226 



THE BEHAVIOUR OF POLYPLOIDS 



lower fertility. It is therefore probable that the occurrence of 

 structural changes in the two separated sets in the normal course 

 of variation is a more important source of differentiation between 

 the chromosome sets, and thus of the evolution of allopolyploids 

 in general. This will be shown more clearly by considering the 

 occurrence of polyploidy within the species. 



(iii) Polyploidy within the Species. Many species have been found 

 to include a series of polyploid forms. In some cases these are 

 indistinguishable from one another, except by distribution, while in 





Fig. 76. — Mitotic metaphases from the three clonal types of Tulipa 

 Clusiana. A. The diploid from Thibet. B. The tetraploid from 

 Chitral. C. The pentaploid from the Mediterranean. Cf. Plate II. 

 (D., unpublished.) 



others they show size differences or slight differences of a general 

 character. 



In some species the multiple forms are autopolyploids, in others 

 they must be allopolyploids, since they have high chromosome 

 numbers and are seed-fertile. In one species, A Ilium schcenoprasum 

 (Levan, 1935 h) chromosom.e behaviour shows that there are both 

 kinds, one a giant auto-tetraploid, the other a smaller allo-tetraploid. 



It might be assumed that such polyploid forms had arisen from 

 their diploid relatives within the species, by hybridisation with a 

 second species followed by selective elimination in the polyploid of 

 all the characters of one parent which distinguished it from the 

 other, i.e., convergence of the two forms, parent and hybrid. 



But it is more plausible to assume that these forms arose as 

 autopolyploids, that is, as giants with free pairing amongst their 

 homologous chromosomes. This condition is still found in certain 



