THE PHYSIOLOGY OF EXERCISE. 3 i 9 



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 imme- 

 diate 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 mus- 

 cles 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 Hal- 

 lerian muscular irritability, till John Reid at a time when experi- 

 ments on living animals were not prohibited in England showed 



