Chap. V. CORRELATION OF GROWTH. 143 



use, and disuse, have, in some cases, played a consider- 

 able part in the modification of the constitution, and 

 of the structure of various organs ; but that the effects 

 of use and disuse have often been largely combined 

 with, and sometimes overmastered by, the natural selec- 

 tion of innate differences. 



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 



