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 live
more and more out of water, and to breathe the air. For these species,
from belonging to distinct families, would have differed to a certain
extent, and in accordance with the principle that the nature of each
variation depends on two factors, viz., the nature of the organism and that
of the surrounding conditions, their variability assuredly would not have
been exactly the same. Consequently natural selection would have had
different materials or variations to work on, in order to arrive at the
same functional result; and the structures thus acquired would almost
necessarily have differed. On the hypothesis of separate acts of creation
the whole case remains unintelligible. This line of argument seems to have
had great weight in leading Fritz Muller to accept the views maintained by
me in this volume.

Another distinguished zoologist, the late Professor Claparede, has argued
in the same manner, and has arrived at the same result. He shows that
there are parasitic mites (Acaridae), belonging to distinct sub-families
and families, which are furnished with hair-claspers. These organs must
have been independently developed, as they could not have been inherited