148 THE MAIN CURRENTS OF ZOOLOGY 



others. The work of E. D. Cope, in particular, led 

 him to attach importance to the effects of mechanical 

 and other external causes in producing variations in 

 animal structure, and he has pointed out many in- 

 stances of use-inheritance. 



Lamarck's theory was founded on two sets of 

 facts those of variation and heredity. These are 

 essential to any theory of transmutation of species. 

 Lamarck undertook to account for variation through 

 the influence of environment during the lifetime of 

 the individual, and the direct inheritance of these 

 purposeful variations was assumed. This is the in- 

 heritance of acquired characters which as we shall see 

 was vigorously opposed by Weismann. 



Darwin. Darwin's theory is based on three sets 

 of facts variation, heredity, and natural selection. 

 Two of the factors are the same as those of Lamarck 

 but they were treated differently by Darwin. La- 

 marck undertook to assign causes for variation. 

 Darwin (Fig. 5) accepted variation and assumed that 

 there is continually occurring in living organisms 

 many small fluctuations in structures. Naturally 

 many of these small, fortuitous, variations would 

 be swamped, but, obviously, some of them are re- 

 tained and improved upon. His reply to the ques- 



