Xii PEOLOGUE 



of the body, show a higher degree of constancy. Indeed, when 

 one takes at random an albino rat of moderate size and com- 

 pares the values for its characters with those given for the body 

 length or body weight in the appropriate tables, one finds to- 

 day in the case of Albinos from our own colony a good agree- 

 ment, between the observed and the table values, for a large 

 number of characters. 



On the other hand, in some characters a distinct shift has 

 occurred. For example, the weight of the thyroid gland in 

 rats from The Wistar Institute Colony is to-day distinctly be- 

 low that given in the tables. 



This compilation is therefore put forth in the hope that the 

 reference data here given can be used to control and to make 

 comparable the results of investigations carried out at different 

 times and places and with different strains, and also with the 

 hope that they may furnish a set of records by which further 

 changes in the albino rat due to continued domestication can 

 be measured. 



The reference tables have been retained largely unaltered 

 because the several determinations were mainly on the same 

 series of rats — and any particular revision for one or more 

 organs would introduce a disharmony in this group of records. 



A few words touching the qualifications of the rat as a labo- 

 ratory animal may be added. 



The larger part of the information contained in this memoir 

 has come from the study of the domesticated albino rat, which 

 is a mutant from the wild Norway, but the general bielogy of 

 all the color varieties of this species is similar. 



The wild Norway rat is cosmopolitan and its albino variety 

 is also found in all countries where small pets are kept. It is 

 omniverous, with a range of diet as wide as that of man. It 

 breeds freely at all seasons but most actively in the spring and 

 early fall. The litters are large and the sexes appear in about 

 the same ratio as in man^ — so that control and test animals 

 of like sex may be had from the same litter — thus giving the 

 closest relationship. 



