ECOLOGICAL 77 



MIGRATION— IN OTHER ANIMALS.— While it is among birds 

 that migration is seen in its most typical form, it is also a custom 

 with many other animals. But there has been an unfortunate 

 tendency to apply the term in a loose way to many movements of 

 animals that are very different from the migrations of birds. Migra- 

 tion in the strict sense, defined in reference to its most typical 

 expression in birds, is a racial custom, enregistered in the animal's 

 constitution, and takes the form of a periodic or seasonal mass- 

 movement between a breeding-place and some other environment in 

 which breeding does not occur. In other words, migration implies 

 that the animals concerned have two haunts in which they regularly 

 live at different seasons of the year, or at different phases of their 

 life. It is illustrated among seals, turtles, toads, by such fishes as 

 salmon, and by such crustaceans as land-crabs. It is a pity that such 

 a clear and convenient term should be blurred by an application 

 to phenomena which it does not fit. The following should be 

 excluded from the rubric of migration, (a) Roammg movements in 

 search of food, whether at short intervals or at different seasons, 

 are not migratory, (b) Mass-movements, of fishes, for instance, that 

 are not related to reproduction, but are instigated by marked 

 changes in the physical conditions, or in the dependent distribution 

 of the food, should not be mixed up with migrations, (c) Even 

 impressive trekking, coerced by increase of numbers and by exhaus- 

 tion or destruction of the food-supply, is not migratory. The march 

 of the Scandinavian lemmings, when they have exhausted the 

 vegetation of a district, is no more a migration than is a devastating 

 swarming of locusts, (d) Also to be excluded are the movements 

 of larval animals from their birthplace to another more suitable 

 environment, as when the larvae of shore animals become pelagic 

 and return to littoral waters as their metamorphosis is or is not 

 being completed. It would be just as unprofitable to apply the term 

 migration to the movement of the May-flies out of the water, or the 

 movement of the liver-fluke's cercariae out of their water-snail host. 

 The word has been similarly misapphed to the striking march of 

 Procession Caterpillars from the pine-trees along the ground — a 

 march that continues until they find suitable soft soil into which 

 to burrow for pupation, (e) Also to be separated off are the various 

 forms of extension of geographical range, which may be very 

 impressive as in the case of the incursions or "invasions" of Sand- 

 Grouse into Britain, or may be, as is more frequent, very gradual 

 and hardly perceptible from generation to generation. The passive 

 diffusion of gossamer-spiders by the wind, or of marine animals by 

 oceanic currents, is certainly not migration. Yet the term is per- 

 sistently applied to the often striking mass-movements of butter- 

 flies and some other insects, which are usually, if not invariably, 

 dispersal-movements. Even Elton calls attention to the way in 



