CHAPTER IX 



THE ORIGIN OF NEW TYPES AMONG 

 ANIMALS 



i 



IN following the steps of the evolution of man and of animals 

 we have seen the conditions under which the most rapid 

 changes occur. We have also seen why it is that during 

 periods of great climatic stress many old forms disappear. But 

 why is it that at the end of such a period we find not only that 

 the old forms have gone, but that their places are amply filled by 

 new forms? Indeed we may almost say that at times of evolu- 

 tionary crises the development of new types is even more marked 

 than the disappearance of the old. The problem of how these 

 new forms originate has a direct bearing upon man's life today, 

 for the laws that apply to species of plants and animals through- 

 out geological history appear also to apply to races of men today 

 and to human ideas and institutions. Let us begin by considering 

 what modern biology has to say on this subject in respect to 

 animals. Then we shall be ready to apply our results to man in 

 the next chapter. 



Charles Darwin filled the world with the idea that natural 

 selection is of the utmost importance in evolution. His work and 

 that of hundreds of his successors have shown that among all the 

 factors that cause selection climate is far the most important, 

 since climate largely controls the food supply and migrations. 

 Darwin believed that very slight differences were enough to give 

 natural selection a free hand. The work of later biologists has 

 shown more and more clearly that this is rarely or never the case. 

 New species do not seem to arise through the cumulative effect 



