368 THE ZOOLOGIST?. 



form varieties, and on the perpetuation of varieties and species 

 by means of natural selection. I then wrote (' Ibis,' 1859, pp. 

 429 — 433) : — "It is hardly possible, I should think, to illustrate 

 this theory better than by the Larks and Chats of North Africa. 

 In all these, in the congeners of the Wheatear, of the Rock Chat, 

 of the Crested Lark, we trace gradual modifications of coloration 

 and of anatomical structure, deflecting by very gentle gradations 

 from the ordinary type, but, when we take the extremes, presenting 



the most marked differences In the desert, where neither 



trees, brushwood, nor even undulations of surface, afford the 

 slightest protection to an animal from its foes, a modification of 

 colour, which shall be assimilated to that of the surrounding 

 country, is absolutely necessary. Hence, without exception, the 

 upper plumage of every bird — whether Lark, Chat, Sylvia, or 

 Sand Grouse — and also the fur of all the smaller mammals, and 

 the skin of all the Snakes and Lizards, is of the uniform isabelline 

 or sand-colour. It is very possible that some further purpose may 

 be served by the prevailing colours, but this appears of itself a 

 sufficient explanation. There are individual varieties of depth of 

 hue among all creatures. In the struggle for life which we know to 

 be going on among all species, a very slight change for the better, 

 such as improved means of escape from its natural enemies (which 

 would be the effect of an alteration from a conspicuous colour to 

 one resembling the hue of the surrounding objects), would 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 colonised by a 

 few pairs of Crested Larks — putting aside the ascertained fact of 

 the tendency of an arid, hot climate to bleach all dark colours — 

 we know that the probability is that one or two pairs would be 

 likely to be of a darker 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 con* 

 tinue for a few hundred years and the dark-coloured individuals 

 would be exterminated, the light-coloured 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 Galerita abyssinica as the typical form ; 





