Darwin's Artificial and Natural Selection 131 
jects, not blending one into another by insensible gradations, 
the range of any one species, depending as it does on the 
range of others, will tend to be sharply defined.” 
Here we have a petztzo principiiz. The sharp definition of 
species, that we started out to account for, is explained by 
the sharp definition of other species! 
A third part of the explanation is that, owing to the rela- 
tive fewness of individuals at the confines of the range dur- 
ing the fluctuations of their enemies, or of their prey, or in 
the nature of the seasons, they would be extremely liable to 
utter extermination. If this were really the case, then new, 
species themselves which, on the theory, are at first few in 
numbers ought to be exterminated. On the whole, then, it 
does not appear that Darwin has been very successful in his 
attempt to meet this objection to the theory. 
Darwin tries to meet the objection, that organs of extreme 
perfection and complication cannot be accounted for by nat- 
ural selection, as follows : — 
“To suppose that the eye with all its inimitable contriv- 
ances for adjusting the focus to different distances, for ad- 
mitting different amounts of light, and for the correction of 
spherical and chromatic aberration, could have been formed 
by natural selection, seems, I freely confess, absurd in the 
highest degree.” 
The following sketch that Darwin gives to show how he 
imagined the vertebrate eye to have been formed is very 
instructive, as illustrating how he supposed that natural se- 
lection acts : — 
“Tf we must compare the eye to an optical instrument, we 
ought in imagination to take a thick layer of transparent 
tissue, with spaces filled with fluid, and with a nerve sensi- 
tive to light beneath, and then suppose every part of this 
layer to be continually changing slowly in density, so as to 
separate into layers of different densities and thicknesses, 
placed at different distances from each other, and with the 
