May 2, 1898.] 



KNOWLEDGE. 



103 



But if other evidence were requisite, there is another 

 reason which would alone suffice to compel us to regard 

 deserts as areas of great antiquity. The habitable parts of 

 all deserts — and it is diilicult for the inexperienced to 

 realize what barren tracts will suffice for the maintenance 

 of animal life — are the dwelling places of many animals 

 whose colour has become specially modified to the needs 

 of their environment. And it will be quite obvious that 

 such modifications of colour, especially when they occur in 

 animals belonging to many widely sundered groups, cannot 

 have taken place suddenly, but must have been due to 

 very gradual changes as the particular species adapted 

 itself more and more completely to a desert existence. 



To obtain an idea of the type of coloration character- 

 istic of the smaller desert animals, the reader cannot do 

 better than pay a visit to the Natural History Museum, 

 where, in the Central Hall, he will find the lower part of a 

 case devoted to the display of a group from the Egyptian 

 desert, mounted, so far as possible, according to their 

 natural surroundings. He may also turn with advantage 

 to the coloured plate of desert finches and larka facing 

 page 380 of the third volume of the " Eoyal Natural 

 History." 



Among such animals may be mentioned the beautiful 

 little rodents respectively known as jerboas and gerbils, 

 together with various birds, such as sand grouse, the cream- 

 coloured courser, the desert lark, desert finches, and desert 

 chat, and also various small snakes and lizards, among 

 the latter being the common skink. Although some of 

 the birds retain the black wing-quills of their allies, in all 

 these creatures the general tone of coloration is extremely 

 pale : browns, fawns, russets, olives, greys, with more or 

 less of black and pink, being the predominant tones ; and 

 how admirably these harmonize with the inanimate sur- 

 roundings one glance at the case in the Museum is sufficient 

 to demonstrate. Very significant among these are the 

 desert finches {Eri/throspi:a), which belong to the brightly 

 coloured group of rose-finches ; one of these specially 

 modified species ranging from the Canaries through the 

 Sahara and Egypt to the Punjab, while the second is an 

 inhabitant of the Mongolian desert. 



Among larger animals a considerable number of the 

 gazelles are desert dwellers, these including the palest- 

 coloured members of the group ; and lions are likewise to 

 a great extent inhabitants of deserts — as, indeed, is true of 

 tawny and pale-coloured animals in general. 



All the animals above mentioned belong, however, to 

 widely spread groups, which are common to the desert 

 tracts of both Africa and Asia, and they do not, therefore, 

 serve to prove the antiquity of any particular desert, as they 

 or their ancestors might have migrated, and probably did 

 migrate, from one desert to another. Birds of such groups 

 are, of course, even more untrustworthy than mammals, 

 owing to their power of flight. And among those referred 

 to, some, such as the sand grouse, can scarcely claim to 

 be regarded as exclusively desert birds, since they are 

 partial to any open sandy plains, like those of the Punjab, 

 or even Norfolk. 



The case is, however, very dififerent with certain of the 

 larger mammals, a notable instance being afforded by the 

 antelopes allied to the South African gemsbok (Oryx). 

 All the members of this group are inhabitants of more or 

 less sandy open districts, and none range eastwards of 

 Arabia, or possibly Bushire. The gemsbok itself, together 

 with the beisa of Eastern and North-Eastern Africa, are 

 inhabitants of districts which do not, for the most part, 

 come under the designation of typical deserts. And we 

 accordingly find that they are by no means very pale 

 coloured animals, while both are remarkable for the bold 



bands of sable ornamenting their faces and limbs. On the 

 borders of the Sahara there occurs, however, a very 

 different member of the group— the white oryx (O. leucnnj.r) 

 —differing from all the others by its curving horns, and 

 likewise by the extreme pallor of its coloration, which is 

 mostly dirty white, with pale chestnut on the neck and 

 undcr-parts. Obviously, this species has been specially 

 modified as reganls coloration for the exigencies of a purely 

 desert existence, and as it is also structurally very different 

 from all its existing kindred, it must clearly be looked upon 

 as a very ancient type, which commenced its adaptation to 

 the surroundings of the Sahara ages and ages ago. The 

 Arabian desert is the home of another species of oryx 

 {(>. Iieatri.r), which, although more nearly allied to the 

 East African beisa, is a much smaller and a much paler 

 coloured creature. In this case also there would seem 

 little doubt that the period when this animal first took to a 

 purely desert existence must have been extremely remote. 



But an even more striking instance is afforded by 

 another antelope remotely connected with the gemsbok, 

 which is an inhabitant of the Sahara and the Arabian 

 desert, and is commonly known as the addax. It is an 

 isolated creature, with no near relation in the wide world, 

 easily to be recognized by its dirty white colour, shaggy 

 mane, and long twisted horns. It must have branched off 

 at a very remote epoch from the gemsbok stock, and 

 affords almost conclusive evidence of the antiquity of the 

 deserts it inhabits, since we have no evidence of the 

 occurrence of allied extinct species in other countries. 



Some degree of caution is, however, necessary in drawing 

 conclusions that ail isolated desert animals have been 

 evolved in the precise districts they now inhabit. A case 

 in point is afforded by the saiga, a pale-coloured antelope 

 without any very near kindred, inhabiting the steppes of 

 Eastern Russia and certain parts of Siberia, where it is 

 accompanied by the hopping Kirghiz jerboa (Alactaga). 

 Now, since fossilized remains of both these very peculiar 

 animals have been discovered in the superficial deposits of 

 the south-eastern counties of England, it is a fair inference 

 that physical conditions similar to those of the steppes 

 (which, by the way, are by no means true deserts) 

 obtained in that part of our own country at an earlier 

 epoch of its history. From their comparatively isolated 

 position in the zoological system, as well as from their 

 occurrence in the strata referred to, both these desert animals 

 evidently indicate very ancient types ; and they accordingly 

 serve to show not only that the semi-desert steppe area 

 formerly had a much greater western extension than at 

 present, but probably also that the existing portion of that 

 area dates from a very remote epoch. Hence they confirm 

 the idea of the early origin of the present deserts of the 

 Old World and their inhabitants. 



It will be gathered from the foregoing that the deserts 

 and steppes of Africa and Asia possess a large number of 

 animals belonging either to speci(-s which have no very 

 near living relatives, or to altogether peculiar genera. In 

 the Arizona desert of the Souoran area of North America 

 it seems, however, to be the case that its fauna is largely 

 composed of animals much more nearly related to those 

 inhabiting the prairie or forest lands of the adjacent 

 districts, of which, in many cases at any rate, they con- 

 stitute mere local races distinguished by their paler and 

 more sandy type of coloration. This is well exemplified 

 by the mule deer, which in the Rocky Mountains is a 

 comparatively dark and richly coloured animal, but be- 

 comes markedly paler on the confines of the Arizona desert, 

 assuming again a more rich coloration when it reaches the 

 humid extremity of the Californian peninsula. Most of 

 the North American mammals, indeed, acquire similar 



