Plates [XII]. and [XIII]. (Between pages 200 and 201, Vol. II.)
Blood relationship between the Vertebrata and the Invertebrata. (Compare vol. ii. pp. [152] and [201].) It is definitely established by Kowalewski’s important discovery, which was confirmed by Kupffer, that the ontogeny of the lowest vertebrate animal—the Lancelet, or Amphioxus—agrees in all essential outlines completely with that of the invertebrate Sea-squirts, or Ascidiæ, from the class of Sea-sacks, or Tunicata. On our two plates, the ascidia is marked by A, the amphioxus by B. Plate [XIII]. represents these two very different animal-forms in a fully developed state, as seen from the left side, the end of the mouth above, the opposite end below. Hence, in both figures the dorsal side is to the right, the ventral to the left. Both figures are slightly magnified, and the internal organisation of the animals is distinctly visible through the transparent skin. The full-grown ascidia (Fig. A 6) grows at the bottom of the ocean, from whence it cannot move, and clings to stones and other objects by means of peculiar roots (w) like a plant. The full-grown amphioxus, on the other hand (Fig. B 6), swims about freely like a small fish. The letters on both figures indicate the same parts: (a) orifice of the mouth; (b) orifice of the body, or porus abdominalis; (c) dorsal rod, or chorda dorsalis; (d) intestine; (e) ovary; (f) oviduct (same as the sperm-duct); (g) spinal marrow; (h) heart; (i) blind-sac of the intestine; (k) gill basket (respiratory cavity); (l) cavity of the body; (m) muscles; (n) testicle (in the ascidia united with the ovary into a hermaphrodite gland); (o) anus; (p) genital orifice; (q) well-developed embryos in the body cavity of the ascidia; (r) rays of the dorsal fin of the amphioxus; (s) tail-fin of the amphioxus; (w) roots of the ascidia.
Plate [XII]. shows the Ontogenesis, or the individual development of the Ascidia (A) and the Amphioxus (B) in five different stages (1-5). Fig. 1 is the egg, a simple cell like the egg of man and all other animals (Fig. A 1 the egg of the ascidia, Fig. B 1 the egg of the amphioxus). The actual cell-substance, or the protoplasm of the egg-cell (z), the so-called yolk, is surrounded by a covering (cell-membrane, or yolk-membrane), and encloses a globular cell-kernel, or nucleus (y), the latter, again, contains a kernel-body, or nucleolus (x); when the egg begins to develop, the egg-cell first subdivides into two cells. By another sub-division there arise four cells (Fig. A 2, B 2), and out of these, by repeated sub-division, eight cells (vol. i. p. [190], Fig. 4 C, D). By fluid gathering in the interior these form a globular bladder bounded by a layer of cells. On one spot of its surface the bladder is turned inwards in the form of a pocket (Fig. A 4, B 4). This depression is the beginning of the intestine, the cavity (d 1) of which opens externally by the provisional larval-mouth (d 4). The body-wall, which is at the same time the stomach-wall, now consists of two layers of cells—the germ-layers. The globular larva (Gastrula), now grows in length. Fig. A 5 represents the larva of the ascidia, Fig. B 5 that of the amphioxus, as seen from the left side in a somewhat more advanced state of development. The orifice of the intestine (d 1) has closed. The dorsal side of the intestine (d 2) is concave, the ventral side (d 3) convex. Above the intestinal tube, on its dorsal side, the neural tube, the beginning of the spinal marrow, is being formed, its cavity still opens externally in front (g 2). Between the spinal marrow and the intestine has arisen the spinal rod, or chorda dorsalis (Notochord) (c), the axis of the inner skeleton. In the larva of the ascidia this rod (c) proceeds along the long rudder-tail, a larval organ, which is cast off in later transformation. Yet there still exist some very small ascidiæ (Appendicularia) which do not become transformed and attached, but which through life swim about freely in the sea by means of their rudder-tail.
The ontogenetic facts which are systematically represented on Plate [XII]. and which were first discovered in 1867, deserve the greatest attention, and, indeed, cannot be too highly estimated. They fill up the gap which, according to the opinion of older zoologists existed between the vertebrate and the so-called “invertebrate” animals. This gap was universally regarded as so important and so undeniable, that even eminent zoologists, who were not disinclined to adopt the theory of descent, saw in this gap one of the chief obstacles against it. Now that the ontogeny of the amphioxus and the ascidia has set this obstacle completely aside, we are for the first time enabled to trace the pedigree of man beyond the amphioxus into the many-branching tribe of “invertebrate” worms, from which all the other higher animal tribes have originated.
If our speculative philosophers, instead of occupying themselves with castles in the air, were to give their thoughts for some years to the facts represented on Plates [XII]. and [XIII]., as well as to those on Plates [II]. and [III]., they would gain a foundation for true philosophy—for the knowledge of the universe firmly based on experience—which would be sure to influence all regions of thought. These facts of ontogenesis are the indestructible foundations upon which the monistic philosophy of future times will erect its imperishable system.
Plate [XIV]. (Between pages 206 and 207, Vol. II.)
Monophyletic, or One-rooted Pedigree of the Vertebrate Animal tribe, representing the hypothesis of the common derivation of all vertebrate animals, and the historical development of their different classes during the palæontological periods of the earth’s history. (Compare Chapter XX. vol. ii. p. [192].) The horizontal lines indicate the periods (mentioned in vol. ii. p. [14]) of the organic history of the earth during which the deposition of the strata containing fossils took place. The vertical lines separate the classes and sub-classes of vertebrata from one another. The tree-shaped and branching lines, by their greater or lesser number and thickness, indicate the approximate degree of development, variety, and perfection, which each class probably attained in each geological period. In those classes which, on account of the soft nature of their bodies, could not leave any fossil remains (which is especially the case with Prochordata, Acrania, Monorrhina, and Dipneusta) the course of development is hypothetically suggested on the ground of arguments derived from the three records of creation—comparative anatomy, ontogeny, and palæontology. The most important starting-points for the hypothetical completion of the palæontological gaps are here, as in all cases, furnished by the fundamental law of biogeny, which asserts the inner causal-nexus existing between ontogeny and phylogeny. (Compare vol. i. p. [310], and vol. ii. p. [200]; also Plates [VIII].-[XIII].) In all cases we have to regard the individual development (determined by the laws of Inheritance but modified by the laws of Adaptation) as short and quick repetitions of the palæontological development of the tribe. This proposition is the “ceterum censeo” of our theory of development.
The statements of the first appearance, or the period of the origin of the individual classes and sub-classes of vertebrate animals (apart from the hypothetical filling in mentioned just now), are taken as strictly as possible from palæontological facts. It must, however, be observed, that in reality the origin of most of the groups probably took place one or two periods earlier than fossils now indicate. In this I agree with Huxley’s views; but on Plates [V]. and [XIV]. I have disregarded this consideration in order not to go too far from palæontological facts.
The numbers signify as follows (compare also Chapter XX. and vol. ii. pp. [204], [206]):—1. Animal Monera; 2. Animal Amœbæ; 3. Community of Amœbæ (Synamœbæ); 4. Ciliated Infusoria without mouths; 5. Ciliated Infusoria with mouths; 6. Gliding worms (Turbellaria); 7. Sea-sacks (Tunicata); 8. Lancelet (Amphioxus); 9. Hag (Myxinoida); 10. Lamprey (Petromyzontia); 11. Unknown forms of transition from single-nostriled animals to primæval fishes; 12. Silurian primæval fish (Onchus, etc.); 13. Living primæval fishes (sharks, rays, Chimæræ); 14. Most ancient (Silurian) enamelled fishes (Pteraspis); 15. Turtle fishes (Pamphracti); 16. Sturgeons (Sturiones); 17. Angular-scaled enamelled fishes (Rhombiferi); 18. Bony pike (Lepidosteus); 19. Finny pike (Polypterus); 20. Hollow-boned fishes (Cœloscolopes); 21. Solid boned fishes (Pycnoscolopes); 22. Bald pike (Amia); 23. Primæval boned fishes (Thrissopida); 24. Bony fishes with air passage to the swimming bladder (Physostomi); 25. Bony fishes without air passage to the swimming bladder (Physoclisti); 26. Unknown forms of transition between primæval fishes and amphibious fishes; 27. Ceratodus; 27a. Extinct Ceratodus from the Trias; 27b. Living Australian Ceratodus; 28. African amphibious fishes (Protopterus) and American amphibious fishes (Lepidosiren); 29. Unknown forms of transition between primæval fishes and amphibia; 30. Enamelled heads (Ganocephala); 31. Labyrinth toothed (Labyrinthodonta); 32. Blind burrowers (Cæciliæ); 33. Gilled amphibia (Sozobranchia); 34. Tailed amphibia (Sozura); 35. Frog amphibia (Anura); 36. Dichthacantha (Proterosaurus); 37. Unknown forms of transition between Amphibia and Protamnia; 38. Protamnia (common primary form of all Amnion animals); 39. Primary mammals (Promammalia); 40. Primæval reptiles (Proreptilia); 41. (Thecodontia); 42. Primæval dragons (Simosauria); 43. Serpent dragons (Plesiosauria); 44. Fish dragons (Ichthyosauria); 45. Teleosauria (Amphicœla); 46. Steneosauria (Opisthocœla); 47. Alligators and Crocodiles (Prosthocœla); 48. Carnivorous Dinosauria (Harpagosauria); 49. Herbivorous Dinosauria (Therosauria); 50. Mæstricht lizards (Mosasauria); 51. Common primary form of Serpents (Ophidia); 52. Dog-toothed beaked lizards (Cynodontia); 53. Toothless beaked lizards (Cryptodontia); 54. Long-tailed flying lizards (Rhamphorhynchi); 55. Short-tailed flying lizards (Pterodactyli); 56. Land tortoises (Chersita); 57. Birds—reptiles (Tocornithes), transition form between reptiles and birds; 58. Primæval griffin (Archæopteryx); 59. Water beaked-animal (Ornithorhynchus); 60. Land beaked-animal (Echidna); 61. Unknown forms of transition between Cloacals and Marsupials; 62. Unknown forms of transition between Marsupials and Placentals; 63. Tuft Placentals (Villiplacentalia); 64. Girdle Placentals (Zonoplacentalia); 65. Disc Placentals (Discoplacentalia); 66. Man (Homo pithecogenes, by Linnæus erroneously called, Homo sapiens.)