THE GROWTH OF BIOLOGICAL IDEAS 165 



ten to show that Vertebrates evolved from kino: crabs. No 

 limit was set to the free play of the imagination when once 

 the idea of evolution had been accepted. Side by side, how- 

 ever, with much that was valueless— and often curiously inter- 

 mingled with it— went a profound study of the comparative 

 anatomy of animals. So complete, indeed, was this study that 

 no problem of major importance was left for solution in the 

 twentieth century. 



Comparative anatomy alone could not provide insight into 

 the causes of evolution. Help came at last from quite an 

 unexpected quarter. It was the rediscovery of Mendelism in 

 1900 that eventually gave the necessary impetus to studies of 

 evolution. It gradually became apparent that the survival of 

 organisms in the struggle for existence might depend on what 

 Mendelian genes they possessed. Those individuals that had 

 genes determining characters favorable to survival would be 

 automatically selected; the rest would perish and leave few 

 or no offspring. It was seen that in any species a very large 

 set of possible combinations of different genes was available, 

 and on these combinations "natural" or automatic selection 

 would operate: there would be survival of the individuals 

 with the fittest genes. But this was not all; it was found that 

 the genes themselves sometimes undergo sudden changes. 

 The cause of this process of mutation is not understood, but 

 it certainly results in the production of new genes; and these 

 behave according to Mendel's rules, generation after genera- 

 tion, until mutation occurs again. Mutation and recombina- 

 tion, then, are thought to provide the material on which 

 Darwin's natural selection can act; but our ideas on the causes 

 of evolution must remain hypothetical until we can demon- 

 strate unequivocally the selection of favorable genes under 

 natural conditions of existence. 



Although we do not know the causes of natural mutation 

 and are, thus, still ignorant of the real cause of evolution, 

 quite a lot is kno^vn about how mutation can be made to 

 occur artificially in the laboratory. In 1927 H. J. MuUer, at 

 the University of Texas, discovered that the rate of muta- 



