Ornithology of Northern Africa. 431 



complexion than the others. These, and such of their offspring 

 as most resembled them, would become more liable to capture 

 by their natural enemies, hawks and carnivorous beasts. The 

 lighter-coloured ones would enjoy more or less immunity from 

 such attacks. Let this state of things continue for a few hun- 

 dred years, and the dark-coloured individuals would be exter- 

 minated, the light-coloured remain and inhabit the land. This 

 process, aided by the above-mentioned tendency of the climate 

 to blanch the coloration still more, would in a few centuries pro- 

 duce the Galerida abyssinica as the typical form. And it must 

 be noted, that between it and the European G. cristata there is 

 no distinction but that of colour. 



But when we turn to Galerida isabellina, G. arenicola, and 

 G. macrorhyncha, we have differences not only of colour but of 

 structure. These differences are most marked in the form of 

 the bill. Now to take the two former first. G. arenicola has 

 a very long bill, G. isabellina a very short one ; the former resorts 

 exclusively to the deep, loose sandy tracts, the latter haunts the 

 hard and rocky districts. It is manifest that a bird whose food 

 has to be sought for in deep sand derives a great advantage from 

 any elongation, however slight, of its bill. The other, who 

 feeds among stones and rocks, requires strength rather than 

 length. We know that even in the type-species, the size of the 

 bill varies in individuals, in the Lark as well as in the Snipe. 

 Now, in the Desert, the shorter-billed varieties would undergo 

 comparative difficulty in finding food where it was not abundant, 

 and consequently would not be in such vigorous condition as 

 their longer-billed relatives. In the breeding-season therefore 

 they would have fewer eggs and a weaker progeny. Often, as 

 we know, a weakly bird will abstain from matrimony altogether. 

 The natural result of these causes would be that in course of time 

 the longer- billed variety would steadily predominate over the 

 shorter, and in a few centuries they would be the sole existing 

 race, their shorter-billed fellows dying out until that race was 

 extinct. The converse will hold good of the stout-billed and 

 weaker-billed varieties in a rocky district. 



Here are only two causes enumerated which might serve to 

 create as it were a new species from an old one, yet they are 

 perfectly natural causes, and such as, I think, must have occurred, 



