6 JOURNAL OF SCIENCE. 
(a characteristic Polynesian genus) is found also in South 
America ; Placostylus is allied to Orthalicus of Chili, Peru, and 
the Solomon Islands; and Vaginulus, a marine pulmonate, 
occurs in India, the Philippines, and in South America. This 
remarkable distribution is very instructive; for as the marine 
shells of the Indo-pacific province have been unable, during the 
whole of the tertiary era, to cross from Polynesia to America, it 
follows that when the ancestors of these land shells crossed, the 
physical geography of the region must have been very different 
from what it is now ; for there it no trace of their having passed 
into South America from the north. 
We see then that the Australian fauna consists of three 
elements. The first is typified by the mammals, and is charac- 
teristically Australian. The second is typified by the birds, and 
is more nearly related to African than to American forms. The 
third is typified by the frogs, and is more nearly related to South 
America than to any other part of the globe. There is alsoa 
fourth element,—the Antarctic.—which I pass over for the 
present. 
Now it is very difficult, or even impossible, to believe that all 
the groups of semi-tropical plants and animals which connect 
Australia, Polynesia, and even the Sandwich Islands with South 
America have travelled down from the north by the present land 
routes, for then we should have to suppose that all had become 
extinct in North America, and certainly we should expect to find 
the connection between Australia and Africa at least as close as 
it is between Australia and South America, which is not the case. 
But even if we got over this difficulty we should still be unable 
to explain the facts. If, for example, the frogs had passed into 
South America by the same route as the birds, both would have 
shown a similarity in their distribution. The assumption that 
the present frogs are mere relics of a formerly more extended 
distribution, and that allied groups have become discontinuous 
through extermination, will not help us. For if all birds were 
now to become extinct north of the equator, we should still find 
the avi-fauna of Australia more nearly related to that of Africa 
than to that of South America ; and it is impossible, by assuming 
any reasonable amount of extermination, to make the distribution 
of birds accord with that of the frogs. The lines of migration of 
frogs must therefore have been different from those of birds. 
Again Mr. Wallace himself allows that salt water is almost a 
complete barrier to the dispersal of frogs,* consequently where 
frogs could pass birds could pass also; and as the former have 
passed between Australia and South America but nt the latter, 
it follows that the two could not have spread together but each 
must have pursued a different route at a different time. And as 
the present shape of the land accounts for the distribution of the 
birds, the distribution of the frogs must have taken place before 
the present groups of birds were in existence. But birds of many 
* Geographical Distribution of Animals, 1., p. 416. 
