562 THE ECONOMIC STATUS OF INSECTS AS A CLASS. 



spread of one species at the expense of the others, but the exact oppo- 

 site, so here also to a degree we find nature arrayed against man, 

 and insects thus play by no means the same part in the destruction of 

 weeds that they do in the destruction of cultivated crops. Neverthe- 

 less, they have an inijjortant function in this direction, and it is safe to 

 say that the benefit which the agriculturist derives from their work in 

 this way is very great. As long ago as the beginning of the century 

 it was pointed out by Sparrman that a region in Africa which had 

 been choked up by shrubs, perennial plants, and hard, half- withered, 

 and unpalatable grasses, after being made bare by a visitation of 

 destructive grasshoppers, soon appeared in a far more beautiful dress, 

 clothed with new herbs, superb lilies, and fresh annual grasses, afford- 

 ing delicious herbage for the wild cattle and game. 



In a similar way Eiley has called attention to the fact that after the 

 great grasshopper invasions of Colorado 'and other Western States in 

 the years 1874 to 1876 there were wonderful changes in the character 

 of the vegetation, the grasshopper devastations being followed by a 

 great prevalence of plants which in ordinary seasons were scarcely 

 noticed. It is true that some of these plants were dangerous weeds, 

 but others were most valuable as forage for the half-starved live stock. 

 Moreover, other plants, and especially short or recumbent grasses, took 

 on a new habit and grew luxuriantly; one species, for example, Era- 

 grostis poceoides, ordinarily recumbent and scarcely noted, grew in pro- 

 fusion to a height of 3£ feet. 



An important, but not generally realized benefit which is derived 

 from the insects may be mentioned under this head, though not strictly 

 belonging here. Kirby showed, seventy-five years ago, that the insects 

 that attack the roots of grasses, such as wireworms, white grubs, etc., 

 in ordinary seasons only devour so much as is necessary to make room 

 for fresh shoots and the product of new herbage, in this manner main- 

 taining a constant succession of young plants and causing an annual 

 though partial renovation of our meadows and pastures, " so that, when 

 in moderate numbers, these insects do no more harm to the grass than 

 would the sharp-toothed harrows which it has sometimes been obliged 

 to apply to hidebound pastures, and the beneficial operation of which 

 in loosening the subsoil these insect borers closely imitate." 



AS POLLENIZERS OF PLANTS. 



It can no longer be doubted that cross fertilization is one of the very 

 most important elements in the progressive development and continued 

 health of the great majority of flowering plants ; and, indeed, that it is 

 with some almost a condition of existence. Opposition to this view, at 

 no time especially strong since the publication of Darwin's great work, 

 has become feebler and more feeble until at the present it is not worth 

 considering. 



Comparative experimentation with self-fertilizing and cross-fertiliz- 



