The identification o f these forces with definite physical agents is still largely speculative, but the time is ripe for an attempt to see how far the theory o f colloidal structures can account for them. In this attempt it will be advisable, following Darlington, to consider separately the forces concerned with the “ internal mechanics” o f the chromosomes, that is to say, forces which act over distances o f the same order as the diameter o f the chromonema, and those concerned with the “ external mechanics,” which act over distances o f the order o f the length o f the

chromonema; we shall postpone till the next section a consideration o f the special forces which bring about the spiral coiling o f chromo­ somes.