However, these facts are best illustrated in animals, and the extent
of the variation, as is well known, is very remarkable in dogs. For
example, there are some dogs very much smaller than others; indeed, the
variation is so enormous that probably the smallest dog would be about
the size of the head of the largest; there are very great variations in
the structural forms not only of the skeleton but also in the shape of
the skull, and in the proportions of the face and the disposition of the
teeth.

The Pointer, the Retriever, Bulldog, and the Terrier, differ very
greatly, and yet there is every reason to believe that every one
of these races has arisen from the same source,--that all the most
important races have arisen by this selective breeding from accidental
variation.

A still more striking case of what may be done by selective breeding,
and it is a better case, because there is no chance of that partial
infusion of error to which I alluded, has been studied very carefully by
Mr. Darwin,--the case of the domestic pigeons. I dare say there may
be some among you who may be pigeon 'fanciers', and I wish you to
understand that in approaching the subject, I would speak with all
humility and hesitation, as I regret to say that I am not a pigeon
fancier. I know it is a great art and mystery, and a thing upon which
a man must not speak lightly; but I shall endeavour, as far as
my understanding goes, to give you a summary of the published and
unpublished information which I have gained from Mr. Darwin.

Among the enormous variety,--I believe there are somewhere about a
hundred and fifty kinds of pigeons,--there are four kinds which may
be selected as representing the extremest divergences of one kind from
another. Their names are the Carrier, the Pouter, the Fantail, and
the Tumbler. In the large diagrams they are each represented in their
relative sizes to each other. This first one is the Carrier; you will
notice this large excrescence on its beak; it has a comparatively small
head; there is a bare space round the eyes; it has a long neck, a very
long beak, very strong legs, large feet, long wings, and so on. The
second one is the Pouter, a very large bird, with very long legs and
beak. It is called the Pouter because it is in the habit of causing its
gullet to swell up by inflating it with air. I should tell you that all
pigeons have a tendency to do this at times, but in the Pouter it is
carried to an enormous extent