OF WASHINGTON, VOLUME XV, 1913 121 



Dryoccetes septentrionis of the western coast and Alaska of America, 

 Xyloterus lineatus of Europe and Xyloterus bivittatus of eastern and 

 western North America, Hylurgops glabratus of Europe and Hylur- 

 gops pinifex of eastern America. There are many others common 

 to two or more countries which superficially seem to be the same 

 things and if so would come under the head of discontinuous 

 distribution of species. 



Some of the most important examples in discontinuous distribu- 

 tion of species are to be found in a leading article by Professor 

 Kellogg in the American Naturalist for March, 1913, on "Distribu- 

 tion and Species-Forming of Ecto-Parasites. " This has reference 

 to the curious and most interesting bird lice on which Professor 

 Kellogg is a special authority. This writer states : " There appears 

 to be a plain tendency for a single parasite species to be common to 

 two or more related host species even though these hosts be so widely 

 separated geographically and so restricted to their separate geo- 

 graphical range that all possible chance of contact between indi- 

 viduals of the different host species seems positively precluded. " 



Numbers of examples are given. Professor Kellogg thinks that 

 this remarkable discontinuous distribution is due to descent from a 

 widely distributed common ancestor. 



Many interesting examples of discontinuous biological islands 

 characterized by the same or very similar species, are to be found in 

 high, isolated mountains or mountain ranges, and low bogy areas, 

 and so-called ice caves, regular caves, etc. Some of the boreal 

 islands are restricted to a few square rods. Whenever in these 

 islands the climatic conditions and the general environments are 

 similar the same or similar genera and species of plants and ani- 

 mals occur, which in many cases are separated by hundreds or even 

 thousands of miles from the nearest area of continuous distribution. 



Some of the causes of discontinuous distribution may have been 

 due to the separation of once continuous land masses, fluctuating 

 climatic conditions of circumpolar areas from similar conditions 

 throughout to radical local differences due to glaciation and ocean 

 currents. There are also a great many examples of artificial or 

 accidental introductions, but I am more and more inclined to the 

 opinion that parallel evolution from a common primitive ancestral 

 base under long continued, similar environments has been a very 

 important factor in establishing what are considered to be the same 

 species, and closely allied species in widely separated areas of the 

 world. Therefore it seems to me that we have in this principle of 

 parallel modification and evolution a simple and plausible explana- 

 tion for many of the puzzling features in the geographic distribu- 

 tion of genera and species of plants and animals. 



