, for these bud-sports do not appear in all
parts of the organism, only in certain buds or parts of it, so that one
part of the zygotic machine would appear to work differently to another.
To discuss this question further would take us too far from our subject.
Suffice it to say that we cannot answer it, any more than we can this
further question of burning interest at the present day, viz. to
what extent and in what manner is the machine itself altered by the
particular way in which it is worked. In connection with this question
we can only submit one consideration: the zygotic machine can, by
its nature, only work once, so that any alteration in it can only be
ascertained by studying the replicas of it which are produced in the
reproductive organs.

It is a peculiarity that the result which we call the ripening of the
generative organs nearly always appears among the final products of the
action of the zygotic machine. It is remarkable that this should be
the case. What is the reason of it? The late appearance of functional
reproductive organs is almost a universal law, and the explanation of it
is suggested by expressing the law in another way, viz. that the machine
is almost always so constituted that it ceases to work efficiently
soon after the reproductive organs have sufficiently discharged their
function. Why this should occur we cannot explain: it is an ultimate
fact of nature, and cannot be included in any wider category. The
period during which the reproductive organs can act may be short as
in ephemerids or long as in man and trees, and there is no reason to
suppose that their action damages the vital machinery, though sometimes,
as in the case of annual plants (Metschnikoff), it may incidentally
do so; but, long or short, the cessation of their actions is always
a prelude to the end. When they and their action are impaired, the
organism ceases to react with precision to the environment, and the
organism as a whole undergoes retrogressive changes.

It has been pointed out above that there is reason to believe that at
the dawn of life the life-cycle was, EITHER IN ESSE OR IN POSSE, at
least as long as it is at the present time. The qualification implied by
the words in italics is necessary, for it is clearly possible that the
external conditions then existing were not suitable for the production
of all the stages of the potential life-history, and that what we
call organic evolution has consisted in a gradua