312 Prof. Dr. G. Pfeffer on the Mutual Rehitions 



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 thej 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. The 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 wliich 

 we know less thoroughly than those already treated of. 

 There is also a universally differentiated pelagic fauna of the 

 warmer seas. The 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 palseontologlsts who do not agree to 

 the limitation of the conception of a universal fauna which I 

 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 

 ihe 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. 



