1915.] 245 



from the presum;ibly common ancestral stock. Moreover other kinds of Pld- 

 lopterus, found on other species of cuckoos from various regions, are all closely 

 related. These cuckoo-parasites form a perfectly definite group of species 

 within the genus Philopterus, so that an expert may say at a glance that any 

 one of them has come from a cuckoo. Similarly, there is another definite group 

 of species of Philopterus found on kingfishers of various genera and of world- 

 wide distribution. Pai'asitos of a number of other bird-groiips (e.g., hawks, 

 ducks, jjigeons) afford well-defined species-groups. To explain these phenomena 

 the writer was forced to the conclusion that the parasites have tended to differ- 

 entiate at a much slower rate than their hosts ; or, to quote the words of 

 Kellogg, who had previously arrived at a similar conclusion, " the parasitic 

 species has persisted unchanged from the common ancestor of the two or more 

 distinct but closely allied bird-species." Harrison suggests that the retardation 

 of differentiation in the parasites may be due to the equable conditions of 

 temperature and nutrition under which they live on the bodies of their hosts, 

 and to the absence of any sharp struggle for existence among them. But, 

 granted that the parasite-species has in many cases been handed down practi- 

 cally unchanged from the common bird-ancestor to the several specifically or 

 generically distinct bird-descendants of the present day, may not this be a 

 means of throAving some light on the vexed question of bird-phylogeny, of the 

 inter-relationships of the various groups of birds ? Such a way of attempting 

 to indicate phyletic relationships may be unconventional, but Harrison con- 

 siders it justifiable in a case like this, where all the ordinary embryological and 

 morphological methods have failed. Important questions naturally arise. How 

 far back does the retardation of differentiation extend ? If ib extend back as 

 far as the point in time when the ancestors of Mallophaga first adopted a 

 parasitic mode of existence, then when was that point ? Harrison hazards the 

 suggestion that it may have been as far back as late Mesozoic time, and gives 

 his reasons for thinking this a possibility. But enough has been written to 

 outline one of the ideas underlying the Mallophagan studies which the writer 

 of these papers is undertaking. Part of the detailed results of those studies is 

 contained in the second paper cited — a systematic work in which various 

 characters previously ignored are called into use. 



Societies. 



The South London Entomological and Natural History Society : 

 Thursday, May 13th, 1915. -Mr. A. E. Tonge, F.E.S., Vice-President, in the 

 Chair. 



Mr. Leeds exhibited aberrations of Polyonimatus icarus, including ab. obsoleta, 

 an asymmetrical specimen near ohsoleta, one with a chocolate banded imderside, 

 and a ? streaked with blue : of Agriades thetis, including a $ withovit orange in 

 margin and bluish clouded, ^ s with aberrant eye-spots below, etc. ; of A. coridon, 

 including dark suffused below, slaty suffused below, $ s with "khaki-coloured" 

 streaks above, and ab. semisyngrapha -. of Coenonympha pamphilus, including 



