OCR Text |
Show LAWS OF VARIATION. 168 CHAP. v. ·n stren thening, and disuse in weakferences, an~ u.s e. 1 . g ns seem to have been more ening a?d du~nnis:~~sg 0if:m~logous parts tend to vary potent In theu e:ffi d. h mologous parts tend to cohere. in the same w~y,:n d p~rts and in external parts someModifications In ar . 1 rt When one part . ffi t softer and Interna pa s. times a ecd l d perhaps it tends to draw nourish-is largely evhe opde. '. . ng parts . and every part of the ment from t e a JOilll ' · h s.t ru.c t·au re wh ic h ca n be saved without detriment tot t e l .ll be saved Changes of structure a an 1ndiV1 ua , WI · b tl d 1 Wl.ll generally affect parts su sequen y e-ear y age 1 t. f velo ed; and there are very many other corre a Ions o pth the nature of which we are utterly unable to ~~:rstand. Multiple parts are variable in number ~nd in structure, perhaps arising from sue~ parts not ~aving been closely specialised to any particular functwn, so that their modifications have not been closely ~hecked b natural selection. It is probably from this same c!use 'that organic beings low in the scal.e of nature ar~ more variable than those which have. theu ~hole organisation more specialised, and are higher ~~ the s?~~:· Rudimentary organs, from being useless, will be dis - garded by natural selection, and hence probably are variable. Specific characters-that is, the char~cter~ whlch have come to differ since the several species o the same genus branched off from a common parentare more variable than generic characters, or. thos~ which have long been inherited, and have not differe within this same period. In these remarks we .have referred to special parts or organs being still vanable, because they have recently varied and thus comet!~ differ· but we have also seen in the second C~ap that th' e same principle applies to the who l e I.n a·I VI dual·' for in a district where many species of any genus are found-that I.s , where there h as b een m uch former CHAP. v. SUMMARY. 169 variation and differentiation, or where the manufactory of new specific forms has been actively at work-there, on ~n average, we now find most varieties or incipient species. Secondary sexual characters are ~1ighly variable, and such characters differ much in the species of the same group. Variability in the same parts of the. or~a~isation has generally been taken ad vantage of In giv1ng 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 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 variable 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. ha.v~ 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 -w~i?h on my view must be a very slow process, requiring a long lapse of time-in this case natural selection may readily have succeeded in givin~ a fixed characte~ to the organ, in however extraordinary a manner It may be developed. Species inheriting nearlv the same constitution from a common parent and e;_posed to similar influences will naturally tend to present a.nalogous variations, and these same species may occaSIOnally revert to some of the characters of their ancient ~rogenitors. Although new and important modificat~ ons rna y not arise from reversion and analogous variatiOn, such modifications will add to the beautiful and harmonious diversity of nature. I |