another, of pouched animals in another, of a
iiar modification of leaves in Australian
shrubs, of peculiar aloes or agaves in America—
are inexplicable on the theory of creation.

Glancing at instincts, marvellous as some are,
they offer, it appears, no greater difficulty than
does corporeal structure, on the theory of the
Natural Selection of successive, slight, but
profitable, modifications. We can thus understand
why nature moves by graduated steps in endowing
different animals of the same clais with their
several instincts. On the view of all the
species of the same genus having descended from
a common parent, and having inherited much in
common, we can understand how it is that allied
species, when placed under considerably different
conditions of life, yet should follow nearly
the same instincts; why the male wrens of
North America, for instance, build " cock-nests"
to roost in, like the males of our distinct kitty-
wrens—a habit wholly unlike that of any other
known bird. On the view of instincts having
been slowly acquired through Natural Selection,
we need not marvel at some instincts being
apparently not perfect, but liable to mistakes, as
when blow-flies lay their eggs in the carrion-
scented flowers of stapelias; nor at many
instincts causing other animals to suffer, as when
ants make slaves of their fellow-ants, when the
larvae of ichneumon flies feed within the live
bodies of caterpillars, and when the nestling
cuckoo ungratefully ejects his legitimate foster-
brethren out of the family nest.

Instincts are as important as bodily structure
for the welfare of each species, under the
conditions of life by which it happens to be
surrounded. Under changed circumstances, it is
possible that slight modifications of instinct
might be profitable to a species; and if it can
be shown that instincts do vary ever so little,
then Mr. Darwin sees no difficulty in Natural
Selection preserving and continually accumulating
variations of instinct to any extent that
may be profitable. His line of argument—and
the whole volume is one long argument—may
be summed up in this: give him an inch, and he
takes an ell. Instincts certainly do vary—for
instance, the migratory instinct varies, both in
extent and direction, and in its total loss. So it is
with the nests of birds, which vary partly in
dependence on the situations chosen and on the
nature and temperature of the country
inhabited, but often from causes wholly unknown to
us. It is thus, he believes, that all the most
complex and wonderful instincts have
originated; although no complex instinct can
possibly be produced except by the slow and
gradual accumulation of numerous slight, yet
profitable, variations, requiring ages upon ages, and
tens of thousands, perhaps hundreds of millions,
of generations to work them out. For Mr.
Darwin assumes such an inconceivably vast
period of lapsed time for the accomplishment of
his theory, that it is simply not eternity, because
it had beginning.

Variations of instinct, thus acquired, become,
in races, habitual and hereditary. Habit and
the selection of so-called accidental variations,
have played important parts in modifying the
mental qualities of our domestic animals. It
cannot be doubted that young pointers will
sometimes point, and even back other dogs, the
very first time that they are taken out; retrieving
is certainly in some degree inherited by
retrievers; as is a tendency to run round,
instead of at, a flock of sheep by shepherds' dogs.
These actions do not differ essentially from true
instincts; for the young pointer can no more
know that he points to aid his master, than the
white butterfly knows why she lays her eggs on
the leaf of the cabbage. How strongly these
habits and dispositions are inherited, and how
curiously they become mingled, is well shown
when different breeds of dogs are crossed. A
cross with the greyhound has given to a whole
family of shepherds' dogs, the lurchers, a
tendency to hunt hares, rendering them invaluable
to poachers. Le Roy describes a dog whose
great-grandfather was a wolf, and this dog
showed a trace of its wild parentage only in one
way—by not coming in a straight line to his
master when called.

To understand how instincts in a state of
nature have become modified by Natural Selection,
let us consider the case of the cuckoo. It
is commonly admitted that the more immediate
and final cause of the cuckoo's instinct is that
she lays her eggs, not daily, but at intervals of
two or three days; so that, if she were to make
her own nest and sit on her own eggs, those
first laid would have to be left for some time
unincubated, or there would be eggs, and young
birds of different ages in the same nest; which
would make the process of laying, hatching, and
rearing the young, inconveniently long and
troublesome. The American cuckoo makes her
own nest, and has eggs and young successively
hatched, all at the same time.

Now, instances can be given of various birds
which have been known occasionally to lay
their eggs in other birds' nests. Let us
suppose that the ancient progenitor of our European
cuckoo had the habits of the American
cuckoo, but that she occasionally laid an egg in
another bird's nest by way of experiment. If
the old bird profited by this occasional habit, or
if the young were made more vigorous by the
mistaken maternal instinct of another bird than
by their own mother's care, encumbered as she
can hardly fail to be by having eggs and young of
different ages at the same time, then the old
birds, or the fostered young, would gain an
advantage. And analogy leads Mr. Darwin to
believe that the young thus reared would be apt
to follow, by inheritance, the occasional and
aberrant habit of their mother, and in their
turn would possibly lay their eggs in other
birds' nests, aud thus be successful in rearing
their young. By a continued process of this
nature, he believes that the strange instinct of
our cuckoo could be, and has been, generated.

To Mr. Darwin, this explanation appears
conclusive; other persons, less under the influence
of a fixed idea, may observe that, with the help