234 EVOLUTION OF TO-DAY. 



(the scallop), belongs to a group very distantly re- 

 lated to the cuttle-fishes ; it has around the edge of 

 its mantle a series of eyes, which can therefore have 

 no inherited likeness either to the cuttle-fish eye or 

 the vertebrate eye. But here, too, is developed the 

 same sort of complicated eye with its various parts, 

 and here the likeness to the vertebrate eye is even 

 more pronunced, the various coats being in almost 

 exactly the same order in the two cases. Since 

 Pecten,the cuttle-fishes, and the vertebrates are very 

 widely separated from each other, we have in these 

 cases three organs very similar to each other, inde- 

 pendently acquired. It is enough to tax one's 

 imagination to believe that such a complicated 

 organ as the eye could ever have been developed 

 by selection of minute variation ; but when we thus 

 find at least three cases where quite similar organs 

 are independently developed, it is plain that we 

 have considerable of a problem. That these organs 

 could have been developed by chance is incredible, 

 and it must be claimed that the conditions requisite 

 for a visual organ are so rigid that all eyes must 

 have developed in this way in order to be of any 

 use. Now this type of eye is not the only type 

 which is found, for Crustacea and insects have visual 

 organs made upon an entirely different plan. To 

 make the matter still more perplexing, we find that 

 some mollusks have eyes built upon the plan of in- 

 sect eyes (Area and Pectunculus). The whole mat- 

 ter is indeed a puzzle. Unquestionably the same 

 sort of eye has been independently acquired in 

 several cases, and since it can hardly be believed 



