he
accumulated amount of deviation would certainly be of an abnormal nature in
comparison with the structure of pigeons living in a state of nature.

I have already alluded to the remarkable fact, that the {192}
characteristic differences between the chief domestic races are eminently
variable: we see this plainly in the great difference in the number of the
tail-feathers in the fantail, in the development of the crop in pouters, in
the length of the beak in tumblers, in the state of the wattle in carriers,
&c. If these characters are the result of successive variations added
together by selection, we can understand why they should be so variable:
for these are the very parts which have varied since the domestication of
the pigeon, and therefore would be likely still to vary; these variations
moreover have been recently, and are still being accumulated by man's
selection; therefore they have not as yet become firmly fixed.

_Fifthly._--All the domestic races pair readily together, and, what is
equally important, their mongrel offspring are perfectly fertile. To
ascertain this fact I made many experiments, which are given in the note
below; and recently Mr. Tegetmeier has made similar experiments with the
same result.[334] The accurate Neumeister[335] asserts that when dovecots
{193} are crossed with pigeons of any other breed, the mongrels are
extremely fertile and hardy. MM. Boitard and Corbie[336] affirm, after
their great experience, that with crossed pigeons the more distinct the
breeds, the more productive are their mongrel offspring. I admit that the
doctrine first broached by Pallas is highly probable, if not actually
proved, namely, that closely allied species, which in a state of nature or
when first captured would have been in some degree sterile when crossed,
lose this sterility after a long course of domestication; yet when we
consider the great difference between such races as pouters, carriers,
runts, fantails, turbits, tumblers, &c., the fact of their perfect, or even
increased, fertility when intercrossed in the most complicated manner
becomes a strong argument in favour of their having all descended from a
single species. This argument is rendered much stronger when we hear (I
append in a note[337] {194} all the cases which I have collected) that
hardly a single well-ascertained instance is known of hybrids between two
true species of pigeons being fertile, _inter se_, or even when crossed
with one of t