814 On Certain Factors of Evolution. 
which might be reared in dark cellars, should be observed for a 
series of generations, to ascertain whether by breeding the eyes 
cannot be restored, and the species by artificial means be induced 
to revert to its ancestral type. The embryology of the cave bee- 
tles, with or without rudimentary eyes, of the eyeless spiders and of 
Myriapods, of the Cæcidotæa, and of the blind crayfish and blind- 
fish should be carefully worked out as regards the presence of 
organs of vision in a rudimentary state, though we should hardly 
expect to find rudimentary eyes in Anophthalmus when larva and 
pupa do not possess them. 
Isolation as a Factor in the Origin of Cave Animals.—When any 
cave, such as Mammoth or Wyandotte, ete., is once colonized by 
emigrants from the upper world, and the colonists becoming adapted 
to the new conditions environing them, have lost their eye-sight, 
or even all traces of eyes, and the new forms thus established 
begin to breed true to their recently acquired characteristics, it 18 
obvious that this process of in-and-in breeding will continue as 
long as the new forms live in total darkness and are isolated from 
the allied forms or their eyed ancestors of the upper world of light. 
Though a subordinate factor, isolation is certainly of no little import- 
ance in securing the stability of the new species and genera. It is 
evident that if no stragglers from the upper world, as species of 
Trechus to interbreed with the cave Anophthalmi, species of Cho- 
leva to cross with Adelops or Bathyscia, or species of Ceuthophilus 
to mix with the true cave Ceuthophili, or species of Myriapods or 
Arachnida to intercross with the cave forms, then the latter will 
tend to remain as fixed as we now find them to be. In the case 
of the crayfish of Mammoth Cave, the normal Cambarus bartoni, 
introduced at times of heavy rains or freshets into the cave, ® 
not seldom found living in company with Orconectes pell 
the blind form, but belonging to a different section of the genus 4 
regards the shape of its gonopods or first male abdominal appen- 
dages, and being of much larger size, it is probably incapable of 
fertilizing the eggs of the blind form, even if the latter, timid 
sensitive to the least disturbance of the water, should allow itself 
to be approached by the larger-eyed form. It is also probable 
that Cæcidotæa stygia is seldom, if ever, brought in contact with 
Asellus communis, which abounds in the pools and streams through- 
