PLEA FOR SYSTEMATIC ATTENTION TO AQUATIC BIOLOGY. 145
For example, as aquatic organisms may be profoundly influenced
by any substances carried by the water either in solution or
suspension, it is of great importance to ascertain whether humic
acid from the decay of land vegetation (from peat bogs, etc.)
and nitrogenous matter from the excreta of land animals (from
manure heaps, etc.) are being brought into any particular piece
of water under observation. As another illustration of the
influence terrestrial organisms have upon the fauna and flora,
of, say, a lake or pond, may be mentioned the cases in which
various kinds of resting stages (e.g. gemmules of sponges, stato-
blasts of Polyzoa, ephippia of Cladocera, winter-buds of some
water plants, etc.) are carried from one piece of water to another
by birds, insects, etc. On the other hand, as already incident-
ally indicated in connection with the question of food-chains,
some land animals may be dependent wholly or in part upon
aquatic animals, and they may also in this way acquire certain
internal parasites (tapeworms, etc.) the earlier stages of which
occurred in their prey.
Altogether, there is evidently much interesting and important
work to be done in tracing the ramifications of the relationships
of aquatic organisms to one another and to terrestrial organisms.
The problems arising out of the relation of aquatic organisms
to their physical environment are very varied, and for the
most part quite different from those which arise in the case
of plants and animals on the land. This is brought about of
course mainly by the medium by which they are surrounded, and
it is consequently of the first importance that the aquatic biologist
should learn as much as possible about the physical and chemical
properties of water. Only with some such knowledge is it possible
to understand why aquatic organisms differ in this and that
particular from land organisms, and to appreciate the wonderful
way in which the special difficulties of under-water life have
been solved and the peculiar properties of water made use of.
Herein to my mind lies a large part of the special attractiveness
of the study of aquatic biology.
Water, however, is only part of the physical environment of
aquatic organisms, at least of those with which we in this part
of the world are likely to have to deal. The influence of the
basin in which the water is held, or the bed over which it flows,
or the shores against which it washes, is also of very considerable