RELATIONS AMONG SPECIES 



383 



Recent. Since about ten thousand years ago there 

 has been gradual warming of the continents. How- 

 ever, the peak ol warming was about six thousand 

 years ago. Since that time there has been a period of 

 coohng. Now there appears to be another warming 

 cycle, but as yet it is not as warm as it was six thou- 

 sand years ago. Ignoring the fluctuations, the over-all 

 warming since the last ice caused northward shifting 

 of floras to present conditions, which approximate 

 those of the latter part of the Pliocene (Figure 19.20). 



INTERCONTINENTAL MOVEMENTS 



The previous discussion of the origin of North 

 American floras and mammals allows a simple ex- 

 planation of intercontinental transfer of organisms 

 by a land bridge, specifically an Asian-North Ameri- 

 can interchange across the Bering Sea area, or the 

 Bering land bridge. Even when all discontinuous 

 intercontinental distributions of closely related life 

 are studied, land bridges can still explain the pat- 

 terns of these distributions. Moreover, there would 



Figure 19.20 Present geofloros. Symbols as in Figure 19.16. 



Figure 19.21 The distribution of the major land bridges: I.Panama, 

 2. Bering, 3. Eurasian, 4. Afro-Eurasian, 5. Australasian. 



have been no need for a large number of land bridges. 

 For example, observations of the patterns of discon- 

 tinuous distributions might cause one to assume land 

 bridges between Africa and South America, between 

 South America and Australia, and between North 

 America and Europe. However, none of these bridges 

 seem to be necessary to explain the present. 



If one assumes no more than the five geologically 

 well-documented land bridges (Figure 19.21), plus 

 knowledge of changing climates and floras, he can 

 formulate a simple and plausible hypothesis of routes 

 of dispersal to explain the present distribution of life 

 on earth. The five land bridges are as follows: (1 ) The 

 Panama land bridge, now open but normally a bar- 

 rier during the Tertiary, is a north-south highway 

 between the Americas. (2) The Bering land bridge 

 is the only other American land bridge, but it now is 

 in the rare state of closure, acting as a barrier to 

 many species. (3) The Eurasian land bridge now is a 

 gigantic interconnection between Europe and Asia. 

 (4) The Afro-Eurasian land bridge even today is a fair 

 highway between Africa and Eurasia. (5) The Aus- 

 tralasian land bridge was an effective barrier through- 

 out most of the Tertiary and still is. 



Another plausible means of dispersal, if it is limited 

 to hypotheses of short-distance intercontinental dis- 

 persal, is rafting. This hypothesis uses the known fact 

 that major rivers tend to accumulate large amounts of 

 debris. Upon such debris vegetation often grows, and 

 because of this growth, the rafts tend to hold together 

 when released into the ocean. On occasion such rafts 

 are encountered at sea and their variety of life is sur- 

 prising. For this reason, it is assumed that organisms 

 can be rafted short distances to new land habitats. 

 Perhaps, in very rare instances, rafts can assume huge 



