ON CERTAIN MUSCLES OF BIRDS. role 213 
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 may have caused it to be acted on by forces somewhat 
similar, and therefore to appear, but only to appear, to have a some- 
what 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 there- 
fore they cannot be any thing 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 Columbe 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 Columbe are 
farther complicated in the same way with regard to the ceca of the 
intestine ; some have ceca, others have not; they must evidently be 
classed with birds possessing ceca. And generally, if exceptions to a 
- Tule are found, when they are in the direction of the loss of any given 
structural peculiarity, such exceptions are not of much 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 
structural character or a combination of characters to turn up which Page 116. 
would give clear indications of the most important divisions of the 
bird class. My search has, to my own mind, been fairly satisfactory 
in its results ; for the classification at which I have arrived appears 
to have a practicability about it which is decidedly promising. 
The oft-named ambiens muscle is, in my idea, the key to the 
whole. In some families it is present, in others absent. By combin- 
ing all those in which it is found into one subclass, to be subsequently 
termed Homalogonate (typically kneed, because the ambiens runs in 
the tendon of the knee), and all those in which it is absent intoa 
second, to be subsequently termed Anomalogonate (abnormally kneed), 
