Evolution of Sieves 139 



criteria of survival from type to type and from age 

 to age. This idea has been emphasized by Professor 

 James Young Simpson in his striking and coura- 

 geous book, Man and the Attainment of Immortality 

 (1922). He points out that the determining factor 

 may be at one time food, at another time family; in 

 one age the criterion is muscle, in another it is brains. 

 There are, of course, permanent indispensables, like 

 health, which must always be winnowed for, if the 

 creature is to live adventurously — one of Nature's 

 lessons that man is apt to forget; but the emphasis 

 shifts, in time and space and type, from speed to 

 cunning, from armor to weapons and vice versa, from 

 vigor of body to a capacity for self-effacement, from 

 the art of lying low to the quality of swiftness. There 

 is obviously an extraordinary variety in the survival- 

 value qualities which may be the subject of Natural 

 Selection or Nature's sifting; but there is another 

 way of looking at the facts — namely, by inquiring 

 into the evolution of the sieves. 



The oldest of sieves is the quest for food ; the living 

 engine must be stoked if it is to continue working. In 

 the case of radium the clock is persistently running 

 down; the overcrowded atom disintegrates slowly 

 into lead, and there is no winding up in the inorganic 

 world as we know it. But it is characteristic of the 

 vital Proteus that it can feed and recuperate itself 

 for the losses of energy implied in all activity. This 

 is the primordial question that Nature asks of her 

 living children: "Can you feed yourself?" That is 

 the first test — the pursuit of food, often under diffi- 



