INTEA-SELECTION OR SELECTION AMONG TISSUES 243 



has continued down to the present day, the strife between the Neo- 

 Lamarckians and the Neo-Darwinians, as the two disputing parties 

 have been called. 



In order to form an independent opinion in regard to this famous 

 dispute, it is, first of all, necessary to examine what actually takes 

 place when an organ is exercised or is left inactive, and further, 

 whether we can assume that the results of this exercise or inaction 

 can be transmitted to descendants. 



That exercise in general has a strengthening, and neglect of it 

 a weakening influence on the relevant organ has long been known 

 and is familiar to all; gymnastics make the muscles stronger, the 

 thickness of the exercised muscle and the number of its fibres 

 increases; the right arm, which is much more used than the left, 

 is capable of performing twenty per cent, more work. Similarly, 

 the activity of glands is increased by exercise, and the glands 

 themselves are increased in size, as are the milk-glands of the cow 

 through frequent milking; and that even the nerve-elements 

 can be favourably influenced by exercise is proved by actors and 

 professors of mnemonics, who have by practice increased their powers 

 of memory to an almost incredible degree. I have heard of a singer 

 who had learned by heart i6o operas; and which of us has not 

 experienced how quickly the capacity for learning by rote can be 

 again increased by practice, even after it has been neglected or left 

 unexercised for a long time 1 



I have always been particularly struck with the practising of 

 a piece of music, with its long succession of periods of different 

 phrase, with its changes in melody, rhythm, and harmony, which 

 nevertheless becomes so firmly stamped on the memory that it can be 

 played, not only consciously, but quite unconsciously, when the player 

 is thinking intensely of other things. It is in this case not the 

 memory alone, but the whole complicated mechanism of successive 

 muscle-impulses, with all the details of fast and slow, loud and soft, 

 that is engraved on the brain elements, just like a long series of 

 reflex movements which set one another a-going. Though in this case 

 we cannot demonstrate the material changes which have taken place 

 in the nervous elements, there can be no doubt that changes have 

 taken place, and that these consist in a strengthening of definite 

 elements and parts of elements. The strengthening causes certain 

 ganglion-cells to give a stronger impulse in a particular direction, and 

 this impulse acquires increasing transmissive power, and so on. 



Our first theoretical insight into these relations came through 

 Wilhelm Roux, who, in 1881, gave expression to what had previously 



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