210 The Mechanistic Conception of Life 



It has been stated that the processes of differentiation and 

 development lead also to the natural death of the individual.^ 

 If we express this in chemical terms it means that the chemical 

 processes which imderlie development also determine natural 

 death. Physical chemistry has taught us to identify two chemi- 

 cal processes even if only certain of their features are kno^vn. 

 One of these means of identification is the temperature 

 coefficient. When two chemical processes are identical, their 

 velocity must be reduced by the same amount if the tempera- 

 ture is lowered to the same extent. The temperature coefficient 

 for the duration of life of cold-blooded organisms seems, how- 

 ever, to differ enormously from the temperature coefficient for 

 their rate of development. For a difference in temperature of 

 10° C, the duration of life is altered five hundred times as much 

 as the rate of development; and, for a change of 20° C, it is 

 altered more than a hundred thousand times as much. From 

 this we may conclude that, at least for the sea-urchin eggs and 

 embryo, the chemical processes which determine natural 

 death are certainly not identical with the processes which 

 underlie their development. T. B. Robertson has also arrived 

 at the conclusion, for quite different reasons, that the process 

 of senile decay is essentially different from that of growth and 

 development. 



1 Weismann showed that infusorians or unicellular organisms in general are 

 immortal, while he assumed that all the other organisms with the exception of their 

 germ-plasm are mortal. Leo Loeb first called attention to the fact that the 

 transplantation of a cancer can be repeated to an imlimited series of generations, 

 and since it is the originally transplanted cancer-cell and the cells derived from it 

 by multiplication that survive, he pointed out that this proved that the principle 

 of immortality must also be granted to cancer-cells (1901). Later he generalized 

 this idea and stated that other cells may be considered immortal in the same 

 sense in which Weismann claimed this for the unicellular organisms. One can 

 indeed well imagine that the same piece of skin might be transplanted through an 

 indefinite series of generations of mice and that such a transplanted piece might 

 outlive an indefinite number of generations of mice in exactly the same way as a 

 cancer cell does. 



The natural death of the metazoa is perhaps a secondary phenomenon due 

 to the cessation of respiratory motions or of the heart beat. This leads to the 

 death of the cells through lack of oxygen. If respiratory motions and circulation 

 could be maintained indefinitely even the metazoa might be found to be immortal. 



