246 



PSYCHE. 



[August 1898. 



many of them, and in all of the cases 

 studied the identity is apparent.* 



To consider now the problem of how 

 this condition comes to exist we may 

 first refer to and put out of the way the 

 few cases of the common occurrence of 

 a bird species on the two continents, 

 as in the case of imported species 

 (domestic fowl, song-birds, the English 

 Sparrow), or in the case of species of 

 circumpolar range. We may take into 

 account, also, the remote possibility of 

 the meeting on mid-ocean islands of 

 American and European maritime birds 

 of different species. Eliminating these 

 few explicable instances of the common- 

 ness of parasite to the two continents, 

 we have left the great bulk of cases to 

 explain in some way which does not 

 presuppose an actual migration from 

 European to American host. 



Now it is to be noted that the several 

 host species to which a parasite species 

 is common are almost always closely 

 allied forms, that is, species of the 

 same genus or representatives of two 

 closely allied genera. For examples, 

 Docophonis pertusus described by Nitzsch 

 from FuUca atra, the European coot, I 

 have taken from Fidica americafia, the 

 American coot; Docop/iorus latifrotis 

 described by Nitzsch from Ciuulus cano- 

 rus I have taken from Coccyzits califor- 

 nicus occidentalis ; A'irmus fuscomarginaius 

 found in Europe on Podiceps I have 

 found in America on Colymbus ; Nirmus 



^ Indeed, I have made new species out of one or two 

 American forms which should evidently be referred to 

 already known European species. 



piceus from the European avocet, Reciir- 

 virostra avoietta, is found in America on 

 the American avocet, Recnnirosira 

 atneiicana ; Lipeiinis fcrox. recorded 

 from two European species of Diomedea 

 I have taken from Diomedea albatrus 

 (California) ; Lipeurus forficulatus de- 

 scribed by Nitzsch from Pelecanus 

 otiocrotalus I have taken from Pelecanus 

 erythrorhynchus (Kansas) and P. cali- 

 fonUcus (California). And so on through 

 the seventeen or eighteen other genera 

 of Mallophaga. There are a few in- 

 stances, it must be said, in which the 

 relationship of hosts is not so close, 

 the always conspicuous example of this 

 condition being the occurrence of Doco- 

 phorus communis through several families 

 of passerine birds. But it may be said 

 almost without qualification that where 

 a parasite species found in Europe has 

 also been found in America its American 

 hosts are the American species repre- 

 senting the European genus, or belong 

 to a genus very closely allied to the 

 European one. On this fact I base my 

 belief that the occurrence of a parasite 

 species common to several hosts under 

 circumstances which do not admit of 

 the migration of the parasites from birds 

 to bird is due to the persistence of the 

 parasite species unchanged from the 

 common ancestor of the two or more 

 now distinct but closely allied bird 

 species. With the spreading of the 

 ancestral species, geographical races 

 have arisen within the limits of the 

 species which have with time and with 

 isolation, caused by newly appearing 



