One of the paleontological facts I have quoted, namely, the late appearance of the mammal class in geology--is particularly important. This most advanced group of the vertebrates comes on the stage in the Triassic period, in the second and shorter half of the organic history of the earth. It is represented only by low and small forms in the whole of the mesozoic age, during the domination of the reptiles. Throughout this long period, which is estimated by some geologists at 8-11, by others at 20 or more, million years, the dominant reptile class developed its many remarkable and curious forms; there were swimming marine reptiles (halisauria), flying reptiles (pterosauria), and colossal land reptiles (dinosauria). It was much later, in the Tertiary period, that the mammal class attained the wealth of large and advanced placental forms that secured its predominance over this more recent period. The many and thorough investigations made during the last few decades into the ancestral history of the mammals have convinced all zoologists who were engaged in them that they may be traced to a common root. All the mammals, from the lowest monotremes and marsupials to the ape and man, have a large number of striking characteristics in common, and these distinguish them from all other vertebrates: the hair and glands of the skin, the feeding of the young with the mother's milk, the peculiar formation of the lower jaw and the ear-bones connected therewith, and other features in the structure of the skull; also, the possession of a knee-cap (_patella_), and the loss of the nucleus in the red blood-cells. Further, the complete diaphragm, which entirely separates the pectoral cavity from the abdominal, is only found in the mammals; in all the other vertebrates there is still an open communication between the two cavities. The monophyletic (or single) origin of the whole mammalian class is therefore now regarded by all competent experts as an established fact. In the face of this important fact, what is called the "ape-question" loses a good deal of the importance that was formerly ascribed to it. All the momentous consequences that follow from it in regard to our human nature, our past and future, and our bodily and psychic life, remain undisturbed whether we derive man directly from one of the primates, an ape or lemur, or from some other branch, some unknown lower form, of the mammalian stem. It is important to point this out, because certain dangerous attempts have been made lately by Jesuitical zoologists and zoological Jesuits to cause fresh confusion on the matter.