FIG. 10-9 Tertiary water gaps (hatching) between Nprth and 

 South America (Mayr 1946). 



and 27 confined to North America ; the two continents 

 had not more than one or two families in common. 

 During the Pleistocene, after the land bridge had been 

 in existence for some time, 22 families were repre- 

 sented on both continents : 7 had dispersed from South 

 to North America, 14 from North to South America, 

 one is of uncertain origin. Some families have be- 

 come extinct. The present faunas of the two conti- 

 nents contain 14 families in common, 15 families 

 found only in South America, and 9 families only in 

 North America. Thus there has been considerable 

 dispersal over the land bridge in both directions, but 

 not a complete exchange or unification of faunas 

 (Simpson 1940). 



The modern fauna of North America is thus de- 

 rived principally from Eurasia and South America, 

 and by autochthonous development. By autochtho- 

 nous we refer to species evolved from very old in- 

 digent types that may or may not be represented by 

 related forms on other continents. The proportion of 

 any local fauna that is derived from one or the other 

 of these sources varies with each geographic area and 

 with each group of animals (Table 10-1 ) . Northward 

 on the continent and in the western mountains, the 

 Eurasian bird element is strongly represented. The 

 South American element becomes greater southward, 

 especially in the lowlands of California, Mexico, and 



Central America. Many Eurasian forms have dis- 

 persed through North America into South America. 

 but no modern forms, at least, of South American 

 origin have been able to invade Asia through North 

 America. 



MIGRATION 



Migration, like dispersal, involves move- 

 ments and the invasion of new areas. Migration, as 

 here defined, dififers from dispersal in that it is a 

 periodic movement back and forth between two areas 

 (but see Urquhart 1958). In contrast, dispersal is a 

 one-way outward movement. Migratory invasions of 

 areas are temporary and repetitive, but invasions re- 

 sulting from dispersal may be permanent. 



Migration is best known in birds, as an invasion 

 of breeding area alternating with an invasion of win- 

 tering area, annually. Representatives of other groups 

 of animals also migrate, particularly mammals, fish, 

 and insects (Heape 1931). Migration may be clas- 

 sified as annual, diurnal, or metamorphic (Clements 

 and Shelford 1939). Annual and diurnal cycles are 

 correlated with the two most pronounced time cycles 

 in the physical environment. Metamorphic migrations 

 are movements from one habitat to another in differ- 

 ent stages of an animal's life cycle. 



Annual migrations 



Annual migrations may involve a change of lati- 

 tude, or altitude, or be more local in extent. 



Latitudinal migrations may traverse only a few 

 miles or may traverse almost from pole to pole. In 

 terms of their occurrence in an area bird species are 

 described as permanent residents, species represented 

 in an area throughout the year even though some in- 

 dividuals migrate: summer residents, species present 

 only during the warmer part of the year, which in- 

 cludes a breeding season that may extend from early 

 spring to late autumn : zvinter visitors, species present 

 only during the winter or non-breeding period ; tran- 

 sients, species ephemerally present only during mi- 



156 Ecological processes and dynamics 



