THE GENERAL CONDITIONS OF LIFE 341 



than they reveal the mechanism of development and life in 

 general ; but they afford a simplification and a sharper formula- 

 tion of the problem, and bring us somewhat nearer to an 

 understanding of it. 



The problem of development and the problem of death contain 

 the same question, namely : Why does living substance continually 

 change during its individual life ? Deeper penetration into the 

 chemism of the living cell will alone be able to reveal the special 

 causes of this phenomenon. 



2 The Question of Physical Immortality 



If natural death be considered from the standpoint just pre- 

 sented, a question which during the last decade has been actively 

 discussed upon the scientific side constantly obtrudes itself, viz., 

 Are there not organisms for which death is not a necessity ? 



Evidently an organism can be imagined the development of 

 which is such that a disturbance that makes impossible the co- 

 operation of the individual parts never appears. This would be 

 the case if the uninterrupted changes that appear during the 

 development of the organism in question form a series composed 

 of members recurring periodically. Such a development could 

 perhaps be represented schematically in the form of the solution 

 of a periodic continued fraction, which, transformed into a 

 decimal fraction, would give a periodic series, while the develop- 

 ment of an organism that is destined to die might be compared to 

 the solution of a definite fraction. Theoretically, such a hypo- 

 thetical organism would necessarily be immortal under external 

 conditions that always remained exactly the same. It is, however, 

 a question whether such organisms really exist. 



Weismann ('82, '84) believes that this question can be 

 answered in the affirmative, and it is interesting to follow his 

 discussion. He finds a fundamental difference between multi- 

 cellular organisms and unicellular Protista. Starting from the 

 thought that the term death can be employed only where a corpse 

 exists afterwards, he considers all multicellular organisms as 

 mortal, and all unicellular organisms as immortal. In multicellular 

 organisms no case is known where sooner or later the body does not 

 die. In unicellular forms, however, this is not true. A unicellular in- 

 fusorian, e.g., never becomes a corpse unless it is the victim of an 

 external catastrophe. It grows and divides into two halves when 

 it has reached a certain size, but each half likewise grows and 

 later divides and so on, and Weismann believes that this continues 

 without end. But since the two halves are wholly alike, and 

 since the species can be maintained only by continued division, 

 a corpse is never found, and a half never dies without external 



