336 DISTRIBUTION AND #TIOLOGY OF THE CRAYFISHES. 
a very ancient continental area, the oldest indigenous 
population of which, in all probability, 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 
@ 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 by far the largest and strongest 
of any of the inhabitants of freshwater, except the Verte- 
brata; and that while 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 Atye 
* 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. 
