204 THE ORIGIN OF SPECIES 



brjonic period, arc identical in structure, and which are 

 necessarily 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 r'^ndered } ermanent by selection. 



Hom,oiogous 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 

 homolc ^ous 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 soma 

 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, accord- 

 ing 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. Isid. GeofEroy St.-Hilaire has forcibly remarked that 

 certain malconformations frequently, and that others 

 rarely, coexist, 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 



