Page - 128 - in The Origin of Species

128 ORIGL^ OF SPECIES 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 a^ is supposed in the diagram to have produced variety a", which will, owing to the principle of divergence, differ more from (A) than did variety a.\ Variety m^ is supposed to have produced two varieties, namely m^ and /^ 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 varie- ties or modified descendants of the common parent (A), will generally go on increasing in number and diverging in char- acter. In the diagram the process is represented up to the ten-thousandth generation, and under a condensed and sim- plified 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, though in itself made somewhat irregular, nor that it goes on continuously; it is far more probable that each form remains for long periods unaltered, and then again undergoes modification. Nor do I suppose that the most di- vergent varieties are invariably preserved; a medium form may often long endure, and may or may not produce more than one modified descendant; for natural selection will al- ways 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 descend- ants from any one species can be rendered, the more places they will be enabled to seize on, and the more their modified progeny will increase. In our diagram the line of succession is broken at regular intervals by small numbered letters mark- ing the successive forms which have become sufficiently dis- tinct to be recorded as varieties. But these breaks are imaginary, and might have been inserted anywhere, after intervals long enough to allow the accumulation of a con- siderable amount of divergent variation.