That purely hypothetical being, the common ancestor of man and apes, is still to be found, and this is the task assigned to palæontology. Whether this science, to which a great future belongs, will ever accomplish the task, is a question which concerns itself. Meanwhile, considering the great palæontological achievements of our day, the discovery of the _Odontornithes_, _Ætosauri_, _Rhamphorynchi_, _Holoptychia_, etc., we need not despair of the possibility of discovering the true link between the world of man and mammals. This purely speculative side of research, this purely scientific mode of treating the descent of man, is no longer satisfied with unproved assertions, but will rather trust to the strenuous labour of future times, and this need not disturb any religious or political convictions. Even if the assumed ancestral type should really be discovered in some geological stratum, yet research will have to overcome immense difficulties, if it is to explain the development of the understanding and of speech, and the growth of independent human intelligence. Yet we must not, on this account, refuse to recognize the possibility of achieving some new discoveries in this direction. To do so would be to stifle the impulse to scientific research, and this would be unworthy of our former intellectual achievements. Let us therefore labour on with courage. In matters which concern ethnology we are constantly shown that even those races of men which are very remote from each other, and of whom it cannot be supposed that they were in earlier times united in one nation, have made the same technical discoveries, and have adopted similar manners and customs and similar religious observances. This allows us to infer that there is a physical and psychical unity of human nature which indeed separates into races and varieties, but not into distinct species. Certain tokens of what is hypothetically the primeval type will predominate even in the progeny which has been modified by a distinct and separate development, and we need not be surprised by reversions to the animal structure, even in man, the ultimate scope of organic development. Nor will the developed culture of man offer any hindrance to such reversions. The theromorphic conditions which we have pointed out in the third chapter of this work, such as the frontal process of the squamous temporal portion, the transverse enlargement of the occipital bone, the pointed ear, etc., occur both in the higher and lower races of man; just as, for example, both in primitive and high-bred races of horses there are reversions to fossil forms in hind toes, cloven hoofs, etc.