The early and middle Proterozoic strata throw no more light upon the early history of animal life than do the Archeozoic strata. The upper or later Proterozoic rocks, however, contain the oldest recognizable animal fossils. Very recently fossil remains of single-celled, shell-bearing Protozoans have been found in northern France, while the upper Proterozoic strata of the Rocky Mountains in Montana, and the Grand Canyon of Arizona have yielded worm tracks, a Molluscoid (brachiopod) and fragments of lower forms of Arthropods. This record, although very meager, clearly proves that animal life was so well advanced by late Proterozoic time, that next to the highest subkingdom was actually represented (see above classification), and that there must have been a long line of simpler and simpler ancestors, probably extending far back into the Archeozoic era. When we stop to consider that Archeozoic and Proterozoic time was fully as long as all succeeding geological time, it is not so surprising that fairly highly developed animals (except Vertebrates) had been evolved before the close of the Proterozoic era. In regard to abundance of fossil animals the oldest (Cambrian) Paleozoic strata stand out in marked contrast to the Proterozoic. Many hundreds of species of animal fossils have been described from Cambrian strata, and a great many others yet remain to be discovered. Cambrian fossils are remarkably numerous, varied in species, and complex in organization (Plate 13). All subkingdoms of animals except the Vertebrates were represented, though usually only by the simpler types in each subkingdom. It is quite generally agreed that no less than 50 per cent of animal evolution had taken place before the beginning of the Cambrian period. The reader should, however, clearly bear in mind that tremendous advances in evolution have taken place since early Cambrian time when not only all forms from lower scale Arthropods to the highest mammals (including man) have evolved, but also when many thousands of species of lower subkingdom animals developed.