CORRELATIVE ADAPTATION. 177 



by parasitic animals, in which, with the aUeration of the 

 aliment and of the alimentary apparatus, especially of 

 the manducatory portions, is usually combined a trans- 

 formation and retrogression, often extending to the total 

 extinction of the locomotive organs, and of the entire 

 segmentation of the body. But, although the limits are 

 difficult to define, the cause of these associated modifi- 

 cations in the locomotive and alimentary apparatus con- 

 sists 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 or- 

 ganism, as is best shown in animals of both sexes cas- 

 trated 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 

 behind or by the side of one another; hence on the di- 

 vision of labour. This Haeckel has designated divergent 



