ol2 Prof. Dr. G. Pfeffer on the Mutual Relations 
were removed, these would be extinguished, and would merge 
themselves into the general circumpolarity. 
The case is the same with the tropical surface-water fauna ; 
the faunas of the West Indies and of Panama were not 
always separate, as they are now, for in pre-Miocene times 
the West Indian overlapped that of Panama and has left its 
traces there to this day. ‘Thus we see that the absolute 
circumpolarity of the tropical surface-water fauna is. present 
but latent, and that it is exhibited as soon as a possibility of 
wider distribution arises. And if we consider aright the 
enormously wide distribution of the uniform tropical fauna 
from the east coast of Africa to the Pacific Islands, we see 
that, if the continent of Africa were to sink, or to be broken 
up into a tropical archipelago, the tropical fauna would spread 
itself over that region also, All that we learn from the 
tropical fauna goes to show that the local gradations, even 
those exhibited by West Africa and tropical West America, 
would disappear if the distribution-barriers were removed. 
And therein the “ universality” of a fauna lies—not in the 
development of an absolutely similar combination at every 
spot in its region, but in the fact that the potentiality to this 
exists, and becomes a reality as soon as the hindering causes 
disappear. ‘lhe development of local faunas in no way affects 
the existence of a contemporaneous and coextensive ‘ uni- 
versal’ fauna, 
Besides these two surface- water faunas there is a universally 
developed subsurface-water and deep-sea fauna, both of which 
we know less thoroughly than those already treated of. 
There is also a universally differentiated pelagic fauna of the 
warmer seas. ‘he works of Keller and Brandt on the Suez 
Canal and the Baltic Canal show us the rapidity with which 
the spreading of a fauna takes place in similar climatic con- 
ditions after the removal of the barriers to distribution. 
Furthermore, the forward and backward displacements of the 
northern and arctic faunas during the Glacial periods are well 
known. 
There are certainly palwontologists who do not agree to 
the limitation of the conception of a universal fauna which | 
have here proposed ; but these must remember one thing—so 
long as they look on fossils as stones they may have an 
opinion with regard to their distribution founded only on their 
observations, but as soon as they begin to see in the fossils 
the living beings of an earlier epoch they must take the 
standpoint of modern biology—that is to say, they must work 
along with biologists and rely upon the well-established 
results of biological observation. 
