28 THE BLIND FISHES OF MAMMOTH CAVE. 
Looking at the case from the standpoint which the facts force 
me to take, it seems to me far more in accordance with the laws of 
nature, as I interpret them, to.go back to the time when the 
region now occupied by the subterranean streams, was a salt and 
brackish water estuary, inhabited by marine forms, including the 
brackish water forms of the Cyprinodontes and their allies (but not 
descendants) the Heteropygii. The families and genera having the 
characters they now exhibit, but most likely more numerously rep- 
resented than now, as many probably became exterminated as the 
salt waters of the basin gradually became brackish and more lim- 
ited, as the bottom of this basin was gradually elevated, and 
finally, as the waters became confined to still narrower limits and 
changed from salt to brackish and from brackish to fresh, only 
such species would continue as could survive the change, and they 
were of the minnow type represented by the Heteropygii, and per- 
haps some other genera of deuce water forms that we have not 
yet discovered. 
In support of this hypothesis we have one species of the family, 
Chologaster cornutus, now living in the ditches of the rice fields of 
South Carolina, under very similar conditions to those under which 
others of the family may have lived in long preceding geological 
times; and to prove that the development of the family was not 
brought about by the subterranean conditions under which some 
of the species now live, we have the one with eyes living with the 
one without, and the South Carolina species to show that a sub- 
terranean life is not essential to the development of the singular 
characters which the family possess. 
That a salt or brackish water fish would be most likely to be 
the kind that would continue to exist in the subterranean streams; 
is probable from the fact that in all limestone formations caves 
are quite common, and would in most instances be occupied first 
with salt water and then brackish, and finally with fresh water so 
thoroughly impregnated with lime as to render it probable that 
‘brackish water species might easily adapt themselves to the 
change, while a pure fresh water species might not relish the solu- 
tion of lime any more than the solution of salt, and we know how 
few fishes there are that can live for even an hour on being 
changed from fresh to salt, or salt to fresh, water. We have also 
the case of the Cuban blind fishes belonging to genera with their 
nearest representative in the family a marine form, and with the 
eae te, =e 5 
