460 University of California Publicatiotis in Zoology [Vol.12 



Summary of Relationships of Certain North American 

 Beavers 



some difficulties to precise statement of relationship 



Gregory (1910, p. 105) has called attention to the stumbling-blocks 

 of the phylogenist, namely the two difficulties, first, of distinguishing 

 between primitive and specialized characters, and second, between 

 resemblances significant of genetic relationship and those indicating 

 only convergence or parallelism. 



That parallelism in evolution may be a principle more widespread 

 and of greater significance than is ordinarily accorded it by taxono- 

 mists has recently been suggested (see Scott, 1913, pp. 649-656, and 

 Hopkins, 1914, p. 187). It should be clearly recognized, however, 

 that the weight of evidence indicates (Scott, 1913, p. 137) that inde- 

 pendent origin of closely similar forms from different stocks in widely 

 separated localities is practically unknown. 



Emphasis should also be laid on the importance: (1) of deter- 

 mination of the order of appearance of diagnostic characters ; and 

 (2) of understanding the general adaptive significance of such char- 

 acters, .so that their broader phylogenetic and systematic value can be 

 appraised (Gregory, 1910, p. 112). We are not now in position to 

 prove in what order the diagnostic characters in the family here con- 

 sidered have appeared, or to state exactly the adaptive significance or 

 insignificance of the characters. The fact that it is difficult to ascribe 

 any adaptive value whatever to many of the characters which are 

 diagnostic between subspecies and species of beavers does, however, 

 suggest that these particular characters may be dependable as indi- 

 cating relationship. Examples of such characters are : Different out- 

 lines of tail ; different outlines of nasal bones ; different degrees of 

 development of median process of interpterygoid fossa ; different 

 shapes of foramen magnum ; different breadths of hamular processes 

 (if pterygoids; different widths of bony palate anteriorly. 



There are possible further difficulties. Since we do not certainly 

 know that the evolution of one form has not taken place somewhat 

 more rapidly than that of another, estimates of time of isolation (or 

 age of a particular form) and closeness of relationship should only 

 cautiously be based on degrees of difference. 



CONSIDERATION OF CERTAIN NORTH AMERICAN FORMS 

 Certain problems remain to be considered regarding the relation- 



