PRESIDENTIAL ADDRESS. 39 



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 

 imago, 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 corresponding part of 

 the retina. The eye is now in the condition in which it l'emains 

 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 some- 

 what 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 efficiency of the 

 organ by increasing the extent of surface in contact with the blood- 

 vessels. The adult condition is attained simply by a continuance of 

 this process ; the foldings of the wall becoming more and more com- 

 plicated, 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 



