INTRODUCTION 3 



The occurrence of senescence in the organic world raises many 

 questions of great interest and importance, not only for the scientist, 

 but in certain aspects for the human race in general. How do young 

 and old organisms differ from each other, and what is the nature 

 of senescence ? Is it a feature of the fundamental processes of life 

 or the result of incidental conditions ? Does it occur in all organ- 

 isms or only in the more complex, more highly differentiated forms ? 

 Does it inevitably lead sooner or later to death, or is a rejuvenes- 

 cence of old organisms or parts possible ? Is the process of senes- 

 cence in a given organism always of the same character, or does it 

 depend upon the environmental conditions ? Is the rate of senes- 

 cence always the same in a particular species, or does it differ in 

 different individuals according to the action of internal or external 

 factors. Many of these questions can be summed up in the one, 

 Can we control senescence? 



In nature the organism resulting from the union of the two 

 sexual cells is young. This fact raises another series of questions. 

 Does rejuvenescence occur somewhere in the course of sexual re- 

 production, or does the germ plasm from which the sex cells arise 

 not grow old ? Are the organisms which result from asexual re- 

 production also young, or is sexual reproduction the only process 

 which gives rise to young organisms? If rejuvenescence occurs, 

 upon what does its occurrence depend and what is its nature? 

 Does it occur in all organisms, or only in certain of them ? Is com- 

 plete rejuvenescence possible, or is the species and the organic world 

 in general undergoing a senescence which will lead to extinction ? 



These are a few of the most important questions which the 

 occurrence of senescence and the processes of reproduction lead us 

 to ask. In the following chapters these and some other questions 

 will be considered in the light of the experimental and observational 

 evidence which we possess. To some of these questions we shall 

 be able to give a definite answer, to some others the answer must 

 be provisional, and some we must leave open for the future to 

 answer, though even here we can indicate the direction in which 

 the facts point. 



The problem of senescence has been discussed many times in 

 the history of biology, and many hypotheses as to its nature have 



