572 THE PK()BLP]M^^ OF HEREDITY AND THEIR BOLTTTIOJST. 



;i wav that some yainotes shall he liiially cxcliuled from partakiti*^- of 

 and traiisiiiittiny- any specitic psirt of the lu^ritaj^e. The theory further 

 demands and by the analogy of what we knoAV otherwise not only of 

 animals and plants, but of physical or chemical laws, perhaps this is 

 the most serious assumption of all — that the structure of the gametes 

 shall admit of their being capable of transmitting any character in any 

 intensity varjang from zero to totality with equal ease, and that 

 gametes of each intensity are all e([ually likely to occur, given a pedi- 

 gree of appropriate arithmetical composition. 



Such an assumption appears so improbable that even in cases where 

 the facts seem as yet to point to this conclusion with exceptional clear- 

 ness, as in the case of human stature, I can not but feel there is still 

 room for reserve of judg-ment. 



However this may l)e, the law of ancestral heredit}- and all modifi- 

 cations of it yet proposed falls short in the respect specified above — 

 that it does not directly attempt to give any account of the distribution 

 of the heritage among the gametes of any one individual. 



Menders conception differs fundamentally from that iuAolved in the 

 lavr of ancestral heredity. The relation of his h3^pothesis to the fore- 

 going may be most easily shown if we consider it first in application 

 to the phenomena resulting from the cross breeding of two pure 

 varieties. 



Let us again consider the case of two varieties, each displaying the 

 same character, but in the respective intensities A and (/. Each gamete 

 of the A variety bears A, and each gamete of the a variety bears a. 

 When thoy unite in fertilization they form the zygote Aa. What will 

 be its characters!' The Mendel ian teaching would reply that this can 

 only be known by direct experiment with the two forms ^1 and //, and 

 that the charactei's ^1 and a perceived in those two forms or vari(>ties 

 need not give an}^ indication as to the character of the zygote Aa. It 

 may displa}' the character ^1 or </ or a character halfwav between the 

 two, or a character beyond A or })elow </. The charactei* of Aa is 

 not regarded as a heritage transmitted to it by A and by a, but as a 

 character special and peculiar to At/, just as NaCl is not a body half- 

 way between sodium and chlorine, or such that its properties can l»e 

 predicted from or easily stated in terms of theirs. 



If a concrete case may help, a tall pea A crossed with a dwarf (/ often 

 produces not a plant having the height of either A or </, but some- 

 thing taller than the pure tall variety ^1. 



But if the case obeys the Mendel ian piinciples — as does that here 

 quoted — then it can be declared, first, that the gametes of Aa will not 

 be bearers of the character proper to A(// but, generally sj^eaking, 

 each ganu^e ^v'ill eitlu^r bear the pui'e .1 character or the }>ure a char- 

 acter. There will in fact be a redisti'ibution of the characters brought 

 in by the gamet(\s which united to form the zygote Aa, such that each 

 gamete of Aa is pure, as the parental gametes were. Secondly, this 



