It appears as if fertility depended on such a
delicate adjustment of the male and female elements to each other, that,
unless constantly kept up by the preservation of the most fertile
individuals, sterility is always liable to arise. This preservation
always occurs within the limits of each species, both because fertility
is of the highest importance to the continuance of the race, and also
because sterility (and to a less extent infertility) is self-destructive
as well as injurious to the species.
So long therefore as a species remains undivided, and in occupation of a
continuous area, its fertility is kept up by natural selection; but the
moment it becomes separated, either by geographical or selective
isolation, or by diversity of station or of habits, then, while each
portion must be kept fertile _inter se_, there is nothing to prevent
infertility arising between the two separated portions. As the two
portions will necessarily exist under somewhat different conditions of
life, and will usually have acquired some diversity of form and
colour--both which circumstances we know to be either the cause of
infertility or to be correlated with it,--the fact of some degree of
infertility usually appearing between closely allied but locally or
physiologically segregated species is exactly what we should expect.
Pages:
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346