CHAPTER 6 

 MIGRATION 



Probably a clearer understanding of migration may be attained if 

 shorter mo\'ements are discussed first. Some movements of animals 

 tend to lead them into new territory, as already indicated in Chap- 

 ter 2. Diurnal-nocturnal migrations are often important; insects and 

 some vertebrates migrate from forest to grassland at night, returning 

 to the forest for the day (Carpenter, 1935), There are also foraging 

 cruises, as in those of the wolf group already mentioned (Olson, 1938, a) . 

 Migration proper is concerned most often with a change of position 

 having to do with adverse seasons, reproduction, or some obscure 

 cause as migratory birds. When the organism does not revisit the 

 starting point, its movements may be termed emigration. When a re- 

 turn journey is made by different individuals or at irregular times the 

 migration may be called return migration or remigration. 



Emigration is usually said to be a consequence of population pres- 

 sure in relation to the food supply, but evidence for this conclusion 

 is very often wanting and in some instances evidently incorrect as in 

 the emigration of certain grasshoppers. Heape distinguishes emigra- 

 tion proper from diffusion, dispersal, and nomadism, though the dis- 

 tinctions do not always hold, especially with what he calls "drift 

 emigration." It is again most frequent in ungulates, but occurs also 

 in rodents and carnivores. A conspicuous example is afforded by the 

 lemming of Europe. This is the most discussed if not the most dra- 

 matic of known mammalian emigrations, particularly in relation to 

 cycles, as indicated in the preceding chapter (Collett, 1895; Elton, 

 1924; Heape, 1931). 



With respect to adverse seasons, regular migrations are most com- 

 monly made with reference to altitude or latitude. The best-known 

 examples are among animals with powers of flight, such as birds, bats, 

 and insects, and among the ungulates, but are bj^ no means confined 

 to these groups. Latitudinal migrations arc so well known for birds 

 as to require no comment ; even the flight routes are fairly well known 

 (Lincoln, 1935). Mammalian migrations are well illustrated by the 



200 



