OCR Text |
Show 112 . NATURAL SELECTION. [CIIAP. IV. tend to produce the greatest number of modified ~escendants · for these will have the best chance of filhng new and 'widely different places in the polity of natur.e : hence in the diagram I have chosen the extreme spec.Ies (A), and the nearly extreme s:pecies _(I), as those ~hi~h have largely varied, and have given nse to new vane~Ies and species. The other nine species (rnarked b~ capital letters) of our original genus, may for a long period _cor;tinue transmitting unaltered descend.ants ; and tlns IS shown in the diagram by the dotted lines not prolonged far upwards from want of space. . . . But during the process of m?di~cation, represented In the diagram, another of our prln?Iples, namely that ~f extinction, will have played an Importa~t part. As _In each fully stocked country, natural selection necessarily acts by the selected form having some a_dvantage in the struggle for life over other forms, there will be a const~nt tendency in the improved descendants of any one speCI~s to supplant and exterminate in each stage of descent thmr predecessors and their original parent. For it should be remembered that the competition will generally be 1nost 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 in1proved state of a ~pecies, as well as the original. parent-sp_ecies itself, will generally tend to become extinct. So It probably will be with many 'vhole collateral lines of descent, which will be conquered by later and improved lines of descent. If, however, the modified offspring of a species get into some distinct country, or becon1e quickly adapted to some quite new station, in which child and parent do not come into competition, both may continue to exist. If then our diagram be assumed to represent a considerable amount of modification, species (A) and all the earlier varieties will have become extinct, having_ been replaced by eight new species (a14 to m14 ); and (1) will have been replaced by six: (n14 to z14 ) new species. But we may go further than this. The original species of our genus were supposed to resemble each other in CHAP. IV.] DIVERGENCE OF CHARACTER. 113 unequal dcgre~s, as is so generally the case in nature; species (A) bmng more nearly related to B C and D than to the other species ; and species (I) mdre 'to G n' K, L, than to the others. These two species (A) and' (I): were also. supposed to be very common and widely diffused spemes, so that they must originally have had some adv~ntage .over most of the other species of the genus, Their modified descendants, fourteen in number at the fourteen-thousandth generation, will probably have inherIted ~orne of t~e same advantages: they have also been modified and Improved in a diversified manner at each stage, of descen~, so as to have become adapted to many related places In the natural economy of their country. It. seems, therefore, to me extremely probable that they Will hav~ taken the places of, and thus exterminated not on.lY: thmr pa;ents (A) and (I), but likewise some of the onginal spemes which were most nearly related to their parents. l-Ienee very few of the original species will have t;ansmitted offspring to the fourteen-thousandth generation.. We. may suppose that only one (F), of the two sp~c!es whiC~ were least closely related to the other nine original spemes, has transmitted descendants to this late stage of descent. . ~e new species in our diagram descended from the ori~Inal eleven. species, will now be fifteen in number. Owing to the divergent tendency of natural selection the e~treme ~~o:rnt of difference in character between sp~cies a and. z Will be muc~ greater than that between the mos~ different of the onginal eleven species. The new SJ?emes, moreover, will be allied to each other in a widely different manner. Of the eight descendants from (A) the ~hree marked a14 , qu,[u' will be nearly related from hav~ ng r~cently branche off from a 10 ; b14 and f 14, from hav-lng diver~e~ at an earlier period from ar., will be in some degree distinct from the three first-named species· and lastly, o1 \ e14 , and ?n14 , will be nearly related one to the other, but from having diverged at 'the first commencement of the process of modification, will be widely differbent from the other five species, and may constitute a su -genus or even a distinct genus. |