Every one who has studied cattle-breeding, or
turned pigeon-fancier, or "pomologist," must have been struck by the
extreme modifiability or plasticity of those kinds of animals and
plants which have been subjected to such artificial conditions as are
imposed by domestication. Breeds of dogs are more different from one
another than are the dog and the wolf; and the purely artificial races
of pigeons, if their origin were unknown, would most assuredly be
reckoned by naturalists as distinct species and even genera.
These breeds are always produced in the same way. The breeder selects a
pair, one or other, or both, of which present an indication of the
peculiarity he wishes to perpetuate, and then selects from the
offspring of them those which are most characteristic, rejecting the
others. From the selected offspring he breeds again, and, taking the
same precautions as before, repeats the process until he has obtained
the precise degree of divergence from the primitive type at which he
aimed.
If he now breeds from the variety thus established for some generations,
taking care always to keep the stock pure, the tendency to produce this
particular variety becomes more and more strongly hereditary; and it
does not appear that there is any limit to the persistency of the race
thus developed.
Men like Lamarck, apprehending these facts, and knowing that varieties
comparable to those produced by the breeder are abundantly found in
nature, and finding it impossible to discriminate in some cases between
varieties and true species, could hardly fail to divine the possibility
that species even the most distinct were, after all, only exceedingly
persistent varieties, and that they had arisen by the modification of
some common stock, just as it is with good reason believed that
turnspits and greyhounds, carrier and tumbler pigeons, have arisen.
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