76 THE ORIGIN OF SPECIES 



vals of time and changes of level during the proeess 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 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, per- 

 haps many times, during the same geological period. 

 Consequently if it were to undergo a considerable amount 

 of modification during the deposition of any one geologi- 

 cal 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 inter- 

 mediate 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 intermediate varieties with 

 either one or both forms. Nor should it be forgotten, as 



