130 ON THE ORIGIN OF SPECIES 



Correlation of Growth. — I mean by this expression 

 that the whole organisation is so tied together during 

 its growth and development, that when slight variations 

 in any one part occur, and are accumulated through 

 natural selection, other parts become modified. This is 

 a very important subject, most imperfectly understood. 

 The most obvious case is, that modifications accumulated 

 solely for the good of the young or larva, will, it may 

 safely be concluded, affect the structure of the adult ; 

 in the same manner as any malconformation affecting 

 the early embryo, seriously affects the whole organisa- 

 tion of the adult. The several parts of the body which 

 are homologous, and which, at an early embryonic 

 period, are alike, seem liable to vary in an allied 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 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 

 natural 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 the union of 

 the petals of the corolla into a tube. Hard parts seem 

 to affect the form of adjoining soft parts ; it is believed 

 by some authors that the diversity in the shape of the 

 pelvis in birds causes the remarkable diversity in the 

 shape of their kidneys. Others believe that the shape 

 of the pelvis in the human mother influences by pres- 

 sure the shape of the head of the child. In snakes, 

 according to Schlegel, the shape of the body and the 

 manner of swallowing determine the position of several 

 of the most important viscera. 



The nature of the bond of correlation is very fre- 

 quently quite obscure. M. Is. Geoffroy St. Hilaire has 





