JJ 



6 GEOGRAPHICAL DISTRIBUTION 



known that wherever there are deserts they are inhaljited by what^ 

 is called a " Desert Fauna " consisting of animals belonging to many 

 Classes — Vertebi^ates and Invertebrates — which are especially adapted 

 to their surroundings ; but hitherto there has been no need to notice 

 this fact in the present work. However, in the Palaearctic Subregion, 

 and especially in its eastern portion, as well as in the Ethiopian 

 Region, next to be treated, deserts are so extensive that some 

 zoologists have been inclined to deem them guides in the matter of 

 Geographical Distribution. For very limited districts there is 

 perhaps no great harm in so doing, though there is always the risk 

 of thereby confounding what botanists have long since seen to be 

 essentially different, namely station and habitat ; but a wholly wrong 

 notion would be conveyed were deserts to be accounted factors in 

 determining the value of geographical areas. These, as Mr. Wallace 

 has laid down (Geogi: Distr. Anim. i. pp. 3 and 4), do not depend on 

 physical features, though physical features may affect them. Further- 

 more, it is observable of Desert Faunas that most of the animals 

 composing them are very nearly related to those Avhich inhabit the 

 country bordeiing on the desert — in some cases the difference 

 between the two is only that of tint and must again be mentioned 

 (Variation), in others it is greater and may extend to stature or 

 the proportional size of various organs, as in Birds in the length and 

 thickness of the Bill. Again, it may be greater still, and instead 

 of regarding the animal as a local race, we have to recognize its 

 specific or generic validity as, among Birds, several Larks, 

 Sparrows, Starlings, and Wheatears, or even as a well-marked 

 form like the COURSERS, a Family like the Sand-Grouse, or an Order 

 such as the Ostrich represents. But it seems clear that the right 

 way to regard these and other inhabiters of the desert is to view 

 them as we do the denizens of the great oceans. We do not 

 determine the Avifaima of Polynesia by the oceanic birds which 

 sweep over its waters or even lodge upon its coral-reefs ; but by the 

 birds Avhich inhabit its islands. Noav oases are to deserts what 

 islands are to oceans, and it is therefore by the dwellei's in the 

 oases that the characteristic fauna of a tract ^ which includes desert 

 must be judged, while that of the wastes Avhich surround them is 

 but the result of local causes. Were it otherwise we should have to 

 recognize a Desert Province (or rather a Region, even) in the Old 

 World, which starting from the mountain range lying to the west- 

 ward of Pekin would stretch in a wide belt over Asia, and crossing 

 Arabia, as well as Africa at its widest part, be terminated only by 



^ Canon Tristram seems to be the first who drew the special attention of 

 ornithologists to Desert Forms, the precise value of which he set forth admirably 

 in his famous remarks [Ibis, 1859, pp. 429-4-33) — too long to be here quoted — 

 on their bearing on the then half- revealed Darwinian hypothesis of Natural 

 Selection. 



