336 GEOGRAPHICAL DISTRIBUTION 
known that wherever there are deserts they are inhabited by what 
is called a “ Desert Fauna” consisting of animals belonging to many 
Classes—Vertebratesand 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 Palearctic 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 (Geogr. 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 which inhabit the 
country bordering 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 Bini. Again, it may be greater stiil, and instead 
of regarding the animal asa 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 Avifauna of Polynesia by the oceanic birds which 
sweep over its waters or even lodge upon its coral-reefs ; but by the 
birds which inhabit its islands. Now oases are to deserts what 
islands are to oceans, and it is therefore by the dwellers in the 
oases that the characteristic fauna of a tract! which includes desert 
must be judged, while that of the wastes which 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 
1 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 (bis, 1859, pp. 429-483)—too long to be here quoted— 
on their bearing on the then half-revealed Darwinian hypothesis of Natural 
Selection. 
