Thus war was declared against the Lamarckian principle of the direct effect of use and disuse, and there arose a strife which 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 160 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? 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.