1897-98.] Prof. D’Arcy W. Thompson on Marine Faunas. 313 
specimens, the doubtfulness of whose identification the describers 
have frankly acknowledged, or from minute species in polymorphic 
groups, where species are distinguished by minute characters and 
very often by characters drawn only from a shell without examina- 
tion of the animal itself. Of the forms which remain after 
deduction of such as these, it seems to me very important to 
discriminate between those whose affinities appertain to the fauna 
of the North Pacific and of the North Atlantic respectively. 
For the fauna of the North Pacific presents many unknown 
problems to us ; hut this we do know, that it contains in part a 
northern circumpolar fauna, and in part a fauna very distinct 
from that of the North Atlantic and peculiarly linked to the 
fauna of the Southern Ocean. And we know a little, though 
not much, of the manner in which this continuity is established, 
along the western shores of the American continent, in waters 
singularly cold for the latitudes under which they lie. There are 
not a few forms that seem to come into the same category as the 
genus Serolis, the penguins, the sea- elephant, the sea-lions, and 
the fur-seals — I might add the giant sea-weed Macrocydis — that, 
from a circumpolar habitat in the Antarctic, seem to creep up to 
varying distances far along the Western American coast, to the 
Galapagos, to California, or even to the northern islands and 
Japan. The work of American naturalists, and, in particular, the 
explorations of the “ Albatross,” have of late years added very 
largely to our knowledge of this area of distribution. 
In this paper I have attempted firstly to weigh the evidence 
for the particular cases that Murray has quoted as examples of 
species represented in both northern and southern seas, though 
absent from the intervening tropical areas : and I think that the evi- 
dence so weighed persuades us to recognise that many, if not most, 
of those alleged cases of identity were extremely dubious, even in 
the eyes of those who first described them, while others, in regard to 
which the describers expressed no doubt, seem dubious at least to me, 
by reason of the insignificant size or comparative poverty in well- 
marked discriminative characters of the organisms themselves. We 
know much more than we did when the Challenger Reports were 
written of the difficulty attending the identification of such forms 
for instance as the Ostracods or the Polyzoa, two groups that 
YOL. XXII. 12/98 X 
