PHYLOGENY 



That is, it could not occur in a world already inhabited by organisms 

 ready to seize upon any beginnings of organic compounds to utilize them 

 as food. But, as Plunkett pointed out ten years earlier, this is an assump- 

 tion for the proof of which no data are available, and it is entirely possible 

 that life is in process of origin on earth at the present time. On the other 

 hand, Plunkett also acknowledged that there is no evidence that this is 

 true. 



EVOLUTION OF THE BACTERIAL CELL 



The problems of the development of the nucleus, mitotic division (cell 

 reproduction), and the evolution of sex are by no means clear. It has al- 

 ready been mentioned that the most primitive bacteria may be more or 

 less equivalent to a single chromosome. Further evolution certainly in- 

 volved the accumulation of a quantity of accessory material. Some author- 

 ities have described the bacteria as cells in which cytoplasmic and nuclear 

 materials are intermingled, while others regard the bacterial cell as an 

 isolated nucleus without cytoplasm. Perhaps the observable facts can be 

 adequately described in either way until much more critical data are 

 obtained. In either case, the evolution of the bacteria must have involved 

 an increase in the quantity of the genie material, and increasing differen- 

 tiation within it. Study of the mechanics of cell division among the bac- 

 teria is hampered by the small size of most bacterial cells. It has long been 

 generally believed that simple fission, without qualitative division of the 

 genes, is the rule among bacteria. This requires a genetic system in which 

 rather few equivalent genes are scattered throughout the cell, so that divi- 

 sion need not result in genetically unlike offspring. Thus occasional muta- 

 tion and the immediate action of natural selection on the mutants would 

 be the only means of evolution. 



Reports of indications of mitotic divisions, that is, qualitatively equal 

 divisions of chromosomes, have been published for many bacteria. Some 

 of the published photographs are quite convincing, and the possibility is 

 open that mitotic division, so fundamental for all of the higher plants and 

 animals, may have been first evolved among the bacteria. Genetic evi- 

 dence also indicates serial arrangement of genes on bacterial chromo- 

 somes. Nothing even suggestive of sexual reproduction, the formation and 

 Fusion of gametes, has ever been observed among the bacteria, but Leder- 

 burg has obtained genetic evid(Mice that it does occm", because recombina- 

 tions of characters occur when different mutant strains of the same species 

 are "crossed." And so it may be that even sexual reproduction was first 

 developed among the bacteria. If so, this is certainly among the most 

 important events in the whole story of e\'olution, for sc^xual reproduction 

 makes possible the accumulation of variabilit\' in the hetero/Agous state 

 (tliat is, the two genes of a pair are different, with the possibility that one 

 may mask the effect of the other). Also, sexual reproduction makes pos- 

 sible the reshufffing of the genes already present and the testing of dif- 

 ferent combinations by natural selection. The result is a much greater 



98 



