Chap. V. Correlated Variation. 1 1 5 



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 

 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 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 m 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 

 lurlasn dog, though here no doubt homology comes into play? 

 With respect to this latter case of correlation, I think it can hardly 



abnZll ^ hat f e tW ° ° rderS 0f mammals whi <* are most 

 S T ?n dGrmal C0Verin - ™, Cetacea (whales) and 



- option f°f abn °5 mal in their teeth ; but there are so many 

 littie X thlS mle ' M Mr ' Mivart ^ remarked, that it has 



l^Tct2Z Q lT ^^ t0 Sh0Wthe importance cf the 



th^cw^^^™^ ^ependently of utility and 



natural selection, than that of the difference between 



i 2 



