provides conclusive evidence of
evolution, for it is most reasonable to regard the "theme" in every case
as a product of common inheritance, while the variations of any theme are
best understood as the results of adaptive changes in various directions.
But the examples have disclosed a larger relation and a principle of wider
scope, as indeed the assignment of all these tribes to the single natural
group of the _carnivora_ implies. These tribes are put together because
comparative anatomy finds that the common characters of all cats are
fundamentally like those of all dogs and bears and seals, and in these
common qualities the carnivora differ from all other mammalia. Does this
mean that the branches which bear respectively the various members of the
several tribes are outgrowths of a single limb of the evolving animal
tree? Science does not hesitate to give an affirmative answer, because, as
in the case of the similar but varying domestic cats, no other explanation
of tribal resemblance in structure seems so reasonable and natural.

So far the examples have been taken from one order of the highest class of
backboned animals, called mammalia. When our survey is extended to other
divisions of this class, additional laws of organic relationship are
discovered. If in a series of evolving generations the line of
modification proceeding from a terrestrial animal like a cat to
semi-aquatic and marine types substantially like an otter and a seal should
be carried further, it will inevitably lead to forms possessing characters
such as those displayed by whales and the related porpoises, dolphins, and
narwhals of the order cetacea. In their make-up all of these animals
clearly possess the general characteristics of mammals, and they
constitute collectively another limb which has sprung from the same stock
as the carnivora, although at an earlier time. This we believe because of
their plan of body and because their peculiar organization fits them even
more perfectly than the seals for aquatic existence that is their only
possible mode of life. In the case of the whales the bony framework of the
fore limb is again like that of the cat's leg, although the whole
structure is a flexible finlike paddle. The hind limb has disappeared as
an efficient organ, but the significant fact is that small rudiments of
hind limbs are present just where corresponding structures are placed in
the seal. These vestiges cannot be reasonably accounted fo