CORRELATIVE ADAPTATION. 177 



pa thy. This may be termed correlative adaptation. It 

 might be supposed that the most perspicuous examples 

 would be afforded by parasitic animals, in which, with 

 the alteration of the aliment and of the alimentary 

 apparatus, especially of the manducatory portions, is 

 usually combined a transformation and retrogression, 

 often extending to the total extinction of the locomo- 

 tive organs, and of the entire segmentation of the body. 

 But, although the limits are difficult to define, the cause 

 of these associated modifications in the locomotive and 

 alimentary apparatus consists less in their reciprocal 

 sympathetic influence than in their simultaneous disuse. 



It is, however, by correlative adaptation that, for in- 

 stance, in the short-beaked races of pigeons, the middle 

 toe and astragalus are shortened, and that in the long- 

 beaked races these organs have shared in the elongation. 

 In the case, however, in which short beaks are combined 

 with short feet, a certain share in the shortening of the 

 feet is also owing to disuse ; while where the pigeon- 

 fancier took pleasure in the elongation of the beak by 

 cumulative selection, the correlative elongation of the 

 foot took place in spite of disuse. The most important 

 group of correlative modifications or adaptations, always 

 using this word in its widest acceptation, relates to the 

 sphere of the sexes. Direct attacks on the generative 

 organs manifest their effects on all the rest of the 

 organism, as is best shown in animals of both sexes 

 castrated for the market or for labour. 



We have already seen that the degree of perfection 

 attained in the orders of the Articulata, Annulosa 

 Vertebrata, and partially in the Radiata also, depends 

 on the integration of the originally similar parts lying 



