Nevertheless, chromosomes appear as loci; in other words, not simply as places in space but as complexes of relations of proximity; genes express differential elements which also characterise an organism in a global manner, and play the role of distinctive points in a double process of reciprocal and complete determination; the double aspect of genes involves commanding several characteristics at once, and acting only in relation to other genes; the whole constitutes a virtuality, a potentiality; and this structure is incarnated in actual organisms, as much from the point of view of the determination of their species as from that of the differenciation of their parts, according to rhythms that are precisely called 'differential', according to comparative speeds or slownesses which measure the movement of actualisation.