372 ON THE ANATOMY OF PASSERINE BIRDS. 
to think that those schizorhinal birds which possess the ambiens 
muscle—or are, in other. words, homalogonatous*—must be retained 
Fig. 3. 
Superior surfacejof skull of Furnarius rufus, to show the schizorhinal form. 
in one great order, the Charadriiformes, until some important 
structural differences are discovered. which necessitate their being 
otherwise arranged. The schizorhinal disposition is most certainly 
one which is a secondary development upon the normal holorhinal 
nares ; and that it has been independently arrived at in two non-related 
orders of the class is proof that it results from most simple causes, 
because the probability that the same complex conformation should 
appear de novo varies inversely as the complexity: the greater the 
elaborateness the less the chance that it, in all its details, comes into 
existence more than once. A still more simple variation is found in 
the number of the carotid arteries, the normal two being reduced to 
Page 451. the left only in certain members of almost every order—in Rhea and. 
the Megapodes among the Galliformes, in Arctica alle and Turniz 
among the Charadriiformes, in Sula fusca and Plotus anhinga among 
the Ciconiiformes, &c. The disappearance here and there of the 
ambiens muscle and of the femoro-caudal, as well as of the colic 
ceca, all come under the same category, as simple operations which 
lose their significance in the determination of affinity in proportion to 
the frequency of their appearance, or to the facility with which they 
are induced, as I would assume. 
The figure of the superior surface of the skull of Furnarius rufus 
(fig. 3), when compared with those of Charadriiform birds in my 
paper above quoted,+ will show the resemblance between the two, as 
far as the point under discussion is concerned. 
* Vide “ Proceedings of the Zoological Society,” 1874, p. 116. (Supra, p. 218.) 
+ “ Proceedings of the Zoological Society,” 1873, p. 34. (Supra, p. 125.) 
