ages supporting the
arm, wrist, and fingers, and those sustaining the leg, ankle, and toes,
with the exception that the toes exceed the fingers in number by one.

[Illustration: SKELETON OF AN ICHTHYOSAURUS.]

[Illustration: A. SKELETON OF ANTERIOR EXTREMITY OF AN EFT.
B. SKELETON OF POSTERIOR EXTREMITY OF THE SAME.]

Yet these animals are far from being the root-forms from which all the
Vertebrata have diverged, as is evidenced from the degree of specialization
which their structure presents. If they have descended from such {178}
primitive forms as Professor Gegenbaur imagines, then they have built up a
secondary serial homology--a repetition of similar modifications--fully as
remarkable as if it were primary. The Plesiosauria--those extinct marine
reptiles of the Secondary period, with long necks, small heads, and
paddle-like limbs--are of yet higher organization than are the efts and
other Amphibia. Nevertheless they present us with a similarity of structure
between the fore and hind limb, which is so great as almost to be {179}
identity. But the Amphibia and Plesiosauria, though not themselves
primitive vertebrate types, may be thought by some to have derived their
limb-structure by direct descent from such. Tortoises, however, must be
admitted to be not only highly differentiated organisms, but to be far
indeed removed from primeval vertebrate structure. Yet certain
tortoises[182] (notably _Chelydra Temminckii_) exhibit such a remarkable
uniformity in fore and hind limb structure (extending even up to the
proximal ends of the humerus and femur) that it is impossible to doubt its
independent development in these forms.

[Illustration: SKELETON OF A PLESIOSAURUS.]

Again in the Potto (Perodicticus) there is an extra bone in the foot,
situated in the transverse ligament enclosing the flexor tendons. It is
noteworthy that in the _hand_ of the same animal a serially homologous
structure should also be developed.[183] In the allied form called the slow
lemur (Nycticebus) we have certain arrangements of the muscles and tendons
of the hand which reproduce in great measure those of the foot and _vice
versa_.[184] And in the Hyrax another myological resemblance appears.[185]
It is, however, needless to multiply instances which can easily be produced
in large numbers if required.

Secondly, with regard to teratology, it is notorious that similar
abnormalities are often found to co-exist in both the pelvic and t