48 THE EVOLUTIONARY THEORY 



ous attempt scientifically to explain the mutation of 

 species.^ His explanation appeared in 1801 and was 

 published in a more elaborate form in 1809. His 

 theory is based upon the well-known fact that animal 

 organs are strengthened and developed by habitual 

 exercise, and suffer degeneration when not used. 

 Without denying the influence of external environment 

 upon the use or non-use of organs, he regarded use 

 and non-use as the actual causes of variation, and 

 maintained that the evolution of species is due to trans- 

 mission to offspring of characters thus acquired. 

 Among the more plausible illustrations which he 

 advanced are those of snakes and giraffes. The con- 

 stant striving of certain animals to pass through narrow 

 crevices was thought by him to cause an attenuation 

 of body, which was inherited and increased through a 

 series of generations. Again, certain animals Hving in 

 arid districts were forced to stretch their necks upward 

 in order to feed on the leaves of trees. This gradually 

 caused, by a similar process of variation, heredity, and 

 accumulation of change, the pecuHarity which most 

 obviously distinguishes giraffes. 



Before going on, two terms should be defined, viz., 

 "factors" of evolution, and "characters." A "factor" 

 of evolution is any cause or condition of variation 

 which may produce or determine the course of organic 

 evolution. Such a factor may be either directly cau- 



1 His theory is expounded by R. H. Lock, Recent Progress, pp. 

 33-37. He gives the arguments pro and con, pp. 59-72. Cf. V. L. 

 Kellogg, Darwinism To-day, pp. 262-274, 290-309. 



