ld 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.

Recognizing that the chromosomes are the physical basis of hereditary
transmission, we can picture to ourselves the transmission of hereditary
characters something as follows: As we have seen, the fertilized egg
contains an equal number of chromosomes from each parent (Fig. 42). Now
when this fertilized cell divides, each of the rods splits len