Page:George Archdall Reid 1896 The present evolution of man.djvu/109

Rh advantages to the individuals distinguished by them." Did such a variation occur as an abnormality it would soon be swamped by interbreeding. But if under stress of scarcity the herd were compelled to supplement its diet with the leaves of the hitherto uneaten plant, it is easy to understand that, other things equal, evolution would occur through the continual survival of those that digested the plant best—and we have no reason to suppose that the other things, keenness of scent, agility, &c., would not on the average be equal, but every reason to suppose that they would. Peculiarities like the above do not spring into the world like Minerva, fully developed, but arise by a slow process of growth.

Mr. Spencer next discusses the question of retrogression—such retrogression, for example, as is exhibited by the eyes of the proteus, an animal now inhabiting dark caverns, but evidently descended from ancestors that lived in the light of day, and possessed well-developed eyes. As usual there are two principal theories whereby it is sought to account for this retrogression; one of which attributes it to the cessation of selection, the other in whole or part to the cessation of use; the one to the progressive lapsing, in consequence of the withdrawal of the preserving influence of selection, of those inborn variations, the accumulation of which, as is supposed by the supporters of this theory, resulted in the fully-developed ancestral eye; the other in whole or part to the progressive lapsing, in consequence of the withdrawal of the preserving influence of use, of those acquired variations, the accumulation of which, as is supposed by the supporters of this theory, resulted in the fully-developed eye. These are the two main positions, but a subsidiary question is—Has Natural Selection—reversed Natural Selection, as it is called in this case—aided Cessation of Selection, or Cessation of Use, in causing retrogression in the eyes of animals to