336 DISTRIBUTION AND AETIOLOGY OF THE CRAYFISHES. 



a veiy ancient continental area, the oldest indigenous 

 population of which, in all j)i'ob ability, is directly de- 

 scended from that which occupied it at the beginning 

 of the tertiary epoch. If Parastacine Crustacea inhabited 

 the southern hemisphere at this period, and subsequently 

 became extinct as marine animals, their preservation in 

 the freshwaters of Australia, New Zealand, and the older 

 portions of South America may be understood. The 

 difficulty of the absence of crayfishes in South Africa * 

 remains ; and all that can be said is, that it is a difficulty 

 of the same nature as that which confronts us when we 

 compare the fauna of South Africa in general with that 

 of Madagascar. The population of the latter region has 

 a more ancient aspect than that of the former; and it 

 may be that South Africa, in its present shape, is of very 

 much later date than Madagascar. 



With respect to the second point for consideration, it 

 is to be remarked that, in the temperate regions of the 

 world, the crayfishes are b}^ far the largest and strongest 

 of any of the inhabitants of freshwater, except the Verte- 

 hrata ; and that Avhile frogs and the like fall an easy prey 

 to them, they must be formidable enemies and com- 

 petitors even to fishes, aquatic reptiles, and the smaller 

 aquatic mammals. In warm climates, however, not only 

 the large prawns which have been mentioned, but Atyce 



* But it must be remembered that we have as yet everything^ to learn 

 respecting the fauna of the great inland lakes and river systems of 

 South Africa. 



