EVOLUTION OF ANIMALS 545 



and Thomas Aquinas (1225-1274), who upheld the evolutionary concep- 

 tion and expressed beliefs in the symbolic nature of the biblical story of 

 creation. 



The first of the modern zoologists to entertain clearly the evolutionary 

 conception was Buffon (1701-1788) but he hesitated to urge his ideas. 

 He was the first to believe in the direct modification of organisms by their 

 environment. Erasmus Darwin (1731-1802), the grandfather of Charles 

 Darwin, recognized the fact of a struggle for existence and also accepted 

 the theory of the inheritance of acquired characters, believing that 

 forces within the organism responding to environmental changes formed 

 the basis for modifications in the organism. Lamarck (1744-1829) 

 made valuable contributions to the field of biology, including the pro- 

 posal of the term biology itself and the use of a tree of life to express 

 phylogenetic relationships among organisms. His most noteworthy 

 contribution was a definite theory of evolution based upon the use and 

 disuse of organs. He believed that necessity in the organism might give 

 rise to new organs and suggested that the use of any organ strengthens, 

 develops, and enlarges it, while a lack of use causes a progressive degen- 

 eration and ultimate disappearance. He also believed that these changes 

 were passed on by heredity — that is, he believed in the inheritance of 

 acquired characters. 



The most famous name in this field is that of Charles Darwin (Fig. 

 376), who lived from 1809 to 1882. His preeminence is indicated by the 

 fact that popularly Darwinism has come to be looked upon as synonymous 

 with evolution, though at the present time many of the ideas which Dar- 

 win advocated are no longer accepted. Darwin's great work was the 

 ''Origin of Species," published in 1859, in which he presented his theory 

 of natural selection. This same theory was arrived at independently by 

 Wallace (1822-1913) who, however, had not such a wealth of observa- 

 tional data to support it as had Darwin. For this reason, though he 

 joined with Darwin in first presenting the theory, he stood aside and 

 permitted Darwin to publish it alone. The publication of Darwin's 

 work excited violent controversy, but since neither he nor Wallace was 

 fitted by disposition effectively to defend the theory in public, that task 

 fell to Huxley (1825-1895) who successfully championed the cause of 

 evolution. 



Since the time of Darwin a flood of contributions has appeared which 

 involve a great many avenues of approach to the subject of evolution. 

 Weismann (1834-1914) supported Darwin's conception of evolution by 

 emphasizing the distinction between germ plasm and somatoplasm and 

 the part played in inheritance by the germ cells. The discovery of chro- 

 mosomes and the development of the field of genetics have provided a 

 physical basis for evolutionary changes. Many of these modern theories 

 will be referred to in discussing the causes and method of evolution. 



