September 4, 1885. j 



SCIENCE, 



195 



their morphological position at the oral end of the 

 body; and, with only one or two exceptions, there is 

 but a single pair of eyes. In Patella, the row of eyes 

 last seen in Solen has become a simple sphere of 

 pigmented cells; and in Haliotis we have also an 

 open sphere, but, instead of the refractive cuticula 

 in front of each cell, there is one combined mass 

 forming a lens, which is purely a secretion, not cellu- 

 lar as in vertebrates. Fissurella goes practically as 

 far as any gastropod, having a closed eye containing 

 a lens, and a transparent epidermal covering acting 

 as a cornea. Both Fissurella and Haliotis have a 

 distinct nerve specialized for sight, which connects 

 the eye with the superior cephalic ganglion. 



In an early stage of the vertebrate embryo, the ante- 

 rior medullary groove divides into three segments, — 

 the fore, mid, and hind brain. The fore-brain sends 

 outwards and laterally a swelling, which increases in 

 size, and passes on to the epidermis; and here an 

 invagination takes place to meet this outward brain- 

 growth. This invagination finally closes, and soon 

 becomes cut off, forming a hollow vesicle, the cavity 

 of which is finally obliterated, and, becoming trans- 

 parent, forms the lens of 



the adult eye. In the 

 mean time the growth 

 from the brain has arched 

 over and above this vesi- 

 cle; and then, folding 

 over laterally, it encloses 

 the lens (fig. .3), which 

 fills up the anterior open- 

 ing of the cavity of this 

 ' secondary optic vesicle.' 

 After the closure is com- 

 pleted by the union of a 

 and 6, there is a double-walled vesicle, the interior 

 wall giving rise to the many layered retina, while the 

 external wall forms the pigment layer of the cho- 

 roidea. 



The evolution of this eye seems simple; for, as soon 

 as it became of importance to its possessor, a corre- 

 sponding stimulation took place in the brain, where 

 sight is without doubt seated. An increase of devel- 

 opment began all along the tract, from the lens to 

 the brain ; and, as this increased, that part of the brain 

 nearest the eye enlarged, and proceeded by steps out- 

 ward in a manner similar to the process now taking 

 place in the development of the eyes of Vertebrata. 

 We then have a stage in which a part of the brain 

 closes over the superior part of the eye, being sepa- 

 rated by a layer of fibres which is the much shortened 

 and flattened primitive optic nerve. The pedicle 

 connecting this advanced part of the brain, which 

 may be looked upon as a ganglion, will now be called 

 the ' secondary optic nerve,' — the optic nerve of the 

 eyes of the adult Vertebrata. Dr. Sharp thus holds, 

 1°, that the lens of the vertebrate eye is homologous 

 with a primitive invaginated eye, such as we find 

 to-day in the gasteropods; 2°, that the layer of optic 

 fibres of the retina is homologous with the primitive 

 optic nerve. 

 In vertebrates as well as invertebrates we frequently 



Fig. 3. — Diagram to illustrate 

 the method hy which the 

 secondary optic vesicle en- 

 closes the lens which should 

 fill up the open end. Eye 

 of vertebrate. 



find blind animals, the near relatives of which have 

 well-developed organs of sight. In these cases the 

 accessory organs are first to disappear, the lens first; 

 and in the lowest forms of degeneration, Branchi- 

 ostoma notably, nothing remains but a slight deposit 

 of pigment on the anterior end of the neural canal. 

 This deposit in Branchiostoma, and a similar deposit 

 in some larval Ascidia, have led Lankester to regard 

 the primitive type of Vertebrata as a transparent ani- 

 mal with eyes sessile in the brain ; but Dr. Sharp's 

 investigations have led him to the opinion that forms 

 so degenerate as these should not be taken as a stand- 

 ard on which to base our conclusions in regard to 

 the origin of the vertebrates. 



CIVILIZATION AND EYESIGHT. 



The discussion following Lord Rayleigh's article 

 upon 'Civilization and eyesight' [Nature, No. 798, 

 p. 340) has resulted in a clear exposition of this in- 

 teresting subject. Rayleigh is of the opinion that the 

 supposed superiority of the savage eye is merely a 

 question of attention, and practice in the interpreta- 

 tion of minute indications ; and that it is comparable 

 with the acuteness of the blind in drawing con- 

 clusions from slender acoustical premises. It is 

 doubtful whether the blind can hear sounds wholly 

 inaudible to others ; and, likewise, it seems impossi- 

 ble for the savage eye, with practically the same aper- 

 ture as the civilized eye, to resolve objects, beyond a 

 certain point, calculable by the laws of optics from 

 the wave-length of light. 



J. Eand Capron [Nature, No. 799, p. 359) suggests 

 that, in considering the question of aperture, the fact 

 that this, though probably following a general rule 

 applicable alike to savages and civilized beings, varies 

 in individual cases, should be taken into account. 

 He mentions an assistant who had a singularly ' sharp ' 

 eye, and could pick out with ease companions to 

 double stars, small satellites, etc., which others saw 

 with difficulty. The pupils of his eyes were always 

 larger than those of most other persons; and he had 

 the peculiar power of being able to read fine print 

 with ease when the gas was turned half on, and it 

 was his habitual custom to read in this way. He sug- 

 gests that there must be something more than a mere 

 ' question of attention and interpretation of minute 

 details,' when a savage can resolve two distant dots 

 into distinctly appreciable personages, as regards sex 

 and garments. 



R. Brudenell Carter urges [Nature, No. 800, p. 386) 

 that there is no necessity for a larger aperture to ex- 

 plain acuteness of vision. The savage might have 

 greater sensitiveness to variations of light, greater 

 sensitiveness to color, and acuteness of vision over a 

 larger retinal area. All these advantages might be 

 conferred by better formation or higher development 

 of the retina ; and such higher development might at 

 once be promoted by exercise, and handed down by 

 descent. He believes that the conditions of town-life 

 are unfavorable to the evolution, and favorable to 

 the degradation, of the eye; and, further, that a mod- 



