160 THE LAW OF BIRTHS AND DEATHS 



that the birthrate continued to increase until the workers 

 learned to protect themselves by combined action, and 

 factory laws were passed to improve their conditions of 

 employment. A similar phenomenon appeared in Japan 

 recently under similar conditions, though the increased 

 prosperity caused by the war appears now to be producing 

 a declining birthrate. Any general principle, no matter 

 how advantageous to a species as a general rule and on 

 the whole, may act with disastrous effect under some 

 unfortunate conjunction of circumstances. 



If we glance along the ascending scale of life from uni- 

 cellular organisms up to man, we note a gradual and con- 

 tinuous modification in the relative importance of the 

 principles which regulate fertility. Among the lowest 

 types asexual reproduction plays the chief part, enabling 

 them to increase with enormous rapidity when circum- 

 stances are favourable, thus forming the ultimate source 

 of the food supply of the higher organisms and the basis 

 of the whole evolutionary scheme. Among the lower 

 types of sexually reproducing animals, although the 

 operation of the principle can be clearly traced, instinct 

 plays the greatest part. It is instinctive reactions to 

 external circumstances which will determine the number 

 of batches of young to be reared annually. Reactions 

 between each species and its enemies will also assist the 

 mutual adjustment of numbers. In the case of the higher 

 animals instinct plays a smaller part and the principle 

 under discussion a much larger part. It is in human 

 society that the principle reaches its full development, 

 where instinct has ceased to constitute a serious regulating 

 factor, and where reason has not yet supplied its place. 



It has been suggested to me that if the fall in the 

 birthrate is the result of a natural law which regulates 



