376 JAMES W. MAYOR. 



Esocidae, is known only from fresh water. "Fossil pike, belong- 

 ing to the existing genus, have been found in the fresh water 

 chalk of Oenigen and the diluvial marl of Silesia" (Gunther, 

 1880, p. 624). 



If then the distribution of Myxidium lieberknhni over both 

 Europe and America dates from the time when Lucius lucius 

 attained that distribution it too must be an old species, and 

 like its host have remained unmodified through a long period. 



A somewhat parallel condition is found in the Mallophaga, 

 the insect parasites of birds, where a very close relation exists 

 between parasite and host. "But it is to be noted that in 

 practically all the cases of the common occurrence of a Mallo- 

 phagan species on two or more host-species, whether these host 

 species are of the same or neighboring regions or restricted to 

 different continents where this commonness cannot be explained 

 by the possibility of a meeting and actual contact of individuals 

 of the different host-species, the distinct host-species are closely 

 allied, that is, are usually both of the same genus. And I 

 believe that the explanation of this condition is that the Mallo- 

 phagan species has persisted unchanged on two or more diverging 

 host-species from their common ancestor. In ancient times, 

 geographical races arose within the limits of the ancestral host- 

 species; these races or varieties have now come to be distinct 

 species, distinguished by superficial differences in color and mark- 

 ings of plumage, etc. But the parasites of the ancient hosts 

 have remained unchanged; the plumage as food, the tempera- 

 ture of the body, practically the whole environment of the 

 insects, have remained the same; there has been no external 

 factor at work tending to modify the parasitic species, and it 

 exists today in its ancient form, common to the newly arisen 

 descendants of the ancient host" (Vernon L. Kellogg, 1908, p. 3). 



PAPERS CITED. 

 Butschli. 



'82 Myxidium Lieberkuhni in Bronn's Kl. Ordn. Bd. I, p. 593. 

 Cohn, L. 



'96 Uber die Myxosporidien von Esox lucius und Perca fluviatilis. Zool. 



Jahrb. Abt. f. Morph. Bd. 9, p. 227. 

 Kellogg, Vernon L. 



'08 Mallophaga in Genera Insectorum. 66 me fascicule. 



