f in each case. By "special tendency" is
meant one the laws and conditions of which are as yet unknown, but which is
analogous to the innate power and tendency possessed by crystals similarly,
to build up certain peculiar and very definite forms.

First, with regard to comparative anatomy. The correspondence between the
thoracic and pelvic limbs is notorious. Professor Gegenbaur has lately
endeavoured[180] to explain this resemblance by the derivation of each limb
from a primitive form of fin. This fin is supposed to have had a marginal
external (radial) series of cartilages, each of which supported a series of
secondary cartilages, starting from the inner (ulnar) side of the distal
part of the supporting marginal piece. The root marginal piece would become
the humerus or femur, as the case might be: the second marginal piece, with
the piece attached to the inner side of the distal end of the root marginal
piece, would together form either the radius and ulna or the tibia and
fibula, and so on.

Now there is little doubt (from _a priori_ considerations) but that the
special differentiation of the limb bones of the higher Vertebrates has
been evolved from anterior conditions existing in some fish-like form or
other. But the particular view advocated by the learned Professor is open
to criticism. Thus, it may be objected against this view, first, that it
takes no account of the radial ossicle which becomes so enormous in the
mole; secondly, that it does not explain the extra series of ossicles {177}
which are formed on the _outer_ (radial or marginal) side of the paddle in
the Ichthyosaurus; and thirdly, and most importantly, that even if this had
been the way in which the limbs had been differentiated, it would not be at
all inconsistent with the possession of an innate power of producing, and
an innate tendency to produce similar and symmetrical homological
resemblances. It would not be so because resemblances of the kind are found
to exist, which, on the Darwinian theory, must be subsequent and secondary,
not primitive and ancestral. Thus we find in animals of the eft kind
(certain amphibians), in which the tarsus is cartilaginous, that the carpus
is cartilaginous likewise. And we shall see in cases of disease and of
malformation what a tendency there is to a similar affection of homologous
parts. In efts, as Professor Gegenbaur himself has pointed out,[181] there
is a striking correspondence between the bones or cartil