Critiques and Addresses

by Thomas Henry Huxley

With much reason the Lemurs are removed altogether from the Primates, under the name of Prosimiae. But I am surprised to find the Sirenia left in one group with the Cetacea, and the Plesiosauria with the Ichthyosauria; the ordinal distinctness of these having, to my mind, been long since fully established.

V. In Professor Haeckel's speculations on Phylogeny, or the genealogy of animal forms, there is much that is profoundly interesting, and his suggestions are always supported by sound knowledge and great ingenuity. Whether one agrees or disagrees with him, one feels that he has forced the mind into lines of thought in which it is more profitable to go wrong than to stand still.

To put his views into a few words, he conceives that all forms of life originally commenced as Monera, or simple particles of protoplasm; and that these Monera originated from not-living matter. Some of the Monera acquired tendencies towards the Protistic, others towards the Vegetal, and others towards the Animal modes of life. The last became animal Monera. Some of the animal Monera acquired a nucleus, and became amoeba-like creatures; and, out of certain of these, ciliated infusorium-like animals were developed. These became modified into two stirpes: A, that of the worms; and B, that of the sponges. The latter by progressive modification gave rise to all the Coelenterata; the former to all other animals. But A soon broke up into two principal stirpes, of which one, a, became the root of the Annelida, Echinodermata, and Arthropoda, while the other, b, gave rise to the Polyzoa and Ascidioida, and produced the two remaining stirpes of the Vertebrata and the Mollusca.

Perhaps the most startling proposition of all those which Professor Haeckel puts before us is that which he bases upon Kowalewsky's researches into the development of Amphioxus and of the Ascidioida, that the origin of the Vertebrata is to be sought in an Ascidioid form. Goodsir long ago insisted upon the resemblance between Amphioxus and the Ascidians; but the notion of a genetic connection between the two, and especially the identification of the notochord of the Vertebrate with the axis of the caudal appendage of the larva of the Ascidian, is a novelty which, at first, takes one's breath away. I must confess, however, that the more I have pondered over it, the more grounds appear in its favour, though I am not convinced that there is any real parallelism between the mode of development of the ganglion of the Ascidian and that of the Vertebrate cerebro-spinal axis.

The hardly less startling hypothesis that the Echinoderms are coalesced worms, on the other hand, appears to be open to serious objection. As a matter of anatomy, it does not seem to me to correspond with fact; for there is no worm with a calcareous skeleton, nor any which has a band-like ventral nerve, superficial to which lies an ambulacral vessel. And, as a question of development, the formation of the radiate Echinoderm within its vermiform larva seems to me to be analogous to the formation of a radiate Medusa upon a Hydrozoic stock. But a Medusa is surely not the result of the coalescence of as many organisms as it presents morphological segments.

Professor Haeckel adduces the fossil Crossopodia and Phyllodocites as examples of the Annelidan forms, by the coalescence of which the Echinoderms may have been produced; but, even supposing the resemblance of these worms to detached starfish arms to be perfect, it is possible that they may be the extreme term, and not the commencement, of Echinoderm development. A pentacrinoid Echinoderm, with a complete jointed stalk, is developed within the larva of Antedon. Is it not possible that the larva of Crossopodia may have developed a vermiform Echinoderm?

With respect to the Phylogeny of the Arthropoda, I find myself disposed to take a somewhat different view from that of Professor Haeckel. He assumes that the primary stock of the whole group was a crustacean, having that Nauplius form in which Fritz Müller has shown that so many Crustacea commence their lives. All the Entomostraca arose by the modification of some one or other of these Naupliform "Archicarida." Other Archicarida underwent a further metamorphosis into a Zoaea-form. From some of these "Zoeopoda" arose all the remaining Malacostracous Crustacea; while, from others, was developed some form analogous to the existing Galeodes, out of which proceeded, by gradual differentiation, all the Myriapoda, Arachnida, and Insecta.

I should, be disposed to interpret the facts of the embryological history and of the anatomy of the Arthropoda in a different manner. The Copepoda, the Ostracoda, and the Branchiopoda are the Crustacea which have departed least from the embryonic or Nauplius-forms; and, of these, I imagine that the Copepoda represent the hypothetical Archicarida most closely. Apus and Sapphirina indicate the relations of these Archaeocarids with the Trilobita, and the Eurypterida connect the Trilobita and the Copepoda with the Xiphosura. But the Xiphosura have such close morphological relations with the Arachnida, and especially with the oldest known Arachnidan, Scorpio, that I cannot doubt the existence of a genetic connection between the two groups. On the other hand, the Branchiopoda do, even at the present day, almost pass into the true Podophthalmia, by Nebalia. By the Trilobita, again, the Archicarida are connected with such Edriophthalmia as Serolis. The Stomapoda are extremely modified Edriophthalmia of the amphipod type. On the other side, the Isopoda lead to the Myriapoda, and the latter to the Insecta. Thus the Arthropod phylum, which suggests itself to me, is that the branches of the Podophthalmia, of the Insecta (with the Myriapoda), and of the Arachnida, spring separately and distinctly from the Archaeocarid root—and that the Zoaea-forms occur only at the origin of the Podophthalmous branch.

The phylum of the Vertebrata is the most interesting of all, and is admirably discussed by Professor Haeckel. I can note only a few points which seem to me to be open to discussion. The Monorhina, having been developed out of the Leptocardia, gave rise, according to Professor Haeckel, to a shark-like form, which was the common stock of all the Amphirhina. From this "Protamphirhine" were developed, in divergent lines, the true Sharks, Rays, and Chimaerae; the Ganoids, and the Dipneusta. The Teleostei are modified Ganoidei. The Dipneusta gave rise to the Amphibia, which are the root of all other Vertebrata, inasmuch as out of them were developed the first Vertebrata provided with an amnion, or the Protamniota. The Protamniota split up into two stems, one that of the Mammalia, the other common to Reptilia and Aves.

The only modification which it occurs to me to suggest in this general view of the Phylogeny of the Vertebrata is, that the "Protamphirhine" was possibly more ganoid than shark-like. So far as our present information goes the Ganoids are as old as the Sharks; and it is very interesting to observe that the remains of the oldest Ganoids, Cephalaspis and Pteraspis, have as yet displayed no trace of jaws. It is just possible that they may connect the Monorhina, with the Sturgeons among the Amphirhina. On the other hand, the Crossopterygian Ganoids exhibit the closest connection with Lepidosiren, and thereby with the Amphibia. It should not be forgotten that the development of the Lampreys exhibits curious points of resemblance with that of the Amphibia, which are absent in the Sharks and Rays. Of the development of the Ganoidei we have unfortunately no knowledge, but their brains and their reproductive organs are more amphibian than are those of the Sharks.