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 (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 jwsse, at 



