284 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



vations of Pick proof of a direct mechanical cause of 

 the structure. I have pointed out the phylogeny of 

 the articulations in the Mammalia in various papers 

 from 1877* to iSSg, 2 and in 1881 I advanced the view 

 that their successive evolution was due to impacts and 

 strains {American Naturalist, July, 1881 ; Origin of the 

 Fittest, p. 373). The opinion of Roux entirely sup- 

 ports my position, and it is further established by the 

 elaborate memoir of Tornier just cited. This author 

 adopts the view that bone- development is controlled 

 by Druck und Zug or impact and strain, and he adds 

 some important considerations to those previously ad- 

 vanced. He asserts that " in all existing Vertebrata 

 true bones may appear as secondary structures, since 

 all of these animals possess bands and threads of con- 

 nective tissue which possess the latent capacity to be 

 changed wholly or in part into cartilage.' 3 Thus is 

 accounted for the development of sesamoid bones in 

 tendons, in which category is included the patella. 

 Tornier also shows that the concave articular face 

 (cotylus) is that to which the flexor and extensor mus- 

 cular insertions are nearest, while the convex face 

 (condyle) is the one most remote from the muscular 

 insertions. 



It must be observed that Tornier adopts the lan- 

 guage of the American Neo-Lamarckians in using the 

 expression "impact and strain." Impact and strain 

 are different modes of motion. "Impact' implies 

 pressure, while "strain " implies a pulling stress, either 

 direct or torsional. It is therefore alleged by Tornier, 

 as it has been by myself, that opposite modes of mo- 



1 Report U. S. Gcol. Survey W. of loot It Meridian, 1875, Vol. IV., p. 277- 

 279. Proceeds. Amer. Philosoph. Soc., 1884, p. 44. 



lAmcr. Journal Morphology, 1889, p. 163. 



