100 THE EEPOET OF THE No. 36 



northern slopes of the mountains are well covered by snow during the winter, and 

 this is not quickly reached and melted by the sun in the spring, thus delaying the 

 season north of the range. During the winter, too, increased cold results, and it is 

 probably this which prevents a farther northward spread of the forms which reacli 

 the southern slopes, by establishing a Avinter temperature which they are unable 

 to withstand. During the summer, northward migrations can and sometimes do 

 occur, but tlie cold of the winter following is always sufficient to destroy these mar- 

 ginal settlements, leaving tlie northern limit of occurrence of these forms where it 

 was before. 



Closely related to the questions of distribution of our native animals and 

 plants, are those of introduced forms of life. The Elmieaf Beetle, which reached 

 this country nearly three-quarters of a century ago at Baltimore, has now spread 

 far to the northward, and how much farther it can go is a question of considerable 

 importance. It thrives in the Upper Austral Zone, but is noticeably absent in 

 the highlands of the Pennsylvania mountain region, though it is present again west 

 of them. To the north it has caused serious loss to the elms of New England, 

 resulting in the appropriation of large sums for spraying of the trees to protect 

 them from its ravages. Careful studies of this pest in Massachusetts show that 

 while a serious menace to the life of the elms iii the southern part of the state and 

 in the river valleys, it becomes of little importance in the higher and northern 

 parts, and many towns which formerly appropriated money for the protection of 

 their elms from this insect have now learned that this was unnecessary, as the- 

 trees would suffer but little at most, in any case. 



With the San Jose Scale similar facts are now coming to light. This pest finds 

 the best conditions for its life in the Lower and Upper Austral Zones, where it has 

 caused the loss of many millions of dollars. Even in the Upper Aiistral .territory 

 of Massachusettes, it is one of the most destructive enemies of the fruit- 

 grower. As we pass into the Transition Zone, however, its ra\;ages become less 

 severe, and by the time the centre of this zone has been reached, it is of only 

 medium imijortance. In this case, it has seemed to those studying this problem 

 that this insect was originally limited by tlic Upper Austral, but has gradually 

 acquired some degree of resistance to lower temperatures and has thus been alile 

 to extend into the Transition Zone. Whether this resistance of cold will continue 

 to develop until it becomes a serious insect in this entire zone is a question which 

 cannot now he answered. At least, it points out the possibility of the 

 acquirement of resistant qualities as a factor which must be taken into considera- 

 tion. The speaker has watched with much interest a small colony of these insects 

 which about fifteen years ago was brought on nursery stock to a point near wliere 

 the Transition Zone meets the Canadian. Here from year to year the insect has 

 reproduced just sufficiently to maintain itself, doing no injury, and " eking out a 

 miserable existence '"* and nothing more. 



The Asparagus Beetle and numerous other examples might here be considered 

 as illustrating the significance of life zones in their relation to the limits of spread 

 of our insect foes, but time for their c(msideration is inadequate. 



With life zones divided by mountain ranges we find that it is not usually the 

 case that the same forms occur on both sides of the barrier. When this does happen, 

 two explanations oflPer themselves. The barrier may be a recent one, at least 

 geologically speaking, having arisen after the zone had been occupied by the forms 

 concerned. Or the barrier may be a less complete one than it was supposed, and 

 these forms have in some way succeeded in crossing it. More often the animals on the 

 two sides are not the same, though they may play similar parts in Nature's economy. 



