Animal life as affected by the natural conditions of existence

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olin,anx

Cornell University

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Preface

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It was Jaeger who once said—but I forget where—that enough had been done in the way of philosophising by Darwinists, and that the task that now lay before us was to apply the test of exact investigation to the hypotheses we had laid down. '

I have long been of the same opinion; but it is in fact a thing much easier said than done. It is infinitely easy to form a fanciful idea as to how this or that fact may be hypothetically explained, and very little trouble is needed to imagine some process by which hypothetical fundamental causes—equally fanciful—may have led to the result which has been actually observed. But when we try to prove by experiment that this imaginary process of development is indeed the true and inevitable one, much time and laborious research are indispensable, or we find ourselves wrecked on insurmountable difficulties.

Nevertheless the step must be taken. The popular cant about ‘ Biogenetic principles and the falsification of Ontogenesis—the laws of inheritance at corresponding periods of life, or the correlation of organs—Ontogeny vi PREFACE.

and Phylogeny—variability and heredity ’—is put out of court as useless, for these are merely axiomatic expressions for a sum of identical or correlative phenomena of which the essential nature is in no way revealed by them. They all still await an intrinsic explanation.

It appears to me that of all the properties of the animal organism, Variability is that which may first and most easily be traced by exact investigation to its efficient causes; and, as it is beyond a doubt the subject around which at the present moment the strife of opinions is most violent, it is that which will be most likely to repay the trouble of closer research.

I have endeavoured to facilitate this task, so far as in me lies, by here presenting a general view of those facts and hypotheses which bear upon the subject and are either of universal significance or, from my point of view, appear to offer favourable subjects for experimental treatment. This list, however, makes no pretension to completeness. If only it should give an impulse to research, on however small a scale, so long as it is systematically conducted and thoroughly carried through—if only it should contribute to extend my own convictions as to the uselessness of casual and disconnected observations, I shall have attained my end.

The immediate occasion of the writing of these papers was a call to deliver a course of twelve lectures at the

Lowell Institute at Boston from October to December 1877.

KARL SEMPER,

WwtrzBurG: Sept. 1879.

THE INFLUENCE OF INANIMATE SURROUNDINGS.

Chapter II

FOOD AND ITS INFLUENCE, The Equivalence of Food to Tissue. The Optimum of Food. Stimulants, Monophagousand Polyphagous Animals. Modifications caused by Food and Change of Food. Changes in Structure and in Colour . 3 7 ° ‘ : . F 40 viii CONTENTS.

Chapter III. The Influence Of Light

PAGE

The Difference between Animals and Plants, General Relations between Light and the Vital Activity of Animals; between Light and Sight. Blind Animals in Caves and at great Depths. Phosphorescence. The Chromatic Function in Fishes. Chromatophores. Pigment and Colour 70

Chapter IV. The Influence Of Temperature

Acclimatisation. Mean Temperature no Guide to Classification. Eurythermal and Stenothermal. A Falling Temperature. Experiments on Infusoria. Chill-Coma. Hybernation. “A Rising Temperature. Its Stimulating Effects. Heat-Coma. Optimum of Temperature. Winter and Summer Generations and Larva-forms. Temperature as affecting the Development of Young Animals. A Constant Temperature. Extreme Climates. Acclimatisation of Parrots. Periodicity. Equable Temperature of the Sea. : . . . e . . I0l

Chapter V. The Influence Of Stagnant Water

Water indispensable to every Form of Existence. Chemical Composition of Water; Saltness of Inland Seas. Fresh-water Animals that can live in the Sea. Insects. Worms in Brine Springs. Mollusca. Bopyrus. Polyps. Plateau’s Experiments on Articulata. Beudant’s on Mollusca. Transition from Branchipus to Artemia. Influence of Volume. Experiments on Lymnaea stagnalis. An Unknown Stimulant. Influence of Oxygen or Air in the Water. Respiration a Function of Protoplasm. Organs specially adapted for Respiration. The Skin; Gills; the Intestine; the Air-bladder,

_ Power of enduring Desiccation. Eggs of Apus and Cypris. 140

CONTENTS. 1x

Chapter VI. The Influence Of A Still Atmosphere

Action of the Lungs. Composition of Air. Trachex in Insects, sii Resistance to a Dry Atmosphere. Desert Snails. Effects of a Saturated Atmosphere. Land Leeches. Land Planarians. Mollusca. Accommodation of Water-breathers to breathing Air, Amphibious Fishes (@obiide). Amphibious Mollusca (Ampullaria). Land Crabs (Birgus latro). Transformation of

the Branchial Cavity intoa Lung. 3 : 5 a . 178

Chapter VII. The Influence Of Water In Motion

Presence of Salts in Fresh Water. The Mechanical Factor. Resistance to Currents. Sedentary Animals. Floating and Swimming Animals. Swimming Organs. Clinging Power of Mollusca. Mechanical Effects of Currents. Erosion of Shells. Corals acted on by Currents produced by Crabs, which form Galls, or Funnels. General Structure of Massive Corals (Porites). Process of Growth; Formation of Reefs as affected by Marine Currents . : . A we. 8 ‘i : - 200

CHAPTER VIII. THE INFLUENCE OF WATER IN MOTION (continued)

The Formation of the Coral Reefs of the Pelew Islands. The Views of Dana and of Darwin disputed. Barrier Reefs and Fringing Reefs. General Form and Structure of the Pelew Reefs. Kriangle. Blocks of Dead Coral. Artificial Canal.° Kossol; the Depth of the Reef. Babelthuap; Slope of the Eastern Reef. Submarine Ridge of Elevations. Theory of Subsidence as explaining the Origin of the Pelew Reefs and Islands. Objections. Co-operation of Currents. Sections of the Reef. Geological Structure—partly Volcanic. Evidences of Upheaval. An Attempt to explain the Structure of these Reefs by Upheaval. Possible Objections answered. Pourtalés Plateau in the West Indian Sea. Mixture of Forms and

Species. . ‘ 230 x CONTENTS.

Chapter IX

CURRENTS AS A MEANS OF EXTENDING OR HINDERING THE : DISTRIBUTION OF SPECIES.

PAGE Migration a Condition of Existence in the Lower Animals. Passive Migration. Currents and Winds as Means of Distribution. ‘Free Crossing inevitable in Water. Transport of Small Animals. Transition of Forms in Land Animals; for example, in the Philippines. Dispersal of Infusoria, Wagner’s Theory of Separation. Some Cases of Resemblance between Animals in Remote Provinces. Conspicuous Effects of Winds. Similarity of Fresh-water Mollusca less great than Darwin assumes. American Astacidz. Facility of Transport of Microscopic Bodies. Currents and Winds as limiting the Distribution of Species. Tendency of Currents to Float off the Objects they carry, to each side. Certain Limitations of Distribution in Eastern Asia and Australia. 7 F ‘ . . - 276

Chapter X. A Few Remarks As To The Influence Of Other Conditions Of Existence

Gravitation; Pneumatic Bones. Air in the Body of Fishes. Modifications in the Skull. Pressure of Resistance. Accommodation to Pressure. Possible Effects of Electricity 317

SECTION IIL

THE INFLUENCE OF LIVING SURROUNDINGS.

Chapter XI. The Transforming Influence Of Living Organisms On Animals

Reciprocal Influences. Parasites on Corals. Mollusca. Sponges. Degeneration of Organs in Parasites. Zulima in and on a Holothurian. Hybridisation. In Confinement. Ina Free State of Nature. Its Effects on Colouring and on Specific Characters 330

CONTENTS.

Chapter XII The Selective Influence Of Living Organisms On Animals

Indirect Selection. Competition for Similar Conditions, Relations of the Pursued and the Pursuer. Transition Forms, in Lamellicorn Beetles. Modifications leading to Physiological Function. The Dorsal Eyes of Onchidiwm. Its Defensive Glands. Protection by Imitation or Mimicry; in Insects. Difficulties with regard to Polyps and Fishes. New Instances amorg Univalve Mollusca in the Philippines. False Mimicry in a Worm in the Mediterranean. Caution as necessary as Research . . ‘ é

NoTEs . a % P ‘ xi

PAGE

LIST OF ILLUSTRATIONS.

MAPS.

I, Sketch Map of the Pelew Islands . ‘ é ~ . 235 IL The Atoll of: Kriangle » © 6 8 © 2 + » 240

WOODCUTS. 1. The Internal Structure of a Chameleon, in its natural position: . . . . . . 4 2. Diagram of the Lungs and Civvalation of Birgus latre, the Palm Crab . 5 ‘ f e * ‘ — 5 3. Section of a Bleak . : 7

4. a, The Bone of a Cat with fies ‘Mavncne: habeus b, the “Hous of a Bird, with Air-cavities instead of Marrow-tubes; ce, the Skull (sawn through) of Bucrevs, with Air-cavities in all the Bones . < ‘ . ‘ oe ae 8 5. a a Flying-Fish; b,a Bat . : . . 9 6. The Branchiz, a to c, of Annelida; d, of a Picseel . s = IS 7. The Skull of the Female Dugong . . » 5 + « WT 8. The Casting of the Skin in Reptiles. . . . .. 20

9. Stages of Casting of the Shell of a Crab . . © @ 21 10. Structure of the Foot of the Gecko . . . : ~ » 22 11. Germinal Layers of the Chick (after Kélliker) . . » 80 12. Sacculina carcini and Thompsonia globosa - ee 13. Section of the Gsophagus and Stomach ofa ies : . 66 14. Larus argentatus . . . . . . . » > 60 15. Myopotamus Coype . . ‘ é a . 61

16. Nestor mirabilis x i ‘ P i . aie 162!

xiv

LIS? OF ILLUSTRATIONS.

. Animals in which Chlorophyll Grains have been detected

. Collozoum inerme (Haeckel) 3 - ! . a Og . Section through Sphenopus Seenateigih a ‘ , a, Proteus from the Adelsberg Grotto, reduced ; 3, “Section of:

a Rudimentary Eye . 3 . . Ages

. a, Pinnotheres Holothuria, nat. size; b, asgenenene Waterlung of the Holothuria . : Fi . :

. Zoea-larva of Pinnotheres Holothurie

23. A blind Cymothoe; in fresh water - : ‘ 3 , . Siredon pisciforme, the Mexican Axolotl . ‘

. Section of a Frog’s Skin . é

. Chromatophores of a Frog’s Skin.

. Rossbach’s Curves of Temperature for the ‘ation ‘of the Contractile Vesicles of the Infusoria . . . .

. The Infusoria experimented on by Rossbach . . Desoria glacialis, the Glacier Flea . ; 5 . . Aphis Beccabunge a, Diplozoon paradoxum ; _ Poly sjetiomaniis epee: tein the Bladder of a Frog . . : .

2. Phasma sp., a Wingless Orthopterous Tassel

. Buplectella. eapegillam . ‘ : J , ‘

. Halobates, new sp.

. Pachydrilus, new ne living i in vite Salt Waters ee. Kissingen . Hlaturus vuleanicus F . . Bopyrus ascendens

. An Oyster from the river Garden 4 in Basilan

. Cordy ylophora lacustris .

. a, Branchipus stagnalis ; b, Avtontn ‘eatin - 5 ’ . Transformation of Artemia salina into A. Milhauseniti . . . Four Shells of Lymnaea stagnalis, all of the same Age and

Brood

. Curve of Volume for Ly yneuen apni

. Curve of Time of Growth of Lymnea stagnalis

. A similar Curve altered by the Effects of Temperature

. a, Anabas scandens ; b, a Tadpole ; ce, ayoung Polypterus from the Nile; d, Embryo of a Shark . ‘ . a ‘ . Gills of esihasac C : : ‘ hs 3 . Portion of the Stomach of a Helothuvied 2 r ‘ . Cobitis fossilis . . A ae

Section through the Lung of ‘ie Embryo of a Pig

. Systems of Trachez sketched in the Outline of the Insects . Geonemertes palaensis, a Nemertean; nat. size . <8 . Lalitrus saltator . . : ‘ : - : : ie

. The Branchial Lungs of Ampullaria

. Gecarvinus rusticola, a Land Crab

. a, Lupea; b, Ocypoda

. Univalves that cling to Rocks aim means sae the F Foot . Various Animals that swim by means of Fins

. Creeping Mollusca, 2

j Diagram of Section through Marit: and Navicella, showing

. Pyrosoma gigas

. Onchidium tonganum, nat size .

. The Shells of Philippine Snails

. Amphidromus macitliferus, Sow.

. Lrocomorpha sp.

. Lemnocephala chilensis, Blanchadl : ‘ ea . Two operculated Fresh-water Snails. : : Sra ties . Various Fresh-water Univalves

, a, an Amceba in a Locomotive State; 3, the same, encpetedl «

. Cypris sp. from the Philippines

. Coregonus hiemalis

. A piece of Wood bored by er tes ie ANS, etiie Biel os

. A piece of solid dinestan; aetiaaly bons tron irsland . Part of a Stock of Campanularia, a Hydroid Polyp, with

LIS£ OF ILLUSTRATIONS, the position of the Operculum

. Section of the Shell of Unio (a Fresh-water Mussel) . Shells with living Animals, but injured by Erosion

. The Inhabitants of Coral Galls .

. Seriatopora hystria, with Galls inhabited iy Rapillosaxck CONUS marsupials .

. A Gall on Seriatopora insti, epeneli . Sideropora palmata, with a Gall or Cyst . . Goniastrea Bournoni, M. Edw., with a Funnel in whidh lives a Cryptochirus coralliodytes, Hiller .

. Diagram of the Growth of a Colony of Porites . & Section through Kriangle; b, through Babelthuap a

Aibukit; c, through Pelelew .

land closed pear-shaped Galls, ene a Sea- ome Pyenogonida . a . .

. Heteropsammia Michelini .

. Heterocyathus philippensis :

. Rhizochilus antipathum, Steenstrup . .

. A Specimen of Carcinus menas, from Heligoland, with three

Individuals of the Parasitic Sacculina carcini . *

. Antipathes filix, Pourtales . .

Xvi

. Spiders which mimic Ants < ‘ ‘ ‘

LIST OF ILLUSTRATIONS.

. Section through a calcareous Sponge, showing the simple central Cavity; 4, the Sponge in an uninjured State (after Haeckel) : : : . 5

. Spongia cartilaginea, Esper, half nat, size.

. Spongia cartilaginea, Esper . : ; : .

. Entoconcha mirabilis, Miller . ‘ A ‘ . ‘

. Two undescribed Species of Eulima . . . .

. Cladognathus dorsalis, Evichson . f . 5 . ‘

. Chalcosoma Atlas, from the Philippines . % * .

. Section of Eyes . ‘ . ‘ sae . Section of the Dorsal lives: of - Guiiiainn verr yealadicin F . Periophthalmus Kocleuteri, a F ish which pursues the Onchidiwmn . Development of the Eyes of Onchidium is H . . . Grasshoppers protected by their likeness to Leaves . ‘ a, Doliops sp., mimics b, Pachyrhynchus orbifer; cv, Dileons curculionides, mimics d, Pachyrhynchussp.; c, Scepastus pachyrhynchoides (a Grasshopper), mimics f, Apocyrtus; g, Doliops sp., mimics h, Pachyrhynchus sp.; i, Phoraspis sp. (a Grasshopper), mimics #, a Coccinella 5 a, Rhysota Antonit, mimicked by 3, co pbetdiinniain: ze Helicarion tigrinus, mimicked by d, Xesta Comings Myxicola infundibulum (after Claparéde) . . . ae

PAGE

. 891

395 401

NATURAL CONDITIONS OF EXISTENCE

AS THEY AFFECT

ANIMAL LIFE.

INTRODUCTION.

No one at the present day disputes the fact that the Darwinian theory has exerted an extensive influence not only on the development of the natural sciences, but in other branches of study ; it would be superfluous here to bring forward any proofs of this. It is equally recognised that it is to this influence that modern zoology owes its most essential pretensions to be regarded as of equal estimation with other sciences. But it may be advisable to pause for a moment at the question, In what way is it that this influence has affected zoology? since in this book we have to deal exclusively with this science.

Darwin showed the possibility of discovering the path which nature struck out in order to produce her endless variety of animal forms, and of detecting the means she has employed in her task. Hence first arose those efforts, so natural in the zoologist, to acquire some comprehension of the succession in time of the different types in the animal kingdom, since all who recognise Darwin’s teaching must regard it not as an arbitrary and lawless assemblage of independent species, but, on the contrary, as a great family of organisms of which the individual members, whether living or extinct, are united by a real, and not merely fanciful, bond of close affinity. The search for the natural genealogy of these families of organisms is one of the grandest of the problems propounded to modern zoology by the great English philosopher.

Naturally enough, in this search, zoologists had recourse to those means and methods which were most familiar to them and which had hitherto been at their disposal. Ever since the time of Cuvier they had been accustomed to discriminate between the different forms of animals and to describe the organs which distinguish and separate them, endeavouring at the same time to detect by them the ideal affinities of animal types. But they were more practised in the use of the scalpel and microscope than in availing themselves of the often highly complicated apparatus and methods of the physiologist, and it is only lately, under the influence of Darwin’s views, that they have begun to enquire into the true and natural affinities of animal types by comparing them together as to form, and by studying their mode of origin. Thus itis that the modern study of animal morphology has arisen, commonly divided into comparative anatomy and embryology ; but at the same time, equally under the influence of Darwin, zoologists began to devise genealogical trees for the different groups of the animal kingdom—sometimes for a whole group, sometimes for a subdivision only—in which they attempted to give graphic expression to such knowledge as they supposed they had acquired of the actual processes which, through constant modifications of the most widely different forms, led finally to the development of the human body.

Of course such pedigrees could not be otherwise than of a somewhat doubtful character. In all zoological investigations, as in almost everything else, a certain influence may be detected which may be termed the personal element. Zoologists are not, as mathematicians are, able to set out from certain immutable axioms, and to calculate from them the forms and origination of animal types with mathematical exactitude ; on the contrary, they are forced to deduce all the laws of their science from observations of phenomena. The mode of carrying

ion these observations, moreover, and consequently the answer which nature gives to the questions put to her, depend

MORPHOLOGICAL PEDIGREES. 3 essentially on the individuality of the observer. Zoologists have hitherto been equally little able to avail themselves of any aids to experiment analogous to those which are abundantly open to the chemist and the physicist, to the physiologist and even to the botanist. In this respect, indeed, zoologists are very badly off—worse off than any other class of scientific enquirers—for until lately they were simply directed to interpret the facts presented to them by nature, without being in a position to formulate their own problems or to force nature by any critical experiments to give a distinct answer to them, Hence any pedigree selected by one or another naturalist, and based on-facts derived from animal morphology, could, and can, only avail to represent those ideas as to the affinities of animals which their author in each case conceives to be the only accurate ones; and hence it must necessarily contain a larger or smaller infusion of subjective fancy and unquestionable error. Thus it is not very surprising to learn that a warm dispute has just broken out as to which group of Invertebrate animals is to be regarded as the closest in affinity to the Vertebrate ; nor that the view, hardly of ten years’ standing, that the Vertebrata are allied to and derived from the Ascidians, is combated, not without strong reason, by another—namely, that they are more nearly allied to the Annelida—without either side having hitherto proved itself victorious. Subjective views inevitably play an important part in every scientific application of the facts of animal morphology.

We may, however, rest satisfied with these illustrations, and proceed to the wider question, bow it was possible that nature should have produced such an immense variety of forms as is exbibited in the animal kingdom, without ever losing the thread of affinity which the zoologist seeks to detect in animal structures and to exhibit in his genealogies or systems. Darwin has here supplied us with the answer. He has shown, as it seems to me in the most satisfactory and exhaustive manner, that two properties inherent in organic beings have contributed, in conjunction with other external natural factors, as means to the accomplishment of this end: First, the power in the parents of transmitting to their descendants their essential morphological and physiological characters— or Inheritance, Secondly, the plasticity of the organism, which enables it, by modifications of its original characters, to accommodate itself to the altered conditions of existence in its successive stages of life— or Adaptation.

Hence it directly follows that the problem of the morpho-

Fia. 1.—Internal structure of a Chameleon in its natural Position, to show the lungs, p, and the long air-sacs, s, proceeding from them.

logist is to learn to distinguish such characters as have been perhaps recently developed by Adaptation, from such as have been transmitted by Inheritance through a long series of generations ; for if he should be incapable of making this distinction he will inevitably fall into many gross errors in his attempts to

CHARACTERS OF ADAPTATION. 5 establish the affinities or the genealogy of an animal. Since the first appearance of Darwin's well-known work, this has in

“hid

Fic, 2.—Diagram of the lungs and circulation of Birgus latro, the Palm Crab. I. The lung-vessels indicated within the outline of the animal—a,, o,, a,, the three upper afferent vessels (veins) ; ¢ 7, the efferent vessel (artery): e br, section of the opening of the arterial gill-vessel; h, heart; hb, pericardium. II. Diagram of section of the same, lettered as above —b/, gills or branchize ; «,, the lower afferent vessel (vein) ; J, the lung cavity, showing the pulmonary villi (tufts) on the inner surface of the wall.

fact been the method of modern zoology; and I do not fear contradiction when I say that we have already made considerable advances in the art of discriminating between those typical attributes which have been preserved by transmission through long series of apparently very different and yet nearly allied species, and those characters of adaptation which here and there arise, as it would seem arbitrarily, and beyond a doubt quite independently of the affinities of animals. The absolute necessity of clearly separating these two groups of characters will be made plain by the following illustration of one special case in point. . i

Everyone knows that the lungs of all the higher vertebrate animals are indispensable to their existence ; no mammal, bird, or even reptile, could live long without this breathing apparatus. Similarly constructed and indispensable organs for ‘breathing air _ occur in many Mollusca and in a few Crustaceans. Now, if a zoologist endeavoured to prove that all animals which have organs adapted to respire air must therefore be closely allied, it would hardly be worth while to point out that his attempt must be hopelessly futile. It suffices, with regard to the example I have adduced, to point to the fact that the lungs of the Vertebrata are connected with the intestinal canal and developed from it; while those of Mollusca and Crustaceans (fig. 2) are nothing more than cavities in the side, which have originated from a lateral invagination of the outer skin. But an organ which, like the lungs of mammals and birds, takes its rise from the intestinal canal, can never have originated in a modification of the outer skin, or epidermis. This proves that the lungs of different groups of animals must have originated independently of each other, and that we are justified in regarding them as, in some degree, characters of adaptation.

Another instance. Jt is now universally admitted that the fore limbs of mammals, the wings of birds, and the pectoral fins of fishes are, morphologically, merely modifications of the same organs, namely, the fore or pectoral limbs. They terminate in man in a hand, in the apes in a hand serving also as a foot, in the horse in a foot only, in birds in an organ of flight, in fish in an organ of swimming. In all these cases the function of the limbs is different, although they are morphologically identical. Furthermore, all fins are not morphologically identical, and if we were to attempt. to regard the pectoral fins of

CHARACTERS OF ADAPTATION. 7 fish as identical with those that occur in manimals, birds, and amphibia, because their function is similar, we should fall into serious error. It would, in the same way, be erroneous to explain the different forms of wings as they occur in mammals (bats, insectivora, and rodents) and reptiles (Draco) by referring them to the same type as the wings of birds. Although the purpose of wings is the same in all the animals named, in each individual species the organ is morphologically different; that is to say, it has originated by the adaptation to the same function of parts that have no anatomical relation. And if, from the occurrence of wings or fins in the different groups of. vertebrata, we attempt to deduce a close affinity, as indicated by those

Fia. 3.—Longitudinal section of a Bleak. s, anterior; s’, posterior portion of the air-bladder ; @, esophagus ; J, nir-passage of the air-bladder.

organs, we shall indeed be gravely mistaken. We may, on the contrary, rather infer from the facts adduced that fins or wings have originated simultaneously and independently in the different groups, since different menibers of the body, in themselves not comparable, have, by adaptation to new conditions of existence, become such organs with similar physiological functions.

Thus, in the examples here given, wings, fins, or lungs must not be considered as hereditary characters, but merely as characters of adaptation which, as they have originated independently, are useless for determining the affinities of the animals that possess them. This conclusion is, however, only partially accurate, as shall now be shown.

In order to do this, we will return to the more striking of the examples here given. There can certainly be no doubt of this—that the lungs of mammals have not been developed by modification from those of land snails; we know on the contrary, or will assume, that the lungs of all the Vertebrata are identical, and to be regarded as. modifications of the air-bladders of the bony fishes (fig. 3), although these organs do not serve, or at any rate do not mainly serve, for respiration. On the contrary, fish breathe by their gills. But the lungs of mammals differ remarkably in structure from those of birds, and yet more from

Fic. 4.—a, the bone of a Cat, showing the marrow-tube; 8, that of a Bird with cavities containing air instead of marrow; c, the skull of a Buceros sawn tarough, Air-cavities traverse every patt of thé bone. Dope Ge) rary res oe Se those of the lower reptiles or the amphibia. In these last they are usually simply capacious sacs opening into the mouth by a very short passage (the trachea); in mammals they exhibit a spongy structure, and often a highly complicated arrangement of extremely long air-tubes; in birds also the lungs have a spongy structure, and connected with them there are always numerous air-cavities which lie partly in the cavity of the body and partly, in the form of canals, deep in the bones of the skull (fig. 4) and of the vertebral column, or penetrate to the end of the extremities, forming what are known as pneumatic bones. Now these differences in the structure of the lungs of

ORGANS OF RESPIRATION. 9 the different Vertebrata go hand in hand with other characters which distinguish the groups ; and although we are not justified in founding the genealogies of Vertebrata exclusively on the character of the lungs, we may regard and use them as an indication of affinity, particularly when we see that the deductions from them coincide with conclusions drawn from other facts. In agreement with this we see that the peculiar re

Fic. 5.—a, Flying-fish (Zxccetus), in which the pectoral fins serve, at least partly, for flight ; 6, Bat, with a membrane extending between the phalanges, limbs, and tail.

construction of the lungs which in birds leads to the development of pneumatic bones is an hereditary attribute characteristic in the highest degree of the whole order of birds, and of great systematic value. It distinguishes Birds as contrasted with Mammals and Reptiles, but nevertheless can and must be conceived of as having originated through modification of a simple organ—perhaps a bladder-shaped lung—which may have been proper to the common ancestor of reptiles and birds alike. A similar modification of the lung might thus be found in such true reptiles as approach most nearly to birds; and in fact we see in the Chameleon (fig. 1) that long thin air-sacs, connected with the semi-spongy lungs, are suspended in-the cavity of the body, and may be directly compared with the large abdominal air-sacs which are found in all birds.! It is evident that, by instituting such comparisons as these, we are tacitly ascribing a character to the lungs of the Vertebrata which differs from that we attributed to them when contrasting them with organs of similar physiological function in land snails and land crustaceans. For in the latter case we considered them, and with’ justice, not as a character inherited from the parent form, and as indicating near affinity, but as a character of adaptation, while it is only among the Vertebrata that they are of real value in estimating the degrees of affinity of the different classes. Thus it is evident that in Vertebrata they possess all the significance of hereditary characters, i.e. of parts which may be made use of for investigating the evolution and modification of these classes—or, as may be, orders—one from another, and for establishing such a natural system of the Vertebrata as may indicate their true affinities. The same result is obtained when the different organs of locomotion of the Vertebrata (wings, fins, legs, feet, and hands) are taken into consideration. So long as the comparison is extended to the whole cycle of the Vertebrata, these seem to have the value merely of characters of adaptation. The whale has fins as efficient as those of the sturgeon or the pike, but I doubt whether a zoologist could be found bold enough to attempt to derive the fins of the whale morphologically from those of fishes. It is quite as unlikely that anyone should undertake to prove that the wings of birds or of bats (fig. 5) could have originated by direct modification of the wing-like fins of flying fishes or of the dermal wings, supported on ribs, of the flying reptiles (Draco). With regard to the higher classes of the Vertebrata all these organs are, beyond a doubt, to be considered merely as characters of adaptation, and so valueless for any determination of their affinity. But if we now turn our attention to the same organs within