Cross and Self-fertilisation 413 



experiments, as we have pointed out 1 , could determine whether cross- 

 fertilisation is in itself beneficial, and self-fertilisation on the other 

 hand injurious; a conclusion which a merely comparative examination 

 of pollination-mechanisms renders in the highest degree probable. 

 Later floral biologists have unfortunately almost entirely confined 

 themselves to observations on floral mechanisms. But there is little 

 more to be gained by this kind of work than an assumption long ago 

 made by C. K. Sprengel that "very many flowers have the sexes 

 separate and probably at least as many hermaphrodite flowers are 

 dichogamous ; it would thus appear that Nature was unwilling that 

 any flower should be fertilised by its own pollen." 



It was an accidental observation which inspired Darwin's experi- 

 ments on the effect of cross and self-fertilisation. Plants of Linaria 

 vidgaris were grown in two adjacent beds ; in the one were plants 

 produced by cross-fertilisation, that is, from seeds obtained after 

 fertilisation by pollen of another plant of the same species ; in the 

 other grew plants produced by self-fertilisation, that is from seed 

 produced as the result of pollination of the same flower. The first 

 w r ere obviously superior to the latter. 



Darwin was surprised by this observation, as he had expected 

 a prejudicial influence of self-fertilisation to manifest itself after a 

 series of generations : " I always supposed until lately that no evil 

 effects would be visible until after several generations of self-ferti- 

 lisation, but now I see that one generation sometimes suffices and 

 the existence of dimorphic plants and all the wonderful contrivances 

 of orchids are quite intelligible to me 2 ." 



The observations on Linaria and the investigations of the results 

 of legitimate and illegitimate fertilisation in heterostyled plants were 

 apparently the beginning of a long series of experiments. These 

 were concerned with plants of different families and led to results 

 which are of fundamental importance for a true explanation of sexual 

 reproduction. 



The experiments were so arranged that plants were shielded from 

 insect-visits by a net. Some flowers were then pollinated with their 

 own pollen, others with pollen from another plant of the same species. 

 The seeds were germinated on moist sand ; two seedlings of the same 

 age, one from a cross and the other from a self-fertilised flower, were 

 selected and planted on opposite sides of the same pot. They grew 

 therefore under identical external conditions ; it was thus possible to 

 compare their peculiarities such as height, weight, fruiting capacity, 

 etc. In other cases the seedlings were placed near to one another in 

 the open and in this way their capacity of resisting unfavourable 

 external conditions was tested. The experiments were in some cases 



1 Ante, p. 408. 2 More Letters, Vol. n. p. 373. 



