Feb.,iS93. distribution of marine animals. 93 



though small in comparison to some other groups, is rich enough in 

 itself to allow conclusions in faunistic problems. 



The problems of the marine fauna are so varied and numerous 

 that we can discuss only a few of them in this place. For con- 

 venience, we might divide them into quantitative and qualitative 

 problems. It is about the former that the combat between Haeckel 

 and Hensen is carried on, i.e., about the uniformity of the distribution, 

 and about the foundation which is given by single captures to general 

 conclusions. It is too early as yet to enter into this question, 

 basing conclusions only on the results of single groups, but it 

 may be affirmed that everything known until now from the different 

 investigators indicates a much more equal composition of the 

 Plankton than was supposed even by Hensen himself. It is certain 

 that the influence of the coasts extends to a considerable distance, 

 that even islands cause modifications, and that currents often change 

 the aspect of the fauna almost at once ; but Hensen is the first to 

 acknowledge the importance of all these factors. 



Setting aside these quantitative problems, there remain the 

 qualitative ones, namely, the nature of the composition of the 

 fauna with regard to the different species, and the geographical 

 occurrence of the single forms. In other -words, are there in the sea as 

 as on the mainland areas of distribution with characteristic inhabitants, or 

 (though certainly the coast has zones of life) are the peculiarly Plankton 

 forms universally distributed in it ? We know for certain that there are 

 forms of life peculiar to the open sea, Plankton animals, par 

 excellence, which are cosmopolitan, and which occur both in the 

 Atlantic and in the Pacific or Indian Ocean, and in very different 

 latitudes. At the last Congress of the German Zoological Society, 

 von Graff exhibited such cosmopolitan forms of the group Turbel- 

 laria, and von Martens, Spengel, Chun, and others seconded him by 

 relating similar facts in the Molluscs, Tornarias, Siphonophores, &c., 

 and this led to an interesting discussion as to the probable continuity 

 of the two oceans in former ages. Among the Medusae we meet with 

 species the distinguishing characters of which are so insignificant 

 that we should, without doubt, consider them as belonging to one 

 species had they not been found in such different regions. On the 

 other hand, we know some forms of Medusae which have been found 

 hitherto only in a certain limited district, which occur in this district 

 regularly, but which have never been seen elsewhere. 



Without impairing the fact that there are cosmopolitan forms 

 which are as regular inhabitants of the Atlantic as of the Pacific 

 Ocean, it is to be expected a priori that we should find in different 

 latitudes at least a different fauna. Since in a different latitude the 

 life-conditions undergo a marked change, and show differences in the 

 temperature, in the movement of the water, in the weather, and 

 so forth, we ought to find a different adaptation and different 

 characters. 



