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Show 450 MR. FORBES ON THE ANATOMY OF THE TODIES. [May 16, be adaptive, or more recently acquired-exhibited by its existing descendants. As most of the Anomalogonatae possess either well-developed caeca, or a tufted oil-gland, whilst all lack the ambiens and accessory femoro-caudal muscles, it may be presumed with some certainty that the ancestor of the group generally possessed both well-developed caeca and a tuft to the oil-gland-the first having disappeared in the Piciformes, the latter in the Passeriformes, and both in the highly specialized Cypseliformes: at the same time it was destitute of both ambiens and accessory femoro-caudal muscles. The existence of Todus therefore exactly substantiates what might have justly been inferred a priori on purely theoretical grounds ; whilst its insular habitat, the small number of species, and their diminutive size are exactly what might have been expected of a very ancient and synthetic form, which has been unable to hold its own, on the larger areas, with more lately developed and highly specialized forms. On the other hand, it is not to be expected, on the doctrine of descent, that any living form, however synthetic, should be exactly intermediate between any other two living groups, because it is nearly certain to have been modified in some points pari passu with those forms to which it (or, rather, its ancestors more or less remote) gave origin. There are structures in other families of the Anomalogonatae- as, e. g., the biceps-slip of the Caprimulgidae, the gluteus quintus of the Coliidae, the vomer and the gluteus primus of several-• which are not represented at all in Todus. These may, of course, have been independently reacquired ; inasmuch as, however, they are all structures met with in the Homalogonatous birds-from some form of which I cannot doubt that the Anomalogonatae are descended-it is more probable that they have been inherited directly from a common ancestor which possessed these along with the other structural characters of the Anomalogonatae. That one or more of such structures should have disappeared in Todus, though present in the hypothetical common ancestor, is in no way surprising. I submit, in conclusion, therefore, (1) That Todus is a much isolated form, with affinities to both the Passeriformes and Piciformes of Garrod. (2) That it cannot be substantiated that Todus is clearly allied to any particular living form of these. (3) That this view may be most correctly expressed by making a group Todiformes, equivalent to Passeri-, Pici-, and Cypseliformes, for the sole reception of the genus Todus. (4) That in all probability Todus, though in some respects much modified and specialized, represents more nearly than any other existing form the common stock from which all the living groups of Auomalogonatous birds have been derived. |