GERMINAL SELECTION. 49 



conceivable example— viz. that of the unifonn change 

 of color on the entire under surface of the wing. 



Suppose, for example, that the ancestral species of 

 a certain forest-butterfly habitually reposed on 

 branches which hung near the ground and were cov- 

 ered with dry or rotten leaves ; such a species would 

 assume on its under surface a protective coloring 

 which by its dark, brown, yellow, or red tints would 

 tend toward similarity with such leaves. If, however, 

 the descendants of this species should be subsequently 

 compelled, no matter from what cause, to adopt the 

 habit of resting on the green-leafed branches higher 

 up, then from that period on the brown coloring would 

 act less protectively than the shades verging towards 

 green. And a process of selection will have set in 

 which consisted first in giving preference only to such 

 persons whose brown and yellow tints showed a ten- 

 dency to green. Only on the assumption that such 

 shades were possible by a displacement in the quanti- 

 tative proportions of the different kinds of biophores 

 composing the determinants of the scales affected, was 

 a further development in the direction of green possi- 

 ble. Such being the case, however, that development 

 had to result; because fluctuations in the numerical 

 proportions of the biophores are always taking place, 

 and consequently the material for germinal selection 

 is always at hand. At present it is impossible to deter- 

 mine exactly the magnitude of the initial stages of the 

 deviations thus brought about and promoted by the 

 sexual blending of characters ; but it may perhaps be 

 ascertained in the future, with exceptionally favorable 

 material. Pending such special observations, however, 

 it can only be said a priori that slight changes in the 

 composition of a determinant do not necessarily condi- 



