PREFACE. 



INTRODUCTORY. 



A cave is a unit of environment so well circumscribed and of such simplicity 

 that we may know its contents, its elements, and its conditions nearly as well as 

 the experimental zoologist knows the contents and conditions of his aquarium. 

 These contents and conditions are of rare uniformity, changing but little from day 

 to night, from season to season, or from decade to decade. The point of chief 

 interest in the cave environment is the total absence of light in all parts except 

 about its mouth. Probably no animals have a more intimate environmental 

 adaptation than those inhabiting caves. This adaptation is largely of color and 

 structure of eye, which modifications are surpassed only by the functional adapta- 

 tion of the tactile apparatus of the blind forms. 



While no one has followed, and although we may not be able to follow in 

 detail, the steps through which the cave animal has acquired this environmental 

 adaptation, a knowledge of the present condition of their unchanging environment 

 gives us a knowledge of what it has been during their entire period of development. 



We know, or can know, what the present stage of their adaptation is. Not in- 

 frequently we know what the condition of the animal was at the start of its cave 

 experiences and enough of the steps along its line of evolution (indicated by the 

 degrees of adaptation reached by different members of the group) to enable us to 

 form so clear a picture of its entire route of evolution that we may conjecture what 

 elements of the environment caused the modifications, and by what process they 

 were brought about. We have, in other words, a long experiment conducted by 

 nature unrolled before us. 



I propose in this work to give an account of the cave as an environment ; to 

 bring together in a revised form the papers on blind and cave vertebrate animals 

 so far published by myself and my students, together with further observations on 

 the species previously considered, to consider the habitat, mode of life, and the 

 origin of the Cuban blind fishes, and to give an account of their eyes. 



My first experience with blind vertebrates was in 1886, when Superintendent 

 Funk sent to Indiana University a living blind fish which had been taken from a 

 well at Corydon, Indiana, and which proved to be a new species, Typhlichthys 

 wyandotte, the only representative of the genus so far taken north of the Ohio 

 River. Later, when a stay in southern California came in prospect, a study of the 

 blind fish, Typhlogobiiis, living under rocks along the base of Point Loma, was one 

 of the first definite plans formed. 



When, in 1890, I returned to Indiana and was once more within reach of the 

 caves, the problem again came up. My laboratory is excellently located for the 

 study of cave faunas, the series of caves to which Wyandotte, Marengo, Mammoth, 



