486 



SLONAKER. 



[Vol. XIII, 



diameter in the mammals. They are about the same length in 

 mammals, birds, and amphibians, while in fishes they are 

 shorter. In birds the diameter of the cones approaches very 

 closely to that found in the reptiles. The following tabulation 

 of measurements compiled from Miiller's descriptions (20) of 

 the rods and cones of different animals will make clear these 

 relations. These measurements are in millimeters. 



The diameter of the rods and cones is of great importance 

 when the sensitiveness of the retina of different animals is con- 

 sidered. Since these sensitive elements always lie as closely 

 together as possible, the animals in which their diameter is 

 small would have more per given area, hence a more sensitive 

 retina. 



Another important difference is the relative number of rods 

 and cones. In mammals and amphibians the rods far surpass 

 the cones in number. In birds the reverse is true, while in 

 reptiles few or no rods are found. In fishes the rods and cones 

 are more equally divided. A few exceptions to this are of 

 great importance in substantiating the theories of the functions 

 of the rods and cones. It has been stated (45) that in the bat 

 and mole there are no cones in the yellow spot and in the 

 rabbit only a few. The same is true of other nocturnal mam- 

 mals which I have examined. I have not been able to demon- 

 strate the presence of cones in the mink, skunk, or rat, while 

 they are present in the squirrels. In the night birds and in 

 the eel very few or no cones have been demonstrated. This 

 accords completely with the theory that the rods function in 



