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Show 152 LAWS OF VARIATION. [CHAP. V. to the whole individual; for in a district where many species of any genus are found-that is, where there has been n1uch former variation and differentiation, or where the Inanufactory of new specific forms has been actively at work-there, on an average, we now find most varieties or incipient species. Secondary sexual characters are hjghly variable, and such characters differ mnch in the species of the same group. Variability in the same parts of the organisation has generally been taken advantage of in giving secondary sexual differences to the sexes of the same species, and specific differences to the several species of the same genus. Any part or organ developed ~o an extr~ordin~ry size or in an extraordinary manner, In companson With the same part or organ in the allied species, must have gone through an extraordinary amount of modification since the genus arose ; and thus we can understand why it should often still be variable in a much high.er degree than other parts; for variation is a longcontinued and slow process, and natural selection will in such cases not as yet have had time to overcome the tend~ncy to further variability and to reversion to a less Ino~Ifie~ state. But when a species with any extraordinanly- developed organ has become the parent of many modified descendants-which on my view must be a very slow process, :equiring a long lapse of time-in this case, natural selection may readily have succeeded in giving a fixed ch~racter to the organ, in however extraordinary a manner It may be developed. Species inheriting nearly the same constitution from a common parent and exposed to similar. in.fiuences will naturally t~nd to present analogous variations, and these same species may occasionally :evert to some of the characters of their ancient progenItors. . Although new and important modifications may not .arise. from .reversion and analogous variation, such modifications Will add to the beautiful and harmonious diversities of nature. Whatever the cause may be of each slight difference in the offspring from their parents-and a cause for each must exist-it is the steady accumulation, through natural CHAP. V.] SUMMARY 153 selection, of such differences, when be;teficial to the ~~dividual, that gives rise to ~11 the m?re Important ~odifications of structure, by which the Innumerable b~Ings on the face of this earth are enabled to struggle w1th each other, and the best adapted to survive. 7* |
