182 'Wisco7ism Academy of Sciences, Arts, and Letters. 



stated, of some of the Crustacea : in some of these it is difficult 

 to make a specific distinction between the fresh-water and 

 marine forms. Most of the environmental conditions in fresh 

 water are so different, however, from those in the sea, that we 

 should expect a fauna to develop itself which would differ 

 widelv from its ancestors. 



It will be noticed that the most pronounced likeness to marine 

 animals, perhaps, is found in the abyssal forms. So far as this 

 is true, it mav be explained, I think, bv the uniformity of condi- 

 tions 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, except 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 com- 

 munication with salt water, and surviving there after the lakes 

 were cut off from tlie sea, and tlioir waters had become fresh, 

 we can see how the animals could gradually adapt themselves 

 to their surroundings, inasmucli as the conditions o:^ light, tem- 

 perature, and food supply would remain with very little change. 



With the limnetic and littoral fauna, however, a very differ- 

 ent condition exists. In our climate the temperature of the sur- 

 face varies during the year from the freezing point to ei^rhty 

 degrees or more Fahrenheit. In shallow lakes, not only is there 

 this variation of the surface, but the lake mav be frozen to the 

 bottom in winter, so that all forms which can not go into a rest- 

 ing stage of some kind are destroyed. The conditions of life are 

 hard, and especial fitness is required in order to make survival 

 possible. In the sea, on the other hand, the conditions 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 generally they are very different from those 

 of the sea. It is perhaps more strange that so many resem- 

 blances remain, and that the forms are so varied as they are. 



To trace out the connection of the individual forms with their 

 marine ancestors is, of course, the work of the specialist in zool- 



