ulations of their feelers, etc. Among Worms, the
different Genera of the Leech Family are combined upon the form of the
disks by which they attach themselves, upon the number and arrangement
of their eyes, upon the structure of the hard parts with which the mouth
is armed, etc. Among Cephalopods, the Family of Squids contains several
Genera distinguished by the structure of the solid shield within the
skin of the back, by the form and connection of their fins, by the
structure of the suckers with which their arms are provided, by the
form of their beak, etc. In every Class, we find throughout the Animal
Kingdom that there is no sound basis for the discrimination of Genera
except the details of their structure; but in order to define them
accurately an extensive comparison of them is indispensable, and in
characterizing them only such features should be enumerated as are truly
generic; whereas in the present superficial method of describing them,
features are frequently introduced which belong not only to the whole
Family, but even to the whole Class which includes them.

X.

There remains but one more division of the Animal Kingdom for our
consideration, the most limited of all in its circumscription,--that
of Species. It is with the study of this kind of group that naturalists
generally begin their investigations. I believe, however, that the study
of Species as the basis of a scientific education is a great mistake.
It leads us to overrate the value of Species, and to believe that they
exist in Nature in some different sense from other groups; as if there
were something more real and tangible in Species than in Genera,
Families, Orders, Classes, or Branches. The truth is, that to study a
vast number of Species without tracing the principles that combine
them under more comprehensive groups is only to burden the mind with
disconnected facts, and more may be learned by a faithful and careful
comparison of a few Species than by a more cursory examination of a
greater number. When one considers the immense number of Species already
known, naturalists might well despair of becoming acquainted with them
all, were they not constructed on a few fundamental patterns, so that
the study of one Species teaches us a great deal for all the rest. De
Candolle, who was at the same time a great botanist and a great teacher,
told me once that he could undertake to illustrate the fundamental
principles of his science with the aid of