CHAPTER XX. 



Laws of Multiplication. 



§ I. Rate of Reproduction and Rate of Increase. — \\'e know 

 much more about the rate at which organisms reproduce, than 

 about the rate at which the number of adults in reahty increases 

 or decreases. The one fact may be ascertained by observation ; 

 the other involves comparative statistics, which are difficult 

 enough to obtain, even for the human species. The rate of 

 reproduction depends upon the constitution of the individual 

 and its immediate environment, including, above all, its nutri- 

 tion. The rate of increase or decrease depends ui)on the wide 

 and complex conditions of the entire animate and inanimate 

 environment, or upon the degree of success in the struggle for 

 existence. 



That there are enormous differences in the rates of repro- 

 duction is very evident. Maupas tells us how a single infu- 

 sorian becomes in a week the ancestor of a progeny only 

 computable in millions, — of numbers which the progeny of a 

 pair of elephants, supposing they all lived their natural term of 

 years, would not attain to in five centuries. Again, Huxley 

 calculates that the progeny of a single parthenogenetic plant- 

 louse — supposed again to live a charmed life — would in a few 

 months literally outweigh the population of China. The geo- 

 metrical ratio of reproduction, so often emphasised, would 

 indeed have startling results if it involved real, and not merely 

 potential, increase. 



That it does sometimes realise itself for short periods or 

 special areas of favourable conditions is well known ; for in- 

 stance, in the periodic plagues of insects, or in the still unmas- 

 tered rabbit pest of Australia. But in the established fauna 

 and flora of a country, without intruded importations or marked 

 climatic changes, the rise and fall of population is seldom 

 emphatic. The rate of reproduction is only one factor in the 



