282 LEISURE-TIME STUDIES. 



nerves and definite nervous acts from the simple exhibitions 

 of sensitiveness we see in the amoeba and its kindred. 

 The problem of the acquirement of sensitiveness by some 

 plants, and even of the power, as exhibited by the Venus's- 

 flytrap (see Fig. 44), of a selective discretion and choice of 

 food, is one which it is difficult even theoretically to investi- 

 gate. We may, therefore, more profitably devote our con- 

 sideration to the origin of nerves and nerve-actions as 

 exhibited in the animal kingdom : the theoretical pathway 

 by which nerve-development has been reached in animal life, 

 if not clearly defined throughout its entire extent, being yet 

 sufficiently plainly marked to give promise of intellectual 

 gain from even a cursory pilgrimage made therein. 



The amoeba's life may be said, as regards its irritability, 

 to be concerned with the reception of external impressions 

 of a simple character, and with responding to these impres- 

 sions by contractions and movements of the protoplasm of 

 its body. How the protoplasm contracts or moves in 

 obedience to the stimuli which play upon its outer parts we 

 do not know, any more than we can describe what takes 

 place in the nerve of a higher animal when an impulse 

 travels through or along its fibres. But there is every reason 

 to believe that molecular movements and activities of like 

 kind which prevail amongst the tissues of living beings at 

 large, are concerned in some special phase of their action 

 with the production and transmission of nerve-force in man. 

 And there similarly exist no grounds for the belief that the 

 molecular actions and forces which affect the protoplasm of 

 nerve-cells and nerve-fibres in man, are in any sense different 

 from those which affect the protoplasm of an amoeba and 

 produce movement in the animalcule's frame. The difference, 

 if it exist at all, is one not of kind, but merely in degree. 



If now, we direct our attention to the observation of 

 animals of higher grade than the amoeba, and compare 

 their acts with those of the animalcule, we may possibly be 

 enabled to explain more definitely the acts of the latter, 

 and at the same time to understand how an advance in the 



