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! 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®. 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*.” 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 Girtner: 
“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’. 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. xxvii. 1908, p. 177. 
2 It is questionable if this was always the case. 
3 Cross and Self fertilisation (1st edit.), p. 55. 
4K. F. Gartner, Versuche ilber die Bastarderzeugung, Stuttgart, 1849, p. 149. 
