142 THE ZOOLOGIST. 



for although this applies to many of the birds, it does not afford a complete 

 explanation. Many of the birds which migrate to the south in winter are 

 replaced by others from the north which live upon similar food. It may be 

 taken for granted that birds and mammals, like plants, have a minimum and 

 a maximum temperature, below and above which they cannot exist, and an 

 optimum temperature that is most favourable to their existence. The 

 migration of birds would be caused by their efforts to keep in the temperature 

 most suitable to their existence. Considering the great and important 

 effect that the cold temperature of the glacial period had upon the distri- 

 bution of animals and plants, it was no doubt at that time that the 

 migration of birds first began. During the warm Miocene period there 

 would be little or no migration. When the temperature of Europe was 

 lowering during the glacial period, the animals and plants would either be 

 exterminated by the cold, or become modified to resist it, or escape the cold 

 by migrating southwards. Plants, not possessing any means of locomotion, 

 would be killed as the cold became too severe for them, while their spread 

 southwards would be favoured. The mollusca and their allies, whose 

 powers of locomotion are also limited, would also be affected in the same 

 way. It is for this reason that plants and testacea are more valuable to the 

 geologist as indications of the prevailing climate than are birds and 

 mammals. Many of the latter were exterminated by the cold of the glacial 

 period, others were protected by thicker coatings of fur (as we find 

 Rhinocerus with wool and Mammoth with fur surviving in Europe after the 

 glacial period), while others were driven southwards. All this shows that 

 the general result was to drive the living things southwards, but birds 

 differed from all other animals generally by returning to the north in spring, 

 and the question is, how did this habit originate ? It began, I think, in 

 this way. After the breeding season, the birds would scatter in all 

 directions, keeping in family groups, or joining with others in flocks, 

 as they do now, returning to their breeding places in spring. When the 

 cold of the glacial period first set in, it would be so gradual that it would 

 hardly be noticed that a change was taking place in the temperature. As 

 the cold increased the birds which wandered southwards would find them- 

 selves in a temperature more suited to them than those who went in any 

 other direction, and their southern range would be gradually extended as 

 the climate became colder, until there would be a considerable distance 

 between their winter haunts and their breeding place, to which they would 

 return every spring. At first birds would find their way backwards and 

 forwards by memory, the homing pigeon shows that by memory they can 

 return from considerable distances over routes they have previously 

 traversed. The birds which went farthest southwards would return more 

 invigorated and in better plumage than those which were more to the 

 north, and would have a considerable advantage over the latter in the 



