-existing. But, independently of a predisposition
to account, if possible, for a series of changes in the organic world by
the regular action of secondary causes, we have seen that in truth many
perplexing difficulties present themselves to one who attempts to
establish the nature and reality of the specific character. And if once
there appears ground of reasonable doubt, in regard to the constancy of
species, the amount of transformation which they are capable of
undergoing may seem to resolve itself into a mere question of the
quantity of time assigned to the past duration of animate existence.

Before entering upon the reasons which may be adduced for rejecting
Lamarck's hypothesis, I shall recapitulate, in a few words, the
phenomena, and the whole train of thought, by which I conceive it to
have been suggested, and which have gained for this and analogous
theories, both in ancient and modern times, a considerable number of
votaries.

In the first place, the various groups into which plants and animals may
be thrown seem almost invariably, to a beginner, to be so natural, that
he is usually convinced at first, as was Linnaeus to the last, "that
genera are as much founded in nature as the species which compose
them."[800] When by examining the numerous intermediate gradations the
student finds all lines of demarcation to be in most instances
obliterated, even where they at first appeared most distinct, he grows
more and more sceptical as to the real existence of genera, and finally
regards them as mere arbitrary and artificial signs, invented, like
those which serve to distinguish the heavenly constellations, for the
convenience of classification, and having as little pretensions to
reality.

Doubts are then engendered in his mind as to whether species may not
also be equally unreal. The student is probably first struck with the
phenomenon, that some individuals are made to deviate widely from the
ordinary type by the force of peculiar circumstances, and with the still
more extraordinary fact, that the newly acquired peculiarities are
faithfully transmitted to the offspring. How far, he asks, may such
variations extend in the course of indefinite periods of time, and
during great vicissitudes in the physical condition of the globe? His
growing incertitude is at first checked by the reflection that nature
has forbidden the intermixture of the descendants of distinct original
stocks, or has, at least, entailed steril