is then entered upon, in farther corroboration
of the instability of species. In order, it is said, that individuals
should perpetuate themselves unaltered by generation, those belonging to
one species ought never to ally themselves to those of another; but such
sexual unions do take place, both among plants and animals; and although
the offspring of such irregular connections are usually sterile, yet
such is not always the case. Hybrids have sometimes proved prolific,
where the disparity between the species was not too great; and by this
means alone, says Lamarck, varieties may gradually be created by near
alliances, which would become races, and in the course of time would
constitute what we term species.[796]

But if the soundness of all these arguments and inferences be admitted,
we are next to inquire, what were the original types of form,
organization, and instinct, from which the diversities of character, as
now exhibited by animals and plants, have been derived? We know that
individuals which are mere varieties of the same species would, if their
pedigree could be traced back far enough, terminate in a single stock;
so, according to the train of reasoning before described, the species of
a genus, and even the genera of a great family, must have had a common
point of departure. What, then, was the single stem from which so many
varieties of form have ramified? Were there many of these, or are we to
refer the origin of the whole animate creation, as the Egyptian priests
did that of the universe, to a single egg?

In the absence of any positive data for framing a theory on so obscure a
subject, the following considerations were deemed of importance to guide
conjecture.

In the first place, if we examine the whole series of known animals,
from one extremity to the other, when they are arranged in the order of
their natural relations, we find that we may pass progressively, or, at
least, with very few interruptions, from beings of more simple to those
of a more compound structure; and, in proportion as the complexity of
their organization increases, the number and dignity of their faculties
increase also. Among plants, a similar approximation to a graduated
scale of being is apparent, Secondly, it appears, from geological
observations, that plants and animals of more simple organization
existed on the globe before the appearance of those of more compound
structure, and the latter were successively formed at more mo