28 i 
Portions of the British Flora . I. 
(1922), dealing with the vegetation of Greenland, regards the present flora as 
post-glacial, and states that of 416 vascular plants recorded, about 60 per 
cent, are ^Duthern types, most of which must have been carried over the sea 
by natural means. 
The extent of former land connexions between Britain and Europe is 
a little doubtful, but even if it were but slight in recent Pleistocene times, 
the distance to be travelled by most of those forms which inhabit only the 
south of England would not be very great. We cannot here enter into the 
question as to how dispersal has been effected in the case of those plants 
held to be indigenous. As Guppy (1893) has pointed out, time has long 
since discounted the various methods, and I would add that the part which 
man has played from earliest times in the introduction of seeds can never 
be truly estimated. We cannot really discriminate between ‘ native’ and 
‘ introduced ’ species if we go far enough back. 
More need not be said to illustrate the divergent'views which are held 
regarding the origin and dispersal of our native flora. Only further 
geological evidence one way or the other will clear the ground for the 
botanist. The problems remain, and to the fundamental question as to the 
effect of glaciation on the plant population at the time it seems impossible 
in the present state of our knowledge to give a definite answer. I should 
not have reverted to these old-standing problems had not plant-distribution 
in all parts of the world recently assumed a new interest and a new 
importance. 
The Present Investigation. 
The promulgation of the hypothesis of ‘ Age and Area ’ by Dr. Willis 
(1915, 1919), which has now been successfully tested in a variety of ways, 
opens up a new standpoint in the study of plant geography. In particular, 
it provides a new angle from which invasions and migrations may be viewed, 
and it is especially from this aspect that the following analysis has been 
made and not from any hope to solve the whole problem of distribution in 
Britain. In order to trace probable lines of invasion and inward spread, 
a cartographic presentation of the facts of distribution seemed desirable, if 
not necessary, and most of the points I wish to bring forward will be offered 
in the form of maps. The first step to tracing the progress of the creation of 
vegetation is to know the proportion in which groups appear in different 
localities , a relation which must be expressed in numbers to be at all tangible} 
We are not here concerned with the origin of new forms, but by ascertaining 
the number of species in different localities we may at least attempt to trace 
the progress of that invasion by which our flora has gradually been 
built up. 
1 A statement of Sir J. D. Hooker. Extracted from a chapter by Dr. Guppy in Willis’s Age 
and Area. Camb. Univ. Press, 1922. 
