358 CHAPTERS ON EVOLUTION. 



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 ascidian 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 inwards as develop- 

 ments from the skin, and unite with an outgrowth from the brain. 

 This outgrowth forms the retina, or nervous network of the eye, whereon 

 the images of things seen are duly received for transmission 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- squirt larva, the retina, on the contrary, 

 forms away below the skin surface, and grows outwards 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 rootstock 

 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 evolu- 

 tion of the vertebrate race proceeded, the tissues became firmer and 

 denser. By " natural selection " or, in other words, by the exer- 

 cise of accommodating power to function the eyed region of the 

 brain would tend to grow more and more towards the body's surface, 

 to receive the rays of light. As development, therefore, advanced, 

 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 inwards, because light-rays strike naturally 

 on the outer surface of the body. But it likewise grows from within 

 outwards 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 explanation, it would consist in an 

 endeavour to account for that affinity between brain and outer sur- 

 face of body which we see to exist. Why the brain should grow 

 outwards, as it does in eye, ear, and nose likewise, 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 epiblast 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 eyes falls to be considered, 

 the affinity of the skin layer and the nervous system is a fact worth 

 noting. It is this truest of relationships which may reasonably 



