1908] KELLOGG — MALLOPHAGA OF THE WORLD 13 



genera in each sub-order are the ones most difficult to define satisfactorily. As a 

 matter of fact the genera INIenopon and Colpocephalum practically run into each 

 other as do the genera Docophorus and Nirmus. Most of the aberrant monotypic 

 genera, on the other hand, are unusually well distinguished. 



It is a matter of note that the species definitions throughout the Mallophaga 

 have to be very plastic. This comes about almost certainly through the fact that the 

 isolation on different host individuals of groups of parasitic individuals is often (cases 

 of nongregarious, solitary hosts for example) considerably in degree, and leads to the 

 restriction of the sAvamping and evening eftects of free inter-crossing. There must 

 be much in-and-in-breeding of successive generations of the parasites on such solitary 

 and monogamous hosts as eagles, for example. (It should be remembered that the 

 Mallophaga are wingless and migrate from host individual to host individual only at 

 times of the actual contact of the host bodies. Mallophaga cannot live off of the 

 host body for more than a few hours or days and as a matter of fact are very rarely 

 [except in hen houses] ever to be found off the host body.) 



But while this isolation of groups of individuals of a Mallophagan species tends 

 to foster and fix many slight variations and hence to break down species limits, the 

 general similarity or identity of environment and food, viz., birds' bodies and feathers, 

 tends to restrain any tendency toward large variation, or at least does not tend to 

 encourage any such modification. Bird's feathers are about the same as food and 

 bird's bodies as habitat whatever the species of bird or its habit. Mallophaga on 

 water birds have no aquatic life at all but live against the dry warm skin of the host, 

 surrounded by the air held among the interlacing feathers. 



How then have the monotypic aberrant genera come to be developed at all ? 

 A problem. And one c[uite beyond my power to answer. Has there been such per- 

 sistent and positive determinate variation that despite the monotony of enviromiient 

 these radically different types have come to exist ? Or have there been considerable 

 mutations or cases of discontinuous variation resulting in the sudden appearance of 

 fixed new forms of markedly different type from their progenitors ? 



Thus even from the tedious work of competition and cataloging one may rescue 

 some stray bits of interest. As a matter of fact if we could really understand the why 

 of the generic relations of our twelve hundred odd Mallophaga we should have the 

 key to the master puzzle. And then how stale and profitless would the rest of our 

 cataloging and scrutinizing of species be! 



