ÆäÀÌÁö À̹ÌÁö
PDF
ePub

Some suppose that races cannot be perpetuated indefinitely even by keeping up the conditions under which they were fixed; but the high antiquity of several, and the actual fixity of many of them, nega tive this assumption. "To assert that we could not breed our cart and race horses, long and short horned cattle, and poultry of various breeds, for almost an infinite number of generations, would be opposed to all experience."

Why varieties develop so readily and devinto so widely under domestication, while they are apparently so rare or so transient in free Nature, may easily bo shown. In Nature, even with hermaphrodite plants, there is a vast amount of cross-fertilization among various individuals of the same species. The inevi table result of this (as was long ago explained in this Journal') is to repress variation, to keep the mass of a species comparatively homogeneous over any aren in which it abounds in individuals. Starting from a suggestion of the late Mr. Knight, now so familiar, that close interbreeding diminishes vigor and fertill ty; and perceiving that bisexuality is ever aimed at in Nature-being attained physiologically in numerous cases where it is not structurally--Mr. Darwin has worked out the subject in detail, and shown how general is the concurrence, either habitual or occasional, of two hermaphrodite individuals in the reproduction of their kind; and has drawn the philosophical infer Volume xvii. (2), 1854, p. 13.

·↑ We suspect that this is not an ultimate fact, but a natural connequence of inheritance--the inheritance of disease or of tendency to dinease, which close interbreeding perpetuates and accumulates, but wide breeding may neutralize or eliminate.

once that probably no organic being self-fertilizes indefinitely; but that a cross with another individual is occasionally-perhaps at very long intervals-indispensable. We refer the reader to the section on the intercrossing of individuals (pp. 96-101), and also to an article in the Gardeners' Chronicle a year and a half ngo, for the details of a very interesting contribution to science, irrespective of theory.

In domestication, this intercrossing may be prevented; and in this prevention lies the art of producing varieties. But "the art itself is Nature," sinco the whole art consists in allowing the most universal of all natural tendencies in organic things (inheritance) to operato uncontrolled by other and obviously inci dental tendencies. No new power, no artificial force, is brought into play either by separating the stock of a desirable variety so as to prevent mixture, or by selecting for breeders those individuals which most largely partake of the pecularities for which the breed Ja valued.'

Wo HCO everywhere around us the remarkablo results which Naturo may bo said to have brought about under artificial selection and separation. Could she accomplish similar results when left to herself? Variations might begin, wo know they do begin, in a wild state. But would any of them be preserved and carried to an equal degree of deviation? Is there any thing in Nature which in the long-run may answer to

1 The rules and processes of breeders of animals, and their results, are so familiar that they need not be particularized. Less is popularly known about the production of vegetable races, We refer our readers back to this Journal, vol, xxvii., pp. 410–112 (May, 1859), for an abstract of the papers of M. Vilmorin upon this subject,

artificial selection? Mr. Darwin thinks that thoro is; and Natural Selection is tho key-noto of his discourse.

As a preliminary, ho has a short chapter to show that there is variation in Nature, and therefore something for natural selection to act upon. Ho readily shows that such mere variations as may be directly referred to physical conditions (like the depauperation of plants in a sterile soil, or their dwarfing as they approach an Alpine summit, the thicker fur of an animal from far northward, etc.), and also those individual differences which we everywhere recognize but do not pretend to account for, are not separable by any assignable line from more strongly-marked varieties; likewise that there is no clear demarkation between the latter and sub-species, or varieties of the higest grado (distinguished from species not by any known inconstancy, but by the supposed lower importance of their characteristics); nor between these and recognized species. "These differences blend into each other in an insensible series, and the series impresses the mind with an idea of an actual passage."

This gradation from species downward is well mado out. To carry it one step farther upward, our author presents in a strong light the differences which prevail among naturalists as to what forms should be admit ted to the rank of species. Some genera. (and theso in some countries) give rise to far more discrepancy than others; and it is concluded that the large or dominant genera are usually the most variable. In a flora so small as the British, 182 plants, generally reckoned as varieties, have been ranked by some botanists as species. Selecting the British genera which

Include the most polymorphous forms, it appears that Babington's Flora gives them 251 species, Bentham's only 112, a difference of 139 doubtful forms. These are nearly the extreme views, but they are the views of two most capable and most experienced judges, in respect to one of the best-known floras of the world. The fact is suggestive, that the best-known countries furnish the greatest number of such doubtful cases. Illus trations of this kind may be multiplied to a great extent. They make it plain that, whether species in Nature are aboriginal and definite or not, our practical conclusions about them, as embodied in systematic works, are not facts but judgments, and largely fallible judgments.

How much of the actual coincidence of authorities is owing to imperfect or restricted observation, and to one naturalist's adopting the conclusions of another without independent observation, this is not the place to consider. It is our impression that species of ani mals are more definitely marked than those of plants; this may arise from our somewhat extended acquaintance with the latter, and our ignorance of the former. But we are constrained by our experience to admit the strong likelihood, in botany, that varieties on the one hand, and what are called closely-related species on the other, do not differ except in degree. Whenever this wider difference separating the latter can be spanned by intermediate forms, as it sometimes is, no botanist long resists the inevitable conclusion. Whenever, therefore, this wider difference can be shown to be compatible with community of origin, and explained through natural selection or in any other way, we are

ready to adopt the probable conclusion; and wo 800 beforehand how strikingly the actual geographical association of related species favors the broader view. Whether we should continue to regard the forms in question as distinct species, depends upon what meaning we shall finally attach to that term; and that depends upon how far the doctrine of derivation can bo carried back and how well it can be supported.

In applying his principle of natural selection to the work in hand, Mr. Darwin assumes, as we havo scen: 1. Some variability of animals and plants in nature; 2. The absence of any definite distinction bo tween slight variations, and varieties of the highest grade; 3. The fact that naturalists do not practically agree, and do not increasingly tend to agree, as to what forms are species and what are strong varieties, thus rendering it probable that there may bo no essential and original difference, or no possibility of ascertaining it, at least in many cases; also, 4. That the most flourishing and dominant species of the larger genera on an average vary most (a proposition which can be substantiated only by extensive comparisons, the details of which are not given); and, 5. That in largo genera the species are apt to be closely but unequally allied together, forming little clusters round certain species-just such clusters as would be formed if wo suppose their members once to have been satellites or varieties of a central or parent species, but to havo attained at length a wider divergence and a specific character. The fact of such association is undeniable; and the-uso which Mr. Darwin makes of it seems fair and natural.

« ÀÌÀü°è¼Ó »