236 EVOLUTION OF MAMMALIAN MOLAR TKK'I'H 



mammals with these original three upper and five lower cusps are 

 derivatives or homogenous. 



Fourthly, it follows that the new cusps of the teeth furnish an 

 example of homoplasy independent of the individual modification. 



Thus, we may say that in the teeth at least homoplasy involves 

 a law of latent or potential homoloyy, without professing to understand 

 what is its significance. 



We should, a priori, expect that if additional cusps were added 

 independently in different families and orders of mammals in different 

 parts of the world, under highly different conditions, the teeth of the 

 higher Mammalia would present very great diversity. As a matter 

 of fact, the new cusps in different families are absolutely uniform up 

 to a certain limit. 1 In the twenty-three orders of placentals and in 

 the seven marsupial families, many of which are adaptively equivalent 

 to orders, the independently developed fourth to eleventh cusps of 

 the upper molars, if so many are developed, are uniform and may 

 be termed homologous ; the eight cusps and folds succeeding the 

 original homogenous three arising, if at all, at similar points and 

 presenting a latent homology or homoplasy. The record in the upper 



molar teeth stands thus : 



HOMOLOGY. 



HOMOGENY HOMOPLASY 



Primitive three cusps common to all Cusps or folds which are or may be 



mammals. independently developed in different 



orders. 



Protocone Hypocone 



Paracone Metaconule 



Metacone Protocol! ule 



Parastyle 

 Mesostyle 

 Metastyle 

 Protostyle 

 Hypostyle 



This expresses the comparison of mammals as a whole. Within 

 many of the orders, such as the Perissodactyla, which arise from six 

 cusped ancestors, the homology is different. 



HOMOLOGY. 



HOMOGENY HOMOPLASY 



Protocone Parastyle 



Paracone Mesostyle 



Metacone Crista 



Hypocone Crochet 



Protoconule Antecrochet, etc. 

 Metaconule 



1 The excess of this limit is in multitubercnlism, or polybunodouty, where cuspules 

 are indefinitely multiplied. 



