740 The American Naturalist. [September, 
congeners, would doubtless become used to life in darkness. 
Their offspring of the first generation might or might not 
suffer some alteration in the visual organs, but doubtless some 
slight degree of physiological change would result; this might 
or might not be latent in the next generation, or it might - 
crop out and become manifested in the first generation, or, if 
not in the first, in the second or third. As soon as the degen- 
eration in the eye-sight began to become fixed by heredity, 
the process must, have gone on rapidly, and, in a few genera- 
tions, perhaps a dozen or twenty, or fifty, rather than many 
hundreds or thousands, or * numberless generations," as most 
writers since Darwin claim. 
Now as deaf mutes already appear to breed true to their in- 
cipient strain or variety, whether congenitally deaf or rendered 
so by disease during the lifetime of either or both parents, it 
seems most probable that animals not at first congenitally 
blind, might have acquired, after having been carried into, 
and after living for some months or even years in darkness, 
the tendency to blindness, and have transmitted to their off- 
spring such first steps in adaptation to their Cimmerian en- 
vironment. It is difficult for any one, it seems to us, not hide- 
bound by theory to imagine any other mode of procedure. 
The steps in the process are these: 1, The change in environ- 
ment from normal conditions to partial or total darkness; 2, 
At first a slight degree of adaptation to such change, if the ani- 
mal survived at all; 3, Becoming gradually habituated to the 
darkness, compensation for the loss of eyesight would result in 
the stimulation of the senses of touch and smell; 4, Mean- 
while the physiological change from loss of eyesight would 
react on the physical structure and the eye would begin to 
degenerate, and very rapidly, after a few generations, the optic 
nerves in some forms, or the optic lobes and nerves in others, 
would disappear, the vestiges of the outer structures of the 
eyes remaining in some forms long after the nervous connec- 
tions between the eyes and the brain had become effaced ; 5, 
Meanwhile, segregation would prevent intercrossing with new- 
comers provided with perfect eyes, and consequently would 
prevent the swamping of the new characters resulting from 
