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Show 114 NATURAL SELECTION. [CHAP. IV. The six descendants from (I) will form t:vo sub-genera or even genera. But as the original species (I) drff~red largely from (A), standing nearly at the extrmne poi~ts of the oriO'inal genus, the six descendants from (I) ~Ill, owing to inheritance, differ considerably from the mght descendants from (A); the two ~rou:ps, more?ver,. are supposed to have gone ?n diverging In ~Iff~rent dire?tion~. The intermediate species, also (and this IS ~ ;ery Imp?Itant consideration), which connected the original species (A) and (I), have all become, excepting (Jf), extinct, a~d have left no descendants. Hence the SIX new species descended from (I), and the eight descended from (A), w~ll have to be ranked as very distinct genera, or even as dis-tinct sub-families. Thus it is, as I believe, that two or more genera are produced by descent, with modification, from two or more species of the same genus. And the two or 1nore parentspecies are supposed to have descen~ed from ~ol?e. on.e species of an earlier genus. In our diagram, th1s IS Indicated by the broken lines, beneath the capital ~etters, ~onverging in sub-branches downwards towards a single P?Int; this point representi.ng a single species, the supposed single parent of our several new sub..:genera and genera. It is worth while to reflect for a moment on the character of the new species F 14 , which is supposed not to have diverged much in character, but to have retained the form of (F), either unaltered or altered only in a slight degree. In this case, its affinities to the other fourteen new species will be of a curious and circuitous nature. Having descended from a form which stood between the two parentspecies ·(A) and (I), now supposed to be distinct and unknown, it will be in some degree intermediate in character between the two groups descended from these species. But as these two groups have gone on diverging in character froin the type of their parents, the new species (F14 ) will not be directly intermediate between them, but rather between types of the two groups ; and every naturalist will be able to bring some such case before his mind. In the diagram, each horizontal line has hitherto been supposed to represent a thousand generations, but each CHAP. IV.] DIVERGENCE OF CHARACTER. 115 may r~pre~ent a n1il~ion or hundred million generations, and likewise a section of the successive strata of the earth's crust including extinct remains. We shall, when we come to our chapter on Geology, have to refer again to this subject, and I think we shall then see that ... the diagram throws light on the affinities of extinct beings, which, though generally belonging to the same orders, or. families, or g~nera, wi~h t~ose now living, yet are often, In some degree, Intermediate In character between existing groups ; and we can understand this fact for the extinct species lived at very ancient epochs when the branching lines of descent had diverged less. I see no reason to liinit the process of modification as now explained, to the formation of genera alone. Il in our diagran1, we suppose the amount of change represen.'ted by each successive group of diverging dotted lines to be very}reat, the forms marked a14 to p 14 , those marked b14 and 14 , and those marked 0 14 to ?n 14 , will form three verv distinct genera. We shall also have two very distinct genera descended from (I) ; and as these latter two genera both from continued divergence of character and fro~ inheritance from a different parent, will differ widely from the three gert~ra descended from (A), the two little groups of genera will form two distinct families or even orders, according to the amount of divergent modification supposed to be represented in the diagram. And the two n.ew families, .o~ orders, will have descended from two speCies of the original genus ; and these two species are supposed to have descended from one species of a still more ancient and unknown genus. We have seen th~t in each country it is the species of t~e larger ~enera ~hi?h oftenest present varieties or incipient spemes. This, Indeed, might have been expected· for as natural selection acts throuO'h one form havinO' som~ ~dv~ntag~ over other forms in the struggle for exi~tence, It Will chiefly act on those which already have some advantage ; and the largeness of any group shows that its species have inherited from a cominon ancestor some advant~ ge in common. Hence, the struggle for the production of new and n1odified descendants, will mainly lie |