CHAP. I. PEIMULA VEKIS. 29 



so here we have two bodies, approximately equal in 

 number, differing in their sexual powers and related to 

 each other like males and females. There are many 

 hermaphrodite animals which cannot fertilise them- 

 selves, but must unite with another hermaphrodite. 

 So it is with numerous plants ; for the pollen is often 

 mature and shed, or is mechanically protruded, before 

 the flower's own stigma is ready ; and such flowers ab- 

 solutely require the presence of another hermaphro- 

 dite for sexual union. But with the cowslip and various 

 other species of Primula there is this wide difference, 

 that one individual, though it can fertilise itself im- 

 perfectly, must unite with another individual for full 

 fertility ; it cannot, however, unite with any other in- 

 dividual in the same manner as an hermaphrodite 

 plant can unite with any other one of the same species ; 

 or as one snail or earth-worm can unite with any other 

 hermaphrodite individual. On the contrary, an indi- 

 vidual belonging to one form of the cowslip in order 

 to be perfectly fertile must unite with one of the other 

 form, just as a male quadruped must and can unite 

 only with the female. 



I have spoken of the legitimate unions as being 

 fully fertile ; and I am fully justified in doing so, for 

 flowers artificially fertilised in this manner yielded 

 rather more seeds than plants naturally fertilised in 

 a state of nature. The excess may be attributed to 

 the plants having been grown separately in good soil. 

 With respect to the illegitimate unions, we shall best 

 appreciate their degree of lessened fertility by the 

 following facts. Gartner estimated the sterility of the 

 unions between distinct species,* in a manner which 

 allows of a strict comparison with the results of the 



' Verauche iiber die Bastarderzcugung,* 1819, p. 216. 



