134 THE STORY OF THE LIVING MACHINE. 



complicated ones without at any moment impair- 

 ing their vigour. It is something as if the steam 

 engine of Watt should be slowly changed by add- 

 ing piece after piece until there was finally pro- 

 duced the modern quadruple expansion engine, 

 but all this change being made upon the original 

 engine without once stopping its motion. 



This gradual construction of the living ma- 

 chines has been called Organic Evolution, or the 

 Theory of Descent. It will be necessary for us, in 

 order to comprehend the problem which we have 

 before us, to briefly outline the course of this evo- 

 lution. Our starting point in this history must 

 be the cell, for such is the earliest and simplest 

 form of living thing of which we have any trace. 

 This cell is, of course, already a machine, and we 

 must presently return to the problem of its origin. 

 At present we will assume this cell as a starting 

 point endowed with its fundamental vital powers. 

 It was sensitive, it could feel, grow, and repro- 

 duce itself. From such a simple machine, thus 

 endowed, the history has been something as fol- 

 lows : In reproducing itself this machine, as we 

 have already seen, simply divided itself into two 

 halves, each like the other. At first all the parts 

 thus arising separated from each other and re- 

 mained independent. But so long as this habit 

 continued there could be little advance. After a 

 time some of the cells failed to separate after 

 division, but remained clinging together (Fig. 45). 

 The cells of such a mass must have been at first 

 all alike; but, after a little, differences began to 

 appear among them. Those on the outside of the 

 mass were differently affected by their surround- 

 ings from those in the interior, and soon the cells 

 began to share among themselves the different 



