684 Studies in the T/ieory of Descent. 



of a warm climate acquire an almost black color- 

 ation (Polyommatus Phlaeas), whilst on the other 

 hand others become lighter (Papilio Podalirius). 



We can thus understand why always certain 

 courses of development are followed, a fact which 

 cannot be completely explained by the nature of 

 the conditions of life which induce the variations. 

 But as soon as we clearly perceive that the quality 

 of the changes essentially depends upon the 

 physical nature of the organism itself, we arrive 

 at the conclusion that species of widely diverging 

 constitutions must give rise to different variations, 

 whilst those of allied constitutions would produce 

 similar variations. But definite courses of de- 

 velopment are thus traced out, and we perceive 

 that from any point of the organic developmental 

 series, it is impossible that any other point can be 

 attained at pleasure. Variation in a definite 

 direction thus by no means necessitates the 

 acknowledgment of a metaphysical develop- 

 mental principle, but can be well conceived as 

 the mechanical result of the physical constitution 

 of the organism. 



The manner in which the dissimilar physical 

 constitution of organisms must arise can also be 

 easily shown, although the first commencement 

 of the whole developmental series, i. e. the oldest 

 living forms must be assumed to have been almost 

 homogeneous in their physical constitution. 

 The quality of the variation is, as said before, 



