the frogs and toads, anourous Batrachians, of which we have at present no relic of any kind linking them on to the Eft group on the one hand, or to reptiles on the other.
The only instance in which an approach towards a series of nearly related forms has been obtained is the existing horse, its predecessor Hipparion and other extinct forms.

But even here there is no proof whatever of modification by minute and infinitesimal steps; _a fortiori_ no approach to a proof of modification by "Natural Selection," acting upon indefinite fortuitous variations.

On the contrary, the series is an admirable example of successive modification in one special direction along one beneficial line, and the teleologist must here be allowed to consider that one {134} motive of this modification (among probably an indefinite number of motives inconceivable to us) was the relationship in which the horse was to stand to the human inhabitants of this planet.

These extinct forms, as Professor Owen, remarks,[129] "differ from each other in a greater degree than do the horse, zebra, and ass," which are not only good _zoological_ species as to form, but are species _physiologically_, _i.e._ they cannot produce a race of hybrids fertile _inter se_.
As to the mere action of surrounding conditions, the same Professor remarks:[130] "Any modification affecting the density of the soil might so far relate to the changes of limb-structure, as that a foot with a pair of small hoofs dangling by the sides of the large one, like those behind the cloven hoof of the ox, would cause the foot of Hipparion, _e.g._, and _a fortiori_ the broader based three-hoofed foot of the Palaeothere, to sink less deeply into swampy soil, and be more easily withdrawn than the more concentratively simplified and specialized foot of the horse.

Rhinoceroses and zebras, however, tread together the arid plains of Africa in the present day; and the horse has multiplied in that half of America where two or more kinds of tapir still exist.