484 Mr. N. À. Vicors on the Natural Affinities 
The chief genera comprised in the S/ruthionide are the Rhea, 
Briss., which unites this family with the last; the Struthio, Linn., 
which having but two toes, and thus carrying the character of 
the group to the extreme, may be considered the type; the 
Casuarius, Briss., Dromiceius, Vieill., and Otis, Linn. Consi- 
derable doubts have arisen as to the present existence of the 
Linneau Didus; and they have been increased by the consi- 
deration of the numberless opportunities that have latterly oc- 
curred of ascertaining the existence of these birds in those situa- 
tions, the Isles of Mauritius and Bourbon, where they were 
originally alleged to have been found. That they once existed 
I believe cannot be questioned. Besides the descriptions given 
by voyagers of undoubted authority, the relics of a specimen 
preserved in the public repository of this country bear decisive 
record of the fact. ‘The most probable supposition that we can 
form on the subject is, that the race has become extinct in the 
before-mentioned islands, in consequence of the value of the 
bird as an article of food to the earlier settlers, and its incapa- 
bility of escaping from pursuit. This conjecture is strengthened 
by the consideration of the gradual decrease of a nearly conter- 
minous group, the Otis tarda of our British ornithology, which, 
from similar causes, we have every reason to suspect will shortly 
be lost to this country. We may, however, still entertain some 
hopes that the Didus may:be recovered in the south-eastern part 
of that vast continent, hitherto so little explored, which adjoins 
those islands, and whence, indeed, it seems to have been origi- 
nally imported into them. I dwell upon these circumstances 
with more particularity, as the disappearance of this group gives 
us some grounds for asserting, that many chasms which occur 
in the chain of affinities throughout nature may be accounted 
for on the supposition of a similar extinction of a connecting 
species. Here we have an instance of the former existence of 
a species 
