DIFFICULTIES. 21? 



connected by a series of transitional links. It is 

 of course plain that no such series of transitional 

 links exists to-day. Two closely related species, in- 

 habiting neighboring localities, are assumed to have 

 descended from a common ancestor, which has be- 

 come differently modified in the two localities, to 

 fit it better for the different conditions there found. 

 It might be expected, perhaps, that the tracts of 

 land intermediate between the localities in question 

 would be inhabited by intermediate varieties or 

 species. This we do not, however, find. The species 

 are usually distinct, and, except in rare instances, it 

 does not happen that a tract of land intermediate 

 between the localities inhabited by related species, 

 is occupied by an intermediate species, even though 

 the physical condition be strictly intermediate. 



The fact that all of these finely graded transitional 

 varieties, which are assumed by natural selection to 

 have lived in the past, no longer live to-day, pre- 

 sents no great drfiiculty. For natural selection 

 and extinction go hand in hand. If any species 

 be assumed to have been derived from an older 

 one by gradual improvement, it follows that all 

 of the intermediate, less-favored varieties must, 

 by the action of natural selection, have been ex- 

 terminated. The result will always be simply the 

 preservation of only the most advanced forms. 

 The development of a new form implies the extinc- 

 tion of the old. Nor could it be expected that, 

 when two contiguous localities are inhabited by 

 related species, the intermediate tract would contain 

 an intermediate species. For this intermediate tract 



