154 TRANSITIONAL VARIETIES. 



be better filled by some modification of some one or more 

 of its inhabitants. And such new places will depend on 

 slow changes of climate, or on the occasional immigration 

 of new inhabitants, and, probably, in a still more important 

 degree, on some of the old inhabitants becoming slowly 

 modified, 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 to see only a few 

 species presenting slight modifications 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 sufficiently distinct to rank as representative species. 

 In this case, intermediate varieties between the several 

 representative species and their common parent, must for- 

 merly have existed 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, when two or more varieties have been formed 

 in different portions of a strictly continuous area, interme- 

 diate varieties will, it is probable, at first have been formed 

 in the intermediate zones, but they will 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 representa- 

 tive species, and likewise of acknowledged varieties), exist 

 in the intermediate zones in lesser numbers than the varie- 

 ties which they tend to connect. From this cause alone 

 the intermediate varieties will be liable to accidental exter- 

 mination ; and during the process of further modification 

 through natural selection, they will almost certainly be 

 beaten and supplanted by the forms which they connect; 

 for these, from existing in greater numbers, will, in the 

 aggregate, present more varieties, and thus be further im- 

 proved through natural selection and gain further advan- 

 tages. 



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

 theory be true, numberless intermediate varieties, linking 

 closely together all the species of the same group, must 

 assuredly have existed ; but the very process of natural 

 selection constantly tends, as has been so often remarked f 



