Sexual Maturity 181 
organism. This brings us back to the question with which we started 
this discussion, viz. what is the relation of these variations in struc- 
ture, which successively appear in an organism and constitute its 
life-history, to the mutational variations which appear in different 
organisms of the same brood or species. The question is brought 
home to us when we ask what is a bud-sport, such as a nectarine 
appearing on a peach-tree? From one point of view, it is simply 
a mutation appearing in asexual reproduction; from another it is 
one of these successional characters (“growth variations”) which 
constitute the life-history of the zygote, for it appears in the same 
zygote which first produces a peach. Here our analogy of a machine 
which only works in one way seems to fail us, 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 appear- 
ance 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 (Metgchnikoff ), 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 
