and constant, and does not depend on sexual peculiarities, then
all naturalists agree in calling them two species; that is what is meant
by the use of the word species--that is to say, it is, for the practical
naturalist, a mere question of structural differences. [1]

We have seen now--to repeat this point once more, and it is very
essential that we should rightly understand it--we have seen that
breeds, known to have been derived from a common stock by selection, may
be as different in their structure from the original stock as species
may be distinct from each other.

But is the like true of the physiological characteristics of animals?
Do the physiological differences of varieties amount in degree to those
observed between forms which naturalists call distinct species? This is
a most important point for us to consider.

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