430 Rev. H. B. Tristram on the 



animal contrives to maintain its existence. In the Desert, where 

 neither trees, brushwood, nor even undulation of surface afford 

 the slightest protection from its foes, a modification of colour, 

 which shall be assimilated to that of the surrounding country, 

 is absolutely necessaiy. Hence, without exception, the upper 

 plumage of every bird, whether Lark, Chat, Sylvian, or Sand- 

 grouse, and also the fur of all the small mammals, and the skin 

 of all the Snakes and Lizards, is of one 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 in 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 escaping 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. 

 Now in all creatures, from Man downwards, we find a tendency 

 to transmit individual varieties or peculiarities to the descend- 

 ants. A peculiarity either of colour or form soon becomes here- 

 ditary when there are no counteracting causes, either from 

 change of climate or admixture of other blood. Suppose this 

 transmitted peculiarity to continue for some generations, espe- 

 cially when manifest advantages arise from its possession, and 

 the variety becomes not only a race, with its variations still more 

 strongly imprinted upon it, but it becomes the typical form of 

 that country. If it be objected that we see many varieties which 

 do not become hereditary, we may reply, that these varieties 

 having experienced changes not advantageous to their means of 

 existence, may from that very cause become extinct. Still there 

 are many which continue, as the Pied Raven of the Faroe 

 Islands, and the Tailless Manx Cat. 



To apply the theory to the case of the Sahara. If the Alge- 

 rian Desert were colonized 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 proba- 

 bility is, that one or two pairs would be likely to be of a darker 



