An amazingly curious link, which connects the reptile with the bird on
the one hand and with the mammal on the other, is the Duck-bill (Fig.
19). This creature, whose home is in Australia, is covered with dense
fur and suckles its young, like a mammal; but, on the other hand, it
lays eggs like the reptile and the bird. The eggs have large yolks,
like those of birds; are hatched by the warmth of the mother’s body;
and when the young is born, it lives on milk drawn from its mother’s
breast. Observe, too, that the mother Duck-bill has no nipples, but
mere depressions in the breast, from which the milk oozes out among the
fur, to be sucked up by the young. Think of a fur-covered, five-toed,
web-footed, duck-billed, flesh-eating, swimming animal, housing itself
in a burrow in the bank of a stream, being born from an egg, like a
bird; formed in part like a reptile, and deriving its early sustenance
by sucking the milk-ooze from its mother’s teatless breast! This link
between reptile, bird and mammal, this crude combination of three forms
of life, shows finished forms in the making. It is the living proof
of the manner in which Nature has accomplished her work—of the steps
by which evolution has advanced. It is what Darwin called “a living
fossil.”
After the Duck-bills came the marsupial mammals—mammals whose young,
born not yet fully developed, are carried for a time in a pouch
attached to the body of the mother. The kangaroo (Fig. 20) belongs to
this class. Here the advance is from an egg-laying mammal to one whose
young is partly formed in the body of the mother. I say partly formed,
for, although the kangaroo is as large as a man, its young, when born,
though it is no larger than the little finger, is still a fœtus, so
imperfectly formed that it must be carried for months in the mother’s
pouch, so that it may complete its development as a babe. Meanwhile,
being unable to feed itself, the mother, by an exercise of her muscles,
forces milk down its throat. Once more Nature, in her forward march,
is blazing a new trail. Life, by employing crude makeshifts and
adaptations, is fashioning for itself a higher mould.
[Illustration: FIG. 18.—THE ARCHAEOPTERYX.