Means of Dispersat. 21 
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 :— 
Percentage Percentage 
in Britain. in Europe. 
Large seeds ie wa 24°5 31°3 
Small seeds dy ier 17°6 12°4 
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 
