l ^ Correlated Variation. Chap. v. 



tion would inevitably tend to preserve those individuals which were 

 born with constitutions best adapted to any country which they 

 inhabited. In treatises on many kinds of cultivated plants, certain 

 varieties are said to withstand certain climates better than others ; 

 this is strikingly shown in works on fruit-trees published in the 

 United States^ in which certain varieties are habitually recom- 

 mended for the northern and others for the southern States ; and as 

 most of these varieties are of recent origin, they cannot owe their 

 constitutional differences to habit. The case of the Jerusalem 

 artichoke, which is never propagated in England by seed, and of 

 which consequently new varieties have not been produced, has even 

 been advanced, as proving that acclimatisation 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 seedlings, with the same precautions, the experiment 

 cannot be said to have been tried. Nor let it be supposed that 

 differences 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 sometimes 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 accumulated through 

 natural selection, other parts become modified. This is a very im- 

 portant 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 of the body which are homologous, and which, at an early 

 embryonic period, are identical in structure, and which are neces- 



