3G2 PROGRESS OF SCIENCE IN THE CENTURY. 



bearers of heredity, and the real builders of the organ- 

 ism." This deliverance is quoted from an essay by 

 Prof. C. O. Whitman, one of the modern leaders, 

 but it will be observed that it leaves the riddle of 

 organisation unread. 



(c) An exceedingly important step was made 

 when it was made clear that new cells arise from the 

 division of pre-existing cells, — a step which may be 

 particularly associated with the names of Goodsir 

 (1845) and Yirchow (1858). Of great importance 

 also was the general rationale of cell-division, which 

 seems to have been suggested independently by R. 

 Leuckart, Herbert Spencer, and Alexander James; 

 it is often referred to as the Leuckart-Spencer prin- 

 ciple, and is based on the fact that in cell-growth the 

 increase of mass or volume outruns the increase of 

 surface. When the cell has, let us say, quadrupled 

 its original mass by growth, it has by no means quad- 

 rupled its surface (the former increasing in spheri- 

 cal cells as the cube, the latter as the square, of the 

 radius) ; physiological difficulties set in, and at '' the 

 limit of growth," the cell divides, halving its mass, 

 and gaining new surface.* But attention has been 

 mainly concentrated on the details of the actual proc- 

 ess of cell-division, which is due, as Prof. Wilson 

 says, to " the co-ordinate play of an extremely com- 

 plex system of forces." Its necessity is clear (on 

 the Leuckart-Spencer principle) as the only feasible 

 mode of growth ; its end is clear — to divide the es- 

 sentials of the mother-cell equally between the 

 daughter-cells; but, in spite of continuous attempts, 

 the actual mechanism of the process remains obscure. 

 Three results seem clear: — (a) the fundamental 



♦ See the writer's The Science of Life, p. 108. 



