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THE MAMMOTH CAVE. 93 
living on the earth were first called into exist- 
ence. But the investigation would involve such 
long and laborious researches, that I doubt 
whether it will ever be undertaken. It has oc- 
curred to me that the final step would be a 
thorough anatomical study of the species found 
in the Cave, with extensive comparison of allied 
species found elsewhere,—next, an investigation 
of the embryology of all of them,—and, when 
fully prepared by such researches, an attempt to 
raise embryos, of the species found in the Cave, 
under various circumstances, different from those 
in which they are naturally found at present. 
“If physical circumstances ever modified 
organized beings, it should be easily ascertained 
here. For my own part, however, I think that 
the blind animals of the Cave would only show 
organs of vision during their embryonic state, in 
conformity with the normal development of the 
respective types to which they belong, and that 
even when placed under a moderate influence 
of light, incapable of injuring them, but sufficient 
to favor the growth of their eyes in the allied 
species provided with them, the young of those 
species peculiar to the Cave would gradually 
grow blind, while the others would acquire per- 
fect eyes; for I am convinced, from all I know 
