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.

Aside from the simple fact that it preserves the race, the most important feature connected with this reproduction is its wonderful fruitfulness.  Since it results from division, it always tends to increase the offspring in geometrical ratio.  In the simplest case, that of the unicellular animals, the cell divides, giving rise to two animals, each of which divides again, producing four, and these again, giving eight, etc.  The rapidity of this multiplication is sometimes inconceivable.  It depends, of course, upon the interval of time between the successive divisions, but among the lower organisms this interval is sometimes not more than half an hour, the result of which is that a single individual could give rise in the course of twenty-four hours to sixteen million offspring.  This is doubtless an extreme case, but among all the lower animals the rate is very great.  Among larger animals the process is more complicated; but here, too, there is the same tendency to geometrical progression, although the intervals between the successive reproductions may be quite long and irregular.  But it is always so great that if allowed to progress unhindered at its normal rate the offspring would, in a few years, become so numerous as to crowd other life out of existence.  Even the slow-breeding elephant would, if allowed to breed unhindered for seven hundred and fifty years, produce nineteen million offspring—­a rate of increase plainly incompatible with the continued existence of other animals.