474 ON OUR PRESENT KNOWLEDGE OF THE ORIGIN OF MAN. 



any secure foundation for these pliyletie hypotheses. Only one impor- 

 tant piece of information is given us, that the oldest mammals of the 

 Mesozoic age, the Paniotlieria and Allotheria of the Trias, were small, 

 lowly organized, for the most part insect eatiug 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 pliyletie unity of the class of mam- 

 mals, because of its common origin from a single extiuct 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 men- 

 tal life of man. The poet Ohamisso 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 counter- 

 part of the same maternal instinct. The slow development of this, in 

 the course of many millions of years, from the Trias period to the pres- 

 ent, 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 suck- 

 ing, the soft palate and afterwards the epiglottis — two organs of the 

 throat that occur only in mammals. In connection with this the mech- 

 anism of breathing was changed; this is shown not only in the minute 

 structure of the lungs, but also in the formation of a complete dia- 

 phragm. Only in mammals does the muscular diaphragm form a 

 complete partition between the throax aud 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 



