Startling as the last assertion may appear to be, it is demonstrably
true, and it alone appears to me sufficient to place beyond all doubt
the structural unity of man with the rest of the animal world, and more
particularly and closely with the apes.

Thus, identical in the physical processes by which he
originates--identical in the early stages of his formation--identical
in the mode of his nutrition before and after birth, with the animals
which lie immediately below him in the scale--Man, if his adult and
perfect structure be compared with theirs, exhibits, as might be
expected, a marvellous likeness of organization. He resembles them as
they resemble one another--he differs from them as they differ from one
another.--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'