728 The American Naturalist. [September, 
his inductions and deductions as to the use and disuse of 
. organs in inducing their atrophy. It is comparatively easy to 
trace the effects of absence of light on animals belonging to 
genera, families, or orders in which eyes are normally almost 
universally present. As we have seen in the list already given 
of non-cavernicolous animals, the eyes are wanting from causes 
of the same nature as have induced their absence in true cave 
animals. No animal or series of generations of animals, 
wholly or in part, loses the organs of vision unless there is a 
physical, appreciable cause for it. While we may never be 
able to satisfactorily explain the loss of eyes in certain deep- 
sea animals from our inability to personally penetrate to the 
abysses of the sea, we can explore caves at all times of day and 
night, of winter and summer; we can study the egg-laying 
habits of the animals, and their embryonic development; we 
can readily understand how the caves were colonized from the 
animals living in their vicinity ; we can nicely estimate the 
nature of their food, and its source and amount, as compared 
with that accessible to out-of-door animals; we can estimate 
with some approach to exactitude the length of time which has 
elapsed since the caves were abandoned by the subterranean 
streams which formed them and became fitted for the abode of 
animal life. The caves in Southern Europe have been ex- 
plored by more numerous observers than those of this coun- 
try, and the European cave fauna is richer than the Ameri- 
can, but the conditions of European cave-life and the effects of 
absence of light and the geological age of the cave fauna are a 
nearly exact parallel with those presented in the pages of my 
memoir. Moreover, the cave-life of New Zealand and the forms 
there living in subterranean passages and in wells, show 
that animal life in that region of the earth has been affected in 
the same manner. The facts seem to point to the origin of the 
cave forms from the species now constituting a portion of the 
present Plistocene fauna; hence they are of very recent 
origin.” 
The advances in our knowledge of cave-life made since 1886 
and 1887, may be referred to under the following heads: 
ME RT 
