SUMMARY AND CONCLUSION 303 



an equatorial base, and ranged far south, not necessarily 

 over continuous land masses south of Africa, although 

 the probability is that such may have been the case. 



We have precisely the same phenomenon among birds 

 as among plants — Northern and Southern groups which 

 are strictly confined either to the Northern or the 

 Southern Hemisphere — north or south of the Equator, 

 which must have started from an equatorial range base 

 and spread north or south towards either Pole. Both 

 Professor Huxley and Professor Parker have described 

 the dominance of these Northern and Southern groups. 

 Of course many groups may attain their highest develop- 

 ment at remote and varying distances from this dividing 

 line, or be common to both sides, or even become 

 temperate or polar with little or no trace remaining of 

 their equatorial origin from remote ancestral forms. 

 As may be readily surmised, this new Law of Dispersal 

 demands a continuous land connection equatorially. 

 But this need not have been a synchronous one. Indeed 

 the distribution of some groups of birds absolutely 

 demonstrates that such was not the case. The elevation 

 necessary to restore Antarctica — say 2,500 fathoms — 

 would also be sufficient to connect the great land masses 

 of the globe equatorially, and to a very great extent in 

 the North Pacific and North Atlantic oceans. But 

 such an elevation again need not have been synchronous ; 

 indeed the probabilities are that the Northern Hemi- 

 sphere continents were united at a much more recent 

 date than those of the Southern Hemisphere. 



If this Law of Dispersal be true, it will explain the 

 absence of southern types from the Northern Hemi- 

 sphere, what Dr. Wallace aptly describes as " the singular 



