March 9, 1900.] 



SCIENCE. 



387 



another by connecting streams : in this 

 way animals or plants introduced at the 

 source of a river may be carried through its 

 whole length. 



From lake to lake, too, seeds, eggs and 

 living animals are carried by water fowl at- 

 tached 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 uni- 

 formity 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 migration between contig- 

 uous bodies of water, and partly by the aid 

 of the winds, currents and water fowls 

 The limnetic fauna and flora is descended 

 either from littoral forms which have grad- 

 ually adapted themselves to limnetic condi- 

 tions, 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 part of the abyssal 

 fauna is descended from littoral forms which 

 have gradually moved into deep water, and 

 have been modified to suit their new en- 



vironment. All the abyssal fauna of the 

 Swiss lakes is supposed to be of this char- 

 acter. 



What I have said thus far applies almost 

 exclusively to lakes of the temperate zones, 

 for it is there that lakes exist in the greatest 

 numbers, and it is upon such lakes that 

 most of the work of investigation has been 

 done. But there are lakes in warmer cli- 

 mates, and we may expect that a thorough 

 study of them will give us much that is 

 new and interesting. A striking example 

 of the extraordinary interest that may be 

 attached to such lakes is Lake Tanganyika 

 in Africa. Some years ago it was reported 

 that a jelly fish was abundant in its waters. 

 This excited the curiosity of zoologists, for 

 the medusa is a marine form, and very rarely 

 is found in fresh water, the most noted case 

 being of the one found in the basin in Re- 

 gent's Park, which is supposed to have been 

 brought with plants from some tropical 

 country. The medusa of Tanganyika is one 

 of four jelly fish known to live in fresh 

 water, and the other examples are very 

 rare. A special expedition was organized 

 to make an exploration of Lake Tangan- 

 yika, and although this work was very im- 

 perfectly done, the results appear to be of 

 great interest. Along with the ordinary 

 lacustrine fresh- water fauna there is a fauna 

 of marine origin, but this marine fauna is not 

 closely related to modern forms. It does, 

 apparently, closely resemble Jurassic forms. 

 Indeed, it is said that were some of the forms 

 referred to a paleontologist, he would not 

 hesitate to say that they belonged to Juras- 

 sic times. Have we here, then, a ' fauna 

 relicta' which dates back to Jurassic times? 

 It is too early to answer this question with 

 any certainty, both because of our imper- 

 fect knowledge of the fauna of Lake Tan- 

 ganyika, and because of our great ignorance 

 of the geology of that part of Africa. But 

 the mere possibility that this may be true 

 is startling, and should incite scientists to a 



