EFFECTS OF INBREEDING ON BODY WEIGHT 5 



Fortunately for this work, many rats in the general stock 

 colonj^, in which there was no inbreeding, exhibited the same 

 characteristics as the rats belonging to the inbred strain. It 

 was evident, therefore, that the unfavorable condition of the 

 animals could not justly be attributed to inbreeding alone. 

 On investigation it was found that all of the rats were suffering 

 from malnutrition due to the character of the food that they 

 received. At the time that these experiments were begun the 

 rat was used as a laboratory mammal in only a few of the larger 

 research institutions in the country, and little was known of 

 the environmental and nutritive conditions best suited to its 

 needs. Following the plan of feeding in general use in other 

 animal colonies, the rats were fed chiefly on bread soaked in 

 milk and on corn; meat and vegetables being given only once 

 each week: Such a diet does not furnish the proportion of food 

 elements that the rat requires if it is to be kept in good physical 

 condition: there is too much starch, too little protein. In the 

 spring of 1911 a radical change was made in the rats' food. 

 Milk and fresh bread Avere eliminated from the diet and ' scrap ' 

 food, consisting of carefully sorted table refuse, was fed once 

 each day; cobcorn being kept in the cages as an extra ration. 

 Such a diet has proved to be a most satisfactory one, and it has 

 been used continually up to the present time, except that dog 

 biscuit has been substituted for cobcorn as extra food supply. 

 This change was made last year following the loss of a consid- 

 erable number of animals through intestinal disturbance caused 

 by the eating of fermented corn. 



A very marked improvement in the general condition of all of 

 the rats in the colony was noted very soon after the diet was 

 changed. The animals gained in size and in weight, sterility 

 almost disappeared, and the average number of young in the 

 litters was increased. From this time on malformations were 

 no longer common, and not a single instance of deformed teeth 

 has been discovered in the thousands of animals that have been 

 bred in the colony during the past five years. Simply by a 

 change in the food characteristics said to typify the 'dire effects 

 of inbreeding' were eliminated, and up to the present time they 



