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THE AMERICAN NATURALIST [Vol. XLIX 



two directions of movement seen in ontogeny and phy- 

 logeny. Accelerated characters are those which hurry 

 forward and appear in successive generations at earlier 

 and earlier stages in the development of the individual; 

 while retarded characters are those which hold back or 

 slow down and appear in later and still later stages in 

 the development of the individual of succeeding gener- 

 ations. We- know that such ontogenetic movement is 

 shown both in embryonic and phylogenic development of 

 the individual; it causes characters to appear in ontogeny 

 out of the order in which they arise in phylogeny ; it gives 

 rise to the heterochrony of Gegenbaur; its rate is meas- 

 ured by comparing one character with all the other char- 

 acters of an individual. 



\ A / 



Quite distinct is what we may call the phyletic move- 

 ment of a character; its rate is measured by comparing a 

 character in individuals of one phylum with the same 

 character in individuals of other phyla. It is illustrated 

 in the comparison of the secondary cusps of the incisor 

 and premolar teeth in Canis, Alopc.r and Vnlpes; in each 

 phylum the same cusp lias its distinctive rate of evolution 

 and thus may appear early in geologic time or late in 

 geologic time. Thus comparison of the phyletic move- 

 ment of the same " character " in various lines of descent, 



