4 DEVELOPMENT AND PURPOSE CHAP. 



The one condition which every successful variation must 

 fulfil is that it should assist its possessors in maintaining 

 their own existence, and in engendering and bringing up 

 young ones after their kind. From this point of view the 

 evolutionist expects to find in every new variation of struc- 

 ture some closer adaptation to the requirements of the 

 living species. But in the animal world, particularly as 

 we ascend the scale, there comes into being one structure 

 which in a sense dominates all the remainder. The central 

 nervous system governs the whole body and therewith 

 determines the use to which every part of it is put. But 

 the growth of the nervous system and the entirety of its 

 functions remains for the biological observer merely the 

 most complex and finely adjusted of all adaptations. It 

 is that structure which by the infinite delicacy of its adjust- 

 ment to the minutest variation of stimulus enables the 

 organism to accommodate itself to a myriad of changes in 

 the outer world, and even to learn from the past and pro- 

 vide for the future. If an object approaches the eyes 

 they blink and so protect themselves. If nevertheless a 

 fragment lodges in the eye a tear rolls down from the 

 lacrimal gland and helps to wash it out. These are very 

 simple instances of reflex adaptation, and they are referred 

 by the biological interpreter to a physical machinery which 

 can in part be traced viz. to a certain plexus of sensory 

 cells, and nerve fibres, ganglion cells, motor nerve fibres 

 and muscles which make up the regular constituent ele- 

 ments of a reflex act. This machinery is part of the 

 hereditary endowment of the individual. It has come to 

 be, according to the evolutionist's interpretation, because 

 those who could not protect their eyes efficiently lost their 

 sight, and left no descendants, because those who had the 

 best eyes which involved the best protecting mechanism 

 prevailed in the struggle. It is in short the product of a 

 series of adaptations to the requirements of the living 

 organism in its given environment. It is, moreover, 

 interpretable as a purely physical process. The details of 

 that process are still in large measure unknown. But 

 there is no reason to doubt that the luminous waves pro- 

 ceeding from an object and impinging on the rods and 



