“The characters of the individual are referable to paired elements (genes) in the germinal matter that are held together in a definite number of linkage groups…. The members of each pair of genes separate when germ cells mature…. Each germ-cell comes to contain only one set…. These principles…enable us to handle problems of genetics in a strictly numerical basis, and allow us to predict…what will occur…. In these respects the theory [of the gene] fulfills the requirements of a scientific theory in the fullest sense.… It is difficult to resist the fascinating assumption that the gene is constant because it represents an organic chemical entity. This is the simplest assumption that one can make at present, and since this view is consistent with all that is known about the stability of the gene it seems, at least, a good working hypothesis.” Thomas Hunt Morgan ( 2022 ).