lis envelope, as a complete Winged Insect, that it has three
distinct regions of the body. Do not the different periods of growth in
this highest order explain the relation of all the orders to each other?
The earliest condition of an animal cannot be its highest condition,--it
does not pass from a more perfect to a less perfect state of existence.
The history of its growth is, on the contrary, the history of its
progress in development; and therefore, when we find that the first
stage of growth in the Winged Insect transiently represents a structural
character that is permanent in the lowest order of its class, that its
second stage of growth transiently represents a structural character
that is permanent in the second order of its class, and that only in the
last stage of its existence does the Winged Insect attain its complete
and perfect condition, we may fairly infer that this division of the
class of Insects into a gradation of orders placing Centipedes lowest,
Spiders next, and Winged Insects highest, is true to Nature.

This is not the only instance in which the embryological evidence
confirms perfectly the anatomical evidence on which orders have been
distinguished, and I believe that Embryology will give us the true
standard by which to test the accuracy of our ordinal groups. In the
class of Crustacea, for instance, the Crabs have been placed above the
Lobsters by some naturalists, in consequence of certain anatomical
features; but there may easily be a difference of individual opinion as
to the relative value of these features. When we find, however, that the
Crab, while undergoing its changes in the egg, passes through a stage in
which it resembles the Lobster much more than it does its own adult
condition, we cannot doubt that its earlier state is its lower one, and
that the organization of the Lobster is not as high in the class of
Crustacea as that of the Crab. While using illustrations of this kind,
however, I must guard against misinterpretation. These embryological
changes are never the passing of one kind of animal into another kind of
animal: the Crab is none the less a Crab during that period of its
development in which it resembles a Lobster; it simply passes, in the
natural course of its growth, through a phase of existence which is
permanent in the Lobster, but transient in the Crab. Such facts should
stimulate all our young students to embryological investigation as a
most important branch of st