1874.] MR. A. H. GARROD ON THE MUSCLES OF BIRDS. H5 



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 poten- 

 tiality 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 aud aquatic habits may 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 thiug else. However similar 

 any individuals of two families which separated off two centuries ago 

 and have never intermarried may be, no one thinks of claiming any 

 nearer relationship for the similar individuals than for the other 

 members of the families. Why 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 Columbce and the Psittaci, in which different genera vary in pos- 

 sessing 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 may be classed 

 with the others in which the ambiens is present. The Columbce 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 much detri- 

 ment 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* 



