3380 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
If the catch mm nets over the stern is seriously affected 
by the action of the propeller, then it is difficult to account 
for the great similarity of the catches in these nets on 
some occasions—as, for example, on April 22nd, 1907, 
when Form 45, which was quoted in last year’s report, 
shows that the two surface nets at Station IV caught 
precisely the same amount of material, and where the lists 
of organisms constituting the catch were almost exactly 
alike, both in species and numbers. Such a close agree- 
ment can scarcely be explained etlicrwise than as due to 
similar sampling of similar populations. That it cannot 
be due to an unusual condition where the surface and 
the deeper zones are at the time alike in population is 
shown by the fact that the weight net on the same occasion 
brought up a catch which differed from the two surface 
hauls. 
3. In Part IL of this investigation last year, we arrived 
at the conclusion that the constitution of the plankton 
from time to time throughout the year is due to the inter- 
action of three factors :— 
(1) The sequence of the stages in the normal life- 
history of the different organisms. 
(2) Irregularities introduced by the interaction of the 
different organisms. 
(3) More or less periodic abnormalities in either time 
or abundance caused by the physical changes 
in the sea which may be grouped together as 
“ weather.” 
Prof. C. A. Kofoid, in his review referred to above 
(Internat. Rev. Hydrobiol. und Hydrogr., December, 
1908, p. 845), seems to find some inconsistency in the use 
of the first and the last of these factors. In case Prof. 
Kofoid, and possibly others, have not entirely understood 
