EVOLUTION I02I 



but in other cases it acts as an accelerator of progress, and it 

 operates in relation to an established System of Nature which 

 becomes increasingly complex. Our point is that while the varying 

 organism may be in itself progressive, the converse of a radio-active 

 element, the sieves may also work in the same integrative direction. 



Sifting in the Himalayan Jungle. — It is always desirable 

 to pass from general discussion to concrete case; and we venture 

 to do this abruptly here, to remind the student that the issue, 

 always one of life and death, sometimes very literally, involves 

 a mortal fray. We have known of two hundred small birds 

 being killed in a single night in a single stack-yard by an unusually 

 hard frost; but such life-or-death modes of the struggle for 

 existence have in temperate coimtries a very moderate occur- 

 rence compared with what happens in the tropics. In Mr. 

 William Beebe's Pheasant Jungles (Putnam's, 1927) we get some 

 glimpses of the frequently terrible severity of the sifting. He was 

 on the track of pheasants, on which he is the greatest authority, 

 in the jungle of Kinchinjunga, when, without warning, there sud- 

 denly descended a terrific burst of hail. The foliage and moss were 

 torn to shreds as by shot. The ferns were lashed fiat. "In a very 

 short time the pellets of ice were piled up three to five inches, and 

 untold numbers of forest creatures must have perished miserably." 

 Two nests that Mr. Beebe had been watching were beaten from 

 their supports and their contents crushed. "Every blossom was in 

 shreds, not a leaf remained whole, and the forest, from the peace 

 and warmth and life of the fuU flush of spring, took on the death-like 

 aspect of winter. Such storms kill dogs, fowls, geese, cattle, and even 

 men; and the destruction of pheasants and their eggs and young 

 must be enormous." 



It is probably true that such very severe natural elimination is 

 of less importance in evolution than the more moderate sifting that 

 we are familiar with in the southern parts of north temperate 

 countries, for there is very little chance of an organism reacting 

 effectively to a cruel hailstorm such as Mr. Beebe describes with his 

 wonted vividness. Mere reduction of numbers is not of much value 

 as a factor in evolution ; it is discriminate selection that counts. The 

 only real sifting effected by the hailstorm would be between those 

 animals that are quick, and those that are slow to find shelter. In 

 less extreme countries we see the sifting at work, quietly and 

 unsensationally, in every hedgerow and shore-pool. 



SEXUAL SELECTION 



Darwin's theory of Sexual Selection is no longer wholly acceptable 

 in the form in which he stated it, but modern criticism has erred 



i 



