chap. xx. THE MAMMOTH AGE. 247 



Europe, and now by the change in the winter temperature 

 of their original home have come to South Africa as mi- 

 grants, apparently out of season in regard to their breeding 

 habits, to mix amongst birds, many of whom are closely 

 allied to, if not congeneric with, themselves. The cause of 

 migration is want of food, not want of warmth. The feathers 

 of a Siberian jay or a Lapp tit are proof against any cold. 



Admitting that the various species of birds that breed in 

 countless thousands in the Arctic regions were once residents 

 there in the days when the climate was much warmer than 

 it is now, we still find some difficulties to explain. In the 

 first place, the Mammoth age does not appear to have been 

 so very much warmer than the present climate. It is said 

 that the remains of the foliage of conifers, such as now exist 

 in Siberia, have been found in the stomachs of frozen mam- 

 moths. In the second place, the question may be fairly 

 asked : How about the 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, 



