itself into two halves, each like the other.  At first all the parts thus arising separated from each other and remained independent.  But so long as this habit continued there could be little advance.  After a time some of the cells failed to separate after division, but remained clinging together (Fig. 45).  The cells of such a mass must have been at first all alike; but, after a little, differences began to appear among them.  Those on the outside of the mass were differently affected by their surroundings from those in the interior, and soon the cells began to share among themselves the different duties of life.  The cells on the outside were better situated for protection and capturing food, while those on the inside could not readily seize food for themselves, and took upon themselves the duty of digesting the food which was handed to them by the outer cells.  Each of these sets of cells could now carry on its own special duties to better advantage, since it was freed from other duties, and thus the whole mass of cells was better served than when each cell tried to do everything for itself.  This was the first step in the building of the machine out of the active cells (Fig. 46).  From such a starting point the subsequent history has been ever based upon the same principle.  There has been a constant separation of the different functions of life among groups of cells, and as the history went on this division of labor among the different parts became greater and greater.  Group after group of cells were set apart for one special duty after another, and the result was a larger and ever more complicated mass of cells, with a greater and greater differentiation among them.  In this building of the machine there was no time when the machine was not active.  At all points the machine was alive and functional, but each step made the total function of the machine a little more accurately performed, and hence raised somewhat the totality of life powers.  This parcelling out of the different duties of life to groups of cells continued age after age, each step being a little advance over the last, until the result has been the living machine as we know it in its highest form, with its numerous organs, all interrelated in such a way as to form a harmoniously acting whole.

[Illustration:  FIG. 46.  A later step in machine building in which the outer cells have acquired different form and function from the inner cells:  ec, the outer cells, whose duties are protective; en, the inner cells engaged in digesting food.]

But a second principle in this growth of the machine was needed to produce the variety which is found in nature.  As the different cells in the multicellular mass became associated into groups for different duties, the method of such division of labor was not alike in all machines.  A city in China and one in America are alike made up of individuals, and the fundamental needs of the Chinaman and the American are alike.