136 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
practically only obtainable in spring or in late autumn; 
and if the gathering be a very large one (like D in our 
Table, p. 174: the September gathering shown in 
column F is unusually large for that time of year) it is 
almost certain to have been taken at the former period, 
say between the middle of March and the middle of May. 
We shall show further on in this paper that some of the 
above statements, descriptive of the planktonic yearly 
cycle in the Irish Sea, do not apply to the seas of the 
Hebrides off the West Coast of Scotland. 
It is evident that in the conclusions we have 
arrived at for the Irish Sea, and which have also been 
demonstrated by other observers in other European seas 
(although it is still uncertain how far they extend 
to other oceans), there are various matters of great 
importance requiring explanation, for example :— 
(1) What is the cause of the spring maximum? 
(2) Even if some physical condition is found to be 
coincident with the great increase of Diatoms, 
we still require to know the origin of the first 
spring Diatoms that gave rise to the others. 
Were they present in the water during the 
winter in small numbers, were they in a resting 
condition on the bottom, or have they been 
carried in from the Atlantic or more Northern 
seas at or about the time of the increase? 
(3) Why do the spring Diatoms disappear? Do 
they die off, sink to the bottom, or migrate out 
of our area? 
(4) Do they remain on throughout the summer in 
deeper water, or in some localities, to any large 
extent? 
