86 A HISTORY OF BIRDS 



the birds themselves are smaller. This increase, however, is 

 not so much due surely to climatic conditions as to adaptation 

 to feeding and other inevitable changes in the environment. 

 In supporting this view we may cite an instance of this kind 

 given by Mr. J. S. Whitaker. Writing in his Birds of Tunisia, 

 he says of one of the crested Larks {Galerida aistata macror- 

 hyncha) it " shows considerable variation in its colouring, ac- 

 cording to the particular district it inhabits. To a certain 

 extent it varies in other ways as well, notably in the size and 

 shape of its bill, the modification of which is presumably 

 brought about by the nature of the soil in which the bird finds 

 its subsistence, a soft sandy soil calling for a long, fine-pointed 

 bill, while a hard gravel or rocky soil requires a shorter and 

 blunter bill." He further points out that the small-beaked 

 group of crested Larks shows a greater variability generally 

 than the large-beaked group. And this he says "is no doubt 

 due to the fact of the former having a wider and more extended 

 range in the Regency (Tunisia) than the latter. The small- 

 billed crested Larks are to be found universally throughout the 

 country, and frequent the hills and high plateaux, as well as the 

 lower ground, whereas the larger-billed birds are confined to 

 certain districts, and are, as a rule, only to be met with in the 

 plains." 



The foregoing facts seem to leave but little room for doubt 

 but that birds are directly affected by climatic conditions, 

 that they react to the stimuli of temperature and moisture, for 

 example, when long continued. Moreover, it seems clear that 

 similar stimuli produce similar results in widely different 

 species, when, be it noted, these are sedentary species, though 

 all are not affected to the same degree. 



But the climatic conditions here referred to are such as are 

 uniform and long-sustained. Climatic conditions, however, are 

 nowhere absolutely stable. Periodic or seasonal changes and 

 sporadic, meteorological joeriods of inclemency occur every- 

 where, and these two factors exercise a very important part 

 on avian life the world over. They play an important part in 

 the geographical distribution of species ; and they further serve 

 to eliminate all but the most perfectly developed individuals, 

 all but those which are exceptionally well adjusted to their en- 

 vironment, or those which can contrive to tide over times of 



