300 THE MIGRATION OF BRITISH BIRDS 



European and Antarctic plants found in all south 

 temperate lands." Now it is impossible to' understand 

 how any area could become " crowded " with plants (or 

 animals) that are retreating from their normal habitat 

 and entering areas where more or less adverse conditions 

 of existence must prevail. Is it not more philosophical 

 to assume that these mountain bases were the points 

 from which the flora started, and that they are the bases 

 on which the remnants of that flora will be preserved, 

 whilst that portion occupying areas beyond them will 

 perish through an adverse change of climate .'' Botanists 

 (and zoologists) are too apt to overlook the fact that no 

 matter how easily a seed (or in the case of an animal, 

 an emigrant) may be transmitted from one region to 

 another, it is perfectly useless as a colonizing or range 

 extending medium, if the region entered is not adapted 

 to its requirements and successful propagation. Breed- 

 ing conditions must therefore always determine and 

 control range extension. 



We have precisely the same phenomena on the 

 mountains of Asia — equatorial range bases and centres 

 of dispersal of that flora which has spread north and 

 south into the Arctic regions on the one hand, and into 

 Australia, New Zealand, and Antarctica on the other. 

 Just as we found to be the case in birds, the highest 

 ranging species are the most widely dispersed ; in the 

 more temperate ranging types the identity is only of a 

 generic value. This brings us to the consideration of 

 such species as we have already described as " Inter- 

 hemisphere." They are species that have ranged north 

 or south from an equatorial base into the Temperate 

 zones only of either hemisphere. The much greater 



