1 64 MENDEL'S LAWS 



latency and patency, and fatal to the hypothesis of segregation. 

 Nevertheless, Professor Cuenot has attempted to surmount the 

 difficulty. 1 He supposes that colour depends, not on a single 

 unit or hereditary tendency, but on two, a colour factor and a 

 colour determiner. According to him, if the one or the other be 

 absent, the individual is an albino. As a fact, it is the factor 

 that is supposed to be absent from the albino, which, it appears, 

 usually carries one determiner and may carry more. According 

 to the latest views the factor is always the same for all the colours 

 of a species, but the determiners (which Bateson has surmised 

 are ferments 2 ) may be as many as there are differently coloured 

 varieties. 3 Colours (i.e. ferments) vary in dominance, the ancestral 

 colour, for example ' wild-grey ' in rabbits and mice, often being the 

 most dominant of all. Consequently, if an albino rabbit or mouse 

 be crossed with some coloured domestic variety (e.g. Belgian 

 rabbit or Japanese waltzing mouse), then, since the albino races 

 seem always to carry -the grey determiner, and the coloured variety 

 carries the factor, the offspring are all grey. If more determiners 

 are carried by the albino, the descendants are not only grey and 

 white, but other colours as well. In this way the reappearance 

 of ancestral traits when varieties are crossed is accounted for in 

 terms, not of latency, but of segregation. 



280. Cuenot's hypothesis of factors and determiners has been 

 used to interpret, not only the reproduction of ancestral colours but 

 also that of all other ancestral peculiarities, for example shape and 

 instinct. Presumably these, also, are due to ferments. But, since 

 both Cuenot's hypothesis and the hypothesis of segregation, which 

 it is intended to support, are merely founded on, not tested by 

 experiment, the repetition of the explanation in any number of 

 instances does not prove them to be true. Apparently, however, 

 mere repetition is all their adherents consider necessary. The facts 

 fit the hypothesis of simple latency and patency at least equally well. 

 Indeed they fit the latter better ; for, if we hold it, we have no ex- 

 ceptions, such as those I have summarized in paragraphs 271 and 

 284, to explain away. Here then we have two rival hypotheses, 

 two interpretations of the same facts. Both of them remain simple 

 guesses, utterly valueless to science, unless they can be tested. " In 

 such a case it is necessary to look for some instance which can be 



1 La loi de Mendel et Vhfrtditt de la pigmentation chez Us souris (2me note). 

 Arch, de ZooL, exp. et gen (4), Notes et revue, pp. xxxiii.-xli. 



2 Mendel's Principles of Heredity, pp. 226-9. 



3 The Progress of Genetics since the Rediscovery of Mendel's Papers, by W. 

 Bateson. 



