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 that the known facts strongly suggest such an inference.  And since species are only congeries of individuals, and every individual came into existence in consequence of preexisting individuals of the same sort, so leading up to the individuals with which the species began, and since the only material sequence we know of among plants and animals is that from parent to progeny, the presumption becomes exceedingly strong that the connection of the incoming with the preexisting species is a genealogical one.

Here, however, all depends upon the probability that Mr. Wallace’s inference is really true.  Certainly it is not yet generally accepted; but a strong current is setting towards its acceptance.

So long as universal cataclysms were in vogue, and all life upon the earth was thought to have been suddenly destroyed and renewed many times in succession, such a view could not be thought of.  So the equivalent view maintained by Agassiz, and formerly, we believe, by D’Orbigny, that, irrespectively of general and sudden catastrophes, or any known adequate physical cause, there has been a total depopulation at the close of each geological period or formation, say forty or fifty times, or more, followed by as many independent great acts of creation, at which alone have species been originated, and at each of which a vegetable and an animal kingdom were produced entire and complete, full-fledged, as flourishing, as wide-spread and populous,