146
THE ESSEX NATURALIST.
importance in the lives of aquatic organisms. So also, although
in a less variable manner, is the influence of the air which bounds
the upper surface of the water. It is indeed the combination
and interaction of the three factors—water, land and air, in all
their varied conditions of composition, situation, temperature,
etc., which produce the main features of the almost infinite
variety of types of physical environment in which aquatic
organisms occur. Even the principal types can be classified in
many different ways. For example, the water may be salt,
brackish or fresh, acid or alkaline, shallow or deep, stagnant or
flowing, etc. But the point which chiefly interests us now is
that each different type of physical environment contains a
characteristic, assemblage of plants and animals, usually referred
to by plant-ecologists as associations. The problems before the
aquatic biologist in this connection are the working out of these
associations in more detail than has yet been done, and the
elucidation of the physical factors involved in the adaptation of
the organisms comprising any association to the conditions of
their particular environment.
To enter into detail on this matter scarcely comes within
the scope of this address, but I may perhaps refer to just one of
the aquatic associations in illustration of the sort of problem
requiring and deserving much further consideration. The
association I have in mind is that varied assemblage of plants
and animals which live, either permanently or periodically, on
or just under the surface of the water. It is, of course, already
known that the number of different forms involved is very con-
siderable, including representatives of the flowering plants,
Liverworts, Algae, Molluscs, Insects, Crustacea, Worms, Hy-
drozoa, etc. Nearly all these organisms show modifications
which enable them to make use of the latent power which resides
in the surface-film, and most of these modifications appear to
depend upon the special arrangement of certain water-repellent
spines, hairs, scales and surfaces. The different organisms use
the surface-film for many purposes, sometimes merely for sus-
pension to save themselves the trouble of constantly swimming,
sometimes to secure food, sometimes to obtain air, sometimes
for pollination, etc. But more information is wanted on all
these matters, and particularly with regard to the exact methods
by which the power of surface-tension is made use of in the
different cases.