IX.] RETROGRESSIVE DEVELOPMENT IN NATURE. 13 



immediate cause of its degeneration, a view which has hitherto 

 actually been held, and which at first seems credible enough 

 and even plausible. 



It is, of course, a well-known fact, although perhaps the 

 subject has hardly been sufficiently studied, that parts which 

 are much used grow larger and more powerful, while those 

 which are seldom exercised become small and weak. Constant 

 gymnastic exercise will immensely increase the size and strength 

 of the muscles of our arms ; while these limbs will lose what 

 strength they once possessed if the muscles are never exerted. 

 The performances of athletes afford us the best examples of 

 the extent to which practice can increase the muscular 

 strength and activity of man ; and, on the other hand, those 

 who work at occupations entailing a sedentary life and lack of 

 exercise plainly show the weakening effects of disuse. Experi- 

 ments prove this still more clearly : when the nerve supplying 

 a muscle is cut, degeneration of the muscle ensues, because its 

 activity is at an end, and the same thing happens with glands, 

 when their functions are disturbed by severing the nerves 

 which supply them. We may accept the general proposition 

 that an organ may be strengthened by exercise, and weakened 

 by a long continued state of inactivity. It is not necessary here 

 to go into the question of how this is brought about, nor has 

 it been as yet completely explained : it is sufficient for our 

 present purpose to know that such is the case. 



Since we may take it for granted that disuse of an organ will 

 lead to its degeneration, even in the life -time of a single in- 

 dividual, may we not also conclude that the gradual disappear- 

 ance of a superfluous structure in the course of generations is 

 due simply to the tendency to degeneration being handed down 

 from one generation to another, and thus gradually intensified 

 to the extent of complete elimination? For supposing disuse 

 to produce infinitely small effects during the life of each in- 

 dividual, yet surely these effects would be cumulative, and in 

 course of generations the organ would gradually diminish in 

 importance, become smaller and weaker, and ultimately dis- 

 appear altogether. 



This explanation, obvious as it may seem to be, cannot be 

 the right one, for there are many facts which are quite incom- 

 patible with it. 



