174 LIFE: ITS NATURE AND ORIGIN 



in its distinctive nature, at the time that growth occurs. In this respect 

 it partakes of the character of a gene or genetic material, in that it 

 affects the character of the individuals and reproduces itself in some 

 degree true to type. But in time it is made over by the nucleus." 



When a race of Paramecium averaging 198 microns in length was 

 mated with another true-breeding race averaging 73 microns, the in- 

 fluence of the cytoplasm was evident in one experiment for 36 gen- 

 erations, for only then did the descendants reach about the same size, 

 even though they all had the same chromosomal content. Referring to 

 experiments of V. Jollos, 20 which indicate that such environmental 

 modifications as acclimatization may be inherited for as much as 800 

 generations, Jennings says: "If they are merely modifications of the 

 cytoplasm one might anticipate that long before so many generations 

 had passed the cytoplasm would have been made over by the nucleus, 

 and its modifications would have disappeared. Yet we do not know 

 that the time required for the nucleus to dominate the cytoplasm 

 would be subject to the same limits in all cases. The question whether 

 inherited environmental modifications are exclusively cytoplasmic, or 

 whether they affect the nucleus, the chromosomes, must be left open 

 for the present." 



It is fortunate that heritable nuclear, genie, symbionic, and cyto- 

 plasmic changes are the exception rather than the rule, for other- 

 wise nature would present a confused Babel of bionts. Before 

 any change can be passed on by heredity, the catalysts or other 

 precursor units involved must themselves be able to run the 

 catalytic and chemical gantlet of the organism and not only sur- 

 vive but also be duplicated. Furthermore, the consequences to 

 the organism, stemming from these heritable changes must be 

 advantageous (and thus possible steps in evolution), or else harm- 

 less or not too detrimental. 



As a counterbalance to the slight probability of the emergence 

 of a markedly advantageous heritable change, we must remember 

 that nature is continually conducting vast multitudes of experi- 

 ments of this kind, over endless eons, and under an enormous 

 range of ever-shifting conditions. But even where a newly devel- 

 oped individual is of a type capable of persisting and of dominat- 

 ing under conditions existing at the time of its emergence, it 

 would still have to face the possibility of being overwhelmed by 

 established forms and predators. 



However, each new form of biont which becomes established 

 tends to add to the amount and especially to the variety of chem- 

 ical substances available to other bionts, either through the prod- 



