Chap. II. THE COMMON OXLIP. 71 



hereafter see good reason to believe. The case of 

 the oxlip is interesting; for hardly any other in- 

 stance is known of a hybrid spontaneously arising 

 in such large numbers over so wide an extent of coun- 

 try. The common oxlip (not the P. datior of Jacq.) 

 is found almost ererywhere throughout England, where 

 both cowslips and primroses grow. In some districts, 

 as I have seen near Hartfield in Sussex and in parts 

 of Surrey, specimens may be found on the borders of 

 almost every field and small wood. In other districts 

 the oxHp is comparatively rare : near my own resi- 

 dence 1 have found, during the last twenty-five years, 

 not more than five or six plants or groups of plants. 

 It is difficult to conjecture what is the cause of this 

 difference in their number. It is almost necessary 

 that a plant, or several plants belonging to the same 

 form, of one parent-species, should grow near the 

 opposite form of the other parent-species; and it is 

 further necessary that both species should be fre- 

 quented by the same kind of insect, no doubt a moth. 

 The cause of the rare appearance of the oxlip in 

 certain districts may be the rarity of some moth, 

 which in other districts habitually visits both the 

 primrose and cowslip. 



FiaaUy, as the cowslip and primrose differ in the 

 various characters above specified, — as they are in a 

 high degree sterile when intercrossed, — as there is no 

 trustworthy evidence that either species, when un- 

 crossed, has ever given birth to the other species or 

 to any intermediate form, — and as the intermediate 

 foi-ms which are often found in a state of nature have 

 been shown to be more or less sterile hybrids of the 

 first or second generation, — we must for the future 

 look at the cowsKp and primrose as good and true 

 species. 



