22 VARIATIONS IN NUMBERS AND HABITAT OF
After living on a yacht for some months, it is, of course,
desirable to clean off the various animals and plants that have
grown on the bottom. Sometimes I can then obtain specimens
not otherwise easy to find. The most remarkable circumstance
I have noticed is that in different years the animals attached to
the bottom may be most strikingly different, though the yacht
may have been at the same places, or very nearly so. One year
the entire surface under water was covered with small Balani,
growing as close to one another as they could ; I estimate that the
total number was something like two millions. Another year
there were few Balani, but the bottom was covered with
Ascidians of the genus Ascidiella. On other occasions the most
striking form was Tubularia larynx, good specimens of which I
have not been able to obtain in the district except from the
bottom of the yacht. In 1899, after lying a long time at Pin
Mill, the bottom was covered by soft, tenacious mud, built up by
the small Amphipod, Jassa pulchella. On other occasions we have
found variable mixtures of the above-named animals. These
facts show what great variations there may be from year to year,
which is thus so marked, because the bottom of the yacht is
always clean to begin with, and animals attached one year cannot
be mixed with those attached another year, as must often happen
on rocks, and stones, and other natural objects.
It will thus be seen that, even in the short period of 10 or 12
years, many noteworthy changes have occurred, and, in some
cases at all events, it seems doubtful if the original conditions
will be restored in years to come. This is certainly indicated by
the fact that in so many cases vast numbers of well preserved
dead shells are met with in situ in places where the same species
are seldom if ever met with alive. Taking all into account, it
certainly appears to me that a considerable number of interesting
animals have become more and more rare, whilst but few have
become more abundant. The first explanation that suggests
itself is that the changes may in some way have been due to
human agency, like so many changes in animals and plants
inland. In some localities an increase in the amount of sewage
discharge may have been the cause; but, though in this respect
the worst place I ever stayed at is the water at the back of
Walton-on-Naze, yet I never saw a better collecting ground for
numerous interesting animals. The use of artificial manures and
gas lime may also have had considerable influence in some