decide how far even the same terms ought to be employed in describing the
eyes of the Cephalopoda and Vertebrata. It is, of course, open to any one
to deny that the eye in either case could have been developed through the
natural selection of successive slight variations; but if this be admitted
in the one case it is clearly possible in the other; and fundamental
differences of structure in the visual organs of two groups might have been
anticipated, in accordance with this view of their manner of formation. As
two men have sometimes independently hit on the same invention, so in the
several foregoing cases it appears that natural selection, working for the
good of each being, and taking advantage of all favourable variations, has
produced similar organs, as far as function is concerned, in distinct
organic beings, which owe none of their structure in common to inheritance
from a common progenitor.

Fritz Muller, in order to test the conclusions arrived at in this volume,
has followed out with much care a nearly similar line of argument. Several
families of crustaceans include a few species, possessing an air-breathing
apparatus and fitted to live out of the water. In two of these families,
which were more especially examined by Muller, and which are nearly related
to each other, the species agree most closely in all important characters:
namely in their sense organs, circulating systems, in the position of the
tufts of hair within their complex stomachs, and lastly in the whole
structure of the water-breathing branchiae, even to the microscopical hooks
by which they are cleansed. Hence it might have been expected that in the
few species belonging to both families which live on the land, the equally
important air-breathing apparatus would have been the same; for why should
this one apparatus, given for the same purpose, have been made to differ,
while all the other important organs were closely similar, or rather,
identical.