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Show 466 RECAPITULATION. CHAP. XIV. doubt their weight. But it deserves especial notice that the more important objections relate to questions on which we are confessedly ignorant; nor do we know how ignorant we are. We do not kno'; all the possible transitional gradations between the simplest and the rnost perfect organs; it cannot b: ~ret~nded t~at we know all the varied means of Distnbution dunng the long lapse of years, or that we know how imperfect the Geological Record is. Grave as these several difficulties are, in my judgment they do not overthrow the theory of descent with modification. Now let us turn to the other side of the argument. Under domestication we see much variability. This seems to be mainly due to the reproductive system being eminently susceptible to .changes in the conditions of life; so that this system, when not rendered impotent, fails to reproduce 'Offspring exactly like the parent-form. Variability is governed by many complex laws,-by correlation of growth, by use ·and disuse, and by the direct action of the physical conditions of life. There is much difficulty in ascertaining how much modification our domestic productions have undergone ; but we may safely infer that the amount has been large, and that modifications can be inherited for long periods. As long as the conditions of life remain the same, we have reason to believe that a modification, which has already been inherited for many generations, may continue to be inherited for an almost infinite number of generations. On the other hand we have. evidence that variability, when it has once come In~o play, does not wholly cease; for new varieties are still. occasionally produced by our most anciently domesticated productions. Man does not actually produce variability; he only CHAP. XIV. RECAPITULATION. 407 unintentionally exposes organic bein()'s t di . f l ·.e o o new con tlons o 11e, and. th. e. n nature acts on the organi.s at I'O n, and caus. es. vanab. ility. But man can and d oes se1 e ct t h e vanat10ns given to him by nature and th 1 h . . ' us accumu ate t em In any desned manner He thus ad t . 1 d . · ap s anima s an plants for his own benefit or pleasure H d h. h · · e may o t Is met odwally, or he may do it uneonsciousl b _ servi· ng the I·n dI' VI' d uals most useful to him at yth yt 'p re 'th e Ime, w~ out any thought of altering the breed. It is cer-tain that he ean largely influence the character of b~·eed b~ selecting, in each successive generation, ina:VIdual differences so slight as to be quite inappreciable by an uneducated eye. This process of selection has b?e~ the great agency in the production of the most distinct and useful domestic breeds. That many of the breeds produced by man have to a large extent the character of natural species, is shown by the inextricable doubts whether very many of them are varieties or aboriginal species. There is no obv~ous reason why the principles which have acted so effimently under domestication should not ?av.e ~cted under nature. In the preservation nf favoured Individuals and .races, during the constantly-recurrent Struggl~ for Existence, we see the most powerful and ever-~ctln.g means of seleetion. The struggle for existenc? Inevitably _follows from the high geometrical ratio of .Inc~ease whwh. is com~on to all organic beings. This high rate of Increase Is proved by calculation, by the effects of a succession of peculiar seasons, and by the results of naturalisation, as explained in the third cha~ter. More. in~ividuals are born than can possibly ~ur;I;e. A grain In the balance will determine which Individ~al shall live and which shall die,-which variety or species shall increase in number, and which shall decrease, or finally become extinct. As the indi- |