.8^. DEATH. 439 



unicellular chain of germ cell-life of which the metazoa are the 

 carriers, may be called potentially immortal, and the metazoa mortal. 

 But all this merely deals with somatic death, and is, so to speak, a 

 biological accident. And so discussion or argument dealing with the 

 possibility of immortal protozoa giving rise to mortal metazoa — by, 

 for instance, some unequal fission separating a hypothetically mortal 

 part of the plasma from a hypothetically immortal, is futile. 



There is left, then, protoplasmic death. It is certain that proto- 

 plasmic death is not a necessary concomitant of protoplasmic life. 

 We know of no means to secure the set of conditions which we call 

 life, without the presence of some small piece of living material ; we 

 have considerable reason to suppose that spontaneous generation 

 does not occur, at least as a normal factor in the organic world ; we 

 know with absolute certainty that it never occurs in metazoa. From 

 this it follows that a direct continuous chain of cell-life binds all 

 animals with at least unicellular animals ; that, for instance, we our- 

 selves are at the end of a simple chain of cell division stretching 

 through an inconceivable number of generations to an inconceivably 

 remote past. So far as experience goes we can certainly say that 

 living matter does not necessarily carry within itself the seeds of 

 death. On the other hand, we can with equal certainty say that 

 death apparently comes sooner or later to all chains of life ; though 

 it may be only that in chains of life, as in single lives, old age brings 

 with it increased chances of death. Thus if we review the procession 

 of animal life in the past history of the earth, we find that always 

 they are the topmost branches that wither ; that sets of animals 

 highest in the scale at any geological period are constantly dying out 

 and being replaced, not by their own descendants, but by the 

 descendants of a younger and fresher stock. I take it that this does 

 not necessarily imply the otherwise improbable conception that death 

 is a necessary incident in life. Just as the older a weapon or an 

 ornament is, invariably it is the more battered or rusted, because, 

 however jealously guarded, increase of years means increase in number 

 of chances of exposure to accident ; so, however nicely adjusted 

 to environment, however constitutionally plastic and ready for 

 adaption to change, lapse of years must almost necessarily bring the 

 one set of unsuitable conditions, the finally fatal moulding of the 

 plastic constitution of an organic stock. 



Exposure to accidents and somatic death are sufficient causes 

 of death in the metazoa. It is not necessary to suppose that their 

 cell protoplasm differs from the cell protoplasm of protozoa in having 

 a limited duration of life. 



Now let us turn to the two cases of practical immortality to see 

 whether there be any special conditions. 



First for the protozoa. It has been too easily assumed that in 

 their case death follows from the gross accidents of life. The 

 protozoa of the Crag, or of the Chalk, or of older beds, are not identical 



