SIMILARITY IN ACQUIRED ORGANS. 233 



is wholly different, with an actual inversion of the 

 elemental parts, and with a large nervous ganglion 

 included within the membranes of the eye. The 

 relations of muscles are as different as it is possible 

 to conceive, and so in other parts. * * * It is 

 of course open to any one to deny that the eye in 

 either case could have been developed through nat- 

 ural selection of successive slight variations, but if 

 this be admitted in the one case it is clearly possible 

 in the other ; and fundamental differences in struc- 

 ture in the visual organs might have been anticipated 

 in accordance with this view of their formation. As 

 two men sometimes independently hit on the same 

 invention, so in the several foregoing cases it ap- 

 pears that natural selection, working for the good of 

 each being, and taking advantage of all favorable 

 variations, has produced similar organs, as far as 

 function is concerned, in distinct organic beings 

 which owe none of their structure in common to 

 inheritance from a common ancestor." 



It is generally admitted that this answer of Dar- 

 win is sufficient to meet the case, at least in part. 

 A visual organ to have any high degree of efficiency 

 must possess certain fundamental parts : must have 

 a dark chamber, a spread-out optic nerve or retina, 

 and a series of lenses to form an image on the ret- 

 ina. Beyond these necessary features there are 

 hardly any common points between the two eyes. 

 But some would still claim that the difficulty is not 

 met. For it has indeed been increased rather than 

 diminished by observation since the above answer 

 of Darwin's was written. Another mollusk, Pecten 



