Birds being of all animals most particularly adapted for extended and rapid locomotion, it became necessary for him to eliminate from his consideration those groups, be they small or large, which are of more or less universal occurrence, and to ground his results on what was at that time commonly known as the order Insessores or Passeres, comprehending the orders now differentiated as Passeriformes, Coraciiformes and Cuculiformes, in other words the mass of arboreal birds.
It was limited, however, to what he called Insessores, being the group upon which that name had been conferred by Vigors (Trans.
At the same time Muller showed himself, his power of discrimination notwithstanding, to fall behind Nitzsch in one very crucial point, for he refused to the latter's Picariae the rank that had been claimed for them, and imagined that the groups associated under that name formed but a third " tribe" - Picarii - of a great order Insessores, the others being (1) the Oscines or Polymyodi - the singing birds by emphasis, whose inferior larynx was endowed with the full number of five pairs of song-muscles, and (2) the Tracheophones, composed of some South-American families.
The final disposition of the " Sub-class Insessores " - all the 2 On the other hand, Muller makes,several references to the labours.
Herein he divided the class A y es into two subclasses, to which he applied the names of Insessores and Grallatores (hitherto used by their inventors Vigors and Illiger in a different sense), in the latter work relying chiefly for this division on characters which had not before been used by any systematist, namely that in the former group monogamy generally prevailed and the helpless nestlings were fed by their parents, while the latter group were mostly polygamous, and the chicks at birth were active and capable of feeding themselves.
Pp. 641-647) the denomination Insessores was changed to Altrices, and Grallatores to Praecoces - the terms now preferred by him being taken from Sundevall's treatise of 1835 already mentioned.
The Insessores are represented by a number of species.