206 THE THEORIES OF EVOLUTION 



cific stimulus, and it is this sensitiveness which deter- 

 mines the ulterior reactions of the organism. 



It is not a somatic change brought about by the ac- 

 tivity of a certain function which constitutes the pri- 

 mary fact. On the contrary, a change in the soma 

 plasm is always preceded by a change in the germ 

 j)lasm. The modified form resulting from the strug- 

 gle between parts, determines and precedes the mod- 

 ified function; this is the absolute antithesis of the 

 Lamarckian maxim that "the function creates the 

 organ." 



The influence of the environment can be only indi- 

 rect. When certain animals assume a different colour- 

 ing in a changed environment, when for instance a 

 butterfly, the Vanessa, presents a different pigmen- 

 tation in different seasons, or when certain arctic ani- 

 mals turn white in winter, this dimorphism is not due 

 to the immediate action of external conditions but to a 

 more complex process by which natural selection sup- 

 plemented by germinal selection produces a protective 

 variation. 



A more accurate explanation could be given of these 

 facts. Certain insects breed twice a vear, once in 

 summer and once in the fall and their caterpillars take 

 on a different colouration in each season. For in- 

 stance, the summer caterpillars of the Lycama pseu- 

 dargiolus living on the white blossoms of Cimicifuga 

 racemosa are white, while the fall caterpillars living 



