ON THE ANATOMY OF THE PARROTS. 253 
The true significance of these facts next requires attention; and 
the principle upon which all attempts at the formation of a satisfac- 
tory genealogy or classification of the suborder can be arrived at must 
be born in mind throughout. It is the following:—An anatomical Page 592. 
character is so much the more or less certain to have been an element 
of the original type or ancestor whence sprang the class, order, family, 
or genus under consideration as it is more or less frequently found in 
the less intimately related minor divisions of the groups under obser- 
vation. An example will make this more clear :—T wo large arteries 
(the carotids), one on each side, run up to supply the head in most 
Pulmonate Vertebrata, as faras I know. In all Mammalia such is 
certainly the case. In many Birds there are, similarly, two carotids, 
though some have only one. It is therefore more than probable that 
the ancestral bird had two carotids, those in which one is absent 
having lost it subsequently. Many Parrots have two carotids; the 
genus Cacatua is characterised by the left only being present: it, 
therefore, has in this respect departed most from the ancestral type. 
Again, other Vertebrata and other Birds with both carotid arteries 
present have them symmetrically placed; many Parrots have symme- 
trical carotids; but in some the left (and the left only) is abnormal in 
being superficial: therefore, from the same considerations, these last 
have differentiated off from the parent stem, and, what is more, this 
’ peculiarity can hardly have occurred on more than one occasion, as it 
is otherwise unique and therefore peculiar and exceptional in origin. 
There is another principle to be remembered, which is that there 
is no such thing as reversion to lost ancestral anatomical characters. 
The genus Cacatua has lost its right carotid, as have the whole family 
of the Passeres and many others. There is not a tittle of evidence 
in favour of the assumption that they or their descendants could ever 
regain that vessel. Its arrested development is a positive act, the 
result of extra forces coming into play in early embryonic life, to 
remove which would require the introduction of a certain definite 
geries of counterbalancing forces superadded to those already in 
action; whilst in the ancestral bird, the persistence of the two 
arteries resulted from the absence of any impediment to their deve- 
lopment. The probability that the ancestral form should be reverted 
to cannot be greater than that an entirely new arrangement should 
be effected. That some domestic excentric varieties should tend in 
some cases to revert to the wild type can have no more bearing on 
the general subject than the similar tendency to exaggeration which 
is not apparent in the feral forms. 
Upon these principles many deductions can be made as to the 
mutual relations of the several genera of the Psittacine suborder. 
For instance, it must be inferred that the ancestral Parrot possessed 
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