76 MODERN IDEAS OF EVOLUTIOX 



of the separate tubes for the several ocelli of the com 

 pound eye were plainly discernible. Let it be 

 observed also that the simple or single eye which 

 culminates in the vertebrate animals probably existed 

 as far back as the compound eye, since we have no 

 reason to suppose that the gastropod and cephalopod 

 molluscs which abounded in the Cambrian age were 

 blind, and their eyes must have been of a type distinct 

 in plan from those of the trilobites. The difference 

 between these two kinds of eyes is not in general 

 principle, but in details of plan. In the one a 

 number of small and comparatively simple eyes are 

 grouped together, radiating from a centre, so as to 

 command a wide range of vision without indistinct 

 ness in any part. In the other there is but one organ 

 of larger size, and with greater complexity in its 

 apparatus for adjustment to distance and direction. 

 Both these types of eye existed in the Cambrian 

 period with all their essential parts, though perhaps 

 the first mentioned had precedence to some small 

 extent in time. In that early period they were sub 

 stantially perfected, in so far at least as vision in 

 water is concerned ; and if this perfection arose by 

 &amp;lt; infinite adjustments, these must have been made in 

 those pre-Cambrian ages in which we have no evi 

 dence of the existence of any creatures requiring to 

 have eyes. Farther, the two types of eyes above re 

 ferred to must have come in independently. The one 

 could not have originated from the other. It is also 



