CHAPTER X. 



RECTIGRADATIONS AS A RESULT OF LATENT OR POTENTIAL 

 HOMOLOGIES IN THE TEETH. 



BY reotigradations I refer to the origin of new cusps or cuspules which 

 appear determinately, definitely, orthogenetically in both the upper 

 and lower teeth quite independently in different orders of mammals, 

 and separated perhaps by vast intervals of time. 



There is some law of predisposition operating here. If it were 

 not for this law the cusps of the teeth of mammals would present 

 an infinite variety of origin, whereas they actually present a singular 

 uniformity of origin except in the multituberculates and other possible 

 exceptions noted in the preceding chapters. It is the modelling of 

 the cusps after they appear which gives the infinite variety. 



We do not know what conditions this " law of predisposition " : 

 we only see evidence of the influence of community of origin or 

 hereditary kinship. 



In 1902 I supposed this law was what E. Hay Lankester meant by 

 Homoplasy, as shown below, but it appears from his letter cited below 

 (p. 239) that I was mistaken Lankester's homoplasy is equivalent to 

 analogous evolution, to parallelism, or convergence. 



1. HOMOPLASY AS A LAW OF LATENT oit POTENTIAL HOMOLOGY. 



(Reprinted under the title given above from The American Naturalist, Vol. XXXVL, 



April, 1902, No. 424, pp. 259-271.) 



My study of teeth in a great many phyla of Mammalia in past times has con- 

 vinced me that there are fundamental predispositions to vary in certain directions ; 

 that the evolution of the teeth is marked out beforehand by hereditary influences 

 which extend back hundreds of thousands of years. These predispositions are 

 arovised under certain exciting causes and the progress of tooth development takes 

 a certain form converting into actuality what has hitherto been potentiality. 



Science, N.S., Vol. VI., No. 146 (Oct. 15, 1897), pp. 583-587. 



Ill previous communications, as shown in the above quotation, I 

 have spoken of the " potential of similar variation," as covering cases 



