II. Paleolithic group, or paleozoic (primary) group of strata : V. Devonian : 10. Pilton : Upper devonian. II. Paleolithic group, or paleozoic (primary) group of strata : V. Devonian : 9. Ilfracombe : Middle devonian. II. Paleolithic group, or paleozoic (primary) group of strata : V. Devonian : 8. Linton : Lower devonian. II. Paleolithic group, or paleozoic (primary) group of strata : IV. Silurian : 7. Ludlow : Upper silurian. II. Paleolithic group, or paleozoic (primary) group of strata : IV. Silurian : 6. Wenlock : Middle silurian. II. Paleolithic group, or paleozoic (primary) group of strata : IV. Silurian : 5. Llandeilo : Lower silurian. I. Archeolithic group, or archeozoic (primordial) group of strata : III. Cambrian : 4. Potsdam : Upper cambrian. I. Archeolithic group, or archeozoic (primordial) group of strata : III. Cambrian : 3. Longmynd : Lower cambrian. I. Archeolithic group, or archeozoic (primordial) group of strata : II. Huronian : 2. Labrador : Upper laurentian. I. Archeolithic group, or archeozoic (primordial) group of strata : I. Laurentian : 1. Ottawa : Lower laurentian. The primordial period falls into three subordinate sections--the Laurentian, Huronian, and Cambrian, corresponding to the three chief groups of rocks that comprise the archaic formation. The immense period during which these rocks were forming in the primitive ocean probably comprises more than 50,000,000 years. At the commencement of it the oldest and simplest organisms were formed by spontaneous generation--the Monera, with which the history of life on our planet opened. From these were first developed unicellular organisms of the simplest character, the Protophyta and Protozoa (paulotomea, amoebae, rhizopods, infusoria, and other Protists). During this period the whole of the invertebrate ancestors of the human race were evolved from the unicellular organisms. We can deduce this from the fact that we already find remains of fossilised fishes (Selachii and Ganoids) towards the close of the following Silurian period. These are much more advanced and much younger than the lowest vertebrate, the Amphioxus, and the numerous skull-less vertebrates, related to the Amphioxus, that must have lived at that time. The whole of the invertebrate ancestors of the human race must have preceded these.