2 
mal in the lower animal. Speaking of the development of the eye¬ 
lids, for example, he remarks that in a six weeks’ human embryo 
these are absent, just as they are in eels and some other fishes, the 
skin of the head being continuous with the anterior layers of the 
cornea. At eight weeks the newly forming lids are seen as small 
buds of mesoblast covered with epiblast above and below. The 
same structural arrangement, permanent in character, is seen in the 
teleostean fishes. In these, little folds of tissue arise from the 
margin of the orbit, project toward the cornea, but do not reach 
that organ. Still later, as the dermal folds in the human fetus 
continue to grow into eyelids, they envelop the cornea, meet and 
adhere at their margins by union of their epithelial surface, so that 
the conjunctiva becomes a closed sac. As we know, separation 
takes place before birth in the human animal, but in our ancestors 
the snakes and in some lacertilia (lizards) the adhesion continues 
during life, these animals being obliged to see through their eyelids, 
as it were. The application of these embryologic facts is seen in 
cases of congenital absence of the lids and congenital ankylo¬ 
blepharon in man. 
If, then, a study of the eyes of the lower animals is likely to 
throw light upon the biology and pathology of the human visual 
apparatus, there is ample justification for the indulgence by the 
practical ophthalmologist in even the desultory observations and 
studies I have been able to make of the ocular organs in birds. I 
have said birds because in them we have the highest type of vision. 
The very existence of every bird depends upon good eyesight with 
which to obtain food and escape enemies. Some mammals, fishes, 
reptiles and amphibia get through life fairly well without eyes, but 
there are no blind bird families. 
As an example of the visual capacity of some birds one has but 
to think for a moment of a hawk poised several hundred yards 
above a meadow in which a field-mouse or a small chicken is hidden. 
In a few seconds after the quarry is sighted it is seized by the bird, 
whose sharp sight has not only detected it, but whose wonderful 
accommodative apparatus permits of a sure and continuous fixation 
from hundreds of meters to less than a meter within an incredibly 
short space of time. Variations in the character of this acute 
vision are seen in many other birds; in the humming-bird that darts 
