DEGENERATION. 387 



body. The eye of the larva likewise disappears, and all that remains 

 to the adult ascidian is a nerve-mass, called by courtesy the " brain," 

 and which serves to regulate the few acts that mark the placid and 

 rooted existence of the race. Attention has been recently directed in 

 a special manner to the resemblance which exists between the eye of 

 the larval sea-squirt and that of vertebrates a statement to be taken 

 along with that which conversely declares the unlikeness of the ascid- 

 ian eye to that of all other invertebrate animals. It is matter of fact 

 that the chief parts of the eye of a vertebrate animal grow inward as 

 developments from the skin, and unite with an outgrowth from the 

 brain. This outgrowth forms the retina^ a nervous network of the 

 eye, whereon the images of things seen are duly received for transmis- 

 sion to brain and sensorium. Now, in invertebrate animals the retina 

 is formed from the skin-layer. This latter method of growth, it has 

 been remarked, is a perfectly natural one. It was to be expected that, 

 as the retina is to be affected in the discharge of its duty by light- 

 rays, it should form on the surface of the body where the light -rays 

 fall. In the vertebrate, and in the sea-quirt larva, the retina, on the 

 contrary, forms away below the skin-surface, and grows outward from 

 the brain. Why is this so ? Professor Ray Lankester maintains that 

 because the ascidian larva is perfectly transparent, the light-rays pass 

 through to its brain-eye, and thus give rise to sensations of sight. 

 Hence, if the original and primitive vertebrate animal or root-stock 

 were like the larval sea-squirt, as we suppose it to have been, its body 

 would be transparent, and its eye or eyes, situated on its brain, would 

 receive light-rays through its clear body. But, as the evolution of the 

 vertebrate race proceeded, the tissues became firmer and denser. By 

 " natural selection " or, in other words, by the exercise of accom- 

 modating power to function the eyed region of the brain would tend 

 to grow more and more toward the body's surface, to receive the rays 

 of light. As development, therefore, proceeded, the mode of growth 

 of the vertebrate eye would be adapted to the exigencies of its new 

 surroundings. Thus, to-day, the vertebrate eye grows from without 

 inward, because light-rays strike naturally on the outer surface of the 

 body. But it likewise grows from within outward as well, because of 

 the ancestral and hereditary tendencies which cause it to repeat in the 

 individual growth the passage to the surface it had to make in the 

 evolution of the race. If one might add a suggestion to such an ex- 

 planation, it would consist in an endeavor to account for that affinity 

 between brain and outer surface of body which we see to exist. Why 

 the brain should grow outward, as it does in eye, ear, and nose like- 

 wise, to connect with the body's surface, and so to form organs of 

 sense, is plain enough. We must bear in mind that the brain itself is 

 formed from the outer layer or epihlast of the larva, and from the 

 same layer which develops into the skin. Brain and skin, to begin 

 with, arise from the same layer. Hence, before even the matter of 



