Ornithology of Northern Africa, 429 



To the accompanying sketches of the sterna of the two species 

 I subjoin their comparative measurements : — 



C desertorum. C. salvim. 

 inch. inch. 



Joint of furculum to posterior end of sternum 1'7 . . 1*4 



Depth of keel -45 . . -375 



Length of keel I'l . . "8 



Span of arch of keel "7 . . '6 



The smaller species I met with only in the southern and 

 south-eastern districts, never in the central or western ; but 

 where it occurred, it by no means supplanted the commoner 

 bird. I found, on consulting Captain Loche, that he had ar- 

 rived independently at the same conclusion as myself, that there 

 were two species, i. e. as species are now made. 



Writing with a series of about 100 Larks of various species 

 from the Sahara before me, I cannot help feeling convinced 

 of the truth of the views set forth by Messrs. Darwin and Wal- 

 lace 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 Varieties, and on the Perpetua- 

 tion 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 modifications of 

 coloration and of anatomical structure, deflecting by very gentle 

 gradations from the ordiuai'y type ; but when we take the ex- 

 tremes, presenting most marked differences. 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 indigenous, or are they de- 

 veloped by climatic and other local causes ? I think the latter 

 alternative almost dem'onstrable in the case of these birds. 

 These differences of structure (I am using the word here in its 

 widest sense, to inchide colour, form, and size) doubtless have a 

 very direct bearing on the ease or difficulty with which the 



* Journ. Proe. Linn. Soc. Zool. III. p. 45. 



