SPECIES DISTRIBUTION ABOUT PLYMOUTH. 315 
much to influence the range of both animals and plants, even if we 
put aside all the direct and intentional works of his carried on in 
each direction. Every interference of his with the natural order 
of things produces effects that extend far beyond his intentions, 
nay, that are sometimes opposed to his desires. The cereals he 
takes with him from place to place always bring with them a host 
of weeds of a certain kind, whilst his sheep carry in their wool 
the seeds of other strangers. Several species follow his footsteps, 
seemingly only because the soil acquires a special fitness for sup- 
porting them after he has come into contact with it. It is easy to 
see how from plants this influence of his must extend to animals 
and insects, and do much to control their range and regulate their 
numbers. He cannot bring a few yards of common into cultiva- 
tion, or fell a small grove of trees, but the work must favour some 
animals and vegetables, and prove as much to the loss of others, 
owing to the complexity of the relations between species in the 
world of nature, to the interdependence and warfare existing 
between so many. [ven a footpath across a field will often induce 
a peculiar vegetation on each side: and here we find such species 
as the swine’s cress (Coronopus Ruelli) and the common knot- 
grass (Polygonum aviculare), plants having wiry stems that lie 
comparatively flat on the surface, so consequently they are well 
fitted to resist the pressure of a passing footstep, though not formed 
to compete successfully with the grasses constituting the matted 
sward of the contiguous old pasture land. 
In numerous cases the entire dependence of certain species on 
others for power to exist is clear. Many animals could not live 
were those weaker ones on which they prey destroyed ; and we 
know that the parasitic insect that infests a certain bird owes to it 
its nutriment, its necessary amount of heat, and its hiding-place. 
On turning to the vegetable kingdom, we, by saying the mistletoe, 
the dodder, the broomrape, and many other plants, are parasitic, 
acknowledge likewise their dependence on other species for their 
support. When we see too the minute vegetable productions com- 
prehended under the words mildew and rust clustering on or 
investing larger and higher organisms, it is clear they-would find 
no place for development were these latter destroyed. 
There are plants whose distribution appears so extremely arbitrary 
that the botanist can only conjecture why this grows here and that 
grows there. We, in our ignorance, on seeing a plant growing in a 
