126 BIOLOGICAL LECTURES. 



Natural selection cannot have affected the coloration of the 

 cave forms, for it can be of absolutely no consequence whether 

 a cave species is white or black. It could only affect the col- 

 oration indirectly in one of two ways : first, as a matter of 

 economy, but since the individual is in part bleached by the 

 direct effect of the darkness, there is no reason why natural 

 selection should come into play at all in reducing the pigment 

 as a matter of economy ; second, Romanes ^ has supposed that 

 the color disappeared through the selection of correlated struc- 

 tures, a supposition he found scarcely conceivable when the vari- 

 ety of animals showing the bleached condition was considered. 



Panmixia cannot account for the discharge of the color, since 

 it returns in some species when they are exposed to the light, 

 and disappears to a certain extent in others when kept in the 

 dark. Panmixia, Romanes thinks, may have helped to discharge 

 the color. In many instances the coloration is a protective 

 adaptation and therefore maintained by selection. " Panmixia 

 might, in such instances, lower the general average to what 

 has been termed the birth mean." Proteus is perhaps such an 

 instance. But in this species the bleached condition has not 

 yet been hereditarily established, and since each individual is 

 independently affected, " the main cause of change must have 

 been of that direct order which we understand by the term 

 ' climatic' " 



Since, however, the bleached condition, which in the first 

 instance is an individual reaction to the absence of light, has 

 become hereditarily established in Amblyopsis, so that the 

 bleaching goes on even when the young are reared in the light, 

 it is evident that in Amblyopsis we have the direct effect of 

 the environment on the individual hereditarily established. 



1 Darwin and after Darwin. 



