Distribution of Marine Animals. 125 



selves over the great oceans is proved by their antiquity, 

 many, like both groups of truly pelagic forms (the Heteropods 

 and Pteropods), ranging back to the Silurian period. 



Their complex distribution is not easily explained. Was 

 the Pleurotoma of Greenland evolved from the same stock as 

 the cones of the tropics, or did each arise from pre-existing 

 forms in the special areas ? Why are the pearl-oysters 

 (Aviculidas) tropical or subtropical, like the giant-clam 

 (Tn'dacna) ? Why should the conditions accompanying the 

 formation of pearls in the former be limited to special regions, 

 even though the presence of certain fishes be necessary ? 



The marine Insecta are comparatively few, and it will 

 suffice to take the two genera described by Dr. Buchanan 

 White* from the collection of the ' Challenger.' Thus five 

 species of Ilalobates occur in the Atlantic, but only one is 

 restricted to it. Six species are found in the Indian Ocean 

 west of long. 100° E., whilst (chiefly) in the West Pacific 

 eight species are met with, of which four are restricted to 

 that region. The metropolis of the genus appears to be the 

 Indian Ocean and West Pacific, for nine out of the eleven 

 known species occur there, and White thinks even originated 

 there, and that currents have carried them eastward. The 

 other genus (Ilalobatodes) is represented only in the Indian 

 Ocean and the China Sea. The Halobatidse are therefore 

 chiefly inhabitants of the warmer seas, and though they have 

 not spread over the whole ocean, they are widely distributed. 



In the class Crustacea the distribution of marine forms is 

 remarkably wide, just as the number of some of the smaller 

 forms like the Copepods swarm in every sea, from pole to 

 pole. Thus a species of the Amphipod Podocerus extends, 

 Mr. Stebbing informs ine, from the waters of New Zealand to 

 7?° 7' N., and another from Tahiti to the Faroes. The 

 higher Crustacea are sensitive to temperature, as is evident 

 from the behaviour of such forms as the shore-crab in summer 

 and winter, and, as Mr. Stebbing observes, by the paucity of 

 species in Arctic, Antarctic, and very deep waters. Yet, as 

 this experienced author states, there are Amphipods and 

 Isopods which abound most and attain their greatest size in 

 Arctic waters. The comparison of the Copepods (Calani &c.) 

 from the feeding-grounds of the right-whale with those in 

 European waters is equally pronounced, the size of the Arctic 

 forms being much greater. Mr. Stebbing mentions that 

 every fresh expedition tends to show the intimate relationship 



* ' Challenger,' vol. vii. pp. 77 & 78. 



