1 8 RETROGRESSIVE DEVELOPMENT IN NATURE. [IX. 



other theory many facts are incapable of explanation, even 

 assuming the possibility of the hereditary transmission of 

 acquired characters, such as those produced by disuse. 



It is clear that degeneration as a result of disuse can only 

 take place in an organ the activity of w^hich depends upon its 

 exercise, so that a real effect is produced b}^ the discharge of 

 function. The act of seeing involves certain chemical changes 

 in the retina of the eye, and perhaps even in the optic nerve, 

 processes which do not take place when the eye is no longer 

 exposed to light. Flying involves metabolism in the muscles 

 which move the wings, and this also ceases when flight is at an 

 end. So that an actual retrogressive influence is exerted on 

 certain parts of the eye and on the muscles, by disuse. But 

 how can the stamens of a plant be affected by the failure or 

 success of their pollen in finding its w^ay to the stigma of 

 another flower ? Yet we know that hermaphrodite flowers 

 sometimes revert to the original condition in which the sexes 

 were separate, and this by the gradual atrophy of the stamens 

 in one flower and the style in another. Whether this particular 

 case is to be explained by the cessation or by the active 

 operation of natural selection, is another question, which we 

 may proceed to consider. 



After the anthers, in the course of evolution, have withered 

 away and disappeared, their stalks (the filaments) remain, 

 and are often of considerable height and thickness. Slowly 

 and very gradually these degenerate also : we find them quite 

 long in some species, in others short, while in others again 

 they have completely disappeared, only reappearing now and 

 then in single instances to remind us that they were once of 

 normal occurrence. It is true that the filaments are no longer 

 useful, but how can this fact have any direct effect in causing 

 them to degenerate ? Their structure remains the same, the sap 

 circulates in them as before and supplies nourishment to them 

 as well as to the petals and the style. From my point of view 

 the matter is intelligible enough ; for the bare filaments which 

 have lost their anthers are in no way essential to the life of the 

 species : natural selection is powerless to affect them and they 

 gradually degenerate. 



Even more striking instances are to be found in the animal 

 kingdom. Why have most of our domestic animals lost their 



