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 
faune 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 primary or a secondary marine 
current, which supplied an abundance of pelagic food indirectly 
necessary for the support of any rich marine fauna. 
