LIFE PROCESSES IN CAPTIVE GRAY RATS 53 



covering the space between the forelimbs (fig. 15). Later the 

 white was extended, and in some cases a second white spot 

 appeared (fig. 16). Matings of these rats produced some off- 

 spring with a long streak of white in the midventral region 

 of the body (fig. 17). In the fifteenth generation a litter was 

 obtained in which three of the individuals, one male and two 

 females, were typical 'Irish' rats in that the four feet, the 

 tip of the tail, and the ventral surface were covered with 

 white hair (fig. 18). Presumably at this point the self factor 

 (H) changed to its allelemorph (h 1 ), a reverse change to that 

 found in experiments made by Castle and Phillips ( '14) with 

 hooded rats where the hooded factor (h) changed to h 1 . When 

 'Irish' rats were inbred they produced some offspring like 

 themselves and others in which the w r hite area was extended 

 up the sides of the body (fig. 19). Descendants of the latter 

 rats had coat patterns showing but slight variations from that 

 in figure 20. Gray hooded rats were kept in the colony for 

 some 3 years, but no further attempts w r ere made to increase 

 the amount of white in the coat through selective breeding. 

 Matings of these rats with black hooded rats from a strain 

 that had bred true to type for many years produced only gray 

 hooded young, indicating that the hooded pattern in the two 

 strains was due to the same gene mutation. 



Some years ago Castle and Phillips ('14) carried out an 

 extensive series of experiments in order to study the effects 

 of selection on the coat pattern of black hooded (piebald) rats. 

 These experiments, which were continued through twenty 

 generations, were divided into two series; in one series selec- 

 tion was made in a plus direction, and in the other series in 

 a minus direction. At the end of the experiments individuals 

 in the plus series were black except for a small area of white 

 on the ventral surface, similar in its position to the spot shown 

 in figure 14. In individuals of the minus series black was 

 restricted to the head region. Castle's ('19) final conclusions 

 regarding the results of this work were that alterations in the 

 coat pattern were not due to changes in the gene for the 

 hooded pattern, but to residual heredity. Selection had uti- 

 lized the minor genetic changes that are occurring continually 



