BIRDS AND EVOLUTION 383 



simply means physiologically that the amount of ferment 

 produced in the feather-germ is insufficient to carry on the 

 oxidation of tyrosin beyond the yellow, or at farthest, the 

 orange state. It may be that inbreeding has accentuated a 

 slight constitutional defect, but it seems more likely that the 

 off-colour character is purely modificational, i.e. impressed 

 on each individual, generation after generation. 



§ 8. Directive Factors : Processes of Selection 



Professor Davenport tells us the story of chickens which 

 he had in a field near the Cold Spring Harbour Institute for 

 Experimental Evolution. Of the 300 there were 240 

 white or black and conspicuous, and 60 spotted and incon- 

 spicuous. In a short time 24 were killed by crows, but only 

 one of these was spotted. This is a simple and clear instance 

 of the operation of discriminate selection. There was 

 survival value in being spotted. In the course of time, had 

 the field been Nature, the race of fowls would have con- 

 tained only spotted and inconspicuous forms. The evolution 

 of the species would be directed by selection. 



Professor Bumpus (1898) relates an interesting observa- 

 tion on the house-sparrow in the United States. After a 

 severe storm 136 were picked up and brought into the 

 laboratory, where 72 revived and 64 died. The survivors 

 and the eliminated were then carefully measured as to length, 

 size of wing, weight, length of head, length of humerus, 

 femur, and tibio-tarsus, width of head, and length of breast- 

 bone. For all but the last of these characters the range of 

 variation was considerably greater in those that succumbed. 

 The extreme variants, e.g. those with longest as well as 

 shortest wing-span, were eliminated. The survivors were a 

 little shorter, lighter, longer in the leg, the humerus, and the 

 breastbone. But the essential characteristic of the survivors 

 was general stability of structure. Now severe storms are of 

 frequent occurrence, and if they do not overpass a certain 

 limit (when all the birds will perish and the elimination will 



