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Show 110 NATURAL SELECTION. [CHAP. IV. varieties or modified descendants, proceed i? -g from. th. e common parent (A), will generally go on In~reasing thn number and diverging in character. In the diagram e process is represented up to the ten thousandth generhation and under a condensed and simplified form up tot e ' . fourteen-thousandth generat1on. But I 1nust here remark that I do not suppose th~t the process ever o-oes on so regularly as is repre~ented In the dia ram, thoggh in itself made somewhat rrregula;r .. I am gfar from thinking that the :rr:ost divergen~ varieties will invariably prevail and multiply : a medium form m·ay often long endure, and may or may not prod~ce more than one modified descendant ; for natural selection will always act according to the nature of the ylaces which are either unoccupied or not per~ectl:y occupied by other beings ; and this will depend on Infinit~ly c?mpl~x relations. But as a general rule, the more di~ersificd In structure the descendants from any one species can ?e rendered, the more pl~ces th~y will be enabl~d to se~ze on and the more thmr modified progeny Will be Incr~ ased. In our diagram the line of succession is bro~en at regular intervals by small numbered letter~ markn;g the successive forms ·which have become suffimently distinct to be recorded as varieties. But these breaks are imaginary, and might have been inserted anywhere, a~ter intervals long enough to have. allowed th~ a.ccumulatlon of a considerable amount of divergent variation. As all the modified descendants from a common and widely-diffused species, belonging to a large genus, wi~l tend to partake of the same advantages which made the~r parent successful in life, they will ~ene:ally go on mult~plying in number as well as diverging In charact.er: this is represented in the diagran1 by the sev~ral diverg~nt branches proceeding from (A). The modified offspring from the latter and more highly improved branches in the. lines of descent, will, it is probable, often take the place of, and so destroy, the earlier and less improved branches: this is represented in the diagram by some of the lower branches not reaching to the upper horizontal lines. In some cases I do not doubt that the process of modification CnAP. IV.] DIVERGENCE OF CHARACTER. 111 will be confined to a single line of . descent, and the number of the descendants will not be increased ; although the amount of divergent modification may have been increased in the successive generations. This case would be represented in the diagra1n, if all the lines proceeding from (.A) were removed, excepting that from a1 to a10 • In the same way, for instance, the English race-horse and English pointer have apparently both gone on slowly diverging in character from their original stocks, without either having given off any fresh branches or races. After ten thousand generations, species (A) is supposed to have produced three forms, a10 , f 10 , and m10 , which, from having diverged in character during the successive generations, 'vill have come to differ largely, but perhaps unequally, from each other and from their common parent. If we suppose the amount of change between each horizontal line in our diagram to be excessively small, these three forms may only still be well-marked varieties ; or they may have arrived at the doubtful category of subspecies ; but we have only 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 into the larger differences distinguishing species. By continuing 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 a14 and ?n14 , all descended from (A). Thus, as I believe, species are multiplied and genera are formed. In a laro-e genus it is probable that more than one species wol-B.d vary. In the diagram I have assumed that a second species (I) has produced, by analogous steps, after ten thousand generations, either two well-marked varieties ( W 10 and Z10 ) or two species, according to the amount of change supposed to be represented between the horizontal lines. After fourteen thousand generations, six new species, marked by the letters n1 "' to z14 , are supposed to have been produced. In each genus, the species, which are already extremely different in character, will generally |