Letters, Announcements, t^c. 481 



convinced of the truth of the \iews set forth by Messrs. 

 Darwin and Wallace in their communications to the Linnean 

 Society, to which my friend Mr. A. Newton last year directed 

 my attention, ' On the Tendency of Species to form Varie- 

 ties, and on the Perpetuation of Varieties and Species by 

 natural means of selection ^■^. 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 modifi- 

 cations 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 diflFerences. 

 Are these extremes, it may be asked, further removed from 

 each other than the Guinea Negro or the Papuan is from the 

 typical Caucasian ? and are these species aboriginal and indi- 

 genous, or are they developed by climatic and other local 

 causes ? I think the latter alternative almost demonstrable 

 in the case of these birds. These diflFerences of structure 

 (I am using the word here in its widest sense, to include 

 colour, form, and size) doubtless have a very direct bearing 

 on the ease or difficulty with which 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 abso- 

 lutely necessary. Hence, without exception, the upper plu- 

 mage 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 

 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 

 escaping from its natural enemies (which would be the effect 

 "* Journ. Proc. Linn. Soc, Zool. iii. p. 45." 



