52 .THE SCOTTISH NATURALIST 



host, and giving rise to a larva generally similar to the adult. 

 They are incapable of maintaining life for more than a couple 

 of days 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 com- 

 position, 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 recognisable 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 }. arasites 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 Rheas 

 have parasites hardly specifically distinct, vvhich are dis- 

 tinguished from all other Mallophaga by a curious asymmetry 

 of the chitinous framework of the head. Consequently, the 

 parasitic history of the group must antedate 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 ; 

 marsupial 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, afford valuable suggestions 

 to the bird-morphologist. 



" Some such suggestions have already been published. 



