The pedigree on p. 177 will show how we may at present approximately conceive the derivation of the twenty orders of Crustacea enumerated on p. 176, from the common primary form of the Nauplius. Out of the Nauplius form-which originally existed as an independent genus-the five legions of lower Crabs developed as diverging branches in different directions, which in the systematic survey of the cla.s.s are united as Segmented Crabs (Entomostraca). The higher division of Mailed Crabs (Malacostraca) have likewise originated out of the common Nauplius form.

The Nebalia is still a direct form of transition from the Phyllopods to the Schizopods, that is, to the primary form of the stalk-eyed and sessile-eyed Mailed Crabs. The Nauplius at this stage gives rise to another larva form, the so-called Zoea, which is of great importance.

The order of Schizopoda, those with cloven feet (Mysis, etc.), probably originated from this curious Zoea; they are at present still directly allied, through the Nebalia to the Phyllopoda, those with foliaceous feet. But of all living crabs the Phyllopods are the most closely allied to the original primary form of the Nauplius. Out of the Schizopoda the stalk-eyed and sessile-eyed Mailed Crabs, or Malacostraca, developed as two diverging branches in different directions: the former through shrimps (Peneus, etc.), the latter through the c.u.macea (c.u.ma, etc.), which are still living and closely allied to the Schizopoda. Among those with stalked eyes is the river crab (cray-fish), the lobster, and the others with long tails, or the Macrura, out of which, in the chalk period, the short-tailed crabs, or Brachyura, developed by the degeneration of the tail. Those with sessile eyes divide into the two branches of Flea-crabs (Amphipoda) and Louse-crabs (Isopoda); among the latter are our common Rock-slaters and Wood-lice.

The second main-cla.s.s of Articulated animals, that of the Tracheata, or air-breathing Tracheate Insects[4] (Spiders, Centipedes, and Flies) did not develop until the beginning of the palaeolithic era, after the close of the archilithic period, because all these animals (in contrast with the aquatic crabs) are originally inhabitants of land. It is evident that the Tracheata can have developed only after the lapse of the Silurian period when terrestrial life first began. But as fossil remains of spiders and insects have been found, even in the carboniferous beds, we can pretty accurately determine the time of their origin. The development of the first Tracheate Insects out of gill-bearing Zoea-crabs, must have taken place between the end of the Silurian and the beginning of the coal period, that is, in the Devonian period.

Gegenbaur, in his excellent "Outlines of Comparative Anatomy,"(21) has lately endeavoured to explain the origin of the Tracheata by an ingenious hypothesis. The system of tracheae, or air pipes, and the modifications of organization dependent upon it, distinguish Flies, Centipedes, and Spiders so much from other animals, that the conception of its first origin presents no inconsiderable difficulties to phylogeny. According to Gegenbaur, of all living Tracheate Insects, the Primaeval Flies, or Archiptera, are most closely allied to the common primary form of the Tracheata. These insects-among which we may especially mention the delicate Day flies (Ephemera), and the agile dragon-flies (Libellula)-in their earliest youth, as larvae, frequently possess _external tracheate gills_ which lie in two rows on the back of the body, and are shaped like a leaf or paint-brush. Similar leaf or paint-brush shaped organs are met with as real water-breathing organs or gills, in many crabs and ringed worms, and, moreover, in the latter as real dorsal appendages or limbs. The "tracheate gills," found in the larvae of many primaeval winged insects, must in all probability be explained as "_dorsal limbs_," and as having developed out of the corresponding appendages of the Annelida, or possibly as having really arisen out of similar parts in Crustacea long since extinct. The present tracheal respiration of the Tracheata developed at a later period out of respiration through the "tracheate gills." The tracheate gills themselves, however, have in some cases disappeared, and in others become transformed into the _wings_ of the Flies. They have disappeared entirely in the cla.s.ses of Spiders and Centipedes, and these groups must accordingly be conceived of as degenerated or peculiarly developed lateral branches of the Fly cla.s.s, which at an early period branched off from the common primary form of Flies; Spiders probably did so at an earlier period than Centipedes. Whether that common primary form of all Tracheata, which in my General Morphology I have named Protracheata, did develop directly out of genuine Ringed worms, or at first out of Crustacea of the Zoea form (Zoepoda, p. 177) will probably be settled at some future time by a more accurate knowledge and comparison of the ontogeny of the Tracheata, Crustacea, and Annelida. However, the root of the Tracheata, as well as that of the Crustacea, must in any case be looked for in the group of Ringed worms.

The genuine Spiders (Arachnida) are distinguished from Flies by the absence of wings, and by four pairs of legs; but, as is distinctly seen in the Scorpion-spiders and Tarantulae, they, like Flies, possess in reality only three pairs of genuine legs. The apparent "fourth pair of legs" in spiders (the foremost) are in reality a pair of feelers. Among the still existing Spiders, there is a small group which is probably very closely allied to the common primary form of the whole cla.s.s; this is the order of Scorpion-spiders, or Solifugae, (Solpuga, Galeodes), of which several large species live in Africa and Asia, and are dreaded on account of their poisonous bite. Their body consists-as we suppose to have been the case in the common ancestor of the Tracheata-of a head possessing several pairs of feelers like legs, of a thorax, to the three rings of which are attached three pairs of legs, and of a hinder body, or abdomen, consisting of many distinct rings. In the articulation of their body, the Solifugae are therefore in reality more closely related to flies than to other spiders. Out of the Devonian Primaeval Spiders, which were nearly related to the Solifugae of the present day, the Long Spiders, the Tailor Spiders, and the Round Spiders probably developed as three diverging branches.

The _Long Spiders_ (Arthrogastres), in which the earlier articulation of body has been better preserved than in Round Spiders, appear to be the older and more original forms. The most important members of this sub-cla.s.s are the scorpions, which are connected with the Solifugae through the Tarantella (or Phrynidae). The small book scorpions, which inhabit our libraries and herbariums, appear as a degenerate lateral branch from the true scorpions. Midway between the Scorpions and Round Spiders are the long-legged Tailor-spiders (Opiliones) which have possibly arisen out of a special branch of the Solifugae. The Pycnogonida, or No-body Crabs, and the Arctisca, or Bear Worms-still generally included among Long Spiders-must be completely excluded from the cla.s.s of Spiders; the former belong to the Crustacea, the latter to Ringed worms.

SYSTEMATIC SURVEY

_Of the 3 Cla.s.ses and 17 Orders of the Tracheata._

------------------+---------------------+----------------------+---------------------- _Cla.s.ses of the_ | _Sub-Cla.s.ses of the_| _Order of the_ | _Two Names of_ _Tracheata._ | _Tracheata._ | _Tracheata._ |_Genera as examples._ ------------------+---------------------+----------------------+---------------------- { { 1. Scorpion spiders { Solpuga { { _Solifugae_ { Galeodes { { { { 2. Tarantella { Phrynus { { _Phrynida_ { Thelyphonus { I. { I. { Long spiders { 3. Scorpions { Scorpio =Spiders= { _Arthrogastres_ { _Scorpioda_ { Buthus { { +Arachnida+ { { 4. Book scorpions { Obisium { { _Pseudoscorpioda_ { Chelifer { { { { 5. Tailor spiders { Phalangium { { _Opilionida_ { Opilio { { { II. { 6. Spinning spiders { Epeira { Round spiders { _Araneae_ { Mygale { { { { _Sphaerogastres_ { 7. Mites { Sarcoptes { { _Acarida_ { Demodex

II. { III.

=Centipedes= { Simple-footed { 8. Simple-footed { Scolopendra { _Chilopoda_ { _Chilopoda_ { Geophilus { { +Scolopendria+ { IV. { 9. Double-footed { Julus or { Double-footed { _Diplopoda_ { Polydesmus +Myriapoda+ { _Diplopoda_ {

{ { 10. Primitive flies { Ephemera { { _Archiptera_ { Libellula { { { { 11. Gauze-wings { Hemerobius { { _Neuroptera_ { Phryganea { V. { { Chewing { 12. Straight-wings { Locusta { _Masticantia_ { _Orthoptera_ { Forficula { { III. { { 13. Beetles { Cicindela =Flies= { { _Coleoptera_ { Melolontha { { +Hexapoda+ { { 14. Bee-wings { Apis { { _Hymenoptera_ { Formica { { { { 15. Bugs { Aphis { VI. { _Hemiptera_ { Cimex { Sucking { { _Sugentia_ { 16. Two-wings { Culex { { _Diptera_ { Musca { { { { 17. b.u.t.terflies { Bombyx

b.u.t.terflies _Lepidoptera_ | Bees | Two-wings _Hymenoptera_ | _Diptera_ | | | | | | Beetles | | | Bugs _Coleoptera_ -------v-----/ | _Hemiptera_ | | | | | Gauze wings | | Straight-wings _Neuroptera_ -------v------/ _Orthoptera_ | | | | | | | | ----------------------v-----------------------------/

Primaeval Flies _Archiptera_ | Scorpions | Double-footed _Scorpioda_ | _Diplopoda_ Tailor Spiders | | | _Opiliones_ | | | | Book Scorpions | | | Mites | _Pseudoscorpioda_ | | | _Acarida_ | | | | | | | -----v-----/ | | | | | | | | | Tarantella | | Weaving Spiders | _Phrynida_ | Simple-footed _Araneae_ | | | _Chilopoda_ | | | | =Centipedes= | | | | +Myriapoda+ -------------v--------/ | | | | | Scorpion Spiders | | _Solifugae_ | | =Spiders.= | | +Arachnida+ | | | | | | =Flies= | | +Insecta Hexapoda+ | | | | | | | --------------------v---------------------/ | Primary Air-breathing Arthropods _Protracheata_ | | Articulated Worms _Coelminthes_

Fossil remains of Long Spiders are found in the Coal. The second sub-cla.s.s of the Arachnida, the _Round Spiders_ (Sphaerogastres), first appear in the fossil state in the Jura, that is, at a very much later period. They have developed out of a branch of the Solifuga, by the rings of the body becoming more and more united with one another. In the true _Spinning Spiders_ (Araneae), which we admire on account of their delicate skill in weaving, the union of the joints of the trunk, or metamera, goes so far, that the trunk now consists of only two pieces, of a head-breast (cephalo-thorax) with jaws, feelers, and four pairs of legs, and of a hinder body without appendages, where the spinning warts are placed. In _Mites_ (Acarida), which have probably arisen by degeneration (especially by parasitism) out of a lateral branch of Spinning Spiders, even these two trunk pieces have become united and now form an unsegmented ma.s.s.

The cla.s.s of _Scolopendria_, _Myriapoda_, or Centipedes, the smallest and poorest in forms of the four cla.s.ses of Arthropoda, is characterized by a very elongated body, like that of a segmented Ringed worm, and often possesses more than a hundred pairs of legs. But these animals also originally developed out of a six-legged form of Tracheata, as is distinctly proved by the individual development of the millipede in the egg. Their embryos have at first only three pairs of legs, like genuine insects, and only at a later period do the posterior pairs of legs bud, one by one, from the growing rings of the hinder body. Of the two orders of Centipedes (which in our country live under barks of trees, in moss, etc.) the round, _double-footed_ ones (Diplopoda) probably did not develop until a later period out of the older flat, _single-footed_ ones (Chilopoda), by successive pairs of rings of the body uniting together.

Fossil remains of the Chilopoda are first met with in the Jura period.

The third and last cla.s.s of the Arthropoda breathing through tracheae, is that of the _Flies_, or _Insects_, in the narrow sense of the word (Insecta, or Hexapoda), the largest of all cla.s.ses of animals, and next to that of Mammalia, also the most important. Although Flies develop a greater variety of genera and species than all other animals taken together, yet these are all in reality only superficial variations of a single type, which is entirely and constantly preserved in its essential characteristics. In all Flies the three divisions of the trunk-head, breast (thorax), and hinder body are quite distinct. The _hinder body_, or _abdomen_, as in the case of spiders, has no articulated appendages.

The central division, the _breast_ or _thorax_, has on its ventral side three pairs of legs, on its back _two pairs of wings_. It is true that, in very many Flies, one or both pairs of wings have become reduced in size or have even entirely disappeared; but the comparative anatomy of Flies distinctly shows that this deficiency has arisen only gradually by the degeneration of the wings, and that all the Flies existing at present are derived from a common, primary Fly, which possessed three pairs of legs and two pairs of wings. (Compare p. 256.) These wings, which so strikingly distinguish Flies from all other Arthropoda, probably arose, as has been already shown, out of the tracheate gills which may still be observed in the larvae of the ephemeral flies (Ephemera) which live in water.

The head of Flies universally possesses, besides the eyes, a pair of articulated feelers, or antennae, and also three jaws upon each side of the mouth. These _three pairs of jaws_, although they have arisen in all Flies from the same original basis, by different kinds of adaptation, have become changed to very varied and remarkable forms in the various orders, and are therefore employed for distinguishing and characterizing the main divisions of the cla.s.s. In the first place, we may distinguish two main divisions, namely, Flies with _chewing_ mandibles (Masticantia) and Flies with _sucking_ mouths (Sugentia). On a closer examination each of these two divisions may again be divided into two sub-groups. Among chewing Flies, or Masticantia, we may distinguish the biting and the licking ones. _Biting flies_ (Mordentia) comprise the most ancient and primaeval winged Flies, the gauzy-winged (Neuroptera), straight-winged (Orthoptera), and beetles (Coleoptera). _Licking flies_ (Lambentia) are represented by the one order of skin-winged (Hymenoptera) Flies. We distinguish two groups of _Sucking Flies_, or Sugentia, namely, those which p.r.i.c.k and those which sip. There are two orders of p.r.i.c.king Flies (Pungentia), those with half wings (Hemiptera) and gnats and blow-flies (Diptera); b.u.t.terflies are the only _sipping_ Flies (Sorbentia), Lepidoptera.

Biting Flies, and indeed the order of _Primaeval Flies_ (Archiptera, or Pseudoneuroptera) are nearest akin to the still living Flies, and include the most ancient of all Flies, the primary forms of the whole cla.s.s (hence also those of all Tracheata). Among them are, first of all, the Ephemeral Flies (Ephemera) whose larvae which live in water, in all probability still show us in their tracheae-gills the organs out of which the wings of Flies were originally developed. This order further contains the well known dragon-flies, or Libellula, the wine-gla.s.s sugar mites (Lepisma), the hopping Flies with bladder-like feet (Physopoda), and the dreaded Termites, fossil remains of which are found even in coal. The order of Gauze-winged Flies (Neuroptera), probably developed directly out of the primaeval Flies, which differ from them only by their perfect series of transformations. Among them are the gauze-flies (Planipennia), caddis-flies (Phryganida), and fan-flies (Strepsiptera).

Fossil Flies, which form the transition from the primaeval Flies (Libellula) to the gauze-winged (Sialidae), are found even in coal (Dictyophylebia).

The order of _Straight-winged Flies_ (Orthoptera) developed at an early period out of another branch of the primaeval Flies by differentiation of the two pairs of wings. This division is composed of one group with a great variety of forms-c.o.c.kroaches, gra.s.shoppers, crickets, etc.

(Ulonata)-and of a smaller group consisting only of the well-known earwigs (Labidura), which are characterised by nippers at the hinder end of their bodies. Fossil remains of c.o.c.kroaches, as well as of crickets and gra.s.shoppers, have been found in coal.

Fossil remains of the fourth order of Biting Flies, _beetles_ (Coleoptera) likewise occur in coal. This extremely comprehensive order-the favourite one of amateurs and collectors-shows more clearly than any other what infinite variety of forms can be developed externally by adaptation to different conditions of life, without the internal structure and the original form of the body being in any way essentially changed. Beetles have probably developed out of a branch of the straight-winged Flies, from which they differ only in their transformations (larva, pupa, etc.).

The one order of _Licking Flies_, namely, the interesting group of the _Bees_, or _Skin-winged Flies_ (Hymenoptera), is closely allied to the four orders of biting Flies. Among them are those Flies which have risen to such an astonishing degree of mental development, of intellectual perfection, and strength of character, by their extensive division of labour, formation of communities and states, and surpa.s.s in this not merely most invertebrate animals, but even most animals in general. This may be said especially of all ants and bees, also of wasps, leaf-wasps, wood-wasps, gall-wasps, etc. They are first met with in a fossil state in the oolites, but they do not appear in greater numbers until the tertiary period. Probably these insects developed either out of a branch of the primaeval Flies or the gauze-winged Flies.

Of the two orders of _p.r.i.c.king Flies_ (Hemiptera and Diptera), that containing the _Half-winged Flies_ (Hemiptera), also called Beaked Flies (Rhynchota), is the older of the two. It includes three sub-orders, viz., the leaf-lice (h.o.m.optera), the bugs (Heteroptera), and lice (Pediculina). Fossil remains of the first two cla.s.ses are found in the oolites; but an ancient Fly (Eugereon) is found in the Permian system, and seems to indicate the derivation of the Hemiptera from the Neuroptera. Probably the most ancient of the three sub-orders of the Hemiptera are the h.o.m.optera, among which, besides the actual leaf-lice, are the shield-lice, leaf-fleas, and leaf-crickets, or Cicadae. Lice have probably developed out of two different branches of h.o.m.optera, by continued degeneration (especially by the loss of wings); bugs, on the other hand, by the perfecting and differentiation of the two pairs of wings.

The second order of _p.r.i.c.king flies_, namely, the _Two-winged Flies_ (Diptera), are also found in a fossil state in the oolites, together with Half-winged Flies; but they probably developed out of the Hemiptera by the degeneration of the hind wings. In Diptera the fore wings alone have remained perfect. The princ.i.p.al portion of this order consists of the elongated gnats (Nemocera) and of the compact blow-flies and house-flies (Brachycera), the former of which are probably the older of the two. However, remains of both are found in the oolitic period. The two small groups of lice-flies (Pupipara) forming chrysales, and the hopping-fleas (Aphaniptera), probably developed out of the Diptera by degeneration resulting from parasitism.

The eighth and last order of Flies, and at the same time the only one with mouth-parts adapted to sipping liquids, consists of _moths_ and _b.u.t.terflies_ (Lepidoptera). This order appears, in several morphological respects, to be the most perfect cla.s.s of Flies, and accordingly was the last to develop. For we only know of fossil remains of this order from the tertiary period, whereas the three preceding orders extend back to the oolites, and the four biting orders even to the coal period. The close relationship between some moths (Tineae) and (Noctuae), and some caddis-flies (Phryganida) renders it probable that b.u.t.terflies have developed from this group, that is, out of the order of Gauze-winged Flies, or Neuroptera.

The whole history of Flies, and, moreover, the history of the whole tribe of Arthropoda, essentially confirms the great laws of differentiation and perfecting which, according to Darwin's theory of selection, must be considered as the necessary results of Natural Selection. The whole tribe, so rich in forms, begins in the Archilithic period with the cla.s.s of _Crabs_ breathing by gills, and with the lowest _Primaeval Crabs_, or Archicaridae. The form of these Primaeval Crabs, which were developed out of segmented worms, is still approximately preserved by the remarkable _Nauplius_, in the common larval stage of so many Crabs. Out of the Nauplius, at a later period, the curious Zoea was developed, which is the common larval form of all the higher or mailed crabs (Malacostraca), and, at the same time, possibly of that Arthopod which at first breathed through tracheae, and became the common ancestor of all _Tracheata_. This Devonian ancestor, which must have originated between the end of the Silurian and the beginning of the Coal period, was probably most closely related to the still living Primaeval Flies, or _Archiptera_. Out of these there developed, as the main tribe of the Tracheata, the cla.s.s of Flies, from the lowest stage of which the _spiders_ and _centipedes_ separated as two diverging branches.

Throughout a long period there existed only the four biting orders of Flies-the Primaeval flies, Gauze-wings, Straight-wings, and the Beetles, the first of which is probably the common primary form of the three others. It was only at a much later period that the Licking, p.r.i.c.king, and Sipping flies developed out of the Biting ones, which retained the original form of the three pairs of jaws most distinctly. The following table will show once more how these orders succeeded one another in the history of the earth.

CLa.s.sIFICATION OF FLIES.

{ { 1. Primaeval winged { M.I. } { { _Archiptera_ { A.A. } { { } { I. { 2. Gauze-winged { M.C. } A. { Biting Flies { _Neuroptera_ { A.A. } { { } =Flies= { _Mordentia_ { 3. Straight-winged { M.I. } =with Chewing= { { _Orthoptera_ { A.D. } =Mouths= { { } { { 4. Beetles { M.C. } +Masticantia+ { { _Coleoptera_ { A.D. } { { { II. { 5. Skin-winged { M.C. } { Licking Flies { _Hymenoptera_ { A.A. } { _Lambentia_ { } } } { { 6. Half-winged { M.I. } { III. { _Hemiptera_ { A.A. } B. { Stinging Flies { } =Flies= { { 7. Tway-flies { M.C. } =with Sucking= { _Pungentia_ { _Diptera_ { A.D. } =Mouths= { { +Sugentia+ { IV. { 8. b.u.t.terflies { M.C. } { Sipping Flies { _Lepidoptera_ { A.A. } { { { } { _Sorbentia_ { { }

_Note._-The difference in the metamorphosis or transformation and in the development of the wings of the eight individual orders of Flies is also specified by the following letters: M.I. = Imperfect Metamorphosis. M.C. = Perfect Metamorphosis. (Compare Gen. Morph. ii.

p. 99.) A.A. = Equal wings (fore and hinder wings are the same, or differ but little). A.D. = Unequal wings (fore and hinder wings very different in structure and texture, occasioned by strong differentiation).

CHAPTER XX.

PEDIGREE AND HISTORY OF THE ANIMAL KINGDOM.

III. VERTEBRATE ANIMALS.

The Records of the Creation of Vertebrate Animals (Comparative Anatomy, Embryology, and Palaeontology).-The Natural System of Vertebrate Animals.-The Four Cla.s.ses of Vertebrate Animals, according to Linnaeus and Lamarck.-Their increase to Nine Cla.s.ses.-Main Cla.s.s 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 Ascidiae.-Origin of the Vertebrate Tribe out of the Worm Tribe.-Main Cla.s.s of Single-nostriled, or Round-mouthed Animals (Hag and Lampreys).-Main Cla.s.s of Anamnionate Animals, devoid of Amnion.-Fishes (Primaeval Fish, Cartilaginous Fish, Osseous Fish).-Mud-fish, or Dipneusta.-Sea Dragons, or Halisauria.-Frogs and Salamanders, or Amphibia (Mailed Amphibia, Naked Amphibia).-Main Cla.s.s 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 palaeontological 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 circ.u.mstance, together with the many high interests which, in other respects, ent.i.tle 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 palaeontology, and Bar'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 Muller'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 cla.s.sical 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, a.n.a.logies must be traced to Adaptation, h.o.m.ologies 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 h.o.m.ology by the supposition of a common transmission by inheritance from a single primary form. On the other hand, the striking differences of these h.o.m.ologous 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 palaeontological 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, Linnaeus' four higher cla.s.ses of animals, viz. Mammals, Birds, Amphibious animals, and Fishes, Linnaeus'

two lower cla.s.ses, Insects and Worms, Lamarck contrasted to the Vertebrata as _Invertebrata_, later also called _Evertebrata_.

The division of the Vertebrata into the four cla.s.ses 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 Bar did almost at the same time from the ontogeny of Vertebrata-that Linnaeus'