22 EVOLUTION AND GENETICS 



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 meni])ers have 

 a|)peared. 



Suppose that evolution "in the open" had taken 

 place in the same way, by means of discontinuous 

 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? 



No one familiar with the entire evidence will doubt 

 for a moment that these four hundred races of Dro- 

 sojihila belong to the same species and have had a 

 common origin, for while they may differ mainly in 

 one thino; they are extremely alike in a hundred 

 other things, and in the general relation of the parts 

 to each other. 



It is in this sense that the evidence from compara- 

 tive 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 any j)articular variations in a continuous 

 series. 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 fi'om comparative anatomy. 



