1897] DISTRIBUTION OF PELAGIC FORAMINIFERA 21 
regions Globigerina dutertrei and pachyderma, together with very 
minute specimens of Globigerina bulloides, appear to be the only forms 
present in the surface waters. The gradual disappearance of the 
tropical species, and their replacement by other species, as the colder 
water to the north and south of the equatorial regions is entered, has 
Fic. 2,—Orbulina universa (d’Orbigny), from the surface. 
always appeared to me rather puzzling, especially when it is re- 
membered that these changes take place in a continuous oceanic 
current, like the Gulf Stream, flowing from the equator towards the 
poles. It sometimes seemed as if the one form slowly passed into 
the other with the changed conditions of surface temperature. 
The same species inhabit all the great oceans, but in the Indian 
and Pacific Oceans certain species appear to predominate, for 
