284 Capt. Lyues on the Ornitholoyy [Ibis, 



But I think that for the fall explanation we must 

 look to future exploration to provide the necessary 

 material. 



Secondly : — The racial distribution of species which 

 presumably owe their origin and existence so far south to the 

 high ground with its more northern climate is remarkable. 

 To explain this phase I have attempted, in Plate XL akb, 

 to show the present relations between altitude and forest- 

 ^irowth in Mauretania. 



Plate XI. a is a map showing the mountain-tracts with 

 special reference to those elevations which, given suitable 

 climate and soil, favour real forest-g)'Owth. Plate XI. h 

 shows (very roughly) the actual tracts of real forest-growth 

 that now exist. 



It is well known that among sedentary forest or woodland 

 species, occurs much of that racial distribution for which no 

 present reason seems to exist : witness, for instance, such 

 near-home instances as the Irish Coal-tit and Jay ; the 

 Nuthatch's absence I'rom the Isle of Wight and abundance 

 in the New Forest ; etc., etc. . . . and being, faute de 

 m'leax^ obliged to confine my remarks to the (-lass perhaps 

 least well suited for the purpose, I have chosen the illustra- 

 tion afforded by the distribution of certain typical resident 

 forest-birds in Mauretania. 



If in the foregoing pages the reader has an impression 

 of vast expanses of forest in Mauratania, this map should 

 disillusion him: the term "well-forested" is always a com- 

 parative one. Even if, here, one may walk in a selected 

 direction for days through forest-growth and tree-horizons, 

 and though both history and recent observation suggest that 

 in bygone centuries the Mauretanian forests were more 

 extensive than now, the idea of comparing them in their 

 past or present to the great subarctic and tropical forests of 

 the world is absurd ; indeed, it is highly improlwble that 

 Mauretania ever possessed forests comparable with those of 

 Central Germany, Flanders, or even our own islands in early 

 Roman days. Perhai)S human agency, in this way, has 



