ANIMAL BEHAVIOR. 301 



Besides the great danger of being deceived by the response, 

 or the lack of response, to stimulus, there are two other 

 insidious sources of error to be guarded against. We habitu- 

 ally assume that intelligence and sensibility rise and fall 

 together. This idea may lead to false conclusions in two 

 directions to overestimating sensibility at the upper end of 

 the scale and underestimating it at the lower. That high 

 sensibility does not imply high intelligence is clear in the case 

 of Clepsine and equally so in almost any other case that might 

 be selected among the lowest segmented animals. That high 

 intelligence does not necessarily imply correspondingly high 

 sensibility is shown by the well-known fact that many animals 

 greatly surpass man in their sense powers. It can be shown, 

 I believe, that the difference in sensibility between higher and 

 lower animals is very much less than is generally supposed. 



The second source of error is the common assumption that 

 the grade of sensibility rises with the structural complexity of 

 the sense organs. This view is likewise untenable. It is true 

 that the sense organs as a rule become more complex in struc- 

 ture as we go up the scale, but this advance in structure is 

 mainly confined to accessory and non-sensory parts, which are 

 either of a protective nature or else concerned in some sub- 

 sidiary function, such as muscular adjustments and regulation 

 of the stimuli. Such improvements in the non-sensory parts 

 may be carried to a high state of perfection and greatly raise 

 the general efficiency of the organ (e.g., the vertebrate eye), 

 without adding much, if anything, to the sensitiveness of the 

 individual sense cells. The sense cells may be multiplied in 

 number and placed in a position of safety and advantage for 

 receiving stimuli, and the stimuli may be strengthened, 

 directed, and otherwise regulated so as to secure the best 

 results ; but all that may obviously not affect the functional 

 power of the cells themselves. We do not know the range of 

 variability in this power, but we do know that the sense cells 

 often vary relatively little in structure, sometimes retaining in 

 the higher forms the same typical features that characterize 

 them in the lower forms. There is no known difference of 

 structure that would warrant the assumption that the dermal 



