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Show 138 LAWS OF , VARIATION. [CHAP. V. lected and no selection be applied, that part (for instanc.e, the comb in the Dorking fowl) or the whole breed w~ll cease to have a nearly uniform character. T~e breed Will then be said to have degenerated. In ~udiment~ry. organs, and in those which have been but l~ttle spemahs~d for any particular purpose, and perhaps In polyn:orphic groups, we see a :r:earlJ: parallel natural case; for In s.uch cases natural selection mther has not or cannot come Into full play, and thus the organisation is Ieft in a fluctuatiJ?-g condition. But what here more especially concerns us Is, that in our domestic animals those points, which at the present time are und~rgoing .rapid chan~e .by continued selection, are also eminently hable to vana~I~n. Look at the breeds of the pigeon ; see what a prodigious amount of difference there is in the beak of the different tumblers, in the beak and wattle of the different carriers, in the carriage and tail of our fantails, &c., these being__ the points I'l:OW mainly attended to by English fancier~. .Even in the sub-breeds, as in the short-faced tumbler, It IS no~ toriously difficult to breed them nearly to perfection, and frequently individuals are born which depart widely from the standard. There may be truly said to be a constant struggle going on between, on the one hand, the tendency to reversion to a less modified state, as well as an innate tendency to further variability of all kinds, and, on the other hand, the power of steady selection to keep the breed true. In the long run selection gains the day, and we do not expect to fail so far as to breed a bird as coarse as a common tumbler from a good short-faced strain. But as long as selection is rapidly going on, there may always be expected to be much variability in the structure undergoing modification. It further deserves notice that these variable characters, produced by man's selection, sometimes become attached, from causes quite unlmown to us, more to one sex than to the other, generally to the male sex, as with the wattle of carriers and the enlarged crop of pouters. Now let us turn to nature. When a part has been developed in an extraordinary manner in any one species, compared with the other species of the same genus, we OHAP. V.] LAWS OF VARIATION. 139 may conclude that this part has undergone an extraordinarY: amount of modification, since the period when the spemes bra~ched. off fr?m the common progenitor of the genus. This period will seldom be remote in any extreme degree? as spe?ies very rarely endure for more than one g:eol?giCa.l period. An extraordinary amount of modificatiOn In;pl~e.s an un?sually large and long-continued amount of vanabihty, .'vlnch has continually been accumulated by natural. sel.e?tion for the benefit of the species. But as the variabihty of the extraordinarily-developed part or org~n has been s_o great and long-continued within a penod not excessively remote, we might, as a general rule, ~xpect still to find more variability in such parts tha_n In other parts of the organisation, which have remauwd for a much longer period nearly constant. And this, I am convinced, is the case. That the strUO'O'le between na~ural select~ on. o.n the one hand, and the t~~dency to reversion and variability on the other hand will in the course of time cease; and that the most abn'ormally developed organs may be made constant, I can see no reason to doubt. Hence when an organ, however abnormal it may .b.e, has been trans:nitted in approximately the same cond1tlon to Inany modified descendants as in the case of the wing of the bat, it must have existed: accordinO' to my theory, fo; an immense period in nearly the sam~ state ; and thus It co~es to ~e no more var~able ~han any other structure. It IS only In those cases In which the modification has been coinparatively recent and extraordinarily ~reat that we ough~ to find th~ gene~ative variability, as It may be called, shll present In a lugh degree. For in this case th~ variability ~ill seldom as yet have been fixed by the c?nt1nued selection of the individuals varying in tli~ r~quired manner _and degree, and by the continued reJ echon of those tending to revert to a former and less modified condition.r The principle included in these remarks may be exten~ ed. It is notorious that specific characters are more varia~le than generic. To explain by a simple example what IS meant. If some species in a large genus of plants had blue flowers and some had red, the colour would be |