The Evolution of Optics. 287 



motion and precise action. 1 suppose there is no organ in the universe 

 responding with such lightning-like precision to such infinitely small 

 and infinitely quick stimulation as the eye of a humming-bird. One is 

 simply appalled and thrilled by such an astonishing miracle. So im- 

 portant is accurate vision to birds of prey that they have two fovea, 

 an explanation of which fact is not at present quite clear. The re- 

 markable adaptive power of life is shown by the change life has had to 

 undertake in the evolution of her eyes. Dr. Alleman has admirably 

 made clear the difference between the construction of the two types of 

 eye, the invertebrate and the vertebrate. The fact seems to show that 

 the perfection of eye required by the vertebrate could not be gained 

 on the invertebrate type or plan, and a complete about-face was under- 

 taken and carried through. It is extremely suggestive that the retina 

 and lens of the invertebrate eye are developed from the epidermal 

 structures, while in the vertebrates the retina is developed from the 

 brain. In other words, in invertebrates the light goes into the brain 

 to affect it, but in the vertebrates the brain comes out to see ! The 

 fact, like many another, shows also that Life, though intelligent and 

 ingenious beyond any human conception, is yet not omniscient. 

 Neither is she omnipotent; she is always working under difficulties 

 and with inexhaustible cunning, doing the best she can with the mate- 

 rials at command. We are at present incapable of catching the least 

 glimpse of a reason why the optic nerves and organs constituting the 

 retina should in vertebrates turn backward and the light be thus 

 forced to pierce the numerous layers of the retina until it reaches the 

 final pigmentary layer into which the rods and cones dip. It looks 

 like a very poor plan indeed, but it is because our little minds are so 

 poor that we think so. It seems to work pretty well perhaps better 

 than any plan we could have devised. You have all heard of the no- 

 ble Castilian who wished he had been present when God created the 

 world he could have given him such excellent advice ! The modem 

 pseudo-scientist is filled with that spirit, and, on the assumption that 

 God is omnipotent and omniscient, the point becomes less vulgarly im- 

 pertinent ; but if the demiurge is neither omnipotent nor omniscient, 

 but works, as he evidently does, under difficulties, then the imperti- 

 nence becomes the most colossal impudence. In illustration of the in- 

 fluence of locomotion upon vision the following instances are note- 

 worthy : 



Sacculina, a degenerate parasitical crustacean, in its early life 

 moves freely about with complex organs of locomotion. It fastens 

 upon the crab's tail and loses its organs of locomotion, losing also its 

 eyes and other organs of special sense. 



The Pinnotheridw, a family of crabs, have vision while moving 



