312 LAWS OF VARIATION. Chap. XXV. 



acquired ; but to this subject I shall presently recur. Again, 

 in many groups of animals the males alone are furnished with 

 weapons, or are ornamented with gay colours ; and these 

 characters manifestly stand in some sort of correlation with 

 the male reproductive organs, for when the latter are de- 

 stroyed these characters disappear. But it was shown in the 

 twelfth chapter that the very same peculiarity may become 

 attached at any age to either sex, and afterwards be exclu- 

 sively transmitted to the same sex at a corresponding age. 

 In these cases we have inheritance limited by both sex and 

 age ; but we have no reason for supposing that the original 

 cause of the variation was necessarily connected with the 

 reproductive organs, or with the age of the affected being. 



In cases of true correlated variation, we are sometimes able 

 to see the nature of the connection ; but in most cases it is 

 hidden from us, and certainly differs in different cases. We 

 can seldom say which of two correlated parts first varies, 

 and induces a change in the other ; or whether the two are 

 the eifects of some common cause. Correlated variation is 

 an important subject for us ; for when one part is modified 

 through continued selection, either by man or under nature, 

 other parts of the organisation will be unavoidably modified. 

 From this correlation it apparently follows that with our 

 domesticated animals and plants, varieties rarely or never 

 differ from one another by a single character alone. 



One of the simplest cases of correlation is that a modification 

 which arises during an early stage of growth tends to influ- 

 ence the subsequent development of the same part, as well as of 

 other and intimately connected parts. Isidore Geoffroy Saint- 

 Hilaire states l that this may constantly be observed with 

 monstrosities in the animal kingdom ; and Moquin-Tandon 2 

 remarks, that, as with plants the axis cannot become mon- 

 strous without in someway affecting the organs subsequently 

 produced from it, so axial anomalies are almost always 



1 'Hist, des Anomalies,' torn. iii. p. on the Morphology of the Cephalous 



392. Prof. Huxley applies the same Mollusca, in ' Phil. Transact.,' 1853, 



principle in accounting for the re- p, 56. 



markable, though normal, differences 2 ' Elements de Teratologic Veg., 



in the arrangement of the nervous 1841, p. 13. 

 system in the Mollusca, in his paper 



