MM 



320 



LAWS OF VARIATION. 



Chap. XXV. 



the initial or primary variations of the several parts were in any- 

 way related : slight modifications or individual differences may 



have been preserved, first in one and 



m 



part 



until the final and perfectly co-adapted structure was acquired 

 but to this subject I shall presently recur. 'Ag. 

 groups of animals 



an, m many 

 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 destroyed these 



characters disapp 



But 



shown in the twelfth chapter 



that the very same peculiarity may become attached at any 

 age to either sex, and afterwards be exclusively transmitted 



by 



hav 



age 



same 



ponding age. In these 



we 



limited by 



elated with, both sex and 



but we have no reason for supposing that the original 

 of the variation was necessarily connected with the repro- 



ductive organs, or with the age of the affected being 



In 



of 



correlated variation, we are sometimes able 



to see the nature of the connexion ; but in most cases the bond 

 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 simul- 

 taneously produced by some distinct 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 domes- 

 ticated animals and plants, varieties rarely or never differ from 

 each other by some single character alone. 



One of the simplest cases of correlation is that a modification 

 which arises during an early stage of growth tends to influence 

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

 and intimately connected parts. Isidore Geoffroy St. Hilaire 

 states 1 that this may constantly be observed with monstrosities 



1 ' Hist, des Anomalies,' torn. iii. p. arrangement of the nervous system in 



392. Prof. Huxley applies the same the Mollusca, iu his great paper on the 



principle in accounting for the remark- Morphology of the Cephalous Mollusca, 



able, though normal, differences in the in 'Phil. Transact.,' 1853, p. 56. 





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