112 THE WONDER OF LIFE 



It seems very likely that not a few of the freshwater 

 animals have migrated gradually from the sea and the sea- 

 shore, through estuaries and brackish water, to rivers 

 and lakes. As the possibiUty of making this transition 

 depends on the physiological constitution of the animal, 

 we can understand that similar forms would succeed in 

 different areas. And this is part of the explanation of 

 the high degree of uniformity seen in the freshwater faunas 

 of widely separated areas. The process of migration may 

 be seen going on at present in the invasion of the Kiel 

 Canal and in some similar cases. Various shrimps and 

 the like go far up certain rivers ; the flounder is found 

 many miles from the sea ; sticklebacks seem to be quite 

 capable of thriving well in either fresh water or salt ; and 

 there are hundreds of similar facts. 



(2) It has been suggested by Credner, SoUas and others 

 that some present-day lakes are dwindling remnants of 

 ancient seas — relict- seas in short. Part of an old sea 

 may become land-locked and be converted in course of 

 time into a freshwater basin. Or it may be that a present- 

 day lake which never was as such part of a sea, may be- 

 come connected with a relict- sea by alterations of land- 

 level and owe part of its fauna to that circumstance. 

 There may have been a somewhat uniform pelagic fauna 

 in the remote past, and that may be part of the explanation 

 of the uniformity of the fauna in freshwater basins widely 

 separated from one another. If the land-locked portion 

 of sea was gradually converted into a freshwater basin, 

 there would be a stern elimination of non-plastic types, 

 and since the conditions of elimination would be much 

 the same everywhere, the result would be uniformity in 

 the survivors. Mr. J. E. S. Moore has brought forward 



