220
THE ESSEX NATURALIST.
such forms belong to the surface association or not. The "acid
test" perhaps in these doubtful cases is whether such organisms,
if animals, feed while at the surface. Another fact to be re-
membered in this connection is that, attached to the undersides
of the water-surface plants, there is usually quite a little world
of miscellaneous organisms such as Polyzoa, Hydrozoa, Protozoa,
Diatoms and other Alga;, as well as the eggs of Insects, Molluscs,
etc. Even the water-surface animals often carry smaller
organisms on their bodies, e.g. Vorticellids, Flagellates, etc., on
Gnat larva;.
I do not propose to argue the point whether the plants and
animals specified constitute an ecological association in the
strict sense of the term. From some points of view it might
be better to regard them as forming parts of other associations,
as indeed has already been done. For example, most of the
water-surface plants belong to that general assemblage of
aquatic plants, possessing floating leaves and other peculiarities
adapting them to a life partly in and partly out of the water,
to which the name Pleuston has been given. On the other
hand the Algae and Bacteria forming filmy scums on the surface
just as evidently belong to the surface-film association called
by Naumann the Neuston. From our present standpoint,
however, it will be sufficient if we realise that the organisms here
referred to as water-surface plants and animals undoubtedly
stand in close association with one another and have come
to occupy a very definite niche in nature's temple. Unfor-
tunately we possess at present only a limited knowledge either
of the inter-relationships of these organisms or of the special
physical problems of their existence. What is wanted to
obtain further information, is mainly more systematic obser-
vation in the field, supplemented, to be sure, by control obser-
vations and experiments in large and small aquaria. But there
is one aspect at least of the conditions of their existence which
is of peculiar importance and to which also, as it happens, a
fair amount of attention has already been given, namely, the
part played by surface-tension in their environment. It is to
this factor in the ecological problems presented by the water-
surface plants and animals that I now wish to direct your
attention ; but, to prevent misunderstanding, I must emphasise
the point that these are not the only aquatic organisms affected