DIFFICULTIES ON THEORY 1G1 



have been formed in the intermediate zones, but they 

 will generally have had a short duration. For these 

 intermediate varieties will, from reasons already as- 

 signed (namely from what we know of the actual 

 distribution of closely allied or representative species, 

 and likewise of acknowledged varieties), exist in the 

 intermediate zones in lesser numbers than the varieties 

 which they tend to connect. From this cause alone 

 the intermediate varieties will be liable to accidental 

 extermination ; 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 variation, and thus be further improved through 

 natural selection and gain further advantages. 



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

 if my theory be true, numberless intermediate varieties, 

 linking most closely all the species of the same group 

 together, must assuredly have existed ; but the very 

 process of natural selection constantly tends, as has 

 been so often remarked, to exterminate the parent- 

 forms and the intermediate links. Consequently evi- 

 dence of their former existence could be found only 

 amongst fossil remains, which are preserved, as we 

 shall in a future chapter attempt to show, in an 

 extremely imperfect and intermittent record. 



On the origin and transitions of organic beings with 

 peculiar habits and structure. — It has been asked by the 

 opponents of such views as I hold, how, for instance, a 

 land carnivorous animal could have been converted 

 into one with aquatic habits ; for how could the animal 

 in its transitional state have subsisted? It would be 

 easy to show that within the same group carnivorous 

 animals exist having every intermediate grade between 

 truly aquatic and strictly terrestrial habits ; and as 

 each exists by a struggle for life, it is clear that each is 

 well adapted in its habits to its place in nature. Look 

 at the Mustela vison of North America, which has 



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