1068 DR. p. CHALMERS MITCHELL ON THE 



slip to III. The pull on the anchor by the contraction of the 

 muscle not only straightens the tarsal joint, but steadies the 

 sustentaculum and therefore assists the action of all the flexors 

 of the toes ; the slip to III acts merely as an accessory to the 

 proper flexor of the third toe. 



I find the Peroneus brevis very difficult to understand. Its 

 i-otating action is often extremely slight, and in its action in 

 flexing the tarsal joint it appears to do little more than assist 

 the much more powerful tibialis anticus, the mechanical arrange- 

 ment of which is more favourable. So far from it being sur- 

 prising to find that it has degenerated or completely disappeared 

 in so many groups, it is remarkable that it should have been 

 preserved at all. In the cases in which it has become stronger 

 and has surpassed the longus, it seems to me that its power of 

 rotation is greater, and in a number of cases, particularly in 

 Passerines, its contraction appears to have the effect of partly 

 flexing the toes and the great toe, as if its action would assist in 

 perching. I am afraid, however, that an equally elaborate 

 investigation of all the muscles of the lower leg would be necessary 

 before the varying conditions of the peroneals could be inter- 

 preted in terms of function. 



On the information at my disposal I find it extremely difficult 

 to associate the conditions of the peroneal muscles with differences 

 in habit that point directly to functional adaptation. On the 

 other hand, with a few, a very few, exceptions to which I shall 

 refer later, there is a close conformity between the condition of 

 the peroneals and what appear to be the most securely founded 

 systematic divisions. Bii^ds seem to have this or that type of 

 peroneal muscle, not because they are arboreal or terrestrial, 

 swimmers or waders, scratchers, pi-edatory or vegetarian, but 

 because it is the type occurring in this or that systematic division. 

 Whatever be their habit they seem to make shift with the type 

 of peroneal which occurs in their group. All the four Legions 

 into which Dr. Gadow groups the Orders of Carinate birds, and 

 most of the orders themselves, show certain members with what 

 I have assumed in this paper to be the generalized, possibly the 

 more ancestral condition of the peroneals, and all show signs of 

 moving in a definite direction away from this generalized con- 

 dition. In the Ratites the longus is dominant but the group has 

 moved away from the central type by extreme reduction or total 

 loss of the brevis. Of the Colymbomorphfe the Penguins show 

 the central type, the Grebes have a weak longus with the slip 

 to III occasionally absent, but the brevis is always rudimentary 

 or absent. In the Petrels the longus has always at least a super- 

 ficial origin, the anchor and slip to III, but the brevis has rela- 

 tively increased in size. In the Pelargomorphte (except Fandion), 

 the longus remains the dominant muscle and always has both the 

 anchor and the slip to III, and the brevis, except in the Falconi- 

 formes, tends to disappear. Some Steganopods, Hei-ons and 

 Ibises, Screamers, and Ducks and Geese remain in the central 

 position. Other Steganopods and Storks and Flamingos have 



