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Show 410 RECAPITULATION. [CHAP. XIV. inhabiting the dark caves of America and Europe. In both varieties and species correlation of growth seems to have played a most important part, so that w~en one.part has been modified other parts ar~ necessarily modified. In both varieties and species reversions to long-lost c~ara?- t ers occur. How I'nexplicable on. the theory ohf crleda tion IdS the occasional appeara~ce of stripes on the s ou . er an .. le s of the several speCI~s of .the horse-g~nus .and In t~en hffbrids 1 How simply IS this fact explained. if we behev.e that these species have descended from a stri~ed progeni~ tor, in the same manner as the several domestic breea,s of pigeon have descended from the blue and barred lock- pi.g eon.' · h · b On the ordinary v1ew of each species. aving een independently created, why should the speCific charac~ers, or those by which the species of the same genu.s differ from each other be more variable than the generic characters in which they all agree? W~y, for instance, ~hould the colour of a flower be more hkely !o vary In any one species of a genus, if the other sp~cies, supposed to have been created independently, have differently coloured flowers than if all the species of the genus have the same coloured flowers? If species are only '\\.,.ell-n1.arked ~arieties of which the characters have be~ome In a high degre~ permanent, we can understand this fact ; for they have already varied since they branched off. from a common progenitor in certain characters, by wh1ch they have come to be specifically distinct from each ot~er; a~d therefore these same characters would be more h~(ely still to be variable than the generic characters whiCh h.ave been inherited without change for an e:r:ormous perwd. It is inexplicable on the theory of cr~atlon why a p~rt developed ip. ~ very unusual manner In any on~ species of a genus, and therefore, as we may natur~lly 1nfe;, of great importance to the species, should be eminently hable to variation · but on my view, this part has undergone, since the se~eral' species branched o~ ~r?m a com~?fi ro enitor, an unusual am_?unt of vana~Ihty and mo I - ~ati~n, and therefore we nnght expect th1s part gen.eraiJy to be still variable. But a part may be developed In t le CHAP. XIV.] RECAPITULATION. 411 most unusual ID:anner, like the wing of a bat, and yet not to be more variable than an:y other structure, _if ~he part be co:nmo~ to many subordinate forms, that Is, If it has b~en Inherited for a very long period ; for in this case it will have been rendered constant by long-continued natural selection; ·Glancing at instincts, marvellous as some are they offer no greater difficulty than does corporeal struct~re on the theory of ·the natural selection of successive slight but profitable modifications. We can thus understand why nature moves by graduated steps in endowino- different animals of the same class with their several i~st1ncts. I have a~tempted to show ho'! much light the principle of grada~Ion throws on .the admirable architectural powers of the hive-bee. Habit no doubt sometimes comes into play in modifying in~tincts ; but it certainly is not indispensable, as we see, 1n the case of neuter insects which leave no pi'Ogeny to inherit the effects of long-co~tinued habit. On the view of all the species of the same genus ~avin.g descend~d from a common parent, and ha!ing nilierited n1uch In common, we can understand how It is that allied species, when placed under considerably different conditions of life, yet should follow nearly the same instincts ; why the thrush of South A1nerica, for instance, lines her nest with mud like our British species. On the view of instincts having been slowly acquired through natural selection, we need not marvel at some instincts being apparently not perfect and liable to mistakes, and at many instincts causing other animals to suffer. If species be only well-marked and permanent varieties, we can at once see why their crossed offspring should follow the same complex laws in their degrees and kinds of resemblance to their parents,-in being absorbed into each other by successive crosses, and in other such points, -as do the crossed offspring of acknowledged varieties. On the other hand, these would be strange facts if species have been independently created, and varieties have been produced by secondary laws. If we admit that the geological record is imperfect in an extreme degree, then such facts as the record gives, |