194 THE MIGRATION OF BIRDS 



three months' night ? Would that be no bar to the 

 wintering of so many birds in the Arctic regions ? But 

 are we sure that there was three months night ? May it 

 not be possible that the obliquity of the earth's axis to 

 its orbit was much less in those ages, and that this was 

 one cause of the comparative mildness of the winters, 

 whilst the summers were for the same reason cool enough 

 for the growth of conifers? An increase in the obliquity 

 of the earth's axis would possibly account for the glacial 

 period which destroyed the mammoths and compelled the 

 birds to migrate. 



It is alleged that many birds leave their winter 

 quarters because in southern climates the heat dries up 

 everything, and lessens the production of insect life. 

 Many of our European birds winter on both shores of 

 the Mediterranean. In wandering through the valleys 

 of Asia Minor, or the mountain-gorges of the Parnassus, 

 or on the islands in the lagoon of Missolonghi, in May 

 and June, I noticed no absence of insect life. On the 

 contrary, insect life appeared to be superabundant. 

 Vegetable life appeared to be threatened by innumerable 

 grasshoppers. One of the prominent features of the 

 district was the countless thousands of beetles which 

 swarmed on every plant ; and, if we may judge from the 

 number and size of their webs, the countries I have 

 named must be a perfect paradise for spiders. That 

 these districts are suitable for the maintenance of insecti- 

 vorous birds during summer is proved by the fact that 

 soon after the Phylloscopi which have wintered there 

 have left, their place is filled with birds belonging to the 

 allied genus Hypolais, so that the olive-trees are well 

 supplied with insect-eating birds both winter and summer. 

 One species of swallow winters in Greece, one is found 

 there all the year round, and three other species are 



