Chap. V. CORRELATED VAKIATION. I47 



seriously to .ifFect the whole org'anization of the adult. The 

 several parts of the body which are homolof^ous, and which, 

 at an early embryonic period, are identical in structure, and 

 which are necessarily exposed to similar conditions, seem emi- 

 nently 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 

 bs 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 the imion of the petals of the corolla 

 into a tube. Hard parts seem to affect the forms of adjoining 

 soft parts ; it is believed by some authors that the diversity in 

 the shape of the pelvis in birds causes the remarkable diver- 

 sity 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 

 Schlegol, the shape of the body and the manner of swallowing 

 determine the position and form of several of the most impor- 

 tant viscera. 



The nature of the bond of correlation is frequently quite 

 obscure. M. Isidore Geoffroy St.-Hilaire has forcibly remarked 

 that certain malconformations very frequently, and that others 

 rarely coexist, without our being able to assign any reason. 

 ^\''hat can be more singular than in cats the relation between 

 complete whiteness with blue eyes and deafness, or between 

 the tortoise-shell color and the female sex? or in pigeons be- 

 tween their feathered feet and skin betwixt the outer toes, or 

 b;^tween the jiresence of more or less down on the young birtl 

 when first hatched, with the future color 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 tliis latter case of correlation, I think it can 

 hardly be accidental, that, if we pick out the two orders of" 

 mammals which are most abnormal in their dermal covering, 

 viz., Cetacca (whales) and Edentata (amindilloes, scaly ant- 



