only the growth of those peculiar parts of animals, which are first wanted
to secure their subsistence, are in general furthest advanced before their
nativity: but some animals come into the world more completely formed
throughout their whole system than others: and are thence much forwarder in
all their habits of motion. Thus the colt, and the lamb, are much more
perfect animals than the blind puppy, and the naked rabbit; and the chick
of the pheasant, and the partridge, has more perfect plumage, and more
perfect eyes, as well as greater aptitude to locomotion, than the callow
nestlings of the dove, and of the wren. The parents of the former only find
it necessary to shew them their food, and to teach them to take it up;
whilst those of the latter are obliged for many days to obtrude it into
their gaping mouths.

IV. From the facts mentioned in No. 2. of this Section, it is evinced that
the foetus learns to swallow before its nativity; for it is seen to open
its mouth, and its stomach is found filled with the liquid that surrounds
it. It opens its mouth, either instigated by hunger, or by the irksomeness
of a continued attitude of the muscles of its face; the liquor amnii, in
which it swims, is agreeable to its palate, as it consists of a nourishing
material, (Haller Phys. T. 8. p. 204). It is tempted to experience its
taste further in the mouth, and by a few efforts learns to swallow, in the
same manner as we learn all other animal actions, which are attended with
consciousness, _by the repeated efforts of our muscles under the conduct of
our sensations or volitions_.

The inspiration of air into the lungs is so totally different from that of
swallowing a fluid in which we are immersed, that it cannot be acquired
before our nativity. But at this time, when the circulation of the blood is
no longer continued through the placenta, that suffocating sensation, which
we feel about the precordia, when we are in want of fresh air, disagreeably