ticated animals and plants, so must there have
existed intermediate forms between all the species of the same group,
not differing more than recognised varieties differ. It must not be
supposed necessary that there should have existed forms exactly
intermediate in character between any two species of a genus, or even
between any two varieties of a species; but it is necessary that there
should have existed every intermediate form between the one species or
variety of the common parent, and likewise between the second species or
variety, and this same common parent. Thus it does not necessarily
follow that there ever has existed <a> series of intermediate sub-varieties
(differing no more than the occasional seedlings from the same
seed-capsule,) between broccoli and common red cabbage; but it is
certain that there has existed, between broccoli and the wild parent
cabbage, a series of such intermediate seedlings, and again between red
cabbage and the wild parent cabbage: so that the broccoli and red
cabbage are linked together, but not _necessarily_ by directly
intermediate forms{309}. It is of course possible that there _may_ have
been directly intermediate forms, for the broccoli may have long since
descended from a common red cabbage, and this from the wild cabbage. So
on my theory, it must have been with species of the same genus. Still
more must the supposition be avoided that there has necessarily ever
existed (though one _may_ have descended from <the> other) directly
intermediate forms between any two genera or families--for instance
between the genus _Sus_ and the Tapir{310}; although it is necessary
that intermediate forms (not differing more than the varieties of our
domestic animals) should have existed between Sus and some unknown
parent form, and Tapir with this same parent form. The latter may have
differed more from Sus and Tapir than these two genera now differ from
each other. In this sense, according to our theory, there has been a
gradual passage (the steps not being wider apart than our domestic
varieties) between the species of the same genus, between genera of the
same family, and between families of the same order, and so on, as far
as facts, hereafter to be given, lead us; and the number of forms which
must have at former periods existed, thus to make good this passage
between different species, genera, and families, must have been almost
infinitely great.

{309} In the _Origin_, Ed. i. p. 28