724 William Patten 



ever, bave but onekiudofdynamopliag'oiisorgan, i.e. cliloropbyll, wbicb 

 absorbs ouly solar energy. 



The fully developed eyes of tbe Vertebrates, and big-ber Inverte- 

 brates, bear tbe sanie relation to tbe isolated ommatidia tbat tbe bigbly 

 developed organs of smeli and bearing do to tbe isolated tactile and 

 olfactory hairs of the lower Invertebrates. In botb cases tbe fiinctional 

 perfection of tbe fully developed organ depends, primarily, upon the 

 aceumulation of tbe constituent elements at definite localities and their 

 coincideut, progressive development ; and secondarily, upon tbe addi- 

 tion of organs for tbe aceumulation or intensificati on of tbe vibrations 

 to be transmitted to tbe seat of perception. In tbe lower forms, as in 

 tbeCoelenterates, tbe sense org-ans, ocelli, otolitbic sacs, olfactory pits, 

 and sense hair papillae, or tentacles, are mainly coufined to a narro w 

 ring surrounding- tbe digestive tract. As we bave said, tbe acquire- 

 ment of certain properties by certain cells of a colouy necessitates the 

 loss in a corresponding degree of otber faculties. But tbis loss is, on 

 the otber band, made good by tbe special development in neigbboring 

 cells of tbose properties lost hy tbe otber, and a loss of tbose it had 

 gained. Tbe dynamopbag-ous, and bylopbagous faculties were tbe first 

 to be specialized. The latter cells were, by the laws governing- segment- 

 ation, confined to tbe so-called vegetative pole, wbile, immediately 

 around them, was formed a ring- of sensory cells, increasing in sensitive- 

 ness according as the vegetative cells lost tbis faculty. As tbis sensory 

 ring was more specialized, its cells became modified in diflferent direc- 

 tions for the reception of various forms of energy. or irritants, giving 

 rise, in dose proximity to each otber, to ocelli, otolitbic sacs, olfactory 

 pits, and sense hair cells. 



The almost universal presence. in ali kinds of eyes, of some re- 

 flecting body showing the greatest variety in structure and origin . in- 

 dicate« tbat it must bave some very important fuuction to perform in 

 the process of vision. Just as pigment, the reflecting body or argen- 

 tea, may be found in two different localities, where the part it has 

 to play must be essentially diflferent. For instance, reflecting membranes 

 may be found outside tbe eye, as in Fisbes, Cepbalopods, Worms etc. 

 or witbin tbe eye, bebind the retinidia. In the first instance they 

 serve, instead of pigment. for tbe exclusion of lateral rays of light. 

 Tbe latter they bave auotber purpose, tbat of retìecting tbe light so tbat 

 it passes a second time tbrough the nerve fibrillae of tbe retinidia ; in 



