500 TRANSACTIONS OF SECTION D. 
animals whose calcarecus deposits would seem to render them highly 
indigestible. 
Very careful investigations have been made and are being made by 
Mr. Borley and Mr. Todd as to the distribution of the fauna of the middle | 
and southern parts of the North Sea, and its relation to the depth of water, 
the varying degree of salinity, and to the texture of the bottom deposits. 
These results, however, have not been published, but I may go as far as to 
say that the inquiry shows that within the area investigated the texture 
of the sea floor has, on the whole, more influence on the distribution of the 
invertebrates of the bottom fauna than has depth, and that depth in the 
area in question seems to have more influence than salinity. 
With regard to the character of the bottom deposits, it has been found 
by Mr. Borley that off shore and on the gently shelving continental coast 
the sea bottom is of a uniform character over wide areas, though on the 
western side it is more patchy; and it has proved possible to divide the 
samples taken into some nineteen main types, each characteristic of one or 
more of the areas into which the region has been split up. Only one or 
two details of this laborious work can be mentioned. One is that the 
texture or degree of coarseness of the ground in various parts of the sea 
is such as to suggest that the distribution of the finer grades of material, 
the finer sands and silts, is greatly influenced by the joint action of 
currents and tides. It is, for instance, known that in the southern part of 
the North Sea the main direction of the bottom current is to the north and 
then to the east; and examination of the deposits shows a regular diminu- 
tion in the proportion of the coarser sands, a regular increase in the pro- 
portion of finer material, as we proceed from the Straits of Dover in a 
north-easterly direction. A remarkable fact in this connexion is the com- 
plete absence of silt from the sandy bottom west of the mouths of the great 
rivers Rhine and Maas. There can be no doubt that the presence of broad 
and shallow stretches of sand on the continental, but not on the English, 
side of the North Sea is one of the factors which has determined the dis- 
tribution of the small plaice, which on the continental shores are so extra- 
ordinarily abundant, and on the English shores are relatively so scarce. 
By means of bottles weighted with shot, so as to have about the same 
specific gravity as the surrounding sea water, Mr. G. P. Bidder has been 
able to trace slow currents moving over the bottom of the sea. The bottles 
are closed, and contain a postcard in many languages, offering a reward to 
whosoever returns the postcard, recording the latitude and longitude of the 
place it was trawled at, to our laboratory at Lowestoft. Attached to the 
neck of the bottle is a copper wire a foot and a half long. This wire trails 
along the bottom, the bottle itself floating about a foot and a quarter above 
the level of the ground. Slowly as the bottles are swept along, yet the 
distance they cover is sufficient to sharpen the free end of the wire to a 
needle point. 
By these and by other methods it has been possible to trace the almost 
imperceptible but steady flow of waters along the bed of the sea. Without 
doubt these currents influence the distribution of the larval and young 
forms of all the creatures which live near the bottom, and especially 
influence the migration of food-fishes in their younger and less active stages, 
when they are swept helplessly along. 
But these bottles have a double lesson to teach us: not only do they 
enable us to chart the slow streaming of the bottom water, but they give us 
to some exterit a measure of the intensity of trawling in the North Sea. 
They have been refished in really surprising numbers. Commercial trawlers 
have retaken them at the rate of 58 per cent. per annum. In one area 
these bottles cast upon the waters were retaken, not after many days, but 
after very few. Out of 390, eighty-five were recovered in six weeks, and 
fifty out of 270 were trawled in five weeks, representing a local intensity 
