IN ANY SINGLE FORMATION. 309 



Tt 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 con- 

 chologists are now sinking many of the very tine 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, main- 

 tain 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 liv- 

 ing 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 re- 

 quired. If we look to rather wider intervals of time, namely, 

 to distinct but consecutive stages of the same great forma- 

 tion, 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 

 tne 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-form until they have been modified 

 and perfected in some considerable degree. According to 

 this view, the chance of discovering 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 spot. Most marine 

 animals have a wide range ; and we have seen that with 

 plants it is those which have the widest range, that oftenest 

 present varieties; so that, with shells and other marine 

 animals, it is probable that those which had the widest 

 range, far exceeding the limits of the known geological 



