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 XII

is not the purpose of this book to deal fully with the principles of stock-breeding, but some phases of the question raised by the preceding chapters might be referred to with interest at least. It cannot have escaped the notice of any one who has read the last few chapters how large have been the parts played by crossing and inbreeding in bringing our British breeds of cattle to their present position. At the same time, it must be admitted that it is not clear that everything accomplished has been due to the action of those two factors. There may still be something more—there may still be the constantly accumulating change which Darwin believed in, although it is hard to think that changes which formerly took thousands and thousands of years may now be induced by man's action in two or three centuries; and there may also be De Vries's mutations or organic "jerks," the results of changed conditions in food, climate, or any other form of environment. So far these questions must be left undecided; but it seems impossible 136