CHAP. X.] IN ANY SINGLE FORMATION. 261 



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 

 palaeontologists 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 we 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 all these tertiary species are specifically 

 distinct, though the distinction is admitted to be very slight ; so 

 that here, unless we believe that these eminent naturalists have 

 been misled by their imaginations, and that these late tertiary 

 species really present no difference whatever from their living 

 representatives, or unless we admit, in opposition to the judgment 

 of most naturalists, that these tertiary species are all truly distinct 

 from the recent, we have evidence of the frequent occurrence of 

 slight modifications of the kind required. If we look to rather 

 wider intervals of time, namely, to distinct but consecutive stages 

 of the same great formation, we find that the embedded fossils, 

 though universally ranked as specifically different, yet are far 

 more closely related to each other than are the species found in 

 more widely separated formations ; so that here again we have 

 undoubted evidence of change in the direction required by the 

 theory ; but to this latter subject I shall return in the following 

 chapter. 



With animals and plants that propagate rapidly and do not 

 wander much, there is reason to suspect, as we have formerly 

 seen, that their varieties are generally at first local ; and that 

 such local varieties do not spread widely and supplant their 

 parent-forms until they have been modified and perfected in some 

 considerable degree. According to this view, the chance of dis- 

 covering in a formation in any one country all the early stages of 

 transition between any two forms, is small, for the successive 

 changes are supposed to have been local or confined to some one 



