s, they, like all other groups
introduced into zooelogical systems, must forever remain arbitrary
divisions, as they have been hitherto. A retrospective glance at the
progress of our science during the past century, in this connection,
may perhaps help us to solve the difficulty. Linnaeus, in his System
of Nature, does not admit Families; he has only four kinds of
groups,--Classes, Orders, Genera, and Species. It was among plants that
naturalists first perceived those general traits of resemblance which
exist everywhere among the members of natural families, and added this
kind of group to the framework of their system. In France, particularly,
this method was pursued with success; and the improvements thus
introduced by the French botanists were so great, and rendered their
classification so superior to that of Linnaeus, that the botanical
systems in which Families were introduced were called natural systems,
in contradistinction especially to the botanical classification of
Linnaeus, which was founded upon the organs of reproduction, and which
received thenceforth the name of the sexual system of plants. The same
method so successfully used by botanists was soon introduced
into Zooelogy by the French naturalists of the beginning of this
century,--Lamarck, Latreille, and Cuvier. But, to this day, the
limitation of Families among animals has not yet reached the precision
which it has among plants, and I see no other reason for the difference
than the absence of a leading principle to guide us in Zooelogy.

Families, as they exist in Nature, are based upon peculiarities of form
as related to structure; but though a very large number of them have
been named and recorded, very few are characterized with anything like
scientific accuracy. It has been a very simple matter to establish such
groups according to the superficial method that has been pursued, for
the fact that they are determined by external outline renders the
recognition of them easy and in many instances almost instinctive; but
it is very difficult to characterize them, or, in other words, to trace
the connection between form and structure. Indeed, many naturalists do
not admit that Families are based upon form; and it was in trying to
account for the facility with which they detect these groups, while they
find it so difficult to characterize them, that I perceived that they
are always associated with peculiarities of form. Naturalists have
established Famili