In the
Monotreme and Marsupial the ear-hole begins to be covered with a
shell of cartilage; we have the beginning of the external ear.
The jaws, which are first developed in the fish, now articulate
more perfectly with the skull. Fat-glands appear in the skin, and
it is probably from a group of these that the milk-glands are
developed. The origin of the hairs is somewhat obscure. They are
not thought to be, like the bird's feathers, modifications of the
reptile's scales, but to have been evolved from other structures
in the skin, possibly under the protection of the scales.
My purpose is, however, rather to indicate the general causes of
the onward advance of life than to study organs in detail--a vast
subject--or construct pedigrees. We therefore pass on to consider
the next great stride that is taken by the advancing life of the
earth. Millions of years of genial climate and rich vegetation
have filled the earth with a prolific and enormously varied
population. Over this population the hand of natural selection is
outstretched, as it were, and we are about to witness another
gigantic removal of older types of life and promotion of those
which contain the germs of further advance.
Pages:
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308