Page 115, 
212 ON CERTAIN MUSCLES OF BIRDS. 
or the reverse; if such were the case we should have to put the 
Ardeide with the Passeres, and to separate the Auks from the Gulls, © 
both of which results would be strongly in opposition to the teaching 
of osteology. It is therefore necessary to look around to find, if 
possible, myological characters which have some definite relations to 
equally well-marked pterylographic, visceral, or osteological pecu- 
liarities. 
Before going further it will be necessary to clearly understand a 
principle which is of much assistance in working out the details of 
classification from a large number of unarranged facts. It is this: 
when any certain structure is found to exist in an unmodified form in 
several clearly separable members of any well-marked larger division 
of the Animal Kingdom, that structure must be considered typical of 
the division; in other words, that structure, or the potentiality for 
producing it, must have existed in the common ancestor of the division 
under consideration; and those of its members who are wanting in 
the particular structure are so because they have lost it in process of 
time, not because the others have separately acquired it; for the pro- 
bability, if it were only a matter of probability, is very little that 
several distinct and different species should separately acquire a single 
identical structure; whilst it is infinitely more likely that several 
distinct species should all lose a common character. That all 
Mammalia should acquire branched horns is improbable; but that 
many which possess branched horns should have them broken off 
whilst rushing through a wood, whatever species they belong to, is 
much more to be expected. 
Employing this argument with regard to the facts under discussion, 
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 there- 
fore be considered typical in birds; it, or the full potentiality for 
acquiring it in time, must have existed in the ancestral bird. Conse- 
quently 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 cate- 
gory. The bird is hatched, as are others, with an incipient poten- 
tiality to develop separate metatarsals; a modification of its early 
nutrition, together with peculiarities in its habits of life, prevent the 
