began, the rate of speed evolution (which depends on structural evolution) was comparatively rapid, but that this rate has been gradually slowing, so that now the race-horses of one decade do not greatly surpass the race-horses of the previous decade. To some extent this, of course, may be due to the fact that to obtain any increment of speed a more than corresponding increment of force must be put forth, as is exemplified by the fact that an oarsman, to increase his speed from two to four miles an hour, must use more than twice the amount of force, and to increase his speed to six miles an hour, must use a still more disproportionate amount of force. But the race-horse surpasses the ordinary horse not so much because it is more powerful, but because it is better shaped for speed, just as the racing boat surpasses in speed the ordinary rowing boat for the same reason. We can hardly conclude that the evolution of the race-horse is now slow because he has nearly reached the perfection of shape for speed, for another of the equidæ, the wild ass, a smaller animal, is said to be swifter. The decreasing rate of evolution, therefore, must be set down to another cause.
If we mate two ordinary horses there is a fair prospect that some at least of the offspring will be as fine, or even finer, animals than either the sire or the dam. But if we mate a Derby winner with a good mare—a better mare than his dam—we shall find that the majority of his progeny are inferior to himself, and that only very exceptionally does he procreate a son or a daughter that can match him for speed. Thus, I venture to say that very few, if any, of the sons or daughters of Ormonde will be his equals. Rapid evolution, therefore, rapidly becomes more slow. Why? Evidently because there is a tendency for the offspring to omit more or less of the latest evolution of the race and revert to the ancestry, sometimes the remote
