Morphology. 71 



allowed that this ought to constitute a crucial test as 

 between the two theories of special design and here- 

 ditary descent. " If physical circumstances," he said, 

 "ever modified organized beings, it should be easily 

 ascertained here.' And eventually he gave it as his 

 opinion, that these fish " were created under the cir- 

 cumstances in which they now live, within the limits 

 over which they now range, and with the structural 

 peculiarities which now characterise them." 



Since then a great deal of attention has been paid 

 to the fauna of this Mammoth cave, and also to the 

 faunas of other dark caverns, not only in the New, 

 but also in the Old World. In the result, the fol- 

 lowing general facts have been fully established. 



(i) Not only fish, but many representatives of other 

 classes, have been found in dark caves. 



(2) Wherever the caves are totally dark, all the 

 animals are blind. 



(3) If the animals live near enough to the entrance 

 to receive some degree of light, they may have large 

 and lustrous eyes. 



(4) In all cases the species of blind animals are 

 closely allied to species inhabiting the district where 

 the caves occur ; so that the blind species inhabit- 

 ing American caves are closely allied to American 

 species, while those inhabiting European caves are 

 closely allied to European species. 



(5) In nearly all cases structural remnants of eyes 

 admit of being detected, in various degrees of obsoles- 

 cence. In the case of some of the crustaceans of the 

 Mammoth cave the foot-stalks of the eyes are present, 

 although the eyes themselves are entirely absent. 



Now, it is evident that all these general facts are in 



