258 



THE AMERICAN NATURALIST [VOL.XLVIII 



lated from the rest of the individuals comprising the par- 

 ticular species represented by it. This brings about cer- 

 tain striking conditions of abundant small variation and 

 subspecific (or intraspecific) distinction, which, however, 

 because of the general similarity of habitat, food and 

 habit, do not tend to grow rapidly into large (specific, 

 generic, family) differences. The hundred or more species 

 of Mallophaga so far recorded from mammals have, until 

 very recently, all been ascribed to two genera, of which one 

 included nearly nine tenths of the total number of kinds. 

 There has been made a beginning — and not a particularly 

 convincing one — at breaking up this inclusive genus 

 (Trichodectes). It is a movement suggested more by the 

 needs of convenience than the needs of expressing a bio- 

 logical situation. Similarly, although not representing 

 so extreme a condition of likeness, the Anoplura, also 

 including about a hundred parasite species (occurring 

 only on mammals) have been, until recently, divided into 

 but half a dozen genera, with the great majority of the 

 species included in one. Certain aberrant forms found 

 on man, the monkeys, the elephant, and on seals and 

 walruses have always made necessary the recognition of 

 four or five quite distinct genera. Attempts, however, are 

 now being made to break up the unwieldly genus Hama- 

 topinus. 



As this paper is, in effect, a continuation of my paper 

 on "Distribution and Species-forming of Ecto-parasites" 

 published in The American Naturalist in March, 1913, 

 which devoted itself to a consideration of the Mallophaga 

 (some 1,400 species as so far known) found on birds, and 

 to the problems presented by their conditions of life and 

 their host and geographic distribution, I can dispense 

 with any further account of the special biology of these 

 parasites by referring the interested reader to this 

 former paper. In it I have set out rather fully the spe- 

 cial structural and habit features of the Mallophaga. 

 Except that the Anoplura take blood, rather than 

 feathers and hair, for food, and have specially modified 



