INHERITANCE OF ACQUIRED CHARACTERS 183 



transmission of " acquired " characters, and is quite as difficult 

 to accept as the latter, while, as Sumner points out, it does not 

 affect the question of the importance of the environment in 

 determining the course of evolution. 



Professor Henslow l believes that the direct action of the 

 environment, in its widest sense, coupled with the responsive 

 power of protoplasm, is the sole and efficient cause of adaptive 

 variations in plants, without any aid from natural selection. 

 This of course implies a firm belief in the inheritance of 

 somatogenic modifications. Although these may be acquired 

 slowly throughout the course of a long series of successive 

 generations, they may become gradually more and more fixed 

 and permanent, until finally they attain a degree of stability 

 which entitles them to be considered as truly blastogenic. 



Professor Eigenmann 2 has arrived at very similar conclusions 

 as a result of his careful study of animals which live in dark 

 caves. A very characteristic feature of such animals is the 

 bleaching which they undergo, due to the loss of pigment. Pro- 

 fessor Eigenmann regards this character as due in the first place 

 to the direct influence of the environment (i.e. the absence of 

 light) upon the individual. He tells us " The bleached condition 

 of animals living in the dark, an individual environmental adap- 

 tation, is transmissible, and finally becomes hereditarily fixed." 

 In Amblyopsis, one of the blind cave fishes, the fixation of this 

 character has become so complete that even when the young are 

 reared in the light they develop without pigment the originally 

 somatogenic character has apparently become blastogenic. In 

 the well known European Proteus a tailed amphibian which 

 lives in subterranean waters on the other hand, it appears that 

 the bleached condition has not yet become hereditarily established, 

 for this animal becomes darker when exposed to the light. Pro- 

 fessor Eigenmann points out that " natural selection cannot 

 have affected the coloration of the cave forms, for it can be of no 

 consequence whether a cave species is white or black." 



We do not know how many generations it may take to effect 

 the fixation of a character acquired slowly under the influence 

 of a constantly repeated or continuous environmental stimulus, 

 but the fact that human beings have not yet learned to speak 



1 "Origin of Plant Structures " (International Scientific Series, Vol. LXXVIL, 

 1895). 



2 " Cave Vertebrates of America. A Study in Degenerative Evolution " (Wash- 

 ington : Carnegie Institution, 11109). 



