And it should be clear to the reader that given these elemental facts of life, given growth and death and reproduction with individual variation in a world that changes, life _must_ change in this way, modification and differentiation _must_ occur, old species _must_ disappear, and new ones appear. We have chosen for our instance here a familiar sort of animal, but what is true of furry beasts in snow and ice is true of all life, and equally true of the soft jellies and simple beginnings that flowed and crawled for hundreds of millions of years between the tidal levels and in the shallow, warm waters of the Proterozoic seas. The early life of the early world, when the blazing sun rose and set in only a quarter of the time it now takes, when the warm seas poured in great tides over the sandy and muddy shores of the rocky lands and the air was full of clouds and steam, must have been modified and varied and species must have developed at a great pace. Life was probably as swift and short as the days and years; the generations, which natural selection picked over, followed one another in rapid succession. Natural selection is a slower process with man than with any other creature. It takes twenty years or more before an ordinary human being in western Europe grows up and reproduces. In the case of most animals the new generation is on trial in a year or less. With such simple and lowly beings, however, as first appeared in the primordial seas, growth and reproduction was probably a matter of a few brief hours or even of a few brief minutes. Modification and differentiation of species must accordingly have been extremely rapid, and life had already developed a very great variety of widely contrasted forms before it began to leave traces in the rocks. The Record of the Rocks does not begin, therefore, with any group of closely related forms from which all subsequent and existing creatures are descended. It begins in the midst of the game, with nearly every main division of the animal kingdom already represented.[9] Plants are already plants, and animals animals. The curtain rises on a drama in the sea that has already begun, and has been going on for some time. The brachiopods are discovered already in their shells, accepting and consuming much the same sort of food that oysters and mussels do now; the great water scorpions crawl among the seaweeds, the trilobites roll up into balls and unroll and scuttle away. In that ancient mud and among those early weeds there was probably as rich and abundant and active a life of infusoria and the like as one finds in a drop of ditch-water to-day. In the ocean waters, too, down to the utmost downward limit to which light could filter, then as now, there was an abundance of minute and translucent, and in many cases phosphorescent, beings.