IntroductiiDi ix. 



referring to i\Ir. Wollaston's admirable remarks on the effects of 

 isolation and exposure to a stormy atmosphere upon the insect world 

 (Variation of Species, p. 70, et seq.), alludes to the efl'ect of the latter 

 conditions as one of the principal causes of the darkening of the 

 plumage of birds. 



With regard to the Ornitholog}' of that part of North-west Africa 

 included in the Western Palasarctic Region and which, roughly 

 speaking, may be said to comprise the greater part of Tunisia, Algeria 

 and Marocco, or that portion lying to the north of the Great Sahara 

 Desert, we are perhaps justified in considering our knowledge fairly 

 well advanced, although we probably still have a good deal to learn 

 concerning it. Regarding, however, the Ornis of the country imme- 

 diately south of the above region, stretching from South Marocco, 

 across the Sahara, to Tripoli and Cyrenaica, we know comparatively 

 little and of some parts absolutely nothing. 



There can be no doubt that the Ornis of the country lying 

 immediately north of the Atlas mountains bears considerable affinity 

 to that of South Europe and the Mediterranean sub-region generally, 

 but, on the other hand, it differs from the latter in possessing many 

 species and forums, which either never occur north of the Mediterranean, 

 or which are only to be met with there accidentally as stragglers. 



Immediately to the south of the Atlas mountains also certain 

 species and forms are to be found, which do not occur north of that 

 range, or are merely met with there accidentally from time to time. 



Although I am not of the opinion held by some ornithologists that 

 there is more difference between the Ornis of the districts north of the 

 Atlas and that immediately to the south of those mountains than 

 between the former and the South European Ornis, it is undoubted 

 that the Atlas range does form a very important natural barrier 

 between the two districts, and still further south in Tunisia there are 

 other, though less important and less clearly defined, natural 

 divisions. 



Confining myself to the Regeuc}', and wishing to give some idea of 

 the varied character of the country, as affecting its Ornis, I may make 

 the following remarks : — 



Tunisia has been subdivided by geographers into three natural 

 divisions or regions, each of these differing from the other two in its 

 climate, hydrography and topography, and consequently in its flora 

 and fauna, viz. : the Northern region comprising that part of the 



