THE VOYAGE OF H.M.S. CHALLENGEE. 
!2.j2 
trawliugs in some spots and the comparatively unsuccessful results in others at the 
same depth and with apparently similar surroundings. From the results of deep-sea 
dredgings and trawlings, up to the present time, there seems no doubt that life is on 
tlie whole more abundant at the bottom near continental shores than at similar depths 
towards the centres of the ocean basins. 
The operations with tow-nets in surface, subsurface, and intermediate waters lead to 
nearly identical conclusions with reference to the pelagic fauna and flora, or Plankton, as 
those with reference to the fauna or flora on the deposits, or Benthos. Sometimes the 
captures in the tow-nets may be very insignificant, while, at a little greater or less 
depth, or at a ditfercnt time of the day, the same nets may yield an abundant harvest. 
Many of the species occur at times in floating banks of vast extent, and at other times 
only a few specimens may be taken at the same locality. On the whole, the Planktonic 
species are more numerous in tropical waters, while in polar waters, although 
tlie species are less numerous, the individuals of the species have often an enormous 
development. The Challenger observations appear to indicate clearly that in warm 
oceanic currents the abundance of life is greater than in the regions of the Sargasso Seas. 
The pelagic fauna and flora are, again, different and probably more abundant along 
coasts affected by river water than in purely oceanic regions. Ascending currents of 
water from the deeper regions near land are sometimes heavily laden with marine 
organisms whose usual habitat is in deep water about the level of the mud-line sur- 
rounding the continental and other coasts. 
Owing to this unequal distribution of organisms in ocean water and on the floor of 
the ocean, it is not possible to arrive at any satisfactory approximation of the total 
number of living organisms or the total amount of organic matter in the sea, but it is 
evident that these must, on the lowest estimate, be enormous. Assuming that the lime- 
secreting organisms were as abundant throughout the whole region as in the path 
followed by his tow-nets, ]\Ir. Murray ^ has estimated that at least sixteen tons of 
carbonate of lime, in the form of shells of living organisms, were present in a mass of 
tro[)ical oceanic water one square mile in extent by 100 fathoms in depth. Hensen has 
even made a praiseworthy attempt to count the number of individuals of each species in 
certain tow-net gatlierings, and from tliese data to estimate the total numbers of each 
species as well as the amount of organic matter in the whole ocean.^ All these calculations 
are interesting and valuable for the time and j)lace of the experiments, but unreliable or 
insuflicient when used as a basis hm any wide general conclusions or deductions. When 
considering the amount of organic matter in the ocean, it must be remembered that a large 
' I'roe. Roy. Soc. K'Un., vol x. p. fjOS. In sonie ten litres of water from the Red Sea, Murray and Irvine recently 
found msp<-ndwl carlxinate of lime (sheila of organisms) equivalent to 51 tons in a mass of ocean water one square mile 
by 1U» fathoms in depth. 
* Menum, “ UelK-rdie Restimmung dea Phuiktons, oder die im Meerc treihenden Materials an Pflauzen und Thieren,” 
Bericht d. Comm. z. wks. Untera. dcrdeutschen Meere in Kiel, 1887. 
