ADJUSTMENT, DIRECT AND INDIRECT 



tion is the concentration of rays of light into a 

 focus upon the retina. Such is the function dis- 

 charged by the lens, and the vitreous and aque- 

 ous humours. Now, while the compound eyes 

 of insects show us that this function c^n be dis- 

 charged in more than one way, a brief consider- 

 ation of the optical conditions in the case would 

 show that it can only be accomplished in a few 

 ways. Not only does the passage of the light 

 directly tend to set up molecular rearrange- 

 ments in the refracting matter which lies before 

 the retina, but out of those rearrangements there 

 are very few which can assist the focalizing pro- 

 cess ; so that natural selection, in preserving the 

 best-refracting eyes, would have but very few 

 directions in which to act. The anterior mem- 

 brane might differentiate into a number of con- 

 verging lenses, as in the higher annulosa, but 

 if such a differentiation did not occur, it is diffi- 

 cult to see how the needful refraction could be 

 secured, save by the differentiation of the succes- 

 sive strata which we call the aqueous, crystal- 

 line, and vitreous humours. This may serve to 

 indicate the course of explanation to be taken. 

 The physical conditions for securing very effi- 

 cient vision being thus limited, and direct adap- 

 tation being such an important factor in the pro- 

 cess, it does not seem at all strange that two eyes 

 quite similar in structure should be independ- 

 ently produced. A precisely similar argument 



87 



