It has often been stated that domestic races do not differ from
each other in characters of generic value. It can be shown that this
statement is not correct; but naturalists differ much in determining what
characters are of generic value; all such valuations being at present
empirical. When it is explained how genera originate under nature, it will
be seen that we have no right to expect often to find a generic amount of
difference in our domesticated races.
In attempting to estimate the amount of structural difference between
allied domestic races, we are soon involved in doubt, from not knowing
whether they are descended from one or several parent species. This point,
if it could be cleared up, would be interesting; if, for instance, it could
be shown that the greyhound, bloodhound, terrier, spaniel and bull-dog,
which we all know propagate their kind truly, were the offspring of any
single species, then such facts would have great weight in making us doubt
about the immutability of the many closely allied natural species--for
instance, of the many foxes--inhabiting the different quarters of the
world. I do not believe, as we shall presently see, that the whole amount
of difference between the several breeds of the dog has been produced under
domestication; I believe that a small part of the difference is due to
their being descended from distinct species. In the case of strongly
marked races of some other domesticated species, there is presumptive or
even strong evidence that all are descended from a single wild stock.
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