(3) When we come to the effects of use and disuse, we reach a much debated ground, and one complicated by the injection of a great deal of biological theorizing, as well as the presence of the usual large amount of faulty observation and inference.
It will be admitted by every one that a part of the body which is much used tends to increase in size, or strength, and similarly that a part which is not used tends to atrophy. It is further found that such changes are progressive in the race, in many cases. Man's brain has steadily increased in size, as he used it more and more; on the other hand, his canine teeth have grown smaller. Can this be regarded as the inheritance of a long continued process of use and disuse? Such a view is often taken, but the Lamarckian doctrine seems to us just as mystical here as anywhere else, and no more necessary. Progressive changes can be satisfactorily accounted for by natural selection; retrogressive changes are susceptible of explanation along similar lines. When an organ is no longer necessary, as the hind legs of a whale, for instance, natural selection no longer keeps it at the point of perfection. Variation, however, continues to occur in it. Since the organ is now useless, natural selection will no longer restrain variation in such an organ, and degeneracy will naturally follow, for of all the variations that occur in the organ, those tending to loss are more numerous than those tending to addition. If the embryonic development of a whale's hind leg be compared to some complicated mechanical process, such as the manufacture of a typewriter, it will be easier to realize that a trivial variation which affected one of the first stages of the process would alter all succeeding stages and ruin the final perfection of the machine. It appears, then, that progressive degeneration of an organ can