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Show 296 IMPEHFECTION OF THE CHAP. IX. generally have been due to geographical .chang~s · re-qu1. n. ng much t1' me. Nor will the c.l osest I.n spe. ction of a 1.c 0rmat 1' on g1. ve a ny 1·dea of the time whiCh Its. depo- s1' tI' on h as consu med. Many instances could b. e g1ven of b ds nl few feet in thickness, representing forma-e o y a . h' k d t I.o ns, e1 s ew her e thousands of feet m t IC· dn e.cs s, ha n· which must have required an enormous pe.rio !Or t mr accumu la tw. n,. yet no one ignorant of this fact would have suspected the vast lapse of time represen~ed by the thinner formation. Many cases. could be g1ve~ of the lower beds of a formation having been upraised, denuded, submerged, and then re-covere~ by the upper beds of the same formation,-facts, showing ~h~t 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 ti~~ and ~hanges 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. L~ell and Dawson found carboniferous beds 1400 feet thick in Nova Scotia, with ancient root-bearin? stra~a, one above the other, at no less than sixty -eight different levels. Hence, when the same species occur at . t?e bottom, middle, and top of a formation, the pro~ab1hty is that they have not lived on the same ~pot 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 modi·f icati· on du n·n g a?y one geological period, a section wou~d not ~robably nwlude 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 CHAP. IX. GEOLOGICAL RECORD. 297 n_o ?olden rule by which to distinguish species and vanetws; 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 0 to be two species, and a third, A, to be found in an underlying bed; even if A were strictly intermediate between Band 0, it would simply be ranked as a third and distinct species, unless at the same time it could be ~ost clos~ly con~ec~ed 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 "-!3 a~d 0, an~ yet might not at all necessarily be stnctly 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 I many palre?ntologists have ~oun.ded 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 sam~ great formation, we find that the embedded fossils though almost universally ranked as specifically different: 0 3 |
