MINOR FAUNAS. 757 



We may distinguish a littoral, a surface, and a deep- 

 water lacustrine fauna. The deep-water forms are chiefly 

 Rhizopods, Turbellarians, Nematodes, Leeches, Chaetopods, 

 Amphipods, Isopods, Entomostraca, a few Arachnids, some 

 insect larvae, and molluscs, and the general opinion is that 

 these are derivable from the shore-fauna of the lake, which 

 includes similar forms, along with a few others, such as the 

 fresh-water sponge and Hydra. On the other hand, the 

 surface lacustrine fauna, consisting of water-fleas, Rotifers, 

 Infusorians, &c., widely and uniformly distributed, is said 

 not to be derivable from the shore forms. In trans- 

 parency, in gregariousness, in nocturnal habit, and in other 

 ways they present a marked analogy with the marine 

 Plankton. How are we to account for their origin and 

 wide distribution ? 



(i.) To explain the uniformity Darwin referred to the birds which 

 carry organisms from watershed to watershed, to the carrying power of 

 the wind, and to changes of land level which bring different river beds 

 into communication. But this is not enough. 



(2.) It seems very likely that some of the fresh water forms have 



migrated from the sea and seashore through brackish water to rivers 



and lakes. As the possibility of making the transition depends on the 



constitution of the animal, it is intelligible that similar forms should 



, succeed in different areas. 



(3.) There seems much force in what Credner and Sollas emphasise 

 that many lakes are dwindling relict-seas of ancient origin. Granted a 

 fairly uniform pelagic fauna, e.g., before Cretaceous times, we can 

 understand that the conversion of land-locked seas into lakes would 

 imply a decimating elimination, and, as the conditions of elimination 

 would be much the same everywhere, the result would be uniformity 

 in the survivors. 



Minor Faunas. 



(a. ) Of Brackish Water. We are warranted in speaking of a 

 brackish-water fauna, because of its uniformity in widely separated 

 regions. It does not seem to be a mere physiological assemblage, vary- 

 ing in each locality, but rather a transition fauna of ancient date, a relic 

 of a littoral fauna once more uniform. The fact is that the power to 

 live in brackish water is not very common ; it runs in families. 



(b. ) Cave fauna. In America, thanks very largely to the labours of 

 Packard, about 100 cave animals are known ; in Europe the number 

 is about 300, the increase being largely due to the occurrence of about 

 100 species of two genera of beetles in European caves. In the famous 

 Mammoth Cave of Kentucky, which has over 100 miles of passages, 

 with streams, pools, and dry ground, there are over 40 different species 



