DISTRIBUTION OF FRESHWATER FISHES. 215 



longitude, and that a main division into an old world and 

 new world fauna is untenable. More especially the Fresh- 

 water fishes, with which we are here solely concerned, have 

 been spread in circum'polar zones, and in a but limited degree 

 from north to south. No family, much less a genus, ranges 

 from the north to the south, whilst a number of families 

 and genera make the entire circuit, and some species more 

 than half of the circuit round the globe within the zone to 

 which they belong. Not even the Cyprinoids and Siluroids, 

 which . are most characteristic of the freshwater fauna of our 

 period, are an exception to this. Temperature and climate, 

 indeed, are the principal factors by which the character of the 

 freshwater fauna is determined ; they form the barriers which 

 interfere with the unlimited dispersal of an ichthyic type, 

 much more than mountain ranges, deserts, or oceans. Hence 

 the tropical zone is an impassable barrier to the northern 

 Freshwater fish in its progress towards the south ; where a 

 similarly temperate climate obtains in the southern hemi- 

 sphere, fish-forms appear analogous to those of the north, but 

 genetically and structurally distinct. 



The similarity which obtains in fishes at somewhat distant 

 points of the same degree of longitude, rarely extends far, and 

 is due to the natural tendency of every animal to spread as 

 far as physical conditions will permit. Between two regions 

 situated north and south of each other there is always a 

 debateable border ground, in parts of which sometimes the 

 fishes of the one, sometimes those of the other, predominate, 

 and which is, in fact, a hand of demarcation. Within this 

 band the regions overlap each other ; therefore, their border 

 lines are rarely identical, and should be determined by the 

 northern and southernmost extent of the most character- 

 istic types of each region. Thus, for instance, in China, 

 a broad band intervenes between temperate and tropical 

 Asia, in which these two faunae mix, and the actual northern 



