EVENING DISCOURSES. 741 
menon, caused both by this and by other organisms. 1t must be of common 
occurrence, and is widespread in the oceans, and it will be admitted that a 
quantitative net hauled vertically through such a Trichodesmium bank 
would give entirely different results from a haul taken, it might be, only a 
mile or two away, in water under, so far as can be determined, the same 
physical conditions, but free from Trichodesmium. [Illustrations shown. | 
Nine nations bordering the North-West seas of Europe, some seven or 
eight years ago, engaged in a joint scheme of biological and hydrographical 
investigation, mainly in the North Sea, with the declared object of throw- 
ing light upon fundamental facts bearing on the economic problems of 
the fisheries. One important part of their programme was to test the 
quantity, distribution, and variation of the Plankton by means of periodic 
observations undertaken four times in the year (February, May, August, 
and November) at certain fixed points in the sea. Many biologists con- 
sidered that these periods were too few and the chosen stations too far 
apart to give reliable results. It is possible that even the original pro- 
moters of the scheme would now share that view, and the opinion has 
recently been published by the American Planktologist, C. A, Kofoid— 
than whom no one is better entitled, from his own detailed and exact 
work, to express an authoritative verdict—that certain recent observa- 
tions ‘ can but reveal the futility of the Plankton programme of the Inter- 
national Commission for the investigation of the sea. ‘The quarterly 
examinations of this programme will, doubtless, yield some facts of value, 
but they are truly inadequate to give any reliable view of the amount and 
course of Plankton production in the sea.’ * That is the latest pronounce- 
ment on the subject, made by a neighbour of yours to the South, who has 
probably devoted more time and care to detailed Plankton studies than 
anyone else on this continent. [Examples were shown of very diversified 
Plankton hauls from neighbouring localities on the same date, or from the 
same locality on adjacent days, to illustrate irregularity in distribution. | 
It is evident from such results that before we can base far-reaching 
generalisations upon our Plankton samples, a minute study of the distribu- 
tion of life in both marine and fresh waters at very frequent intervals through- 
out the year should be undertaken. Kofoid has made such a minute study 
of the lakes and streams of Illinois, Marsh of those of Wisconsin, and similar 
intensive work is now being carried out at several localities in Europe. 
Too little attention has been paid in the past to the distribution of many 
animals in swarms, some parts of the sea being crowded and neighbouring 
parts being destitute of such forms, and this not merely round coasts and 
in the narrow seas, but also in the open ocean. For example, some species 
of Copepoda and other small crustacea occur notably in dense crowds, and 
are not universally distributed. This is true also of some of the Diatoms, and 
also of larger organisms. Many naturalists have remarked upon the banks 
of Trichodesmium, of Medusae and Siphonophora, of Salpae, of Pteropods, 
of Peridinians and of other common constituents of the Plankton. Cleve’s 
classification into Tricho-Plankton (arctic), Styli-Plankton (temperate), and 
Desmo-Plankton (tropical) depends upon the existence of such vast swarms 
of particular organisms in masses of water coming into the North Atlantic 
from different sources. 
It is possible that in some parts of the ocean, far from land, the Plankton 
may be distributed with the uniformity supposed by Hensen. It is important 
to recognise that at least three classes of locality exist in the sea in relation 
to distribution of Plankton :— 
(1) There are estuaries and coastal waters where there are usually 
1 Internationale Revue der Hydrobiologie und Hydrographie, vol. i., p. 846, 
December 1908. 
