ORIGIN OF CAVE FAUNA. 15 



poor when found in the caves, and will never be able to establish themselves in 

 them. On the other hand, there are no reasons why fishes detecting their prey 

 either by smell or touch should not be capable of colonizing caves. The cat-fishes 

 and Amblyopsidae belong to the latter class. It is surprising that more cat-fishes 

 have not established themselves in caves. Among the Amblyopsidae, even those 

 with functional eyes depend on touch and vibrations for their food. Chologaster 

 has well-developed tactile organs and poor eyes. It is found chiefly at the mouths 

 of underground streams, but also in the underground streams themselves. The 

 tactile organs are not different in kind from those of other fishes, and their high 

 development is not more marked than their development in the barbels of the cat- 

 fishes. The characters which distinguish Chologaster as a fish capable of secur- 

 ing its food in the dark are emphasized in Typhlickthys, and the tactile organs 

 are still more highly developed in Amblyopsis. The eyes of the last two genera 

 are so degenerate that it is needless in this connection to speak of degrees of degen- 

 eration. On account of the structure of their eyes and their loss of protective 

 pigment, they are incapable of existence in open waters. With the partial and 

 total adaptation to underground existence in the Amblyopsidae and their negative 

 reaction to light, it is scarcely possible that for this family the idea of accidental 

 colonization can be entertained for a moment. Their structure is not as much 

 due to their habitat as their habitat is due to their structure and habit. 



Typhlogobiiis lives in the holes of shrimps, under rocks, on the coast of southern 

 California. It is a living example of the origin of blind forms in dark places 

 remote from caves. Here again the "accidental" idea is preposterous, since no 

 fish could by accident be carried into the devious windings of the burrows they 

 inhabit. Moreover, a number of related species of gobies occur in the neighbor- 

 hood. They live ordinarily in the open, but always retreat into the burrows of 

 crustaceans when disturbed. The origin of the blind species by the gradual 

 change from an occasional burrow seeker to a permanent dweller in the dark, 

 and the consequent degeneration of the eye, is evident here at once. Among 

 insects the same process and the same results are noted. We have everywhere the 

 connection of diurnal species with nocturnal, dark-loving, and blind forms, a tran- 

 sition, the result of habit entered into with intent, but no evidence of such a con- 

 nection as the result of accident ; also numerous instances of daylight species 

 being swept into caves, but no instance of one establishing itself there. 



Attention has been called to the difference in the time of origin of the aquatic 

 and non-aquatic cave-dwellers. The latter are later immigrants. They neces- 

 sarily arrived after some channels had been cleared of water through the stream 

 burrowing into still lower channels. The non-aquatic forms are derived, in 

 part at least, by migrations from the twilight forms that may have developed with 

 the twilight region, and in part they are active immigrants of stereotropic 

 or negatively phototropic forms like the Spelerpcs. Some of them, like the myrio- 

 pods, may even be accidentally brought in with their food and habitat, 1 but even 

 here the active voluntary immigration is, at least, as probable as the accidental one. 



Species widely distributed over a continuous environment may have become 

 distributed from one center of development. The same may be said of the species 

 found in distant, discontinuous environments where it can be shown that the dis- 

 continuity is of recent origin. The same can not be said of species distributed in 



1 Decaying logs have been carried into and are found in various parts of the Mitchel] caves. 



