The improvement is by no means generally due to crossing different breeds; all the best breeders are strongly opposed to this practice, except sometimes among closely allied sub-breeds.

And when a cross has been made, the closest selection is far more indispensable even than in ordinary cases.

If selection consisted merely in separating some very distinct variety and breeding from it, the principle would be so obvious as hardly to be worth notice; but its importance consists in the great effect produced by the accumulation in one direction, during successive generations, of differences absolutely inappreciable by an uneducated eye--differences which I for one have vainly attempted to appreciate.