EFFECTS OF INBREEDING AND CROSSBREEDING. 9 



ciable extent. Most but not all of the matings in the control stock 

 were made between immature guinea pigs. A single pair has been 

 kept in each pen as already stated, except for cases in the early genera- 

 tions. 



Large wooden pens, 23 by 16 by 29 inches on the inside, were used 

 until 1916. They had got into rather bad condition by this time and 

 had become infested with bedbugs. They were replaced in Decem- 

 ber, 1916, by metal pens, 16 by 14 by 24 inches, with removable trays 

 for ease in cleaning. Whether the improvement in cleanliness and 

 freedom from bedbugs compensates for the smaller amount of room 

 for exercise and the greater difficulty in maintaining an even tem- 

 perature in winter is not certain. In any event, experiments con- 

 ducted in different years must be compared with due caution. 



The guinea pigs have been given oats and fresh water every day. 

 Green feed is supplied three times a week and hay once a week when 

 the pens are cleaned. Green oats and fresh grass in spring and sum- 

 mer, and cabbage and kale in fall and winter have been used most 

 successfully as green feed. The greatest difficulty in procuring good 

 green feed comes in late winter and early spring, after other winter 

 conditions have depressed the stock. All records indicate that the 

 stock is in the poorest state at this time. Conditions are also better 

 as a rule in the early part of summer and in fall than during the hot 

 weather in July and August. 



THE DATA RECORDED. 



All litters are recorded on the day on which they are born, except 

 that litters born on Sunday or a holiday are recorded on the next day. 

 At birth the date, pen, number, sex, color, and weight are recorded 

 for each young one. Drawings are made of the coat pattern of each 

 guinea pig in a rubber-stamp outline. The variety of the colors, 

 including intense and dilute agouti, black with red, yellow, or cream 

 spotting, and albinism, together with the almost endless variety in 

 the tricolor pattern, make color and pattern an almost certain means 

 for the identification of individuals. Ear punches are also used as a 

 help to identification. All mated animals are identified at death to 

 insure that no confusion has occurred in the matings. Each of the 

 young is weighed at the ages of 3, 13, 23, and 33 days as well as at 

 birth. Up to 1916 the mated animals were weighed when 1 year old. 

 Since 1917 more frequent weighings have been made. 



THE CHARACTERS STUDIED. 



Possible effects of inbreeding have been looked for in age of matur- 

 ity, fertility, rate of growth, mortality among the young, resistance 

 to tuberculosis, sex ratio, the production of monstrosities, and coat 

 color. 



106851°— 22— Bull. 1090 2 



