54 EVOLUTION AND GENETICS 



ity, but it may also come about by individuals within 

 the same area ripening their germ cells at different 

 times, etc. 



If species arise in this way we avoid the difficulty 

 raised by Bateson concerning the origin of sterilit}^ 

 of hybrids w^hose parents have arisen through muta- 

 tion. Moreover the difficulty is not one 23eculiar to 

 the mutation theory, but applies in large part to any 

 theory that postulates the origin of one species from 

 another. 



If then we dismiss the problems that have grown 

 out of the historical definitions of species, and turn 

 directly to an examination of the origin of new vari- 

 ations we shall find that some progress has been 

 made since Darwin wrote, and that this new knowl- 

 edge supports Darwin's view that the variations 

 shown by animals and plants furnish materials for 

 a theor}^ of evolution. There is evidence that new 

 characters suddenlv arise bv mutation both in do- 

 mesticated and in wild types, and that these varia- 

 tions are inherited in the same way as are the differ- 

 ences present in wild tyj^es that distinguish them 

 from one another. In addition there are variations 

 (fluctuations) due to the action of the environment 

 on the developing individual. These are not inherited 

 and cannot therefore take part in evolution. These 

 statements will be discussed in later chapters after 

 Mendel's laws, and some other laws of hereditv dis- 

 covered since his time, have been examined. 



