152 THE LAW OF BIRTHS AND DEATHS 



extraordinary rapidity when conditions are favourable. 

 They thus form the ultimate source of the food supply of 

 the higher organisms, and the basis of the whole scheme 

 of organic evolution. Sexual genesis is a regulating 

 form of reproduction suitable for the higher organisms. 

 As we glance along the ascending scale of life we see 

 that it gradually supersedes asexual genesis and becomes 

 the exclusive method of reproduction among the higher 

 animals and man. The above diagram can be applied to 

 the higher animals by simply following Nature and obliter- 

 ating the part which deals with asexual reproduction. 



The term " nutrition " is used in a very broad sense. 

 It must not be understood as applying merely to the 

 quantity of food. Among simple organisms the reactions 

 to the environment will be correspondingly simple, and 

 survival-capacity will be decided mainly by a few simple 

 factors, the quantity and quality of the food and the 

 temperature. But as the organism grows more complex 

 in adaptation to an increasingly complex environment, 

 the reactions will obviously become more complex also. 

 And the term " nutritive conditions " in the case of the 

 higher organisms must be taken to embrace all those 

 varied and subtle factors which affect vitality and which 

 are summarised by the nervous system. From the diagram 

 we may perceive that the optimum point for fertility 

 and the optimum point for vitality are very different 

 things, and they may vary from practical identity to 

 extreme divergence in different varieties, or even between 

 different individuals of the same variety. Thus there is 

 opportunity for an enormous range of variation between 

 varieties and individuals even when exposed to the same 

 environment. 



Whilst discussing asexual reproduction we may con- 

 veniently inquire what are the laws which govern ovula- 



