PALEONTOLOGY ZORRITOS FORMATION 21 



beyond the possible introduction of a few exotic or other- 

 wise interesting relationships. 



The treatment of a fractional fauna such as that here con- 

 sidered is more a matter of judgment than of tabulation 

 and derivation of percentages. The system of expressing 

 fauna! relationships through percentages is faulty under the 

 best circumstances if the specific determinations or the align- 

 ment of affinities is at all loose, and so much variation is 

 encountered in both of these matters that the evidence of a 

 group of species needs to be tempered by a differential 

 weighing of the strength of individual determinations and 

 the degree of closeness in relationship shown by separate 

 forms. Could systematic paleontology be standardized the 

 student would be freed of the necessity for examining closely 

 the results obtained by others, but the element centering 

 about the personal equation is inevitable in its effect on the 

 character of a study, and the student must take separate 

 account of determinations if his work is to be balanced. So 

 much depends on the drawing of specific lines that a work 

 in which it is necessary to make close distinctions is sure to 

 reflect more the character of the worker than that of the 

 fauna in matters which hinge on minute determinations. 

 And it is obvious that anything verging on predisposition is 

 dangerous. 



Although in the present work the major elements will be 

 expressed in the form of percentages, their value must be 

 considered to lie in the preponderance of affinity they reveal 

 rather than in any intrinsic quality of the figures themselves. 

 It is obvious that the smaller relationships, such as those 

 with the recent faunas, can not bear much weight for two 

 reasons which hinge on the size of the fauna: first, the 

 incompleteness of the assemblage makes an attempt at 

 expressing proportions probably false, and second, the per- 

 centages obtained in dealing with smaller total numbers of 

 fossils are out of proportion with the ideas the numbers 

 should actually express. In a group of forty species two 



