HISTORICAL DEVELOPMENT OF ZOOLOGY 451 



Origin of Species, in which the theory of natural selection was 

 elaborated. The main features of the theory have already 

 been outlined in Chapter X. Darwin was a prolific writer; 

 some of his books are On the Variations of Animals and Plants 

 under Domestication, The Descent of Man, and The Expres- 

 sions of the Emotions in Man and the Lower Animals. In all 

 of these natural selection is applied. 



Among the contemporaries who aided Darwin in his work, 

 and who were in turn influenced by him, may be mentioned 

 the names of Herbert Spencer (1820-1903), the philosopher 

 and author of the Principles of Biology; Ernst Haeckel (born 

 1834), the brilliant author of General Morphology ; Thomas 

 H. Huxley (18251895), a great teacher and popularize! 1 of 

 zoology, and author of important works ; and, finally, August 

 Weismann (born 1834), who aroused skepticism as to the 

 hereditary transmission of acquired characters, and who has 

 made important supplementary contributions to the theory 

 of natural selection. 



It is impossible to overestimate the influence of the selec- 

 tion theory in the progress of modern zoology. It has thrown 

 new light on all fields of activity, so that it may be said that 

 a new science of zoology has arisen. The importance of care- 

 ful experiment has been appreciated as never before. A host 

 of investigators have sprung up, whom to merely mention 

 would transcend the limits of this work. The work of a very 

 few men has been referred to in the preceding chapters, in the 

 effort to give some picture of the state of the science of 

 zoology to-day. Everywhere zoologists are endeavoring to 

 learn the truth about animals, trying to understand better 

 the problem of animal life, and the greatest problem of all, 

 towards which the solution of all others is directed, the 

 problem of man and his relation to the universe. 



