SEASONAL LIFE 85 



birds there must be some other governing cause at work 

 demanding this change, but this so far has eluded dis- 

 covery. 



The naturalist Allen, to whom we have already referred, 

 showed that in the birds of North America there is increase in 

 size from the southern towards the northern regions, where, in 

 the northern forms, this increase is correlated with an appreci- 

 able decrease in colour. This increase in size from the south 

 northwards appears to be true not only of individuals of the 

 same species, but also, as a rule, the largest species of each 

 genus are to be met with in the north. But there are some 

 exceptional cases, however, in which the increase in size is 

 in the opposite direction. This apparent contradiction Allen 

 suggests may be explained by assuming that the largest forms 

 are found in the area from which the species took its origin — 

 that is, if the species arise in the north then the northern forms 

 will be largest, if in the south then the reverse is the case. 

 The increase in size northward is, however, less common than 

 in the opposite direction. In species whose breeding area 

 covers a wide range of latitude the northern birds are not only 

 smaller, but have a different coloration, as in the Common 

 Quail, Meadow Lark, Purple Crackle, Red-winged Blackbird, 

 Blue Jay, Crow, Titmouse, and numerous species of Finches, 

 Warblers and Thrushes, the variation amounting to as much 

 as from 10 to 15 per cent, of the average size of the species. 



Possibly this decline in size as the species wanders further 

 from its area of origin is due to the fact that the new conditions 

 encountered during the migration grow less favourable as the 

 distance between the ancestral home is increased. That is to 

 say, those which remain in the area of origin have become so 

 completely adapted to their environment that an increased size 

 has become possible. And the huge bulk attained by certain 

 island forms which have become largely freed from the struggle 

 for existence seems to support this view. 



But Allen further contends that in birds the peripheral 

 parts — the beak, claws and tail — also increase towards the 

 south. That in passing from New England to Florida the 

 beak in slender-billed forms becomes larger, longer, more at- 

 tenuated and more decurved ; while in short-beaked forms the 

 southern individuals have larger and thicker beaks, although 



