GEOGRAPHIC DISTRIBUTION" 



211 



greatly increases the possibilities of such means of 

 transoceanic extension of species. Continental com- 

 merce will likewise facilitate the spread of insects 

 across continents. Insects which enter the hold of a 

 lading ship can exist through a reasonable sea voyage. 

 Their establishment in the new country depends upon 

 the finding of suitable food, and the number and sex 

 of the individuals of the species transported. It is 

 possible that in the merchandise or the packing sur- 

 rounding merchandise the majority of successful trans- 

 portations are made. Especially is this true in the 

 case of scale insects, since nursery stock furnishes not 

 only means of transportation, but at the same time sus- 

 tenance. The female scale, of limited powers of loco- 

 motion, has in a state of nature limited distribution. 

 Tiider present commercial conditions the spread of 

 these insects has been unusually great. The San Jose 

 scale was known in this country in 1S8S in California 

 only. Through commercial traffic it now exists in more 

 than thirty States of the Union. 



Deductions. --In this study of distribution, light is 

 thrown upon the subject of land relationships. It is 

 fair to suppose that an island fauna, similar to the 

 adjoining mainland, and whose similarity cannot be 

 accounted for on the ground of transportation, can be 

 accounted for on the ground of a previous land con- 

 nection of the island with the mainland. If the fauna 

 of an island differs materially from that of the near- 

 est mainland, this seems to indicate a separation of 

 great antiquity, or possibly a distinct separation of 

 the two land areas from the beginning. The fauna of 

 Madagascar, for instance, differs widely from the op- 



