no DEVELOPMENT AND PURPOSE CHAP. 



only to the point of signalling to us certain changes in 

 the environment. Wherever in any species this signalling 

 apparatus is adequate to the maintenance of the stock at 

 a sufficient rate, no further development is to be expected 

 from natural selection and the laws of inheritance. Hence 

 in the lower orders of the organic kingdom where the rate 

 of reproduction is very high, and it is possible for very 

 large numbers to perish in immaturity without the destruc- 

 tion of the stock, a very low grade of accuracy in response 

 may secure the survival of a sufficient fraction of those 

 born to carry on the life of the species. As the rate of 

 reproduction diminishes, the individual acquires a higher 

 value, and the necessity for judicious action becomes more 

 imperative. Greater powers of perception and inference 

 are evolved, and the fine structures of eye and ear come 

 into being. But these grow up by the increasing speciali- 

 sation of structures that are originally rude, and limitation 

 is written over every page of their history. Thus the ear 

 is only susceptible to the impulse of aerial waves of a 

 certain length and frequency. To other waves differing 

 from these only in quantity it is deaf. The eye begins 

 to respond to transverse waves of a certain length, and 

 there arises in our consciousness the sensation of a dark 

 red. As the wave lengths diminish, the colours change 

 till they reach the violet and then again it is dark. We 

 have no sense organ to respond to electrical waves as such. 

 We can see nothing distinctly that does not subtend a 

 certain definite angle upon the retina, and the optimist who 

 told us that man had not 



the microscopic eye 

 For the plain reason man is not a fly, 



wrote before the days of bacteriology. Could man by 

 direct perception have seen the microbe in the infected 

 substance, the history of medicine would have been very 

 different. In place of this means of combating disease, 

 man has only some indirect and very imperfect perceptual 

 signals the disgust at putrefying substance, the aversion 

 to the spectacle of disease, the fear of infected persons, the 

 early preference for cooked food, the aversion to close 



