OCR Text |
Show 118 NATURAL SELECTION. CHAP. IV. 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 know 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 a2 , which will, owing to the principle of divergence, differ more from (A) than did variety a1• Variety m1 is supposed to have produced two varieties, namely m2 and s2 , 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 varieties or modified descendants, proceeding from the common parent (A), will generally go on increasing in number and diverging in character. In the W-agram the process is represented up to the tenthousandth generation, and under a condensed and simplified form up to the fourteen-thousandth generation. But I must here remark that I do not suppose that the process ever goes on so regularly as is represented in the diagram, th~ugh in itself made somewhat irregular. CHAP. IV. DIVERGENCE OF CHARACTER. 119 I am far from thinking that the most divergent varieties will invariably prevail and multiply: a medium form may often long endure, and may or may not produce more than one modified descendant ; for natural selection will always act according to the nature of the places which are either unoccupied or not perfectly occupied by other beings; and this will depend on infinitely complex relations. But as a general rule, the more diversified in structure the descendants from any one species can be rendered, the more places they will be enabled to seize on, and the more their modified progeny will be increased. In our diagram the line of succession is broken at regular intervals by small numbered letters marking the successive forms which have become sufficiently distinct to be recorded as varieties. ~ut these breaks are imaginary, and might have been Inserted anywhere, after intervals long enough to have allowed the accumulation of a considerable amount of divergent variation. As all the modified descendants from a common and widely-diffused species, belonging to a large genus, will ten~ to partake of the same advantages which made therr pa~ent. suc~essful in life, they will generally go on multiplyi~g .In number as well as diverging in characte~: this IS represented in the diagram by the seve:al diverge~t branches proceeding from (A). The modified offspnn~ from ~he later and more highly improved branches In the lines of descent, will, it is probab~ e, often ta~e 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 will be confined to a single line of descent, and the number of the descendants will not be increased; although the amount |