ents.
The proof that all the members of any given group of animals, or plants,
had thus descended, would be ordinarily considered sufficient to entitle
them to the rank of physiological species, for most physiologists
consider species to be definable as "the offspring of a single primitive
stock."

But though it is quite true that all those groups we call species MAY,
according to the known laws of reproduction, have descended from a
single stock, and though it is very likely they really have done so,
yet this conclusion rests on deduction and can hardly hope to establish
itself upon a basis of observation. And the primitiveness of the
supposed single stock, which, after all, is the essential part of the
matter, is not only a hypothesis, but one which has not a shadow of
foundation, if by "primitive" be meant "independent of any other living
being." A scientific definition, of which an unwarrantable hypothesis
forms an essential part, carries its condemnation within itself;
but, even supposing such a definition were, in form, tenable, the
physiologist who should attempt to apply it in Nature would soon find
himself involved in great, if not inextricable, difficulties. As we have
said, it is indubitable that offspring TEND to resemble the parental
organism, but it is equally true that the similarity attained never
amounts to identity, either in form or in structure. There is always a
certain amount of deviation, not only from the precise characters of a
single parent, but when, as in most animals and many plants, the sexes
are lodged in distinct individuals, from an exact mean between the two
parents. And indeed, on general principles, this slight deviation seems
as intelligible as the general similarity, if we reflect how complex the
co-operating "bundles of forces" are, and how improbable it is that, in
any case, their true resultant shall coincide with any mean between
the more obvious characters of the two parents. Whatever be its cause,
however, the co-existence of this tendency to minor variation with the
tendency to general similarity, is of vast importance in its bearing on
the question of the origin of species.

As a general rule, the extent to which an offspring differs from its
parent is slight enough; but, occasionally, the amount of difference is
much more strongly marked, and then the divergent offspring receives the
name of a Variety. Multitudes, of what there is every reason to believe
are such varietie