II. SIZE OP THE BODY AND ORGANS 101 



at G 6 and G 7 in response to the unfavorable conditions then 

 acting. 



To summarize the foregoing: In captivity for ten genera- 

 tions' the gray rat shows a constant body length on age, but 

 a weight-length relation which differs from that for the wild 

 Norway, and agrees at G 10 with that for the Albino. The>e 

 captive Grays, then, are rats which grow normally in length, 

 but tend to become fatter as the generations progress. 



In these captive Grays the weights of the brain, hypophysis, 

 thyroid, and suprarenals have been followed from GI to (!,. 



Brain weight. As to these values in the ancestors of this 

 series little can be said. The brain weight for the male 

 ancestors was that expected for the wild gray Xorways, 

 reference table 187, but for the female ancestors it was 2.5 

 per cent low. The GI values are definitely lower, as shown 

 iii chart 14. This drop at G! is followed by a recovery in 

 weight, which runs into a sag at G 7 and G 8 and then again 

 recovers. On the average, both sexes show brains 5 per cent 

 or less below the standard for the wild gray Norway, but 

 within the ten generations there has been no progressive 

 diminution in weight. It appears, therefore, that the con- 

 ditions of captivity cause an immediate drop (Gi) in brain 

 weight, but within ten generations this drop does not further 

 increase. 



Hypophysis weight. When compared with the data for 

 the wild strain, the observations on the captive Grays show 

 a tendency to an increase in weight, the values for the males 

 running 7 per cent and for the females 11 per cent above 

 those for the wild strain. Thus, although there is a sag in 

 the graphs at G and G 7 and no progressive change in weight, 

 there appears to be in the hypophysis a slight inei-a>- in 

 weight as the result of captivity. So far as this goes it is a 

 shift toward the relations in the Albino, but it <!.- n. 

 far enough to produce the very heavy female hypophysis 

 characteristic of the Albino. 



v Thyroid v//////. The thyroid gland is the only organ thus 

 far found in which there is a proirre>-ivr lo- s in weiirht. In 



