28 INHERITANCE OF COAT-PIGMENTS AND COAT-PATTERNS 



The objection may be made that a nose spot as such is not inherited, 

 any more than an eye spot or a rump spot (see page 25) ; that in a spotted 

 race a certain average amount of pigmentation is an inherited condition, 

 but the distribution of this pigment is wholly a matter of chance. To test 

 this point, comparison has been made with the young of certain sires in Series 

 H (table 31). In the case of the litters recorded in this table, neither parent 

 bore a nose spot, nor came from a nose-spot family. The extent of the pig- 

 mentation of the parents is measured roughly by the number of the typical 

 spot-areas which were pigmented (see columns 2, 3, 4, 7, and 10, table 31). 

 For comparison with the nose-spot series, see columns 3 and 6 of tables 28, 

 29, and 30. It will be observed that the extent of the pigmentation is similar 

 in the nose-spot and head-spot series, being in both cases close to an average 

 of three spots to an individual. The percentage of nose-spot young, how- 

 ever, is much lower in the eye-spot than in the nose-spot series, being 16.3 

 per cent in the former as compared with 40 to 50 per cent in the latter. 

 That the nose-spot marking does occur quite frequently among spotted 

 guinea-pigs not selected for that character is shown clearly by table 31. 

 That its occurrence, however, is more frequent when selection is made for 

 the character is evident from tables 28 to 30. 



In table 31, as in tables 28 to 30, it will be observed that nose-spot young 

 have fewer other spots than do their brothers and sisters that lack nose 

 spots. The average difference is from half to two-thirds of a spot. This 

 means that a nose spot takes the place, to some extent, of pigmentation 

 elsewhere, and it is in part a matter of chance whether the pigmentation is 

 located on the nose or elsewhere. But chance is not the only element (if 

 we may so speak of chance) entering into the matter, for nose-spot parents 

 clearly produce more nose-spot yotmg than do other parents transmitting 

 a like amount of pigmentation. How, then, are we to account for the fact 

 indicated by our observations that, while other patterns are unfixable, the 

 nose-spot pattern is in part at least fixable? At present we can not accoimt 

 for it, but a consideration of familiar facts concerning mammalian develop- 

 ment may help us in shaping a hypothesis. 



The production of hair and skin pigments in guinea-pigs is the exclusive 

 function of the ectoderm, as shown by Leo Loeb ('97) and confirmed by 

 Castle (:o5). In spotted guinea-pigs the limits of the pigment spots are 

 very precisely defined at birth and these limits, so far as we have been able 

 to observe, are never subsequently transgressed in the slightest degree. 

 Areas which are white at birth remain white ever afterward;* areas which 



*We leave out of consideration for the present the "peripheral" pigmentation which 

 albinos as well as spotted animals may possess (see Castle, ;o5). This is not fully developed 

 at birth. 



