1916 ENTOMOLOGICAL SOCIETY. 101 



<iiid again two explanations are possible. ^\'llel■e two similar but diifercnt forms 

 occur, one on each side of a barrier, it has been suggested that a common ancestor 

 of the two had established itself over the entire zone before the barrier was formed, 

 and that development on the two sides since has been along sufficiently different 

 lines to produce. different species. The Peach Borer east of the Rocky Mountains, 

 and its close relative, the Pacific Coast Peach Borer, on the western coast, are con- 

 -sidered an example of this. Many forms, however, show little close relationship 

 but much similarity to Old World forms, and here geology steps in to provide an 

 oxjilanation. 



There is much evidence that in past ages the northern part of the world's 

 surface was much warmer than it now is, and also that there were more or less 

 complete land connections between Europe and Xorth America on the east, and 

 Asia and Alaska on the west. It is noticeable that many forms of life in the north- 

 (>astern part of this country find their closest relatives among European forms, 

 and similarly that many of our w^estern forms closely resemble those of Xorth- 

 luistern Asia. From these facts it seems at least probable that differences in the 

 life of the same zone fouiul on two sides of a north and south barrier may be 

 accounted for as being the results of migration from the two opposite ends of the 

 Eurasian continent. 



Life zones then mean, not the areas continuously inhabited by a certain list of 

 forms, but territory having fixed standards, which meeting the needs of animals 

 and plants, able to live under sucli standards, can be populated by them if means 

 of access is i^rovided. 



It has been said that certain places in Africa are perfectly fitted for some 

 American forms of life. If this be correct, such American forms once placed there 

 would establisli themselves and thrive in their new home, the only difficulty being 

 that of getting them across the ocean in the first place. This may remain a diffi- 

 culty for years, but, so far as Xorth America is concerned, the arrival of new 

 forms from other countries is not only possible, but is actually occurring, and if 

 favorable conditions are found on arrival, or, in other words, if proper life zone 

 conditions and proper food are at hand, the establishment of new animals and 

 plants in our land is certain. 



Some of these arrivals in the past have been desirable, but certain it is that 

 many have proved veritable pests. It is stated that about seventy-five of our one 

 hundred worst insect pests are of foreign origin, and, in spite of all systems of in- 

 spection and care, new ones somehow creep in and establish themselves before we 

 are aware of their presence. 



To prevent this seems hopeless under our present methods, and the recent de- 

 velopment of the nursery business, bringing in millions of all kinds of plants from 

 all parts of the world, harboring insects many of which it may be difficult or im- 

 possible to find by any inspection, raises the question whether it would not be wise 

 to absolutely prevent the importation of all plants from foreign countries, in order 

 to protect ourselves from the pests of other lands which otherwise might join forcn^s 

 with those already here, in the destruction of our crops. 



