THE ENERGY OF EVOLUTION. 491 



On the other hand, it is equally well known that disuse 

 produces diminution of muscular tissues, and through 

 it, a reduction in the quantity of the harder tissue 

 (bone, chitin, etc.) to which it is attached (as muscu- 

 lar insertions, etc.). It was the observation of such 

 well-known phenomena as these that led Lamarck to 

 advance his doctrine of evolution under use and dis- 

 use, and which has led many others to give their ad- 

 herence to such a view. 



Thus much for cell-growth. Another class of mod- 

 ifications of a similar kind may be found in the parts 

 of an organism which consist of a complex of cells, or 

 tissues. Thus the lumen of a small artery is enlarged 

 under the influence of pressure when it is compelled 

 to assume the function of a larger vessel through the 

 interruption of the latter. A part of an internal or 

 external skeleton which is fractured will form an arti- 

 ficial joint at the point of fracture, if the adjacent sur- 

 faces are kept in motion. Marey {^Animal Mechanism, 

 pp. 88-8g) says, "After dislocations the old articular 

 cavities will be filled up and disappear, while at the 

 new point where the head of the bone is actually placed, 

 a fresh articulation is formed, to which nothing will be 

 wanting in the course of a few months, neither articu- 

 lar cartilages, synovial fluid, nor the ligaments to re- 

 tain the bone in place." I have given some illustra- 

 tions of this fact,^ which have come under my observa- 

 tion, and which have an important bearing on the 

 origin of the articulations of the vertebrate skeleton as 

 I have traced them throughout geological time. I 

 have, as I think, conclusively shown that these varied 

 structures have been produced by impacts and strains, 



iPage 275 and Proceeds. Amer. Philos. Soc, 1892, p. 285. 



