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is represented merely by a single ganglion, and there are no sense organs. The notochord is absent. In

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Clavelina, a compound Ascidian, showing the anatomy of an adult member of the colony, and two members in early stages of development (produced asexually). The members of the colony are connected together by the stolon, which consists of an outer tube of ectoderm continuous with the outer layer of the individuals ; and an inner tube of endoderm, which is a direct prolongation of the so-called epicardium."

a, Anus; ad, Dorsal nerve cord; at, Atrial aperture; br, Branchial aperture; cl, Cloaca; dl, Dorsal languets; e, Endostyle; ep, "Epicardium"; g, Genital ducts; h, Heart; i, Intestine; k, Sub-neural gland; n, Nerve ganglion; o, Ovary; oe, Esophagus; p, Pharynx, with numerous gillslits; s, Stomach; st, Stolon; t, Testis.

(Clavelina is more primitive than most Ascidians in retaining the dorsal nerve cord in the adult condition.)

fact, there is nothing in the adult Ascidian to indicate Vertebrate affinities. (Fig. 35)

In their development, however, we find that they pass through the stage of a free-swimming larva, like a tadpole, which possesses a swimming tail, a nerve cord, a notochord, and an eye and ear. (See Fig. 8.) This larva after a time becomes fixed; the tail shrivels up and is absorbed; the nervous system becomes reduced to a single ganglion; the ear and eye become aborted, and the pharynx enlarges. In fact, Ascidians are degenerate animals, which in their larval stage stand at about the same level as Amphioxus-if anything somewhat higher. Some Ascidians, such as Appendicularia, never get beyond the larval stage, and are found as minute free-swimming pelagic animals, with only one pair of gill clefts.

Concerning the zoological position of Ascidians, it is accepted by every one that they are Vertebrates, which is proved by their development. Mistakes have, however, been made in assuming that they must be ancestral, and this mistake was made by Darwin himself. If ancestral, they should stop at what is a transitional stage in the development of higher animals, but they do not do this. The resemblance is not between the adult Ascidian and the embryo Vertebrate, but between the embryo Ascidian and the embryo Vertebrate. This indicates descent from some common ancestral stock, but not the descent of one group directly from the other.

Amphioxus and Appendicularia are the simplest existing forms with the characteristic vertebrate structure, and represent the nearest approach yet

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made towards the determination of the ancestry of Vertebrates. This suggests that the Vertebrate stem arose very early, and must not be derived from groups such as annelids, lobsters, spiders, and seaurchins, each of which has been claimed as the ancestor of Vertebrates in recent years. Any attempt to get further back than this is mere speculation. The only available evidence is that of embryology, and that this is trustworthy we have abundant proof as regards the later stages of development. The Ascidian tadpole is shown by Appendicularia to be a possible adult, and indeed to correspond very closely to an existing adult animal. With regard to the earlier stages of development we have seen in a former lecture* that this will also apply, and that adult organisms are known corresponding to the several stages in the early development of Amphioxus which may possibly represent ancestral forms.

Thus, by the aid of anatomy, palæontology, and embryology we are able to define fairly clearly the broad lines of Vertebrate ancestry; and the conclusion we arrive at is the usual one, viz., to emphasize the uselessness of search for directly intermediate links between existing forms, and to drive back the origin of this, the highest group of animals, to times earlier than those of the oldest fossiliferous rocks; to show that our only hope of obtaining information concerning these first progenitors depends on the extent to which their existing descendants have preserved the record in their own development, and on our skill in deciphering this record.

* See page 114.

THE DESCENT OF MAN.

And now we turn to the last stage in our inquiry, the zoological position of man.

Man is distinctly an animal-i.e., neither a plant nor a mineral-requiring organised food, for which he is dependent on other animals or on plants. He is distinctly a vertebrate, as proved by his backbone, the relations of his nervous system, brain, heart, and sense-organs. Further, he clearly belongs to marnmals-the presence of hairs instead of scales or feathers would alone be sufficient to show this; but he also possesses all the other characteristics of the group-viz., two condyles to the skull, seven cervical vertebræ, and a left aortic arch. Of the different groups of mammals, it is allowed on all hands that he is most closely allied to that of monkeys. The general shape of his body; the form of his limbs; the number and nature of his fingers and toes; the power of pronation and supination of the fore-arm; the shape of his head; the structure and size of his brain, and the form of his teeth, all prove this incontestably.

Further inquiry shows this correspondence to be a very close one. It is seen in every detail. of structure of the human body, bone for bone, muscle for muscle, nerve for nerve, and even tooth for tooth. Man and monkey can be compared, and the most exact correspondence pointed out. This correspond

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ence is so close that it is almost impracticable to find any constant points of difference of any value whatever. It has indeed been shown by Professor Huxley that the anatomical differences between man and the higher monkeys are markedly less than those between the higher and lower monkeys.

Again, as to development, man, like all other animals, commences as a single cell, and passes through a gill-cleft stage indistinguishable from that of other Vertebrates. He has, like other mammals, at one time two aortic arches, a right and a left, but during development he loses the right arch. His brain, eyes, and limbs are all formed in the same manner as in other mammals.

Rudimentary organs are also present in man, and are of the utmost value, because they are only explicable on the supposition that man is descended from some ancestor in which these organs were in a functional condition. For instance, the intrinsic muscles of the ear are present in man in an incompletely developed form, and in a condition in which they can be of no use to their possessors. So also with the platysma muscle, which, while extensively developed in some Vertebrates, such as horses, is in a comparatively rudimentary condition in man, and of scarcely any functional value. Again, the "wisdom teeth" form another example of rudimentary organs, since they are always cut long after the others, and sometimes never pierce the gums at all.

The tendency to reversion is also met with in man, and is seen in the more or less complete presence of

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