520 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



origin of the peculiar dentition of the Glires, 1 and a 

 third on the mechanical origin of the dentition of the 

 Amblypoda. 2 In 1889 I discussed the mechanical 

 causes of the structures of the elbow and other joints 

 in the Artiodactyla and the origin of the peculiar in- 

 tervertebral articulations in that order. 3 In the same 

 year I published a resume of the work done on this 

 subject with reference to the Mammalia. 4 Since that 

 time important contributions to the subject have been 

 made by Ryder, Osborn, Wortman, Dall, Jackson, 

 and others, to which reference will now be made. 



Hyatt says as a result of his exhaustive studies 

 of the phylogeny of the Cephalopoda 5 ; "The ac- 

 tion of physical changes takes effect upon an irrit- 

 able, plastic organism which necessarily responds to 

 external stimulants by an internal reaction or effort. 

 This action from within upon the parts of organisms 

 modifies their hereditary forms by the production of 

 new growths or changes, which are therefore adapted 

 or suitable to the conditions of the habitat, and are 

 therefore physiologically and organically equivalent to 

 the physical agents and forces from which they directly 

 or indirectly originated. In so far then, as causes and 

 habits are similar, they probably produce representa- 

 tion or morphological equivalence in different series of 

 the same type in similar habitats : and in so far as 

 they are different, they probably produce the differen- 



1 American Naturalist, January, 1888, p. 3. 



2 Proceeds. Amer. Philosoph. Soc., 1888, p. 80. 



3 American Naturalist, March, 1889. 



4 The American Journal of Morphology, September, 1889, pp. 137-277, " On 

 the Mechanical Causes of the Development of the Hard Parts of the Mam- 

 malia." 



5 The Genesis of the Arietidce, by Alpheus Hyatt ; Smithsonian Contribu- 

 tions to Knowledge, and Memoirs of the Museum of Comparative Zodlogy, Vol. 

 XVI., No. 3, 1889. 



