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. c. 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 inccpialities in the rates of multi- 

 plication in the various lines of the cell-descendants of 

 the serm 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 performs 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 



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