Marsh — The Planl-lon of Fresh Water Lahes. 183 



ogy and botany. It may be noticed, however, tliat the present 

 popuhition of our lakes has come since the glacial period, in fact 

 the lakes themselves only date from that period. So far, then, 

 as the fauna and flora pass from one body of water directly to 

 another, we may assume that the present animals and plants 

 are descended from those that were pushed south by the ice, and 

 that as the ice retreated they followed again towards the north. 



Currents carry organisms from one part of a lake to another, 

 and from one lake to another by connecting streams : in this way 

 animals or plants introduced at the source of a river may be car- 

 ried through its whole length. 



From lake to lake, too, seeds, eggs, and living animals are car- 

 ried by water fowl attached to their feathers or in the mud upon 

 their feet. This is not simply from one lake to its neighbor, but 

 many of these birds take long flights before alighting, so that 

 the organisms are scattered over a wide stretch of territory. It 

 is in this way, probably, that we can account for the uniformity 

 in the fauna and flora of the lakes and the wide distribution of 

 some of the forms. Where conditions are similar, then, we may 

 expect likeness in the fauna and flora. As we have seen already, 

 temperature is the great controlling factor in distribution, so 

 that in lakes of the same latitude or the same elevation, other 

 conditions being equal, of which the principal is depth, we may 

 expect close similarity in fauna and flora. 



We may assume, then, that the littoral fauna and flora have 

 had their origin from neighboring bodies of water, and that as 

 the ice retreated, the lakes were populated, partly by direct mi- 

 gration between contiguous bodies of water, and partly by the 

 aid of the winds, currents, and water fowl. The limnetic fauna 

 and flora is descended either from littoral forms which have 

 gradually adapted themselves to limnetic conditions, or from 

 pelagic forms, which, in bays where the water was less salt or 

 brackish, have become adapted to the conditions of fresh water, 

 and have been distributed by the same agencies as the littoral 

 forms. 



Part of the abyssal fauna is descended from marine forms 

 directly, as in the "fauna relicta" of the Scandinavian lakes, and 

 in the case of some of the animals in our Great Lakes. Another 



