Self-fertilisation 419 



species reaches the stigma rather later than that of another species, 

 the latter does not effect fertilisation. 



Darwin showed that the fertilising power of the pollen of another 

 variety or of another individual is greater than that of the plant's 

 own pollen ^ This has been demonstrated in the case of 3Iimuhis 

 luteiis (for the fixed white-flowering variety) and Iberis nmbellata 

 with pollen of another variety, and observations on cultivated 

 plants, such as cabbage, horseradish, etc. gave similar results. It is, 

 however, especially remarkable that pollen of another individual of 

 the same variety may be prepotent over the plant's own pollen. This 

 results from the superiority of plants crossed in this manner over 

 self-fertilised plants. " Scarcely any result from my experiments has 

 surprised me so much as this of the prepotency of pollen from a 

 distinct individual over each plant's own pollen, as proved by the 

 greater constitutional vigour of the crossed seedlings"^." Similarly, 

 in self-fertile plants the flowers of which have not been deprived 

 of the male organs, pollen brought to the stigma by the wind or by 

 insects from another plant effects fertilisation, even if the plant's own 

 pollen has reached the stigma somewhat earlier. 



Have the results of his experimental investigations modified the 

 point of view from Avhich Darwin entered on his researches, or not ? 

 In the first place the question is, whether or not the opinion ex- 

 pressed in the Orchid book that there is "Something injurious" 

 connected with self-fertilisation, has been confirmed. We can, at 

 all events, affirm that Darv.in adhered in essentials to his original 

 position ; but self-fertilisation afterwards assumed a greater im- 

 portance than it formerly possessed. Darwin emphasised the fact 

 that "the difference between the self-fertilised and crossed plants 

 raised by me cannot be attributed to the superiority of the crossed, 

 but to the inferiority of the self-fertilised seedlings, due to the 

 injurious effects of self-fertilisation^." But he had no doubt that in 

 favourable circumstances self-fertilised plants were able to persist 

 for several generations Avithout crossing. An occasional crossing 

 appears to be useful but not indispensable in all cases ; its sporadic 

 occurrence in plants in which self-pollination habitually occurs is 

 not excluded. Self-fertilisation is for the most part relatively and 

 not absolutely injurious and always better than no lertilisation. 

 " Nature abhors perpetual self-fertilisation* " is, however, a pregnant 



1 Cro^K and Self fertilisation, p. 391. ' Ibid. p. 397. » Ibid. p. 437. 



* It is incorrect to say, as a writer has lately said, that the aphorism expressed by 

 Darwin in 1859 and 18G2, " Nature abhors perpetual self-fertilisation," is not repeated in 

 his later works. The sentence is repeated in Cross and Self fertilisation (p. 8), with the 

 addition, "If the word perpetual had been omitted, the aphorism would have been false. 

 As it stands, I believe that it is true, though perhaps rather too strongly expressed." 



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