CHAPTER XXIV. 



THE BUILDING OF REFLEX ARCS. 



Assuming the foregoing origin of the innervation of the skin, I 

 submit that between this rudimentary process and the building 

 of sensori-motor arcs in the spinal cord and brain there is a field, 

 almost unlimited, for initiative in the construction of new forms 

 of animal life. The former is nothing without the latter. To leave 

 it without proceeding further is to leave it " in the air " as military 

 writers sajf. The formation of Receptors, then, both in the skin 

 field and in the higher sense-organs, leads of necessity to the forma- 

 tion, multiplication and co-ordination of reflex arcs. As in an 

 imperfectly organised telephone service after many a repeated 

 stimuli or " rings " the messages begin to reach their destinations, 

 and as by practice the operators better and better learn their 

 business, so the impulses passing through receptors and nerve- 

 fibrils become organized into more or less efficient systems of arcs, 

 and response is secured to them by some effector of gland or muscle. 

 It is not true of man alone that practice makes perfect. 



A certain feature of higher animals which distinguishes them 

 from lower must be remembered, and that is that among them the 

 individual becomes increasingly important. Speaking generally, 

 the latter are born and die in large groups, and their lives resemble 

 those of their group more closely than in the former. The struggle 

 of the individual is vividly pictured by Professor Woods Jones in 

 his description of the baby of the perfected arboreal animals. He 

 shows how they and the roaming Ungulates and Pelagic Cetacea 

 cannot indulge in large families, and that it is only those forms which 

 have a safe retreat for their young which can avoid reduction of the 

 size of their families, and how the higher apes still more resemble 

 in these respects mankind, as we know it. For the proper study 

 of the " synthesis of the individual " organism this essential fact 

 must be kept in mind. 



Some Illustrations. 



It will be expected of course that for the claim here advanced 

 on behalf of the predominant influence of the nervous system in the 

 initiative of the evolutionary process some experimental or other 



