290 Matthews .— The Distribution of certain 
floral diversity employed by Colgan ( 1901 ) is the ‘ratio which the total 
of species not common to both areas bears to the total flora of the two 
areas combined \ This index for Ouse and Thames is o-8co, for Thames 
and Channel 0-764, for Channel and Peninsula 0-730, for Peninsula and 
Ouse 0-901. The greatest difference is between the groups of the Peninsula 
and the Ouse provinces. 
Another striking feature of the results presented in Table II is the pro¬ 
nounced and fairly regular decrease in numbers from the several maxima, 
whether we take the figures as they stand or in proportion to the group to 
which they belong, and it is suggested that we may here be dealing with 
more than one invasion. The presentation of the facts in this form pro¬ 
vides, indeed, some definite criterion by which to examine possible lines of 
invasion, as Willis ( 1920 ) has done for New Zealand. 
Diagram 3. The 37 plants of the Ouse province are widely distributed 
in Central and South Europe. Nearly all occur in Germany and France, so 
the European distribution does not suggest that they should be more pre¬ 
valent in the east of England than in the south. But a migration from the 
east is suggested from the range of these Ouse species within England 
itself, and here ecological factors probably play an important part in deter¬ 
mining distribution. The introduction of seed and the subsequent germina¬ 
tion and establishment of the plant are different things. The former must 
be happening frequently, the latter rarely, for competition in an area already 
clothed with vegetation is great. Selection will follow, and so plants are 
most likely to obtain a foothold in that area where local conditions are most 
suitable. Once established in the country, the chances for inward spread 
will be much greater from the English stations than by means of recurrent 
introductions from the Continent. Diagram 3 illustrates this spread, a pro¬ 
nounced coastal tendency being obvious, for the group possesses a number 
of littoral forms and a large proportion of species which favour the dry, 
sandy, or chalky soils found in some of the south-eastern counties. To this 
may be added the fact that birds as possible carriers of seeds commonly 
migrate along the coasts of a country. Thus, if Norfolk represents the 
centre of distribution in England, dispersal has not been uniform in all 
directions. But, if time is a condition of wide distribution, as it must be, 
then the small number of Ouse species which have reached the Peninsula 
may be explained by the fact that they have lacked the time to spread so 
far. Mathematical precision cannot be expected, and a few outliers are not 
surprising. Only two Ouse species, Lythrum Hyssopifolia , L., and Hypo- 
chaeris maculata , L., are absent from the Thames and Channel and occur 
in the Peninsula area. This may be due, though not necessarily, to separate 
introductions from continental sources, yet, if such occurrences were at all 
frequent, it is unlikely that the regular decrease from the maximum would 
be obtained. 
