1436 
THE VOYAGE OF H.M.S. CHALLENGER. 
shallow zones the percentage of the species procured in these zones which pass through 
several zones of depth is veiy low, while in the deeper zones the percentage of species 
procured in these zones which pass through several zones of depth is rather high. These 
facts are evidently to be accounted for by the changes in the physical conditions 
which take place between the depths of 100 and 500 fathoms. When once animals 
have accommodated themselves to deep-sea conditions there are few barriers to further 
vertical or horizontal migration, hence the same species is often recorded in several of 
the deeper zones. An examination of Table I. page 1430 shows that the percentage of 
new species from the shallower zones, which extend into the deeper zones, is relatively 
large. This is probably due to a large number of the Challenger’s dredgings and 
trawlings being in rather deep water within these zones and on muddy bottoms. In all 
probability the deejD sea w’^as peopled by continuous migrations downwards and seawards 
from about the mud-line. If we suppose these migrations to have taken place from 
the mud-line at many different periods of time and from many different parts of the 
world, then it may be thus possible to account for the relatively large number of genera 
m the deep sea in proportion to the number of species. 
Diminution in 
TOE Number of 
Species of 
Eentoos with 
Distance from 
Shore. 
When we take into consideration the horizontal as well as the vertical distribution, 
it would appear that in all the tropical and sub-tropical regions of the ocean the number of 
individuals and species living on the sea-bed very distinctly diminishes with the distance 
from the shore, and especially with the distance from continental shores. This statement 
would be universally true for all regions of the ocean were not the continental conditions 
pushed far from land by the presence of floating ice in the great Southern Ocean, and 
in some similar regions of the northern hemisphere. At first sight it might appear that 
this diminution in the number of species resulted from the greater depth at the seaward 
stations. This cannot, however, be the sole explanation. When the captures at the same 
or similar depths are compared from those stations within and those beyond 300 miles 
from land, it is found that the near-shore stations yield much the larger number of 
specimens and sjiecies. Stations in depths between 1000 and 2000 fathoms near shore 
give, on the average, 121 specimens and 39 species per haul ; the remote stations beyond 
300 miles from land and between the same limits of depth yielded only 21 specimens and 
10 species per haul. In making this comparison, those stations were selected in which 
the conditions as to depth and other circumstances were nearly similar, excepting always 
the distance from a continental shore. 
When the results of the trawlings and dredgings on the different kinds of deposits 
are compared, similar indications are obtained with reference to the relatively greater 
abundance of both individuals and species close to continental shores — the Terrigenous 
