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Show 346 DARWINIAN A. those arrangements inN ature to secure cross-fertilization in the species, either constantly or occasionally, which are so general, so varied and diverse, and, we may add, so exquisite and wonderful, that, once propounded we see that it must be true.1 What else, indeed is ~he meaning and use of sexual reproduction? Not ~imply increase of numbers ; for that is otherwise . effectually provided for by budding propagation in plants and many of the lower animals. There are plants, indeed, of the lower sort (such as diatoms), in which the whole multiplication takes place in this way, and with great rapidity.' These also have sexual reproduction ; but in it two old individuals are always destroyed to make a single new one ! Here propagation diminishes the number of individuals fifty per cent. Who can suppose that such a costly process as this, and that all the exquisite arrangements for crossfertilization in hermaphrodite plants, do not subserve some most important purpose? How and why the union of two organisms, or generally of two very mi- 1 Here an article would be in place, explaining the arrangements in Nature for cross-fertilization, or wide-breeding, in plants, th1·ough the ·agency, sometimes of the winds, but more commonly of inse,cti:l ; the m~re so, since the development of the principle, the appreciation of its importance, and its confirmation by abundant facts, are mainly due to Mr. Darwin. But our reviews and notices of his early work "On the Contrivances in Nature for the Fertilization of Orchids by Means of Insects," in 1862, and his various subsequent papers upon other parts of this subject, are either to otechnical or too fragme~tary or special to be here reproduced. Indeed, a popular essay IS now hardly needed, since the topic has been fully presented, of late years, i~ the current popular and scientific journals, and in common educatwnal works and text-books, so that it is in the way of becoming a part-and a most inviting part-of ordinary botanical instruction. DURATION OF SPECIES. 34:7 nute portions of them, should reenforce vitality, we do not know, and can hardJy conjecture. But this must be the meaning of sexual reproduction. The conclusion of the matter, from the scientific point of view, is, that sexually-propagated varieties or races, although liable to disappear through change, need not be expected to wear out, and there is no proof that they do ; but, that non-sexually propagated varieties,_ though not especially liable to change, may theoretically be expected to wear out, but to be a very long time about it. II. .l)o Species wear out? and if not, why not f TI-IE question we have just been considering was merely whether races are, or may be, as enduring as species. As to the inherently unlimited existence of species themselves, or the contrary, this, as we have said, is a geological and very speculative problem. Not a few geologists and naturalists, however, have concluded, or taken for granted, that speqies have a natural term of existence-that they culminate, decline, and disappear through exhaustion of specific vitality, or some equivalent internal . cause. As might be expected from the nature of the inquiry, the facts which bear upon the question are far from decisive. If the fact that species in general have not been interminable, but that one after another in long succession has become extinct, would seem to warrant this conclusion, the persistence through immense periods of no incon- |