LAWS OF VARIATION 117 



We shall presently see that simple inheritance often gives the false 

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

 that variations of structure arising in the young or larvae natu- 

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

 parts which are homologous, and which, at an early embryonic 

 period, are identical in structure, and which are necessarily ex- 

 posed 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 be- 

 lieved by some anatomists to be homologous with the limbs. These 

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

 pletely 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 kid- 

 neys. 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 Saint-Hilaire has forcibly remarked that certain mal- 

 conformations 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 deafness, or between the tortoise-shell color 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 

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

 the teeth in the naked Turkish dog, though here no doubt homol- 

 ogy comes into play? With respect to this latter case of correla- 

 tion, 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, 



