258 Mr. L. Harrison on 



or the equator, in the Old World or New, carry the same 

 types of parasite. I have shown that opportunities of in- 

 vading a new host are limited. The final question I wish to 

 discuss is : Whether it is possible for parasites to reach and 

 thrive upon hosts not of their proper group, and so to 

 vitiate any general theory based upon their distribution? 

 I admit freely that they can invade, and have invaded, 

 other than their true hosts, and I admit that they can thrive 

 upon these new hosts. Bird-parasites have been found living 

 upon mammals, marsupial parasites on carnivores ; a species 

 of the Petrel type, undoubtedly originally parasitic upon 

 Petrels, has become established as a normal parasite of Skuas ; 

 Goniocotes gigas, a parasite of the genus Numida, will be 

 found on domestic Fowls almost anywhere. But I submit 

 that these cases are few, and are almost always capable of 

 detection. 



I have now put before you the main points to which I 

 \vish to direct your attention, and I will briefly recapitulate 

 them. The Mallophaga are a group of insects with a long- 

 standing history of parasitism, which, from their biological 

 conditions, have tended to be handed down from parent to 

 offspring in such a manner as to be associated always with 

 definite host groups, and which have evolved at a much 

 slower rate than their hosts. These facts made it quite 

 evident to me, when I began some six years ago to work at 

 Mallophaga, that the group should be useful in connection 

 with the very vexed question of bird-phylogeny. 



I am sure that, even in a gathering of ornithologists, I 

 may say that very little is known about the inter-relations of 

 the bird orders. We can easily divide birds up into a 

 number of perfectly natural groups, but I think that few in 

 this room would care to answer the question as to whether 

 a Crow, say, was more nearly related to a Hawk or to a 

 Duck. Ordinary morphological and embryological methods 

 have broken down badly as far as birds are concerned, and 

 the fossil record is woefully inadequate. This is my excuse, 

 if excuse be required, not for attempting to classify birds 



