Another mode of isolation is brought about by the variety--either owing
to habits, climate, or constitutional change--breeding at a slightly
different time from the parent species. This is known to produce
complete isolation in the case of many varieties of plants. Yet another
mode of isolation is brought about by changes of colour, and by the fact
that in a wild state animals of similar colours prefer to keep together
and refuse to pair with individuals of another colour. The probable
reason and utility of this habit will be explained in another chapter,
but the fact is well illustrated by the cattle which have run wild in
the Falkland Islands. These are of several different colours, but each
colour keeps in a separate herd, often restricted to one part of the
island; and one of these varieties--the mouse-coloured--is said to breed
a month earlier than the others; so that if this variety inhabited a
larger area it might very soon be established as a distinct race or
species.[48] Of course where the change of habits or of station is still
greater, as when a terrestrial animal becomes sub-aquatic, or when
aquatic animals come to live in tree-tops, as with the frogs and
Crustacea described at p.
Pages:
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272