PROLOGUE XI 



great importance, and recognition of them can thus faciUtate the 

 interpretation of differences found in two successive investiga- 

 tions or in those made in other laboratories where other strains 

 of rats have been used. The tables and data offer then base 

 hne values to which those of the controls used in any investiga- 

 tion may be referred. 



Further, the individuals in the group of controls, no matter 

 how carefully chosen, will differ from the individuals in the 

 test group. They cannot therefore be directly compared with 

 one another for the values of any character under considera- 

 tion, but the determination of their respective deviations from 

 the corresponding table values furnishes at once a basis of com- 

 parison. 



As to these table values themselves, one hastens in the first 

 instance to disclaim the suggestion that they furnish standards 

 of the same type as are to be had in the physical sciences. In 

 the very nature of the case such accuracy and constancy is un- 

 attainable, for all animals at all times are in a state of flux. 

 The data must be statistical and always carry a probable error. 



Really all that can be said in the way of apology for the use 

 of the values here given, as basal, is that "once upon a time" 

 there was a group of albino rats living under moderately favor- 

 able conditions which had the various characters with the values 

 here recorded. 



All the present tables might therefore be replaced by new 

 ones which would show a somewhat different series of values, 

 and if two such sets of tables were available, the best criterion 

 for preference would lie, I believe, in the degree to which the 

 values given approximated a position intermediate between the 

 values reported at the time from different laboratories and for 

 different strains. 



In discussing the use of the reference tables, emphasis has 

 been laid on the cases in which the observed values are for one 

 reason or another distinctly different from the table values. 

 This is particularly common in the case of the glandular sys- 

 tem and in the storage organs, while on the other hand, skeletal 

 relations, such as the lengths of the limb bones to the length 



