had such habits and structure best developed would be the most securely
reared. The first step towards the acquisition of the proper instinct
might have been mere unintentional restlessness on the part of the young
bird, when somewhat advanced in age and strength; the habit having been
afterwards improved, and transmitted to an earlier age. I can see no more
difficulty in this than in the unhatched young of other birds acquiring the
instinct to break through their own shells; or than in young snakes
acquiring in their upper jaws, as Owen has remarked, a transitory sharp
tooth for cutting through the tough egg-shell. For if each part is liable
to individual variations at all ages, and the variations tend to be
inherited at a corresponding or earlier age--propositions which cannot be
disputed--then the instincts and structure of the young could be slowly
modified as surely as those of the adult; and both cases must stand or fall
together with the whole theory of natural selection.

Some species of Molothrus, a widely distinct genus of American birds,
allied to our starlings, have parasitic habits like those of the cuckoo;
and the species present an interesting gradation in the perfection of their
instincts. The sexes of Molothrus badius are stated by an excellent
observer, Mr. Hudson, sometimes to live promiscuously together in flocks,
and sometimes to pair. They either build a nest of their own or seize on
one belonging to some other bird, occasionally throwing out the nestlings
of the stranger. They either lay their eggs in the nest thus appropriated,
or oddly enough build one for themselves on the top of it. They usually
sit on their own eggs and rear their own young; but Mr. Hudson says it is
probable that they are occasionally parasitic, for he has seen the young of
this species following old birds of a distinct kind and clamouring to be
fed by them. The parasitic habits of another species of Molothrus, the M.
bonariensis, are much more highly developed than those of the last, but are
still far from perfect. This bird, as far as it is known, invariably lays