* Here an article would be in place, explaining the arrangements in Nature for cross-fertilization, or wide-breeding, in plants, through the agency, sometimes of the winds, but more commonly of insects; the more 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 too technical or too fragmentary or special to be here reproduced.  Indeed, a popular essay is now hardly needed, since the topic has been fully presented, of late years, in the current popular and scientific journals, and in common educational works and text-books, so that it is in the way of becoming a part—­and a most inviting part—­of ordinary botanical instruction. use of sexual reproduction?  Not simply 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 cross-fertilization in hermaphrodite plants, do not subserve some most important purpose?  How and why the union of two organisms, or generally of two very minute portions of them, should reenforce vitality, we do not know, and can hardly 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

Do Species wear out? and if not, why not?

The 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 species 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.