Chap. XV. OCCASIONALLY INTERCROSSING. 91 



having experimented on tliem : even the peasants of Liguria 

 say that cabbages must be prevented " from falling in love " 

 with each other. In the orange tribe, Gallesio 14 remarks that 

 the amelioration of the various kinds is checked by their con- 

 tinual and almost regular crossing. So it is with numerous 

 other plants. 



Nevertheless some cultivated plants can be named which rarely 

 intercross, as the common pea, or which never intercross, as 1 

 have reason to believe is the case with the sweet-pea (Latliyrus 

 odoratus) ; yet the structure of these flowers certainly favours 

 an occasional cross. The varieties of the tomato and aubergine 

 (Solanum) and pimenta {Pimenta vulgaris?) are said 15 never 

 to cross, even when growing alongside each other. But it should 

 be observed that these are all exotic plants, and we do not know 

 how they would behave in their native country when visited by 

 the proper insects. 



It must also be admitted that some few natural species 

 appear under our present state of knowledge to be perpetually 

 self-fertilised, as in the case of the Bee Ophrys (0. apifera), 

 though adapted in its structure to be occasionally crossed. The 

 Leersia oryzoides produces minute enclosed flowers which cannot 

 possibly be crossed, and these alone, to the exclusion of the 

 ordinary flowers, have as yet been known to yield seed. 16 A 

 few additional and analogous cases could be advanced. But 

 these facts do not make me doubt that it is a general lav/ of 

 nature that the individuals of the same species occasionally in- 

 tercross, and that some great advantage is derived from this act. 

 It is well known (and I shall hereafter have to give instances) 

 that some plants, both indigenous and naturalised, rarely or 

 never produce flowers ; or, if they flower, never produce seeds. 

 But no one is thus led to doubt that it is a o-eneral law of nature 



o 



that phanerogamic plants should produce flowers, and that 

 these flowers should produce seed. When they fail, we believe 

 that such plants would perform their proper functions under 

 different conditions, or that they formerly did so and will do 

 so again. On analogous grounds, I believe that the few flowers 



14 'Teoria della Riproduzione Vegetal,' 1816, p. 12. 



15 Verlot, ' Des Varie'te's,' 1865, p. 72. 



16 Duval-Jouve, 'Bull. Soc. Bot. do France,' torn, x., 1863, p. 191. 



