FRESH-WATER ANIMALS 79 



glance at a map of the world will show that the 

 area covered by the ocean is much greater than 

 that occupied by land the proportion being about 

 three to one. Moreover, only a very small por- 

 tion of the land is taken up by rivers and ponds ; 

 so that the area over which fresh-water forms can 

 exist is necessarily much more restricted than that 

 inhabited by marine animals. 



Again, geologists show us that the land is sub- 

 ject to constant change in level and in extent, 

 owing to upheaval or depression ; and these oscil- 

 lations, especially those of depression, must cause 

 great disturbance, or even local annihilation, of the 

 fresh-water fauna. By upheaval on the other 

 hand, shallow water marine areas may be cut off 

 from the sea, and converted into brackish marshes, 

 and ultimately into fresh-water ponds ; the animals 

 inhabiting them being either killed off, or else 

 adapting themselves to the altered environment, and 

 becoming converted into fresh-water forms. 



From these and other similar considerations it 

 may be concluded that fresh-water animals must, 

 with few exceptions, have been derived from marine 

 forms, and my chief object this evening is to con- 

 sider the various modifications, either in structure or 

 life-texture, which marine animals undergo in 

 consequence of becoming adapted to fresh-water 

 life. 



Why should marine animals strive to work their 

 way up rivers ? Why should they not stop in the 

 sea ? The reason is not far to seek, and is to be 

 found in that struggle for existence we hear so 



