After a thousand
generations, species (A) is supposed to have produced two fairly
well-marked 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 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.
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