CORRELATIVE ADAPTATION. 177 
pathy. 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 
