LIMITS OF HYBRIDITY. STRUCTURE OP FLOWER. 293 



a third generation in like manner. But there is no instance in 

 which a hybrid race, which has thus originated in the intermix- 

 ture of two species really distinct, has ever been continued 

 without intermixture, beyond the fourth or fifth generation. The 

 plant, when not fertile by itself, may bear seed, if its stigma be 

 .sprinkled with the pollen of one of its parent species ; and its 

 pollen may be fertile, when placed on the stigma of either of 

 these. In this manner, a race intermediate between the hybrid 

 and one of the parent species is produced ; and this is continued 

 longer, just in proportion as it is caused to approach the pure 

 breed, by a successive intermixture of this kind. The end of all 

 hybrid races, produced between species really distinct, appears to 

 be, therefore, that either the race becomes soon extinct, which 

 it will do if kept separate, or it merges into one of the parent 

 races, if continued by intermixture with either of them. This 

 principle affords a valuable test for determining what really are, 

 and what are not, distinct species ; for if a hybrid race can be 

 produced between them, which continues to be fertile of itself, the 

 probability is strong, that they are only varieties. Cultivators 

 of flowers are constantly in the habit of producing such new 

 races, between the different varieties of many plants, for 

 instance, the South American Amaryllis and the Calceolaria; 

 both these species are very much disposed to spontaneous varia- 

 tion; and, by selecting the most beautiful of the new races, 

 which spontaneously originate from their seeds, and causing these 

 to produce hybrids, a still larger amount of variety, both in form 

 and colour, may be obtained. These hybrids are of equal fertility 

 with their parents, since the latter are not separated by any 

 really essential difference. 



455. Having now considered the general structure and offices 

 of those organs of Phanerogamia, which are most interesting to 

 the Physiologist, from their connection with the important func- 

 tion of Reproduction, we shall notice those parts of the flower 

 which are less essential to this object, but which commonly excite 



