HIGHWAYS 01? DISPERSAL. 39 



records for the last forty-six — i. e., 1859 to 1905. During this period it has 

 become a common resident in all of the country wherein it has been at all able 

 to gain a foothold. It has gone from habitat to habitat, from one climatic or 

 topographic area to another, without undergoing any marked change; it 

 everywhere has shown itself to be a plastic form, well fitted to undertake the 

 extensive migrations that it has so successfully accomplished. 



Occurrence in Europe. 



During the years in which L. decemlineata was spreading eastward over 

 the United States and lower Canada, everywhere proving a serious pest to the 

 potato crop, the people of western Europe were intently watching its prog- 

 ress and dreading the time when it might be transported to their lands, where, 

 in the thickly populated countries, its presence would prove even more disas- 

 trous than in the United States and Canada. So great was the apprehension 

 felt that many German States published popular information about this pest, 

 and German schoolmasters were required to instruct the school children con- 

 cerning it. The French government likewise published an elaborate bulletin 

 describing this beetle and its ravages. 



As anticipated, it was transported to Europe in 1875 or 1876, probably 

 in the holds of vessels, appearing in England, Sweden, and Germany ; but 

 such prompt and effectual measures were taken to suppress it that it was 

 soon exterminated. Since then, although frequently introduced, as far as 

 I can learn, it has not been able to gain a foothold in any continental 

 country. It has been introduced at various times into England and Ireland, 

 and it was reported in 1900 to have become established in the Pyrenees 

 Mountains. None of these introductions, however, have been able to spread 

 over more than a few acres, and have all been promptly exterminated. 



The dissemination of this beetle has ceased for the time being. The 

 Atlantic Ocean has proven a hard barrier to cross, but it has crossed it in 

 the past and will do so again and again, until finally it will probably become 

 established as a member of the fauna of Europe. When this happens, it will 

 spread, as it did in this country, until it is found in all the countries of 

 Europe in which it is possible for it to live. 



The Highways of Dispersai. and Rate of Movement. 



This dispersal, covering a large area of country of diverse climatic and 

 topographic conditions, affords an opportunity in a specific case for examin- 

 ing closely some phases of migration. 



There is no reason to suppose that the dispersion of organisms from their 

 center of origin is ever uniform in all directions. We know, in fact, that it 

 is more pronounced in some directions than in others, owing to various 

 directive factors — winds, temperature, and topographic features — such as 

 those natural highways which afford lines of least resistance to migration. 



