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. 
(1) 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 
