West Indian
plants is no doubt due to the Gulf Stream and constant surface drift of
warm water in this direction, while others have been brought by the annual
cyclones which sweep over the intervening ocean. The great number of
American migratory birds, including large flocks of the American golden
plover, with ducks and other aquatic species, no doubt occasionally bring
seeds, either in the mud attached to their feet or in their stomachs.[59]
As these causes are either constantly in action or recur annually, it is
not surprising that almost all the species should be unchanged owing to the
frequent intercrossing of freshly-arrived specimens. If a competent
botanist were thoroughly to explore Bermuda, eliminate the species
introduced by human agency, and investigate the source from whence the
others were derived and the mode by which they had reached so remote an
island, we should obtain important information as to the dispersal of
plants, which might afford us a clue to the solution of many difficult
problems in their geographical distribution.

_Concluding Remarks._--The two groups of islands we have now been
considering furnish us with some most instructive facts as to the power of
many groups of organisms to pass over from 700 to 900 miles of open sea.
There is no doubt whatever that all the indigenous species have thus
reached these islands, and in many cases the process may be seen going on
from year to year. We find that, as regards birds, migratory habits and the
liability to be caught by violent storms are the conditions which determine
the island-population. In both islands the land-birds are almost
exclusively migrants; and in both, the non-migratory groups--wrens, tits,
creepers, and nuthatches--are absent; while the number of annual visitors
is greater in proportion as the migratory habits and prevalence of storms
afford more efficient means for their introduction. {274}

We find also, that these great distances do not prevent the immigration of
some insects of most of the orders, and especially of a considerable number
and variety of beetles; while even land-shells are fairly represented in
both islands, the large proportion of peculiar species clearly indicating
that, as we might expect, individuals of this group of organisms arrive
only at long and irregular intervals.

Plants are represented by a considerable variety of orders and genera, most
of which show some special adaptation for dispersal by wind