164 Mr. H. M. Bernard on 



discharge their contents at the surface), tliat the animal king- 

 dom owes its many visual organs. It is legitimate to assume 

 that the laden wandering-cells would flock in excess towards 

 the source of the brightest ligiit. At such points complications 

 would arise between these invaders and the more stationary 

 tissues which bar the way. Out of such complications, I be- 

 lieve, eyes have arisen. The sensory nerves in those parts of 

 the skin most strongly and frequently illuminated become 

 associated in different ways with these complications, either 

 with the struggling crowd of wandering-cells collected in 

 excess at such bright spots, and set in commotion Avhenever 

 light falls on them, or with other cells into which these 

 wandering-cells have discharged their contents to over- 

 crowding, and which, on this account, practically become 

 equally restive whenever subjected to light-stimulus. For it 

 is important to note that these granules appear to make the 

 same endeavours under light-stimulus to leave the cells in 

 which they iind themselves and travel towards the light, as do 

 tiie wandering-cells themselves to escape from the Metazoan 

 body. 



We may enumerate some of the different types of eye 

 which can be thus accounted for. 



1. Simple epidermal cells associated with epithelial sensory 

 (tactile) cells become filled with granules to overcrowding. 

 The escape of these granules at the exterior is hindered by 

 the excess of slime to which they themselves have contributed, 

 'i hese granules, crowding forward whenever stimulated by 

 light, but unable to escape fast enough, exert a lateral pressure 

 upon the adjacent sensory cells. The excess of slime pro- 

 duced by the continual crowding forward of the granules may 

 result in the formation of vitreous bodies or lenses. Eyes 

 arising in this way occur, for instance, in some MoUusks. 



2. The wandering-cells may be arrested by, or collect in 

 excess round, epithelial glands which have sunk below the 

 surface. This would be especially the case round gland- 

 cells or glands which contained glassy or refractive contents 

 in the line of light, such blight points having, according to the 

 theory, an especial attraction lor the wandering-cells. The 

 strivmg of the wandering-cells either to push on through, or to 

 pass on their contents to, these cells would again be appreciated 

 by sensory nerves. Further, assuming, and there is ground for 

 the assumption, that the granules in many cases contribute to 

 the formation of the slimy or refractive secretions in such glands, 

 these secretions w-ould, owing to the excessive supply of 

 granules, tend to develop abnormally, and thus help to form 

 a more efficient dioptric apparatus than the simple gland 



