A considerable part of the difficulty in recognising a variable species
in our systematic works, is due to its varieties mocking, as it were,
some of the other species of the same genus. A considerable catalogue,
also, could be given of forms intermediate between two other forms,
which themselves must be doubtfully ranked as either varieties or
species; and this shows, unless all these forms be considered as
independently created species, that the one in varying has assumed some
of the characters of the other, so as to produce the intermediate form.
But the best evidence is afforded by parts or organs of an important
and uniform nature occasionally varying so as to acquire, in some
degree, the character of the same part or organ in an allied species. I
have collected a long list of such cases; but
here, as before, I lie under a great disadvantage in not being able to
give them. I can only repeat that such cases certainly do occur, and
seem to me very remarkable.
I will, however, give one curious and complex case, not indeed as
affecting any important character, but from occurring in several
species of the same genus, partly under domestication and partly under
nature. It is a case apparently of reversion. The ass not rarely has
very distinct transverse bars on its legs, like those on the legs of a
zebra: it has been asserted that these are plainest in the foal, and
from inquiries which I have made, I believe this to be true. It has
also been asserted that the stripe on each shoulder is sometimes
double. The shoulder stripe is certainly very variable in length and
outline. A white ass, but _not_ an albino, has been described without
either spinal or shoulder-stripe; and these stripes are sometimes very
obscure, or actually quite lost, in dark-coloured asses. The koulan of
Pallas is said to have been seen with a double shoulder-stripe. The
hemionus has no shoulder-stripe; but traces of it, as stated by Mr.
Blyth and others, occasionally appear: and I have been informed by
Colonel Poole that the foals of this species are generally striped on
the legs, and faintly on the shoulder. The quagga, though so plainly
barred like a zebra over the body, is without bars on the legs; but Dr.
Gray has figured one specimen with very distinct zebra-like bars on the
hocks.
With respect to the horse, I have collected cases in England of the
spinal stripe in horses of the most distinct breeds, and of _all_
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