The Modern Idea of Evolution 



19 



Does selection play any role in evolu- 

 tion, and, if so, in what sense? Does 

 the elimination of the unfit influence 

 the course of evolution, except in the 

 •negati\'e 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. 



"If through a mutation a character 

 appears that is neither advantageous 

 or disadvantageous, but indifferent, 

 the chance that it may become estab- 

 lished in a race is extremely small, al- 

 though by good luck such a thing may 

 occur rarely. It makes no difference 

 whether the character in question is a 

 dominant or a recessive one, the chance 

 of its becoming established is exactly 

 the same. If through a mutation a 

 character appears that has an injurious 

 effect, however slight this may be, it 

 has practically no chance of becoming 

 established. 



"If through a mutation a character 

 appears that has a beneficial influence on 

 the individual, the chance that the 

 individual will survive is increased, not 

 only for itself, but for all of its des- 

 cendants that come to inherit this char- 

 acter. It is this increase in the num- 

 ber of individuals, possessing a particular 

 character, that might have an influence 

 on the course of evolution." 



If the word variation be substituted for 

 the word mutation in the preceding three 

 paragraphs, the statement would ap- 

 pear to be a good outline of Darwin's 

 own position. The question of how 

 much Morgan's view differs from Dar- 

 win's is, then, a question of the differ- 

 ence between Darwin's variations and 

 Morgan's mutations. To the reviewer, 

 the difference appears one rather of 

 words than of facts. The mutations 

 in Drosophila, which Morgan de- 

 scribes at some length, are many of 

 them exactly the kind of changes which 

 Darwin described as variations. 



A DIFFERENCE OF WORDS 



If this is a fair statement, then 

 Darwin's work has not been supplanted 



to anything like the extent that is some- 

 times supposed. A decade ago, when 

 the mutationists were young and en- 

 thusiastic, they gave the impression 

 that they were about to make a great 

 change in the status of natural selec- 

 tion. Darwin's variations were chal- 

 lenged as not being inheritable (as a 

 fact , many of them were not) and muta- 

 tions were put forward as the real 

 basis of evolution. After ten years of 

 study, it appers that mutation and 

 variation mean practically the same 

 thing. A lot of mere fluctuations have 

 been thrown out, but the concept of 

 mutations as described by Dr. Morgan, 

 while more definite, is not radically 

 different from that of variations which 

 Darmn recognized. That the two par- 

 ties should now stand so nearly on 

 common ground (whether they recog- 

 nize it or not) argues well for the 

 validity of the ideas they hold. 



Next, does selection of a certain kind 

 of variations lead to the probable 

 appearance of further variations in the 

 same direction''! Dr. Morgan thinks 

 not. The only role of selection is to 

 multiply the numbers of some favorable 

 variation, thus giving it a chance to 

 become established and crowd out the 

 older forms. His conclusion follows: 



"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 Men- 

 del's Law. 



"The causes of the mutations that 

 give rise to new characters we do not 

 know, although we have no reason to 

 suppose that they are due to other than 

 natural processes. 



"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 in- 

 dividuals 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 popu- 

 lation makes some further results more 



