106 CORRELATED VARIATION. [CHAP. V. 



much to both means combined, is an obscure question. That habit 

 or custom has some influence, I must believe, both from analogy 

 and from the incessant advice given in agricultural works, even ia 

 the ancient Encyclopaedias of China, to be very cautious in trans- 

 porting animals from one district to another. And as it is not 

 likely that man should have succeeded in selecting so many breeds 

 and sub-breeds -with constitutions specially fitted for their own 

 districts, the result must, I think, be due to habit. On the other 

 hand, natural selection Avould 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 varie- 

 ties are habitually recommended 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 

 someone 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 acci- 

 dental 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. 

 Ws shall presently see that simple inheritance often gives the false 



