PEDIGREE AND HISTORY OF THE ANIMAL KINGDOM.
III. VERTEBRATE ANIMALS.
The Records of the Creation of Vertebrate Animals (Comparative
Anatomy, Embryology, and Palæontology).—The Natural System of
Vertebrate Animals.—The Four Classes of Vertebrate Animals,
according to Linnæus and Lamarck.—Their increase to Nine
Classes.—Main Class of the Tube-hearted, or Skull-less Animals
(the Lancelet).—Blood Relationship between the Skull-less Fish
and the Tunicates.—Agreement in the Embryological Development
of Amphioxus and Ascidiæ.—Origin of the Vertebrate Tribe out of
the Worm Tribe.—Main Class of Single-nostriled, or
Round-mouthed Animals (Hag and Lampreys).—Main Class of
Anamnionate Animals, devoid of Amnion.—Fishes (Primæval Fish,
Cartilaginous Fish, Osseous Fish).—Mud-fish, or Dipneusta.—Sea
Dragons, or Halisauria.—Frogs and Salamanders, or Amphibia
(Mailed Amphibia, Naked Amphibia).—Main Class of Amnionate
Animals, or Amniota.—Reptiles (Primary Reptiles, Lizards,
Serpents, Crocodiles, Tortoises, Flying-Reptiles, Dragons,
Beaked Reptiles).—Birds (Feather-tailed, Fan-tailed,
Bush-tailed).
Not one of the natural groups of organisms—which, we have designated as
tribes, or phyla, on account of the blood-relationship of all the
species included in them—is of such great and exceeding importance as
the tribe of Vertebrate Animals. For, according to the unanimous opinion
of all zoologists, man also is a member of the tribe; and his whole
organization and development cannot possibly be distinguished from that
of other Vertebrate animals. But as from the individual history of
human development, we have already recognized the undeniable fact that,
in developing out of the egg, man at first does not differ from other
Vertebrate animals, and especially from Mammals, we must necessarily
come to the conclusion, in regard to the palæontological history of his
development, that man has, historically, actually developed out of the
lower Vertebrata, and that he is directly derived from lower mammals.
This circumstance, together with the many high interests which, in other
respects, entitle the Vertebrata to more consideration than other
organisms, justifies us in examining the pedigree of the Vertebrata and
its expression in the natural system, with special care.
Fortunately, the records of creation, which must in all cases be our
guide in establishing pedigrees, are especially complete in this
important animal tribe, from which our own race has arisen. Even at the
beginning of our century Cuvier’s comparative anatomy and palæontology,
and Bär’s ontogeny of the Vertebrate animals, had brought us to a high
level of accurate knowledge on this matter. Since then it is especially
due to Johannes Müller’s and Rathke’s investigations in comparative
anatomy, and most recently to those of Gegenbaur and Huxley, that our
knowledge of the natural relationships among the different groups of
Vertebrata has become enlarged. It is especially Gegenbaur’s classical
works, penetrated as they are throughout with the fundamental principles
of the Theory of Descent, which have demonstrated that the material of
comparative anatomy receives its true importance and value only by the
application of the Theory of Descent, and this in the case of all
animals, but especially in that in the Vertebrate tribe. Here, as
everywhere else, analogies must be traced to Adaptation, homologies to
Transmission by Inheritance. When we see that the limbs of the most
different Vertebrata, in spite of their exceedingly different external
forms, nevertheless possess essentially the same internal structure;
when we see that in the arm of a man and ape, in the wing of a man or a
bird, in the breast fins of whales and sea-dragons, in the fore-legs of
hoofed animals and frogs, the same bones always lie in the same
characteristic position, articulation and connection—we can only explain
this wonderful agreement and homology by the supposition of a common
transmission by inheritance from a single primary form. On the other
hand, the striking differences of these homologous bodily parts proceed
from adaptation to different conditions of existence. (Compare Plate
IV.)
Ontogeny, or the individual history of development, like comparative
anatomy, is of especial importance to the pedigree of the Vertebrata.
The first stages of development arising out of the egg are essentially
identical in all Vertebrate animals, and retain their agreement the
longer, the nearer the respective Vertebrate animal forms, when fully
developed, stand to one another in the natural system, that is, in the
pedigree. How far this agreement of germ forms, or embryos, extends,
even in the most highly developed Vertebrate animals, I have already had
occasion to explain (vol i. pp. 306-309). The complete agreement in form
and structure, for example, in the embryos of a man and a dog, of a bird
and a tortoise, existing in the stages of development represented on
Plates II. and III., is a fact of incalculable importance, and furnishes
us with the most important data for the construction of their pedigree.
Finally, the palæontological records of creation are also of especial
value in the case of these same Vertebrate animals; for their fossil
remains belong for the most part to the bony skeleton, a system of
organs which is of the utmost importance for understanding their general
organization. It is true that here, as in all other cases, the fossil
records are exceedingly imperfect and incomplete, but more important
remains of extinct Vertebrate animals have been preserved in a fossil
state, than of most other groups of animals; and single fragments
frequently furnish the most important hints as to the relationship and
the historical succession of the groups.
The name of _Vertebrate Animals_ (Vertebrata), as I have already said,
originated with the great Lamarck, who towards the end of the last
century comprised under this name, Linnæus’ four higher classes of
animals, viz. Mammals, Birds, Amphibious animals, and Fishes, Linnæus’
two lower classes, Insects and Worms, Lamarck contrasted to the
Vertebrata as _Invertebrata_, later also called _Evertebrata_.
The division of the Vertebrata into the four classes above named was
retained also by Cuvier and his followers, and in consequence by many
zoologists down to the present day. But in 1822 Blanville, the
distinguished anatomist, found out by comparative anatomy—which Bär did
almost at the same time from the ontogeny of Vertebrata—that Linnæus’
class of Amphibious animals was an unnatural union of two very different
classes. These two classes were separated as early as 1820, by Merrin,
as two main groups of Amphibious animals, under the names of Pholidota
and Batrachia. The _Batrachia_, which are at present (in a restricted
sense) called Amphibious animals, comprise Frogs, Salamanders, gilled
Salamanders, Cæcilia, and the extinct Labyrinthodonta. Their entire
organization is closely allied to that of Fishes. The _Pholidota_, or
Reptiles, on the other hand, are much more closely allied to Birds. They
comprise lizards, serpents, crocodiles, and tortoises, and the groups of
the mesolithic Dragons, Flying reptiles, etc.
In conformity with this natural division of Amphibious animals into two
classes, the whole tribe of Vertebrate animals was divided into two main
groups. The first main group, containing Amphibious animals and Fishes,
breathe throughout their lives, or in early life, by means of gills, and
are therefore called _gilled Vertebrata_ (Branchiata, or Anallantoida).
The second main group—Reptiles, Birds, and Mammals—breathe at no period
of their lives through gills, but exclusively through lungs, and hence
may appropriately be called Gill-less, or _Vertebrata with lungs_
(Abranchiata, or Allantoida). However correct this distinction may be,
still we cannot remain satisfied with it if we wish to arrive at a true
natural system of the vertebrate tribe, and at a right understanding of
its pedigree. In this case, as I have shown in my General Morphology, we
are obliged to distinguish three other classes of Vertebrate animals, by
dividing what has hitherto been regarded as the class of fishes into
four distinct classes. (Gen. Morph. vol. ii. Plate VII. pp. 116-160.)
The first and lowest of these classes comprises the _Skull-less_ animals
(Acrania), or animals with _tubular hearts_ (Leptocardia), of which only
one representative now exists, namely, the remarkable little Lancelet
(Amphioxus lanceolatus). Nearly allied to this is the second class, that
of the _Single-nostriled_ animals (Monorrhina), or _Round-mouthed_
animals (Cyclostoma), which includes the Hags (Myxinoida) and Lampreys
(Petromyzonta). The third class contains only the genuine Fish (Pisces):
the Mud-fishes (Dipneusta) are added to these as a fourth class, and
form the transition from Fish to Amphibious animals. This distinction,
which, as will be seen immediately, is very important for the genealogy
of the Vertebrate animals, increases the original number of Vertebrate
classes from four to eight.
In most recent times a ninth class of Vertebrata has been added to these
eight classes. Gegenbaur’s recently published investigations in
comparative anatomy prove that the remarkable class of _Sea-dragons_
(Halisauria), which have hitherto been included among Reptiles, must be
considered quite distinct from these, and as a separate class which
branched off from the Vertebrate stock, even before the Amphibious
animals. To it belong the celebrated large Ichthyosauri and Plesiosauri
of the oolitic and chalk periods, and the older Simosauri of the Trias
period, all of which are more closely allied to Fish than to Amphibious
animals.
These nine classes of Vertebrate animals are, however, by no means of
the same genealogical value. Hence we must divide them, as I have
already shown in the Systematic Survey on p. 133, into four distinct
main-classes or tribes. In the first place, the three highest classes,
Mammals, Birds, and Reptiles, may be comprised as a natural main-class
under the name of _Amnion animals_ (Amnionata). The _Amnion-less
animals_ (Anamnionata), naturally opposed to them as a second
main-class, include the four classes of Batrachians, Sea-dragons,
Mud-fish, and Fishes. The seven classes just named, the Anamnionata as
well as the Amnionata, agree among one another in numerous
characteristics, which distinguish them from the two lowest classes
(the single-nostriled and tubular-hearted animals). Hence we may unite
them in the natural main group of _Double-nostriled_ animals
(Amphirrhina). Finally, these Amphirrhina on the whole are much more
closely related to those animals with round mouths or single nostrils
than to the skull-less or tube-hearted animals. We may, therefore, with
full justice class the single and double-nostriled animals into one
principal main group, and contrast them as _animals with skulls_
(Craniota), or _bulbular hearts_ (Pachycardia), to the one class of
_skull-less animals_, or animals with _tubular hearts_. This
classification of the Vertebrate animals proposed by me renders it
possible to obtain a clear survey of the nine classes in their most
important genealogical relations. The systematic relationship of these
groups to one another may be briefly expressed by the following table.
A.
=Skull-less Animals= 1. Tubular hearts 1. Leptocardia
(+Acrania+)
{ _a._ Single nostriled
{ animals { 2. Round-mouths 2. Cyclostoma
B. { _Monorrhina_ {
=Animals with= {
=Skulls= { b. Double { { 3. Fish 3. Pisces
(+Craniota+) { nostriled { I. Non-Amnionate { 4. Mud-fish 4. Dipneusta
{ animals { Anamnia { 5. Sea-dragons 5. Halisauria
or { { { 6. Batrachians 6. Amphibia
{ _Amphirrhina_ {
=Thick Hearts= { { II. Amnionate. { 7. Reptiles 7. Reptilia
(+Pachycardia+) { { Amniota { 8. Birds 8. Aves
{ { { 9. Mammals 9. Mammalia
The only one representative of the first class, the small _lanceolate
fish_, or Lancelet (Amphioxus lanceolatus) (Plate XIII. Fig. _B_),
stands at the lowest stage of organization of all the Vertebrate
animals known to us. This exceedingly interesting and important animal,
which throws a surprising light upon the older roots of our pedigree, is
evidently the last of the Mohicans—the last surviving representative of
a lower class of Vertebrate animals, very rich in forms, and very highly
developed during the primordial period, but which unfortunately could
leave no fossil remains on account of the absence of all solid skeleton.
The Lancelet still lives widely distributed in different seas; for
instance, in the Baltic, North Sea, and Mediterranean, where it
generally lies buried in the sand on flat shores. The body, as the name
indicates, has the form of a narrow lanceolate leaf, pointed at both
extremities. When full grown it is about two inches long, of a white
colour and semi-transparent. Externally, the little lanceolate animal is
so little like a vertebrate animal that Pallas, who first discovered it,
regarded it as an imperfect naked snail. It has no legs, and neither
head, skull, nor brain. Externally, the fore end of the body can be
distinguished from the hinder end only by the open mouth. But still the
Amphioxus in its internal structure possesses those most important
features, which distinguish all Vertebrate animals from all Invertebrate
animals, namely, the spinal rod and spinal marrow. The _spinal rod_
(Chorda dorsalis) is a straight, cylindrical, cartilaginous staff,
pointed at both ends, forming the central axis of the internal skeleton,
and the basis of the vertebral column. Directly above the spinal rod, on
its dorsal side, lies the _spinal marrow_ (medulla spinalis), likewise
originally a straight but internally hollow cord, pointed at both ends.
This forms the principal piece and centre of the nervous system in all
Vertebrate animals. (Compare above vol. i. p. 303.) In all Vertebrate
animals without exception, man included, these important parts of the
body during the embryological development out of the egg, originally
begin in the same simple form, which is retained throughout life by the
Amphioxus. It is only at a later period that the brain develops by the
expansion of the fore end of the spinal marrow, and out of the spinal
rod the skull which encloses the brain. As these two important organs do
not develop at all in the Amphioxus, we may justly call the class
represented by it, _Skull-less animals_ (Acrania), in opposition to all
the others, namely, to the _animals with skulls_ (Craniota). The
Skull-less animals are generally called _tubular-hearted_ (Leptocardia),
because a centralized heart does not as yet exist, and the blood is
circulated in the body by the contractions of the tubular blood-vessels
themselves. The Skulled animals, which possess a centralized,
thick-walled, bulb-shaped heart, ought then by way of contrast to be
called _bulbular-hearted_ animals (Pachycardia).
[Illustration: _Ascidia (A.) and Amphioxus (B.)_ Pl. XII.
E. Haeckel del. Legesse sc.]
Animals with skulls and central hearts evidently developed gradually in
the later primordial period out of those without skulls and with tubular
hearts. Of this the ontogeny of skulled animals leaves no doubt. But
whence are these same skull-less animals derived? It is only very lately
that an exceedingly surprising answer has been given to this important
question. From Kowalewsky’s investigations, published in 1867, on the
individual development of the Amphioxus and the adhering Sea-squirts
(Ascidia) belonging to the class of mantled animals (Tunicata), it has
been proved that the ontogenies of these two entirely different looking
animal-forms agree in the first stage of development in a most
remarkable manner. The freely swimming larvæ of the Ascidians (Plate
XII. Fig. _A_) develop the undeniable beginning of a spinal marrow (Fig.
5 _g_) and of a spinal rod (Fig. 5 _c_), and this moreover in entirely
the same way as does the Amphioxus. (Plate XIII. Fig. _B_.) It is true
that in the Ascidians these most important organs of the Vertebrate
animal-body do not afterwards develop further. The Ascidians take on a
retrograde transformation, become attached to the bottom of the sea, and
develop into shapeless lumps, which when looked upon externally would
scarcely be supposed to be animals. (Plate XIII. Fig. _A_.) But the
spinal marrow, as the beginning of the central nervous system, and the
spinal rod, as the first basis of the vertebral column, are such
important organs, so exclusively characteristic of Vertebrate animals,
that we may from them with certitude infer the true blood relationship
of Vertebrate with Tunicate animals. Of course we do not mean to say by
this, that Vertebrate animals are derived from Tunicate animals, but
merely that both groups have arisen out of a common root, and that the
Tunicates, of all the Invertebrata, are the nearest blood relations of
the Vertebrates. It is quite evident that genuine Vertebrate animals
developed progressively during the primordial period (and the skull-less
animals first) out of a group of worms, from which the degenerate
Tunicate animals arose in another and a retrograde direction. (Compare
the more detailed explanation of Plates XII. and XIII. in the Appendix.)
Out of the Skull-less animals there developed, in the first instance, a
second low class of Vertebrate animals, which still stands far below
that of fish, and which is now represented only by the Hags (Myxinoida)
and Lampreys (Petromyzonta). This class also, on account of the absence
of all solid parts, could, unfortunately, as little as the Skull-less
animals leave fossil remains. From its whole organization and ontogeny
it is quite evident that it represents a very important intermediate
stage between the Skull-less animals and Fishes, and that its few still
existing members are only the last surviving remains of a probably very
highly developed animal group which existed towards the end of the
primordial period. On account of the curious mouth possessed by the Hags
and Lampreys, which they use for sucking, the whole class is usually
called _Round-mouthed_ animals (Cyclostoma). The name of
_Single-nostriled_ animals (Monorrhina) is still more characteristic.
For all Cyclostoma possess a simple, single nasal tube, whereas, in all
other Vertebrate animals (with the exception of the Amphioxus) the nose
consists of two lateral halves, a right and a left nostril. We are
therefore enabled to comprise these latter (Anamnionata and Amnionata)
under the heading, _double-nostriled_ animals (Amphirrhina). All the
Amphirrhina possess a fully developed jaw-skeleton (upper and under
jaw), whereas it is completely wanting in the Monorrhina.
Apart also from the peculiar nasal formation, and the absence of jaws,
the Single-nostriled animals are distinguished from those with double
nostrils by many peculiarities. Thus they want the important sympathetic
nervous system, and the spleen which the Amphirrhina possess. Of the
swimming bladder, and the two pairs of legs—which all double-nostriled
animals have, at least in their embryonic conditions—not a trace exists
in the Single-nostriled animals, which is the case also in the
Skull-less animals. Hence, we are surely justified in completely
separating the Monorrhina, as we have separated the Skull-less animals,
from the Fishes, with which they have hitherto been erroneously classed.
[Illustration: _Ascidia (A.) and Amphioxus (B.)_ Pl. XIII.
E. Haeckel del. Lagesse sc.]
We owe our first accurate knowledge of the Monorrhina, or Cyclostoma, to
the great zoologist, Johannes Müller of Berlin; his classical work on
the “Comparative Anatomy of the Myxinoida” forms the foundation of our
modern views on the structure of the Vertebrate animals. He
distinguished two distinct groups among the Cyclostoma, which we shall
consider as sub-classes.
The first sub-class consists of the Hags (Hyperotreta, or Myxinoida).
They live in the sea as parasites upon other fish, into whose skin they
penetrate (Myxine, Bdellostoma). Their organ of hearing has only one
annular canal, and their single nasal tube penetrates the palate. The
second sub-class, that of Lampreys, or Prides (Hyperoartia, or
Petromyzontia) is more highly developed. It includes the well-known
Lamperns, or Nine-eyes, of our rivers (Petromyzon fluviatilis), with
which most persons are acquainted. They are represented in the sea by
the frequently larger marine or genuine Lampreys (Petromyzon marinus).
The nasal tube of these single-nostriled animals does not penetrate the
palate, and in the auricular organ there are two annular canals.
SYSTEMATIC SURVEY
_Of the 4 Main-classes, 9 Classes, and 26 Sub-classes of Vertebrata._
Gen. Morph. vol. ii. Plate VII. pp. 116-160.
======================================================================================
I. =Skull-less= (+Acrania+), or =Tube-hearted= (+Leptocardia+).
Vertebrata without head, without skull and brain, without centralized heart.
--------------------------------------------------------------------------------------
1. =Skull-less= I. Tube-hearted { 1. Lancelet 1. Amphioxus
+Acrania+ _Leptocardia_ {
--------------------------------------------------------------------------------------
II. =Animals with skulls= (+Craniota+) and with =thick-walled hearts= (+Pachycardia+).
Vertebrata with head, with skull and brain, with centralized heart.
======================================================================================
_Main-classes_ | _Classes_ | _Sub-classes_ | _Systematic Name_
_of the Skulled_ | _of the_ | _of the_ | _of the_
_Animals._ | _Skulled Animals._ | _Skulled Animals._ | _Sub-classes._
--------------------------------------------------------------------------------------
2. =Single-Nostriled= { II. Round mouths { 2. Hags, or Mucous 2. Hyperotreta
+Monorrhina+ { _Cyclostoma_ { Fish (Myxinoida)
{ { 3. Lampreys, or 3. Hyperoartia
{ { Pride (Petromyzontia)
{ III. Fish { 4. Primæval fish 4. Selachii
{ _Pisces_ { 5. Ganoid fish 5. Ganoides
{ { 6. Osseous fish 6. Teleostei
{
{ IV. Mud-fish { 7. Mud-fish 7. Protopteri
3. =Non-amnionate= { _Dipneusta_ {
{
+Anamnionata+ { { 8. Primæval 8. Simosauria
{ V. Sea-dragons { dragons
{ _Halisauri_ { 9. Snake-dragons 9. Plesiosauria
{ { 10. Fish-dragons 10. Ichthyosauria
{
{ VI. Batrachians { 11. Mailed Batrachians 11. Phractamphibia
{ _Amphibia_ { 12. Naked Batrachians 12. Lissamphibia
4. =Amnion= { VII. Reptiles { 13. Primary reptiles 13. Tocosauria
=Animals= { _Reptilia_ { 14. Lizards 14. Lacertilia
+Amnionata+ { { 15. Serpents 15. Ophidia
{ { 16. Crocodiles 16. Crocodilia
{ { 17. Tortoises 17. Chelonia
{ { 18. Flying reptiles 18. Pterosauria
{ { 19. Dragons 19. Dinosauria
{ { 20. Beaked reptiles 20. Anomodontia
{
{ VIII. Birds { 21. Long-tailed 21. Saururæ
{ _Aves_ { 22. Fan-tailed 22. Carinatæ
{ { 23. Bush-tailed 23. Ratitæ
{
{ IX. Mammals { 24. Cloacal animals 24. Monotrema
{ _Mammalia_ { 25. Pouched animals 25. Marsupialia
{ { 26. Placental animals 26. Placentalia
----------------------------------------------------------------
9. Mammals
_Mammalia_
8. Birds |
_Aves_ |
| |
7. Reptiles |
_Reptilia_ |
5. Sea-dragons | |
_Halisauria_ \-------v--------/
| |
Osseous fish | =Amnion Animals=
_Teleostei_ 4. Mud-fish | +Amniota+
| _Dipneusta_ | |
| | | |
| | | 6. Batrachians
Ganoid fish | | _Amphibia_
_Ganoidei_ | | |
| | | |
| \--------------v--------------/
| |
| Vertebrate animals breathing through lungs
| _Amphipneumones_
| |
| |
\------------v-----------------/
Primæval fish Selachii
3. Fishes _Pisces_
=Double-nostriled= +Amphirrhina+
| 2. Round-mouthed
| _Cyclostoma_
| |
| |
\--------------------v-------------/
=Single-nostriled= Monorrhina
=Animals with skulls= +Craniota+
1. Tube-hearted |
_Leptocardia_ |
| |
| |
\---------v------/
_Ascidiæ_ |
Sea-barrels | =Skull-less Animals=
_Thaliacea_ | +Acrania+
| | =Vertebrate Animals=
| | +Vertebrata+
\----v----/ |
| |
=Tunicate Animals= |
+Tunicata+ |
| |
| |
\-----------v---------/
|
Worms
_Vermes_
All existing Vertebrate animals, with the exception of the Monorrhina
and Amphioxus just mentioned, belong to the group which we designate as
Double-nostriled animals (Amphirrhina). All these animals possess (in
spite of the great variety in the rest of their forms) a nose consisting
of two lateral halves, a jaw-skeleton, a sympathetic nervous system,
three annular canals connected with the auricular sac, and a spleen.
Further, all Double-nostriled animals possess a bladder-shaped expansion
of the gullet, which, in Fish, has developed into the swimming bladder,
but in all other Double-nostriled animals into lungs. Finally, in all
Double-nostriled animals there exist in the youngest stage of growth the
beginnings of two pairs of extremities, or limbs, a pair of fore legs,
or breast fins, and a pair of hinder legs, or ventral fins. One of these
pairs of legs sometimes degenerates (as in the case of eels, whales,
etc.), or both pairs of legs (as in Cæciliæ and serpents) either
degenerate or entirely disappear; but even in these cases there exists
some trace of their original beginning in an early embryonic period, or
the useless remains of them may be found in the form of rudimentary
organs. (Compare above, vol. i. p. 13.)
From all these important indications we may conclude with full assurance
that all double-nostriled animals are derived from a single common
primary form, which developed either directly or indirectly during the
primordial period out of the Monorrhina. This primary form must have
possessed the organs above mentioned, and also the beginning of a
swimming bladder and of two pairs of legs or fins. It is evident, that
of all still living double-nostriled animals, the lowest forms of sharks
are most closely allied to this long since extinct, unknown, and
hypothetical primary form, which we may call the Primary
Double-nostriled animals (Proselachii). We may therefore look upon the
group of primæval fish, or Selachii, to which the _Proselachii_ probably
belonged, as a primary group, not only of the Fish class, but of the
whole main-class of double-nostriled animals.
The class of _Fish_ (Pisces) with which we accordingly begin the series
of Double-nostriled animals, is distinguished from the other six classes
of the series by the swimming bladder never developing into lungs, but
acting only as a hydrostatic apparatus. Agreeing with this, we find that
in fish the nose is formed by two blind holes in front of the mouth,
which never pierce the palate so as to open into the cavity of the
mouth. In the other six classes of double-nostriled animals, both
nostrils are changed into air passages which pierce the palate, and thus
conduct air to the lungs. Genuine fish (after the exclusion of the
Dipneusta) are accordingly the only double-nostriled animals which
exclusively breathe through gills and never through lungs. In accordance
with this, they all live in water, and both pairs of their legs have
retained the original form of paddling fins.
SYSTEMATIC SURVEY
_Of the 7 Legions and 15 Orders of the Fishes._
---------------------------------------------------------------------------------
_Sub-classes_ | _Legions_ | _Orders_ | _Examples_
_of_ | _of_ | _of_ | _from_
_Fishes._ | _Fishes._ | _Fishes._ | _the Orders._
---------------------------------------------------------------------------------
{
{ { 1. Sharks Sharks, dog-fish
{ I. Transverse { _Squalacei_
A. { mouths { 2. Rays Spiked rays, electric
=Primæval= { _Plagiostomi_ { _Rajacei_ rays, etc.
=Fish= {
+Selachii+ { II. Sea-Cats { 3. Sea-Cats Chimæra, Calorrhynchias
{ _Holocephali_ { _Chimæracei_
{ { 4. Buckler-heads Cephalaspidæ, Placoderma,
{ III. Mailed Ganoid { _Pamphracte_ etc.
{ Fish { 5. Sturgeons Spoon-sturgeons, sturgeons,
{ _Tabuliferi_ { _Sturiones_ sterlet, etc.
{
{ { 6. _Efulcri_ Double-finned
B. { IV. Angular-scaled { 7. _Fulcrati_ Palæoniscus, bony pike,
=Ganoid= { Ganoid Fish { etc.
=Fish= { _Rhombiferi_ { 8. _Semæopteri_ African finny pike, etc.
+Ganoides+ {
{ V. Round-scaled { 9. _Cœloscolopes_ Holoptychius, Cœlacanthides,
{ Ganoid Fish { etc.
{ _Cycliferi_ { 10. _Pycnoscolopes_ Coccolepida, Amiadæ,
etc.
{ VI. Osseous Fish {
{ with an air { 11. Herring species Herrings, salmon, carp,
{ passage to the { _Thrissogenes_ etc.
C. { swimming { 12. Eel species Eels, snake eels, electric
=Osseous= { bladder { _Enchelygenes_ eels, etc.
=Fish= { _Physostomi_ {
+Teleostei+ {
{ VII. Osseous Fish { 13. _Stichobranchii_ Perch, wrasse, turbot,
{ without an air { etc.
{ passage to the { 14. _Plectognathi_ Trunk fish, globe fish,
{ swimming { etc.
{ bladder { 15. _Lophobranchii_ Pipe fish, sea horses,
{ _Physoclisti_ { etc.
----------------------------------------------------------------
PEDIGREE OF THE NON-AMNIONATE CRANIOTA.
Plectognathi Anura
| Lophobranchia Peromela |
| | | |
| | | Sozura
\-------v-----/ Labyrinthodonta | |
| | | |
Stichobranchia | | |
+Physoclisti+ \--v-----/ |
| | |
Enchelygenes | Ganocephala Sozobranchia
| | +Phractamphibia+ +Lissamphibia+
| | | |
\--v---/ | |
| \-------v-------/
| |
Thrissogenes +Amphibia+
+Physostomi+ Semæopteri |
+Teleostei+ | |
| | Protopteri |
| Fulcrati | | | Plesiosauria
Pycnoscolopes | | | | Icthyosauria |
| | | | | | |
Cœloscolopes Efulcri | | | | |
+Cycliferi+ +Rhombiferi+ | | | |
(Cycloganoides) (Rhomboganoides) | | | |
| | | | | |
\---------v--------/ | | \----v-----/
| +Dipneusta+ | |
Placoderma | | Simosauria
Sturiones | | | +Halisauria+
| | | | |
Cephalaspidæ | Rajacei \-----------v--------/
| | | |
| | | Amphipneumona
\-----v-------/ | |
| | |
Pamphracti | |
+Tabuliferi+ | |
(Placoganoides) | Chimæracei |
+Ganoides+ | +Holocephali+ |
| | | |
Squalacei | | |
| | | |
\--------v---------/ | |
| | |
+Plagiostomi+ | |
| | |
\-------------v---------------------/
|
+Selachii+
+Fish+
+Amphirrhina+
| Cyclostoma
| +Monorrhina+
| |
\-------v---------/
|
+Craniota+
Genuine fish are divided into three distinct sub-classes, namely,
Primæval fish, Ganoid fish, and Osseous fish. The oldest of these, where
the original form has been most faithfully preserved, is that of the
_Primæval fish_ (Selachii). Of these there still exist Sharks (Squali),
and Rays (Rajæ), which are classed together as cross-mouthed fishes
(Plagiostomi), and the strange and grotesquely formed Sea-cats, or
_Chimæracei_ (Holocephali). These primary fish of the present day, which
are met with in all seas, are only poor remains of the prevailing animal
groups, rich in forms, which the Selachii formed in the earlier periods
of the earth’s history, and especially during the palæolithic period.
Unfortunately all Primæval fish possess a cartilaginous, never a
completely osseous skeleton, which is but little, if at all, capable of
being petrified. The only hard parts of the body which could be
preserved in a fossil state, are the teeth and fin-spikes. These are
found in the older formations in such quantities, varieties, and sizes,
that we may, with certainty, infer a very considerable development of
Primæval fish in those remote ages. They are even found in the Silurian
strata, which contain but few remains of other Vertebrata, such as
Enamelled fish (and these only in the most recent part, that is, in the
upper Silurian). By far the most important and interesting of the three
orders of Primæval fish are Sharks; of all still living double-nostriled
animals, they are probably most closely allied to the original primary
form of the whole group, namely, to the Proselachii. Out of these
Proselachii, which probably differed but little from genuine Sharks,
Enamelled fish, and the present Primæval fish, in all probability,
developed in one direction, and the Dipneusta, Sea-dragons, and Amphibia
in another.
The _Ganoid_, or _Enamelled fish_ (Ganoides), in regard to their anatomy
stand midway between the Primæval and the Osseous fish. In many
characteristics they agree with the former, and in many others with the
latter. Hence, we infer that genealogically they form the transition
from Primæval to Osseous fish. The Ganoids are for the most part
extinct, and more nearly so than the Primæval fish, whereas they were
developed in great force during the entire palæolithic and mesolithic
periods. Ganoid fish are divided into three legions according to the
form of their external covering, namely, Mailed, Angular-scaled, and
Round-scaled. The _Mailed Ganoid fish_ (Tabuliferi) are the oldest, and
are directly allied to the Selachii, out of which they originated.
Fossil remains of them, though rare, are found even in the upper
Silurian (Pteraspis ludensis of the Ludlow strata). Gigantic species of
them, coated with strong bony plates, are found in the Devonian system.
But of this legion there now lives only the small order of Sturgeons
(Sturiones), including the Spade-sturgeons (Spatularidæ), and those
Sturgeons (Accipenseridæ) to which belong, among others, the Huso, which
yields isinglass, or sturgeon’s sound, and the Caviar-sturgeon, whose
eggs we eat in the shape of caviar, etc. Out of the mailed Ganoid fish,
the angular and round-scaled ones probably developed as two diverging
branches. The _Angular-scaled Ganoid fish_ (Rhombiferi)—which can be
distinguished at first sight from all other fish by their square or
rhombic scales—are at present represented only by a few survivors,
namely, the Finny Pike (Polypterus) in African rivers (especially the
Nile), and by the Bony Pike (Lepidosteus) in American rivers. Yet during
the palæolithic and the first half of the mesolithic epochs this legion
formed the most numerous group of fishes. The third legion, that of
_Round-scaled Ganoid fish_ (Cycliferi), was no less rich in forms, and
lived principally during the Devonian and Coal periods. This legion, of
which the Bald Pike (Amia), in North American rivers, is the only
survivor, was especially important, inasmuch as the third sub-class of
fish, namely, Osseous fish, developed out of it.
_Osseous fish_ (Teleostei) include the greater portion of the fish of
the present day. Among these are by far the greater portion of marine
fish, and all of our fresh-water fish except the Ganoid fish just
mentioned. This class is distinctly proved by numerous fossils to have
arisen about the middle of the Mesolithic epoch out of Ganoid fish, and
moreover out of the Round-scaled, or Cycliferi. The Thrissopidæ of the
Oolitic period (Thrissops, Leptolepis, Tharsis), which are most closely
allied to the herrings of the present day, are probably the oldest of
all Osseous fish, and have directly arisen out of Round-scaled Ganoid
fish, closely allied to the existing Amia. In the older Osseous fish of
the legion called _Physostomi_, as also in the Ganoides, the swimming
bladder throughout life was connected with the throat by a permanent air
passage (a kind of windpipe). This is still the case with all the fish
belonging to this legion, namely, with herrings, salmon, carp, shad,
eels, etc. However, during the chalk period this air passage, in some of
the Physostomi, became constricted and closed, and the swimming bladder
was thus completely separated from the throat. Hence there arose a
second legion of Osseous fish, the _Physoclisti_, which did not attain
their actual development until the tertiary epoch, and soon far
surpassed the Physostomi in variety. To this legion belong most of the
sea fish of the present day, especially the large families of the
Turbot, Tunny, Wrasse, Crowfish, etc., further, the Lock-jaws
(Plectognathi), Trunk fish, and Globe-fish and the Bushy-gills
(Lophobranchi), viz., Pipe-fish, and Sea-horses. There are, however,
only very few Physoclisti among our river fish, for instance, Perch and
Sticklebacks; the majority of river fish are Physostomi.
Midway between genuine Fish and Amphibia is the remarkable class of
_Mud-fish_, or _Scaly Sirens_ (Dipneusta, or Protopteri). There now
exist only a few representatives of this class, namely, the American
Mud-fish (Lepidosiren paradoxa) in the region of the river Amazon, and
the African Mud-fish (Protopterus annectens) in different parts of
Africa. A third large Salamander-fish (Ceratodus Fosteri) has lately
been discovered in Australia. During the dry season, that is in summer,
these strange animals bury themselves in a nest of leaves in the dry
mud, and then breathe air through lungs like the Amphibia. But during
the wet season, in winter, they live in rivers and bogs, and breathe
water through gills like fish. Externally, they resemble fish of the eel
kind, and are like them covered with scales; in many other
characteristics also—in their internal structure, their skeleton,
extremities, etc.—they resemble Fish more than Amphibia. But in certain
features they resemble the Amphibia, especially in the formation of
their lungs, nose, and heart. There is consequently an endless dispute
among zoologists, as to whether the Mud-fish are genuine Fish or
Amphibia. Distinguished zoologists have expressed themselves in favour
of both opinions. But in fact, owing to the complete blending of
characteristics which they present, they belong neither to the one nor
to the other class, and are probably most correctly dealt with as a
special class of Vertebrata, forming the transition between Fishes and
Amphibians. The still living Dipneusta are probably the last surviving
remains of a group which was formerly rich in forms, but has left no
fossil traces on account of the want of a solid skeleton. In this
respect, these animals are exactly like the Monorrhina and the
Leptocardia. However, teeth are found in the Trias which resemble those
of the living Ceratodus. Possibly the extinct Dipneusta of the
palæolithic period, which developed in the Devonian epoch out of
primæval fish, must be looked upon as the primary forms of the Amphibia,
and thus also of all higher Vertebrata. At all events the unknown forms
of transition—from Primæval fish to Amphibia—were probably very like the
Dipneusta.
A very peculiar class of Vertebrate animals, long since extinct, and
which appears to have lived only during the secondary epoch, is formed
by the remarkable _Sea-dragons_ (Halisauria, or Enaliosauria, also
called Nexipoda, or Swimming-footed animals). These formidable animals
of prey inhabited the mesolithic oceans in great numbers, and were of
most peculiar forms, sometimes from thirty to forty feet in length. From
many and excellently preserved fossil remains and impressions, both of
the entire body of Sea-dragons as well as of single parts, we have
become very accurately acquainted with the structure of their bodies.
They are usually classed among Reptiles, whilst some anatomists have
placed them in a much lower rank, as directly allied to Fish.
Gegenbaur’s recently published investigations, which place the structure
of their limbs in a true light, have led to the surprising conclusion
that the Sea-dragons form quite an isolated group, differing widely both
from Reptiles and Amphibia as well as from Fish. The skeleton of their
four legs, which are transformed into short, broad, paddling fins (like
those of fish and whales) furnishes us with a clear proof that the
Halisauria branched off from the main-stock of Vertebrata at an earlier
period than the Amphibia. For Amphibia, as well as the three higher
classes of Vertebrata, are all derived from a common primary form, which
possessed only _five_ toes or fingers on each leg. But the Sea-dragons
have (either distinctly developed or in a rudimentary condition as parts
of the skeleton of the foot) more than five fingers, as have also the
Selachians or Primæval fish. On the other hand, they breathed air
through lungs, like the Dipneusta, although they always swam about in
the sea. They, therefore, perhaps, in conjunction with the Dipneusta,
branched off from the Selachii, but did not develop into higher
Vertebrata; they form an extinct lateral line of the pedigree, which has
died out.
The more accurately known Sea-dragons are classed into three orders,
distinct enough one from the other, namely, _Primæval Dragons_, _Fish
Dragons_, and _Serpent Dragons_. The _Primæval Dragons_ (Simosauria) are
the oldest Sea-dragons, and lived only during the Trias period. The
skeletons of many different genera of them are met with in the German
limestone known as “Muschel-kalk.” They seem upon the whole to have been
very like the Plesiosauria, and are, consequently, sometimes united with
them into one order as Sauropterygia. The _Serpent Dragons_
(Plesiosauria) lived in the oolitic and chalk periods together with the
Ichthyosauria. They were characterised by an uncommonly long thin neck,
which was frequently longer than the whole body, and carried a small
head with a short snout. When their arched neck was raised they must
have looked very like a swan; but in place of wings and legs they had
two pairs of short, flat, oval-paddling fins.
The body of the _Fish Dragons_ (Ichthyosauria) was of an entirely
different form; these animals may be opposed to the two preceding orders
under the name of Fish-finners (Ichthyopterygia). They possessed a very
long extended body, like a fish, and a heavy head with an elongated,
flat snout, but a very short neck. Externally, they were probably very
like porpoises. Their tail was very long, whereas it was very short in
the members of the preceding orders. Also both pairs of paddling fins
are broader and show very different structure from that seen in the
other two orders. Probably the Fish Dragons and Serpent Dragons
developed as two diverging branches out of the Primæval Dragons; but it
is also possible that the Plesiosauria alone originated out of the
Simosauria, and that the Ichthyosauria were lower off-shoots from the
common stock. At all events, they must all be directly, or indirectly
derived from the Selachii, or Primæval fish.
The succeeding classes of Vertebrata, the _Amphibia_ and the _Amniota_
(Reptiles, Birds, and Mammals), owing to the characteristic structure
which they all exhibit of five toes to each foot, may all be derived
from a common primary form, which originated from the Selachii, and
which possessed five toes on each of its four limbs. When we find a less
number of toes than five, we can show that the missing ones must have
been lost in the course of time by adaptation. The oldest known
Vertebrata with five toes are the _Batrachias_ (Amphibia). We divide
this class into two sub-classes, namely, mailed Batrachians and naked
Batrachians, the first of which is distinguished by the body being
covered with bony plates or scales.
The first and elder sub-class of Amphibia consists of the _Mailed
Batrachians_ (Phractamphibia), the oldest land living Vertebrata of
which fossil remains exist. Well-preserved fossil remains of them occur
in the coal, especially of those with _Enamelled heads_ (Ganocephala),
which are most closely allied to fish, namely, the Archegosaurus of
Saarbruck, and the Dendrerpeton of North America. There then follow at a
later period the gigantic _Labyrinth-toothed animals_ (Labyrinthodonta),
which are represented in the Permian system by Zygosaurus, but at a
later period, more especially in the Trias, by Mastodonsaurus,
Trematosaurus, Capitosaurus, etc. The shape of these formidable
rapacious animals seems to have been between that of crocodiles,
salamanders, and frogs, but in their internal structure they were more
closely related to the two latter, while by their solid coat of mail,
formed of strong bony plates, they resembled the first animals. These
gigantic mailed Batrachians seem to have become extinct towards the end
of the Triassic period. No fossil remains of mailed Batrachia are known
during the whole of the subsequent periods. However, the still living
blind Snakes, or _Cæciliæ_ (Peromela)—small-scaled Phractamphibia of the
form and the same mode of life as the earth-worm—prove that this
sub-class continued to exist, and never became completely extinct.
The second sub-class of Amphibia, the _naked Batrachia_ (Lissamphibia),
probably originated even during the primary and secondary epochs,
although fossil remains of them are first found in the tertiary epoch.
They are distinguished from mailed Batrachia by possessing a naked
smooth, and slimy skin, entirely without scales or coat of mail. They
probably developed either out of a branch of the Phractamphibia, or out
of the same common root with them. The ontogeny of the three still
living orders of naked Batrachia—the gilled Batrachia, tailed
Batrachia, and frog Batrachia—distinctly repeats the historical course
of development of the whole sub-class. The oldest forms are the gilled
Batrachia (Sozobranchia), which retain throughout life the original
primary form of naked Batrachia, and possess a long tail, together with
water-breathing gills. They are most closely allied to the Dipneusta,
from which, however, they differ externally by the absence of the coat
of scales. Most gilled Batrachia live in North America: among others of
the class is the Axolotl, or Siredon, already mentioned. (Compare above,
vol. i. p. 241.) In Europe the order is only represented by one form,
the celebrated “Olm” (Proteus anguinus), which inhabits the grotto of
Adelsberg and other caves in Carinthia, and which, from living in the
dark, has acquired rudimentary eyes which can no longer see (vol. i. p.
13). The order of Tailed Batrachia (Sozura) have developed out of the
gilled Batrachia by the loss of external gills; the order includes our
black and yellow spotted land Salamander (Salamandra maculata), and our
nimble aquatic Salamanders (Tritons). Many of them—for instance, the
celebrated giant Salamanders in Japan (Cryptobranchus Japonicus)—still
retain the gill-slits, although the gills themselves have disappeared.
All of them, however, retain the tail throughout life. Tritons
occasionally—when forced to remain in water always—retain their gills,
and thus remain at the same stage of development as gilled Batrachia.
(Compare above, vol. i. p. 241.) The third order, the _tailless_ or
_frog-like Batrachia_ (Anura), during their metamorphosis, not only lose
their gills, with which in early life (as so-called tadpoles) they
breathe in water, but also the tail with which they swim about. During
their ontogeny, therefore, they pass through the course of development
of the whole sub-class, they being at first _Gilled Batrachia_, then
_Tailed Batrachia_, and finally _Frog-like Batrachia_. The inference
from this is evidently, that _Frog-like Batrachia_ developed at a later
period out of _Tailed Batrachia_, as the latter had developed out of
_Gilled Batrachia_ which originally existed alone.
In passing from the Amphibia to the next class of Vertebrata, namely,
Reptiles, we observe a very considerable advance in the progress of
organization. All the double-nostriled animals (Amphirrhina) up to this
time considered, and more especially the two larger classes of Fish and
Batrachia, agree in a number of important characteristics, which
essentially distinguish them from the three remaining classes of
Vertebrata—Reptiles, Birds, and Mammals. During the embryological
development of these latter, a peculiarly delicate covering, the _first
fœtal membrane_, or _amnion_, which commences at the navel, is formed
round the embryo; this membrane is filled with the amnion-water, and
encloses the embryo or germ in the form of a bladder. On account of this
very important and characteristic formation, we may comprise the three
most highly developed classes of Vertebrata under the term
_Amnion-animals_ (Amniota). The four classes of double-nostriled animals
which we have just considered, in which the amnion is wanting (as is the
case in all lower Vertebrate animals, single-nostriled and skull-less
animals), may on the other hand be opposed to the others as _amnion-less
animals_ (Anamnia).
The formation of the fœtal membrane, or amnion, which distinguishes
reptiles, birds, and mammals from all other Vertebrata, is evidently a
very important process in their ontogeny, and in the phylogeny which
corresponds with it. It coincides with a series of other processes,
which essentially determine the higher development of Amnionate animals.
The first of these important processes is the _total loss of gills_, for
which reason the Amniota, under the name of _Gill-less animals_
(Ebranchiata), were formerly opposed to all other Vertebrate animals
which breathed through gills (Branchiata). In all the Vertebrate already
discussed, we found that they either always breathed through gills, or
at least did so in early life, as in the case of Frogs and Salamanders.
On the other hand, we never meet with a Reptile, Bird, or Mammal which
at any period of its existence breathes through gills, and the
gill-arches and openings which do exist in the embryos, are, during the
course of the ontogeny, changed into entirely different structures,
viz., into parts of the jaw-apparatus and the organ of hearing. (Compare
above, vol. i. p. 307.) All Amnionate animals have a so-called cochlea
in the organ of hearing, and a “round window” corresponding with it.
These parts are wanting in the Amnion-less animals; moreover, their
skull lies in a straight line with the axis of the vertebral column. In
Amniotic animals the base of the skull appears bent in on the abdominal
side, so that the head sinks upon the breast. (Plate III. Fig. _C_, _D_,
_G_, _H_.) The organs of tears at the side of the eye also first develop
in the Amniota.
The question now is, When did this important advance take place in the
course of the organic history of the earth? When did the common ancestor
of all Amniota develop out of a branch of the Non-amniota, to wit, out
of the branch of the Amphibia?
To this question, the fossil remains of Vertebrata do not give us a very
definite, but still they do give an approximate, answer. For with the
exception of two lizard-like animals found in the Permian system (the
Proterosaurus and Rhopalodon), all the fossil remains of Amniota, as yet
known, belong to the _secondary_, _tertiary_, and _quaternary epochs_.
With regard to the two Vertebrata just named, it is still doubtful
whether they are genuine reptiles, or perhaps Amphibia of the salamander
kind. Their skeleton alone is known to us, and even this not perfectly.
Now as we know nothing of the characteristic features of their soft
parts, it is quite possible that the Proterosaurus and Rhopalodon were
non-amnionate animals more closely allied to Amphibia than to Reptiles;
possibly they belonged to the transition form between the two classes.
But, on the other hand, as undoubted fossil remains of Amniota have been
found as early as the Trias, it is probable that the _main class of
Amniota_ first developed in the Trias, that is, in the beginning of the
Mesolithic epoch. As we have already seen, this very period is evidently
one of the most important turning points in the organic history of the
earth. The palæolithic fern forests were then replaced by the pine
forests of the Trias period; important transformations then took place
in many of the classes of Invertebrata. Articulated marine lilies
(Colocrina) developed out of the plated ones (Phatnocrina.) The
Autechinidæ, or sea-urchins with only twenty rows of plates, took the
place of the palæolithic Palechinidæ, the sea-urchins with more than
twenty rows of plates. The Cystideæ, Blastoideæ, Trilobita, and other
characteristic groups of Invertebrata of the primary period became
extinct. It is no wonder that transforming conditions of adaptation
powerfully influenced the Vertebrate tribes also in the beginning of the
Trias period, and caused the origin of Amniotic animals.
If, however, the two Lizard and Salamander-like animals of the Permian
system, the Proterosaurus and Rhopalodon, are considered genuine
Reptiles, and consequently the most ancient Amniota, then the origin of
this main class must necessarily have taken place in the preceding
period, towards the end of the primary, namely, in the Permian period.
However, all other remains of Reptiles, which were formerly believed to
have been found in the Permian and the Coal system, or even in the
Devonian system, have been proved to be either not remains of Reptiles
at all, or to belong to a more recent date (for the most part to the
Trias). (Compare Plate XIV.)
The common hypothetical primary form of all Amniotic animals, which we
may call _Protamnion_, and which was possibly nearly related to the
Proterosaurus, very probably stood upon the whole midway between
salamanders and lizards, in regard to its bodily formation. Its
descendants divided at an early period into two different lines, one of
which became the common primary form of Reptiles and Birds, the other
the primary form of Mammals.
Of all the three classes of Amniota, _Reptiles_ (Reptilia, or Pholidota,
also called Sauria in the widest sense), remain at the lowest stage of
development, and differ least from their ancestors, the Amphibia. Hence
they were formerly universally included among them, although their whole
organization is much more like that of Birds than Amphibia. There now
exist only four orders of Reptiles, namely,—Lizards, Serpents,
Crocodiles, and Tortoises. They, however, form but a poor remnant of the
exceedingly various and highly developed host of Reptiles which lived
during the Mesolithic, or Secondary epoch, and predominated over all
other Vertebrata. The immense development of Reptiles during the
Secondary epoch is so characteristic that we could as well name it after
those animals as after the Gymnosperms (p. 111). Twelve of the
twenty-seven sub-orders, given on the accompanying table, and four of
the eight orders, belong exclusively to the secondary period. These
mesolithic groups are marked by an asterisk. All the orders, with the
exception of Serpents, are found fossilized even in the Jura and Trias
periods.
[Illustration: _Pl. XIV._
[Horizontal axis:]
Branches,
Classes,
and Sub-Classes,
of the Vertebrate
Stem.
Prochordata
Evertebrate
Forefathers
of the
Vertebrate
Skull-less
(Acrania)
or
Tube-hearted
Fish,
(Leptocardia)
Single
nostrilled
(Monorrhina)
or
Round-mouthed
Fishes
(Cyclostoma)
Anamnia { Paired-nostrilled forms or Amphirrhina
{ with gills, without Amnion.
Fish, Pisces
Primeval
Fish,
Selachii.
Enamelled
Fish,
Ganoides.
Bony
Fish,
Teleostei.
Mud
Fish,
Dipneusta.
Sea-Dragons,
Halisauria.
Frogs and
Newts,
Amphibia.
Amniota { Paired nostrilled or Amphirrhina
{ with Amnion, without gills.
Reptiles, Reptilia.
Primæval
Reptiles,
Tocosauria.
Lizards,
Lacertilia.
Snakes,
Ophidia.
Crocodiles,
Crocodilia.
Tortoises,
Chelonia.
Flying
Reptiles,
Pterosauria.
Dragons,
Dinosauria.
Billed
Reptiles,
Anomodontia.
Birds,
Aves.
Suckling animals, Mammalia.
Billed
Animals,
Monotrema.
Pouched
Animals,
Marsupialia.
Placental
Animals,
Placentalia.
[Vertical axis:]
Cenolithic or
Tertiary Epoch.
Pliocene
Age.
Miocene
Age.
Eocene
Age.
Mesolithic or
Secondary Epoch.
Chalk
Period.
Jurassic
Period.
Triassic
Period.
Palæolithic or
Primary Epoch.
Permian
Period.
Coal
Period.
Devonian
Period.
Archilithic or
Primordial Epoch.
Silurian
Period.
Cambrian
Period.
Laurentian
Period.
[Legend:]
Single or
MONOPHYLETIC PEDIGREE
of the Stem of the
BACK-BONED ANIMALS
Based on Palæontology.
Relative lengths of the 5
Epochs in per centages.
Quarternary Epoch 0.5
Tertiary Epoch 2.3
Secondary Epoch 11.5
Primary Epoch 32.1
Primordial Epoch 53.6
------
_Total_ 100.0]
In the first order, that of _Primary Reptiles_, or _Primary Creepers_
(Tocosauria), we class the extinct _Thecodontia_ of the Trias, together
with those Reptiles which we may look upon as the common primary form of
the whole class. To the latter, which we may call _Primæval Reptiles_
(Proreptilia), the Proterosaurus of the Permian system very probably
belongs. The seven remaining orders must be considered as diverging
branches, which have developed in different directions out of that
common primary form. The Thecodontia of the Trias, the only positively
known fossil forms of Tocosauria, were Lizards which seem to have been
like the still living monitor lizards (Monitor, Varanus).
SYSTEMATIC SURVEY
_Of the 8 Orders and 27 Sub-orders of Reptiles._
(Those groups marked with * became extinct even during the Secondary
Period.)
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| _Sub-orders_ | _Systematic Name_ | _A Generic Name_
_Orders_ | _of_ | _of the_ | _as_
_of Reptiles._ | _Reptiles._ | _Sub-orders._ | _an example._
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I. =Primary= { 1. Primæval reptiles 1. Proreptilia * (Proterosaurus?)
=Reptiles= {
+Tocosauria+ { 2. 2. Thecodontia * Palæosaurus
{ 3. Cleft-tongued 3. Fissilingues Monitor
{ 4. Thick-tongued 4. Crassilingues Iguana
II. =Lizards= { 5. Short-tongued 5. Brevilingues Anguis
+Lacertilia+ { 6. Ringed lizards 6. Glyptodermata Amphisbæna
{ 7. Chameleons 7. Vermilingues Chamæleo
{ 8. Adders 8. Aglyphodonta Coluber
{ 9. Tree serpents 9. Opisthoglypha Dipsas
III. =Serpents= { 10. 10. Proteroglypha Hydrophis
+Ophidia+ { 11. Vipers 11. Solenoglypha Vipera
{ 12. Worm serpents 12. Opoterodonta Typhlops
{ 13. Amphicœla 13. Teleosauria * Teleosaurus
IV. =Crocodiles= { 14. Opisthocœla 14. Steneosauria * Steneosaurus
+Crocodilia+ { 15. Prosthocœla 15. Alligatores Alligator
{ 16. Sea tortoises 16. Thalassita Chelone
V. =Tortoises= { 17. River tortoises 17. Potamita Trionyx
+Chelonia+ { 18. Marsh tortoises 18. Elodita Emys
{ 19. Land tortoises 19. Chersita Testudo
{ 20. Long-tailed 20. Rhamphorhynchi * Rhamphorhynchus
VI. =Flying= { Flying lizards
=Reptiles= { 21. Short-tailed 21. Pterodactyli * Pterodactylus
+Pterosauria+* { Flying lizards
{ 22. Giant dragons 22. Harpagosauria * Megalosaurus
VII. =Dragons= { 23. Elephantine 23. Therosauria * Iguanodon
+Dinosauria+* { dragons
{ 24. Dog-toothed 24. Cynodontia * Dicynodon
VIII. =Beaked= { 25. Toothless 25. Cryptodontia * Udenodon
=Reptiles= { 26. Kangaroo reptiles 26. Hypsosauria * Compsognathus
+Anomodontia+* {
{ 27. Bird reptiles 27. Tocornithes * (Tocornis)
Of the four orders of reptiles now existing, and which, moreover, have
alone represented the class since the beginning of the tertiary epoch,
that of _Lizards_ (Lacertilia) is probably most closely allied to the
extinct Primary Reptiles, and especially through the monitors already
named. The class of _Serpents_ (Ophidia) developed out of a branch of
the order of lizards, and this probably not until the beginning of the
tertiary epoch. At least we at present only know of fossil remains of
serpents from the tertiary strata. _Crocodiles_ (Crocodilia) existed
much earlier; the Teleosauria and Steneosauria belonging to the class
are found fossil in large quantities even in the Jura; but the still
living alligators are first met with in a fossil state in the chalk and
tertiary strata. The most isolated of the four existing orders of
reptiles consists of the remarkable group of _Tortoises_ (Chelonia);
fossils of these strange animals are first met with in the Jura. In some
characteristics they are allied to Amphibia, in others, to Crocodiles,
and by certain peculiarities even to Birds, so that their true position
in the pedigree of Reptiles is probably far down at the root. The
extraordinary resemblance of their embryos to Birds, manifested even at
later stages of the ontogenesis, is exceedingly striking.
The four extinct orders of Reptiles show among one another, and, with
the four existing orders just mentioned, such various and complicated
relationships, that in the present state of our knowledge we are obliged
to give up the attempt at establishing their pedigree. The most
deviating and most curious forms are the _Flying Reptiles_
(Pterosauria); flying lizards, in which the extremely elongated fifth
finger of the hand served to support an enormous flying membrane. They
probably flew about, in the secondary period, much in the same way as
the bats of the present day. The smallest flying lizards were about the
size of a sparrow; the largest, however, with a breadth of wing of more
than sixteen feet, exceeded the largest of our living flying birds in
stretch of wing (condor and albatross). Numerous fossil remains of them,
of the long-tailed Rhamphorhynchia and of the short-tailed Pterodactylæ
are found in all the strata of the Jura and Chalk periods, but in these
only.
Not less remarkable and characteristic of the Mesolithic epoch was the
group of _Dragons_ (Dinosauria, or Pachypoda). These colossal reptiles,
which attained a length of more than fifty feet, are the largest
inhabitants of the land which have ever existed on our globe; they lived
exclusively in the secondary epoch. Most of their remains are found in
the lower cretaceous system, more especially in the Wealden formations
of England. The majority of them were fearful beasts of prey (the
Megalosaurus from twenty to thirty, the Pelorosaurus from forty to fifty
feet in length). The Iguanodon, however, and some others lived on
vegetable food, and probably played a part in the forests of the chalk
period similar to that of the unwieldy but smaller elephants,
hippopotami, and rhinoceroses of the present day.
The _Beaked Reptiles_ (Anomodontia), likewise also long since extinct,
but of which very many remarkable remains are found in the Trias and
Jura, were perhaps closely related to the Dragons. Their jaws, like
those of most Flying Reptiles and Tortoises, had become changed into a
beak, which either possessed only degenerated rudimentary teeth, or no
teeth at all. In this order, if not in the preceding one, we must look
for the primary parents of the bird class, which we may call Bird
Reptiles (Tocornithes). Probably very closely related to them was the
curious, kangaroo-like Compsognathus from the Jura, which in very
important characteristics already shows an approximation to the
structure of birds.
The class of _Birds_ (Aves), as already remarked, is so closely allied
to Reptiles in internal structure and by embryonal development, that
they undoubtedly originated out of a branch of this class. Even a glance
at Plates II. and III. will show that the embryos of birds at a time
when they already essentially differ from the embryos of Mammals, are
still scarcely distinguishable from those of Tortoises and other
Reptiles. The cleavage of the yolk is partial in the case of Birds and
Reptiles, in Mammals it is total. The red blood-cells of the former
possess a kernel, those of the latter do not. The hair of Mammals
develops in closed follicles in the skin, but the feathers of birds and
also the scales of reptiles develop in hillocks on the skin. The lower
jaw of the latter is much more complicated than that of Mammals; the
latter do not possess the quadrate bone of the former. Whereas in
Mammals (as in the case of Amphibia) the connection between the skull
and the first neck vertebra is formed by two knobbed joints, or
condyles, in Birds and Reptiles those have become united into a single
condyle. The two last classes may therefore justly be united into one
group as Monocondylia, and contrasted to Mammals, or Dicondylia.
The deviation of Birds from Reptiles, in any case, first took place in
the mesolithic epoch, and this moreover probably during the Trias. The
oldest fossil remains of birds are found in the upper Jura
(Archæopteryx). But there existed, even in the Trias period, different
Saurians (Anomodonta) which in many respects seem to form the transition
from the Tocosauria to the primary ancestors of Birds, the hypothetical
Tocornithes. Probably these Tocornithes were scarcely distinguishable
from other beaked lizards in the system, and were closely related to the
kangaroo-like Compsognathus from the Jura of Solenhofen. Huxley classes
the latter with the Dinosauria, and believes them to be the nearest
relations to the Tocornithes.
The great majority of Birds—in spite of all the variety in the colouring
of their beautiful feathery dress, and in the formation of their beaks
and feet—are of an exceedingly uniform organization, in much the same
way as are the class of insects. The bird form has adapted itself on all
sides to the external conditions of existence, without having thereby in
any way essentially deviated from the strict hereditary type of its
characteristic structure. There are only two small groups, the
feather-tailed birds (Saururæ) and those of the ostrich kind, which
differ considerably from the usual type of bird, namely, from those with
keel-shaped breasts (Carinatæ), and hence the whole class may be divided
into three sub-classes.
The first sub-class, the _Reptile-tailed_, or _Feather-tailed Birds_
(Saururæ), are as yet known only through a single, and that an
imperfect, fossil impression, which, however, in being the oldest and
also a very peculiar fossil bird, is of great importance. This fossil is
the Primæval Griffin, or Archæopteryx lithographica, of which as yet
only one specimen has been found in the lithographic slate at
Solenhofen, in the Upper Jura system of Bavaria. This remarkable bird
seems on the whole to have been of the size and form of a large raven,
especially as regards the legs, which are in a good state of
preservation; head and breast unfortunately are wanting. The formation
of the wings deviates somewhat from that of other birds, but that of the
tail still more so. In all other birds the tail is very short and
composed of but few short vertebræ; the last of these have grown
together into a thin, bony plate standing perpendicularly, upon which
the rudder-feathers of the tail are attached in the form of a fan. The
Archæopteryx, however, has a long tail like a lizard, composed of
numerous (20) long thin vertebræ, and on every vertebra are attached
the strong rudder-feathers in twos, so that the whole tail appears
regularly feathered. This same formation of the tail part of the
vertebral column occurs transiently in the embryos of other birds, so
that the tail of the Archæopteryx evidently represents the original form
of bird-tail inherited from reptiles. Large numbers of similar birds
with lizard-tails probably lived during the middle of the secondary
period; accident has as yet, however, only revealed this one fossil.
The _Fan-tailed_, or _Keel-breasted birds_ (Carinatæ), which form the
second sub-class, comprise all living Birds of the present day, with the
exception of those of the ostrich kind, or Ratitæ. They probably
developed out of Feather-tailed Birds during the first half of the
secondary period, namely, in the Jura or Chalk period, by the hinder
tail vertebræ growing together, and by the tail becoming shortened. Only
very few remains of them are known from the secondary period, and these
moreover only out of the last section of it, namely, from the Chalk.
These remains belong to a swimming bird of the albatross species, and a
wading bird like a snipe. All the other fossil remains of birds as yet
known have been found in the tertiary strata.
The _Bushy-tailed_, or _Ostrich-like Birds_ (Ratitæ), also called
_Running Birds_ (Cursores), the third and last sub-class, is now
represented only by a few living species, by the African ostrich with
two toes, the American and Australian ostrich with three toes, by the
Indian cassowary and the four-toed kiwi, or Apteryx, in New Zealand. The
extinct giant birds of Madagascar (Æpyornis) and the New Zealand
Dinornis, which were much larger than the still living ostriches, also
belong to this group. The Birds of the ostrich kind—by giving up the
habit of flying, by the degeneration of the muscles for flying resulting
from this, and of the breast bone which serves as their support, and by
the corresponding stronger development of the hinder legs for
running—have probably arisen out of a branch of the Keel-breasted birds.
But possibly, as Huxley thinks, they may be the nearest relations of the
Dinosauria and of the Reptiles akin to them, especially of the
Compsognathus; at all events, the common primary form of all Birds must
be looked for among the extinct Reptiles.