370 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
variations in the structural arrangement of the protoplasmic ele- 
ments of living organisms could have become established, and sub- 
sequently developed in succeeding generations, in the constantly 
changing environment (climatic and otherwise) to which these organ- 
isms must have been exposed. It certainly seems necessary, that the 
modes of energy which, by their action on the living elements of 
protoplasm had caused its molecular modifications, should have con- 
tinued to act on these elements for considerable periods of time, in 
order that these beneficial variations should be established and 
become hereditary. This objection, if valid, would seem seriously 
to affect the soundness of the foundations on which the theory of 
natural selection rests. This difficulty, however, is one capable of 
being satisfactorily met; for there is good reason to suppose that, in 
spite of the adverse influences to which primitive organisms must 
have been subjected, certain of the forces acting upon their living 
protoplasm have been continuously in operation; such, for instance, 
as that form of energy we call light, which we may suppose by its 
constant action on these elements gradually changed their molecular 
structures, and adapted them to its own specific mode of action. 
To illustrate our meaning we may take, as an example, the develop- 
ment of structures such as those which enter into the formation 
of the eyes of two different classes of animals, viz, mollusks and 
vertebrates. 
It seems probable that these structures were derived from a com- 
mon ancestral stock, for they both consist of similar tissues adapted to 
concentrate a definite mode of energy on a specialized form of nervous 
elements, which, in conjunction with work performed by corre- 
sponding cerebral matter, gives rise to visual sensations. There is, 
however, a difference in the arrangement of the internal structures 
of the eyes of mollusks and vertebrates, especially in those tissues 
which are concerned in the adjustment of the focus of the eyes to 
near and distant objects, and also in its nervous apparatus. The 
question is: How are we to account for these differences, supposing 
the eyes of these creatures to have been evolved from a common 
ancestral stock 2 
It seems unlikely that the delicate tissues entering into the for- 
mation of the eyes of vertebrates and mollusks have been built up on 
similar lines by the play of chance variations in their protoplasmic 
elements, produced in response to the action of a constantly varying 
environment. Even supposing slight identical beneficial changes 
in the living matter of these structures had thus been effected, this 
action must have been persistent, otherwise these molecular changes 
would soon have become obliterated; but, as above stated, it would 
be different supposing light acted continuously and directly on the 
protoplasmic elements, so as to change its molecular structure and 
