TABLES FORMULAS 3 



All of the longer tables are based on formulas. These formu- 

 las are those for the graphs which most closely fit the observed 

 values and their utility lies in giving precision to the values 

 obtained and in making possible interpolations : as a rule how- 

 ever they cannot be used for extrapolation. In this connection 

 determinations of the normal variability are always wanted, yet 

 although this need has been met in a measure, it is far from 

 being satisfied. 



The significance and general use of these tables has already 

 been set forth in the Prologue and need not be dealt with here. 



One use of the tables when these are based on age may how- 

 ever be emphasized. The comparison of the experimental re- 

 sults obtained on animals with the corresponding results on man 

 has heretofore been difficult because of the absence of a good 

 basis for comparison. We have found reason to assume that in 

 the case of the rat the postnatal span of life of three years is 

 approximately equivalent to the span of ninety years in man 

 or to put it another way, that the rat grows thirty times as fast 

 as man. This ratio appears to hold for fractions of the span of 

 life, as well as for the entire span. All of the data for the Al- 

 bino, based on postnatal age, may therefore be compared with 

 the corresponding data for man, if the tune intervals are taken 

 as one for the rat to thirty for man. 



Finally it is desirable to explain here a seeming inconsistency 

 in the arrangement of the material presented. In the Preface 

 the statement is made that Part I deals with the Albino rat, 

 while Part II deals with the Norway. So far as all of the im- 

 portant tables and records are concerned this statement does 

 not need revision. 



It has been found however in arranging the literature that it 

 would prove most useful to include in Part I all of the incidental 

 and general observations on the wild Norway, on the ground 

 that these applied to the entire species, and to reserve for Part 

 II the more precise data which apply to the wild Norway, as 

 contrasted with the domesticated Albino. 



The reader therefore will find in the literature cited in Part 

 I papers referring to M. decumanus, M. norvegicus and Epimys 



