==Reproduction.==--Reproduction is the primary factor in this process of machine building, heredity and variation being simply phases of reproduction. The living machine has developed by natural processes, all other machines by artificial methods. Reproduction is the one essential point of difference between the living machine and the others which has made their construction by natural processes a possibility. What, then, is reproduction? Reproduction is in all cases at the bottom simple division. Whether we consider the plant that multiplies by buds or the unicellular animal that simply divides into two equal parts, or the larger animal that multiplies by eggs, we find that in all cases the fundamental feature of the process is division. In all cases the organism divides into two or more parts, each of which becomes in time like the original. Moreover, when we trace this division further we find that in all cases it is to be referred back to the division of the cell, such as we have described in a previous chapter. The egg is a single cell which has come from the parent by the division of one of the cells in the body of the parent. A bud is simply a mass of cells which have all arisen from the parent cells by division. The foundation of reproduction is thus in all cases cell division. Now, this process of division is dependent upon the properties of the cell. Firstly, it is a result of the assimilative powers of the cell, for only through assimilation can the cell increase in size, and only as it increases in size can it gain sustenance for cell division. Secondly, it is dependent, as we have seen, upon the mechanism of the cell body, and especially the nucleus and centrosome. These structures regulate the cell division, and hence the reproduction of all animals and plants. We can not, therefore, find any explanation of reproduction until we have explained the mechanism of the cell. The fundamental feature, of nature's machine building is thus based upon the machinery of the nucleus and centrosome of the organic cell.