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 ? 



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 



