204 Variations and Orthoplasy 



eye muscles in each generation allowed a more flattened 

 position without impairment of vision ; this position was 

 selected for its utility, particularly when correlated with 

 variations in the direction of the protective colouring ; and 

 with the accumulated variations in bodily position and in 

 coloration, thus screened by the eye adjustments, the sole as 

 we find him attained perfection. This, at any rate, — what- 

 ever we may say as to the leading role thus assigned to 

 vision in this series of adaptations, — shows how a single 

 accommodation of great utility may screen not only coinci- 

 dejit variations, but also correlations of variations, by which 

 a species' characters and habits of life are in a remarkable 

 way transformed. 



In this case it is only the variations in the eye position 

 that can in any sense be called ' coincident ' with the mod- 

 ifications acquired by the individual soles by the use of 

 their eyes; but the other great systems of variations, in 

 bodily position and in coloration, are just as truly screened 

 and developed by these modifications. 



Another instance may be cited, which is due to the writer's 

 observation, and which is accordingly submitted with some 

 misgiving to the scrutiny of expert ornithologists. 



The gray parrot {Psittacus erythacns, Linn.) from the 

 west of Africa has, in company with all individuals of 

 his kind, a very strong upper mandible which is curved 

 sharply downward at the extremity, so that its very sharp 

 point is directed downwards about at right angles to the 

 line of the whole beak. It seems that this curving of the 

 mandible downward subserves the bird great utility as a 

 sort of third foot. In getting down from his perch he 

 constantly extends his head, rests the beak upon the 

 bottom of the cage, and then alights on one foot, while 



