Freshwater Crustacea of the Indian Archipelago. 241 



local species in varying numbers and variety, or, at least, 

 such as have a more restricted area of distribution. To these 

 forms we will apply the designation " regional " freshwater 

 animals. The question now arises whence these latter are 

 derived, and how their more limited distribution is to be 

 explained. 



In connexion with this question various considerations 

 have to be taken into account, which at present render it 

 impossible to return a comprehensive answer. For instance, 

 it is difficult to account for the fact that Apus and Branchipus 

 are absent from the Indo-Malayan region, although the eggs 

 of both genera are so exceptionally constituted for being 

 carried about from place to place. 



It is above all among Vertebrates, Mollusks, and Mala- 

 costracous Crustacea that the forms occur which are not 

 universally distributed. I need but refer to Urodeles, which 

 with the exception of the Ccecilians are absent from the 

 tropics and the southern hemisphere; or to the regional 

 occurrence of Ganoids, Dipnoi, Siluroids, Labyrinthici, and 

 other freshwater fish. Allusion may also be made to the 

 Astacidse, Palasmonidas, and freshwater Brachyura, while the 

 absence of Asellidse and Gammaridas in the Indo-Malayan 

 region may likewise be pointed out. 



Now Vertebrates, Mollusks, and the Crustaceans mentioned 

 are all animals of some size, which for this reason alone could 

 not or could hardly be carried about. The same applies to 

 their eggs, which are not suited for passive dissemination. 

 Herein we already have a conspicuous reason for their to a 

 certain extent emphatically regional distribution. 



Here and there we find the idea expressed that the above- 

 mentioned denizens of fresh water, to which we have applied 

 the term " universal," are of especially high geological 

 antiquity, which may be taken as a partial explanation of 

 their wide distribution. 



In many cases this may certainly be correct. I shall, 

 moreover, attempt to show directly that as a matter of fact, 

 as opposed to the more ancient forms, there are also more 

 recent freshwater animals, which evidently derive their 

 origin from the sea. But to contrast the whole of the 

 " universal " freshwater forms, as being geologically the more 

 ancient, with the " regional " ones, as geologically more 

 recent, would undoubtedly, in such general terms, be incorrect. 

 Why among universal forms Spongilla, for instance, and many 

 Entomostraca or Bryozoa are to be considered historically 

 more ancient than perhaps Apus, Branchipus, Asellus, 

 Gammarus, Dipnoi, Ganoidei, and Urodeles, it is certainly 

 impossible to perceive. Lower organization of a species is 



