{Trilobites VII. Arthropods, e.g. {Crabs, lobsters {So-called "sea scorpions" {Insects {Ostracoderms {Fishes VIII. Vertebrates, e.g. {Amphibians {Reptiles {Birds {Mammals (including man) Before entering into a brief but rather systematic discussion of some of the most important types of animals which lived during geological time, it may be well for the reader to have in mind some of the most important conclusions which have been reached as a result of the study of the fossil animal records. These conclusions may be summarized as follows: 1. Animal life existed many millions of years ago. 2. Not only the animals of to-day, but also those of any given geological period, directly descended from those of preceding geological periods. 3. Animal life has undergone continuous change since its introduction upon the earth, so that each group of strata, representing a particular geological age, contains a characteristic assemblage of fossil animals. 4. Many of the changes in the history of animals have been progressive or evolutionary, so that strata of early geological time contain distinctly more primitive or lower order forms than the strata of late geological time. But, while the line of evolution has been maintained without a break, culminating in man, there have been many offshoots of a retrogressive nature. 5. Even as far along in geological time as the early Paleozoic era, the highest subkingdom--vertebrates--had no representative whatever. In other words, all the important subdivisions of animal life from a little below fishes to man have been evolved since about the close of the Ordovician period. 6. Any species of animal which ever became extinct has never been known to reappear, and literally tens of thousands of species are known to have become extinct. 7. No species like those now living are found in the more ancient strata, such being confined to the strata of relatively recent geological dates. 8. While more and more highly organized animals have continuously been evolved, many of the earlier and simpler types have persisted, a remarkable case in point being the single-celled animals called foraminifers which may be traced, without very notable change, through the tens of millions of years of geological time from the late Proterozoic era to the present day. 9. Many species have been able to maintain themselves practically without change through long stretches of geological time, while others have had only very brief existence.