any secure foundation for these phyletie hypotheses. Only one important piece of information is given us, that the oldest mammals of the Mesozoic age, the Pantotheria and Allotheria of the Trias, were small, lowly organized, for the most part insect eating animals that represent the derivation from older vertebrates, reptiles or amphibia. There is nothing in this to contradict the idea that the entire class of mammals, from the oldest monotremes to man, is monophyletic; that all members of it can be traced back to a single common stem-form.
This positive conviction of the phyletic unity of the class of mammals, because of its common origin from a single extinct stem-group, is now shared by all expert zoologists, and I hold it to be one of the greatest advances of modern zoology. No matter what system of organs we compare in the various mammalian orders, we everywhere find this typical agreement in the essential characters of their structure, both minute and gross. Only among mammals is the skin covered with true hairs, from which fact Oken named this class the "hairy animals." Only in this class is generally found that remarkable kind of nurture, the nourishment of the newborn child with the milk of the mother. Here lies the physiological source of that highest form of maternal love which has exercised such a significant influence upon the family life of various mammals, as well as upon the culture and higher mental life of man. The poet Chamisso justly says of this:
Only the loving mother, only she
Who nurtures from its birth the child she bears,
Knows the true joy that we call happiness,
Created by the love she never spares.
If the Madonna seems to us the most sublime and pure prototype of this human maternal love, yet we perceive on the other hand in the "ape love," in the excessive tenderness of the ape mother, the counterpart of the same maternal instinct. The slow development of this, in the course of many millions of years, from the Trias period to the present, goes hand in hand with an important series of transformations. For the adaptation of the new-born mammal to suckling involved a series of changes not only in its own body but in that of its mother. While in the skin of the mother the mammary glands developed through the irritation and differentiation of a group of ordinary skin glands, there was formed in the mouth of the child, by the act of sucking, the soft palate and afterwards the epiglottis—two organs of the throat that occur only in mammals. In connection with this the mechanism of breathing was changed; this is shown not only in the minute structure of the lungs, but also in the formation of a complete diaphragm. Only in mammals does the muscular diaphragm form a complete partition between the throax and the abdomen. In all other vertebrates the two cavities remain openly connected. Also in the bony framework of the body, and especially in the skull, do we find results of these important transformations. Much the most important
