PEARSE. — THE REACTIONS OF AMPHIBIANS TO LIGHT. 201 



a similar period in the dark. Mechanical stimulation serves to initiate 

 reactions which are directed by light, but it produces no marked 

 changes in phototropism. Fatigue makes the photic responses more 

 difficult to induce in some cases (e. g. Cryptobranchus), but does not 

 alter their character. These few examples are typical and will serve 

 to illustrate the influence of many factors on the photic reactions of 

 amphibians. In general it may be said that, while various factors may 

 give rise to changed phototropic responses in some individuals, the 

 same factors may be without apparent influence in others. No stimu- 

 lus, with the possible exception of decreased temperature (Torelle, 

 ■ 03) has been demonstrated to produce uniform changes in the light 

 responses of amphibians. The internal causes which produce negative 

 reactions in one species, or even in one individual of a species, while 

 the same external conditions call forth positive reactions in other 

 species or individuals, is practically an untouched field as far as the 

 amphibians are concerned. The careful study of such a form as Die- 

 myctylus, which undergoes marked changes in habitat during its life, 

 ought to throw light on at least one aspect of this matter. 



The next subject that deserves consideration is the nature of the 

 photoreceptors upon which the sensitiveness of amphibians to light 

 depends. There are at least two sets of nerve terminations which are 

 open to photic stimulation, those of the retina and those of the skin. 

 The investigation of the responses produced by light received through 

 these two sets of endings is involved in considerable difficulty, for we 

 are obliged to refer constantly to judgments formed through the 

 human eye. We are able to form opinions as to the direction, inten- 

 sity and color of light, and to judge the form, size, color, position, and 

 movement of illuminated objects as they appear through our own eyes, 

 but we have no conception of how these things appear when they are 

 seen through the eyes of an amphibian, except as we can interpret its 

 actions, and the problem becomes even more difficult when we attempt 

 to consider the reception of light through the skin. There is some 

 evidence that nervous connections exist in amphibians between these 

 two kinds of photoreceptors and this complicates the matter still 

 farther. Englemann ('85) observed that retinal changes were induced 

 in the eyes of frogs by illuminating the skin. Furthermore, Fick ('90) 

 found that the same changes took place after the optic nerves had been 

 cut, and connections, if they exist, must therefore take some other 

 course, in part at least, than that through the second nerve. 



The eyes of amphibians are adapted for use in both air and water, 

 and are hence not finely adjusted for visual discrimination in either 

 medium. Binocular vision cannot be present, as the eyes are placed 



