THE STRUGGLE FOR EXISTENCE 393 
offered for competition among plants. Of several plants 
of the same kind, growing side by side, where there is 
room for but one full-grown one, all may be stunted, or 
one may develop more rapidly than the others, starve them 
out, and shade them to death. Of two plants of different 
kinds the hardier will crowd out the less hardy, as ragweed, 
pigweed, and purslane do with ordinary garden crops. 
Weeds like these are rapid growers, stand drought or 
shade well, will bear to be trampled on, and, in general, 
show remarkable toughness of organization. 
Plants which can live under conditions that would be 
fatal to most others will find much less competition than 
the rank and file of plants are forced to encounter. Lichens, 
growing on barren rocks, are thus situated, and so are the 
fresh-water plants, somewhat like pond-scum in their struc- 
ture, which are found growing in hot springs at tempera- 
tures of 140°, or in some cases nearly up to 200°. 
463. Examples of Rapid Increase. — Nothing but the 
opposition which plants encounter from overcrowding or 
from the attacks of their enemies prevents any hardy kind 
of plant from covering all suitable portions of a whole 
continent, to the exclusion of most other vegetable life. 
New Zealand and the pampas of La Plata and Paraguay, 
in’ South America, have, during the present century, fur- 
nished wonderful examples of the spread of European 
species of plants over hundreds of thousands of square 
miles of territory. The newcomers were more vigorous, 
or in some way better adapted to get on in the world 
than the native plants which they encountered, and so 
managed to crowd multitudes of the latter out of 
existence. 
