of the Arctic and the Antarctic Faunas. 319 
America through the researches of the ‘ Albatross,’ but there 
still remains too much to be worked up to allow us to suggest 
reasons why a subsurface-water connexion between the 
similar forms of north and south has not been discovered 
on the western shores of tropical America. Possibly such a 
connexion may some day be established for one or other 
group of animals. If, personally, I doubt this, it is for two 
reasons. In the Panama province there is certainly no coral- 
reef formation, but there is a surface-water fauna of tropical 
character, and in times not very long gone by there was really 
coral-formation. The reasons for a more or less marked 
suppression of the subsurface-water fauna by the tropical 
surface-fauna may hold good in this case also to a greater or 
less degree. 
Secondly, corresponding to the remarkably equable climate, 
we find, on the west coast of America from the temperate 
southern to the temperate northern zone, a fauna of nearly 
homogeneous character, interrupted only in the narrow pro- 
vince of Panama. In "general character it may be described 
as a cool-water fauna, but it has undergone quite remarkable 
local differentiation. This fauna springs, apparently, from 
the southern hemisphere ; and thus, probably in ancient days, 
possibly before the time of the separation of the faunas, 
this fauna of southern origin gradually conquered its present 
region—that is to say, it crowded out more or less the members 
of the universal fauna. 
Thirdly, Agassiz expressly mentions the poverty of the 
depths examined by him on the ‘ Albatross.” He accounts 
for this by the fact that the currents on the tropical shores 
of West America waft in a comparatively limited quantity of 
pelagic organisms, which would afford food for the inhabitants 
of the deep sea. As the animal world of the deep sea is 
wholly, and that on the slopes of the continents partly, 
dependent economically on the pelagic fauna, we have here a 
reason of great importance, and one which would apply in 
the main to West Africa also. In the same way, it must 
be taken into account that the extreme narrowness of the 
continental slope affords anywhere the opportunity for an 
interruption of faunal continuity. 
I cannot omit to refer here to a fact which may be brought 
carried out by the great African rivers.” Probably the bottom of the 
continental slope shows these river-deposits in a still higher degree, so 
that we may havea similar state of affairs as, for instance, on the east coast 
of temperate South America, where the mouths of the Rio Negro and 
Rio La Plata form barriers between the South Brazilian and the Pata- 
gonian littoral and continental faunas. 
22* 
