410 Scientific Proceedings^ Royal Dublin Society. 



factors tliat produce the single comb, the mnemonics rr and 2^P are com- 

 mandeered instead, and thus are made use of as posiiive factors. And not only 

 does this happen with the single comb ; it happens with other combs also. In 

 the chess-board scheme displaying the presence and absence solution of the 

 problem to be found botli in Professor Bateson's and in Professor Punnett's 

 books, tlie constitutions Brpp (rose) and rrpP (pea) are given : and unless 

 the small letters represent isositive factors, these two combs are produced by 

 only half a factor — a thing which, so far, lias not been found possible, for it 

 would mean that only one parent is necessarj'. Thus, when the case is dealt 

 with by the presence and absence theory, the assumption that tlie rose and 

 pea combs are each produced by single factors fails again. 



Since the facts of the case which the presence and absence theory was first 

 set up to explain are not as they were taken to be, tlie theory itself comes 

 under suspicion, and the suspicion is deepened when symbols are used 

 mnemonically at one time and positively at another. A little furtlier con- 

 sideration will show the theory to be imsound on its own merits, and will 

 bring out the nature of the fallacy. 



Doubt has already been raised as to whether a factor could segregate from 

 its own absence. It was raised upon the statement that "the two factors" 

 (i.e. for rose and pea) " belong to distinct allelomorphic pairs, and each in 

 the gametogenesis of the heterozygote segregates from its own allelomorpli, 

 which is simply the absence of the factor in question." There is no question 

 about a factor " segregating " from its own allelomorph, that is, vacating a 

 position which its allelomorph is about to occupy. The question is, Can a 

 factor's allelomorph be its own absence, and can the factor segregate from 

 its absence ? — that is. Can a factor segregate from no factor at all ? Unless the 

 word be used figuratively for what has taken the place of tlie absent factor, 

 the action suggested is impossible. For, when a factor is removed from any 

 position, its place must be taken by something else — and as yet we know of 

 nothing that can do so but anotlier factor — and the only "segregation " possible 

 must take place with that something else. If a book be taken from its shelf, 

 we may say that its absence is left — that the book lias "segregated" from its 

 absence — but we can only say so figuratively, for wiiat is really left is air 

 and dust; and, when the book is put back again, we may say that it takes 

 the jplace of its absence, but we can only say so figuratively. 



In dealing with the application of the presence and absence theory to 

 Mendel's peas, Professor Punnett writes' : — " On this theory tlie dominant 

 character of an alternative pair owes its dominance to the presence of a factor 



' Punnett, 13. 31. 



