94 BULLETIN OF THE 



taneous operation of light from two directions, and the ad^'antage to 

 vision secured through the more favorable relation of the retina to the 

 direction of the newly admitted light ; and there are, in addition, some 

 hitherto unexplained anatomical features which gain by this hypothesis a 

 reasonable explanation. 



Changes similar to those imagined above might possibly have accom- 

 panied a gradual shifting in the position of the original lens (compare 

 Figs. 25-29), rather than the substitution of a new lens. Such a shift- 

 ing, from whatever cause, might have concentrated the light upon one 

 portion of the retina at the expense of remaining parts. The less-favored 

 parts might have been degraded in functional importance, and might 

 have atrophied. So far not much difficulty would be encountered in 

 appreciating the assumed conditions ; but how the light, acting through 

 the original, though shifted, lens, could have afforded any advantage 

 which would have been competent to initiate an inversion, or' to carry 

 forward such a process when once begun, is not so easy to comprehend. 



In considering the development of " post-nuclear " eyes, however, it 

 will be possible to show how such a migration on the part of the lens 

 may have been an important factor in the process of inversion. 



The structure of ocelli with "post-nuclear " bacilli, both in the adult 

 condition and in such stages of development as are at present known, is 

 only conditionally referable to what has been assumed above as the 

 primitive state of the eye, and the development is not so easily explained 

 as that of eyes with pre-nuclear bacilli. 



The difficulty depends partly upon the uncertainty as to the exact 

 changes through which the eye passes in its ontogeny. Further study 

 will unquestionably soon determine this in a more satisfactory way. 

 But even when it has been definitely established that the retinal layer 

 either does or does not become inverted, it will not even then follow that 

 the relations of the two types to each other, and to a primitive antece- 

 dent condition, will at once become evident. One naturally looks for a 

 development of both types from a common origin, and, for a time at 

 least, along a common line. 



If the retina is inverted, a general comparison with the retina of "pre- 

 nuclear" eyes becomes possible ; but the bacilli cannot be strictly homol- 

 ogous, since they do not occupy equivalent ends of the retinal cells. 



If the existence of an inversion were established, a common line of 

 development could be fairly maintained; the "post-nuclear" type must 

 then be considered less modified, as far as regards the retina, than the 



