262 CONCLUDING REMARKS CHAP. VI. 



quired by various plants independently of inheritance 

 from a common progenitor, and that it can be acquired 

 without any great difficulty that is, without any very 

 unusual combination of circumstances. 



It is probable that the first step towards a species 

 becoming heterostyled is great variability in the length 

 of the pistil and stamens, or of the pistil alone. Such 

 variations are not very rare: with Amsinckia specta- 

 ~bilis and Nolana prostrata these organs differ so much 

 in length in different individuals that until experiment- 

 ing on them, I thought both species heterostyled. The 

 stigma of Gesneria pendulina sometimes protrudes far 

 beyond, and is sometimes seated beneath the anthers; 

 so it is with Oxalis acetosella and various other plants. 

 I have also noticed an extraordinary amount of differ- 

 ence in the length of the pistil in cultivated varieties of 

 Primula veris and vulgaris. 



As most plants are at least occasionally cross-fer- 

 tilised by the aid of insects, we may assume that this 

 was the case with our supposed varying plant; but 

 that it would have been beneficial to it to have been 

 more regularly cross-fertilised. We should bear in 

 mind how important an advantage it has been proved 

 to be to many plants, though in different degrees and 

 ways, to be cross-fertilised. It might well happen that 

 our supposed species did not vary in function in the 

 right manner, so as to become either dichogamous or 

 completely self-sterile, or in structure so as to ensure 

 cross-fertilisation. If it had thus varied, it would never 

 have been rendered heterostyled, as this state would 

 then have been superfluous. But the parent-species of 

 our several existing heterostyled plants may have been, 

 and probably were (judging from their present consti- 

 tution) in some degree self-sterile ; and this would have 

 made regular cross-fertilisation still more desirable. 



