92 THREE CRUISES OF THE '' BLAKE." 



passing richness. In a similar way, we may expect to find in 

 the track of the Pacific equatorial current the most favorable 

 conditions for the support of a rich and varied marine fauna. 

 The " Challenger " found, perhaps, no richer dredging fields 

 than off the coast of Japan, which lie directly in the track of 

 the Japanese stream ; the fauna of the Kuro Siwo may be con- 

 sidered as the Pacific equivalent of the Florida and Caribbean 

 fauna. 



In past geological times the effect of the currents in deter- 

 mining the distribution of the marine invertebrates must have 

 been as marked as it is at the present day. As long as we had 

 a great equatorial current running practically unbroken round 

 the world, and only slightly deflected by the continental islands 

 of Central America and of the East Indies, which stood in 

 the path of this equatorial belt, it was natural that we should 

 have a very extensive geographical range for all the tropical 

 marine forms. It was only after the complete shutting off 

 or comparative isolation of the Atlantic from the Pacific that 

 different physical conditions began to exist simultaneously, 

 which were of the greatest importance in reducing the supply 

 of food to the animals on the west coast of the continental bar- 

 riers, and in extending towards the north, as far as the tempera- 

 ture would allow, a supply of food far more abundant than that 

 with which the fauna of the eastern coast was supplied before 

 such a break of continuity existed. As this separation of the 

 Atlantic and Pacific probably took place late in the cretaceous 

 period, and was perhaps not completed till the middle tertiary, 

 we shall naturally expect to find the marine fauna of the earlier 

 geological periods of the Old and the New World to be very 

 similar, and consisting of many identical species. These older 

 faunae flourished on the shores and continental shelves which 

 were washed either by the equatorial currents, or by branches 

 extending- both north and south along* the then existing" con- 

 tinents and continental islands ; and where we now find rich 

 fossiliferous deposits we may feel assured that the beds at the 

 time of their formation were either formed along a continental 

 shelf, or lay in the track of a j^rimary or a secondary marine 

 current, which supplied an abundance of pelagic food indirectly 

 necessary for the support of any rich marine fauna. 



