No. I.] THE SENSORY CLUBS. 295 



or response to light in organisms which we cannot regard as 

 sensitive to light. Susceptibility to the influence of light is 

 not a property of the organism as a mass, but of the substance 

 of its cells, and the first step in the evolution of a sense of 

 vision is the specialization, in certain differentiated cells, of a 

 property which inheres, to a lesser degree, in all the cells. 



The aggregation of these cells in a special sense organ is 

 only one more step in the process, and there is no difficulty 

 in understanding that every one of the incipient stages in the 

 evolution of eyes may have related to light. 



The case of hearing is very different. There is no constant 

 environment of sound as there is of light, nor does sound have 

 any definite physiological influence apart from its action on 

 sense organs. In these it does not act on the sensitive proto- 

 plasm, but it excites their tactile sensibility by variations in 

 their contact with more massive bodies which are thrown into 

 vibration by the waves of sound. 



It is not probable that the incipient stages in the evolution 

 of hearing organs were acquired for the purpose of hearing, 

 and it is, to say the least, not impossible that the auditory 

 organs of some animals may have arisen by the modification 

 of sense organs which, while acquired for a different function, 

 have proved, incidentally, to be sensitive to vibrations of 

 sound. 



• As the soft, watery, homogeneous medusae float in the 

 ocean their specific gravity approaches more nearly to zero than 

 that of any other animal with the power of voluntary motion, 

 and their mobile, loaded, sense clubs and sense vesicles are 

 well adapted for giving them the sensation of weight. 



It is hard for us with our heavy bodies to appreciate the 

 value of the sensation of weight or the necessity of special 

 sense organs for perceiving weight, but a little reflection will 

 show that we owe to it the difference between upwards and 

 downwards and sideways ; the perception of existence in space. 



Gravity is the most constant of all the phenomena of the ex- 

 ternal universe, and the sensation of weight may possibly be 

 the basis of all conscious life, the substratum which underlies 

 all other sensations. 



