CORRELATED VARIATION 155 



kidney-bean has been often cited for a similar purpose, and 

 with much greater weight; but until someone will sow,' dur- 

 ing a score of generations, his kidney-beans so early that a 

 very large proportion are destroyed by frost, and then collect 

 seed from the few survivors, with care to prevent accidental 

 crosses, and then again get seed from these seedlings, with 

 the same precautions, the experiment cannot be said to have 

 been tried. Nor let it be supposed that differences in the con- 

 stitution of seedling kidney-beans never appear, for an ac- 

 count has been published how much more hardy some seed- 

 lings are than others; and of this fact I have myself ob- 

 served striking instances. 



On the whole, we 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 some- 

 times overmastered by, the natural selection of innate 

 variations. 



CORRELATED VARIATION 



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 accu- 

 mulated through natural selection, other parts become modi- 

 fied. This is a very important subject, most imperfectly 

 understood, and no doubt wholly dift'ercnt 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 larvre nat- 

 urally 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 struc-_ 

 ture, and which are necessarily exposed to similar condi- 

 tions, 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 be- 

 lieved by some anatomists to be homologous with the limbs. 

 These tendencies, I do not doubt, may be mastered more or 



