ANATOMY 17 



that the ancestral bird-stock did possess well-developed caeca, there- 

 fore all those birds which are now found without casca must have 

 lost them either phylogenetically or even during their embryonic 

 development. In fact, we find in embryos of such birds as 

 have, when adult, only very small or rudimentary caeca, that the 

 germs of these organs are, in the embryo, just as well developed as 

 in birds with long ca^ca ; but these organs, in a Pigeon for instance, 

 do not grow any further. They are in early life stopped in their 

 development, and thus remain in a rudimentary state. Again, in 

 all those birds which are completely devoid of ceeca, their suppres- 

 sion is simply carried out to the extreme. We cannot therefore, as 

 has been done sometimes, separate Birds into those with and those 

 without caeca : this is especially wrong, as there exist many forms, 

 which, although undoubtedly allied to each other, differ greatly in 

 the presence or absence of these organs. If we want to use the 

 cajca as a differentiating character, we must consider their quality, 

 and enquire whether those organs are functional and well developed, 

 or are they now without function ? Consequently birds with 

 rudimentary caeca have to be grouped together with those which 

 have no c^ca, although the ancestors of both had functional caeca ; 

 and since we know that these organs stand in close correlation with 

 the nature of the food, we are enabled to weigh their taxonomic 

 value. Hence it is probable that the Owls are related to the caeca- 

 possessing Nightjars, and that the caecaless Macrochires (like Swifts) 

 are a recent oftshoot of the latter, while it is impossible to assume 

 that the Owls are descendants of the Diurnal Birds-of-Prey. 



The modifications of the Carotid Arteries have enabled Prof. 

 Fuerbringer to draw a very ingenious and valid conclusion as 

 to the probable original centre of the Parrots. While the Aus- 

 tralian, Oriental, and African Parrots exhibit almost every possible 

 modification of these arteries, from the most primitive to the most 

 specialised conditions, the American Parrots possess only the right 

 deep carotis and a left superficial carotis, an arrangement which is 

 a decidedly recent, not primary feature. Hence the conclusion that 

 the American Parrots are a branch of the Palaeotropical stem ; but 

 however fascinating such speculations are, we must not forget that 

 they hardly ever amount to definite proofs. 



Supp'osing we divide Birds into two classes (A and B), according 

 to the presence or absence of the Ambiens muscle. As a second 

 differentiating character let us take the functional or fully developed 

 (a) and the absent or functionless state of the C^CA (b) ; and as a 

 third character the presence (a) or absence (/3) of an Aftershaft. 

 Then using the ambiens as the principal, and the aftershaft as the 

 tertiary diff"erentiating feature, and indicating presence or absence 

 by the signs + and - respectively, we get the following eight 

 divisions : — ■ 



