84 ON HEREDITY. [II. 



organs. It is well known that Lamarck attempted to explain 

 the structure of the organism as almost entirely due to this 

 principle alone. According to his theory the long neck of the 

 giraffe arose by constant stretching after the leaves of trees, 

 and the web between the toes of a water-bird's foot by the 

 extension of the toes, in an attempt to strike as large a surface 

 of water as possible in swimming. There can be no doubt that 

 those muscles which are frequently used increase in size and 

 strength, and that glands which often enter into activity become 

 larger and not smaller, and that their functional powers increase. 

 Indeed, the whole effect which exercise produces upon the 

 single parts of the body is dependent upon the fact that fre- 

 quently used organs increase in strength. This conclusion 

 also refers to the nervous system, for a pianist who performs 

 with lightning rapidity certain pre-arranged, highly complex, 

 and combined movements of the muscles of his hands and 

 fingers has, as Du Bois Reymond pointed out, not only exer- 

 cised the muscles, but also those ganglionic centres of the 

 brain which determine the combination of muscular movement. 

 Other functions of the brain, such as memory, can be similarly 

 increased and strengthened by exercise, and the question to be 

 settled is whether characters acquired in this way by exercise 

 and practice can be transmitted to the following generations. 

 Lamarck's theory assumes that such transmission takes place, 

 for without it no accumulation or increase of the characters in 

 question would be possible, as a result of their exercise during 

 any number of successive generations. 



Against this we may urge that whenever, in the course of 

 nature, an organ becomes stronger by exercise, it must possess 

 a certain degree of importance for the life of the individual, 

 and when this is the case it becomes subject to improvement 

 by natural selection, for only those individuals which possess 

 the organ in its most perfect form will be able to survive. 

 The perfection of form of an organ does not however 

 depend upon the amount of exercise undergone b}'' it during 

 the life of the organism, but primarily and principally upon 

 the fact that the germ from which the individual arose was 

 predisposed to produce a perfect organ. The increase to which 

 any organ can attain by exercise during a single life is bounded 

 by certain limits, w^hich are themselves fixed by the primary 



