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THE AMEBIC AX NATURALIST [Vol. XL VI 



in the sense of Waagen, who proposed this inter-specific 

 term in 1869. Finally they reach a sufficiently impor- 

 tant phase to designate the stage as a species. 20 



These new characters were first (1891) termed "defi- 

 nite variations"; subsequently (1907 ) 27 the term "recti- 

 gradations" was applied to them. Rectigradation is 

 merely a designation for the earliest discernible stages 

 of certain absolutely new characters. It involves no 

 opinion nor hypothesis as to genesis ; it is a simple mat- 

 ter of observation. Eef erring to the figure (p. 274) of 

 the upper grinding teeth of the horse, the majority of 

 the fourteen characters have been observed to arise as 

 rectigradations. 



Quite different is the allometron. This is a new desig- 

 nation for the continuous change of proportion in an ex- 

 isting character which may be expressed in differences 

 of measurement. Since 1902 and especially during the 

 past year the behavior of allometrons has been very 

 carefully investigated by myself and by my colleague, 

 Dr. W. K. Gregory. 



Rectigkadatiox == a qualitative change, the genesis 

 of a new character. 



Allometrox = a quantitative change, the genesis 

 of new proportions in an ex- 

 isting character. 



The distinction between a rectigradation and an allo- 

 metron is readily grasped: when the shadowy rudiment 

 of a cusp or of a horn first appears it is a rectigradation; 

 when it takes on a rounded, oval or flattened form this 



M This sentence may be contrasted with that of Punnett (op. cit., p. 15) : 



