INHERITANCE OF WHITE-SPOTTING IN RABBITS. 25 



factors do sometimes change, leading to the formation of multiple 

 allelomorphs; (2) that the action of single factors is not limited to any 

 particular part of the organism, but may affect parts apparently 

 unrelated; (3) that the total number of factors concerned in the genesis 

 of even the simplest organisms must be very great; and (4) that in 

 what should theoretically be "pure lines" (asexually reproducing 

 organisms, Jennings) genetic changes are constantly occurring. Prag- 

 matically, then, genetic variability by minute gradations is a reality, 

 precisely as Darwin assumed it to be, and this fact allows races to be 

 altered steadily and permanently by selection, either natural or 

 artificial, as Darwin also assumed was the case. The hypothesis 

 that stable organic forms come into being only suddenly, by abrupt 

 changes from preexisting forms and not by gradual modification — this 

 hypothesis, the "mutation theory" as commonly understood, is not 

 substantiated. 



RELATION OF DUTCH TO ENGLISH. 



It remains to consider the genetic relations to each other of Dutch 

 and English spotting. Dutch, as we have seen, behaves as a reces- 

 sive in crosses with self-pigment ed races; English, on the other hand, 

 beha^-es as a dominant. Dutch marking appears to result from a 

 simple deficiency of pigmentation, as if in development the pigment 

 supply failed at an extremity or at an embryological point of finishing- 

 off . When Dutch marking is reduced to its lowest point of expression 

 by selection or crossing, the only white visible is found at the tip of 

 the nose, or on the toes of a fore-foot, or as a spot in the middle of 

 the forehead. English spotting, on the contrary, appears to result 

 from some positive inhibiting force, some agency which uses up the 

 pigment-forming materials here and there in the epidermis and con- 

 verts them into an end-product not colored but white. That English 

 individuals possess all the agencies necessary for full pigment formation 

 is shown by the fact that English parents may produce fully pigmented 

 (self) young as recessives, which then produce only self young if mated 

 with each other. 



We have seen that there occur different forms of Dutch spotting, 

 which apparently behave as allelomorphs, but which tend to become 

 less distinct, one from another, when thej'^ are associated in the same 

 zygote. It would seem probable that they represent quantitatively 

 different stages of reduction in the amount of some substance carried 

 in the germ-cell. But undoubtedly this substance, whatever it is, is 

 located in the chromatin, since the defect is transmitted equally 

 through egg and sperm. There are also qualitative differences among 

 the different forms of Dutch, as for example between "dark" and 

 "tan" Dutch, in one of which the white collar is more in evidence, 

 while in the other it is the head markings that are more in evidence. 

 Probably, then, the different forms of Dutch are variants of a single 



