ORGANIC EVOLUTION — MACNAMAKA. 371 



to adapt it to its own specific mode of action. The resemblance of 

 the tissues of the eyes of vertebrates and mollusks would thus be 

 referable to an identical force or cause. The more and more complex 

 eyes of vertebrates would be something like the deeper and deeper 

 impression of light on a substance, which, being organized, possesses 

 a special aptitude for receiving it. 1 



In many unicellular and some invertebrate beings, red spots of 

 coloring matter may be seen on their outer surface. These are known 

 as "eye-spots," for in some of them lens-like structures exist which 

 are analogous to those of the eyes of the higher orders of animals. 

 There is reason to suppose that by the action of light on the substance 

 forming these eye-spots, organisms possessing these structures are 

 enabled to distmguish light from darkness. Animals having more 

 highly developed eye spots seem to be sensitive to alterations 

 in the intensity of light; their rudimentary organs of vision may 

 therefore, in a vague way, assist these organisms to guide the move- 

 ments of their bodies. 2 



There must have been very many stages in the evolution of eye- 

 spots into structures such as those which constitute the eyes of 

 mollusks arid vertebrates, and some of these stages may be traced 

 from one to another through the ascending order of beings. Each 

 stage consisting in the purposive adaptation of the structures entering 

 into the formation of the eye to the requirements of each order of indi- 

 viduals. Beyond this natural selection is no longer operative, because 

 a further specialization of structures entering into the construction 

 of the organ of vision would not assist this particular order of beings 

 in their struggle for existence. It would, for instance, be of no advan- 

 tage to a scallop, as it is to human beings, to possess a complex 

 arrangement of structure adapted to instantaneously focus its eyes on 

 near and distant objects. 3 



So far as our knowledge extends regarding existing orders and 

 species of animals, we do not find any indications of sudden changes 

 taking place in the structures entering into the construction of their 

 organs of vision. On the other hand, we can account for their 

 undoubted progressive development by supposing their eyes to have 

 been evolved by the continued action of light on living matter, which 

 under the operation of the laws of natural selection has gradually 

 been molded into a form adapted to respond to this mode of energy. 

 In other words, the action of light on the living purposive elements of 



i Creative Evolution, by H. Bergson, p. 73. It is as Prof. Crampton remarks (Doctrine of Evolution, 

 p.31) that organisms are in a true sense complicated chemical mechanisms adapted to meet the conditions 

 under which they must operate. 



2 The rudimentary eyes of these lower kinds of beings have been developed from the living protoplasm 

 of the outer layers of their body (somatic) cells by chemico-physica! action, the color producing enzymes 

 of thisspecialized form of matter being stimulated and brought into action by energy derived from sun- 

 light, 



» The Evolution of Living Purposive Matter, by N. C. Macnamara, p. 32. 



