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.
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