810 The Gejietics of the Dutch Rabbit 



With regard to (a) I agree with Castle that a true breeding strain 

 can be readily established, and I have had one in my possession for 

 some years. With regard to the establishment of (b) and (c) my 

 experiments are not yet complete, though at the same time the evi- 

 dence I have accumulated supports the suggestion made above. It is 

 however possible that another true-breeding form of Spotted Dutch 

 exists va^rying about grade 14, but at present my data are not sufficient 

 to justify the assertion of its existence. Moreover in the case of the 

 Typical Dutch (c) I have not yet succeeded in obtaining a strain in 

 which the pigmented area is entirely free from white in every in- 

 dividual. Though the area of pigmentation varies about that shewn in 

 grade 7, many individuals shew small blurred patches of white on the 

 dorsal surface (cf PI. XI, Fig. 1). Whether a strain of Typical Dutch can 

 be obtained without these small patches is a question now under in- 

 vestigation. In any case the evidence is not sufficient to connect these 

 small patches definitely with the Spotted and White Dutch characters. 



Further, I should mention that Typical Dutch animals may be hetero- 

 zygous for Spotted Dutch, though perhaps the grade of pigmentation is 

 somewhat lower in the heterozygote than in the homozygote. Crossed 

 with White Dutch however both Typical and Spotted Dutch give rise to 

 intermediate forms. The i^2 generation exhibits segregation, the parental 

 types reappearing together with intermediates. The cross between 

 White and Typical Dutch for example produces F^ animals indistinguish- 

 able in appearance from Spotted Dutch (cf PI. XI, Figs. 3 and 4). 

 Genetically however they are distinct, for such ^i animals bred together 

 have produced among their progeny both White and Typical Dutch. 



Assuming then the existence of the three strains {a), (b), and (c) we 

 may turn to the higher grades of pigmentation, viz. grades 1-6. My 

 experiments have yielded evidence for the existence of a factor (P) 

 which, when added to any one of the three forms (a), (b), or (c) greatly 

 increases the development of the pigmentation. When added to the 

 White Dutch for example it brings up the pigmentation to a grade 

 characteristic of Typical Dutch i; when added to Spotted Dutch, animals 

 approximating to grade 3 result; and when added to Typical Dutch it 

 leads to the production of animals which are almost, or even quite self- 

 coloured. An interesting point in connection with P is its relation to 

 the colouration of the iris. Dutch rabbits, particularly those of a low 



1 It is interesting here to recall that Baur obtained Fj animals near Typical Dutch on 

 crossing his blue-eyed whites with albino ; cf. Einfiihrung in die experimentelle Vererbungs- 

 lehre, 1914, p. 75. 



