/ol. xxxvi.] 50 



off the body of the host, and usually die fixed by their 

 mandibles to the feathers. Transference to a new host can 

 only take place at actual bodily contact. 



Owing to the fact that these insects have lived for a very 

 long time under very equable conditions, on a nutrition 

 of epidermal products which varies little in chemical 

 composition, and at a body-temperature which remains 

 practically uniform, they exhibit a condition of " retarded 

 evolution/' Parasites of any group of birds, such as Crows, 

 Kingfishers, Hawks, Plovers, or Petrels, are recognizable as 

 such, whether their host-origin be known or not. The only 

 reasonable explanation of this condition is that parasites of 

 these groups have evolved from parasites of their ancestral 

 stocks. And, as they have evolved at a slower rate, the 

 gaps to be bridged are smaller in the case of the parasites 

 than in that of the hosts. 



That the Mallophaga have a long-standing history of 

 parasitism is proved by the fact that the Ostrich and the 

 llheas have parasites hardly specifically distinct, which are 

 distinguished from all other Mallophaga by a curious 

 asymmetry of the chitinous framework of the head. 

 Consequently, the parasitic history of the group must ante- 

 date the isolation of these birds in the Ethiopian and 

 Neotropical regions. 



Although opportunities of invading a new host are limited, 

 it is admitted that straggling can, and does, take place. 

 Bird-parasites have been found living on mammals ; marsu- 

 pial parasites on carnivores ; and a Petrel-infesting species 

 has become established as a normal parasite of Skuas. But 

 such cases are few, and are almost always capable of 

 detection. 



Although birds have been split up into obviously natural 

 ordinal groups, the inter-relation of these groups is not 

 understood, and no satisfactory characters of phyletic value 

 have been found. A closer study of bird-parasites may, 

 from the conditions outlined, aff'ord valuable suggestions to 

 the bird-morphologist. 



Some such suggestions have already been published. The 



