theory of selection. Unfortunately, however, this theory encounters insuperable difficulties as soon as it tries to step from the free-sailing air-balloon of probabilities upon the hard ground of realities. Nothing is easier than to ridicule the doctrine of natural and sexual selection. So much the more earnestly will the seeker for truth seize any means that can contribute anything to the solution of the problem. Is it not now a most promising coincidence that the higher beings exhibit in exercise such a self-improving machinery as we have recognized in the aggregate of life?
From these remote distances of research, which are the peculiar metaphysics of our time, come with me into a blacksmith's shop. The lad who lifts the hammer for the first time to-day soon becomes tired in spite of his splendid muscular foundation. He sweats; and, when he takes a horseshoe from the master's hands, he burns his fingers. Two years later he can, without sweating, perform the trick illustrating the mechanical theory of heat of pounding cold iron red-hot, and is not afraid to touch the hot metal. What has happened? First, the lad's arms have increased in compass, their muscles in tension to the highest capacity of contraction. If we could have weighed the muscles of his arms at the beginning of his apprenticeship, and could weigh them now, we should find that they had grown heavier; as also, according to Edward Weber, the muscles of the right side of the body are heavier than those of the left. The muscles are also the most perfect power-machines—not only in that when active they make the most complete use of the consumed matter; not only in that, according to Herr Heidenhain, their strength in particular instances increases with the service demanded of them—but they are distinguished above all machines made by man in that by frequent labor-service they become stronger and more capable of enduring further labor. It does not need to be proved that the effect of exercise on the muscles is immediate and local, and not transmitted through the favorable influence of bodily exertion on the general organism. Even the Greeks found fault with the disproportionate degree to which boxers trained their arms only, and runners their legs; and our pugilists and ballet-dancers are illustrations of the same. Under some circumstances the local results of exercise may be destructive to the whole, as when the muscles of the heart suffer hypertrophy in consequence of excessive resistance in some part of the circulation.
On the other hand, the surgeon knows only too well that the muscles of a stiffened or sprained joint, or of one that has been confined with bandages, become wasted, as do likewise muscles the nerves of which have been cut or that have been otherwise disabled.
The part is known which the latter fact, falsely interpreted by the older physiologists, played in the question of what was called the Hallerian muscular irritability, till John Reid—at a time when experiments on living animals were not prohibited in England—showed
