CASTLE AND ALLEN. — THE HEREDITY OP ALBINISM. 615 



potliesis of gametic purity, and which therefore require justification. Con- 

 strued in the strictest sense, the doctrine of gametic purity is untenable. 

 We cannot accept that interpretation of it which requires that the gametes 

 formed by an individual be the precise equivalents in all respects of the 

 respective gametes which united to form that individual. Mendel him- 

 self would not have assented to such an interpretation, for in the latter 

 part of his original paper ('66) he clearly states the important principle 

 that a composite character may undergo resolution into its elements in 

 consequence of crossing. This allows a 2^o.'>'t of a complex character to 

 pass into one gamete, while the remaining parts pass into another ; in 

 other words, it makes possible the formation of mosaic gametes, into 

 whose composition the dominant and recessive characters may both enter 

 in varying degree. All gametes which contain any portion of a dominant 

 or of a recessive character associated with its opposite, are in reality 

 mosaic; yet, if a gamete essentially recessive contains only truces of the 

 dominant character, it may be convenient to recognize this fact in its 

 designation, which we do by calling it an impure recessive. 



In guinea-pigs the impurity of recessives tainted with the dominant 

 character is commonly visible. Ordinary white guinea-pigs with pink 

 eyes, though they invariably produce albinos when bred inter se, have a 

 greater or less amount of sooty black pigment in the skin and hair of 

 their ears, nose, and feet, showing the presence of a trace of the domi- 

 nant character. Rarely is it possible to obtain an animal free from this 

 visible taint, and even when obtained, we are informed by breeders, such 

 individuals are likely to produce offspring with a certain amount of pig- 

 ment on their ears or feet. 



The so-called Himalayan rabbit is another illustration of a mosaic 

 with a predominantly recessive (albino) character, in which the dominant 

 (pigment-forming) character is localized precisely as in the impure al- 

 bino guinea-pig, namely, at the extremities. Himalayan rabbits have 

 brownish-black noses, ears, feet, and tails, being elsewhere snowy white 

 and having pink eyes. They breed true inter se, yet, according to 

 Darwin ('76, p. 114), may occasionally produce a silver-gray animal, 

 in which the pigment is not restricted to the extremities. This condi- 

 tion must result from liberation of the dominant character from the strict 

 localization which it has in ordinary individuals and which it must have 

 also in the gametes that produce them. 



In mice, on the other hand, impurity in recessive individuals is not 

 visible, though doubtless sometimes present. So long as the breeder 

 wishes only to obtain white mice, it makes no dillerence what the ances- 



