384 THE BIOLOGY OF BIRDS 



be indiscriminate), the tendency will always be towards the 

 survival of those of more stable constitution. 



There are many modes and levels of Natural Selection, 

 and Darwin always laid emphasis on its manifold and subtle 

 operation. It is useful to distinguish lethal selection, which 

 works by eliminating the relatively less fit to given conditions 

 of life, from reproductive selection which works through the 

 predominant increase of those that are at once relatively 

 fit and relatively fecund. 



In his shrewd realisation of the subtlety of many selective 

 processes, Darwin saw, for instance, what some of his 

 successors have missed, that the sifting need not involve a 

 sudden cutting off of the relatively less fit, since a shortened 

 life and a less successful family will in the long run bring 

 about the same result as a drastic pruning. 



Darwin also realised, what some of his successors have 

 missed, that even slight peculiarities may be of critical 

 moment when tested in relation to the complex web of life 

 in which the creature has its being. This is of great im- 

 portance in regard to the general progressiveness of evolution 

 — that new departures or variations are sifted in reference to 

 a slowly elaborated and firmly established system of inter- 

 relations. There has been an evolution of sieves as well as 

 of the sifted. And that way progress lies. 



As a modern instance of artificial selection we may take 

 Raymond Pearl's experiments (1915) towards improving the 

 winter production of eggs in fowls. The capacity for laying 

 well in winter seems to behave like a Mendelian character, 

 depending upon two factors or genes, one of which is sex- 

 linked. 



For a time the method pursued was that of selecting out 

 hens with a high egg record, but this did not yield very 

 promising results. What proved effective, however, was 

 selection based on the egg-laying performance of the progeny. 

 The reason for this is plain, that a bird might be itself a high 

 layer without having the quality of high fecundity as an 

 integral part of its hereditary make-up. A sure diagnosis of 

 the hereditary constitution can only be made by means of the 



