474 Evolution and Distribution of Fishes 



A partial explanation of the above difference is doubt- 

 less to be found in the earlier occupation of S. America by 

 evolving members of the above families. But much more 

 we believe, depends on the great changes that took place 

 in the Central African region after these fishes had reached 

 parts of it. And to these we can now give attention. 



If then, over the mid-African area there existed in 

 early Eocene times fairly extensive systems of swamps, 

 lakes, and connecting rivers with their streams, such as 

 largely characterize wide sections of It and other continents 

 still, these would receive and shelter a varied freshwater 

 fauna of greater or less specific richness. If we may judge 

 from parallel cases of Brazil, of the east central United 

 States and of some Indian localities, this fauna would con- 

 sist of genera and species of Infusoria that showed varied 

 structure and habits, and from one of which has evolved 

 Trichodina pedicidiis already mentioned (p. 465) that 

 established parasitic relation, and spread over African, 

 Indian and possibly over Brazilian territory. Freshwater 

 sponges, of ancient origin, would occur over shells, rocks, 

 or sunken lumber. These, with advancing investigation, 

 are now known to represent not a few world-wide species, 

 but numerous specific variation-forms, that are for some 

 species of wide distribution, for others of localized endemic 

 nature. But all are everywhere referable to the great 

 freshwater class Spongilllnae. Of these the central African 

 region had its share. 



The freshwater medusae of Brazil, of Central Africa, 

 of India, and of the United States all belong to the Hydrlda 

 or Corynlda, that have wholly or In part fixed freshwater 

 hydrold stages, as well as the reproductive crystalline bells. 

 None have been found as marine organisms, and there is 

 absolutely no evidence, that they are marine derivatives. We 

 would readily accept however that from these simple fresh- 

 water organisms the increasingly complex Corynlds, Sertu- 

 larians, and Campanularians branched off into marine life, 

 and this at a period that was immensely antecedent to the 

 Eocene or the Cretaceous. The freshwater ancestors, and 

 the more complex marine derivatives are alike left alive 

 still in their appropriate habitats. 



