416 



The Biology of Flowers 



different varieties. "It is really wonderful what an effect pollen 

 from a distinct seedling plant, which has been exposed to different 

 conditions of life, has on the offspring in comparison with pollen from 

 the same flower or from a distinct individual, but which has been long 

 subjected to the same conditions. The subject bears on the very 

 principle of life, which seems almost to require changes in the 

 conditions 1 ." 



The fertility — measured by the number or weight of the seeds 

 produced by an equal number of plants — noticed under different 

 conditions of fertilisation may be quoted in illustration. 



Crossing under very similar conditions shows, therefore, that the 

 difference between the sexual cells is smaller and thus the result of 

 crossing is only slightly superior to that given by self-fertilisation. Is, 

 then, the favourable result of crossing with a foreign stock to be 

 attributed to the fact that this belongs to another systematic entity or 

 to the fact that the plants, though belonging to the same entity were 

 exposed to different conditions? This is a point on which further 

 researches must be taken into account, especially since the analysis 

 of the systematic entities has been much more thorough than 

 formerly 2 . We know that most of Linnaeus's species are compound 

 species, frequently consisting of a very large number of smaller or 

 elementary species formerly included under the comprehensive term 

 varieties. Hybridisation has in most cases affected our garden and 

 cultivated plants so that they do not represent pure species but a 

 mixture of species. 



But this consideration has no essential bearing on Darwin's point 

 of view, according to which the nature of the sexual cells is in- 



1 More Letters, Vol. n. p. 406. 



2 In the case of garden plants, as Darwin to a large extent claimed, it is not easy to 

 say whether two individuals really belong to the same variety, as they are usually of hybrid 

 origin. In some instances (Petunia, Iberis) the fresh stock employed by Darwin possessed 

 flowers differing in colour from those of the plant crossed with it. 



