.312 



J. K. MORTON 



From these results it is clear that interbreeding is taking place between the two 

 species in these populations. At the Blackball Rocks, in particular, the structure of the 

 colony is as would be expected if a proportion of Fj hybrids (mode at 14-15) occurred, 

 together with a large number of back-crosses with O. spinosa (mode at 16-18). The struc- 

 ture of the Marsden colony is less variable, and may represent a more advanced state of 

 introgression in which O. spinosa has been largely eliminated, though a high proportion 

 of plants belonging to the other parent still remain. A clearer indication of the degree 

 of interbreeding can be gained from the pictorial scatter diagrams in Figs. 5 and 6. 



