260 ABSENCE OF INTERMEDIATE VARIETIES [CHAP. X. 



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 forma- 

 tion composed of beds of widely different mineralogical com- 

 position, we may reasonably suspect that the process of deposition 

 has been more or less interrupted. Nor will the closest inspection 

 of a formation give us any idea of the length of time which its 

 deposition may have consumed. Many instances could be given 

 of beds only a few feet in thickness, representing formations, 

 which are 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 even 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 not have been suspected, had not the trees been preserved : 

 thus Sir C. Lyell and Dr. 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 occurs a,t the bottom, middle, and top of a 

 formation, the probability is that it has not lived on the same 

 spot during the whole period of deposition, but has disappeared 

 and reappeared, perhaps many times, during the same geological 

 period. Consequently if it were to undergo a considerable amount 

 of modification during the deposition of any one geological for- 

 mation, a section would not include all the fine intermediate 

 gradations which must on our theory have existed, but abrupt, 

 though perhaps 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 the closest 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 older and underlying bed ; even 

 if A were strictly intermediate between B and C, it would simply 





