Differentiation, migration, and mimicry. 360 



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 Galerita isabelfaia, 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, which 

 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 relations. 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 

 longest-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 still hold good of the stout-billed and weaker- 

 billed varieties in a rocky district. 



11 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, 

 and are possibly occurring still. We know so very little of the 

 causes which, in the majority of cases, make species rare or 

 common, that there may be hundreds of others at work, some 

 even more powerful than these, which go to perpetuate and 

 eliminate certain forms ' according to natural means of se- 

 lection.' " 



It would appear that those species in continental areas are 

 equally liable to variation with those which are isolated in limited 

 areas, yet that there are many counteracting influences which 

 operate to check this tendency. It is often assumed, where we 

 find closely allied species apparently inter-breeding at the centre 



