260 
THE VOYAGE OF H.M.S. CHALLENGEE. 
entirely similar. Of course, I do not take into consideration such forms as pass their 
existence on the bottom of the deep-sea or at the surface of the ocean. It appears 
scarcely probable that the shore fauna of two regions so far separated from each other as 
the arctic and antarctic seas, has any direct exchange of forms at the present epoch, so 
as to allow the same species, in its larval or adult state, to pass from one pole to the 
opposite without settling at interjacent regions. In brief, I do not believe that at the 
present epoch the arctic shallow-water fauna can directly originate from the antarctic, or 
vice versa. 
On the other hand, it is a fact that the two faunae in question resemble each other 
very closely, and, with regard to the Holothurids, that several forms occur in the arctic 
sea which are most closely allied to those in the antarctic. I am inclined to suppose 
that the progenitors of. these Holothurids have had a much wider distribution during a 
past period, that altered physical conditions, a keener struggle for existence, &c., under 
the tropic and the temperate zones have effected their extinction, or their migration 
towards the polar seas, or even produced changes in their organisation and general appear- 
ance so marked, that their descendants which still remain in the tropical zones present 
themselves as species distinct from the original, and finally that the polar seas with their 
more uniform physical conditions, allowed them and their descendants to live there and 
to develop slowly but continuously after almost the same plan. 
The genus Psolus offers an instructive example of forms which are distributed over 
all seas from the Arctic Ocean to the Antarctic, and which are so very little differentiated 
that we scarcely acknowledge them as distinct species. Thus the northern species, 
Psolus squamatus and Psolus fabricii pass imperceptibly into Psolus operculatus, 
Psolus complanatus, and several other tropical or subtropical forms, which in their turn 
are replaced towards the antarctic regions by Psolus antarcticus and Psolus epliippifer, 
&c. But it must be observed that all these forms of Psolus, though they apparently 
present great similarities, are nevertheless distinguishable, though the differences may or 
may not be of specific value. It appears pretty evident that they are all descendants 
from the same ancestors, which may have had their origin in the polar seas or in the 
tropic or subtropic oceans, and that they, in their wide dispersion, have sustained very 
well the influence of altered and very various physical conditions in different regions of 
the world. But, of course, different physical conditions and an altered mode of life have 
caused some small deviations in internal and external organisation. 
With regard to the remaining species of Holothurids mentioned above, I think the 
same is or has been the case. Forms intermediate between those in the arctic and 
those in the antarctic seas, are either still living in interjacent regions, having undergone 
alterations of a more or less adaptive nature, or they have succumbed. 
As a fact, numerous arctic forms of animals are “ circumpolar,” and among the Holo- 
thurioidca, Myriotrochus rinckii, Chirodota IcBvis, Cucumaria calcigera, Cucumaria 
