294 BROOKS. [Vol. X. 



"We cannot here cite physiological experiments for, much as 

 they are to be desired, none have yet been made either by us 

 or by others. Our interpretation rests, therefore, on structural 

 analogy alone, the value of which in estimating the nature of 

 sense organs we have already set forth." 



The pages which follow this extract show that the analogy 

 upon which the authors rely is for the most part with sense 

 organs which we have only the same reason for regarding as 

 auditory organs ; that is, although their structure seems to be 

 adapted for hearing, there is little experimental evidence that 

 they serve this purpose. 



It is very probable that the sense vesicles of molluscs, 

 Crustacea, the brachiopods, doliolum, and many other inverte- 

 brates, and of many medusae may give sensations of sound, but 

 it by no means follows that this is their only or their primary 

 function. 



Darwin and Dohrn have shown that many organs which now 

 perform a definite specialized function were acquired for a dif- 

 ferent purpose, and afterwards came to perform, in a secondary, 

 incidental way what at last became their chief function. 



It is easy to understand that organs of vision may have been 

 useful for vision from the first and may have been evolved 

 directly for this purpose ; that all the incipient stages in their 

 history may have related to light, but it is very hard to believe 

 that this is true of hearing organs. 



All organisms which live in the light are exposed to its 

 action for a considerable part of their lives, and its violent and 

 rapid waves produce molecular and chemical changes in their 

 structure. They do not act upon the organism as a mass, but 

 they affect its most intimate structure. Their influence is in 

 no sense psychological, for while they come from the outer 

 world their action is identical with that of chemical changes 

 which are set up within the body, although the first beginnings 

 of sensation, no doubt, consisted of the production by natural 

 selection of adaptive adjustments to constant external influences, 

 as distinguished from those of internal origin. 



As light is among the most persistent elements in the ex- 

 ternal environment, we have such phenomenon as heliotropism 



