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Show 1874.] MR, A. H. GARROD ON THE MUSCLES OF BIRDS. 1 15 the ambiens muscle is present in many not closely related birds. It is found in genera so distant as Struthio, Gallus, Musophaga, Cuculus, Anser, Aquila, Ciconia, and Thalassidroma. This muscle must therefore be considered typical in birds ; it, or the full potentiality for acquiring it in time, must have existed in the ancestral bird. Consequently those birds in which it is absent may be set down as having possessed the muscle in their ancestral form, as having lost it, and, what is more, as having lost all power ever to recover it-because the probability that exactly the same structure should be reproduced as the result of the influence of forces different from those by which it first originated, especially when acting on the body modified upon its previous condition, is infinitely little. I find no tendency to atavism in any structure once fully specialized. The modification of the tarso-metatarse of the Penguin cannot be included in the same category. The bird is hatched, as are others, with an incipient potentiality to develop separate metatarsals ; a modification of its early nutrition, together with peculiarities in its habits of life, prevent the metatarsals from blending into a cylindrical bone; and so they take on a semi-ancestral form. Therefore, and nevertheless, the Penguin is no nearer the Reptilia than any other bird. It is a true bird, derived from the Avian ancestor only, which is the same thing as saying that it has no special Reptilian affinities, although its terrestrial and aquatic habits m a y have caused it to be acted on by forces somewhat similar, and therefore to appear, but only to appear, to have a somewhat similar conformation. The same argument applies to all the members of the class. The Ostrich and Tinamou are no nearer to reptiles than is the Sparrow or the Parrot; they are birds, and therefore they cannot be any thing else. However similar any individuals of two families which separated off two centuries ago and have never intermarried m a y be, no one thinks of claiming any nearer relationship for the similar individuals than for the other members of the families. W h y then should it be said that some birds are Reptilian and others not ? Reptiles and birds can never have interbred, therefore there can be no relationship between them. To return to the subject. There are some families of birds, such as the Columba and the Psittaci, in which different genera vary in possessing or not having the ambiens muscle developed. Those in which it is absent must, from previous considerations, have lost it since the families differentiated off; and therefore those families m a y be classed with the others in which the ambiens is present. The Columba are further complicated in the same way with regard to the caeca of the intestine ; some have caeca, others have not; they must evidently be classed with birds possessing caeca. And generally, if exceptions to a rule are found, when they are in the direction of the loss of any given structural peculiarity, such exceptions are not of m u c h detriment to an argument if other conditions are favourable. But positive exceptions, such as the reappearance of a lost character in minor divisions in the major division of which it is supposed to be absent, are not to. be allowed under any consideration whatever. For nearly the last two years I have been on the watch for a 8* |
