AutiusT 1, 1893.] 



KNOWLEDGE 



157 



Perhaps the most interesting indication of an old land 



surface is that sujJsested by the migration of the orange- 

 legged hobby {Falni diiutrt'iisis). Breeding in Eastern 

 Siberia, Mongolia, and North China, it migrates to winter 

 quarters in India and South-East Africa. Its suggested 

 route is by the Maldive Islands, the Chagos Archipelago, 

 and the Saya de Malha Banks, and in this passage it is 

 supposed to be following an ancestral habit acquired when 

 India was connected with Africa, and the migration could 

 be performed by land. 



Such an Indo-African continent has also been suggested 

 by the geologist from the study of the rocks of the two 

 areas, and by the zoologist from a study of their faunas. 



In the Indian rocks, forming what is knowTi as the 

 Gondwana system, an assemblage of organic remains are 

 met with, presenting such a remarkable similarity to those 

 of South Africa and Australia that Dr. Blandford has been 

 led to advocate the existence of an expanse of land con- 

 necting India with these continents during the Gondwana 

 period. And naturalists studying the present faunas, and 

 finding a striking similarity between that of India and that 

 of Madagascar and the adjacent islands, likewise require an 

 Indo-African continent to explain their facts. The name 

 Lemuria has been proposed for this continent, since it is 

 supposed to have been the original home of the family 

 LemuridiP, now di\'ided by the Indian Ocean into the two 

 separate regions of Madagascar and South Africa on the 

 one hand, and South Asia on the other. It is to be 

 remarked, however, that the migration of the orange-legged 

 hobby, and the distribution of the Lemurid», require 

 a much more recent land connection than the ancient 

 Gondwana continent of Dr. Blandford. 



Coast migration, again, has suggested the existence of 

 former coast lines. Thus the knot, the bar-tailed godwit, 

 and the grey plover migrate up the east coast of England 

 as far north as Spurn Point, and then strike across the sea 

 eastwards. They are supposed to be following an old 

 coast line once reaching from Spurn Point to Denmark or 

 Holland ; and geologists have certainly given us a restored 

 Britain, in which our eastern sea-board was united to the 

 Continent. Yet the northern coast of this uniting land 

 was far away to the north of Spurn Point. Still, if we 

 suppose the submergence of the land was gradual, and 

 from the south northwards, there may have been a time 

 when the coast, line, after trending north to Spurn Head, 

 turned eastward to the Continent. And if this phase in 

 the physical geography of the land was a comparatively 

 permanent one, the peculiar line of migration followed by 

 the grey plover might have been learned. It seems a more 

 reasonable supposition, however, that birds which had 

 learned to follow a coast line would continue to follow a 

 ijretduaUy alterini/ coast line, rather than take to the sea. 



It is interesting to note that birds, in migrating over 

 continental areas, make lai'ge use of valleys and water- 

 courses, and less frequently of mountain ranges and passes. 

 And just as many of the species inhabiting warmer 

 regions retire north to breed, so others ascend the moun- 

 tains for the same purpose. Thus in India, for example, 

 birds retire to the hill stations for coolness, just as do the 

 human inhabitants. 



Like other writers on the subject, Mr. Dixon attributes the 

 origin of migration to lack of food and unsuitableness of 

 temperature ; and he considers that these causes have been 

 largely brought into operation by the climatal changes of the 

 glacial period. Birds, he considers, have only been subject 

 to one glacial period, for he rejects the endence for any of 

 them between that known as the Great Ice Age and Permian 

 times. Migration, as we see it now, commenced with the 

 glacial epoch, Mr. Dixon illustrates the initiation of 



migration by taking the spotted fly-catcher as an example. 

 During a mild period it was a resident species in one un- 

 broken area from the Arctic woodlands to the Pacific. As 

 the cold period came on. the spotted fly-catcher was driven 

 further and further south. Then they began to leave their 

 northern haunts in autumn, and gradually, as the cold 

 increased, their journey was lengtliened. As glaciation 

 ceased, they began to move north agam, and also to journey 

 north in spring to breed, returning south in winter, as they 

 had learned during glaciation. Migration, then, consisted 

 at first of very small journeys, gradually increasing as the 

 ages rolled on. 



In relation to the ornithological axiom, that " the birds 

 which go the furthest north to breed go the furthest south 

 to winter," Mr. Dixon expresses the belief that the habit 

 indicates an ancient migration from pole to pole. Many 

 birds which breed in tlie Arctic regions extend their 

 winter migration far south of the Equator, as the sauder- 

 ling, which reaches the Malay Archipelago, Cape Colony, 

 and Patagonia ; and the curlew sandpiper, which flies as far 

 south as Australia for the winter ; and quite possibly 

 some species may even yet perform the journey from pole 

 to pole. Thus Mr. Hudson, in "The Naturalist m La 

 Plata," mentions certain species which come from the 

 Arctic regions to winter on the Pampas, and of which 

 species individuals also reach the same area from Antarctic 

 regions. May not some of these be birds which have 

 actually passed from the Arctic to the Antarctic regions ? 

 And Mr. Dixon suggests that these flights for vast 

 distances southwards are due to inherited love of home ; 

 for he believes that during the glaciation of the northern 

 hemisphere the species were transferred from the north 

 polar basin to the more genial south ; and now, when 

 their home and breeding place is in the milder northern 

 regions, they show their hankering after the old home by 

 flying as far south during the northern winter as circum- 

 stances permit. 



Emigration, as distinct from migration, refers to the 

 colonizing movements of birds by which they isermanently 

 leave their old haunts. And Mr. Dixon considers that 

 the emigration of birds has had a vast influence on the 

 origin of species. It leads to isolation of a number of 

 individuals from the main body of the species, and the 

 absence of intercrossing with the latter is supposed to be 

 sufficient to allow of their developing into a new variety, 

 and finally species, under their new and slightly diflerent 

 conditions. 



Besides the great north and south migrations, by which 

 our winter and summer visitors come and go, great east 

 and west migrations in spring and autumn have been 

 established. And, strange to say, the species moving in 

 these latter are to a very large extent such as have always 

 been looked upon as residents. Starlings, larks, sparrows, 

 buntings, and many others leave Britain in immense 

 numbers in autumn, and their place is taken by similar 

 numbers of the same species from the Continent. In the 

 spring the two streams are reversed, the one returning to 

 Britain from the Continent, and the other passing back to 

 the Continent from Britain. Almost all our native birds 

 take part, more or less, in these migrations. 



"With very few exceptions," runs the report of the 

 British Association Committee, " the vast majority of our 

 British birds, such as are generally considered habitual 

 residents — the young invariably, the old intermittingly — 

 leave these islands in the autumn, their place being taken 

 by others, not necessarily of the same species, coming 

 from more northern latitudes, or from districts of Eastern 

 Europe. . . . These immigrants on the approach of spring 

 leave, moving back to the Continent on the same lines, 



