lyo Introduction to Botany. 



discussion to state at once that the devices of flowers are 

 chiefly for the purpose of securing the cooperation of two 

 individuals in the production of offspring, and that the off- 

 spring thus produced are stronger and better equipped 

 for the struggle for existence than those which arise by 

 the division of a single individual. 



This has been clearly shown by experiments by Darwin, 

 which were carried out through a period of many years, 

 and in which a large number of species was employed. 

 These experiments show that if the egg is continually fer- 

 tilized by the sperm from the same flower (self fertilization), 

 or from another flower of the same plant (cross fertiliza- 

 tion in a narrow sense), through a number of generations, 

 the resulting individuals are smaller and weaker, and pro- 

 duce smaller and a less number of seeds than those indi- 

 viduals which have for an equal number of generations 

 resulted from the fertilization of eggs by sperms which 

 have been brought from the flowers of different plants 

 (cross fertilization in a more exact sense), provided the 

 plants taking part in the cross fertilization were not de- 

 scended from the same immediate ancestors, and were 

 therefore not closely related. 



Darwin concluded from his experiments "that the mere 

 act of crossing by itself does no good. The good depends on 

 the individuals which are crossed differing slightly in con- 

 stitution, owing to their progenitors' having been subjected 

 during several generations to slightly different conditions, 

 or to what we call in our ignorance spontaneous variation." 



It having been ascertained that cross fertilization as 

 compared with self fertilization is of immense advantage 

 to the species, the manifold devices which have been 

 evolved to prevent self fertilization and insure cross fertili- 

 zation are subjects of great interest and meaning, and the 



