Fig. 109.—The Gill-clefts of a Shark (upper fig.) Compared with Those of the Embryonic Chick (to the left) and Rabbit.
Of a similar nature are the rudimentary teeth in the jaws of the embryo of the whalebone whale (Fig. 110). The adults have no teeth, these appearing only as transitory rudiments in the embryo. It is to be assumed that the teeth are inheritances, and that the toothless baleen whale is derived from toothed ancestors.
Fig. 110.—The Jaws of an Embryonic Whale, Showing Rudimentary Teeth.
If we now turn to comparative anatomy, to classification, and to the geographical distribution of animals, we find that it is necessary to assume the doctrine of descent in order to explain the observed facts; the evidence for evolution, indeed, becomes cumulative. But it is not necessary, nor will space permit, to give extended illustrations from these various departments of biological researches.
The Human Body.—Although the broad doctrine of evolution rests largely upon the observation of animals and plants, there is naturally unusual interest as to its teaching in reference to the development of the human body. That the human body belongs to the animal series has long been admitted, and that it has arisen through a long series of changes is shown from a study of its structure and development. It retains marks of the scaffolding in its building. The human body has the same devious course of embryonic development as that of other mammals. In the course of its formation gill-clefts make their appearance; the circulation is successively that of a single-, a double-, and a four-chambered heart, with blood-vessels for the gill-clefts. Time and energy are consumed in building up rudimentary structures which are evanescent and whose presence can be best explained on the assumption that they are, as in other animals, hereditary survivals.
Wiedersheim has pointed out more than one hundred and eighty rudimentary or vestigial structures belonging to the human body, which indicate an evolutionary relationship with lower vertebrates. It would require a considerable treatise to present the discoveries in reference to man's organization, as Wiedersheim has done in his Structure of Man. As passing illustrations of the nature of some of these suggestive things bearing on the question of man's origin may be mentioned: the strange grasping power of the newly born human infant, retained for a short time, and enabling the babe to sustain its weight; the presence of a tail and rudimentary tail muscles; of rudimentary ear muscles; of gill-clefts, etc.
Antiquity of Man.—The geological history of man is imperfectly known, although sporadic explorations have already accumulated an interesting series, especially as regards the shape and capacity of skulls. The remains of early quarternary man have been unearthed in various parts of Europe, and the probable existence of man in the tertiary period is generally admitted. As Osborn says, "Virtually three links have been found in the chain of human ancestry." The most primitive pre-human species is represented by portions of the skull and of the leg bones found in Java by the Dutch surgeon Dubois in the year 1890. These remains were found in tertiary deposits, and were baptized under the name of Pithecanthropus erectus. The structural position of this fossil is between the chimpanzee, the highest of anthropoid apes, and the "Neanderthal man." With characteristic scientific caution Osborn says that the Pithecanthropus "belongs in the line of none of the existing anthropoid apes, and falls very near, but not directly, in the line of human ancestry."