There is yet left to be noticed perhaps the most wonderful of all the ideas in this first memoir by Huxley.  In the course of describing the two foundation membranes of the Medusae he remarks: 

“It is curious to remark, that throughout, the outer and inner membranes appear to bear the same physiological relation to one another as do the serous and mucous layers of the germ:  the outer becoming developed into the muscular system, and giving rise to the organs of offence and defence:  the inner on the other hand appearing to be more closely subservient to the purposes of nutrition and generation.”

In the whole range of science it would be difficult to select an utterance more prophetic of future knowledge than these few words.  Huxley had been reading the investigations of Von Baer into the early development of back-boned animals.  He had learned from them the great generalisation, that the younger stages of these animals resemble one another more closely than the adult stages, and that in an early stage in the development of all these animals the beginning of the embryo consists of two layers of cells, in fact of two foundation-membranes, one forming specially the wall of the future digestive canal, the other forming the most external portion of the future animal.  In these days nothing could have seemed a remoter or more unlikely comparison than one instituted between Medusae and the embryonic stages of back-boned animals.  But Huxley made it, not allowing the evidence brought before his reason to be swamped by preconceived ideas.  At the time he did no more than to make the comparison.  It was much later that the full importance of it became known, when more extended work on the embryology of vertebrates and of the different groups of the invertebrates had made it plain that the two foundation-membranes of Huxley occur in all animals from the Medusae up to man.  In the group of Coelenterata the organisation remains throughout life as nothing more than a folding in and folding out of these membranes.  The early stages of all the higher animals similarly consist of complications of the two membranes; but later on there is added to them a third membrane.  Thus the group that Huxley gathered together comprises those animals that as adults remain in a condition of development which is passed through in the embryonic life of all higher animals.  The immense importance of this conclusion becomes plain, and the conclusion itself seems obvious, when seen in the light of the doctrine of descent.  The group of Coelenterata represents a surviving, older condition in the evolution of animals.  Huxley himself, when on the Rattlesnake, regarded evolution only as a vague metaphysical dream, and he made the comparison which has been described without any afterthought of what it implied.  In this we have the earliest authentic instance of the peculiar integrity of mind which was so characteristic of him in his dealings with philosophy and tradition.  He never allowed any weight of authority or any apparent disturbance of existing ideas to alter the conclusions to which his reason led him.  This intellectual courage made him fitted to be the leader in the battle for evolution and against traditional thought, and we shall find again and again in consideration of his work that it was the keynote of his life.