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 larvse naturally tend 

 to affect the structure of the mature animal. The 

 several 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 liable to vary in a like 

 manner : we see this in the risdit 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 

 Sehlegel, the form of the bodv and the manner of swallow- 

 ing determine the position and form of several of the 

 most important viscera. 



