THE LAMARCKIAN THEORY 237 



cessors fell the task of discussing, supplementing, and 

 developing the masters' theories. 



Lamarck's main thesis is the influence exercised on 

 living things by their environment and their mode of 

 life. "In animals and in plants," Lamarck writes, 

 "whenever the condition of habitat, exposure, climate, 

 nutrition, mode of life, etc., are modified, the charac- 

 ters of size, shape, relations between parts, coloura- 

 tion, consistency and, in animals, agility and industry 

 are modified proportionately." 2 * 



The new races and varieties created artificially by 

 breeders are a striking illustration of this statement. 

 The environment, however, does not exert its influence 

 on plants and animals in an identical manner. The 

 immediate action of external factors is more notice- 

 able in plants, among which it creates remarkable 

 differences between individuals of the same species. 

 "As long as the Ranunculus aquatilis is immersed in 

 water, its leaves are all finely divided and their di- 

 visions are capillaceous ; whenever the stems reach 

 above the surface the leaves which develop in the air 

 are longer and simply lobed. Individuals growing 

 in marshy soil, where they are not covered by water, 

 have short stems and none of the leaves are divided 

 into capillaceous segments. This gives rise to Ra- 

 nunculus heredaceus which botanists regard as a dis- 

 tinct species.' 



" 3 



2 Philosophie Zoologique, Vol. I, p. 227. 

 8 Vol. I, p. 231. 



