CHAP. V ] CORRELATED VARIATION. 107 



appearance of correlation. One of the most obvious real cases is, 

 that variations of structure arising in the young or Lrvae naturally 

 tend to affect the structure of the mature animal. The several 

 parts of the body which are homologous, and which, at an early 

 embryonic period, are identical in structure, and which are neces- 

 sarily exposed to similar conditions, seem eminently liable to vary 

 in a like manner : we see this in the right and left sides of the 

 body varying in the same manner ; in the front and hind legs, and 

 even in the jaws and limbs, varying together, for the lower jaw is 

 believed by some anatomists to be homologous with the limbs. 

 These tendencies, I do not doubt, may be mastered more or less 

 completely by natural selection ; thus a family of stags once existed 

 with an antler only on one side ; and if this had been of any great 

 use to the breed, it might probably have been rendered permanent 

 by selection. 



Homologous parts, as has been remarked by some authors, tend 

 to cohere ; this is often seen in monstrous plants : and nothing is 

 more common than the union of homologous parts in normal struc- 

 tures, as in the union of the petals into a tube. Hard parts seem 

 to affect the form of adjoining soft parts : it is believed by some 

 authors that with birds the diversity in the shape of the pelvis 

 causes the remarkable diversity in the shape of their kidneys. 

 Others believe that the shape of the pelvis in the human mother 

 influences by pressure the shape of the head of the child. In 

 snakes, according to Schlegel, the form of the body and the 

 manner of swallowing determine the position and form of several 

 of the most important viscera. 



The nature of the bond is frequently quite obscure. M. Is. 

 Geoffroy St. Hilaire has forcibly remarked, that certain malcon- 

 formations frequently, and that others rarely, co-exist, without 

 our being able to assign any reason. What can be more singular 

 than the relation in cats between complete whiteness and blue 

 eyes with deafness, or between the tortoise-shell colour and the 

 female sex ; or in pigeons between their feathered feet and skin 

 betwixt the outer toes, or between the presence of more or less 

 down on the young pigeon when first hatched, with the future 

 colour of its plumage ; or, again, the relation between the hair and 

 teeth in the naked Turkish dog, though here no doubt homology 

 comes into play? With respect to this latter case of correlation, 

 I think it can hardly be accidental, that the two orders of 

 mammals which are most abnormal in their dermal covering, viz., 

 Cetacea (whales) and Edentata (armadilloes, scaly ant-eaters, 

 <fec.), are likewise on the whole the most abnormal in their teeth ; 

 but there are so many exceptions to this rule, as Mr. Mivart has 

 remarked, that it has little value. 



I know of no case better adapted to show the importance of the 



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