ii HISTORICAL ii 



resemble their parents more than other members of the 

 species. Parents possessing a favourable variation tend 

 to transmit that variation to their offspring, to some in 

 greater, to others in less degree. Those possessing it in 

 greater degree will again have a better chance of survival, 

 and will transmit the favourable variation in even greater 

 degree to some of their offspring. A competitive struggle 

 for existence working in combination with certain princi- 

 ples of variation and heredity results in a slow and con- 

 tinuous transformation of species through the operation 

 of a process which Darwin termed natural selection. 



The coherence and simplicity of the theory, sup- 

 ported as it was by the great array of facts which Darwin 

 had patiently marshalled together, rapidly gained the 

 enthusiastic support of the great majority of biologists. 

 The problem of the relation of species at last appeared 

 to be solved, and for the next forty years zoologists and 

 botanists were busily engaged in classifying by the light 

 of Darwin's theory the great masses of anatomical facts 

 which had already accumulated and in adding and classi- 

 fying fresh ones. The study of comparative anatomy 

 and embryology received a new stimulus, for with the ac- 

 ceptance of the theory of descent with modification it be- 

 came incumbent upon the biologist to demonstrate the 

 manner in which animals and plants differing widely in 

 structure* and appearance could be conceivably related 

 to one another. Thenceforward the energies of both 



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