256 Mr. L. Harrison on 



epidermal products which varies little in chemical com- 

 position, at a body temperature which remains practically 

 uniform, and without any complication of the nature of a 

 struggle for existence, they exhibit a condition that I have 

 elsewhere referred to as "retarded evolution." They have 

 not evolved as fast as their hosts. The Pigeons of the world 

 include a very varied assemblage of birds^ ranging from 

 large, almost flightless, forms, such as Goura, to tiny Doves 

 such as Stictopelia. They are parasitized by species of 

 Mallophaga belonging to five genera, two of Amblycera and 

 three of Ischnocera. In connection with one of the latter, 

 LipeuruSy we have the remarkable phenomenon of L. columba, 

 passing for the present as a single species, occurring on 

 practically all the Pigeons of the world ; while the remain- 

 ing genera from Pigeons, though they have produced a 

 considerable number of species, nevertheless present a very 

 distinct facies which enables us to detect them as Pigeon- 

 parasites, even when taken straggling upon other hosts. 

 The same thing holds true for any other group of birds. 

 Parasites of Crows, of Kingfishers, of Hawks, of Plovers, of 

 Petrels, are recognisable as such, whether their host origin 

 be known or not. 



This condition can have only one reasonable explanation. 

 Just as everyone in this room is convinced that each of the 

 larger groups of birds has been derived from one common 

 ancestral stock, so we must believe, if we examine the 

 evidence in more detail than I am able to submit to you 

 to-night, that the parasites of these groups have also 

 evolved from the parasites of the ancestral stock. And the 

 point I wish to impress upon you is, that they have evolved 

 at a slower rate. 



This statement implies that the Mallophaga took to a 

 parasitic mode of life at a very early period, and I wish to 

 suggest to you the grounds upon which I base my opinion. 

 Upon the marsupial fauna isolated in the Austro-Malayan 

 region occurs a family of Amblycera, of primitive two-clawed 

 parasites, which is very closely related to the lowest and 

 most generalised bird-infesting genera. No member of the 



