THE AQUATIC VERTEBRATES 1055 



light as those with eyes, and the entire skin seems equally sensitive 

 to light. The sight of the nocturnal fishes is worse than that of 

 owls, and their eyes are but little used. The sight of the posi- 

 tively heliotropic fishes, on the other hand, is good, their eyes are 

 large and they depend on their eyesight for food. They capture 

 living food and are frequently pelagic in habit. Small-mouthed 

 fishes depending on their eyes for food will not take food that is 

 at rest. Small fragments of meat falling through the water will 

 readily be seized but will not be picked up from the bottom. The 

 great variety of artificial flies and gyrating baits are man's adapta- 

 tions to the fact that some large-mouthed fishes also select their 

 food by sight. 



Depth and the Bottom, and Adjustments to Them. No system- 

 atic study of the bottoms of our lakes has been made and it is 

 hence unknown how extensive the abysmal fauna is. In fishes 

 ranging in deep water the adjustment is probably due not so much 

 to the depth itself, as to the things that go with depth. Pressure 

 increases one atmosphere with each thirty feet in depth, plants 

 disappear beyond a few feet, and with the plants necessarily disap- 

 pear all the animals (fish food for the most part), that are associ- 

 ated with the plants. 



The character of the bottom is not a simple element like tem- 

 perature, light, or current. There is a graduation from mud to 

 gravel and rock and each of these may be weed-covered or bare. 

 But whether the bottom is mud, gravel, or rock depends on cur- 

 rent. That certain species are found principally on one bottom or 

 another is certain, but that the adjustment is to the character of 

 the bottom and not to the current and food that go with it is 

 doubtful. 



The Biological Environment and Adjustment to It. Food is the 

 controlling factor in the local distribution of fishes within any 

 unit, as chemical composition and temperature are controlling 

 factors in the geographical distribution among the different 

 units. Food itself is dependent on other food and this ultimately 

 on depth, nature of bottom, current, and the other elements of the 

 physical environment. For the most part the food of the young is 

 essentially different from the food of the adult of the same species, 



