Here, then, we have the foundation of nature’s method of building animals and plants of the higher classes.  In the machinery of the cell she has a power of reproduction which produces an increase in geometrical ratio far beyond the possibility for the surface of the earth to maintain.

==Heredity.==—­The offspring which arise by these processes of division are like each other, and like the parent from which they sprung.  This is the essence of what is called heredity.  Its significance in the process of machine building is evident at once.  It is the conserving force which preserves the forms already produced and makes it possible for each generation to build upon the structures of the earlier ones.  Without it each generation would have to begin anew at the beginning, and nothing could be accomplished.  But since this principle brings each individual to the same place where its parents stand, and thus always builds the offspring into a machine like the parent, it makes it possible for the successive generations to advance.  Heredity is thus like the power of memory, or better still, like the invention of printing in the development of civilization.  It is a record of past achievements.  By means of printing each age is enabled to benefit by the discoveries of the previous age, and without it the development of civilization would be impossible.  In the same way heredity enables each generation to benefit by the achievements of its ancestors in the process of machine building, and thus to devote its own energies to advancement.

The fact of heredity is patent enough.  It has been always clearly recognized that the child has the characters of its parents, and this belief is so well attested as to need no proof.  It is still a question as to just what characters may be inherited, and what influences may affect the inheritance.  There are plenty of puzzling problems connected with heredity, but the fact of heredity is one of the foundation stones of biological science.  Upon it must be built all theories which look toward the explanation of the origin of the living machine.

This factor of heredity again we must trace back to the machinery of the cell.  We have seen in the previous pages evidence for the wonderful nature of the chromosomes of the cells.  We can not pretend to understand them, but they must be extraordinarily complex.  We have seen proof that these chromosomes are probably the physical basis of heredity, since they are the only parts of each parent which are handed down to subsequent generations.  With these various facts of cell division and cell fertilization in mind, we can reach a very simple explanation of fundamental features of heredity.  The following is an outline of the most widely accepted view of the hereditary process.