92 



THE CRITERIA OF LIVINGNESS 



of reproduction ! Perhaps we are looking back to near the 

 beginning of organic life when we see the fragmentation 

 of a protoplasmic corpuscle which has grown too large to 

 be a successful unity. It cannot be gainsaid that the division 

 of a cell remains one of the mysteries of the world. Professor 

 Bateson writes (1913, p. 39): "I know nothing which to 

 a man well trained in scientific knowledge and method brings 

 so vivid a realisation of our ignorance of the nature of 

 life as the mystery of cell-division. . . . It is this power 

 of spontaneous division which most sharply distinguishes 

 the living from the non-living. . . . The greatest advance 

 I can conceive in biology would be the discovery of the 

 instability which leads to the continual division of the cell. 

 When I look at a dividing cell I feel as an astronomer might 

 do if he beheld the formation of a double star: that an 

 original act of creation is taking place before me.'' 



In most cases the cell divides into two precisely similar 

 daughter-cells, this being associated with an exceedingly 

 complicated division of the nucleus, which secures that each 

 of the two daughter-cells gets a meticulously precise half 

 of the chromatin material of the original nucleus. But 

 the difficulty of the problem is increased by the fact that 

 a cell may also divide into two dissimilar halves, as appears 

 to happen in certain modes of inheritance. In exceptional 

 cases among multicellular organisms the process of cell- 

 division is simpler and more direct, and in some unicellular 

 organisms it is very simple. It is probable that the com- 

 plicated methods of cell-division which are now the rule 

 are the results of a long process of evolution, and that the 

 fundamental characteristic is simply division. But why 

 should the protoplasmic unit divide? Spencer, Leuckart, 

 and James pointed out independently that, as a cell of 



