THE GENETIC NATURE OF TAXONOMIC DIFFERENCES 



257 



heterogenetically (cf. p. 72), and a small F2 was obtained on selfing. 

 Among this F2 was a triploid apparently formed by the fertilization of 

 an unreduced diploid ovule by a haploid pollen grain; the latter pre- 

 sumably contained a mixture of speciosa and pubescens chromosomes, 



ph + sp 

 so that the triploid can be symboUzed as ph + sp -\ . The 



triploid was then crossed again with pubescens, and gave a tetraploid 

 P. floribunda P. verticil lata 





p. kewensis 





'Of. 



somatic 



"^r- »• doubling 



Zx 





4x 



Fig. 112. The Origin of Primula Kewens/s.— The drawings show typical mitotic 

 figures, those of P. verticillata and the diploid hybrid being early anaphases, the 

 others metaphases. 



(From Darlington, after Newton and Pellew.) 



by fertiHzation of an unreduced triploid gamete. This tetraploid would 

 then contain two sets of pubescens chromosomes, one set of speciosa, 

 and one set of mixed speciosa and pubescens. It exactly resembled the 

 natural species G. tetrahit, and was perfectly fertile when crossed with 

 it or when selfed, but was infertile with either G. pubescens or speciosa. 

 It is clear that the crosses had synthesized the species G. tetrahit. 



3. The Analysis of Natural Polyploid Species 



It is usually not possible to reverse the process by which artificial 

 polyploid species are made, and to break down a species into two com- 

 ponents. This has, however, been done by Wettstein^ in the moss 



1 Wettstein 1932. 



