55 [Vol. xxxvi. 



Mr. Seth-Smith : One interesting point about bird-lice is 

 the extraordinary rapidity with which they multiply if the 

 bird is unhealthy. If you look at a perfectly healthy bird, 

 you will perhaps find a few lice, but as soon as that bird 

 gets out of condition you will find it is simply swarming 

 with them. They increase at an enormous rate as soon as 

 a bird gets into a low condition of health. 



Mr. Harrison : With regard to the question of con- 

 vergence which has come up more than once to-night, I do 

 not think any serious student of these parasites who really 

 examines the evidence closely (it is unfortunate I cannot 

 possibly put all this evidence in front of you) would insist 

 very much upon convergence as a serious factor in deciding 

 relations in general. Here and there convergence in the 

 past may lead to erroneous results in the present. We 

 practically know nothing about Mallophaga yet. Where 

 I go into a group and work it intensively, I find all the 

 ideas held previously are grossly wrong. The actively- 

 moving Liotheids are supposed to be able to effect an 

 easy transference to a new host. I quite agree with 

 one speaker that they are not so useful for my purpose, 

 because we do not understand them at present_, and it is 

 possible they have succeeded in more irregular transference. 

 Yet, even among the Liotheids, there is evidence of the 

 same condition. The Pelicans possess a peculiar Liotheid 

 genus, which has adopted a specialised habitat in the gular 

 pouch, and has its tracheal system specially modified to 

 withstand the periods of submersion which it must undergo. 

 It seems to me more reasonable to suppose that the species 

 of this genus have descended from the parasites of the 

 Pelican ancestral stock, rather than that they have been 

 produced by some process of convergence. The question of 

 straggling, which Mr. Cummings has adduced, I quite 

 admit. There are a reasonable number of proved instances, 

 but, so far, we have detected them. With regard to the 

 question of retarded evolution, I am quite willing to admit 

 that at the ultimate ends of the evolutionary branches, there 

 may be varying rates of evolution, but I maintain that the 

 general rate of evolution has been slower. No parasite of 



