128 CORRELATED VARIAtiOtf. 



varieties have not been produced, has even been advanced, 

 as proving that acclimatization cannot be effected, for it 

 is now as tender as ever it was ! The case, also, of the 

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

 with much greater weight; but until some one will sow, 

 during 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 seed- 

 lings, with the same precautions, the experiment cannot be 

 said to have been tried. Nor let it be supposed that differ- 

 ences in 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, 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 vari- 

 ations. 



CORRELATED VARIATION. 



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 modi- 

 fied. 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 real obvious cases is that 

 variations of structure arising in the young or larvae natur- 

 ally tend to affect the structure of the mature animal. The 

 several parts 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 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 



