igo Kelerate. 



in Fl (AABBCCDDEEFFGgHH). These he mated inter se, and thev 

 produced only agoutis (AABBCCDDEEFFGGHH and AABBCCDDEEFF 

 GgHH) and blacks (AABBCCDDEEFFggHH). 



In the third and lourth generation also, nothing but agoutis and 

 blacks were produced. Morgan says that if his hypothesis had been true, 

 there should have been some albinos produced, so that he abandons this 

 hypothesis. But it does not seem to mean that he now accepts the purity 

 of gametes. 



In mating together spotted with solidcoloured animals, M. obtained 

 solid-coloured 3'oung, which, on being mated inter se produced solid-coloured 

 and spotted ones. The spotted ones however, presented a much greater 

 variability than the spotted ones used as parents. Some of them were 

 very much darker, in Morg.\n's own words: "Uniform coat has encroached 

 on the spotted coat." From these results, Morgan concludes "that the 

 relation of spotted coat to uniform is far more complicated than the 

 mendelian ratio (3 : i ? H) requires, and that hybridizing introduces a new 

 factor or modifies the olcl one. The spotted coat may, in fact, be said to 

 have been contaminated by the cross, so that in most cases segregation, 

 if the process can be said, in fact, to be one of segregation, is less com- 

 plete than before". 



Now, I think, that the error made by Morgan, in his interpretation 

 of the facts, is the old one, formerly made by so many authors, namely, 

 that of speaking about and working with the qualities of the organisms 

 instead of with the genetic factors (genen), which by their cooperation 

 cause these qualities to appear. It is clear, that when at the outset we 

 look upon "solidcoloured" and "spotted" as a single pair of "unit-characters" 

 or "alellomorphs", we must expect three solid coloured against one, spotted 

 like the original, in the second generation, and in this case we can only 

 say with Morgan, that this relation is "Non-mendelian". But we are 

 gradually learning to approach such a case from an other direction. The 

 fact itself, that in the second generation, there exist far more diverse types 

 than the two original ones, we now know to prove, that the two original 

 types did not differ in the presence or absence of one single factor, but 

 in more, and we devise breeding-tests to analyze this case further, and 

 to get to know all these factors separately- The remarkable thing in these 

 crosses of solid-coloured and spotted mice (or rats), is certainly, that when 

 we add together all the young having some white hair, produced in F,, 

 we find that they constitute exactly one fourth the number of the young. 

 This proves that at least the case is simple in so far, that all solid-coloured 

 animals must have a factor absent from all spotted ones. In the absence 

 of this factor, which I have called L, the animals may have only few white 

 hairs, or they may be more or less heavily marked with white. The 

 experiments of Cuenot have amply proved that this more or less heavy 

 pigmentation is due to the presence or absence of several different genetic 

 factors, which we will for a moment call W, X, Y and Z. An animal 

 having all these factors, but not 1 (an IWXYZ) will then be nearly 

 solid-coloured, having e. g. only a white tip to its tail and a white spot 

 on its belly, an IWXYz will have more white, an IWXyz still more, and 

 finally an Iwxyz will have only very little pigmented area left (for simpli- 

 city's sake, and because the relation of these factors has in no case yet 

 been fully worked out, I will not enter into speculations as to the nature 

 of an animal having other combinations of these factors). But, though it 

 is certain that animals, differing in the possession or non-possession of 



