BREEDING SYSTEMS 

 TABLE 22 



RESULTS OF POLLINATING THE TYPE VARIETY (FROM 

 KNYSNA) OF STREPTOCA RP US REXII WITH EQUAL 

 MIXTURES OF ITS OWN POLLEN AND THAT OF 

 OTHER VARIETIES AND SPECIES IN ORDER OF 

 INCREASING DISTANCE OF RELATIONSHIP (LAW- 

 RENCE, UNPUBLISHED) 



The Breakdown of Control 



The second consequence of the relationship of the mating system 



to a hybridity optimum is that, in so far as we expect the hybridity 



optimum to change, so we must expect the mating system to change. 



Mating systems must be built up and broken down with clianging 



conditions. Of these the conditions of cultivation in plants are the 



most easily verifiable. The wild tomato growing in Peru is, as we 



saw, regularly cross-pollinated. In England there is no pollinating 



insect — a common result of acclimatization. In consequence the 



cultivation of tomatoes, especially in glasshouses, has been contingent 



on the acquirement of a property of automatic self-pollination : a 



property which has therefore been developed by the preference 



of the grower for plants setting most fruit, and of nature for 



plants setting most seed. Hence the property we saw of modern 



glasshouse tomatoes, which have flowers whose structures ensure 



self-pollination, at least with a little shaking. Outbreeding has 



been replaced by inbreeding. 



Primula sinensis has been cultivated in glasshouses since 1824. Being 

 hcterostyled, it presumably had, at first, a strong outbreeding 

 mechanism, more rigorous than the tomato. Now the seed which 

 the glasshouse grower saves is always from pin plants because, in 

 dropping off, the corolla self-pollinates the pin flowers, and even 



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