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Show 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 observation. An example will make this more clear :-Two large arteries (the carotids), one on each side, run up to supply the head in most Pulmonale 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 symmetrical 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 development. 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 species ; 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. |