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 the members of which
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 being." A scientific
definition, of which an unwarrantable hypothesis forms an essential part,