II. The group of lowly-organized fishes. These are said to have been "probably" derived from the aquatic worm (I), and they are described to have been as lowly organized as the lancelet, which is a known fish of negative characters, without brain, vertebral column, or heart, presenting some affinities with the Ascidians, which are invertebrate, hermaphrodite marine creatures, permanently attached to a support, and consisting of a simple, tough, leathery sack, with two small projecting orifices. The larvæ of these creatures somewhat resemble tadpoles, and have the power of swimming freely about. These larvæ of the Ascidians are said to be, in their manner of development, related to the Vertebrata in the relative position of the nervous system, and in possessing a structure closely like the _chorda dorsalis_ of vertebrate animals.[36] Here, again, it is apparent that a group of lowly-organized fish-like animals, of which there are no remains, have been constructed by a process of scientific reasoning from a certain class of marine creatures that are known. As a matter of pure theory, there can be no serious objection to this kind of construction, especially if it is supported by strong probabilities furnished by known facts. But when a theory requires this kind of reasoning in order to establish an important link in a chain of proofs, it is perfectly legitimate and necessary criticism that we are called upon to assume the former existence of such a link; and, indeed, the theorists themselves, with true candor and accuracy, tell us that they are arguing upon probabilities from the known to the unknown, or that a thing "must have existed" because analogies warrant the assumption that it did exist. In a matter so interesting, and in many senses important, as the evolution theory of man's descent, it is certainly none too rigid to insist on the application of the ordinary rules of belief.