146 THE MAIN CURRENTS OF ZOOLOGY 



ditions are the two principal means which nature has 

 employed in giving existence to all her productions.' 7 

 When surrounding conditions are stable the evolu- 

 tionary process reaches a stage of equilibrium, but, 

 when the environment varies (or new needs arise), 

 the new conditions of life impress themselves on the 

 plastic organisms and they become altered to better 

 adapt themselves to these new conditions. His 

 theory as set forth was comprehensive and included 

 the evolution of the human body as well as all other 

 organisms. 



Adaptation of organisms to environment, in gen- 

 eral and in detail, producing changes of structure 

 through use and disuse of organs and the direct in- 

 heritance of the modifications is a simplified state- 

 ment of Lamarck's theory. 



Until Lamarck was fifty years of age he was a 

 botanist and had secured a lasting reputation in that 

 field, but, in 1894, when the Jar dm des Plantes was 

 reorganized he was appointed to a position in Zoology 

 in charge of the invertebrates. Apparently, it was 

 the observations he made in connection with these 

 new duties that led to the formulation of his theory. 

 At this time the belief was current that species are 

 unalterable. The dogma of the fixity of species 



