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one； and the species undergoing modification will have had to live on the same area throughout this whole time。 But we have seen that a thick fossiliferous formation can only be accumulated during a period of subsidence； and to keep the depth approximately the same； which is necessary in order to enable the same species to live on the same space； the supply of sediment must nearly have counterbalanced the amount of subsidence。 But this same movement of subsidence will often tend to sink the area whence the sediment is derived； and thus diminish the supply whilst the downward movement continues。 In fact； this nearly exact balancing between the supply of sediment and the amount of subsidence is probably a rare contingency； for it has been observed by more than one palaeontologist； that very thick deposits are usually barren of organic remains； except near their upper or lower limits。
It would seem that each separate formation； like the whole pile of formations in any country； has generally been intermittent in its accumulation。 When we see； as is so often the case； a formation composed of beds of different mineralogical composition； we may reasonably suspect that the process of deposition has been much interrupted； as a change in the currents of the sea and a supply of sediment of a different nature will generally have been due to geographical changes requiring much time。 Nor will the closest inspection of a formation give any idea of the time which its deposition has consumed。 Many instances could be given of beds only a few feet in thickness； representing formations； elsewhere thousands of feet in thickness； and which must have required an enormous period for their accumulation； yet no one ignorant of this fact would have suspected the vast lapse of time represented by the thinner formation。 Many cases could be given of the lower beds of a formation having been upraised； denuded； submerged； and then re…covered by the upper beds of the same formation； facts； showing what wide； yet easily overlooked； intervals have occurred in its accumulation。 In other cases we have the plainest evidence in great fossilised trees； still standing upright as they grew； of many long intervals of time and changes of level during the process of deposition； which would never even have been suspected； had not the trees chanced to have been preserved： thus； Messrs Lyell and Dawson found carboniferous beds 1400 feet thick in Nova Scotia； with ancient root…bearing strata； one above the other； at no less than sixty…eight different levels。 Hence； when the same species occur at the bottom； middle； and top of a formation； the probability is that they have not lived on the same spot during the whole period of deposition； but have disappeared and reappeared； perhaps many times； during the same geological period。 So that if such species were to undergo a considerable amount of modification during any one geological period； a section would not 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…important to remember that naturalists have no golden rule by which to distinguish species and varieties； they grant some little variability to each species； but when they meet with a somewhat greater amount of difference between any two forms； they rank both as species； unless they are enabled to connect them together by close intermediate gradations。 And this from the reasons just assigned we can seldom hope to effect in any one geological section。 Supposing B and C to be two species； and a third； A； to be found in an underlying bed； even if A were strictly intermediate between B and C； it would simply be ranked as a third and distinct species； unless at the same time it could be most closely connected with either one or both forms by intermediate varieties。 Nor should it be forgotten； as before explained； that A might be the actual progenitor of B and C； and yet might not at all necessarily be strictly intermediate between them in all points of structure。 So that we might obtain the parent…species and its several modified descendants from 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 to rank them all as distinct species。
It is notorious on what excessively slight differences many palaeontologists have founded their species； and they do this the more readily if the specimens come from 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 the rank of varieties； and on this view we do find the kind of evidence of change which on my theory we ought to find。 Moreover； if we look to rather wider intervals； namely； to distinct but consecutive stages of the same great formation； we find that the embedded fossils； though almost universally ranked as specifically different； yet are far more closely allied to each other than are the species found in more widely separated formations； but to this subject I shall have to return in the following chapter。
One other consideration is worth notice： with animals and plants that can propagate rapidly and are not highly locomotive； there is reason to suspect； as we have formerly seen； that their varieties are generally at first local； and that such local varieties do not spread widely and supplant their parent…forms until they have been modified and perfected in some considerable degree。 According to this view； the chance of discovering in a formation in any one country all the early stages of transition between any two forms； is small； for the successive changes are supposed to have been local or confined to some one spot。 Most marine animals have a wide range； and we have seen that with plants it is those which have the widest range； that oftenest present varieties； so that with shells and other marine animals； it is probably those which have had the widest range； far exceeding the limits of the known geological formations of Europe； which have oftenest given rise； first to local varieties and ultimately to new species； and this again would greatly lessen the chance of our being able to trace the stages of transition in any one geological formation。
It should not be forgotten； that at the present day； with perfect specimens for examination； two forms can seldom be connected by intermediate varieties and thus proved to be the same species； until many specimens have been collected from many places； and in the case of fossil species this could rarely be effected by palaeontologists。 We shall； perhaps； best perceive the improbability of our being enabled to connect species by numerous； fine； intermediate； 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； and dogs have descended from a single stock or from several aboriginal stocks； or； again； whether certain sea…shells inhabiting the shores of North America； which are ranked by some conchologists as distinct species from their Eur