It should now be understood that Nature, like man, produces divergences from the parental stock, and isolates them by various effective agencies. Natural Selection ever carefully watches over these processes, eliminating the unfit variations and selecting the useful ones. Variation, Heredity, Environment, Isolation, and Natural Selection, having been acting and reacting through the ages, have produced, from a common parental stock, all the innumerable divergent and adaptive forms of living creatures that can be traced through the geologic strata to those on the surface of the earth to-day; have produced, therefore, _man_ also as the inflorescence of the topmost branch of the tree of life. SECTION VI. EVOLUTION OF MAN. EVOLUTION OF MAN. The detailed study of the development (ontogenetic and phylogenetic) of man is so vastly intricate and extensive a subject that it will be impossible in a work of this character to do more than refer to it in brief outline. One of the best and simplest methods to approach the study of such a subject is to acquire some idea of the _development of the frog_. The fertilized frog’s egg, which is the starting point in the life of a new frog, is deposited in water and hatched by the warmth of spring. After fertilization the egg or cell divides into two cells, these two into four, the four into eight, the eight into sixteen, and so on till a large number of small cells, associated together, is formed (Fig. 10). These cell phases represent different stages in the life of the growing frog. It is thus seen that the frog, in its earliest stages, consists of nothing but many small cells. These cells, through the mysterious powers of heredity, are going to differentiate as development proceeds into the various organs (groups of various kinds of cells) that form the tadpole and finally the frog. For further stages in the development of the egg (oösperm) towards the tadpole, works upon Comparative Embryology should be consulted. At length the tadpole is hatched from the egg, and then soon swims about in the water (Fig. 17).