Spatially Determined Reactions 201 



two retinas, there appears to be a secondary fovea in each 

 eye, so placed as to suggest that it serves binocular vision, 

 while the primary fovea is used for monocular vision. Con- 

 vergence, the turning of the eyes toward each other to bring 

 the two images of an object on the central part of the retinas, 

 which is an important aid to human estimations of distance, 

 is also necessarily lacking in animals without binocular 

 vision. A third factor in our own perceptions of distance, 

 the accommodation of the crystalline lens, that is, the altera- 

 tion of its convexity through the pull of the accommodation 

 muscle to enable it to focus objects at different distances, 

 has been carefully studied in connection with the lower 

 animals by Beer. \ Through experiments on the refractive 

 powers of eyes dissected from the dead animal, he reached the 

 conclusion that no invertebrates but cephalopods have the 

 power of accommodation.^ It is rudimentary or lacking also in 

 some members of the fish, lizard, crocodile, snake, and mam- 

 mal families. In cephalopods, fishes, amphibians, and most 

 reptiles, the process of accommodation does not involve a 

 change in the form of the lens, but an alteration in the dis- 

 tance between the lens and the retina. The device of in- 

 creasing the curvature of the lens for vision of near objects 

 appears first in certain snakes, and is found throughout the 

 higher vertebrates (18). 



Where accommodation does not exist, as in most inverte- 

 brates, it is possible to trace other arrangements for adapting 

 vision to the distance of the object seen. Thus in com- 

 pound eyes, part of the eye may be adapted to near vision and 

 part to far vision. This is suggested by the fact that some 

 of the little tubes, or ommatidea, of which the compound eye 

 is composed, diverge from each other by a less angle than 

 others, indicating that they are suited to the reception of more 

 nearly parallel rays. In insects with both simple and com- 



