Vol.. 6, 1920 
ZOOLOGY: M. M. METCALF 
433 
cates, for example, that at the time when Patagonia was connected with 
Antarctica there was, some bar to free migration between the northern 
part of South America and Patagonia, this bar being probably a shallow 
sea stretching across the middle of South America from ocean to ocean. 
Toads of the genus Bufo are abundant in tropical and temperate America, 
as they are in tropical and temperate Asia and the East Indies, but they 
have not reached Australia either by way of Antarctica or via New Guinea. 
The Bufos of South America carry Zelleriella parasites. Why did they 
not migrate to Australia by way of Antarctica carrying their Zelleriellas 
with them? The Leptodactylidae are a southern family having arisen 
apparently in the Patagonian region. The toads are a northern family. 
It seems clear that the toads were not present in Patagonia at the time 
when this region was connected with Australia by way of Antarctica. 
Apparently the shallow sea across the middle of South America separated 
the toads in the north from the Leptodactylids in the south. As Patagonia 
let go of Antarctica by subsidence of the intervening land area, there was 
a probably concomitant rise of land through what is now middle South 
America and the northern and southern portions of this continent came 
together. This allowed the toads and Leptodactylidae to meet, and the 
toads which are hospitable to all genera of Opalinidae accepted the 
Zelleriella parasites from the Leptodactylids. But it was now too late 
for the toads to migrate to Australia, the Antarctic route being barred 
by a wide stretch of newly formed ocean. 
Numerous other problems of distribution are similarly illuminated by 
the study of the Anura and their Opalinid parasites. Other groups of 
Anuran parasites besides the Opalinidae should be studied with reference 
to problems of distribution, and every group of animals and of plants 
should similarly be studied with their parasites. In this way many 
questions of genetic relationship can be settled beyond doubt and many 
problems of geographical distribution and of migration routes can be 
solved. The method is one which should be used to the full because of the 
definiteness of the conclusions to which it leads. In all likelihood this 
method of study will in time settle the question of former land connection 
between Africa and northern South America as it has settled the question 
of former land connection between South America and Australia. The 
study of my own African and South American Opalinid material is not 
yet complete, so I do not discuss this problem here. 
This paper is written merely to illustrate and emphasize the value of 
the method of studying relationships between groups of animals and plants, 
and their geographical distribution and migration routes, by means of 
comparison of their parasites and comparison of the distribution of the 
hosts with that of their parasites. The results in many cases are sure 
to be as surprisingly convincing as they have proven to be in the study 
of the Anura and their Opalinidae. 
