OLD AGE AND NATURAL DEATH 



done, but it simply has not been done as yet.' In due course it 

 was done, and the strain was with us until 1939. Tissue-culture 

 has other evidence to offer us of death. We are told that one 

 of the last experiments of Thomas Strangeways was to cultivate 

 the connective-tissue cells surviving in a sausage — as neat a 

 demonstration as one could wish of the tenacity of the vita 

 propria and the half-truth that is legal death. So let us submit 

 yet another zoological simile of common speech to the censor- 

 ship of our new wisdom. The earth stirs over MendePs grave 

 when we say that two people are as like as two peas. Many fish, 

 moreover, never drink. ''As dead as mutton"* is likewise super- 

 annuated by the march of time; and those whose most pressing 

 fear it is that they will be lowered living into their graves can 

 have their doubts resolved: they will be. 



(The so-called '"immortahty" of the Protozoa is like that of 

 the tissues: not an immortality of cells but an indeterminate- 

 ness of cell lineages. Obviously the cell lineages of protozoa are 

 in some cases immortal or indeterminate, for otherwise they 

 could hardly be with us to-day. But does this immortality 

 depend upon the performance of an occasional act of nuclear 

 reconstitution, or can protozoa thrive for ever by the mere act 

 of dividing asexually into two? The matter has long been 

 controversial.^ Some of the early investigators believed that, in 

 default of such ^rejuvenation"*, a protozoan lineage must under- 

 go a microcosmic cycle of growth, maturity, decay and death, 

 exactly like the cell population of higher organisms. Others 

 believed that vegetative fission would suffice. When it came to 

 be known that the former opinion was founded at least in part 

 on the use of faulty techniques of cultivation, the latter dis- 

 possessed it. But Jennings^ is now inclined to doubt whether 

 asexual fission is in itself enough, and the more recent genetic 



^ Cf. H. S. Jennings, Problems of Ageing, ed. E. V. Cowdry, 2nd ed., 

 pp. 24-46, Baltimore, 1942. 



2 Journal of Experimental Zoology, 99, p. 15, 1945. 



27 



