Portions of the British Flora. /. 
28 5 
Table I. 
Occupyi)ig not more than 
I. 
Seven 
vice-counties 
2 . 
Fourteen 
yy 
Twenty-one 
9 y 
4 - 
Twenty-eight 
9 9 
5 * 
Thirty-five 
yy 
6 . 
Forty-two 
yy 
/• 
Forty-nine 
y 9 
8 . 
Fifty-six 
yy 
9 - 
Sixty-three 
y y 
10. 
Seventy-one 
yy 
Number of species. 
129 1 
55 \ 
184 
21 } 
18 ( 
39 
11 
21 
101 
11 ) 
14 
3 \ 
n 
8 
The figures form a decreasing series like those obtained by Willis for 
endemic species, but since they relate to ‘ wides’ the principle of £ Age and 
Area ’ suggests recent arrival, not recent origin, for the great majority of the 
plants concerned. This is only a broad conclusion, and results which per¬ 
tain to the mass may not apply to the individual. This is one of the chief 
objections brought against Willis’s hypothesis, and it is often forgotten that 
the theory is not intended to be applied to single cases and then to draw 
comparisons. Yet, if it is applicable at all, it is necessary to offer some 
explanation of those cases which do not conform to the general law. In 
the present series, the figures which refer to ten degrees of rarity do not 
produce a very smooth curve, although roughly they follow the hollow 
pattern type. But to the perfect working out of the ‘ Age and Area ’ prin¬ 
ciple there seems to be some disturbing factor. To discover these disturbing 
causes is one of the problems confronting the plant-geographer. Relics 
from past climatic successions, or commingling of more recent assemblages 
arriving from different directions, or some outstanding ecological differentia¬ 
tion of the flora may be suggested. Competition is perhaps the most 
potent factor influencing plant distribution. I have already shown ( 1922 ), 
in connexion with a flora having a high percentage of relics, that the ‘ Age 
and Area’ principle is not particularly obvious until the relic species are 
eliminated. 
But if this English portion of our flora, considered broadly, is at an 
early stage of invasion, greater precision regarding migrations may be 
obtained by submitting the rarest class (nearly 50 per cent, of the total as 
here arranged) to further analysis, since the rarest species will be, in general, 
the latest arrivals, and the area or areas showing concentration may provide 
a clue to the direction from which they came. Accordingly, the 129 mem¬ 
bers of the rarest class have been mapped separately. The result is shown 
in Diagram 2, where the actual number of species is given for each vice¬ 
county. 1 There is a considerable number of outlying species in the north 
1 From my list of data these 129 species occupy, on the average, 3*3 vice-counties. Taking the 
London Catalogue numbers or those of Druce’s List they occupy 3.5 vice-counties. 
