DIFFICULTIES OF THE THEORY 141 



parent, must formerly 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 differ- 

 ent portions of a strictly continuous area, intermediate 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 

 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 varieties, and thus be further improved 

 through natural selection and gain further advantages. 



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, to exterminate the parent-forms and the inter- 

 mediate links. Consequently evidence of their former existence 

 could be found among fossil remains, which are preserved, as we 

 shall attempt to show in a future chapter, in an extremely imper- 

 fect and intermittent record. 



ON THE ORIGIN AND TRANSITION OF ORGANIC BEINGS WITH 

 PECULIAR HABITS AND STRUCTURE 



It has been asked by the opponents of such views as I hold, 

 how, for instance, could a land carnivorous animal have been con- 

 verted into one with aquatic habits; for how could the animal in 

 its transitional state have subsisted? It would be easy to show that 

 there now exist carnivorous animals presenting close intermediate 

 grades from strictly terrestrial to aquatic habits; and as each exists 

 by a struggle for life, it is clear that each must be well adapted to 

 its place in nature. Look at the Mustela vision of North America, 

 which has webbed feet, and which resembles an otter in its fur, 

 short legs, and form of tail. During the summer this animal dives 

 for and preys on fish, but during the long winter it leaves the frozen 

 waters, and preys, like other pole-cats, on mice and land animals. 



