1865.] Jenkins on Strata identified by Organic 'Remains. 625 



theory of descent with modification, who are also advocates of the 

 orthodox maxim of " Strata identified by organic remains," will say — 

 " But if these species came into being first, for instance, in the 

 Miocene seas of Europe, and emigrated thence south-eastwards as the 

 climate of Europe changed, they should, according to your views, have 

 undergone some alteration by the way ; — in other words, they should 

 by this time have been ' modified ' into new species." The corollary 

 that would be drawn from this is that the ' descent with modification' 

 doctrine is incompatible with the other. 



In reality, however, it is only the strongest and most stable of the 

 Miocene species that did withstand the wear and tear of their long 

 voyage ; the great majority of them were " modified " into the 

 " representative species," as Professor Forbes called them, which now 

 exist in sub-tropical regions, their Miocene ancestors being extinct. 

 The more hardy species which did contrive to exist unchanged (such 

 as the one represented in Plate, fig. 3) remain as witnesses to prove 

 the emigration, while the more changeable forms, by having become 

 altered so much as to be considered new and representative species, 

 bear testimony to the effect that is so often wrought upon species by 

 a change in the " conditions of existence," and also to the probable 

 truth of the theory of descent with modification. 



A very interesting example of a variation which may be due to a 

 process of modification has occurred to me during the examination of 

 Miocene Mollusca from different parts of the world. In 1863, I 

 described and figured a Tertiary shell from Java (Plate, fig. 8), which 

 appeared to be the young of the recent Oliva utriculus ; but although 

 I had a large number of specimens, not one of them seemed to have 

 reached the adult state. In other respects there was scarcely the 

 most fractional difference to be detected between the fossil and the 

 young of the recent species, but the former differed very markedly, of 

 course, from full-grown specimens of the latter (Plate, fig. 9). Since 

 then I have met with another series of specimens of the same species 

 from the Miocene strata of Crete, exactly corresponding with those 

 from Java, and all presenting the characters of the young of Oliva 

 utriculus, but unassociated with any specimens comparable to the 

 adult state of that species. Now it appears probable, that during the 

 Miocene and later Tertiary periods this shell never got beyond the 

 young state of the existing species ; and that its present excessive 

 growth and development, owing probably to very favourable conditions, 

 may be but a portion of a course of modification that will ultimately 

 produce a sufficient divergence in the two forms to entitle them to 

 rank as distinct species. In fact, if we were not so well acquainted 

 with all the stages of growth of the recent form, they would be con- 

 sidered so now. 



Having used the term " representative species," it is necessary to 

 define it. Those species, then, are " representative " which appear to 

 hold a place in one zoological province, or geological formation, cor- 

 responding to that held by the species they represent in an equivalent 

 province or formation. Sometimes the representative species are 

 clearly very closely allied to those they represent ; but it frequently 



