322 THE STEUCTUKE OF FLOWERS. 



If flowers, unlike the preceding, are normally very self- 

 fertile, as Ipomcea and Mimulus proved to be, then it appears 

 that intercrossing supplies a remarkable stimulus, and the 

 intercrossed beat the self-fertilised for a time. Sooner or 

 later, however, the effect of the stimulus gradually dis- 

 appears, and self-fertilisation reasserts itself. Thus with 

 Ipomcea purpurea Mr. Darvvin raised crossed and self- ferti- 

 lised plants for ten generations ; and the heights of the 

 latter were 24, 21, 32, 14, 25, 28, 19, 15, and 21 p.c.,* respec- 

 tively, less than the crossed. Grouping these into threes, 

 the ratios become 100 : 74-3 ; 100 : 77-6 ; 100 : 81-6. That is to 

 say, the intercrossed w^ere steadily declining ; for if the self- 

 fertilised be regarded as 100, then the ratios of these to 

 the crossed appear as follows : 100 : 134 ; 100 : 129 ; 100 : 121. 

 Similarly with regard to fertility, the ratio of that of the 

 intercrossed plants to the self-fertilised was for the first and 

 second generations as 100 : 93 ; for the third and fourth, as 

 100 : 94 ; for the fifth, as 100 : 106 ; and the eighth, as 

 100 : 113. Hence the self-fertilised were superior. 



Mimulus luteus gave analogous results. The crossed 

 plants {i.e. offspring of crossings) surpassed the self-ferti- 

 lised until the fourth generation, when several plants of the 

 latter assumed a taller character, with whiter blossoms. 

 This self-fertilising form "increased in the later self-ferti- 

 lised generations, owing to its great self-fertility, to the com- 

 plete exclusion of the original kinds." f " It transmitted its 

 character faithfully, and as the self-fertilised plants consisted 

 exclusively of this variety, it was manifest that they would 

 always exceed in height the crossed plants." % 



* These numbers correspond to the first nine years. The tenth gives 

 46; but Mr. Darwin thinks this number to have been accidental (p. 41). 

 t Cross and Self Fertilisation, p. 67. 

 X Ibid., p. 70. 



