Means of Dispersal. 2 1 



and Lemming inhabited Salisbury Plain, and the Arctic 

 Birch and Bearberry grew in the lowlands of South Devon. 

 The Temperate flora has returned again ; but the fact that 

 the whole, or nearly the whole, of our plants have been 

 compelled at least twice, probably many more times, to 

 migrate long distances, shows that the British flora as it 

 now exists must be a flora highly specialised for dispersal. 

 In this respect it is probably more specialised than any 

 tropical flora, which has been developed in an unvarying 

 climate, but under a struggle for existence more violent to 

 the individual. 



We should expect to find, therefore, that the British 

 flora consists of a selection of the more mobile plants of 

 Europe, without the accompanying sedentary forms. As 

 the best illustration of what is meant, we may take the 

 proportions of plants with minute seeds and of plants with 

 large seeds to the total number, in orders represented both 

 in the flora of Britain and in that of Europe ; the numbers 

 not including plants that have seeds, either large or small, 

 modified in special ways for dispersal over long distances. 

 The approximate percentages are as follows : — 



The composites, which at first sight appear to form an 

 order particularly adapted for dispersal, constitute, how- 

 ever, a much smaller proportion of the British than of the 

 continental plants. This, I believe, is due to the general 

 deficiency in our flora of prairie vegetation — the majority 

 of the composites are prairie species, and until the last 

 thousand years Britain, while possessing a temper- 

 ate climate, was mainly woodland, so that there 



