a law of wider application than
this, that a body impelled by two forces takes the direction of their
resultant. But living bodies may be regarded as nothing but extremely
complex bundles of forces held in a mass of matter, as the complex
forces of a magnet are held in the steel by its coercive force; 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" he 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 inv