l 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.

Fritz Muller argues that this close similarity in so many points
of structure must, in accordance with the views advanced by me, be
accounted for by inheritance from a common progenitor. But as the vast
majority of the species in the above two families, as well as most
other crustaceans, are aquatic in their habits, it is improbable in the
highest degree that their common progenitor should have been adapted for
breathing air. Muller was thus led carefully to examine the apparatus in
the air-breathing species; and he found it to differ in each in several
important points, as in the position of the orifices, in the manner in
which they are opened and closed, and in some accessory details. Now
such differences are intelligible, and might even have been expected, on
the supposition that species belonging to distinct families had slowly
become adapted to liv