30 The Problems 



Lastly, it is impossible to be presented with the fact 

 that in Mendelian cases the cross-bred produces on an 

 average equal numbers of gametes of each kind, that is to 

 say, a symmetrical result, without suspecting that this fact 

 must correspond with some symmetrical figure of distribu- 

 tion of those gametes in the cell-divisions by which they are 

 produced. 



At the present time these are the main conceptions 

 though by no means all arising directly from Mendel's 

 work. The first six are all more or less clearly embodied 

 by him, though not in every case developed in accordance 

 with modern knowledge. The seventh is not a Mendelian 

 conception, but the facts before us justify its inclusion in 

 the above list though for the present it is little more than 

 a mere surmise. 



In Mendelian cases it will now be perceived that all 

 the zygotes composing 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 pre-existing 

 characters. It is now evident that in such cases each 

 several progenitor need not be brought to account in 

 reckoning the probable characters of each descendant ; 

 for the gametes of cross-breds are differentiated at each 

 successive generation, some parental (Mendeliau) characters 

 being left out in the composition of each gamete produced 

 by a zygote arising by the union of bearers of opposite 

 allelomorphs. 



When from these considerations we return to the 

 phenomena comprised in the Law of Ancestral Heredity, 

 what certainty have we that the same conceptions are not 

 applicable there also ? 



