The Evolution of Man, V.2 by Ernst Haeckel (comprehension books .TXT) 📕

Chapter 2.

16). The agreement in the structure of the branchial gut of the Enteropneusts, Tunicates, and Vertebrates was first recognised by Gegenbaur (1878); it is the more significant as at first we find only a couple of gill-clefts in the young animals of all three groups; the number gradually increases. We can infer from this the common descent of the three groups with all the more confidence when we find the Balanoglossus approaching the Chordonia in other respects. Thus, for instance, the chief part of the central nervous system is a long dorsal neural string that runs above the gut and corresponds to the medullary tube of the Chordonia. Bateson believes he has detected a rudimentary chorda between the two.

Of all extant invertebrate animals the Enteropneusts come nearest to the Chordonia in virtue of these peculiar characters; hence we may regard them as the survivors of the ancient gut-breathing Vermalia from which the Chordonia also have descended. Again, of all the chorda-animals the Copelata (Figure 2.225) and the tailed larvae of the ascidia approach nearest to the young Balanoglossus. Both are, on the other hand, very closely related to the Amphioxus, the Primitive Vertebrate of which we have considered the importance (

Chapters

2.16 and 2.17). As we saw there, the unarticulated Tunicates and the articulated Vertebrates must be regarded as two independent stems, that have developed in divergent directions. But the common root of the two stems, the extinct group of the Prochordonia, must be sought in the vermalia stem; and of all the living Vermalia those we have considered give us the safest clue to their origin. It is true that the actual representatives of the important groups of the Copelata, Balanoglossi, Nemertina, Icthydina, etc., have more or less departed from the primitive model owing to adaptation to special environment. But we may just as confidently affirm that the main features of their organisation have been preserved by heredity.

We must grant, however, that in the whole stem-history of the Vertebrates the long stretch from the Gastraeads and Platodes up to the oldest Chordonia remains by far the most obscure section. We might frame another hypothesis to raise the difficulty--namely, that there was a long series of very different and totally extinct forms between the Gastraea and the Chordaea. Even in this modified chordaea-theory the six fundamental organs of the chordula would retain their great value. The medullary tube would be originally a chemical sensory organ, a dorsal olfactory tube, taking in respiratory-water and food by the neuroporus in front and conveying them by the neurenteric canal into the primitive gut. This olfactory tube would afterwards become the nervous centre, while the expanding gonads (lying to right and left of the primitive mouth) would form the coeloma. The chorda may have been originally a digestive glandular groove in the dorsal middle line of the primitive gut. The two secondary gut-openings, mouth and anus, may have arisen in various ways by change of functions. In any case, we should ascribe the same high value to the chordula as we did before to the gastrula.

In order to explain more fully the chief stages in the advance of our race, I add the hypothetical sketch of man's ancestry that I published in my Last Link [a translation by Dr. Gadow of the paper read at the International Zoological Congress at Cambridge in 1898]:--

COLUMN 1 : CHIEF STAGES. COLUMN 2 : ANCESTRAL STEM-GROUPS. COLUMN 3 : LIVING RELATIVES OF ANCESTORS.

STAGES 1 TO 5. PROTIST ANCESTORS. UNICELLULAR ORGANISMS.

1 to 2. Protophytes. : 1. Monera. Without nucleus. : Chromacea. (Chroococcus.) Phycochromacea.

1 to 2. Protophytes. : 2. Algaria. Unicellular algae. : 2. Paulotomea. Palmellacea. Eremosphaera.

3 to 5. Protozoa. : 3. Lobosa. Unicellular (amoebina) rhizopods. : 3. Amoebina. Amoeba Leucocyta.

3 to 5. Protozoa. : 4. Infusoria. Unicellular. : 4. Flagellata. Euflagellata. Zoomonades.

3 to 5. Protozoa. : 5. Blastaeades. Multicellular hollow spheres. : 5. Catallacta. Magosphaera, Volvocina, Blastula.

STAGES 6 TO 11. INVERTEBRATE METAZOA ANCESTORS.

6 to 8. Coelenteria, without anus and body-cavity. : 6. Gastraeades. With two germ-layers. : 6. Gastrula. Hydra, Olynthus, Gastremaria.

6 to 8. Coelenteria, without anus and body-cavity. : 7. Platodes I. Platodaria (without nephridia). : 7. Cryptocoela. Convoluta, Proporus.

6 to 8. Coelenteria, without anus and body-cavity. : 8. Platodes II. Platodinia (with nephridia). : 8. Rhabdocoela. Vortex, Monotus.

9 to 11. Vermalia, with anus and body-cavity. : 9. Provermalia. (Primitive Worms.) Rotatoria. : 9. Gastrotricha. Trochozoa, Trochophora.

9 to 11. Vermalia, with anus and body-cavity. : 10. Frontonia. (Rhynchelminthes.) Snout-worms. : 10. Enteropneusta. Balanoglossus, Cephalodiscus.

9 to 11. Vermalia, with anus and body-cavity. : 11. Prochordonia. Chorda-worms. : 11. Copelata. Appendicaria. Chordula-larvae.

STAGES 12 TO 15. MONORHINA ANCESTORS.

Oldest vertebrates without jaws or pairs of limbs, single nose. : 12. Acrania I. (Prospondylia.) : 12. Amphioxus larva.

Oldest vertebrates without jaws or pairs of limbs, single nose. : 13. Acrania II. More recent. : 13. Leptocardia. Amphioxus.

Oldest vertebrates without jaws or pairs of limbs, single nose. : 14. Cyclostoma I. (Archicrania.) : 14. Petromyzonta larvae.

Oldest vertebrates without jaws or pairs of limbs, single nose. : 15. Cyclostoma II. More recent. : 15. Marsipobranchia. Petromyzonta.

COLUMN 1 : GEOLOGICAL PERIODS. COLUMN 2 : ANCESTRAL STEM-GROUPS. COLUMN 3 : LIVING RELATIVES OF ANCESTORS.

Silurian. : 16. Selachii. Primitive fishes. Proselachii. : 16. Natidanides. Chlamydoselachius. Heptanchus.

Silurian. 17. Ganoides. Plated-fishes. Proganoides. : 17. Accipenserides. (Sturgeons.) Polypterus.

Devonian. : 18. Dipneusta. Paladipneusta. : 18. Neodipneusta. Ceratodus. Protopterus.

Carboniferous. : 19. Amphibia. Stegocephala. : 19. Phanerobranchia. Salamandrina. (Proteus, triton.)

Permian. : 20. Reptilia. Proreptilia. : 20. Rhynchocephalia. Primitive lizards. Hatteria.

Triassic. : 21. Monotrema. Promammalia. : 21. Ornithodelphia. Echidna. Ornithorhyncus.

Jurassic. : 22. Marsupalia. Prodidelphia. : 22. Didelphia. Didelphys. Perameles.

Cretaceous. : 23. Mallotheria. Prochoriata. : 23. Insectivora. Erinaceida. (Ictopsida +.)

Older Eocene. : 24. Lemuravida. Older lemurs. Dentition. 3. 1. 4. 3. : 24. Pachylemures. (Hyopsodus +), (Adapis +).

Neo-Eocene. : 25. Lemurogona. Later lemurs. Dentition. 2. 1. 4. 3. : 25. Autolemures. Eulemur. Stenops.

Oligocene. : 26. Dysmopitheca. Western apes. Dentition. 2. 1. 3. 3. : 26. Platyrrhinae. (Anthropops +), (Homunculus +).

Older Miocene. : 27. Cynopitheca. Dog-faced apes (tailed). : 27. Papiomorpha. Cynocephalus.

Neo-Miocene. : 28. Anthropoides. Man-like apes (tail-less). : 28. Hylobatida. Hylobates. Satyrus.

Pliocene. : 29. Pithecanthropi. Ape-men (alali, speechless). : 29. Anthropitheca. Chimpanzee. Gorilla.

Pleistocene. : 30. Homines. Men, with speech. : 30. Weddahs. Australian negroes.

Our task of detecting the extinct ancestors of our race among the vast numbers of animals known to us encounters very different difficulties in the various sections of man's stem-history. These were very great in the series of our invertebrate ancestors; they are much slighter in the subsequent series of our vertebrate ancestors. Within the vertebrate stem there is, as we have already seen, so complete an agreement in structure and embryology that it is impossible to doubt their phylogenetic unity. In this case the evidence is much clearer and more abundant.

The characteristics that distinguish the Vertebrates as a whole from the Invertebrates have already been discussed in our description of the hypothetical Primitive Vertebrate (

Chapter 1.

11, Figure 1.98 to 1.102). The chief of these are: (1) The evolution of the primitive brain into a dorsal medullary tube; (2) the formation of the chorda between the medullary tube and the gut; (3) the division of the gut into branchial (gill) and hepatic (liver) gut; and (4) the internal articulation or metamerism. The first three features are shared by the Vertebrates with the ascidia-larvae and the Prochordonia; the fourth is peculiar to them. Thus the chief advantage in organisation by which the earliest Vertebrates took precedence of the unsegmented Chordonia consisted in the development of internal segmentation.

The whole vertebrate stem divides first into the two chief sections of Acrania and Craniota. The Amphioxus is the only surviving representative of the older and lower section, the Acrania ("skull-less"). All the other vertebrates belong to the second division, the Craniota ("skull-animals"). The Craniota descend directly from the Acrania, and these from the primitive Chordonia. The exhaustive study that we made of the comparative anatomy and ontogeny of the Ascidia and the Amphioxus has proved these relations for us. (See

Chapters

2.16 and 2.17.) The Amphioxus, the lowest Vertebrate, and the Ascidia, the nearest related Invertebrate, descend from a common extinct stem-form, the Chordaea; and this must have had, substantially, the organisation of the chordula.

However, the Amphioxus is important not merely because it fills the deep gulf between the Invertebrates and Vertebrates, but also because it shows us to-day the typical vertebrate in all its simplicity. We owe to it the most important data that we proceed on in reconstructing the gradual historical development of the whole stem. All the Craniota descend from a common stem-form, and this was substantially identical in structure with the Amphioxus. This stem-form, the Primitive Vertebrate (Prospondylus, Figures 1.98 to 1.102), had the characteristics of the vertebrate as such, but not the important features that distinguish the Craniota from the Acrania. Though the Amphioxus has many peculiarities of structure and has much degenerated, and though it cannot be regarded as an unchanged descendant of the Primitive Vertebrate, it must have inherited from it the specific characters we enumerated above. We may not say that "Amphioxus is the ancestor of the Vertebrates"; but we can say: "Amphioxus is the nearest relation to the ancestor of all the animals we know." Both belong to the same small family, or lowest class of the Vertebrates, that we call the Acrania. In our genealogical tree this group forms the twelfth stage, or the first stage among the vertebrate ancestors (

Chapter 2.

20). From this group of Acrania both the Amphioxus and the Craniota were evolved.

The vast division of the Craniota embraces all the Vertebrates known to us, with the exception of the Amphioxus. All of them have a head clearly differentiated from the trunk, and a skull enclosing a brain. The head has also three pairs of higher sense-organs (nose, eyes, and ears). The brain is very rudimentary at first, a mere bulbous enlargement of the fore end of the medullary tube. But it is soon divided by a number of transverse constrictions into, first three, then five successive cerebral vesicles. In this formation of the head, skull, and brain, with further development of the higher sense-organs, we have the advance that the Craniota made beyond their skull-less ancestors. Other organs also attained a higher development; they acquired a compact centralised heart with valves and a more advanced liver and kidneys, and made progress in other important respects.

We may divide the Craniota generally into Cyclostoma ("round-mouthed") and Gnathostoma ("jaw-mouthed"). There are only a few groups