2 INTRODUCTION 



in each instance something of the conditions under which they 

 have been obtained. It follows therefore that they must not 

 be expected to possess the precision of physical or chemical 

 determinations. Nevertheless, so long as the values here pre- 

 sented are not mistaken for absolute standards representing 

 ideal or final determinations, they may be used with advantage. 



Most of the matter presented is taken from researches al- 

 ready published in full, but in a few instances data from work 

 in progress have been included also. In the latter instance the 

 author's name is followed by (MS with date). 



In a few of the tables already published, mainly from our 

 own laboratory, it has been found necessary to make correc- 

 tions, so that when the tables here printed do not agree with 

 the originals, it is to be assumed that the changes are due to 

 revision. 



Owing to the absence of data for the normal animal or to 

 the failure of the authors to express their results in a quantita- 

 tive form, much of the literature which is cited does not appear 

 in the text. Such papers however often contain valuable in- 

 formation on either the Albino or Norway rat and the citation 

 of them in the bibliography serves to indicate the range of the 

 studies in which this animal has been used. 



Extensive reference tables have been computed for the various 

 characters only as these appear under normal conditions, while 

 the modifications which may be experimentally induced in these 

 characters are merely mentioned or presented very briefly. 



In a number of cases the results are represented by both 

 graphs and tables. The purpose of the graphs is merely to 

 furnish a general view of the form of change which occurs, while 

 for the exact values, the tables must always be consulted. In 

 those tables which are based on size, the body length of the 

 rat, because it is least subject to incidental variations, is the 

 measurement to which the others have been referred. 



It is recognized however that some of the characters are func- 

 tions of age and in that case it is of course necessary to know the 

 age of the animal in order to obtain satisfactory results. 



