VI. Biology. 1. Dispersal of Animals. 59" 



the flora so largely dependent upon the latter ; species, genera 

 and even families found in forests, are in some respects different 

 from those met with in deserts, so that a forest-fauna, steppe-fauna, 

 or mountain-fauna, may be spoken of. 



The animal life of fresh water has a relatively non-distinctive- 

 stamp ; of the larger groups, there is scarcely one which can be 

 called a special fresh water type. This fauna is composed largely 

 of forms which are partly undoubted land types, partly marine :. 

 it is a peculiarly borrowed and mongrel collection. The numerous. 

 Pulmonata, Insects and Spiders, living in fresh water 

 are derived from the terrestrial fauna, which, however, , gives, 

 absolutely no Bird or Mammal as a constant inhabitant, although not 

 a few of these creatures spend a part, or even the most, of their 

 time there. A fair number of forms is furnished by Reptiles,, 

 most of which however, come on land now and then (Crocodiles, 

 Turtles) . Amphibia are almost all fresh water in the larval form ; 

 in the adult state many are, for the most part or entirely so. The fresh- 

 water fauna has drawn upon the sea also. Numerous Pish, whole 

 families of which belong almost exclusively to rivers and lakes, 

 a number of branchiate Gastropods, Lamellibranchs, 

 Crustacea, Chaetopods, Polyzoa, Platy helminths, 

 some Coelentera, not a few Rhizopoda, etc. A very few of 

 a mammalian group modified for a sea life, the Whales, live in 

 fresh water. Leeches, Rotifers, Infusorians, groups 

 which are fresh water as well as marine, are so common in the former 

 that they should perhaps be regarded as fresh-water types. The 

 modifications which terrestrial or marine forms have undergone by 

 the transition, are usually not very important. The land animals 

 generally remain air-breathing, and the modification is practically 

 limited to that necessitated by the difference in mode of locomotion; 

 the alterations in the marine forms are also non-essential.* 



The class Pisces, the phylum Mollusc a, the Crustacea, 

 Chsetopoda, the Polyzoa, Brachiopoda, Platyhel- 

 minthia, Echinoderma, Coelent er a, Porif er a, Rhiz o - 

 poda and Radiolaria are obviously marine groups. Of these 

 the Brachiopoda, Echinoderma, and Radiolaria exclusively, and the 

 Coelentera and Porifera with few exceptions, belong to the sea. The 

 sea has received a large contribution from the terrestrial fauna, 

 especially as regards the Vertebrata. Two orders of Mammalia 

 the Whales and the Sirenia, the former with many genera and species, 

 have adapted themselves entirely to a marine life, and their structure 

 has undergone correspondingly important modifications ; a third 

 Mammalian order, the Seals, are also distinctly marine, though they 



* It is very notewortliy that many fresh-water forms do not pass through a free- 

 STvimming larral stage, whilst their relatives in the sea do (Astaous, Lamellibranchs, 

 Oligoohsets). 



