The Sense of Sight in Spiders 297 



that the four pairs of eyes form four pairs of images differing from 

 each other in size. As a rule we shall find that the largest eyes 

 form the largest images. The question at once occurs, are all 

 eyes equally sensitive notwithstanding that they form images 

 differing in size, or are the larger eyes more sensitive than the 

 smaller ones .^ The surest way to find the answer to this question 

 is to determine the minimum angle of vision for each eye. But 

 how are we to do this when we do not even know with sufficient 

 exactitude the distance at which under normal conditions a spider 

 recognizes another of the same species. The experiments of the 

 Peckhams, in spite of their ingenuity, still admit of too great 

 range for error, to be utilized in a study of the normal angle of 

 vision. Of what advantage, then, would be a similar experiment 

 but with some of the eyes blackened with paint ^ It could serve 

 merely to control another method, a method of comparative 

 morphology. We have to start from the proposition that the 

 physiology of the nervous system is analogous in the other animals 

 and man, a proposition which few are disposed to admit, but here 

 the experience gathered in many fields and from observations 

 made on different animals, comes to our aid, an experience that 

 leaves scarcely any room for doubt that the stimulation of a single 

 nerve-ending transmits to the central nervous system a single 

 sensation only, whether or not the stimulus itself is a simple one 

 or in reality composed of many contemporaneous stimuli. Thus, 

 as is well known, in order to perceive two pin-pricks as two dis- 

 tinct sensations, it is necessary that they should be applied to 

 two separate nerve-endings as otherwise the sensation is that of a 

 single prick and again, when the image of two stars falls on only 

 one cone of the retina of the unaided human eye, the eye perceives 

 but a single star. These are well known facts which justify us 

 in saying that in the spider's eye two rods must be stimulated by 

 light rays in order that the image of two points should be produced. 

 But here the analogy ends. How strong the effect produced and 

 whether the corresponding image in the brain is of the same kind 

 as in man, we cannot know. We cannot know whether a spider 

 sees colors as we do, whether green appears to it in the same way 

 as it does to us, although we do know from the experiments of 



