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Show 260 IMPE~FEOTION OF THE [OBAP. IX. any one geological period, a section w?uld n~t probably include all the .fine intermediate gradations which must on my theory have existed between them, but abrupt, though perhaps very slight, changes of form. . It is all-in1portant ~o rem~m?er ~hat na~urahsts h~;e no golden rule by whiCh to distinguish spemes and va~Ieties · they grant s01ne little variability to each species, but ~hen they 1neet with a somewhat greater amount of difference between any two forins, they rank both as species, unless they are enable~ to connect th~m together by close intermediate gradations. And this fro~ the reasons just assiO'ned we can seldom hope to effect In any one geological s~ction. Supposing .B and 0 to. be two species, and a third, A, ~o be fou:t;d In an underlying bed.; even if A were strictly IntermedJ~te betwe~n .B and 01 1t would simply be ranked as a thud and distinct species, unless at the same time it could be most closely connected with either one or both fonns by intermediate varieties. Nor should i.t be forgotten, as before explained, that A might be the actual progenitor of Band 0, and yet might not at all nec.essarily be strictly intermediat.e betwe~n them in all points of structure. So that we might obta1n the parent-species and its several modified descendants froin the lower and upper beds of a formation, and unless we obtained numerous transitional gradations, we should not recognise their relationship, and should consequently be compelled t? rank them all as d~stinct ~pecies: It is notonous on what excessively shght differences many palreontologists have founded their species ; and they do this the more readily if the specimens come fr01n different sub-stages of the same formation~ Some experienced conchologists are now sinking many of the very fine species of D'Orbigny and others into t~e Tank or varieties ; and on this view we do find the lnnd of evidence of change which on my theory we ought to find. Moreover, if we look to rather wide intervals, nan1ely, the distinct but consecutive stages of the same great formation, we find that the embedded fossils, though almost universally ranked as specifically different, yet are ~ar more closely allied to each other than are the specieS CHAP. IX.l GEOLOGIOAL RECORD. 261 fou~d in more widely separated formations ; but to this subJeCt I shall hav.e to r~tu~n in the foll.owing chapter. One other consideration Is worth notice: with animals and pla1_1ts that ca?- propagate rapidly and are not highly locomotive, there Is reason to suspect as we have formerly seen, ,that their vari~ti~s are gen'orally at first local; and that suc.h local varieties do not spread widely and supplant thmr parent-forms until they have been modified and J?er~ected in some consi.derabl~ deg-ree. According to this VIew, the chance of discovering In a formation in any one country ~11 the early stages of transition between any two forms, Is small, for the successive changes are supposed ~o hav~ been local or confined to some one spot. Most marin~ annuals ~a':"e a wide range ; and we have seen that ~Ith plants It Is those which have the widest range, that oftenest present varieties· so that with shells and other ~arine animals, it is probably those which have had the Wid~st range, far exceeding the limits of the know?- geol?giCal formations of Europe, which have oftenest given. rise, first t? loca~ varieties and ultimately to new spemes; ar:d this again would greatly lessen the ?hance of our ben~g able to trace the stages of transition In any on,e geologwal formation . . It should no~ be forgotten, that at the present day, With perfect spemmens for examination, two forms can seldom be connected by intermediate varieties and thus hroved to be the same species, until many specimens have een, colle?ted from many places ; and in the case of fossil ;emes this could rarely be ~ffected by palreontologists. ~ shall, perhaps, best percmve the improbability of our bein~ enabled to connect species by numerous, fine, intermedJ. ate, fossil. links; by asking ourselves whether, for instance, geologists at some future period will be able to prove, that our different breeds of cattle sheep horses :~1 dogs ?~ve descended from a. single st~ck or f~om sev: h 11 a!>Or1g1n.al stocks ; or, again, whether certain seas e s Inhabiting the shores of North -America which are Eanked by some conchologists as distinct spe~ie~ from their 0 uropea~ ~epresentatives, and by other conchologists as nly varieties, are really varieties or are, as it is called, |