h have come from any distant regions; so that, in this instance, the
influence of man has surpassed that of all the other causes which tend
to disseminate plants to remote districts. Of nearly 1600 British
flowering plants, it is supposed that about 300 species are naturalized;
but a large proportion of these would perish with the discontinuance of
agriculture.

Although we are but slightly acquainted, as yet, with the extent of our
instrumentality in naturalizing species, yet the facts ascertained
afford no small reason to suspect that the number which we introduce
unintentionally exceeds all those transported by design. Nor is it
unnatural to suppose that the functions, which the inferior beings,
extirpated by man, once discharged in the economy of nature, should
devolve upon the human race. If we drive many birds of passage from
different countries, we are probably required to fulfil their office of
carrying seeds, eggs of fish, insects, mollusks, and other creatures, to
distant regions: if we extirpate quadrupeds, we must replace them not
merely as consumers of the animal and vegetable substances which they
devour, but as disseminators of plants, and of the inferior classes of
the animal kingdom. I do not mean to insinuate that the very same
changes which man brings about, would have taken place by means of the
agency of other species, but merely that he supersedes a certain number
of agents; and so far as he disperses plants unintentionally, or against
his will, his intervention is strictly analogous to that of the species
so extirpated.

I may observe, moreover, that if, at former periods, the animals
inhabiting any given district have been partially altered by the
extinction of some species, and the introduction of others, whether by
new creations or by immigration, a change must have taken place in
regard to the particular plants conveyed about with them to foreign
countries. As, for example, when one set of migratory birds is
substituted for another, the countries from and to which seeds are
transported are immediately changed. Vicissitudes, therefore, analogous
to those which man has occasioned, may have previously attended the
springing up of new relations between species in the vegetable and
animal worlds.

It may also be remarked, that if man is the most active agent in
enlarging, so also is he in circumscribing the geographical boundaries
of particular plants. He promotes the migration of some, he retar