XI.] PROBLEMS OF THE DAY. 75 



be eternal. The immortality of unicellular organisms and 

 of germ-cells is, as I said years ago, not absolute, but po- 

 tential ; for they are not, like the gods of ancient Greece, 

 compelled to live for ever. Thus we are told that Ares 

 received a wound which would have proved fatal to any 

 mortal, but although he roared as loud as ten thousand bulls, 

 he could not die. The organisms in question can, and the ma- 

 jority of them do die, but a part of each lives on. But is it one 

 and the same substance which continues to live ? Does not life, 

 here and everywhere else, depend on assimilation, that is on 

 a constant change of material ? What then is immortal ? Appa- 

 rently not a substance at all, but a certain form of motion. The 

 protoplasm of unicellular beings possesses such an arrange- 

 ment in its chemical and molecular structure, that the cycle of 

 material which makes up life is ever repeating itself, and can 

 always begin afresh so long as the external conditions remain 

 favourable. In this respect it may be compared to the circu- 

 lation of water on the earth. Water evaporates, is condensed 

 into cloud, falls to the earth as rain, only once more to evapo- 

 rate, and thus the cycle repeats itself. And just as there exists 

 no inherent cause in the physical and chemical nature of water, 

 which interrupts this circulation, so in the physical nature of 

 the protoplasm of unicellular beings there is nothing which 

 puts an end to the cycle of existence, —that is fission, growth 

 by assimilation, and then fission again. It is this property 

 which I have called immortality, and in organic nature it is the 

 only real immortality to be met with. It is a purely biological 

 conception, and must be distinguished from the immortality of 

 non-living, that is of inorganic, matter. 



If then this real immortality is simply a cyclical movement 

 conditional on certain physical properties of protoplasm, why 

 should it be inconceivable that this property, under certain cir- 

 cumstances, should alter to some extent, so that the phases of 

 metabolic activity should not exactly repeat themselves, but 

 after a certain number of cycles should come to an end, result- 

 ing in death t All living matter varies, and why is it incon- 

 ceivable that variations of protoplasm should arise which, while 

 fulfilhng better certain functions advantageous to the indivi- 

 dual, should be associated with a metabolism that does not 

 exactly repeat itself a metabolism that sooner or later comes 



