it may be heterozygous for the + purpling factor when it will throw the dilute red (tinged white), or it + may be heterozygous for either or both of the two colour factors (cf. p. + <a href="#page44">44</a>), in which case it will throw whites. Of the + picotees which come in such a family, therefore, some will give picotees, + tinged whites, and whites, others will give picotees and tinged whites + only, others will give picotees and whites only, while others, again, and + these the least numerous, will give nothing but picotees. The new variety + is already fixed in a certain definite proportion of the plants; in this + particular instance in 1 out of every 27. All that remains to be done is + to pick out these plants. Since all the picotees look alike, whatever + their breeding capacity, the only way to do this is to save the seed from + a number of such plants <i>individually</i>, and to raise a further + generation. Some of them will be found to breed true. The variety is then + established, and may at once be put on the market with full confidence + that it will hereafter throw none of the other forms. The all-important + thing is to save and sow the seed of separate individuals separately. + However alike they look, the seed from different individuals must on no + account be mixed. Provided that due care is taken in this respect no long + and tedious process of selection is required for the fixation of any + given variety. Every possible variety arising from a cross appears in the + F<sub>2</sub> generation if only a sufficient <!-- Page 155 --><span + class="pagenum"><a name="page155"></a>{155}</span>number is raised, and + of all these different varieties a certain proportion of each is already + fixed. Heredity is a question of individuals, and the recognition of this + will save the breeder much labour, and enable him to fix his varieties in + the shortest possible time.</p> + + <p>Such cases as these of the sweet pea throw a fresh light upon another + of the breeder's conceptions, that of purity of type. Hitherto the + criterion of a "pure-bred" thing, whether plant or animal, has been its + pedigree, and the individual was regarded as more or less pure bred for a + given quality according as it could show a longer or shorter list of + ancestors possessing this quality. To-day we realise that this is not + essential. The pure-bred picotee appears in our F<sub>2</sub> family + though its parent was a purple bicolor, and its remoter ancestors whites + for generations. So also from the cross between pure strains of black and + albino rabbits we may obtain in the F<sub>2</sub> generation animals of + the wild agouti colour which breed as true to type as the pure wild + rabbit of irreproachable pedigree. The true test of the pure breeding + thing lies not in its ancestry but in the nature of the gametes which + have gone to its making. Whenever two similarly constituted gametes + unite, whatever the nature of the parents from which they arose, the + resulting individual is homozygous in all respects and must consequently + breed true. In deciding questions of purity it is to the gamete, and not + to ancestry, that our appeal must henceforth be made. <!-- Page 156 + --><span class="pagenum"><a name="page156"></a>{156}</span></p> + + <p>Improvement is after all the keynote to the breeder's operations. He + is aiming at the production of a strain which shall combine the greatest + number of desirable properties with the least number of undesirable ones. + This good quality he must take from one strain, that from another, and + that again from a third, while at the same time avoiding all the poor + qualities that these different strains possess. It is evident that the + Mendelian conception of characters based upon definite factors which are + transmitted on a definite scheme must prove of the greatest service to + him. For once these factors have been determined, their distribution is + brought under control, and they can be associated together or dissociated + at the breeder's will. The chief labour involved is that necessary for + the determination of the factors upon which the various characters + depend. For it often happens that what appears to be a simple character + turns out when analysed to depend upon the simultaneous presence of + several distinct factors. Thus the Malay fowl breeds true to the walnut + comb, as does also the Leghorn to the single comb, and when pure strains + are crossed all the offspring have walnut combs. At first sight it would + be not unnatural to regard the difference as dependent upon the presence + or absence of a single factor. Yet, as we have already seen, two other + types of comb, the pea and the rose, make their appearance in the + F<sub>2</sub> generation. Analysis shows that the difference between the + walnut <!-- Page 157 --><span class="pagenum"><a + name="page157"></a>{157}</span>and the single is a difference of two + factors, and it is not until this has been determined that we can proceed + with certainty to transfer the walnut character to a single-combed breed. + Moreover, in his process of analysis the breeder must be prepared to + encounter the various phenomena that we have described under the headings + of interaction of factors, coupling and repulsion, and the recognition of + these phenomena will naturally influence his procedure. Or again, his + experiments may show him that one of the characters he wants, like the + blue of the Andalusian fowl, is dependent upon the heterozygous nature of + the individual which exhibits it, and if such is the case he will be wise + to refrain from any futile attempt at fixing it. If it is essential it + must be built up again in each generation, and he will recognise that the + most economical way of doing this is to cross the two pure strains so + that all the offspring may possess the desired character. The labour of + analysis is often an intricate and tedious business. But once done it is + done once for all. As soon as the various factors are determined, upon + which the various characters of the individual depend, as soon as the + material to be made use of has been properly analysed, the production and + fixation of the required combinations becomes a matter of simple + detail.</p> + + <p>An excellent example of the practical application of Mendelian + principles is afforded by the experiments which Professor Biffen has + recently carried out in Cambridge. <!-- Page 158 --><span + class="pagenum"><a name="page158"></a>{158}</span>Taken as a whole + English wheats compare favourably with foreign ones in respect of their + cropping power. On the other hand, they have two serious defects. They + are liable to suffer from the attacks of the fungus which causes rust, + and they do not bake into a good loaf. This last property depends upon + the amount of gluten present, and it is the greater proportion of this + which gives to the "hard" foreign wheat its quality of causing the loaf + to rise well when baked. For some time it was held that "hard" wheat with + a high glutinous content could not be grown in the English climate, and + undoubtedly most of the hard varieties imported for trial deteriorated + greatly in a very short time. Professor Biffen managed to obtain a hard + wheat which kept its qualities when grown in England. But in spite of the + superior quality of its grain from the baker's point of view its cropping + capacity was too low for it to be grown profitably in competition with + English wheats. Like the latter, it was also subject to rust. Among the + many varieties which Professor Biffen collected and grew for observation + he managed to find one which was completely immune to the attacks of the + rust fungus, though in other respects it had no desirable quality to + recommend it. Now as the result of an elaborate series of investigations + he was able to show that the qualities of heavy cropping capacity, + "hardness" of grain, and immunity to rust can all be expressed in terms + of Mendelian factors. Having once analysed his material <!-- Page 159 + --><span class="pagenum"><a name="page159"></a>{159}</span>the rest was + comparatively simple, and in a few years he has been able to build up a + strain of wheat which combines the cropping capacity of the best English + varieties with the hardness of the foreign kinds, and at the same time is + completely immune to rust. This wheat has already been shown to keep its + qualities unchanged for several years, and there is little doubt that + when it comes to be grown in quantity it will exert an appreciable + influence on wheat-growing in Great Britain.</p> + + <div class="figright" style="width:45%;"> + <a href="images/177.png"><img style="width:100%" src="images/177.png" + alt="Fig. 30. Curves to illustrate the influence of selection." title="Fig. 30. Curves to illustrate the influence of selection." /></a> + <span class="sc">Fig.</span> 30. + + <p class="cenhead">Curves to illustrate the influence of selection.</p> + </div> + + <p>It may be objected that it is often with small differences rather than + with the larger and more striking ones that the breeder is mainly + concerned. It does not matter much to him whether the colour of a pea + flower is purple or pink or white. But it does matter whether the plant + bears rather larger seeds than usual, or rather more of them. Even a + small difference when multiplied by the <!-- Page 160 --><span + class="pagenum"><a name="page160"></a>{160}</span>size of the crop will + effect a considerable difference in the profit. It is the general + experience of seedsmen and others that differences of this nature are + often capable of being developed up to a certain point by a process of + careful selection each generation. At first sight this appears to be + something very like the gradual accumulation of minute variations through + the continuous application of a selective process. Some recent + experiments by Professor Johannsen of Copenhagen set the matter in a + different light. One of his investigations deals with the inheritance of + the weight of beans, but as an account of these experiments would involve + us in the consideration of a large amount of detail we may take a simple + imaginary case to illustrate the nature of the conclusions at which he + arrived. If we weigh a number of seeds collected from a patch of plants + such as Johannsen's beans we should find that they varied considerably in + size. The majority would probably not diverge very greatly from the + general average, and as we approached the high or low extreme we should + find a constantly decreasing number of individuals with these weights. + Let us suppose that the weight of our seed varied between 4 and 20 + grains, that the greatest number of seeds were of the mean weight, viz. + 12 grains, and that as we passed to either extreme at 4 and 20 the number + became regularly less. The weight relation of such a collection of seeds + can be expressed by the accompanying curve (Fig. 30). Now if we select + for <!-- Page 161 --><span class="pagenum"><a + name="page161"></a>{161}</span>sowing only that seed which weighs over 12 + grains, we shall find that in the next generation the average weight of + the seed is raised and the curve becomes somewhat shifted to the right as + in the dotted line of Fig. 30. By continually selecting we can shift our + curve a little more to the right, <i>i.e.</i> we can increase the average + weight of the seeds until at last we come to a limit beyond which further + selection has no effect. This phenomenon has been long known, and it was + customary to regard these variations as of a continuous nature, + <i>i.e.</i> as all chance fluctuations in a homogeneous mass, and the + effect of selection was supposed to afford evidence that small continuous + variations could be increased by this process. But Johannsen's results + point to another interpretation. Instead of our material being + homogeneous it is probably a mixture of several strains each with its own + average weight about <!-- Page 162 --><span class="pagenum"><a + name="page162"></a>{162}</span>which the varying conditions of the + environment cause it to fluctuate. Each of these strains is termed a + <b>pure line</b>. If we imagine that there are three such pure lines in + our imaginary case, with average weights 10, 12, 14 grains respectively, + and if the range of fluctuation of each of these pure lines is 12 grains, + then our curve must be represented as made up of the three components</p> + +<table class="nobctr" summary="Stable populations." title="Stable populations."> +<tr><td class="qspcsingle" align="center"> A </td><td class="qspcsingle" align="center"> </td><td class="qspcsingle" align="center"> fluctuating </td><td class="qspcsingle" align="center"> between </td><td class="qspcsingle" align="center"> 4 </td><td class="qspcsingle" align="center"> and </td><td class="qspcsingle" align="center"> 16 </td><td class="qspcsingle" align="center"> with </td><td class="qspcsingle" align="center"> a mean of </td><td class="qspcsingle" align="center"> 10</td></tr> +<tr><td class="qspcsingle" align="center"> B </td><td class="qspcsingle" align="center"> </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> 6 </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> 18 </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> 12</td></tr> +<tr><td class="qspcsingle" align="center"> C </td><td class="qspcsingle" align="center"> </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> 8 </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> 20 </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> " </td><td class="qspcsingle" align="center"> 14</td></tr> +</table> + + <div class="figright" style="width:45%;"> + <a href="images/179.png"><img style="width:100%" src="images/179.png" + alt="Fig. 31. Curves to illustrate the conception of pure lines in a population." title="Fig. 31. Curves to illustrate the conception of pure lines in a population." /></a> + <span class="sc">Fig.</span> 31. + + <p class="cenhead">Curves to illustrate the conception of pure lines in + a population.</p> + </div> + + <p>as is shown in Fig. 31. A seed that weighs 12 grains may belong to any + of these three strains. It may be an average seed of B, or a rather large + seed of A, or a rather small seed of C. If it belongs to B its offspring + will average 12 grains, if to A they will average 10 grains, and if to C + they will average 14 grains. Seeds of similar weight may give a different + result because they happen to be fluctuations of different pure lines. + But within the pure line any seed, large or small, produces the average + result for that line. Thus a seed of line C which weighs 20 grains will + give practically the same result as one that weighs 10 grains.</p> + + <p>On this view we can understand why selection of the largest seed + raises the average weight in the next generation. We are picking out more + of C and less of A and B, and as this process is repeated the proportion + of C gradually increases and we get the appearance of selection <!-- Page + 163 --><span class="pagenum"><a name="page163"></a>{163}</span>acting on + a continuously varying homogeneous material and producing a permanent + effect. This is because the interval between the average weight of the + different pure lines is small compared with the environmental + fluctuations. None the less it is there, and the secret of separating and + fixing any of these pure lines is again to breed from the individual + separately. As soon as the pure line is separated further selection + becomes superfluous.</p> + + <p>Since the publication of Darwin's famous work upon the effects of + cross and self fertilisation, it has been generally accepted that the + effect of a cross is commonly, though not always, to introduce fresh + vigour into the offspring, though why this should be so we are quite at a + loss to explain. Continued close inbreeding, on the contrary, eventually + leads to deterioration, though, as in many self-fertilised plants, a + considerable number of generations may elapse before it shows itself in + any marked degree. The fine quality of many of the seedsman's choice + varieties of vegetables probably depends upon the fact that they had + resulted from a cross but a few generations back, and it is possible that + they often oust the older kinds not because they started as something + intrinsically better, but because the latter had gradually deteriorated + through continuous self-fertilisation. Most breeders are fully alive to + the beneficial results of a cross so far as vigour is concerned, but they + often hesitate to embark upon it owing to what was held <!-- Page 164 + --><span class="pagenum"><a name="page164"></a>{164}</span>to be the + inevitably lengthy and laborious business of recovering the original + variety and refixing it, even if in the process it was not altogether + lost. That danger Mendelism has removed, and we now know that by working + on these lines it is possible in three or four generations to recover the + original variety in a fixed state with all the superadded vigour that + follows from a cross.</p> + + <p>Nor is the problem one that concerns self-fertilised plants only. + Plants that are reproduced asexually often appear to deteriorate after a + few generations unless a sexual generation is introduced. New varieties + of potato, for example, are frequently put upon the market, and their + excellent qualities give them a considerable vogue. Much is expected of + them, but time after time they deteriorate in a disappointing way and are + lost to sight. It is not improbable that we are here concerned with a + case in which the plants lose their vigour after a few asexual + generations of reproduction from tubers, and can only recover it with the + stimulus that results from the interpolation of a sexual generation. + Unfortunately this generally means that the variety is lost, for owing to + the haphazard way in which new kinds of potatoes are reproduced it is + probable that most cultivated varieties are complex heterozygotes. Were + the potato plant subjected to careful analysis and the various factors + determined upon which its variations depend, we should be in a position + to remake continually any good potato without <!-- Page 165 --><span + class="pagenum"><a name="page165"></a>{165}</span>running the risk of + losing it altogether, as is now so often the case.</p> + + <p>The application of Mendelian principles is likely to prove of more + immediate service for plants than animals, for owing to the large numbers + which can be rapidly raised from a single individual and the prevalence + of self-fertilisation, the process of analysis is greatly simplified. + Even apart from the circumstance that the two sexes may sometimes differ + in their powers of transmission, the mere fact of their separation + renders the analysis of their properties more difficult. And as the + constitution of the individual is determined by the nature and quality of + its offspring, it is not easy to obtain this knowledge where the + offspring, as in most animals, are relatively few. Still, as has been + abundantly shown, the same principles hold good here also, and there is + no reason why the process of analysis, though more troublesome, should + not be effectively carried out. At the same time, it affords the breeder + a rational basis for some familiar but puzzling phenomena. The fact, for + instance, that certain characters often "skip a generation" is simply the + effect of dominance in F<sub>1</sub> and the reappearance of the + recessive character in the following generation. "Reversion" and + "atavism," again, are phenomena which are no longer mysterious, but can + be simply expressed in Mendelian terms as we have already suggested in + Chap. VI. The occasional appearance of a sport in a supposedly pure + strain is <!-- Page 166 --><span class="pagenum"><a + name="page166"></a>{166}</span>often due to the reappearance of a + recessive character. Thus even in the most highly pedigreed strains of + polled cattle such as the Aberdeen Angus, occasional individuals with + horns appear. The polled character is dominant to the horned, and the + occasional reappearance of the horned animal is due to the fact that some + of the polled herd are heterozygous in this character. When two such + individuals are mated, the chances are 1 in 4 that the offspring will be + horned. Though the heterozygous individuals may be indistinguishable in + appearance from the pure dominant, they can be readily separated by the + breeding test. For when crossed by the recessive, in this case horned + animals, the pure dominant gives only polled beasts, while the + heterozygous individual gives equal numbers of polled and horned ones. In + this particular instance it would probably be impracticable to test all + the cows by crossing with a horned bull. For in each case it would be + necessary to have several polled calves from each before they could with + reasonable certainty be regarded as pure dominants. But to ensure that no + horned calves should come, it is enough to use a bull which is pure for + that character. This can easily be tested by crossing him with a dozen or + so horned cows. If he gets no horned calves out of these he may be + regarded as a pure dominant and thenceforward put to his own cows, + whether horned or polled, with the certainty that all his calves will be + polled. <!-- Page 167 --><span class="pagenum"><a + name="page167"></a>{167}</span></p> + + <p>Or, again, suppose that a breeder has a chestnut mare and wishes to + make certain of a bay foal from her. We know that bay is dominant to + chestnut, and that if a homozygous bay stallion is used a bay foal must + result. In his choice of a sire, therefore, the breeder must be guided by + the previous record of the animal, and select one that has never given + anything but bays when put to either bay or chestnut mares. In this way + he will assure himself of a bay foal from his chestnut mare, whereas if + the record of the sire shows that he has given chestnuts he will be + heterozygous, and the chances of his getting a bay or a chestnut out of a + chestnut mare are equal.</p> + + <p>It is not impossible that the breeder may be unwilling to test his + animals by crossing them with a different breed through fear that their + purity may be thereby impaired, and that the influence of the previous + cross may show itself in succeeding generations. He might hesitate, for + instance, to test his polled cows by crossing them with a horned bull for + fear of getting horned calves when the cows were afterwards put to a + polled bull of their own breed. The belief in the power of a sire to + influence subsequent generations, or telegony as it is sometimes called, + is not uncommon even to-day. Nevertheless, carefully conducted + experiments by more than one competent observer have failed to elicit a + single shred of unequivocal evidence in favour of the view. Until we have + evidence based upon experiments which are capable of <!-- Page 168 + --><span class="pagenum"><a name="page168"></a>{168}</span>repetition, we + may safely ignore telegony as a factor in heredity.</p> + + <p>Heterozygous forms play a greater part in the breeding of animals than + of plants, for many of the qualities sought after by the breeder are of + this nature. Such is the blue of the Andalusian fowl, and, according to + Professor Wilson, the roan of the Shorthorn is similar, being the + heterozygous form produced by mating red with white. The characters of + certain breeds of canaries and pigeons again appear to depend upon their + heterozygous nature. Such forms cannot, of course, ever be bred true, and + where several factors are concerned they may when bred together produce + but a small proportion of offspring like themselves. As soon, however, as + their constitution has been analysed and expressed in terms of Mendelian + factors, pure strains can be built up which when crossed will give + nothing but offspring of the desired heterozygous form.</p> + + <p>The points with which the breeder is concerned are often fine ones, + not very evident except to the practised eye. Between an ordinary Dutch + rabbit and a winner, or between the comb of a Hamburgh that is fit to + show and one that is not, the differences are not very apparent to the + uninitiated. Whether Mendelism will assist the breeder in the production + of these finer points is at present doubtful. It may be that these small + differences are heritable, such as those that form the basis of + Johannsen's pure lines. In this case the breeder's outlook is <!-- Page + 169 --><span class="pagenum"><a name="page169"></a>{169}</span>hopeful. + But it may be that the variations which he seeks to perpetuate are of the + nature of fluctuations, dependent upon the earlier life conditions of the + individual, and not upon the constitution of the gametes by which it was + formed. If such is the case, he will get no help from the science of + heredity, for we know of no evidence which might lead us to suppose that + variations of this sort can ever become fixed and heritable.</p> + +<hr class="full" /> + +<p><!-- Page 170 --><span class="pagenum"><a name="page170"></a>{170}</span></p> + +<h3>CHAPTER XV</h3> + +<p class="cenhead">MAN</p> + + <div class="figcenter" style="width:40%;"> + <a href="images/189.jpg"><img style="width:100%" src="images/189t.jpg" + alt="Fig. 32. Normal and brachydactylous hands." title="Fig. 32. Normal and brachydactylous hands." /></a> + <span class="sc">Fig.</span> 32. + + <p class="cenhead">Normal and brachydactylous hands placed together for + comparison. (From Drinkwater.)</p> + </div> + + <div class="figright" style="width:40%;"> + <a href="images/190.jpg"><img style="width:100%" src="images/190t.jpg" + alt="Fig. 33. Radiograph of a brachydactylous hand." title="Fig. 33. Radiograph of a brachydactylous hand." /></a> + <span class="sc">Fig.</span> 33. + + <p class="cenhead">Radiograph of a brachydactylous hand.</p> + </div> + + <p>Though the interest attaching to heredity in man is more widespread + than in other animals, it is far more difficult to obtain evidence that + is both complete and accurate. The species is one in which the + differentiating characters separating individual from individual are very + numerous, while the number of the offspring is comparatively few, and the + generations are far between. For these reasons, even if it were possible, + direct experimental work with man would be likely to prove both tedious + and expensive. There is, however, another method besides the direct one + from which something can be learned. This consists in collecting all the + evidence possible, arranging it in the form of pedigrees, and comparing + it with standard cases already worked out in animals and plants. In this + way it has been possible to demonstrate in man the existence of several + characters showing simple Mendelian inheritance. As few besides medical + men have hitherto been concerned practically with heredity, such records + as exist are, for the most part, records of deformity or of disease. So + it happens that most of the <!-- Page 171 --><span class="pagenum"><a + name="page171"></a>{171}</span>pedigrees at present available deal with + characters which are usually classed as abnormal. In some of these the + inheritance is clearly Mendelian. One of the cases which has been most + fully worked out is that of a deformity known as brachydactyly. In + brachydactylous people the <!-- Page 172 --><span class="pagenum"><a + name="page172"></a>{172}</span>whole of the body is much stunted, and the + fingers and toes appear to have two joints only instead of three (cf. + Figs. 32 and 33). The inheritance of this peculiarity has been carefully + investigated by Dr. Drinkwater, who collected all the data he was able to + find among the members of a large family in which it occurred. The result + is the pedigree shown on p. <a href="#page173">173</a>. It is assumed + that all who are recorded as having offspring were married to normals. + Examination of the pedigree brings out the facts (1) that all affected + individuals have an affected parent; (2) that none of the unaffected + individuals, though sprung from the affected, ever have descendants who + are affected, and (3) that in families where both affected and unaffected + <!-- Page 173 --><span class="pagenum"><a + name="page173"></a>{173}</span>occur, the numbers of the two classes are, + on the average, equal. (The sum of such families in the complete pedigree + is thirty-nine affected and thirty-six normals.) It is obvious that these + are the conditions which are fulfilled in a simple Mendelian case, and + there is nothing in this pedigree to contradict the assertion that + brachydactyly, whatever it may be due to, behaves as a simple dominant to + the normal form, <i>i.e.</i> that it depends upon a factor which the + normal does not contain. The recessive normals cannot transmit the + affected condition whatever their ancestry. Once free they are always + free, and can marry other normals with full confidence that none of their + children will show the deformity.</p> + + <div class="figcenter" style="width:76%;"> + <a href="images/192.png"><img style="width:100%" src="images/192.png" + alt="Fig. 34. Pedigree of brachydactylous family." title="Fig. 34. Pedigree of brachydactylous family." /></a> + <span class="sc">Fig.</span> 34. + + <p class="cenhead">Pedigree of Drinkwater's brachydactylous family. The + affected members are indicated by black and the normals by light + circles.</p> + </div> + +<p><!-- Page 174 --><span class="pagenum"><a name="page174"></a>{174}</span></p> + + <p>The evidence available from pedigrees has revealed the simplest form + of Mendelian inheritance in several human defects and diseases, among + which may be mentioned presenile cataract of the eyes, an abnormal form + of skin thickening in the palms of the hands and soles of the feet, known + as tylosis, and epidermolysis bullosa, a disease in which the skin rises + up into numerous bursting blisters.</p> + + <p>Among the most interesting of all human pedigrees is one recently + built up by Mr. Nettleship from the records of a night-blind family + living near Monpelier in the south of France. In night-blind people the + retina is insensitive to light which falls below a certain intensity, and + such people are consequently blind in failing daylight or in moonlight. + As the Monpelier case had excited interest for some time, the records are + unusually complete. They commence with a certain Jean Nougaret, who was + born in 1637, and suffered from night-blindness, and they end for the + present with children who are to-day but a few years of age. Particulars + are known of over 2000 of the descendants of Jean Nougaret. Through ten + generations and nearly three centuries the affection has behaved as a + Mendelian dominant, and there is no sign that long-continued marriage + with folk of normal vision has produced any amelioration of the + night-blind state. <!-- Page 175 --><span class="pagenum"><a + name="page175"></a>{175}</span></p> + + <div class="figcenter" style="width:45%;"> + <a href="images/194.png"><img style="width:100%" src="images/194.png" + alt="Fig. 35. Pedigree of a hæmophilic family." title="Fig. 35. Pedigree of a hæmophilic family." /></a> + <span class="sc">Fig.</span> 35. + + <p class="cenhead">Pedigree of a hæmophilic family. Affected (all + males) represented by black, and normals of both sexes by light + circles. (From Stahel.)</p> + </div> + + <p>Besides cases such as these where a simple form of Mendelian + inheritance is obviously indicated, there are others which are more + difficult to read. Of some it may be said that on the whole the + peculiarity behaves as though it were an ordinary dominant; but that + exceptions occur in which affected children are born to unaffected + parents. It is not impossible that the condition may, like colour in the + sweet pea, depend upon the presence or absence of more than one factor. + In none of these cases, however, are the data sufficient for determining + with certainty whether this is so or not.</p> + + <p>A group of cases of exceptional interest is that in which the + incidence of disease is largely, if not absolutely, restricted to one + sex, and so far as is hitherto known the burden is invariably borne by + the male. In the inheritance of colour-blindness (p. <a + href="#page117">117</a>) we have already discussed an instance in which + the defect is rare, though not <!-- Page 176 --><span class="pagenum"><a + name="page176"></a>{176}</span>unknown, in the female. Sex-limited + inheritance of a similar nature is known for one or two ocular defects, + and for several diseases of the nervous system. In the peculiarly male + disease known as hæmophilia the blood refuses to clot when shed, and + there is nothing to prevent great loss from even a superficial scratch. + In its general trend the inheritance of hæmophilia is not unlike that of + horns among sheep, and it is possible that we are here again dealing with + a character which is dominant in one sex and recessive in the other. But + the evidence so far collected points to a difference somewhere, for in + hæmophilic families the affected males, instead of being equal in number + to the unaffected, show a considerable preponderance. The unfortunate + nature of the defect, however, forces us to rely for our interpretation + almost entirely upon the families produced by the unaffected females who + can transmit it. Our knowledge of the offspring of "bleeding" males is as + yet far too scanty, and until it is improved, or until we can find some + parallel case in animals or plants, the precise scheme of inheritance for + hæmophilia must remain undecided.</p> + + <p>Though by far the greater part of the human evidence relates to + abnormal or diseased conditions, a start has been made in obtaining + pedigrees of normal characters. From the ease with which it can be + observed, it was natural that eye-colour should be early selected as a + subject of investigation, and the work of Hurst and others <!-- Page 177 + --><span class="pagenum"><a name="page177"></a>{177}</span>has clearly + demonstrated the existence of one Mendelian factor in operation here. + Eyes are of many colours, and the colour depends upon the pigment in the + iris. Some eyes have pigment on both sides of the iris—on the side + that faces the retina as well as on the side that looks out upon the + world. Other eyes have pigment on the retinal side only. To this class + belong the blues and clear greys; while the eyes with pigment in front of + the iris also are brown, hazel, or green in various shades according to + the amount of pigment present. In albino animals the pigment is entirely + absent, and as the little blood-vessels are not obscured the iris takes + on its characteristic pinkish-red appearance. The condition in which + pigment is present in front of the iris is dominant to that in which it + is absent. Greens, browns, or hazels mated together may, if heterozygous, + give the recessive blue, but no individuals of the brown class are to be + looked for among the offspring of blues mated together. The blues, + however, may carry factors which are capable of modifying the brown. Just + as the pale pink-tinged sweet pea (Pl. IV., 9) when mated with a suitable + white gives only deep purples, so an eye with very little brown pigment + mated with certain blues produces progeny of a deep brown, far darker + than either parent. The blue may carry a factor which brings about + intensification of the brown pigment. There are doubtless other factors + which modify the brown when present, but we do not yet know enough of the + <!-- Page 178 --><span class="pagenum"><a + name="page178"></a>{178}</span>inheritance of the various shades to + justify any statement other than that the heredity of the pigment in + front of the iris behaves as though it were due to a Mendelian + factor.</p> + + <p>Even this fact is of considerable importance, for it at once suggests + that the present systems of classification of eye-colours, to which some + anthropologists attach considerable weight, are founded on a purely + empirical and unsatisfactory basis. Intensity of colour is the criterion + at present in vogue, and it is customary to arrange the eye-colours in a + scale of increasing depth of shade, starting with pale greys and ending + with the deepest browns. On this system the lighter greens are placed + among the blues. But we now know that blues may differ from the deep + browns in the absence of only a single factor, while, on the other hand, + the difference between a blue and a green may be a difference dependent + upon more than one factor. To what extent eye-colour may be valuable as a + criterion of race it is at present impossible to say, but if it is ever + to become so, it will only be after a searching Mendelian analysis has + disclosed the factors upon which the numerous varieties depend.</p> + + <p>A discussion of eye-colour suggests reflections of another kind. It is + difficult to believe that the markedly different states of pigmentation + which occur in the same species are not associated with deep-seated + chemical differences influencing the character and bent of the + individual. <!-- Page 179 --><span class="pagenum"><a + name="page179"></a>{179}</span>May not these differences in pigmentation + be coupled with and so become in some measure a guide to mental and + temperamental characteristics? In the National Portrait Gallery in London + the pictures of celebrated men and women are largely grouped according to + the vocations in which they have succeeded. The observant will probably + have noticed that there is a tendency for a given type of eye-colour to + predominate in some of the larger groups. It is rare to find anything but + a blue among the soldiers and sailors, while among the actors, preachers, + and orators the dark eye is predominant, although for the population as a + whole it is far scarcer than the light. The facts are suggestive, and it + is not impossible that future research may reveal an intimate connection + between peculiarities of pigmentation and peculiarities of mind.</p> + + <p>The inheritance of mental characters is often elusive, for it is + frequently difficult to appraise the effects of early environment in + determining a man's bent. That ability can be transmitted there is no + doubt, for this is borne out by general experience, as well as by the + numerous cases of able families brought together by Galton and others. + But when we come to inquire more precisely what it is that is transmitted + we are baffled. A distinguished son follows in the footsteps of a + distinguished father. Is this due to the inheritance of a particular + mental aptitude, or is it an instance of general mental ability displayed + in a field rendered attractive by early association? We have <!-- Page + 180 --><span class="pagenum"><a name="page180"></a>{180}</span>at present + very little definite evidence for supposing that what appear to be + special forms of ability may be due to specific factors. Hurst, indeed, + has brought forward some facts which suggest that musical sense sometimes + behaves as a recessive character, and it is likely that the study of some + clean-cut faculty such as the mathematical one would yield interesting + results.</p> + + <p>The analysis of mental characters will no doubt be very difficult, and + possibly the best line of attack is to search for cases where they are + associated with some physical feature such as pigmentation. If an + association of this kind be found, and the pigmentation factors be + determined, it is evident that we should thereby obtain an insight into + the nature of the units upon which mental conditions depend. Nor must it + be forgotten that mental qualities, such as quickness, generosity, + instability, etc.,—qualities which we are accustomed to regard as + convenient units in classifying the different minds with which we are + daily brought into contact,—are not necessarily qualities that + correspond to heritable units. Effective mental ability is largely a + matter of temperament, and this in turn is quite possibly dependent upon + the various secretions produced by the different tissues of the body. + Similar nervous systems associated with different livers might + conceivably result in individuals upon whose mental ability the world + would pass a very different judgment. Indeed, it is not at all impossible + <!-- Page 181 --><span class="pagenum"><a + name="page181"></a>{181}</span>that a particular form of mental ability + may depend for its manifestation, not so much upon an essential + difference in the structure of the nervous system, as upon the production + by another tissue of some specific poison which causes the nervous system + to react in a definite way. We have mentioned these possibilities merely + to indicate how complex the problem may turn out to be. Though there is + no doubt that mental ability is inherited, what it is that is + transmitted, whether factors involving the quality and structure of the + nervous system itself, or factors involving the production of specific + poisons by other tissues, or both together, is at present uncertain.</p> + + <p>Little as is known to-day of heredity in man, that little is of + extraordinary significance. The qualities of men and women, physical and + mental, depend primarily upon the inherent properties of the gametes + which went to their making. Within limits these qualities are elastic, + and can be modified to a greater or lesser extent by influences brought + to bear upon the growing zygote, provided always that the necessary basis + is present upon which these influences can work. If the mathematical + faculty has been carried in by the gamete, the education of the zygote + will enable him to make the most of it. But if the basis is not there, no + amount of education can transform that zygote into a mathematician. This + is a matter of common experience. Neither is there any reason for + supposing that the superior education of a <!-- Page 182 --><span + class="pagenum"><a name="page182"></a>{182}</span>mathematical zygote + will thereby increase the mathematical propensities of the gametes which + live within him. For the gamete recks little of quaternions. It is true + that there is progress of a kind in the world, and that this progress is + largely due to improvements in education and hygiene. The people of + to-day are better fitted to cope with their material surroundings than + were the people of even a few thousand years ago. And as time goes on + they are able more and more to control the workings of the world around + them. But there is no reason for supposing that this is because the + effects of education are inherited. Man stores knowledge as a bee stores + honey or a squirrel stores nuts. With man, however, the hoard is of a + more lasting nature. Each generation in using it sifts, adds, and + rejects, and passes it on to the next a little better and a little + fuller. When we speak of progress we generally mean that the hoard has + been improved, and is of more service to man in his attempts to control + his surroundings. Sometimes this hoarded knowledge is spoken of as the + inheritance which a generation receives from those who have gone before. + This is misleading. The handing on of such knowledge has nothing more to + do with heredity in the biological sense than has the handing on from + parent to offspring of a picture, or a title, or a pair of boots. All + these things are but the transfer from zygote to zygote of something + extrinsic to the species. Heredity, on the other hand, deals with the + <!-- Page 183 --><span class="pagenum"><a + name="page183"></a>{183}</span>transmission of something intrinsic from + gamete to zygote and from zygote to gamete. It is the participation of + the gamete in the process that is our criterion of what is and what is + not heredity.</p> + + <p>Better hygiene and better education, then, are good for the zygote, + because they help him to make the fullest use of his inherent qualities. + But the qualities themselves remain unchanged in so far as the gamete is + concerned, since the gamete pays no heed to the intellectual development + of the zygote in whom he happens to dwell. Nevertheless, upon the gamete + depend those inherent faculties which enable the zygote to profit by his + opportunities, and, unless the zygote has received them from the gamete, + the advantages of education are of little worth. If we are bent upon + producing a permanent betterment that shall be independent of external + circumstances, if we wish the national stock to become inherently more + vigorous in mind and body, more free from congenital physical defect and + feeble mentality, better able to assimilate and act upon the stores of + knowledge which have been accumulated through the centuries, then it is + the gamete that we must consult. The saving grace is with the gamete, and + with the gamete alone.</p> + + <p>People generally look upon the human species as having two kinds of + individuals, males and females, and it is for them that the sociologists + and legislators frame their schemes. This, however, is but an imperfect + view to <!-- Page 184 --><span class="pagenum"><a + name="page184"></a>{184}</span>take of ourselves. In reality we are of + four kinds, male zygotes and female zygotes, large gametes and small + gametes, and heredity is the link that binds us together. If our lives + were like those of the starfish or the sea-urchin, we should probably + have realised this sooner. For the gametes of these animals live freely, + and contract their marriages in the waters of the sea. With us it is + different, because half of us must live within the other half or perish. + Parasites upon the rest, levying a daily toll of nutriment upon their + hosts, they are yet in some measure the arbiters of the destiny of those + within whom they dwell. At the moment of union of two gametes is decided + the character of another zygote, as well as the nature of the population + of gametes which must make its home within him. The union once affected + the inevitable sequence takes its course, and whether it be good, or + whether it be evil, we, the zygotes, have no longer power to alter it. We + are in the hands of the gamete; yet not entirely. For though we cannot + influence their behaviour we can nevertheless control their unions if we + choose to do so. By regulating their marriages, by encouraging the + desirable to come together, and by keeping the undesirable apart we could + go far towards ridding the world of the squalor and the misery that come + through disease and weakness and vice. But before we can be prepared to + act, except, perhaps, in the <span class="correction" title="Original reads `simples'." + >simplest</span> cases, we must learn far more about them. At present we + are woefully ignorant <!-- Page 185 --><span class="pagenum"><a + name="page185"></a>{185}</span>of much, though we do know that full + knowledge is largely a matter of time and means. One day we shall have + it, and the day may be nearer than most suspect. Whether we make use of + it will depend in great measure upon whether we are prepared to recognise + facts, and to modify or even destroy some of the conventions which we + have become accustomed to regard as the foundations of our social life. + Whatever be the outcome, there can be little doubt that the future of our + civilisation, perhaps even the possibility of a future at all, is wrapped + up with the recognition we accord to those who live unseen and + inarticulate within us—the fateful race of gametes so irrevocably + bound to us by that closest of all ties, heredity.</p> + +<hr class="full" /> + +<p><!-- Page 187 --><span class="pagenum"><a name="page187"></a>{187}</span></p> + +<h3>APPENDIX</h3> + + <p>As some readers may possibly care to repeat Mendel's experiments for + themselves, a few words on the methods used in crossing may not be + superfluous. The flower of the pea with its standard, wings, and median + keel is too familiar to need description. Like most flowers it is + hermaphrodite. Both male and female organs occur on the same flower, and + are covered by the keel. The anthers, ten in number, are arranged in a + circle round the pistil. As soon as they are ripe they burst and shed + their pollen on the style. The pollen tubes then penetrate the stigma, + pass down the style, and eventually reach the ovules in the lower part of + the pistil. Fertilisation occurs here. Each ovule, which is reached by a + pollen tube, swells up and becomes a seed. At the same time the fused + carpels enclosing the ovules enlarge to form the pod. When this, the + normal mode of fertilisation, takes place, the flower is said to be + <b>selfed</b>.</p> + + <p>In crossing, it is necessary to emasculate a flower on the plant + chosen to be the female parent. For this purpose a young flower must be + taken in which the anthers have not yet burst. The keel is depressed, and + the stamens bearing the anthers are removed at their base by a <!-- Page + 188 --><span class="pagenum"><a name="page188"></a>{188}</span>pair of + fine forceps. It will probably be found necessary to tear the keel + slightly in order to do this. The pistil is then covered up again with + the keel, and the flower is enclosed in a bag of waxed paper until the + following day. The stigma is then again exposed and dusted with ripe + pollen from a flower of the plant selected as the male parent. This done, + the keel is replaced, and the flower again enclosed in its bag to protect + it from the possible attentions of insects until it has set seed. The bag + may be removed in about a week after fertilisation. It is perhaps hardly + necessary to add that strict biological cleanliness must be exercised + during the fertilising operations. This is readily attained by + sterilising fingers and forceps with a little strong spirit before each + operation, thereby ensuring the death of any foreign pollen grains which + may be present.</p> + + <p>The above method applies also to sweet peas, with these slight + modifications. As the anthers ripen relatively sooner in this species, + emasculation must be performed at a rather earlier stage. It is generally + safe to choose a bud about three parts grown. The interval between + emasculation and fertilisation must be rather longer. Two to three days + is generally sufficient. Further, the sweet pea is visited by the + leaf-cutter bee, <i>Megachile</i>, which, unlike the honey bee, is able + to depress the keel and gather pollen. If the presence of this insect is + suspected, it is desirable to guard against the risk of admixture of <!-- + Page 189 --><span class="pagenum"><a + name="page189"></a>{189}</span>foreign pollen by selecting for + pollinating purposes a flower which has not quite opened. If the standard + is not erected, it is unlikely to have been visited by <i>Megachile</i>. + Lastly, it not infrequently happens that the little beetle + <i>Meligethes</i> is found inside the keel. Such flowers should be + rejected for crossing purposes.</p> + +<hr class="full" /> + +<p><!-- Page 191 --><span class="pagenum"><a name="page191"></a>{191}</span></p> + +<h3>INDEX</h3> + + <div class="contents"> + <div class="stanza"> + <p><i>Abraxas grossulariata</i>, <a href="#page99">99</a></p> + <p>"Acquired" characters, <a href="#page14">14</a></p> + <p>Adaptation, <a href="#page143">143</a></p> + <p>Agouti mice, <a href="#page50">50</a></p> + <p>Albino mice, <a href="#page50">50</a></p> + <p>Albinos, nature of, <a href="#page53">53</a></p> + <p><i>Amauris</i>, <a href="#page144">144</a></p> + <p>Analysis of types, <a href="#page156">156</a></p> + <p>Ancestral Heredity, Law of, <a href="#page13">13</a></p> + <p>Andalusian fowls, <a href="#page70">70</a></p> + <p>Axil colour in sweet peas, <a href="#page93">93</a></p> + </div> + + <div class="stanza"> + <p>Bateson, W., <a href="#page14">14</a>, <a href="#page29">29</a>, <a href="#page55">55</a>, <a href="#page116">116</a>, <a href="#page132">132</a>, <a href="#page141">141</a></p> + <p>Biffen, R. H., <a href="#page157">157</a></p> + <p>Blue Andalusian fowls, <a href="#page71">71</a></p> + <p>Brachydactyly, <a href="#page171">171</a></p> + <p>Bryony, <a href="#page120">120</a></p> + <p>Bush sweet peas, <a href="#page63">63</a></p> + </div> + + <div class="stanza"> + <p>Castle, <a href="#page132">132</a></p> + <p>Cattle, horns in, <a href="#page86">86</a>, <a href="#page166">166</a></p> + <p>Colour, nature of, in flowers, <a href="#page48">48</a></p> + <p>Colour-blindness, <a href="#page117">117</a></p> + <p>Combs of fowls, <a href="#page33">33</a>, <a href="#page43">43</a></p> + <p>Correns, C., <a href="#page29">29</a>, <a href="#page120">120</a></p> + <p>Coupling of characters in gametes, <a href="#page93">93</a></p> + <p>Cuénot, <a href="#page50">50</a>, <a href="#page119">119</a></p> + <p>"Cupid" sweet peas, <a href="#page62">62</a></p> + <p>Currant moth, <a href="#page99">99</a></p> + </div> + + <div class="stanza"> + <p>Darwin, C., <a href="#page10">10</a>, <a href="#page65">65</a>, <a href="#page147">147</a>, <a href="#page163">163</a></p> + <p>De Vries, H., <a href="#page15">15</a>, <a href="#page29">29</a>, <a href="#page141">141</a></p> + <p>Discontinuity in variation, <a href="#page14">14</a></p> + <p>Dominant characters, <a href="#page18">18</a></p> + <p>Doncaster, L., <a href="#page99">99</a></p> + <p>Drinkwater, H., <a href="#page172">172</a></p> + <p>Dutch rabbits, <a href="#page60">60</a></p> + </div> + + <div class="stanza"> + <p>Eggs, <a href="#page2">2</a></p> + <p>Environment, influence of, <a href="#page137">137</a></p> + <p><i>Euralia</i>, <a href="#page144">144</a></p> + <p>Evolution, <a href="#page10">10</a>, <a href="#page85">85</a>, <a href="#page139">139</a></p> + <p>Eye, in primulas, <a href="#page55">55</a></p> + <p>Eye-colour, in man, <a href="#page176">176</a></p> + </div> + + <div class="stanza"> + <p>Factor, definition of, <a href="#page31">31</a></p> + <p>Factors, interaction of, <a href="#page42">42</a></p> + <p>Fertilisation, <a href="#page3">3</a></p> + <p>Fertilisation, self- and cross-, <a href="#page163">163</a></p> + <p>Fixation of varieties, <a href="#page153">153</a></p> + <p>Fluctuations, <a href="#page138">138</a></p> + <p>Fowls, coloured from whites, <a href="#page49">49</a>, <a href="#page73">73</a></p> + </div> + + <div class="stanza"> + <p>Galton, <a href="#page13">13</a>, <a href="#page179">179</a></p> + <p>Gametes, nature of, <a href="#page6">6</a></p> + <p>Gregory, R. P., <a href="#page55">55</a>, <a href="#page93">93</a></p> + </div> + + <div class="stanza"> + <p>Hæmophilia, <a href="#page176">176</a></p> + <p>Hardy, G. H., <a href="#page147">147</a></p> + <p>Heterozygote, definition of, <a href="#page28">28</a></p> + <p>Heterozygote, of intermediate form, <a href="#page68">68</a></p> + <p><i>Hieracium</i>, <a href="#page27">27</a>, <a href="#page132">132</a></p> + <p>Himalayan rabbits, <a href="#page60">60</a></p> + <p>Homostyle primulas, <a href="#page56">56</a></p> + <p>Homozygote, definition of, <a href="#page28">28</a></p> + <p>Hooded sweet peas, <a href="#page89">89</a></p> + <p>Horses, bay and chestnut in, <a href="#page167">167</a></p> + <p>Hurst, C. C., <a href="#page62">62</a>, <a href="#page176">176</a>, <a href="#page180">180</a></p> + </div> + + <div class="stanza"> + <p>Immunity in wheat, <a href="#page158">158</a></p> + <p>Individuality, <a href="#page135">135</a></p> + <p>Inhibition, factors for, <a href="#page74">74</a>, <a href="#page108">108</a></p> + <p>Intermediates, <a href="#page125">125</a></p> +<!-- Page 192 --><span class="pagenum"><a name="page192"></a>{192}</span> + </div> + + <div class="stanza"> + <p>Johannsen, W., <a href="#page160">160</a></p> + </div> + + <div class="stanza"> + <p>Lop-eared rabbits, <a href="#page132">132</a></p> + </div> + + <div class="stanza"> + <p>Mendel, <a href="#page8">8</a>, <a href="#page17">17</a>, <a href="#page26">26</a>, <a href="#page132">132</a></p> + <p>Mental characters, <a href="#page180">180</a></p> + <p>Mice, inheritance of coat colour in, <a href="#page50">50</a></p> + <p>Mimicry, <a href="#page143">143</a></p> + <p><i>Mirabilis</i>, <a href="#page151">151</a></p> + <p>Morgan, T. H., <a href="#page116">116</a></p> + <p>Mulattos, <a href="#page129">129</a></p> + <p>Mutation, <a href="#page83">83</a>, <a href="#page138">138</a></p> + </div> + + <div class="stanza"> + <p>Nägeli, C., <a href="#page26">26</a></p> + <p>Natural selection, <a href="#page11">11</a>, <a href="#page140">140</a>, <a href="#page142">142</a>, <a href="#page149">149</a></p> + <p>Nettleship, E., <a href="#page175">175</a></p> + <p>Night-blindness, <a href="#page175">175</a></p> + </div> + + <div class="stanza"> + <p><i>Pararge egeria</i>, <a href="#page132">132</a></p> + <p>Parkinson, J., <a href="#page122">122</a></p> + <p>Pea comb, <a href="#page33">33</a></p> + <p>Peas, coloured flowers in, <a href="#page24">24</a></p> + <p>Peas, tall and dwarf, <a href="#page18">18</a></p> + <p>Pigeons, <a href="#page86">86</a></p> + <p>Pin-eye in primulas, <a href="#page55">55</a></p> + <p><i>Pisum</i>, <a href="#page17">17</a></p> + <p>Primulas, <a href="#page31">31</a>, <a href="#page55">55</a>, <a href="#page68">68</a>, <a href="#page93">93</a></p> + <p>Pollen, <a href="#page3">3</a></p> + <p>Pollen of sweet peas, <a href="#page92">92</a></p> + <p>Pomace fly, <a href="#page115">115</a></p> + <p>Population, inheritance of characters in a, <a href="#page147">147</a></p> + <p>Presence and Absence theory, <a href="#page35">35</a></p> + <p>Pure lines, <a href="#page162">162</a></p> + <p>Purity of gametes, <a href="#page24">24</a></p> + <p>Purity of type, <a href="#page155">155</a></p> + </div> + + <div class="stanza"> + <p>Rabbits, <a href="#page53">53</a>, <a href="#page60">60</a></p> + <p>Ratios, Mendelian—</p> + <p class="i2">3 : 1, <a href="#page20">20</a></p> + <p class="i2">9 : 3 : 3 : 1, <a href="#page25">25</a>, <a href="#page34">34</a></p> + <p class="i2">9 : 3 : 4, <a href="#page51">51</a></p> + <p class="i2">9 : 7, <a href="#page49">49</a></p> + <p>Ray, John, <a href="#page143">143</a></p> + <p>Recessive characters, <a href="#page19">19</a></p> + <p>Repulsion between factors, <a href="#page90">90</a></p> + <p>Reversion, <a href="#page59">59</a>, <a href="#page165">165</a></p> + <p class="i2">in rabbits, <a href="#page59">59</a></p> + <p class="i2">in sweet peas, <a href="#page62">62</a></p> + <p class="i2">in fowls, <a href="#page65">65</a></p> + <p class="i2">in pigeons, <a href="#page65">65</a></p> + <p>Rose comb, <a href="#page33">33</a></p> + </div> + + <div class="stanza"> + <p>Saunders, E. R., <a href="#page54">54</a>, <a href="#page122">122</a></p> + <p>Seeds, nature of, <a href="#page4">4</a></p> + <p>Segregation, <a href="#page22">22</a></p> + <p>Selection, <a href="#page162">162</a></p> + <p>Sheep, horns in, <a href="#page76">76</a></p> + <p>Silky fowls, <a href="#page30">30</a>, <a href="#page105">105</a></p> + <p>Single comb, <a href="#page32">32</a></p> + <p>Species, nature of, <a href="#page150">150</a></p> + <p>Species, origin of, <a href="#page11">11</a></p> + <p>Speckled wood butterfly, <a href="#page132">132</a></p> + <p>Spermatozoa, <a href="#page3">3</a></p> + <p>Sports, <a href="#page147">147</a></p> + <p>Staples-Browne, R., <a href="#page66">66</a></p> + <p>Sterility, <a href="#page151">151</a></p> + <p>Sterility in sweet peas, <a href="#page93">93</a></p> + <p>Stocks, double, <a href="#page122">122</a></p> + <p>Stocks, hoariness in, <a href="#page54">54</a></p> + <p>Sweet pea, colour in, <a href="#page44">44</a>, <a href="#page79">79</a></p> + <p class="i2">history of, <a href="#page82">82</a></p> + <p class="i2">inheritance of hood in, <a href="#page89">89</a></p> + <p class="i2">inheritance of size in, <a href="#page62">62</a></p> + </div> + + <div class="stanza"> + <p>Telegony, <a href="#page167">167</a></p> + <p>Thrum-eye in primulas, <a href="#page55">55</a></p> + <p>Toe, extra toe in poultry, <a href="#page76">76</a></p> + <p>Tschermak, E., <a href="#page29">29</a></p> + </div> + + <div class="stanza"> + <p>Unit-character, definition of, <a href="#page31">31</a></p> + </div> + + <div class="stanza"> + <p>Variation, <a href="#page14">14</a>, <a href="#page137">137</a>, <a href="#page139">139</a></p> + </div> + + <div class="stanza"> + <p>Walnut comb, <a href="#page33">33</a></p> + <p>Weismann, A., <a href="#page13">13</a></p> + <p>Wheat, beard in, <a href="#page74">74</a></p> + <p class="i2">experiments with, <a href="#page157">157</a></p> + <p>White, dominant in poultry, <a href="#page72">72</a></p> + <p>Wilson, J., <a href="#page168">168</a></p> + </div> + + <div class="stanza"> + <p>Yellow mice, <a href="#page119">119</a></p> + </div> + + <div class="stanza"> + <p>Zygotes, nature of, <a href="#page5">5</a></p> + </div> + </div> + +<hr class="full" /> + +<h2>Notes</h2> + +<hr class="short" /> + +<div class="note"> + <p><a name="footnote1" href="#footnotetag1">[1]</a> Cf. note on p. <a + href="#page171">171</a>.</p> + + <p><a name="footnote2" href="#footnotetag2">[2]</a> It has been found + convenient to denote the various generations resulting from a cross by + the signs F<sub>1</sub>, F<sub>2</sub>, F<sub>3</sub>, etc. F<sub>1</sub> + on this system denotes the first filial generation, F<sub>2</sub> the + second filial generation produced by two parents belonging to the + F<sub>1</sub> generation, and so on.</p> + + <p><a name="footnote3" href="#footnotetag3">[3]</a> Hurst's original + cross was between a Belgian hare and an albina Angora, which <span + class="correction" title="Original reads `turned to out be'.">turned out + to be</span> a masked Dutch.</p> + + <p><a name="footnote4" href="#footnotetag4">[4]</a> The Spot is an almost + white bird, the colour being confined to the tail and the characteristic + spot on the head.</p> + + <p><a name="footnote5" href="#footnotetag5">[5]</a> The reader who + searches florists' catalogues for these varieties will probably + experience disappointment. The sweet pea has been much "improved" in the + past few years, and it is unlikely that the modern seedsman would list + such unfashionable forms.</p> + + <p><a name="footnote6" href="#footnotetag6">[6]</a> It is to be + understood that wherever a given factor is present the plant may be + homozygous or heterozygous for it without alteration in its colour.</p> + + <p><a name="footnote7" href="#footnotetag7">[7]</a> It should be + mentioned that as the shape of the pollen coat, like that of the seed + coat, is a maternal character, all the grains of any given plant are + either long or else round. The two kinds do not occur together on the + same plant.</p> + + <p><a name="footnote8" href="#footnotetag8">[8]</a> For the most recent + discussion of this peculiar case the reader is referred to Professor + Castle's paper in <i>Science</i>, December 16, 1910.</p> + + <p><a name="footnote9" href="#footnotetag9">[9]</a> <i>Paradisus + Terrestris</i>, London, 1629, p. 261.</p> + +</div> + +<p> </p> +<p> </p> +<hr class="pg" /> +<p>***END OF THE PROJECT GUTENBERG EBOOK MENDELISM***</p> +<p>******* This file should be named 28775-h.txt or 28775-h.zip *******</p> +<p>This and all associated files of various formats will be found in:<br /> +<a href="http://www.gutenberg.org/dirs/2/8/7/7/28775">http://www.gutenberg.org/2/8/7/7/28775</a></p> +<p>Updated editions will replace the previous one--the old editions +will be renamed.</p> + +<p>Creating the works from public domain print editions means that no +one owns a United States copyright in these works, so the Foundation +(and you!) can copy and distribute it in the United States without +permission and without paying copyright royalties. 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