1444 
THE VOYAGE OF H.M.S. CHALLENGER. 
A-'TEKoIDK.\ ok 
THK Northern 
AND SoCTUERN 
Hf;Mi-<rnEREs. 
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, etc., under the tropic and 
the temperate zones have effected their extinction, or their migration towards the polar seas, or even 
proeiuced changes in their organisation and general appearance 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 to the Antarctic, and 
which are so very little differentiated that we scarcely acknowledge them as distinct species. Thus the 
northern species, Psolus sqiiamatus and Psolus fabricii, pass imperceptibly into Psolus operculatus, Psolus 
complanntus, and several other tropical or subtropical forms, which in their turn are replaced tow’ards the 
Antarctic regions by Psolus antarcticus and Psolus ephippifer, &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 con- 
ditions and an altered mode of life have caused some small deviations in internal and external organisation.” ^ 
Agassiz writes : “ I am unable to distinguish the specimens of this species \_Echinocardium flavescens^ 
collected at the Cape of Good Hope from the northern ones. There seems to be a number of northern 
sjHicies of Spatangoids which extend to the Cape ; among them are to be mentioned also Brissopsis lyriferUy 
Upatangm raschi, and Schizaster fragilis.” ^ 
SiJiDEN writes ; “ So far as at present known this genus (Stichaster) appears to be confined to the temperate 
and frigid zones ; and most of the species are probably limited bathymetrically to the Littoral zone, only two 
occurring in the Continental zone and one in the Abyssal zone. . . . Lopliaster was hitherto known only from 
the North- Atlantic area. The Challenger obtained examples of a form which I consider referable to the 
genus from the South I’acific, thus adding another example to the remarkable list of representative forms 
found in the temperate and sub-frigid zones of tlie Northern and Southern hemispheres respectively. . . . For 
a long time the genus Cribrella was thought to be represented by a single species only, and to bo confined to 
the northern portion of the Atlantic. Within the last twenty years a number of other species have been 
discovered, and the genus is found to have a wide area of distribution in the Southern as well as in the 
Northern tenii)erate and sub-frigid zones. In the higher latitudes north of the Equator Cribrella is the only 
representative of tlie family, but in its approach towards the southern pole it is accompanied by the allied 
genus Perknastcr. Its bathymetrical range is greater than that of any other genus of Echinasteridae, it being 
the only repre-sentative of the family in the Abyssal zone . . . Since the discovery of this genus 
[/Wicc/fas/er] in the North Atlantic area a number of forms have been referred to it, which indicate a wide 
range of distribution. Although I am inclined to think that the whole of the species now ranked as 
Peflirellaater may not ultimately be retained in the genus, the occurrence of representative forms in the 
higher latitudes of the Northern and Southern hemispheres is fully established.” ® 
Cari-enter writes : “Closely allied to the North Atlantic species [of Comatulse] are those occurring at 
Kerguelen and Heard Island, together with a couple of forms inhabiting the Strait of Magellan. This 
' Zool. Cball. Exp., pt zxzix. |)p. 21»9, 260. ■ Zool. Cliall. Exp., pt. ix. p. 175. 
^ Zool. Chall. Exp., pt. li. pp. 430, 458, 459, 540, 557. 
