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chances are much against absolute equality. In the result let 

 us say that 52 molecules pass into one daughter-cell and 

 48 into the other. Before the next fission these two cells 

 have to grow to normal size — that is, they must be provided 

 with foods from which each factor can draw its appropriate 

 nourishment. All parts of the cell grow at approximately equal 

 rates, and the whole having to be doubled, each factor increases 

 in that proportion. Like the halving process, this process of 

 doubling will of course be inexact, but as there is no reason 

 for assuming a tendency either to excess or defect, an exact 

 doubling must be postulated on an average. The selected 

 factor therefore on the attainment of maturity consists of 

 104 molecules in the one cell and 96 in the other. On a second 

 division of the two cells into four there can again be no equality. 

 The selected factor in one of the four cells now produced must 

 consist of more than 52 molecules, let us say 53, and in one 

 other of less than 48, it may be 46, the other two being inter- 

 mediate. On maturity these numbers become 106 and 92, and 

 so on for subsequent divisions. We see then a constant, natural, 

 and inevitable tendency to divergence in respect of this par- 

 ticular factor. And what is true of this imaginary factor is 

 also true of all the many real factors that constitute a real 

 cell. In proportion as the number of molecules in each factor 

 is larger, the argument is the stronger, and although more 

 difficult to follow, it is not really altered if the molecules 

 composing each factor be of many different kinds. These cells 

 are produced in immense numbers, and there must contem- 

 poraneously exist among those of a single generation a great 

 variety of combinations of all the factors, each of which has an 

 equal chance of being called on by fertilisation to take part in 

 the continuance of the race. 



Briefly the argument is that germ-cells are individuals, and 

 like all individuals they differ, even the twin cells, which result 

 from the fission of a single cell, not being exactly similar, for 

 we know that " no two creatures in nature are alike " ; and 

 owing to the method of reproduction by fission, there is a 

 continuous tendency towards a differentiation which accumulates 

 from generation to generation. This tendency appears to me 

 to be not so much a fortuitous as a necessary and universal 

 cause of variation in the animals which at intervals arise from 

 the germs. The hypothesis seems to accord well with the 



