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Show 120 NATURAL SELECTION. CHAP. IV. of divergent modification may have been increased in the successive generations. This case would ~e rcpresentecl . th diagram if all the lines proceeding from (A) In e ' 1 10 I th were removed, excepting that from a to a . n e same way, for instance, the English race-horse and English pointer have apparent~y bo~h. gone on slo:Vly diverging in character from the~r orJginal stocks, Wlth-t either having given off any fresh branches or races. ou After ten thousand generations, species (A) is su pp~sed to have produced three forms, a 10 , f 1 ~, and m 10 , whi~h, from having diverged in character dunng the successive generations will have come to differ largely, but perhaps unequally, 'from each other and from their common parent. If we suppose the amount of change bet;veen each horizontal line in our diagram to be excessively small these three forms may still be only well-marked varieties; or they may have arrived at the doubtful category of sub-species; but we h.ave ?nly to suppose the steps in the process of modification to be more numerous or greater in amount, to convert these three forms into well-defined species : thus the diagram illustrates the steps by which the small differences distinguishing varieties are increased in to ~he. larger differences distinguishing species. By contmu1ng the same process for a greater number of generations (as shown in the diagram in a condensed and simplified manner), we get eight species, marked by the letters between a 14 and m 14, all descended from (A). Thus, as I believe, species are multiplied and genera are formed. In a large genus it is probable that more than one species would vary. In the diagram I have assumed that a second species (I) has produced, by analogous steps, after ten thousand generations, either two w~llmarked varieties ( w10 and z10 ) or two species, according to the amount of change supposed to be represented be- CHAP. IV. DIVERGENCE OF CHARACTER. 121 tween the horizontal lines. After fourteen thousand generations, six new species, marked by the letters n 14 to z 1 \ are supposed to have been produced. In each genus, the species, which are already extremely different in character, will generally tend to produce the greatest number of modified descendants; for these will have the best chance of filling new and widely different places in the polity of nature : hence in the diagram I have chosen the extreme species (A), and the nearly extreme sp~cies (I), as those which have largely varied, and have given rise to new varieties and species. The ot~~r nine species (marked by capital letters) of our onginal genus, may for a long period continue transmitting unaltered descendants; and this is shown in the diagram by the dotted lines not prolonged far upwards from want of space. . But d~ring the process of modification, represented In th~ di~gram? another of our principles, namely that of extinction, will have played an important part. As in each fully stocked country natural selection necessarily acts by the selected form having some advantage in the struggle for life over other forms, there will be a constan~ tendency in the improved descendants of any one spemes to s_upplant and exterminate in each stage of desc~nt their predecessors and their original parent. For It should be remembered that the competition will generally be most severe between those forms which are most nearly related to each other in habits, constitution, and structure. Hence all the intermediate forms between the earlier and later states, that is between the less and more improved state of a species, as well as the original parent-species itself, will generally tend to become extinct. So it probably will be with many whole collateral lines of descent, which will be conquered by later and improved lines of descent. If, however, the G |