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' net, and the ordinary dredger.
A word or two on the using of '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 object of the latter and its doubtful accuracy, young fish trawls.
From this to the chemical composition of sea water, the total salt about 3·5 per cent. but variable: the proportions of the various salts do not appear to differ, thus the chlorine test detects the salinity quantitatively. Physically plankton life must depend on this salinity and also on temperature, pressure, light, and movement.
(If plankton only inhabits surface waters, then density,
