COLOR AND COAT CHARACTERS. 21 



treated as a unit; yet, in the guinea-pig, differences in the agouti factor 

 can be seen. Recently Morgan (1911) has reported on a cross between 

 gray-bellied agouti mice and light-bellied agouti mice, and although the 

 numbers given are small, it is quite clear that each form acts as a unit, 

 and that the gray-bellied agouti is recessive. Although Morgan does 

 not state it, it would appear that the difference between gray-bellied 

 agoutis and light-bellied agoutis is not a difference in separable belly- 

 ticking factors. The difference is probably a difference between two 

 kinds of agouti, in which the peculiarity of one agouti is a weakened 

 restricting power and the consequent appearance of black on belly hairs, 

 whereas the other agouti is a more powerful restrictor and therefore 

 gives yellow or light belly, without the usual black in the belly hair. 



It has been found expedient to treat all kinds of agouti as one, 

 whether found in the wild, tame, or hybrids. This treatment of the 

 ticking factor has been adhered to, because all forms of agouti have 

 some qualities in common, and whatever the agents may be that cause 

 the exclusion of black or brown from a part of the hair, the qualitative 

 effect of the agents appears the same, but the quantitative effect varies. 

 To be concrete, all the agouti animals have a factor which restricts 

 black or brown in the subapical band of the ticked hair, but the amount 

 of this restriction differs, particularly when a wild agouti or a hybrid- 

 bearing wild agouti is contrasted with the tame. The common qualities 

 of all agoutis are as follows: 



(1) All restrict black or brown on the individual hairs in the sub- 

 apical band, giving each dorsal hair a barred appearance. 



(2) Any agouti expresses itself more powerfully on the belly than 

 on the back, restricting black more in this region. 



(3) Any agouti is epistatic to the non-agouti condition, and allelo- 

 morphic to the absence of agouti. 



But to class all agoutis together, without a thorough consideration 

 of their differences, would be a superficial method of treatment. From 

 an examination of many tame agoutis the conclusion is reached that 

 these never show the condition which the wild agouti presents in some 

 pure wild animals and in some hybrids. These differences are briefly 

 as follows : 



(a) The very weak restricting power, which some wild individuals 

 and some hybrids show, is unknown in tame guinea-pigs. This differ- 

 ence in the restricting power may be readily seen from measurements 

 of the yellow subapical band, for the greater the power to restrict black 

 or brown, the broader the yellow band from which these pigments are 

 excluded. The narrowest yellow band on a mid-dorsal hair of a tame 

 agouti animal measures about 2 mm. The yellow band of a hybrid 

 or wild agouti may measure as small as 1 mm. In a number of cases 

 the wild agouti was so powerless to restrict black in young hybrids that 

 yellow was not visible at all in the dorsal hairs, and only very slightly 



