190 THE SURVIVAL OF THE UNLIKE. [vil. 



more or less continuous areas, and because the natural 

 equilibrium or tension is disturbed. This may all be 

 summed up by saying that it is a question of readjust- 

 ment following some disturbance. You will now want 

 to ask how long before this readjustment will have 

 become complete, and equilibrium be again restored, 

 and the enemies in great measure disappear, from lack 

 of food and opportunity. So long as cultivation or 

 disturbance continues, so long perfect readjustment 

 can not occur. Therefore I expect that we shall 

 always find new enemies coming upon us. But, to a 

 considerable degree, readjustment does occur. The 

 potato beetle is less abundant than formerly. It has 

 spread itself over the potato area of the country, and 

 it can not now propagate itself so rapidly as it did in 

 the sixties. It has taken to other plants, but it has 

 met competitors, and is held in check. Enemies have 

 also appeared. Nearly all invaders have their seasons 

 or cycles of great prosperity, and corresponding periods 

 of comparative obscurity. When they are first run- 

 ning over a country they enjoy their greatest license. 

 This is the case in many invasions now progressing 

 in this new country, where the rapid clearing of the 

 land, the great extension of commerce, and the plant- 

 ing and sowing of enormous areas, afford almost 

 unlimited prosperity to a new incui'sion. (Compare 

 Essay VIII.) The introduction of the phylloxera and 

 downy -mildew into Europe, where they still spread with 

 unabated fury, are examples of new enemies running 

 riot in old and stable countries. 



Insects or fungi or weeds are often held in subjec- 

 tion, like a smoldering fire, by the pressure of circum- 

 stances, as weather, the presence of enemies, or other 



