402 PROCEEDINGS OF THE AMERICAN ACADEMY. 



it turned in most cases toward the larger area. Out of 32 trials fol- 

 lowing the first 8, 22 were in this direction. By the time the first 8 

 trials had been made the frog had been in the warm room for a half 

 hour or more, and it seemed reasonable to assume that the change 

 in its reactions was due to its having become warmed up to the room 

 temperature. Further observations tended to confirm this conclusion ; 

 and this agrees with the results obtained by Torelle (:03), who says, 

 (p. 487): "A rise in the temperature to 30° C. accelerates the rate of 

 the positive response. A lowering of the temperature to 10° C. 

 produces movements away fi'om the light." 



IV. General Considerations and Discussion. 



Conclusions as to the degree of the ability possessed by the vari- 

 ous animals studied to form images, as inferred fi-om the results of 

 the experiments, have been stated in connection with the description 

 of the experiments upon each animal employed. It still remains to 

 consider certain general phases of the subject : to examine the rela- 

 tionship existing between the natural habits of the animals and their 

 reactions to the two lights ; to determine, in so far as possible, what 

 part their phototropism — especially in its relation to luminous areas 

 of different sizes — plays in their ordinary activities ; and to attempt 

 to trace, as well as can be done with the limited data at hand, the 

 probable steps which have led to the development of highly specialized 

 organs capable of forming richly detailed images of external objects. 



It is a noticeable fact that those forms which showed by their re- 

 actions the most evidence of discrimination between the two lights 

 (viz., Vanessa, Ranatra, and the two species of frogs) are all positive 

 in their ordinary phototropic responses. On the other hand, the 

 earthworm, the land planarian, the mealworm, the sow bug, and the 

 cockroach — all of which showed little or no ability to discriminate 

 between the different areas, but responded almost entirely according 

 to the relative intensities of the lights — are all ordinarily negative- 

 When one remembers that all the animals in the second group live 

 either in the ground, or beneath stones, logs, or similar objects, or in 

 other dark places, the nature of their responses is not surprising. For, 

 whatever view may be held as to the way in which adaptation may 

 have come about, it is undeniable that organisms are adapted to their 

 ordinary conditions of life ; consequently it would not be expected that 

 those which live in darkness, or in very dim light, would be so likely 

 to be provided with image-forming organs, since for these they would 

 have little or no use. The ability to distinguish light from darkness is^ 



