The facts, showing that with certain diseases the period of inheritance
occasionally or even frequently advances, are important with respect to the
general descent-theory, for they render it probable that the same thing would
occur with ordinary modifications of structure. The final result of a long
series of such advances would be the gradual obliteration of characters proper
to the embryo and larva, which would thus come to resemble more and more
closely the mature parent-form. But any structure which was of service to the
embryo or larva would be preserved by the destruction at this stage of growth
of each individual which manifested any tendency to lose its proper character
at too early an age.

Finally, from the numerous races of cultivated plants and domestic animals, in
which the seeds or eggs, the young or old, differ from one another and from
those of the parent-species;--from the cases in which new characters have
appeared at a particular period, and afterwards been inherited at the same
period;--and from what we know with respect to disease, we must believe in the
truth of the great principle of inheritance at corresponding periods of life.

SUMMARY OF THE THREE PRECEDING CHAPTERS.

Strong as is the force of inheritance, it allows the incessant appearance of
new characters. These, whether beneficial or injurious,--of the most trifling
importance, such as a shade of colour in a flower, a coloured lock of hair, or
a mere gesture,--or of the highest importance, as when affecting the brain, or
an organ so perfect and complex as the eye,--or of so grave a nature as to
deserve to be called a monstrosity,--or so peculiar as not to occur normally
in any member of the same natural class,--are often inherited by man, by the
lower animals, and plants. In numberless cases it suffices for the inheritance
of a peculiarity that one parent alone should be thus characterised.