422 The Biology of Flowers 



although only in a slight degree. If the favourable effects of sexual 

 reproduction, according to Darwin's view, are correlated with change 

 of environment, it is quite possible that this is of less importance in 

 plants which die after ripening their seeds (" hapaxanthic ") and 

 which in any case constantly change their situation. Objections which 

 are based on the proof of the prevalence of self-fertilisation are 

 not, therefore, pertinent. At first sight another point of view, which 

 has been more recently urged, appears to have more weight. 



W. Burck 1 has expressed the opinion that the beneficial results 

 of cross-fertilisation demonstrated by Darwin concern only hybrid 

 plants. These alone become weaker by self-pollination ; while pure 

 species derive no advantage from crossing and no disadvantage from 

 self-fertilisation. It is certain that some of the plants used by 

 Darwin were of hybrid origin 2 . This is evident from his statements, 

 which are models of clearness and precision ; he says that his Ipomoea 

 plants "were probably the offspring of a cross 3 ." The fixed forms of 

 this plant, such as Hero, which was produced by self-fertilisation, and 

 a form of Mimulus with white flowers spotted with red probably 

 resulted from splitting of the hybrids. It is true that the phenomena 

 observed in self-pollination, e.g. in Ipomoea, agree with those which 

 are often noticed in hybrids ; Darwin himself drew attention to this. 



Let us next call to mind some of the peculiarities connected with 

 hybridisation. We know that hybrids are often characterised by 

 their large size, rapidity of growth, earlier production of flowers, 

 wealth of flower-production and a longer life ; hybrids, if crossed 

 with one of the two parent forms, are usually more fertile than 

 when they are crossed together or with another hybrid. But the 

 characters which hybrids exhibit on self-fertilisation are rather 

 variable. The following instance may be quoted from Gartner : 

 "There are many hybrids which retain the self-fertility of the 

 first generation during the second and later generations, but very 

 often in a less degree ; a considerable number, however, become 

 sterile."' But the hybrid varieties may be more fertile in the 

 second generation than in the first, and in some hybrids the fertility 

 with their own pollen increases in the second, third, and following 

 generations 4 . As yet it is impossible to lay down rules of general 

 application for the self-fertility of hybrids. That the beneficial in- 

 fluence of crossing with a fresh stock rests on the same ground — a 

 union of sexual cells possessing somewhat different characters — as 

 the fact that many hybrids are distinguished by greater luxuriance, 



1 Burck, "Darwin's Kreuzungsgesetz...," Biol. Centralbl. xxvm. 1908, p. 177. 



2 It is questionable if this was always the case. 



3 Cross and Self fertilisation (1st edit.), p. 55. 



4 K. F. Gartner, Versuche ilber die Bastarderzeugung, Stuttgart, 1849, p. 149. 



