Chap. V.] CORRELATED VARIATION. 115 



earily 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 

 structures, 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 motber 

 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 (armaclilloes, scaly ant-eaters, &c), 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 

 laws of correlation and variation, independently of utility and 

 therefore of natural selection, than that of the difference between 



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