2 95 
Portions of the British Flora . /. 
which invasion it belongs. In the rarest class there are only two such 
species, but in the whole group, as shown above, there are as many as 53. 
The meeting and commingling of plants following different migratory paths 
then becomes so pronounced that it obscures any particular movement 
which may be applicable to certain assemblages considered separately. The 
distribution of the flora taken as a whole does not apparently follow the 
‘ Age and Area’ law, not because the law fails, but because it is obscured. 
Discussion. 
The data brought together in the preceding pages raise numerous 
questions about the distribution of plants in England, and I have presented 
the facts, I hope, without unduly pressing any particular conclusion, for the 
problem of distribution, even in a small area of the earth’s surface, is 
eminently complex. No attempt can be made to deal with all the questions 
that arise. The study developed with the aid of the ideas underlying 
Willis’s theory of distribution, and in broad outline that hypothesis seems 
applicable to the limited portion of the British flora which is under review. 
To discover invasions has been my chief aim, and while it would be rash to 
conclude that a definite number of migrations distinct in time and place has 
been fully demonstrated, there is nevertheless evidence to show that more 
than one invasion from continental sources has shared in the building up of 
our native flora. As to the time of these invasions there is little to guide 
us, although a consideration of all the evidence would suggest that they are 
comparatively recent. As to place, there seems reason to believe that 
plants have reached our shores generally along the coastal area from the 
Bristol Channel to the Wash. Certain areas of establishment are observable, 
and these show a geographical alliance with continental centres of distribu¬ 
tion. Once established in England further spread seems to be effected from 
the English stations, although the possibility of recurrent introduction from 
the Continent cannot be entirely ruled out. The coastal tendency is a feature 
of the distribution of the smaller groups which have been studied in detail, 
and may be related to the ecological class of the species, and possibly to 
dispersal by birds which possess coasting habits. Hence species need not 
radiate uniformly from their centre of origin in the country. 
But, allowing for many modifying factors, the results which the present 
analysis has produced seem capable of explanation along lines suggested 
by the principle of distribution which Willis has styled ‘Age and Area’. 
An important modification is the exclusion on geological grounds of the 
boreal element in the flora, a procedure which is justified by the terms of 
the law itself. 
Again, there is a small assemblage of species in North Somerset and 
Gloucester, few of which come within the boundaries of the areas I have 
