412 PROCEEDINGS OF THE AMERICAN ACADEMY. 



and " vision " would not cover the responses to light perceived by the 

 integument, and it would perhaps be better to employ for the whole 

 series the terms "photo-reception" and "photo-reaction," which have 

 recently been proposed (see Beer, Bethe u. Uexkiill, '99, and Nuel, :04). 



Although, as has been said, this classification cannot be taken as 

 representing a genetic series, nevertheless it does indicate in a rough 

 way the steps which have probably led up to the possession of the 

 highest type of vision. The classes as here established are far from 

 distinct, and especially is this true of the last two, which necessarily 

 must depend in part upon each other. However, the form of response 

 outlined under Type G is in general more primitive than that under 

 D. It depends upon a definite phototropic reaction, and as more 

 acute vision is gained phototropism becomes inhibited more and more 

 until it apparently disappears. 



No experiments were made to test color perception and its relation 

 to image-formation. This is an extremely difficult field, since it is. 

 well-nigh impossible to get an objective criterion as to whether animals 

 perceive color as it is interpreted by the human eye, or whether colors 

 represent to them merely differences in light intensity. 



V. Summary. 



In the study of the reactions of certain animals to two lights of 

 different areas, the one 41 cm. square, the other only one ten thousandth 

 as much (for all practical purposes a point), the two lights were always 

 adjusted so that they gave an equal intensity of light at a plane mid- 

 way between them — the plane of experimentation. This intensity 

 varied from about 5 candle meters to 1.25 candle meters. A summary 

 of the principal results obtained is as follows : 



1. The earthworm {Allolobopkora foetida) was negative in its re- 

 sponse to either of the two lights used separately ; to the two lights 

 used simultaneously it was indifferent, turning almost exactly as 

 many times in one direction as in the other. Allolobophora therefore 

 apparently responds only to the intenxttn of the light. 



2. Bipallum keivense was similarly negative to either light used 

 singly. To the two lights operating at the same time it was nearly 

 indifferent, but showed a slightly larger number of turnings away from 

 the larger light ; this may indicate a slight ability to discriminate 

 between the lights, and if so, it is probably owing to the arrangement 

 of the eyes around the periphery of the semicircular head. 



3. The mealworm (larva of Tenehrio molitor) was decidedly negative 

 in its reactions to the lights employed separately. As inferred from its 

 reactions to the two lights acting simultaneously, — the turnings to 



