278 ABSENCE OF 'NTERMEPIATE VARIETIES [Chap. X. 



pccted, had not the trees heen 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 proba- 

 bility 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 ifc 

 were to undergo a considerable amount of modification during the 

 deposition of any one geological formation, a section would not in- 

 clude 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 ? 

 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 intermediate 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 intermediate between them in all respects. 

 So that we might obtain the parent-species and its several modified 

 descendants from the lower and upper beds of the same formation, 

 and unless we obtained numerous transitional gradations, we should 

 not recognise their blood-relationship, and should consequently rank 

 them as distinct species. 



It is notorious on what excessively slight differences many palae- 

 ontologists have founded their species ; and they do this the more 

 readily if the specimens come from different sub-stages of the same 

 formation. Some experienced conchologists are now sinking many 

 of the very fine species of D'Orbigny and others into the rank of 

 varieties; and on this view Ave do find the kind of evidence of 

 change which on the theory we ought to find. Look again at the 

 later tertiary deposits, which include many shells believed by the 

 majority of naturalists to be identical with existing species ; but 

 some excellent naturalists, as Agassiz and Pictet, maintain that ail 

 these tertiary species are specifically distinct, though the distincticn 



