dividual organ
undergoes in its passage from the embryonic to the adult condition, or
from the simple and incomplete to the complex and perfect. 2. Ascending
metamorphoses, including those changes of form manifested in the same
adult organism by the several parts of which it consists--those parts
being typically identical or homologous, such as the parts of the
flower, or, in animals, the vertebrae, &c. 3. Collateral metamorphoses,
comprising those permutations of form and function manifested in
homologous organs in the different groups of organisms, classes, orders,
genera, species, &c.

Thus, in the first instance, we have a comparative examination of the
form of each or any separate part of the same individual at different
epochs in its life-history; in the second we have a similar comparison
instituted between the several parts of the same organism which
originally were identical in appearance, but which have in course of
evolution altered in character. In the third form we have the
comparative view not of one organ at different times, nor of the several
parts of one organism, but of the constituent elements pertaining to
those aggregates of individuals to which naturalists apply the terms
classes, orders, &c.

In successive metamorphosis we have a measure of the amount of change
and of the perfection of structure to which each separate organ attains.

In ascending metamorphosis we have a gauge of the extent of alteration
that may take place in the several homologous organs under existing
circumstances.

In collateral metamorphosis, in the same way, we have an illustration of
the degree of change possible in aggregates of organisms under existing
circumstances.

Now it is clear that from an investigation of all three classes just
mentioned, we shall be able to gain an idea of those points which are
common to all parts, to all individuals or to all aggregates, and those
that are peculiar to some of them, and, by eliminating the one from the
other, we shall arrive at conclusions which will be more or less
generally accurate or applicable, according to the ability of the
student and the extent to which the comparative analysis is earned. It
is thus that morphologists have been enabled to frame types or standards
of reference, and systematists to collocate the organisms they deal with
into groups. These standards and groups are more or less artificial
(none can be entirely natural) in proportion to the amount of k