V INTERACTION OF FACTORS 49 



for G {ccGgBB), and albinos without G {ccggBB). 

 These albinos are, as it were, like photographic plates, 

 exposed but undeveloped. Their potentialities may 

 be quite different, although they all look alike, but 

 this can only be tested by treating them with a colour 

 developer. In the case of the mice and rabbits the 

 potentiality for which we wish to test is the presence 

 or absence of the factor G, and in order to develop 

 the colour we must introduce the factor C. Our 

 developer, therefore, must contain C but not G. In 

 other words, it must be a homozygous black mouse 

 or rabbit, CCggBB. Since such an animal is pure 

 for C it must, when mated with any of the albinos, 

 produce only coloured offspring. And since it docs 

 not contain G the appearance of agoutis among its 

 offspring must be attributed to the presence of G 

 in the albino. Tested in this way the F^ albinos 

 were proved, as was expected, to be of three kinds : 

 (i) those which gave only agouti, i.e. which were 

 homozygous for G ; (2) those which gave agoutis 

 and blacks in approximately equal numbers, i.e. 

 which were heterozygous for G ; and (3) those which 

 gave only blacks, and therefore did not contain G. 



Though albinos, whether mice, rabbits, rats, or 

 other animals, breed true to albinism, and though 

 albinism behaves as a simple recessive to colour, yet 

 albinos may be of many different sorts. There are 

 in fact just as many kinds of albinos as there arc 

 coloured forms — neither more nor less. And all 

 these different kinds of albinos may breed together, 

 transmitting the various colour factors according to 

 the Mendelian scheme of inheritance, and yet the 

 visible result will be nothing but albinos. Under 



