The Descent of Man

it will suffice to call to mind the common pheasant,
duck, and house-sparrow. The cases under
this class graduate into others. Thus the
two sexes when adult may differ so slightly,
and the young so slightly from the adults, that
it is doubtful whether such cases ought to come
under the present, or under the third or fourth
classes. So again the young of the two sexes,
instead of being quite alike, may differ in a
slight degree from each other, as in our sixth
class. These transitional cases, however, are
few, or at least are not strongly pronounced, in
comparison with those which come strictly under
the present class.
The force of the present law is well shewn in
those groups, in which, as a general rule, the
two sexes and the young are all alike; for when
in these groups the male does differ from the
female, as with certain parrots, kingfishers,
pigeons, etc., the young of both sexes resemble
the adult female. (2. See, for instance, Mr.