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.

Tria.s.sic. : 21. Monotrema. Promammalia. : 21. Ornithodelphia. Echidna.

Ornithorhyncus.

Jura.s.sic. : 22. Marsupalia. Prodidelphia. : 22. Didelphia. Didelphys.

Perameles.

Cretaceous. : 23. Mallotheria. Prochoriata. : 23. Insectivora.

Erinaceida. (Ictopsida +.)

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

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

Oligocene. : 26. Dysmopitheca. Western apes. Dent.i.tion. 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.

CHAPTER 2.21. OUR FISH-LIKE ANCESTORS.

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 cla.s.s 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 of the former in existence now, but they are very interesting, because in their whole structure they stand midway between the Acrania and the Gnathostoma. They are much more advanced than the Acrania, much less so than the fishes, and thus form a very welcome connecting-link between the two groups. We may therefore consider them a special intermediate group, the fourteenth and fifteenth stages in the series of our ancestors.

(FIGURE 2.247. The large marine lamprey (Petromyzon marinus), much reduced. Behind the eye there is a row of seven gill-clefts visible on the left, in front the round suctorial mouth.)

The few surviving species of the Cyclostoma are divided into two orders--the Myxinoides and the Petromyzontes. The former, the hag-fishes, have a long, cylindrical, worm-like body. They were cla.s.sed by Linne with the worms, and by later zoologists, with the fishes, or the amphibia, or the molluscs. They live in the sea, usually as parasites of fishes, into the skin of which they bore with their round suctorial mouths and their tongues, armed with h.o.r.n.y teeth. They are sometimes found alive in the body cavity of fishes (such as the torsk or sturgeon); in these cases they have pa.s.sed through the skin into the interior. The second order consists of the Petromyzontes or lampreys; the small river lamprey (Petromyzon fluviatilis) and the large marine lamprey (Petromyzon marinus, Figure 2.247). They also have a round suctorial mouth, with h.o.r.n.y teeth inside it; by means of this they attach themselves by sucking to fishes, stones, and other objects (hence the name Petromyzon = stone-sucker). It seems that this habit was very widespread among the earlier Vertebrates; the larvae of many of the Ganoids and frogs have suctorial disks near the mouth.

The cla.s.s that is formed of the Myxinoides and Petromyzontes is called the Cyclostoma (round-mouthed), because their mouth has a circular or semi-circular aperture. The jaws (upper and lower) that we find in all the higher Vertebrates are completely wanting in the Cyclostoma, as in the Amphioxus. Hence the other Vertebrates are collectively opposed to them as Gnathostoma (jaw-mouthed). The Cyclostoma might also be called Monorhina (single-nosed), because they have only a single nasal pa.s.sage, while all the Gnathostoma have two nostrils (Amphirhina = double-nosed). But apart from these peculiarities the Cyclostoma differ more widely from the fishes in other special features of their structure than the fishes do from man. Hence they are obviously the last survivors of a very ancient cla.s.s of Vertebrates, that was far from attaining the advanced organisation of the true fish. To mention only the chief points, the Cyclostoma show no trace of pairs of limbs.

Their mucous skin is quite naked and smooth and devoid of scales.

There is no bony skeleton. A very rudimentary skull is developed at the foremost end of their chorda. At this point a soft membranous (partly turning into cartilage), small skull-capsule is formed, and encloses the brain.

The brain of the Cyclostoma is merely a very small and comparatively insignificant swelling of the spinal marrow, a simple vesicle at first. It afterwards divides into five successive cerebral vesicles, like the brain of the Gnathostoma. These five primitive cerebral vesicles, that are found in the embryos of all the higher vertebrates from the fishes to man, and grow into very complex structures, remain at a very rudimentary stage in the Cyclostoma. The histological structure of the nerves is also less advanced than in the rest of the vertebrates. In these the auscultory organ always contains three circular ca.n.a.ls, but in the lampreys there are only two, and in the hag-fishes only one. In most other respects the organisation of the Cyclostoma is much simpler--for instance, in the structure of the heart, circulation, and kidneys. We must especially note the absence of a very important organ that we find in the fishes, the floating-bladder, from which the lungs of the higher Vertebrates have been developed.

When we consider all these peculiarities in the structure of the Cyclostoma, we may formulate the following thesis: Two divergent lines proceeded from the earliest Craniota, or the primitive Craniota (Archicrania). One of these lines is preserved in a greatly modified condition: these are the Cyclostoma, a very backward and partly degenerate side-line. The other, the chief line of the Vertebrate stem, advanced straight to the fishes, and by fresh adaptations acquired a number of important improvements.

(FIGURE 2.248. Fossil Permian primitive fish (Pleuracanthus Dechenii), from the red sandstone of Saarbrucken. (From Doderlein.) I Skull and branchial skeleton: o eye-region, pq palatoquadratum, nd lower jaw, hm hyomandibular, hy tongue-bone, k gill-radii, kb gill-arches, z jaw-teeth, sz gullet-teeth, st neck-spine. II Vertebral column: ob upper arches, ub lower arches, hc intercentra, r ribs. III Single fins: d dorsal fin, c tail-fin (tail-end wanting), an a.n.u.s-fin, ft supporter of fin-rays. IV Breast-fin: sg shoulder-zone, ax fin-axis, ss double lines of fin-rays, bs additional rays, sch plates. V Ventral fin: p pelvis, ax fin-axis, ss single row of fin-rays, bs additional rays, sch scales, cop p.e.n.i.s.

FIGURE 2.249. Embryo of a shark (Scymnus lichia), seen from the ventral side, v breast-fins (in front five pairs of gill-clefts), h belly-fins, a a.n.u.s, s tail-fin, k external gill-tuft, d yelk-sac (removed for most part), g eye, n nose, m mouth-cleft.)

The Cyclostoma are almost always cla.s.sified by zoologists among the fishes; but the incorrectness of this may be judged from the fact that in all the chief and distinctive features of organisation they are further removed from the fishes than the fishes are from the Mammals, and even man. With the fishes we enter upon the vast division of the jaw-mouthed or double-nosed Vertebrates (Gnathostoma or Amphirhina).

We have to consider the fishes carefully as the cla.s.s which, on the evidence of palaeontology, comparative anatomy, and ontogeny, may be regarded with absolute certainty as the stem-cla.s.s of all the higher Vertebrates or Gnathostomes. Naturally, none of the actual fishes can be considered the direct ancestor of the higher Vertebrates. But it is certain that all the Vertebrates or Gnathostomes, from the fishes to man, descend from a common, extinct, fish-like ancestor. If we had this ancient stem-form before us, we would undoubtedly cla.s.s it as a true fish. Fortunately the comparative anatomy and cla.s.sification of the fishes are now so far advanced that we can get a very clear idea of these interesting and instructive features.

In order to understand properly the genealogical tree of our race within the vertebrate stem, it is important to bear in mind the characteristics that separate the whole of the Gnathostomes from the Cyclostomes and Craniota. In these respects the fishes agree entirely with all the other Gnathostomes up to man, and it is on this that we base our claim of relationship to the fishes. The following characteristics of the Gnathostomes are anatomic features of this kind: (1) The internal gill-arch apparatus with the jaw arches; (2) the pair of nostrils; (3) the floating bladder or lungs; and (4) the two pairs of limbs.

The peculiar formation of the frame work of the branchial (gill) arches and the connected maxillary (jaw) apparatus is of importance in the whole group of the Gnathostomes. It is inherited in rudimentary form by all of them, from the earliest fishes to man. It is true that the primitive transformation (which we find even in the Ascidia) of the fore gut into the branchial gut can be traced in all the Vertebrates to the same simple type; in this respect the gill-clefts, which pierce the walls of the branchial gut in all the Vertebrates and in the Ascidia, are very characteristic. But the EXTERNAL, superficial branchial skeleton that supports the gill-crate in the Cyclostoma is replaced in the Gnathostomes by an INTERNAL branchial skeleton. It consists of a number of successive cartilaginous arches, which lie in the wall of the gullet between the gill-clefts, and run round the gullet from both sides. The foremost pair of gill-arches become the maxillary arches, from which we get our upper and lower jaws.

The olfactory organs are at first found in the same form in all the Gnathostomes, as a pair of depressions in the fore part of the skin of the head, above the mouth; hence, they are also called the Amphirhina ("double-nosed"). The Cyclostoma are "one-nosed" (Monorhina); their nose is a single pa.s.sage in the middle of the frontal surface. But as the olfactory nerve is double in both cases, it is possible that the peculiar form of the nose in the actual Cyclostomes is a secondary acquisition (by adaptation to suctorial habits).

A third essential character of the Gnathostomes, that distinguishes them very conspicuously from the lower vertebrates we have dealt with, is the formation of a blind sac by inv.a.g.i.n.ation from the fore part of the gut, which becomes in the fishes the air-filled floating-bladder.

This organ acts as a hydrostatic apparatus, increasing or reducing the specific gravity of the fish by compressing or altering the quant.i.ty of air in it. The fish can rise or sink in the water by means of it.

This is the organ from which the lungs of the higher vertebrates are developed.

(FIGURE 2.250. Fully developed man-eating shark (Carcharias melanopterus), left view. r1 first, r2 second dorsal fin, s tail-fin, a a.n.u.s-fin, v breast-fins, h belly-fins.)

Finally, the fourth character of the Gnathostomes in their simple embryonic form is the two pairs of extremities or limbs--a pair of fore legs (breast-fins in the fish, Figure 2.250 v) and a pair of hind legs (ventral fins in the fish, Figure 2.250 h). The comparative anatomy of these fins is very interesting, because they contain the rudiments of all the skeletal parts that form the framework of the fore and hind legs in all the higher vertebrates right up to man.

There is no trace of these pairs of limbs in the Acrania and Cyclostomes.

Turning, now, to a closer inspection of the fish cla.s.s, we may first divide it into three groups or sub-cla.s.ses, the genealogy of which is well known to us. The first and oldest group is the sub-cla.s.s of the Selachii or primitive fishes; the best-known representatives of which to-day are the orders of the sharks and rays (Figures 2.248 to 2.252).

Next to this is the more advanced sub-cla.s.s of the plated fishes or Ganoids (Figures 2.253 to 2.255). It has been long extinct for the most part, and has very few living representatives, such as the sturgeon and the bony pike; but we can form some idea of the earlier extent of this interesting group from the large numbers of fossils.

From these plated fishes the sub-cla.s.s of the bony fishes or Teleostei was developed, to which the great majority of living fishes belong (especially nearly all our river fishes). Comparative anatomy and ontogeny show clearly that the Ganoids descended from the Selachii, and the Teleostei from the Ganoids. On the other hand, a collateral line, or rather the advancing chief line of the vertebrate stem, was developed from the earlier Ganoids, and this leads us through the group of the Dipneusta to the important division of the Amphibia.

(FIGURE 2.251. Fossil angel-shark (Squatina alifera), from the upper Jura.s.sic at Eichstatt. (From Zittel.) The cartilaginous skull is clearly seen in the broad head, and the gill-arches behind. The wide breast-fin and the narrower belly-fin have a number of radii; between these and the vertebral column are a number of ribs.)

The earliest fossil remains of Vertebrates that we know were found in the Upper Silurian (Chapter 2.18), and belong to two groups--the Selachii and the Ganoids. The most primitive of all known representatives of the earliest fishes are probably the remarkable Pleuracanthida, the genera Pleuracanthus, Xenacanthus, Orthocanthus, etc. (Figure 2.248). These ancient cartilaginous fishes agree in most points of structure with the real sharks (Figures 2.249 and 2.250); but in other respects they seem to be so much simpler in organisation that many palaeontologists separate them altogether, and regard them as Proselachii; they are probably closely related to the extinct ancestors of the Gnathostomes. We find well-preserved remains of them in the Permian period. Well-preserved impressions of other sharks are found in the Jura.s.sic schist, such as of the angel-fish (Squatina, Figure 2.251). Among the extinct earlier sharks of the Tertiary period there were some twice as large as the biggest living fishes; Carcharodon was more than 100 feet long. The sole surviving species of this genus (C. Rondeleti) is eleven yards long, and has teeth two inches long; but among the fossil species we find teeth six inches long (Figure 2.252).

From the primitive fishes or Selachii, the earliest Gnathostomes, was developed the legion of the Ganoids. There are very few genera now of this interesting and varied group--the ancient sturgeons (Accipenser), the eggs of which are eaten as caviare, and the stratified pikes (Polypterus, Figure 2.255) in African rivers, and bony pikes (Lepidosteus) in the rivers of North America. On the other hand, we have a great variety of specimens of this group in the fossil state, from the Upper Silurian onward. Some of these fossil Ganoids approach closely to the Selachii; others are nearer to the Dipneusts; others again represent a transition to the Teleostei. For our genealogical purposes the most interesting are the intermediate forms between the Selachii and the Dipneusts. Huxley, to whom we owe particularly important works on the fossil Ganoids, cla.s.sed them in the order of the Crossopterygii. Many genera and species of this order are found in the Devonian and Carboniferous strata (Figure 2.253); a single, greatly modified survivor of the group is still found in the large rivers of Africa (Polypterus, Figure 2.255, and the closely related Calamichthys). In many impressions of the Crossopterygii the floating bladder seems to be ossified, and therefore well preserved--for instance, in the Undina (Figure 2.254, immediately behind the head).