408 Scientific Proceedings, Royal Dublin Society. 



crossing in the case when the factors for rose and pea are absent. The con- 

 stitution of the walnut comb is written RRPP. That of rose ought to be 

 written RR ; but, in order to indicate that the factor for pea has no hand in 

 the case, i.e., is absent, the letter p is added, and the constitution is written 

 RRpp. Similarly, to indicate the absence of the factor for rose in its produc- 

 tion, the constitution of pea is written rrPP. Following these precedents 

 the constitution of single comb ought to be written, say, SSrrppi — SS to denote 

 the factor for single comb, and rrpp the absence of the factors for rose and 

 pea — but it is written rrpp. By writing it thus we are not told what does 

 produce the single comb, but what does not; and the danger of it and 

 lip changing their significance in the course of manipulation is increased by 

 the absence of a positive symbol of some kind to represent the factor for the 

 single comb. 



Divided into separate paragraphs to make the reasoning clear. Professor 

 Bateson's statement of the case is as follows: — 



1. "A rose comb is not due to an elemental factor which can segregate 

 from the pea comb factor. 



2. " The two factors belong to distinct allelomorphic pairs, and each in 

 the gametogenesis of the heterozygote segregates from its own allelomorph, 

 which is simply the absence of the factor in question. 



3. " The single comb contains neither R nor P. 



4. " The rose comb is a single comb modified by the presence of R, while 

 the pea comb is produced by the presence of P. 



5. " We may tlierefore describe the rose as R no P, and the pea as P no R. 



6. " It is convenient to use capital letters for dominants, and small letters 

 for recessives, the rose being written thus, Rp, and the pea rP. The walnut 

 comb is the RP, while rp gives the single. "^ 



Thus tlie first two paragraphs affirm rose and pea to be due each to 

 single factors. 



The second paragraph states that these factors "segregate" from their own 

 absences. This could be understood if the word " absence " were used 

 metaphorically for real factors alternative to rose and pea ; but it is not 

 easy to think of such a thing as a factor segregating from nothing, or, 

 at any rate, from something which is not present. 



As already mentioned, the third paragraph tells what the single comb 

 does not contain, but not what it does contain. 



In the fourth paragraph the rose and pea combs become due to something 

 more than R and P, namely, to the effect of the factor for single comb plus 



' Bateson's " Mendel's Principles of Heredity," p. 66. 



