DISPERSAL AND MIGRATIONS 149 



dividing countries where the coast-Hnes of each 

 are visible from either shore (as in parts of the 

 English Channel, for instance), but such is not 

 the case. Land birds everywhere evince a 

 rooted objection to extend their area by cross- 

 ing expanses of water, and a great many 

 instances could be given where narrow seas 

 which might be crossed in half-an-hour's easy 

 flight have effectually arrested dispersal. But, 

 on the other hand, we know that in not a few 

 cases the range of a species is divided by such 

 waterways. We have, for instance, the narrow 

 seas that divide our islands from the continent 

 of Europe and from each other, the Mediter- 

 ranean, the Japan Sea, many parts of the China 

 Sea, Behring Strait, Bass Strait, and so forth. 

 Now, as we have already stated, land birds 

 do not apparently ever increase their area over 

 waterways — seas and straits — so that the natural 

 inference in such cases of sea-divided areas of 

 dispersal is that the birds must have entered 

 those countries when no seas were barring the 

 way. In not a few cases we know that at 

 some comparatively recent geological epoch dry 

 land actually did take the place of what is now 

 shallow seas. This fact is further confirmed in 

 another way. Wherever two countries are 



