the
least doubt regarding the characters of the classes Mammalia, Aves, or
Reptilia; nor does the question arise whether any thoroughly well-known
animal should be placed in one class or the other. Again, there is
a very general agreement respecting the characters and limits of
the orders of Mammals, and as to the animals which are structurally
necessitated to take a place in one or another order.

No one doubts, for example, that the Sloth and the Ant-eater, the
Kangaroo and the Opossum, the Tiger and the Badger, the Tapir and
the Rhinoceros, are respectively members of the same orders. These
successive pairs of animals may, and some do, differ from one another
immensely, in such matters as the proportions and structure of their
limbs; the number of their dorsal and lumbar vertebrae; the adaptation
of their frames to climbing, leaping, or running; the number and form
of their teeth; and the characters of their skulls and of the contained
brain. But, with all these differences, they are so closely connected in
all the more important and fundamental characters of their organization,
and so distinctly separated by these same characters from other animals,
that zoologists find it necessary to group them together as members
of one order. And if any new animal were discovered, and were found to
present no greater difference from the Kangaroo and the Opossum, for
example, than these animals do from one another, the zoologist would not
only be logically compelled to rank it in the same order with these, but
he would not think of doing otherwise.

Bearing this obvious course of zoological reasoning in mind, let us
endeavour for a moment to disconnect our thinking selves from the mask
of humanity; let us imagine ourselves scientific Saturnians, if you
will, fairly acquainted with such animals as now inhabit the Earth, and
employed in discussing the relations they bear to a new and singular
'erect and featherless biped,' which some enterprising traveller,
overcoming the difficulties of space and gravitation, has brought from
that distant planet for our inspection, well preserved, may be, in a
cask of rum. We should all, at once, agree upon placing him among the
mammalian vertebrates; and his lower jaw, his molars, and his brain,
would leave no room for doubting the systematic position of the new
genus among those mammals, whose young are nourished during gestation by
means of a placenta, or what are called the 'placental mammal