and 'Genera'; while the last are finally broken up into the smallest
assemblages, which are distinguished by the possession of constant,
not-sexual, characters. These ultimate groups are Species.

Every year tends to bring about a greater uniformity of opinion
throughout the zoological world as to the limits and characters of
these groups, great and small. At present, for example, no one has 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