ORGANIC EVOLUTION — PHYSICAL 49 



mass ; for instance, the mass of cells constituting a man 

 owes its particular shape, the human shape, to certain 

 definite inequalities which occur in the rates of multi- 

 plication in the lines of the cell-descendants of the 

 germ whence the man is derived. Were there no 

 inequalities in the rates of multiplication, did each line 

 of cell-descendants multiply at the same rate, a solid, 

 spherical mass of cells must result, whereas, owing to 

 their unequal but definite rates of multiplication, the 

 shape of the mass is irregular (i. e. not spherical), but 

 nevertheless definite for man and every species of ani- 

 mal. An ox, therefore, differs in shape from a man 

 mainly because the inequalities in the rates of multi- 

 plication in the various lines of the cell-descendants of 

 the germ whence it is derived are different from the 

 inequalities in the rates of multiplication in the lines of 

 cells that spring from a human germ ; but it resembles 

 other oxen in shape, because the inequalities in the 

 rates of cell-multiplication are much the same. Some- 

 times, however, though rarely, this or that cell-de- 

 scendant of the germ reverts to the remote unicellular 

 ancestral type, in so far that it does not multiply at a 

 rate bearing a definite proportion to the rates of multi- 

 plication of its other co-descendants of the germ, but 

 at a rate that has no definite proportion to them, and 

 is only proportionate to its supply of nutriment and 

 powers of assimilation. There then results the " mor- 

 bid " condition which is known as a " tumour," in which 

 the cell-descendants of a cell which has so reverted to 

 the remote ancestral type form a more or less spherical 

 mass which neither bears a definite proportion to the 

 whole mass of the cell-community ,'nor perfoims definite 

 functions beneficial to it, and is therefore an encum- 

 brance or worse. But this tendency of cells to revert 

 to the ancestral unicellular type, to multiply at a rate 

 that is only proportionate to the supply of nutriment 



