188 THE SURVIVAL OF THE UNLIKE. [\ni. 



pests come from Europe, but most of our familiar weeds 

 come from that country also. Is it possible that Old 

 World species are inherently more vicious than our 

 own? I think not. The species of the two countries 

 probably possess no constant differences in disposition. 

 They are only what their environments make them to 

 be. Perhaps over half of our plant diseases and nox- 

 ious insects are of Old World origin, but the indigenous 

 species are equally as assiduous and ambitious. Among 

 native insects we number such species as the potato - 

 beetle, the round-headed and flat-headed apple-tree 

 borers, the grasshoppers and Rocky Mountain locusts, 

 the plant-lice, rose-chafer, the army-worm and the 

 chinch-bug. Among plant diseases there are the black- 

 rot and downy -mildew of the grape, plum -knot, pear- 

 blight and peach -yellows. These are no inconsiderable 

 enemies. 



The nativity of an enemy counts for nothing. It 

 is the opportunity which it enjoys for spreading itself 

 rapidly which determines the degree of its noxious- 

 ness. This opportunity lies in the environment or 

 external conditions under which it finds itself. The 

 insect or plant cannot spread until it is placed in new 

 conditions ; either the plant must be new to the condi- 

 tions or the conditions must be new to the plant. A 

 given area may be filled to the utmost with the plants 

 and animals of a region, but an entirely different or 

 foreign plant or animal may gain a foothold without 

 dislodging any of the present occupants, because it can 

 fit itself into chinks which they are not fitted to occupy. 

 An area may be full of corn, and yet grow a few 

 cow -peas between. A region may be densely clothed 

 with forest, and yet vines will grow up the tree trunks. 



