72 HELEN DEAN KING 



substitutes that from time to time were added to the diet in 

 order to vary it, such as alfalfa, Unseed and cottonseed meal, 

 proved very injurious to the rats and very materially affected 

 their growth and fertility. For some time, therefore, the food 

 given the animals has been largely in the nature of an experiment, 

 and it has not even yet been possible to work out a ration that 

 produces as rapid and vigorous growth and that is as favorable 

 to reproduction as was the 'scrap' focd given previously. 



Extremes of temperature, either of heat or of cold, have a 

 very marked effect on the body growth of the rat, as they have 

 on that of mice (Sumner, '09), and many of the animals in the 

 later generations of the inbred strain suffered considerably from 

 this cause. During the excessive cold of the winter of 1917-1918 

 it was impossible to keep the colony house above the freezing 

 point for days at a time, and in consequence the rats ceased 

 growing at a normal rate and many of them developed pneu- 

 monia. The periods of intense heat experienced during the 

 summer of 1918 also had a very deleterious effect on the vitality 

 and on the body growth of the rats. As a result of the combined 

 action of these various factors, all inimical to growth as well as to 

 leproduction, the rats of the eighteenth to the twenty-fifth 

 generations were severely handicapped, and they did not increase 

 in body weight as rapidly, nor did they attain as great a maxi- 

 mum body weight, as did the individuals of the earlier gener- 

 ations. That this decrease in the size of the inbred animals 

 was caused by unfavorable conditions of environment and of 

 nutrition, and not by continued inbreeding, is shown conclusively 

 by the fact that the body weights of hundreds of rats in the 

 outbred-stock colony were just as seriously affected by these 

 adverse conditions as were those of the inbred rats, as will be 

 shown later. 



Data showing the average body weights at different ages of 

 179 males and of 130 females belonging in the sixteenth to the 

 twenty-fifth generations of the A series of inbred rats are given 

 in table 1 and in table 2: similar data for 117 males and for 180 

 females belonging in the same generations of the B series of 

 inbreds are given in table 3 and in table 4. 



