yet the cause of gravitation is even to-day a mere speculation, and no link has yet been discovered to connect the three phases of attraction, gravitation between masses, cohesion and adhesion between molecules, and chemical affinity between atoms. The fact of evolution is universally recognized, much is known and more is a matter of speculation as to how and why evolution has taken place, but why an organism tends to resemble its progenitor and why it tends to vary are as unknown as before the days of Darwin and Wallace. So the Periodic Law is, after all, a mere statement of fact. But it is a statement which has already exerted vipon chemistry an influence as great as that of gravitation and evolution in their respective fields, and is destined more and more to become the very foundation of chemistry. We may hope and confidently expect that it will eventually lead us to the real constitution of matter.
