ld state. As the
affinity between wolf, jackal, fox, and dog, cannot fail to attract
the notice of the most superficial observer; so he may ask if they do
not all really belong to one species, modified by varieties of
climate, food, and education? If answered in the negative, he would
want to know what constitutes a species, little thinking that this
question, apparently so simple, involves one of the nicest problems in
natural history. Difference of form will scarcely avail us here, for
the pug, greyhound, and spaniel, are wider apart in this respect, than
many dogs and the wild animals just named. It has often been said that
these varieties in the dog have arisen from artificial habits and
breeding through a long succession of years. This seems very like mere
conjecture. Can the greyhound be trained to the pointer's scent or the
spaniel to the bulldog's ferocity? But admitting the causes assigned
to be adequate to the effects, then the forms would be temporary, and
those of a permanent kind only would serve our purpose. Of this nature
is the shape of the pupil of the eye, which may be noticed somewhat
particularly, not merely to make it plain to those who have never
thought on the subject, but with the hope of leading them to
reflections on this wondrous inlet to half our knowledge, the more
especially as the part in question may be examined by any one in his
own person by the help of a looking-glass. In the front of the eye
then, just behind the transparent surface, there is a sort of curtain
called the _iris_, about the middle of which is a round hole. This is
the pupil, and you will observe that it contracts in a strong light,
and dilates in a weaker one, the object of which is to regulate the
quantity of light admitted into the eye. Now the figure of the pupil
is not the same in all animals. In the horse it is oval; in the wolf,
jackal, and dog, it is round, like our own, however contracted; but in
the fox, as in the cat, the pupil contracts vertically into an
elongated figure, like the section of a lens, and even to a sort of
slit, if the light be very strong.

This is a permanent character, not affected, as far as is at present
known, by any artificial or natural circumstances to which the dog has
been subjected. Naturalists, therefore, have seized upon this
character as the ground for a division of animals of the dog kind, the
great genus _Canis_ of Linnaeus, into two groups, the diurnal and
nocturnal; not to