MENDEL'S DISCOVERY xxxi 



not combine In successive generations, but may be shown 

 by an appropriately arranged experiment gradually to 

 become divided between two groups of individuals. 

 Existing information as to the occurrence of these two 

 forms in Nature is given an arithmetical precision we 

 did not possess before ; furthermore we are led back to 

 a fascinating picture of the distribution of the antecedents 

 of ' thrum-eye ' and ' pin-eye ' among the germ-cells, and 

 of their unions in fertilization. 



In order to do justice to Mendel and because of the 

 value and interest of the inference, it is necessary to 

 explain in few words and as simply as possible what is 

 the nature of this picture of events we can never hope to 

 see with the microscope. The fertilized germ or zygote 

 from which all the higher animals develop is a single cell 

 of which the essential or nuclear elements are contributed 

 equally by the two parents. It has been proved by 

 observation that these essential parts of germ-cells (or 

 gametes) are reduced by half as a preparation for the 

 reception of a fresh half in fertilization or zygosis. Thus 

 it is that the starting-point of the individual is a single 

 cell and not a double cell. What Van Beneden and 

 others have seen with the microscope in the gamete and 

 the fertilized germ, Mendel's discovery enables the mind 

 to see in the material germinal basis of certain single 

 hereditary characters. Here, too, the material precursor 

 of a character at the starting-point of the individual is 

 made up of two parts, the allelomorphs, one from each 

 parental gamete. In the case of yellow and green 

 cotyledoned peas originally investigated by Mendel, we 

 may call the allelomorphs of the yellow character and the 

 green character respectively Y and G. In the first cross 

 between a pure yellow and a pure green pea it is obvious 

 that all the gametes of the one parent would carry Y and 



