WEISM ANN'S ESSA YS UPON HEREDITY. 7 



If natural selection, operating on the material of 

 minute quantitative variations in the cells, not only 

 produced a quantitative distinction between mortal 

 and immortal, but put a premium on death, why 

 should not immortality some day reappear? If 

 immortality is lost because it proved under certain 

 conditions "a useless luxury," under other con- 

 ditions it might reappear. Professor Weismann 

 answers The line between mortal and immortal is 

 less sharp than we might suppose. For the immor- 

 tal cells, which do not die, can be killed ; and the 

 mortal cells do recover the power of indefinite, if 

 not infinite, reproduction, where long life is neces- 

 sary for the good of the species. The duration of 

 life is seen to be controlled by the good of the 

 species, and long life, where it is so needed, is 

 secured by an increase of the number of cell- 

 generations in the soma. 



If we further ask Why, then, may not this 

 increase continue till the line between reproductive 

 and somatic, immortal and mortal disappears ? 

 the answer is, that there is no reason, except that 

 we cannot imagine conditions under which such an 

 extension would be for the good of the species. A 

 Tithonus, endowed with immortality but not 

 exempted from the wear and tear of life, is as 

 useless to the species as he is burdensome to him- 

 self. The soma being in its nature vulnerable, 

 would, if immortal, become of less and less use to 



