232 



EVOLUTION OF MAMMALIAN MOLAR TEETH 



admitted that the common ancestors of the Osseous Fishes and Mammalia 

 had a skull of decidedly undifferentiated character, with a much less 

 amount of differentiation than is observed in the skulls of either of 

 these groups. It is only in so far as they have parts represented 

 in the common ancestor that we can trace homog<:ny in these groups ; 

 and yet the homology of a vast number of bones in the skull of the 

 two is discussed and pointed out." Suppose, accordingly, that in the 



formation of dermal rooting 



paracone 



metacone 



crochet 

 -postfossette 



prefossette -| 



antecrochet 



protocone- 



FIG. 212. Molar tooth of an Upper Miocene rhinoceros 

 ( T< /' ocerns), showing origin of secondary folds. 



bones in different orders of 

 tishes a pair of bones corre- 

 sponding in position to the 

 parietals should arise inde- 

 pendently, or that in the 

 evolution of the teeth cusps 

 should arise independently 

 having the same form and posi- 

 tion, what criterion should 

 be applied ? All such struc- 

 tures are habitually regarded 

 as homologous, yet it is apparent that they are not derivatives of each 

 other and therefore not homogenous or homologous in the strictest 

 sense. 



Such cases of independent evolution of apparently homologous organs 

 I recently proposed J to signify as potential, or latent homology, borrowing 

 the term " latent " from Galtoii as indicative of a germinal rather 

 than of a patent or adult character, and the physical term " potential " 

 as expressing the innate power or capacity to develop a certain organ. 

 But my colleague, Prof. Edmund B. Wilson, pointed out to me that 

 such cases were almost exactly covered by the original definition of 

 the word " homoplasy " by Lankester ("70, p. 42), as shown in the 

 subjoined quotations from his essay : 



When identical or nearly similar forces, or environments, act on two or more 

 parts of an organism which are exactly or nearly alike, the resulting modifications" 

 of the various parts will be exactly or nearly alike. Further, if, instead of 

 similar parts in the same organism, we suppose the same forces to act on parts 

 in two organisms, which parts are exactly or nearly alike and sometimes homo- 

 genetic, the resulting correspondences called forth in the several parts in the 

 two organisms will be nearly or exactly alike. I propose to call this kind of 

 agreement homoplastic or homoplasy. 3 . . . What is put forward here is this : that 

 under the term "homology," belonging to another philosophy, evolutionists have 



1 In a communication before the National Academy of Science, Nov. 13, 1901. 



2 Italics are mine. 



:i At this time Lankester accepted Herbert Spencer's Lamarckian views. Subsequently 

 he abandoned the mechanical inheritance theory for the pure natural selection theory. 



