32 INHERITANCE OF COAT-PIGMENTS AND COAT-PATTERNS 



CONCLUSIONS. 



The results of selection brought to bear upon the coat-pattern are seem- 

 ingly very different in rats and in guinea-pigs, yet a careful analysis of the 

 facts shows the results to be not so dissimilar in the two cases as they at 

 first thought appear. 



In both rats and guinea-pigs we can by selection increase or decrease at 

 will the average extent of the pigmented areas. In both rats and guinea- 

 pigs the extent of the pigmented areas varies continuously, and out of these 

 continuous variations permanent modification of the pigmentation can be 

 secured. 



Reduction in the total amount of the pigmentation is attended in rats 

 by restriction of the pigment to very definite areas, whereas in guinea-pigs 

 it may be distributed in any or in all of a series of spots. Herein lies the 

 whole difference between the two cases. When in rats we select for reduced 

 pigmentation, we get animals with a narrow or interrupted back-stripe and 

 with a less extensive hood; when in guinea-pigs we make a similar selection, 

 we get animals with fewer or less extensive spots. We can not in guinea- 

 pigs decide arbitrarily -which areas shall be pigmented (except, possibly, in 

 the case of nose spots), any more than in rats we can at the same time 

 increase the extent of the hood and decrease that of the back-stripe. 



In rats, we have as a result of pigment reduction a series of coat-patterns, 

 each breeding true within certain limits; in guinea-pigs, the fluctuation in 

 the extent of the pigmented areas is probably no greater than in rats, but 

 because the pigmented areas do not disappear in as definite an order during 

 pigment reduction, we have no constant coat-patterns. Nevertheless there 

 is every reason to suppose that different degrees of pigmentation are in- 

 herited in Mendelian fashion in guinea-pigs, precisely as they are in rats. 

 If the pigment reduction followed a definite course in guinea-pigs, as it does 

 in rats, this would be easily recognizable in the coat-pattern. As it is, 

 measurement of the extent of the pigmented areas would be necessary to 

 make it apparent. This we have not undertaken to do in the case of guinea- 

 pigs; we have merely taken account of the regions pigmented, not of their 

 extent. This probably explains in part why regression is observed in the 

 selection experiments with guinea-pigs, but not with uniformity in those 

 with rats. In guinea-pigs we attempted by selection to restrict the number 

 of the pigmented areas ; this was found to be impossible except as it occurred 

 incidentally to reduction in the total amount of pigmentation. The regres- 

 sion occurred in number of pigmented areas, not, so far as we know, in the 

 total amount of the pigmentation. We have no doubt, however, that such 

 regression would be found to occur in cases in which extreme variates were 

 selected. We have found it so in selecting black-eyed white guinea-pigs, 

 those with no pigment except in the eye. Almost invariably the young of 



