Chap, ix.] OF THE SENSE OF SIGHT. 421 



marvellous of the eye in man and the superior vertebrates I It 

 has been praised as a perfect instrument, necessarily the 'wort of 

 an intelligent artificer anxious to adapt means to ends, and so 

 forth. We now know that, considered as an optical apparatus, 

 the eye is a tolerably good instrument, but by no means perfect. 

 Moreover, comparative anatomy and embryology prove to us 

 that, spite of its complicated construction, the eye is like all the 

 other organs, merely the result of a slow labour of perf ectionment 

 and accommodation. 



In effect, the rudimentary eye is only a simple drop of black 

 pigment, reposing on nervous elements. Such it is in the inferior 

 medusse, in which we find at 

 the base of the tentacles pig- 

 mentary spots. These spots 

 are the first rudiment of the 

 eyes : for sometimes we meet 

 therein with crystalline rods. 

 In other cases the construc- 

 tion of the optical apparatus 

 takes an additional step, and Fm. 78. 



■ma AT\nni^n^»» ,•« n,n ,„„„„^„ Marginal bodies of acraapodal medusae : j4, Pc- 



we encounter m the masses u^u rwctlluca; B, Charyhdm marmpixUs: 



of niirTnaTi+ a tTaTiancroTit a-r,A o, free part of the edge of the animal placed 



01 pigment a transparent ana between the scolloped fringes of the dials : b, 



rpfrapt.iTrn hnrl-n- l'^\a 7«^ pedmieles ; c, canal thereof ; tJ, ampulla ; e, 



reiractive DOay ^J!lg. (H). sac with crystals; /, pigment; j, tody in 



In certain species, nervous *o™ofi™s. 

 bundles penetrate manifestly into the capsule.^ 



Many planaries have first of all in the embryonary state pig- 

 mentary spots in the place where are to be at a later period eyes 

 with rods or crystalline cones. The absence or the pi-esence of the 

 organs of vision is often subordinated to the kind of life. Eyes, 

 for example, are sometimes lacking in the annelids which live in 

 the darkness. These eyes have not, in this case, been the object 

 of selection or difEerentiation, for they are useless. 



' For all questions of evolutive anatomy, consult especially the Manuel 

 d' Anaiomie Gompwrie of Gegenbaur, one of the few treatises composed in 

 harmony with the results of transformism. 



