26 LIFE: ITS NATUEE, OEIGIN AND MAINTENANCE 



other parts of the body, the cells of which they excite to activity. These 

 substances have received the general designation of ' hormones ' (op/zaw, to 

 x>"stir up), a term introduced by Professor Starling. Their action, and indeed 

 their very existence, has only been recognised of late years, although the 

 part which they play in the physiology of animals appears to be only 

 second in importance to that of the nervous system itself ; indeed, main- 

 tenance of life may become impossible in the absence of certain of these 

 hormones. 



Before we consider the manner in which the nervous system serves to 

 Part played by tlie co-ordinate the life of the cell-aggregate, let us see how 

 nervous system in it has become evolved. 



S^rSaSISr 06 f The first ste P in the P roc ess was taken when 

 Evolution of a ner- certain of the cells of the external layer became 

 vous system. specially sensitive to stimuli from outside, whether 



caused by mechanical impressions (tactile and auditory stimuli) or 

 impressions of light and darkness (visual stimuli) or chemical impres- 

 sions. The effects of such impressions were probably at first simply 

 communicated to adjacent cells and spread from cell to cell throughout the 

 mass. An advance was made when the more impressionable cells threw 

 out branching feelers amongst the other cells of the organism. Such 

 feelers would convey the effects of stimuli with greater rapidity and 

 directness to distant parts. They may at first have been retractile, in this 

 respect resembling the long pseudopodia of certain Rhizopoda. When they 

 became fixed they would be potential nerve-fibres and would represent the 

 beginning of a nervous system. Even yet (as Boss Harrison has shown), 

 in the course of development of nerve-fibres, each fibre makes its appear- 

 ance as an amoeboid cell-process which is at first retractile, but gradually 

 grows into the position it is eventually to occupy and in which it will become 

 fixed. 



In the further course of evolution a certain number of these specialised 

 cells of the external layer sank below the general surface, partly perhaps 

 for protection, partly for better nutrition : they became nerve-cells. They 

 remained connected with the surface by a prolongation which became an 

 afferent or sensory nerve-fibre, and through its termination between the 

 cells of the general surface continued to receive the effects of external 

 impressions ; on the other hand, they continued to transmit these impres- 

 sions to other, more distant cells by their efferent prolongations. In the 

 further course of evolution the nervous system thus laid down became 

 differentiated into distinct afferent, efferent, and intermediary portions. 

 Once established, such a nervous system, however simple, must dominate 

 the organism, since it would furnish a mechanism whereby the individual 

 cells would work together more effectually for the mutual benefit of the 

 whole. 



It is the development of the nervous system, although not proceeding 

 in all classes along exactly the same line, which is the most prominent 



