I. The group of marine animals described as resembling the larvæ of existing Ascidians; that is to say, an aquatic animal in the form of a grub, caterpillar, or worm, which is the first condition of an insect at its issuing from the egg. These assumed progenitors of the Vertebrata are reached, according to Mr. Darwin, by "an obscure glance into a remote antiquity," and they are described as "apparently" existing, and as "resembling" the larvæ of existing Ascidians. We are told that these animals were provided with branchiæ, or gills, for respiration in water, but with the most important organs of the body, such as the brain and heart, imperfectly or not at all developed. This simple and crude animal "we can see," it is said, "in the dim obscurity of the past," and that it "must have been the early progenitor of all the Vertebrata."[35] It is manifest that this creature is a mere hypothesis, constructed, no doubt, by the aid of analogy, but existing only in the eye of scientific imagination. Why is it placed in the water? For no reason, apparently, but that its supposed construction is made to resemble that of some creatures which have been found in the water, and because it was necessary to make it the progenitor of the next group, the lowly-organized fishes, in order to carry out the theory of the subsequent derivations. It might have existed on the land, unless at the period of its assumed existence the whole globe was covered with water. If it had existed on the land, the four subsequent forms, up to and including the Marsupials, might have been varied to suit the exigencies of the pedigree without tracing the descent of the Marsupials through fishes and the Amphibians.