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lows, therefore, that the natural range of cultivated species is 

 widely extended in every direction, and in the teeth of the very 

 barriers which naturally would have held them rigidly in check. 

 Plant-feeding insects in general follow the natural distribution of 

 their specific food. Experience has shown that as this natural 

 food becomes a cultivated crop they increase. As the cultivation 

 of the crop is spread along natural lines of distribution, they fol 

 low it. When, however, by artificial selection, hardy varieties 

 of the crop plant have developed, and the range becomes thus 

 extended along what may be termed unnatural lines, with certain 

 species, at least, and within certain limits with them, their insect 

 enemies will naturally be unable to follow them. The result will 

 be, theoretically, natural selection with the insects trying to catch 

 up with the results of artificial selection with the plants. 



It will be interesting to follow the geographical distribution in 

 the United States of a few important insect enemies of cultivated 

 crops, to see what geographical limits apparently exist with the 

 insects, which do not exist with the crops on which they feed. 



My attention was first called to this matter by the somewhat 

 peculiar spread in the east of Asp idiot us perniciosus. This in 

 sect, the original home of which is unknown but which may be 

 South America or Australia, made its first appearance in the 

 vicinity of San Jose, California. It spread rather slowly north 

 and south through that peculiar life-zone on the Pacific coast of 

 our country which combines throughout its entire extent forms 

 belonging to the boreal and upper Sonoran regions. Brought 

 across the entire country to New Jersey upon nursery stock, it 

 spread rapidly through two large nurseries, and then for a num 

 ber of years was sent out, ignorantly, upon nursery stock to the 

 North, South, East and West, probably to nearly all of the thou 

 sands of customers of two of the most prominent nursery firms 

 in the country. Not until the summer of 1893 was its presence 

 in the East recognized by entomologists. Traffic in infested stock, 

 however, had been going on for five or six years, and it was soon 

 ascertained that the insect had taken a foothold at a number of 

 points. Many of these points have been definitely located, and 

 the occurrences of the insect studied. The species occurs upon 

 all deciduous fruit trees. Apples and pears, and to a certain ex 

 tent peaches, are, as every one knows, extensively grown in por 

 tions of the transition life-zone ; in certain of these localities, in 

 fact, fruit-growing is a great industry, and we may accept it as 

 practically certain that nursery stock was sent to many points in 

 this zone. The facts so far collected, however, fail to reveal a 

 single locality within the limits of this zone in which the insect 

 has established itself. Many points have been found in the up 

 per austral, others occur in the austro-riparian and lower Sono- 



