Marcq. 1902.] 



KNOWLEDGE. 



55 



8;iiJ, the siin]ili'St uud lUDst obvious way of L-udowinji; au 

 ordinarv four-limbed luauiiiial with the power of takiu-jf 

 fl_vinj,'-ieai>s i.s by the develoiiuieut of lateral expansions of 

 skin. Similarly, the only easily conceivable method by 

 which a jirimitive short-liiubed and many-toed hoofed 

 uuinunal could bi- eonverted into one cut out for sj)eed, 

 like a hoi'se or a i;azelle, is by reducing the number of the 

 dii,'its and iuereasin-; the leiiLTth of the lower .seL,'meuts of 

 the limbs. Accordingly, we find parallelism in this 

 resjK'ct between the horses and the zebras on the one 

 hand, and the gazelles, autelojx's, and deer on the 

 other. 



But the parallelism is by no means enact in this latter 

 case, as indeed would be naturally expected if tlie lines of 

 evolution were distinct ; and the structure of the lower 

 portion of the limbs of a horse differs essentially from the 

 same part in a ga/.clle. 



Neither is the parallelism exact in the ease of the two 

 groups of tlying-sipiirrels. In the flying-squirrels of 

 Europe and Asia, such as the one deincted on the left side 

 of the plate, the flyiug-membraue, or parachute, is merely 

 a lateral expansion of the ordinary skin of the body, which 

 ext*'nds outwards between the limbs and termiiiatfs at Ihe 

 wrist and aukle. In addition to the two lateral mem- 

 branes, there is a mirrow aud inconspicuous one passing 

 from each cheek along the front of the shoulder to the 

 front of wrist ; aud another, at least iu the larger forms, 

 connecting the twohiud-legs aud involving the base of the 

 tail. 



In general characters the parachute of the scale-tailed 

 tlying-siiiiirrels of Africa conforms to the above type; aud 

 a superficial observer might say that the two were in all 

 respects similar. A closer examination will, however, 

 reveal the fact that the )iarachute in this group is siip- 

 ))orted by a jnocesa of cartilage projecting like a yard-arm 

 from the ellww and extending to the edge of the mem- 

 brane. As this is present in all the flying scale-tails (as 

 we may c-all them for short, especially as they have no 

 right at all to the title of squirrels) and absent in all the 

 true tlyiug- squirrels, it evidently indicates an important 

 difference between the two groups. 



A further important distinction between them is afforded 

 by the presence on the under surface of the basal portion 

 of the tail of a series of overlapping horny scales, from 

 which the African group takes both its popular title 

 of scale-tail, and its scientific name of Anomaliirus. 

 Evidently these scales are intended to aid iu supporting 

 the animals as they climb the boughs or stems of trees, 

 and are thus strictly analogous to the stiff tail-feathers of 

 woodi>eckers. 



Another important difference between the two groups is 

 to be found in the structure of the crowns of their cheek- 

 teeth. In ordinary squirrels the grinding surfaces of these 

 teeth are surmounted by simple tubercles, which in some 

 causes may be elongated into ridges. And a similar type 

 of tooth-structure obtains iu most of the flying-sijuirrels 

 of Europe and Asia, although iu the species shown in the 

 plate the structure has become somewhat more complicated 

 owing to the taller crowns of these teeth. In the scale- 

 tails, on the other hand, a totally different type of 

 tooth-structure obtains, the crowns of the molars being 

 divided by transverse folds of enamel, after a fashion 

 recalling th;it which obtains in certain South American 

 rodents. 



To the anatomist these differences are sufiicient to render 

 it (piite certain that the scale-tailed flying-squirrels are, at 

 most, but very remotely coiniected with their uon-scaled 

 namesakes of the northern hemisphere. The uou-scientific 

 person might, however, say that the "yard-arin" in the 

 parachute and the scales on' the tail are features which 



have been develoj>ed concomitantly with the acquisition of 

 the jiarachute itself in certain species of flying-squirrels, 

 and that, like the difTereuces in the structure of the teeth, 

 are of no particular importance one way or the other in 

 regard to the affinities of the animals in which they 

 occur. 



A few years ago it would have been impossible to pro- 

 duce absolutely decisive evidence as to the futility of such 

 specious arguments. Recently, however, there has been 

 discovered on the west coast of Africa— that home of 

 strange and primitive types of animal life — a rodent 

 looking not unlike a large dormouse, which is really the 

 " grandlathiM- " of all the flying scale-tails. For this 

 creature (known as ZitilcereUa), although without a para- 

 chute, has scales on its tail like Anomaliirus, and teeth 

 of the same type as the latter. Whether it is the actual 

 form from which the flying scale-tails are descended, or 

 whether it is itself a descendant of such ancestral form, 

 may be left an open (piestiou, as it is one of no practical 

 importance. But it may be taken as certain that the 

 flying scale-tails, of which, by the way, there are two 

 distinct generic types (Anomalurus aud Idiurus). are the 

 specialised descendants of a creature closely allied to, if 

 not identical with, Zenlcerella. It may further be affirmed 

 with certainty that the evolution of the flying from the 

 non-flying scale-tails has taken place in Africa. Whether, 

 however, Zenkerella itself is au aboi-iginal African type, 

 or au immigrant into the dark continent from the 

 north, is a question not easy to answer at the present 

 time. 



Although the flying-squirrels of Europe and Asia have 

 been known from time immemorial, their pedigree is not 

 so easy to trace as is that of the scale-tails. Probably 

 they were evolved from uou-flyiug squirrels at au earlier 

 date than that at which Anomaliirus branched off from 

 Zenkerella (or its prototype), as they appear to be repre- 

 sented by teeth in some of the earlier Tertiary deposits of 

 Europe. It is therefore quite probable that even the 

 generic types from which they trace their descent have 

 died out. Nevertheless, it may be considered practically 

 certain that they are descendants more or less nearly allied 

 to the true squirrels of the genus Scmrus. Their pedigree 

 is therefore wholly distinct from that of their reputed 

 cousins the scale-tailed flying-squirrels of Equatorial 

 Africa. 



In appearance the true flying-squirrels, of which there 

 are three distinct generic types, are very similar to 

 ordinary squirrels, as indeed they are iu their habits ; 

 their long flying leaps, during which they half float in the 

 air by the aid of the parachute, being only an extension of 

 the bounds taken by the ordinary red squirrel in its 

 passage from tree to tree. Many of them are even more 

 beautifully coloured than ordinary squirrels. Compared 

 with the latter, flying-squirrels are more strictly nocturnal 

 animals ; and their shrill scream is familiar to all travellers 

 in the wooded districts of the Himalaya as they are 

 attracted by the light of the camp-fire. 



The smallest members of the group are the pigmy 

 flying-squirrels, typified by Sciuropterus volans of Eastern 

 Europe aud Siberia, aud represented iu North America 

 by the closely allied .S'. voliicella. They are pretty little 

 creatures, with soft velvety fur, and enormous staring 

 black eyes. In all the pigmy flying-squirrels the mem- 

 brane connecting the hind-legs aud the base of the tail is 

 absent ; but, in compensation, the tail itself is broad, flat, 

 and laterally expanded, so as to form an eflicient aid in 

 flight. 



The typical and larger flying-squirrels, formerly known 

 as Pteromys but now called Petaurista, are confined to 

 Europe aud Asia, having no transatlantic representative! 



