February 1, 1895. 



KNOWLEDGE. 



27 



which will not be soon filled up. But it is not only by his 

 coUeagiies, and in the scientific world that his loss will be 

 keenly felt. His kindly disposition, his many acts of 

 unostentatious benevolence, and his readiness to take 

 trouble for the good of others, endeared him to a wider 

 circle, which, but for his habits of assiduous labour, would 

 have been wider still. 



THE SMALLEST FLYING SQUIRREL. 



By E. Lydekker, B.A.Cantab., F.R.S. 



IT is a somewhat remarkable fact in natural history 

 that while all the true flying mammals, that is to say 

 bats, belong to a single ordinal group, and, for all 

 we know to the contrary, may have been derived 

 from one original ancestral stock, this is very far 

 from being the case with those creatures coming under the 

 popular designation of flying squirrels. As we have had 

 occasion to point out in a previous article published in this 

 journal, bats have their fore-legs and fingers specially 

 modified for the support of their peculiar leathery wings, 

 and are able to fly in the same manner as birds ; whereas 

 all the flying squirrels have no such modification of the 

 skeleton, and are merely enabled to take long flying leaps 

 from tree to tree by a parachute -like expansion of the skin 

 of the sides of the body, which is supported between the 

 fore and hind limbs of each side, and in some cases extends 

 1 etween the latter to embrace the root of the tail. 



In popular natural history the term " flying squirrel " is 

 taken to include certain mammals endowed with this kind 

 of spurious flight inhabiting North America, Asia, Africa, 

 and Australia. So like, indeed, in general external appear- 

 ance are all these creatures, that it is not surprising to find 

 them all confounded under one general title. When, 

 however, they are examined with the critical eye of an 

 anatomist, they are foimd to arrange themselves in three 

 main groups, two of which are much more nearly allied to 

 one another than is the third to either of them.* This 

 third group, which is confined to Australia and some of 

 the neighbouring islands, has really no right to the name 

 of flying squirrels at all, seeing that it belongs to the order 

 of pouched or marsupial mammals, and has teeth of a 

 totally difl'erent type to those characterizing all the rodent 

 order, of which the flying squirrels properly so called are 

 members. Accordingly, since these Australian creatures 

 are allied to the ordinary phalangers of the same region, 

 they are more properly spoken of as flying phalangers ; and 

 by this name we shall allude to them in the course of the 

 present article. 



Since these flying phalangers belong to the marsupial 

 order, while the flying squirrels are rodents, it will of 

 com'se be perfectly evident that they have no sort of 

 genetic connection with one another, and hence that 

 their flying membranes have been developed quite 

 independently. If this were all it would be a very 

 remarkable instance of that " parallelism in development " 

 to which we have alluded in previous articles, f seeing how 

 strikingly similar in external appearance are the members 

 of the two groups. The marvel does not, however, by any 

 means rest here ; since it has been shown fairly conclusively 

 that the three genera into which the flying phalangers are 

 divided by zoologists have been evolved independently of 

 one another from as many non-volant forms. For instance, 

 the great flying phalanger /'PetaMroiftes^, measuring upwards 



* We purposely omit mention of the ilying lemur (Galeopithuus), 

 which may perhaps also at times be called a Hying squirrel. 

 t Republished in " Life and Rock," by the present writer. 



of twenty inches to the root of the tail, is so closely allied 

 to the climbing, crescent- toothed phalanger [PseiulocJiims) 

 that there can be no reasonable doubt of its having originated 

 from that genus. On the other hand, the smaller squirrel 

 flying phalanger and its allies of the genus Petaurm are 

 equally closely related to another non-volant genus known 

 as Gymnobelideus. Finally, the tiny little creature known as 

 the pigmy flying phalanger (Acrolmtes), which is not so 

 large as a good-sized mouse, resembles the little pen-tailed 

 phalanger (Dintwcurus') of New Guinea in having the hairs 

 of the tail arranged in rows on the two sides after the 

 manner of the vanes on a feather ; and it may accordingly 

 be inferred that the flying type has been evolved from the 

 one which can only climb. 



We have here, therefore, not only the parallelism of the 

 parachute of the flying phalangers to that of the flyutg 

 squirrels, but likewise three independent instances of 

 parallelism in development among the flying phalangers 

 themselves. We must further call the reader's especial 

 attention to the great difference in bodily size between the 

 great and the pigmy flying phalanger, since this difference 

 is precisely paralleled among the African flying squirrels. 



Leaving the phalangers, we pass on to the flying squirrels 

 preparatory to the consideration of the species forming the 

 special subject of the present article. As we have said, all 

 the flying squirrels in the zoological (but not in the popular 

 sense of the term) belong to the rodent order, the distinctive 

 characteristics of which have been pointed out in an article 

 recently published in this journal, under the title of " The 

 Home of the Rodents." It must not, however, be supposed 

 that all flying squirrels belong to a single genus, or even 

 to a single family. As a matter of fact, they may be 

 assigned to two distinct famihes. What we may call the 

 northern or typical flying squirrels range over a part of 

 Europe, Asia, and North America, and belong to the great 

 family of Sciwidce, which likewise includes ordinary 

 squirrels as well as marmots, chipmunks, or ground-squirrels, 

 prairie-marmots, etc. In all these forms the parachute is 

 supported by a rod of cartilage projecting like a yard-arm 

 from the outer side of the wrist, and there is another 

 expansion of skin connecting the fore limbs with the neck, 

 while there may be a third between the hind-legs and the 

 root of the tail. The whole of these flying squirrels are 

 characterized by the complex structure of their molar teeth ; 

 and as their skulls differ considerably from those of other 

 members of the family, they must be regarded as consti- 

 tuting a sub-family group by themselves. Flying squirrels 

 of this group, as we learn from paleontology, have existed 

 since a comparatively early epoch in the Tertiary period, 

 and we are consequently unable to affiliate them with any 

 of the genera of ordinary squirrels ; it is, indeed, quite 

 likely that they have originated from a totally extinct 

 genus or genera. Hence, it is impossible to say whether 

 the three genera into which they are divided have all 

 taken origin from one non-volant form, or whether, as in 

 the case of the flying phalangers, the power of flight has 

 been separately evolved in each of the three generic 

 groups. 



Of the three genera in question, the one known as 

 SciuroiJti'nis includes the lesser flying squirrels, all of 

 which have the crowns of their molar teeth comparatively 

 low, and the parachute of moderate width, and not inclu- 

 ding any portion of the tail. Having one representative 

 in North America, and a second in north-eastern Europe 

 and Siberia, the lesser flying squu-rels are mainly charac- 

 teristic of India and the Malayan coimtries. While some 

 of the larger kinds measure as much as twelve inches 

 from the nose to the root of the tail, in the pigmy flying 

 squirrel of Cochin China and Arakan the length of the 



