150 THE STORY OF THE LIVING MACHINE. 



tends to increase the offspring in geometrical 

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

 lar animals, the cell divides, giving rise to two 

 animals, each of which divides again, producing 

 four, and these again, giving eight, etc. The 

 rapidity of this multiplication is sometimes incon- 

 ceivable. It depends, of course, upon the inter- 

 val of time between the successive divisions, but 

 among the lower organisms this interval is some- 

 times 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, although the 

 intervals between the successive reproductions 

 may be quite long and irregular. But it is al- 

 ways so great that if allowed to progress unhin- 

 dered at its normal rate the offspring would, in 

 a few years, become so numerous as to crowd 

 other life out of existence. Even the slow-breed- 

 ing elephant would, if allowed to breed unhin- 

 dered for seven hundred and fifty years, produce 

 nineteen million offspring a rate of increase 

 plainly incompatible with the continued existence 

 of other animals. 



Here, then, we 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 pos- 

 sibility for the surface of the earth to maintain. 



Heredity. The offspring which arise by these 

 processes of division are like each other, and like 



