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Show CHAP IV.] DIVERGENCE OF CHARACTER. 109 ations (represented by the outer dotted lines) being preserved and accumulated by natural selection. When a dotted line reaches one of the horizontal lines, and is there marked by a small numbered letter, a sufficient amount of variation is supposed to have been accumulated to have formed a fairly well-marked variety, such .as would be thought worthy of record in a systematic work. The Intervals between the horizontal lines in the diagram, Inay represent each a thousand generations; but it would have been better if each had represented ten thousand generations. After a thousand generations, species (A) is supposed to have produced two fairly wellmarked varieties, namely a1 and m1 • These two varieties will generally continue to be exposed to the same conditions which made their parents variable, and the tendency to variability is in itself hereditary, consequently they will tend to vary, and generally to vary in nearly the same manner as their parents varied. Moreover, these two varieties, being only slightly modified forms, will tend to inherit those advantages which made their common parent (A) more numerous than most of the other inhabitants of the same country; they will likewise partake of those more general advantages which made the genus to ·which the parent-species belonged, a large genus in its own country. And these circumstances we kno'v to be favourable to the production of new varieties. If, then, these two varieties be variable, the most divergent of their variations will generally be preserved during the next thousand generations. And after this interval, variety a1 is supposed in the diagram to have produced variety a\ which will, owin~ to the principle of divergence, differ more from (A) than did variety a1• Variety m1 is supposed to have produced two varieties, namely m\ and s'\ differing from each other, and more considerably from their common parent (A). We may ·continue the process by similar steps for any length of time ; some of the varieties, after each thousand generations, producing only a single variety, but in a more and more modified condition, some producing two or three ·varieties, and some failing to produce any. Thus the |
