390 MEANS OF CROSS-FERTILISATION. CHAP. X 



if they are crossed with pollen brought from a plaiit 

 growing under somewhat different conditions, or 

 descended from one thus growing, their offspring 

 would profit greatly. It cannot be admitted, under 

 our present state of knowledge, that self-fertilisation 

 continued during many successive generations is evei 

 the most beneficial method of reproduction. 



The Means which favour or ensure Flowers "being fer- 

 tilised with Pollen from a distinct Plant. We have seen 

 in four cases that seedlings raised from a cross between 

 flowers on the same plant, even on plants appearing 

 distinct from having been propagated by stolons or cut- 

 tings, were not superior to seedlings from self-fertilised 

 flowers ; and in a fifth case (Digitalis) superior only 

 in a slight degree. Therefore we might expect that 

 with plants growing in a state of nature a cross between 

 the flowers on distinct individuals, and not merely 

 between the flowers on the same plant, would generally 

 or often be effected by some means. The fact of bees 

 and of some Diptera visiting the flowers of the same 

 species as long as they can, instead of promiscuously 

 visiting various species, favours the intercrossing of 

 distinct plants. On the other hand, insects usually 

 search a large number of flowers on the same plant 

 before they fly to another, and this is opposed to cross- 

 fertilisation. The extraordinary number of flowers 

 which bees are able to search within a very short space 

 of time, as will be shown in a future chapter, increases 

 the chance of cross-fertilisation ; as does the fact 

 that they are not able to perceive without entering a 

 flower whether other bees have exhausted the nectar. 

 For instance, H. Miiller found* that four-fifths of the 



* ' Die Befruchtung.' &c. p. 311. 



