FIELD STUDY IN ORNITHOLOGY. 471 



.spicuoiTS color to one resembling the line of the snrronndino- objects), 

 wonld give the variety that possessed it a decided advantage over the 

 typical or other forms of the species. - - - To apply the theory to 

 the case of the Sahara. If the Algerian desert were colonized by a 

 few pairs of crested larks — imfti^Ji' aside the ascertained fact of the 

 tendency of an arid, hot climate to bleacdi all dark colors — we know 

 that the probability is that one or two pairs wonld be likely to be of a 

 darker complexion than the others. These and snch of the offspring 

 as most resemlded them wonld become more liable to captnre by their 

 natural enemies — liawk^ and carnivorous beasts. The lighter colored 

 ones would enjoy more or less immunity from such attacks. Let this 

 state of things continue for a few hundred years and the dark-colored 

 individuals would be exterimated, the light-colored remain and inherit 

 the land. This process, aided by the above-mentioned tendency of the 

 climate to bleach the coloration still more, would, in a few centuries, 

 produce the GaJerida aht/.ssiiiica as the typical form ; audit must be 

 noted that between it and the European (J. cristatu there is no distinc- 

 tion but that of color. 



"But when we turn to Galerida isaheUlna, 0. arenicola, and G. mac- 

 rorhyncha, we have differences lu^t oidy of color, but of structure. 

 These differences are most marked in the form of the bill. Now, to 

 take the two former first, G. areniroJa has a very long bill, G. isabdlina 

 a very short one. The former resorts exclusively to the deep, loose, 

 sandy tracts; the latter haunts the hard and rocky districts. It is man- 

 ifest 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 comi)arative difficulty 

 in finding food where it was not abundant, and consequently Avould not 

 be m such vigorous condition as their longer-billed relation. In the 

 breeding season, therefore, they would have fewer eggs and a weaker 

 progeny. Often, as we know, a weakly bird will abstain from mat- 

 rimony 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 Mould be the sole exist- 

 ing race, their shorter-billed fellows dying out until that race is extinct. 

 The converse will still 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 ])erfectly nat- 

 ural causes, and snch as I think nuTSt have occurred and are possibly 

 occurring still. We know so very little of the causes which, in the major- 

 ity of cases, make species rare or common that there may be hundreds 

 of others at work, some even more powerful than these, wliich go to per- 

 petuate and eliminate certain forms ' according to natural means of 

 selection.'" 



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 infiuences which operate to check 

 this tendency. It is often assumed, where we find closely allied species 

 apparently interbreeding at the center of their area, that the blend- 

 ing of forms is caused by the two races commingling. Judging from 



