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SCIENCE. 



[N. S. Vol. XI. No. 271. 



shore. The explanation seems to be this : 

 because of its slight depth the environment, 

 even far from shore, is favorable to the 

 growth of littoral forms. Then there is good 

 reason for thinking that the winds have a 

 profound effect on its waters, thus thor- 

 oughly mixing limnetic and littoral waters, 

 and consequently causing a similar ming- 

 ling of the organisms. Lake Winnebago 

 has the characteristics, in many respects, 

 of an enormously overgrown puddle. I do 

 not say this, however, to show a lack of 

 respect for this lake, for from a practical 

 standpoint it is a most valuable possession 

 to the state. It can support, and does sup- 

 port, an enormous number of fish. Few 

 lakes can compare with it in productiveness. 



All the inhabitants of fresh water are, of 

 course, descendants of marine forms. In 

 some cases the modifications have been very 

 great but in others they are hardly to be 

 distinguished from their salt-water rela- 

 tives. This is true, as has already been 

 stated, of some of the Crustacea : in some of 

 these it is difiicult to make a specific dis- 

 tinction between the fresh-water and marine 

 forms. Most of the environmental condi- 

 tions in fresh water are so different, how- 

 ever, from those in the sea, that we should 

 expect a fauna to develop itself which 

 would differ widely from its ancestors. 



It will be noticed that the most pro- 

 nounced likeness to marine animals, per- 

 haps, is found in the abyssal forms. So far 

 as that is true, it may be explained, I think, 

 by the uniformity of conditions existing in 

 the depths of lakes. The temperature 

 varies but a few degrees from one end of the 

 year to another, and such currents as exist 

 are slow and almost imperceptible. The 

 abyssal fauna of a lake is subject to nearly 

 the same conditions as that of the sea, ex- 

 cept for the difference in the composition of 

 the water. If, as has been supposed, the 

 deep-water fauna of the Scandinavian lakes 

 is descended directly from the deep-water 



fauna of the sea, coming from the sea into 

 lakes having a communication with salt 

 water, and surviving there after the lakes 

 were cut off from the sea, and their waters 

 had become fresh, we can see how the 

 animals could gradually adapt themselves 

 to their surroundings, inasmuch as the con- 

 ditions of light, temperature and food sup- 

 ply would remain with very little change. 



With the limnetic and littoral fauna, how- 

 ever, a very different condition exists. In 

 our climate the temperature of the surface 

 varies during the year from the freezing 

 point to eighty degrees or more Fahrenheit. 

 In shallow lakes, not only is there this va- 

 riation of the surface, but the lake may be 

 frozen to the bottom in winter, so that all 

 forms which can not go into a resting stage 

 of some kind are destroyed. The conditions 

 of life are hard, and especial fitness is re- 

 quired in order to make survival possible. 

 In the sea, on the other hand, the con- 

 ditions even of the littoral and pelagic fauna 

 and flora are much more uniform. It is 

 not strange that the fresh-water animals 

 and plants are of few kinds, and that gen- 

 erally they are very different from those of 

 the sea. It is perhaps more strange that 

 so many resemblances remain, and that the 

 forms are so varied as they are. 



To trace out the connection of the indi- 

 vidual forms with their marine ancestors 

 is, of course, the work of the specialists in 

 zoology and botany. It may be noticed, 

 however, that the present population 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 bodj' of water directly 

 to another, we may assume that the pres- 

 ent 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 



