70 ABSENCE OF INTERMEDIATE VARIETIES [Chap. X. 



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 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 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 be 

 ranked as a third and distinct species, unless at the 

 same time it could be closely connected by interme- 

 diate varieties with either one or both forms. Nor 

 should it be forgotten, as before explained, that A 

 might be the actual progenitor of B and C, and yet 

 would not necessarily be strictly iatermediate between 



