248 LIFE : OUTLINES OF GENERAL BIOLOGY 



reflex is never exhibited more than a few times \sithin a short period; 

 the creatures soon cease to answer back. 



Much less frequent than reaction to a shadow is reaction to a 

 sudden increase of Hght; but there are instances of animals that 

 move towards the more illumined part of an aquarium lighted from 

 above, though a reverse movement is commoner. Our point is 

 simply that long before there was any "seeing" there was exquisite 

 sensitiveness to light and shade. This discrimination is of obvious 

 importance to animals that live in darkness or are active at night, 

 as also to others which cannot be active except in the light. Of 

 great interest, in insects especially, is the relation between the 

 intensity of the illumination and the tone of the muscles; but this 

 is a very difficult question. 



When a simple animal moves from an illumined to a shaded part 

 of an aquarium lighted from above, or conversely, it is illustrating 

 the light and shade reflex ; but a higher level of reaction is seen when 

 an animal moves towards or away from the source from which the 

 light comes. This "phototactic" movement usually implies some- 

 thing in the way of a genuine eye; yet it is sometimes exhibited by 

 creatures that have not risen to anything higher than an ej^e-spot. 

 In certain cases there is no actual movement from one place to 

 another, but merely an adjustment of the body so that both sides 

 are equally illumined, or equally in the shade. But it was a great 

 step when the eye began to be used like a compass, enabling the 

 animal to steer, either automatically or tentatively, towards an 

 illumined object. In other words, it was a great step when sight 

 b-'came directive. It is very instructive to contrast the circum- 

 ambulating, circuitous track of an animal moving in total darkness 

 with the straight course it pursues when there is an illumined object 

 to serve as a guiding star. 



The next great step in eye evolution was the formation of an 

 image, the visual perception of form. This implied the differentiation 

 of a lens and a retina, and not too simple a retina. Towards the 

 seeing of shapi's there were doubtless many steps, such as the per- 

 ception of moving objects without discrimination of their precise 

 form, and the recognition of obstacles in the path without per- 

 ception of their precise contour. Of much importance must have 

 been the evolution of some method of accommodation, that is to 

 say, some way of adjusting the focus of the eye for objects at 

 different distances, or for the variously distant parts of the same 

 object. This may be effected by altering the distance of the lens 

 from the retina, as is illustrated by marine worms, the sea-snails 

 called heterojwds, the cuttlefishes or squids, most of the true fishes, 

 and the amphibians. In reptiles, birds, and mammals the method is 

 quite different, for accommodation is effected by altering the 

 curvature of the lens. 



