592 MR. A. H. GARROD ON THE [Nov. 3, 



be borne in mind throughout. It is the following : — An anatomical 

 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 : — Two large arteries 

 (the carotids), one on each side, run up to supply the head in most 

 Pulmonate Vertebrata, as far as 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 characterized 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 

 series 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 

 two carotids, running symmetrically in front of the neck, and that the 

 ambiens muscle was present, as was the furcula and the tufted oil- 

 gland. The intestinal caeca and gall-bladder must have been absent or 

 lost very early, as must the postacetabular portion of the tensor fasciae 

 muscle* ; for they are none of them to be found in any existing spe- 

 cies ; whilst the beak, tongue, crop, and rectrices must have possessed 

 the characteristic features, which are not found to vary to any 

 important extent. The pterylosis of the suborder forms a consider- 

 * Vide P. Z. S. 1873, p. 628. 



