6 



INTRODUCTION 



suits 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 Albino, 

 based on postnatal age, may therefore be compared fairly with 

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

 as one for the rat to thirty for man. 4 



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 jat, 

 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. 



4 As an example of the comparison of the rat with man in respect to certain 

 changes which are related to age the observations on the percentage of water in 

 the brain may be quoted Donaldson ('10) : 



TABLE 1 



Comparison of the percentage of water in the encephalon of man and the albino rat 



at corresponding ages 



W = Weisbach, 1868 K = Koch and Mann, '09 



In table 1 the data for man, collected from various studies, are compared with 

 data for the rat on the assumption that the conditions in the rat brain at any 

 age will be represented by those in the human brain at that age multiplied by 

 thirty. 



