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little digit is the thumb, and that the other two are Nos. 
2 and 3 respectively, is unsupported by evidence, he 
seems to me to be stating what is undeniable. When 
he goes further and argues that they are Nos. 3, 4, 5, 
he is flying in the face of facts. In the embryos of 
the Swift and Tern several good observers have seen 
a fourth unmistakable metacarpal on the ulnar side 
(ze. the side on which in our hand the little finger is) 
and in the embryo of that extraordinary South 
American bird, the Hoatzin, there is a remnant on 
the same side of a fourth finger though the meta- 
carpal has disappeared. There are, then, only two 
alternatives : the surviving digits are either 1, 2, 3, or 
2, 3,4. Now the Emeu has only one, the central one 
of the three, and all analogy would lead us to believe 
that this is the third of the original five and not the 
second, since, when reduction proceeds very far with 
the digits of birds’ feet, or with those of the fore or 
hind feet of mammals, they are lost, as far as can be, 
symmetrically, not in lopsided fashion. And since it 
is the ulnar side of the wing on which mainly the 
strain falls in flight, it is not likely that all the weak- 
ening would go on on this side and all the strength- 
ening on the other. Moreover, in the embryo Hoatzin 
there has been found beyond the so-called “thumb,” 
besides vaguely suggestive cartilage, a bone, small yet 
solid and well defined, that may be a trace of the true 
thumb that has disappeared In any case, the bird’s 
1 See Leighton, 7u/t's College Studies I1T., on“ The Develop- 
ment of the Wing of Sterna Wilsonii,” and W. K. Parker, 
Trans. Zool. Soc. Part 2, April, 1891, on “ The Morphology of 
Opisthocomus Cristatus.” 
