94 BULLETIN OF THE 
taneous operation of light from two directions, and the advantage to 
vision secured through the more favorable velation 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 
