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Show 326 DARWINISM CHAJ>. of the species or race, yet he clearly saw that it was not always and necessarily advantageous. ~e sa_ys : "The mo t important conclusion at which I have arnved 1s, that the mere act of intercrossing by itself does no goo~. . The. good depends on the individuals w~ich are c~ossed dr~ermg shghtly in constitution owinO' to therr progemtors havmg been sub- ]. ected durincr 'several crenerations to slightly different con-b b . 1 1 ditions. This conclusion, as we shall hereafter sec, 1s c osc y connected with various important physiological problems, such as the benefit derived from sliO'ht changes in the conditions of life." 1 Mr. Darwin has also badduced much direct evidence proving that slight changes in the c~ndi~i~ns of life ~rc beneficial to both animals and plants, mamtammg or re. tonng their viO'our and fertility in the same way as a favonrahle cross se~ms to restore it.2 It is, I believe, by a carefnl consideration of these two classes of facts that we shall find the clue to the labyrinth in which this subject has appeared to involve us. Supposed E'Vil Res~tlts of Close InterbTeeding. Just as we have seen that intercrossing is not necessarily good, we shall be forced to admit that close inter~reedi:1 g is not necessarily bad. Our finest breeds of dorr_w~tlC ~mm~1h have been thus produced, and by a careful statistical mqmry Mr. George Darwin has shown that the most constant and long- continued intermarriages among the British aristo:::racy have produced no prejudicial results. The rabbits on P?rto Santo are all the produce of a single female; they have hved on the same small island for 4 7 0 years, and they still abound there and appear to be vigorous and healthy (see p. 161 ). We have, however, on the other hand, overwhelming evidence that in many cases, among our domestic animals and cultivated plants, close interbreeding does produce bad results, and the apparent contradiction may perhaps be explamcd on the same general principles, and under similar limitatim:s, a were found to be necessary in defining the value of mtcrcrossing. It appears probable, then, _that it i~ not .interbreeding in itself that is hurtful, but mterbreedmg without 1 Cross- and Self-Fertilisation, p. 27. 2 Animals and Plants, vol. ii. p. 145. XI THE SPECIAL COLOURS OF PLANTS 327 rig~d selection or some change of conditions. Under nature, as m th~ case of the ~orto Santo rabbits, the rapid increase of these amrnals would m a very few years stock the island with a full population, and thereafter natural selection would act powerfully in the preservation only of the healthiest and the most fertile, and under these conditions no deterioration would occur. Among the aristocracy there has been a co_nstant selecti~n of beauty, which is generally synonymous Wlt~ h~alth, while any_ constitutionnJ infertility has led to the extmctwn of the family. -With domestic animals the selection practised is usually neither severe enou(j'h nor of the right . kind.. There is no natural struOb'cbr le forb cxi tcnce ' but certam pomts of form and colour characteri stic of the breed are consi~ered essential, and thus the mo.,t vigorous or the most fert1le are not always those which are selected to continue the stock. In nature, too, the pccies always extends over a larger area and consists of much greater number., and thus a di:fferenc~ of _constitt_ltio~ oon ari. cs in different parts of the area,. whiC!l 1s wantmg m the limited numbers of pure bred domestiC ammals. From a consideration of these varied facts we conclude that an occasional disturbance of the oruanic equ~l~brium is what i~ essential to keep up the vigourb and fert1hty of any orgamsm, and that this eli turbance may be equally well produced either by a eros between individuals of somewhat different constitutions, or by occasional . lio-ht changes in the conditions of life. :Now plants which h~ve great powers of clisper. al enjoy ;1, constant chancre of conditions, and can, therefore, exist permanently, or at ball events, for very long periods, without intercrossino·; while those which have limited powers of dispersal, and ~re restricted to a comparatively small and uniform area, need an occasional cross to keep up their fertility and general vigour. \V e should, therefore, expect that those groups of phtnts whi ch are adapted both for cross- :mel self-fertilisation, which have howy flowers and possess great powers of seecl-disper al, would be the most :tbundant and most widely distributed; nud thi we find to be the case, the Compo. itm possessing all the e characteristics in the highest degree, and hei ng the most generally abundant group of plants with conspicuous flowers in all parts of the world. |