158 THE AMERICAN NATURALIST [Vol. XLVII 



groups and family strains each isolated on its host island 

 or succession of self-reproducing islands. 



But the change of plumage markings, or of bill shape 

 or even of food and flight habit, of the separating host 

 kinds splitting off from a common ancestor need mean 

 nothing much for the parasite. So although in time we 

 come to have, derived from a common ancestor, an Amer- 

 ican avocet and a European one, an American coot and a 

 European one, we do not have an American avocet- or 

 coot-parasite and a European avocet- or coot-parasite. 

 But the parasite of the common avocet or coot ancestor 

 of the two present bird species remains unchanged and 

 is thus a single species common to the two geographically 

 separated, specifically distinct, never-meeting, host 

 species. 



If this is a true explanation for the commonness of a 

 parasite of two separated host species, it is likely also 

 the explanation of the larger phenomenon of the general 

 faithfulness of certain parasite species or genera to cer- 

 tain bird groups — families, or even orders. I do indeed 

 believe that it is a commonness of the genealogy rather 

 than a commonness of adaptation that is the chief ex- 

 planation of this restriction of certain parasite groups to 

 certain host groups. It is in my eyes an unusually clear 

 example of the potency of heredity. There is more 

 nature than nurture in the upbringing of the Mallophaga. 



Isolation and inheritance, then, are the two evolution 

 factors especially concerned in the species-forming and 

 the distribution of the Mallophaga. Adaptation seems 

 to play a very subordinate part. And this is a rather 

 unusual condition in insect biology. The plasticity of 

 insect nature combined with the stresses of insect life 

 and the necessary shifts for a living, have resulted in pro- 

 ducing among the insects some of the most striking ex- 

 amples of adaptive evolution to be found in the kingdom 

 of life. In the face of this fact, this little group of para- 

 sites may have by the very exceptionality of its evolu- 

 tionary behavior, an enhanced interest for us ! 



