ABSENCE OF INTERMEDIATE VARIETIES 349 



case, a formation composed of beds of widely different min- 

 eralogical composition, 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 igno- 

 rant 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 up- 

 raised, 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 accumula- 

 tion. 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 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 reap- 

 peared, 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 geo- 

 logical formation, a section would not include all the fine 

 interme diate gradatTotis~which must ofi our IHeotr ijl^ve ex- 

 iste d.'but abrupt, tho ugh 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 be- 

 tween any two forms, they rank both as species, unless they 

 are enabled to connect them together by the closest inter- 

 mediate gradations; and this, from the reasons just assigned, 

 we can seldom hope to effect in any one geological section. 



