No. 7.] ROBINS — NATURAL SELECTION. 415 



by descent, with modifications under natural selection, must still 

 be considered as sub judice. Many related topics of the greatest 

 significance have met but partial, incidental, and quite un- 

 satisfactory treatment. Among these neglected topics is the 

 question of the genesis of individual organs by gradual modifica- 

 tion under natural selection. For example, how has the eye of 

 the vertebrates reached its highest development ? What succes- 

 sive stages of increasing efficiency connect that vague sensibility 

 to sonorous vibrations which may be conceived as inherins; to 

 the whole sarcodous mass of a rhizopod, with the highly complex 

 and efficient auditory apparatus of a man ? From what common 

 structures and along what lines of development have homoloo-ous 

 organs, which subserve different functions, descended ? If we 

 trace back the genealogy of the van of a bat and of the paddle 

 of a porpoise, in what sort of structure will they meet ? And 

 what, from that structure, have been the causes and the courses 

 of a divarication so wide ? Full discussion of a hundred such 

 questions is the necessary but as yet unattempted preliminary to 

 any conclusive deliverance respecting the origin of specific forms 

 through heredity and adaptation. For every specific form re- 

 sults from the integration of a multitude of organs, each of 

 which is of great complexity. The whole, therefore, comprises 

 parts so numerous, and involves correlations so intricate, that 

 no understanding, however comprehensive, can grasp it, before 

 the parts have been separately submitted to exhaustive study. 



When the formal attempt shall be made to account for the 

 formation of complex organs " by the accumulation of innumer- 

 able slight variations, each good for the individual possessor," 

 the following principles will emerge as the necessary conditions 

 to the solution of the problems presented. 



1st. The function which each organ sub3erves must exist in a 

 rudimentary condition in the ancestral germ, or both the func- 

 tion and the organ will be absent from every individual descen- 

 dant. No slight variation can account for the first appearance 

 of a new function. The interval between the absence of a func- 

 tion and its presence, in however small amount, is infinitely 

 greater than between its most rudimentary and its most com- 

 plete manifestation. " Numerous, slight, successive modifica- 

 tions " may account for the development of that which is insig- 

 nificant until it shall attain commanding proportions and interest, 

 but they can never cause that which is non-existent to arise into 

 being. 



