68 Life and Death, Heredity and Evolution 



I wish to emphasize that this is a picture of the present 

 situation of affairs; that it gives the prevailing theory of 

 these matters. If you will read the address of President 

 Pearl of the American Society of Naturalists, published in 

 the American Naturalist (Feb. 1917), you will find that this 

 is the theory which he presents. I have no doubt that if 

 there are any experimental workers in heredity in this audi- 

 ence, this is the theory which they maintain. 



Now, -as I remarked at the end of the last chapter, this 

 situation of affairs presents great difficulties for the theory 

 of evolution, leading indeed logically to the conclusion 

 that evolution does not occur. And even when we add to 

 these results the observed cases of sudden change of charac- 

 ters, which are called mutations, it becomes extremely diffi- 

 cult to see how evolution can occur. For most if not all of 

 these mutations, as is well known, consist in defects and 

 losses ; and it is difficult to believe that evolution has occurred 

 by repeated losses, although attempts have been made to 

 maintain even that paradoxical theory (Bateson, 1914; 

 Davenport, 1916). 



Moreover, there are certain facts about organisms which 

 it seems impossible to explain by the appearance of sudden 

 extensive mutations. In the organisms that we have been 

 describing we find that the hereditary differences between 

 the races are as minute as can possibly be detected by the 

 most refined methods ; they run down to the very limits of 

 visibility with the microscope. Such are the differences be- 

 tween the diverse races of Paramecium (Figure 22); such 

 those between the races of Difflugia (Figures 19 to 21). It 

 is clear that such differences cannot have been produced by 

 "saltations" by mutations of large extent. And the same 

 condition of affairs is found in higher organisms ; the differ- 

 ences between Jordan's 200 races of Draba verna were so 



