. L'écologie animale. L'écologie animale. FIG. 10-9 l'eau tertiaire des lacunes (l'éclosion) entre Nprth et l'Amérique du Sud (Mayr, 1946). et 27 confiné à l'Amérique du Nord ; les deux continents n'avait pas plus d'une ou deux familles en commun. Au cours du Pléistocène, après le pont terrestre avait été en existence pendant un certain temps, 22 familles ont été représenté sur les deux continents : 7 étaient dispersés du Sud au Nord, l'Amérique du Nord au 14 Amérique du Sud, l'un est d'origine incertaine. Certaines familles ont devenir éteintes. La présente la faune des deux conti- composantes contiennent 14 familles en commun, 15 familles présentes uniquement dans

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. Animal Ecology. Animal ecology. 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 tempo

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