There was no wild American
stock. Yet in the times of the mastodon and megatherium, at the dawn of
the present period, wild-horses--certainly very much like the existing
horse--roamed over those plains in abundance. On the principle of
original and direct created adaptation of species to climate and other
conditions, why were they not reproduced, when, after the colder
intervening era, those regions became again eminently adapted to such
animals? Why, but because, by their complete extinction in South
America, the line of descent was there utterly broken? Upon the
ordinary hypothesis, there is no scientific explanation possible of
this series of facts, and of many others like them. Upon the new
hypothesis, "the succession of the same types of structure within the
same areas during the later geological periods ceases to be mysterious,
and is simply explained by inheritance." Their cessation is failure of
issue.
Along with these considerations the fact (alluded to on page 98) should
be remembered that, as a general thing, related species of the present
age are geographically associated. The larger part of the plants, and
still more of the animals, of each separate country are peculiar to it;
and, as most species now flourish over the graves of their by-gone
relatives of former ages, so they now dwell among or accessibly near
their kindred species.
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