m the
standpoint of adult anatomy and from embryological data; and we
have seen through the vertebrate series a common structure
underlying wide diversity in external appearance and detailed
anatomy. We have seen a certain intermediateness of structure in the
frog, as compared with the rabbit and dog-fish, notably in the skull
and skeleton, in the circulation, in the ear, and in the reduced
myomeres; and we have seen that the rabbit passes in these
respects, and in others, through dog-fish- and frog-like stages in its
development, and this alone would be quite sufficient to suggest that
the similarities of structure are due to other causes than a primordial
adaptation to certain conditions of life.

Section 44. It has been suggested by very excellent people that these
resemblances are due to some unexplained necessity of adherence
to type, as though, the power that they assume created these
animals originally, as they are now, coupled creative ability with a
plentiful lack of ideas, and so perforce repeated itself with impotent
variations. On the other hand, we have the supposition that these are
"family likenesses," and the marks of a common ancestry. This is the
opinion now accepted by all zoologists of repute.

Section 45. It must not be for a moment imagined that it is implied
that rabbits are descended from frogs, or frogs from dog-fish, but that
these three forms are remote cousins, derived from some ancient
and far simpler progenitor. But since both rabbit and frog pass through
phases like the adult condition of the dog-fish, it seems probable that
the dog-fish has remained more like the primordial form than these
two, and similarly, the frog than the rabbit.

Section 46. Hence we may infer that the mammals were the last of
the three groups, of which we have taken types, to appear upon the
earth, and that the fishes preceded, the amphibia. Workers in an
entirely independent province, that of palaeontology, completely
endorse this supposition. The first Vertebrata to appear in the fossil
history of the world are fishes; fish spines and placoid scales
(compare dog-fish) appear in the Ordovician rocks. In the coal
measures come the amphibia; and in the Permo-triassic strata,
reptile-like mammals. In the Devonian rocks, which come between the
Silurian and the coal measures, we find very plentiful remains of
certain fish called the dipnoi, of which group three genera still survive;
they display, in numberles