. Animal Ecology. Animal ecology. Paleogeography Upper Cretaceous, showing land bridges and epeiric seas (Ross 1951). present tropical fauna and flora was greater than it is at the present time. Another difficulty that southern species would en- counter on the bridge would be the very long days of the summer and the very short ones of the winter. Tropical species are adjusted to fairly equal photo- periods at all seasons of the year. Seasonal differences in photoperiod are due to the inclination of the earth's axis and there is no positive evidence that this inclina- tion has changed during ge
. Animal Ecology. Animal ecology. Paleogeography Upper Cretaceous, showing land bridges and epeiric seas (Ross 1951). present tropical fauna and flora was greater than it is at the present time. Another difficulty that southern species would en- counter on the bridge would be the very long days of the summer and the very short ones of the winter. Tropical species are adjusted to fairly equal photo- periods at all seasons of the year. Seasonal differences in photoperiod are due to the inclination of the earth's axis and there is no positive evidence that this inclina- tion has changed during geological time. Although it appears very likely that the Bering land bridge was an important route of dispersal between Asia and North America, considerably more study is required before we will satisfactorily understand how this was accomplished by various kinds of animals. Centers of origin The tracing of dispersal routes presupposes a starting point where the taxonomic group, whatever its size, first evolved. These starting points are called centers of origin. Various criteria for determining centers of origin have been suggested (Savage 1958), but caution must be exercised in applying them (Cain 1944). Of the many criteria proposed, the following two are especially important, although neither one is infallible: 1. Location of greatest differentiation of THE type or the GREATEST VARIETY OF ENDEMIC RACES, SPECIES, AND GENERA, INCLUDING PRIMITIVE FORMS OR FOSSILS. The older a group is and the longer it has occurred at a particular location, the more chance it has had to radiate into different habi- tats, become isolated, and evolve into new varieties. However, a shift of climate or a drastic change in physiography may render an original locality unhabit- able and the group moves elsewhere. Also, a group losing its vitality may contract its range into some area other than the one in which it originated. 2. Continuity and convergence of lines of DISPERSAL. Dispersal from a center
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