No. 555] SPECIES-FORMING OF ECTO-P ABASHES 157 



The American and old world dippers or water ouzels are 

 in the same case. The same Mallophagan species occurs 

 on both an old world and a new world kinglet. One para- 

 site species is common to two old world and two new 

 world crows. Practically all of those isolated bird spe- 

 cies found only on the Galapagos Islands are infested by 

 Mallophaga that occur on related new world or even old 

 world hosts. 



Now, removing all cases of even an imaginable rare 

 possible contact of bodies between these related but spe- 

 cifically distinct hosts, such as might occur in birds of 

 circum-polar range, or in gregarious maritime kinds 

 meeting on common mid-ocean islands, or in kinds occa- 

 sionally exported by man from their normal range, etc., 

 and there are still left many cases of this commonness 

 of a parasite species to two or more usually rather closely 

 related host species of quite distinct geographic range. 

 How can this actual condition be explained? 



I can see but one answer. That is, that the parasite 

 species has been handed down practically unchanged to 

 the present specifically and even generically distinct sev- 

 eral bird species from their common ancestor of earlier 

 days. The parasite species dates from the days of this 

 ancestor. With the splitting up of the ancient host spe- 

 cies due to geographic wandering and isolation of groups 

 of its individuals, and their gradual divergence in plu- 

 mage, color and pattern, shape of bill or toes or wings, 

 caused partly by adaptation and partly by the simple per- 

 sistence of chance variations fostered by the isolation 

 and inbreeding, there has been no equivalent evolution- 

 ary divergence of the isolated groups of individuals of 

 the parasite species. No adaptive changes have been 

 necessary for it. It has indeed been broken into isolated 

 groups of individuals, but no more than is normal to its 

 life under conditions less novel. I have already pointed 

 out the large variability that occurs within every Mal- 

 lophagan species caused by the separation, more or less 

 complete and persisting, of its individuals into little 



