172 ABSENCE OR IJARITY Chap. VI. 



still more important doffrco, on some of the old inhabitants be- 

 coming slowly moditied, with the new forms thus produced 

 and the old ones acting and reacting on each other. So that, 

 in any one region and at any one time, we ought only to sec 

 a few species j)resenting slight modiiications of structure in 

 some degree permanent ; and this assuredly we do see. 



Secondly, areas now continuous must often have existed 

 within the recent period as isolated portions, in which many 

 forms, more especially among the classes which unite for each 

 birth and wander much, may have separately been rendered 

 suiKciently distinct to rank as representative species. In this 

 case, intermediate varieties betw^een the several representative 

 species and their common parent, must have existed formerly 

 within each isolated portion of the land, but these links during 

 the process of natural selection will have been supplanted and 

 exterminated, so that they will no longer be found in a living 

 state. 



Thirdly, wlien two or more varieties have been formed in 

 diflerent j)ortions of a strictly continuous area, intermediate 

 varieties Avill, it is probable, at first have been formed in the 

 intermediate zones, but they Avill generally have had a short 

 duration. For these intermediate varieties will, from reasons 

 already assigned (namely, from what we know of the actual 

 distribution of closely-allied or representative species, and like- 

 Avise of acknowledged varieties), exist in the intermediate zones 

 in lesser numbers than the varieties which they tend to con- 

 nect. From this cause alone the intennediate varieties will 

 be hable to accidental extemiination ; and during the process 

 of further modification through natural selection, they will al- 

 most certainly lie beaten and supplanted by the forms which 

 they connect ; for these from existing in greater numbers will, 

 in the aggregate, present more variation, and thus be further 

 improved through natural selection and gain further advan- 

 tages. 



Lastly, looking not to any one time, but to all time, if 

 my theory be true, numberless intermediate varieties, linking 

 closely together all the species of the same grouji, must assur- 

 edly have existed; but the very process of natural si'leetion 

 constantly tends, as has been so often remarked, to exterminate 

 the parent-forms and the intermediate links. Consequently 

 evidence of their former existence could be found only among 

 fossil remains, Avhich are preserved, as we shall in a future* 

 chapter attempt to show, in an extremely imperfect and inter- 

 mittent record. 



