ORGANIC EVOLUTION—-MACNAMARA. 871 
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.! 
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 distinguish 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.? 
There must have been very many stages in the evolution of eye- 
spots into structures such as those which constitute the eyes of 
mollusks and 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.® 
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 
1Creative 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 oi these lower kinds of beings have been developed from the living protoplasm 
of the outer layers of their body (somatic) cells by chemico-physical action, the color producing enzymes 
of thisspecialized form of matter being stimulated and brought into action by energy derived from sun- 
light. 
8 The Evolution of Living Purposive Matter, by N. C. Macnamara, p. 32. 
