EVENING DISCOURSES. 739 
This method of collecting samples of the surface fauna of the sea in any 
required quantity per day or hour from an ocean liner going at full speed 
was suggested to me by Sir John Murray of the Challenger Expedition, 
and was first practised, I believe, by Murray himself in crossing the 
Atlantic. I have since been able to make similar traverses of several of 
the great oceans, in addition to the North Atlantic—namely, twice across 
the Equator and through the South Atlantic, between England and South 
Africa, and four times through the Mediterranean, the Red Sea, and 
the Indian Ocean to Ceylon—and no doubt other naturalists have done much 
the same. The method is simple, effective, and inexpensive; and the 
gatherings, if taken continuously, give a series of samples amounting 
to a section through the surface layer of the sea, a certain volume of water 
being pumped in continuously through the bottom of the ship, and strained 
through the fine silk nets, the mesh of which may be the two-hundredth of an 
inch across, before passing out into the sea again. In examining with a 
microscope such a series of gatherings across an ocean, two facts are brought 
prominently before the mind: (1) the constant presence of a certain 
amount of minute living things; (2) the-very great variation in the 
quantity and in the nature of these organisms. [Illustrations showing this 
were given. | 
Such gatherings taken continuously from an ocean liner give, however, 
information only in regard to the surface fauna and flora of the sea, 
including many organisms of fundamental importance to man as the 
immediate or the ultimate food of fishes and whales and other useful 
animals. [Examples were shown. | 
It was therefore a great advance in Planktology when Professor Victor 
Hensen (1887) introduced his vertical quantitative nets which could be 
lowered down and drawn up through any required zones of the water. 
The highly original ideas and the ingenious methods of Hensen and his 
colleagues of the Kiel School of Planktology—whether all the conclusions 
which have been drawn from their results be accepted or not—have at the 
least inaugurated a new epoch in such oceanographic work; and have 
inspired a large number of disciples, critics, and workers in most civilised 
countries, with the result that the distribution of minute organisms in the 
oceans and the fresh waters of the globe are now much more fully known 
than was the case twenty, or even ten, years ago. But perhaps the 
dominant feeling on the part of those engaged in this work is that, not- 
withstanding all this activity in research and the mass of published litera- 
ture which it has given rise to, much still remains to be done, and that 
the Planktologist is still face to face with some of the most important 
unsolved problems of biology. 
It is only possible in an address such as this to select a few points for 
demonstration and for criticism—the latter not with any intention of dis- 
paraging the stimulating work that has been done, but rather with the 
view of emphasising the difficulties, of deprecating premature conclusions, 
and of advocating more minute and more constant observations. 
The fundamental ideas of Hensen were that the Plankton, or assem- 
blage of more or less minute drifting organisms (both animals and plants) 
in the sea, is uniformly distributed over an area where the physical con- 
ditions are approximately the same, and that by taking a comparatively 
small number of samples it would be possible to calculate the quantity 
of Plankton contained at the time of observation in a given sea area, and 
to trace the changes of this Plankton both in space and time. This was 
a sufficiently grand conception, and it has been of great service to science 
by stimulating many workers to further research. In order to obtain 
answers to the problems before him, Hensen devised nets of the finest 
silk of about 6,000 meshes in the square centimetre, to be hauled up from 
3B2 
