62 COMMUNITY FUNCTIONS— DYNAMICS OF BIOTIC FORMATION 



birds, mammals, and fishes, and of a few reptiles, amphibians, and 

 insects and other invertebrates. Practically always, the movement 

 is into a new habitat for reproduction and out again. Some degree of 

 change of place frequently accompanies reproductive activities. In 

 some instances the distance concerned is small, but if it is into a dif- 

 ferent habitat or set of conditions, it is significant and quite com- 

 pletely homologous to that of the salmon or of birds which travel long 

 distances to breeding places. 



Birds are the most noteworthy migrants, and the distances traveled 

 by many well-known species are remarkable. With the approach of 

 spring in the northern hemisphere, they leave their winter homes in 

 the grassland of Patagonia, the tropical forest of Venezuela, the 

 swamps of Louisiana, or elsewhere, as the case may be, and pass 

 northward. The Kaibab deer journeys from the pinyon and sage- 

 brush to the montane forest for the birth of the young. The salmon, 

 shad, and other fish go many miles to places suitable for spawning. 

 Moreover, the behavior of the black bass which moves from its feed- 

 ing grounds in aquatic vegetation, to a sandy beach is essentially 

 similar. The rose beetle (Microdactylus) leaves its food plant to 

 deposit eggs among grasses. The force that initiates and directs such 

 movements is probably similar in all cases, and the local migrations 

 are sufficiently simple to permit experimental study. 



There are many local recurrent annual migrations of insects from 

 hibernation quarters to breeding and feeding grounds, and the re- 

 verse, especially between forest edge and grassland, or water and 

 adjacent land. In proportion to size, these flights of small insects are 

 considerable, a few hundred yards for a chinch bug being comparable 

 to miles for a deer. Metamorphic migration, such as the return of 

 the adult to air from water, is typically annual and recurrent, but 

 may be seasonal when two generations occur in one season. Further- 

 more, recurrently migratory species shift their breeding grounds and 

 their winter rookeries with the mass of associated species, but this 

 does not introduce new features into biotic migration. Diurnal recur- 

 rent migration characterizes many insects that move from forest edge 

 to open country, and vice versa, with day and night (Carpenter, 1935) . 

 It is also typical of plankton, which moves down and up in both sea 

 and fresh water in accordance with the alternation of day and night. 



Non-recurrent migrations are perhaps best illustrated by range 

 extension. The Virginia deer has advanced northward several hun- 

 dred miles since the settlement of eastern North America. It has 

 taken the place of the woodland caribou in the forests about the 

 Great Lakes and for some distance northward. The opossum has 



