REPRODUCTION IN UNICELLULAR ORGANISMS 261 



In Pandorina the state of things is still the same as in single-celled 

 organisms, for each cell is still all in all, each can bring forth the 

 whole, none dies from physiological causes involved in the course of 

 development, and they are therefore ' immortal ' in the sense stated. 

 But iu Volvox the ' individual ' dies when it has given off its repro- 

 ductive cells, because here the contrast between germ-cells and body 

 has developed. Only the body is mortal in the sense of being subject 

 to natural death ; the germ-cells possess the potential immortality of 

 the single-celled animals, and it is necessary that they should possess 

 it if the species is to continue to exist. 



From this alone it does not seem quite clear why the body or 

 soma should be subject to death, and when I first endeavoured to 

 arrive at clearness in regard to these matters I tried to find out why 

 a natural death of the body was necessitated by the course of 

 evolution. I did not at once discover the true explanation, but 

 without delaying to discuss my mistakes I shall proceed to expound 

 what I believe to be the true reason. It lies simply in the fact, which 

 we shall inquire into later on in more detail, that every function and 

 every organ disappears as soon as it becomes superfluous for the 

 maintenance of the particular form of life in question. The power of 

 being able to live on without limit is useless for the somatic cells, and 

 thus also for the body, since these cannot produce new reproductive 

 cells after those that had been present are liberated; and with this 

 the individual ceases to be of any value for the preservation of the 

 species. What advantage would it be to the species if the Volvox 

 balls were to continue living for an unlimited time after the repro- 

 ductive cells were developed and had been liberated? Obviously 

 their further fate can have no influence whatever in determining or 

 preserving the characters of the species, and it is quite indifferent to 

 the continuance of the species whether and. how long they go on 

 living. Therefore the soma has lost the capacity which conditions 

 endless continuance of life and continued renewal of body-cells. 



In regard to these' views it has been asked jeeringly, how 

 ' immortality,' if it were really a property of the Unicellulars and of 

 undifferentiated cell-colonies, could be lost, as if the world, which we 

 believe to be everlasting, should give up its everlastingness. But the 

 jeer recoils on the superficial outlook which is unable to distinguish 

 between the immortality dreamed of by the poets, religious and 

 secular, and the real power that certain forms of life have to resist 

 being permanently exhausted by their own metabolism. That we 

 should call this 'immortality' does not seem to me to require any 

 apology, for the right has always been conceded to science to transfer 



