DAME NATURE AND HER CHILDREN 
bivorous and graminivorous mammals greatly exceed 
in numbers the flesh-eaters; they can get their food 
more easily, for they do not have to use speed, wit, 
strength, or prowess in order to obtain it. How rare 
are the weasels, compared with their prey of rats 
and mice and birds and squirrels and rabbits! Yet 
the weasels have goodly families each season. If 
man had not been a miscellaneous feeder, could he 
have overspread the earth as he has done? If an 
animal can eat only fish, it must keep near the water; 
if it can eat only nuts, it must keep near the woods; 
if it subsists upon mosquitoes, it must live near the 
marshes; if grass is its only diet, its range is limited 
to certain zones and certain seasons. 
The farmer finds it much more difficult to check 
or exterminate certain plants or weeds than others. 
The common milkweed and the Canada thistle defy 
his plough because the parent roots are beyond its 
reach; they creep horizontally through the soil, and 
send up their shoots at short intervals. To exter- 
minate the plants, you must remove the parent 
roots. Looked at in the light of the doctrine of 
natural selection, it would seem as if these two 
plants had learned through experience to avoid the 
plough by diving deeper into the soil and establish- 
ing permanent parent roots there. This method or 
habit baffles the plough completely. What other 
enemy or circumstance could have so driven them 
into the ground? In a region unvisited by the 
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