TRANSACTIONS OP SECTION K. 577 



again on quite different soil in North Cornwall, so that the serpentine soil has 

 nothing to do with its present distribution. 



Chance introductions of seeds during thousands of years explain the exist- 

 ing peculiarities of geographical distribution in a way that no changes of sea or 

 land or climate will do. Our alpine flora consists largely of survivors from a 

 colder period ; the rest of our flora, on the other hand, is constantly being added 

 to by chance introductions from the nearest continental shore. That is why the 

 Atlantic element, and the eastern element, though not consisting to any great 

 extent of maritime plants, are confined mainly in Britain to areas within a few 

 miles of the coast. Seeds are evidently brought from the Continent and scat- 

 tered broadcast over certain coastal districts, and they grow and spread where 

 soil and climate are suitable. But the Post-Glacial Period has been so short that 

 the process is still incomplete, and the slow spreading inland has only as yet 

 extended a few miles. We can still fix the point or points of introduction. 



The most striking elements in the British flora, except the arctic and alpine 

 species, have a marked coastal distribution. The plants found correspond to 

 those of the land opposite (in which they are often inland, as well as coastal). 

 Thus the Cornish plants and those of S.W. Ireland contain a large Pyrenean 

 element; Norfolk plants correspond to those of the opposite shore of the 

 North Sea; even two or three American plants are found on the coasts facing 

 America. 



All the evidence seems, therefore, to point to a steady change and increase in 

 our flora, due to occasional introductions. These introductions are, I think, 

 now mainly due to birds driven by exceptional gales. But herds of migrating 

 bison, deer, and horse have played their part, especially when the Straits of 

 Dover were much narrower or non-existent. Packs of wolves which hunted the 

 large game, foxes, cats, and especially raptorial birds which waited for and 

 struck down the tired migrants, must also have assisted. Fences and the 

 destruction of wild animals have probably rendered the process far slower than 

 formerly ; but it still goes on, as anyone can see who notes the constant occurrence 

 of seedling oaks miles from the nearest tree. 



If I am right, therefore, there is no such thing as a native plant in Britain. 

 Our flora has been swept away like that of Krakatoa ; but we have arrived 

 at a much later stage of the re-peopling in our islands. It seems to me far more 

 interesting to watch this process of introduction, change, and spreading than to 

 enter into speculations as to what species shall be listed as 'natives,' 'denizens,' 

 or 'colonists.' No such differences exist; it is all a question of degree. 



Britain for several thousand years has been receiving colonists from all 

 sources, and the process still goes on. The oldest element in our flora, the 

 alpine, occurs on nearly all our mountains; for it once occupied the intervening 

 areas, and it does not greatly depend on conditions of soil. The limestone, 

 aquatic, and Lusitanian flora, on the other hand, are more recent introductions ; 

 they can never have occupied continuous areas, and their present distribution is 

 full of singular anomalies. These three elements of our flora are steadily grow- 

 ing in importance, whilst the alpine element is stationary, or tends to die out. 



The Chairman (Professor Weiss) then read a letter from Dr. Alfrkd 

 Russel Wallace, F.R.S., to Mr. Clement Eeid, in which the following 

 passages occur : ' I have read your paper on British plants and the Glacial 

 Period with great interest, mainly because you support my views of the great 

 powers of distribution of plants over the ocean, not only for a few tens of 

 miles, but for many hundreds and even, in rare cfvses, thousands. 1 really 

 wish you would look up and read again my discussion of the Flora of the 

 Azores, in my " Island Life." In this case there is absolutely no doubt that 

 the whole of its plants have been gradually introduced during the latter half 

 of the Tertiary Period over a width of ocean of about a thousand miles by such 

 causes as you mention, while the absence of all those genera whose seed 

 could not have passed by those means, completes the proof. . . . But while, 

 therefore, I quite agree with your argument as to the fact, of the very large 

 number of our species which have been so derived since^the Glacial Period, I 

 cannot accept your view that the whole has been so introduced, for several 

 reasons. It is certain that temperature is only one of many, very many, factors 

 that determine the distribution of species; and it is also certain that at the 

 1911. P P 



