The wide distribution of useful animals and plants from a very remote epoch has, no doubt, been a powerful cause of modification, because the particular breed first introduced into each country has often been kept pure for many years, and has also been subjected to slight differences of conditions.

It will also usually have been selected for a somewhat different purpose in each locality, and thus very distinct races would soon originate.
The important physiological effects of crossing breeds or strains, and the part this plays in the economy of nature, will be explained in a future chapter.
_Concluding Remarks._ The examples of variation now adduced--and these might have been almost indefinitely increased--will suffice to show that there is hardly an organ or a quality in plants or animals which has not been observed to vary; and further, that whenever any of these variations have been useful to man he has been able to increase them to a marvellous extent by the simple process of always preserving the best varieties to breed from.

Along with these larger variations others of smaller amount occasionally appear, sometimes in external, sometimes in internal characters, the very bones of the skeleton often changing slightly in form, size, or number; but as these secondary characters have been of no use to man, and have not been specially selected by him, they have, usually, not been developed to any great amount except when they have been closely dependent on those external characters which he has largely modified.
As man has considered only utility to himself, or the satisfaction of his love of beauty, of novelty, or merely of something strange or amusing, the variations he has thus produced have something of the character of monstrosities.

Not only are they often of no use to the animals or plants themselves, but they are not unfrequently injurious to them.

In the Tumbler pigeons, for instance, the habit of tumbling is sometimes so excessive as to injure or kill the bird; and many of our highly-bred animals have such delicate constitutions that they are very liable to disease, while their extreme peculiarities of form or structure would often render them quite unfit to live in a wild state.
In plants, many of our double flowers, and some fruits, have lost the power of producing seed, and the race can thus be continued only by means of cuttings or grafts.