104 HELEN DEAN KING AND HENRY H. DONALDSON 



Using the data for the wild gray rat as standards, the 

 effects of captivity for ten generations on the gray Norway 

 rat, as shown by the foregoing data, may be formulated as 

 follows : 



1. Body length on age is not modified. ; 



2. Body weight on body length increases progressively and 

 at G 10 has the relation found in the Albino. 



3. In the organs : 



a. An initial drop in weight (ancestors to GJ apparently 

 occurs in the brain, thyroid, and suprarenals. It cannot be 

 determined in the gonads or bones, as the GI values are not 

 available. 



b. Progressive loss in weight is shown by the thyroid the 

 only organ behaving this way. This is probably due to the 

 character of the food. 



c. A slight increase in weight is shown by the hypophysis. 



d. No progressive loss in weight between GI (or G 4 ) and 

 G 10 occurs in the brain, t hypophysis, suprarenals, gonads, or 

 bones. 



e. A shift in the weight values toward those characteristic 

 for the Albino is shown by the brain, hypophysis, thyroid, 

 and suprarenals. This point cannot be determined in the 

 case of the bones. .,,, 



4. Unfavorable conditions, represented by louse infection, 

 delousing, high temperature, and poor care (chart 1), are 

 associated with low values in the graphs, except in the cases 

 of body length and of the testes. Table 26 gives the genera- 

 tions in which low values appear in the several instances. 

 In the seven cases in which a ^ponse occurs it always ap- 

 pears at G 7 and, with the exception of the brain, always at G 6 , 

 but in two instances, the ovaries ; and the bones, it is seen 

 also at G 5 and in another two instances, brain and supra- 

 renals, at G 8 . The interpretation of the two extreme groups 

 is that, in the case of the ovaries and the bones, unfavorable 

 conditions can modify the weight even after growth is well 

 advanced, while in the case of the brain and suprarenals the 

 modification is either through the young themselves or even 

 through effects on the parents. 



