"The thoracic, intercostal, or internal mammary artery of birds... is found to arise at slightly different relative positions--from a point at the base of the inferior pectoral artery to a point near the base of the coracoid or sternoclavicular artery, and in some instances both of these vessels have a common root from the subclavian artery. Such differences are found to be of common occurrence within several orders of birds. In the Galliformes and the Passeriformes there appears to be a graded series in the sites of attachment of the thoracic artery from a lateral to a medial position. As a result of these observations, numerical values can be assigned to the site of attachment of the intercostal or thoracic artery, and these values may come to be used as an index in specific levels of evolution.... "The medial migration of the thoracic artery appears to have some phylogenetic significance as yet not understood." The six types of thoracic arteries described in Glenny's classification were distinguished as follows (Glenny, 1955:544): "Type 1: attachment to the pectoral stem lateral to the axillary. "Type 2: attachment to the subclavian between the axillary and coracoid. "Type 3: attachment to the subclavian at the base of the coracoid. "Type 4: attachment to the subclavian, but with a common root for both the coracoid and thoracic. "Type 5: attachment to the subclavian medial to both the axillary and coracoid. "Type 6: two separate thoracic arteries are present; the primary thoracic is the same as type 1 above, while the secondary thoracic is the same as type 3 or type 4 above." Possibly the thoracic artery has undergone migration but apparent differences in its origin might well be due to differences in other vessels of the thoracic area. Additionally, there seems to be no reason to assume that the lateral position of the thoracic artery is the primitive one, or that the medial is the derived position, as is implied by the phrase "medial migration." Although the lateral site of attachment (type 1) is predominant in the lower orders of birds, and the medial attachment is found primarily in Passeriformes, a fact which may indicate that type 1 is the more primitive, it must nevertheless be kept in mind that a sequence of a single morphological character does not necessarily represent the phylogenetic sequence of the character itself (see Mayr, 1955:41). Also, a given arterial arrangement might be independently derived more than once. If such has been the case, similarities in arterial arrangements in different taxa would sometimes be "chance similarities," that is to say, "resemblance in characteristics developed in separate taxa by independent causes and without causal relationship involving the similarity as such" (Simpson, 1961:79).