286 LAWS OF VARIATION. Ohap. XXIV. 



grow, even in. young children, near old-standing inflamed 

 surfaces or fractured bones by an increased flow of blood to 

 the part. When Hunter inserted the spur of a cock into the 

 comb, which, is well supplied with blood-vessels, it grew in 

 one case spirally to a length of sis inches, and in another case 

 forward, like a horn, so that the bird could not touch the 

 ground with its beak. According to the interesting observa- 

 tions of M. Sedillot, 14 when a portion of one of the bones of 

 the leg of an animal is removed, 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 

 that of the tibia. Kow, it is at first difficult to believe that 

 increased weight acting on a straight bone could, by alternately 

 increasing and diminishing the pressure, cause the blood to 

 flow more freely in the vessels which permeate the periosteum 

 and thus supply more nutriment to the bone. Nevertheless 

 the observations adduced by Mr. Spencer, 15 on the strengthen- 

 ing of the bowed bones of rickety children, along their con- 

 cave sides, leads to the belief that this is possible. 



The rocking of the stem of a tree increases in a marked 

 manner the growth of the woody tissue in the parts which are 

 strained. Prof. Sachs believes, from reasons which he assigns, 

 that this is due to the pressure of the bark being relaxed in 

 such parts, and not as Knight and H. Spencer maintain, to 

 an increased flow of sap caused by the movement of the 

 trunk. 16 But hard woody tissue may be developed without 

 the aid of any movement, as we see with ivy closely attached 

 to an old wall. In all such cases, it is very difficult to distin- 

 guish between the effects of long-continued selection and those 

 which follow from the increased action of the part, or directly 

 from some other cause. Mr. H. Spencer 17 acknowledges 

 this difficulty, and gives as an instance the thorns on trees 



14 'Comptes Rendus,' Sept. 26th, 16 Ibid., vol. ii. p. 269. Sachs, 

 1864. p. 539. 'Text-book of* Botany,' 1875, p. 734. 



15 H. Spencer, 'The Principles of 1; Ibid., vol. ii. p. 273. 

 Biology,' vol. ii. p. 243. 



