136 CORRELATED VARIATION. 



the constitution of seedling kidney-beans never appear, for 

 an account has been published how much more hardy some 

 seedlings are than others; and of this fact I have myself 

 observed striking instances. 



On the whole, Ave may conclude that habit, or use and 

 disuse, have, in some cases, played a considerable part in 

 the modification of the constitution and structure; but 

 that the effects have often been largely combined with, and 

 sometimes overmastered by, the natural selection of innate 

 variations. 



CORRELATED VARIATIOi^r. 



I mean by this expression that the whole organization is 

 so tied together, during its growth and development, that 

 when slight variations in any one part occur and are accu- 

 mulated through natural selection, other parts become 

 modified. This is a very important subject, most 

 imperfectly understood, and no doubt wholly different 

 classes of facts may be here easily confounded together. 

 We shall presently see that simple inheritance often gives 

 the false appearance of correlation. One of the most 

 obvious real cases is, that variations of structure arising in 

 the young or larvae naturally tend to affect the structure of 

 the mature animal. The several parts which are homo- 

 logous, 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 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 per- 

 manent 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 homolo- 

 gous parts in normal structures, as in the union of the 



