296 



LAWS OF VARIATION. 



Chap. XXIV. 



inserted the spur of a cock into the comb, which is well supplied 



with blood-vessels, it g 



spiral dir 



length of six inches, and in another case forward, like a ho 



that the bird could not touch the 



ound with its beak 



whether Mr. Herbert Spencer's view of the exudation of 

 matter due to increased movement and pressure 



But 



will 



for the 



augmen 



ted 



fully 



of bones, ligaments, and espe- 



ially of internal glands and nerves, seems doubtful 



ding to the interesting observations of M. Sedillot. 



Ac 



por 



of 



bone of 



leg: or fore-arm of 



t> 



emoved and 



placed by g 



is 



the associated bone 



enlarges till it attains a bulk equal to that of the two bones of 

 which it has to perform the functions. This is best exhibited 

 in dogs in which the tibia has been removed • the companion 

 bone, which is naturally almost filiform and not one-fifth the size 

 of the other, soon acquires a size equal to or greater than the 

 tibia. Now, it is at first difficult to believe that increased weight 

 acting on a straight bone could, by alternately increased and 

 diminished pressure, cause nutritive matter to exude from the 

 vessels which permeate the periosteum. Nevertheless, the obser- 



adduced by Mr. Sp 



15 



the strengthening of 



bowed bones of rickety children, along their concave sides 

 to the belief that this is possible. 



Mr. H. Spencer has also shown that the ascent of the sap ii 

 trees is aided by the rocking movement caused by the wind 

 and the sap strengthens the trunk " i 



a 



proportion to the stress 

 be borne ; since the more severe and the more repeated the 



greater must be the exudation from the vessels 

 rounding tissue, and the greater the thickening of 



the 



" this tissue by secondary deposits." 16 But woodv trunks may 

 be formed of hard tissue without their having be'en subjected 

 any movement, as we see with ivy closely attached to old 



In all these cases, it is very difficult to disentangle the 



walls 



effects of long-continued selection ft 



those 



ised action or movement of the part. Mr. H. Sj 



quent 



„17 



acknowledges this difficulty, and gives as an instance the spines 



14 

 15 

 16 



< Comptes Rendus,' Sept. 26th, 1864, p. 539. 

 ' The Principles of Biology/ vol. ii. p. 243. 

 Idem, vol. ii. p. 269. V Idem, vol. ii. p. 273, 



i 



pa 



k 



is 



