272 Animal Life and Intelligence. 



for example — the otolith (which is generally single) is 

 retained in a free position by ciliary action. In aquatic 

 creatures an organ for the appreciation of changes of 

 motion might be of more service than an auditory organ. 

 And if one be permitted to speculate, one may surmise that 

 the sense of hearing may be a refinement of the sense 

 through which changes of motion are appreciated. First 

 would come a sense of movements of the organism in the 

 medium through the stimulation of the sense-hairs by 

 the relative motion of the otolith ; then these sense-hairs, 

 with increased delicacy, might appreciate shocks in the 

 medium; and, eventually, those more delicate shocks 

 which we know as auditory waves. In this way we might 

 account for the fact that in the vertebrates the same 

 organ, through different parts of its structure, appreciates 

 both change of motion and auditory vibrations. And thus 

 the organs in the invertebrata which are generally regarded 

 as auditory, and for which has been suggested the function 

 of reacting to changes of motion, may, in truth, subserve 

 both purposes — may be organs in which the differentiation 

 I have hinted at is taking place. 



Sight, like hearing, is a telaesthetic sense. Through it 

 we become aware of certain vibratory states of more or less 

 distant objects. The medium by means of which these 

 vibrations are transmitted is not, as in the case of hearing, 

 the air, but the aether which pervades all space. The rate 

 of transmission is about 186,000 miles in a second. That 

 which answers in vision to pitch in hearing is colour. The 

 lowest, or gravest, light-tone to which we are sensitive is 

 deep red, where the number of vibrations per second is 

 about 370 billions (370,000,000,000,000). The highest, or 

 most acute, light-tone is violet, with about 833 billion 

 vibrations in a second. If white light be passed through 

 a prism, the rays are classified according to their vibration- 

 periods, and are spread out in a spectrum, or band of 

 rainbow colours. But different individuals vary, as we 

 shall presently see, in their sensibihty to the lowest and 



