diversity of species so considerable and so singularly ordered that
instead of being able to arrange them, like the groups, in a single
simple linear series under the form of a regular graduated scale,
these very species often form around the groups of which they are
part lateral ramifications, the extremities of which offer points
truly isolated.

"There is needed, in order to change each internal system of
organization, a combination of more influential circumstances, and
of more prolonged duration than to alter and modify the external
organs.

"I have observed, however, that, when circumstances demand, nature
passes from one system to another without making a leap, provided
they are allies. It is, indeed, by this faculty that she has come to
form them all in succession, in proceeding from the simple to the
more complex.

"It is so true that she has the power, that she passes from one
system to the other, not only in two different families which are
allied, but she also passes from one system to the other in the same
individual.

"The systems of organization which admit as organs of respiration
true lungs are nearer to systems which admit gills than those which
require tracheae. Thus not only does nature pass from gills to lungs
in allied classes and families, as seen in fishes and reptiles, but
in the latter she passes even during the life of the same
individual, which successively possesses each system. We know that
the frog in the tadpole state respires by gills, while in the more
perfect state of frog it respires by lungs. We never see that nature
passes from a system with tracheae to a system with lungs.

"_It is not the organs, i.e., the nature and form of the parts of
the body of an animal, which give rise to the special habits and
faculties, but, on the contrary, its habits, its mode of life, and
the circumstances in which individuals are placed, which have, with
time, brought about the form of its body, the number and condition
of its organs, finally the faculties which it possesses._

* * * * *

"Time and favorable circumstances are the two principal means which
nature employs to give existence to all her productions. We know
that time has for her no limit, and that consequently she has it
always at her disposition.

"As to the circumstances of which she has need (_besoin_) and which
she em