LAWS OF MULTIPLICATION. 261 



CHAPTER XX. 

 LAWS OF MULTIPLICATION. 



I. Rate of Reproduction and Rate of Increase. — We know 

 much more about the rate at which organisms reproduce, than about 

 the rate at which the number of adults in reality increases or decreases. 

 The one fact may be ascertained by observation; the other involves 

 comparative statistics, which are difficult enough to obtain, even 

 fpr the human species. The rate of reproduction depends upon 

 the constitution of the individual and its immediate environment, 

 including, above all, its nutrition. The rate of increase or decrease 

 depends upon 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 reproduction 

 is very evident. Maupas tells us how a single infusorian 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 cen- 

 turies. Again, Huxley calculates that the progeny of a singe par- 

 thenogenetic plant-louse — supposed again to live a charmed life — 

 would in a few months literally outweigh the population of China. 

 The geometrical ratio of reproduction, so often emphasized, would 

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

 potential, increase. 



That it does sometimes realize itself for short periods or special 

 areas of favorable conditions is well known; for instance, in the peri- 

 odic plagues of insects, or in the still unmastered 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 numerical strength of the species or in 

 its increase. The common tapeworm produces myriads of embryos, 

 but these have only one chance in eighty-five millions (it is said) of 

 succeeding. Many common and numerous animals reproduce very 

 slowly. That some species are on the increase, for example, bac- 

 teria, under the unprecedentedly favorable conditions which our 

 recent "industrial progress" affords, while other species are on the 

 decrease, for example, many birds, is certain; but the rate of repro- 

 duction is not a direct condition in either case. 



