304 THE MIGRATION OF BRITISH BIRDS 



want of reciprocity in the migrations of northern and 

 southern types of vegetation." This Flora is a dominant 

 southern one — a flora that has almost entirely spread 

 from a base south of the Equator. One or two extreme 

 northern outliers of this southern flora are to be met 

 with on the Equator, and from such a base a few others 

 have emigrated as far north as California, India, China, 

 and the Philippines ; but these are in every case ex- 

 ceptional, and are species or genera that should properly 

 be excluded from that dominant southern flora. Dr. 

 Wallace attempts to account for the "curious inability " 

 of this southern flora to penetrate into the Northern 

 Hemisphere by the totally different distribution of land 

 in the two hemispheres ; but dispersal, so long as it is 

 normal, will overcome such difficulties, or at least over- 

 come them to such an extent as to show some signs of the 

 intrusion of organisms into adjoining areas, as we have 

 already had abundant testimony. We can explain this 

 absence of southern types from the Northern Hemisphere 

 by one way, and one way alone, namely, that they were 

 developed from a range base south of the Equator, and 

 that the Law of Dispersal inexorably keeps those types 

 to the Southern Hemisphere, and will continue to do so 

 notwithstanding a change to adverse conditions which 

 could have one effect only, their total extermination, 

 just as we have seen to be the case with a once domi- 

 nant northern fauna and flora. If we take the flora of a 

 Northern zone, say between the loth and 40th parallels 

 of North Latitude, and compare it with a Southern zone 

 of equal limits, say between the loth and 40th parallels 

 of South Latitude, we shall find the same inability of a 

 dominant northern flora to establish itself in the south ; 



