22 



PRINCIPLES OF ANIMAL BIOLOGY 



Then came Charles Darwin (Fig. 12) grandson of Erasmus Darwin, 

 with indisputable evidence of evolution. Cuvier was no longer living 

 to combat the theory, and Darwin had laboriously collected a mass of 

 supporting facts which could not be set aside. These he published 

 in a series of books beginning with Origin of Species in 1859, and continu- 

 ing to the end of his life in 1882. With them he proposed several theories 

 to account for evolution which appeared so plausible that the whole 

 thinking world was convinced of the fact of the mutability of species. 

 The chief of these theories was that of natural selection, according to 

 which, in the struggle for existence, the fittest survive. The theory of 



Fig. 13.— Thomas Henry Huxley, 1825-1895. 



natural selection, as will be pointed out later, is still commonly held, 

 though with diminished significance. 



The doctrine of evolution was not accepted without opposition. 

 However, with the aid of the powerful Thomas Huxley (Fig. 13), who 

 spread the idea in forceful lectures among biologists and non-biologists 

 aUke, its victory was complete. With regard to the fad of evolution 

 there are now no dissenters among thinking people. Whatever contro- 

 versy still exists is concerned with the causes and method of evolution; 

 evolution itself is admitted by all. 



Genetics. — It will be noted that the theory of natural selection, 

 1)3' means of which Darwin sought to explain evolution, contained nothing 

 to account for the origin of changes in species, but only for their freserva- 

 tion. So strongly did the theory of evolution and the theory of natural 

 selection appeal to the imagination that another field of investigation, 



