FROM LAMARCK TO ST. HIL.\IRE 233 



the influence of light upon plants, directly upon 

 the coloring of animals, and upon the develop- 

 ment and degeneration of eyes ; and the influences 

 of heat. The main influences come under the law 

 of use and disuse, for he believes that Nature 

 effects her changes not directly but through the 

 reaction of animals to their environment. He thus 

 differs widely from Buff on: *'Lack of employ- 

 ment of an organ becoming constant under the 

 influence of certain habits, gradually impover- 

 ishes the organ and ends by causing it to dis- 

 appear entirely." 



In the discours preliminaire of the Philosophie 

 Zoologique, Lamarck outlines his work as di- 

 vided into three parts. The first is to treat of the 

 subject in general, of methods of research, of 

 artificial distinctions raised by man in classifica- 

 tion, of the real meaning of the term 'species,' of 

 the proofs of the 'de gradation' of organization 

 from one end to the other of the animal scale, of 

 the influences of environment and habit as causes 

 favoring or arresting the development of animals, 

 of the natural order and classification of animals. 

 In this first section is to be expanded his whole 

 theory of Evolution, which we will examine later. 

 In the second part he considers the essential phe- 

 nomena and physiological conditions of life, of 

 'orgasme' and irritability, of the peculiarities of 

 cellular tissue, of the conditions of spontaneous 



