200 PROCEEDINGS OF THE AMERICAN ACADEMY. 



while the Salientia are positive. Such reactions are easily conceived 

 to be of benefit to the different species under their ordinary conditions 

 of environment, but whether the different types of reactions have 

 arisen as the result of natural selection in the development of each 

 species, or whether they are due to structural peculiarities which limit 

 each species to certain stereotyped reactions and have hence caused 

 it to frequent a particular habitat, or whether they have been brought 

 about by other factors, are open questions. The negatively photo- 

 tropic reactions of the nocturnal species would serve to bring them into 

 places of concealment during the day. The positive reactions of the 

 more diurnal forms would lead them toward the water (a large illum- 

 inated area) and thus facilitate their escape from pursuing enemies, or 

 would take them into the bright sunlight, where insests were abun- 

 dant and their hunger would be satisfied. 



Under artificial conditions light has been shown to have a directive 

 influence on the movements of all the amphibians which have been 

 made the subject of experiment, but it does not follow that the pres- 

 ence of light will induce motor reactions in all these species, and there 

 is, in fact, great variation between the different forms in this respect. 

 For example, Cryptobranchus is strongly photokinetic and becomes 

 restless when suddenly illuminated, while Necturus is comparatively 

 indifferent to such stimulation. This photokinetic quality is appar- 

 ently little developed in frogs and toads, though they are strongly 

 phototropic. Generally speaking, there seems to be no correlation 

 between the photokinesis and the phototropism of amphibians. 



A given individual of any species is seldom consistently positive or 

 negative in its phototropism, even when the conditions of light stimu- 

 lation are uniform. This may be due to the influence of internal 

 factors which bring about changes in the physiological state of the 

 animal, or to external stimuli other than light which exert a modifying 

 influence. Some of these modifying factors will be briefly considered, 

 as far as they apply to the amphibians. Broadly speaking, the habits 

 of the different forms are correlated with their phototropic responses 

 and the species which are most truly terrestrial (Bufo americaiius and 

 Rana sylvatica) are most strongly positive, while the typical aquatic 

 forms (Cryptobranchus allegheniensis and Necturus maculosus) are as 

 decidedly negative. Therefore any variation from the conditions 

 found in the normal habitat of a species might involve changes which 

 would alter its ordinary phototropic responses. Previous exposure in 

 light or dark does not usually exert a marked influence on the photic 

 reactions of the toad, but some individuals were found to be positive 

 after having been in the light, though they were negative after passing 



