138 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
assuming too much to say that the limit of size has been attained by 
any bird of flight, it is yet very evident that the difficulties of flight 
increase very rapidly with increase of size, — hence the correlation be- 
tween gigantic stature and loss of flight. It is a noteworthy fact that 
the forms attaining the maximum size in their respective groups are 
generally flightless, e. g. Cnemiornis, Notornis, Didus, ete. Since flight- 
less forms have originated in comparatively recent times wherever the 
conditions were favorable, so undoubtedly they arose in the past, and 
only amid unusually favorable conditions and stable environment could 
these ancient flightless forms persist. That existing ‘Ratite’ birds 
were long ago differentiated from the parent stock, or that they arose 
independently, is indicated by the great differences between forms sepa- 
rated by considerable stretches of water. In view of the parallel devel- 
opment of the horse and rhinoceros in Europe and America, it would 
hardly seem necessary to suppose a unity of origin for Struthious birds ; 
moreover the palzeontological history of the class is so fragmentary that 
phylogenetic arrangements of the birds can be regarded as little more 
than guesses. As to the characters of the Ratite, the absence of a keel 
to the sternum and the slight angle between the scapula and coracoid are 
purely degenerate features without the slightest taxonomic value; and 
the ‘ Ratite’ type of skull is a generalized skull having resemblances to 
that of the reptiles. The characters in which Hesperornis resembles the 
ostrich are generalized characters, such as one would be surprised not to 
find in so early a bird; its shoulder girdle is unique among birds and 
decidedly reptilian, while the foot is the most highly specialized swim- 
ming foot known. ‘That this bird is the direct descendant of any land 
bird is incredible. As for the tinamous, their skull and pelvis of a very 
generalized type prevents us from regarding them as recent derivatives ; 
they are in fact ‘ hold-overs’ in a region noted for the number of curious 
forms it contains, indicating the persistence of a few very old species in 
the midst of a more advanced yet not strictly modern fauna.” ? 
1 The above quoted remarks are from some notes which Mr. Lucas very kindly 
took the pains to write out by way of comment on the present paper before it was 
finally prepared for press; and the writer has great pleasure here in acknowl- 
edging his indebtedness to this source for many helpful criticisms and suggestions. 
Reference should be made also to Mr. Lucas’s review of Professor Thompson’s 
paper “On the Systematic Position of Hesperornis,” published in The Auk, Vol. 
VIII. p. 804 (1891), as well as to the comments of Dr. J. A. Allen in the same 
journal, Vol. XV. p. 70 (1898), which brings the literature of /esperornis down to 
date. For a copious bibliography of the distribution of recent birds, see Mr, P. L. 
Sclater’s address before the Second Ornithological Congress at Budapest, May, 
1891. 
