306 Alexander Petrunkevitch 



are more sensitive than the smaller ones of the same individual; 

 we may be sure that the eyes of the young spiderling are less 

 sensitive than the same eyes in the adult; we may be sure of the 

 proportional sensitiveness of the eyes; we may be sure that changes 

 take place in the directions of the eye-axes and of the direction 

 of these changes as well as of the time when they take place; and 

 all this goes to show unmistakably the existence of a perfect 

 balance between function and structure in the eyes as well as a 

 remarkable degree and an extraordinary rapidity of adaptation. 

 Thus when the' spiderling first begins to lead an independent life, 

 it finds in its eyes an organ already sufficiently perfect to be relied 

 upon in the struggle for existence. 



The changes in the angles of the eye-axes may well be looked 

 upon as an adaptation. I have also tried to show that the slower 

 growth of the eye-group as compared with that of the cephalo- 

 thorax, is to be considered advantageous. The way in which these 

 changes came about is also well understandable but when we ask 

 ourselves in what way in the phylogeny was the increasing acuity 

 of vision accomplished, we have to depend for the answer upon 

 theoretical considerations. We know that in the individual this 

 increasing acuity of vision is brought about as a consequence of 

 the slower growth of the retina elements compared with that of 

 the lens. Is it then a retarding influence of which nature took 

 advantage in order to accomplish its own end, that was responsible 

 or was it a possible capacity on the part of the lens to grow more 

 rapidly than the retina .? Or were perhaps both factors at work } 

 And at what time in the phylogenetic development did the eyes 

 diff'erentiate into smaller and larger and why did they not all 

 reach the same degree of perfection ? What was it that arrested 

 the progress toward perfection or is further perfection possible 

 and still in process of attainment ^ All these are questions before 

 which we stand without answer as before a door behind which 

 treasures lie concealed. To find the key to that door would mean 

 to understand adaptation. For a long time science remained 

 satisfied with the explanation afforded by the principle of selection 

 and the attempt was made to apply this principle to all the phe- 

 nomena of organized life. But the very ease with which it answers 



