TRANSACTIONS OF SECTION D. 847 
retina, and by the formation of folds of skin around the eye, which become 
the iris and the eyelids respectively. 
Each stage in this developmental history is a distinct advance, physiologically, 
on the preceding stage, and, furthermore, each stage is retained at the present day 
as the permanent condition of the eye in some member of the group Mollusca. 
The earliest stage, in which the eye is merely a slightly depressed and slightly 
modified patch of skin, represents the simplest condition of the Molluscan eye, and 
is retained throughout life in Solen. The stage in which the eye is a pit, with 
widely open mouth, is retained in the limpet; it is a distinct advance on the 
former, as through the greater depression the sensory cells are less exposed to 
accidental injury. 
The narrowing of the mouth of the pit in the next stage is a simple change, 
but a very important step forwards. Up to this point the eye has served to dis- 
tinguish light from darkness, but the formation of an image has been impossible. 
Now, owing to the smallness of the aperture, and the pizmentation of the walls of 
the pit which accompanies the change, light from any one part of an object can only 
fall on one particular part of the inner wall of the pit or retina, and so an image, 
though a dim one, is formed. This type of eye is permanently retained in the 
Nautilus. 
The closing of the mouth of the pit by a transparent membrane will not affect 
the optical properties of the eye, and will be a gain, as it will prevent the entrance 
of foreign bodies into the cavity of the eye. 
The formation of the lens by deposit of cuticle is the next step. The gain here 
is increased distinctness and increased brightness of the image, for the lens will 
focus the rays of light more sharply on the retina, and will allow a greater quantity 
of light, a larger pencil of rays from each part of the object, to reach the corre- 
sponding part of the retina. The eye is now in the condition in which it remains 
throughout life in the snail and other gastropods. Finally the formation of the 
folds of skin known as iris and eyelids provides for the better protection of the eye, 
and is a clear advance on the somewhat clumsy method of withdrawal seen in 
the snail. 
The development of the vertebrate liver is another good but simpler example. 
The most primitive form of the liver is that of Amphioxus, in which it is present as 
a simple saccular diverticulum of the intestinal canal, with its wall consisting of a 
single layer of cells, and with bloodvessels on its outer surface. The earliest stage 
in the formation of the liver in higher vertebrates, the frog for instance, is practically 
identical with this. In the frog the next stage consists in folding of the wall of 
the sac, which increases the etticiency of the organ by increasing the extent of 
surface in contact with the bioodvessels. The adult condition is attained simply 
by a continuance of this process; the foldings of the wall becoming more and more 
complicated, but the essential structure remaining the same—a single layer of 
epithelial cells in contact on one side with bloodvessels, and bounding on the other 
directly or indirectly the cavity of the alimentary canal. 
It is not always possible to point out the particular advantage gained at each 
step even when a complete developmental series is known to us, but in such cases 
as, for instance, in Orbitolites, our difficulties arise chiefly from ignorance of the 
particular conditions that confer advantage in the struggle for existence in the case 
of the forms we are dealing with. 
The early larval stages in the development of animals, and more especially those 
that are marine and pelagic in habit, have naturally attracted much attention, since 
in the absence, probably inevitable, of satisfactory paleontological evidence, they 
afford us the sole available clue to the determination of the mutual relations of the 
large groups of animals, or of the points at which these diverged from one another. 
In attempting to interpret these early ontogenetic stages as actual ancestral forms, 
beyond which development at one time did not proceed, we must keep clearly in 
view the various disturbing causes which tend to falsify the ancestral record; such 
as the influence of food yolk, or of habitat, and the tendency of diminution in size 
‘to give rise to simplification of structure, a point of importance if it be granted 
