294 THE GROWTH OF THE BRAIN. 



remembered that nutrition has been left out of account. 

 In any given case, the abundance of the blood supply 

 to the encephalon can be expressed only in a vague 

 way, as an inference from the size of the heart and blood 

 vessels. Taking such facts into consideration, it becomes 

 intelligible that there should be within us a vast number 

 of nutritive variations, and that between the states of 

 "well " and " ill," which are clearly appreciated, there is 

 a long series of undefined conditions. Many of these 

 changes, dependent both on the influences from without 

 and ' within the organism, are subject to rhythmic 

 variations. 



Of the external stimuli, those of light, sound, and 

 temperature undergo a rhythmic change in intensity 

 corresponding to the alternation of day and night, and 

 such changes are still more clearly marked in the nutri- 

 tive variations to which the central system is subjected. 

 These physiological rhythms are habits of organic 

 activity which have been found advantageous, and 

 which in man are highly developed. As habits, they 

 are open to modification, but owing to their fundamental 

 character they probably change only with great slowness. 



All the lesser cycles of the individual are encompassed 

 by the one great period of growth, beginning with the 

 fertilisation of the ovum and ending with somatic disso- 

 lution. Inevitable and rooted in the constitution of 

 the organism, this places the termini between which 

 the other changes fall. Yet there is, perhaps, a cycle 

 beyond this if, taking man collectively, we attempt a 

 wider view, and put the species in the place of the 

 individual. Thus it is not to be forgotten that to 

 anticipate a termination of our species on the earth 

 is but looking forward, as we look backward to its 

 earliest appearance, and within the limits of this 

 greatest period must lie the record of humanity. 



