142 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
Australia (Pliocene of Queensland) furnishes ground for believing that 
a form of wingless bird arose in Australasia at some time previous 
to the detachment of the present New Zealand and adjacent islands 
from the mainland. If the stock really had an uninterrupted Tertiary 
history, it is of course immeasurably removed from any existing Carinate 
forms, or other Ratite. The moas are supposed to have enjoyed their 
period of culmination during the Pliocene, when they flourished prodi- 
giously and covered the land, but thereafter they suffered great mortality. 
Their extinction was certainly not caused by the encroachment of natu- 
ral enemies, any more than was the case with Zpyornis ; nor can their 
decline be reasonably attributed to physical or climatic changes of which 
we have no evidence. Their decadence not being traceable to external 
influences, we can only interpret it as the result of some inherent cause 
or causes, — taken together with the retarded action of natural selec- 
tion, — such as are frequently seen to follow in the wake of hypertrophy 
among various groups. 
We cannot pass from this subject without calling attention to Captain 
Hutton’s remarkably ingenious explanation of the crowding together of 
so many varieties of Struthious birds in the limited area of New Zea- 
land, and the unequal distribution of species between the two islands. 
What appear at first sight to be unparalleled or anomalous features of 
distribution are all consistently explained on the theory of a simple 
order of geographical changes, namely, alternating elevation and subsi- 
dence of land masses. ' Two periods of subsidence and one of elevation 
are sufficient to account for all the phenomena, according to Captain 
Hutton’s hypothesis. His interpretation, with which Mr. Wallace 
heartily concurs, is concisely summarized by the latter author as fol- 
lows: “First, we must suppose a land connection with some country 
inhabited by Struthious birds, from which the ancestral forms might be 
derived ; secondly, a separation into many considerable islands, in which 
the various distinct species might become differentiated ; thirdly, an 
elevation bringing about the union of these islands to unite the distinet 
species in one area; and fourthly, a subsidence of a large part of the 
area, leaving the present islands with the various species crowded 
together.” ! 
To revert finally and in few words to the origin of 2pyornis, it is plain 
that, if its ancestors reached the island from Africa as flightless birds, 
the migration must have taken place not later than the Eocene, since no 
mammals initiated since the Cretaceous are found in Madagascar. The 
1 Loc. cit. p. 479. 
