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 yaclit. 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 

 alwa3S 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 liave 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 



