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 its 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 luider 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, 

 while 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 Loven 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. Loven'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 antarctic 

 invertebrates. It was therefore of the greatest interest when 

 Pourtales dredged in the deep water of the Straits of Florida 

 the little Rhizocrinus 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 

 Caribbean 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 felt in finding well- 

 known Scandinavian 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 the North At- 

 lantic, and an equal number extend to the arctic regions ; 

 eight are Australian, and four belong also to the Red Sea. 



