MU. C. DAKWIN OX SPECIFIC DTFFEREXCES IN PRIMULA. 437 



allied species cannot be crossed, or can only be crossed with the 

 greatest difficulty. We are led to this conclusion still more 

 forcibly by considering the great diiference which often exists in 

 the facility of crossing reciprocally the same two species ; for it is 

 clear in this ease that the result must depend on the nature of the 

 sexual elements, the male element of the one species acting freely on 

 the female element of the other, but not so in the reversed direction. 

 And now we see that this same conclusion is independently and 

 strongly fortified by considering the illegitimate unions and off- 

 spring of trimorphic and dimorphic plants. In so complex and 

 obscure a subject as hybridism it is no slight gain to arrive at a 

 definite conclusion, namely, that we must look exclusively to 



a functional difference in the sexual elements, as the cause of 

 the sterility of species when first crossed, and of their hybrid 

 offspring. It was this consideration which led me to make so 

 many and such laborious observations as have been recorded in 

 this paper, and which justify, I think, their publication. 



On the Specific Difference between Pxifnula veris, Brit. Fl. (var. 

 officinalis of Linn.), P. vulgaris, Brit. Fl. (var. acauJis^ Linn.), 

 and P. elatior, Jacq. ; and on the Hybrid Nature of the com- 

 mon Oxlip. With Supplementary Bemarks on naturally-pro- 

 duoed Hybrids in the genus Verhascum, By Charles Dau- 



wM, M.A., F.K.S., F.L.S., &c. " 



[Read March 19, 1868.] 



The claim of the above three forms (namely, the common Cowslip, 

 Primrose, and Bardfield Oxlip) to be ranked as distinct species has 

 been discussed at greater length than that of almost any other 

 plants. Linnaeus considered them varieties, as do some of the 

 most distinguished botanists at the present day ; whilst others 

 who have carefully studied these plants do not doubt that they 

 deserve to be ranked as distinct species. The following observa- 

 tions show, I think, that the latter view is correct ; and they fur- 

 ther show that the common Oxlip, which is found in most parts 

 of England, is a hybrid between P. veris and vulgaris. 



The Cowslip differs so conspicuously in general appearance 

 from the Primrose, that nothing need here be said with respect to 

 their external characters *. But some less-ob\dous differences de- 



♦ 



Eev 



of the capsules and seed, in * Ann. & Mag. of Nat. Hist.' 2nd series, toI. ii. (1848), 

 p. 164. 



