338 ABSENCE Of mrEEMBtoTATM VARIETIES 



whole pile of formation in any country, has generally been 

 intermittent in its accumulation. "When we see, as is so 

 often the case, a formations composed of beds of widely 

 different mineralogical composition, we may reasonably 

 suspect that the process of deposition has been more or less 

 interrupted. Nor will the closest inspection of a forma- 

 tion give us any idea of the length of time which its depo- 

 sition mav have consumedo Many instances could be 

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

 formations which are elsewhere thousands of feet in thick- 

 ness, 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 repre- 

 sented by the thinner formation. Many cases could be 

 given of the lower beds of a formation having been up- 

 raised, denuded, submerged, and then recovered 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 evi- 

 dence in great fossilized 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 

 1,400 feet thick in Nova Scotia, with ancient root-bearing 

 strata, one above the other, at no less than sixty-eight dif- 

 ferent levelSo Hence, when the same species occurs at 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 

 formation, a section would not include all the fine inter- 

 mediate 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 



