2 DEVELOPMENT AND HEREDITY. 



to have been caused. He applied his conception 

 inaccurately, and it was generally found to be too 

 limited or too indefinite to be intelligibly applicable 

 to the known facts. It is worthy of remark, however, 

 that Lamarck's theory was one which attempted 

 to explain the origin of variations, and though, 

 as such, it has been abandoned, yet it has never 

 been supplanted. Darwin based his theory on the 

 fact of variation, supposing it to be universal, and 

 not at first attempting to account for it further than 

 to point, out that in some unknown way it was made 

 more pronounced by a change of environment. The 

 great strength of Darwin's theory was " natural 

 selection"; that is, the universal struggle for exist- 

 ence, and the survival of those individuals whose 

 "fortuitous'' variations made them fittest for their 

 environment. Later in his study of variations, 

 Darwin adopted to some extent a modification of 

 Lamarck's view of the origin of variations. Thus 

 the use and disuse of organs was admitted as a 

 factor of evolution, and it was thought there was 

 some ground for believing that all variation was 

 due in some way to changing environment. Pro- 

 fessor W. K. Brooks embodied this view in his 

 mechanical theory of heredity,^ and the idea obtained 

 largely among other students of the subject, that 



1 Heredity, \V. K. Brooks, Baltimore, 1883. 



