THE WAY OF THE WILD 



51 



Man is one of the slowest of animals to multiply, yet under 

 good conditions his numbers may double in twenty-five years ; 

 indeed this rate has been maintained in this country because 

 the population of the United States has doubled four times in 

 the last century, with four wars to reduce numbers. If this ratio 

 could continue for another hundred years, we should have by that 

 time no less than fourteen hundred millions of people in this 

 country, making a denser population than that of China to-day. 1 



Few wild animals are known but will breed faster than man, 

 and it takes but slight exercise of the imagination to see how 

 reproduction might go on, were there nothing to check it, until 

 there would no longer be even standing room on earth for the 

 animals alone, to say nothing of their food. 



The possible rate of increase of plants is indeed enormous. 

 It is said that the common pigweed ripens from three to four 

 thousand seeds, and a large plant of purslane as many as a 

 million, explaining one reason why they are such troublesome 

 weeds. Plants that seed thus freely are exceedingly difficult of 

 eradication, especially if the seeds are hardy. 2 



Plant lice are still more prolific than weeds. Dr. S. A. Forbes, 

 state entomologist of Illinois, is authority for the statement that a 

 single corn-root aphis is capable of producing ninety-eight young, 

 and that sixteen generations are possible in a single season. At 

 half this rate of increase he computes that if the successive off- 

 spring of a single female and her descendants for a single season 

 could be put upon an acre of land at Cairo at the southern end 

 of the state and placed as thick as they could stand, then on 

 top of this set another acre, and so on without crushing till the 

 end of the season, and if then the column could be tipped to 



1 Showing the extent to which social, economic, and political considerations 

 will shortly turn upon our power to feed our people, and that in turn upon 

 questions of land fertility. 



2 The cocklebur ripens two seeds in one bur. One of these is larger than 

 the other and under equal conditions will germinate first. This weed, there- 

 fore, has two distinctly separate chances of propagation with respect to con- 

 ditions of germination alone. 



