ediate between the placental
mammals and the birds? and does not nature indicate their true position
by the position which she assigns to them in the geologic scale? The
birds are oviparous; and between the extrusion of the egg and the
development of the perfect young bird they have to hatch it into life
during a long period of incubation. The marsupiata are not oviparous,
for their _eggs_ want the enveloping shell or skin; but they, too, are
extruded in an exceedingly rudimentary and foetal state, and have to
undergo in the pouch a greatly longer period of _incubation_ than that
demanded by nature for any bird whatever. The young kangaroo is
extruded, after it has remained for little more than a month in the
womb, as a foetus scarcely an inch in length by somewhat less than half
an inch in breadth: it is blind, exhibiting merely dark eye spots; its
limbs are so rudimentary, that even the hinder legs, so largely
developed in the genus when mature, exist as mere stumps; it is unable
even to suck, but, holding permanently on by a minute dug, has the
sustaining fluid occasionally pressed into its mouth by the mother. And,
undergoing a peculiar but not the less real process of incubation, the
creature that had to remain for little more than a month in the
womb,--strictly thirty-nine days,--has to remain in the mother's pouch,
ere it is fully developed and able to provide for itself, for a period
of eight months. It is found to increase in weight during this hatching
process, from somewhat less than an ounce to somewhat more than eight
pounds. Now, this surely is a process quite as nearly akin to the
incubation of egg-bearing birds as to the ordinary nursing process of
the placental mammals; and on the occult but apparently real principle,
that the true arrangement of the animal kingdom is that which we find
exemplified by the successive introduction of its various classes and
orders in the course of geologic history, should we not anticipate a
point of time for the introduction of the marsupiata, intermediate
between the widely-distant points at which the egg-bearing birds and the
true placental mammals appeared? Ranged at once chronologically, and by
their mode of reproduction, the various classes of the vertebrata would
run, did we accept the suggested reading, as follows:--First appear
cold-blooded vertebrates (fishes), that propagate by eggs or
spawn,--chiefly by the latter. Next appear cold-blooded vertebrates
(reptiles