404 PLANT PHYSIOLOGY 



The first organisms experienced no such thing as natural death, 

 which has appeared in the course of evolution along with the in- 

 crease in specialization. Death is not an inevitable and neces- 

 sary accompaniment of life, but is the price paid for differenti- 

 ation. 



The Cause of Death. — With this summary of the known facts 

 concerning death, it may be well to inquire into the reasons why, 

 in differentiated forms, the soma cells should die. Though death 

 is found only in differentiated cells, that does not explain why it 

 is found even there. The following theories have been brought 

 forward by various individuals to explain natural, physical death : 



1. The theory of the Hebrews and of the Christian Church was 

 that death is a penalty for the original sin. "The wages of sin 

 is death." As a bare statement this has little scientific weight, 

 although a careful analysis might show it to be in accord with 

 the previous discussion. Differentiation is to many people a 

 form of sin. Galileo, among others, was one who nearly died as 

 a result of excessive differentiation. 



2. Turning to the more seriously considered theories of death, 

 that of Weismann (1881) may be mentioned. He proposed that 

 death was an adaptation advantageous to the species, and was 

 hence preserved by natural selection; it had survival value in the 

 struggle for existence. This theory is now considered absurd and 

 is cited as one of the worst examples of post-Darwinian logic. 



3. Metchnikoff (1908) held that death was the result of intoxi- 

 cation arising from the absorption of putrefying products in the 

 intestine. That this is not the cause of death, has been shown by 

 rearing animals aseptically without preventing their death. 



4. Jickeli (1902) held that death is due to the accumulation of 

 poisons within the cells owing to the lack of complete metabolism; 

 as time passes these accumulations increase until death ensues. 



5. Verworn, Hertwig (1903), and Minot (1908) considered that 

 death was due to the changes in the proportion of nucleus and 

 cytoplasm in the body cells as described above. Such changes 

 certainly do occur as an accompaniment of death, but whether 

 they are the cause, or the result, is another question. 



6. Conklin (1913) and Child (1915) have associated senescence 

 and death with changes in the rate of metabolism. This theory 

 is to be connected with the accumulations considered by Jickeli 

 and the changes in the ratio of nucleus to cytoplasm as discussed 



