Next to the ears, the jaws and teeth (as was to be expected from the variability of these organs in allied animals), are most affected by degeneracy. This is particularly true of the vertebrates; especially the mammals, as might have been anticipated from their phylogeny or line of descent. At the head of the vertebrates is man; at the foot is the lancelet (amphioxus), most akin to those semi-vertebrates the ascidians, who, in their larval phase, are higher than when adult and whose life history excellently illustrates that potent phase of evolution, degeneracy.1
The lancelet2 has a spinal cord inclosed in a soft semi-cartilaginous canal (the notochord). It is practically destitute of a brain. The cerebral vesicle which represents this, is a plain cavity without true subdivision into ventricles. There is no cranium and the median eye is a mere pigment spot with which it is able to