1894.] Subterranean Fauna of North America. 743 
that the sense of hearing is, with little doubt, nearly, if not 
quite, obsolete (p. 128). 
While, then, Weismann claims that there is a cessation of 
natural selection in the case of cave animals, another writer, 
Lankester, in a brief note in Nature, asserts that the blindness of 
eave animals is due to natural selection, remarking: * This in- 
stance can be fully explained by natural selection acting on con- 
genital fortuitous variations. Many animals are thus born with 
distorted or defective eyes, whose parents have not had their 
eyes submitted to any peculiar conditions. Supposing a number 
of some species of Arthropod or fish to be swept into a cavern or 
to be carried from less or greater depths in the sea, those indi- 
viduals with perfect eyes would follow the glimmer of light 
and eventually escape to the outer air or to the shallower 
depths, leaving behind those with imperfect eyes to breed in 
the dark place. A natural selection would thus be effected. 
In every succeeding generation (bred in the dark place) this 
would be the case, and even those with weak but still seeing 
eyes would, in the course of time, escape, until only a pure 
race of eyeless or blind animals would be left in the cavern or 
deep sea." 
This explanation seems, however, vague and speculative, as 
well as inadequate, when we compare the kind of natural selec- 
tion here invoked with such direct, powerful and readily 
appreciated factors as partial or total darkness (no plants being 
able to grow in caves, and only a very scanty fauna); added 
to the disease of organs whose very existence was originally 
due to the stimulus of light, and where, were it not for their 
enforced isolation, the swamping effects of crossing with eyed 
forms would constantly tend to prevent the permanent exist- 
ence of blind or eyeless forms. Besides, how can the varia- 
tions be fortuitous when the overshadowing and all-prevailing 
influence is darkness, this cause inducing a change primarily 
in a single organ, and, in a single sense, due to a single cause, 
urging the variation in a determinate way? Indeed, it may 
be questioned whether variations are ever “ fortuitous” in the 
sense that they can arise independently of and are not con- 
trolled by the ever active forces of nature. 
49 
