1 91 9-] Geographical Distribution and Migration. 387 



Gradual contraction among non-migratory species Avill 

 eventually produce interrupted distribution, extermination, 

 or isolation. Of the first of these conditions Sitta canadensis 

 occurring in Corsica, China, and America, Cyanopica cyanus 

 in Spain and Eastern Asia, and Pyrrhocorax pyrrhocorax 

 with its reported isolated colony in Abyssinia, afford good 

 examples. 



Isolation will in its turn most assuredly produce differ- 

 entiation. In these three above-quoted cases, there can be 

 little doubt that the isolated colonies emanated from the 

 same parental stock and that they primarily emigrated from 

 the same area. As in Mesopotamia we find derelict remains 

 of ancient civilization, such as the banks of some Babylonian 

 canal, cropping up at sometimes great intervals and only 

 giving us a general clue to a once-huge work, so we find 

 among some species, derelict groups or forms cropping up 

 in widely-separated parts of the world as landmarks of some 

 bygone migration or continuous distribution. 



Such gradual movements as are outlined above, w^hen 

 undertaken by what are commonly believed to be resident 

 species, represent in fact incipient migration or movements 

 from which a strong migratory habit has since developed in 

 other species. 



2. Periodic and regular migration. 

 We see periodic and regular migration effecting changes in 

 breeding-area in certain species of Palaearctic birds. We find 

 the Bee-eater {Merops apiaster) taking advantage of South 

 African conditions and establishing breeding-colonies there 

 (Stark and Sclater, 'Fauna of South Africa, Birds,' iii. p. 59). 

 That this species breeds regularly in Algeria and Egypt is 

 beyond question, and it seems possible that it also breeds 

 in the northern Sahara (Novit. Zool. xviii. 1911, p. 524, 

 XX. 1913, p. 60). It is not then surprising to find them 

 nesting in South Africa, where conditions are more favour- 

 able than in North Africa. But it is not inferred that this 

 bird breeds twice a year, once in its normal summer haunts 

 and agraiu in its winter haunts. It is more likelv that the 



