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polyploid species (i.e., creative) and from polyploid to diploid 

 species (i.e., emergent), according to geological conditions. This 

 mechanism of alternating creative and emergent evolution in asso- 

 ciation with other secondary processes such as hybridization and 

 chromosome mutations of the Oenothera lata type would also serve 

 to explain the origin of the Tribes and Families of the Order 

 Resales, including the numerous extinct species eliminated by 

 natural selection." (I independently reached similar conclusions, 

 about the same time, as set forth in "Nature," April lo, 1926). 

 It ma}- be added that if polyploids are built up as postulated, they 

 have two advantages as sources of divergent t}pes. One is that if 

 mutations occur once in so often, there is more chance for them 

 in a polyploid with its greatly increased number of genes. The 

 other is tliat such mutations, which might be injurious or fatal 

 in a diploid may survive in a polyploid, until such time as circum- 

 stances favor some type of plant which they represent. On the 

 question whether hybridization has played a large part in the 

 evolution of roses, opinions dififer, though no one disputes the 

 fact that very numerous hybrids occur in nature. Blackburn and 

 Harrison, in the paper already cited, conclude that the irregular 

 polyploids arose through hybridization ; and would also explain 

 the regular or balanced polyploids in the same manner. In the 

 latter case they suppose that the plants attained fertility "simultan- 

 eously with, and as a direct consequence of, a doubling in their 

 chromosome complements". They dispute many of Hurst's find- 

 ings in detail, and especially stress the case of R. i\.nlsoui, which 

 Hurst calls a true hexaploid species, but which they assert to be 

 a hybrid between R. pirn pin elli folia and R. tomentosa, the latter 

 the pollen parent. Such a hybrid, if I understand Hurst's notation, 

 should apparently come to nothing, but Blackburn and Harrison 

 state that theoretically it ought to have 14-1-7 as its somatic number 

 of chromosomes, whereas it actually has 42. (This number 42 

 agrees with the view that it is a hexaploid species). They add: 

 "Obviously chromosome doubling by some means or other has 

 occurred, but, what is most noteworthy, although the reciprocal 

 hybrid is unbalanced and sterile, this is balanced and fertile. Thus 

 we have generated before us, by the union of an tgg with 14 

 chromosomes and a pollen grain with 7, a fully fertile hexaploid 

 rose." In other words, a veritable species has been produced by 

 hvbridization. 



