SPECIES DISTRIBUTION ABOUT PLYMOUTH. ol? 
tions benefits any species he tends simply by giving each plant a 
much larger space of ground than it would have had to itself in a 
wild condition. Indeed we know, however rich he may make the 
soil, his seedlings will form but weakly plants if he allow them 
either to grow up thickly together, or to become choked by weeds ; 
the stronger killing the weaker, till, by a process of natural selection, 
or, as Mr. Herbert Spencer would say, “the survival of the fittest,” 
only the most vigorous remain at last to bear seed, though with 
powers weakened by the contest that they have had to carry on. 
When we see how every available inch of ground is appropriated 
by one form of wild vegetation or another, and bear in mind that 
notwithstanding this there are recently-introduced weeds which are 
rapidly becoming common, it is apparent that in some cases these 
intruders can only have obtained their footing through dispossessing 
other species of the ground they now occupy. Buxbaum’s speed- 
well (Veronica Buxbaumit) is now as common in cultivated fields 
in our neighbourhood as either of the two closely allied field 
speedwells, V. agrestis and V. polita. Yet so comparatively re- 
cently as 1841 the Rev. W. S. Hore wrote of it (in vol. 1 of the 
Phytologist”): “@ccurs both in Devon and Cornwall, but not abun- 
dantly. It appears limited to the fields which have been recently 
ploughed, and disappears in a season or two.” Occupying as it 
does just the same spots as V. agrestis and polita, it must since its 
introduction have had much to do in controlling the numbers and 
checking the increase of each; and the contention now going on 
in our fields between these three closely allied speedwells may be 
considered to furnish a striking illustration of the following remarks 
of Darwin: “As the species of the same genus usually have, 
though by no means invariably, much similarity in habits and 
constitution, and always in structure, the struggle will generally be 
more severe between them, if they come into competition with 
each other, than between the species of distinct genera.” * 
The common sallow (Salix cinerea) affords a good example of a 
species possessing a great power of increase through its capability 
of adapting itself to very diversified conditions, and its having its 
numerous seeds furnished with down, fitted for carrying them over 
a wide surface and lodging them in all kinds of situations, so that, 
although it is pre-eminently a shrub of the swampy woodland 
valleys, it helps to form many a hedge-row, be the situation moist 
* Page 59. 
