12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



One quotation, showing his effective use of hosts and parasites to- 

 gether as sources of evidence, may well be given here. After dis- 

 cussing again the Crayfish and their external parasites, the temno- 

 cephaloids, he writes : 



The acceptance of Matthew's hypothesis of four separate dispersal streams of 

 crayfishes from the northern hemispliere potamobiids [northern crayfish] into 

 Madagascar (through Africa), Australasia, New Zealand and South America 

 implies : 



The presence in the past of temnocephaloids upon northern potamobiids, for 

 which there is no evidence. 



The extinction of both crayfishes and temnocephaloids in Africa, where there 

 is no evidence that either ever existed and no obvious or plausible reason .... 

 why either or both should have become extinct. 



The general distribution in the past of both crayfish and their parasites in 

 the tropical belt, for which again there is not positive evidence. Moreover, since 

 opportunity has been afforded for the southern crayfish to migrate into the 

 tropical belt, and since they have not done so to any marked degree, it would 

 seem that the tropics do not afford a congenial environment for crayfishes. 



The extinction of temnocephaloids upon Asiatic and North American potamo- 

 biids, for which there is no evidence, and which should not, I think, be assumed 

 without some justification or explanation. 



These considerations seem to me to rule out Matthew's hypothesis completely. 

 If the parastacids [southern crayfish] have been derived from potamobiids, the 

 only possibility seems to be that such derivation took place in America, and that 

 the parastacids, as such, first appeared in South America, and must have reached 

 the other southern land masses by a southern route of dispersal, carrying their 

 temnocephaloid parasites with them. 



In 1928 two more papers liy Harrison appeared. One (1928) 

 presents important additional evidence as to antarctic zoogeography. 

 A genus of lowly segmented worms, Stratiodrilus (one of the 

 Histriobdellidae) occurs on fresh water crayfish in Tasmania, in New 

 South Wales, and in Uruguay, and in this paper Harrison describes a 

 fourth species on a crayfish from Madagascar. He discusses the family 

 Histriobdellidae and its three genera and he prophesies that one or 

 more species of the southern genus Sfrafiodrilus will be discovered 

 on the gills of other South American crayfish (Parastaciis) and of 

 New Zealand crayfish (ParaiicpJiros). 



The second of Harrison's papers in this year (1928) is an excellent 

 general review of the whole host-parasite method. He had not learned 

 of von Ihering's thorough-going use of this method of illuminating 

 problems of genetic relationships of hosts, of geographical distribution 

 of both hosts and parasites, and of former intercontinental connec- 

 tions. Also he failed to realize the extent of Kellogg's appreciation 

 of the wide applicability of the host-parasite method. Harrison's own 

 realization of the broad value of such data apparently came from 

 reading two of my papers and from correspondence with me in the 

 year 1921, a correspondence which, though brief, was very valuable 



