{307} It is interesting to find the argument from sterility given so prominent a place. In a corresponding passage in the _Origin_, Ed. i. p. 480, vi. p. 659, it is more summarily treated. The author gives, as the chief bar to the acceptance of evolution, the fact that "we are always slow in admitting any great change of which we do not see the intermediate steps"; and goes on to quote Lyell on geological action. It will be remembered that the question of sterility remained a difficulty for Huxley. {308} Similar statements occur in the Essay of 1842, p. 24, note 1, and in the _Origin_, Ed. i. p. 299. As I suppose that species have been formed in an analogous manner with the varieties of the domesticated 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