210 THE CONTROL OF LIFE 



life are such that there is a continuous sifting of the 

 novelties that crop up. The variants with advantageous 

 variations tend to form the surviving type ; those with 

 disadvantageous variations tend to be eliminated. The 

 result is what Darwin described as " the preservation 

 during the battle of life of varieties which possess any 

 advantage of structure, constitution, or instinct." 



We have already referred to an interesting calculation, 

 which we owe to Professor Punnett, that if in a popula- 

 tion of 10,000 animals in a district there are 10 with an 

 advantageous novelty that gives them 5 per cent, of a 

 better chance than their neighbours, then, if sifting goes 

 on persistently and consistently, the new departures 

 will represent the species in a hundred generations. The 

 process works on the whole automatically, but it is 

 more than mechanical, since we cannot give a mechanical 

 account of variation and heredity, and since organisms 

 often take a hand in their own evolution — for instance, 

 in selecting or changing their surroundings. 



There is an overlooked aspect of Nature's sifting which 

 is of great interest to man. In the course of ages of 

 evolution living creatures enter into complex relations 

 with one another, they form a web of life, they establish 

 a system of linkages. Thus, it was a great step when 

 flowers and insects got linked up together. There is 

 an external registration of steps of progress, and this 

 forms part of the sieve by which future new departures 

 are sifted. It seems likely that we have here one of the 

 reasons why there has been persistent advance in evolu- 

 tion, why slipping down the rungs of the ladder is 



