

ON HEREDITY. 83 



unintelligible hypothesis, which is only deduced from the facts 

 which it attempts to explain.' 



We therefore naturally ask whether the hypothesis is really 

 necessary for the explanation of known facts. 



At the first sight it certainly seems to be necessary, and it 

 appears rash to attempt to dispense with its aid. Many phenomena 

 only appear to be intelligible if we assume the hereditary trans- 

 mission of such acquired characters as the changes which we 

 ascribe to the use or disuse of particular organs, or to the direct 

 influence of climate. Furthermore, how can we explain instinct 

 as hereditary habit unless it has gradually arisen by the accumula- 

 tion, through heredity, of habits which were practised in succeeding 

 generations 1 



I will now attempt to prove that even these cases, so far as they 

 depend upon clear and indubitable facts, do not force us to accept 

 the supposition of the transmission of acquired characters. 



It seems difficult and well nigh impossible to deny the transmis- 

 sion of acquired characters when we remember the influence which 

 use and disuse have exercised upon certain special organs. It is 

 well known that Lamarck attempted to explain the structure of 

 the organism as almost entirely due to this principle alone. Accord- 

 ing 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 oppose 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 func- 

 tional powers increase. Indeed, the whole effect which exercise 

 produces upon the single parts of the body is dependent upon the 

 fact that frequently used organs increase in strength. This con- 

 clusion also refers to the nervous system, for a pianist who per- 

 forms 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 exercised the 

 muscles, but also those ganglionic centres of the brain which deter- 

 mine 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 



G 2 





