CHAP. 1.] INTRODUCTION. 177 
of peculiar creatures are few, yet sufficient to give a 
marked character to it, whilst the other portions of 
its population are derived from the higher zones, and 
must be regarded as colonists. As we descend deeper 
and deeper in this region, its inhabitants become more 
and more modified, and fewer and fewer, indicating 
our approach towards an abyss where life is either 
extinguished, or exhibits but a few sparks to mark 
its lingering presence.”’ ? 
Forbes pointed out that the groups of animals 
having their maximum development in these several 
zones are thoroughly characteristic, and that groups 
of representative forms occupy the same zones all 
over the world, so that on examining an assemblage 
of marine animals from any locality, it is easy to tell 
from what zone of depth they have been procured. 
At all periods of the earth’s history, there has been 
the same clear definition of zones of depth, and fossil 
animals from any particular zone are in some sense 
representative of the fauna of the corresponding zone 
at_the present day. We can, therefore, usually tell 
with tolerable certainty to which zone of depth a 
particular assemblage of fossils is to be referred. 
Although we must now greatly modify our views 
with regard to the extent and fauna of the zone of 
deep-sea corals, and give up all idea of a zero of 
animal life, still we must regard Forbes’ investiga- 
tion into the bathymetrical distribution of animals 
as marking a great advance on previous knowledge. 
His experience was much wider than that of any 
other naturalist of his time; the practical difficulties 
in the way of testing his conclusions were great, and 
* Edward Forbes, Natural History of the European Seas, p. 26. 
C 
