lation in the group, then you will hold the true thread
which connects all the productions of nature; you will have a just
idea of its progress, and you will be convinced that the most simple
of its living productions have successively given existence to all
the others.

"_The series which constitutes the animal scale resides in the
distribution of the groups, and not in that of the individuals and
species._

"I have already said[166] that by this shaded graduation in the
complication of structure I do not mean to speak of the existence
of a linear and regular series of species or even genera: such a
series does not exist. But I speak of a quite regularly graduated
series in the principal groups, _i.e._, in the principal system of
organizations known, which give rise to classes and to great
families, series most assuredly existing both among animals and
plants, although in the consideration of genera, and especially in
that of species, it offers many lateral ramifications whose
extremities are truly isolated points.

"However, although there has been denied, in a very modern work, the
existence in the animal kingdom of a single series, natural and at
the same time graduated, in the composition of the organization of
beings which it comprehends, series in truth necessarily formed of
groups subordinated to each other as regards structure and not of
isolated species or genera, I ask where is the well-informed
naturalist who would now present a different order in the
arrangement of the twelve classes of the animal kingdom of which I
have just given an account?

"I have already stated what I think of this view, which has seemed
sublime to some moderns, and indorsed by _Professor Hermann_."

Each distinct group or mass of forms has, he says, its peculiar system
of essential organs, but each organ considered by itself does not follow
as regular a course in its degradations (modifications).

"Indeed, the least important organs, or those least essential to
life, are not always in relation to each other in their improvement
or their degradation; and an organ which in one species is atrophied
may be very perfect in another. These irregular variations in the
perfecting and in the degradation of non-essential organs are due to
the fact that these organs are oftener than the others submitted to
the influences of external circumstances, and give rise to a