ORIGIN OF CAVE LIFE. 99 



Eigenmann (1900, 57) first applied this principle to the occurrence 

 of blind fishes in caves: 



On account of the structure of their eyes and their loss of protective pigment 

 they are incapable of existence in open waters. Their structure is not so much due 

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



Davenport (1903, 19), discussing the same group, remarked: 



Moreover, this family includes species that are structurally especially fitted for cave 

 life, even when they occur in regions where there are no caves and never have been 

 any. They shun the light, and live in crevices and under stones. Their bodily conditions 

 fit them for cave life, and when, in their constant search for dark holes, some of them 

 succeeded in getting into caves, they naturally thrived there. 



Davenport (1903, 19), in explaining the origin of certain beach 

 fauna, stated this principle of adaptation as follows: 



* * * that the structui"e existed first and a fitting environment was sought or 

 fallen into by the species having the peculiar bodily condition. Thus the adaptive re- 

 sult is, on this theory, not due to a selection of structure fitting a given environment, 

 but, on the contrary, a selection of an environment fitting a given structure. 



The principle applies to the origin of cave animals in general as 

 logically as I believe it does in the case of Amblyopsidae and of the 

 beach fauna. 



Blind and highly modified cave species do not suddenly arise from 

 typical out-doors forms which some accident or even the struggle for 

 existence has caused to enter a cave. It is, as expressed by Davenport 

 (1903, 21), a segregation into the fittest environment. Animals do not 

 possess degenerate eyes and lack pigment because they are cave ani- 

 mals. The eyes have in many cases degenerated and the color disap- 

 peared before they entered caves. They are cave animals because their 

 eyes are degenerate and because they lack pigment. The greater the 

 degree of depigmentation and degeneration of the eyes the more the 

 species has become confined to the dark and the greater its tendency 

 to subterranean existence. 



Cave animals have not arisen by accidental isolation. They are 

 isolated in caves and other subterranean abodes because they are unfit 

 for a terranean life and caves are among the possible habitats. 



To sum up, this voluntary migration into caves falls into two classes. 

 Although the accidental origin of cave species from strays which are 

 not structurally adapted to life in such situations is preposterous, cave 

 animals do sometimes arise from the collecting within the mouths of 

 caves of forms more or less adapted to life in partial darkness. These 

 animals making their homes in the mouths of caves may become further 

 modified and, gradually entering farther and farther into the recesses 

 of the cave, may finally become highly specialized and typical cave 



