The sutures
in the skulls of young mammals have been advanced as a beautiful adaptation
for aiding parturition, and no doubt they facilitate, or may be
indispensable for this act; but as sutures occur in the skulls of young
birds and reptiles, which have only to escape from a broken egg, we may
infer that this structure has arisen from the laws of growth, and has been
taken advantage of in the parturition of the higher animals.
We are profoundly ignorant of the cause of each slight variation or
individual difference; and we are immediately made conscious of this by
reflecting on the differences between the breeds of our domesticated
animals in different countries, more especially in the less civilized
countries, where there has been but little methodical selection. Animals
kept by savages in different countries often have to struggle for their own
subsistence, and are exposed to a certain extent to natural selection, and
individuals with slightly different constitutions would succeed best under
different climates. With cattle susceptibility to the attacks of flies is
correlated with colour, as is the liability to be poisoned by certain
plants; so that even colour would be thus subjected to the action of
natural selection. Some observers are convinced that a damp climate
affects the growth of the hair, and that with the hair the horns are
correlated. Mountain breeds always differ from lowland breeds; and a
mountainous country would probably affect the hind limbs from exercising
them more, and possibly even the form of the pelvis; and then by the law of
homologous variation, the front limbs and the head would probably be
affected.
Pages:
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315