ANATOMY 17 
that the ancestral bird-stock did possess well-developed ceca, there- 
fore all those birds which are now found without caeca 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 cxca, that the 
germs of these organs are, in the embryo, just as well developed as 
in birds with long ceca; but these organs, ina 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 czca, 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 cca: 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 
ceca 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 cxeca have to be grouped together with those which 
have no ceca, although the ancestors of both had functional cxea ; 
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 ceca- 
possessing Nightjars, and that the czecaless Macrochires (like Swifts) 
are a recent offshoot 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 Paleotropical stem ; but 
however fascinating such speculations are, we must not forget that 
they hardly ever amount to definite proofs. 
Supposing 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 Caca (b); and asa 
third character the presence (a) or absence (f) of an AFTERSHAFT. 
Then using the ambiens as the principal, and the aftershaft as the 
tertiary differentiating feature, and indicating presence or absence 
by the signs + and — respectively, we get the following eight 
divisions :— 
2 
