and, since the differences of sex are comparatively slight, or, in other
words, the sum of the forces in each has a very similar tendency, their
resultant, the offspring, may reasonably be expected to deviate but
little from a course parallel to either, or to both.

Represent the reason of the law to ourselves by what physical metaphor
or analogy we will, however, the great matter is to apprehend its
existence and the importance of the consequences deducible from it. For
things which are like to the same are like to one another; and if; in a
great series of generations, every offspring is like its parent, it
follows that all the offspring and all the parents must be like one
another; and that, given an original parental stock, with the
opportunity of undisturbed multiplication, the law in question
necessitates the production, in course of time, of an indefinitely
large group, the whole of whose members are at once very similar and
are blood relations, having descended from the same parent, or pair of
parents. 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