Vlll , PROLOGUE 



modifying factor and we know at present several characters in 

 which the Albino differs from the Norway or the pied varieties. 

 While there are differences between the pied and Albino varie- 

 ties which require further study, they seem to be trifling in 

 comparison with the more conspicuous modifications which have 

 already appeared in the domesticated albino strain as con- 

 trasted with the wild Norway. 



During the past few years several laboratories have reported 

 Albinos of large size, even larger than the wild Norways as 

 commonly found. Here there comes to view the increase in 

 body size which is so common a response in mammals under 

 domestication. 



There is a change also in the physiology of the Albino. 

 Puberty now appears earlier in the domesticated Albino, not 

 only in comparison with the wild Norway, but also in compari- 

 son with our records of 1915. Undoubtedly there are other in- 

 stances of changes of this type not yet noted. Such changes 

 mean that there is a drift in the characters of these domesticated 

 rats, and suggest that after a century more of domestication, 

 records of the structure and functions of the albino rat might 

 distinctly differ from those with which we are here presented. 

 Such a drift is sometimes designated as orthogenesis. 



Returning to the difficulties which arise in the use of the 

 tables here given, it is pertinent to enquire as to the significance 

 of these, when, for example, it becomes necessary to deal with 

 a rat one hundred days old, but which has a body weight two 

 and a half times as large as the value tabulated for that age. 

 Plainly, the age-weight relations do not agree with the values 

 in the table. 



Without going into details, inappropriate here, it may be 

 stated in general that such an overgrown rat will .have, in the 

 case of characters related to body length or body weight, ap- 

 proximately those values which go with its size, despite the 

 fact that it is young for its size, but these in turn will be modified 

 where they are also correlated with age. 



Pursuing the matter a step further, it seems highly probable 

 that the number of cells in the nervous system is characteristic 



