12 BLIND VERTEBRATES AND THEIR EYES. 



THE ORIGIN AND DISPERSAL OF CAVE ANIMALS. 



It has been shown that many cave animals have good eyes. Epigean animals 

 with degenerate or no eyes are not rare, hence the origin of the cave fauna and of 

 the blind fauna are two distinct questions. This was first recognized by H. Gar- 

 man and indorsed by Eigenmann and by Hamann. Other writers have usually 

 confused the two questions, and indeed they may become one when they concern 

 an animal that has become blind concomitantly with its cave colonization. A 

 consideration of the forms that are not found in caves, and the reasons why they 

 are not found there, is in this connection possibly more illuminating than the 

 direct consideration of the cave forms. 



Caves may have become populated by one of the four following processes : 

 (i) Animals may by accident have been carried into caves. 



(2) Animals may, step by step, have colonized caves, becoming adapted 



to the environment as successive generations gradually entered 

 deeper and deeper recesses of the caves. 



(3) Animals which had elsewhere become adjusted to do without light 



may have gathered voluntarily in caves. 



(4) Animals may have developed along with the development of the caves. 



First process : This process was imagined by Lankester to operate as follows : 



Supposing a number of some species of arthropod or fish to be swept into a cavern or to be 

 carried from less to greater depths in the sea, those individuals with perfect eyes would follow the 

 glimmer of light and eventually escape to the outer air or 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 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. 



While this is a possible mode of origin of cave animals, and even of blind ones, 

 it is highly improbable that many or even any animals depending, as he supposes, 

 on their eyes have thus come to first colonize the cave. Fishes are annually swept 

 into caves, but these are not able to permanently establish themselves in them. 

 To do this the fish must have peculiar habits, special methods of feeding and 

 mating, before an accidental colonization can become successful, and if they are 

 so adapted for a cave existence, they would probably voluntarily colonize the caves, 

 without waiting for an accident. 1 The Amblyopsidae are a small family of fishes, 

 8 species being known. They form a very small part of the large fish fauna about 

 the North American caves. But since 6, possibly 7, of the species of this family 

 are cave dwellers, and only one of the numerous other fishes is permanently at 

 home in the caves, we must suppose, if the theory under consideration is the correct 

 one, that the accident of being carried into caves happened to 6 or 7 out of 8 of the 

 Amblyopsidae, and to only 1 of all the other fishes about the caves. The absurdity 

 of this supposition is self-evident. A comparison of the abysmal fauna with the 

 pelagic and shore faunas would probably give us similar results. 



1 A distinction ought possibly to be made between the aquatic cave animals that will be discussed under the 

 "fourth process," and non-aquatic forms. Non-aquatic cave animals are later immigrants of caves. These must 

 either be voluntary recruits from the twilight fauna about the entrance of the cave or they must have become other- 

 wise adjusted to live in the dark. There is no difficulty in accounting for the presence of Myriopoda on this score 

 nor for the other forms habitually found under bark and under rocks. Myriopods are everywhere abundant in 

 the caves of North America and they (if any animals) may have accidentally been carried into caves with sticks 

 of wood or trunks of trees. 



