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THE AMERICAN NATURALIST [Vol. XLIX 



Cope we note in every instance that as soon as a cusp 

 passes beyond the rudimentary stage it is apt to be ob- 

 served and used in definition. 



So far as we know both rectigradations and allometrons 

 arise continuously, definitely or determinate^, and so 

 far as we have observed they arise adaptively or in an 

 adaptive direction from the very beginning. 



Inheritance. The germinal separability of the " least- 

 characters" known as rectigradations is well illustrated 

 in the case of the " pli caballin," a delicate fold of enamel 



which the French systematic writers a century ago selected 

 as a specific "character" by which the horse (E. cabal- 

 lus) could invariably be distinguished from the ass (E. 

 asinus). They little knew how very ancient and stable 

 this minute character is. We see it strongly developed in 

 the Miocene Mcrychippus. We do not know whether it 



