IV. The Amphibians. Here we come to what is now a very numerous group, of which it is said that the first specimens received, among other modifications, the transformation of the swim-bladder of their fish progenitors into an air-breathing lung. We are told that from the fishes of the last preceding group (III) "a very small advance would carry us on to the Amphibians."[38] But whether the advance from an animal living in the water and incapable of existing out of that element, to an animal capable of living on the land as well as in the water, was small or large, we look in vain, at present, for the facts that constitute that advance. V. The Ancient Marsupials. These were an order of mammals such as the existing kangaroos, opossums, etc., of which the young, born in a very incomplete state of development, are carried by the mother, while sucking, in a ventral pouch. They are supposed to have been the predecessors, at an earlier geological period, of the placental mammals, namely, the highest class of mammals, in which the embryo, after it has attained a certain stage, is united to the mother by a vascular connection called the _placenta_, which secures nourishment that enables the young to be born in a more complete state. There is a third and still lower division of the great mammalian series, called the Montremata, and said to be allied to the Marsupials. But the early progenitors of the existing Marsupials, classed as the Ancient Marsupials, are supposed to constitute the connection between the Amphibians and the placental mammals; that is to say, an animal which produced its young by bringing forth an egg, from which the young is hatched, became converted into an animal which produced its young from a womb and nourished it after birth from the milk supplied by its teats, the young being born in a very incomplete state of development and carried by the mother in a ventral pouch while it is sucking. The steps of variation and development by which this extraordinary change of structure, of modes of reproduction and formation of organs, as well as habits of life, took place, are certainly not yet discovered; and it is admitted, in respect to forms "now so utterly unlike," that the production of the higher forms by the process of evolution "implies the former existence of links binding closely together all these forms."[39] In other words, we are called upon to supply by general reasoning links of which we have as yet no proof. VI. The Quadrumana and all the higher (or Placental) Mammals. These are supposed to stand between the implacental mammals (V) and the Lemuridæ (VII). The latter were a group of four-handed animals, distinct from the monkeys, and "resembling the insectivorous quadrupeds." But the gradations which would show the transformation from the implacental Marsupials to the placental Quadrumana are wanting.