I do not in the least mean by this that our faith in mechanistic methods and conceptions is shaken. It is by following precisely these methods and conceptions that observation and experiment are every day enlarging our knowledge of colloidal systems, lifeless and living. Who will set a limit to their future progress? But I am not speaking of tomorrow but of today; and the mechanist should not deceive himself in regard to the magnitude of the task that still lies before him. Perhaps, indeed, a day may come (and here I use the words of Professor Troland) when we may be able ‘to show how in accordance with recognized principles of physics a complex of specific, autocatalytic, colloidal particles in the germ cell can engineer the construction of a vertebrate organism’; but assuredly that day is not yet within sight of our most powerful telescopes. Shall we then join hands with the neovitalists in referring the unifying and regulatory principle to the operation of an unknown power, a directive force, an archaeus, an entelechy, or a soul? Yes, if we are ready to abandon the problem and have done with it once and for all. No, a thousand times, if we hope really to advance our understanding of the living organism.