USE OF TABLES 6 



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, be- 

 cause it is least subject to incidental variations, is the measure- 

 ment 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. 



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 ob- 

 tained and in making possible interpolations : as a rule however 

 they cannot be used for extrapolation. In this connection de- 

 terminations of the normal variability are always wanted, yet 

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

 satisfied. 



Since heretofore tables of this nature have not been commonly 

 available, a word as to their use is in order. 



There exist now and there will probably continue to appear 

 strains of the Albino having physical peculiarities related to the 

 locality in which they are bred: e.g., a relatively short tail. 

 The treatment of such an instance by the use of the tables is 

 considered in the paragraph which follows. 



As has been stated, the tabular values here given apply to the 

 stock strain reared at The Wistar Institute and furnish data from 

 which deviations found in other -local strains can be measured. 

 In all experimental work it is now generally agreed that the 

 control and the test animals should be taken from the same lit- 

 ter, and the determinations of any modification made within the 

 litter the results for the several litters being given the same 

 statistical weight in the subsequent computations. While this 

 procedure might at first seem to render the reference tables su- 

 perfluous, yet to compare the results from two laboratories work- 

 ing with different local strains, having according to the example 

 chosen different normal tail lengths, a series of reference values 



