90 ON HEREDITY. 



rendered myopic in the course of a life-time by continually looking 1 

 at objects from a very short distance, even when no hereditary 

 predisposition towards the disease can be shown to exist. Such a 

 change would of course appear more readily if there was also a 

 corresponding predisposition on the part of the eye. But I should 

 not explain this widely spread predisposition towards myopia as 

 due to the transmission of acquired short-sightedness, but to the 

 greater variability of the eye, which necessarily results from the 

 cessation of the controlling influence of natural selection. 



This suspension of the preserving influence of natural selection 

 may be termed Panmixia* for all individuals can reproduce them- 

 selves and thus stamp their characters upon the species, and not 

 only those which are in all respects, or in respect to some single organ, 

 the fittest. In my opinion, the greater number of those variations 

 which are usually attributed to the direct influence of external 

 conditions of life, are to be ascribed to panmixia. For example, the 

 great variability of most domesticated animals essentially depends 

 upon this principle. 



A goose or a duck must possess strong powers of flight in the 

 natural state, but such powers are no longer necessary for obtaining 

 food when it is brought into the poultry-yard, so that a rigid selec- 

 tion of individuals with well-developed wings, at once ceases among 

 its descendants. Hence in the course of generations, a deterioration 

 of the organs of flight must necessarily ensue, and the other 

 members and organs of the birVl will be similarly affected. 



This example very clearly indicates that the degeneration of an 

 organ does not depend upon its disuse ; for although our domestic 

 poultry very rarely make use of their wings, the muscles of flight 

 have not disappeared, and, at any rate in the goose, do not seem 

 to have undergone any marked degeneration. 



The numerous and exact observations conducted by Darwin 

 upon the weight and measurement of the bones in domestic fowls, 

 seem to me to possess a significance beyond that which he attributed 

 to them. 



If the weight of the wing-bones of the domestic duck bears 

 a smaller proportion to the weight of the leg-bones than in the 

 wild duck, and if, as Darwin rightly assumes, this depends not 

 only upon the diminution of the wings, but also upon the increase 

 of the legs, it by no means follows that this latter increase in 



