40 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 
" presence and absence," which need not necessarily involve any 
assumption as to the nature or constitution of either the factor or 
its allelomorph, though it does provide an indispensable symbolic 
method of denoting the difference between them. 
On the other hand, the reversionary change by which the Moss- 
Rose reproduced the Cabbage-Rose by bud-variation involves 
the absence of the factor M in a single locus of one of the chromo- 
somes, either by somatic segregation with a reduction division, or 
by a reverse mutational change in the locus concerned. In any case 
it is evident that in view of Morgan's discoveries bud-mutations 
take on a new importance, and the case of the Moss-Rose is clearly 
one of considerable genetic significance ; for in a simple way it seems 
to narrow down to a fine point the difficult and usually complex 
problem of the origin of a definite mutation, and may bring us within 
measurable distance of the possibility of tracing the origin to a certain 
cause. 
For the present, however, we must be content to work and wait 
patiently for the genetic and cytological facts, which alone can offer 
even an approximate solution. 
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Crepin, F. 1892. Bull. Soc. Bot. Belg., xxxi. pt. 2, p. 73. 
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Darwin, C. 1893. Var. Animals and Plants Dom., ed. 2, i. pp. 404-406. 
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