TUE 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 ^hich 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 



