THE WEST INDIAN FAUNA. 15 
regions is far greater than that between the inhabitants of the 
same regions in the temperate or arctic provinces. This is 
readily explained by the circumstance that the cold water of the 
abyssal regions, with 1ts characteristic animals, approaches nearer 
the shore as we go north within the continental region, so that 
the littoral fauna of the arctic circle lies practically under the 
same conditions of temperature as the abyssal in the tropics, or 
the continental in the temperate zones. That is, the divisions 
of these faunal regions are to be determined more by tem- 
perature than by depth, although of course the temperature 
depends upon the depth and upon the currents of the ocean. 
Below a depth of seven to eight hundred fathoms, correspond- 
ing to a temperature of 40° F., we pass into the abyssal regions, 
whilé upon the continental slope at a depth of about 150 fath- 
oms we reach the lower limit of the littoral region. 
One of the first points noted by Lovén in reference to the 
few deep-sea types occasionally brought up from various quar- 
ters of the Atlantic was their wide geographical range ; and he 
first distinctly formulated the theory of the uniformity of an 
abyssal fauna extending in the Atlantic from the arctic to the 
antarctic regions, with a somewhat modified fauna at the two 
poles, — a theory which has been slightly changed by later deep- 
sea explorations. Lovén's theory seemed to give a most natural 
explanation of the marked similarity, often noticed by vari- 
ous naturalists, between a number of the arctic and antaretie 
invertebrates. It was therefore of the greatest interest when 
Pourtalés dredged in the deep water of the Straits of Florida 
the little Rhizoerinus discovered by Sars on the coast of Norway, 
and when subsequent explorations of the * Blake" brought to 
light a large number of boreal types in the deep water of the 
Otifbean district, and off our eastern coast. Professor Smitt, 
who examined our collection of the Bryozoa from the West 
Indian district, speaks of the interest he telt in finding well- 
known- Seandinavian forms among these tropical and antarc- 
tic types. The range of many of the Bryozoa is very wide. 
More than ten Caribbean species are found in thé North At- 
lantie, and an equal number extend to the arctic regions ; 
eight are Australian, and four belong also to the Red Sea. 
