XII. TYPES OF DISTRIBUTION. 519 



species (and to which alone, some of them) now seem 

 specially adapted ; insular and sub-insular climates dif- 

 fering so decidedly from those experienced on lands of 

 wide continuity. — And the supposition assumes also an 

 absolute distinctness of species, their non-transition, and 

 their original descent from single or dual ancestors ; 

 highly important questions, yet too unsettled to admit of 

 being so dogmatically answered, and so summarily' dis- 

 posed of, for the convenience of an untried hypothesis. 



Moreover, there are various facts of distribution which 

 accord better with the supposition of maritime and even 

 atmospheric transport, than with that of land transport ; 

 allowing a more probable and more extensive influence 

 to the former agencies, than the hypothesis of land- 

 continuity appears willing to leave room for. Under 

 existing knowledge, it would be very tedious and scarcely 

 jiracticable to demonstrate the notion as a fact, that the 

 coast-line flora of Britain is richer than the inland flora 

 on an equal area, after excluding the strictly littoral 

 plants from the former. Leaving out of count the 

 alpines and humanly introduced plants, the most local 

 species appear to occur on or near the coast in more 

 than due proportion to those found inland only. If so, 

 it would seem more reasonable to suppose those species 

 transported to their present sites over the sea or through 

 the air, than to refer their introduction to an overland 

 route, and consequently more distant date. WhUe it 

 may still be true, that a former land- continuity, no 

 longer existent, had largely contributed to the present 

 flora of Britain, before the introduction of some sub- 

 sequently sea-borne, bird-carried, or wind-wafted species, 

 to augment its numbers. 



