NO. 8 PARASITES — METCALF I3 



to me. But his grasp of the importance of parasites as indicating 

 relationships of hosts was reached independently of von Ihering and 

 Kellogg and much antedated my own. The following quotation shows 

 Harrison's grasp of the wide extent of the usefulness of the host- 

 parasite method : 



The ostriches of Africa and the rheas or nandus of South America are 

 commonly supposed by ornithologists to have arisen from quite distinct stocks. 

 But their lice are so similar, and so different from all other bird-lice, that these 

 must have evolved from a common ancestor, and so also must the birds them- 

 selves. Evidence derived from lice is confirmed by the cestode and nematode 

 parasites of the two groups of birds. Thus a phylogenetic relationship may be 

 established by means of parasites. Equally, a supposed relationship may be 

 refuted. Their lice prove that the penguins are in no way related to any northern 

 group of aquatic birds, but belong in an ancient complex which includes the 

 tinamous, fowls and pigeons ; that the kiwis of New Zealand are modified rails, 

 and not struthious birds at all ; that the tropic-birds are not steganopodes but 

 terns, and so on. A third use is to refute suggestions of convergent resemblance, 

 which are often very lightly made, and which are so exasperating to the zoo- 

 geographer since they are usually incapable of either proof or disproof. Lepto- 

 dactylid frogs are found in South America and Australia. Did they evolve 

 separately, or are they derived from common ancestors? The herpetologist 

 cannot say with any certainty, but the parasitologist discovers that they share 

 a genus, Zelleriella, of ciliate protozoan parasites, and must have had common 

 origin. This same example will serve to illustrate a fourth use for the host- 

 parasite relation. The genus Zelleriella can, and does, infest frogs other than 

 Leptodactylids. It is not found, however, anywhere except in Australia, South 

 and Central America, so that its distribution affords strong presumptive 

 evidence that South America and Australia have been joined in past time in 

 some way which excluded the northern land masses. 



These examples indicate the nature of the host-parasite relation, and its 

 possible usefulness. 



In 1926 Harrison discussed before the Australian Association for 

 the Advancement of Science " The Composition and Origins of the 

 Australian Fauna, with Special Reference to the Wegener Hypoth- 

 esis." This paper, in press hut still unpublished in 1928, I have, of 

 course, not seen. 



S. J. Johnston, of Sydney, Australia, had heard Harrison present 

 before the Sydney University Science Society his first discussion of 

 the biting lice ( Alallophaga) of birds as furnishing evidence of the 

 genetic relationships of their hosts (Harrison 191 1) and two years 

 later Johnston (1913) wrote of the frog trematodes of Europe, Amer- 

 ica, Australia and Asia and their bearing upon possible former con- 

 nections between these now separate lands. He concluded that the 

 trematodes of Australian frogs find their nearest relatives in those of 

 Asiatic frogs, and Grobbelaar, writing in 1922 upon African frogs 



