DISTRIBUTION OF FEESHWATEE FISHES. 213 



continents of Africa, South America, and Australia, and their 

 present distribution is evidently the consequence of their 

 wide range in palfeozoic and secondary epochs ; the proof of 

 their high antiquity can be found in their fossil remains only. 

 For, though the Siluroids have a still greater range, their wide 

 distribution is of comparatively recent date, as the few fossil 

 remains that have been found belong to the tertiary epoch. 

 The rapidity of dispersal of a type depends entirely on its 

 facility to accommodate itself to a variety of physical condi- 

 tions, and on the degree of vitality by which it is enabled 

 to survive more or less sudden changes under unfavourable 

 conditions ; proof of this is afforded by the family of Silu- 

 roids, many of which can suspend for some time the energy 

 of their respiratory functions, and readily survive a change of 

 water. 



To trace the geological sequence of the distribution of an 

 ichthyic type, and to recognise the various laws which have 

 governed, and are still governing its dispersal, is one of the 

 ultimate tasks of Ichthyology. But the endeavour to establish 

 by means of our present fragmentary geological knowledge 

 the divisions of the fauna of the globe, leads us into a maze 

 of conflicting evidence ; or, as Mr. Wallace truly observes, 

 " any attempt to exhibit the regions of former geological ages 

 in combination with those of our own period must lead to 

 confusion." Nevertheless, as the different types of animals 

 found at the present day within a particular area have 

 made their appearance therein at distant periods, we should 

 endeavour to decide as far as we can, in an account of 

 the several zoo-geographical divisions, the following ques- 

 tions : — 



1. Which of the fishes of an area should be considered to 

 be the remnailts of ancierut types, probably spread over much 

 larger areas in preceding epochs ? 



