Biologists are agreed that a certain stage in the organization of the nervous system--the germs of which, we saw, are visible in the quivering of an amoeba, and probably in plants as well as animals--must be reached before consciousness is manifest. Obscurity still hangs round the stage at which mere irritability passes into sensibility, but so long as the continuity of development is clear, the gradations are of lesser importance. And, for the present purpose, there is no need to descend far in the life-scale; if the psychical connection between man and the mammals immediately beneath him is proven, the connection of the mammals with the lowest invertebrate may be assumed as also established. Speaking only of vertebrates, the brain being, whether in fish or man, the organ of mental phenomena, how far does its structure support or destroy the theory of mental continuity? In Man's Place in Nature, and its invaluable supplement, the second part of the monograph on Hume, this subject is expounded by Huxley with his usual clearness. In the older book he traces the gradual modification of brain in the series of backboned animals. He points out that the brain of a fish is very small compared with the spinal cord into which it is continued, that in reptiles the mass of brain, relatively to the spinal cord, is larger, and still larger in birds, until among the lowest mammals, as the opossums and kangaroos, the brain is so increased in proportion as to be extremely different from that of fish, bird, or reptile. Between these marsupials and the highest or placental mammals, there occurs "the greatest leap anywhere made by Nature in her brain work." Then follows this important statement in favour of continuity. "As if to demonstrate, by a striking example, the impossibility of erecting any cerebral barrier between man and the apes, Nature has provided us, in the latter animals, with an almost complete series of gradations from brains little higher than that of a Rodent to brains little lower than that of Man." After giving technical descriptions in proof of this, and laying special stress on the presence of the structure known as the "hippocampus minor" in the brain of man as well as of the ape--in the denial of which Owen cut such a sorry figure, Huxley adds: