--And, though these differences and resemblances cannot be
weighed and measured, their value may be readily estimated; the scale
or standard of judgment, touching that value, being afforded and
expressed by the system of classification of animals now current among
zoologists.
A careful study of the resemblances and differences presented by animals
has, in fact, led naturalists to arrange them into groups, or
assemblages, all the members of each group presenting a certain amount
of definable resemblance, and the number of points of similarity being
smaller as the group is larger and 'vice versa'. Thus, all creatures
which agree only in presenting the few distinctive marks of animality
form the 'Kingdom' ANIMALIA. The numerous animals which agree only in
possessing the special characters of Vertebrates form one 'Sub-Kingdom'
of this Kingdom. Then the Sub-kingdom VERTEBRATA is subdivided into
the five 'Classes,' Fishes, Amphibians, Reptiles, Birds, and Mammals,
and these into smaller groups called 'Orders'; these into 'Families'
and 'Genera'; while the last are finally broken up into the smallest
assemblages, which are distinguished by the possession of constant,
not-sexual, characters.
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