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<pre>
The Project Gutenberg EBook of A Critique of the Theory of Evolution, by
Thomas Hunt Morgan
This eBook is for the use of anyone anywhere at no cost and with
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re-use it under the terms of the Project Gutenberg License included
with this eBook or online at www.gutenberg.org
Title: A Critique of the Theory of Evolution
Author: Thomas Hunt Morgan
Release Date: December 17, 2009 [EBook #30701]
Language: English
Character set encoding: ISO-8859-1
*** START OF THIS PROJECT GUTENBERG EBOOK CRITIQUE OF THEORY OF EVOLUTION ***
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Transcriber's note:
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<td>
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
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</table>
<h3>Princeton University</h3>
<h3>THE LOUIS CLARK VANUXEM FOUNDATION<br />
LECTURES FOR 1915-1916</h3>
<p><br style="clear:both" /></p>
<hr class="short" />
<h2>The Louis Clark Vanuxem Foundation
of Princeton University</h2>
<p>was established in 1912 with a bequest of $25,000 under the will of
Louis Clark Vanuxem, of the Class of 1879. By direction of the executors
of Mr. Vanuxem's estate, the income of the foundation is to be used for a
series of public lectures delivered in Princeton annually, at least one
half of which shall be on subjects of current scientific interest. The
lectures are to be published and distributed among schools and libraries
generally.</p>
<p>The following lectures have already been published or are in
press:</p>
<blockquote class="b1n">
<p>1912-13 The Theory of Permutable Functions, by Vito Volterra</p>
<p>1913-14 Lectures delivered in connection with the dedication of the
Graduate College of Princeton University by Emile Boutroux, Alois Riehl,
A. D. Godley, and Arthur Shipley</p>
<p>1914-15 Romance, by Sir Walter Raleigh</p>
<p>1915-16 A Critique of the Theory of Evolution, by Thomas Hunt
Morgan</p>
</blockquote>
<p><br style="clear:both" /></p>
<hr class="short" />
<h3>LOUIS CLARK VANUXEM FOUNDATION</h3>
<h1>A CRITIQUE</h1>
<p class="cenhead">OF THE</p>
<h1>THEORY OF EVOLUTION</h1>
<p class="cenhead">BY</p>
<h2>THOMAS HUNT MORGAN</h2>
<p class="cenhead">PROFESSOR OF EXPERIMENTAL ZOOLOGY IN<br />
COLUMBIA UNIVERSITY</p>
<p class="cenhead">LECTURES DELIVERED AT PRINCETON UNIVERSITY<br />
FEBRUARY 24, MARCH 1, 8, 15, 1916</p>
<p class="cenhead">PRINCETON UNIVERSITY PRESS<br />
PRINCETON<br />
LONDON: HUMPHREY MILFORD<br />
OXFORD UNIVERSITY PRESS<br />
1916</p>
<p><br style="clear:both" /></p>
<hr class="short" />
<p class="cenhead">Copyright, 1916, by<br />
<span class="sc">Princeton University Press</span><br />
Published October, 1916</p>
<div class="figright" style="width:10%;">
<a href="images/copyright_page.png"><img style="width:100%" src="images/copyright_page.png"
alt="Publisher's Mark" title="Publisher's Mark" /></a>
</div>
<div style="clear: both"></div>
<p><br style="clear:both" /></p>
<hr class="full" />
<p><!-- Page v --><span class="pagenum"><a name="pagev"></a>{v}</span></p>
<h3>PREFACE</h3>
<p>Occasionally one hears today the statement that we have come to
realize that we know nothing about evolution. This point of view is a
healthy reaction to the over-confident belief that we knew everything
about evolution. There are even those rash enough to think that in the
last few years we have learned more about evolution than we might have
hoped to know a few years ago. A <i>critique</i> therefore not only
becomes a criticism of the older evidence but an appreciation of the new
evidence.</p>
<p>In the first lecture an attempt is made to put a new valuation on the
traditional evidence for evolution. In the second lecture the most recent
work on heredity is dealt with, for only characters that are inherited
can become a part <!-- Page vi --><span class="pagenum"><a
name="pagevi"></a>{vi}</span>of the evolutionary process. In the third
lecture the physical basis of heredity and the composition of the germ
plasm stream are examined in the light of new observations; while in the
fourth lecture the thesis is developed that chance variation combined
with a property of living things to manifold themselves is the key note
of modern evolutionary thought.</p>
<p class="address"><span class="sc">T. H. Morgan</span></p>
<p class="author"><i>July, 1916</i></p>
<p><br style="clear:both" /></p>
<hr class="full" />
<p><!-- Page vii --><span class="pagenum"><a name="pagevii"></a>{vii}</span></p>
<h3>TABLE OF CONTENTS</h3>
<table class="nobctr" summary="Contents." title="Contents." style="width:50%">
<tr><td colspan="4" style="text-align:center; padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER I</td></tr>
<tr><td colspan="4" style="text-align:center; padding-top: 1.5ex; padding-bottom: 1.5ex;"> A REVALUATION OF THE EVIDENCE ON
WHICH THE THEORY OF EVOLUTION
WAS BASED</td></tr>
<tr><td class="qspcsingle" colspan="3" style="vertical-align:top;"> </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <span class="scac">PAGE</span></td></tr>
<tr><td class="qspcsingle" colspan="3" style="vertical-align:top;"> <span class="sc">Preface</span> </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#pagev">v</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> 1. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">Three Kinds of Evolution</span> </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page1">1</a>-<a href="#page7">7</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> 2. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">The Evidence for Organic Evolution</span> </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page7">7</a>-<a href="#page27">27</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> a. </td><td class="qspcsingle" style="vertical-align:top;"> The Evidence from Comparative Anatomy </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page7">7</a>-<a href="#page14">14</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> b. </td><td class="qspcsingle" style="vertical-align:top;"> The Evidence from Embryology </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page14">14</a>-<a href="#page23">23</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> c. </td><td class="qspcsingle" style="vertical-align:top;"> The Evidence from Paleontology </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page24">24</a>-<a href="#page27">27</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> 3. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">The Four Great Historical Speculations</span></td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page27">27</a>-<a href="#page39">39</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> a. </td><td class="qspcsingle" style="vertical-align:top;"> The Environment </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page27">27</a>-<a href="#page31">31</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Geoffroy St. Hilaire</td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> b. </td><td class="qspcsingle" style="vertical-align:top;"> Use and Disuse </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page31">31</a>-<a href="#page34">34</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> From Lamarck to Weismann</td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> c. </td><td class="qspcsingle" style="vertical-align:top;"> The Unfolding Principle </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page34">34</a>-<a href="#page36">36</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Nägeli and Bateson</td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> d. </td><td class="qspcsingle" style="vertical-align:top;"> Natural Selection </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page36">36</a>-<a href="#page39">39</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Darwin</td></tr>
<tr><td colspan="4" style="text-align:center; padding-top: 1.5ex; padding-bottom: 1.5ex;">
<!-- Page viii --><span class="pagenum"><a name="pageviii"></a>{viii}</span>
CHAPTER II</td></tr>
<tr><td colspan="4" style="text-align:center; padding-top: 1.5ex; padding-bottom: 1.5ex;"> THE BEARING OF MENDEL'S DISCOVERY ON
THE ORIGIN OF HEREDITY CHARACTERS</td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> 1. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> Mendel's First Discovery—Segregation </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page41">41</a>-<a href="#page52">52</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> 2. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> Mendel's Second Discovery—Independent Assortment</td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page52">52</a>-<a href="#page59">59</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> 3. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> The Characters of Wild Animals and Plants
Follow the Same Laws of Inheritance as do
the Characters of Domesticated Animals and
Plants </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page59">59</a>-<a href="#page84">84</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> a. </td><td class="qspcsingle" style="vertical-align:top;"> Sexual Dimorphism </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page61">61</a>-<a href="#page64">64</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Eosin eye color of Drosophila </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page61">61</a>-<a href="#page62">62</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Color of the Clover Butterfly, Colias philodice</td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page62">62</a>-<a href="#page63">63</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Color of Papilio turnus </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page63">63</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Color pattern of Papilio polytes </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page63">63</a>-<a href="#page64">64</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> b. </td><td class="qspcsingle" style="vertical-align:top;"> Duplication of parts </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page65">65</a>-<a href="#page66">66</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Thorax of Drosophila </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page65">65</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Legs of Drosophila </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page65">65</a>-<a href="#page66">66</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> c. </td><td class="qspcsingle" style="vertical-align:top;"> Loss of characters </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page66">66</a>-<a href="#page68">68</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> "Eyeless" of Drosophila </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page66">66</a>-<a href="#page67">67</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Vestigial wings of Drosophila </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page67">67</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Bar eye of Drosophila </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page67">67</a>-<a href="#page68">68</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> d. </td><td class="qspcsingle" style="vertical-align:top;"> Small changes of characters </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page68">68</a>-<a href="#page70">70</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> "Speck" </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page68">68</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Bristles of "club" </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page70">70</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> e. </td><td class="qspcsingle" style="vertical-align:top;"> Manifold effects of same factor </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page71">71</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> f. </td><td class="qspcsingle" style="vertical-align:top;"> Constant but trivial effects may be the
product of factors having other vital
aspects </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page73">73</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;">
<!-- Page ix --><span class="pagenum"><a name="pageix"></a>{ix}</span>
g. </td><td class="qspcsingle" style="vertical-align:top;"> Sex-linked inheritance </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page75">75</a>-<a href="#page80">80</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> in Drosophila ampelophila </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page75">75</a>-<a href="#page76">76</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> in the wild species D. repleta </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page76">76</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> in man </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page77">77</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> in domesticated Fowls </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page77">77</a>-<a href="#page78">78</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> in the wild moth, Abraxas </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page78">78</a>-<a href="#page80">80</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> h. </td><td class="qspcsingle" style="vertical-align:top;"> Multiple allelomorphs </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page81">81</a>-<a href="#page84">84</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> in the wild Grouse Locust </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page81">81</a>-<a href="#page83">83</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> in domesticated mice and rabbits </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page83">83</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> in Drosophila ampelophila </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page84">84</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> 4. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">Mutation and Evolution</span> </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page84">84</a>-<a href="#page88">88</a></td></tr>
<tr><td colspan="4" style="text-align:center; padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER III</td></tr>
<tr><td colspan="4" style="text-align:center; padding-top: 1.5ex; padding-bottom: 1.5ex;"> THE FACTORIAL THEORY OF HEREDITY
AND THE COMPOSITION OF THE
GERM PLASM</td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> 1. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">The Cellular Basis of Organic Evolution and Heredity</span></td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page89">89</a>-<a href="#page98">98</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> 2. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">The Mechanism of Mendelian Heredity Discovered in the Behavior of the Chromosomes</span></td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page98">98</a>-<a href="#page102">102</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> 3. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">The Four Great Linkage Groups of Drosophila Ampelophila</span></td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page103">103</a>-<a href="#page118">118</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> a. </td><td class="qspcsingle" style="vertical-align:top;"> Group I. </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page104">104</a>-<a href="#page109">109</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> b. </td><td class="qspcsingle" style="vertical-align:top;"> Group II. </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page109">109</a>-<a href="#page112">112</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> c. </td><td class="qspcsingle" style="vertical-align:top;"> Group III. </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page112">112</a>-<a href="#page115">115</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> d. </td><td class="qspcsingle" style="vertical-align:top;"> Group IV. </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page115">115</a>-<a href="#page118">118</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> 4. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">Localization of Factors in the Chromosomes</span></td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page118">118</a>-<a href="#page142">142</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> a. </td><td class="qspcsingle" style="vertical-align:top;"> The Evidence from Sex Linked Inheritance </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page118">118</a>-<a href="#page137">137</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;">
<!-- Page x --><span class="pagenum"><a name="pagex"></a>{x}</span>
b. </td><td class="qspcsingle" style="vertical-align:top;"> The Evidence from Interference </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page137">137</a>-<a href="#page138">138</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> c. </td><td class="qspcsingle" style="vertical-align:top;"> The Evidence from Non-Disjunction </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page139">139</a>-<a href="#page142">142</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> 5. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">How Many Genetic Factors are there in the Germ-plasm of a Single Individual?</span></td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page142">142</a>-<a href="#page143">143</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> 6. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">Conclusions </span> </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page144">144</a></td></tr>
<tr><td colspan="4" style="text-align:center; padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER IV</td></tr>
<tr><td colspan="4" style="text-align:center; padding-top: 1.5ex; padding-bottom: 1.5ex;"> SELECTION AND EVOLUTION</td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> 1. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">The Theory of Natural Selection</span> </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page145">145</a>-<a href="#page161">161</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> 2. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">How has Selection in Domesticated Animals and Plants brought about its Results?</span></td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page161">161</a>-<a href="#page165">165</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> 3. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">Are Factors Changed through Selection?</span></td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page165">165</a>-<a href="#page187">187</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> 4. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">How does Natural Selection Influence the course of Evolution?</span></td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page187">187</a>-<a href="#page193">193</a></td></tr>
<tr><td class="qspcsingle" style="vertical-align:top;"> 5. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">Conclusions </span> </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page193">193</a>-<a href="#page194">194</a></td></tr>
<tr><td class="qspcsingle" colspan="3" style="vertical-align:top;"> <span class="sc">Index </span> </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page195">195</a>-<a href="#page197">197</a></td></tr>
</table>
<p><br style="clear:both" /></p>
<hr class="full" />
<p><!-- Page 1 --><span class="pagenum"><a name="page1"></a>{1}</span></p>
<h3>CHAPTER I</h3>
<p class="cenhead">A REVALUATION OF THE EVIDENCE ON WHICH THE THEORY OF EVOLUTION WAS BASED</p>
<p>We use the word evolution in many ways—to include many different
kinds of changes. There is hardly any other scientific term that is used
so carelessly—to imply so much, to mean so little.</p>
<p class="cenhead"><span class="sc">Three Kinds of Evolution</span></p>
<p>We speak of the evolution of the stars, of the evolution of the horse,
of the evolution of the steam engine, as though they were all part of the
same process. What have they in common? Only this, that each concerns
itself with the <i>history</i> of something. When the astronomer thinks
of the <i>evolution</i> of the earth, the moon, the sun and the stars, he
has a picture of diffuse matter that has slowly condensed. With
condensation came heat; with heat, action and <!-- Page 2 --><span
class="pagenum"><a name="page2"></a>{2}</span>reaction within the mass
until the chemical substances that we know today were produced. This is
the nebular hypothesis of the astronomer. The astronomer explains, or
tries to explain, how this evolution took place, by an appeal to the
physical processes that have been worked out in the laboratory, processes
which he thinks have existed through all the eons during which this
evolution was going on and which were its immediate causes.</p>
<p>When the biologist thinks of the evolution of animals and plants, a
different picture presents itself. He thinks of series of animals that
have lived in the past, whose bones (fig. 1) and shells have been
preserved in the rocks. He thinks of these animals as having in the past
given birth, through an unbroken succession of individuals, to the living
inhabitants of the earth today. He thinks that the old, simpler types of
the past have in part changed over into the more complex forms of
today.</p>
<p>He is thinking as the historian thinks, but he sometimes gets confused
and thinks that he is explaining evolution when he is only describing it.
<!-- Page 3 --><span class="pagenum"><a name="page3"></a>{3}</span></p>
<div class="figcenter" style="width:34%;">
<a href="images/fig_001.jpg"><img style="width:100%" src="images/fig_001.jpg"
alt="Fig. 1." title="Fig. 1." /></a>
<p class="poem"><span class="sc">Fig. 1.</span> A series of skulls and
feet. Eohippus, Mesohippus, Meryhippus, Hipparion and Equus. (American
Museum of Natural History. After Matthews.)</p>
</div>
<p><!-- Page 4 --><span class="pagenum"><a name="page4"></a>{4}</span></p>
<p>A third kind of evolution is one for which man himself is responsible,
in the sense that he has brought it about, often with a definite end in
view.</p>
<p>His mind has worked slowly from stage to stage. We can often trace the
history of the stages through which his psychic processes have passed.
The evolution of the steam-boat, the steam engine, paintings, clothing,
instruments of agriculture, of manufacture, or of warfare (fig. 2)
illustrates the history of human progress. There is an obvious and
striking similarity between the evolution of man's inventions and the
evolution of the shells of molluscs and of the bones of mammals, yet in
neither case does a knowledge of the order in which these things arose
explain them. If we appeal to the psychologist he will probably tell us
that human inventions are either the result of happy accidents, that have
led to an unforeseen, but discovered use; or else the use of the
invention was foreseen. It is to the latter process more especially that
the idea of <i>purpose</i> is applied. When we come to review the four
great lines of evolutionary thought we <!-- Page 5 --><span
class="pagenum"><a name="page5"></a>{5}</span>shall see that this human
idea of purpose recurs in many forms, suggesting that man has often tried
to explain how organic evolution has taken place by an appeal to the
method which he believes he makes use of himself in the inorganic
world.</p>
<div class="figcenter" style="width:34%;">
<a href="images/fig_002.jpg"><img style="width:100%" src="images/fig_002.jpg"
alt="Fig. 2." title="Fig. 2." /></a>
<p class="poem"><span class="sc">Fig. 2.</span> Evolution of pole arms.
(Metropolitan Museum. After Dean.)</p>
</div>
<p><!-- Page 6 --><span class="pagenum"><a name="page6"></a>{6}</span></p>
<p>What has the evolution of the stars, of the horse and of human
inventions in common? Only this, that in each case from a simple
beginning through a series of changes something more complex, or at least
different, has come into being. To lump all these kinds of changes into
one and call them evolution is no more than asserting that you believe in
consecutive series of events (which is history) causally connected (which
is science); that is, that you believe in history and that you believe in
science. But let us not forget that we may have complete faith in both
without thereby offering any explanation of either. It is the business of
science to find out <i>specifically</i> what kinds of events were
involved when the stars evolved in the sky, when the horse evolved on the
earth, and the steam engine was evolved from the mind of man.</p>
<p>Is it not rather an empty generalization to say that any kind of
change is a process of evolution? At most it means little more than that
you want to intimate that miraculous <!-- Page 7 --><span
class="pagenum"><a name="page7"></a>{7}</span>intervention is not
necessary to account for such kinds of histories.</p>
<p>We are concerned here more particularly with the biologists' ideas of
evolution. My intention is to review the evidence on which the old theory
rested its case, in the light of some of the newer evidence of recent
years.</p>
<p>Four great branches of study have furnished the evidence of organic
evolution. They are:</p>
<div class="poem">
<div class="stanza">
<p>Comparative anatomy.</p>
<p>Embryology.</p>
<p>Paleontology.</p>
<p>Experimental Breeding or Genetics.</p>
</div>
</div>
<p class="cenhead"><i>The Evidence from Comparative Anatomy</i></p>
<p>When we study animals and plants we find that they can be arranged in
groups according to their resemblances. This is the basis of comparative
anatomy, which is only an accurate study of facts that are superficially
obvious to everyone.</p>
<p>The groups are based not on a single difference, but on a very large
number of resemblances. Let us take for example the group of vertebrates.
<!-- Page 8 --><span class="pagenum"><a name="page8"></a>{8}</span></p>
<div class="figcenter" style="width:40%;">
<a href="images/fig_003.jpg"><img style="width:100%" src="images/fig_003.jpg"
alt="Fig. 3." title="Fig. 3." /></a>
<p class="poem"><span class="sc">Fig. 3.</span> Limb skeletons of
extinct and living animals, showing the homologous bones: 1,
salamander; 2, frog; 3, turtle; 4, Aetosaurus; 5, Pleisiosaurus; 6,
Ichthyosaurus; 7, Mesosaurus; 8, duck. (After Jordan and Kellogg.)</p>
</div>
<p>The hand and the arm of man are similar to the hand and arm of the
ape. We find the same plan in the forefoot of the rat, the elephant, the
horse and the opossum. We can identify the same parts in the forefoot of
the lizard, the frog (fig. 3), and even, though less certainly, in the
pectoral fins of fishes. Comparison does not end here. We find
similarities in the skull and back bones of these same animals; in the
brain; in the digestive system; in the heart and blood vessels; in the
muscles.</p>
<p>Each of these systems is very complex, but <!-- Page 9 --><span
class="pagenum"><a name="page9"></a>{9}</span>the same general
arrangement is found in all. Anyone familiar with the evidence will, I
think, probably reach the conclusion either that these animals have been
created on some preconceived plan, or else that they have some other bond
that unites them; for we find it difficult to believe that such complex,
yet similar things could have arisen independently. But we try to
convince our students of the truth of the theory of evolution not so much
by calling their attention to this relation as by tracing each organ from
a simple to a complex structure.</p>
<p>I have never known such a course to fail in its intention. In fact, I
know that the student often becomes so thoroughly convinced that he
resents any such attempt as that which I am about to make to point out
that the evidence for his conviction is not above criticism.</p>
<div class="figcenter" style="width:31%;">
<a href="images/fig_004.jpg"><img style="width:100%" src="images/fig_004.jpg"
alt="Fig. 4." title="Fig. 4." /></a>
<p class="poem"><span class="sc">Fig. 4.</span> Drosophila ampelophila.
a, Female and b, male.</p>
</div>
<p>Because we can often arrange the series of structures in a line
extending from the very simple to the more complex, we are apt to become
unduly impressed by this fact and conclude that if we found the complete
series we should find all the intermediate steps and that they have
arisen in the order of their <!-- Page 10 --><span class="pagenum"><a
name="page10"></a>{10}</span>complexity. This conclusion is not
necessarily correct. Let me give some examples that have come under my
own observation. We have bred for five years the wild fruit fly
Drosophila ampelophila (fig. 4) and we have found over a hundred and
twenty-five new types that breed true. Each has arisen independently and
suddenly. Every part of the body has been affected by one or another of
these mutations. For instance many different kinds of changes have <!--
Page 11 --><span class="pagenum"><a name="page11"></a>{11}</span>taken
place in the wings and several of these involve the size of the wings. If
we arrange the latter arbitrarily in the order of their size there will
be an almost complete series beginning with the normal wings and ending
with those of apterous flies. Several of these types are represented in
figure 5. The order in which these mutations occurred bears no relation
to their size; each originated independently from the wild type.</p>
<div class="figcenter" style="width:33%;">
<a href="images/fig_005.jpg"><img style="width:100%" src="images/fig_005.jpg"
alt="Fig. 5." title="Fig. 5." /></a>
<p class="poem"><span class="sc">Fig. 5.</span> Mutants of Drosophila
ampelophila arranged in order of size of wings: (a) cut; (b) beaded;
(c) stumpy; (d) another individual of stumpy; (f) vestigial (g)
apterous.</p>
</div>
<p><!-- Page 12 --><span class="pagenum"><a name="page12"></a>{12}</span></p>
<p>The wings of the wild fly are straight (fig. 4). Several types have
arisen in which the wings are bent upwards and in the most extreme type
the wings are curled over the back, as seen in figure 54 (g), yet there
is no historical connection between these stages.</p>
<p>Mutations have occurred involving the pigmentation of the body and
wings. The head and thorax of the wild Drosophila ampelophila are grayish
yellow, the abdomen is banded with yellow and black, and the wings are
gray. There have appeared in our cultures several kinds of darker types
ranging to almost black flies (fig. 20) and to lighter types that are
quite yellow. If put in line a series may be made from the darkest flies
at one end to the light yellow flies at the other. These types, with the
fluctuations that occur within each type, furnish a complete series of
gradations; yet historically they have arisen independently of each
other.</p>
<p>Many changes in eye color have appeared. As many as thirty or more
races differing in eye <!-- Page 13 --><span class="pagenum"><a
name="page13"></a>{13}</span>color are now maintained in our cultures.
Some of them are so similar that they can scarcely be separated from each
other. It is easily possible beginning with the darkest eye color, sepia,
which is deep brown, to pick out a perfectly graded series ending with
pure white eyes. But such a serial arrangement would give a totally false
idea of the way the different types have arisen; and any conclusion based
on the existence of such a series might very well be entirely erroneous,
for the fact that such a series exists bears no relation to the order in
which its members have appeared.</p>
<p>Suppose that evolution "in the open" had taken place in the same way,
by means of <i>discontinuous</i> variation. What value then would the
evidence from comparative anatomy have in so far as it is based on a
continuous series of variants of any organ?</p>
<p>No one familiar with the entire evidence will doubt for a moment that
these 125 races of Drosophila ampelophila belong to the same species and
have had a common origin, for while they may differ mainly in one thing
they are extremely alike in a hundred other things, and <!-- Page 14
--><span class="pagenum"><a name="page14"></a>{14}</span>in the general
relation of the parts to each other.</p>
<p>It is in this sense that the evidence from comparative anatomy can be
used I think as an argument for evolution. It is the resemblances that
the animals or plants in any group have in common that is the basis for
such a conclusion; it is not because we can arrange in a continuous
series any particular variations. In other words, our inference
concerning the common descent of two or more species is based on the
totality of such resemblances that still remain in large part after each
change has taken place. In this sense the argument from comparative
anatomy, while not a demonstration, carries with it, I think, a high
degree of probability.</p>
<p class="cenhead"><i>The Evidence from Embryology</i></p>
<p>In passing from the egg to the adult the individual goes through a
series of changes. In the course of this development we see not only the
beginnings of the organs that gradually enlarge and change into those of
the adult animal, but also see that organs appear and <!-- Page 15
--><span class="pagenum"><a name="page15"></a>{15}</span>later disappear
before the adult stage is reached. We find, moreover, that the young
sometimes resemble in a most striking way the adult stage of groups that
we place lower in the scale of evolution.</p>
<p>Many years before Darwin advanced his theory of evolution through
natural selection, the resemblance of the young of higher animals to the
adults of lower animals had attracted the attention of zoölogists and
various views, often very naïve, had been advanced to account for the
resemblance. Among these speculations there was one practically identical
with that adopted by Darwin and the post-Darwinians, namely that the
higher animals repeat in their development the <i>adult stages</i> of
lower animals. Later this view became one of the cornerstones of the
theory of organic evolution. It reached its climax in the writings of
Haeckel, and I think I may add without exaggeration that for twenty-five
years it furnished the chief inspiration of the school of descriptive
embryology. Today it is taught in practically all textbooks of biology.
Haeckel called this interpretation the Biogenetic Law. <!-- Page 16
--><span class="pagenum"><a name="page16"></a>{16}</span></p>
<div class="figcenter" style="width:27%;">
<a href="images/fig_006.jpg"><img style="width:100%" src="images/fig_006.jpg"
alt="Fig. 6." title="Fig. 6." /></a>
<p class="poem"><span class="sc">Fig. 6.</span> Young trout (Trutta
fario) six days after hatching. (After Ziegler.)</p>
</div>
<p>It was recognized, of course, that many embryonic stages could not
possibly represent ancestral animals. A young fish with a huge yolk sac
attached (fig. 6) could scarcely ever have led a happy, free life as an
adult individual. Such stages were interpreted, however, as
<i>embryonic</i> additions to the original ancestral type. The embryo had
done something on its own account.</p>
<p>In some animals the young have structures that attach them to the
mother, as does the placenta of the mammals. In other cases the young
develop membranes about themselves—like the amnion of the chick
(fig. 7) and mammal—that would have shut off an adult animal from
all intercourse with the outside <!-- Page 17 --><span class="pagenum"><a
name="page17"></a>{17}</span>world. Hundreds of such embryonic
adaptations are known to embryologists. These were explained as
adaptations and as falsifications of the ancestral records.</p>
<div class="figcenter" style="width:21%;">
<a href="images/fig_007.jpg"><img style="width:100%" src="images/fig_007.jpg"
alt="Fig. 7." title="Fig. 7." /></a>
<p class="poem"><span class="sc">Fig. 7.</span> Diagram of chick
showing relations of amnion, allantois and yolk. (After Lillie.)</p>
</div>
<p>At the end of the last century Weismann injected a new idea into our
views concerning the origin of variations. He urged that variations are
germinal, i.e. they first appear in the egg and the sperm as changes that
later bring about modifications in the individual. The idea has been
fruitful and is generally accepted by most biologists today. It means
that the <!-- Page 18 --><span class="pagenum"><a
name="page18"></a>{18}</span>offspring of a pair of animals are not
affected by the structure or the activities of their parents, but the
germ plasm is the unmodified stream from which both the parent and the
young have arisen. Hence their resemblance. Now, it has been found that a
variation arising in the germ plasm, no matter what its cause, may affect
any stage in the development of the next individuals that arise from it.
There is no reason to suppose that such a change produces a new character
that always sticks itself, as it were, on to the end of the old series.
This idea of germinal variation therefore carried with it the death of
the older conception of evolution by superposition.</p>
<p>In more recent times another idea has become current, mainly due to
the work of Bateson and of de Vries—the idea that variations are
discontinuous. Such a conception does not fall easily into line with the
statement of the biogenetic "law"; for actual experience with
discontinuous variation has taught us that new characters that arise do
not add themselves to the end of the line of already existing characters
but if they affect the adult characters <!-- Page 19 --><span
class="pagenum"><a name="page19"></a>{19}</span>they change them without,
as it were, passing through and beyond them.</p>
<table class="nobctr"><tr><td style="width:50%; vertical-align:top;">
<div class="figright" style="width:59%;">
<a href="images/fig_008.jpg"><img style="width:100%" src="images/fig_008.jpg"
alt="Fig. 8." title="Fig. 8." /></a>
<p class="poem"><span class="sc">Fig. 8.</span> Diagram of head of
chick A and B, showing gill slits, and aortic arches; and head of fish
C showing aortic arches. (After Hesse.)</p>
</div>
</td><td style="width:50%; vertical-align:top;">
<div class="figleft" style="width:59%;">
<a href="images/fig_009.jpg"><img style="width:100%" src="images/fig_009.jpg"
alt="Fig. 9." title="Fig. 9." /></a>
<p class="poem"><span class="sc">Fig. 9.</span> Human embryo showing
gill slits and aortic arches. (After His; from Marshall.)</p>
</div>
</td></tr></table>
<p>I venture to think that these new ideas and this new evidence have
played havoc with the biogenetic "law". Nevertheless, there is an
interpretation of the facts that is entirely <!-- Page 20 --><span
class="pagenum"><a name="page20"></a>{20}</span>compatible with the
theory of evolution. Let me illustrate this by an example.</p>
<div class="figcenter" style="width:21%;">
<a href="images/fig_010.jpg"><img style="width:100%" src="images/fig_010.jpg"
alt="Fig. 10." title="Fig. 10." /></a>
<p class="poem"><span class="sc">Fig. 10.</span> Young fish, dorsal
view, and side view, showing gill slits. (After Kopsch.)</p>
</div>
<p>The embryos of the chick (fig. 8) and of man (fig. 9) possess at an
early stage in their development gill-slits on the sides of the neck like
those of fishes. No one familiar with the relations of the parts will for
a moment doubt that the gill slits of these embryos and of the fish
represent the same structures. When we look further into the matter we
find that young fish also possess gill slits (fig. 10 and 11)—even
in young stages in their development. Is it not <!-- Page 21 --><span
class="pagenum"><a name="page21"></a>{21}</span>then more probable that
the mammal and bird possess this stage in their development simply
because it has never been lost? Is not this a more reasonable view than
to suppose that the gill slits of the embryos of the higher forms
represent the adult gill slits of the fish that in some mysterious way
have been pushed back into the embryo of the bird?</p>
<div class="figcenter" style="width:24%;">
<a href="images/fig_011.jpg"><img style="width:100%" src="images/fig_011.jpg"
alt="Fig. 11." title="Fig. 11." /></a>
<p class="poem"><span class="sc">Fig. 11.</span> Side views of head of
embryo sharks, showing gill slits.</p>
</div>
<p>I could give many similar examples. All can be interpreted as
embryonic survivals rather than as phyletic contractions. Not one of them
calls for the latter interpretation.</p>
<p>The study of the cleavage pattern of the segmenting egg furnishes the
most convincing evidence that a different explanation from the one stated
in the biogenetic law is the more probable explanation. <!-- Page 22
--><span class="pagenum"><a name="page22"></a>{22}</span></p>
<div class="figcenter" style="width:36%;">
<a href="images/fig_012.jpg"><img style="width:100%" src="images/fig_012.jpg"
alt="Fig. 12." title="Fig. 12." /></a>
<p class="poem"><span class="sc">Fig. 12.</span> Cleavage stages of
four types of eggs, showing the origin of the mesenchyme cells
(stippled) and mesoderm cells (darker); a, Planarian; b, Annelid
(Podarke); c, Mollusc (Crepidula), d, Mollusc (Unio).</p>
</div>
<p>It has been found that the cleavage pattern has the same general
arrangement in the early stages of flat worms, annelids and molluscs
(fig. 12). Obviously these stages have never been adult ancestors, and
obviously if their resemblance has any meaning at all, it is that each
group has retained the same general plan <!-- Page 23 --><span
class="pagenum"><a name="page23"></a>{23}</span>of cleavage, possessed by
their common ancestor.</p>
<p>Accepting this view, let us ask, does the evidence from embryology
favor the theory of evolution? I think that it does very strongly. The
embryos of the mammal, bird, and lizard have gill slits today because
gill slits were present in the embryos of their ancestors. There is no
other view that explains so well their presence in the higher forms.</p>
<p>Perhaps someone will say, Well! is not this all that we have contended
for! Have you not reached the old conclusion in a roundabout way? I think
not. To my mind there is a wide difference between the old statement that
the higher animals living today have the original adult stages telescoped
into their embryos, and the statement that the resemblance between
certain characters in the embryos of higher animals and corresponding
stages in the embryos of lower animals is most plausibly explained by the
assumption that they have descended from the same ancestors, and that
their common structures are embryonic survivals. <!-- Page 24 --><span
class="pagenum"><a name="page24"></a>{24}</span></p>
<p class="cenhead"><i>The Evidence from Paleontology</i></p>
<p>The direct evidence furnished by fossil remains is by all odds the
strongest evidence that we have in favor of organic evolution.
Paleontology holds the incomparable position of being able to point
directly to the evidence showing that the animals and plants living in
past times are connected with those living at the present time, often
through an unbroken series of stages. Paleontology has triumphed over the
weakness of the evidence, which Darwin admitted was serious, by filling
in many of the missing links.</p>
<p>Paleontology has been criticised on the ground that she cannot pretend
to show the actual ancestors of living forms because, if in the past
genera and species were as abundant and as diverse as we find them at
present, it is very improbable that the bones of any individual that
happened to be preserved are the bones of just that species that took
part in the evolution. Paleontologists will freely admit that in many
cases this is probably true, but even then the evidence is, I think,
still just as valuable and <!-- Page 25 --><span class="pagenum"><a
name="page25"></a>{25}</span>in exactly the same sense as is the evidence
from comparative anatomy. It suffices to know that there lived in the
past a particular "group" of animals that had many points in common with
those that preceded them and with those that came later. Whether these
are the actual ancestors or not does not so much matter, for the view
that from such a group of species the later species have been derived is
far more probable than any other view that has been proposed.</p>
<p>With this unrivalled material and splendid series of gradations,
paleontology has constructed many stages in the past history of the
globe. But paleontologists have sometimes gone beyond this descriptive
phase of the subject and have attempted to formulate the "causes", "laws"
and "principles" that have led to the development of their series. It has
even been claimed that paleontologists are in an incomparably better
position than zoölogists to discover such principles, because they know
both the beginning and the end of the evolutionary series. The retort is
obvious. In his sweeping and poetic vision the paleontologist may fail
completely to find out the nature of <!-- Page 26 --><span
class="pagenum"><a name="page26"></a>{26}</span>the pigments that have
gone into the painting of his picture, and he may confuse a familiarity
with the different views he has enjoyed of the canvas with a knowledge of
how the painting is being done.</p>
<p>My good friend the paleontologist is in greater danger than he
realizes, when he leaves descriptions and attempts explanation. He has no
way to check up his speculations and it is notorious that the human mind
without control has a bad habit of wandering.</p>
<p>When the modern student of variation and heredity—the
geneticist—looks over the different "continuous" series, from which
certain "laws" and "principles" have been deduced, he is struck by two
facts: that the gaps, in some cases, are enormous as compared with the
single changes with which he is familiar, and (what is more important)
that they involve numerous parts in many ways. The geneticist says to the
paleontologist, since you do not know, and from the nature of your case
can never know, whether your differences are due to one change or to a
thousand, you can not with certainty tell us anything about the
hereditary units <!-- Page 27 --><span class="pagenum"><a
name="page27"></a>{27}</span>which have made the process of evolution
possible. And without this knowledge there can be no understanding of the
causes of evolution.</p>
<p class="cenhead">THE FOUR GREAT HISTORICAL SPECULATIONS</p>
<p>Looking backward over the history of the evolution theory we recognize
that during the hundred and odd years that have elapsed since Buffon,
there have been four main lines of <i>speculation</i> concerning
evolution. We might call them the four great cosmogonies or the four
modern epics of evolution.</p>
<p class="cenhead"><span class="sc">The Environment</span></p>
<p class="cenhead"><i>Geoffroy St. Hilaire</i></p>
<p>About the beginning of the last century Geoffroy St. Hilaire, protégé,
and in some respects a disciple of Buffon, was interested as to how
living species are related to the animals and plants that had preceded
them. He was familiar with the kind of change that takes place in the
embryo if it is put into new or changed surroundings, and from this
knowledge he concluded that as the surface of the <!-- Page 28 --><span
class="pagenum"><a name="page28"></a>{28}</span>earth slowly
changed—as the carbon dioxide contents in the air altered—as
land appeared—and as marine animals left the water to inhabit it,
they or their embryos responded to the new conditions and those that
responded favorably gave rise to new creations. As the environment
changed the fauna and flora changed—change for change. Here we have
a picture of progressive evolution that carries with it an idea of
mechanical necessity. If there is anything mystical or even improbable in
St. Hilaire's argument it does not appear on the surface; for he did not
assume that the response to the new environment was always a favorable
one or, as we say, an adaptation. He expressly stated that <i>if</i> the
response was unfavorable the individual or the race died out. He assumed
that <i>sometimes</i> the change might be favorable, i.e., that certain
species, entire groups, would respond in a direction favorable to their
existence in a new environment and these would come to inherit the earth.
In this sense he anticipated certain phases of the natural selection
theory of Darwin, but only in part; for his picture is not one of strife
within and without <!-- Page 29 --><span class="pagenum"><a
name="page29"></a>{29}</span>the species, but rather the escape of the
species from the old into a new world.</p>
<p>If then we recognize the intimate bond in chemical constitution of
living things and of the world in which they develop, what is there
improbable in St. Hilaire's hypothesis? Why, in a word is not more credit
given to St. Hilaire in modern evolutionary thought? The reasons are to
be found, I think, first, in that the evidence to which he appealed was
meagre and inconclusive; and, second, in that much of his special
evidence does not seem to us to be applicable. For example the monstrous
forms that development often assumes in a strange environment, and with
which every embryologist is only too familiar, rarely if ever furnish
combinations, as he supposed, that are capable of living. On the
contrary, they lead rather to the final catastrophe of the organism. And
lastly, St. Hilaire's appeal to sudden and great transformations, such as
a crocodile's egg hatching into a bird, has exposed his view to too easy
ridicule.</p>
<p>But when all is said, St. Hilaire's conception of evolution contains
elements that form the <!-- Page 30 --><span class="pagenum"><a
name="page30"></a>{30}</span>background of our thinking to-day, for taken
broadly, the interaction between the organism and its environment was a
mechanistic conception of evolution even though the details of the theory
were inadequate to establish his contention.</p>
<p>In our own time the French metaphysician Bergson in his <i>Evolution
Creatrice</i> has proposed in mystical form a thought that has at least a
superficial resemblance to St. Hilaire's conception. The response of
living things is no longer hit in one species and miss in another; it is
precise, exact; yet not mechanical in the sense at least in which we
usually employ the word mechanical. For Bergson claims that the one chief
feature of living material is that it responds favorably to the situation
in which it finds itself; at least so far as lies within the possible
physical limitations of its organization. Evolution has followed no
preordained plan; it has had no creator; it has brought about its own
creation by responding adaptively to each situation as it arose.</p>
<p>But note: the man of science believes that the organism responds today
as it does, because at <!-- Page 31 --><span class="pagenum"><a
name="page31"></a>{31}</span>present it has a chemical and physical
constitution that gives this response. We find a specific chemical
composition and generally a specific physical structure already existing.
We have no reason to suppose that such particular reactions would take
place until a specific chemical configuration had been acquired. Where
did this constitution come from? This is the question that the scientist
asks himself. I suppose Bergson would have to reply that it came into
existence at the moment that the first specific stimulus was applied. But
if this is the answer we have passed at once from the realm of
observation to the realm of fancy—to a realm that is foreign to our
experience; for such a view assumes that chemical and physical reactions
are guided by the needs of the organism when the reactions take place
inside living beings.</p>
<p class="cenhead"><span class="sc">Use and Disuse</span></p>
<p class="cenhead"><i>From Lamarck to Weismann</i></p>
<p>The second of the four great historical explanations appeals to a
change not immediately connected with the outer world, but to one within
the organism itself. <!-- Page 32 --><span class="pagenum"><a
name="page32"></a>{32}</span></p>
<p>Practice makes perfect is a familiar adage. Not only in human affairs
do we find that a part through use becomes a better tool for performing
its task, and through disuse degenerates; but in the field of animal
behavior we find that many of the most essential types of behavior have
been learned through repeated associations formed by contact with the
outside.</p>
<p>It was not so long ago that we were taught that the instincts of
animals are the inherited experience of their ancestors—lapsed
intelligence was the current phrase.</p>
<p>Lamarck's name is always associated with the application of the theory
of the inheritance of acquired characters. Darwin fully endorsed this
view and made use of it as an explanation in all of his writings about
animals. Today the theory has few followers amongst trained
investigators, but it still has a popular vogue that is widespread and
vociferous.</p>
<p>To Weismann more than to any other single individual should be
ascribed the disfavor into which this view has fallen. In a series of
brilliant essays he laid bare the inadequacy of the supposed evidence on
which the inheritance of <!-- Page 33 --><span class="pagenum"><a
name="page33"></a>{33}</span>acquired characters rested. Your neighbor's
cat, for instance, has a short tail, and it is said that it had its tail
pinched off by a closing door. In its litter of kittens one or more is
found without a tail. Your neighbor believes that here is a case of cause
and effect. He may even have known that the mother and grandmother of the
cat had natural tails. But it has been found that short tail is a
dominant character; therefore, until we know who was the father of the
short-tailed kittens the accident to its mother and the normal condition
of her maternal ancestry is not to the point.</p>
<p>Weismann appealed to common sense. He made few experiments to disprove
Lamarck's hypothesis. True, he cut off the tails of some mice for a few
generations but got no tailless offspring and while he gives no exact
measurements with coefficients of error he did not observe that the tails
of the descendants had shortened one whit. The combs of fighting cocks
and the tails of certain breeds of sheep have been cropped for many
generations and the practice continues today, because their tails are
still long. While in Lamarck's time there <!-- Page 34 --><span
class="pagenum"><a name="page34"></a>{34}</span>was no evidence opposed
to his ingenious theory, based as it was on an appeal to the acknowledged
facts of improvement that take place in the organs of an individual
through their own functioning (a fact that is as obvious and remarkable
today as in the time of Lamarck), yet now there is evidence as to whether
the effects of use and disuse are inherited, and this evidence is not in
accord with Lamarck's doctrine.</p>
<p class="cenhead">THE UNFOLDING PRINCIPLE</p>
<p class="cenhead"><i>Nägeli and Bateson</i></p>
<p>I have ventured to put down as one of the four great historical
explanations, under the heading of the unfolding principle, a conception
that has taken protean forms. At one extreme it is little more than a
mystic sentiment to the effect that evolution is the result of an inner
driving force or principle which goes under many names such as
Bildungstrieb, nisus formativus, vital force, and orthogenesis.
Evolutionary thought is replete with variants of this idea, often naïvely
expressed, sometimes unconsciously implied. Evolution once meant, in <!--
Page 35 --><span class="pagenum"><a name="page35"></a>{35}</span>fact, an
unfolding of what pre-existed in the egg, and the term still carries with
it something of its original significance.</p>
<p>Nägeli's speculation written several years after Darwin's "Origin of
Species" may be taken as a typical case. Nägeli thought that there exists
in living material an innate power to grow and expand. He vehemently
protested that he meant only a mechanical principle but as he failed to
refer such a principle to any properties of matter known to physicists
and chemists his view seems still a mysterious affirmation, as difficult
to understand as the facts themselves which it purports to explain.</p>
<p>Nägeli compared the process of evolution to the growth of a tree,
whose ultimate twigs represent the living world of species. Natural
selection plays only the rôle of the gardener who prunes the tree into
this or that shape but who has himself <i>produced</i> nothing. As an
imaginative figure of speech Nägeli's comparison of the tree might even
today seem to hold if we substituted "mutations" for "growth", but
although we know so little about what causes mutations there is no reason
for <!-- Page 36 --><span class="pagenum"><a
name="page36"></a>{36}</span>supposing them to be due to an inner
impulse, and hence they furnish no justification for such a
hypothesis.</p>
<p>In his recent presidential address before the British Association
Bateson has inverted this idea. I suspect that his effort was intended as
little more than a <i>tour de force</i>. He claims for it no more than
that it is a possible line of speculation. Perhaps he thought the time
had come to give a shock to our too confident views concerning evolution.
Be this as it may, he has invented a striking paradox. Evolution has
taken place through the steady loss of inhibiting factors. Living matter
was stopped down, so to speak, at the beginning of the world. As the
stops are lost, new things emerge. Living matter has changed only in that
it has become simpler.</p>
<p class="cenhead"><span class="sc">Natural Selection</span></p>
<p class="cenhead"><i>Darwin</i></p>
<p>Of the four great historical speculations about evolution, the
doctrine of Natural Selection of Darwin and Wallace has met with the most
widespread acceptance. In the last <!-- Page 37 --><span
class="pagenum"><a name="page37"></a>{37}</span>lecture I intend to
examine this theory critically. Here we are concerned only with its
broadest aspects.</p>
<p>Darwin appealed to <i>chance variations</i> as supplying evolution
with the material on which natural selection works. If we accept, for the
moment, this statement as the cardinal doctrine of natural selection it
may appear that evolution is due, (1) <i>not</i> to an <i>orderly</i>
response of the organism to its environment, (2) <i>not</i> in the main
to the activities of the animal through the use or disuse of its parts,
(3) <i>not</i> to any innate principle of living material itself, and (4)
above all <i>not</i> to purpose either from within or from without.
Darwin made quite clear what he meant by chance. By chance he did not
mean that the variations were not causal. On the contrary he taught that
in Science we mean by chance only that the particular combination of
causes that bring about a variation are not known. They are accidents, it
is true, but they are causal accidents.</p>
<p>In his famous book on "Animals and Plants under Domestication", Darwin
dwells at great length on the nature of the conditions that <!-- Page 38
--><span class="pagenum"><a name="page38"></a>{38}</span>bring about
variations. If his views seem to us today at times vague, at times
problematical, and often without a secure basis, nevertheless we find in
every instance, that Darwin was searching for the <i>physical causes of
variation</i>. He brought, in consequence, conviction to many minds that
there are abundant indications, even if certain proof is lacking, that
the causes of variation are to be found in natural processes.</p>
<p>Today the belief that evolution takes place by means of natural
processes is generally accepted. It does not seem probable that we shall
ever again have to renew the old contest between evolution and special
creation.</p>
<p>But this is not enough. We can never remain satisfied with a negative
conclusion of this kind. We must find out what natural causes bring about
variations in animals and plants; and we must also find out what kinds of
variations are inherited, and how they are inherited. If the
circumstantial evidence for organic evolution, furnished by comparative
anatomy, embryology and paleontology is cogent, we should be able to
observe evolution going on at <!-- Page 39 --><span class="pagenum"><a
name="page39"></a>{39}</span>the present time, i.e. we should be able to
observe the occurrence of variations and their transmission. This has
actually been done by the geneticist in the study of mutations and
Mendelian heredity, as the succeeding lectures will show.</p>
<p><br style="clear:both" /></p>
<hr class="full" />
<p><!-- Page 40 --><span class="pagenum"><a name="page40"></a>{40}</span></p>
<h3>CHAPTER II</h3>
<p class="cenhead">THE BEARING OF MENDEL'S DISCOVERY
ON THE ORIGIN OF HEREDITARY
CHARACTERS</p>
<p>Between the years 1857 and 1868 Gregor Mendel, Augustinian monk,
studied the heredity of certain characters of the common edible pea, in
the garden of the monastery at Brünn.</p>
<p>In his account of his work written in 1868, he said:</p>
<blockquote class="b1n">
<p>"It requires indeed some courage to undertake a labor of such a
far-reaching extent; it appears, however, to be the only right way by
which we can finally reach the solution of a question the importance of
which cannot be over-estimated in connection with the history of the
evolution of organic forms."</p>
</blockquote>
<p>He tells us also why he selected peas for his work:</p>
<blockquote class="b1n">
<p>"The selection of the plant group which shall serve for experiments of
this kind must be made with all possible care if it be desired to avoid
from the outset every risk of questionable results."</p>
<p>"The experimental plants must necessarily <!-- Page 41 --><span
class="pagenum"><a name="page41"></a>{41}</span></p>
<p>1. Possess constant differentiating characters.</p>
<p>2. The hybrids of such plants must, during the flowering period, be
protected from the influence of all foreign pollen, or be easily capable
of such protection."</p>
</blockquote>
<p>Why do biologists throughout the world to-day agree that Mendel's
discovery is one of first rank?</p>
<p>A great deal might be said in this connection. What is essential may
be said in a few words. Biology had been, and is still, largely a
descriptive and speculative science. <i>Mendel showed by experimental
proof that heredity could be explained by a simple mechanism. His
discovery has been exceedingly fruitful.</i></p>
<p>Science begins with naïve, often mystic conceptions of its problems.
It reaches its goal whenever it can replace its early guessing by
verifiable hypotheses and predictable results. This is what Mendel's law
did for heredity.</p>
<p class="cenhead"><span class="sc">Mendel's First Discovery—Segregation</span></p>
<p><!-- Page 42 --><span class="pagenum"><a name="page42"></a>{42}</span></p>
<div class="figcenter" style="width:31%;">
<a href="images/fig_013.jpg"><img style="width:100%" src="images/fig_013.jpg"
alt="Fig. 13." title="Fig. 13." /></a>
<p class="poem"><span class="sc">Fig. 13.</span> Diagram illustrating a
cross between a red (dark) and a white variety of four o'clock
(Mirabilis jalapa).</p>
</div>
<p>Let us turn to the demonstration of his first law—the law of
segregation. The first case I choose is not the one given by Mendel but
one worked out later by Correns. If the common garden plant called four
o'clock (Mirabilis jalapa) with red flowers is crossed to one having
white flowers, the offspring are pink (fig. 13). The hybrid, then, is
intermediate in the color of its flowers between the two parents. If
these hybrids are inbred the offspring are white, pink and red, in the
proportion of 1:2:1. All of these had the same ancestry, yet they are of
three different kinds. If we did not know their <!-- Page 43 --><span
class="pagenum"><a name="page43"></a>{43}</span>history it would be quite
impossible to state what the ancestry of the white or of the red had
been, for they might just as well have come from pure white and pure red
ancestors respectively as to have emerged from the pink hybrids.
Moreover, when we test them we find that they are as pure as are white or
red flowering plants that have had all white or all red flowering
ancestors.</p>
<p>Mendel's Law explains the results of this cross as shown in figure
14.</p>
<p>The egg cell from the white parent carries the factor for white, the
pollen cell from the red parent carries the factor for red. The hybrid
formed by their union carries both factors. The result of their combined
action is to produce flowers intermediate in color.</p>
<p>When the hybrids mature and their germ cells (eggs or pollen) ripen,
each carries only one of these factors, either the red or the white, but
not both. In other words, the two factors that have been brought together
in the hybrid separate in its germ cells. Half of the egg cells are white
bearing, half red bearing. Half of the pollen cells are white bearing,
half red <!-- Page 44 --><span class="pagenum"><a
name="page44"></a>{44}</span>bearing. Chance combinations at
fertilization give the three classes of individuals of the second
generation.</p>
<div class="figcenter" style="width:31%;">
<a href="images/fig_014.jpg"><img style="width:100%" src="images/fig_014.jpg"
alt="Fig. 14." title="Fig. 14." /></a>
<p class="poem"><span class="sc">Fig. 14.</span> Diagram illustrating
the history of the factors in the germ cells of the cross shown in Fig.
13.</p>
</div>
<p>The white flowering plants should forever breed true, as in fact they
do. The red flowering plants also breed true. The pink flowering plants,
having the same composition as the hybrids of the first generation,
should give the same kind of result. They do, indeed, give this result
i.e. one white to two pink to one red flowered offspring. <!-- Page 45
--><span class="pagenum"><a name="page45"></a>{45}</span></p>
<div class="figcenter" style="width:34%;">
<a href="images/fig_015.jpg"><img style="width:100%" src="images/fig_015.jpg"
alt="Fig. 15." title="Fig. 15." /></a>
<p class="poem"><span class="sc">Fig. 15.</span> Diagram illustrating a
cross between special races of white and black fowls, producing the
blue (here gray) Andalusian.</p>
</div>
<p>Another case of the same kind is known to breeders of poultry. One of
the most beautiful of the domesticated breeds is known as the Andalusian.
It is a slate blue bird shading into blue-black on the neck and back.
Breeders know that these blue birds do not breed true but produce white,
black, and blue offspring. <!-- Page 46 --><span class="pagenum"><a
name="page46"></a>{46}</span></p>
<div class="figcenter" style="width:31%;">
<a href="images/fig_016.jpg"><img style="width:100%" src="images/fig_016.jpg"
alt="Fig. 16." title="Fig. 16." /></a>
<p class="poem"><span class="sc">Fig. 16.</span> Diagram showing
history of germ cells of cross of Fig. 15. The larger circles indicate
the color of the birds; their enclosed small circles the nature of the
factors in the germ cells of such birds.</p>
</div>
<p>The explanation of the failure to produce a pure race of Andalusians
is that they are like the pink flowers of the four o'clock, i.e., they
are a hybrid type formed by the meeting of the white and the black germ
cells. If the whites produced by the Andalusians are bred to the blacks
(both being pure strains), all the offspring will be blue (fig. 15); if
these blues are inbred they will give 1 white, to 2 blues, to 1 <!-- Page
47 --><span class="pagenum"><a name="page47"></a>{47}</span>black. In
other words, the factor for white and the factor for black separate in
the germ cells of the hybrid Andalusian birds (fig. 16).</p>
<div class="figcenter" style="width:20%;">
<a href="images/fig_017.jpg"><img style="width:100%" src="images/fig_017.jpg"
alt="Fig. 17." title="Fig. 17." /></a>
<p class="poem"><span class="sc">Fig. 17.</span> Diagram of Mendel's
cross between yellow (dominant) and green (recessive) peas.</p>
</div>
<p>The third case is Mendel's classical case of yellow and green peas
(fig. 17). He crossed a plant belonging to a race having yellow peas with
one having green peas. The hybrid plants had yellow seeds. These hybrids
inbred gave three yellows to one green. The explanation <!-- Page 48
--><span class="pagenum"><a name="page48"></a>{48}</span>(fig. 18) is the
same in principle as in the preceding cases. The only difference between
them is that the hybrid which contains both the yellow and the green
factors is in appearance not intermediate, but like the yellow parent
stock. Yellow is said therefore to be dominant and green to be
recessive.</p>
<div class="figcenter" style="width:32%;">
<a href="images/fig_018.jpg"><img style="width:100%" src="images/fig_018.jpg"
alt="Fig. 18." title="Fig. 18." /></a>
<p class="poem"><span class="sc">Fig. 18.</span> Diagram illustrating
the history of the factors in the cross shown in Fig. 17.</p>
</div>
<p>Another example where one of the contrasted characters is dominant is
shown by the cross of Drosophila with vestigial wings to the wild type
with long wings (fig. 19). The F<sub>1</sub> flies have long wings not
differing from those of the wild fly, so far as can be observed. When two
such flies are inbred there result three long to one vestigial. <!-- Page
49 --><span class="pagenum"><a name="page49"></a>{49}</span></p>
<div class="figcenter" style="width:33%;">
<a href="images/fig_019.jpg"><img style="width:100%" src="images/fig_019.jpg"
alt="Fig. 19." title="Fig. 19." /></a>
<p class="poem"><span class="sc">Fig. 19.</span> Diagram illustrating a
cross between a fly (Drosophila ampelophila) with long wings and a
mutant fly with vestigial wings.</p>
</div>
<p><!-- Page 50 --><span class="pagenum"><a name="page50"></a>{50}</span></p>
<p>The question as to whether a given character is dominant or recessive
is a matter of no theoretical importance for the principle of
segregation, although from the notoriety given to it one might easily be
misled into the erroneous supposition that it was the discovery of this
relation that is Mendel's crowning achievement.</p>
<p>Let me illustrate by an example in which the hybrid standing between
two types overlaps them both. There are two mutant races in our cultures
of the fruit fly Drosophila that have dark body color, one called sooty,
another which is even blacker, called ebony (fig. 20). Sooty crossed to
ebony gives offspring that are intermediate in color. Some of them are so
much like sooty that they cannot be distinguished from sooty. At the
other extreme some of the hybrids are as dark as the lightest of the
ebony flies. If these hybrids are inbred there is a continuous series of
individuals, sooties, intermediates and ebonies. Which color here shall
we call the dominant? If the ebony, then in the second generation we
count three ebonies to one sooty, putting the hybrids with the ebonies.
If the dominant is the sooty then we count three <!-- Page 51 --><span
class="pagenum"><a name="page51"></a>{51}</span>sooties to one ebony,
putting the hybrids with the sooties. The important fact to find out is
whether there actually exist three classes in the second generation. This
can be ascertained even when, as in this case, there is a perfectly
graded series from one end to the other, by testing out individually
enough of the flies to show that one-fourth of them never produce any
descendants but ebonies, one-fourth never any but sooties, and one-half
of them give rise to both ebony and sooty.</p>
<div class="figcenter" style="width:35%;">
<a href="images/fig_020.jpg"><img style="width:100%" src="images/fig_020.jpg"
alt="Fig. 20." title="Fig. 20." /></a>
<p class="poem"><span class="sc">Fig. 20.</span> Cross between two
allelomorphic races of Drosophila, sooty and ebony, that give a
completely graded series in F<sub>2</sub>.</p>
</div>
<p><!-- Page 52 --><span class="pagenum"><a name="page52"></a>{52}</span></p>
<p class="cenhead"><span class="sc">Mendel's Second Discovery—Independent Assortment</span></p>
<p>Besides his discovery that there are pairs of characters that disjoin,
as it were, in the germ cells of the hybrid (law of segregation) Mendel
made a second discovery which also has far-reaching consequences. The
following case illustrates Mendel's second law.</p>
<p>If a pea that is yellow and round is crossed to one that is green and
wrinkled (fig. 21), all of the offspring are yellow and round. Inbred,
these give 9 yellow round, 3 green round, 3 yellow wrinkled, 1 green
wrinkled. All the yellows taken together are to the green as 3:1. All the
round taken together are to the wrinkled as three to one; but some of the
yellows are now wrinkled and some of the green are now <!-- Page 53
--><span class="pagenum"><a name="page53"></a>{53}</span>round. There has
been a recombination of characters, while at the same time the results,
for each pair of characters taken separately, are in accord with Mendel's
Law of Segregation, (fig. 22). The second law of Mendel may be called the
law of independent assortment of different character pairs.</p>
<div class="figcenter" style="width:31%;">
<a href="images/fig_021.jpg"><img style="width:100%" src="images/fig_021.jpg"
alt="Fig. 21." title="Fig. 21." /></a>
<p class="poem"><span class="sc">Fig. 21.</span> Cross between
yellow-round and green-wrinkled peas, giving the 9: 3: 3: 1 ratio in
F<sub>2</sub>.</p>
</div>
<p>We can, as it were, take the characters of one organism and recombine
them with those <!-- Page 54 --><span class="pagenum"><a
name="page54"></a>{54}</span>of a different organism. We can explain this
result as due to the assortment of factors for these characters in the
germ cells according to a definite law.</p>
<div class="figcenter" style="width:26%;">
<a href="images/fig_022.jpg"><img style="width:100%" src="images/fig_022.jpg"
alt="Fig. 22." title="Fig. 22." /></a>
<p class="poem"><span class="sc">Fig. 22.</span> Diagram to show the
history of the factor pairs yellow-green and round-wrinkled of the
cross in Fig. 21.</p>
</div>
<p>As a second illustration let me take the <!-- Page 55 --><span
class="pagenum"><a name="page55"></a>{55}</span>classic case of the combs
of fowls. If a bird with a rose comb is bred to one with a pea comb (fig.
23), the offspring have a comb different from either. It is called a
walnut comb. If two such individuals are bred they give 9 walnut, 3 rose,
3 pea, 1 single. This proportion shows that the grandparental types
differed in respect to two pairs of characters.</p>
<div class="figcenter" style="width:31%;">
<a href="images/fig_023.jpg"><img style="width:100%" src="images/fig_023.jpg"
alt="Fig. 23." title="Fig. 23." /></a>
<p class="poem"><span class="sc">Fig. 23.</span> Cross between pea and
rose combed fowls. (Charts of Baur and Goldschmidt.)</p>
</div>
<p>A fourth case is shown in the fruit fly, where an ebony fly with long
wings is mated to a grey fly with vestigial wings (fig. 24). The <!--
Page 56 --><span class="pagenum"><a
name="page56"></a>{56}</span>offspring are gray with long wings. If these
are inbred they give 9 gray long, 3 gray vestigial, 3 ebony long, 1 ebony
vestigial (figs. 24 and 25).</p>
<div class="figcenter" style="width:31%;">
<a href="images/fig_024.jpg"><img style="width:100%" src="images/fig_024.jpg"
alt="Fig. 24." title="Fig. 24." /></a>
<p class="poem"><span class="sc">Fig. 24.</span> Cross between long
ebony and gray vestigial flies.</p>
</div>
<p><!-- Page 57 --><span class="pagenum"><a name="page57"></a>{57}</span></p>
<p>The possibility of interchanging characters might be illustrated over
and over again. It is true not only when two pairs of characters are
involved, but when three, four, or more enter the cross.</p>
<div class="figcenter" style="width:32%;">
<a href="images/fig_025.jpg"><img style="width:100%" src="images/fig_025.jpg"
alt="Fig. 25." title="Fig. 25." /></a>
<p class="poem"><span class="sc">Fig. 25.</span> Diagram to show the
history of the factors in the cross shown in Fig. 24.</p>
</div>
<p>It is as though we took individuals apart and put together parts of
two, three or more individuals by substituting one part for another. <!--
Page 58 --><span class="pagenum"><a name="page58"></a>{58}</span></p>
<p>Not only has this power to make whatever combinations we choose great
practical importance, it has even greater theoretical significance; for,
it follows that the individual is not in itself the unit in heredity, but
that within the germ-cells there exist smaller units concerned with the
transmission of characters.</p>
<p>The older mystical statement of the individual as a unit in heredity
has no longer any interest in the light of these discoveries, except as a
past phase of biological history. We see, too, more clearly that the
sorting out of factors in the germ plasm is a very different process from
the influence of these factors on the development of the organism. There
is today no excuse for confusing these two problems.</p>
<p>If mechanistic principles apply also to embryonic development then the
course of development is capable of being stated as a series of
chemico-physical reactions and the "<i>individual</i>" is merely a term
to express the sum total of such reactions and should not be interpreted
as something different from or more than these reactions. So long as so
little is known of the actual processes involved in <!-- Page 59 --><span
class="pagenum"><a name="page59"></a>{59}</span>development the use of
the term "individuality", while giving the appearance of profundity, in
reality often serves merely to cover ignorance and to make a mystery out
of a mechanism.</p>
<p class="cenhead"><span class="sc">The Characters of Wild Animals and Plants Follow the Same Laws of Inheritance as do the Characters of Domesticated Animals and Plants.</span></p>
<p>Darwin based many of his conclusions concerning variation and heredity
on the evidence derived from the garden and from the stock farm. Here he
was handicapped to some extent, for he had at times to rely on
information much of which was uncritical, and some of which was
worthless.</p>
<p>Today we are at least better informed on <i>two</i> important points;
one concerning the <i>kinds</i> of variations that furnish to the
cultivator the materials for his selection; the other concerning the
modes of inheritance of these variations. We know now that new characters
are continually appearing in domesticated as well as in wild animals and
plants, that these characters are often sharply marked <!-- Page 60
--><span class="pagenum"><a name="page60"></a>{60}</span>off from the
original characters, and whether the differences are great or whether
they are small they are transmitted alike according to Mendel's law.</p>
<p>Many of the characteristics of our domesticated animals and cultivated
plants originated long ago, and only here and there have the records of
their first appearance been preserved. In only a few instances are these
records clear and definite, while the complete history of any large group
of our domesticated products is unknown to us.</p>
<p>Within the last five or six years, however, from a common wild species
of fly, the fruit fly, Drosophila ampelophila, which we have brought into
the laboratory, have arisen over a hundred and twenty-five new types
whose origin is completely known. Let me call attention to a few of the
more interesting of these types and their modes of inheritance, comparing
them with wild types in order to show that the kinds of inheritance found
in domesticated races occur also in wild types. The results will show
beyond dispute that the characters of wild types are inherited in
precisely <!-- Page 61 --><span class="pagenum"><a
name="page61"></a>{61}</span>the same way as are the characters of the
mutant types—a fact that is not generally appreciated except by
students of genetics, although it is of the most far-reaching
significance for the theory of evolution.</p>
<p>A mutant appeared in which the eye color of the female was different
from that of the male. The eye color of the mutant female is a dark eosin
color, that of the male yellowish eosin. From the beginning this
difference was as marked as it is to-day. Breeding experiments show that
eosin eye color differs from the red color of the eye of the wild fly by
a single mutant factor. Here then at a single step a type appeared that
was sexually dimorphic.</p>
<p>Zoölogists know that sexual dimorphism is not uncommon in wild species
of animals, and Darwin proposed the theory of sexual selection to account
for the difference between the sexes. He assumed that the male preferred
certain kinds of females differing from himself in a particular
character, and thus in time through sexual selection, the sexes came to
differ from each other. <!-- Page 62 --><span class="pagenum"><a
name="page62"></a>{62}</span></p>
<div class="figcenter" style="width:33%;">
<a href="images/fig_026.jpg"><img style="width:100%" src="images/fig_026.jpg"
alt="Fig. 26." title="Fig. 26." /></a>
<p class="poem"><span class="sc">Fig. 26.</span> Clover butterfly
(Colias philodice) with two types of females, above; and one type of
male, below.</p>
</div>
<p>In the case of eosin eye color no such process as that postulated by
Darwin to account for the differences between the sexes was involved; for
the single mutation that brought about the change also brought in the
dimorphism with it.</p>
<p>In recent years zoölogists have carefully studied several cases in
which two types of female are found in the same species. In the common
clover butterfly, there is a yellow and a white type of female, while the
male is yellow (fig. 26). It has been shown that a single factor
difference determines whether the female <!-- Page 63 --><span
class="pagenum"><a name="page63"></a>{63}</span>is yellow or white. The
inheritance is, according to Gerould, strictly Mendelian.</p>
<div class="figcenter" style="width:26%;">
<a href="images/fig_027.jpg"><img style="width:100%" src="images/fig_027.jpg"
alt="Fig. 27." title="Fig. 27." /></a>
<p class="poem"><span class="sc">Fig. 27.</span> Papilio turnus with
two types of females above and one type of male below.</p>
</div>
<p>In Papilio turnus there exist, in the southern states, two kinds of
females, one yellow like the male, one black (fig. 27). The evidence here
is not so certain, but it seems probable that a single factor difference
determines whether the female shall be yellow or black.</p>
<p>Finally in Papilio polytes of Ceylon and India three different types
of females appear, <!-- Page 64 --><span class="pagenum"><a
name="page64"></a>{64}</span>(fig. 28 to right) only one of which is like
the male. Here the analysis of the breeding data shows the possibility of
explaining this case as due to two pairs Mendelian factors which give in
combination the three types of female.</p>
<div class="figcenter" style="width:31%;">
<a href="images/fig_028.jpg"><img style="width:100%" src="images/fig_028.jpg"
alt="Fig. 28." title="Fig. 28." /></a>
<p class="poem"><span class="sc">Fig. 28.</span> Papilio polytes, with
three types of female to right and one type of male above to left.</p>
</div>
<p>Taking these cases together, they furnish a much simpler explanation
than the one proposed by Darwin. They show also that characters like
these shown by wild species may follow Mendel's law. <!-- Page 65
--><span class="pagenum"><a name="page65"></a>{65}</span></p>
<div class="figcenter" style="width:25%;">
<a href="images/fig_029.jpg"><img style="width:100%" src="images/fig_029.jpg"
alt="Fig. 29." title="Fig. 29." /></a>
<p class="poem"><span class="sc">Fig. 29.</span> Mutant race of fruit
fly with intercalated duplicate mesothorax on dorsal side.</p>
</div>
<p>There has appeared in our cultures a fly in which the third division
of the thorax with its appendages has changed into a segment like the
second (fig. 29). It is smaller than the normal mesothorax and its wings
are imperfectly developed, but the bristles on the upper surface may have
the typical arrangement of the normal mesothorax. The mutant shows how
great a change may result from a single factor difference.</p>
<p>A factor that causes duplication in the legs <!-- Page 66 --><span
class="pagenum"><a name="page66"></a>{66}</span>has also been found. Here
the interesting fact was discovered (Hoge) that duplication takes place
only in the cold. At ordinary temperatures the legs are normal.</p>
<div class="figcenter" style="width:21%;">
<a href="images/fig_030.jpg"><img style="width:100%" src="images/fig_030.jpg"
alt="Fig. 30." title="Fig. 30." /></a>
<p class="poem"><span class="sc">Fig. 30.</span> Mutant race of fruit
fly, called eyeless; a, a' normal eye.</p>
</div>
<p>In contrast to the last case, where a character is doubled, is the
next one in which the eyes are lost (fig. 30). This change also took
place at a single step. All the flies of this stock however, cannot be
said to be eyeless, since many of them show pieces of the
eye—indeed the variation is so wide that the eye may even appear
like a normal eye unless carefully <!-- Page 67 --><span
class="pagenum"><a name="page67"></a>{67}</span>examined. Formerly we
were taught that eyeless animals arose in caves. This case shows that
they may also arise suddenly in glass milk bottles, by a change in a
single factor.</p>
<p>I may recall in this connection that wingless flies (fig. 5 f) also
arose in our cultures by a single mutation. We used to be told that
wingless insects occurred on desert islands because those insects that
had the best developed wings had been blown out to sea. Whether this is
true or not, I will not pretend to say, but at any rate wingless insects
may also arise, not through a slow process of elimination, but at a
single step.</p>
<p>The preceding examples have all related to recessive characters. The
next one is dominant.</p>
<div class="figcenter" style="width:31%;">
<a href="images/fig_031.jpg"><img style="width:100%" src="images/fig_031.jpg"
alt="Fig. 31." title="Fig. 31." /></a>
<p class="poem"><span class="sc">Fig. 31.</span> Mutant race of fruit
fly called bar to the right (normal to the left). The eye is a narrow
vertical bar, the outline of the original eye is indicated.</p>
</div>
<p><!-- Page 68 --><span class="pagenum"><a name="page68"></a>{68}</span></p>
<p>A single male appeared with a narrow vertical red bar (fig. 31)
instead of the broad red oval eye. Bred to wild females the new character
was found to dominate, at least to the extent that the eyes of all its
offspring were narrower than the normal eye, although not so narrow as
the eye of the pure stock. Around the bar there is a wide border that
corresponds to the region occupied by the rest of the eye of the wild
fly. It lacks however the elements of the eye. It is therefore to be
looked upon as a rudimentary organ, which is, so to speak, a by-product
of the dominant mutation.</p>
<p>The preceding cases have all involved rather great changes in some one
organ of the body. The following three cases involve slight changes, and
yet follow the same laws of inheritance as do the larger changes.</p>
<div class="figcenter" style="width:31%;">
<a href="images/fig_032.jpg"><img style="width:100%" src="images/fig_032.jpg"
alt="Fig. 32." title="Fig. 32." /></a>
<p class="poem"><span class="sc">Fig. 32.</span> Mutant race of fruit
fly, called speck. There is a minute black speck at base of wing.</p>
</div>
<p><!-- Page 69 --><span class="pagenum"><a name="page69"></a>{69}</span></p>
<p>At the base of the wings a minute black speck appeared (fig. 32). It
was found to be a Mendelian character. In another case the spines on the
thorax became forked or kinky (fig. 52b). This stock breeds true, and the
character is inherited in strictly Mendelian fashion.</p>
<div class="figcenter" style="width:31%;">
<a href="images/fig_033.jpg"><img style="width:100%" src="images/fig_033.jpg"
alt="Fig. 33." title="Fig. 33." /></a>
<p class="poem"><span class="sc">Fig. 33.</span> Mutant race of fruit
fly called club. The wings often remain unexpanded and two bristles
present in wild fly (b) are absent on side of thorax (c).</p>
</div>
<p>In a certain stock a number of flies appeared <!-- Page 70 --><span
class="pagenum"><a name="page70"></a>{70}</span>in which the wing pads
did not expand (fig. 33). It was found that this peculiarity is shown in
only about twenty per cent of the individuals supposed to inherit it.
Later it was found that this stock lacked two bristles on the sides of
the thorax. By means of this knowledge the heredity of the character was
easily determined. It appears that while the expansion of the wing pads
fails to occur once in five times—probably because it is an
environmental effect peculiar to this stock,—yet the minute
difference of the presence or absence of the two lateral bristles is a
constant feature of the flies that carry this particular factor.</p>
<p>In the preceding cases I have spoken as though a factor influenced
only one part of the body. It would have been more accurate to have
stated that the <i>chief</i> effect of the factor was observed in a
particular part of the body. Most students of genetics realize that a
factor difference usually affects more than a single character. For
example, a mutant stock called rudimentary wings has as its principle
characteristic very short wings (fig. 34). But the factor for rudimentary
wings also produces other <!-- Page 71 --><span class="pagenum"><a
name="page71"></a>{71}</span>effects as well. The females are almost
completely sterile, while the males are fertile. The viability of the
stock is poor. When flies with rudimentary wings are put into competition
with wild flies relatively few of the rudimentary flies come through,
especially if the culture is crowded. The hind legs are also shortened.
All of these effects are the results of a single factor-difference.</p>
<div class="figcenter" style="width:23%;">
<a href="images/fig_034.jpg"><img style="width:100%" src="images/fig_034.jpg"
alt="Fig. 34." title="Fig. 34." /></a>
<p class="poem"><span class="sc">Fig. 34.</span> Mutant race of fruit
fly, called rudimentary.</p>
</div>
<p>One may venture the guess that some of the specific and varietal
differences that are <!-- Page 72 --><span class="pagenum"><a
name="page72"></a>{72}</span>characteristic of wild types and which at
the same time appear to have no survival value, are only by-products of
factors whose most important effect is on another part of the organism
where their influence is of vital importance.</p>
<p>It is well known that systematists make use of characters that are
constant for groups of species, but which do not appear in themselves to
have an adaptive significance. If we may suppose that the constancy of
such characters may be only an index of the presence of a factor whose
<i>chief</i> influence is in some other direction or directions, some
physiological influence, for example, we can give at least a reasonable
explanation of the constancy of such characters.</p>
<p>I am inclined to think that an overstatement to the effect that each
factor may affect the entire body, is less likely to do harm than to
state that each factor affects only a particular character. The reckless
use of the phrase "unit character" has done much to mislead the
uninitiated as to the effects that a single change in the germ plasm may
produce on the organism. Fortunately, the expression "unit character"
<!-- Page 73 --><span class="pagenum"><a name="page73"></a>{73}</span>is
being less used by those students of genetics who are more careful in
regard to the implications of their terminology.</p>
<p>There is a class of cases of inheritance, due to the XY chromosomes,
that is called sex linked inheritance. It is shown both by mutant
characters and characters of wild species.</p>
<p>For instance, white eye color in Drosophila shows sex linked
inheritance. If a white eyed male is mated to a wild red eyed female
(fig. 35) all the offspring have red eyes. If these are inbred, there are
three red to one white eyed offspring, but white eyes occur only in the
males. The grandfather has transmitted his peculiarity to half of his
grandsons, but to none of his granddaughters.</p>
<p><!-- Page 74 --><span class="pagenum"><a name="page74"></a>{74}</span></p>
<div class="figcenter" style="width:26%;">
<a href="images/fig_035.jpg"><img style="width:100%" src="images/fig_035.jpg"
alt="Fig. 35." title="Fig. 35." /></a>
<p class="poem"><span class="sc">Fig. 35.</span> Diagram showing a
cross between a white eyed male and a red eyed female of the fruit fly.
Sex linked inheritance.</p>
</div>
<p>The reciprocal cross (fig. 36) is also interesting. If a white eyed
female is bred to a red eyed male, all of the daughters have red eyes and
all of the sons have white eyes. We call this criss-cross inheritance. If
these offspring are inbred, they produce equal numbers of red eyed and
white eyed females and equal numbers of red eyed and white eyed males.
The ratio is 1: 1: 1: 1, or ignoring sex, 2 reds to 2 whites, and not the
usual 3:1 Mendelian ratio. Yet, as will be shown later, the result is in
entire accord with Mendel's principle of segregation.</p>
<p><!-- Page 75 --><span class="pagenum"><a name="page75"></a>{75}</span></p>
<div class="figcenter" style="width:27%;">
<a href="images/fig_036.jpg"><img style="width:100%" src="images/fig_036.jpg"
alt="Fig. 36." title="Fig. 36." /></a>
<p class="poem"><span class="sc">Fig. 36.</span> Diagram illustrating a
cross between a red eyed male and white eyed female of the fruit fly
(reciprocal cross of that shown in Fig. 35).</p>
</div>
<p>It has been shown by Sturtevant that in a wild species of Drosophila,
viz., D. repleta, two varieties of individuals exist, in one of which the
thorax has large splotches and in the <!-- Page 76 --><span
class="pagenum"><a name="page76"></a>{76}</span>other type smaller
splotches (fig. 37). The factors that differentiate these varieties are
sex linked.</p>
<div class="figcenter" style="width:31%;">
<a href="images/fig_037.jpg"><img style="width:100%" src="images/fig_037.jpg"
alt="Fig. 37." title="Fig. 37." /></a>
<p class="poem"><span class="sc">Fig. 37.</span> Two types of markings
on thorax of Drosophila repleta, both found "wild". They show sex
linked inheritance.</p>
</div>
<p>Certain types of color blindness (fig. 38) and certain other abnormal
conditions in man such as haemophilia, are transmitted as sex linked
characters.</p>
<p><!-- Page 77 --><span class="pagenum"><a name="page77"></a>{77}</span></p>
<table class="nobctr"><tr><td style="width:50%; vertical-align:top;">
<div class="figright" style="width:52%;">
<a href="images/fig_038a.jpg"><img style="width:100%" src="images/fig_038a.jpg"
alt="Fig. 38A." title="Fig. 38A." /></a>
<p class="poem"><span class="sc">Fig. 38, A.</span> Diagram
illustrating inheritance of color blindness in man; the iris of the
color-blind eye is here black.</p>
</div>
</td><td style="width:50%; vertical-align:top;">
<div class="figleft" style="width:52%;">
<a href="images/fig_038b.jpg"><img style="width:100%" src="images/fig_038b.jpg"
alt="Fig. 38B." title="Fig. 38B." /></a>
<p class="poem"><span class="sc">Fig. 38, B.</span> Reciprocal of cross
in Fig. 38 a.</p>
</div>
</td></tr></table>
<p>In domestic fowls sex linked inheritance has been found as the
characteristic method of transmission for at least as many as six
characters, but here the relation of the sexes is in a sense reversed.
For instance, if a black Langshan hen is crossed to a barred Plymouth
Rock cock (fig. 39), the offspring are all barred. If these are inbred
half of the daughters are black and half are barred; all of the sons are
barred. The grandmother has transmitted her color to half of her
granddaughters but to none of her grandsons.</p>
<p><!-- Page 78 --><span class="pagenum"><a name="page78"></a>{78}</span></p>
<table class="nobctr"><tr><td style="width:50%; vertical-align:top;">
<div class="figright" style="width:61%;">
<a href="images/fig_039.jpg"><img style="width:100%" src="images/fig_039.jpg"
alt="Fig. 39." title="Fig. 39." /></a>
<p class="poem"><span class="sc">Fig. 39.</span> Sex-linked inheritance
in domesticated birds shown here in a cross between barred Plymouth
Rock male and black Langshan female.</p>
</div>
</td><td style="width:50%; vertical-align:top;">
<div class="figleft" style="width:61%;">
<a href="images/fig_040.jpg"><img style="width:100%" src="images/fig_040.jpg"
alt="Fig. 40." title="Fig. 40." /></a>
<span class="sc">Fig. 40.</span> Reciprocal of Fig. 39.
</div>
</td></tr></table>
<p>In the reciprocal cross (fig. 40) black cock by barred hen, the
daughters are black and the sons barred—criss-cross inheritance.
These inbred give black hens and black cocks, barred hens and barred
cocks.</p>
<p><!-- Page 79 --><span class="pagenum"><a name="page79"></a>{79}</span></p>
<p>There is a case comparable to this found in a wild species of moth,
Abraxas grossulariata. A wild variation of this type is lighter in color
and is known as A. lacticolor. When these two types are crossed they
exhibit exactly the same type of heredity as does the black-barred
combination in the domestic fowl. As shown in figure 41, lacticolor
female bred to grossulariata male gives grossulariata sons and daughters.
These inbred give grossulariata males and females and lacticolor females.
Reciprocally lacticolor male by grossulariata female, <!-- Page 80
--><span class="pagenum"><a name="page80"></a>{80}</span>(fig. 42) gives
lacticolor daughters and grossulariata sons and these inbred give
grossulariata males and females and lacticolor males and females.</p>
<table class="nobctr"><tr><td style="width:50%; vertical-align:top;">
<div class="figright" style="width:68%;">
<a href="images/fig_041.jpg"><img style="width:100%" src="images/fig_041.jpg"
alt="Fig. 41." title="Fig. 41." /></a>
<p class="poem"><span class="sc">Fig. 41.</span> Sex-linked inheritance
in the wild moth, Abraxas grossulariata (darker) and A. lacticolor.</p>
</div>
</td><td style="width:50%; vertical-align:top;">
<!-- Page 81 --><span class="pagenum"><a name="page81"></a>{81}</span>
<div class="figleft" style="width:68%;">
<a href="images/fig_042.jpg"><img style="width:100%" src="images/fig_042.jpg"
alt="Fig. 42." title="Fig. 42." /></a>
<span class="sc">Fig. 42.</span> Reciprocal of Fig. 41.
</div>
</td></tr></table>
<p><!-- Page 82 --><span class="pagenum"><a name="page82"></a>{82}</span></p>
<div class="figcenter" style="width:31%;">
<a href="images/fig_043.jpg"><img style="width:100%" src="images/fig_043.jpg"
alt="Fig. 43." title="Fig. 43." /></a>
<p class="poem"><span class="sc">Fig. 43.</span> Four wild types of
Paratettix in upper line with three hybrids below.</p>
</div>
<p>It has been found that there may be even more than two factors that
show Mendelian segregation when brought together in pairs. For example,
in the southern States there are several races of the grouse locust
(Paratettix) that differ from each other markedly in color patterns (fig.
43). When any two individuals of these races are crossed they give, as
Nabours has shown, in F<sub>2</sub> a Mendelian ratio of 1: 2: 1. It is
obvious, therefore, that there are here at least nine characters, any two
of which behave as a Mendelian pair. These races have <!-- Page 83
--><span class="pagenum"><a name="page83"></a>{83}</span>arisen in nature
and differ definitely and strikingly from each other, yet any two differ
by only one factor difference.</p>
<div class="figcenter" style="width:31%;">
<a href="images/fig_044.jpg"><img style="width:100%" src="images/fig_044.jpg"
alt="Fig. 44." title="Fig. 44." /></a>
<p class="poem"><span class="sc">Fig. 44.</span> Diagram illustrating
four allelomorphs in mice, viz. gray bellied gray (wild type) (above,
to left); white bellied gray (above, to right); yellow (below, to
right); and black (below, to left).</p>
</div>
<p>Similar relations have been found in a number of domesticated races.
In mice there is a quadruple system represented by the gray house mouse,
the white bellied, the yellow and the black mouse (fig. 44). In rabbits
there is probably a triple system, that includes the albino, the
Himalayan, and the black races. In <!-- Page 84 --><span
class="pagenum"><a name="page84"></a>{84}</span>the silkworm moth there
have been described four types of larvae, distinguished by different
color markings, that form a system of quadruple allelomorphs. In
Drosophila there is a quintuple system of factors in the sex chromosome
represented by eye colors, a triple system of body colors, and a triple
system of factors for eye colors in the third chromosome.</p>
<p class="cenhead"><span class="sc">Mutation and Evolution</span></p>
<p>What bearing has the appearance of these new types of Drosophila on
the theory of evolution may be asked. The objection has been raised in
fact that in the breeding work with Drosophila we are dealing with
artificial and unnatural conditions. It has been more than implied that
results obtained from the breeding pen, the seed pan, the flower pot and
the milk bottle do not apply to evolution in the "open", nature "at
large" or to "wild" types. To be consistent, this same objection should
be extended to the use of the spectroscope in the study of the evolution
of the stars, to the use of the test tube and the balance by the chemist,
of the galvanometer by the physicist. All these <!-- Page 85 --><span
class="pagenum"><a name="page85"></a>{85}</span>are unnatural instruments
used to torture Nature's secrets from her. I venture to think that the
real antithesis is not between unnatural and natural treatment of Nature,
but rather between controlled or verifiable data on the one hand, and
unrestrained generalization on the other.</p>
<p>If a systematist were asked whether these new races of Drosophila are
comparable to wild species, he would not hesitate for a moment. He would
call them all one species. If he were asked why, he would say, I think,
"These races differ only in one or two striking points, while in a
hundred other respects they are identical even to the minutest details."
He would add, that as large a group of wild species of flies would show
on the whole the reverse relations, <i>viz.</i>, they would differ in
nearly every detail and be identical in only a few points. In all this I
entirely agree with the systematist, for I do not think such a group of
types differing by one character each, is comparable to most wild groups
of species because the difference between wild species is due to a large
number of such single differences. The characters <!-- Page 86 --><span
class="pagenum"><a name="page86"></a>{86}</span>that have been
accumulated in wild species are of significance in the maintenance of the
species, or at least we are led to infer that even though the visible
character that we attend to may not itself be important, one at least of
the other effects of the factors that represent these characters is
significant. It is, of course, hardly to be expected that <i>any</i>
random change in as complex a mechanism as an insect would improve the
mechanism, and as a matter of fact it is doubtful whether any of the
mutant types so far discovered are better adapted to those conditions to
which a fly of this structure and habits is already adjusted. But this is
beside the mark, for modern genetics shows very positively that adaptive
characters are inherited in exactly the same way as are those that are
not adaptive; and I have already pointed out that we cannot study a
single mutant factor without at the same time studying one of the factors
responsible for normal characters, for the two together constitute the
Mendelian pair.</p>
<p>And, finally, I want to urge on your attention a question that we are
to consider in more detail in the last lecture. Evolution of wild <!--
Page 87 --><span class="pagenum"><a name="page87"></a>{87}</span>species
appears to have taken place by modifying and improving bit by bit the
structures and habits that the animal or plant already possessed. We have
seen that there are thirty mutant factors at least that have an influence
on eye color, and it is probable that there are at least as many normal
factors that are involved in the production of the red eye of the wild
fly.</p>
<p>Evolution from this point of view has consisted largely in introducing
new factors that influence characters already present in the animal or
plant.</p>
<p>Such a view gives us a somewhat different picture of the process of
evolution from the old idea of a ferocious struggle between the
individuals of a species with the survival of the fittest and the
annihilation of the less fit. Evolution assumes a more peaceful aspect.
New and advantageous characters survive by incorporating themselves into
the race, improving it and opening to it new opportunities. In other
words, the emphasis may be placed less on the competition between the
individuals of a species (because the destruction of the less fit does
<!-- Page 88 --><span class="pagenum"><a name="page88"></a>{88}</span>not
<i>in itself</i> lead to anything that is new) than on the appearance of
new characters and modifications of old characters that become
incorporated in the species, for on these depends the evolution of the
race.</p>
<p><br style="clear:both" /></p>
<hr class="full" />
<p><!-- Page 89 --><span class="pagenum"><a name="page89"></a>{89}</span></p>
<h3>CHAPTER III</h3>
<p class="cenhead">THE FACTORIAL THEORY OF HEREDITY AND
THE COMPOSITION OF THE GERM PLASM</p>
<p>The discovery that Mendel made with edible peas concerning heredity
has been found to apply everywhere throughout the plant and animal
kingdoms—to flowering plants, to insects, snails, crustacea,
fishes, amphibians, birds, and mammals (including man).</p>
<p>There must be something that these widely separated groups of plants
and animals have in common—some simple mechanism perhaps—to
give such definite and orderly series of results. There is, in fact, a
mechanism, possessed alike by animals and plants, that fulfills every
requirement of Mendel's principles.</p>
<p class="cenhead"><span class="sc">The Cellular Basis of Organic Evolution and Heredity</span></p>
<p>In order to appreciate the full force of the evidence, let me first
pass rapidly in review a <!-- Page 90 --><span class="pagenum"><a
name="page90"></a>{90}</span>few familiar, historical facts, that
preceded the discovery of the mechanism in question.</p>
<div class="figcenter" style="width:38%;">
<a href="images/fig_045.jpg"><img style="width:100%" src="images/fig_045.jpg"
alt="Fig. 45." title="Fig. 45." /></a>
<p class="poem"><span class="sc">Fig. 45.</span> Typical cell showing
the cell wall, the protoplasm (with its contained materials); the
nucleus with its contained chromatin and nuclear sap. (After
Dahlgren.)</p>
</div>
<p>Throughout the greater part of the last century, while students of
evolution and of heredity were engaged in what I may call the more
general, or, shall I say, the <i>grosser</i> aspects of the subject,
there existed another group of students who were engaged in working out
the minute structure of the material basis of the living organism. They
found that organs such as the brain, the heart, the liver, the lungs, the
kidneys, etc., are not themselves the units of structure, but that all
these organs can be reduced to a simpler unit that repeats itself a <!--
Page 91 --><span class="pagenum"><a
name="page91"></a>{91}</span>thousand-fold in every organ. We call this
unit a cell (fig. 45).</p>
<p>The egg is a cell, and the spermatozoon is a cell. The act of
fertilization is the union of two cells (fig. 47, upper figure). Simple
as the process of fertilization appears to us today, its discovery swept
aside a vast amount of mystical speculation concerning the rôle of the
male and of the female in the act of procreation.</p>
<p>Within the cell a new microcosm was revealed. Every cell was found to
contain a spherical body called the nucleus (fig. 46a). Within the
nucleus is a network of fibres, a sap fills the interstices of the
network. The network resolves itself into a definite number of threads at
each division of the cell (fig. 46 b-e). These threads we call
chromosomes. Each species of animals and plants possesses a
characteristic number of these threads which have a definite size and
sometimes a specific shape and even characteristic granules at different
levels. Beyond this point our strongest microscopes fail to penetrate.
Observation has reached, for the time being, its limit.</p>
<p><!-- Page 92 --><span class="pagenum"><a name="page92"></a>{92}</span></p>
<div class="figcenter" style="width:26%;">
<a href="images/fig_046.jpg"><img style="width:100%" src="images/fig_046.jpg"
alt="Fig. 46." title="Fig. 46." /></a>
<p class="poem"><span class="sc">Fig. 46.</span> A series of cells in
process of cell division. The chromosomes are the black threads and
rods. (After Dahlgren.)</p>
</div>
<p>The story is taken up at this point by a new set of students who have
worked in an entirely different field. Certain observations and
experiments that we have not time to consider <!-- Page 93 --><span
class="pagenum"><a name="page93"></a>{93}</span>now, led a number of
biologists to conclude that the chromosomes are the bearers of the
hereditary units. If so, there should be many such units carried by
<i>each</i> chromosome, for the number of chromosomes is limited while
the number of independently inherited characters is large. In Drosophila
it has been demonstrated not only that there are exactly as many groups
of characters that are inherited together as there are pairs of
chromosomes, but even that it is possible to locate one of these groups
in a particular chromosome and to state the <i>relative position</i>
there of the factors for the characters. If the validity of this evidence
is accepted, the study of the cell leads us finally in a mechanical, but
not in a chemical sense, to the ultimate units about which the whole
process of the transmission of the hereditary factors centers.</p>
<p>But before plunging into this somewhat technical matter (that is
difficult only because it is unfamiliar), certain facts which are
familiar for the most part should be recalled, because on these turns the
whole of the subsequent story.</p>
<p><!-- Page 94 --><span class="pagenum"><a name="page94"></a>{94}</span></p>
<div class="figcenter" style="width:26%;">
<a href="images/fig_047.jpg"><img style="width:100%" src="images/fig_047.jpg"
alt="Fig. 47." title="Fig. 47." /></a>
<p class="poem"><span class="sc">Fig. 47.</span> An egg, and the
division of the egg—the so-called process of cleavage. (After
Selenka.)</p>
</div>
<p>The thousands of cells that make up the cell-state that we call an
animal or plant come from the fertilized egg. An hour or two after
fertilization the egg divides into two cells (fig. 47). Then each half
divides again. Each <!-- Page 95 --><span class="pagenum"><a
name="page95"></a>{95}</span>quarter next divides. The process continues
until a large number of cells is formed and out of these organs mould
themselves.</p>
<div class="figcenter" style="width:27%;">
<a href="images/fig_048.jpg"><img style="width:100%" src="images/fig_048.jpg"
alt="Fig. 48." title="Fig. 48." /></a>
<p class="poem"><span class="sc">Fig. 48.</span> Section of the egg of
the beetle, Calligrapha, showing the pigment at one end where the germ
cells will later develop as shown in the other two figures. (After
Hegner.)</p>
</div>
<p>At every division of the cell the chromosomes also divide. Half of
these have come from the mother, half from the father. Every cell
contains, therefore, the sum total of all the chromosomes, and if these
are the bearers of the hereditary qualities, every cell in the body, <!--
Page 96 --><span class="pagenum"><a name="page96"></a>{96}</span>whatever
its function, has a common inheritance.</p>
<p>At an early stage in the development of the animal certain cells are
set apart to form the organs of reproduction. In some animals these cells
can be identified early in the cleavage (fig. 48).</p>
<p>The reproductive cells are at first like all the other cells in the
body in that they contain a full complement of chromosomes, half paternal
and half maternal in origin (fig. 49). They divide as do the other cells
of the body for a long time (fig. 49, upper row). At each division each
chromosome splits lengthwise and its halves migrate to opposite poles of
the spindle (fig. 49 c).</p>
<p>But there comes a time when a new process appears in the germ cells
(fig 49 e-h). It is essentially the same in the egg and in the sperm
cells. The discovery of this process we owe to the laborious researches
of many workers in many countries. The list of their names is long, and I
shall not even attempt to repeat it. The chromosomes come together in
pairs (fig. 49 a). Each maternal chromosome mates with a paternal
chromosome of the same kind. <!-- Page 97 --><span class="pagenum"><a
name="page97"></a>{97}</span></p>
<div class="figcenter" style="width:34%;">
<a href="images/fig_049.jpg"><img style="width:100%" src="images/fig_049.jpg"
alt="Fig. 49." title="Fig. 49." /></a>
<p class="poem"><span class="sc">Fig. 49.</span> In the upper row of
the diagram a typical process of nuclear division, such as takes place
in the early germ cells or in the body cells. In the lower row the
separation of the chromosomes that have paired. This sort of separation
takes place at one of the two reduction divisions.</p>
</div>
<p>Then follow two rapid divisions (fig. 49 f, g and 50 and 51). At one
of the divisions the double chromosomes separate so that each resulting
cell comes to contain some maternal and <!-- Page 98 --><span
class="pagenum"><a name="page98"></a>{98}</span>some paternal
chromosomes, i.e. one or the other member of each pair. At the other
division each chromosome simply splits as in ordinary cell division.</p>
<div class="figcenter" style="width:33%;">
<a href="images/fig_050.jpg"><img style="width:100%" src="images/fig_050.jpg"
alt="Fig. 50." title="Fig. 50." /></a>
<p class="poem"><span class="sc">Fig. 50.</span> The two maturation
divisions of the sperm cell. Four sperms result, each with half
(haploid) the full number (diploid) of chromosomes.</p>
</div>
<p>The upshot of the process is that the ripe eggs (fig. 51) and the ripe
spermatozoa (fig. <!-- Page 99 --><span class="pagenum"><a
name="page99"></a>{99}</span>50) come to contain only half the total
number of chromosomes.</p>
<div class="figcenter" style="width:33%;">
<a href="images/fig_051.jpg"><img style="width:100%" src="images/fig_051.jpg"
alt="Fig. 51." title="Fig. 51." /></a>
<p class="poem"><span class="sc">Fig. 51.</span> The two maturation
divisions of the egg. The divisions are unequal, so that two small
polar bodies are formed one of these subsequently divides. The three
polar bodies and the egg are comparable to the four sperms.</p>
</div>
<p>When the eggs are fertilized the whole number of chromosomes is
restored again.</p>
<p class="cenhead"><span class="sc">The Mechanism of Mendelian Heredity Discovered in the Behavior of the Chromosomes</span></p>
<p>If the factors in heredity are carried in the chromosomes and if the
chromosomes are definite structures, we should anticipate that there
should be as many <i>groups</i> of characters as there are kinds of
chromosomes. In only one <!-- Page 100 --><span class="pagenum"><a
name="page100"></a>{100}</span>case has a sufficient number of characters
been studied to show whether there is any correspondence between the
number of hereditary groups of characters and the number of chromosomes.
In the fruit fly, Drosophila ampelophila, we have found about 125
characters that are inherited in a perfectly definite way. On the
opposite page is a list of some of them.</p>
<p>It will be observed in this list that the characters are arranged in
four groups, Groups I, II, III and IV. Three of these groups are equally
large or nearly so; Group IV contains only two characters. The characters
are put into these groups because in heredity the members of each group
tend to be inherited together, i.e., if two or more enter the cross
together they tend to remain together through subsequent generations. On
the other hand, any member of one group is inherited entirely
independently of any member of the other groups; in the same way as
Mendel's yellow-green pair of characters is inherited independently of
the round-wrinkled pair.</p>
<p><!-- Page 101 --><span class="pagenum"><a name="page101"></a>{101}</span></p>
<table class="nobctr" summary="Groups I-IV." title="Groups I-IV.">
<tr><td class="spacsingle" style="width:25%; vertical-align:top;"><i>Group I</i><br />
Abnormal<br />
Bar<br />
Bifid<br />
Bow<br />
Cherry<br />
Chrome<br />
Cleft<br />
Club<br />
Depressed<br />
Dot<br />
Eosin<br />
Facet<br />
Forked<br />
Furrowed<br />
Fused<br />
Green<br />
Jaunty<br />
Lemon<br />
Lethals, 13<br />
Miniature<br />
Notch<br />
Reduplicated<br />
Ruby<br />
Rudimentary<br />
Sable<br />
Shifted<br />
Short<br />
Skee<br />
Spoon<br />
Spot<br />
Tan<br />
Truncate intensifier<br />
Vermilion<br />
White<br />
Yellow
</td><td class="spacsingle" style="width:25%; vertical-align:top;"><i>Group II</i><br />
Antlered<br />
Apterous<br />
Arc<br />
Balloon<br />
Black<br />
Blistered<br />
Comma<br />
Confluent<br />
Cream II<br />
Curved<br />
Dachs<br />
Extra vein<br />
Fringed<br />
Jaunty<br />
Limited<br />
Little crossover<br />
Morula<br />
Olive<br />
Plexus<br />
Purple<br />
Speck<br />
Strap<br />
Streak<br />
Trefoil<br />
Truncate<br />
Vestigial
</td><td class="spacsingle" style="width:25%; vertical-align:top;"><i>Group III</i><br />
Band<br />
Beaded<br />
Cream III<br />
Deformed<br />
Dwarf<br />
Ebony<br />
Giant<br />
Kidney<br />
Low crossing over<br />
Maroon<br />
Peach<br />
Pink<br />
Rough<br />
Safranin<br />
Sepia<br />
Sooty<br />
Spineless<br />
Spread<br />
Trident<br />
Truncate intensifier<br />
Whitehead<br />
White ocelli
</td><td class="spacsingle" style="width:25%; vertical-align:top;"><i>Group IV</i><br />
Bent<br />
Eyeless
</td></tr></table>
<p><!-- Page 102 --><span class="pagenum"><a name="page102"></a>{102}</span></p>
<p>If the factors for these characters are carried by the chromosomes,
then we should expect that those factors that are carried by the same
chromosome would be inherited together, provided the chromosomes are
definite structures in the cell.</p>
<div class="figcenter" style="width:27%;">
<a href="images/fig_052.jpg"><img style="width:100%" src="images/fig_052.jpg"
alt="Fig. 52." title="Fig. 52." /></a>
<p class="poem"><span class="sc">Fig. 52.</span> Chromosomes (diploid)
of D. ampelophila. The sex chromosomes are XX in the female and XY in
the male. There are three other pairs of chromosomes.</p>
</div>
<p>In the chromosome group of Drosophila, (fig. 52) there are <i>four</i>
pairs of chromosomes, three of nearly the same size and one much smaller.
Not only is there agreement between the number of hereditary groups and
the number of the chromosomes, but even the size relations are the same,
for there are three great groups of characters and three pairs of large
chromosomes, and one small group of characters and one pair of small
chromosomes. <!-- Page 103 --><span class="pagenum"><a
name="page103"></a>{103}</span></p>
<p class="cenhead"><span class="sc">The Four Great Linkage Groups of Drosophila Ampelophila</span></p>
<p>The following description of the characters of the wild fly may be
useful in connection with the account of the modifications of these
characters that appear in the mutants.</p>
<p>The head and thorax of the wild fly are grayish-yellow, the abdomen is
banded with alternate stripes of yellow and black. In the male, (fig. 4
to right), there are three narrow bands and a black tip. In the female
there are five black bands (fig. 4 to left). The wings are gray with a
surface texture of such a kind that at certain angles they are
iridescent. The eyes are a deep, solid, brick-red. The minute hairs that
cover the body have a very definite arrangement that is most obvious on
the head and thorax. There is a definite number of larger hairs called
bristles or chaetae which have a characteristic position and are used for
diagnostic purposes in classifying the species. On the foreleg of the
male there is a comb-like organ formed by a row of bristles; it is absent
in the female. The comb is a secondary sexual character, and it is, so
far as known, functionless. <!-- Page 104 --><span class="pagenum"><a
name="page104"></a>{104}</span></p>
<p>Some of the characters of the mutant types are shown in figures <span
class="correction" title="Original reads `52, 53, 54, 55'.">53, 54, 55,
56</span>. The drawing of a single fly is often used here to illustrate
more than one character. This is done to economize space, but of course
there would be no difficulty in actually bringing together in the same
individual any two or more characters belonging to the same group (or to
different groups). Without colored figures it is not possible to show
many of the most striking differences of these mutant races; at most dark
and light coloring can be indicated by the shading of the body, wings, or
eyes.</p>
<p class="cenhead"><i>Group I</i></p>
<p>In the six flies drawn in figure 53 there are shown five different
wing characters. The first of these types (a) is called cut, because the
ends of the wings look as though they had been cut to a point. The
antennae are displaced downward and appressed and their bristle-like
aristae are crumpled.</p>
<div class="figcenter" style="width:33%;">
<a href="images/fig_053.jpg"><img style="width:100%" src="images/fig_053.jpg"
alt="Fig. 53." title="Fig. 53." /></a>
<span class="sc">Fig. 53.</span> Group I. (See text)
</div>
<p>The second figure (b) represents a fly with a notch in the ends of the
wings. This character is dominant, but the same factor that <!-- Page 105
--><span class="pagenum"><a name="page105"></a>{105}</span>produces the
notch in the wings is also a recessive lethal factor; because of this
latter effect of the character no males of this race exist, and the
females of the race are never pure but hybrid. Every female with notch
wings bred to a wild male, will produce in equal numbers notch winged
daughters and daughters with normal wings. There will be half as many
sons as daughters. The explanation of <!-- Page 106 --><span
class="pagenum"><a name="page106"></a>{106}</span>this peculiar result is
quite simple. Every notch winged female has one X chromosome that carries
the factor for notch and one X chromosome that is "normal". Daughters
receiving the former chromosomes are notched because the factor for notch
is dominant, but they are not killed since the lethal effect of the notch
factor is recessive to the normal allelomorph carried by the other
chromosome that the daughters get from their father. This normal factor
is recessive for notch but dominant for life. This same figure (b) is
used here to show three other sex linked characters. The spines on the
thorax are twisted or kinky, which is due to a factor called "forked".
The effect is best seen on the thorax, but all spines on the body are
similarly modified; even the minute hairs are also affected. Ruby eye
color might be here represented—if the eyes in the figure were
colored. The lighter color of the body and antennae is intended to
indicate that the character tan is also present. The light color of the
antennae is the most certain way of identifying tan. The tan flies are
interesting because they have lost the positive heliotropism <!-- Page
107 --><span class="pagenum"><a name="page107"></a>{107}</span>that is so
marked a feature in the behavior of D. ampelophila. As this peculiarity
of the tan flies is inherited like all the other sex linked characters,
it follows that when a tan female is bred to a wild male all the sons
inherit the recessive tan color and indifference to light, while the
daughters show the dominant sex linked character of their father, i.e.,
they are "gray", and go to the light. Hence when such a brood is
disturbed the females fly to the light, but the males remain behind.</p>
<p>One of the first mutants that appeared in D. ampelophila was called
rudimentary on account of the condition of the wings (c). The same
mutation has appeared independently several times. In the drawing (c) the
dark body color is intended to indicate "sable" and the lighter color of
the eyes is intended to indicate eosin. This eye color, which is an
allelomorph of white, is also interesting because in the female the color
is deeper than in the male. In other cases of sex linked factors the
character is the same in the two sexes.</p>
<p>In the fourth figure (d) the third and fourth longitudinal veins of
the wing are <i>fused</i> into <!-- Page 108 --><span class="pagenum"><a
name="page108"></a>{108}</span>one vein from the base of the wing to the
level of the first cross-vein and in addition converge and meet near
their outer ends. The shape of the eye is represented in the figure as
different from the normal, due to another factor called "bar". This is a
dominant character, the hybrid condition being also narrow, but not so
narrow as the pure type. Vermilion eye color might also be here
represented—due to a factor that has appeared independently on
several occasions.</p>
<p>In the fifth figure (e) the wings are shorter and more pointed than in
the wild fly. This character is called miniature. The light color of the
drawing may be taken to represent yellow body color, and the light color
of the eye white eye color.</p>
<p>In the last figure (f) the wings are represented as pads, essentially
in the same condition that they are in when the fly emerges from the pupa
case. Not all the flies of this stock have the wings in this condition;
some have fully expanded wings that appear normal in all respects.
Nevertheless, about the same percentage of offspring show the pads
irrespective of <!-- Page 109 --><span class="pagenum"><a
name="page109"></a>{109}</span>whether the parents had pads or expanded
wings.</p>
<p>The flies of this stock show, however, another character, which is a
product of the same factor, and which is constant, i.e., repeated in all
individuals. The two bristles on the sides of the thorax are constantly
absent in this race. The lighter color of the eye in the figure may be
taken to indicate buff—a faint yellowish color. The factor for this
eye color is another allelomorph of white.</p>
<p>There are many other interesting characters that belong to the first
group, such as abnormal abdomen, short legs, duplication of the legs,
etc. In fact, any part of the body may be affected by a sex-linked
factor.</p>
<p class="cenhead"><i>Group II</i></p>
<p>In the first figure (a) of figure 54 that contains members of Group II
the wings are almost entirely absent or "vestigial". This condition arose
at a single step and breeds true, although it appears to be influenced to
some extent by temperature, also by modifiers that sometimes appear in
the stock. Purple <!-- Page 110 --><span class="pagenum"><a
name="page110"></a>{110}</span>eye color belongs in Group II; it
resembles the color of the eye of the wild fly but is darker and more
translucent.</p>
<div class="figcenter" style="width:34%;">
<a href="images/fig_054.jpg"><img style="width:100%" src="images/fig_054.jpg"
alt="Fig. 54." title="Fig. 54." /></a>
<span class="sc">Fig. 54.</span> Group II. (See text.)
</div>
<p>In the second figure (b) the wing is again long and narrow and
sometimes bent back on itself, as shown here. In several respects the
wing resembles strap (d) but seems to be due <!-- Page 111 --><span
class="pagenum"><a name="page111"></a>{111}</span>to another factor,
called antler, insufficiently studied as yet.</p>
<p>In the third figure (c) the wings turn up at the end. This is brought
about by the presence of the factor called jaunty.</p>
<p>In the fourth figure the wings are long and narrow and several of the
veins are unrepresented. This character, "strap", is very variable and
has not yet been thoroughly studied. On the thorax there is a deep black
mark called trefoil. Even in the wild fly there is a three pronged mark
on the thorax present in many individuals. Trefoil is a further
development and modification of this mark and is due to a special
factor.</p>
<p>In the fifth figure (e) the wings are arched. The factor is called
arc. The dark color of the body, and especially of the wings, indicates
the factor for black.</p>
<p>The sixth figure (f) shows the wings "curved" downwards. In addition
there is present a minute black speck at the base of each wing, due to
another factor called speck.</p>
<p>In the seventh figure (g) the wing is truncate. Its end is obliquely
squared instead of <!-- Page 112 --><span class="pagenum"><a
name="page112"></a>{112}</span>rounded; it may be longer than the body,
or shorter when other modifying factors are present. The mutation that
produces this type of wing is of not infrequent occurrence. It has been
shown by Muller and Altenburg that there are at least two factors that
modify this character—the chief factor is present in the second
chromosome; alone it produces the truncate wing in only a certain
percentage of cases, but when the modifiers are also present about ninety
percent of the individuals may show the truncate condition of the wing.
But the presence of these factors makes the stock very infertile, so that
it is difficult to maintain.</p>
<p>In the eighth figure (h) the legs are shortened owing to the absence
of a segment of the tarsus. The stock is called dachs—a nickname
given to it because the short legs suggested the dachshund.</p>
<p class="cenhead"><i>Group III</i></p>
<p>In figure 55, (a), a mutant type called bithorax is shown. The old
metathorax is replaced by another mesothorax thrust in between the normal
mesothorax and the abdomen. It <!-- Page 113 --><span class="pagenum"><a
name="page113"></a>{113}</span>carries a pair of wings that do not
completely unfold. On this new mesothorax the characteristic arrangement
of the bristles is shown. Thus at a single step a typical region of the
body has doubled. The character is recessive.</p>
<div class="figcenter" style="width:33%;">
<a href="images/fig_055.jpg"><img style="width:100%" src="images/fig_055.jpg"
alt="Fig. 55." title="Fig. 55." /></a>
<span class="sc">Fig. 55.</span> Group III. (See text.)
</div>
<p>The size of the adult fly of D. ampelophila varies greatly according
to the amount of nourishment obtained by the larva. After the fly emerges
its size remains nearly constant, as in many insects. Two races have,
<!-- Page 114 --><span class="pagenum"><a
name="page114"></a>{114}</span>however, been separated by Bridges that
are different in size as a result of a genetic factor. The first of
these, called dwarf, is represented by figure 55, (b).</p>
<p>The race is minute, although of course its size is variable, depending
on food and other conditions. The same figure shows the presence of
another factor, "sooty", that makes the fly very dark. Maroon eye color
might be here represented, due to still another factor.</p>
<p>In the third figure (c) the other mutation in size is shown. It is
called "giant". The flies are twice the size of wild flies. An eye color,
called peach, might here be represented. It is an allelomorph of
pink.</p>
<p>In the fourth figure (d) the mutant called dichaete is shown. It is
characterized by the absence of two of the bristles on the thorax. Other
bristles may also be absent, but not so constantly as the two just
mentioned. Another effect of the same factor is the spread-out condition
of the wings. The very dark eye color in this figure may be taken to
indicate the presence of another factor, "sepia", which causes the eyes
to assume a brown color that <!-- Page 115 --><span class="pagenum"><a
name="page115"></a>{115}</span>becomes black with age. Most of the other
mutations in eye color that have occurred tend to give a lighter color:
this one, which is also recessive, makes the eye darker.</p>
<p>In the fifth figure (e) the color of the darkest fly is due to a
factor called ebony, which is an allelomorph of sooty.</p>
<p>In the sixth figure (f) the wings are beaded, i.e., the margin is
defective at intervals, giving a beaded-like outline to the wings. This
condition is very variable and much affected by other factors that
influence the shape of the wings. The lighter eye color of the drawing
may be taken to represent pink.</p>
<p>In the seventh figure (g) the wings are curled up over the back. This
is a recessive character.</p>
<p class="cenhead"><i>Group IV</i></p>
<p>Only two mutants have been obtained that do not belong to any of the
preceding groups; these are put together in Group IV. It has been shown
that they are linked to each other and the linkage is so close that it
has thus far been impossible to obtain the dominant recessive. <!-- Page
116 --><span class="pagenum"><a name="page116"></a>{116}</span>One of
these mutants, called "eyeless" (fig. 56, a, a<sup>1</sup>), is
variable—the eyes are often entirely absent or represented by one
or more groups of ommatidia. The outline of the original eye, so to
speak, is strongly marked out and its area might be called a rudimentary
organ, if such a statement has any meaning here.</p>
<div class="figcenter" style="width:21%;">
<a href="images/fig_056.jpg"><img style="width:100%" src="images/fig_056.jpg"
alt="Fig. 56." title="Fig. 56." /></a>
<span class="sc">Fig. 56.</span> Group IV. (See text.)
</div>
<p>The other figure (b) represents "bent", so called from the shape of
the wings. This <!-- Page 117 --><span class="pagenum"><a
name="page117"></a>{117}</span>mutant is likewise very variable, often
indistinguishable from the wild type, yet when well developed strikingly
different from any other mutant.</p>
<p>This brief account of a few of the mutant races that can be most
easily represented by uncolored figures will serve to show how all parts
of the body may change, some of the changes being so slight that they
would be overlooked except by an expert, others so great that in the
character affected the flies depart far from the original species.</p>
<p><i>It is important to note that mutations in the first chromosome are
not limited to any part of the body nor do they affect more frequently a
particular part. The same statement holds equally for all of the other
chromosomes. In fact, since each factor may affect visibly several parts
of the body at the same time there are no grounds for expecting any
special relation between a given chromosome and special regions of the
body. It can not too insistently be urged that when we say a character is
the product of a particular factor we mean no more than that it is the
most conspicuous effect of the factor.</i> <!-- Page 118 --><span
class="pagenum"><a name="page118"></a>{118}</span></p>
<p>If, then, as these and other results to be described point to the
chromosomes as the bearers of the Mendelian factors, and if, as will be
shown presently, these factors have a definite location in the
chromosomes it is clear that the location of the factors in the
chromosomes bears no spatial relation to the location of the parts of the
body to each other.</p>
<p class="cenhead"><span class="sc">Localization of Factors in the Chromosomes</span></p>
<p class="cenhead"><i>The Evidence from Sex Linked Inheritance</i></p>
<p>When we follow the history of pairs of chromosomes we find that their
distribution in successive generations is paralleled by the inheritance
of Mendelian characters. This is best shown in the sex chromosomes (fig.
57). In the female there are two of these chromosomes that we call the X
chromosomes; in the male there are also two but one differs from those of
the female in its shape, and in the fact that it carries none of the
normal allelomorphs of the mutant factors. It is called the Y
chromosome.</p>
<p>The course followed by the sex chromosomes and that by the characters
in the case of sex <!-- Page 119 --><span class="pagenum"><a
name="page119"></a>{119}</span>linked inheritance are shown in the next
diagram of Drosophila illustrating a cross between a white eyed male and
a red eyed female.</p>
<div class="figcenter" style="width:36%;">
<a href="images/fig_057.jpg"><img style="width:100%" src="images/fig_057.jpg"
alt="Fig. 57." title="Fig. 57." /></a>
<p class="poem"><span class="sc">Fig. 57.</span> Scheme of sex
determination in Drosophila type. Each <i>mature</i> egg contains one
X, each mature sperm contains one X, or a Y chromosome. Chance union of
any egg with any sperm will give either XX (female) or XY (male).</p>
</div>
<p><!-- Page 120 --><span class="pagenum"><a name="page120"></a>{120}</span></p>
<div class="figcenter" style="width:46%;">
<a href="images/fig_058.jpg"><img style="width:100%" src="images/fig_058.jpg"
alt="Fig. 58." title="Fig. 58." /></a>
<p class="poem"><span class="sc">Fig. 58.</span> Cross between white
eyed male of D. ampelophila and red eyed female. The sex chromosomes
are indicated by the rods. A black rod indicates that the chromosome
carries the factor for red; the open chromosome the factor for white
eye color.</p>
</div>
<p><!-- Page 121 --><span class="pagenum"><a name="page121"></a>{121}</span></p>
<p>The first of these represents a cross between a white eyed male and a
red eyed female (fig. 58, top row). The X chromosome in the male is
represented by an open bar, the Y chromosome is bent. In the female the
two X chromosomes are black. Each egg of such a female will contain one
"black" X after the polar bodies have been thrown off. In the male there
will be two classes of sperm—the female-producing, carrying the
(open) X, and the male-producing, carrying the Y chromosome. Any egg
fertilized by an X bearing sperm will produce a female that will have red
eyes because the X (black) chromosome it gets from the mother carries the
dominant factor for red. Any egg fertilized by a Y-bearing sperm will
produce a male that will also have red eyes because he gets his (black) X
chromosome from his mother.</p>
<p><!-- Page 122 --><span class="pagenum"><a name="page122"></a>{122}</span></p>
<p>When, then, these two F<sub>1</sub> flies (second row) are inbred the
following combinations are expected. Each egg will contain a black X (red
eye producing) or a white X (white eye producing) after the polar bodies
have been extruded. The male will produce two kinds of sperms, of which
the female producing will contain a black X (red eye producing). Since
any egg may by chance be fertilized by any sperm there will result the
four classes of individuals shown on the bottom row of the diagram. All
the females will have red eyes, because irrespective of the two kinds of
eggs involved all the female-producing sperm carry a black X. Half of the
males have red eyes because half of the eggs have had each a
red-producing X chromosome. The other half of the males have white eyes,
because the other half of the eggs had each a white-producing X
chromosome. Other evidence has shown that the Y chromosome of the male is
indifferent, so far as these Mendelian factors are concerned.</p>
<div class="figcenter" style="width:34%;">
<a href="images/fig_059.jpg"><img style="width:100%" src="images/fig_059.jpg"
alt="Fig. 59." title="Fig. 59." /></a>
<p class="poem"><span class="sc">Fig. 59.</span> Cross between red eyed
male and white eyed female; reciprocal cross of Fig. 58.</p>
</div>
<p><!-- Page 123 --><span class="pagenum"><a name="page123"></a>{123}</span></p>
<p>The reciprocal experiment is illustrated in figure 59. A white eyed
female is mated to a red eyed male (top row). All the mature eggs of such
a female contain one white-producing X chromosome represented by the open
bar in the diagram. The red eyed male contains female-producing X-bearing
sperm that carry the factor for red eye color, and male-producing Y
chromosomes. Any egg fertilized by an X-bearing sperm will become a red
eyed female because the X chromosome that comes from the father carries
the dominant factor for red eye color. Any egg fertilized by a Y-bearing
sperm will become a male with white eyes because the only X chromosome
that the male contains comes from his mother and is white producing. <!--
Page 124 --><span class="pagenum"><a name="page124"></a>{124}</span></p>
<p>When these two F<sub>1</sub> flies are inbred (middle row) the
following combinations are expected. Half the eggs will contain each a
white producing X chromosome and half red producing. The female-producing
sperms will each contain a white X and the male-producing sperms will
each contain an indifferent Y chromosome. Chance meetings of egg and
sperm will give the four F<sub>2</sub> classes (bottom row). These
consist of white eyed and red eyed females and white eyed and red eyed
males. The ratio here is 1:1 and not three to one (3:1) as in other
Mendelian cases. But Mendel's law of segregation is not transgressed, as
the preceding analysis has shown; for, the chromosomes have followed
strictly the course laid down on Mendel's principle for the distribution
of factors. The peculiar result in this case is due to the fact that the
F<sub>1</sub> male gets his single factor for eye color from his mother
only and it is linked to or contained in a body (the X chromosome) that
is involved in producing the females, while the mate of this
body—the Y chromosome—is indifferent with regard to these
factors, yet active as a mate to X in synapsis. <!-- Page 125 --><span
class="pagenum"><a name="page125"></a>{125}</span></p>
<div class="figcenter" style="width:22%;">
<a href="images/fig_060.jpg"><img style="width:100%" src="images/fig_060.jpg"
alt="Fig. 60." title="Fig. 60." /></a>
<p class="poem"><span class="sc">Fig. 60.</span> Diagram of sex
determination in type with XX female and XO male (after Wilson).</p>
</div>
<p>In man there are several characters that show exactly this same kind
of inheritance. Color blindness, or at least certain kinds of color
blindness, appear to follow the same scheme. A color blind father
transmits through his daughters his peculiarity to half of his grandsons,
but to none of his grand-daughters (fig. 38A). The result is the same as
in the case of the white eyed male of Drosophila. Color blind women are
rather unusual, which is expected from the method of inheritance of this
character, but in the few known cases where such color blind women have
married normal husbands the sons have inherited the peculiarity from the
mother (fig. 38B). Here again the result is the same as for the similar
combination in Drosophila. <!-- Page 126 --><span class="pagenum"><a
name="page126"></a>{126}</span></p>
<div class="figcenter" style="width:33%;">
<a href="images/fig_061.jpg"><img style="width:100%" src="images/fig_061.jpg"
alt="Fig. 61." title="Fig. 61." /></a>
<p class="poem"><span class="sc">Fig. 61.</span> Spermatogenesis in
man. There are 47 chromosomes (diploid) in the male. After reduction
half of the sperm carry 24 chromosomes (one of which is X) and half
carry 23 chromosomes (no X).</p>
</div>
<p>In man the sex formula appears to be XX for the female and XO for the
male (fig. 60), and since the relation is essentially the same as that in
Drosophila the chromosome explanation is the same. According to von
Winiwarter there are 48 chromosomes in the female and 47 in the male
(fig. 61). After the extrusion of the polar bodies there are 24
chromosomes in the egg. In the male at one of the two maturation <!--
Page 127 --><span class="pagenum"><a
name="page127"></a>{127}</span>divisions the X chromosome passes to one
pole undivided (fig. 61, C). In consequence there are two classes of
sperms in man; female producing containing 24 chromosomes, and male
producing containing 23 chromosomes. If the factor for color blindness is
carried by the X chromosome its inheritance in man works out on the same
chromosome scheme and in the same way as does white eye color (or any
other sex linked character) in the fly, for the O sperm in man is
equivalent to the Y sperm in the fly.</p>
<p> </p>
<p>In these cases we have been dealing with a single pair of characters.
Let us now take a case where two pairs of sex linked characters enter the
cross at the same time, and preferably a case where the two recessives
enter the cross from the same parent.</p>
<p>If a female with white eyes and yellow wings is crossed to a wild male
with red eyes and gray wings (fig. 62), the sons are yellow and have
white eyes and the daughters are gray and have red eyes. If two
F<sub>1</sub> flies are mated they will produce the following classes.
<!-- Page 128 --><span class="pagenum"><a
name="page128"></a>{128}</span></p>
<div class="figcenter" style="width:46%;">
<a href="images/fig_062.jpg"><img style="width:100%" src="images/fig_062.jpg"
alt="Fig. 62." title="Fig. 62." /></a>
<p class="poem"><span class="sc">Fig. 62.</span> Cross between a white
eyed, yellow winged female of D. ampelophila and a red eyed, gray
winged male. Two pairs of sex linked characters, viz., white-red and
yellow-gray are involved. (See text.)</p>
</div>
<p><!-- Page 129 --><span class="pagenum"><a name="page129"></a>{129}</span></p>
<table class="nobctr" summary="Groups I-IV." title="Groups I-IV.">
<tr><td class="spacsingle" style="width:25%; text-align:center;"> Yellow<br />White</td><td class="spacsingle" style="width:25%; text-align:center;"> Gray<br />Red</td><td class="spacsingle" style="width:25%; text-align:center;"> Yellow<br />Red</td><td class="spacsingle" style="width:25%; text-align:center;"> Gray<br />White</td></tr>
<tr><td class="spacsingle" colspan="2" style="text-align:center;"> <a href="images/$ubrace.png"><img src="images/$ubrace.png" class="middle" style="height:3ex; width:6em" alt="brace" /></a><br />99.%</td><td class="spacsingle" colspan="2" style="text-align:center;"> <a href="images/$ubrace.png"><img src="images/$ubrace.png" class="middle" style="height:3ex; width:6em" alt="brace" /></a><br />1.%</td></tr>
</table>
<p>Not only have the two grandparental combinations reappeared, but in
addition two new combinations, viz., grey white and yellow red. The two
original combinations far exceed in numbers the new or exchange
combinations. If we follow the history of the X chromosomes we discover
that the <i>larger classes</i> of grandchildren appear in accord with the
way in which the X chromosomes are transmitted from one generation to the
next.</p>
<p>The <i>smaller classes</i> of grandchildren, the exchange combinations
or cross-overs, as we call them, can be explained by the assumption that
at some stage in their history an interchange of parts has taken place
between the chromosomes. This is indicated in the diagrams.</p>
<p>The most important fact brought out by the experiment is that the
factors that went in together tend to stick together. It makes no
difference in what combination the members of <!-- Page 130 --><span
class="pagenum"><a name="page130"></a>{130}</span>the two pairs of
characters enter, they tend to remain in that combination.</p>
<p>If one admits that the sex chromosomes carry these factors for the
sex-linked characters—and the evidence is certainly very strong in
favor of this view—it follows necessarily from these facts that at
some time in their history there has been an interchange between the two
sex chromosomes in the female.</p>
<p>There are several stages in the conjugation of the chromosomes at
which such an interchange between the members of a pair might occur.
There is further a small amount of direct evidence, unfortunately very
meagre at present, showing that an interchange does actually occur.</p>
<p>At the ripening period of the germ cell the members of each pair of
chromosomes come together (fig. 49, e). In several forms they have been
described as meeting at one end and then progressively coming to lie side
by side as shown in fig. 63, e, f, g, h, i. At the end of the process
they appear to have completely united along their length (fig. 63, j, k,
l). It is always a maternal and a paternal <!-- Page 131 --><span
class="pagenum"><a name="page131"></a>{131}</span>chromosome that meet in
this way and always two of the same kind. It has been observed that as
the members of a pair come together they occasionally twist around each
other (fig. 63, g, l, and 64, and 65). In consequence a part of one
chromosome comes to be now on one side and now on the other side of its
mate.</p>
<div class="figcenter" style="width:32%;">
<a href="images/fig_063.jpg"><img style="width:100%" src="images/fig_063.jpg"
alt="Fig. 63." title="Fig. 63." /></a>
<p class="poem"><span class="sc">Fig. 63.</span> Conjugation of
chromosomes (side to side union) in the spermatogenesis of Batracoseps.
(After Janssens.)</p>
</div>
<p><!-- Page 132 --><span class="pagenum"><a name="page132"></a>{132}</span></p>
<p>When the chromosomes separate at the next division of the germ cell
the part on one side passes to one pole, the part on the other to the
opposite pole, (figs. 64 and 65). Whenever the chromosomes do not untwist
at this time there must result an interchange of pieces where they were
crossed over each other.</p>
<div class="figcenter" style="width:26%;">
<a href="images/fig_064.jpg"><img style="width:100%" src="images/fig_064.jpg"
alt="Fig. 64." title="Fig. 64." /></a>
<p class="poem"><span class="sc">Fig. 64.</span> Scheme to illustrate a
method of crossing over of the chromosomes.</p>
</div>
<p>Janssens has found at the time of separation <!-- Page 133 --><span
class="pagenum"><a name="page133"></a>{133}</span>evidence in favor of
the view that some such interchange probably takes place.</p>
<p> </p>
<p>We find this same process of interchange of characters taking place in
each of the other three groups of Drosophila. An example will show this
for the Group II.</p>
<div class="figcenter" style="width:16%;">
<a href="images/fig_065.jpg"><img style="width:100%" src="images/fig_065.jpg"
alt="Fig. 65." title="Fig. 65." /></a>
<p class="poem"><span class="sc">Fig. 65.</span> Scheme to illustrate
double crossing over.</p>
</div>
<p>If a black vestigial male is crossed to a gray long-winged female
(fig. 66) the offspring are gray long. If an F<sub>1</sub> female is
back-crossed to a black vestigial male the following kinds of flies are
produced:</p>
<p><!-- Page 134 --><span class="pagenum"><a name="page134"></a>{134}</span></p>
<table class="nobctr" summary="Groups I-IV." title="Groups I-IV.">
<tr><td class="spacsingle" style="width:25%; text-align:center;"> Black<br />vestigial</td><td class="spacsingle" style="width:25%; text-align:center;"> Gray<br />long</td><td class="spacsingle" style="width:25%; text-align:center;"> Black<br />long</td><td class="spacsingle" style="width:25%; text-align:center;"> Gray<br />vestigial</td></tr>
<tr><td class="spacsingle" colspan="2" style="text-align:center;"> <a href="images/$ubrace.png"><img src="images/$ubrace.png" class="middle" style="height:3ex; width:6em" alt="brace" /></a><br />83%</td><td class="spacsingle" colspan="2" style="text-align:center;"> <a href="images/$ubrace.png"><img src="images/$ubrace.png" class="middle" style="height:3ex; width:6em" alt="brace" /></a><br />17%</td></tr>
</table>
<p>The combinations that entered are more common in the F<sub>2</sub>
generations than the cross-over classes, showing that there is linkage of
the factors that entered together.</p>
<p>Another curious fact is brought out if instead of back-crossing the
F<sub>1</sub> female we back-cross the F<sub>1</sub> male to a black
vestigial female. Their offspring are now of only two kinds, black
vestigial and gray long. This means that in the male there is no
crossing-over or interchange of pieces. This relation holds not only for
the Group II but for all the other groups as well.</p>
<p>Why interchange takes place in the female of Drosophila and not in the
male we do not know at present. We might surmise that when in the male
the members of a pair come together they do not twist around each other,
hence no crossing-over results.</p>
<p><!-- Page 135 --><span class="pagenum"><a name="page135"></a>{135}</span></p>
<div class="figcenter" style="width:46%;">
<a href="images/fig_066.jpg"><img style="width:100%" src="images/fig_066.jpg"
alt="Fig. 66." title="Fig. 66." /></a>
<p class="poem"><span class="sc">Fig. 66.</span> Cross between black
vestigial and gray long flies. Two pairs of factors involved in the
second group. The F<sub>1</sub> female is back crossed (to right) to
black vestigial male; and the F<sub>1</sub> male is back crossed to
black vestigial female (to left). Crossing over takes place in the
F<sub>1</sub> female but not in the F<sub>1</sub> male.</p>
</div>
<p>Crossing-over took place between white and yellow only once in a
hundred times. Other characters show different values, but the same value
under the same conditions is obtained from the same pair of characters.
<!-- Page 136 --><span class="pagenum"><a
name="page136"></a>{136}</span></p>
<div class="figcenter" style="width:46%;">
<a href="images/fig_067.jpg"><img style="width:100%" src="images/fig_067.jpg"
alt="Fig. 67." title="Fig. 67." /></a>
<p class="poem"><span class="sc">Fig. 67.</span> Map of four
chromosomes of D. ampelophila locating those factors in each group that
have been most fully studied.</p>
</div>
<p><!-- Page 137 --><span class="pagenum"><a name="page137"></a>{137}</span></p>
<p>If we assume that the nearer together the factors lie in the
chromosome the less likely is a twist to occur between them, and
conversely the farther apart they lie the more likely is a twist to occur
between them, we can understand how the linkage is different for
different pairs of factors.</p>
<p>On this basis we have made out chromosomal maps for each chromosome
(fig. 67). The diagram indicates those loci that have been most
accurately placed.</p>
<p class="cenhead"><i>The Evidence from Interference</i></p>
<p>There is a considerable body of information that we have obtained that
corroborates the location of the factors in the chromosome. This evidence
is too technical to take up in any detail, but there is one result that
is so important that I must attempt to explain it. If, as I assume,
crossing over is brought about by twisting of the chromosomes, and if
owing to the material of the chromosomes there is a most frequent
distance of internode, then, when crossing over between nodes takes place
at same level at a-b in figure 68, the region on <!-- Page 138 --><span
class="pagenum"><a name="page138"></a>{138}</span>each side of that
point, a to A and b to B, should be protected, so to speak, from further
crossing over. This in fact we have found to be the case. No other
explanation so far proposed will account for this extraordinary
relation.</p>
<div class="figcenter" style="width:33%;">
<a href="images/fig_068.jpg"><img style="width:100%" src="images/fig_068.jpg"
alt="Fig. 68." title="Fig. 68." /></a>
<p class="poem"><span class="sc">Fig. 68.</span> Scheme to indicate
that when the members of a pair of chromosomes cross (at a-b) the
region on each side is protected inversely to the distance from
a-b.</p>
</div>
<p>What advantage, may be asked, is there in obtaining numerical data of
this kind? It is this:—whenever a new character appears we need
only determine in which of the four groups it lies and its distance from
two members within that group. With this information we can predict with
a high degree of probability what results it will give with any other
member of any group. Thus we can do on paper what would require many
months of labor by making the actual experiment. In a word we can predict
what will happen in a situation where prediction is impossible without
this numerical information. <!-- Page 139 --><span class="pagenum"><a
name="page139"></a>{139}</span></p>
<p class="cenhead"><i>The Evidence from Non-Disjunction</i></p>
<p>In the course of the work on Drosophila exceptions appeared in one
strain where certain individuals did not conform to the scheme of sex
linked inheritance. For a moment the hypothesis seemed to fail, but a
careful examination led to the suspicion that in this strain something
had happened to the sex chromosomes. It was seen that if in some way the
X chromosomes failed to disjoin in certain eggs, the exceptions could be
explained. The analysis led to the suggestion that if the Y chromosome
had got into the female line the results would be accounted for, since
its presence there would be expected to cause this peculiar
non-disjunction of the X chromosomes.</p>
<p>That this was the explanation was shown when the material was
examined. The females that gave these results were found by Bridges to
have two X's and a Y chromosome.</p>
<p>The normal chromosome group of the female is shown in figure 52 and
the chromosome group of one of the exceptional females is shown in figure
69. In a female of this kind <!-- Page 140 --><span class="pagenum"><a
name="page140"></a>{140}</span>there are three sex chromosomes X X Y
which are homologous in the sense that in normal individuals the two
present are mates and separate at the reduction division. If in the X X Y
individual X and X conjugate and separate at reduction and the unmated Y
is free to move to either pole of the spindle, two kinds of mature eggs
will result, viz., X and XY. If, on the other hand, X and Y conjugate and
separate at reduction and the remaining X is free to go to either pole,
four kinds of eggs will result—XY—X—XX—Y. As a
total result four kinds of eggs are expected: viz. many XY and X eggs and
a few XX and Y eggs.</p>
<div class="figcenter" style="width:26%;">
<a href="images/fig_069.jpg"><img style="width:100%" src="images/fig_069.jpg"
alt="Fig. 69." title="Fig. 69." /></a>
<p class="poem"><span class="sc">Fig. 69.</span> Figure of the
chromosome group of an XXY female, that gives non-disjunction.</p>
</div>
<p>These four kinds of eggs may be fertilized either by female-producing
sperms or <!-- Page 141 --><span class="pagenum"><a
name="page141"></a>{141}</span>male-producing sperms, as indicated in the
diagram (fig. 70).</p>
<div class="figcenter" style="width:46%;">
<a href="images/fig_070.jpg"><img style="width:100%" src="images/fig_070.jpg"
alt="Fig. 70." title="Fig. 70." /></a>
<p class="poem"><span class="sc">Fig.</span> 70. Scheme showing the
results of fertilizing white bearing eggs (4 kinds) resulting from
non-disjunction. The upper half of the diagram gives the results when
these eggs are fertilized by normal red bearing, female producing
sperm, the lower half by normal, male producing sperm.</p>
</div>
<p>If such an XXY female carried white bearing Xs (open X in the
figures), and the male <!-- Page 142 --><span class="pagenum"><a
name="page142"></a>{142}</span>carried a red bearing X (black X in the
figures) it will be seen that there should result an exceptional class of
sons that are red, and an exceptional class of daughters that are white.
Tests of these exceptions show that they behave subsequently in heredity
as their composition requires. Other tests may also be made of the other
classes of offspring. Bridges has shown that they fulfill all the
requirements predicted. Thus a result that seemed in contradiction with
the chromosome hypothesis has turned out to give a brilliant confirmation
of that theory both genetically and cytologically.</p>
<p class="cenhead"><span class="sc">How Many Genetic Factors are there in the Germ-plasm of a Single Individual</span></p>
<p>In passing I invite your attention to a speculation based on our maps
of the chromosomes—a speculation which I must insist does not
pretend to be more than a guess but has at least the interest of being
the first guess that we have ever been in position to make as to how many
factors go towards the makeup of the germ plasm. <!-- Page 143 --><span
class="pagenum"><a name="page143"></a>{143}</span></p>
<p>We have found practically no factors less than .04 of a unit apart. If
our map includes the entire length of the chromosomes and if we assume
factors are uniformly distributed along the chromosome at distances equal
to the shortest distance yet observed, viz. .04, then we can calculate
roughly how many hereditary factors there are in Drosophila. The
calculation gives about 7500 factors. The reader should be cautioned
against accepting the above assumptions as strictly true, for
crossing-over values are known to differ according to different
environmental conditions (as shown by Bridges for age), and to differ
even in different parts of the chromosome as a result of the presence of
specific genetic factors (as shown by Sturtevant). Since all the
chromosomes except the X chromosomes are double we must double our
estimate to give the <i>total</i> number of factors, but the half number
is the number of the different kinds of factors of Drosophila. <!-- Page
144 --><span class="pagenum"><a name="page144"></a>{144}</span></p>
<p class="cenhead"><span class="sc">Conclusions</span></p>
<p>I have passed in review a long series of researches as to the nature
of the hereditary material. We have in consequence of this work arrived
within sight of a result that seemed a few years ago far beyond our
reach. The mechanism of heredity has, I think, been
discovered—discovered not by a flash of intuition but as the result
of patient and careful study of the evidence itself.</p>
<p>With the discovery of this mechanism I venture the opinion that the
problem of heredity has been solved. We know how the factors carried by
the parents are sorted out to the germ cells. The explanation does not
pretend to state how factors arise or how they influence the development
of the embryo. But these have never been an integral part of the doctrine
of heredity. The problems which they present must be worked out in their
own field. So, I repeat, the mechanism of the chromosomes offers a
satisfactory solution of the traditional problem of heredity.</p>
<p><br style="clear:both" /></p>
<hr class="full" />
<p><!-- Page 145 --><span class="pagenum"><a name="page145"></a>{145}</span></p>
<h3>CHAPTER IV</h3>
<p class="cenhead">SELECTION AND EVOLUTION</p>
<p>Darwin's Theory of Natural Selection still holds today first place in
every discussion of evolution, and for this very reason the theory calls
for careful scrutiny; for it is not difficult to show that the expression
"natural selection" is to many men a metaphor that carries many meanings,
and sometimes different meanings to different men. While I heartily agree
with my fellow biologists in ascribing to Darwin himself, and to his
work, the first place in biological philosophy, yet recognition of this
claim should not deter us from a careful analysis of the situation in the
light of work that has been done since Darwin's time.</p>
<p class="cenhead"><span class="sc">The Theory of Natural Selection</span></p>
<p>In his great book on the <i>Origin of Species</i>, Darwin tried to do
two things: first, to show that the evidence bearing on evolution makes
<!-- Page 146 --><span class="pagenum"><a
name="page146"></a>{146}</span>that explanation probable. No such great
body of evidence had ever been brought together before, and it wrought,
as we all know, a revolution in our modes of thinking.</p>
<p>Darwin also set himself the task of showing <i>how</i> evolution might
have taken place. He pointed to the influence of the environment, to the
effects of use and disuse, and to natural selection. It is to the last
theory that his name is especially attached. He appealed to a fact
familiar to everyone, that no two individuals are identical and that some
of the differences that they show are inherited. He argued that those
individuals that are best suited to their environment are the most
probable ones to survive and to leave most offspring. In consequence
their descendants should in time replace through competition the less
well-adapted individuals of the species. This is the process Darwin
called natural selection, and Spencer the survival of the fittest.</p>
<p>Stated in these general terms there is nothing in the theory to which
anyone is likely to take exception. But let us examine the argument more
critically. <!-- Page 147 --><span class="pagenum"><a
name="page147"></a>{147}</span></p>
<div class="figcenter" style="width:30%;">
<a href="images/fig_071.jpg"><img style="width:100%" src="images/fig_071.jpg"
alt="Fig. 71." title="Fig. 71." /></a>
<p class="poem"><span class="sc">Fig.</span> 71. Series of leaves of a
tree arranged according to size. (After de Vries.)</p>
</div>
<p>If we measure, or weigh, or classify any character shown by the
individuals of a population, we find differences. We recognize that some
of the differences are due to the varied experiences that the individuals
have encountered in the course of their lives, i.e. to their environment,
but we also recognize that some of the differences may be due to
individuals having different inheritances—different germ plasms.
Some familiar examples will help to bring home this relation.</p>
<p>If the leaves of a tree are arranged according to size (fig. 71), we
find a continuous series, but there are more leaves of medium size than
extremes. If a lot of beans be sorted out <!-- Page 148 --><span
class="pagenum"><a name="page148"></a>{148}</span>according to their
weights, and those between certain weights put into cylinders, the
cylinders, when arranged according to the size of the beans, will appear
as shown in figure 72. An imaginary line running over the tops of the
piles will give a curve (fig. 73) that corresponds to the curve of
probability (fig. 74).</p>
<table class="nobctr"><tr><td style="width:50%; vertical-align:top;">
<div class="figright" style="width:61%;">
<a href="images/fig_072.jpg"><img style="width:100%" src="images/fig_072.jpg"
alt="Fig. 72." title="Fig. 72." /></a>
<p class="poem"><span class="sc">Fig.</span> 72. Beans put into
cylindrical jars according to the sizes of the beans. The jars arranged
according to size of contained beans. (After de Vries.)</p>
</div>
</td><td style="width:50%; vertical-align:top;">
<div class="figleft" style="width:61%;">
<a href="images/fig_073.jpg"><img style="width:100%" src="images/fig_073.jpg"
alt="Fig. 73." title="Fig. 73." /></a>
<p class="poem"><span class="sc">Fig.</span> 73. A curve resulting from
arrangement of beans according to size. (After de Vries.)</p>
</div>
</td></tr></table>
<p>If we stand men in lines according to their height (fig. 75) we get a
similar arrangement. <!-- Page 149 --><span class="pagenum"><a
name="page149"></a>{149}</span></p>
<table class="nobctr"><tr><td style="width:50%; vertical-align:top;">
<div class="figright" style="width:65%;">
<a href="images/fig_074.jpg"><img style="width:100%" src="images/fig_074.jpg"
alt="Fig. 74." title="Fig. 74." /></a>
<span class="sc">Fig.</span> 74. Curve of probability.
</div>
</td><td style="width:50%; vertical-align:top;">
<div class="figleft" style="width:76%;">
<a href="images/fig_075.jpg"><img style="width:100%" src="images/fig_075.jpg"
alt="Fig. 75." title="Fig. 75." /></a>
<p class="poem"><span class="sc">Fig.</span> 75. Students arranged
according to size. (After Blakeslee.)</p>
</div>
</td></tr></table>
<p>The differences in size shown by the individual beans or by the
individual men are due in part to heredity, in part to the environment
<!-- Page 150 --><span class="pagenum"><a
name="page150"></a>{150}</span>in which they have developed. This is a
familiar fact of almost every-day observation. It is well shown in the
following example. In figure 76 the two boys and the two varieties of
corn, which they are holding, differ in height. The pedigrees of the boys
(fig. 77) make it probable that their height is largely inherited and the
two races of corn are known to belong to a tall and a short race
respectively. Here, then, the chief effect or difference is due to
heredity. On the other hand, if individuals of the same race develop in a
favorable environment the result is different from the development in an
unfavorable environment, as shown in figure 78. Here to the right the
corn is crowded and in consequence dwarfed, while to the left the same
kind of corn has had more room to develop and is taller.</p>
<p><!-- Page 151 --><span class="pagenum"><a name="page151"></a>{151}</span></p>
<div class="figcenter" style="width:31%;">
<a href="images/fig_076.jpg"><img style="width:100%" src="images/fig_076.jpg"
alt="Fig. 76." title="Fig. 76." /></a>
<p class="poem"><span class="sc">Fig.</span> 76. A short and a tall boy
each holding a stalk of corn—one stalk of a race of short corn,
the other of tall corn. (After Blakeslee.)</p>
</div>
<p><!-- Page 152 --><span class="pagenum"><a name="page152"></a>{152}</span></p>
<div class="figcenter" style="width:49%;">
<a href="images/fig_077.png"><img style="width:100%" src="images/fig_077.png"
alt="Fig. 77." title="Fig. 77." /></a>
<p class="poem"><span class="sc">Fig.</span> 77. Pedigree of boys shown
in Fig. 76. (After Blakeslee.)</p>
</div>
<p>Darwin knew that if selection of particular kinds of individuals of a
population takes place the next generation is affected. If the taller men
of a community are selected <i>the average</i> of their offspring will be
taller than the average of the former population. If selection for
tallness again takes place, still taller men will <i>on the average</i>
arise. If, amongst these, selection again makes a choice the process
would, he thought, continue (fig. 79).</p>
<p><!-- Page 153 --><span class="pagenum"><a name="page153"></a>{153}</span></p>
<div class="figcenter" style="width:33%;">
<a href="images/fig_078.jpg"><img style="width:100%" src="images/fig_078.jpg"
alt="Fig. 78." title="Fig. 78." /></a>
<p class="poem"><span class="sc">Fig.</span> 78. A race of corn reared
under different conditions.</p>
</div>
<p>We now recognize that this statement contains an important truth, but
we have found that it contains only a part of the truth. Any one who
repeats for himself this kind of selection experiment will find that
while his average class will often change in the direction of his
selection, the process slows down as a rule rather suddenly (fig. 80). He
finds, moreover, that the limits of variability are not necessarily
transcended as the process continues even although the average may for a
while be increased. More tall men may be produced by selection of this
kind, but the tallest men are not necessarily any taller than the tallest
in the original population. <!-- Page 154 --><span class="pagenum"><a
name="page154"></a>{154}</span></p>
<div class="figcenter" style="width:33%;">
<a href="images/fig_079.jpg"><img style="width:100%" src="images/fig_079.jpg"
alt="Fig. 79." title="Fig. 79." /></a>
<p class="poem"><span class="sc">Fig.</span> 79. Curves showing how
(hypothetically) selection might be supposed to bring about progress in
direction of selection. (After Goldschmidt.)</p>
</div>
<p>Selection, then, has not produced anything new, but only more of
certain kinds of individuals. Evolution, however, means producing more
new things, not more of what already exists.</p>
<p>Darwin seems to have thought that the range of variation shown by the
offspring of a given individual about that type of individual would be as
wide as the range shown by the original population (fig. 79), but
Galton's work has made it clear that this is not the case in a general or
mixed population. If the offspring of individuals continued to show, as
Darwin seems to have thought, as wide a range on each side of their
parents' size, so to speak, as did the original population, then it would
follow that <!-- Page 155 --><span class="pagenum"><a
name="page155"></a>{155}</span>selection could slide successive
generations along in the direction of selection.</p>
<div class="figcenter" style="width:46%;">
<a href="images/fig_080.jpg"><img style="width:100%" src="images/fig_080.jpg"
alt="Fig. 80." title="Fig. 80." /></a>
<p class="poem"><span class="sc">Fig.</span> 80. Diagram illustrating
the results of selection for extra bristles in D. ampelophila.
Selection at first produces decided effects which soon slow down and
then cease. (MacDowell.)</p>
</div>
<p>Darwin himself was extraordinarily careful, however, in the statements
he made in this connection and it is rather by implication than by actual
reference that one can ascribe this <!-- Page 156 --><span
class="pagenum"><a name="page156"></a>{156}</span>meaning to his views.
His contemporaries and many of his followers, however, appear to have
accepted this <i>sliding scale</i> interpretation as the cardinal
doctrine of evolution. If this is doubted or my statement is challenged
then one must explain why de Vries' mutation theory met with so little
enthusiasm amongst the older group of zoölogists and botanists; and one
must explain why Johannsen's splendid work met with such bitter
opposition from the English school—the biometricians—who
amongst the post-Darwinian school are assumed to be the lineal
descendants of Darwin.</p>
<p>And in this connection we should not forget that just this sort of
process was supposed to take place in the inheritance of use and disuse.
What is gained in one generation forms the basis for further gains in the
next generation. Now, Darwin not only believed that acquired characters
are inherited but turned more and more to this explanation in his later
writings. Let us, however, not make too much of the matter; for it is
much less important to find out whether Darwin's ideas were vague, than
it is to make sure that our own ideas are clear. <!-- Page 157 --><span
class="pagenum"><a name="page157"></a>{157}</span></p>
<p>If I have made several statements here that appear dogmatic let me now
attempt to justify them, or at least give the evidence which seems to me
to make them probable.</p>
<p>The work of the Danish botanist, Johannsen, has given us the most
carefully analyzed case of selection that has ever been obtained. There
are, moreover, special reasons why the material that he used is better
suited to give definite information than any other so far studied.
Johannsen worked with the common bean, weighing the seeds or else
measuring them. These beans if taken from many plants at random give the
typical curve of probability (fig. 74). The plant multiplies by
self-fertilization. Taking advantage of this fact Johannsen kept the
seeds of each plant separate from the others, and raised from them a new
generation. When curves were made from these new groups it was found that
some of them had different modes from that of the original general
population (fig. 81 A-E, bottom group). They are shown in the upper
groups (A, B, C, D, E). But do not understand me to say that the
offspring of each bean gave a different mode. <!-- Page 158 --><span
class="pagenum"><a name="page158"></a>{158}</span></p>
<div class="figcenter" style="width:31%;">
<a href="images/fig_081.jpg"><img style="width:100%" src="images/fig_081.jpg"
alt="Fig. 81." title="Fig. 81." /></a>
<p class="poem"><span class="sc">Fig.</span> 81. Pure lines of beans.
The lower figure gives the general population, the other figures give
the pure lines within the population. (After Johannsen.)</p>
</div>
<p><!-- Page 159 --><span class="pagenum"><a name="page159"></a>{159}</span></p>
<p>On the contrary, some of the lines would be the same.</p>
<p>The result means that the general population is made up of definite
kinds of individuals that may have been sorted out.</p>
<p>That his conclusion is correct is shown by rearing a new generation
from any plant or indeed from several plants of any one of these lines.
Each line repeats the same modal class. There is no further breaking up
into groups. Within the line it does not matter at all whether one
chooses a big bean or a little one—they will give the same result.
In a word, the germ plasm in each of these lines is pure, or homozygous,
as we say. The differences that we find between the weights (or sizes) of
the individual beans are due to external conditions to which they have
been subjected.</p>
<p>In a word, Johannsen's work shows that the frequency distribution of a
pure line is due to factors that are extrinsic to the germ plasm. It does
not matter then which individuals in a pure line are used to breed from,
for they all carry the same germ plasm.</p>
<p>We can now understand more clearly how <!-- Page 160 --><span
class="pagenum"><a name="page160"></a>{160}</span>selection acting on a
general population brings about results in the direction of
selection.</p>
<p>An individual is picked out from the population in order to get a
particular kind of germ plasm. Although the different classes of
individuals may overlap, so that one can not always judge an individual
from its appearance, nevertheless on the whole chance favors the picking
out of the kind of germ plasm sought.</p>
<p>In species with separate sexes there is the further difficulty that
two individuals must be chosen for each mating, and superficial
examination of them does not insure that they belong to the same
group—their germ plasm cannot be inspected. Hence selection of
biparental forms is a precarious process, now going forward, now
backwards, now standing still. In time, however, the process forward is
almost certain to take place if the selection is from a heterogeneous
population. Johannsen's work was simplified because he started with pure
lines. In fact, had he not done so his work would not have been
essentially different from that of any selection experiment of a pure
race of animals or plants. Whether Johannsen <!-- Page 161 --><span
class="pagenum"><a name="page161"></a>{161}</span>realized the importance
of the condition or not is uncertain—curiously he laid no emphasis
on it in the first edition of his "Elemente der exakten
Erblichkeitslehre".</p>
<p>It has since been pointed out by Jennings and by Pearl that a race
that reproduces by self-fertilization as does this bean, automatically
becomes pure in all of the factors that make up its germ plasm. Since
self-fertilization is the normal process in this bean the purity of the
germ plasm already existed when Johannsen began to experiment.</p>
<p class="cenhead"><span class="sc">How Has Selection in Domesticated Animals and Plants Brought About Its Results?</span></p>
<p>If then selection does not bring about transgressive variation in a
general population, how can selection produce anything new? If it can not
produce anything new, is there any other way in which selection becomes
an agent in evolution?</p>
<p>We can get some light on this question if we turn to what man has done
with his domesticated animals and plants. Through selection, <!-- Page
162 --><span class="pagenum"><a name="page162"></a>{162}</span>i.e.,
artificial selection, man has undoubtedly brought about changes as
remarkable as any shown by wild animals and plants. We know, moreover, a
good deal about how these changes have been wrought.</p>
<p>(1) By crossing different wild species or by crossing wild with races
already domesticated new combinations have been made. Parts of one
individual have been combined with parts of others, creating new
combinations. It is possible even that characters that are entirely new
may be produced by the interaction of factors brought into
recombination.</p>
<p>(2) New characters appear from time to time in domesticated and in
wild species. These, like the mutants in Drosophila, are fully equipped
at the start. Since they breed true and follow Mendel's laws it is
possible to combine them with characters of the wild type or with those
of other mutant races.</p>
<p>Amongst the new mutant factors there may be some whose chief effect is
on the character that the breeder is already selecting. Such a
modification will be likely to attract attention. Superficially it may
appear that the <!-- Page 163 --><span class="pagenum"><a
name="page163"></a>{163}</span>factor for the original character has
varied, while the truth may be that another factor has appeared that has
modified a character already present. In fact, many or all Mendelian
factors that affect the same organ may be said to be modifiers of each
other's effects. Thus the factor for vermilion causes the eye to be one
color, and the factor for eosin another color, while eosin vermilion is
different from both. Eosin may be said to be a modifier of vermilion or
vermilion of eosin. In general, however, it is convenient to use the term
"modifier" for cases in which the factor causes a detectable change in a
character already present or conspicuous.</p>
<p><!-- Page 164 --><span class="pagenum"><a name="page164"></a>{164}</span></p>
<div class="figcenter" style="width:41%;">
<a href="images/fig_082.jpg"><img style="width:100%" src="images/fig_082.jpg"
alt="Fig. 82." title="Fig. 82." /></a>
<p class="poem"><span class="sc">Fig.</span> 82. Scheme to indicate
influence of the modifying factors, cream and whiting. Neither produces
any effect alone but they modify other eye colors such as eosin.</p>
</div>
<p>One of the most interesting, and at the same time most treacherous,
kinds of modifying factors is that which produces an effect <i>only</i>
when some other factor is present. Thus Bridges has shown that there is a
factor called "cream" that does not affect the red color of the eye of
the wild fly, yet makes "eosin" much paler (fig. 82). Another factor
"whiting" which produces no effect on red makes eosin entirely white.
Since cream or whiting may be carried by red eyed flies without their
presence being seen until eosin is used, the experimenter must be
continually on the lookout for such factors which may lead to erroneous
conclusions unless detected. As yet breeders have not realized the
important rôle that modifiers have played in their results, but there are
indications at least that the heaping up of modifying factors has been
one of the ways in which <!-- Page 165 --><span class="pagenum"><a
name="page165"></a>{165}</span>highly specialized domesticated animals
have been produced. Selection has accomplished this result not by
changing factors, but by picking up modifying factors. The demonstration
of the presence of these factors has already been made in some cases.
Their study promises to be one of the most instructive fields for further
work bearing on the selection hypothesis.</p>
<p>In addition to these well recognized methods by which artificial
selection has produced new things we come now to a question that is the
very crux of the selection theory today. Our whole conception of
selection turns on the answer that we give to this matter and if I appear
insistent and go into some detail it is because I think that the matter
is worth very careful consideration.</p>
<p class="cenhead"><span class="sc">Are Factors Changed Through Selection?</span></p>
<p>As we have seen, the variation that we find from individual to
individual is due in part to the environment; this can generally be
demonstrated. Other differences in an <!-- Page 166 --><span
class="pagenum"><a name="page166"></a>{166}</span>ordinary population are
recognized as due to different genetic (hereditary) combinations. No one
will dispute this statement. But is all the variability accounted for in
these two ways? May not a factor itself fluctuate? Is it not <i>a
priori</i> probable that factors do fluctuate? Why, in a word, should we
regard factors as inviolate when we see that everything else in organisms
is more or less in amount? I do not know of any <i>a priori</i> reason
why a factor may not fluctuate, unless it is, as I like to think, a
chemical molecule. We are, however, dealing here not with generalities
but with evidence, and there are three known methods by means of which it
has been shown that variability, other than environmental or
recombinational, is not due to variability in a factor, nor to various
"potencies" possessed by the same factors.</p>
<p>(1) By making the stock uniform for all of its factors—chief
factors and modifiers alike. Any change in such a stock produced by
selection would then be due to a change in one or more of the factors
themselves. Johannsen's experiment is an example of this sort.</p>
<div class="figcenter" style="width:13%;">
<a href="images/fig_083a.jpg"><img style="width:100%" src="images/fig_083a.jpg"
alt="Fig. 83a." title="Fig. 83a." /></a>
<p class="poem"><span class="sc">Fig.</span> 83 a. Drosophila
ampelophila with truncate wings.</p>
</div>
<p>(2) The second method is one that is <!-- Page 167 --><span
class="pagenum"><a name="page167"></a>{167}</span>capable of
<i>demonstrating</i> that the effects of selection are actually due to
modifiers. It has been worked out in our laboratory, chiefly by Muller,
and used in a particular case to demonstrate that selection produced its
effect by isolating modifying factors. For example, a mutant type called
truncate appeared, characterized by shorter wings, usually square at the
end, (fig. 83a). The wings varied from those of normal length to wings
much shorter (fig. 83b). For three years the mutant stock was <!-- Page
168 --><span class="pagenum"><a name="page168"></a>{168}</span>bred from
individuals having the shorter wings until at last a stock was obtained
in which some of the individuals had wings much shorter than the body. By
means of linkage experiments it was shown that at least three factors
were present that modified the wings. These were isolated by means of
their linkage relations, and their mutual influence on the production of
truncate wings was shown.</p>
<div class="figcenter" style="width:32%;">
<a href="images/fig_083b.jpg"><img style="width:100%" src="images/fig_083b.jpg"
alt="Fig. 83b." title="Fig. 83b." /></a>
<p class="poem"><span class="sc">Fig.</span> 83 b. Series of wings of
different length shown by truncate stock of D. ampelophila.</p>
</div>
<p><!-- Page 169 --><span class="pagenum"><a name="page169"></a>{169}</span></p>
<p>An experiment of this kind can only be carried out in a case where the
groups of linked gens are known. At present Drosophila is the only animal
(or plant) sufficiently well known to make this test possible, but this
does not prove that the method is of no value. On the contrary it shows
that any claim that factors can themselves be changed can have no
finality until the claim can be tested out by means of the linkage test.
For instance, bar eye (fig. 31) arose as a mutation. All our stock has
descended from a single original mutant. But Zeleny has shown that
selection within our stock will make the bar eye narrower or broader
according to the direction of selection. It remains to be shown in this
case how selection has produced its effects, and this can be done by
utilizing the same process that was used in the case of truncate.</p>
<p>Another mutant stock called beaded (fig. 84), has been bred for five
years and selected for wings showing more beading. In extreme cases the
wings have been reduced to mere stumps (see stumpy, fig. 5), but the
stock shows great variability. It is probable here <!-- Page 170 --><span
class="pagenum"><a name="page170"></a>{170}</span>as Dexter has shown,
that a number of mutant factors that act as modifiers have been picked up
in the course of the selection, and when it is recalled that during those
five years over 125 new characters have appeared elsewhere it does not
seem improbable that factors also have appeared that modify the wings of
this stock.</p>
<div class="figcenter" style="width:33%;">
<a href="images/fig_084.jpg"><img style="width:100%" src="images/fig_084.jpg"
alt="Fig. 84." title="Fig. 84." /></a>
<p class="poem"><span class="sc">Fig.</span> 84. Two flies showing
beaded wings.</p>
</div>
<p>(3) The third method is one that has been developed principally by
East for plants; also by MacDowell for rabbits and flies. The <!-- Page
171 --><span class="pagenum"><a name="page171"></a>{171}</span>method
does not claim to prove that modifiers are present, but it shows why
certain results are in harmony with that expectation and can not be
accounted for on the basis that a factor has changed. Let me give an
example. When a Belgian hare with large body was crossed to a common
rabbit with a small body the hybrid was intermediate in size. When the
hybrid was crossed back to the smaller type it produced rabbits of
various sizes in apparently a continuous series. MacDowell made
measurements of the range of variability in the first and in the second
generations.</p>
<p class="cenhead"><i>Classification in relation to parents based on skull lengths and ulna
lengths, to show the relative variability of two measurements and
of the first generation (F<sub>1</sub>) and the back cross (B. C.)</i></p>
<table class="allbctr" summary="Results of MacDowell" title="Results of MacDowell">
<tr><td class="allb" colspan="2"> CHARACTER </td><td class="allb" style="text-align:right;"> GENERATION </td><td class="allb" style="text-align:right;"> -13</td><td class="allb" style="text-align:right;"> -12</td><td class="allb" style="text-align:right;"> -11</td><td class="allb" style="text-align:right;"> -10</td><td class="allb" style="text-align:right;"> -9</td><td class="allb" style="text-align:right;"> -8</td><td class="allb" style="text-align:right;"> -7</td><td class="allb" style="text-align:right;"> -6</td><td class="allb" style="text-align:right;"> -5</td><td class="allb" style="text-align:right;"> -4</td><td class="allb" style="text-align:right;"> -3</td><td class="allb" style="text-align:right;"> -2</td><td class="allb" style="text-align:right;"> -1</td><td class="allb" style="text-align:right;"> 0</td><td class="allb" style="text-align:right;"> 1</td><td class="allb" style="text-align:right;"> 2</td><td class="allb" style="text-align:right;"> 3</td><td class="allb" style="text-align:right;"> 4</td><td class="allb" style="text-align:right;"> 5</td></tr>
<tr><td class="nob" style="text-align:center;"> Length of </td><td class="rightb" rowspan="2" style="vertical-align:middle;"> <a href="images/$lbrace.png"><img src="images/$lbrace.png" class="middle" style="height:5ex; width:0.7em" alt="brace" /></a></td><td class="nob" style="text-align:center;"> F<sub>1</sub> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td></tr>
<tr><td class="nob" style="text-align:center;"> skull </td><td class="nob" style="text-align:center;"> B.C. </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 3</td><td class="vertb" style="text-align:right;"> 6</td><td class="vertb" style="text-align:right;"> 4</td><td class="vertb" style="text-align:right;"> 13</td><td class="vertb" style="text-align:right;"> 18</td><td class="vertb" style="text-align:right;"> 42</td><td class="vertb" style="text-align:right;"> 32</td></tr>
<tr><td class="nob" style="text-align:center;"> Length of </td><td class="rightb" rowspan="2" style="vertical-align:middle;"> <a href="images/$lbrace.png"><img src="images/$lbrace.png" class="middle" style="height:5ex; width:0.7em" alt="brace" /></a></td><td class="nob" style="text-align:center;"> F<sub>1</sub> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> </td></tr>
<tr><td class="nob" style="text-align:center;"> ulna </td><td class="nob" style="text-align:center;"> B.C. </td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 3</td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 4</td><td class="vertb" style="text-align:right;"> 4</td><td class="vertb" style="text-align:right;"> 12</td><td class="vertb" style="text-align:right;"> 11</td><td class="vertb" style="text-align:right;"> 20</td><td class="vertb" style="text-align:right;"> 26</td><td class="vertb" style="text-align:right;"> 17</td><td class="vertb" style="text-align:right;"> 19</td></tr>
</table>
<p class="cenhead"><i>same table continued</i></p>
<table class="allbctr" summary="Results of MacDowell" title="Results of MacDowell">
<tr><td class="allb" colspan="2"> CHARACTER </td><td class="allb" style="text-align:right;"> GENERATION </td><td class="allb" style="text-align:right;"> 6</td><td class="allb" style="text-align:right;"> 7</td><td class="allb" style="text-align:right;"> 8</td><td class="allb" style="text-align:right;"> 9</td><td class="allb" style="text-align:right;"> 10</td><td class="allb" style="text-align:right;"> 11</td><td class="allb" style="text-align:right;"> 12</td><td class="allb" style="text-align:right;"> 13</td><td class="allb" style="text-align:right;"> 14</td><td class="allb" style="text-align:right;"> 15</td><td class="allb" style="text-align:right;"> 16</td><td class="allb" style="text-align:right;"> 17</td><td class="allb" style="text-align:right;"> 18</td><td class="allb" style="text-align:right;"> 19</td><td class="allb" style="text-align:right;"> 20</td><td class="allb" style="text-align:right;"> 21</td><td class="allb" style="text-align:right;"> 22</td><td class="allb" style="text-align:right;"> 23</td><td class="allb" style="text-align:right;"> 24</td><td class="allb" style="text-align:right;"> 25</td></tr>
<tr><td class="nob" style="text-align:center;"> Length of </td><td class="rightb" rowspan="2" style="vertical-align:middle;"> <a href="images/$lbrace.png"><img src="images/$lbrace.png" class="middle" style="height:5ex; width:0.7em" alt="brace" /></a></td><td class="nob" style="text-align:center;"> F<sub>1</sub> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 8</td><td class="vertb" style="text-align:right;"> 5</td><td class="vertb" style="text-align:right;"> 10</td><td class="vertb" style="text-align:right;"> 7</td><td class="vertb" style="text-align:right;"> 3</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td></tr>
<tr><td class="nob" style="text-align:center;"> skull </td><td class="nob" style="text-align:center;"> B.C. </td><td class="vertb" style="text-align:right;"> 38</td><td class="vertb" style="text-align:right;"> 34</td><td class="vertb" style="text-align:right;"> 16</td><td class="vertb" style="text-align:right;"> 16</td><td class="vertb" style="text-align:right;"> 8</td><td class="vertb" style="text-align:right;"> 4</td><td class="vertb" style="text-align:right;"> 3</td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td></tr>
<tr><td class="nob" style="text-align:center;"> Length of </td><td class="rightb" rowspan="2" style="vertical-align:middle;"> <a href="images/$lbrace.png"><img src="images/$lbrace.png" class="middle" style="height:5ex; width:0.7em" alt="brace" /></a></td><td class="nob" style="text-align:center;"> F<sub>1</sub> </td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 5</td><td class="vertb" style="text-align:right;"> 3</td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> 7</td><td class="vertb" style="text-align:right;"> 3</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 1</td></tr>
<tr><td class="nob" style="text-align:center;"> ulna </td><td class="nob" style="text-align:center;"> B.C. </td><td class="vertb" style="text-align:right;"> 18</td><td class="vertb" style="text-align:right;"> 15</td><td class="vertb" style="text-align:right;"> 12</td><td class="vertb" style="text-align:right;"> 13</td><td class="vertb" style="text-align:right;"> 15</td><td class="vertb" style="text-align:right;"> 11</td><td class="vertb" style="text-align:right;"> 5</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 4</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td></tr>
</table>
<p>He found that the variability was smaller in the first generation than
in the second <!-- Page 172 --><span class="pagenum"><a
name="page172"></a>{172}</span>generation (back cross). This is what is
expected if several factor-differences were involved, because the hybrids
of the first generation are expected to be more uniform in factorial
composition than are those in the second generation which are produced by
recombination of the factors introduced through their grandparents.
Excellent illustrations of the same kinds of results have been found in
Indian corn. As shown in figure 85 the length of the cob in F<sub>1</sub>
is intermediate between the parent types while in F<sub>2</sub> the range
is wider and both of the original types are recovered. East states that
similar relations have been found for 18 characters in corn. Emerson has
recently furnished further illustrations of the same relations in the
length of stalks in beans.</p>
<p><!-- Page 173 --><span class="pagenum"><a name="page173"></a>{173}</span></p>
<div class="figcenter" style="width:31%;">
<a href="images/fig_085.jpg"><img style="width:100%" src="images/fig_085.jpg"
alt="Fig. 85." title="Fig. 85." /></a>
<p class="poem"><span class="sc">Fig.</span> 85. Cross between two
races of Indian corn, one with short cobs and one with long cobs. The
range of variability in F<sub>1</sub> is less than that in
F<sub>2</sub>. (After East.)</p>
</div>
<p><!-- Page 174 --><span class="pagenum"><a name="page174"></a>{174}</span></p>
<p>A similar case is shown by a cross between fantail and common pigeons
(fig. 86). The latter have twelve feathers in the tail, while the
selected race from which the fantails came had between 28 and 38 feathers
in the tail. The F<sub>1</sub> offspring (forty-one individuals) showed
(fig. 87) between 12 and 20 tail feathers, while in F<sub>2</sub> the
numbers varied between 12 and 25. Here one of the grand-parental types
reappears in large numbers, while the extreme of the other grand-parental
type did not reappear (in the counts obtained), although the
F<sub>2</sub> number would probably overlap the lower limits of the race
of fantail grandparents had not a selected (surviving) lot been taken for
the figures given in the table.</p>
<div class="figcenter" style="width:34%;">
<a href="images/fig_086.jpg"><img style="width:100%" src="images/fig_086.jpg"
alt="Fig. 86." title="Fig. 86." /></a>
<p class="poem"><span class="sc">Fig.</span> 86. Cross of pigeon with
normal tail P<sub>1</sub> and fantail P<sub>1</sub>; F<sub>1</sub>,
bird below.</p>
</div>
<p><!-- Page 175 --><span class="pagenum"><a name="page175"></a>{175}</span></p>
<div class="figcenter" style="width:35%;">
<a href="images/fig_087.jpg"><img style="width:100%" src="images/fig_087.jpg"
alt="Fig. 87." title="Fig. 87." /></a>
<p class="poem"><span class="sc">Fig.</span> 87. Cross of normal and
fantail pigeons. (See Fig. 86.) The F<sub>2</sub> range is wider than
that of F<sub>1</sub>. The normal grand-parental type of 12 feathers
was recovered in F<sub>2</sub> but the higher numbers characteristic of
fantails were not recovered.</p>
</div>
<p><!-- Page 176 --><span class="pagenum"><a name="page176"></a>{176}</span></p>
<p>The preceding account attempts to point out how I should prefer to
interpret the problem of selection in the light of the most recent work
on breeding. But I would give a very incomplete account of the whole
situation if I neglected to include some important work which has led
some of my fellow-workers to a very different conclusion.</p>
<div class="figcenter" style="width:31%;">
<a href="images/fig_088.jpg"><img style="width:100%" src="images/fig_088.jpg"
alt="Fig. 88." title="Fig. 88." /></a>
<p class="poem"><span class="sc">Fig.</span> 88. Scheme to show classes
of hooded rats used by Castle. (After Castle.)</p>
</div>
<p>Castle in particular is the champion of a view based on his results
with hooded rats. Starting with individuals which have a narrow black
stripe down the back he selected for a narrower stripe in one direction
and for a <!-- Page 177 --><span class="pagenum"><a
name="page177"></a>{177}</span>broader stripe in the other. As the
diagram shows (fig. 88) Castle has succeeded in producing in one
direction a race in which the dorsal stripe has disappeared and in the
other direction a race in which the black has extended over the back and
sides, leaving only a white mark on the belly. Neither of these extremes
occurs, he believes, in the ordinary hooded race of domesticated rats. In
other words no matter how many of them came under observation the extreme
types of his experiment would not be found.</p>
<p>Castle claims that the factor for hoodedness must be a single
Mendelian unit, because if hooded rats are crossed to wild gray rats with
uniform coat and their offspring are inbred there are produced in
F<sub>2</sub> three uniform rats to one hooded rat. Castle advances the
hypothesis that factors—by which he means Mendelian
factors—may themselves vary in much the same way as do the
characters that they stand for. He argues, in so many words, that since
we judge a factor by the kind of character it produces, when the
character varies the factor that stands for it may have changed. <!--
Page 178 --><span class="pagenum"><a name="page178"></a>{178}</span></p>
<p>As early as 1903 Cuénot had carried out experiments with spotted mice
similar to those of Castle with rats. Cuénot found that spotted crossed
to uniform coat color gave in F<sub>2</sub> a ratio of three uniform to
one spotted, yet selection of those spotted mice with more white in their
coat produced mice in successive generations that had more and more
white. Conversely Cuénot showed that selection of those spotted mice that
had more color in their coat produced mice with more and more color and
less white. Cuénot does not however bring up in this connection the
question as to how selection in these spotted mice brings about its
results.</p>
<p>Without attempting to discuss these results at the length that they
deserve let me briefly state why I think Castle's evidence fails to
establish his conclusion.</p>
<p>In the first place one of the premises may be wrong. The three to one
ratio in F<sub>2</sub> by no means proves that all conditions of
hoodedness are due to one factor. The result shows at most that one
factor that gives the hooded types is a simple Mendelian factor. The
changes in this type may be caused by modifying factors <!-- Page 179
--><span class="pagenum"><a name="page179"></a>{179}</span>that can show
an effect only when hoodedness is itself present. That this is not an
imaginary objection but a real one is shown by an experiment that Castle
himself made which furnishes the ground for the second objection.</p>
<p>Second. If the factor has really changed its potency, then if a very
dark individual from one end of the series is crossed to a wild rat and
the second generation raised we should expect that the hooded
F<sub>2</sub> rats would all be dark like their dark grandparent. When
Castle made this test he found that there were many grades of hooded rats
in the F<sub>2</sub> progeny. They were darker, it is true, as a group
than were the original hooded group at the beginning of the selection
experiment, but they gave many intermediate grades. Castle attempts to
explain this by the assumption that the factor made pure by selection
became contaminated by its normal allelomorph in the F<sub>1</sub>
parent, but not only does this assumption appear to beg the whole
question, but it is in flat contradiction with what we have observed in
hundreds of Mendelian cases where no evidence for such a contamination
exists. <!-- Page 180 --><span class="pagenum"><a
name="page180"></a>{180}</span></p>
<p>Later Castle crossed some of the extracted rats of average grade
(3.01) from the plus series to the same wild race and got F<sub>2</sub>
hooded rats from this cross. These F<sub>2</sub> hooded rats did not
further approach the ordinary range but were nearer the extreme selected
plus hooded rats (3.33) than were the F<sub>2</sub>'s extracted from the
first cross (2.59). Castle concludes from this that multiple factors can
not account for the result. As a matter of fact, Castle's evidence <i>as
published</i> does not establish his conclusion because the wild rats
used in the second experiment may have carried plus modifiers. This could
only be determined by suitable tests which Castle does not furnish. This
is the crucial point, without which the evidence carries no
conviction.</p>
<p>Furthermore, from Castle's point of view, these latest results would
seem to increase the difficulty of interpretation of his first
F<sub>2</sub> extracted cross, and it is now the first result that calls
for explanation if one accepts his later conclusion.</p>
<p>These and other objections that might be taken up show, I think, that
Castle's <!-- Page 181 --><span class="pagenum"><a
name="page181"></a>{181}</span>experiment with hooded rats fails entirely
to establish his contention of change in potency of the germ or of
contamination of factors, while on the contrary they are in entire accord
with the view that he is dealing with a case of modifying factors.</p>
<div class="figcenter" style="width:31%;">
<a href="images/fig_089.jpg"><img style="width:100%" src="images/fig_089.jpg"
alt="Fig. 89." title="Fig. 89." /></a>
<p class="poem"><span class="sc">Fig.</span> 89. Races of Paramecium.
(After Jennings.)</p>
</div>
<p>Equally important are the results that Jennings has obtained with
certain protozoa. Paramecium multiplies by dividing across in the <!--
Page 182 --><span class="pagenum"><a
name="page182"></a>{182}</span>middle, each half replacing its lacking
part. Both the small nucleus (micronucleus) and the large nucleus
(macronucleus) divide at each division of the body. Jennings found that
while individuals descended from a single paramecium vary in size (fig.
89), yet the population from a large individual is the same as the
population derived from a small individual. In other words, selection
produces no result and the probable explanation is, of course, that the
different sizes of individuals are due to the environment, while the
constancy of the type is genetic. Jennings found a number of races of
paramecium of different sizes living under natural conditions. The
largest individual of a small race might overlap the smallest individual
of other larger races (fig. 89); nevertheless each kind reproduced its
particular race. The results are like those of Johannsen in a general
way, but differ in that reproduction takes place in paramecium by direct
division instead of through self-fertilization as in beans, and also in
that the paramecia were probably not homozygous. Since, however, so far
as known no "reduction" takes place in <!-- Page 183 --><span
class="pagenum"><a name="page183"></a>{183}</span>paramecium at each
division, the genetic composition of parent and offspring should be the
same. Whether pseudo-parthenogenesis that Woodruff and Erdmann have found
occurring in paramecium at intervals involves a redistribution of the
hereditary factors is not clear. Jennings's evidence seems incompatible
with such a view.</p>
<div class="figcenter" style="width:32%;">
<a href="images/fig_090.jpg"><img style="width:100%" src="images/fig_090.jpg"
alt="Fig. 90." title="Fig. 90." /></a>
<p class="poem"><span class="sc">Fig.</span> 90. Stylonychia showing
division into two. (After Stein.)</p>
</div>
<p>More recently one of Jennings's students, Middleton, has made a
careful series of selection experiments with Stylonychia (fig. 90) in
which he selected for lines showing more rapid <!-- Page 184 --><span
class="pagenum"><a name="page184"></a>{184}</span>or slower rates of
division. His observations seem to show that his selection separated two
such lines that came from the same original stock. The rapidity of the
effects of selection seems to preclude the explanation that
pseudo-parthenogenesis has complicated the results. Nevertheless, the
results are of such a kind as to suggest that they were due to selection
of vegetative (somatic) differences and that no genetic change of factors
was involved, for his conclusion that the rapidity with which the effects
gained by long selection might be suddenly reversed when selection was
reversed is hardly consistent with an interpretation of the results based
on changes in the "potencies" of the factors present.</p>
<p>Equally striking are the interesting experiments that Jennings has
recently carried out with Difflugia (fig. 91). This protozoon secretes a
shell about itself which has a characteristic shape, and often carries
spines. The opening at one end of the shell through which the protoplasm
protrudes to make the pseudopodia is surrounded by a rim having a
characteristic pattern. The protoplasm contains <!-- Page 185 --><span
class="pagenum"><a name="page185"></a>{185}</span>several nuclei and in
addition there is scattered material or particles called chromidia that
are supposed to be chromatic in nature and related to the material of the
nuclei, possibly by direct interchange.</p>
<div class="figcenter" style="width:19%;">
<a href="images/fig_091.jpg"><img style="width:100%" src="images/fig_091.jpg"
alt="Fig. 91." title="Fig. 91." /></a>
<p class="poem"><span class="sc">Fig.</span> 91. Difflugia Corona.
(After Cash.)</p>
</div>
<p>When Difflugia divides, part of the protoplasm protrudes from the
opening and a new shell is secreted about this mass which becomes a
daughter individual. The behavior of the nucleus and of the chromidia at
this time is obscure, but there is some evidence that their materials may
be irregularly distributed <!-- Page 186 --><span class="pagenum"><a
name="page186"></a>{186}</span>between parent and offspring. If this is
correct, and if in the protozoa the chromatin has the same influence that
it seems to have in higher animals, the mode of reproduction in Difflugia
would be expected to give little more than random sampling of the germ
plasm.</p>
<div class="figcenter" style="width:16%;">
<a href="images/fig_092.jpg"><img style="width:100%" src="images/fig_092.jpg"
alt="Fig. 92." title="Fig. 92." /></a>
<p class="poem"><span class="sc">Fig.</span> 92. Races of Difflugia.
(After Leidy.)</p>
</div>
<p>Jennings was able by means of selection to get from the descendants of
one original individual a number of different types that themselves bred
true, except in so far as selection could affect another change in them.
In this connection it is interesting to note that Leidy <!-- Page 187
--><span class="pagenum"><a name="page187"></a>{187}</span>has published
figures of Difflugia (fig. 92) that show that a great many "types" exist.
If through sexual union (a process that occurs in Difflugia) the germ
plasm (chromatin) of these wild types has in times past been recombined,
then selection would be expected to separate certain types again, if, at
division, irregular sampling of the germ plasm takes place. Until these
points are settled the bearing of these important experiments of Jennings
on the general problem of selection is uncertain.</p>
<p class="cenhead"><span class="sc">How Does Natural Selection Influence the Course of Evolution?</span></p>
<p>The question still remains: Does selection play any rôle in evolution,
and, if so, in what sense? Does the elimination of the unfit influence
the course of evolution, except in the negative sense of leaving more
room for the fit? There is something further to be said in this
connection, although opinions may differ as to whether the following
interpretation of the term "natural selection" is the only possible
one.</p>
<p><!-- Page 188 --><span class="pagenum"><a name="page188"></a>{188}</span></p>
<div class="figcenter" style="width:25%;">
<a href="images/fig_093.jpg"><img style="width:100%" src="images/fig_093.jpg"
alt="Fig. 93." title="Fig. 93." /></a>
<p class="poem"><span class="sc">Fig.</span> 93. Evolution of
elephant's skulls. (After Dendy.)</p>
</div>
<p>If through a mutation a character appears that is neither advantageous
nor disadvantageous, but indifferent, the chance that it may become
established in the race is extremely small, although by good luck such a
thing may occur rarely. It makes no difference whether the character in
question is a dominant or a <!-- Page 189 --><span class="pagenum"><a
name="page189"></a>{189}</span>recessive one, the chance of its becoming
established is exactly the same. If through a mutation a character
appears that has an <i>injurious</i> effect, however slight this may be,
it has practically no chance of becoming established.</p>
<div class="figcenter" style="width:34%;">
<a href="images/fig_094.jpg"><img style="width:100%" src="images/fig_094.jpg"
alt="Fig. 94." title="Fig. 94." /></a>
<p class="poem"><span class="sc">Fig.</span> 94. Evolution of
elephant's trunk. (After Lull.)</p>
</div>
<p>If through a mutation a character appears that has a <i>beneficial</i>
influence on the individual, the chance that the individual will survive
is increased, not only for itself, but for all of its <!-- Page 190
--><span class="pagenum"><a name="page190"></a>{190}</span>descendants
that come to inherit this character. It is this increase in the number of
individuals possessing a particular character, that might have an
influence on the course of evolution. This gives a better chance for
improvement by several successive steps; but not because the species is
more likely to mutate again in the same direction. An imaginary example
will illustrate how this happens: When elephants had trunks less than a
foot long, the chance of getting trunks more than one foot long was in
proportion to the length of trunks already present and to the number of
individuals; but increment in trunk length is no more likely to occur
from an animal having a trunk more than one foot long than from an animal
with a shorter trunk.</p>
<p>The case is analogous to tossing pennies. At any stage in the game the
chance of accumulating a hundred heads is in proportion to the number of
heads already obtained, and to the number of throws still to be made. But
the number of heads obtained has no influence on the number of heads that
will appear in the next throw. <!-- Page 191 --><span class="pagenum"><a
name="page191"></a>{191}</span></p>
<div class="figcenter" style="width:34%;">
<a href="images/fig_095.jpg"><img style="width:100%" src="images/fig_095.jpg"
alt="Fig. 95." title="Fig. 95." /></a>
<p class="poem"><span class="sc">Fig.</span> 95. Evolution of
elephant's trunk: above Maeritherium, in the middle Tetrabelodon (After
Lancaster); below African elephants (After Gambier Bolton).</p>
</div>
<p><!-- Page 192 --><span class="pagenum"><a name="page192"></a>{192}</span></p>
<p>Owing then to this property of the germ plasm to duplicate itself in a
large number of samples not only is an opportunity furnished to an
advantageous variation to become extensively multiplied, but the presence
of a large number of individuals of a given sort prejudices the probable
future result.</p>
<p>The question may be raised as to whether it is desirable to call
selection a <i>creative</i> process. There are so many supernatural and
mystical implications that hang around the term creative that one can not
be too careful in stating in what sense the term is to be used. If by
creative is meant that something is made out of nothing, then of course
there is no need for the scientist to try to answer such a question. But
if by a creative process is meant that something is made out of something
else, then there are two alternatives to be reckoned with.</p>
<p>First, if it were true that selection of an individual of a certain
kind determines that new variations in the same direction occur as a
consequence of the selection, then selection would certainly be creative.
How this could occur might be quite unintelligible, but of course it <!--
Page 193 --><span class="pagenum"><a name="page193"></a>{193}</span>might
be claimed that the point is not whether we can explain how creation
takes place, but whether we can get verifiable evidence that such a kind
of thing happens. This possibility is disposed of by the fact that there
is no evidence that selection determines the direction in which variation
occurs.</p>
<p>Second, if you mean by a creative process that by picking out a
certain kind of individual and multiplying its numbers a better chance is
furnished that a certain end result will be obtained, such a process may
be said to be creative. This is, I think, the proper use of the term
creative in a mechanistic sense.</p>
<p class="cenhead"><span class="sc">Conclusions</span></p>
<p>In reviewing the evidence relating to selection I have tried to handle
the problem as objectively as I could.</p>
<p>The evidence shows clearly that the characters of wild animals and
plants, as well as those of domesticated races, are inherited both in the
wild and in the domesticated forms according to Mendel's Law.</p>
<p>The causes of the mutations that give rise <!-- Page 194 --><span
class="pagenum"><a name="page194"></a>{194}</span>to new characters we do
not know, although we have no reason for supposing that they are due to
other than natural processes.</p>
<p>Evolution has taken place by the incorporation into the race of those
mutations that are beneficial to the life and reproduction of the
organism. Natural selection as here defined means both the increase in
the number of individuals that results after a beneficial mutation has
occurred (owing to the ability of living matter to propagate) and also
that this preponderance of certain kinds of individuals in a population
makes some further results more probable than others. More than this,
natural selection can not mean, if factors are fixed and are not changed
by selection.</p>
<p><br style="clear:both" /></p>
<hr class="full" />
<p><!-- Page 195 --><span class="pagenum"><a name="page195"></a>{195}</span></p>
<div class="poem">
<div class="stanza">
<p class="i6"><b>INDEX</b></p>
</div>
<div class="stanza">
<p>Abnormal abdomen <a href="#page109">109</a></p>
<p>Abraxas <a href="#page78">78</a>-<a href="#page81">81</a></p>
<p>Allantois <a href="#page17">17</a></p>
<p>Allelomorphs <a href="#page83">83</a>-<a href="#page84">84</a></p>
<p>Altenburg <a href="#page112">112</a></p>
<p>Amnion <a href="#page16">16</a>-<a href="#page17">17</a></p>
<p>Andalusian fowl <a href="#page45">45</a>, <a href="#page46">46</a></p>
<p>Annelids <a href="#page22">22</a></p>
<p>Antlered wing <a href="#page111">111</a></p>
<p>Apterous wing <a href="#page11">11</a></p>
<p>Arc wing <a href="#page111">111</a></p>
<p>Aristae <a href="#page104">104</a></p>
</div>
<div class="stanza">
<p>Bar eye <a href="#page67">67</a>, <a href="#page108">108</a>, <a href="#page169">169</a></p>
<p>Bateson <a href="#page18">18</a>, <a href="#page34">34</a>, <a href="#page36">36</a></p>
<p>Beaded wing <a href="#page11">11</a>, <a href="#page115">115</a></p>
<p>Beans <a href="#page147">147</a>-<a href="#page149">149</a>, <a href="#page157">157</a></p>
<p>Belgian hare <a href="#page171">171</a></p>
<p>Bent wing <a href="#page116">116</a></p>
<p>Bergson <a href="#page30">30</a>, <a href="#page31">31</a></p>
<p>Bildungstrieb <a href="#page34">34</a></p>
<p>Biogenetic law <a href="#page15">15</a>, <a href="#page18">18</a>, <a href="#page19">19</a>, <a href="#page21">21</a></p>
<p>Biometricians <a href="#page156">156</a></p>
<p>Bird <a href="#page21">21</a>, <a href="#page23">23</a></p>
<p>Bithorax <a href="#page65">65</a>, <a href="#page112">112</a>, <a href="#page113">113</a></p>
<p>Black body color <a href="#page111">111</a>, <a href="#page133">133</a></p>
<p>Blakeslee <a href="#page152">152</a></p>
<p>Bridges <a href="#page114">114</a>, <a href="#page143">143</a>, <a href="#page163">163</a></p>
<p>British Association <a href="#page36">36</a></p>
<p>Brünn <a href="#page40">40</a></p>
<p>Buff eye color <a href="#page109">109</a></p>
<p>Bufon <a href="#page27">27</a></p>
</div>
<div class="stanza">
<p>Castle <a href="#page176">176</a>-<a href="#page180">180</a></p>
<p>Cat <a href="#page33">33</a></p>
<p>Cell <a href="#page90">90</a>, <a href="#page91">91</a></p>
<p>Chance variations <a href="#page37">37</a></p>
<p>Chick <a href="#page16">16</a>, <a href="#page17">17</a>, <a href="#page20">20</a></p>
<p>Chromatin <a href="#page184">184</a></p>
<p>Chromosome group of Drosophila <a href="#page102">102</a></p>
<p>Chromosomes <a href="#page91">91</a>, <a href="#page95">95</a>, <a href="#page96">96</a>, <a href="#page98">98</a>, <a href="#page130">130</a>, <a href="#page131">131</a>, <a href="#page132">132</a></p>
<p>Cleavage <a href="#page21">21</a>, <a href="#page22">22</a>, <a href="#page94">94</a></p>
<p>Clover butterfly <a href="#page62">62</a></p>
<p>Club wing <a href="#page69">69</a>, <a href="#page70">70</a>, <a href="#page108">108</a></p>
<p>Colias philodice <a href="#page62">62</a></p>
<p>Color blindness <a href="#page77">77</a>, <a href="#page125">125</a></p>
<p>Comb of Drosophila <a href="#page103">103</a></p>
<p>Combs of fowls <a href="#page33">33</a>, <a href="#page54">54</a></p>
<p>Comparative anatomy <a href="#page7">7</a>, <a href="#page8">8</a>, <a href="#page9">9</a>, <a href="#page14">14</a></p>
<p>Corn <a href="#page150">150</a>, <a href="#page153">153</a>, <a href="#page172">172</a></p>
<p>Correns <a href="#page41">41</a></p>
<p>Cosmogonies <a href="#page27">27</a></p>
<p>Cream eye color <a href="#page163">163</a>, <a href="#page164">164</a></p>
<p>Crepidula <a href="#page22">22</a></p>
<p>Criss-cross inheritance <a href="#page78">78</a></p>
<p>Crossing over <a href="#page131">131</a>-<a href="#page133">133</a></p>
<p>Cuénot <a href="#page178">178</a></p>
<p>Curled wing <a href="#page115">115</a></p>
<p>Curved wing <a href="#page111">111</a></p>
<p>Curve of probability <a href="#page149">149</a></p>
<p>Cut wing <a href="#page11">11</a>, <a href="#page104">104</a></p>
</div>
<div class="stanza">
<p>Dachs legs <a href="#page112">112</a></p>
<p>Dahlgren <a href="#page62">62</a></p>
<p>Darwin <a href="#page15">15</a>, <a href="#page24">24</a>, <a href="#page28">28</a>, <a href="#page32">32</a>, <a href="#page35">35</a>-<a href="#page37">37</a>, <a href="#page64">64</a>, <a href="#page145">145</a>, <a href="#page146">146</a>, <a href="#page152">152</a>, <a href="#page154">154</a>-<a href="#page156">156</a></p>
<p>Dendy <a href="#page188">188</a></p>
<p>De Vries <a href="#page18">18</a>, <a href="#page147">147</a>, <a href="#page156">156</a></p>
<p>Dexter <a href="#page170">170</a></p>
<p>Dichaete <a href="#page114">114</a></p>
<p>Difflugia <a href="#page184">184</a>-<a href="#page187">187</a></p>
<p>Discontinuous variation <a href="#page13">13</a></p>
<p>Disuse <a href="#page31">31</a></p>
<p>Drosophila ampelophila <a href="#page10">10</a>, <a href="#page12">12</a>, <a href="#page13">13</a>, <a href="#page48">48</a>-<a href="#page50">50</a>, <a href="#page60">60</a>, <a href="#page75">75</a>, <a href="#page84">84</a>, <a href="#page85">85</a>, <a href="#page93">93</a>, <a href="#page100">100</a>, <a href="#page103">103</a>, <a href="#page119">119</a>, <a href="#page155">155</a>, <a href="#page162">162</a>, <a href="#page169">169</a></p>
<!-- Page 196 --><span class="pagenum"><a name="page196"></a>{196}</span>
<p>Drosophila repleta <a href="#page76">76</a></p>
<p>Duplication of legs <a href="#page109">109</a></p>
<p>Dwarf <a href="#page114">114</a></p>
</div>
<div class="stanza">
<p>East <a href="#page170">170</a>, <a href="#page172">172</a></p>
<p>Ebony <a href="#page50">50</a>, <a href="#page55">55</a>, <a href="#page56">56</a>, <a href="#page115">115</a></p>
<p>Egg <a href="#page91">91</a>, <a href="#page94">94</a></p>
<p>Elephant <a href="#page191">191</a></p>
<p>Elephants' skulls <a href="#page188">188</a></p>
<p>Elephants' trunks <a href="#page190">190</a></p>
<p>Embryology <a href="#page13">13</a>-<a href="#page23">23</a></p>
<p>Emerson <a href="#page172">172</a></p>
<p>Environment <a href="#page27">27</a></p>
<p>Eosin eye color <a href="#page61">61</a>, <a href="#page107">107</a>, <a href="#page163">163</a></p>
<p>Erdmann <a href="#page183">183</a></p>
<p>Evolution Creatrice <a href="#page30">30</a></p>
<p>Evolution—three kinds of <a href="#page1">1</a>, <a href="#page2">2</a>, <a href="#page4">4</a></p>
<p>Eye color <a href="#page13">13</a></p>
<p>Eyeless <a href="#page66">66</a>, <a href="#page115">115</a></p>
</div>
<div class="stanza">
<p>Factorial theory <a href="#page89">89</a></p>
<p>Factors of Drosophila <a href="#page143">143</a></p>
<p>Fantails <a href="#page172">172</a>, <a href="#page175">175</a></p>
<p>Fertilization <a href="#page91">91</a></p>
<p>Fish <a href="#page16">16</a>, <a href="#page20">20</a>, <a href="#page21">21</a></p>
<p>Flatworms <a href="#page22">22</a></p>
<p>Fluctuations <a href="#page12">12</a></p>
<p>Forked bristles <a href="#page106">106</a></p>
<p>Fowl <a href="#page77">77</a></p>
<p>Fused veins <a href="#page107">107</a>, <a href="#page108">108</a></p>
</div>
<div class="stanza">
<p>Galton <a href="#page154">154</a></p>
<p>Geneticist <a href="#page26">26</a></p>
<p>Germ-plasm <a href="#page142">142</a></p>
<p>Geoffroy St. Hilaire <a href="#page27">27</a></p>
<p>Giant <a href="#page114">114</a></p>
<p>Gill-slits <a href="#page20">20</a>, <a href="#page21">21</a>, <a href="#page23">23</a></p>
<p>Groups I, II, III, IV <a href="#page100">100</a>-<a href="#page118">118</a></p>
</div>
<div class="stanza">
<p>Haeckel <a href="#page15">15</a></p>
<p>Haemophilia <a href="#page77">77</a></p>
<p>Heliotropism <a href="#page106">106</a>, <a href="#page107">107</a></p>
<p>Himalyan rabbits <a href="#page83">83</a></p>
<p>History <a href="#page1">1</a>, <a href="#page6">6</a></p>
<p>Hoge <a href="#page66">66</a></p>
<p>Horse, evolution of <a href="#page6">6</a></p>
</div>
<div class="stanza">
<p>Indian corn <a href="#page172">172</a>, <a href="#page173">173</a></p>
<p>Interference <a href="#page137">137</a>, <a href="#page138">138</a></p>
</div>
<div class="stanza">
<p>Janssens <a href="#page132">132</a></p>
<p>Jaunty wing <a href="#page111">111</a></p>
<p>Jennings <a href="#page161">161</a>, <a href="#page181">181</a>-<a href="#page184">184</a>, <a href="#page186">186</a></p>
<p>Johannsen <a href="#page156">156</a>, <a href="#page157">157</a>, <a href="#page159">159</a>-<a href="#page161">161</a>, <a href="#page166">166</a>, <a href="#page182">182</a></p>
</div>
<div class="stanza">
<p>Lamarck <a href="#page31">31</a>-<a href="#page34">34</a></p>
<p>Langshan <a href="#page77">77</a></p>
<p>Leaves <a href="#page147">147</a></p>
<p>Leidy <a href="#page186">186</a></p>
<p>Lethal <a href="#page105">105</a></p>
<p>Linkage groups <a href="#page103">103</a></p>
<p>Lizard <a href="#page23">23</a></p>
<p>Localization of factors <a href="#page118">118</a></p>
</div>
<div class="stanza">
<p>MacDowell <a href="#page155">155</a>, <a href="#page170">170</a>, <a href="#page171">171</a></p>
<p>Macritherium <a href="#page191">191</a></p>
<p>Mammal <a href="#page16">16</a>, <a href="#page21">21</a>, <a href="#page23">23</a></p>
<p>Man <a href="#page20">20</a>, <a href="#page77">77</a>, <a href="#page125">125</a>, <a href="#page126">126</a></p>
<p>Map of Chromosomes <a href="#page136">136</a></p>
<p>Maroon eye color <a href="#page114">114</a></p>
<p>Mendel <a href="#page40">40</a>, <a href="#page41">41</a>, <a href="#page52">52</a>, <a href="#page89">89</a></p>
<p>Mendelian heredity <a href="#page39">39</a></p>
<p>Mendel's law <a href="#page41">41</a>-<a href="#page59">59</a>, <a href="#page64">64</a>, <a href="#page124">124</a></p>
<p>Mendel's second law <a href="#page52">52</a></p>
<p>Mesenchyme cells <a href="#page22">22</a></p>
<p>Mesoderm cells <a href="#page22">22</a></p>
<p>Metaphysician <a href="#page30">30</a></p>
<p>Mice <a href="#page33">33</a>, <a href="#page178">178</a></p>
<p>Middleton <a href="#page183">183</a></p>
<p>Miniature wing <a href="#page108">108</a></p>
<p>Mirabilis <a href="#page42">42</a></p>
<p>Modifiers <a href="#page163">163</a>, <a href="#page164">164</a>, <a href="#page170">170</a>, <a href="#page171">171</a></p>
<p>Molluscs <a href="#page22">22</a></p>
<p>Mouse <a href="#page83">83</a></p>
<p>Muller <a href="#page112">112</a>, <a href="#page167">167</a></p>
<p>Mutations <a href="#page35">35</a>, <a href="#page39">39</a>, <a href="#page84">84</a></p>
</div>
<div class="stanza">
<p>Nägeli <a href="#page34">34</a>, <a href="#page35">35</a></p>
<p>Natural Selection <a href="#page36">36</a>, <a href="#page145">145</a>, <a href="#page146">146</a>, <a href="#page187">187</a>-<a href="#page194">194</a></p>
<!-- Page 197 --><span class="pagenum"><a name="page197"></a>{197}</span>
<p>Nisus formativus <a href="#page34">34</a></p>
<p>Non-disjunction <a href="#page139">139</a>-<a href="#page142">142</a></p>
<p>Notch wing <a href="#page104">104</a>-<a href="#page106">106</a></p>
<p>Nucleus <a href="#page91">91</a></p>
</div>
<div class="stanza">
<p>Origin of Species <a href="#page35">35</a>, <a href="#page145">145</a></p>
<p>Orthogenesis <a href="#page34">34</a></p>
</div>
<div class="stanza">
<p>Paleontology <a href="#page24">24</a>-<a href="#page27">27</a></p>
<p>Papilio polytes <a href="#page63">63</a></p>
<p>Papilio turnus <a href="#page63">63</a></p>
<p>Paramecium <a href="#page181">181</a>, <a href="#page182">182</a></p>
<p>Paratettix <a href="#page81">81</a></p>
<p>Peach eye color <a href="#page114">114</a></p>
<p>Pea comb <a href="#page54">54</a></p>
<p>Pearl <a href="#page161">161</a></p>
<p>Peas <a href="#page47">47</a></p>
<p>Pigeons <a href="#page172">172</a>, <a href="#page174">174</a>, <a href="#page175">175</a></p>
<p>Pink eye color <a href="#page114">114</a>, <a href="#page115">115</a></p>
<p>Planarian <a href="#page22">22</a></p>
<p>Plymouth Rock <a href="#page77">77</a></p>
<p>Podarke <a href="#page22">22</a></p>
<p>Polar bodies <a href="#page126">126</a></p>
<p>Pole arms <a href="#page5">5</a></p>
<p>Protozoa <a href="#page181">181</a></p>
<p>Pseudo-parthenogenesis <a href="#page183">183</a></p>
<p>Purple eye color <a href="#page109">109</a></p>
<p>Purpose <a href="#page4">4</a></p>
</div>
<div class="stanza">
<p>Rabbits <a href="#page83">83</a>, <a href="#page170">170</a></p>
<p>Rats <a href="#page176">176</a>-<a href="#page180">180</a></p>
<p>Reduction division <a href="#page182">182</a></p>
<p>Reproductive cells <a href="#page96">96</a></p>
<p>Ruby eye color <a href="#page106">106</a></p>
<p>Rudimentary organ <a href="#page116">116</a></p>
<p>Rudimentary wing <a href="#page70">70</a>, <a href="#page71">71</a>, <a href="#page107">107</a></p>
</div>
<div class="stanza">
<p>Sable body color <a href="#page107">107</a></p>
<p>Science definition of <a href="#page6">6</a></p>
<p>Segregation <a href="#page41">41</a></p>
<p>Selenka <a href="#page94">94</a></p>
<p>Sepia eye color <a href="#page13">13</a>, <a href="#page114">114</a></p>
<p>Sex chromosomes <a href="#page118">118</a></p>
<p>Sex linked inheritance <a href="#page75">75</a>, <a href="#page118">118</a>-<a href="#page130">130</a></p>
<p>Sexual dimorphism <a href="#page62">62</a></p>
<p>Sheep <a href="#page33">33</a></p>
<p>Single comb <a href="#page54">54</a></p>
<p>Sooty body color <a href="#page50">50</a>, <a href="#page114">114</a>, <a href="#page115">115</a></p>
<p>Speck <a href="#page68">68</a>, <a href="#page69">69</a>, <a href="#page111">111</a></p>
<p>Spencer <a href="#page145">145</a></p>
<p>Spermatozoön <a href="#page91">91</a>, <a href="#page98">98</a></p>
<p>Stars, evolution of <a href="#page6">6</a></p>
<p>St. Hilaire <a href="#page27">27</a>-<a href="#page30">30</a></p>
<p>Strap wing <a href="#page110">110</a>, <a href="#page111">111</a></p>
<p>Stumpy wing <a href="#page11">11</a></p>
<p>Sturtevant <a href="#page76">76</a>, <a href="#page143">143</a></p>
<p>Stylonychia <a href="#page183">183</a></p>
<p>Survival of the fittest <a href="#page146">146</a></p>
<p>Systematist <a href="#page85">85</a></p>
</div>
<div class="stanza">
<p>Tails <a href="#page33">33</a></p>
<p>Tan flies <a href="#page106">106</a>, <a href="#page107">107</a></p>
<p>Tetrabelodon <a href="#page191">191</a></p>
<p>Trefoil <a href="#page111">111</a></p>
<p>Truncate wing <a href="#page111">111</a>, <a href="#page112">112</a>, <a href="#page167">167</a>, <a href="#page168">168</a></p>
</div>
<div class="stanza">
<p>Unfolding principle <a href="#page34">34</a></p>
<p>Unio <a href="#page22">22</a></p>
<p>Unit character <a href="#page74">74</a>, <a href="#page75">75</a></p>
<p>Use <a href="#page31">31</a></p>
</div>
<div class="stanza">
<p>Variation discontinuous <a href="#page13">13</a></p>
<p>Vermilion eye color <a href="#page108">108</a>, <a href="#page163">163</a></p>
<p>Vestigial wing <a href="#page11">11</a>, <a href="#page55">55</a>, <a href="#page56">56</a>, <a href="#page109">109</a>, <a href="#page133">133</a></p>
<p>Vital force <a href="#page34">34</a></p>
</div>
<div class="stanza">
<p>Wallace <a href="#page36">36</a></p>
<p>Walnut comb <a href="#page54">54</a></p>
<p>Weismann <a href="#page17">17</a>, <a href="#page31">31</a>-<a href="#page33">33</a></p>
<p>Wilson, E. B. <a href="#page125">125</a></p>
<p>Wingless <a href="#page67">67</a></p>
<p>Winiwarter <a href="#page126">126</a></p>
<p>White eye color <a href="#page13">13</a>, <a href="#page75">75</a>, <a href="#page119">119</a>-<a href="#page130">130</a></p>
<p>Whiting eye color <a href="#page163">163</a>, <a href="#page164">164</a></p>
<p>Woodruff <a href="#page183">183</a></p>
</div>
<div class="stanza">
<p>Yellow body color <a href="#page108">108</a>, <a href="#page133">133</a></p>
<p>Yolk sac <a href="#page16">16</a>, <a href="#page17">17</a></p>
</div>
<div class="stanza">
<p>Zeleny <a href="#page169">169</a></p>
</div>
</div>
<pre>
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