The Descent of Man

Professor Owen, relying chiefly on the structure
of the brain, has divided the mammalian
series into four sub-classes. One of these he
devotes to man; in another he places both the
marsupials and the Monotremata; so that he
makes man as distinct from all other mammals
as are these two latter groups conjoined. This
view has not been accepted, as far as I am aware,
by any naturalist capable of forming an independent
judgment, and therefore need not
here be further considered.
We can understand why a classification founded
on any single character or organ—even an
organ so wonderfully complex and important
as the brain—or on the high development of
the mental faculties, is almost sure to prove
unsatisfactory. This principle has indeed been
tried with hymenopterous insects; but when
thus classed by their habits or instincts, the
arrangement proved thoroughly artificial. (3.
Westwood, 'Modern Classification of Insects,'