564 THE CAUSES OF EVOLUTION 



in marine snails of the genus Thais in Puget Sound. These 

 live on rocky shores and cannot cross sand or go into deep water. 

 Since their young are hatched as miniature snails there is no chance 

 for dispersal as free-swimming larvse. Each rocky point or other 

 area that is isolated from other suitable places has its particular 

 variety. Another example is seen in the snails of the genus Par- 

 tula, studied by Crampton in Tahiti. In such cases evolution 

 proceeds in a manner which is not so different from the Darwinian 

 scheme, since the isolation of certain individuals might be called a 

 form of selection, although the characters evolved cannot be 

 regarded as necessarily useful. 



Evolution by Hybridization 



From the account of complex Mendelian hybrids that was given 

 in the chapter on Genetics, it is possible to understand how 

 evolution might occur by the appearance of characteristics that 

 are new only in the sense that they are new combinations. Thus, 

 if a black, short-haired guinea-pig is bred with a white, long- 

 haired individual, there will be in the second filial generation 

 (Fig. 247, p. 469) a theoretical ratio of one white, short-haired indi- 

 vidual to fifteen of other types. Neither the short hair nor the 

 white hair is new, but the combination of short and white hair 

 might be something that had never occurred before. If such a 

 new type of guinea-pig were thus produced under domestication, 

 or if a new combination thus appeared in nature and was perpet- 

 uated, this might be called evolutionary modification. Although 

 there would seem to be a Umit to the amount of change that could 

 arise in this manner, the Dutch botanist, Lotsy, has argued for 

 the importance of such hybridization in evolution. The English 

 geneticist, Bateson, even went so far as to maintain, whether 

 seriously or as a stimulus to a more critical attack upon the prob- 

 lem, that recombination and loss of characters might account for 

 all evolution. 



Organic evolution is, therefore, l)ut one aspect of the historic 

 process of change which has occurred throughout nature in the 

 past, which is happening in the present, and which it is presumed 

 will go on happening in the future. The universe is not static but 

 dynamic, that is, it is a " going concern," if we wish to put the 

 idea in the language of the street, In the physical sciences cl 



