176 Dynamic Evolution 



is a cause back of those variations, and a 

 statement that they are fortunate variations 

 implies that some fortunate circumstances 

 set that cause into operation. What is that 

 cause? Or, if we do not know the cause, 

 what set the cause into operation? 



An animal body is composed of working 

 cells and breeding cells, the working cells 

 being derived from the breeding cells by 

 differentiation. When two germ cells unite 

 to form the first breeding cell, that cell multi- 

 plies rapidly by segmentation, accompanied 

 by the growth of the individual cells to a 

 normal size after each division. As working 

 cells begin to form by the differentiation of 

 part of the breeding cells, the rate of cell 

 division declines and the percentage rate of 

 growth of the new animal becomes less rapid. 

 This decline in the rate of growth coming 

 from the decline in the rate of cell division 

 is beautifully illustrated by Minot in Age, 

 Growth, and Death. Thus, according to 

 Minot's tables, the cell divisions on a child 

 from birth to one year of age are frequent 



