20 PROBLEMS OF GENETICS 



form should always, in all the diversities of soil and situation 

 which they inhabit, be unable to exist? Some one may fancy 

 that the hybrids which are found in the situations occupied by 

 both forms are this original parental species. But nothing can 

 be more certain than that these plants are simply heterozygous 

 combinations made by the union of gametes bearing the characters 

 of diurna and vespertina. 11 For they may be reproduced exactly 

 in Fi or in later generations of that cross when it is artificially 

 made; when bred from their families exhibit palpable phenomena 

 of segregation more or less complex; and usually, if perhaps not 

 always, they are partially sterile. 12 In a locality on the Norfolk 

 coast that I know well, there is a strip of rough ground chiefly 

 sand -bank, which runs along the shore. This ground is full of 

 vespertina. Not a hundred yards inland is a lane containing 

 diurna, and among the vespertina on the sand-bank are always 

 some of the hybrid form, doubtless the result of fertilisation 

 from the heighbouring diurna population. Seed saved from these 

 hybrids gave vespertina and hybrids again, having obviously been 

 fertilised by other vespertina or by other hybrids, and I have 

 no doubt that such hybrid plants if fertilised by diurna would 

 have shown some diurna offspring. The absence of diurna 

 in such localities may fairly be construed as an indication that 

 diurna is there at a real disadvantage in the competition for 

 life. 



But if, admitting this, we proceed to consider how the special 

 aptitude of vespertina is constituted, or what it is that puts 

 diurna at a disadvantage, we find ourselves quite unable to 

 show the slightest connexion between the success of one or the 



11 A discussion of this subject with references to literature is given by Rolf e, 

 in an excellent paper on "Hybridisation viewed from the standpoint of Systematic 

 Botany" (Jour. R. Hort. Soc, XXIV, 1900, p. 197). He concludes: "The simple 

 fact is that the two plants (L. diurna and vespertina) are thoroughly distinct in 

 numerous particulars, and affect such different habitats that in some localities 

 one or the other of them is completely wanting. But when their stations are 

 adjacent they hybridise together very readily, and it is here that these intermediate 

 forms occur which have puzzled botanists so much." The same paper contains 

 valuable information concerning several cognate illustrations. 



12 In only two cases have I seen such plants (both females) completely sterile. 



