As regards the great majority of physiological characteristics, there is
no doubt that they are capable of being developed, increased, and
modified by selection.

There is no doubt that breeds may be made as different as species in
many physiological characters. I have already pointed out to you very
briefly the different habits of the breeds of Pigeons, all of which
depend upon their physiological peculiarities,--as the peculiar habit
of tumbling, in the Tumbler--the peculiarities of flight, in the
"homing" birds,--the strange habit of spreading out the tail, and
walking in a peculiar fashion, in the Fantail,--and, lastly, the habit
of blowing out the gullet, so characteristic of the Pouter. These are
all due to physiological modifications, and in all these respects these
birds differ as much from each other as any two ordinary species do.

So with Dogs in their habits and instincts. It is a physiological
peculiarity which leads the Greyhound to chase its prey by sight,--that
enables the Beagle to track it by the scent,--that impels the Terrier
to its rat-hunting propensity,--and that leads the Retriever to its
habit of retrieving. These habits and instincts are all the results of
physiological differences and peculiarities, which have been developed
from a common stock, at least there is every reason to believe so. But
it is a most singular circumstance, that while you may run through
almost the whole series of physiological processes, without finding a
check to your argument, you come at last to a point where you do find a
check, and that is in the reproductive processes. For there is a most
singular circumstance in respect to natural species--at least about some
of them--and it would be sufficient for the purposes of this argument
if it were true of only one of them, but there is, in fact, a great
number of such cases--and that is, that, similar as they may appear to