first one was made by Lamarck, who, taking the really different ideas
of descent and evolution as one, made use of the hypothesis of
transmutation; thus becoming the pioneer of Darwinism. The other attempt
was made by Darwin in his theory of natural selection, or struggle, for
existence, and is called the "Theory of Selection."

In defining our problem, therefore, we find ourselves under the influence
of a scientific law of development. The simple problem which we started
from has developed into a trinity of problems and attempts at solution. The
simple question of the origin of species led us into the dilemma of a
_generatio aequivoca_, or a descent; the hypothesis of a descent led to the
dilemma of a heterogenetic conception, or an evolution; and the hypothesis
of an evolution rendered necessary the attempt at explaining this
evolution, and showed Darwin's method of explaining it by his selection
theory. It will be well for the reader to keep distinctly in mind the
difference between these problems and theories, in following our
investigations, even if we cannot arrange our historical sketch according
to the natural principle of division arising from these differences.

For it lies in the nature of the question itself, that {28} these theories,
in their historical progress, did not appear singly, but together. Those
who inclined to the theory of a descent of species could claim for it the
attention of scientific investigators only after having also made the
attempt at conceiving this descent in a concrete way, and according to
certain analogies of observation. The only analogy of the kind appeared in
the sphere of individual development and individual differences on the one
hand, and in that of closely related characters of allied species on the
other; and thus led of itself to the evolution theory. As soon as the
naturalists thought they had found the causes of such an evolution of the
species, they naturally placed these causes in the foreground of their
demonstrations, and erected upon them the structure of their entire theory;
thus treating descent, evolution and selection as one single and
indissolubly connected theory. But this manner of treating the question had
also its dangers, which have already caused a great deal of confusion and
misunderstanding, as well as much unprofitable controversy. Often friends
and enemies of the theories placed that which was in favor of the theory of
descent to the credit of the