On Certain Factors of Evolution. 809 
sont sous les eaux, n’a plus, comme |’Aspalax, que des vestiges de 
Porgane de la vue; vestiges qui sont couverts et cachés de la même 
maniére, 
“Voici une considération décisive, relativement à la question 
que j’agite actuellement. 
“ La lumière ne pénètre point partout ; conséquemment, les ani- 
maux qui vivent habituellement dans les lieux où elle n’arrive pas, 
manquent d'occasion d’exercer l’organe de la vue, si la nature les 
en a munis, Or, les animaux qui font partie d’un plan d’organisa- 
tion, dans lequel les yeux entrent nécessairement, en ont df avoir 
dans leur origine. Cependant puisqui’on en trouve parmi eux qui 
sont privés de usage de cet organe, et qui wen ont plus que des 
vestiges cachés et recouverts, il devient évident que Vappauvrisse- 
ment et la disparition même de l’organe dont il s’agit sont des 
résultats, pour cet organe, d’un défaut constant d’exercice (2d edit., 
i, p: 241).” 
In his “ Origin of Species” Darwin, after claiming that “natural 
selection would constantly aid the effects of disuse” in the case of 
moles and the burrowing rodents, then remarks in regard to cave 
animals: “ As it is difficult to imagine that eyes, though useless, 
could be in any way injurious to animals living in darkness, I 
attribute their loss wholly to disuse” (p. 142). On the next page 
he writes: “By the time an animal had reached, after numberless 
generations, the deepest recesses, disuse will on this view have more 
or less perfectly obliterated its eyes, and natural selection will often 
have effected other changes, such as an increase in the length of the 
antenne or palpi, as a compensation for blindness.” 
It may be that the struggle for existence goes on even in the 
darkness of caves, and that the “ fittest” of the limited population 
survive by reason of their adaptation to their untoward surround- 
ings. How adverse to life of any sort caves are may be realized 
when we consider that only the lowest plants, and only a very few of 
those, live in caves, Without doubt the germs of fungi and the 
seeds of the higher plants are carried into the caves by freshets in 
subterranean streams and through sink-holes. Why, in spite of 
darkness, we should not find more fungi even, and why one or 
two of the green alge should not flourish in the pools and brooks 
of caves, or why the seeds of the higher plants should not germi- 
