238 INITIATIVE IN EVOLUTION 



plants, some to the trees alone, some to the trees and land, some 

 to the land by night and trees by day, and some for ever and wholly 

 to the land — is it probable that any process of selection of suited 

 structures with countless ages of trial and error, could have deter- 

 mined these changes of habit and habitat ? At least one may 

 claim that the balance of probabilities is heavily against that view, 

 and that the forging of reflex -arcs, with all it means to the career 

 of an individual, affords a more intelligible hypothesis, and that 

 this is strongly supported by modern discoveries and doctrines 

 arising from the work of physiologists, as will appear later. 



The Place of the Nervous System in Evolution. 



The constitution of the nervous system is conditioned by 

 conduction, its fundamental and primary function. Its processes 

 consist in the transmission of impulses from receptive fields to 

 effective reactions through devious paths in a region which, even 

 to-day, is a jungle, with many further secrets for physiology to 

 reveal. From this point of view the nervous system may be looked 

 at as a clearing-house and storehouse of impulses on their way in, 

 on their way through, and on their way out. If so, the making of new 

 reflex-arcs is a process which has gone on simultaneously with the 

 formation of receptors in the skin, the higher sense-organs and such 

 deep structures as muscles, and that of effectors of infinite variety 

 — and these are called conveniently adaptations. When we hear 

 from Professor Sherrington that the afferent fibres with their 

 private paths which enter the spinal cord outnumber three times 

 those which leave it, and that those of the cranial nerves should 

 be added, so that the afferent fibres may be reckoned as five times 

 more numerous than the efferent, we get a vivid idea of the funda- 

 mental importance of the formation and compounding of reflex- 

 arcs into systems. Without that the most sensitive receptors and 

 the widest range of structures and organs, small and great, would 

 be as nothing and things of naught. 



A neurone is the anatomical, as the reflex-arc is the functional 

 unit of a central nervous system. Just as it is profitless to consider 

 apart the engines and body of a motor car, as working machine, 

 so is it to picture neurones and reflex-arcs separately in the living 

 nervous system except for the purpose of an ideal construction. 

 In common with the organs and structures of higher animals they 

 have to pass, as historical structures, through the stages of initiation, 

 repetition of rudimentary function, and selection by trial and error, 

 till the " canalizing force of habit " issues in rudimentary and 

 increasingly efficient effectors. It is in this final stage where the 



