THE LIVING MACHINE BUILDING FACTORS. 173 



ratio. In the simplest case, that of the uni- 

 cellular animals, the cell divides, giving rise to 

 two animals, each of which divides again, pro- 

 ducing four, and these again, giving eight, &c. 

 The rapidity of this multiplication is sometimes 

 inconceivable. It depends, of course, upon the 

 interval of time between the successive divisions, 

 but among the lower organisms this interval is 

 sometimes not more than half an hour, the result 

 of which is that a single individual could give 

 rise in the course of twenty-four hours to sixteen 

 million offspring. This is doubtless an extreme 

 case, but among all the lower animals the rate is 

 very great. Among larger animals the process 

 is more complicated ; but here, too, there is the 

 same tendency to geometrical progression, al- 

 though the intervals between the successive 

 reproductions may be quite long and irregular. 

 But it is always so great that if allowed to 

 progress unhindered at its normal rate the off- 

 spring would, in a few years, become so numer- 

 ous as to crowd other life out of existence. Even 

 the slow-breeding elephant would, if allowed to 

 breed unhindered for seven hundred and fifty 

 years, produce nineteen million offspring a rate 

 of increase plainly incompatible with the con- 

 tinued existence of other animals. 



Here, then, M r e have the foundation of nature's 

 method of building animals and plants of the 

 higher classes. In the machinery of the cell 

 she has a power of reproduction which produces 

 an increase in geometrical ratio far beyond the 

 possibility for the surface of the earth to main- 

 tain. 



