178 CORRELATED VARIATION. [Chap. V. 



together. We shall presently see that simple inheri- 

 tance often gives the false appearance of correlation. 

 One of the most obvious real cases is, that variations of 

 structure arising in the young or larva3 naturally tend 

 to affect the structure of the mature animal. The sev- 

 eral parts of the body which are homologous, and which, 

 at an early embryonic period, are identical in structure, 

 and which are necessarily exposed to similar conditions, 

 seem eminently Hable 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 homolo- 

 gous 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 ren- 

 dered 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 au- 

 thors 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 

 Sehlegel, the form of the body and the manner of swal- 

 lowing determine the position and form of several of the 

 most important Tiseera. 



