314 SCOTT'S LAST EXPEDITION [June 
In the killer whale may be found a seal, in the seal a 
fish, in the fish a smaller fish, in the smaller fish a copepod, 
and in the copepod a diatom. If this be regular feeding 
throughout, the diatom or vegetable is essentially the 
base of all. 
Light is the essential of vegetable growth or metabolism, 
and light quickly vanishes in depth of water, so that all 
ocean life must ultimately depend on the phyto-plankton. 
To discover the conditions of this life is therefore to go 
to the root of matters. 
At this point came an interlude — descriptive of the 
various biological implements in use in the ship and on 
shore. The otter trawl, the Agassiz trawl, the ' D 9 net, 
and the ordinary dredger. 
A word or two on the using of i D ' nets and then 
explanation of sieves for classifying the bottom, its 
nature causing variation in the organisms living on it. 
From this he took us amongst the tow-nets with 
their beautiful silk fabrics, meshes running 180 to the 
inch and materials costing 2 guineas the yard — to the 
German tow-nets for quantitative measurements, the ob- 
ject of the latter and its doubtful accuracy, young fish 
trawls. 
From this to the chemical composition of sea water, the 
total salt about y$ per cent, but variable : the proportions 
of the various salts do not appear to differ, thus the 
chlorine test detects the salinity quantitatively. Physi- 
cally plankton life must depend on this salinity and also 
on temperature, pressure, light, and movement. 
(If plankton only inhabits surface waters, then density, 
