of Edinburgh, Session 1883 - 84 . 529 
Hexactinellida, Brachiopoda, Stalked Crinoids and other Ecliino- 
derms^ &c. 
As already mentioned, the transitional area is that which now 
shows the greatest variety in respect to biological and physical 
conditions, and in past time it has been subject to the most frequent 
and the greatest amount of change. The animals now living 
in this area may be regarded as the greatly modified descendants of 
those which have lived in similar regions in past geological ages, and 
some of whose ancestors have been preserved in the sedimentary 
rocks as fossils. On the other hand, many of the animals dredged 
in the abysmal regions are most probably also the descendants of 
animals which lived in the shallower waters of former geological 
periods, but descended into deep water to escape the severe 
struggle for existence which must always have obtained in those 
depths affected by light, heat, motion, and other conditions. Having 
found existence possible in the less favourable and deeper water, 
they may be regarded as having slowly spread themselves over the 
floor of the ocean, but without undergoing great modifications, 
owing to the extreme uniformity of the conditions and the absence 
of competition. Or we may suppose that in the depressions which 
have taken place near coasts, some species have been gradually 
carried down to deep water, have accommodated themselves to the 
new conditions, and have gradually migrated to the regions far from 
land. A few species may thus have migrated to the deep sea during 
each geological period. In this way the origin and distribution of 
the deep-sea fauna in the present oceans may in some measure be 
explained. In like manner, the pelagic fauna and flora of the ocean 
is most probably derived originally from the shore and shallow water. 
During each period of the earth’s history a few animals and plants 
have been carried to sea, and have ultimately adopted a pelagic mode 
of life. 
Without insisting strongly on the correctness of some of these 
deductions and conclusions, we present them for the consideration 
of naturalists and geologists, as the result of a long, careful, but as 
yet incomplete, investigation. 
