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. 
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.? 
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. : 
