364 A HISTORY OF BIRDS 



checked by natural selection. And no better instances of this 

 can be found than are to be met with in the skull. Take, for 

 example, a bone like the squamosal. In the more primitive 

 forms, like the Struthious birds, this bone is superficial, and, 

 save in the Apteryx, does not appear on the inner wall of the 

 brain case ; and in the Penguins, the most primitive of the 

 Neognathae, this bone is more loosely attached to the outer 

 wall of the skull than in any other birds. But as we proceed 

 from the more primitive to the more specialised forms this bone 

 is found absorbing more and more of the underlying bony 

 tissue until finally it comes to take a large share in the forma- 

 tion of the inner wall of the brain case. That is to say, it 

 passes from the position of a purely superficial bone to take up 

 a much more intimate relation with the neighbouring bones 

 whereby it comes, as it were, to enter into the very warp and 

 woof of the skeleton of the cranium. In the Passeres it dis- 

 plays still further changes. Having long since passed from the 

 position of a superficial scale to that of a bony plate whose 

 entire inner surface is in contact with the brain, it has now 

 begun to change its shape. In its more primitive condition it 

 is roughly of the shape of a Phrygian cap; and from this 

 shape it becomes more and more elongated, till finally it forms 

 but a narrow crescentic band running along the outer border 

 of the frontal and contributing to form the posterior border of 

 the orbit. 



In these changes then we have a progressive evolution re- 

 sulting, apparently, from some inherent tendency within the 

 squamosal. They cannot be influenced by, or determined by, 

 the strains and stresses incident to the struggle for existence, 

 for this bone, like all the cranial bones of the bird's skull, as a 

 separate entity ceases to exist before the bird is able to fend for 

 itself. That is to say, within a week or two of birth all the 

 bones of the cranium have fused to form one homogeneous 

 bony tissue. Each successive generation of birds then seems to 

 have contributed something towards this transformation, but 

 each stage has had to be passed through within an incredibly 

 short space of time, and before the bird had been subjected to 

 any of the many factors which go to make up the struggle for 

 existence. One seems justified in concluding then that this 

 evolution of the squamosal has taken place independent of any 



