Such lowly forms were destitute of any hard parts and could leave no evidence of their existence in the record of the rocks.  And of their supposed descendants we find so few traces in the pre-Cambrian strata that the first steps in organic evolution must be supplied from such analogies in embryology as the following.  The fertilized ovum, the cell with which each animal begins its life, grows and multiplies by cell division, and develops into a hollow globe of cells called the blastosphere.  This stage is succeeded by the stage of the gastrula,—­an ovoid or cup-shaped body with a double wall of cells inclosing a body cavity, and with an opening, the primitive mouth.  Each of these early embryological stages is represented by living animals,—­the undivided cell by the protozoa, the blastosphere by some rare forms, and the gastrula in the essential structure of the coelenterates,—­the subkingdom to which the fresh-water hydra and the corals belong.  All forms of animal life, from the coelenterates to the mammals, follow the same path in their embryological development as far as the gastrula stage, but here their paths widely diverge, those of each subkingdom going their own separate ways.

We may infer, therefore, that during the pre-Cambrian periods organic evolution followed the lines thus dimly traced.  The earliest one-celled protozoa were probably succeeded by many-celled animals of the type of the blastosphere, and these by gastrula-like organisms.  From the gastrula type the higher sub-divisions of animal life probably diverged, as separate branches from a common trunk.  Much or all of this vast differentiation was accomplished before the opening of the next era; for all the subkingdoms are represented in the Cambrian except the vertebrates.

Evidences of pre-Cambrian life.  An indirect evidence of life during the pre-Cambrian periods is found in the abundant and varied fauna of the next period; for, if the theory of evolution is correct, the differentiation of the Cambrian fauna was a long process which might well have required for its accomplishment a large part of pre-Cambrian time.

Other indirect evidences are the pre-Cambrian limestones, iron ores, and graphite deposits, since such minerals and rocks have been formed in later times by the help of organisms.  If the carbonate of lime of the Algonkian limestones and marbles was extracted from sea water by organisms, as is done at present by corals, mollusks, and other humble animals and plants, the life of those ancient seas must have been abundant.  Graphite, a soft black mineral composed of carbon and used in the manufacture of lead pencils and as a lubricant, occurs widely in the metamorphic pre-Cambrian rocks.  It is known to be produced in some cases by the metamorphism of coal, which itself is formed of decomposed vegetal tissues.  Seams of graphite may therefore represent accumulations of vegetal matter such as seaweed.  But limestone, iron ores, and graphite can be produced by chemical processes, and their presence in the pre-Cambrian makes it only probable, and not certain, that life existed at that time.