64 



KNOWLEDGE 



[March, 1902. 



lasts most of the day ; but they arc also noted as fomm;:; 

 iu small parties, flock after floclc, for several hours in 

 succession, and imaccompanicd by any other kind of birds.' 



In the last week of Au;,'ust there is a decided southward 

 movemeut of the swallows in Scotland and the North of 

 England, while in Ireland there is a movemeut to the 

 coast in Au<j;ust. There is no evidence that these birds 

 actually quit the country, and they jn-obably wait some 

 time iu the southern counties before crossing the Channel. 

 In Se]>tember the southern movement becomes general 

 throughout the country, and in the early days of that 

 mouth the swallows begin to cross the Channel. The 

 emigration reaches its maximum near the end of Sep- 

 tember, and by the middle of October it is practically over 

 as far as Scotland and the north of England are concerned. 

 But the swallows continue to leave the south if England 

 until the middle of November, while stragglers are often 

 seen later, especially in the south-west. A December 

 swallow, however, is a very rare occurrence. 



The swallows depart iu much the same way as they 

 come, excej>t that they are, of course, more numerous, and 

 they leave in perhaps larger bands. As far as it has been 

 observed, the " emigration usually takes the form of the 

 continuous passage of small parties, not exceeding a score, 

 and as this may last for hours, vast numbers thus depart." 

 Swallows have, however, been observed " on the south 

 coast to assemble in thousands and fly away en ma8sc,hnt 

 this is only occasionally recorded." It seems that in some 

 cases old and young birds migrate separately, and iu others 

 that they are proportionately mixed. The time of day 

 at which they migrate is also varied. Swallows cross the 

 English Channel by many routes, although there are 

 certain much-used points of departure. 



Little need be said about the swallows which visit us on 

 their passages to and fro from the countries to the north 

 and south of us. In the spring this migration sets in at 

 the end of April, reaches its maximum about the middle 

 of May, and may be prolonged until nearly the middle of 

 June. Many of the earlier of these migrants reach our 

 coasts iu company with our own swallows. The migration 

 is almost entirely confined to the east coast, and the North 

 Sea is crossed before the northern limit of the mainland 

 is reached. A few swallows may reach Northern Europe 

 from the west of Scotland. After spending the summer in 

 Scandinavia these swallows begin to return with their 

 young along our east coasts on their way to Africa, iu the 

 middle of Sej^tember. The passage is well maintained 

 during the rest of the month, and is jirolonged by a few 

 birds to the first or second week in October. On their 

 arrival on our shores these birds mix with our own 

 swallows, and trace is lost of them, but it is more than 

 likely that our milder climate induces many of them to 

 linger, and if this is really the case, the lateness of migra- 

 tion on the east coast, as compared to that on the west 

 coast, would be accounted for. 



It will be seen from the foregoing brief summary, that 

 Mr. Clarke has now jjut us into possession of a very com- 

 plete history of the migrations of the skylark and the 

 swallow, as observed in the British Islands. Beyond 

 their intrinsic value and their great interest, such clear 

 statements of facts will go far to solve the many im- 

 portant problems connected with the migrations of birds. 



LIKE AND YET UNLIKE: THE FLYING- 

 SQUIRRELS OF ASIA AND AFRICA. 



By R. Lydekker. 



Despite the repetition of the statement as to their 

 essential structural difEerence iu almost every work on 



popular natural history issued to the public, few persons, 

 save those who have made anatomy a special study, can 

 be induced to believe tliat swallows and swifts are not 

 closely allied birds. And it may be presumed that an 

 e(jual degree of incredulity will prevail in the minds of 

 most i)eople when they are told that the two animals whose 

 portraits are given in the ]>late accomjninying this article 

 have no sort of intimate relationship, being in fact much 

 more widely sundered from one auother than are such 

 apparently dissimilar creatures as a squirrel and a beaver. 

 An instance of this incredulity has indeed been actually 

 published with regard to the animal shown on the right 

 side of the plate, which is one of the so-called African 

 flying-squirrels, or, as they might be better termed, scale- 

 tailed squirrels. Now this particular species of the group 

 was sent home from Central Africa by Emin Pasha in the 

 eighties, and described and figured under the name of 

 Anomalurue pusillus by Mr. Thomas, of the British Museum, 

 in 1887 and 1888. Three years later his figure (the one 

 here reproduced) appeared in Major Oasati's " Ten Years 

 in Equatoria," with the following remarks : — 



"The flying squirrel (Mboma) lives iu the forests, almost 

 always upon the branches of the trees, whence it throws 

 itself, expanding the membrane which joins the feet to the 

 body, like a parachute. The skin is used as an ornament. 

 I think it is identical with one very common in the Island 

 of Ceylon, which is almost tame." 



The extraordinary misconception as to the affinities of 

 the creature displayed in the last sentence of this quota- 

 tion will be apparent when we say that the scale-tailed 

 squirrels — whether furnished with a flying-membrane or 

 not — are absolutely restricted to Equatoi'ial Africa, where 

 not a single representative of the true flying-squirrels of 

 Asia and Europe exists. 



The reason why these two very dissimilar groups of 

 animals are regarded in popular estimation as closely allied 

 is, of course, due to the fact that both are furnished with 

 expansions of skin by means of which they are enabled 

 to take flying leaps from bough to bough. Such flying- 

 membranes are developed in very few mammals, and the 

 popular idea is that the presence of such a membrane 

 must necessarily imply intimate affinity between all the 

 forms in which it occurs. Hence not only are the African 

 flying scale-tailed squirrels associated with the typical 

 flyiug-sciuirrels, but the stUl more widely-separated flying- 

 phalaugers of Australasia are likewise regarded as members 

 of the same grouj). 



In making such associations the public fail to recognise 

 that similar structures may be produced iu totally different 

 groups of animals owing to their living under similar 

 special conditions, or having peculiar habits of the same 

 nature. In external appearance rodents belonging to 

 different families, such as squirrels aud dormice, may be 

 very much alike ; and if cei'tain members of each grouj) 

 had acc|uired the same mode of Ufe as the flying-squirrels, 

 their similarity would probably have become still more 

 noticeable. For unless the whole skeleton of the fore- 

 limbs be so modified as to form a wing, as iu bats, it is 

 difficult to see how ordinary mammals could be endowed 

 with the power of taking flying-leaps save by the develop- 

 ment of an expanse of skiu along the sides of the body in 

 the manner which obtains in the true flying-squirrels, the 

 scale-tailed flying-squirrels, the flying-phalangers, aud, it 

 may be added, the flying-lemurs. 



The development of flying-membranes in all these four 

 groups of mammals has, iu fact, taken place quite inde- 

 pendently, and affords an interesting example of what is 

 knowu as parallelism iu development. Such parallelisms 

 are due, so to speak, to the poverty of possibilities iu the 

 way of modification of auimal structures. As akeady 



