232 EVOLUTION OF TO-DAY. 



animals not closely related do not possess similar 

 organs, the same function being differently per- 

 formed. But it is not impossible to find instances 

 where organs bearing the greatest similarity are 

 found in animals so distantly related as to preclude 

 the idea of inheritance as an -explanation, and to 

 make it necessary to assume that they are indepen- 

 dently acquired. Probably the best illustration that 

 can be selected is found in the eye of the vertebrates 

 and mollusks. These two groups of animals are 

 very widely separated from each other, and any 

 organs which they possess in common, except those 

 found in all animals, must have been independently 

 acquired, since they separated from each other in 

 the ancient pre-silurian times. Now the higher 

 members of the mollusks (cephalopods) possess eyes 

 which bear the greatest similarity to those of verte- 

 brates, each having sclerotic, retina, choroid, vitreous 

 humor, aqueous humor, and lens. The correspond- 

 ence at first sight seems complete, and remember- 

 ing the remarkable complexity of the organ, together 

 with the fact that the various parts are only of use 

 when the others are present, it becomes a marvellous 

 thing to find such an organ independently developed 

 in two different cases. This difficulty Darwin has 

 answered as follows : " Beyond a superficial resem- 

 blance, there is hardly any real similarity between 

 the eyes of cuttle-fishes and vertebrates. * * * The 

 crystalline lens in higher cuttle-fishes consists of two 

 parts placed one behind the other like two lenses, 

 both having a very different structure and disposi- 

 tion to what occurs in the vertebrates. The retina 



