es simply by bringing together a number of animals
resembling each other more or less closely, and, taking usually the name
of the Genus to which the best known among them belongs, they have given
it a patronymic termination to designate the Family, and allowed the
matter to rest there, sometimes without even attempting any description
corresponding to those by which Genus and Species are commonly defined.

For instance, from _Canis_, the Dog, _Canidae_ has been formed, to
designate the whole Family of Dogs, Wolves, Foxes, etc. Nothing can be
more superficial than such a mode of classification; and if these
groups actually exist in Nature, they must be based, like all the other
divisions, upon some combination of structural characters peculiar to
them. We have seen that Branches are founded upon the general plan of
structure, Classes on the mode of executing the plan, Orders upon the
greater or less complication of a given mode of execution, and we shall
find that form, as _determined by structure_, characterizes Families. I
would call attention to this qualification of my definition; since, of
course, when speaking of form in this connection, I do not mean those
superficial resemblances in external features already alluded to in
my remarks upon Parallel or Collateral Types. I speak now of form as
controlled by structural elements; and unless we analyze Families in
this way, the mere distinguishing and naming them does not advance our
science at all. Compare, for instance, the Dogs, the Seals, and the
Bears. These are all members of one Order,--that of the Carnivorous
Mammalia. Their dentition is peculiar and alike in all, (cutting teeth,
canine teeth, and grinders,) adapted for tearing and chewing their
food; and their internal structure bears a definite relation to their
dentition. But look at these animals with reference to form. The Dog is
comparatively slender, with legs adapted for running and hunting his
prey; the Bear is heavier, with shorter limbs; while the Seal has a
continuous uniform outline adapted for swimming. They form separate
Families, and are easily recognized as such by the difference in their
external outline; but what is the anatomical difference which produces
the peculiarity of form in each, by which they have been thus
distinguished? It lies in the structure of the limbs, and especially in
that of the wrist and fingers. In the Seal the limbs are short, and the
wrists are on one continuous line w