multiplied, since introduced from the Old World not long ago.  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.

Here also comes in that general “parallelism between the order of succession of animals and plants in geological times, and the gradation among their living representatives” from low to highly organized, from simple and general to complex and specialized forms; also “the parallelism between the order of succession of animals in geological times and the changes their living representatives undergo during their embryological growth,” as if the world were one prolonged gestation.  Modern science has much insisted on this parallelism, and to a certain extent is allowed to have made it out.  All these things, which conspire to prove that the ancient and the recent forms of life “are somehow intimately connected together in one grand system,” equally conspire to suggest that the connection is one similar or analogous to generation.  Surely no naturalist can be blamed for entering somewhat confidently upon a field of speculative inquiry which here opens so invitingly; nor need former premature endeavors and failures utterly dishearten him.

All these things, it may naturally be said, go to explain the order, not the mode, of the incoming of species.  But they all do tend to bring out the generalization expressed by Mr. Wallace in the formula that “every species has come into existence coincident both in time and space with preexisting closely-allied species.”  Not, however, that this is proved even of existing species as a matter of general fact.  It is obviously impossible to prove anything of the kind.  But we must concede