On Certain Factors of Evolution. 817 
inhabiting the darker regions, and such a case as this, remarkable 
as it would appear, does not affect the general rule, that animals 
living in total darkness and never living in twilight, nor inter- 
crossing with twilight forms, are eyeless, or at least blind. 
Nor does the case of Hadencecus, the cave cricket, with well- 
developed eyes and brains, affect the argument ; for this is essen- 
tially a twilight form, though migrating to regions of total darkness 
and abounding there. The same may be said of the cave species of 
Ceuthophilus. A parallel case may be that of Chologaster as 
compared with Amblyopsis, the former living out of caves in ditches 
as well as in wells and caves. 
J udging by the following statement, so eminent a naturalist as 
Professor Semper denies that heredity acts in the case of the mole. 
He says; : 
“ This almost total blindness in the mole is the result solely of 
complete degeneration of the optic nerve, so that the images which 
are probably formed in the eye itself can never be transmitted to 
the animal’s consciousness. Occasionally, however, the mole even 
can see a little, for it has been found that both optic nerves are not 
ways degenerate in the same individual, so that one eye may 
remain in communication with the brain while the other has no 
connection with it. In the embryo of the mole, however, and 
without exception, both eyes are originally connected with the brain 
by well-developed optic nerves, and so theoretically efficient. This 
may indeed be regarded as a perfectly conclusive proof that the 
blind mole is descended from progenitors that could see ; it would 
Seem, too, to prove that the blindness of the fully grown animal is 
the result not of inheritance, but of the directly injurious effects of 
darkness on the optic nerve in each individual.” } 
It may be objected, however, that each mole certainly inherits a 
tendency to weakness and atrophy of the optic nerves, just as the 
children of consumptive or strumous parents inherit a tendency to 
those diseases, and that when the conditions are favorable the disease 
manifests itself. We know there have been many generations of 
blind or partially blind moles, and it would be strange if heredity 
did not ata certain age act in such a case, and would not for at 
least a few generations even if the moles were kept out of the dark- 
1 Animal Life, etc., pp. 79, 80. 
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