THE BUILDING OF REFLEX ARCS 239 



triumphs of selection have been won, and where their undeniable 

 value and interest has led some exponents of the distributional 

 laws of genetics to disregard, or accept as data, the early and for- 

 mative stages. Theirs is a mental state which resembles that of 

 Darwin, who, for once in a moment of haste, declared the question 

 of the origin of life to be rubbish. 



In the foregoing consideration of the formation of receptors 

 of the skin it was assumed that certain common stimuli of the 

 environment hammer out for themselves paths in the nerve-fibrils 

 of the skin and by ceaseless repetition lay down not only the receptor, 

 which may be called the terminus a quo, but also the afferent fibres 

 which ultimately find their way into the grey matter of the cord 

 and brain. That this is the initial stage of the construction of the 

 higher nervous system can hardly be denied. But it carries the 

 problem of the synthesis of the organism but a little way unless 

 it be coincident with the construction of new reflex-arcs and their 

 co-ordination into systems. Till this stage be reached in a rudi- 

 mentary form the most cunning and exact adaptations and struc- 

 tures, or, as they may be broadly called effectors, will not advance 

 the efficiency of the organism in the smallest degree. If the receptor 

 be the terminus a quo the effector is the terminus ad quern. This is 

 so obvious that it may be waved aside as a truism not worth the 

 notice of a zoologist concerned with the major problems of biology. 

 It may seem to challenge in a highly speculative region and manner 

 the labours of the biometrician and Mendelian, but, if fairly met 

 it no more encroaches on their territory than do the labours of the 

 engineers who invented the first and crudest chassis of a motor 

 car upon the elaborate and brilliant ingenuity, taste and skill of 

 the coachbuilders who turn out the " body " of a sumptuous Rolls- 

 Royce of 1920. But the latter would never have " arrived " if the 

 former had not made his slow and arduous trials and errors and 

 final success. So here, as in many other subjects, a truism has its 

 use. If the biometrician and Mendelian will only abstain from 

 erecting notice-boards to proclaim " No thoroughfare here," we 

 shall not be put down as trespassers or poachers on their ground 

 and may range at large in certain fields of speculation. 



Some Neural Phenomena. 



Among numerous phenomena of nervous reactions discovered 

 by the research of physiologists certain have a close bearing on the 

 formation of receptors, afferent fibres and reflex-arcs, especially 

 those of Delay, Summation, Fatigue, Block or Resistance, Localiza- 

 tion, Facilitation and Inhibition. 



