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 radiates in all 

 directions in which conditions are favorable and until 

 insurmountable barriers are encountered. Lines of 

 dispersal may be readily distinguished where one or 

 more ta.xonomic characters can be traced from primi- 

 tive or generalized types through more and more 

 specialized types the greater the distance involved. 

 However, once a species filters through a break in a 

 barrier and invades an extensive, new, and favorable 

 habitat, there may be increased evolution of new types, 

 and a secondary dispersal center formed. This has 

 happened repeatedly in the geological past, so there is 

 often difficulty in distinguishing which center is the 

 original one for a group. 



154 Ecological processes and dynamics 



