PACIFIC EXPLORATION: H. A. PILSBRY 
431 
existing genera were represented in the Oligocene. If Pacific faunas 
were derived as has often been claimed, from waifs drifted thither on 
'natural rafts' or carried by birds from the continents, such migration 
must have stopped as an effective factor in colonization before Tertiary 
times, for otherwise it seems unaccountable that all dominant Tertiary 
snails of the continents are absent, as with our present knowledge, 
they appear to be. There is small reason for beheving that supposed 
means of mollusk transportation which have failed during the three or 
four milKon years of Cenozoic time, were any more eiB&cient before 
that. It appears that here, as on the continents, their dispersal has 
been chiefly by their own known powers of locomotion. 
It is difficult to escape the conviction that the archipelagoes of today 
are superposed upon folds of vastly greater extension, now lost by sub- 
sidence, upon which the Pacific snails were evolved, and which served 
as paths for their migrations. It may be inferred that in the Mesozoic 
or earlier, continental connections, or close proximity, gave the snails 
of that time access to the Pacific lands. It is not necessarily inferred 
that at any one time an area of continental proportions existed. It is 
of course heterodox to doubt the absolute permanence of oceanic basins, 
but I must confess that my faith in the good old doctrine has been 
shaken. It seems conceivable that the doctrine of isostatic balance is 
not incompatable with the idea that some former areas of low gravity, 
perhaps intermediate between that of the continents and the sea floors, 
have been depressed in late geological times. 
The limits of my space do not allow discussion of much evidence 
favoring the distribution of mid-Pacific land snails by former land 
extensions. The great amount of differentiation among the primitive 
Pacific snails indicates a very long period of evolution— far longer 
than any estimate of the duration of the present volcanic islands. The 
affinities of island faunas are highly suggestive, even though so imper- 
fectly known, of distribution along certain trends. The trade winds 
have always blown from eastward, yet there is absolutely no snail of 
a distinctively American group known in the Pacific faunas. 
Some light may be thrown upon the subject of inter-island distri- 
bution or migration by a brief view at close range of a limited area. In 
the Hawaiian Islands, where alone I have had personal experience, no 
large species of land snail is found living on more than one island, but 
adjacent islands, such as Molokai, Maui and Lanai, have closely related 
species. The tree snails and allied ground snails inhabit mountain 
forests, mainly above 1000 feet, but before the destruction of the lower 
forests they came down to less than half that elevation, though not into 
