204 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the light is received through the skin alone, no such potency is shown 

 by the more refrangible rays. The differences observed in the first 

 case may therefore be interpreted as being due to stimulation received 

 through the eyes, and we may conclude that the power of color per- 

 ception, as distinct from light perception, is present in the eyes but 

 absent in the skin. It is not certain, however, that these differences, 

 which are supposedly due to differences in wave length, are not, after 

 all, brought about by intensity differences. 



Generally speaking, the parts of the central nervous system are 

 segmentally arranged throughout the vertebrate series. Each neural 

 segment is, however, capable of carrying on only the comparatively 

 simple reflex actions which are concerned with the somatic segment 

 which it controls. The complex reactions which involve correlated 

 movements in different regions of the body depend upon correlation 

 centres, and, the higher we go in the vertebrate scale, the more these 

 centres become localized toward the anterior end of the nervous tube. 

 A spinal eel is able to swim in a normal manner (Bickell, '97), but in 

 the higher vertebrates spinal reactions show less correlative power, and 

 there is a correspondingly greater importance attached to those reac- 

 tions which are controlled through the brain. The fact that spinal 

 fishes react to light (Parker, :03b), while spinal amphibians do not, is 

 therefore perhaps to be expected and may be interpreted as new evi- 

 dence of the progressive anterior localization of functions in the nervous 

 system of vertebrates. However, Sherrington (:06, p. 9) has called 

 attention to. the fact that only stimuli of a particular kind will evoke 

 certain reflexes. He was easily able to induce the croak reflex in a 

 spinal frog by certain forms of stimulation, but he could not evoke it by 

 others, and he also found that the scratch reflex could be called forth 

 in spinal dogs by certain forms of tactual stimulation only. It is 

 therefore possible that spinal amphibians may yet be induced to give 

 photic reactions under some new method of stimulation. As far as 

 the present evidence goes, however, the myelencephalon, as well as 

 the cord, is essential for photic responses in which the skin is the 

 receptor. 



In the reactions of many organisms the ultimate direction of 

 locomotion is determined by making many random movements and 

 following such of them as lead away from conditions unfavorable to 

 the organism or into conditions better adapted to its existence. Other 

 organisms do not make great use of this method, but usually move 

 directly toward or away from the source of stimulation, and Loeb ('90) 

 has given the name of tropism to such responses. The light reactions 

 of amphibians are characteristically tropic in nature, and, as has been 



