Page - 176 - in The Origin of Species

176 ORIGIN OF SPECIES inherited, and have not differed within this same period. In these remarks we have referred to special parts or organs being still variable, because they have recently varied and thus come to dift'er; but we have also seen in the second chapter that the same principle applies to the whole indi- vidual; for in a district where many species of a genus are found—that is, where there has been much former variation and differentiation, or where the manufactory of new specific forms has been actively atwork—in that district and amongst these species, ws now find, on an average, most varieties. Secondary sexual characters are highly variable, and such characters differ much in the species' of the same group. Variability in the same parts of the organisation has gener- ally been taken advantage of in giving secondary sexual differences to the two sexes of the same species, and specific differences to the several species of the same genus. Any part or organ developed to an extraordinary size or in an extraordinary manner, in comparison 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 vari- able in a much higher degree than other parts; for variation is a long-continued and slow process, and natural selection will in such cases not as yet have had time to overcome the tendency to further variability and to reversion to a less modified state. But when a species with any extraordinarily- developed organ has become the parent of many modified descendants—which on our viewmustbe avery slow process, requiring a long lapse of time—in this case, natural selection has succeeded in giving a fixed character to the organ, in however extraordinary a manner it may have been developed. Species inheriting nearly the same constitution from a com- mon parent, and exposed to similar influences, naturally tend to present analogous variations, or these same species may occasionally revert to some of the characters of their ancient progenitors. Although new and important modifications may not arise from reversion and analogous variation, such modi- fications will add to the beautiful and harmonious diversity of nature. Whatever the cause may be of each slight difference be-