Simpson (1944:12) and others have emphasized that different parts of organisms evolve at different rates. Beecher (1951b:275) in stating that "... the hind limb is very similar in muscle pattern throughout the Order Passeriformes and seems to have become relatively static after attaining a high level of general efficiency ..." implies that the muscle pattern of the leg must be one of long standing and slow change. This concept was emphasized by Hudson (1937) who found but little variation in muscle pattern among members of the several families of passerine birds. The concept is further confirmed by the present investigation. The intricate patterns of origin and of insertion seem to remain almost the same throughout the order in spite of adaptive radiation which has occurred.
Two major differences in patterns of leg-musculature, however, were found among the species studied, and these differences are significant since they are consistent between subfamilies. The muscles involved are the _m. obturator externus_ and the _pars interna_ of the _m. gastrocnemius_.
The _m. obturator externus_ is bipartite, consisting of dorsal and ventral parts, in the passerine species studied by Hudson (1937) and in all of the species examined by me except the ploceids and the cardueline finches. In the ploceids and cardueline finches this muscle is undivided and resembles in its position, origin, and insertion only the ventral portion of the muscle found in the other birds studied. It is difficult to imagine what advantage or disadvantage might be associated with the bipartite or with the undivided condition. The action of this muscle is to rotate the femur (right femur clockwise, left femur counterclockwise), and certainly the greater mass of the bipartite muscle could lend greater strength to such action. The possible significance of this is discussed below.
Abd. dig. IV _M. abductor digiti IV_ Acc. _M. accessorius semitendinosi_ Add. long. _M. adductor longus et brevis_ Anterolat. can. Anterolateral canal of hypotarsus Anteromed. can. Anteromedial canal of hypotarsus Bic. fem. _M. biceps femoris_ Bic. loop Loop for _m. biceps femoris_ Ext. cot. External cotyla Ext. dig. l. _M. extensor digitorum longus_ Ext. hal. l. _M. extensor hallucis longus_ Fem. tib. ext. _M. femorotibialis externus_ Fem. tib. int. _M. femorotibialis internus_ Fem. tib. med. _M. femorotibialis medius_ F. dig. l. _M. flexor digitorum longus_ F. hal. brev. _M. flexor hallucis brevis_ F. hal. l. _M. flexor hallucis longus_ F. p. et p. d. II _M. flexor perforans et perforatus digiti II_ F. p. et p. d. III _M. flexor perforans et perforatus digiti III_ F. per. d. II _M. flexor perforatus digiti II_ F. per. d. III _M. flexor perforatus digiti III_ F. per. d. IV _M. flexor perforatus digiti IV_ Gas. _M. gastrocnemius_ Iliacus _M. iliacus_ Il. tib. _M. iliotibialis_ Il. troc. ant. _M. iliotrochantericus anticus_ Il. troc. med. _M. iliotrochantericus medius_ Il. troc. post. _M. iliotrochantericus posticus_ Int. cot. Internal cotyla Isch. fem. _M. ischiofemoralis_ Midmed. can. Midmedial canal of hypotarsus Obt. ext. _M. obturator externus_ Obt. int. _M. obturator internus_ P. ant. _Pars anticus_ P. ext. _Pars externa_ P. int. _Pars interna_ P. med. _Pars media_ P. post. _Pars posticus_ Per. brev. _M. peroneus brevis_ Per. long. _M. peroneus longus_ Pirif. _M. piriformis_ Plan. _M. plantaris_ Posterolat. can. Posterolateral canal of hypotarsus Posteromed. can. Posteromedial canal of hypotarsus Sar. _M. sartorius_ Semim. _M. semimembranosus_ Semit. _M. semitendinosus_ Tib. ant. _M. tibialis anticus_ Tib. cart. Tibial cartilage
