THE PROBLEMS OF HEREDITY AND THEIR SOLUTION. 577 



the iibove list, though for the prcsmt it is litth^ more tiiiiii a mere sur- 

 mise. ' , 



In MeudeUan eases it will now 1>e pcreeivt'fl that all the zygotes 

 eoniposing the population consist of a limited number of possible 

 types, each of definite constitution, bearing gametes also of a limited 

 and definite number of types, and definite constitution in respect of 

 preexisting characters. It is now evident that in such cases each sev- 

 eral i)rogenitor need not ))e ])i'ought to account in reckoning the prob- 

 able characters of each descendant; for the gametes of crossbreds arc 

 difi;'erentiated at each successive generation, some parental (Mendelian) 

 characters being left out in the composition of each gamete produced 

 by a zygote arising ))y the union of l)earers of opposite allelomorphs. 



When from these considerations we return to the phenomena com- 

 prised in the law of ancestral heredity, wliat certainty have we that 

 the same conceptions are not applicable there also^ 



It has now been shown that the question whether in tlie crossbred 

 zygotes in general the characters l)lend or are nmtually exclusive is an 

 entirely subordinate one, and distinctions with regard to the essential 

 nature of heredity l)ased on these circumstances ])ecome irrelevant. 



In the case of a population presenting continuous variation in regard 

 to, say, stature, it is easy to see how ])urity of the gamt>tes in respect of 

 any intensities of that character might not in ordinary circumstances 

 be capable of detection. There are doubtless more tiian two pure 

 gametic forms of this character, but there may ([uite conceiva])ly be 

 six or eight. AVhen it is remem])ered that each heterozygous combi- 

 nation of any two may lune its own appropriate stature, and that 

 such a character is distinctly dependent on external conditions, the 

 mere fact that the obser\ed curves of stature give "chance distribu- 

 tions'' is not surprising and may still be comi)atible with purity of 

 gametes in respect of certain pure types. In peas {I\ .saflvu///)^ for 

 example, from MendeFs work wa know that the tall forms and the 

 extreme dwarf forms exhil)it gametic purity. 1 liave seen at Messrs. 

 Sutton's strong evidence of the same nature in the case of the tall 

 sweet pea {LatJti/i'us odordfux) and th(^ dwarf or procuml)ent ''Cupid" 

 form. 



But in the case of the sweet pea we know at h'ast one pure form of 

 definitely intermediate height, and in the case of /*. .satlvinii there are 

 many. ^Yhen the extreme types breed togetlnn- it will be remembered 

 the heterozygote commonly exceeds the taller in height. In th(> next 

 generation, since there is in the case of extremes, so nuicli margin 

 between the types of the two pure forms, the return of the ofi'springto 

 the three forms, of whicli two are homozygous and one heterozygous, 

 is clearly perceptil)le. 



If, however, instead of pure extreme varieties, we were to take a 

 pair of varieties differing normally by only a foot or two, we might, 



SM 1902 37 



